It's all Plain to Sea

[justwait]

The 4-H Self Reliant Club of Depoe Bay has taken on the project to build an “Off the Grid” Dome Solar Greenhouse that will grow vegetables all year long. They have chosen to use Mr. Hobbit’s SHCS concepts.

They have acquired the land and finances to design, test and build a series of Domes to determine how to accomplish this at the lowest cost.
The plan is to build a 22 ft Dome, improve the design, then build another 22 ft Dome, improve the design and keep repeating the process. It is planned to build up to 10 Domes.

The temperature is mild in Depoe Bay, Oregon. The Average temperature in January is 33.8 F and the average temperature in July is 57.6 F. There is not a major need for cooling, but heating.

The 4-H Club has its own backhoe and front-end loader tractor; so digging will not be a problem.


The project will be trying different designs. Our first design thoughts are:
1. Eight plenums, with every two (2) plenums with 6 tubes.

2. Each set of two (2) plenums and 6 tubes are laid out in four (4) circles, with diameters of 22, 18, 14, and 10 feet. Each set of tubes is one (1) foot above the other and 2 feet apart.

3. One high speed in line fan will be exhausting the plenums. The fan will be varible speed with dampers to able to change the CPM for each circle for testing.

4. Three (3) temperature sensor strings will be install between each set of circles to monitor the temperature of the earth between the circles.



To help to determine the correct CPM, we had to developed are own spreadsheet.

We invite all to share their knowledge and imagination as we enter the final stages of planning and begin construction of the first design.
Since this is a research project, we plan to take post pictures, designs, and test results as the project moves forward.

Any experience, ideas, comments or questions are welcome about our design.

[mrhobbithhnet]

justwait,

Great project! Glad to see you here, working out the details.

First up, I've never seen anyone do a series of circular rings, each with it's own plenum set. That should help resolve a number of issues wrt to a balanced flow through all tubes and maximizing the tubing/mass exposures.

The problem still exists where the top layer of tubing in each plenum series will be the favored flow (it's the closest to the fan...) effectively starving the flow through the bottom row of tubes You might be looking at one of the plenums being a pair of 6 inch boxes with the fan in one, the tubes in the other and a connection at the bottom. That way you could, say, blow in one plenum top, and feed to, say, the bottom of the other plenum, pressurizing from the bottom. Then the corner to corner layout of input and output to the tubes/plenum is established.

I can also see there only being a need for a double set of plenums opposite the floor space when the tubing lengths are long. The shorter runs could have the input and output plenums back to back on one side only. Looking at the largest diameter (max tube length of 35' or so), I'd say that you only need one set of plenums per ring.

With a system like this using many small fans rather than one large one, you also have a opportunity to explore some VERY cost effective fan selections. The small a fan gets, the more affordable they seem to be wrt to the cost per cfm.

There is a possibility that you could cross connect all of the inputs to one master plenum across the plenums on one side and then 'tune' the flow to each ring with adjustable dampers. If you did that, you'd have an amazing opportunity to change the mass/air flow ratio dynamically or on an ad hoc basis through each ring just by changing a damper... creating more flow based on the soils ability to absorb more heat (ie: the perimeter ring will probably cool faster than the center ring, so could get more air than the center ring) If you had a fan in each plenum, and they were variable speed (dc brushless computer fans are I believe?), then you could control their on/off and speed operation with a small PLC with temperature sensing inputs. Then you'd have perfect control of max heat exchange and power conservation (should you be thinking of running the whole thing solar with a small array and battery pack (for night running) - more on that, 'cause I do believe you can set up the plenums to do the night heat exchange without running the fans simply by opening the tubing to the frost flow at night)

Looks like a great project, and I'm glad to have the opportunity to keep an eye on it. Good luck... and seeing how Depoe is on my way, more or less, to Portland and Eugene, you might see me drop in too. That would be great if it's possible...
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[justwait]

I am a little confused, as I understand your concept, the objective is to get the highest temperature air and most humid air flow through all the tubes at a speed that would allow the maximum transfer by lowering the dew point and by normal heat transfer.

I now understand and agree that the plenum series closest to the fan will always be favored. This brings up a whole series of questions.

1. How does one get the airflow at an equal rate through all of the tubes in ANY system without dampers?

2. If it is possible to change the fan speeds, is it better to have the rate at 2 CFS or higher at 4 CFS?

3. Is the optimum rate of CFS a function of the length of tube?

4. If the air is being pushed at rate of 2.53 CFS with a tube circle 13 ft long and spending 5.12 second in the tube, is that as effective as air being pushed at a rate of 6.52 CFS with a tube circle 33 ft long and spending 5.05 seconds in the tube?

5. Does it not make any difference of how long the air spends in the tube?


We are recruiting only teenagers to become 4-H members for this fall, so they can learn this technology by doing. We will not be breaking ground until September or October. We would welcome you and anyone interested to come and visit the project when ever it is convenient.

[Hex]

Hi John and Justwait,
I like the alternative tube layout, it looks like it should be quite flexible in terms of control. It`ll be interesting to see how well the airflow behaves with multiple plenums.
I settled for the single intake central plenum with multiple 360 degree horizontal outlets to take advantage of the aerodynamic shape of the dome but mine is a lot smaller than 22ft Heightwise there probably isn`t a lot of difference, i have about 10ft overall height with the kneewall and ring foundation. You plan to build 10 domes? i wish i had the space.

Wishing you the best of british for your dome/shcs project.

[mrhobbithhnet]

Good questions... I'll see what I can do.

[justwait]

Based on your suggestions, we will be adding temperature-sensing tubes. (see above diagram) between all the circles. Our present idea is to have a number of capped plastic tubes place vertically spaced half way between each of the circles. These tubes will be have the ability to be uncapped at the top and have a temperature sensors place to any depth up to 3 feet. The temperature sensors will be move from tube to tube and circle to circle to monitor the earth temperatures between the circles as we change the speed of the fans.

Some questions we would appreciate some input:

1. What distance should be between the sensing tubes?

2. Should we monitor each half of the circle or the whole circle at a time?

There will be running four (4) fans, one per circle. Each fan can be independently controlled. These fans can be turned on/off based on temperature or humidity.

1. Is better of turn the fans on/off based on temperature or humidity?
2. In what sequence?

[mrhobbithhnet]

[justwait]

Hi John

1. We are going to take your suggestion and install three (3) temperature sensor strings between each set of circles at all three levels. We will do it for only ½ of each circle.

2. To do testing on different CPM vs the earth temperatures as measured by the sensor strings, we are considering use a high-speed variable speed fan to exhaust the four (4) circles with a damper to control the airflow for each circle. (see diagram above)

Any opinion?

[mrhobbithhnet]

justwait,

Looks good...

Glad you see the sense in some cheap thin wall 1/2" poly irrigation tubing for conduit. Be sure to fish some string into your air tubes too so you can fish some sensor strings in there too. Just be sure you install the conduit with good long sweeps and no tight kinks when you bring it all up to the surface and you'll be fine ($20 for 100ft). I use that stuff fishing CAT6 LAN cable all over the place, works fine. The trick is to fish a pull line into it before the install, don't kink it and then pull your sensor strings in with a little wire pulling gel. To do a real cheap analog system, you can use just cheap K type thermocouples and 8 wire thermostat cables to wire 4 of them all up in one string you can pull into place.

If you are going to be doing manual damper adjustments, then you really have to get a manometer for air speed measurement. You can get yourself a handheld instantaneous wind speed, temp and RH meter from Kestral and you're set with an ad hoc monitoring system. You got anyone on your team with HVAC savvy and experience? They'll get the picture....

If your soil is more or less porous, you would be better off pushing the air in with a single blower and connected manifolds. That way the tubes are pressurized and some of the gh air can infiltrate past the tubing and into the soil and condense out directly in the sub soil area for a little bit better heat exchange. Not to mention that squirrel cage fan blowers are typically designed to work best as pressurizing devices, not suction devices.

If your project gets some tech backing and funding, you can exchange all of the analog for digital and go for a real time digital online monitoring and logging setup later - assuming you already have the wiring conduit in place. That would be sweet! Someone could do a nice grad-level thesis with that one...

The real advantage with a bunch of small fans on each circle is that you have a chance down the road to do real simple computer controlled automatic feed back controls to change the fan speed and air speed during the day and the season. A single fan leaves you with a single source of air and expensive damper controls if it turns out automated variable air speed settings is really going to make a big difference.

But for simplicities sake, and to just get some research data a single fan would get the job done with less up front design time and expense. Just experiment a bit and find the fan and damper settings that work for you and you are good to go. And you won't have to get involved picking up on digital control circuits... that is a time and money sucker even if you're a fast learner.

And another thing, I do believe you only need fan housings on one side of the circles... with the input and exhaust back to back. No use splitting it all up with a lot of real short pipes and twice as much assembly rigmarole.

BTW, it's not CPM, it's CFM (Cubit Feet per Minute)
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[justwait]

John, thanks for your comments

We agree with you about finding some grad student or developer to design, conduct, and record the results of our dome experimenting. We will be approaching Oregon State University to try to find a grad student to work with us.

All of the first dome’s control system will be manual while we try to find relationship between air flow, temperature, relative humidity, length of tubes, distance between tubes, size of the tubes, etc. We are rethinking about making the size tubes in each circle different maybe one circle with 3” tubes vs 4” tubes. Another consideration is making the distance between the circles different, for example 1 ft, vs 2 ft, vs 3 ft, vs 4 ft. As we determine these relationships, we will be looking for a firm or individual to develop a digital control system that would use these relationships to optimize the running of a dome.






Our next design objective is to explore the placing of underground our umbrella/berm arrangement. The umbrella can create a huge subterranean thermal reservoir that soaks up the sun’s energy during summertime and stores it for winter heating. Hopefully the dry earth under an insulating/water-shedding umbrella could store enough free heat from the summertime to warm the greenhouse through the entire winter. The umbrella is made with plastic sheeting and Styrofoam. The exact combination of sheeting and Styrofoam is still being investigated.



We will place an umbrella/berm arrangement over the north wall and the top of the dome. The hot air will be taken from the top to the dome and run outside of the dome, under the umbrella/berm, to each circle’s plenum. There will be four of these tubes. (see above)

Another umbrella/berm arrangement is planned for the dome foundation.

The foundation will made by pouring a 24” high “paperfoamcrete” wall with a treated 2 X 6 bolted on top for the dome to sit upon. We will place an underground umbrella and berm on the outside to this wall. This umbrella/berm arrangement will drain the rain away from the dome. Under this umbrella/berm arrangement 2 more circles of 6 tubes will be place. These circles will be outside of the dome, 3 feet deep, but under the umbrella/berm. There length of the tubes will be 40 ft and 46 ft. The purpose of these circles is to vent and store the excess heat during the summer. These two circles will be on a thermostat that only is used when the excess heat in the dome needs to be vented to the outside.

Opinions?

[mrhobbithhnet]

[justwait]

As we examine the your above CAD diagram, we are now confused. We have proposed a concentric circular UACT system with 6 tubes each with a separate fan from the intake plenum to the exhaust plenum, which is across the circle, with a Total Tubing Effective Area SQFT= .45 (see above Plan A)

We think that you have proposed a concentric circular UACT system with 3 tubes each with a separate fan with the intake plenum and the exhaust plenum back to back with a Total Tubing Effective Area SQFT = .22 (see above Plan B)

Are you proposing a system with 12 tubes, with a Total Tubing Effective Area = .91

As you know the Total Tubing Effective Area SQFT effects the fan CFM we use.

Can you clear up our confusion?

[mrhobbithhnet]

[justwait]

[mrhobbithhnet]

Justwait,

No time to answer fully, on the road now. You've got some of the details pretty mixed up in your thinking. Sorry if there isn't enough on the site of a quality to make crystal clear for you.


But some of your questions and conclusions indicate a real need to reread the SHCS explanations OR I've got the descriptions all wrong (likely possible in some areas but not in the basic stuff)

For starters, the air exchange rate always refers to how much gh air is moving underground... a 5x exchange rate means there is 5x times the volume of the gh moving through the SHCS every hour. THis is the minimum to get a decent ROI BTW... not the design goal. ROI increases all the way up to 20X. I would be designing for a max up there in the 20X rate and comfortably able to do at least 10X.

I'll have more time online in a few days when I get back home...
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