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Solar energy technologies seek to transform the energy from our Sun into forms that humans can use and store. Though humans have been using the Sun’s heat energy to their benefit since antiquity, modern society has increasingly focused on generating electric power using sunlight. In addition to technological advancements, changing attitudes towards renewable energy and the development of a regulatory structure have catalyzed the incorporation of solar power into the world’s energy portfolio.
The beginnings of solar power generation can be traced back to the late 19th century with several fundamental discoveries in physics, such as the photovoltaic effect and, later, the photoelectric effect. The first solar array dates back to 1883. However, because solar power could not compete in power production with other technologies of the time, such as coal, solar was not seriously considered as a large-scale power source until the 1950s when researchers from Bell Labs developed the modern solar cell. Since then, solar power technologies have transitioned from the research laboratory to practical uses on satellites, rooftops, and utility-scale solar farms, ultimately becoming the mainstream power source it is today.
The Sun as a Resource
By combining hydrogen and helium atoms through nuclear fusion, our Sun is able to produce light at a variety of different wavelengths. However, power emission is not evenly distributed over the whole spectrum. Instead, the Sun’s power distribution follows that of blackbody radiation (as shown in Figure 1), concentrating most of its power emission over the infrared range, which is invisible to our eyes.
Figure 1. Solar radiation spectrum on Earth. Source: Robert A. Rohde via Wikimedia Commons, CC BY-SA 3.0
Back-of-the-envelope calculations estimate that over less than two hours, the energy of sunlight striking the Earth’s surface surpasses yearly worldwide energy consumption. However, more detailed analyses of solar energy availability reveal that solar irradiance is not evenly distributed geographically. Factors such as elevation, local weather patterns, and atmospheric variations can significantly impact how much solar irradiance reaches a given location. By studying these effects and calculating their impact, solar resource maps can be generated that allow for strategic solar energy resource planning.
Photovoltaic Solar Power
The solar panels commonly seen lining the roofs of commercial or residential buildings utilize the photovoltaic effect to generate electric power from sunlight. By absorbing photons (the elementary particle of light), photovoltaic (PV) materials allow for electrons (the elementary particle of charge) to freely move and be extracted. This flow of electrons creates a current and is how electric power is generated.
Many factors impact the number of electrons that can be generated and successfully extracted, which are expressed in the power conversion efficiency value of solar cells. This figure represents how much of the solar irradiance over the cell is ultimately converted into electric power and strongly depends on the PV material being used. Currently, the theoretical maximum power conversion efficiency for a single solar cell is 33.7%, meaning that about one-third of solar irradiance can be converted into electric power in the most ideal situations.
Two main types of PV solar power generation technologies exist: flat panel and concentrating. While both types use the photovoltaic effect to generate electric power, flat panel technologies operate under the direct illumination of the Sun, whereas concentrating technologies use lenses or mirrors to focus sunlight onto solar cells. By focusing sunlight, solar cells in concentrating technologies are exposed to a higher solar power density (power per area) than cells in flat panel technologies. This allows concentrating technologies to use smaller solar cells for similar energy output as flat panel technologies, though they are generally built using more expensive high-efficiency PV materials.
Concentrating technologies are highly dependent on strong direct illumination, making them appropriate for use only in certain geographical areas (see Figure 2). Additionally, concentrating technologies require significant investment to cover the costs of the large mirror or lens arrays used to focus light. Concentrating technologies are generally only used by utility companies, while flat panel technologies are widely seen at the consumer, commercial, and utility scales. The vast majority of new PV installations at all scales are flat panel applications.
Figure 2. Annual average direct solar illumination, 1998-2009. Source: National Renewable Energy Laboratory
The development of silicon-based solar cells, used in both flat panel and concentrated PV, has benefitted from extensive research into the properties of silicon and silicon processing done by the integrated circuit industry. Currently, silicon is found within solar panels in both its polycrystalline and monocrystalline forms (poly-Si and mono-Si, respectively). This has allowed silicon to compete in price by using comparatively cheaper poly-Si, and in performance by using the more efficient (albeit more expensive) mono-Si. Ultimately, early research and market presence have allowed silicon to dominate the PV materials market, giving it around 90% of the market share.
Despite silicon’s dominance, other promising new PV technologies have emerged recently. For specialized applications like concentrating or space satellite PV power generation, solar cells made of alloyed elements from group III and V of the periodic table have allowed for higher power conversion efficiencies to be attained. In these applications, solar panels on satellites, the need for high power conversion efficiencies may necessitate the increased costs of working with these materials.
For broader applications, “thin film” PV technologies can replace silicon with materials that are hundreds of times thinner. Because silicon cells require significant energy to produce, thin film cells are more economical not only in reduced resource usage due their thin profile, but also in their simpler processing. Although these cells tend to have lower efficiencies, their lower costs make the “Energy Payback Time” (EPBT) for thin film PV systems shorter than that of silicon systems.
The “cutting-edge” of solar PV technologies include materials with unique properties that have not yet been fully commercialized. For example, perovskite solar cells could someday enable the creation of solar cells from simple liquid solutions, significantly lowering processing costs. Similarly, organic solar cells are composed of organic light absorbers, could be used to create flexible solar cells.
Solar tracking technologies may be used to align mirrors, lenses, or panels with the current position of the sun, and is seen in most concentrating and high-end flat panel PV systems. In concentrating PV systems, tracking is necessary to make sure direct sunlight is being properly focused on the solar cell being used. Alternatively, tracking in flat panel systems is used to make sure sunlight reaches the solar cell at 90 degrees relative to the surface of the panel, allowing for the highest possible absorption of light.
Concentrating Solar Power
Though it shares some similarities with concentrating PV power generation, concentrating solar power (CSP) uses a radically different mechanism to generate electric power. Instead of using mirrors to focus sunlight on solar cells, CSP uses them to focus sunlight onto elements that are used as heat sources to power steam turbines. Generally, these heat sources take the form of a liquid that is heated by concentrated sunlight and can boil water to generate steam. Power generation then results from the pressurized steam rotating a turbine, ultimately generating electricity like many other power sources. Common heat transfer liquids include synthetic oils, molten salts, and liquid metals.
The power conversion efficiency of CSP technologies depends on both the receiver system’s efficiency in converting solar power into heat, and the efficiency of the generator turbine. Because these efficiencies have inverse relationships with temperature (specifically that receiver efficiency decreases with temperature while turbine efficiency increases), the ultimate power conversion efficiency of the system depends directly on the temperature of the heat source. This relationship (shown in Figure 3) presents additional limits on maximum power conversion efficiency based on the concentration ratio, a geometry-dependent design metric.
Figure 3. Solar-to-work efficiency curves per concentration ratio. Source: MarcChambon via Wikimedia Commons, CC BY-SA 3.0
Current CSP technologies include trough, tower, and dish designs. Troughs use curved mirrors to focus sunlight on a receiver placed within the mirrors’ focal point. This receiver usually takes the form of a tube containing the liquid heat source which is pumped through the CSP facility. In power tower designs, flat mirrors surround a receiver tower through which the heat source liquid is pumped. These flat mirrors are then aligned to focus sunlight and heat the receiver within the tower. Finally, dish designs utilize arrays of mirrors to focus sunlight on a specific spot on the dish assembly which contains the heat source.
The use of liquid heat sources provides some CSP systems with the possibility of heat energy storage. Unlike standalone PV systems, which offer no option to store excess generated power, some CSP systems can reroute their heat source to heat storage units. By doing so, these CSP systems allow for solar-generated power to be distributed to the grid even when its primary source of energy (the Sun) is unavailable, such as at night or in cloudy conditions.
Solar Thermal Energy
Where CSP can be thought of as high-temperature collection of solar thermal energy, other technologies have been developed for temperatures under 600 degrees C. These technologies do not seek to convert solar power into electric power, but to instead replace some electricity usage with thermal energy (heat) harvested from sunlight.
By installing solar thermal collectors, buildings can be provided with amenities like hot water or room heating. Different collector designs make their installation more appropriate to given applications; however, these applications universally share the use of direct sunlight without conversion to electricity and do not require any concentrating mirrors or lenses. Ultimately, because of its adaptability and low cost, low-temperature solar thermal energy is geared towards serving individuals and communities.
Solar Power Economics
All of these different options in PV materials, system types, and scales lead to a variety of different cost scenarios. The Levelized Cost of Energy is a calculation that provides a standardized dollar value per unit of power generated that can be used to compare the costs of different power generating technologies. Because the calculated costs are provided in a range of extremes, this measure also allows for differentiation between power technologies to be achieved, something valuable given the variety of solar PV archetypes that exist. However, despite other factors playing a minor role, this metric shows that scale (the size of an installation) remains the most important consideration when investing in solar power. This means that although PV systems can be installed at residential, commercial, industrial, or utility levels, having larger economies of scale makes the investment a much cheaper option in the long run.
The Levelized Cost of Energy of CSP power generation shows a technology with promise given that its $/MWh figure matching that of nuclear energy. However, they are still more expensive than PV systems. CSP plants are multi-million (or multi-billion) investments that require significant backing from utility companies, private enterprises, and/or national governments.
Solar Power Policy
Like most renewable energy sources, the development of solar power technologies is heavily influenced by observed changes to the Earth’s climate due to fossil fuel energy production. In an effort to curb greenhouse gas emissions, the renewable energy movement has embraced solar power as a way of generating electricity without significant harmful emissions. Additionally, the modular nature of solar power can facilitate decentralized power distribution, meaning that solar power can serve “off the grid” communities and make consumers less beholden to utilities.
Solar power’s first steps into the mainstream were based on reactive economic measures stemming from the 1973 oil crisis. The Public Utility Regulatory Policies Act of 1978 (PURPA) was enacted to decrease the dependence of the United States on foreign energy resources. The most pertinent points related to solar power in PURPA were the establishment of a market for “non-utility” power producers and a push to incorporate renewable energy resources into utility companies’ energy portfolios.
Although PURPA set a framework for renewable energy policy in the US, more explicit policies have since been enacted to drive utilities and other power purchasers toward greater use of renewables in their generation portfolios. For example, renewable portfolio standards (RPS) set percentage targets for the share of electricity that must come from renewable resources in a certain jurisdiction by a certain date. So far, no RPS policies have been enacted at the federal level, but a majority of states have developed their own RPS via legislation.
To regulate the relationship between utilities and non-utility power producers such as solar power producers, several regulatory frameworks have been developed, including net metering, feed-in tariffs, and power purchase agreements (PPA). Net metering allows power consumers who install solar power generators to sell unused excess power back to utilities. This is done by utilities providing consumers with next-generation power meters that can track the amount of electricity consumed and going back out into the grid. The specifics of net metering vary from state to state, and in some instances state net metering policies only allow customers to “sell” back to the grid in exchange for credits against future electricity purchases. This effectively prevents such consumers from being long-term net power exporters and limits the economically beneficial size of consumer-owned solar facilities.
Feed-in tariffs build on net metering by establishing a monetary price for the power sold back to utilities. In the US, these tariffs are either imposed by state governments or are the result of a voluntary agreement between the utility company and the power consumer-producer. PPA setups involve consumers working with developers to host solar power-generating equipment on their property over a set period of time. Any power generated by the solar installation is then sold by the developer directly to the consumer, usually at a lower price than from their utility company. PPAs can also be done directly with the utility, as is the case with most large, ground-mounted solar installations.
To further incentivize the adoption of solar power technologies, different federal and state subsidies have been implemented over the years to lower installation and capital costs for consumer-producers and utilities. These incentives, which generally take the form of either cash discounts or tax rebates are enacted to drive demand for solar technologies by reducing capital costs. For example, the Residential Renewable Energy Tax Credit (also known as the Investment Tax Credit) provides a 30% discount off solar power systems installed by the end of 2019, with an annually declining discount value between 2019 and the credit’s expiration at the end of 2021.
Duke SciPol, "Science Module: Solar Power" available at http://scipol.duke.edu/content/science-module-solar-power (03/21/2018).