Great project, not just doing it but monitoring the data.
Makes me wonder about the energy cost of losing (i.e. replacing) a 1/2 gal of milk vs the energy cost of a gas cycle fridge. I know food wastage is a huge resource lost but I’ve always overlooked the fact that energy is a huge one of the resources.
(pulling random data points from around the internet)
It takes approximately 255 liters of water to make 250ml of milk. So 1020 liters per liter of milk. There's 3.78542 liters per gallon. So that makes about 1930 liters per half gallon milk.
510 gallons of water for 0.5 gallons of milk.
It takes between 2500 and 5000 joules of energy per liter of tap water depending on the water source (2000 times that for bottled water). So up to 9,650,000 joules of energy per half gallon of milk for the water(about 2680 Wh).
1.25 to 1.44 kg of feed to produce a liter of milk (0.2645 gallons). To produce 1kg of wheat requires between 500 and 4,000 liters of water. But then you need to transport the feed to where the cows are...
Then you have to transport the milk. How far is it, and what sort of fuel is used? What sort of bottle is used? If it's come across the country via truck in a plastic bottle, that doesn't get recycled, that could be a lot of fuel. How long was the milk refrigerated before it reaches a home fridge?
There's 4 million joules to make a PET 1 liter plastic bottle. So about 7.5 million joules for a half gallon one(2 kilo watt hours). Often the transportation uses more than that for the bottle, but varies greatly. One amount mentioned is 4 million joules per liter.
That's 4 kilo watt hours just for the bottle.
There's lots more to it, but I guess 1-2 days of solar power production from a 5kW solar system for a half gallon of milk.
Kudos to the author! The best thing about this project is that it does not require any tricky mechanical modifications to the fridge itself. Mucking around with wires, inverters, and panels is scalable - I could build this myself using parts I already have in the event of the Apocalypse.
Some of the comments about heat pipes, heat transfer between freezer and fridge compartments etc. miss this point.
I got trapped in a fascinating rabbit hole like this a while ago trying to create hot water without (significant) batteries. The best solution seemed to be coupling the DC from the panels more or less directly to the heating element of the hot water cylinder.
Once you have a few of these lash-ups powering your new age off-grid sanctuary, it sounds like divvying up the power from the panels becomes the next major challenge.
Creating hot water is much easier and much cheaper. You run a copper or stainless steel pipe with water, paint it matte black, and put it in an enclosure with a transparent side. The sun will hear it up. It's quite popular in Greece and Italy.
Just get a solar water heating kit. They're quite common here in South Africa & are pretty much considered a compulsory first step for a solar installation since most existing houses are elec water heaters.
Some people also run them in sequence with an electric one. i.e. Solar one first & that feed the elec one. Meaning that you'll always have guaranteed hot water, near double the capacity but the solar one picks up most of the work.
Using this in a disaster recovery situation is one possibility I've considered, since these are increasingly parts that people have on hand or that are much easier to get distributed.
Awesome idea and writeups. Something I didn’t see mentioned as a consideration was an in-ground installation... not that I know it would benefit, and I know it’d be a whole heap of extra logistics. But root cellars and pit refrigeration were things before the appliance was invented.
I've always felt he was one of the more interesting personalities on the tech scene. Surprised he doesn't have a bigger following than he does, though I think maybe he likes it that way.
Thermal masses are a super cool way (ha!) to store energy, and I'm surprised they don't get more attention, both in the media, and in terms of research / investment. Heating and cooling accounts for somewhere around half(!) of residential energy use [1], which could be easily time-shifted with some very cheap thermal mass and some economic incentive to do so.
This is actually used in some commercial buildings in places where energy is cheaper at certain times of the day (for example, at night) [2], by freezing ice when energy is cheap, and thawing it when it isn't.
That being said, although this is a super cool project (I have the same inverter, and I made a similar control board ;)), this thermal mass doesn't seem like it would be particularly practical in most cases. Water has a relatively high specific heat capacity of 4.186J/gC, but given the narrow range of temperatures acceptable for a fridge, this doesn't end up being very much - only 79Wh per degree Celsius that the fridge is allowed to swing. If you consider 1C - 6C "acceptable", you only end up storing 395Wh. This is about 30-40% of the capacity of a $100-$200 "deep discharge" lead-acid battery, and is also a much wider range than most consumers would be used to (and may result in frozen veggies, for example).
In order to make this more practical, you really want something that can freeze around fridge temperature. For the same amount of water used above, freezing and thawing the water would store 6,308Wh(!), around 16x as much. If you could get something that freezes at 3C/4C with a similar heat of fusion to water, you could have a much smaller thermal battery that lasts _much_ longer, without the substantial temperature swings you see with your current design.
You need to take into account efficiencies of getting the cold into the fridge too. It takes around 10-15 hours of runtime to cool the thermal mass down from 5 to 0.5 degrees C (at 14C exterior temp), and the fridge needs 120 watts to run. Measured this way, the thermal battery is storing ~1200-1800 watt-hours.
But then, if it were powered from batteries, there would be significantly more power needed to fully charge the batteries and maintain good health -- my 860AH battery bank (4 deep cycle batteries) needs at least 1kwh input to charge up from 12v to full).
Another way to come at the question is, how many batteries are typically specced out to power an offgrid fridge, and banks costing 10-20k dollars are not at all uncommon, though they're also shared with other household needs.
I was subletting for a year from a family of six (who were away themselves). Their fridge was extremely oversized for the amount of stuff I stored in it, so I filled up two thirds of the volume with refilled water bottles.
It has the added benefit of stabilizing the temperature a lot. I recall reading somewhere (who knows, maybe one of the earlier blog entries here) that turning on a cooling element costs a lot of energy too on top of it just running, to the point where that effect on energy savings should not be underestimated.
All in all I saved quite a bit on my energy bill with quite a simple hack.
A material that can be frozen would be better, because it takes a lot of extra heat to melt a frozen material and so more cold could be stored.
However, this needs a material that freezes at a higher temperature than water, and most such are oils, which are less dense and so store less cold overall.
I've heard about projects where they store heat underground - not sure if they use a certain medium or groundwater or something, but they basically pump heat into the ground during summer (and cold out into the HVAC) and vice-versa in winter. I guess ground itself - whatever is down there - is good enough. See also the London underground for an undesired underground heat storage.
But yeah, back in the day they would get blocks of ice from nearby lakes and put them underground, it'd last all year.
Check the temperature of where you keep the milk. Some parts of the fridge are warmer than others. Lots of fridges now have spots for milk in the door, but that's probably the warmest spot. Move it to the top back where the cold comes from the freezer if your fridge is like mine.
A couple of weeks is unremarkable here. I get a carton from the local store. They sell a lot of milk so it likely hasn’t been in the pipe line for long before I get it.
Looking at other comments, and FWIW this is non-organic whole milk.
I think a lot depends on where you bought the milk from. Our current grocery store milk last much longer than the milk from a previous grocery store. It probably has something to do with temperature in their refrigerator or something.
> Using a supercapacitor to provide power while shutting down on loss of solar power, instead of the current few minutes of use of batteries.
I wonder why this is necessary. I've also seen it mentioned in the wiki, but no explanation.
Isn't a compressor the only electrical thing in a fridge? I know you must be careful with leaving some time between restarts, but why can't you just shut it down immediately on power loss?
> Fridge0 does not need a big battery bank, since it stores cold in its thermal mass and runs only when solar power is available.
> But, at least a mimimal battery is needed to run the computer control. Probably on the order of 5 watts for a computer like a raspberry pi; this could be reduced more with a more embedded computer like an arduino.
> Enough battery to run the fridge for a minute or two is also needed. Consider what happens when the fridge is running, and the sun goes behind a rain cloud. Suddenly, there's not enough power to run it, but it's still turned on. The inverter will try to run with whatever small solar power is still coming in, but it's not enough. Some inverters might manage a clean power off, probably accompanied with loud beeping. Other inverters might fail in more interesting ways.
> To deal with this situation, the computer needs to check the incoming solar power frequently, and power off the fridge if there's not enough. The battery is needed to keep the inverter and fridge running until that happens.
Basically, it sounds like the answer is because with a solar panel, there are states that are between "off" and "on," you rather just get an amount of power proportional to the amount of sunlight, and there's a concern that those states could cause problems if you don't provide a more constant power supply and active switching to fully shut off the power when your solar panel is not producing enough power for the "on" state.
I thought so too. Even if they’re all mechanical, it usually makes sense to keep the fan running across the evaporator to prevent moisture buildup (which will then sit in a place susceptible to freezing). It can also be more efficient to push this residual cold into the vessel.
My car’s AC gets funky if I don’t remember to turn off the AC a few minutes before shutting down.
Probably the best cold battery is to have a freezer compartment that's full of salt water which freezes at several degrees below 0, and some kind of adjustable heat link between the freezer and the fridge.
As far as peanut oil goes, it's actually not that much lighter than water by volume (91%), but it has half the specific heat, so it stores 45% as much heat per unit volume.
I couldn't find a reliable source for the phase-change energy for peanut oil, but it would have to be pretty high to make up for the difference.
Any luck with phase change materials for more dense « cold » banking?
You do say « However, this needs a material that freezes at a higher temperature than water, and most such are oils, which are less dense and so store less cold overall. », but I wouldn’t underestimate the enthalpy of melting.
Depending on your climate, a long heat pipe would be interesting to move heat outside when it’s cool enough outside (even if only during a few hours overnight). But that’s getting complicated to fabricate oneself.
Oops, edit: you have it outside already. How do you prevent freezing everything ? (Again, climate dependent).
One thought, if this is combined with a freezer and there is a pumped heat exchanger between that and the fridge, you can effectively use the freezer as the battery.
Small boats used systems like this: freezing blocks of ice to provide convective cooling to a cooler so that things stay cold without the engine running.
A lot of people hated them enough (and the gas used while sailing) to replace it with solar based systems.
Regular refrigerators use the same principle, albeit with a mechanism much simpler than a heat pump. They have holes between the freezer and refrigerator compartments and use a fan to push cold air from the freezer to the refrigerator. A heat pump would make that exchange much more efficient.
One of the interesting possible uses for 'thermal mass', with photovoltaics, is as an energy storage method for cooling purposes. I've seen whitepapers and prototypes of systems designed to implement the following for office building air conditioning, with no batteries, and no energy drawn back from the grid:
1) Build a big PV array
2) Use the surplus energy from the PV array during the day to chill water into a big block of ice.
3) Use a standard chilled-water-loop air conditioner system to run the AC chillers off the meltwater from the giant block of ice.
4) Repeat next day.
In this example the big block of ice becomes the energy storage method (surplus kWh used to freeze water), and is 'consumed' when it melts.
if you find this interesting Joey (the creator) is regularly posting updates on this and his other off-grid projects, as well as working through the core ideas with others on Scuttlebutt ( https://www.scuttlebutt.nz/ )
Could you not section of the part of the freezer with the cooling elements, and fill with water.
You could then cool that below zero increasing the effectiveness of the thermal mass.
You'd also get some freezer space into the bargain.
The only issue would be passively moving cold into the fridge proper.
I know greenhouses use wax cylinders that expand in heat to open louvre vents, I wonder whether something like that would work.
For putting the thermal mass above the food, could a workaround be to put a heat sink on the bottom of the lid, and connect it to the thermal mass with an equally heat conductive material?
Also, before I saw what the heat sink was, I wondered if you could use shotcrete on the walls of the freezer, to act as the thermal mass.
Would it be possible to run a comparison vs. some of the ultra-efficient mini-fridges out there? I had previously seen a few folks on youtube showing some of their setups with existing mini-fridges.
I live out on some land with just solar power and batteries for stretches of time and am weighing refrigeration options.
[+] [-] gumby|7 years ago|reply
Makes me wonder about the energy cost of losing (i.e. replacing) a 1/2 gal of milk vs the energy cost of a gas cycle fridge. I know food wastage is a huge resource lost but I’ve always overlooked the fact that energy is a huge one of the resources.
Thanks for writing this up!
[+] [-] throwaway12iii|7 years ago|reply
It takes approximately 255 liters of water to make 250ml of milk. So 1020 liters per liter of milk. There's 3.78542 liters per gallon. So that makes about 1930 liters per half gallon milk.
510 gallons of water for 0.5 gallons of milk.
It takes between 2500 and 5000 joules of energy per liter of tap water depending on the water source (2000 times that for bottled water). So up to 9,650,000 joules of energy per half gallon of milk for the water(about 2680 Wh).
1.25 to 1.44 kg of feed to produce a liter of milk (0.2645 gallons). To produce 1kg of wheat requires between 500 and 4,000 liters of water. But then you need to transport the feed to where the cows are...
Then you have to transport the milk. How far is it, and what sort of fuel is used? What sort of bottle is used? If it's come across the country via truck in a plastic bottle, that doesn't get recycled, that could be a lot of fuel. How long was the milk refrigerated before it reaches a home fridge?
There's 4 million joules to make a PET 1 liter plastic bottle. So about 7.5 million joules for a half gallon one(2 kilo watt hours). Often the transportation uses more than that for the bottle, but varies greatly. One amount mentioned is 4 million joules per liter.
That's 4 kilo watt hours just for the bottle.
There's lots more to it, but I guess 1-2 days of solar power production from a 5kW solar system for a half gallon of milk.
[+] [-] agumonkey|7 years ago|reply
[+] [-] beachy|7 years ago|reply
Some of the comments about heat pipes, heat transfer between freezer and fridge compartments etc. miss this point.
I got trapped in a fascinating rabbit hole like this a while ago trying to create hot water without (significant) batteries. The best solution seemed to be coupling the DC from the panels more or less directly to the heating element of the hot water cylinder.
Once you have a few of these lash-ups powering your new age off-grid sanctuary, it sounds like divvying up the power from the panels becomes the next major challenge.
[+] [-] bufferoverflow|7 years ago|reply
https://c8.alamy.com/comp/D24NWW/solar-hot-water-glass-tube-...
[+] [-] Havoc|7 years ago|reply
https://en.wikipedia.org/wiki/Solar_water_heating
Some people also run them in sequence with an electric one. i.e. Solar one first & that feed the elec one. Meaning that you'll always have guaranteed hot water, near double the capacity but the solar one picks up most of the work.
[+] [-] joeyh|7 years ago|reply
[+] [-] sixstringtheory|7 years ago|reply
TIL about the Yakhchāl (https://en.m.wikipedia.org/wiki/Yakhchāl) which I found searching for an example of a pit style. Also that 7-Eleven sprang from an icehouse company in Texas (https://en.m.wikipedia.org/wiki/Ice_house_(building)).
[+] [-] adrianN|7 years ago|reply
[+] [-] war1025|7 years ago|reply
[+] [-] snazz|7 years ago|reply
(for those of you who are unaware: https://www.urbandictionary.com/define.php?term=A%20really%2...)
[+] [-] crazy2be|7 years ago|reply
This is actually used in some commercial buildings in places where energy is cheaper at certain times of the day (for example, at night) [2], by freezing ice when energy is cheap, and thawing it when it isn't.
That being said, although this is a super cool project (I have the same inverter, and I made a similar control board ;)), this thermal mass doesn't seem like it would be particularly practical in most cases. Water has a relatively high specific heat capacity of 4.186J/gC, but given the narrow range of temperatures acceptable for a fridge, this doesn't end up being very much - only 79Wh per degree Celsius that the fridge is allowed to swing. If you consider 1C - 6C "acceptable", you only end up storing 395Wh. This is about 30-40% of the capacity of a $100-$200 "deep discharge" lead-acid battery, and is also a much wider range than most consumers would be used to (and may result in frozen veggies, for example).
In order to make this more practical, you really want something that can freeze around fridge temperature. For the same amount of water used above, freezing and thawing the water would store 6,308Wh(!), around 16x as much. If you could get something that freezes at 3C/4C with a similar heat of fusion to water, you could have a much smaller thermal battery that lasts _much_ longer, without the substantial temperature swings you see with your current design.
[1] https://www.eia.gov/tools/faqs/faq.php?id=96&t=3 [2] https://en.wikipedia.org/wiki/Ice_storage_air_conditioning
[+] [-] joeyh|7 years ago|reply
You need to take into account efficiencies of getting the cold into the fridge too. It takes around 10-15 hours of runtime to cool the thermal mass down from 5 to 0.5 degrees C (at 14C exterior temp), and the fridge needs 120 watts to run. Measured this way, the thermal battery is storing ~1200-1800 watt-hours.
But then, if it were powered from batteries, there would be significantly more power needed to fully charge the batteries and maintain good health -- my 860AH battery bank (4 deep cycle batteries) needs at least 1kwh input to charge up from 12v to full).
Another way to come at the question is, how many batteries are typically specced out to power an offgrid fridge, and banks costing 10-20k dollars are not at all uncommon, though they're also shared with other household needs.
[+] [-] vanderZwan|7 years ago|reply
It has the added benefit of stabilizing the temperature a lot. I recall reading somewhere (who knows, maybe one of the earlier blog entries here) that turning on a cooling element costs a lot of energy too on top of it just running, to the point where that effect on energy savings should not be underestimated.
All in all I saved quite a bit on my energy bill with quite a simple hack.
[+] [-] dbaupp|7 years ago|reply
composition
Water is the easy choice.
A material that can be frozen would be better, because it takes a lot of extra heat to melt a frozen material and so more cold could be stored.
However, this needs a material that freezes at a higher temperature than water, and most such are oils, which are less dense and so store less cold overall.
This is an open research area.
[+] [-] Cthulhu_|7 years ago|reply
But yeah, back in the day they would get blocks of ice from nearby lakes and put them underground, it'd last all year.
[+] [-] raphinou|7 years ago|reply
[+] [-] sweden|7 years ago|reply
It has a shelf life of months, it's highly popular in southern Europe.
[+] [-] 0xcafecafe|7 years ago|reply
https://www.scientificamerican.com/article/experts-organic-m...
[+] [-] dylanz|7 years ago|reply
[+] [-] Cyph0n|7 years ago|reply
[+] [-] komali2|7 years ago|reply
[+] [-] nightbrawler|7 years ago|reply
[+] [-] unknown|7 years ago|reply
[deleted]
[+] [-] tgb|7 years ago|reply
[+] [-] gumby|7 years ago|reply
Looking at other comments, and FWIW this is non-organic whole milk.
[+] [-] blktiger|7 years ago|reply
[+] [-] linase789|7 years ago|reply
I wonder why this is necessary. I've also seen it mentioned in the wiki, but no explanation.
Isn't a compressor the only electrical thing in a fridge? I know you must be careful with leaving some time between restarts, but why can't you just shut it down immediately on power loss?
[+] [-] lambda|7 years ago|reply
> Fridge0 does not need a big battery bank, since it stores cold in its thermal mass and runs only when solar power is available.
> But, at least a mimimal battery is needed to run the computer control. Probably on the order of 5 watts for a computer like a raspberry pi; this could be reduced more with a more embedded computer like an arduino.
> Enough battery to run the fridge for a minute or two is also needed. Consider what happens when the fridge is running, and the sun goes behind a rain cloud. Suddenly, there's not enough power to run it, but it's still turned on. The inverter will try to run with whatever small solar power is still coming in, but it's not enough. Some inverters might manage a clean power off, probably accompanied with loud beeping. Other inverters might fail in more interesting ways.
> To deal with this situation, the computer needs to check the incoming solar power frequently, and power off the fridge if there's not enough. The battery is needed to keep the inverter and fridge running until that happens.
Basically, it sounds like the answer is because with a solar panel, there are states that are between "off" and "on," you rather just get an amount of power proportional to the amount of sunlight, and there's a concern that those states could cause problems if you don't provide a more constant power supply and active switching to fully shut off the power when your solar panel is not producing enough power for the "on" state.
[+] [-] Scoundreller|7 years ago|reply
My car’s AC gets funky if I don’t remember to turn off the AC a few minutes before shutting down.
[+] [-] newnewpdro|7 years ago|reply
I tend to keep a number of filled water jugs occupying any otherwise unused space in mine since I don't keep them brimming with food stuffs.
It smooths out the temperature swings from opening the doors, and generally results in the compressor running less as a result.
Edit: it also buys more time before food spoils in power outages.
[+] [-] rmtech|7 years ago|reply
[+] [-] joeyh|7 years ago|reply
[+] [-] aidenn0|7 years ago|reply
I couldn't find a reliable source for the phase-change energy for peanut oil, but it would have to be pretty high to make up for the difference.
[+] [-] vram22|7 years ago|reply
https://news.ycombinator.com/item?id=18772237
[+] [-] Scoundreller|7 years ago|reply
You do say « However, this needs a material that freezes at a higher temperature than water, and most such are oils, which are less dense and so store less cold overall. », but I wouldn’t underestimate the enthalpy of melting.
Depending on your climate, a long heat pipe would be interesting to move heat outside when it’s cool enough outside (even if only during a few hours overnight). But that’s getting complicated to fabricate oneself.
Oops, edit: you have it outside already. How do you prevent freezing everything ? (Again, climate dependent).
[+] [-] komali2|7 years ago|reply
They don't. If it's too cold outside for too long, everything will freeze.
[+] [-] starbeast|7 years ago|reply
[+] [-] Scoundreller|7 years ago|reply
A lot of people hated them enough (and the gas used while sailing) to replace it with solar based systems.
[+] [-] edaemon|7 years ago|reply
[+] [-] walrus01|7 years ago|reply
1) Build a big PV array
2) Use the surplus energy from the PV array during the day to chill water into a big block of ice.
3) Use a standard chilled-water-loop air conditioner system to run the AC chillers off the meltwater from the giant block of ice.
4) Repeat next day.
In this example the big block of ice becomes the energy storage method (surplus kWh used to freeze water), and is 'consumed' when it melts.
https://www.greentechmedia.com/articles/read/ice-energy-laun...
https://cleantechnica.com/2016/11/25/time-shift-excess-solar...
[+] [-] Scoundreller|7 years ago|reply
[+] [-] sn9|7 years ago|reply
[+] [-] masukomi|7 years ago|reply
[+] [-] benj111|7 years ago|reply
Could you not section of the part of the freezer with the cooling elements, and fill with water. You could then cool that below zero increasing the effectiveness of the thermal mass.
You'd also get some freezer space into the bargain.
The only issue would be passively moving cold into the fridge proper. I know greenhouses use wax cylinders that expand in heat to open louvre vents, I wonder whether something like that would work.
[+] [-] sthgrau|7 years ago|reply
Also, before I saw what the heat sink was, I wondered if you could use shotcrete on the walls of the freezer, to act as the thermal mass.
[+] [-] wpietri|7 years ago|reply
[+] [-] nowarninglabel|7 years ago|reply
I live out on some land with just solar power and batteries for stretches of time and am weighing refrigeration options.
[+] [-] DanBC|7 years ago|reply
I wonder what testing they've done to be able to say that a warm fridge isn't a source of alarm, other than "the milk doesn't go bad"?
[+] [-] davidbanham|7 years ago|reply