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There's more to energy than joules

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Energy is not only about quantity. Its quality is equally important. This minute lecture, inspired by an idea from Camatini & Kester, introduces the concept of exergy, the quality measure for an amount of energy. It uses the example of space heating with various methods: electric heating, fossil-fired boiler and heat pumps.

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There's more to energy than joules

1. 1. Energy and exergy explainedMinute LecturesA representation of energyA good analogy often helps to understand the concepts of physicsSo let’s represent energy by little menThe number of men is equivalent tothe amount of energyThe 1st law of thermodynamics says:during energy conversion, thetotal amount of energy remains the same.Or: no men can ever disappear (or appear)
2. 2. Energy and exergy explainedMinute LecturesA representation of exergyAnother basic concept is “exergy”, or the “quality of energy”.This can be represented by the height at which the men arestanding.If the energy is heat, exergy islinear with temperature.Just like height, exergy is relative to a reference level. Thisreference is the outside atmosphere, which we assume to be at0° Celsius or 273.15° Kelvin.
3. 3. Energy and exergy explainedMinute LecturesThe 2nd law of thermodynamics: the more men falldown, the fewer of them will be able to climb upagainOr: when men move to a lower platform, somebecome discouraged. They are still capable ofwalking on a flat surface or falling down to a lowerplatform, but they lack the courage to climb up to ahigher level again without a helping hand. Atground zero, all men are discouragedHeight = exergy = “courage”
4. 4. Energy and exergy explainedMinute LecturesThe total amount of courage of the men on a certainplatform is the exergy level of that platform. This isalso called the Carnot factor (C) calculated by:Cplatform = 1 – (Tatmosphere / Tplatform)Electricity, natural gas, oil and all other fuels, have aCarnot factor ‘1’. Indeed, their energy can(theoretically) be converted to heat at any desiredtemperature. They are on the highest exergy levelThe exergy level C
5. 5. Energy and exergy explainedMinute LecturesHeating a roomExample: the heating of a roomE(C,J) is electrical energy with a Carnot factor C and anamount of energy J (from Joule).QT(C,J) is heat at a temperature T, with a Carnot factor Cand an amount of energy JNow let’s compare heating a room with an electric radiator,to heating a room with a heating element containing hotwater at 65°C
6. 6. Energy and exergy explainedMinute LecturesBy an electric radiatorC1 = 1C2 = 1 – (273.15/293.15) = 0.07
7. 7. Energy and exergy explainedMinute LecturesBy a heating element with water at 65°CC1 = 1 – (273.15/338.15) = 0.19C2 = 1 – (273.15/293.15) = 0.07
8. 8. Energy and exergy explainedMinute LecturesEnergy LossesNot all the men that landed at the exergy level of the room will staythere, unfortunately. Through cracks and chinks, some of them willleave the room and fall down into the atmosphere.
9. 9. Energy and exergy explainedMinute LecturesWhen the room is heated by a hot water radiator,the hot water comes from a boiler.Boiler (1/2)
10. 10. Energy and exergy explainedMinute LecturesThe boiler converts the fuel energy into combustion heat(1200°C), reducing the exergy level.The combustion heat is transferred to hot water – aprocess by which exergy goes further downIn the conversion, some energy is lost.Boiler (2/2)
11. 11. Energy and exergy explainedMinute LecturesThe electrical powerfor a radiator comesfrom a power stationPower station (1/2)
12. 12. Energy and exergy explainedMinute LecturesIn a power station Fuel creates heat in a boiler, which is then transferred tosteam. The exergy level goes down (some men becomediscouraged) This process causes some loss of energy (some men fall intothe atmosphere) Electricity is generated with the steam. The exergy level goesup. But not all the men will crawl up again; some of them arediscouraged and will stay were they are. It is impossible toconvert 100% of the heat energy into electrical energyPower station (2/2)
13. 13. Energy and exergy explainedMinute LecturesIs it possible to take energy out of the atmosphere? Yes,but remember: at atmosphere level, all men arediscouraged, so they will need some help.Example: a heat pump.The heat of the atmosphere is transferred to a medium atlow pressure = a life buoy. This buoy is taken up to ahigher pressure and higher temperature by acompressor, driven by electrical energy. On average,one unit of electrical energy can draw three units ofheat out of the atmosphereHeat pump (1/2)
14. 14. Energy and exergy explainedMinute LecturesHeat pump (2/2)
15. 15. Energy and exergy explainedMinute LecturesRound-upDuring an energy conversion process, no energy can belost.But it is not only the amount of energy that counts, the“quality level” or “exergy” is just as important.Exergy is a measure that indicates to what degree energyis convertible to other forms of energy.It is impossible to take energy from a lower exergy level toa higher exergy level without a price.
16. 16. Energy and exergy explainedMinute LecturesLinks and references• Heat pumps and their contribution to energyconservation, Ed E Camatini and T Kester,Noordhoff, Leyden, 1976, ISBN 9028600566• Why thermal power plants have a relatively lowefficiency>>