Thermal characteristics of supercritical CO2 gas turbine cycles integrated with renewable methanol generation systems

Energy storage is crucial for reducing the imbalances between energy demand and generation and thus an important asset to increase the share of renewable energy. This work proposes an efficient, zero-pollution, self-contained integrated energy system for energy conversion and storage. The integrated energy system comprises a supercritical CO2 gas turbine, an electrolyzer, and a methanol synthesis unit. With this system, excess renewable electrical energy is stored by producing hydrogen in the electrolyzer and converting it into methanol in the methanol synthesis unit. The renewable methanol is used as a fuel in a supercritical CO2 gas turbine system with direct combustion, to generate electrical energy and heat on demand. This paper investigates the supercritical CO2 gas turbine systems with the aim of achieving high thermal efficiency. The thermal characteristics of the supercritical CO2 gas turbine cycle are assessed and the influence of different design and operating parameters is determined. To assess the thermal characteristics of supercritical CO2 gas turbines a thermodynamic model is developed in MATLAB. To calculate the properties of CO2 and CO2/H2O mixture CoolProp is used. In an environment of recirculating CO2, the methanol is oxidized using the stored oxygen from the electrolysis. High pressure and high temperature gases (CO2/H2O mixture) expand in the gas turbine, generating mechanical energy. Later, the combustion products, H2O and CO2 are separated from the recirculating CO2 at the cold side of the regenerator and stored to be reused for fuel production. The influence of various operating conditions, such as (i) compressor outlet pressure, (ii) gas turbine inlet temperature, (iii) compressor isentropic efficiency, (iv) gas turbine isentropic efficiency, (v) regenerator temperature difference, and (vi) pressure drop in heat exchangers, is determined. The results of the investigation show that high thermal efficiency of the supercritical CO2 gas turbine (above 60 %) can be achieved. Hereby, the low compression work needed to compress the fluid near the critical point to the desired pressure is essential to achieve this high efficiency. The supercritical CO2 gas turbine is a crucial component of the integrated energy system for energy conversion and storage, which generates electricity with high efficiency and zero pollutants. The mass and energy flow balances of the supercritical CO2 gas turbine and the renewable fuel generation lead to a self-contained closed energy system with high round-trip efficiency.