The evaluation of a solar–driven aqua–ammonia diffusion absorption heating and cooling cycle

Abstract

An ever-increasing demand for energy with virtually no decline in our dependence on fossil fuels puts the world’s non-renewable resources under great pressure. This problem necessitates careful consideration of alternative methods of energy conversion and usage by investigation of novel and more sustainable methods or new applications for old ideas. Using solar thermal energy to drive an absorption cycle is one such new use for technology which has been around for almost a century. Low values for coefficient of performance (COP) and high capital cost are large drawbacks of such systems. However due to negligible operating costs the investment can be regained back quickly. There are several possible mixtures to use as working fluid of which aqua-ammonia is selected for diffusion absorption systems as well as solar-driven variants. This is due to the low temperatures that can be achieved in the evaporator for refrigeration purposes. Another factor for its use in a simulation is the availability of accurate thermo-physical property correlations. The simulation of such a cycle is required in order to prove its feasibility in a domestic setting within South Africa. After thorough investigation of cycle configurations and alternative working fluids, the study focusses on simulating a zero-order model of a solar-powered diffusion absorption cycle for refrigeration purposes. Climate data of Potchefstroom over a 22-month period is used in order to establish boundary values for temperatures and along with certain system requirements, parameters are set for the overall cycle. Varying performance and efficiency is also investigated. By following a systems-CFD approach the cycle is represented as a thermal-fluid system network of components connected by nodes to closely resemble reality. Where previous studies have neglected zero-order requirements or adequately accounted for irreversibility’s, here the conservation equations have been applied with the most terms used and the least amount of assumptions made in order to achieve the greatest accuracy. Several steps are followed in order to evaluate the cycle as the title suggests. The diffusion absorption refrigerator (DAR) cycle performance is evaluated when using helium or hydrogen as auxiliary gas. A slight increase in COP is found when using helium, but it is not sufficient to justify the cost. A secondary simulation of an alternate dual-pressure cycle using a pump is done as feasibility comparison with the same parameters as the diffusion cycle. It was found that the second cycle is not acceptable due to high evaporator temperatures needed to ensure liquid enters the pump instead of partially evaporated solution. This would greatly increase the work input required for what essentially becomes a compressor. Optimisation of the DAR is evaluated by simulating the use of a rectification column and the effects of different design points on overall performance. Meteorological data for Potchefstroom, South Africa is used to perform a yearly analysis on the simulated cycle and to specify a suitable design point. The use of a radiative cooling system as heat sink for the system is then investigated and incorporated into the system model. Finally, the performance characteristics of the simulated DAR cycle are discussed, verified and compared with available data from similar research. It is shown that a 40% solution aqua-ammonia-hydrogen cycle driven by 526 kW of solar thermal energy at 130°C and a system pressure of 1.5 MPa can easily achieve a COP over 0.4 with an air-cooled absorber at 40°C and a water-cooled condenser at 35°C. A 231 kW refrigeration capacity at an average evaporator temperature of –20°C is achieved, satisfying the requirements for a domestic refrigeration system.