Helping a museum set up an off-grid system to power a couple of sculptures

Rich@prg-llc.com · February 2015

Hello!

I know enough about solar to be dangerous (have taken a week-long course and set up a couple of little systems over the years.) I'm on an environmental committee and so was recently asked to help put together the equipment to power a couple of sculptures. Once I price a system, an engineer will draw it and produce stamped drawings. But I want to ask a few questions to get the most economical system rather than turning the engineer over to the problem to spec whatever he feels like--they don't seem to care about pricing too much and the budget is tight!

BTW--if any professionals want to give me some assistance, they are happy to provide a letter of donation/appreciation and recognition in the misc. materials.....

Basically, I need to power 540 watts, 45 amps, of lights and need to have the capacity to keep them on for six hours (really, for two nights since they will be left on for 3 hours)

I have access to 4 of these 175 watt 24 volt panels, donated so we will use them: http://africanenergy.com/new/wp-cont.../sharp-175.pdf

Biggest challenge, the panels are going to be 50 foot away on top of a pergola, so they stay out of the "view shed"

Here is what I'm envisioning now--would love input on how to do it better (or right!?)

4 of the 175W panels, sent 40 foot over 6 gauge wire, to an enclosure with a MPPT charge controller (thinking MorningStar Tristar 30), to charge up a Rolls Battery Bank using two deep cell batteries with a 12 volt, 428 Ah configuration ( http://www.wholesalesolar.com/battery-banks.html )

So here are some questions I would love some help with:

1. How do I control the lights? Since they are 45 amps, I'm guessing I need a 60 amp controller for a safety factor. I can't find one that would turn the lights on and off (the Xantrex C12 looks perfect but at only 12 amps too small). Does anyone know a load controller that will do 60 amps with a light sensor and timer?

2. Enclosures (this is all outside) are crazy expensive--any idea of the best way to find an enclosure that would pass the permitting process and hold the batteries/controllers as economically as possible?

3. Is this wire size good enough? It seems to be the best I can find for a reasonable price that I can bury.....wisdom on that whole things would be excellent.

4. Am I correct in thinking I can go from 24 volt panels to 12 volt battery if I use a MPPT controller? Can I then hook up a load controller directly from the 12 volt battery system to the 12 volt light system?

5. Am I anywhere near correct that I can use the Morningstar 30 for this? I don't see how the amps get over 30 going in to the battery, just going out.

I appreciate any help you can give me!! Rich

BB. · February 2015

Welcome to the forum Rich!

I am going to make this short and sweet (and probably still too long of post). Please ask questions, because I cannot tell from here what your experience level is.

First... The loads. Using a Kill-a-Watt type meter for AC, or a DC AH/WH meter, or even a DC Current Clamp Meter like this one from Sears (good enough for our needs).

Next--Look at conservation. For lighting, focused LEDs can be much more efficient than just about any small light source out there. And look very closely at your required illumination levels. Our eyes react to light in a logarithmic scale. More or less, a 2x fact change in illumination is just about imperceptible. And a 10x change will swamp the smaller light source.

The reason I am suggesting this--Very roughly, off grid power costs around $1 to $2+ per kWH--Or almost 10x the cost of utility power (capital costs, labor, battereis every 3-7 years or so, electronics every 10+ years, etc.).

If you can reduce 540 Watts of lighting to 250 Watts, or even 100 Watts or less--You will be greatly reducing the cost of your off grid power system.

Next--You want 3 hours of lighting per night? Is this seasonal (summer only, or do you want solar in deep winter?). Roughly, where will the system be installed--Most relatively "sunny" places will average ~4 hours of sun per day for 9 months of the year... In winter, you may be looking at 3 to 2 or less hours of sun.... And there are streaks of bad weather--No sun for week or week(s). Backup genset.

Once you have the above nailed down, we can very easily "do the math" and figure out your system needs.

To rough out a system based on what you have asked for (lots of guess work). The numbers will be a bit conservative--So the system will work out of the box.

First the battery bank, 2 days of storage and 50% maximum discharge (very common off grid requirement).

540 Watts * 1/12 volt battery bank * 3 hours per night * 2 nights * 1/0.50 maximum battery discharge (longer life) = 540 AH @ 12 volt battery bank

Next, sizing the solar array to the battery bank. Two methods, first is based on the 5% to 13% or so rate of charge:

540 AH * 14.5 volts charging * 1/0.77 panel+controller losses * 0.05 rate of charge = 508 Watts of solar panel minimum (2 days a week, summer seasonal, etc.)
540 AH * 14.5 volts charging * 1/0.77 panel+controller losses * 0.10 rate of charge = 1,017 Watts nominal (daily use 9+ months a year)
540 AH * 14.5 volts charging * 1/0.77 panel+controller losses * 0.13 rate of charge = 1,322 Watts "cost effective maximum"

Next, based on how much sun you get for your region... Assuming 4 hours per day (use a genset during bad weather in winter):

540 Watts * 3 hours * 1/0.61 DC off grid system eff * 1/4 hours per night = 664 Watt array minimum (less sun, fire up the generator as needed).

Next... sending 45 amps of 12 volts DC any distance requires a lot of copper cable... Using a generic voltage drop calculator for 50 feet one way rung and 0.5 volt drop for wiring:

http://www.calculator.net/voltage-drop-calculator.html

1/0 copper cable (pretty heavy/expensive chunk of copper cable)--Think heavy welding cable:

Voltage drop: 0.44
Voltage drop percentage: 3.67%
Voltage at the end: 11.56

Round about way of asking if you can use 120 VAC wiring... There are lots of LED lamps out there that run very nicely on 120 VAC--Yes, you have the cost of an AC inverter (and about 85% efficiency)--But the ease of wiring can make it worth the extra cost... Say you wan to send 540 Watts 100 feet one way run @ 120 VAC (or 4.5 amps). Same voltage drop calculator with 3% voltage drop (3.6 volts):

16 AWG:
Voltage drop: 3.61
Voltage drop percentage: 3.01%
Voltage at the end: 116.39

A lightweight 16 AWG extension cord will carry the same wattage 2x farther.

I will stop here... Some things to answer, and more to think about.

Once you get the loads (and 12 VDC vs 120 VAC question) nailed down, we can look at touching the system sizing and start designing for hardware requirements.

-Bill

Rich@prg-llc.com · February 2015

Hey Bill--great things to think about. You can't probably dumb it down enough for me! As noted, once I get if figured out from the perspective that we can do this economically, my rough equipment list is going to a real engineer for stamped drawings (and corrections), so at least I'm not a danger to myself or others! But want to get a handle on the most economical method ---what and where to buy--before sending it to them.

This is in Charlotte, NC.

First, on the loads: The sculptures are actually light sculptures that are designed to attract bugs. You can see previous installations here:
http://brandonballengee.com/projects/love-motels/ They need a particular type of LED UV lights which have been field tested--so I don't have any leeway on that. It's going to be 184 foot of these:http://www.flexfireleds.com/content/...spec-sheet.pdf. By my calculations, 534 DC watts, 45 amps. They need to be on 3 hours per evening, but they want to have a battery bank capable of providing 6 hours in case of a day without sun. If there are two days without sun, the sculptures will simply stay dark until there is sun again--part of the lesson of being off-grid.

So they are already 12 volt DC. I could invert, but then would have to use a transformer to get back to DC anyway...seems like a shame but need to figure out this wire cost issue.

Because this is an art project, we can't stick the panels next to the sculpture--they call that interfering with the viewshed; we'll be 50 ft away. Someone wisely suggested going to 48volts with an MPPT. If it's possible economically, I'm thinking I should send the power DC at 48 volts to the enclosure box with batteries and controllers, and then get the last ten foot to the lights with 12 volt DC. Would #6 underground wire handle that 40'? So it has to be copper? Guess I was being naive thinking something like this would work (#6 underground-rated wire): http://www.lowes.com/Search=?storeId=10151&langId=-1&catalogId=10051&N=0&newSearch=true&Ntt=%236+uf+wire#!

As noted, we already have four of these panels, donated and in the shop: http://africanenergy.com/new/wp-cont.../sharp-175.pdf I believe (and of course may very well be completely wrong!) we can wire these (two series of two in parallel) for 48 volts and create adequate production. So I'm feeling like we can generate the power and send enough to the sculpture but I might need to think about a bigger battery bank. Would love suggestions on that!

As you can see, the lights are 12V DC, so I need to get to that. Would you suggest that I keep the batteries at a higher voltage and change it between the batteries and the lights? How? Also, know of a load controller that could flip the lights on at dark and leave them on for 3 hours (and handle 45 amps?)

As noted, would love any suggestion on economical weatherproof enclosures, batteries, controllers, wire!

The artist has previously always just plugged these in, but would love to get them off-grid as part of the environmental message--hence the scramble to make a tight budget work. Giving up because the load is too high will be a (plugged-in) failure!

BB. · February 2015

The link to your UV Leds is bad (the ... link shortening is also the link). I found the pull sheet here?

http://www.flexfireleds.com/content/Colorbright-UV-LED-strip-light-spec-sheet.pdf

Regarding the engineer and stamped drawings -- Generally those are required for building permits and are structural engineers, unless you are going for an electrical engineer with a PE cert.

And, in any case, I would suggest you do as much work as you can to keep the engineering costs low and because you will probably end up knowing more about the installation than the engineer.

Is this seasonally powered? Using PV Watts for a fixed array titled to latitude (~35 degrees from horizontal), the "hours of sun" per day average:

Month
Solar Radiation
(kWh/m²/day)

1
4.04

2
4.46

3
5.30

4
5.89

5
5.79

6
5.77

7
5.55

8
5.59

9
5.26

10
5.23

11
4.25

12
3.74

Year
5.07

So seasonal power usage can affect the design.

Note that Lead Acid Batteries do not like to sit for days/weeks partially discharged... More or less, they sulfate faster when stored below ~75% state of charge. So, letting discharge to 50% and sitting for a few days or week+ of d weather is not a good idea.

LiFePO4 batteries would be better for storage at partial charge--But they are much more expensive and a little bit "exotic" at this point (not as many people have experience with these batterries and the support hardware).

Got to go now--But you will probably have to do several paper designs and see what ends up being the most cost effective/practical to install.

Got to go...

-Bill

Helping a museum set up an off-grid system to power a couple of sculptures

Comments

Categories