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I recently wondered about how to store heat, yes, heat. Granted, typical wall insulation is fairly good at slowing down the transfer of heat (usually from your home to the outside air) but I wanted something more “passively active” if that makes any sense. Apparently, it’s a well-known concpet among interested folks that you can store heat gain in water with the express purpose of allowing that retained heat to be released when the surrounding air is colder than the water in the tank. This idea is often utilized in greenhouses to help regulate temperature when the air temperature drops drastically at night. As I had no clue how to do this, I did some research.

Though I have no plans to try it–in large part because it would be really weird to put a huge IBC tote or two in the basement and I would probably end up divorced–I wondered how effective one or two of the 275 gallon IBC totes would be at regulating the temperature of our basement. Of course, I actually have to BUY a tote to even attempt it! Anyway, I’m not about to do the math either but let’s say the basement is roughly 700 square feet or so of living space. A typical IBC tote is about a four foot cube and would fit quite nice in the middle of the basement, if you ask me.

Now, a large part of how this would work is because of relative temperatures, that is, how cold is the outside-the-house air, how warm is the basement air, how warm can the tote water get when being actively heated (by a fireplace, for example), how fast does heat transfer from the house to the outside air, and probably a few other factors I didn’t even consider. The point is that there are a whole bunch of factors at work and if I had paid more attention in Engineering school I could probably do the math but, these days, that sounds like entirely too much effort and planning. 😉

Is Water The Best Storage Medium?

The first thing I had wondered about was whether water is the best medium to use, maybe rocks or bricks would be better? Turns out water is the best option for DIY home use. Someone else posed a similar question here and following are a few excerpts of answers:

“…If you look up the “specific heat” value for those materials (a quick Internet search will find it), you will see that water has a remarkably high specific heat, much higher than nearly anything. This means it can store more heat for a given volume and would be your best choice. Another factor to consider is that convection currents in water allow heat to move around more quickly than in a solid, which is another vote for water…”

“…To compare rock and water for heat storage, you need to know the heat capacity and the density of the two materials. Comparing water to stone, it takes a little over 4kJ of energy to raise 1kg of water by 1deg C. In contrast, granite takes a little less than 1 kJ to raise 1 kg by 1 deg c. Granite is about 3 times more dense than water, so for a given volume, it still stores less than 75% of the energy of water…”

“…But there is something else to consider…. gravel weighs more than the same volume of water. A container when filled with gravel will weigh about 2.5 times more than if the same container were filled with water. So when a container that holds 1 kg of water were allowed to cool 1°C, it will release about 4000 Joules of heat (as explained above). But if you fill the same container with gravel it will weigh about 2.5 kg, so it will store (800J/kg x 2.5kg =) 2000 Joules of heat energy. This is still only about half the heat energy that the same volume of water will store…”

And if you want more numbers to verify that water is the best medium for most of us, here’s the specific heat capacity of water (and other substances) as well as the specific heat capacity of common building materials. So, the short answer is it’s water or bust for me!

How to Heat the Water?

Originally, my thought was to just set the totes in the middle of the basement and let the heat from the fireplace radiate into the water and gradually heat it that way. Apparently, there are products designed to accept passive heat, but not from the fireplace… from sunlight, such as these Sun-Lite Thermal Storage Tubes:

The problem is that they need access to sunlight to make them useful so that constraint makes their placement a concern (that is, south-facing windows only). But, seeing as though most homes are more window glass than not, you could probably make it work if you really wanted to.

Is There a Better Way?

I wanted a more active solution and happened upon two sites that explained the concept of using solar gain for heat, including to heat water. As it turns out, I would have been better off using 10 or more 55-gallon drums as heat collectors rather than one or two large IBC totes. This article on Multi Tank Heat Storage provides quite a bit of information regarding what I wanted to do and even considers how to use the system for hot, pressurized water too. It’s a long read so plan ahead.

I also found this article on DIY Solar Heat Storage Systems t0 be an interesting read. While less about thermal heat storage (the end of the article does discuss it some) there are some interesting ideas on building design and even utilizing an attached greenhouse for passive heat.

Sadly, this wasn’t quite what I was looking for. Then I remembered this video by Engineer775:

Yeah, that’s more like it! Granted, the purpose was to make hot water for bathing and the like, but why not utilize the idea for heating water that can be used as a thermal heat storage? I’d bet that over the course of the day you could get the temperature of the water inside a typical IBC tote quite hot with constant heating from a fireplace, but I’m really just speculating here.

And just to show this concept isn’t new, here’s an article from 1978 Mother Earth News titled Make Your Fireplace Work For You that’s more along the lines of what I was thinking about.

Is Water the ONLY Thermal Mass to Rely On?

The short answer is that to be most effective your entire home should utilize the thermal mass concept (in the form of other substances like concrete, brick, tile, earth, etc) to store, release, and regulate temperature changes not only at night but throughout the day. In fact, large thermal masses can be quite effective at helping to cool the surrounding area by allowing heat transfer from the air to the thermal mass (and thus cool) as well as for heat, which is what I wanted in the first place.

What Did I Learn?

I learned that I could do what I wanted–to use IBC totes to store heat–but there are far more efficient options such as using many smaller barrels (e.g., 55 gallon drums) instead and that I can either passively heat the water using sunlight if necessary as well as to actively heat the water using a heat source such as a fireplace with a little creativity mixed in.

What do you think? Useful, plausible, cost-effective? Am I missing something completely and, more importantly, has anyone tried it? I would love to hear about success stores.