Aug 7 2008
Even though most of the energy of the earth would not be present without the sun, only a few forms of power are considered to be solar power. In the context of renewable energy, solar power is associated with the harnessing of the sun's present emissions of heat or light.
Solar power, besides providing heat and light, also causes the wind that we feel here on Earth. Winds are created when various layers of the atmosphere absorb different amounts of heat and therefore expand differently. This creates regions of lower and higher pressure, resulting in masses of air that circulate both at ground level and at higher altitudes.
Solar power is also responsible for fossil fuels such as petroleum and coal. These substances are the result of large masses of decayed plant matter, which during their lifetime, absorbed solar energy. Fossil fuels are merely concentrated stores of the solar energy that these plants had while alive.
Power from the sun comes to the Earth as heat and light. This heat and light are the effect of the Sun's constant nuclear fusion of hydrogen nuclei. The process of fusion produces helium nuclei along with large amounts of energy. This energy is expressed as electromagnetic radiation (light is a specific frequency range of this radiation) as well as radiated temperatures of more than 6,100 degrees C. This is actually fairly cool compared with the corona and core of the sun that burn at several million degrees C. A small fraction of these extreme levels of energy that are released by the Sun come into contact with the Earth. The average amount of energy that contacts the Earth's surface in a day is 200 W/m2. This means that the average home has more than enough roof space to produce enough electricity to supply all of its power needs. In fact, each day, more energy reaches the Earth from the sun than would be consumed by the global population in 27 years.
Why is Solar Power Renewable?
Solar power is renewable as long as the sun keeps burning the massive amount of hydrogen it has in its core. Even with the sun expending 700 billion tons of hydrogen every second, it is expected to keep burning for another 4.5 billion years. Therefore, technically, solar power is not a completely renewable power source because it will be depleted in 4.5 billion years.
Are There Different Types of Technologies Associated with Solar Power?
There are a variety of types of technologies associated with solar power. These technologies can be divided into two groups. The first group are those that use the sun to generate heat, called solar thermal technologies. Solar thermal technologies include solar concentrator power systems, flat plate solar collectors, and passive solar heating. The other group of solar power technologies directly convert solar radiation into electricity through the photoelectric effect by using photovoltaics (also known as PV).
Solar Thermal Technologies
- Concentrating solar power systems generate electricity with heat. Concentrating solar collectors use mirrors and lenses to concentrate and focus sunlight onto a receiver mounted at the system's focal point. The receiver absorbs and converts the sunlight into heat. This heat is then transported by means of a heated fluid (either water or molten salt) through pipes to a steam generator or engine where it is converted into electricity.
- Flat plate solar collectors are usually large flat boxes with one or more glass covers. Inside the boxes are dark colored metal plates that absorb heat. Air or liquid, such as water, flows through the tubes and is warmed by heat stored in the plates. These systems are particularly useful for providing hot water to households--and 83% of households in Israel were using solar collectors by 1994. As of 1992, over 4.5 million buildings in Japan were using solar hot water systems.
- Passive solar heating design methods use features such as large south-facing windows and building materials that absorb the sun's thermal energy. Passive solar methods can be used to greatly lower heating bills and can even be used to cool a building using natural ventilation. The simplest and perhaps most common of the passive solar technologies is referred to as direct solar gain. A direct gain system includes south-facing windows and a large mass, usually of composed of stone, brick, or concrete, placed within the space to receive the most direct sunlight in cold weather and the least direct sunlight in hot weather. The result is that in cold weather the large thermal mass in the room absorbs solar energy and radiates heat throughout the room. During warmer times, due to its strategic placement away from the windows most concentrated light, the thermal mass absorbs only the warm air already in the room. This leaves the air cooled in warmer seasons and heated in cooler seasons.
Solar thermal technologies come in various sizes. There are small portable solar cookers that utilize a parabolic concentrating disk to cook food and boil water. There are also large centralized solar power plants, known as "power towers", that use many acres of mirrors to collect and focus the sun's power. This focused heat turns water into steam that is used to power a generator. The "Solar One" and "Solar Two" power plants, both with 10 MW capacities, are examples of these large scale solar thermal power plants. The "Solar Two" produces enough power for 10,000 households. Engineers hope to build larger versions in the future with capacities of 30-200 MW.
One beneficial and not immediately apparent application of solar thermal technology is the use of sunlight to cool buildings. Solar thermal energy is used to cool buildings in two ways. The first is by using absorption cooling devices that run on a normal refrigerator cycle by condensing and evaporating a refrigerant fluid. The second method uses dessicant cooling systems, which use a drying agent to absorb water vapor, reduce humidity, and cool the air through evaporation.
The second main method for capturing the sun's energy is through the use of photovoltaics. Photovoltaics (PV) utilize the sun's photons or light to create electricity. PV technologies rely on the photoelectric effect first described by French physicist Edmund Becquerel in 1839.
The photoelectric effect occurs when a beam of UV light, composed of photons (quantized packets of energy), strike one part of a pair of negatively charged metal plates. This causes electrons to be "liberated" from the negatively charged plate. These free electrons are then attracted to the other plate by electrostatic forces. This flowing of electrons is an electrical current. This electron flow can be gathered in the form of direct current (DC). This DC can then be inverted into alternating current (AC), which is the electrical power that is most commonly used in buildings.
Why Aren't There More Solar Panels or Big Solar Plants Being Used Today?
There are actually more solar panels and big solar plants being used today than ever before. The PV industry is experiencing annual growth rates of around 25% with higher growth rates in countries such as Japan, where it is currently growing at 63%. However, solar power has clearly not met its full potential. There are a few reasons for this under-exploitation of solar panels and big solar plants.
The main reason for the lack of mass exploitation of solar power technologies is economic. In order for widespread generation of electricity using solar panels to be feasible it needs to be economically advantageous. In order for solar panels to be an economically viable choice for the production of electricity, production costs must go down and efficiency of the final product must go up. It is difficult to find funding to fuel the projects that are necessary to increase the amount of electricity that can be produced for a certain price, when the current technology is not already adequately efficient.
The absence of mass consumer demand for solar technologies is a hidden factor behind the lack of wide-spread solar power production. If there is a demand for a product, there will be people that will supply that product at a cost that fulfills that demand. If there is enough consumer demand, economic and efficient solar power technologies will be developed and exploited more quickly.
What Happens When the Sun Doesn't Shine?
The amount to which a period of little or no sunlight will effect a home using solar power varies greatly depending on the physical location of a particular home and the nature of the solar system being used. For instance, if the home uses PV and solar thermal, and is also connected to the standard electricity grid, a period of no sunlight will simply mean relying on grid power. On the other hand, homes that are not connected to grid power must either be able to rely on other energy producers, such as a fuel cell, a wind turbine, a diesel generator, or on a supply of electricity stored in batteries. For some solar technologies, such as passive solar applications that utilize a large thermal mass, stored power in batteries or power from a standard utility cannot serve as a backup.
How Do Different Solar Technologies Effect the Environment?
During operation, PV and solar thermal technologies produce no air pollution, little or no noise, and require no transportable fuels. One environmental worry with solar technologies is the lead-acid batteries that are used with some systems. This is a concern especially in developing countries where proper disposal and recycling is not always available. The impact of these lead batteries is lessening however as batteries become more recyclable, batteries of improved quality are produced and better quality solar systems that enhance battery lifetimes are created.
A second environmental concern with solar technologies is the difficulty or recycling heavy metals such as cadmium, which are used in PV cells. Just as there is a large worry about the large amount of discarded personal computers that may pile up and leach cadmium, mercury, and lead into the environment, there is a worry that the cadmium used in discarded PV panels may also be an environmental threat. Since the use of cadmium sulfide in the production of PV panels is on the rise (replacing the more expensive silicon)this is an issue that should be considered.
Pollution Prevented by Using Solar Technologies
Since the environmental impact of solar technologies is relatively small, it is perhaps more beneficial to take a look at the enormous amount of pollution that is prevented due to the use of solar technologies. The US EPA has developed a solar environmental benefits calculator which computes, based on the amount of electricity produced by a PV system and the geographic location of that system, the amount of nitrogen oxide (NOx), sulfur dioxide (SO2), and carbon dioxide (CO2) that is prevented from being emitted each year. A similar calculator is provided by BP Solar.
The amount of emissions that can be prevented through the use of a small PV system is surprising. For example, if in Iowa, a relatively small 500 watt PV system was installed, emissions of 4 lbs. of NOx, 8 lbs. of SO2, and 6,733 lbs. of CO2 would be avoided annually. At the same location, if a modest 66 gallon solar hot water system was installed, an additional 18 lbs. NOx, 37 lbs. SO2, and 8,546 lbs. of CO2 would be avoided annually.
How Much Does Solar Power Cost?
Currently solar power is more expensive than other methods of producing electricity. However, utilities using fossil fuels and nuclear are able to provide a lower price, in part, because of government subsidies and incentives as well as the avoided cost of pollution control, and NOx credits in some places. It is also important to remember that as supplies of fossil fuels continue to be depleted their price will increase. Solar technologies on the other hand will become less expensive as they evolve into more efficient forms. With solar power, along with some batteries for backup, one is also paying for the extra reliability with their increased resistance to the simple line failures of standard utility electricity.
There are different parts of the whole system to consider when looking at price. There is the price per watt of the solar cell, price per watt of the module (whole panel), and the price per watt of the entire system. It is important to remember that all systems are unique in their quality and size, making it difficult to make broad generalizations about price. The average PV cell price was $2.01 per peak watt in 1999 and the average per peak watt cost of a module was $3.62 in the same year. The module price however does not include the design costs, land, support structure, batteries, an inverter, wiring, and lights/appliances. With all of these included, to buy a full system it can cost anywhere from $7 per watt to $20 per watt. So, for example, if you wanted to put in a 10 kilowatt-hour per day system in an area with on average 5 hours of sun a day, you will need a 2 kilowatt system. At $7 a watt it would cost about $14,000. With most average homes drawing from 1 kilowatt to 2 kilowatts, a system of this size would offset a significant portion of load during hours of maximum sunlight, maintenance free for 15-20 years.
What Percentage of Solar Power Currently is Part of the Electricity Mix in the US?
The amount of solar power that is currently part of the electricity mix in the U.S. is quite small. According to the Annual Energy Review of 1999 provided by the EIA, 0.076 quadrillion BTU's of energy were produced by solar power. This is about 0.1% of the overall 72.523 quadrillion BTU's produced in the U.S. This percentage is dwarfed by the 57.673 quadrillion BTU's, or 80% of the total, produced using fossil fuels. Coal alone produced 52% of the electricity produced in the US in 1999. From an environmental perspective, this is troubling, since coal is the most potent emitter of lead and mercury as well as a leading emitter of CO2, NOx, and SO2. The process of mining the coal is itself harmful in a number of ways. 95% of acidic mine drainage is a product of coal mining along with 18.8 million metric tons of methane (CH4) each year. All of this damage is done even before coal is burned!