Why Solar

Aren't Solar Energy Systems too expensive?
The argument often put forward against the use of solar heating systems is that they are not economical. This often culminates in a flat out rejection of renewable energy. But have you ever asked yourself whether the expensive tile you have used on the floor has any utility? Or the new aluminum wheel rims on your car are economical? Or the electric geyser you are using is energy efficient? Other than that, you cannot forget that solar heating systems provide an important contribution to the use of environmentally friendly energy.

The inexpensive prices for conventional energy sources conceal the real facts. The consequential costs for environmental and health damages caused by their use (the so-called "external costs") are not included in their price and have to be payed for by the general public. You also have to consider, that the price for conventional energy sources will increase considerably in the near future, due to these resources running short.

The sun, however, supplies its energy free of charge. T he relatively high initial investment at first sight suggests that the systems are, in general, very expensive. But from the time of installation of the system on, there are no more operating costs, except for the negligible maintenance costs. Whoever invests in a solar system is also investing in the future.


Active environmental protection

CO² emissions from heating systems producing
With the installation of a solar system, one is actively contributing to the lowering of environmentally harmful CO² emissions. A solar system has an unequivocally positive CO² balance compared to conventional systems.

In contrast, the use of a solar system combined with efficient energy technology with the lowest possible energy consumption is environmentally ideal.

The period of energetic amortization (the time until the solar heating system has produced as much energy as was needed to manufacture the system) on a thermal solar heating system is between half a year and two and a half years. In comparison to that, conventional systems never pay back energetically. In order to make a certain amount of energy available, they need an even larger amount of primary energy.

Solar PV Systems

Photovoltaics: Solar Electricity and Solar Cells in Theory and Practice

The word Photovoltaic is a combination of the Greek word for Light and the name of the physicist Allesandro Volta. It identifies the direct conversion of sunlight into energy by means of solar cells. The conversion process is based on the photoelectric effect discovered by Alexander Bequerel in 1839. The photoelectric effect describes the release of positive and negative charge carriers in a solid state when light strikes its surface

.

How Does a Solar Cell Work?

Solar cells are composed of various semiconducting materials. Semiconductors are materials, which become electrically conductive when supplied with light or heat, but which operate as insulators at low temperatures.

Over 95% of all the solar cells produced worldwide are composed of the semiconductor material Silicon (Si). As the second most abundant element in earth`s crust, silicon has the advantage, of being available in sufficient quantities, and additionally processing the material does not burden the environment. To produce a solar cell, the semiconductor is contaminated or "doped". "Doping" is the intentional introduction of chemical elements, with which one can obtain a surplus of either positive charge carriers (p-conducting semiconductor layer) or negative charge carriers (n-conducting semiconductor layer) from the semiconductor material. If two differently contaminated semiconductor layers are combined, then a so-called p-n-junction results on the boundary of the layers.

Model of a crystalline solar cell

Flat Plate Collectors (FPC) based Solar Water Heaters

Characteristics of a Solar Cell

The usable voltage from solar cells depends on the semiconductor material. In silicon it amounts to approximately 0.5 V. Terminal voltage is only weakly dependent on light radiation, while the current intensity increases with higher luminosity. A 100 cm² silicon cell, for example, reaches a maximum current intensity of approximately 2 A when radiated by 1000 W/m².

The output (product of electricity and voltage) of a solar cell is temperature dependent. Higher cell temperatures lead to lower output, and hence to lower efficiency. The level of efficiency indicates how much of the radiated quantity of light is converted into useable electrical energy.

Different Cell Types

One can distinguish three cell types according to the type of crystal: monocrystalline, polycrystalline and amorphous. To produce a monocrystalline silicon cell, absolutely pure semiconducting material is necessary. Monocrystalline rods are extracted from melted silicon and then sawed into thin plates. This production process guarantees a relatively high level of efficiency.

The production of polycrystalline cells is more cost-efficient. In this process, liquid silicon is poured into blocks that are subsequently sawed into plates. During solidification of the material, crystal structures of varying sizes are formed, at whose borders defects emerge. As a result of this crystal defect, the solar cell is less efficient.

If a silicon film is deposited on glass or another substrate material, this is a so-called amorphous or thin layer cell. The layer thickness amounts to less than 1µm (thickness of a human hair: 50-100 µm), so the production costs are lower due to the low material costs. However, the efficiency of amorphous cells is much lower than that of the other two cell types. Because of this, they are primarily used in low power equipment (watches, pocket calculators) or as facade elements.

 

Material
Level of efficiency in % Lab
Level of efficiency in % Production
Monocrystalline Silicon
approx. 24
14 to17
Polycrystalline Silicon
approx. 24
13 to15
Amorphous Silicon
approx. 24
5 to7

From the Cell to the Module

In order to make the appropriate voltages and outputs available for different applications, single solar cells are interconnected to form larger units. Cells connected in series have a higher voltage, while those connected in parallel produce more electric current. The interconnected solar cells are usually embedded in transparent Ethyl-Vinyl-Acetate, fitted with an aluminum or stainless steel frame and covered with transparent glass on the front side.

The typical power ratings of such solar modules are between 10 Wpeak and 100 Wpeak. The characteristic data refer to the standard test conditions of 1000 W/m² solar radiation at a cell temperature of 25° Celsius.

Natural Limits of Efficiency

In addition to optimizing the production processes, work is also being done to increase the level of efficiency, in order to lower the costs of solar cells. However, different loss mechanisms are setting limits on these plans. Basically, the different semiconductor materials or combinations are suited only for specific spectral ranges. Therefore a specific portion of the radiant energy cannot be used, because the light quanta (photons) do not have enough energy to "activate" the charge carriers. On the other hand, a certain amount of surplus photon energy is transformed into heat rather than into electrical energy. In addition to that, there are optical losses, such as the shadowing of the cell surface through contact with the glass surface or reflection of incoming rays on the cell surface. Other loss mechanisms are electrical resistance losses in the semiconductor and the connecting cable. The disrupting influence of material contamination, surface effects and crystal defects, however, are also significant.

Single loss mechanisms (photons with too little energy are not absorbed, surplus photon energy is transformed into heat) cannot be further improved because of inherent physical limits imposed by the materials themselves. This leads to a theoretical maximum level of efficiency, i.e. approximately 28% for crystal silicon.

Surface structuring to reduce reflection loss: for example, construction of the cell surface in a pyramid structure, so that incoming light hits the surface several times. New material: for example, gallium arsenide (GaAs), cadmium telluride (CdTe) or copper indium selenide (CuInSe²).

Tandem or stacked cells: in order to be able to use a wide spectrum of radiation, different semiconductor materials, which are suited for different spectral ranges, will be arranged one on top of the other.

Concentrator cells: A higher light intensity will be focussed on the solar cells by the use of mirror and lens systems. This system tracks the sun, always using direct radiation.

MIS Inversion Layer cells: the inner electrical field are not produced by a p-n junction, but by the junction of a thin oxide layer to a semiconductor.

Grätzel cells: Electrochemical liquid cells with titanium dioxide as electrolytes and dye to improve light absorption.

 

Thermal Systems (for hot water)From History

1455 B.C. - A.D. 1419
During the reign of the Egyptian King Amenhotep III there were "sound statues" in the temples. The sun shining on the statues heated up the air inside them. This heating caused warm air to rise up through the statues. The sound came about when the air passed through the apertures. This effect occurred when the morning sun was shining on the statues. It became a morning signal. It is also recorded that in the burial chamber of Zari Memnon, son of Amenhotep III, the wellknown song of an artificial bird was actually caused by the early morning sun. In China descriptions have been found dating from the Han dynasty. These show concave bronze or copper mirrors that were used by the "Sun-kindler" to light the sacrificial lamps. Later attempts to harness solar energy include the story of the burning of the Roman fleet in 212 B.C According to Johannes Tetzes, a thirteenth-century writer, Archimedes set fire to the enemy
fleet using burning glasses made of small square movable mirrors on a hinge system. When these were positioned to face the rays of the sun, the rays were reflected towards the Roman fleet. At a distance of a bowshot the fleet was set on fire and destroyed after the sails had been ignited. Whether this story is true or not, it is a fact that solar devices were developed and built early in history. As the number of them grew, man's mythic relationship with the sun altered. Early religious and cultural attitudes and belief in the sun began to disappear, whereas by the seventeenth century there was a greater focus on science than on superstition and magic.

1300 - 1600
The Renaissance - an age of science and art - brought forth many solar-energy inventions. One of the most original solar inventions was built by Salmon de Caus in France. He used the sun to heat air to pump water in his "sun engine". Although this was a very simple mechanical use of solar energy, it was another 200 years before the "sun engine" was rediscovered.

1750 _ 1800
Renaissance use of solar energy was mostly in the form of "toys" with no practical application. This trend however took a turn in the latter half of the eighteenth century when solar furnaces capable of smelting iron, copper, and other metals were constructed out of burnished iron, glass lenses, and mirrors. These solar furnaces were in use in Europe and the Middle East. One of them was designed by the French scientist: Antoine Lavoisier. It achieved temperatures of 1,750°C and was made up of one lens with a diameter of 130 cm and a secondary lens with a diameter of 20 cm.

1820 _ 1830
During this period several hot-air engines were developed. The famous two- cylinder Stirling air engine was ideal for solar use, even though it was not originally developed for this purpose. A wondrous selection of such machines was built during the next hundred years. They drove everything from printing- presses and electric light to distillation processes. In 1826 the Swedish engineer John Ericsson invented a hot-air engine. He used a 300 horsepower version to power a paddle-steamer. Later he modified the engine and ran it using solar energy. The Swiss scientist Horace de Saussure is credited with inventing the world’s first solar collectors or solar hot box 1767.

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