Abstract
Low-grade heat is abundantly available as solar thermal energy and as industrial waste heat. Non concentrating solar collectors can provide heat with temperatures 75–100 °C. In this paper, a new system is proposed and analyzed which enhances the electrical coefficient of performance (COP) of vapour compression cycle (VCC) by incorporating low-temperature heat-driven ejectors. This novel system, ejector enhanced vapour compression refrigeration cycle (EEVCRC), significantly increases the electrical COP of the system while utilizing abundantly available low-temperature solar or waste heat (below 100 °C). This system uses two ejectors in an innovative way such that the higher-pressure ejector is used at the downstream of the electrically driven compressor to help reduce the delivery pressure for the electrical compressor. The lower pressure ejector is used to reduce the quality of wet vapour at the entrance of the evaporator. This system has been modelled in Engineering Equation Solver (EES) and its performance is theoretically compared with conventional VCC, enhanced ejector refrigeration system (EERS), and ejection-compression system (ECS). The proposed EEVCRC gives better electrical COP as compared to all the three systems. The parametric study has been conducted and it is found that the COP of the proposed system increases exponentially at lower condensation temperature and higher evaporator temperature. At 50 °C condenser temperature, the electrical COP of EEVCRC is 50% higher than conventional VCC while at 35 °C, the electrical COP of EEVCRC is 90% higher than conventional VCC. For the higher temperature heat source, and hence the higher generator temperatures, the electrical COP of EEVCRC increases linearly while there is no increase in the electrical COP for ECS. The better global COP indicates that a small solar collector will be needed if this system is driven by solar thermal energy. It is found that by using the second ejector at the upstream of the electrical compressor, the electrical COP is increased by 49.2% as compared to a single ejector system.
Highlights
Vapour compression cycle (VCC) is a commonly used technology for cooling and air conditioning [1]
The simple ejector refrigeration system has the issue of stability when used with solar thermal energy [7] and to solve this, many researchers have studied a system combining ejector refrigeration and vapour compression cycle [8]
After P10 has been determined, the enthalpies at state 11 and 12 are calculated by using working fluid properties based on the saturation pressure at P10 (P10 = P11 = P12) which are directly entered in the Engineering Equation Solver (EES) code
Summary
Vapour compression cycle (VCC) is a commonly used technology for cooling and air conditioning [1]. Wang et al [12] reported experimental results on an EERS for an automobile case such that the exhaust of automobile supplied waste heat and the compressor was run by engine power They designed the system such that it could operate in three modes: (i) as conventional VCC, (ii) as an ERS, (iii) as a hybrid-EERS. In this research, a new system is proposed which extracts heat from the waste stream at two levels; in high pressure (HP) generator and in low pressure (LP) generator such that the heat still available in the waste stream released by an HP generator is used in an LP generator to drive an LP ejector This novel ejector enhances the vapour compression cycle (EVCC) is described .
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