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. Regarding commercial and residential use, they move residual warmth in the air or ground from outside a building to inside. Heat pumps can extract low-grade heat needed to create useful heat transfer into the desired area. This can be 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. This is because 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. Faults are rare but can happen. The heat pipe system can be buried up to 100m underground so difficult to access if there is a fault. Also, 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). This is because they do not require bore holes or underground systems with nearby waterways, lakes or ponds. They do function better in higher temperatures though and so one downfall of the ASHP is that air is the most seasonally diverse source. In winter, when heat is required the most, there is less available heat to be transferred. In Summer it is 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. The UK, however, has a pretty stable and agreeable range of temperature throughout the year, so they can operate at a high efficiently.

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. This is because 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. This was as an insurance against loss of a heating device for installed buildings. Modern ASHPs, however, 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 developing heat pump design, especially regarding large capacity heating. That was, however, until Kensa Heat pumps, based in the UK started to manufacture heat pumps. These pumps have a capacity up to 24kW for single phase electricity supplied homes. This is a lot larger than the previous common 12kW capacities. The larger output models, however, require a larger initial start-up input. Particularly in older properties, this can have side effects, such as lights flickering when they come on. Also, if a power supply is shared with others, this could become an installation obstacle, as the demand for, and on, these units is understandably very high.

“We need to stop living in poorly insulated, high heat-loss homes using radiators containing water at 70°C and 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. This is because 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 fact 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. This is done 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. This allows for short spells of insulation reductions with minimal heat loss (windows opening etc.). This also again increases the surface area the heat must emit from. In existing buildings this may be prohibitive and more complicated to achieve. This may be 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 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. 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 enough.

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, (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. On average, though, heat pumps will work at a 3.5 or 350% efficiency. This means that for every unit of energy put into the pump as electrical energy, 3.5 units of heat energy are put into the home. This is in comparison to 0.96, or 96% average rating of a gas boiler. This shows the potential benefit a gas consumer can have by converting to 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.