Optimizing a 1.4 MW combined heat and power system: Efficiency in the light of high-temperature heat exchanger performance

Manufacturing companies consuming significant amounts of electrical energy and heat, while simultaneously possessing renewable fuel resources, are seeking the best solutions for the future. However, relatively popular organic Rankine cycle (ORC) systems have not conquered the market in the category of small cogeneration systems. It seems that the reasons are high investment costs and relatively low efficiency of electricity production. In an effort to support the development of the Polish small and medium enterprise sector, this paper presents a thermodynamic analysis of a 1.4 MW total power combined gas–steam system, based on the Brayton cycle with an external combustion chamber and a Rankine cycle. The proposed system is intended for a company in the timber industry, hence the fuel is post-production waste wood. A parametric analysis was carried out to determine the characteristics of both component cycles to assess their contribution to the efficiency of a system, taking into account parameters of the high-temperature heat exchanger of a topping cycle as a key element influencing the entire system’s performance. Toluene is used as a working fluid in the steam Rankine cycle. The study involves the effects of excess air ratio (λ), heat transfer surface area (AHHEX) and compression ratio (rp). The obtained results point to the regenerative Brayton cycle as more efficient compared to ORC. Its efficiency increases with a decrease in λ and rp, and is predicted to reach 20% for the assumed limiting temperature of 950 °C, λ=5 and rp=3. The calculated efficiency of the combined cogeneration unit under these conditions will be 32%.