Solar energy can be converted into electricity, used to heat water and heat buildings through passive solar building design.

The concept of using solar energy is not new. No one can say for sure when humans first used the sun’s heat to help achieve their goals. Solar may be the oldest example of renewable energy. Our primitive ancestors may well have used the warmth of the sun to dry belongings that had become wet. In Roman times solar energy was used on a commercial scale with heat from the sun being used to extract salt from seawater.

Today when we use the term solar power we are normally referring to the production of electricity or the provision of hot water for washing or bathing.

Electrical Power

There are two ways that electricity can be created from sunlight. Either directly through the use of photovoltaic cells or indirectly using heat from the sun to convert water into steam to turn turbines.

Solar panels use semiconductors exhibiting a photovoltaic (PV) effect to convert sunlight into electricity. Solar panels are commonly installed on the roofs of buildings. This makes their position fixed and limits their efficiency somewhat. Placing panels on more than one side of a sloping roof can increase the amount of electricity produced during the day. But this does not get away from efficiency issues relating to the individual panels that will only receive an optimal level of sunlight for part of the day. Commercial scale solar power facilities attempt to overcome this inefficiency by rotating the panels to track the sun through the day thus receiving an increased level of sunlight over the course of the day.

PV cells are not able to overcome the limitations of physical properties of light.

In common with many other renewable energy sources such as wind or wave power storage of excess electricity needs to be considered in solar power. Clearly the solar panels will produce the most electricity during the day when in sunlight. The performance of PV cells will be reduced on overcast days and they are not able to produce electricity at night. Arguably this pattern is the exact opposite of the demand pattern for domestic use. Many homes will be empty during the day when the occupants are at work. A household’s peak demand for electricity is likely to be when homeowners return in the evening. Activities such as cooking, watching television or performing chores are performed under artificial light. To overcome the misalignment of demand and potential supply a self sufficient home solar energy system must be able to store the electricity produced during the day and release it to match demand. Electricity is stored via large rechargeable batteries. Limitations relating to capacity and efficiency of these batteries place a further constraint on solar power systems.

Heliostat Power Plants

Sunlight can also be converted into electricity indirectly using concentrated solar power (CSP). Solar power tower plants or heliostat power plants focus rays of sunlight to generate steam. Early solar furnaces converted water into steam to turn turbines in much the same way as old-fashioned coal or gas powered power stations. Newer designs use molton salts which have a high heat capacity and allow electricity to be generated when the sun is not shining.

The world’s largest Heliostat power plant is the PS20 in Spain which is capable of producing 20MW. Two much larger plants are currenlty under construction. The Crescent Dunes Solar Energy Project and Ivanpah Solar Power Facility both in the US are due to be completed in 2013 and will produce 110MW and 392MW respectively.

Solar hot water system

Using the sun to heat water is a relatively Lo tech form of renewable energy. Using the suns power in this way overcomes some of the issues of storage that are present in electricity generating systems. Water is a natural store of heat. As long as the hot water is stored in a well insulated tank can remain warm for hours. The capability of the heating facility is limited by the size of tank and quality of insulation.

Solar hot water systems can be easily integrated into conventional gas or electrically heated hot water systems. This enables the sun to be used to provide free energy on sunny days. Conventional methods can be used to heat the water on overcast days or during high demand.

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