Energy Conversion

FORMS OF ENERGY

Chemical Thermal Mechanical Electrical

(nuclear, electromagnetic)

ENERGY CONVERSION

• Chemical -> thermal combustor

• Thermal -> mechanical heat engine

• Mechanical -> electricalgenerator

HEAT TRANSFER

• Thermal energy flows from high Temperature to low Temperature

• A HEAT EXCHANGER brings the hot and cold media in contact without mixing

• To move thermal energy from low temperature to high temperature we need to expend some energy (typically mechanical energy)

ENERGY FROM FOSSIL FUELS

• HEAT

• POWER GENERATION

• CO-GENERATION

(Simultaneous generation of heat and power)

1st law of thermodynamics

You can’t get something for nothing!

(Energy can be transformed from one form to another but cannot be created or destroyed)

2nd law of thermo

You can’t even “break even”

(It is not possible convert ALL of the thermal energy derived from a high temperature source to mechanical energy; some thermal energy must be rejected to a low temperature sink)

Efficiency of conversion

hot

cold

in

out

in

outin

inengineheat T

T

Q

Q

Q

QQ

Q

W

11

Definitions Carnot cycle (reversible)(maximum) efficiency

Therefore:

- add heat at as high a temperature as possible

- reject heat at as low a temperature as possible

Practical considerations:

• Temperature of heat source: Combustion: Adiabatic flame temperature? (Less in practice because AFT is for Q = 0)

• Large heat sinks:Atmosphere, oceans. Lakes. ~ ambient temperatures, Tcold = 10 C = 283 K

• Limitations of materials of construction, T, P of working fluids, Thot = 600 C = 873 K

For Tcold = 10 C = 283 KThot = 600 C = 873 K

68.0873

2831maximum

We get less in real cycles (irreversible)

Power Plant with Simple Vapour Cycle

The efficiency of a power plant:

~ 35% for modern plants

inenergychemical

outenergyelectricalplantpower

producedenergyelectricalnet

requiredenergyChemicalRATEHEAT

i.e., the inverse of efficiency, usually expressed as kJ/kWh or Btu/kWh

1 kWh = 1 kJ/s * 3600 s = 3600 kJ

So,

a HEAT RATE of 10,500 kJ/kWh means 34.3% efficiency

η= 3600/10,500 = 0.343

producedenergyelectricalnet

requiredenergyChemicalRATEHEAT