23 March 2010
It's a kind of magic
How does SolarMagic technology boost today's photovoltaic solar system efficiency?
Solar power is one of the most promising forms of renewable energy on the market and, due to high Government incentive plans combined with the projection of increasing conventional electricity costs, more home owners than ever are going solar and installing photovoltaic (PV) system on their roofs.
With today's PV system prices, the return of investment is usually met in seven or eight years. Incentives and PV systems are guaranteed for 20 years or more.
The return on investment is guaranteed by the amount of energy produced on an annual basis, therefore it's important to maximise the energy yield with efficient, reliable and well maintained PV systems.
However, many solar installation owners remain unaware of the degree to which the array might be failing to realise its full potential due to partial or temporary shading.This article analyses the problem of average losses in electricity production caused by partial shading of a photovoltaic system. It introduces the advantages of a distributed maximum power point tracking system (MPPT) at panel level and discusses the results of different real case studies with SolarMagic technology.
Partial shading effect
A minimum shading condition on the PV system due to trees, chimneys or other obstructions, known as 'mismatch', can cut the power output of the entire solar array significantly. The actual impact of energy yields due to partial shading mismatch is difficult to predict with a simple calculation because it depends upon many factors, such as internal module cell interconnections, module orientation, PV module array series-parallel connections and the configuration of the inverter.
A PV module is realised by a series connection of cells called strings. Each string is protected by a bypass diode that prevents damage through overheating if one or more cells are shaded or defective. These strings are connected in series or parallel to get, respectively, higher voltage or higher current out of the panel.
A PV array is defined as a structure made of a number of parallel strings of series connected PV modules. The maximum voltage of a string must be lower than the maximum input voltage rating of the inverter.
Under a partial shading condition, the current of the non shaded cells flows through the bypass diode of the shaded section. These non homogeneous conditions of a PV array lead to a V-P electrical curve with multiple peaks. Figure 1 shows a standard grid tied configuration with centralised MPPT function, where two panels of one string are shaded. The centralised MPPT would not be able to set the dc voltage to maximise the output power of both strings. At high dc voltage (M1), the MPPT maximises the output power of the unshaded string. At lower dc voltage (M2), the MPPT will maximise the output power of the shaded string: the shaded panels are bypassed by bypass diodes and the unshaded panels of this string will deliver their full current.
Multiple MPPs of the array may lead to additional losses in a centralised MPPT configuration as the maximum power point tracker can be confused and stop on a local maximum point and settle in a sub optimal point of the V-P characteristic.
Different case studies and field tests have proven that the partial shading effect has a severe impact on energy yields of a PV system. The shading effect can be mitigated by using a distributed MPPT control.
To maximise the energy output of each solar PV panel in the array, National Semiconductor has developed SolarMagic technology, which enables each solar panel to produce the maximum energy, regardless of whether other panels in the array are underperforming due to mismatch. SolarMagic technology monitors and maximises the energy harvest of individual solar PV panels through advanced algorithms, combined with mixed signal technology. In this way, up to 50% of the energy that would otherwise be lost due to mismatches can be recouped.
The SolarMagic Power optimiser is designed to be quickly and easily installed within the traditional architecture of a solar array. The system has two strings connected in parallel of n modules, for simplicity the picture shows only 3 PV modules per string, however typically a string is built from 5 to 12 modules in series to get a string voltage of 500 to 800V.
String A has all modules that don't have irradiation mismatch, each module will present the same characteristic and will be equally irradiated.
String B has grouped all modules that have different characteristics or irradiation mismatch due to shading, tilt orientation or higher potential of dirt accumulation. The output of each module is connected at the input of a SolarMagic Optimiser (SMO) module. The output of each SMO is connected in series, like the modules of string A.
The SMO module has highly efficient integrated power circuitry with a maximum power point algorithm that maximises the output power of each PV module. It guarantees the same output current on the entire string minimising hot spot problems and internal by pass mode. Each SMO module will adjust its output voltage to match the total bus voltage.
The result is that the entire PV system will present an I-V curve with a single maximum power point, simplifying the operation of the central inverter and maximising the energy yield of systems affected by mismatch.
SolarMagic allows those parts of the solar array still in full sunlight to continue working at full capacity, despite the shading of one panel. The key benefits of Solar Magic technology are:
* It can recapture up to 50% of energy lost through shading
* It can enable a 10 to 40% increase in energy production
* Compensation for panel to panel power mismatch
* Dirt, dust, debris and bird droppings will no longer significantly degrade PV array performance, and
* Panel ageing effects are minimised
Installation flexibility is another key advantage of Solar Magic. It makes possible using multiple orientations/tiles and different module sizes/shapes, mixing and matching string lengths, easing replacement of a failed module and permitting use of shorter/longer strings than normal.
From analogue to solar
National Semiconductor is committed to deliver analogue semiconductor products targeting electronic applications focused on quality of life. One of the most promising is Solar Magic. Because the return on investment is guaranteed by the amount of energy produced on an annual basis, it is important to maximise the energy yield with efficient, reliable and well maintained PV systems.
Author profile:
Michele Sclocchi is Business Development, Renewable Energy, for National Semiconductor Europe.
Author
Michele Sclocchi
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