Dear reader,
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.
Simulation Study of Different Storage Geometries
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.
Geothermal Storage for Heating and Cooling
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.
Open Data for a Storage with Solar Thermal Collectors
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.
Further Information
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:
The next issue of our newsletter will be published on April 5, 2023. Until
then, feel free to follow us on LinkedIn where we share smaller use cases and
information.
Best regard,
Your heatbeat team
