Abstract
Seasonal heat storage is considered as one of the key elements on the path to a low-emission economy. Embedded in local district heating networks, they raise the share of renewable energies and balance out highly fluctuating supplies of e.g. solar systems or windmills. The technology of seasonal heat storage can be described as almost technically mature, with well-established concepts and some systems being in operation for a considerable time. Nevertheless, the operating experience gained to date also revealed two critical problems. On the one hand, even smallest leakages in sealing foils led to irreparable breakdowns. On the other hand, heat loss in the marginal areas was revealed as a key deficiency, preventing the technology from advancing towards global marketability. This study presents an experimental approach to address these two key issues in the field of seasonal energy storage. Two small-scale laboratory tests were carried out to test paraffin wax as a completely novel component in the marginal area of seasonal storages. This is based on two material properties: As hydrophobic and mobile medium, the warmed and molten paraffin should actively seal the fissures and holes in the event of leakage. Additionally, the latent heat storage properties of the paraffin wax should increase the systems’ total storage capacity and reduce lateral heat losses via its low thermal conductivity. With retardation periods from 2.5 to 4 hours, the results show an effective phase change effect of the paraffin wax, which reduces energy losses and allows to buffer short-term, intensive loading and unloading processes. By storing up to 138 kJ/kg energy in the paraffin wax, increased capacities of application-scale pit storages by up to 40.70 MWh are to be expected. Additionally, the self-healing features could be successfully demonstrated: With only small losses of between 1.5 and 17%, the paraffin wax effectively sealed artificially incised leaks. Thereby, the mechanism was most effective for local defects. Following these positive demonstrations of feasibility, technical design questions still remain, which concern prevention of deformation of the paraffin wax. Once solved, this new component can then provide a path for further optimization of seasonal heat storage technologies.
Highlights
Seasonal heat storage has evolved as a promising strategy for storing thermal energy from fluctuating sources over long periods
Even experiments with target temperatures below the melting point of the used paraffin wax show significant retardation effects and a storing/recycling of thermal energy. This can be attributed to the paraffin wax composition, as the technical quality employed in the experiments here is not a highly purified material
A Paraffin wax as self-sealing insulation material of seasonal sensible heat storage systems natural cooling curve as applied in the experiments does not properly represent the conditions of intermittent storage discharging in an application case, the findings prove that cooling is delayed by the energy recovered from the paraffin wax solidification
Summary
Seasonal heat storage has evolved as a promising strategy for storing thermal energy from fluctuating sources over long periods. In contrast, features the utilization of temperature changes [13,14,15,16] In this context, large seasonal storage systems are generated via borehole fields (Borehole Thermal Energy Storage [17, 18]) or wells in aquifers Long-term thermal storage efficiency strongly depends on a competent and reliable technique that minimizes lateral heat loss from the basin [2, 25] This means, the storage media needs to be embedded in a stable waterproof shell of low thermal conductivity. As these are not resistant against the high structural loads of the overlying storage media, most of the previously constructed storage systems are insulated only at the top and side walls [1, 29, 30]
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