Lotte World Tower: Freezing the night to use during the day

At Lotte World Tower in Jamsil, Seoul, they run the air conditioning at full blast even in the middle of a hot summer day, but the actual chillers are kept idle. It's not that the air conditioners aren't on; they were turned on the previous night. Ice frozen overnight in underground thermal storage tanks gradually melts during the day, producing chilled water. As water freezes and melts, the thermometer stays fixed at 0 degrees Celsius, while heat moves in and out at a rate of 80 kilocalories per kilogram. Ice packs the same amount of cooling power into a tank that would require six to eight times the volume if using chilled water. That cool breeze blowing down from the 123rd floor is actually the sound of last night's ice melting.