Domestic space and water heating accounts for approximately 18% of the UK’s total greenhouse gas (GHG) emissions with around 80% of domestic heat currently provided by natural gas (www.is.gd/sabise). Although significant reductions are expected by improving building insulation, continued reliance on natural gas will not be compatible with target reductions in GHG emissions. It is expected that electricity generated by renewable sources will need to provide most of our energy needs.
Currently, renewables generate 40% of the UK’s electricity (www.is.gd/uniped). Producing the other 60%, while also meeting increased demand from electrification of transport, domestic heating and other applications, will be extremely challenging. The UK’s peak domestic heat demand (www.is.gd/sabise) has been estimated at 170 GW. The challenge looks even more daunting when the energy storage needed to provide this heat when needed, from intermittent renewable sources, is also considered.
Heat pumps will play a major role in addressing this challenge. Firstly, they effectively multiply the amount of heat that electricity provides by two to four times by extracting low-temperature heat from the environment and upgrading it to useful high-temperature heat, typically hot water, that can then be used to heat buildings. Heat pumps are most economical when they are installed as part of large systems supplying heat to hundreds of homes. This means that they are well suited to providing a distributed solution to the intermittent supply from renewables. Insulated underground storage tanks can be used to store hot water when there is a surplus of renewable energy. If there are longer periods when wind and solar are unable to meet demand, then a secondary boiler can be used to produce the required heat. This boiler can burn biogas or biofuel.
The transition of UK homes and businesses to heat pump heating has already started. Last year, OE highlighted a large-scale ‘water source’ heat pump scheme (see OE issue 7, or www.is.gd/unicub) using heat extracted from the River Clyde. The Princess Royal Stand at Cheltenham Racecourse has also installed 'ground source' heat pumps (see box), while in November, a project to install ‘ground source’ heat to 364 flats in Sunderland launched (www.is.gd/yimuha). Furthermore, BBC News reported in January 2020 that new rules are to be introduced to ensure all new homes built in Scotland use renewable or low-carbon heating (www.is.gd/buwuno).
A refrigerator is just a specific application of a heat pump, used to cool a space rather than increase the temperature of a heat source. The term ‘heat pump’ is typically only used for machines intended to supply heat at an elevated temperature and ‘refrigerator’ is used for machines used to extract heat from a cold space. The distinction in terminology only relates to this intended use; they are identical in the way that they operate, and it is possible to use one machine to perform both functions at the same time, for example, providing a cold space for food storage while also heating water.
Most heat pumps and refrigerators use a single-stage, closed-cycle that has four basic components: a compressor; condenser; expansion valve; and evaporator. A working fluid arrives at the pump as a gas where it is compressed, causing it to condense in the condenser. As the fluid condenses, heat is given off at a high temperature. The working fluid, now a liquid, then passes through an expansion valve causing the pressure to drop so that it evaporates in the evaporator. As the fluid evaporates, heat is taken from the environment and stored in the fluid.
If you’re not clear about the difference between heat and temperature, just remember that heat is a form of energy. Temperature on the other hand is the state of matter. When different substances increase in temperature by the same amount, they store different amounts of heat.
The coefficient of performance (COP) for a refrigerator is the heat extracted by the evaporator divided by the work done by the pump. The COP for a heat pump is the heat expelled by the condenser divided by the work done by the pump. In both cases, performance declines when there are large differences in the environmental temperatures at the evaporator and condenser.
LAND, SEA, AIR
A heat pump’s evaporator can be placed in the ground (‘ground source’), in a body of water (‘water source’) or even in the air (‘air source’). Wherever the heat is extracted from, the idea is to use environmental heat, which comes from the sun heating the surface of the Earth. Geothermal energy, where high temperature rocks are used to superheat steam for power generation, is different as it is produced by tapping into the heat at the core of the Earth, and it doesn’t require a heat pump to upgrade the heat because it is hot enough already.
The heat source used impacts on the design of the heat pump due to two parameters – the temperature of the source and its heat transfer. Ground source heat pumps use the ground to heat their evaporators. Typically, this doesn’t mean that the evaporator itself is buried under the ground but rather a ground loop (pictured), which uses water as a heat transfer fluid, is buried in boreholes or trenches. A heat exchanger then transfers the heat from the ground loop to the evaporator. The ground loop is buried at a depth where the ground temperature is very stable, meaning they deal with the least temperature variation.
The air normally varies in temperature significantly more than bodies of water. In terms of heat transfer, flowing water transfers heat the most quickly. Water source heat pumps, therefore, require less surface area to exchange heat between the evaporator and the water. Air source heat pumps, meanwhile, require a very large surface area and often a fan to blow air through the heat exchanger. A major advantage with water source is that the water can be pumped into the heat pump. This means that all the plant is located in a central location and is easily accessible for maintenance.
A QUESTION OF SCALE
Manufacturers such as Kensa produce heat pumps aimed at the individual homeowner. Individual installations can be economical for rural dwellings as a replacement for fuel oil, but it is, however, typically more expensive than a mains gas supply boiler. For larger installations, costs are reduced by approximately 30-50%. This is because some of the fixed costs of a project, such as moving the drilling equipment to and from the site, are spread across a greater number of properties. In addition, there are economies of scale achieved from drilling more boreholes and supplying more ground source heat pumps.
Smaller installations may also find it more difficult to economically provide sufficient heat storage or biofuel-powered axillary boilers to redress intermittency of renewable electricity sources. This can mean placing additional energy storage demand on the electrical grid as it transitions to a fully-renewable system. Biofuels could be used to generate electricity, but this is a highly-inefficient use compared to heating, both in terms of capital costs and energy efficiency. Heat storage using both water and phase change is developing rapidly, making individual installations increasingly competitive.
“Heat pumps can't compete on price with gas boilers for individual homes under current utility pricing, but will change with the ability to load shift and apply time variable tariffs,” says Bean Beanland, chairman of the Ground Source Heat Pump Association. “In addition, I anticipate changes to the spark gap between the price of gas and electricity over coming months and years. If the government chooses not to bite this particular bullet, it will demonstrate a fundamental refusal to get to the nub of the issue. They are most efficient for larger buildings and district heating systems if the DH scheme is an ambient loop with distributed heat pumps in each target building.”
There is also the question of the central heating system to which the heat pump will be attached. For new builds, the heating system can be matched to the heat pump but for retrofits this can be problematic. Christian Engelke, technical director at heating systems maker Viessmann, notes: “It is tempting, but not economical and not even necessarily environmentally-friendly, to take the specification of the incumbent gas boiler and simply replace it with a high temperature heat pump. This product may deliver a flow temperature of 90°C from a source temperature of 40°C, for example, yet it is not operating efficiently and may have a COP of around just 2.5. It’s not how heat pumps are designed to run and there is often a lack of understanding of the building’s heat requirement.”
A major advantage of heat pumps, when used at the district heating scale, is their ability to store heat and to integrate with auxiliary boilers at minimal additional cost. Current systems use auxiliary boilers, powered by natural gas, to deal with peak heating demand. These will be easily transitioned to biogas and supplemented by heat storage to provide fully-renewable heat that doesn’t increase peak electricity demand.
BOX: HEAT PUMPS AT CHELTENHAM GRANDSTAND
In November 2015, iconic horse racing venue Cheltenham Racecourse opened the Princess Royal Stand, as part of a £45 million transformation. The 6,500 capacity, five storey grandstand features restaurants, bars, public viewing areas and the royal box. It replaced a 1920’s building and was officially opened by Princess Anne.
The Jockey Club, which runs Cheltenham Racecourse, wanted the new facility to be less reliant on fossil fuels. Building services consultant BaileyGomm therefore opted for ground source heat pumps, using Viessmann Vitocal 300-G units, installed by G-Core, a specialist in the design, installation, commissioning, and maintenance of ground source, water source, and air source heat pump systems. The heating system took three months to install and commission and is situated in the main boiler plant room of the new grandstand.
Two ground source heat pumps work together in a master/slave configuration, where the lead 42.8 kW heat pump works in conjunction with a 28.8 kW unit, to provide 100% of the building’s cooling requirements and 20% of the heating. The remainder of the heat support is provided by two 1 MW gas boilers, which provide a top up heating requirement to the heat pump delivered base-load, as and when required.
In total, 16 boreholes draw heat from the ground, which is then stored in a 950 litre heating buffer. The pumps can signal to the boilers when a top up is needed.