Photovoltaics

he U.S. Department of Energy works to provide clean, reliable, affordable solar electricity for the nation through its research programs in photovoltaic (PV) energy systems. The following pages explain the "how's" and "why's" of PV. Whether you are a student, builder, consumer, engineer, or researcher, there is something here for you.

Photovoltaic technology makes use of the abundant energy in the sun, and it has little impact on our environment. Photovoltaics can be used in a wide range of products, from small consumer items to large commercial solar electric systems.

Our goal is to ensure that photovoltaic energy systems make an important contribution to the energy needs of our nation and the world. In these pages, you will learn about DOE's R&D in photovoltaic energy systems.

Advancement in Solar Energy Technology

The atmosphere, oceans and land mass of the Earth absorbs enough energy from the sun in one hour to power the entire planet for one year. Surely we are clever enough to capture some of this magnificent force and use it to fuel our environment.
Solar energy and its use can be divided into two areas. Those are static or passive solar energy collection and dynamic, or perhaps better termed, kinetic solar energy collection and use.
An example of passive solar energy collection would be building a house so that the windows face the morning sun in colder climates. An even more rudimentary example would be that of an alligator sunning himself on the edge of the water. In both cases the sun’s energy is simply absorbed for warmth. And the simplest use of solar energy is as the very daylight we walk about in. Our Earth automatically uses the power of the sun in millions of ways. Not the least of which is photosynthesis by plants for production of oxygen for our atmosphere. Ours is an inherently rechargeable renewable world, provided we use our resources such as solar energy wisely.
To that end, we must examine dynamic solar energy collection for the production of warmth and light.
When you walk though almost any shopping mall built in the last twenty years you will probably notice a flood of bright natural light all around you. Most large malls and department stores are built with double paned insulated windows that allow light to enter yet keep heating or cooling locked inside. But what happens when the sun follows its arc away from those windows? Active solar lighting can use mirrors that track with the sun’s movement and then reflect light into fiber optic cable that can carry that light into any part of our same department store.

We can create transfer warmth through various forms of solar thermal energy. Since the 1950s it has not been uncommon to see simple glass paned boxes filled with copper pipes used to help heat water for swimming pools and boilers. These low temperature collectors are fine for space heating but there are far more effective ways to heat water with the sun’s rays and put that water to work.
High temperature parabolic shaped mirrors can heat water to far greater temperatures than made possible by our simple rooftop hot boxes. In fact bowl and trough type mirrors can boil water to steam which in turn uses a turbine to generate electricity for heating, air conditioning and general power supply. When properly applied, this concentrated solar power can supply 50% of the power needs for a modern factory. Concentrated Solar Power is one half of our method for creating electricity from the sun’s radiant energy.

he most commonly thought of use and form of solar energy conversion is that of relying upon solar voltaic cells. These solar cells are also called photovoltaic. First developed in the 1880s, photovoltaic cells rely upon the electronic reaction of certain key elements to the Sun’s rays so as to produce a tapable flow of electrons that are in turned used to create current flow. In short photovoltaic cells turn sunlight into energy. And lest we think we are so clever for figuring out how to do this, consider that plants have been turning sunlight into energy for millions of years.
Advances in the development of photovoltaic cells have increased drastically since the oil shortages of the 1970s. This is primarily due to development of silicon technologies. Crystalline silicon cells when working in conjunction with CSP (concentrated solar power) as supplied by parabolic mirrors have improved output from Photovoltaic cells by a factor of 50 since their more basic development in 1954. Increases in demand and subsequent increases in production have lowered the price of solar cells to the point that they are now almost competitive with wind power technology and like their low emissions wind counterparts are far less costly than nuclear power.
Development, deployment and economics

Solar Electric power as supplied by huge banks of photovoltaic cells is providing billions of watts of power throughout the world.

Thin film solar cells

The high cost of crystalline silicon wafers (they make up 40-50% of the cost of a finished module) has led the industry to look at cheaper materials to make solar cells.

The selected materials are all strong light absorbers and only need to be about 1micron thick, so materials costs are significantly reduced. The most common materials are amorphous silicon (a-Si, still silicon, but in a different form), or the polycrystalline materials: cadmium telluride (CdTe) and copper indium (gallium) diselenide (CIS or CIGS).

Each of these three is amenable to large area deposition (on to substrates of about 1 meter dimensions) and hence high volume manufacturing. The thin film semiconductor layers are deposited on to either coated glass or stainless steel sheet.

The semiconductor junctions are formed in different ways, either as a p-i-n device in amorphous silicon, or as a hetero-junction (e.g. with a thin cadmium sulphide layer) for CdTe and CIS. A transparent conducting oxide layer (such as tin oxide) forms the front electrical contact of the cell, and a metal layer forms the rear contact.

Thin film technologies are all complex. They have taken at least twenty years, supported in some cases by major corporations, to get from the stage of promising research (about 8% efficiency at 1cm2 scale) to the first manufacturing plants producing early product.

Amorphous silicon is the most well developed of the thin film technologies. In its simplest form, the cell structure has a single sequence of p-i-n layers. Such cells suffer from significant degradation in their power output (in the range 15-35%) when exposed to the sun.

The mechanism of degradation is called the Staebler-Wronski Effect, after its discoverers. Better stability requires the use of a thinner layers in order to increase the electric field strength across the material. However, this reduces light absorption and hence cell efficiency.

This has led the industry to develop tandem and even triple layer devices that contain p-i-n cells stacked one on top of the other. In the cell at the base of the structure, the a-Si is sometimes alloyed with germanium to reduce its band gap and further improve light absorption. All this added complexity has a downside though; the processes are more complex and process yields are likely to be lower.

In order to build up a practically useful voltage from thin film cells, their manufacture usually includes a laser scribing sequence that enables the front and back of adjacent cells to be directly interconnected in series, with no need for further solder connection between cells.

As before, thin film cells are laminated to produce a weather resistant and environmentally robust module. Although they are less efficient (production modules range from 5 to 8%), thin films are potentially cheaper than c-Si because of their lower materials costs and larger substrate size.

However, some thin film materials have shown degradation of performance over time and stabilized efficiencies can be 15-35% lower than initial values. Many thin film technologies have demonstrated best cell efficiencies at research scale above 13%, and best prototype module efficiencies above 10%. The technology that is most successful in achieving low manufacturing costs in the long run is likely to be the one that can deliver the highest stable efficiencies (probably at least 10%) with the highest process yields.

Amorphous silicon is the most well-developed thin film technology to-date and has an interesting avenue of further development through the use of "microcrystalline" silicon which seeks to combine the stable high efficiencies of crystalline Si technology with the simpler and cheaper large area deposition technology of amorphous silicon.

However, conventional c-Si manufacturing technology has continued its steady improvement year by year and its production costs are still falling too.

The emerging thin film technologies are starting to make significant in-roads in to grid connect markets, particularly in Germany, but crystalline technologies still dominate the market. Thin films have long held a niche position in low power (<50W) and consumer electronics applications, and may offer particular design options for building integrated applications.

Opportunity Keeps On Driving To Endeavour Crater

Opportunity continues to make good progress toward Endeavour crater as solar energy levels improve.

On Sol 2281 (June 24, 2010), the rover completed over 70 meters (230 feet), driving east/southeast.

On Sol 2283 (June 26, 2010), the rover headed 57 meters (187 feet) to the northeast to avoid some large ripples.

The rover drove again on Sol 2286 (June 29, 2010), covering over 70 meters (230 feet) to the east.

As of Sol 2286 (June 29, 2010), solar array energy production has improved to 354 watt-hours, atmospheric opacity (Tau) was 0.295 and the solar array dust factor is 0.577.

Total odometry is 21,408.21 meters (21.41 kilometers, or 13.30 miles).

Shrinking the cost for solar power

One of the big problems with solar power has been that it costs more than electricity generated by conventional means. But some experts think that, under certain circumstances, the premium for solar power can be erased, without subsidies or dramatic technical breakthroughs.

A sufficiently large solar thermal power plant (also called concentrated solar power, or CSP) could potentially generate electricity at about the same cost as electricity from a conventional gas-burning power plant, experts say.

It's not easy. The plant would also have to come with a large energy storage system, be built next to others and be located close to users. To date, no one has completed a facility that comports to all of these parameters, said Fred Morse, an energy analyst who has studied the issue.

"Solar thermal is available at much more attractive prices than solar photovoltaic. The land mass isn't huge, but it does take a while to build these," said Stephan Dolezalek, a managing partner and co-head of the clean tech practice at venture firm Vantage Point Venture Partners, an investor in Bright Source Energy, which builds solar thermal plants and components.

Both Dolezalek and Jiang Lin, who heads up the China Energy Group at the Lawrence Berkeley National Laboratory, said that solar thermal is likely the most promising technology in the entire alternative-energy field right now.

When asked when solar thermal can hit parity, Lin responded "now."

Thermal by the numbers
Conventionally generated electricity ranges between 5 and 18 cents per kilowatt hour (the amount of money to get a kilowatt of power for an hour) but in most places it's below 10 cents, according to the Energy Information Agency. Solar thermal costs around 15 to 17 cents a kilowatt hour, according to statistics from Schott, a German company that makes solar thermal equipment.

A solar thermal plant would need a facility to store the heat harvested in the day by its sunlight-concentrating mirrors so that the heat could be used to generate electricity at night. "You need the kind of system that can run in the evening," Morse said. At some sites, such as Nevada Solar One, excess heat is stored in molten salt and released at night to run the turbine.

The plant, ideally, should be capable of generating about 300 megawatts of electricity. Those plants can churn out electricity at about 13 cents a kilowatt.

That's still a relatively high price, so utilities would need to group two, three or more 300-megawatt plants together to share operational resources, Morse said. "They could share control rooms or spare parts," he said. That would knock the price closer to 11 cents a kilowatt hour.

"Under 10 cents is sort of the magic line," he said.

Dolezalek puts it another way: the plants need to be around 500 megawatts in size. Most solar thermal plants right now aren't that big. The 22-year-old thermal plant in California's Mojave Desert is 354 megawatts. Utility company Southern California Edison is erecting a 500-megawatt plant scheduled to open in 2009.

By 2014, solar thermal plants located in the Southwest could crank out nearly 3 gigawatts of power, estimated Travis Bradford of the Prometheus Institute for Sustainable Development, a nonprofit based in Cambridge, Mass. That's enough for about 1 million homes.

Costs can then be reduced further by building the plants close to consumers. It costs about $1.5 million per mile for transmission lines, according to statistics from Acciona Solar Power, which owns solar thermal plants. Solar thermal plants work best in arid deserts that get little rainfall. Since some of the fastest-growing cities in the world are located in sun belts, that's less of a problem than it used to be.

But getting to that point isn't easy. Land-use hearings and permits can drag on for years while construction costs rise. The amount of land required can be an issue too: the 354-megawatt plant in California occupies 1,000 acres. Larger plants would need more land, while smaller plants result in higher costs per kilowatt hour.

Even if all of these factors could be completely optimized, solar thermal power plants would likely not produce electricity at a level that would compete with coal plants. Coal plants, however, will likely be hit with carbon taxes in the near future, which will make solar thermal more competitive. Still, at less than 10 cents a kilowatt, solar thermal would be competitive with electricity from gas-powered plants.

Utilities will also likely work hard to lower the costs of solar thermal in the coming decades, Morse added. Utilities are under mandates to increase their renewable energy sources. Citizen groups often complain about wind turbines and the wind doesn't blow at a constant, predictable rate. Several companies are intent on tapping heat from under the surface of the earth to generate power. Geothermal power, however, works best only in certain locations.

"There is an enough flat, unproductive land in the U.S. to power the U.S.," Morse said. "We just don't have the wires to get there. Eisenhower built the national highway system. Some president will build the national grid."

SOLAR ELECTRICITY EXPLAINED

Solar electricity is created by using Photovoltaic (PV) technologyby converting solar energy into solar electricity from sunlight. Photovoltaic systems use sunlight to power ordinary electrical equipment, for example, household appliances, computers and lighting. The photovoltaic (PV) process converts free solar energy - the most abundant energy source on the planet - directly into solar power. Note that this is not the familiar "passive" or Solar electricity thermal technology used for space heating and hot water production.

A PV cell consists of two or more thin layers of semi-conducting material, most commonly silicon. When the silicon is exposed to light, electrical charges are generated and this can be conducted away by metal contacts as direct current (DC). The electrical output from a single cell is small, so multiple cells are connected together and encapsulated (usually behind glass) to form a module (sometimes referred to as a "panel"). The PV module is the principle building block of a PV system and any number of modules can be connected together to give the desired electrical output.

PV equipment has no moving parts and as a result requires minimal maintenance. It generates solar electricity without producing emissions of greenhouse or any other gases, and its operation is virtually silent.

What is PV power used for?

PV systems supply solar electricity to many applications in the UK, ranging from systems supplying power to city buildings (which are also connected to the normal local solar power network) to systems supplying power to garden lights or to remote telecom relay stations.

The main area of interest in the UK today is grid connect PV systems. These systems are connected to the local solar electricity network. This means that during the day, the solar electricity generated by the PV system can either be used immediately (which is normal for systems installed on offices and other commercial buildings), or can be sold to one of the electricity supply companies (which is more common for domestic systems where the occupier may be out during the day). In the evening, when the electrical system is unable to provide the electricity required, power can be bought back from the network. In effect, the grid is acting as a Solar electricity energy storage system, which means the PV system does not need to include battery storage.

Grid connect PV systems are often integrated into buildings. PV technology is ideally suited to use on buildings, providing pollution and noise-free solar power without using extra space. The use of photovoltaics on buildings has grown substantially in the UK over the last few years, with many impressive examples already in operation.

PV systems can be incorporated into buildings in various ways. Sloping rooftops are an ideal site, where modules can simply be mounted using frames. Photovoltaic systems can also be incorporated into the actual building fabric, for example PV roof tiles are now available which can be fitted as would standard tiles. In addition, PV can also be incorporated as building facades, canopies and sky lights amongst many other applications.

Stand-alone photovoltaic systems have been used for many years in the UK to supply solar electricity to applications where grid solar power supplies are unavailable or difficult to connect to. Examples include monitoring stations, radio repeater stations, telephone kiosks and street lighting. There is also a substantial market for PV technology in the leisure industry, with battery chargers for boats and caravans, as well as for powering garden equipment such as solar electricity fountains. These systems normally use batteries to store the solar power, if larger amounts are required they can be combined with another source of power - a biomass generator, a wind turbine or diesel generator to form a hybrid power supply system.

PV technology is also widely used in the developing world. The technology is particularly suited here, where electricity grids are unreliable or non-existent, with remote locations often making PV power supply the most economic option. In addition, many developing countries have high solar radiation levels year round.

Electricity from: Solar Energy

The ultimate source of much of the world's energy is the sun, which provides the earth with light, heat and radiation. While many technologies derive fuel from one form of solar energy or another, there are also technologies that directly transform the sun's energy into electricity.

The sun bathes the earth in a steady, enormous flow of radiant energy that far exceeds what the world requires for electricity fuel.

Since generating electricity directly from sunlight does not deplete any of the earth's natural resources and supplies the earth with energy continuously, solar energy is a renewable source of electricity generation. Solar energy is our earth's primary source of renewable energy.

There are two different approaches to generate electricity from the sun: photovoltaic (PV) and solar-thermal technologies.

* Initially developed for the space program over 30 years ago, PV, like a fuel cell, relies upon chemical reactions to generate electricity. PV cells are small, square shaped semiconductors manufactured in thin film layers from silicon and other conductive materials. When sunlight strikes the PV cell, chemical reactions release electrons, generating electric current. The small current from individual PV cells, which are installed in modules, can power individual homes and businesses or can be plugged into the bulk electricity grid.

* Solar-thermal technologies are, more or less, a traditional electricity generating technology. They use the sun's heat to create steam to drive an electric generator. Parabolic trough systems, like those operating in southern California, use reflectors to concentrate sunlight to heat oil which in turn creates steam to drive a standard turbine.

Two other solar-thermal technologies are nearing commercial status. Parabolic dish systems concentrate sunlight to heat gaseous hydrogen or helium or liquid sodium to create pressurized gas or steam to drive a turbine to generate electricity. Central receiver systems feature mirrors that reflect sunlight on to a large tower filled with fluid that when heated creates steam to drive a turbine.


What are the environmental impacts?

PV systems operate without producing air, water or solid wastes.

When constructed as grid-connected central station systems, they require significant land, which can impact existing ecosystems. Nevertheless, most PV installations come in the form of distributed systems that use little or no land since the panels are installed on buildings.

Manufacturing PV cells involves the generation of some hazardous materials. Nonetheless, appropriate handling of these small quantities of hazardous material reduces risks of exposure to humans and to the environment.

Like PV, solar-thermal technologies generate zero air emissions, though some emissions are created during the manufacture of both technologies. Water use for solar thermal plants is similar to amounts needed for a comparably sized coal or nuclear plants.

The biggest concern with solar technologies may be land use...

...since five acres of land are often needed for each megawatt of capacity. PV can eliminate the land use impacts by integrating the generators into building construction, eliminating the need for dedicating land use to PV generation.