The Bright Side of Solar Power

Every day the sun generates more energy than the planet's 5.9 billion people would consume in 27 years, according to the National Renewable Energy Lab, a branch of the Department of Energy. Talk to any engineer, and he will tell you that solar power is a cleaner and more plentiful resource than most grid power. Yet carriers have not even begun to tap into this resource for site power.

Why isn't solar used more widely? Traditionally, cost and doubts about reliability have prevented carriers from considering it seriously. But with gradual decreases in the price of solar energy and tax incentives for those who use it, the cost could be comparable to grid power in the future. Likewise, when designed properly, a solar-powered site can be very reliable.

COST CONCERNS Engineers always have favored solar power for remote cell sites, but its high price has kept carriers from using it in places where grid power is accessible. With a gradual but continual increase in the cost of grid power and some small victories in the solar arena, you may want to take another look at what makes solar a bright idea for the future.

Solar power is clean, renewable and reliable, according to Justin Mizany, Solar Depot marketing manager. Soon it will be economical, too. Currently, a single solar-powered kilowatt hour costs about 13.6 cents. When regular maintenance is factored in, the price goes up to 20 cents per kilowatt hour, which is not unreasonable compared with traditional power, which costs 12 cents to 22 cents, depending on the state your site is in. But these solar-cost projections are over a site's average life, up to 30 years on average with proper maintenance. Carriers don't want to wait 30 years to see a return on their investments. When viewed over a more practical period of three years, solar costs about 10 times more than grid power.

Part of what makes solar power so expensive is the cost of materials. Solar photovoltaic (PV) panels are made of high-grade silicon, which sells for $80 to $120 a kilogram. But with silicon refining plants getting on-line and more production facilities cropping up, the price of materials is expected to decrease.

Mizany also said the price of grid power and solar power could be on a level playing field in five to 10 years with an increased demand for traditional electricity and not enough new power plants being built.

One way carriers could save money with solar power is by taking advantage of many states' solar tax incentives. For instance, California and Hawaii give tax credits to residents and businesses that use solar power. Other savings can come in the form of rebates. In California, legislators recently passed the Debt Metering Law, which requires Pacific Gas & Electric and other local public utilities to purchase power back from individuals using solar or alternative power. A rebate program from the California Energy Commission gives those using solar power a cash rebate for selling power back to the grid.

Today solar is used to power telecommunications sites only in extreme rural areas, and sometimes maintenance is done remotely through computers. Here again is where solar sites can be expensive. However, Mike Brennan, Northern Power vice president of wireless power, said that remotely monitored systems are cost-effective when compared with the cost of traveling to a site that only can be reached by helicopter or on foot.

Also, in most cases, solar-powered cell sites don't include the added expense of air-conditioned shelters, said Bob Nuner, Northern Power marketing representative.

RELIABILITY Rural Cellular does not use solar power because of its cost, said Kyle Gruis, Rural Cellular engineering manager. He also believes he could not rely on it, a common fear among engineers.

Northern Power's Brennan said with an appropriately sized battery bank, a site with little sun still could receive and store enough solar energy to power a cell site reliably.

Most solar vendors agree that the key to any solar site is a good design. You must design a system specifically for your site, according to its location and load. Engineers always design a system to be larger than what a network will need during the poorest solar-generating months of winter based on average historical data for the area. It acts as system insurance.

Next, the amount of power needed during sustained inclement weather is assessed. Every solar-system engineer must ask how long it will take to recharge the system from its lowest possible state of charge (SoC) to an acceptable level. Also, the solar array must be able to recharge the battery while continuing to serve the system load.

Brennan's engineers typically size a system so that it can recharge the battery from 20% SoC up to 90% within 30 days, while continuing to service the load full time. However, each carrier is able to specify what it wants in SoC turnaround.

Tom Felter, GTE Wireless systems technician, maintains and monitors a solar-powered repeater in the California desert.

"If you've got good battery strength, solar is quite reliable," he said. "I've had no problems."

Felter's repeater has a load of about 12A at 24V and transmits at 5W ARP for four carriers. It is a small application, but solar is not limited to small applications. Sites can be as big or as little as the situation dictates. As with all things, the bigger the site, the higher the costs.

Some carriers in remote locations have bought hybrid systems to offset reliability issues. To make sure there is always enough power to a site, you can use systems that use both PVs and fossil-fuel-driven generators. Hybrids often are able to offer up to 1,700W without additional battery banks because of the added power source.

With a hybrid solution, you can completely recharge your battery every day with a backup system such as fossil fuel or wind-generated power. Instead of programming the generator to start only when the battery reaches a low SoC, many hybrids run the generator automatically every day to recharge the battery 100%.

In a Solar Depot cell site off the coast of California, the PV panels received plenty of sun during the day. Because the area commonly had high wind gusts at night, a hybrid system used the wind for power during the night. With two alternative sources at its disposal, the site was nearly always at optimum SoC.

MORE IMPROVEMENTS Besides cost and reliability, carriers have had other complaints about solar power. Some say solar-powered sites require extra attention. However, Felter said maintenance on GTE Wireless' solar repeater is no more or less difficult than with traditionally powered sites. Because it involves no moving parts, solar actually can reduce maintenance costs.

Another concern is the room that is required for bulky solar panels. Depending on a site's load, its footprint could be quite large and costly. Gary Jones, Applied Power industrial sales representative, said today's radios use less power, which in turn means solar-powered sites need fewer PV panels than they have in the past.

Other improvements anticipated for solar are newer, less-expensive materials and the use of more colors of the spectrum that could increase efficiency by three or four times. But, solar power will have to wait a little longer for these breakthroughs.

The environmental benefits of using solar power are obvious, said Solar Depot's Mizany. Currently, more than 50% of all electricity in the United States comes from coal. Unlike coal and oil, solar power causes no pollution and will never run out.

Rural Cellular's Gruis said as an engineer he wants the same thing from power solutions that end-users want from service: long life, low cost, small size and reliability. Solar is beginning to meet these requirements. If costs continue to decline, solar power may shine in the future.

When the sun is down, a solar-powered site's battery bank discharges power. Once the sun rises, the battery discharges less power as more "sun hours" (the standard used to size solar arrays) become available. Usually at midday the photovoltaic array produces more energy than the network needs. This is when sun hours convert into energy and begin to recharge the battery. The battery continues to charge through daylight hours, then discharges power at nightfall. The battery bank is a critical part of the solar-powered cell site. It stores all excess solar energy accumulated during the day and discharges it, whether it's overcast or nighttime.

The battery bank often is referred to in "days of autonomy," which means how long the system can continue to support the network when no solar energy is produced. For Northern's 50W to 400W Telesol system, for example, a minimum of five days of system autonomy is specified. Carriers can request more or less depending on their needs.