Heat Pumps, Making Use of Low Grade Heat

What are Heat Pumps?

 

Heat pumps are any device which transfers heat from one place to another. In regard to commercial and residential use, they move warmth in the air or ground from outside a building to inside. Heat pumps can extract their low-grade heat, needed to create useful heat transfer into the desired area, from any source above -30°C, although they work better the higher the temperature of the source.

Traditionally, ground and flowing water can be used, as they keep a mostly constant temperature all year round, no matter what the conditions. These methods of obtaining heat, however, require an increased initial investment. There is a very small risk if anything were to turn faulty, for example, the heat pipe system can be buried up to 100m underground, or if refrigerant were to leak into the waterways it could cause environmental damage.

Sourcing from the air

Air source heat pumps (ASHP) are becoming more widely used in the UK, as they are elsewhere in the world. They are essentially reversed air-conditioners. They are the easiest to install and the most cost-effective forms of heat pumps (circa £7000 as opposed to GSHP, circa £18-£23,000) to install, as they do not require bore holes or underground systems or nearby waterways or lakes/ponds. They do function better in higher temperatures though, and so one downfall of the ASHP is that when the heat is required most in winter, there is less available heat to be transferred and in the summer, vice versa (although they can be reversible and used to cool in the Summer). As a result the seasonal performance factor (SPF) is 10-30% worse than ground source heat pumps (GSHP) especially on windy and cold days, but the UK has a pretty stable and agreeable range throughout the year for them to operate at a high efficiency.

Working in the UK

 

The UK’s temperature range normally sits between 0°C and 15°C, which increases the SPF compared to other countries, as constant temperatures can raise the efficiency of a heat pump especially if it can stay above 0°C. Due to the intermittent nature of the air source, in the past ASHPs have required a fossil fuel powered backup system as an insurance against loss of a heating device for installed buildings, but modern ASHPs do not require this and can operate efficiently to provide hot water and space heating for most reasonably well-insulated homes.

For residential properties, our single phase supply has had very little investment from manufacturers in pushing the envelope in regards to heat pump design, especially in regards to large capacity heating. That was, however, until Kensa Heat pumps, based in the UK started to manufacture heat pumps with a capacity up to 24kW for single phase electricity supplied homes. This is a lot larger than the previous common 12kW capacities, however the larger output models require a larger initial start-up input, which in older properties in particular, can have side effects, such as lights flickering when they come on and if a power supply is shared with others could become an installation obstacle, as the demand of these units is understandably very high.

“We need to stop living in poorly insulated, high heat-loss homes using radiators containing water at 70°Cand start constructing super insulated buildings that are heated using water at 35°C, through under floor and wall heating systems”

– Andy McCrea, Renewable Energy, 2013

Fitting and efficiency

Due to the fact that any heat pump circulates heated output at a temperature of around 35°C, the traditional radiators are inefficient at distributing the heat to the room, as it has a relatively small surface area. At these lower temperatures, larger radiators or a network of under floor heating is required to distribute the heat evenly and effectively. This means installation and retrofitting of heat pumps can be labour intensive and not suited to certain buildings (particularly listed buildings). They also require a better base level of insulation as they cannot allow for as large a heat loss as the conventional 70°C wet central heating systems.

A new construction can most efficiently use the pumped heat by designing heat outlet pipes in the core of any building (between walls of two rooms or floors of two levels). This also means there is less pipe network required as the rooms can use the same heat source. It is best paired with high thermal retaining materials and buildings, allowing for short spells of insulation reductions with minimal heat loss (windows opening etc.). This again increases the surface area the heat has to emit from. In existing buildings this may be prohibitive and more complicated to achieve, due to any existing infrastructure, leading to expenses mounting and financial inviolability becoming a problem. Commonly retrofits overcome this issue by using existing piping but fitting special air radiators which better circulate the lower level warmth produced by the heat pumps.

Benefits to current gas consumers of switching

If we assume for an example that 1 kWh of electricity is 15p, and 1kWh of gas is 5p, heat pumps as an example can be said to convert 1kWh into 3.5 (worked out using the COP, see below) units of useful heat, where the traditional heating system works at a 1:1 ratio of 1kWh to 1 unit of heat. Therefore the one unit of electricity which the heat pump would use, could actually become a more efficient method of heating the desired area, so long as insulation is sufficient.

To work out the coefficient of performance (COP), which is the formula used to assess the efficiency of the system in question, there is a simple formula:

Direct ground heat source pumps, which is when the refrigerant is directly circulated into the ground, can work at up to 500%, or 5.0 efficiency levels at certain times of the year, but on average heat pumps will work at a 3.5 or 350% efficiency. That is to say for every unit of energy put into the pump as electrical energy, 3.5 units of heat energy are put into the home, in comparison to 0.96, or 96% average rating of a gas boiler. This shows the potential benefit a gas consumer can have by integrating a heat pump (~250%~). The difference in actual heat output, however, may mean that a property wanting to swap between the two supplies may have to insulate before installation to gain the full possible improvements.

Investment and Incentives

The government has just increased the Renewable Heat Incentive (RHI) for this kind of technology to encourage increased uptake in the UK. If you are thinking of installing a system like this, now is the time! Read more on the RHI here: https://www.renewableenergyhub.co.uk/heat-pumps-information/heat-pumps-cost-and-savings.html

 

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