Performance comparison of large-scale thermal energy storage and hydrogen as seasonal storage for achieving energy autarky in residential districts with different renovation levels

Abstract

This work investigates the potential of large-scale thermal energy storage (TES) and hydrogen as seasonal storage technologies in achieving the energy autarky in renewable (RE) districts. The study focuses on the case of Austria, and considers two possible evolutions of the building stock thermal energy demand, a high-demand scenario (BAU) and a low demand scenario (BEST). A quasi-steady state model is implemented in MATLAB to calculate, on the basis of the energy demand for space heating and domestic hot water preparation, the size of required generation and storage technologies. Two seasonal storage paths are considered: a large-scale TES path, and an electrical path with a green hydrogen storage, coupled with an electrolyser and a fuel cell. Both storage solutions are considered on the district level. In both investigated paths, the thermal load is assumed to be provided by a heat pump (HP). In the TES path, a large-scale HP is used to provide the required heat demand to the district or to the seasonal TES for charge. In the hydrogen-path, a decentral (building-wise) HP is used. The conversion of green hydrogen in the fuel cell provides to the local grid the electricity that allows to operate the HP also in absence of electricity from RE sources. In both paths an air-source and a groundwater HP are considered to investigate the effect of seasonality also on the COP. The RE generation system is based on PV, whose extension to achieve the full autarky in the district is calculated. The results showed for both building stock scenarios a lower PV extension required to reach energy autarky in the TES path than in the hydrogen path: the hydrogen path requires a PV extension of up to +95 % in the BAU scenario and + 82 % in the BEST scenario, for the air source HP, compared to the TES case. The main reason is the low round trip efficiency of the hydrogen production and conversion in the winter season. In the BEST scenario, the lower energy demand in the winter season with respect to the BAU scenario allows a reduction in the relative difference of PV field extension between the TES and hydrogen path, but the first remains the most efficient. However, detailed modelling of the TES considering its geometry, size and envelope is necessary to assess its effective performance, considering the complex dynamics involved. A relevant outcome of the study is the energy demand reduction: compared to the BAU scenario, the BEST scenario allows a reduction of the required PV installed of 39 % in the TES path, and 44 % in the hydrogen path. The seasonal pattern of the COP in the air-source HP plays a significant role in the hydrogen path, where the groundwater HP operates in winter at higher COP. In the TES path this difference is less relevant, as the HP operates mainly in summer to charge the TES (with higher COP in the air-source HP), while in winter a large share is covered by TES discharge. • Two building stock scenarios are studied to assess the impact of energy efficiency measures on thermal energy demand. • In both scenarios, the required thermal load is met by a heat pump powered by PV electricity. • Two seasonal storage strategies are considered: water-based thermal energy storage and hydrogen storage. • Scope of the work is to assess the required PV field size and storage capacity to achieve full autarky. • Results show that the thermal storage solution requires less PV area, but its design affects the autarky target.

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