Surviving Winter With a PV System


By Windy Dankoff

The winter season puts photovoltaic systems to the test. The electrical demands are highest while the least amount of sunshine is available. Here are some suggestions for maintenance, plus some tricks-of-the-trade to minimize or eliminate the use of a backup generator.

CHECKLIST

PHOTOVOLTAIC ARRAY: Inspect/tighten mounting bolts & wiring, test output, tilt for winter angle.

TRACKERS: oil bearings, check mounting bolts and shock absorber action.

GAS GENERATORS, WIND GENERATORS: Consult the supplier or instruction manual.

BACKUP CHARGING SYSTEM: Be sure that it is wired, tested and ready in advance.

CHARGE CONTROLLER: Check regulator voltage settings, check voltmeter accuracy with digital meter. If batteries may reached a temperature below 55?F, they should be allowed to rise to a higher voltage (14.8V min. on a 12V system). If your charge control has a "temperature compensation" feature, then this will happen automatically. If it has an external temperature sensor, be sure that it is attached to a battery. If it does not have this but is adjustable, you may raise the voltage by hand, and lower it again in the Spring (to 14.3V). If your controller is not adjustable, keep the batteries warmer.

BATTERIES (Lead-Acid types): Test each cell/each battery with digital voltmeter or hydrometer to spot potential failures and check need for equalization. Set up equalization charge if necessary (typically, an 8-hour moderate overcharge after the batteries have reached full charge). Wash away accumulated moisture and dust from battery tops (use baking soda solution to neutralize acid deposits). Clean, or replace corroded terminals. Coat all terminal compoents with petroleum jelly, preferably while they are disassembled. This will prevent future corrosion. Check water levels & refill with distilled or deionized water. Inspect venting (check for insect nests in vent pipes).

WIRING: Check for proper wire sizing, tight connections, fusing, safety.

GROUNDING/LIGHTNING PROTECTION: Install/inspect ground rods and connections, ground wiring.

LOADS/APPLIANCES: Check for "phantom loads" and innefficient usages. Examples: Wall cube transformers and TVs with remote control that use power all the time they plugged in. Does your furnace thermostat hold your inverter on 24 hrs/day? -- See below!

LIGHTS: Look for blackening incandescent bulbs, consider more efficient Halogen or fluorescent replacements. Replace blackened fluorescents. Clean the dust from light bulbs and fixtures.

INVERTERS: Check adjustments, settings, connections.
NOTE: Inverters with battery charging function should have charge voltage set around 14.5 (or 29) volts if a generator is to be used for charging. See your manual. Add accessory temperature probe if appropriate.

WATER SUPPLY: Check freeze-protection, pump maintenance, pressure tank pre-charge.

BATTERY TEMPERATURE

Lead-Acid storage batteries lose about 25% of their storage capacity at 30?F. If fully discharged, they can freeze at 20?, and be destroyed. Summer heat is also destructive. For these reasons batteries should be protected from outdoor temperature extremes. Batteries can be safe indoors, if installed in accordance with the National Electrical Code.

FREEZE PROTECTION & HEAT TAPES

Electric heat tapes are a popular way to prevent water pipes from freezing under mobile homes, on solar water heaters, in well sheds and other places where they may be exposed to cold. Where heat tapes are a necessary evil, follow these tips to MINIMIZE THEIR ENERGY USAGE:

INSULATE!!! Use foam pipe jacketing, fiberglass, ANYTHING that insulates, and PLENTY OF IT! Be sure cold air and moisture are sealed out.

USE LESS HEAT TAPE than recommended, with fewer, wider-spaced coils. With extra insulation, you won't need much heat.

USE AN INVERTER that is efficient for running SMALL loads like your heat tape. Or, consider the following.

CONVERT HEAT TAPES TO 12 OR 24 VOLTS! If you're not afraid to cut and splice, here's how to make a low voltage heat tape:

Buy a conventional FLAT heat tape.

(2) For 12V, measure 1/10 of its length from the thermostat end and CUT. For 24V, use 1/5 of its length.

(3) Strip the cut end and connect the two inner wires together using the barrel of a crimp terminal. Be careful, the wires are thin and delicate. Protect the end with silicone sealant and/or tape.

You now have a low voltage tape. It will draw the same wattage PER FOOT as the original. The thermostat (if present) will work, but the neon indicator light won't. The remaining tape may be cut into more low voltage tapes by splicing lamp cord to one end, and tying the other end together (using crimp connectors). Add an external thermostat* because heat tape thermostats sometimes turn on as high as 50?. One line voltage thermostat can switch several tapes on and off.

FURNACES AND CONTROLS

THERMOSTAT CIRCUITS and POWER USAGE: Most central heating systems use a low voltage circuit through a wall thermostat, to tell the furnace when to turn on and off. The low voltage is derived from a small transformer which is powered constantly. It consumes only a few watts, but in an alternative energy system that may be a significant load -- if it is the only AC device that's running, it is adding constant additional draw just to keep the inverter "up". That amounts to the wintertime energy output of 1 or 2 PV panels, costing over $300 each, plus battery capacity to match!

If yours is a system where the inverter spends most of its time turned off (relatively little AC power usage) it is worth the small modification of adding a line voltage thermostat * to your furnace circuit. Have it installed IN THE AC LINE to the furnace controls. (Also bypass the original thermostat.) This way when heat is not needed, all power is cut to the furnace transformer. A small "limit switch" thermostat may also be added to sense heat in the furnace and keep the blower on until residual heat is exhausted. Material cost of these modifications is under $50 and the wiring is simple.

LOW TEMPERATURE SETTING: When nobody's home, you may need your furnace only to prevent your home from reaching freezing temperature (so that water pipes, fixtures and bottles won't freeze). Most heating thermostats stop at 55?F, but a lot of fuel may be saved if the temp can be lowered to 40?. or less. Electric power is saved too, since the furnace blower will run much less. The thermostat listed below will set down to 40?.

CONCLUSION

Being your own power company has its rewards AND its responsibilities. Extra attention paid in preparation for winter time will reward you with greater energy independence for years to come. If you are uncertain about working on your system, contact your dealer or a qualified electrician.

* A "line voltage thermostat" is one that is designed to handle power directly from 120 vac. An example is Honeywell T498A1778 available from W. W. Grainger, stock #2E831. It has a 40-80 F range.