In short, the building is heated and cooled using an off-peak electricity system. For cooling, they freeze big blocks of ice during the night (off-peak) and then use the ice to cool the building during the day (peak).
That’s all good, but they lose me at this quote:
*It also cuts down on pollution. An ice-cooling system in the Credit Suisse offices at the historic Metropolitan Life tower in Manhattan is as good for the environment as taking 223 cars off the streets or planting 1.9 million acres of trees to absorb the carbon dioxide caused by electrical usage for one year.*How does off-peak energy usage cut down on emissions? Isn’t it just shifting the energy usage from one time to another? Indeed, wouldn’t this example cost more energy, just by entropy in the additional heat exchange and energy lost to imperfect thermal storage?
Here at the Nardo household, we have an off-peak heating system. Ideally, the heat pump and hot water heater should only run at night, and during the day we get our heat from a big tank of hot water that was heated overnight. It cuts back on the monthly electric bills quite a bit. And in the community, less peak electric usage means less necessary capacity for the generators. But I never considered it “green”, because I don’t see how it uses less electricity.
It mainly depends on what type of generation supplies the base load, vs. what supplies the peak load.
Where I live, base load is supplied by fairly dirty coal fired plants. Peak load is supplied by much cleaner gas fired plants…so I would expect shifting usage off-peak to be enviromentally misguided.
If you live where base load is supplied by nuke or hydro plants, then off-peak usage adds less carbon load. (though the “greeness” of the sources I used as an example is debateable)
Finally, the peak load must be distributed. When you double the current in a transmission line, you slightly more* than quadruple the transmission losses. The same is more-or-less true of transformers etc. Thus by transferring load off-peak, net distribution losses are reduced.
*It would be esactly 4X if the resistance were constant, But at higher current, the lines run slightly warmer, thus the resistance increases a bit.
Thanks, Kevbo. Those reasons make sense to me. Especially the last one, regarding higher resistance based on higher current through the transmission lines.
There’s also the fact that off-peak use results in a more efficient use of generator resources. If you keep increasing peak usage, pretty soon you have to build new generators. Building a new generator is certainly not cheap or clean, as far as energy use goes. Increasing off-peak use doesn’t require the construction of new generation facilities.
Depending on geographic constraints, peak energy may have to travel further on average, increasing transmission losses.
In the case of an ice cooling system, you use less energy because the waste heat from refrigeration is produced at night, when you don’t really care and you can just vent it to the atmosphere, and the cool air is used during the day. Compare this to an active refrigerator running during the hottest part of the day. You have to do extra work to pump the heat generated by the compressor out of the building as well as working against heat entering from outside (or generated by other sources).
Another minor (probably very minor) effect: In the same way as above, power plants should be more efficient at night, since they’re heat engines and the heat sink (the atmosphere) is cooler, then.