One of key elements of the geothermal thermal pump is the soil heat exchanger. Exactly thanks to this element the thermal pump receives low-potential heat. Correctly designed and established soil heat exchanger pledge of effective and reliable operation of the soil thermal pump.
There is a set of designs of heat exchangers for obtaining warmth from soil and ground waters. All of them can be classified on three main types:
- Horizontal heat exchangers (collectors);
- Vertical heat exchangers, so-called "probes";
- And basket and spiral type heat exchangers.
Horizontal heat exchanger of the geothermal thermal pump
Installation of the horizontal heat exchanger is simplest and does not demand considerable monetary expenses. A shortcoming is the big occupied space on which there should not be subsequently structures and trees.
The apportion of such heat exchanger is carried out on depth below level of a promerzaniye of soil, usually from 1 to 3 m depending on the geographical district and soil type.
The top layers of earth accumulate solar heat and, in fact, are accumulators of solar energy. Therefore the temperature of the top layers of earth is not uniform within a year and has seasonal fluctuations that in turn influences efficiency of the thermal pump in operation term. For example on depth of 2 m the temperature of the soil fluctuates from 7 °C to 13 °C in a current of year.
Soil temperature depending on depth
I will eat heat from each meter of a soil collector depends on a set of factors. Such as: depth of laying, type and humidity of soil, zateneny platforms under the heat exchanger etc. On the average value makes 20 W/m. For more detailed calculations it is necessary to address to experts for the detailed geological analysis of the soil.
The step of laying of pipes should not be less than 0,7 m for effective work of a collector. It is recommended to use a contour of the general long no more than 150 m because of big hydraulic resistance. At application of several contours it is necessary to try, what each of them was approximately identical are long.
Example of calculation of the horizontal heat exchanger
For an example we will calculate the area occupied by a horizontal collector. Let’s say that the thermal pump in capacity of 10 kW is necessary for the house.
The removed capacity of contours of the soil heat exchanger of the thermal pump is calculated concerning capacity and RUBBISH of the chosen thermal pump on a formula:
Ре = Pн * (1 – 1/rubbish), kW
Where Rn – rated power of the thermal pump, RUBBISH – factor of transformation. Calculation apply to one of modes according to the EN 14511 standard (usually admit point B0/W35 where 0 °C – heat-carrier temperature on an entrance in the evaporator, 35 °C – giving temperatures to heating system). As an example we will take the thermal pump Nibe F1145-10 with parameters at B0/W35: capacity – 9,95 kW and RUBBISH – 5,03.
Ре = 9,95 * (1 – 1/5,03) = 7,97 kW;
The necessary is long the horizontal heat exchanger of the thermal pump, is equal to the relation of necessary capacity to the removed capacity of one meter of a pipe:
L= Pe/q, m
Where q – we accept 20 W/m (average value for horizontal collectors).
L= 7,97/0,02= 398,5 m
For our thermal pump 4 soil contours on 100 m everyone will be optimum. What will learn what area to occupy such collector it is necessary to increase this number by size of a step of laying of pipes (we accept a step of equal 0,7 m) S = 400 * 0,7 = 280 m?.
The vertical soil heat exchanger – "Probe"
The temperature of soil is deeper than 20 meters is stable for all year and is equal 8-10 °C, it is supported thanks to geothermal energy of a subsoil of Earth. For obtaining this energy use vertical soil heat exchangers called "Probes" which immerse in wells depth of 20-300 m and in diameter of 120-200 mm. Usually use a plastic pipe in diameter from 32 mm. In a well place one or two loops of a probe and space between soil and a pipe fill with bentonite or other solution with high heatconductivity.
Teplosjem from the vertical heat exchanger is higher, than at horizontal and 50 W/m are accepted on the average. However real value can strongly will cause a stir, and depends on humidity of breed and availability of ground waters.
Breed waters
Teplosjem, W/m
Dry sedimentary rocks
20
Dry sand, gravel
25
Damp sand
35-40
The stony soil sated with water, damp clay
40-50
Limestone
50-60
Granite
60-80
Ground waters
80-100
Heat of a subsoil of the earth though is a renewable source of energy, nevertheless thermal regeneration (renewal) occurs not so quickly as we spend warmth of soil. Therefore all data are provided taking into account 1800 business hours of the thermal pump in a year. According to researches in the first 2-3 years the soil temperature round heat exchangers sharply decreases, however every year fall of temperature decreases. All this leads to decrease in efficiency of the thermal pump. If to use the thermal pump more than 1800 hours in a year, there is a risk of considerable decrease in temperature round a well or even a promerzaniye of some sites. It can lead to a prosedaniye of some layers of earth and destruction of pipes of the heat exchanger or to more dangerous consequences for a microclimate of soil and structures near a well. For the best regeneration of the soil it is recommended to submit to the summer period additional heat in the heat exchanger, for example from solar collectors or to use the thermal pump for cooling, thereby warming up a probe.
Calculation are long the vertical soil heat exchanger it is made similarly with horizontal collectors. For earlier chosen thermal pump:
L= Pe/q = 7,97/0,05 = 159,4 m
It can be both one 160 m a well, and three wells on 55 m. When using several probes it is necessary to drill wells on the greatest possible distance from each other (not less than 6 m). For more effective work it is recommended to drill smaller quantity of wells.
Other soil heat exchangers of geothermal thermal pumps
Basket and Spiral type heat exchangers were separately classified.
They unite in themselves properties of horizontal heat exchangers and the way of installation reminding vertical heat exchangers. Such heat exchangers keep within on depth to 5 m. Exist as some other updatings of soil heat exchangers of geothermal thermal pumps.
