[TriLUG] OT: thermodynamics of A/C question

[David Both]

My 20 year old HVAC unit died several weeks ago. I replaced it with a new unit 
of the same make (20 years, after all) with double the efficiency. My first full 
month of usage was June and my electric usage was reduced by over 60%. I also 
added zoning so that the upstairs, especially the bonus room over the garage, 
where I have my many computers is nicely cooled without having to use the very 
inefficient room air-conditioner.

My point is that new units are very efficient and represent a simple way to 
improve your overall efficiency and reduce energy usage. If your old unit is old 
enough, then you will have to do it soon anyway. And appropriate zoning is a way 
to improve your efficiency since it moves the cool air to where it is needed and 
you are not cooling areas that are already cool enough.

I have no idea whether any of these other methods work, although I suspect that 
spraying the radiator coils with cool water would help as it would absorb much 
of the heat from the coils. But the water would have to be cooler than the coil 
temperature (not too hard these days) or it would add heat to the coils. No 
math, just a bit of thermodynamics from my college semiconductor classes.

Of course keeping your HVAC unit shaded also helps considerably.

To get to your original question, regardless of the thermal transfer medium, 
Maxwell and his daemons work best with greater thermal differentials. Therefore 
cooling when the outside air is coolest makes for greater efficiency than when 
it is hotter. The problem is that you need cooling the most when the outside air 
is hottest.

Passive solar homes use thermal heat sinks to help with this. They absorb heat 
during the hottest part of the day and then radiate it in the cool(er) parts of 
the day. This is simply a thermal capacitor that works best with hot days and 
cool nights.

The problem I see with cooling a lot in the AM is that there is no thermal sink 
to be cooled during this time and release its heat during the cool parts of the 
day and then to absorb it from the air during the hot parts. So you would be too 
cool in the coolest parts of the day and too hot in the hot parts of the day.

Just my $0.02.

On 07/04/2012 07:29 AM, Joseph Mack NA3T wrote:
> On Wed, 4 Jul 2012, Jeremy Portzer wrote:
>
>> But Joe, the condenser unit already has a "radiator" to cool the compressed
>> freon: - heat sink, fins to transfer the heat to the air, and a large fan,
>> just like in your car.  What's the benefit of an extra cycle of water in
>> addition to that?
>
> My initial posting was to ask
>
> "if you're moving a fixed number of BTU/day, is it better to move them when
> the air is cooler, eg early morning 5am, rather than running the A/C full
> blast mid afternoon, when the outside air is hottest"
>
> The assumption was that the A/C is a Carnot machine and in the early morning
> the condenser would be operating at a lower temperature. From the Carnot cycle
> efficiency formula, I expected I'd get 50% more efficiency.
>
> No-one had an answer to this, and none of us know if an A/C operates at Carnot
> cycle efficiency (I suspect it doesn't), but we've had fun discussing other
> aspects of A/C as a result.
>
> However if it's more energy efficient to have your condenser at a lower
> temperature, then you want your condenser operating against the lowest
> temperature heat sink. Air isn't great for transferring heat. If you had the
> condenser in a countercurrent heat exchanger with water, then the condenser
> would be operating at air temp, 80-100degF rather than at 160deg.
>
>> And of course we've already discussed the cooling tower systems that
>> commercial A/C's use and the pros/cons of those:  they aren't cost-effective
>> for small-scale systems.
>
> evaporative cooling has been discussed. A couple of people have talked about
> sprinkler systems to cool the condenser in home A/Cs, so some people think it
> can be scaled down. No-one has shown that evaporative cooling can't be scaled
> down. It might be true, but I haven't seen any numbers showing it. Whenever
> someone says "it can't be done" and has no numbers or proof, I assume they
> just don't want to do bother doing it and are making excuses.
>
>> To get back to the spirit of the thread however, another easy to take action
>> you can do to improve your condenser's efficiency is to simply clean the
>> fins, so that they can transfer heat between the freon and air more
>> efficiently. See for example this link:
>> http://blog.srmi.biz/energy-saving-tips/residential-air-conditioning-aircon-ac/cleaning-condenser-coils/
>> .
>
> they tell you that the fins are dirty even if they look clean. Strike 1. They
> don't have numbers for effectiveness ($, efficiency) of their suggestion.
> Strike 2. I expect this is an infomercial flogging their books.
>
> Joe
>

-- 

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"I'd put my money on the sun and solar energy. What a source of power! I hope we 
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  - Thomas Edison, in conversation with Henry Ford and Harvey Firestone, 1931

*********************************************************
David P. Both




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David P. Both, RHCE
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