Energy, exergy and exergoeconomic analysis and optimisation of the scale-up of a combined ammonia-water absorption pilot plant producing electricity and refrigeration

A low-temperature heat driven ammonia-water combined absorption cycle is studied in this paper from a thermodynamic and exergoeconomic point of view. The work is based on an absorption chiller prototype to which a partial admission turbine has been added in parallel to the cooling production line for the production of electricity. First, the performance of the pilot plant is analysed in the design point and in a base case characterised by a hot source temperature of 100 degrees C, intermediate source temperature of 25 degrees C and cold source temperature of 10 degrees C. Exergoeconomic performance is assessed for two different cost of the fuel thermal input. Being the small scale of the plant very penalising, the scale-up of the plant to a size 25 times bigger is evaluated. Unit cost of products in this case is strongly reduced, mainly because of the constant cost and increased efficiency of the turbine. Parametric studies are carried out on the temperature of the sources and on the size of components to assess how these parameters influence the performance of the cycle. Finally, an optimisation aiming at minimizing the unit cost of products is performed on the size of heat exchangers for the base case working point and fixing constant the size of the turbine. The optimised cycle layout allows reaching a unit cost of produced cooling of 10.14 $/GJ (0.036 $/kWh) and a cost of produced electricity of 40.45 $/GJ (0.145 $/kWh) if the cost of the thermal input is neglected. When a thermal input cost of 15 $/GJ is considered, the calculated cooling and electricity cost are 66.7 $/GJ (0.24 $/kWh) and 106.8 $/GJ (0.384 $/kWh) respectively. In both cases the optimal vapour split ratio between cooling and power production lines is found to be around 0.47.