DESIGNING A SOLAR SYSTEM: BASIC INFORMATION (UK)

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• The cells used in photovoltaic modules convert all wavelengths within the visible light spectrum to DC electricity but are optimised for the wavelengths that occur most commonly.

• In the UK we receive approximately five times more energy from the sun in June & July than we do in December & January.

• The unit of energy for insolation (incoming solar radiation or energy from the sun) is the kilowatt hour per square meter per day (kWh/m²/day). The UK mainland receives at best a mean daily insolation of around 5kWh/m²/day and at worst a mean daily insolation of around 1kWh/m²/day. As these are mean values it must be anticipated that actual values will at times be higher or lower than these figures.

• The unit "kWh/m²/day" is also referred to as the "equivalent sunshine hour" (ESH).

• For peak performance a solar module should face the brightest part of the sky. Most modules are installed at a fixed azimuth and tilt angle in order to maximise their annual energy output.

• Tracking systems may be utilised to improve system performance (by up to 30% for 2-axis tracking) but are expensive and a potential source of unreliability in systems which would otherwise have no moving parts.

• At high tilt angles most modules are self-cleaning, however, if bird droppings are likely to be a problem then "bird spikes" should be employed to disuade birds from perching along the top of the module. This is most effectively achieved by drilling holes along the module support frame's top edge and passing plastic cable ties through them. Once tightened the end that would normally be cut off is left pointing upwards.

• Solar modules are made up of many "cells" manufactured from various forms of silicon. The greater the light intensity falling on these cells the greater the current produced (light intensity and output current are proportional). However, the voltage produced is not proportional to light intensity but rises very quickly in low light ensuring that charging can take place.

• Partial shadowing of a module should be avoided at all costs as the effect is a disproportionate reduction in power output. The cells in a module are in long series strings, where the current passing through each cell is the same, the effective output is thus determined by the cell with the lowest output.

In order to determine the size of a suitable module to meet a specific load requirement we need the following information:

1. LOAD - worst case daily figure expressed in ampere hours per day (Ah/day).

2. VOLTAGE - nominal system voltage (12, 24, 48, etc).

3. ESH - the worst case equivalent sunshine hours per day value must be selected for the site, azimuth and tilt angle chosen.

Using this information you can calculate the System Amps (SA) using the simple formula below (where SA=the module(or array) current at peak power - Ipp). A safety factor of 1.2 is incorporated to account for various losses including cabling, diodes, battery charge efficiency, module production tolerances and ageing.

 Once the SA has been calculated then a module (or modules) with the required Ipp can be specified. The total number of modules required in the system is also a function of the required nominal voltage. The current requirement is met by the number of modules in parallel whilst the voltage requirement is met by the number of modules in series.

 AZIMUTH - Modules should face true south in the Northern Hemisphere and true north in the Southern Hemisphere.

 TILT ANGLE - For systems providing power all year round with a fixed module the appropriate tilt angle above horizontal may be chosen from the following table in order to maximise the annual energy output.

•  BATTERY SIZING - The battery in a photovoltaic system is of equal importance to any other element, if it fails the system fails. For a continuous load it serves to provide electrical power every night when module output is zero as well as during days when module output is below the average for the time of year. For systems providing power all year round the size of a lead acid battery is calculated taking the following into account:

• 1) Worst case daily load.

• 2) Site latitude - higher latitudes generally have longer periods of below average insolation during the winter months and thus require      greater reserve capacity.

• 3) Temperature derate - battery capacity is reduced at low temperatures.

• 4) Depth of discharge - the deeper we discharge a battery the fewer cycles it will give to this depth during its life.

• REGULATION & BLOCKING DIODES - If at any time during the year the daily module output exceeds the daily current drawn from the battery then a regulator is required. If the module is to remain permanently connected to the battery (except during periods of regulation) then a blocking diode is required. All BECO regulators incorporate a blocking diode.

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