| Insolation: What is it? The term INSOLATION means the number of hours of useable sunlight at your location that a Solar Electric System will be able to convert into electrical energy. "How many hours of sunlight do I have at my location?" Here are three maps showing average insolation in Canada, USA and Mexico (courtesy of NREL). SESP will provide detailed parametric data for your specific location. This data is necessary to engineer your system. If the sun is at a right angle or "normal", from the face of the solar panel, it will receive the most available direct sunlight. If the sun is off to the side of the panel, casting a shadow completely across the face of the solar panel, it will not receive any direct sunlight and minimal power will be produced. Solar cells and solar panels only receive direct sunlight for certain angles. Therefore useable sunlight is only available for a few hours a day. Solar Cell basics Solar Panels are made up of many individual Solar Cells. They are connected in series and parallel within the panel to increase the voltage and current to useful levels. An array is several panels connected together. Solar panels will product power only along the I-V curve shown below. What that means to the Solar Electric System owner is that if the panel gets shorted out, they will go to the Short Circuit level of current (Isc) and no higher. You won't get extremely larges amounts of current like what public utility power could provide. This means they are much safer to use. It also means that if your batteries are low, the panels will product more current and less current when the batteries are full. If a panel is open circuited, such as if your batteries are charged up and don't need any more power, they go up to a moderate level of Voltage (Voc) and stop producing current. Either short circuited, Isc, or open circuited (Voc) conditions won't hurt your panels. They just sit there waiting to provide you with lots of free power again.
Figure 1: Typical Current vs. Voltage curve of a solar panel.
Panels should be placed 6 inches off roof surfaces. Excessive temperatures will be reduce the voltage cells operate at. In very hot climates like Arizona, this must be taken into considerations when calculating a Solar Electric System size and you should design your system for 120% or more of the quantity of panels that you would need in a more moderate climate.
Figure 2: Effect of hot ambient temperatures on Solar Panel output. Cloudy days During cloudy days, the sun intensity is greatly reduced. The important thing to ask for a Solar Electric System owner is "Will I get any power out of my solar panels during a cloudy day?". Fortunately, you will. During a partially cloudy day you will receive about 50% of your normal levels. During a completely overcast day, you will receive about 30% of a clear day.
Figure 3: Effect of reduced solar intensity on Solar Panel output. Days of Autonomy: After a few days of socked-in overcast days, your battery bank will be depleted of stored energy. Another important parameter that must be decided when designing a Solar Electric System is "How many Days of Autonomy do I want?". The normal range is 1-5 days. This means the number of days of storage that you have to supply your power needs without sunlight or before you have to start up the stand-alone generator. The higher the number of days, the more expensive the system will cost. If you want a system with 99 or 100% availability, the system will be VERY expensive.
Figure 4: % availability equates to the number of days of power stored in your battery bank that will supply your house during overcast days. SESP recommends the addition of a Stand-alone generator. The generator will charge your battery bank during extremely long periods of overcast days and supply power to your home for special high load situations . Modern inverters that SESP recommends include a battery charger and automatic power transfer switch so that your Solar Electric System will be effectively 100% available but at a much lower cost. SESP also has a very good source of reliable good quality Stand-alone generators. Please check out our Kits & Products section for generators or contact us directly. Stand-alone generators carry with them certain disadvantages.
Figure 5: Sun passes higher in the summer and lower in the winter. Array tilt angle Everyone knows that the sun is higher in the sky on a Summer day and arches lower across the horizon on a winter day. The important question for a new Solar Electric System owner to ask is "What angle do I tilt the Solar Panels to get the most power out them?" There are three options:
Figure 6: Tilt angle is number of degrees up from horizontal. 1. Set at a fixed pitch. The best pitch is true south and angled up at an angle equal to your Latitude. (Your Latitude is the 0 degrees at the equator and 90 degrees at the north pole.) Example: If your house is located at 33 degrees north, such as in Phoenix, tilt the panels up at 33 degrees above the horizon. If you fix the panels at 15 degrees above or below your latitude you will lose 3% of your available power. Reference: a 4/12 pitch roof is 18.4 degrees up from horizontal. A 12/12 pitch roof is 45 degrees up from horizontal. 2. Change the pitch twice a year. If you set the panels 15 degrees greater than you latitude during the summer and 15 degrees less than your latitude in the winter (2 changes a year), you will gain 4.5% additional power over a fixed pitch. Question for the Solar Electric System owner to ask is "When do I change the panel pitch?" The optimal time the change the pitch is at the equinoxes. At the spring equinox (March 20) tilt the panels up 15 degrees above your latitude and at the at the autumnal equinox (September 23) lower the panels to 15 degrees below your latitude. Hint: Wouldn't that be a great idea to design your house roof to be due south and at the desired winter panel tilt angle of your latitude minus 15 degrees!
3. Moving axis tracker. Check out the Sunflower trackers! (photo courtesy of solar design associates.) Solar Panels can be mounted on moving axis trackers that will automatically follow the sun through its path during the day. They come in 1-axis and 2-axis types. A 1-axis tracker will give you a power gain of 32% and a 2-axis tracker will give you a power gain of 37% over a fixed tilt at you latitude. SESP would suggest you consider three issues: 1. Trackers are difficult to mount on your roof. Trackers need an open space directly around them for safety. The Sunflowers shown above are very artistic, but the space is then unavailable for other uses. If you want to take advantage of your south facing roof, trackers are impractical. 2. Trackers significantly add to the maintenance of your Solar Electric System. Fixed solar panels require very little maintenance over their life of over 25 years. Trackers will usually require a lot more maintenance and repair during the 25 years. 3. Panels should be relatively close to your batteries and inverters. Power from your Solar Electric panels to the inverter is low voltage/high current. Power line loss is proportional to the current squared (P=I2R). This means long distances will cause significant power losses and the use of very large and expensive wiring. Your solar panels must be located close to the Battery Charge Controller, Batteries and Inverter. In a normal situation, 25 feet is a long distance. SESP can calculate your power loss for your individual situation to determine if this is an issue. Which type or brand of Solar panel should I use? Forget all the terms and details. The bottom line to remember is the Cost per Kilowatt ($/KW or $/W) at the same system voltage. How much energy are you getting out of a solar electric panel depends on how many hours of sunlight shines on the solar panels. (remember the discussion on Insolation above?) Since the sun is only available for a few hours per day, you will need to store the energy for when the sun is not energizing your solar panels. The solar panels slowly feed power directly (Through your battery charge controller) into your batteries for later use. What SESP has unfortunately found that smaller wattage panels usually cost more per Watt ($/W) than higher wattage panels. Sometimes special sales or deals occur and you should keep your eyes open for them. SESP has very thoroughly researched out the different solar panel manufacturers, established distribution relationships and can provide them at a very competitive price. SESP has researched all the available designs and manufacturers and recommends Siemens Single Crystal solar panels. They have the highest efficiency, highest fill factor, longest warranty, lowest leakage and most importantly, the lowest cost per Kilowatt-hour ($/KWH). Please check out our Kits & Products section. How much roof area will be needed? In order to determine how much roof area your Solar Electric System will need, you need to know the system size and your panel efficiency.
Figure 7: The amount of space required on your roof is directly proportional to the size of your system. A typical family size system is 1-2 KW. Larger systems require more roof space. SESP will assist you in calculating the area you will require for your system. The second parameter needed to determine the the roof space is the solar cell efficiency. For the same power, a higher efficiency solar cell design will require less roof space. Of the three of the most common designs of solar cells (Single Crystal (14-16%), Polycrystalline (12-14%), Amorphous (4-5%)) Single Crystal solar cells have the most efficiency and therefore use the least roof space. Low efficiency Amorphous solar cells (~4-5%) can require as much as 5 times as much roof space as Single Crystal cells (14-16%). (With all this said, remember when selecting a solar cell design, the most important detail is cost per W, $/W, second is guarantee and third is roof space).
Figure 8: Lower efficiency solar cells requires more space, and probably cost, than high efficiency solar cells. (Remember $/Watt is the bottom line!). Batteries? Batteries are your fuel tank. They store your energy for use when you want to use it. Remember you own all aspects of your power generation plant. This means battery quality, size and maintenance are critical. Again, with batteries the bottom line to remember is Cost per ampere-hour ($/A-H). An Amp-hour is how many Amperes a battery can supply for an hour. Or, a reduced number of amperes for a longer number of hours. Example: If a 220 Amp-hour battery is discharged (used) at a rate of 220 Amps, it will have enough charge for one hour. On the other hand, if you are using 22 Amps, the battery will last 10 hours. Deep cycle Lead-Acid batteries presently the best choice for most Solar Electric Systems. Specialized systems may work better with other types of batteries, such as, NiCad, Gel-cell, etc. SESP will be glad to discuss these special situations with you and give you a recommendation on the type of batteries to use. For most home and cabin systems, deep cycle Lead-Acid (DCLA) batteries are the best choice today. Other systems, e.g., remote mountain communications repeater or light beacon, may be better suited for different kinds of batteries. Fuel cells will soon be available which greatly change everything we use batteries for. But, they have not yet come of age to the general public. Keep your ears open for news of fuel cells advancements. All batteries all have a limited lifetime. With DCLA batteries, the depth of discharge directly affect their life. Systems are usually set at 75 to 80% DOD. At that level and at a daily discharge cycling, DCLA batteries will last approximately 5 years. With less frequent discharge cycle, that lifetime will be longer. Battery manufacturers are seeing Solar Electric System DCLA batteries lasting typically 5 years. SESP will definitely use the Depth of Discharge (DOD) parameter in your LCC and battery sizing design calculations. Figure 9: A battery bank typically discharged down 75% will last approximately 1900 cycles. A battery bank discharged down 50% will last approximately 2700 cycles. DCLA batteries require periodic maintenance. They are filled with an electrolyte which is sulfuric acid and water. During the recharge cycle, the water is dissociated into hydrogen and oxygen gas and needs to be vented safely outside. They should only be refilled with distilled water monthly and never allow the Lead plates to be exposed to air. Adding tap water will quickly contaminate the cells and cause them to fail. What Appliances should be powered by a Solar Electric System? Appliances used solely for heating are more cost effectively and more efficiently fueled by Liquid Propane or natural gas. Thermal electric heating appliances such as electric ranges, electric ovens, electric water heaters, electric clothes dryers, toasters, etc., are very inefficient and comprise a major portion of your energy consumption.
Figure 10: Typical power Pareto and fuel recommendations chart.
Why have a Solar Electric System when Public Utility power is so close or already in your housing development? Utility rates are quickly on the rise. California's recent rolling blackouts were well publicized. They also clearly called that they will not be able to keep up with the demands caused by growth. Public Utility companies have split the rates you pay for Public Utility power into On-peak and Off-peak rates. Last years rates were approximately $0.18 for On-peak and $0.07 for Off-peak consumption. These rates are clearly on the rise. The data below was obtained from a Southern California Edison report on future utility rates.
Figure 11: On-peak and off-peak Public Utility rates. (Courtesy of SCE). With utility power readily available, a Solar Electric System doesn't have to be sized to supply 100% of your power needs. It only has to be sized to eliminate the exorbitant On-peak rates.
Figure 12: Power consumption curve from year to year. Highest usage is when summer cooling is required.
Figure 13: A grid-connected Solar Electric System will cut down the high On-peak consumption times. Excess Solar Electric System power can even be sold back to the Public Utility Company. Figure 13 shows that the excessive over threshold, On-peak hours, are mostly if not completely satisfied by the Solar Electric System. You will only be paying the lower Off-peak power you need to make up the power your Solar Electric System isn't supplying. When considering procuring a Solar Electric System, the On-peak rates is what you should compare it with. Net Metering Net metering is offered by almost all states. It is when you are generating more electrical power than you consume, the utility company will buy it back. SESP has performed cost analysis on this and found that it is not cost effective. Utility companies pay a low amount of money for power sold back to them. Therefore, the benefit you will gain is in eliminating the On-peak consumption and not on the money will you make selling power back to the utility company. SESP will design your Solar Electric System to satisfy your load as closely as possible with very little excess within your budget. Check out your latest utility bill!
That money is your own and not the Public Utility companies! You could be paying these fees to the procurement of your own power generation equipment! Utility lines cost between $10,000 to $20,000 per mile to put in. When building the ultimate dream house, you are required to pay that fee or forced to live in a "development". A Solar Electric System gives you freedom to pick the perfect site. Wind
power. If you are fortunate to live in an area that has consistent high prevailing winds, another great power generation opportunity to supplement your solar panels are wind generators. (see wind intensity map courtesy of NREL and Pacific Northwest Laboratory) Modern wind generators are highly efficient, long lasting and available from 120W to 10KW. For example, an AIR403tm wind generator running for 7 hours a day at an output of 400 Watts generates 2800 Watt-hours which is equivalent to the output of five 120W Solar Panels under an average of 5 hours of Insolation per day. During a cloudy windy day, your solar electric panels will have reduced output but your wind generator will be producing full output. SESP always recommends taking advantage of your local renewable natural resources. Hint: Wouldn't a solar panel and a wind generator make an ideal combination for your sailboat! SESP would be glad to engineer a Solar Electric System for your Sailboat or RV. Check out our Kits and Products section for some great buys.
Energy efficient, modern appliances.
Standard incandescent lights in most homes are 100 or 60 W. Modern compact fluorescent lights, like the one pictured above, emit the same number of Lumens (Light intensity) using only 8 or 14 W. They are also guaranteed by the manufacturer for 7 or more years. SESP also has deals on compact fluorescent lights in the Kits & Products section.
Modern new appliances are much more power efficient. Appliances made even only 5 or more years ago can be very inefficient. Many Public Utility companies and city governments are offering a refrigerator buy-back program. A typical "Energy Star" refrigerator uses approximately 1300 Watt-hours a day. A modern Conservtm or Sunfrosttm refrigerator will use only 480 Watt-hours in a day!
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