First Off-Grid Solar Freezer (for Dummies)
“Building Your First Solar-Powered Off-Grid Freezer (for Dummies).”
Jeffery Yago, is an electrical engineer who writes for the website, Back Woods Home. In one article, he talks about how to set up a freezer in a remote back woods cabin without grid electric power. He does a nice job breaking it down to the level where us non-engineers can understand the issues. I’ll summarize my synopsis here and link to a very useful handout from Grape Solar (see age 4) aimed at rank beginners who need to compute their power requirements. (Hat tip to TallestManOnEarth and my neighbor, Gerry.)
Danfoss is an OEM manufacturer that has made a highly efficient, very dependable compressor used by many companies to run their small refrigerator / freezer products marketed for RVs, boats and back-woods cabins. It runs on 12 Volts DC and uses a brushless motor making it ultra-dependable. It is ideal to run off a bank of deep cycle batteries.
Freezers build around this compressor are expensive ($1,000 +) but very energy efficient and durable. The 12 Volt DC requirement makes it possible to run off batteries without requiring the equipment to convert DC battery power to AC (an “inverter”). So the expense of the freezer is partially offset by a much less-expensive, more-efficient electrical solar system.
I’ll focus on one model, this 8 cubic foot chest freezer: the SunDanzer CDF225. It is just under 4’ long x 3’ tall, and has about double the capacity of the 4 cubic foot freezer found in most American kitchens. Cost is about $1,400. (I know. I know. Ouch!!)
So we have decided to put this freezer in our hypothetical remote cabin and power it from batteries. The batteries will be recharged by solar panels. What kind of system will be needed to support this freezer?
Refresher: A couple of basic electrical terms.
Amps. Electricity can be envisioned as a river of electrons flowing through a wire. Amps describes how many electrons per second are going through the wire. Amps measures the flow of current.
Volts. How much pressure the electrons are under as they flow through the wire. When a battery “pushes” electrons out into a wire we want to know two things: how many electrons per second (Amps), and at what pressure (Volts).
Amp-hours. Describes how many electrons a battery can store.
How much “juice” does this freezer require?
Yago includes a table that gives the power requirement for several models during summer and winter. Warmer days make the compressor run more. For this model:
Summer: 40 amp-hours/day.
Winter: 36 amp-hours/ day.
(~3 amps of current per hour)
An amp-hour is a measure of charge capacity–how much electricity a battery can store. Knowing that our freezer needs 40 amp-hours/day lets us calculate how many batteries we need.
Choosing our batteries
Lets assume that we have occasional rainy or cloudy days where little sunlight falls on our solar panels. Lets choose battery capacity large enough to run the freezer for 4 cloudy days.
40 amp-hours/day x 4 days = 160 amp-hours.
Unfortunately, deep cycle lead acid batteries are damaged if discharged past 50%. So to have 160 amp-hours of stored electricity we will need to buy 320 amp-hours (or more) of battery capacity.
We must choose batteries designed for deep repeated discharging. Car batteries will give out after only a few months if used this way. Golf cart batteries work the best. 6V Trojan T-105 golf cart batteries, $148 each. (Thanks again Tall.)
Each of these 6 Volt batteries has 225 amp-hours storage capacity. 2 of them hooked together in series, will give the needed 12 Volts DC current, but still only 225 amp-hours—not quite enough stored electricity to run our freezer for 4 cloudy days. To get 320 amp-hours capacity, we’ll have to buy 4 batteries. Here is how to hook them up.
Diagram: one golf cart battery
Diagram: Two 6-Volt batteries hooked together in this way (in series) behave just like a single 12 volt battery which is what our freezer needs. But they have only 225 amp-hour capacity—not quite enough.
Diagram: Four golf cart batteries can store more than enough juice (450 amp-hours) to power our freezer for 4 days, at 12 Volts, without discharging below 50% capacity.
How many solar panels do we need to charge the batteries?
How much current does one 100 Watt solar panel produce? Solar panels are marketed by their peak power output in ideal conditions. Since solar panels all pump out electrons at a “pressure” of about 16 Volts, the current produced by one 100 Watt panel, during one hour of full sunlight, is given by:
100 Watts / 16 volts = 6 amps of current
During a sunny summer day, a single panel makes 6 amps/hour x 6 hours of sunlight / day for a total of 36 amps of current per day. Recall though that our freezer needs 40 amp-hours / day. So one 100 Watt panel is not quite enough. We will need 2 panels–a 200 Watt system.
The addition of a second solar panel will allow putting more amp-hours of charge into the battery than will be consumed by the freezer that day. Having extra amp-hours going into the battery on a sunny day will carry us through those days without sun.
Hooking it all together using a charge controller.
This system uses the Morningstar-SS-10L-12V-SunSaver-Charge-Controller available at Amazon for $56. The places to hook the wires to the solar panels, batteries and freezer are all labeled.
TLC for batteries
Batteries are damaged if overcharged. Since our solar panels don’t “know” when the batteries are full, we must use a “charge controller” to shut of current flow when they are full.
Likewise, batteries useful life is reduced when they are discharged too deeply. For golf cart batteries, consider a 50% discharge the limit.
However, like horses pulling a heavy cart, the strain put on each horse depends on how many horses are sharing the load. Adding extra batteries in the system makes them all last longer as it does not force them to discharge so deeply. In a prolonged grid-down situation, replacement batteries will probably not be available.
And if the grid is available, using the grid (rather than the battery) to run the freezer, similarly prolongs battery life. (This is why Yago includes the grid connected 12 Volt battery charger in the wiring diagram above.)
Solar Kits from Costco
Grape Solar is a reputable company that makes small off-grid solar kits. The 200W kit, enough for our freezer, is available at Costco for $650.
This kit comes with an inverter (used to convert DC to AC current) something that is NOT needed for this refrigerator. But it conveniently includes connecting cables. Each panel has an aluminum mounting frame on it. The user will need to build a mounting rack. (Face the panels to the south, and tilt them up according to your latitude—38 degrees where I am in Virginia.)
For those of a more DIY bent who would prefer to not use a kit, individual solar panels can be purchased at Costco for ~$140 each and the charge controller (12V-10Amp) from Amazon for $56. Connecters, wires and the batteries would need to be purchased separately.
Off-grid freezers are not cheap.
For the 8 cubic foot SunDanzer CDF225 discussed above:
$1,400 for the freezer
$ 600 for 4 batteries
$ 650 for a solar kit with 2 panels and some lumber to mount the panels.
$ 2,600 total
The next smaller sized freezer, the 5 cubic foot SunDanzer 165 is less expensive and requires a less expensive system to support it.
$ 950 for the freezer,
$ 300 for 2 batteries,
$ 140 for 1 solar panel (or this 100 Watt kit)
$ 56 for the Morningstar 12V-10Amp charge controller
$ 20 for some wiring and connectors
The Ice House at Monticello
This cost makes me reflect on a recent trip to Monticello, the plantation home of Thomas Jefferson. Three centuries ago he had an ice-house, a mammoth brick and mortar cylindrical structure dug deep into the ground under a shady side of his house. During the winter, slaves would go down to the river and cut surface ice with hand saws and carry it back on horse drawn carts. If the ice-house could be filled by March, they would have refrigeration until late August.
The ice-house allowed the household to preserve game greatly improving the nutritional quality of their diet. Nutrition and sanitation are the twin pillars of our ability to resist infectious diseases and to remain productively healthy.
Compared to Monticello, a “mere” $2,600 for a solar freezer doesn’t sound quite so bad.
I wonder if there isn't a way to do without the batteries and charge controllers and use the 'coldness' of the freezer for the storage? I guess you might need to beef up the insulation.
'i've been using a solar freezer for about 5 years. i use the GMAT marine batteries from cabella's and they last about 3 years. cost of the batteries equals out to what one would pay for the same amount of electricity…so someone has this figured out. the batteries right now are the weak link and it's almost like the ptb are keeping it that way.
i've been thinking about experimenting with the idea of letting the sun (via the sundanzer)cool the freezer at night and if well insulated it should carry thru the night til it gets another boost from the sun. the heat from the condenser still has to be vented away from the freezer.once the food is frozen that mass should keep things cool. also things like freezing food first in a reg freezer or putting warm foods in only during a nice sunny day.
my entire house works on this principle, of being so well insulated, that i can run the ac or furnace just twice in a 24 hour period.
so yes i think some other options are very possible, and i can verify sandpuppies costs analysis.
remember if you live where it snows, put your panels where you can scrape them off regularly
correction let the sundanzer cool during the day
Wow, thanks for sharing your research on this topic. This is on my list and I have briefly looked into it before. This info takes the edge off; I have not found a straight path for this exact issue of keeping a freezer going off grid. Thank you
James_knight_chauser and ferralhen, Good questions. I took this issue to my friend Gerry who is a engineer, and he pointed me to some OEM spec sheets that mostly went over my head.
But I did get this much:
1. Yes, it is possible to connect a solar panel directly to several of the Danfoss (now Secop) compressors as they are designed to work over a range of voltages from 12 V to 40 V. Theoretically, this could avoid the need for charge controllers and batteries.
2. And, he agreed that one way to keep a direct solar freezer cold during hours without sunlight is called "thermal banking." Basically, you put jugs of water in the freezer and freeze them solid when the sun is up. Then during the night, the melting of the ice absorbs heat and keeps the freezer cool. Here is a graph of this happening. An engineer could compute the exact number of jugs of water needed to keep a specific freezer cool for a specific number of hours…..
But there are two problems with thermal banking: 1. Space given to water jugs is space not available to store food. And these freezers are little to start with. 2. Once the ice completes its melting, the cooler warms. And if the next couple of days are cloudy, your food thaws too, …. then spoils….. It doesn't seem to me that thermal banking is dependable enough for food storage that is essential to survival.
3. It takes a big surge of current for a fraction of a second to start a compressor. A direct solar setup would have to have enough solar panels to supply this PEAK STARTING current.
Graph of starting current: Left axis is amps of current and bottom is time in milliseconds.
4. Every time a cloud passes over, solar panels stop producing current. The compressor would stop, then need to restart again, over and over, wearing it out prematurely.
A battery bank would supply A) the starting current surge, B) a continuously current even when sunlight was intermittent, and C) enough current to keep the freezer running through 4 days without sunlight.
An important distinction in Yago's setup above is to think of the freezer as battery driven. And, the batteries as charged by solar panels.
amorphous solar panels work when it's cloudy and that is why i have a combination of those and polycrystalline panels. i'm thinking about ac from these compressors, for just one room to sleep in.
ultimately, i keep thinking supply chain disruption and where am i vulnerable.
Thanks for putting this together Sand_Puppy. I have the panels and related components, but I'm waiting until we get our new roof on before installing them and buying the batteries. I will be taking this through an DC/AC converter which will reduce overall efficiency, but allow me to use a standard AC freezer and also use the system for the occasional charge of other household electronics. Another way to reduce the load of the batteries is to add extra insulation to the freezer or keep it in a root cellar or basement. I have considered burying mine below grade, but have not looked into the technical challenges of that yet.
One note on efficiency. Standard front door refrigerators and freezers loose a lot of cold air every time the door is opened because cold air sinks and with a front door freezer there is nothing to hold it in. It spills out each time the door is opened forcing the compressor to go to work. Top door freezers are much better in this regard because the cold air settles into the freezer itself rather than spilling out. One simple way to avoid thermal loss with a top door chest freezer is to open the door very slowly which limits the vortex that can draw cold air up with the door and allow it to spill out. Crack the door slightly and wait a second for the pressure to equalize then slowly draw it open. Also, know what you want before you open it so that you don't keep it open longer than necessary. It is a small gain, but in a energy-constrained future the small gains will make all the difference. As my Dad used to say to me when I was a kid, "Close that door, you're letting the bought air out."
years ago I investigate using low voltage DC applicances for off-grid or grid down scenerio. I've reached the conclusion that switching from AC to low voltage DC is not the best option
1. Low voltage DC loses a lot of energy in cabling. To power a 120W device at 12V requires 10 Amps of power. With at AC, 120W requires about 1 AMP. Thus for applicances that need a lot of power, very heavy gauge cabling is needed to be installed. The cost of the heavy cabling is expensive. Even with the heavy gauge cabling there will still be a large loss because the much higher current will result in higher resistance losses. Power loss = I^2R. Power loss increases at square of the current load. 10 times the current and the power loss is 100 times.
2. Low voltage DC system can be prone to fire if the cable heats up or begins to arc. AC power switches off 120 times per second which provides a sufficient time to quench an arc.
3. In a long term grid down, it make become impossible to source replacement parts locally. Few People will have Low voltage DC appliciances. However there will be excessive amount of AC applicances that can probably obtained for free in abandon homes.
4. An off grid AC power system is more flexible since you can power just about every electrical device since they are all designed to use AC Power. Its much more difficult to find DC appliances that manufactures always charge a premium compared with DC systems. In addition to a refriderator perhaps you want to power up a laptop/desktop PC, well pump, clothes washer, cordless drill battery rechargers, gas stove/oven (electric controlled oven thermostat), window fans, LED lighting, etc. While it may be possible to source all these devices that except DC power, its fair easier to source them with AC power inputs.
If you choose to go with a DC system, I would recommend looking at using 36VDC or 48VDC systems instead of 12VDC since it would cut the amount of current needed by 1/3 or 1/4, which would reduce cabling costs and improve overall system efficiency. Danfoss also sells 24VDC and 48VDC compressors:
One last comment is that its possible that the PV panel may not recieve sufficient light to keep the batteries charged. Perhaps the region experices extended overcast for many days which causes a deep cycle drain, or complete depletes the battery, or snow coverage obscures the PV panels. One option is to incorporate a portable inverter generator with an electric start that can automatically start and recharge the battery as well supply power to run refrigerator. The electric start inverter generator can be started using a microcontroller (ie Arduino) or a SBC such as a raspberry pi. I believe some charge controllers or AC inverters also offer auto-generator start to recharge the batteries if the PV panel cannot provide sufficient power if your not the DIY electronics type.
"1. Yes, it is possible to connect a solar panel directly to several of the Danfoss (now Secop) compressors as they are designed to work over a range of voltages from 12 V to 40 V. "
I don't think so,and here is why.
1. The PV panel may not always provide sufficient voltage. During partly cloudy days or when the sun angle is large relative to the PV panel, the output voltage can be significantlly lower. for instance, a 12V Panel may produce only 8 or 9 volts. This will either cause the motor to stall or put excessive stress on the motor as the windings draw more current and saturate the windings. Motors (DC or AC) are subject to saturation which cause input power to be dissipated at heat instead of mechanical work. This is a bit complex to explain. In general practice, a motor should not be operated at low at a voltage below its rated input. Operating a motor below its operating voltage will sigificantly reduce its operating lifespan.
2. DC/AC Motors under load will pull a current surge when starting up. The PV panels may not be able to source sufficient power to start up the motor. Especially if the Panel aren't providing 100% of the thier output because of the sun angle.If the motor fails to start, its possible that the current flowing will saturate the windings and begin overheating them, damaging the motor.
3. The Batteries also serve to provide power at night and during overcast. I am certain that a direct tied PV refrigurator system will have exteremely poor temperature regulation resulting in frequent food spoilage, especially in periods of overcast which exceeds thermal banking measures.