What's cooler than being cool?
January 8, 2019 9:10 AM Subscribe
"Imagine something really cool, like a fridge connected to a powerwall, powered entirely by solar panels. What could be cooler than that?
How about a fridge powered entirely by solar panels without the powerwall? " - joeyh
"My offgrid, solar powered, zero-battery-use fridge has sucessfully made it through spring, summer, fall, and more than enough winter.
I've proven that it works. I've not gotten food poisoning, though I did lose half a gallon of milk on one super rainy week. I have piles of data, and a whole wiki documenting how I built it. I've developed 3 thousand lines of control software. It purrs along without any assistance"
"My offgrid, solar powered, zero-battery-use fridge has sucessfully made it through spring, summer, fall, and more than enough winter.
I've proven that it works. I've not gotten food poisoning, though I did lose half a gallon of milk on one super rainy week. I have piles of data, and a whole wiki documenting how I built it. I've developed 3 thousand lines of control software. It purrs along without any assistance"
I was thinking about this exact thing lately.
Now, forgive me since I'm not an engineer and don't actually know anything about physics, but a refrigerator is mostly a way of using heat to cool something down, right? Is there a reason you can't skip (or augment) the electricity side of things and use the solar heating directly?
posted by ragtag at 9:18 AM on January 8, 2019 [2 favorites]
Now, forgive me since I'm not an engineer and don't actually know anything about physics, but a refrigerator is mostly a way of using heat to cool something down, right? Is there a reason you can't skip (or augment) the electricity side of things and use the solar heating directly?
posted by ragtag at 9:18 AM on January 8, 2019 [2 favorites]
I'm live coding my fridge
:)
Similar tech has been deployed at scale for HVAC: Ice Energy Inc.
posted by gwint at 9:26 AM on January 8, 2019
:)
Similar tech has been deployed at scale for HVAC: Ice Energy Inc.
posted by gwint at 9:26 AM on January 8, 2019
Now, forgive me since I'm not an engineer and don't actually know anything about physics, but a refrigerator is mostly a way of using heat to cool something down, right?
I'm also not an engineer, so maybe I don't know what I'm talking about... but as far as I know, refrigerators mostly manipulate pressure to effect their cooling. They compress refrigerant, which would heat it, but then they allow that heated, compressed refrigerant to radiate away some heat, and then finally allow the refrigerant to go back to normal pressure- which cools it. I don't think there's a part of this cycle where adding heat from the outside will help.
posted by a snickering nuthatch at 9:34 AM on January 8, 2019 [4 favorites]
I'm also not an engineer, so maybe I don't know what I'm talking about... but as far as I know, refrigerators mostly manipulate pressure to effect their cooling. They compress refrigerant, which would heat it, but then they allow that heated, compressed refrigerant to radiate away some heat, and then finally allow the refrigerant to go back to normal pressure- which cools it. I don't think there's a part of this cycle where adding heat from the outside will help.
posted by a snickering nuthatch at 9:34 AM on January 8, 2019 [4 favorites]
Now, forgive me since I'm not an engineer and don't actually know anything about physics, but a refrigerator is mostly a way of using heat to cool something down, right?
Refrigerators and (most) air conditioners are best understood as heat exchangers. They take heat away from one place (inside the fridge/house) and dump it in another place (behind the fridge/outside the house). The process is made more effective by exploiting the thermal properties of gas expansion and condensation.
You're not really using heat to cool things, but you'll inevitably release heat into the room as a byproduct of the exchange itself, in addition to the waste heat produced by the compressor pump.
You could probably use a solar heater rather than photovoltaic panels to power the compressor, if you wanted like a literally steampunk refrigerator or something, but I'm not sure whether that would be any more efficient than an electrical compressor.
posted by tobascodagama at 9:34 AM on January 8, 2019 [3 favorites]
Refrigerators and (most) air conditioners are best understood as heat exchangers. They take heat away from one place (inside the fridge/house) and dump it in another place (behind the fridge/outside the house). The process is made more effective by exploiting the thermal properties of gas expansion and condensation.
You're not really using heat to cool things, but you'll inevitably release heat into the room as a byproduct of the exchange itself, in addition to the waste heat produced by the compressor pump.
You could probably use a solar heater rather than photovoltaic panels to power the compressor, if you wanted like a literally steampunk refrigerator or something, but I'm not sure whether that would be any more efficient than an electrical compressor.
posted by tobascodagama at 9:34 AM on January 8, 2019 [3 favorites]
There is something called an adsorption refrigerator that can do it with heat alone, but they can't get as cold as a compressor fridge, especially down to freezing temps unless you're already in a very cold room. But they're useful in some places like RVs.
A solar powered adsorption fridge would also not work at night, natch.
posted by JoeZydeco at 9:35 AM on January 8, 2019 [5 favorites]
A solar powered adsorption fridge would also not work at night, natch.
posted by JoeZydeco at 9:35 AM on January 8, 2019 [5 favorites]
Adding a small vent from outside, well insulated and filtered, might help you meet your winter needs if it's below zero where you are in the winter. No reason you need to fight your furnace.
posted by mhoye at 9:48 AM on January 8, 2019
posted by mhoye at 9:48 AM on January 8, 2019
A fridge doesn't use "heat" to cool something; it uses energy to do work (move a piston which causes a gas to alternately expand/contract) and heat is the byproduct.
The goal is to use only solar energy to do the work. Collecting this energy can happen in two ways.
First off, it should be noted, just how much energy the sun gives. The average amount the sun gives to the earth is 164 watts per square meter, averaged over 24 hours, and I think I've read that high sunlight places like Nevada in summer can get up to 900 watts per square meter during noon.
Whether we convert this energy into electricity (which is then converted into work, I.E. a solar panel generating electricity to run a compressor) or convert it directly into work (the steampunk idea tobascodagama mentioned), there is still going to be a loss due to inefficiency.
Solar panels are currently somewhere around 22% conversion efficiency. There will then be some loss as that electricity is rconverted to work in the electric motor, but I think motors are something like 90% efficient. So, we're likely looking at a total system efficiency of around 20%, which would be like 180 watts per square meter at noon in Nevada.
Now to do the steampunk version... a super-efficient heat engine (such as a Stirling Engine) could be used with a parabolic reflector to collect solar heat energy and convert it into mechanical work. That mechanical work (turning a driveshaft) could be used to directly power a compressor. Stirling engines are way more efficient than internal combustion engines, but they still are unlikely to get past (or even close to) 50% efficiency. I think 30-40% is more normal for them.
So, yes, it is possible and slightly more efficient to steampunk it, and do a direct solar energy to mechanical power conversion. But you'd have to slap a large engine in the apex of a giant parabolic mirror to do so. It would look kind of like these things.
posted by weed donkey at 9:50 AM on January 8, 2019 [3 favorites]
The goal is to use only solar energy to do the work. Collecting this energy can happen in two ways.
First off, it should be noted, just how much energy the sun gives. The average amount the sun gives to the earth is 164 watts per square meter, averaged over 24 hours, and I think I've read that high sunlight places like Nevada in summer can get up to 900 watts per square meter during noon.
Whether we convert this energy into electricity (which is then converted into work, I.E. a solar panel generating electricity to run a compressor) or convert it directly into work (the steampunk idea tobascodagama mentioned), there is still going to be a loss due to inefficiency.
Solar panels are currently somewhere around 22% conversion efficiency. There will then be some loss as that electricity is rconverted to work in the electric motor, but I think motors are something like 90% efficient. So, we're likely looking at a total system efficiency of around 20%, which would be like 180 watts per square meter at noon in Nevada.
Now to do the steampunk version... a super-efficient heat engine (such as a Stirling Engine) could be used with a parabolic reflector to collect solar heat energy and convert it into mechanical work. That mechanical work (turning a driveshaft) could be used to directly power a compressor. Stirling engines are way more efficient than internal combustion engines, but they still are unlikely to get past (or even close to) 50% efficiency. I think 30-40% is more normal for them.
So, yes, it is possible and slightly more efficient to steampunk it, and do a direct solar energy to mechanical power conversion. But you'd have to slap a large engine in the apex of a giant parabolic mirror to do so. It would look kind of like these things.
posted by weed donkey at 9:50 AM on January 8, 2019 [3 favorites]
In some ways, the ideal cooling is thermoelectric. No pumps, gases, rods, motors, grease, or anything else. Alas, it's about one quarter as efficient.
posted by cowcowgrasstree at 10:00 AM on January 8, 2019
posted by cowcowgrasstree at 10:00 AM on January 8, 2019
Very interesting, and especially cool since joeyh is the one mefite I've actually met in person!
posted by TedW at 10:08 AM on January 8, 2019 [1 favorite]
posted by TedW at 10:08 AM on January 8, 2019 [1 favorite]
joeyh: I am definitely stealing this idea for UG student projects.
posted by biffa at 10:22 AM on January 8, 2019 [1 favorite]
posted by biffa at 10:22 AM on January 8, 2019 [1 favorite]
Seems simple enough. Not sure that it would make sense for on-grid applications where a loss of power is a rare event, but sure, with enough thermal mass this seems like a perfectly reasonable concept. I can't seem to find any specifics on his design, but what I'd do myself is have an exchanger that basically freezes a big block of water into ice, and then a thermostatically-controlled fan that blows air over the ice and into the refrigeration chamber. There would need to be another sensor that could tell when the water was fully frozen; I'd probably have an ohmmeter positioned near the bottom of the ice chamber which would detect the increase in electrical resistance when the water froze and close the compressor circuit. This wouldn't be strictly necessary, but it would save wear & tear on the compressor. Depending on your power budget you could go with a peltier plate instead which would be less efficient but have no moving parts and could be allowed to just run whenever the sun was out.
I'd also include a small battery just to smooth out power fluctuations due to things like passing clouds, but again if you use a plate chiller this would be less important. I would also try to run the whole thing off of DC, if at all possible.
The design in the article looks like it was based around just a modified chest freezer, mind you. I wonder what kind of mileage you'd get from just buying one that was way larger than you needed and filling it most of the way with bottles of water. If you set it for 1°C, how long would it take to come up to 8°C? Using ice instead of cold water is better because the heat of fusion of water is way higher than the specific heat of water, but if you wanted a fridge rather than a freezer you'd need to separate the ice from the refrigeration chamber and use a thermostat to move heat from the ice to the refrigeration chamber as needed.
posted by Anticipation Of A New Lover's Arrival, The at 10:50 AM on January 8, 2019 [2 favorites]
I'd also include a small battery just to smooth out power fluctuations due to things like passing clouds, but again if you use a plate chiller this would be less important. I would also try to run the whole thing off of DC, if at all possible.
The design in the article looks like it was based around just a modified chest freezer, mind you. I wonder what kind of mileage you'd get from just buying one that was way larger than you needed and filling it most of the way with bottles of water. If you set it for 1°C, how long would it take to come up to 8°C? Using ice instead of cold water is better because the heat of fusion of water is way higher than the specific heat of water, but if you wanted a fridge rather than a freezer you'd need to separate the ice from the refrigeration chamber and use a thermostat to move heat from the ice to the refrigeration chamber as needed.
posted by Anticipation Of A New Lover's Arrival, The at 10:50 AM on January 8, 2019 [2 favorites]
Ok, now do it with a mini-split heat pump....
I kid, I kid, one day I'll run that experiment myself on a tiny, tiny house, that I insulate and vapor barrier like whoa, maybe in Florida.... with a lot of panels... sigh.
This is really impressive and I'm glad to see the data play out. Grats on a great project and thanks for the presentation of the same.
posted by RolandOfEld at 10:55 AM on January 8, 2019
I kid, I kid, one day I'll run that experiment myself on a tiny, tiny house, that I insulate and vapor barrier like whoa, maybe in Florida.... with a lot of panels... sigh.
This is really impressive and I'm glad to see the data play out. Grats on a great project and thanks for the presentation of the same.
posted by RolandOfEld at 10:55 AM on January 8, 2019
ICE FROM FIRE! ICE IS CIVILIZATION! THAT'S WHY WE CAME! THAT'S WHY WE'RE HERE!
posted by rude.boy at 11:04 AM on January 8, 2019 [3 favorites]
posted by rude.boy at 11:04 AM on January 8, 2019 [3 favorites]
Seems simple enough. Not sure that it would make sense for on-grid applications where a loss of power is a rare event, but sure, with enough thermal mass this seems like a perfectly reasonable concept.
But it shows quite nicely that it would not take much to introduce a lot more solar on to the grid, and run our entire HVAC load on it, without any need for battery storage.
Remember this stuff is a lot EASIER on a larger base of loads, because refrigerator thermostats don't march in lockstep, and it does not take a lot of effort to affirm programatically that they will break step with each other and only sync to the solar power source.
This is because HVAC is a form of energy storage.
posted by ocschwar at 11:20 AM on January 8, 2019 [2 favorites]
(Us EEs already know this, but this demo should be enough to explain to laymen.)
posted by ocschwar at 11:20 AM on January 8, 2019 [1 favorite]
posted by ocschwar at 11:20 AM on January 8, 2019 [1 favorite]
I'm busy burying a line for my solar powered gravity fed water system today, but I look forward to a nice plate of beans about my fridge later!
posted by joeyh at 11:24 AM on January 8, 2019 [8 favorites]
posted by joeyh at 11:24 AM on January 8, 2019 [8 favorites]
Depending on your power budget you could go with a peltier plate instead which would be less efficient but have no moving parts and could be allowed to just run whenever the sun was out.
This sounds neat and I want to learn more about it. Does that mean you could do without the controller?
posted by ragtag at 11:34 AM on January 8, 2019
This sounds neat and I want to learn more about it. Does that mean you could do without the controller?
posted by ragtag at 11:34 AM on January 8, 2019
Hey now, no need to yell. Of course it's possible to incorporate thermal mass into a building's HVAC system, this is done all the time for large commercial buildings (where the inherent mass of the building itself is factored into HVAC loading) and while it's not SOP for residential it's certainly possible. Storage heaters are common in areas that use demand-based electricity pricing, and Passivhaus designs often go well beyond that and incorporate thermal mass and solar heat gain into the design of the building itself. And of course that could be extended to food refrigeration as well.
What it comes down to is cost effectiveness, really. Assuming that you're starting with a pretty efficient
house with a solar array on it to begin with (or else why bother?) does it cost more to incorporate enough thermal mass to allow you to run the HVAC and food refrigeration off of regular old self-supply, or does it cost more to add a Powerwall? You will already need a critical loads panel and an inverter that is capable of islanding and self-supply. The marginal cost of adding a Powerwall to that (there are other battery backup products but Powerwall is the cheapest AFAIK) is around $6,000. I genuinely don't know what the marginal cost of a thermal mass-based HVAC and refrigeration system would be—maybe it would be cheaper! Of course, if you have a Powerwall you can run other critical loads as well, like your internet and some lights and a few outlets here and there, so we might be talking about one Powerwall vs. two, which reduces the marginal cost further.
Just some food for thought.
posted by Anticipation Of A New Lover's Arrival, The at 11:51 AM on January 8, 2019 [1 favorite]
What it comes down to is cost effectiveness, really. Assuming that you're starting with a pretty efficient
house with a solar array on it to begin with (or else why bother?) does it cost more to incorporate enough thermal mass to allow you to run the HVAC and food refrigeration off of regular old self-supply, or does it cost more to add a Powerwall? You will already need a critical loads panel and an inverter that is capable of islanding and self-supply. The marginal cost of adding a Powerwall to that (there are other battery backup products but Powerwall is the cheapest AFAIK) is around $6,000. I genuinely don't know what the marginal cost of a thermal mass-based HVAC and refrigeration system would be—maybe it would be cheaper! Of course, if you have a Powerwall you can run other critical loads as well, like your internet and some lights and a few outlets here and there, so we might be talking about one Powerwall vs. two, which reduces the marginal cost further.
Just some food for thought.
posted by Anticipation Of A New Lover's Arrival, The at 11:51 AM on January 8, 2019 [1 favorite]
haskell controlled offgrid fridge
haskell controlled offgrid fridge
haskell controlled offgrid fridge
posted by pracowity at 11:53 AM on January 8, 2019 [3 favorites]
haskell controlled offgrid fridge
haskell controlled offgrid fridge
posted by pracowity at 11:53 AM on January 8, 2019 [3 favorites]
ragtag, take a look at the Wikipedia page for thermoelectric cooling. Common applications include mini fridges and image sensor cooling. The effect can also be used in reverse for cogeneration by scavenging waste heat and turning it into electricity. It's not super efficient but depending on your application that doesn't always matter.
posted by Anticipation Of A New Lover's Arrival, The at 11:54 AM on January 8, 2019 [1 favorite]
posted by Anticipation Of A New Lover's Arrival, The at 11:54 AM on January 8, 2019 [1 favorite]
Maybe I am thick but I don't really understand what the point of this is.
My understanding of the goal for this project is to power a cheap refrigerator with solar panels and no electric battery. Full stop.
Goal does not include increased efficiency or reduction of environmental impact.
If this is accurate then I can contribute my own simple non solar system. A cheap mini fridge on my sailboat that is charged when the boat is at the Marina. The Fridge has its freezer compartment (which is inside the main compartment and is basically the evaporator) packed with those phase change gel bags you use on ankle sprains and for coolers and a gallon jug of water. Upon unplugging the Fridge is swaddled in 2" styrofoam. Everything stays cold for a minimum of 2 days in the summer.
posted by Pembquist at 12:39 PM on January 8, 2019
Also, when people talk about cost I wish there was a way to account for the externals that are not reflected in that cost. I am pretty sure "cheap" often means much more costly if you factored in the actual comet trail of the thing that is cheap. The open dumping of silicon tetrachloride from solar panel manufacture in China would be an example.
posted by Pembquist at 12:47 PM on January 8, 2019
posted by Pembquist at 12:47 PM on January 8, 2019
haskell controlled offgrid fridge
Keeping your beer cold. With monads.
posted by ocschwar at 1:00 PM on January 8, 2019 [3 favorites]
Keeping your beer cold. With monads.
posted by ocschwar at 1:00 PM on January 8, 2019 [3 favorites]
There is something called an adsorption refrigerator that can do it with heat alone
-- JoeZydeco
Very early refrigerators used a methane gas flame to cool, as unintuitive as that sounds. Einstein even built one.
There was a Bob and Ray (comedians from long ago) skit where a scientist is asked how you can cool with fire, and of course the scientist has trouble explaining it because he doesn't have a clue.
posted by eye of newt at 1:03 PM on January 8, 2019 [2 favorites]
-- JoeZydeco
Very early refrigerators used a methane gas flame to cool, as unintuitive as that sounds. Einstein even built one.
There was a Bob and Ray (comedians from long ago) skit where a scientist is asked how you can cool with fire, and of course the scientist has trouble explaining it because he doesn't have a clue.
posted by eye of newt at 1:03 PM on January 8, 2019 [2 favorites]
. I wonder what kind of mileage you'd get from just buying one that was way larger than you needed and filling it most of the way with bottles of water.
That's what he does, it's part of how he cuts the batteries out.
posted by zabuni at 1:19 PM on January 8, 2019 [2 favorites]
That's what he does, it's part of how he cuts the batteries out.
posted by zabuni at 1:19 PM on January 8, 2019 [2 favorites]
Very early refrigerators used a methane gas flame to cool
My camper's fridge is the absorption type, with 3 modes. all three use heat, but the source is different. one is 120V AC one is 12V DC, and the other uses a tiny (pilot light size or smaller) propane flame.
posted by ArgentCorvid at 2:02 PM on January 8, 2019 [1 favorite]
My camper's fridge is the absorption type, with 3 modes. all three use heat, but the source is different. one is 120V AC one is 12V DC, and the other uses a tiny (pilot light size or smaller) propane flame.
posted by ArgentCorvid at 2:02 PM on January 8, 2019 [1 favorite]
Also, when people talk about cost I wish there was a way to account for the externals that are not reflected in that cost. I am pretty sure "cheap" often means much more costly if you factored in the actual comet trail of the thing that is cheap. The open dumping of silicon tetrachloride from solar panel manufacture in China would be an example.
Well there are LCAs but putting a meaningful cost to each line is pretty difficult. Same goes for fossil fuel externalities of course, which seem likely to be a lot bigger.
posted by biffa at 2:12 PM on January 8, 2019 [1 favorite]
Well there are LCAs but putting a meaningful cost to each line is pretty difficult. Same goes for fossil fuel externalities of course, which seem likely to be a lot bigger.
posted by biffa at 2:12 PM on January 8, 2019 [1 favorite]
→ ICE FROM FIRE
May I introduce the Crosley Icyball of the late 1920s?
posted by scruss at 2:28 PM on January 8, 2019 [1 favorite]
May I introduce the Crosley Icyball of the late 1920s?
posted by scruss at 2:28 PM on January 8, 2019 [1 favorite]
The general principle here is that the overwhelming bulk of domestic electricity consumption is dedicated to heating and cooling things, and that it's a lot simpler and less expensive to store heat than it is to store the electricity required to manipulate it.
This principle has been well understood since domestic electricity supplies first became a thing. Off-peak electric hot water services are its most common embodiment.
It's more expensive to shut a large coal-fired thermal electricity generating station down overnight and ramp it up again in the morning than it is to keep it ticking over at low output all night. In an era where electricity supply systems were dominated by exactly that kind of generation plant, the marginal cost of the electricity produced overnight was essentially zero. Some demand needed to be created for that power to make it saleable, and off-peak storage hot water services did exactly that.
Now that we're moving into a new era where the contribution from weather-dependent power sources is increasing, we're seeing new patterns emerge around the most economical times to generate power. Rather than wring our hands about how horribly "unreliable" solar and wind plant is, it makes more sense to me to respond to its output patterns in much the same way as we did to coal plant forced to idle overnight: with thermal energy storage at point of end use.
Phase-change materials incorporated into building interiors can already time-shift the demand for active cooling to make it match the output of solar PV panels better. Storage hot water services already exist, many of them driven by heat pumps instead of brute-force resistive heaters for better efficiency. And I can see no good reason why phase-change food refrigerators shouldn't join them in the market. Personally I want that a lot more than I want a fridge with pretensions of being a web browser.
posted by flabdablet at 3:48 PM on January 8, 2019 [7 favorites]
This principle has been well understood since domestic electricity supplies first became a thing. Off-peak electric hot water services are its most common embodiment.
It's more expensive to shut a large coal-fired thermal electricity generating station down overnight and ramp it up again in the morning than it is to keep it ticking over at low output all night. In an era where electricity supply systems were dominated by exactly that kind of generation plant, the marginal cost of the electricity produced overnight was essentially zero. Some demand needed to be created for that power to make it saleable, and off-peak storage hot water services did exactly that.
Now that we're moving into a new era where the contribution from weather-dependent power sources is increasing, we're seeing new patterns emerge around the most economical times to generate power. Rather than wring our hands about how horribly "unreliable" solar and wind plant is, it makes more sense to me to respond to its output patterns in much the same way as we did to coal plant forced to idle overnight: with thermal energy storage at point of end use.
Phase-change materials incorporated into building interiors can already time-shift the demand for active cooling to make it match the output of solar PV panels better. Storage hot water services already exist, many of them driven by heat pumps instead of brute-force resistive heaters for better efficiency. And I can see no good reason why phase-change food refrigerators shouldn't join them in the market. Personally I want that a lot more than I want a fridge with pretensions of being a web browser.
posted by flabdablet at 3:48 PM on January 8, 2019 [7 favorites]
I use four golf-cart deep-cycle batteries to save current from 400 watts of electricity coming from four solar panels. The batteries cost about $500.00, the panels, controller and inverter cost me about $800.00. Otherwise I'd have to pony up twenty to thirty thousands of dollars to have power run to my place. My simple system works, keeps my fridge running, gives me power for my laptop and lights -- but my needs are few.
posted by Agave at 4:10 PM on January 8, 2019 [6 favorites]
posted by Agave at 4:10 PM on January 8, 2019 [6 favorites]
I can't seem to find any specifics on his design, but what I'd do myself is have an exchanger that basically freezes a big block of water into ice, and then a thermostatically-controlled fan that blows air over the ice and into the refrigeration chamber.
I'd build sheets of water into horizontal evaporator plates, on the basis that the thermal conductivity of ice is miserable. To get decent thermal coupling between the ice and the circulating air, you'd probably want more surface area than you'd get from a single large block.
The freezer compartment could use brine rather than fresh water, to achieve a similar heat of fusion at a lower temperature.
I would expect that the main engineering problem with this design would be dealing with frost buildup in the circulating air channels. It might be better to use a liquid-based secondary heat exchanger rather than air circulation to move heat from the refrigeration compartment into the phase change storage block, and auto-defrost that exchanger's cold plate in much the same way that standard fridges defrost their evaporators.
posted by flabdablet at 4:33 PM on January 8, 2019 [2 favorites]
I'd build sheets of water into horizontal evaporator plates, on the basis that the thermal conductivity of ice is miserable. To get decent thermal coupling between the ice and the circulating air, you'd probably want more surface area than you'd get from a single large block.
The freezer compartment could use brine rather than fresh water, to achieve a similar heat of fusion at a lower temperature.
I would expect that the main engineering problem with this design would be dealing with frost buildup in the circulating air channels. It might be better to use a liquid-based secondary heat exchanger rather than air circulation to move heat from the refrigeration compartment into the phase change storage block, and auto-defrost that exchanger's cold plate in much the same way that standard fridges defrost their evaporators.
posted by flabdablet at 4:33 PM on January 8, 2019 [2 favorites]
A friends camp in the woods had an extremely efficient beverage fridge. It was a chest freezer, but instead of being connected to any type of electricity it instead had a pipe that dripped water in from a small spring located slightly uphill. It then (once full) ran out a simple drain pipe into a gully where it eventually joined up with a minor stream. Beer and soda cans float and they were kept at a chilly 35-40F year round until it got too cold for the water to flow and instead when you wanted a chilled beverage you had to just put it outside for half an hour to cool. Working fridge for beers and soda with essentially zero environmental impact (the water would have flowed into the gully eventually anyways) but a bit difficult to replicate in different places.
posted by koolkat at 1:25 AM on January 9, 2019
posted by koolkat at 1:25 AM on January 9, 2019
Some friends have a big tank they use to collect rainwater for domestic use. By big think 7,000 gallons or so (roughly 15 feet in diameter and 6 feet high.) I have spent quite a few idle hours imagining how to efficiently use that mass of water as a thermal battery for heating/cooling... It's a fun thought experiment that, sadly, I have yet to convince friend is worth trying to implement.
posted by From Bklyn at 4:03 AM on January 9, 2019
posted by From Bklyn at 4:03 AM on January 9, 2019
I've crunched the numbers on using a rainwater collection system (or a swimming pool) as a heat source/sink for space heating and cooling. It doesn't really work out very well except as a sort of temporary "turbocharger" that you can dump a lot of heat into very quickly, and then let it equilibrate more slowly. The amount of heat that you'd put into it, if you are not running a cooling tower or something to discharge the heat into the air, will eventually raise the temperature to the point where the efficiency of the system goes down and it stops being worthwhile. The only reason to do this, is if heating the water to the breakeven temperature is desirable for its own reasons (like heating a swimming pool).
What you can do, if you have a cheap source of nonpotable water, is dump heat into it and then mix a lot of water and atmospheric air together, which evaporates some of the water and also removes a bunch of heat. It does so at the 'cost' of requiring additional water, but if water is cheap or free, well, that's not a problem.
There are some crafty solutions that homebrewers (most down in Aus / NZ where you can't just use your tapwater as once-through coolant) have come up with, involving barrels stacked on top of each other, where the bottom barrel actually contains the water, and the top barrel is basically a cooling tower arrangement (with a sprinkler head to spray the water, and then some fans to blow air upwards through the downward-falling water mist) to cool the return water before it trickles into the tank. You can maintain the water temperature as low as 20F below ambient air this way, which would make an air conditioner more efficient using an air-to-water Freon loop instead of air-to-air. (This is how big building AC systems work, for exactly this reason.)
posted by Kadin2048 at 9:29 AM on January 9, 2019 [2 favorites]
What you can do, if you have a cheap source of nonpotable water, is dump heat into it and then mix a lot of water and atmospheric air together, which evaporates some of the water and also removes a bunch of heat. It does so at the 'cost' of requiring additional water, but if water is cheap or free, well, that's not a problem.
There are some crafty solutions that homebrewers (most down in Aus / NZ where you can't just use your tapwater as once-through coolant) have come up with, involving barrels stacked on top of each other, where the bottom barrel actually contains the water, and the top barrel is basically a cooling tower arrangement (with a sprinkler head to spray the water, and then some fans to blow air upwards through the downward-falling water mist) to cool the return water before it trickles into the tank. You can maintain the water temperature as low as 20F below ambient air this way, which would make an air conditioner more efficient using an air-to-water Freon loop instead of air-to-air. (This is how big building AC systems work, for exactly this reason.)
posted by Kadin2048 at 9:29 AM on January 9, 2019 [2 favorites]
It's also how big building AC systems become the canonical hosts for Legionella and god knows what other slightly less hideous organisms. Warmed recirculated water + constant influx of airborne pollutants = nutrient bath.
posted by flabdablet at 9:43 PM on January 9, 2019
posted by flabdablet at 9:43 PM on January 9, 2019
Anybody interested in thermal energy storage will probably be interested in the pumped heat energy storage systems originally developed by the now-defunct Isentropic Ltd.
The key insight required for understanding this technology is that Carnot efficiency, the maximum theoretical efficiency with which a heat engine can convert heat energy into mechanical energy, cuts two ways; the Carnot efficiency for a heat engine is in fact the reciprocal of the maximum coefficient of performance for a heat pump working at comparable temperatures. Which means that a mechanical system that works as a heat pump while storing energy, and a heat engine while releasing it, is not as ridiculously inefficient as experience with fuelled heat engines would have you intuit; given careful attention to reducing mechanical losses, the overall round trip efficiency for pumped heat energy storage compares favourably with that of quite good chemical batteries.
The actual energy storage component is nothing more complicated than gravel and thermal insulation, and since the cost of the relatively expensive insulation rises in proportion to the square of an element's radius while the cost of gravel is roughly proportional to its cube, the bigger you make these things the cheaper they get per stored megajoule.
posted by flabdablet at 10:02 PM on January 9, 2019 [1 favorite]
The key insight required for understanding this technology is that Carnot efficiency, the maximum theoretical efficiency with which a heat engine can convert heat energy into mechanical energy, cuts two ways; the Carnot efficiency for a heat engine is in fact the reciprocal of the maximum coefficient of performance for a heat pump working at comparable temperatures. Which means that a mechanical system that works as a heat pump while storing energy, and a heat engine while releasing it, is not as ridiculously inefficient as experience with fuelled heat engines would have you intuit; given careful attention to reducing mechanical losses, the overall round trip efficiency for pumped heat energy storage compares favourably with that of quite good chemical batteries.
The actual energy storage component is nothing more complicated than gravel and thermal insulation, and since the cost of the relatively expensive insulation rises in proportion to the square of an element's radius while the cost of gravel is roughly proportional to its cube, the bigger you make these things the cheaper they get per stored megajoule.
posted by flabdablet at 10:02 PM on January 9, 2019 [1 favorite]
... if you are not running a cooling tower or something to discharge the heat into the air,
That's where the system (in this case) had a chance in that the nights are significantly cooler and that becomes an 'outlet' for the heat banked during the day - as 'heating' at night or at least heat pump run in reverse. It still wasn't quite enough as after three days, I think it was, if the night-time air didn't drop far enough below day-time temps (into the low 70's/20's C - a delta-T of 40 deg. from day-time temps) the tank would just turn into a soup-pot.
The system is 'closed' to air, flabdablet, making Legionella a non-issue. The pumped-heat energy storage system is interesting. When systems scale up, it's interesting how efficiencies 'change' - I wonder if this isn't a fact that will play a bigger and bigger role in the re-jiggering of energy use.
posted by From Bklyn at 12:43 AM on January 10, 2019
That's where the system (in this case) had a chance in that the nights are significantly cooler and that becomes an 'outlet' for the heat banked during the day - as 'heating' at night or at least heat pump run in reverse. It still wasn't quite enough as after three days, I think it was, if the night-time air didn't drop far enough below day-time temps (into the low 70's/20's C - a delta-T of 40 deg. from day-time temps) the tank would just turn into a soup-pot.
The system is 'closed' to air, flabdablet, making Legionella a non-issue. The pumped-heat energy storage system is interesting. When systems scale up, it's interesting how efficiencies 'change' - I wonder if this isn't a fact that will play a bigger and bigger role in the re-jiggering of energy use.
posted by From Bklyn at 12:43 AM on January 10, 2019
I've crunched the numbers on using a rainwater collection system (or a swimming pool) as a heat source/sink for space heating and cooling. It doesn't really work out very well except as a sort of temporary "turbocharger" that you can dump a lot of heat into very quickly, and then let it equilibrate more slowly.
Could work really well for cooling if you used an air-sink or (better) ground-sink heat pump to cool the tank aggressively whenever you had free energy available from PV panels to do that - possibly helped by the cold side of another heat pump attached to the house hot water service - and then used thermostatically controlled circulation of cold tank water to establish a comfortable temperature indoors.
This is the same kind of time shift for peak solar generation that you can get with phase change panels, but if you already needed the big tank for storing the house's water supply you might beat phase change material on cost. You'd also be able to cool some rooms more aggressively than others, which could be very helpful for food storage.
posted by flabdablet at 2:29 AM on January 10, 2019 [1 favorite]
Could work really well for cooling if you used an air-sink or (better) ground-sink heat pump to cool the tank aggressively whenever you had free energy available from PV panels to do that - possibly helped by the cold side of another heat pump attached to the house hot water service - and then used thermostatically controlled circulation of cold tank water to establish a comfortable temperature indoors.
This is the same kind of time shift for peak solar generation that you can get with phase change panels, but if you already needed the big tank for storing the house's water supply you might beat phase change material on cost. You'd also be able to cool some rooms more aggressively than others, which could be very helpful for food storage.
posted by flabdablet at 2:29 AM on January 10, 2019 [1 favorite]
The pumped-heat energy storage system is interesting.
Even more so since a test unit has apparently just been hooked into the UK grid.
posted by flabdablet at 5:13 AM on January 10, 2019 [1 favorite]
Even more so since a test unit has apparently just been hooked into the UK grid.
posted by flabdablet at 5:13 AM on January 10, 2019 [1 favorite]
Highview Power claims 60-75% efficiency for a thermal energy storage process based on liquefaction of air.
posted by flabdablet at 5:25 AM on January 10, 2019
posted by flabdablet at 5:25 AM on January 10, 2019
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posted by infini at 9:15 AM on January 8, 2019 [2 favorites]