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Heat from Ice?

A novel HVAC system has been implemented to fully utilize ambient energy from the Colorado outdoor environment. Our design specifically takes advantage of the clear skies and large daily variations in outdoor temperature throughout much of the American West. Similar to traditional passive heating and cooling strategies, we capitalize on solar energy and natural temperature variations to heat and cool, using thermal storage to manage the mismatched timing between building needs and environmental opportunities. Unlike traditional passive systems, we use mechanical systems to actively manage energy transfers.

HVAC System Schematic of Thermal Storage, Radiant Panels and BIPV-T

At the core of the system are a water-to-water heat pump and two tanks for hot and cold water storage. Water from the tanks is used to heat and cool the house. During mild weather, the tanks can be heated or cooled by the environment using heat exchangers located on the backs of the PV modules. Solar radiation during the day will heat the hot tank to meet building heating needs at night. Thermal radiation to the sky at night, coupled with cool outdoor temperatures, will cool the cold tank to meet building cooling needs during the day.

For most of the year, though, the ambient energy needs a boost to meet the building heating and cooling requirements. The heat pump connected between the hot and cold tanks will heat the hot tank and cool the cold tank at very high efficiency. The storage tanks enhance the efficiency of the system compared to a conventional heat pump by optimizing the time – and temperatures – at which energy is transferred to and from the outdoor environment. For example, in winter, the main heating needs occur at night when it is coldest outdoors. However, the efficiency of the heat pump is higher when it is warmer outside. Similarly, in summer, the main cooling needs occur during the day when it is hottest outdoors. Unfortunately, the efficiency of the air conditioner is higher when it is cooler outdoors. In both cases, the storage tanks in our system allow us to bridge the gap between day and night to improve system efficiency. The cold tank even includes encapsulated ice to expand the thermal storage capacity with a relatively small tank volume.

The storage tanks also allow optimization of the interaction between building loads and power sources, both from the sun and from the electric utility. Energy storage is a well-known challenge and common component of solar energy systems. Today, residential utility customers seldom see opportunities for thermal storage – energy costs the same at night as it does during the day. However, as utilities look to further improve the match between grid loads and energy resources, our design offers opportunities for distributed storage in conventional homes as well as solar homes. Our team is also exploring novel control strategies, including online models and forecasting, to improve the utilization of the energy storage.

Interior Rendering of Architecturally Integrated Heat Exchangers

Hot and cold water from the tanks is circulated to heat exchangers to provide heating and cooling to the individual rooms. All of these heat exchangers are located in the engineering spine of the house, but are also in thermal communication with the adjacent rooms. Some of the heat exchangers are conventional coils with a fan to circulate air to the rooms. In addition, some heat exchangers are architecturally integrated into the interior design in the form of radiant panels or exposed tubing. While radiant heating has a long history, from the old cast iron radiators to the modern radiant floor systems, radiant cooling is rare in residential applications, largely due to the problems with condensation on cold surfaces. However, Colorado’s dry climate allows us to use these energy-efficient systems that might not be applicable in other climates.