Solar power for remote communication systems

Gather the data needed to make business decisions on employment of solar power systems at remote sites. Articles in MRT (1) have made a good case for powering remote sites with solar energy, but most communication business owners still ask, “Is it for me?” Because of recent advances in both solar panel and battery efficiencies, “alternate energy” power is increasingly the better option for those remote sites not served by reliable commercial power systems. As environmental restrictions tighten, fuel-powered systems will become even more costly.

This article discusses building an energy budget and ways to calculate, with reasonable accuracy, the total solar insolation available at your site. Major system components are briefly discussed and a list of information and supply sources is provided.

Power budget Creating a communications system power budget is an exercise in detail. If you choose to use an engineered or turnkey system, budget information is still required by the contractor that builds the solar system. The time you invest in preparing the power budget document is money saved in consultant fees. The power budget, in its simplest form, records the amount of energy used (in watts) by your communications system components. If the technical data provided with your equipment seems fuzzy, the component chassis usually has a label that lists the power consumed.

In extreme cases, such as with used or surplus equipment, I have taken the fuse value and the voltage rating and calculated a wattage for the unit. While this is sloppy at best, it does give you a figure to work with on your power budget calculations. Take the time to calculate the use time of transmitters vs. receivers and the difference in power consumed.

If the bulk of the remote site equipment will be powered by alternating current, do not forget to add a percentage to account for conversion loss within an inverter system. Even modern inverter systems may suffer as much as a 10% conversion loss. In the case of dc-only systems, allow for at least 5% loss, including inefficiencies in cables. Do not forget to include energy used for lights, fans, test equipment and any sanitation devices. Now, even with all of that that data recorded, we are not quite finished yet.

This data now must be expressed in watt-hours. Quite simply, it is a question of “How many watts did your equipment use, and for how many hours did your equipment operate?” For example, if you run a 100W light bulb for a full day, you have used 2,400 watt-hours. This is where billing records become invaluable in determining the actual amount of power consumed.

It is possible to build a solar power system and add additional panels and batteries if your original calculations were off a bit. In real life, this may be the less expensive alternative to buying too much in the beginning. You can find sample work sheets for this power budget process at www.sunelco.com/home.html, one of many excellent solar information “pages” on the World Wide Web. Other companies listed offer software, both IBM- and MacIntosh-compatible, to perform these chores. Once you have determined the total watt-hours used by your equipment, you can begin to calculate the solar insolation of your site.

Solar insolation is a measure of the amount of usable solar energy falling on your site. In theory, this is 1,367Wm2. This theoretical value is not obtainable for most of the world, because potential energy available is affected by latitude, angle of declination, eccentricity, obliquity, apparent seasonal movement of the sun and most of all, weather. (See the sidebar on page 18 if you are a rocket scientist and want to do the math.)

Latitude and total hours of sunlight are the real factors to worry about. Altitude has more to do with weather than solar gain. Your latitude can be obtained at a local airport or, in the United States, topographical maps usually show latitude. For really remote sites, a hand-held Global Positioning System (GPS) unit will provide acceptable accuracy in determining latitude. Cross-reference the latitude figures with a naval or aeronautical almanac to derive the mean hours of daylight, winter and summer.

If your library doesn’t have these reference materials, check with your local airport. The flight controllers or fixed base operator should be able to give local sunrise and sunset times for December 21 and June 21, the shortest and longest days of the year, respectfully. Weather, and resultant cloudy days vary widely, so its best to check with your regional weather forecast center for the percentage of cloudy days in your region.

With these figures (total power budget in watt-hours, solar insolation for your site and the percentage of cloudy days) in hand, you are ready to size the system you need to power your equipment. Panel size (in watts) 3 available solar energy should more than equal your calculated power budget. For example, a 50W panel, used on a sunny, summer day may yield as much as 500 watt-hours. Be sure to add a percentage to account for cloudy days where panel output is low or reduced due to loss of sunlight.

Equipment selection The type of components you choose will determine the real cost of power to your site. A basic system comprises a solar panel, a controller (regulator) and a battery to store the collected energy. More complex systems have multiple battery banks, smart controllers, inverters, and even tracking mounts to maximize the energy collected per panel. Each of these components plays an important role in the final system cost.

Sites located within 508 north or south of the equator, with generally sunny weather, can use fixed panel arrays, which allow for a seasonal “tilt” factor. Some site owners I have worked with have elected to use tracking mounts for their panels. These motorized frames move during the day and point the panels toward the sun for maximum power collection. This is an important consideration if your site has limited space. The down side to this efficiency is the increased maintenance workload and the potential for wind or storm damage.

Speaking of damage, the panels you purchase should contain “bridging diodes” which allow the panel to produce electricity even if damaged or shadowed. This is an important consideration if your site is subject to storms or vandalism.

Controllers or regulators are required on all but the most basic of systems. Do not leave an unattended solar panel connected to a battery! Controllers act to regulate the voltage and current fed into your battery bank. This same controller can switch between battery banks, provide equalization voltages and, if the battery bank is “full,” divert power to an alternate load, such as a resistive heater. Some “smart” controllers will track battery bank energy input and output. Energy flow and rate information is valuable for maintaining the long-term health of your system. Advanced controllers will send this energy information back to your office via a telephone line, radio link or SCADA system as part of your overall site environmental monitoring.

Flooded-cell, lead-acid batteries generally represent the best value for storage capacity, ease of maintenance, useful life and size. Smaller installations, or those with extremely cold climates, will usually require a different battery type. Any flooded-cell, lead-acid battery will generate hydrogen gas as a byproduct of normal charging and the equalization process. Venting of this explosive gas is essential and required by the National Electric Code, OSHA and the National Building Code. A sealed, acid-proof container with outside venting will not only meet code requirements, but it will keep corrosive gasses away from equipment. If you are planning to perform the installation of a solar system yourself, contact a battery professional in your area prior to any purchase.

Koehler teaches at the University of Alaska, Anchorage.

Reference 1. “Powering the New Wave of Wireless Networks,” N.E. Stevens, Mobile Radio Technology, April, 1996

Applied Power, 1210 Homann Drive, SE, Lacy, WA 98503, 360-438-2110 Direct Power and Wind, 3455 A Princeton, NE, Albuquerque, NM, 87107-2027, 505-889-3585, [email protected] Photocomm, 7681 E. Gary Road, Scottsdale, AZ 85260-3469, 800-223-9580, [email protected] SES Solar Electric Specialties, P.O. Box 537, Willits, CA 95490-0537, 800-344-2003, www.solarelectric.com Siemens Solar Industries, 4650 Adohr Lane, Camarillo, CA 93012, 805-482-6800, www. solarpv.com http://syssrv9.nrel.gov/ National Renewable Energy Lab (basic information) www.alt-energy.com/aeetoc.htm (worksheets and more information) www.aral.com/enrenew.htm (the mother of all solar link pages!) www.sunelco.com/home.html (worksheets, new and used equipment) And visit the Jade Mountain site, www.jademountain.com/index.htm, for fun, if for no other reason. Their catalog seems to list everything from solar panels (new and used) to kerosene powered refrigerators to LED arrays that save energy.