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Micro inverters vs string inverters - which works best?
Posted by Paul Hutchens on 19 April 2011 at 8:45 am
The yield and efficiency of solar PV panels is a constant topic of debate. Talk of kWp and kWh as well as percentage efficiencies of solar PV modules rings out at energy clubs across the land.
This is just the headline information though. Just as an obsession with a car's top speed or the time it takes to get from 0 to 60 does not really relate to how long it takes to complete a journey - it does not relate to how well the "system" performs and how much useful energy is actually transmitted.
The inverter is the main component of the transmission process. Firstly, it goes without saying, that the inverter needs to be chosen carefully. To install the best one, you need to consider the size of the PV array, the type of modules, orientation and pitch, location and potential shading problems.
Many inverter manufacturers have made significant developments including the inclusion of multiple Maximum Power Point (MPP) tracking which is the automatic adjustment of electrical load to achieve the greatest possible power harvest, during moment to moment variations of light level, shading, temperature, and photovoltaic module characteristics.
PV cells have a single operating point where the values of the current and voltage of the cell result in a maximum power output. MPP trackers utilize some type of control circuit or logic to search for this point and thus to allow the converter circuit to extract the maximum power available from a cell.
Therefore multiple MPP trackers allow this to be done at multiple points in the solar array which will maximise yield even if part of the array is shaded.
Micro inverters, such as those from Cambridge based Enecsys, take this a step further.The micro inverter is a compact unit, installed at the back of each solar module. It converts DC to AC power for connection to the electricity grid without the need for a string or central inverter.
Apparently a micro inverter maximizes the power harvested from each solar module and therefore the entire PV array. This is accomplished by performing MPP tracking for each module ensuring maximum harvest of energy even under partially shaded conditions. Degradation in the performance of any one module, due to module mismatch, shadows from trees or chimneys does not degrade the performance of other modules and therefore has minimal impact on the power harvested from the PV array.
According to Enecsys this results in improved power harvest of 5% to 20% from the solar PV system. The system is also claimed to be safer as power conversion from DC to AC is done at each solar module, high voltage DC wiring is eliminated and a lifespan of up to 25 years is quoted.
As you would expect there is a cost implication to this and, although pricing for the UK is not formally available yet, micro inverters are expected to work out more expensive than string inverters. At Ecobuild I was told of a new version which effectively shares one micro inverter between 2 PV modules which may bring pricing to a par with string inverters.
In conclusion there are advantages to micro inverters but there may be a cost penalty which needs to be taken into consideration. I would also suggest that string inverters, particularly with advances in technology such multiple MPP trackers, would probably still be suitable for many applications.
More information about solar PV and inverters from YouGen
If you have a question about anything in the above blog, please ask it in the comments section below.
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