First post / project

System

Hi everyone. I am new here and had a few questions. I have been doing a ton of reading about PV systems and would like to ask for some opinions on some decent quality components for a beginners initial foray into PV.

What I have in mind is to purchase 2 small panels to power a 12v 1.2 amp fan motor from an old HP DL380 server. This is all just to get a good basic understanding of PV systems and how they work before I attempt a larger project.

From my reading I have figured out that I would need:

Panels: 2 12v rated at least .6 amps a piece
I wanted two, so I can get some experience with different wiring configurations ie..series or parallel )

Batteries: I figured one 12v 24AH would do so I could run the fan for a full day or so. (Would there be any benefit in wiring up 2 6 volt batts instead of a single 12v?)

Can you recommend a place to get good cheap batteries for this type of beginner project ?

Controllers: I was looking at the Morningstar SunGuard

I was wanting to run the fan from the battery but am not sure how to keep the the panel charging the battery instead of running the fan during the day? Maybe I do not understand well enough how batteries work...? Or do I need some sort of voltage regulator to accomplish the task.

I have been looking into how solar lights work and i think they do essentially what i am looking for but not for the load that I would have from this fan...

If you have any ideas or opinions and could just fill in some useful info that would be great. Will the parts I am looking at work? Have I over looked anything?

Thanks
Tommy

I have no real background in electrical work other than occasionally helping to wire houses when I was growing up. So, I have some basic knowledge as I have been a systems engineer for the past several years and worked with building PCs for the last 18 years for myself. I am technical minded person but want to make sure I have good understanding before I commit to do anything.

BB.

Re: First post / project

You will never get as much power as you expect from a solar/battery system--if you where just to take "ratings" and take into account reality...

Basically, a simple lead acid storage battery is about 80% efficient.

A lead acid storage battery should not be discharged more than 50% for long life (and immediately--withing hours or a day--recharged to above 75% to prevent sulfates from hardening).

A solar panel (+95% efficient controller) is about 77% of the STC rating.

An inverter is around 80-85% efficient.

A couple of good places to look for solar radiation data:

http://rredc.nrel.gov/solar/pubs/redbook/ (PDF data with good graph showing how weather/seasons affect solar output)

http://rredc.nrel.gov/solar/codes_algs/PVWATTS/version1/ (designed to give Grid Tied system estimates)

And using variations of V=IR, P=IV=I^2 * R=V^2 / R will give you the algebra.

Understanding (typically) parallel adds current and series adds voltage. Solar panels are just specialized "batteries". And understanding where you have conservation of current (PWM controllers) vs conservation of Power (MPPT controllers).

Wind-Sun (our forum host) has some nice FAQ's sprinkled around their web store... Go to the battery page and read the battery FAQ. Go to the inverter page, and read the inverter FAQ, etc.

http://store.solar-electric.com/

Believe it or not--it may be fun to play with small panels, motors, etc... But the actual behaviors are pretty well known--so don't spend too much on the pieces when your real need to for a full fledged solar system that does useful work for you (for example, moving air would be better just placing air vents and/or wind turbines in your roof rather than buying very expensive solar air movers--which are not usually large enough to move much air).

Another place I would suggest for solar devices is to go to a Home Depot/Lowes type store and get a solar powered motion detector light. Won't be too expensive--and you can take it apart and do your panels+batteries+load testing.

-Bill

System2

Re: First post / project

Thanks for the insight.

I will go and pick up a solar fan or flood light kit to take apart and play with..I guess i could always wire in more solar panels to whatever if i need or want to make more power though be it on a smaller scale than what most people are doing.

I appreciate the links i will add them to the reading list.

ultimately i am wanting to build a system to power my home PC with solar power and batteries if it is feasible ..

BB.

Re: First post / project

Realistically, powering a home PC is highly variable in power requirements... My old laptop can run on 20-30 watts average 12 hours per day (less than one watt on standby). That would be 400 Watt*Hours per day.

A high powered PC with big monitor and a laser printer--you may be talking about a couple hundred watts 24 hours per day (assuming worst case). Or a system about 20x that required to power a laptop. Or 4,800 Watt*Hours per day.

If you don't have a kill-a-watt meter yet--it would be worth it to properly size your power requirements.

Doing an Off-Grid pure solar+batteries for the laptop system--maybe $2,000-$4,000 (price depends...).

For powering a large workstation--$20,000-$40,000+... (very SWAG numbers--depends on where you live, seasonal variations, specific requirements, automatic generator support, etc.).

In the end, unless you have special needs (such as no grid power), or live in area with very poor power (ice storms, floods, tornadoes, huricanes, lots of brownouts/blackouts)--you would be better off spending on Conservation, a small UPS, and real grid tied system to generate power.

A full solar system usually only makes sense if you plan on a 15-25+ year system life/use. If this is for supporting random outages over the next few years--a small generator with UPS (look for a nice used UPS system from a data center than you can replace the batteries to get it running reliably) probably makes more sense.

-Bill

System2

Re: First post / project

Is there a real explanation as to why some systems would be wired as 6 volts for 12 24 or 48?

I guess I am looking for why the differences in voltage instead of a standard voltage...this flood light i picked up at home depot is a 6 volt system....

BB.

Re: First post / project

Generally, the higher the voltage, the lower the current losses and smaller the wire sizes...

General systems are 12, 24, 48 volt. Above 48 volt, the NEC and other safety issues come into play (High DC voltage with high currents are quite dangerous).

For smaller systems and panels--(like a LED light source)--whatever the cheapest components can be put together at the lowest cost. LED's don't need very high voltages or current--so a 6 volt system is OK.

For larger systems, when your current is above a 100 amps--that requires some very large gauge wires to run (and large electronics)... So, for systems under 1,000 watts--12 volts is workable. For systems over 4kW--pretty much you are limited to 48 volt systems.

12 volt system components tend to be cheaper (because of volume) and 48 volt are popular for large systems. You could have a 36 volt system too--but not many people choose this voltage (sort of common for some marine uses?)--so it is hard to find system components.

For solar charge controllers--between 12 and 48 volts--they are all current limited.... So, a 60 amp controller can control ~720 watts at 12 volts--but the same controller can manage ~2,880 watts with a 48 volt battery bank.

And then you run into some other issues once in a while--for example, many wind turbines will output a reasonable amount of power into a 12 volt battery bank--but need darn near a hurricane to pump power into a 48 volt battery bank.

There are trade-offs everywhere... And many times the rules of thumb for system design sort of take care of the major issues (like recommending a battery bank to be roughly 6x your daily load--takes into account the size of the solar panels, min/max current to charge the battery bank, minimum amount of deep cycling for long life, low current draw vs capacity for efficient use of battery bank, etc.).

It all depends on your power needs and system design.

-Bill

System2

Re: First post / project

So, you are kind of like Yoda around here aren't you..?

niel

Re: First post / project

gee, you've come a long way yada yada yoda. may the photons be with you.:p:D

stimpy17

Re: First post / project

A good source for cheap 6 volt batteries is Sam's Club. They have golf cart batteries for around 60 FRN's. Let's face it, we'll screw-up some where and I'd rather not fry an upper end storage battery.