Remeha installs its first Fusion Hybrid system
Gas-fired absorption heat pumps now supply most of the space heating and support the DHW system at a care home in Banbury. Ken Sharpe was given the chance to look at the project.
It was abut the middle of 2013 when Mike Hefford, head of renewable technologies with Remeha Commercial, started thinking of combining gas-fired air-to-water absorption heat pumps with condensing boilers and a specially designed control system to maximise the use of renewable energy. The concept was dubbed Fusion Hybrid, and it has been referred to several times in MBS.
Now he has an installation to his name. Surprisingly, perhaps, it is a refurbishment scheme rather than a new project. It is for a care home in Banbury run by The Orders of St John Care Trust, a not-for-profit care provider which operates some 70 care homes. Lake House has 43 residents. They have single rooms and communal facilities.
What makes Lake House particularly well suited to being heated by air-to water heat pumps is that its heating system was designed for a maximum flow temperature of 42°C; the system does not use LST radiators. That flow temperature ensures that the radiators and pipework all have the low surface temperature that is required in a care home.
Hot water is provided by a direct gas-fired water heater. However, the opportunity has been taken to preheat the incoming mains water using the absorption heat pumps when they have spare capacity.
The home was opened in 1991, and space heating was previously provided by three gas-fired condensing boilers with a total output of 180 kW, the design heat load at -3°C ambient. However, the building is heated continuously, with just a small amount of night setback, so the maximum heat output is only seldom required. The boilers were suffering problems with their heat exchangers, and one was defunct.
Remeha had previously done work for another home run by The Order of St John Care Trust and was therefore known by the chief engineer Graham Hipwell. He had budget available for Lake House and was very keen to maximise the use of renewable energy.
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| The buffer vessel (left) is heated directly by the heat pumps, and the flow temperature is topped up as required by the two boilers to maximum of 42°C. |
The renewable component of the new installation comprises three gas-fired absorption heat pumps, each with an output of 35 kW. They directly heat the water in a 1000 l storage vessel to a 50°C.
Heat is then transferred to the heating system by a coil in the cylinder and blended with return water to achieve the required flow temperature. The flow temperature is controlled according to the heating load, and can be as low as 30°C. The heat pumps use flow temperature as their control parameter, and the lead heat pump is rotated at intervals of 200 hours.
The storage vessel is also used to preheat incoming mains water before its temperature is topped by the direct-fired water heater. The incoming water passes through water softeners before being preheated, as water softeners are not tolerant of high temperatures. The water heater has an output of 48 kW, so there can be a significant load for the heat pumps when the heating requirement is low.
Two 45 kW Quinta Pro gas-fired condensing boilers support the heat pumps by boosting the flow temperature as required. Each boiler can modulate down to 18% of full output.
Lake House being a care home, the changeover to the new system had to be achieved with minimum disruption. The actual changeover was accomplished in just half a day on what turned out to be a mild day in early autumn. Remeha managed the whole job as a turnkey project.
The two new boilers were installed using a cascade kit and connected to the existing stainless-steel flue with short runs of plastic flue. Gas pipe is in stainless steel with pressfttings. Connections to the heating system use carbon-steel pipes, again with press fittings.
The heat pumps are mounted on a concrete plinth in a yard just outside the boiler house. They are badged units using ammonia/water as the refrigerant. Modifications carried out to suit Remeha’s requirements include different controls and low-noise fans with inverted drives. The heat pumps can modulate down to 50%.
This being the first installation of a new concept, it is difficult to predict energy and cost-savings . However, with the heat pumps having a seasonal efficiency of about 140%, Mike Hefford expects gas consumption to be reduced by 20 to 35% and a payback of around four to five years. The heat pumps have an life of 18 to 20 years.
Supporting the decision to install a Fusion Hybrid system is a wealth of analysis by Mike Hefford.
With flow/return temperatures of 82/71°C and 65°C off the heat pumps, which is OK for an ambient temperature of 6°C and above, heat pumps could be expected to meet a third of the heating demand, with the rest met by boiler plant.
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| The three gas-fired absorption heat pumps are in a yard just outside the boiler house. |
Drop the flow/return to 70/50°C, and the heat pumps could deliver 65°C down to -0.5°C ambient and meet two thirds of the heat demand.
With flow/return temperatures of 60/40°C, heat pumps could deliver 65°C down to -3°C ambient and not require integration with gas-fired boilers.
The efficiency of heat pumps falls off as the ambient temperature falls. With a flow temperature of 50°C, the following efficiencies might be expected.
• 155% at 7°C ambient.
• 140% at 0°C ambient.
• 120% at -7°C ambient.
Ever the engineer, Mike Hefford is eyeing up the adjacent laundry room with its large tumble driers. He wants to suggest redirecting the exhaust air from the driers over the heat pumps.
Remeha Commercial’s parent company is based in Holland, and that company has been involved with some 200 installations of heat pumps — but not based on the Fusion Hybrid approach. The Lake House project is receiving a lot of interest and, therefore, visits from the company’s engineers based in Holland.
