Products - Ground Source Heat Pumps
Ground and Air Source Heating
A few metres below ground level the earth remains at a constant temperature of about 10 - 15 degrees in the UK. You can make use of this fact to transfer this latent heat to your home using a ground source heat pump. Heat pumps draw heat from the ground via a ground collector (slinky) or a ground probe (bore hole).
Heat pumps work in much the same way as a fridge but in reverse. In a fridge the heat is transported from inside to outside, while a heat pump takes heat stored below the underground frost line and transports it via the heating system to the house interior. Some systems are designed to work in reverse as well, cooling the interior in hot weather.
Heat pumps work by extracting the inexhaustible free supply of solar energy stored naturally either in the ground, air or water and utilizing it for domestic heating and hot water. Solar energy in this form is an unlimited resource that produces no harmful emissions and requires no transportation or supply pipelines.
Air source heat pumps use the same principle but extract the heat from the air, rather than the ground. As such their installation is much simpler and cheaper but the available heat is not constant.
These systems require power to drive them, but in an efficient system one kilowatt of energy can generate three kilowatts of heat.
Although the installation of a ground source heat pump to an existing building can be disruptive it has the following advantages:
» Low Operating Cost: provide domestic water heating essentially free or at substantial savings.
» Low Maintenance Costs: fewer mechanical components, making them more reliable, easier to service and less prone to failure. The ground heat exchanger and its associated piping has an expected life of over 50 years.
» Durability: last much longer than conventional HVAC systems because they are protected from harsh outdoor weather.
» Low Noise: systems are among the quietest ever designed.
» Reduced Carbon Emissions: do not require external venting, as fossil fuel systems do, so they do not pollute the air.
How do they work?
Ground source explained
The heat from the earth is a valuable, renewable resource that we can tap into without damage. The technology behind heat pumps, whilst not conventional, can be easily explained.
At a depth of 1metre below the surface the ground remains at a relatively constant temperature of 12°C throughout most of the UK. This energy can be harvested from the ground either by placing horizontal ground loops in a series of 1-metre deep trenches or by placing the ground loops in vertical boreholes.
Very cold fluid is pumped through the ground loops where its temperature is increased by the surrounding warm ground. The heat pump unit extracts this energy from the fluid and uses it to warm water for heating and domestic use.
A heat pump can genuinely reduce energy bills by around 70% compared to oil. When run efficiently at about 45°C, they can produce four or even five times as much power as they use, this is called Co-efficient of Performance (COP) and means that for each kilowatt of energy provided to the heat pump 5 kilowatts of energy will be provided to your house. Unfortunately, with older properties, the ideal level of insulation required for a Ground Source Heat Pump to be fully effective during the very coldest days of the year, cannot always easily be achieved. The harder a Ground Source Heat Pump has to work the less efficient it is. Therefore, the COP will drop and in some cases you may have to supplement the worst days with an auxiliary system, such as an existing oil boiler for example.
The basic principles of the Ground Source Heat Pump system begin with the absorption of heat from the heat source (rock, ground or lake). This is through a closed collector system containing water mixed with vegetable based antifreeze. The collector fluid then emits its heat to the refrigerant in the heat pump's evaporator. The refrigerant vaporizes, and is then compressed. The refrigerant, with its increased temperature, is led into the condenser where it emits its energy to the heat medium circuit, and therefore provides heating to the home or the hot water cylinder or both.
For horizontal ground loops, of all the closed collector heat sources mentioned, using a lake as the heat source is the most preferable option, as it has the best heat transfer qualities. Installing the collector in a lake or any permanent water source will also drastically reduce the necessary collector pipe length. In short, the wetter the better. Installs using closed loop collector pipes in the ground also follow these rules; collectors in wet sandy soil or wet clay are always positive. It is dry media, which is the least suitable; as the least effective transferor of heat the pipe lengths required can increase hugely. This is because they take longer to disperse the cold and gain degrees from the earth.
When a lake or sufficient ground for loops is unavailable, drilling boreholes becomes an option. In the UK drilling can be an expensive exercise due to the unpredictable make-up of ground. Drilling companies have to be prepared for many eventualities and are reluctant to provide fixed price quotations.
Advantages
» Consistent temperature below ground throughout the year providing a high CoP
» Full integration with solar systems
» Boreholes only require a small ground area to harvest energy
Air source explained
Air source heat pumps are often a practical alternative, as they require no digging or drilling and fewer plumbing connections are used during installation, making them cost effective to install. The units extract energy from the air and will operate in temperatures down to -20°C. Inevitably the CoP for air source units will be slightly lower than for ground source heat pumps, however they will still make a considerable contribution to reducing energy consumption by up to 50% when compared to conventional boiler systems.
An Air Source Heat Pump, also known as an Air to Water Heat Pump, works in precisely the same way as a Ground Source Heat Pump, except they take the heat out of the outside air, instead of the ground. This might sound strange in the winter months, but given that the refrigerant in a heat pump boils at around -20°C, even the coldest weather feels hot to a heat pump!
Some Air Source Heat Pumps in particular are designed for radiator based heating systems and can be used together with most electric boilers, oil-fired boilers or equivalent if docking is required. Some systems (usually Swedish) will work to outdoor air temperatures of minus fifteen degrees centigrade meaning that no extra heat source will be required due to the UK's milder climate. The heat pump includes an advanced control system for optimal control. This is started by a signal from another controller, return line sensor or thermostat. Some high quality pumps can also be controlled by a specially designed controller. This connects and disconnects the additional heat source whilst also controlling switching between room heating and hot water heating.
The main advantage of an Air Source Heat Pump is that you do not have the cost and disruption of laying the ground arrays. A disadvantage is that the Air Source Heat Pump only goes up to a maximum of 14kw capacity; a larger output than this and the fans become so large due to the air volume requirement the machines become impossible to site in most locations. Also Air Source Heat Pumps are not quite as efficient in the coldest weather, and are not capable of passive cooling. Having said that, in the south of England, where recent average winter temperatures have been around 6°C, this is not such a great problem. Indeed, it is arguable that the Air Source Heat Pump is more efficient in spring than a Ground Source Heat Pump as it is likely that the temperature underground will have cooled to less than 6°C due to the effect of having freezing water running through the pipes throughout the winter months whilst the outside temperature will be significantly higher. For pool owners, an Air Source Heat Pump is usually the cheapest option for pool heating. In the winter months you could expect an average COP of 3.5 /1, and in the summer an average of around 8/1.
Advantages
» Lower capital expenditure to achieve energy savings
» No ground works required
» Full integration with solar systems
Exhaust air explained
These units extract the latent energy from the internal air of a property and use the recovered heat to recharge both the under floor heating and domestic hot water. By doing so they also provide a means of controlling ventilation within the dwelling. The exhaust air unit is best suited to apartments and smaller houses with low heat loss and will often provide an excellent solution for house builders wanting to meet the requirements of The Code for Sustainable Homes.
Advantages
» The heat pump is sited within the property
» Controls the ventilation
» Helps achieve Level 3 of The Code for Sustainable Homes in apartment dwellings and Level 4/5 when linked to solar
Under floor Heating
A heat pump operates most efficiently when the temperature difference between the heat source and the heat demand is smallest. For that reason they are most suited to powering under floor heating systems which typically run at a temperature of 36-40 degrees rather than radiator based heating systems which typically run at 70 degrees plus.
Most ground source system companies also supply and install under floor heating and can advise on your total system requirements.
Commercial Heat Pumps
Geothermal heat pumps are also used in non-residential buildings, but the variety of loads and load patterns in these applications make it difficult to specify rules of thumb for capacity per unit of building area, or quantity of heat exchanger required per unit of heat pump capacity. In commercial applications a field of bore holes is drilled. Bore holes are spaced 15-20 feet apart and generally at 150-200 feet deep per ton of cooling.
Grants
The DTI's Low Carbon Buildings Programme provides grants for microgeneration technologies for householders, community organisations, schools, the public sector and businesses. The programme is managed by the Energy Saving Trust.