Show us on the doll where the PV system touched you.

A few weeks ago Suthenboy expressed a strong opinion on the effectiveness of photovoltaic (PV) power systems, or solar electricity[1]. Reading between the lines I surmise he had a bad experience with one once.

I cannot deny Suthenboy’s lived experience but I can present an alternative experience. I’ve been living in my off the grid[2] PV-powered cabin for over 20 years.

I’ve designed four off the grid PV power systems: two for cabins and two for recreational vehicles. The largest is a one kilowatt PV array for a neighbor’s camp. All four systems work perfectly except for my neighbor’s because he doesn’t maintain his battery bank. He’s probably going to install utility power this summer which doesn’t bother me because he’ll certainly make me a good offer for his big PV array.

How It Works

If you were promised there would be no math then you can skip the next paragraph.

A PV array is composed of several photovoltaic panels. A PV panel is composed of several photovoltaic cells. When illuminated by bright sunlight each PV cell produces about 0.5 volts[3] of electromotive force with an amperage[4] proportional to the cell’s area. My cabin’s ancient PV array consists of eight panels. Each panel has 33 cells. The cells are connected in series so the voltage adds up to (33 cells) * (0.5 volts) = 16.5 volts. Each cell puts out about 2 amps of current to a single panel provides (16.5 volts) * (2 amps) = 33 watts[5] of power. With eight panels my PV array puts out (8 panels) * (33 watts) = 264 watts. My cabin’s PV array is tiny by modern standards. These days you can get a single PV panel with more power than my entire array.

But you can ignore the details and think of a PV array simply as a free source of battery bank charging power because a PV system of the type I’m describing is more accurately called a battery bank system. The battery bank extends power into the nighttime. The battery bank expands the consciousness of one’s energy usage. The battery bank is vital to the PV system.

The battery bank is composed of one or more deep cycle batteries. My battery bank has two that look like car starter batteries but are designed to be charged and discharged (cycled) many times. Car starter batteries aren’t designed to be cycled and won’t last long in a battery bank application.

In a modern PV system the battery bank powers a single device: the inverter. The inverter converts low-voltage DC[6] power from the battery bank into high-voltage AC[7] power like the kind that comes out of a wall socket. A modern inverter can be plumbed into a home with standard AC wiring without having to make any wiring changes.

How It Works II: The Diagram

This diagram can be used as an actual schematic for a PV system because all the parts and connections are shown. Power flows from right to left. Blue lines are AC power. Black and red lines are DC power, black is negative (minus) and red is positive (plus). The equipment to the right of the battery bank is the “charge” section from which power comes. The equipment to the left of the battery bank is the “load” section to which power goes. The independent charge and load sections mean half the system still works while the other half is down for whatever reason.

At the upper right corner is a PV array consisting of two PV panels wired in parallel. Simple PV systems use 12-volt deep cycle lead-acid batteries and PV panels sized to charge such batteries. More panels can be added to the array as long as they’re wired in parallel, plus-to-plus and minus-to-minus.

The PV array is connected to a PV Charge Controller which ensures that the PV array doesn’t overcharge the battery bank.

The PV array is usually not the only battery bank charging source. Nearly all PV systems have a backup generator for long stretches of cloudy weather. A gasoline generator and a battery charger are shown in the lower right corner. My backup generator is a 1KW Honda.

If the site has sufficient wind then a windmill is an excellent additional charging source. Windmills come in AC and DC varieties; the one on the diagram is DC. A windmill needs its own charge controller.

The plus outputs and minus outputs of the battery charger and charge controller(s) are connected together to make a single positive/negative wire pair. The positive wire is connected to a fuse (or breaker) that prevents the battery bank from exploding in case of a short in the charge section. The negative wire is connected to a current shunt that is used by the “Charge Meter” to calculate the amperage coming from the charge section.

A modern DC electric meter shows voltage, amperage, wattage, and cumulative watt-hours. This PV system design has two meters, one for the electricity coming in from the charge section and one for the electricity going out to the load section.

The other sides of the charging section’s fuse (plus) and current shunt (minus) are connected to the battery bank.

In the middle of the diagram is a battery bank consisting of two lead-acid batteries wired in parallel. Like the PV array, additional batteries can be added as long as they’re connected in parallel, plus-to-plus and minus-to-minus. The battery bank includes a desulfator which is a clever circuit that puts a high-frequency pulse over the battery bank leads. The pulse encourages any sulfur crystals that may be forming on the batteries’ lead plates to dissolve back into the acid. A desulfator increases a battery bank’s life many times.

The load section is a mirror-image of the charge section. A fuse and a current shunt are connected to the battery bank. The other sides of the fuse and current shunt are connected to the inverter which in this design is the only DC-powered device. The inverter turns low-voltage DC power into high-voltage AC power. The AC output of the inverter is wired into the household AC distribution box.

If the house has utility power then a special synchronizing inverter is required. A synchronizing inverter synchronizes its AC output with the AC output of the utility. A synchronizing inverter will also turn itself off if the utility power is out. This is a safety measure so that linemen working on the utility wires outside won’t be electrocuted by unexpected sources of battery power.

Modern PV systems often don’t have a battery bank and dump excess power on the grid. This runs the electric meter backwards, effectively using the grid as a battery bank, storing power during the day and drawing it back again at night.

Maintenance

A PV system is remarkably stable. There’s little that can go wrong.

If a PV panel is well-built and the cells protected from the elements then the panel will last a long time.[8] I bought my panels used in 1990 and they were about five years old at the time. They still work fine.[9] The only maintenance is sweeping snow off of them in the winter.

A well-maintained battery bank can last a long time too. Thanks to the desulfator my first set of batteries lasted 20 years before they simply refused to take a charge. My batteries have always been the sealed maintenance-free type.

One time my generator battery charger stopped working so I replaced it.

My neighbor’s camp is at the top of a hill in a clearing and he has had instances of his charge controller and inverter getting fried by nearby lightning ground strikes. Lightning protectors work by shunting the power into the ground. I don’t know of a way to protect equipment when the lightning surge is coming up from the ground.

Footnotes

[1] Electricity is really hard to describe. An approachable, but bad, conceptual model is “Electricity is the movement of electrons through a conductor (wire).”

[2] The “grid” is the telephone and electric companies’ wiring and infrastructure. A location enjoying these services is “on the grid”.

[3] Voltage (volts) is a measure of force. Electrons are compelled to move along a conductor when they’re subject to a voltage differential. The higher the voltage the faster electrons move.

[4] Amperage (amperes or amps) is a measure of flow. From an amperage, one can calculate the number of electrons per second passing a point on a conductor.

[5] Power (watts) is calculated by multiplying voltage and amperage. High power applications are measured in thousands of watts (kilowatts) or millions of watts (megawatts).

[6] Direct Current (DC) electricity is produced by a battery, PV array, or the power supply/charger of most common electronic devices. There’s a positive wire and a negative wire. DC electricity is generally low voltage most commonly 24 volts or less.

[7] Alternating Current (AC) electricity is produced by generators large (nuclear plant) and small (gasoline backup) and inverters. AC electricity is distributed on the grid and comes out of your home’s wall socket. AC electricity alternates positive/negative voltage on the two wires quickly, 60 times a second in the U.S. AC electricity is generally high voltage with 120 volts and 240 volts being most common in the U.S.

[8] This article from 2010 is about testing a 30-year-old PV panel of the same model I have in my PV array. My PV panels are a few years newer than the one in the article and not in such good shape:

https://www.greenbuildingadvisor.com/article/testing-a-thirty-year-old-photovoltaic-module

[9] I recently measured 234 watts coming from my PV array in a high-power use situation. My record is 262 watts in a high-power use situation in 2009. I’ve never done a maximum-power test on my PV array.