Technology
Water-water heat pumps

Vertical ground collector. The vertical ground collector is made of vertical pipe segments placed in boreholes and connected at the bottom with a U-shaped fitting. The pipes are filled with a glycol/denatured alcohol/spirit water solution which - circulating inside them – is warmed by the ground and delivers the collected heat to the heat pump. In the evaporator of the heat pump the heat is extracted from the solution, that is the solution is cooled down, and afterwards the solution returns to the ground. The particular boreholes are connected with one another in series and form a loop. The Tichelmann system is also employed to use the boreholes more evenly. The horizontal sections which connect the boreholes must be placed at a depth of 1.4-1.5 metres under the surface of the ground. The distance between the boreholes should be 5-15 metres. The depth of the boreholes is from 30 to 200 metres. An approximate value of the energy extracted from the ground is about 50 W per linear metre of the pipe. The advantages of such a collector are as follows: high efficiency and low susceptibility to changes in external temperature (at a depth of 10 metres and below the temperature is stable all the year round and amounts to 10°C). Furthermore, the vertical ground collector requires much less space to be built than the horizontal collector, but it is, however, much more expensive.


Vertical ground collector

Horizontal ground collector. It is made of horizontally placed pipes filled with a glycol/denatured alcohol/spirit water solution. The pipes are placed at a depth of 1.2-1.5 metres below the ground level every 0.5-1 metre. Up to 4 pipes can be placed in one trench, but the vertical distance between the pipes should not be shorter than 30 centimetres. The amount of energy extracted from the ground depends - to a large extent - on its type and dampness. It ranges from 10 to 35 W per linear metre of the pipe according to the following rule: the more cohesive and damper the ground, the higher its energetic efficiency. If the future user has a pond or a lake, they can be used as a very good source of heat. Not only is the energetic efficiency higher (which means that the collector can be shorter), but also the costs of the investment are - to a considerable degree - reduced compared to the costs connected with earthworks. A polyethylene coil pipe in an easy way can be placed at the bottom of a pond or a lake. In most cases ponds with an area of 1000-2000 square metres and a minimum depth of 1.5- 2.5 metres are sufficient.


Horizontal ground collector


Horizontal collector in a pond/lake

Slinky ground collector. The slinky collector works in a similar way to the horizontal collector. The collector sections have the form of spiral coils placed in trenches 10-15 linear metres long. The slinky collectors are an alternative to the horizontal collectors. Digging wide trenches between ten and twenty metres long is less difficult than digging narrow ones. The length of trenches is 30% shorter. The distance between the sections should not be shorter than 4 metres. The length of pipes for the horizontal slinky collectors needs to be increased by 30%, because they extract heat from one square metre of the ground less efficiently. The width of the trench should make it possible to place pipe coils flat at the bottom of it – as a general rule, the trench should be 1-1.2 metres wide at the bottom. Thus every 10 linear metres of the trench requires 111 linear metres of the pipes. Because of the flow resistance it is recommended that the length of a single loop should not exceed at the most 200 linear metres per loop. Solar collectors are recommended with this solution. Solar radiation energy insufficient to heat tap water (over 45°C) is used by the heat pump and increases its efficiency level. The quick regeneration of the ground energy at the end and after the end of the heating season is an additional advantage. Before the heating season the supply temperature can be even 3°C higher. A minimum recommended surface of solar collectors is 2 m2 per 10 kW of the pump cooling power (optimally 4 m2 per 10 kW).


Slinky ground collector


How to place a loop

Water heat pumps. They extract heat from deep water. The water circulates in two or more wells: it is sucked in from an intake well by a deep-well pump, then it is delivered to a heat pump and from there it is carried through a discharge well to groundwaters. There is usually one intake well, but there can be several discharge wells. A minimum output of the intake well should be 1.5 to 2.5 m3 per hour. The depth depends on local conditions and theoretically it is up to 30 metres, but practically it does not exceed 15 metres. This is so because the cost of raising the water from a depth greater than 15 metres is too high. The well system is advantageous first and foremost because its efficiency coefficient is higher compared to the ground heat pump system. Its value exceeds 4, because the temperature of the deep water is always higher than of the ground at a depth of 1 metre and it does not drop below 5-8 degrees centigrade. The intake and discharge wells can be made relatively easily and inexpensively. It is enough to make a borehole such as is made in case of a traditional drilled well. The cost of delivering the deep water to the heat pump is also low. Moreover, the requirements regarding the surface necessary to carry out the system are small. However, this system has some drawbacks. It is necessary to do the tests to assess the output of the intake well and to evaluate the quality of the groundwater. When the depth or the output of the well exceed the values established (30 metres or 15 m3 per hour respectively), one needs to obtain a water permit, but the investor rarely decides to do that. When these values are exceeded, another variant of the bottom heat source is usually selected. If the deep water is chemically aggressive (which is established on the basis of a chemical analysis), an appropriate system of filters might be needed.