I am considering using solar to power several 3000’s on a 70’ tower, rather than trying to run an electrical line to the tower. Is this a practical approach in your opinion?
Depending where you are, I think you can use solar power. The ePMP 3000 AP draws 25 watts max continuous per AP. In my opinion, the best way to configure a solar arrangement is to charge a battery bank with the solar panels, then invert the power to 110v AC, and use PoE to run the radios. This also has the benefit of being able to run lights or test equipment at the site when you need to work there. I assume that you have a back haul that you need to power also, plus a switch or router, so you’ll have to add that power requirement in also. Be careful with your power calculations, and take lots of margin so your site can stay operating through days of bad weather, and in the shortest days of winter.
I am of a different mind from DaveClelland, Direct DC works just fine and I can prove it since all of my towers are setup this way now. Charging the batteries is not the problem, its the inverting to 120v just to switch-mode convert back to 29.8v. This actually wastes a lot of the battery run-time capacity unnecessarily . We used to do this but could not get enough storage to last the needed time (I like to sleep through the night and just get an email about a power issue that I dont have to deal with asap).
The 25w/AP is actually under stated, plan for 45w if you are in a cold zone, I am in Saskatchewan so I plan for 45w/AP just in case the heaters are needing to run (which does happen even after startup). If you are in a more temperate location, plan for 30w as these do use more power as you load them up and add functions (DPI). The idea is to do worst case usage not best or some middle ground.
We commonly use 24v to power our towers and buck or boost to meet the need, but I do suggest that to get optimal run time from the batteries that you choose the battery voltage based on most loads voltage with the exception being if your 48v loads equal 50% or more of the 24v loads then go 48v. That said, planning battery capacity for 5 days of no sun/almost no sun is a must and if you can get more AH of storage at 24v than at 48v, then go 24v and boost convert to 48v, the added current demands will be offset by the extra available capacity.
Solar sizing is based on a few factors but these are the important ones: total watts needed, total available solar time and if you have room to split the array into two or three smaller arrays each facing a different direction to get more solar time.
The idea to split the arrays into different directions is to get off the batteries as soon as possible, stay off them as long as possible and only take what is needed if needed. This will also increase the total solar time you can generate for and give longer run-times during poor solar weather. But adding a wind turbine to a site also helps offset the solar needs by providing opportunistic charging/ system support, you need only enough to carry your loads, but every little bit helps!
You need at least c/20 for your batteries capacity to recharge and the number of solar hours available on the shortest day of winter will give you your needed recharge capacity. Divide the battery AH by 20 to get the battery charge rate, but then divide by 2 to get the 50% recovery rate since you should not go below 50% DoD if possible. Now add the loads to this to ensure minimum charge plus loads current is met.
So a simple calc for one of my towers would be:
battery AH = 850
three arrays facing E-S-W = 6.85hrs on shortest day
battery recharge rate (50%AH/20) = 21.25
loads (4xAP@45w/AP, Switch+router=200w, 100w for PTP,cameras and monitoring) = 480w, plan 500w = 20.84a @ 24v
need 42.1A @24v, battery charges at 29.6v = 1246w, divide by 2 arrays (only two can work at the same time due to light angles), need 623w per array, or 2x 380w panels (yes you could use 320w, but the higher the wattage, the higher the VoC, which is more important than panel amps) in series per sub-array with each sub-array paralleled to each other before the charge controller. Assuming 24v panels, the VoC will be approx 40v. this gives an array VoC of 80v with a peak VoC of 96v, or 2.5 - 3 times the battery voltage. The controller will turn this extra voltage into available current that will charge your batteries. On peak periods, you will see more than 1500w but that will not be long enough to count on.
Choose your solar charge controller, dont go cheap here, get a good MPPT controller with logging. We have both MidniteSolar and MorningStar and are going MorningStar in the future for all of our charge controller needs. Victron is also good, but does have its quirks so please research and choose based on your needs.
We use Packetflux’s Sitemonitor and Gigabit Sync injectors with a Syncbox-Jr for all our setups as this give both tower shack security and system monitoring plus can automatically start a remote start generator if its the 2-wire type, add more modules to get more functions or inputs/outputs. We also use the 8 port power injector to provide power for IP cameras for surveillance.
There are other options available through other companies, tycon, but best bag for the buck we found was Packetflux.
We angle our panels to make the most power in the winter (the perpendicular solar angle during the majority of winter, is more vertical than 45deg) and are not optimal for summer, but we get 18hrs of solar time in the summer so the trade off it power vs time.
Most switches and routers are actually 12v DC inside with a few exceptions and notable inclusions being POE switches usually need 12 and 48v common negative. A simple buck converter for 12v and a boost converter for 48v keeps everything happy.
Some of this will not make immediate sense and will be counter intuitive, but if you dig a bit into how solar systems work, the differing goals between off-grid (the application you want) vs grid-tied (what you normally want for a house) and the storage capacity and under charging of the storage batteries (deficit charging) you will start to see how this actually works.
If you need help designing a setup, PM me and I can help get you started in the right direction.
This is definitely the direct DC connect manual! Thank you, Douglas!
not exactly the manual but definitely a fair synopsis that should get anyone with some electronics experience moving in the right direction. Of course there is way more to consider and each site seems to be slightly different, so I engineer each site to its intended use with some extra for future needs. Some of this planning is straight forward like adding for 3x ptp links despite only one being active on setup, others is not intuitive because it depends on total possible user counts per site. I have sites that will only service 60 customers, so 4 APs is not needed, but another site can see over 300 so planning for 8x APs is important. It is good to start small and add later, but also plan for smaller sectors and horn antenna use for better gain and better SNR/sector. Adding to a solar system later is very expensive and will take up more resources, better to plan for it ahead of time and have the capacity in the power budget, especially when one is buying $6000 850AH batteries that have a 2 month lead time and each of the three batteries weight over 300lbs!
Doug and Dave, thank you for the [manual :>) ]. I am in Florida so cold is not a concern, just the usual daytime considerations during the winter months. I am thinking the same for our backhaul system and using solar there also, although 110V will probably be available there. TBC.
if you have mains AC, you can still run a DC site, just use an appropriately sized DC power supply to provide the power needed to maintain the batteries and recharge them after an outage, I have two such sites right now and the telephone system still uses this model to keep land lines and cell towers active. It is very common to run a DC system even connected to AC mains. This also removes issues with adding generators and possible other power sources since its all DC so will share the load reasonably equally. Servers and network equipment do have DC options available.
As for location considerations, Florida sites can still take advantage of east and west (even south-east/south-west) facing sub-arrays for winter months, to increase the amount of solar time available. Again the idea is to minimize on-battery time and maximize recharge opportunity time.