After our last issue, we received very positive feedback from a reader. We are happy to respond to his request for an issue on seasonal storage in district heating networks in our current newsletter. Especially for heat sources whose availability varies significantly in different seasons, it is worthwhile to consider storage systems that balance this discrepancy. The most well-known example of this are solar thermal systems, which have a high output in summer but low heat demand in the system. One commonly used technology are thermal pit storages but also other types of storages, especially borehole systems are getting new attention due to the new challenges in the heat transition. We have found three recent articles that focus on seasonal storage and its integration into district heating systems. We would like to briefly present the research results.
In the first article, A. Tosatto et al. investigate two different storage concepts in a simulation study. They investigate a cylindrical storage tank and a storage tank integrated in a shallow pit. Both storages have the same volume of 200.000 m³. This large volume translates into a cylinder with a diameter of 63.57 m and a height of 63 m, as well as a pit with a diameter between 176.58 m (top) and 141.94 m (bottom). The pit is 10 m high. The height of the pit is 10 m. The two concepts differ in geometry and the underground construction work and space required as a result. In the presented study, the storage tank is fed by a heat source in summer. In addition to the direct use of the storage tank, a heat pump allows the storage tank to be further discharged when the temperature in the storage tank is no longer sufficient to provide the temperature for the district heating network.
The storage tank and the heat pump are modeled as an equation-based one-dimensional system. In order to investigate the heat losses compared to the ground in more detail, a finite element method is applied in this case. If only the storage efficiency is considered, the simulation study shows a significantly better efficiency for the cylindrical storage tank, which is mainly due to the relationship between surface and volume and thus the lower heat losses. However, if the heat pump is also considered in its different operating modes and the available temperatures, the efficiencies of both systems converge. This study showed us once again how important a multi-criteria evaluation of complex energy systems is. In addition to the energy evaluation, the economic and planning components must also be taken into account.
The second article by M. Fiorentini et al. presents a geothermal heat storage system. In the use case, the heat storage is coupled to district heating networks as well as cooling networks. In many of our projects, we also see an effort to make more use of the synergies between heating and cooling demand. We see a possible application of the presented storage variant not only in separate networks but also in district heating networks of the 5th generation.
In summer, the compression chiller works towards the ground and can thus operate with a good efficiency and heats up the ground. The heated ground can then be used in winter as a heat source for heat pumps in district heating networks. Using a simulation model, different configurations between 158 - 466 probes with a respective depth between 53.4 m and 91.7 m are tested. The maximum power of the system is given as about 3 MW for heating and 1 MW for cooling.
The aim of the study is not only to determine the technical parameters and the operation of the system with the simulation study, but also to optimize the storage system according to economic criteria. For example, the model showed that the optimal size of the system is correlated to the level of the CO2 price. The higher it is, the larger the optimal seasonal storage, since larger storage can reduce CO2 emissions. The study runs a large number of sensitivities. Again, it becomes clear that a discrete consideration of a valuation variable is often not useful.
I. Sifnaios et al. provide measurement data of the storage system over a period of six years along with their paper (see: https://github.com/PitStorages/DronninglundData). The storage system in Dronninglund, Denmark, has been the subject of several investigations and simulation studies, but often different methods have been used for pre- and post-processing of the measurement data. In order to harmonize this, the measurement data is provided online. The paper gives an overview of the existing measurement data points and also provides an overview of the quality of the data. Furthermore, a proposal is made on how missing data can be replaced.
The storage in Dronninglund is thermal pit storage with a capacity of 60,000 m³. The solar collectors have an area of 37,573 m² and charge the storage tank in summer. The thermal energy in the storage tank can be used directly in the district heating networks or via an absorption heat pump (driven by biomass boilers) as a heat source. The measured storage efficiency is reporten with over 90 %.
A great initiative by I. Sifnaios et al. that we are happy to share here.
The selected articles show that a multi-criteria evaluation is particularly important in the selection and design of seasonal storage systems. High investments in the storages make it necessary to also consider the operation over the entire life cycle at an early stage. Simulations are the appropriate tool here. Many thanks at this point for the great feedback and the suggestion for this topic. As always, we recommend all articles in full length:
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