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Fixed Wireless Equipment

When Solar Power Leaves You in the Dark

Renewable energy is great, but using it may force you to redesign your tower site.

[February 4, 2008]

On the ISP-Wireless list in January, LM posted a problem:

When there is sun it charges the batteries, at night we work off batteries. If no sun, we run the generators for a while, then run off batteries. So we need to have 12 hours of run time off batteries. We need our batteries to last a few years, five would benice. We need to provide about 500 to 700 watts per site.

[GM warned] "Batteries are very fussy how they are treated in a deep-cycle environment. Specific charge rates, voltages and times for different battery chemistries are critical to long battery life. . . The best strategy for your batteries is probably to run your bulk charging in the early morning off the generator, then top off (absorption phase) using solar during the rest of the day. A generator is a very expensive way to charge a battery once you get past the bulk phase."

[LN advised] "If you need to run 500 to 700 Watts over night and through low sun periods you will need a truly massive solar array to catch those batteries up. The thing that hurts them is a large depth of discharge and then running in a low charge state for too long. Most of the deep discharge batteries can only take deep discharges (more than 50% capacity) in the hundreds or low thousands of times, thus you have to try and keep them charged. A normal automotive battery is done after about 50 such cycles.

For charging we WAY over do it. I have a constant draw of only 10W per unit (4 radios) and have a maximum of 4 units per site, yet I have 800W of Solar arrays and still, during the period from November to January the batteries might need a catchup boost from a generator we keep on site.

Any time you are involved in Solar as the primary power source you have to reduce, reduce, and reduce some more for your power draw. We do our best so that under a totally overcast day, with barely a flicker form the panels we cover the equipment demand, so that we only lose charge during the night. Any sort of break in the clouds will have the panels sending 30A into the 24V batteries. I should note that if I let my batteries get dragged right down it will take over 36 hours of full on 30A to bring them back to topped up. I don't get nearly enough sun until late February, but from March to October, we can pretty much relax.

There is no magic to power, even Solar. You start with your load and then size your array and batteries based on that load and your Sun numbers for your area. I also have to point out the Solar numbers are an average and could be WAY out for your actual site. I have one site that is at just the right height and it always has a cloud hanging over it. We can be reaching for sunglasses and it is socked in. Luckily it is a single system (4 radios) with 800W of solar, which I routinely do so that I can expand at will.

So, others will chime in, but your system will either need to find less power hungry devices and/or a very large Solar array and some big batteries.

[CE agreed] "Up here in Alaska we have a number of public safety trunking radio sites that are completely off of the power grid and they are indeed a challenge. In the winter the huge solar panel arrays barely generate 65 watts at local noon on a clear day versus about 3,000 watts at the same time during the summer."

[DB concluded] "As others have said, the key thing is to reduce your power consumption. Nothing else will help, IMHO. 700W is just massive for solar power,especially at northern latitudes.

If you can find gear that performs multiple tasks it'll help with your power draw, also removing any DC-AC-DC conversions. This summer, I replaced : 2 wireless bridges, an Ethernet hub, a temp/voltage sensing box, a remote boot device, and the DC-DC converter needed to power the hub and the sensor. These were all replaced with a single embedded Linux board that takes 24V DC direct from the batteries, does its own temperatureand voltage monitoring, has its own watchdog timer, and has three radios plugged in, therefore removing the need for Ethernet and the hub at the site. Power consumption is much lower while performance and reliability are much better, and I couldn't be happier."

Asked what the Linux board was and where he got it, DB replied:

"http://www.gateworks.com/products/avila.php

I ordered the boards from Microcom : http://store.microcom.us/"

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