Photovoltaic System Design

Where possible photovoltaic cells should be considered as part of the installation. The cells give an output whenever there is light, the higher the light level the higher the voltage generated by the cells. If the aim is to peak lopp; that is generate electricity only during daylight hours then all that is required are for the cells to be connected to a synchronising inverter which is connected to the 'mains' and the necessary G59/G83 protection unit. The form filling that goes with the installation is typical of that produced by politicians and big business, it is difficult to fill in so that in the end you have to employ the services, at great expense, of a so called specialist - a form filling specialist! Don't you just hate politicians and big business; the terms rip off, over a barrel and short and curlies comes to mind.

There is an additional consideration, an average household uses approximately 6kWhrs (units) of electricity each day unless the dwelling is heated by electricity which is foolish as it is the most expensive form of heating available, if the minimum hours of daylight per day during the year is taken as eight hours for England and the output of photovoltaic cells is taken as 90W per square metre then the installation can be designed to have electricity generated by the photovoltaic cells throughout the year with export to the grid available when the hours of light per day exceed the design minimum.

The calculation, before beginning the calculation it is necessary to state the dc to dc conversion efficiency and the dc to ac inverter efficiency, both are taken at 92.5%; literature will state a maximum efficiency for each of these as 95% or even better however that efficiency is achieved at only one point of the operating curve of the converter/inverter therefore the 92.5% is a more realistic figure. Thus to achieve an 6kWhr output over a 24 hour period batteries will be required to store the electricity generated during the daylight hours, those batteries will need to be charged at the appropriate voltage although the output from the photovoltaic cells will vary according to the light level to cater for this variation a dc to dc converter with a fixed output voltage is used, the output from the dc to dc converter will charge the batteries and supply the dc to ac inverter, during the night when the photovoltaic cells are not producing electricity the dc to ac inverter is supplied from the batteries.

This double conversion efficiency is calculated by multiplying the converter and inverter efficiences together thus 0.925 x 0.925 which equals 0.856 rounded to three decimal places or 85.6%.

To achieve a daily total of 6kWhrs the cells must generate a total of 6 ÷ 0.856 which equals 7.01kWhrs rounded to two decimal places.

Therefore in each hour of daylight the cells must generate 7.01 ÷ 8 which equals 0.877kW rounded up to three decimal places which equals 877W.

The average output of a photovoltaic cell is 90W per square metre thus the area of photovoltaic cells required is 877 ÷ 90 which equals 9.75 square metres rounded up to two decimal places.

If it is assumed that the usage of electricity is even throughout the day; which we all know is untrue, much more is used during the daylight hours; then the batteries will need to be sized for two thirds of the daily use capacity, this will oversize the batteries which will give a margin of safety. Thus the batteries will need to have a capacity of 4kWhrs, since batteries are sized in Ahrs (ampere hours) the capacity may be determined by dividing the Watt hours by the battery voltage.

Due to the minimum daylight hours being used for the above calculations there will be an excess during the other days of the year which will amount to a total 1557 hours then the excess power will total [90 x 9.75 x 1557 x 0.856] ÷ 1000 which equals 1169kWhrs for sale back to the grid which considering your electricity is free is a bonus. Since to get the maximum infeed tariff you have to utilise an approved contractor it is suggested that you do not export to the grid as the money you would make doing so will be lost to the so called approved contractor. Remember approved does not mean competant and that the approved contractor will likely demand a maintenance contract which will almost certainly cost more than any monies that may be earned from exporting electricity. Utilise the excess electricity to power hot water storage to heat your home thereby reducing the gas or oil bill during those parts of the year where excess electricity is available, or in the summer months for air conditioning.