heat storage system fudge factors

[Stephen Nuchia]

Hi, I'm working up a new prototype of a heat storage stove. I'm looking at some "boiler blanket" insulation material that is some kind of high-temp fiber matrix with aerogel embedded in it. Details are alibaba-obscure. Anyway, they provide a thermal conductivity versus temperature curve that I can live with, if I'm applying it correctly. That is, I calculate the surface area of the heat storage cell and the temperature difference, assume the blanket at rated thickness is handling the entire delta-T, and make the simplifying assumption that the wedge geometry is irrelevant (1 inch blanket on a 10-inch cylinder, say).

My question is, what pragmatic factors should I be considering for an application like this? The entire design is on the cusp of economic impracticality and I'd like to get the insulation system cost nailed down in as few iterations as possible so a little help from someone with real experience would be appreciated.

 

[IRstuff]

"on the cusp of economic impracticality"

I don't doubt that. A vacuum insulated thermal cooker can be purchased for well less than $100:

http://www.ebay.com/bhp/thermal-cooker?_trksid=p2045573.m2388

And they provide about 6 hours of useful cooking time.

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[Stephen Nuchia]

Thanks but that's not exactly what I'm doing. And vacuum insulation is too fragile to meet the needs of my target audience. Even in a heavy sleeve it's too vulnerable to shock.

 

[IRstuff]

"Even in a heavy sleeve it's too vulnerable to shock."

And what do you think will happen your aerogel?

Nevertheless, I not sure why you think a stainless steel vacuum vessel is fragile.

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[Stephen Nuchia]

Ok, I'll admit it, I'm old enough to have broken a lot of aluminized-glass lunchbox thermos bottles in grade school. Stainless would probably be durable enough, though thermal resistance will go way down if it's dented to the point the inner and outer walls touch, and that would create a safety problem. As for the areogel durability, solid pieces would not be at all durable but the blanket material looks like it would be, the areogel beads are small and floating in a fiber matrix. The manufacturer doesn't publish a shock rating though.

I think I ran the numbers for radiant heat transfer across a simple vacuum annulus and it was worse than what I needed. But I'll run it again, the people who make those could easily make what I need. Getting a modified sample might be expensive but product development usually is.

 

[racookpe1978]

A confounding issue is the "structural" assembly. A vacuum chamber is held at the top symmetrically, and the inner cylinder is suspended inside the outer cylinder fairly securely. (And yes, they do break because the inner and outer bottle walls are (used to be) made of glass.)

the gel-type insulation is very, very effective but it has absolutely NO "strength against anything, even a breeze. Works OK in an attic for example, because it sits in a calm space with nothing to support, and is well-supported underneath the whole area. Even inside a wall, the aerogel insulation tends to "settle" into the bottom of the wall under its own "weight". Loose spun insulation blankets are less efficient (conduct more heat than aerogel) but at least they stay in place. Aerogel has to "sprayed" in place, so the inner and outer "liners" have to be positioned correctly and kept in place while the gel is "inserted". Obviously, this can be done, but you have to think through the assembly process completely. Painting? Coating? Extreme heat later in the process of assembly - as if your were going to use thermo-setting plastic - will "melt" the aerogel.

So, assume you have an inner "liner" and and outer "liner" of some stiff material. the supports between the two (since the insulation between the two parts has no strength at all) will be a thermal conduction path that will lower your thermal efficiency.

 

[Stephen Nuchia]

Thank you racookpe1978, I had been fretting over the mechanical supports I'll need and the thermal path they represent. Steel isn't too bad, it's strong and has high yield strength for a given cross section, but the cooking interface needs to be aluminum (maybe, that's what I've been prototyping with anyway) and in a dirty environment I'd worry about dissimilar metals corroding.

I tried calcium silicate, thinking it would be mechanically stable enough that I wouldn't need so much structure and it is, but it's not really very good insulation. Residential fiberglass outperformed it, but of course can't bear any loads.

I thought briefly about ceramics but aerogel seems more promising ... but not if it's going to crumble when the device is handled.

This is the stuff I'm thinking of trying next:

https://www.alibaba.com/product-det...pm=a2700.7724838.2017115.1.4ba432afMqLSVD&s=p

 

[racookpe1978]

The aerogels I've seen (handled) were in a demonstration "attic" where the gel chemicals were "sprayed" (squirted) out into an attic: Very, very thin tenuous spider-web-like "cloud" that was many times less strong than a commercial expanding foam type insulation. So, a lack of strength didn't matter (and low weight was good too).

Of course, part of the aerogel thermal resistance is that very thin nature: There is little thermal conduction through the spiderweb, but the gel traps the air and prevents convection from surface to surface, and the material reduces (somewhat) radiation transfer between the two surfaces.

 

[IRstuff]

Perhaps you should explain what you are trying to build. Your initial description fits my thermal cooker to a T. Unless you are planning on have chimpanzees doing the cooking, I just can't see why you might think that anything you're looking at will work anywhere near as well as the stainless steel vacuum system. When one looks at products such as this, one has to consider why there are dozens of examples all using a vacuum system, and none using any other insulation. It's unlikely, after decades, if not centuries, of looking for a solution that dozens of companies have picked a stupid or impractical solution. Now, if the objective is simply lower cost, then the thermal performance must be sacrificed. See this for a discussion on design retained heat cookers (RHCs).

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[Stephen Nuchia]

IRstuff, in the systems you've suggested a heated charge of food is enclosed in an insulated chamber and cooks by retained heat over a period of time. This is mainly suitable for wet cooking and requires a conventional heat source to provide the initial heat.

In primitive conditions (and in some cases for novelty value) something analogous is done with "hot rocks". Masonry ovens heated by a fire burned for some time prior to cooking and removed before introducing the food charge, which is cooked by the heat retained in the structure is another common mechanism.

The system I'm working on uses a phase change material in an insulated cylinder, with a removable insulated cover on one face and an optimized heat transfer surface on that face. It is intended to be charged by a solar concentrator (eg parabolic mirror) and store the heat for some time, then used like a hot plate to cook foods in traditional cooking vessels at some convenient time and place independent of the concentrator availability.

Quick description here:

https://drive.google.com/open?id=1b1eGfmh5-gRCSaiWZAvc5zuSZbM8hzGrXQnmWV0kkF4

(poster from IEEE GHTC 2015)

Briefly, my aim is to support traditional modes of cooking without traditional open combustion heat sources and to do so affordably and safely in a resource-constrained environment.

I appreciate the interest and input very much!

.

 

[IRstuff]

OK, seems to me that nothing has changed. In fact, the constraints are even higher, since you have a potentially much longer storage time, and you'll have much higher losses and inefficiencies, which means that a vacuum storage is all the more beneficial

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[Stephen Nuchia]

Looks like I might still want to use that "boiler blanket" or something comparable for the lid insulation, disk-shaped vacuum-insulated structures aren't a thing that sounds practical.

How can I calculate (or estimate) the heat loss through a vacuum flask? Other than the neck (and possibly other structural members) the primary mechanism is radiation and that's not dependent on thickness of the gap, right? It doesn't have a thermal conductivity bulk property the way solid insulating materials do.

I guess I just need to find test results for actual examples?

 

[IRstuff]

You can use the Stefan-Boltzman Law:

https://en.wikipedia.org/wiki/Stefan–Boltzmann_law

Polished stainless steel would have an emissivity on the order of 0.08. Assuming an inside temperature of 227 C and an outer temperature of -23C would result in 266 W/m^2 of loss.

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[racookpe1978]

IRStuff: You assigned an outside temperature of -23C for the S-B approximation. Would it not be "near-room-temperature" of 20-25 C rather than extreme winter cold? (If outdoors, seems like you'd have to determine the probable market for near-Arctic conditions, simple no-air-conditioning 20-30 C, or potential desert 30-35 C)

 

[IRstuff]

It's what I do for worst case design. As it is, who knows whether the PCM can even handle 227C?

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[Stephen Nuchia]

Thanks! I saw a version of that computation but without real-world application context or numbers. Having somebody who knows what they are doing turn the crank is reassuring.

The surface area is only about 0.3m2, ambient for most of my market will be around 20c worst-case (tropical and semi-tropical) and 250C or so internal is a good point to use for feasibility purposes, it's enough above the melting point of the phase-change material to serve as a "fully charged" number. If I had vacuum over the whole surface losses would be in the 100W area, or 360 kJ/hr. More than I'd like but within the feasible zone. Calculated losses using 35mm of "boiler blanket" are lower but costs are higher and I'm less confident of the real-world validity of that calculation anyway.

I'm concerned about the surface temperature near the neck/lid interface. Both the flask neck and the inner surface of the lid capsule will be conducting there and it could reach hazardous temperatures. Maybe the flask neck could be formed a centimeter or two below the cooking surface and something like a silicone ring installed there to separate the two.

Time to go into the lab. Not sure how I'm going to go about building a prototype with vacuum insulation, maybe my welder will have some ideas. Thank you again for sharing your experience.