Energy Har vesting
1 Combined Heat and Power (CHP)
2 MicroCHP
3 Stirling Engines
4 Heat Pumps
5 Small scale energy har vesting
1
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 har vest these
different forms of energy.
2
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
Sustainable Energy — without the hot air, David JC MacKay
4
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
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) ]
= $/kWh - [ ($/Btu) * (Btu / kWh) ]
OR
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 = 105.5 MJ
1kWh = 3412 Btu
6
Example
Calculate the spark spread if gas costs 1.9 c/MJ and
electricity costs 21 c/kWh.
7
Gas fired CHP
http://youtu.be/uXLUoqzlT2k
8
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
What is the idea?
10
11
Stirling engine
Alpha engine
http://en.wikipedia.org/wiki/Stirling_engine
12
13
Alpha engine
http://en.wikipedia.org/wiki/Stirling_engine
14
Beta engine
http://en.wikipedia.org/wiki/Stirling_engine
15
16
Beta engine
http://en.wikipedia.org/wiki/Stirling_engine
17
http://en.wikipedia.org/wiki/File:Stirling_Cycle_color.png
18
Stirling Efficiency
http://www.robertstirlingengine.com/principles.php
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 cur ve 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
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
http://www.robertstirlingengine.com/principles.php
20
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
http://www.robertstirlingengine.com/principles.php
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
Which gas is best?
The one with the smallest constant volume molar heat
capacity.
http://hyperphysics.phy-astr.gsu.edu/hbase/tables/heatcap.html
23
Model high temp stirling engine
24
An application of the Stirling engine
25
Stirling engines are also in use in other energy
areas, especially in generation of renewable energy.
26
27
There are other microCHP technologies
28
29
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
http://en.wikipedia.org/wiki/File:Refrigeration.png
http://en.wikipedia.org/wiki/File:RefrigerationTS.png
31
Coefficient of Performance (COP)
- The ratio of useful heat movement to work input
ΔQhot
Thot
≤
ΔA
Thot − Tcool
ΔQcool
Tcool
=
≤
ΔA
Thot − Tcool
COPheating =
COPcooling
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
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
33
http://heatexchanger-design.com/category/air-source-heat-pump/
Reverse cycle - Cooling mode
http://www.geo4va.vt.edu/A3/A3.htm
34
Reverse cycle - Heating mode
http://www.geo4va.vt.edu/A3/A3.htm
35
http://en.wikipedia.org/wiki/Heat_pump
36
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
38
Vibration
The simplest of these devices utilise the piezo effect
http://en.wikipedia.org/wiki/File:SchemaPiezo.gif
39
http://www.piezo.com/prodeh0nav.html
40
41
Thermal
The common thermal energy har vesting
device is a thermo electric generator (TEG).
These devices us the Seebeck effect
The Seebeck coefficient
http://en.wikipedia.org/wiki/File:Seebeck_effect_circuit_2.svg
42
ΔV
S=−
ΔT
43
http://www.micropelt.com/applications/te_power_plus.php
44
RF harvesting
http://www.powercastco.com/products/powerharvester-receivers/
45