FEATURE FOCUS: SUSTAINABILITY
Why schools need to learn to like heat pumps
While heat pumps remain a subject of controversy in the UK, NIGEL AYLWIN-FOSTER, Director of ReEnergise, explains why they are to be welcomed in school estates – when the time is right
refrigerant running though the heat pump. The heat pump acts like a fridge in reverse, using a pump and compressor to increase further the temperature of the refrigerant. The heat is then transferred from the hot refrigerant to the water in the building’s heating system. Horizontal ground arrays, just a metre below the surface, may well allow a cheaper installation. However, to achieve the equivalent level of heat collection more space is required and so for larger systems, or those with limited land availability as is typical in schools, a horizontal array may not be practical. Some electrical power is required to run the heat pump, but its heat output is 3 to 4 times that input. This ratio is known as its Coefficient of Performance (CoP). The more efficient the heat distribution system within the target building(s), the lower the required temperature of the heating water and the higher the CoP will be.
Nigel Aylwin-Foster H
eat pumps are a well-known, proven and reliable technology. They are central to the drive to decarbonise buildings by replacing fossil fuelled based systems. Unlike fossil fuel heating plant, they generate no carbon emissions on site, where installed; and they run on electricity, meaning that as the grid decarbonises, their net operating carbon emissions will become zero. They are typically between 3 to 4 times more efficient than gas and oil boilers. They are already installed and working well in many energy-leaky, listed buildings around the UK, perhaps the most challenging target. So why the controversy? Let’s start by reviewing how heat pumps work. Then we can discuss pros and cons.
Ground Source Heat Pumps (GSHP) Below the surface layer, the ground remains at a constant temperature of about 12 degrees Celsius all year round. GSHPs makes use of this consistent temperature by extracting heat and enhancing it to provide hot water for heating and domestic hot water infrastructure. GSHP systems are served by either a vertical borehole or horizontal ground array. Boreholes may be drilled to 150-250 metres deep, depending on the specific geological conditions. Fluid (diluted glycol) is then circulated in continuous closed loop pipes within these boreholes to absorb the ground heat. The harvested heat is transferred from the fluid to a
Water used to heat hot water systems may need to be boosted at times to raise the temperature to protect against Legionella. Heat can also be transferred back into the ground to allow a GSHP system to be set up for cooling as well as heating. Systems will need to be designed with cooling in mind at the outset. This will increase the capital cost but also increase the efficiency and life of the ground array.
The heat pumps will usually have a design life of 25 years. However, the supporting infrastructure – the ground array and associated pipework in the district heat network – will have a design life of at least twice that and possibly
100 years. This means that when a heat pump needs replacing it can be connected into the existing ground array and the repeat capital outlay will be much less than for the original conversion project.
Water Source Heat Pumps
A body of flowing water, such as a river, lake or even the sea can also be used as the initial source of energy for the same heat pumps as used for a GSHP system. The closed loop pipes are connected to heat collectors, in the form of fins, blades or pond mats (coiled loops), which are sunk or installed on the bottom of the water source and act as the heat collectors. Perhaps counter-intuitively, this can even work in winter when the surface of the lake may be frozen, as there is still adequate thermal energy in the water below.
Installing a water-source system is usually cheaper than installing an equivalent ground array, because the extent of groundworks can be somewhat less. Correctly designed, the system may also be more efficient than a GSHP because the source – the water – can be at a higher temperature than the ground and heat can be replenished more quickly.
Open and Closed Loops
As an alternative to the closed loop ground and water source collectors described, it is possible to collect the heat for these heat pumps using an open loop system. Water from the heat source is pumped directly to a heat exchanger, which heats the heat pump input loop, and the system then operates as a closed loop.
Gatton Hall at Royal Alexandra & Albert School_ Grade 2 listed building served by a GSHP 40
www.education-today.co.uk October 2024
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