Solar

Solar energy has a popular appeal due to its relatively low cost, ease of installation along with the free availability of the renewable heat source.

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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Photovoltaic Cells are more commonly referred to simply as Pv's. Pv's are another form of solar collector, instead of harvesting the suns energy and turning it into heat energy like the solar thermal panels, Pv's convert the energy into usable electricity.

A typical Pv cell has a surface area in the region of 1.5 mtr sq and an output ranging from 125 watts to 220 watts depending upon the make and model of the particular Pv cell. On there own one Pv does nothing worthwhile, however when they are grouped together the collective power can be considerable, there is no limit to the number of Pv cells than can be grouped into a solar array.

Typical domestic Pv arrays are usually no less than 2.3KWp (2300 watts) @ peak output. To obtain this output 15 x 150 Pv cells would be linked together taking up an area of about 23 sq metres. On new build homes this area is usually made available by utilising the south facing pitched roof.

A Pv cell looks very different to a solar water heating cell and is much more sophisticated and manufactured in laboratory type conditions. They have the appearance of a sheet of very dark glass with a hatched silver colour grid inlaid into them, they are quite light at 17 kg each, and often weigh less than the tiles they replace, so when mounted cause no structural impact upon the roof. Larger and typically commercial installations require the Pv's to be mounted onto proprietary "A Frames" at ground level or on-top off office buildings.

Until fairly recently it was not practically possible to install PV cells into the domestic /commercial properties because the power providers would not except the "spillage" this is unused electricity generated by the Pv array, back into the grid. Happily, today this has changed and the power providers will buy back any surplus spillage electricity and allow it into the national grid.

The electricity produced can be used to power virtually anything, fridges, freezers, televisions, washing machine, water heater immersion elements and all manner of things. A well designed Pv installation should dramatically reduce the electrical consumption of the home and during daytime provide sufficient power to cover the base load requirements of the dwelling completely.

The Components of an Installation

A modern efficient Pv system has three main components, solar Pv cell, an inverter, and a device that manages the energy generated.
Solar Pv Cells.

Photovoltaic cells are packaged together into convenient modules to produce specific voltage and currant during daylight hours. These packages typically have outputs of 2.3KWp, 3.5KWp, 4.6KWp, 6.1KWp and 8.0KWp for the domestic market. Commercially virtually any size output can be attained, however design and installation are extremely complex and sophisticated.

The Inverter

All solar Pv cells generate DC electricity. This must be converted into AC electricity before it can be used within the home and conveyed around the household electrical circuits. A device call a grid connected (or grid tied) inverter is used to do this. Grid tied inverters are manufactured by specialist companies and have to undergo rigorous testing before they are allowed into the market as safety is of paramount concern.

Energy Manager

The energy manager is another essential piece of the jigsaw, its job is to ensure that the solar Pv system is as economical and efficient as possible and to prevent any surplus power generated by the solar Pv system being lost to the grid during periods when energy production is greater than energy demand (such as on a bright sunny day when everybody is out and demand is low). This device, called EMMA (energy and micro-generation manager),

Solar Pv systems have no moving parts, are modular (so they can be sized to match power requirements on any scale) they are reliable, and easily installed. Pv cells are a life long product, most top quality manufacturers guarantee the performance output for at least the first 20-25 years of operation, and generally they predict an operational life well in excess of 30+ years.

Not all European countries, as is the case here, within the Channel Islands, have adopted the Building Energy Ratings System, this is an officially regulated method of calculating the energy efficiency of your home, however, in areas where BER exists, Pvs can make a very significant improvement upon the buildings BRE rating.

Unlike solar thermal systems, 100% of the output from the solar Pv systems can be used to displace electricity purchased off the grid and to thereby generating enhanced energy credit ratings. This energy rating credit, (which in parts of the United Kingdom and Ireland is 2.7 times the output from the Pv system) is subtracted from primary electric consumption figures when calculating a building energy rating. As a result of this, even a modest Pv installation will have a significant positive effect on your buildings BER.

Another positive benefit, is the requirement for all new buildings to meet higher energy efficiency levels, to comply with Building Regulations. In countries such as Ireland these regulations require that a minimum of 10 KWhr/msq/year of thermal energy or 4 KWhr/m2/year of electrical energy must come from renewable sources. This requirement can be very easily and economically met by a modest Pv installation.

DIY pre-packed kits containing all the required materials including the controller and installation instructions are available from us via our web site DIY purchasing section (due for launch November 2008) for approximately £4,800.00 per KWp, plus gst and/or vat as aplicable and delivery.

Installation of these Pv systems is relatively straight forward and can be undertaken by any reasonably competent DIY enthusiast, however we advise that a professional qualified electrician be engaged to make the final connections to the fuse board. As described previously the Pv collectors can be mounted onto the south facing part of the roof or at low level on property "A Frames". The Pv modules are then linked by a pair of plug and play DC cables back to the inverter, and then onto the main fuse-board, this final section of work is best undertaken by a qualified electrician.

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Flat Plate Solar Panels

Typically these simple panels come in aluminium or plastic box form with a surface area of around 1.5 - 2.5 metres square, the top is glazed with special solar glass and the box construction is well insulated to retain the suns heat. They can be either integrally mounted into the roof, or more commonly fixed over the existing roof tiles.

Sunlight passes through the solar glazing and strikes the absorber plate, which heats up, changing solar energy into heat energy. The heat is transfered to liquid passing through pipes firmly attached to the absorber plate. Absorber plates are commonly painted with "selective coatings" which help to absorb of UV radiation, but are reflective to infrared so they retain much better than simple black paint. Absorber plates are typically manufactured from sheet copper and use copper pipework to transfer the heating medium from the collectors to the hot water cylinder or swimming pool heat exchanger. 

Evacuated tube array
This solar installation in Madeira is used to provide heating for the external swimming pool. The evacuated air collectors are mounted onto A frames at 30 degrees for optimum energy collection.

Overheating of the transfer liquid during high insolation periods has historically always been a problem with old style solar thermal collectors. This problem has now been overcome by an innovative design of high efficiency collectors from a Spanish manufacturer. These new collectors do not have any fluid inside them unless they are actually working, IE when there is a demand from the hot water cylinder. Likewise, having no fluid when not actually working they avoid the risk of freezing up in winter.

The way they operate is remarkably simple, the special solar collector is mounted onto the roof in the normal manner and two small pipes, being a flow in, and a return out, drop down inside the building to the solar water heater cylinder usually located within the airing cupboard. A temperature sensor within the hot water tank monitors the stored water temperature and another sensor located inside the solar collector monitors the panel temperature, when the panel temperature is 4 Degrees Celsius or more than the stored hot water temperature a special twin circulation pump forces water up and into the panel where the heat energy is harvested and the return pipe delivers it back to the hot water tank.

When the hot water tank is satisfied and at temperature or if the weather clouds over and solar radiation is lost the panel sensor stops the circulation pump, and the transfer fluid drains back by gravity into the solar store tank. When further solar energy is available the whole cycle starts all over again.

The system is simple and reliable, no antifreeze is required and the collectors cannot overheat so the installation is far safer and superior to any basic old style flat plate collection system.

The average cost of the new self draining system, installed into an average home is in the region of £5000.00. 

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