Performance Analysis and Optimization of an Organic Rankine Cycle Coupled to a Fresnel Linear Concentrator for Various Working Fluids

The aim of this study is to model, optimize and analyze the performance of a micro power solar of 3 kW in order to meet the priority needs of Mauritania rural people in electric power and contribute to the reduction of greenhouse gases. The proposed system is composed of a Fresnellinear concentrator which is coupled to an Organic Rankine cycle with a regenerator. Several working fluids (R134a, R152a, R290 and R717) were used for better optimizing the system. Thus, many thermodynamic and physical parameters which are influent on the performance of the overall system were analyzed. This analysis shows that the R152a and R134a fluids are the best candidates for the applications of ORC solar at low temperatures. Indeed, a 5°C superheating, the overall system efficiency, the energy and exergy efficiency of the ORC cycle has been respectively improved to 7.7%, 16.1% and 14.4%. The Optimization made on the overall system allowed a 26% and 12% reduction of the surface of the concentrator and the volume flow out of the micro-turbine respectively. The minimum surface and volumetric flow rate required to produce 3 kW is respectively 21.25 m 2 and 21.421 m 3 /h, this last result is achieved from the operating conditions: working fluid R152a, evaporation temperature which is 90°C and the direct normal radiation 1800 W/m 2 .


INTRODUCTION
The lack of energy means the fact of not having access to necessary energy services. Most people in developing countries are affected by this tragic phenomenon especially people from rural areas. Actually, the basic needs (Niez, 2010) are lighting, cooking energy, heating and cooling system. According to the Niez (2010), about 22% of the world's population have no access to electricity and 85% of these people live without electricity in rural developing countries areas and the majority of these people are in sub-Saharan Africa and South Asia. The same way as the African countries, Mauritanian rural area was only 3.9% electrified in 2015. However, there are several renewable energy solutions such as the solar power, geothermal power, biomass and the rejected industrial heat, all these can be exploited to meet the priority needs of these populations and also reduce emissions of greenhouse gas. According to Solar Gis Mauritania has respectively a global horizontal and direct normal sunshine average of 2300 kWh/m 2 and 2000 kWh/m 2 and this proves once again that solar powers plants are good opportunities to increase access to electricity in these remote areas. According to the works of Kuo et al. (2009) the efficiency of photovoltaic modules commercially available is lower of 10 to 15% than that of solar thermal modules which means that CSP technology is more competitive than photovoltaic technology. The conversion of these solar resources into electricity is not economically profitable by the conventional Rankine cycle because of their low temperature heat (Chen et al., 2010). Among the various energy conversion cycles at low temperature, the organic Rankine cycle is however, less complex and easily maintained with a competitive cost of investment contrary to the other cycles (Chen et al., 2010).
The performance and economic profitability of an organic Rankine cycle solar are related to the thermophysical properties of the working fluid (Bruno et al., 2008;Stijepovic et al., 2011). Analyzed the relationship between the properties of working fluids and the economic and thermodynamic profitability of an ORC solar cycle from a theoretical and analytical point of view. Their results showed that poor choice could lead to a less efficient and costly cycle. Several authors have studied the performance of different working fluids in order to select the optimum working fluid for the organic Rankine cycle solar (Wang et al., 2011). Analyzed the performance of different working fluids as a function of the operating conditions, in particular the evaporation pressure and the condensation temperature. Chen et al. (2010) summarized the selection criteria for working fluids and studied the influence of these properties on the performance of the ORC solar (Bertrand et al., 2008). Analyzed the thermodynamic characteristics and performance of 20 working fluids of a low temperature ORC cycle. Fluids with a critical temperature above 75°C were studied (Wang et al., 2012). Studied 13 working fluids for an ORC cycle in order to optimize the total exchange surface of the exchangers. Studied the performance of an ORC solar system for electricity production using working fluids with low Global Warming Potential (GWP).
This study was interested in standard ORC solar system i.e., without recuperator. In recent years ORC solar system with recuperator have been developed in order to increase the performance of ORC systems. Several authors have studied the performance of different working fluids for this type of system (Darvish et al., 2015). Compared the energy and exergy performance of several working fluids for an ORC cycle with recuperator (Desideri et al., 2016). Evaluated experimentally and compared the performance of different working fluids in an ORC solar system with recuperator. Fluids with a critical temperature greater than 150°C were studied. To our knowledge, the performances of working fluids whose critical temperature is less than 150°C of an ORC solar system with recuperator have never been studied. The aim of this work is to study the performance and optimization of a microcentral solar power plant of 3 with recuperator kW,working with critical temperature working fluids ranging from 80 °C to 130 °C.

Materials:
System description: This present ORC solar system is made of a linear Fresnel concentrator, a diaphragm type pump 4-5 and three heat exchangers: Evaporator (6-1), condenser (4-3) and recovery (3-2) and a microturbine/Scroll compressors, as shown in Fig. 1. Indeed, the micro-turbine illustrated here is similar to that proposed in the study of Quoilin et al. (2008) and Lemort (2006). The functioning scheme is as following: the pump supplies the working fluid to the evaporator where it is heated and vaporized by the heat transfer fluid, which is synthetic oil, coming from the solar collector. The enthalpy of the high-pressure steam produced in the micro-turbine is then converted into work. The Low pressure steam at the outlet of the micro-turbine is directed to the condenser where it is liquefied for starting a new cycle. The whole process ( Fig. 2) described above is shown in the diagram T-s in Fig. 3 and 4. The thermodynamic modeling of the ORC cycle was performed using the energy balance and mass for four shortlisted working fluids. The theoretical performances and the thermodynamic and environmental properties of different preselected fluids were evaluated and compared under certain assumptions. This work also studies the performance of the ORC model coupled to a Fresnel linear concentrator.

Models:
Model of the ORC cycle: The various components of the ORC cycle are open systems that exchange matter permanently. Each cycle component is associated with a corresponding control volume. The application of the principle of conservation of mass and energy of a volume control between time t to t + ∆tcan be reduced to the following equations: Most of the thermal machines in permanent diet with unidirectional flow and changes in kinetic and potential energy are often negligible; the Eq. (1) is simplified yet: Equation (2) is particularly used in the study of ORC cycle. Indeed, with the components that exchange only heat with the outside environment, W ̇ = 0 and (2) become: Contrary to that, for the adiabatic transformations where there will be production of mechanical energy, ̇ Q = 0 and (3) become: The equation of the destruction of exergy flow or(flow of irreversibility) ORC cycle for a steady state can be expressed as follows: For the supposed adiabatic components, this equation can be reduced to: Energy and exergy equations of ORC cycle: For both adiabatic components considered here, (the) pump and the turbine, work exchanged with the outside which is given by Eq. (4). Relaxation in the micro-turbine: Flow of irreversibility (exergy destroyed in the micro-turbine): Mechanical pump power: Flow irreversibility (Exergy destroyed in the pump): For both heat transfer components, there is no work exchange with the outside so that the equation is used (3) for the amount of heat exchanged with the outside: Thermal power received by the working fluid (evaporator): Flow of irreversibility (The energy conveyed between the inlet and the outlet of the evaporator): Thermal power rejected the cold sink (condenser): Flow of irreversibility (The energy conveyed between the inlet and the outlet of the condenser):  Thermal power regained in recovery: Flow of irreversibility (The energy conveyed between the inlet and the outlet of the recuperator): Determination of ORC cycle performance: ORC system performance is determined by the following equations: Mechanical power: Thermal efficiency (performance under the first law of thermodynamics): The total reversibility of the cycle: Performance under the second law of thermodynamics: ܶ , ܶ ‫ܶݐ݁‬ Are the reference temperatures the cold source and hot source respectively. The flow volume of output of the turbine Vt2 determines the size of the turbine and the influence on the system cost. Therefore, the working fluids with low volume flow are preferred for economic reasons: Here, m ̇ and ρ 2 are the mass flow rate and volumetric mass at the point of state two.
Model optical of concentrator and the linear thermal receiver: The solar collector considered for this study was modeled by a performance-based model NOVA-1 developed by the company Solar Novatec. The overall performance of the sensor is the product of the optical concentrator performance by the thermal efficiency of linear receiver: The model of the optical performance, which is defined as follows, models the optical concentrator: 23) ߟ is the maximum optical efficiency of the concentrator. In this hub, this yield is 0.67. ‫⊥ܭ‬ and ‫ܭ‬ // are factors of corrections, the first is called transversal and the other one is called longitudinal. These factors are respectively based on a transverse angle (Θ⊥) and a longitudinal angle (Θ //) and are determined from Table  1.
The linear receiver is modeled as follows: The thermal efficiency of the linear receiver is defined by the Eq. (20): P loss is the dissipated power by the receiver and DNI is the direct normal radiation.
The dissipated heat power Ploss is calculated by the following formula: With ‫ݑ‬ 0 and ‫ݑ‬ 1 are coefficients which give respectively to 5.6 10 -2 ܹ.݉ -2 ‫ܭ.‬ -1 2.13 10 -4 ܹ.݉ -2 ‫ܭ.‬ -2 and DT is the temperature difference between the coolant temperature and the temperature of outside air.

The global model of the Solar Organic Rankine Cycle (SORC):
The coupling of solar collector model and the model of the ORC machine allows one hand, to calculate the thermal power transported by the coolant and on the other hand, to evaluate the surface of the hub while meeting the operating conditions nominal ORC machine. Thermal power carried by the coolant fluid: Here, Sc is the total area of concentrator mirrors and the DNI isthe direct normal radiation.
By neglecting losses the load between the receiver and the evaporator, we can write: From Eq. (22) and (23) the total area of the hub is determined by the formula: Overall performance of the solar ORC model: The overall yield of the ORC solar systemis calculated as follows: ߟ ௌைோ = ߟ ைோ . ߟ (29) ߟ ௌைோ is the performance of the first law of thermodynamics the ORC cycle. For simplicity reasons, several reasonable assumptions were implemented by thermodynamic modeling. These assumptions are listed as following: • Evaporation temperature 80°C • Condensing temperature 30°C • Mechanical micro-turbine Yield 63% • Isentropic efficiency of the micro-turbine 70% • Pump Efficiency 80% • Reference temperature 25°C The Synthetic oil at 80°C is provided by a Fresnel linear concentrator. The condenser is cooled by air ambient in a surrounding area and the whole is supposed to be located in a hot rural area in Mauritania where the monthly average ambient temperature is about 30°C. The source temperature may vary between 60°C and 110°C (Table 2 and 3).

RESULTS AND DISCUSSION
In total, four pure organic working fluids were chosen as potential candidates, as shown in Table 1. Table 4 lists the performance results of the ORC solar system for a power of 3 kW.
Overall performance of the ORC solar system: Thermal efficiency characterizes the performance of the ORC machine. Figure 5 shows that when the inlet temperature of the turbine increases the thermal efficiency also increases. From Table 4 it varies between 5.4% (R134a) to 5.77% (R717). Figure 6 shows that when the inlet temperature of the turbine increases the energy efficiency also increases. The exergy efficiency of the ORC cycle assesses the potential to recover the noble energies and the level of irreversibility of the different components. A maximum exergetic efficiency of 10.27% was obtained by using R717 and 152a (9.86%). The R134a has the lowest exergy efficiency (9.61%) this is due to its high share of Fig. 9: Overall efficiency, from the collector and thermal vs turbine inlet temperature irreversibility, which is 13.56 kW. The irreversibility and exergy efficiency are inversely proportional in ORC cycle. Figure 7 shows that when the inlet temperature of the turbine increases the total rate of irreversibility decreases and beyond the critical temperature of each fluid, the latter begins to lose these thermo physical characteristics and this entails a increased irreversibility rates. The evaporator and the condenser contribute 78% and 14% of the total irreversibility of the cycle, respectively, followed by the turbine (Fig. 8).
The overall yield of the ORC solar system varies from 3.72% (R134a) to 3.98% (R717) this is due to the thermal efficiency of the ORC cycle which is low, ranging from 5.4% (R134a) at 5.77% (R717), compared to the solar concentrator efficiency varying in a range of 68.6% to 69.1% (Fig. 9). Figure 10 shows that when the inlet temperature of the turbine increases Overall Performance also increases From Fig. 9 it is clear that the inlet temperature of the turbine significantly influence the thermal efficiency and (the) exergy efficiency and consequently the overall performance. So it is always necessary to work with higher temperatures (in order) to ensure maximum performance of the cycle, without forgetting that these operating temperatures should be around 10-15°C below the working fluid critical temperature (Delgado-Torres and Garcia-Rodriguez, 2007) Influence of ambient temperature: The ambient temperature influences strongly the overall performance (Fig. 11)   Influence of heat input from the heat source: Heat input in an ORC solar system is important. It determines the surface of the solar concentrotor and defines a substantial portion of the cost of the system. Therefore, the fluids which have minimum amount of heat are preferred. From Table 4 the heat input required to produce 3 kW varies 51.01 kW (R7171) to 54.95 kW (R134a). Figure 12 shows that the heat input requirement is reduced when the inlet temperature of the turbine increases.
Influence the implantation site on the solar system ORC: The implantation site of a solar system ORC greatly influences the performance and cost of the system. The heat input required to produce a net working 3 kW depends exclusively on the direct normal radiation received at site. When DNI is high at the implantation site, the surface of the solar collector decreases (Fig. 13). This causes a reduction in the cost of the overall system and the cost of the space occupied by the system. From Table 2, the area needed to produce 3 kW varies from 32.78 m 2 (R717) 35.02 m 2 (R134a).
Influences the operating pressures and the turbine flow rate: High pressure in an ORC cycle requires a robust system and increases the cost of the components.
To overcome this problem, it would be wise to use two micro-turbines in series where parallel to divide the pressure at the inlet of the turbine. This is advantageous since the power system will be higher. In return, the system becomes expensive due to the increase of micro turbines numbers. From the Table 4 we see that the fluid R152a and R134a have low-pressure values in the condenser while the fluids R290 and R717 have the highest pressures in the evaporator greater than 2.5 MPa (Maizza and Maizza, 1996). From the economic point of view, the volume flow leaving the turbine is important because it determines the size and cost of the microturbine. As shown in Table 4, the fluid R717 and R152a have the lowest rates. A low volume flow rate also reduces the size of the pump and reduces losses of the connecting pipes loads. In general, the volume flow leaving the turbine is inversely proportional to the turbine inlet temperature, which can be illustrated in Fig. 14. Security and environmental impact: Some substances, mostly refrigerants deplete the ozone layer and / or contribute to global warming. Because of their adverse effects, there is a need to choose those that have less harmful effects on the environment. The fluids R717 and R290 are low GWP R152a and R134a monitoring whose GWP is a little high (Table 2). According to the classification ASHAR (Table 3) R134a is the best candidate (A1) non-flammable nor toxic followed by R152a (A2) flame retardant and nontoxic. The R290 (A3) more flammable than R152a and nontoxic. R717 (B2) bit flammable and toxic.
Performance of the ORC solar system with and without overheating of 5°C: With an overheating of 5°C, the solar ORC overall performance improved slightly (Table 4). It reached its maximum of 4.27% (R717) with overheating compared to 3.96% (R717) in the case without overheating (corresponding to an increase of 7.7%). Despite the increase in overall yield of the solar ORC cycle thermal and exergy efficiency improved by 16.1% and 14.4% respectively. This results in a decrease in the amount of heat required (Qe) of 51.93 kW to 45.37 kW (Corresponds to a 14.4% reduction) and therefore the surface of the concentrator required to produce a net work of 3 kW is lowered 32.78 m 2 to 30.51 m 2 (7.4% reduction). Beyond the critical temperature of each working fluid, with or without overheating the thermal efficiency and exergy decrease. The reason for this decrease is due to the growth of irreversibility flow (I tot) and the amount of heat (Q e) of the high evaporation temperature Te cycle ( Fig. 7 to 13).

Optimization of some operating parameters of the solar system ORC:
The aim is to optimize the surface of the solar concentrator and the volume flow of the outlet turbine for economic reasons.
The optimization of the surface of the solar concentrator will reduce the cost of the optical system and in turn a decrease in overall area occupied by the solar system ORC. On Fig. 15 to 19 the arrows are directed towards areas of unfavorable operating while the optimal operating zones are identified by the points. Figure 15 shows that the areas where the solar yield ORC is maximum (5.041%) correspond to a surface of the concentrator ranging from 20 m 2 to 26 m 2 over 30.51 m 2 to 32.78 m 2 before optimization (26% The optimum area to maximize the overall efficiency and minimize the surface of the concentrator is observed at a 21.25 m 2 surface and a minimal amount of heat 41.25 kW (Fig. 16). This amount of heat was obtained with a direct normal insolation of 1800 W/m 2 (Fig. 17). The volume flow out of the turbine determines the cost of the micro turbine. In order to optimize it we must on the one hand, minimize the mass flow rate of the cycle and secondly to ensure that the temperature of the turbine inlet does not exceed the critical temperature of the working fluid. By minimizing the mass flow we at the same time maximize the overall performance and the parallel volume flow is also optimized. The maximum overall efficiency is obtained with mass flow and volume ranging from 0.16 kg/s to 0.14 kg/s and 24 m 3 /h respectively (Fig. 18). The most favorable area to minimize the volume flow and maximize the overall performance is observed to 21.424 m 3 /h (Fig. 19) with a minimum rate of 0.14 kg/s at 90°C (Fig. 20).

CONCLUSION
This study focuses on the analysis of performance and the optimization of an organic Rankine cycle coupled to a Fresnel linear concentrator. The thermodynamic modeling of organic Rankine cycle was obtained by using the energy and mass balance for four preselected working fluids with operating temperatures between 80°C to 130°C. The model of the solar collector used for this study is a commercial model developed by the company Novatec Solar.
The influence of several thermodynamic and physical parameters on the performance of the overall system were analyzed for different working fluids.
Following this analysis the R152a fluids and R134a appears as potential candidates for solar applications at low temperature ORC, followed by the fluids R290 and R717 that offer excellent performance but require safety precautions, because of their flammability and toxicity respectively.
The evaporation temperature and ambient strongly influence the overall performance, thermal and Exergy system efficiency. Parallel to the increase of the 48°C for R152a R717 fluids the overall system performance degrades. The performance of the ORC solar system without and with overheating has been studied for various working fluid. With an overheating of 5°C the overall efficiency of the solar system ORC, energy and exergy efficiency of the ORC cycle has been improved to 7.7, 16.1 and 14.4%, respectively.
The optimization of the surface of the concentrator and the volume flow of micro-turbine output have led to a reduction of 26 and 12%, respectively. This is very advantageous to minimize the investment cost of the overall system. The surface and the minimum flow rate acquired to produce 3 kW is 21.25 m 2 and 21.424 m 3 /h respectively. This is achieved for the operating conditions: working fluid R152a and the evaporation temperature is 90°C and the direct normal radiationis 1800.