Once seen mostly as an interesting curiosity, photovoltaic arrays are gaining rapid acceptance as renewable energy sources. Analysts report that revenues for the photovoltaic industry grew from $10.6 billion in 2006 to $17 billion in 2007 and expect growth of 26% for 2009, despite widespread economic gloom. This forecast is equivalent to installing some 7.1GW of generating capacity.
At the same time, panel prices are set to fall by 20 - 30% as the global supply of polycrystalline silicon doubles. Recent shortages in polysilicon supplies have driven the emergence of thin-film panels built using materials such as amorphous silicon, cadmium telluride, and copper indium gallium selenide. At around $2.50/W to $1.60/W, these materials are half to one-third as expensive as polysilicon panels, but are also as much as 50% less efficient.
Key factors that drive the uptake of photovoltaic arrays include the role of government ‘green energy’ grants together with incentives such as the ability to sell excess capacity to the national grid. A recent survey by the UK’s Centre for Alternative Technology shows that no less than 91% of UK households would consider installing photovoltaic arrays, especially if they can access European-style feed-in tariffs for selling excess energy. In addition to economic considerations, it’s also attractive for companies to promote carbon neutrality.
Despite considerable periods of overcast conditions, one square metre of level ground in London captures an average 1,000kWh of light energy every year, while tilting that area to point southwards at 35 degrees improves capture by about 11%. This consideration naturally suits installations on south-facing pitched roofs. Techniques to further improve energy capture include installing tracking systems to orient the array to the brightest area of sky and adding optical concentrators to focus light upon the array.
It is also important to select arrays that function well across a range of lighting conditions. Representative high-efficiency designs that use mono-crystalline silicon are available that deliver 185W/m2 for an irradiance level of 1,000W/m2, with power output falling by 50% for an equivalent reduction in light level.
Another approach that appears in some high-end arrays is the use of multi-junction cells that capture photon energy more efficiently. In the near future, arrays will be available that at least double efficiencies from the 10 - 18% level that’s typical of today’s devices. For instance, a new class of metamorphic semiconductors and cell architectures recently achieved more than 40% efficiency in laboratory tests, while nanotechnology promises yet greater efficiency improvements with lower production costs.
Switchgear considerations
The switchgear that connects photovoltaic arrays to the distribution system is an aspect that deserves special consideration. A typical installation comprises a number of panels, wired in series, to raise the output level from a single array’s 30-60Vdc to 500-900Vdc. This open-circuit voltage remains relatively constant while the available current level varies according to the irradiance level. Individual panels may generate 2 to 7A and be wired in parallel to provide currents of up to 300A. This power feeds an inverter that converts dc to line-frequency ac and drives a transformer to provide galvanic isolation. The transformer’s output may supply local circuits and/or couple into the grid.
It is essential to isolate the photovoltaic arrays at source and to provide a master-switch facility at the inverter’s input. The level 1 switches make it possible to disconnect individual sets of paralleled arrays without affecting other arrays, while the level 2 switch safely disconnects all power sources.
In addition, each switch box requires protection using high-speed semiconductor-protection fuses with breakdown voltages of at least 1,000Vdc, plus varistors to guard against transients and other over-voltage faults. All of these elements must withstand high-voltage, high-current switching into the capacitive and inductive loads that inverters present and, of course, meet appropriate regulatory requirements.
Newly available from Switchtec, the CVF series of connection boards from Telergon are specifically designed to provide level 1 and level 2 switching and protection within photovoltaic installations. Five level 1 configurations support eight to 13 arrays at 6 – 12A per circuit together with their fusing and over-voltage protection devices. Four variants of the level 2 connection board handle five or seven inputs and fusing at up to 40A per input, with or without over-voltage protection.
Easy to install, these IEC60364-7-712 compliant switchboards employ polyester and wall-mounting enclosures that are stabilised to resist degradation from exposure to UVA and most chemicals over the temperature range -30ºC to +120ºC. They offer environmental protection to IP65 for level 1 and IP55 for level 2 applications.
Where greater level 2 switching capacity is necessary – for example, in on/off and changeover configurations, the S5000-DC series of four-pole switches handles currents from 80 to 800A.
To assist customers with their designs, Switchtec offers application engineering expertise and full laboratory testing facilities for its range of more than 7,000 specialist components.
Since this article was published, we received the following from Mr Kristen Cadman of Marlow, Buckinghamshire:
You quote a number of figures in the article which do not match the ones I use from respectable sources.
On the ground, bright sunshine gives just over a kW per square metre, of which about half is visible light. Most of the other half is infra-red. These figures are interesting to people who want hot water from their
panels and already assume that the panel is pointing at the sun.
For photovoltaic, your figure of 185 W/m2 is believable and not that much more than my mono-crystalline panel when pointing at the sun. With 1500 hours of bright sunshine per year at Kew, it is difficult to explain capturing 1000 kWh of energy per square metre. 280 kWh sounds like a very good top target. In reality, at every step the figure goes down. The World Meteorological Organisation defines sunshine as over 120 W/m2.
It is my view that 91% of households are being misled with regard to the benefits of having solar panels. The truth is that large panels cannot be mounted on south facing roofs and track the sun which turns through far too great an angle in summer and is very low over the horizon for an average of an hour and a quarter per day in December. The large investment gives a trickle of power which can run a few highly efficient devices.
To illustrate this, in Kew, a 1 m2 panel on average in December would power a 2400 W kettle for the three minutes it takes to boil enough water for two cups of coffee and to wash them up with the leftover hot water. An alternative use for this power (say 10000 Watt. minutes) might be a couple of low energy (11 W) bulbs for the whole evening, listen to the radio, charge a cell phone and even use an efficient TV or computer for a short time.