Solar energy

Solar energy


Solar energy is energy from the sun. It supports life on Earth and drives the Earth's weather. Solar energy predominantly arrives in the form of infrared, visible and ultraviolet light, and is either returned back to space or is absorbed. Nearly all of the absorbed energy is converted directly to heat, with a small but important fraction converted to chemical energy, such as in ozone production, photosynthesis or photovoltaic energy production.

Solar energy also broadly describes technologies that utilize sunlight. These technologies are diverse and date back millennia. The Greeks, Native Americans and Chinese warmed their buildings by orienting them toward the sun. In Europe, farmers used elaborate field orientation and thermal mass to increase crop yields during the Little Ice Age. Modern solar technologies continue to harness the sun to provide water heating, daylighting and even flight.

Solar power from a parabolic reflector.


The amount of solar energy available to the Earth in one minute exceeds global energy demand for a year.

Solar power generally describes technologies that convert sunlight into electricity and in some cases thermal or mechanical power. In 1866, the French engineer Auguste Mouchout successfully powered a steam engine with sunlight. This is the first known example of a solar powered mechanical device. Over the next 50 years inventors such as John Ericsson, Charles Tellier and Frank Shuman developed solar powered devices for irrigation, refrigeration and locomotion. The progeny of these early developments are concentrating solar power plants.

The modern age of solar power arrived in 1954 when researchers at Bell Laboratories developed a photovoltaic cell capable of effectively converting light into electricity. This breakthrough marked a fundamental change in how power is generated. Since then solar cells efficiencies have improved from 6% to 15% with experimental cells reaching efficiencies over 40%. Prices on the other hand have fallen from $300 per watt to less than $3 per watt.

The utilization of solar energy and solar power spans from traditional technologies that provide food, heat and light to electricity which is uniquely modern. Solar energy is used in a wide variety of applications, including:
Heat (hot water, building heat, cooking, process heat)
Lighting (daylighting, hybrid lighting, daylight savings time)
Electricity generation (photovoltaics, heat engines)
Transportation (solar car, solar plane, solar boat)
Desalination
Biomass (wood, biofuel)
Clothes drying

Energy from the Sun


Solar power as it is dispersed on the planet and radiated back to space.

The Earth receives 174 petawatts (PW) of solar radiation at the upper atmosphere. While traveling through the atmosphere, 6% of the incoming solar radiation (insolation) is reflected and 16% is absorbed. Average atmospheric conditions (clouds, dust, pollutants) further reduce insolation by 20% through reflection and 3% through absorption. The absorption of solar energy by atmospheric convection (sensible heat transport) and by the evaporation and condensation of water vapor (latent heat transport) drive the winds and the water cycle.

Atmospheric conditions not only reduce the quantity of light reaching the Earth's surface but also affect the qualities of light by altering its spectrum and diffusing approximately 20% of the incoming light.After passing through the Earth's atmosphere approximately half the insolation is in the visible electromagnetic spectrum with the other half mostly in the infrared spectrum, and a small part of ultraviolet radiation.Upon reaching the surface, sunlight is absorbed by the oceans, earth and plants. The energy captured in the oceans drives the thermohaline cycle. As such, solar energy is ultimately responsible for temperature driven ocean currents such as the thermohaline cycle and wind driven currents such as the Gulf Stream. The energy absorbed by the earth in conjunction with that recycled by the Greenhouse effect warms the surface to an average temperature of approximately 14°C.The solar energy captured by plants and other phototrophs is converted to chemical energy via photosynthesis. All the food we eat, wood we build with, and fossil fuels we use are products of photosynthesis.


Annual average insolation at the top of Earth's atmosphere (top) and at the surface (bottom). The black dots represent the land area required to replace the total primary energy supply with electricity from solar cells.

The map on the top shows how solar radiation at the top of the earth's atmosphere varies with latitude. The bottom map shows annual average ground level insolation. For example, in North America the average insolation at ground level over an entire year (including nights and periods of cloudy weather) lies between 125 and 375 W/m² (3 to 9 kWh/m²/day).At present, photovoltaic panels typically convert about 15% of incident sunlight into electricity; therefore, a solar panel in the contiguous United States on average delivers 19 to 56 W/m² or 0.45 - 1.35 kWh/m²/day.

Types of technologies

Many technologies use solar energy. Some classifications of solar technology are active, passive, direct and indirect.
Active solar systems use electrical and mechanical components such as tracking mechanisms, pumps and fans to process sunlight into usable outputs such as heating, lighting or electricity.
Passive solar systems use non-mechanical techniques of controlling, converting and distributing sunlight into usable outputs such as heating, lighting, cooling or ventilation. These techniques include selecting materials with favorable thermal properties, designing spaces that naturally circulate air, and referencing the position of a building to the sun.
Direct solar generally refers to technologies or effects that involve a single conversion of sunlight which results in a usable form of energy.
Indirect solar generally refers to technologies or effects that involves multiple transformations of sunlight which result in a usable form of energy.

Architecture and Urban planning


The Zion National Park Visitor's Center incorporates several aspects of solar design.

Solar architecture controls the use of the sun to provide comfortable temperatures, lighting and air quality. The basic elements of solar architecture are building orientation, proportion, thermal mass and window placement. The solar architecture and design process tailors these elements to the local climate and environment.

The oldest principle of solar architecture is building orientation. The entire building can be positioned and angled to be oriented towards or away from the sun, overshadowing from other structures or natural features can be avoided or used, and the building can be set into the ground using earth sheltering techniques.
As a general rule, a solar building's axis should run lengthwise east to west and the structure should be twice as long as wide.
Windows facing the equator should be equal to 5-7% of the building's floor space.If heating is a concern, window area facing away from the equator should be minimized.
The thermal mass in the building should be sized to smooth out temperature swings.
Spaces can be designed to naturally circulate air. Cooling elements such as a solar chimney can be incorporated to help with ventillation.
Lighting quality and energy use are strongly influenced by window design. In cold climates insulated glazing with low-emissivity coatings can maximize solar gain and reduce heat losses by 30-50%. In hot climates low-emissivity coatings on the outside of window panes can be used to reduce and control solar gain.

The albedo of an object indicates the percentage of light it reflects. Asphalt has an albedo of around 10% while the average albedo of the Earth is 30%.Urban heat islands (UHI) are metropolitan areas with significantly higher temperatures than the surrounding environment. These higher temperatures are the result of urban materials such as concrete and asphalt which 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, California called for the planting of ten million trees, the reroofing of almost 5 million homes and painting one-quarter of the roads. These measures are estimated to reduce urban temperatures by approximately 3°C. The projected costs of such a program are approximately $1 billion. The annual savings from reduced air-conditioning costs are estimated at $170 million with an additional yearly health benefit of $360 million in smog-reduction savings.

Water heating

Solar water heaters, on a rooftop in Jerusalem, Israel

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 thereafter 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 lost in the 1920s. That being said, the recent price spikes and erratic availability of conventional fuels is renewing interest in solar heating technologies.Solar water heating is highly efficient (up to 86%) and is particularly appropriate for low temperature (25-65 °C) applications such as domestic hot water, heating swimming pools and space heating. The oldest and simplest type of solar water heater is a black water tank which is exposed to the sun. These are called batch systems but there are many other configurations. Some configurations are designed to heat water to high temperatures while other are designed for economy. A solar pond is a pool of salt water that collects and stores solar energy. Solar ponds were first proposed by Dr. Rudolph Bloch in 1948 after coming across reports of a lake in Hungary in which the temperature increased with depth. This effect was due to salts in the lake's waters which created a "density gradient" that prevented convection currents. A prototype was constructed in 1958 on the shores of the Dead Sea near Jerusalem.The pond consisted of layers of water that successively increased from a weak salt solution at the top to a high salt solution at the bottom. This solar pond was capable of producing temperatures of 90 °C in its bottom layer and had an estimated solar to electric efficiency of 2%. Current, representatives of this technology include a 150 KW pond in En Boqeq, Israel, and another used for industrial process heat at the University of Texas El Paso.