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Power Plant Exergy Flows and Destruction
27Fuel 92
Stack 2
Steam 43
7Shaft Power 32
2Steam 3
Other Losses 1
Cooling Water 1
Turbine
Condenser
Combustion
2065
HeatTransfer
100C
300C
200C
400C 1000C800C600C
Pre
ssur
e, b
ar
Enthalpy
Pressure/Enthalpy Diagram for Water
1
0.01
0.1
10
100
1000
Water + Steam
Water
Steam50C
Gas Turbine – Exergy Flows and Destruction
Air in
Compressor, 15x, 85% efficient
Gas in 92
Shaft power 59 Shaft power out 32
Turbine Inlet Temperature 1000 C
Turbine, 85% efficient
Heat out 1654 115
531
8
Home Furnace Exergy Flows and Destruction
27 Heat Transfer58
Fuel 92
Combustion
Heat to Building 6
Exhaust 1
Heat Pump
Heat Engine in reverse
Takes in heat at low (usually environment) temperature, uses mechanical work to produce heat at higher temperature(s).
Heat Pump
Heat in = qin
Heat out = qout
Pump energy in = w
For maximum efficiency case, no entropy change, so:
qin /T0 = qout /T
But: w = qout – qin
So: w = (T – T0 )/T * qout
Or: COP = qout /w = T/(T – T0 )
Heat Pump
Ideal COP much higher than actual – why?
Heat transfer requires ΔT (at input and output)
Compressor inefficiency
Superheating of fluid
Expansion valve losses
R22 Heat Pump
Heat reservoir at 10C
Heat pumped into air at 30C
Compressor efficiency 70%
Evaporator temperature 0C
Condenser temperature 40C
No frictional losses
Heat Pump
Pump
Mechanical Work, w
Heat in from environment 10C
Heat out, q = 4.9*w 30C
Exergy - nil
Exergy = q(303 – 283)/3030.066 * q, or 0.32*w
Exergy Flows and Destruction
Condenser 25
Evaporator 13
Power in 100
Valve 10 Compressor 20
0.6 MPa,0 C
0.6 MPa,0 C
1.55 MPa,71 C
1.55 MPa,40 C
Heat at 30 C 32
Heat at 10 C 0
Modified Heating System
Building
Engine HeatPump
100 35
10
70
10555
MechanicalPower
Gas Heat from Environment
Heat Heat
“Renewable” Energy
Not so much renewable as very long-lasting
Solar, thermal and photovoltaic
Hydro power
Wind
Biomass
Ocean Waves
Tides
Geothermal?
Energy from Waste?
Solar Energy
Black-body radiation at 6000K
high-grade energy, but the source appears very small in the sky
Solar thermal
accept exergy loss by conversion to heat
use heat directly or make steam and thus power
Solar PV
direct conversion, limited efficiency
Hydro
Solar energy converted to gravitational potential – effectively pure exergy
Also
dense, incompressible, low-viscosity fluid
already runs in confined channels
can be stored (up to a point)
It doesn’t get better than this?
may be in the wrong place
using it has some adverse consequences
Wind
Solar energy converted to kinetic energy – pure exergy
But:
light, compressible fluid
flows not controllable and not very predictable
not confined
Biomass
Solar energy stored as chemical energy
High exergy content, but -
slow collection system
spread over large areas
use is combustion or conversion followed by combustion
Ocean Waves
Solar energy converted to wind and then to ocean waves
In principle a low exergy source, provided by a dense fluid, but:hostile environment
chemically (corrosion)
mechanically (very large unpredictable motion possible)
fouling (weed, barnacles…)
Tides
Earth’s rotational energy converted to flow of ocean water
The moon provides a gravitational field gradient, causing water to build up toward and away from the moon – earth rotates under the bulges
Low exergy energy – available from flows of a dense fluid at predictable times
Limited choice of sites
Hostile environment
Geothermal Energy
Heat in the Earth’s crust
Source – radioactive decay in the crust, heat from the core.
Low average flux, 100kW per (km)2
Relatively low temperature
Use for heating directly
Electric power – may use Organic Rankine Cycle, rather than steam cycles.
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