Geothermal Heating Explained
The use of ground source heat pumps to provide heating and cooling is commonly referred to a "geothermal heating". This is in fact not strictly true, as geothermal heating is the term for energy extracted from extremely deep geothermal energy probes drilled down many hundreds, if not thousands of metres into the earths crust to extract energy from super heated steam, to power turbines to generate large quantities of electricity, or supply district heating systems to whole towns.
Iceland, is currently one of the world leaders in this technology, and is currently operating several geothermal power stations, and adding to the number at ever increasing rates. It is Iceland's goal, to eventually become an nett energy exporter into Europe, and offer an alternative to gas currently being supplied from Russia.
Within the domestic / commercial market place ground source heat pumps (GSHP) are generating temperatures more suited to central heating and hot water production, rather than super heated steam. The systems are based around a number of collection mediums, surface energy collection, from shallow trenches dug into gardens or open agricultural land containing "slinky" collectors, or more comonly geothermal energy probes drilled down into the bedrock and thermal springs below the sites, and occaisionally from streams and lakes.
Typically the energy probes are drilled anything between 60 and 250 metres into the bedrock, the deeper the probe the greater the energy extraction level. The energy probe holes are cased with a high grade steel liner from the surface down to the stable bedrock, and then "toed" into the bed-rock about a metre to provide a good seal to avoid surface water entering the ground water system. Once the geothermal probe has been installed into the probe pocket the top of the probe is capped off with a proprietary rubber boot with stainless steel sealing clamps to avoid surface water or foreign debris entering the probe pocket.
Where Does The Energy Come From?
GSHP systems draw their energy from the stable ground temperatures that exist below the surface, within the Channel Islands, South of England and Northern France ground temperatures are usually relatively stable at around 12 degrees Celsius. The soil surrounding the "slinky" energy collector field or the ground water around the energy probe acts as the heat source in winter and the heat sink (dumping ground) in summer. The beauty of a combined heating / cooling system is that energy extracted in winter to provide heating is replenished by warmth being dumped from the house /office cooling system in summer. Thus making the renewable energy system even more renewable.
Where Else Could We Collect The Energy From?
In addition to surface energy collection it is also possible to collect energy from rivers, small streams, ponds, lakes even the sea.
Energy collection from any of the above means is a complex and specialist task, the engineering of these systems is much more complex than simple surface energy collection, however the potential energy extraction rate is considerably greater and in many cases limitless. A large commercial site in close proximity to a river or the sea could easily fulfill all of its heating and cooling requirements without any need for auxiliary back-up fossil fuel boilers or expensive refrigerant based air conditioning units.
How Well Will it Perform?
GSHP systems are in excess of 500% efficient, creating over 5 times the amount of energy they consume, this factor of efficiency is called COP (coefficient of performance). Nibe of Sweden currently hold the world record on COP performance at 5.03. Virtually all domestic GSHP units will incorporate a small back-up electric cartridge heater that can be automatically brought into operation to offer a boost during sustained abnormally cold conditions, from experience we find that these boost heaters are extremely rarely called upon due to our moderate climate.
A well designed installation will provide all of the domestic heating and hot water requirements and operate at a fraction of the cost of any fossil fuel installation.
What Does an Installation Consist Of?
The Geothermal Ground Source Heat Pump system has three main components: (1) the collection medium, either a slinky collector field or a geo-energy probe installation, (2) the GSHP unit, and (3) a heating and hot water system, the cooling option is usually a "bolt-on" additional package to the primary system.
1 -The Collection Medium
The GSHP energy collection pipework is specially manufactured 40mm SR11 40mm plastic pipe and comes in large coil, the pipes thin wall allows fast and efficient energy transfer between either the collection medium and the thermal transfer brine. As with all pipework systems, the joints are the weakest link. To provide maximum reliability and performance we recommend "electro-fusion" welded fittings, these high quality specialist fittings are designed to weld the pipe and fitting into one union, giving the pipe and fitting the same robust integrety and making the installation virtually indestructible and totally leak proof.
Standard mechanical fittings are cheap and readily available, but are suspect to fracture and leakage especially if the collection field is over planted with trees and shrubs. As the roots extend down they can cause the pipework to be susceptible to "heave" and eventually cause a leakage of the geothermal brine, any loss of brine pressure will cause the whole system to shut down.
Why does the Brine not Freeze up?
The thermal transfer brine that circulates between the GSHP and the energy collection field is a mixture of vegetable grade antifreeze and potable water, the mixture strength depends upon the individual system dynamics. On average the quantity of brine mixture to water is between 33% and 45%.
Does the Brine go Off?
A major problem than can exist within the collection field is bacterial growth, systems need to be very carefully installed by skilled engineers with knowledge of the potential pitfalls of bacterial contamination. GSHP systems can ground to a halt within a few months if bacterial growth contaminates the brine medium. The effects of even mild bacterial build up can be severe, it is not uncommon for the whole brine medium to turn into a thick "soup" that can be costly to be rectified.
2 - The Ground Source Unit
GSHP units within the domestic sector of the market come in two options, option one is a combined GSHP with integral hot water tank or option two (usually for the larger house) is a free standing GSHP with a matched hot water accumulator tank.
The combined unit will typically have a 160 litre hot water tank inbuilt, and its about the size of your regular vertical fridge freezer. This unit is usually 240v single phase and has an output capacity of between 4 and 12Kw, these combined GSHP's will happily heat a new built home up to 350 sq/mtrs (3,500 ft) with two bathrooms.
For larger properties, or homes with 3, 4 or more bathrooms where the hot water demand is greater, a stand alone GSHP with a matched hot water accumulator tank is the favoured design option. The VPA hot water accumulator tanks come in various sizes from 200 to 450 litres as standard, and for large installations in modules or 500 litres that can be coupled together.
For county properties often located far from the core of the main electricity grid and served by overhead power lines, 3 phase units maybe required as the starting currant of the scroll compressor is shared over the three phases and reduces the drag off the grid. A typical 12Kw 3-phase unit would have a starting currant of only 7.3 amps a phase making little effect upon the electrical network.
The GSHP unit requires no actual maintenance, being factory assembled all of the components are tested and checked before it leaves the manufacturers, with only a couple of moving parts and now flues or flames to worry about, the unit is maintenance free. Also, being all electric, it requires no ventilation or special conditions so often found as a pre-requisite with fossil fuel appliances.
Can I Check How Much Its Costing to Operate?
Most good quality units have a programme accessible though the front end that will allow you to see how many hours the compressor has run, multiply this by the input power of the GSHP, and you have the number of Kilowatts consumed, multiply this again by the electrical tariff, and you have the actual cost of operating the unit. Some sceptics will argue that there's other costs associated with operation such as power consumed by the brine pump and heating circulation pumps etc, this is true, however the amount of energy consumed annually pales into significance when the savings are analyzed as a percentage of what a fossil fuel system costs to operate. If of course you want really accurate results, the operation manual will give the wattage of both pumps and again it a mathematical exercise to multiply it by the hour run log.
After several years of monitoring installations and annual fuel bills regularly see savings in excess of 80% over fossil fuel installations. With an anticipated working life of 25-30 years the environmental and economic benefits of installing such systems cannot be ignored.
3 - The Heat Emitter System
GSHP installations will work with almost any type of heating system from, underfloor heating, radiators, warm air heating, and swimming pools.
Before we discuss individual forms of heating its worth for a moment considering how a conventional domestic heating and hot water system works, typically a time-clock or programmer is installed that controls the fossil fuel boiler and heating system. Most installations roughly fall into a similar bracket, with the heating coming on at 6.00 am and off at 9.00am, it stays off most of the day and comes on again at 4.00pm and then off again at 11.00pm thus operating in bursts for about 10 hours a day, with slightly longer running times over the week-ends and public holidays. For a system to operate in this manner the installation has to be sized to allow rapid heat up from cold, thus larger than actually required plant is installed and the systems operate at very high temperatures. Most domestic heating systems operate on a 82 Deg C flow temperature and a 71 deg C return temperature.
A GSHP installation works in a completely different manner. The heat pump is very often sized at only 25% of the output of a fossil fuel boiler, However, it operates 24 hours a day, slowly and gently trickle charging heat into the home via the heat emitter. The GSHP installation also has an outside weather sensor that constantly monitors the external temperature, and modulates the temperature of the heating water, either up or down, on colder days the water enters the system warmer, and on warm days it enters cooler, this term is known as weather compensation.
Underfloor Heating Systems
Almost any underfloor heating system can successfully operate with a GSHP, however most "packaged" UFH systems are designed to be connected to high temperature fossil fuel systems and require the flow temperatures to be lowered at the manifolds and have all manner of controls and pumps to facilitate this. Geo-floor is a bespoke UFH system specifically designed by our design department for use with GSHP units. It is suited to any floor application and is designed to operate directly from the heat pump and has been engineered to offer both both increased comfort levels and economics. For further information please look at our underfloor heating section.
Radiator heating falls into two categories, existing installations and new installations. When re-using existing installation's which generally occurs when an existing heating installation is converted from expensive fossil fuel heating to low cost geothermal heating the installation is of course already in place. It is important to fully flush any debris from the existing system as carbon debris and sludge will quickly choke the filters on the heat pump.
When re-connecting to any existing installation it is critical to assess the quality and performance of the installation, very often the home owner can be of assistance in his area, its worth asking how they feel the existing system performs, often they will identify areas that do not get as warm as they would like or highlight radiators that do not work. The installation should also be checked for iron oxide debris and sludge, this seriously effects the performance of the heating system and if left unaddressed will cause the GSHP to trip out due to low flow.
Warm Air Systems
GSHP units can be used very effectively to heat warm air system, the new high velocity systems such as the Unico system perform extremely well and offer an alternative to the old style low volume warm air heating systems so prevalent in the 1970's. These systems are particularly beneficial in restoration projects where lifting old floors and chasing into walls would damage the character and structure of the building, another benefit is they offer comfort cooling in summer as standard.
Swimming Pool Heating
GSHP's are ideal for providing low cost "top-up" heating within the shoulder months of the swimming season, of course systems can be designed to accommodate both small and large outdoor or indoor domestic and commercial heating applications where primary heating is required all year around. Within the domestic market place, the relatively small output from the GSHP can be used to assist in heating the pool. As there is very little demand during the shoulder and summer months for house heating, the excess capacity can be utilised to provide heating for the domestic outdoor pool.
It must me noted that the added benefit during the shoulder months is due to the lower demand of the home heating system, the GSHP in these installations must only be considered as a bonus to extending the swimming season, also a good quality pool cover is essential to avoid excessive night time losses. If constant pool water temperatures are required then a bespoke heat pump pool heating system must be utilised.
Potential Other Uses
There are many uses, here are a few that we cannot cover in detail, these include;
- Large Scale Commercial Hot Water Heating For - Hotels, Sports Pavilions, Distilleries, Centrally Serviced Apartment Blocks, Factories
• Agricultural Uses Including - Nursery Plant Warming, Glass House Heating, Flower and Seed Prorogation.
• Sports & Community Centre Heating & Cooling Systems
• Church Heating
• Sports Field Defrosting
• Computer Room Cooling
• Water Theme Parks
if you require more information please contact us.