Solar ejector refrigeration systems: benefits and latest developments

As recalled by the Czech and Portuguese authors of a recent article in the International Journal of Refrigeration ⁽¹⁾, ejector refrigeration is a technology with many advantages. In addition to its very simple design and near-zero maintenance, its most decisive advantage is the possibility of using low-grade thermal energy to drive the refrigeration system, instead of electricity, as in conventional vapour compression technology. The authors mention that systems using this technology can save up to 80% of electrical energy compared to conventional vapour compression refrigeration.

 

Ejector cooling is not the only heat driven option; there are also other technologies on the market, such as absorption and adsorption systems, which are less suitable for small capacity systems with increased complexity and higher cost. Vapour compression refrigeration coupled with PV source is only competitive when there is no need for electricity storage and only because of the recent availability of very low-cost photovoltaic panels on the market.

 

One of the crucial factors affecting the performance of an ejector refrigeration system is the geometry of the ejector ⁽¹⁾. The authors have tested the influence of a variable geometry ejector (VGE) design on the performance of a small-scale, 1.5 kW nominal capacity solar heat driven ejector air-conditioning system using R600a as a refrigerant under real working conditions.

 

Since under variable operating conditions (e.g. solar radiation, ambient temperature), fixed geometry ejector performs poorly, the objective of their study was to prove the benefit of the VGE concept. In the tested VGE, the area ratio through a movable spindle (SP) and the nozzle exit position (NXP) can be adjusted in order to respond to the operating conditions. The results showed very stable operation of the cooling cycle during the experiments. Both NXP and SP had considerable influences on the cooling cycle performance. Optimal NXP and SP were identified. However, the optimum point depended strongly on the operating conditions.

 

The results clearly highlight the benefit of using the VGE design over the fixed geometry. The COP improvement was 24% with a maximum of 0.29 corresponding to a cooling capacity of 1.6 kW when compared to a fixed geometry ejector. The study also confirmed the importance of using a variable-speed pump in the ejector cycle.

 

Another factor influencing the performance of ejector systems is the choice of refrigerant. In another very recent IJR article ⁽²⁾, Chinese researchers have theoretically analysed the performance of a solar-driven ejector refrigeration system with low-GWP working fluids. The study is based on the solar radiation data in summer in Beijing and Guangzhou.

The hybrid system was composed of an ejector refrigeration system as the upper stage and a vapor compression system as the bottom stage. It was found that HFO-1234ze(Z) presented the highest COP, followed by HCFO-1233zd(E) and HC-600. These three refrigerants showed higher COP than that of HFC-245fa used as the baseline.

 

⁽¹⁾ Van Nguyen V., Varga S., Soares J., Dvorak V., Oliveira A. C., Applying a variable geometry ejector in a solar ejector refrigeration system, International Journal of Refrigeration, May 2020. Available in FRIDOC, free of charges for members.

⁽²⁾ Xuehui W., Yuying Y., Bo L., Xinyue H., Neng G., Prospect of solar-driven ejector-compression hybrid refrigeration system with low GWP refrigerants. September 2020. Available in FRIDOC, free of charges for members.