Northern Arizona's Most Trusted Solar Expert:
Northern Arizona Wind & Sun
Phone: 1-800-383-0195 | (928) 526-8017
Why they are an Expert...
Northern Arizona Wind & Sun is a family owned business. We started up in the solar electric business officially in 1979 when the company was formed.
Our family has been in Arizona since 1885, aside from a few excursions overseas in WW2 and Vietnam. Most of that has been in the Grand Canyon area, and many of the old timers contributed to the history (and some made history) of that area. We recently turned over nearly all of our family historical material, documents, photos etc. to the University of Northern Arizona. Quite a bit of it is now available on the internet. There is a lot a material, including letters and diaries from Edith Bass, one of the first women to live at the Grand Canyon, and the first white woman to have a child at Grand Canyon. It is not well indexed, so the easiest way to find it is to go to www.nau.edu and do a search on LAUZON and/or BASS.
Solar Power Basics
Sunlight—solar energy—can be used to generate electricity, provide hot water, and to heat, cool, and light buildings.
Photovoltaic (solar cell) systems convert sunlight directly into electricity. A solar or PV cell consists of semiconducting material that absorbs the sunlight. The solar energy knocks electrons loose from their atoms, allowing the electrons to flow through the material to produce electricity. PV cells are typically combined into modules that hold about 36 cells. Anywhere from 2 to about 16 of these modules are mounted in PV arrays. Some larger systems use multiple PV arrays. PV arrays can be used to generate electricity for a single building or, in large numbers, for a power plant.
A power plant can also use a concentrating solar power system, which uses the sun's heat to generate electricity. The sunlight is collected and focused with mirrors to create a high-intensity heat source. This heat source produces steam or mechanical power to run a generator that creates electricity.
Solar water heating systems for buildings have two main parts: a solar collector and a storage tank. Typically, a flat-plate collector—a thin, flat, rectangular box with a transparent cover—is mounted on the roof, facing the sun. The sun heats an absorber plate in the collector, which, in turn, heats the fluid running through tubes within the collector. To move the heated fluid between the collector and the storage tank, a system either uses a pump or gravity, as water has a tendency to naturally circulate as it is heated. Systems that use fluids other than water in the collector's tubes usually heat the water by passing it through a coil of tubing in the tank.
Many large commercial buildings can use solar collectors to provide more than just hot water. Solar process heating systems can be used to heat these buildings. A solar ventilation system can be used in cold climates to preheat air as it enters a building. And the heat from a solar collector can even be used to provide energy for cooling a building.
A solar collector is not always needed when using sunlight to heat a building. Some buildings can be designed for passive solar heating. These buildings usually have large, south-facing windows. Materials that absorb and store the sun's heat can be built into the sunlit floors and walls. The floors and walls will then heat up during the day and slowly release heat at night—a process called direct gain. Many of the passive solar heating design features also provide daylighting. Daylighting is simply the use of natural sunlight to brighten up a building's interior.
Volt-Amps vs Watts
You often see inverters rated in volt-amps (VA or va) instead of watts. For a perfect resistance, such as a heating element, watts and VA are essentially the same. However, many appliances, especially motors, are not a "perfect" load. Thus, the watts actually used may be less than the VA - but the inverter sees the VA. The motor or appliance may be using only 100 watts, but the inverter sees an "apparent" power of 150 VA.
The difference is due to the fact that in a perfect resistance, the voltage and current are perfectly in sync, but in a load with capacitance or inductance, the current may lag (or lead) the voltage waveform. This results in a higher apparent load to the inverter. In the first few milliseconds of a motor starting, the two may be nearly 90 degrees out of phase - this is the reason for the very heavy startup currents for AC motors. It is also the reason why inverters must be oversized if using large AC motors (but not DC motors).
Blocking and Bypass Diodes
Many older books and articles recommend using blocking diodes to prevent reverse current flow back through the panel at night ("dark current"). Many others do not (including us, mostly).
It actually depends on the situation, but as a general rule in 12 volt systems, you will lose more power from diode losses than you will from leakage back into the panel at night. The situation gets much worse at higher temperatures with crystalline panels. All regulators (charge controls) have built-in blocking circuits.
About the only case where a blocking diode might be needed is with small thin-film panels, such as the Unisolar US-5 or Siemens ST5, where the panel is connected directly to the battery. Diode losses are much less in higher voltage systems, such as 48 volts.
Blocking diodes and bypass diodes are NOT the same thing. Often the actual diode type is the same, but serve different purposes. The two diodes at the END are the blocking diodes, which prevent reverse current flow at night. These are seldom needed in systems with controllers, as nearly all charge controllers have a built in method of preventing reverse current.
The diodes ACROSS each panel are the bypass diodes. Most medium and larger panels come with these already installed. The purpose of bypass diodes is to shunt the current around a shaded, weak, or damaged panel. If the full current passes through a shaded or weak panel, overheating and damage may occur. Bypass diodes are not needed on 12 volt systems, optional on 24 volt, and should always be used on 36 volt or higher systems. See also our tip on shading. For another perspective, see the Sandia Labs IEEE paper on blocking diodes (PDF file). Bypass diodes may be ordered from our online webstore at 8 Amp bypass diode
Charge Control & Battery Charger Settings for Batteries
Nearly all batteries used in solar and backup systems are Lead-Acid. There are some differences that you should be aware of to get optimum performance and life from your batteries. Some charge controls have settings to choose between "flooded" and "Sealed" batteries. This has caused quite a bit of confusion with the newer AGM (absorbed glass mat) batteries. Since gelled need to be treated more gently than the flooded or AGM (such as Concorde PVX/LifeLine), most controllers make the assumption that all sealed batteries, are GELLED. Not so! - All of the AGM batteries that we sell should use the "Flooded" setting if there is no setting for AGM (unless recommended specifically otherwise by the manufacturer), which is typically about 2/10ths of a volt higher than the voltage setting for gelled. Also, if your charge controller has a choice, AGM batteries do not need nearly as much equalization as flooded, and most only need equalization about 2 or 3 times a year. Since AGM batteries do not have liquid, there is no stratification in the cells.
Theft and Vandalism
Because solar panels are somewhat expensive, they are a target for theft in some areas. There is no sure cure for this, but using theft-proof hardware, such as bolts that require special wrenches can help. Often the best prevention measure for both theft and vandalism is to make the solar panels less obvious, especially if they are visible from a major road or highway. It is also a good idea not to "advertise" them - such as one county in California did, by placing large signs up at a roadside rest stop bragging about their use. The panels were gone in a few weeks.
It's not like there are hordes of Panel Thief Rings roaming the countryside, but it is something you should be aware of. Theft resistant hardware may be ordered from our online store at Nuts & Bolts.
Shade losses in PV panels
All panels, regardless of what the advertising says, will lose considerable output even if only partly shaded. Some, such as the Unisolar panels, lose less in partial shade, but the output is still reduced. In crystalline panels, when a cell is shaded, it essentially "turns off" that cell, turning it into a high resistance. If a single cell in a panel is shaded, it can reduce or even completely cut off the output of the panel. In some cases, it can also result in overheating of the cell as the unshaded cells try to force current through the high resistance cell. Unisolar panels are less susceptible due to built-in bypass diodes on the cells, but will still lose up to most or all of their power with 15-20% shading due to the voltage dropping below the battery voltage.