Questions & Answers
How does solar work?
In a typical polycrystalline cell the bulk of the material is silicon doped with a small quantity of boron to give it a positive or p-type character. A thin layer on the front of the cell is doped with phosphorous to give it a negative or n-type character. The interface between these two layers contains an electric field and is called a pn junction. The PV panels produce DC Electricity.
Light consists of particles called photons. When light hits the solar cell, some of the photons are absorbed in the region of the junction, freeing electrons in the silicon. If the photons have enough energy the electrons will be able to overcome the electric field at the junction and are free to move through the silicon and into an external circuit. As they flow through the external circuit they give up their energy as useful work, (turning motors, lighting lamps, etc.) and return to the solar cell.
The photovoltaic process is completely solid state and self-contained. There are no moving parts and no materials are consumed or emitted.
What can photovoltaics do?
Virtually any electric power need can be met by an appropriately designed PV power system. This includes power for lighting, pumping, refrigeration, radio transmission, etc. The only limitation is the cost of the equipment and occasionally the size of the PV array.
What does PV cost?
Although this depends greatly on the application there are some general guidelines to give you an idea of the cost. Grid-tied systems containing 100 watts or more of PV will generally cost between $4.50 and $8.00 per watt of installed PV. Smaller systems will be more expensive on a per-watt basis. Off-grid systems generally cost $15.00 to $20.00 per square foot of living space. The PV modules will be between 1/3 and 1/2 of the total system cost. Each watt of PV of your array will generally produce between 2 and 6 watt-hours of energy per day depending on the season and location. Using typical borrowing costs and equipment life, the life cycle cost of PV generated energy generally ranges from $0.06 (for ideal grid-tied) to $0.20/kWh (for complex off-grid). Occasionally low power applications, such as an outdoor light, may be cost effective only a few feet from the power line. The current incentives being offered greatly reduce the cost of a system; oftentimes by 60%!
Is PV difficult to use?
Although making PV cells and modules requires advanced technology they are very simple to use. PV modules are generally low voltage DC devices (although arrays of PV modules can be wired together for higher voltages) with no moving or wearing parts. Once installed, a PV array generally requires no maintenance other than an occasional cleaning (and even that is not imperative). Some PV systems do contain storage batteries which can require some watering and maintenance.
What is the environmental impact?
Photovoltaics are probably the most benign method of power generation known. They are silent, produce no emissions, and use no fuel other than sunlight. The production of photovoltaics of course varies among manufacturers. BP Solar makes extensive use of recycled materials and even uses waste from other industries as raw material. While some hazardous materials (primarily strong inorganic acids and bases) are used in solar cell manufacturing, these substances are not released into the environment. BP Solar PV technology is based on silicon, the second most common element on the earth's surface and is non-toxic when used in PV modules.
How well does solar work for heating?
While it is technically possible to use electricity produced by a PV array to heat a home or heat water, it is one of the least efficient methods to do so and does not make economic sense. If hot water is desired it can usually be produced more affordably by a solar thermal system (which uses heat absorbing panels filled with water or a glycol solution).
Does PV work in the cold?
Yes, very well in fact. Contrary to most people's intuition, PV actually generates more power at lower air temperatures, other factors being equal. This is because PVs are really electronic devices and generate electricity from light, not heat. Like most electronic devices, PVs operate more efficiently at a cooler temperature.
In temperate climates, PVs will generate less energy in the winter than in the summer, but this is due to shorter days, lower sun angles and greater cloud cover, not cooler temperatures.
Does it work in cloudy weather?
PVs do generate electricity in cloudy weather although their output is diminished. In general, the output varies linearly down to about 10% of the normal full sun intensity. Since flat plate PVs respond to a 180 degree window, they do not need direct sun and can even generate 50-70% of their expected output under a slight overcast condition. PV electric production is roughly proportional to sunlight intensity.
Indoor light levels, even in a bright office are dramatically lower than outdoor light levels, typically by a factor of several hundred or more. PVs are designed for outdoor use and will generally not produce useful power at these light levels since they are optimized full sunlight.
What else do I need in my PV system?
Although a PV system can be as simple as a module and a load, such as a direct driven fan, most PV systems are designed to supply power whenever it is needed and so must include batteries to store the energy generated by the PV array. Grid-tied PV systems use only PV panels, disconnects, and an inverter. Off-grid systems with batteries also need electronic devices to control their charging or limit the discharging of the batteries. Since PVs and batteries are inherently DC devices, larger systems usually include DC/AC inverters to supply AC power in standard voltages and frequencies. This enables the use of standard appliances in the system. Otherwise special DC appliances must be used. On the electrical side, protective devices such as diodes, fuses, circuit breakers, safety switches, and grounds are required to meet electrical code standards. In general, PV systems also require mounting hardware to support and elevate the PV modules and wiring to connect the PV modules and other components.
Why are solar cells so inefficient?
This is a matter of comparison. Modern single junction mass produced solar cells are about 18% efficient. This is a little more than half of the theoretical maximum efficiency for such devices. Multi-junction cells could theoretically achieve efficiencies of up to 50% and laboratory cells have achieved over 30% efficiency. The challenge is to increase the efficiency while reducing the cost.
Since the “fuel” is free, efficiency is not the major factor limiting PV today. Usually more than enough area is available to generate the energy required. Cost is usually the limiting factor today.
In comparing PVs with other methods of energy generation, it is important to start from the same point. Since all fossil fuels originally got their energy from the sun, if one were to measure electrical generation from fossil fuels relative to the original solar energy source the efficiency would be a fraction of a percent! In this comparison, PVs win hands down. For example, what is the efficiency of a car?
The effectiveness of tracking depends a lot on the climate and the application. Areas with a lot of haze or clouds won't get much benefit from trackers because the light is scattered. Also, applications where the load is the same in every month will also derive little benefit because tracking doesn't improve the performance of the system very much under worst case (usually winter) conditions. Under ideal conditions, trackers improve PV output per day up to 40% but they add to system complexity and expense and are not generally as strong as fixed mounted systems. The use of a tracking system is generally limited to applications where the increased output matches the increased demand (such as livestock watering) in drier areas.
How long will my PV system last?
In general, the PV modules are the longest lived component of a PV system. Top quality modules are designed to last at least 30 years. Most carry a 25 year performance warranty and a 10 year workmanship warranty. However, SunPower panels carry a 25 year workmanship and performance warranty. Innovative Energy only installs panels that are designed to withstand the harsh cold and icy conditions we experience. All panels are designed to withstand snowloads in excess of 100 lbs/ft2 and 1 inch hail at terminal velocity.
Batteries will last 7 years at best (high quality industrial types). Smaller sealed units will typically last 3 to 5 years. Automotive batteries are poorly matched to the characteristics of PV systems and will generally only last 12 to 18 months.
What about breakage?
The most reliable, longest lived PV modules use a glass substrate much like a tempered glass windshield. The modules we work with use a low-iron tempered glass that is laminated with layers of plastics. This construction is very durable but given a strong enough impact, it will break. If the glass is shattered or punctured the module will eventually fail due to water getting into the solar cells and causing corrosion. It may take years for the module to completely fail (produce no power). On the other hand, if the module is damaged in such a way that the two electrical connections between any give pair of cells are both severed there will be no path for the current and the module will have no output.
In summary, given enough force anything will break. The most effective protection against vandalism, theft and other catastrophe is property casualty insurance