Thermodynamic Cycles for CSP

Transcription

1 Thermodynamic Cycles for CSP D. Yogi Goswami, Ph.D, PE Distinguished University Professor Director, Clean Energy Research Center University of South Florida, Tampa, Florida Editor-in-Chief, Solar Energy Journal

2 Advantages Can be integrated with fossil fuels Thermal Energy Storage Challenge Cost

3 Common Thermodynamic Cycles used are Rankine Cycle Brayton Cycle Stirling Cycle

4 Basic Thermodynamic Cycles for Solar Power Common Thermodynamic Cycles used are Rankine Cycle Brayton Cycle Stirling Cycle 4

5 Power Cycle Temperatures and Efficiencies 5

6 New Cycles for Power & Other Applications Temperatures below C Organic Rankine Cycle (ORC) Supercritical ORC Temperatures above C Supercritical CO 2 cycle Supercritical CO 2 cycle with bottoming cycles Combined Cycles for Power & Other Applications Combined power/cooling cycles Other combined cycles (e.g. power/desalination) 6

7 7 Temperatures below C Many examples of Low/Medium temperature sources: < C Geothermal, Waste heat, Low Conc. Solar Collectors etc. ORC is usually considered for these sources We have analyzed an alternative Supercritical ORC

8 8 Background ORC, SORC 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.

9 9 Background Supercritical Organic Rankine cycle Advantages: o Simple configuration o Better thermal match with the heat source o Higher efficiency T-S diagram of a supercritical cycle with the temperature profile of the hot brine.

10 10 Background Important parameters Source temperature Working fluid o CO 2 o Refrigerants o Mixtures Pressure ratio Sink temperature o Lower sink temperature improves the efficiency o Limited by the ambient conditions Chen et al. 2010

11 11 SORC Power Cycle Analysis Operating conditions for SORC analysis Heat source: C C Sink temperature: 20 0 C Turbine efficiency: 85% Pump efficiency: 85% Vapor fraction at expander outlet >95% Pinch temperature: C Layout of the cycle used for the simulations

12 12 Working Fluid Selection Fluid selection criteria Critical temperature Critical pressure Environmental concerns o o o Ozone Depletion Potential (ODP) Non-flammable, Stable in the temperature and pressure range

13 13 Potential Fluids Fluid Critical Temperature ( 0 C) Critical Pressure (Bar) R134a R R143a R R R

14 14 Optimum operating pressure 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.

15 Thermal efficiency at optimum pressure 15

16 SUPERCRITICAL CARBON DIOXIDE POWER CYCLE 16

17 Critical properties of some fluids 17

18 Characteristics of s-co 2 around the critical point 18

19 Characteristics of s-co 2 around the critical point Thermal conductivity of water at 305K is At the atmospheric pressure and the same temperature, the thermal conductivity of air is given as

20 Supercritical CO 2 Power Cycle Wright et al (2011) 20

21 Simple S-CO 2 Brayton Cycle 21

22 S-CO 2 Recompression Brayton Cycle 22

23 S-CO 2 Partial cooling Brayton Cycle 23

24 Validating the model 24

25 Combined s-co 2 -ORC cycles 25

26 Combined simple s-co 2 -ORC cycles The efficiency of the simple S-CO 2 configuration without the bottoming cycle under same operating condition is obtained as

27 Combined recompression s-co 2 -ORC cycles The efficiency of the recompression S-CO 2 configuration without the bottoming cycle under same operating condition is obtained as

28 Combined Partial cooling s-co 2 -ORC cycles 28

29 Combined Partial cooling s-co 2 -ORC cycles The efficiency of the partial cooling s-co2 configuration without the bottoming cycle under same operating condition is obtained as

30 Performance of the cycle at different temperatures 30

31 S-CO 2 power cycle in CSP plants Wright et al (2011) 31

32 COMBINED CYCLES FOR POWER AND OTHER APPLICATIONS 32

33 Goswami Cycle (Combined Power & Cooling) 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 33

34 Result da=[5.96 bar, 150 C, 0.22 kg NH3/kg solution, 150 C]. db=[33.62 bar, 150 C, 0.52 kg NH3/kg solution, 85.6 C]. dc=[10.56 bar, 150 C, 0.23 kg NH3/kg solution, 150 C]. Pareto front of cooling and first law efficiency with respect to net work output. 34

35 Modified Goswami cycle for combined power and cooling DES 8 REC 10 SH EXP SHX CON 6 13 SEV 7 SP 1 16 RSC 14 REV 15 ABS 17 EVA Schematic diagram of single-stage combined absorption cycle with series flow arrangement 35

36 1. For low- and mid-temperature applications Cycle Simulation: Effect of Generator Temperature 36

37 1. For low- and mid-temperature applications 37

38 Combined SORC Power-RO Desalination Cycle

39 Comparison of the optimized condition for ORC-RO and SORC-RO system using low grade heat sources R245fa R152a Solar Field Output (kw) Heat to Water (kj/kg) Cycle Efficiency 15.86% 13.47% Solar Collector Area (m 2 ) Solar radiation to water (kj/kg) HTF Flow Rate (kg/s) HTF Temperature Range ( C) Fresh Water Production (kg/s) Operation Pressure (MPa) Recuperator or not Yes No 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

40 Possible Combined Desalination Cycles Once-Through Heat Source Recirculating Heat Source Heat Heat Heat Heat Boiler Boiler Power Cycle Power Cycle Steam TVC MVC MED MED MED RO Fresh Water Many Combinations and Configurations

41 41 Dry Cooling Dry cooling using ambient air can increase the condensation temperature by 15 0 C 25 0 C That will reduce thermal efficiency by ~ 20-25% for low/medium source temperatures Passive cooling techniques can reduce the sink temperature by 15 0 C 25 0 C Ground Coupling Night sky radiation

42 42 Ground Coupling 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

43 43 Ground-coupled Dry Cooling Air Cooled Condenser coupled with EAHE

44 44 Ground-coupled Dry Cooling Cool water with ground coupling instead of air Water may also be cooled by night sky radiation

45 45 Dry cooled condenser with Ground Coupling and Nocturnal Cooling Air as the cooling medium in the condenser

46 46 Dry cooled condenser with Ground Coupling and Nocturnal Cooling Water as the cooling medium in the condenser

47 47 Current study Earth-air-heat-exchanger (EAHE) Mass flow rate: 80 kg/hour Pipe diameter: 25 cm Pipe length: m Depth: 1-4 m Location: Las Vegas Average annual temperature: C Heat source temperature: C Working fluid in SRC: R134a Ambient air temperature and underground temperature at different depths

48 48 Effect of depth Performance of EAHE improved with depth Ambient air can be used directly during colder weather Outlet air temperature for different depths Efficiency of SRC for different depths of EAHE

49 49 Effect of length As length increases Outlet temperature and daily variations decrease Efficiency increases with length Improvement is negligible after 50 m Annual variation of the outlet air temperature at different pipe lengths Annual variation of SRC efficiency for different lengths of EAHE

50 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

51 Thank You

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