Sizing and design of a Solar System
Many factors will influence the extent of your solar system. You may wish to generate enough power to cover all your needs but find that certain constraints limit your ambitions.
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When designing a system, the first things a professional designer will consider will be the sunlight levels for your area (insolation) and your total power requirement. The optimum performance of a photovoltaic panel is obtained when it’s correctly aligned to the sun when the sun is directly overhead. This usually equates, as a fixed mounting, to an alignment of around latitude + or – 15 degrees. Even though it may be visible all day, there may only be around six hours of full sun, due to reflection off your panel and the amount of atmosphere the light has to pass through. This will naturally be least when the sun is directly overhead, called solar noon. |
A good rule of thumb to remember when selecting the site for your panels or array is to try and find a spot that is unshaded between the hours of 10 A.M. to 2 P.M. on your hemisphere’s shortest day. Don’t forget that even the seemingly inconsequential shading from a tree branch can cause a substantial reduction in generated power.
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To offset the effects of low insolation, you may be able to install additional panels, larger panels with a higher output or panels designed to track the sun’s passage across the sky, thereby maximising correct orientation (although the depth of the atmosphere cannot be overcome). Concentrator panels, with a lens arrangement designed to better concentrate weak sunlight onto the cells are another alternative option. Unfortunately these options introduce one of the biggest constraints on a system’s size: that of your budget. Solar panels are not cheap. |
Many countries have compensation schemes and grants available designed to encourage solar power generation and the enthusiastic uptake in certain countries, particularly Germany and Japan, has caused a panel supply shortage. As the market increases and supply improves, prices should come down.
Solar panel output is measured in watts and is usually supplied at a nominal 12 volts although this may well be up to 17 volts effective output. Panels can be wired in series (+-+-) to increase voltage, parallel (++--) to increase amperage. Series/parallel wiring, where sets of panels already wired together in series are wired together in parallel. This serves to increase both voltage and amperage.
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Be aware also of the distance between the various components of your system when choosing the nominal DC voltage. The greater the distance, the greater the voltage drop and a higher voltage will travel further than a low one around the same cabling. 24 or 48v nominal systems will avoid having to use more efficient cabling, especially if your batteries are a considerable distance from the solar panels. The easiest way to calculate the total amount of electrical load you currently use is to check your electricity bill, where the amount you’ve used over your billing period will be expressed in kWh. |
Alternatively, you will have to laboriously list the loading of all your electrical equipment using either the data given in the user manual or on the labels usually affixed to the back or underside of the appliance. If your appliance is rated in amps, find the wattage by multiplying the voltage by the amps. You may wish to consider reducing the total load by investing in low wattage energy saving equipment. Reasonably priced meters are available if you are in any doubt.
Batteries are rated by the amount of current they can supply over a period of hours i.e. in amp hours (ah). E.g. a 300 ah battery will be able to supply 15 amps for 20 hours or 30 amps for 10 hours. Don’t forget to consider the drain of a 120 or 240 volt AC system on a low voltage DC battery bank. For example, to calculate the drain on a nominal 24 volt DC system using an inverter to supply an appliance with a load of 4 amps at 120 volts AC for 3 hours per day, use the following calculation: divide the voltage of the load (120v) by the battery nominal voltage (24v). This gives 5. Multiply this by the 120v AC amp hour load (4 amps for 3 hours = 12ah), 5 x 12, to give a total drain on your nominal 24 volt DC system of 60 amp hours. Try to ensure you design in enough amp-hour capacity to take account of any involving bad weather periods. An additional one-fifth capacity is thought to be sufficient to cover this eventuality.
It is possible to run a completely DC system although not entirely practical as although many appliances now come with DC alternatives, they may be difficult to obtain and priced at a premuim. DC lighting is readily obtainable though. If you have alternative power for high load appliances, a smaller system solely for DC components may be an option. Otherwise you are going to need an inverter to convert the DC supply to your chosen 120 or 240v AC supply. Your inverter should be capable of coping with the power surges caused when starting certain appliances, especially those incorporating high-powered motors. The minimum surge rating will be roughly twice that of the continual wattage your system is calculated at. Inverters do break down occasionally so it is advisable to have a DC powered light in the room where the inverter and batteries are located.
Finally, if you are able to produce more energy than you require, it may be possible to export the excess back to your local utility company. Many utility companies are exploring the practicalities of localised generation of “green” power and have put in place the mechanisms by which they can buy back surplus. They will be able to advise you on what equipment and metering arrangements will need to be installed.
Designing and installing a solar powered system does involve a considerable amount of expertise and should not be undertaken without first researching the subject and preferably taking professional advice. Mistakes can be very costly to rectify. See also Cidnetwork for sustainable design