# Could we make a practical solar fridge?

Staff Emeritus
In another thread, @Grinkle asked about a solar fridge project. In the end, he decided against it, but it set me to wondering if it could be done for people living off grid.

At this point, I really miss our beloved Jim Hardy. Jim would have brainstormed with me, then he might have gone in his back yard and prototyped the idea. So what do PF engineers familiar with refrigeration have to say about the following? Ping @russ_watters and others.

1. I would start with a chest type freezer, divided into two compartments upper/lower. Let's call them freeze/cold. An insulated partition divides the two. Remember that the top loading design saves a lot of energy because cold air is not spilled on the floor when the door opens. The object is not to make a freezer. The object is to keep the cold compartment cold enough for food storage.
2. A hole in the partition divides the two, and a sliding door covers up a portion of the hole to make the hole area adjustable.
3. The freezer's cold plate (where the evaporator coils are), should be in the lower half of the chest freezer. That is unlike those in this schematic of a commercial freezer that appear to use the whole vertical space for evaporator coild.

4. The compressor motor would be the BLDC type. Variable speed. The BLCD controller would be directed by a microcontroller with custom logic.
5. A solar panel would supply power to the motor. No inverter needed. No MPPT controller. No grid connection. Assume a daily profile of solar power available to look like this:

6. The idea is to use the freezer compartment as energy storage. It would be filled with jugs of water to freeze. The cold compartment stores food like a refrigerator. To be successful, the freezer compartment needs to store enough energy to keep the cold compartment cold for say 3 days without solar power.
7. According to this source, a medium size chest freezer uses 230 kWh per year. That's 0.63 kWh, or 2.3 million joules per day. Pretty low. 3 days without power means about 2 kWh energy storage needed. If we froze 30 kg of water in the lower compartment, and the latent heat of ice is 336000 J/kg, the total energy storage is about 10 million joules. So, 30 kg appears to provide enough storage to cover 4 days with zero solar power; that's long enough for many bad weather systems to pass.
8. In rough numbers, a 150 watt PV panel could provide 0.63 kWh or more per day.
9. A bit of antifreeze in the water should prevent expansion due to freezing. That makes it similar to so-called blue ice that they sell for ice chests. Would that negate the benefit of the high latent heat of freezing? I don't know.
10. Inspiration for this idea comes from the refrigeration I had on my boat. It had a cold plate and two compartments. However the two were side by side, and the divider was vertical. I'm not sure how significant vertical versus horizontal is.
11. The CEO of my boat's system told me that max efficiency could be attained with a variable speed compressor motor that spins 24/7 and never cycles on/off. But he also said, customers didn't like constant noise, and they called customer service if it didn't cycle on/off. So the version he markets used the less efficient constant speed cycling compressor motor.
12. So, my strategy is to pair the variable speed compressor motor with the variable quantity of solar power during the day. The micro controller would measure PV panel voltage, and current, and vary the motor speed to draw maximum power available from the PV panel. At night, the motor would just stop. It would freeze the water, then lower the temperature of the ice for additional storage. A second control loop would open/close the door covering the hole between the two compartments, regulating cold side temperature. [Maybe a muffin fan could replace the door.]
13. Only if cold side temperature approached freezing, would the whole system cycle off. (That would be @Grinkle 's case because he wanted to put it outdoors in a location where ambient temperature in winter is below freezing.)
14. In rough numbers, a medium size chest freezer costs about $250, and a 150 w solar PV panel also costs about$250. So a target cost of $500 for the whole system is not crazy. If I had such a solar fridge system, I would call it the Jim Hardy Solar Refrigerator. Richard Crane, BillTre, phinds and 2 others ## Answers and Replies Science Advisor Gold Member A bit of antifreeze in the water should prevent expansion due to freezing. That makes it similar to so-called blue ice that they sell for ice chests. Would that negate the benefit of the high latent heat of freezing? I don't know. I would want to use the latent heat with a material that has a phase change near the operating temp to minimize the space needed for storage. It's a lot of energy. Maybe just deal with the volume change in water? hmmm27, BillTre, The Bill and 2 others Gold Member Is a shell approach feasible? Instead of an upper / lower configuration, install a shell around the inside of the freezer and stack ice-packs inside the shell, water or blue-ice or anti-freeze or whatever, inside plastic bags or hard containers. The shell material to keep the heat-sink liquid containers stacked in place and separate from the food in main inner compartment could be made of light metal grill plates. This would better match the existing coil design of the freezer you show, I am assuming that coil placement is typical of such freezers. It would also keep the temperature more uniform than having all of the cold heat-sink liquid at the bottom, and room could be kept to allow for expansion if the liquid freezes. BillTre, The Bill and anorlunda Staff Emeritus This would better match the existing coil design of the freezer you show I like that. Less modification as you said. The shell barrier could be closed cell foam. Do modern freezers use induction motors for the compressor? If yes, then that needs to be swapped out for a new BLDC motor of at least the same HP. I'm usually not a hands on type of guy, but this is beginning to sound like a DIY project that I could succeed at. I'll give it thought when I return to Florida for the winter. No worries about freezing temperatures in Florida, so I could leave it outside. The Bill The Bill Another option is to use a nonimaging solar heat collector (maybe something similar to the evacuated glass tube collectors on solar water heaters) to power an absorption refrigerator. https://en.m.wikipedia.org/wiki/Absorption_refrigerator Adding a fair bit of phase change thermal mass as suggested upthread would probably still be a good idea here. sophiecentaur and anorlunda Science Advisor Gold Member Also, assuming it works OK, I like the idea of using water for thermal storage in some structure that allows the user to configure, maintain, or repair that part themselves. Exotic materials may be good for the spec sheet and initial sales, but wouldn't it be nice if a guy with some RTV could fix their leaky freezer instead of buying parts from the internet? For example, why have a separate compartment when you could just say something like "put 5 liters of water/ice in it to maintain temperature stability"? Let them figure out what kind of container it should be in. This could empower the guy in Toronto to do something different than the guy in Kenya. Last edited: anorlunda Gold Member Do modern freezers use induction motors for the compressor? If yes, then that needs to be swapped out for a new BLDC motor of at least the same HP. Some reading up on this flagged for me that swapping out a compressor involves dealing with the coolant in the system, one needs to discharge and recharge the loop, and this requires special equipment I think. I also found a link to some DC compressors that seem to have right controls (speed control, temp sensors etc) integrated into them. https://www.danfoss.com/en/products...n/mobile-dc-cooling-compressors/#tab-overview Swapping out the compressor might be phase 2? I think one can install the thermal mass, run it from an inverter-AC setup, measure power savings vs no-thermal-mass installed, and have good confidence via calculation how much power savings can be had by doing the compressor swap to DC, which I expect is a lot more effort than putting thermal mass in the fridge, but perhaps that's where most of the fun is. anorlunda Staff Emeritus Some reading up on this flagged for me that swapping out a compressor involves dealing with the coolant in the system, one needs to discharge and recharge the loop, and this requires special equipment I think. Uh oh. I never took one of those apart. I assumed, perhaps naively that the motor driving the compressor was separate, not integrated. I visualized swapping motors not compressors. Like I said, I miss my friend Jim Hardy. Swapping out the compressor might be phase 2? I like the idea of a phased experiment. 1. Data logger project. Measure and log power consumption, plus several temperature points. 2. Unmodified freezer. Test the idea of a frozen water thermal mass being able to extend the tolerance for power outages. 3. Modify the freezer controls to make it run at refrigerator temperatures rather than freezing temperatures. 4. Repeat (2) with a non-frozen water thermal mass. 5. Add a barrier to create a central second refrigerator compartment. Freeze the perimeter water jacket and repeat the experiment. 6. Swap the motor (and compressor???), add a microcontroller to control speed and intercompartment door. Repeat the experiment. 7. Replace grid power with solar power. Repeat the experiment. 8. Write it all up as a DIY article. 9. Donate the whole rig to a hiking club that would be amused at having a cold beer stop at some remote place far from the grid. Richard Crane and Grinkle Science Advisor Gold Member Completely recharging a refrigerant system is best done by a professional. In many places (probably the whole USA) you can't even purchase the refrigerant without showing documentation of your training. In addition to the refrigerant, there is the dessicator cartridge that must be replaced once the system is opened to the atmosphere. Then a vacuum pump is used to purge the air from the system before introducing the new refrigerant. The amount of refrigerant is critical, too little and it won't get cold enough... or at all, too much and you risk damaging the compressor when it tries to compress the liquid refrigerant. Let the pros handle this approach! Cheers, Tom anorlunda Gold Member I would want to use the latent heat with a material that has a phase change near the operating temp to minimize the space needed for storage. Perhaps use salt water to lower the freezing point. Then you can store frozen items as well. too little and it won't get cold enough I have an air conditioner that has leaked some of its refrigerant and it has a reduced capacity but freezes solid very quickly. BoB hmmm27 Science Advisor Gold Member I don't know why, but I know a system that has lost some of the refrigerant can get colder than normal. Mentor I don't know why, but I know a system that has lost some of the refrigerant can get colder than normal. And we know this how? Seems pretty counterintuitive... Science Advisor Homework Helper And we know this how? Seems pretty counterintuitive... I think if the refrigerant pressure is such that the phase change is closer to ambient temperature that the overall efficiency can be maximized. This will put you closer to the ideal "Carnot" refrigerator. But you are screwed if you need to go to WARP 6 because the sun came out and ambient T went up.. berkeman Gold Member And we know this how? Seems pretty counterintuitive... Lower pressure of the refrigerant allows for a lower temperature. Less material however means less heat moved per unit time so you can't remove all of the refrigerant to get to really low temps. BoB DrClaude Science Advisor Gold Member Seems pretty counterintuitive... Yes it does. Science Advisor Gold Member berkeman Mentor I have an air conditioner that has leaked some of its refrigerant and it has a reduced capacity but freezes solid very quickly. Same thing will happen with a refrigerator or dehumidifier. The reduced volume and pressure allows all the coolant to boil to vapor in the first few coils so they ice up; and then the remaining coils contain only vapor which doesn’t do much cooling. With a full charge the coolant vaporizes more slowly across all the coils, absorbing heat from the entire surface. NTL2009, berkeman, jrmichler and 1 other person Mentor In order to have something to compare to, here is some energy data for the Frigidaire refrigerator that we bought in 2006. The energy sticker says 479 kWh per year, with similar refrigerators in a range of 391 to 484. It's one of the least efficient models available at the time. I put a Kill-A-Watt on it at one time, and measured 10.97 kWh in 237 hours, for average power consumption of 46 watts. Typical consumption while running is about 120 watts. The running power consumption varies, and occasionally reaches 165 watts. A temperature data logger placed inside showed the temperature varying between 37 and 40 deg F, at about 1.3 hours per cycle. The data logger agreed with the average power consumption in that the refrigerator ran at a duty cycle of about 38%. The total measured power consumption is about 405 kWh per year (1.1 kWh per day), which is less than the energy sticker rating of 479 kWh per year. I found a web site that said that refrigerators are rated at an ambient temperature of 32 deg C (90 deg F). Our house is typically 71-72 deg F during the heating season, and 78-79 deg when the air conditioning is running. The currently available efficiency champion is apparently a Sun Frost RF19, for which they claim 1.02 kWh per day (372 kWh per year) at 90 deg F ambient, and 0.77 kWh per day (281 kWh per year) at 70 deg F ambient. It all makes me wonder what would happen if I glued two inch thick foil face polyiso insulation to the outside of my refrigerator. It won't happen because it would not fit in the space and my wife would not allow it anyway. An extremely energy efficient refrigerator has another advantage. In case of a power outage, it stays cold longer. Nugatory, DaveE, berkeman and 1 other person Staff Emeritus The currently available efficiency champion is apparently a Sun Frost RF19, for which they claim 1.02 kWh per day Thanks. Useful information. That Sun Frost is a 16 cubic foot front loading refrigerator. I'm looking at 7 cubic foot top-loading chest-type freezers. Checking a couple of brands at Home Depot; they claim about 250 kWh/year. Pretty amazing that the Sun Frost is nearly able to match that. It must be extremely efficient. Staff Emeritus Sigh, It's so hard to have an original idea in today's world. I just found a solar freezer advertised for$1699, sans the PV panel. It claims 0.63 kWh/day energy draw, and a temperature range of -18C to +10C which means freezer or refrigerator.

That's a very steep price, but it sounds like it might be just what I proposed to do, except the compartmented system to make it ride out lack of solar power for several days.

https://www.homedepot.com/p/Unique-...d6620dd60fa615c14ab56ef6f02deddf&gclsrc=3p.ds

the compartmented system to make it ride out lack of solar power for several days
Just a thought
There is that thing about the density of +4 degree water, does this apply to water with antifreeze added too?
Based on this there might be a nice way to automatically maintain a stable temperature if the storage compartment is on the top.
Although this would kill any possibility of temperature adjustment.

Staff Emeritus
There is that thing about the density of +4 degree water, does this apply to water with antifreeze added too?
Other comments have discouraged me from considering antifreeze. The reason is the high heat of fusion as water freezes. That is a large amount of energy compared to cooling a few more degrees.

Gold Member
That is a large amount of energy compared to cooling a few more degrees.
When you use frozen pure water (tap water etc.) your freezing point is pretty close to 0°C. Foods often contain salts. Their melting point is a bit below 0°C. Inside your cooler your hamburgers are melting to keep your ice solid.

If you add some salt or other antifreeze to you ice packs you get a freezing point lower than common foods. Now you freezer pack melts to keep the hamburgers frozen.

You definitely want frozen cooling material to leverage heat of fusion but antifreeze in moderation will be helpful.

BoB

berkeman and Tom.G
The reason is the high heat of fusion as water freezes. That is a large amount of energy compared to cooling a few more degrees.
I meant it as a way to automatically transport heat between compartments while maintaining a constant temperature, but this requires further thoughts.
Since water is densest around +4°C, if the lower compartment is kept at that temperature then any heat picked up there would be transferred up => the temperature would be maintained.
Usually you don't need any lower than that for everyday usage.
But this requires antifreeze to keep it the transfer liquid fluid in the upper compartment, and I don't know if the water+antifreeze would also have this +4°C rule.

Vigardo

Despite it would be much less efficient than a properly tuned compressor and coolant gas, it is much simpler.

Staff Emeritus
That's a fair question. There are some Peltier coolers on the market. But I've never seen that scaled up to the size of a home refrigerator or freezer. Does anybody know why?

Here is a Peltier cooler for $99. It cools down to 40F (4C) Here is a compressor cooler for$239. It cools down to 12F (-11C)

Peltier modules are inexpensive, and might be well suited to this solar application.

Vigardo
Vigardo
You´re right they are really cheap. Thus, I bet their very low efficiency should be the reason:. In this link their efficiency is discussed. It seems not easy to calculate, as it depends on thermal difference, but I think it can´t be over 5 or 10% in practical designs.

Homework Helper
That's a fair question. There are some Peltier coolers on the market.

FYI there is a youtube guy who built a couple of intersting devices.

One is a solar desiccant-based air conditioner (no electricity).

This is basically a swamp cooler which uses sun to dry calcium chloride which predries the air. I think it was not as successful as expected , but very interesting.
The other is just a homemade Peltier cooler

Vigardo, OmCheeto and anorlunda
Gold Member
There are some Peltier coolers on the market. But I've never seen that scaled up to the size of a home refrigerator or freezer. Does anybody know why?
They are simple, but quite inefficient. Also very non-linear; good for relatively controlled environments. For small coolers the simplicity wins out. For big coolers they're too expensive. Whenever I hear of a peltier cooler, my first focus is on the waste heat, the hot side heat sink, and the size of the power supply.

My crude rule of thumb for the TEC coolers I worked with in the laser business was the rule of 3rds; to move 10W of heat you needed to input 20W of electricity and dissipate 30W to the environment. This sounds awful and doesn't agree with the efficiency specs you read on the TEC data sheets. But when you take into account all of the other thermal issues, it may be pessimistic but not ridiculous.

You'll find them in the back seat of pickup trucks, but I don't think they've penetrated the sailboat or RV market yet.

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jrmichler, anorlunda and Vigardo
Mentor
I once had one of those TEC mini refrigerators in my office. It pulled a steady 3 amps from a 12 V power supply, so the average power was 36 watts. It was similar to this screen shot from Amazon:

Compare to the average 46 watts for the low efficiency full size refrigerator discussed in Post #18. And the 9.3 cubic foot freezer in Post #20, which draws an average 26 watts. That's why thermoelectric cooling has never become popular - the efficiency is too low.

anorlunda
Gold Member
Another option is to use a nonimaging solar heat collector (maybe something similar to the evacuated glass tube collectors on solar water heaters) to power an absorption refrigerator.
I think that could be a good way to go. Absorption fridges were popular for camping because they could use gas, 12V or mains, depending on the circumstances.

But what about using a totally thermoelectric (Seebec / Peltier) system, with a solar furnace beamed onto a large area of Peltier panels (pretty cheap and modular) and some thick cable, routed round the house, to similar panels in the fridge?

The temperature of the hot source should be as high as possible, consistent with the panel not getting over-cooked (<320C or less for cheap units). That could be taken care of with a bimetal strip to defocus or deflect the incident sunlight. Efficiency could be a problem but hot weather would supply more cooling than cold - which is the right way round.

Any system with interrupted power supply could use water / ice as a thermal store but, in a static situation, a water tank wouldn't be a problem.