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The use of Shallow Geothermal Energy systems for more sustainable cities

4 maj 2021

In our future smart cities and societies energy will play an important role in securing sustainability and giving us a chance to meet up to the challenges of climate neutrality. Patricia Monzó, Sara Eriksson and Joakim Hjulström work at Swecos Geoenergy group – here are their thoughts on how Geothermal energy will contribute.

High energy efficiency, synergies and energy circular economy will be crucial aspects to achieve the European goal of climate neutrality by 2050. Up to now buildings of different purposes, such as residential, commercial, office, leisure, educational, etc. and with different energy demands have been acting as independent actors. In future urban areas, buildings are thought to be part of a dynamic system, in which energy will be exchanged between them. Moreover, waste heat from surrounding facilities such as industrial or underground facilities, etc. will be reused in urban areas at a highly efficient rate. Additionally, local energy sources, like solar, wind, geothermal, biomass along with electric and thermal storage will have a significant role in the energy mix of cities. Electrification of current fossil-fuel dependent processes will also be a key parameter of shaping future urban areas.

Within this context, the European commission has pointed shallow geothermal energy (SGE) out as one of the renewable energy sources in the energy mix to accomplish decarbonization of heating and cooling (H&C) in cities. SGE provides sustainable and low-CO2 heating and cooling by harnessing the energy beneath our feet.

Main applications of SGE systems to decarbonize H&C in cities are:

  •  Supply of heating and cooling in buildings

The temperature of the ground from 5 m down the soil surface to a depth of ca. 300 – 400 m is relatively stable over the year as compared to the air. The ground temperature depends upon the climatic conditions of the site. For example, the 100-m-depth average ground temperature in Stockholm is around 8°C, and in the south of Europe, Valencia, 20°C.

As compared to outdoor temperatures, the ground temperature is higher in the winter, which makes the ground attractive as a heat source for heating applications. In those cases, the ground is usually connected to a heat pump system. The heat pump moves the heat from the ground to the building while lifting it to useful heating and domestic hot water temperatures. In the summer, when outdoor temperatures are too high, the ground can be used as a heat sink for cooling applications. Figure 1 shows how ground temperature varies with depth and how heat flows travels in the surrounding  ground of a SGE while supplying heating  or cooling in a building.

Figure 1: Schematic representation of the ground temperature as a function of depth and heat flows in the surrounding ground of SGEs while supplying heating (left top) and cooling (right bottom) in a building, www.offentligafastigheter.se (2021/04/21)

Combining heating and cooling in a SGE systems creates already a synergy! While supplying heat in the building (i.e. ground heat extraction), the ground temperature decreases which will favor a higher efficient cooling process in the next summer. In the summer, the heat is moved from the building to the ground, i.e. the building waste heat is preserved in the ground for the next winter. This process will also impact positively with a higher performance than if the system wouldn’t have been benefited from the heat injection in summer.

Direct used of the ground temperature for preheating or cooling applications will result in higher efficiency. Moreover, when connecting buildings to the same thermal grid, synergies of heating and cooling demands can be easily found resulting in a simultaneous supply of heating and cooling with very high efficiency.

Most common SGE systems are closed-loop (CL) of vertical borehole heat exchangers (BHEs) (Figure 2 left top) and open-loop (OL) systems that couples with existing groundwater reservoir in aquifers  (GWHE) (Figure 2 right top). SGE systems that create a thermal energy storage are identified as TES. If the TES is carried out via BHEs is named BTES while TES in aquifer identifies as ATES. A new trend is to use existing caverns (abandoned or natural) to create thermal energy storage identified as CTES (Figure 2 left bottom). SGE that couple with existing water reservoir at lakes or sea are identified as WHEs (Figure 2 right bottom).

Figure 2: Schematic representation of most common SGE systems (www.offentligafastigheter.se (2021/04/21)

  • Thermal energy storage (TES)

The ground has a relatively slow thermal response, i.e. the heat stored as sensible heat (or cold) remains at acceptable useful temperature levels after a given period, typically over a or several season/s. This fact makes the ground attractive for underground storage applications. Within the context of urban areas, two main applications of SGE systems are identified:

  • Seasonal thermal storage in the 5th Generation District Heating (5G DH) networks. The 5G DH is based on decentralized plants (usually heat pumps) that are connected to low temperature heating (12-20°C) and/or cooling (2-12°C) grids. In the 5G DH, H&C can be interchanged between buildings and usually requires seasonal thermal storage to keep the temperature balanced in the grids.
  • In urban areas SGE systems would act as a storage of waste heat from nearby source. Examples of waste heat are exhausted heat from refrigeration machines in supermarkets, data centers, industrial processes, or underground transportation. Examples of waste heat sources are highlighted in Figure 3, text is embodied in an orange rectangle. The temperature of waste heat can vary between a few degrees to 95°C (high temperature application of SGE systems). The temperature of the storage and its final application will dictate if the heat/cold could be used in direct application or with the help of a heat pump.
Figure 3: Overview of shallow geothermal energy systems in urban area. Simplified version of the original figure. Source: MUSE – Managing Urban Shallow Geothermal Energy – General, https://geoera.eu/projects/muse3/ (2021/03/25).

Aspects that make SGE systems attractive to decarbonize H&C in cities

  • Highly efficient and mature technology with low CO2 emissions and to be carbon-neutral by 2050.
  • It is a reliable source that harness the heat locally and, in principle, it can be installed at any location.
  • It has a high flexibility to connect with other energy systems for both thermal and electricity sides and being an important part of heat circular economy in urban areas.
  • It is economically viable (pay-back time 10-15 years) with low maintenance and operation cost.
  • Space in urban area has a high value. SGE systems take a relatively small surface area and keep the space free either in the roof or in the surrounding area of buildings.

Example of SGE systems in emblematic buildings in Sweden, in which Sweco was involved are shown in Figure 4. Next time passing by these places may you experience them from a different perspective!

Figure 4 (a): Aquifer Thermal Energy Storage – ATES that supplies heating and cooling at Arlanda airport, Stockholm
Figure 4 (b): Borehole Thermal Energy Storage – BTES under DNs huset, Stockholm

Find more information about SGE systems in Swecos Urban Insight-report BEYOND THE TIPPING POINT: FUTURE ENERGY STORAGE

Or contact Patricia Monzó, Sara Eriksson and Joakim Hjulström at Sweco Geoenergy group.

References

Offentliga fastigheter. 2017. Guide för geoenergi, http://www.offentligafastigheter.se/publikationer/energi.297.html (2021/04/23)

MUSE – Managing Urban Shallow Geothermal  Energy. 2021. Fact sheet of shallow geothermal energy  concepts, Fact Sheet 00, https://geoera.eu/projects/muse3/ (2021/03/25)

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Författare
Jenny Carlstedt arbetar på Sweco med IT för samhällsutveckling och skriver gärna om utmaningarna med informationsflöden och behovet av digitalisering för våra framtida smarta samhällen.

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