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 | Frequently Asked Questions
WHAT IS AN INVERTER?
An inverter changes DC voltage, nearly always from batteries, into standard household AC voltage so that it can be used by common tools and appliances. Essentially, it does the opposite of what a battery charger or "converter" does. DC is usable for some small appliances, lights, and pumps and is usually suitable for small home or cabin systems, RV's and boats. However, nearly all larger home systems should include an inverter.
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| Although some DC appliances are available, with the exception of lights there is not a wide selection - and many are expensive and/or poorly made compared to their AC cousins. The most common battery voltage inputs for inverters are 12, 24, and 48 volts DC - a few models from some companies are also available in other voltages. There is also a special line of inverters (Trace) called a "utility intertie", which does not usually use batteries - the solar panels or wind generator feed directly into the inverter and the inverter output is tied to the grid power. The power produced is either sold back to the power company or (more commonly) offsets a portion of the power used. These inverters usually require a fairly high input voltage - 48 volts or more.
HOW DOES AN INVERTER WORK?
An inverter takes the DC input and runs it into a pair (or more) of power switching transistors. By rapidly turning these transistors on and off, and feeding opposite sides of a transformer, it makes the transformer think it is getting AC. The transformer changes this 12, 24, or 48 volts "alternating DC" into 115 volts AC at the output. Depending on the quality and complexity of the inverter, it may put out a square wave, a "quasi-sine" (sometimes called modified sine) wave, or a true sine wave. Square wave inverters are usually only suitable for running some type of electrical tools and motors and incandescent lights. Quasi-sine (modified sine, modified square) wave inverters have more circuitry beyond the simple switching, and put out a wave that looks like a stepped square wave - it is suitable for most standard appliances, but may not work well with some electronics, such as bread makers. Also, some of the chargers used for battery operated tools (such as Makita) may not shut off when the battery is charged, and should not be used with anything but sine wave inverters unless you are sure they will work. Sine wave inverters put out a wave that is the same as you get from the power company - in fact, it is often better and cleaner. Sine wave inverters can run anything, but are also more expensive than other types. The quality of the "modified sine" (actually modified square wave), Quasi-sine wave, etc. can also vary quite a bit between inverters, and may also vary somewhat with the load. The very bottom end put out a wave that is nothing but a square wave, and is too "dirty" for all but universal motor driven tools, coffee makers, toasters, and other appliances that have only a heating element. One solution to the problem of a few small appliances not working well with modified sine wave inverters is to get a large standard inverter, and a small (such as the Exeltech 250 watt) true sine wave for use only with that equipment. This would also allow you to keep the small appliance (such as an answering machine) powered up without having to run the larger inverter full time.
WHAT ARE PHOTOVOLTAICS AND HOW DO THEY WORK?
Photovoltaics is the process by which sunlight is converted into electricity. We can easily explain how the Photovoltaic effect produces a flow of electrons. In short, electrons are excited by particles of light and find the attached electrical circuit the easiest path to travel from one side of the cell to the other. Envision a piece of metal such as the side panel of a car. As it sits in the sun the metal warms. This warming is caused by the exciting of electrons, bouncing back and forth creating friction and therefore heat. The solar cell merely takes a percentage of these electrons and directs them to flow in a path. This flow of electrons is, by definition, electricity.
The basic idea is simple, Photovoltaic modules (solar panels) convert sunlight into electricity. Wire conducts the electricity to batteries where it is stored until needed. On the way to the batteries, the electrical current passes through a controller (regulator) which will shut off the flow when the batteries become full. For some appliances, electricity can be used directly from the batteries. This is "direct current" and it powers "DC" appliances such as car headlights, flashlights, portable radios, etc. To run most appliances found in the home, however, we need to use "alternating current" or "AC", the type which is found in wall sockets. This we can produce utilizing an inverter which transforms DC electricity from the batteries into AC. The inverter's AC output powers the circuit breaker box and the common outlets in your home.
ARE PHOTOVOLTAICS COST-EFFECTIVE?
Yes, PV is cost effective in the right location. By this we mean where the extension of utility lines are a major factor. We use the figure of half a mile as a rule of thumb for cost effectiveness, yet rates vary substantially from site to site. This half-a-mile figure is only a rule of thumb. If you haven't already, get a quote from your local power company.
If you are on utility power at present - PV is not a cost effective move. Utility power is much cheaper than PV power. Why? Because we have not yet begun to pay for the externalities of fossil fuel and nuclear generating plants. When this country begins to pay for the sulfur emissions which cause acid rain, global warming and nuclear waste disposal, to name a few, we will see power costs increase. With this in mind we need to ask and answer the question again. We believe, over the working life of a PV system, it can very well be a cost effective move. It all depends on the real price increases of utility power, 2, 5, 10 and more years from today.
CAN I PUMP WATER FROM A WELL WITH SOLAR POWER?
Solar power and water pumping are a natural. Generally, water is needed most when the sun shines the brightest. Solar modules generate maximum power in full sun conditions when we typically need larger quantities of water. Because of this "sun-synchronous" matching, solar is an economical choice over windmills and engine-driven generators for most locations where we do not have utility power. Owners of solar pumping systems enjoy a reliable power system that requires no fuel and very little attention.
WHERE DO SOLAR PUMPING SYSTEMS WORK?
Solar pumping systems work anywhere the sun shines.
The intensity of light varies greatly throughout the day. Morning and afternoon sunlight is less intense because it is entering the earth's atmosphere at a high angle and passing through a greater cross section of atmosphere, which reflects and absorbs a portion of the light.
We measure sun intensity in equivalent full sun hours. One hour of full sun is roughly equivalent to the sunlight on a clear summer day at noon.
These light or insulation levels also vary seasonally. Fortunately, most needs for water correspond with the sunniest seasons of the year - spring, summer and fall. One of the advantages of providing additional sources of water for livestock use is increased forage utilization. On large tracts some areas are not fully utilized because they are too far from water. As you know, livestock will only travel so far between feed and water.
Small to medium solar electric pumping systems are easily portable. By mounting the solar system on an axle or trailer, a system can be moved from well to well. This increases the economic return of a system by increasing the seasons of use. It may also correspond with the rotation of grazing areas.
HOW DOES THE SUN POWER A PUMP?
The photovoltaic effect produces a flow of electrons. Electrons are excited by particles of light and find the attached electrical circuit the easiest path to travel from one side of the solar cell to the other. Envision a piece of metal such as the side panel of a car. As it sits in the sun, the metal warms.
This warming is caused by the excitation of electrons, bouncing back and forth, creating friction, and therefore, heat. The solar cell merely takes a percentage of these electrons and directs them to flow in a path. This flow of electrons is, by definition, electricity. Photovoltaics or solar electric cells convert sunlight directly into electricity. This electricity is collected by the wiring in the module, then supplied to the DC pump controller and motor, which, in turn, pumps water when-ever the sun shines. At night, or in heavy cloud conditions, electrical production and pumping ceases.
CAN I POWER A WATER PUMP FROM A WIND GENERATOR?
Yes, but solar water pumping systems have many advantages over windmill water pumpers. Though the initial cost of solar powered systems can be similar to that of windmills (however, in many cases far less!), the lifetime costs are much lower. Windmills must be used where there is a steady, constant wind for maximum results while solar powered pumps operate anywhere the sun shines. Solar pumping systems can be installed in less than a day by an individual or small crew and can be portable, while windmills (because of the need to erect a tower) can take a larger crew a much longer time to install. Windmills are secured to the ground and are stationary. Solar powered water pumping systems are the modern day, upgraded version of the windmill - using natural resources to deliver water in off grid locations.
WHAT IS A BATTERY?
A battery, in concept, can be any device that stores energy for later use. A rock, pushed to the top of a hill, can be considered a kind of battery, since the energy used to push it up the hill (chemical energy, from muscles or combustion engines) is converted and stored as potential kinetic energy at the top of the hill. Later, that energy is released as kinetic and thermal energy when the rock rolls down the hill. Common use of the word, "battery," however, is limited to an electrochemical device that converts chemical energy into electricity, by use of a galvanic cell. A galvanic cell is a fairly simple device consisting of two electrodes (an anode and a cathode) and an electrolyte solution. Batteries consist of one or more galvanic cells.
A battery is an electrical storage device. Batteries do not make electricity, they store it, just as a water tank stores water for future use. As chemicals in the battery change, electrical energy is stored or released. In rechargeable batteries this process can be repeated many times. Batteries are not 100% efficient - some energy is lost as heat and chemical reactions when charging and discharging. If you use 1000 watts from a battery, it might take 1200 watts or more to fully recharge it. Slower charging and discharging rates are more efficient. A battery rated at 180 amp-hours over 6 hours might be rated at 220 AH at the 20-hour rate, and 260 AH at the 48-hour rate. Typical efficiency in a lead-acid battery is 85-95%, in alkaline and NiCad battery it is about 65%.
Practically all batteries used in PV and al but the smallest backup systems are Lead-Acid type batteries. Even after over a century of use, they still offer the best price to power ratio. A few systems use NiCad, but we do not recommend them except in cases where extremely cold temperatures (-50 F or less) are common. They are expensive to buy, and very expensive to dispose of due the the hazardous nature of Cadmium. We have had almost no direct experience with the NiFe (alkaline) batteries, but from what we have learned from others we do not not recommend them - one major disadvantage is that there is a large voltage difference between the fully charged and discharged state. Another problem is that they are very inefficient - you lose from 30-40% in heat just in charging and discharging them. Many inverters and charge controls have a hard time with them. It appears that the only current source for new cells is from Hungary.
It is important to note here that ALL of the batteries commonly used in deep cycle applications are Lead-Acid. This includes the standard flooded (wet) batteries, gelled, and AGM. They all use the same chemistry, although the actual construction of the plates etc can vary considerably. NiCads, Nickel-Iron, and other types are found in some systems, but are not common due to their expense and/or poor efficiency.
WHAT BATTERY DO I USE?
Starting (sometimes called SLI, for starting, lighting, ignition) batteries are commonly used to start and run engines. Engine starters need a very large starting current for a very short time. Starting batteries have a large number of thin plates for maximum surface area. The plates are composed of a Lead "sponge", similar in appearance to a very fine foam sponge. This gives a very large surface area, but if deep cycled, this sponge will quickly be consumed and fall to the bottom of the cells. Automotive batteries will generally fail after 30-150 deep cycles if deep cycled, while they may last for thousands of cycles in normal starting use (2-5% discharge).
Deep cycle batteries are designed to be discharged down as much as 80% time after time, and have much thicker plates. The major difference between a true deep cycle battery and others is that the plates are SOLID Lead plates - not sponge. Unfortunately, it is often impossible to tell what you are really buying in some of the discount stores or places that specialize in automotive batteries. The popular golf cart battery is generally a "semi" deep cycle - better than any starting battery, better than most marine, but not as good as a true deep cycle solid Lead plate, such the L-16 or industrial type. However, because the golf cart (T-105, US-2200, GC-4 etc) batteries are so common, they are usually quite economical for small to medium systems.
Marine battries Most marine batteries are usually actually a "hybrid", and fall between the starting and deep-cycle batteries, while a few (Rolls-Surrette and Concorde, for example) are true deep cycle. In the hybrid, the plates may be composed of Lead sponge, but it is coarser and heavier than that used in starting batteries. It is often hard to tell what you are getting in a "marine" battery, but most are a hybrid. "Hybrid" types should not be discharged more than 50%. Starting batteries are usually rated at "CCA", or cold cranking amps, or "MCA", Marine cranking amps - the same as "CA". Any battery with the capacity shown in CA or MCA may not be a true deep-cycle battery. It is sometimes hard to tell, as the terms marine and deep cycle are sometimes overused. CA and MCA ratings are at 32 degrees F, while CCA is at 0 degree F. Unfortunately, the only positive way to tell with some batteries is to buy one and cut it open - not much of an option.
WHAT IS A CHARGE CONTROLLER?
A charge controller is a regulator that goes between the solar panels and the batteries. Regulators for solar systems are designed to keep the batteries charged at peak without overcharging. Meters for Amps (from the panels) and battery Volts are optional with most types. Some of the various brands and models that we use and recommend are listed below.
Most of the modern controllers have automatic or manual equalization built in, and many have a LOAD output. There is no "best" controller for all applications - some systems may need the bells and whistles of the more expensive controls, others may not.
These are some of the charge controllers that we recommend at this time for all systems. Exact model will depend on application and system size and voltage.
Trace
C12, C40, TC25 or TC60
Morningstar
Prostar and SunSaver
Using any of these will almost always give better battery life and charge than "on-off" or simple shunt type regulators.
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