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Comparison of cases B and C indicates that the recuperator increases the cycle efficiency (from 7.8% to 8.9%) but decreases the output power (from 71 KW down to 68 KW). This is explained by a higher heat source temperature at the exhaust of the evaporator, which reduces the amount of heat recovered, and the output power. In summary, in heat recovery applications, the output power, and not the efficiency should be maximized, and a recuperator will generally decrease the performance. 4.2. Comparison between working fluids This section aims at comparing the most commonly used working fluids for three typical applications: - The first application corresponds to an evaporating temperature of 85°C and to a condensing temperature of 20°C. These temperature levels are typical of a geothermal application. - The second application corresponds to an evaporating temperature of 150°C and to a condensing temperature of 30°C, which could correspond to a low-temperature solar collector. - The third application corresponds to an evaporating temperature of 280°C and to a condensing temperature of 100°C, which is typical of biomass CHP plant. Four different fluids are considered because they seem to be the most common in ORC applications (see tables 1 and 2): R134a, R245fa, n-pentane, and silicon oil. The selected silicon oil is octamethylcyclotetrasiloxane (‘D4’) and its thermodynamic properties are calculated according to Colonna’s Multiparameter Equation of State (Colonna, 2004). The simplified model introduced in section 4.1 with its parameters is used (still assuming 80% effectiveness for the recuperator). A new performance indicator, the back work ratio (BWR) is defined as the ratio between the works consumed by the pump and produced by the expander. The density at the exhaust of the expander is also considered in order to evaluate the required size of the equipment. Table 3 gives the cycle performance as a function of the fluid and of the considered application. For high temperatures, the only computed fluid is D4, the other ones being in supercritical state. Table 3 Cycle performance for 3 different applications Tcd = 20°C Tev = 85°C Tcd = 30°C Tev = 150°C Tcd = 100°C Tev = 280 °C Fluid R134a R245fa n-pentane D4 R245fa n-pentane D4 D4 Pev Pcd [bar] [bar] 29.28 5.73969 8.92 1.28839 4.16 0.62557 0.04541 0.0009533 33.79 1.80767 15.91 0.84297 0.50238 0.001985 8.04243 0.08718 ηcycle BWR 10.6% 10.8% 11.7% 2.9% 11.5% 1.6% 10.3% 0.0% 16.4% 8.0% 18.1% 3.9% 15.6% 0.1% 18.6% 2.2% ρex,exp [kg/m3] 26.2 6.775 1.803 0.007966 8.598 2.055 0.01437 0.483 Table 3 indicates that R134a and R245fa have a good comparative levels. They also show the highest back work ratio. N-pentane shows good performance for the second case but with a lower vapor density than R134a and R245fa. The low density becomes prejudicial for the Silicon Oil at low temperature: it is for example 61 times lower than the density of R245fa at a condensing temperature of 30°C, which would lead to oversized expander and condenser. 5. EXPANDER Performance of the ORC system strongly correlates with those of the expander. The choice of the machine strongly depends on the operating conditions and on the size of the system. Two main types of machines can be distinguished: the turbo and positive displacement types. Similarly to refrigeration performance at low temperaturePDF Image | Technological and Economical Survey of Organic Rankine Cycle Systems
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