Off-Grid, Solar-Powered, Zero-Battery Refrigerator
notechmagazine.com
notechmagazine.com
Take boots, for example. He earned thirty-eight dollars a month plus allowances. A really good pair of leather boots cost fifty dollars. But an affordable pair of boots, which were sort of OK for a season or two and then leaked like hell when the cardboard gave out, cost about ten dollars. Those were the kind of boots Vimes always bought, and wore until the soles were so thin that he could tell where he was in Ankh-Morpork on a foggy night by the feel of the cobbles.
But the thing was that good boots lasted for years and years. A man who could afford fifty dollars had a pair of boots that'd still be keeping his feet dry in ten years' time, while the poor man who could only afford cheap boots would have spent a hundred dollars on boots in the same time and would still have wet feet.
This was the Captain Samuel Vimes 'Boots' theory of socioeconomic unfairness.”
― Terry Pratchett, Men at Arms
Most rich people can lay out the $50 for the good boots.
Most poor people don’t turn rich.
Most poor people can’t wait to save up $50 for the good boots; they need boots now.
This all points to the lesson that wealth and riches aren't the same thing. Rich people are rich because they have wealth, even poor people can amass riches, say by winning the lottery, but can't be called wealthy until they actually have wealth.
A person can become wealthy off the efforts of other people, but if we want wealth for all, then it's society itself that must hold the wealth. This was Karl Marx's conclusion, but all attempts to build a society capable of holding wealth for all failed.
So we're stuck with trickle-down, the idea that the surplus wealth of the wealthy is good enough for all.
Because your current system is portable and well known in a place where land use and other rights are less solid?
Because no one around you has the technical capacity to update / maintain / repair the system?
Because if you invest in something in a less developed area - someone may come and take it (yes, this includes the govt and various officials who may start talking about permitting etc etc).
Interestingly, this same question is asked and answered at the country scale for major industries. Why ship unrefined / unprocessed product that takes up 10x the volume when you could process locally for the whole country? Because the market doesn't trust the systems in that country well enough to invest $5B that has a 20 year payback even if its "easy math".
I don't feel I have enough of a grasp to summarize it. And I'm not saying it's bad or good in particular. I'm just saying, expect serious culture shock in this area of culture. You'll come away with a greater understanding of why attempted Western interventions have been less successful than expected, if nothing else.
For example, if he orders a truckload of gravel he has to watch the entrance like a hawk and be ready to rush out the instant the truck arrives, because if he doesn't the truck will just dump the gravel right in the middle of their entrance and block everybody in until he can round up people to shovel it into wheelbarrows by hand. The idea that you might want to ask where to dump the load is not something that will ever cross the truck driver's mind. The project manager won't think about it either unless my father-in-law asks him explicitly where he is planning to store the gravel he just ordered. It's these kinds of problems day in and day out that makes reconstruction so difficult.
Learning how to think ahead is something we take for granted in the west, but it's not an innate human skill. It has to be taught. Worse, if you never learned it yourself then you won't notice that your kids lack the skill. It's a difficult cycle to break.
Also, plastic bottles are surprisingly popular in Mali. Children beg tourists primarily for bottles and pens.
Ice takes a lot of energy to freeze. That means your freezer is going to be really inefficient for a long period of time once the power kicks back on. Also, any other food you put in the freezer is a thermal mass as well. It might be the last to thaw but it could also be the last to freeze if you deplete the "battery" and have to start the process over.
I'm surprised he didn't invest in further insulating the freezer. Deep freezes are not well insulated and waste more cold than your typical cheap foam beer cooler.
At southern or tropical latitudes often 90% of the product would melt before it made it to market. Insulation was primitive (saw dust).
This is why Tudor ended up in debtors prison before he got the business model right.
Well, yes, but that means it also takes a the same amount of energy to thaw. That's the whole idea: large heat capacity to create a temperature buffer.
It is also not immediately obvious whether the losses when freezing ice would be less efficient than energy losses due to the heat produced by charging up a battery, and the leaking of charge. If you take all energy loss into account, the questions of efficiency can have counter-intuitive results[0].
[0] https://www.lowtechmagazine.com/2014/06/thermal-efficiency-c...
A lot of times during the really hot midday all I can think of is making ice and letting it melt during the night so it can cool the side of the bed.
If the freezer is next to your bedroom then the bad insulation becomes a good thing because you may save on air conditioning.
I've often wondered if there's not meaningful efficiency to be gained in piping the coolant to remote coils outdoors like we do for A/Cs (with a control system to vent the heat indoors when heating is desired, or even use atmospheric cooling instead of the compressor when the outside temp is low enough).
Assuming illumination is constant in timescales of minutes (so thousands of electrical AC cycles), one can observe the following conundrum: a solar panel produces DC electrical power, while most consumer devices assume AC power input. So when designing (or choosing) an inverter one can make 2 choices in theory.
If one draws on average half the current a solar panel can supply, the other half of the energy will simply recombine in the solar cell, heating it up.
I will assume a resistive load (as a well designed product should not reflect energy back in the grid).
An AC grid voltage is sinusoidal, and for a resistive load the current is also sinusoidal (the relation of course being U=RI)
The power is P=UI=RI^2. so the power is also oscillating sinusoidally, centered about the average power.
This seems to forces us to choose between the following 2 options:
1) have the inverter use all the incoming power (requiring electrical energy storage within the inverter: energy storage capacitors or inductors, which is expensive) or
2) have the inverter draw half the solar panel power (the other half is wasted as heat in the ), or have 2 solar panels (half the energy again wasted) to reach the same output AC power as 1); the inverter can theoretically convert the DC power to AC this way
But in reality there is a third way, if you don't mind having 2 outlets where the AC voltages come out in quadrature. If two AC outlets deliver the same average power with the voltages in quadrature, then the powers will be in counterphase, so no energy storage capacitors or inductors are needed (cheaper inverter per watt), and no energy is wasted (full utilization of the solar panels). The only downside is that you can't deliver the total power to one and the same device. But consider some high-way stop convenience store in the desert with multiple refrigerators, say an even number of off-grid refrigerators, then who cares if half of the refrigerators use AC power in counterphase with the other refridgerators?
A similar reasoning could be used for houses in a street, where the even houses and the odd houses are powered in counterphase (voltages in quadrature), which would substantially decrease the costs for inverters.
Redesigning them so that in addition to AC, the devices have terminals for DC would probably not add that much to the cost. And off-grid may be niche, but partially because it's inconvenient to connect some of the larger loads that may require AC to your solar panels: if your going to buy energy buffering AC inverters for the house anyway, and don't understand or want to think about the price gradient for the energy buffer, people will obviously choose the setup you describe.
In practice, isn’t most of that storage used as part of a boost converter to get a higher output voltage than you input? You either need to switch capacitors between series and parallel or feed a step-up transformer with low-voltage, high-current A/C, which ends up being enough copper wire to get heavy and expensive.
but given a desired AC output wattage implies a specific output power, cycled at twice the line frequency, so with only 1 outlet you need to either throw away half the solar power energy, or have storage capacitors or inductors to store energy for the timescale of the line frequency (which is much much longer than the timescale for using capacitors or inductors for merely stepping up / down the voltage.
for example an ideal inverter drawing 10 kW DC and delivering 10 kW AC at 50Hz line frequency will by definition require an energy store of 10 kW / 2 50Hz = 100 J,
square waves would also work, but would still require 2 outlets, and would require the downstream equipment to tolerate it.
This is dual to using a full-wave bridge rectifier to get DC from AC, where a half-wave rectifier is simpler but needs energy storage to ride through the negative half of the cycle.
it would be nice if a flexible (supporting all operating points in the power square) 1 DC-in 2 AC-out quadrature voltage storage-less inverter had an open-source design.
[One can not make a single outlet sinusoidal without equivalent storage capacitors / inductors, its simple mathematics, constant DC average power in - clean AC power out = power stored and released or simply wasted in sinusoidal oscillating fashion (regardless of implementation).
to avoid storing energy, while requiring clean sinusoidal output, 2 AC outlets is the lowest number of output outlets that admit an exact solution since the sum of 2 power sinusoids in counterphase result in a constant output power.]
the power operating point square is delimmited by 0 and 1/2 total power for each AC outlet.
https://superuser.com/a/912689
It probably isn't going to damage any modern equipment, but some equipment may randomly shut off or not work correctly.
a lot of the material effort that was put in to them would be wasted if we replace them before they break down
and if we keep using them, and keep opting for the single AC outlet inverter, we are wasting energy storage elements at volume of 0.01 Joule per Watt (think of the total PV energy production at least in residential solar [I would be uncomfortably surprised if commercial solar parks don't use this already, but then again that would require the grids to be split in an in-phase grid and out-of-phase grid, which would also be new knowledge for me...])
this also affects adoption, as less expensive storage-less inverters would decrease the time until the solar set-up pays itself back!
consider 4 phases in 90 degree turns (I wish HN had a button to render LaTeX or so when a reader wants it on demand):
V_0 = V/2 sin(w t + 0 pi / 2)
I_0 = I sin(w t + 0 pi / 2)
V_1 = V/2 sin(w t + 1 pi / 2)
I_1 = I sin(w t + 1 pi / 2)
V_2 = V/2 sin(w t + 2 pi / 2)
I_2 = I sin(w t + 2 pi / 2)
V_3 = V/2 sin(w t + 3 pi / 2)
I_3 = I sin(w t + 3 pi / 2)
Now outlet "in" (in-phase to avoid clashing with "I") is across terminal 0 and 2, and outlet "qu" (quadrature) is across terminals 1 and 3, so the current and and voltages are:
Vin = V/2 sin(w t + 0 pi / 2) - V/2 sin(w t + 2 pi / 2)
= V/2 (sin(w t) - ( - sin(w t) ) )
= V/2 (2 sin( w t) ) = V sin(w t)
Iin = I sin(w t)
Vqu = V/2 sin(w t + 1 pi / 2) - V/2 sin(w t + 3 pi / 2)
= V/2 (sin(w t + 1 pi / 2) - ( - sin(w t + 1 pi / 2) ) )
= V/2 (2 sin( w t + 1 pi / 2) ) = V sin(w t + 1 pi / 2)
= V cos(w t)
Iqu = I sin(w t + 1 pi / 2) = I cos(w t)
and the powers flowing through the outlets are:
Pin = Vin Iin = V I sin(w t) sin(w t) = V I sin^2(w t)
Pqu = Vqu Iqu = V I cos(w t) cos(w t) = V I cos^2(w t)
so the total power is
Ptot = V I (sin^2(w t) + cos^2(w t)) = V I
constant total power out, just like constant DC power into the inverter so no need for storage capacitors / inductors.
QED
It makes sense for solar parks to use 3 - phase because the back-bone power distribution of the grid is 3 phase.
Do you know if the inverters from solar park panels to 3-phase use storage capacitors to convert to 3 phase?
Do you know if any residential inverters are commercially available that split into 4 phase (2 AC outlets) like I describe, without unnecessary storage capacitors / inductors?
according to:
https://www.energycentral.com/c/cp/solar-photovoltaic-pv-pow...
>China now leads in total solar energy capacity followed by Europe with 114 GW.
>Remarkably, 64 percent of solar systems in the EU are installed on rooftops, 26 percent of them residential, 18 percent commercial and 20 percent industrial.
so residential solar in Europe is 26% of 114GW = 29.64 GW
so a line frequency of 50Hz in europe implies single AC outlet inverters need an energy store of 0.01 Joule per Watt (in the US only 0.008333... J per W, because they have 60Hz)
This amounts to 296.4 MJ (mega joule) of capacitor / inductor energy storage, elements whose raw materials must be sourced, must be built, must be bought by the consumer aand which are potential points of failure (what is not present can not break down). That could have been avoided. Which we still can avoid for future inverters, without replacing all our consumer electronics with DC consumer electronics.
Whoa, you're right! Assuming a perfect power factor and perfect balancing on the loads, of course. Your four-phase scheme is very ingenious! I'm sorry I didn't appreciate this at first.
But here's a thing I'm not understanding: yes, the powers sum to a constant. But the way an inverter produces real sinewave power (as opposed to modified square wave) is to PWM an H-bridge to ramp the voltage up and down, filtering it with an inductor (and usually a capacitor, and maybe more than one of each, but those are inessential). An inductor's time integral of voltage, and thus its average voltage drop (disregarding losses to winding resistance, hysteresis, eddy currents, etc.), must be zero to keep its current finite. So the output side of the inductor has the same average voltage as its input side. So, for example, a 25% PWM duty cycle produces 25% of the full-scale output voltage.
However, in your four-phase scheme, when one of the phases (let's say in) is at ±25% output voltage and thus 6.25% peak output power, the other (qu) is at 93.75% output power and thus ±96.8% peak output voltage. This implies that, during that part of the wave, the active in MOSFET needs to be on 25% of the time, while the active qu MOSFET needs to be on 96.8% of the time. That means that between 18.7% and 25% of the time, both MOSFETs are on, so the two phases are actually shorted to each other (though not on the load side of the inductors). Is that okay? I guess it means that the current generated by the solar cells is being shared between the two phases during that time.
Still, it makes me worry that an imbalance of loads or power factors between the four phases could produce some kind of hazardous condition.
You say that the four-phase system doesn't need any energy-storage elements. To a first approximation you're right, again assuming well-balanced, power-factor-corrected loads on the phases: there's no need to store energy harvested close to the zero-crossing for, assuming 50 Hz, the average 5 milliseconds until it can be released close to the peak. 5 milliseconds is a long time, so these storage or filtering elements need to be quite large, 5 millijoules per watt (I think you dropped a factor of ½ in your calculation there, presumably calculating for a full half-cycle instead of a quarter cycle). By contrast, if you're PWMing a sine wave with a PWM frequency of 100 kHz --- a reasonable thing to do with modern IGBTs or power MOSFETs --- the filtering elements only need to store the energy for a maximum of 10 microseconds, and less if the PWM duty cycle is above the minimum. 10 microseconds is 10 microjoules per watt, 500 times smaller. So you only need 0.2% of the energy storage elements you need for what you're describing as typical current systems.
(All that happens if your power factors or loads are imbalanced is that the inverter isn't drawing a consistent amount of current, so potentially the panels become less efficient.)
A thing I'm not sure about is the junction capacitance of photovoltaic cells. A photovoltaic cell is a diode, reverse-biased in normal use, and reverse-biased diodes have a junction capacitance; we'd expect their enormous junctions to have significantly larger junction capacitance than the pF-scale capacitances we see in small-signal diodes. How large is the ½CV² = 50 nJ/microfarad (at V=0.316 V) energy-storage capacity of the PV panel itself? In particular, is it much larger or much smaller than the 5 mJ/W = 5 ms number you'd need for a single-phase inverter to not be wasteful?
As for your questions about the current designs of power-generation inverters and residential inverters, no, I don't know about them. Typically, in both the US and here in Argentina, one side of a normal power outlet is "neutral" (see http://amasci.com/amateur/whygnd.html for the reasons around this) and violating that expectation might cause some problems --- notably, electric shocks from the outside of Edison-screw-type lightbulb sockets.
However, I don't think you need to violate that expectation; you've described a four-phase system, but I think you get the same advantages with a two-phase system, with the phases in quadrature just as you proposed, but with the two phases sharing a neutral wire. The voltage from neutral ("ground") to the in terminal would be V sin(ωt), much as before, while the qu terminal would be at V sin(ωt + ½π), which is to say, V cos(ωt) --- both relative to the neutral wire. In effect you need only a single H-bridge with four MOSFETs (or IGBTs) and two inductors to produce the two voltages, controlled with a scheme slightly different from the usual H-bridge scheme, because it makes sense to have one side of the H-bridge turned on (in, say) while the other side is turned off.
The thing is, lots of appliances do accept DC input (most electronics and probably all inverter-based stuff like washing machines and aircos). If only there was a standard for DC they could adhere to. Then they could add yet another adhesive to proudly proclaim the feat.
I haven't seen any truly high quality data on voltage safety, due to the obvious ethical issues with performing properly controlled experiments. It's possible the reason you see 120v in every home but 120v DC almost nowhere is inertia rather than safety.
AC in houses is an artifact of the difficulty (until recently) of stepping up/down voltage for DC.
Stepping AC up/down is relatively easy, all you need is a transformer. I'd say the invention of the transistor / IC made it possible for DC. But it took a while to perfect those designs - wall warts changed from transformers in the 1990s?
Something to look at from that period are hobby-grade RC (radio control) cars. Most used NiCad battery packs, and had some extreme amounts of power behind the motors (which were all brushed DC - BLDC was in the future). These motors pulled a lot of amperage (550 and 750 can styles), which the battery packs could deliver, however, there weren't motor controllers small enough to control that much power.
So instead - pretty much up until the 1990s at some point - hobby RC cars used a "resistor speed controller" - something like this one (also known as a "mechanical speed controller":
https://www.rcecho.com/GUIDANCE-FOR-ELECTRIC-POWERED-RC-CAR-... (scroll down a bit to see an explanation)
Basically it was a multi-tapped high-power resistor (or multiple smaller value high-power resistors) that was tapped in a "variable rheostat" manner with a switch operated by a servo. You would usually have three speeds - high (direct to battery), medium, and low; the resistor would "bleed off" excess current as heat (boy, did they get hot!). Yes, it was inefficient, but it was small, robust, and simple to repair or replace.
Of course, there usually wasn't a "reverse gear" (though I am sure someone hacked something together back then). Most of the time, this wasn't a real issue in the hobby - you spent most of your time going forward.
Such controllers actually have a long history - the earliest electric cars used a similar system (just much larger resistors - usually open coil):
http://www.sunrise-ev.com/controllers.htm
http://www.twinkletoesengineering.info/wells_auto_museum/bak...
(some designs also used multi-tapped battery packs for speed control - some in addition to resistors)
Early electric golf carts used them as well (smaller than the electric car version, but still open coil):
https://www.golfcarcatalog.com/golf-cart-blog/put-spring-in-... (scroll about half-way down to see it)
In both cases, switching was done either mechanically, or using large relays or contactors. While it is very inefficient, it is also fairly robust if designed right. Which is why it is still used in a lot of automobiles (though this is rapidly changing with newer models using electronic PWM control) - where?
The AC/heater blower motor! On many cars, there's a "resistor pack" that plugs into the control switch/knob for setting the speed of the blower, and it looks virtually the same as ever - here's one for an older vehicle:
https://www.opgi.com/cutlass/G990031/
But here's one a bit newer:
https://www.turnermotorsport.com/p-11492-final-stage-unit-bl...
About the only difference is the addition of a heat sink. Newer models from even more recent vehicles don't look much different, and they all work on the same principle. They are usually installed in the blower duct work, so that the air rushing by keeps them cool. Unfortunately, if they are designed improperly, or they don't get enough air cooling (or the fan motor dies) - they can heat up extremely hot and melt or catch the car (ductwork - which is usually plastic) on fire! This is especially true if the fan is on "medium" or "low" speeds and the motor seizes (maximum current draw); high speed wouldn't be a problem because the load would short things out and hopefully a fuse would blow (though - not always - sometimes the "fuse" is the wire itself!). This would cause the resistors to get extremely hot - glowing red even - and can cause a fire. I'm certain more than one automotive fire has started this way.
Today, though, thanks to low cost and highly efficient mosfets - and BLDC motors - more and more cars are implementing true variable speed blowers, and using more efficient motors as well. This comes at a cost of more complexity and (depending on how it's implemented) more difficult to repair/replace control and motor systems, but they tend to be safer, and more efficient (this isn't really an issue with ICE vehicles, but very important on electrics for obvious reasons).
the 12V DC vs 120V AC is an apples oranges comparison, with 12V AC and 120V DC it's the other way around. both would be false comparisons.
why would voltage safety tests be unethical? why would one actually test flesh, instead of the theoretical safety models?
The reason we have AC everywhere is simple, historically it was easier to step up and down with transformers (which don't work with DC), but nowadays DC-DC converters are a solved problem.
I don't know if it's only anecdotal or if there is some actual study to verify it, but the typical reasoning on why AC is "safer" than DC is that AC has a "zero-crossing" point, whereas DC (obviously) does not. Why is this considered "safer" (again, possibly anecdotal)?
Because if you accidentally contact DC at a high enough voltage to shock you, your muscles contract - and stay contracted. AC, on the other hand - at least at the relatively low frequencies typically used (50/60 Hz) - crosses a "zero point" where the voltage is "zero" - and lets your muscles relax - briefly - long enough to be able to move away from the current (or in worst case - ungrip your hands).
Again, I don't know if any study has been done on this potential "mythological" reasoning (I would be surprised if there hasn't) - but that's usually the reasoning given.
Well, Edison did it that way because it made for better PR for people to watch criminals or elephants being killed by deadly ac, than to have them read papers on theoretical safety models.
In the case of shock and electrocution DC is _less dangerous_ than low frequency AC (sub ~1kHz). The "let go" currents for DC are several times higher than that of low freq AC, meaning it requires a higher DC voltage to prevent someone from being able to let go. The same is true for the currents where danger of injury and death can occur. DC is still safer than low freq AC.
This has been scientifically tested numerous times in both ethical and non-ethical ways. Here is a paper that shows actual numbers for "let go" currents and dangerous currents vs frequency (from DC up to 10kHz): http://www.wright.edu/~guy.vandegrift/wikifiles/Electric%20s...
In particular look at Fig 3 on page 3 of the above PDF. (One really interesting thing to note in this paper is that women have lower "let go" and dangerous current levels!)
However! There is another factor here where AC can be safer than DC. Fire safety! It is much more difficult to prevent DC from arcing and potentially caused fires than AC. This is because the zero crossover of AC which you mentioned generally causes any arcs to quickly extinguish at lower voltages. DC doesn't cross zero volts and will produce far more arcing at the same voltage.
This is why if you look at the ratings for switches, relays, plugs, etc the DC rating is always much lower than the AC rating.
what makes you think identical lengths of identical cable with the same resistance R powering identical loads R_L will dissipate more heat when carrying DC than AC?
the total resistance of the pair of wires R and the load R_L form voltage divider
in the DC case: P_cable=RI^2
in the AC case: P_cable=R*(I_RMS)^2
they should dissipate the same heat, you may want to brush up:
https://en.wikipedia.org/wiki/Root_mean_square
>For alternating electric current, RMS is equal to the value of the direct current that would produce the same average power dissipation in a resistive load.
and
>Because of their usefulness in carrying out power calculations, listed voltages for power outlets (e.g., 120 V in the USA, or 230 V in Europe) are almost always quoted in RMS values, and not peak values.
The only association with higher resistance losses would be when using low voltages but high currents... and even then the resistance losses would be equally high with low voltage high current AC since the fraction of energy dissipated in the cable versus the load is the same in both cases I^2 R / R_L
So the issue isn't that running your house on dc is less efficient; it's that running your house on 12 volts is less efficient.
There are. 12v automotive (13.9v) is one. Then comes 24v, a standard in aviation. There was once a push for 48v in cars so that air conditioners and braking systems could be made all-electric, but it never became widespread.
https://jalopnik.com/everything-you-need-to-know-about-the-u...
In any case, neither system appears to have defined a plug standard, which IMO is the real issue as far as supporting end user applications, and where USB and the 12V cigarette lighter plug have been such winners. It's only ever going to be useful for installed applications (RV fridges and the like) if it's something you have to wire in.
Modern power electronics are not just more flexible, smaller, and lighter, they're more efficient as well.
They're more common in industrial equipment, but a lot of them there are being converted to run off VFDs, which of course internally run on dc.
But thinking of air conditioners, fridges, washing machines, central vacs, etc; those are all typically shipped today with a two-phase plug but could use 3-phase. OTOH, if they're not actually driving the motor with the 3-phase and are all just rectifying the power and generating their own waveform with a VFD then there's no point; they should have an option to accept DC.
If the panels are small enough, you could get a pretty good approximation to a sinusoidal output from the array of panels.
The shuttering system doesn't actually have to be shutters. Anything that can block panels with the right timing would do. You could probably do something with rotating discs with holes or slots in them, where the phase between adjacent discs can be adjusted to control have often the holes or slots align to let light through.
The exact solution isn't there but there are DC fridges for sale and examples of the set-up you require for solar charging. So this is a solved problem.
Shenzhen is well ahead in the hardware innovation game. If you have a hardware idea look there first.
I'm assuming that he used an AC fridge / freezer with an inverter because he already had one. Besides, I've never seen a large inexpensive DC fridge before. They just don't enjoy the same economies of scale.
A lot of PSUs can do this too; I just bought a Mean Well RSP-1500-48 which specifies its input voltage as AC 90-264V, or DC 127-370V. This means I can run it straight from the traction battery in my car (200V DC) if I care to...
That ensures that the phase-change material is always completely frozen after a long period of being powered, and keeps the fridge at a good refrigeration temperature.
Tetradecane paraffin seems like a good starting point. It's not always easy to find phase-change data on heavier organic molecules, even plain old alkanes, but here are a few in about the right range:
5.8°C n-tetradecane C14H30
5.0°C 2-methyl hexadecane C17H36
4.8°C 2-methyl heptadecane C18H38
2.5°C 2-ethyl octadecane C20H42
0.6°C 3-methyl octadecane C19H40
Neat project though, and I might build one if the reliability improves.
The spikes to 12C or so that then slowly drop back down are generally the fridge being filled with groceries and taking time to cool back down. If you measured your conventional fridge you might see similar events.
I did have the arm board hang twice this summer. Once I was on vacation, luckily it froze with the fridge turned off (so it didn't freeze down to -5C which is where the dumb thermostat is set), and the fridge was fine for the 24 hours or so until I got home, creeping up to maybe 10C. I need to find a more reliable embedded computer.
Edit: Okay, found it [1]. For 2.7KWh it is Height 929mm, Width 313mm, Depht[sic] 329mm. That's large but definitely within acceptable usage.
Follow up question: What are stacking limitations? How much clearance do you need between different two modules or wall and module? Can you stack vertically?
[1] https://www.bluesky-energy.eu/en/salt-water-battery-aib-tech...
A similar one would be a hot water cylinder that only heats up when the sun is shining.
And maybe something that fills scuba bottles to store electricity as compressed air. etc.
The best and most reliable method of non-battery storage of energy is the traditional water tower. It used to be standard practice for buildings to maintain water storage tanks on their roofs. Mate a water tank with some solar panels and you can create an energy storage system without heat, high pressures, chemicals, high voltage or even moving parts (the pump has only one moving part).
Mind you, I don't discharge the system past 23.5v ever, and I check my electrolyte levels weekly adding distilled water as needed, but lead acid are cheap, effective, long lasting, and recyclable with minimal effort. The only thing against them is maintenance, and they are big and heavy. My house doesn't seem to care.
See the chart in [0], where 100% discharge nets 200 cycles, 50% gives you ~500, 30% is ~1200. One would expect that if you discharge 50%, thats half a cycle, and therefore get 400 cycles, but in reality you get 500.
That said, this thread has descended from a comment about apocalyptic scenario planning and this general principle of maintaining batteries makes sense. Learning how to build and repair lead acid batteries doesn't seem to be beyond the realms of possibility.
A good MPPT charge controller will handle stepping up and down the voltage as need to provide optimum life.
https://batteryuniversity.com/learn/archive/advancements_in_...
I disagree with the 1000 cycles concept. Modern battery controllers are very good at preserving oldschool batteries. I don't see every off-grid house replacing their batteries every few years, just as I don't see automotive or marine batteries die so quickly.
Assuming off grid prepper mentality is being used.
Are you sure about this? Lead-acid batteries are highly recyclable, but lithium cells are straightforward to recycle, too. I know that Tesla has a recycling center set up at their gigafactory.
Water towers have to be built on site, which can be difficult. Not to mention, to get appreciable storage, you have to build a really tall tower or a really big one. This is difficult too. Especially if you are in a location where the soil shifts or you get high winds. Because now your tower has to survive the elements.
There is definitely a strong case against water towers for distributed energy storage.
Similar system from the age of steam:
However there's something about living off grid that makes one want to simplify, and batteries are one of the big, expensive, and limited life time components of a solar system. If you can reduce the load on, and the required capacity of your battery bank, then you have improved reliability significantly.
As you say, pumped hydro is certainly a great storage system if the situation permits. It just takes a very large storage system to store as much as batteries. If you have a pond or lake and an easy supply of water it would certainly be my choice.
On the other hand, so is a big chest freezer!
https://en.wikipedia.org/wiki/Solar_water_heating#Energy_pro...
[Silly to ignore equipment costs, sure, but if you have other uses for PV power ... using PV for heating water isn't necessarily nuts.]
So in sunny days I let the water tank heat as much as possible(usually around 70-80C), and can go 3-4 days without even needing to turn it on. We are 4 in the household and it holds nearly 100liters of water. It is a kind of battery :)
I find it ironic that this sort of thing is so overlooked with solar/wind power on the electric grid. Consumer electricity prices should be allowed to rise and fall with the availability of solar and wind. Then, you can heat one hot water tank only when electricity is cheap, and also use that tank to heat your house in winter. In summer, a separate tank can be chilled when power is cheap, and used to cool the house.
Or even simpler, a pile of rocks will work.
With such a scheme, the need for grid batteries would be reduced enormously. Instead grids have the problem of what to do with the extra power when there's too much of it.
For example, the power goes out around here often. I've noticed it can take up to two days for the house to cool down. The water in the hot water tank is hot enough for a comfortable shower for 3 days.
Water/rock batteries are cheap as dirt, no maintenance, no fire risk, etc.
> NO TECH MAGAZINE – We believe in progress and technology
> No Tech Magazine hosts all links and updates from Low-tech Magazine. We refuse to assume that every problem has a high-tech solution.
Is it a parody site? Is it a "Low Tech Magazine" mirror site? Or is it sister site operated by low-tech magazine fans, just with a slightly different altitude towards technology, hence the joke?
https://git.joeyh.name/index.cgi/joey/house.git/tree/src/Sun...
The first rule of operating an off grid cold chain is to always, always, always keep the freezer full to the top with ice packs.
The ice packs will of course be needed to pack coolers to distribute the vaccines, but they are also your backup system. A freezer filled to the brim with ice packs will be able to bridge a 3 or 4 day interruption in fuel (or failed generator, or burned up wiring, or security evacuation, or, etc, etc, etc), and save hundreds of thousands of dollars of vaccines from loss, not to mention possibly human lives.
Ice rocks.
For AC applications, though, water is not the perfect heat storage medium. The efficiency would be better if the freezing point was higher. But then again, I do not know of any material with melting point at +10 celsius, heat capacity close to water and yet, would be having a reasonable cost compared to water.
While a fridge works by extracting the heat from the stuff you put in your fridge and heating your home with it, decreasing the load on the rest of your heating system before even accounting for the compressor inefficiencies.
Kinda like when hotels put their ice machines in some tiny room on each floor: that room gets really hot because it’s tap water in and ice out.
Point being, that low-tech fridge (ice box connected to outside) is thermodynamically inefficient if you’re heating inside, while an electric fridge is an efficient heat pump.
In a cabin with poor insulation and “limitless” wood, go for it. But for a modern structure, it’s not fully thought out.
What would really make sense is a fridge that moves heat outside in hot climates.
When it's cold outside, and you want to heat inside, you want to use indoors as the heatsink. Even if it's -20 outside and +20C inside, you can still heat the inside by removing heat from something +10C and making it -20C.
If you put liquid water into your electric freezer, a fridge pumps heat OUT of the water to freeze it and exhausts it indoors. A net benefit.
While the heat tube system moves the heat from the water outside when you really want to move that enthalpy into the home. But it instead moves that heat outside. That's bad when you're trying to heat inside. It's just throwing away energy.
But, if you have a ground source heat pump as your home's heat (sourcing from 45° ground temps) - wouldn't it be more efficient to use the outside air as a source for a fridge?
But if we're comparing to resistive heating, an air-source heat pump near/below freezing points, propane or even natural gas, the economics move in favour of the electric fridge.
[0] https://en.wikipedia.org/wiki/Stirling_cycle [1] http://www.cruisersforum.com/forums/f115/coleman-stirling-po...
Seems a bit excessive. Why does it need a computer at all?
That said, I think the 3k lines include adapters for talking to the sensors, etc. The algorithm itself is about 250 lines: https://git.joeyh.name/index.cgi/joey/house.git/tree/src
Oh, also, Haskell - nice going fridge fellow.
I can understand downvoting heated expressions, but I don't understand why a sincere question can be downvoted.
Perhaps because it comes off like a criticism not a question.
> 3000 lines of control software? Seems a bit excessive. Why does it need a computer at all?
It really means,
> 3000 lines of control software is unbelievable. It's excessive, it doesn't even need a computer at all!
Sure, it's criticism. But I don't see any bad faith argument here. So I guess "Just asking questions" doesn't apply here.
And from the Guideline,
> Please don't post shallow dismissals, especially of other people's work. A good critical comment teaches us something.
So it's probably the reason for downvoting.
But consider the fact that other readers have already responded to the question/criticism with more information on the author's intention, I think downvoting this is unnecessary.
I meant exactly what I said, there was no subtext. I wanted to know the answer and it was provided by icebraining.
If I had intended to say that the number of lines were in actual fact excessive I would have omitted the word seems.
In that article it was suggested that getting the exact size and shape of the compressor elements was more fiddly and troublesome than "it was worth". So this start-up's value proposition was making it way easier to develop very well balanced refrigeration / heat pump designs... which in turn are significantly more efficient than current state of the art in mass production.
Given that the writer of the article doesn't appear to have space issues, it would be cool to see a homebuilt unit with much higher insulation using an existing freezer to mine for parts. I really like the redneck Yeti coolers that people build.
A more trivial modification might be to put strips of plastic across the opening so that less air interchange occurs when you put stuff in or out.
There are a lot of ways of getting heating to 60C and doing this. I have always thought that a system could use evacuated solar tubing to generate the heat.
1kw of panels is a lot tho. Not going to work for many van life situations.
Probably most people do not need that thing, too.
It seems to be a cult object of prosperity.
It just happened a few years ago while travelling a lot (by train), but then I found that idea interesting and never bought one, just to see if I "can do it" without.
After a few years I do not even know what I would store in a refrigerator.
Yes, of course, I am living in a city, with everything I need in markets around me, but it really just takes a little bit of planning and thinking to not need that thing even in remote places, I did that, it worked.
These sorts of strange delusional comments, followed up by a claim that they can’t understand normal perspectives on things like “what would you do with a fridge”, are surprisingly common on HN. I have no idea what sort of thinking goes into a comment like this, but I suspect it’s sort of role play where they imagine the world would be such a great place if only everybody else also prescribed to their fringe view on fridges.
> conforming to a standard; usual, typical, or expected.
Having a fridge is entirely normal, and to claim otherwise would require quite an extraordinary justification.
If you wanted to question whether people really need them, that would be a much more reasonable line of inquiry. But for anybody posting here to claim that they cannot understand why people would want to have one is frankly not believable.
Where did I say that?
Also to keep salad and vegetables longer fresh.
Can you describe in more detail how you are coming around of this?
Because where I live, food gets spoiled in hours without refrigeration and living without cold water/beverages/ice seems downright tortuous.
Perhaps I'm conforming to a cult of prosperity. Or perhaps I'm supporting local business, helping the environment by reducing food transportation, eating healthily, eating good food, and meeting local people with similar interests.
I think there's a tendency people have, especially prevalent on Hacker News, to do something unusual and cool, and then assume everyone else isn't doing it because they're stupid/conformist/etc. It's great to do something off the beaten path, but don't go that extra step and start making assumptions about people.
I'll also add that in the past, I've lived in some poor neighborhoods, and even very poor people who certainly aren't in any "cult of prosperity" in the US usually have refrigerators. I think this has more to do with the cost savings associated with buying bulk food than to do with any "cult of prosperity".
[1] https://en.wikipedia.org/wiki/Community-supported_agricultur...
Edit: Actually, while this idea sounds good, a fridge requiring 1 million joules for a rainy day would need 10,000kg @ 10 meters, even before considering efficiency losses.