I'm having trouble picturing what you're describing. Cisterns can come in all sorts of designs (underground or aboveground, for example?), and what are you picturing with "incorporating" it into a wall? How do you plan to get the heat from the water into the greenhouse and stop it from being lost to the outside? Will this be heated with the greenhouse effect (I.e., a transparent glazing)? What's up with the white paint? Are you trying to reflect light away? Then how are you planning to use 'active solar heat'?

More details would be useful.

Sorry,

Maybe this will clarify a bit. The room would not have glazing all over it. I would put enough glazing on it that I could see out a bit and get some passive solar gain in the winter. In the summer I have a deciduous tree outside so the glazing could be partially shaded. In other words I plan to leave some of the walls intact. It would just end up being a room with lots of glass.

underground or aboveground, for example?

I was thinking above ground.

what are you picturing with "incorporating" it into a wall?

Imagine a rock wall. They have to be really thick to be stable. Now imagine that the rock wall is hollow and made of cement so it could hold water. If its say 3 ft. tall it could be the lower 1/2 of one wall of the greenhouse and the wall would continue up from there. It could have glazing if I wanted extra passive solar gain there or be solid.

How do you plan to get the heat from the water into the greenhouse and stop it from being lost to the outside?

There are three options with this heat transfer.
1) allow the heat to diffuse into the greenhouse passively
2) blow air over the length of the cistern so there would be a inlet at one end of the wall and an outlet at the other
3) blow air through plumbing in the water that would also run the length of the cistern to transfer heat

There would be some loss to the outside but I was thinking of insulating it. We have so many sunny days that I am not sure that would really be a big deal anyway.

Will this be heated with the greenhouse effect (I.e., a transparent glazing)?

I was thinking of using 'active solar' in that I would construct a solar panel with liquid running through it that could either go on the roof or be set outside. If I had enough heat I could just turn it off and I think it could also be used to cool the water in the summer if the glazing was removed and it was exposed to the coolish night air.

What's up with the white paint?

Since we have really intense summer heat and sun I would likely paint the outside of the greenhouse walls (including the cistern) where there wasn't glazing white so they wouldn't passively collect heat. The only heat collection for the cistern would be done with the solar panel.

Thanks for your response, sorry again that I wasn't specific enough.

I've done exactly what you are asking:
http://njhurst.com/~njh/blog/20061118
http://njhurst.com/~njh/blog/20061209
http://njhurst.com/~njh/blog/20061210

The biggest problem you'll have is the outward force on the tank. Mine is made from plywood with a wood frame and it bows considerably when full. I don't care because I know it will take that, but concrete is quite brittle and will fail catastrophically. So make sure you have some way of transferring the force from one wall to the other. Stainless steel cable is probably your only practical choice, normal steel will rust, plastic is too stretchy, glass rope too hard to handle and aluminium not compatible with concrete. Alternatively, make smaller separate compartments and link them with through holes.

Passive transfer of heat to the water is ineffective, blowing air over the tank will just evaporate water and cool the air and tank down, blowing through tubes would work, but holding the tubes down is tricky. I use a car radiator and it works a charm.

An external trickle based water heater would also be a good way to collect more heat. I will do this next may when winter approaches (waiting on some other projects to finish first).

I doubt your summers are as hot as ours, and I have found that that one radiator is perfectly adequate for that greenhouse (indeed my next plan is to air condition the house with the same setup). A bit of removable shadecloth would be more effective, especially if it is easy to adjust.

As I keep fish (like stoneunhenged's AI complex, I use fish to provide an endless supply of fertilizer and lunch) I don't want my water getting much above 30C, but if you just want to store warmth or coolth, I suggest you insulate your tank and seal it to allow for a much larger temperature swing.

If you can find them, a commonly available rainwater tank here in Melbourne is the waterwall:
http://www.waterwall.com.au/

This would be a 1 stop solution to what you want (assuming no fish).

Ah, you're planning on using a solar water heater for heat collection, not the cistern itself (like I was picturing)? Now this is starting to make more sense.

You'll need a good bit of area of solar exposure for this to be effective, of course. A single panel on the roof won't be enough. I'm assuming that you have an opaque roof, and thus wouldn't be wasting that precious winter sunlight into the greenhouse by blocking it with your collector, because that would be silly ;)

As for cooling: sure, your method should work. However, don't discount one of the fastest ways to lose heat from water: let it evaporate. Just have a setup that encourages your cistern to vent when in "summer" mode. You want wind to blow over the surface of the water. You could combine the methods if necessary.

As for getting heat from the cistern into the greenhouse, that's hard to say without looking at your particular situation. I'd start with passive and only move to active circulation if you discover that it's not losing enough heat overnight. Often that's not a problem; barrels often lose their heat too quickly. The more water you have, the slower it'll use up its heat store, so the more likely you'll need active heat transfer.

Ah, Nathan beat me to the punch with a much better post. ;)

Hmm, another idea for water cooling, if the aforementioned ways prove insufficient: A cooling tower. That's what industry uses, and it'll cool down water in no time. The type that I'm most accustomed to to basically create a sheltered rainstorm, giving the water huge surface area for evaporation as it falls.

Probably overkill, though. :)

karen: a small version of the cooling tower idea is the bong cooler:http://en.wikipedia.org/wiki/Bong_cooler
popular in overclocking circles.

The most effective way to cool a ghse here in the TX Panhandle is to use cool cells - fancy word for evaporative cooler except picture a much wider and taller cooling surface - ours is the width of our ghse and 3' tall. Most of us mount our cool cells on the south end of our ghses (long way runs north/south) and have exhaust fans on the north end. That is what we use on ours and what other commercial growers around here use.

Heat - heat storage in the water heater and then pumped through tubes places under benches (if floor is insulated to prevent heat loss into the cement) is an efficient heating method, but works better if there is at least a liner of some sort that creates a "quasi" dead air space at the top and sides of the area.

Search "solar blankets" on here and also look for some posts from Orchiddude.

Good luck and enjoy you ghse/sunroom.

Mary

gmatx: I'm not convinced that evaporative cooling is as good a deal as my heat exchanger method - for a start I get heat back at night.

Nathan,

During the time you need cooling here in the TX Panhandle, you sure don't need any hint of heating - I promise you!

Mary

Thanks for all the comments folks.

Nathan - I looked at your blog and I must say I am a bit jealous. Nice GH. Maybe mine will be as cool someday. I had thought of using an old radiator as a heat exchanger. Sounds like it works great - I will plan to try it.
I am not sure quite what you mean about transfering the weight / force from the water to another wall. I was thinking of making the cistern out of ferrocement. There is a great website on them . . . well many actually but here's one of them
http://www.ferrocement.net/index.phpinc=3&mfm=detail&sel=exa
Anyway it is basically just a concrete tank reinforced with scrap iron. They are used all over the world for safe water storage as far as I can tell. All the plumbing would have to be put in as the cistern was built but I had sort of planned on that anyway.
I am not sure you are right about the blowing water over the tank only cools air. Yes as water evaporates it takes heat but if the water is significantly warmer than the air it should humidify and heat it up I think. Kind of like when hurricanes go over a warm ocean and gain energy . . .

Do you think that the radiator could be used in reverse in the summer so that it radiated heat outside at night to cool off the water in the cistern in addition to the radiating solar panel. Also how do you get the radiator fans to work on AC power? Do you run an inverter or just have a battery?

Mary - you are right about not needing extra heat up here in the summer. I know evaporative cooling is really efficient with our low humidity but water is a very limited resource around here too. I was thinking of using an evaporative cooler as a back up in case my heat load got too big. The other thing with evaporative cooling is that the equipment gets really gummed up if you have hard water like I do. I am planning to fill the cistern with collected rainwater but given our low rainfall I am not sure that I wouldn't use it all up if I used it for evaporative cooling exclusively.

Karen - actually my problem isn't so much keeping the greenhouse warm as cool I think. It warms up pretty well already in the winter with just the one set of glass doors facing south so I am thinking I will leave the roof with no glazing thus plenty of room for my solar collecting panel. In the winter it stays warm during the day but cools off a lot at night. Right now I run a small electric heater at night to keep my seedlings going. I think the cistern will even out the temperatures. It is the summer I am really worried about. I hope the water cools enough at night to dump the heat load from the day. . .

More comments would be great & or see my next post with more questions.

Kate

gtmax: certainly, but nights are presumably cooler than days, so precooling your storage at night can pay off during the day.

kate: It doesn't matter what you use, if you go a for a long flat wall like I have, there is a lot of force trying to bend it. (I think it works out be be equivalent to putting 2 tonnes in the middle of the beam) Beam strength needs to go up by a factor of 8 or 16 every time you double it (depending on configuration), so a 4 m stretch requires at least 64 times stiffer material than a 1m stretch. Tanks are usually made round for this reason.

Warm water certainly warms the air, but warm air does not warm the water significantly. So you'll never get warm water to start with. (Except for things like the sun heating it)

I actually run my radiator fan continuously as it has been getting down to 10C at night whilst being 'too hot' during the day. Better would be to have two radiators, one inside, one outside, and use the outside one to cool the water. Ideally you would have a set of these, with the fans controlled in order to maximise the expected utility the next day. A computer can read the weather forecast and make decisions accordingly.

I happened to have a 300VA 12V transformer getting dusting in a cupboard so I just used that, but you can buy a 12V DC power supply for less than $40. An old computer power supply may be sufficient (sometimes the 12V supply doesn't work properly unless the 5V supply is used though). A battery and a 100W solar panel might be sufficient depending on how smart your controller is.

Evaporative cooling is a good backup, and indeed I effectively automatically switch over to using evaporative cooling as when the roof line gets too hot the autovent opens and draws dry air in across the tank surface (under the creek).

I think you should be able to remove almost all your electrical heating demand using a tank with radiator approach, especially if you find a way to increase your collection area: uninsulated attics can get quite hot on cold sunny days, you could trickle water over a metal roof protected above and below with plastic film.

With the radiator do you use a pump to circulate the water or just rely on the passive thermal circulation?

Wow computer controls. Do you have that with yours? I wasn't thinking anything so sophisticated. Maybe someday. I will just have to wire it so that it would be possible. So much to think about

I use a pump. thermosyphoning would not be efficient. When considering energy use, the correct measure to use is the COP - coefficient of performance. This is the thermal energy output / electrical energy input. In my case it is somewhere around 100 - for every watt of electricity I use, I get around 100W of heating or cooling. For comparison, a split cycle refrigerative air conditioner gets a COP of between 2 and 3. So although I use a pump and fan, I'm getting a lot more benefit from that pump and fan than someone using a resistance heater or similar (which have a COP of 1 for heating and 0 for cooling).

I don't think a computer is required - I would get most of the desired behaviour with a simple thermostat. I just want to see how much better I can do using predictive control.

My fan is also inefficient - it has small blades spinning at high speed. More efficient would be to use a large bladed, slow moving fan. If radiators were cheaper I might just hang lots of them along the roof line without any fan.

The fan doubles as a HAF, the pump also a) aerates the water for the fish b) feeds my bog garden c) my living wall, d) my rockface e) refills my fertigation system. Eventually it will also cool and heat my house and workshop, drive a large trickle collector and make my sandwitches. Pretty good value for $100 a year in electricity.

OK nathan,

I was right with you until the very end, the bit about a fan and a pump making a sandwich.

Then you lost me and I had visions of a Wallace and Grommet movie.

Or am I wearing The Wrong Trousers today?

;)

I get what you where saying about the walls now. You where thinking I should connect the walls of the cistern together on the inside. It would surely help structural integrity. I am not sure why a round cistern would have any less force on it . . . it seems to me that a round surface is only really stronger in compression (pressure from the outside). Would all the force of the water be transfered to the corners though?? I have access to old oil drilling pipe here which is fairly sturdy. I could potentially sink one of those deep in the ground (or several if neccesary).

Kate: you may have noticed that industrial facilities which store huge quantities of fluids in aboveground tanks use cylindrical tanks. For example:

http://images.google.com/images?hl=en&ie=ISO-8859-1&q=industrial+tanks&btnG=Search+Images

You are correct -- there is no less force. You could have a tiny pipe reaching up a hundred feet, and the pressure at the base of the pipe would be the same as the pressure at the base of a hundred foot high dam. The only relevant factors in determining pressure are fluid density, gravity, and height.

However...

Even though the force on a cylindrical cistern is the same as for a rectangular one, this doesn't mean that the shape will withstand pressure equally well. A circle is the minimum stress shape in this situation (all of the force will be an even tensile stress around the breadth of the container -- that's why using rings of a high tensile strength material, like steel, on a barrel makes it very difficult to rupture). Not all shapes are created equal for every situation (that's why the arch was such a big discovery for Roman architecture!).

Or put another way, if you make a rectangular tank that is not strong enough, it will become a circular tank.