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A Non-Technical Intro to Solar Energy

Sunlight—solar energy—can be used to generate electricity, provide hot water, and to heat, cool, and light buildings. With its many uses and systems, solar energy can be applied to a variety of different systems.

Photovoltaic (PV), or solar cell, systems convert sunlight directly into electricity. A solar or PV cell consists of semiconducting material that absorbs the photons from the sunlight. This solar energy knocks electrons loose from their atoms, allowing the electrons to flow through the material to produce electricity. PV cells are typically combined into modules that hold about 40 cells. About 10 of these modules are mounted in PV arrays. PV arrays can be used to generate electricity for a single building or, in large numbers, for a power plant. A power plant can also use a concentrating solar power system, which uses the sun's heat to generate electricity. The sunlight is collected and focused with mirrors to create a high-intensity heat source. This heat source produces steam or mechanical power to run a generator that creates electricity.

Source: www.nasa.gov

Solar water heating systems for buildings have two main parts—a solar collector and a storage tank. Typically, a flat-plate collector—a thin, flat, rectangular box with a transparent cover—is mounted on the roof, facing the sun. The sun heats an absorber plate in the collector, which, in turn, heats the fluid running through tubes within the collector. To move the heated fluid between the collector and the storage tank, a system either uses a pump or gravity, as water has a tendency to naturally circulate as it is heated. Systems that use fluids other than water in the collector's tubes usually heat the water by passing it through a coil of tubing in the tank.

Many large commercial buildings can use solar collectors to provide more than just hot water. Solar process heating systems can be used to heat these buildings. A solar ventilation system can be used in cold climates to preheat air as it enters a building. In addition, the heat from a solar collector can even be used to provide energy for cooling a building.

A solar collector is not always needed when using sunlight to heat a building. Some buildings can be designed for passive solar heating. These buildings usually have large, south-facing windows. Materials that absorb and store the sun's heat can be built into the sunlit floors and walls. The floors and walls will then heat up during the day and slowly release heat at night—a process called direct gain. Many of the passive solar heating design features also provide daylighting. Daylighting is simply the use of natural sunlight to brighten up a building's interior.

When starting to investigate a solar electric installation, begin by contacting your local utility. There are electrical safety requirements that must be met to ensure the personal safety of utility line service personnel.

Major Components

There are four major components to a stand-alone, grid-independent solar electric system: solar panels, charge controllers, batteries, and inverters. All of these components are necessary to have a functioning Solar Electric (PV) system. There are two major components for a grid-tied system—solar panels and inverters.

The solar panel is the basic building block of the system. This is the DC power generation source for your battery charger or energy source. If you have several solar modules wired together, you have created a solar array. The size of the solar array determines the amount of power or energy that will be produced. Your location is also a factor in the amount of energy produced. If you live in Florida, Southern California, or Texas you will produce more than if you live in Oregon, Maine, or Maryland. In general, your local weather (in other words, the cloudy days), and your location (if you live closer to the equator) dictate the amount of energy your system will produce.

Charge controllers come in many different sizes and types. They all basically do the same thing. The charge controller prevents the solar panel or array from overcharging your battery.

Batteries are the energy storage for your system. Without batteries there is no way to store the energy your solar panels produce during the day. Typically, loads receive their power from batteries instead of directly from the output of a solar panel. A solar panel produces a high voltage that will damage electronics if loads are powered directly. A common application for solar panels directly powering a load is water pumping. Instead of storing energy you store water. This way you can pump during the day and have water all night. Batteries will provide you with the energy you need at night.

The last major component is the inverter. The inverter converts the DC energy stored in your batteries and turns it into the AC power you use in your home. Inverters are rated by wattage and the quality of their output. You can use a 50-watt inverter that plugs into your car's 12-volt outlet to power a computer, or you could have a 4,000 to 11,000 watt inverter system that powers your home. These major components can be put together in many different ways. Minor components like wire, disconnects, circuit breakers, and fuses are also needed for a complete system.

Now that you know what the major components are, how are these components used in systems?

Stand-Alone or "Cabin" Systems

Source: www.energy.ca.gov

Solar---Charge Controller---Battery---Inverter---AC Loads
or
Solar---Charge Controller---Battery---DC Loads

A stand-alone solar system is just as it sounds. It is not connected to the utility or other types of charging sources. This type of system is used when utility power is not present and is too costly to bring in from the nearest pole. If you have a shed set off from the house, a cabin in the mountains, or a summer home by the lake that is without power, this type of system can often be very cost-effective. When compared to bringing in the power lines, the initial cost can be less. However, you have to know your loads and have the system designed correctly, since you do not have utility power for backup.

Utility-Tied System
Solar---Inverter---Utility

This system uses an inverter that does not require batteries. During the day, the power generated is used at the installation site or fed back into the utility. This concept is often referred to as Net Metering. If you are producing more power than you are using, your meter can even spin backwards. Due to the simplicity of the system, it has the lowest cost per watt. The downfall of this system is that when the utility grid fails the system will shut down.

Battery Backup System
Utility---Battery Charger---Batteries—Inverter---AC Loads

This is a system that does not involve solar power, but provides uninterrupted power for critical power applications. This system uses an inverter that has a built-in battery charger. It will charge batteries and hold them at 100%, waiting for a power outage or a brownout. Your critical loads will never see the power outage. Computers, home health equipment, and lights will continue to operate when the utility grid fails. This is a system that is great for areas where power is lost for short periods of time. The limit on this system is the amount of battery capacity that you have. The larger the batteries, the longer your run-time will be.

Utility-Tied Battery Backup System with Solar
This system operates on the same principle as the Battery Backup System. The difference is the addition of solar. The solar is used to charge your battery bank. When the batteries are full, the excess power is fed back into the grid. In the event of an outage, your critical loads are powered by the system, and the solar panels continue to charge the batteries. The benefit of this system is that you have the ability to sell power back and have the peace of mind that your critical loads will continue to operate. The drawback is the cost per watt is higher than a Utility-Tied System.

See the article, "Solar Energy: Frequently Asked Questions" for additional information.