Integration of a flue gas condensing heat pump within a combined cycle: Thermodynamic, environmental and market assessment

Although considered as the bridging technology to a decarbonized economy, in the current energy market scenario Combined Cycle Gas Turbine (CCGT) must deal with more demanding requirements of efficiency and flexibility, and often they are not profitable enough to avoid mothballing or closure. The purpose of this work is to propose an innovative solution to enhance the plant flexibility, integrating a combined heat and power CCGT with a flue gas condensing heat pump. The paper firstly evaluates the thermodynamic performances of two possible plant layouts coupled with a District Heating Network, illustrating the respective advantages and disadvantages under different operating conditions. Then, in order to assess the viability of the investment the best solution is selected and a thermoeconomic analysis is performed using as benchmark the same CCGT integrated with a natural gas fed Heat Only Boiler in, a cheaper state of art solution but with a higher carbon footprint. Different energy market scenarios are assessed, considering the effect of different parameters on the integrated CCGT's final profitability. On the environmental side, the heat pump allows reaching an higher global efficiency (average annual efficiency increase up to 5 percentage points), and reducing the annual CO2 emission (-13.9% in the best case). On the economic side, the higher heat pump capital expenditure, with respect to the heat only boiler limits the field of viability of the investment to high gas to electricity price ratios. In particular, the minimum ratio between the cost of natural gas and the average cost of electricity that make the solution viable was found to be between 0.7 and 0.8, a ratio envisaged in case of large RES production share in the grid network. The proposed layout can be of interest for already existing or new CCGT coupled with an heat demand in the temperature range (80-150 degrees C) as a power to heat solution able to increase the overall system efficiency.

Automated summary

This research aims to propose an innovative solution to enhance the flexibility of CCGT plants by integrating them with a flue gas condensing heat pump. The heat pump increases overall efficiency and reduces CO2 emissions, but its higher capital expenditure limits the viability of the investment, only becoming feasible when the ratio between natural gas prices and electricity prices is high.