It's all Plain to Sea

[jellis]

Hey everyone!
Two semi-quonset, low-profile greenhouses, each 20'x100' in size, have fallen into my lap as it were. I say that they have fallen into my lap even though I paid for them because I am located in a very, very remote portion of the country here in Boulder, Utah ("the other Boulder"). Someone in town had these in boxes, never assembled, so I got lucky in that shipping anything out here is a nightmare (the only semi company that comes this way makes the trip once a month).
Just to set the scene a little bit, these things are being installed in a location that is fully off the grid (i.e., conventional heating/cooling is NOT an option) about a mile down a 4x4 road which includes a creek crossing, 10 miles out the Burr Trail from Boulder itself. It's difficult to get more remote in the lower 48. I think that I am Zone 7a here, right under 6000 feet elevation. It can easily get to 95-100 everyday in the summer and in the low teens at night in the depth of winter (and, of course, a few nights at or below 0F). I do, however, get an almost infinite amount of intense sun.
The upshot is that the property I'm putting these on has an infinite amount of sand--pure, dry blow-sand from eroding sandstone formations--so I can dig, backfill, and generally play as much as I want. There is also a little front-loading bobcat on the property for doing the earth moving, so renting equipment is not necessary (nor is it much of an option given my remote location).

I will also say that I have a degree in Bioengineering (basically Chemical Engineering) so my tendency is to spend a lot of upfront time modeling, researching, and calculating before I start moving any dirt. I spent my last few dimes on these greenhouses and I want to make damn sure I get the most bang for my buck. I think that it is also worth noting that my family runs ClimateMaster, one of the world's largest geothermal heat pump manufacturers. I don't know their engineers but I can certainly grab a few ears when needed, and I think this whole technology could use a few ears from what I'm reading.

So far I have done a thorough heat-loss analysis, so I have a really good idea of what kind of heat storage I need to maintain any given greenhouse temperature setpoint throughout the winter (on average). What I'm having a problem with is determining the thermal storage EFFICIENCY of these systems. That is to say, what would be ideal is the ability to look up the solar insolation at my given location, use that to calculate the energy input into the greenhouse, and, using an efficiency percentage for the SCHS, determine how much energy is being stored/removed from the soil on a daily basis (on average).

I think that what I'm really asking for is what some of you have described as the "missing link" for taking this technology big time: The ability to take a well-described system and implement it in another location at any scale using a simple correction factor. However, I don't think this is really too hard; it seems like there is plenty of research out there, it just needs put in a useable form (i.e., what engineers like myself do). I am certainly having trouble getting my hands on the research, however. The titles seems very enticing! Anyone have access to university accounts for obtaining academic journal articles? I can't believe taxpayers fund this research and you have to pay to read the results--total BS!!!

What I will tell you so far is this: From the one article I've managed to get thanks to our hobbit friend (NUMERICAL SIMULATION OF SOIL HEAT EXCHANGER-STORAGE SYSTEMS FOR GREENHOUSES) I have calculated the efficiency of the system by looking up solar insolation values at the location the test greenhouse was constructed (Quebec, CA) for the end of April when the validation data was collected. Depending upon the configuration, the overall system (greenhouse as solar capturing device, SCHS as heat transfer device) has an efficiency of 18-26%.
The problem I have is with the solar insolation values: horizontal, vertical, or latitudinal surface? They aren't too different in April, but they vary wildly in December. That really throws the energy inputs for a ride. I know there are journal articles detailing simple greenhouses as solar collectors (% efficiency given solar insolation of a known value), and this is what I really need to move forward.

Are there other data sets, using different SCHS setups, to compare with? I see lots of people building these things but not lots of data to toy around with. The psychometric charts and whatnot I can handle (or, like I said, can find people who handle them everyday), but I need actual data!!!

Thanks,
~Josh

[randy]

jellis: I also have been challenged with this. I have been working with a spreadsheet in which I attempt to model the energy balance. My operation remains under construction. The greenhouse is closed-in and there is no operational SHCS or heating. I have been informally monitoring the temperature and have found that my model is very close to actual day-time temps. My greenhouse is oriented east-west and I have a method of averaging the insolation throughout the day. You would orient your greenhouses north-south but the concept may be applicable.

[jellis]

Randy,
You've got quite the project!

I would definitely like to see your spreadsheet and energy balance, especially if you know it's reasonably accurate. My greenhouse will be oriented east-west as well since I plan on building a backfilled/insulated north wall. I am very interested to see what things look like once you get your system up and running!

What is the air velocity in your pipes? I haven't seen any discussion about Reynolds number, but turbulent flow is incredibly important for heat transfer. I worry about the recommended 5 greenhouse volumes/hour, for me it gives an insufficient velocity to achieve fully-developed turbulent flow. I need 2.4 ft/s minimum which gives 740 cfm.
For comparison the one journal article I have access to states 4 m/s (13.2 ft/s) as the ideal velocity. That's a big variation!
In this regard, my complication is the fact that I'm off-grid: blowing power is not simply a function of how much I spend on a blower, but also in the infrastructure to power it. Anyone have advice/recommendations on 12V high-capacity blowers?

[randy]

What effect would the ADS pipe corrugation have on turbulance, even at the lower air velocity? How does contact time inside the pipe affect the models? It would seem we want sufficient sensible heat transfer to achieve dew point then latent heat transfer takes over and the real power of the SHCS kicks in.

[jellis]

The corrugation has a large effect on turbulence, taken into account via various "pipe roughness" estimators such as Manning's or Darcey's coefficients. The effects, unfortunately, are non-linear and are not derivable from first principles (i.e., it requires a test setup and experimental data collection to develop the coefficient values at various Reynolds numbers). At any rate, for heat transfer purposes the corrugations are very good (increase in surface area AND increase in Nusselt number due to the increase in vortices, etc. that are part of a turbulent flow regime). The down side is pressure requirements increase dramatically, which is to say, static pressure increases/flow rate decreases from your blower.
In other words, there's no good way to know if your velocity is laminar or turbulent in a 4" corrugated plastic pipe using the basic Reynold's number equation, but I would try and shoot for turbulent flow in a SMOOTH pipe using the basic equation to make sure that I had great turbulent flow in a corrugated pipe.

As for the heat transfer, I am still figuring that out. It seems pretty simple, actually, but I haven't pulled out my old text books to grind through it. You've got an air flow rate of a given enthalpy (which takes into account both the temperature and humidity), and what you're looking for is the pipe system to either condense out water (cooling mode) or evaporate in water (heating mode) to the air stream. So, setup the equations using a bad scenario, say maybe 34 degree air at 20% humidity, and see what happens at your flow rates and pipe configurations. The one journal article I've read set the convective heat transfer coefficient in the pipes to h=23w/m2/k, but that was for an air velocity of 4 m/s--much higher than most systems I'm sure.

Let me know about that spreadsheet....

Thanks,
~Josh