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EE-VERT
© 2009 The EE-VERT consortium
Project acronym: EE-VERT
Project title: Energy Efficient VEhicles for Road Transport – EE-VERT
Grant Agreement Number: 218598
Programme: Seventh Framework programme, Theme 1.1, Greening
Contract type: Collaborative project
Start date of project: 1 January 2009
Duration: 36 months
Deliverable D2.1.1:
POWER GENERATION REPORT
Authors: Organisation Name
MIRA David Ward
MIRA Bob Simpkin
CRF Carlo D‘Ambrosio
BOSCH Marcus Abele
LEAR Antoni Ferré
FH-J Manuela Midl
FH-J Raul Estrada Vazquez
UPT Ion Boldea
BEE Sever Scridon
VTEC John Simonsson
Reviewers: Organisation Name
VTEC John Simonsson
FH-J Raul Estrada Vazquez
FH-J Hubert Berger
CRF Carlo D‘Ambrosio
Dissemination level: Public
Deliverable type: Report
Work Task Number: WT2.1
Version: 1.0
Due date: 31.12.2009
Actual submission date: 23 December 2009
Date of this Version 23 December 2009
EE-VERT
© 2009 The EE-VERT consortium
Consortium Members
Organisation Abbreviation Country
MIRA Limited MIRA GB
Volvo Technology AB VTEC SE
Centro Ricerche Fiat Società Consortile per Azioni CRF IT
Robert Bosch GmbH Bosch DE
LEAR Corporation Holding Spain SLU Lear ES
Engineering Center Steyr GmbH & Co KG /
MAGNA Powertrain
ECS AT
FH-JOANNEUM Gesellschaft mbH FH-J AT
Universitatea ―Politehnica‖ din Timisoara UPT RO
SC Beespeed Automatizari SRL BEE RO
EE-VERT Deliverable D2.1.1 23 December 2009
Document history
Revisions:
Version Description Planned
date
Actual
date
0.0 Draft document outline – sent to contributors 28/09/2009
0.1 First internal version of deliverable with all contributions 10/12/2009
0.2 Incorporating revisions suggested by John Simonsson 15/09/2009
0.3 Incorporating revisions suggested by Raul Estrada Vazquez 18/12/2009
1.0 First version of deliverable 23/12/2009
A brief summary
Despite improvements in modern vehicles, a considerable amount of energy is still wasted due to the
lack of an overall on-board energy management strategy. Further electrification of auxiliary systems
promises energy and efficiency gains but there is an additional need for a coordinated approach to the
generation, distribution and use of energy.
The objective of work task 2.1 is the development of high efficiency power generation concepts,
including their operational strategies, within EE-VERT‘s system approach. Also the integration of
innovative systems will be studied, such as solar panels or thermo-electric generators to recover
energy from exhaust gases. Energy recovery from waste energy e.g. thermo-electric generators to
recover energy from exhaust gases can make a significant contribution to energy efficiency and CO2
reduction. This is particularly important for the overall system when different energy sources are
combined.
This deliverable reports on the power generation concepts investigated by EE-VERT and needed for
the operation of an advanced electrical power net aimed at generating and reusing energy at a very
high level of efficiency.
EE-VERT Deliverable D2.1.1 23 December 2009
Version 1.0 Page 2
Contents
A BRIEF SUMMARY ..................................................................................................................... 1
1 INTRODUCTION ................................................................................................................... 4
1.1 BACKGROUND .................................................................................................................... 4
1.2 PURPOSE ............................................................................................................................ 5
1.3 SCOPE ................................................................................................................................ 5
1.4 PROCEDURE OF WORK ........................................................................................................ 5
2 REQUIREMENTS .................................................................................................................. 7
2.1 BASIC FUNCTIONAL REQUIREMENTS ................................................................................... 7
2.2 REQUIREMENTS FROM AN ARCHITECTURE POINT OF VIEW................................................... 8
2.3 REQUIREMENTS FROM THE STORAGE SELECTION ................................................................ 9
2.4 SUMMARY - REQUIREMENTS FOR POWER GENERATION COMPONENTS ............................... 11
3 POWER GENERATION CONCEPTS ................................................................................. 14
3.1 GENERATOR ..................................................................................................................... 14
3.1.1 State of the art .............................................................................................................. 14
3.1.2 Two selected solutions for the future ............................................................................. 21
3.1.3 Characterization of the proposed solutions ................................................................... 22
3.1.4 Decision ....................................................................................................................... 25
3.1.5 Selected Electric Alternator: IPM- Lundell configuration ............................................. 25
3.2 SOLAR PANELS ................................................................................................................. 28
3.2.1 Current Vehicle Applications for PV Panels ................................................................. 28
3.2.2 Solar Panel Performance ............................................................................................. 29
3.2.3 Types of solar cell ........................................................................................................ 35
3.2.4 Panel Electrical Characteristics ................................................................................... 36
3.2.5 Possible Sizes of PV Panel on the Alfa Romeo 159 Reference Car ................................ 40
3.2.6 Possible use of PV panels within EE-VERT .................................................................. 41
3.2.7 Energy Saving Calculation ........................................................................................... 42
3.3 WASTE HEAT RECOVERY .................................................................................................. 44
3.3.1 Technology overview .................................................................................................... 44
3.3.2 On vehicle heat recovery management .......................................................................... 48
3.3.3 Current vehicle applications for TEG ........................................................................... 49
3.3.4 TEG Performance ........................................................................................................ 51
3.3.5 DC/DC requirements for TEG management .................................................................. 53
3.3.6 Possible use of TEG within EE-VERT ........................................................................... 54
3.3.7 TEG energy saving calculation ..................................................................................... 54
3.4 DC/DC CONVERTER ......................................................................................................... 59
3.4.1 Basic architectures ....................................................................................................... 59
3.4.2 Baseline ....................................................................................................................... 62
3.4.3 MIPEC (Multi-input Power Electronic Converter) ........................................................ 66
3.4.4 MIPEC working operation............................................................................................ 70
3.5 CONCLUSIONS .................................................................................................................. 74
4 SYSTEM INTEGRATION AND MANAGEMENT ............................................................. 76
4.1 SYSTEM CONCEPT............................................................................................................. 76
4.1.1 Basic EE-VERT approach ............................................................................................ 76
EE-VERT Deliverable D2.1.1 23 December 2009
Version 1.0 Page 3
4.1.2 EE-VERT architecture with the power generation components ..................................... 77
4.2 OPERATION STRATEGY ..................................................................................................... 78
4.3 LINK TO HYBRID VEHICLES ............................................................................................... 78
4.4 IMPACT ON SAFETY RELEVANT APPLICATIONS .................................................................. 79
4.5 IMPACT ON COMMERCIAL VEHICLES ................................................................................. 81
5 CONCLUSIONS AND OUTLOOK ...................................................................................... 86
REFERENCES .............................................................................................................................. 87
EE-VERT Deliverable D2.1.1 23 December 2009
Version 1.0 Page 4
1 INTRODUCTION
1.1 Background
The electrical system in conventional vehicles consists of a single electrical power bus, a generator
mechanically linked to the engine, an energy storage device (usually a 12 V lead acid battery) and
many different loads. In present-day vehicles, even in those regarded as the state-of-the-art electrical
power is generated with little knowledge of the actual loads. In general, the energy required for
auxiliary systems (e.g. power steering, water pump, oil pump) is generated and consumed
continuously, regardless of demand. Similarly, the energy generation for the vehicle‘s electrical
system operates continuously.
Currently the generator is responsible for the conversion of mechanical energy from the vehicle‘s
engine to electrical energy for the power distribution network. Due to the current operating conditions
the generator can only be optimised selectively for special operating points. Today it has to fulfil its
task over the whole range of engine speed with only a low average efficiency. With a new machine
concept and enhanced control and regulation methods a significant improvement of efficiency is
possible. These enhanced control and regulation methods have to be integrated in the vehicle overall
system approach.
Auxiliaries 5%
Rolling friction 10%
Aerodynamic resistance 7%
Drive train 2%
Brake energy 8%
Fu
el
de
ma
nd
10
0%
Exhaust gases
Cooling
system,
thermal
radiation
22%
40%
Idling (6%)
Drive
(94%)
Idling losses 5%
Thermal
energy
(62%)
Driving
(25%)Driving resistance (17%)
Auxiliaries during Idling 1%
Mechanical energy
(32%)
Fig. 1.1.1 Energy flow diagram of a conventional vehicle with diesel engine [4]
Fig. 1.1.1 shows the energy flow diagram with average values for a medium-class passenger car with
a diesel engine on the NEDC. The combustion engine converts less than one third of the chemical
energy supplied by the fuel into mechanical energy which can be used to propel the vehicle and
overcome frictional losses and aerodynamic drag as well as drive the auxiliaries such as pumps and
the generator. The predominant part, more than two-thirds of the chemical energy, is converted into
heat and will be released to the environment via the engine cooling system and the exhaust system.
Only about 17 % of the chemical energy is used to overcome the actual driving resistances.
Despite improvements in modern vehicles, a considerable amount of energy is still wasted due to the
lack of an overall on-board energy management strategy. Further electrification of auxiliary systems
promises energy and efficiency gains but there is an additional need for a coordinated approach to the
generation, distribution and use of energy.
EE-VERT Deliverable D2.1.1 23 December 2009
Version 1.0 Page 5
1.2 Purpose
The objective of work task 2.1 is the development of high efficiency power generation concepts,
including their operational strategies, within EE-VERT‘s system approach. Also the integration of
innovative systems will be studied, such as solar panels or thermo-electric generators to recover
energy from exhaust gases. Energy recovery from waste energy e.g. thermo-electric generators to
recover energy from exhaust gases can make a significant contribution to energy efficiency and CO2
reduction. Nevertheless this is strongly dependent on energy management and the mission profile.
Additionally, the link to hybrid vehicles will be studied, since power generation technologies are also
important for hybrid vehicles. Since hybrid vehicles use still combustion engines, the technology of
energy recovery from waste energy is also applicable to hybrid vehicles. Hence, the application of
thermo-electric generators or other EE-VERT power generation concepts for hybrid vehicles will also
be investigated within WT2.1. A special focus will also be given to the possible application of EE-
VERT‘s smart power electronic sub-systems for hybrid vehicles. They could lead to cost advantages
for hybrid vehicles.
Also the link to safety relevant applications will be studied. For safety-relevant applications predictive
diagnosis algorithms will be considered along with reaching a high quality and certainty of service for
power systems.
Finally, the link to commercial vehicle applications will be studied.
1.3 Scope
The central EE-VERT approach is the electrification of auxiliary systems, and supplying their energy
through high efficiency electrical power generation and reuse of waste energy. The EE-VERT concept
considers the combination of several different approaches to energy saving within an overall energy
management strategy (thermal and electrical). The approaches include:
Greater efficiency in energy generation with a new concept for the electrical generator and an
optimised operation strategy;
Energy recovery from waste energy such as waste heat recovery or an optimised braking
energy recuperation with a temporarily increased generator output power with up to 8kW at a
higher voltage level;
Energy scavenging from unused and new sources of energy, for example the use of solar
cells.
The increased electrification of auxiliary systems with optimised operation promises efficiency gains.
But this can only be accomplished if the energy generation and distribution is optimised and adapted
to the current driving conditions and the power demands.
1.4 Procedure of work
Fig. 1.4.1 shows the procedure of work. To support the work a list of components (LoC) has been
generated which was used to identify the partner contributions to each power generation component.
The intention was to have a list with all of the components and their development status. On the basis
EE-VERT Deliverable D2.1.1 23 December 2009
Version 1.0 Page 6
of this spreadsheet the working groups and the responsible partners have been determined.
Furthermore, the activities for every component within the consortium have been determined.
Collection of
partner
contributions
Definition of
working
groups
Definition of
responsible
partners
Selection of
components
for EE-VERT
Fig. 1.4.1 Procedure of work
EE-VERT develops a system that will optimise the energy performance and efficiency of
conventional vehicles in general. But special attention will be given to validate the results on a
reference car, an Alfa Romeo 159 1.9JTD. Hence, the LoC contains information on whether a
component is available in the reference car and how it is realised (mechanical or electrical or if there
are any special implementations we have to consider). Furthermore, the partner contributions are
listed and what activities can be undertaken within the project consortium (Fig. 1.4.2).
Power generation Power generation Power generation Voltage converter System control
Optimised generator with
recuperation capabilitySolar panels Reuse of thermal energy DC/DC converter / power interface ECU or dSpace
Reference car
Available in the
reference car
Y= Yes
N= NOY N N N
State of the art in
the reference car
Fixed voltage regulation (14 V).Fixed
LRC time (3s). LRC time cut off =
3000 RPM.
Activities Stage
Concept study C x x x x x
Simulation S x x x x x
Development D x - - x x
Prototyping P x x - x x
Test bench T x x - x x
Vehicle V x - - x x
Partner
contributionsPM
MIRA 4 C C C C
CRF 30 C,P,V - C C,P,V
BOSCH 49 C,S,D,P,T,V - - C,S C, T, V
LEAR 14 - C,S,P,T C, S C,S,D,P,T,V
UPT 49 C,S,D,P - - C,S,D,P
Bee 13 C,S,D,P C,S,D,P
Working groups
Responsibel
partnerBosch Mira CRF Lear CRF, Lear
Working group Mira, CRF, Bosch, UPT, Bee Mira, Lear Mira, CRF, Lear Mira, CRF, Bosch, Lear, UPT, Bee CRF, Bosch, Lear
Working groups and responsible partners
WT2.1 Power generation
Role of beneficiaries and contributions to activities
Classification
Picture
Component
What we are able to do within the project consortium
Fig. 1.4.2 List of components with the partner contributions to each power generation component
EE-VERT Deliverable D2.1.1 23 December 2009
Version 1.0 Page 7
2 REQUIREMENTS
2.1 Basic functional requirements
In conventional vehicles a 14 V low voltage power net distributes the supply voltage for the auxiliary
systems and loads via direct connections. The generator contains a voltage regulator to obtain a
constant voltage for charging the vehicle battery, so the full potential of this equipment is not utilised
for most of the time. Also, some systems e.g. ECUs require higher voltage stability to function
properly and regulate this internally in the ECU. Furthermore, there is no coordination between these
systems with respect to current consumption.
ECU #1AlternatorR Battery
Load #1BMS
Voltage
Stabilizer
ECU #N
Voltage
Stabilizer
Load #NStarter
CommunicationCommunication NetworkNetwork
14 V 14 V PowernetPowernet
R Alternator regulator
BMS Battery Monitoring System
Fig. 2.1.1 Diagram of a standard power net architecture [3]
Fig. 2.1.1 shows the diagram of a standard power net architecture [3]. As discussed in D1.3.1 [3], the
EE-VERT concept is focused on introducing new architectures for conventional vehicles with better
energy efficiency both at generation and at consumption stages. Functional requirements for this
improved electric architecture within a conventional vehicle, i.e. maintaining the low voltage power
net, are the following:
assure the same degree of functionality and availability of the standard power net;
provide a stable 14 V power net, protected against load dump and similar glitches
(including cranking) to allow the hardware requirements and design of ECUs to be
simplified and lead to an ECU cost reduction;
improve the efficiency of electrical power generation;
add brake energy recovery up through the generator to reduce the generator mechanical
power request to the engine; currently there is some rotation of the generator during
motor braking but with only a small amount of benefit;
include energy recuperation from other sources;
improve electrical energy storage (charge/discharge) capability (both in size and peak
demand);
convert mechanically driven auxiliaries to electrically driven for better fuel economy, by
using recovered energy and by improved control of them;
EE-VERT Deliverable D2.1.1 23 December 2009
Version 1.0 Page 8
introduce an overall electrical energy management approach for power net management
with the potential for temporarily switching off systems that are not necessary at that
specific time;
allow on the easy addition of a start and stop system.
2.2 Requirements from an architecture point of view
The architectures overview in D1.3.1 [3] has shown some of the possible solutions and has
highlighted critical issues to be considered. Particular attention must be paid to:
the energy flow for engine cranking;
the stability of the 14 V power net voltage in every vehicle condition;
the efficiency of energy generation and energy recovery;
the availability of suitable energy storage for energy recovery.
As proposed in §1.1 of D1.3.1 [3] the EE-VERT concept for power generation is based on two main
assumptions:
Possibility of using multiple power sources: To improve the overall efficiency of the
vehicle, new and improved generation sources may be introduced. In all cars, alternators with
braking recuperation capabilities will be incorporated. Also, other sources may be (optionally)
introduced including waste heat recovery, solar cells and grid connection. The characteristics
of these sources are highly variable: some of them may be available at any time, but with
varying efficiency depending on operating conditions, others are only available under certain
conditions. For instance, if a grid connection is installed in the vehicle it will be available
only when the car is parked in the vicinity of a charging point.
Generation is decoupled from the conventional power net for consumers. Instead, an
energy converter device is introduced. This energy converter device is a multiple-input power
electronics converter (MIPEC). In this way, each generation device can be used in an efficient
manner because each source may be conditioned for an optimized power output. Also, MIPEC
gives more freedom to implement management strategies for saving energy during the vehicle
operation by avoiding current consumption by systems that are not in active use at the time.
It is also necessary to have a storage element with high power / high energy capabilities in order to
store the energy produced when it can be generated with low energy losses / high efficiency. The
stored energy should be used when generation is not recommended due to low efficiency. Due to the
unbalancing of power generation capabilities, it is recommended that the storage element is connected
directly to the output of the generator, as discussed in D1.3.1.
In order to store this energy, two possibilities have been considered: Lithium-Ion battery and
ultracapacitor. If a battery is used, it could be more beneficial to regulate the generator output current
(externally controlled current level) to charge the battery more optimally, within certain fixed limits,
of course, for the voltage level. Furthermore, it is possible to provide a high quality supply voltage for
the more sensitive systems by means of a switch between the low voltage node and the starter and the
starter battery. The switch is opened during cranking. This gives the possibility to save some energy
EE-VERT Deliverable D2.1.1 23 December 2009
Version 1.0 Page 9
and expensive protection components on each single device. The resulting general architecture is
shown in Fig. 2.1.1. A detailed explanation is given in section 4.1 of this report.
Fig. 2.2.1 EE-VERT architecture
2.3 Requirements from the storage selection
Storage device
The recommended general EE-VERT architecture is shown in Fig. 2.2.1. One important question is
which high voltage storage device type to use. With an ultracapacitor the system has to start charging
on a low voltage level and increase the voltage during charging of the ultracapacitor. At the beginning
of a braking phase the car has a high velocity and therefore a high kinetic energy but the generator has
to start on a low voltage level. So the power from the generator is limited due to the low voltage. At
the end of the braking phase the ultracapacitor is already charged and works on a higher voltage level
but the kinetic energy of the car is lower and restricted.
If a battery is used instead of an ultracapacitor the system can have a higher voltage level during the
whole braking phase and therefore is able to receive more energy from the generator. Furthermore, a
battery allows the supply of electrified auxiliaries when the vehicle is stationary enabling the engine
to be turned off. Hence, a battery on the higher voltage level is presumably the better choice. But a Li-
Ion battery has a higher weight and with an aimed recuperation power of 8 kW it has to be able to
tolerate a charging current of up to 200 A or even more (dependent on the system voltage level).
Hence, current Li-Ion battery technology was reviewed to see if it is able to fulfil the given
requirements. Specific questions included:
Which system voltage level on the generator side is adequate?
Can the Li-Ion technology provide a lifetime of up to 5 years under the boundary conditions?
Is the Li-Ion battery able to handle a charging current of up to 200 A (8 kW charging power)?
What is the weight and required space for the necessary Li-Ion battery?
To clarify these questions it was agreed during the plenary meeting in Graz in June 2009 to convene a
task force to investigate specific aspects of the architecture, especially the storage device to be used
(Li-Ion battery or ultracapacitor) and the operational voltage range of the higher-voltage bus. The task
force comprised MIRA, VTEC, Lear, CRF and Bosch.
Storage
Alternative
energy
sources
Load # 1
Volt . stab .
Load # X + 1
Load # X
Volt . stab .
Load # N S
Lead acid
Battery High
Power
Loads G
Low voltage power net
DC/DC
converter
High voltage power net
EE-VERT Deliverable D2.1.1 23 December 2009
Version 1.0 Page 10
Key constraints on the architecture included:
The ability to permit the estimated maximum recuperation of up to 8 kW on each recuperation
event in NEDC (though the effect on drivability must be considered);
The storage device must last a comparable time to the life of the vehicle (typically 8–10 years,
but at least 5 years);
The voltage level in the ―power‖ bus must be within ―safe‖ limits for human working (e.g.
Low Voltage Directive < 50 V AC, < 75 V DC; previous ―42 V‖ specifications < 60 V DC;
and [19]).
Main recommendation
The main recommendations from the task force activities are to use a Lithium-Ion battery operating at
40 V DC (nominal) with a capacity of 64 Ah for the main solution. The main reasons were:
Most flexible solution: 40 V 64 Ah Li-Ion ―power‖ battery; 40 V is a good compromise
between power capability and safety (<60 V);
Charge current capability: 64 Ah are necessary with one currently available Li-Ion technology
to charge a current of 200 A;
Permits electrified auxiliary operation during stop phase, for instance electrified air-
conditioning;
Possible downsizing of 12 V ―energy‖ battery, if a voltage modified starter is connected to the
high voltage level and not the 14 V power net;
Present-day mass and size of Li-Ion pack is an acknowledged issue, but this is expected to
improve due to the impetus of current of research programmes into battery technology.
Maximum recuperation power and voltage level
With an aimed recuperation power of maximum 8 kW the storage device has to be able to tolerate a
charging current of up to 200 A. The proposed 40 V Li-Ion battery is based on the manufacturer
LiFeBatt 8 Ah lithium iron phosphate cells with 12 cells in series giving a nominal voltage of 39.6 V
(3.3 V/cell) and 8 cells in parallel to accept a maximum charge rate of 200 A (25 A/cell). This results
in a maximum charge power of 8.8 kW and a 64 Ah capacity. The maximum charging voltage is 3.65
V/cell, giving a maximum battery charging voltage of 43.8 V. The battery includes cell balancing
electronics that operate during charging. Warning signals are available to the control system: High
voltage 3.85 V and low voltage 2.4 V as well as temperature warnings. The high and low warning
signals can be used to provide an outer loop control. When the high voltage signal is triggered the
charging current should be limited to 1 A by the system management and the generator regulator.
Lifetime of the battery
The ―high‖ capacity leads to an acceptable lifetime for the battery. With charging and discharging
calculations it has been estimated that the lifetime of the battery in EE-VERT‘s recuperation approach
is about 8.5 years for 15000 kilometres travelled per year and 6.4 years for 20000 kilometres travelled
per year respectively. More information on this is reported in the quarterly report for quarter 3 of
2009. Since the intended lifetime is at least 5 years – which comes from the comparable lifetime
requirements of lead acid batteries in conventional passenger cars – the 40 V DC 64 Ah battery is
sufficient and offers an additional optimisation potential for downsizing the 12 V battery.
Weight and required space
EE-VERT Deliverable D2.1.1 23 December 2009
Version 1.0 Page 11
The physical size is approximately 600x400x210 mm and a weight of around 30 kg. With CRF it has
been clarified that it is possible to integrate a battery with this dimensions into the demonstrator car.
Baseline solution
The main recommendation for EE-VERT is to use a Li-Ion battery due to the reasons above.
Nevertheless an ultracapacitor could be used in a baseline architecture solution, if only brake energy
recuperation is required. In this case it is not possible to store energy from different sources over a
longer period due to the low energy level of an ultracapacitor. Note that the 12 V lead-acid battery
which is still in the system could not be used to store this energy because the operational strategy of
this battery is to keep the state of charge always near to 100 %. With the low energy level it is also not
possible to supply electrified auxiliaries during stand-still phases. Hence, an ultracapacitor can only
fulfil the requirements for a baseline braking recuperation solution.
Subchapter 2.3 provides only a summary of the task force results. More information is reported in the
EE-VERT report for quarter 3 of 2009. As a consequence of the task force results, some important
component requirements have been defined. They are summarised in the following section.
2.4 Summary - Requirements for power generation components
This subchapter summarises the requirements identified for the power generation components. They
have been derived from sections 2.1 – 2.3. These requirements are necessary to develop adequate
power generation solutions. The main further activities in EE-VERT and the following basic
requirements are based on the architecture in Fig. 2.2.1.
Boundary requirements
power generation is decoupled from the conventional power net;
inputs from multiple power sources (braking energy recuperation, solar cells, waste heat
recovery);
voltage level below 60 V direct current in order to avoid additional safety means for personal
safety [19];
deployment of a high power storage device – recommendation is a Li-Ion battery;
high braking energy recuperation power generation with a maximum of between 4 and 8 kW;
Li-Ion battery
The recommended Li-Ion battery has the following characteristics:
voltage level of 40 V (nominal voltage of 39.6 V – due to the number of cells);
charging power up to 8 kW that corresponds to 200 A at 40 V;
connected directly to the output of the generator;
lifetime should be higher than 5 years;
dimensions have to allow the integration in the vehicle;
warning signals: low cell voltage level = 2.4 V; high voltage level = 3.85 V;
DC/DC converter
decoupling power generation from conventional power net;
provide a stable 14 V voltage for the conventional consumers;
output power on 14 V must be the same as the current generator output power on 14 V,
because the 14 V loads will not be modified in EE-VERT;
EE-VERT Deliverable D2.1.1 23 December 2009
Version 1.0 Page 12
multi input port to link a generator, a solar panel and a thermal generator to the power supply;
the efficiency should as high as possible in the main operation points;
lifetime = vehicle lifetime;
The main challenge is to design a unit that is able to link several different voltage levels of the power
net and accommodate inputs from a generator, a solar panel and a thermal generator, whilst delivering
high efficiency. Hence, a multi-input power converter (MIPEC) is proposed.
Generator
Since a Lithium-Ion battery is recommended in the EE-VERT architecture a well regulated voltage
and current respectively is required for charging the battery. Key points for the generator:
at 40 V the maximum efficiency should be as high as possible but greater than 70 % at 3000-
4000 rpm (larger than a conventional generator);
the maximum output power should be 8 kW at 8000 rpm (using a 3:1 drive ratio);
at 35 V the maximum efficiency should be as high as possible but greater than 75 % with a
maximum output of 7.0 kW (in case the battery has a low state of charge level);
at 47 V the maximum output power should be 9 kW;
voltage range has to be between 28.8 V (lowest cell voltage 2.4 V and 12 cells in series, for
instance after a long stand-still period) and 43.8 V (highest cell voltage 3.65 V and 12 cells in
series);
interface to send control signals to and from the generator;
dimensions that allow integration into a vehicle;
lifetime = vehicle lifetime;
If an ultracapacitor is used, the generator would not have to provide a constant voltage. Instead the
generator would have to provide a voltage with a range from 0 V to the maximum charging voltage.
Solar panels
Solar panels, consisting of interlinked solar cells, are able to generate electricity from sunlight. To
maximise the power output high efficiency cells should be selected and the largest surface area of
panel used. The output will fall with temperature of the panel and if any of the cells are shaded or
covered by dirt.
the roof is the most suitable area where approximately 1m2 is available;
the typical maximum power output that can be expected is 150 W at a voltage of 20 V and a
current of 10 A for instance; but this is quite flexible and can be decided by how many cells
are series and parallel coupled; the voltage level is selectable and should be chosen to be less
than 60 V;
Waste heat recovery
Thermo electric devices (TEGs) are able to generate electricity from heat. Also other solutions like
steam turbines are possible to convert heat energy to electrical energy.
EE-VERT Deliverable D2.1.1 23 December 2009
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DEVICE VOLTAGE CURRENT (PEAK) POWER (PEAK)
Generator (for Li-Ion battery architecture) 28 – 44 V 200 A 4 – 8 kW
Generator (for ultracapacitor architecture) 0 – <60 V 200 A 4 – 8 kW
DC/DC converter
Input: see
generator
Output: see
lead-acid
battery
See generator See generator
Waste heat recovery <60 V
(e.g. 0 – 20 V)
Power / Voltage
(e.g. 35 A) 750 W
Solar Cell <60 V
(e.g. 0 – 20 V)
Power / Voltage
(e.g. 10 A) 200 W
Table 2.4.1: Characteristics of power sources and storage elements for near-future conventional
electrical vehicle systems
Table 2.4.1 gives a summary of the characteristics identified for power sources and storage elements
of near-future conventional electrical vehicle systems. More information is to be found in chapter 3.
The next chapter describes the development of adequate power generation concepts for the different
energy sources and the development of the DC/DC converter.
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3 POWER GENERATION CONCEPTS
3.1 Generator
3.1.1 State of the art
Motivation
The present feasibility study is prompted by:
the larger electrical power demand over the entire speed range, on automobiles;
the low efficiency of electrical energy production on automobiles is no longer acceptable as
the power level increases;
the need to reduce energy (fuel) consumption and pollution from automobiles by intelligent
strategies; for instance to store and reuse vehicle braking energy.
Scope
The scope of the feasibility study is to investigate competitive electric alternators (with their control)
capable of higher power at higher efficiency for moderate cost and weight additions.
Introduction
The new electric accessories on automobiles - such as electric drives for air conditioning power
assisted steering, electrically heated catalytic converters, various ventilators, fuel pump, electric
brakes, electric throttle - require increased average (and peak) electrical power from the on board
alternator Fig 3.1.1 [7]. There is also a demand for the introduction of innovative power electronics
for all these electric loads to secure intelligent energy use. The emerging 40 V / 14 V bus is one way
to reduce power electronics costs for the growing electric power loads and enable more energy
retrieval from regenerative braking.
Fig. 3.1.1 Average automobile electrical power;
Fig. 3.1.1 shows the average automobile electrical power since 1970. But note that the average
electrical power consumption of a typical car on the NEDC is between 200 W and 350 W because
most of the electrical loads are turned OFF. Fig. 3.1.2 shows the typical (existing) Lundell alternator.
Fig. 3.1.3 gives the performance requirements and limitations of the Lundell alternator at 14 V.
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Fig. 3.1.2 Typical (existing) Lundell alternator
Fig. 3.1.3 Performance requirements and limitations of the Lundell alternator at 14 V
For the 40 V the Lundell generator with switched mode rectifier (SMR) – boost DC-DC converter
with high MOSFET or 3 MOSFET SMR within the existing power diode rectifier – is shown to
almost double the power produced at high speeds and 14 V, with 6-7 % more efficiency [9, 10], at 40
V, but also, at idle engine speed about 15 % more power is produced with the same losses
(efficiency), [9, 10]. However, the additional costs of power electronics are likely to more than double
the costs of the existing alternator to meet the scope, but with twice the power (at high speeds).
Fig. 3.1.4 The system structure/Lundell alternator with 3-MOSFET SMR
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Fig. 3.1.4 – 3.1.8 show the performance improvement of Lundell alternator with 3 MOSFET SMR
and independent control of MOSFETs:
Fig 3.1.4: The system structure;
Fig 3.1.5: The principle of 3 MOSFET SMR operation;
Fig 3.1.6: Modulation patterns for the MOSFETs;
Fig 3.1.7: Power enhancement versus speed;
Fig 3.1.8: Power enhancement and current increase at idle speed.
Fig. 3.1.5 The principle of 3 MOSFET SMR operation (Vag-phase to ground voltage)
Fig. 3.1.6 Modulation patterns for the MOSFETs
Fig. 3.1.7 Power enhancement versus speed/Lundell alternator
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Fig. 3.1.8 Power enhancement and current increase at idle speed for Lundell alternator
To summarize the 3 MOSFET SMR solution Fig. 3.1.4 – 3.1.8 [9] illustrate key results on a small
power alternator with 37sR m , 120sL H , ( ) 10,716sV RMS V , at 1 180f Hz , field
current 3.6Fi A , operating at 14 VDC power bus.
Though the power is doubled at high speeds, the increase in power at idle speed, within the
thermal limit, is still small, 10-12 %.
The same solution (SMR with 3 MOSFETs) has been applied to the permanent magnet –
reluctance synchronous machine (PM-RSM) and a 66 % increase of power at idle speed was claimed
[12] from idle engine speed. A summary of the results is shown in Fig. 3.1.9 – 3.1.12. The Figures
show the PM-RSM with 3 MOSFET SMR plus diode rectifier control as an automotive alternator:
Fig 3.1.9: The rotor topology and emf waveform;
Fig 3.1.10: Power enhancement with engine speed (3:1 transmission ratio);
Fig 3.1.11: Efficiency vs. speed;
Fig 3.1.12: Power versus id, iq currents at 1800 rpm alternator speed (600 rpm, idle engine
speed) – F is the best operation point [12]: SMR is here the single MOSFET device after the
diode rectifier.
Fig. 3.1.9 The rotor topology and emf waveform for PM-RSM
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Fig. 3.1.10 Power enhancement with engine speed (3:1 transmission ratio)/PM-RSM
Fig. 3.1.11 Efficiency vs. speed for PM-RSM
Fig. 3.1.12 Power versus id, iq currents at 1800 rpm for PM-RSM
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The results seem to suggest that the PM-RSM produces the required 66 % power enhancement from
the idle speed in contrast to the 3 MOSFET SMR Lundell alternator. However, we should mention
that the PM-RSM, even if designed with 150 % emf (at maximum speed), still remains uncontrollable
at high speeds, when emf > Vdc, when the DC-DC converter is faulty (off). So, in fact, a single switch
DC-DC converter (1 switch SMR) is required for load control (and dumping) at high speeds.
If we were to use only the single switch SMR, the maximum power at idle speed will be of 2.6 kW
(point C in Fig. 3.1.12) and not 4.5 kW as for the 3 MOSFET SMR (points G or F in Fig. 3.1.12).
Consequently the power increase at idle engine speed with single switch SMR is again only 10-15 %
as for the Lundell machine with 3 MOSFET SMR.
So in fact, to retain the large (66 %) power increase at idle speed, with IPMSM, we need the 3
MOSFET SMR, operational up to 66 % of maximum speed; above that speed load control has to be
achieved through a one switch SMR.
Fig. 3.1.13 Proposed full speed range controllable alternator system with PM-RSM
Unfortunately 4 MOSFETs are required and at high speeds most part of the short-circuit current losses
remain for zero load. The 4 MOSFET switches use simplified control circuitry and sensorless
simplified control hardware/software, but still their cost is substantial.
Other existing improvements on Lundell generator
Use thin sheet copper wire in the field coil with an on-rotor DC converter to boost the field
current to the level needed by the smaller number of turns (sheets) per coil. As the space
filling factor increases from 0.6 to 0.74, for the same rotor temperature, 10-15 % more air gap
flux density is obtainable; consequently 10-15 % more power at idle speed is expected. [11],
provided magnetic saturation is not too heavy.
Also the improvement of filling factor in the stator slots will result in lower copper losses and
better efficiency.
An initial proposal to place PMs on the ―claw poles‖ was not followed through, despite of the
potential advantages evident at low speeds mainly due to the reduction of stator inductance.
Other proposed alternator solutions:
Heteropolar hybrid (DC excited plus PMs) rotor synchronous generator [14, 15]; all
heteropolar DC. excited solutions, with PMs along same (d) axis imply a few times larger
rotor copper losses which reduce the efficiency notably though the machine reactance is
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decreased; as the Lundell machine uses 2p=12, 14, 16, 18 poles, its size is notably smaller
than all smaller number of poles heteropolar rotor synchronous generator (SG) solutions.
Increasing the number of heteropolar rotor DC excited poles leads to further rotor copper
losses per Nm of torque. So 4-6 poles should be a limit.
Induction generators with inverter-controlled parallel capacitors (or full inverter) for machine
excitation have been proposed but shown to be less practical than the Lundell machine system
in costs, with an only incremental increase in efficiency [16].
Switched reluctance generators have also been proposed for the scope but they are totally
power electronics dependent; they have been found notably costlier than the Lundell
generator system, while offering only an incremental increase in efficiency [17].
Unless the alternator is used also as a starter and torque assistant for the ICE, when a full
power PWM converter is required, we are stuck with DC excited or interclaw permanent
magnet (IPM) rotor synchronous machine (PM-RSM) with diode rectifier and switched mode
rectifier to control the alternator power output at reasonable costs.
Improving further on Lundell alternator is a first choice in this case as the technology is there
and at low cost and volume/power, burdened still by the somewhat reduced (but still up to 70
%) efficiency up to 6000 rpm alternator speed (3:1 belt transmission ratio ) in up to date
versions.
Up to date Lundell alternator performance
Fig. 3.1.14 shows an exemplary state-of-the-art Lundell alternator (Bosch H8 Li-X Series, 2.5 kW,
115/180 V at 1800/6000 rpm generator speed, 3/1 transmission, (maximum generator speed 18.000
rpm) 2p=16, air gap=0.3 mm, stack length: 0.037 m, Dis=106 mm, Dos=143.2 mm, weight: 7.1 kg)
performance with speed.
Fig. 3.1.14 Lundell alternator performance (Bosch H8-Li-x Series)
It should be noticed that the efficiency decreases drastically above alternator speed 6000 rpm and up
to 18000 rpm (6000 rpm-maximum engine speed), but it is about 70 % (max) below 6000 rpm. This is
caused mainly by large core losses due to flux density space harmonics and large stator current along
axis d to keep the unity power factor condition as imposed by the diode rectifier, and, perhaps mainly,
due to inevitably increased skin effect in the stator coils.
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3.1.2 Two selected solutions for the future
To try to meet the higher power levels required by the future automobiles, with both a 14 V and a 40
V power bus, with lower losses and reasonable initial costs (Euro/Watt) the conclusion of the
thorough investigation of the existing (Lundell) and proposed car alternator systems, that two
solutions stand out as the most competitive (and practical).
1. Hybrid (PM) claw pole (Lundell) alternator
2. PM-RSM alternator with 4 MOSFET SMR control
In view of the above, the two proposed solutions are illustrated in Fig. 3.1.15.
a) b)
Fig. 3.1.15 Proposed alternator solutions: a) Hybrid (surface PM) claw pole alternator; b) PM-RSM
(Ld/Lq>3)
16 pole hybrid (PM) Lundell alternator rotor
q=1 slot/pole/phase (48 slots), air gap: 0.3 mm
Improved fill factor stator winding
Lower core loss higher flux-density stator laminations
Increased stator slot area
PM-RSM 2p=8 poles (48 slots); air gap=0.4 mm
q=2, y/τ=5/6 which is the stator coil chording factor
2 rotor segmental skewing
Increased air gap flux density
Lower inductance;
Fig. 3.1.16 Proposed control system for hybrid (SPM) Lundell alternator
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Fig. 3.1.17 Proposed control system for PM-RSM alternator
Fig. 3.1.16 and Fig. 3.1.17 show the proposed control systems for the hybrid (PM) Lundell alternator
and for the PM-RSM alternator.
3.1.3 Characterization of the proposed solutions
The hybrid (PM) claw pole alternator
It keeps the present geometry of claw pole rotor alternator but the rotor pole surface is
machined such that the total magnetic air gap is more than doubled. The PMs placed on the
rotor poles (Fig. 3.1.15) are 0.4-0.5 mm thick (the present air gap is 0.35 mm).
It is inherent that, because the magnetic saturation influence in p. u. is reduced, the PMs could
produce up to half the maximum air gap flux density with an equivalent 50 % inductance; that
is, with about the same (or 50 %more) DC field current and mmf the same total air gap flux
density or more is produced.
For emf control up and down, the field current should be now ±, so a four quadrant chopper is
required for fast field current control. This way, when the speed increases the field current
will become smaller positive and even negative if the load decreases.
For load dumping the PM field is drastically reduced by negative field current, until the emf
becomes small enough so that the diodes in the rectifier do not conduct anymore.
With PMs on the claw poles the machine saliency decreases, which is good in terms of
magnetization requirements and stator copper losses, for almost unity power factor operation
imposed by the diode rectifier.
The PMs, glued to the rotor surface (against centrifugal and attraction forces), will put the
claw poles at a larger distance from stator (radially) and thus the claw pole eddy current
losses should be smaller.
To further decrease the copper losses in the stator, the stator winding will be made with a
larger filling factor, the slots will be larger in area by using a larger saturation (low core loss,
flux density lamination core/0.18 mm thick, Hiperco 50), and by increasing the slot area and
thus of stator outer diameter (by 12-14 mm).
The reduction of the machine inductance Ld≈Lq to about 60 % of its initial value should
allow for more DC current in the battery at idle engine speed (1800 rpm for generator speed).
A 40 % increase in current would imply a notable increase in stator copper losses unless the
stator resistance is not reduced by at least 30-40 % by methods indicated above. But at least
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40 % more power can be produced at higher efficiency and perhaps less than 25 % more
stator weight.
However, the placing of PMs on the rotor claws, the presence of a 4 quadrant DC/DC
converter to control the field current, the over-voltage protection at high speed and the PM
eddy current losses are the main disadvantages of this solution.
The PM-RSM alternator with SMR
The PM-RSM will be mostly likely a 4 (8) pole 3 flux barrier/pole, bonded NdFeB PM rotor
machine with q=2 slots/pole/phase in the stator.
To control power:
o At low speeds a 3 MOSFET SMR which acts as a voltage booster on positive current
polarity will basically provide one phase active at a time and some overlapping of
phases. The system works as a constant voltage source.
o At high speeds when the 3 MOSFETs (S1, S2, S3) SMR is idle, the single switch (S4)
SMR has to be activated to control power as a constant current source.
Though there are 4 MOSFETs, the power source for their control is unique; however
their control is independent.
o There is no need for a position sensor to control the 3 MOSFET SMR as only the zero
crossing of stator currents is required which corresponds to observing phase to
ground phase voltage polarity change for positive value on the off-state MOSFET.
Fig. 3.1.18 Control angles of 3 MOSFET SMR (phase a);Vag-phase to
ground voltage
o The angle δ is crucial for controlling the DC current; for maximum power at idle
speed δ ≈45˚, ε =5˚;
o Also, the levels of Vag (during ε) and during Φ angle (Vov) are allowing for degrees
of freedom in control. For stator Vov+Vbase ≈V and Vag≈1/2V, or so. Given the
complexity of phenomena a trial and error method will lead to a practical solution for
a given IPMA;
o For large speed the single MOSFET (S4) SMR becomes active;
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Fig. 3.1.19 Single switch (S4) SMR operation at high speeds
Fig. 3.1.20 Single switch (S4) SMR operation
Typical DC input and DC output current relationships with single switch SMR are shown in Fig.
3.1.21 for various duty cycles and speeds.
Fig. 3.1.21 DC input and output currents of single switch SMR dependency on speed and duty
cycle of S4
o The single switch (S4) SMR retains control of output current at all speeds down to zero
by 100 % duty cycle, that is short circuiting the machine. This however implies notable
losses at zero output current in contrast to the Lundell generator.
So the combined usage of 3 MOSFET (for low speed) and single MOSFET SMR (for high speeds)
seems to be required to control power along all speed range, up and down.
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3.1.4 Decision
Comparing the two proposed competitive solutions based on additional cost in power electronics (4
MOSFETs) on the part of PM-RSM alternator system and its rotor mechanical fragility above 6000
rpm, we hereby decide to go on only with the hybrid (PM) claw pole (Lundell) alternator. Even here,
after thorough modelling and design efforts we decided to drop the surface PM Lundell alternator
configuration because it needs a 4 quadrant (instead of today‘s two quadrant) chopper in the field
current control at more excitation power. We proceeded to investigate thoroughly, then the interclaw
PM rotor (IPM) Lundell alternator which needs only a 1 quadrant DC/DC converter to control the
field current, albeit at higher power(200 W, level) and which does not require special over voltage
protection at very high speed with zero field current.
3.1.5 Selected Electric Alternator: IPM- Lundell configuration
Preliminary analytical design models and dynamics and control models for the IPM Lundell
alternators have been performed. In detail this is described in D2.1.2 [20]. In this report D2.1.1 only
the results are given.
Permanent magnets are placed only between rotor claws (Fig 3.1.22c) in a, say, si-luminum enclosure,
the air-gap is increased to 0.8 mm (from 0.3 mm), the voltage is 40 V, number of turns per coil is 8.
Moreover the stator interior diameter is brought back to 106 mm (unchanged rotor of the existing 2.5
kW, 14 V alternator). Permanent magnet dimensions and characteristics:
30 mm x 10 mm x 9.7 mm
Br_20=1.13 T (remanent flux density at 20 C)
Hc_20=860000 A/m (field strength at 20 C)
Fig. 3.1.22 Permanent magnets placement options: on shaft, a); on claw poles (SPM), b); between the
rotor claws (IPM).
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The following figures show some of the characteristics.
Fig. 3.1.23 Output power of IPM Lundell alternator at 40 V (g6ipm)
Fig 3.1.23 shows the output power at 40 V over the alternator speed range.
Fig. 3.1.24 Efficiency of IPM Lundell alternator at 40 V (g6ipm)
Fig 3.1.24 shows the efficiency characteristic at 40 V over the alternator speed range.
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Fig. 3.1.25 Efficiency of IPM Lundell alternator at 40 V (g6ipm)
Fig 3.1.25 shows the efficiency characteristic at 40 V over the alternator speed range.
Out of the simulation results and investigations at this point EE-VERT favours the solution ―g6ipm‖
(project-internal identifier for IPM Lundell alternator) as it offers the multiple advantages presented
above. The machine g6ipm though with existing rotor (Dis=107 mm) but with interpole magnets and
Dos=137+14=151 mm stator outer diameter seems capable of more than 7.5 kW of power at 40 V.
Also the power at 2000 rpm is above 3 kW. The maximum efficiency is around 80 - 85 %. No over-
voltage protection for zero current excitation is necessary as the PM flux in the air-gap is very small
(see above Fig. 3.1.22 flux at zero field current). Finally, the one quadrant field circuit chopper is
maintained though at higher power. The solution g6ipm with surface PMs (SPM Lundell alternator)
also produces 8.5 kW at 42 V but with Dis=106+10=116 mm and Dos=137+14+10=161 mm, but it
requires a 4 quadrant chopper for field current bi-directional control and a dedicated over voltage
protection (at high speeds and zero field current).
The proposed alternator (IPM Lundell alternator (g6ipm)) fulfils the requirements for high efficiency
and high output power during braking phases (30% of power can be attributed to the presence of
IPMs). However additional effort will be made to further improve it during the design optimization
stage of the project .
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3.2 Solar Panels
It is proposed that solar panels, also known as photovoltaic (PV) panels that comprise a series of
linked solar cells should be used as an additional source of power for vehicles. Whilst the power
output from these panels is relatively low, typically 50-100 W for the surface areas available on the
roof of passenger cars, they provide continuous power, provided the sun is out, even when the vehicle
is turned off. This means that the panel can provide a new source of energy when the vehicle is
stationary. This power could be used to recharge the battery or combat quiescent drain over longer
periods. Additionally, the available solar power can be used to provide cabin pre-conditioning whilst
the vehicle is parked in the sun, with a subsequent reduction in AC load when the vehicle is started. It
has been calculated that the reference vehicle requires about 350 W of power for the electrical
devices. A power output of around 100 W from the PV panel is a significant proportion of this load.
A brief description of how solar panels work can be found in section 2.1.4 of deliverable D1.1.1 [1].
3.2.1 Current Vehicle Applications for PV Panels
The use of such panels in automotive applications is not new, having been offered by Mazda in the
past and by the VW group companies as an optional fit on their high-end vehicles. For instance, a
solar panel is available on the Audi A8 as an option feature at ~€1350. The main use of such panels is
to power the Heating and Ventilation (HVAC) blower whilst the vehicle is parked up in the sun. This
function operates all year round (whilst the sun is out), so in the summer cooler ambient air is used to
displace hotter cabin air, and in the winter the airflow, albeit much reduced, can help alleviate misting
issues.
Normally the PV panel is integrated into a sunroof (Fig
3.2.1). In this way the panel is mounted onto an existing
accessory feature (no additional packaging in the roof panel
keeps implementation costs low) and the sunroof retains tilt
and slide capability. However, as the panels are normally
opaque, no transmitted light is possible through the sunroof,
into the cabin, a feature that may be off-putting for some
customers. Webasto Solar are one of the leading European
suppliers (www.webasto-solar.de/en/).
Fig. 3.2.1
Audi include the following in their glossary explanation of their solar powered sunroof;-
Even in very low sunlight, light-sensitive elements under the glass sunroof panel produce electricity to
power the ventilator inside the vehicle. Even when the ignition is switched off, the interior will be
supplied with a continuous flow of fresh air and temperature levels inside the vehicle can be reduced
by as much as 20°C with the outside air that is cleaned as is passes through the dust and pollen filter.
This kind of ventilation does not put any additional demands on the car‘s battery. This preliminary
cooling lets the air conditioner cool the interior to the desired temperature with little energy and use of
the ventilator.
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More recently, Toyota has announced that the Prius hybrid will also be available with a Kyocera solar
panel roof option. These panels are not part of a sunroof but part of the main roof section.
3.2.2 Solar Panel Performance
The ability of a given panel to generate electricity is a combination of many factors but principally:-
Size of panel
Amount of solar irradiance in the physical location
Topography of terrain
Type of solar cells
System losses
Panel Size
The size of panel is dependant on a number of factors that include:-
Type (size) of cell and number of cells required (total peak power output expected)
Cost of panel vs. price customer is expected to pay
Size of roof space available
Topography of roof (although some curvature of the thin silicon cells can be accommodated)
Solar Irradiance within Europe
Broadly the power available from the sun is related to the latitude, with higher latitudes having less
ability to generate photovoltaic electricity than lower latitudes. The European Community covers a
broad range of latitudes, from Sweden and Norway in the north to Spain and Greece in the south. Fig.
3.2.2 shows a map of potential electricity generation from a 1 kW(peak) panel. It assumes a horizontal
panel and a performance ratio of 0.75. Performance ratio is a quantity, defined namely by the
European Communities (JRC/Ispra), which represents the ratio of the effective energy produced
compared with the energy which would be produced by a "perfect" system. The performance ratio
includes the array losses such as shading, PV conversion, mismatch, wiring, and the system losses
including inverter efficiency and storage/battery.
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PVGIS © European Communities, 2001-2008
Fig. 3.2.2 Map of potential solar power output
The data is taken from the PVGIS1 website (http://re.jrc.ec.europa.eu/pvgis/index.htm). The
background database represents the period 1981-1990, and has been computed by interpolation and
modelling of 580 meteorological measurements over Europe. In order to understand the year round
application of PV panels, the PVGIS data has been examined in more detail for the four European
cities shown in Table 3.2.1:-
Latitude Longitude (E) Hy, horizontal (kWh)
Göteborg, SE 57°41‘48‖ 11°59‘12‖ 917
Graz, AT 47°04‘06‖ 15°26‘30‖ 1154
Tarragona, SP 41°07‘06‖ 1°14‘42‖ 1493
Athens, GR 37°58‘48‖ 23°43‘00‖ 1600
Table 3.2.1 Irradiation data for 4 European cities
It can be seen that for the yearly Irradiance totals, there is ~75 % improvement for Athens compared
with Göteborg. How these yearly totals break down into monthly solar irradiance figures are shown in
Fig. 3.2.3 below. As expected, the most southerly city, Athens, has the highest peak value although
the winter months are very similar to Tarragona. Graz and Göteborg have similar summer peaks that
are around 75 % the Athens value. However, the minimum available solar energy in winter falls to
1 Photovoltaic Geographical Information System
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20% of the Athens value. This is probably due to the differences in daylight between the two cities. In
summer, the sun is less intense in the northern latitudes, but the length of the solar day is
approximately 2 hours longer in Göteborg than it is in Graz – in winter the solar days are very short in
northern latitudes.
Also shown in the graph are the mean maximum monthly temperatures (for 2008) which, when
combined with the level of solar activity, are suggestive of overall cooling demand.
0
10
20
30
40
50
60
70
80
90
100
0
50
100
150
200
250
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Ave
rage
Te
mp
era
ture
-°C
Sola
r Ir
rad
ian
ce -
kWh
/Mo
nth
Monthly Solar Irradiance and Average Monthly Maximum Temperatures
Goeteborg, Hy
Graz, Hy
Athens, Hy
Tarragona - Hy
Goeteborg, °C
Graz, °C
Tarragona, °C
Athens, °C
Fig. 3.2.3 Monthly solar irradiance and average monthly maximum temperatures
Local topography – The effect of shading
Fig. 3.2.4 The effect of shading on solar panels
Clearly the geographic location of the panel has a large effect on performance but this is also affected
by the local topography. For instance, adjacent mountains will have a shadowing effect when the sun
is at low inclination. Additionally, using the panel in built-up areas, especially larger cities, will have
a negative affect, again due to the shadowing influence of near-by buildings.
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PV modules are very sensitive to shading. Unlike a solar thermal panel which can tolerate some
shading, many brands of PV modules cannot even be shaded by the branch of a leafless tree.
Fig. 3.2.5 The effect of shading – reduction of PV module power
Shading obstructions can be defined as soft or hard sources. If a tree branch, roof vent, chimney or
other item is shading from a distance, the shadow is diffuse or dispersed. These soft sources
significantly reduce the amount of light reaching the cell(s) of a module. Hard sources are defined as
those that stop light from reaching the cell(s), such as a blanket, tree branch, bird dropping, or the like,
sitting directly on top of the glass. If even one full cell is hard shaded the current of that module will
drop to half of its unshaded value. If one cell in a series connection is shaded, the output current of the
panel will drop significantly. The weakest cell defines the panel‘s output current. The current, which
is generated by the other not shaded cells, is dissipated in the weak cell – cell gets hot! The output
voltage is affected only by temperature. To get rid of the pure series connection, bypass diodes are
used. If enough cells are hard shaded, the module will not convert any energy and will, in fact,
become a tiny drain of energy on the entire system.
Partial-shading even one cell of a 36-cell module for example, will significantly reduce its power
output. Because all cells are connected in a series string, the weakest cell will bring the others down to
its reduced power level. Therefore, whether ½ of one cell is shaded, or ½ a row of cells is shaded as
shown above (Fig. 3.2.5), the power decrease will be the same and in this case 50 %.
When a full cell is shaded, it can act as a consumer of energy produced by the remainder of the cells,
and the module is protected through bypassing diodes. The module will route the power around that
series string. If even one full cell in a series string is shaded (Fig. 3.2.5) it will likely cause the module
to reduce its power level to ½ of its full available value. If a row of cells at the bottom of a module is
fully shaded the power output may drop to zero. The best way to avoid a drop in output power is to
avoid shading whenever possible though, in practice, bypass diodes should be implemented.
PV panel system losses
Although a solar conversion efficiency of ~30 % is theoretically possible for a silicon based solar cell,
the actual efficiency in cells found on the market is around 15-23.5 %.
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The graph below (Fig. 3.2.6) shows the expected monthly energy output (Ey, horizontal kWh) for a 100
W2 panel operating at 14 % efficiency for the four European cities cited in Fig 3.2.6. The large
difference between the energy that is incident in these locations compared with what can be
realistically converted into electricity using PV panel is clear.
0
25
50
75
100
125
150
175
200
225
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
kWh
/Mo
nth
Solar Irradiance Available vs Estimated 100W PV Panel Output
Goeteborg, Hy
Goeteborg, Ey
Graz, Hy
Graz, Ey
Athens, Hy
Athens, Ey
Tarragona - Hy
Tarragona, Ey
Fig. 3.2.6 There are a number of factors that impact panel efficiency:-
Inclination angle
Ideally a PV panel would be mounted at an inclination angle that maximises the panels surface
exposure to the sun. This is a geometric effect and will vary with latitude. For instance, the optimum
angle in Göeteborg is 38°, whilst in Athens it is 30°C. Clearly, for automotive applications, the panel
has to be horizontally mounted, ideally on the roof to minimise shading effects.
Typically roof mounting accounts for around a 14% decrease in performance compared with
mounting at the optimum angle.
Thermal effects
From section 3.2 it can be seen that there is more solar energy at lower latitudes. At lower latitudes
the temperatures are also warmer and thus the demand for cooling is greater. Unfortunately, PV
panels become less efficient as they get hotter (panels generate electricity from light, not heat). But
when a vehicle is moving the wind helps cooling the panel. Using local ambient data, the graph below
shows how the efficiency of a PV panels differs for various European locations. Thermal effects differ
from 6% loss to 9% loss between the latitude extremes.
Reflectance effects
2 A 100 W panel, at ~1m
2 is the maximum size that could be expected on a vehicle
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Not all the light energy incident on a solar cell is absorbed, some is reflected away. Whilst anti-
reflective coatings can minimise these losses, there is a geometric factor that increases or decreases
reflective losses, these being slightly worse at higher latitudes. The graph below (Fig. 3.2.7) shows,
for a given anti-reflective coating, how the panel losses are effected by latitude. Older anti-reflective
coatings worked for only one specific wavelength. More recent developments work for a range of
wavelengths, greatly minimising these angular losses. Note that some of the most efficient anti-
reflective coatings may not be acceptable from a customer aesthetics view point.
0
10
20
30
40
50
60
70
0
1
2
3
4
5
6
7
8
9
10
Goeteborg Nuneaton Graz Turin Tarragona Athens
Lati
tud
e -
°
Loss
-%
Town
PV Panel Losses as a Function of Latitude
Loss due to Temp
Loss due to Ang Reflect
Latitude
Fig. 3.2.7 PV panel losses as a function of latitude
Other losses
Other losses between the panel and the load are primarily due to the voltage converter that is often
needed (see section 4) and cabling. These losses can account for additional losses in the order of 10 %
in power output. The output of the PV panel is proportional to the solar irradiance, so cloud cover will
reduce the system output. Typically, the output of any solar module is reduced to 5 to 20 % of its full
sun output when it operates under cloudy conditions.
Total system losses
Taking the various losses above into account, the overall system losses of a typical PV panel under
full sunlight are shown in the graph below. It shows that actual losses tend to be fairly flat, primarily
as the increase in thermal losses with decreasing latitude is counteracted by the lower angular
reflectance losses.
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0
10
20
30
40
50
60
70
0
10
20
30
40
50
60
70
Goeteborg Nuneaton Graz Turin Tarragona Athens
Lati
tud
e -
°
Loss
-%
Town
Overall PV Panel Losses as a Function of Latitude
Overall System Losses
Latitude
Fig. 3.2.8 Overall PV panel losses as a function of latitude
Conditions where little or no current is generated
There are several environmental conditions under which the PV panel would not generate a
meaningful amount of current.
Night time
Excessive cloud
Day time, northern latitudes in winter
Snow/ice build-up
Parked in garage, covered car park etc
Heavily shaded areas (e.g. adjacent to large buildings)
3.2.3 Types of solar cell
Table 3.2.2, taken from Section 2.1.4 of [1], summarises the main types of solar panels available.
Table 3.2.2 Types of solar panels
Thin film solar cells can be made transparent, overcoming some of the obscuration issues associated
with crystalline silicon used in sun roof applications. Unfortunately the efficiencies are quite low at
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less than 10 %. Because of the limited surface on vehicles high cell efficiencies are essential.
Consequently monocrystalline or poly-crystalline cells are the best choice for passenger car
applications.
Typical cell sizes are as follows;-
MCSi 100x100 mm 1.5 Wp ~16 % efficient
MCSi 125x125 mm 3.24 Wp ~23.5 % efficient
PCSi 156x156 mm ~3.75 Wp ~14-16.5 % efficient
MCSI 156x156 mm ~3.89 Wp ~15-17 % efficient
3.2.4 Panel Electrical Characteristics
Typical characteristics of a crystalline silicon solar panel are shown in Fig. 3.2.9. The graph
demonstrates how the DC characteristics change as a function of solar irradiance and with
temperature. Increases in cell temperature increase current slightly, but drastically decrease voltage.
Maximum power is derived at the knee of the curve.
Fig. 3.2.9 PV panel I/V characteristic for different temperatures and irradiances
Battery Interaction
Solar panels are often used to provide trickle charging for batteries, normally at 12 V or 24 V. For
these applications, additional precautions are required. For instance, for a 12 V lead-acid battery the
following is required
DC/DC conversion: voltage step down from >17 V to 14 V
Regulation: This is required to ensure efficient and safe battery charging
Reverse current protection: If a solar panel is connected directly to a battery it will charge
whilst the sun is out but once the sun goes in the battery can then discharge back through the panel. A blocking diode could be used to prevent this.
Load control: to prevent battery overcharging
Temperature compensation: the charging voltage is adjusted according to the temperature
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A typical charging algorithm for instance for a 12 V lead-acid battery would be similar to the
following Fig. 3.2.10.
Fig. 3.2.10 Typical charging algorithm
Clearly at night or under cloudy conditions no charging is available. The first stage is the Bulk
Charge, where current is sent at a maximum safe rate until the battery has been nearly fully charged
(about 80 % - 90 % of maximum). The charger then switches to the Absorption Charge, where the
voltage is constant and the current reduces gradually according to the resistance of the battery as it
become charged. Although the current drops, the charger produces maximum voltage during
absorption charging, at around 14 V.
The third and final stage in the charging process is the Float Charge, where the charging voltage is
reduced to around 12.8 to 13.2 volts, often called the trickle charge. At this voltage the emission of
hydrogen gas is maintained at a minimum for safety reasons. The purpose of this stage is to maintain
the battery at full charge until it is needed for use.
Some regulators use Pulse Width Modulation as a more efficient charging method so as to maintain
the battery at its maximum state of charge whilst minimising adverse sulfation by pulsing the battery
charging voltage at high frequency.
Maximum Power Point Tracking (MPPT)
MPPT is, when by certain means the load to the panel is continuously adjusted to match its internal
impedance (which changes with irradiance), in order to always achieve proper impedance matching.
This impedance transformation can be achieved by integrating a converter, with special control for
MPPT.
It is clear from the Volts/Amps characteristics shown in Fig 3.2.11 that a number of factors affect the
output from the panel. Having a ‗standard, DC/DC converter with a device with such a characteristic
means that some typical output has to be chosen and deviation away from this will lead to losses in
efficiency.
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Cu
rre
nt
Voltage10V 15V ~17V
BatteryVoltageRange
MaximumPowerPoint
Fig. 3.2.11 Volts/Amps characteristics and MPPT
The diagram shows how MPPT can overcome this. The battery typically operates at 10-15 V worst
case, Vmp is of the order 17 V for Pmax in a typical solar panel configuration. The MPP changes, as the
cell/panel output current reduces with irradiance, and the cell/panel voltage reduces with temperature.
MPPT assures by means of impedance matching, that maximum available power is transferred to the
load. At the same time, the voltage/current provided to the battery can be adjusted to some extent.
Keeping the voltage of the panel at a FIXED voltage, reduces the freedom for MPPT – a properly
MPP tracked PV panel will behave as a power source, with voltage limit. Peak efficiencies of ~97 %
are achievable when using such tracking algorithms.
To date, a number of MPPT algorithms have been proposed in the literature [21], including perturb-
and observe method, open- and short-circuit method, incremental conductance algorithm, fuzzy logic
and artificial neural network. Among these, three are being considered to be implemented in EE-
VERT: perturb-and observe method, open- and short-circuit method and incremental conductance
algorithm. The other controls need complex algorithms that lead to high cost of implementation.
Perturb-and-observe (P&O) method is dominantly used in practical PV systems for the MPPT
control due to its simple implementation, high reliability, and tracking efficiency. The perturbation in
the PV output power is accomplished by periodically changing (either increasing or decreasing) the
reference current, with a small amount. Fig. 3.2.12 illustrates the flow chart of the MPPT algorithm.
EE-VERT Deliverable D2.1.1 23 December 2009
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Fig. 3.2.12 Flow chart of the perturb-and-observe (P&O) MPPT algorithm [22]
From the plot of PPV versus VPV, two possible operating regions, A and B, can be defined (Fig.
3.2.13). The current operating point location can be determined by a perturbation in the PV output
power. For instance, if the PV controller increases the reference for the converter output power by a
small amount, and then detects the actual output power. If the output power is indeed increased, it will
increase again until the output power starts to decrease, at which the controller decreases the reference
as shown in Fig. 3.2.13.
.
Fig. 3.2.13 PV output power curve with respect to the PV output voltage [22]
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The open- and short-circuit current method for MPPT control is based on measured terminal
voltage and current of PV arrays. By measuring the open-circuit voltage or short circuit current in
real-time, the maximum power point of the PV array can be estimated with the predefined PV current-
voltage curves. This method features a relatively fast response, and do not cause oscillations in steady
state. However, this method cannot always produce the maximum power available from PV arrays
due to the use of the predefined PV curves that often cannot effectively reflect the real-time situation
due to PV non-linear characteristics and weather conditions. Also, the online measurement of open-
circuit voltage or short-circuit current causes a reduction in solar panel output power.
The main task of the incremental conductance algorithm is to find the derivative of PV output
power with respect to its output voltage that is dP/dV. The maximum PV output power can be
achieved when its dP/dV approaches zero. The controller calculates dP/dV based on measured PV
incremental output power and voltage. If dP/dV is not close to zero, the controller will adjust the PV
voltage step by step until dP/dV approaches zero, at which the PV array reaches its maximum output.
The main advantage of this algorithm over the P&O method is its fast power tracking process.
However, it has the disadvantage of possible output instability due to the use of a derivative
algorithm. Also, when insulation is weak, the differentiation process is noisy and the algorithm has a
poor performance.
3.2.5 Possible Sizes of PV Panel on the Alfa Romeo 159 Reference Car
The available roof space on the Alfa Romeo 159 is shown as the shaded area in the picture Fig 3.2.14.
This equates to a size approximately 1277x907 mm (or 1.16 m2).
Fig. 3.2.14 Available roof space on the Alfa Romeo 159
Given the possible cell sizes listed in Section 4, the options in Table 3.2.3 are available with this open
area, assuming that the full roof curvature can be accommodated by any particular panel:-
Cell
Type
Cell size
(mm x mm)
Number of
cells (Length)
Number of
cells (Width)
Total
Cells
Potential
Peak Power
(Wp)
~Cell Cost
(€)*
MCSI 100 x100 12 8 96 144 310
MCSI 125 x 125 9 7 63 204 440
PCSi 156 x 156 8 5 40 148 295
MCSI 156 x 156 8 5 40 156 340
Table 3.2.3 Options with given cell sizes
* Approximate cost for the cells (not ‗made up‘ panel)
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It can be seen from a power production point of view that the high efficiency mono-crystalline cell
offers the best prospect, with some 200 W(peak) being theoretically available. However, from the
discussions on system losses covered in Section 4, ~150 W(peak) delivered to the point of need is more
realistic. Note, at 14.0 V, this equates to 10.7 A.
3.2.6 Possible use of PV panels within EE-VERT
There are several benefits that a solar cell would offer to an overall vehicle energy management
strategy.
Vehicle Off - Battery: Low State of Charge
If the vehicle is switched off after heavy use of the battery the battery will be in a state of low charge.
Combined with quiescent drain, after a period of time parked up, this may cause subsequent starting
issues for the customer. Clearly use of a solar panel, given the right environmental conditions, could
go some, or all, the way in overcoming this issue. Consequently, it would be prudent for any PV panel
implementation strategy to first examine the battery state of charge, and if low, direct all power to
recharging prior to any other activity, such as cabin cooling.
Vehicle Off - Battery: Quiescent Drain
Even with the vehicle switched off, there are sufficient electrical loads to draw a small, continuous
current, from the battery. This is of the order 0 - 10 mA but can vary from vehicle to vehicle. Over a
long period of time (e.g. parking at an airport whilst on holiday) can lead to a discharged battery.
Consequently, it would be prudent for any PV panel implementation strategy to include some small
part of the available current to be directed to the battery to overcome quiescent drain issues.
Vehicle On – Lower Alternator Load
With the vehicle switched on, ~13.7 A is available to the electrical circuit that would otherwise have
to be generated by the engine via the alternator.
Vehicle Off – Cabin-Preconditioning
Most OEMs offering PV panels as an option use the current generated when the vehicle is powered
off to ventilate the interior cabin using the HVAC blower. This offers a number of benefits. Even for
brief periods parked up in the sun, vehicle interiors can become uncomfortably hot. With a high solar
load, the panel will also be generating a high current and thus interior cooling will be maximised, with
cooler exterior air drawn in and vented back out. The added benefit is that as the existing ducting is
used, the very hot air that would otherwise accumulate in the ducting whilst the vehicle is parked is
also expelled. Thus when the occupant turns on the HVAC upon entering, cooler air is immediately
available, both cabin ambient and that expelled from the ducting, rather than the slug of very hot air
normally experienced.
If sufficient pre-cooling is available, then the A/C system will have to do less work as the cabin air
and interior structure has already been pre-cooled. This could lead to a reduction in the maximum
specified load for the A/C system and the possibility of down-sizing of the compressor.
Alfa Romeo 159 – Expected Airflow
The airflow versus current graph for the reference vehicle is shown in Fig. 3.2.15.
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0
20
40
60
80
100
120
140
0 5 10 15 20 25 30
Air
Flo
w -
l/s
Current - A
Alfa 159 HeVAC Performance - Vent Flow vs Blower Current
Flow (Vent)
+ Blower Setting 1
+ Blower Setting 2
+ Blower Setting 3
+ Blower Setting 4
+ Blower Setting 5
+ Blower Setting 6
Fig. 3.2.15 Reference car HVAC performance
Also indicated are the approximate blower settings on the control panel. From the possible peak
output from a full roof panel described in Section 7, it can be seen that if 150 Wpeak can be generated,
then 10.7 A corresponds roughly to blower setting between 3 and 4 and ~70 l/s airflow (peak). It is
expected that ambient outside are drawn into the vehicle at this rate would have a significant impact
on cabin pre-cooling and hence the maximum load on the A/C system that occurs at start-up.
Impact on HVAC Strategy
The introduction of a cabin cooling approach will have an impact on how HVAC strategy is
implemented on the vehicle in the following ways:
on key-off, outside air must be selected (cabin cooling will be meaningless if re-
circulating air is selected!);
on key off, the HVAC sets face vent mode;
the HVAC blower bearings will have to be upgraded to take into account continuous
running ( unfortunately this will incur additional cost);
ideally a blower with a brushless motor would be used as this is more efficient than a
brushed one (again at additional cost).
3.2.7 Energy Saving Calculation
Reduced Alternator Loading
With the vehicle running, power delivered from the PV panel is power that is not generated by the
engine via the alternator. As an approximation, it has been assumed that for all vehicles, 0.1 litre of
fuel is saved for every 100 W the alternator is saved from generating.
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159 3.2 V6
Gasoline
159 1.9l
Diesel
Fuel consumption (l/100 km) 12.2 5.9
Peak solar power (W) 150 150
Fuel saving, (l/100 W) 0.1 0.1
Fuel saving (l) 0.150 0.150
~Fuel saving, % decrease (peak) 1.2 2.5
Table 3.2.4 Potential fuel saving possible with a PV panel
Table 3.2.4 shows that a solar panel could make a significant contribution to reducing fuel
consumption of up to 2.5 % for the reference vehicle during normal driving.
Reduced AC Demand
The AC system is the largest ancillary load on the engine which is seen as an increase in fuel
consumption for the customer. For a typical EU vehicle, fuel consumption increases by 10 % when
AC is used [5] and represents some 3.2 % of total fuel used by the European automotive fleet. It was
also shown that reducing AC power load by 30 % could save some 2.5 billion litres per annum within
Europe.
The effect of cabin pre-cooling on reduced energy demand has been studied in depth by a number of
organisations, particularly the National Renewable Energy Laboratory (NREL) in the US. They have
investigated panel ventilation as part of a wider AC load reduction strategy. It was shown [6] that
running the blower in a Jeep Grand Cherokee with 81 watts of energy reduced the interior temperature
by 6.9°C (by comparison, just having the sunroof open 6cm reduced the temperature by only 2°C. A
similar study conducted on a Cadillac STS with just a 17 W panel was able to reduce interior
temperatures by 5.6°C and seat temperatures by 5-6°C. As part of a wider load reduction strategy
(solar reflective glazing and IR reflective paints) they were able to show that A/C load could be
reduced by 30 %, representing ~26 % improvement in fuel used for the A/C.
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3.3 Waste heat recovery
3.3.1 Technology overview
It is proposed that a heat recovery device should be used as an additional source of power for vehicles.
Around 30 % of the fuel potential energy is transformed into available mechanical energy in a
conventional engine. Most of the potential energy is transformed into thermal energy (Fig.
1.1.1).Different technologies are available to recover the wasted thermal energy. The thermal energy
may be converted to mechanical, thermal or to electrical energy. The focus of EE-VERT is mainly on
thermoelectric conversion and the application of available technology solutions to evaluate the
effective fuel saving.
Thermal to thermal
Engine warm-up can be sped up adding a heat exchanger between exhaust gas and engine water
temperature, transferring heat from the gas to the water. Obviously the engine friction is mainly
related to the oil temperature; so the heat from exhaust gas must reach the engine oil through water.
When the engine is hot, the heat exchanger is bypassed and exhaust gas reaches the muffle via the
traditional direct pipe.
Improved engine warm-up on NEDC cycle means improved engine combustion and less emission
during the cycle. The difference between cold and hot NEDC cycle in terms of fuel consumption is
about 10 %. A fuel reduction may be expected if during the initial part of cold homologation NEDC
(600 s), engine warm-up is sped up.
This kind of device may also speed up cabin heating at cold start if heated water is also routed to the
cabin heater. This also gives a relevant contribution to the vehicle comfort.
Heat transfer may vary according to:
o exhaust gas temperature
o heat exchanger efficiency.
Typically a high exhaust gas temperature is required (500-600 °C), so the heat exchanger must be
placed just after the catalyst devices with a proper exhaust pipe layout. If temperature is too low, the
heat exchanger efficiency is reduced.
Moreover the heat exchanger must not be a source of excessive pressure drop in the exhaust pipe;
otherwise engine performance and efficiency may be greatly affected. Greater pressure drop means
higher engine pumping losses.
It must be noted that such kind of device for fuel saving just works when the engine is cold. As soon
as the engine is hot, the bypass circuit is activated. The device can not be exploited during the
remaining part of the mission.
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Fig. 3.3.1 Fast warm up architecture
Thermal to mechanical
A significant reduction of the fuel consumption may be reached through the recuperation of the
engine waste heat by means of a Rankine bottom cycle. The system produces mechanical power and
is based on the principle of the steam engine: a fluid is heated by the exhaust gas to form steam which
is conducted into an expansion device producing mechanical power that can be used to assist the main
engine or generate electrical power for hybrid vehicle or to drive auxiliaries.
The following scheme in Fig 3.3.2 shows the layout of a typical Rankine cycle applied to the engine
exhaust gas. A system based on a Rankine cycle is made of four elements: a pump, a steam generator,
an expander (in this case a turbogenerator with electric generator), a condenser.
Water 1 bar @ 85°C
Steam
Engine
Steam
generator
Exhaust gas
Water cooled
Condenser
Steam 100 bar @ 450°C
Steam 1 bar @ 100°C
Water 100 bar @ 100°C
V1
V2
Fig. 3.3.2 Typical Rankine cycle applied to engine exhaust gas
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A thermodynamic analysis must be conducted for each component of the Rankine bottom cycle to
find out the optimum thermodynamic working points, i.e. to find out the best values for the minimum
pressure, the maximum pressure and the superheating temperature of the cycle.
The ideal thermodynamic efficiency of the cycle depends on the maximum pressure and temperature
of the cycle. The following graph Fig 3.3.3 shows the thermodynamic efficiency of a superheated
steam cycle, without extractions or reheating, as a function of the maximum pressure; for the
calculation, a minimum pressure of 1 bar and a superheating temperature of 450°C have been
assumed.
-
0,05
0,10
0,15
0,20
0,25
0,30
0,35
0,40
0 20 40 60 80 100 120 140
Pmax [bar]
id
Pmin=1bar, Tmax=450°C
Fig 3.3.3 Isentropic efficiency of steam Rankine cycle as function of cycle maximum pressure
As is well known, efficiency increases when maximum pressure increases, as the ideal work also
increases.
Another way to recover thermal energy to electric energy via first mechanical energy is the
application of a turbogenerator integrated in the gas exhaust pipe. For example such a device may
exploit a water cooled switched reluctance generator for instance coupled to an exhaust gas driven
turbine. It is capable of operating in exhaust temperatures, at high speeds, delivering a typical shaft
power of 6kW. Such kind of system is in development among vehicle suppliers: the TIGERS system
is shown in Fig. 3.3.4.
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Fig. 3.3.4 Turbogenerator with switched reluctance integrated generator
Thermal to electric (thermoelectric)
Thermoelectric conversion allows direct energy conversion from thermal to electric, being a suitable
heat recovery technology for the EE-VERT concept. Converted electrical energy may be managed
within the EE-VERT powernet concept: energy may be stored or used for low voltage or high voltage
loads through the DC/DC converter.
Thermoelectric technology is based on the well-known Seebeck effect. The basic element is the
thermoelectric cell (Fig 3.3.5). Typically efficiency ranges from 4 % to 13 %. Low efficiency is not a
big weakness since the wasted heat energy is in the order of several kW and a low efficiency would
therefore still generate considerable electric power. Cell types are developed for low temperature or
high temperature operation.
A typical cell performance (WATRONIX source) is shown in Table 3.3.1. Depending on temperature
difference between the sides, a certain cell voltage and available current is produced. To produce a
meaningful electric power several cells must be used. An output power target may be about 150 W. In
such case, taking into account an average cell efficiency of 10%, 1.5 kW thermal power has to be
available.
Only part of the thermal power is converted into electric power (150 W); the remainder must be
dissipated (about 1.35 kW). In such a device the main problem is to assure proper heat dissipation,
proper cooling of cold cell side and proper temperature difference between cell sides. Otherwise
efficiency can fall drastically. If auxiliaries are to be introduced to manage the TEG (Thermo Electric
Generator) (like water circuit, water pumps, …), their contribution must be taken into account for the
overall electric energy balance.
Fig. 3.3.5 Low temperature cell
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COLD HOT dT U (V) I (A) Q (W)
30 60 30 0.77 0.44 0.34
30 80 50 1.29 0.75 0.97
30 100 70 1.8 1.01 1.82
30 120 90 2.31 1.24 2.86
30 140 110 2.82 1.46 4.12
30 160 130 3.31 1.66 5.49
30 180 150 3.8 1.84 6.99
30 200 170 4.27 2 8.54
Table 3.3.1 TEG typical performance
Conclusion
Among the described technologies to recover waste heat energy, TEG is recommended be considered
in the new EE-VERT approach to the electric power net, since being a source of electric energy
always available during the whole vehicle mission. A motivation to this conclusion is given below.
Thermal to thermal conversion has no impact on the electric powernet and has a limited impact on
vehicle mission (cold start phase); nevertheless it must be noted that it has a potential in fuel saving.
Thermal to mechanical conversion has no impact on electric powernet. A further conversion stage
must be installed (electric generator) to produce electric energy, lowering overall efficiency. The
development of a Rankine cycle has already been addressed in other EU projects. The integration of
such a source of energy with an electric generator, has to be considered in the overall electric energy
management. A turbogenerator with integrated electric generator is a promising solution, though
impact on engine management must be verified.
It must be noted that all the above mentioned systems affect the pressure drop along the exhaust pipe.
A solution in the design process must be found which limits the pressure drop and typically must
include an electronically controlled full flow by-pass on the exhaust pipe that ensures the desired
proportion of exhaust gas is delivered to the heat recovery device as determined by the control system.
Development of new devices for thermal energy recovery is out of scope of EE-VERT. The scope of
EE-VERT is the integration and management on the vehicle powernet of new existing promising
electric generation solutions to check their contribution during NEDC and real driving conditions.
3.3.2 On vehicle heat recovery management
This paragraph describes the best way to manage heat recovery, regardless of the kind of technology
used for the conversion.
On a vehicle there are two main sources of thermal energy: engine water and engine exhaust gas. The
overall amount of energy and the allocation between them depend on engine type (gasoline, diesel,
methane,…) and engine calibration.
During engine warm-up it is better to avoid to taking heat from the engine water. Engine warm-up
must be as fast as possible to reach best thermal engine working condition regarding fuel consumption
and emissions. This condition is reached when the engine water temperature is above approximately
80°C. When the engine has reached its best working conditions, heat can be taken from the water with
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no relevant effects on fuel consumption and emissions performance. Obviously not so much heat so
that the engine water temperature is lowered too much. State of the art engine thermal management
does not need engine fan ventilation during the NEDC. Engine thermal management is optimized to
get the engine warm as fast as possible and to not produce so much heat that engine fan engagement is
needed. In short, heat can be taken from water in order to avoid engine fan engagement.
So during the NEDC cycle a thermoelectric device placed on the engine water circuit will recover
heat for a really small part of the cycle.
Moreover the engine water circuit heat is used for cabin heating during the cold season. Cabin
comfort may be affected if during cold start heat is not available as soon as possible to the cabin.
Exhaust gas heat energy is indeed really wasted energy and can be used as soon as it is available. The
only recommendation is to avoid taking heat from the exhaust pipe before the precatalyst and catalyst
converter: lowering the exhaust gas temperature there may lead to lower catalyst efficiency. Heat
must be taken before the muffle.
In case of thermal recovery from water, heat is available at about 90°C. In case of thermal recovery
from the exhaust gas, heat is available at 300-600 °C. It is clear that having available heat at higher
temperature is a more suitable source for energy conversion.
3.3.3 Current vehicle applications for TEG
Fig. 3.3.6 and Fig. 3.3.7 show vehicle applications in development. Systems under development claim
interesting figures of electric power production (up to 600 W) but just in well defined vehicle
conditions. The contribution on NEDC cycle and in real use must be assessed.
BMW Thermoelectric exhaust gases recovery system Hi-Z 1kW generator for class 8 trucks
Amerigon CCSTM Vehicle Seat ApplicationGM design for TE heat recovery
Fig. 3.3.6 Ongoing development
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Fig. 3.3.7 BMW and DLR (Deutsches Zentrum für Luft- und Raumfahrt) thermal-electric prototype
BSST has started to develop a high efficiency thermoelectric waste energy recovery system for
passenger vehicle applications in November 2004 under a contract awarded by the U.S. Department
of Energy Freedom Car Office. The goal of the effort is to reduce fuel consumption by converting
exhaust gases into electricity using a Thermoelectric Generator Module (TGM). This project has
presented prototype simulation results, see Fig. 3.3.8. Depending on engine load, different amounts of
recovered electrical energy are available. 500 W are available at 130 km/h. NEDC speed is mainly
around 50 km/h with an output power of 120 W; such output is still interesting compared to the
electrical load request of the vehicle (about 240 W), allowing some fuel saving. A fuel economy of 1
% is claimed over the NEDC cycle by BSST. The system has been installed on a BMW vehicle as
shown in Fig. 3.3.6 for testing.
Fig. 3.3.8 BSST 500 Watt TEG module simulation
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On trucks the waste heat amount is larger than on a passenger car and recuperation can reach high
values. In Fig. 3.3.9 a MACK application is shown with a claimed nominal electric generation of 1
kW.
Fig. 3.3.9 1 kW TEG for trucks
Thermoelectric devices are also used to exploit the reverse principle: producing cold or warm air
supplying electric energy to the cell, as shown in the Amerigon seat application (Fig. 3.3.6). Such
type of application may be greatly exploited in hybrid and full electric vehicles, where no heat is
available from an internal combustion engine.
3.3.4 TEG Performance
The ability of a given TEG to generate electricity is a combination of many factors but principally:
Number of cells
Output voltage regulation
Amount of available heat
Cell side temperature control and heat dissipation
Type of cell
Number of cells
Typically TEG cells are placed around the external side of the exhaust pipe in a ring configuration
(Fig. 3.3.10), assuring the same temperature for all the cells. The number of cells depends on the
required power output. State of the art shows a sustainable trade-off of performance versus cost with a
power output of at least 200 W. Such power output requires about 30-50 cells. A suitable cell package
must be designed.
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Cells
insulator
Heatsink
Fig. 3.3.10 Cell package layout
Output voltage regulation
The number of cells in series determines the output voltage of the TEG. A DC/DC or active
electronic device is always needed for connection of the TEG to the vehicle powernet. The voltage
regulation is similar to the regulation for solar panels with maximum power point tracking (MPPT),
see section 3.2.4 for more information about MPPT.
Amount of available heat
There is typically lots of heat power available on the vehicle exhaust pipe. The amount may vary from
3 kW (engine idle) up to 100 kW at full load on a medium passenger car. On a truck it may be as
much as 450 kW. It is costly to design a device for optimised operation over the whole of such a wide
power range. Therefore, the TEG must be designed to control the heat flow through the section of the
pipe fitted with cells. A suitable by-pass for the exhaust gas must be available to control heat flow and
assure proper operation in the desired range (typical 5-30 kW), as shown in a prototype installation in
Fig. 3.3.11.
Fig. 3.3.11 Installation of a prototype TEG
Cell side temperature control and heat dissipation
A cell can be irreparably damaged from over temperature on the hot side. A Waltronic module
(ibnC1_127.08hts) has an optimal working temperature on the hot side of 200°C and is destroyed at a
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temperature of 232°C. Just a few °C deviations from the optimal working temperature cause cell
damage.
Exhaust gas control flow is also relevant in order to control the hot side temperature.
Typically also a thermal insulator is placed between the pipe and the hot side of the cell to lower the
contact temperature and assure reliability.
A temperature sensor must be present on the hot side to take recovery actions in case of over
temperature or in a more complex way to control the temperature within a desired optimal range to
assure a high efficiency.
To assure energy conversion, the cell cold side must be cooled. A heatsink is used in case of air
cooling. To reach better cooling performance, water cooling is used on vehicles with a more complex
device package. Water cooling has an impact on fuel consumption. If the engine water cooling circuit
is used, impact on pipes, pressure drops and engine water request must be taken into account. The
engine water circuit is typically not suitable for such cooling due to high water temperature (> 80°C).
A new cooling circuit may be required with a lower water temperature (40°C) which will have
significant impact on pipes, radiator and the introduction of a new electric water pump.
Ongoing research projects claims TEG output performances without taking into account related
factors with an impact on fuel consumption (additional cooling request to engine load, weight…).
Within the EE-VERT approach all these factors will be modelled and evaluated to get the TEG net
power contribution to the vehicle.
DC/DC
heat source 400°C
Thermal insulator To lower the contact temperature to 200 ° C
Heatsink
Cell
Temperature sensor
Fig. 3.3.12 TEG layout with control device and thermal management
3.3.5 DC/DC requirements for TEG management
The connection from cells to DC/DC converter is quite challenging. To work at the optimal working
point of each cell, each cell should be connected directly to a DC/DC input stage. Design optimization
requires putting some cells together in series in order to reduce harness complexity and DC/DC cost.
Putting cells together in series connection has the risk to force some cells to work in a non-optimal
working point. Moreover additional electronic components must be introduced to avoid that one
damaged cell will lead to cell strip open circuit; devices like diodes must be introduced in parallel to
each cell in order to allow by-pass of broken cells, i.e. similar as described for solar cells. The DC/DC
converter should also monitor the power generation in order to check for cell damage and warn the
overall energy management system.
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3.3.6 Possible use of TEG within EE-VERT
TEG is recommenced to be integrated in the EE-VERT power net concept. TEG development is out
of scope of the project, but EE-VERT can benefit from the results of other on going projects fully
devoted to TEG development (e.g. HEATRECAR). Furthermore if a TEG device will be available for
proper vehicle installation from suppliers, it will be tested on a test bench and on a test vehicle, to
assess the benefit during the NEDC cycle and in real use.
TEG device will be considered in the simulation activities. Reasonable hypothesis will be made about
real energy recovery, based on information available on state of the art and a TEG simulator block
will be integrated.
A TEG device is able to continuously produce electric power; the amount depending on the engine
load. During the cold start, the TEG is not able to work at full performance. As soon as the exhaust
gas reaches the nominal working point, TEG produces nominal output power.
From an energy management point of view, a TEG device seems to be a very good complement to
brake energy recovery. The combination of TEG and brake energy recovery allows a source of
continuous ―free‖ energy, in most vehicle conditions.
During engine cut-off, there is no engine load and consequently there is no heat production. TEG
generation drops, but energy level is restored by brake energy recuperation. Engine cut-off is read as
an intention of the driver to slightly slow down, so application of a slight brake force (controlled by
the new alternator concept) is feasible and has negligible impact on vehicle driveability. During
vehicle off condition there is no contribution from the TEG to the electric power net. Although the
exhaust pipe temperature may still be high just after an engine shut down, no exhaust gas flow is
available and no efficient heat exchange is possible.
3.3.7 TEG energy saving calculation
Experimental measurements in the exhaust gas pipe temperature
The reference vehicle (Alfa Romeo 159 1.9 jtdm) has been equipped with temperature sensors along
the exhaust pipe (Figure 1). Sensors are placed just before and after the pre-catalyst device. Another
couple of temperature sensors are placed before and after the DPF (Diesel Particulate Filter). The last
sensor is placed before the muffle.
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ENGINE
CAT
DPF MUFFLE(e)(e)(i) (e)(i)(i)
(i)
(i)
32
22
37
121187
(i)
6
5
Figure 1. Temperature sensor layout on test vehicle; (i) internal to pipe; (e) on pipe surface
5 Catalyst gas inlet
6 Catalyst gas outlet
7 DPF gas inlet
8 DPF gas outlet
11 Exhaust gas at middle DPF-muffle
12 Muffle gas inlet
37 External pipe at DPF outlet
32 External pipe at middle DPF-muffle
22 External pipe at muffle inlet
Table 1. Temperature sensor list
The scope of the temperature sensors setup is to monitor the gas temperature to check if it is suitable
for a TEG device.
The results are shown in Fig. 3.3.13 for a cold NEDC cycle. The exhaust gas temperature at the
engine outlet is not too high: it reaches about 400°C at high vehicle speed. This behaviour is typical of
efficient diesel engines where the heat losses are lower compared to gasoline engines, i.e. in a
gasoline engine a higher exhaust gas temperature may be expected.
The pre-catalyst device produces a reduction of about 20°C and a filtering of gas temperature bursts.
There is a further reduction in temperature along the pipe before it reaches the DPF. The DPF
produces the largest reduction in temperature of about 170°C during an ECE cycle and 260°C during
an EUDC cycle.
An acquisition was also done on a part of the EE-VERT cycle (Fig. ). The temperature trend is similar
to NEDC, but another issue has been highlighted: the DPF regeneration phase. The DPF regeneration
process is based on combustion on particulate, so exhaust gas temperature increases a lot. This
process may happen during the real use of the vehicle. In the acquisition it happened at the beginning
of the EE-VERT mission. The exhaust gas temperature may reach up to 600°C. It is clear that an
exhaust pipe by-pass device is really needed in that case to avoid to damage the TEG. Furthermore a
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coordination with engine control unit is recommended to coordinate the DPF regeneration process
with the TEG management for improved reliability.
No devices should be inserted before the pre-catalyst and the DPF in order to avoid lowering the pre-
catalyst and the DPF efficiencies.
The TEG may be inserted before the muffle. The available gas temperature is however low (<200°C)
and thermoelectric conversion may be less efficient. It must also be considered that thermoelectric
cells are mounted around the exhaust pipe: so further temperature reduction must be considered when
passing from exhaust gas to pipe surface.
Fig. 3.3.13 Exhaust gas temperature along the exhaust pipe.
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Fig. 3.3.14 EE-VERT cycle acquisition
It could be useful to add thermal insulation to the exhaust pipe to avoid heat dissipation. This is a
typical trick used in cogeneration (generation of electrical and hot thermal energy from an engine)
application. In actual production cars, heat is considered as a disturbance to system performance and it
must be dissipated as much as possible to allow vehicle full performance and availability. Therefore
also passive dissipation is desired in actual production cars.
Since heat is now considered as a source of energy, heat must not be dissipated. Heat must be trapped
and released in the thermal conversion device. Therefore, insulation must now be considered.
Reduced alternator load
The contribution of the TEG device leads to reduction of the alternator load and consequently of fuel
consumption. From the previous description and facts related to a real application, a contribution to
the vehicle electrical supply may be expected, but not full coverage to overall vehicle electric demand,
due to:
Heat available at low temperature (~170°C);
Heat available after 600 sec in cold NEDC cycle;
TEG cooling auxiliaries request still not considered in state of the art TEG declared
performance. Power claims are gross values. Net values must be considered, subtracting TEG
auxiliaries power request.
Simulations have been performed with the following assumptions:
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nominal TEG performance as shown in Fig. 3.3.8;
flat road: vehicle speed is assumed to be related to the engine load in such a case;
reduced efficiency during engine warm up;
efficiency of the DC/DC converter: 90 %;
50 W reduced output power due to TEG auxiliaries: additional water pump.
Fig. 3.3.15 Simulation of TEG electric generation during NEDC cycle
The final simulation result is shown in Fig. 3.3.15. The TEG contribution is able to support just a part
of the vehicle electric request. The average power supplied is 50 W. Considering 240 W as the vehicle
electric request, about 20 % of the alternator load may be replaced.
Basic requirements for a TEG application
Requirements have been defined for TEG application on a vehicle for an effective fuel saving:
Exhaust gas pipe insulation;
TEG placed after the DPF;
Added exhaust gas pressure drop <200 mbar.
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3.4 DC/DC converter
3.4.1 Basic architectures
As previously explained, a first option to augment the efficiency of the system is to have generation
(i.e. the alternator) decoupled from the conventional power net for consumers. In this way, the
generation device could be used in an efficient manner. In particular, it may be conditioned for
optimized power output (i.e. selecting an output voltage not limited by the power net voltage).
This option defines, as explained in D1.3.1, a first basic scenario (baseline) for optimizing the electric
architecture of a conventional car (Fig. 3.4.1).
ECU #1AlternatorR Battery
Load #1BMS
Voltage
Stabilizer
ECU #N
Voltage
Stabilizer
Load #NStarter
CommunicationCommunication NetworkNetwork
14 V 14 V PowernetPowernet
R Alternator regulator
BMS Battery Monitoring System
Fig. 3.4.1 Currently power net architecture of a conventional passenger car
Firstly, the alternator may be optimized by using a higher voltage as described in section 3.1. As
consequence, an energy converter (DC/DC converter) should be used to transfer the energy from the
generator to the consumer power net. Depending on system requirements, the DC/DC converter has
different characteristics: unidirectional / bi-directional and different topologies such as buck / boost /
buck-boost. In this section we will discuss different DC/DC concepts depending on the system (i.e.
depending on electric architecture).
As explained in section 2.2., the electronic architecture also requires having a storage element with
high power / high energy capabilities in order to store the energy produced when it can be generated
with low energy losses / high efficiency. The energy stored should be used when generation is not
recommended due to low efficiency.
In order to store this energy, two possibilities have been addressed: Lithium-Ion battery and
ultracapacitor. If an ultracapacitor is used, the generator does not need a well regulated constant
voltage. If a Lithium-Ion battery is used a well regulated constant voltage is needed. The resulting
general architecture is shown in Fig. 3.4.2.
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Gene - rator
DC / DC
Storage
Load # 1
Volt . stab .
Load # X + 1
Load # X
Volt . stab .
Load # N Starter
Lead acid battery
LV HV
Fig. 3.4.2 EE-VERT architecture - baseline
The DC/DC converter characteristics are determined by the voltage and current characteristics of the
generator (and therefore of the storage) element chosen. For the baseline version, the most suitable
DC/DC converters are listed in Table 3.4.1.
ARCHITECTURE INPUT RANGE TYPE DIRECTIONALITY
Generator with
Li-Ion battery 28 – 47 V Buck
Unidirectional
(although some topologies are
intrinsically bi-directional)
Generator with
ultracapacitor 0 – <60 V Buck-boost
Unidirectional
(although some topologies are
intrinsically bi-directional)
Table 3.4.1 Characteristics of DC/DC converter for baseline architecture
However, this solution represents only a limited enhancement of current systems. The capability of
energy recovery is higher than for systems with one unique power net but most of the energy is still
wasted (as shown in Figure 1.1.1) or, in other words, is left unused.
Therefore, to improve the overall efficiency of the vehicle, it is necessary to recuperate / harvest
energy from other sources that may be available. For instance, sources such as waste heat recovery,
solar cells and possibly also AC grid connection. Some of these sources are available at any time, but
with varying efficiency, depending on operating conditions, while others are only available under
certain conditions. For instance, grid connection is available only when car is parked in the proximity
of a charging point.
In order to integrate these power sources on the architecture, different options are possible. Since
auxiliary sources (i.e. all mentioned except braking energy recovery) will be considered as optional
elements in the vehicle and they generally provide a limited power to the system, a straightforward
solution is to have an independent DC/DC converter for each of them and connect it to one of the
power nets (Fig. 3.4.3).
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Fig. 3.4.3 EE-VERT architecture – Baseline with add-on power sources
If the external DC/DC converter is connected to the consumer power net, it is recommend having a
main DC/DC converter that is bi-directional in order to allow recharging of both batteries.
However, this solution is not completely satisfactory since, if an external DC/DC is plugged to the
system, the common operation point of the vehicle may not be optimal for the different subsystems.
Therefore, it is interesting to develop a more compact solution able to support all these energy sources
while providing reliability, flexibility and a unique control. A possible solution is to use a multi-input
power electronics converter (MIPEC), directly interfacing the available energy sources.
Fig. 3.4.4 EE-VERT architecture – MIPEC with alternative power sources
The different sources —the alternator, the solar cells, thermoelectric devices or grid connections —
have different voltage and current characteristics. In general, one source (the alternator) will be
preferred before others but, in some situations, a simultaneous combination of sources is appropriate
for optimal energy/economic use. Therefore, multiple-input power converters (MIPECs) are required
to enable multiple-source technology. An ideal MIPEC could accommodate a variety of sources and
combine their advantages automatically, such that some of the inputs are interchangeable. Such a
converter could also take advantage of the local environment, e.g., in some areas solar power would
be readily accommodated, in others, grid power may be especially inexpensive.
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3.4.2 Baseline
In the baseline solution, a mono-input DC/DC converter is selected for current control. Two options
are possible, depending on the alternator and the storage:
Buck (from HV to LV)
Buck-Boost (from 0-HV to LV)
Buck Converter
For energy transfer from HV (28 – 47 V) to 14 V, the classical buck converter topology is the straight
forward solution. However, the efficiency of classic buck converter is low. To improve the efficiency
several buck converters can be paralleled and the driving signals shifted. This kind of converters is
called interleaving DC/DC converter (also called multiphase synchronous DC/DC converter).
The general structure of interleaving DC/DC converters is shown in Figure 3.4.5. They consists of n
converters in parallel (each with its own inductor) sharing the output filter capacitor.
Fig. 3.4.5 Interleaving DC/DC converter with n phases
For a step-down conversion, i.e., for an interleaving Buck DC/DC converter the resulting circuit is
shown in Figure 3.4.6. The diodes which are normally used to re-circulate the current through the
inductors L have been replaced in the figure below by power MOSFETs [23]. This is called
synchronous rectification. The advantage of the synchronous rectification is the lower voltage drop on
the transistors when they are conducting.
Fig. 3.4.6 Interleaving buck DC/DC converter with n phases [23]
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This converter has bi-directional capability and high efficiency (generally >95%) can be achieved. No
isolation is required between the batteries and therefore, topologies with galvanic insulation are
unnecessary. The output current is the sum of the phase currents.
There are several advantages of using the interleaving buck converters. One of these is the reduction
of the total current ripple. This is shown in Fig. 3.4.7 which presents the maximum current ripple as a
function of the number of phases for an interleaved buck converter. The current ripple is filtered by
the output capacitor which can be much smaller than for one phase. This also gives a possibility to
shift the technology of the output capacitor. Instead of bulky electrolytic capacitors some ceramic
capacitors can be used. This fact has the consequence of reducing the power loss in the output
capacitors because the ceramic capacitors have much lower values of the equivalent series resistance.
Fig. 3.4.7 Interleaving buck DC/DC converter with n phases
The interleaving converter can be implemented using off-the-shelf inductors but there are other
possibilities to realize the inductors. One possibility is to use coupled inductors as shown in Fig. 3.4.8.
In coupled inductors the current slope of one inductor is affected by the voltage across the other
inductors. The main advantage in using coupled inductors is that the current ripple is reduced in
comparison with the case of the uncoupled inductors. If the current ripple is lower, the frequency of
the MOSFETs control can be reduced thus lowering the switching losses in power transistors.
Fig. 3.4.8 Interleaving buck DC/DC converter with coupled inductors
Buck-boost Converter
The buck–boost converter is a type of DC-DC converter that has an output voltage magnitude that is
either greater than or less than the input voltage magnitude. The output voltage is adjustable based on
the duty cycle of the switching transistors. The most usual non-inverting synchronous topology
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consists of a buck (step-down) converter followed by a boost (step-up) converter. Such a non-
inverting buck-boost converter may use a single inductor that is used as both the buck inductor and
the boost inductor.
Fig. 3.4.9 Buck-boost DC/DC converter
This converter can be used as a buck, boost or non-inverting buck-boost converter by selecting the
operating mode and using appropriate control circuitry.
1) Standard control operates all four MOSFETs during each switching cycle see (Fig. 3.4.10 c). This
type of operation generates the classical buck-boost waveforms (i.e. the converter works in buck-
boost mode). In this mode, the RMS current through the inductor and MOSFETs is significantly
higher than that of a standard buck or boost converter. This increases both the conduction and
switching losses in the classical buck-boost. Operating all four switches simultaneously also increases
gate-drive losses, which can significantly lower efficiency at lower output currents.
The physical size of the inductor must also be larger to accommodate the extra current without
saturating. Furthermore, as the output capacitor must carry the full output current during the PWM on-
time (D), and the charge current during the PWM off-time, the output capacitor must have low
equivalent series resistance (ESR).
2) The second buck-boost control scheme reduces losses by only operating two MOSFETs per switch
cycle. Referring to Fig. 3.4.10, this control scheme operates in three distinct modes. When Vin is
greater than Vout, the converter opens SW4 and closes SW3. It then controls SW1 and SW2 as a
classical buck converter. When Vin is below Vout, the control circuitry opens SW2 and closes SW1.
It then controls SW3 and SW4 as a classical boost converter. This control mode has several
operational and control problems around the transition region between the buck and boost modes. The
solution is to operate as a classical buck-boost mode during the transition region. In this operating
mode, as explained, all four switches are operational. Therefore, we have a transition region with
significant efficiency drop due to the increased switching losses and increased RMS currents.
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SW1
SW2
SW3
SW4
(a) Buck mode (Vin >> Vout)
(b) Boost mode (Vin << Vout)
(c) Buck-Boost mode (Vin > Vout)
Fig. 3.4.10 Buck-boost DC/DC converter operating modes
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3.4.3 MIPEC (Multi-input Power Electronic Converter)
For conventional vehicles, the need to reduce fuel consumption and emissions has boosted the search
of ways to optimize the energy used and wasted in the car. To achieve this goal, new and improved
generation sources should be introduced, such as braking recuperation, waste heat recovery, solar
cells and grid (AC power) connection.
In particular, some of these sources have very promising features. For instance, the TEG device is
able to continuously produce electric power; the amount depending on engine load. During the cold
start, the TEG is not able to work at full performance but, as soon as the exhaust gas reaches the
nominal working point, the TEG produces nominal output power. From an energy management point
of view, the TEG device can be integrated very well. The combination of TEG and brake energy
recovery allows a source of continuous ―free‖ energy, in most vehicle conditions.
However, to use these sources, a device able to combine the available sources is needed. Multiple-
input power electronic converters (MIPEC) have recently been developed to interface more than one
power source with a load, especially in the field of renewable sources. By using these devices it is
possible to diversify the energy sources so that the power system availability can be increased. In
automotive, MIPECs have not been used until the advent of Hybrid and Electric vehicles and the use
of high-voltage batteries for supporting traction [24]. In conventional vehicles, this strategy has not
been yet used.
High-level design
In EE-VERT architecture, the MIPEC should fulfil the following requirements:
power sources may have different power and voltage / current variation range;
power sources can deliver power to the load individually or simultaneously;
current flowing from any source / storage element to any load / storage element is possible
and controllable; set up the appropriated energy flow path and transformation strategy to
combine the multiple sources in one single power net line;
select the appropriate sources / storages supply configuration based on vehicle conditions for
energy efficiency;
maximize efficiency conversion on ―non-free‖ energy (i.e. energy obtained from fuel-
consumption);
Maximum Power Point Tracking (MPPT) for PV and TEG arrays;
design should be as compact as possible while maintaining flexibility on input types.
The general circuit topology for a MIPEC fulfilling these requirements is shown in Fig. 3.4.11. It is
based on two principal stages: a Smart Source connection/selection Bays (SSB) and the multi-input
DC/DC converter itself. The aim of SSB is to connect-disconnect at appropriate time and system
conditions, based on Energy Management strategy, the different sources/storage devices into the
power converter. In some cases, depending on the actual sources and storages devices connected and /
or the topology of the MIPEC, the SSB may be eliminated since its function is not needed and / or is
already done by the switches in the MIPEC.
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Fig. 3.4.11: Multi-input power electronic converter with smart sources connection bays
This flexible connection by means of SSB, allows the MIPEC to select among different generation
sources and storage devices to set up the more convenient power structure to satisfy the vehicle
electrical net condition, while maintaining the targets originally assigned to it. The Energy
Management algorithm (developed later in the EE-VERT project) is in charge of setting up those
targets.
Smart Sources Bays (SSBs)
The aim of SSB is to connect-disconnect at appropriated time and system conditions, based on Energy
Management strategy, the different sources/storage devices into the power converter. The SSB main
characteristics are listed below:
SSBs must be generally bi-directional blocking controlled.
SSBs work in long time ‗ON‘ conditions at high current ratings.
SSBs work at relatively low frequency.
SSB can be implemented as IGBT or series MOSFET and diode pair depending of voltage
and power of the circuit.
The low level driver acts over the SSB and the converter in order to set up the appropriated
configuration for the conversion mode and vehicle condition. The SSB technology depends on
power/dynamics of application (IGBT, MOSFET, SCR, etc.) and directionality of the power / energy
source (an alternator is unidirectional while a battery is intrinsically bi-directional). Fig. 3.4.12: shows
an example of a bi-directional SSB based on MOSFET technology as proposed in [25].
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Fig. 3.4.12: Bi-directional SSB based on MOSFET technology [25]
DC/DC Converter
As mentioned before, two kinds of qualitatively different inputs are present in a vehicular
environment: for sources such as thermoelectric generator or solar cells, only unidirectional power
flow is needed. Storage elements such as batteries or ultracapacitors require bi-directional power flow.
For the realization of the converter, several options are available, depending on the topology.
Combination strategies include sharing the output filter capacitor, sharing some switches and energy
transfer inductor and capacitor, and sharing a magnetic core [24]-[27]. These input combination
methods are shown in Fig. 3.4.13.
.
(a) (b) (c)
Fig. 3.4.13: MIPEC topologies. (a) sharing output filter capacitor
(b) sharing inductor, switch and/or capacitor, (c) sharing magnetic core [26]
Variant 1: Output filter capacitor sharing
This implementation is based on interleaving topology, i.e., the one used in mono-input converters.
This topology is typically used in hybrid / electric vehicle applications since they are easily matched
with requirements for energy flow and operation modes.
In hybrid / electric vehicles with two storage elements (battery and ultracapacitor), these topologies
are generally used to supply the inverter for traction load. In this case, a bi-directional buck-boost
converter is usually implemented such that the converter acts as step up converter (boost converter)
for one mode of operation and as step down converter (buck converter) for other mode of operation.
Each power source is connected to the DC-link by means of this bi-directional converter. Step up
mode of operation is used in order to transfer energy from each power source to the DC-link, where as
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step down operation is used to charge both ultracapacitor and battery storage system and to recover
the braking energy. Main features are:
1. Interleaved single cells sharing output filter
2. Cells based on simplest buck-boost topology
3. Variant suitable for:
Non isolated requirements
Bi and mono directional power flow
Non-overlapped input voltage sources ranges
4. Drawbacks:
No isolation
Low rate of simplification/integration (high cost)
Low-medium power rates
Output O-ring needed
Variant 2: Inductor, switch and/or capacitor sharing
This implementation is typically used in combined (wind and solar) renewable energy installations in
the form of a unidirectional buck-boost converter. Main features are:
1. Common power cells sharing power plant components
2. Cells based on simplest buck-boost topology
3. Variant suitable for:
Non isolated requirements
Bi & mono directional power flow
Overlapped input voltage sources ranges
4. Drawbacks:
No isolation
Low-medium power rates
SSBs and its control play a key role assuring operating conditions at the input.
Variant 3: Magnetic core sharing
This implementation is preferred for developments such as battery chargers, i.e., incorporating a
connection to the grid for recharging the battery. Main features are:
1. Common Secondary cells sharing power plant components.
2. Cells based on simplest isolated buck-boost topology
3. Variant suitable for:
Isolated requirements
Bi and mono directional power flow
High voltage applications
Un-overlapped input voltage sources ranges
Symmetrical topology
4. Drawbacks:
Cost due to number of components
Transformer characteristics are key to achieve high efficiency
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Fig. 3.4.14: MIPEC structure based on Magnetic core sharing
3.4.4 MIPEC working operation
In order to understand the system operation with the MIPEC, let us consider a version with four
power sources: generator with regenerative braking, thermoelectric generator, photovoltaic generator
and grid connection and one storage element (an Li-ion battery). The generator and the battery are
directly connected and working at voltage higher than the power net voltage (in order to improve
efficiency of generation and recuperation).
The different working operation modes of the MIPEC are directly linked to vehicle status. In
conventional electrical systems, the power supply necessities during driving phases were covered by
the alternator as main provider. The alternator charge regulator was not directly associated to specific
driving phases like traction or coasting/breaking mode. Therefore, the electrical energy was converted
independently of the state of the machine. In new alternator designs, as the proposed, the possibility to
partly regenerate the brake energy of the vehicle, known as recuperation, will be possible.
In systems like BMW microhybrid [28], a system named BER implements this function. In this mode,
the moment of inertia drives the engine with engaged gear. Therefore, the rotational speed of the
alternator is not powered by fuel consumption, but by the vehicle inertia moment, i.e. the already
‗paid‘ energy. The BER system controls the rotator exciting current to an alternator output of 14.8 V
on-board voltage. This implicates over-covering of the electrical consumers resulting in recuperative
charging of the battery.
However, two issues should be considered: firstly, the conversion of mechanical to electrical energy
strongly depends on the charge acceptance of the battery at the moment of the voltage increase. As the
charge acceptance is inversely proportional to the state-of-charge (SOC), the state-of-charge has to be
regulated to a certain level below 100 % SOC while driving. Secondly, the efficiency of the
conversion of energy depends on engine regime. By being aware of these situations, it is possible to
find appropriate time zones / vehicle state where it is more efficient to use energy from a different
source.
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Depending on status of primary inputs, the alternator, the thermoelectric generator and grid are
connected as inputs to the common input MIPEC. Due to different conduction cases of switches, the
primary converter can be operated in buck, boost and buck-boost modes for different values of input
voltages.
Figures 3.4.15 and 3.4.16 shows an example of using the MIPEC to efficiently control the different
sources to provide demanded current to the loads, by choosing the most suitable combination of
power sources at any driving situation (acceleration, braking, and steady-state) for a NEDC drive.
In PARKING MODE (engine OFF), the vehicle loads generate almost no consumption. In this
scenario, battery is recharged from available sources: energy from a solar cell (if available) or energy
from grid plug-in may be used to fill completely the battery and provide energy to the loads (like car
access system, alarm or fans for controlling vehicle interior temperature). Also, it may be recharged
from LV powernet if needed or the other way around as well, i.e. the Li-ion could be used to charge
the 14V battery.
LV powernet
CAN-bus
Grounding
Supervisor
HV source /storage
Thermoelectric source
Photovoltaic source
Electric Grid
LV powernetThermoelectric source
Photovoltaic source
Electric Grid
CAN-bus
Grounding
Supervisor
HV source /storage
(a) (b)
LV powernet
CAN-bus
Grounding
Supervisor
Thermoelectric source
Photovoltaic source
Electric Grid
HV source /storage
(c)
Fig. 3.4.15: MIPEC in PARKING MODE (a) recharge from grid. (b) recharge from solar cell. (c)
recharge from LV power net (using LV battery, external charger or external solar cell)
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In DRIVE MODE (engine ON), energy for supplying loads is initially provided by the battery. It is
assumed that vehicle starts with battery at high SOC (since has been recharged when is parking by
means of a solar cell or grid connection). Therefore, at start and low speeds (low efficiency regime for
the alternator) the vehicle electrical system is supplied with electrical energy from the battery. If an
extra power source is available, this energy may be used to support the current demand by combining
them through the primary converter, therefore reducing somewhat the battery current. However, just
during the cold start, the TEG is not able to work at full performance. As soon as exhaust gas reaches
the nominal working point, TEG produces nominal output power.
While driving, the alternator may be used in braking to recuperate energy using a regenerative braking
system to recharge the battery. Thus, the alternator control system may accomplish approximately
zero discharging / charging of the battery while driving, in order to keep it at partial state-of-charge
allowing the system to recuperate braking energy. In this way, the alternator generates electrical
power with decreased power demand on the internal combustion engine.
At specific regimes, where alternator / TEG are highly efficient, it may be used to provide current lo
loads and, depending on the SOC of the battery, to sustain the battery SOC or, if needed, to recharge
it. In general, however, direct operation of the alternator should be kept at a minimum to reduce fuel
consumption. In this regime, thermal TEG is more efficient, so this energy may be used to support the
current demand by combining them through the primary converter, therefore reducing somewhat the
alternator current or helping for a fast recharge of the battery. Or, if the load is low, the system may
use the remaining energy to recharge the HV battery.
LV powernetThermoelectric source
Photovoltaic source
Electric Grid
CAN-bus
Grounding
Supervisor
HV source /storage
LV powernetThermoelectric source
Photovoltaic source
Electric Grid
CAN-bus
Grounding
Supervisor
HV source /storage
(a) (b)
Fig. 3.4.16: MIPEC in DRIVING MODE: (a) Direct mode conversion (b) Direct mode conversion
plus recharge from harvesting sources.
Using this strategy, system variables depending on an exemplary driving pattern in NEDC is shown in
Fig. 3.4.17. With this strategy, the power demand of the alternator related to fuel consumption is
reduced. The total amount of this reduction depends on several factors such as the statistics of the
driving phases, the actual efficiency of power electronic modules (including the alternator and the
MIPEC) and, especially, on the charge acceptance of the storage systems. A more detailed evaluation
of this fuel reduction is addressed later in EE-VERT project.
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Fig. 3.4.17: System variables depending on an exemplary driving pattern with MIPEC
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3.5 Conclusions
The objective of this part of the EE-VERT project was the development of adequate power generation
concepts for the EE-VERT approach.
In 3.1 a new generator concept was introduced and explained. In 3.2 and 3.3 innovative systems like
solar panels and thermo-electric generators have been studied. In 3.4 especially the integration and
combination of the different power generation technologies have been investigated with an adequate
DC/DC converter topology.
The generator is currently responsible for the conversion of mechanical energy from the vehicle‘s
engine to electrical energy for the power distribution network, and is therefore a crucial component
for the overall energy efficiency. But the average efficiency is very low. In addition, the capability of
the current regenerative braking is limited due to the voltage level of 14 V and the characteristics of
the lead-acid battery.
To overcome the current limitations a new generator concept was identified and selected. The
introduction of the new generator concept with permanent magnets, a higher voltage level and an
adequate storage technology greatly improves the regenerative braking capabilities. Brake energy
recuperation is now possible with up to 8 kW.
Due to the EE-VERT architecture with decoupled power generation and consumption the generator
can now be operated with more flexibility and has a higher efficiency especially during standard
operation.
The alternator concept is so promising that the EE-VERT consortium has started to develop a
prototype generator. It keeps the present geometry of the claw pole generator but with integrated
permanent magnets to improve the characteristics.
Regarding the photovoltaic study several benefits that a solar panel would offer to an overall vehicle
energy management strategy were identified. Hence, it is proposed that solar panels should be used as
an additional source of power for vehicles since they are able to deliver around half of the basic
power net demand for passenger cars which is a significant proportion (!). Also for cabin-
preconditioning PV panels offer a number of benefits. Especially since power delivered from PV
panels is power that is not generated by the engine.
Since two thirds of the chemical energy supplied by the fuel is converted into heat energy it is
proposed that a heat recovery device should be used as an additional source of power for vehicles.
Different technologies are available to recover thermal wasted energy but the study has shown that the
focus of EE-VERT should be on thermo-electric conversion. Among the described technologies
clearly TEG must be considered in the new EE-VERT approach, being a source of electric energy
always available during the whole vehicle mission. EE-VERT will benefit here from other ongoing
projects, like HEATRECAR for instance.
For the combination of all the power generation concepts a new DC-DC converter design with high
efficiency has been developed and proposed with multiple inputs with different voltage levels.
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This chapter has shown that there are additional concepts for power generation that could be of
benefit for the EE-VERT approach. All the investigated concepts will be a part of the EE-VERT
system. In this stage of the EE-VERT project it is planned that the new generator concept, solar panels
and the DC/DC converter concept will be integrated into the demonstrator car while the thermo-
electric device will be at least investigated via simulations in the system context.
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4 SYSTEM INTEGRATION AND MANAGEMENT
The following chapter describes how the energy generation sources are integrated in the EE-VERT
approach, the system concept and the operation strategy. Furthermore, the link to hybrids, the impact
on safety relevant applications and the impact on commercial vehicles will be considered.
4.1 System concept
4.1.1 Basic EE-VERT approach
The central EE-VERT concept is to combine several different approaches to energy saving within an
overall energy management strategy (thermal and electrical). The approaches include:
Greater efficiency in energy generation with a new concept for the electrical generator and an
optimised operation strategy;
Improved efficiency in energy consumption, through electrification and demand-oriented
operation of auxiliary systems and the use of more efficient electrical machines such as
brushless DC motors;
Energy recovery from wasted energy such as waste heat recovery or an optimised braking
energy recuperation with a temporarily increased generator output power with up to 8 kW at a
higher voltage level;
Energy scavenging from unused and new sources of energy, for example the use of solar cells
on the roof of the vehicle.
The increased electrification of auxiliary systems with an optimised operation promises efficiency
gains. But this can only be accomplished if the energy generation and distribution is optimised and
adapted to the current driving conditions and the power demands.
Solar
panels
Waste
heat
Electric
energy
Generator
Brake energy
recuperation
Engine
lubrication
system
Engine
cooling
system
Air
conditioning
system
Oil
pu
mp
Wa
ter
pu
mp
Va
cu
um
pu
mp
Ste
eri
ng
pu
mp
…
A/C
co
mp
res
so
r
Po
wer
ge
nera
tio
nC
om
po
nen
tsS
ys
tem
Electrified auxiliaries
Fig. 4.1.1 The basic EE-VERT approach [4]
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Fig. 4.1.1 shows the basic EE-VERT approach. Electric energy is recovered from regenerative
braking, waste heat and solar cells. The (engine) auxiliaries are mainly driven by the recovered
electric energy. This strategy leads to the following benefits:
Auxiliary systems can operate on a demand-oriented basis and fulfil their tasks in an
optimised way;
Less mechanical power demand on the engine;
Less engine drag torque, because the generator and the electrified auxiliaries runs only on
demand and not continuously, leading to a higher capability for braking energy recuperation.
The optimised operation of auxiliary systems allows the thermal engine management and the air
conditioning system to be enhanced. This leads to additional benefits for the fuel demand and for the
convenience of vehicle users.
4.1.2 EE-VERT architecture with the power generation components
In order to achieve the necessary flexibility in power generation and consumption the system concept
requires a change in the electrical architecture to permit the integration of multiple generation,
actuation and storage devices with different optimal operating voltages and usage profiles.
Consequently, it is necessary to divide the electrical distribution system into two parts: one electrical
distribution system for the existing standard loads (14 V) and an electrical distribution system on a
higher voltage level (40 V) for efficient energy recovery and for high power loads (e.g. electric power
steering). The subsystems are connected by a DC/DC converter and incorporate different energy
storage elements. The EE-VERT architecture is shown in Fig 4.1.2.
Fig. 4.1.2 EE-VERT architecture with different new power sources
The EE-VERT architecture will deploy a distributed network of smart components, whose
characteristics are co-ordinated to optimise their interaction and their efficiency. In addition to this, it
is crucial to manage the use of different types of energy, such as electrical, mechanical or thermal
energy, which means that an overall vehicle optimisation and management concept needs to be
developed. This is the task of work package 3. Generally the power generation and the power
consumption are decoupled. If a bi-directional or unidirectional DC/DC converter is necessary
G
DC / DC
Storage
Waste heat recovery
Solar cells
Load # 1
Volt . stab . Load # X + 1
Load # X
Volt . stab . Load # N Starter Lead acid
battery
Low voltage power net
AC power 110/220V
High
Power
Loads
High voltage power net
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depends on the kind of high power loads on the higher voltage level and on the configuration of
power from other solar panels and waste heat recovery as well and how this is configured.
4.2 Operation strategy
The decoupling of the power generation and the power consumption which is one of the basic EE-
VERT concepts leads to a new flexibility in the system management. Furthermore, due to that there
are several different energy sources for the electrical network – discussed and investigated in chapter
3 - it is possible to control the power generation in a way that the efficiency is maximised in every
operation point. EE-VERT will combine the contributions from different energy sources in the best
way.
The operation strategy will deal with optimising the fuel consumption especially during NEDC but
also in real-life missions. For that, it is considered to switch on the alternator only when the cost of
electrical power generation, in terms of fuel, is low, i.e. during braking recuperation and stop phases
as much as possible. The optimization performances for the operation strategy will have as a main
constraint the feasible operative area of the storage components. These components will provide the
electrical power whether the alternator is switched off and their operative area can be established as a
function of the state-of-charge.
The calculations and simulations made in function of the storage requirements and the designed
generator have shown that on NEDC there will be enough energy to switch on the alternator only at
the low cost generation phases.
Further developments in the operations strategy will be made in WT3.2 ―Optimisation Strategies‖ and
in WT3.5 ―Systems Integration‖. This work tasks will continue to use the results and work of WT2.1.
4.3 Link to hybrid vehicles
Transferability of the solutions
A hybrid vehicle has a relatively high potential to reduce CO2 emissions but it presently requires cost-
intensive and drastic technical modifications. In the past hybrid vehicles have penetrated the
automotive market relatively slowly in Europe. Hence, hybrid vehicles can provide only a slowly
increasing effect for the reduction of CO2 emissions in the short and medium term. This is one of the
reasons why EE-VERT is focusing on marketable solutions (in respect of vehicle integration, costs
and reliability) for conventional vehicles. Additional functions and benefits especially for
convenience, and taking into account the transferability of the solutions (e.g. for hybrid or electric
vehicles) will guarantee a high market acceptance of the solutions to be developed.
Auxiliary units and power management
Hybrid vehicles typically require auxiliary units that must be modified to operate with electric power
as the ICE is switched off during EV driving:
Electrically powered steering (EPS or EHPS)
Vacuum pump for braking assistance (passenger car)
Air compressor for brake system (commercial vehicle)
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Water pump for cooling motor, inverter and DC/DC converter
Water pump and possibly AC compressor for cooling the battery pack
Actuator to operate clutch and automatic transmission - depending on hybrid topology (e.g.
electrical oil pump)
During pure electric vehicle mode, brake energy recuperation by electric generator and stop/start
operation the listed auxiliaries are required to be fully functional. The designer can decide whether the
larger loads are placed on the high voltage system, otherwise these systems will draw electrical power
from the low voltage system.
Additionally the comfort and infotainment systems may also be activated and draw electrical power
from the low voltage system. An intelligent power management can decide or control what equipment
is available during different operation modes.
It is evident that hybrid vehicles will benefit from the availability of more efficient, electrically driven
auxiliary units.
EE-VERT’s power generation components
The link to hybrids is bi-directional. As described in subchapter 3.1 the EE-VERT generator is
optimised to deliver a high electrical power especially during braking phases. This is an operation
strategy that is used already in hybrid vehicles. Here EE-VERT benefits from the experiences in
hybrid vehicles. But due to the different boundary and system conditions the generator for EE-VERT
is based on the standard claw pole generator. It is only generating energy while an electrical machine
for hybrid vehicles has to be a generator and an electrical motor. Therefore, the EE-VERT generator
is not directly transferable to a hybrid car. The EE-VERT generator is optimised especially for the
recuperation phases and to deliver a high efficiency for power generation in standard operation mode
in conventional vehicles.
EE-VERT solutions which are transferable to hybrid vehicles are the waste heat recovery and the
usage of solar panels. Both technologies are adaptable to hybrid architectures. Especially the
developed DC/DC converter operation strategy (see subchapter 3.4) is usable for hybrid vehicles. The
DC/DC converter combines several energy sources. This is highly interesting for hybrid vehicles as
well. Different is however the voltage level. The voltage level has to be adapted to hybrid
architectures since the voltages in hybrids are usually higher.
4.4 Impact on safety relevant applications
A brief overview of these issues is given here since the subject of safety-relevant applications will be
studied in detail in WT3.3 which starts in October 2010.
Top-level safety requirements
The following top-level requirements were identified in D1.3.1 [3] as applying to power net
architectures and energy management strategies:
Energy management strategies shall consider the requirements for availability of power supply to
safety-related functions.
Energy transfer shall take place only from non-critical to critical power supply busses.
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Any safety-related power net or energy management function shall have a safety level, such as
SIL or ASIL, associated with it. The SIL or ASIL associated with power net and energy
management functions shall consider the SIL or ASIL of all safety-related vehicle functions that
depend on those functions.
Power net hardware shall be analysed to determine which faults could lead to a dangerous failure,
and which proportion of those failures are covered by a diagnostic capability.
The power net shall have sufficient fault tolerance and redundancy with the safety requirements of
the functions it is supplying.
Energy management strategies shall be able to provide ―state of health‖ information on the power
supplies to safety-related functions that require this information, and shall be able to provide (for
example, through prognostic functions), ―early warning‖ of impending disruption to the power
supply to safety-related functions.
Furthermore one of the key findings of the previous EASIS project was to require redundant power
supplies to certain safety-related functions.
EE-VERT is a key enabling technology
The power generation strategy and electrical architecture proposed in EE-VERT is a key enabling
technology to achieve such a redundant power supply strategy since it incorporates a number of levels
of redundancy and dissimilarity including:
Two separate power nets with differing voltage levels;
Redundant storage devices (Li-Ion battery and/or supercap and conventional battery);
Energy generation and recovery from multiple sources;
Multi-input DC/DC converter.
In respect of the requirements of safety-relevant functions, further developments in the EE-VERT
project need to consider:
The design of the multi-input DC/DC converter should support energy management strategies that
permit prioritising of available energy to critical functions.
The design of the multi-input DC/DC converter should also consider a modular approach such
that an additional output bus can be added for critical supplies if needed (see for example
Architecture 7 of [3]). This could either be an additional output block within a single DC/DC
converter (provided there is adequate independence) or a separate additional converter for the
critical power bus.
The design of the multi-input DC/DC converter should ensure that a failure in a non-critical bus
cannot affect a critical bus.
Provision for health monitoring and diagnostics should be provided in the DC/DC converter and
in control and management aspects of the power supply.
Conclusion
With two energy storages, several power generation sources and two decoupled sub nets a first simple
consideration offers that at least two failures are necessary to disconnect a safety relevant application
from the power supply.
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Fig. 4.4.1 Fault tolerant power supply from several connection points
Fig 4.4.1 shows that it is possible with the EE-VERT architecture to supply a safety relevant load
from different connection points. Therefore, the safety level of the EE-VERT power net is much
higher than the safety level of the current conventional 14 V power net. A more detailed safety study
including the system management and the operation strategy will be done in WT3.3.
4.5 Impact on commercial vehicles
Buses are the commercial vehicles that are in focus in the EE-VERT project. In [2] a Volvo 7700 city
bus was chosen as reference vehicle for the commercial vehicles. Buses are the focus of this
paragraph as well although most of what is written here is applicable also for trucks.
Much of what is said in previous paragraphs about conventional vehicles is applicable also for
commercial vehicles. A few important differences in the prerequisites for commercial vehicles
compared to conventional vehicles which influence the components and power net architecture are
listed below:
● The mass is bigger for commercial vehicles. This leads to more available kinetic energy to
recuperate when braking since the available kinetic energy is proportional to vehicle mass.
● The power load sum of mechanical and electrical auxiliaries are higher in commercial
vehicles. However, the total power load of auxiliaries generally does not increase as much as
the kinetic energy available to recuperate.
Note: The most power consuming auxiliaries, which today normally are mechanically driven,
are air compressor, power steering, oil pump, water pump, motor cooling fan and A/C
compressor.
● Commercial vehicles generally obtain significantly more mileage and / or operational hours
per year.
Volvo Bus Corporation currently produces and sells a full hybrid 7700 city bus, see [18]. In this bus
some of the auxiliaries are electrified such as the air compressor, the A/C compressor and the steering
are electro-hydraulic. The kinetic energy recuperated as electric power in this hybrid bus covers the
need, with margin, of all the currently electrified auxiliaries for a typical city driving cycle. It is most
likely that there will be a surplus of electric energy even with all auxiliaries electrified. In a full hybrid
this surplus of electric energy can be used for propulsion. For a long-distance coach it will probably
40 Vdc
subnet G DC/DC
coupling
14 Vdc
subnet
Additional
power sources
Safety relevant system
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be quite different though, i.e. the recuperated electric energy will not be sufficient to drive all
auxiliaries if electrified. The balance between recuperated electric energy and the auxiliaries load
might be better for an inter-city like route. More intercity bus like routes will therefore also be
investigated for the chosen reference bus. The same argument of energy balance should be applicable
also for a truck application with an optimum somewhere between the two extremes of a refuse truck
and a long-haul truck.
Power net architecture
Despite the differences listed above between commercial and conventional vehicles the power net
architecture presented in section 4.1.2 with small modifications, is considered to be the most
promising also for commercial vehicles. The major change proposed is an increase in voltage level,
see discussion about voltage level below.
Storage
No decision on storage type, battery or supercapacitor, has been taken at the moment for commercial
vehicles. Cost, especially to predict future cost in larger quantities, is difficult. Both battery and
supercapacitor solutions seem quite expensive at the moment.
A solution with a combination of storage types is possible. In this scenario the supercapacitor
interfaces the generator and a DC/DC converter is inserted between supercapacitor and battery. Here
the high power loads are connected to the battery where the voltage level is more stable. This solution
is however discarded at the moment due to the extra complexity and hardware that would be needed
and the therefore inevitable cost increase.
It will be investigated further in the project which storage type to use. Below a few advantages for the
different types are listed.
Battery Supercapacitor
Very beneficial if start-stop functionality is
included. A battery makes it easy to run
electrified auxiliaries that are needed during the
stop phase
Lower voltage swing less stringent
requirements needed for high power auxiliaries
and the DC/DC converter
Lower power losses in storage device no extra
cooling should be needed
Longer life-time is expected. This is especially
good considering the higher mileage compared to
for a conventional vehicle
Table 4.5.1 Comparison of advantages with a battery or a supercapacitor storage
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Voltage level
The baseline, based on the information in the table below, is to choose 600 V in the high voltage node
of the power net architecture.
40V 600V
<60VDC according to [19]
o No galvanic isolation is necessary in
DC/DC between high voltage and low
voltage node
o Safety restrictions less severe, i.e. less
isolation of cables is necessary etc
Slightly cheaper Battery Management Unit
(BMU). This since there will be fewer cells in
series which means less voltage nodes for the
BMU to handle
Slightly higher efficiency of DC/DC is expected
It is expected that at the end of the EE-VERT
project many electrified auxiliaries will be
available at this voltage level in the future. They
will be optimised for conventional vehicles
though.
Lower currents at higher voltage
o ―Reasonable‖ cable dimensions:
Generator to battery and inside
generator.
Note: With 40V option and assumed
peak recuperation of 40-120kW the peak
currents would be in the order of 1-3kA
o Weight and cost is lower for power
cables to high power electrified
auxiliaries
o Cable power losses will be lower
o Generator with a little bit higher
efficiency
o Auxiliaries with a little bit higher
efficiency are expected
Advantage for Volvo: A few electrified
auxiliaries are already available from the hybrid
bus project at this voltage level. Standardisation
at one voltage level leads to lower prices
Table 4.5.2 Comparison of advantages with a high voltage node voltage of 40 V or 600 V
Generator
The efficiency will be a little higher for commercial vehicles due to the choice of a high voltage level
of 600V. The impact of the significantly higher power levels in commercial vehicles on for instance
generator mass and cooling arrangements need to be investigated.
Solar panels
The potential for solar generation on commercial vehicles seems to be significant. The highest
potential exists for buses due to the big area available on top of the roof. The costs for solar cells /
panels are currently quite high but the cost might drop in the future so this is planned to be
investigated further in the project.
Typical truck
For a ―typical truck‖ an area in the order of 2.5m2 should be available for solar cells placement on top
of the cab. Given the possible cell sizes listed in Section 3.2.3, the following options are available
with this open area, assuming that the full roof curvature can be accommodated by any particular
panel.
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Cell
Type
Cell size
(mm x
mm)
Number of
cells (Length)
Number of
cells
(Width)
Total
Cells
Potential
Peak Power
(Wp)
~Cell Cost
(€)*
MCSI 100 x100 12 20 240 360 775
MCSI 125 x 125 9 16 144 466 1005
PCSi 156 x 156 7 13 91 336 670
MCSI 156 x 156 7 13 91 355 775
Table 4.5.3 The potential result with solar panels covering 2.5m2 of the roof of a vehicle
* Approximate cost for the cells (not ‗made up‘ panel)
It can be seen that the high efficiency mono-crystalline cell offers the best prospect, with some 460
W(peak) being theoretically available. However, from the discussions on system losses covered in
Section 3.2.2, ~345 W(peak) delivered to the point of need is more realistic.
It should be noted, however, that many cab roofs have a skylight for driver comfort (both visual and
airflow) and many trucks have aerodynamic fairings added to boost fuel efficiency. Clearly such
additions would make the use of solar panels impractical in these cases.
Typical bus
Due to their large size, buses offer the best possibility of panel coverage. However, like for the truck,
the roof space is sometimes taken up with additional features, particularly skylights for coaches and
air conditioning packs for many buses and coaches. Again this would limit coverage.
The roof of a typical bus, such as the Volvo 7700 (12 m version), covers an area of approximately
12m x 2.2m = 26.4m2. In the calculation below it is assumed that around 80 % of this area, ~21m
2, is
available for solar panels. Given the possible cell sizes listed in Section 3.2.3, the following options
are available with this area.
Cell
Type
Cell size
(mm x
mm)
Number of
cells (Length)
Number of
cells
(Width)
Total
Cells
Potential
Peak Power
(Wp)
~Cell Cost
(€)*
MCSI 100 x100 93 21 1953 2930 6310
MCSI 125 x 125 75 17 1275 4131 8900
PCSi 156 x 156 60 14 840 3150 6190
MCSI 156 x 156 60 14 840 3267 7140
Table 4.5.4 The potential result with solar panels covering 21m2 of the roof of a vehicle
Again it can be seen the high efficiency mono-crystalline cell offers the best prospect, with some 4.1
kW(peak) being theoretically available. However, from the discussions on system losses covered in
Section 3.2.2, ~3.1 kW(peak) delivered to the point of need is more realistic.
Waste Heat Recovery
Both thermoelectric generation and thermal to mechanical and then from mechanical to electrical
generation with a generator look promising also for commercial applications. Details about these
technologies can be found in section 3.3.
EE-VERT Deliverable D2.1.1 23 December 2009
Version 1.0 Page 85
DC/DC converter
A DC/DC fitting into the architecture described in section 4.1.2 is foreseen also for commercial
vehicles. If it is to be unidirectional or bi-directional are left unanswered at the moment. Decision on
some implementation details such as converter topology will be left to suppliers.
EE-VERT Deliverable D2.1.1 23 December 2009
Version 1.0 Page 86
5 CONCLUSIONS AND OUTLOOK
The project ―EE-VERT‖ is concerned with improving the energy efficiency of conventional vehicles.
The central concept is the electrification of auxiliary systems, and supplying their energy by recovered
energy from new sources or wasted energy such as recuperation of braking energy, waste heat
recovery or solar cells.
This report has described the EE-VERT concept to reduce the fuel consumption in conventional
vehicles especially by generating electrical energy from different sources. Therefore, this report has
presented the power generation concepts investigated by EE-VERT and needed for the operation of an
advanced electrical power net aimed at generating and reusing energy with a very high efficiency.
The objective of work task 2.1 was the development of adequate power generation concepts with high
efficiency including their integration and operation strategies within EE-VERT‘s system approach.
Also the integration of innovative systems has been studied, such as solar panels and thermo-electric
generators to recover energy from exhaust gases. Thereby, EE-VERT is focusing on marketable
technologies for conventional road vehicles with the potential for a fast market launch and market
penetration. The link to hybrid vehicles and to commercial vehicles was considered and furthermore a
brief overview of the impact on safety relevant applications was given.
Finally, the work and results of work task 2.1 has contributed to deliver adequate power generation
concepts with high efficiency for the EE-VERT concept.
EE-VERT Deliverable D2.1.1 23 December 2009
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