The share of renewable energies in German district heating networks is about 13 % and must be increased rapidly in the next years. In past newsletters, we have already presented some research on this topic (e.g., on the integration of heat pumps in district heating networks, integration of waste heat from data centres or a municipal heat planning for the city of Stockholm). However, many more studies exist that investigate possible renewable heat sources for 4th generation district heating networks. Possible sources can be categorized as solar thermal, waste heat, geothermal, and biomass. Each of these heat sources are considered by the European Union to have a "Technology Readiness Level" rating of 9, meaning they can be economically deployed on a large scale.

A new research paper by A.M. Jodeiri, M.H. Golfsworthy, S. Bugga, and M. Cozzini of the Institute for Renewable Energy (EURAC Research) in Bolzano, Italy provides an overview of the potential currently being exploited of the four above mentioned categories. Role of sustainable heat sources in transition towards fourth generation district heating – A review, a total of 140 research papers investigating the integration of renewable energies were evaluated (freely available at: https://www.sciencedirect.com/science/article/pii/S1364032122000843?via%3Dihub). In addition to the advantages of each category, Jodeiri et al. also elaborate on the challenges and disadvantages. They address not only the technical, but also the non-technical aspects. They derive the need for seasonal storage from their detailed literature review. As a prerequisite for the integration of renewable energies, the authors mention the reduction of supply temperatures, as well as the reduction of heat losses in the network, i.e. a transformation of district heating networks into the 4th generation. From an economic point of view, the following overarching conclusions can be drawn: (a) additional costs are incurred to reduce the temperature level of buildings, (b) investment in infrastructure is necessary to enable low temperatures, (c) investment in renewable energy development must be made, and (d) operating costs are reduced compared to current systems.

Solar thermal in district heating networks

Non-concentrating solar collectors are often used to integrate solar thermal in district heating networks. The water heated in the collectors is used directly in the supply or for return boosting. The authors identified 260 large-scale installations (more than 350 kW or 500 m²). The (planned) solar fraction of coverage is between 30 and 90 % for the systems studied. The temperatures of the systems range from 40 °C (return boost) to reach 95 °C (direct use in the supply). Based on different studies, the authors show that the central integration of flat-plate collectors is economically more advantageous than decentralized integration. The use of vaccum tube collectors is particularly suitable for high temperatures but entails additional investments. A combination of flat-plate and vaccum tube collectors can offer advantages, because the flat-plate collectors have a high yield at high irradiation (midday), while the tube collectors convert energy more evenly throughout the day. Solar thermal systems are almost always combined with seasonal storages. The advantages of solar thermal are high availability with low maintenance. In addition, these systems can be combined very well with other renewable and non-renewable sources. The challenges lie especially in the short- and long-term fluctuating supply. Solar thermal is particularly available when heat demand is low. In addition, there are high investments and a large space requirement for the collectors.

Waste heat in heating networks

The use of waste heat is divided by the authors into industrial waste heat, urban waste heat and district heating networks. However, Jodeiri et al. write that in between 30-60% of energy consumption is dissipated as waste heat into the environment. In the EU, the amount of waste heat that can realistically be used is estimated to be up to 750 TWh/year, which is about 25 % of the heat demand of all buildings in the EU. The authors identify a particularly high potential of low temperature waste heat in the range of 30 - 100 °C. Most of the potential (about 50 %) is about 40 °C and thus requires heat pumps to use it for heating and domestic hot water in existing buildings. Urban waste heat is mostly present at even lower temperatures in the range of 20 - 40 °C. It is generated where rooms or processes need to be cooled (food, data centers, large buildings) but also waste heat from wastewater treatment plants has a high potential. These waste heat sources also require heat pumps to utilize them. The biggest challenges of using waste heat are that waste heat can also be fluctuating and at different temperatures. Another factor is the contractual relationship between the district heating network operator and the waste heat supplier. It is often difficult to give guarantees over longer periods. In addition, there is still little binding law, especially for urban waste heat. Waste heat utilization is a low-investment option for integrating sustainable energy when used directly. The use of heat pumps makes the use of waste heat more expensive.

Geothermal energy in heat networks

The authors divide geothermal potential into three categories based on the temperature level that can be achieved: high enthalpy (> 180 °C), medium enthalpy (100 - 180 °C), and lower enthalpy (< 100 °C). Most of the 240 geothermal systems installed in district heating networks in Europe are small (0.5 - 2.0 MW), with a few systems reaching up to 50 MW capacity. In particular, the low enthalpy sources often need to use heat pumps. Geothermal energy is often used as a base load in heating networks, this is due to the high investment but relatively low operating costs. In addition, the geothermal potential is available all year round. Depending on the technology used, however, regeneration times must be considered. This is especially the case for closed and near-surface systems. A major disadvantage of geothermal energy is the investment-intensive development of the source. Depending on the location and the technology used (especially open processes), the environmental impact can be more pronounced.

Biomass in heating networks

A widely used possibility to use renewable energies in heating networks is the direct utilization (combustion) of biomass. Currently, about 8% of the heat demand in district heating networks is covered by biomass. Boilers as well as combined heat and power plants are used for this purpose. This possibility is also already used in 3rd generation heat networks (higher temperatures). Despite the higher temperatures, the integration of biomass in 4th generation heat networks makes sense, according to Jodeiri et al.. The lower heat losses reduce fuel use, which can often be more cost-intensive than the systems outlined above. Biomass fuel is a significant challenge for biomass. The lower energy content means that transportation distances for biomass must be kept to a minimum. Land competition (in both rural and urban areas) is also listed as an important factor.

Seasonal heat storage as a key element

The implementation of seasonal storage plays a crucial role in the integration of renewable energy. Seasonal storages can be implemented as pit thermal energy storages, gravel-water storages, aquifers, and boreholes. As mentioned above, seasonal storages are indispensable especially for the integration of solar thermal systems. But also, for waste heat a higher potential is often recognizable in summer than in winter and therefore opposite to the heat demand. In addition to the space required for seasonal storage, the high investment and the associated risk of long depreciation periods are a particularly high risk. This risk is more pronounced in urban areas with high land prices. The planning and implementation of seasonal storage needs a close dynamic consideration. Often the temperatures in winter in seasonal storages are not sufficient for direct heating and must be combined with heat pumps, again increasing the investment risk.

Further information

Especially for the detailed literature review on use cases in the EU and worldwide, we recommend the full-length article by Jodeiri et al. The article is freely available (https://www.sciencedirect.com/science/article/pii/S1364032122000843?via%3Dihub). The next issue of our newsletter will be published on June 1, 2022. Until then, feel free to follow us on LinkedIn where we share smaller use cases and information.