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Comment by beachy | Hacker News Reader

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.

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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.

https://c8.alamy.com/comp/D24NWW/solar-hot-water-glass-tube-...

I've seen it done in Northern Spain with just a long black PVC tube in a spiral pattern on top of a roof. Worked well enough for three showers/day, even during the winter.

I've seen this occasionally in the deserts of the United States, though usually in poorer communities and neighborhoods. Perhaps electricity is cheaper there, since that's where it's grown (solar farms).

PVC and ABS aren't recommended for hot water, nor are they particularly well suited to sustained UV exposure. It'll work in a pinch, but if building something permanent with the luxury of options I'd ignore those.

I don't know, I just know it's been working untouched for over a decade. They don't use the water for drinking or cooking, though.

Given the fallen prices of photovoltaic panels, it is frequently cheaper to use them rather than to build a passive collector, particularly if you have to deal with freezing weather and already have some photovoltaic infrastructure in place.

Photovoltaic panels still cost €0.19 per peak watt, wholesale (https://www.solarserver.de/service-tools/photovoltaik-preisi...). Lowe's lists 50 feet of 3-inch corrugated black plastic flex pipe at US$31.98 (https://www.lowes.com/pd/ADS-3-in-x-50-ft-Corrugated-Solid-P...), which works out to 1.16 m² and thus 1160 peak watts, which is 2.7¢ per peak watt — 9¢ per peak watt if you figure that the collector is only 25% efficient (this depends on the temperature). Currently US$1 ≈ €0.874, so that's €0.079 per peak watt.

That pipe is HDPE, and corrugated to boot, so it can go through quite a few freeze/thaw cycles before it starts leaking. It's true that it won't provide you hot water when it's freezing outside unless you put it under glass or at least plastic, though, and that drives the cost up.

I was expecting to find that you were ridiculously wrong, but actually it seems like the costs for a passive solar collector that can produce hot water in freezing weather actually might be about equal, or in any case no less than half the cost of photovoltaic. Bravo! And thank you for the insight!

I've just recently learned about parabolic reflectors concentrating solar energy on water pipes: https://en.m.wikipedia.org/wiki/Parabolic_trough

But I guess they must be rotated to follow the sun, which is more complicated than just pipes on the roof.

Using non-imaging optics, trough concentrators up to a factor of five or ten suns can work through a substantial part of the day with only seasonal tracking (i.e. twice a year you go out and change their angle by hand).

The very first solar power plant, in Egypt in I think about 1910, was actually a steam plant that worked on precisely this principle. Photovoltaic is cheaper in most of the world than a steam engine now, though.

this is how it's done in Israel for decades.

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.

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.

Yep thanks for that, they're popular here as well (as well as simple variants like a big coil of black plastic irrigation pipe lying on the ground).

In my case I'm trying to put all my investment into a heap of solar panels, as that's a more general purpose infrastructure.

Considered the black pipe stuff...but I'm not convinced the plastic doesn't leach nasty crap into the water.

I assume you have gas heating & cooking then? If they're elec its usually cheaper to buy new ones than up-spec the solar PVs.

Heating's optional around here but we use LPG (propane) for cooking. Cooking's the last big hurdle, wife refuses to consider a wood burner.

You can get systems that use heat exchanges - the water that goes through the black pipes circulates closed and heats up a separate potable water supply.

https://www.energy.gov/energysaver/heat-exchangers-solar-wat...

That's clever. We were considering it for heating a pool. Iei lay the pipes on a hot roof in the sun

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.

That is a cool idea! If people are piecing together the infrastructure for their lives, the best and most resilient technologies probably involve wiring together with a screwdriver and a pair of pliers existing stuff - basic things that we already have lying around in warehouses today. Fridges, solar panels, inverters, hot water cylinders, raspberry pis, SSRs etc.

I like the way that your project is a kind of glue that hooks up some of these things. I could really see people shipping and using a product like this.

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