I have a question about my specific situation and a more general question regarding air conditioning. I live in Austin, TX where we have high heat and high humidity for several months out of the year. I bought a small metal outbuilding for use as a workshop in which I lined the walls with unfaced fiberglass insulation and covered over that with 1/8" hardboard. The structure has no windows and no vents. I was wondering the best way to cool the structure in the summer. I was wondering if there were a way to dessicate the air that was cheaper than running a portable A/C. Is there? Is there a cheap way to build my own Einstein-Szilard system to cool it using solar heat?
That last question goes to my more general question. What are the options for running an A/C on waste heat?
All efficient heat powered AC relies on absorbtion refrigeration techniques. It’s theoretically possible to use a sterling cycle engine to run a mechanical compressor, but I’m not aware of any non-research systems that have made that work.
The most common absorpttion is based on ammonia as the refrigerant, with water as the absorber. Ammonia is so soluable in water, that the water acts to “pump” the ammonia vapor oyut of the evaporator. Once saturated with ammoniq, the water and ammonia can be seperated by heating the water. There are examples of batch systems, as well as continuous process systems. This is used for RV refrigerators, and large industrial systems. Domestic ammonia based systems are largely obsolete due to concerns in the event of refrigerant leakage.
It is also possible to use water as the refrigerant, with molecular sieve material (AKA zeolyte) as the absorber. Water is much safer in a domestic enviroment than ammonia, but the solid dessicant presents issues with effective heating and cooling, and doesn’t flow, so only works in a batch mode…but two absorbers can be “ping ponged” to provide continuous cooling if needed.
The payback time on such systems would likely constitute several of your lifetimes, and exceed the expected service life of the system. If you can use groundwater cooling, that is probably the most cost effective setup. (considering both operating cost and initial investment).
I’m not sure if this is what you were getting at or not but another idea if your handy would be to take a long rubber hose filled with water and hooked into a loop with a pump. Half the hose inside, the other half burried fairly deep outside (probably 5 or 6 feet I would guess). It’ll absorb the hot air from inside and dump it into the earth outside. I was going to suggest running the hose along the ceiling due to heat rising and cold falling, but if it’s really damp and the hose get’s cold enough it’s going to condensate and make a big mess. That’s something you’ll have to find a way to deal with, someway to collect the condensation and drain it back outside.
It depends on specific local geology and climate, among other factors, but in general, yes, you don’t have to go very far down before the insulating properties of the ground have stabilized the temperature to a more or less constant year-round average; for temperate climes, it’ll probably be somewhere between 50 and 60 degrees or so, IIRC.
An easy way to find out is to run your cold water for a few minutes than take the temp. You have a water heater hooked up to the faucet, but not a water cooler.
Here’s a site that might be helpful. I don’t know what humidity levels you’re dealing w/, but evap. coolers are effective up to about 40% and they also make “piggyback” units that combine evap. cooling w/ A/C.
I’m not so sure about this. I stayed at a resort in the Phoenix area. There was a very prominent sign in each room to the effect of don’t let the cold water run trying to get really cold water, it is not going to happen. It seems that the earth gets so warm around there that the heat in transferred into the “cold” water pipes making them fairly warm water pipes. The sign went on to say that if you wanted cold water they would bring you ice.
The other problem I see is even if the ground is cold, just how much heat can you transfer in before you reach equilibrium? Once the ground near your water pipe approaches the temp of the water being pumped out of your shed, heat transfer will stop. :mad: :smack: It would be a real bitch if you system worked great for 3 days and then had to cycled off for 4 days to allow the ground to cool off.
Now if you have access to well water, you could build a system that pump cold well water up into a radiator and blow the warm air across the radiator. The heat will transfer into the cold water cooling the air. The warmer water will return back into the well. This will work great and be cheap unless to many people in the area all start doing this. If this happens, the temp of the ground water may get raised so the system is no longer fully effective. My father told me that people in Kansas City used to cool their house this way until so many people were doing it the
I’m not so sure about this. I stayed at a resort in the Phoenix area. There was a very prominent sign in each room to the effect of don’t let the cold water run trying to get really cold water, it is not going to happen. It seems that the earth gets so warm around there that the heat in transferred into the “cold” water pipes making them fairly warm water pipes. The sign went on to say that if you wanted cold water they would bring you ice.
The other problem I see is even if the ground is cold, just how much heat can you transfer in before you reach equilibrium? Once the ground near your water pipe approaches the temp of the water being pumped out of your shed, heat transfer will stop. :mad: :smack: It would be a real bitch if you system worked great for 3 days and then had to cycled off for 4 days to allow the ground to cool off.
Now if you have access to well water, you could build a system that pump cold well water up into a radiator and blow the warm air across the radiator. The heat will transfer into the cold water cooling the air. The warmer water will return back into the well. This will work great and be cheap unless to many people in the area all start doing this. If this happens, the temp of the ground water may get raised so the system is no longer fully effective. My father told me that people in Kansas City used to cool their house this way until so many people were doing it the well water turned warm.
Your posts suggests that geothermal heat exchange isn’t backed up by sound science.
More likely the frost line in Phoenix is only one or two feet below the surface, so that’s as far as they dig when running a water main. It’s a pretty stable 50-60 degrees when you go ten feet into the ground in almost every part of the country.
I never said that. My point was that I believe that the heat penetrates into the ground more than just a few inches or feet. Water mains are buried way deeper than a foot or so. The previous posts were talking about 5 or 6 feet, a 10 foot deep ditch is twice as deep as what was being discussed. You are moving the goal posts here.
Also frost and Phoenix Arizona are mutually exclusive terms. If you ask someone from Phoenix about frost they will give you a WTF look. You are going to have to dig a long way to find a frost line in Phoenix. Like maybe to Flagstaff.
Furthermore you have not addressed what happens if you dump BTUs into the ground faster than they can dissipate. As the temp of the ground near the water pipe approaches the temp of the water in the pipe, the rate of heat exchange will slow down and when the temp is equal it will stop.
I’m not saying it won’t work, but you will have to lay enough pipe covering enough ground so that the system won’t reach equilibrium. That might take a whole lot of pipe, and a metric buttload of ditches.
The Austin ISD has experimented with geothermal cooling. (here’s a short positive blurb (PDF) from the company that sells the system). However, with the exception of Menchaca ES (mis-spelled in the article), most of the school have had trouble using the geothermal cooling exclusively. They don’t really know why it hasn’t worked as planned but one reason may be because schools are used a lot after hours and those hours may not have been planned for. Menchaca is a rural school that sees much less after hours use. Anyway, the point is, it’s not that easy to construct an adequate geothermal cooling system down here.
I’m not going to argue that Phoenix is heven on earth, but the OP is in Austin. I have a friend that lives in Austin. he claims that it is not hell, but you can see it from there.
tremorviolet reading your link, that system uses well water, like what I described in my first post, not coiled tubing buried in the ground.
Rick hit the nail on the head. I do HVAC work, including GeoThermal. In an “indirect” system methanol (or some other material) is sent into the earth where the earth “accepts” the heat, sending back cool liquid to pick up heat from the house. (through a complicated process)
There is a lot of engineering behind this----the total of amount of heat (Btus) that need to be transferred from the house (a complicated task in itself), the pumping capacity of the compressor & it’s cooling capacity (for the “direct” side of the system), the amount of fluid needed & correct pump, the amount of feet of pipe needed in the earth etc etc etc.
Even then it is COMMON for systems to reach what Rick calls “equilibrium.” Many manufacturers put in “soaker fields” which is a perforated PVC pipe that is in the trench and comes up up out of the ground to a standard garden hose fitting. When the earth gets too hot/dry (which only a service technician can figure out as it is a complicated thing to describe here) the homeowner hooks up a hose and cools the trench with cold water via the perforated hose. (which leaches the heat into the earth and cools the trench) I’ve seen the hose run for hours and hours and hours.
So you need to know, among other things:
The heat load in the structure
The heat transfer qualities of the transfer medium (methanol, water etc)
The heat transer of the piping (rubber hose etc)
The correct pump needed. (flow rate, velocity etc)
The amount of feet needed to transfer all the Btus, which will determine,
The size of the trench
There are more things to consider, but you get the drift.
The possibility that you will sink a rubber tube into the ground and have the earth accept the Btus in the structure is slim IMO.
You may get some heat transer, but find that you reach the point that the earth won’t accept any more heat, (length of tube, not enough water etc etc) and the structure is still blazing hot.
Or, you may have little transfer. (Poor transfer of the rubber hose, not enough pump, water etc)
At any rate, if you want to have fun experimenting, go for it. If this will cost real money, and you have to have it right, this is NOT a DIY project.
So then, back to my original problem. What would be the cheapest way to cool my shed or at least dessicate the air without installing a window and window unit?
The sign went on to say that if you wanted cold water they would bring you ice.
WTF look. You are going to have to dig a long way to find a frost line in Phoenix. Like maybe to Flagstaff. 
