Session: 12-07-01: Engineered Materials for Thermal Control

Paper Number: 172722

Multi-Season Passive Variable Insulation for Buildings Using Magnetic Thermal Diodes. 

Variable thermal insulation offers potential for energy reduction as compared to the traditional static insulation case, especially in regions that experience large diurnal temperature swings. Variable insulation would allow buildings to exchange heat flow between the inner and outer surfaces of the building when local conditions are favorable, enabling passive cooling during a cold summer night or passive heating during a warm winter day while still blocking heat flow at other times. Active variable envelope insulation methods  consume electricity, may require complicated sensing and actuation methods to operate effectively, and cannot operate in power outage scenarios. Passive variable insulation (PVI) would use temperature responsive devices to switch between thermally conducting ON and thermally insulating OFF states. It is well-known that fluidic thermal diodes with preferential heat transmission direction can achieve effective single-season PVI (either heating or cooling mode). However, to maximize the uptake of the technology, a PVI system would ideally be able to operate in multiple seasons (i.e., would provide both the “cold summer night” and “warm winter day” advantages). 

Here, we present proof-of-concept experiments of a thermal circuit to assist in regulating the building temperature in multiple seasons, achieving both passive building heating and passive building cooling at different desired times. The PVI thermal circuit consists of two magnetic thermal diodes oriented in anti-parallel. These centimeter-scale magnetic thermal diodes exhibit forward-mode mechanical linear oscillations due to an unstable balance between magnetic and mechanical forces. The thermal energy transferred during the oscillations leads to diode-level thermal conductance rectification ratios of 6 in air with gravity-independent performance and durability over >10,000 cycles. These oscillations are controlled by the temperature-dependent magnetic properties of gadolinium, which has a convenient Curie temperature for use in building thermal regulation. Our experiments show that a prototype PVI circuit using these magnetic diodes enables thermal regulation with a narrow 4 oC comfort band and circuit-level thermal switching ratio of 3. Thermal modeling considering year-round thermal conditions in different U.S. climate zones for a south-facing wall with prescribed interior  shows that the PVI circuit offers up to 47% reduction in building envelope HVAC thermal loading with base-case thermal parameters and future growth opportunity with optimal parameters and improved diode thermal rectification. The PVI circuit can also assist in regulating the building  during cold-weather or warm-weather power outage scenarios if the diurnal  changes are relatively large; in contrast, active systems would require backup power to function during an outage. Thus, this work demonstrates the potential for magnetic thermal diodes in passive variable insulation systems to reduce the energy consumption needed for building thermal management.

Presenting Author: Geoff Wehmeyer Rice University

Presenting Author Biography: Geoff Wehmeyer is an assistant professor in Mechanical Engineering at Rice University. He received his B.S. in Mechanical Engineering from the University of Texas at Austin in 2013 and his Ph.D. in Mechanical Engineering from the University of California, Berkeley in 2018 before joining the faculty at Rice. His research group develops thermal devices and studies thermal properties of materials for improved thermal management in aerospace, semiconductor, and energy applications. His research has been recognized with a NSF CAREER Award (2022) and NASA STMD Early Career Faculty award (2019), and he was awarded the 2021 Sophia Meyer Farb Prize for Teaching and the 2024 School of Engineering Tenure-track Teaching + Research Award at Rice.

Authors:

Lorenzo Castelli Rice University
Monisha Vijay Kumar Rice University
Geoff Wehmeyer Rice University