Rooftop solar photovoltaic (PV) solar systems are mushrooming around the world. In South Africa they have gained significant ground as users look to supplement Eskom power, combat load shedding or move off the grid altogether.
Over the past year, system components have grown more mature, capable and reliable. At the same time solar PV systems have become more price competitive and viable, particularly when seen in the light of spiralling electricity rates.
While small-scale solar systems are increasingly sold in hardware stores and builders’ supply depots, users should resist the temptation to effect a DIY installation. Here are just some of the reasons why:
Although most roofs can support the added weight of a solar energy system, some can’t. It takes a professional – preferably a structural engineer – to check the condition of the rafters and assess the capability of the roof to safely support the added dead load of the solar array, the mounting rack and the temporary live load imposed by the installation crew. (Unsurprisingly, the latter calculation is often omitted by DIY’ers.)
While the type of solar panels (be wary of thin film modules which have a lower yield) and the number to be installed will need to be professionally selected and calculated, it’s important to choose a manufacturer that will back its products with an optimum performance guarantee of 80% over a 25 year period and offer panels with an expected lifetime of over 30 years.
Note that the orientation (north–facing) and angle of inclination of the solar array are both critical for optimum performance. So is the spacing between solar arrays to allow channels for electrical cabling. Ideally, the array should be located near the main electrical service board if a grid-tied system is being installed.
Grid-tied PV systems should be interconnected by a licensed electrician while solar hot water systems (often employed in tandem with a solar PV system) should be installed by a licensed plumber.
Choosing a solar PV inverter also requires expert intervention. The inverter converts solar energy to electricity and will also have to be expertly sized and selected. Many inverters are energy wasters. Choose one that isn’t.
In most grid-tied installations, the building’s electrical demand may be determined and used to specify the inverter needed for the solar PV system. In other cases, however, the estimated peak array output is used as the basis for specifying the inverter.
In a remote or stand-alone solar PV system installation, the average daily electric load of the building needs to be calculated first. The building’s electric demand should include the Watt demand of all AC loads running at the same time, plus the wattage from the surge of starting motors, and all DC loads operating simultaneously. This demand should be further increased by a factor of 1.2 to 1.4 in order to account for inverter losses.
In both grid-tied and remote site situations, the initial estimate of the inverter’s capacity may be impacted by future plans, such as increasing the size of the PV array. This needs to be accommodated at the planning stage by a professional PV system engineer.
Eskom produces electricity that is a true sine wave. A modified sine wave inverter produces a slightly squared off electrical waveform, but some computers, power tools, refrigerators and most other powered equipment can use this generated electricity.
On the other hand, pure sine wave inverters produce a true sine wave that is the same as Eskom’s which is needed by high-end audio and other specialised equipment that are electrically sensitive, such as life support systems.
Importantly, in grid-tied installations the inverter must be able to be shut down rapidly in situations where utility power goes down. This is called anti-islanding and is a safety function for any Eskom personnel and electricians who may be working on the lines in the area.
An off-grid system will require storage batteries (the number and capacity are critical) and – usually – a backup diesel or petrol generator to take up the slack on cloudy days.
There are any number of economic studies that reveal how this calculations should be made and more about the long-term value of installing a solar PV system. But in the main, these studies are only as good as the input data. Make sure you are professionally advised when working with the numbers.