What is the best way to use solar heating to cool a living space?

In summary,The sun heats the "hot-box" container filled with air from outside, to a certain temperature. Once the temperature is reached, a one-way check valve is opened, and the air is admitted into a water-immersed copper coil. The air is then cooled by the water, and finally escapes to atmospheric pressure. The amount of time it takes the hot-box to stop pushing air (1atm) depends on the friction of air in the copper tube, and size of the hot-box. When the hot-box reaches 1atm, or close-to-it*, the valve to the copper tube is closed, and the two
  • #1
Glurth
59
0
I have an airtight container, the "hot-box", filled with air from outside.
I close the two valves to the outside air, and let the sun heat it up.
Once it reaches a certain temperature/pressure: I open/unlock a one-way check-valve that leads to a shaded/water-immersed coil of copper tubing. The tube is open to (indoor) air on the other end.
For simplicity, let’s say the copper coil is long enough that hot & high-pressure air from the container, while passing through it, will be cooled to room-(outside)-temperature by the time it reaches the open end.
When the air finally escapes from the tube, it will expand from high-pressure to atmospheric pressure, further cooling the gas.
The amount of time it takes the hot-box chamber to stop pushing air (1atm), depends on the friction of air in the copper tube, and size of the hot-box.
When the hot-box reaches 1atm, or close-to-it* (no longer pushing high pressure air); we close/lock the valve to the copper tube, and open the two valves (top and bottom) exposing it to open air so the hot air inside will be replaced with fresh air, and we can repeat the process.

Is all of this actually correct? Am I forgetting/ignoring something important? Like; will significant pressure be lost due to friction in tube? Or, do I need to consider the air already in the tube if << the air in hotbox?
If this does work, I assume such a device like this already exists, what is it called? ( I saw similar, but not the same, devices here, http://en.wikipedia.org/wiki/Solar_air_conditioning)

If I wanted to use this as a small cooling unit (which would probably require a round-robin array of heating chambers, with temperature controlled valves for each, leading into to the coil, and so forth), will the cooling effect even be noticeable? Or should I be asking, how big or hot would my hot-box need to be for the effect to be noticeable? (I live in rural high desert: powerful sun, low atmospheric pressure, lots of space)
I'm getting a bit lost trying to figure out what’s going to happen to the pressure in the hot-box chamber (and the tube's output), when the air is leaving via the open end of the copper tube, AND still being heated by the sun. E.g. will the decrease in pressure be linear, or exponential? *I think this info will be important in determining when a "cycle" should end, and the hot-box flushed with fresh air.

Thanks in advance for your feedback!
 
Engineering news on Phys.org
  • #2
This is roughly the basic idea of a fridge or machines that produce liquid air, just in the first step the heating gets replaced by compression as this is faster and does not heat the medium so much.
 
  • #3
Indeed, AC's and Fridges are where the idea originated. I'm trying to make it even more KISS, and reduce electric use to powering valves open and closed, rather than running a compressor.

But is it too simple to work WELL, and be worth the effort of making it? (I live off grid, and need to save my PV generated electric for lights and computers, so "worth the effort" has a much lower threshold than for most: just needs to cost less than new PV cells to power an AC unit, and mounting equipment.) Size of those hot-boxes will really effect price. How can I determine how big I'll need to make them?
 
  • #4
I doubt you get much out of it. Air has a tiny density, has a bad absorption both for visible and infrared light (so heating a lot of it takes a while), and I guess you won't achieve high pressures. Your cooling pipe cannot be placed inside (otherwise the setup heats your room), so a part of the temperature drop just goes to the difference between outside and inside air temperature before any cooling effect starts. And your overall setup would be another heat bridge where heat gets conducted to the inside.
 
  • #5
>>has a bad absorption both for visible and infrared light
That's why I would make the"hot-boxes" out of metal, and paint them black. Perhaps even add mylar reflectors to direct more sunlight on them.

>>and I guess you won't achieve high pressures
I think I see your point, this is indeed probably the biggest issue. Just to confirm I have PV=nRT right, I would need to DOUBLE the air temp (in Kelvins), to get just 2ATM of pressure? That does indeed seem difficult (75F -> 620F!)

>>so a part of the temperature drop just goes to the difference between outside and inside air temperature before any cooling effect starts
I'm actually thinking about using a small outside POND that I have, for cooling the tube. The pond will be the average daily temp, which includes those cold desert nights. Evaporation will also help keep it cooler than hot daytime air (and probably even cooler than my house, which does NOT have any AC yet.)

>>And your overall setup would be another heat bridge where heat gets conducted to the inside.
Do you mean the copper pipe will thermally conduct the heat from the hotbox to inside? This I don't understand, won't any given section of the tube be the temp of the air inside that section of tube? If we can cool the heated air inside the tube down to outside-temp, won't the section of tube containing that cooled air also be the outside-temp?
 
  • #6
That's why I would make the"hot-boxes" out of metal, and paint them black. Perhaps even add mylar reflectors to direct more sunlight on them.
I would paint the bottom and make the top out of glass - that way, you trap more light. And convection works better if you heat it at the bottom.
I think I see your point, this is indeed probably the biggest issue. Just to confirm I have PV=nRT right, I would need to DOUBLE the air temp (in Kelvins), to get just 2ATM of pressure? That does indeed seem difficult (75F -> 620F!)
Indeed.

Do you mean the copper pipe will thermally conduct the heat from the hotbox to inside? This I don't understand, won't any given section of the tube be the temp of the air inside that section of tube? If we can cool the heated air inside the tube down to outside-temp, won't the section of tube containing that cooled air also be the outside-temp?
Unless the pond is directly at the house, there is a pipe in between that can get hotter than the pond temperature. And your pipe needs some connection to the wall, which might not be 100% airtight, and so on.
 
  • #7
What about circulating the pond water through a heat exchanger in the house?
 
  • #8
Did you ever consider a cooling tower style of cooler? A tall black chimney will have a vertical flow of air through it. The air flowing in at the bottom is drawn over a 'wick', wetted by the pond water and in good thermal contact with the pond. The hotter the sun, the stronger the updraft and the more evaporative cooling.
No moving parts either. Top up the pond occasionally.
The chimney / tower may need to be quite tall, of course but I have seen a reference somewhere to such a system in PF which works. I will try to find it.
 
  • #9
I thought about evaporative too, but it would depend on the size of the pond and the frequency if rain or whatever method keeps the pond full... Wouldn't want to empty the pond I assume, as they are off the grid, they may use a well for water, which would use power to fill back up...
 
  • #10
Evaporation could be a problem; true. For a pond that's not too large, there's always the possibility of using plant cover. Plants can be good at regulating temperature by efficient use of water by transpiration.

Alternatively, what about a reflective floating cover?
 
  • #11
Air heaters

The tank could be fitted internally with finned air heaters powered by photovoltaic cells.This will increase tank pressure substantially.Small dia orfices at the outlet end will reduce the temperature.
 
  • #12
I've heard of "swamp coolers" before, but never heard of one using a chimney to power airflow like that. But I'm a bit confused about that idea, won't the sun on the black chimney make the air hot, rather than cool? Is the ouput \ top of the chimney supposed to feed into the area I'm cooling?

I DO use a well for water, so there IS a power cost to pump it up, but pumps are one of the things I don't mind using my PV electric power for. Of course I'd rather power pumps differently, but that's another post.
 
  • #13
I think the Pr drop inside the box will be close to exponential anyway as it will take longer to keep increasing the Pr due to heat than the Pr drop because of opening the valves.
I think there are 2 main problems for this approach to be able to produce a useful cooling effect:
-Air as the refrigerant. Reason why they have been using other gases to increase efficiency of refrigeration systems instead of air.
-Open refrigerant circuit.
Perhaps you can still use solar energy for the heating and use a system similar to absorption refrigerators http://en.wikipedia.org/wiki/Absorption_refrigerator. Having a close refrigerant circulation circuit, will allow you to use a much more efficient refrigerant than air.
 
  • #14
If you want to go down the refrigeration route then use a 12v compressor-based system, fed from a large PV array. Proper compressor systems have by far the best performance coefficient. Even doing it this way, the problem is that you need to shift 1kW worth of solar energy falling on every sq metre of pond area with what the PV array can produce. Allowing for inefficiency of the whole refrigeration power chain, that means a pretty massive area of PV.
It is far better to stop the sunlight from falling on the pool in the first place - if that's practicable.
 
  • #15
"...Proper compressor systems have by far the best performance coefficient..."
That's why they are used more than anything else. Apparently he wants to use sunlight directly in the first place. Since he wants it for lightly cooling a room and not refrigerate goods, perhaps he does not need so much sun power to get the cooling effect he needs. On the other hand, the conversion from sunlight to electricity in the PV, is pretty inefficient anyways and most of the sunlight power will be lost right there. Maybe its better to use sunlight to directly heat the absorption refrigerator system. It could be less expensive also. I don't know.
 
  • #16
Great input, thank you!
I read that wiki article on the absorption refrigerator, but I don't think I'm quite clear on it yet: I see how the thermodynamics of it are supposed to work, but I'm not quite clear how the fluids and gases are moved around between the different chambers, does it use a pump, or does the heat gradient itself generate this flow, like a convection cell?
So you think I can buy a commercial Absorption Fridge that runs off, say... propane, and convert it to heat the internal-chamber with solar reflectors instead? I'll need fans to get the cool air into the room I want it, but that should be pretty low power usage. I can even recirculate back to the cooling unit over and over, for better cooling over time. I like this solution.Regarding the pond, which actually has yet to be dug, it's going to be a DEEP and long, kind of like a stream (minimal surface), with lots of plants growing on the banks, and floating potted plants in the water itself to provide SOME shade to reduce evaporation. It may not be enough.

Regarding PV: We have high winds in this area, so mounting a PV panel, costs about as much as the PV panel itself, maybe even more! This can get really pricey really quick, which is why I'm shying away from PV where possible.
 
  • #17
-No pump required to circulate the refrigerant, heat does the job.
-Yes, I think you can buy a commercial unit and heat the system with sunlight reflectors, etc. I would suggest you build a temp control system of some kind controlling the amount of sunlight to avoid the thing getting overheated and over pressurized which can make it explode. An overpressure release valve will also be a good idea. Generally speaking, provide safety means to avoid overpressure of the refrigerant which might occur by overheating, leading to explosions.
-I agree, you can circulate the air using a simple fan which takes very little power to operate.
-PVs are expensive and inefficient in energy conversion. I would avoid using them if not really needed.
 
  • #18
Glurth said:
I've heard of "swamp coolers" before, but never heard of one using a chimney to power airflow like that. But I'm a bit confused about that idea, won't the sun on the black chimney make the air hot, rather than cool? Is the ouput \ top of the chimney supposed to feed into the area I'm cooling?
No, this would be a method to cool the pond. Or a smaller amount of cooling water.

sophiecentaur said:
If you want to go down the refrigeration route then use a 12v compressor-based system, fed from a large PV array. Proper compressor systems have by far the best performance coefficient. Even doing it this way, the problem is that you need to shift 1kW worth of solar energy falling on every sq metre of pond area with what the PV array can produce. Allowing for inefficiency of the whole refrigeration power chain, that means a pretty massive area of PV.
It is far better to stop the sunlight from falling on the pool in the first place - if that's practicable.
Why do we have to cool the pond now?
 
  • #19
"...but I'm not quite clear how the fluids and gases are moved around between the different chambers, does it use a pump, or does the heat gradient itself generate this flow, like a convection cell?..."
Check kerosene or propane refrigerators. They work very similar to compressor driven ones, that is they have a close refrigerant circulation circuit evaporator and so on. They don't operate based on heating air; but the refrigerant inside the close circulation refrigerant system which when boils in the evaporator, absorbs heat from the surroundings. The chambers (I guess) refer to the evaporator, condenser and so on, not the cooling chambers inside the machine where the food is placed and air circulates by convection.
 
  • #20
If your area is not very humid, swamp coolers work alright for cooling. All they are are a box with some absorbant material through which water flows, usually from top to bottom. At one end of the box there is a fan which blows through the box into the room. People usually mount them in windows and they have a small hose which feeds the top of the box.

As i mentioned they are good in hot, dry climates, not humid ones, so your usage may vary.

Have a great morning! :)
 
  • #21
Refrigeration cycles that don't use a compressor are unbelievably poor performers. Peltier junctions or so-called 'absorption style' units have their place in appropriate circumstances but when you want to shift serious amounts of heat on Earth, they are useless. On a small scale, a far better solution for camping and boating fridges ( and even freezers- I have one) PV cells driving a compressor system win hands down.
It could be that a large solar collector could yield the right sort of power at the right high temperature to work an absorption cycle. Calculations needed here and throughout this project.
Personally, I like the swamp cooler idea, driven by a tall convection chimney. There is no problem having a hot chimney 'cos the air drawn into the bottom is at ambient temperature.
The only running cost is in replacing evaporated water. To estimate feasibility of this style of cooling, you compare the Latent Heat of evaporation of the water with the amount of heat that needs to be shifted (total solar heat on pond surface per day and required min / max temperatures you want.)
Loads of fun experimenting to be had.
 
  • #22
  • #23
Many thanks for all the suggestions and input.

Regarding primary purpose: I'd just like to help keep my home office a bit below ambient outside temperature. Ideally the cooler air will enter the room near my computer's fan intake, to help keep it from overheating. My comfort is a less important, but existing, goal as well.

Regarding "the pond": We certainly don't need to try and keep it cool, other than shade. I think everyone is rightly concerned about the pond's rate of evaporation in my climate, but that is a different post. I was just throwing it's presence out there as an available heat sink.

I have a few specific questions:
"Refrigeration cycles that don't use a compressor are unbelievably poor performers."
While I see that EFFICIENCY may be poor, that's OK if it just means I need more direct sunlight (I have lots of that for free). It's the cost of buying and mounting the PV cells (plus charge controllers, and all the additional circuitry) that adds up to the BIG $$. So rather than power efficiency, I need to consider my "Cost Efficiency" in terms of TOTAL INITIAL COST vs FINAL COOLING EFFECT. Given this fact, do you stick by your statement, Sophie? (I must certainly grant that the PV output is far more flexible, as I can power ANY electric device, rather than JUST a cooling device.)

Swamp coolers/black chimney: I really like this idea. Swamp coolers work great in this area of low atmospheric pressure and low humidity. A pump (small: ounces per minute) for the water supply will be all the electricity I need, perhaps I can buy a commercial unit, and disable the fan, using the black chimney instead. Am I correct in my assumption that this little pump will require much less power than an AC's compressor?

I could use a bit more explanation of how the black chimney actually works. I mean SOME of the air in there must get hotter, in order to rise and pull in fresh air, right? Is this heating negligible because fresh air is always being pulled in (more heating directly relates to more fresh air)? Or does the expansion of the heated air itself counteract the addition of heat, keeping temperature the same? Design wise; Do I want the chimney to be a constant width, or to taper/expand as it goes up? Do I want to shoot for particular height/width ratio?

Thanks again for all the input!
 
  • #24
" Am I correct in my assumption that this little pump will require much less power than an AC's compressor?"

Yes. Much less.

Have a great day! You're gettin' there! :)
 
  • #25
Just a few points. The best solution is very application specific. Swamp coolers are useless in UK and very good in Oz outback (I have heard).
The PV / compressor system needs no charge controller because batteries are not needed. You may need a 'smart' system to supply the starting current for a compressor motor but it can be home brewed. Cooling is only needed when the sun shines.
The cheapest solution must involve a solar heated chimney and evaporation cooling. The hot air is all in the top section. Air is at ambient temp as it is drawn in at the bottom. A wide, shiny half cylindrical reflector would double / triple heat input to a vertical black chimney. Evaporative cooling produces 2,000kJ for every kg of water evaporated.
You have to involve actual figures if you want to have an optimal solution here.
 
  • #26
"The hot air is all in the top section."

I thought the output, for the air to be cooled by the swamp cooler, came FROM the top section.
Or do I misunderstand, is there more than one output?

Good point regarding hooking it up to only run when the sun shines, as separate from my existing power infrastructure. Though single-purposing it like this eliminates some of the flexibility electric power provides. I'll look for some DC powered commercial swamp-coolers (fan included) tonight, and let you know what I find. This might just be the simplest-setup route, not sure about cost yet.

Why do they call them "swamp-coolers", when the worst place for them to work is in a swamp?
 
  • #27
Glurth said:
"The hot air is all in the top section."

I thought the output, for the air to be cooled by the swamp cooler, came FROM the top section.
Or do I misunderstand, is there more than one output?

Good point regarding hooking it up to only run when the sun shines, as separate from my existing power infrastructure. Though single-purposing it like this eliminates some of the flexibility electric power provides. I'll look for some DC powered commercial swamp-coolers (fan included) tonight, and let you know what I find. This might just be the simplest-setup route, not sure about cost yet.

Why do they call them "swamp-coolers", when the worst place for them to work is in a swamp?

I think the reason they call them swamp coolers is because the poor people in America live in areas surrounded by swamps, and swamp coolers are the most energy/cost efficient way to keep cool.

As an experiment tonight, I suggest you take a spritz bottle with you, as you go to bed.
Once in bed, cover yourself, with a single bed sheet.
Then, using your spritz bottle, spritz your sheet, somewhat thoroughly.
Then, and only then, will you realize how freaking efficient swamp coolers are.

ps. This is why only poor people use such devices, as you will wake up a few hours later, and have to spritz all over again. It is not conducive to a good nights sleep.
 
  • #28
@glurk
I see your confusion here. This is because you have two different applications. For cooling the pond, ambient air flows over the pond surface (with an area of wick to cool by contact). To cool a room, some simple heat exchanger is called for. This is a step up in complexity and would be best with some moving parts. For a big chimney system there would be sufficient power to drive a turbine. But an office has electrical power so a turbine is hardly worth while.
 
  • #29
"...Evaporative cooling produces 2,000kJ for every kg of water evaporated..."
These numbers although apparently huge, don't mean much. To properly quantify the cooling effect Watts (Energy/Unit of time) is a more proper unit to use. Its not how much energy is absorbed; but how much and how fast it is absorbed. It could take a week to evaporate 1Kg of water, in which case the cooling effect will be nothing as the heating due to other sources will surpass that considerably. The key is to make the Energy transfer in a way that more heat is evacuated and faster than what is coming in.
Close circulation refrigeration achieves that. The more refrigerant is evaporated per unit of time and the faster it circulates the more the heat absorption at the expense of the energy consumed to do that, of course.
 
  • #30
TechFan said:
sophiecentaur said:
... Evaporative cooling produces 2,000kJ for every kg of water evaporated.
...
These numbers although apparently huge, don't mean much.
That number means a lot to me, converted to watt-hours. It tells me I have to evaporate 1 kg of water per hour, to be equivalent to my 500 watt electric air conditioner, which keeps me quite comfortable on 100+ °F days, and 80+ °F nights.
... It could take a week to evaporate 1Kg of water
It probably would, if you had a bucket of water sitting on the porch.
, in which case the cooling effect will be nothing as the heating due to other sources will surpass that considerably. The key is to make the Energy transfer in a way that more heat is evacuated and faster than what is coming in.
Close circulation refrigeration achieves that. The more refrigerant is evaporated per unit of time and the faster it circulates the more the heat absorption at the expense of the energy consumed to do that, of course.

I have an experiment going on in my kitchen at the moment. It started about 4 hours ago. The only thing I've determined, is that I've apparently built a psychrometer. The sheet still feels cool to the touch... :cry:

≈80% humidity...

data:
area of evaporative material: 1.25 m2
mass of dry material: 0.16 kg
initial mass of wet material: 0.31 kg
final mass of wet material: 0.17 kg
maximum ΔT: 4°F

conclusions:
tent shaped wet towels in a humid kitchen, will eventually, dry out.
Om is really enjoying an infinite amount of time off.
:biggrin:

next project:
tomato cage tower of evaporative doom...
(with aluminum foil)
(and waxed paper)
(and a 5 watt muffin fan, salvaged from my experiment from 20 years ago)
 
  • #31
"... like kids
playing on the seashore ...
finding smoother pebbles ..."

oMMCheeto - i think you've got the idea...

Have a great afternoon! :)
 
  • #32
"...That number means a lot to me, converted to watt-hours. It tells me I have to evaporate 1 kg of water per hour, to be equivalent to my 500 watt electric air conditioner, which keeps me quite comfortable on 100+ °F days, and 80+ °F nights..."
Exactly, when time is mentioned it has meaning, without it there is no meaning. That's what I meant.
"...It probably would, if you had a bucket of water sitting on the porch..."
Exactly, it depends where you have it. It was just an example to show time its important and without it there is no meaning to that number. That was exactly what I was saying.
 
  • #33
I think the Einstein–Szilard refrigerator hasn't been mentioned before, but I'm not sure how practical that is.

It cools with just a heat source and a cooling part (->pond), without moving parts.
 
  • #34
mfb said:
I think the Einstein–Szilard refrigerator hasn't been mentioned before, but I'm not sure how practical that is.

It cools with just a heat source and a cooling part (->pond), without moving parts.

But the "pond" is to be a heat source (we want to take heat out of it) and not a heat sink in this case. You would need another (bigger) pond to act as the heat sink. :wink:
 
  • #35
TechFan said:
"...That number means a lot to me, converted to watt-hours. It tells me I have to evaporate 1 kg of water per hour, to be equivalent to my 500 watt electric air conditioner, which keeps me quite comfortable on 100+ °F days, and 80+ °F nights..."
Exactly, when time is mentioned it has meaning, without it there is no meaning. That's what I meant.
"...It probably would, if you had a bucket of water sitting on the porch..."
Exactly, it depends where you have it. It was just an example to show time its important and without it there is no meaning to that number. That was exactly what I was saying.

Taken on its own, of course, it's 'only a number' and you are right to question my bringing it up in the first place. I only introduced the figure to show that there need be very little water loss to achieve cooling. If you needed to evaporate 1kg of water to cool 10kg by 1°C then it would be a lousy system. A similar figure could be quoted to account for why they use the fluids they do use in normal refrigerant cycles. No one would use air (which was an early suggestion in the thread) because you need to work very hard to make up for the fact that a state change wouldn't be used in an 'air cycle' system.

Using water evaporation is a very common low tech form of cooling. You can get specially made pot containers to give you cool water in hot countries, which work very well on that principle. A bit of forced draft will increase evaporation rate significantly (the wind does the job but you need it to be blowing when the sun is strongest and that doesn't happen always.) As everyone who hangs out wet washing knows, it is easy to get rid of several hundred g of water in a short time. People in wet clothes die of hypothermia in high winds. The whole phenomenon is very significant and the numbers are very favourable in some conditions.

Your "500W" refrigeration system is very costly to run (if it does, in fact, run continuously) and is unlikely to be an optimum (sole) solution to your house-cooling requirements (although it's the most convenient, of course). My home is the UK (cooling is seldom a problem) and I have just returned from a holiday in Sicily, with baking Sun, almost overhead, all day. People use AC all the time and everywhere and it worried me. I joined in the system of course; well you can't change the world when you're on holiday. But there must be better ways. I notice that roofs are not insulated and upstairs ceilings feel very warm. People do make good use of external shutters, which are closed for most of the day but they are a bit 'antisocial'.

The trouble with any discussion of this sort is that words like 'efficiency' are not always relevant. A point already made is that 'overall cost efficiency' is really what counts and the natural resources in any particular place are the dominant factors. We have all been selling our own pet solutions - which is what PF is all about - but none of us is necessarily 'right' of 'wrong' about choices - only the Physics involved is right or wrong.
 

Similar threads

Replies
25
Views
3K
Replies
4
Views
4K
  • General Engineering
Replies
23
Views
2K
Replies
5
Views
821
  • Mechanical Engineering
Replies
2
Views
216
Replies
4
Views
866
Replies
1
Views
745
Replies
20
Views
3K
  • Engineering and Comp Sci Homework Help
Replies
8
Views
1K
  • Electrical Engineering
Replies
3
Views
480
Back
Top