-This paper presents a new cycle for utilising the energy of low -temperature heat sources that combines the Organic Rankine Cycle (ORC) with the Trilateral Flash Cycle (TFC) via a cascade condenser . The performance of the combined cycle that allows different working fluids to be used in the two circuits is compared with those of the simple ORC and TFC cycles for a heat -source of 120oC by using R152a as the working fluid in the three cycles . Compared to the simple ORC , the results show that the combined cycle gives more power and a higher the rmal efficiency. Compared to the simple TFC, the results show that the cycle gives less power , but a higher thermal efficiency . The cycle’s performance with R1234yf in the ORC circ uit and R152a in the TFC circuit is also compared with that with R152a in both circuits at the same condenser temperature . The results show that the R1234yf/R152a pair gives more power than R152a alone, but less efficiency. The model developed for the cycle uses Microsoft Excel as the modelling platform and determine s the fluid properties with VBA functions . The functions are validated with published data for the TFC model with three suitable organic working fluids which are R134a, R245fa, and R1233zd . Keywords : Low-temperature heat sources , ORC, TFC , combined cycle, Excel 1. Introduction The mounting concerns about climate change s caused by the large -scale use of fossil -fuels for electricity generation have inspired intensive research for the development of new technologies for utilising low and moderate temperature energy sources such as solar and geothermal energy and waste heat from industries. One of the promising technologies in this respect is the organic Rankine cycle (ORC) [1,2 ]. Unlike the conventional Rankine cycle that requires very high temperatures to raise the steam to the required pressure , the ORC can adapt to the low-temperature heat sources by using an organic working fluid instead of steam. L ike the conventional cycle, though, the efficiency of the simple ORC deteriorates if the temperature of the heat source is reduced or that of the condenser is increased. Various modifications to simple ORC have been considered to improve its efficiency, such as reheating, regeneration, and recuperation [3]. However , such m odifications do not address a main drawback of the cycle which is the mismat ch between the temperature of the heating source and that of the working fluid during the heat -addition process in the evaporator since the temperature of the working fluid remains constant while that of the heating source drops [4]. A newly proposed cycle that can solve this problem is the trilateral flash cycle (TFC) in which the working fluid is not allowed to vaporise during the heat recovery process , but directly taken as saturated liquid to exp and in a two -phase expander [4,5 ]. Since the technology of TFC expanders is relatively immature compar ed to the conv entional turbines , they are costlier and less efficient [3,5]. The literature on the ORC and TFC analyses is large and numerous previous researchers developed theoretical models for comparing the performance of the ORC and TFC from thermodynamics and thermoeconomic viewpoints or compared them with other cycles such as the Kalina cycle. For example, Bidgoli and Yanagihara [1 ] analy sed an ORC system for recover ing the waste heat from the intercoolers of the compression units of a large processi ng plant. By using Aspen HYSYS , they analysed the performance of various working fluids including R123, n -butane, n -pentane, hexane, and n - heptane. Their results showed that a net power of up to 40 MW could be generated with R123. Wolf et al. [2 ] investigated a solar powered ORC that uses pure iso -pentane and two iso -pentane/CO 2 zeotropic mixtures . Modelling of t he system was done by using EES and all thermodynamic properties we re determined by using REFPROP . By conducting both exergy and exergo -econom ic analyses of the investigated unit , they showed that the unit was capable of co -producing approximately 30 kW electricity and 160 kW district hea ting with a n exergetic efficiency exceeding 60%. Therefore, they concluded that the unit was able to compete with existing renewable power generating systems in terms of specific cost of electricity. 150-2 Some studies investigated the combination of ORC an d vapour-compression or vapour -absorption refrigeration cycles. For example, Sun et al. [ 6] investigated the ORC combined with the absorption refrigeration cycle (ARC) and the ejector refrigeration c ycle (ERC) to recover the waste heat from the low -temperature flue gas for generating both powe r and cooling for external uses . Their results with R113 as the ORC working fluid showed that the net power output, the refrigerating capacity and the resultant exergy efficiency of the ORC -ARC combination are all higher than those of the ORC -ERC combination for evaporation temperatures of the basic ORC exceeding 153°C. Toujani et al. [7] combined the ORC with the vapour compression refrigeration (VCR) cycle for c ogeneration applications. Three configurations were examined in terms of the net power, refrigeration capacity and overall efficiency. They used n - hexane as working fluids for the ORC and R600 for the VCR cycle . Their results obtained by using the EES soft ware show that, for a hot spring of 1000 kW, the cycle can provide simultaneously, a maximum net work of 17 kW and a maximum net cooling capacity of 160 kW and an overall coefficient of the order of 0.3. The above literature review and the recent and more comprehensive review conducted by Jiménez -García et al. [8] indicate that a cycle that combines the TFC in the high -temperature circuit (HTC) and the ORC in the low - temperature circuit (LTC) has not been considered before. While minimising the mismatch be tween the working fluid and the heat source for the ORC circuit , this cycle enables different organic fluids to be used in the HTC and LTC that suit the high and low temperature ran ges better than a single fluid and gives cogeneration systems more flexibility than a sing le ORC or TFC. The present paper contributes to knowledge by presenting a thermodynamic evaluation of this new cycle. From another perspective, the above literature review shows that most researchers used commercial software for their studies . However, the use of general -purpose software can encourage independent researchers and engineering students to contribute to the development of innovative systems for utilising low-temperature heat sources . In th is respect, the present paper uses Microsoft Excel as the modelling platform with special VBA functions to determine the thermodynamic p roperties of the working fluids . The functions are verified by comparing the estimations of the TFC model with published data for three organic working fluids , which are R134a, R1233zd, and R245fa . 2. The OR C, TFC, and the combined cycle Figure 1 shows the T-s diagrams of the simple ORC and TFC. The ORC system has the same components as those of the conventional steam -turbine power plant which are the evaporator (boiler), the turbine, the condenser, and the pump. The difference between the ORC and TFC systems is that the TFC system heats the workin g fluid without going into the vaporisation process so that the hot pressurised fluid expands in a two -phase expander. Organic fluids are used in the ORC and TFC instead of water because they have higher boiling pressures and, therefore, suit the low - tempe rature applications better. As Figure 1 .b shows, the TFC leads to a uniform temperature glide between the heat source and the working fluid which minimises the losses during the heat -transfer process and improves the performance of the system. According to [3], TFC system s can provide 50% more work than ORC system s for the same energy input, but they need sophisticated expanders to adequately handle the liquid -phase presence d uring the expansion process . Figure 2.a shows a schematic diagram of the proposed combined -cycle system in which the heat source heat s the working fluid of the TFC circuit first and then heats the working fluid in the ORC circuit. After expanding in the two - phase expander to produce power, the working fluid of the TFC circu it is condensed by the cooler working fluid of the ORC circuit in a cascade condenser . The liquefied fluid is then pumped by the TFC pump back into the heater where it is heated to the saturation temperature. The ORC system shown on Figure 2.a is a recuper ative system in which the superheated fluid exiting the turbine is used to heat the cold fluid exiting the pump in an internal heat -exchanger (IHEX). Accordingly, the initially superheated fluid enters the condenser of the ORC as saturated vapour. In the condenser, the working fluid is cooled by the cold -sink fluid to the saturated -liquid state before being pumped into the cold side of the IHEX. After the IHEX, the fluid is heated by the heat source to the state of saturated liquid before entering the casca de condenser where it is heated by the condensing fluid of the TFC circuit until it becomes saturated vapour. 150-3 (a) (b) Fig. 1: T-s diagrams of: (a) the simple ORC and (b) the simple TFC Fig. 2: Schematic and T-s diagrams of the combined cycle Figure 2.b shows the T-s diagram of the combined cycle . Unlike the TFC in which the pinch point occurs at the exit point of the heating source, that of the combined cycle occurs in the middle of the heating process which forces the heating source to exit the system at a higher temperature than that of the TFC. Although this limits the amount of energy recovered and the work produced by the ORC circuit, the cycle can still produce more power than a simple ORC because of the additional work produced by the TFC circuit. Compared to the simple TFC, the combined cycl e may not be able to produce more power , but it can have a higher thermal efficiency by replacing the two -phase expander with the single -phase turbine and by allowing two different working fluids to be used in the TFC and ORC circuits . Two fluids suit the low-temperature and high -temperature heat -transfer parts better than a single fluid and lead to a better overall performance of the system . 2 3 4 T 2 3 4 T s s ΔTpp ΔTpp 1 1 (a) (b) 8s 4s ΔTpp ΔTcc 2c 4b 1 2 3 4 T 1 6 7 5 8 s 2b G Pump Pump Condenser Heater Expander 6 7 Turbine 2 3 1 4 Cascade condenser 5 8 IHEX Heater 2b 2c 4 b Heat source Cooling water 150-4 3. Verification of the VBA functions for fluid properties The Excel -based models developed for the present analyses determine the thermal fluid properties by usin g a special Excel add -in developed with VBA [9]. The functions related to the present analyses , which are those of the refrigerants group, simply store and interpolate the data given by ASHRAE [ 10] for saturat ed liquids and saturated vapours . The specific volume of superheated refrigerants is determined by the Redlich -Kwang equation and the functions that determine the ir enthalpy and entropy use ideal -gas equations in which the specific heat is determined at an adjusted pressure by multiplying the actual pressure by a “compressibility factor” for which an average value of 0.5 is adopted [ 11]. Although this factor extends the usefulness of these functions to supercritical conditions, the accuracy of the functions needs to be verified. In what follows, the functions are verified by com paring the results of the model developed for the simple TFC with the data p rovided by Lai et al . [12] who conducted a thermo dynamics analysis of the cycle by using the REFPROP software. Lai et al . [12] fixed the temperature and the mass flow rate of heat source at 80oC and 4.16 kg/s, respectively, t he heat sink temperature to 30oC, the condenser inlet temperature to 37oC, and used an isentropic efficiency of 0.7 for the pump. Figure 3 shows the Excel -based model devel oped for the TFC using their data. Fig. 3: The Excel -aided model for the TFC using R134a with the data of Lai et al. [ 12] The sheet consists of three blocks of cells. The first block on the left side of the sheet stores the specified d ata, while the second block calculates the various temperature s, pressures, and fluid properties . The third block on the right side of the sheet determines the overall parameters that include the total amount of recovered heat (Q_TFC), the net power produced by the system (Worknet_TFC), and the overall energetic and exergetic efficiencies, η_TFC and ε_TFC, respectively. The model uses R134a as the working fluid, but the name of the fluid is stored as a variable so that the sheet can be used for other fluids. Lai et al . [12] determined the specific work of the expander by following the method described by Trædal [13]. The formula bar in Figure 3 shows the function used to determine the efficiency of the nozzle according to this method . They also fixed the operating temperature at the inlet o f the pump ( T2) at 35oC. In the present analysis , T2 is not fixed but evaluated by the model. For their analysi s, Lai et al . [12] selected four working fluids that are commonly used in low -temperature power generation systems, which are R134a, R236fa, R245fa, and R1233zd. Since R236fa is not supported by present add -in, comparison is made with their results for three fluids which a re R134 a, R245fa, and R1233zd. Figure 4 and Figure 5 compare the results obtained by Lai et al . [12] and by the present model . According to both models , R245 fa and R1233zd have comparable power and thermal efficiency, but R134a has considerably lower value s. In general, the figures show a close agreement between the estimations of the two models. 150-5 (a) (b) Fig. 4: Variations of the net power and thermal efficiency of the TFC with the evaporation temperature as given by Lai et al . [12] (a) (b) Fig. 5: Variations of the net power and thermal efficiency of the TFC with the evaporation temperature as obtained by the present model 4. Comparison of the combined cycle with the ORC and TFC Two other models for the simple ORC and for the combined cycle have been developed i n order to compare the performance of the combined cycle to those of the ORC and TFC . Figure 6 shows the model dev eloped for the combined cycle in which t he first block of cel ls on the left side stores the specified data, while the second and third blocks in the middle perform the calculations for the TFC and ORC circuits, respectively. The fourth block on the right side of the sheet determines the overall cycle parameters. The names of the working fluids in the TFC a nd ORC circuits are stored as variables so that the model can be used for different fluid pairs, but R152a is used in both circuits for the sake of comparison . In this section, t he three cycles will be compared on the basis of the data given by Yari et al. [5] for a heat source at 120oC with the data shown on Table 1. The temperature of the TFC fluid entering the expander, T6, is taken as 110oC and the temperature difference in the cascade -condenser is taken as 3oC. The temperature of the fluid in the TFC circuit in the cascade condenser is specified as 83oC so that the temperature of the fluid in the ORC circuit is 80oC, which is the optimum evapor ator temperature for the simple ORC as shown by Yari et al. [5] . Figure 6 shows the results of the combined cy cle. 7111519 65 67 69 71 73 75Net power (kW) Evaporator temperature (oC) R245fa R134a R1233zd 11.41.82.22.63 65 67 69 71 73 75Efficiency (%) Evaporator temperature (oC) R245fa R134a R1233zd 150-6 Fig. 6: Excel -aided model for the combined using R152 a with the data shown on Table 1 Table 1 : Values of the parameters used for validating the ORC model (Yari et al. [5]) Parameter Value Ths [oC] 120 hsm [kg/s] 100 ΔTpp [K] 10 ηp (%) 85 ηt [%] 85 Table 2 gathers the results obtained by the three models. The table figures show that the lowest exit temperature for the heat source is that of the TFC , which is 50oC. Both the ORC and the combined cycle produce d higher exit temperatures, but the highest exit temperature is that of the combined cycle, which is 80.24oC. Therefore, both the amounts of energy recovered and energy rejected to the cooling water in the condenser by the combined cycle are the lowest amon g the three cycles. A lower heat input means that there is more energy left in the heat source that can be recovered for cogeneration and a lower rate of heat rejection means that the capacity of the cooling system is reduced. The table figures also show t hat the combined cycle achieved the highest thermal efficiency of 9.026% compared to 7.466% for the simple TFC and 8.153% for the simple ORC. The power produced by the combined cycle is less than that of the simple TFC by 31.3%, but more than that of the simple ORC by 3.3% . Table 2 : Comparison of the thermodynamic performance of the ORC, TFC, and combined cycle TFC ORC Combined cycle Exit temperature of heat source, Thsout (oC) 50 77.41 80.24 Amount of recovered heat, Qtotal, (kW) 29708 18074.17 16872.18 Amount of rejected heat, Qout, (kW) 27489.92 16600.63 15349.28 Power produced , Wtotal, (kW) 2218.085 1473.536 1522.897 Overall thermal efficiency (%) 7.466 8.153 9.026 Overall exergetic efficiency (%) 38.83386 25.798 26.66263 150-7 5. Comparison of the combined cycle with the ORC at various cascade temperatures The comparison of the three cycles presented in the previous section did not take into consideration an important advantage of the combined cycle, which is the allowance to use different working fluids in the TFC and ORC circuits. This section analyses the performance of the combined cycle with R152a and R1234yf as a pair with the same data of Table 1 . The critical temperature of R1234fy is lower than the temperature of the heat source and Yari et al. [5] show ed that it can achieve a higher thermal efficiency in the lower -temperature range than R152a . Therefore, it is used in the ORC circuit while R152a is used in the TFC circuit. Figure 7 compares the power and thermal efficiency of the combined cycle using this fluid pair with those of R152a in both circuits and the simple ORC at various tempera tures in the cascade condenser . (a) (b) Fig. 7: Comparison of (a) the power and (b) thermal efficiency of the combined cycle using R152a/R1234yf and R152a/R152a with those of the simple ORC Figure 7 .a shows that the power of the combined cycle with both fluid pairs is higher than that of the simple ORC at low values of the cascade temperature, i.e., whe n the contribution of the TFC c ircuit is domina nt. Although the cycle’s power decreases by incre asing the cascade temperature, t he figure shows that the power of the combined cycle with the R152a/R1234yf pair remains higher than that of the R152a/R152a pair and the simple ORC over the whole range of cascade temperatures. However, Figure 7.b shows that the thermal efficiency of the combined cycle with the R152a/R152a pair remains higher than that of the R152a/R1234yf pair . While the thermal efficiency of the simple ORC increases significantly as the cascade temperature increases , Figure 7 .b shows that the efficiency of the co mbined cycle increases slowly but remains high er than that of the simple ORC for all the temperatures below 80oC, which the optimum evaporator temperature for the simple ORC. Below 80oC, both the power and thermal efficiency of the combined cycle with the two fluid pairs are higher than those of the simple ORC. 6. Conclusion This paper presents a new cycle for the utilisation of low -temperature heat sources for power generation that combines the organic Rankine cycle (ORC) and the trilateral flash cycle (TFC) . The performance of the combined cycle i s compared to those of the two basic cycles for a heat source available at 120oC. Two performance para meters are used for comparison which are the power produced by the cycle and the thermal efficiency of the cycle. The results obtained with R152a as the working fluid show that the combined cycle gives more power than the simple ORC and higher thermal efficiency . Compared to the simple TFC, the combined cycle produce s less power but achieves a higher thermal efficiency . The results also show that the exit temperature of the heating source for the combined cycle is high enough to be utilised fo r cogeneration. The models developed for the analyses of the three cycles use Excel as s modelling platform with special VBA functions to determine th e thermodynamic p roperties of the organic working fluid . The Excel -based platform enables 05001000150020002500 60 65 70 75 80 85 90Power (kW) Cascade temperature (oC) Simple ORC R152a/R152a R152a/R1234yf 45678910 60 65 70 75 80 85 90Thermal efficiency (%) Cascade temperature (oC) Simple ORC R152a/R152a R152a/R1234yf 150-8 the analytical model to be developed in a compact form. By extending the analytical model to include the economic and environmental factors , it can be used for multi-objective optimis ation analyses of the cycle [14, 15].