Burying a Water Container for Ground Cooling System, Can I Use Plastic?

I don't think it will matter, since the heat will only move into the surrounding soil, as fast as the surrounding soil can dissipate it...regardless of the vessel material.

However, is moving heat is a primary concern, you may want to consider several smaller containers, (in series), rather than one large one.

Any junkyards around?

I'm thinking old car radiators. Corrosion resistant, and made to dissipate heat.

I remember this guy from a few months ago, its a Rube Goldberg set up, but, you made a good point. he also has to consider temp at various soil depths, and how fast the soil surrounding his heat exchanger will dissipate heat. the design blows , his exchanger will rapidly equalize in shallow depths. depending on soil temps he would need an exchanger roughly the size of the space he's attempting to condition, so a gallon of exchanger wont cool a 10x10 room

Yeah. I guess I'm missing something, since he's indicating that the corroded drum worked, before it corroded.

All I keep seeing is a buried drum full of hot water.

A radiator would be particularly effective, yes, but I'm looking to use the most basic materials possible, so this thing can be of most use in developing world type scenarios. There's more water bottles than radiators, and they're a lot cheaper.

Good idea tho.

0.51 k - (W/mK) for plastic and and 43 k - (W/mK).So, the answer is LOTS, and lots!

Thermal conductivity myths and facts: Copper, plastic, air, and water |

As you suspect, the efficiency will be very poor.

The paper has ONLY one flaw heat is NOT conducted to air since air is not still and a convection is generated by the density differences. This natural convection was totally forgotten by the author. What counts is not only the conductivity but the THERMAL resistance which is proportional to thickness/(area*conductivity) and this has to be compared with convection. In general convection is the highest resistance so that the stuff for a thin wall duct plays a totaly secondary role.

For instance a ss wall with 16 w/m°K thick 1mm has a resistance per sqm = 1e-3/16 = 6.25 E-5 °K/sqm/W. Convection (natural) is in the range between 8 and 15 W/sqm°K so tha the resistance in °K/sqmW= 1/15= 6.67E-2. It is 1067x higher!

If cooper is used it will affect only the smallest part so that.....

Heat transfer is a tricky technology.

the steel will give up a LOT more

How?

I understand that steel is a better conductor of heat, but how would steel move the heat away, faster than the surrounding earth can absorb it?

Since everything is buried, it seems like the critical factor is how fast the soil can remove heat, not the material of the vessel.

Soil and water are given. It's the transfer rate of the container that governs the amount of heat removed from the water into the dirt.

Yes, denser, wetter dirt will help.

Are you bubbling the air through the water inside the container? Is that where the heat exchange comes from?

the buubling is carrying heat (heated air) from the warm space. than transferring that heat to the water (bong) , the bubbling water transfers heat to the walls and in the case of steel creates an evaporative and splashing (distributing water) on the walls to give its heat up to the walls

Ok, I guess I missed that part.

How about some real numbers for us to work with?

How big is the space you are trying to heat/cool and what is the temperature differential you are tying to reach between inside and outside temperatures?

What is your ambient ground temp?

How much electrical power are you using to run the system?

WIthout this info no one here can make any educated level of guess as to what you are trying to accomplish and to whether your design comes anywhere close to meeting said goal.

> How big is the space you are trying to heat/cool

In this case it's a smallish hut, about 4m x 5m x ~3m high.

> what is the temperature differential you are tying to reach between inside and outside temperatures?

It's going to average mid thirties outside in summer, I guess it'd be nice to hit low 20s inside.

> What is your ambient ground temp?

Haven't had a chance to measure it yet, I'll be digging holes next couple days. The general average a meter deep most places most times of year should be 10-16C.

> How much electrical power are you using to run the system?

I've got a 120w pv panel, but it should only take 7-9 watts of mechanical power to put 3 litres of air per second through the 20cm water column. It really comes down to efficiencies of motor and pump.

> WIthout this info no one here can make any educated level of guess as to what you are trying to accomplish and to whether your design comes anywhere close to meeting said goal.

Like I say, I'm really just looking for the thermal properties of the materials in question. Whether it works or not is a matter for direct experimentation.

GA. Lots of surface area is a more attractive quality than the thermal conductivity of the medium. The least desirable shape is a sphere, having the lowest specific surface and a cylindrical drum isn't far off it.

Bubbling air through the water and spalshing may create some evaporative cooling, but essentially this will be maore a humidifier than a cooler.

The dry air from the "room" will become saturated and then there will be no additional effect.

You would be better building a simple evaporative air conditioner (Coolgardie safe style) than this. [If you are in Aus outback, then you will understand Coolgardie safe.]

Some old homesteads still have their evaporators, built with timber and charcoal blocks to increase surface area.

Also consider, if you have any reasonable water source (whether drinkable or not) then submerged pipe in there will provide good heat exchange, just like they do to pre-cool the artesian water before running it into the poly lines around the properties.

From the initial test results in the link I posted:

http://urbangreenhouse.blogspot.com.au/2011_03_01_archive.html

it's clear that there is a cooling effect. Humidification may well be an issue, I'll need to test it to find out. The system wasn't working at the point the tank had corroded, for obvious reasons, but the vacuum cleaner motor had failed by that point anyway.

Let's not worry about whether the thing will work or not, that's for me to test and I'll post more results to the forum as I have them. I just need to know the numbers for whether 1.5mm of polycarb will significantly impede the flow of heat out of the drum relative to 1mm of steel. Sand on the outside, water inside (the tank is pretty much full, so it's not splashing so much as just sitting there). Both at let's say 16C.

Yes, it will! Plastic is 100 times less effective at transferring heat than steel!

Read the website I posted earlier.

0.51- (W/mK) for plastic and and 43- (W/mK) for steel. So, the answer is LOTS, and lots less.

Notice where the decimal point is.

Yes, but how much is that in real terms when the materials are only a mm thick?

Exactly!

It won't matter what the material is, as long as you surround it with an insulator, (dirt), you will only get what the dirt will give you....or take away, as the case may be.

You can make it out of anything, and you will notice little difference, because it is the dirt that is, (supposedly), doing the cooling.

Well, if it was 2cm of polystyrene or similar it would make a fair difference, but I suspect you're right that at the mm scale it really all comes down to the other factors in the system.

But an actual number would make me more confident in going ahead with the water bottle. I could work it out, but no idea what formula to use.

I like your sig, btw.

Fredski, you're now being aggressive to the point of facile name calling. Please stop. CR4 has always had a nice vibe, and it's not appreciated being called stupid for asking a simple material properties question.

this is not aggression, I think your plan is a joke if you're going to waste our time you'll need thicker skin than you're currently displaying

If you feel that following this thread is a waste of your time, there's an obvious solution for that...

Maybe some other contributors are not a waste of time.

The formula I would use, would be to bury a small plastic water bottle, and stick a thermometer in it...or just do it full scale with the large water bottle. It will either work, or it won't, and regardless of formulas, you're going to have to test it anyway.

With a thin walled material, I think you will find the difference between steel and plastic, negligible. It's the dirt that's acting as your heat sink.

The one potential problem, (and I don't know how deep these are buried), is the weight of the dirt, collapsing the water bottle.

A side by side comparison would give the most comprehensive answer, yes, especially given that the bottle has more surface area per volume with the handle bit. But I'm a little pressed for time.

I'd be surprised if it collapsed, they're pretty robust things.

Well I think to work within the parameters you have set here, I would try to find a dozen or so of 10' sections of pvc(preferably 12">), and bury them in the ground vertically a few meters apart...The temperature difference of 5-15ºC is not a lot to work with, so a large supply of cool water is necessary because the temperature difference is quickly compromised....maybe start with one and keep cloning the system until satisfactory results are achieved....The average ground temp should be around 15ºC(59ºF), so the wider the temperature difference, the more your efforts will be noticed....

this is the design he's working with

Oh, boy. Bacteria are going to love that!

looks like a high school science project to grow slime

That would be more effective, but as the data from the initial prototype shows, even with just a 20 liter tank in 24.5 C sand, and not even at a full meter depth, the water didn't increase by more than a half degree after eight hours of constant use.

I wasn't expecting it to be that effective, to be honest.

That was in pure sand tho, which is quite a bit more thermally conductive than other kinds of soil, I'll have to test it in other situations to see what the dynamic is.

So optimisation isn't my main concern at the moment, more how to make the thing as simple to construct and implement as possible, and solving issues such as corrosion.

The greater the temperature difference between the makeup air and the cool water sump, the greater the rate of heat transfer....Your model does not appear to have been testing the actual working temperatures expected here....I would anticipate a greater transfer of heat for a similar time period....To maintain the sump temperature as low as possible will require a larger quantity of water storage....this is all dependent on the heat gain experienced in the space you are attempting to cool...If you could determine the heat load of the building, you could then determine the cooling necessary to overcome it....If you for instance have a 10k btu heat load, you can figure out what amount of cooling is required by the methods you are using, and save a lot of time possibly by eliminating your trial and error methods....determining cfm and temperature drop over a defined period of time will give you the btu's being produced.... Btus/hr = CFM X 1.08 X temperature rise.

> Your model does not appear to have been testing the actual working temperatures expected here

The only numbers I have so far are from that first prototype, in which the vac motor was significantly heating the air; ~55c rather than the ambient temp of about 38C, from memory. So it was doing about twice the heat load that it otherwise would. The current version uses a motor+pump which won't add any heat to the air.

> I would anticipate a greater transfer of heat for a similar time period

How do you mean?

> To maintain the sump temperature as low as possible will require a larger quantity of water storage

More water will certainly help, but given that the water temp didn't rise during the day hardly at all, it seems the soil is doing pretty much all the heat shedding, and the amount in question, to surround a 20l cylindrical drum, seems to be enough.

> If you could determine the heat load of the building, you could then determine the cooling necessary to overcome it

Yes, but given that this is an open source design which will have to be as helpful as possible to as many people as possible, there's not going to be any one particular building in question.

Basically as long as the thing is reasonably effective overall, I'll just take what cooling I can get. So far I'm happy with how effective it's turned out to be.

For my take on this going with the cheap as possible concept in mind would be to use a number of parallel garden hoses buried as deep as you can get them on a spacing of 1 or so meters apart for the ground source heat sink system.

That would give you the most exposed surface area for the least amount of water being circulated.

Inside I would use a simple used radiator or other similar tube and fin type heat exchanger system with a small fan and pump to run the system.

If you have the power and skills to spare I would look at converting a old dehumidifier to work as a simple ground source air conditioner unit by circulating the water returning from the buried hoses over the hot side of the heat pump system before pumping it back into the ground loop.

If done right you would only need the dehumidifier and its one fan for the cooling and heat pump system and a small pump to move the water for the other side taking the heat away.

By the numbers this would give you the greatest amount of heat transfer and hot to cold side temperature differential for the least amount of energy being used.

That's my take on how to make this work using second hand/cheap materials and scavengeable items.

Fair call, but still a bit above the level of resource I'm aiming for. I generally try to only use stuff which can be found in scrap heaps and corner stores, and costing less then $20 or so.

Old dehumidifiers etc aren't what you'd call rare, but they're also not everywhere, especially in the developing world.

With regards to using hose instead of a drum, for practicality reasons I'd like to keep the system an open loop and just cycle the air, with the water staying in place. Garden hose would be narrow enough that the water would probably get pumped into the room. It'd also greatly reduce the rate of air flow.

Using a fatter hose, such as 40mm corrugated, would work better and I'll give it some thought, but I'd still be concerned about the water getting raised at least enough that it'd no longer be a meter deep where the cooler soil is.

The only way to get any level of dependable heat transfer from the ground is to have a large surface areas exposed to as much earth as possible. A single bucket is a large mass with a small relative surface area and small relative volume of exposed earth to dump its heat into.

A garden hose however has a large surface area that is exposed to a large relative mass of earth for the small volume of water it holds yielding maximum heat transfer for the least volume of water being used.

As far as heat energy transfer goes using a simple liquid to air heat exchanger (radiator and small fan) to transfer the heat from the room back to the cooled water which is working as the primary heat transfer fluid between the room and the ground rather than air is by far the most efficient and effective way to go.

I'll make one more attempt here.

Let's assume that you have a 1 mm thick plastic jug holding your water.

An equivalent thickness of steel jug would be 100 mm.

<unsubscribe>

Cool, I'll consider the question effectively answered.

Next stop, finding a source of water cooler bottles in Alice Springs.

Cheers.

Sort of true but you have to factor in localized thermal conductivity of the ground as well which even with a steel container doesn't transfer heat all that much better once the first few inches are thermally saturated. Typical dry earth only has a thermal conductivity of 1.5 which is about 3 - 4 times that of typical HD plastics that are used for water lines and 1/10 that of steel.

Thermal conductivity values of common materials.

Apparently despite its less than optimum heat transfer characteristics HD plastics are still used in ground loops for geothermal everywhere so obviously it works for transferring heat to or from an earth based heat sink/source.

That and a tank system just does not offer the surface area advantage that a hose can.

Say I have a buried tank 1 foot in diameter and 2 feet tall. By volume it has 11.75 gallons of water in it and a total surface area of 1131 square inches.

Now compare that to a common 3/4" 100 foot long garden hose. It holds 2.3 gallons and has a surface area of 3770 square inches. It has 20% of the volume but 3 1/3 more exposed surface area to transfer heat with.

Leaving the technical answers to those who have already made their contributions, there are several free or very inexpensive sources for containers. If you contact users of: poly drums, 5 - 55 gallon preferably closed head; poly pails with lids, 1-10 gal; plastic carboys, 2-55 gal; and other plastic containers you could develop a source for them. On some there might be a deposit or demurrage charge from the supplier so avoid them. Also those that were used for food products are also easier to clean enough to use. If only chemical containers are available clean them thoroughly and make sure any small residual will not harm your other process equipment.

Look for the cheapest source with the highest quality containers in the volume range you want.

Good Luck, Old Salt

I would bury a long length, or several lengths connected in parallel, of underfloor heating pipe, which is made of a special plastic to allow the hot water to heat the flooring....

I have no idea if its available where you are, or what it costs, but it should resist corrosion far better than most metals.....

As the ground appears to be possibly corrosive, I resisted in mentioning either stainless steel or copper piping.....though both are available here with a thin plastic coat to protect from corrosion when underground! I have no idea as to what the heat transfer is like as they are mainly used for underground gas supply on private properties....

Radiator inhibitor will stop the drum rusting. Castrol do small tins of concentrate that fall inside your budget range.

If you insist on using scrap, look at a way of joining several cool drink cans together, even using plastic. They will eventually corrode but then they are easily sourced. I pay a lot more for water than cola and you say you will use water bottles so I am guessing that there will be cola cans available in the same rubbish heap.

Jim

out of interest, why are you even bothering to run the air through a drum of water?

If the below ground temperature is low enough, which it must be as it is keeping the water in the drum cool, why don't you just run the air through about 30 metres of garden hose buried deep enough in the ground to cool the air.

Hot air will go in from the room, pass through the pump into the ground, cool and then pass back into the room. No need for drums at all.

Much smaller hole to dig.

So what you are saying is you want to half ass the whole project and get little to no measurable returns due to not wanting to put for the effort involved to build it in a way that would actually work and be supportable by any basic level of applied engineering?

Personally if I was you I would just put a small window fan blowing air in from outside on the shaded side of the building and call it good.

I think I am going to go and join Lyn on the CR4 park bench making snarky remarks about whomever walks by. I feel like my input will be more constructive that way.

LOL!!

Sadly also true.

Well put my friend!!

@jimrat

How long does radiator inhibitor last? Does it fume? Might be a possibility... Would I assume keep the water sterile as well.

Drinks cans would be good in terms of high surface area and more conductive again than steel. 15 cans would have about the same surface area as a 20l drum, but a quarter the volume.

Getting 15 cans wouldn't necessarily be the hardest thing in the world, but attaching them all in such a way as they didn't leak would be a bit tricky, compared to just plugging two bits of hose into a drum.

@old salt

I'll look into some of those, cheers.

@tcmtech

Like I say, hose would be more effective and just as easy, but would likely result in the water getting pumped out of it by the airflow.

sugarandfat-

The nice thing about most of the containers mentioned is that most of them have either or both 3/4"npt breather vent holes and 2"npt drain bungs with break-out 3/4"npt plug holes to insert tube-to-hose adapters into. With these features it is easier and less technical to connect the adjacent containers to each other.

Good Luck, Old Salt

Plastic is a very poor conductor of heat. I wouldn't think it would be a very good choice. I would look into treating the drums to prevent rust with a good coat of paint.

Re: Burying a Water Container for Ground Cooling System, Can I Use Plastic?

A small tip for you. Soil in Au is extremely acidic. Coating the outside of the metal drum with an epoxy paint or resin will reduce the corrosion. To aid the reduction in corrosion, back fill the hole with small rocks in the bottom of the hole, install the tank/drum, and reinstate the hole with small gravel/stones surrounding the tank then shovel in some washed river sand, to fill the voids around the loose gravel/stones.

Make sure the hole is about 300mm wider than the tank, giving 150mm clearance around the tank. This will aid cooling and stop the tank corroding.

Tank 1000mm diameter. Hole diameter, 1300mm. Stone depth on the bottom of the hole, 100 - 300mm. Level off with washed river sand. Install tank centrally to the hole. Fill surround with 20mm stone. Fill stone voids with washed river sand. Fill tank top with stone and sand, if required.

It can even be done with a cement mix of 14 sand to 1 cement, replacing the gravel/stone around the tank. It is easily removed at a later date if needed. This cement bound sand mix will help cooling and help prevent corrosion from the surrounding soil. Just cost and what is available to you will determine what you opt for.

@tcmtech

> So what you are saying is you want to half ass the whole project

This is always amuses/infuriates actual engineers, including my father, but.. yes.

I'm designing predominantly for the developing world where people have very limited access to materials and resources, minimal tools, and not a lot of higher education. So, yeah, I make stuff as basically as I possibly can because if I make it that it'll only work if it's nicely made, then all those billions of people not in a position to nicely make it aren't going to have access to the thing. I under engineer, and then improve the bits that explode. This I feel is more helpful to more people than leaving things over engineered.

If I was just making stuff for my own personal use then I would do everything completely differently.

> and get little to no measurable returns

At this point I'm going to have to ask everyone still here to read through the link in the original post, and check the data I got from that prototype.

@old salt

> The nice thing about most of the containers mentioned is that most of them have either or both 3/4"npt breather vent holes and 2"npt drain bungs

Bonus. I'm probably just going to cut some 40mm holes and push through PVC pipe to fit, maybe some resin if needed. Will keep nicer options in mind tho.

@rixter

> I would look into treating the drums to prevent rust with a good coat of paint.

Painting the inside of a small tank is a bit tricky, also at some point two layers of paint will add as much insulation as having used plastic instead.

@stevewicks

> what stops the water becoming toxic and delivering something bad like legionnaires disease?

50 ml hydrogen peroxide added every two weeks. Cheap and easy to get, breaks down to water and oxygen. Ultimately I'd prefer something more set-and-forget like a tube thing of copper and another metal which strips antimicrobial copper ions into the water, but that's going to have to be prototyped in and of itself when I get the chance.

@jp76

Not sure how to read that..?

@IQ

> To aid the reduction in corrosion, back fill the hole with small rocks in the bottom of the hole, install the tank/drum, and reinstate the hole with small gravel/stones

Good idea. Where this is going tho I unfortunately don't have the luxury of a large hole. It's extremely rocky to the point of being smallish deposits of silt and soil on a big rock. In the end I'm going to only be able to dig down about a half meter and then place a mound of soil and rocks on top, contained by mudbricks (which fortunately we have a bunch of here from a building project).

Will the acidity damage plastic?

@everyone

With regards to why I don't just dig a trench and lay tens of meters of hose; because so far it looks like the ground tank system is better in every way:

One small hole instead of a thirty meter trench, which as I say wouldn't be possible here, and wasn't possible at my mum's small suburban place for the first prototype.

One effectively free container instead of tens of meters of surprisingly expensive pipe and or hose.

More effective and efficient heat exchanger.

Less back pressure, probably, so can get away with a smaller pump and less energy.

On the down side, water has to be kept sterile but as I say that's not a huge problem and I'm looking to improve it further. And may humidify the air, tho the only way to find out for sure is to try it and see. Personally I'm of the impression that cooling warm air with cold water should drop the dew point and actually dehumidify it, but only actual measurement will tell.

And, ultimately, it's an experiment. If I only ever did things the way experts assured me was the best way to do things I'd never come up with anything new or useful. http://solarflower.org has some other things I've been working on and documenting.

Ok...

Please read through the what I've posted here:

http://solarflower.org/?content=othertech&lang=en&tech=airconditioner

and tell me which bit of those results you take exception to and demonstrates that it doesn't work.

I'm fully open to peer review, I'm just not sure which apparent systemic dysfunction you're referring to.

Okay. How much power does the vacuum cleaner motor air pump draw and what range of service life are you getting from one?

> How much power does the vacuum cleaner motor air pump draw and what range of service life are you getting from one?

Way too much and way too short (although longer than I would've thought).

The vac was only ever meant as a quick and easy stand in for that first prototype. The one at my mum's would now be happily wind turbine powered:

(http://youtu.be/0fAAdwDEMbM)

if it hadn't been for the last minute discovery that her roof brickwork is dangerously unstable and not something you'd want to attach a wind turbine to. Or stand under, frankly.

Here there's not enough wind, but plenty of sun. So I have an old PV panel which I'll be attaching directly to an old 12v drill, which will drive a bellows pump similar to the one in the vid above but smaller for higher revs.

The only significant backpressure on the system should be putting 3 litres per second of air through 20cm of water, which is about 9 watts mechanical. Apparently.

Why bother? This Guy gets off by looking at himself in a mirror.

I'm gone, see you around!

The acidity will not affect the plastic tank, but the plastic tyoe may affect the water if you want to consume it. PVC, (Chlorines). PTFE in sunshine can be dangerous too. Hence steel is better and disapates heat easier.

Definitely won't be touching the water, and none of the system will be in direct sunlight.

...however the system will deliver a quantity of hydrogen peroxide vapour into the living space once every two weeks. The thing about this stuff is that it really doesn't care which biology it destroys. So please make sure that no CR4 reader can enter the building while the system is running and until it has been sufficiently ventilated after it stops.

Passing the air through an underground hose network seems to be the way to go, eliminating the water stage completely along with the biological hazards that go with it. Good luck with it all, for luck will be needed. It's time to unsubscribe from this thread on the grounds of not wanting to associate with such a design (if one might call it such by exaggeration).

<unsubscribes>

I don't see why you are trying to power the system. If you used the chimney effect to vent hot air and draw in fresh air from the cooler subsoil area, percolated through water or not, you have a simple passive system.

Jim

P.S. your chimney/s could be made from plastic drink bottles.

Solar chimneys are good tech, but pretty much need to be integrated into the building during construction. Also since they suck rather than blow the rest of the building needs to be relatively sealed to ensure as much air as possible is coming in through your tunnels.

If I was building a house I would very probably put one in.

I read with interest the different comments, may I make one myself.

If the intention is good " to design/concept a solution for those who have nothing" the approach is on my opinion a cut and try one which is of course possible only if the developer has not the skills to analyse it on paper.

I know, it is extremely difficult to make the conceptional work with the constrain to have in fact NOTHING of what is usually available however only tests do not lead to the solution in general only to one solution for specific conditions.

The thread did not stay on the initial path because as I noticed several times participants do go further than the asked question many times without knowing the reasons for it. This is a peculiarity of CR4 we know better and more and we give a lot of counsels which in fact many times are not what the OP asked for and we are surprised many times disturbed that he does not look at our "precious" participation with more respect.

If you look at the answers ONLY a FEW consider the question: can I use plastic instead of steel? The rest is about his concept a.s.o. And after it we say he looks in his mirror because he does not accept what we gave. Funny behavior.

Indeed.

In my experience the simple task of selecting what will work best involves looking at the whole problem not just one single part of the system.

For example.

I am looking for a simple insulating material I can put on my house windows to improve the overall heat retention of the house being its -20 F outside and I like to have it at +72 F inside. Oh yea I ma lazy and my entire front door is missing but I figure spraying something on my windows will be easier than plugging the huge hole in the wall where my door should be.

Any ideas on how to improve the windows to make up for the total lack of a door in my house?

You have said no to a number of ideas/materials (garden hose...), on the premise that these materials are to expensive or rare; at least for the 3rd world audience you are designing for.

But if that is the case, how do you figure that: "Here there's not enough wind, but plenty of sun. So I have an old PV panel which I'll be attaching directly to an old 12v drill", is any better of an option?

In my experience 12v drill motors don't grow on trees, and my garden produces absolutely NO PV panels.

> You have said no to a number of ideas/materials (garden hose...)

Garden hose is fine in terms of availability (or at least fine enough), but at 18mm at the fattest it puts significant amounts of backpressure on the system. 40mm corrugated hose and similar solves that problem, is a bit harder and more expensive to get, but the main issue is that I'm still concerned it's going to be too narrow that the airflow will to some degree pump the water upwards and out of the cool soil zone. But I have been giving it some thought as an option.

Ultimately tho the issue is that I've got no need to swap out the drum design, because it works absolutely fine. If I've appeared dismissive of solutions it's because there's no problem there that needs solving.

> 12v drill motors don't grow on trees

No they don't but I do need to draw a line somewhere as to the minimum availability of resource that I have any chance of working with. A drill as a tool is needed to make everything I've designed so far, it would obviously be more accessible still if that sort of thing wasn't necessary but there's just no way, frankly, to get that low tech and still be able to do things like generate electricity and the rest of it.

> and my garden produces absolutely NO PV panels.

Which is largely why I've not worked with them til now. The only reason I'm looking at using one with this is because if I can get away with a 20-30 watt panel (the one I got given for free has all the glass frontage smashed) which I can attach directly to the motor without a battery, charge controller etc, then that at least opens up more possibilities for more people. Lots of people don't have pv panels, but lots do, so making this available to them might not be helping every person on the planet, but it's helping more than zero, and is therefore worth my time.

People here are reacting like I've come begging for my design to be overhauled, and then turned up my nose at the response.

I do appreciate the input, but I never actually asked for that, I only asked a simple question about plastic vrs steel as a conductive surface.

I wish you well with your adventure. While there are many things that I might have done differently this is ultimately your "little project".

Those suggesting burried length of hose seem to have missed your intention of turning the container interior into a "splashing evaporator" to achieve heat exchange into the air.

My gut feel is that plastic would be prefered, since the interior will be intermittantly wet and the fluid levels will vary over time. A plastic drum burried will still be there in 10 years while steel will barely last a year. (Did a similar thing years ago for grey water for gardens around home.)

You will be familiar with the 20L square plastic drums used for chemical concentrates and such. Drums of that size have threaded outlets in the bottom and the threaded cap fitting. If you are using rural grade poly line (much cheaper than hose) you can get barbed fittings for a few cents (literally only $0.30c or so each) that will screw and seal into those. UNDER NO CIRCUMSTANCE use old chemical drums, no matter how many times they have been rinsed. Rinsing is to enable safe transportation to recycling, not for safe re-use!!!!!!!

You will need to top up fluid frequently and you will need to be VERY CAREFUL about the bugs that will breed in that space. Local (Australian) regulations would prohibit this type of system for space where people would be even intermittantly working due to a number of factors. (Especially no physical isolation between the evaporator water and the delivered air.)

I suspect that your power drill will fail very quickly. I've used power drills on continuous duty trying to wire brush things for painting and succeeded in releasing the magic grey smoke that is essential inside all electrical items. (Once released they never work again!)

An aquarium pump might be a better alternative, or else a 12V tyre inflation pump from the servo.

Have you considered having the evaporator above ground and pumping the water? If your tank is underground and you pump the water (solar panel and small pond pump) up inside the room and have it trickle through a pile of stones or over some corrugated iron. Those pond pumps are designed for continuous duty.

In central Australia you will definately need to be deeper than you are indicating to get good thermal output/heat exchange (Or else have the container underneath the structure so that the soil above is shaded.) Soil temps in central Qld (St George) are surprisingly hig even to 300mm below surface.

> turning the container interior into a "splashing evaporator" to achieve heat exchange into the air.

I'm hoping it won't act as an evaporative cooler, I'd rather not humidify the air and have to top op the water. My feeling is that introducing say 25C air into 10-16C water will immediately evaporate some of the water, but then as the air temp and dew point quickly comes down to the water temp that most, if not all, if not more than that moisture will then condense out again.

It is of course completely likely that I'm wrong about this, but as it's a somewhat complicated process the only way to find out is in practice. I'm hoping it'll just be a standard heat exchanger with the water only acting to conduct heat between the air and the container wall.

I worked out that if 100% of the energy extracted from 28C air by 16C water went into evaporating the water it would do, with the configuration I'll be using, about 200ml in eight hours, giving about 280 liters of water vapour, or 0.56% of a 50 cubic meter space.

That's a fair bit more than I'd prefer, but that's every bit of energy going into evaporating the water, none for heating it, and none leaving the container. Given that with the first prototype and 55C air the water only increased by half a degree, and that heating water as you know is significantly easier than vaporising it, I'm somewhat hopeful that not a lot of evaporation is going to take place.

Science!

> My gut feel is that plastic would be prefered

Yeah that's the way I'm going to go. I also worked out how much a water cooler bottle would shed in this situation and it's something like 200 watts per Kelvin, or about ten times what's going to be going in.

> If you are using rural grade poly line (much cheaper than hose) you can get barbed fittings for a few cents

I can hopefully get away with just chucking two lengths of pvc pipe through holes drilled in the top, but if not then as you say.

> UNDER NO CIRCUMSTANCE use old chemical drums, no matter how many times they have been rinsed.

Word.

> you will need to be VERY CAREFUL about the bugs that will breed in that space.

Hydrogen peroxide kills pretty much everything, and since it doesn't fume and there'll be no contact with the water I can use aggressively high amounts of it.

> Local (Australian) regulations would prohibit this type of system

Yup, and everything else I've ever done. Another reason the developing world is going to be more viable, tho obviously I'll not be taking any chances with people's health.

> I suspect that your power drill will fail very quickly.

Probably. Aquarium pumps won't do the volume (2-3 l/s), compressors are little tricky to come by. I'm thinking maybe extractor fan motors or similar, which are meant to be run long term. When the drill blows it will be easy enough to swap it out for something better.

> Soil temps in central Qld (St George) are surprisingly hig even to 300mm below surface.

I'm looking at a meter minimum. Here I can only go 50cm down until rock, so will put a little meter radius hill on top of that. Still need to measure the actual temp here, will be digging holes over the net couple days.

I don't know what all the carfufil is about with peroxide nor the hassle of third worlds. It is made out here from assumed 1st world folks, the 3rd world knows nothing. They happen to be streaks ahead in knowledge.

No 1. At least 80% of 3rd world folks know that a table spoonful of bleach in 25 litres of water will produce 25 litres of drinkable water. So simply add some bleach to your system to kill germs.

Look at your water under a microscope in the assumed 1st world. You may be shocked as to what is in it in chemicals, bugs, and narcotics. Dare to look. Or go and live in a third world country and learn. Very educational.