Refrigerator power

Moe · November 2008

Let's see if I have these off-grid refrigerator usage/requirement calculations down.

An 18 cu ft Energy Star refrigerator I looked at is rated to use 382 kWh per year.

That's an average of 382,000 watt-hours/365 days = 1,047 watt-hours per day or

1,047 watt-hours ÷ .85 inverter efficiency ÷ .85 battery charge efficiency = 1,449 watt-hours of solar input per day

So I go to http://rredc.nrel.gov/solar/pubs/redbook/PDFs/OH.PDF and check out Dayton OH with Tilt = Latitude

Looks like we get 2.4 kWh/m^2/day average in December. A Kyocera 135W panel is roughly 1 m^2 and with a claimed efficiency of .16 (according to Kyocera), on average, it should produce 2,400 x .16 = 384 watt-hours per day in Dayton OH in December.

1,449 watt-hours requirement ÷ 384 = 3.77 or 4 whole 135W panels (540W), on average, to run this refrigerator in Dayton OH in December.

But worse case, we only get 1.7 kWh/m^2/day in December, which comes out to 5.33 or 6 whole 135W panels (810W), worse case, to run this refrigerator in Dayton OH in December. Given a backup generator, might it be more economical to not size the panels for worse case?

I realize sizing for December will result in a lot of excess capacity, especially in July, but that can be used for fans in the hotter months.

I guess by the rule of thumb I'd need 1449 watt-hours x 6 days ÷ 12 VDC = 725 amp-hours of batteries as well, just for the refrigerator.

Thanks!

icarus · November 2008

Re: Refrigerator power

You also have to figure in a reserve,, especially for December. The size of the reserve sort of depends on how low you are willing to discharge your batteries before they can get charged,,,either by the sun or by the generator.

What is it you are trying to achieve? Is your goal to reduce your footprint, or to be energy self sufficient? You are right about sizing for the worst case (December), and that it may make more sense to size for say October/March?

Our November is usually quite poor, but our January is often quite spectacular, what with no snow, open water in November, but Snow covered ice, coupled with very cold temps in January, giving us (on those good days), spectacular yield.

Tony

Cariboocoot · November 2008

Re: Refrigerator power

Very good calculations.
And then there is reality.
I run a 'frige off solar. It draws 240 watts +/- when cycling. When it goes into defrost mode, that more than doubles. This is followed by heavy current usage when it then has to cool back down. 'Average' usage figures are one thing; like you already know, you have to plan for peaks and valleys.
You also have to know for a fact what solar exposure <I>you</I> will be getting, not just the figures for your area. For instance, at my cabin we are supposed to get 8 hours of daylight in winter. Ha ha. Not even in mid-summer do you get that much, because the mountains block most of it.
I have 700 watts of panels on the roof, and on a good day I get 2.8 kWhrs total. The average in bright sun, summer conditions is more like 2.4. In winter it drops to unreasonable.
So you plan for worst-case, but you may find you're spending more on panels than it would cost to run a generator.
The un-erring rule of solar planning is: PLAN FOR EXPANSION!

icarus · November 2008

Re: Refrigerator power

Why don't you turn off the auto-defrost mode on your fridge, and defrost the old fashioned way. Some fridges have that option, others you may have to cut the wire to the heater.

Tony

Moe · November 2008

Re: Refrigerator power

The goal of being off-grid is both, smaller footprint and self-sufficiency to an extent. I realize LP isn't "self-sufficient" but it is also a consideration for refrigeration.

In the referenced PDF file, there are three historical values for each month, minimum insolation (my "worst case"), average, and maximum insolation.

As far as reserve, I edited the post and added the 6 day RoT for batteries, but added no reserve of panels. Could use the generator (and do laundry) while charging.

mike95490 · November 2008

Re: Refrigerator power

If you end up with humid conditions, and the coils frost up, don't you end up with a mess if the defrost heater has been disabled ?

And you can always "plan" on 2x weekly use of genset in winter, instead of another 2KW of PV panels for those short, cloudy, winter days. But it does not make you independent.

Heck, there ought to be a way, in winter, to get some frosty outside air to your fridge coils, and cool it with 35F air, instead of indoor 70f air. But it would help heat the house too.

niel · November 2008

Re: Refrigerator power

"Heck, there ought to be a way, in winter, to get some frosty outside air to your fridge coils, and cool it with 35F air, instead of indoor 70f air."

mike,
you know i've pondered that too, but the problem would exist for the warm summer months of increased outside temps over my a/c temps inside. in the winter i won't let in cold air to my kitchen just to cool my refrig with as that's counterproductive to having a warm cozy kitchen when it's frigid outside too. these new refrigs don't have the coils out in the back anymore either making it difficult to place that colder air to it via a duct of some sort and keep that air isolated from the rest of the desired warmer air in the home. anybody have any insight into this as this could drastically cut winter refrig operational costs?

mike95490 · November 2008

Re: Refrigerator power

Gonna have to build a couple of plenum chambers, one for the clothes dryer, intake from the warm attic, and a blockable one for the fridge - outdoors. And in the summer, if the fridge can dump it's heat outdoors, not in the kitchen, that's gotta help too.

icarus · November 2008

Re: Refrigerator power

Unless you live in a VERY humid climate, defrosting a fridge is not that big a deal. I have the auto-defrost off on my fridges and every month or so, (more often in the summer) I defrost. Takes about 1/2 hour, and it does warm things up a bit, but it has to be more energy efficient than running defrost cycle.

The biggest advance has been more and better insulation, bottom door freezers, and chest freezers as well as better compressors. The old fashioned grocery store open topped "coffin box" freezer were (are) remarkably efficient given their open top, because they use the density of the cold air in the box to it's advantage. An upright fridge or freezer suffers dramatically because as soon as you open the door, the dense cold, dry, air "falls out" to be replaced with warm moist air. The moisture condenses on the coils and then freezes.

If someone would design a good top loading fridge that would allow you get to the stuff without moving stuff it would be great.

Tony

RWB · November 2008

Re: Refrigerator power

Nice break down. Gotta remember all the inefficiency's to end up with a realistic number.

crewzer · November 2008

Re: Refrigerator power

Moe,

I think you’re on the right track. But, there are few more “inefficiency” factors to add to the mix. The entire list should look something like this:

~85% for the inverter
~85% for the batteries
~97% for the wring
~96% for the MPPT charge controller
~85% for the PV modules

So, figure on ~60% “end-to-end” efficiency for an “off-grid” system. To deliver 1.05 kWh/day net in December, the system will have to be rated at 1.75 kWh/day. With an average of 2.4 hrs/day of insolation, the PV array will need to be rated at ~730 W. YMMV.

Another factor to consider is seasonal performance. My Energy Star fridge is rated at ~1.23 kWh/day. It uses as little as 1.0 kWh/day in the winter, and as much as 1.6 kWh/day in the summer. Our two college-age sons are usually home for much of the summer…

My fridge story: http://forum.solar-electric.com/showthread.php?t=449

HTH,
Jim / crewzer

JESSICA · November 2008

Re: Refrigerator power

icarus wrote: »

The biggest advance has been more and better insulation, bottom door freezers, and chest freezers as well as better compressors....
Tony

Icarus:

Why are "bottom door freezers" better than top mounted freezers?
Can you explain?

icarus · November 2008

Re: Refrigerator power

Heat rises,,,cold settles. I'm not sure why we have evolved to top freezers. It seems you get into the freeze less often, so having it on the bottom makes sense for convenience as well.

Also the idea of putting the compressor and the condenser coils UNDER the cabinet makes no sense either! At least with the propane fridges the boiler and the condenser are behind the cabinet.

Tony

PS "Better" being a relative term,,, more efficient usually. When you think about it, a fridge is just a large insulated box, the better insulated the easier it is to keep the heat out/cold in. The real work of the fridge is extracting the heat from the contents and sending that heat outside the cabinet. (Fridges don't make cold/heat per sey, but MOVE heat from place to place). Once the heat is removed, keeping it out is easy,,,relatively speaking

Moe · November 2008

Re: Refrigerator power

This helps tremendously, Jim! Thank You!

That's especially true for the thread about your new frig. It's also good to note the seasonal change in refrigerator power consumption due to residential temperature, a real plus in winter when PV output is down. I have a couple of questions though.

First, certainly the watts output by a MPPT charge controller must be less than watts input, and it must be less than 100% efficient. I have no problem with that. However, when it comes to amps of charging current, in winter, when panel temperature is lower and panel voltage is higher (particularly with 17+ Vmp STC panels), and when at the same time, battery discharge is likely to be deeper and thus the voltage difference between panel and battery greatest, the MPPT controller should offer its greatest "gain" over non-MPPT controllers, right? So how can you account for that when it comes to efficiency?

This leads into the second question, and that's where do you get 85% efficiency for the PV modules? I was using roughly one minus 85% (16%) times the kWh/m^2/day insolation. Although hours of daylight go up in the summer, so does temperature, resulting in lower voltage and less power. Insolation history doesn't account for that, so it seems efficiency should, and be seasonal.

I'm beginning to plan the retirement cabin and off-grid electrical power sure opens up a lot of beautiful property, reasonably priced except for the cost of bringing in power. We're experienced at splurging off-grid (boondocking in our Airstream travel trailer with two Honda EU2000 generators, 80 pounds of LP, and 60 gallons of water) as well as austerely conserving off-grid (camping at anchor for several days in our small sailboat with one 85AH battery (no charging source), 12 ounces of butane, and 10 gallons of water).

Like you had, we currently have two older 21-22 cu ft frigs that need replacing (as well as WAY more house than we need). But the absorption frig in the RV shows us 8 cu ft, the size of a Dometic/Servel LP frig, is too small for long-term "comfort" even though it only uses about a pound of LP per day. So we're considering compromising with a 15-18 cu ft (< 30" wide) Energy Star. Your estimate of 730W of panels aligns with posts by others using 800W for larger ES frigs, so it sounds like a pretty reliable estimate. I really appreciate it.

mike95490 · November 2008

Re: Refrigerator power

Moe wrote: »

This leads into the second question, and that's where do you get 85% efficiency for the PV modules? I was using roughly one minus 85% (16%) times the kWh/m^2/day insolation. Although hours of daylight go up in the summer, so does temperature, resulting in lower voltage and less power. Insolation history doesn't account for that, so it seems efficiency should, and be seasonal.

That's because the PV nameplate power is taken in laboratory conditions, when the panel is at 70F, with a big flash gun. Flash ! 1 sun of power, peak reading meters record the PV outputs, and they get sorted to the 200, 205, 210, 215 watt piles. When it's on the roof, in the sun for more than a couple of minutes, the dark panels warm up, and their voltage drops. That is 75-80% of the nameplate, depending on local conditions.

Also, under test, the panels are exactly perpendicular to the sun, unless you have tracking arrays, you only get perpendicular for about 5 minutes of the day. Not perpendicular, less power.

Hope that helps, Mike

icarus · November 2008

Re: Refrigerator power

"But the absorption frig in the RV shows us 8 cu ft, the size of a Dometic/Servel LP frig, is too small for long-term "comfort" even though it only uses about a pound of LP per day."

My 8 cuft dometic(s) use ~ 1500btu/hour,~50 hours per gallon. We figure that ours run ~ 25-30% of the time in the summer, 15-20% in the winter. (An average of 1 gallon per week, works out close to your 1# per day,, .75 ppd) We are very happy with the size of the fridge, although we do run two during those times when we are island bound during freeze up or break up. We also use a very small dometic turned up full as a freezer from time to time.
I have added 2" of styrofoam around the cabinet, as well as a temperature activated fan on the condenser to make them run as efficiently as possible. As I have stated elsewhere, you pay a premium for a LP fridge over a conventional fridge, but if you buy a good used LP fridge you can do pretty well. As I have also stated if I was building from scratch I MIGHT consider using a good hi-ef conventional for full time. For a cabin used part time I would go LP.

We use 60# (12gal) of propane about every 5-8 weeks, for fridge, cookstove and water heater.

Tony

Moe · November 2008

Re: Refrigerator power

Tony, the cabin will be for full-time, and the 8 cu ft Dometic/Servel is still a possibility. We bought the Airstream planning to full-time in it for awhile, until some medical problems came up that prevent a lot of travel and keep me working in the meantime. We've done okay with 8 cu ft in it, and I'm sure with practice, could get better at it. If using propane for heat, hot water, clothes drying, and backup generator, hauling in a bit more for refrigeration shouldn't be a problem.

There are thousands, if not tens of thousands, of Dometic RV absorption refrigerators now under recall for cracks that can let ammonia leak and hit the flame. Dometic is blaming the cracks on a higher wattage 120VAC element they went to in the late 1990s and the recall puts them back to the original 325W element. If that IS the problem, it shouldn't be an issue for their residential refrigerators, which seldom if ever get used on 120VAC. The RV units switch to it whenever plugged in to campground or generator power (unless overridden).

Mike, thanks for the explanation of the 85% PV efficiency. It make sense and does help. So it looks like the Kyocera 135W efficiency should be 16% advertised x 80% at NOCT (47ºC), x 85% due to average angle of 32º away from perpendicular ≈ 11% overall PV efficiency?

When it comes to refrigerator configuration efficiency, download the Energy Star product spreadsheet, delete the top 3 rows, then sort the data by Volume (first) and Configuration, with header row checked. You'll see for most refrigerator sizes, at least the ones I checked, bottom freezer models use significantly more power annually than top freezer models. That probably is due at least in part with the bottom freezer being adjacent to the hot compressor and condenser compartment.

crewzer · November 2008

Re: Refrigerator power

Moe,

Always glad to help. I'm glad you found my fridge reports to be useful!

First, certainly the watts output by a MPPT charge controller must be less than watts input, and it must be less than 100% efficient. I have no problem with that. However, when it comes to amps of charging current, in winter, when panel temperature is lower and panel voltage is higher (particularly with 17+ Vmp STC panels), and when at the same time, battery discharge is likely to be deeper and thus the voltage difference between panel and battery greatest, the MPPT controller should offer its greatest "gain" over non-MPPT controllers, right? So how can you account for that when it comes to efficiency?

Since the efficiency is W out / W in, I just use ~96% year ‘round.

This leads into the second question, and that's where do you get 85% efficiency for the PV modules? I was using roughly one minus 85% (16%) times the kWh/m^2/day insolation. Although hours of daylight go up in the summer, so does temperature, resulting in lower voltage and less power. Insolation history doesn't account for that, so it seems efficiency should, and be seasonal.

As Mike indicated, full PV module output is based on Standard Test Conditions (STC). This includes a module cell temp of 25C. However, cell temp under full insolation is ~30C to ~35C above ambient, so PV modules rarely operate at 25C during the day.

85% “operational” efficiency is a good rule of thumb for PV modules. As you suggest, it can be as low as 75% on a very hot summer day if the modules are mounted close to a roof, and it can be as high as 95% on a very cold winter day. Again, YMMV.

Finally, I recommend you not get too hung up on module efficiency specs. The STC spec is a good reference number for estimating performance.

HTH,
Jim / crewzer

icarus · November 2008

Re: Refrigerator power

"There are thousands, if not tens of thousands, of Dometic RV absorption refrigerators now under recall for cracks that can let ammonia leak and hit the flame. Dometic is blaming the cracks on a higher wattage 120VAC element they went to in the late 1990s and the recall puts them back to the original 325W element. If that IS the problem, it shouldn't be an issue for their residential refrigerators, which seldom if ever get used on 120VAC. The RV units switch to it whenever plugged in to campground or generator power (unless overridden)."

Moe<

You are correct about many of the Dometics being recalled. The following is a link to perhaps the best Absorption fridge folks that I know of. They also have a very useful forum.

http://www.rvmobile.com/

"Out of Level. If the cooling unit is operated in a stationary, out of level position (on any heat source), it will eventually become permanently damaged. Before we go any further, there is one more ingredient inside the cooling unit: sodium chromate. The ammonia solution inside the cooling unit is a mild corrosive, and sodium chromate is mixed with the ammonia solution (ammonia and water) to neutralize the corrosive effects of the solution, protecting the inner pipes of the cooling unit.

Since the cooling unit depends greatly on the effects of gravity for moving the liquids and gases inside, running it off level and stationary causes these liquids and gases to collect in unwanted areas and not be recycled back to the boiler. The liquid level inside the boiler begins to drop and become weaker. Eventually, the water in the ammonia solution begins to vaporize with the ammonia and leave the boiler. At some point, the boiler becomes dry and the temperature rises rapidly inside. The sodium chromate which was once in solution with the ammonia solution is left behind and begins to burn and permanently change state from a powder into a sort of sludge that will eventually plug the perk tube. If left to cook long enough, the sodium chromate will become as hard as steel. If the cooling unit were "saved" from this out of level condition by being leveled, or the heat source turned off, any sodium chromate that had changed state would not return to a powder in solution with the ammonia solution. This makes it possible to ruin a cooling unit a little at a time."
(From RVmobil)
Another way to get good quality Dometics (or others as well) is to buy or salvage for a free a fridge with a blown cooling unit. These can often be had for free. New quality cooling units can be had for way less than the price of a new fridge. Installation is fairly easy.

Another option, if you are looking for security is to by a fridge from a good rebuilder that offers a warrantee. I have bought several over the years from a Re-builder in Monction NB with good results. I also know that RVmobil has a good rep as well.

Tony

PS Another advantage of the new electronic control Dometics vs the Danbys or others is that the gas goes off when not needed, rather than to low flame. In the winter, our old fridge would burn on low flame for weeks waiting for the t-stat to call for high flame to cool the unit, all the time burning LP wastefully. The new style, ignites when needed, shuts off when not needed. (Does require 12vdc however)

mike95490 · April 2009

Re: Refrigerator power

Refrigerators
Last reviewed: May 2009 www.ConsumerReports.org

This article is the archived version of a report that appeared in the May 2009 Consumer Reports Magazine. Visit our refrigerators page to see the very latest information on refrigerators.

In May and June, stores hold sales to make room for new fridges, so it's a good time to get a deal. We tested 80 models in four categories to help you find a refrigerator
that delivers high performance and efficiency at a good price:

--

So - the online version is out, and has efficiency listings.
Mike

niel · April 2009

Re: Refrigerator power

again see post 55 for my reply on this thread, http://forum.solar-electric.com/showthread.php?t=9

i have decided to put this whole thread into the energy use and conservation area as they tend to parallel some on the same subject matter. i will not combine them however, as they are both large, but be aware of both threads.

lotek · April 2009

Re: Refrigerator power

Dayton Ohio, My sister-inlaw lives there, and its FREEZING that time of yr.
Just put the frig on the porch and you wont have a power bill for it.
Back in the day. when I lived in the north Ga mountains, we did this, and had to put the milk inside the house at night to keep it from freezing. The house would get down to around 45 at night after the fire whent out, but warmed rapidly after the cook stove was stoked for morning coffee.

RCinFLA · April 2009

Re: Refrigerator power

I found defroster defeating was not worth the trouble. Depends where you live.

I rigged a switch to allow it to be shut off. In Florida, after hurricane, without air conditioner, it took only a day before cooling suffered and refrig ran longer to keep temp down.

Mine goes to defrost every 16 hours and cycle last for 20 minutes. It draws 1000 watts when the defrost heater is on but during the 20 minute cycle the heater cycles on and off. The actual heater cummulative ON time total was only 5 minutes for each 20 minute cycle.

Your inverter has to supply the peak load for the heater but if it runs the refrig it has the capability since the compressor start current is usually higher then the defrost heater.

System2 · November 2009

Re: Refrigerator power

Ah, the sweet relief of my cranky 3 year old Consul propane fridge getting the boot,to be replaced with the most efficient 12/24v DC refrigerator on the market. Made by Sunfrost and available in sizes from 4 to 19 cu ft it also comes as a 120v version. www.sunfrost.com.(USA)

In my research, every Energy Star rated conventional 120v AC fridge is a power hog in an off grid sytems and requires a huge investment in panels and batteries and anyone considering one should look closely at their decision. Better to spend a little more and buy a high efficency model and save on the cost of PV panels and batteries.

Best site I've been to that delves into the inner workings of propane fridges is
www.UniqueOffGrid.com (Canada).

Off grid for 17 years and lovin' it.

Windsun · November 2009

Re: Refrigerator power

SUNSHINECOASTER wrote: »

Ah, the sweet relief of my cranky 3 year old Consul propane fridge getting the boot,to be replaced with the most efficient 12/24v DC refrigerator on the market. Made by Sunfrost and available in sizes from 4 to 19 cu ft it also comes as a 120v version.

Most efficient on paper perhaps, but we stopped selling SunFrost about 8 years ago because of their inflated claims, and the fact that you can run a much cheaper Energy Star rated off the shelf refrigerator (such as Whirlpool) for a lot less money overall. And you don't have to put up with SunFrosts' poor construction.

A SunFrost RF19 can easily cost over $4000, which nowadays can pay for quite a bit of solar panel. They made sense when solar was $10-12 per watt, but at $3 per solar, not so much. The one single thing that SunFrost has going for it is that DC models are available for setups with no AC power available.

mike95490 · November 2009

Re: Refrigerator power

SUNSHINECOASTER wrote: »

Ah, the sweet relief of my cranky 3 year old Consul propane fridge getting the boot,to be replaced with the most efficient 12/24v DC refrigerator on the market. Made by Sunfrost

Don't be a sockpuppet & and don't dispose of the propane unit quite yet

icarus · November 2009

Re: Refrigerator power

While I have never been a big fan of Consuls, I agree with Windsun and Mike. Do a bit of research to see if that is the way you wish to go.

Tony

PS Do you know Mark and Rosemary Lennox in PW?

tallgirl · November 2009

Re: Refrigerator power

My back-of-envelop "panel costs" equation for a 450KWh / year Energy Star fridge is

450KWh AC / year / 365 days per year = 1.23KWh AC / day
1.23KWh AC / day / 0.75 (DC to AC derating) = 1.64KWh DC / day
1.64KWh / day / 5.1 hours insolation per day = 320 watts

320 watts @ KD210GX for $2.81 per watt is $899.

Add balance of system components, markup and labor and you're still WELL below the cost of a SunFrost.

And the 450KWh / year Energy Star model is going to be larger and not look weird.

Dave Angelini · November 2009

Re: Refrigerator power

Sunshine,

This is a big change for you! The hard part will not be the energy use but rather having to run a generator alot more. In your location there is not the option of adding more panels to suppliment. There just is not enough dayliight and too many days of that will cause some big changes after 17 years. It is hard for gridtie people to understand because they just see the math.

Take a look at the link below for the 18 cu. ft. Crystal Colds. The guy's below service the whole west coast of N. America. The web site is really good for gas reefers of all makes. Good Luck !

http://www.gasrefrigeration.net/cabin_refrigerators.htm

icarus · November 2009

Re: Refrigerator power

For all you absorption fridge users (or maybe users) out there, here is a link to what I think is the best fridge advice on the continent. They host a forum for like this one for LP absorption fridges of all types, going back to the early Kerosene Servels right up to the most modern Dometics.

They rebuild and sell fridges, and the guy who moderates the forum is pretty sharp as to all the ins and outs of these fridges. His tips have save me tons of aggravation over the years. Handy source for parts as well.

http://rvmobile.com

Find the forum and read though some of it.

T

Dave Angelini · November 2009

Re: Refrigerator power

icarus wrote: »

Find the forum and read though some of it.

T

Nice forum and very useful! Thanks! But, if they don't sell/service the big ones like the Crytal Colds they are missing what I "think the last OP" was so happy to dispose. The small 8 cu. ft. types are not realistic (for long) with users especially the female ones. About half of my customers have gas reefers and the 18 cu. ft.
Crytals are pretty much bullit proof and work in hot environments and up at 9,000 feet as well.

BTW the Amish make the Crytal Colds and them put othe names on them as well.

Refrigerator power

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