Heat pumps take useful heat from the natural environment for use inside buildings, normally as space heating. Even though the temperature of the heat source may be cooler than the inside of the building, heat pumps use a special refrigerant which is evaporated into a vapour by the heat source (low pressure, cold gas), then this vapour is compressed by an electrically driven compressor, then this higher pressure ‘hot’ gas is condensed inside the building to be heated, then the pressure of this high pressure, cool liquid is reduced back to low pressure through a throttle valve, and then the process starts again.
This clever thermodynamic process allows the temperature from the outside source to be increased so it is suitable to use for heating inside a building. The principle is the same as a fridge turned inside-out, so instead of taking the heat from the small volume of air inside the fridge and dumping it into the main kitchen via the condenser on the back of the fridge, a heat pump takes low-grade heat from the external environment and delivers it to the building at a higher temperature, via underfloor heating or radiators. Obviously by using the external environment as a heat source a heat pump can provide an almost limitless amount of heat to the building.
The process is shown diagrammatically below.
Of course nothing comes for free, and in the case of a heat pump it uses electricity to drive the compressor to compress the refrigerant. In a well-designed water-source heat pump system for every 1 kWh of electrical energy consumed 4 or 5 kWh of thermal energy is produced – this would be a so-called ‘Coefficient of Peformance’ or CoP of 4 or 5. This can make it extremely attractive if you have a suitable water body next to a building you want to heat.
Note that some heat pumps can also work in reverse as an air conditioner and will benefit from the same energy-amplification effect – in this case it behaves exactly like a giant fridge where the inside of the building is the inside of the fridge. This is especially attractive for commercial buildings with large air conditioning loads in the summer and large heating bill in winter; a heat pump can deliver benefits year-round.
To encourage this technology, heat pump projects in the UK are supported by the Government’s Renewable Heat Incentive (RHI). For a single domestic property, this pays around 19 pence per kWh of heat delivered by the heat pump, guaranteed for 7 years. For larger domestic or commercial projects, the rate is tiered depending on usage but equates to around 9 pence per kWh, guaranteed for 20 years.
Of course, there are also environmental benefits: heat pumps have a smaller carbon footprint than fossil-fueled heating systems, and by using a green electricity supply can become a 100% renewable form of heating.
Types of heat pump
There are three fundamental types of heat pump; air-source, ground-source and water-source. Note that Renewables First only work with water-source heat pumps, though as described below these are the Rolls-Royce of heat pumps anyway.
Air-source heat pumps
Air-source heat pumps take their heat from ambient air, so utilise a fan to suck air into a heat exchanger to extract the heat and evaporate the refrigerant inside the heat pump. A well designed air-source heat pump can have a Coefficient of Performance (CoP) of 2.5 to 3. Although this is good, it must be remembered that fuel for heat (often gas) is about a third of the price of electricity, so financially a CoP of at least three is required to break even. A typical air-source heat pump is shown below and can be seen to look the same as a typical air conditioning unit.
Ground-source heat pumps
Ground-source heat pumps benefit from the steadier and warmer temperatures available from the ground compared to ambient air. They use boreholes or buried coils of tubing to extract the heat and this is piped back to a the heat pump. Well-designed ground-source systems can have CoPs of 2.5 to 4, sometimes even better. At the upper end of these CoPs the finances of a ground–source heat pump can get very attractive, particularly where cooling is also required during the summer.
Water-source heat pumps
Water-source heat pumps use a static or flowing water source as the heat source, so typically a river, canal or lake, though wells and the sea can also be used. The amount of heat that can be extracted from water is much higher than the ground, and in the case of flowing water there is no risk of depleting the resource of heat because it is being continuously replenished. There are ‘open-loop’ and ‘closed-loop’ water-source heat pumps. In the former case the water passes through the evaporator heat exchanger inside the heat pump but there is a risk of blocking the heat exchanger with fine debris present in the water. To overcome the risk of blocking, closed-loop systems are often used which use a heat exchanger in the water body to transfer the heat via a ‘transfer fluid’ to the heat exchanger inside the heat pump. Because of the higher ‘specific heat capacity’ of water and the steadier temperatures found in water bodies compared to air source, the CoPs that can be obtained from water-source heat pumps typically range from 3 to 5.5 in well-designed systems.
The downside of a water-source heat pump is the added complexity of extracting heat from water and dealing with water-borne debris, plus the more stringent regulatory controls that are applied to the ecologically important surface waters in the UK. This is where Renewables First’s expertise comes into play; from our position as the UK leading hydropower company we are experts in handling water and meeting stringent regulatory requirements. Coupled to that our proven technical and engineering expertise ensures that heat pump systems designed, specified and delivered by us will always push towards the highest Coefficients of Performance (CoPs) possible, ensuring a good return on investment for our clients.
So what makes a good water-source heat pump site?
The water source
Obviously access to water – so ideally the property to be heated should be close to and be able to access a running stream or river, or a canal or lake, or possibly a well or the sea. Running water is better because the cooler water discharged from the heat pump is washed downstream and replaced by warmer water, whereas static water bodies tend to cool down (or heat up if the heat pump is cooling rather than heating). For industrial sites, process water can be a valuable source of waste heat, and the relatively high temperatures will improve the heat pump efficiency.
Static water bodies can still be used though provided they are large enough. Where wells are used these are basically an existing borehole, so the same as for boreholes there would be an abstraction borehole (the well) and a second discharge borehole would be needed, typically 10 metres away. The discharge could be a soakaway.
The sea can also be used and is ultimately the largest thermal store on the planet. The downside is that a long abstraction and discharge pipe is normally needed and this has associated pumping losses, plus the pipework must be protected against extreme storm conditions, and all of the components need to be protected against corrosion from the salty water. Seawater can however be an attractive option, provided it can be accessed affordably, particularly for larger-scale commercial projects.
The heat load
Heat pumps work best if they run continuously and at maximum output. Unlike gas boilers, they don’t like constantly turning on and off and don’t like operating on very low part-loads. Bearing in mind the range of source water temperatures over the seasons, and the out-of-phase nature of heating loads increasing when it gets colder when the source water is also often colder, this shows that the job of a water-source heat pump, and the system designer, is not an easy one. It is vitally important to understand the thermal loads and get the system designed and specified early to ensure efficient operation into the future.
Another key consideration is the output water temperature. Domestic hot water (DHW) is generally used at around 60° C, and must be stored at at least this temperature to avoid the risk from legionella bacteria. Conventional radiator-based space heating systems operate on flow temperatures of 60 – 70° C, whereas underfloor heating systems run at a much cooler 35° C. Water-source heat pumps operate with the higher CoPs if the temperature difference between the heat source and the load is minimised, so for a high performance system it is better for the water-source heat pump to only provide heat to the space heating and not the DHW, and even then steps should be taken to reduce the flow temperature of the space heating system. This can be done (preferably) by using underfloor heating which is generally recognised as being a much more comfortable way of heating a house anyway, but isn’t always a realistic option for existing properties. Generally speaking, radiators can be used with heat pump systems but the area of radiators will need to be increased by approximately 50% to account for the lower operating temperatures from a heat pump system. You may think that only increasing the radiator area by 50% won’t be enough, but remember that the radiators would be heated continuously rather than intermittently in a conventional heating system.
This is a good summary of the water-source heat pump specification that we would recommend:
- Use a large thermal store / buffer tank to reduce heat pump cycling.
- Design the heat pump to work down to 5° C external river temperature
- Use underfloor heating where possible or increase the radiator area by 50%.
If you are considering a heat pump you should also take the opportunity to improve the building’s overall thermal efficiency if it is not up to current standards in order to maximise the benefits.
This ensures that the heat pump will be operating at near optimum conditions almost all of the time and delivering a high CoP, which ensures significantly lower overall heating bills for the customer.
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