Solar Cooling Using Variable Geometry Ejectors

Solar heat driven cooling systems are an attractive concept. The need for cooling is associated with high ambient temperatures. Ejector based cooling systems have been in existence for some time but have not gained widespread use due to their low performance and difficulty of control. Furthermore, although ejectors are well suited to steady state operation, they do not couple well to varying solar conditions. However, ejectors offer a robust and reliable design which demonstrates flexibility to a range of refrigerants and most importantly, a low electrical power requirement. Variable geometry ejectors help to overcome the present objections of ejectors by taking advantage of cooler ambient conditions and allowing continued operation at elevated ambient temperatures. This paper describes and compares performance maps generated by fixed and variable geometry ejectors and hints at control strategies for each. INTRODUCTION The ejector principle has been known for over 100 years, originally developed for evacuating air from condensors of steam engines. Low pressure steam was extracted from a boiler to power the ejector which then produced a moderate vacuum. It was a natural progression for steam based ejectors to be used to draw vapour from an evaporator and thus produce cooling effect. Ejectors were typically applied to cooling applications when there was a ready source of low pressure steam and were commonplace in ships and hotels from around 1910 to 1930. They typically provided cooling with no maintenance over extended periods which made them popular. The development of chlorinated fluorocarbon refrigerants in the 1930s allowed electrically driven heat pumps to be deployed to locations such as houses where a steady source of heat may not have been available but electricity was. The superior thermal performance and safe operation of heat pumps were other notable benefits. Heat pump cooling systems dominate the air-conditioning market to the present day. However, concerns over electricity network peak loading, ozone depletion and high