The renewable heating system currently being installed at the National Museum of Flight was designed in house, based on the specification provided. To achieve the desired result and an annual space heating demand of over 383,400 kilowatt hours (kWh), Ecoliving have installed three 75kW Dimplex SI 75 TE ground source heat pumps (GSHPs). These heat pumps source their heat from 35 borehole collectors and having raised the temperature through the refrigeration cycle, hot water is stored in a 1000 litre buffer tank and distributed to the hangars via 424m of highly insulated district heating pipework connecting to over 4,000m2 of underfloor heating throughout both hangars.
Underfloor Heating Distribution System
The first stage of installing the underfloor heating distribution system is putting the gas protective membrane in place. Then, 16,000m of underfloor heating pipework is applied between two layers of reinforcing mesh and is then fastened using wire ties. The concrete floor was then poured in sections and left to dry before being polished. The advantage of underfloor heating with a concrete slab floor in commercial spaces, is that it is more energy efficient in the long term. Once heated to the desired temperature, it requires very little energy to maintain the temperature as it is a radiant heat emitter meaning it is more efficient than conventional radiators.
Bore Hole Installation and District Heating Network
In order to generate sufficient heat to supply both hangars, 35 vertical boreholes have been drilled to depths of 70-180m – more than originally designed. Drilling on this particular site presented its’ challenges. This was more complex than originally anticipated, as the initial site investigation suggested a competent rock head would be found at around 15m. This was the case, however there were four different geological substrates identified below this point within the borehole field; red stone, grey stone, ash and a soft volcanic band.
The differing substrates did not directly upset the drilling, but it caused the borehole to lose rigidity easily once the drilling apparatus was removed. The soft volcanic rock especially, would fold in on itself, causing the borehole to back-fill making it very difficult to insert fragile ground collectors to the required depth. This could happen a number of times before successful insertion of the collector pipework, therefore custom-designed collector weights were sourced to assist the collector installation. Each weight was specially manufactured, 1.5m long and 60Kg in weight. These were attached to the bottom end of the collectors, helping to pull them down through any crumbled substrate to the required depth.
In the end, this was very effective at overcoming the challenging substrate. However, some boreholes were unable to meet the required depth, meaning less heat is able to be retrieved from these boreholes. This resulted in a redesign of the borehole field and a new total of 35 boreholes, varying from 70-180m in depth, 5 more than originally planned.
Once all the boreholes have been drilled and the collectors have been inserted correctly, flow and return pipes from the borehole collector pipes are run in trenches 1m deep and come together at central manifolds before passing into the plant room.
Over 424m of highly insulated district heating pipework delivers the usable heat from the heat pumps in the plant room to the hangars as well as returning the cool flow to be reheated.