PROCEEDINGS, Twenty-Ninth Workshop on Geothermal Reservoir EngineeringStanford University, Stanford, California, January 26-28, 2004SGP-TR-175

ELECTRICITY PRODUCTION BY GEOTHERMAL HYBRID – PLANTSIN LOW-ENTHALPY AREAS

T. Kohla,b; R. Speckb, 1

aInstitute of GeophysicsETH Zurich,

8093 Zurich, Switzerlande-mail: kohl@ig.erdw.ethz.ch

bGeowatt AGDohlenweg 28

8050 Zurich, Switzerland

1 now at: Electrowatt-Ekono Ltd., Hardturmstr. 161, 8037 Zürich, Switzerland, e-mail: raphael.speck@ewe.ch

ABSTRACT

In Central Europe, the utilization of low enthalpyreservoirs for electricity production has not yet fullydeveloped. Reliable technological experience willonly be obtained from future projects. Currently, theutilization is still at an experimental stage and thefailure of only a single project could easily havedramatic consequences for the continuation of otherprojects due to limited public interest and funding.Due to the imponderability of pure geothermalproduction we propose a new hybridization concept.Herewith, the probability of a success will beincreased and geothermal energy can becomeattractive for private funding. In contrast to existinghybridization plants using geothermal to preheatworking fluids of conventional plants we consider asuperheating from additional fuels of a geothermalvapor cycle. Therewith, an optimum temperature canbe adjusted that is less dependent on the geothermalsource temperature but that is generally considerablyhigher than most geothermal plants. Suchhybridization offers the advantage of a smoothtransfer from conventional systems and allows anearly arbitrary combination of different energyfuels. On the basis of existing data from the EuropeanEGS location in Soultz-sous-Forêts, economicestimations have been performed. Based on properthermodynamic evaluation, our quantitative analysisdemonstrates the advantage of such utilization, ifalready pure water is superheated. Such hybridcombination would produce electricity to anoverwhelming part from geothermal resources. Usingeven organic working fluids, such plants even start tobecome economic at T~100°C under typical legalEuropean conditions.

INTRODUCTION

Geothermal activities in Central Europe are mainlyrestricted to low enthalpy utilization, lacking high-temperature resources. The successful applicationsand spread of geothermal low-enthalpy production inmany countries prove highlights the potential for thislow-enthalpy use. Electricity production in thesegeological settings has however not been carried out.The required reservoir temperatures (>200°C) andfluid circulation rates (>50 l/s) present a strongembarrassment for exploitation in these areas. A mostsignificant obstacle for faster geothermal powerdevelopment are the high drilling costs and theexpensive experimental program. In these low-temperature settings depth ranges of >5 km need tobe penetrate. But also the effects of hot, highlymineralized brines require developing cost-intensivelogging and experimental (i.e. packer for hydraulicand stress measurement, sampling technologies) toolsto evaluate the conditions at these depths. At the HotDry Rock (HDR) project in Soultz-sous-Forêts(France) experience for the production of geothermalfluids from greater depth ranges has been gained overthe last 10 years. This project is conducted as jointeffort of the European Union and individualparticipation of individual countries. It alsorepresents the culmination of European EnhancedGeothermal Systems (EGS) activities starting morethan 30 years ago. Due to the successful findings ofthe last project phase, exploring the conditions of a~3.5 km deep reservoir, a 5 km deep reservoir will becreated in the current project phase leading to ascientific pilot plant. Currently, further EGS activitiesin Europe are planned with new projects inSwitzerland and Germany. In spite of its success thefinancial engagement from industrial funds is still

remaining on a low level. Financial support isoverwhelmingly provided by government funds;industry's reluctance to enter the Europeangeothermal arena may be explained by a potentiallyhigh risk of failure.

In this perspective, new ideas should be evaluatedand more closely inspected. By combining furtherenergy sources to geothermal, potentially lowertemperature fluids could be extracted and shallowerreservoirs could be exploited, thus reducingexploration costs and geological risk. Kohl et al.(2002) investigated the possibility of a hybridizationof geothermal energy by superheating a binary water-vapor cycle. This paper describes now especially thepossibilities and conditions of using Organic RankineCycles (ORC) and biomass as external fuel.

HYBRIDIZATION CONCEPT

General IdeaGeneral electricity production is enabled by steam-based Rankine cycles, a well-established technologyfor the conversion of high-temperature heat intoelectricity. For their application to a geothermalsource the necessary temperature and pressure shouldbe high enough so that, with the help of heatexchangers, pressurized steam is produced at theentrance of the steam turbine. If the geothermalsource has lower temperatures than needed fordriving a steam-based Rankine cycle – conditionswidely met in low-enthalpy areas – electricityproduction can be commonly performed by OrganicRankine cycles (ORC), which make use of anorganic-based working fluid with a lower boilingpoint than water.

From an energy efficiency viewpoint an optimizedenergy production needs the use of appropriate heatsources optimized for the temperature / pressurerequirements of the turbines. This brings to mind ahybrid approach, where more than one heating sourceis employed for a given application in order to bestexploit a technology. Possible geothermal hybridscenarios are either the utilization of geothermalenergy for pre-heating water in conventionallyoperated power plants or superheating steam ofcirculating working-fluids. In a detailed investigationof the pre-heating concept, Bruhn (2002) shows thatno major CO2 reductions can be achieved and theirapplication is rather restricted to situations whenexisting plants need to be renovated. Parallel for theconsiderations of DiPippo et al. (1978) we willinvestigate subsequently a hybrid power generationscheme based on a superheating concept.

Figure 1 illustrates a possible GeoHybrid system withenergy transfer accomplished by means of three heatexchangers: a preheater, a steam generater, and a

steam super-heater. The energy source for the firsttwo heat exchangers is geothermal; the energy sourcefor the third one need to supply base energy and –when selecting a renewable source – will be mostlikely from biomass.

Preheater

Figure 1. GeoHybrid geothermal power generationscheme (adapted from Kohl et al., 2002)

Kohl et al. (2002) have mentioned the possibleadvantages of the proposed hybrid power generationscheme:

• It allows the use of more energy efficientand cost effective steam-based Rankinecycles, eliminating the time and cost neededfor developing other less-establishedconversion technologies.

• It offers the flexibility of determining theoptimal steam temperature, independent ofthe geothermal source temperature, addingmore reliability to the system design becauseof the uncertainty associated with thegeothermal source temperature at a givendepth.

• It provides a system concept for thecombined generation of electricity and heatthat can be designed for a decentralizedapplication or scaled-up to a centralizedapplication. It creates therewith a transitionpath to a 100% renewable electricitygeneration when the energy source for thesuper-heater is a renewable-based fuel.

System Layout for water-steam working fluidThe following considerations are based oncalculations performed by Speck (2002) and Kohl etal. (2002). We applied data from the European HDRsite Soultz-sous-Forêts project. During a 1997 long-term circulation test in a 3'200-3'800 m deepreservoir flow rates of 25 l/s were established attemperatures approaching 150°C (Baumgärtner et al.,1998). Already drilling but also subsequent hydraulicexperiments (Kohl et al., 1997) have shown that thetectonic situation of the Rhine Graben seems to bevery favorable for high subsurface permeabilities.

The nearly identical hydraulic characteristics of theboreholes GPK1 and GPK2 on the 3.5 km reservoirleads to the assumption that additional productionand injection wells could linearly increase the totalflow rate in the reservoir. In the following, weassume a total flow rate of 100 kg s-1 (i.e. 4 times themeasured circulation rate) which is realized by 4boreholes (i.e. two times the borehole layout of the1997 circulation test), producing constantly 150°Ctemperatures over a considered time period of 20 yr..produced from additional boreholes. From a purelyenergetic perspective, such 3.5 km deep reservoircould already be used for electricity production bybinary plants, however at a considerably bad energyconversion factor.

Kohl et al. (2002) have presented the application ofthe GeoHybrid concept to these settings. For a small~2 MWe plant, external fuel of 1.9 MWt should beadded to the 9.5 MWt obtained from the geothermalcycle. Depending on the pressure level of the binarycycle the efficiency could be improved. Kohl et al.(2002) have assumed a turbine with minimum inletpressure of 3 bar requiring minimum temperatures ofthe evaporator of 134°C. The turbine pressurefurthermore has also an impact on the necessaryreinjection temperatures that are as high as 128°C for3 bars.

New calculation toolBased on earlier experience and application, a new,more flexible calculation tool was developed. By thistool, it is now possible to combine variablegeothermal production data to a variable andincreasing technological database. Therewith,optimum configuration can be much easier assessed.More in detail, we now specify the followinggeothermal field data:

• production rate and temperature• number of wells with borehole depth to

estimate the drilling costs (Kitsou et al.,2000)

• local temperature gradient (to calculateeffects when deepening a borehole)

• injection pressure to assess the pumpingpower for circulating fluid

Each technical device is characterized at least bycosts and efficiency. Still, this calculation tool isbased on the same energy transfer calculations asbefore. We assume that the errors introduced areacceptable within the present stage of development.

Besides the technical installations, the new tool alsoallows to calculate the electricity costs as function ofa single variable. This way, it becomes possible toestimate the influence of

• production temperature• borehole depth• production rate• turbine inlet pressure

For economic calculations, estimations of installationand further investments and of annual operation andamortization costs need to be supplied. Furthermore,the complementary utilization of the waste heat canbe specified. Activating this option allows generallyreducing electricity production costs. The externalfuel required by the hybridization approach iscurrently implemented as costs per kWht. This priceshould cover all additional charges (installations, fuelcosts, ...).

The most significant change to the earlier stagerepresents however the integration of organicworking fluids

Figure 2. New GeoHybrid input scheme, used laterfor thermodynamic calculation

Organic Working fluidsIt is a well-known fact that organic working fluidshave lower efficiency for energetic transfer thanwater. In contrast, their operating points are moreadequate to geothermal production data. In the newcalculation scheme we assumed the validity and thesame principles of super-heating water vapor. Theindividual fluid properties have been includedaccording to the NIST database.

Currently, Hexane and Isobutane are implemented inthe database, but will be continuously extended. Itmay be noted that the following results arepreliminary, based on the validity of abovementioned assumptions. Further and more detailedstudies are under way. However more sophisticatedcalculation schemes for complete power plantinstallations will not drastically change thesestatements. It is estimated that the results aresubmitted to a maximum error of 10-20%.

EXAMPLES OF UTILIZATION

In the following different typical operating points forwater, Hexane and Isobutane are assumed. Thegeothermal cycle is assumed to produce a flow rate of100 l s-1 by three boreholes at a variable temperature.Injection pressure of 6 MPa is assumed. Optimumturbine pressures are investigated, however no systemis to undergo a minimum turbine pressure of 2 bars.Superheating temperatures for water is assumed to be450°C, 234°C for Hexane and 134°C for Isobutane.Condenser pressures of 0.05 bar for water, 0.2 bar forHexane and 4.0 bar for Isobutane are assumed.

Appropriate total investment costs for installationsand three 3km well of 50 mio € are assumed, anumber that is rather conservative. The real costsmight be lower.

Table 1 illustrates the working points for pure water-vapor mostly for a fixed turbine pressure of 2 bars.Speck (2002) and Kohl et al. (2002) already showedthat its application depends strongly on this pressurevalue. Minimum geothermal production temperaturesof 130°C would be required for this system. Atincreasing temperatures, the system becomes moreefficient, starting at an overall efficiency, η, of14.4%. Possible net electric power production wouldstart at minimum temperatures at 1.5 MWe. As notedearlier, the strong portion of geothermal is obvious.

Table 1: Hybridization concept when superheatingthe working fluid "water vapor"T[°C]

ps

[bar]Pel

[MWe]Pgeo/Pext

[MWt]geotherm. %

η

140 2 1.5 8 / 281%

14.4%

160 2 4.0 18 / 481%

17.7%

180 2 6.7 29 / 781%

18.7%

200 2 9.3 39 / 981%

19.1%

220 2 11.8 49 / 1281%

19.4%

The possible improvements for power productionusing organic working fluids are illustrated in Table 2(Hexane) and Table 3 (Isobutane). The organicworking fluids generate more power than water dueto the increased heat transfer in the steam generator,but have a slightly lower efficiency and decrease thegeothermal percentage of the power generation.The minimal required temperature for powergeneration with a water cycle is 140°C. Hexanerequires at least 120°C.

The application of an isobutene cycle could startalready at temperatures as low as 80°C but generatesat this temperature only a low power. At productiontemperatures of 220°C, a net power production of 22MWe is reached.The application of the right working fluid needs to becarefully planned and adjusted for individual sites. Apossible plant size of >5MWe is possible atconsiderable important portion of geothermal energy.

Table 2: Hybridization concept with Hexane asworking fuidT[°C]

ps

[bar]Pel

[MWe]Pgeo/Pext

[MWt]geotherm. %

η

120 2 3.3 18 / 11 63%

11.4%

140 2 6.2 31 / 19 63%

12.3%

160 3.5 8.0 35 /17 67%

15.3%

180 6 9.4 38 / 1573%

17.8%

200 7 13.3 53 / 18 75%

18.7%

220 9 17 68 / 2078%

19.7%

Table 3: Hybridization concept with Isobutane asworking fluidT [°C] ps

[bar]Pel

[MWe]Pgeo/Pext

[MWt]geotherm. %

η

80 8 0.7 12 / 669%

3.9%

100 10 2.9 20 / 871%

7.3%

120 12 3.7 29 / 1074%

9.5%

140 20 5.9 36 / 881.7%

13.5%

160 25 9.7 58 / 986%

14.4%

180 25 14.0 82 / 1386%

14.7%

200 25 18.4 107 / 1786%

14.9%

220 25 22.0 126 / 2186%

15%

Figure 3. Dependency of geothermal production data on generation costs of electricity with isobutane as workingfluid. The scenario assumed a payback time of 20 yrs, total investments of 42.4 M€ and a re-injectionpressure of 4 MPa.

In addition to these thermodynamic behaviors, weroughly estimated the possible costs when externallysuperheating a binary cycle with Isobutane workingfluids. Figure 3 illustrates the potential dependencyof the described Isobutane considerations onproduction temperature (at a fixed production flowrate 100 l s-1) and on production rate (at a fixedtemperature of 130°C).

When assuming both, a fixed production flow rate of100 l s-1 and a fixed production temperature of130°C, the Isobutane hybridization would yield costsof ~9 €ct/kWhe. Clearly, these costs depend stronglyupon the geothermal production data. Especially,larger flow rates will reduce costs nearly linearly.Note, that this scenario, assumes external fuel costsof 2 €ct/kWht.

CONCLUSIONS

Hybridization of geothermal power shows stronglybeneficial aspects in many ways. The explorationcosts of low-temperature resources (Temperaturegradient < 40 K km-1) could be drastically reduced atlower target temperatures. Our considerationsdemonstrate the viability and effectiveness ofhybridization. This way, steam-based Rankine cyclescould become a cost-efficient solution taking

advantage of much higher Carnot efficiencies athigher steam temperatures.

Especially, when using biomass as superheater two"green" band-energies are most convenientlycombined. Biomass generally suffers from highproduction costs that present a major setback forthese energy sources. However, when hybridizedwith geothermal energy, much smaller portion ofbiomass would be needed to produce the sameamount of electricity. Power plants in the order of5 MWe could become conceivable even in areas witha relative low geothermal gradient. We estimated thatthe amount of 2 MWt typically required for such asmall unit requires a wooden area of 6x6 km2, an areathat is easily identifiable even in the stronglypopulated areas of Central Europe. Future studies arenecessary to pinpoint further installations that couldserve as database for decision-making institutions.Especially, further non-governmental funding couldbe easier assessed for the application of geothermalhybridization since it will strongly reduce the risk ofgeothermal energy production.

REFERENCES

Baumgärtner, J. et al., 1998. Circulating the HDRreservoir at Soultz: maintaining productionand injection flow in complete balance.,Proceedings of the 23rd Workshop onGeothermal reservoir Engineering,, StanfordUniversity, pp. 11-20.

Bruhn, M., 2002. Hybrid geothermal–fossilelectricity generation from low enthalpygeothermal resources: geothermal feedwaterpreheating in conventional power plants.Energy, 27: 329–346.

DiPippo, R., Khalifa, H.E., Correia, R.J. and Kestin,J., 1978. Fossil Superheating In GeothermalSteam Power Plants, Society of AutomotiveEngineers. Inc., San Diego, pp. 1095-1101.

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