Solar Energy: How PV panels work - and how to install them

Monday, August 9, 2010

Photovoltaic panels require little maintenance and last a long time, making them an excellent choice for a farmer

by RON MACDONALD

Photovoltaic panels or modules convert radiation energy from the sun into direct current electricity through the use of silicon semiconductors. These PV panels have no moving parts and require very little maintenance throughout their lifespan, which makes them an excellent choice for a farmer.

PV panels are generally composed of a load-carrying member for mounting and protection from the elements, silicon semiconductors, an inverter, a charge controller and solar batteries. In this article, I will give a basic description of these PV panel components, as well as the theory behind PV and the best location for mounting a PV panel. 

Silicon semiconductors convert the sun's energy into electricity. In short, when the photons from the sun pass through the PV panel, they disturb some of the electrons found in the silicon semiconductor. These disturbed electrons then become part of the current in the panel's electric circuit.

The current is conducted through wires to a charge controller and a battery, or to the electrical system of the home through an inverter. The charge controller prevents the battery from over-charging through voltage regulation. It also can prevent the battery from over-discharging through a low-voltage disconnect. The inverter converts the DC output of the panel or output of the battery to AC which is the standard electricity form of a wall socket.

There are two main types of silicon semiconductors – monocrystalline and multicrystalline – and their advantages and disadvantages are shown in Figure1.
There are also two types of inverters – micro and central. Micro inverters are installed on the back of each module, whereas a central inverter is a separate entity from the modules and may invert many different PV systems. Micro inverters are becoming very popular due to the advantages shown in Figure 2.

PV Panels can be either mounted on the ground or on a southern-facing roof.

If mounting on a barn, it is important to determine whether or not the roof, due to age or moisture from animals, can sustain the increased loading of the panel, so you should consult a structural engineer if necessary. PV Panels can also be mounted onto a ground tracking system, which will follow the direction of the sun throughout the day. For tracking systems to work optimally, the panel must have clear exposure on the eastern and western sides. The use of a tracking system can increase the power production of a PV module by 25 per cent.

PV panels should have clear southern exposure during the peak hours of sunlight, 10 a.m. to 3 p.m. As mentioned, for tracking systems, PV panels should also have clear eastern and western exposure. In order to maximize the PV effect, the panels should be mounted perpendicular to the direction of the sun, which is approximately equal to the latitude of the proposed location. For a location in southern Ontario, this value varies from 42 to 45 degrees from the horizontal. Fortunately, a variation of 15 degrees either way will not significantly reduce power output.   During the winter months, tilt is important for snow accumulation. A lower tilt will accumulate more snow than a higher one. Figure 3 shows a comparison of increasing or decreasing the tilt of a PV module to power output. Natural Resources Canada has also developed maps showing the estimated PV electricity production for over 3,500 Canadian municipalities (see the website: https://glfc.cfsnet.nfis.org/mapserver/pv/index_e.php).

PV modules are also very sensitive to shading. The cells in an array are more often than not connected in series and therefore a shaded cell would act as a resistor. For example, if one cell is 100 per cent shaded, it will reduce the output of the entire array by 100 per cent. So it is important to choose a site that will incorporate shade analysis.

It is also important to note that, in the winter, the sun is lower in the sky than in summer.

Therefore, objects will cast longer shadows. To mitigate this effect, most PV panels can be mounted to the roof of a house, keeping in mind chimney location or other possible objects that may cast a shadow.

The majority of the energy converted from the sun in a PV system is emitted as heat. Unfortunately, at higher temperatures, the efficiency of PV panels begins to decrease.

Therefore, it is important to dissipate this heat. For rooftop mounting, the panels are mounted on a rack which leaves a small space between the panels and the roof. This is left open to encourage convective heat dissipation to cool the panels. 

Finally, PV modules have no moving parts and require very little maintenance.

Warranties range from 20 to 30 years for a relatively good quality module with an average lifespan of more than 40 years. The charge controls or inverters will generally only last about 10 years and the batteries will need to be replaced approximately every five to 10 years. BF

Ron MacDonald, P.Eng., is an agricultural engineer with Agviro Inc. in Guelph.

References:
http://www.solarpowerfast.com/build-solar-panel/monocrystalline-solar-panels/
http://www.solarpowerfast.com/build-solar-panel/polycrystalline-solar-panels/
http://photovoltaics.sandia.gov/docs/PVFEffIntroduction.htm
http://www.thesolarguide.com
http://www.cmhc-schl.gc.ca/en/co/maho/enefcosa/enefcosa_003.cfm
http://www.omafra.gov.on.ca/english/engineer/facts/sol_elec.htm#4
http://www.energyalternatives.ca/PDF/Photovoltaic%20Systems%20
Introduction.pdf
http://www.omafra.gov.on.ca/english/engineer/ge_bib/photo.htm
http://www.homesolarandwindinfo.com/solar-micro-inverter/