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Training Module COMBINED HEAT AND POWER PROJECT ANALYSIS CLEAN
ENERGY PROJECT ANALYSIS COURSE SPEAKERS NOTES
This document provides a transcription of the oral presentation
(Voice & Slides) for this training module and it can be used as
speaker's notes. The oral presentation includes a background of the
technology and provides an overview of the algorithms found in the
RETScreen Model. The training material is available free-of-charge
at the RETScreen International Clean Energy Decision Support Centre
Website: www.retscreen.net.
SLIDE 1: Combined Heat and Power Project Analysis
This is the Combined Heat and Power Project Analysis Training
Module of the RETScreen Clean Energy Project Analysis Course. In
this presentation, we examine the use of combined heat and power
systems, such as the plant shown in this photo.
Slide 1
SLIDE 2: Objectives
This module has three objectives. These are, first, to review
the basics of combined heat and power, or CHP, systems; second, to
illustrate key considerations in combined heat and power project
analysis; and third, to introduce the RETScreen Combined Heat and
Power Project Model.
Slide 2
SLIDE 3: What do Combined Heat and Power (CHP) systems
provide?
CHP systems provide both electricity and heat, and are sometimes
called cogeneration systems as a result. The electricity can be
used for loads on or near the site, or can be fed into the electric
grid. The electric generation equipment in the CHP system is
usually driven by the combustion of fuel, such as gas, diesel, or
biomass. Part of the combustion heat can not be used to generate
electricity; the combined heat and power system recovers this heat,
which would otherwise be wasted, and distributes it to nearby heat
loads, such as buildings and industrial processes.
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SLIDE 3: What do Combined Heat and Power (CHP) systems provide?
(cont.)
A CHP system has a number of attractive attributes other than
its ability to provide heat and power. These include increased
efficiency, reduced waste and reduced emissions. Normally the
production of electricity is not especially efficient, due to heat
rejected to the environment. By making use of this wasted heat, the
overall system efficiency is improved, and more useful energy is
produced per unit of fuel. This in turn reduces total greenhouse
gas and other pollution emissions.
Since heat is not easily transported over long distances, CHP
plants are normally located near heat loads, and thus dispersed
geographically. This often means that electricity is produced
closer to the ultimate load than is the case with centralized power
production, in which large plants service loads that may be quite
distant. This so-called distributed generation can reduce losses
incurred in the transmission of electricity.
Combined heat and power developments can provide a source of
energy, and heat in particular, for district energy systems. In
district energy systems, a central plant distributes hot water or
steam, and sometimes chilled water, to the buildings in its
vicinity. This arrangement is more efficient than locating
autonomous heating and cooling equipment in each building.
Some CHP systems also provide cooling. The CHP system may drive
conventional refrigeration equipment using a portion of its power
output, or it may turn its heat output into cooling using
absorption refrigeration equipment or desiccant dehumidifiers. In
this way, increased demand for cooling during hot periods provides
a heat load to make up for reduced space heating requirements.
SLIDE 4: CHP System Motivation
Electricity generation is inherently inefficient: one-half to
two-thirds of the energy input to a typical generation system is
wasted as heat, rather than turned into electricity.
This is the motivation for combined heat and power systems:
locate an electric power generating system near a heat load that
will utilize the heat rejected from the generation equipment. This
increases overall efficiency, from the range of 25 to 55% to the
range of 60 to 90%, depending on the equipment and the application.
The heat, which would otherwise be lost, can be used for industrial
processes, space heating, water heating, cooling, or other
purposes.
The financial motivation for CHP plants often stems from the
high value of the electricity produced. Electricity is more
versatile and easily transported than heat, and thus commands a
higher price. If a plant must be built to furnish heat to a load,
it can make sense to invest slightly more in the plant, so that it
provides both heat and electricity.
The figure on this slide shows that the quantity of heat wasted
by thermal conversion plants is enormous: around 25,000 TWh
worldwide in the year 2002. The figure also shows that fossil
fuels, specifically coal, oil, and natural gas, drive the majority
of the worlds electricity generation, so improving efficiency can
have major economic and environmental benefits.
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SLIDE 5: The CHP Concept
This concept of using the waste heat from power generation
equipment is illustrated by the diagram on this slide. The input to
the system is 100 units of energy, embodied in fuel. This operates
the power system, which drives an electrical generator. The output
of this generator is 30 units of electrical energy. If this
equipment were not part of a CHP system, the efficiency of the
system would be 30%, and 70% of the energy embodied by the fuel
would be wasted as heat and exhaust. By adding a Heat Recovery
Steam Generator (HRSG), however, 55 units of useful heat can be
extracted from this waste stream, and fed to a heating load. (The
heat recovery efficiency is around 75 to 80%, and the overall
efficiency is therefore 85%.)
Slide 5
SLIDE 6: CHP Description: Equipment and Technologies
Combined heat and power systems make use of waste heat from
power generation equipment. An essential component, therefore, is
power generating equipment that rejects heat at a temperature high
enough to be useful. Most CHP plants employ gas turbines, steam
turbines, or reciprocating engines to drive their electrical
generators, although more exotic technologies such as fuel cells
are occasionally seen.
A jet engine is an example of a gas turbine: a stream of inlet
air is compressed, heat is added, generally by combustion of a
gaseous or liquid fuel, and then the high pressure outlet stream
turns a reaction turbine at high speed. The reaction turbine in
turn drives a generator.
A steam turbine is driven by high-pressure steam, generated in a
boiler. Water in the boiler is raised to its boiling point by the
addition of heat from combustion of one of a wide range of
fuels.
A gas turbine and a steam turbine can be placed in series in a
so-called combined cycle plant. The hot exhaust gases of the gas
turbine are used to generate steam for the steam turbine. In terms
of power generation, this technology can achieve 55% efficiency;
this is one of the most efficient combustion-based power generation
technologies.
Reciprocating engines are well known to most people as the power
plants in gasoline-fuelled cars, which rely on the spark-ignited
Otto cycle, and diesel-fuelled trucks, which rely on the high
temperatures achieved during compression to cause ignition.
Following ignition, the expansion of the combusting mixture drives
a piston.
These are the power generating technologies used in CHP plants.
A second essential component of these plants is the waste heat
recovery unit, which collects the waste heat from the power plant
so that it can be delivered to heat loads. The nature of the waste
heat recovery system depends on the type of power plant, the
required temperature, and the characteristics of the heat load.
Most applications requiring high temperature heat will demand
steam, which can be produced using the hot exhaust gases of a gas
turbine or reciprocating engine; this requires a heat recovery
steam generator. Steam turbines can sometimes provide low-grade
steam at their exhaust port; otherwise it can be bled from the
turbine as necessary. When only low temperature heat is available,
it can be used to preheat water destined for steam generation.
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SLIDE 6: CHP Description: Equipment and Technologies (cont.)
Low temperature heat can be conveniently transported from the
plant to loads in streams of hot water. A heat exchanger recovers
the waste heat from the power generation equipment. The hot water
can be used directly, such as for domestic hot water, or for space
heating. The temperature of the water can be raised somewhat by a
heat pump, if necessary.
CHP plants will often include dedicated heating systems to
supply heat in combination with or in addition to that available
from the power plant. These include boilers and furnaces.
Where a cooling load is located near the CHP plant, cooling
equipment may be included as part of the system. This may be vapour
compression cycle refrigeration equipment, such as conventional
chillers or heat pumps, that are driven by a portion of the CHP
plants power output. Alternatively, absorption refrigeration
systems and desiccant dehumidifiers make use of the heat output of
the plant.
SLIDE 7: CHP Description (cont.): Fuel Types
CHP systems can, depending on the equipment used, operate on a
wide variety of fuels. Fossil fuels, such as natural gas, diesel,
and coal, are undoubtedly the most common fuels. Although the
combustion of fossil fuels has negative environmental consequences,
CHP plants are considered clean energy technologies due to their
high efficiency.
CHP systems can also run on biomass, as seen in the upper photo
on this slide. Biomass includes wood bark, shavings and chips;
biogas emitted by decomposing animal or plant waste; agricultural
byproducts; and purpose-grown energy crops such as poplar and
switchgrass. A byproduct of sugar-cane refining, called bagasse, is
widely used in Brazil and elsewhere.
Landfills emit methane and other combustible gases as they
decompose. A landfill can be capped and the emitted gas captured.
This gas can run a CHP plant.
At certain locations heat from below the earths crust is
available near the surface. This can give rise to volcanoes and
geysers, such as the one pictured on this slide. This heat can also
be used in CHP plants.
Hydrogen is produced using electricity or by processing
conventional fossil fuels. It is one way to store electrical
energy, generated by hydroelectric plants or wind turbines, for
example. It can be combusted, but is particularly useful as a fuel
for certain types of fuel cells. It is, therefore, a potential fuel
for CHP systems.
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SLIDE 8: CHP Description (cont.): Applications
CHP systems exist at a variety of scales. There are small
systems for single buildings, such as the greenhouse seen on this
slide. Larger systems serve commercial and industrial buildings or
complexes, including the city hall shown here. Very large systems
are found near large heat loads, such as industrial processes and
whole communities utilizing district energy systems, as illustrated
by the plant shown at the bottom of this slide.
Slide 8
SLIDE 9: District Energy Systems
A heat distribution system transports heat from the heating
plant to the locations where it is required. These locations may be
within the same building as the plant or, in the case of a district
heating system, may be a cluster of buildings located in the
vicinity of the plant. The consumers are often grouped in clusters
of public, commercial, and residential buildings located within a
few hundred meters of each other. A network of insulated piping,
buried 60 to 80 cm below the surface of the ground, conveys water
away from the plant at temperatures up to 130 degrees Celsius and
returns the water, cooled to temperatures of 40 to 80 degrees, back
to the plant for reheating. The pipes need not be buried below
frost line since the pipes are insulated and contain circulating
heated water.
District heating systems can be made to transport chilled water
for building cooling during summer. When they do such double
service, the piping networks are referred to as district energy
systems.
A district energy system creates a sizeable heat load for a CHP
plant. With the waste heat of the power generation equipment
substituting for the output of individual heating and cooling
systems located in each of the buildings, overall efficiency is
improved. Furthermore, compared with autonomous distributed heating
and cooling plants, the centralized district energy system can
offer better emissions controls, more advanced safety systems,
better comfort, and operating convenience.
The initial costs of a district heating system are high.
District heating is easiest to integrate into newly constructed
communities and requires a high level of planning and
organization.
Slide 9
SLIDE 10: CHP System Costs
CHP system costs vary widely from installation to installation.
For example, the table on this slide lists the installed costs of
various power generating technologies, many of which can be found
in combined heat and power projects. However, these installed costs
do not include many of the other initial costs that may be
associated with the CHP system: heating equipment, cooling
equipment, electrical interconnection, access roads, and district
energy piping. More importantly, the initial costs say nothing
about the recurring costs, which may be even more significant.
These include fuel, O&M, and equipment replacement and
repair.
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SLIDE 11: CHP Project Considerations
The success of a CHP project is influenced by a number of risk
factors.
First, for equipment that is designed to be used with a
particular fuel, there must be a reliable, long-term supply of that
fuel.
Second, construction of the project must be carefully managed to
keep capital costs from escalating beyond their budgeted
levels.
Third, the demand for heat and power has to be positively
correlated. An on-site heat load requiring a large fraction of the
annual heat output of the plant must be present; otherwise the
waste heat will not be productively employed. Conversely, if the
electricity cannot all be consumed on site, then the long-term sale
of electricity onto the grid must be negotiated. The power plant
should be sized such that its heat output is sufficient to meet the
heating base load; the heat output of the system is typically 100%
to 200% of the electricity output. Excess heat, especially during
summer when space heating loads decline, can be used for cooling
through absorption chillers.
Finally, the relative cost of electricity and fuel, especially
natural gas, strongly influences the financial viability of a
project. The difference between the price of electricity sold by a
generator and the price of the fuel used to generate it, adjusted
for equivalent units, is called the spark price spread. It must be
positive for the operation of the CHP plant to be financially
attractive.
Slide 11
SLIDE 12: Single Buildings Example: Canada
Single buildings requiring both heat and a reliable power supply
are excellent candidates for CHP plants; these include hospitals,
schools, commercial buildings, and agricultural buildings. The CHP
plant can provide cooling as well. The photos on this slide show
the reciprocating engine power plant and exhaust heat recovery
steam generator used in a hospital located in Ontario, Canada.
Slide 12
SLIDE 13: Multiple Buildings Examples: Sweden and the USA
Clusters of buildings can be served by a single central heating,
cooling, and power plant. This is often done for universities,
industrial and commercial complexes, communities, and hospitals. In
their most developed form, these are district energy systems. The
pictures on this slide show a CHP district energy plant in Sweden
and the 25 MW gas turbine used in a CHP plant at the Massachusetts
Institute of Technology, in Cambridge, Massachusetts, USA.
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SLIDE 14: RETScreen Industrial Processes Example: Brazil
Industries with a high constant heating or cooling demand are
good candidates for CHP developments. This is especially true for
industries that produce waste material that can be used as a fuel,
such as the bagasse byproduct of the sugar cane refinery seen in
this photo. The diagram shows how the waste heat from a gas turbine
combined cycle power plant can satisfy an industrial heat load.
Slide 14
SLIDE 15: Landfill Gas Examples: Canada and Sweden
Landfills produce methane as waste decomposes. The landfill can
be capped and the gas collected for use as a fuel in cooling,
heating, or power projects. The diagram on this slide shows
schematically the use of landfill gas. The photo shows a Swedish
CHP plant fuelled with landfill gas and connected to a district
heating system.
Slide 15
SLIDE 16: RETScreen CHP Project Model
The RETScreen CHP Project Model provides an analysis of the
energy production, life-cycle costs, and greenhouse gas emissions
for a CHP system located anywhere in the world. All combinations of
power, heating, and cooling systems can be analysed. Virtually all
power generating technologies and fuels are permitted, and
different operating strategies can be employed. There are even
tools for landfill gas production and district energy systems. The
software is available in many different languages, and it is
possible to change from one language to another with a simple
switch; another switch permits the use of either metric or imperial
units. Other tools include a currency switch, user defined fuel and
electricity rate calculators, and a unit conversion tool among
others.
Slide 16
SLIDE 17: RETScreen CHP Project Model (cont.)
RETScreen can model systems using virtually any combination of
heating, cooling, and power generation. This is illustrated by the
diagram on this slide, which shows the input of fuel to the system,
the flow of heat and electricity, and the production of heat,
cooling, and power. (Project types include: Heating only; Power
only; Cooling only; Combined heating and power; Combined cooling
and power; Combined heating and cooling; and Combined cooling,
heating and power.)
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SLIDE 18: RETScreen CHP Project Model for Heating Systems
The RETScreen CHP Project Model accommodates systems
incorporating multiple heat sources. For example, the diagram on
this slide shows the average heating load, cooling load, and power
generation for each month of the year for a particular CHP system.
The heating load peaks in the winter months and the cooling load
peaks in the summer months. The heating load is supplied by three
different systems: a base load heating system, an intermediate load
heating system, and a peak load heating system. The peak load heat
source runs relatively infrequently and therefore should be
characterized by low capital costs, even if that results in
somewhat elevated operating costs. In contrast, the base load will
usually be met by waste heat recovery, which entails minimal
operating costs.
Slide 18
SLIDE 19: RETScreen CHP Project Model for Cooling Systems
The RETScreen CHP Project Model can similarly accomodate plants
incorporating multiple cooling systems. On this slide, the plant
includes a base load cooling system and a peak load cooling system.
Different technologies could be used for base and peak load
cooling; one could be an absorption chiller and the other a vapour
compression cycle chiller, for example.
Slide 19
SLIDE 20: RETScreen CHP Project Model for Power Systems
The RETScreen CHP Project Model permits multiple power
generation sources as well. Here a base load power, intermediate
load power, and peak load power generator are included. The user
can select the operating strategy for the power generation
equipment: for example, in this system, the user can require the
intermediate load power generator to operate at maximum power all
the time, at the level that supplies the power load that is in
excess of the base load, or at the level that satisfies the heat
load.
Slide 20
SLIDE 21: RETScreen CHP Energy Calculation
In overview, the RETScreen CHP energy calculation starts by
estimating load and demand curves over the course of the year.
These will vary depending on whether heating, cooling, power
generation, or a combination of these are involved. Then the
equipment characteristics, and in particular their fuel
requirements per unit of load, are defined. Finally RETScreen
calculates the energy delivered and the corresponding fuel
consumption.
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SLIDE 22: Example Validation of the RETScreen CHP Project
Model
The overall validity of the RETScreen CHP model was established
in tests conducted by an independent consultantFVB Energy Inc.and
by numerous beta testers from industry, utilities, government and
academia. It was also compared with other models and measured data.
For example, the table on this slide shows good agreement between
one part of the model, the steam turbine performance calculations,
and the output of a GE Energy process simulation software called
GateCycle.
Slide 22
SLIDE 23: Conclusions
CHP systems make efficient use of heat that would otherwise be
wasted. In this way, they reduce the fuel required to meet a
combination of heat and power loads. Since these loads are normally
met by fossil fuels, this reduces greenhouse gas emissions. With
minimal input from the user, RETScreen can determine demand and
load duration curves, energy delivered, and fuel consumption for
various combinations of heating, cooling, and power systems. It
thus significantly reduces the time, effort, and cost of conducting
preliminary feasibility studies.
Slide 23
SLIDE 24: Questions?
This is the end of the Combined Heat and Power Project Analysis
Training Module in the RETScreen International Clean Energy Project
Analysis Course.
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