Effect of Working Fluid on Characteristics of Organic Rankine Cycle with Medium Temperature Geothermal Water

Abstract

The total installed geothermal power plant capacity at year-end 2023 was 16,335 MW, while the forecast for the installed capacity in 2025 is 19,331 MW. In Croatia, several medium-temperature geothermal resources (geothermal water) with temperatures from 90 to 200 °C exist, by means of which it is possible to produce electricity in binary plants, with the Organic Rankine Cycle (ORC) or with the Kalina cycle. In earlier studies, the authors presented the results of an energy-exergy analysis of geothermal sources at Velika Ciglena (170 °C), Lunjkovec-Kutnjak (140 °C), Babina Greda (125 °C), and Rečica (120 °C), aiming to determine which binary plant is more suitable for the environmental conditions in Croatia. The calculations indicate that the plant with ORC is thermodynamically superior to the one with the Kalina cycle for all geothermal sources. Taking into account the typical challenges faced by new technologies during their initial implementation, the authors recommend using the ORC plant for all medium-temperature geothermal sources. Literature on ORC applications mainly addresses working fluid selection, unit and plant optimization, and modifications to enhance thermodynamic efficiency or net power output. While many studies on working fluid selection exist, each geothermal source is unique due to its specific temperature and local cooling fluid (water or air). As a result, this paper presents the findings of an analysis on the influence of working fluids on the thermodynamic performance of an ORC system, focusing on the Lunjkovec-Kutnjak Geothermal Power Plant with a geothermal water temperature of 140 °C. As the working fluid, the next are analyzed: isopentane (C₅H₁₂), isobutene (C₄H₁₀), isohexane (C₆H₁₄), R114 (C₂Cl₂F₄), R141B (C₂H₃Cl₂F), and R142B (C₂H₃Cl₂F₂). In respect to cycle efficiency and net power, all working fluids are equally favorable, but R601a (isopentane) with low ALT, ODP, and GWP, favorable upper and lower pressure, is the most suitable fluid for ORC with a medium-temperature geothermal source.

1. Introduction

Today, a large part of electricity is still produced using fossil fuels, which have a limited lifespan and negative effects on the environment, especially CO₂ emissions (greenhouse effect and global warming). Therefore, the requirement to reduce the consumption of fossil fuels with simultaneous satisfaction of the growing needs for electricity is a difficult task. In such a situation, renewable energy sources represent the most promising solution in the future, whereby geothermal energy can play a significant role, as energy is contained in the Earth’s interior [1].

Today, geothermal energy is used indirectly to produce electricity and, to a greater degree, directly to heat spaces, greenhouses, swimming pools, for medical purposes (balneology), in fish farming, and in various industrial processes [2].

A standard international classification of geothermal resources has not yet been defined. The most commonly used classification of geothermal resources is based on the temperature of the geothermal fluid: low-temperature (<100 °C), medium-temperature (100–200 °C), and high-temperature sources (>200 °C) [1].

The most important way of using high-temperature geothermal sources (>200 °C) is the production of electricity. The most economical and simplest way of producing electricity is from sources of dry saturated water vapor in geothermal power plants using dry saturated steam [1].

In high-temperature geothermal reservoirs, where hot water is the dominant fluid, the hot water evaporates, and the steam is released to the turbine to drive the electrical generator. There exist single- and double-evaporation geothermal power plants, depending on the temperature of geothermal water [1].

If the temperature of the geothermal water is medium (100–170 °C), the heat from the water is used to evaporate another, so-called secondary fluid (organic isopentane, isobutane, isopropane, etc., or inorganic as a mixture of ammonia and water), which has a much lower boiling point than water [1].

The total installed capacity of geothermal power plants in the world at year-end 2023 was 16,335 MW, which is an increase of 208 MW considering the year 2022. The leaders in the use of geothermal energy for electricity production are the United States (3900 MW), Indonesia (2418 MW), Philippines (1952 MW), Turkey (1691 MW), New Zealand (1042 MW), Kenya (985 MW), Mexico (976 MW), Italy (916 MW), Iceland (754 MW), and Japan (576 MW) [3]. For comparison, the total installed capacity worldwide at the end of 2020 for direct geothermal utilization was 107,727 MW [4].

Croatia has a centuries-old tradition of using geothermal energy from natural sources for medical purposes and for bathing. Now used for heating, in the production of fruits and vegetables in greenhouses, and for the subsequent industrial heat treatment of such produced products (drying, pasteurization), etc. There are several medium-temperature geothermal sources with temperatures of geothermal water in the range of 90–200 °C, from which it is possible to produce electricity [5]. In late 2018 Croatia’s first geothermal power plant started operation: Geothermal Power Plant “Velika Ciglena” with 17.5 MW of power is the largest plant with Organic Rankine Cycle (ORC) in the EU, Figure 1.

Binary plants, utilizing either ORC or the Kalina cycle, are prominent for electricity production from medium-temperature geothermal sources. References [6,7,8,9,10] compare these two cycles for geothermal fields in Croatia based on energy-exergy analysis results. In all instances, the ORC demonstrated superior thermodynamic performance compared to the Kalina cycle, making it the recommended option for geothermal power plants aimed at electricity generation in Croatia.

The working fluid is vital for the ORC, influencing system efficiency, operating conditions, environmental impact, and economic viability. Today, at ORC, a whole series of substances from the group of Hydrocarbons (HCs), Perfluorocarbons (PCFs), Chlorofluorocarbons (CFCs), Hydrofluorocarbons (HFCs), Hydrofluoroolefins (HFOs), etc., are used as pure working fluids or zeotropic mixtures [1]. When selecting a working fluid while meeting suitable values for the ozone depletion potential (ODP), global warming potential (GWP), atmospheric lifetime (ALT), and other thermo-physical properties, it is necessary to take into account that it has not been banned from use by the Copenhagen Amendment (effective as of 1994) to the Montreal Protocol (signed in 1987, effective as of 1989) [1].

This paper analyzes the six most frequently used working fluids for Organic Rankine Cycles with medium-temperature energy sources into electricity at the Lunjkovec-Kutnjak Geothermal Power Plant: isopentane (C₅H₁₂), isobutene (C₄H₁₀), isohexane (C₆H₁₄), R114 (C₂Cl₂F₄), R141B (C₂H₃Cl₂F), and R142B (C₂H₃Cl₂F₂). While all fluids show similar cycle efficiency and net power, R601a (isopentane) is the most suitable choice due to its low ALT, ODP, and GWP, as well as favorable pressure conditions.

2. Geothermal Potential in Croatia for Electricity Production

The increase in temperature per unit of depth is called the temperature gradient or geothermal gradient, and it is important for the possibility of using geothermal energy. The average value of the geothermal gradient for Europe is 0.03 °C/m, while for Croatia it is from 0.018 °C/m (Adriatic coast and islands, Dinarides) to 0.049 °C/m (Pannonian area), Figure 2. In the “Dinarides” basin, the average heat flux is 29 mW/m², while in the “Pannonian” basin it is much greater: 76 mW/m² [5].

Given that the average temperature gradient and heat flux in the Pannonian Basin significantly exceed the European average, it is anticipated that, in addition to the existing geothermal fields, new geothermal fields will be discovered in this region.

Croatia’s geothermal potential consists of approximately 30 geothermal fields (Figure 3), classified into three categories: medium-temperature sources (100–200 °C), low-temperature sources (65–100 °C), and those below 65 °C [5]. Of these, around 20 fields are actively utilized, primarily for space heating and bathing [5]. Only the Velika Ciglena field generates electricity at the Geothermal Power Plant ‘Velika Ciglena’.

In addition to the Velika Ciglena geothermal field, where the aforementioned geothermal power plant is installed, there are several other medium-temperature geothermal sources with temperatures in the range of 100–200 °C (Figure 3), by means of which it is possible to produce electricity: Molve 1 (98 °C), Molve 2 (180 °C), Kalinovac (180 °C), Slatina (183 °C), Lunjkovec-Kutnjak (144 °C), Beničanci (127 °C), Ferdinandovac (125 °C), Babina Greda (121 °C), Rečica-Karlovac (120 °C), Madarinci (117 °C), and Bizovac (113 °C). The electricity generation from these geothermal sources is possible using the binary plants, either with the ORC or with the Kalina cycle.

Binary geothermal power plants with ORC (organic working media in the secondary closed circuit work according to the Rankine cycle) and with the Kalina cycle (the mixture of water and ammonia as working fluid in the secondary circuit, which evaporates in a wide range of temperatures) are described in detail in [1,2].

Previous studies [6,7,8,9,10] conducted an energy-exergy analysis to compare these two cycles for various medium-temperature geothermal resources, including Velika Ciglena, Lunjkovec-Kutnjak, Babina Greda, and Rečica.

Configurations of the proposed binary plants with ORC and Kalina cycle are presented in Figure 4 and Figure 5 and are chosen in such a way that they are very close to the performed, type-designed plants of the leading world manufacturers.

Main components of a binary power plant with an ORC are presented on the power plant scheme in Figure 4a. Heat transfer from geothermal fluid to the organic working fluid is carried out in the preheater and evaporator. Condensate of organic fluid is heated from state 4 to state 5 (to the boiling point) in the preheater while it transforms to dry saturated steam (state 1) in the evaporator. The expansion of steam in the turbine (from state 1 to state 2) provides mechanical work to drive the electric generator. The exhaust steam from the turbine is condensed in an air-cooled condenser by forced convection of air. The feed pump returns condensate to the preheater and evaporator on the initial pressure (from state 3 to state 4). The T-s diagram of ORC is presented in Figure 4b.

Main components of a binary power plant with a Kalina cycle are presented in the power plant scheme in Figure 5a [6]. The working fluid is a mixture of ammonia and water, which has a variable concentration. In the evaporator, the basic mixture with primary concentration (state 1) evaporates by means of geothermal water heat. Generated wet steam (state 2), due to different evaporation temperatures of ammonia and water, consists of a steam portion with increased ammonia concentration and a liquid portion with reduced ammonia concentration. Extracted in the separator steam with increased ammonia concentration (state 1), it expands in the turbine (from state 3 to state 5), performing useful mechanical work to drive the electric generator. The boiling liquid with reduced ammonia concentration (state 4) from the separator flows to the high-temperature preheater, where it preheats the basic ammonia-water mixture condensate. After the high-temperature recuperator, the liquid (state 6) passes through the throttle valve, where its pressure is reduced to the pressure of the turbine exhaust (state 7). Reduced liquid together with exhaust steam is introduced to the mixer, forming wet steam of basic ammonia concentration (state 8). The wet steam first passes through the low-temperature preheater, where a part of its heat has transferred to the condensate from the condenser, so the heat that must be rejected by the air stream during condensation is reduced (state 9 to state 10). The feed pump pumps condensate on the initial pressure value (state 11), and it first passes through the low-temperature preheater, after that through the high-temperature preheater (state 12 to state 1), and finally, it is introduced to the evaporator (state 1). Figure 5b presents a T-s diagram of the Kalina cycle.

Detailed calculations of the plants are provided in [6,7,8,9,10], outlining significant characteristics such as the use of air-cooled condensers in both systems, an average annual air temperature of 15 °C, and the working fluid in the ORC being isopentane, while the Kalina cycle utilizes a water-ammonia mixture that varies in composition during operation. The turbine isentropic efficiencies are 0.85 for the ORC (dry turbine) and 0.75 for the Kalina cycle (wet turbine), with feed pump isentropic efficiencies at 0.8 and pinch point values not falling below 5 °C. These thermodynamic calculations were performed using a binary cycle model based on ORC and Kalina cycles, developed from [1,2,11,12,13,14,15,16,17,18,19]. The thermodynamic properties of the working fluids were determined with the REFPROP^® program Version 8.0 [20], while exergy analysis was conducted in accordance with [1,21,22,23,24,25,26,27]. For the exergy balance, the dead state conditions were set at T₀ = 298.15 K and p₀ = 101,325 Pa.

In the case of all analyzed medium-temperature geothermal resources, the ORC is thermodynamically better than the Kalina cycle, as presented in

Table 1. The comparison of geothermal power plants with ORC and Kalina cycle in cases of medium-temperature geothermal fields in Croatia [6,7,8,9,10].

Geothermal Field	Geothermal Fluid			Organic Rankine Cycle (ORC)			Kalina Cycle
	Flow Rate [kg/s]	Input Temp. [°C]	Output Temp. [°C]	Cycle Net Power [kW]	1st Law Effici. [%]	2nd Law Effici. [%]	Cycle Net Power [kW]	1st Law Effici. [%]	2nd Law Effici. [%]
Velika Ciglena	83.0	170	69	5270	14.1	52	3949	10.6	44
Lunjkovec- Kutnjak	64.87	140	80	2225.5	13.5	46.2	2101.4	12.8	43
Babina Greda	93.97	125	70	2509.9	11.5	46.3	2317.4	10.7	42.7
Rečica	94.38	120	80	1964.1	12.4	35.9	1872.4	11.8	34.2

. The explanation is given in [6,7,8,9,10]. Thus, for all medium-temperature geothermal sources in Croatia (Velika Ciglena, Lunjkovec, Ferdinandovac, Babina Greda, Rečica, etc.), the application of the binary plants with ORC is proposed.

3. Working Fluids for ORC

In ORC, working fluids are pure refrigerants or hydrocarbons [28,29] or their mixtures [29,30]. The organic working fluids are different from water, and depending on the slope of the saturation vapor curve in the T-s diagram, they can be classified as dry, isentropic, or wet fluid, Figure 6 [28]. Wet fluids usually need to be superheated, while dry or isentropic fluids do not need to be superheated.

The effect of various working fluids on ORC characteristics is mainly investigated on low-temperature heat sources such as solar energy [31,32], biomass [33], waste heat [34,35], and ocean energy [36]. This paper deals with the selection of the most suitable fluid(s) for medium-temperature geothermal ORC.

The properties of the working fluid strongly affect the techno-economic characteristics of an ORC, and a bad selection can lead to a low-efficiency and expensive plant. In much research, the characteristics of different working fluids are analyzed, which should be fulfilled for some ORCs. Properties of the working fluid, candidate for ORC, are presented in

Table 2. Properties of the ideal working fluid for ORC [37,38,39,40].

	Properties	Reason
1.	As low as possible dew point pressure at the point of initial condensation but greater than atmospheric pressure:	- to minimize condenser cost per unit of heat transfer surface area; - to avoid leakage of air into the system
2.	Nearly vertical saturated vapor line in a T-s diagram:	- to avoid excessive superheat in heat exchangers (small heat transfer coefficient); - to avoid condensation in the turbine (loss due to condensation and blade erosion)
3.	Large specific enthalpy drop in the turbine:	- to maximize cycle thermodynamic efficiency; - to minimize working fluid flow rate; - if specific enthalpy drop becomes too large, less efficient and more costly multi-stage turbines (optimal drop exists)
4.	Reasonably low turbine inlet pressure:	- to minimize heat exchanger cost per unit of heat transfer surface area
5.	A small heat of evaporation and a matching heat resource:	- to maximize energy utilization
6.	A low specific heat:	- to avoid a high load for condenser
7.	Small specific volume at the turbine exit (turbine size factor):	- to keep small turbine size
8.	Large thermal conductivity coefficients:	- to minimize heat transfer surface area due to higher convective heat transfer coefficients
9.	Low viscosities:	- to minimize frictional pressure drops; - maximize convective heat transfer coefficients
10.	Appropriate freezing point:	- below the lowest operating temperature in the cycle
11.	Sufficiently high boiling temperature under atmospheric pressure:	- to avoid more stringent requirements for the selection of the condenser.
12.	Chemical stable under working conditions:	- to avoid decomposition, resulting in material corrosion and possible detonation
13.	Low both ODP and GWP:	- due to environmental protection
14.	A high flash point:	- to avoid flammability
15.	A low toxicity:	- due to the personnel protection from contamination in case of a fluid leakage
16.	Low cost, essentially non-fouling, non-corrosive	- due to low plant cost and its maintenance

[37,38,39,40].

Table 3. Properties of the working fluids considered in the paper [31,32].

Working Fluid	Physical Data				Safety Data- ASHRAE 34 Safety Group	Environmental Data
	Molecular Mass, M [kg/kmol]	Boiling Point, Tbp [°C]	Critical Point			ALT [yr]	ODP [-]	GWP [100 yr]
			Tcrit [°C]	pcrit [MPa]
hexane	86.18	68.71	234.67	3.034	n.a.	n.a.	n.a.	n.a.
R114	170.92	3.6	145.7	3.257	A1	300	1	10,000
R142b	100.5	−9.12	137.11	4.06	A2	17.9	0.07	2310
R141b	116.95	32.05	204.35	4.212	A2	9.3	0.12	725
R600a	58.12	−11.7	134.7	3.63	A3	12 ± 3	0	4
R601a	72.1	27.8	187.2	3.78	A3	12 ± 3	0	4 ± 2

outlines the thermodynamic and physical characteristics of potential working fluids for the ORC utilizing medium-temperature geothermal sources. The molecular mass provides insight into the fluid’s density, and the critical point indicates the applicable temperature and pressure ranges. Environmental considerations are addressed through the ozone depletion potential (ODP), global warming potential (GWP), and atmospheric lifetime (ALT). Furthermore, the ASHRAE refrigerant safety classification assesses the safety level of each fluid [1].

The ideal working fluid, which simultaneously meets all of the requirements, does not exist. The working fluid must be selected from potential candidates on a relative basis.

4. Case Study—Geothermal Power Plant “Lunjkovec-Kutnjak”

The geothermal field at Lunjkovec-Kutnjak is located in Koprivnica-Križevci County, within the municipalities of Legrad and Mali Bukovec. This geothermal reservoir is characterized as a closed hydrogeological system with the following features:

• Resource Classification: It belongs to medium-temperature geothermal resources, which do not have natural replenishment. To maintain the sustainability of the geothermal system, the exhausted geothermal fluid will be reinjected into the reservoir during exploitation.
• Depth and Thickness: The reservoir is situated at a depth of 2010 m, with an average thickness of 117.5 m.
• Well Configuration: There are two wells in the system: a production well and an injection well.
• Production Well Parameters: At the production well, the temperature at the mouth is 140 °C, the pressure is 16 bar, and the flow rate is 70 L per second, achieved using a submerged pump.

In the proposed ORC plant, geothermal fluid cools from 140 °C to 80 °C, transferring heat to the working fluid. The cooled geothermal water will then be used for direct applications, such as heating buildings, greenhouses, and swimming pools. Economic analyses typically favor combined heat and power (CHP) generation for improved efficiency and profitability [41].

Due to insufficient cooling water for the water-cooled condenser at the Lunjkovec-Kutnjak geothermal field, air-cooled condensers are utilized. Thermodynamic calculations are based on an average annual air temperature of 15 °C, with a pinch point value maintained above 5 °C.

The presumed isentropic efficiencies are as follows: turbine (0.85 for dry turbine), pump (0.80), and fan (0.75).

Calculations for various working fluids (referenced in Table 3) are conducted using a mathematical model of the ORC based on energy and exergy balance equations for the cycle components, presented in Appendix A. Optimization of key cycle parameters, particularly the upper pressure, aims to maximize specific work output, as discussed in [7]. The thermodynamic properties of the working fluids are determined using the REFPROP program [20].

The performance results for the ORC using six working fluids are summarized in

Table 4. The calculation results (performances) of ORCs for selected working fluids from Table 3 (Temperature of geothermal water, $t_{1\mathrm{g}\mathrm{w}}=$ 140.00 °C; Condensation temperature, $t_{3\mathrm{w}\mathrm{f}}=$ 25.00 °C).

Working Fluid	Performances of ORC
	Type of Working Fluid	Cycle Net Power, ${\mathit{P}}_{\mathbf{n}\mathbf{e}\mathbf{t}}$ [kW]	Mass Flow of Working Fluid, ${\dot{\mathit{m}}}_{\mathbf{w}\mathbf{f}}$ [kg/s]	1st Law (2nd Law) Efficien., ${\mathit{\eta }}_{1\mathit{s}\mathbf{t}\mathbf{L}\mathbf{a}\mathbf{w}}$ ${(\mathit{\eta }}_{2\mathit{n}\mathit{d}\mathbf{L}\mathbf{a}\mathbf{w}})$ [%]	$\mathbf{Turbine} \mathbf{Power}, {\mathit{P}}_{\mathbf{T}}$ [kW]	$\mathbf{Pump} \mathbf{Power}, {\mathit{P}}_{\mathbf{P}}$ [kW]	$\mathbf{Fan} \mathbf{Power}, {\mathit{P}}_{\mathbf{F}}$ [kW]	Heat Rejected, ${\mathit{Q}}_{\mathbf{c}\mathbf{o}\mathbf{n}\mathbf{d}}$ [kW]	Upper Pressure, ${\mathit{p}}_{\mathbf{m}\mathbf{a}\mathbf{x}}$ [bar]	Lower Pressure, ${\mathit{p}}_{\mathbf{m}\mathbf{i}\mathbf{n}}$ [bar]
hexane	dry	2227.46	35.13	12.56 (37.97)	2068.12	17.72	141.61	14,250.48	2.9	0.3
R114	dry	2099.55	97.08	12.76 (35.79)	2354.61	113.75	141.30	14,219.36	15.8	2.1
R142b	isentropic	2129.37	68.64	12.94 (36.30)	2413.88	143.51	141.01	14,189.83	22.0	3.4
R141b	isentropic	2093.71	60.64	12.72 (35.69)	2265.87	30.80	141.36	14,225.14	5.8	0.8
R600a- isobutane	dry	2135.21	39.26	12.97 (36.40)	2449.44	173.29	140.95	14,184.05	23.0	3.5
R601a- isopentane	dry	2077.32	35.89	12.62 (35.41)	2263.19	44.34	141.52	14,241.37	7.0	0.9

.

5. Discussion

This paper compares six potential working fluids for the ORC utilizing a medium-temperature geothermal source. The comparison focuses on thermodynamic performance metrics, including net power output, mass flow rate, plant efficiency, and the power requirements of the turbine, pump, and fan, as well as rejected heat and cycle pressures. Additionally, the analysis evaluates the physical properties and environmental impacts of each fluid.

Developed a computer program based on the mathematical model of the plant presented in Appendix A (mainly equations of mass and energy balances for each component), allowing a complete overview of the plant operation in different operating conditions for a single working fluid. Optimization of the upper (maximum) pressure of the cycle gives the optimal upper pressure of the cycle, which meets the conditions of the minimum temperature difference in the pinch point (Δt_PINCH,min) between the geothermal water and the working fluid. It gives maximum net power and thus 1st Law efficiency.

If the output temperature of the geothermal fluid in a model is fixed (for secondary consumers), the heat exchanged in the preheater/evaporator will be constant. With a change in the upper pressure of the cycle, the evaporation temperature also changes, and thus the mass flow of the working fluid that can be obtained by exchanging heat. Figure 7. At lower pressure, the evaporation process starts earlier, resulting in a higher mass flow rate of working fluid vapor with the same amount of heat but at the same time a lower temperature.

The final temperature of the geothermal water is fixed; therefore, the heat transferred in the preheater-evaporator is also constant. Furthermore, by increasing the upper pressure of the cycle, the evaporation temperature of the working fluid increases, which causes a decrease in its mass flow (Figure 8).

The obtained steam parameters are significantly higher at higher upper cycle pressures, which is more favorable in terms of net power. It can be seen that a higher net power is obtained with a lower mass flow of the working fluid, but at higher parameters. Also, an increase in the upper pressure of the cycle causes an increase in the required power of the pump and a decrease in the required power of the air condenser fan. The effect of changing the required power of the pump and fan on the net power of the plant is almost constant, i.e., a decrease in the required power of the fan to the same extent causes an increase in the required power of the pump. Due to this fact, the lines of net power and turbine power are parallel.

Another quantity that has a significant impact on the operation of the plant is the environmental temperature (air temperature), as presented in Figure 9 for the case of isopentane as a working fluid. By reducing the environmental temperature, the condensation temperature of the working fluid also decreases, which in all analyzed cases has a favorable effect on the cycle. A decrease in the environmental temperature by 20 °C (summer-winter difference) in the best case causes an increase in net power of almost 27%. An increase in the environmental temperature causes a significant drop in the net power obtained for the same upper cycle pressure. This case can be optimized by reducing the upper pressure of the cycle, which increases the previously obtained net power.

By utilizing the heat accumulated in the geothermal water, i.e., by decreasing the final temperature of the geothermal water in the cycle, the useful power of the plant increases. For example, in the winter mode of operation, with a lower environmental temperature, it will be necessary to increase only the upper pressure of the cycle and leave the final temperature of the geothermal water at a temperature that would meet the level required for the hot water heating system.

In the summer mode of operation, when the environmental temperature is higher and there is no need for hot water heating, it is possible to reduce the final temperature of the geothermal water more than in the winter mode, thus obtaining a higher net power. A decrease in the final temperature of the geothermal water causes a decrease in the upper pressure of the cycle. This case does not have an important impact on the net power, but the task of the plant control is always to achieve the optimal operating mode.

From the previous, it can be seen that the operation of the geothermal plant is affected by a large number of parameters; therefore, for optimal operation, advanced control methods are needed that will enable maximum utilization of the potential at all times.

In the following, the characteristic diagrams for each working fluid will be presented, which, from a technological point of view, show the most favorable working fluid for the ORC described by the previously presented mathematical model (see Appendix A).

The pump power for each working fluid changes depending on the change in the cycle upper pressure. As can be seen from the diagram in Figure 10, the process with isohexane is the least sensitive due to the low upper pressure of the cycle (2.9 bar). Cycles whose optimal operation is at higher pressures will have a steeper characteristic, and the change in pump power for the corresponding change in the cycle’s upper pressure will be significantly higher.

Regarding the change in fan power, the most favorable characteristic is again the cycle with isohexane, which means that, compared to the other presented characteristics, the change in the upper pressure of the cycle will have a significantly smaller effect on the change in fan power (Figure 11).

Figure 12 presents changes in the net power of the plant depending on the change in the cycle upper pressure.

Figure 13, as well as Table 4, presents ORC 1st Law efficiency for the considered working fluids. It can be seen that the cycle with isobutane has the highest efficiency (12.97%), followed by the cycle with R142b (12.94%), R114 (12.76%), R141b (12.72%), isopentane (12.62%), and isohexane (12.56%). Also, Table 4 presents the 2nd Law efficiency. As expected, 2nd Law efficiency is significantly higher than 1st Law efficiency, which is also a consequence of optimizing the upper cycle pressure. The values are as follows: isohexane (37.97%), isobutane (36.40%), R142b (36.30%), R114 (35.78%), R141b (35.69%), and isopentane (35.41%). The values are in a narrow range of change, and the order for different working fluids is slightly different than for 1st Law efficiency.

The results presented in Table 3 and Figure 13 show that the geothermal power plant utilizing ORC with six selected working fluids (properties listed in Table 2) achieves an efficiency ranging from 12.56% to 12.97%. The net power output varies between 2093.71 kW and 2227.46 kW, with mass flow rates from 35.13 kg/s to 97.08 kg/s.

The power ranges for the turbine, pump, and fan are as follows:

• Turbine: 2068.12 kW to 2449.44 kW
• Pump: 17.72 kW to 173.29 kW
• Fan: 140.95 kW to 141.61 kW

Key cycle parameters include upper and lower pressures, which range from 2.9 bar to 23.0 bar and from 0.3 bar to 3.5 bar, respectively. The heat rejected from the working fluid to the cooling fluid (air) varies in accordance with changes in plant efficiency.

When selecting the most suitable working fluid from Table 2, several key factors should be additionally considered. The turbine size factor [42,43] and evaporator pressure (upper cycle pressure) [42] are crucial. It is preferable for the working fluid to have high latent heat, high density, and low liquid-specific heat [44,45]. Additionally, the design condensation temperature should typically be above 300 K, along with careful consideration of the critical temperature [42] and the fluid’s freezing point. An acceptable pressure range is also essential [46]. Finally, the working fluid must exhibit chemical stability, be non-corrosive, and be compatible with engine materials [47,48].

The Ozone Depletion Potential (ODP) and Global Warming Potential (GWP) indicate a substance’s potential to contribute to ozone degradation and global warming. According to the ASHRAE refrigerant safety classification, the selected working fluids should ideally be non-flammable and non-toxic. Additionally, the availability and cost of the working fluids are critical factors in the selection process.

6. Conclusions

This paper examines the potential of six distinct working fluids for generating electricity through an Organic Rankine Cycle (ORC) utilizing a medium-temperature geothermal source, with a case study of the Geothermal Power Plant “Lunjkovec-Kutnjak”. The ORCs employing the selected working fluids demonstrate a plant 1st Law efficiency between 12.56% and 12.97%, resulting in net power outputs ranging from 2093.71 kW to 2227.46 kW. Second Law efficiency is significantly higher than 1st Law efficiency, and it changes in the very narrow range of 37.97% to 35.41%. In this respect, all working fluids are equally favorable. Also, all working fluids are satisfactory with regard to evaporator pressure (upper cycle pressure), latent heat, density, specific heat, design condensation pressure (temperature), specific volume at the turbine exit (the turbine size factor), the critical temperature, the freezing point, pressure range, chemical stability, non-corrosiveness, and compatibility with engine materials (Table 2). So it is necessary to set additional criteria such as environmental impact.

After that, the following fluids are not recommended:

• R114, R142b, and R141b have high ALT (Atmospheric Lifetime), GWP (Global Warming Potential), and ODP (Ozone Depletion Potential);
• Although the working fluids, such as isohexane and isobutane (R600a), are the fluids with the highest plant efficiency and plant net power, they have a high upper pressure or low lower pressure.

In conclusion, working fluid R601a (isopentane) (low ALT, ODP, and GWP; favorable upper and lower pressure, etc.) is the most suitable fluid for ORC with a medium-temperature geothermal source. The final confirmation should be given by the techno-economic analysis (the complexity and cost of the plant).