Comment by crazy2be | Hacker News Reader

Comment by crazy2be | Hacker News Reader

Thermal masses are a super cool way (ha!) to store energy, and I'm surprised they don't get more attention, both in the media, and in terms of research / investment. Heating and cooling accounts for somewhere around half(!) of residential energy use [1], which could be easily time-shifted with some very cheap thermal mass and some economic incentive to do so.

This is actually used in some commercial buildings in places where energy is cheaper at certain times of the day (for example, at night) [2], by freezing ice when energy is cheap, and thawing it when it isn't.

That being said, although this is a super cool project (I have the same inverter, and I made a similar control board ;)), this thermal mass doesn't seem like it would be particularly practical in most cases. Water has a relatively high specific heat capacity of 4.186J/gC, but given the narrow range of temperatures acceptable for a fridge, this doesn't end up being very much - only 79Wh per degree Celsius that the fridge is allowed to swing. If you consider 1C - 6C "acceptable", you only end up storing 395Wh. This is about 30-40% of the capacity of a $100-$200 "deep discharge" lead-acid battery, and is also a much wider range than most consumers would be used to (and may result in frozen veggies, for example).

In order to make this more practical, you really want something that can freeze around fridge temperature. For the same amount of water used above, freezing and thawing the water would store 6,308Wh(!), around 16x as much. If you could get something that freezes at 3C/4C with a similar heat of fusion to water, you could have a much smaller thermal battery that lasts _much_ longer, without the substantial temperature swings you see with your current design.

[1] https://www.eia.gov/tools/faqs/faq.php?id=96&t=3 [2] https://en.wikipedia.org/wiki/Ice_storage_air_conditioning

Interesting we made similar control boards!

You need to take into account efficiencies of getting the cold into the fridge too. It takes around 10-15 hours of runtime to cool the thermal mass down from 5 to 0.5 degrees C (at 14C exterior temp), and the fridge needs 120 watts to run. Measured this way, the thermal battery is storing ~1200-1800 watt-hours.

But then, if it were powered from batteries, there would be significantly more power needed to fully charge the batteries and maintain good health -- my 860AH battery bank (4 deep cycle batteries) needs at least 1kwh input to charge up from 12v to full).

Another way to come at the question is, how many batteries are typically specced out to power an offgrid fridge, and banks costing 10-20k dollars are not at all uncommon, though they're also shared with other household needs.

I was subletting for a year from a family of six (who were away themselves). Their fridge was extremely oversized for the amount of stuff I stored in it, so I filled up two thirds of the volume with refilled water bottles.

It has the added benefit of stabilizing the temperature a lot. I recall reading somewhere (who knows, maybe one of the earlier blog entries here) that turning on a cooling element costs a lot of energy too on top of it just running, to the point where that effect on energy savings should not be underestimated.

All in all I saved quite a bit on my energy bill with quite a simple hack.

Using a solid thermal mass is discussed on the linked wiki: https://fridge0.branchable.com/thermal_mass/

composition

Water is the easy choice.

A material that can be frozen would be better, because it takes a lot of extra heat to melt a frozen material and so more cold could be stored.

However, this needs a material that freezes at a higher temperature than water, and most such are oils, which are less dense and so store less cold overall.

This is an open research area.

I've heard about projects where they store heat underground - not sure if they use a certain medium or groundwater or something, but they basically pump heat into the ground during summer (and cold out into the HVAC) and vice-versa in winter. I guess ground itself - whatever is down there - is good enough. See also the London underground for an undesired underground heat storage.

But yeah, back in the day they would get blocks of ice from nearby lakes and put them underground, it'd last all year.

Yeah, here's an example of one in Alberta, Canada: https://www.dlsc.ca/borehole.htm

Some of the things I found crazy when reading about it is that they can get the ground temperature up to 80 degrees celsius by the end of the summer, and then the ground stores enough heat (without just dissipating off to the environment) that it can provide a majority of the heating for 52 houses over the course of a cold Alberta winter.

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