What Is Solar Energy?

What Are Photovoltaics?

Introduction to Solar Energy

We have always used the energy of the sun as far back as humans have existed on this planet. As far back as 5,000 years ago, people "worshiped" the sun. Ra, the sun-god, was considered the first king of Egypt. In Mesopotamia, the sun-god Shamash was a major deity and was equated with justice. In Greece there were two sun deities, Apollo and Helios. The influence of the sun also appears in other religions - Zoroastrianism, Mithraism, Roman religion, Hinduism, Buddhism, the Druids of England, the Aztecs of Mexico, the Incas of Peru, and many Native American tribes.

We know today, that the sun is simply our nearest star. Without it, life would not exist on our planet. We use the sun's energy every day in many different ways.

When we hang laundry outside to dry in the sun, we are using the sun's heat to do work - drying our clothes.

Plants use the sun's light to make food. Animals eat plants for food. Decaying plants and organisms hundreds of millions of years ago produced the coal, oil and natural gas that we use today. So, a fossil fuel is actually sunlight stored millions and millions of years ago.

Indirectly, the sun or other stars are responsible for ALL our energy. Even nuclear energy comes from a star because the uranium atoms used in nuclear energy were created in the fury of a nova - a star exploding.

Solar Hot Water

In the 1890s solar water heaters were being used all over the United States. They proved to be a big improvement over wood and coal-burning stoves. Artificial gas made from coal was available to heat water, but it cost many times the price we pay for natural gas today. And electricity was even more expensive if you even had any in your town!

Many homes used solar water heaters. In 1897, 30 percent of the homes in Pasadena, Calif., just east of Los Angeles, were equipped with solar water heaters. As mechanical improvements were made, solar systems were used in Arizona, Florida, and many other sunny parts of the United States.

By 1920, tens of thousands of solar water heaters had been sold. By then, however, large deposits of oil and natural gas were discovered in the western United States. As these low cost fuels became available, solar water systems began to be replaced with heaters burning fossil fuels.

Today, solar water heaters are making a comeback. There are more than half a million of them in California alone! They heat water for use inside homes and businesses. They also heat swimming pools.

Panels on the roof of a building contain water pipes. When the sun hits the panels and the pipes, the heat of sunlight warms them.

That warmed water can then be used in a swimming pool.

Solar Thermal Electricity

Solar energy can also be used to make electricity.

Some solar power plants, like the one in the picture to the right in California's Mojave Desert, use a highly curved mirror called a parabolic trough to focus the sunlight on a pipe running down a central point above the curve of the mirror. The mirror focuses the sunlight to strike the pipe, and it gets so hot that it can boil water into steam. That steam can then be used to turn a turbine to make electricity.

In California's Mojave desert, there are huge rows of solar mirrors arranged in what's called "solar thermal power plants" that use this idea to make electricity for more than 350,000 homes. The problem with solar energy is that it works only when the sun is shining. So, on cloudy days and at night, the power plants can't create energy. Some solar plants, are a "hybrid" technology. During the daytime they use the sun. At night and on cloudy days they burn natural gas to boil the water so they can continue to make electricity.

Another form of solar power plants to make electricity is called a Central Tower Power Plant, like the one to the right - the Solar Two Project.

Sunlight is reflected off 1,800 mirrors circling the tall tower. The mirrors are called heliostats and move and turn to face the sun all day long.

The light is reflected back to the top of the tower in the center of the circle where a fluid is turned very hot by the sun's rays. That fluid can be used to boil water to make steam to turn a turbine and a generator.

This experimental power plant is called Solar II. It was re-built in California's desert using newer technologies than when it was first built in the early 1980s. Solar II will use the sunlight to change heat into mechanical energy in the turbine.

The power plant will make enough electricity to power about 10,000 homes. Scientists say larger central tower power plants can make electricity for 100,000 to 200,000 homes.

Solar Cells or Photovoltaic Energy

We can also change the sunlight directly to electricity using solar cells.

Solar cells are small, square-shaped panel semiconductors made from silicon and other conductive materials. They are manufactured in thin film layers. When sunlight strikes a solar cell, chemical reactions release electrons, generating electric current. Solar cells are also called photovoltaic cells - or PV cells for short - and can be found on many small appliances, like calculators, toys and even hats.

Inpidual PV cells are arranged together in a PV module and the modules are grouped together in an array. Some of the arrays are set on special tracking devices to follow sunlight all day long.

The electrical energy from solar cells can then be used directly. It can be used in a home for lights and appliances. It can be used in a business. Solar energy can be stored in batteries to light a roadside billboard at night. Or the energy can be stored in a battery for an emergency roadside cellular telephone when no telephone wires are around.

There are two primary PV markets. Off-grid systems are used where the cost of a PV system is cheaper than stringing electrical power lines long distances from the local utility. Grid-connected PV systems usually cannot compete directly with the cost of utility-produced power. Because of state incentives and federal tax credits, many people are considering grid-connected PV systems. If the PV system provides more power than the home or business uses, additional electricity is fed back into the grid for other people to use. This effectively spins an electricity meter backward in what is known as "net metering."

Incentives offered to homeowners and small businesses is helping develop a more robust PV industry in the United States. Additional, growing demand for PV cells, along with competition, can help drive down the per watt price of PV cells while, at the same time, create new jobs.

Photovoltaics or solar cells can be purchased in two formats: as a stand-alone module that is attached to your roof or on a separate system, or using integrated roofing materials with dual functions - that as a regular roofing shingle and as a solar cell making electricity.

Because they do not produce polluting air emissions or water effluents, solar PV systems are prime candidates for supplying electricity at locations where such environmental impacts are unacceptable. For example, in parks and places where preserving high levels of environmental quality is important.

Does My Home or Building Get Enough Sun?

Except for occasional foggy weather along the central and northern coasts, California has great sunshine almost year round.

The image above shows the relative strength of solar irradiation in the state.

Orientation

How do you determine where your home or building site is a good place for a PV system? It's relatively simple.

The property or roof where the PV system will be installed should have clear, unobstructed access to the sun for most of the day and be free from shade.

The best orientation for a PV system is on a south-facing roof.

If your location looks promising, a PV provider can trace the sun's path for you and determine whether your home or business would benefit from a PV system.

Typically, composition shingle roofs are the easiest to work with.

Shading

Shading is a critical issue for PV performance. The PV array needs to be located where it will have access to the sun and where it will not be in the shadow of other building elements (e.g., facades and parapets, mechanical/ plumbing elements (including vent pipes and flues), and landscaping (when at full growth).

Shading constraints from neighboring properties are common as well, especially given the urban setting for many commercial and multifamily buildings. Thus when considering possible shade problems, look both at your site and at buildings that shade your site.

• Not all shade on a site is a problem - remember that if you will be mounting your PV array on the rooftop of a 5-story building, then the 5-story neighboring building that shades your lot at ground level will not be a shade problem for your system.
• Particularly for nearby property to the South, consider whether the site will be developed with a structure taller than your own.

Full or partial shading of the panels inhibits the production of electricity... "because the solar cell with the lowest illumination level determines the operating current for all of the cells wired in that series." (Energy Design Resources web site) Solar professionals describe this effect of partial shading as similar to kinking a garden hose, where the reduced size of the opening allows only a small amount of water through. Because of this effect, a relatively small amount of shading can create a disproportionate reduction in electricity production. Note: Thin film PV products are less affected by shading and overcast days than are crystalline PV modules.

It may be helpful to perform a shading analysis of your site and building outline to calculate (or to optimize) space with good solar exposure. PV professionals can also help you in this area.

How Much Shade-Free Space Do You Need?

Remember the rough sizing requirements for PV: You'll need about 100 sq. ft. for each kilowatt (kW) of system capacity for crystalline technologies and 175 sq. ft. for each kilowatt of thin film PV products.

For example, a 10 kW system with crystalline modules would require 10 kW x 100 sq. ft. per kW = 1000 sq. ft of unshaded area.

A 30 kW system using thin film product (e.g., building-integrated roofing material) would require 30 kW x 175 sq. ft per kW = 5250 sq. ft. of rooftop or other space.

Consider these estimates to be minimums for the shading analysis.

Source: Government of California, USA