Solar Powered Site App Notes

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Solar powering of equipment is easily done but it is not cheap. Solar powering of a site can easily double the expense of building the site.


Step 1 - Determine Power Consumption

A minimal installation will require one backhaul and one access point. With each drawing approx 8 watts, the total continual power consumption will be in excess of 16 watts(24/7/365). Rounding up is a safe way to approach all solar design.

Round up the power consumption to a "more than" realistic safe level. We will use 20 watts for this example.

Step 2 - Determine Size of Solar Panel

Next decision is how much panel to buy. A bare bare bare minimum of 10 times the continual power consumption is required. Remember that during the winter in the Northern hemisphere, the higher latitudes have shorter days. On December 21, in Northern Utah, the sun hits the panels at about 9:30 AM and leaves them around 4:30 PM. That leaves 7 hours of charging, but when the total energy actually produced is graphed, a half sine wave curve is produced showing very little energy at the start and end of the charging day. In reality, (non tracking) panels are only producing fully rated power for an hour or two during the middle of the day.

Most people that have chosen to use the 10X factor for telecom applications go back and add panels. Only in very sunny locations without snow and frost will this work.

If you don't want to be out in the snow with a generator during extended cloudy periods, 20 times is a much more safe figure.

In this example, a 20x factor will be chosen. That yields a 400 watt panel array. Normally solar panels can be found at around $2.50watt or less (3/30/2011). So, $1000 minimum will be spent on the panels.

Step 3 - Determine How Much Energy to Store

Next decision is how much battery to buy. How may days can you go without any solar charging? That depends on climate, weather, latitude, seasonal patterns, topography etc. One week of autonomous operation is a minimum. 20 watts continual draw times 24 hours = 480 watt hours per day. Again, round up to 500 watt hours/day times 7 days is 3500 watt hours you need to store. Storage batteries can be had for as little as 20 cents per watt hour but 30 cents is a mor common figure. So the minimum cost for this battery will be around $1050.

If you have to scrimp, do not scrimp on the battery. A rule of thumb is that half your money will be spent on the panel and half on the battery bank. The 10 X system outlined above will work OK only in very sunny climates with light winter and cloudy seasons.

In the Rocky Mountain region of the U.S. 20 times power and 2 weeks of battery is more of a practical minimum. Even at that, there may be one day a winter where an aux generator may be needed to top off the battery.

Step 4 - Select System Voltage

Note that nothing has been said about voltage. That is because the design and implementation of a PV solar powering system does not depend on voltage. You only need to define the amount of POWER you need to produce and the amount of ENERGY you need to store. Once you have defined that then you simply choose the best voltage for your system. Motorola Canopy products have the lowest power consumption at approx 24 volts. They will operate over a voltage range of about 10 to 32 volts but the lowest consumption comes around 24-26 volts. This is a very convenient voltage for solar systems as most are built in increments of 12 volts. 12,24,36,48 etc. Controllers are widely available for these voltages.

Divide the watt hours you want to store by the system voltage to determine the amp hours of the battery. 3500/24=aH

Telemetry to monitor the voltage of the system is a must. The RMS BND product does not consume much power and will help you to keep an eye on the system to predict an outage. The only true downside is that an ethernet hub will have to be added to allow the connection of the telemetry board. That will consume a little power.

Step 5 - Select Battery Chemistry

Battery selection is important in that battery technology varies greatly. Any battery with a liquid electrolyte will freeze in the cold weather when discharged. Sometimes a deep discharge is unavoidable. Deep cycle lead acid batts are not very good in the cold weather. Auto batteries will not take the daily nighttime discharge cycles for long. Avoid gel cells totally. EnerSys Genesis batts are designed for operation when frozen.

Flooded batts generally have an operating temperature range of 30F to 130F with greatly reduced life for upper temps. They are generally good for 3500 cycles if only discharged 30% or less. So they are good for 10 years if you can keep them temperature controlled. Unfortunately solar sites don't have the power for air conditioning and heating so generally flooded cells are out. That includes sail boat batts, golf cart batts and car batts.

Ni-Cads are worse for the number of charge/discharge cycles. Typically 1000-2000 cycles at a 20% depth of discharge. They also need to be in a temperature controlled environment.

Valve Regulated Lead Acid (VLRA) Absorbed Glass Mat (AGM) take temperature extremes much better than flooded cells or Ni-Cads. Generally they will work to 5F to 120F. Upper temps are still a problem but they continue to work well when most other technologies are frozen solid. They are not stellar performers as to the number of cycles they can do. Similar to Ni-Cad in that respect.

Step 6 - Charge Controller

Choose a high quality charge controller. This will make the most of the energy available and ensure the batteries are as charged as possible whenever there is any solar energy available.

Step 7 - Talk to your Accountant

Panels age, especially amorphous panels. Monocrystaline solar panels will produce full rated power for more than 10 years. Generally they can be counted on for good service out to 20 years. Polycrystaline solar panels will age a little faster, 15 years is a good rule of thumb. Amorphous start to age as soon as you start to use them. At the 10 year mark they may only be producing 50% of the original value.

Likewise, batteries age fast too. In a non temperature controlled environment, count on changing them every 5 years. Heat is the enemy. If you can keep them cool, you might get a few more years.

In any event, have your accountant expense the whole system if you can. At a bare minimum try to convince them that a 5 year depreciation schedule is realistic. Start a sinking fund to replace the batts in 5 years and the panels in 10 years.

(Spoiler Alert, mild political opinion follows): Contrary to what environmental pundits are wont to claim, PV solar is not a good choice for being green. There are hazardous and toxic components and chemicals used in the production of PV products and batteries. Moreover, they produce a significant waste stream due to their relatively short life cycle. They cannot be recycled (the PV panels that is). Furthermore, dollar for dollar, they produce a pittance of power compared to other technologies. Wind is much more friendlier but is not nearly as reliable and in some areas is wholly impractical for powering a Canopy site.

Turnkey Solar Power Systems

If you would rather leave the design and building of the solar powering systems to others, several companies are creating Motorola Canopy specific systems. Sunwize (source of photo at head of this page)

--Genesis Extreme Environment Batteries

--Xantrex Solar Charge Controller

Typical Site Power loads:

I am running the following:

  • 1x RB532
  • 1x RB333
  • 1x XR5
  • 2x XR2
  • 1x TR5a
  • 1x RB192
  • 1x SuperRMS2

= .3710A @ 12VDC or 4.452W continuous.