INTRODUCTION TO RENEWABLE ENERGY
course code IE3320
subject: THERMAL POWER CONVERSION
N. Woudstra
e-mail: n.woudstra@wbmt.tudelft.nl
Course material:
•
Renewable energy; second edition, Godfrey Boyle, Open
University; ISBN 0-19-926178-4
chapter 2 section 2.9 and 2.10 (solar thermal energy)
chapter 9 section 9.1, 9.2 and 9.3 (geothermal energy)
•
Copies of slides (blackboard)
Additional material:
•
Fundamentals of Engineering Thermodynamics; third
edition, Moran & Shapiro, ISBN 0-471-97960-0
THERMODYNAMIC LAWS
thermodynamic laws are based on experience
FIRST LAW:
‘energy can neither be lost nor originated
from the nothingness’
[basis for energy balances]
SECOND LAW:
‘work can be transformed completely into heat,
but heat cannot be transformed completely into work’
[basis for definition of the thermodynamic
temperature scale (Kelvin scale), entropy,
exergy, ……etc.]
SECOND LAW OF THERMODYNAMICS
“work can be converted completely into heat, but
heat cannot be converted completely into work”
hot reservoir
at temperature TH
efficiency of thermal power cycle:
th 
QH
thermal
power cycle
QC
cold reservoir
at temperature TC
W
W QH  QC

QH
QH
efficiency of a reversible cycle depends only
on temperatures of heat transfer to and from
the system
the thermodynamic temperature scale (Kelvin
scale) has been defined such that:
QH  QC TH  TC
T

 1 C
Q1
TH
TH
 T 
then: work from a reversible power cycle becomes:
Wrev   1  C   QH
 TH 
TC
the term 1 
is generally known as the Carnot efficiency
TH
th, rev. 
SECOND LAW OF THERMODYNAMICS
efficiency of a reversible cycle: th, rev.  1 
hot reservoir
at temperature TH
because of losses within the cycle the
efficiency of an irreversible cycle will always
be lower,
QH
thermal
power cycle
QC
cold reservoir
at temperature TC
TC
TH
W
therefore:
or:
with:
th, irrev.
th, irrev.  1 
TC
TH
 TC 
 ex, intern.   1  
 TH 
ex, intern. 
then:
work from an irreversible power cycle becomes:
Wirrev
Wrev
 T 
Wirrev  ex, intern.   1  C   QH
 TH 
in general temperatures of heat transfer to and from power cycles are not constant
this complicates the application of this equation
TEMPERATURE OF HEAT TRANSFER
heat transfer to fluid without phase
change (liquid or gas)
T
temperature increase due to heat
transfer
T
Q
heat transfer to fluid with phase change
(from liquid to gas)
evaporation
(phase change)
Q
temperature increase in liquid phase
and gas phase
constant temperature during
evaporation (phase change)
WORK FROM HEAT OF A FLUID FLOW

as: Wrev   1 

TC 
  QH
TH 
then the reversible work from an infinitesimal
amount of heat becomes:
 TC 
dWrev   1    dQH
 TH 
 T 
and from a fluid flow with varying temperatures: Wrev    1  C   dQH
 TH 
suppose: temperature of cold reservoir is ambient temperature T0
 T0 
   1    dQH
TH 
in 
out
then:
Wrev
the thermodynamic equivalent temperature of heat transfer to the fluid TH is
by definition:
Wrev
out
 T0 
 T 
  1    QH    1  0   dQH
TH 
 TH 
in 
WORK FROM HEAT OF A FLUID FLOW
Wrev
out
 T0 
 T 
  1    QH    1  0   dQH
TH 
 TH 
in 
it appears that the equivalent temperature of heat transfer TH can be
determined with the following (simple) equation:
TH 
H hout  hin

S sout  sin
in case of ideal gas with constant specific heat (without phase change)
this equation can be transformed into:
TH 
hout  hin Tout  Tin

T
sout  sin
ln out
Tin
this equation can also be used to determine TC when transfer from the
system occurs at varying temperatures:
TC 
hout  hin Tout  Tin

T
sout  sin
ln out
Tin
POWER FROM THERMAL POWER CYCLE
work from a thermal power cycle can be determined by applying the following
equation:
TC
Wirrev  ex, intern.  (1 
TH
)  QH
as power P is work per unit of time, and Q a heat flow, then the
following equation results for the power from a thermal power cycle:
Pcycle  ex, intern.  (1 
TC
)  QH
TH
when heat is generated in a boiler, the heat flow to cycle becomes:
QH  th, boiler  m, F  LHVF
with: th, boiler 
LHVF 
thermal efficiency boiler (-)
lower heating value of fuel (kJ/kg)
efficiencies (internal efficiency as well as boiler efficiency) strongly depend on
type and scale of the considered system:
th, boiler  0.90 à 0.95
for large steam boilers:
for large steam turbine cycles: ex, intern.  0.80
EXERGY OF HEAT
definition:
exergy is the maximum theoretical work that can be derived from an
amount of energy (in a well defined system), using the environment
as reservoir for heat (and matter)
exergy of heat at temperature T:
 T 
ExQ  Wrev   1  0   Q
 T 
(according to the definition the temperature of the cold reservoir is T0 )
the exergy of heat from a fluid flow = reversible work from the heat that
can be extracted from the fluid flow:
 T 
ExQH  Wrev   1  0   QH
 TH 
THERMAL POWER CYCLE
simple steam cycle
1
1 = boiler
2 = turbine
2
3 = condenser
1
4 = condensate
pump
7
2
5 = deaerator
8
6
6 = feedwater
pump
7
5
3
4
9
10
8
3
6
5
4
7 = heat sink
8 = cooling water
pump
THERMAL POWER CYCLE
standard steam cycle
standard steam cycle
•
superheated steam
•
without steam reheat
•
without feedwater
preheat
T
life steam conditions:
p3  40 bar ; T3  530 C
h h
3513.1  129.3
TH  3 1 
s3  s1 7.1774  0.4353
 502 K (229 C)
TC  Tcond  30 C (303 K)
3
TH
Tcond.
1
5
Wirrev
T
 ex, intern.  (1  C )
QH
TH
with: ex, intern.  0.75
303
th  0.75  (1 
)  0.297
502
th 
2
4
x minimum
s
THERMAL POWER CYCLE
low temperature saturated steam cycle
low temperature steam cycle
•
saturated steam
•
without steam reheat
•
without feedwater preheat
life steam conditions:
p3  3.0 bar ; T3  134 C
h h
2724.7  129.3
TH  3 1 
s3  s1 6.9909  0.4353
 396 K (123 C)
TC  Tcond  30 C (303 K)
T
Wirrev
T
 ex, intern.  (1  C )
QH
TH
with: ex, intern.  0.75
303
th  0.75  (1 
)  0.176
396
th 
2
TH
Tcond.
3
1
5
4
x minimum
s
THERMAL POWER CONVERSION
AND RENEWABLE ENERGY
HEAT SOURCES:
(biomass
combustion
gasification + combustion)
geothermal heat
low temperature heat
solar heat
(low temperature heat (flat plate collectors))
high temperature heat
parabolic concentrators
solar central receiver (solar tower)
SOLAR HEAT
SOLAR HEAT
Concentrated solar
radiation enables
high temperature of
heated liquids
SOLAR HEAT
SOLAR ELECTRIC GENERATING SYSTEM
(SEGS)
SOLAR HEAT
GEOTHERMAL HEAT
GEOTHERMAL HEAT
Natural steam from the production wells power the turbine generator.
Steam is condensed by evaporation in the cooling tower and pumped down an
injection well to sustain production.
DRY STEAM
POWER PLANTS
Steam plants use hydrothermal fluids that are primarily steam.
The steam goes directly to a turbine, which drives a generator that produces
electricity.
This is the oldest type of geothermal power plant. It was first used at Lardarello
in Italy in 1904, and is still very effective (>700 MWe).
Steam technology is used today at The Geysers in northern California, the
world's largest single source of geothermal power.
FLASH STEAM
POWER PLANTS
Hydrothermal fluids above 360ºF (182ºC) can be used in flash plants to make
electricity.
Fluid is sprayed into a tank held at a much lower pressure than the fluid,
causing some of the fluid to rapidly vaporise, or "flash."
The vapour then drives a turbine, which drives a generator.
BINARY-CYCLE
POWER PLANTS
Most geothermal areas contain moderate-temperature water (below 200 ºC).
Energy is extracted from these fluids in binary-cycle power plants. Hot
geothermal fluid and a secondary (hence, "binary") fluid with a much lower
boiling point than water pass through a heat exchanger. Heat from the
geothermal fluid causes the secondary fluid to flash to vapour, which then
drives the turbines.
Moderate-temperature water is by far the more common geothermal resource,
and most geothermal power plants in the future will be binary-cycle plants.