Thermodynamic Pros/Cons coating light metal tanks with rubber

[Steven Bolgiano]

Hi Folks,
I promise there is an actual question at the end of this note.
I couldn't see where to attach images on this forum page, so here is a link to a rough illustration and a photo of the project site in progress (the cabinet for the digester is partially built)...
http://terradesic.org/hybrid-anaerobics-work-on-progress/

I am assembling an Anaerobic Digester which are two, horizontal, 55 gal. connected drums. They are heated with solar heated water, solar heated forced air, and direct sunlight that comes through an acrylic "quonset hut roof" that mounts on top of the insulated Digester cabinet, containing the drums.
The solar heated water phase is delivered via a pattern of 3/4 copper pipes, directly under the tanks.
the solar water heating collectors are directly adjacent to the Digester Cabinet.
There is no heating components inside the drums.

As an Anaerobic System its important there are no air leaks, so ... I coated welds with first a specialty neoprene by 3M. And then followed up with a Rubber based black "Pond Coating" that I consider a a tough material. But so far I've just spot covered on weld areas, but my question is to whether or not cover the entire drum.
It will certainly inhibit corrosion, but I am concerned it will create too much of an insulation barrier, rather than without it and the copper directly transfers its heat to the steel drums, filled with slurry.

SO ... In considering going ahead and coating the entire tank with the Pond Coating ... (and probably no thicker than 4 - 6 mil) ... with the tanks contained in the insulated cabinet, being heated below with the solar heated water-in-copper (3/4") ... also working with (or against) the proposed rubber coating, is the solar heated air fan forced into digester cabinet (from a small green house, adjacent to digester, opposite side from solar water...

Question:
Will the black rubber coating act as an insulator, and significantly inhibit efficient transfer of heat?

How much less efficient is a rubber coating, than the metal in a steel drum) (assume equal thickness for each) ... in terms of a general percentage descriptor?

Once the heat transfers through the rubber and steel, and begins to establish a temperature in the combo off water and waste materials (initially cow manure, but transitions to food waste) ... I assume the rubber transfers heat equally both into and away from the slurry contained in the tanks, so there is no increased insulation advantage with the rubber? (it gains and loses heat at the same rate, either direction, go in or dissipating out?)

thanks in advance for all feedback and information!
Steven

 

[berkeman]

I couldn't see where to attach images on this forum page

Use the "Attach files" link in the lower left of your Edit window to attach PDF or JPEG files to posts...

 

[jrmichler]

How much less efficient is a rubber coating, than the metal in a steel drum) (assume equal thickness for each) ... in terms of a general percentage descriptor?

A rubber coating is several times less efficient than an equal thickness of steel, HOWEVER that's not the whole story. If you have a slurry inside a rubber coated steel drum that is heated by hot air on the outside, the total heat transfer is controlled by the total of all of the following:

1) Heat transfer film coefficient (thermal resistance) between slurry and rubber
2) Thermal resistance of the rubber
3) Thermal resistance of the steel
4) Heat transfer film coefficient (thermal resistance) between steel drum and air

That's four thermal resistances in series. The largest of those resistances is #4, and it is typically larger than the other three combined.

A digester does not normally need a lot of heat added or removed, so heat transfer is not normally a limiting factor.

 

[Steven Bolgiano]

Thanks Jrimichler!
Your response has really helped me to understand at least what the right questions are I need to ask myself ... plus a bit more!
I posted a quick video of the site on YouTube for you to get some better details:


So skipping right to that, tell if this makes sense:

• Because solar heat is essentially limited to various amounts of partial daytime (less the cloudy days) , ... the highest priority is to deliver heat to the drums as efficiently as possible.
  So the rubber coating immediately is an inhibitor of that, no matter the level it will always be "less efficient", where the priority is to maximize efficiency.
  So that answers my question. No full rubber coating.
• My next priority is to be able to control, modulate, and conserve digester heat levels at proper temperature ranges, and with minimum fluctuation.
  I would say the digester cabinet's 2 inch walls (1" wood, 1" foam board), and the .095" acrylic curved top, ... and the weather's wind/sun/temps, ... besides good instrumentation, plays the biggest part in that priority. I have some thermo-switch and valve control ideas, but I'd really enjoy talking with anyone who has their own thoughts about it,

Thanks again so much. Even though I am totally unprepared to do those calcs you began to describe, this gave me the principal factors to consider.
My experience designing/constructing large poultry farm scale digesters, ... is that the parasitic load is very significant, as to the heat needed to maintain either a mesophilic or a thermopilic heat level. And this also limits the geographic regions from where anaerobic digestion is excluded altogether because of cooler weather.
I see food waste as a serious community pollutant, putting dangerous pathogens into the surrounding soils and then aquifers.
To apply AD has a feasible solution, it must be scale-able, affordable, and be available to a wider region of the public, than currently limited to warmer regions. A hybrid Solar Powered Anaerobic Digester.

thanks!
s.

 

[jrmichler]

The rubber coating is normally only a minor factor in the heat transfer. Typically, the most important factor is the film coefficient for heat transfer from air to a surface.

Are you sure that you need solar heat? What climate(s) are you designing for? What is the temperature of the slurry entering the digester? Anaerobic digestion is an exothermic process. How much heat is generated per unit mass of slurry? What is the difference in productivity of a mesophilic vs thermophilic system? If you sufficiently insulate the system, do you need external heat? Note that internal heat gain is proportional to the system volume, while heat loss is proportional to total surface area. Doubling the linear dimensions results in eight times the volume and four times the surface area. Therefore, larger systems that are heated with internal heat will run warmer than smaller systems, while larger system heated by solar will be more difficult to heat.

How much methane is generated? Do you have a plan for dealing with the methane? Are your proposed systems large enough to use the methane to generate power, with the waste heat going into the slurry? Does an energy balance show this to be feasible?

 

[Steven Bolgiano]

Exothermic refers to the release of heat. But actually the anaerobic digester process is a microbiologic process. You are literally raising livestock... Methonogens.
They require the right environment and foodstock, like any livestock.
The exothermic takes place when utilizing the methane produced. ... Which can vary in volume and quality produced according to the specific make up of the food waste.
You also have to maintain the correct C/N ratio (carbon/nitrogen 30:1) ... mixing in straw, paper, etc.
25kgs of food waste will produce 1.3 - 2 cubic meters of gas.
Methenagens are most productive at the thermophilic levels which is 120° to about 140° f.
The mesophilic level is about 80° f to 100° f. It's not as productive but it has a smaller parasitic load on the system. Most waste treatment plants run on the mesophilic levels. But here's the rub. They don't try to turn a profit from that system. They are concerned with health and decontamination issues not microenergy production. Running at thermophilic levels is more productive, but very hard to accurately engineer the thermodynamics of the generators and the boilers and the radiation pipes in the tanks. Typically you see these types of systems run in Europe where there is financial support for the systems, and they run with amazing micro circuitry and instrumentation.

Any slide rule monkey can design a system that works in the laboratory or in the theoretical world,...
Anaerobic digestion thrives in Asia, and parts of Africa, where the temperatures remain warm throughout the year. But I'm trying to design a system that serves a common community good, a broad geographic region, and for as much of an entire year's seasons as possible.
Relying on the gas production from an anaerobic digester to support its self-heating needs, AND utilize the gas to generate value through electricity or heating barns or greenhouses is just not possible over a great deal of North American communities.
This project I'm doing is a big step down from the 800 cubic meter system I previously built. It's much easier to identify critical data as valid on the system that big as opposed to a small system whose critical variables can be too small to record.

If you look at the video I sent you it's a an extremely complicated thermodynamic calculation for all the variables I've introduced.
So the forum feedback I got was really helpful and breaking down the problem to priorities, and making sure those specific functions were met at the priority level, and then follow behind with the remaining design assignments.

So yes in order to create a home scale digester, that is financially feasible, and will operate in the colder weather, over a broader geographic region, then in my opinion you need solar. I play with hydrogen production too as a supplemental heat resource (interesting but a long story!)

I think readers should keep in mind that with anaerobic digestion there are three options, either the goal is for waste treatment, or for energy production, and in rare cases both. But when we're talking about a home scale digester, you're talking about just enough methane production to save up and use to heat the greenhouse and help out a little bit with the digester itself. They're still will not be enough gas production to completely support the digester during the cooler months. But even if it could support the digester with its own
gas production, The main goal would be met which would be to neutralize the food waste's harmful microorganisms and transitions into a odorless environmentally compatible fertilizer.

Sincere apologies for this long note, I just realized how long it got!

Thanks so much for this forum, it's pretty amazing ,
Steven

 

[jrmichler]

There are two ways to maintain an exothermic system at a temperature above ambient:

1) Add external heat, such as solar, then insulate enough to keep it warm until the sun shines again.
or
2) Use a heat exchanger to warm the incoming material using heat from the outgoing material, then insulate until the internal heat is enough to keep it at the desired temperature.

You still have not said what climate you are designing for, but here are some approximate numbers for my climate in Wisconsin, USA, at latitude almost 46 degrees north:

Summer: Approximately 16 hours of daylight, sun is high enough for useful heat 8 hours, cloud cover 40% of time equals useful sun for 4.8 hours per average day. A full week with heavy overcast is not unusual. Mean ambient temperature is about 70 deg F.

Winter: Approximately 8 hours of daylight, sun is high enough for useful heat 4 hours, cloud cover 60% of time equals useful sun for 1.6 hours per average day. A full week with heavy overcast is not unusual. Mean ambient temperature is about 10 deg F.

A solar heated system needs enough insulation to get through periods of no sun, plus enough solar panel area to heat it up when the sun shines. Because of the cube square law mentioned above, maintaining temperature in the desired range is many times easier in an 800m^3 system than a home scale system.

 

[Steven Bolgiano]

Thanks! I think you nailed those principals down nicely!
So as I said, the goal is to make this technique available to as many communities as possible, and in fact expand what have been limited to warmer places, farther into northern cooler areas.

And we made a lot of assumptions on the consumer side of the equation .
We took what we assumed to be an average suburban backyard, and from that decided on an assumed square footage acceptable to place a system on the property.
We then use the daily average output of food waste by a small family, to calculate a cubic meter volume for the tank (using a retention value of 30 - 40 days).

So we took the square footage of the site, subtracted the footprint of a digester, and divided up the remaining space between a greenhouse and a solar hot water heater.

And then when it's operating, observe the temperature data from various combinations of the heating mediums... Including the weather data of course.
The location is in Eastern shore Maryland.
We don't know if it can sustain year round.
Common knowledge would say it will not.
But, we know that with a Digester it is possible to operate in cooler conditions, just not make much gas, ... methonogens with a very diminished but living colony.

So information from this is hoped to provide the answer to your question, ... "Where?".

But you sure nailed the "vitals" in your post.

We've already played around with photo switches ... both "Turn On with sun" and switches that don't turn on circulation pumps until there is no sun. And this is where your points on insulation and temperature retention come into play. By taking certain portions of the solar resource and holding it until after dark where it will be introduced into the tank heaters.

Thanks again!
s.