January 31, 2014 6:20 pm

Solar-powered homes

The Solar House, a £1m five-bedroom property in Leicestershire, where heat from the sun is stored underground for use in winter©Caplin Homes

The Solar House, a £1m five-bedroom property in Leicestershire, where heat from the sun is stored underground for use in winter

Harnessing the sun to warm our homes is nothing new. As Socrates declared, during a wood-fuel shortage in fifth-century BC Greece, “the southern side of a house should be built higher to catch the rays of the winter sun, and the northern side lower to prevent cold winds finding ingress”.

With fuel bills rising, many scientists say it is blazingly obvious where we should be looking for our energy needs – the sun. “The solar energy hitting the earth is an average 1.366Kw per sq metre, roughly equivalent to a three-bar electric heater over every sq metre of the earth’s surface,” says astronomer Dr John Mason of the British Astronomical Association.

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The problem is that most of that energy hits the earth in summer. In winter, when we need power for heating, we have less sunshine and shorter days. So far, in the UK at least, zero-carbon buildings – a standard to which it is proposed all new UK homes should be built from 2016 – have been designed with a mix of renewable energies, such as solar, plus wind or biomass (plant and animal waste material).

But last autumn what is claimed to be Britain’s first year-round, solar-powered house was completed. It was certainly about time. Switzerland, not renowned for a surfeit of sunshine, has had solar-powered homes for at least two decades.

“It won’t be long before energy costs and mortgage costs are equal for many homeowners,” says Michael Goddard, director of Caplin Homes, who has built the £1m five-bedroom Solar House on his 42-acre country estate near the village of Great Glen, Leicestershire. “We’re trying to save people exorbitant energy costs. Reducing carbon emissions is an added bonus but we’re not trying to save the planet,” he says.

Still, the home’s green credentials were helpful in gaining planning permission to build on greenbelt land next to Goddard’s own early 20th-century mansion, which he describes as “the polar opposite of an eco-house, with draughty sash windows”.

The downstairs living area of the Solar House

The downstairs living area of the Solar House

His new neighbours will live in a very 21st-century wooden home which is, by contrast, highly insulated, airtight and has a high-tech ventilation and heat-recovery system. The 358 sq metre, south-facing property incorporates technical principles similar to those built to the energy-efficient Passivhaus design standard.

However, the Solar House does not reach this strict German standard, which Goddard, an Oxford physics graduate, dismisses as “too prescriptive and too expensive”. He estimates that the premium for his zero-carbon house “is less than 8 per cent of the build cost and perhaps only 4 per cent of the sale price”. In 2013, the average UK household spent £1,364 on energy.

The average UK home is half the size of the Solar House. Thus, with the prospect of zero annual energy bills, at least for heating, lighting and hot water, the extra costs could be quickly recovered.

The house makes use of an “innovative mix” of existing sustainable technologies costing an estimated £45,000 – about £37,000 more than a conventional heating system. The main cost is the “earth energy bank” (EEB), which stores surplus solar energy as heat and releases it when needed.

When I visit the Solar House on a grey winter’s day, the outside temperature is 7C. Inside, however, it is a comfortable 21C, thanks to the underfloor heating.

The EEB is the foundation of the property. “We called it a bank because we deposit heat in and withdraw it when needed,” says Goddard, leading me on to the flat roof via a nifty staircase that lowers at the flick of a switch. He points towards 28 south-facing solar panels tilted at 20 degrees. “These are hybrid panels – PV [photovoltaic] and thermal – that generate electricity and capture heat energy in copper pipes filled with a water-glycol [antifreeze] mix.”

Goddard explains how the heated liquid is pumped below ground into plastic pipes, which sit in a matrix of shallow bores, 1.5 metres deep and 1.5 metres apart. These bores are filled with a highly conductive mixture of “porridge-like” consistency that protects the pipes from earth movement. Heat is conducted away from the pipes into the earth where it is stored.

“Earth is not only free, it’s also a very, very poor thermal conductor, but it has a high thermal capacity, which makes it an ideal place to store heat,” says Goddard. “Over summer, the earth gets warmer and warmer. The earth energy bank is insulated above and at the sides, so the heat doesn’t escape.”

When this heat is needed, it is withdrawn from the ground using an “off-the-shelf” ground source heat pump powered by electricity generated by the PV panels. “By charging up the earth with surplus heat, [the pump] functions more efficiently than it would do otherwise,” says Goddard. “Ground source heat pumps often work well in the first year, but after that, as the warmth in the earth is depleted, they become less efficient.”

Surplus electricity from the hybrid PV-thermal solar panels is used to further charge up the EEB with hot water from an immersion heater.

The government’s feed-in tariff scheme should mean that electricity generation when the sun is shining will financially offset usage from the National Grid when it isn’t.

While efficient top-of-the-range kitchen appliances rated A++ will keep demand relatively low, the solar panels are not intended to supply all the home’s electricity consumption, just its “primary” needs, such as heating, hot water and (energy-efficient) LED lighting. This is the requirement for a zero-carbon home.

There are small – 400w – electric radiators in the bedrooms and a wood-burning stove in the living room, but these are for “reassurance” and to offer “a focal point,” says Goddard. Although he is confident they won’t be needed to provide heat, the truth is that anyone interested in purchasing this innovative house would be buying an as yet unproven experiment.

Its performance is monitored independently at Leicester’s De Montfort University, with the data gathered remotely and uploaded on the internet. The monitoring probes have recorded an increase in soil temperature in the EEB of 2C per week during the summer. “So far it seems to be ‘charging up’ well,” says Dr Andrew Wright, of the Institute of Energy and Sustainable Development, who is overseeing the study. “Assuming the occupants don’t make excessive use of electrical appliances . . . their energy bills could be zero.”

Wright continues: “Lots of houses have solar panels, although they’re mostly PV or thermal, unlike these hybrid ones. The key difference with this house is the inter-seasonal heat storage – if it works, it could be a model for mass housing. There’s no reason why we couldn’t use it on a large scale on many newbuilds.”

John Cotterill, the home’s architect, has also drawn up plans for more affordable two- and three-bedroom, solar-powered homes.

Although there are other forms of inter-seasonal heat storage – such as that pioneered by Swiss company Jenni Energietechnik, where surplus solar energy is stored in enormous, insulated hot water tanks – Goddard describes his company’s method, by comparison, as “dirty engineering” that is “cheap and easy, quick and uncomplicated”.

A significant first for the Solar House is the use of SolarWall technology. On the south-facing wings, two rectangular areas of dark brown, corrugated steel look as if they could be design elements of this modern, angular home. On closer inspection, the panels are full of small holes.

“Air is heated by the sun and drawn into the house,” says Goddard. On warm days, the hot air is vented out and cool air drawn in from the north face of the house. Such technology is used in many commercial buildings worldwide, but this is its first use in the UK’s domestic sector.

The manufacturers claim the two 12 sq metre panels of SolarWall will provide a thermal capacity of 14Kw and save 7,000KWh of energy annually. The average cost of a kWh of natural gas – the most popular heating fuel in UK homes – is about 4.5p, meaning a saving of more than £300 per year. The cost of installing this technology in a home is estimated to be £1,000.

Rolf Disch’s Heliotrope, a rotating solar house, in Offenburg, Germany

One building company, CA group, claims that its “solar air heating has the lowest capital cost, highest known efficiency and the quickest return on investment of any active solar technology in the world”.

Passive heating for the Solar House – the kind that Socrates would have been more familiar with – is achieved by a large area of south-facing, triple-glazed windows. Shading and insulated blinds protect the interior from overheating in the summer.

The ancients might be surprised it has taken the human race so long to fully utilise the power of our nearest star in home design but, thanks to some pioneering efforts, we are finally getting there.

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World’s solar-power pioneers

A rotating wooden home by US company Solaleya©B. Thoby

A rotating wooden home by US company Solaleya

Josef Jenni built his first fully solar-powered house in Oberburg, central Switzerland, in 1989. His company, Jenni Energietechnik, has since built a fully solar-powered apartment block.

In 1994 German architect Rolf Disch built a rotating solar house in Freiburg, Germany, called the Heliotrope. The design captures more than enough solar energy to heat and power the 200 sq metre home where Disch lives with his wife. It features PV and thermal panels, a woodpellet boiler and composting toilets. His company now specialises in constructing “plus energy” buildings that harness more solar energy than they use. Disch has also designed a 59-home “solar settlement” in Freiburg and two other Heliotropes, in Bavaria and Offenburg.

US group Solaleya builds rotating wooden homes that follow the sun. They can be designed to be zero carbon, relying on a mix of renewables. Solaleya’s properties have been built at various latitudes, from Australia to Russia.

The interior of the rotating home©B. Thoby

The interior of the rotating home

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