• Energy Research

Abstract An Organic Rankine Cycle (ORC) model is presented in this paper to easily, quickly, and inexpensively evaluate the performance potentials of various working fluids. When given molecular structure of working fluid, the normal boiling temperature, critical properties, liquid density and ideal gas heat capacity can be obtained via the existing group contribution methods (GCMs). Other properties required in the ORC model are calculated out by the thermodynamic relationships with the estimated properties of GCMs. Based on the calculated properties, four basic processes of the ORC including compression, evaporation, expansion and condensation are modeled. Meanwhile, the cycle parameters of 21 potential working fluids for typical ORC operating conditions are obtained from the molecular structures by the developed model. Compared with the REFPROP, the model shows sufficient accuracy for engineering purposes. The relative errors of thermodynamic properties and cycle parameters are less than 10% for most of working fluids. It is concluded that the proposed model can estimate the ORC characteristics of any pure working fluid only based on its molecular structure. Thus, a large amount of working fluids formed by the combination of groups can be directly screened by this model, and the optimal working fluids can be identified for a quick assessment in engineering field.

Abstract An Organic Rankine Cycle (ORC) model is presented in this paper to easily, quickly, and inexpensively evaluate the performance potentials of various working fluids. When given molecular structure of working fluid, the normal boiling temperature, critical properties, liquid density and ideal gas heat capacity can be obtained via the existing group contribution methods (GCMs). Other properties required in the ORC model are calculated out by the thermodynamic relationships with the estimated properties of GCMs. Based on the calculated properties, four basic processes of the ORC including compression, evaporation, expansion and condensation are modeled. Meanwhile, the cycle parameters of 21 potential working fluids for typical ORC operating conditions are obtained from the molecular structures by the developed model. Compared with the REFPROP, the model shows sufficient accuracy for engineering purposes. The relative errors of thermodynamic properties and cycle parameters are less than 10% for most of working fluids. It is concluded that the proposed model can estimate the ORC characteristics of any pure working fluid only based on its molecular structure. Thus, a large amount of working fluids formed by the combination of groups can be directly screened by this model, and the optimal working fluids can be identified for a quick assessment in engineering field.

Abstract This paper experimentally investigated the thermal performance of the developed solar box cooker with different reflector configurations. All tests were carried out under Bahir Dar (Ethiopia) prevailing weather condition. Two standard tests, namely stagnation test and sensible heat test were adopted in order to evaluate the thermal performance of the cooker. The highest maximum plate temperature of 148.7 °C can be achieved by the cooker with tracking reflector at optimal angle. In order to evaluate the performance of the cooker, the following performance indexes were employed, thermal efficiency, cooking power and two figures of merit, namely the ratio of optical efficiency to heat loss factor (F1) and the product of heat exchange efficiency factor and optical efficiency (F2). According to the experimental data, the highest computed value F1 of 0.154°C m2/W, can be obtained from the cooker with tracking reflector at optimal angle. The thermal efficiencies achieved by Cooker I, II and III are 37.24%, 32.27% and 33.89%, respectively. Thermal efficiency of the cooker was improved by 9.6% by tracking reflector at optimal angle instead of with fixed reflector. Furthermore, the thermal efficiency was also improved by 15.4% by using aluminum cooking vessel in place of using stainless steel vessel. It can be concluded from the experiments that effective utilization of all solar radiation intercepted on the reflector, and the use of cooking vessel having a reasonably good thermal diffusivity contribute significant roles to improve the cooking performance of the cooker.

Abstract This paper experimentally investigated the thermal performance of the developed solar box cooker with different reflector configurations. All tests were carried out under Bahir Dar (Ethiopia) prevailing weather condition. Two standard tests, namely stagnation test and sensible heat test were adopted in order to evaluate the thermal performance of the cooker. The highest maximum plate temperature of 148.7 °C can be achieved by the cooker with tracking reflector at optimal angle. In order to evaluate the performance of the cooker, the following performance indexes were employed, thermal efficiency, cooking power and two figures of merit, namely the ratio of optical efficiency to heat loss factor (F1) and the product of heat exchange efficiency factor and optical efficiency (F2). According to the experimental data, the highest computed value F1 of 0.154°C m2/W, can be obtained from the cooker with tracking reflector at optimal angle. The thermal efficiencies achieved by Cooker I, II and III are 37.24%, 32.27% and 33.89%, respectively. Thermal efficiency of the cooker was improved by 9.6% by tracking reflector at optimal angle instead of with fixed reflector. Furthermore, the thermal efficiency was also improved by 15.4% by using aluminum cooking vessel in place of using stainless steel vessel. It can be concluded from the experiments that effective utilization of all solar radiation intercepted on the reflector, and the use of cooking vessel having a reasonably good thermal diffusivity contribute significant roles to improve the cooking performance of the cooker.

A theoretical study on a combined power and ejector refrigeration cycle using zeotropic mixture isobutane/pentane is carried out. The performances of different mixture compositions are compared. An exergy analysis is conducted for the cycle. The result reveals that most exergy destruction happens in the ejector, where more than 40% exergy is lost. The heat exchange in generator causes the second largest exergy loss, larger than 28%. As the mass fraction of isobutane changes ranges from 100% to 0%, the relative exergy destruction of each component is also changing. And mixture isobutane/pentane (50/50) has the maximum exergy efficiency of 7.83%. The parametric analysis of generator temperature, condenser temperature and evaporator temperature for all the mixtures shows that, all these three thermodynamic parameters have a strong effect on the cycle performance.

A theoretical study on a combined power and ejector refrigeration cycle using zeotropic mixture isobutane/pentane is carried out. The performances of different mixture compositions are compared. An exergy analysis is conducted for the cycle. The result reveals that most exergy destruction happens in the ejector, where more than 40% exergy is lost. The heat exchange in generator causes the second largest exergy loss, larger than 28%. As the mass fraction of isobutane changes ranges from 100% to 0%, the relative exergy destruction of each component is also changing. And mixture isobutane/pentane (50/50) has the maximum exergy efficiency of 7.83%. The parametric analysis of generator temperature, condenser temperature and evaporator temperature for all the mixtures shows that, all these three thermodynamic parameters have a strong effect on the cycle performance.

Abstract Overall energy efficiency remains a large and unexploited resource in combined cooling, heating, and power systems driven by solar energy. This study aims to explore the configuration effects on such systems using parabolic trough collector and organic Rankine cycle (ORC) technologies. The configurations are concisely clarified into sequential and parallel connections, in which a single-effect absorption chiller and a heat exchanger are considered for cooling and heating, respectively. A comprehensive assessment framework is proposed by establishing the thermodynamic performance, system size, and economic models. Under reasonable thermodynamic boundary conditions, the optimal operational parameters are obtained via a Pareto frontier solution for such a system, with an ORC of 200 kW. Promising technical solutions and enhancement potential are justified and quantified by means of system simulations and comparison on the annual time scale.

Abstract Overall energy efficiency remains a large and unexploited resource in combined cooling, heating, and power systems driven by solar energy. This study aims to explore the configuration effects on such systems using parabolic trough collector and organic Rankine cycle (ORC) technologies. The configurations are concisely clarified into sequential and parallel connections, in which a single-effect absorption chiller and a heat exchanger are considered for cooling and heating, respectively. A comprehensive assessment framework is proposed by establishing the thermodynamic performance, system size, and economic models. Under reasonable thermodynamic boundary conditions, the optimal operational parameters are obtained via a Pareto frontier solution for such a system, with an ORC of 200 kW. Promising technical solutions and enhancement potential are justified and quantified by means of system simulations and comparison on the annual time scale.

Abstract U-type evacuated glass tubular solar collectors are increasingly used for solar-assisted heat pump system for space heating, and it is an ideal choice as an evaporator for this system. The temperature of the working fluid in such kind of collectors is commonly lower than that of the ambient air. However, it was rarely reported in literatures. In this paper, the thermal performance of the U-type evacuated glass tubular solar collector is studied experimentally under conditions where the temperature of the working fluid is lower than that of ambient air. The collector which consists of 16 U-type evacuated glass tubes is tested. Thermal efficiencies of such solar collector are obtained for the typical conditions with the solar irradiance of 375 W/m 2 , 435 W/m 2 , 535 W/m 2 , 675 W/m 2 and 735 W/m 2 generated by a solar simulator, and the mass flow rate of 0.02 kg/s, 0.04 kg/s, 0.06 kg/s, 0.08 kg/s, 0.1 kg/s. The results suggest that the thermal efficiency is higher at negative reduced temperature and it is positively correlated with mass flow rates and solar irradiances, as well as negatively correlated with inlet temperatures. Additionally, taking into account the error effects, the lower solar irradiance is, the higher sensitivity to changes of the mass flow rate and the solar irradiance efficiency changes will have. The results could be a guideline for the application of U-type evacuated glass tubular solar collector in solar-assisted heat pump system for space heating.