Solar thermal system with back up from rocket stove water heater

Hello,

This is my first post on the forum and I'm hoping for some advice on hot water! My question is: Can a rocket stove water heater be used to top up a solar thermal system when there is not enough hot water produced by the solar thermal collectors in the winter? I saw this great video explaining rocket stove water heaters (https://permaculturenews.org/2013/05/20/how-to-build-a-rocket-stove-mass-water-heater/) and was wondering if that water could be added directly to a hot water storage tank, which would also be connected to the solar thermal collectors?

Thanks!

Absolutely it can, either direct RMH boiler fluid to storage or via a heat exchanger.

How is your solar thermal system arranged. Closed loop to collector, drainback, etc. How is your storage arranged, open tank or pressurized, etc.?

People get bothered and in a safety tizzy over the conjoined RMH and fluid heat exchanger. I think it has to do with the high combustion chamber temps and assumption of a closed loop, but nobody ever said the exchanger has to be in the chamber and you wouldnt generally want it there anyhow.

(These same folks may just have gas lines run to a furnace or other combustion or simply live near someone who does. Whole and several city blocks are leveled quite often and i know of no diy use of boiler that has ever done that.
In that case we could call into question their safety decisions as these things explode and cause fires all the time, more often than diy rocket boiler and maybe on scale, but who would risk potentially explosive gas and ignition systems to be installed under or next to a living space?)

Now that that is out of the way and assuming you are responsible for your own safety, what do you need it to do and what do you have in place?

You can ensure the safety of the system by making the portion of your water heat transfer medium that is directly exposed to fire/hot exhaust gasses an open tank (i.e. Steam can escape at ambient pressure) heated from below and sides within an insulated masonry bell, which will also absorb heat and continue to transfer it to the tank after the fire has gone out...

 Then a closed loop exchanger as simple as a copper coil, submerged in the open tank can collect heat from the heated body of water and transfer it to your existing storage through the heat exchanger in that tank.  Simply add a pair of manual or electronically controlled 3 way valves at the intake and exit points from your solar storage tank's heat exchanger.  

 If your solar thermal is not a passive circulation system, you can place the valves such that the rocket stove heat exchange loop is pumped by the same pump as your existing solar thermal loop.

If you have any specific questions about the idea I present I'll do my best to help you come up with an optimized design compatible with your existing solar system, as there are a number of specifics to consider based on the design and setup you already have.

For instance, you would not want to mix water from an open tank with your solar loop if that is connected to and directly heating your potable water...

 If they are separate, and there is a heat exchanger in your solar hot water storage tank, then you could have that be directly using the water in the rocket heater's "open vat".  However, if your solar system uses antifreeze or antifreeze treated water as the heat transfer medium, then you might have issues with evaporation a and maintaining the correct balance of antifreeze, and having to have the extra volume of freeze proof heat transfer fluid to fill the tank.

 

I'm working on the same type of problem.  I have a drain back solar hot water system that is half complete, and I want to tie it into a Batch Box Rocket Mass Heater that is yet to be built.  My solution was this:  Since I already have the need for inputs to my Differential Temperature Controller, I could dedicate one of the available inputs to the daybed/bench thermal mass of the Rocket Mass Heater.  By looping pex water line though the thermal mass, (say, in the upper third) I could effectively use the 5-55 gallon poly drums to moderate the temperature of the bench.  Should the temperature of the bench become warmer than that set point, a circulator pump will turn on and move cooler water through the thermal mass and bring the temperature down.   If I chose to, I could burn extra batches in the heater and top off my hot water storage.  I'm not really sure what a comfortable temperature is for a cobb bench.  I haven't heard it mentioned anywhere yet.

I'm also looking for this. I'm planning a thermal battery using PCM's fed year round by solar panels, and I need a backup. Electric boiler backup was my initial choice but RMH is more interesting. Is there plans, trials or anything that shows how maximize heat transfer from the combustion to the water ? I've seen many setups, none of which seem to be very efficient.

The idea would be to transfer as much heat to the water as possible and send the exhaust in a solid mass under the tank.

The system is already set for over temp/pressure protection so if steam is generated it will not be a risk.

Sidney,
I suppose it matters if you are looking for high temperature water storage.  Since I have a lot of small children around, I don't want anything too hot.  I'm not really looking to raise my water temperature up much over 100 degrees F, since my solar water system is intended as a pre-heater for a conventional system.  If you want to go hot, I would expect your best bet is to construct a coil on the outside of your barrel, or even more daring, to put the coil on the "Outside" of the heat riser inside the barrel.

Hi Thomas,

The reservoir is large enough that a complete batch burn theoretically would raise the water temperature about 43F (based on an 6" batch burn estimates). The tank is going to be out of the way and insulated of even if a toddler reaches it, it would just be warm to the touch.

I have seen this setup. but I think there would be to much heat loss from the barrel.

On the other hand, If I insulate the barrel and install coils in between the riser and barrel that might work.

I'd only need to make sure I keep the flue gases hot enough to get outside.

Sidney,
Not sure what to tell you except that from my understanding insulating the barrel will defeat it's purpose and interfere with the exhaust flow.  If the purpose of your rocket stove is just to heat water and not to heat the space the heater resides in then maybe you could re-purpose an old natural gas water heater tank.  The concave bottom of the tank will funnel hot flue gas up through the flue of the tank, heating the water in the tank.  In this case you would want to vent the flue gasses directly out of the living space. The difficulty I see is in the construction of your heat exchanger.  One last thought, Make your riser out of two steel pipes, one inside the other.  Cap it off top and bottom with two steel donuts with a fitting on the top and bottom. This is your boiler.
The idea is to run the flue gas up through the riser, with the riser itself being the heat exhanger.   Fluid Dynamics is not my forte, but I can imaging that if you made this tall enough, you could extract a lot of heat from the flue gas before it exits the riser.  You would not need a barrel in this case.  Just insulate the outside of the riser.  
I encourage you to check out my post

Solar Water Heater / Rocket Mass Heater Fusion

https://permies.com/t/76265/rocket-stoves/Solar-Water-Heater-Rocket-Mass

I'm looking to have as my number one priority, a heated space,  secondly, a comfy daybed/bench, and thirdly, a way to preheat my domestic hot water in the winter time when I don't get much out of my solar thermal collector.

Thomas Tipton wrote:Sidney,
Not sure what to tell you except that from my understanding insulating the barrel will defeat it's purpose and interfere with the exhaust flow.

Not quite. The exposed barrel releases a lot of the heat in the room. The cooling effect on the gases makes them flow down. If you insulate the barrel and there is no other means of cooling the gases, they will not flow as well if at all. But running cold water though copper pipes inside will cool the gases as well, thus keeping the flow.  The tricky part will be to balance the system. If to much heat is extracted, there will not be enough heat to exit the flu gases out. It to little heat is extracted, the flow wont be fast enough and I'll loose the heat outside out the chimney. If I understand correctly, the flu gases at the output of the chimney should be around 70F.

Thomas Tipton wrote: If the purpose of your rocket stove is just to heat water and not to heat the space the heater resides in then maybe you could re-purpose an old natural gas water heater tank.  The concave bottom of the tank will funnel hot flue gas up through the flue of the tank, heating the water in the tank.  In this case you would want to vent the flue gasses directly out of the living space. The difficulty I see is in the construction of your heat exchanger.

I've seen this in one of the posts. I've been looking for a older fuel or propane hot water tank (no natural gas here) but they are few and far and most of them are jammed (hard water here, all wells) so this could work if I can find a tank.

Thomas Tipton wrote:  One last thought, Make your riser out of two steel pipes, one inside the other.  Cap it off top and bottom with two steel donuts with a fitting on the top and bottom. This is your boiler.
The idea is to run the flue gas up through the riser, with the riser itself being the heat exhanger.   Fluid Dynamics is not my forte, but I can imaging that if you made this tall enough, you could extract a lot of heat from the flue gas before it exits the riser. You would not need a barrel in this case.  Just insulate the outside of the riser.  

You cant do that without compromising the efficiency of the system. The riser has to be insulated as much as possible. Once out of the riser you can do something like that. But it makes the cost of the build very much more expensive. Using steel in this setup is know to have corrosion and failures. Using stainless steal solves that problem, but increases cost dramatically as only decent grade of SS will work.

Thomas Tipton wrote:
I encourage you to check out my post

Solar Water Heater / Rocket Mass Heater Fusion

https://permies.com/t/76265/rocket-stoves/Solar-Water-Heater-Rocket-Mass

I'm looking to have as my number one priority, a heated space,  secondly, a comfy daybed/bench, and thirdly, a way to preheat my domestic hot water in the winter time when I don't get much out of my solar thermal collector.

I wish I could use the barrel setup. The way my house is divided I would end up with a very hot room and the rest of the house would progressively be colder as you got further away and at least 2 rooms would be very cold. The house is heated with HVAC and a fuel oil furnace. There is no easy way to move the heat around the house without major renos. Using the HVAC is the most cost effective way. I'm hoping to reduce the amount of fossil fuel I use to the bare minimum. This means eventually eliminating it from the heating system. I'm almost done insulating the house. Its old and was leaking way to much. It still would be air tight but much much better.

Sid

Sid,
I think I understand your situation better.  If I were you, I would be looking at ways to put a hot water radiator, (like from a car?)  in the hot side plenum of your air handling system and feed it from whatever kind of rocket heater you come up with.  I see two fundamental differences in our systems.  I need mine to heat water slowly at a lower temperature range,  and you need yours to heat a lot of water very quickly.  That's the trick, isn't it?  To get fire to give it's heat to the water as efficiently as possible without breaking the bank.

Ok.  I have one more idea.

You build the rocket stove with a heavily insulated riser.  But you don't use a barrel, you divert the hot gasses 180 degrees back down though a double walled cylindrical water jacket like I mentioned before.  This way the water jacket cools the gasses and causes them to constrict, just as the barrel would have done, but this way you are gathering the heat in the water and siphoning the heat away to your storage system.  The gasses are then drawn down through the water jacket and diverted out of the living space, perhaps through a cob bench to gather residual heat. This way you get the efficiency of a complete burn in the riser, plus, the extraction of heat to the water jacket.  Will it work?  I don't know.

On my BB Rocket the exhaust from the 5" riser is targeted to heat a 45,000 btu/hr 'wraparound' woodstove boiler (ex Clearview 750). Any remaining heat generated warms a single skin brick bell and exits low down the bell  via a masonry chimney.

It heats the water well and charges the bell that then holds onto useful heat for around 12 hours or so.

Thomas,

I've been pondering the water jacket solution for a while. It is the most efficient way to extract the heat, but for a permanent set up the material cost are high. I could use soft steel in very cheap, easy to replace design though. That would be a good compromise.

Using a rad in the plenum was the idea all along. It is common practice in outside wood boilers. All of the higher efficiency wood boilers pass the gases in tube instead of a water jacket. It is in reading about these systems that I found that mild steel and even 304 and 316 SS have issue long term. The material that seams to have the best performance is 409 SS. But it all boils down to cost vs maintenance I guess.

There are some critical aspects to this kind of setups though. The system has to be open so there is no pressure built up and having a large enough water mass so that it take several burns to get the water temps to reach set maximum are very important safety solutions. Wood boilers have  170 to 180 F set max temp for that reason. Since this is not going to be force air driven, there has to be enough mass in the tank to support 1 full batch burn without boiling after the water is already at set max temp.

It sounds like you've pretty much got it figured out.  For overheating protection, I used a normally closed solenoid valve that opens when my storage tanks hit a set point.  When the valve opens, it dumps cold supply water into the system to bring it down, excess water is lost down the drain.

Good luck with your build.  Maybe you could start your own thread on here and document it as you go.

I got a few questions still, the bulk of the hydraulics I got a handle of, pretty much. It is the RMH part of it that is still a grey zone. I posted a new thread asking some questions but I haven't had a reply yet. I'm ready and I have time to make some trials. But I wish some of my questions where answered. haven't had a reply yet.

Sid,
I just wanted to get back with you about an idea I had.  Unfortunately, the batch box I was looking at is way to big for my space, and I'm getting too old to want to be crouching down on the floor to load the burn chamber.  I think I'm going to compromise and go for a J-tunnel since it would be easier to feed,  but I still want an 8 inch diameter profile to heat the size of cob bench I'd like and that's what puts the top of the barrel uncomfortably close to my basement ceiling. (wood)

I have yet to see anyone attempt a single burn chamber/tunnel, dual riser configuration.  what do you think?

Could dual risers, either both in one barrel, or one in each of two barrels serve the needs of one larger burn chamber?

Thomas Tipton wrote:Sid,
I just wanted to get back with you about an idea I had.  Unfortunately, the batch box I was looking at is way to big for my space, and I'm getting too old to want to be crouching down on the floor to load the burn chamber.  I think I'm going to compromise and go for a J-tunnel since it would be easier to feed,  but I still want an 8 inch diameter profile to heat the size of cob bench I'd like and that's what puts the top of the barrel uncomfortably close to my basement ceiling. (wood)

There are many different solutions possible for that problem. I'll get back to that in a bit...

Thomas Tipton wrote:

I have yet to see anyone attempt a single burn chamber/tunnel, dual riser configuration.  what do you think?

Could dual risers, either both in one barrel, or one in each of two barrels serve the needs of one larger burn chamber?

Theoretically it is possible, but it would be technically challenging and prone to malfunction specially in a single barrel. Even in a 2 barrel configuration, any small change can and will trigger an unbalanced flow. The complexity of fluid dynamics requires fine calculations and flow control provisions. Without the proper math, you would need a lot of trial an error testing and you can't assume it is good simply because you had a few good runs.

Remember that fluids always takes the path of least resistance this means that the slightest difference in one of the paths then to favor one over the other.

It is best to stay within the KISS principle. In the link I posted in my thread (http://batchrocket.eu/en/) there are designs that would make the burn chamber more accessible. Instead of using a barrel, you can use a masonry bell. You can insulate the top of the barrel since your not going to be cooking. You can install a simple heat shield/deflector made of cheap sheet metal. You can use convection to control the ceiling's exposure to heat.  

Check out this thread too for more info: https://permies.com/t/41635/Top-Questions-Rocket-Mass-Heaters

Check out this Peter Van De Berg's masonry build with an insulated top  

Sid

Sid,

How's your build planning going?  Thanks to the input provided by Peter Berg, Glenn Herbert and others around here, my build plans have taken some pretty wild turns.  I'm no longer considering a barrel, but rather, a masonry bell to accompany my mass bench, which will be built mostly of cement blocks and patio pavers.  I won't be using stove pipe for the bench channels anymore either.  I'll be going with a stratification chamber that will take advantage of the elevated flue cavity in my defunct fireplace to pre-heat the chimney flue.  I agree with you about the pex.  When I was going to do a massive cob bench I thought it would be best to bury a large coil of pex into the mass, but now I think it will be better to sandwich a system of copper pipe between the first layer of patio pavers and the second layer of natural sandstone slabs I will be using to top my bench.

For my heat riser I propose using 12" and 8" sonotube for the mold.  My intention is to treat the sonotube to make it waterproof, then shred enough 2600 degree mineral wool blanket to fill the full length of the cavity.   I then have to spritz the shredded mineral wool with sodium silicate (water glass) until it is well moistened and pack it, somewhat loosely, into the void.  I will then have to infuse the mineral wool/sodium silicate mass with carbon dioxide to cure the sodium silicate and make it hard.  Once that is done, and I can remove the rigidized riser from the sonotube form, I will use a 3000 degree refractory cement and smear it inside the riser, pushing it into any voids and smoothing the surface out to leave a nicely glazed (hot face) on the interior of the riser.  I could do the same for the outside of the riser, though I do not know if there would be any benefit, other than to giving it a tough outer skin.  

Let me know what you've come up with for your situation.

Hey Thomas, Good to hear from you.

I got the burn chamber and riser basic designs from Peter Berg. I have an idea for a secondary burn chamber that uses a vortex to reduce the amount of ashes going to the rest of the system. So, I plan to build prototypes with relatively thin steel. The first being a replica of peter berg's batch system. Then I'll build prototypes and I'll have a known good system to compare it with.

For my final build for the home I want to get a vacuum formed Riser sleeve for the riser

and I want to build the burn chamber with fire brick and insulation brick as a base. As for the heat exchanger to water, I'm looking at high efficiency wood boilers for ideas. There are a few designs that might work. My sister is a mechanical engineer, and I'll ask her to check them for flaws. but basically the most efficient way is multiple small tubes. like in Peter'g webpage. The thing that I need to consider is what happens when there is a power outage. There must be fail safes in the system and I think I can use wax thermostats to generate circulation by convection to keep the water from boiling.

Will I'll be prototyping, I want to try to make carbon foam from bread in a manner that is usefully for a racket mass heater. If I succeed in my prototypes, I'll try it in my shop. I have an automotive repair shop that I want to install one in. This one will be my test bed for different ideas.

Right now I'm looking for building materials. I'm in my slow season right now so I don't have any money to spare, so I'm hunting old food stoves, chimneys etc. Even old oil furnaces if I can find one. Their combustion chamber is usually vacuum formed ceramic fiber. I am also getting dimensions for cutting my sheet metal so I have the least amount of joints as possible.

I've also looked into moving heated air to areas of the house that are isolated by running the furnace air intake around the "barrel" so it sucks the heated air and blow it around the house.

As I ponder a solution for the house I want to get the mass heater done in the shop for next winter. The problem at the shop is space. At the moment there are very little space not used. I need to re arrange what can be moved around to accommodate the masse heater and not have it impede the work space. The rocket stove is not to much of an issue, but there is no room for a bench.

Sid

"I have an idea for a secondary burn chamber that uses a vortex to reduce the amount of ashes going to the rest of the system."

Sounds excellent!  I would love to hear more about this.  I think everyone would like to see a way to concentrate the ash deposit in a controlled fashion.

Regarding those riser sleeves.  I've seen those used before to good effect.  They were utilized in fashion whereby a smaller cylinder was nested half way into a larger cylinder allowing them to be stacked.  I didn't get prices on those, and I wasn't certain if they came large enough for me to build an 8" diameter riser.  I assumed they were pricey so I conjured up the way I described earlier.  

Wax gaskets.... interesting.  Electro-mechanical valves that open or close when current is applied also open or close when current is removed.  

I have never heard of carbon foam before and I took some time to read up on it last night.  I'm not really sure what to do with that.  It seems like pretty cool stuff, but is it practical?  Keep us informed.

I like the ideal of creating a shroud around your barrel to whisk away heat to your HVAC system.  Why not?  As long as you can keep the draft from negatively impacting the air flow to your burn chamber, I don't see a problem with that.  

Regarding space issues, I saw one fellow build his mass bench under his shop workbench.  It wasn't possible to sit on it, but it was there, doing it's thing.

You have a lot of great ideas and I appreciate having someone to bounce ideas off of.  

Good luck on your build/s.