Energy Harvesting1 Combined Heat and Power (CHP)2 MicroCHP3 Stirling Engines4 Heat Pumps5 Small scale energy harvesting

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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.

CHPThe process of producing electricity usually produces waste heatThis heat can be collected to provide district heatingOr it can be used to generate more electricitySimilarly a boiler producing heat (ie steam, water etc) can produce waste heat which can be harvested to produce electricity

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Sustainable Energy — without the hot air, David JC MacKay

CHP can be applied to large scale electricity generation plantOr smaller CHP units can be installed in large commercial or industrial sitesIn Australia the former would normally utilise coal and the latter gas

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Spark spreadCHP is only a viable option if you have1 A use for the heat2 A use for the electricity3 A large enough spark spreadSpark Spread = Cost of Electricity - [ (Cost of Gas) * (Btu to kWh conv) ]

= $/kWh - [ ($/Btu) * (Btu / kWh) ]

Spark Spread = [ (Cost of Electricity) * (kWh to Btu conv) ] - Cost of Gas

= [ ($/kWh) * (kWh/Btu) ] - $/Btu

OR

1 therm = 100,000 Btu = 100cf natural gas = 1 Ccf = 29.3 kWh = 105.5 MJ

1kWh = 3412 Btu

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ExampleCalculate the spark spread if gas costs 1.9 c/MJ and electricity costs 21 c/kWh.

Gas fired CHP

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http://youtu.be/uXLUoqzlT2k

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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.

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What is the idea?

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Stirling engine

Alpha enginehttp://en.wikipedia.org/wiki/Stirling_engine

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Alpha enginehttp://en.wikipedia.org/wiki/Stirling_engine

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Beta enginehttp://en.wikipedia.org/wiki/Stirling_engine

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Beta enginehttp://en.wikipedia.org/wiki/Stirling_engine

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http://en.wikipedia.org/wiki/File:Stirling_Cycle_color.png

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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

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 dynow 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.

http://www.robertstirlingengine.com/principles.php

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Mechanical energyThe total work (Wnet) is equal to sum of the positive recovered work during expansion and the supplied negative work during the compression.

Wnet = Wexp + WcompWnet = ∫exp PdV + ∫comp PdV where P = nRT / V

Wnet = ∫exp (nRTmax / V) dV + ∫comp (nRTmin / V) dV Wnet = nR (Tmax - Tmin) ln Vmax / Vmin

http://www.robertstirlingengine.com/principles.php

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Supplied heatDuring the isothermal expansion phase the supplied heat is equal to the recovered work during this phaseQexp = ∫exp PdVQexp = nR Tmax ln Vmax / Vmin

During isochoric heating, we have to provide heatQheat = 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

http://www.robertstirlingengine.com/principles.php

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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 beη = 1 - Tmin / Tmax

23http://hyperphysics.phy-astr.gsu.edu/hbase/tables/heatcap.html

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

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Model high temp stirling engine

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An application of the Stirling engine

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

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There are other microCHP technologies

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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 air-conditioner.

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http://en.wikipedia.org/wiki/File:Refrigeration.png http://en.wikipedia.org/wiki/File:RefrigerationTS.png

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Coefficient of Performance (COP)- The ratio of useful heat movement to work input

whereΔQcool - 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).

COPheating =ΔQhot

ΔA≤

ThotThot − Tcool

COPcooling =ΔQcool

ΔA≤

TcoolThot − Tcool

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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.

http://greenedmonton.ca/taxonomy/term/97 http://heatexchanger-design.com/category/air-source-heat-pump/

34http://www.geo4va.vt.edu/A3/A3.htm

Reverse cycle - Cooling mode

35http://www.geo4va.vt.edu/A3/A3.htm

Reverse cycle - Heating mode

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http://en.wikipedia.org/wiki/Heat_pump

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• 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

Small scale energy harvesting

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VibrationThe simplest of these devices utilise the piezo effect

http://en.wikipedia.org/wiki/File:SchemaPiezo.gif

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http://www.piezo.com/prodeh0nav.html

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Thermal

http://en.wikipedia.org/wiki/File:Seebeck_effect_circuit_2.svg

The common thermal energy harvesting device is a thermo electric generator (TEG).

These devices us the Seebeck effect

S = −ΔVΔTThe Seebeck coefficient

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http://www.micropelt.com/applications/te_power_plus.php

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RF harvesting http://www.powercastco.com/products/powerharvester-receivers/