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

Advantages• Can be integrated with fossil fuels• Thermal Energy Storage

Challenge• Cost

Common Thermodynamic Cycles used are

Rankine Cycle

Brayton Cycle

Stirling Cycle

Basic Thermodynamic Cycles for Solar Power

4

Common Thermodynamic Cycles used are

Rankine Cycle

Brayton Cycle

Stirling Cycle

Power Cycle Temperatures and Efficiencies

5

New Cycles for Power & Other Applications

6

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)

•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

7

Temperatures below 3000C

• 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

• Advantages:

o Simple configuration

o Better thermal match with

the heat source

oHigher efficiency

9

Supercritical Organic Rankine cycle

T-S diagram of a supercritical cycle with the

temperature profile of the hot brine.

Background

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

11

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

12

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

13

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

14

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

15

Thermal efficiency at optimum pressure

16

SUPERCRITICAL CARBON DIOXIDE

POWER CYCLE

17

Critical properties of some fluids

Characteristics of s-CO2 around the critical point

18

Characteristics of s-CO2 around the critical point

19

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 .

20

Supercritical CO2 Power Cycle

Wright et al (2011)

Simple S-CO2 Brayton Cycle

21

S-CO2 Recompression Brayton Cycle

22

S-CO2 Partial cooling Brayton Cycle

23

Validating the model

24

Combined s-CO2-ORC cycles

25

Combined simple s-CO2-ORC cycles

26

The efficiency of the simple S-CO2 configuration without the bottoming cycle under sameoperating condition is obtained as 0.4507.

Combined recompression s-CO2-ORC cycles

27

The efficiency of the recompression S-CO2

configuration without the bottoming cycleunder same operating condition is obtained as0.4932.

Combined Partial cooling s-CO2-ORC cycles

28

Combined Partial cooling s-CO2-ORC cycles

29

The efficiency of thepartial cooling s-CO2configuration without thebottoming cycle undersame operating conditionis obtained as 0.4959.

Performance of the cycle at different temperatures

30

31

S-CO2 power cycle in CSP plants

Wright et al (2011)

32

COMBINED CYCLES FOR POWER

AND OTHER APPLICATIONS

Goswami Cycle (Combined Power & Cooling)

33

• 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

Result…

34

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].

35

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

36

1. For low- and mid-temperature applications

Cycle Simulation: Effect of Generator Temperature

37

1. For low- and mid-temperature applications

Combined SORC Power-RO Desalination Cycle

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

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

•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

41

Dry Cooling

42

• 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

43

Ground-coupled Dry Cooling

Air Cooled Condenser coupled with EAHE

44

Cool water with ground

coupling instead of air

Water may also be cooled

by night sky radiation

Ground-coupled Dry Cooling

45

Dry cooled condenser with

Ground Coupling and Nocturnal Cooling

Air as the cooling medium in the condenser

46

Dry cooled condenser with

Ground Coupling and Nocturnal Cooling

Water as the cooling medium in

the condenser

47

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

48

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

49

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

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

Thank You