Abstract: Urban areas are both contributors to and the main victims of the effects of climate change and will be confronted with drastic changes in terms of heat supply (heating and cooling) and urban climate in the years to come. Moreover, conventional technologies for heat supply are future-proof only to a limited extent due to their greenhouse gas emissions from the combustion of organic substances and/or due to the more vulnerable supply situation during high power demand peaks or negative feedbacks with the urban climate. Against this background, seasonal underground thermal storage combined with the use of urban heat sources and sinks can take on an important function in future urban heat supply systems. Such storage systems are technologically well developed at least for feed-in temperatures below 25 °C. However, for a broad application, a large-scale, spatially quantitative utilisation management for the subsurface space is essential, e.g. to ensure the future inner-city supply of heat and water even in expectable extreme weather situations. Numerical models and technological approaches for the productive thermal activation of entire city districts are available in principle, but cities and municipalities are still called on to define the regulatory and economic framework conditions for the implementation of thermal “sponge cities”.

Initial situation and perspectives of a climate-neutral heat supply in urban areas

It can be regarded as largely certain that cities worldwide are both significant contributors to and strongly affected by climate change and will be increasingly so in the future. Cities are significant contributors due to their high demand for so-called “grey“ energy, which describes the energy required to build infrastructures, and the equally high demand for energy in the narrow sense to ensure operation in the mobility and heating sectors (heating and cooling). In particular, the heat supply (heating and cooling) usually represents a large part of the urban final energy demand and can only be reduced in the medium to long term, especially in the area of “heating”, by further increasing the use of “grey” energy, e.g. through thermal insulation measures, etc. As long as the urban energy demand is not covered by climate-neutral energy sources, cities will significantly contribute directly and indirectly to greenhouse gas (GHG) emissions or, in reverse, are an important key to mitigating climate change. It must be taken into account that due to the global trend towards urbanisation, the importance of the role of cities in climate change mitigation measures will continue to grow.

At the same time, the economic and social functionality of large cities and megacities will be greatly affected by climate change and its impacts, and they are also much more vulnerable to energy supply problems than rural areas. A number of scientific studies give an impression of the extent and speed of change for major European cities for the year 2050, e.g. it has been impressively forecasted that in just less than 30 years, Madrid, for instance, will be climatically more like Marrakech of today and London’s future climate will be more like Barcelona‘s of today. For the future climatic conditions of cities and megacities located in lower latitudes, there are not even any current analogues and there are justified doubts as to whether or under which boundary conditions urban functionality or tolerable living conditions can be maintained there at all.

Against this background, it becomes evident that there is an urgent need for urban adaptation measures against thermal stresses and/or risks also with regard to large cities located in higher latitudes that already today show significant excess mortality, increased morbidity and a reduction in economic performance with increasing heat waves. In this context, concepts for risk minimisation do not only have to take into account the climatic changes that are considered unavoidable today in cities that are particularly affected by pronounced heat and cold islands. It is also essential to take precautions against a longer-term shortfall in the supply of electrical energy in the range of days, e.g. due to special weather conditions or other disruptive disturbances in the electrical power supply. In addition, the transformation of the heat supply must be implemented predominantly in existing building structures, during operation and economically under uncertain legal boundary conditions, as well as in a socially acceptable manner with a high level of social sensitivity in this field.

Solution concepts and available technologies

A critical analysis of the transformation concepts discussed so far with regard to the future heat supply of urban areas, which are generally oriented towards an energetic optimisation combined with the reduction of GHG emissions as complete as possible – reveals a clear need for discussion as to whether the tasks outlined above can be adequately addressed with the technologies favoured today alone (in summary, thermal building insulation, expansion of centralised and decentralised heat pump systems plus district and local heating networks and increased use of green H2 as a heat source). The main discussion points and questions in this regard can be summarised as follows:

  • How high are the costs for the provision and operation of a heat-led “back-up” heat supply system (usually CHP units or even more decentralised units) in order to ensure the heat supply, which is then actually mainly based on electrical energy, in the event of a long-term low electricity supply?
  • What quality level of thermal insulation in buildings still contributes to a reduction of GHG emissions, is economically feasible and socially acceptable, respectively? What is the effect of thermal insulation in cooling of buildings and what are the thermal and energetic effects of air conditioning systems in this context?
  • Which quantities of green H2 are economically feasible in the next 2-3 decades and where will the heat supply be positioned according to “merit-order” criteria?
  • What concepts exist to be able to thermally influence the urban outdoor area during heat and cold waves?

Without entering into an in-depth discussion on these individual points under global aspects here, it is foreseeable for Germany that sustained and economically expensive misallocations are likely to occur in the transformation of the heat supply system if the dynamic changes in the urban climate are not taken into account. That is why in addition to the consideration of corresponding climate and urban climate forecasts and impact analyses, e.g. on the urban temperature increase due to building air conditioning systems, new ways of heat supply with the use of seasonal thermal storage and buffer systems, virtually “thermal sponge cities”, must be pursued.

Seasonal thermal storage systems can significantly reduce the size of conventional “back-up” systems and, with a suitable operation management approach supported by weather forecasts, can significantly reduce the annual performance factor and thus the electrical power demand of heat pump systems. Furthermore, being part of the coupling between the temporally fluctuating electricity and heating sectors, they can play an important role in the stability of the electricity grid.

Likewise, in combination with thermally activatable façades, they represent in principle an alternative to conventional thermal insulation systems, which are based solely on reduced thermal conductivity and thus lower heat transfer, but which do not allow for a seasonal shift in the heat supply and have fundamental deficits as thermal shielding against heat waves.

Moreover, unlinke the use of green H2, which can only be produced with relatively low energy efficiency, seasonal heat storage systems do not require the probably long development of an international production capacity and a corresponding transport network. Instead, seasonal heat storage systems could be installed successively with the transformation of the heat supply system in different capacities, depending on the level and pattern of the demand and the utilisation potential.

Possibly the most important argument for the increased use of seasonal thermal heat storage in the medium and long term is its potential in combination with thermally modified elements of urban infrastructure (building façades, public squares and streets or footpaths, which then act as large-scale heat exchangers), which besides providing heat also positively influences the urban climate. Unlike alternative technologies for cooling urban buildings (usually conventional air conditioning systems, heat-repelling façades and windows) or entire outdoor areas of urban neighbourhoods (e.g. through intensified greening, optimised albedo of urban surfaces), seasonal heat storage systems do not cause any counterproductive side effects such as an increased of air humidity, the undesired multiplication of mosquitos (risks of greening) or significant heat emissions into the environment (consequences of intensified use of air conditioning and partly change of the urban albedo).

The technology of underground seasonal heat storage can be roughly divided into open and closed systems with regard to the use of groundwater and their mode of operation. In the case of closed systems (so-called borehole thermal energy storage, or BTES), heat is injected into or extracted from the geological subsurface in a so-called geothermal probe, using a heat exchanger fluid that circulates in a U-tube in a borehole, for example. Usually a cluster of borehole heat exchangers is used for this purpose, preferably in hydraulically low-permeability sediments. In so-called open systems, the groundwater is directly pumped from or injected to aquifers (so-called aquifer thermal energy storage, or ATES) and the heat exchange takes place at the terrain surface. The technical readiness level (TRL) of such systems is 9 – which corresponds to “proven technology“ – for feed-in temperatures of up to 25 °C. Such ATES systems are considered standard technology in the Netherlands and in Denmark.

However, it must also be stated that, with the exception of the Netherlands and Denmark, the use of seasonal heat storage systems as part of the transformation of the heating sector towards climate neutrality only plays a subordinate role in most urban heat supply concepts, despite the fundamental systemic advantages outlined above. The reasons for this are manifold but can be roughly summarised as follows:

  • Above-ground seasonal sensible (e.g. water storage) or latent (e.g. ice heat storage) heat storage systems require a relatively large volume, which usually is only available to a limited extent in inner-city areas and also causes very high investment costs, which again are currently not considered economically justifiable.
  • Underground heat storage facilities (usually ATES and BTES systems) are considered relatively uncertain in terms of their performance and capacity due to the natural heterogeneities of the geological subsurface. In practical implementation, however, regulatory reservations are even more limiting due to fears of damaging the surface layers of aquifers when installing BTES systems. In the case of ATES systems, there is a widespread assumption that immediate, adverse hydrochemical and microbiological changes would be triggered in the aquifers, at least at injection temperatures of more than 25 °C.

With regard to the current economic evaluation of above-ground seasonal heat storage facilities, it can indeed be stated that only ice heat storage facilities are considered profitable as latent heat storage with a significantly larger “energy content” per volume during phase change, especially if they are to provide a high cooling capacity. Large above-ground water reservoirs, on the other hand, which act as seasonal sensitive heat reservoirs, require very large volumes and sound insulation. That is why they are at best feasible only if large areas are available for the construction of artificial lakes, if no great importance is assigned to ecological aspects in the medium term (e.g. with the settlement of flora and fauna) and if the nevertheless low temperature ranges in winter can be used sensibly for exergetic purposes.

The classification and evaluation of BTES and ATES plants, on the other hand, is more complex. It is true that the efficiency of BTES and ATES plants is influenced by the geological conditions on site. Generic numerical scenario analyses show, for example, that the effectiveness and efficiency of ATES and BTES plants mainly depends on the mode of operation (e.g. usability of seasonal heat sources, feed-in and feed-out temperature, temporal distribution of heat demand, etc.) and only subordinately on changes in the geological boundary conditions (within the framework of their natural variance or heterogeneity).
It is also clear from these studies that a consistent concept is still lacking nationally and in many cases internationally for an evaluation of seasonal underground heat storage with regard to the benefits in terms of climate protection and the potentially negative hydrochemical, microbiological and faunistic impacts on groundwater. Instead, at present, either the energetic benefit of seasonal underground heat storage is emphasised or any hydrochemical, microbiological or faunistic changes in aquifers are considered potentially harmful.

Furthermore, due to the prevalent lack of suitable hydraulic 3-D models of the geological subsurface, sweeping and mostly oversized restriction zones are usually defined by regulation for underground seasonal heat storage. If these restriction zones are simply projected as a two-dimensional area from the top of the terrain into the subsurface, then the necessary spaces for the construction and operation of underground seasonal heat storage facilities below urban areas are indeed severely limited.

Opportunities and challenges in the future use of the geological subsurface as a seasonal heat storage facility

Therefore, a controlled large-scale thermal and hydraulic utilisation management of the geological subsurface in urban areas will be a prerequisite for hydraulic and thermal sponge cities. A concept for a corresponding utilisation management, which is currently being tested, assigns a so-called utilisation space as well as an impact space to each energetic and material utilisation of the geological subsurface (e.g. heat storage and/or groundwater extraction or injection) by means of numerical approaches. Depending on the use and the geological boundary conditions, quantifiable volumetric changes in thermal, hydraulic, geomechanical, hydrochemical and microbiological/faunal impacts then result. An interesting and significant result of applying this approach is that the underground spaces significantly affected by e.g. thermal use are indeed small in relation to a considered total near-surface geological volume (e.g. defined as depth down to 400 m below ground level). This means that with a suitable management approach, underground uses (e.g. water and heat supply) can often be well compatible. In addition, such a spatial management approach naturally also prevents negative interactions between the same types of use such as overlapping of thermal reservoirs with different temperature levels.

In order to be able to manage the geological subsurface in the future in terms of thermal and hydraulic “sponge city“ approaches and the associated resilience optimisations, the following measures must be initiated promptly by the municipalities due to the dynamics of climate change:

  • Construction (if not already available) of a resilient 3-D geological and hydrogeological model of the urban subsurface as well as of a numerical use management model.
  • Establishment (if not yet available) of digital twins on the current and future demand for water and heat supply (heating & cooling) with sufficient spatial and temporal resolution, as well as the development of the urban climate.
  • Establishment and use of a numerical platform for scenario analyses with regard to “best case” and “worst case” estimates of the future heat demand (heating and cooling) and the future development of the urban climate with various constructural options for action and the controlled thermal and hydraulic management of the geological subsurface.
  • Development of suitable regulatory approaches as well as economic business models, e.g. for the sustainable multifunctional use of the geological subsurface.
  • Definition, implementation and monitoring of transformation paths towards sponge cities based on the digital twin models.

Prof. Dr. Andreas Dahmke
Christian-Albrechts-Universität zu Kiel Institut für Geowissenschaften Kompetenzzentrum Geo-Energie

Dr. Sönke Bohm
Christian-Albrechts-Universität zu Kiel Institut für Geowissenschaften Kompetenzzentrum Geo-Energie