Off-grid PV setup for cabin

[Cynnergy]

Hi all,

I was hoping to get some advice on what you think about this PV model data? (broken link removed).

Specifically, I am trying to tell if I can power this fridge (http://www.homedepot.ca/product/10-cubic-feet-refrigerator-white/989324 - 297kWh/year Energy Star rating) on a 1kW system year-round in this location.

The data just doesn't seem right to me. At tilt=latitude, 32 kWh/kW in December, but only 115 kWh/kW in July? We are in a rainforest here, and the sun (if it comes out) is only above the horizon for max. 8hrs/day in January. July is almost always sunny and the sun is out for 16 hrs/day.

I'd appreciate your thoughts.

 

[woodgeek]

The short answer is I believe it.

To get 16 hours of output in one day you would need a 'tracker', the panel would need to rotate to follow the sun as it moves across the sky from east to west.

Say a panel puts out its rated power when the sun is at the point in the sky which corresponds to the line perpendicular to the panel. Otherwise, if the sun's rays make an angle A with that perpendicular line, the panel only puts out cosine(A). So, the output of a panel is a nice smooth cosine hump, roughly 12 hours wide, with a peak in the middle at hour 6. The area under the curve (the energy harvest) is the same as getting rated output for 6 hours.

Of course, the above is only exactly true for the sun on the equinoxes, with your lat tilt panel, and no atmospheric scattering or clouds. If partly cloudy, you can actually get MORE output early and late in the day than you would without clouds, from reflected light. But still, on a perfectly clear day, you get about 6 hours of max output, period. Times 30 = 180 hours in a month. Your numbers are saying 80% of days are cloudy in January, but only 35% of them are cloudy in July.

 

[woodgeek]

To answer your other question....if your fridge needed 297 kWh/year, that is the same as a 813W constant load, and needs 25 kWh in a 31 day month. In an off-grid setup, you lose energy in your charge controller (PV to batt), and you lose power in the batt (round trip eff), and then again in the inverter (DC to AC). I know for grid-tie, they tell you to figure on losing 22% (i.e. 78% eff). My guess is that a battery will lose another 15-20%, because it charges at 14V, and discharges at 12V, so you only get 12/14ths of the power back out, in round figures. So, roughly speaking you get 0.78*12/14 in an off-grid system, about 0.67.

So in your ave December with 32 hours of potential, you get 32*0.66 = 21 kWh on average. So, to be assured to make 25 on average, you would need to go about 25% bigger, 1.25 kW.

In terms of battery size, the rule of thumb is to charge at somewhere between C/20 and C/10. That is, it would take between 10 and 20 hours of your max output to fully charge. If we say 15 hours, then you need a battery bank that stores 15*1.25 = 19 kWh. A deep discharge battery the size of a car battery is 80 amp-hours, or 80 Ah * 12V = 1 kWh (if fully cycled, which you never do), and cost $80. Your off-grid system would want ~20 of these (or fewer larger batteries....some folks like 'golf-cart' size). Cheap batteries would then run $1600, call it $2k with wiring, and would last 3-5 years. Or about $400/year if you got 5 years from them.

To protect your batteries, you would never want to discharge them past 25%, so your bank would have a real storage capacity of 15 kWh, or 15 days of runtime (with inefficiencies). Longer runs of cloudy days would need a backup gen. You would get a genny that is several times more powerful than your PV array, say 5 kW, and you could do a 4-5 hour run to charge the bank to refill 15 days of fridge run time.

You can see from your yearly table that December sucks with 32 hours of potential. In July you make 3X the power, enough to run 2.5 kW of loads continuous with above setup! If you don't have such loads, you are 'wasting' that potential power, and still kinda paying for it.

So one alternative would be to downsize by 50%, to a 700 W system with a 10 battery (800Ah) bank. Now the fridge only has 7 days of power (if totally cloudy), and you are prob aok March thru October, 7 mos. And you have to do a 2 hour genny run every time there is a full cloudy week or two otherwise. The 1.25 kW system would only need the occasional genny run in Dec and Jan.

Of course the 800W figure is an estimate. If the cabin ran cooler in the winter, and the door got opened a lot less (less occupancy) than the usage would also follow a seasonal curve, and then the 1.25 kWh would be way oversized.

 

[Cynnergy]

Woodgeek you are awesome, thank you for the advice I was surprised when I saw the figures, but your explanation makes a lot of sense.

I have thought about just unplugging the fridge during the winter months - we have a 'cold cupboard' in the cabin that will probably do just fine for storing cured meats, cheese, veggies, preserves, etc. I'm a little worried about the milk (I don't think it would be cold enough to keep it for long), but we could try canned milk or UHT milk. Or just bring enough for the weekend - a pint should be enough for tea/coffee/scrambled eggs I guess. I have 7 quarts of carrots canned for making quick carrot and lentil soup. We have canned fish too, but it might be worth canning some meats for stews, pies, etc so that I don't have raw meat in the cold cupboard at variable temps.

The only downside is no freezer, which I use a lot for leftovers, bread, etc. It helps cut down on food waste with going back and forth so much.

At the moment, we are on 24/7 generator power, so I will probably buy the fridge, plug it in and get some real-life figures using the Efergy. The cabin is unheated and the fridge unopened when we are not there, so hopefully it will use less than the energuide rating.

 

[Cynnergy]

For interest - the model does take into account some inefficiencies in the system (see below). So using your figures but taking out the 22% inefficiencies (already included in the figures) with a Dec rating of 32 kWh * 12/14 = 27 kWh, so I'll have 2kWh left over for one CFL light while we're there that month on a 1 kW system . No idea what we'd do with all of the power in the summer though - an electric range for the canner I guess .

"Photovoltaic (PV) electricity generation potential for grid-connected photovoltaic systems without batteries (in kWh per kilowatt of photovoltaic installed power capacity) was estimated from the insolation models for all fixed surface orientations for each grid cell using a performance ratio of 0.75. The performance ratio quantifies and takes into account overall system losses due to operation under non-ideal conditions: climatic factors, inverter operation and so on."

 

[jharkin]

Where did you get this 12/14 idea?

When charging the battery does not hold 14v. It will slowly ramp from 12 to 14.xx in the CC phase then go to CV to top off and equalize the cells. Then it stabilizes around 13v off charge.
During discharge, unless the load is large it will similarly ramp down from 13ish to 12. An unloaded battery at 12v is completely dead.


For sizing, going up to 50% ave DoD is fine, and I believe offgrid folks aim for a 3+ day reserve. They usually use a rule of thumb thatpv output should be 5-13% of the bank capacity, to insure enough capacity to recharge in a day.

 

[Cynnergy]

Sorry jharkin, you lost me at CC and CV . ? I think CC = charge controller, but CV?

I don't know about just a 3 day reserve, we have a lot of rainy days here - how much power can you get on a cloudy rainy day anyway?

What would happen if we didn't have a genny? I assume the charge controller would just shut things down and the fridge would shut off. In a cabin at an ambient temp of say, 7 or 45, this maybe isn't such a big deal as long as the freezer is empty. As long as it's an auto-defrost freezer it shouldn't have ice in it that will drip everywhere, right? Is there anything else bad that could happen?

 

[jharkin]

cc= constant current, cv = constant voltage.

The charge method for lead acid is called CC/CV because what is done is at first you just push a lot of amps and monitor the voltage until it gets up to a certainly level (depends on the type, usually between 14.1 and 14.8v). This is called the bulk charge stage and is all that a typical cheap auto parts stove car battery charger does. It leavs the battery 80-90% full.

The second stage is the constant voltage stage, called "absorption" Now you hold the voltage at 14.x and just feed as many amps as the battery will take, and the amps will slowly fall to zero as it reaches 100% full and the cells equalize.


http://jgdarden.com/batteryfaq/carfaq9.htm#stages

 

[woodgeek]

I defer to your knowledge Jeremy....but I was under the impression there was a cycle loss for energy in a battery, and that that manifests as different voltages in and out.....usually the charge/current gets recovered (except for leakage/standby). I was using round numbers ( ) and estimating a 6-8% energy loss.

What do you think the energy loss is?