Abstract
Ejector refrigeration is a promising technology for the integration into solar driven cooling systems because of its relative simplicity and low initial cost. The major drawback of such a system is associated to its relatively low coefficient of performance (COP) under variable operating conditions. In order to overcome this problem, an advanced ejector was developed that changes its geometrical features depending on the upstream and downstream conditions. This paper provides a short overview of the development process and results of a small cooling capacity (1.5 kW) solar driven cooling system using a variable geometry ejector. During the design steps, a number of theoretical works have been carried out, including the selection of the working fluid, the determination of the geometrical requirements and prototype design. Based on the analysis, R600a was selected as working fluid. A prototype was constructed with two independent variable geometrical factors: the area ratio and the nozzle exit position. A test rig was also assembled in order to test the ejector performance under controlled laboratory conditions and to elaborate a control algorithm for the variable geometry. Ejector performance was assessed by calculation of cooling cycle COP, entrainment ratio and critical back pressure. The results show that for a condenser pressure of 3 bar, an 80% increase in the COP was obtained when compared to the performance of a fixed geometry ejector. Experimental COP values varied between 0.4 and 0.8, depending on operating conditions. Currently the cooling cycle is being integrated into a solar driven demonstration site for long term “in situ” assessment.
Highlights
Most worldwide existing cooling systems are driven by electricity
Ejectors are simple in construction, have a low initial cost, and can be operated using a wide range of refrigerants when compared to alternatives such as an absorption refrigerator
The results indicated that a one-degree increase in the condenser decreased the corresponding area ratio about 5.3% on average
Summary
Most worldwide existing cooling systems are driven by electricity. Their increase over the last years is so dramatic that, in many European countries the peak of electricity consumption is switching from winter to summer. An ejector with fixed geometry only works with good performance in a rather limited range of operating temperatures/pressures (near design condition). The sensitivity of rA on the downstream conditions was again the FIGURE 3 | The effect of generator temperature on the entrainment ratio for the six selected working fluids and for constant Te = 10°C and Tc = 35°C. highest for water. The reader is referred to Varga et al (2013c) Based on this analysis, it was concluded that an ejector using R600a can operate with good performance for a wide range of operating conditions under moderate pressures, and can be an attractive choice for a small capacity air conditioner running on solar thermal energy, as long as special attention is paid to the fluid flammability.
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