CPD Programme
Pre-Heated Cold Water
21.3kW GSHP,
4.7 CoP (G0/W35)
35°C 25°C
Buffer Tank
Figure 5 – Space Heating (Under-Floor) Application
Operation period Ground type:
poor underground and dry sediment
normal
underground and water-saturated sediment
60 W/m 50 W/m consolidated rock 84 W/m 70 W/m
Table 2 – Practical heat pump extraction rates per metre run of vertical bore hole (see EN 15450:2007 for full detail)
And so using the same 10kW load as
before the extraction rate would be just under 60W/m and so the vertical borehole would need to be 10000/60 = 167 metres deep. Practically, two bore holes of approximately 85m deep or three at 60 metres are likely to be used. In recent years there have been a growing number of ground source loops being integrated into the piles of buildings. Whether the ground loop is open or comprises horizontal trenches or deep bore holes, it is always recommended to conduct a geological survey.
Optimising operational performance
The key aspects to ground source heat pumps delivering economic and environmental benefits is control of its operation. The Coefficient of Performance (COP) of the heat pump is a function of the ground loop temperature and the load into which the thermal output of the heat pump is being discharged – the closer the two temperatures are the greater the COP and COP may be calculated from: COP = thermal heat delivered (kW) / electrical power supplied (kW).
As the temperature of the ground loop is relatively constant, unlike air source heat pumps, seasonal variations in performance with a ground source heat pump will be less than for air source. For comparison purposes the performance of heat pumps is frequently measured on a seasonal basis using the
www.cibsejournal.com
Figure 4 – ‘Slinky’ coil being installed
seasonal performance factor (SPF ) and SPF = heat energy delivered (kWh/ season) / electrical energy supplied (kWh/season).
As electricity is being consumed and thermal energy is being generated, the financial benefit offered by ground source heat pumps is a function of the electricity tariff and the cost of the fuel being used as a primary source of heat (gas, LPG, oil or electricity). In a properly installed GSHP the heating energy costs should be below that of any other centrally supplied fuel. The better COP is achieved in applications
where the ground source heat pump is integrated into a heating system using low temperatures such as under-floor heating, such as the one shown in Figure 5. Another application to ensure low flow
and return water temperatures to the heat pump condenser is to use the heat pump to pre-heat hws feed water where, using an indirect cylinder as an interface, the ground source heat pump can operate in conjunction with a direct-fired water heater or a hot water calorifier as shown in Figure 6. The schematic shows the ground source
heat pump load side temperatures being controlled to 35°C flow and 25°C return. At these operating conditions temperatures
1 800 h 2 400 h 25 W/m 20 W/m
35°C 35°C
Flow to Under-floor Heating Manifold
21.3kW GSHP,
4.7 CoP (G0/W35)
35°C 25°C
Return from Heating Circuit 25°C Indirect Cylinder
30°C
Direct-Fired Water Heater
Anti-Legionella Pump
Hot Water to Outlets
60°C
Cold Water Inlet
Figure 6 – Cold Water Pre-Heat Applications
within the pre-heat cylinder can rise to 25°C. This reduces the amount of fuel required in the primary heating appliance to raise the hot water service to 60°C with a subsequent reduction in energy costs and carbon emissions. Additionally the use of a the indirect heater
will provide some buffer storage – this is beneficial as it is not recommended [6] that ground source heat pumps start up more than three times in a one hour period. If a pre- heat cylinder is employed then particular care must be taken when determining operational requirements to avoid legionella. In under-floor heating applications, if the
size of the heating array is relatively large, holding a high volume of water, this may act as buffer storage and could mitigate the need for the buffer vessel. This needs careful consideration and depends on the under-floor heating zoning, and a buffer tank may be needed and detailed calculations are needed to determine the requirements to prevent heat pump cycling. © Tim Dwyer and Yan Evans 2010
References
1. BS EN 45011:1998 - General requirements for bodies operating product certification systems. 1998
2. Survey, B. G., Ground source heat pumps - Development of GeoReports for potential site characterisation. http:// www.bgs.ac.uk/reference/gshp/gshp_ report.html Accessed
3. BS EN 15450:2007 - Heating systems in buildings — Design of heat pump heating systems. 2007
4. Heat Pumps - A guidance document for designers - BG 7/2009. BSRIA, 2009
5. BS EN 14511:2004 - Air conditioners, liquid chilling packages and heat pumps with electrically driven compressors for space heating and cooling. 2004
6. Evans, Yan, Baxi - internal notification. 2010.Evans, Yan
April 2010 CIBSE Journal
73
10°C
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