Solar Wifi Station

System · September 2009

Hi,
I am a newbie here..

Hopefully someone will be able to answer my questions. Im putting in a wireless internet antenna on a hill on my property so that I can get internet where I am. I live in central Texas. The antenna radio pulls around 1 amp or less and the point to point wireless antenna / radio pulls about the same, so Im going with a 3 amps just to be safe.

What would be the appropriate solar panel wattage that I would need?
What batteries and other equipment/parts would I need?

I calculated that I would be on the internet approx. 6-8 hours a day especially in the evening and night and on the weekends probably 8 hours a day. I have looked all over for someone that has done something similar and couldnt find very much. Any help would be appreciated!

mike95490 · September 2009

Re: Solar Wifi Station

1) how do you instruct the remote radio, when to be on, and off, or do you want to run it 24/7 ?

BB. · September 2009

Re: Solar Wifi Station

Armed101,

First we need to know your loads... You said 3 amps--I am going to guess that is at 12 VDC.

You said you want to use it 8 hours per day... You can use a digital timer to turn off the power, or possibly some remote control.

For the moment, lets assume 8 hours per day at 3 amps--if you want to use it 8 hours per day, just multiply the solar panel rating and the battery capacity by 3.

To start, a nice place is the PV Watts program. Put in 1 kW (1,000 watts) as the panel size (minimum for program). Assume 0.61 derating (off-grid power with flooded cell battery, no inverter). And fixed array mounted at Latitude. I will guess Abilene is close to you.

The results:

[FONT=Fixedsys]Results

Month     
Solar Radiation (kWh/m2/day)     
AC Energy (kWh)     
Energy Value ($ @ 9.7 ¢/kWh)

1      4.80           89        8.63   
2      5.44           90        8.73   
3      6.06          106       10.28   
4      6.43          108       10.48   
5      6.12          105       10.19   
6      6.17           99        9.60   
7      6.29          102        9.89   
8      6.22          101        9.80   
9      5.72           91        8.83   
10     6.10          106       10.28   
11     5.24           91        8.83   
12     4.57           85        8.25  
====================================== 
Year 5.76 hrs of sun  1172kWhr $113.68   [/FONT]

The minimum collection is 85 kWhrs per month in December per 1,000 watts of solar panel. Your calculations (keeping things simple):

85kWhrs / 30 days per month = 2.833 kWhrs per day or 2,833 Watt*Hours per day in December per 1,000 watts of panels

You need:

12 volts * 3 amps * 8 hours per day = 288 Watt*Hours per day

Solar panel wise--you will need (remember, the system derating of 0.61 gives us pretty conservative results for an "average" December):

288 WH per day / 2,833 WH per 1,000W of panel = 102 watts of solar panel

Battery size--usually we recommend 3 days of "no sun" and 50% maximum discharge (for longer battery life). Battery rating is in Amp*Hours (look for 20 Hour Rating "deep cycle" storage battery, not a "car/marine battery"):

288 WH * 1/12 volts * 3 days * 1/0.50 = 144 Amp*Hour 12 volt battery

That will give you some idea of the system capacity.

You can visit our host (Northern Arizona Wind & Sun) for their recommended products (they stand by the brands they sell). But you are welcome to purchase anywhere (and things like batteries, you may want to purchase locally anyway).

For this small of system--you can probably look at getting a PWM solar charge controller (MPPT is nice--but expensive and better for larger systems---200 watts or 400 watts of solar panels minimum depending on controller size).

A big issue is the system needs to not fully discharge the battery bank (below 20% state of charge, but ideally not below 50% state of charge for longer battery life--see battery FAQ for details why). So--you will either have to keep an eye on the system or set up some sort of disconnect so that the batteries are protected.

A Digital Timer you can possibly use is this one.

You should also have hydrometer to measure specific gravity on the battery bank (if flooded cell type) once in a while to make sure everything is OK.

If you go with AGM type batteries (sealed)--you cannot use a hydrometer and must use either a DVM or, ideally, a Battery Monitor... However, a Battery Monitor may be too expensive for you to justify if you use a small/cheap battery bank (although--a BM is very highly recommended--much more accurate and easier to use than anything else).

Questions?

-Bill

System2 · September 2009

Re: Solar Wifi Station

Thanks for answering. I did some thinking and i believe that my total system amps are going to be 2.5 amps at 12 volts. In my calculations I came up with 12 hrs max that I would be using it per day so.... Doing the math comes out to be 360Watt Hours per day. Im close to austin, tried to use the PV Watts program and it seems to be way over my head for me to use. Im really confused about it.... The 12 hours might be way overkill on this system as I might use it sometimes for around 6 - 8 hrs or so. Maybe I should start out small and then work up to a bigger system if need be... What do you think? I think that I will definitly be able to save power if I shut the system down when not in use, which I think there is something out on the market that will allow me to shut the power to the radios off wirelessly, which will be nice since If I have power cycle everything if the system is down I dont have to Hike up the Hill 1500 ft to the tower... HeHe

So if I do the math again with 8 hrs im at 240 Watt Hours per day... Also you stated that I might be able to get a power monitor to monitor the power in the batteries, I wonder if there is something wirelessly that will relay this data to the house, sure would be nice! I did do some research on my exact location and found this data on the RREX Renewable Resource Energy Explorer...

Concentrating Solar Collectors: Direct Normal Irradiance (DNI): 4.82 kWh/m sq
Horizontal Flat Plate Collectors: Global Horizontal Irradiance (GHI): 4.82 kWh/m sq
Tilted Flat Plate Collectors: Latitude Tilt Irradiance (TILT): 5.18 kWh/m sq
Wind Speed at 50m Above Ground: 4.91 m/s
Air Temperature at 10m Above Ground: 17.84 degree C
Earth Skin Temperature: 18.36 degree C
Cooling Degree Days Above 10 degrees Celcius: 3189 degree days
Heating Degree Days Below 18 degrees Celcius: 1157 degree days
Atmospheric Pressure: 97.54 kPa
Relative Humidity: 71.14 %

Is any of this relative to inputing into the PV WATTS Program?
Im going to need a way to hook up the radios what type of converter to make sure the proper amps are getting to the devices?
So it is very important to have a large Amp Hour battery capacity and the solar panels have to be large enought to top the batteries off at least every 3 days in the winter? So basically in the summer I wouldnt run into a issue but during the winter I might have a problem if my solar panels arent big enough?

I am starting to understand all of this I believe just need a little bit more help...
Sorry to be such a Noob but all of this stuff is new to me and im learning. Im sure that once I get it figured out I will be able to tackle any type of solar project a whole lot easier... Hopefully!

BB. · September 2009

Re: Solar Wifi Station

Armed101 wrote: »

...Maybe I should start out small and then work up to a bigger system if need be... What do you think?

It is difficult to scale the batteries... You would be better off sizing them for your needs (or a bit larger) and live with them for 3-6 years. And replace them with larger/smaller batteries then.

I think that I will definitly be able to save power if I shut the system down when not in use, which I think there is something out on the market that will allow me to shut the power to the radios off wirelessly, which will be nice since If I have power cycle everything if the system is down I dont have to Hike up the Hill 1500 ft to the tower... HeHe

Yes, conservation really saves money with solar RE systems--they are a pretty expensive source of power.

So if I do the math again with 8 hrs im at 240 Watt Hours per day... Also you stated that I might be able to get a power monitor to monitor the power in the batteries, I wonder if there is something wirelessly that will relay this data to the house, sure would be nice!

Some battery monitors have a digital interface--but that will cost you more power to encode and send.

Some battery monitors also have programmable alarm contacts. Set the alarm contact to turn on at 50% capacity... It could turn off your repeater (to save energy) or turn on a bright RED blinking LED pointing at your home (for example).

I did do some research on my exact location and found this data on the RREX Renewable Resource Energy Explorer...
...
Is any of this relative to inputing into the PV WATTS Program?

Not really, the PV Watts program has better data internally to predict the power generated for your location.

Im going to need a way to hook up the radios what type of converter to make sure the proper amps are getting to the devices?

If the repeater is 12 volts--you may not need anything. Converters do "waste" power--so if you can avoid them--it would be better.

However, you have to be careful... A solar RE battery system can have the voltage range from 10.5 to 15.5 volts. Many 12 volt devices are designed for cars where the voltage ranges from 11-14.2 volts or so... The 15+ volts has been known to blow many "car type" 12 volt devices.

If you need 120 VAC -- There are some very nice AC inverters out there that will work fine. You would have to get slightly more solar panels to make up for the inverter losses (typically 85% efficient or so).

So it is very important to have a large Amp Hour battery capacity and the solar panels have to be large enought to top the batteries off at least every 3 days in the winter? So basically in the summer I wouldnt run into a issue but during the winter I might have a problem if my solar panels arent big enough?

You got it! You are a slave to the weather. Some winters I have lots of clear skies and good power generation. Other winters, I have a week of "black skies" and little solar power generation.

The typical flooded cell Lead Acid battery does not like to sit for more than a few hours or a day at below ~75% state of charge. The sulfates begin to harden and the battery capacity is reduced (see Battery FAQ).

I am starting to understand all of this I believe just need a little bit more help...
Sorry to be such a Noob but all of this stuff is new to me and im learning. Im sure that once I get it figured out I will be able to tackle any type of solar project a whole lot easier... Hopefully!

We all started at the same place.

-Bill

dwh · September 2009

Re: Solar Wifi Station

BB. wrote: »

If the repeater is 12 volts--you may not need anything.

-Bill

It won't be. He's going to need an inverter. The ISP's unit will likely be something like this:

http://www.motorola.com/Business/US-EN/Business+Product+and+Services/Wireless+Broadband+Networks/Point-to-Multipoint+Networks/Licensed+Point-to-Multipoint+Solutions/PMP+49400_US-EN?vgnextoid=12ea4779ec6cf110VgnVCM1000008806b00aRCRD

The customer owned repeater back to the house...well if he uses something decent it will be like this (which I've used many times to shoot long links and really like a LOT):

http://www.netgate.com/product_info.php?cPath=31_61&products_id=564

Either way, both units will be powered by 802.3af Power-over-Ethernet, which is 15w (max) @ 48vDC. They use PoE injectors at the bottom so that only a single Ethernet cable (per unit) has to run up the pole. The little standalone PoE injectors normally use a wall-wart for power and look something like this:

http://www.demarctech.com/products/reliawave-poe/dt-poe-01.html

I very much doubt the ISP installer is going to be happy if he sees some home-made 12v-48v converter/PoE injector. Mostly likely he'll just demand 120v and that's that.

http://en.wikipedia.org/wiki/Power_over_Ethernet

[It's worth noting that 802.3at was ratified a couple of weeks ago - that bumps the max wattage up to 25w @ 48vDC over a single Ethernet pair (50w using both spare pairs).]

vcallaway · November 2009

Re: Solar Wifi Station

Take a look at this device.

I have something similar setup at work. It is used with a wifi access point in bridge mode. We use it to link our office with the guard shack.

I don't remember the specs, it was setup quite some time ago. I think it uses an 80 watt panel and two 12v SLA batteries. I'm not the one that set it up so I'm not sure.

I was asked to look into devices like this for setting up wireless cameras. I'm still working on the camera end so I don't know what our power needs are yet.

dwh · November 2009

Re: Solar Wifi Station

Nifty.

"Solar Input Voltage
12V - 36Vmax @ 5A max
(up to 85W 12V or 170W 24V solar panel)"

Not bad.

"Maintenance Voltage
13.8+/-0.3V
27.3 +/- 0.5V"

Make sure you use a battery that likes that voltage.

Solar Wifi Station

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