Architectural Technology - Solar Energy

The sun is just one of billions of stars, yet it is the powerhouse of every living thing and of the earth itself.

Fossil fuels are being used at such a rate that resources will be exhausted some time in the second half of this century. Nuclear power stations, once considered to be a good alternative, have proved hazardous. Of all the alternatives to fossil fuels, solar energy is one of the cleanest and safest. 

About 30% of the solar radiation reaching the earth is reflected by the atmosphere, and another 20% is absorbed. This leaves 50% that passes through to the earth's surface. But this is still equivalent to the output of about 170 million of the world's largest power stations.

Solar energy is evident in heat and light from the sun. Even in Ireland solar energy has more potential than most people realise.

Harnessing Solar Energy
Three technologies have evolved to harness solar energy.

• Passive solar design
• Active solar heating
• Photovoltaics

Solar energy can be exploited directly in various ways. For heating, passive solar design and active solar systems are the main methods.

Passive Solar Design
Passive solar architecture is a design approach rather than the active use of a specific technology or device. The fabric, orientation and layout of the building are manipulated to achieve maximum solar gains and minimise the need for artificial lighting, heating and ventilation.

Passive solar design principles can be incorporated into new homes at no extra cost while bringing substantial benefits in energy savings and comfort. Passive solar design involves designing buildings so that they make the best use of the energy freely available from the sun in the form of heat, daylight or wind. By using simple passive solar design measures, the heating needs of an individual house can be reduced by up to 30 percent at no extra cost.

Heating
South-facing surfaces absorb more solar radiation in winter and less in summer when compared to surfaces with east or west orientations

Throughout the year, solar gains through west and south-west glazing are very similar to those through glazing facing east and south-east. When solar radiation strikes any material part, it is absorbed, transformed into heat and stored in the mass of the material. The material heats up progressively by conduction as the heat diffuses through it. 

Materials with high heat storage capacity such as concrete, brick and water heat up and cool down relatively slowly. Thermal insulating materials such as glass fibre and foam, usually because of their open or cellular structure, form poor heat stores and diffuse heat very badly.

Lighting
Daylighting design involves the provision of natural daylight in the interiors of buildings to reduce or eliminate daytime use of artificial lights, thereby offering substantial savings in energy use and reducing environmental damage. Various devices are now available to capture daylight and direct it deep into buildings and to reduce excessive light levels near glazing, providing a more uniform spread of natural light. Some of these such as prismatic glazing reflective blinds or shading systems can be more easily applied in the case of existing buildings. A wide range of specially treated glazing materials which can control the intensity and optical properties of natural light and heat flows through windows is now available.

Active Solar Energy
A second type of solar system, sometimes known as active solar technology, involves the installation of a solar collector device, which is typically a metal box structure containing an absorber.

The solar collector absorbs the sun's heat to provide space or, more commonly, water heating. A correctly sized unit can provide around half of a household's hot water needs over a year.  

Around 2,500 square meters of solar thermal collectors have been installed in Ireland. This represents just 0.2% of the estimated practical solar heating resource available now (ESBI Report).
Active solar heating systems are perhaps the most familiar methods of harnessing solar energy. These use solar panels to collect heat from the sun. Sunshine heats water flowing through flat collecting panels, which work like radiators in reverse - they absorb radiation to heat water. Such a system can provide hot water directly or for use in a central heating system. Solar panels provide clean, CO² free heat, which, can be used to provide domestic hot water, to heat swimming pools and to provide service hot water for commercial buildings such as hotels and hospitals. In Ireland, a correctly sized  unit can provide 60 percent of a household's hot water needs over a year. The panels are usually placed on the roof of the house and angled to catch as much direct sunshine as possible. Ideally solar collectors should face south however, an orientation up to 45° east or west of due south will not significantly decrease performance. It is estimated that current solar collector installations represent just 0.2% of the estimated practical solar heating resource available.

Measuring the resources
A visual inspection of a site will be needed to assess the best  location for a solar panel and whether there will be an impact from shading from other buildings. As most solar heating applications only require collectors to be faced anywhere from south-east to south-west, a large proportion of existing buildings will have roof orientations suitable for solar energy systems. In order to collect as much solar radiation as possible, a surface must be tilted towards the sun. How much it should be tilted is dependent upon the latitude and at what time of year most solar collection is required.

Solar water systems
There are several types of solar collector used for water heating, the most popular of which is the flat plate water collector. This consists of a layer of glazing over a black absorber plate through which water is passed to be heated by the sun. A solar water heating system for an individual household will have an area of about 3-4m². Another device is the evacuated tube, able to heat water to much higher temperatures. These are tubes, similar in appearance to long fluorescent lamps, consisting of an evacuated glass tube with a heat absorbing plate and a heat pipe running up the centre. An array of 20 to 30 tubes is normally used for an individual house.

System installation
These types of system can be supplied and installed on a building relatively easily, either by professional contractors or indeed anyone with good plumbing skills. They are usually mounted on the roof of a building and need to be firmly secured in a leak-proof manner. Solar heating systems need to be able to withstand the impact of the elements. Water can cause corrosion to metal parts, and high winds can damage the structures and crack the modules. It is the expansion and contraction which may cause cracking. Most of these risks can be overcome with a well designed system. Because the panels are exposed to the elements an anti-freeze solution must be added to the water in the primary circuit.

Photovoltaics

A third way in which solar energy can be harnessed in buildings is through the use of photovoltaic technology. Photovoltaic systems use semiconductor materials to convert sunlight to electricity. They are used in consumer goods such as solar watches and calculators. In Dublin, parking metres are powered by PV panels. They can also be integrated in building structures or used in larger scale electricity generation.  

Solar power and the environment
Passive and active solar heating techniques have few environmental impacts other than from a visual perspective. The environmental impact is similar to those for any new building or alteration of an existing building.

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