ejection system, low-grade heat, working fluid
Selection of the working fluid for the refrigeration or solar air-conditioning system is the crucial problem because of the strong influence of the thermodynamic fluid properties on the system efficiency. Moreover, the working fluid should fulfil the environmental criteria such as zero ODP (Ozone Depletion Potential) and as low GWP (Greenhouse Warming Potential) as possible. Therefore the natural fluids are thought as the best option. From presented in literature analysis it can be concluded that isobutane is the best option as a working fluid for ejection system in application to air-conditioning since it offers the highest COP. Other important issue in ejection cycle analysis is the operating parameters. Thermal energy use as the motive energy for ejection cycles makes this cycle comparable with absorption system. In absorption system the temperature of the heat source tg in most cases exceeds 100 C. With decreasing of the generator (motive) temperature the efficiency of absorption system also decreases due to vanishing of the difference of strong and weak solutions concentrations. Therefore temperature tg < 80 C can be considered as the minimum value at which the absorption system can still operate. In contrast, for ejection systems for most of working fluids temperature of the motive fluid at the level of 100 C is not required. Also, there is no physical limitation for operation of the ejection system at lower temperatures. Therefore the operation of the ejection cycle below tg < 80 C can be considered as very attractive and in this range of the motive temperatures the ejection cycles becomes truly competitive in comparison with the absorption refrigeration systems. The paper deals with numerical analysis of operation of the ejection systems driven by low grade heat. The aim of the paper is comparison of the operating parameters of the system for natural refrigerant isobutane with other popular in air-conditioning systems refrigerant R134a and one of the most promising fluid from HFO group HFO1234. Three types of HFO1234 has been investigated, such as 1234yf, 1234ze(E) and 1234ze(Z). Analysis shows the promising feature of the HFO1234 applied in ejection system. i.e. geometric similarity of ejectors designed for isobutane and HFO1234. This proves that HFO1234 can be substitution of isobutane due to its flammable and explosive conditions.