Solar panels convert sunlight directly into electricity.
As a means of offsetting electricity from coal fired or other power stations, they are not very cost effective, being expensive to build and install, and having a lot of embodied energy, due to production processes.
Innovations, such as the development of titanium oxide films for PV cells, and installation systems that avoid aluminium frames, may reduce both cost and embodied energy.
Installing panels in large sheets clusters on factory roofs may be a cheaper way of rolling solar panels out across cities initially, than in small lots on house roofs.
What are Solar Panels?
Solar Panels. Alternative Technology Association
Solar (photovoltaic) panels convert energy in the form of light from the sun into electrical energy. Between 4 and 22 percent of the energy falling on a panel is actually converted to usable electrical energy. The rest is reflected or turned into heat. These panels should not be confused with those used for solar water heating, which simply use the sun's energy to heat water directly.
How do solar panels work?
How Solar Panels Work Diagram. City of West Torrens
How much electricity do solar panels generate per square metre?
Renewable Energy Comes Of Age. Christopher Darker. Sixth International Permaculture Conference & Convergence. 1996
For the production of electricity via the 'photo-voltaic' effect, we utilize the ultra-violet and visible light waves with panels realizing a conversion efficiency of between 10 to 15 percent. In other words, between 100 to 150 watts of electricity can be generated per square metre area.
How much do solar panels cost?
Cost Dulls Appeal of Solar Panels: Experts. Tim Dick. Sydney Morning Herald. 2004.6.16
Solar power panels are a central part of the Federal Government's energy white paper, but they are expensive. The NSW Sustainable Energy Development Authority says solar panels cost about $20,000 for a 1 kilowatt system. As Archicentre's David Lawrence, there are plenty of other ways houses could be made more energy-efficient, such as double-glazing windows and shutting off rooms to reduce heating requirements.
Fast Solar Energy Facts. Solarbuzz. 2004.6
A residential solar energy system typically costs about [US] $8-10 per Watt. Where government incentive programs exist, together with lower prices secured through volume purchases, installed costs as low as $3-4 watt - or some 10-12 cents per kilowatt hour can be achieved [around 30,000 hours at full capacity]. Without incentive programs, solar energy costs (in an average sunny climate) range between 22-40 cents/kWh for very large PV systems.
An average crystalline silicon cell solar module has an efficiency of 15%, an average thin film cell solar module has an efficiency of 6%. Thin film manufacturing costs potentially are lower, though.
Approximately 45% of the cost of a silicon cell solar module is driven by the cost of the silicon wafer, a further 35% is driven by the materials required to assemble the solar module.
Solar Energy (photovoltaic) prices have declined on average 4% per annum over the past 15 years. Progressive increase in conversion efficiencies and manufacturing economies of scale are the underlying drivers.
Alternative Energy Critical Technology Review Group S7 (PDF). Siti Hajar A. Bakar, Daniel Francis, Matthew Blyde, Samuel West. University of Newcastle
Solar power’s high initial investment need is a definite drawback. Prices for grid interactive systems can start from as little as $2000 for a pair of solar panels and a small inverter. For a system capable of running an average suburban home would cost around $20,000 to $25,000, and would include a solar array of 1.5 to 2 kilowatts and an inverter. This still makes purchasing entirely from the grid (fossil fuel source) more cost effective.
Thin Film Solar Panels
Solar Film. The Osgood File. American Communications Foundation. 2002.8.28
A newly developed super-thin solar film promises to revolutionize solar energy.
One day soon you may be able to roll a super-thin solar film onto your roof to harness the sun's free energy. The film, being developed by University of Arizona scientists, would roll out just like wallpaper and be a lot easier to handle than traditional, bulky solar panels. And, it would be considerably cleaner to manufacture and dispose of than thin-film solar panels that involve the use of heavy metals, like tellurium and selenium, in their production process.
Scientists believe that thin film panels are the wave of the future. Because
of thin films' ability to be placed on walls, windows and other surfaces,
they're apt to be more widely used than the big, bulky panels typically
associated with solar energy. But the challenge has been in making a thin
film that is as efficient and durable as silicon. And then there's the
matter of pollution – some thin-film materials may be too polluting.
Thin-film technology has been developed using cadmium, telluride and selenide,
but there are environmental concerns about the manufacture and disposal
of these heavy metals. But now there's a new development, which could lead
to clean and efficient thin film solar panels.
Professor Neal Armstrong and a team of chemists and optical scientists at the University of Arizona, Tucson, are trying to develop molecules out of organic compounds – like carbon and hydrogen – that would arrange themselves into a super-thin film about 100 nanometers thick, or one-thousandth the thickness of a human hair. Organic materials currently are used in car stereo displays and for cell phones. These devices emit light using 100-nanometer-thick organic films that carry high-current densities.
It works like this: sunlight releases a stream of electrons that can be absorbed by certain materials. Organic molecules, like the carbon and hydrogen that would make up the solar film, have to be well ordered to receive the stream of electrons and transport them efficiently. On their own, they tend to line up in the wrong direction. Armstrong's team is working on getting them to line up the right way – vertically instead of horizontally – and is using heat to get them to cooperate. Once lined up properly, they can be crystallized into a liquid and then, like paint, rolled out or sprayed on to flexible, transparent plastic, or even onto building materials themselves. Armstrong and his team will be working on this over the next three years and expect it'll be another 10 years or so before consumers have easy access to these inexpensive, toxin-free organic solar panels.
Renewable Energy Comes Of Age. Christopher Darker. Sixth International Permaculture Conference & Convergence. 1996
The two overwhelming advantages of thin film panels is their low embodied energy and the ease and speed at which they can be produced, which is in a roll-to-roll process similar to newsprint.
Within three years, a thin film panel will repay its original 'energy debt' (the amount of energy embodied in its production), and the operational lifespan of the new 'UNI-SOLAR' modules is in excess of 20 years.
Crystal cell solar panels, on the other hand, are far less suited to mass production, as they are made by a tedious, energy-intensive batch process that cancels out the positive environmental aspects of the first seven years of the panel's operation!
Other major advantages of thin film panels are that they can be manufactured without glass, so they are extremely durable and some are flexible; their power output is stable irrespective of temperature variation, and the Uni-solar panels that I have here on display have a parallel cell architecture so they are shadow-tolerant.
Solar Roof. Infolink. 2003.7.22
Solar Roof, by Advanced Energy Systems, is created by bonding an amorphous PV laminate to roofing panels.
Architects and designers are no longer constrained by the traditional solar module which is restricted by unsightly and bulky framing system.
Amorphous silicon modules have a significantly lower embodied energy. Combine this with not requiring an aluminum frame and galvanized steel mounting brackets the embodied energy is even lower.
Suitable for residential grid connected systems to large commercial and remote off grid systems. It can be installed by roofing contractors and can be simply connected by the electrical contractor.
Tolerant of low light conditions like cloudy days due to the triple junction structure which captures light from a wider spectrum of light (blue, green & red). With bypass diodes across each cell, Solar Roof will outperform alternatives in partial shading conditions.
Suited to higher temperatures experienced in the Australian environment.
In real world conditions will produce up to 25% more power per specified watts than alternative modules.
Very durable due to the polymer cover that includes DuPont’s Tefzel film. Can be walked over or pelted by stones. The array will even continue to operate if pierced by a projectile.
Solar Panel Energy Payback Period
How long does it take for solar panels to supply the same amount of energy that went into their production, installation and eventual disposal?
Energy Payback of Roof Mounted Photovoltaic Cells. Colin Bankier and Steve Gale. Energy Bulletin. 2006.6.16
The likely energy payback of a typical domestic sized rooftop grid connected PV cell is approximately four years. Larger utility PV cell power stations would have a much longer energy payback period.
Aluminium frames on modules account for a large amount of the embodied energy [but] are no longer necessary in newer models.
Solar Panels and Embodied Energy - Sun Real Renewable Energy Systems
A commonly asked question about solar panels is "How long does it take to make back the energy used in the manufacture of the panel?" This energy (otherwise known as embodied energy) is produced by a Siemens panel in about 18 months. So that gives another 23.5 years of free electricity before the warranty even runs out! Other brands of solar panels would have similar amounts of embodied energy.
The Energy Intensity of Photovoltaic Systems - Andrew Blakers and Klaus Weber, Centre for Sustainable Energy Systems, Australian National University, October 2000
The use of photovoltaic systems on a large scale in order to reduce fossil fuel consumption and greenhouse gas emissions requires that the energy associated with the construction, operation and decommissioning of PV systems be small compared with energy production during the system lifetime.
At present the energy payback time for PV systems is in the range 8 to 11 years, compared with typical system lifetimes of around 30 years. About 60% of the embodied energy is due to the silicon wafers. As the PV industry reduces production costs and moves to the use of thin film solar cells the energy payback time will decline to about two years.
The total energy requirement to produce a PV panel is 1,060 kWh/m2. In Sydney the useable panel output will be 153 kWh/m2/year [0.42 kWh/m2/day], giving an energy payback time (EPBT) for the panel of 6.9 years. After mounting in an open field or on a roof the EPBT will be 11.5 or 8.3 years respectively. These energy payback times are well short of the likely system lifetime of 30 years.
The most important parameter in the determination of the energy payback time of a PV system is the thickness of the crystalline silicon layer and the method of mounting the panels. Other important parameters are:
- The presence or absence of aluminium components such as panel frames;.
- The efficiency of the solar cell.
- Solar radiation availability.
- The ability to use recycled silicon wafers or aluminium.
- The details of the cell fabrication process.
Fossil fuel use during PV system operation and decommissioning is negligible. Virtually all of the fossil fuel energy and carbon dioxide production associated with PV systems arises from the initial production and installation of the system.
Solar Panel Materials
What are the different kinds of silicon that solar panels are made from?
US Department of Energy
Silicon is the second-most abundant element in the Earth's crust. However, to be useful as a semiconductor material in solar cells, silicon must be refined to a purity of 99.9999%.
In single-crystal silicon, the molecular structure is uniform, because the entire structure is grown from the same crystal. This uniformity is ideal for transferring electrons efficiently through the material. To make an effective PV cell, however, silicon has to be "doped" with other elements to make it n-type and p-type.
Semicrystalline silicon, consists of several smaller crystals or grains, which introduce boundaries. These boundaries impede the flow of electrons and encourage them to recombine with holes to reduce the power output of the solar cell. However, semicrystalline silicon is much less expensive to produce than single-crystalline silicon. Researchers are trying to minimize the effects of grain boundaries.
To create silicon in a single-crystal state, molten silicon is put in contact with a single crystal "seed". The silicon adapts to the pattern of the single-crystal seed as it cools and gradually solidifies.
The resulting single-crystal rods are sliced or sawn to form thin wafers. Such sawing, however, wastes as much as 20% of the valuable silicon as sawdust, known as "kerf." The resulting thin wafers are then doped to produce the necessary electric field. They are then treated with a coating to reduce reflection, and coated with electrical contacts to form functioning PV cells.
With "ribbon" growth, the silicon is cast directly into wafer sheets. This technique does not waste much material, but the quality of the material is not as high.
Multicrystalline silicon devices are generally less efficient than those of single-crystal silicon, but they can be less expensive to produce.
Molten silicon may be directly cast into a mold and allowed to solidify into an ingot. The starting material can be a refined lower-grade silicon, rather that the higher-grade semiconductor grade required for single-crystal material. The cooling rate is one factor that determines the final size of crystals in the ingot and the distribution of impurities. The mold is usually square, producing an ingot that can be cut and sliced into square cells that fit more compactly into a PV module. (Round cells have spaces between them in modules, but square cells fit together better with a minimum of wasted space).
Amorphous silicon does not have the structural uniformity of single- or multicrystalline silicon. Small deviations in this material result in defects such as "dangling bonds," where atoms lack a neighbor to which they can bond. These defects provide places for electrons to recombine with holes, rather than contributing to the electrical circuit. Ordinarily, this kind of material would be unacceptable for electronic devices, because defects limit the flow of current. But amorphous silicon can be deposited so that it contains a small amount of hydrogen, in a process called "hydrogenation." The result is that the hydrogen atoms combine chemically with many of the dangling bonds, essentially removing them and permitting electrons to move through the material.
Amorphous silicon absorbs solar radiation 40 times more efficiently than does single-crystal silicon, so a film only about 1 micrometer—or one one-millionth of a meter—thick can absorb 90% of the usable light energy shining on it. This is one of the chief reasons that amorphous silicon could reduce the cost of photovoltaics. Other economic advantages are that it can be produced at lower temperatures and can be deposited on low-cost substrates such as plastic, glass, and metal. This makes amorphous silicon ideal for building-integrated PV products, and the leading thin-film PV material.
Amorphous silicon is common in solar-powered consumer devices that have low power requirements, such as wristwatches and calculators.
The electrical output of amorphous silicon cells decreases up to 20% over a period of time when first exposed to sunlight, before the material stablises.
US Department of Energy
"Thin-film" cells are deposited in very thin, consecutive layers of atoms, molecules, or ions. Thin-film cells have many advantages over their "thick-film" counterparts. For example, they use much less material—the cell's active area is usually only 1 to 10 micrometers thick, whereas thick films typically are 100 to 300 micrometers thick. Also, thin-film cells can usually be manufactured in a large-area process, which can be an automated, continuous production process. Finally, they can be deposited on flexible substrate materials.
As with other thin films, advantages include the savings of material, monolithic device design, use of inexpensive substrates, and manufacturing processes that are low temperature and possible over large areas.
Gallium arsenide (GaAs) is a compound semiconductor: a mixture of two elements, gallium and arsenic. Gallium is a by-product of the smelting of other metals, notably aluminum and zinc, and it is rarer than gold. Arsenic is not rare, but it is poisonous.
Gallium arsenide and its alloys allow cell designers greater control over the material's structure, to push efficiencies closer and closer to theoretical levels.
But the single-crystal GaAs substrate is expensive. GaAs cells are used primarily in concentrator systems, in which a typical concentrator cell measures only about 0.25 cm2 in area but can produce ample power at high concentrations.