A Theoretical and Experimental Study of HFE-7000 in a Small Scale Solar Organic Rankine Cycle As a Thermofluid

Abstract

Renewable energy technologies and sources have been playing a key role in reducing reliance on fossil fuels and significantly reducing CO 2 emissions and its footprint. EU initiative of generating 50% of the energy needs through sustainable sources by 2050 needs a direct response in terms of providing applied solutions to realize this target on time. Solar energy is one of the major and abundantly renewable energy sources which are free and clean. Solar energy can be utilized by means of solar Photovoltaic (PV) or solar collectors. Concentrating solar collectors supply thermal energy from medium to high grade where as non-concentrating collectors (flat plate) delivers low-grade thermal energy. The use of thermofluids with boiling temperatures lower than the water, allows the operation of low grade solar thermal systems on an Organic Rankine Cycle (ORC) to generate both mechanical and heat energy. At the same time, the selection of appropriate thermofluid is an important process and has a significant effect both on the system performance and the environment. Conventional thermofluids such as Chlorofluorocarbons (CFCs) and Hydrochlorofluorocarbons (HCFCs) have high ozone depletion (ODP) and high global warming (GWP) potential. It is therefore important to investigate novel and environmentally friendly thermofluids to address environmental impacts as global warming and ozone layer depletion. Hydrofluoroethers (HFEs) are non-ozone depleting substances and they have relatively low GWP. Therefore, HFEs can be used as a replacement for CFCs and HCFCs. In this study, a solar ORC is designed and commissioned to use HFE 7000 as a thermofluid. The proposed system consists of a flat-plate solar collector, a vane expander, a condenser and a pump where the collector and the expander are used as the heat source and prime mover of the cycle respectively. The performance of the system is determined through energy analysis. Then, a mathematical model of the cycle is developed to perform the simulations using HFE-7000 at various expander pressure. Experimental data indicated that the efficiency and the net mechanical work output of the cycle was found to be 3.81% and 135.96 W respectively. The simulation results showed that increasing the pressure ratio of the cycle decreased the amount of the heat that is transferred to HFE 7000 in the collector due to the increased heat loss from the collector to the environment. Furthermore, net output of the system followed a linear augmentation as the pressure ratio of the system increased. In conclusion, both the experimental and theoretical research indicates that HFE 7000 offers a viable alternative to be used efficiently in small scale solar ORCs to generate mechanical and heat energy.

Application of the Multi-Vane Expanders in ORC Systems—A Review on the Experimental and Modeling Research Activities

“…The exergy analysis showed that the expander is the second-largest source of exergy losses in the system. Further experimental analysis and modeling results related to this expander were presented in [85]. Modeling results indicated that the pressure of the gas at the inlet to the expander has a significant impact on the efficiency of the ORC system and the efficiency of the solar collector.…”

“…At Bournemouth University (England), ORC systems adopting multi-vane expanders and driven by solar heat were tested. The research results were published in [84][85][86]. In [86], the results of experimental tests on an ORC test stand adopting a modified multi-vane pneumatic motor featuring a maximum power of 800 W and a maximum rotational speed of 4000 rpm were presented.…”