Very Simple Cooling System for Growing Spaces and Homes.

And, of course, it would serve equally well as a heater if the air temp is less than the ground temp.

It may also be a good idea to have the water saline so nothing breeds in it and everyone gets legionair's disease.

No need for saline water.......it will breed stuff too. Just add some bleach about once a month. Actually, I'm not sure if you need water at all, just running air through underground piping would cool it.

Looks like a good idea for air recirculation. Sounds obvious, but I'll mention it anyway..............A few well placed roof vents will go a long way toward eliminating hot air buildup.

Wait a minute!!! If you run some piping underground, with an inlet at ground level, combined with some roof vents to exhaust hot air, you won't even need a wind turbine. Just the hot air exiting from the roof will pull in cooler air.

Bleach would be better, yes.

I should've stated that I'm looking for something that'll work with air temperatures of 35C and above.

Without the water you'd be dumping into the ground, which not conducting well or circulating, and with a poor interface with the pipe, even if you used copper or aluminium, the air would need to spend quite a while to drop enough heat.

To get circulation through, the air at the top of the space would need to be hotter than outside, so it'd be unlikely that by the plants would be significantly colder. It'd probably be more effective to have the same air recirculating. The problem then however is not enough CO2...

I'm not sure of the cost, but I wonder if, as part of your system, you could introduce dry ice..........it would kill two birds with one stone. Maybe a block of this every couple of days combined with a simple box fan.

I would certainly do the trick, but I'm looking for a setup which is a bit more ghetto, with as little in the way of consumables as possible. This would be mostly for micro scale urban farming and developing communities.

Does your garden have to be enclosed? I worked in the tobacco fields when I was a kid. To keep the plants from burning, they used shade cloth.

Tobacco's pretty good with heat, edible plants generally aren't. You can certainly get away with quite a bit with just shade cloth, but if you can't regulate your air temp it only takes a couple days in the high thirties to fry pretty much anything.

I don't know what you're growing. Here in NC it gets that hot every day during the summer. I'm able to grow lettuce in the shade. Also, in sun, corn, tomatoes, radishes, peppers, zucchini, carrots......most anything as long as I keep it watered.

If heat is your problem you are simply compounding it (and adding expense) by enclosing the growing space in an insulated greenhouse.

The shade cloth idea is nice, but if community food is the product then you may do even better with strategically aligned shade rows - perennial bush crops like currants for example are tall enough to create partial shade conditions without taking up too much root space. Perennial crops also help to conserve moisture in the soil, and shrubs or bushes are good habitat for beneficial insect communities (eg parasitoid wasps). To reduce your watering and pest control inputs.

Or build an arbor and let beans and peas grow over it to provide a shade row, if there's no space for perennials.

Near coastal areas where humidity is high evaporative cooling may not be as efficient as in less humid areas.

A couple more bits of info I should've mentioned from the start:

It's not been my project so much as some research I'm doing for a guy aiming to set up hydroponic growing spaces in Burma. I did initially bring up shade with him, but he's pretty adamant about enclosed environmental control, the air temp is just too high, including overnight, and he also wants the ability to regulate co2 and humidity.

I'm also looking at this as a form of basic airconditioning for living spaces. Here in Oz today it's 39º, humid, and we have no aircon. I'm thinking about things that are cheap and easy and don't involve building ordinances or aggravating the landlord.

Regarding geothermal rates of heat transfer, it's not quite a standard rig as the heat doesn't initially go into the earth, but rather the water, which then has all night to drop into the soil. Or even not, if you wanted to use that heat to warm the air at night.

I did initially think of evaporative/mist cooling, and in a lot of applications its a pretty good option. For this tho I'd be concerned about it overly humidifying the air. Edible plants seem to like about 30-40%, depending. Misting can be used to dehumidify, but it's a bit more complicated, and I think this idea would be a bit simpler.

And somewhat off topic, regarding boosting CO2; was thinking about either open flame or having a compost heap inside the space. Both are completely counter productive in terms of heat, but apparently CO2 is quite a big deal for plant growth, so might be worth it...

And merry xmas to yall.

Also I made an error; 2x3x4 meters of air would be balanced by 8 litres of water, not 16.

The Qld Education Dept are using this type of heat exchanging for cooling classrooms in some schools, I'm told. ( I tried to get hold of the guy who told me, but no responce today)

He's using large dia. plastic plumbing in the air reticulation, laid in trenches at 3m depth.

I did question the thermal conductivity of this medium with him but he said that it worked.

Immediate thought with the guy in Burma is that he'll need to use shade as well, so's not to have too much heat load and thus 'cook' the plants.

Maybe he needs to come here and see the plant propagation shadehouses in areas around Townsville and Cairns. Maybe here on the Gold Coast too.

My mate had a hydroponics veg farm at Freeman's Waterhole and he only needed to keep out the 'varmits'. A great Fararday Cage for the elimination of unwanted cellphone calls too.

MERRY CHRISTMAS AND BEST FOR NEW YEAR, EVERYONE.

This is all starting to beg the question; if it's so easy to have free, unlimited cooling in summer and heating in winter, why isn't everyone doing it?

'Initial investment, efficiency, space' and last not the least the 'payback period'.

For hydroponic growing I have seen root misting. It helps to avoid having the water go sour on you. All the nutrients are in the mist, and the roots are just hanging in the air. I guess the plant is supported by a screen. This way you are not misting the whole room.

As far as geothermal or any system, it's all about the calories. A drum of water at a certain temp has a certain amount of calories; to change that takes calories coming or going, and those changes follow thermodynamic rules. Look into BTUs and what it takes to cool the room, and the BTU capacity of various systems available.

Also look into state change. The real magic in thermodynamics happens at state change, like when water evaporates. Absorbing heat is one thing, but when state change happens, a lot of BTUs are on the move.

An interesting point to consider is a room with elevated CO2 will become a mosquito magnet. A commercial device here uses a small propane flame to make CO2, and a small fan to trap the mosquitoes in a screen trap. I believe Israel has used greenhouses with elevated CO2 to achieve remarkable growth. Keep in mind, elevated CO2 is risky for breathing.

I agree with mike, you need to analyse how much energy needs to be removed from the air and then go from there. However, I suggest using metric rather than imperial measurements. Here are the critical values you need to know normalized to volume rather than mass or moles as they are normally stated:

• Specific Heat Water ρ_water 4179.6 J.l.K°^-1 you will use this to calculate how much the temperature of the water will rise when you dump the energy from the room into the water.
• Specific Heat of Air ρ_air 1,297 J.m^-3K°^-1 you will use this to calculate how much energy needs to be removed.
• Volume of Air V_air 24 m³ again you will use this to calculate how much energy needs to be removed.
• Temperature Rise ΔT_air UNKNOWN this you will have to calculate or measure yourself and is critical in calculating the amount of energy that needs to be removed from the air.

If you get stuck trying to do the calculations the we can certainly help provided you supply all the information needed, in particular the temperature rise in the room on the hottest of days.

Indeed. That's what I was looking up in Wolfram Alpha to get the figure of 3000:1 water thermal mass vrs air.

I think tho the real critical values are going to be to do with the heat gain/loss of the growing space walls and between the water container and ground. A system like this probably has enough factors that the only real way to analyse its effectiveness is to build the thing and see what happens.

Which I think I might. I'm sidetracked at the moment with a solar project, but once that's done I might take a crack at this in terms of home aircon. I'm pretty sure I can get something going for about $20 in materials and maybe a week's work. How well it actually works remains to be seen...

You might want to try building a small say 2m³ meter greenhouse and seeing what sort of temperature rise you get. Once you have an idea of that the rest is fairly simple. I also like the idea of home air-conditioning using the concept. If you do go ahead with it would please post it in this thread as I am interested in what you ultimately find out.

You may also want to have a look at the Australian Bureau of Meteorology's Solar Radiation Map. The image below shows the chart for yesterday and shows that the amound of energy reaching the ground can vairy wildly.

It will give you an insight into how much solar energy is actually reaching the ground. You may then be able to take the results from a test structure and combine it with a known solar radiation intensity to calculate the sort of temperature gain you will get.

create shade by growing large shubs and trees around your house that will do the trick and save the wind turbine.

crm

I've been measuring the ground temperature here at a depth of 90 cm in shaded sand over sandstone.

Full sciency results here: http://urbangreenhouse.blogspot.com/2011/01/ground-temperature-data.html

but the short version is that seemingly regardless of air temperature (got between 21 and 35.5 C (96 F)) and time of day, the ground temp doesn't really budge from 21.5 C (probably cooler outside of mid summer).

So that's good news, most people have their air con set to 24-27 C, which is also about what most edible plants like.

Just because the temperature is 21.5°C doesn't mean that it will stay at that when you start dumping heat into it.

A more important test would be to find out how much heat you can sink into the surrounding ground without the temperature rising above a useful level.

What I suggest would be to burry a small say 20 litre container in the ground to the depth you intend to use then fill it with boiling water. Then as it cools take regular temperature readings of both the water in the container and the surrounding ground at say distances of 1, 10 and 50 cm from the container.

The figures will then give you an idea of the thermal conductivity of the soul as well as its capacity to absorb heat and ultimately allow you to calculate how large a buried container will be required to keep your greenhouse cool.

Yeah good call. Probing at various distances might be a bit of work with all the digging and monitoring, but I'll definitely measure how fast the container loses it's heat.

The cooling is kind of working on two basic principles, there's the heat loss to the soil, but also the thermal mass of the water itself.

I pulled a 20 litre oil tin out of a skip which by my calculations would contain an equivalent thermal mass to 60 cubic meters of air, or half the volume of the house.

So let's say the air temp is 35 C and the ground 21. There's twice as much thermal mass of air, so if it loses 5 degrees the water will pick up ten (after all the air has been cycled through), giving both the air and water a temp of 30 C.

Then it comes down to the rate of loss of heat from the water into the ground, and gain from the air into the house. But even if the water is only losing heat half as fast as it's gaining it from the air (a figure I'm completely making up) then both temps come down to 25. Which is perfect. Also the day would only be 35 for a few hours.

But of course this is all speculation and therefore not worth much. I'll be posting actual data here and to the blog as I get it.

One of the problems you are going to run into is the rate the energy is transferred into the gound which is proportional to the temperature differential and thermal conductivity of the medium that separates the source and sink.

With the buried metal drum/can the thickness of the metal is thin enough to ignore so that just leaves the temperature differential and thermal conductivity of the ground, that's why I suggested measuring the temperature at a range of distances from the buried drum/can.

So it goes like this, if you have water at 35°C that you wish to cool to 30°C buy dumping the energy into the soil that is at 21°C. This would give you an energy transfer rate of say X Joules per second and take Y seconds to achieve. However, the soil is going to heat up so let's assume that the ground temperature has now risen to 28°C which halves the temperature differential. Now the rate at which energy is going to be absorbed into the ground is only ½ X and it will take 2 Y seconds to achieve the target 30°C water temperature.

This is where things start to get complicated because the soul will definitely not stay at 21°C for very long when you start dumping heat into and it may rapidly reach a point where the energy transfer becomes ineffective.

Something you may wish to think about as an addition is a secondary radiator that could be used at night when the air temperature is low. You could then use this at night to get rid of any excess energy in the water by radiating it into the air. You could also use this to cool the ground surrounding your buried drum so that the next day you start with a fresh batch of cold ground rather than ground that is still warm from the previous days cooling. This would make things a little more complex but you can pick up a programmable logic controller for under US$100.00 that would be more than capable of controlling the flow of water and a couple of old car radiators would probably enough to use for the night time radiators.

What you say is true.

I've done a bit more calculation, figuring the volume of sand with the same thermal mass as 20 litres of water. Quartz sand has 2.6 times the density of water, but only about a 5th the heat capacity, so holds half as much heat by volume, giving 40 litres of sand.

This would mean the water would be balanced by a mass of sand extending about 8cm from the container surface. Which sounds reasonable.

The variable conductivity given the temperature gradient would of course still hold.

I did think of a way of using cooler night time air temperatures as a sink, and like you suggest the issue is selective interaction with the air. ie you want to lose heat to the night air, but not gain it from the day.

The easiest way I can think to do this is with a heat pipe, which only conducts in one direction and only kicks in at a settable temperature. All you need to do is draw a near vacuum in a sealed copper tube containing a small amount of water. If you drop the pressure to 0.03 atmospheres the water's boiling point drops to 24 C, which means as soon as the container temp is higher than that the pipe water starts boiling, and if the outside air temp is lower than 24 the steam condenses, drops its heat, and runs back down the pipe.

If the tank is cooler or the air warmer, nothing gets conducted. The kick in temp is set by the pressure, which can be fine tuned with a vacuum pump and a pressure gauge.

This kind of design would be ideal for places where the night time temp is lower than the ground temp. It's a little trickier to construct, but not by much.

That sounds like an interesting concept, using the latent heat of vaporization of water to conduct heat away from the greenhouse. However, I'm not that sure how easy it is going to be to set up but I would be interested to know what happens.

Personally I'm a control systems engineer by trade so I would err on the use of something like a Programmable Logic Controller and a couple of two way valves that can be used to reverse the flow of water plus a couple of old car radiators that could be used at night to cool the water you are using as the heat sink. The good thing about using PLCs is that you can reprogram than to do just about anything so if your system evolves over time you can reprogram the PLC to allow for the changes, but that's just me and my control system background.

Anyway, I'm very interested in your results so please post them here.

That approach would yield good results, yes.

I have a personal fetish for extreme ease of construction, scrap materials, and no electrics. The things I'm building aren't so much for my own use, I'm trying to generate design approaches which can be easily replicated by pretty much anyone anywhere, particularly the developing world.

Results posted as they come in, and may well do an instructable on this if it turns out.

Can you move 3000 liters of air per minute through the system? I am guessing that is how much air you will need to move to keep the greenhouse at a comfortable level.

A vacuum cleaner pump will apparently do about 3000 litres a minute, tho that's not usually through several meters of garden hose and a barrel of water...

The airflow needed would depend on a bunch of factors, not sure I could easily put a figure on it, even knowing the volume of the space. But 3000 l/m sounds a bit high...

You have three heat transfer problems to figure out:

1 Heat transfer from ambient temperature to the greenhouse. This is a combination of conduction transfer through the wall plus the radiation effect of the sun on the walls and through the glass.

2 Heat transfer from the circulated air into the water. For better heat transfer, small bubbles are preferred to large bubbles. Sewer plants use diffusers for O2 transfer into waste water, but a low tech solution would be shower heads or small holes in a pipe to have an averaage bubble size of 1/8" (2mm) or smaller.

3 Heat transfer from the water tank into the ground. Large surface area between water and soil. Geothermal systems use long coils of buried pipe instead tanks to have a larger surface area to reject heat and to expand the area of heat rejection so you do not create a localized hot spot.

My 3000 liter/minute (100 cubic feet per minute) air flow is a guess based on other ventilation experience and my unfamiliarity with metric conversions. I think with this air flow rate through a buried tank at 22C and an outdoor temperature of 35C, you could probably maintain the greenhouse under 30C.

What you say is true.

1. This is something that will just need to come out in the wash. I'll hopefully be building a greenhouse based on this design: http://urbangreenhouse.blogspot.com/2010/11/here-is-first-basic-design-for.html in the next couple months. I could attempt to calculate the heat in and out, but there's just too many factors, so I'll just make it and see what happens. I'll be wanting to keep out about 70% of the light tho, as this is around the optimum for plant growth.

It's also going to be completely determined by local conditions. I'll be doing this in the Nelson bays, which has lots of sunlight but not terribly high air temperatures.

But as the prototype is going into a house, this probably won't really be an issue for now.

2. I'll probably just stretch a bit of nylon stocking or something over the end of the pipe, so the air comes out frothy.

3. That's kind of the point of this system; that the heat is absorbed by the water during the day, rather than the soil. Most of the transfer out of the water would happen overnight, when there's plenty of time to do so. If the water is getting saturated, just use more.

Regarding air flow, what air volume would 3000 l/m be for? Since this design would cost practically nothing to build, multiple implementations could be running in the one space, until enough air was being pumped.

In a setting where people are involved, you would change out the entire volume of air 3 times or so per hour

I think one of the big questions is how are you going to let the light in & keep the heat out?

Surely the required air flow rate is arbitrary? Most greenhouses have no cooling systems at all, other than ventilation and shade, and do fine. This would be to optimise growing conditions and protect against hot/cold spikes, so surely whatever you can get out of the thing is a bonus, rather than a set rate.

If you see what I mean.

This all started with me looking at materials which filter out IR while letting through white light. They do exist but for various reasons are a little impractical. So rather than try to filter the light I'll probably aim to exclude most of it (speckled white paint? Not completely sure at this stage) and then cool the air. Most plants grow best with about 300 watts per square meter, or about 30% of full.

The air flow rate would depend on the number & type of plants & being able to keep the co2 levels optimum [high]. So you have this conflict when the outside temp is high, [which is the point of the design] the need to limit air from outside the building so the cooler will be effective & the need to keep the exchange of vital gases flowing...

you are in effect trying to bodge together an Evaporative Cooler & hoping that the earth will keep the temperature of the water low enough to increase the effect. Thing is most of the cooling comes from the phase change of the water

have you had a good look at what a traditional greenhouse does? Are there any in the area? The number of greenhouses in the area is probably a good indication of the economic viability of the concept

Don't reinvent the wheel unless you have to :D

CO2 is one of the advantages of having a completely enclosed growing space, in that you can increase it through growing mushrooms, composting, integrated animal farming etc.

It's not really an evaporative cooler, the energy flow will probably be almost entirely conductive. (I think I might be making these terms up...) The water may evaporate a little, but probably not much at 24 degrees C.

There are no greenhouses anywhere here because it's too hot...

Why would you need cooling at 24c?

you must mean 42?

I mean the ground water will be 24. It will obviously be heated by the airflow, but the point would be that this minimised as much as possible, by using more water.

http://urbangreenhouse.blogspot.com/2011/03/testing-which-diffuser-has-smallest.html

Quick and dirty comparison of bubble size and flow rate with a couple of diffuser options.

I think what you need to do is break up the problem into three parts:

• Earth Absorption Capacity: Basically this is how much energy and how fast it can be transferred into the earth surrounding you're subterranean storage tank .
• Air to Water Transfer Rate: This is a measure of how much energy and how fast it can be transferred from the hot air into the water which is the intermediate energy storage medium.
• Greenhouse Excess Energy: This is a measure of how much excess/deficit energy there is in the air of the greenhouse and will ultimately govern the design of the entire system.

However, you have to go about things in reverse. First you need to work out how much energy the soil can absorb then once you have that you can work out how to transfer the energy from the air to the water and then finally when you have those worked out you can use the information to design a system that can sink the excess energy from the greenhouse.

If I were doing it the first thing I would do would be to set up a few experiments as follows:

• Earth Absorption Capacity: To work this out I would take a smaller metal container like the one you intend to use and bury it in the ground . Then fill it with water and stick in an immersion heater that you know the output of to a reasonable degree of accuracy. You would then need to monitor the temperature of the water in the tank over at least 24 hours and preferably longer, in fact the longer the better. Provided your emersion heather doesn't boil the water in your container the figures from this experiment will give you an empirical idea of how much energy can be absorbed by the earth an ultimately the size and capacity of the heat sink and transfer system
• Air to Water Transfer: The next experiment would be similar to that in the video link that you provided except using hot air of a known temperature, humidity and volume then playing about with it till you can maximize the rate you can heat to the water. This will ultimately govern the design of the air to water heat transfer system.
• Greenhouse Energy: Finally I would build a small version of your greenhouse and again measure the temperature inside the structure over a period of several days. This will ultimately give you the energy input that your system will need to sink and thus the size and flow rates of the rest of the system.

It's like any engineering problem, you need to break it down into workable chunks then experiment with each of these independently until you have a good working understanding of what's going on. You can then put these together to give you your final design and be fairly confident that it will work.

There is one engineering law that's worth mentioning when you do scaled experiments and that's the square cube law.

Let us for the moment look at how this would affect the characteristics of the subterranean tank.

If you were to double the size of the tanks dimensions you would effectively be doubling the surface area of the tank and therefore doubling the rate at which it could transfer energy into the surrounding soil. However, you would be increasing the volume of water in the tank by a factor of four fold which means that you effectively have twice the intermediate energy storage capacity or looking at if the other way half the rate at which you can transfer the energy from the water to the earth.

Now the square cube law is going to crop up numerous times during the design of this cooling system, especially if you are going to try the scaled experiments I recommended. I would therefore highly recommend developing a good understanding of the square cube law because if you don't it really easy to suddenly find that you figures are out by a factor of two and that can really wreck a project.

Good call.

> take a smaller metal container like the one you intend to use and bury it

I'm going to fill the 20 l drum (which I buried yesterday) and take periodic temperature readings. I'll be heating the water on the stove and it'll take about 3 trips to fill it, so the water will cool some before I can start measuring, but it should still be quite a bit hotter than the target working range.

> using hot air of a known temperature, humidity and volume

That would be the proper way of doing it, yes. I only have a week left here, I think I'll just plug this into the house and measure the in and out air temperatures. It won't give me full play-withability, but should yield some useful data. When I get a chance I'll start playing properly with different diffusers etc.

> build a small version of your greenhouse

The plan is to do a full size one when I'm back in New Zealand, if I can find some people interested. It's not super hot there, so the cooler probably won't be essential (tho they do get frosts in winter), I can measure heat build up tho.

> the square cube law

Yes and no. I've done some quick volume analysis in my cad package and it's a little more complicated than that. It's not really surface area, but volume of earth at a set radius.

For example, I made a simple cylinder with 20 litres volume, then a larger cylinder (with bevelled edges, for accuracy) offset from the smaller by 4 cm (which is an arbitrary amount, but gives about the same volume of soil.)

inner vol 20 litres
inner surface area 4140 cm2
outer volume (soil within 4 cm) 21 L

I then doubled the height of the inner to double the volume, and made another outer still with an offset of 4 cm:

inner vol 40 L
inner surface 7040 cm2
outer vol 35 L

Then the same, but doubled the original on all axis, and re-offset by 4cm:

inner vol 160 L
inner surface 16600 cm2
outer vol 77 L

(Rounded to 3 s.f.)

So, comparing the first and the last, 8 times the volume (cubed), but the ratio of water to available soil (assuming 4 cm is the distance the heat will travel, could be anything in reality) has gone from about 1:1 to about 2:1.

So 8 times the water volume only effectively halves the soil volume. Therefore it does get harder to sink the heat with larger containers, but is a long way off the square cube relationship.

And doubling the volume on one axis makes almost no difference to the ratio at all.

Unless I have this horribly wrong, which is entirely possible.

Just realised how obviously dumb that whole section on the square cube relationship is. Please ignore everything I said after 'yes and no'. That'd be great.

The square cube law only applies when you scale things up in three dimensions not just a single dimension like you did by doubling the length. In that case you are increasing the surface area by slightly less than two while doubling the volume.

An example of where the square cube law does apply would be in the load bearing columns of a building. If you wanted to make a building that was twice as big in all directions then you would be increasing the volume and therefore mass of material in the building by a factor of 8. However, the cross sectional area of the vertical load bearing columns would only have increased by a factor of 4 so in effect you would have double the load the columns would have to take and this is where you can get caught.

One thing that worries me about your experiment with the hot water in the can is that you are not going to know the absolute capacity of the soil to absorb energy, just a rate of transfer over a range of temperatures which shouldn't be too difficult to calculate because the rate of transfer in this case is going to be proportional to the temperature difference between the soil and water. You may be able to get some sort of idea of the absolute limit the soil can absorb by monitoring both the water and soil temperature at a range of distances and depths from the buried container, but it's not going to give you the full picture.

The thing that worries me is the absolute capacity of the surrounding soil to transfer energy to its surrounding environment and if you keep pumping in heat sooner or later you are going to reach a point where the soul is not going to be able to absorb any more energy and your water temperature will start rising. This is where I think you are going to run into problems as it's where geothermal power plants have in the past. I know they are working in reverse but they have had problems because they have depleted the energy stored in the rock faster than it can be replaced from the surrounding rock and ultimately the have shut down due to lack of return. However, I do remember where an abandoned geothermal power station was brought back to life by fracturing the rock around the wells thus dramatically increasing the area they were collecting energy from.

I would highly recommend getting some sort of immersion heater and a temperature probe that you can use not only to monitor the temperature of the water in the container but the surrounding soil to a reasonable depth. What I would expect you to see is a steady climb in the temperature of the water in the container and surrounding soil until you reach the souls limit to absorb energy at which point the temperature in the container is going to start rising more rapidly. This is critical as it will give you the numbers you need to know if just a buried drum is going to be capable of sinking all the energy you need on a permanent basis or whether you are going to need to add some sort of sub‑soul piping to increase the area of transfer.

If you don't you run the risk of having something like the geothermal power stations that works well initially but slowly and steadily loses efficiency as the temperature of the surrounding soil rises over a period of anything from weeks to years.

Don't get me wrong, I think you have a workable idea here but you may have to lay a grid of polly pipes buried about 0.6 to 1 metres under the ground to ultimately have enough surface area to not reach the limit of the soils capacity to absorb energy and ultimately transfer it to the surrounding subterranean environment.

Of course you could always just suck it and see which considering that the whole idea is somewhat of an experiment isn't out of the question and you could always add a subterranean grid of polly pipe later if it's needed.

It's up to you but I'm the sort of pedantic engineer that likes to get it right the first time. That's because coming back to retrofit something can not only cost a lot more than installing it at the beginning but can also damage your reputation as an engineer.

Yes a roll of drip tubing buried is going to work much better than a 20 liter [5gallon] bucket, much more surface area, much bigger thermal sink.

what kind of heat exchanger would you use in the green house?

> The square cube law only applies when you scale things up in three dimensions

Yep. I just got a bit confused and failed to observe that that's exactly what's going on here.

But then I guess my next potentially naive question would be, is this actually working against me?

If I double a container on all axis I end up with 8 times the volume, but it's still the same amount of energy going in, so the temperature rise is an eighth as much, and I then have 4 times the surface area/soil volume to put it out into...
no?

> a point where the soul is not going to be able to absorb any more energy

Yes. But overnight at the least there'll be no extra heat coming in, and quite possibly it'll start sucking air that's cooler than the general soil temp. I'm assuming, depending on soil type etc, that heat will generally build up during the day, but there'll be enough time overnight to shed it all again. This clearly remains to be seen.

Another option is to have a switchable valve, so that when the air starts to cool down, rather than drawing in warm air from the top of the greenhouse or home, it sucks in cool outside air, which will tap off some of the heat from the water.

> I would highly recommend getting some sort of immersion heater

That'd be nice to have at some point, but is not currently available. I'll hopefully be able to get similar data just running the thing, measuring input and output air temperatures.

> you may have to lay a grid of polly pipes

Starting to get back towards existing technologies. Obviously they're that way because they actually work, but most of the point of this here is that it'd be vastly simpler to build and implement.

> get it right the first time.

meh. This not being my main project (it's about third) I'm happy to potter away at it in various forms over the next wee while. And seeing as I'm leaving here in a week and have my god so many things to get done in that time, it's going to have to be a bit slapdash.

But it's good to know how I should be proceeding, so I can at least vaguely aim in that direction.

> what kind of heat exchanger would you use in the green house?

None, just circulating the air itself.

the sq law works against you as you end up with 8 times the volume & only twice the surface area you won't be able to transfer significant amounts of heat

in the end the phase change [evaporation] of the water will have the biggest effect

Ah, didn't think about frosts.

This system would also protect against frosts.

That's probably worth mentioning.

RESULTS!

Actually got all this last week, but most of it was being gathered on the day I was flying out of Australia, so was gathered while running round packing, and this is the first chance I've had to get it down properly.

I did two basic experiments.

1.

Filled the 20 litre drum (30 cm tall, deepest point 1.1 m, shallowest 80 cm, in dry sand over sandstone) with hot water. This was in the form of 3 x 7 litres bought to boiling on the stove and poured into the tank through the connector hose. Should've measured the water temp at start and end of filling (took a minute or two to get it through the funnel) but I'm going to assume it was about 90 C. The ground temp was around 24.5 C.

So:

First 7 L volume at 9.40 am, second at 9.57, third at 10.12. Measured tank temp at 10.15 and was 77 C. This is a total of just on 5.75 mega joules.

Full data:

time water temp (C)

10:15 77
10:30 74
10:50 70
11:05 67
11:20 65
11:30 64
11:45 63
12:00 60
12:15 59
12:35 57
12:45 56
13:00 56
13:40 53
14:00 52
14:30 50
15:00 48
15:35 46
16:35 43
17:00 43
17:45 41
18:45 39
20:00 38
21:00 36
23:20 34

00:15 33
08:20 29
11:25 28
13:15 28
14:00 28
16:00 27
17:15 27
18:30 27
23:35 26

08:45 25

Graph!

And 2:

More relevant to the actual cooling system, I plugged the whole thing together and ran it for about 8 hours. The setup was: air being sucked from the top of the house through about 9 meters of 18mm garden hose into the air pump housing, which was a vacuum cleaner pump and motor inside a tupperware container with the attached electronics.

This meant the air was passed over the motor itself, which added a lot of heat to it. Dumb for a proper installation, but good for fully testing the system.

Air flow was about 2 litres / second, in that it took 20 seconds to fill a 40 litre bag.

The air then passed through 3 m hose, bubbled through about 30 cm vertical of water, and exited into the house through another 3 meters hose.

..........Air In....Air Out.....Air House
11:15.......50......24...........30
11:30.......52......24.5........30
12:05.......54......24.5........31
12:40.......53.5....24..........32
13:25.......55.5....24..........29
14:40.......50......24.5........28
15:20.......49......24.5........27
16:20.......46......25...........26
17:05.......48......24.5........26
19:10.......45.5....24.5........25

'Air in' being the temp coming out of the hose from the pump, ie just before it enters the tank. Pretty high due to the motor, the general air temp was about 34-36.

Air out is the air coming directly out of the tank.

Air house is the what came out into the house. At about 13:00 I wrapped the whole last length of hose in insulant and moved it out of the sun. I was surprised by how much heat the air picked up through only a couple meters of rubber hose, but there you go.

Sorry for the delay in replying but things have been a bit hectic here over the last week.

The results of the first experiment are pretty much what I would have expected to see and if you know the surface area of the water that was in contact with the walls of the container you should be able to work out the thermal conductivity of the system which you would then be able to use to calculate how much and fast the heat could be dumped into the surrounding earth.

However, in experiment 2 there is a major problem and that is the humidity of the air returning to the house from the water container. What you have there is a pretty good evaporative cooler rather than heat exchanger and you need to take into account the enthalpy of the air going into the container with the enthalpy of the air coming out to get an idea of the amount of energy being transferred. Otherwise you are not comparing apples with apples.

What happens when you bubble hot air through cold water is the water evaporates into the air and cools it due to the latent heat of vaporization but raises the relative humidity of the air in the process. In a closed system like yours this is ultimately going to cause a problem because the air will soon become saturated with water and you will end up with much less heat transfer than in your experiment .

Actually thinking about it evaporation of the water into the air as it's bubbled through is going to be a major problem. In the beginning you will get what appears to be really effective cooling, that's till the air that is being recycled becomes saturated with water and then and only then will you start transferring heat into the water. This is going to be a big problem because plants need to be able to evaporate water from their leaves so while they like humid climates they can't cope with a climate that has a relative humidity of 100% all the time. You will also have problem with condensation overnight when the outside air temperature drops, not to mention building the perfect environment for the growth of moulds and or bacteria.

However, I think there is a solution and that is something akin to the way a boiler or heat exchanger works. Basically you have container of water that has lots of pipes running through it. You then blow the air through these pipes and transfer the heat energy to the water which is then both temporarily stored in the water and transferred to the surrounding earth whenever the soil temperature is lower than the water temperature. This way the water never comes in direct contact with the air so you don't have a problem with evaporation. In a low pressure system like this copper would be the best material to construct both the heat exchanger and buried container because it has both a very good thermal conductivity and resistance to corrosion

It's going to make the system somewhat more complex and it may be simpler to just lay a grid of polly pipes in the ground that you blow the air through directly, but if you have the air coming into direct contact with the water you are going to have evaporation and while that will cool the air initially in a closed system it is going to have problems with air that is saturated with water.

I must apologize for not thinking of this earlier as it's going to have a major effect on the design of your system, but I still think experiment 1 shows that you have the basics of a workable system. You're just going to have to work out a different way of transferring the energy in the air to the water as bubbling it through isn't going to work.

Quite possibly, yes, but I'll need to measure the humidity levels to be sure.

It also seems possible that the warm air will cause some evaporation, but as that then gets remixed with the still quite cold water an amount of it will recondense, dropping the heat and vapour into the water.

Keep in mind that the 50-55 C air was a result of the motor, and in a real setting you'd probably be looking about mid thirties at most, and cooling during the day as the system takes effect.

But I'll have to monitor the thing in action and see what actually happens.

If possible, can we move this over to the thread at http://cr4.globalspec.com/thread/67521 ? Just so it's in one place.

So I guess the two most striking things are that even with a fairly basic interface between the air and water (vague attempt at a diffuser, not really) the air dropped all it's heat, even at 55 C.

And that even with only 20 litres of water, which was only ever meant as a quick test, the water temp didn't really come up over the ground temp. True that 2 litres of air per second isn't much (I could've turned the motor up a lot more, but didn't want to 'splode it) but at an average of 50 C, the air was twice the temp above 24 that it would probably usually be running, so is therefor the same energy as 37 C at 4 litres per second, which is quick enough to replace 50 cubic metes of air in about 3.5 hours.

Which aint bad, frankly. It aint bad at all...

Thoughts?

Could;d be these links might be of interest to you.

http://www.infolink.com.au/c/SOLAIR/Solarventi-Solar-Panels-from-Solair-n867100

http://solarventi.com/