FEATURE FOCUS: SUSTAINABILITY GSHP
sophisticated systems than the outgoing oil and gas plant and it’s hard to envisage cost parity being achieved. That’s the main reason why there are currently such generous grant schemes available for the public sector, including schools and colleges. In particular, look out for the Public Sector Decarbonisation Scheme (PSDS). PSDS is the main grant-funding programme for improving energy efficiency and phasing out fossil fuel heating plants from public sector sites. Phase 4 has just been launched, with a final application deadline of 25 November 2024.
The water may be drawn from a water source as above or pumped from an underground aquifer. It requires more powerful pumps than for closed loops and tends to be used on the largest systems or in special circumstances.
Air Source Heat Pumps (ASHP) An ASHP works in the same manner as a GSHP except that it draws its initial energy from the air, rather than the ground. This has pros and cons:
· The installation is invariably cheaper than a GSHP as there is no requirement for extensive groundworks.
· The installation process is less intrusive, and the plant can usually more easily be accommodated in a smaller space.
· The efficiency is not as good as a GSHP at the time when it is most needed (cold days), mainly because the source of energy – the air – does not maintain the consistent 10-12 degrees Celsius that the ground maintains. This means that an ASHP will have a lower seasonal CoP than a GSHP.
Heating hot water will need to be boosted by means of an electrically powered immersion heater similar to GSHPs.
The design life of an ASHP unit is around 20 years.
The net effect is that an ASHP can be a good choice for heating an energy efficient building in which there is not a significant requirement for also providing hot water. However, a single ASHP is unlikely to be a good candidate for heating the older, less efficient building stock that tends to form the backbone of school estates; especially if these are boarding houses which require significant amounts of hot water. In whole life terms a GSHP solution will usually be more cost-effective than an equivalent ASHP solution.
Pros and Cons
Working in the sector, we often hear ‘our buildings aren’t suitable for heat pumps’; ‘we’ll
October 2024
need to change all the radiators’. But this is nearly always not the case in practice. A heat pump can heat any enclosed space: the issue is to get the heat in more quickly than it’s escaping. A heat pump’s temperature output is generally lower than fossil fuel systems. They will therefore not make radiators as hot, so will need to run for longer hours rather than the constant heat up and cool down cycles that typify fossil fuel systems. They will maintain a more constant, even temperature: operated correctly, they will provide a more comfortable environment than gas boilers. Indeed, they are often the preferred system in listed buildings. Sometimes the introduction of heat pumps may entail increasing the size of the emitters but more often it’s a case of changing the heating hours and ensuring the system is operating correctly.
During installation it is also rather more important to ensure that the site-specific design is correct, compared to what has often happened in the past when installing fossil fuel systems. The heat pump must be adequately sized to meet the demand. For a GSHP, the ground array must also be appropriately sized to extract enough heat from the ground whilst allowing the ground to regenerate its heat, from the sun. The result of a poorly sized array can be frozen ground and an inability to deliver the required hot water. Likewise, for a water-sourced system it’s essential that heat is extracted at a rate compatible with natural replenishment. But equally, the system should not be over-sized: heat pumps are usually rather more expensive to install, especially where a ground array is required. Over-sizing, which may have given assurance in the fossil fuel era, will prove an unnecessary drain on the school’s capital outlay. Furthermore, if the installation is being grant-funded (e.g. via the PSDS) there are checks and balances built into the award process to encourage correct sizing. There is no denying that heat pumps are expensive to install, especially if the engineering diligence discussed above is to be observed. Capital outlay may come down as systems proliferate around the UK, but these are more
Relative operating costs will depend on the difference in price between grid gas and electricity (known as the spark-spread), or whatever other source of electricity the heat pump is using. For example, if gas is 4 times cheaper to procure than the power for the heat pump and the heat pump’s SCoP is only 3, then the heat pump will cost more to operate. But the converse also applies, and we can reasonably expect the spark spread to reduce in coming years, whilst heat pump SCoPs will increase. The implication is that heat pumps will increasingly become cheaper to run than the outgoing fossil fuel plant.
Facilities staff do need to understand the process and not expect, or try, to achieve the same operating temperatures as fossil fuel systems: so there is a training burden during the transition, or whenever new facilities staff join the school. Do not expect staff familiar with oil and gas systems to be instantly OK with heat pumps.
In summary, heat pumps are an essential component in the transition to net-zero, both in new-builds and in retrofits. Specified, designed, installed, operated and maintained correctly they will prove to be cost-effective and efficient, and will become the mainstay of the transition in the education sector. But this is no place for shortcuts or rushing the job. Frankly, if the job cannot be done properly with the time or money available then a school is better off waiting until a later opportunity and instead opting for one more round of fossil fuel boilers.
Contact Nigel at unigel@reenergisegroup. com if you have any queries about the detail in the article or have a school heat pump experience you’d like to share.
Two small GSHPs plus thermal storage (School Boarding House)
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