Thermal utilisation of ground energy in conjunction with thermally active building systems or TABS provides an ideal heating and cooling solution. On the one hand, it can make a contribution to environmental protection; and on the other operating costs are reduced through the use of this sustainable energy solution. A distinction is made between horizontal and vertical collectors in the ground as seen in the image below.
From a depth of approx. 10 metres, ground temperatures are relatively constant over the seasons, leading to more stable operating conditions for vertically oriented components. Energy piles are the preferred option in cases where pile foundations are required for the building. Otherwise, ground probes are more cost-effective.
Slotted walls are only applicable in special cases and can only offer limited depth. Their thermal performance is similar to that of the horizontally oriented components described below. Horizontal components are worth considering in cases where extensive ground excavation work is required so that pipes or pipe registers can be laid in the soil or within a blinding layer relatively cost-effectively.
Unlike vertical components, the long-term performance of ground collectors is affected by temperature fluctuations. The performance of floor slab cooling systems is affected by possible thermal coupling with the basement so that in both cases closer consideration is required. For these reasons, ground collectors are usually used for ’heat pump heating mode’, and floor slab cooling systems for free cooling or ’heat pump cooling mode’ (refrigerating machine). The performance potential of a ground collector located adjacent to a building would be inadequate for space cooling during the summer while using a floor slab cooling system as a thermal absorber in winter involves frost risk for the foundation.
In addition to these heat exchangers, short or long-term ground stores (thermal energy storage according to VDI 4640) can be used, although these involve significantly more excavation and insulation work.
Basic Design Principles
Proper design of ground heat exchanger systems requires an understanding of the thermal interrelationships in the ground near the surface. Any design calculations should, therefore, be preceded by a geological survey in order to determine geological ground conditions at the site. From these survey results, the different thermal ground parameters required for an accurate calculation of the ground heat exchanger configuration can be derived.
For large systems that have to provide high security of supply, complex simulation calculations are recommended. These can provide insights into sustainable operation, possible effects on the geothermal balance of adjacent land, and any chemical/physical changes in the ground or groundwater.
For the design of ground collectors, floor slab cooling systems or foundation storage systems, ISO EN 13370 ”Heat transfer via the ground – calculation methods” and other guidelines can be used. This standard deals with the heat transfer of floor slabs, including thermally-active slabs, via the ground.
By modifying the system parameters (pipe registers, insulation, geometric dimensions of the building etc.) and system management variables, statements about the thermal performance of a floor slab cooling system during the summer can be derived.
VDI guideline 4640 ”Thermal use of the underground” can be used for estimating system performance in heating mode. The following tables provide an overview of the thermal extraction performance of ground types.
Accurate modelling and more detailed calculations are also recommended in the following cases:
- Deviations of heat pump operating times from those mentioned above
- Higher heating energy demand for hot water generation
- Ground effect of heat input through space or commercial cooling or solar thermal recharging (annual balance method)
- Strong groundwater influence (drift velocity between 10 m/a and 150 m/a).
The above-mentioned guide values for thermal extraction performance are not necessarily directly transferable to summer operation. The following factors may lead to differences between extraction and input performance:
- Starting from an undisturbed ground temperature of more than 10 °C, in heating mode the ground adjacent to the probes or pipes may cool down as far as freezing point. This temperature difference is greater than the thermally useful range in summer operation. For space cooling, the water temperature should not exceed approximately 17 °C, so that soil temperature has to be lower.
- In winter mode, an ice shield will form around the probe or pipe that influences heat conduction. In summer mode, heat conduction is characterised by moist or dry soil.
- Soil layers near the surface are subject to strong climatic influences, so that classic ground collectors that are not located below buildings should not actually be called geothermal, but solar thermal components. For floor slab cooling systems, these climatic influences only affect input performance in the edge zone, but on the whole, the efficiency of this type of component is determined by soil characteristics, including groundwater.
Differences in Temperature Lift Between Heating & Cooling Mode
For the purpose of estimating the long-term performance of ground probes and energy piles, it is therefore recommended to reduce the thermal extraction performance values quoted in VDI 4640 by 30 %.
For floor slab cooling systems, the following guide values can be used, based on theoretical considerations and practical experience including measurements:
According to the current state of knowledge, the following design recommendations can be given for floor slab cooling systems:
- Specific input performance is strongly dependent on the groundwater level. Saturation, due to high groundwater levels of soil layers below the foundation increases heat conduction. This can lead to long-term cooling outputs that are similar to ceiling performance with concrete core activation, or floor performance with underfloor cooling.
- The pipe spacing should not exceed 15 cm.
- Regeneration phases during periods when the system is switched off or during periods with reduced or no cooling demand (cool summer days) improve performance potential.
- Performance may be higher if basement spaces are thermally coupled. However, if basement temperatures rise, long-term performance will be reduced (similar to the effect of increasing soil temperature).