Energy

Solar-Powered Housing

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© Atelier SoA architectes; C. Hein

The “living tower,” a project designed by SoA architects, is an ecological energy-producing habitat, where space is divided between housing, offices, and agricultural production.
> www.ateliersoa.fr

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The house of the future will be solar-powered and self-sufficient!” You might be forgiven for thinking that this is some idealistic ecologist from the 1970s speaking, but actually it's Christophe Ménézo, coordinator of the “Solar Energy Team” at Cethil,¹ in Lyon, and his reasoning is firmly rooted in the twenty-first century: “Given the context of restrictions on greenhouse gas emissions and the uncertainty with regard to fossil fuel resources, inevitably, solar energy will be used in the future both to provide electricity, heating, and cooling (air conditioning) for housing, and to move towards energy self-sufficiency or even positive energy (producing energy in surplus of its own needs) for homes, apartment blocks, and offices, or even entire neighborhoods.” In other words, houses and buildings must cease to simply be energy consumers–they must become increasingly efficient energy producers. In fact, on September 30, 2005, Cethil and French electricity company EDF created a common laboratory. The research conducted in this lab concerns “high energy efficient buildings” and is wholly dedicated to finding better ways of responding to building energy needs.

overcoming hurdles

In France, for instance, dwelling and office buildings output around 90 million tons of CO₂ per year, out of a total of 385 million tons. The aim of the national “Climate 2004” plan is to divide France's emissions by four by the beginning of 2050, and promote the use of solar and other renewable energies. However, few economic players seem interested, whether they be consultants, research departments, manufacturers, or installers. “In the 1970s there were as many as 60-70 manufacturers of solar thermal collectors. There are considerably fewer today,” Ménézo observes. Another handicap is the extremely slow rate of housing renewal (1% per year), since it is far harder to equip an existing building with solar energy than to include it in a new building. Policy makers don't seem concerned either. “In Spain, all new building plans must include solar energy equipment,” points out Jean-Bernard Saulnier, who is in charge of the issue at the National Solar Energy Institute (Ines).² “That's far from being the case in many French regions that receive high levels of sunshine,” despite a number of tax incentives.

new materials, new uses

Nonetheless, the technological projects being cooked up in order to design zero-energy housing (i.e., producing as much energy as is consumed, or consuming no fossil fuels) or even surplus-energy housing (producing more energy than is consumed) certainly have substantial appeal. These include photovoltaic solar energy³–producing electricity–and thermal solar energy⁴–used to warm up or cool down buildings. “In the last twenty-five years, the cost price per watt of photovoltaic electricity has dropped significantly, from over one hundred euros in 1975 to around two euros today,” points out Jean-Claude Muller, a research engineer at Iness.⁵ The most promising approach, from a technical and industrial viewpoint, relies on crystalline silicon, an abundant, perfectly stable and non-toxic material which has conquered over 93% of the market. “In the future, for cells with industrial conversion efficiencies⁶ of as much as 16-17% for large areas, we will see a reduction in the thickness of the wafers and above all in costs,” Muller continues. Unless, of course, silicon is replaced by something else.

One possible successor to silicon could be “thin-film” photovoltaic cells, exemplified by copper indium diselenide (CIS) with a 2 micrometer-thick absorber layer, as opposed to 200 micrometers for silicon. “Using this technique, we can attain efficiencies of around 19% in the laboratory, as compared to 25% for silicon,” says Daniel Lincot, director of Leca⁷ and deputy director of Irdep.⁸ “For photovoltaic modules, efficiencies of nearly 12% can be obtained, which is approaching the efficiency of polycrystalline silicon modules. The advantage of CIS is that it is potentially cheaper, because of its thin-film technology. Using electrolysis to make such cells, something Irdep is working on, should enable further improvements.” There is one small obstacle: Indium is a rare element. However, it is abundant enough to allow large-scale development of this approach.

Another potential rival to silicon is nanostructured materials, which are still at a stage of conceptual validation. And then there are photovoltaic polymers, which are also “at the basic research stage, but which have every likelihood of becoming an attractive and credible solution in the future,” according to Saulnier. These semiconductors, which are reasonably cheap to produce, degradable (of crucial importance given the requirements of sustainable development), and easy to handle because of their flexibility, are exclusively made of organic materials. However, their level of efficiency is currently only in the range of 3-5% and their lifetime is restricted to 500-1000 hours. But in the long term, provided their aging process can be better understood and controlled, it is expected that they will fulfill the destiny anticipated for them, namely, to produce one watt for less than one euro.

flexible solutions

There's the same flurry of activity going on in the field of thermal solar energy, where efforts are concentrated on “boosting efficiency levels of the glass sheets which trap heat,” explains Gérard Guarracino.⁹ And there's a bright spot on the horizon: The thermal collectors of the future will no longer be a gloomy black. “We're working on making them in various colors (gray, dark blue, green, etc.) so as to make it easier for architects to incorporate solar collectors into building façades.”

Then again, why not kill two, or rather three, birds with one stone? The aptly named hybrid photovoltaic-thermal components, PV/T, are slowly but surely gaining ground in the race to simultaneously provide electricity, heating, and cooling. The ingenious idea underpinning these multifunctional collectors, which can operate year-round, is to make full use of the 80 to 85% of heat normally lost to the atmosphere by recovering it and using it for air conditioning and water heating. Several prototypes are in development. This is known as energy cogeneration, or even tri-generation, since one idea currently being investigated involves combining this concept with others by exploiting the cells' higher temperature in warmer weather and using this heat in an air-conditioning system. In other words, replacing mechanical compressors with refrigeration systems which are exclusively solar and which consume twenty times less energy. Another option, which is still at the experimental stage, is to cool down air by combining solar energy and evaporation. This system uses dehumidification/rehumidification of outside air (the humidity is removed by the warmth produced by the solar collectors), which causes the temperature of the incoming air to fall and ensures a satisfactory constant humidity level inside the building.

All these initiatives concern electricity production, but when it comes to the storage of electricity, “we need to find something much cleaner than traditional lead batteries, which are disastrous for the environment,” says Saulnier. “One promising recent development is lithium storage batteries.” As for heating and cooling, “the main barrier remains the length of time it can be stored,” explains Ménézo. “Our aim is for a storage time period of over one or two weeks, rather than one day, which is what hot water tanks, for instance, offer nowadays.” According to Ménézo, there are some encouraging prospects, such as the use of “components that incorporate phase change materials (PCM) encapsulated inside partition walls or thermochemical processes.” PCM are materials with a high heat of fusion which, by melting and solidifying at certain temperatures, are capable of storing or releasing large amounts of energy.

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© CNRS-ENTPE

Installing solar collectors (right) on façades of buildings (left) is one solution that promotes the incorporation of renewable energies into housing and reduces energy consumption.

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If we are to move beyond fossil-based energy, solar energy, which is by definition intermittent, will not be the only option. It will be reinforced by other alternative sources of energy and innovative systems, such as geothermal energy, biomass, wind turbines, and fuel cells. We are thus likely to see increasing use of Canadian or Provençal wells–geothermal systems that make use of the ground's relative thermal inertia.¹⁰ How do they work? Air intended for ventilation flows through a network of tubes buried underground where it is either warmed up (in winter–this is the Canadian well) or cooled down (in summer–the Provençal version), and then is used to regulate the temperature of rooms in a building. These systems can be bolstered by “energy piles,” which are U-shaped pipes equipped with geothermal probes through which glycol water (i.e., water plus antifreeze) flows. This way, the heat stored underground during summer months can be used all winter long, while the relative coolness of the ground in summer can be used to cool the house down. Research is being carried out into other methods of ventilating houses naturally without consuming electricity, depending on their location. For instance, there's the “solar chimney,” which incorporates a component that captures solar radiation and thereby warms up the surrounding air. The air then rises up the chimney, moving at a speed of 30-60 km/h, reducing the pressure inside the house, which enables air drawn up from a Canadian or Provençal well to flow naturally throughout the building.  

societal change

And then there's the “wind chimney,” which uses the prevailing wind, guided and accelerated in the upper part of the chimney to reduce pressure inside the duct and thereby ventilate the whole house.

Of course, a house wouldn't need to be equipped with all these various technologies in order to be self-sufficient. Two or three of them would be adequate. However, all these local sources need to be managed efficiently through an intelligent monitoring and control system which, by gauging a room's dynamic characteristics, can anticipate the requirements for warmth, cooling, and electricity according to climate conditions indoors and out, and communicate with the various components which produce and store energy. As pointed out by Saulnier, “we need to learn how to share and distribute such resources throughout a neighborhood, and eventually an entire town, as well as deal with the problem of connecting with the national grid.”

There is no shortage of solutions in view, but all of them imply new directions as well as attitudes in technology, society, architecture, and politics. Only genuine political determination, coupled with adequate funding, will enable renewable energies to take off. At the end of 2005, solar photovoltaic energy accounted for a mere 0.01% of energy consumption in France.

Philippe Testard-Vaillant