Wednesday, July 02, 2008

Dynamic Solar Arrays (Patent Pending)

Okay enough with the teases! Here it is:

A photovoltaic PV array owner can control the power output of the array, allowing the array owner to maximize power production when they want to.

The primary determinate of how much power a solar PV panel generates is how much sun it receives. But there is a secondary determinate, namely the operating temperature of the panel. Generally this works against PV array owners, since once they put the array of panels in the sun, the panels heat up and produce less power. But circulating a cool fluid through the panel can bring the panel temperature down boosting output.

While the change in power output per degree is small, ~0.5% per degree C for silicon, it adds up. Consider that a panel placed in the sun will normally reach an operating temperature 10-15 degrees C above the ambient temperature. Simply by cooling the panel to ambient temperature one can get 5-7% more power output from an array. But why stop at cooling the array to the ambient temperature? The best sunlight, and hence prime PV array locations, is often found in hot places like deserts in the US southwest. In such places array temperature can top 60 degrees C (140 F), meaning that if one wished to cool the panels to 10 degrees C (50 F) one could produce 25% more power.

Getting more power from an asset is always good, but perhaps even more interesting to an array operator, like a utility, would be the ability to control the power output from an array by controlling the temperature of the panels over a wide range of temperatures. Instead of just one reservoir at either the ambient temperature or the cold temperature, the array operator could have both reservoirs and mix the fluids in a controlled fashion (and circulate it around the array) to either increase or decrease output in this “top” 20-25% of capacity as desired.

On a clear day, the utility/array operator could run the plant at one temperature during most of the day, but drop the temperature during the peak demand period to maximize the power production at peak. Alternately if the day is partly cloudy, the utility could raise and lower the temperature of the array to maintain a steady power output despite clouds that block 20%+ of the sunlight. (A cooler panel produces more power at a given light level, but you still need light.)

Finally, if one wants to exercise control of the panel output over a greater percentage of its capacity one could add a hot temperature reservoir (above ambient temperature) and mix it in to raise the temperature of the array. While this last option may appear counter-intuitive to many people working in the solar industry (as it implies purposely reducing the efficiency of the array) it might be a useful option for a utility that has multiple generating assets to balance against variable demand. The hot temperature reservoir would allow a utility to smoothly ramp (up/down power) from the PV array over 50% of the array’s total capacity.

I’ve used temperatures consistent with using water as the working fluid in the above examples, although clearly any fluid can be used. The costs of creating a dynamic solar array system will be the cost of plumbing the PV array, the pumps & energy to circulate the fluid, and maintaining at least two reservoirs at different temperatures. One will also get relatively high volumes of low grade heat (the few degrees the fluid may rise passing from the input to the output of the array) which might used to condition the fluid returning to the reservoirs.