The DR Byen building, now under construction in Copenhagen, will produce most of its cooling through an innovative deep water storage system. The winter cold is stored in a thermally and hydraulically balanced porous groundwater aquifer, at a depth of approximately 65 feet. The ground water is cooled down to 2 to 3 degrees C by using free cooling (cooling by means of an air cooler) in the winter, and the water is returned to the underground storage. The ground water moves so slowly - and the ground insulates so well - that the water keeps cold for months. By undercooling the ground water the system can almost double the cooling effect of the ground water in the summer. The building also makes extensive use of natural ventilation, by pulling in cool night air through highly placed windows to precool the building on warm summer days. Utilizing groundwater and external air for cooling the building will save an estimated 75% in comparison with regular compression cooling. (figure by Cowi A/S)