Open systems This type is characterised by the fact that the main heat carrier, ground water, flows freely in the un-derground, and acts as both a heat source/sink and as a medium to exchange heat with the solid earth.Main technical part of open systems are ground-water wells, to extract or inject water from/to water bearing layers in the underground (aquifers").In most cases, two wells are required (doublette"), one to extract the groundwater, and one to re-inject it into the same aquifer it was produced from.

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To choose the right system for a specific installa-tion, several factors have to be considered: Geology and hydrogeology of the underground (sufficient permeability is a must for open systems), area and utilisation on the surface (horizontal closed systems require a certain area), existence of potential heat sources like mines, and the heating and cooling characteristics of the building(s).

In the design phase, more accurate data for the key parameters for the chosen technology are necessary, to size the ground system in such a way that optimum perfor-mance is achieved with minimum cost.

The individ-ual types of ground systems are described in more detail on this and the following pages.

Burkhard Sanner The ground system links the heat pump to the underground and allows for extraction of heat from the ground or injection of heat into the ground.

These systems can be classified generally as open or closed systems, with a third category for those not truly belonging to one or the other.

Open systems: Groundwater is used as a heat carrier, and is brought directly to the heat pump.

Between rock/soil, ground water, and the heat pump evaporator is no barrier, hence this type is called open".

Closed systems: Heat exchangers are located in the underground (either in a horizontal, vertical or oblique fashion), and a heat carrier medium is circulated within the heat exchangers, transport-ing heat from the ground to the heat pump (or vice versa).

The heat carrier is separated from the rock/soil and groundwater by the wall of the heat exchanger, making it a closed" system.