Integration of daytime radiative cooling and solar heating for year-round energy saving in buildings

Building heating and cooling account for a substantial portion of global energy consumption and greenhouse gas emissions. Current solutions often focus on single-function technologies (either heating or cooling), limiting their effectiveness across diverse climates and seasons. This research addresses this limitation by developing a dual-mode device capable of both heating and cooling, adapting to varying weather conditions for optimized energy efficiency. The high energy consumption in buildings, exceeding 30% of global final use and contributing to 10% of global greenhouse gas emissions, presents a significant challenge. In the US alone, annual building energy costs surpass $430 billion, with space heating and cooling consuming roughly 48% of this total. Projected growth in heating and cooling demand (79% and 83%, respectively, from 2010-2050) underscores the urgent need for innovative, sustainable solutions. Existing passive systems, such as solar heating and radiative cooling, suffer from limitations due to fluctuating weather patterns. The varying heating and cooling degree days across different US climate zones highlight the need for adaptive, switchable systems that can efficiently manage indoor thermal environments year-round. This study aims to develop a novel dual-mode device that addresses these challenges, offering significant energy savings through dynamic adaptation to climate changes.

The paper reviews existing literature on radiative cooling and solar heating technologies for building applications. It highlights the limitations of single-mode approaches and the need for dual-mode systems that can switch between heating and cooling based on climatic conditions. Existing research on radiative cooling materials, selective absorbers, and thermal management strategies is discussed, laying the groundwork for the proposed dual-mode device. The authors cite various studies on the challenges of efficient heat transfer and the need for improved thermal contact conductance in such systems.

The study presents a dual-mode device consisting of rotary actuators and a thin-film polymer composite with integrated solar heating and radiative cooling functionalities. The key innovations involve electrostatically controlled thermal contact conductance to minimize thermal resistance between the device and the building envelope. The methodology includes material selection (polyimide film, zinc film with copper particles, silver film, and polydimethylsiloxane), fabrication techniques (electrodeposition, evaporation), and characterization of optical and thermal properties. Electrostatic control is achieved by applying high voltage to the electrode, utilizing Maxwell pressure to enhance thermal contact. The performance of the dual-mode device was evaluated experimentally using a Peltier-based system, measuring both heating and cooling power densities under outdoor conditions. Building energy simulations were conducted using EnergyPlus to assess the potential energy savings of the dual-mode device across different US climate zones. The simulation model considers various parameters such as building type, climate data, and energy loads, comparing the energy savings of the dual-mode system against single-mode heating-only and cooling-only systems.

The dual-mode device achieved a cooling power density of up to 71.6 W/m² and a heating power density of up to 643.4 W/m² (over 93% solar energy utilization). The electrostatic control of thermal contact conductance significantly improved the performance, increasing both heating and cooling power densities. Building energy simulations demonstrated a potential 19.2% reduction in heating and cooling energy consumption in the United States if the dual-mode device were widely adopted. This represents 1.7 times the savings of a cooling-only approach and 2.2 times the savings of a heating-only approach. The analysis of 16 US cities, representing diverse climate zones, showed that the dual-mode device outperforms single-mode systems in almost all locations. The experimental results demonstrate the effectiveness of the electrostatic method in reducing thermal contact resistance and enhancing both heating and cooling performance, with the system able to switch between modes almost instantaneously. The results show a significant improvement in energy utilization compared to systems without electrostatic control.

The findings demonstrate the feasibility and effectiveness of integrating daytime radiative cooling and solar heating in a single, dynamic device for year-round building energy efficiency. The significant energy savings predicted by the building energy simulations highlight the potential impact of this technology on reducing building energy consumption and greenhouse gas emissions. The electrostatic control of thermal contact, a key innovation in this work, offers a practical and efficient solution to a common challenge in thermal management systems. The success of the dual-mode device, adaptable to diverse climate zones, emphasizes the potential for significantly reducing energy reliance on fossil fuels for building heating and cooling. The study opens avenues for future research focusing on system optimization, material improvements, and integration with smart grid technologies for enhanced energy management.

This research successfully developed and demonstrated a dual-mode device capable of both heating and cooling, offering significant energy savings for buildings. Electrostatic control of thermal contact significantly enhanced performance. Building energy simulations project substantial national energy savings, highlighting the potential for widespread impact. Future work should focus on optimizing system design, exploring advanced materials, and integrating the device with smart grids.

The study's building energy simulations relied on a specific model and set of assumptions. The actual energy savings in real-world applications may vary depending on factors such as building design, occupancy patterns, and local climate conditions. Long-term durability and maintenance requirements of the dual-mode device remain to be fully investigated. The experimental evaluation was conducted at a specific location and time; further testing across various environmental conditions is warranted.