Technology Journal of Energy JET Volume 8 (2015) p.p. 11-24 Issue 3, November 2015 Typology of article 1.01 www.fe.um.si/en/jet.html THE EXPLOITATION OF ULTRA-LOW-ENTHALPY GEOTHERMAL ENERGY IN AN ORC PROCESS IN COMBINATION WITH RES AND A HEAT PUMP IZKORIŠČANJE ZELO NIZKOENTALPIJSKE GEOTERMALNE ENERGIJE V ORC PROCESU V KOMBINACIJI Z OVE IN Keywords: renewable energy resources, ORC process, energy analysis, economic analysis The paper presents a model of a binary-cycle hybrid power plant to be located in Topolšica, Šalek Valley, Slovenia. It is based on an ORC process and utilises several different renewable energy sources: geothermal energy, solar energy, and wood biomass energy. The portion of geothermal energy is so low that it is used in the system as a source of energy for a heat pump, thus increasing the inlet temperature of the ORC process. The model of the hybrid ORC power plant dealt with in this paper is intended only for electricity production. R Corresponding author: Urška Novosel, University of Maribor, Faculty of Energy Technology, Laboratory for Theromechanics, Applied Thermal Energy Technologies and Nanotechnologies, Tel.: +386 7 620 2 213, Mailing address: Hočevarjev trg 1, SI-8270 Krško, Slovenia, E-mail address: urska.novosel@um.si 1 University of Maribor, Faculty of Energy Technology, Laboratory for Theromechanics, Applied Thermal Energy Technologies and Nanotechnologies, Hočevarjev trg 1, SI-8270 Krško, Slovenia 2 Kostak, d.d., constructional/civil engineering company, Development, Leskovška cesta 2a, SI-8270 Krško, Slovenia 3 University of Maribor, Faculty of Energy Technology, Koroška cesta 62a, SI-3320 Velenje, Slovenia S TOPLOTNO ČRPALKO Urška NovoselR, Jurij Avsec1, Ivana Tršelič2, Sonja Novak3 Abstract JET 11 Urška Novosel,Jurij Avsec, Ivana Tršelič, Sonja Novak JETVol. 8 (2015) Issue 3 The energy source used first is geothermal water from a well, entering the heat pump to heat the heat pump working medium. Solar energy and wood biomass energy are sources of heat flow, additionally heating the ORC process working medium. The paper presents thermodynamic and economic analyses of this potential system. Povzetek Članek predstavlja model binarne hibridne elektrarne, ki bi bila postavljena v Topolšico, Šaleška dolina, Slovenija. Osnova je ORC proces, ki uporablja več različnih obnovljivih virov energije -geotermalno energijo, sončno energijo in energijo lesne biomase. Delež geotermalne energije je tako nizek, da jo v sistemu uporabimo kot vir energije za uporabo v toplotni črpalki in tako povečamo vstopno temperaturo v ORC proces. Model hibridne ORC elektrarne v članku je predviden samo za proizvodnjo električne energije. Energetski vir, ki je najprej uporabljen, je geotermalna energija iz vrtine, ki vstopa v toplotno črpalko, kjer segreje delovni medij toplotne črpalke. Sončna energija in energija lesne biomase sta izvora toplotnega toka, ki še dodatno segrejeta delovni medij ORC procesa. V članku sta predstavljeni termodinamična in ekonomska analiza takšnega potencialnega sistema. 1 INTRODUCTION Global warming is a current crucial issue in the world and requires immediate action in order to reduce it. New, clean power generation technologies have to be explored. One very good option is the exploitation of renewable energy sources using hybrid technologies. The so-called binary processes to increase system efficiency by exploiting the maximum amount of energy are used for the exploitation of low-enthalpy geothermal energy. According to the cycle, [1]. An organic Rankine process (ORC) was chosen as a geothermal power plant model, i.e. a binary process, similar to the Rankine vapour power cycle in terms of thermodynamics, except that hydrocarbons are used as the working medium in the ORC instead of vapour, [2], reaching the boiling temperature sooner and, consequently, being more suitable for processes of power generation from low-temperature sources, [3]. The working medium chosen on the basis of thermodynamic properties for a specific system receives heat, evaporates and expands through the turbine, and then it is condensed and returns to the evaporator with a supply pump, [1]. 2 SYSTEM DESCRIPTION AND STRUCTURE The geothermal power plant model is essentially an ORC system, in which the working medium is heated by various RES. First, we exploit geothermal energy through a heat pump, then solar energy and finally wood biomass energy. Three different RES will be used for the system with the advantage being to utilise the available RES potential as much as possible. This will provide 12 JET The exploitation of ultra-low-en thalpy geothermal energd in anOpC pro cess in combination with RES and a heat pu mp greater operational reliability of the system and, consequently, reduce the oscillation of electricity production. Figure 1 shows the heat pump/ORC system diagram with all its elements. Figure 1: Heat pump hybrid system diagram Geothermal energy enters the system as a medium that heats the heat pump medium in the heat pump evaporator. Then we look at the ORC process and see that in State 1 the working medium enters the heat exchanger, which is in fact a heat pump condenser, where it receives the heat from the heat pump working medium and passes to State 2. The working medium then enters the evaporator from State 2, where it is additionally heated by the heat from a solar subsystem. Immediately after this stage, a temperature sensor is placed in the system, which measures the temperature of the working medium. If the sensor detects a lower temperature than desired, valve V2 closes and valve V1 opens, thereby allowing the working medium to pass into another evaporator to be additionally heated by the heat from the wood biomass subsystem, where it is heated to the desired temperature, which is State 3. In State 3, the working medium enters the steam turbine, where it expands to State 4. The ORC process working medium then enters the condenser, where it releases the heat to the coolant, i.e. water in our case, and passes to State 5. In the last step of the cycle, the working medium passes through the pump from State 5 to State 1, [4]. The correct working fluids must be chosen for both the ORC process and the heat pump, because the choice of the working fluid has a great impact on the efficiency of the processes. According to [5], R245fa is an optimum choice of the working medium. Considering its thermodynamic properties and environmental characteristics, it is suitable for a number of applications in industry, [6]. R134a is chosen as the heat pump working medium, also widely used in similar processes, [7]. JET 13 Urška Novosel,Jurij Avsec, Ivana Tršelič, Sonja Novak JETVol. 8 (2015) Issue 3 3 INPUT DATA AND DIAGRAMS In system modelling, some parameters need to be defined, some are assumed and some are obtained from the data. The basis for the ORC process calculation is the p-h diagram in Figure 2, [7], showing the changes to the states of R245fa in the ORC process. 150 200 250 300 350 400 450 500 650 600 650 Enthalpy [kJ/kg] Figure 2: p-h diegoem R245fe The input data was provided by two geothermal wells in the area of Topolsica (Salek Valley, Slovenia). The total average geothermal water flow from the two wells is 50 l/s and the maximum temperature 40 °C, [8]. The collected input data for the ORC system is indicated in Table 1. 14 JET The exploitation of ultra-low-en thalpy geothermal energd in anOpC pro cess in combination with RES and a heat pu mp Table 1: Data for the ORC system calculation Parameter Value Parameter Value cp,w , cp,geo 4.18 kJ/kgK H 3.4 kWh/kg G 1,250 kWh/m2 ATgeo 8 K hi 255 kJ/kg ÛTw 8 K h2 310 kJ/kg Vgeo 50 l/s h3 500 kJ/kg n^ nt 0.8 h4 468 kJ/kg pgeo 1,250 kg/m3 hs 250 kJ/kg pwm 1,350 kg/m3 Before carrying out the ORC system calculation, the calculation of the heat pump has to be provided. The result of the heat pump calculation is used as the input data for the calculation of the ORC process indicated below. The geothermal water temperature is too low to enter the ORC system and, therefore, it has to be heated. We decided to install a heat pump using R134a as a working medium. The Solvay program was used for the calculation of the heat pump performance. Table 2 contains the input parameters, output parameters, and results of the calculation. Figure 3 illustrates the p-h diagram of the heat pump, [7]. Table 2: Heat pump parameters Element, parameter Value Evaporator T = 30 °C, superheating 5 K, Qgeo = 2,090 kW Condenser T = 90 °C, subcooling 0 K, QHP = 3,040 kW R134a mass flow m R134a = 27.22 kg/s Point A (see Figure 3) p = 7.7 bar, T = 35 °C, h = 419.88 kJ/kg, s = 1.7311 kJ/kgK Point B (see Figure 3) p = 32.44 bar, T = 104.33 °C, h = 454.52 kJ/kg, s = 1.7449 kJ/kgK Point C (see Figure 3) p = 32.44 bar, T = 90 °C, h = 343.08 kJ/kg, s = 1.4391 kJ/kgK Point D (see Figure 3) p = 7.7 bar, T = 30 °C, h = 343.08 kJ/kg, s = 1.4775 kJ/kgK Compressor efficiency 85% Compressor power WHP = 943 kW JET 15 Urška Novosel,Jurij Avsec, Ivana Tršelič, Sonja Novak JETVol. 8 (2015) Issue 3 Figure 3: p-h diegoem R134e 4 THERMODYNAMIC AND ECONOMIC CALCULATION The following equation was used to calculate the geothermal water heat flow: Q geo _ Vgeo ' pgeo ' cp,geo ' ^Tgeo (4.1) This heat flow enters the heat pump in order to heat R134a in the heat pump evaporator and R134a then heats R245fa (see Figure 1). Examining the diagram in Figure 2, we calculated how much heat flow was required from the heat pump working medium to change State 1 to State 2. For further calculation, Equation 4.2 is used: (4.2) It was assumed that the total heat flow was transferred to the R245fa working medium - heat exchangers have the efficiency of 100%. Equation 4.3 is used to calculate the working medium mass flow. h2 - h5 (4.3) In the next step, we will calculate how much additional heat flow is required to heat R245fa to the desired 130 °C (see Figure 2). The additional heat flow is divided into two components: the heat flow from the solar subsystem and the heat flow from the wood biomass subsystem, as can be described by Equation 4.4: wm wm wm 16 JET The exploitation of ultra-low-en thalpy geothermal energd in anOpC pro cess in combination with RES and a heat pu mp Q add = Q s + Q WB (4-4) The value of the additional heat flow to reach State 3 (see Figure 2) will be calculated by using Equation 4.5: Qadd = mwm '(h3 - h2) (4-5) Furthermore, Equation 4.5 will be used to calculate the turbine power by using enthalpies in States 3 and 4 rather than enthalpies in States 2 and 3 (see Figure 2). When calculating the turbine power, the isentropic efficiency of 80% has to be taken into account, [4]. A similar calculation is made for the condenser heat flow. The enthalpy difference used in Equation 4.5, is now the difference between States 4 and 5 (see Figure 2), [7]. The next step is the calculation of the water mass flow by taking into consideration the equality as in Equation 4.2; nevertheless, the water heat flow is now equal to the ORC system condenser heat flow. Equation 4.6 is used to calculate the water heat flow: Qw = mw • cp,w • ATw (4.6) Water mass flow may be calculated according to Equation 4.3 by taking the values and data for water. The necessary pump power to raise the pressure from 2.5 to 20 bar is calculated according to Equation 4.5 by using, however, the enthalpy difference between States 1 and 5 (see Figure 2). The COP number of the heat pump will be calculated using Equation 4.7. COP = (4.7) Whp Finally, the ORC system thermodynamic efficiency is calculated according to Equation 4.8: Wt - Wp nTD = - t - p (4.8) Q HP + Q add Furthermore, the system contains a solar subsystem. Various parabolic trough solar collectors are used, more specifically the PTMx-36 model manufactured by Soltigua. The value of solar irradiation on a horizontal surface (G) in the area of the potential site is taken as the input data. The basis for the calculation using all the equations required to calculate the solar subsystem complies with [9]. The wood biomass system starts operating as soon as solar irradiation is insufficient for the operation of the solar subsystem. From the data and parameters, it is possible to calculate how much heat can be obtained annually from the solar subsystem, whereas the remaining amount of the total annual heat needed must be obtained from the wood biomass system. On the basis of the required amount of heat from wood biomass, we can calculate the quantity of wood chips needed per year for such a system; see Equation 4.9. mwch = (4.9) H •Hb JET 17 Urška Novosel,Jurij Avsec, Ivana Tršelič, Sonja Novak JETVol. 8 (2015) Issue 3 An economic analysis of the ORC process will also be carried out. In order to calculate certain economic indicators, the input data selected on the basis of experience, by the companies, according to the laws and guidelines in Slovenia, is needed. The estimate of revenues and expenditure was made on the basis of initial investment costs, operating costs and maintenance costs, as well as potential revenues and the revenues arising from energy savings. The initial investment costs were divided into funding investment, construction work, equipment with installation, and unforeseen work. Funding investment is divided into costs for land and costs for documentation and engineering. The costs for land are assessed on the basis of previous experience, namely €5/m2, whereas the costs for documentation and engineering work are assessed to 2% of the costs for solar collectors and 7% of the costs for the other equipment. The construction costs are divided into the costs of geological surveys, costs of well excavation, costs of soil preparation and costs of the facility and ancillary buildings. The costs of equipment and installations are mainly estimated on the basis of previous experience with certain data provided by manufacturers (Siemens, Hurst Boiler Inc., Soltigua). Regarding unforeseen work, the costs are assessed to 5% of the costs of equipment and installations. The operating costs comprise all the data on the disbursements foreseen for the purchase of goods and services, which are not of an investment nature since they are consumed within each accounting period. The operating costs are divided into direct costs of production, administrative and general costs, as well as expenses arising from sales and distribution. The direct costs of production are divided into the costs of wood chips, estimated at €75/t (since a large quantity of wood chips is required, a 25% discount may also be taken into consideration), and the geothermal water consumption costs. Administrative and general expenses are estimated at 8% of staff costs. Costs of sales and distribution, however, are divided into staff costs, i.e. costs of salaries for four full-time employees and power distribution costs. Maintenance costs are estimated at 5% of investment costs. The revenues are estimated according to the price of power generated from renewables, in particular from geothermal energy, for medium-sized units in Slovenia. The estimated useful life of a power plant taken into consideration was 25 years. The revenues arising from energy saving are estimated in accordance with the price of CO2 coupons, given that wood biomass may be regarded as CO2 neutral. For the sake of comparison, lignite was taken instead of wood chips. All the values described above are summarised in Table 3, [7]. Table 3: Economic peoemutuoa Investment costs Category Costs (€) Funding investment Land 525,990 Documentation and engineering 632,572 Construction Geological research 150,000 Soil preparation 500,000 Wellbore 1,300,000 18 JET The exploitation of ultra-low-en thalpy geothermal energd in anOpC pro cess in combination with RES and a heat pu mp Building and supply buildings 100,000 Equipment with installation Heat pump 2,081,230 Pumps 199,925 Heat exchangers 377,086 Steam turbine 600,000 Biomass boiler 1,203,218 Solar collectors 11,726,000 Pipelines and tubes 1,225,000 Unforeseen work 870,623 Operating and maintenance costs Category Costs (€) per year Direct production costs Wood chips 1,032,304 Geothermal water consumption 40,000 Administrative and general expenditures 7,520 Sales and distribution expenditures Staff costs 94,000 Distribution expenditures 350,000 Maintenance costs 1,074,582 Revenues and energy savings Category € per year Energy savings 134,426 Category Revenues (€) Total revenues in 25 years of operation 26,503,716 5 RESULTS The geothermal water inlet temperature is very low (40 °C), and it had to be preheated in a heat pump. The geothermal water heat flow that heated R134a in the heat pump amounts to 2,090 kW. As a result, the ORC system inlet heat flow amounts to 3,040 kW. This heat flow directly defines the mass flow of the ORC system working medium (R245fa), which is 55.27 kg/s. The desired turbine inlet temperature was 130 °C, which means that a large amount of additional heat flow has to be supplied to the system from the solar subsystem chosen as the first option. The quantity of additional heat flow is 10,501.3 kW. If the solar subsystem is not capable of producing such heat flow (depending on solar irradiation and weather conditions), R245fa as the working medium will be additionally heated in the wood biomass subsystem so as to finally reach 130 °C. The expansion of R245fa occurs in the turbine to a state of 2.5 bars and 73 °C in the range of overheated vapour (see Figure 2). Considering Figure 1, the working medium has to get to State 5 in which it enters the pump. It reaches State 5 by passing through the condenser in which it moves through a mixed area to the liquid phase. In the condenser, 12,048.9 kW of heat flow is removed from the water. The water used as a refrigerant must have its inlet temperature lower than the outlet temperature of the working medium in State 5. The water inlet temperature is 15 °C and outlet temperature 23 °C, which means that a relatively high water mass flow is required: 390.3 kg/s. The necessary power of the pump in the ORC system is 276.35 kW. JET 19 Urška Novosel,Jurij Avsec, Ivana Tršelič, Sonja Novak JETVol. 8 (2015) Issue 3 The total input heat flow is 13,541.3 kW, of which 22.45% from the heat pump and 77.55% is from additional sources. In the condenser, 12,048.9 kW of heat flow is released. Electrical power generated from the ORC system amounts to 1,768.64 kW, with 15.63% (276.35 kW) of it consumed by the pump and, therefore, the net recovered electrical power amounts to 1,492.3 kW. Another option is that the pump takes the power for its operation from the grid. Thermal efficiency is the ratio between the net generated power and the invested heat flow; it amounts to 11.02%. Figure 4 shows the power flows in the ORC system. The COP of a heat pump as the ratio of heat output to the amount of energy input of a heat pump amounts to 3.22. Qcond Figure 4: ORCayatvm prwvoflow diegoem To provide additional heating of the R245fa working medium in the ORC system, 10,501.3 kW of heat flow is needed. This is a rather large amount of heat and as a consequence, 286 units of PTMx-36 solar collectors are needed and approximately 5.5 hectares of land for their installation. The solar subsystem calculation was made on the basis of a fixed solar irradiation value. However, we know that the amount of solar irradiation changes throughout the day and, therefore, another subsystem is installed in the ORC system, i.e. the wood biomass system, which in our case requires quite a large quantity of wood chips per year. Since we know that we need approximately 10.5 MW of additional heat flow, we need 18,434 tons of wood chips per year for this purpose. The wood biomass system would operate mostly in winter and at night, when there are low levels of solar irradiation or none at all. All the above-described calculated values are indicated in Table 4. Table 4: Celrvletirs ovavlta Parameter Value Parameter Value m w 390.3 kg/s Q HP 3,040 kW mwch 18,434 t Qs 23,452 MWh m wm 55.27 kg/s qwb 50,141 MWh 20 JET The exploitation of ultra-low-en thalpy geothermal energd in anOpC pro cess in combination with RES and a heat pu mp N 286 S 55,198 m2 Q 73,593 MWh WP 276.35 kW Q add 10,501.3 kW Wt 1,768.64 kW Qcond , Qw 12,048.9 kW Htd 0.1102 Q geo 2,090 kW COP 3.22 The results of the economic analysis are not encouraging since the costs exceed the revenues. The investment was estimated at €21,491,644. The funding of the system with the estimated lifetime of 25 years would be provided through own sources. The cash flow plan was calculated on the basis of the estimated expenses and revenues (see Table 3). The ORC system has a negative cumulative cash flow, as shown in the diagram in Figure 5. Figure 5 shows a negative range throughout the diagram. By the 10th year of the operation, the slope of the graph is slightly less steep, because higher state subsidies for power production from renewables were taken into consideration. After the 10th year of the operation, however, the slope of the graph is even steeper, which means that a 100% return on investment cannot be expected due to the fact that the revenues are too low in relation to the current expenditure or in other words: the annual revenues fail to reach or exceed the annual revenues resulting in losses increasing one year after another. Period (years) Figure 5: Cumulative investment cash flow 6 CONCLUSION Currently, the use of RES is strongly encouraged. One of the options for their use is described in this paper: a system using three different renewable energy sources in combination with a heat pump. The temperature of the geothermal water from a wellbore, taken as the input data, is JET 21 Urška Novosel,Jur¡J Avsec, Ivana Tršelič, Sonja Novak JEVV.8 8 (2015) Issue 3 too low and had to be heated in a heat pump. The total input heat flow into the ORC process from the heat pump amounts to 22.45%, which is much more than if connecting the heat from geothermal water directly to the ORC process. However, we know that the temperature increases with the depth of the well, so it would probably make sense to choose a deeper well. The ORC system was modelled solely for electricity production and as a result, a considerable amount of additional heat flow had to be brought into the system from the solar subsystem and, consequently, many solar collectors are needed. If the desired amount of heat flow is not provided by the solar subsystem, a wood biomass subsystem is used to ensure the same required amount of additional heat flow. This system is not economically feasible. The ORC system with recuperation would be much more feasible, or geothermal water with such temperature would be used for heating and cooling systems. This paper presents an example of electrical energy production from a very low-enthalpy geothermal source in combination with other renewable energy sources and a heat pump. From an environmental aspect and, in view of the consequences of global warming, the use of these sources of energy will be crucial for electricity production in the near future. References [1] R. DiPippo: Gurthuomel Prwuo Plesta: Principles, Applications¡ Coju Studiva esd Essiorsmustel Impect¡ Butterworth-Helnemann - Elsevier, Third Edition, 2012 [2] D. Walraven, B. Laenen, W. D'haeseleer: Crmpeoiars rf thuomrdysemic cyclua fro prwuo porductirs form lrw-tumpuoetuou gurthuomel huet aruocua¡ Energy Conversion and Management, 66, 220-233, 2013 [3] S. Qsoilin, M. Van Den Broek, S. Declaye, P. Dewallef, V. Lemort: TuchsT-ucTSTmic auosuy rf Oogesic Reskisu Cyclu (ORC) ayatuma¡ Renewable and Sustainable Energy Reviews, 22, 168-186, 2013 [4] U. Novvsel: Optimel uau rf lrw-usthelpy gurthuomel usuogy, Master thesis: University of Maribor, Faculty of Energy Technology, 2013 [5] S. Masheiti, B. Agnew, S. Walker: As Eseluetirs rf R134e esd R245fe ea thu Wrokisg Fluid is es Oogesic Reskisu Cyclu Esuogizud form e Lrw Tumpuoetuou Gurthuomel Esuogy Sruocu, Journal of Energy and Power Engineering, 5, 392-402, 2011 [6] E. Gso, H. Wang, S. Zhang: Fluid auluctirs fro e lrw-tumpuoetuou gurthuomel rogesic Reskisu cyclu by usuogy esd uxuogy¡ Asian-Pacific Power and Energy Engineering Conference (APPEEC), Chengdu, China, March 28-31, 2010 [7] GeoSJJ: Thuomrdysemic eselyaia rf lrw-tumpuoetuou gurthuomel aruocua - A gurthuomel-JTleo-biTmeJJ istugoetirs rf thu Trprlšice gurthuomel apoisg esd Ecrsrmic mrdullisg rf lrw-tumpuoetuou gurthuomel usuogy - Trprlšice, Šelušne selluy, Project GeoSEE, WP4, Act. 4.3, prepared by: UM, KSSENA, January, 2014 [8] A. Lapanje: Gurtuomelsi sioi ausuosu is JUšuoTšzhTdsu Slrsusiju, Dravograd, RRA Koroška, regionalna razvojna agencija za Koroško, Geološki zavod Slovenije, 2007, retrieved from: http://www.rra-koroska.si/files/Geotermalni_viri_S_in_SV_Slovenije_ web.pdf (March 2015) [9] Soltigsa: PTMx Peoebrlic Torugh Crlluctro - Tuchsicel dete ahuut¡ Internal material from Soltigua™. Company website: http://www.soltigua.com/ (March 2015) 22 JET The exploitation of ultra-low-en thalpy geothermal energd in anOpC pro cess in combination with RES and a heat pu mp Nomenclature (Symbols) (Symbol meaning) cp Specific heat capacity [kJ/kgK] G Solar irradiation [kWh/m2] h Specific enthalpy [kJ/kg] H Heating value [kWh/kg] m Mass flow [kg/s] N Number of collectors [-] p Pressure [bar], [Pa] Q Heat [kWh] Q Heat flow [kW] s Specific entropy [kJ/kgK] S Surface area [m2] T Temperature [°C], [K] AT Temperature difference [K] V1, V2 Valves V Volume flow [m3/s] W Power [kW] Ü Efficiency [-] p Density [kg/m3] (Subscripts) (Subscript meaning) add Additional b Boiler cond Condenser geo Geothermal HP Heat pump p Pump s Solar t Turbine TD Thermodynamic JET 23 Urška Novosel, Jurij Avsec, Ivana Tršelič, Sonja Novak JETVol. 8 (2015) Issue 3 w WB wch wm (Abbreviations) COP ORC RES Water Wood biomass Wood chips Working medium (Abbreviation meaning) Coefficient of performance Organic Rankine cycle Renewable energy source(s) 24 JET