Today’s solar inverters try to optimise the entire array from a central vantage point, which severely limits their effectiveness.
A typical solar photovoltaic (PV) installation consists of a number of carefully matched panels that are connected in series strings and then put in parallel with other series strings. The direct current (DC) generated by this array subsequently feeds into an inverter at the side of the array.
The inverter has two basic functions: to provide the grid connection in combination with the DC to alternating current (AC) conversion; and to optimise the PV array by finding its best operating point. To be more specific, for any given irradiance, temperature and cell type, there is only one voltage-current pair for which a PV cell produces maximum power. Operating a PV cell at different voltage and current levels reduces the efficiency of the system and leaves valuable energy unharvested.
Inside panels there are again strings of cells and it is not uncommon to end up with hundreds of PV cells connected in series within these arrays. Similar to Christmas lights, if one cell in this link fails to produce current, the entire string drops out. This is exactly what happens when partial shading or soiling caused by debris occurs on PV installations.
Article continues below…As a remedy, modern panels have integrated bypass diodes that reroute the current around heavily impaired sections of the strings. However, by rerouting the current around significant sections of the string, they lose the potential energy contribution of these cells. Worse, it could lower the overall voltage of that entire string. And because the inverter requires a maximum operating point for the cells, it will now have to decide if it should optimise the voltage for the impaired string or maximise the energy harvest from the unimpaired strings. In most cases the inverter will choose to optimise for the unimpaired strings, causing the energy harvest of the impaired string to drop to near zero.
The result is that just 10 per cent shading on a PV system can cut its energy production in half. This phenomenon has to do with an extreme sensitivity of today’s solar architecture to mismatches in the PV system. Clearly, after more than 30 years, solar energy management is due for a major overhaul.
National Semiconductor’s SolarMagic technology monitors and maximises the energy harvest of each individual panel and optimises the energy flow through the array.
Solar systems fitted with SolarMagic can compensate for up to 50 per cent of the lost power due to mismatches and impaired cells. A power optimiser distributes intelligence down to the panel level allowing each panel to operate at its maximum power point regardless of the different conditions apparent along a string.
Ralf J. Muenster is Director of the Renewable Energy Segment at National Semiconductor.
