Types of technologies
There are many technologies for harnessing solar energy. Applications span through the residential, commercial, industrial, agricultural and transportation sectors. Solar energy can be used to produce food, heat, light and electricity. The flexibility of solar energy is manifest is a wide variety of technologies.
Agriculture and horticulture
Main article: Greenhouse
Photosynthesis is an incredibly important photochemical interaction. Most life on earth depends on the ability of plants to photosynthesize light. While easy to overlook, the efficient use of solar energy is a founding principle of agriculture. This principle takes special priority during times of shortage as seen during the Little Ice Age when Medieval European farmers employed elaborate field orientation and thermal mass techniques to convert as much sunlight into food as possible Agricultural advancement continues optimizing the availability of light by carefully timing planting cycles, tailoring row orientation, staggering heights between rows, varying plant density, and many other methods.
Human communities have used plant material, especially wood, as fuel for many millennia. Today biomass is considered as a form of renewable energy but not as solar energy because the production is indirect. Fossil fuels were also originally created through plants using solar energy. The use of fossil fuels is not considered to be renewable energy.
Greenhouses carefully control the use of solar heat and light to grow specialty crops. Primitive greenhouses were first used during Roman times to grow cucumbers for the Roman emperor Tiberius. In the 16th century the first modern greenhouses were built in Italy to conserve tropical plants that explorers returned with. Greenhouses remain an important part of modern horticulture. The largest greenhouse complex in the world is in Willcox, Arizona where 106 hectares of tomatoes and cucumbers are entirely grown under glass.
Architecture and urban planning
Main articles: Passive solar building design and Urban heat island
Solar design can provide practical lighting, comfortable temperatures, and improved air quality by tailoring building orientation, proportion, window placement, and material components to the local climate and environment. As climate varies by region so too will the features of solar-designed buildings. In the words of the first century Roman architect Vitruvius:
“We must begin by taking note of the countries and climates in which homes are to be built if our designs for them are to be correct. One type of house seems appropriate for Egypt, another for Spain...one still different for Rome, and so on with lands and countries of varying characteristics. This is because one part of the Earth is directly under the sun's course, another is far away from it, while another lies midway between these two....It is obvious that designs for homes ought to conform to diversities of climate.”
The difference in temperature profiles between “urban heat Islands” and less-developed surrounding areas demonstrates the challenge in mitigating such impacts.
Urban heat islands (UHI) are metropolitan areas with higher temperatures than the surrounding environment. These higher temperatures are the result of urban materials such as asphalt and concrete that have lower albedos and higher heat capacities than the natural environment. A straightforward method of counteracting the UHI effect is to paint buildings and roads white and plant trees. A hypothetical "cool communities" program in Los Angeles has projected that urban temperatures could be reduced by approximately 3 °C after planting ten million trees, reroofing five million homes, and painting one-quarter of the roads. The estimated cost of the cool communities program is approximately US$1 billion, with an annual benefit estimated at $170 million resulting from reduced air-conditioning costs alone. An additional $360 million in health costs could be saved annually by the associated reductions in smog.
The history of lighting is dominated by the use of natural light. The Romans recognized the Right to Light as early as the 6th century and English law echoed these judgments with the Prescription Act of 1832. It wasn't until the 20th century that artificial lighting took over as the main source of interior illumination. The 1973 oil and 1979 energy crises brought attention to conservation measures such as natural lighting but interest waned on both occasions with the restoration of energy supplies. Approximately 22 percent (8.6 EJ) of the electricity used in the United States is for lighting. When daylighting techniques are appropriately applied, natural light can supply interior lighting for a significant portion of the day
Daylighting systems collect and distribute sunlight to provide interior illumination. These systems directly offset energy use by replacing artificial lighting and indirectly offset energy use by reducing cooling loads. Although difficult to quantify, the use of natural lighting also offers physiological and psychological benefits compared to artificial lighting. Daylighting design carefully selects window type, size, and orientation and may consider exterior shading devices as well. Individual features include sawtooth roofs, clerestory windows, light shelves, skylights and light tubes. These features may be incorporated into existing structures but are most effective when integrated in a solar design package that accounts for factors such as glare, heat gain, heat loss and time-of-use. Architectural trends increasingly recognize daylighting as a cornerstone of sustainable design.
Hybrid solar lighting (HSL) is an active solar method of using sunlight to provide illumination. Hybrid solar lighting systems collect sunlight using focusing mirrors that track the sun. The collected light is transmitted via optical fibers into a building's interior to supplement conventional lighting. In single-story applications, these systems are able to transmit 50 percent of the direct sunlight received
Daylight saving time (DST) utilizes solar energy by matching available sunlight to the time of the day in which it is most useful. DST shifts electricity use from evening to morning hours thus lowering evening peak loads and the higher costs associated with peaking electricity. In California, winter season DST has been estimated to cut daily peak load by 3 percent and total electricity use by 3400 MWh. DST has been estimated to reduce early spring and late fall peak loads by 1.5 percent and total daily electricity use by 1000-2000 MWh. DST, like other solar energy technologies, has not proven successful in all regions.
Solar thermal applications make up the most widely used category of solar energy technology. These technologies use heat from the sun for water and space heating, ventilation, industrial process heat, cooking, water distillation and disinfection, and many other applications
Solar water heaters face the equator and are angled according to latitude to maximize solar gain.
Main articles: Solar hot water and Solar combisystem
Solar hot water systems use sunlight to heat water. Commercial solar water heaters began appearing in the United States in the 1890s. These systems saw increasing use until the 1920s but were gradually replaced by relatively cheap and more reliable conventional heating fuels. The economic advantage of conventional heating fuels has varied over time resulting in periodic interest in solar hot water; however, solar hot water technologies have yet to show the sustained momentum they had until the 1920s. Recent price spikes, erratic availability of conventional fuels, and other factors are renewing interest in solar heating technologies. Approximately 14 percent (15 EJ) of the total energy used in the United States is for water heating. In many climates, a solar heating system can provide 50 to 75 percent of domestic hot water use.
As of 2005, the total installed capacity of solar hot water systems is 88 GWth and growth is 14 percent per year. China is the world leader in the deployment of solar hot water systems with 80% of the market. Israel is the per capita leader in the use of solar hot water with 90 percent of homes using this technology. In the United States heating swimming pools is the most successful application of solar hot water.
Solar water heating technologies have high efficiencies relative to other solar technologies. Performance will depend upon the site of deployment, but flat-plate and evacuated-tube collectors can be expected to have efficiencies above 60 percent during normal operating conditions. In addition, solar water heating is particularly appropriate for low-temperature (25-70 °C) applications such as swimming pools, domestic hot water, and space heating. The most common types of solar water heaters are batch systems, flat plate collectors and evacuated tube collectors