MUSE – Open-loop systems requirements & advantages

Radovan Cernak, 5 October 2020

Groundwater is a valuable natural source especially for drinking water. Additionally, using ground water for energy extraction is common in many countries for both heating and cooling. In general, these so called open-loop systems are more efficient than closed-loop systems. This is based on the fact that water is the best carrier of thermal energy (very high specific heat capacity 4180 J/kgK). Having this in mind, we can say that water is not only essential for drinking purposes but is also an important energy source. The concept of open loop system can be applied in surface water systems too, but this is not in the focus of our MUSE project.

Groundwater open-loop systems withdraw water from wells and deliver its heat (or cold) to the heat pump. Afterwards the water is reinjected back to the aquifer via a second well (figure 1). Small installations, such as suitable for single-family houses using around 1 L/s, generally have one well doublet. For larger installations doublet well systems are used – several for production and approximately an equal number of wells for reinjection. Depending on the aquifer productivity and the user demand, system sizes can vary widely. Depth of wells can range from only a few meters, up to more than 100 meters. Based on hydrogeological settings different distances between pumping and reinjection wells are designed, as a general rule down-stream of the groundwater flow.

Benefits of open-loop systems

Open-loop systems are often preferred to closed-loop systems due to higher inlet temperatures and higher efficiency. With open systems, a powerful heat source can be exploited at comparably low cost. Especially when using a shallow aquifer, the drilling costs – and therefore a major part of the installation costs – are cheaper than for borehole heat exchangers. Groundwater can be used for direct cooling without a heat pump (“free cooling”) as well. The efficiency of free cooling is very high and is therefore of great interest.

Keep limiting factors in mind

Limiting factors are typically connected to natural conditions, like insufficient permeability of the aquifer (to allow production of the desired amount of groundwater with little drawdown) or poor groundwater chemistry from technological point of view (e.g. high iron of manganese content, causing problems with scaling, clogging and corrosion). However, proper planning and a careful selection of materials can diminish negative consequences of poor groundwater chemistry.

Furthermore, the groundwater is often primarily a source of drinking water and utilization as energy source is secondary. Existing drinking water wells or other water rights have to be taken into account in the planning phase. Even though open loop systems generally return the water to the ground, negative influences could occur, due to the altered temperature in the reinjection well.  Under such circumstances, a closed-loop system may or should be considered as an alternative.

Environmental impacts arise usually only in case of an unsustainable use of the groundwater, meaning a very high change in temperature. The temperature change might lead to an alteration of the groundwater chemistry and might disturb the ecological balance of the groundwater. The relevant permits depend on local conditions and regulations stated by authorities.

Figure 1. Open-loop system installations using groundwater from a suitable aquifer (left) and surface water from a lake (right). © GRETA – INTERREG Alpine Space ERDF

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