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Cold local heating from groundwater wells - opportunities and challenges

Cold local heating networks, which are fed from renewable sources, are a sustainable way of supplying energy. One promising source is groundwater, which can be utilised by geothermal well systems.
This article analyses the basics, advantages and challenges of using groundwater for cold local heating networks.

Basics of cold local heating networks:


Cold local heating networks operate at temperatures between 0 and 30°C and do not require pipe insulation. The surrounding ground can serve as an additional heat source, which reduces the dimensioning of the primary source. These networks use a two pipe network with supply and return pipes and can provide both heating and cooling with just one thermal network. One advantage is the ability to balance heating and cooling requirements between different buildings.

We have already described the topic of cold local heating in this blog - and we compare the advantages and disadvantages of the different network types here.

Definition and utilisation of groundwater:

Groundwater, part of the natural water cycle, is formed by the seepage of rain, snow or hail and collects in cavities in the ground. In addition to drinking water utilisation, groundwater is used in agriculture for irrigation and in systems for heat and cold generation.
Heat energy is extracted from the groundwater via the well systems and fed into the local heating network. 

 

Suction and swallowing wells:

A so-called water-water system usually consists of one or more suction and absorption wells, which provide groundwater in the required quantity. In a suction well, the water is drawn from the depths and transported via pipes to an energy centre, where it transfers its temperature to a heat exchanger. The groundwater itself does not come into contact with the heating circuit and remains chemically unchanged. Thermally, however, the groundwater changes its temperature as a result of this process - depending on whether it releases heat, downwards, or absorbs heat, upwards. 
The well water then flows back into the groundwater via an absorption well. It is important to note that the groundwater also has a flow direction and the extraction well must therefore be located before the absorption well. This prevents water that has already been heated from being drawn in again, which would result in double heating of the groundwater and lower efficiency for the local heating network. 
The use of groundwater from wells is not only suitable for single-family homes, but also for neighbourhoods - in which case several suction and absorption wells are required.

Requirements for well construction:

A good aquifer in the drilling area is crucial. This is a layer of rock in which the groundwater collects - it is bounded at the bottom by impermeable layers. The aquifer must be sufficiently thick and permeable. The groundwater should be extracted as close to the surface as possible (no deeper than 20 metres below ground level) in order to ensure the efficiency of the pump - also with regard to its power requirements. A pumping test clarifies the groundwater yield of the site and the required dimensions of the well shaft.

Important planning data for the groundwater source:

The depth of the groundwater, the thickness of the water layer, the temperature of the source and the permitted temperature spread are essential for planning. This data makes it possible to calculate the amount of heat that can be technically extracted and the number and dimensions of the wells required. As planners, we also need to know how many cubic metres of water can be extracted per hour and per year.

Legal framework for well drilling and groundwater utilisation in Germany

The use of groundwater for geothermal purposes is subject to authorisation in many federal states and must be requested from the local water authority (usually the district office). Both water and mining regulations apply. (However, the German Heat Pump Association recommends - when asked for advice by the Federal Government in 2023 - that mining law should only be applied to deep geothermal energy and near-surface geothermal energy should be organised exclusively under water law). 

The information systems of the federal states can be used to find out in advance whether geothermal systems are generally eligible for authorisation. Site assessment maps show whether or not restrictions are to be expected at the selected site (e.g. Bavarian Energy Atlas, Hessian State Agency for Nature Conservation, Environment and Geology (HLNUG)).

The regulations currently differ from state to state. In Bavaria, for example, operators of a groundwater heat pump require a permit under water law, which must be applied for at the local district office. Sometimes an expert opinion from a private water management expert (PSW) is required for this authorisation under water law, sometimes the official expert from the water management office is responsible. In most cases, construction supervision by a geologist is also required. After completion, the system is often inspected by a water management expert (PSW). 

The permitted temperature spread, i.e. the difference between the temperature of the groundwater when it is extracted and when it is fed back into the system, is also usually specified. This also varies from location to location.

The temperature spread is given in Kelvin [K] (0°C corresponds to 273.15 K), whereby the temperature difference in degrees Celsius and Kelvin is the same. If the spread is 5°C, this also corresponds to 5K.

Fortunately, local drilling companies and contractors are usually very familiar with the situation and can provide advice and support. Drilling a well is a rather complex matter, not only from a legal point of view, and is therefore in good hands with a drilling company or well builder.

Efficiency and monitoring of the well system:

A shallower depth and greater thickness of the aquifer lead to lower investment costs and higher efficiency. The constant temperature of the groundwater makes it a very efficient source. Once the system has been commissioned, regular monitoring and adjustments to the control technology can further increase efficiency.

Challenges:

  • Quality of the well water: Suspended particles and pebbles can damage the system, resulting in increased maintenance requirements.
  • Costs and uncertainties: Test drilling is expensive and not always successful.
  • Authorisation procedures: Legal requirements are complex and can delay project realisation.
  • Climate change: Droughts and changes in groundwater levels are difficult to predict and can affect long-term utilisation.

Chances:

  • Constant temperatures: High efficiency thanks to almost constant water temperatures.
  • Combination with other renewable sources: Using PV systems to power the pumps increases sustainability and can feed the well pumps and heat pumps.
  • Flexible energy supply: Suitable for heating and cooling
  • Free cooling: Thanks to the stable cool source, the cold local heating network can cool buildings without requiring the heat pump (and its electricity) - a cost-effective option for air conditioning buildings.
  • No regeneration required: In contrast to geothermal probes, the groundwater source does not need to be regenerated.

Summary: Efficient source for heating networks

Groundwater wells as an energy source for cold local heating networks are an extremely efficient source, as the temperature of the groundwater is consistently stable throughout the year. However, development and maintenance are relatively complex and therefore both installation and operation are somewhat more expensive. In cities such as Munich, groundwater is currently mainly used for cooling, which means that the temperature of the groundwater tends to increase overall. Care must be taken here to ensure a balance in which the heat from the groundwater is also utilised within a heating network. Balanced utilisation can then be easily achieved over the course of the year, but is rather difficult to achieve seasonally.
In the long term, these systems offer a reliable and environmentally friendly energy supply that can cover both heating and cooling requirements.