Solar-driven refrigeration systems with focus on the ejector cycle

University dissertation from Stockholm : KTH

Abstract: Interest in utilizing solar-driven refrigeration systems for air-conditioning or refrigeration purposes has grown continuously. Solar cooling is com-prised of many attractive features and is one path towards a more sus-tainable energy system. Compared to solar heating, the cooling load, par-ticularly for air-conditioning applications, is generally in phase with solar radiation.The objective of this thesis is to establish a fundamental basis for further research and development within the field of solar cooling. In this thesis, an overview of possible systems for solar powered refrigeration and air-conditioning systems will be presented. The concept of the ‘Solar Cool-ing Path’ is introduced, including a discussion of the energy source to the collector, and choice of cooling cycle to match cooling load. Brief infor-mation and comparisons of different refrigeration cycles are also pre-sented.The performance of solar cooling systems is strongly dependent on local conditions. The performance of a solar divan air-conditioning system in different locations will therefore be compared in this thesis. Solar cooling systems can be efficiently operated in locations where sufficient solar ra-diation and good heat sink are available.A solar-driven ejector refrigeration system has been selected as a case study for a further detailed investigation. A low temperature heat source can be used to drive the ejector refrigeration cycle, making the system suitable for integration with the solar thermal collector. Analysis of the solar driven ejector system is initiated by steady state analysis. System performance depends on the choice of working fluid (refrigerant), oper-ating conditions and ejector geometry. Results show that various kinds of refrigerants can be used. Also, thermodynamic characteristics of the re-frigerant strongly influence the performance of the cycle. An ejector re-frigeration cycle using natural working fluids generates good perform-ance and lower environmental impact, since traditional working fluids, CFC’s and HFC’s are strong climate gases. Further on, exergy analysis is used as a tool in identifying optimum operating conditions and investi-gating losses in the system. Exergy analysis illustrates that the distribu-tion of the irrervsibilities in the cycle between components depends strongly on the working temperatures. The most significant losses in the system are in the solar collector and ejector. Losses in the ejector pre-dominate over total losses within the system.In practice, the cooling load characteristic and solar radiation are not constant. Therefore, a dynamic analysis is useful for determining the characteristics of the system during the entire year, and dimensioning the important components of the solar collector subsystem, such as storage tanks.The final section of the thesis will deal with the ejector, the key compo-nent of the ejector refrigeration cycle. Characteristics of the actual ejector are shown to be quite complicated and its performance difficult to de-termine solely through theoretical analysis. Suggested design procedures and empirical equations for an ejector are offered in this thesis. Prelimi-nary test results for one fixed ejector dimension using R134a as the re-frigerant are also included.