Optimum Size of ORC Cycles for Waste Heat Recovery in Natural Gas Compressor Stations

Abstract

The paper investigates the optimum size and potential economic, energetic and environmental benefits of ORC applications, as bottomer section in natural gas compressor stations. Since typical installations consist of multiple gas turbine units in mechanical drive arrangement, operated most of the time under part-load conditions, the economic feasibility of the ORC can become questionable even though the energetic advantage is indisputable. Depending on mechanical drivers profile during the year the optium size of the bottomer section must be carefully selected in order not to overestimate its design power output. To achieve this goal a numerical optimization procedure has been implemented in the Matlab environment, based on the integration of a in house-developed calculation code with a commercial software for the thermodynamic design and off-design analysis of complex energy systems (Thermoflex). Thus the optimal ORC design power size is identified in the most generic scenario, in terms of compressors load profile, installation site conditions (i.e. ambient conditions and carbon tax value) and gas turbine models used as drivers. Two different objective functions are defined aiming at maximize the CO2 savings or the net present value. Different case studies are shown and discussed to prove the potential of the developed code. The comparison among the case studies highlights, chiefly, the influence of yearly mechanical drivers profile, part-load control strategy applied and carbon tax value on the ORC techno-economic feasibility.