Power-to-Heat-to-Storage: A Technology Comparison

Abstract

The transition to renewable energy sources, such as wind power, photovoltaics, or hydropower, results in a stronger focus on electricity also for heat generation. This focus increases the need for efficient power-to-heat processes, in particular in industry. However, the time-dependent heat demand is often in conflict with the fluctuating electricity supply from renewable sources. This problem can be solved by the integration of thermal energy storage. However, the efficient integration of thermal energy storage and highly efficient power-to-heat processes, such as heat pumps, is still an open question, in particular, for high-temperature applications. This work compares three power-to-heat processes: electrical heater, vapor-compression heat pump, and inverse-Brayton heat pump, and their combination with various storage technologies. The analysis is based on thermodynamic process modeling. Assessment criteria are typical thermodynamic parameters such as exergetic efficiency, but also technological aspects. The technological aspects include technical limits and challenges in the combination of power-to-heat processes and thermal storage. At this, typical challenges stem from realizing an efficient heat transfer from power-to-heat process to storage. Key factors are, e.g., whether additional heat transfers to and from secondary fluids are necessary as well as the number and types of phases of the fluids/materials involved. Our study shows the highest power-to-heat exergetic efficiencies for the vapor-compression heat pump. However, the vapor-compression heat pump is also subject to the most severe limitations that are due to the working fluid. Also, integration with thermal storage is more challenging for vapor-compression heat pumps than for other power-to-heat processes. Combining a vapor-compression heat pump and thermal storage usually requires an additional heat transfer that produces further exergy losses and decreases the power-to-heat-to-storage exergetic efficiency. Thus, the most efficient power-to-heat-to-storage technology is case-specific and can be identified by the presented analysis method based on fundamental thermodynamic and technological aspects.