Combined Cycle for Power Generation and Refrigeration Using Low Temperature Heat Sources

Introduction

Rankine cycle, whose working fluid has a constant boiling temperature and pressure, is generally used for power generation. The cycle does not have a temperature profile that closely matches the heat source along the path of heat exchange, and this causes higher exergy destruction. Kalina (1984) developed a new cycle making use of a binary fluid (ammonia and water) that boils over a range of temperatures, enabling a closer match with the temperature profile of finite thermal capacity heat source during heat transfer, and thus reducing the exergy destruction. El-Sayed & Tribus (1985) made a theoretical comparison of the Kalina cycle with the Rankine cycle. A number of new cycles, which use low or medium temperature heat sources such as solar energy, geothermal energy or energy from the waste, were designed (Goswami et al., 2004) in order to solve the problem of energy consumption and conversion. Zheng et al (2006) proposed a combined cycle using Kalina cycle utilizing high temperature heat source. Zhang et al (2004) and Zhang & Lior (2007) proposed several novel refrigeration cogeneration systems which generate power alongside, based on ammonia-water working fluid, but can only be used with higher temperature heat resources. A combined power and refrigeration cycle was studied by Wang et al (2008) showing various parameters which affect the cycle performance significantly and the potential of optimization of combined cycles. Work by Jawahar et al (2013) shows higher performance of the generator absorber heat exchange based absorption power and cooling cycle as compared to simple cycles. A novel ammonia – water binary mixture thermodynamic cycle, capable of producing both power and refrigeration, with an advantage of a single low temperature heat source, was proposed by Goswami (Goswami., 2002; Lu & Goswami., 2003; Tamm et al., 2004). This cycle can be used either as a bottoming cycle utilizing waste heat from a conventional power cycle or as an independent cycle using low temperature heat sources such as solar and geothermal energy. Typical working conditions of 400 K boiler temperature superheated to 410 K and an ambient at 280 K yield better first law efficiency than conventional power cycles (Tamm et el., 2004). The strength of this cycle lies in improved heat source utilization. It exhibits much higher second law efficiencies than conventional power cycles. Hasan et al (2002) have done first and second law thermodynamic analysis of Goswami’s cycle using solar heat source. Vidal et al (2006) did the exergy analysis of combined cycle proposed by Goswami using Hasan et al (2002) parameters. The cycle was simulated as a reversible as well as an irreversible cycle to show the effect of irreversibility in each component.

The cycle proposed by Goswami (Goswami, 2002; Lu & Goswami, 2003; Tamm et al., 2004) is for combined power and refrigeration, with the limitation that the ratio of refrigeration capacity to power output cannot be varied to a greater extent. The cycle proposed by Goswami does not give sufficient amount of refrigeration required for fixed value of power for the present problem, even at optimum working conditions (Lu & Goswami, 2003) based on maximum refrigeration output. This is because the refrigeration potential, obtained by expanding the vapour through the turbine to low temperatures, is limited due to the required minimum dryness fraction. Due to the requirement of minimum dryness fraction of the working fluid at the turbine outlet for a fixed value of turbine outlet pressure, the refrigeration capacity is also restricted for the given mass flow rate.