Techno-economic analysis of power‑to‑heat‑to‑power storage for a residential building

Sammanfattning: Despite the share of renewable energies worldwide is increasing, which can help in reducing the CO2 emissions, their unpredictability has become a problem due to the mismatch between generation and demand. Among the different alternatives to solve this problem, energy storage is a very interesting solution. Depending on the aim of the storage, there are two types: intra‑day or seasonal. The former corresponds normally to a highly efficient and high‑cost storage, such as the Li‑ion; whilst the latter is a low efficient and low‑cost storage. An example of this second type of storage is the power‑to‑heat‑to‑power storage, whose efficiency is mainly determined by the heat‑to‑power converter, and it can be increased if the waste heat produced in the converter is reused in a combined heat and power system.Given that the residential sector represents a large amount of the global energy use (both electricity and heat), this study has considered a power‑to‑heat‑to‑power storage in a fully electrified residential building with a PV installation in order to increase self‑consumption and reduce the cost. Both the heating, electricity and cooling demand are supplied by the system.As this storage technology is currently under an early stage of development, this project aims to understand the main challenges for this storage and the advantages over a very well settled technology, such as the Li-ion. In order to achieve this objective, a model has been created in Matlab. A parametric study has been conducted in which optimum sizing of the components for several scenarios have been considered, as a means to identify the most important parameters that could hinder the feasibility of the power‑to‑heat‑to‑power storage system.From the optimization it was concluded that the scenarios with a thermally driven heat pump for cooling, resulted in larger installations leading to higher cost due to the low coefficient of performance. Regarding the other scenarios which consider an electrical heat pump for cooling, this technology can surpass the Li-ion performance for heat‑to‑power efficiencies over 20 %. In these cases, the feasibility is clearly hindered by the cost per energy capacity, which must be below 5 €/kWh and could be achieved with silicon; and the cost per power capacity that must be around 300 €/kW. An example of a heat‑to‑power converter could be the TPV technology which is a solid‑state converter, whose efficiency is currently around 30 % and is expected to reduce its cost up to 300 €/kW. In smaller systems, in which the stand‑by heat losses have more impact over the system’s feasibility due to the larger surface to volume ratio, it is imperative to reduce these heat losses, as well as reduce the cost per energy and power capacities. In addition, it is remarkable that there is no significant improvement when increasing the heat‑to‑power efficiencies over certain values. To finish, as this technology increases its feasibility when implemented in large systems, further studies should be done in the industrial and tertiary sector.