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Thermodynamic Cycles for CSP
D. Yogi Goswami, Ph.D, PEDistinguished University Professor
Director, Clean Energy Research Center
University of South Florida, Tampa, Florida
Editor-in-Chief, Solar Energy Journal
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Advantages• Can be integrated with fossil fuels• Thermal Energy Storage
Challenge• Cost
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Common Thermodynamic Cycles used are
Rankine Cycle
Brayton Cycle
Stirling Cycle
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Basic Thermodynamic Cycles for Solar Power
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Common Thermodynamic Cycles used are
Rankine Cycle
Brayton Cycle
Stirling Cycle
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Power Cycle Temperatures and Efficiencies
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New Cycles for Power & Other Applications
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Temperatures below 3000C
Organic Rankine Cycle (ORC)
Supercritical ORC
Temperatures above 6000C
Supercritical CO2 cycle
Supercritical CO2 cycle with bottoming cycles
Combined Cycles for Power & Other Applications
Combined power/cooling cycles
Other combined cycles (e.g. power/desalination)
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•Many examples of Low/Medium temperature
sources: < 3000C
Geothermal, Waste heat, Low Conc. Solar Collectors
etc.
•ORC is usually considered for these sources
•We have analyzed an alternative
Supercritical ORC
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Temperatures below 3000C
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• Organic Rankine Cycle (ORC): Similar
to steam Rankine cycle but with
organic working fluid with low
boiling and critical points.
• Supercritical Organic Rankine Cycle
(SRC): Working fluid is pressurized
above its critical pressure and heated
to supercritical state.
8
Background – ORC, SORC
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• Advantages:
o Simple configuration
o Better thermal match with
the heat source
oHigher efficiency
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Supercritical Organic Rankine cycle
T-S diagram of a supercritical cycle with the
temperature profile of the hot brine.
Background
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Important parameters
• Source temperature
• Working fluid
o CO2
o Refrigerants
o Mixtures
• Pressure ratio
• Sink temperature
o Lower sink temperature improves the efficiency
o Limited by the ambient conditions
10
Background
Chen et al. 2010
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Operating conditions for SORC analysis
• Heat source: 1000C – 2000C
• Sink temperature: 200C
• Turbine efficiency: 85%
• Pump efficiency: 85%
• Vapor fraction at expander outlet
>95%
• Pinch temperature: 7-90C
SORC Power Cycle Analysis
Layout of the cycle used for the
simulations
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Working Fluid Selection
Fluid selection criteria
• Critical temperature
• Critical pressure
• Environmental concerns
o Ozone Depletion Potential (ODP)
o Non-flammable,
o Stable in the temperature and pressure range
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Potential Fluids
Fluid Critical Temperature (0C) Critical Pressure (Bar)
R134a 101.05 40.6
R32 78.11 57.8
R143a 72.71 37.6
R218 71.87 26.4
R125 66.02 36.2
R170 32.18 48.7
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Fluid with the lowest critical temperature (R170) had the highest
optimum pressure while fluid with the highest critical temperature
(R134a) had a lower optimum pressure.
Optimum operating pressure
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Thermal efficiency at optimum pressure
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SUPERCRITICAL CARBON DIOXIDE
POWER CYCLE
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Critical properties of some fluids
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Characteristics of s-CO2 around the critical point
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Characteristics of s-CO2 around the critical point
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Thermal conductivity of water at 305K is 618.41 . At the atmospheric pressure and the same temperature, the thermal conductivity of air is given as 26.355 .
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Supercritical CO2 Power Cycle
Wright et al (2011)
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Simple S-CO2 Brayton Cycle
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S-CO2 Recompression Brayton Cycle
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S-CO2 Partial cooling Brayton Cycle
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Validating the model
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Combined s-CO2-ORC cycles
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Combined simple s-CO2-ORC cycles
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The efficiency of the simple S-CO2 configuration without the bottoming cycle under sameoperating condition is obtained as 0.4507.
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Combined recompression s-CO2-ORC cycles
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The efficiency of the recompression S-CO2
configuration without the bottoming cycleunder same operating condition is obtained as0.4932.
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Combined Partial cooling s-CO2-ORC cycles
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Combined Partial cooling s-CO2-ORC cycles
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The efficiency of thepartial cooling s-CO2configuration without thebottoming cycle undersame operating conditionis obtained as 0.4959.
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Performance of the cycle at different temperatures
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S-CO2 power cycle in CSP plants
Wright et al (2011)
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COMBINED CYCLES FOR POWER
AND OTHER APPLICATIONS
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Goswami Cycle (Combined Power & Cooling)
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• Uses mixed working
fluids
• Overcomes pinch point
problem
• Condensation is by
absorption
• Removes the turbine exit
temperature constraint
• Can be designed for all
power to all cooling and
any combination of power
and cooling
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Result…
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Pareto front of cooling and first law efficiency withrespect to net work output.
dA=[5.96 bar, 150 C, 0.22 kg NH3/kgsolution, 150 C].
dB=[33.62 bar, 150 C, 0.52 kg NH3/kgsolution, 85.6 C].
dC=[10.56 bar, 150 C, 0.23 kg NH3/kgsolution, 150 C].
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EXP
SH
RSC
EVA
13
14
15
11
12CON
16
SHX
SP
ABS
RECDES
1
23
4
7
6
5
8
9
17
10
SEV
REV
Schematic diagram of single-stage combined absorption cycle with series flow arrangement
Modified Goswami cycle for combined power and cooling
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1. For low- and mid-temperature applications
Cycle Simulation: Effect of Generator Temperature
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1. For low- and mid-temperature applications
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Combined SORC Power-RO Desalination Cycle
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Comparison of the optimized condition for ORC-RO
and SORC-RO system using low grade heat sources
the most energy efficiency MED-Double absorption heat pump combined system has the heat
to water consumption is 108kJ/kg and solar energy to water consumption is 142 kJ/kg
R245fa R152a
Solar Field Output (kW) 586.33 685.69
Heat to Water (kJ/kg) 53.11 62.11
Cycle Efficiency 15.86% 13.47%
Solar Collector Area (m2) 1020 1065
Solar radiation to water (kJ/kg) 92.39 96.47
HTF Flow Rate (kg/s) 6.651 2.903
HTF Temperature Range (°C) 124.5-150 87-150
Fresh Water Production (kg/s) 11.04 11.04
Operation Pressure (MPa) 2.2 5.3
Recuperator or not Yes No
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Possible Combined Desalination Cycles
Once-Through Heat Source
Boiler
MED
Heat
Steam
Fresh Water
Power
Cycle
MED
Heat
MVC
Heat
Power
Cycle
RO
Boiler
MED
Heat
TVC
Recirculating Heat Source
Many Combinations and Configurations
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•Dry cooling using ambient air can increase the
condensation temperature by 150C – 250C
• That will reduce thermal efficiency by ~ 20-25% for
low/medium source temperatures
• Passive cooling techniques can reduce the sink
temperature by 150C – 250C
Ground Coupling
Night sky radiation
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Dry Cooling
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• Earth-air-heat-exchanger (EAHE) have been used for air-conditioning
of buildings and greenhouses
• EAHE may be coupled with SORC condenser
• Water may also be cooled by ground coupling
• If water is used, it can also be cooled by night sky radiation
Ground Coupling
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Ground-coupled Dry Cooling
Air Cooled Condenser coupled with EAHE
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Cool water with ground
coupling instead of air
Water may also be cooled
by night sky radiation
Ground-coupled Dry Cooling
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Dry cooled condenser with
Ground Coupling and Nocturnal Cooling
Air as the cooling medium in the condenser
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Dry cooled condenser with
Ground Coupling and Nocturnal Cooling
Water as the cooling medium in
the condenser
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Current study
Earth-air-heat-exchanger (EAHE)
• Mass flow rate: 80 kg/hour
• Pipe diameter: 25 cm
• Pipe length: 25-100 m
• Depth: 1-4 m
• Location: Las Vegas
• Average annual temperature: 19.50C
• Heat source temperature: 1500C
• Working fluid in SRC: R134a
Ambient air temperature and underground
temperature at different depths
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Effect of depth
Outlet air temperature for different depths Efficiency of SRC for different depths
of EAHE
• Performance of EAHE improved with depth
• Ambient air can be used directly during colder weather
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Effect of length
Annual variation of the outlet air
temperature at different pipe lengths
Annual variation of SRC efficiency for
different lengths of EAHE
• As length increases
• Outlet temperature and daily variations decrease
• Efficiency increases with length
• Improvement is negligible after 50 m
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Conclusions
• Recent research on thermodynamic
cycles has opened up new possibilities for
CSP
New working fluids
Higher efficiencies
New combined outputs
• Potential for additional R & D
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Thank You
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