Low-Energy Building Cooling Systems

Global and national concerns about energy consumption and carbon emissions are leading to tighter standards for buildings, the largest energy-consumption sector in the U.S. and other countries. In many climates, space cooling requires significant energy. Cooling systems pump heat out of cool indoor spaces into warmer ambient conditions. Even though this temperature difference may be small, a chiller or air conditioner must produce very cold refrigerant to absorb heat from indoor air or from chilled water in the heat pump’s evaporator and then must elevate the refrigerant temperature to a very high level to reject the heat to outdoors in the condenser.

This project studies one approach to improving the efficiency of cooling systems. Radiant cooling, via pipes embedded in floor slabs, increases heat-transfer surface area, making it possible to raise the temperature of the chilled water and the refrigerant in the evaporator. Making use of heat storage in the slab permits pre-cooling at night, when outdoor temperatures are lower and the refrigerant in the condenser can be at lower pressure and temperature. When combined with model-based predictive control to take advantage of further efficiencies associated with running the heat pump’s compressor and auxiliaries at lower speeds, this approach provides substantial reductions in cooling energy consumption. Lab tests at MIT have shown savings of about 20-25%.

This work has been performed by former MIT PhD student and postdoctoral researcher Nick Gayeski and now by MIT PhD student Tea Zakula., directed by Masdar Institute professor Peter Armstrong and by Les Norford.