The ironic fact of solar PV electricity generation is, the more sunlight the panels get, the more electricity they make BUT the hotter they become.   The hotter they become, the lower the efficiency, which directly affects power output.   So, on a ht day in any Australian city, the ambient temperature might be 38 degrees but the panels are around 60-75 degrees.   

Thankfully, there is a simple solution : Cooling the panels with mists of rain water to maintain a hysteresis less than 10 degrees above ambient can deliver between 12-20% more power depending on the weather, your location, and the quality of the panels installed on your home.

Technical Details : 

So, the solar panel temperature affects the maximum power output directly.   As solar panel temperature increases, its output current increases exponentially while the voltage output is reduced linearly. Since power is equal to voltage times current this property means that the warmer the solar panel the less power it can produce.

If you look at the manufacturer's data sheet for your solar panels, you will see a term called "temperature coefficient Pmax".   For example the temperature coefficient of a Suntech 190 W (monocrystalline) solar panel is –0.48%. What this means is that for each degree over 25˚C … the maximum power of the panel is reduced by 0.48%.

So on a hot day in the summer – where solar panel temperature on the roof might reach 45˚C or so – the amount of electricity would be 10% lower.

Conversely, on a sunny day in the Spring, Autumn, or even Winter – when temperatures are lower than 25˚C – the amount of electricity produced would actually increase above the maximum rated level.

Therefore, in most Southern Australian climates (Adelaide, Melbourne, Sydney, Perth) – the days above and below 25˚C would tend to balance each other out. However, in locations closer to Darwin, Townsville, Broome etc, the problems of heat loss could become substantial over the full year and warrant looking at alternatives.   An I-V curve shows all of a solar panel’s possible operating points (current/voltage combinations) at a given cell temperature and light intensity (irradiance). “I” is the amperage/current. Increases in cell temperature will slightly increase a solar panel’s current while significantly decreasing voltage

Maximum power in watts is shown at the knee of the curve. Most I-V curve charts use optimal operating cell temperature of 25° C, but when is it only ever 25° C.  To calculate the true power generated from a battery/array system without a charge controller, check the amperage produced during operating battery voltage (12, 24 or 48 volts) and actual cell temperatures.   

While it is important to know the temperature of a solar PV panel to predict its power output, it is also important to know the PV panel material because the efficiencies of different materials have varied levels of dependence on temperature.   Therefore, a PV system must be engineered not only according to the maximum, minimum and average environmental temperatures at each location, but also with an understanding of the materials used in the PV panel.   The temperature dependence of a material is described with a temperature coefficient. 

For polycrystalline PV panels, if the temperature decreases by one degree Celsius, the voltage increases by 0.12 V so the temperature coefficient is 0.12 V/C. There is a general equation for estimating the voltage of a given material at a given temperature, however, the bottom line is that the is a benefit for running the whole system at a temperature just above ambient.

So, the solar panel temperature affects the maximum power output directly. As solar panel temperature increases, its output current increases exponentially while the voltage output is reduced linearly. Since power is equal to voltage times current this property means that the warmer the solar panel the less power it can produce. The power loss due to temperature is also dependent on the type of solar panel being used.

Typically, solar panels based on monocrystalline and polycrystalline solar cells will have a temperature coefficient in the –0.44% to -.50% range. Sunpower (Monocrystalline) does the best in this regard with a temperature coefficient of –0.38%. It is also the most efficient commercially available solar panel – making it an excellent choice for high temperature areas.

Amporphous Silicon does a bit better. For example, the Sanyo HIT hybrid cells and bifacial cells, which consist of a layer of monocrystalline silicon covered with a thin coating of amorphouse silicon have a lower temperature coefficient of –0.34% - making them another good choice for people looking for high efficiency solar panels in areas closer to the equator.

The best so far in terms of dealing with high temperatures are the Cadmium Telluride solar panels – with a temperature coefficient of –0.25%. However, while they are good with dealing with temperature changes – they are not as efficient at converting sunlight into electricity.

What Is It You Are Researching Then ?:

I have developed a pair of thermometers feeding temperature data from the underside of one panel and another from ambient air temperature to an ATtiny85 microprocessor.   The software allows me to turn on or off a small 50W submersible pump located within a rain water tank.  In the ON condition, the pump will send rain water to the roof where small misters provide water to the surface of the panels.   Evaporation of the water creates a cooling effect and thus, the panel cools.  We now have one unit that uses an additional small PV panel to charge a 12 Volt SLA battery and that battery powers the pump and the micro controller so the whole system is independent of main power.

As a side effect, the panels remain cleaner for longer without the build up of dust that can scatter sun light instead of allowing the panels to convert the light into power.

Application : 

I am aiming to install the system in domestic markets only at this stage.   The system uses around 40L or rain water each day, however, about 60% of this is recovered as it runs off the roof and back into the rain water tank.   We would also like to develop a portable unit for mobile homes and caravans.

Cost : 

So far, our system has been installed in 4 homes in Adelaide, and has proven to be effective in each case.   Total cost of the system to date is around $300 (depending on the number of panels and the orientation) which provides a return on the investment within the first 6 months.   Rain water is best because the tap water contains calcium which eventually builds up a sheen on the surface of the panels.   The customer needs to have a rain water tank or at least a large drum of rain water.

Why Invest In Us : 

We are a very small backyard operation at the moment and while we have funded the preliminary research ourselves, we are now at a point that we can confidently prepare a portable demonstration unit on a trailer to show customers in real time the benefit of the hardware to help them make more power from their expensive investment, keep it clean, and maximise the return on their investment by making as much power as possible from the sun.   We are also interested in keeping the development of this device in Australia - far too much of our light manufacturing and smarts gets shifted off shore because of cheaper manufacturing.   This WILL NOT happen with our project.

Budget : 

We are aiming to spend the $2600 in the following ways;

(a) Purchase a pair of 12V 120W mono crystalline Solar Panels for $640 each.

(b) Purchase a suitable 2.4W Solar Water Pump (with panel) to pump water from the rain water tank on the trailer to the mister sprays for $100.

(c) Purchase a registered second hand 6x4 Trailer and refit with a small rain water tank, a simulated roof and a temperature monitor micro controller package for $1200.   The pair of panels would be mounted to the simulated roof.

What If You Are Over Supported : 

In the event that we attract more than $2600 of investment, we will put the additional funds towards developing a template for the mass production of the temperature monitor and pumping circuit (IN AUSTRALIA)