Energy Harvesting. 1 Combined Heat and Power (CHP) 2 MicroCHP 3 Stirling Engines 4 Heat Pumps 5 Small scale energy harvesting

Transcription

1 Energy Harvesting 1 Combined Heat and Power (CHP) 2 MicroCHP 3 Stirling Engines 4 Heat Pumps 5 Small scale energy harvesting 1

2 Often industrial (and domestic) processes will create waste or unwanted energy. Energy can also be extracted from many parts of our environment. Different technologies are required to harvest these different forms of energy. 2

3 CHP The process of producing electricity usually produces waste heat This heat can be collected to provide district heating Or it can be used to generate more electricity Similarly a boiler producing heat (ie steam, water etc) can produce waste heat which can be harvested to produce electricity 3

4 Sustainable Energy without the hot air, David JC MacKay 4

5 CHP can be applied to large scale electricity generation plant Or smaller CHP units can be installed in large commercial or industrial sites In Australia the former would normally utilise coal and the latter gas 5

6 Spark spread CHP is only a viable option if you have 1 A use for the heat 2 A use for the electricity 3 A large enough spark spread Spark Spread = Cost of Electricity - [ (Cost of Gas) * (Btu to kwh conv) ] OR = $/kwh - [ ($/Btu) * (Btu / kwh) ] Spark Spread = [ (Cost of Electricity) * (kwh to Btu conv) ] - Cost of Gas = [ ($/kwh) * (kwh/btu) ] - $/Btu 1 therm = 100,000 Btu = 100cf natural gas = 1 Ccf = 29.3 kwh = MJ 1kWh = 3412 Btu 6

7 Example Calculate the spark spread if gas costs 1.9 c/mj and electricity costs 21 c/kwh. 7

8 Gas fired CHP 8

9 The recovery of this waste heat in a CHP plant utilises reasonably mainstream technology. CHP technology can be downsized for smaller industrial and domestic situations. It is then often called microchp. The most common method of heat recovery in this arena is the Stirling engine. 9

10 What is the idea? 10

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12 Stirling engine Alpha engine 12

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14 Alpha engine 14

15 Beta engine 15

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17 Beta engine 17

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19 Stirling Efficiency The force exerted on the piston is F = S x P where S is the surface of the piston and P the instantaneous pressure. Over a short time (dt) the work is equal to the instantaneous force times the displacement of the piston (dy). dw = F x dy = S x P x dy now S x dy = dv, so dw = P x dv This equation describes the surface under each curve. The work is positive under the expansion curve as dv>0 and negative under the compression curve as dv<0. The total work for one cycle is the area under the expansion curve decreased by the area under the compression curve, ie the area of the loop. The efficiency can be calculated by the ratio of recovered mechanical energy (Wnet) and supplied heat (Qtot). Wnet = Wexp + Wcomp remember Wcomp will be negative Qtotal = Qheat + Qexp 19

20 Mechanical energy The total work (Wnet) is equal to sum of the positive recovered work during expansion and the supplied negative work during the compression. Wnet = Wexp + Wcomp Wnet = exp PdV + comp PdV where P = nrt / V Wnet = exp (nrtmax / V) dv + comp (nrtmin / V) dv Wnet = nr (Tmax - Tmin) ln Vmax / Vmin 20

21 Supplied heat During the isothermal expansion phase the supplied heat is equal to the recovered work during this phase Qexp = exp PdV Qexp = nr Tmax ln Vmax / Vmin During isochoric heating, we have to provide heat Qheat = ncv (Tmax - Tmin) where Cv is the constant-volume molar heat capacity of the gas when heated from Tmin to Tmax. The total provided heat is : Qtotal = ncv (Tmax - Tmin) + nr Tmax ln Vmax / Vmin 21

22 Stirling cycle efficiency η = Wnet / Qtot η = [R (Tmax - Tmin) ln Vmax / Vmin] / [Cv (Tmax - Tmin) + R Tmax ln Vmax / Vmin] Stirling regenerator The limit will be η = 1 - Tmin / Tmax 22

23 Which gas is best? The one with the smallest constant volume molar heat capacity. 23

24 Model high temp stirling engine 24

25 An application of the Stirling engine 25

26 Stirling engines are also in use in other energy areas, especially in generation of renewable energy. 26

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28 There are other microchp technologies 28

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30 Heat pumps These devices work on the principle that it is more efficient to move heat from one place to another, rather than to create heat. The principle is the same used in a fridge or airconditioner. 30

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32 Coefficient of Performance (COP) - The ratio of useful heat movement to work input COP heating = ΔQ hot ΔA T hot T hot T cool COP cooling = ΔQ cool ΔA T cool T hot T cool where ΔQcool - amount of heat extracted from a reservoir at temperature Tcool, ΔQhot - amount of heat delivered to a reservoir at temperature Thot, ΔA - work done by compressor. All temperatures in kelvin(k). 32

33 Heat pumps may be Air Source Heat Pumps (ASHP) or Ground Source Heat Pumps (GSHP) They can be used as space heaters, water heaters or as a means of generating electricity from heat

34 Reverse cycle - Cooling mode 34

35 Reverse cycle - Heating mode 35

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37 Small scale energy harvesting Mechanical Energy vibration, mechanical stress and strain Thermal Energy waste energy from furnaces, heaters, and friction sources Light Energy sunlight or room light Electromagnetic Energy RF and low frequency electromagnetic fields Natural Energy wind, water flow, ocean currents Human Body mechanical and thermal energy naturally generated from humans and animals Other Energy from chemical and biological sources 37

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39 Vibration The simplest of these devices utilise the piezo effect 39

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42 Thermal The common thermal energy harvesting device is a thermo electric generator (TEG). These devices us the Seebeck effect The Seebeck coefficient S = ΔV ΔT 42

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45 RF harvesting 45