Effect of cooling charge air on the gas turbine performance and feasibility of using absorption refrigeration in the “Kelanitissa” power station, Sri Lanka

Abstract: One of the drawbacks of the gas turbine is that performance drops rapidly when ambient air temperature increases. This is a major drawback for gas turbines operated in a tropical country like Sri Lanka. In Colombo, commercial capital of Sri Lanka where this study was carried out, the ambient temperature typically varies between 25 0C and 32 0C.   The Kelanitissa gas turbine plant has single shaft gas turbines (GE MS5001 R) operated in open cycle which use diesel as fuel (designed for dual fuel) at a designed heat rate of 13,980 kJ/kWh and an electrical efficiency of 25.8%. The designed exhaust temperature is 513 0C.   In this study, Kelanitissa gas turbine unit was used for assessment of the performance with the changes in ambient air temperature. Two approaches were used to study this phenomenon. Firstly, the performance parameters were calculated by using actual data acquired by the operation history of the power plant. Secondly, the performance was analyzed using thermodynamic principles. Then results of the two approaches were compared.   The present performance values of the studied gas turbine, when compared to designed values, showed a very poor performance due to predominantly high ambient air temperatures. Originally designed for an efficiency of 25.8%, the maximum efficiency achieved at 33 0C was only 21.2%. This translates into a 4.6 %-point reduction in efficiency at 33 0C ambient temperature.   Estimated cooling load for the proposed inlet air cooling is 679.87 RT. Cost per unit cooling load of the reference 2-stage direct-fired absorption system is $751-721 (according to 600RT-700RT). For the worst case scenario the value of $751 per RT and exhaust system constituting 98% of the cost of a market-ready direct fired system (Broad Inc., 2008) can be used. This results in $736/RT as the cost for an absorption chiller system driven by exhaust heat. Total cost for the 679.87 RT system is $ 500,370.72 (Rs. 65Mn).   Payback period of the project is 11 years but the present value after 19 years is exceeding the project cost. Present value for 19 years is Rs. 65.86 Mn. Bringing down the temperature from an average of 270C to the ISO value of 15 0C would give Rs. 6 Mn of annual savings.