However, even though solar energy is highly renewable, and available in an inexhaustible supply, its potential is limited. The energy available is diffused by cloud cover and it is ineffective at night. Great care has to be taken when designing solar thermal water heating installations as systems can overheat in peak conditions and become potentially highly dangerous.

The sun provides more than 10,000 times the total energy required by the human race. Almost all energy ultimately comes from the sun, Coal, Oil, Gas and peat are fossil fuels. They are the remains of plants that have captured and stored up energy over millions of years from the sun, when they are burnt they then release this energy for use in all types of applications.

The graph SE1 below shows the percentage of energy generated by the sun each year in comparison to global reserves and annual global energy consumption. It is very clear that worldwide energy consumption is just a tiny fraction in comparison to the solar incident on the earth.

Earth Energy Reserves

How Much Solar Energy Falls on the Earth.

The amount of solar energy that falls upon the Earth depends upon a number of factors, location, season and weather conditions to name but a few, this energy is called "Insolation". The word insolation refers to the amount of solar energy reaching the Earth's surface per square metre, and is often referred to as KW/m2.

The largest solar radiation values, as expected, are concentrated closest to the sun, in Earth's case, this is in the equatorial region, towards the poles and lower latitudes the suns rays strike the Earth obliquely, and are therefore more diffused and emit a lower level of radiation energy. It can be seen that for a given segment of insolation, the area that is covered in the tropics is very much smaller that at the poles. In other words, the same amount of the suns energy that strikes the Earth's Surface at the poles is much weaker and more dissipated than at the equator. Another factor the effects the amount of insolation are the atmospheric conditions, that the radiation has to pass through. The further the sun's rays have to travel, the greater the loss, clouds, airborne dust particles and pollution all effectively weaken the strength of the insolation.

Because the northern hemisphere tilts away from the sun in winter, and tilts towards the sun in summer, this effects changes between summer and winter.

How Big is This Effect in The Channel Islands.

On a cloudless day, facing directly into the sun at mid day, mid winter insolation levels are in the region of 50% less than summer levels. However, because the sun is much lower in the sky in winter the available solar energy is spread out over a greater area. Therefore any solar installation receives considerably less energy per square metre in winter than in summer.

In terms of insolation levels a value of 5 in not considered high during summer, however as an average annual insolation level it would be considered as being very high. For example; Central Australia is known for being a very hot and sunny location, the Channel Islands may have record levels of sunshine within the UK, but as it is in the Northern hemisphere suffers from the "solar spread" levels of insolation.

In the Channel Islands between July 2007 and August 2008 the Islands received 2389.1 hours of sunshine, with the average annual rate of 6.54 Hrs/day

This offers a potential insolation rate of 560 Watts per metre square.

Average isolation levels at the following locations are;

Central Australia = 5.89 KWhr/msq/day - Very High

Channel Islands = 3.69 KWhr/msq/day - Good

Dublin Ireland = 2.56 KWhr/msq/day - Moderate

Other effects that can mitigate the amount of solar energy collected are poor design and workmanship. Being in the northern hemisphere causes solar design engineers some dilemmas, in winter the insolation rates are low but in summer they are moderate to high, so how do we collect sufficient energy in winter and avoid over collection in the summer.

A thermal balance is very important, and very special attention needs to be paid to the safety of the installation. An over sized solar thermal (water heating) array can be potentially a bomb waiting to go off in summer. Water temperatures can easily exceed 100 Celsius at atmospheric conditions and temperatures of in excess of 250 degrees Celsius have been recorded in even the simple basic domestic installations that are held under pressure.

If, under these conditions, a pipe to fracture the water would "flash" and turn to steam and effectively explode causing serious damage and injury to any structure or persons in the vicinity. Therefor overheating and temperature control are critical to the same effective design of a solar water heating system.

What are The Main Uses of Solar Technology.

The uses of solar technology are many and varied, primarily they are divided into two main categories, solar thermal (fluid or air heating) and solar pv (electrical generation). Both options are based on proven technologies and becoming more widely available, however the cost of solar Pv remain high presently.

Solar Thermal.

Sometimes referred to as active solar, it is a term employed to convert solar energy into usable heat. The main types of energy collection are either fluid to fluid or fluid to air transfer.

The main component in any solar thermal collection installation is the "solar collector". These collectors come in two basic designs, Flat Plate Collectors or Evacuated Tube Collectors. The collector is the energy absorber, and usually mounted on the roof of the building well away from any solar shading, a special thermal transfer fluid is circulated through the collectors and is heated by the suns energy, this energy is then imparted to any number of uses.

The heated fluid can be used to warm anything from domestic or commercial sanitary hot water systems to swimming pools or even central store heating systems. Presently, a cost efficient system is not available to store solar energy in sufficient quantity long term, to provide a complete solution to home or office heating demands, however, in conjunction with other renewable energy solutions it plays a significant part in the battle to reduce global energy consumption and green house emissions.

Solar warm air collection systems are just entering the market place, these systems typically operate at lower efficiency than their solar thermal sisters, as air is a less efficient transfer medium than any liquid. Considering this technology is very new, the potential exists to provide an efficient heating system, there are some advantages in this type of system, as the air systems can potentially produce heat earlier and later in the day than any liquid system. This means that they may produce more usable energy over a full cycle heating season than a fluid based system. Further research in this field is currently being undertaken.

Also, unlike some more basic liquid filled solar systems, air systems do not freeze, and any small air leaks from the collection or distribution system will not significantly effect the systems operation or performance.

Solar PV (Photovoltaic)

This is the capture and conversion of direct light energy into electricity by a semiconductor device known as a photovoltaic cell. Large groups of cells are clustered together to form an array. A basic domestic solar array of 2.3KWp output would typically comprise of about 15 cells, each cell being approximately 1.5 metres square in surface area.

Pv collectors due to there low power to area conversion rate do take up a sizable area, however, on new build installations they can easily be integrated into the south facing roof structure in place of the roof tiles. For existing properties they can be mounted onto surface brackets above the slates or even at low level on proprietary "A" Frame's.

As with solar thermal installations location is critical, any form of shading must be avoided as it dramatically effects the performance.

Pv collectors are an expensive item, typically a basic installation of, say 2.3KWp can cost in the region of £10,000 to design and install. Once installed however, the solar array will produce reliable amounts of electricity year in, year out. Also having virtually no moving parts it requires next to no annual general maintenance and the system will payback the capital sum many times over, during its operational life.

Commercial applications for Pv are endless, they are ideally suited to installation into shopping centres, hospitals, hotels and in fact anywhere where there is a steady annual energy demand. The payback period for commercial installations is usually quicker than in domestic application's as businesses pay higher commercial tariffs for the off grid electricity, compared to small domestic users.

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