Did you know that in 15 minutes the sun radiates as much energy as mankind consumes in all forms, during an entire year? Did you know that in one day the sun radiates enough energy on the United States to power the entire nation for a year and a half? Not only that, but it does it every day - for free. Solar power is a safe, clean and renewable energy resource that will no doubt play a vital role in powering our future.
How can the energy from the sun be harnessed? And how can we put that energy to practical use? Harnessing the sun’s power is accomplished through the use of a photovoltaic system. Basically, the word “photovoltaic” is used to describe a device, which when exposed to the sun’s radiation, creates electrical power. It’s abbreviated as “PV”. PV systems incorporate the use of PV modules, (which are also commonly referred to as “solar panels”), to generate energy from sunlight and inverters which convert that power and safely deliver it to the utility grid for our use.
When properly designed, a PV system not only helps our environment, but it makes economical sense as well. Owners of PV systems make an important contribution to protecting our environment while enjoying the economic advantages of their “clean” electricity at the same time.
In the following sections you will learn more about how to plan your own PV system and why the inverter is such an important component.
The sun - a Reliable Source of Energy
The United States is well suited for the use of solar power. Most of the nation is exposed to high levels of irradiation every day. Even in parts of the nation confronted with rainy summers and dark winter months, solar power can still be put to use economically.
Depending on the location, the average annual amount of energy PV modules will be exposed to totals between 950 to 2,150 kilowatt hours per square meter (kWh/m2). This is a lot considering that 1,000 kWh/m2 is equal to the energy of about 25 gallons of heating oil. So you can see that the energy potential is already there and PV systems are an excellent way to put it to work for you.
Ample Sun throughout the Nation
The sunlight that your solar system is exposed to is sometimes “direct” or unobstructed by clouds. At other times the sunlight is “diffused”, that is, filtered to some degree either by clouds or the atmosphere in the more northern parts of the country. Solar technology can utilize either form of sunlight. The seasons, elevation and angle of the sun also affect the usable amount of energy. In the northwest for example, the amount of diffused sunlight caused by clouds is relatively high. However, even diffused sunlight can be effectively harnessed to produce electricity by using a well-designed photovoltaic system.
PV Modules – the Cooler the Better
It seems counterintuitive because PV modules are made to be mounted in the sun, but the fact is that they perform better when cooler – in fact, the ideal temperature is right around 25°C.
This means that PV systems up in the clear air and cool temperatures of the mountains will perform better than a system of the same size located in the desert. The amount of direct sunlight at the equator for example, is much higher than in the latitudes in North America; however, the high ambient temperatures heat the modules up and therefore reduce the overall system performance considerably. The power loss is approximately 0.4 % per °C for common PV modules.
So although the sunlight is weaker in North America as compared to South America, the temperatures are cooler making the PV modules more efficient. This compensates for the lower intensity of the sun.
Types of Solar Systems
The sun delivers its energy in two forms: heat and light.
Solar systems can efficiently convert either form into power for practical use.
When many people hear the term “solar system” they think of solar hot water, where the sun is used to heat water for swimming pools or domestic use. This is accomplished by exposing the water to the sun’s heat prior to using it. We also make use of the sun’s heat by orienting windows towards the south (in the northern hemisphere) to take advantage of the sun’s warmth in the winter. This is called “passive” solar. This term is used to describe methods of using the sun’s energy indirectly, such as through bio-mass or heat pumps.
The term solar power system on the other hand, typically describes only those systems that convert sunlight into heat (solar thermal energy) or electricity (photovoltaics). Photovoltaics and solar thermal energy are not in direct competition - quite the contrary: they ideally complement each other and can be combined well. Many specialized companies provide innovative solutions for this purpose.
How Photovoltaics Work
As solar power becomes increasingly popular, more and more solar panels can be seen on the roofs of homes and businesses alike. These solar panels employ one of the most environmentally friendly methods for producing electricity: “photovoltaic”. The term photovoltaic, or PV, is used to describe something that creates electricity when exposed to sunlight. Solar panels, or PV modules, are made up of several solar cells. Each cell is comprised of materials which have photovoltaic properties.
Photovoltaic technology is actually quite simple:
Electricity can be produced by solar cells whose principal component consists of a semiconductor that is typically made of silicon. A semiconductor consists of a material that cannot be classified as an isolator or a conductor and whose electrical properties can be influenced by adding foreign substances (doping). The solar cells comprise two adjoining semiconductor layers that are equipped with separate metal contacts and have each been doped, thus creating an “n” layer (n = negative) with a surplus of electrons and below that, a “p” layer (p = positive) with an electron deficiency. Due to the difference in concentration, the electrons flow from n into the p area, thus creating an electrical field, or “space charge zone”, inside the semiconductor structure.
The Photovoltaic Effect
The upper “n” layer in a solar cell is so thin that the photons from sunlight can penetrate it and can only discharge their energy to an electron once they are in the space charge zone. The electron that is activated in this manner follows the internal electrical field and thus travels outside of the space charge zone and reaches the metal contacts of the “p” layer. When an electrical load is connected, the power circuit is closed: the electrons flow across the electrical load to the solar cell’s rear contact and then back to the space charge zone. This effect is called the “photovoltaic effect” (derived from ‘‘Phos’’, the Greek word for light and the name of the physicist Alessandro Volta). An inverter, the “heart” of the system, converts the direct current (DC) produced by the solar cells into alternating current (AC).
From the Cells to the Module
The sun radiates approximately 1000W per square meter, so a 10 x 10 cm solar cell is exposed to nearly 10 watts of radiated power. Depending on the quality of the cell, it can produce an electrical output of 1 - 1.5 watts. To increase the output, several cells are combined and connected to a PV module. The connection of several PV modules is also referred to as a PV array. You can learn more about photovoltaics using the menu bar on the upper right-hand side.
How Solar Thermal Energy Works
Solar thermal energy is the use of solar energy to produce heat.
This is an effect you’re familiar with if you’ve ever gotten into your car after it has been parked in the sun on a hot summer day. Solar thermal energy works in the same way except that the heat generated is put to practical use to heat water or space heating.
In addition, by using a solar thermal system, you make an effective contribution in preserving our energy reserves and environmental protection by reducing CO2 emissions.
A Simple Principle that Integrates Easily
The solar collectors absorb the sun’s rays, convert them to heat and transfer the heat to a heat-transfer fluid. (The heat-transfer fluid is typically a glycol and water mixture in regions where seasonal freezing in a concern.) The heat-transfer fluid is then pumped into a heat exchanger located inside the water storage tank where it heats the water.
After releasing its heat via the heat exchanger, the heat-transfer fluid flows back to the collectors to be reheated. The controller keeps the heat-transfer fluid circulating whenever there is heat available in the solar collectors. In the winter, a boiler serves as an alternate heat source. Solar thermal systems can be integrated into existing hot water systems with relative ease.
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