C2 South City Business Centre, Tallaght, Dublin 24.
Phone: (353) 01 4031200
Fax: (353) 01 4137777
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Contents
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1. General
1. Ground Collector
1.1 Collector Design
1.2 Material
1.3 Placement & Assembly
1.4 Depth & Spacing
1.5 Collector Headers & Distribution Headers
2. Heat Transfer Medium
2.1 Medium Selection
2.2 Medium Concentration
2.3 Flow Velocity
2.4 Collector Inspection
3. Extracted Heat Capacity
3.1 Maximum Extracted Capacity
3.2 Calculetion Example
4. Collection Selection & Sizing
4.1 Determination of HP Heating Capacity
4.2 Maximum operation time of the Collector
4.3 Collector Circulating Pump
5. Installation Regulations
6. Collector Trench Cover
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General
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This information material reflecting the state of the art has been compiled to assist the designer in calculating and designing the equipment used to extract heat from a ground collector. Any findings included herein are based on hands-on experience, measurements, research and simulations performed by manufacturers, universities and institutes of geophysics in Switzerland, Germany and Sweden.
The body of soil represents an enormous storage of heat. In dependance on the soil humidity, site accessibility and geographic position and despite the low specific heat of soil, 2 m3 of soil with 1 m2 of surface area can deliver as much as 70 kWh of heat during the heating season. A small part of the accumulated heat comes from the earth depth, but the bulk is produced by the sun (98%), warm air and rain. This heat can be extracted with a ground collector and used as a source of power for heat pumps.
When extracting the energy from soil, the soil temperature around the collector drops. The temperature drop corresponds to a certain amount of extracted heat, defined as the collector specific capacity (W/m2).
When the energy extraction from the collector is intensive, the temperature inside the collector decreases. To achieve an optimal Performance Factor of the HP, the temperature of the soil body, and consequently the temperature of the medium inside the collector, should be the highest achievable. This objective can be attained by reducing the amount of heat extracted from the body of soil (here, an important role is played by the climate).
The experience tells us that, when some basic conditions are observed, the heat extraction from the body of soil does not impact on vegetation (the most pronounced impact may rest in about a two week delay of growth onset in the springtime).
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1. Ground Collector
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Collector Design
The ground collector consists of pipes with the diameters of 20 to 40 mm; the pipes are buried horizontally and in a regular pattern.
1.2 Material
The material opted for is polyethylene, since it offers sufficient strength, elasticity and low drag; polyethylene (PE) is stable (50 years of service life) and resists corrosion.
1.3 Placement & Assembly
The ground collector can be placed into a naturally grown soil with leveled surface. PE hoses are laid on a bed of sand.
1.4 Depth & Spacing
PE hoses are buried 0.8 to 1.2 m deep and no closer to buildings and other structures than 1 m. The individual runs of hoses are spaced in dependance on the hose diameters
(the larger the diameter, the larger the distance).
1.5 Collector Headers & Distribution Headers
The headers must offer free access any time (they may be installed in a light well or the like). Each branch has to enable independent closing.
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2. Heat Transfer Medium
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2.1 Medium Selection
The media have to comply with environmental regulations introduced to protect groundwater and soil. Therefore the following criteria must be observed:
-Data delivered by the manufacturer
-Medium properties affecting the amount of heat transferred and consequently the Performance Factor
-Viscosity, since viscosity exerts major influence on the system's resistance to flow and thus impacts on the input power required for the circulating pump
-The chosen medium must exhibit constant viscosity even during prolonged operation and is not to induce corrosion in other parts of the equipment
2.2 Medium Concentration
The ratio of an anti-freeze agent to the water will affect the medium properties. A higher content of the agent will increase the viscosity and consequently the resistance to flow of both the pipelines and the evaporator. That is why the concentration has to be adjusted (in view of the HP evaporation temperature) so high that the medium resists freezing, but so low that the system's resistance to flow stays as low as possible.
2.3 Flow Velocity
The medium flow rate stipulated by the HP manufacturer has to be strictly observed. Even the tiniest reduction of the flow rate will lower the flow velocity and thus the amount of heat transferred from the ground collector to the evaporator; consequently, the HP heating capacity will drop.
2.4 Collector Inspection
Each component of the ground collector has to be periodically checked for tightness, chemical stability and the freezing point of the anti-freeze mixture.
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3. Extracted Heat Capacity
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3.1 Maximum Extracted Capacity
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3.2 Calculation Example
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In dependance on the soil composition and the climatic zone each 1 m2 of soil can deliver as much as 70 kWh during the heating season. When deciding about the collector surface area, we shall have to take into account the overall number of operation hours and the overall amount of ground heat needed (influenced by the envisaged HP purpose).
Example:
-temperate climatic zone
-soil: damp, loamy/sandy, normal amount of sunshine
-extracted capacity: 50 kWh/m2
Extracted capacity:
-HP operating in the monovalent mode for 2000 hours during the heating season.
-If so, the extracted capacity will be: 50 kWh/m2 / 2000 h = 25 W/m2
Collector surface:
Mean cooling capacity (Qom) of HP is 8.35 kW at B0/W35 (these parameters correspond to a family house heating system featuring the temperature drop of 50/40 °C and the heat consumption of 10 kW)
F = 8.350 W / 25 (W/m2) = 334 m2
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4. Collector Selection & Sizing
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The dynamics of HP operation require optimal matching of the heating system both to source side and to the consumption side of the process. Both sides are closely coupled to the HP in its capacity as a heat transfer device.
The content of water in soil, the soil composition and the total sun exposure of the soil are factors directly impacting on the HP operation.
Provided the soil is: dapm, loamy/sandy, with normal amount of sunshine we can expect these values of extracted heat capacity:
20 - 30 W / m2
In case any of the major factors is less than normal, the amount of heat being extracted from 1 m2 of soil will have to be reduced.
Provided the soil is: stony, dry, shaded
we cannot rely on higher extracted capacity than:
8 - 12 W/m2
Provided the soil is: sandy, wet, shaded
we can count on:
30 – 35 W/m2
The collector surface area will be calculated like this:
Cooling capacity Qom (W) / extracted capacity (W/m2)
Keep in mind that the mean heat flow is inversely proportional to the heat carrier (medium) temperature; to the HP outlet temperature; and to the climatic conditions at the installation site.
The temperature profile encountered in the utilized soil (0.8 to 1.2 m deep) will correspond to the mean temperatures experienced during the heating season.
4.1 Determination of HP Heating Capacity
The HP heat capacity has to make up for the heat loss of the building. An undersized HP would have to be operated for prolonged periods of time and particularly at low outside temperatures such prolonged operation would overload the ground collector. If the inflow of heat into the soil around the collector is not allowed sufficient time, the soil around the collector hoses will cool down and the collector and HP capacities will deteriorate.
4.2 Maximum Operation time of the Collector
The specific values of extracted capacity quoted in item 3 guarantee that the HP will offer trouble-free performance when operated in the monovalent mode. The period of trouble-free operation proven in practice extends over 2000 hours.
4.3 Collector Circulating Pump
The circulating pump used to drive the collector loop flow has to be carefully sized, since the mean differential temperature, the flow velocity and the medium properties are of major importance.
Keep in mind that a high input power of the circulating pump deteriorates the Performance Factor of the entire HP facility. The number of branches and their lengths have to be optimized. Only then it will be possible to opt for the best low-input-power circulating pump.
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5. Installation Regulations
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A heating facility based on the ground collector will require a certain volume of construction jobs.
- The surface envisaged for the placement of the collector will have to be leveled and cleaned, large stones removed; in case the ground is really stony, the collector will be laid on a bed of loose material and covered with a protective layer (of fine-grained material) no less than 15 cm thick
- The collector will be buried 0.8 to 1.2 m deep and no closer to the buildings, the site borderline, etc. than 1 m
- The collector should not be placed under parking lots, stairs and entrance gates
- The pipe must be embedded solely in natural grown soil
- In case the soil is partly natural and partly landfilled, the pipes will be placed only to one of these types of soil.
- The landfilled soil may be prone to subsiding and the collector may thus be damaged
- Damaged pipes have shorter lifetimes and therefore should be avoided; pipes should be properly protected even during transportation (never dragged, etc.)
- When being placed the pipe is unwound from a reel; never bend the pipe so that its wall is damaged; in case the pipe breaks, repair it immediately with an electrically welded sleeve; threaded joints may only be used where easily accessible
- PE hoses may act as mediators and transfer the soil tension; that is why the threaded joints have to be re-tightened from time to time
- The ground collector should be buried in a continuous start-to-end process
- The collector headers and distributor headers installed in light wells and shafts will be protected from dewing with steam-proof insulation
- In case the connection pipeline is buried in ground, the individual pipes will be spaced by 0.5 m or more
- The hose bending radii and other parameters stated by the manufacturer must be duly observed
- The ground collector and the related equipment shall be pressure tested by
3 bars for 10 hours.
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6. Collector Trench Cover
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When backfilling the collector trench with soil, use caution not to damage the hose. The backfilling material shall be devoid of any sharp particles. The material surrounding the hoses shall be compacted, 30 cm thick, and free from voids.
To provide for future collector inspection, make a layout drawing and enclose it with the installation plan.
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