The building contents Bill Nettles mentioned also adsorb water that needs to
evaporate and then re-condense in the A/C evaporator when the A/C comes back
on. This also slows down the return to "comfortable conditions" that some
people will complain about. This may or may not cost extra money. Had the
A/C been left on, and if the moisture would have just been infiltration,
then leaving the A/C on would have had to condense that infiltrated water
anyway. So running the A/C all the time does not save money from the cost
of energy itself, but keeping the humidity by running the A/C 24/7 reduces
the uncomfortable time if people must occupy the space during the "catch-up
time." The "humidity catch-up" can grossly increase the catch-up time more
than what is caused by the temperature catch-up. The heat of vaporization
of water is clearly many times larger than the heat capacity of air.
On the other hand, letting the humidity increase can have other costs that
are not directly energy costs. As I mentioned in an earlier post, the
higher humidity can cause various kinds of damage to equipment and to
structures. This can be direct water damage, or can be the stress of
cycling the humidity. Remember that when the A/C shuts off, say for an
evening, that everything in the cooled space is cool. The space will warm
up, and you would think this would result in a lowering of the relative
humidity. But water vapor, with a molecular weight of 18, can diffuse into
the space at a higher rate than the thermal leakage. If the humidity rises
fast enough such that the walls, furniture, equipment surfaces end up at a
temperature lower than the dew point, you get direct water condensation on
every cold thing in the space. We have actually had occasions in which the
A/C unit in our science building failed during a humid night, and we arrived
the next morning to find water condensing and running down the inside walls.
This also means it was condensing on/in any equipment that was turned off,
and therefore not warmer than ambient.
Another thing the James Mackey experiment does consider is the off-peak
electric meters that are becoming more common. My house is not yet this
way, but our campus buildings and many public buildings around here are
paying off-peak rates at night. If the A/C shuts off at night, and the
catch-up occurs after 6:00 AM, then the catch-up is using electricity billed
at peak rates whereas the electricity saved during the night would have been
off-peak. Even if the nightly shutdown saves some energy, it might actually
cost more money. This seems to be the case at my church.
Although one might argue that saving energy is more important than saving
money, a big part of our electric-grid and generation problem is satisfying
peak needs. Nightly shutdowns that shift some of the off-peak consumption
to on-peak consumption are not necessarily a move in the right direction.
Someone else mentioned ice-making at night. We have several campus
buildings in operation that make ice off-peak and then melt that ice during
the day in order to move in the direction of shifting some of our cooling
load into off-peak hours.
Although I appreciate attempts to simulate energy usage and possible savings
by doing experiments like James Mackey described, the full picture is pretty
complicated and not easily simulated in the lab. We did not anticipate
water running down our walls and losing the salt windows in our FT-IR until
it happened. That was expensive. Also, so far, from viewing our actual
electricity bills, it appears we are saving money at the church by keeping
the temperature constant 24/7 rather than setting it back at night. This
doesn't even count any savings that can come from reduced organ or piano
maintenance, etc.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Chair, Division of Natural and Applied Sciences
Bluffton University
1 University Drive
Bluffton, OH 45817