Colin Pavey, sales manager for CTC, the renewables division for Enertech, offers a guide to both ground source and air source heat pumps to assist with making an informed choice on which technology to select.
The principles of how air source and ground source heat pumps work, as well as their benefits, are very straightforward. Both work on the same refrigerant circuit technology, extracting energy from a natural, readily available source, recovering it from the refrigerant and making it available to a heating system, all driven by an electrically driven compressor. Ground source heat pumps absorb this heat from the earth, while air source heat pumps draw in heat from the surrounding atmosphere. The output from the heat pump is then used for space heating and, where required, provision of DHW.
Take ground source heat pumps and their efficiency. The temperature of the ground is fairly constant when measured from a borehole or ground loop with an average of around 7-8°C, so the CoP figures will remain steady throughout the year.
With air source heat pumps, the efficiency will change considerably throughout the year with CoP figures varying according to the ambient temperature. When the temperature outside is low, more electrical input is required and the efficiency falls, but when the air is warmer, efficiency will increase.
Year-round performance
Where the number of frost days is considerable and the atmosphere is humid, air source units tend to be less efficient, due to frost build up on the evaporator and the need for de-icing. Units with large evaporators, such as those offered by CTC, have lower temperature drop across the evaporator, leading to fewer de-icing cycles and better year-round energy performance.
In coastal locations, where ground frost is more infrequent and the air temperature rarely falls below 3°C, the average air temperature in the winter can be above the 7-8°C of the typical ground loop system. An air source heat pump will have to make less of an increase to achieve the 35°C flow temperature than the ground source unit. Therefore, during a heating season the coastal air source equipment will be more efficient than ground source system.
CoP can be in the range of 3-4 for ground units, with a fairly consistent 'supply' temperature from the ground collector into the evaporator - usually in the range 7- 10°C. The CoP of air source units can vary considerably, with 2-4 being typical with ambient temperatures above say 2°C.
One of the main considerations of ground source equipment is that the initial capital costs will be higher than other methods as it requires excavation work, plus the costs of the loops, manifolds and will always require pumps to be in constant operation.
When compared with ground source units, the installation of air source heat pumps is a much easier process. Usually they are sited adjacent to or slightly away from a building with simple flow and return pipework linked to the heating system. This in turn means the overall purchase cost and installation of air source units is lower than their ground source counterparts.
Possible nuisances
When siting units, steps should be taken to avoid possible nuisance which may arise from noise generated by the fan.
Required flow temperatures are the other significant variable on the CoP. Heat pumps are most efficient when operating at the lowest possible flow temperature, with a sensible ceiling, from the energy efficiency point of view, of 50°C. The emitter system must be designed to accommodate this. Underfloor heating is generally the preferred heating option, as this usually requires low flow temperatures from 30-45°C, but oversized or forced air radiators can also be used. Where DHW is to be produced, the cylinder must have a longer coil, specifically designed for the lower flow temperature. A system and regime for legionella control must be incorporated.
Water to water heat pumps
Ground source heat pumps are also referred to as water-to-water heat pumps, which better describes their operation. There are three types of collector - horizontal loops, boreholes and lake collectors. Horizontal loops require a large amount of suitable land, while boreholes can be drilled in relatively confined spaces.
The design of the array of horizontal loops is critical, and will be specific to the ground. With bore holes, the geology of the area has to be taken into account and a specialist survey is strongly advised. Different rock types have varying thermal characteristics, heat transfer and conductivity. Local conditions, such as elevation and positioning also have a bearing, as does the ground water level. With either option, seeking the advice of experienced specialists is highly recommended.
Where the groundwork needed for water to water heat pumps is either impractical or otherwise unfeasible, air source heat pumps can be a good choice.
The fan and compressor circuit parts of an air source heat pump - such as the EcoAir range from CTC - is normally held in a single cased unit, sited outside.
Because of the ease of siting and lack of need for collector circuits, air source units are often suitable for retrofit applications or new builds - offering a flexible solution that will suit most projects.
Air units with a large evaporator offer the most efficient solution, not only to extract more energy from the air, but also to reduce icing. When temperatures are a degree or two above freezing, with high humidity, there is a risk of freezing on the evaporator. The unit then has to switch to de-icing mode, losing output and impacting efficiency.
The large evaporators favoured by CTC and proven in Scandinavian conditions as well as in the UK, ice less frequently and drain more rapidly, offering a better solution during difficult low temperature/damp conditions.
It is usual for heat pumps to be sized slightly below the total load, to avoid excessive cycling and this dictates that some form of back-up heating must be included.
As with all heating appliances, good control is paramount, as is careful integration of components, to avoid conflict between components and to optimise efficiency. CTC's EcoLogic EXT controls include fully weather compensated flow temperature control and will control other parts of the whole system.
With both types of heat pump now readily available in the UK, specifiers and contractors are able to choose the solution that is most suitable for the individual application.
And, if both technologies are fully understood, then advising customers and securing a contract should be easy to achieve.