Today there are three forms of commercially available solar cells - monocrystalline, polycrystalline and thin-film. Each of these technologies has its benefits and demonstrated strengths. Due to their higher efficiency and longer useful life monocrystalline cells have been the technology most favoured by industry, however, the trend and future seems to lie with thin-film which typically has demonstrated lower cost per watt, cell efficiencies and life expectancy.
PV cells operate at a relatively stable voltage while the direct currents they produce varies with light intensity. The performance of solar cells under different light conditions is demonstrated by the I-V curve, the relation between the current and voltage produced. The efficiency of this relationship is the ratio of the power output to the power of the sunlight striking the cell. In practical terms, the solar module with the highest efficiency will have the smallest surface area producing the highest amount of useful energy over a given period of time.
Rated vs. actual energy output
The optimum power of a cell is referred to as its maximum power point (MPP) where the combination of the highest current and voltage, usually 17 VDC, is achieved under standard test conditions (STC) of 1000 w/m2 of light and cell temperature of 25° C. Seldom do the modules operate under a clear blue sky and operating temperatures of the cells are generally in the range of 30° C warmer than the ambient air which reduces their output. Photovoltaic modules are typically rated according to their wattage output, under STC.
If a solar module generates a rated 4.4 amps @ 17 volts this equates to a 75 watt rating. In a nominal 12 V battery system, however, the module continues to generate 4.4 amps though at a lower 12 - 13 volts producing a maximum of only 57 watts. Good photovoltaic system design will use the rated amps - the power actually delivered to the battery bank.
What affects PV performance?
Light intensity - The rated power of a solar module is made at a standard test condition of 1000 w/m2 light intensity. Their power is directly related to this intensity, thus on a fully overcast day a solar module can be expected to reach about one-tenth of its rated power.
Technology - Different cells have varying degrees of efficiency and operating characteristics. Crystalline modules usually operate in the 0.5 -0.6 V range, however, thin film cells are generally higher.
Cell size - The power generated by the cell is proportional to its size - more sunlight hits a larger cell thus more power is generated.
Seasonality - Most regions of Canada receive at least 2200 hours of sunlight each year. The distribution of this sunlight is of course highly variable with November and December yielding less than 1/3 the direct sunlight that is available in July.
Temperature - With the exception of amorphous modules, photovoltaic cell efficiency improves producing higher currents in cold temperatures. In fact, a modules peak power and voltage will improve by 0.3 - 0.5% for every one degree Celsius below 25° C.
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