Application of Exergy Analysis

Value and Limitations

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 Losses – 1st Law

Fuel 100

Exhaust 5

Heat to Building 95

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

Pump

Mechanical Work

Heat in from environment T0

Heat out T

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 )

Basic Heat Pump Cycle

Heat out

Heat in

Power inValve Compressor

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

Modified Heat Pump Cycle

Heat out

(more) Heat in

Power inExpander Compressor

Power out

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

Modified Heating System Exergy Flows and Destruction

Building

Engine HeatPump

92 35

1

0

74

52 28

“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

Solar Thermal

Inclusion in combined cycle power system

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.

Energy From Waste

Only “renewable” if the source is.

Mostly, it isn’t.

Combustion process.

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