The Schatz Solar Hydrogen Project photovoltaic array.
Solar electric, or photovoltaic (PV), power systems convert sunlight directly into electricity. You see these systems on home and business rooftops, powering traffic signs and billboard lights, and integrated into large commercial building facades. In the developing world they are providing more and more people with electricity for lights and other basic needs. In addition, they provide power in many places that you don't see, places too remote for conventional sources of power. These include telecommunications and remote monitoring and control applications. Today, solar electric power systems are becoming more affordable and accessible to the homeowner and small business owner.
The two main types of PV systems are grid-connected systems and stand-alone systems. Grid-connected systems are connected to the electric power grid. Stand-alone systems are often installed in remote locations where there are no electric power lines. These systems are independent of the electric power grid.
Most of the systems being installed on residences and businesses today are grid-connected systems. These systems effectively use the electric power grid instead of batteries to "store" solar energy via an arrangement called net metering. Power generated by a grid-connected system will serve on-site loads first, and any excess power will be fed to the grid. When the solar generated electricity is not sufficient to meet on-site demands (like at night), electric power is drawn from the grid.
The main components of a solar electric system are typically the photovoltaic (PV) modules and an inverter. The PV modules convert sunlight directly into electricity. The electricity generated is direct current (DC) electricity. Most common electrical loads require alternating current (AC) electricity. An inverter is used to convert the electricity from DC to AC.
Grid-connected systems require nothing more than PV modules and an inverter. However, if you want your system to be able to provide back-up power when the electric power grid is down (i.e. due to a black-out), then you need a battery bank to store electricity and a charge controller to regulate the charge on the batteries. Stand-alone PV systems typically require batteries and a charge controller as well.
Grid-connected systems provide power to residences and businesses. These small, dispersed systems are referred to as distributed generation. Grid-connect PV power systems can also be installed on a large scale as central station power plants.
Stand-alone applications often provide power to remote locations, such as a remote cabin or telecommunications site. However, stand-alone systems can also be found in the middle of big cities powering traffic warning signs. In these cases, it is sometimes cheaper to put in a stand-alone PV system to provide power than it is to dig up the streets and extend the existing power lines.
In the developing world, PV systems are commonly used to provide residential power for lights and TV's, power to medical clinics, power for vaccine refrigeration, and power for water pumping.
People are commonly putting in PV systems today that can meet all of their domestic electricity needs. The required system size depends on your electricity usage and your location (how much sun you get). The average residential electricity consumption in California is about 18 kilowatt-hours (kWh) per day. With the installation of reasonable energy efficiency measures (and assuming the use of natural gas, propane, or wood for space and water heating ), a homeowner in our area should be able to reasonably lower their electricity consumption to between 5-10 kWh/day. Even in Arcata, one of the less sunny locations in California, it is feasible to generate this much energy using a modestly sized rooftop solar system.
The installed cost of a residential grid-connected PV system will typically run about $7 to $10 per Watt before rebates and tax credits.
Yes, certainly. Conservation and energy efficiency are better for your pocket book and better for the environment. Installing energy efficiency measures is cheaper and more cost-effective than installing a solar electric system, so you should always invest in efficiency first. Try to lower your electricity use as much as you can before buying a PV system. Then, when you do buy a PV system, you can buy a smaller system and still meet all your electricity needs.
Stand-alone PV systems have been cost competitive with other stand-alone type power systems for many years. However, grid-connected systems have also become more economically attractive in recent years. This is due to in part to cost reductions and efficiency improvements, and to increases in the cost of conventional electricity sources. It is also due to the economic incentives that are now being offered for the purchase of grid-connected systems.
In California, rebates and tax incentives can cut the installed cost of a grid-connected PV system in half. Coupled with net metering, grid-connected PV systems can pay for themselves, though paybacks are typically rather long (10, 15, 20 or even 30 years depending on your electricity use characteristics, location, system cost and utility rate choice).
Net metering is a billing procedure utilized by electric utilities that allows a customer to generate excess electrical power, sell it back to the grid, and get credit toward their bill. This allows customers to use the electric power grid to effectively "store" excess solar electricity.
Net metering is allowed on a state-by-state basis, and the rules vary among states. Contact your state public utilities commission or utility company to find out how net metering applies to you.
California has one of the most lucrative incentive programs in the country, and it has greatly spurred the market for grid-connected systems. The California Energy Commission offers rebates for grid-connected systems < 30 kW in size. These rebates were set at $4/watt in February of 2003, decreased to $3.80/watt in July of 2003, and they continue to decrease by 20¢/watt every six months thereafter. The State of California also offers a tax credit for both grid-connected and stand-alone PV systems. This credit started at 15% of the installed cost (less any rebates). It drops to 7.5% from 2004-2006, and ends in 2006.
PV systems manufactured today are safe and reliable. They are expected to last 20 to 30 years or more. There are many examples of older PV systems that have already lasted at least 20 years, and the technology has improved in the meantime. Most PV module manufacturers offer warranties that cover their products for 10 to 20 years or more. Systems may degrade over time, but will still put out a high percentage of the initial rated power output.
Yes, it can. The solar energy resource is abundant. Each day, the amount of solar energy that reaches the earth is greater than the entire world's total annual energy use. The average amount of solar energy received in the US is approximately 5 kWh/m2/day. In 2006, US electricity consumption was 3.8 trillion kWh. Based on these figures and assuming the efficiency of a standard rooftop PV system, the area required to meet the electricity needs of the entire US with solar energy is about 10,000 square miles, or barely 1% of US land area.
However, this grossly simplifies things. It is likely that a renewable energy future will rely on a diverse array of renewable energy resources, like solar, wind, small scale hydro, bio-fuels, ocean energy, and others. Because of the availability of these other resources, and because sunlight is not equally available everywhere and fluctuates in availability over time, solar electricity will not be the only renewable resource we use.
