INTRODUCTION TO PASSIVE SOLAR WATER HEATING
The UK receives solar radiation equivalent to the output of 1,000 power stations. The sun provides most of our renewable energies, by driving wind, wave and water systems, and sustaining plant-life. Solar energy may be used more directly to provide significant quantities of domestic hot water. An average household's yearly hot water supply requires around 3000kWh. Over 1000kWh of energy is received every year in the British Isles on each square metre of surface: that is 60% of the solar radiation found at the equator. Even on cloudy days we receive energy from diffused sunlight. The amount of solar energy reaching the roof of the average UK house would provide many times our heating and hot water needs, though summer levels are obviously higher than winter, and households' requirements vary. More than 42,000 solar domestic water heating systems are estimated to be used currently in the UK. The average solar hot water system can meet around half the user's annual household needs, and provide at least 20 years of useful service.
THE TECHNOLOGY
The system needs to absorb, retain, transfer and store energy as efficiently as possible. The main component in a solar water heating system is the collector. The two basic types of solar hot water collectors are flat plate collectors and evacuated tube collectors. Many flat plate collectors use an absorber plate with a specially developed black coating to maximise the collection of solar energy whilst simultaneously limiting re-radiation of energy back to the atmosphere. Evacuated tube collectors use an evacuated glass tube to enclose each pipe and its associated absorber plate. Convection losses are almost eliminated by the vacuum in the tube, making this type of collector more efficient than the flat plate type, especially at high temperatures. A translucent sheet covering the absorber, plus insulation at the back, minimise heat loss. Transfer of heat to the domestic hot water system may be via solar fluid flowing through a tube attached to the absorber plate, or heat pipes integral with the solar plates to fluid contained in a manifold at the top of the collector, connected in turn to the storage cylinder by a heat exchanger. The solar fluid usually contains a non-toxic anti-freeze solution. Most UK systems circulate the fluid between collectors and tank by means of a small pump, requiring also a temperature differential controller and sensors. This enables convenient positioning of the collectors. A thermosiphon system (naturally circulating) requires the tank to be sited above the collectors. A 'direct' system, where the water passing through the collectors supplies the hot water system, is not usually appropriate in the UK due to risk of freezing.
ENVIRONMENTAL ASPECTS
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It makes sense to harness this clean and free energy thereby reducing our dependency on expensive and polluting fossil fuel. Emissions of carbon dioxide (the main cause of accelerated climate change), may be reduced by around 350-1,300kg per year for each domestic water-heating system, depending on its efficiency, and what it replaces. Combustion of fossil fuels produces other harmful gases: sulphur dioxide and nitrogen oxides, and particulates. If every house in the UK was equipped to collect solar energy, about 40 million megawatt hours annually could be saved.
THE COSTS
Each development has different requirements for hot water, which will govern the type of collector chosen and collector area required. A typical domestic system could use collector array units covering 3-4 square metres. Ideally panels should go on an unshaded slope or pitched roof facing south, although SE to SW is acceptable. The cost of a solar water-heating system supplying 1,000-1,500kWh of heat per year would be around £1,000-£2,500 (DIY) or £2,000-£4,000 for a professionally installed system. A well-designed system should provide the major part (about 80%) of hot water during summer months and a useful contribution at other times, giving an overall saving of up to 60% of the hot water bill. Savings will depend on what fuel is being replaced, and the efficiency of the system. A 1995 DTI survey reported average savings of approximately £200 per year.
INTRODUCTION TO PHOTOVOLTAIC CELLS AND THEIR USE
Solar energy is the source of all life on Earth. Most of the energy available to us radiates from the sun. It provides us with food energy through plant photosynthesis and provides the heat that we need to survive. Trapped solar energy is released when we burn fossil fuel reserves and the sun drives the earth's weather systems which provide renewable forms of energy like wind, solar and wave power. It is now widely recognised that utilising the sun's energy can offer real alternatives to burning finite resources of fossil fuels or endangering future generations by relying on dangerous technologies such as nuclear power. Solar energy has long been used for space heating utilising passive solar design, and for water heating through the use of solar water panels but one of the most exciting areas of development has come in the form of the photovoltaic cell which can convert the sun's energy directly into electricity. Note the following: • In half an hour enough of the sun's energy reaches the Earth's surface to meet the World's energy demand for a year. • The sun produces 400,000,000,000,000 Terawatts of power. • The world's average energy consumption is around 14 TW . • Just one square cm of the Sun's surface burns with the brightness of 232,500 candles. • All the Earth's oil, coal and wood supplies would fuel the Sun for only a few days.
PHOTOVOLTAIC USES
Photovoltaic cells are nothing new. In 1883 a New York electrician constructed one of the first cells from thin wafers of Selenium which eventually came into widespread use in photographic exposure meters. These early cells were very inefficient and it wasn't until the 1950's that research into the effect of light on semiconductors gave birth to the modern solar cell. The first of these new PV cells were used to power space satellites, although the stringent specifications made them very expensive. Nowadays, most people are familiar with solar powered calculators and watches and as developments over the last couple of decades have brought prices down, we have seen increasingly cost effective applications of this technology. PV systems presently supply electricity all over the world. It has proved to be the most cost effective form of electricity generation in many remote locations around the world where there is no mains electricity. Such examples include telecommunications, lighting for remote dwellings, water pumping and refrigeration in developing countries, and a whole host of consumer products powered by the sun. The caravan, trucking and boating industries have also long used small PV systems to charge their batteries, sometimes in conjunction with small wind turbines. Lately, more and more cost effective systems have emerged with PVs now powering whole houses in the form of solar roofs which are linked to the existing electricity grid in case there is insufficient power during low light periods. The Oxford Solar House pictured here was one of the first British grid connected solar houses and over the course of a year actually generates more than it consumes selling the excess back into the national grid.
The reasons for using photovoltaic cells are the following:
• Power from the sun is clean, silent, limitless and free.
• Photovoltaics release no CO2, SO2, or NO2 gases which are normally associated with burning finite fossil fuel reserves and don't contribute to global warming.
• Photovoltaics are now a proven technology which is inherently safe as opposed to other dangerous electricity generating technologies.
• As a result they will leave no legacy for our grandchildren to inherit other than a sustainable energy technology.
PLACING OF SOLAR / PHOTOVOLTAIC PANELS
On a clear summer day Britain receives almost as much energy from the sun per square metre as Africa. The problem is that over a year we only receive a fraction of that because of our changeable climate. The amount of electricity which a photovoltaic collector can produce depends on the angle at which it is tilted towards the sun. It stands to reason that they are most efficient when the sun falls directly on the face of the panel but usually a south facing array is sufficient. The angle of tilt is sometimes varied from winter to summer to meet the angle of the sun. There are tracking devices which will keep the panels facing into the sun though it is generally thought that even though these offer a possible 40% increase in output over the year they introduce moving parts in an otherwise almost fail-safe system. An array mounted on a south facing roof in Britain will on average produce around 700 kilowatt-hours for every kW of panels installed. The south of Britain will have a slightly higher value as will those places that benefit from good weather.