2004 Martin and Goswami

7/29/2019 2004 Martin and Goswami

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Analysis of experimental power and cooling production in a

combined power and cooling cycle

C. L. Martin1, D. Y. Goswami

1*

1Solar Energy and Energy Conversion Laboratory, Department of Mechanical and Aerospace Engineering,

University of Florida, P.O. Box 116300, Gainesville, Florida 32611-6300, USA

Abstract

A novel combined power-cooling thermodynamic cycle, for use with low-temperature, sensible heatsources, is under experimental investigation. In this power-cooling cycle, absorption condensation is usedto regenerate the working fluid. This allows the expander exhaust temperature to drop significantly belowthe temperature at which absorption is taking place. This is a departure from pure working fluid, Rankine

cycle operation and is the source of cooling. Experiments have been performed that show the power-cooling concept to be valid, however, minimum cooling temperatures were higher than expected. This isattributed to lower than expected expander performance. Experimental results give the impression thatthe expander efficiency drops as cooler exhaust temperatures are approached. Heat transfer losses are the

probable reason rather than measurement error. With improvements to expander efficiency, however,

practical cooling temperatures with simultaneous power output can be achieved.

Keywords: Thermal energy conversion, Cooling, Absorption power cycle

1. Introduction

The thermo-chemical compression system of absorption refrigeration cycles is used to createa heat-driven alternative to the mechanical vapor compression cycle. It has also beenincorporated into a power cycle, and was initially studied by Maloney and Robertson [1]. The

cycle uses a binary mixture of ammonia-water as the working fluid and absorption condensation

to regenerate the working fluid. The use of an ammonia-water working fluid was also proposedfor use in a power cycle by Kalina [2], though absorption condensation was not employed. In

these cycles, boiling takes place over a range of temperatures at constant pressure. This reduces

heat transfer related losses by providing a better thermal match with sensible heat sources.

Goswami [3,4] has proposed a power cycle that takes advantage of these boiling characteristics,and capitalizes on the possible vapor temperature drop across the expander when absorption

condensation is employed. This creates a heat-driven cycle that can simultaneously produce

power and cooling.In this combined power and cooling cycle, low temperature heat (60- 100 C) is used to

drive a thermodynamic cycle based around the properties of a binary working fluid, ammonia-

water. Power is produced by expansion of a high pressure vapor through an expander andcooling comes from the sensible heating of the expander exhaust. Absorption condensation is

used to regenerate the working fluid, this allows the expander exhaust temperature to be

* Corresponding author. Fax: +1-352-392-1071.E-mail address:[email protected] (D. Y. Goswami).


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Nomenclature

Pinlet Expander inlet pressurePexit Expander exhaust pressure

xvr Rectified vapor ammonia mass fraction

expander Expander isentropic efficiency

significantly below the temperature at which absorption is taking place. It differs from pure

working fluid Rankine cycle operation, where the limiting expander exhaust temperature is the

vapor condensation temperature, subcooling effects aside. Therefore, in the power-coolingcycle, it is possible to expand the vapor to sub-ambient temperatures, to produce a low

temperature stream that can be used for cooling.

This cycle has been the focus of previous theoretical and experimental investigations [5-7],however, experimental power and cooling production had not taken place. Recent investigations

have been performed to experimentally show the potential for simultaneous outputs of

mechanical work and sub-ambient cooling by pursuing the temperature drop possible across theexpander. This paper presents the experimental results and an analysis of the experimental

performance.

2. Cycle Operation

Figure 1 is the flow schematic for the experimental setup of the combined power and cooling

cycle. Basic-concentration fluid is drawn from the absorber and pumped to high pressure via thesolution pump. Before entering the boiler, the basic solution recovers heat from the returning

weak solution in the recovery heat exchanger. In the boiler, the basic solution is partially boiled

to produce a two-phase mixture; a liquid, which is relatively weak in ammonia, and a vapor with

a high concentration of ammonia. This two-phase mixture is separated in the separator, and theweak liquid is throttled back to the absorber. The vapors ammonia concentration is increased by

cooling and condensate separation in the rectifier. Heat can be added in the superheater as thevapor proceeds to the expander, where energy is extracted from the high-pressure vapor as it is

throttled to the system low-pressure. To capitalize on any cooling available from the exhaust, an

additional heat exchanger would follow the expander, however, this has not been implemented

experimentally and is not shown in the figure. The vapor rejoins the weak liquid in the absorberwhere, with heat rejection to the cooling water, the basic solution is regenerated.

2.1 Cooling Production

In the combined power and cooling cycle, the vapor temperature exiting the expander can besignificantly below ambient conditions; cooling can be obtained by sensibly heating the expanderexhaust. The temperature drop possible across the expander is due to the fact that the working

fluid is a binary mixture, and at constant pressure the condensing temperature of an ammonia

rich vapor can be below the saturation temperature for a lower concentration liquid. This is best

illustrated with a binary mixture, phase equilibrium diagram, Figure 2. The low concentration


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ThermocoupleFlow Measurement

Fluid Sampling Port

Pressure TransducerP

S

Key

T

F

Throttle

Recovery Heat

Exchanger

TBoiler

S

F

T

Heat

Source

F

T

P T T

Separator

F

Cooling

Water

ST T

Rectifier

T T S

F

T

Absorber

T P

T PP

T

Cooling

Water

T

Expander

P

T

T

F

P

Condensate

Superheater

Solution

Pump

Fig. 1. Experimental power-cooling cycle schematic. Note that a cooling heat exchanger following the expander

was not implemented for this study.

-20

0

20

40

60

80

100

120

0 0.2 0.4 0.6 0.8 1

Ammonia Mass Fraction

Temperature[C]

Pressure = 0.203 MPaVapor

Liquid

Two-Phase

Basic solution

in absorber

Expander

exhaust

Fig. 2. Ammonia-water phase equilibrium diagram highlighting the source of cooling temperatures.

saturated liquid state represents the basic solution exiting the absorber, while the high

concentration vapor is typical of the expander exhaust conditions. This shows how it is possible

for the vapor to be expanded to a temperature below that at which absorption is taking place.According to the equilibrium diagram, to maximize this temperature difference the basic solution

should be low in ammonia concentration and the vapor should be high. Also, partial


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condensation of the expander exhaust would cause an additional decrease in vapor temperature.

This is possible since ammonia becomes saturated upon expansion.

Consideration of the vapor entropy is an alternative viewpoint that provides additionalinsight. Minimization of the exhaust temperature also implies a minimization of the vapor

entropy at expander exhaust, assuming constant exit pressure. From this consideration an

efficient expander is an obvious feature for low temperatures, but even an ideal device wouldonly maintain the vapor entropy from inlet to exhaust. Therefore, expander inlet conditions

should be considered. For a binary vapor mixture, entropy decreases with increasing pressure,

increasing concentration, and decreasing temperature. The limit of these conditions, while stillmaintaining vapor, would be saturated, pure ammonia.

3. Experimental Setup

To investigate the performance of this concept, an experimental study of the power and

cooling cycle is underway. A boiling-absorption loop, used for previous work [7], has been

modified and was used for this study. In the experimental setup, the boiler and recovery units

were implemented with brazed, flat-plate heat exchangers. The rectifier consisted of anotherflat-plate heat exchanger and a packed-bed entrainment separator. Absorption took place in a

pressure vessel equipped with internal heat exchange coils and a bubbling tube to introduce thevapor into the weak liquid. Superheating was achieved by a variable-temperature heating tape

that was wrapped around a portion of the vapor plumbing. Heat for boiling came from hot water

circulated by a separate system. Cooling water for heat rejection and rectification was produced

with a separate water chiller.The expander was a single-stage, partial admission turbine originally for use in an air-cycle

cooling system. Its nozzles are sonic, i.e. they have a converging section only. Loading was

performed with a custom eddy-current brake connected directly to the expander shaft.Instrumentation was installed according to Figure 1. A combination of manual recording and

computer data acquisition were used for data collection. Liquid concentration measurements

were obtained by analyzing a syringe sample with a gas chromatograph. Vapor measurementswere problematic with this method, so concentration was determined using thermodynamic

property data, an assumption of saturated vapor at rectifier exit, and the measured temperature

and pressure at this location.

As indicated in Figure 1, the mechanical power generated by the expander is not directlymeasured. Parameters such as power output and expander efficiency are currently derived from

thermodynamic property measurements at expander inlet and exhaust. Direct power output

measurements have been attempted by measuring the reaction torque of the expander. However,due to its low output power and high rotational speed (approximately 40,000 rpm) the torque is

correspondingly small and difficult to measure accurately. Thus far, these measurements have

not been used.

4. Experimental Results

Experiments have been performed to not only provide a demonstration of the temperature

difference of Figure 2, but also to explore the effects of the expander inlet conditions on exhaust

temperature. Through successive stages of rectification and superheating, the data of Figure 3

was collected. A nominal vapor ammonia concentration of 0.993 [kg/kg] was maintained by


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controlling pressure and temperature at rectifier exit. Expander inlet temperature was then varied

by adding heat in the superheater. All other parameters that could have an effect on expander

performance were held constant within the limits of the experimental setup. The average valuesof other system parameters are presented in Table 1.

Table 1.Average parameter values for the testing of Figures 3 and 4.

Expander Inlet Pressure: 0.516 MPa

Expander Exit Pressure: 0.208 MPa

Vapor Flow Rate: 0.00299 kg/sVapor Concentration: 0.993 kg/kg

Rectifier Inlet Temp.: 83.4 C

Rectifier Exit Temp.: 41.7 CAbsorber Temp.: 31.4 C

20

21

22

23

24

25

26

27

28

29

30

35 40 45 50 55Expander Inlet Temp. [C]

ExpanderExitTemp.

[C]

20 %

30 %

25 %expander=

35 %

Fig. 3. Experimental test results.

60

70

80

90

100

110

120

130

35 40 45 50 55

Expander Inlet Temp. [C]

Power[W]

35 %

30 %

25 %

20 %

= expander

Fig. 4. Expander power output based on measured thermodynamic properties.


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The power and cooling cycle concept is demonstrated by the difference in temperature

between the expander exhaust and the average temperature at which absorption is taking place.The average absorber temperature for all of the testing was 31.4 C. Clearly, all of the results in

Figure 3 fall below this temperature, providing a measurement of the temperature difference in

Figure 2.Mechanical power was simultaneously produced during these experiments, and the computed

values are presented in Figure 4. All of the indicated shaft power was consumed in the eddy-

current brake and dissipated as heat. Again, the power outputs in Figure 4 are based on thecomputed enthalpy change determined from the thermodynamic property measurements

mentioned previously.

5. Analysis

The experimental results verify the power-cooling concept by measuring the temperature

difference that is the source of cooling. However, this is not the same as demonstrating

practicality since the minimum temperature achieved was only 21.9 C. Assuming a standardchilled water temperature of 7.2 C and a 5 C temperature glide, a maximum practical expander

exhaust for cooling production would need to be 2.2 C. Reasons for this lower-than-expectedperformance center around poor expander performance, Figures 3 and 4. Also in these figures,

the expander efficiency does not appear to remain constant as expected. This section examines a

few of the possible reasons for the measured performance.

5.1 Saturated Vapor

As mentioned, the expander is a partial admission dynamic turbine which was not designedto expand a condensing working fluid. If enough flow were to condense it would alter the

momentum transfer in the turbine and there would be a corresponding drop in efficiency.

However, when the amount of condensation is examined for the experimental conditions, theeffect should be small. For example, Figure 5 presents the expected expander exit quality values

for simulated conditions at various expander efficiencies. As can be seen, even for an isentropic

expansion, the minimum expected exit quality does not go below 0.97 for these conditions.

Balje [8] provides information about the dynamic turbine efficiency penalty to be expectedfrom operation with two-phase flow. For the expected range of exhaust qualities, the efficiency

factor is 0.95 or higher and therefore, not severe. Unless there were substantially more

condensed flow than expected, condensed flow does not explain expander performance.


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0.97

0.975

0.98

0.985

0.99

0.995

1

35 40 45 50 55

Expander Inlet Temp. [C]

EquilibriumExh

austQuality

100 %

70 %

30 %

Pinlet/Pexit = 0.516 MPa/0.208 MPa

xvr= 0.993

expander =

Fig. 5. Expected expander exhaust quality for several isentropic efficiencies.

5.2 Enthalpy Determination

For pure component fluids near saturation conditions, the sensitivity of temperature to

enthalpy changes is poor due to their isothermal phase change. This could introduce significanterror in temperature-based enthalpy measurements. For the ammonia-water binary mixture,

however, condensation does not take place isothermally, even for a very high ammonia

concentration vapor. Figure 6 is a temperature-enthalpy diagram for two fluids, pure ammoniaand a nearly-pure ammonia-water mixture, at a pressure typical of the exhaust conditions for the

expander. The curves are similar except near the saturated vapor-two phase mixture boundary,

and that condensation begins for the mixture in the equivalent sensible heating range for the pure

fluid. Figure 6 indicates that expansions resulting in qualities of 0.97 or greater fall within arange where the sensitivity of temperature to enthalpy changes is good.

-25

-15

-5

5

15

25

35

-200 0 200 400 600 800 1000 1200 1400

Enthalpy [kJ/kg]

Temperature[C]

Mixture Vapor Quality = 1

0.98

0.97

0.990.993 Ammonia Concentration

Pure Ammonia

Pressure = 0.208 MPa

Fig. 6. T-h diagram for an ammonia concentration and pressure typical of expander exhaust conditions. Pure

ammonia data included for reference.


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5.3 Heat Transfer

Since it appears that there is minimal effect on efficiency due to saturated flow and that

temperature is a good indicator of thermodynamic state, heat transfer across the expander could

account for the measured results. Judging from Figure 4, a heat transfer loss of only 50 W couldaccount for the entire range of expander efficiencies.

A direct measure of the power output would resolve this issue, however, due to the design of

the experimental expander and its small capacity, it has not been conducive to doing so. Afixture to measure reaction torque has been constructed and some experimentation has been

performed. However, due to the low power output and high-speed operation of the expander, the

reaction torque is correspondingly small and difficult to measure independently. Therefore, even

with careful construction and measurement technique, the data thus far have not been useable.

6. Conclusion

The power-cooling concept has been shown to be valid, however, practical coolingtemperatures have not yet been achieved. Reasons for this lie not with the power-cooling

concept but with the poor expander performance. The analysis provided suggest that heattransfer to or from the working fluid is the probable explanation for the measured performance

compared to the other effects considered. With improvements to expander efficiency, cooler

exhaust temperatures should be obtained, as suggested by the trend lines of Figure 3.

References

[1] Maloney J, Robertson R. Thermodynamic study of ammonia-water heat power cycles, reportno. CF-53-8-43. Oak Ridge, TN: ORNL, 1953.

[2] Kalina A. Combined cycle system with novel bottoming cycle. ASME J. Eng. Gas TurbinesPower 1984;106:737-742.

[3] Goswami DY. Solar thermal power: status of technologies and opportunities for research.In:Heat and Mass Transfer 95, Proc. of 2

ndASME-ISHMT Heat and Mass Transfer Conf.,

Tata-McGraw Hill Publishers, New Delhi, India, 1995:57-60.

[4] Goswami DY. Solar thermal power technology: present status and ideas for the future.Energy Sources 1998;20:137-145.

[5] Lu S, Goswami DY. Optimization of a novel combined power/refrigeration thermodynamic

cycle. ASME J. Sol. Energy Eng. 2003;125:212-217.[6] Tamm G, Goswami DY, Lu S, Hasan AA. Novel combined power and cooling

thermodynamic cycle for low temperature heat sources, part I: theoretical investigation.ASME J. Sol. Energy Eng. 2003;125:218-222.

[7] Tamm G, Goswami DY. Novel combined power and cooling thermodynamic cycle for lowtemperature heat sources, part II: experimental investigation. ASME J. Sol. Energy Eng.

2003;125:223-229.

[8] Balje OE. Turbomachines a guide to design, selection, and theory. New York: John Wiley& Sons Inc., 1981.

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2004 Martin and Goswami