Ground and air generated heat

The ground and air around a development site can be used as a source of heat for new buildings via a heat pump

Pipes under the tarmac in the playground heat this school in the winter. Photo by Fulcrum

This is a device that moves heat energy from one place to another and from a lower to a higher temperature, or visa versa. Heat pumps are available as both heating only or reverse cycle heating/cooling systems and are classified according to the type of heat source and the heat distribution medium used.

Typical systems use a refrigeration cycle with electricity as the energy input driving the process. They are generally more suitable for heating applications that use lower temperatures, such as underfloor heating. The efficiency of heat pumps is measured in terms of COP (coefficient of performance). For example, a COP of 3 would provide 3kW of useful heat output for every 1kW of electrical input. For a heat pump to be considered better than an efficient gas boiler in carbon emission terms it needs a COP of around 3 or above.

There are various types of heat pump, each with different sources and sinks. For example, for a water-to-air heat pump, the source could be ground water, and the sink the air in a conditioned space within a building. The lower the temperature difference (seasonally) between the average source and sink temperature, the greater the efficiency of the system, the higher the COP and the lower the CO2 emissions. For this reason heat pumps can generally only be considered as a ‘pre-heating’ method for producing higher temperature process heat such as domestic hot water (as otherwise carbon emission efficiency would be unacceptably low) though technology advances may soon overcome this constraint.

Applications include space heating and cooling, pre-heating domestic hot water, heat recovery and dehumidification in both domestic and industrial sectors. Types of heat pumps include:

• air-to-air: the most common type of heat pump. An example of this is a domestic refrigerator or heating mode in a reverse cycle split system air conditioning unit. Air-to-air heat pumps have a typical seasonal COP of between 2.5 and 3.
• water-to-air: these rely on water as the heat source and air as the sink, to transmit heat or coolth to the conditioned space. The COP of water-to-air heat pumps will depend on the temperature of the source water, but typically lies between 2.8 and 3.7.
• water-to-water: these are typically used to heat water, for example using a source of ground water at 11°C to provide the energy input for underfloor heating. The COP of water-to-water heat pumps will depend on the source water temperature and the temperature to which it is being raised, but is typically in the range of 3 and 5.
• air-to-water: these are a major possibility for use in existing housing as they are a direct boiler replacement option with similar COPs to air to air units.

Ground source heating and cooling systems above 20Kw output must be considered environmentally unharmful in line with Environment Agency guidance. In order to be approved, the Environment Agency states that the systems must be in thermal balance between how much heat is needed and how much cooling is required - heat (and cold) is a possible pollutant.

An interseasonal thermal storage system is therefore the ground source system most likely to get regulatory approval in the future. Because the source is warmed, it will also have, by definition, the best COP (typically between 6 and 12). The use of interseasonal thermal storage systems, managed at the municipal scale, is seen by some as one of the most exciting urban possibilities at the city scale to comply with possible new zero carbon construction standards at the best economic cost.

Many different ground source energy systems are available, but the main types involve utilising the natural thermal conditions of the ground, large bodies of underground water, or large bodies of surface water.

The earth absorbs a large proportion of incident solar radiation, which keeps the ground/groundwater in the UK at a stable temperature of around 11-12 degrees C throughout the year. This is warmer than the mean winter air temperature and cooler than the mean summer air temperature.

This heat/coolth can be harnessed using a water-to-air or water-to-water heat pump connected to a ground heat exchanger, in one of the following combinations:

• closed loop ground coupled heat exchange. This is typically a network of pipes laid either horizontally or vertically in the ground, through which a liquid is pumped in a continuous loop. The low-grade heat contained within the ground is extracted via the liquid and can be converted to higher-grade heat (water temperature of 35-65°C) by a heat pump. In the summer, the cooler ground enables the extraction of cooler-than-ambient liquid, which can be used to cool buildings. It is therefore better to consider this technology where there is not a significant flow to groundwater.
• open loop groundwater heat exchange. Borehole water is extracted from and rejected to an aquifer for heat exchange as above. If the heat rejected during summer cooling operation is stored in one part of an aquifer and coolth from winter heating operation stored in another more than 50 metres away with flow direction changed seasonally, the system is known as aquifer thermal energy storage (ATES) and is the most efficient of the heat pump-based systems.
• closed loop surface water heat exchange. This system uses surface water bodies such as lakes and rivers for heat exchange and is likely to have a lower COP than an open loop groundwater system as the lake/river source is likely to be colder in winter and warmer in summer than a ground water source. It may not get Environment Agency consent as rejected heat or coolth in water is considered to be a pollutant as it can affect eco-systems.

Air source heating and cooling systems are most appropriate at the building scale - as individual systems for dwellings and buildings or as communal systems for blocks. They require an adequate building size to justify installation.

The Westway Beacons project is the first example in the UK of Aquifer Thermal Energy Storage (ATES) for space heating and comfort cooling. The development of 126 social rented homes benefits from active cooling to combat increasing summertime temperatures as part of a low carbon, low-cost approach.

Ground source heating systems are most appropriate as building scale individual systems for homes, while groundwater (aquifer) source heating and cooling can be applicable to larger mixed use buildings or communal systems for blocks or neighbourhood schemes. They can be used to retrofit existing mixed use buildings as a low carbon solution. They require space for horizontal ground loop (lowest carbon savings), suitable ground conditions for boreholes or vertical loops (better carbon savings) or an available suitable body of water (aquifer) as a heat sink (best carbon savings especially where cooling is also required).

A variant of ground source heating systems is interseasonal heat transfer (IHT). Pipes are placed below tarmac to heat or cool liquid. This can be effective as a tarmac surface in direct sunshine will often be 15° C warmer than the air temperature at the same time.The heat is then transferred to thermal banks below the building for release when needed. This enables transfer of heat (or cooling) between day and night and summer and winter. IHT may be effective in places where there are large areas of tarmac such as school playgrounds, car parks or roads.

Howe Dell school in Hertfordshire has been fitted with such a system and it is currently being monitored in use to judge effectiveness.

Solar heat may also be collected from flat roofs for storage in thermal banks.

Cost

The average capital costs for a ground source heat pump system would be between £800 and £2,750 per kWt. This cost reflects the capital cost when applied to a Part L1a 2006 compliant home and is based on figures from the 2008 Communities and Local Government research. The same research also undertook an economic cost and benefit analysis of each technology. This found the value of saving in energy costs were £807 per tonne of CO2 saved.

Priority: develop a low carbon and renewable energy portfolio
Tags: energy, neighbourhoods, buildings and spaces