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whereas R13 and R123 had a better performance in recov- ering a low-temperature waste heat. Hung et al.11 compared the efficiencies of ORCs using cryogens such as benzene, ammonia, R11, R12, R134a, and R113 as working fluids. The system efficiency increased for dry fluids and decreased for wet fluids. The isentropic fluids achieved an approximately constant value for high turbine inlet temperatures, and isen- tropic fluids were most suitable for the low temperature of waste heat recovery. Hettiarachchi et al.4 presented a cost- effective optimum design criterion for ORCs utilizing low- temperature geothermal heat sources. They used the ratio of the total heat exchanger area to the net power output as the objective function to optimize the ORC using the steepest descent method. They observed that the choice of working fluid could greatly affect the power plant cost. Wei et al.12 considered the system performance analysis and optimiza- tion of ORCs using R245fa as a working fluid. They analyzed the thermodynamic performances of an ORC system under disturbances. They found that maximizing the use of ex- haust heat improves the system net power output. Under high ambient temperatures, the system output performance deteriorated. Dai et al.13 used a genetic algorithm as a new optimization method for waste heat recovery in ORCs. The optimum performances of ORCs with different working flu- ids were compared and analyzed under the same waste heat conditions. They compared ammonia, butane, isobutene, R11, R123, R141b, R236ea, R245ca, R113, and water. The ORC system working with R236ea had higher exergy effi- ciency under the same given waste heat condition. Dai et al.13 also proved that for the working fluids with a nonnega- tive slope of saturation vapor curves, the turbine inlet tem- perature should be kept as low as possible above the boiling point of the working fluid, and the ORC system with satu- rated vapor at the turbine inlet would produce the greatest turbine power. In this paper, R245fa was selected as a working fluid in ORC. Its basic properties are shown in Table 1. R245fa is considered a dry working fluid (dT/dS 0). For such fluids after expansion, the superheated vapor obtained is what eliminates the problem of liquid droplets in the expansion machine. Moreover, the superheating apparatus is not needed. The sensible heat resting in the expanded vapor can be used for preheating the liquid working fluid. The ORC used solar thermal energy from solar collec- tors, so the maximum temperature of the working fluid was approximately 90 °C. The temperature in the con- denser was approximately 25 °C. THE EXPANSION MACHINE Compared to water, the organic working fluids have relatively low enthalpy difference between high pressure and expanded vapor. This leads to higher mass flows compared with water. The application of larger turbines because of the higher mass flow reduces the gap losses compared with a water-steam tur- bine with the same power. The efficiency of an ORC turbine is up to 85%.14 Turbines are available mostly in a power range above 50 kWe. For smaller ORC systems, volume expansion machines are used. A very promising machine is the scroll expander, which is a modification of a compressor that is commonly used for air conditioning technologies. Scroll ex- panders are positive displacement machines with a typical volume ratio value between 2 and 3 and an isentropic effi- ciency of 75%.15 In the ORC presented in this paper, the multivane volume machine was used as an expansion machine. The mass flow of the R245fa working fluid in the cycle was 160 kg/hr. The maximum pressure in the inlet of the expan- sion machine was 950 kPa. The pressure after expansion was approximately 140 kPa. THE ORC SYSTEM PERFORMANCE The ORC with internal heat exchanger was built and tested. The system received energy from solar collectors, which were put on the roof of the building. The collector area was approximately 38 m2. The angle between the collector surface and the roof was 45°. The average energy per year was approximately 525 kWh/m2. This value was obtained from weather data ana- lyzed for the city Wroclaw (in Poland) in 2 yr of observation. The efficiency of the collectors was 0.4–0.6 and was dependent on the absorber temperature. The maximum heat input to the system was approximately 10 kW. The solar collectors with vacuum insulation were used as a heat source for the ORC system. The working medium (solar fluid) flows through copper tubes placed in vacuum glass tubes (double-wall glass tubes with vacuum between their walls). The inner wall of the glass tube was covered with an absorbent coat (aluminum nitrite) characterized by high absorption (95%) and minimum solar radiation emission (5%). The applied absorber enables the absorp- tion of diffuse radiation. The glass tubes are placed over a highly reflective compound parabolic concentrator (CPC) parabolic mirror. The mirror absorbs sunrays for which the incidence angle is unfavorable and ensures that the entire absorbent surface is used to collect solar radiation. Twenty solar collectors were connected parallel (in four batteries). In the solar installation, Tyfocor LS was used. Tyfo- cor LS is a ready-to-use, reversibly evaporative, special heat transfer fluid based on 1,2-propylene glycol for solar heating installations equipped with evacuated tubular collectors. The solar fluid flows through the hydraulic separator directly to the evaporator in the ORC system. Excess heat was utilized in a domestic hot water boiler. The maximum temperature of the solar fluid was kept near 90 °C. Figure 1 shows the scheme of the ORC system. The working fluid (R245fa) evaporates in the evaporator—the shell-tube heat exchanger. Then the vapor flows to the ex- pansion machine—the multivane volume expander. After expansion, the working fluid flows to the internal heat ex- changer (the plate type), where the liquid phase is preheated. Then the working fluid condenses in the condenser— the plate heat exchanger. The condenser is cooled in a closed water system cooperating with the cooling tower. The heat source for the ORC is a battery of 20 solar collectors. The fluid circulation in all cycles (the Table 1. Properties of R245fa. Chemical name Molecular formula Molecular weight Critical pressure Critical temperature Critical density 1,1,1,3,3-Pentafluoropropane CF3CH2CHF2 134.05 g/mol 3640 kPa 427.20 K 517.0 kg/m3PDF Image | Solar-Powered Organic Rankine Cycle Engine Performance
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