Peter B. Onde

Gary:
Safety is not a big concern when heating water in a closed loop system, if done properly. Most new houses here has closed loop water heater. You need an expantion tank of propper size at the highest point of the loop. And you need one or two blow out valves.

RMH for greenhouse is something lots of people do. There are lots of experiences to learn from. I don't see that as an major challenge.

What IS a challenge will be to move away from the central principal of a RMH where a metal barrel is heating the room where the heater is located. The process to transfer most of the heat to the inhouse mass is the challenge here.

Thanks for linking to the stacked barrel design. I'll read up on that.


Mike:
The insurance company does not care what is safer or not. All they care about is the certification of the fireplace.

I'm not willing to go outside every 15 minutes. Thats why I'm looking for a batchbox design. (Even better would be an autofeeder.) With a large enough batchbox I'm hoping to fill it up a couple of times each day, (except for the coldest days) and let the mass do the heating of the house between each fill up.

I'm hoping that there is a balance for which the heater heats the water, without loosing the draft. If exhaust temperature gets below 80C, it won't contribute much to heating the water. Hence there will be some heat to make the draft. I'm thinking that if water pump is controlled based on water temperature (aiming for 90-100C), and there are two coils around the heat riser, with a valve controlling if water should flow through both coils or just the upper one depending on exhaust temperature, the system should be both efficient and ensure propper draft. That is one of the things I'm hoping a layer of firebricks between the heat riser and the coil of waterpipes will help with. I'm thinking the rest heat from heating the water will go to the green house. I'm hoping for one big heater rather than two (one for the house, one for the greenhouse).

William:
Steam is scary! I see your idea, but I think it's important to make the water circulate so fast that it won't turn into steam - temperature dependent waterpump. Efficiency using steam would be a lot better though. I'm thinking of using sand as the indoor storage mass. Steam would make it posible to reduce the size of the storage, while storing the same amount of energy. But I don't want to take the risk of using steam. I want to keep the water near boiling, And if it boils, let the pressure out.

I will not have the feeder opening in the greenhouse. I've seen RMH's that has been working perfectly for a long time suddenly smoke back. To minimize the damage of such event, I want the feed to be in a separate building. But I can have that building very close to the greenhouse so that the heat riser can go inside the greenhouse.

In my case there is no options for where to build the greenhouse, unless I accept the floor to be a 30+ degree slope. It has to go where ground is flat, 6m from the house, 2-3m above the mass in the basement of the house. The heater has to either go between these buildings or around the corner of the house, leaving it 3m from the house, 6m from the greenhouse, and 1.5m below the mass in the basement. Having the heater placed low may be an advantage when moving heat into the mass.

An outdoor wood furnace is an option. It's expensive though. And while living in a place where weather doesn't cooperate in regards to keeping wood dry during transport. Hence it's necessary to have a large firewood storage in the same building as the heater. (That way the heater also helps drying the firewood.)

What I really want is a DIY outdoor furnace, clean burning with firewood storage.

Heating the greenhouse, I'm sure the RMH will do fine. It's even possible to build the heater house next to the greenhouse so that the barrel gets inside the greenhouse. The challenge will be to get most of the heat into the mass under the house - that's where most of the energy is needed.

I'm thinking of various ways to achieve that - getting more energy into the circulating water than into the room where the heater is. One way would be to cover the barrel within a coil og waterpipes. (Putting the pipes inside the barrel I think will be too hot for the piping.) Then I could put isolation outside the coil. But then the barrel probably will be overheated. One way to deal with that problem could be to NOT use a metal barrel, but rather build a "barrel" of firebricks. Then make a coil of piping around and over this "brickbarrel", and lots of isolation outside this. This may introduce another issue - In a regular RMH he barrel has a cooling effect on the exhaust. This effect may be crucial to the airflow through the burn chamber. How can that be solved? Maybe by using a metal barrel without top at the bottom and extend with a "brickbarrel" on top of that?

Does anyone have experience with isolating the barrel of a RMH? Or stacking two barrels on top of each other? How did this affect the heater?

I'd like to have a batchbox RMH. But I'm afraid that something will fail someday. So is the insurance company - if the house burns down as a result of not using a certified fireplace, they will not cover.
So I figured I'd like the stove outside (in a separate building), and the mass in the basement of the house. In this case, maybe a rocket is not the right path to follow?

What I want to heat is the house, hot water, and a planned greenhouse. The greenhouse will come 6m from the main house. I'm thinking that if I place the "heaterhouse" between those two, it could heat both the house and the greenhouse? Maybe the exhaust tubing can go through the greenhouse, while circulating water can heat the hot water and the mass in the basement of the house? The downside is that this requires a pump. Another potential downside is that the heaterhouse will get extremely hot, while just a fraction of the heat reaches the mass in the basement. Is there a way to transfer most of the heat to the water? Is it just a matter of how fast the water circulates and the number turns the pipes goes around the heater?

Is this all just a stupid idea?

Steve Farmer wrote:

Peter B. Onde wrote:@Steve
Pressure can move the boiling point, but it takes a major leap in pressure to move the freezing point significantly.

Sure, so use a pressure cooker to get 115C on the hot side and use ambient ground or lake temp for 15C on the cold side. Sorted.

And when you don't want to be that close to the modules max temperature? (with durability in mind)

----

When it comes to those 230C (450F) max units I haven't fond any data telling their electricity production. They cost 15 times as much as the 125C max units. Do they produce 15 times as much electricity in a real world application? What I consider real world application is cold side kept somewhat above outside temperature (about 10C), and hot side kept as high as possible without reducing the units lifespan.  There are also some 180C (350F) max units out there. They cost 5 times the 125C. Do they produce 5 times as much electricity?

From the information I've read, I get the impression that the differences between these modules are the material used to seal the unit and cable isolation material. If that's true, their performance should be much the same at any given delta-T. From the data on the 125C unit, I find a formula for electric power produced to be:
P = 0.0004672339 * dT^1.915394

That makes this table:
cold - hot - dT - power
-20 - 95 - 115 - 4,14
-10 - 95 - 105 - 3,47
0 - 95 - 95 - 2,87
10 - 95 - 85 - 2,32
20 - 95 - 75 - 1,82
30 - 95 - 65 - 1,39

-20 - 115 - 135 - 5,62
-10 - 115 - 125 - 4,85
0 - 115 - 115 - 4,14
10 - 115 - 105 - 3,47
20 - 115 - 95 - 2,87
30 - 115 - 85 - 2,32

-20 - 150 - 170 - 8,74
-10 - 150 - 160 - 7,79
0 - 150 - 150 - 6,88
10 - 150 - 140 - 6,03
20 - 150 - 130 - 5,23
30 - 150 - 120 - 4,49

-20 - 170 - 190 - 10,82
-10 - 170 - 180 - 9,76
0 - 170 - 170 - 8,74
10 - 170 - 160 - 7,79
20 - 170 - 150 - 6,88
30 - 170 - 140 - 6,03

-20 - 180 - 200 - 11,94
-10 - 180 - 190 - 10,82
0 - 180 - 180 - 9,76
10 - 180 - 170 - 8,74
20 - 180 - 160 - 7,79
30 - 180 - 150 - 6,88

-20 - 210 - 230 - 15,60
-10 - 210 - 220 - 14,33
0 - 210 - 210 - 13,11
10 - 210 - 200 - 11,94
20 - 210 - 190 - 10,82
30 - 210 - 180 - 9,76


As can be seen, if you get 30C on the cold side, a hot side of 210C makes 7 times more electricity than 95C on the hot side.
If you get -20C on the cold side, a hot side of 210C makes 3,76 times more than the hot side of 95C.

To me it seems like a bunch of the cheap ones will produce way more electricity than the expensive ones for the same money. And the colder you can make the cold side, the better value for money the cheap modules get.

I don't fully understand the propane fridge. My understanding is that it needs a very high temperature at one small area to run. With a RMH this could be achieved by placing this area on top of the barrel. The rest of the cooling system will probably need to be as cold as possible - be exposed to the outside temperature rather than the RMH.

To my understanding a propane fridge can be placed in a 20C environment and will create 5C inside the box. That is a delta-T of 15C. IF that means it will be able to maintain the same delta-T when temperature outside the box changes, it may work for my application. To reach my goal in a +3C environment my "fridge" just needs a delta-T of 8C. From there it should just be a matter of scaling to reach a sufficient

If it's so that the propane fridge will aim for a temperature of 5C (thermostat disabled) independent of environment temperature, maybe another refrigerant in the system could be the solution?

Close to 5C seems easy, yes. The goal is at least 10C colder than that < -5C

@Matt, William
What I want? Get useful energy to the household using the resources available on the property - wood.
Sure there are other resources as well. There is a lake 300m away. They say the ice on this lake is never safe. I assume this means the ice never gets thick. So far I think of this more for fishing than energy.
There are two SMALL rivers here. Comparing weather statistics with the flow I see, tells me they should be able to produce the power needed for the household over a year. Problem is that when I first moved here they were dry. And I bet they will freeze in January. So using these as a source of energy will require a lot of buffering.
Wind is to be found on a top 400m away or by the lake.
Sun is useful April - October.

My needs are:
Heating the house October - April without being a fulltime firewood handler. I think a rocket mass heater combined with a large tank (5000L?) of water and radiant floor heating will be the solution. This will require water pumps.
Electricity to fridge, freezer, water pumps, laptop, ligths. Solar covers this April - October.

So, what my plan does not yet cover by the resources available at the property is electricity October - April, and my heating plan depends on this. This is why I started the search for ways to make rocket powered electricity. As stated, I've been looking at sterling, steam, and TEGs before I figured woodgas was the way to go. Then I came across these "cheap" TEGs that seems to solve "everything" if I can figure a way to cool them below -5C when the outside temperature is around 0C. I'm thinking there may be a way to do that makes the TEG-approach better than woodgas. I just don't know how, so I'm asking for ideas.

@Steve
Pressure can move the boiling point, but it takes a major leap in pressure to move the freezing point significantly. When the desired temperature range is from -5C (or less) to 95C, antifreeze is a better approach than pressure change.

@Frank
Sure there are TEGs that handles higher max temp. What do you find for cost / watt and life span of these high temp TEGs?
As mentioned, at first I wrote off TEGs as too expensive. Then I found these low temp modules for less than  $1/watt. I don't know much about these modules. Actually ebay is the most scientific source of information in this case! They claim to work with 150C delta-T, in the range from -60C to +125C, which is 185C, way more than 150C.
Claimed output:
delta-T : generated power
20C : 0,21w
40C : 0,66w
60C : 1,12w
80C : 2,00w
100C : 3,21w

So the output seems to be exponential to increasing delta-T. If it continues like that all the way to delta-T = 150C one could actually put an TEG powered compressor/heat pump to cool/heat the TEG and would end up having a machine that powers it self and creating energy from nothing! Who dare to say this is not a fantastic machine? (Never mind this machine only works in theory based on guesswork based on the data provided by a seller on ebay who probably has no knowledge of the product sold.) I ordered a dozen of them. 60 days of shipping.


Well. The topic for this thread. How to use energy from wood/air/ground/compost/.... to provide the best possible conditions for this TEGs when outside temperature is around 0C? Hot side is easy: water heated to 95C by a rocket. What about the cold side? (Ground temperature here is about +4C.)

I'm planning to have a spring loaded feeder tray to push the firewood in. That way I'm hoping to have it burning maybe 10 hours per load. Burn time should just be a matter of length of the tray.

The thing with water is that you can't really get the 100C range without freezing or boiling. Glycol will help. But then, how do you cool the water when outside temp is above freezing? Even though outside temps can reach -30C here, most days has a temperature around -1C - +3C.

Hello forum. My first posting here.
I've recently moved to a 400 years old house, 15 minutes drive from a small village. Then winter came and I realized something has to be done to improve heating before next winter. Here temperatures can reach -30C (-22F). The sun diapered behind a near by mountain about 15th of November. This property produces more dead wood than I'll ever be able to make use of.
So I started searching for ways heat with wood without making myself a full time firewood worker and discovered the rocket mass heater, and then I discovered this forum, which has given me a lot of great reading for weeks. Then I got to think that this stove should be able to produce electricity as well. So I looked into variations of sterling engine and realized they were too small to make useful energy. Next up was steam engines, which scared me off. I touched TEGs and learned they were expensive. So I figured woodgas was the most realistic way to go - until today.

Today I came across cheap TEGs - less than $.8 / watt on ebay. These are small modules. 4x4cm, cost $2.5 and claim to produce 3.2w. 100pcs of these would generate quite some useful electricity. The catch is the conditions required to do this: Delta-T of 100C (212F), and max temp of 120C (248F) on the hot side, min temp of -60C (-76F) on cold side. If delta-T is dropped to 60C (140F) the electric power is cut to a third. I'd guess that delta-T > 100C would make quite a bit more electric power.

I assume that it's not a good idea to aim for the max temp using these TEGs. I'm thinking about 95C (203F) on the hot side could be reasonable for the lifespan of the TEG. When making a RMH to produce hot water, 95C (203F) could be a reasonable output temp for looping water. Then the cold side is wanted below -5C (23F), -20C (-4F) would be a dream. On the coldest few days the outside temp here should be able to cool a heatsink on the TEGs fairly well. But I'd like this thing to work even in the warmer days of winter, say in temperatures around 5C (41F). I've seen gas powered refrigerators (which I assume can be converted to rocket powered), but never freezers. Is it possible to meet the TEGs cooling needs of the TEGs with the energy of the firewood?