My fan-coil active thermal mass greenhouse is finally operating!

I begin to wonder if "passive solar only" is adequate for the cold, shoulder seasons in your locale.

Have you considered directly adding heat to the system by other means?

Old, pioneer farm buildings around here had indoor wooden storage tanks for water, and set inside each was a wood-fired immersion heater that kept the water thawed for livestock.

In a setup like yours, an occasional stick fire that adds heat to the water mass might just be the ticket.

Hi Mike,

I'm looking into the phase change materials; thanks for pointing that out! Many of the substances the melt in the right temperature range are dangerous or fairly expensive, but there are a few that might work.

I'm particularly interested in "Paraffin 16-Carbons" which melts at 62 degrees F, but I can't find a source for it. Does anybody know what it is used for or if it is known under a different name? The only paraffin I can find for sale is a sold at room temperature.

Would it be helpful to use two different substances, one melting at a lower temperature (say 50 F) and another at a higher one (say 80 F) to keep the temperatures in a certain band?

Douglas, I've been looking into some type of immersion heater. I've also thought of having a heavily insulated rocket stove outside of the greenhouse envelope and pumping water through the thermal mass, so that it does not take up space inside.

I think multiple phase change materials could be a neat option.  If your greenhouse is always cycling between 40 and 60 degrees, then a phase change at 50 would take care of you year round.

But if it cycles between 40-60 in Oct, Nov, Feb and Mar but it's 30-50 in Dec and Jan, maybe a second phase change at 40 would be worthwhile.  I think two is only a benefit if one won't handle all your needs.

Each material will hold the temp at it's phase change until the volume of material is all frozen or thawed.

If you're temp swing is from 60-80 and your phase change is at 62, it will nearly always be melted and will only kick on when your temperature bottoms out.  So it would bring your final temp swing to 62-80 (or maybe 78).  If you pick a material that phases at 75, it will mostly be frozen so your swing would probably be 60-75.  So I'm presuming that picking one hear the midpoint of your swing (or slightly below) would be best.

But you may be onto something with a high and low phase.  I really don't know.

I haven't done this myself so this is a modest amount on conjecture plus a bit of engineering background all mixed together.

I think glycerin melts/freezes around 62 degrees so that might be an option.  I'm not sure where to get that paraffin you found.

I think glycerin melts/freezes around 62 degrees

This was driving me bonkers. On the one hand, there were lots of references online to this fact. On the other, I found lots of references to people using glycerin as antifreeze, and trying to freeze their soap making glycerin with no luck. It couldn't possibly be both . . . except that it can. A mixture of water and glycerin has a very low freezing point. Even ten percent of water lowers the freezing point below the freezing point of water! The same appears to be true of Polyethylene glycol 600

Mike, you probably already knew this, but it is pretty strange!

One of the things I'm wondering is if just one material could lead to a situation where I have a tipping point and two "steady states," such that the temperature could get "trapped" below or above it.

In other words, if I had a material that melted at 55F, it might keep greenhouse temperatures quite low during the winter months, while not doing much to slow the fall in nighttime temperatures. Once the thermal hump had been overcome in the spring, the daytime temperature would shoot up, since there would be proportionally less water available to slow the rise.

Is this a valid concern?

It is a hot, sunny day, about 70 degrees. Inside the greenhouse it is 90 degrees, even with the door and vent propped open. The tanks have already got over 68 degrees, and that is with the inefficient fan. Over the next few days I imagine that the tank temperature will get over 80 F, and stop having a cooling effect on the greenhouse. Obviously a case for increased thermal storage; cold weather will be coming back at the end of the week, and it would be great to store more of that heat.

All the phase change materials I've looked into so far have been prohibitively expensive, alas. So I might have to just build more tanks.

I'm thinking if you had enough phase change material, it would hopefully trap the temperature.  Keeping the greenhouse at a constant 62 degrees.  Yay!

Adding more water to hold more heat is a direct improvement.  Double the water, double the storage.  That's great until the greenhouse is nothing but tanks of water :)  That's why I brought up the phase change stuff.  It can give you 60x more storage in the same space.  

I think glycerin is a byproduct of biodiesel making.  So if you can find it that way, it might be affordable.  Maybe.  I dunno...

The greenhouse made it through a three day spell of cold, cloudy, snowing weather without freezing! The last night dropped down to 13 degrees.

I got the more efficient fan installed, and it obviously moves a lot more air; there is now a noticeable breeze in the greenhouse. However, the inflow/outflow temperatures are still only showing a three degree difference; I can't figure out why this would be. It does not seem possible since the new fan is probably moving more than twice the air that the old one did.

Now I have a new problem; some voracious field mice moved in and destroyed more than 100 seedlings and plants in a couple of nights. They dug tunnels through flats full of soil blocks and chomped the leaves off of the bigger seedlings, as well as digging for squash and wheat seeds in the pots. I've set traps and caught two mice. I can report that the peppermint and clove oil I tried didn't help at all; I really used a lot of it on some high value flats, and they just tunneled right through anyway.

I'm also trying to figure out what would be the most efficient way of skimming or soaking some of the oil slick off the surface of the water, and what media I should use for a small biofilter/grow bed. The filter would be located such that the outfall from one of the higher tanks fell into it and drained through it into the bed. I want to use some sort of media which will adsorb some of the contaminants while at the same time providing a substrate for some (non-edible) plants that will pull nutrients and more contaminates out of the water. I want to clean the water up enough to be able to discharge it onto a wood chipped area in my yard; changing and throwing away some filtration media is easier than disposing of hundreds of gallons of water. And whatever it is has to be cheap.

Hmmm, that is curious.  What are the temps you're seeing?  Air into fan, water in, water out and air out?

If I remember correctly, 70F air, 50F water going in, 53F water coming out.

The valve on the pump may have got bumped at some point so that more water is going through than before, but this would be hard to quantify; I've got a pipe on the outlet to prevent splashing which makes it hard enough to measure the temperature, let alone the flow rate. In any case it can't be TOO much more water, because at a certain point the pump flow rate overwhelms my bulkhead fittings, and I initially set it to be just below that rate.

Does the air feel like it's coming out of the whole radiator or is most of the air coming through in one portion?  Does the radiator feel like it has different temps on the surface?  I'm kind of grasping at straws here.

IF the air is maxed out (I'd be surprised) then that mean you can pump more water through to harvest more heat.  I think...

Seems like it is coming through fairly evenly, though it is hard to say.

I'm going to keep a close eye on the overall performance of the system, which might be a better indicator than isolated temperatures; tonight will be a great test, its going to drop down around 13 degrees again after a cloudy day below freezing with no solar gain.

How much oil are we talking about? Just a light sheen? Can you blot it up with a bit of cotton towel, let it dry, and use it as a firestarter?

Some of the fine folks here may recoil in horror and spontaneously combust (kidding!), but I don't think a few drops of hydrocarbon on active soil are a grave concern. Soil bacteria will take care of it. It's the junk we add (tetraethyl lead, holy cr*p!) that are the persistent hazards.

You can remove oil slicks from water with a drum

SpillPro drum skimmers have aluminium frames, which are lightweight for easy transport and handling. Their rotating drum is made from high density polyethylene, making them resistant to impact and abrasion.

A pneumatic or hydraulic power source turns the drum. Oil sticks to its surface and is scraped off into a collection trough. The same power source pumps the recovered oil to a collection tank onshore, or on vessel.

spill pro oil skimmer

or a simpler system that has a stainless steel thin film between 2 rollers. Surface oil belt skimmer

So the greenhouse performed pretty well. It didn't get above freezing all day yesterday and there was heavy cloud cover/snow. Then after at 1 in the morning the clouds cleared off and the temperature dropped to 14 degrees. Inside, it only hit 36 degrees. There was still some heat in the tanks which could have been released a bit faster to improve things, but on the whole I'm fairly pleased; with a radiant foil insulating layer that can be used at night I should be doing fairly well.

The temperatures on the tanks are still puzzling. I took some very careful measurements. The inlet temperature is 46F, the outlet is 50F. The greenhouse air is 65 F and very humid, judging by heavy condensation. I can't figure it out. One thing I'm going to do is see how many degrees of heat the inlet tank manages to accumulate throughout the day, which is another test of efficiency.

The oil isn't that much; just a floating scum on the surface. I'll see what I can do with paper towel. And I'd tend to agree about soil life being able to consume hydrocarbons.

I think I got the mice under control; no damage for three days now. Maybe there were just two of them. Amazing how much damage they can do!

Gilbert Fritz wrote:The inlet temperature is 46F, the outlet is 50F. The greenhouse air is 65 F and very humid, judging by heavy condensation.

Is the radiator condensing as well?  It should be dripping like crazy.

Yes, it is dripping steadily. The inlet hose is also covered with condensation.

The airflow does seem to be more concentrated than I'd like; how could I spread it more evenly through the radiator?

Ideally the transition from the fan pipe to the radiator would let the air spread out.  Air tends to seek the easiest path so that must be through the spot that is concentrated.  Maybe that's why the bigger fan didn't yield a clear benefit?  Maybe both were successfully putting all the heat they could into that small part of the radiator and neglecting the rest of the potential heat transfer area.  

Another way, albeit crude, would be to make baffles in the box prior to the radiator to guide the air from the duct over to other parts of the radiator.

Another way, albeit even more crude, would be to partially or fully block the air flow as it exits that part of the radiator so that it has to spread to other parts of the radiator in order to get back into the room.  This would be the easiest to try and see what happens.

I'm glad it's dripping, otherwise that could indicate other mysterious issues.

Radiator fans are set up to pull air through the radiator. I think the vacuum would distribute the air flow more evenly through the fins. You could try hanging the radiator from the ridge and the fan pull the air down to the floor. The water flow in the radiator is also normally from the bottom to the top. Your set up is simple for installation to have the outfall to the tank but it may not be fully using the elements capacity. Possibly mounting the radiator above the inlet tank and pumping the water into the bottom of the radiator then letting the current inlet line drain to the high tank. The fan could then be beside the pump simplifying the electrical and blow the air down on the floor for more heat distribution.

Well, I found that there was a gap where a lot of air was bypassing the fins. I taped it up . . . no difference to water temperatures. (Today there was only a one degree difference!!! It varies between 4 and 1 degrees different, probably depending on something I'm not measuring, like the amount of humidity in the air.)

I'll probably try some of your ideas next, Hans.

I'm rather disappointed I can't get any more performance out of it so far.

Hey Gilbert, I made something similar recently and I think it's working.  Maybe you can see something different between our designs that can give you some ideas for yours?
https://permies.com/t/145886/Active-Heat-Battery#1139825

Getting ready to start working on this again.

If I reversed the flow through the radiator and turned up the flow from the pump, would I be in danger of damaging the radiator? How much pressure can they take? I ask because a small pressure release valve is mounted on the top tank, and if I moved the inflow to the bottom tank the water would go through the fins before reaching it.

Automotive radiators are designed to operate under quite high pressure.  If the outlet is not blocked there should be no problem

The description of my pump says that it is "150PS." What does that mean in terms of horsepower? Trying to figure out how much pressure it could generate.

Well, here it is, a new year and new thoughts about the greenhouse.

The greenhouse has a lot of problems.

The radiator/fan system can't move enough heat into and out of the tanks. This is probably for a number of reasons:

I can't turn the pump all the way up because the radiator pressure release valve pops open.

The radiator/fan is undersized.

The connection between the radiator and the fan is inefficient

I don't like the open tanks. They get full of junk. Also, the radiator contaminated them with oil/antifreeze (even though I tried really hard to clean them up.) The use up the space inside really inefficiently. And I can't really raise fish in them; if the system was working, the temperature would fluctuate too much. In any case, I probably won't be ready to raise fish for a long time. The space underneath them is good for the mice and voles to live under. Trapping mice isn't fun, and anyhow, the ants eat up all the peanut butter before I can catch any mice.

I don't like growing things in pots. They dry out/run out of nutrients really fast. Of course, there's ways around that, but still . . .

The greenhouse is full of mice and voles who eat the plants and tunnel through things.

After a greenhouse has been set up for a while, it starts to feel unhealthy and yucky inside. And of course with rodents there is a Hanta concern.

The greenhouse is overbuilt to raise greens or overwinter brassicas. It is underbuilt to grow tropicals. So it falls between two stools.

During the summer, we don't really need any heat storage; I could do better by just blowing the heat out the door, and not spending money running a pump.

Soooooo . . ..

Time to do some thinking.

What do you all think? Would a radiator like this work better? https://www.instructables.com/Free-Air-Conditioning/

Yes that design or just using recycled base board heating units in the peak of your greenhouse would allow you to use the full capacity of the pump.  To maximize growing space close your tanks without any space underneath and build planting beads on top. That eliminates evaporative heat loss and water loss. The system then maintains soil temperature with a slow transfer. To warm the air the baseboard unit could be lowered or have an additional unit for that purpose. I can see as units could be obtained adding one on each side and one in the walkway below the tanks. Water circulating from the lowest to the highest and then returning to the tanks. If the  water is warmer than the air the heat will rise if it is colder than the air the cold will fall, so the air circulation is passive and you can use the fan to move heat outside beyond the systems capacity.  
I am considering using black black PVC waste water plumbing and blue plastic barrels in such an arrangement. This would have the advantage of radiant heat transfer more than air heat transfer but would still have air heat transfer.

Any thoughts on how I would go about building an efficient radiator/fan combo from scratch? I'm particularly wondering how to put some type of radiator inside a long length of piping so that I can blow air through it.

I imagine that putting an old baseboard heater inside a pipe wouldn't work; the airflow wouldn't work out properly.

Gilbert, it's a tough lesson when things don't work out as planned. Everything looks bleak, as if you've wasted your time, and the rodents own the day.

But I don't think it's a waste, not at all.

I have to say I admire your energy, and your tenacity. And I'm willing to bet real money that you will make v2.0 work.

Keep on' truckin'!

Gilbert Fritz wrote:Any thoughts on how I would go about building an efficient radiator/fan combo from scratch? I'm particularly wondering how to put some type of radiator inside a long length of piping so that I can blow air through it.

I imagine that putting an old baseboard heater inside a pipe wouldn't work; the airflow wouldn't work out properly.

The fins would have to be soldered as radiuses around the water pipe instead of across it as in the baseboard heater. Water flow would come out the end the air is going in.
How large an air duct would you want to use?  You could use multiple water pipes with fins wrapped around them and then bundled together so that each pipe would supply 2 fins.

I was thinking that a fan-coil will always be more efficient at transferring heat into the water from the air, rather than transferring heat from the water into the air, because when the air is warmer condensation will transfer heat to the colder water, whereas there will be no corresponding evaporation from the coil when it is warmer than the air.

That being the case, I suppose it would be important to have a fairly high temperature differential to improve heat transfer to the air?

In which case, would a smaller tank of water (which would have more extreme temperature swings) be better than a larger tank which only moved a few degrees?

Would something like this work for me? https://www.instructables.com/Hollis-homemade-AC/

Gilbert you've identified a bunch of the "not working" parts. Is this green house oriented with it's long aspect north-south or east-west?

Your first photo shows tanks and equipment sitting on top of a bunch of concrete blocks and pavers. I'm currently reading "The Chinese Greenhouse" by Dan Chiras and I've read Oehler's greenhouse book in the past. How do you feel about ditching the "higher tech" stuff and insulating the north wall reasonably well, and putting all that concrete stacked against it on the inside as "thermal mass". If you get enough winter sun, you could put some pipes through that thermal mass and down into the dirt below your beds with pipes and rocks.

Several greenhouse books I've read suggest that if the south side is your long access, you'll be further ahead to insulate the entire east and west walls as well. That's certainly what Dan Chiras is claiming. The Chinese also use rolled "curtains" on the outside of the south wall at night - another concept that I'd heard before. Maybe exercise your high-tech mojo in figuring out how to automate a curtain system?

No, I haven't tried this yet. I'm in a *really* cloudy winter environment and the clouds bounce what little light gets through around from all angles. Codes will prevent me from experimenting on a large enough scale that it's likely to work without spending more money on permits/sign-offs than I'm willing to spend to test something in a climate it wasn't intended for. That said, I'm getting desperate enough to be trying to figure out how I can make 2 structures soooo... close together I can open the doors between them to get the effect of one large space, but be technically under the 10' x 10' un-permitted structure rules.

Hi Jay,

I'm going to be trying some of that. I'm putting together a large insulated blanket that I can roll over the house at night, and then roll back during the day, leaving it up on the North side. And yes, the greenhouse is lined up East-West, with the long sides to the South and North.

The reason I'm trying to avoid using passive thermal mass, whether tanks of water or stacks of bricks, is that they take up precious space; active mass (fan-coil) can store more heat in a smaller space. Also, due to the curve of the wall, stacking up thermal Mass against it is difficult.

Also, the greenhouse is bermed into a slight North-facing slope. I know this isn't ideal for solar capture, but: It was the only sunny space available, and also overheating, even in the winter, is a big problem in Colorado. The exact opposite of what you're dealing with.

I thought about running pipes into the underlying soil, but this would have required burying massive amounts of insulation, which I didn't want to do. So right now I'm trying to isolate any thermal mass from the cooler soil.

Got everything inside (tanks, pots, heat exchanger) torn apart and hauled out. Had to disassemble an end-wall to get them out. I got a 330 gallon IBC delivered and will move it inside once I get a spot leveled out for it.

Very cool project. I would not say this is a failure but that you are in the middle of problem solving.

I haven't seen mentioned, is the roof just a single layer of poly? A double layer will make a big difference with heat retention. It can be down with a fan (electricity) or spacers.

You were talking about phase change materials. Coconut oil changes at 76 degrees. I've seen studies done in Asia where they are trying to use it to cool homes (in walls) or vehicles (oil put in roof). It would regulate the highs better and the phase change would kick in a temperate much more preferable to tropicals, keeping it near that temperature longer. Main problem I see with it is it's 1.40$ a pound to buy here in US. If you got direct ship from Asia (alibaba) it would be less I don't know how much.

I'm in the process of brainstorming my own greenhouse. We have many cloudy winter days here in north georgia. I believe instead of relying on sun to heat water I would heat the water with wood (which I can get free on my property).

Keep the updates coming!

Thanks for the encouragement Kevin!

I'm going to be attaching an insulated tarp that I can roll over the structure at night and roll off during the day; should give me some of the same effect as double poly. I will be putting a double layer on the end-walls.

I've thought about using coconut oil, trying to figure out a way to get it cheaply has been difficult so far.

I got the IBC tank inside the greenhouse and the endwall closed back up. The tank looks awfully large inside there!

Gilbert Fritz wrote: The tank looks awfully large inside there!

The bane of small greenhouses! Is your plan for the IBC to be closed or open topped? An IBC frame is pretty strong, so theoretically, you could put planters on top of it, but as you mentioned earlier, the down-side of that is nutrient and water needs if the pots are small. At the very least, if I have to grow in pots I try to get ones that are a minimum of 1 foot tall and put punky wood and chicken shit in the bottom, under decent dirt and I *always* put a reservoir under them.
I've forgotten what your inside area specs are - is it as large as 120 sq. feet? Larger?