Acta Polytechnica Vol. 43 No. 112003 Prospects for Geothermal Energy Conversion through a Hybrid Combined Cycle Power Plant L. Boszorm6nyi, G. Boszorm6nyi The demandfor tnore intensiue utilization of energy sources is getting more important uith theforthcorni,ng European llnion membership of the Slotak Republic. lnch of resources and poor exploitaition of aaailable resources urn be a uery dfficuLt problem for energy policl. It zs irnportant to use technical solutions to minimize or eliminate this problem. The most benef,cial progress could be achieaed in the Koiice basin where geothermal energl could haae effectiue and tnulti-purpose use. Keywords: cornhi,ned cycle, geotlrcrnal energl, heat puntp, biornass. I Introduction Slovakia is extremely poor in fbssil fuels and is con- sequently 90 7o dependent on imported energy sourtes. This is all the mor"e significant because the national economy has been assessed to be three times more dependent on energ'y than the average of the EU economies. Intensified use of local renewable sources and raising efliciency should therefore be major priorities of the energy and environmental policy. Since there are good conditions for conversion of biomass to energy, its utilization should be as usual as that of hvdro energ'y. It is necessary to focus on usage of solar and geo- thermal energy since they are currently the least utilized energy sources (6 Vo and 2 Vo), if we do not take into accouttt the almost negligible use of wind energy. While solar energy is suitable for use in projects in the range of some kW, due to its low concentration, the porver output from geothennal projects is significantly highea and can be used for major projects. The idea of a geothermal project in Ko5ice to utilize about 100 MW of geothermal output and to provide the city with 2500 TJ/year from 8 geothermal doublets is undoubtedly one of the boldest initiatives of its type in the world today. It could change the energy balance of the country in a relatively short period of time. As presented in [3] and [6], it seems that the company GEOTERM is prepared to solve the problems connected with exploitation and transportation of geothermal water into the area of TEKO (the company that owns the heating plant in Ko5ice). Now it is the turn of this company to find a proper design to utilize this source that would improve its efficiency in comparison with the simple utilization of fossil fuel. The results of the study [4] show that this seems to be unreal in the case of dircct use o1'geothermal heat in the District Heating Network (DHN). It is possible to increase the competitiveness of direct use through a heat pump that would intensi$ the utilization of the disposable enthalpy potential. 'fhe way to improve the e{ficiency of the geothermal project in Ko5ice is by changing the basic philosophv. All the possible proposals should be technical- ly and economically analyzed and compared in a feasibility study. The aim to replace a part of TEKO that is reaching the end of its working life with a combined cycle CHP plant leads us to think of a non-traditional use of geothermal resources to support combined heat and power production. This could lead to an increase in power output for constant fossil fuel consumption or a decrease in fossil fuel consumption for constant power output. This concept requires the construc- tion of a combined cycle power plant of a new generation - a combined cycle power plant with integrated geothermal sup- port or a hybrid combined cycle power plant for heat and power production. -fhe principle of a hybrid geothermal power plant is already knorvn. There is only one operating plant of this type. It is located in Honey Lake, California and has a power output of 35 NfW. It is a steam rycle power plant that diflers fron.r the standard design by its source of energy, which is wood waste (biomass). For heating the feed rvater 22 kg/s of geothermal water of I l8 'C are used. Using these parameters the maximum geotherffral output can be 8 MW. 2 The idea of a hybrid combined cycle power plant with heat and Power production and integrated geothermal support The design of a combined cycle power plantwith integrat- ed geothermal support may have many variants. The main idea of this hybrid concept is to use geothermal energy to heat the feed water in the steam cycle, which is more advantageous than mere direct use in the DHN for the following reasons: o The temperature of the condensate in the steam cycle is 30K lower than the temperature of the returning primary water in the DHN, which allows significantly better use of the enthalpy ofsecondary geothermal water, in our case by 507o. This means that the same geothermal output can be obtained from half of the geothermal doublets. This leads to a saving of 50 Vo. o The use of this output is much higher because power production is provided throughout the year, while heating has a seasonal character. Such integration of a geothermal source into a combined cycle power plant can be combined with direct supply of the 3t Acta Polytechnica Vol. 43 No. ll2001 _i___J L- 1t--- 4 geothermal doubleG Fig. l: Principal scheme of a combined cycle power plant with heat extraction including integrated geothermal support - winter period DHN. This seems to be advantageous also in the case of the geothermal project in Kodice. The idea of a hybrid combined cycle power plant for heat and power production could be based on the same particular processes for each variant. The principle of these processes is shown in Frg. l. The stream of the secondary geothermal water *sccn would be divided into two parts. One of them mgrys would be used for heating the feed water (condensate) in the heat exchanger HEC between the condenser C of the steam turbine and vapor extractor VE. The second part mpp1,1 would be used directly in the DHN. After mixing rhe rerurn- ing flows that were cooled down to different degrees, the heat will be pumped fiom the resulting flow, using the heat pump HP for indirect utilization in the DHN. The heating water in the condenser of the heat pump can be overheated as required in the waste heat boiler \4lHB, as assumed in [l]. It seems to be more reasonable to use the extracted steam from the low-pressure stage of steam turbine STL for heating in the heat exchangers HEI and HE2 in spite of the decrease in geothermal output that can be used for heating the feed water in the winter period. In the evaporator of the heat pump the secondary geothermal water can be cooled to such an extent that it can be used for cooling the condenser C of the steam turbine. Part of the heat losses can branch awav 38 from condenser C. The implementation of this arrangement would bejustified especially in the summer period. Iinally, in the waste heat boiler WHB the secondary geothermal water can be heated by the heat losses from the outgoing flue gases at the same temperature as is assumed in the case of only direct utilization in the DHN. The flue gases can be cooled down in this way below the dew point and in addition to the sensible heat also the latent heat ofthe flue gases can branch away. This is conditional on solving many technical problems that can increase the costs. The heat losses of the waste heat boiler and condense6 which constitute a substantial part of the heat losses for standard design of the combined cycle, will be accumulated through the geothermal water being re-injected into the earth's crust. It would be better to use this water in recreational facilities or in agriculture (this was not assumed in Fig. l). The temperature of the returning secondary water must correspond to the demand for optimal re-injection of primary water. The parameters of the steam cycle should be adjusted to the described processes. The power output ofthe steam cycle would be determined by the amount of condensate and indirectly by the flow of secondary geothermal water used for heating the feedwater. Considering the high demands for the effectiveness ofconversion ofnatural gas to powel when the Acta Polytechnica Vol. 43 No. 112003 maximum geothermal heat would be used, such a stream is expected when there are at least two pressure levels ofsuper- heated steam. The heat pump being integrated into the combined heat and power production system plays a key role in its eflective function with the geothermal source. Therefore its design must be adjusted to this requirement. The first results show that the heat pump should be designed ar least in rwo srages, but it is necessary to analyze whether the economic con- tribution of a more effective three-stage design would not justif, the higher costs. If the machinery of the combined cycle power plantwere on a single shaft with the machinery of the heat pump, the costs would be reduced. In the case of the concept described here, progressive energy technologies enable very effective use of geothermal heat in comparison with only direct use in the DHN, especially in rhe summer period (see in Fig. 2). The operation of direct use in the DHN and of steam extraction can be stopped durir-rg summer, when there is lower demand for temperature and lower heat. consumption is needed for heating the feed water. 'lhis would bring an increase in the power output of the steam turbine and would free a significant geothermal source of high temperature that could be used for absorption cooling in absorption chiller AC and then for warming up the heating water in heat exchanger HE3. The necessary thermal output in the DHN can be obtained from the geothermal source using heat pump HP and heat exchanger HE3 operating rogerher. In the condenser of the heat pump the heating water would be warmed to 60 'C by the heat pumped from the second- ary geothermal water that would be cooled in the heat exchangers HEC and HE3. This water would then be over- heated to the desired temperature in the heat exchanger by the secondary geothermal water previously used for ab- sorption cooling. Absorption chillers are commonly used for air con- ditioning, which is justified only for higher ambient temperatures of relatively short duration. In this case, it would be more rational to use this cooling capacity for cooling the air entering the compressor in the heat exchanger HEA, since this would even operate for an ambient temperature of 15'C. A decrease in air temperature by l0 K results in an increase in power output of the gas turbine GT by 6%. This absorption chiller can be effectively used as power source for consumption peaks, since the ambient temperature is higher for load peaks. Such un-traditional conversion ofgeo- thermal heat to power would be achieved, when heat losses from absorption cooling could be accumulated through the secondary geothermal water. The heat losses of absorption refrigerating systems constitute approximately 170 Vo of rhe thermal input used for the operation of the system, and branching them away is expensive. )r!-J1 rJnl.' 2 h€otexdEngpr L_l Fig. 2: Principal scheme of a combined cycle power plant with heat extraction including integrated geothermal support - summer perloo 39 Acta Polytechnica Vol. 43 No. ll200\ 3 Prospects for integrating the geothermal source of the KoSice basin into the structure of a hvbrid combined cycle power plant The results of the analysis show that a geothermal source could be used in a hybrid combined cycle power plant based on a gas turbine with 260 MW power output (ISO requirement) without overheating. If the capacity of 4 geo- thermal doublets instead of the planned 8 doublets were to be utilized and the stream of secondary geothermal water ,irsccn =240 kg/s were ro be divided in the rario tit.pgy:mpyp = 5:3, the power output of the steam turbine in the winter period would be approximately 120 MW and in the summer period for 15 "C ambient temperature approx. 135 MW. The total thermal ourpur would be 150 MW in the winter period and 45 MW in the summer period. In the summer period the hear fiom fossil fuel (natural gas) would be used only for power production. The power required for compression in the heat pump is about 7 MW. If the total unused capaciry of the geothermal source (about 12 MW) were to be used for absorption cold production, the air at the inlet of the compressor would be cooled by 12K. For an ambient temperature of 20 'C this would result in an increase in power output by l9 MW. This indirect conversion of geothermal heat is much more eflicient than technologies for direct conversion (for example the Organic Rankine Cycle or Kalina cycle), which operate with llVo efficiency. The described concept would enable rhe use of about 90 MW of geothermal output in the summer period for heat- ing the feed wateq for warming up the heating water and for absorption cooling. The required thermal outpur of the city for domestic warm water is about 45 MW. The main contribution of the operation of a hybrid com- bined cycle power plant with heat extraction that would replace the TEKO block that is reaching the end of its working life, would be that more rhan 2500 TJ/year fiom 4 doubiets in the DHN only during the processei of heating would be utilized, as against using less thermal energy from 8 geothermal doublets in the case of only direct use. In comparison with the aim described in [3] this involves a saving on investment costs of about 25 mil. USD, and operating costs could be reduced by 50 %a.The selling price of geothermal heat could be significantly lowe4 and money would become available for investing in the rebuilding of TEKO. Apart from its use in heating processes, the secondary geothermal water would also be used in cooling processes. The useful cooling effect in the condenser of the steam turbine and in the absorption refrigerating system would have positive effect on the economic efficiency of the operat- ion of the plant. If the heat losses were used and,/or the amount of consumed geothermal heat were measured at the input of the source, the economic efficiency could be even higher. At all events, the thermal load of the environment would be lower and through the accumulation of heat losses the operating life of the mains would be longer. Frnally, a hybrid combined cycle power plant with the de- scribed structure would be environmentally much cleaner 40 than any source based on fossil fuel. Therefore rhis plant could have strong position on a liberal energy market. 4 The idea of a hybrid combined cycle power plant based on a combination of natural gas, biomass and geothermal heat In consequence of intensive multi-purpose utilization of geothermal heat in the Ko5ice basin, four of the eight geo- thermal doublets would be fi'eed according to the described concept. The need for a more significant proportion of re- newable sources in the energy balance of Slovakia morivates a search for a reasonable use ofthis capacity, if possible close to the geothermal mains. Since there is no demand for more thermal energy from households in this location, we have to consider o conversion of geothermal heat into power, o utilization of geothermal heat in technology processes. The second alternative raises some important issues. There is a requirement to reduce emissions from power pro- duction. Direct conversion of geothermal heat to power is attractive fiom this point of view. The anticipated temperature level of the geothermal heat (about 125 oC) is too low to consider direct conversion to power. Indirect conversion through ORC technology or a Kalina cycle could be implemented with low efficiency and high costs. Moreove4 large amounts of the special medium used in these technologies would be a potential treat to the environment. For this reason, conversion of geothermal heat is recommended in a hybrid combined cycle power plant designed, as described above. In this case only power would be produced and therefore integration of the hear pump is not justified. Due to the high demand for natural energy it would be suitable to extend the principle of hybrid power plants to another renewable source. Erst of all we have to consider biomass. The principal scheme for an alternative design for a hy- brid combined cycle power plant based on a combination of natural gas, biomass and geothermal heat is shown in Frg. 3. Its structure is designed to make the conversion of natural gas as effrcient as possible. For these reasons, processes of low pressure and high pressure evaporation that cause most singificant losses due to irreversibilities are moved fiom the waste heat boiler WHB to an independent boiler BB where biomasswill be burned. Consequently, the partial efficiency of conversion ofnatural gas to power could reach a value higher than 60 Vo. Use of geothermal heat for heating the feed water in rhe heat exchanger HEC and for absorption cooling in absorp- tion chillers AC would have equal importance. In our case it could also be used for drying the biomass before burning in the drier BD. In the energy balance ofsuch a hybrid power plant, bio- mass would be dominant, The power output of the gas tur- bine should be lower and that of the steam turbine should be highe4 as described above. Another alternative would be advantageous when the steam cycle operates in the basic load and a gas turbine operates in the half-peak load. Acta Polytechnica Vol. 43 No. 1/2003 --->- h€at axcfiangot plenB greohcmal doubl€ts Fig. 3: Principal scheme of a hybrid combined cycle power plant based on a combination natural gas, biomass and geothermal heat 5 Conclusions The hybrid combined cycle power plant based on the described concept would be the largest plant of this type. About 90 mil. m' of natural gas could be saved, which would lead to a reduction of CO, by 220 000 Vyear. The real savings would be higher when taking into account the emissions produced by the plant being replaced. The construction of such a unique plant would contribute to effective conversion of fossil fuel energy to useful energ'y forms using geothermal energy. This would be a chance to improve the credibility of Slovak energy policy, since progress in cogeneration systems and more intensive use of renewable sources are priorities. The specification of the optimal variant of this concept and also the overall evaluation of its power supply, economic and environmental potential should form the subject of a very serious feasibility study. The results could positively affect the possible privatization of TEKO. In spite of the possible contribution of the described con- cept to environmental policy, Slovak engineers have shown surprisingly little interest, less than that shown by engineers in foreign countries. The concept was presented at the sev- enth World Renewable Energy Congress inJuly 2002 in Koln, Germany and at the World Sustainable Energy Day in March 2002 in Wels, Austria. Hybrid combined cycle power plants are a new generation of combined cycle power plants. They allow effective use of fossil and renewable sources. Consequently they allow power production with fewer environmental impacts than any standard plant. It is therefore desirable to move toward implementing concepts that have until now only been dis- cussed. KoSice basin has advantageous natural conditions to implement such a concept. While closure of the nuclear power plant at Jaslovsk6 Bohunice and of all the brown coal-based power plants in Hungary is being planned, this opportunity should not be ignored. More intensive utilization of local renewable sources would then become a reality. More- ovet the chance to fulfill the strategic aims of the EU in the field of energy and environmental protection and to apply for financial support would not be wasted. References lll Boszorm6nyi, L., Boszorm6nyi, G.: Ztlsoboaanie Koitc geo- tenruilnou energiou. Magazin Energia, Vol. 3., 2001, No.3. t2l Boszorm6nyi, L., Boszorm6nyi, G.: Thc Perspechaes of Geotherrutl Energy Utiliuttion in District Heating Slstem of Koiire Cif. World Renewable Energy Congress VII, PERGAMON,2OO2. t3] Geotermilrn energin pre centnilne uisoboaanic teplom a mcste Koii,ce. GEOTERM Ko5ice, Ko5ice 1999. l4l Rieimie ruihra.d1 zasnraljch zlrojou tepln. a TEKO Koi'ixe. Vfskumnf fstav energeticky EGU Bratislava, Bratislava, i996. 4l Acta Polytechnica Vol. 43 No. 112003 t5] Tuschy, l.: Thennisclu Hybridkraftuerhe atr l(rafterzeug- Technical University of Ko5ice ung au; Nfufurtanpnatunairme. VDI-Verlag, Dtiseldof Faculty of Civil Engineering 2001. Vysoko5kolskii 4 [6] Vtifia, O., Beilovskf, V., Schustr, P., Pavlas, P.: Vyulirt 042 0l Ko5ice, Slovak Republic geotemui,ln{ch zdrojd, pro uisobwdni teplzrn a elzktrirhou erwr- gi{ tt Koiittch. Acta Mechanica Slovaca, Ko5ice, 2000, Ing. Gabriel Boszorm6nyi No.3. e-mail: G.Boszormenyi@sh.cvut.cz Doc. Ing. Ladislav Bosz