SOLAR ARRAY and POWER TRACKERS

We recommend a solar array created from individual solar cells as opposed to one made of prefabricated solar panels. It enhances the students' learning and can result in a lighter solar array. Cells can be bought from either Siemens or ASE Americas. 

Both sell the terrestrial-grade cells that are permitted in most solar challenges, and the cost for terrestrial-grade cells are much lower than space-grade cells, though terrestrial-grade is less efficient. Each solar cell should produce .5 volts at about 3 amps at peak sunlight. The number of cells to use depends on their size and the allowable solar area for the rules of an event. Solar cells should be wired in series on a panel and should be divided into several zones. For example, if you have 750 solar cells, you might want to wire 3 sets of 250 cells, each zone producing about 125 volts. If one zone fails, two other zones are still producing power. 

The solar array voltage does not need to match the system voltage of the motor if you use power trackers. Power trackers convert the solar array voltage to the system voltage. They are essential in a solar car. Be sure to verify with the power tracker vendor the necessary array voltage to feed the power trackers. If the car drives underneath shade, the power trackers automatically adjusts the power to match system voltage, allowing the system to run as efficient as possible. Power trackers are available from AERL.

WHAT IS A SOLAR CELL

A solar cell converts solar energy to electrical energy. Photons in sunlight provide the energy that moves electrons from one layer of a semi-conducting metallic wafer to another. The movement of the electrons creates a current.

Solar cells are devices which convert solar energy directly into electricity.  The most common solar cells function by the photovoltaic effect.  Photo- means light and -voltaic means electrical current or electricity.  (light-electricity)  A solar cell supplies direct current (DC) electricity that can be used to power DC motors and light bulbs among other things.  Solar cells can even be used to charge rechargeable batteries so that electricity can be stored or transported for later use when the sun is not available.

There are primarily two types of cells used today, silicon and gallium arsenide, which come in several different grades and varying efficiencies. The satellites that orbit the earth typically use gallium arsenide, while silicon is more commonly used for Earth based (terrestrial) applications.

Photovoltaic cell semiconductor layers

Stock class solar cars use commerically available terrestrial grade silicon cells. Numerous individual cells (approaching 1000) are combined to form the "solar array". Depending on the electric motor used to drive the car, these arrays generally work between 50 and 200 volts, and can provide up to around 1000 watts of power. The intensity of the sun, cloud cover, and temperature affect the array's output.

Open class solar cars can use any type of solar cell and many teams use the space grade cells. These cells are generally smaller and much more expensive than the conventional silicon cells. They also are more efficient.  Photovoltaic cells are a relatively technology. Their development and use has come about as part of the technology developed for space travel and satellite communication systems.

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.

At this junction, an interior electric field is built up which leads to the separation of the charge carriers that are released by light. Through metal contacts, an electric charge can be tapped. If the outer circuit is closed, meaning a consumer is connected, then direct current flows.  Silicon cells are approximately 10 cm by 10 cm large (recently also 15 cm by 15 cm). A transparent anti-reflection film protects the cell and decreases reflective loss on the cell surface.

Photovoltaics: Solar Electricity and Solar Cells in Theory and Practice

Photovoltaic cell construction

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.

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. The manufacturer's standard warranty of ten or more years is quite long and shows the high quality standards and life expectancy of today's products.

THE SOLAR CAR ARRAY

The solar array is the vehicle's only source of power during the cross-country Rayce. The array is made up of many (often several hundred) photovoltaic solar cells that convert the sun's energy into electricity. Teams use a variety of solar cell technologies to build their arrays. The cell types and dimensions of the array are restricted by the Regulations, depending on the vehicle size and class.

The cells are wired together to form strings. Several strings are often wired together to form a section or panel that has a voltage close to the nominal battery voltage. There are several methods used to string the cells together, but the primary goal is to get as many solar cells possible in the space available. The solar cells are very fragile and can be damaged easily. Teams protect the cells from both the weather and breakage by encapsulating them. There are several methods used to encapsulate cells and the goal is to protect the cells while adding the least amount of weight.

The power produced by the solar array varies depending on the weather, the sun's position in the sky, and the solar array itself. On a bright, sunny day at noon, a good solar car solar array will produce well over 1000 watts (1.3 hp) of power. The power from the array is used either to power the electric motor or stored in the battery pack for later use.

Solar Car Array - or cells arrangement

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.

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