Photovoltaic (PV) describes the electrochemical process of using the energy delivered in photons of light to create an electric current. Photo means light, and voltaic means related to producing electricity. (A photon is a particle of electromagnetic energy.) When a photon of light strikes one side of a solar cell, the photon’s energy causes an electron to jump to the other side of the cell. From there, the electrons travel through a circuit to where the electricity performs the desired work, such as lighting a lamp or charging a battery. A PV panel, or module, is made up of many small PV cells wired together to produce the desired voltage and power. The cells are assembled into a sturdy frame and covered by a strong, clear, waterproof elastomer or thermoplastic layer that is resistant to breakdown by UV rays from the sun. Each module can range in power output from 10 to over 200 watts. Two or more PV modules wired together are called an array. The size of the array needed for any given task depends upon the power requirements of the particular site. Arrays can be installed on a roof or a ground-mounted rack.
Solar Power Potential
Depending on the technology and materials used, commercially available PV cells convert light energy to electrical energy with an efficiency rate of between 8 and 18 percent. Current work in laboratories is producing cells with efficiencies of over 30 percent.
The material in PV cells that converts the energy of photons to electricity is called a semiconductor. The most common semiconductor used in cells today is the element silicon (as in Silicon Valley; not silicone, as in tub and tile caulk).
However, other materials, such as germanium, gallium, and cadmium can also be used. Organic solar cells using polymers are also being developed.
Semiconductor materials can be formulated in a solid crystal for use in rigid cells, or in sintered form, where powdered semiconductor material can be sprayed onto flexible products, such as roof shingles (see Space Requirements, page 134) and clothing. Each technology has advantages and disadvantages in terms of cost, efficiency, and flexibility. Multicrystalline silicon PV cells are the most common type used for stationary residential and commercial installations and have an efficiency of about 16 percent.
Assuming an average efficiency of 15 percent, a PV panel can deliver about 14 watts per square foot (150 watts per square meter). Keeping your panels aimed directly at the sun throughout the year and throughout the day will increase overall output. You can adjust panel position manually, but an easier and more reliable way is to incorporate a tracking system that automatically moves the array to the optimum position
How Much Power?
On average throughout the world, solar energy striking Earth from directly overhead delivers about 1,000 watts of energy per square meter, or about 93 watts per square foot. In reality, though, how much energy your solar collector “sees” and absorbs depends on several factors:
• Your location on the planet
• Season
• Temperature
• Level of cloudiness
• Light reflection from the ground to the solar panel
• Angle of incidence between the collector and the sun (see page 130)
• How much light the panel reflects away from itself On a clear day, you can expect the sun to deliver between 700 and 1,400 watts of raw solar energy to each square meter of solar collector area that is aimed directly at the sun.
AC vs. DCI
The power produced by PV systems is direct current (DC), not the alternating current (AC) you need to power your home. The conversion from DC to AC is handled through an electronic device called an inverter.
Planning for a PV System
You can determine how much electricity you use simply by looking at your electric bill, which tells you how many kilowatt-hours (kWh) you used during the past month. Some bills present daily usage, and some utilities let you look at how much energy you’re using at any given time by way of a “smart meter” that may even have a Web interface.
As you examine your electrical use, you may find patterns where consumption during certain times of the year is greater than at other times. But the general rule is to size your PV system for average daily use, knowing that you will make more than you need on some days and less on others. Solar electric systems can be expanded over time, so you can start small and add more as your budget allows. Making your home ready for renewables requires long-term planning; if you prepare for future expansion, there will be less work and expense when the time comes to upgrade.
Assessing Your Site
Let’s say that your home uses an average of 20 kWh of electricity each day, and you want to supply 50 percent of that electricity using solar power. Your solar-generating capacity needs to provide 10 kWh per day. Depending upon where you live, there are some months when you can count on the sun more than other months. It would probably not be practical to size your system to generate 10 kWh on cloudy days, because on sunny days you would have far more than you need. So let’s keep things simple and use a sunny day as an example to design a solar power system.
How Much Sun?
Once you’ve determined how many kilowatt-hours you want to generate, the next step is to understand how many hours of sunlight you can expect each day at your location. The greatest power output occurs during peak sun hours when the sun is high in the sky, typically spanning the three hours on either side of noon (that is, 9:00 a.m. to 3:00 p.m.). Outside of that ideal “solar window,” power production starts to fall off unless you have a tracking system that allows the array to follow the daily movement of the sun across the sky. The intensity of sunlight will vary throughout the year (unless you’re on or near the equator), so the actual power output of your PV panels will vary further with the seasons.
You can find average daily sun hours from a local weather station, or you can research weather data online through the National Climate Data Center or the National Renewable Energy Lab’s Renewable Resource Data Center (see Resources). The latter website includes an analysis tool called PV Watts that helps you predict the output of a PV system in your area. These tools are a good place to start, but you still need to perform a solar site survey to assess the daily and seasonal solar resource available at your specific site.
PV and Peak Demand
Peak demand is the period of time when the utility needs to deliver the greatest amount of power to its customers. For example, if peak demand is at 5:00 p.m. in the summertime — when everyone comes home to air conditioning, electric cooking, and water heating — a west-facing PV array will help offset the demand on other power plants that may be close to full production capacity. The overall power output of the PV system may be substantially less than with a south-facing array, but the utility’s objectives (which are likely different from yours) are achieved. This approach is often a better solution than building a new power plant which would be needed for only a few hours a day.
Solar Power Potential
Depending on the technology and materials used, commercially available PV cells convert light energy to electrical energy with an efficiency rate of between 8 and 18 percent. Current work in laboratories is producing cells with efficiencies of over 30 percent.
The material in PV cells that converts the energy of photons to electricity is called a semiconductor. The most common semiconductor used in cells today is the element silicon (as in Silicon Valley; not silicone, as in tub and tile caulk).
However, other materials, such as germanium, gallium, and cadmium can also be used. Organic solar cells using polymers are also being developed.
Semiconductor materials can be formulated in a solid crystal for use in rigid cells, or in sintered form, where powdered semiconductor material can be sprayed onto flexible products, such as roof shingles (see Space Requirements, page 134) and clothing. Each technology has advantages and disadvantages in terms of cost, efficiency, and flexibility. Multicrystalline silicon PV cells are the most common type used for stationary residential and commercial installations and have an efficiency of about 16 percent.
Assuming an average efficiency of 15 percent, a PV panel can deliver about 14 watts per square foot (150 watts per square meter). Keeping your panels aimed directly at the sun throughout the year and throughout the day will increase overall output. You can adjust panel position manually, but an easier and more reliable way is to incorporate a tracking system that automatically moves the array to the optimum position
How Much Power?
On average throughout the world, solar energy striking Earth from directly overhead delivers about 1,000 watts of energy per square meter, or about 93 watts per square foot. In reality, though, how much energy your solar collector “sees” and absorbs depends on several factors:
• Your location on the planet
• Season
• Temperature
• Level of cloudiness
• Light reflection from the ground to the solar panel
• Angle of incidence between the collector and the sun (see page 130)
• How much light the panel reflects away from itself On a clear day, you can expect the sun to deliver between 700 and 1,400 watts of raw solar energy to each square meter of solar collector area that is aimed directly at the sun.
AC vs. DCI
The power produced by PV systems is direct current (DC), not the alternating current (AC) you need to power your home. The conversion from DC to AC is handled through an electronic device called an inverter.
Planning for a PV System
You can determine how much electricity you use simply by looking at your electric bill, which tells you how many kilowatt-hours (kWh) you used during the past month. Some bills present daily usage, and some utilities let you look at how much energy you’re using at any given time by way of a “smart meter” that may even have a Web interface.
As you examine your electrical use, you may find patterns where consumption during certain times of the year is greater than at other times. But the general rule is to size your PV system for average daily use, knowing that you will make more than you need on some days and less on others. Solar electric systems can be expanded over time, so you can start small and add more as your budget allows. Making your home ready for renewables requires long-term planning; if you prepare for future expansion, there will be less work and expense when the time comes to upgrade.
Assessing Your Site
Let’s say that your home uses an average of 20 kWh of electricity each day, and you want to supply 50 percent of that electricity using solar power. Your solar-generating capacity needs to provide 10 kWh per day. Depending upon where you live, there are some months when you can count on the sun more than other months. It would probably not be practical to size your system to generate 10 kWh on cloudy days, because on sunny days you would have far more than you need. So let’s keep things simple and use a sunny day as an example to design a solar power system.
How Much Sun?
Once you’ve determined how many kilowatt-hours you want to generate, the next step is to understand how many hours of sunlight you can expect each day at your location. The greatest power output occurs during peak sun hours when the sun is high in the sky, typically spanning the three hours on either side of noon (that is, 9:00 a.m. to 3:00 p.m.). Outside of that ideal “solar window,” power production starts to fall off unless you have a tracking system that allows the array to follow the daily movement of the sun across the sky. The intensity of sunlight will vary throughout the year (unless you’re on or near the equator), so the actual power output of your PV panels will vary further with the seasons.
You can find average daily sun hours from a local weather station, or you can research weather data online through the National Climate Data Center or the National Renewable Energy Lab’s Renewable Resource Data Center (see Resources). The latter website includes an analysis tool called PV Watts that helps you predict the output of a PV system in your area. These tools are a good place to start, but you still need to perform a solar site survey to assess the daily and seasonal solar resource available at your specific site.
PV and Peak Demand
Peak demand is the period of time when the utility needs to deliver the greatest amount of power to its customers. For example, if peak demand is at 5:00 p.m. in the summertime — when everyone comes home to air conditioning, electric cooking, and water heating — a west-facing PV array will help offset the demand on other power plants that may be close to full production capacity. The overall power output of the PV system may be substantially less than with a south-facing array, but the utility’s objectives (which are likely different from yours) are achieved. This approach is often a better solution than building a new power plant which would be needed for only a few hours a day.
