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

Buildings consume over 30% of total global final energy and are responsible for about 10% of global greenhouse gas emissions. In the U.S., annual building energy costs exceed $430 billion, with roughly 48% devoted to space heating and cooling. Heating and cooling demands are expected to rise significantly due to climate change and population growth. A key challenge is that most passive solar heating or radiative cooling solutions are static and optimized for limited conditions, while real-world climates exhibit strong diurnal, seasonal, and regional variability. Many U.S. locations require both heating and cooling across the year (e.g., Durham, NC). This study addresses the need for an adaptable, renewable, and switchable building envelope that can alternate between solar heating and radiative cooling to improve year-round energy efficiency.

Prior work established high-performance, but largely single-function, technologies for solar-thermal harvesting and passive radiative cooling. Selective solar absorbers and receivers enable efficient solar-thermal conversion (e.g., plasmonic absorbers and concentrator systems), while engineered photonic and polymer coatings achieve sub-ambient radiative cooling by reflecting solar irradiance and emitting strongly in the 8–13 μm atmospheric window. Reported materials include hierarchically porous polymers, PDMS-coated metal structures for all-day cooling, randomized glass/polymer hybrids, and infrared-transparent/visible-opaque textiles. However, these solutions are typically static and optimized for either heating or cooling. Thermal interface methods (e.g., welding, thermal interface materials) can reduce contact resistance but are unsuitable for frequently switchable systems. The literature thus motivates a reconfigurable platform that integrates both optical functionalities with low thermal resistance and robust, reversible thermal contact.

Device concept: A dual-mode thin-film polymer composite integrates side-by-side solar heating and radiative cooling regions and is translated by rollers/actuators to expose the desired mode. The cooling side reflects most sunlight and maximizes mid-IR emission within the 8–13 μm atmospheric window for sub-ambient cooling. The heating side absorbs most solar energy while suppressing thermal radiation loss via a selective absorber to deliver heat flux to the building envelope or heat exchanger. Electrostatically controlled thermal contact: To minimize thermal contact resistance during frequent attach/detach operations, a polyimide (PI) substrate capable of retaining static charge is used. Applying high voltage establishes Maxwell pressure that increases both macro- and microscopic contact areas, greatly reducing interfacial thermal resistance. Infrared thermography on samples placed over a constant-temperature copper plate quantified contact conductance as a function of applied voltage. The current during operation (~0.07 mA) remains within safe limits. The PI retains charge for days under ambient conditions, reducing the need for continuous high voltage; a reverse bias releases the film for switching. Material design and fabrication: The common substrate is PI (≈25 μm) for flexibility, smoothness, and mechanical robustness. Heating side: a copper film serves as electrode and base; a ~1 μm Zn layer is electroplated and then partially replaced by galvanic reaction with CuSO₄ to create Cu/CuOx nanoparticle clusters (~1 μm agglomerates), yielding a plasmonic selective absorber. Cooling side: a ~300 nm Ag mirror is deposited and overcoated with PDMS (~10–110 μm), which is highly transparent in the visible and strongly emissive in the mid-IR, while maintaining high solar reflectance. Optical properties (UV–NIR–MIR reflectance and emittance) were measured with integrating spheres and FTIR; morphology and composition were characterized by SEM and XPS. Rolling endurance (100 cycles) was evaluated for optical stability. Performance measurement: An outdoor Peltier-based setup at Duke University (Durham, NC; Oct 24, 2019) maintained the sample plate at ambient temperature via PID control to minimize convection losses. Heat flux (positive for heating, negative for cooling) was recorded with a heat flux sensor; ambient solar irradiance, humidity, and temperature were logged for model comparison. Six 15-minute cycles alternated between heating and cooling; thermal contact conditions were varied: (i) non-electrostatic PI, (ii) PI with no external voltage, and (iii) PI with a 2 kV bias. Switching could be motorized or manual, with minimal thermal inertia due to thin films, enabling rapid equilibration (<10 s). A calibrated factor accounted for sensor path resistance differences. Building energy simulations: EnergyPlus v9.2 modeled annual energy impacts across 16 representative U.S. climate-zone cities. Baseline commercial building archetypes followed DOE meta-model definitions, using TMY3 weather data and standard internal setpoints. Scenarios included heating-only, cooling-only, and dual-mode envelopes using empirically derived device performance. Annual energy savings (GJ) and percentages relative to baseline heating and cooling loads were computed and mapped across climate zones.

- Electrostatic thermal contact: Applying high voltage increased average thermal contact conductance up to ~9.5×10^2 W/m²K at 2 kV, reducing the sample–substrate temperature difference to ~0.4 °C. PI retained charges for at least 3 days under ambient conditions (20 °C, 40% RH), enabling intermittent recharging; operating current was ~0.07 mA. - Heating performance: Average heat fluxes during outdoor tests were 442 ± 13.3 W/m² (non-electrostatic PI) and 548 ± 16.4 W/m² (PI, zero external voltage), corresponding to ~70% and ~83% of model predictions (633 ± 25.2 and 662.2 ± 48.7 W/m²). With 2 kV applied, heating reached 643.4 W/m², utilizing over 93% of the modeled solar input. - Cooling performance: Average cooling fluxes were 41.1 ± 1.3 W/m² (non-electrostatic PI) and 52.5 ± 1.6 W/m² (PI, zero external voltage), ~58% and ~66% of modeled values (76.1 ± 15.5 and 79.5 ± 16.1 W/m²). With 2 kV applied, measured cooling reached ~71.6 W/m², closely matching the model. - Switching dynamics: Mode switching was near-instantaneous, with system thermal equilibrium achieved in <10 s due to low thermal mass. - Building energy impact: Mapping across 16 U.S. climate-zone cities showed solar heating benefits are greater in northern regions, while radiative cooling is more impactful in southern regions; cooling-only savings slightly exceed heating-only savings. The dual-mode envelope outperformed single-mode approaches in almost all zones. Annual savings from simulations totaled ~236 GJ across baseline heating (548 GJ) and cooling (681 GJ), equating to ~12.9% reduction; this represents ~1.7× cooling-only (138 GJ) and ~2.2× heating-only (106 GJ). The abstract notes a potential 19.2% U.S.-wide heating and cooling energy reduction if widely deployed.

The study demonstrates that a single, reconfigurable envelope material can adapt to seasonal and diurnal variations by switching between high solar-thermal absorption and high mid-IR emissivity states, directly addressing the challenge of static, monofunctional solutions under dynamic climates. Electrostatic enhancement of thermal contact is crucial: by substantially increasing interfacial conductance, both heating and cooling fluxes approach theoretical limits, enabling reliable real-world performance and rapid response. Simulations confirm that combining both modalities delivers greater annual savings than either heating-only or cooling-only strategies across diverse U.S. climates, aligning device capabilities with geographically varying loads. The results highlight the importance of co-optimizing optical design, mechanical flexibility, and interfacial heat transfer to realize practical, year-round building energy reductions.

This work introduces and validates a flexible, rollable, dual-mode building envelope that integrates daytime radiative cooling and solar heating, augmented by electrostatically controlled thermal contact. Outdoor tests achieved up to ~71.6 W/m² cooling and ~643.4 W/m² heating with rapid switching and high utilization of modeled energy inputs. Energy modeling across U.S. climate zones indicates dual-mode operation significantly outperforms single-mode approaches, with modeled annual savings on the order of ~12.9% of heating and cooling energy (and potentially higher with broader deployment). Future research should explore integration with thermal energy storage to capture surplus solar gains, advanced control algorithms for autonomous mode selection, expanded multimodal functionality, durability under soiling/humidity, and system-level integration with smart grids and heat exchangers for zero-energy building applications.

- Electrostatic contact requires periodic charging; surface charge persistence can diminish over time and may be affected by humidity and soiling, necessitating maintenance or control strategies. - High voltage is required to establish Maxwell pressure, though the current is low; practical implementations must ensure electrical safety, insulation, and reliability. - Reported outdoor tests were performed at a single site and date; broader climatic and seasonal performance should be validated experimentally. - Building energy simulations used standard archetypes and did not include thermal storage; inclusion of storage or advanced control could alter savings estimates. - Thin-film durability under repeated rolling, environmental exposure, and long-term optical stability warrants extended testing.