Cogeneration versus combined cycle to enhance the feasibility of small gas turbines

Abstract

Small gas turbines (up to 40 MWe) are used in combined heat and power (cogeneration) industrial applications as an efficient way to produce both electricity and useful heat. However, due to both the high costs of fuel and the relative large investment required even for a small machine, such devices do not reach their economic feasibility, being necessary the use of incentives, as feed-in tariff systems. In this work, an organic Rankine cycle (ORC) is proposed as a way to recover the waste heat from the flue gases of the turbine, resulting in a gas turbine/ORC combined cycle. Although the electric efficiency is improved, being increased from 38.4% in CHP gas turbine to 46.3% in combined cycle configuration, the levelised cost of electricity (LCOE) increases from 80.4 /MWhe to 100 /MWhe, both well above the pool market electricity price (around 72 /MWhe, once it is levelised). In order to obtain a lower value of LCOE, different arrangements have been tested, coming up with two possible solutions. Both of them use a supercritical ORC and recover the condensation heat for cogeneration. One of the solutions (GT/RORC/HRX) uses a recuperator, in such a way that the flue gases of the gas turbine leave the heat recovery gas generator (HRGG) at high temperature, enabling a new heat recovery for cogeneration. The other solution (GT/ORC) does not include a recuperator, so flue gases leave the HRGG at 123ºC, not allowing a further heat recovery from them. The value of LCOE in the GT/RORC/HRX configuration is 63.8 /MWhe, whereas in the GT/ORC is 60.5 /MWhe, both well below the pool price. In order to select the best option, the exergy of the recovered heat has been evaluated in both configurations, resulting 12 MW in the GT/RORC/HRX arrangement and 13.25 MW in the GT/ORC. The difference is due to the higher temperature of the organic fluid (316ºC) at the inlet of the condenser in the second configuration compared to the first one (99ºC), being the condensing temperature the same (85ºC) in both arrangements. In conclusion, the conversion of all the heat recovered from the flue gases of the gas turbine into electricity is not enough to enhance the economic feasibility of the turbine. On the other hand, the use of all the recovered heat as useful heat in a pure cogeneration scheme does not allow the turbine to reach the feasibility, neither. The optimal solution is a hybrid one where an ORC is used to convert the heat from the flue gases into electricity and the useful heat is the heat released in the condenser. Using such arrangement, the overall electricity production increases from 37.8 MW in the simple cogeneration gas turbine (baseline) to 47.7 MW and the recovered heat decreases from 46.5 MW to 37 MW.