Introduction

As a result of the rapid increase in the population and the technological developments in our country and the world, the environmental impacts and sustainability of the energy sources are becoming more important. In this regard, the use of renewable energy resources in energy conversion systems is increasing [1,2,3,4]. Turkey has a good condition with its renewable energy sources such as wind, geothermal, and solar energy. Simav geothermal field is one of the usable energy sources. Geothermal heat sources with low temperature are used with binary cycles since the electricity conversion efficiency is higher than the steam power plants [5]. The Organic Rankine Cycle (ORC) is the one type of the binary cycle which is used an organic working fluid [6].

In the literature, the power plants cycle types were analyzed according to resources to the energy and exergy analysis [7, 8]. Luo et al. [7] analyzed the energy efficiencies of the single-flash and binary cycle by using mid–low-temperature geothermal resource and waste heat resource. They stated that if the energy resource water temperature is higher than 403.15 K, the flash steam power plant is more effective than a binary cycle. Yari [8] investigated the energy and exergy efficiencies of the single-flash, double-flash and flash-binary, simple ORC, ORC with an internal heat exchanger, regenerative ORC, and regenerative ORC with an internal heat exchanger geothermal power plants for three working fluids (R-113, R-123, and n-Pentane). The results show that the exergy destruction in the single-flash plant is higher than the double-flash and flash-binary plants. The energy efficiencies of the binary cycles changed between 6.362 and 15.35%. It is mentioned that the highest energy efficiency is obtained for the binary cycle with a regenerative ORC with an internal heat exchanger and R-123 as the working fluid. Basaran and Ozgener [9] investigated the energy and exergy efficiencies of the binary geothermal power plant for twelve different working fluids. As a result of the thermodynamical analysis, they mentioned that the dry-type fluids (R-236ea, R-600, R-600a, R-227ea) are more efficient than the wet-type fluids (R-143a, R-415A, R-290, R 413A). Heberle and Brüggemann [10] investigated the most effective working fluid for combined geothermal energy power and heat cycle according to the exergy analysis. They mentioned that the isobutane and R-227ea were efficient working fluids for series circuit and parallel circuit, respectively. Zhou et al. [11] investigated the hybrid solar–geothermal power plants for Australian geological and climatic conditions by using the economic and exergy analysis. They mentioned that the hybrid solar–geothermal power plants have more potential and economic benefits than stand-alone geothermal and stand-alone solar power plants. The results showed that the net power output value of the geothermal–solar power plant is higher than the stand-alone ORC [12, 13]. Arslan and Yetik [14] optimized supercritical ORC-binary cycle by using artificial neural network (ANN). Some studies have investigated the performance of the power plants [15,16,17].

Sonsaree et al. [18] and Cioccolanti et al. [19] investigated the performance of a small-scale solar ORC plant. Nouri et al. [20] analyzed the combined heat and power system with Brayton, Rankine, and ORC from the viewpoint of energy and exergy. Sheshpoli et al. [21] analyzed the waste heat recovery from a hybrid system using recuperative ORC. They used two different working fluids and waste heat of hybrid vapor compression refrigeration cycle and fuel cell for analyses. Sadeghi et al. [22] used a modified artificial bee colony algorithm for the performance analysis and optimization of ORC.

In this study, the solar energy-aided ORC for energy production from geothermal energy in Simav was compared with geothermal-powered ORC. The solar energy-aided, geothermal-powered ORC and geothermal-powered ORC were evaluated by means of energy and exergy analysis from the thermodynamics point of view.

Simav geothermal field

Simav geothermal field is located in the southern part of the Simav graben system (39  latitude, 28 .4′ longitude) at Kutahya province in western Anatolia of Turkey. And it is one of the most important geothermal fields in Turkey. The wellhead temperature is up to 420.85 K in the Simav geothermal field. Between 1985 and 2008, General Directorate of Mineral Research and Exploration (MTA) has drilled ten deep wells ranging in depth from 169 to 725 m [14, 15, 23, 24]. Simav geothermal field and wells’ location in the field are shown in Fig. 1. Well properties of Simav geothermal region are given in Table 1.

Fig. 1
figure 1

Location of the Simav geothermal fields and wells [14, 15, 23, 24]

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Table 1 Well properties of Simav geothermal region [14, 15, 23, 24]
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Design of solar and geothermal energy integrated Organic Rankine Cycle

Geothermal-powered ORC integrated solar energy system is given in Fig. 2. As shown in Fig. 1, the solar energy was integrated into the ORC through the thermal energy storage (TES) tank. Thus, the solar energy was given the ORC with the aid of TES whole day equally. The Therminol VP-1 and molten salt were used as working fluid in solar tube collector and TES, respectively. The thermodynamic properties of Therminol VP-1 and molten salt were calculated using the thermodynamic properties equations in the literature [25]. The heat of geothermal energy was transferred to the working fluid with the heat exchanger (H.E. 1). The heated working fluid enters the heat exchanger (H.E. 2) and takes the heat of salt. Then, the working fluid enters the turbine where power is produced. It enters the air-cooled condenser and leaves as the saturated liquid. The R-600a was used as a working fluid in ORC. The ORC parameters are designed parametrically according to the geothermal energy heat source temperature. The properties of the system units and system parameters are given in Tables 2 and 3. The thermodynamic properties of R-600a were determined by Reference Fluid Thermodynamic and Transport Properties Database (REFPROP) [26]. The solar radiation values of Simav were calculated by using global radiation values of Simav (Kutahya). The global radiation values for Simav region were used in the study. These values in monthly basis are given in Table 4 [28].

Fig. 2
figure 2

The flow diagram of the solar and geothermal-powered ORC

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Table 2 Properties of system units [23, 25, 27]
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Table 3 System parameters
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Table 4 The global radiation values of Simav [28]
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Method

There are some assumptions such as kinetic and potential energy effects are negligible and the reference state is 298.15 K and 101.325 kPa in the thermodynamic analysis. The governing energy equations of the geothermal and solar energy integrated ORC were obtained as follows:

The rate of useful heat is gained for the collector [27];

$$\dot{Q}_{\text{Coll}} = I \cdot A_{\text{Coll}} \cdot \eta_{\text{Coll}}$$
(1)

The rate of heat transfer input from the sun is given by;

$$\dot{Q}_{\text{sun}} = \frac{{\dot{Q}_{\text{Coll}}}}{{\eta_{\text{Coll}}}}$$
(2)

Mass of the heat transfer fluid is calculated as;

$$\dot{m}_{\text{terminol}} = \frac{{\dot{Q}_{\text{Coll}}}}{{\left({T_{{ 3 {\text{b}}}} - T_{{ 3 {\text{a}}}}} \right) \cdot c_{\text{p, terminol}}}}$$
(3)

The power output of the turbine can be calculated as;

$$\dot{W}_{\text{T}} = \dot{m}_{\text{wf}} \cdot \left({h_{{ 2 {\text{b}}}} - h_{{ 2 {\text{a}}}}} \right)$$
(4)

The electric power of the generator can be calculated as;

$$\dot{W}_{\text{G}} = \eta_{\text{G}} \cdot \dot{W}_{\text{T}}$$
(5)

The power consumption occurring in the pump can be calculated as;

$$\dot{W}_{\text{P}} = \dot{m}_{\text{wf}} \cdot \left({h_{{ 2 {\text{d}}}} - h_{{ 2 {\text{c}}}}} \right)$$
(6)

The energy of the geothermal heat exchanger is given by;

$$\dot{Q}_{\text{gf}} = \dot{m}_{\text{gf}} \cdot \left({h_{{ 1 {\text{b}}}} - h_{{ 1 {\text{a}}}}} \right) \cdot \eta_{\text{HE,1}} = \dot{m}_{\text{wf}} \cdot \left({h_{{ 2 {\text{e}}}} - h_{{ 2 {\text{d}}}}} \right)$$
(7)

The energy balance of the salt heat exchanger can be calculated by;

$$\dot{m}_{\text{salt}} \cdot \left({h_{{ 4 {\text{b}}}} - h_{{ 4 {\text{a}}}}} \right) \cdot \eta_{\text{HE,2}} = \dot{m}_{\text{wf}} \cdot \left({h_{{ 2 {\text{e}}}} - h_{{ 2 {\text{d}}}}} \right)$$
(8)

The net power output of the system is given as;

$$\dot{W}_{\text{net}} = \dot{W}_{\text{G}} - \dot{W}_{\text{P}}$$
(9)

The energy efficiency of the system is calculated as;

$$\eta = \frac{{\dot{W}_{\text{net}}}}{{\dot{Q}_{\text{s}} + \dot{Q}_{\text{gf}}}}$$
(10)

The exergy balance equation for steady systems is given by the following equation:

$$\dot{E}_{{{\text{X}}_{\text{heat}}}} - \dot{E}_{{{\text{X}}_{\text{work}}}} + \dot{E}_{{{\text{X}}_{\text{m,i}}}} - \dot{E}_{{{\text{X}}_{\text{m,o}}}} = \dot{E}_{{{\text{X}}_{\text{dest}}}}$$
(11)

where the exergy terms occurred by heat, work, solar radiation exergy, and mass flow are given as follows [27]:

$$\dot{E}_{{{\text{X}}_{\text{heat}}}} = \sum \left({1 - \frac{{T_{0}}}{{T_{\text{k}}}}} \right) \cdot \dot{Q}_{\text{k}}$$
(12)
$$\dot{E}_{{{\text{X}}_{\text{work}}}} = \dot{W}$$
(13)
$$\dot{E}x_{\text{s}} = A_{\text{Coll}} \cdot I \cdot \left({1 + \left({1/3} \right) \cdot \left({T_{0}/T_{\text{sun}}} \right)^{4} - \left({4/3} \right) \cdot \left({T_{0}/T_{\text{sun}}} \right)} \right)$$
(14)
$$\dot{E}_{{{\text{X}}_{\text{m,i}}}} = \sum \dot{m}_{\text{i}} \cdot \psi_{\text{i}}$$
(15)
$$\dot{E}_{{{\text{X}}_{\text{m,o}}}} = \sum \dot{m}_{\text{o}} \cdot \psi_{\text{o}}$$
(16)

here, ψ indicates the physical exergy term and is given as:

$$\psi = \left({h - h_{0}} \right) - T_{0} \cdot \left({s - s_{0}} \right)$$
(17)

where h is enthalpy, s is entropy, and the subscript zero indicates properties of fluids at the dead state. The sun temperature (Tsun) is 5800 K [2].The exergetic efficiency of the system is then calculated by the following equation;

$$\varepsilon = 1 - \frac{{\dot{E}_{{{\text{X}}_{\text{m,total}}}}}}{{\dot{E}_{{{\text{X}}_{\text{m,i}}}}}} = \frac{{\dot{W}_{\text{net}}}}{{\left({A_{\text{Coll}} \cdot I \cdot \psi_{\text{s}}} \right) + \left({\dot{m}_{\text{gf}} \cdot \left({\psi_{{ 1 {\text{a}}}} - \psi_{{ 1 {\text{b}}}}} \right)} \right)}}$$
(18)

Results and discussion

Handling the operating parameters as T2e = 390 K, Acoll = 10,000 m2, R-600a, the change of the energy efficiency (η) with different geothermal fluid outlet temperatures was obtained as shown in Fig. 3.

Fig. 3
figure 3

The variation of η versus T1b

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Figure 3 shows that the energy efficiency values of the solar energy-aided, geothermal-powered ORC increase by the decrease in the T1b. The energy efficiency of the proposed system ranges between 14.52 and 14.54%. The energy efficiency of geothermal power plant is constant at 14.56%, while the T1b value is increasing. Taking T1b = 353.15 K, T2e = 390.15 K, the change of energy efficiency (η) with different solar collector areas (Acoll) was obtained as shown in Fig. 4.

Fig. 4
figure 4

The variation of η versus Acoll

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Figure 4 shows that the energy efficiency values of the solar energy-aided, geothermal-powered ORC increase by the decrease in the solar collector area (Acoll). The energy efficiency of the proposed system ranges between 14.50 and 14.53%. Taking T1b = 353.15 K, Acoll = 10,000 m2, the change of energy efficiency (η) with the working fluid temperature after the first heat exchanger (T2e) was obtained as shown in Fig. 5.

Fig. 5
figure 5

The variation of η versus T2e

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Figure 5 shows that the energy efficiency values of the solar energy-aided, geothermal-powered ORC increase by the increase in working fluid temperature after the first heat exchanger (T2e). The energy efficiency of the proposed system ranges between 12.56 and 14.56%. The highest energy efficiency value is calculated for the geothermal-powered ORC. For the same system parameters, the energy efficiency value of the solar energy-aided, geothermal-powered ORC is calculates as 14.53%. Taking T2e = 390 K, Acoll = 10,000 m2, the change of exergy efficiency (ε) with different geothermal fluid outlet temperatures was obtained as shown in Fig. 6.

Fig. 6
figure 6

The variation of ε versus T1b

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Figure 6 shows that the exergy efficiency values of the solar energy-aided, geothermal-powered ORC and geothermal-powered ORC increase by the decrease in the T1b. The exergy efficiency of the proposed system ranges between 58.24 and 70.91%. Taking T1b = 353.15 K, T2e = 390.15 K, the change of exergy efficiency (ε) with different solar collector areas (Acoll) was obtained as shown in Fig. 7.

Fig. 7
figure 7

The variation of ε versus Acoll

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Figure 7 shows that the exergy efficiency values of the solar energy-aided, geothermal-powered ORC increase by the decrease in the solar collector area (Acoll). The exergy efficiency of the proposed system ranges between 58.30 and 61.08%. Taking T1b = 353.15 K, Acoll = 10,000 m2, the change of exergy efficiency (ε) with the working fluid temperature after the first heat exchanger (T2e) was obtained as shown in Fig. 8.

Fig. 8
figure 8

The variation of ε versus T2e

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Figure 8 shows that the exergy efficiency values of the solar energy-aided, geothermal-powered ORC increase by the increase in working fluid temperature after the first heat exchanger (T2e). The exergy efficiency of the proposed system ranges between 52.81 and 64.22%. The highest exergy efficiency value is calculated for the geothermal-powered ORC as 64.22%. For the same system parameters, the exergy efficiency value of the solar energy-aided, geothermal-powered ORC is calculates as 61.08%. Taking T1b = 353.15 K, Acoll = 10,000 m2, the change of net energy production of the systems with the working fluid temperature after the first heat exchanger (T2e) was obtained as shown in Fig. 9.

Fig. 9
figure 9

The variation of Epro versus T2e

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Figure 9 shows that the net electrical energy production values of the solar energy-aided, geothermal-powered ORC increase by the increase in working fluid temperature after the first heat exchanger (T2e). The net electrical energy production of the proposed system ranges between 146,362 and 171,385 kWh. The highest net electrical energy production value is calculated for solar energy-aided, geothermal-powered ORC. For the same system parameters, the net electrical energy production value of the geothermal-powered ORC is calculated as 169,128 kWh.

Conclusions

In this study, the solar- and geothermal energy-powered ORC was designed by using the well properties of Simav geothermal field. The net power output, energy, and exergy efficiencies of the geothermal-powered ORC and solar-integrated, geothermal-powered ORC were determined. The solar energy was integrated into the power plant using a thermal energy storage unit.

The most effective system parameters were determined as T1b = 333.15 K, T2e = 390.15 K, and Acoll = 10,000 m2. The electrical and exergetic efficiencies of the solar energy-aided, geothermal-powered ORC were calculated as 14.54% and 67.84%, respectively. For the same conditions, the electrical and exergetic efficiency of the geothermal-powered ORC was calculated as 14.56% and 70.91%, respectively. The net electrical energy production of the solar energy-aided, geothermal-powered ORC and the geothermal-powered ORC was calculated as 302,575.1 kWh and 305,713.5 kWh, respectively. The decrease in energy efficiencies of solar- and geothermal-powered ORC, in comparison with the energy efficiencies of geothermal-powered ORC, changes between 0.02 and 0.04%. These decrease ratios for the exergy efficiencies were determined as between 2.76 and 5.92%. By the integration of the solar energy to the power cycle, the net electrical energy production increases in a range between 1.03% and 1.3%.

The obtained results show that it is not possible to decide whether the installation of the solar-integrated system is efficient or not. Because the change between the two handled system efficiencies is relatively small. Besides, the net power output value of the solar-integrated, geothermal-powered ORC is higher than a geothermal-powered ORC.

Since solar energy is a renewable energy source, there is no payment for energy source during the lifetime of the system and it also has no effect on the environment. Therefore, it can be taken into consideration to invest for a system such that. However, it would be better to decide by using different analysis methods which design is the best. For this aim, economic analysis of the proposed systems should be performed. The determination of levelized cost of energy seems as a good choice in which the costs of operating, maintenance, installation, and selvage are included.