Application of Exergy Analysis. Value and Limitations

Transcription

1 Application of Exergy Analysis Value and Limitations

2 Power Plant Exergy Flows and Destruction Stack 2 Other Losses 1 Fuel Steam 43 7 Shaft Power 32 Combustion Heat Transfer Turbine Steam 3 2 Condenser Cooling Water 1

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5 Pressure/Enthalpy Diagram for Water C 400C 600C 800C 1000C Pressure, bar 10 1 Water 100C 200C Water + Steam C Steam 0.01 Enthalpy

6 Gas Turbine Exergy Flows and Destruction Gas in 92 Turbine Inlet Temperature 1000 C Air in Heat out 16 Shaft power 59 Shaft power out 32 Compressor, 15x, 85% efficient Turbine, 85% efficient

7 Home Furnace Losses 1 st Law Exhaust 5 Fuel 100 Heat to Building 95

8 Home Furnace Exergy Flows and Destruction Combustion Exhaust 1 Fuel Heat Transfer 58 Heat to Building 6

9 Heat Pump Heat Engine in reverse Takes in heat at low (usually environment) temperature, uses mechanical work to produce heat at higher temperature(s).

10 Heat Pump Heat out T Mechanical Work Pump Heat in from environment T 0

11 Heat Pump Heat in = q in Heat out = q out Pump energy in = w For maximum efficiency case, no entropy change, so: q in /T 0 = q out /T But: w = q out q in So: w = (T T 0 )/T * q out Or: COP = q out /w = T/(T T 0 )

12 Basic Heat Pump Cycle Heat out Valve Compressor Power in Heat in

13 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

14 Modified Heat Pump Cycle Heat out Power out Expander Compressor Power in (more) Heat in

15 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

16 Heat Pump Mechanical Work, w Heat out, q = 4.9*w 30C Exergy = q( )/ * q, or 0.32*w Pump Heat in from environment 10C Exergy - nil

17 Exergy Flows and Destruction Heat at 30 C MPa, 40 C 1.55 MPa, 71 C Condenser 25 Valve 10 Compressor 20 Evaporator 13 Power in MPa, 0 C Heat at 10 C MPa, 0 C

18 Modified Heating System Building Heat Heat Gas 10 Engine Mechanical Power Heat Pump 70 Heat from Environment

19 Modified Heating System Exergy Flows and Destruction Building Engine Heat Pump 0

20 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?

21 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

22 Solar Thermal Inclusion in combined cycle power system

23 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

24 Wind Solar energy converted to kinetic energy pure exergy But: light, compressible fluid flows not controllable and not very predictable not confined

25 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

26 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 )

27 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

28 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.

29 Energy From Waste Only renewable if the source is. Mostly, it isn t. Combustion process.

30 Exergy Analysis Shows actual location of loss Sometimes implies what change would improve things Improvement actually possible may be limited by material properties or other practicalities