Technische Universität Berlin
Institut für Energietechnik
Low temperature cycles –
Concepts and prospects
Felix Ziegler
TU Berlin, Institut für Energietechnik
Fachgebiet Maschinen- und Energieanlagentechnik
Silke Köhler
GFZ GeoForschungsZentrum Potsdam, Germany
Present affiliation: RWE Power AG, Germany
Power
Production of Power:
Optimisation
Other options:
Basic thoughts
Cold
Heat
Triangular Cycle
temperature
heat source
heat sink
entropy
Fits in heat source / heat
sink characteristics
Sets upper limit
Realisation:
Gas Turbine with
Isothermally Cooled
Compressor
Rankine Cycle
Kalina Cycle
(sorption power cycle)

 T2e T1 1 T1 (T2e T2a ) 
 T2e 
T1
 g  
  1 
ln 

g
2

2
T2e
 T2e

 T2e T2 a  T2a 

W
Q2
0,15
120°C

W
Q2
100°C
0,1
T2e
supply temperature T 2i
80°C
T2a
T1=31°C
g=0,7
DT=5K
0,05
T1
Q1
0
40
60
80
100
return temperature T 2o [°C]
120
COST
Specific heat turnover:


Q
in
  Qout
Euse
Use of low temperature heat sources:
The heat exchangers will cause a large fraction of the first cost.
The total turnover of heat may serve as a partial cost indicator.

Specific heat turnover:

2

1
40
Power plant
 el 35
30
supply temperature T 2i
80°C
25
100°C
20
120°C
15
10
40
60
80
100
return temperature T 2o [°C]
120
Rankine Cycle (Organic Working Fluid)
1
2
4
5
2 isentropic compression
4 constant pressure heat addition
preheater, evaporator
KP
5 isentropic expansion
1 constant pressure heat
rejection desuperheater, condenser
Te
4
temperature
3
2
1
pu
5
Tc
6
entropy
pl
Real Cycle (Organic Working Fluid)
1
2
3
4
5
6
2 irreversibility in the pump
3 pressure drop
KP
4 pressure drop
5 irreversibility in the turbine
6 pressure drop
1 pressure drop
Te
2
1
5
Tc
6
entropy
pl
4
temperature
3
pu
ideal cycle
real cycle
Heat Transfer Diagram ORC
Constraints
Tb,in
temperature
e
brin
DTmin,in
3
Te
4
Tb,out
Free variables
 Working fluid
 Te
 DTmin,in, DTmin,out
5
Tc
1
6
DTmin,out 2
TCW,in cooling water TCW,out
Qout
transferred heat
 Brine: Tb,in, mass flow rate,
specific heat capacity
 Cooling medium: TCW,in,
TCW,out, specific heat capacity
Qin
Return Temperature Brine, ORC & Kalina
Optimised for work output
125
125
Kalina
100
return temperature brine (°C)
return temperature brine (°C)
ORC
75
50
25
air cooling
water cooling
125°C
75
50
25
air cooling
water cooling
0
100°C
100
150°C
175°C
200°C
(R290) (RC318) (R600a) (R600)
(i-C5)
initial temperature brine, (working fluid)
100°C
125°C
150°C
175°C
initial temperature brine
200°C
Overall Efficiency ORC & Kalina
Optimised for work output
10%
10%
Kalina
ORC
8%
overall efficiency
overall efficiency
8%
6%
4%
6%
4%
2%
2%
air cooling
water cooling
air cooling
water cooling
0%
0%
100°C
125°C
150°C
175°C
200°C
(R290) (RC318) (R600a) (R600)
(i-C5)
100°C
125°C
150°C
175°C
initial temperature brine
initial temperature brine, (working fluid)
Pgen  PDHpump  PFeedPump  PCWpump
Pnet
sy s  

 brine c b Tb,in  T0 
m
Qbrine
200°C
Conclusions of Comparison
Both systems are suitable for power production from low
enthalpy reservoirs
With given constraints from heat source and heat sink
 ORC cool the brine more
 Kalina reach higher thermal efficiency
 High parasitic loads at ORC, especially for air cooling
 ORC are more sensible to changes of heat sink
Suitability of the systems
 Kalina KCS34 up to 150 °C brine or CHP
 ORC from 150 °C brine temperature
Improvements
 Supercritical ORC may improve thermal efficiency
 Other sorption power cycles may improve cooling of brine
Additional options:
COGENERATION (HEAT AND POWER)
COLD PRODUCTION
Goal:
Increase of cost effectiveness
Geothermal Heat and Power
(no “real” cogeneration!)
Serial
electrical
energy
T
Parallel
electrical
energy
heat to district
heating system
waste heat
power plant
T
heating station
brine
waste heat
power plant
brine out
T
s
brine
s
QH
 brine temperature > temperature
for heating purposes
 Not necessarily simultaneous
production
Additional constraints due to
heating demand!
Outlet temperature brine
Mass flow rate brine
brine out
heating station
T
QH
heat to district
heating system
 brine temperature 
temperature for heating
Subsystems compete
Cold production
Heat driven chiller:
COPc 
Q0
Q2
T2i
Q2
T2o
T1
Q0
T0
Q1
COPc
COPc

T 0 1  T 1 ln T 2i  g




T 1  T 0   T 2i  T 2o  T 2o 

T 0  T 2i  T 1  1 T1 (T2i  T2o )  g


2
T 1  T 0  T 2i 2 T2i

T2i
Q2
T2o
T1
Q0
T0
Q1

COPc
T 0 1  T 1 ln T 2i  g




T 1  T 0   T 2i  T 2o  T 2o 
1,25
COP c
120°C
1
100°C
T2i
Q2
T2o
T0
80°C
T0=9°C
T1=31°C
g=0,7
DT=5K
0,5
T1
Q0
supply temperature T 2i
0,75
Q1
0,25
0
40
60
80
100
return temperature T 2o [°C]
120

Specific heat turnover:
2
2
COPC

8
Chiller
c
7
6
80°C
supply temperature T 2i
5
100°C
4
120°C
3
40
60
80
100
return temperature T 2o [°C]
120
Conclusions
Power production:
Free Variables




Layout
Working fluid (medium, composition)
Upper process temperature
More free parameters with Kalina
Cooling from low grade heat is also an attractive option
Sorption seems to be a key technology
Future requirements:
- Technical experience
- Cost: Heat exchange
Comparison of chiller with power plant:
COPc
COPc

T0

T 1  T 0
 10 
Value
Specific heat
turnover 
heat
5 ct/kWh
1
1
cooling energy
10 ct/kWh
2/COPc + 2
5
electrical energy 20 ct/kWh
2/ - 1
20
Kalina KCS 34 Layout
6’’
desorber 6
turbine generator
G
separator
5
6’
7
8
preheater
4
9
10
LT-recuperator
3
HTrecuperator
down
hole
pump
production
well
injection
well
basic solution
rich vapor
poor solution
heat
sink
11
2
1
feed absorber cooling water
pump
pump
Heat Transfer Diagram Kalina
Tb,in
DTmin,in
6
6’
temperature
ne
bri
desorber
Tb,out DTmin,in
8
5
preheater
4
10
11
HT-preheater
3
absorber
LT- preheater
1
DTmin,out
TCW,in cooling water
Qout
TCW,out
Qre
Qin
transferred heat
Neustadt-Glewe
Constraints
net capacity
to grid
T
brine
98°C
110 m³/h
power plant
generator capacity
210 kWel
s
heating station
6 MWth geothermal
11 MWth total
Tb,out
 Brine temperature,
mass flow rate
 Heat sink temperature
 Heating capacity in district
heating system
Free variable
 Portion of brine through plant
/ upper process temperature
Objective functions
 Generator capacity ~ Pmech
 Cooling of the brine Tb,out
 Resource Utilization Factor
heat to district
heating system
RUE (overall exergetic
efficiency)
 heatingsy stem  hHS  hHS,0  T0  sHS  sHS,0 
Pexergy
m
t  Pp products
RUE 
 brine  hin  h0  T0  sin  s0 
exergy brine
m
Results of Optimisation
 Brine 35 kg/s, 98 °C
 District heating system (assumed) 50 kg/s, 70/55, 3.1 MWth
 Working medium power plant Perflourpentane, water cooling 15/20
1.2
200
T bout (°C)
portion of brine through plant
1
RUE
P mech (kW)
150
T, P_mech
0.8
0.6
100
0.4
50
0.2
0
0
30
50
70
90
upper process temperature (°C)
  const  m
 brine c  Tb,in  Tb,out 
Q
in
30
50
70
upper process temperature (°C)
90
ORC Layout
evaporator
turbine generator
4
G
3
preheater
heat
sink
down
hole
pump
5
2
1
production
well
injection
well
feed
pump
condenser
cooling
water
pump
Heat Transfer Diagram Kalina
Constraints
Tb,in
DTmin,in
6
6’
temperature
ne
bri
desorber
Tb,out DTmin,in
8
10
HT-preheater
3
absorber
LT- preheater
1
DTmin,out
TCW,in cooling water
Qout
2
TCW,out
Qre
Qin
transferred heat
Free variables






5
preheater
4
11
 Brine: Tb,in, mass flow rate,
specific heat capacity
 Cooling medium: TCW,in,
TCW,out, specific heat capacity
Composition basic solution
Mass flow rate basic solution
Pressure desorption
Pressure absorption

Q
des

Q
re
Optimisation of Lorentz Cycle

heat source
heat source
heat source
temperature
temperature
temperature
w 
heat sink
Tb,r
heat sink
entropy
heat sink
entropy
heat source
entropy

heat source

heat source
temperature
temperature
temperature
w 
heat sink
entropy
Tb,r 
heat sink
entropy
heat sink
entropy