One-watt Air Conditioner

[Swelters]

I'm interested in micro- or mini- air conditioners, less than 4000 BTU/hr, which I think have an important place in any energy-efficient future of the tropics. These are not available in the marketplace, the minimum size being 9000 BTU/hr.

If this interests you, I'd be happy to correspond, some preliminary ideas are at:

http://www.thaivisa.com/forum/blog/swelters/index.php?

Thanks,

Swelters

[Phil Conners]

I won't pretend to know much about aircon systems, but it was always my understanding that an under dimensioned AC is considerably more expensive to run than a over dimensioned....

[Pink Mist]

not wanting to sound insulting, but do you realise what 1 watt is??

A 1 watt resistor is tiny.

[Swelters]

I won't pretend to know much about aircon systems, but it was always my understanding that an under dimensioned AC is considerably more expensive to run than a over dimensioned....

For humid (tropical) areas oversized machines are undesirable because the compressor cycles on and off because the temperature-driven thermostat tells it that it's done its job and the moisture removal is accordingly less effective. Moisture removal (ie lowering the humidity from say 70 percent to say 40 percent) is what you really want, more than "cooling". Ideally, an aircon should be going all the time, just barely keeping up with the load.

But my aim is not room or house aircon, I'm trying to go for big savings by working with "personal space" only.

America has gone the route of energy-effective whole house conditioning, which is in my view a disaster.

Swelters

PS in regard to the query about whether I know what one watt is, I do; the machine described is a bench model, not an operational prototype.

Edited January 28, 2007 by Swelters

[chownah]

A refrigerator is an aircon consuming less than 4000 watts....isn't it?

Edit: Sorry, I mean that a refrigerator is an aircon that produces less thant 4,000 BTU/hr of cooling...isn't it?

Chownah

Edited January 28, 2007 by chownah

[Swelters]

A refrigerator is an aircon consuming less than 4000 watts....isn't it?

Edit: Sorry, I mean that a refrigerator is an aircon that produces less thant 4,000 BTU/hr of cooling...isn't it?

Chownah

Chownah, how is it that every time I come up with an idea that takes a week of thought, you come up with it in a few seconds? But keep in mind that 4000 watts of cooling doesn't take 4000 watts of power to produce

Swelters

[Basil B]

not wanting to sound insulting, but do you realise what 1 watt is??

A 1 watt resistor is tiny.

um, yes... SMD or THD???

The control circuitry probably consumes more power...

What next? nanowatt A/C ?

"So Watt"

[chownah]

A refrigerator is an aircon consuming less than 4000 watts....isn't it?

Edit: Sorry, I mean that a refrigerator is an aircon that produces less thant 4,000 BTU/hr of cooling...isn't it?

Chownah

Chownah, how is it that every time I come up with an idea that takes a week of thought, you come up with it in a few seconds? But keep in mind that 4000 watts of cooling doesn't take 4000 watts of power to produce

Swelters

I'm not following your train of thought here....can you expand on it?

If you want a micro aircon unit you could try running water coils through a refrigerator...the rate at which you pump the water could vary the amount of cooling performed and you could quantify it by recording the outlet and inlet temperatures at the refrigerator along with the flow rate...you could then calculate the amount of energy consumed. This would be a very simple test bed device to supply various amounts of cooling effort...once you have determined how much cooling effort you need for whatever configuration of "personal space" you design then you could start to design the properly sized unit to supply it.

Another source for this sort of aircon equipment would be drinking fountain water coolers or office water coolers. Have you researched how many BTU's per hour a typical human body creates during different states of activity and rest?...or how much water is exhaled or prespired for the corresponding activity or rest state?....just some ideas you might want to toy with.....I'm looking forward to updates on your progress.

Chownah

[elkangorito]

I won't pretend to know much about aircon systems, but it was always my understanding that an under dimensioned AC is considerably more expensive to run than a over dimensioned....

For humid (tropical) areas oversized machines are undesirable because the compressor cycles on and off because the temperature-driven thermostat tells it that it's done its job and the moisture removal is accordingly less effective. Moisture removal (ie lowering the humidity from say 70 percent to say 40 percent) is what you really want, more than "cooling". Ideally, an aircon should be going all the time, just barely keeping up with the load.

But my aim is not room or house aircon, I'm trying to go for big savings by working with "personal space" only.

America has gone the route of energy-effective whole house conditioning, which is in my view a disaster.

Swelters

PS in regard to the query about whether I know what one watt is, I do; the machine described is a bench model, not an operational prototype.

I really think that you're pushing poo uphill.

...and what exactly is your 'personal space'? As I see it, the only way you can have a 'personal space' is to enclose yourself in something. If you plan only to dehumidify this space, it will still get pretty hot. If you have to much supply air, using such low power will not provide enough dehumidification. But good luck trying & have fun.

But what you said earlier was highlighting a typical problem. Many people take a 'stab in the dark' about the size of an air conditioner. If it's to small, it'll work its freckle off & cost a fortune to run. If it's too big, the chances are that the compressor will have a shortened life because it will 'short cycle'. If these 'on/off' cycles are too short (say 2 minutes on, 2-3 minutes off) & the thermostat temp differential is small (say +/- 0.5 degree), the compressor will not get enough cooling & so slowly get hotter.

Home insulation, sensible construction techniques & correct a/c selection will be the only solution to efficient a/c.

Another alternative to the DX (direct expansion) unit is a chiller. They are available in small sizes but not in Thailand. You can definitely get them from Italy & Australia. Very efficient compared to a DX unit.

Edited January 28, 2007 by elkangorito

[Naam]

"For humid (tropical) areas oversized machines are undesirable because the compressor cycles on and off because the temperature-driven thermostat tells it that it's done its job and the moisture removal is accordingly less effective. Moisture removal (ie lowering the humidity from say 70 percent to say 40 percent) is what you really want, more than "cooling". Ideally, an aircon should be going all the time, just barely keeping up with the load."

*****

= correct! oversizing aircons is nothing but a waste of money, both when investing and running (no matter what kind of fairy tales are spread in Thailand).

[Swelters]

I really think that you're pushing poo uphill.

...and what exactly is your 'personal space'? As I see it, the only way you can have a 'personal space' is to enclose yourself in something. If you plan only to dehumidify this space, it will still get pretty hot. If you have to much supply air, using such low power will not provide enough dehumidification. But good luck trying & have fun.

tsk, tsk, from the man with a mind like an open parachute.

But thanks for the thoughtful comments. Question: why should low power not dehumidify? Suppose I spend the day using some low power to make some liquid nitrogen, I could certainly dehumidify some evening air with that, couldn't I? And what's wrong with enclosing yourself in something? Isn't that what a house is? (Or maybe something to stop your papers from blowing around, I'm not so sure what houses are).

Check that parachute!

Swelters

[chownah]

Swelters,

I googled "how many watts does the human body burn" and got alot of good hits.....bottom line is that a good estimate of the 24 hour per day average is 100 watts. Some people's averages would be higher and some would be lower. A personal space cooler would probably be used while being relatively inactive so you might be able to design for less but since there are so many design variable possible and since you don't know the efficiencies of any of the systems to be used at this point 100 watts should serve as a good estimate....so if you wanted to sleep in your own air conditioned coffin for 8 hours, then your cooler would have to dissipate about 800 watt-hours of heat or be able to generate and store enough cool water (for instance) to absorb 800 watt-hours of heat during that 8 hour period. Is this the kind of stuff you need to know?

Chownah

[Crossy]

Swelters,

I googled "how many watts does the human body burn" and got alot of good hits.....bottom line is that a good estimate of the 24 hour per day average is 100 watts. Some people's averages would be higher and some would be lower. A personal space cooler would probably be used while being relatively inactive so you might be able to design for less but since there are so many design variable possible and since you don't know the efficiencies of any of the systems to be used at this point 100 watts should serve as a good estimate....so if you wanted to sleep in your own air conditioned coffin for 8 hours, then your cooler would have to dissipate about 800 watt-hours of heat or be able to generate and store enough cool water (for instance) to absorb 800 watt-hours of heat during that 8 hour period. Is this the kind of stuff you need to know?

Chownah

Ok, best to use ice as the heat (cold) storage medium.

Latent heat of fusion for water is 334kJ / kg. Meaning that to convert 1kg of ice at 0C to 1kg of water at 0C it must absorb 334kJ of energy.

800 Watt hours equates to 800J x 3600s (1 Watt = 1J per second) = 2880 kJ

So to provide our 800 Watt-Hours we need 2880/334 = 8.6 kg of melting ice, seems possible.

BUT

You need to provide ventilation (or you may run into death problems) so you're going to lose some of your cold in the ventilated air, initially you're going to need to cool the air in your personal space to whatever you deem an acceptable temperature, and you'll be losing cold through the walls, floor and ceiling. Obviously the amount of heat you lose through the walls etc. depends upon the material and area of the surfaces (I'll let someone else do those sums ).

Edited January 29, 2007 by Crossy

[chownah]

Ice is nice but the energy used per unit of heat expelled is greater if the difference between the heat sink and the load being cooled is greater. This means that per equivalent amounts of heat expelled it will take more energy to expell heat from water at 0 deg C than it does to expell heat from water at 10 deg C. If you look at the coefficient of efficiency (I think thats what its called) for refrigeration equipment I think that you will see that it is rated at a certain temperature difference and if you can get a performance curve for the refrigeration cycle for that system you will see that the greater the difference in temperature the less the efficiency will be.

Bottom line....for maximum efficiency don't make anything any colder than it has to be.

At least I think that this is correct although its been a long time since I studied this stuff.

Chownah

P.S. I'm trying to dredge up some old memories on this subject and I'm wondering if anyone out there can comment on this: Is it true that since ice is a more ordered state than water if you create ice and then allow it to melt you are losing energy through/as entropy?

Chownah

Edited January 29, 2007 by chownah

[backflip]

Why not use a simple Peltier device? Strap them to your wrists or your forehead, and you'll be cool.

[Crossy]

Bottom line....for maximum efficiency don't make anything any colder than it has to be.

Agreed, but ice has the nice charateristic that it absorbs energy without warming up making the rate of absorbtion more predictable.

Using water means that for every 4.2kJ you absorb each 1kg of water warms up by 1C, so if you use 8.6kg of water and pump our 800 watt hours into it the temperature of the water will rise by 80C, you need a LOT (like 10x) more water to keep the temp rise manageable.

PS I think I got the sums right there, it has been a LONG time since school physics

Commercial off-peak aircon units (that store cold at night using cheap electricity) use ice as the storage medium.

I do enjoy these thought experiments, we all know that the practical solution is to go and buy an aircon unit from HomePro :D

[Phil Conners]

"For humid (tropical) areas oversized machines are undesirable because the compressor cycles on and off because the temperature-driven thermostat tells it that it's done its job and the moisture removal is accordingly less effective. Moisture removal (ie lowering the humidity from say 70 percent to say 40 percent) is what you really want, more than "cooling". Ideally, an aircon should be going all the time, just barely keeping up with the load."

*****

= correct! oversizing aircons is nothing but a waste of money, both when investing and running (no matter what kind of fairy tales are spread in Thailand).

Interesting. My AC units all have "moisture removal" setting. Using that would it matter if the AC is over dimensioned?

[chownah]

Why not use a simple Peltier device? Strap them to your wrists or your forehead, and you'll be cool.

Very very inefficient unless you want to cook something at the same time that you are cooling something so you at least get the use of the heat wasted....maybe heat water in a tank...strapped to your hips????

Chownah

[chownah]

Bottom line....for maximum efficiency don't make anything any colder than it has to be.

Agreed, but ice has the nice charateristic that it absorbs energy without warming up making the rate of absorbtion more predictable.

Using water means that for every 4.2kJ you absorb each 1kg of water warms up by 1C, so if you use 8.6kg of water and pump our 800 watt hours into it the temperature of the water will rise by 80C, you need a LOT (like 10x) more water to keep the temp rise manageable.

PS I think I got the sums right there, it has been a LONG time since school physics

Commercial off-peak aircon units (that store cold at night using cheap electricity) use ice as the storage medium.

I do enjoy these thought experiments, we all know that the practical solution is to go and buy an aircon unit from HomePro :D

86 kilos of water wouldn't necessarily be prohibitive...I was thinking of a refrigerator that was emptied of shelves and custom fitted with a big water tank...or if the refrigerator could keep up with the load then you wouldn't even need the tank but just a coil. Then you would have two water lines running to the "personal space" a cold water supply and a return line.

Also, if the personal space was mostly used for sleeping then it might be that you would require more cooling initially but as you go into a deep sleep your metabolic rate would slow and your body temperature would fall and the cooling load would gradually diminish....perhaps a water bed filled with chilled water if properly designed might provide that diminishing cooling ability to match your bodies circadian rhythm.

If the refrigerator was located higher than the "personal space" could a thermo-syphon system be designed so that no pump would be required?...I think so.

Interesting that the commercial units use ice....this at least shows that the difference in cost for power is greater than the increase in energy required per unit of cooling......of course I could be wrong on my COE comments but I think its correct....I'm still hoping someone can comment on the possible effect of enthalpy.

Chownah

[mlsk]

But my aim is not room or house aircon, I'm trying to go for big savings by working with "personal space" only.

Just "personal space" only? Try chucking an ice pack down your shirt, and once its melted, change it.

Get those ice packs that are not too bulky.

This is the most portable personal cooling system, and it works both indoors and outdoors too.

You will be surprised how cooling it feels.

Madmike

[phibunmike]

Swelters,

I googled "how many watts does the human body burn" and got alot of good hits.....bottom line is that a good estimate of the 24 hour per day average is 100 watts. Some people's averages would be higher and some would be lower.

Chownah

100 watts sounds like a good figure. There are lots of tables around showing energy usage in calories (kilo-calories) for different people with different activities, so to convert use this:

The food calorie is a kilocalorie --- the heat that will raise the temperature of one kilogram of water one degree Celsius. It is equal to 4186 joules. A diet of 2400 calories per day, equal to 100 calories per hour, is equivalent to 116 joules per second, or 116 watts.

As Chownah suggested, use a small fridge - but leave the door open. Detach the cooling circuit at the back and stick it outside (or put it in your hot water tank).

Cheers,

Mike

[Swelters]

Thanks for the wonderfully creative and technically capable thinking, Chownah, Crossy, all. Rather fun to challenge oneself with high school physics, eh? I was going the daytime "ice machine" direction(ie the opposite of the load transfer strategy), or just start out with a block of ice delivered like they used to have in my grandmother's house in Boston Mass in the 1940s. The Iceman Cometh!

I'm figuring April-May sleeping time as the target need, the 100 watt per person load is the goal here. If the conditioned air is fresh air, then the ventilation issue is solved. A 2 inch pipe delivering cool dehumidified air at 2 m/s would provide about 300 watts of cooling, about 3 kg ice per hour.

The ice pack alternative is a challenging alternative, one wonders why this solution is not more heard of. Maybe it's too much cooling applied to too little area of the body.

A mosquito net (which I fancy if only for esthetic reasons) provides a natural enclosure, but you'd want one that doesn't breathe much in order to retain the conditioned air.

I'm off for a stint in the wat; no speakee, no emailee for a while, but thanks much for the responses and I'll look forward to resuming the topic in a couple of weeks if there is interest.

Swelters

[pete_r]

P.S. I'm trying to dredge up some old memories on this subject and I'm wondering if anyone out there can comment on this: Is it true that since ice is a more ordered state than water if you create ice and then allow it to melt you are losing energy through/as entropy?

Chownah

I think you're right Chownah. One of the rare memories I have from thermodynamics is that if you consider the system "Water + Surroundings", the entropy variation will always be positive during a freezing + melting cycle, according to the second law of thermodynamics (meaning that you are increasing the overall entropy and disorder of the universe). But I'm not sure how it works if you consider the system "Water" only.

[chownah]

A somewhat off topic but also somewhat related can be found at:

http://www.straightdope.com/mailbag/mfancool.html

The title: Is fanning yourself energy-efficient?

It does an analysis on whether the effort required to fan oneself with a hand fan creates more bodily heat generation than the amount of heat it removes......the answer.....it depends.

Chownah

[Naam]

Interesting. My AC units all have "moisture removal" setting. Using that would it matter if the AC is over dimensioned?

there might be quite some savings with this setting depending how the manufacturer has designed your AC. a year ago in a rented home i detected the rather intelligent setup for the moisture removal setting. compressor runs whereas the fan of the inside unit is slowed down to an absolute minimum thus creating a very low temperature at the evaporator. the lower that temperature the higher the dehumidification. but that works only till ice forms on the evaporator. once a sensor detects that both fan and compressor are stopped by the chip that is connected to the sensor till the ice melts and turns into condense water which drains. after a few minutes of standstill the whole procedure is repeated.

if the AC does not have that function try to run the fan on the lowest speed to achieve best dehumidification. but that is a sword with two edges if distance between inside and outside unit (length of piping) is too much (something done in Thailand all the time). in this case -unfortunately- energy savings = zero.

Edited January 30, 2007 by Dr. Naam

[Naam]

Why not use a simple Peltier device? Strap them to your wrists or your forehead, and you'll be cool.

or a few ice cubes in the underpants

[phibunmike]

A somewhat off topic but also somewhat related can be found at:

http://www.straightdope.com/mailbag/mfancool.html

The title: Is fanning yourself energy-efficient?

It does an analysis on whether the effort required to fan oneself with a hand fan creates more bodily heat generation than the amount of heat it removes......the answer.....it depends.

Chownah

Drinking cold beer is energy-efficient if one ignores the negatively entropic (positively enthalpic?) refrigrator (and serving staff, and energy involved in cap removal).

I suspect that the potential energy released by pouring the beer into a glass goes someway towards compensating for the energy required to swallow the beer, but not completely.

Such is life

[Beardog]

Personally I think if you were to find or make The silencing chamber(USED IN GET SMART-On Television) in the 1960's in the U.S. & put a bag of ice in it & seal yourself in It might provide enough to keep you cool in a hibernative state.........the only drawback I could see is suffocating due to lack of oxygen.

Seriously if you get a decent ac at homepro or wherever & run it for 1 hour on sleep mode at night with a fan running it will be enough to cool you off till you pass out & the fan would take care of the rest. A hel_l of a lot easier to sell any farang.

Or The Peltier device as it has already proven it does cool your body temp already. Not to be insulting, but I think you would make as much in Pattaya opening another beer bar!

Beardog