[Greenbuilding] Central vs window AC

[John Straube]

The reasons for the ratio of sensible to latent heat are in the physics of air, not government regs.
Get a psychrometric chart and draw on the process in terms of enthalpy (total energy).
Drying to 40%RH requires me to cool to just above 40F.
Now I need to reheat that air to get it to 70F, or I will overcool the space.  This is a normal one-speed fixed output air conditioner.
A dehumidifier uses the same techniques and components but puts the heat back in the room, thereby drying the air down to 40% RH and heating it to over 70F.
Hence, the recommendation for dehumidifiers if you have a building that requires more than about 30% dehumidification.
You cant buy a small AC unit that puts arbitrary amounts of heat back into the air and the remainder to the outside.  We have built several, and they are undergoing field trials in a numerous locations (because the problem in real life is more significant and widespread than computer simulation show).

If you reduce airflow through the AC, the air gets colder, and this drags more moisture out the limit is 32F, at which point frost forms and quickly blocks it off.  Because the refrigerant coming in is colder than when it leaves, one can only cool to about 40F air and not freeze the coil, so it is difficult to go much lower by slowing flow further.  Some systems use sophisticated controls and even IR frost sensors to get as low as possible, and these can get to a SHR of perhaps 0.60.

The easiest current answer is to get a really good efficient variable output AC unit, like a ductless split, run them slow (low airflow, low capacity) and then add back heat for the hours needed. As ductless units often can run as heat pumps too, it is feasible (not yet implemented) that one could run on cool and dehumidify for say 2 hours, then, as the indoor temperature drops to say 70 from 74, run on heat pump for 20 minutes to boost temperature to 74, then begin the cooling again.  People would complain about this "large" temperature swing, but the energy requirements would be very very low.  And this could all be done with a SEER26 (COP=5+) $1500 mini-split that can do all your heating in the winter.

Enthalpy controlled ventilation is a common technique which reduces cooling needs exactly in the cool dry hours that are not a big problem.  Often they only work when the outdoor temperature is below 60F (and air at 50F and 100%RH- rainy cool night- actually has more humidity in it than a 70F 50%RH room). As experience tells us, if you routinely have night temperatures below 60F, and it is dryish, there are lots of cooling techniques that work.  Surprisingly, the efficiencies of compression based AC is now so high, that economizers often use more energy to run the fan through filters ducts and dampers, than the AC uses when it is 60F outside. We know regularily have to check if the economizer saves energy and numerous times it will not without a major redesign of the economizer ductowrk.


On 12-06-10 7:21 AM, nick pine wrote:
> John Straube <jfstraube at gmail.com> writes:

>>  Almost all air conditioning systems have a sensible heat ratio of
>> about 0.70 to 0.75. This means that 3/4 of the energy goes into
>> cooling the air and 1/4 goes into removing moisture.
>
> I wonder why. Something to do with government regulations? Why not
> cool 70 F air to 40 for dehumidification and reheat it to 70 and
> exhaust the latent heat outdoors?
>
>> In Philly or New York, if you control solar gain (shade) and have
>> decent insulation and airtightness, the ratio required is more
>> along the lines of 0.30 to 0.60. So, normal systems cause problems
>> with high humidity. ASHRAE has spent a ton of money in the last
>> while on research to try and identify and solve this problem.
>
> Why not just reduce the airflow? When I put duct tape over some of
> the air over some of the air outlets of a window AC, the output temp
> dropped and it became a better dehumidifier.
>
>> In weather that is dry enough (eg lets say that air would have less
>> moisture in it than it would at 75F and say 40%RH) it is so cool
>> outside that night ventilation works
>
> Air at 75 F and 40% RH has vapor pressure Pa =
> 0.4e^(17.863-9621/(460+75)) = 0.355 "Hg, approximately, and humidity
> ratio w = 0.62198/(29.921/Pa-1) = 0.00746, which seems a lot more
> than "dry enough" for human comfort. The object is comfort, no?
> ASHRAE says still air at 80 F with humidity ratio w = 0.0120 is
> comfortable. Higher humidity can be comfortable with a ceiling fan.
>
>> This is Gennaro's New Mexico example. Works great. Does not work in
>> NYC, Philly, even Chicago.
>
> I'd agree, if you were to say it does not work 100% of the time.
> Let's quantify this. My simulations say smart ventilation can work
> for all but 2 weeks in August in an airtight brick row house in
> Phila, with significant savings, compared to AC without night
> ventilation. Big buildings often have night ventilation with enthalpy
> economization. Who makes one of these
> http://www.youtube.com/watch?v=wexdNx_StRc in a nicer package?


--
>
Prof. John Straube, Ph.D., P.Eng.
Faculty of Engineering
Dept of Civil Engineering / School of Architecture

www.buildingscience.com