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Case study: solar-assisted heat pump system

Liz Nickels

Minus 7 installed a hybrid energy harvesting system at a charity housing development for the elderly in London.

The Walthamstow and Chingford Almshouse Charity development runs retirements homes in London. One of its properities in Chingford houses eight tenants within seven apartments. It also has a community room, kitchen and toilet room. The combined floor space is 365m2.

The charity decided to install a Minus 7 hybrid energy harvesting system, a solar-assisted heat pump technology in the housing development. The system was chosen to provide hot water and space heating and uses two solar energy processors (SEP), two 4.6m3 thermal stores and a 3m3 cold store. 

The system is made up of an endothermic roofing system, a solar energy processor and a thermal store. It uses an endothermic tile plank to form a weather-tight interlocking roofing system, made from uniform profile, aluminium extrusion, dressed in a powder-coated, hard-wearing finish. The endothermic tile planks are flooded with a heat transfer fluid which absorbs heat energy and solar thermal energy from the ambient surroundings which is then processed and sent to the thermal store to be drawn off as required

The southern facing roof has 40m2 of active endothermic tile planks along with 45m2 of inactive Minus 7 roofing. There are three tanks (two hot stores and one cold store) and two 9kW SEP plants sited in a fenced off area at the back of the building delivering up to 200kWh/day each. The heat from these thermal stores reaches the building through insulated pipes running under the paving and entering the building in the community room. Here it is divided into two rings: one for upstairs and one for downstairs.

Each apartment and the communal space is fitted with a heat transfer unit (HTU) in a kitchen cupboard that extract heat from the thermal ring main and distribute it through the underfloor heating circuits. In addition the HTU provides a preheat to the domestic hot water system of 25°C which is then topped when it reaches the hot water cylinder. 

“The project provides affordable and sustainable heat provision with whole-life cost benefits and low maintenance costs,” says Mark Wozencroft, MD of Minus 7. “The building owners are also generating electricity on site to further offset their carbon footprint. This project demonstrates that buildings with reduced environmental impact during construction or during their lifetimes are economically viable for private residences. Aesthetically speaking, the building blends in well into the environment as the Minus 7 interlocking tile plank looks similar in appearance to a slate finish.”

The initial assessment in 2009 was that apartments of this size would need between 40 and 50kWh/day of heat to keep them at a liveable temperature of 21°C. Subsequently the size of the roof collectors and SEP was calculated to provide this; however, the installation unfortunately coincided with worst winter in the UK for 30 years and since then, changes have been made.

“Essentially the system was designed to provide temperatures of up to 21°C within each apartment, but demand exceeded this so that the energy stores became depleted of heat and residents were frequently switching over to the backup system, which used peak electricity at obvious cost to the residents,” said Liz Abbott, clerk to the directors, at Walthamstow and Chingford Almshouse Charity. “This was combined with the coldest winter in 30 years – 2009/10 which understandably exacerbated the problem. Following this, there were numerous problems with the system failing usually because of problems with flow meters and pumps, but also other electronic failures, to relays for example.”

The charity agreed to have Minus 7 upgrade the system which included insulating the energy stores, renewing flow sensors and installing new temperature sensors and circuit boards. Another important modification made was to limit the amount of heat residents can take from the system. “As a result if residents wish to run the heating at more than 21°C they will be paying for the additional heat. This work has taken place and the system is more reliable as a consequence,” confirms Abbott. 

A new, more efficient, solar energy processor was also installed in October 2012. “It’s considerably more efficient than the older SEP, we should see an improvement in the fuel costs as a consequence, and fewer days of residents backup heating required,” says Abbott. “The ability to monitor the operation of the system via the internet offers reassurance that the system is working correctly, and informs us when residents heating is going onto backup and is an asset to the management of the property.”

Abbott is interested to see the cost effectiveness of the system over the next 12 months. “The costs up to the point of having the new SEP installed were £9.91 per week, per apartment. Now that the new SEP is installed we are forecasting a cost of slightly over £6 per week, per apartment,” says Abbott.

A further improvement made by Minus 7 is the ability for users to remotely monitor the system. Prior to remote monitoring users such as the charity would have been unable to determine when resident’s heating had switched to backup and issues over costs may have arisen. “The remote monitoring allows us to establish whether the stores are at the correct temperature, check the historic temperatures of roofs, etc., which assists us in establishing when periods of heating backup are required (or not) thereby assisting with the management of complaints and also costs,” said Wozencroft.

“The Minus 7 system is still relatively new and therefore our engineers continue to make small but significant improvements to efficiency and reliability, passing these advancements onto the customer. The development at Walthamstow and Chingford Almshouse Charity is evidence of this.”

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Energy efficiency  •  Energy infrastructure