Need Advice on Solar Panel System to Power BTS Unit

[MarielVox]

Hey Everyone,

I am trying to work out a solar panel system to power a remote GSM site. I'm new to this forum and would really appreciate advice from some experts. I am an engineer but I never had the chance to study solar panels and so a lot of this is new to me, but I do have a general understanding of different systems.

I need to hook up solar panels to a battery bank that will power a BTS unit. I'm looking for a 48V system and I'm only looking for panels since my guy has a battery and rectifier already. However, I'm pretty sure we also need a charge controller as well, and I'm not sure which model would work best in this situation. The system needs to be installed in a pretty remote area. If we're looking to generate 3-4kWH, will we need backup for power during the night? I think we would probably want a backup option regardless. And, I think diesel is a difficult fuel to transport to the station location. Are there other types of backup power available that would work in this situation?

Any advice on different parts, makes or models would be greatly appreciated.

Thanks for your help!

Cheers,
Mariel

[Mike90250]

diesel is generally, the densest fuel. Solar PV is (pardon the pun) very light.

So, you would size the system by calculating your power needed in a 24 hr period. ____KW for 24 hours = _____KWh

How many cloudy days do you want to store power for ? 2? 3? 5?

More than 2, and you get into some huge battery arrays. I'd prefer to steer you to a 1 cloudy day, and the generator starts on the 2nd cloudy day. Exercises the genset, and you get by with a small battery bank.

What's security like at the site ? Armed guards?, city police ? night vision cam?
You will have $30,000 or so, of fragile glass PV panels hanging out. They can be fitted into a trailer or just carried away one at a time.

What is a BTS unit ?
Do you need 48VDC, or AC 120V, 240V ?

As the batteries charge and wane, the voltage will vary between 445V and 60V, so if you direct wire any thing, it should be able to withstand 65VDC

[MarielVox]

We're working with on average about 7 hours of daylight per day annually. We need this thing to run constantly because a BTS is a component of a cellphone network that is on 24/7. I'm guessing that means a big battery supply. I don't think security is an issue - it's more just getting this thing up and running.

We're looking for 48V DC and 3-4kWh according to the request for this stuff.

[Sunking]

I have designed several hundred of them for remote GSM and CDMA sites. What do you need to know?

It is extremely expensive. You are looking at a cost depending on Location from $75,000 on the low end up to $200,000 on the high end if outside the dessert southwest with a multi-carrier radio.

For back up power there are only two options: diesel and LP. Diesel is pretty much only used for gensets of 75 Kva and larger. You will be looking at something in the ball park of 5 to 15 Kva genset using LP

[MarielVox]

We are trying to put together a system for a client thats basically a 48V DC solar panel set connected to a switch, which is connected to a battery bank, rectifier and BTS. I guess our client already has a battery and a rectifier and just needs panels. They're looking for a 3-4kWh system but I'm pretty sure they don't know a lot of specifics. I think we might need a charge controller but I'm not sure, and I'm not exactly sure what make/model of solar panel is going to be reliable and work in the system. I dont think it is a problem to be super expensive, I just need to figure out how to put together this system.

[Sunking]

We're working with on average about 7 hours of daylight per day annually. We need this thing to run constantly because a BTS is a component of a cellphone network that is on 24/7. I'm guessing that means a big battery supply. I don't think security is an issue - it's more just getting this thing up and running.

We're looking for 48V DC and 3-4kWh according to the request for this stuff.

You cannot use your average solar insolation for a battery system. You have to use December insolation Sun Hours, or whatever your areas shortest insolation is. For example if you use an average like 5 hours, and your winter insolation is 3 hours, you will destroy the batteries or be running the generator every day to make up for the shortage. Not good.

You first need to nail down exactly how many watt hours the radio uses in a day. All the ones i work with are 24 volts and a single carrier, 3 sector radio consumes and average of 300 watts. So 300 watts x 24 hours = 7200 watt hours. So to generate 7200 usable watt hours your panels must generate 1.5 x 7200 wh = 10,800 wh minimum every day. So if your winter insolation is 3 hours your panel wattage has to be at least 10,800 wh / 3 h = 3600 watts.

For the battery in a very remote location you want a minimum 5 day reserve capacity which only gives you 2.5 days of real capacity before you go beyond the 50% DOD rate which you never want to exceed So if using a 24 volt system you need a minimum reserve capacity of 5 days x 10,800 wh = 54,000 wh or 54 Kwh. At 24 volts that is 54,000 wh / 24 volts = 2250 AH battery stack.

But here is the deal, this is a mission critical site, and you may want a 10 day reserve time, so double the battery size.

As for the generator size, you need a special monitor circuit for the batteries. So when they reach 50% DOD the generator comes on to fully recharge the batteries in 8 to 10 hours run time. So if you have 54 Kwh batteries at 50% DOD you need to generate 40.5 Kwh in 8 hours so you would need a 5 Kva LP generator to do the job with a 200 amp commercial grade rectifier.

You will also need a MPPT solar charge controller. The size is based on panel wattage and battery voltage. With the above example using 3600 watt solar panel and 24 volt battery system you need a minimum 3600 watts / 24 volts = 150 amps. So you are looking at two 80 amp charge controllers with 1800 watts input from the panels. In other words 2 1800 watt solar panel systems feeding a common battery. At 48 volts just 1 80 amps charge controller.

[MarielVox]

Interesting. Say they wanted to use solar power during the day and then switch back to the mains at night? They already have a base station established, they just need to switch over to solar to power some of it.

[Sunking]

If they have commercial power, it is insane to even consider a solar battery system.

[MarielVox]

I don't think it is commercial or at least readily available because the location is so remote that they have like donkeys carrying diesel up to power this station. Whether that is only when it fails or not I am not sure, but if diesel is only for backup, I think it fails a lot. I am going to try to get on the phone with their engineer tomorrow and squeeze a little more info out of him. Because I know they want solar, but based on your response, I think I need to getting a better understanding from their end. Are there any specifics in particular you think I should be sure to ask him?

[Sunking]

Well yes you need to know:

• How many watt hours per day the site needs so the panel, battery, and generator calculations can be done.
• How many day reserve time do the batteries need with 5 minimum.
• Are the batteries they are going to use are they standard Telecom stationary batteries or true deep cycle application batteries. This one is important because stand stationary telecom batteries are designed and made for emergency back up only. At best maybe 100 to 300 cycles in them (less than year is pressed into cycle service) This application requires a true deep cycle battery like a Surrette 2 KS33P 2 volt 1766 AH cells. These are true 10 year cells with 10,000 cycles if properly cared for.

Lastly is logistics. If no roads for heavy equipment is available, you will need helicopter lifts to get the heavy equipment and personnel in. Donkeys will not work with generators, equipment cabinets, fuel tanks, and 4000 pound battery stacks.

I hope you know what you are getting yourself into.

[Mike90250]

I don't think it is commercial or at least readily available because the location is so remote that they have like donkeys carrying diesel up to power this station. Whether that is only when it fails or not I am not sure, but if diesel is only for backup, I think it fails a lot. I am going to try to get on the phone with their engineer tomorrow and squeeze a little more info out of him. Because I know they want solar, but based on your response, I think I need to getting a better understanding from their end. Are there any specifics in particular you think I should be sure to ask him?

any feedback yet ?

At the very least, adding solar, at $6 watt, can strech the diesel fuel and save the donkeys !

[matamba]

powering gsm bts with solar panels and inverter for 24hrs operation

please i'll like to know how possible it will be for me to power a gsm bts with solar panels connected to a 15kva inverter for 24hrs operation. please ill want the normal 2-3 ac to function with it as well for 24hrs. please my email address is matamba2@googlemail.com

[Sunking]

please i'll like to know how possible it will be for me to power a gsm bts with solar panels connected to a 15kva inverter for 24hrs operation. please ill want the normal 2-3 ac to function with it as well for 24hrs. please my email address is matamba2@googlemail.com

I assume you mean 2 to 3 ton of air conditioning? It can be done if money is no object, but I have never seen or heard of a tower shelter building being ran off solar with air conditioning. All BTS sites I have ever seen with solar power and worked with are outdoor cabinets.

Shelter with AC the AC is on 50% of the time in winter. With a 3 Sun Hour day at 2 tons of AC with a 12 SEER would require a 6000 watt solar panel just to run the AC unit, plus another 4000 to 6000 for the radio. I don't think even ATT or VZW would pay that kind of money for a remote cell site. I could be wrong, but I have worked for these guys and I do not think they would go for it.