Techno-economic optimization of high-temperature heat pumps for waste heat recovery

Heat Pumps will play a crucial role in decarbonizing the heat supply, however, the progressive phase-out of environmentally impactful fluids and relevant cost of investments are today limiting the spread of power-to-heat technologies, especially for supply temperatures beyond 100 °C. This paper aims to carry out a comprehensive assessment, investigating the optimal design and most relevant features in different market scenarios. Since iso-butane (R600a) has demonstrated an interesting potential for such an application, it was selected for the purpose of this paper. First, the best heat pump sizing and the contribution of each component are discussed in detail from the techno-economic perspective, through optimization of both efficiency and cost of investment highlighting the greatest impact of evaporator. Eventually, a case study is considered to investigate the viability of replacing gas-fired boilers with HPs to supply heat up to 120 °C. A sensitivity analysis is performed highlighting that the large and centralized heat generation units working almost continuously are generally more attractive. Moreover, this work establishes that when performing a sizing procedure or a sensitivity analysis, it is essential to account for the dependency of retail prices to the yearly energy consumption. The national trends of non-household retail prices are here provided for different countries. A market indicator is here defined, summarizing the market competitiveness of heat pumps vs boilers, combining the gas, and electricity cost, weighted on COP and boiler efficiency, in one single term. This indicator remained stable over the period 2017–2021, characterized by volatile energy market conditions, demonstrating the robustness of the economic conclusions of this paper. Sweden presents the best profitability (up to a net present value of 2.25 M€ for each MW of installed thermal capacity), France and Poland shows intermediate results (ca. 0.6 M€/MW), while in Italy and Germany, the worst markets, the minimum Heat Pump sizes to reach positive profits (ca 0.1 M€/MW) are respectively 1.5 MW and 4 MW.