Solar Power refers to useful energy derived directly from sunlight. When most people consider solar power, they think of photovoltaic (PV) panels that are used to generate electricity. Other ways to harness solar energy also exist, and some are even more practical or efficient than solar PV — particularly solar thermal and passive solar applications.
By virtue of sunlight's wide geographical distribution, solar power is one of the few energy sources that is relatively easy to generate on any scale from utility grade down to personal residence or even for handheld devices such as calculators or cell phones.
Type of Solar Strategies:
Solar photovoltaic (PV) is the only direct sunlight-to-electricity technology available. It is also the most portable and can be deployed at almost any scale. Basically, solar PV modules/panels are composed of cells with special semiconductors in them that release electrons when light hits them. The free electrons are then induced to flow, creating an electrical current. To create a solar PV system, multiple cells are wired together to create modules, and then multiple modules are wired together to create an array with a specified capacity for generating electrical power.
Solar thermal is one of the most efficient and least expensive technologies, but isn't widely deployed. The concept of solar thermal is to convert sunlight directly to heat energy through the use of collector plates. Basic solar thermal techniques are commonly used in solar water heating systems, but more advanced techniques, such as focusing or reflecting sunlight, are increasingly applied to large-scale solar thermal power plants to create steam that drives traditional turbines and generators.
Passive solar uses good design and thermal materials to reduce energy needs for a building without relying on any active mechanical systems. Through appropriate siting, windows, insulation, thermal mass, and other techniques, passive solar design works with the local climate and geography to collect and distribute heat in cool seasons and block heat in warm seasons.
Distributed solar refers to the scale of solar application rather than to a specific technology. Most electricity is generated at large central generating facilities located some distance from consumers. Solar and some other forms of renewable energy, however, can also be generated close to communities, individual homes and businesses, or directly to electric cars, streetlights or devices. This "distributed" generation reduces transmission loss and bottlenecks along power lines and also decreases risk of blackouts. An increasing number of states either require or provide incentives for distributed renewable energy generation.
Photovoltaics (PV) are devices that convert sunlight to usable electricity via the photovoltaic effect, a phenomenon that causes certain materials to release electrons when they absorb light. Once the electrons are attracted by an electric field, they flow in an electric current. The basic component of PV panels (or "modules") is the solar cell.
How PV cells work
PV consists of solar cells encapsulated in a protective panel, usually with a glass face. The cells themselves are made of a semiconductor material typically in a thin, crystal structure some 0.5 to 350 micrometers thick depending on the technology.
The amount of power generated by a solar cell is a function of how much sunlight is absorbed by the solar panel. The sunlight-to-electricity efficiency of the cell, however, is determined by two key factors:
The crystal structure of the semiconductor — perfect crystal conducts electrons better than lower grade crystal or non-crystal.
The semiconductor's "band gap," which refers to the amount of energy required to free an electron from the semiconductor atoms — semiconductors with band gaps that optimally match up with the energy levels of incoming photons have the potential to be more efficient than others.
How a PV system works
A solar panel consists of multiple solar cells wired in a particular fashion. Each cell has a relatively small output, typically less than 0.5 volts, and a certain amount of current, typically 1 to 2 amps. The cells are wired in series until a desired panel voltage is reached, and then multiple strings are wired in parallel until the desired panel output current is reached. This is the power output of the panel in ideal lighting conditions. For example, if each cell output is 0.5 volts at 1.5 amps, then 48 cells may be wired in series, and five strings of 48 series cells are wired in parallel. The result is a panel of 240 cells whose output is 24 volts at 7.5 amps, resulting in a 180-Watt panel.
While a single panel may be used in remote applications to power a light or similar load, for typical power production a group of panels are wired together into an array. As with the wiring for solar cells, the panels are wired in series until a desired voltage is reached, and then multiple strings are wired in parallel until the desired output current, and thus power, is reached. For example, 20 of the panels in the above example (24 volts at 7.5 amps) may be wired in series as a single string, and then two strings can be wired in parallel. The result is an array of 40 panels whose output is 480 volts at 15 amps, resulting in a 7200-watt system — providing the amount of power consumed by one average Arizona home.
The power from an array is then usually passed through an inverter, if necessary, and fed into a building's power panel, into the power grid, or into a backup system of batteries or other storage.
Power inverters are devices that convert direct current (DC) power to alternating current (AC) power. All solar panels produce DC power, yet most appliances, and indeed the entire nation's power grid, runs on AC. Thus, the output of a solar array usually goes through an inverter before it can be used. An exception would be a customized system that supplied DC devices such as LED lights or a bank of batteries.
While in years past inverters were a major source of power loss, modern inverters have greater than 90 percent efficiency, meaning that for every 1000 watts of DC power going in, at least 900 watts of AC power comes out. While this is a relatively minor reduction in power, it explains why discrepancies often exist when discussing the system size or production of an installation.
Each model of inverter has a different "ideal" voltage and amperage input range for the most efficient conversion of DC to AC power. For this and other practical reasons — such as heat dissipation and redundancy in case one fails — multiple small inverters are often used for a large-scale array rather than a single big inverter. While this multi-inverter strategy may not apply for many small home installations, it does for nearly every solar installation at ASU.
Factors Affecting Production
Many factors can degrade optimal power production from solar PV arrays to varying degrees. These include the following, arranged from most significant to least significant. Note that different technologies may vary in their sensitivity to some factors.
Solar Radiation at Site — large effect for all technologies. Generally, the American Southwest is a solar 'hotspot,' but solar radiation in the U.S. degrades fairly quickly moving farther north and east. Solar panels in the Southwest routinely produce more power annually than the same panels in the Northeast.
Mounting and tracking accuracy — large effect for all panels. Panels that aren't facing the sun as much as possible, aren't producing up to their potential. PV panels reach around 99 of capability when within 7-10 degrees of perfect alignment. Concentrators require much greater precision.
Time of day and seasonal angle of the sun — large effect. Early morning and late evening light strikes solar panels obliquely, producing lower output. Similarly, seasonal variation in the angle of the sun can be nearly 50 degrees, while maximum output occurs within a range of 7 to 10 degrees from ideal. This is why dual-axis trackers, which compensate for time-of-year variation, add about 40 percent to the annual production of a system.
Clouds — large effect, depending on their depth. Some technologies are more resilient than others.
Panel Efficiency — large effect. Most solar cell efficiencies currently range from around 5 to 22 percent in commercial modules. Laboratory efficiencies are higher, but they are still limited by theoretical efficiency limits on each technology, which are usually less than 30 percent conversion. Multijunction cells exceed these limits, reaching about 40 percent, while concentrators can further that production by increasing the amount of sunlight reaching the cells.
High Temperatures — large effect in extreme climate. Production losses for extreme temperatures typically range between 0.2 and 0.6 percent per degree C above the ideal temperature for a given panel. Crystalline silicon panels are more sensitive to high heat than thin film panels.
Partial Shade — effect varies widely. The loss from shade — such as caused by overhanging tree limbs or rooftop chimneys — depends on the technology. Crystalline silicon is highly sensitive, often causing an entire panel or array to stop producing when only part of the panel or array is shaded; shade can also create temperature gradients across a panel that leads to cracking. Thin films usually lose production only in the areas shaded and not elsewhere and they are not usually damaged by thermal gradients or hot spots.
Inverter Efficiency — moderate effect. Modern inverters typically have relatively high efficiencies, in excess of 90 percent for DC-to-AC conversion. A system that produces power directly for DC devices, however, will have up to a 10 percent production advantage over systems converting to AC.
Dust — minimal effect. In most areas, rainfall is sufficient to clean PV systems and maintain high output with a loss of less than 5 percent. In dry climates such as Arizona's, however, heavy dust can accumulate in summer, resulting in a 15 to 20 percent observed loss. Solar thermal systems are more sensitive than PV systems to dust and typically must be cleaned more frequently.
Age — minimal effect. Panels usually show a 0.5 percent degradation in output per year, though warranties typically allow for up to 1% per year for 20 to 25 years.
Staebler-Wronski Effect — no effect for most. This phenomenon only affects amorphous silicon (a-si) cells, which can lose 20 percent of their initial output within the first month or two of operation in the sun. New a-si panels are typically rated on the stabilized output after this loss occurs.
At Arizona State University, the majority of research concerning solar is housed within ASU LightWorks. LightWorks is a multidisciplinary effort to leverage ASU's unique strengths, particularly in renewable energy fields including artificial photosynthesis, biofuels, and next-generation photovoltaics. LightWorks will eventually include other light-based projects in fields ranging from health care to computing and beyond. Visit the ASU LightWorks website to find out more information on recent accomplishments and news within ASU's solar research.
Additionally, ASU has a variety of faculty and other professionals who specialize in solar research. Use the ASU research profile portal to find researchers associated with solar at ASU.
Solar thermal systems do not rely on advanced material properties, crystals, or specific photon energies as do solar PV systems. They simply take advantage of the heat from sunlight to warm something. Low and medium temperature systems are used to directly heat swimming pools or hot water for human use. High temperature systems usually concentrate sunlight to create steam for electric power production — or to power a heat engine.
Direct Water Heating
The sun has long been used to provide hot water for personal use and to warm swimming pools. This technology is considered one of the most cost-effective solar options. More recently, solar water heating has been configured in scaled-up versions for commercial purposes and for "district heating" to provide hot water for multiple families or neighborhoods.
Solar water heaters have evolved in many variants through the years, but the operating principle is generally the same. Water is directed through panels that may be mounted on a rooftop or placed in another sunny location. After the sun heats the water, it is directed to an insulated storage tank for later use. Portable versions of solar water heaters use the sun's heat to move the water from panel to tank.
The typical solar water heater today is a flat plate collector that consists of a series of tightly coiled black metal pipes mounted above a matte black backing and covered with a glass top. The black pipes and backing provide high thermal conductivity and good heat transfer to the water. Solar water heaters are commonly paired with fueled water heaters to provide backup when the solar heat isn't sufficient or available.
A variation on the common model, called an evacuated tube solar collector, uses a series of vacuum-sealed tubes that enclose thin water pipes. This system produces a significant efficiency increase for cloudy environments and moderate or hot climates, but comes with a higher cost and requires more frequent maintenance.
Concentrated Solar Power
When solar thermal energy is used to generate electricity, a second technology is needed to convert the heat into a useful energy that powers a generator. Most of these systems work best at a utility scale because they rely on traditional steam turbine and generator technology with heat exchangers. Only dish systems currently have the potential to work at much smaller scales. The main solar thermal technologies for electricity generation include dish, parabolic trough, and power tower.
Dish systems use a reflective device —a parabolic dish of mirrors — to focus light onto a power conversion unit — usually a Stirling engine — that is mounted at the focal point. The focused sunlight raises the temperature of the engine's heating chamber, which then drives pistons that power a generator to produce electricity. To be most efficient, solar dish systems require dual axis tracking to maintain optimum alignment with the sun.
Due to their relatively small size and self-contained architecture, dish systems can be used for distributed generation, though current versions would provide excessive electricity for a typical home. A good example of a utility-scale dish system is located at the Maricopa Solar Project adjacent to the Agua Fria Generating Station in Peoria, Ariz.
Parabolic trough systems — also known as linear concentrator systems — use rectangular mirrors in a parabolic shape that are placed in a long linear configuration facing the sun. The mirrors focus sunlight on a tube filled with a thermal transfer fluid — usually an oil — that flows throughout the array. The focused sunlight heats the fluid, which is then used to vaporize water to power a conventional steam turbine driven generator. Tracking devices keep the mirrors focused on the sun throughout the day.
Because trough systems are fluid-based, they can provide some energy storage to produce electricity after sunset — and advantage over solar PV systems. Unlike PV systems, however, trough systems, can require considerable amounts of water to cool the thermal side of the plant, which can be a problem in desert environments like Arizona. A good example of a trough system is the Saguaro Concentrating Solar Power plant near Tucson, Ariz.
Power tower systems, also known as "solar central receivers," consist of a tall tower surrounded by a large array of sun-tracking mirrors, called heliostats. The heliostats reflect sunlight onto a receiver on top of the tower that contains a thermal transfer fluid. As with trough systems, the sunlight heats the fluid, which then boils water that drives a conventional steam turbine driven generator. Also similar to trough systems, power towers can store heat for producing electricity at night and they typically have high water use unless air cooling is used. Good examples of power towers are the Solar One and Solar Two projects designed by the U.S. Department of Energy in the 1980s and 1990s. These 10 MW generators were successfully demonstrated in the Mojave Desert near Barstow, Calif., and have since been decommissioned.
Solar power technology is a relatively new field, and rapid progress is being made to increase the efficiency of each conversion type. Several different photovoltaic (PV) technologies and substances currently exist and are described below, organized by their different "generations" of PV technology.
First Generation — Crystalline
Crystalline silicon was the first PV technology to be successfully commercialized and is what most people think of when they think of solar panels. The silicon crystal on the semiconductor that converts light to electricity is a layer typically 350 micrometers thick, but it accounts for about 50 percent of the cost of the panels. Although an older solar technology than others, mono and polycrystalline technologies dominate currently installed PV systems.
Two common types of crystalline solar cells:
Monocrystalline Cells contain slices of a single large silicon crystal. This gives them the highest efficiency of any single band-gap solar technology, but it also makes them the most expensive.
Polycrystalline Cells contain slices of a multi-crystal block. The crystal boundaries reduce their efficiency somewhat, so they are typically a bit less efficient than monocrystalline cells, but also less expensive.
Second Generation — Thin Film
Thin film PV technologies are characterized by their very thin active layer, as little as 0.5 microns thick. Compared to traditional crystalline technologies, this is a very small amount of material and amounts to only about 5 percent of the panel cost, thus greatly reducing the overall price tag. Thin film cells generally having a lower efficiency than crystalline, but the low cost of manufacturing and certain physical characteristics of thin films tends to make them more versatile. They are used extensively for applications ranging from hand-held calculators to rooftop solar panels.
Refinements to thin film technology include the following:
Amorphous Silicon is one of the oldest solar PV technologies, consisting simply of silicon without any crystal structure. While it is very cheap and easy to produce, amorphous silicon is also very inefficient. This is the type of material that is found in some hand-held calculators.
Cadmium Telluride (CdTe) is the most commonly used of the thin film technologies. Its conversion efficiency tends to be less than that of crystalline silicon cells, but it has better resistance to heat. This is the technology most pursued to achieve the sought-after "$1 per watt" PVs that could make solar power competitive with other fuels.
Copper Indium Gallium (di)selenide (CIGS) is a more technically complex thin film than CdTe, but with similar heat resistance. Its potential advantages compared to CdTe are that it doesn't contain the toxin cadmium and it has shown potential in the laboratory to be more efficient.
Third Generation — Concentrators, Multijunction and Beyond
Researchers are tackling a variety of third generation PV technologies for two main reasons:
To find ways to surpass the theoretical maximum efficiency of current solar cells, which is only 29 percent
To make better use of PV materials that are expensive and potentially scarce.
Current third generation PV options include the following:
Solar Concentrators use mirrors or lenses to focus sunlight on high efficiency solar cells, such as multijunction cells. The intensely focused light increases the amount of electrons that can be converted to electricity.
Multijunction PV Cells essentially combine two or more solar cells, one overlaid upon the other, with each designed to catch a different bandwidth of light. This technology has the potential to capture more electrons and greatly increase efficiency in converting sunlight to electricity.
Testing and Certifications
Solar panels are designed, tested and certified to be exceptionally resilient, so they can perform well over time while potentially withstanding excessive heat, cold, wind, torrential rain and the impact of hail among other hazards. Today most commercial PV panels are backed by 25 year warranties.