Solar shadowing is like blocking a water pipe. If one cell is fully shadowed, it does not just block current through that cell, it blocks the entire string. Because of this, only a few shadowed cells can block your total solar output. If partially shadowed, (e.g. by light cloud) the loss is proportional to the amount of sunlight being blocked. The higher quality solar modules produce some output unless clouding is severe. It is, however, rare to have zero during ‘daylight hours’.
Solar Shadowing – reducing the losses – bypass diodes partially assist
A totally basic 12-volt solar module has 60 cells. Any cell that is totally shadowed produces no current. The loss, however, is far greater. This is because short wires interconnect those cells end-to-end. It’s like having 60 short water pipes. Partially or fully blocking just one affects the other 59. With solar, the module only supplies the current of the least producing cell. If a module has only some cells fully shaded, those cells may overheat.
To limit this, all but the most basic (12 volt) solar modules have three strings, each of 20 cells. Each string has a diode that, if activated, provides a path for unshaded strings. This is not a perfect solution. It slashes output by over one-third. Worse, the diodes are not reliable. If fails it will not protect sub-strings. The failed diode may then prevent them from working.
The solution is simple. Avoid cheap solar modules. They are rarely a bargain.
Solar Shadowing – reducing the losses – the more effective ways
The output of the lowest cell limits overall output in basic systems. Worse, in multiple module systems, shadowing one limits output of all. Fortunately, technology can now confine losses to the area shaded.
Solar Shadowing – reducing the losses – power optimisers & micro-inverters.
Power optimisers attach to individual existing solar modules. They harvest maximum dc energy by optimising the modules’ dc output. In addition, doing so eliminates power mismatch and decreases the effect of shadowing losses.
The power optimisers can be inbuilt or fitted separately. They were initially available for a limited range of modules. This is now changing as the concept has been proven to work well.
This concept is generally similar to that above – but at ac not dc.
An inverter changes dc power to ac power. For example, with stand-alone solar, the modules’ dc output is usually inverted to 110 or 230 volts ac following intermediate battery charging. Such charging is typically at 12-48 volts dc.
Pic: Enphase micro-inverter
The micro-inverter concept has no central inverter. Instead, a micro-inverter is included within (or attached to) each solar module. Regardless of partial shadowing, each solar module outputs 110 or 230 volts ac – albeit of varying current.
Shadowing cannot be compensated by a micro-inverter. A micro-inverter, however, usefully limits its effect to the module shadowed. This enables solar to be used where partial shadowing is unavoidable.
Moreover, there is a possible downside for stand-alone solar systems. It is that high ac output does not preclude battery charging. It does, however, limit installation to certified electricians in some countries (such as Australia).
Solar Shadowing – reducing the losses
Our books cover shadowing issues in greater depth. For example, Solar That Really Works! is for boats, cabins and RVs. Solar Success is for homes and properties. This book also covers the legal issues involved. Caravan & Motorhome Electrics has all you need to know in the RV solar and general electrical area.