Solar Electric Systems
The first questions and answers on this list by Mark Fitzgerald. (Used with permission.)
Q. What is PV?
A. PV is short for photovoltaics (photo=light, voltaics=electricity). PV is a semiconductor-based technology used to convert light energy into direct current (dc) electricity, using no moving parts, consuming no conventional fuels, and creating no pollution.
Q. What does PV cost? Aren't PV systems expensive?
A. Everything is relative. Using a PV system can be more expensive than buying power from the local utility, through the electrical outlet in your wall. However, it is dramatically less expensive than running a power line to a site currently without service (off-grid homes, more than 0.25 mile [or 0.4 kilometer] away from power, or a mountain-top communications system). PV modules can cost less than US$5/watt, in quantity, but that is only one part of a system cost. A system could include design costs, land, support structure, batteries, an inverter, wiring, and lights/appliances. The total system cost could be as low as US$7/watt or as much as US$20/watt or more, depending on the complexity. Every application is unique, and generalizations on cost are difficult to make.
Q. What is BOS?
A. BOS stands for "Balance of Systems." This refers to everything in a PV system except for the PV modules (design, land, wiring, electronics, loads/appliances).
Q. How much PV do I need for my house?
A. How much PV you need depends on your power loads and their duty cycles. I you wanted to completely replace your current electrical purchases from the utility with a PV system, you could look at your kWh usage on your electric bills for a year, calculate a daily average, and divide that by the number of average daily sun hours for your location. (3600 kWh/yr divided by 365 days/yr equals approximately 10 kWh/day, divided by 5 sun-hours per day (for locations in middle America), equals 2 kW. This would indicate that a 2-kW system would, over the course of an average year, produce enough energy to replace the power you are currently using.
However, if you design an energy efficient home, you could cut the annual electricity usage dramatically, reducing the size of the system. In the real world, the majority of home systems range from 1 kW to 2 kW. Where you live, if you are on the grid or off, and how you live, will dictate the size of your system, and its ultimate cost and value.
Q. How long will PV last?
A. PV modules have been tested in controlled settings and in the field, with results showing module lifetimes in excess of 20 years. Other system components have varied lifetimes (batteries can last 2-15 years, and power electronics are the most sensitive components).
Q. What kinds of PV are available?
A. The majority of power modules in use since 1955 are made of single- or multicrystalline silicon, though several manufacturers are producing large quantities of amorphous silicon power modules. Most solar-powered consumer products use thin-film amorphous silicon PV. Satellites and other space applications have used single-crystal silicon, single-crystal gallium arsenide, and test systems of thin-film materials. Several companies are manufacturing thin-film modules of cadmium telluride (CdTe) and copper indium dieseling (CuInSe2, or CBS), but these are mostly pilot production at this time and are not available in commercial quantities.
Q. What makes up a PV system?
A. PV system comprises the PV modules and the balance of systems (support structure, wiring, storage, power electronics).
Q. Where is PV used?
A. PV is used in space, in consumer products, in remote communications, in village power systems, in traffic signs and lights, in cathodic protection systems, in roadside emergency call boxes, in grid-connected systems (residential or grid support), in remote homes, and many other applications.
Q. What are the markets for PV?
A. The largest market for PV today is in developing countries, in village power and remote communications systems (estimates indicate that more than 2 billion people world wide have no access to conventional electric power; if they have electricity, they use batteries or diesel generators). There are projections of large markets for utility grid support and for building-integrated PV systems in developed countries.
Q. How long has PV been around?
A. The photovoltaic effect was first recognized by Edmund Bacquerel, in France, in 1839. Scientists made solar cells of selenium in the 1880s. And, modern PV technologies were developed at Bell Labs and RCA Labs in the mid 1950s.
Q. I am interested in making my home less dependent on local power companies, plus helping out the environment a bit. Knowing that I don't have any solar panels at the moment, and that I live in Michigan, what would be your recommendation? And how would the solar panels work with my current power system?
A. The best place to spend your energy dollar is in efficiency improvements. Although not as glamorous as putting PV or a Wind generator on your roof, reducing your electrical consumption is the first step towards energy independence.
1) Change over your lighting from "standard" incandescent to compact fluorescent (CF) and halogen bulbs. You can get 75 watts worth of light with 20 watts of electricity with a CF. Changing you lighting can reduce your electrical demand by 10-20%.
2) Replace aging appliances with either top of the line electric models or switch to gas models. A typical older refrigerator can use 100kwhours per month. We have models that use less than 20 kw/hours. Switching from an electric hot water heater to a gas tankless style saves as much as $30 per month. You can gradually move towards this goal by replacing one appliance a year and maybe one lightbulb a month, or take a home improvement loan and do it all at once!
3) Once you are using electricity in the most efficient manner possible, then you can talk about adding Renewable energy to the mix. Conceptually, either PV panels or a wind generator charges a bank of batteries, which supplies electricity to an inverter. The inverter transforms the Direct Current electricity from the battery to 120 AC (household current). The inverter will interface with the Utility Co., and sell back excess power to them, running your meter backward. Start with powering your garage or outdoor lighting, and gradually add panels and/or wind power until energy independence is achieved. Just keep in mind that every dollar you spend on efficiency saves about $3-5.00 on this stage.
Q. What does PV cost? Aren't PV systems expensive?
A. Everything is relative.
Q. Does it make sense to heat water with photovoltaics?
A. A number of people have set me straight on why it doesn't make any sense to heat water with a heater element connected to a PV array. Basically, it comes down to efficiency and ability. For about $2000, you could purchase four 75 watt PV BP Solar Modules. This would give you 300 watts, or 1020 Btu's (1 watt = 3.4 Btu). Essentially, it's too inefficient and slow, and not cost effective to heat water using PV arrays. (Andrew Burke)
However, if you already have solar modules, it might make perfect sense to use them for water heating. Most people size their solar array for the lowest amount of sunshine during winter months. Consequently, they have much more power than they need during the summer. Voltage regulators prevent batteries from overcharging during these times but also waste what could be considerable power. Why not use a controller with load divert (like the $120 30A Flexcharger) and direct the extra power into water heating (or solar cooling, water pumping, etc.)?
Q. I am interested in your solar panel for powering my IBM PC110. Do you include a diode to prevent back current on your solar panel? - Andy Tenka, 7/4/97
A. We don't recommend using diodes with solar modules this small - you lose more power during the day with a diode than you would lose at night without one.
Q. I plan to move soon, and hope to build my dream home. As part of this, solar power is potentially exciting. Unfortunately, last night I printed your FAQs on it, and my wife read it. She keyed on the line that utility power is cheaper right out of the outlet. I'm interested in any studies you have done comparing costs that I can use to justify PV as possible main power source for my home, or just as emergency power. (Scott Kirby)
A. There is no way to economically beat 10 cents for a 1000 watts of electricity. The things to keep in mind are:
1) The utilities are able to provide that electricity to us only because of massive subsidies and tremendous damage to our environment. We pay a dime, but our grandchildren?
2) The cost of electricity is only going to increase. Fossil fuel sources are finite and are only going to become more scarce. Nuclear cleanup, storage of spent waste and "mothballing" of old plants is going to cost more than a Trillion dollars (that's with a "t", 1000 billion dollars, and a billion is 1000 million).
3) Solar and wind sources will not run out of fuel, do not pollute, do not put future generations at risk for our short-term pleasure. You are invulnerable to the rising costs of other electricity sources or even simple line failures.
