Green's function simulation method for earth-air heat exchangers

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Montana State University - Bozeman, College of Engineering


Earth-air heat exchangers (EAHXs), or earth-tubes, decrease building heating/cooling loads by pre-conditioning supply air in underground pipes. Air circulates underground to exchange heat with the surrounding soil before entering the building. The concept is fairly simple, but the field currently lacks fundamental information of the energy interactions in common EAHX installations. This identifies the need of a model framework inclusive of all EAHX design considerations, time-dependent climate conditions, and EAHX types to support the completion of this fundamental information. The EAHX types in this study include installations under locations free of structures (under yard) and under foundational slabs (under slab). This research develops the groundwork of this framework through a versatile model using the Green's Function method and numeric integration. The Green's Function method incorporates the majority of time-dependent heat transfer mechanisms surrounding EAHXs through long and short time solution components. The long time component calculates the initial soil temperature distribution in the under yard, non-radiant under slab, and radiant under slab installations. The under yard simulations were successfully validated using experimental soil temperature data from around the United States. The non-radiant under slab temperatures produced unrealistic results in some locations, but the novel Green's Function method in this location has significant potential for under slab EAHX applications. Results from the long time solution feed into the short time solution as a space-dependent initial condition. The short time solution uses a finite difference approach to calculate the heat transfer along the EAHX length. This method was validated using computation fluid dynamics with good agreement. The two components work together to quickly simulate a large number of EAHX installations. The research includes an example optimization procedure to demonstrate the framework's versatility. It successfully optimized the EAHX lengths for 95% effectiveness in cooling on the hottest day of the year in 15 locations around the United States.




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