best use for a smallish PV system ???

al128 · June 2008

hi there,

I managed to pick up 750w of panels for cheap ... while reading/catching up on PV - allow me a Q.

what is the best use for a smallish PV system ???? - i gather many of you have started out small some time long ago :-)

cheers and thx for sharing your thoughts/experiences

al

icarus · June 2008

Re: best use for a smallish PV system ???

The best use would for you to ship them to me!

Seriously, describe your situation a bit more? Are you on grid, do you have a cabin somewhere, RV.?

750 watts would be a bit small for a grid tie system to be cost effective. Barring that, you would be into a battery system, which if you are gird tied, may not make sense either, unless you need a UPS for something.

Give us some more to go on.

Icarus

SolarJohn · June 2008

Re: best use for a smallish PV system ???

Your question is a little hard to answer without more information, so I'll tell you about my situation and what I do.

I have 425 watts of PV and about 675ah of battery capacity. I use the system to its fullest capacity to power my refrigerator and freezer. I've designed my system to use battery power as much as possible, and automatically switch to grid power when the batteries are discharged to 70 or 80%.

I've described my system in greater detail here:
http://solarjohn.blogspot.com/2008/02/ive-automated-my-off-grid-pv-system.html

John

n3qik · June 2008

Re: best use for a smallish PV system ???

I have 690 watts of solar panels with 330 AH of batteries. This system is used to power my laptop, modem, router, printer, gas heater, all lights at night and cat's water fountain. When there is enough sun my homemade geothermal cooling is switch from the mains to the solar. You don't have go as big as John and I did, start small. Like a single 55AH battery and a charge controller to power some LED night lamps.

al128 · June 2008

Re: best use for a smallish PV system ???

icarus wrote: »

The best use would for you to ship them to me!

Seriously, describe your situation a bit more? Are you on grid, do you have a cabin somewhere, RV.?

Icarus

i am grid tied (but with real threats of programmed blackouts down here in Stgo, Chile).

I remember, some 8 yrs ago energy was simply turned off for 2 hrs. per day (rotative scheme) due to depleted water behind dams.

to be honest, i bought the panels as they were too sweet a deal to pass up (about U$ 2.5 / watt - used).

my plan is to babystep into a system by moving over all exterior lighting to solar (on a single 110ah bat.) to keep all errors on the cheap side

... and then probably invest in a bigger bank for fridge/freezer and hopefully gen. lighting duty

how does that sound?
al

icarus · June 2008

Re: best use for a smallish PV system ???

A couple of quick thought,,

Since you are not inclined to ship them to me,,,,, here is my opinion.

Your 750 watts translates to ~60 amp@ 12vdc. You 120 ah battery will be way too small both to charge from these panels and to run much loads. Depending on your sun you should harvest~250 ah/day=~3kh not counting charging losses, inverter losses etc.

If your not going to go grid tie, then add some battery, and an inverter. A 300watt suresine is a good buy @~$300. Solar John and N3qik have some good ideas as to how to tie into you house wiring (safely) without doing a grid tie inverter. I am currently designing a similar system, but I'm struggling with how to keep the battery as small as possible, and to use the power directly on the loads. The problem is trying to figure out is how to feed multiple circuits from an inverter/transfer switch without multiple switches and wire runs in an existing building. (Thoughts John?)

Your 250 ah/day certainly would go along way to reducing your loads. If you want a UPS then you are going to need a bigger battery system. The problem with any hybrid (battery/inverter) system is that the batteries add considerably to the cost, with argueably little benefit. Hence why most people go grid tie.

What does your utility think of grid tie solar? Do they have any incentives?
That might help you decide.

Keep us posted,
Icarus

niel · June 2008

Re: best use for a smallish PV system ???

remember, there is grid-tie with battery backup. more battery ah is still needed though.

Telco · June 2008

Re: best use for a smallish PV system ???

Hmm, with the threat of programmed blackouts, think I'd concentrate on a larger battery set, and use the panels to do a general charge on them. Then, use them to primarily power the fridge, even if it means picking up more panels. Sucks to see a lot of food go down the tubes and nothing you can do about it, as I just saw last week when the deep freeze failed. We've had way too many localized power outages in my area as there's a TON of construction down the street, and I think that one of them was just too much for my freezer. Luckily we were about to make a stockup trip, so I was able to move most of the food over to the regular fridge, but I still lost a box of pork chops, about 10 pounds of potatoes, and some dairy products. Coulda been a lot worse, as the stockup trip was going to be for half a cow at the local butcher shop.

al128 · June 2008

Re: best use for a smallish PV system ???

icarus wrote: »

A couple of quick thought,,

Since you are not inclined to ship them to me,,,,, here is my opinion.

Your 750 watts translates to ~60 amp@ 12vdc. You 120 ah battery will be way too small both to charge from these panels and to run much loads. Depending on your sun you should harvest~250 ah/day=~3kh not counting charging losses, inverter losses etc.

If your not going to go grid tie, then add some battery, and an inverter. A 300watt suresine is a good buy @~$300. Solar John and N3qik have some good ideas as to how to tie into you house wiring (safely) without doing a grid tie inverter. I am currently designing a similar system, but I'm struggling with how to keep the battery as small as possible, and to use the power directly on the loads. The problem is trying to figure out is how to feed multiple circuits from an inverter/transfer switch without multiple switches and wire runs in an existing building. (Thoughts John?)

Your 250 ah/day certainly would go along way to reducing your loads. If you want a UPS then you are going to need a bigger battery system. The problem with any hybrid (battery/inverter) system is that the batteries add considerably to the cost, with argueably little benefit. Hence why most people go grid tie.

What does your utility think of grid tie solar? Do they have any incentives?
That might help you decide.

Keep us posted,
Icarus

well - my first step (outdoor lights only) would be on a 120ah battery ... I would just use 1-2 panels (75-150w). -> babysteps ... just a learning experience ;-)

what I am trying to achieve later on... how to cool a freezer down to extreme cold temps (way below the -18C they run normally) - while having sun at noon hopefully down to -25C - and then let the freezer cycle up to -18C again over night w/out needing large bat's ...

kindalike use the energy "right here and now" instead of buying expensive storage facilities (aca batteries) ... a freezer as diversion load - could this work?

whattya think?

thx again
al

BB. · June 2008

Re: best use for a smallish PV system ???

We had a similar discussion here before--and you are right in aiming at -18C as your "high temperature"... During WWII, the US Department of Defense (Department of War?) did a study and found that even raising the temperature to -15C, measurable decreases in stored food quality was observed over time.

I currently have my freezer set to its minimum temperature, and use a timer to avoid operation between noon and 6pm Mon-Fri (my peak Time Of Use pricing time).

It works OK--but I am sort of wondering if I experience more freezer burn/freeze drying of food because of this or not (freezer temperatures drop a lot when "max cooling", causing the "warm" food to sublimate water onto the "cool" packaging.

Your cooling would only be for ~5 hours or so per day--and I would expect the thermal cycling extremes to be even wider than mine--and you to possibly expect even more "freezer burn" potential.

Also, my freezer is not a frost free model--so I have do defrost timers to worry about... If you get/have a frost free model--this sort of 5 hours on 19 hours off cycle may play havoc with the whole auto defrost cycle.

Sorry--probably more questions here than answers--But I would certainly try and factor the "extra" battery costs over their life-time, and see if the food quality issues (and savings of bulk/sale buys) vs battery costs makes paper sense too.

You are, possibly, in the Los Angeles CA. region--another reason grid tie makes sense--much cheaper "power costs" vs off-grid battery system.

-Bill

icarus · June 2008

Re: best use for a smallish PV system ???

Interesting idea. My initial intuition (with no evidence to back it up!) is that you may be running things quite inneficiently with such a set up. It seems the energy require to "over cool" might be more than the gain. That said, it may beat other "diversion loads", I have no idea.

The second thought is, can you indeed cool a deep freeze to such temps, or is there a natural limit that you might be up against. The issue with a freezer is that you are dealing with thermal mass. The internal temp of a fridge is really long cycling. (assuming it is full), so that the kind of temp swings you are looking at are probably not realistic on a daily basis. As I think of this as I write, perhaps then your idea does make some sense, but instead of dropping from -18 to -25, you might drop from say -18 to -20 and then shut it off. Some experimentation would be in order.

I know that during power outages, keeping the freezer closed will keep things frozen for several days if you are careful.

Icarus

niel · June 2008

Re: best use for a smallish PV system ???

won't work if you go using it much during the non-sunny periods as the food temps will rise again with no way to freeze it again. though super cooling helps in its preservation and may buy some outage time, the power needed in super cooling it will most likely be as high as the battery inefficiencies and all other losses maintaining a more constant lower cold temp. also remember the heat gains through the insulation will be much higher too so maintaining a really low temperature is a big energy consumer.

al128 · June 2008

Re: best use for a smallish PV system ???

like many of you - I havent thought this "project" completely through yet... (but nevertheless thx for the informed interchange of thoughts/opinions).

I am not necesarily saying that I will go black-and-white as in "5hrs ON - 19 hrs OFF" on the freezer. Though this is the basic idea .... I hope to get a "head-start" temp-wise, to avoid unnecessary battery use ... and to be honest - the idea sounds smart, at least in theory - the freezer would be kindalike a thermal battery - one that I already have which makes the idea so much sweeter ;-) .

The most probably scenario would be to cool the freezer down, then harvest a couple of hours w/ low temps and then start to cycle again, but at a lighter pace (would also be in the night, where amb. temps are lower.)

What I dont really know is how to implement a system that would cool for say 5 hrs straight to -20 or -22C (numbers made up) and then would restart at -18C and keep it there.

... and I am most intrigued by the chest freezer to fridge conversion thread discussed somewhere here.

I found a local made chest freezer that can be run both as a freezer and a fridge ... but I havent gotten any consumption figures yet.

keep them ideas coming :cool:

cheers
al

best use for a smallish PV system ???

Comments

Categories