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 | Technical Tips |
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| Insolation Map
The numbers listed on this map are the average worst case insolation hours. Insolation hours provide a way of predicting the output of a solar module at a specific location. This data has typically been gathered over a number of years from weather stations located throughout the US. For example, if you have an Kyocera-60 solar module that produces 3.5A peak power in a location that has 3 hours of insolation, it can be said that the Kyocera-60 will produce 10.5AH a day. A word of caution. This insolation information should be used only for estimates. Solar systems should not have a final design based on this information. This map does take into account small climate changes and may not be 100% accurate for all locations. Evergreen Auxiliary Power has a database that has specific cities listed with accurate insolation data. Please allow us to verify any critical design before purchasing.
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| | PLANNING & SIZING A SOLAR ELECTRIC SYSTEM
In sizing a PV system the first two factors we work from are the sunlight levels or insolation values from your area and the daily power consumption of your electrical loads.
Insolation
Insolation or sunlight intensity is measured in equivalent full sun hours. One hour of maximum, or 100% sunshine, received by a module equals one equivalent full sun hour. Even though the sun may be above the horizon, for example, 14 hours a day, this site may only receive six hours of equivalent full sun. Why? For two main reasons. One is reflection due to a high angle of the sun in relationship to your array. The second is also due to the high angle and the amount of the earth's atmosphere the light is passing through. When the sun is straight overhead the light is passing through the least amount of atmosphere. Early or late in the day the sunlight is passing through much more of the atmosphere due to its position in the sky.
Our sun trackers can help reduce reflectance but cannot help with the increased atmosphere in the sun's path.
Because of these factors our most productive hours of sunlight are from 9:00 a.m. to 3:00 p.m. around solar noon. Before and after these times we are making power but at much lower levels.
When we size solar modules, we take these equivalent worst case full sun hour figures per day and average them over a given period. See the chart above.
We like to work with two figures here: average annual equivalent full sun hours and average winter equivalent full sun hours. In most locations in the United States winter yields the least sunlight because of shorter days and increased cloud cover, as well as the sun's lower position in the sky.
Remember when selecting a site for your solar modules to pick a spot that is clear of shade from a minimum of 10 A.M. to 2 P.M. on December 21st. Even a limb from a deciduous tree will substantially reduce power output. Many solar sites are quite uncomplicated in terms of shading and aspect. You may already have a good idea of where the sun appears in the morning and disappears in the evening, as well as how low it swings in the winter sky. If your site is partially shaded, it may be necessary to determine exactly where the best placement of modules will be. We do have site analysis tools. If you need a more sophisticated site analysis, please contact us.
CALCULATING POWER CONSUMPTION
After determining the amount of solar radiation available, we must next determine the size of the load that we are supplying with power. The unit of measure for sizing is either watthours or amphours. We normally use watthours because it applies to both A C and DC circuits.
The procedure is the same for all systems, regardless of whether the load is a telecommunications repeater or a house. What we need to end up with is a figure of the average daily watthours consumed. This will allow us to determine how many modules will be needed to produce the power and how many batteries will be needed to store the power.
The table on this page is an analysis of energy usage for a representative small home. We have itemized each appliance and its individual run time per day and per week. We then summed the watt hours of all the individual units for a total daily watthour figure. Making up a chart such as this will allow you to understand where your power is going and may give you ideas for how to reduce your loads in the most effective manner.
Incorrectly assessing loads can end up being frustrating and expensive. Underestimating your loads can lead to major system inadequacies. Overestimating will lead to excess capacity. While many of our hybrid systems have a range of flexibility in provi ding power, some systems do not. But both problems can be avoided by careful assessment of loads.
Volts x Amperes = Watts
Watts x Hours of Use = Watthours
THE WHOLE HOME APPROACH
When considering energy efficiency it is important to consider the home as a system. Most loads are related to each other. For example: a well insulated house requires not only less heating and cooling but also less energy to distribute and circulate this conditioned air. Correctly placed windows can not only heat the home, but can also contribute a great deal of natural light, thus reducing both heating and lighting requirements. The home that is designed from the ground up with energy efficiency in mind will require much less of a photovoltaic power system.
Trying to utilize photovoltaics to power the conventional American home with its conventional appliances can be an unnecessarily expensive project. Reflection on these costs has prompted most of our customers to look first to conservation to reduce the ir loads. This is a cost effective move even for those still on utility power. For those going with PV, it can mean a much smaller and less expensive system.
Most of the houses which we have powered with PV do not appear noticeably different from conventional houses in terms of comfort and convenience. Some people do decide to adapt their life style when producing their own energy, and most of these changes have to do with simply being more conscious of shutting off loads not in use. The largest change of being your own utility is the responsibility that this entails. Almost without exception, however, the increased independence that this decision brings is cited by PV home owners as a great source of satisfaction.
Powering Your Heating Loads
Photovoltaic systems and the power they produce are best suited and most economical for operating motors, pumps, electronic equipment, lighting and the like.
PV's are not recommended to run your heating loads
Appliances such as toasters and microwaves are not a problem because of the low running times. Yet electric ranges, water heaters or baseboard heaters simply require enormous amounts of power, and can not be run by photovoltaics in an economically effective manner.
To power these loads we recommend thermal solar systems for space and water heating. In cloudy weather, wood and gas, either natural or propane, run these appliances efficiently and economically. In many systems we recommend propane for cooking, water heating and sometimes refrigeration.
Solar Water Heating
Different solar technologies are often confused. While the conversion of sunlight to electricity is photovoltaics, the collection of radiant energy to produce heat is Solar Thermal.
We do not utilize photovoltaics to create heat as this is an unnecessarily complex, very indirect and inefficient way to do so. "Heating with electricity", as Amory Lovins has put it, "is like cutting butter with a chainsaw." The direct capture of solar radiation by heating a black collection surface, however, can be a very cost effective and efficient way to produce hot air or hot water. We do not deal with solar thermal space heating. As sensible and efficient as this technology can be, it require s a good deal of on-site engineering and is the province of solar architects.
Solar water heating for household uses can also be complex, but it can also be quite simple. We do carry one water heater, and feel that it is one of the most reliable because of the simplicity and ingenuity of design. This modular unit can be found in the Water Heating section.
Solar water heating can also be supplemented by propane heat as demonstrated by the instantaneous heaters also in this section.
Solar vs. Wind vs. Hydro Power
How do PV's compare to other alternative power sources? Wind generating plants require a good steady wind at regular intervals over the four seasons. If you have a site where you have this resource, power production will not be a problem.
Hydroelectric generators are another option. These small generator requires a healthy flow of water with good vertical drop throughout the year.
Two points to look at are the site specific nature of these power sources and the difference in moving parts. Solar electricity many times has the advantage with both factors, sunlight being fairly universal and PV's having no moving parts to wear and eventually fail.
A combination of systems often work the best. Many times when the clouds reduce your solar output, wind or hydro systems are performing at full power.
IMPORTANT CONSIDERATIONS
System Voltage Selection
12 or 24 VOLT?
The nominal voltage of your system is usually determined by the system size. Small to medium systems, where most loads are DC, or a few loads are AC through an inverter, lend themselves to 12 volts nicely. Many lights and small appliances can be found at this voltage and efficiencies are high.
On the down side, 12 volt suffers from high line loss problems. The solar modules and loads cannot be far from the battery bank.
24 volt systems are suggested for medium to large systems. With 24 volts we have less wire loss problems and larger inverters are available. 24 volt DC appliances are more rare than 12 volt units. For this reason we lean heavily toward AC loads from these larger inverters. This simplifies wiring of the home to conventional AC wiring which exists in most homes and which any electrician can wire economically.
With the increased efficiency of AC lighting and the unlimited variety of low cost AC appliances, 24 volt systems, as well as 48 for large systems, have many advantages.
AC or DC?
The AC versus DC debate goes back to at least the time of Mr. Edison and Mr. Westinghouse. High voltage AC has the advantage of being efficiently conducted over very long distances with relatively low transmission losses. AC has thus become the standard for industry and domestic usage.
DC is generally used in low voltages, where transmission efficiencies are low. In some cases however, DC does have the advantage of efficiency in operation; as much as twice that of AC for some applications. A disadvantage of DC is that many appliances and equipment in 12 Volt DC versions are hard to find and are expensive.
Both have their advantages. In telecomm and water pumping systems, we generally use all DC. In home systems we typically run all or the majority of loads with AC power. For maximum efficiency certain specific loads can easily be powered by DC circuits. Cabins or RV's use mostly DC and can use regular gauge wire because of small loads and short transmission distances.
The Importance of High Efficiency
Using the best available technologies can save you money by saving energy. Such appliances often provide better service than outdated and inefficient technologies. These newer designs often cost more initially than their cruder counterparts but can have impressively short payback times. The importance of high efficiency appliances becomes doubly important for someone providing their own electricity. For example: a high efficiency refrigerator might be run with three 60 watt PV modules where as a conventional refrigerator might necessitate an additional 9 modules and additional battery capacity. This extra generating and storage capacity will cost many times the investment of the more efficient unit. An additional benefit is that more efficiency means less run time and less wear and tear on components. In the case of the refrigerator this can mean a life span twice that of the conventional unit.
Ghost Loads
Small loads that are not easily discernible but that can consume considerable amounts of power each day are termed "ghost loads". Examples include "instant on" circuitry in a television, wall cube transformers for answering machines, and electronic typewriters. These types of loads can sneak far more than their fair share of power. If not anticipated, located and dealt with, ghost loads can waste a substantial quantity of power.
"Power cubes" or "wall cubes" that plug into outlets to convert AC to DC for electronic equipment contain small transformers which can waste incredible amounts of power. A unit for a boom box, for instance, might consume 17 watts of power 24 hours a day, even though the actual electronic circuitry uses only 7 watts.
These kinds of loads are difficult to detect with an AC amp meter. The best way to find them is to shut every load in the home "off", and then shut down all circuits at the breaker box. Using a DC amp meter on the main battery cable, monitor each circuit as they are turned on one by one. If there is a ghost, it will appear.
There are two ways to deal with these troublesome loads. The first, easiest, and most costly method is to accept them. Accept the fact that your inverter will never go into standby mode, add on to your array to compensate for this power consumption.
The second method minimizes power consumption. Place switches on the appliances that run unnecessarily, turning them truly off and on when required. For small, but necessary, loads p; consider operating these at 12 or 24 volts DC. If this is not possible, a second, smaller, inverter can be installed to run select loads more efficiently. By doing this, the inverter can return to a no-load or idle mode, where it uses very little power.
Common Appliances with Ghost Loads
Electric clocks, Clocks built in to appliances (microwaves, ranges, telephones), Ni-Cad Battery chargers, Ground Fault Interrupting Receptacles and breakers (GFCI), Cordless telephones, Answering machines and low voltage appliances that can also utilize AC power.
ELECTRICITY FOR BEGINNERS
Electricity can be thought of as a flow of electrons through a conductor, generally wire. This flow is often compared to the flow of water through a pipe.
In this analogy, if you wish to have increased flow through the pipeline, you will need either a bigger pipe or you will have to push the water (or electricity) through at a more rapid rate. To push water through a pipeline at high speed requires high pressure. Pressure in water is measured in psi, pounds per square inch. You can envision water under high pressure squirting out very rapidly from a nozzle, such as a fire hose, with enough speed and force (power) to carry it to great heights or to do the work of knocking someone off their feet if they get in the way. Similarly, the "pressure" of electron flow is called voltage and is measured in volts. Generally speaking, the higher the voltage of an electrical current, the more force behind it.
The amount of flow at a given pressure is determined by the size of the cross-section of the pipe. If you were to open a spigot twice as big as another with the water in both at the same pressure, twice the amount of water will flow from the larger. The amount of flow in electricity is called amperage or "current" and is measured in amperes, or "amps" for short. Taking our analogy further, a battery stores electricity much as a water tower stores water. The taller this tower, the higher the pressure present at its base. If you open a valve at the base, water will flow out at a high pressure. In the same way, if you flip a switch connecting batteries to a load, electricity begins to flow. The higher the voltage of a battery bank, the greater the "pressure" of the electron flow. And just as with a tower of water, as electricity is drained from the battery, the pressure (voltage) slowly drops.
Most of the water available in such a tower is available from 45 to 60 psi. Once drained below 40 psi, usage will rapidly deplete the supply at an ever decreasing pressure. In the same way, a nominal 12-volt battery has most of its stored electricity available from just below 12 volts to 12.6 volts. When drained below 12 volts, little amperage remains.
Just as a pump designed to fill such a tower would need to be able to produce at least 60 psi (that is, be able to lift 138 feet) so does a solar PV module need to be able to produce at least 15 or 16 volts in order to charge a 12 volt battery.
Electrical power (the ability to do work) is a function of pressure (voltage) and amount (amperage). Double either one and you double the power the current is carrying through the circuit. The rule "VOLTS MULTIPLIED BY AMPERES EQUALS WATTS" defines this relationship. The watt is the measure of the power of electricity and will be our basic unit of measure for determining the size of our electrical loads.
A one watt load that is powered for one hour will consume one watthour of power. A 100 watt load powered for 2 hours will consume 200 watthours. And so on.
A 100 watt load could consist of a 12 volt appliance drawing 8.3 amperes or it might consist of a 120 volt appliance drawing .83 amperes. If the 120 volt, 100 watt unit is run for one hour it will consume .83 amperehours. And so on.
Another unit of measure that you will come across is the kilowatt. A kilowatt is 1000 watts. A kilowatthour could result from a 100 watt load being powered for 10 hours or a 1000 watt load being powered for 1 hour.
NOTE: the terms 110 volt, 117 volt and 120 volt all refer to the same common household AC current.
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| THIS PAGE IS ALWAYS UNDER CONSTRUCTION, PLEASE CHECK BACK AT A LATER DATE FOR MORE TECHNICAL INFORMATION
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