Solar energy

Solar energy can be used in two ways, either to produce electricity or to produce heat or hot water, or obviously to do both. When solar energy is used to produce electricity it is generally referred to as Solar Photovoltaic (PV) Energy, and when used to produce heat or hot water it is generally referred to as Solar Thermal Energy.
Solar energy originates within the depths of our sun. The sun endures a continuous stream of thermo-nuclear explosions as hydrogen atoms are fused into helium atoms. We encounter the resultant energy as radiation that strikes the surface of the earth. Solar panels convert this solar radiation into useful electrical energy which is then stored in batteries for our use, or as a direct heat source. Enough solar radiation strikes the earth every day to meet earth's energy needs for an entire year. Solar panels help us harvest this energy and convert it into usable energy (electricity or heat) to meet the everyday needs of modern life.
Solar PV Electricity
Photovoltaic technology on a small scale is probably familiar to most people throughout the world. It is used to power calculators, road signs, toys and phone chargers for example. This takes light from the sun and then uses it to run the appliance. Photovoltaic panels for electricity work on the same principle, just at a larger scale.
Sunlight is all you need to get electricity from the panels. It is an unlimited resource that’s never going to stop shining. Although more electricity is produced on sunny days they work well on overcast days too.
There are lots of benefits of a PV system. It is easy to install, needs virtually no maintenance and is estimated to last 40 years.
The main disadvantage is the cost which is currently higher than for other renewable energy sources (if you don’t take the environmental costs into consideration). Some countries in Europe, such as Germany and Spain, have feed-in-tariffs which pay a premium rate for electricity generated (or in some cases just the amount exported). This makes the cost of installing a system much more attractive.
How does solar electricity work?
• Photovoltaic systems use cells, consisting of one or two layers of semi-conducting material, to convert solar radiation into electricity. The semi-conducting material is generally silicon, which is the second most abundant element on earth after oxygen.
• light shines on the cell creating an electric field across the layers • this causes electrons to flow creating electricity
• on its own each cell only creates a tiny bit of electricity, but connected together to form panels, which are linked together to form a system, they create useful amounts
• panels are mounted on the roof or on a frame
• an inverter converts the direct current (DC) to alternating current (AC – or mains equivalent) electricity which is suitable for running appliances
• grid connected systems can export electricity they don’t use to the grid, and import it from the grid when there is not enough sunlight
• off-grid systems store excess electricity in a bank of batteries
• off-grid systems can be used in conjunction with other sources of power such as biomass boilers, wind or hydro turbines.
How green are solar PV panels?
Solar panels do not produce any carbon dioxide as they generate electricity. They do take energy to produce, but the time it takes a PV panel to produce as much energy as is needed to manufacture it tends to range from one and a half to three years. As panels are expected to last for 40 years, this makes them an effective way of getting green electricity.
Is solar electricity suitable for my home?
Ideally a photovoltaic system should face between south east and south west, and be free of shade. For best performance they should be angled at 30 to 40 degrees – although you will still catch a reasonable level of sunlight at angles of 20 – 50 degrees.
Solar panels weigh quite a bit, so your roof must be strong enough to hold them. If you need to re-roof, you can do so using solar tiles. These are more expensive than panel systems, but if you are re-roofing anyway, it can be more cost-effective to re-roof with solar tiles than to use conventional tiles and put panels on top.
Which solar PV panels work best?
The photovoltaic industry is developing fast, so it’s worth asking a few suppliers what they recommend.
Crystalline silicon technology is the most commonly used today and the most efficient at converting sunlight into electricity. It consists of thin slices of silicon cut from a single crystal (monocrystalline) or from a block of crystals (polycrystalline). However, it is expensive to manufacture, so it costs more.
Monocrystalline performs most efficiently with modules typically converting 15 per cent of solar radiation into electrical energy, and with the best products reaching 21.5 per cent. The higher price reflects this. Polycrystalline silicon converts between 8 and 12 per cent of solar radiation into electricity.
Thin film technology involves depositing very thin layers of photosensitive materials onto a low-cost backing, such as glass, stainless steel or plastic. This technology is cheaper and more suitable for mass production. However, the efficiency rates are correspondingly lower.
All types of PV system are measured according to their peak power rating which is measured in kWp (kilowatt peak). This is a guide to how much power the module produces under standard test conditions: it measures the power produced under 1kW per m2 of light. The more efficient the module, the smaller the array needed.
How close to peak power a PV module performs will depend on the intensity of light shining on it. It also varies between makes of module.
What size / cost for solar PV panels?
Size of system will depend on which type of PV cell you choose, how good your site is, where within the world you live and how much electricity you want to generate. As a rough guide, a 1kWp system will generate an average of 850kWh of power in Northern Europe. Most domestic systems are between 1.5 and 3 kWp.
Solar Thermal Energy:
Solar water heating (known as solar thermal) captures the free heat from the sun and uses it to heat up water for use in the home. It’s a simple process:
• panels on your roof absorb heat from the sun – they are known as the collector
• the water in the panels heats up
• this hot water is pumped through a coil in your cylinder
• which transfers the heat to the water in the cylinder
How much hot water from solar thermal panels?
Within Northern Europe Solar thermal panels should provide most of your hot water from April to September, and make a worthwhile contribution in the months on either side of that period. The claims made regarding how much of your hot water production can be generated by solar thermal energy vary but a number of solar panel suppliers promise 60 to 70 per cent of annual hot water usage heated by the sun.

The reality will depend on a variety of factors:
• How much interest you take in how the system works and adapt to make the most of the free hot water (ie having showers in the evening rather than the morning). The sun isn’t as reliable as a timer clock.
• The size of your accumulator tank makes a difference. Many accumulators only hold enough water for a day’s supply of hot water, so a day or two of cloud and rain will mean that you will have to produce your hot water using an alternative heat source, such as a biomass boiler system.
• If your control panel does not allow you to programme the hot water and central heating separately, you may not get the maximum benefit from the solar panels when the heating is turned on.
• If you have an electric shower it won’t use your solar hot water.
• If your dishwasher and / or washing machine are cold-fill they will still have to heat the water using electricity, and won’t use your solar hot water.
Is solar thermal suitable for my home?
The ideal situation for solar panels is facing due south, although they are effective facing anywhere between south east and south west. As a rough guide you will need between 1 and 2 m2 of panels per person.
Most panels are mounted on a roof, but they can also be mounted at ground level. It is important that they get direct sunlight. To get the best results they should be at an angle between 20 and 50 degrees from horizontal (most pitched roofs fall within this bracket).
Solar panels are compatible with most existing hot water systems, although if you may well need a new accumulator tank that is tall and thin, with two coils, and ideally big enough to hold two days worth of hot water. The pipes coming from the solar panels are then led into a coil at the bottom of the tank. This coil then heats up the water in the accumulator tank, before the pipe then continues out from the tank and up onto the roof again. This then means that the pipes which have the liquid being heated by the sun is then a ´closed system´.
If your present system is gravity fed, it will need a control (such as a valve and pump) for the hot water circuit, if the panels are to work effectively in winter when the biomass boiler for example is running for central heating.
What type of solar panel is best?
There are two types of solar thermal panel: flat plate panels and evacuated tubes
Flat plate panels consist of an absorber plate in an insulated metal box. The top of the box is glass or plastic, to let the sun’s energy through, while the insulation minimises heat loss. Lots of thin tubes carry water through the absorber plate heating it up as it passes through.
Instead of a plate, Evacuated Tube collectors have glass tubes containing metal absorber tubes, through which water is pumped. Each tube is a vacuum (the air is ‘evacuated’ hence the name), which minimises heat loss.
Recent Swiss research into solar thermal panels also gave clear indications of where the best and worst performing panels are made. The most effective flat plate collectors are made in Austria and Germany and the best evacuated tubes in Switzerland and Northern Ireland. Six of the 10 worst performing evacuated tube collectors are made in China. Its tests show that the best performing collectors are more than twice as efficient as the worst ones.
Evacuated tube panels are generally more expensive than flat plate panels, but you need a smaller surface area.
The cost effectiveness of how your solar thermal energy could be will obviously depend on the size of the system that you intend to use. However, to maximise benefit from obtaining and using solar thermal energy, a system should be capable of producing around 50 to 70 per cent of all water heating that is not associated with space heating.
Many solar panel manufacturers can today offer standard systems. Besides heating heater with solar panels you can also today buy standard packages allowing you to combine wood burning and pellet boilers for example. The manufacturers have also been able to develop a so called ‘drive package’, consisting of all system equipment (controls, circulation pump, expansion tank, safety valves etc). That way both the sale of the system and the installation work are facilitated.

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