Ambient shared ground loops with ground source heat pumps overcome overheating and provide passive cooling

The summer of 2018 ranks as one of the hottest on record. We are not complaining, it’s been lovely, but overheating is certainly an issue that needs to be faced when looking at the design of multiple occupancy buildings. 

With the NHS reporting record numbers of people attending A&E this summer, extreme heat is being held accountable for increased emergency admissions for respiratory problems, particularly those affected by the associated higher air pollution, and conditions heightened by dehydration.

Indeed, hot on the heels of the Royal College of General Practitioners (RCGP) decision to cease its investments in all fossil fuel companies, the RCGP’s Chair, Professor Helen Stokes-Lampard, has warned that the prospect of hotter summers and increased air pollution as a consequence of climate change could result in a summer crisis on a parallel with cold-related hospital admissions and excess winter deaths:

We already face a seasonal crisis every winter that threatens to destabilise our national health service, but with our summers forecasted to become hotter and hotter, we risk the emergence of a second seasonal crisis, and the NHS will simply be unable to cope.

The connection between the choice of a building’s heating system with climate change, air pollution (read Kensa’s blog), and health is becoming of increasing concern, hence Government’s Clean Air Strategy and London’s Cleaner Heat Cashback incentivising the retrofit of combustion heating systems with renewable alternatives like ground source heat pumps.

However even in buildings which comply with the recommendations of the Cleaner Heat Cashback (a gas or LPG boiler that is at least 90% energy efficient and has NOx emissions of 40mg/KWh or less, or a renewable heating technology, a hybrid system, CHP or heat recovery), overheating can be an issue. If a central plant district heating system (aka heat network) is used, the method of transferring heat throughout the building using these heating systems is in fact raising the temperature within the building, resulting in further overheating. So even if the technology being used to provide heat may be energy efficient and reduce climate change and air pollution, within the building it can still worsen the overheating issue, and could be contributing to associated health problems.

Causes of overheating

Modern apartment blocks are well insulated and have large amounts of glazing which make them prone to overheating. One common form of heating uses a conventional central plant system, which relies on circulating hot water typically at around 65°C to 85°C through a network of pipes around the building; because the system doesn’t know when any one flat might want heating or hot water this has to be maintained hot 24/7.

The pipes are insulated, but they still leak some heat into the building; in summer this can cause the building to overheat. Over time, the pipe insulation degrades and often parts of it get damaged, making the problem worse, and of course these heat losses also have to be paid for in energy bills.

Shared ground loop arrays

An emerging form of district ground source heat pump system known as “Ambient Shared Ground Loop Arrays” is making a big impact on climate change and air pollution, at the same time as providing low cost heat for households and a return for investors through the Non Domestic Renewable Heat Incentive (RHI).

Connecting multiple homes and businesses together with a low temperature shared ground loop array (typically borehole ground arrays) coupled to individual Kensa Shoebox heat pumps in each property provides a technically robust district heating system, whilst allowing simplified billing and administration as each heat pump is connected to the individual property’s own electricity supply. Each heat user therefore pays for the heat they have consumed naturally through their own independent electricity bills.

Overcoming overheating

The shared ground loop system circulating at ambient temperatures avoids the common overheating problems associated with district heating losses entirely. The ambient ‘heat’ circulating the building is low temperature (-5°C to 20°C), and the higher grade heat for each dwelling is produced at the point of use i.e. inside the Kensa Shoebox heat pump inside the flat, and only when required so heat losses are minimised. The pipes are usually still insulated but only to prevent condensation, not to retain heat.

As such, there is no heat loss from the system to contribute to overheating in risers and corridors. Furthermore, as the cold side infrastructure extends all of the way to the individual apartment, it is relatively straightforward to add cooling to the system.

Passive cooling

Passive cooling uses the low temperature of the ground loop to cool the property via a fan coil, which can be located in the main living rooms or bedrooms to provide comfort cooling in the summer. As passive cooling bypasses the heat pump the only running cost is for the circulating pump and the distribution fan, estimated around £20 per year, which is a fraction of the cost of air conditioning.

As buildings become increasingly airtight and well insulated, and with climate change making hotter summers more likely, the need for cooling in new build developments is increasing. Shared ground loops could therefore offer significant benefits over and above providing the lowest cost of heating – especially in highly glazed flats.

The coolest bit is that any energy taken out of the building during the cooling process helps to recharge the ground array; this will increase the efficiency of the heat pump when providing hot water through the summer, and also store enough energy in the ground to improve the efficiency of the heat pump when heating the building the following winter.

The savings on heating costs will offset the running cost of cooling in the summer, so the cost of cooling is negligible, unlike regular air conditioning where you pay to run a heat pump only to throw this heat energy out of the window.

Passive cooling works in individual properties, or communal areas within blocks of flats, but gets really interesting when scaling up to larger ambient shared ground loop arrays connecting multiple buildings with a mix of dwellings and commercial buildings, as the benefit of balancing the heating and cooling loads can offer the opportunity of not only efficiency improvements, but also reducing the capital cost of the ground array as the balanced load means borehole depths can be reduced.

If higher cooling loads are required, such as glazed offices or in server rooms, then cooling optimized heat pumps can be used to provide active cooling, but connecting these to the same ambient temperature loops means you again have the benefit of using the waste heat to re-charge the ground array to improve the overall efficiency of the system rather than just throwing it away.

To help with the installation of passive cooling systems Kensa has developed a Passive Cooling module and Control Centre wiring centre which when linked to Kensa’s Shoebox range of Ground Source Heat Pumps provides the components for a cost effective cooling system.

For more information on ground source heat pumps and passive cooling, visit www.kensaheatpumps.com/cooling

 

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