Thermodynamic modeling of different configurations ((1) cycle A; (2) cycle B, and (3) cycle C).
The new ORC-VCC combined system is analyzed. It is a new system that can be operated in four modes depending on the type of energy. The novelty of the system appears essentially in the development of new ORC-VCC combination architecture, the lowering of the condensation temperature, the possibility of cold production by the ORC cycle affected by the pumping phase, preheating of fluid cycle using the VCC cycle fluid, and new configurations based on the integration of heat recovery systems to improve overall system performance. In addition, each installation mode has several configurations depending on the recovery points that will be integrated later, besides its adaptation to any energy source, where we can use biomass, solar, and heat rejects of industry at low temperatures (60–130°C). This system can produce under and above zero temperature. Although, due to its architecture, it is also characterized by many combination of selection fluid for the ORC and VCC cycles, it is not necessary to have the same working fluid as in the classic systems. In this study, three configurations are examined and studied in terms of energy efficiency mainly the performance of each configuration including net power, refrigeration capacity and overall efficiency, the thermal efficiency for ORC.
- organic Rankine cycle
- vapor compression cycle
- desalination of seawater
According to the International Energy Agency (IEA), solar power will be the fastest-growing source of energy in the future. The growth rate of solar energy can reach more than 12% . Many countries today are making decisions to put political strategies in the use of renewable resources. For that, many studies were done all over the world, Asia [2, 3], Africa [4, 5], and America , whose objectives are to determine the energy potential and to choose the political strategies to improve the solar energy potential. In Europe , the Commission communication to the European Parliament and the Council for new European energy policies set out in 2014  a target to reach 20% energy efficiency by 2020 and 30% by 2030. In fact, researches are looking for technologies that can be used to mitigate global warming around the world and reduce CO2 emissions . Energy shortage problems are faced in all over the world and becoming more acute in all fields [9, 10] (metallurgy, chemical, electrical and mechanical sectors). So, the world is facing two energy challenges: increase production to meet energy needs and reduce CO2 emissions issued by industrial plants. For that, the utilization of renewable energy becomes a political duty and not a strategic choice to solve energy problems. Among these renewable resources used is the solar energy: Sunil Kumar  published a review as a synthetic fruit of studies done on energy analysis. He presented the various solar energy systems used in solar drying [12, 13], solar air conditioning [14, 15], solar refrigeration , solar water heating , and solar cooking . These systems have been operated by solar photovoltaic techniques  and solar thermal energy used for heat and power generation [20, 21, 22].
The new ORC-VCC combined system is developed. As it is shown in Figures 1–4, we wanted to design a new architecture for multi-objective optimization. It is a new system that can be operated in four modes depending on the type of the produced energy, namely, the electric energy, refrigeration, poly-generation, and water desalination. The four developed modes are:
Mode 1: cold production. Figure 1 illustrates the basic architecture of the system. It receives a heat flow from an external renewable source in the boiler so that the ORC cycle can be run in order to deliver a mechanical work at the turbine; this work is transmitted totally to the VCC cycle compressor (turbo system compressor). This system provides us a refrigeration quantity at the evaporator as illustrated in the figure.
Mode 2: electricity power. Figure 2 shows the basic installation. It also receives a quantity of heat from an external renewable source in the boiler to have mechanical work at the turbine; it is partially transmitted to the VCC cycle compressor. On the other hand, the power supplied by the VCC cycle evaporator is totally exploited by the ORC cycle condenser. So this mode of operation requires a renewable source and provides us an electric power.
Mode 3: cogeneration (cold production and electricity power). Figure 3 presents the basic architecture. It receives an external renewable source in the boiler. Through this source, it allows us to have mechanical work at the turbine: this is partially transmitted to the VCC cycle compressor as mode 2. The power provided by the VCC cycle evaporator is partially operated by the condenser ORC cycle. So this operation mode requires a renewable source and offers an electric power by ORC cycle and cooling capacity by the VCC cycle.
Mode 4: tri-generation and desalination of seawater are illustrated in Figure 4. This configuration has four circuits: an ORC cycle circuit that is represented by the red color, a circuit of the VCC cycle in blue, a circuit in purple color of the desalinated seawater, and a red circuit of the heated water. We will couple the system with a limited renewable energy source.
In addition, each installation mode has several configurations depending on the recovery points that will be integrated later, besides its adaptation to any energy source, where we can use biomass, solar, and heat rejects of industry at low temperatures (60–130°C). This system could produce a negative and a positive cold. Although, due to its architecture, it is also characterized by many combinations of selection fluid for the ORC and VCC cycles, it is not necessary to have the same working fluid as the classic systems.
The main purpose of this presented study is to analyze the performance of a new system that combines the steam compression cycle and the Rankine cycle for tri-generation (electricity, cold, hot) as well as the desalination of water. This system uses a low-temperature heat source such as solar energy, heat from industrial waste, and biomass.
The objectives of this study are:
Architectural development of the basic system
Development of improvement configurations
Energy analysis and choice of fluids
The impact of operating parameters on energy performance
In this study, we will develop a new ORC combination with the VCC system in order to make cogeneration and tri-generation with a negative temperature cold (−10°C, 0°C), as well with a positive temperature cold (0°, 10°C). Three new configurations are examined and studied in terms of energy efficiency, namely, the performance of each configuration including net power, refrigeration capacity and overall efficiency, the thermal efficiency for ORC, and the coefficient of performance for VCC. The working fluids are n-hexane for ORC and R600 for VCC.
2. System description
As illustrated in Figure 5, the configuration consists of four circuits: an ORC cycle circuit represented by the red color, a circuit of the VCC cycle which is in blue, a circuit in purple color of the desalinated seawater, and a red circuit in the heated water. We will couple our facility with a limited renewable energy source which is thermal photovoltaic center, at low temperature (100–130°C). Our approach is to lower the condensing temperature between −10 and 10°C of the ORC cycle so that the delivered work can be increased. So, a cold part produced by VCC will be dedicated to condense the fluid of the ORC cycle. For this, we will integrate an exchanger regenerator1 which is used to condense the ORC fluid by a quantity of cold produced laying vapor phase VCC side.
2.1 Desalinated water circuit description
First, the seawater is pumped by a pump PMP1 and preheated by the exchanger 3. Then, it will be evaporated at constant pressure in the boiler by a solar collector. After having saturated steam, the latter passed the condensation phase using the regenerator 2 in order to equate the water at a hot temperature which is equal to that of the evaporation. With integrity of exchanger H2, we started the first phase of cooling the salty water and chaffered sanitary water. In the end, for the desalted water to complete this phase of cooling, also we have to warm the water out of the sea, using an exchanger H3.
2.2 Circuit description of heated domestic water
This is the simplest circuit in our loop. It is enough the sanitary water enters the exchanger H3 to become hot thanks to the quantity of heat provided by the desalted water.
So our system produces electricity due to the mechanical work obtained by the turbine, a refrigeration quantity by the evaporator 1, and de-watered water obtained using two serial transformations (evaporation, condensation) and produces hot water by the exploitation of the hot quantity from the de-watered water.
3. The different configurations developed for ORC-VCC combination
3.1 Configuration A
The cycle A is the basic configuration. We will combine the two ORC and VCC cycles without any recovery for cogeneration. As shown in Figure 6, the only combination is made at the heat exchanger H1 which serves as the condenser of the ORC cycle fluid. This configuration allows having cogeneration with positive or negative cold according to our needs.
The operating principle is described as follows:
First, the ORC cycle fluid enters the boiler in order to heat it up to 100°C by a renewable external source (biomass, industrial and solar thermal discharge, etc.). It suffices that the fluid reaches a saturated vapor phase; it enters a turbine to generate a mechanical work. This phase allows the fluid to pass from the high pressure to the low pressure. After this phase, a condensation phase is necessary to make the fluid in a liquid state. For our application, the condensation is done at low temperature which requires a cold external source. For this, we combined the ORC cycle condenser with the VCC cycle evaporator by integrating a H1 exchanger. For this configuration, after condensation, the fluid goes to the pumping phase.
In addition, the VCC cycle operation is the inverse of those ORC cycle. The VCC fluid is compressed with a mechanical compressor and then condensed at a temperature of 30°C. In this configuration, after this phase, the fluid is released directly by an expansion valve. Then it is evaporated in two phases.
3.2 Configuration B
For cycle B, we kept the same basic architecture as in cycle A, except that we will integrate an H2 exchanger. This exchanger is mounted just after the pumping phase of the ORC cycle. Seeing that the temperature obtained at the pumping point is almost the same as the temperature of condensation which varies between −10 and 10°C, the idea is to exploit this temperature to make the sub-cooling of the VCC cycle to improve its performance. Cycle B is shown in Figure 7, and it is also developed to make cogeneration with a negative cold.
3.3 Configuration C
As shown in Figure 8, cycle C is used also for cogeneration. Unlike the conventional ORC cycle, which is used only for electricity production at the turbine state, cycle C allows the generation of electricity and cold in the ORC cycle. So, the configuration C is used to produce negative cold, positive cold, and electricity.
We will use the heat quantity at low temperature following the pumping step in the ORC cycle in order to produce a positive cold at 18°C for air conditioning. For this reason, we will integrate the H3 exchanger for the heat transfer between the ambient air and the ORC cycle fluid.
4. Mathematical modeling and validation of the model
4.1 Thermodynamic modeling
During this study, we treated the thermodynamic equations as well as the resolution by a calculation program developed by the EES software. Also, this software allows us to realize the different curves and tables presented in the study.
Table 1 illustrates the different thermodynamic models used throughout the work and the different configurations for cogeneration mode.
|Coefficient of performance|
|Overall performance of ORC-VCC||Net work|
|Overall performance of the system||(1 and 2)|
|Efficacity||(1 and 2)|
|Exchangers for cogeneration||Exchanger 1|
|Exchangers for tri-generation|
4.2 Validation of the model
The approach followed to validate our model is based on two procedures of the developed model. The ORC and the VCC are validated, respectively, in Sections 4.2.1 and 4.2.2.
4.2.1 ORC validation
The model developed for the ORC is tested with the results by Saleh et al. , which is the most appropriate configuration to validate the current model using the similar working applied fluid. The comparative results are illustrated in Table 2. These results show a small deviation of 2.09% concerning the thermal efficiency. It is worthy to notice that certain changes in the developed model are made for an appropriate comparison. Specifically, the condensation temperature was 40°C and the isentropic efficiency at 85%.
4.2.2 VCC validation
In this section, the operation of the VCC cycle is enabled. Nasir and Kim  are selected for the validation. Some changes in the model are made to have an appropriate comparison against the literature. Indeed, the temperature of the condenser is set to 30°C. Table 3 includes the validation results along with the COP for cooling. We selected three fluids for validation, which are R245fa, R123, and R134a. Table 3 shows the margin of error between Ref.  and our model. The error results for R245fa, R123, and R134a are, respectively, 0.6, 0.44, and 0.92%. These margins are acceptable to give their low values.
5. Selection of the working fluid
The choice of the working fluid for an ORC or VCC cycle is an important criterion to improve the cycle performances. Generally, there are three families of organic fluids.
Figure 9 shows these three classes on a T-S diagram. The distinction between these different types essentially depends on the slope between the saturation temperature and the isentropic variation (ΔT/Δs). If a negative slope is said, the fluid is wet, such as H2O, NH3, and R134a. For a positive slope, we speak of a dry fluid such as benzene and pentane. In cases where the slope is infinite, it is said that this fluid is isentropic like R600 and R600a.
For the ORC cycle is to have fluid hot admits a weak latent heat in the evaporator, in order to minimize the quantity received by the boiler. Thus, a low latent heat in the condenser minimizes the amount of cold delivered by the VCC cycle. In addition, we are looking for a fluid with a positive slope to avoid vapor having less than 0.95 of steam rate.
We guarantee the elimination of the oxidation effect in the turbine especially that we will make it lower concerning the condensation temperature to −10°C. Based on these criteria and conditions mentioned above, it is necessary to choose a dry or isentropic ORC cycle fluid. We choose the n-hexane; the chemical formula is C6H14. The thermophysical characteristics of this fluid are presented in Table 4.
|Fluid||Critical temperature (°C)||Critical pressure (bar)||MW (kg/kmol)|
The R600 is selected as a working fluid for the VCC cycle. It is a hydrocarbon of formula C4H10 crude which is found in the gas status under normal conditions of temperature and pressure. The physical characteristics of this fluid are presented in Table 4. Furthermore, our choice is toward the use of n-hexane for the ORC cycle. This choice is essentially due to the steam rate which is equal to 1 even when the condensation temperature is lowered to a low degree. This allows us to have a margin of confidence and turbine safety (avoid the effect of oxidation). During our study, we chose the R600 as a working fluid for the VCC cycle. This fluid is characterized by its robustness in the market, so it is used in recent years in several researches. In addition, we find that the environmental damage is minimal.
6. System settings and boundary conditions
To reassure the efficiency and rentability of the system, it is necessary that we set some parameters and define their limits. For example, the network and refrigeration capacity must be always positive. Also, to guarantee the safety of the turbines, it is necessary that the vapors’ quantity must be more than 95%.
The boundary conditions are shown in Table 5.
7. Results analysis and discussion
The main purpose of this study is to analyze the performance of a new system that combines the steam compression cycle and the Rankine cycle for tri-generation (electricity, cold, hot).
In the previous section, we presented the different designs of the system. This system has an energy autonomy. It needs only the solar temperature “Th.” For that reason, we will focus our work on the impact of solar temperature.
First of all, we started with the mass flow analysis of each circuit in order to have the mass potential of our mini power plant.
Figure 10 resumes the evaluation of the different values of the flow rate for each circuit. The different ratios of the mass flow rates are R1, R2, R3, and R4. The shape of different curves is of positive exponential form. It can be seen that the variation between the four curves is constant in function of Th. The three ratios R1, R3, and R4 represent small variations of the ratios of the flow rates as a function of Th between them. Consequently, the geometries of different constituent bodies are coherent in terms of dimensions. In contrast, the ratio R2 is a large margin of variation compared to the other ratios.
This variation in flow rates does not depend only on the temperature of the solar collector Th, but it also depends on the temperature of the boiler Tg. For this, we have made surfaces of each flow ratio with two temperatures Th and Tg as shown in Figure 11.
In addition, it can be noticed that the net quantity of the hot water is delivered by the system.
Figure 12 illustrates the evolution of different mass flow rates as a function of the heat delivered Qb by the boiler. The four flows are varied proportionally with Qb. It is noted that the mass flow has a large positive slope with respect to the other mass flow rates. This allows us to interpret that the geometry of the ORC cycle is very important in relation to the different cycles. Also, this cycle promotes significant power.
It is constant that the lowest mass flow rate corresponds to the mass flow rate that does not exceed 0.18 kg/s, which means that the desalinated water is installed at a low power.
In energy potential term provided by our installation, Figures 13 and 14 indicate the net work and the amount of cold produced as a function of the hot source Qb. It is observed that the two quantities Wnet and Qev are proportional to Qb. It is possible to obtain a net work of maximum value equal to 14 kW and a maximum amount of cold equal to 75 kW.
Figure 15 shows the evolution of the ORC thermal efficiency as a function of the generator temperature Tg and the vaporization temperature of the cold Tev.
It is observed that the ORC efficiency is proportional to Tg and inversely proportional to Tev; this is justified by the first principle of thermodynamics. A better efficiency noted is 0.21 for Tev = −10°C and Tg = 95°C.
7.1 Technical-economic analysis and investment costs
The investment costs can be estimated from specialized works [31, 32, 33, 34, 35, 36] where they are generally presented in the form of charts or tables. Often these abacuses represent the cost taking into account the influence of parameters such as pressure, temperature, material or manufacturing, assembly, transport, etc.
We have undertaken to gather technical and economic data of the components used in the VCC and ORC cycles (compressors, condensers, evaporators) in order to develop technical-economic and then exergo-economic con-elations. This task is not easy since often the data is discrete and the interpolations or extrapolations are not conclusive due to the nonlinearity of the cost according to the parameters used by the manufacturer.
7.1.1 Compressor cost
For compressors, the price depends on the type of compressor, the power, and the volume swept, while for motor compressors the price is given according to the type, the power, and flow of the heat transfer fluid at the condenser.
Compressor cost = f (type, power, volume swept).
Motor compressor cost = f (type, power, flow rate of the coolant at the condenser).
So for each model corresponding to a certain type of compressor, a function of the following form is established:
Compressor cost = a. (Power)b. (Volume swept)c
Determine the coefficients a, b, and c.
We are able to express by an analytical approach the price of a certain type of compressor, knowing the technical characteristics.
Generally correlations have been developed to determine the investment costs of each organ.
For the compressor, the correlations used are :
10.3 < cyl < 30.6 [cm3]
835 < QEV < 2650 [W]
475 < W < 1225 [W]
7.1.2 Evaporator cost
For evaporators the price depends on the type of evaporator, the power, the exchange surface, the flow of the heat transfer fluid at the evaporator, the number of fans, and the pitch of the fins.
Evaporator price = function (type, power, surface, flow, no. of fans, no fins).
For the evaporator, the correlations used are :
850 < QEV < 5500 [W]
685 < DEV < 3325 [m3/h]
7.1.3 Condenser cost
The price of a condenser is given according to the type of condenser, the power, the exchange surface, the flow of heat transfer fluid, and the number of fans.
Price = function (type, power, surface area, heat transfer fluid flow, no. of fans).
For the condenser, the correlations used are :
3500 < QCD < 20,000 [W]
0.6 < DCD < 2 [m3/h]
The energy performance of power and refrigeration cogeneration and tri-generation through an organic Rankine cycle (ORC) with a vapor compression cycle (VCC) by a new combination systematic is examined. We can use a low-temperature energy source. Two cases of refrigeration and cogeneration are analyzed, including cases of cogeneration (−10, 10°C) and congelation (0, −17°C).
The effects of the system parameters include the condensation and vaporization temperatures for ORC and VCC, and the efficiency E on performance such as thermal efficiency, specific refrigeration, and net work output and global system performance are investigated.
According to the analysis and the investigation carried out during this study, the main interpretations retained are:
The results show that operating parameters have a significant effect on performance. This effect differs from one use case to another (positive or negative refrigeration) and according to the installed configuration (cycles A, B, and C).
The three configurations developed which were based on the integration of recovery exchangers noted improvements in overall performance. These improvements also differ from one cycle to another, which makes it possible to say that the spot of integration of the exchangers is an effect on the performances.
The results show that for cogeneration with negative cold, among the three configurations that we have developed, cycle B is preferable in which it has a better energy performance.
|COPvcc||coefficient of performance for the vapor compression cycle|
|COPs||coefficient of performance for the overall system|
|Cicp||investment cost of the compressor|
|CiEV||investment cost of the evaporator|
|Cyl||piston compressor displacement|
|CiCD||investment cost of the condenser|
|DCD||heat transfer fluid flow rate at condenser|
|DEV||heat transfer fluid flow rate at evaporator|
|+ H2. H1||cycle with exchangers 1 and 2|
|m1||mass flow rate for ORC (kg/s)|
|m2||mass flow rate for VCC cycle (kg/s)|
|m3||mass flow rate for heated water|
|m4||mass flow rate for seawater desalination|
|ORC||organic Rankine cycle|
|Rm||mass flow ratio|
|Rpp||pressure ratio for pump|
|Rpc||pressure ratio for compressor|
|Tev||vaporization temperature for VCC cycle (°C)|
|Tcit||critical temperature (°C)|
|Th||temperature in the panel solar (°C)|
|Tg||temperature in the boiler for ORC cycle|
|Tcond||condensation temperature for organic Rankine cycle|
|Tsh||overheating temperature for organic Rankine cycle|
|Qb||boiler heat input (kW)|
|Qh2||heat input for the exchanger 2 (kW)|
|Qev1||the power of the evaporator 1 (kW)|
|Qev2||the power of the evaporator 2 (kW)|
|Qev||the overall power evaporated by the VCC cycle (kW)|
|QEV||heat flow exchanged at the evaporator|
|QCD||heat flow exchanged at the condenser|
|VCC||vapor compression cycle|
|Wcom||working fluid pump power consumption (kW)|
|Wexp||expander work output (kW)|
|Wnet||net work output for overall system (kW)|
|Wpump||working fluid pump power consumption (kW)|
|WT||mechanical work of the turbine (KW)|
|Wc||mechanical work of the compressor (kW)|
|η is1||compressor isentropic efficiency|
|η is2||expander isentropic efficiency|
|ηpump||working fluid pump isentropic efficiency|
|ΔTPinch||pinch temperature (°C)|
|3||boiler output and expander inlet|
|13||evaporator 1 inlet|
|14||evaporator 1 outlet|