Long-term heat storage opportunities of renewable energy for district heating networks Free

By 2050, Denmark must be carbon dioxide (CO₂) neutral (Energistyrelsen, 2022). This is already a hard goal to reach and means that technologies must be shifted from current and known solutions into  new and more innovative solutions, methods and ways of thinking (Sørensen and Torfing, 2022). To reach a carbon dioxide neutral energy grid, renewable energy sources are a big part of it within all energy sectors (De La Peña et al., 2022). One main issue, which seems to still be the argument for keeping fossil fuels alive, is about when the wind is not blowing; what is the meaning of the whole system (Xydis, 2013a)? Or when the sun is not shining? Thus, this paper will focus on storage solutions for renewable energy sources, that can be utilised in times when excess renewable energies are produced and save this valuable energy for later use when renewable energy production is low (Xydis, 2015), enabling a more stable supply of renewable energy within the district heating (DH) sector even when the wind is not blowing or the sun is not shining. Figure 1 shows the concept of storing renewable energies and how they later can be utilised  for DH purposes.

There is another problem that arises in the DH sector that makes the need for storing renewable energies even greater. For instance, when collecting sunlight, the most favourable conditions are typically found during the summer, especially in Nordic countries like Denmark – due to the minimum available sunlight during winter (Graell and Xydis, 2022). However, there is not a big demand for heating during the summer months. This means if solar panels (SP) are producing excess without being connected to a thermal energy storage solution (TESS), the excess renewable energy produced must be let out in the air as heat-waste, which is simply a waste of clean energy. On top of that, process-wise, there is a waste of energy spent on the conversion; therefore, the waste of resources is immense.

This study's practical significance is found in its guidance for selecting efficient thermal storage solutions, optimising renewable energy conversion and making informed decisions about resource allocation and grid stability in the renewable energy sector, while also highlighting potential areas for future research in sustainable energy technologies. The originality of this study resides in its thorough analysis of the effectiveness of thermal storage systems and practical knowledge of resource allocation and grid stability in the renewable energy industry. It is a useful contribution to the area since it also provides new possibilities for research into sustainable energy solutions.

It has been difficult to find comparable research similar to that conducted here – that is, when an exploratory research design is used (Labaree, 2013). Thus, there is very less knowledge about the process to be outlined, and this study will thus be conducted as an exploratory study. Exploratory studies are very good when it comes to gaining and establishing background knowledge about particular topics (Lynch and Nowosenetz, 2009). Moreover, exploratory studies often generate new hypotheses and open questions that require further research and can potentially lead to new and more studies (Alves et al., 2008). However, exploratory studies provide the researcher with insight but do not necessarily make it possible to draw definitive conclusions or possibly only allow for tentative conclusions. This type of study lacks standards and it can therefore be difficult to determine how to approach a problem, thus the researcher is also more or less free to choose which approach or approaches they should take on the problem (Labaree, 2013).

Data gathering will mainly be based on quantitative data from Danish government websites, scientific articles and official government reports. A large amount of quantitative data will be used; the preliminary data processing will be done in Excel and is not shown to save space; thus the data visualised in this study will be the processed data. The main resource of data will be from Energinet (2022b) and Mey and Diesendorf (2018), where data are available for energy production, both live and historical. Furthermore, data from Solvarmedata.dk (2022) will be utilised, where historical data also will be collected.

The main objectives of this study can be succinctly stated as follows.

• To investigate the energy production effectiveness of several thermal storage options, such as borehole thermal energy storage (BTES) and tank thermal energy storage (TTES), while considering any potential constraints and needs for each solution.

• To determine if it is feasible to turn solar and wind energy into thermal energy, with an emphasis on any possible benefits of doing so while utilising heat pumps.

• To assess the effectiveness of this energy conversion process by contrasting the thermal energy output of the Silkeborg solar thermal generating plant in Denmark with the electricity produced by wind and sun sources in the DK1 (Jutland) production region.

• To think about the possibility of using thermal energy storage systems to deal with swings in grid energy demand, providing an alternative to exporting surplus energy and investigating possibilities.

• To consider the potential for utilising TESSs to address fluctuations in energy demand within the grid, offering an alternative to exporting excess energy and exploring potentials.

Additionally, this work is supposed to be seen from a high-level perspective, showing what is possible and what are the opportunities. Therefore, it will not be investigated down to the smaller details in different processes whose factors could affect the processes later described. However, results from this study should be seen more from the perspective of showing which opportunities look exciting, which have potential and where could it make sense to conduct future work (Johnstone et al., 2021).

Furthermore, some delimitations will be considered for this study. This includes the amount of power production sources that are considered. The energy storage solutions that are being compared will only be related to thermal energy storage and not to all known thermal storage solutions. There are several reasons for this – for example, some other energy storage solutions such as pumped hydro storage do not have ideal conditions in a country like Denmark, due to the relatively flat terrain. Hydro storage's configuration by definition consists of at least two reservoirs at different heights. The water discharge from the upper to the lower reservoir generates power by using a turbine (Rehman et al., 2015). Therefore, since Denmark's highest point is 171 m above sea level (Ejer Bavnehøj Møllehøj), the conditions cannot be considered ideal. Moreover, some assumptions will be made to estimate the losses in processes later described as well as assumptions for the transmission of energy will be made.

Not all renewable energy gathered can be directly utilised for DH purposes. As seen in Figure 2 SP are directly picking up thermal energy that can be put into DH, but wind energy and photovoltaic (PV) panels convert renewable energies to electrical energy (Koroneos et al., 2013), which cannot directly be utilised for DH purposes, and they are highly dependent by the outside temperature and meteorological conditions (Xydis, 2013b). Thus, the electrical energy collected must be transformed into thermal energy to be utilised in DH (Balaman and Selim, 2016). Transforming one energy form into another inevitably involves a loss of energy (Davis et al., 2020), which makes it important to consider how renewable energies are used and prioritised according to the usage.

When thinking of storage solutions for energy purposes, one of the first things that come to mind is batteries (Luo et al., 2019). However, in this case, batteries are not nearly sufficient for storing energy for long periods – for example, up to half a year, as the example in this study contains long-term and high-capacity solutions (Kebede et al., 2016). The problem is that the time horizon for all battery solutions/options is within hours–days (Lamp and Samano, 2022; Manohar et al., 2012). The duration has not improved much over the last decade, something which is not durable in terms of a DH systems perspective.

As of today, different long-term thermal storage solutions are in place and operating and are being utilised within DH systems. Some of the most known and developed systems are TTES, pit thermal energy storage (PTES), aquifer thermal energy storage (ATES) and BTES (Dahash et al., 2019). Most of these are water-based, as water is non-toxic, has good heat conductivity properties (Esfe and Afrand, 2020), has a high heat capacity and, most importantly, is cheap (Energistyrelsen, 2013). In Table 1, a comparison of the heat storage solutions are presented.

This study is based on the municipality of Silkeborg, since the utility company (Silkeborg Forsyning) as of 2017 has the largest operating SP for collecting thermal energy (Wittrup, 2017). The key figures of the solar heating plant in Silkeborg can be seen in Table 2.

Data on exactly how much thermal energy was produced is available on Solvarmedata.dk (2022). Figure 3(a) illustrates the thermal energy production over a 3-year period, beginning on 1 January 2019 and ending on 31 December 2021 as Solvarmedata.dk (2022) allows for data extraction over a 3-year period. Figure 3(b) shows an average of the thermal energy production over the same 3-year period. It can be observed that, surprisingly, April and May (at least for 2019 and 2020) were the months with the highest heat production from SP. The reason for this is that during the last 2 years, those two months saw the highest number of sunny days. In 2021, the peak was noticed in June, and May 2021 was rather rainy. The overall heat production for 2019 and 2020 was on average also higher compared with 2021 (Table 3).

Figure 3(a) shows the fluctuations in the annual thermal energy production and the same has been listed in Table 3.

In addition to data concerning the thermal energy production of the solar panel plant in Silkeborg, it is also possible to see the electricity production of renewable energy sources per municipality at Energinet (2022a). Thus, the same has been done for data collection of renewable electricity generated for Silkeborg, as seen in Figure 4(a) over a 3-year period from 1 January 2019 to 31 December 2021. The monthly average for the 3 year period is shown in Figure 4(b).

Once again, there are fluctuations in the energy production from wind and solar depending on  the time of year. It can be graphically seen that overall, in 2019 and 2020, wind and solar energy were higher compared with 2021. The total energy is listed in Table 4 to summarise the yearly power production.

From the data in Table 4, it is clear that in the municipality of Silkeborg, there is much less renewable electricity available than renewable thermal energy. Thus, bearing in mind the conversion loss described previously, it would not make sense to convert  the renewable electricity in Silkeborg into thermal energy and store it, since there is such a large production of direct thermal energy from the SP. Electricity can be used most of the time directly.

Because of the aforementioned distribution of produced energy in Silkeborg, the rest of the paper  will be based on a theoretical approach to the amount of excess, conversion and storage of renewable electricity production. By looking at the power produced from onshore wind and solar for the DK1 area, the data for DK1 can be seen over a 3-year period in Figure 5(a) and the average of each month in Figure 5(b).

In 2020, 4.3% of the electricity generated was curtailed because there were fluctuations in demand and flexibility of the grid and in 2019 1.3% of the electricity generated was curtailed for the same reasons (Energinet, 2021). As the paper will be based on a theoretical approach to the excess electricity produced in DK1, an assumption will be made that 0.01% of the excess energy produced can be stored as thermal energy and allocated for Silkeborg. Table 5 shows 0.01% corresponding energy.

To compare the different methods used to collect energy and what the final output would be for consumers in a DH network, a journey from power generation, through transmission, conversion and storage, has been mapped. Figure 6 shows the collection and the journey of thermal energy directly from SP that is put into a storage system and then transferred to consumers. The collection and journey of wind and solar power from windmills and solar cells, shown in Figure 7, is similar to that for thermal energy shown in Figure 6, but it faces some different challenges.

In Table 6, it can be seen that if the energy from both energy collection methods is combined and stored in the same place, an equivalent from the initial thermal input energy can be reached.

It can also be seen from Figure 7, that converting electricity into thermal energy to store gives lower efficiencies than harvesting solar energy directly. However, as seen in Table 6, it can still  make sense to combine excess energy from wind and solar cells with thermal energy instead of letting it go to waste.

In this study, the numbers used as input and output are based on what amount of energy is actually produced from the given solutions. However, one should bear in mind that these are not true inputs as a true input would be the amount of solar radiation from the sun or the speed of the wind. Naturally, there will be a loss from these values to what is produced from a solar panel or a wind turbine/wind farm, ultimately affecting the true efficiencies (Astolfi et al., 2021). This has not been taken into consideration as the paper mainly investigates possibilities and the specific numbers and processes in detail from input to output.

It should also be taken into consideration that all the different thermal storage solutions assessed in this work have their limitations. For instance, while BTES might appear as the most efficient option according to the analysis, establishing a BTES solution necessitates ideal earth conditions, substantial funding, a minimal groundwater flow and specific heat conduction capabilities of the earth (Mahon et al., 2022). Not only does BTES have requirements or limitations like this, but the same goes for the other solutions, which also have specific requirements and boundary conditions that need to be met before establishing a thermal storage solution. The cheapest, however, and one with the least requirements will be TTES (Nuñez and Zaversky, 2022).

In the case of converting wind or solar power into thermal energy, as shown in Figure 7, the conversion efficiency has been set as η = 0.9, which is usually the efficiency of the type of boilers typically used in DH (Hoback, 2022; Weiss et al., 2021). However, it could be interesting to consider if the power could be utilised  in a better way if a heat pump was utilised at this stage instead, either boosting the energy for storage or, instead of storing it, sending it directly out in the DH grids. However, this would extend the study and could therefore be seen as a perspective for investigating in future work.

Four different thermal storage solutions have been mapped in this study; however, there are already more existing solutions for storing thermal energy which would also be interesting to investigate in future work, such as mine thermal energy storage or phase change materials. Once again, to limit the scope of this study, the analyses of TESSs have been discarded.

In a DH network, the study investigates the use of various thermal energy collecting techniques, such as SP, wind and solar power. It demonstrates that these techniques can produce as much energy when used together as the initial thermal input energy. It should be emphasised, though, that using electricity to generate heat is less effective than using the sun's energy directly. Due to the losses involved in storing heat in water in big steel tanks, BTES seems to be the most effective thermal storage technique, whereas TTES is the least effective. The study also highlights the potential of thermal energy storage methods to balance out variations in the energy system, making them a workable substitute for exporting surplus energy.

The research has its limitations, including its dependence on energy output figures rather than actual inputs like solar radiation or wind speed. Since the focus of this study is possibilities, intricate input-to-output mechanisms are not thoroughly investigated. Each thermal storage option has certain prerequisites and constraints, and BTES, while effective, needs optimal earth conditions, significant investment, limited groundwater flow and particular heat conduction capabilities. Other thermal energy storage options, such as mine thermal energy storage or phase change materials, are not included in this study but may be considered in future research.

Policymakers should think about encouraging the deployment of effective thermal storage options, with an emphasis on BTES, in light of the findings. Investigating the use of heat pumps in energy conversion procedures may also improve overall energy efficiency. When the local potential for thermal energy production surpasses that of wind and solar power, policies should encourage the use of specialised facilities, such as the Silkeborg solar thermal generating facility. Given the varying demand on the energy grid, methods for storing extra wind and solar energy in thermal energy storage systems should be investigated. To increase the variety of possibilities for sustainable energy storage, future regulations should also fund research into other thermal storage techniques.

On the basis of the data collected in Section 4, it is seen that the thermal energy produced by the solar thermal generation plant in Silkeborg exceeds the produced renewable power generated from wind and solar in Silkeborg. Therefore, a theoretical approach was taken to compare it to assumed excess power production from onshore wind and solar in the whole DK1 production area. Silkeborg's assumed excess power allocation was set at 0.01%. This resulted in efficiencies ranging between η = 0.739 and 0.765 from input to output, with efficiencies of the solar thermal generation plant in Silkeborg ranging from η = 0.864 to 0.895.

In both cases, BTES seemed to be the most efficient solution, since the losses connected to storing heat in earth or rock using boreholes seemed to be the least. In contrast, TTES was shown to be the least efficient, due to the losses associated with storing heat in water in large steel tanks.

When comparing the outputs of the solar thermal generation plant in Silkeborg with the power generation from wind and solar in DK1, it becomes evident that utilising the already installed and dedicated DH plant in Silkeborg is the most logical choice in this scenario. However, a crucial point can be made that even though there is more potential for the solar thermal generation plant in Silkeborg, storing power generated from wind and solar in TESSs could be an option due to the fluctuations of demand in the energy grid. Thus, it can be seen as an alternative to exporting energy when there is excess production, and it can possibly be utilised in other energy sectors than initially intended.