Solar Electric (Photovoltaic, PV)
On an initial cost basis, solar electric remains out of many people's budget, although in areas with higher costs for electric, they can pay off over their lifetimes. However, the availability of subsidies or tax breaks can make solar electric affordable, and with the on-going research and improvement in manufacturing, the price of the solar cells themselves is expected to go down dramatically.Solar electric systems come in two basic varieties: those connected to the power grid and those not connected (off-grid). PV panels were once confined to mostly off-grid systems because the initial cost of connecting to the power grid is often higher than installing solar electric. While those installations continue to occur, grid-tied PV systems are now, by far, the most common. Two things happened to cause this change: in addition to it now being generally legal to hook your PV system up to the grid, most states now have a net-metering law, which essentially says you can run your meter backwards when you generate more than you use (ie they're buying your excess electric at the same rate you pay for it). Some states (and some other countries as well) offer additional incentives, with a few of them being quite generous.
With net metering, you don't ever get a check if you generate more than you use, so there systems are usually sized so that you generate no more than you use. The advantage of net-metering is that you get to use the grid as a battery.1 Typically, any credit for excess power you have expires in 12 months.
Solar System components
- Solar cells, which turn sunlight into direct current electric, typically 12 volts, the same as car batteries. Solar cells come in three varieties themselves, single crystal, polysilicon, and amorphous, in general order of decreasing both the electric conversion efficiency4 and cost. The power generation ability of a solar cell is given in watts, and is the power it can generate at maximum sunlight, and often only as long as the cell itself stays under a maximum temperature. Solar cells are usually sold in panels and rated for their maximum output, so for the most part the type of solar cell can be ignored. Lower efficiency cells will require more panels, and hence more roof area to generate the same power, and so may not be cheaper per generated watt than a higher efficiency unit.
- Inverters turn the low voltage DC electric generated by the solar cells and convert it to the 120 Volts AC that is the standard plug power in the United States. In doing so, the inverter uses some of the power generated by the solar cell to run itself, and so lowers the overall efficiency of the system , but typically less than 5%. In grid-tied inverters, the PV panels are usually strung in series and then sent to the inverter as high voltage DC (at least 120V, often more). Off-grid inverters must use the same DC voltage as the battery bank, which these days if often 24V or 48V. Higher voltage is generally more efficient, and can utilize smaller wires sizes, because the power loss in the wire is due to current. In some off-grid applications, some of the load is put on 12VDC appliances and lighting (the same kind used in RVs), and so bypasses the inverter completely. The downside of this, is that you have to use very heavy gauge (expensive) wire to not get large losses in the wire. As the cost of PV systems has gone down, 12VDC usage has largely dissapeared. There are now also inverters that can work both grid-tied and off-grid, allowing a grid tie system to switch to battery backup when the power goes out.
- Batteries store excess power for use when there is no other. In an off-grid system, this is anytime the sun isn't shining, and so those systems usually need a large (and expensive) bank of batteries. In a system connected to the power grid, batteries aren't needed at all, and are only used for those who want a backup power source during power outages, which can be a significant feature if your home experiences frequent power outages. Typical batteries are just an improved version of car batteries, and are both expensive, short lived, and easy to ruin3. There are unfortunately no good batteries out there, and none are likely to be available in the near future.
In addition to these main components, a system requires racks to hold the solar panels and wiring to link it all together. The overall efficiency of the system can be improved somewhat by installing the solar cells on tracking racks, which follow the sun during the day.
What you Will Get
How much electric you generate will depend on the same criteria that all solar power does: how many clear days you get, how long the sun is up, whether the solar cells avoid shading2 and whether they are facing the sun directly (with true south being the best if tracking mounts are not used). Solar electric systems are rated by their maximum output, and are typically somewhere between a half a kilowatt to six kilowatts, with systems in the 1-2 kilowatt range being generally adequate for a house with a reasonable amount of electric appliances. For example a 1kw system that receives 5 hours of full sunshine a day will generate 5kwh of electric in a day, while a 2kw system getting the same 5 hours will generate 10kwh a day.
To determine how much electric you need, look at how much electric you use from a current electric bill and divide by the number of days to get the amount of power used in one day. If your house has electric hot water, and electric stove or other large power users, you will find that even if you are very conservative, you will use a very large amount of electric. For example it takes about 3.5kwh to heat 30 gallons of water, which is a typical amount of hot-water to use in a day (although an experienced water miser can do much better!).
For a more conservative number, take the power rating of all your appliances ignoring hot water, electric stoves and electric heaters and multiply each one's power rating by how many hours a day that appliance will be used to get the total power used (in kilowatt-hours). For example a 800watt blow dryer used 5 minutes a day uses only about .07kwh, while a top energy star refrigerator uses about 1.2kwh a day, and a 200w computer on for four hours a day uses .8kwh a day. When they are all added up, it is often the case that a 1-2kw system producing between 4 and 10kwh a day is of adequate size.
In most cases, the limiting factor on the size of a PV system isn't need, but budget and/or roof space.
Cost
Solar system costs can vary much, and like any other product, the costs
change over time. As a ballpark estimate, a solar electric system costs
anywhere from $6000 to $10,000 per 1kw installed, with grid-connected systems
being at the much cheaper end due to having no batteries. Not only are
batteries expensive, but they also required a charge controller, a more
expensive inverter, and significantly more complex wiring. Larger systems
lower the cost somewhat, although the cost of the solar panels is fixed.
Lifetime & Reliability
While single crystal and polysilicon cells should last a long time-at
least 30 years, amorphous cells have a tendency to lose efficiency over time
resulting in lower power output.
BIPV Systems
Building Integrated Photovoltaic systems integrate the solar
cells into a building material, thereby lowering the overall cost somewhat.
While this technique is generally used in large commercial construction, there
are two roofing products intended for home use, both using amorphous cell
mounted on a roofing material: metal roofing in one case, and asphalt
composition roofing in the other. Anyone considering these systems should
check their expected lifetimes with the manufacturer.
Solar Electric & Sustainability
Most solar electric systems are sized to be the minimum acceptable due
to the initial high system cost, forcing homeowners to use other methods for hot
water, cooking and space heating and cooling. With combinations of passive
and active solar, most space heating and cooling as well as hot water can be
provided, but often some kind of backup is needed, and just as often that backup
is either natural gas or propane.
Historically, this compromise has been acceptable, but as the price of solar cells comes down, it begins to make more sense to use solar electric as the backup power, because unlike fossil fuels it can be produced sustainably. (But of course sustainability is always a complex analysis: we must include the energy to produce the solar cell, and we can't ignore alternative ways of producing "natural gas", for example a bio-gas generator using cow manure as a fuel is sustainable as long as the manure is produced sustainably).
Resources
www.findsolar.com directory of installers and state by state incentives run by the American Solar energy society.
www.dsireusa.org database of state incentives for renewables and efficiency
The New Solar Electric Home, Joel Davidson, AATC publications 1987.
Notes
1: but it isn't actually a battery: the power you get back is generated by the whatever mix of energy sources the power company uses, for which the current national average is around 70% fossil fuel. Still, the excess PV electric you export, has theoretically avoided using those fossil fuels during the time you're exporting power, so other than line losses moving the power around, and other complexities of power generation, you are almost using the grid as a battery.
2: shading, even a tiny amount reduces the output of a PV system dramatically. This is because all the cells in the panel are in a string like old-style Christmas lights: when one goes out, the rest are taken down with it, and when multiple panels are strung in series (as they typically are in grid-tied systems), the entire string is taken out. One solution that has emerged to this is attaching a tiny inverter to every PV panel.
3: newer batteries, although still the same lead-acid technology used in cars, apparently will last quite a long time if properly cared for.
4: current typical efficiencies are anywhere from 6% to 15%, with values slowly creeping up every year. The cost per watt tends to be about the same for all technologies.