Performance investigation of organic Rankine-vapor compression refrigeration integrated system activated by renewable energy

. In this article, the performance and working ﬂ uid selection for an organic Rankine cycle-vapor compression refrigeration (ORC – VCR) integrated system activated by renewable energy is investigated. The performance of the system is described by the system coef ﬁ cient of performance (COP S ), and the refrigerant mass ﬂ ow rate per kilowatt refrigeration capacity ( _ _ m _ total ). Twenty-three pure substances are proposed as working ﬂ uids for the integrated system. The basic integrated system performance is assessed and compared using the proposed working ﬂ uids. The basic VCR cycle works between 35 and 0 ° C, while the basic ORC works between 35 and 100 ° C. The impacts of different operating parameters such as the evaporator, the boiler, and the condenser temperatures on the ORC – VCR system performance are also examined. The results show that the cyclopentane accomplished the highest system performance under all investigated operating conditions. Accordingly, among the examined 23 working ﬂ uids, cyclopentane is the most appropriate working ﬂ uid for the integrated system from the viewpoints of environmental concerns and system performance. Nevertheless, due to its high ﬂ ammability, further restrictions should be taken. The basic integrated system COP S , refrigeration effect, and the corresponding _ m total utilizing cyclopentane are 0.654, 361.3 kW, and 0.596 (cid:1) 10 (cid:3) 2 kg/(s kW), respectively.


Introduction
Low-grade thermal energy such as geothermal energy, solar energy, low-temperature waste heat from industrial plants, and exhaust gases from engines and turbines extensively exists in the world. Besides their renewable nature, they are also considered as free and clean energy sources since there is no additional direct carbon emission. Most of these heat sources cannot be used efficiently by the traditional power machines. In order to make better usage of low-temperature heat sources, researches on the combination of refrigeration and power systems have been conducted since the 1990s. Many systems arrangements have been suggested and inspected in the previous decade. These systems can transform low-grade heat to beneficial cooling or power energy. An organic Rankine cycle (ORC) driven by renewable energy and waste heat may be combined with vapor compression refrigeration (VCR) system for production of refrigeration or electricity [1,2].
The working fluids performance in an organic Rankine cycle-vapor compression refrigeration (ORC-VCR) integrated system is considerable. Numerous studies have been carried out to select the best fluid for the integrated system [3][4][5][6][7]. Saleh [4] suggested 10 substances as fluids for an ORC-VCR combined system. The results exhibited that R600 is the best fluid for the combined system. A parametric study and a regression analysis for a combined ORC and a cascade refrigeration system using natural refrigerants as working fluids were performed by Lizarte et al. [8]. The highest system coefficient of performance (COP S ) value was 0.79. The performance and working fluid selection for a VCR-ORC system were examined by Asim et al. [9]. Based on thermodynamics, R600a-R123 was chosen as the fluid pair for the integrated system. They concluded that the COP of the system was improved from 3.10 to 3.54 compared with that of the VCR cycle subsystem. Cihan [10] performed a theoretical analysis of a combined system with R600, R600a, R245fa, and pentane as working fluids. The results showed that R601 is the most appropriate fluid for the combined system. Li et al. [11] evaluated the performance of an ORC-VCR system using R1270, R600, R290, and R600a. The results indicated that butane is the best fluid for the system with COP S of 0.47. Aphornratana and Sriveerakul [12] examined the performance of a combined system. With R134a, for a condenser temperature (T cond ) of 35°C and an evaporator temperature (T eva ) of À10°C, the COP S was 0.125. Bu et al. [13,14] analyzed a combined system utilizing R245fa, R123, R600, R600a, R290, and R134a as working fluids. They concluded that R600a is the most appropriate fluid for the system. Han et al. [15] investigated experimentally an integrated power refrigeration system that utilizes an ammonia-water binary fluid. The COP S was 0.47 with cooling output of 11.7 kW. Wang et al. [16] performed an experimental study and theoretical analysis for an ORC-VCR system. The system attained a COP S value of approximately 0.5. Molés et al. [17] inspected an ORC-VCR system utilizing two working fluids for the ORC and two different fluids for the VCR. The results showed that the most suitable fluid for the power subsystem is R1336mzz(Z), while R1234ze(E) is the best fluid for the cooling subsystem. Nasir and Kim [18] examined the performance of seven working fluids, in an ORC-VCR system driven by low-grade thermal energy. They found that R600a is the most appropriate fluid for VCR cycle and R134a for ORC. Li et al. [19] performed energetic analysis for an ORC-VCR system using different working fluids. They concluded that R134a is the best fluid for the combined system. Kim and Perez-Blanco [20] performed a theoretical analysis for an ORC-VCR system using different working fluids. They concluded that R600 and R600a attained the highest system performance.
In this paper, the performance analysis of ORC-VCR integrated system for refrigeration or power production running with various working fluids is conducted. The inspected system is powered by a low-grade renewable heat source like waste heat or geothermal heat at around 115°C. Twenty-three common and new pure hydrofluorocarbons (HFCs), hydrocarbons (HCs), fluorocarbons (FCs), hydrofluoroolefins (HFOs), and hydrofluoroethers (HFEs) are suggested and assessed as working fluids for the integrated system. The inspected substances are R161, RC318, butane (R600), pentane (R601), isobutane (R600a), isopentane (R601a), hexane (R602), R152a, perfluoropentane (C5F12), R236fa, R245ca, R236ea, R245fa, RE245cb2, isohexane (R602a), R1234ze(E), RE245fa2, RE170, RE347mcc, R365mfc, heptane (R603), octane (R604), and cyclopentane. The performance of the integrated system was assessed by the performance parameters, i.e., COP S and the refrigerant total mass flow rate per kilowatt refrigeration capacity ( __ m _ total ). The impacts of some operating parameters like the condenser, evaporator, and boiler temperatures on the system performance were also studied. 2 The system description and selection of working fluid Figure 1 illustrates a scheme of the inspected integrated system, which includes two subsystems: the ORC, specified as 3-4-6-7-8-3, and the VCR cycle, specified as 1-2-3-4-5-1. The ORC contains turbine, pump, evaporator, and condenser. The VCR cycle consists of a compressor, an evaporator, a condenser, and a throttle devise. The features of the proposed system are as follows: (a) use the same fluid for both subsystems, (b) the turbine output power is equal to the compressor power, and (c) the system uses one condenser for the two subsystems. Two flow regulators were utilized to regulate flexibly the mass flow rate of the working fluid to VCR and ORC subsystems.
The working fluid selection for the integrated system is very important. An ideal fluid must accomplish both maximum system performance and lowest environmental concerns. Hydrochlorofluorocarbons (HCFCs) and chlorofluorocarbons (CFCs) are ozone-depleting fluids. Therefore, FCs and HFCs are used as alternative fluids for combined systems, ORC, and VCR cycle because they have zero ozone depletion potential (ODP) [1,21]. FCs and HFCs have high global warming potential (GWP), accordingly their use are controlled. Therefore, researches are still ongoing for alternative fluids, which may have lesser environmental concerns. As one of the proposals, HCs are considered as alternative fluids. HCs are environmentally friendly, have superior thermophysical properties, and have very low GWP [22]. The HCs are highly soluble in mineral oils, environmentally friendly, and chemically stable, but they are flammable. However, with proper safety protections, flammability will not be considered the largest challenge against HCs. HFEs are low toxic, nonflammable, have very low GWP, short atmospheric lifetime (ALT), and zero ODP; accordingly, they have been suggested as working fluids for thermal systems [23]. Moreover, many HFOs were recommended as working fluids due to their low environmental impacts [1,17].
The outline of temperature-entropy (T-s) diagram is a necessary property for fluid categorization. The fluids are categorized as isentropic, dry, and wet. For wet fluids, condensation takes place during the expansion in the turbine. This might be a reason for turbine blade erosion. Conversely, in the case of isentropic and dry fluids, there is  Table 1 [24,25].  Figure 2b shows the T-s diagram of the integrated system. The processes in the system that are shown in Figure 2b can be described for each subsystem. The VCR cycle: Processes (1-2s and 1-2a) are isentropic and actual  compression processes, process (8-2a-3) is an adiabatic mixing process, process (3-4) is a condensation process, process (4-5) is a throttling process, and process (5-1) is a vaporization process of the refrigerant through the evaporator. With respect to the ORC, processes (4-6 S and 4-6 a ) are isentropic and actual pumping, process (6 a -7) is vaporization process of the working fluid across the boiler, and processes (7-8 S and 7-8 a ) are isentropic and actual expansion in the turbine.

System energy analysis
The next assumptions are assumed to simplify the system modeling: (i) the system runs at a steady state, (ii) saturated states are supposed at the boiler, condenser, and evaporator exits, (iii) there is no heat loss in the pipelines, (iv) the pressure loss in the pipelines are neglected, and (v) flow losses, for example, the friction losses impacts and actual compression and expansion processes are taken into account by utilizing compressor, turbine, and pump efficiencies. The mathematical model of the integrated system displayed in Figure 1 is presented in the next sections.
Concerning the VCR cycle, the required power for the compressor, _ W comp , can be calculated as follows: where _ m VCR is the mass flow rate of the fluid in the VCR, h 1 is the specific enthalpy at the compressor entrance, h 2s and h 2a are the isentropic and actual specific enthalpies at the compressor outlet, respectively, and h comp is the compressor isentropic efficiency.
The heat transfer rate to the refrigerant through the evaporator,_ _ Q _ eva , can be expressed as where h 1 and h 5 are the specific enthalpies at the outlet and entrance of the evaporator, respectively, in kJ/kg. The VCR cycle coefficient of performance, COP VCR , is expressed as Concerning the ORC, the power output from the turbine, _ _ W _ turb , is just sufficient to power the compressor: The working fluid mass flow rate in the ORC,__ m _ ORC , can be expressed as where h 7 is specific enthalpy at the turbine entrance, h 8s is the isentropic specific enthalpy at the exit of the turbine, and h turb is the turbine isentropic efficiency. The rate of heat transfer in the boiler,_ _ Q _ boil , can be written as where h 6a is the actual specific enthalpy at the boiler inlet, and h 7 is the specific enthalpy at the boiler outlet.
The required power to the pump, _ W pump , can be expressed as where h 6s and h 6a are the isentropic and actual enthalpies at the pump outlet, respectively, h 4 is the enthalpy at the entrance of the pump, and h pump is the efficiency of the pump.
The thermal efficiency of the ORC, h ORC , is represented as The heat transfer rate from the fluid in the condenser, _ Q cond , can be expressed as The COP S of the integrated ORC-VCR system can be written as The working fluid total mass flow rate per kW cooling capacity, _ m total , in kg/(s · kW) is expressed as A computer program is constructed to compute the performance of the integrated system using various fluids under different operating parameters and to examine the impacts of many working conditions on the performance of the system. The NIST REFPROP 9.1 database [26] was applied to get the properties of the investigated fluids. The basic values of the integrated system operating conditions and their ranges are specified in Table 2. The uppermost temperature of the boiler (T boil ) was kept constant at 100°C, which is permitted to use renewable energy heat source at ∼115°C.
A performance comparison of the basic integrated system utilizing all inspected fluids is presented in Table 3. Additionally, the T-s diagram type, cooling effect ( _ Q eva ), the power output from the turbine, and the actual quality at the turbine outlet (x 8a ) are also specified in Table 3. The outcomes in Table 3 were gotten utilizing the basic values of the operating conditions as specified in Table 2. It is detected from the results in Table 3 that cyclopentane has the highest COP S , _ Q eva , and the lowest _ m total values. These values are 0.654, 361.3 kW, and 0.596 Â 10 À2 kg/(s · kW), respectively. Conversely, perfluoropentane with the uppermost molecular mass accomplishes the lower most COP S , _ Q eva , values and the uppermost _ m total . These values are 0.43, 63.57 kW, and 3.2 Â 10 À2 kg/(s · kW), respectively. Accordingly, from the energetic analysis viewpoint, cyclopentane may be considered as a promising fluid for the integrated system.
The impacts of some selected working parameters such as T cond , T eva , and T boil on the performance of the integrated system are explained in the next sections. In each subsection, the parameter whose impact is inspected varied within the range listed in Table 2

Parameters
Basic values Ranges

Evaporator temperature impact on the integrated system performance
The alterations of COP S and__ m _ total with T eva using some selected investigated fluids in the basic integrated system are shown in Figure 3. As observed from Figure 3a, the COP S increases with the increase of T eva for all fluids. The alteration of T eva has no impact on ORC. Thus, h ORC is unchanged with the variation of T eva . The evaporator saturation pressure increases with the increase of T eva for all fluids. This leads to decline of _ _ W _ comp with constant T cond . Conversely, the increase of T eva enhances the refrigeration effect. Both impacts enhance the COP VCR . Based on equation (10), this results in improvement in the COP S as T eva increases. As T eva varies from À15 up to 15°C, the COP S improves nearly 230% for all examined fluids.
The variations of _ m total as function of the T eva for the selected fluids in the basic system are exhibited in Figure 3b. As T eva increases and with the assumption of fixed _ m VCR , the required _ W comp declines. According to the assumption_ _ W _ comp ¼_ _ W _ turb ; _ W turb decreases as T eva increases. The turbine-specific work is kept constant as T eva changes. Based on equation (5), due to reducing _ W t and fixed turbine-specific work, _ m ORC must decline as T eva rises. Therefore, _ m total decreases as T eva increases, as shown in Figure 3b. The alteration of _ m total with T eva is almost linear, as seen in Figure 3b. With the change of T eva from À15°C to 15°C, the average decline of _ m total is approximately 49% for all investigated fluids. Among the 23 investigated fluids, cyclopentane accomplishes the uppermost COP S and the lowest _ m total for all inspected T eva values. Conversely, perfluoropentane attains the lowest COP S and the highest _ m total for all inspected T eva values.

Boiler temperature impact on the integrated system performance
The influence of T boil on the basic integrated system performance using some investigated fluids is shown in Figure 4. The COP S alterations with the variations of T boil are shown in Figure 4a. The COP S enhances as T boil improves. The enhancement in T boil has no influence on the COP VCR as_ _ Q _ eva and_ _ W _ comp are kept constant. The turbine-specific work rises as T boil increases. Since it is assumed that _ _ W _ comp ¼_ _ W _ turb , and the truth that the _ _ W _ comp is kept constant as T boil increases, _ m ORC should reduce as T boil increases. The increase of T boil results in enhancement of the specific heat added to the boiler. The trends of both _ m ORC and boiler-specific heat with T boil lead to decrease of _ Q boil as T boil increases. With the constant _ W turb , decline of _ Q boil , and the increase of T boil and based on equation (8), h ORC is enhanced. This results in the development of COP S , as shown in Figure 4a. As seen from Figure 4a, the COP S for all examined fluids at 100°C T boil are approximately double of those at 60°C T boil . Figure 4b shows _ m total as function of T boil for some investigated fluids in the basic system. With the assumption that _ m VCR is kept constant and _ m ORC declines as T boil increases, _ m total reduces as T boil increases, as shown in Figure 4b. Figure 4 also shows that among all inspected fluids, cyclopentane attains the uppermost COP S and the lowermost _ m total for all examined T boil . Conversely, perfluoropentane accomplishes the lowest COP S and the highest _ m total . Figure 5 shows the T cond impact on the performance of basic integrated system. Figure 5a shows the alteration of COP S against T cond for some examined fluids. As observed from Figure 5a, T cond has a great impact on the COP S . This is because of the impact of T cond on the ORC and VCR subsystems. The rejected heat is constrained by T cond , which is an additional restriction to improve the system efficiency in addition to T boil . Huge values of total heat rejected are unwanted to achieve large efficiencies in the two subcycles. Both pressure and enthalpy at the exit of the compressor increase with the enhancement of T cond with constant temperature and pressure at the entrance of the compressor. This results in a decline of _ Q eva , increase of _ W comp , and decline of COP VCR . Moreover, the increase of T cond leads to increase of _ _ W _ t because of the assumption of _ W comp ¼ _ W rurb and accordingly the _ Q eva increase of _ m ORC to attain the assumption. The increase of _ m ORC results in increase of _ Q boil . But the rate at which _ Q boil increases is greater than that of _ W t , which results in the decline of h ORC . Based on equation (10), the decrease of both COP VCR and h ORC results in decrease of COP S . As noticed in Figure 5a, the COP S declines as T cond increases for all inspected fluids. As T cond changes from 25 to 50°C, COP S declines by nearly 69% for all examined fluids. In comparison to all studied fluids, cyclopentane achieves the maximum thermal efficiency, while perfluoropentane attains the lowest thermal efficiency for all inspected T cond . With T cond equal to 35°C and the basic values for the remaining operating conditions, COP S utilizing cyclopentane is larger than that those of perfluoropentane by about 34.3%.

Condenser temperature impact on the integrated system performance
The change of _ m total with T cond for all inspected fluids in the basic integrated system is shown in Figure 5b. As T cond increases, the required _ W comp increases, and to achieve the assumption of _ W comp ¼ _ W rurb (Eq. (4)), the _ m ORC should be increased. With the increase of _ m ORC and with fixed _ m VCR , _ m total increases as T cond increases. The common tendency in Figure 5b is increase of _ m total with the increase of T cond for all inspected fluids. In comparison to all inspected fluids, the lowermost _ m total was achieved by the bottommost molecular mass fluids. Conversely, the fluids with the uppermost molecular mass attained the highest _ m total for all inspected T cond . At T cond equal to 50 and 25°C, _ m total values in the case of perfluoropentane  are nearly 6.1 and 5.1 times those of cyclopentane, respectively. As T cond changes from 25 to 50°C using cyclopentane, _ m total increases by almost 132%. To summarize, among all inspected fluids, cyclopentane achieves the highest COP S and the lowest _ m total under all inspected working parameters. Conversely, perfluoropentane attains the lowest COP S and the highest _ m total under all inspected working parameters. Therefore, cyclopentane may be considered the most convenient fluid for the integrated system. Cyclopentane is strongly flammable, which is the main challenge contrary to its usage. However, with additional safety cautions, the flammability will not be the problem in using cyclopentane.
The results show that the highest thermal efficiency and the lowest mass flow rate were achieved by cyclopentane. Among the 23 inspected fluids, cyclopentane is the best working fluid for the integrated system to recapture low-grade renewable energy with a temperature between 75 and 115°C. Since cyclopentane is highly flammable, supplementary precautions must be taken. The subsequent results were acquired using cyclopentane as working fluid. The COP S and _ m total values at 100°C boiler temperature equal 2.18 and 0.57 times, respectively, those at boiler temperature of 60°C. The COP S and _ m total values at 25°C condenser temperature equal 2.84 and 0.43 times, respectively, those at condenser temperature of 50°C. The COP S and _ m total values at 15°C evaporator temperature equal 3.26 and 0.47 times those at evaporator temperature of À15°C, respectively. When the condenser temperature equals 25°C and the remaining parameters at their basic values, the highest COP S and the corresponding _ m total are 1.05 and 0.44 Â 10 À2 kg/ (s · kW), respectively.