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GROUND SOURCE HEAT PUMPS

(Often referred to as "Geothermal" or abbreviated to GSHP)

A heat pump is a system that uses a refrigeration-style compressor to transfer heat from outside to inside, in order to heat offices or homes. Heat pumps can take heat from the air, water or ground. Heat pumps that use the outside air temperature are generally inefficient – this is because the air loop attracts condensation, and this quickly freezes, building up a thick layer of insulating ice, forcing the heat pump to work harder and harder.  For this reason, ground source heat pumps are the choice of preference. Ground source heat pumps are very efficient – in fact you will get 3-4 units of heat for every unit of electricity supplied to the heatpump – if you are heating with electricity, a ground source heat pump will quarter your heating bill!!

Ground Source Heat Pumps (GSHPs) can use the ground, streams, wells or boreholes to supply the heat. Heat gained from running water or ground water is the most efficient of all.

 
Navitron WRB05 - 5kW output Ground Source Heat Pump with cover removed £950 (see below for specifications)

 
Navitron WRB09 - 9kW output Ground Source Heat Pump with cover removed £1900 (see below for specifications)

Basic description of the component parts of a GSHP:

 1 A heat pump packaged unit: Water-Water type. (approx. the size of a small fridge) containing two cold water connections and two heated water connections.

2. The heat source which is usually a closed loop of plastic pipe containing water with glycol or common salt to prevent the water from freezing. This pipe is buried in the ground in vertical bore holes or horizontal trenches. The trenches take either straight pipe or coiled (Slinky) pipe, buried about 1.5 to 2m below the surface. A large area is needed for this.

3. The heat distribution system. This is either underfloor heating pipes or conventional radiators of large area connected via normal water pipes.

4. Electrical input and controls. The system will be require an electrical input energy, three-phase being preferred, but single phase is perfectly adequate for smaller systems. A specialised controller will be incorporated to provide temperature and timing functions of the system.

This type of installation offers many advantages.

a) The water-water heat pump unit is a sealed and reliable self contained unit.

b) There are no corrosion or degradation issues with buried plastic pipes.

c) The system will continue to provide the same output even during extremely cold spells.

d) The installation is fairly invisible. i.e. no tanks or outside unit to see.

e) No regular maintenance required.

 Some tips

 The efficiency of any system will be greatly improved if the heated water is kept as low as possible. For this reason, underfloor heating is preferred to radiators. It is vital to ensure that the underfloor layout is designed to use low water temperatures. i.e. plenty of pipe and high flow-rates. Heat pumps have a different design emphasis to boiler systems. A mixing valve should not be used.

Most underfloor systems use zone valves that reduce the flow-rate. . To maintain the correct flow-rate through the heat pump a buffer tank is suggested.

If radiators are to be used, they must be large enough. Double the normal sizing (as used with a boiler) is a good starting point.

Whilst this type of heat pump installation could provide all the heating needs, it is common practice, and often economic sense to have a back-up boiler linked to the system to cope with the very cold periods.

Electric back-up is not ideal. This is putting a high load on the Mains supply at a time of peak demand. At this time the power station's net fuel efficiency is lower.

The ground pipe system must be planned carefully, especially as it will be there for well over 50 years. Any mistakes may be too difficult or costly to rectify later. The highest energy efficiency will result from systems that do not go below freezing point, therefore, the bigger the pipe system/ ground area, the better, however, this is costly and gives diminishing returns.

The pressure drop in the pipes should be compatible with standard low-head pumps.

Weather compensation will greatly improve the annual energy efficiency, by reducing the heated temperature to the minimum required, dependent on outside temperature. Most heat pumps incorporate this in the controller.

If you want to keep energy efficiency high, try to keep the heated water temperature as low as possible. Try to keep some zone valves fully open and control the temperature down by carefully adjusting the weather compensation controller. If you don't have weather compensation, simply adjust the water temperature as low as possible such that adequate heating is attained.

If domestic hot water is provided by the heat pump, have a big enough cylinder such that the water can be stored at a slightly lower temperature. Avoid "thermal store" type systems. They require temperatures higher than heat pumps can efficiently provide

Heat Pump compressors like to run for long periods. Stop-starts should be minimised. The use of Buffer tanks, correctly set thermostat differentials and correctly positioned cylinder sensors will all help to maximise run periods.

Noise could be a problem if not considered properly. Be cautious at the design stage and this problem should be eliminated.

How does it work?

The earth's surface acts as a huge solar collector, absorbing radiation from the sun. In the U.K, several metres below the surface, the ground maintains a constant temperature of 11 to 13°C. In the winter this temperature is warmer than the air above it. GSHPs are used to extract this heat and transfer it to a building, where heat is required.

  In the summer months the ground temperature is cooler than the air on the surface.The function of a GSHP can be reversed and used as a cooling mechanism, drawing heat out of a building. For every unit of electricity used to pump the heat, 3-4 units of heat are produced.  

There are three important elements to a GSHP system:

 Ground loop

Lengths of plastic pipe are buried in the ground, either in a borehole or a horizontal trench. The pipe is a closed loop, which is filled with a water/antifreeze mixture. This mixture circulates in the pipe, absorbing heat from the ground.

 Horizontal trenches are drilled to a depth of 1 to 2 metres and can cost less than boreholes, but require a greater area of land. Placing coiled piping in horizontal trenches will enhance the performance compared with straight piping.

 A borehole is drilled to a depth of between 10 and 100 metres and will benefit from higher ground temperatures than the horizontal trench, although installation costs will be greater.

 Heat pump

The heat pump works by promoting the evaporation and condensation of a refrigerant tomove heat from one place to another. A heat exchanger transfers heat from the water/antifreeze mixture in the ground loop to heat and evaporate refrigerants, changing them to a gaseous state.

 A compressor is then used to increase the pressure and raise the temperature at which the refrigerant condenses. This temperature is increased to approximately 40°C. A condenser gives up heat to a hot water tank, which then feeds the distribution system.

 Heat distribution system

Because GSHPs raise the temperature to approximately 40°C they are most suitable for underfloor heating systems, which require temperatures of 30 to 35°C, as opposed to conventional boiler systems, which require higher temperatures of 60 to 80°C. GSHPs can also be combined with radiator space heating systems and with domestic hot water systems. However top-up heating would be required in both cases in order to achieve temperatures high enough for these systems. Some systems can also be used for cooling in the summer.

Sizing

Sizing of the heat pump and the ground loops is essential for the operation of the system. If sized correctly a GSHP can be designed to meet 100% of space heating requirements. Please note that sizing is a job for specialists and heating needs should be properly assessed. The sizing of a system is very sensitive to heat loads and should therefore be installed into properties with high-energy efficiency standards, particularly new build. It is a good idea to explore ways of minimising space heating and hot water demand by incorporating energy efficiency measures.

Installation

The Installation of a GSHP should be carried out by a trained engineer. At present the UK market is small and there is currently no network of accredited installers as with other technologies.

 We would therefore recommend you to ask for references and follow these up. Manufacturers and suppliers should also be able to provide trained engineers but geographical limitations may increase installation costs.

Installation costs

The typical cost of a professionally installed GSHP ranges from about £1,200 to £1,700 per kW of peak heat output. This includes the cost of the distribution system. Vertical borehole systems would be at the higher end of this scale, due to greater installation costs. A typical 8kW system would therefore vary between £9,600 to £13,600. Please note that costs will vary from property to property. However, the installation work involved amounts to basic labour – so by carrying out most of the groundwork yourself, it is possible to fit these systems for a fraction of that price. The Navitron Heat pump is available in two sizes – 5kW (ground source to air) and 9kW (ground source to water). The prices are listed below – see how much you can save!! 

Running and maintenance costs

Coefficient of Performance (CoP) is an indicator of the efficiency of a GSHP system. This indicates the number of units of heat output for each unit of electricity used to power the equipment. Typical CoPs would range between 2.5-4.5. There are some exaggerated claims - but these will apply only for temperature differences of 3-4 degrees.

The highest COP will be obtained if you use the heat for underfloor heating or air heating, because it works at a lower temperature (30-35°C) than a radiator system (45-50°C). Dependent on the size of the system installed, the heat distribution system chosen and the resulting CoP, GSHPs can be a cheaper form of space heating than oil, LPG and electric storage heaters. It is, however, slightly more expensive than natural gas – assuming that you are using grid electricity. Why not use the excess electricity from a water turbine to power a heat pump?

 GSHP technology is low in maintenance as systems have very few moving parts. Systems can have an operating life of over 40 years.

 Environmental benefits/ impacts

Significant carbon dioxide savings can be gained by displacing fossil fuels. Even compared to the most efficient gas or oil condensing boilers, a well-designed heat pump with CoP of 3 to 4 will reduce emissions by 30-35%. Further carbon savings can be made if the electricity used to power the pump comes from a renewable energy source such as photovoltaics or a renewable electricity tariff.

 Points to consider

• What type of heat distribution system is required? (underfloor heating or radiators)

• Is there adequate space for installation of the ground loop?

• What would be most suitable, a borehole or trench? Is the ground material appropriate?

 SPECIFICATIONS OF NAVITRON GSHP MODELS

 download user's manual   download controller manual