Investigations of pit thermal energy storages in district heating systems

In recent years, there has been a growing interest in the use of pit thermal energy storages (PTES), which is mainly attributable to their low cost and high efficiencies. However, the existing knowledge derives primarily from short-term performance monitoring of seasonal storages and a small number of scientific studies. Specifically, the effect of storage geometry, soil conditions, and storage operation remains largely uninvestigated, although they potentially have a large impact on performance. Thus, this thesis aimed to investigate methods for assessing storage performance, determining the effect of storage design and soil conditions, and quantifying the economic impact of PTES.

In order to assess the performance of PTES systems, a thorough review of key performance indicators regarding efficiency and stratification was conducted. Initially, the most common energy efficiency expressions were compared, and a recommendation for handling the internal energy change was provided. Moreover, exergy efficiency was found to be the only efficiency indicator that accounted for heat losses and mixing in the storage. Regarding stratification, exergy destruction was recommended as an indicator that can evaluate stratification without being biased by differences in heat losses.

The performance of PTES was further investigated by assessing the impact of the geometry in terms of the slope of the storage sides and the aspect ratio of the lid. It was found that, due to a smaller surface area, changing the slope of the side walls from 26° (typical PTES side-wall slope) to 44° could reduce the total heat losses by 20%. Additionally, a square-shaped PTES had 9% less heat loss compared to a rectangular one.

Moreover, the impact of groundwater on PTES performance was investigated by simulating the heat transfer in the soil domain. It was found that groundwater could increase heat losses toward the ground by up to 60%, compared to a case without groundwater. Furthermore, the increase in groundwater temperature was investigated since it is subject to regulation in many countries. It was found that if the groundwater table was at a depth of at least 25 m, the groundwater temperature could be maintained below 20 °C for a seasonal PTES. However, this was not feasible for the short-term PTES operation.

The economic impact of PTES was investigated using the Danish city of Viborg as a case study. Since it has become common practice to install electricity-based generation technologies in distinct heating grids, a system using an air-to-water heat pump, an electric boiler, and a PTES was compared to a system without a PTES. It was found that the PTES could reduce the levelized cost of heat by 10%, with a payback period of 5.1 years. Furthermore, if the PTES charge temperature was reduced from 90 °C to 80 °C, the cost of heat could be decreased by an additional 4%.

Furthermore, the impact of thermal energy storage (particularly PTES) was investigated at a country level, with a focus on Denmark. It was found that heat storage enabled higher installation of renewable technologies (i.e., 35% higher PV capacity and 10% higher wind capacity) compared to scenarios without heat storage. In parallel, heat storages allowed for utilizing excess electricity (through power-to-heat technologies), leading to 53% lower curtailment levels and ultimately to a 2.4 €/MWh lower average heat price. Last, it was demonstrated that only by utilizing heat storage systems could carbon neutrality be achieved for the energy system by 2050.