Photovoltaics

There is a seductive simplicity about solar photovoltaic (PV) technology. No complex machinery is needed, no fossil fuels, power stations or electricity pylons, no energy is wasted in cooling towers or distribution networks. A classic solar PV installation has no moving parts. It is silent, unobtrusive and needs little maintenance. PV cells can be integrated into elegant façades without architectural embarrassment. A PV installation can take up much less space than its solar air or water heating analogues, even though it will

Centre Alternative Technology Wales
Monocrystalline and polycrystalline solar PV panels at the Centre for Alternative Technology Wales (Reproduced with permission from Centre for Advanced Technology)

usually also need a rectifier to convert the low voltage direct current electricity it produces into mains voltage alternating current before it can be utilised. And while most other microgeneration technologies are firmly rooted in earlier centuries, PV is a child of the space age, and very much one of the key technologies for the twenty-first century.

As always, such simplicity comes at a price. Efficiency of conversion is still low. Even the best PV cells currently available commercially can utilise less than 40% of the sunlight falling onto them - and these are far too expensive to use on the large-scale (see below). Fifteen per cent or less conversion is more typical. Cells must be kept cool to reach even these levels; cooling adds to complexity. Cell manufacture is an energy intensive process; indeed, some researchers have suggested that most PV cells on the market will still not produce more useable energy during their lifetime than was consumed in their manufacture. Some of the materials used in some cells are very scarce potentially toxic and need particular care during manufacture and disposal/recycling. Power is only available when the sun is providing energy: like wind and other forms of solar energy, PV is only a practical option for many buildings when a grid connection or some other form of energy store is available (see Chapter 15). Despite these drawbacks, however, the inherent attractions of PV means that it will always be worth considering when determining the best alternative energy technology mix for any particular project.

Physicists discovered that a particular class of material would generate electricity direct from sunlight as far back as the nineteenth century. These were the materials we now know as semiconductors, but the first true PV cell, which used selenium, was only about 1% efficient. In the 1950s, however, Bell Laboratories in the USA was researching semiconductors, looking for better performance than could be achieved by germanium, the first semiconductor to be used on a commercial scale. Silicon was to take over once the practical problems of mass producing high purity silicon crystals had been cracked. However, during the research process it was discovered that silicon with certain specific impurities was much more efficient at converting sunlight to electricity than selenium.

Nevertheless, conversion efficiency was still only 6% or so, and the cells were expensive. It was not until the Soviets launched Sputnik 3 in 1957 that real interest in the new technology appeared. Sputnik used an array of PV cells for power; conversion efficiency might have been low but the vast sea of solar energy in which the satellite swam meant that plenty of power was available whenever the satellite was out of the Earth's shadow. Western research was galvanised by this Soviet first. Generous government funds became available, and virtually every satellite and spacecraft launched into permanent orbit in the last 50 years has spread fragile silicon arrays to mark its successful insertion. The International Space Station has the largest PV array ever assembled in space, currently (2007) spanning more than 70 m.

These first generation cells, also known as silicon-wafer based solar cells, are relatively robust units. Two layers of single crystal or monocrystalline silicon form the heart of the cell. One is deliberately 'doped' with a small percentage of phosphorus - this is known as N-type silicon. P-type silicon, doped with boron, forms the second layer. The two layers are sandwiched between a metallic back contact sheet and a fine metallic contact grid on the upper face. As silicon is basically a shiny silver metal that would naturally reflect a lot of the sunlight falling on it, an anti-reflective coating is applied before the whole cell is sealed below a protective glass cover plate. With an effective anti-reflective coating, one that cuts losses due to reflection to less than 5%, and an efficient front contact grid that blocks as little light as possible, conversion efficiencies as high as 15% are now possible.

Monocrystalline silicon is sliced from highly refined very pure cylindrical ingots, so to minimise waste the slices are usually left circular. A cheaper alternative is polycrystalline or multicrystalline silicon. Made from less refined square ingots, polycrystalline silicon cells are less efficient than the monocrystalline alternative, but cover a greater area of a normal rectangular PV panel. These two types of cells still account for the vast majority of solar PV capacity installed, but they remain expensive, and research is now focussed on bringing the cost per kilowatt down to more competitive levels.

One of the most significant developments to come onto the market recently is the SunPower A-300 monocrystalline cell. This has a unique rear contact design that reveals the 5% of the face of the cell normally covered by the contact grid and permits the use of thicker metal in the grid, which reduces resistive losses. Claimed efficiency tops 20%, and the A300 is said to be cheaper and easier to mass-produce than the conventional alternative.

Other results so far include the development of cells based on new semiconductors, or on much thinner films of silicon-based semiconductors such as amorphous silicon, or a combination of both. Generally classified as second generation cells, these are significantly cheaper to manufacture than the first generation cells, which usually more than makes up for their lower efficiency - only about 8% in the case of amorphous silicon. Some thin film cells use only 1% or so of the expensive silicon that goes into silicon wafer-based cells, saving massive amounts of energy during manufacture. And although overall efficiency is low, thin film cells do work better than silicon wafer cells in low light conditions, and are generally more robust and vandal-proof. One thin film technology now available on the commercial scale is based on cadmium telluride (CdTe), which is easier to deposit on substrates than most of the alternatives. The perceived potential toxicity of elemental cadmium means that this technology has attracted some controversy, although research to date indicates the risks are low. Efficiency is above 10%.

Significantly higher conversion efficiencies, approaching 20%, can be achieved by multi-layered thin film composites. These have a more complex operating model than the basic silicon cell, and can be tuned more precisely for particular end uses. Both copper indium selenide (CIS) and copper indium gallium selenide (CIGS) cells are now in production, although there are long-term concerns about indium supplies. CIGS cell development is focused on replacing as much of the indium with the much more available gallium as possible. Both CIS and CIGS cells can achieve efficiencies of around 11%. Thin film composites can even be deposited on flexible materials such as polymer roofing and textiles.

Moving to conversion efficiencies above 20% requires completely new technologies. Many, such as quantum dot modified photovoltaics, have achieved conversion efficiencies above 40% in the laboratory but are still in the development phase. One technology that is available, albeit at a very high price, is based on gallium arsenide (GaAs) multi-junction cells. Their ability to absorb nearly all the solar spectrum allows them to achieve conversion efficiencies approaching 40%, but their cost means that they have been used almost exclusively in the aerospace sector so far. However, alternative ways of using GaAs

Roof Membranes With Photovoltaics
'Flexible' solar PV is now a reality (Reproduced with permission from United Solar Ovonics)
Photovoltaic Roofs
Roofing membranes incorporating PV cells open up new possibilities (Reproduced with permission from United Solar Ovonics)

cells are now coming to the fore, and the technology is very promising (see below). At London's Imperial College, for example, ' quantum well' GaAs cells, which promise to be significantly cheaper than their predecessors, have been developed - these, marketed under the trade name QuantaSol' are aimed squarely at the concentrating PV field (see below) and are said to be a cost-effective solution for generating electricity on the larger scale.

Cells based on organic/polymer materials and dyes appear to offer the possibility of acceptable conversion efficiencies and massive reductions in production costs, but their developers have still not solved the problems of ultra-violet degradation and intolerance of high temperatures. Research into the manufacture of much cheaper silicon cell variants is also ongoing.

Other third generation PV technologies include photoelectrochemical cells (PEC), and those that use nanotechnology, such as ormasil. These promise conversion efficiencies in excess of 60%, but there are formidable problems to be overcome before any of these technologies become realistic options in practice.

PV cell conversion efficiency is calculated as the amount of energy produced by a square metre of cell at 25°C exposed to a standard solar radiation, which is taken as the solar radiation falling on a square metre of the Earth's surface at the equator at noon on a clear day at the spring or autumn equinox. Conveniently, this is 1,000 watts. Thus, a square metre of cell with a conversion efficiency of 15% will produce 150 watts of peak power. In practice, of course, the solar radiation falling on a solar PV panel located away from the equator will be far less than this for almost all the time. Research suggests that the effective capacity factor of a fixed PV array will rarely exceed 20%. Thus a rule of thumb figure would be that a square metre of typical PV cell would produce no more than 250 kWh in a year.

w0E S

West South East

-90°-75°-60°-45°-30°-15° 0° 15° 30° 45° 60° 75° 90°

West South East

-90°-75°-60°-45°-30°-15° 0° 15° 30° 45° 60° 75° 90°

Solarcentury

Typical PV performance chart (Reproduced with permission from Solarcentury.com)

(90) (90) (90) (90) (W) (90) (90) (90) (90) (90) (M) (w)(W)

solarcentury

Typical PV performance chart (Reproduced with permission from Solarcentury.com)

Crystalline PV cells are usually made up into modules containing either 36 or 72 individual cells; modules are assembled into PV panels and arrays. Module efficiency is usually 1 to 3% lower than theoretical cell efficiency in practice, due to reflection off the cover glass and temperature variations.

Extracting more useable power from the solar radiation available by increasing the conversion efficiency of the PV cell is one option. An alternative is to maximise the amount of radiation falling on the cell. Fixed arrays are usually orientated as close to true south as possible (see Chapter 9), although deviations of as much as 30° east or west are acceptable. Angle to the horizontal is usually the same as the latitude, unless maximum winter electricity output is a priority - in which case the angle should be latitude plus 15°.

Greater energy capture can be achieved by using some of the power generated to drive a single or dual axis tracking mechanism that will keep the PV array pointing towards the sun from dawn to dusk. A single axis east-west tracking installation can yield up to nearly 30% more energy in a year than a fixed array. A dual axis set-up, which also tilts the array to follow the sun's changing altitude, can produce around 40% more, both at the cost of higher capital and maintenance costs. Alternatively, so-called passive trackers are now becoming available for solar PV installations. These use the solar-induced expansion of a low boiling point compressed gas fluid (the same principle used in glasshouse ventilation actuators) to keep the panels pointing at the sun, and viscous dampers to minimise wind shake.

Solar Tracker Mechanism
A passive solar tracking mechanism (Reproduced with permission from Leonard G.)

Adding mirrors or lenses to concentrate the sun's rays on the array produces a heliostat concentrator (HC). In areas where cloudless skies are common, this approach improves the overall performance of all PV cells, thereby reducing the area needed for any specific output. It can also make it easier to justify the use of such high-performance cells as gallium arsenide. In 2006 Australia announced it was planning to build a GaAs-based HCPV installation in northwest Victoria, which, with a projected 154 MW output, would be ten times larger than any other PV plant anywhere else in the world. A conventional flat plate PV installation would use 1,000 times as much PV cell material as an HCPV installation, it is claimed, on the basis that each GaAs cell receives 500 times as much sunlight and is twice as efficient as a silicon cell.

A SolFocus concentrating PV module (Reproduced with permission from SolFocus)

On the smaller scale, companies such as SolFocus in the US are now offering developed HCPV panels made up of 16 mirror-based concentrators focussing sunlight onto GaAs cells only 10 mm square. Overall panel efficiency is 17%, and the panel is claimed to be capable of generating 205 W of 40 V electricity at peak. Most if not all HCPV installations include some form of solar tracking as they rely on direct sunlight rather than diffuse light. An alternative approach has been adopted by Soliant of California. A combination of mirrors and lenses is used to concentrate sunlight onto the cells, which are mounted in small twin axis tracking units. Modules of 35 of these currently under development measure 700 mm by 2500mm, generate around 500 W, and are said to be 24% efficient. Soliant hopes to have the technology available in mid-2009.

Rooftop Dual Axis Solar Tracker
Individual dual axis tracking features in this concentrating PV module under development by Solaire (Reproduced with permission from Solaire)

The Australian installation will be some 35° from the Equator, but it will be in a rural location where there is no risk of shading from adjoining trees, buildings or geographical features. Overshadowing can seriously limit the performance of a PV installation, more so than with solar air or water heating collectors. Low-rise buildings in dense urban locations with many tall buildings would not be fruitful locations for PV. Where buildings are much the same height then a rooftop installation would be indicated, and where the building stands well clear of others - say in an office or industrial park - façade-integrated arrays might be the best option, particularly where the building is relatively tall and the roof area proportionally small.

Photovoltaic Facade
Façade mounted PV is a practical option in the right location (Reproduced with permission from United Solar Ovonics)

Like all solar technologies, PV is most effective on buildings whose peak power demand is during daylight hours. Residential buildings will need some form of energy store (see Chapter 15), most commonly by connection to the national grid. The latter is not always a straightforward option. Alternatively, the power from the PV panels can be used exclusively to drive the fans and pumps of a solar air- or water heating installation, or a space cooling and ventilation system, or an integrated system that performs both heating and cooling functions. These normally need most power when the sun is shining brightest, so PV is the obvious provider. Many pumps, fans and ventilators now come with integrated

PV panels, greatly simplifying installation. Surplus power could also be converted into heat and passed into a thermal store.

Pool Pump Air Conditioner Fence Cover
Solar PV powered ventilation is now readily available (Reproduced with permission from Monodraught)

Offices, schools, hospitals and the like can utilise PV electricity as a substitute for, or supplement to, power from the national grid. Converting the low voltage direct current output from a PV cell to mains voltage alternating current requires a rectifier, which at best will be 90% efficient. In many modern buildings much of the electrical energy consumed will be used to power desktop computers - which will convert the mains voltage AC into low voltage DC via internal transformers, which have a typical efficiency of around 40%. So a significant percentage of the electricity generated by the PV cells will be lost as heat: useful in the winter, a nuisance in the warmer months. It would make sense for buildings of the future to have separate low voltage DC circuits powered by PV that would be used directly by computers, low voltage lighting and the like to maximise efficiency.

For all types of PV cell, output is inversely proportional to cell temperature, so keeping PV arrays cool should be a priority. Air is the usual medium; natural ventilation is usually adequate, even in the sunniest locations, although ventilation fans powered by the array are a realistic alternative. In winter the heated air can be a valuable asset: it can be used to back up a space heating system or passed through an air/water heat exchanger to boost domestic hot water supplies. Even in summer the solar energy contained in the cooling air can be tapped for space cooling purposes (see Chapter 13), or captured in a seasonal thermal store (see Chapter 15). Large-scale HCPV installations may be more effective with water cooling - concentrating sunlight by up to 500 times produces very high temperatures at the cells, and although air cooling can work well with smaller HCPV arrays, water cooling is usually the best answer for large installations.

Many recent buildings combine both solar air/water heating collectors and PV cells on their roofs or façades (see Chapter 16). Rain screen cladding and curtain walling systems are available with integrated PV - these are sometimes dubbed 'active cladding', and can actually be cheaper than some forms of conventional cladding. Glazing with semitransparent silicon-wafer cells laminated into it is on the market. Cell spacing can be tuned to provide the optimum balance between daylight transmission and electricity generation. ' Transparent' thin film cells have been produced and are being offered by some manufacturers, although the contact grids are still visible. High-performance roofing membranes are

Translucent Cell
'Translucent' solar PV glazing continues to improve (above and below) (Reproduced with permission from Solarcentury.com)
Translucent Cell
(Reproduced with permission from Centre for Advanced Technology)

available with flexible thin film amorphous silicon cells laminated in. Solar tiles and slates are another possibility, as are PV arrays integrated into shading louvres. Options like these should go a long way to dispelling any lingering architectural objections to PV arrays.

A typical small office development of 1,000 m2 floor area will consume up to 200,000 kWh per annum. Assuming that one square metre of monocrystalline silicon PV array can yield 250 kWh annually, meeting say 10% of the building's needs through PV will require at least an 80 m2 installation. If cheaper technology is preferred, perhaps in the form of a roofing membrane, then two or three times this area might be needed.

Output from PV arrays will decline with time, but how fast, and how long the array will continue to function is still the subject of heated debate. Silicon-based cells can be contaminated by iron or oxygen from the environment; the materials used for encapsulation can be attacked by heat, moisture and UV radiation. The oldest Earth-based solar modules have been in operation for more than three decades without obvious signs of widespread distress, so confidence in the long-term performance of PV cells is still high.

Solar PV is too often seen as an architecturally acceptable symbol of commitment to a green agenda. A small but visible PV array is a quieter alternative to a token rooftop wind turbine, one with fewer structural implications. Size itself is no guarantee that the PV array is not just another greenwash. What matters is the way the output from the array is utilised. If it is genuinely reducing the building's demand for energy from fossil fuels and/or providing an emergency back-up, solar PV is a valid option.

Solarcentury
Manchester's CIS tower shows what PV can do in the right context (Reproduced with permission from Solarcentury.com)

This page intentionally left blank

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook


Post a comment