Alternate working fluids and their propertiesfor high temperature plants

Jyoti Swaroop Repaka07010042

Praveen Jain07d10002

April 10, 2010

Department of Mechanical Engineering,Indian Institute of Technology Bombay.

Abstract

This is a part of the course ME 306 - Applied Thermodynamicstaught by Prof. Uday N. Gaitonde. The topic is Alternate workingfluids and their properties for high temperature plants. (Topic ID-268).

Water has long been used as the working fluid for high-temperatureplants. But the increasing need for efficiency, higher power output andwider range of applications of working fluids have created a need foralternate working fluids. This report investigates the various alterna-tives available. 1

1Typeset in LATEX.

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Contents

1 Introduction 1

2 Water as a working fluid 1

3 Various other choices of working fluids 2

4 General constraints in the selection of working fluids 3

5 Carbon Dioxide 3

6 Organic and Titanium Based working fluids 4

7 BZT Fluids Siloxanes 5

8 Alkali Metals 6

9 Mercury 69.1 Schiller power station . . . . . . . . . . . . . . . . . . . . . . . 69.2 Nuclear-heated mercury power plant . . . . . . . . . . . . . . 7

10 Organic Rankine Cycle working fluids 7

List of Figures

1 A mercury-steam power plant. . . . . . . . . . . . . . . . . . . 62 The NASA nuclear mercury turbine system. Note the extra

cooling and lubricating loop using polyphenyl ether. . . . . . . 73 Plant layout for organic rankine cycles. . . . . . . . . . . . . . 8

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1 Introduction

The working fluid in a machine is the pressurized gas or liquid which actuatesthe machine.[6] Examples include steam in a steam engine, air in a hot airengine and hydraulic fluid in a hydraulic motor or hydraulic cylinder.

It absorbs thermal energy and converts and provide work output in aturbine. For a specified power output, the required size and obtained per-formance of a thermodynamic power system releasing mechanical work arevery dependent upon the properties of the working fluid.

2 Water as a working fluid

Water is an excellent working fluid for high-temperature industrial heatpumps due to its favourable thermodynamic properties and the fact thatit is plentiful, low in cost, nontoxic, chemically stable, and relatively noncor-rosive. Water has mainly been used as a working fluid in open and semi-openMVR systems, but there are also a few closed-cycle compression heat pumpswith water as working fluid. Typical operating temperatures are in the rangefrom 80◦C to 150◦C. 300◦C has been achieved in a test plant in Japan, andthere is a growing interest in utilising water as a working fluid, especiallyfor high- temperature applications. The major disadvantage with water asa working fluid is that the low volumetric heat capacity (kJ/m3) of water.This requires large and expensive compressors, especially at low tempera-tures. In addition, water has a relatively large change in specific enthalpywhen it vaporizes at ordinary steam generator pressures, which tends to limitthe mass flow rate for a desired power plant output. The properties of liq-uid water and water vapor are also such that the back work ratios achievedare characteristically quite low, and the techniques of superheat, reheat, andregeneration can be effective for increasing power plant efficiencies.[7]

Water is less satisfactory insofar as some other desirable working fluidcharacteristics are concerned. For example, the critical temperature of wateris only 374.14◦C, which is about 225◦C below the maximum allowable tur-bine inlet temperatures. Accordingly, to achieve a high average temperatureof heat addition and realize the attendant higher thermal efficiency, it maybe necessary for the steam generator to operate at supercritical pressures.This requires costly piping and heat exchanger tubes capable of withstand-ing great stresses. Another undesirable characteristic of water is that itssaturation pressure at ordinary condenser temperatures is well below atmo-spheric pressure. As a result, air can leak into the system, necessitating theuse of special ejector pumps attached to the condenser or de-aerating feed

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water heaters to remove the air.Although water has some shortcomings as a working fluid, no other single

working fluid has been found that is more satisfactory overall for large electri-cal generating plants. Still, vapor power cycles intended for special uses mayemploy working fluids that are better matched to the application at handthan water. Cycles that operate at relatively low temperatures may performbest with a refrigerant such as ammonia as the working fluid. Power systemsfor high-temperature applications may employ substances having desirableperformance characteristics at these temperatures. Moreover, water may beused together with some other substance in a binary vapor cycle to achievebetter overall performance than could be realized with water alone.

3 Various other choices of working fluids

• Carbonic acid / CO2 [9]

• Organic and Titanium Based

• BZT fluids eg. Siloxanes [3]

• FC-72 [13]

• Ethanol [13]

• Zeotropic working fluids

• Ammonia Maximum temperatures attainable are 58◦C-78◦C

• HCFCs and HFCs

• Petrol and alcohol

• Mercury

• Compressed air

• Alkali Metals[8]

• Other organic working fluids

• Ether and Chloroform Low boiling points and hence are not used inhigh temperature plants.

• Carbon Disulphide Low boiling point, 46.3◦C and hence cannot beused in high temperature plants

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4 General constraints in the selection of work-

ing fluids

The following parameters are considered, while selecting working fluid for aplant.[10]

1. Molecular weight.

2. Critical temperature.

3. Critical pressure.

4. Acentric factor.

5. Boiling point.

6. Auto-ignition temperature.

7. Vapor pressure of the liquid.

8. Heat of vaporization.

9. Heat capacity of the liquid.

10. Heat capacity of ideal gas.

11. Cost.

12. Toxicity to humans.

13. Eco-friendly nature.

Based on the above parameters, the advantages and propertiesof some of the working fluids are now discussed in detail.

5 Carbon Dioxide

Many Hot Dry Rock geothermal energy plants have started utilizing super-critical CO2 instead of water.[9] Such plants run at approximately 200◦C andpressure of a few hundred bars.Favorable properties of CO2 include the following:

1. Large expansivity, which would generate large density differences be-tween the cold CO2 in the injection well and the hot CO2 in the pro-duction well, and would provide buoyancy force that would reduce thepower consumption of the fluid circulation system.

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2. Lower viscosity, which would yield larger flow velocities for a givenpressure gradient.

3. CO2 would be much less effective as a solvent for rock minerals, whichwould reduce or eliminate scaling problems, such as silica dissolutionand precipitation in water-based systems.

While the thermal and hydraulic aspects of a CO2 - EGS (Enhanced geother-mal system) system look promising, major uncertainties remain with regardto chemical interactions between fluids and rocks. An added advantage inusing CO2 is: Fluid losses are an unavoidable, and loss of water would be dis-advantageous and costly, whereas fluid loss in an EGS plant running on CO2

would offer the possibility of geologically storing this greenhouse gas. CO2

may be an attractive heat transmission fluid not only for high-temperatureresources used for electricity generation, but may offer even greater benefitsfor direct heat applications of lower-temperature geothermal resources.

6 Organic and Titanium Based working flu-

ids

Examples include:

1. Biphenyl

2. Biphenylmethane

3. Naphthalene

4. Isoquinoline

5. Titanium Tetrabromide

6. Titanium Tetraiodide

These working fluids which represent a promising cleaner alternative tofossil fuel heating were assessed thermodynamically as potential working flu-ids for high temperature mechanical heat pumps by C.Zamfirescu et al.[12]Their results have been summarized:

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Biphenyl Forms saturated vapor condensate during compression.Biphenyl Methane Not suitable in vapor compression heat pumps.Naphthalene Forms saturated vapor condensate during compression.Isoquinoline Not suitable in vapor compression heat pumps.Titanium Tetrabromide High energetic and exergetic COPs.Titanium Tetraiodide High energetic and exergetic COPs and can obtain temperatures of 650/circC and COP¿3.

This suggests that TiI4 is a promising working fluid for a heat pumprunning at high temperatures.

7 BZT Fluids Siloxanes

It is demonstrated theoretically that fluids composed of molecules of sufficientcomplexity may display isentropes that are concave in the pV thermodynamicdiagram in a limited region on the vapour side, in the vicinity of the criti-cal point. As a consequence, it is physically possible that supersonic wavesevolving in that region behave in a counter-intuitive fashion: compressionsare smooth and isentropic while expansions are steep and non-isentropic.These effects may be exploited in designing turbomachinery for better effi-ciency and compactness.[3] The related phenomena and the complex fluidsthat support them are known nowadays as BZT, from the names of the threeresearchers who pioneered the field.[2]

Γ ≡ V 3

2c2

(δ2p

δv2

)s

(1)

where v is the specific volume, p is the pressure and c2 = V 2(δP/δv)s isthe zero-frequency speed of sound. A fluid to be BZT has to have a thermo-dynamic region where Γ is negative. Various studies have been done on theperformance with various BZT working fluids for a range of parameters, andit has been observed that Siloxanes perform greatly. 2

2A siloxane is a chemical compound composed of units of the form R2SiO

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8 Alkali Metals

Alkali metals such as lithium, sodium and potassium are very efficient work-ing fluids. They are generally used with the heat sources in a range above800K-1250K. However, because these metals are highly corrosive, the heatpipe envelopes have to be fabricated from high temperature refractory metals.[8]

9 Mercury

Liquid mercury has been proposed as a working fluid for a heat pipe typeof cooling device for spacecraft heat rejection systems or radiation panels.The choice of mercury for a working fluid may seem bizzare as its bothexpensive and extremely poisonous. But owing to sound thermodynamicreasons, mercury has been used.[1] Mercury has a higher boiling point thanwater, i.e. at 357◦C.

9.1 Schiller power station

The Schiller power station at Portsmouth, New Hampshire, USA was thelast binary mercury/steam plant built. The site has two decommissionedGE Mercury Turbines and two B&W Mercury/Steam boilers. It currentlyuses three 50 MW coal-fired steam boilers built in 1950s. One of these isbeing replaced with a fluidized-bed boiler to burn whole-tree wood chips andother clean low-grade wood materials. The Schiller station consisted of two7.5MW mercury units and one 25MW steam turbine. The rated capacity was40MW. It was equipped to run on either Bunker-C fuel oil or bituminous coal.

Figure 1: A mercury-steam power plant.

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9.2 Nuclear-heated mercury power plant

NASA designed a nuclear-heated mercury power plant intended to generatelarge amounts of electricity in space. The reactor is cooled with a mixture ofsodium and potassium, (NaK) which heats a mercury boiler. The mercuryvapour drives a turbo-alternator and is then condensed and subcooled bya secondary NaK heat rejection loop which transfers the waste heat to aradiator for rejection to space. The design has an exotic potassium/mercuryboiler. It is a spiral stucture designed to fit in a small toroidal space ina cylindrical spacecraft. The heat exchanger tubes are made of tantalum,which is readily wetted by mercury, making for good heat transfer. The netpower output was 37 kW.

Figure 2: The NASA nuclear mercury turbine system. Note the extra coolingand lubricating loop using polyphenyl ether.

10 Organic Rankine Cycle working fluids

In the last years, large endeavors have been made to extend the market shareof renewable energies. Power and heat cogeneration by solid biomass is one ofthe most interesting options for a sustainable and reliable energy supply dueto its high availability.[4] Electrical power is usually generated in processesbased on the Rankine with water as a working fluid. The ORC processuses an organic working fluid instead of water.[5] In contrast to water, theexpansion in the turbine ends for most organic fluids not in the wet steamregime but in the gas phase above condenser temperature. Thus, often aninternal heat exchanger is used to improve efficiency.[11]

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Figure 3: Plant layout for organic rankine cycles.

Toluene has the highest vaporization enthalpy, but at a low temperaturelevel. Thus, toluene shows worst efficiency of the alkylbenzenes. Butylben-zene shows highest efficiency. It has the lowest maximum and minimum pro-cess pressure, vaporizes at maximum process temperature and its condensertemperature is only slightly lifted. OMTS vaporizes at higher temperatureas toluene, but its vaporization enthalpy is significantly lower. It has thelowest efficiency of the selected fluids.

As expected, efficiency rises with maximum process temperature. Butyl-benzene gains 6.5% points and OMTS 4.7 % points from 523K to 623K. Ingeneral, efficiency difference between the fluids rises with higher maximumprocess temperature.

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References

[1] Unusual working fluids, 2008.

[2] Bethe, H. A., and Teller, E. Deviations from thermal equilibriumin shock waves.

[3] Colonna, P., Guardone, A., and Nannan, N. R. Siloxanes:A new class of candidate bethe-zel’dovich-thompson fluids. Physics ofFluids 19, 8 (2007), 086102.

[4] Curran, H. M. Use of organic working fluids in Rankine engines.NASA STI/Recon Technical Report N 80 (Sept. 1979), 30917–+.

[5] Drescher, U., and Brggemann, D. Fluid selection for the organicrankine cycle (orc) in biomass power and heat plants. Applied ThermalEngineering 27, 1 (2007), 223 – 228.

[6] Eastop, and McConkey. Applied Thermodynamics for EngineeringTechnologists, 5th ed. Prentice Hall, Singapore, 1993.

[7] Moran, M. J., and Shapiro, H. N. Fundamentals of EngineeringThermodynamics, 5th ed. John Wiley & Sons Ltd, England, 2006.

[8] Morris, James F. (Fairview Park, O. Heat pipes containing alkalimetal working fluid.

[9] Pruess, K. Enhanced geothermal systems (egs) using co2 as workingfluid–a novel approach for generating renewable energy with simultane-ous sequestration of carbon. Geothermics 35, 4 (2006), 351 – 367.

[10] S., G. Selection of working fluids for high-temperature heat pumps.Fuel and Energy Abstracts 36 (September 1995), 367–367(1).

[11] Saleh, B., Koglbauer, G., Wendland, M., and Fischer, J.Working fluids for low-temperature organic rankine cycles. Energy 32,7 (2007), 1210 – 1221.

[12] Zamfirescu, C., Dincer, I., and Naterer, G. Performance evalu-ation of organic and titanium based working fluids for high-temperatureheat pumps. Thermochimica Acta 496, 1-2 (2009), 18 – 25.

[13] Zhang, X. M. Experimental study of a pulsating heat pipe using fc-72,ethanol, and water as working fluids. Experimental Heat Transfer 17, 1(2004), 47 – 67.

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