Energy–Efficiency & Energy Conservation

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Paulina Bohdanowicz, Miroslav Petrov,

Dep. of Energy Technology,Royal Institute of Technology

Energy–Efficiency & Energy Conservation

Miro PetrovKTH – Dept. of Energy Technology, Energy & Environment Course, 4A1613

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Introduction

• Inevitable interaction between energy production & utilization and the ENVIRONMENT chemical effluents !

• The general problem is that:the Earth is gradually being brought outof its established equilibrium !


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Overview

How to attack the problem?

• Increase of energy conversion efficiency, • Change of energy conversion methods,• Change of primary or secondary energy source.

• Defining EFFICIENCY:• How many steps between input and product?

Product / Input


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Energy-Efficiency ChainPrimary energy source. Fuel mining.

Fuel conditioning and transport.

Primary energy conversion: Power Plant.

Transport of secondary (useful) energy form.

Secondary energy conditioning / distribution.

Final energy conversion (utilization) at end-user site.

losses

losses

losses

losses

losses


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Supply and Demand side

• Energy efficiency can be improved on both:supply side (primary and secondary

conversion and distribution) and

demand side (final conversion at end-user),

and/or byfewer and/or improved conversion steps !


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Natural Examples (1):Photosynthesis

How high is the efficiency of conversion of solar radiation to biomass?

Around 80x1012 W of solar power can be used for photosynthesis, but:

20% of the total insolation reaching the plant’s leaf is reflected away, 40% (50% of the rest) has a wavelength unsuitable for photosynthesis,30.8% (77% of the rest) is a ”quantum” loss during photosynthesis,3.7% (40% of the rest) is a respiration loss for the plant, thus only5.5% of the original solar power can ideally be converted to biomass.

Further practical losses (climate, shading, water shortage, damage, etc.) brings the final efficiency down to around 0.5% – 1.5% for temperateclimate regions, and around 1% - 3% for equatorial regions.


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Natural Examples (2): Food Chain

• Perfect example of energy-efficiency chain in itsbasics: the FOOD CHAIN.

• Solar energy – (photosynthesis) – biomass/ /vegetation/flora – (cellulose to meat andcellulose to power) – biomass/fauna – (meat to meat and meat to power) – fauna...

The overall efficiency is poorly low!

[Humans are the most useless and malicious fauna on Earth!]


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Natural Examples (3): Hydropower

• How much is the efficiency of energy conversionfrom solar to hydropower potential ?

• The evaporation of 1 kg/s water by solar energy at 1 bar requires ~ 2500 kW!

• The hydropower potential of 1 kg/s water at 1000 m elevation is ~ 10 kW!

• The overall efficiency of energy conversion is ~ 0.4%!

• Very few conversion steps but very low overall efficiency!!!


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Natural Examples (4): Summary

Efficiency of solar power conversion to:

• Biomass photosynthesis ~ 0.5 - 3%, • Wind ~ 2%,• Hydropower ~ 0.4%,• Wave power – even less,• Fossil fuels – can anybody calculate?

• HOWEVER: energy concentration rises!

Concentration

of energy


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”Value” of energy

• The notion of ”ordered” and ”disordered”energy.

• Exergy (availability or ”workability” of energy).

• Concepts of energy, exergy & entropy…


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”Ordered” and ”Disordered”?

• HEAT is ”disordered” (low-value) energy, described by Brown motion of molecules...

• Electricity (ordered motion of charges), and kinetic or potential energy of fluids or solid objects, are ”ordered” (high-value) energy forms...

• Transformation of ”ordered” energy into”disordered” energy is easy, the reverse is very difficult, thus the notion of ”value” !!


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Exergy and irreversibility

• Transformation of ”ordered” energy into”disordered” energy is a natural process, with always 100 % efficiency !

• Conversion of ”disordered” into ”ordered”energy requires expenditures and is alwaysfar less than 100 % efficient !!!

• This discrepancy is incorporated in the concepts of EXERGY & ENTROPY.


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Energy, Exergy, Entropy

• Energy cannot be produced or lost, onlytransformed from one form to another !

• Exergy can be lost during energy transformations, but cannot be produced !!!

• Entropy cannot be lost, but can be produced and transfered during energy transformations !!!


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Destruction of Exergy (1)

• Inefficiency (loss) within any energy conversion process manifests itself in the form of unavoidable and usually undesiredtransformation of high-value energy intolow-value HEAT.

• This is destruction of exergy or productionof entropy !


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Destruction of Exergy (2)

• Examples: Electrical energy losses in electrical conductors (conversion to heat), friction losses (kinetic energy into heat)...

• Heat exchange through large temperaturedifferences is also destruction of exergy (loss of ”energy value” in terms of dumping high temperature to low such, even if the energy balance is correct)...


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Destruction of Exergy (3)• Another example: A common water heater

is using an oil burner to heat the water up to 80oC. No energy is lost from the water tank (perfect insulation). However, there is a big exergy loss here, because the final temperature of the water is far lower thanthe temperature of the combustion gases, meaning that we have lost the ability of the fuel to be converted to electricity. Exergy is destroyed !


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Primary energy conversion

• Conversion of primary energy to electro-mechanical energy can proceed along a ”conventional” or ”non-conventional” path:

1. Thermodynamic cycle (thermal method, heat engine), with combustion of the fuel.

2. Direct conversion methods (thermal non-conventional), with combustion of the fuel.

3. Non-thermal methods (fuel cell, solar cell).


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Energy matrix

Hydro

sourceHydrosource


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Thermal energy conversion

• Thermal methods of energy conversionalways aim at the transformation of heat into electromechanical energy, providedthat the raw primary energy (fuel, nuclear, solar, etc...) is firstly converted to heat.

• Such thermal methods are: (a) heat engines, or (b) direct thermal conversion.


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Conversion in Heat Engines

• Energy conversion path from raw energy supplyto demand side:Fuel – (chemical reaction) – heat – (heat exchange) – working fluid – (potential energy of working fluid into kinetic energy via expansion) -(kinetic energy of working fluid to kinetic energy of solid mass via pistons or rotating blades) –mechanical power – (mechanical drivetrain) –electrogenerator – (electromagnetic process) –electrical power - (transformer) – (transmission of electricity) - (transformer) – end user…


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Heat Engines (1)

• All energy conversion processes implylosses, and in the heat engines there are too many conversion steps.

• Overall efficiency is thus limited, boththeoretically & practically.

• The theoretical limit is CARNOT, relevant to all types of thermal energy conversion....


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Heat Engines (2)

• The practical limit is the impossibility to achieve very high temperatures, plus the impossibility to reach the Carnot valuewithin the given temperature range.

• ηheat engine << ηcarnot << 100 %

• ηcarnot= 1- Tlowest/Thighest


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Heat Engines (3)

• Higher efficiency can be achieved by:(a) High & steady temperature of heat input,(b) Low & steady temper. of heat rejection,(c) Minimizing exergy loss in combustion and

friction processes, (d) Minimizing exergy loss by minimizing

temper. differences in heat exchangers,(e) Combinations of different heat engines...


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Heat Engines (4)• Nevertheless, heat engines rule, because they are the most

developed and the cheapest option to implement in all scales !

An example of a heat engine (small biomass-fired steam cycle power plant) with electrical efficiency of maximum around 30% …


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Heat Engines (5)

…and another heat engine (gasoline car IC engine), with efficiency of around 20% or less…


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Heat Engines (6)

…And another heat engine (large stationary gas or diesel IC engine), the most efficient single cycle - up to 50%


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Heat Engines (7)

…And yet another one, the least efficient heat engine we use today!


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Direct Thermal conversion

• Attempting to circumvent or avoid some of the conversion steps in heat engines.

• Still limited by Carnot, but shouldtheoretically bring the practical efficiencycloser to the Carnot limit within the cycle.

• Several different physical phenomena canbe exploited to convert heat directly intoelectricity or thrust...


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MagnetoHydroDynamics

• Under development since long time.• Uses a high-temperature high-speed

ionized fluid (plasma) flowing in magnetic field to convert kinetic energy directly into DC electricity.

• Practical single-cycle efficiency is same or lower than that of common heat engines.

• Very promising in combination with bottoming steam or gas cycles (η~50-65%).


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MagnetoHydroDynamics


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ElectroHydroDynamics

• Comparatively well developed, but not considered as promising.

• Uses a middle-to-low-temperature ionized fluid flowing between electrodes to convert kinetic energy directly into DC el.

• Practical efficiency is quite low, but theoretically (with gas fluid) should reach 50-60%, very close to its Carnot limit.


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ElectroHydroDynamics


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Thermoionic power

• Absolutely no moving parts!• Uses the effect of electron discharge

from a hot metal surface and electron absorption on a cold metal surface, producing directly DC electricity.

• Theoretical efficiency reaches very close to the Carnot limit.

• Practical efficiency around 20%.


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Thermoionic power


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Infrared Voltaics

• Absolutely no moving parts!• Converts infrared (longwave) radiation

directly into DC electricity in a special photovoltaic cell (IR photoelectric effect).

• Theoretical efficiency reaches very close to the Carnot limit, but decreases at very high temperatures.

• Practical efficiency of more than 70% is possible (however not achieved yet).


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Infrared Voltaics


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Thermoelectricity

• Absolutely no moving parts! Simplest of all.• Converts heat directly into DC electricity

via the well-known thermoelectric effect (thermocouple between a hot junction and a cold junction).

• Theoretical efficiency should be very close to the Carnot limit, but cannot be achieved due to unavoidable losses.

• Practical efficiency is very low (~10-20%).


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Thermoelectricity


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Non-thermal conversion

• Allows for circumvention of the Carnotlimit!!! Promises efficiencies near 100%...

• Converts energy of higher value (higher than heat) directly into DC electricity via various physical phenomena.

• Representatives are:(a) the fuel cell (electrochemistry),(b) the solar cell (photoelectricity).


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Fuel Cells

• Comparatively well developed, commercialisation is just round the corner.

• Various types with different advantages…• Converts high-value chemical energy

directly into DC electricity via a simple electrochemical process.

• Theoretical efficiency > 80%• Practical efficiency can reach 50-55%.


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Fuel Cells


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Solar Cells

• Comparatively well developed, commercialisation is under way.

• Various types with different advantages…• Converts highest-value energy (photons)

directly into DC electricity via a simple photoelectric process (photovoltaics, PV).

• High theoretical efficiency, but• Very low practical efficiency (10-20%).


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Solar Cells


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Energy Conversion Summary


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Transmission & Storage

• Transmission of energy also involves losses.(on average 7% of all electricity is lost in transmission networks, can be as high as 30% in poor electrical grids!)

• Variations in demand can be shaved out by energy storage, allowing the energy supply to operate at steady level.

• Storage will also introduce losses…


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Energy Storage

• Should be as reversible as possible, also as cheap as possible.

• Should have high energy density per unit mass and per unit volume.

• Should be easily scalable and safe…• Recovery efficiency for the storage =

energy out / energy in


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Energy Storage options

• Chemical energy: Fuel – used every day ! • Electrochemical: Batteries.• Mechanical: (a) Hydropower, • (b) Flywheels, (c) Stress & strain,

(d) Compressed Air.• Thermal: Latent and Sensible heat.• Electromagnetic: Electromagnetic fields.


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Chemical Energy Storage

• Fuel – nothing new! The future ”hydrogen society” would be a typical example of the importance of energy storage in the form of fuel, especially if fuel cells are used instead of heat engines. However, hydrogen has verylow energy density per unit volume and is problematic to store...


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Electrochemical Storage

• Batteries – nothing new. Further development is welcome.

• Improvement of energy density is required, both per unit mass and per unitvolume.

• Recovery efficiency around 70-90%.


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Mechanical Energy Storage (1)

• Hydropower – nothing new. Stores potential energy of water mass in the field of gravitational forces. The energy density is actually quite low.

• Energy density ~ g∆h per kg water.

• Recovery efficiency is around 75-90%.


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• Flywheel – stores kinetic energy in rotating mass.

• Energy density = ½R2Gω2 per kg mass,

where RG is radius of gyration and ω is angular velocity.

• Recovery efficiency suffers in practice from serious friction losses.


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• Stress & Strain – stores potential energy of elastic forces (wind-up toy).

• Energy density = 0.5Eε2 = 0.5σ2/E per unit volume, where E is elastic modulus and σ, εare stress and strain, respectively. The energy density per unit mass for steel is ~ 100 J/kg, a miserable value.

• Recovery efficiency is close to 100%.


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• Compressed Air – stores potential energy of a pressurised gas. Promising technology, experimental projects exist.

• Stored energy = mCvT = pV/(γ-1)

• Recovery efficiency can be generally high (80% or more), but depends too much on local parameters…


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Thermal Energy Storage

• Latent or Sensible Heat – stores heat in solid objects or fluids, with or without phase change. Using latent heat is a promising way to increase sharply the energy density, especially per unit mass (example: steam accumulators).

• Recovery efficiency depends on dissipation and exergy losses during storage period...


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Electromagnetic Storage

• Electromagnetic field – stores electro-magnetic energy (interchangeable) in a capacitor or a coil.

• Stored energy = 0.5CV2 = 0.5LI2, where C is capacitance, L is inductance.

• Superconducting electromagnetic storage has a future, with expected recovery efficiency close to 100%.


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Energy Utilization

• Industry.

• Transport.

• Public services and buildings.

• Residential (private) buildings.


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Defining the Demand

• Lighting and mechanical power. • Heating, cooling & air conditioning.

• It would be best to “produce” all these energy forms (that the demand side requires) together in a co-generation plant as close as possible to the end users… !


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Industry (1)

• The industry uses large amounts of all types of final energy forms.

• Most of the electricity is needed to drive electric motors.

• Most of the heat demand is a middle- or high-temperature process steam or gas.

• Large potential for decreasing of losses and pollutant emissions.


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Industry (2)

Energy intensity of some major industries (primary energy cost as % of total product cost)

25%30%

40%

50%15%

16%

25%10%

aluminium

iron&steel

cement

gypsum

glass

synthetic fibres

paper&board

food preparation


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Pollution from IndustryCO2 emissions from industry, IEA (1988)

29%5%

8%

2%4%3%5%6%16%

22%

chemicals

non-ferousnon-metalic

transport equipmentmachinery

textilesfood production

pulp&paperother

iron&steel


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Transport (1)

• The transport sector uses high-quality fuels (liquid, gas or electricity) as raw energy input.

• Which are the most suitable points for attacking losses and inefficiencies??? And which vehicles to attack first?

• Answer: Not only the engine, but also: Miro Petrov

KTH – Dept. of Energy Technology, Energy & Environment Course, 4A161362

Transport (2)

• Aerodynamic drag and wheels-to-road friction,

• Vehicle mass, • Shaving the varying power output from the

engine by any kind of energy buffers (hybrid cars!),

• Storage of deceleration energy for use in subsequent acceleration (again a buffer).


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Pollution from Transport (1)

CO2 emissions from transportation, IEA (1988)

road82%

air13%

rail3%

other2%


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CO2 emissions from road vehicles, IEA Europe (1988)

passenger gasoline

57%

passenger diesel

9%goods gasoline

5% goods diesel29%


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NOx emissions, OECD (1986)

transport54%

other24%

industry22%

CO emissions, OECD (1986)

transport89%

other10%

industry1%


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Public & Private Sector (1)

• The demand for electricity and heating for a typical private house in Northern Europe is around 20-25% and 75-80% respectively, out of the total annual energy need of the house.

• These match perfectly to the thermal efficiency of small-scale distributed CHP generation units…


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Identifying and diminishing losses:

• Less electricity usage for heating,• More efficient lighting,• More efficient electric appliances,• More efficient refrigerators, • Better insulation of the house…


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• Back to the dilema: what is inefficiency?

• Example 1: Everybody has a fridge, which works in a room heated to 20oC, while the machine struggles to maintain ~4oC inside. Would you save anything if you place the fridge out on the balcony in winter?? The fridge also helps to heat the room (discharged heat), so this is not a real loss…


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• Back to the dilema: what is inefficiency?

• Example 2: You use electric heating, and you use low-efficient bulbs for lighting. Are the losses in the bulbs really losses? Not in this case, because the inefficiency of the lamps (converting electricity to heat instead of light) actually helps to warm-up the house and saves heating…


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Material used from:

• Decher, R. “Energy Conversion: systems, flow physics and engineering”, Oxford University Press, 1992, ISBN: 0-19-507959-0.

• Decher, R. “Direct Energy Conversion: fundamentals of electric power production”, Oxford University Press, 1994, ISBN: 0-19-509572-3.

• “Energy Efficiency and the Environment”, Energy and the Environment Series, published by the International Energy Agency, 1990, ISBN: 92-64-13561-8.

• Dunn, P.D. “Renewable Energies: sources, conversion and application”, Peter Peregrinus Ltd., London, 1986.

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Energy–Efficiency & Energy Conservation