Rocket Mass Heater to heat a fish tank in aquaponics

Hi to fellow fire & water investigators – your comments on the following would be appreciated

The Fish Tank Challenge 
How to hold water in a fish tank (1000 litres or 225 gallons) to a temperature range of 20-23 deg C (73 deg F) in a well  insulated but unheated greenhouse, when outside temperatures fall to 0 deg C (32 deg F) at night and 6 deg C (43 deg F) during the day. This fish tank is part of an aquaponics system under development.

Proposal - Rocket Mass Heater for heating fish tank water /b]

[li]Install a RMH in the Greenhouse with a Cob bench in which water reticulation pipes are embedded in loops between the  top surface of the Cob Bench and the RMH’s 150mm (6 inch) exhaust ducting embedded low in the Cob Bench.
The fish tank water to be pumped through the water pipes (with pressure relief valves) to pick up the residual heat held by the Cob – then back to the fish tank.
Fish tank to sit on top of the Cob Bench to absorb heat
Cob Bench sits on a concrete floor in the insulated building and will warm the floor around the fish tank.
Ambient temperature in the Greenhouse will be held up by heat radiated from the Cob and the concrete floor.[/li]
[li][/li]

[b]Unknowns – where I need advice1.

[li]What are the temperature gradients in the Cob at different points along its length & at different heights above the ducts from the RHM when the RMH is operating.
How long is the heat retained in the bench and how do the temperatures & gradients change as the cob cools after a period of RHM burning ends
Can the water reticulation be  through cob that is not hot enough to cause localized boiling & the risk of explosions – can safety pressure valves be incorporated.
Will heat transfer up into the fish tank from the cob bench it sits on
What temperature controllability would there be for the  Fish Tank water
Will the cob hold together with the piping heating & cooling[/li]
[li][/li]

Temperatures – what has been measured by other experimenters???
From Ianto Evans & Leslie Jackson’s book & many blogs read my understanding is that temperatures are likely to be about

[li]Burn Box  1000 deg C
Top of the Burn Riser to enter the Barrel – about 1000 deg C
Exhaust gases exiting the Barrel & entering the ducts to the cob bench -  450 deg C
Exhaust after say 7 to 9 metres of duct embedded in the Cob Bench –  39 deg C
Temperature on the surface of the Cob Bench just after the exhaust gas enters  the Bench 39 deg C (100 deg F)[/li]
[li][/li]

but the keys to this water heating idea seem to be

[li]Temperature gradients along the Cob Bench at different heights ???
Rate at which temperature gradients degrade at different points in the Cob Bench after the RMH burn ends ???
Whether steam will blow the bench sky high
Whether I would have slow cooked fish[/li]
[li][/li]

Comments, advice and humour from past experiments would be greatly appreciated.
I dont want to create an explosive water heater or boil fish

Until next time
Creeky (in the bones and in the mind)

I've had a similar idea, and I think it would work.

My idea was to have the cob form the shell of the fish tank (with a waterproof liner that is good to such a temperature).

With your cob-water pipe heat exchanger, you could plumb different distances from the burn pipe and test out the temperatures. I don't think there will be a standard specific heat for your cob as each mixture will be a bit different.

Hi Tamo42 & other "experimenters with fire & water"

Have been searching various sites for peoples experience in using water heating pipes in the cob bench, and have only come up with one thread so far -  Canyon was looking at "pipes in your bench" back in 2007 & 2008 - on the site "Rocket Stoves" "Experimenters Corner" on http://donkey32.proboards.com/index.cgi

Have added a post to "Experimenters Corner" to see if Canyon pursued the idea.

Will keep you updated - lets hope we can uncover other intrepid "experimenters with fire & water" who are keen to explore the theme

Till next time - see Ya!
Creeky (in body & mind - & I also have a Creek in my back yard)

I would worry the water would get to hot and cook the fish. with electric heat, it could be turned off. although you COULD play with the temperatures before adding the fish to water tank. and i suppose you could have a door to close on the stove to control the air intake. so yea, that does sound neato. I would love to see some glass incorperated into the cob tank! i picture a cob wall you can look into and see the fish. seems like it would be easy enough to even have a few drains plumbed into the bottom?

I also want to do what you are describing.  I'm in S.E. MI so it sounds like my temps will be close to yours.  I do have a southern facing slope I could build it into that I have considered.

Just a thought, but how about you extend the cob of the RMH and make a small pond with it, so that an entire wall is the RMH itself. Then recirculate the water from that heater pond through to the actual fish tank. This should allow for better temperature management and more thermal mass in the greenhouse itself. You should also be able to build this in as part of a natural filtering apparatus as well, filling the heated pond with filtering plants.

What I am planning to do is build a seconf tank ala Monster Fish forums and insulate it well. This tank will be heated by my rocket stove with a heat exchanger. Then I will use a small pump and tempature contrller to circulate the heated water under the gravel beds and DWC tanks. This will be able to run off a small PV panel and provide very fiine grain control of the tempeture of the fish.

I saw this very same thing done with a conventinal wood stove at an small aquaponics installation outside of Boise, ID. It prevents the chance of poaching your fish and killing of the plants if you let the heater run too long. Since the heated water does not mix with the slightly acidic aquaponics water you do not have to worry about copper/brass/galvanized pipe or fittings poisining your fish either.

Hi fellow experimenters with fire & water

Thanks KentUB for your ideas on temperature control - I will look up Monster Fish forums to try & understand it a little better & will be back in touch

Regards
Creeky (Creaky in both body & mind)

I thought maybe you had looked at it already as most Aquaponics people have visited. A lot of guys are making very large tanks as large as 500+ gallons with Plywood and fiberglass or another option is to line it with EDPM or Butyl. Although the ply/fiberglass tanks are the strongest.

I also had a similar thought in regards to the RMH and aquaponics. I considered having the greenhouse slightly below ground level with the RMH pipes running beside and under it (maybe even multiple RMH's since they're so fuel efficient). My thought on it being sunken a bit being that the earth will act as insulation and more thermal mass to keep greenhouse temps more stable. It also now occurs to me that at least in the float type of aquaponics (not sure which method you're using) you could probably use Cob (plus a liner) with the pipes under it to warm the water as well (similar to what you're suggesting with the pipes). While I can't offer you first hand experience my impression is that the greater thermal mass that you use the greater the amount of total warmth and longer it will last. I think basically you're just going to have to figure out how far to keep the pipes from each other. At any rate I'm really interested in the topic so please continue to share anything you learn!

A given volume of water will store +3x the amount of theraml energy as the same given volume of cob. If you are using DWC (Deep Water Culture) you would have a HUGE thermal mass and really avoid thermal swings. NFT on the other hand loses and gains heat very quickly due to the thin film running down the channels.

My thoughts are that you are better insulating the fish tanks and DWC beds rather than build them out of cob. cob would naturally draw off heat. Also in the setup I am building the DWC beds are elevated on cider blocks so I do not need to bend over so much. AS cob is ~40% heavier the support structure would have to be stronger and more expensive.

I'd be interested to see your system when you have it set up KentUB and hear about any other insights you have! (Modified post since I didn't read your other posts well enough apparently  )

Creeky,  Interesting idea.  I have plans to start some sort of aquaculture system in a greenhouse sometime in the next year, and had wondered how to maintain winter temps for both the plants and fish.  I didn't note as to whether you plan on using floating mats over a deep tank or a gravel bed with fill/drain or continuous trickle system.  But here may be a couple of thought to ponder.  (and if anyone sees flaws in my thought process, please speak up)  As Kent and a couple others mentioned you could run the exhaust from the RMH under/around the tank, which could be made out of block or cob with a waterproof liner.  This would be the probably option if using floating mats.  OR if you go the gravel bed route, you could run the exhaust around the tank or build the regular RMH with an oversized bed/couch with the minor modification of incorporating a small raised bed on top of the couch, you'd need a waterproof liner for the gravel bed and a few plumbing modifications to the couch design.  The mass would continuously radiate the heat up to the roots of the plants, the water surrounding the roots would recycle back to the fish tank and both of those masses would warm the surrounding air keeping the room warm.    This thing looks good in my brain, can anyone see where it wouldn't work?
Sorry for the length of this post.

I wonder also if venting the RMH or at least part of it into the greenhouse for additional CO2 would be beneficial to the plants or end up as a disaster? Any thoughts?