Passive Solar

Passive solar technology, technically a form of green building, relies on the shape, orientation and location of structures to improve their energy efficiency. Passive solar requires no moving parts, instead using specifically designed architectural elements that encourage natural heating during the winter and cooling during the summer. Implementing passive solar tecniques adds little to nothing to the cost of building a structure but results in reduced operational costs and equipment demand.⁴

Passive solar design integrates various elements, most generally including an aperture, an absorber, thermal mass, and systems that manage distribution and control.⁵   The aperture serves as the solar "collector," using a transparent, south-facing material (usually windows) to allow plentiful sunlight into the building. The absorber is the dark surface of the storage element (i.e. a wall or floor) that soaks up the sun's heat, and the thermal mass is the area below or behind the absorber that stores that heat. Distribution and control systems include strategically-placed ducts and radiant floors.

Photovoltaic                       

A photovoltaic cell, also called a solar cell, is a device that directly converts sunlight. Photovoltaic solar panels are made of either silicon cells or thin films, also called a semiconductor wafer, that is positively charged on one side and negative on the other. Solar cells convert light into electricity using the photovoltaic effect, whereby incoming sunlight knocks loose atoms in the semiconductor, and electrical conductors pick up the electrons to form an electrical circuit.⁶

For solar cells, a thin semiconductor wafer is specially treated to form an electric field, positive on one side and negative on the other. When light energy strikes the solar cell, electrons are knocked loose from the atoms in the semiconductor material. If electrical conductors are attached to the positive and negative sides, forming an electrical circuit, the electrons can be captured in the form of an electric current -- that is, electricity

familiar examples are the small solar cells that are used to power calculators and watches. However, PV works on a larger scale, for instance by combining 40 cells into a single module and 10 modules into a single PV array.  PV arrays can be mounted on a tracking device to face the sun directly for greater efficiency. About 10 to 20 PV arrays can provide enough power for a household. A household solar system can be grid connected or stand alone, depending whether the user is connected to a power grid or has a battery.

Photovoltaic power is not currently economical for power plants; PV power plants are few and small. The top ten (as of July 2009) have a combined capacity approximately half the size of a typical coal-burning power plant.^{7 8} Many of them are in Spain, which has been encouraging solar development with large, but politically volatile, subsidies for solar power plants up to 60 MW. The subsidy is up to .45 Euros per KW-hr; by comparison, typical US retail prices are $.13 per KW-hr .⁹ Newer generations of PV claim to be much cheaper to manufacture. ¹⁰

In the United States, Southern California Edison is working to install more than 500 megawatts of PV panels on California warehouses. This project is expected to come online by 2011, and the company claims this power plant will produce solar power at $3.50 per installed watt (half the national average cost of solar). ¹¹

Solar thermal 

The two most popular types of solar thermal are solar water heaters and solar thermal power plants.  Solar water heaters use storage tanks and solar collectors to collect the solar power to heat the water used in buildings and swimming pools. The most common collector is called a flat plate collector, which usually has a thin, flat, rectangular box on the roof of the building with a transparent cover facing the sun. The plate is painted black to absorb the heat and then is transferred through tubes carrying fluid attached to the box. Other types of solar water heaters include evacuated-tube solar collectors and integral collector-storage systems.

Concentrated Solar Power Plants                   

Solar thermal power plants capture sunlight as heat and use it to run steam turbines (the same technology used by coal and nuclear plants, only with a different heat source.) Solar thermal power plants are most efficient at a large scale, which requires a large area of land; this is a major constraint for solar thermal power plants.  The three most famous solar power plants, SEGS, Solar One and Solar Two, are located in southern California.

The two main technologies are "parabolic trough" and "Solar tower".  Parabolic troughs are long mirrored troughs which focus sunlight on a tube full of liquid, generally heat transfer oil. This oil is pumped back to the main unit where it boils water to operate the steam turbine.  Solar towers work by focusing heat on a large tower full of molten salt;  this salt is used to boil the water that operates the steam turbine. The molten salt technology is also used by nuclear reactors.¹²

Solar towers have an efficiency advantage- the molten salt can reach heats of 650 degrees C, whereas the heat transfer oil breaks down above temperatures of 400 degrees. Higher temperature steam results in more efficient energy generation. However, parabolic trough technology is several years ahead- SEGS,  Nevada Solar One, and Andasol 1 are three currently operating trough plants, while there are no active solar towers operating.

There are other technologies being developed to use concentrated solar power: Stirling Energy Systems has a parabolic dish with a Stirling engine at the focus. Stirling engines are highly efficient, sealed systems which merely require a heat source and a heat sink to function. Stirling engines do not scale up well, so the approach is to have a large number of units in parallel. A typical Stirling engine would  produce 25 KW (roughly 33 horsepower) or less, while most coal power plants are in the 750 MW range (750,000 KW) There are also troughs (the Kimberlina solar plant) which heat water directly to produce steam, instead of heating an intermediate fluid. ¹³

Advantages

• Solar power is a renewable resource that won’t be used up and serves as a constant and consistent power source.
• Solar power is environmentally friendly and creates no pollution. Unlike fossil fuels, solar power emits neither pollutants nor greenhouse gases.
• Photovoltaics are silent and have no moving parts, and operate efficiently at any scale. Placed in a residential area (the roof of a house, for instance) they do their work inconspicuously.
• Solar panels and solar cells usually have a long lifespan and require little maintenance. Even though the purchase of the panels and solar cells is usually expensive, the operation and maintenance is easy and cheap. Therefore, users can save money in the long run.

Disadvantages  

One of the major disadvantages of solar has been its relatively high cost per unit of energy compared to conventional sources of energy.  For example the installation cost of solar panels is between $7 to $9 per watt, so a 5 kW system would cost on the order of $35,000-$45,000 and an 8 kW system would be anywhere from $56,000 to $72,000. Some subsidies exist, from utility companies or governments; sometimes as much as 50% of the cost is subsidized. However, even with these subsidies it is difficult for solar panels to pay for themselves. ¹⁴

Reliability and consistency is another disadvantage of solar power. Solar power generation requires intense direct sunlight; systems produce very little energy on cloudy days and, of course, none at night. The farther from the equator, and the farther from midsummer, the less efficient the solar energy system.

Other disadvantages come from technical barriers. The materials used to create a solar panel are constantly exposed to other things besides photon atoms. The constant bombardment of ultraviolet (UV) rays and other solar radiation often causes the panels themselves to deteriorate.¹⁵ Additionally, the inverters used to convert electricity into a usable form typically have much shorter lives than the solar cells themselves.¹⁶

Finally, solar cells are a poor fit for vehicles, which use a large portion of the fossil fuels burnt in the United States. The amount of power available to a car is limited by the amount of sunlight striking it; one recent solar vehicle generated 1850 watts, or approximately 2.5 horsepower. ¹⁷

Japan

As a forerunner in developing solar power, the solar industry boomed in Japan thanks to the policy support where fossil fuels are not cheap and solar energy is competitive. In 2003, Japan generated half of all the world's solar power, built 44% of all new solar energy equipment, and installed five times as much new solar power capacity as the U.S.¹⁸

As one of the earliest promoters of solar power, the Japanese government provided subsidies to households who adopted solar power between 1994 and fiscal 2005. The subsidy system was scrapped for financial reasons. However, in late 2008, Japan’s Ministry of Economy, Trade and Industry requested 23.8 billion yen (approximately $24 million) in budget outlays for fiscal 2009 to revive subsidies to promote the use of solar power systems at private homes. The subsidies would cover about 10% of purchase costs. ¹⁹

Europe

Besides of a relative high fossil fuel price, which makes solar energy competitive in Europe, a feed-in tariff system stimulates the development of solar power in Europe. Feed-in tariff guarantee a solar generator a guaranteed power sales price, coupled with a purchase obligation by electric utilities.

The renewable energy thrived since a decade ago in Germany, when a new coalition of Germany’s Social Democrats and Green set up a framework to promote solar, wind and other renewable. Utilities are required to buy clean energy at above-market rates. Germany is now the leading producer of solar energy and solar panels. It produces half of the world’s solar power, twice as much as its nearest rival, Japan, and four times third-placed United States. It produced 3.78 gigawatts in 2007.²⁰

The "Feed-in Law" in Germany permits customers to receive preferential tariffs for solar generated electricity depending on the nature and size of the installation. Under the new tariff structure introduced in 2004, the base level of compensation for ground-mounted systems can be up to 45.7 euro cents/kWh. PV installations on buildings receive higher rates of up to 57.4 euro cents/kWh.²¹ However, annual feed-in tariff declines in Germany. As Germany reviews its Renewable Energy Law, political consensus has centered on feed-in tariff declines from 8-10% depending on system size and type. Last year, the German Parliament was revising solar subsidies to cut the feed-in tariff by as much as 10 percent. The change would modify the rates mandated by a law that requires utilities to buy all the solar electricity generated at a fixed price for 20 years. Utilities are paying between 38 and 54.2 euro cents per kilowatt-hour for German solar installations in 2008. A 10 percent drop would equal to between 34.2 euro cents to 48.8 euro cents for 2009 projects.²²

Recently , other countries, such as Spain revised incentive terms for solar power as well. In 2007, the European market grew 87% to 2,157 MW , led by explosive growth in Spain (to 640 MW) and consistent development of the bedrock German market (reaching 1328 MW).²³  

United States

Most of the policies that impact the solar industry in the United States are created at the State level. The most important State market is California. Over 80% of grid connected installations in 2003 took place in California and New Jersey is emerging as the next solar friendly State.²⁴

The United States has been lagging behind in solar power development. In late 2008, Florida City passed the first solar feed-in tariff in the US. Under this program, Gainesville Regional Utilities will buy all of the electricity produced by solar PV systems at a guaranteed rate per kilowatt-hour for the next 20 years.²⁵

China

The Chinese government gives a big push on its solar industry development. According to China’s new Renewable Energy Law, the state will increase production capacity of solar cells to 15 megawatts(MW) each ,with a cumulative capacity of 53 MW.²⁶

The new policy boosts China’s solar industry. Two large solar power plants will be built in the western plateau provinces of Qinghai and Yunnan in 2009. Beijing has announced plans to build a “solar street” where buildings, streetlights, and other features will run entirely on energy from the sun.^{27 28}

Resources

EcoBusiness links : Provides a comprehensive list of solar power producers  

NREL Solar Technologies Program: Supports research and development on photovoltaics and solar thermal energy.