Off-Grid Alternative Energy Systems: A Basic Overview

Courtesy of Bruce Baldwin, The Solar Guy
ROC 201760
thesolarguy@hotmail.com

To select a solar electric system for your home or RV requires a fundamental knowledge of the basic components which comprise the system and an understanding of how they work together. A solar panel, because it generates either 12 or 24 Volts DC, is a relatively safe and simple electrical device, which along with the rest of the system is governed not by magic but by basic physical law; despite what you might see on the Internet there are no "black boxes" which will allow you to run your 2,000 square foot cabin forever on 2 car batteries! The following is a quick overview of the process.

First, you should understand the basics of Voltage and Current. Voltage is the force produced by an electrical source, while Current is the amount of electricity flowing through a circuit. Voltage is measured directly, while current is measured in series between a source and a load while flowing3;without flow, technically there is no current. The amount of electricity contained in a battery bank is known as Potential Current, as it is available as soon as there is flow.

Many people better understand voltage and current through a water analogy: Voltage is equivalent to water pressure, while Current equates to water volume (amount of water passed though a pipe). Like in a battery, the amount of water in a reservoir represents potential volume, but until it passes through an open pipe, there is no flow.

Current is measured in Amps and Watts. Current is either Direct (DC) or Alternating (AC). DC is a continuous flow, while AC shuts on and off 60 times a second (this is referred to as 60 cycles or 60 Hertz [60 Hz.]). 120/240 volts AC is the standard in the U.S. for household electricity. Solar systems utilize both DC (the panels and batteries) and AC (inverter/generator power).

Because of friction in the wiring, the lower the DC voltage (and to a lesser degree AC) the larger the wire needed to move electricity a given distance. DC falls off rapidly with distance, and wire tables should be consulted which define the wire size needed to transport the desired voltage and current over the number of feet in the circuit. Due to the fact that AC is on and off an equal length of time, and is usually at a higher voltage than DC in a solar system, smaller/longer wire sizes can be used on the AC side of the system.Undersize wire will reduce both the voltage and current available as the far end of a circuit, and in severe cases will cause the wire to overheat and burn, resulting in fire danger and possible damage to connected equipment.

Solar System Components

Solar Panels (also known as Photovoltaics or "PV Modules") produce direct current electricity, or DC, when exposed to sunlight. Often this is nominally 12 volts DC, the standard used in cars and RVs. Larger systems may be designed for 24 volts DC, or even 48 volts DC. Usually, this means combining the same solar modules in pairs for 24 volts, or groups of four to get 48 volts. Solar panels are sized and priced by the watt, generally panels in a system will be from 75 to 175 watts each. Panels must be mounted within 20 degrees of true south, and can be either fixed or tilt mount to increase efficiency during the winter. Solar panels operate in complete silence and usually are extremely long lasting devices3;most panels are guaranteed for 20 or 25 years!

Panel types and sizes can be freely intermingled, it is perfectly OK when adding power to wire a 110 watt and a 150 watt panel to your existing 50 watt panel (assuming your charge controller, breakers and batteries are up to the task). You should be cautious about used panels and check them carefully for both voltage AND current, it is entirely possible for a panel to be outputting voltage with little or no current.

Solar panels are installed in groups of 1 to 12 modules on a solar mount, which in turn attaches to a building, to the roof of an RV, or atop a metal post in the yard. Together this is called a solar array. Each solar module is wired to the other modules in that array by sunlight tolerant solar interconnect wiring. Several arrays may be wired to a solar combiner box where they are all connected to heavier wires taking the power to the battery and equipment room.

Wind Generators are another source of renewable energy, however, in Yavapai and Coconino counties there is not sufficient wind on a steady basis year-round to be the primary source of DC power. Although the wind in this area may seem seasonally strong, the long-term average is only 7 MPH, whereas wind generators need 15 MPH average to be economically feasible. Wind generators can also be noisy and troublesome; nonetheless, some people prefer them as a backup or additional source of power.

A Charge Controller is a small wall mounted unit whichreceives the power from the solar array or wind generator, and controls the flow of power to the battery. To prevent battery damage from overcharging, the charge control automatically cuts back or stops the charge when batteries become full. A charge control may have meters or lights to show the status of the charging process. In the higher price ranges, charge controllers (such as the Outback MX-60) are available as "sun-trackers", and can increase the efficiency of a solar system by as much as 40% in winter and/or cloudy weather by manipulating the output voltage and current.

This DC power is received by and stored in Deep Cycle Batteries, which can instantly supply large surges of stored electricity as needed to start or run heavy power appliances that the solar panels alone could not power. This DC power is either used directly through a circuit breaker or fuse for DC appliances, or is used to power an AC inverter. Sometimes a small solar powered RV or cabin may have no inverter, and use only DC wiring and appliances.

Rather than being used singly, in the typical solar system batteries are connected together to form a Battery Bank, contain numerous batteries connected either in parallel (for increased current capacity), series (to increase voltage of the bank), or both (increasing voltage and current). For instance, two 12 volt, 100 amp batteriesconnected in parallel equate to a 12 volt, 200 amp battery bank, while the same batteries connected in series equate to a 24 volt, 100 amp battery bank. As you can see, to run a 24 volt, high current capacity bank will require twice as many batteries as the 12 volt equivalent.

Batteries are available in standard (vented) lead-acid, which must be vented to the outside because of hydrogen outgassing while charging; or sealed batteries such as sealed lead-acid, Absorbed Gas Mat (AGM) or "gel-cells". Lead acid batteries must be handled with extreme care, as they can explode if exposed to a spark or open flame.

The life span of deep cycle batteries is directly affected by their care and state of charge. Lead-acid batteries normally are rated for approximately 400 discharge cycles (running them down to no less than 50% capacity, than fully charging them). Taking them down below 50%, or not returning them to a full charge each time, will shorten their life span. They must be maintained regularly (watered, cleaned and charged) to remain in good condition. They should never be drawn completely down to empty. Because of these needs, maximizing the life of lead acid batteries requires some direct action by the owner.

Sealed batteries, especially AGM (Absorbed Gas Mat), are often rated for double the number of discharge cycles (400-800 or more), thus offsetting the initial higher cost. Because the batteries are sealed, there is no outgassing of flammable hydrogen gas while charging, and no acid mess to corrode cables, enclosures, or your hands and eyes! While they still must be monitored for charge levels, there is no other maintenance required.

Standard automotive batteries, while inexpensive, are not made for deep cycle use and their life span will be very short (as few as 25 discharge cycles).

Remember that batteries are considered a consumable device, and will require replacement at some point. Carefully balancing the number and type of batteries with the generating capacity of the system as well as scrupulous maintenance will help get the most life out of the bank. Because batteries are affected by the weakest link in the chain (a single bad battery in a bank will drag down every other battery), all the batteries in a bank will require replacement at once.

The high power capability of a battery bank can be a fire hazard just like utility company power, so fuses or circuit breakers on every circuit connected to a battery are essential. Battery size is chosen for both surge power requirements and for the amount of reserve power needed. Batteries may contain both acid and a great deal of stored energy, therefore requiring care and knowledge to safely install.

The AC Power Inverter uses a transformer and circuitry to convert the 12 or 24 volt DC current from the battery bank into 120 volt AC current, the same as utility power for standard household lights, outlets, and appliances. Short, heavy cables with a large fuse or circuit breaker carry battery power to the inverter. After conversion to AC, power from the inverter connects into the circuit breaker box of the house in place of utility lines. The house breaker box routes power to lights, appliances, and outlets of the house. Inverters are available up to 5,000 watts, and some may be "stacked" for 120/240 Volts AC use.

Inverters are available as either "Modified Sine-Wave", which electronically simulates the 60 cycle current, or "Pure Sine-Wave", which produces current identical to utility power. Appliances with motors (ceiling fans, refrigerators) will run more quietly and efficiently on pure sine wave inverters, and may hum or buzz when used with modified sine-wave power.Sensitive electronics such as stereos and LCD/Plasma TVs often require sine wave power to prevent long-term damage. Some rechargeable tools cannot be used with modified sine inverters (those with separate "wall-wart" transformers are usually OK), and inexpensive variable speed devices (hair dryers, floor fans) should be used with caution.

Inverter use requires that batteries be maintained near full charge as much as possible.? If the charge level is low, numerous options are available to recharge the batteries in order to keep the whole system working. A battery charger plugs into 120 volt AC from the generator producing low voltage DC to charge the battery. The generator is shut down after the batteries have been recharged.

An Inverter/Charger is an inverter that also has a battery charger and transfer relay built in. When the input terminals of a standby inverter/charger receive power from an outside source of AC (a generator or utility power) the inverter stops producing AC power from the batteries, and instead passes generator or utility AC power straight through to the house. At the same time it uses the generator to recharge the batteries. Some standby inverters even auto-start the generator when batteries need charging. A separate battery charger can be used instead of (or in addition to) a standby inverter/charger.

Please note that in our experience inexpensive off-brand inverters such as those found in truck stops and variety stores have a practical failure rate close to 100% over time and make sense only if you are using low power AC devices on an intermittent basis.

An Engine Generator producing 120 volt (and sometimes 240 volt) AC power is usually part of the system. This is a second source of AC power and a backup for charging the batteries when there is a shortfall in solar or wind power, a temporary need for large appliances, additional power for construction or visitors, or in case of breakdown of other equipment. Generators are powered by internal combustion engines fueled by gasoline, diesel, propane or natural gas, and thus must be located outside. For reasons of health and safety, it cannot go in a house or basement. By nature, generators are noisy and polluting and must be refueled on an ongoing basis, adding considerably to the long term costs. Less expensive generators also have a tendency to break down, and can be expensive to repair.

120 volt AC power from the generator goes through a circuit breaker, then is wired into the power room to run battery chargers as well as supply the AC power to the house whenever the generator runs. If you have a standby (battery charger) inverter, starting the generator automatically starts the charging process.

Some generators also produce 12 volt DC from a separate outlet. This can be used to charge the batteries directly, although the current is usually limited to around 8 amps, practical only for charging 1-2 batteries at a time.

Fuses and Circuit Breakers are necessary in all DC wiring between the batteries and other power system components described, but are not shown in the drawing. This prevents fires and equipment damage in event of a malfunction. Breakers might each be a separate component, or might be built into a powercenter. In contrast, the AC breaker box for household wiring is part of the house wiring, not usually included with power generating equipment.

Meters, like the gas and temperature gauges in a car, are useful in both everyday operation of the system and are invaluable in troubleshooting problems should they occur. Solar charge indicating meters are often built in the charge control to confirm the charging process instantly. Other meters show how much power is being consumed, and confirm approximately how much power is available. These battery system monitors can be located in the power room, or remotely mounted for easier checking.

As you can see, many factors go into calculating the total Run Time of a solar system, but the largest influence will be the total solar watts available, the size of the battery bank, the size of the loads used with the system and the willingness to run a generator to make up for shortfalls associated with appliance use and weather. In order to perform as expected, systems must be carefully balanced; 20 batteries with a single solar panel will not work properly, nor will an overabundance of panels with an insufficient battery bank. If budget is a concern, it is always better to start out with a small, balanced system, expanding it as funds become available.