arXiv subjects

Jyotirmoy Mandal

Publications and source records attributed to Jyotirmoy Mandal.

11 recordsLinked to original sources

arXiv2026

Tropospheric Ozone Formation Potential and Related Design Considerations for Radiative Coolers

Recently, radiative coolers have been widely explored for reducing cooling loads or lowering temperatures in buildings, and at urban scales as a heat-mitigation measure. However, the potential impacts of radiative cooler deployment on the chemical composition of the atmosphere remain largely unexplored. A defining feature of recently-designed radiative coolers is their high ultraviolet (UV) reflectance, which is required for sub-ambient cooling under strong sunlight. Yet, wide adoption of such UV-reflective radiative coolers could substantially increase the UV actinic flux in the atmosphere above. This, in turn, may affect tropospheric ozone concentrations, particularly in urban atmospheres with high NOx concentrations. Here, as a case study, we use a 0-dimensional photochemical box model, constrained by field measurements of meteorological conditions and chemical concentrations in the urban environment of Houston, Texas, to explore the potential impact of the widespread use of UV-reflective radiative coolers on ozone concentrations. Our calculations show that complete deployment of radiative coolers may increase tropospheric ozone levels by as much as 30% during specific meteorological conditions in Houston. Informed by the wavelength-dependent modelling results, we propose specific designs, namely pigmented radiative coolers with different UV reflectances, and UV-absorptive visible-reemitting fluorescent radiative coolers, that could minimize negative ozone formation while retaining appreciable cooling performance. Our results motivate further study on the effects of widespread deployment of radiative cooling designs like superwhite roof paints on air quality, and materials that simultaneously minimize adverse photochemical impact and maximize cooling performance.

physics.ao-ph↗

arXiv2026

A Disordered Photonic Medium Enabling Ultrabroadband Light Scattering and Selective Longwave Infrared Emission

A surface that selectively emits heat in the long-wave infrared (LWIR) can enable passive cooling in hot environments while retaining partial radiative insulation in cold conditions. However, its cost-effectiveness, practical deployment, and fundamentally, the optical design, remain limited by the reliance on metal reflectors. To overcome this limitation, here we use an absorption-scattering competition factor to establish design guidelines for enhancing reflection or absorption in disordered media across the ultrabroadband ultraviolet-to-far-infrared range. Based on electromagnetic simulations and the optical constants of real materials, we then propose disordered photonic media with a layered, multiscale scattering architecture which, unlike typical scattering designs, simultaneously attains ultraviolet-to-far-infrared reflection and selective LWIR emission. We validate this approach by developing a metal-free selective emitter that exhibits high LWIR emittance (0.88), strong solar reflectance (0.97), and low thermal emittance outside the LWIR (0.49), independent of substrates. Field tests, supported by theoretical modelling, show both enhanced radiative cooling and seasonal thermoregulation performance relative to a state-of-the-art broadband radiative cooler. By expanding the spectral functionality of disordered scattering media as a scalable and low-cost optical materials platform across the solar-to-thermal infrared waveband, this work provides a pathway towards improved energy savings and thermal comfort through passive radiative thermal management.

physics.optics↗

arXiv2026

How do sub-bandgap reflectors affect the performance of PV modules?

Sub-bandgap reflectors (SBR) can reduce the temperature of photovoltaic (PV) modules by reflecting the near-infrared region of the solar spectrum with photon energies smaller than the electronic bandgap of the solar cell absorber material. We consider an ideal SBR, which reflects 100 % of non-harvestable low-energy photons but does not alter the reflectivity of the PV module for usable high-energy photons, and estimate how reducing the module temperature with the SBR affects the annual and the cumulative energy yield of silicon PV modules for six locations in North America and Europe. An ideal SBR would increase the annual energy yield between 1.0 % and 1.5 % for open-rack mounted modules and between 1.6 % and 2.4 % for close-roof mounted PV modules. Whether a non-ideal SBR provides a benefit in actual deployments strongly depends on the location and the optical properties of the coating. Beyond effects on the instantaneous power conversion efficiency and hence the annual energy yield, reducing the temperature by a SBR might also reduce the degradation and increase the overall lifetime of the PV module. By describing degradation using a simple Arrhenius approach using typical activation energies between 0.4 eV and 0.8 eV, we find that an ideal SBR increases the cumulative energy yield over 30 years between 2.2 % and 4.0 % for an open-rack mounted PV module in Princeton, New Jersey, USA.

physics.app-ph↗

arXiv2025

When Energy-Efficiency May Not Yield Positive Climate Impact -- The Case of Adaptive Radiative Coolers

The climate impact of building envelopes is often quantified using their energy savings and CO2 emission reduction benefits. However, building envelopes also trap solar and thermal infrared heat, which is dissipated as a direct heating penalty into our warming planet. For static or adaptive envelopes that passively heat buildings by radiatively retaining heat, these two effects are antagonistic. Yet, their net effect remains unexplored. In this study, we compare the emission reductions benefit, and direct heating penalty of two classes of roof envelopes, traditional and adaptive radiative coolers (TRCs and ARCs). Calculations for buildings in different urban climates show that relative to TRCs like cool roofs, ARCs like smart roofs may have a net heating impact on earth well past this century. Thus, despite their relative energy savings and CO2 emissions reductions benefits, adaptive envelopes on roofs have a negative climate impact relative to traditional cooling designs. Our findings are generalizable across climates and a range of building envelopes and call for a rethinking of how sustainability is quantified for building envelopes, and of material and architectural design for buildings.

physics.app-ph↗

arXiv2025

Radiative Cooling and Thermoregulation of Vertical Facades with Micropatterned Directional Emitters

We demonstrate a micropatterned directional emitter (μDE) with an ultrabroadband, azimuthally selective and tailorable emittance across the thermal wavelengths and over wide angles. The μDE can enable a novel and passive seasonal thermoregulation of buildings by reducing summertime terrestrial radiative heat gain, and wintertime loss. We show several types of μDE, such as metallic, white and transparent variants, made using low-cost materials and scalable manufacturing techniques that are already in large-scale use. Furthermore, we show that its directional emittance can be geometrically tailored to sky-view factors in different urban scenarios. Outdoor experiments show that μDEs stay 1.53-3.26°C cooler than traditional omnidirectional building envelopes in warm weather, including when they are sunlit. In cold weather, μDEs can be up to 0.46°C warmer. Additionally, μDEs demonstrate significant cooling powers of up to 40 Wm-2 in warm conditions and heating powers of up to 30 Wm-2 in cool conditions, relative to typical building envelopes. Building energy models show that μDEs can achieve all-season energy savings similar to or higher than those of cool roofs. Collectively, our findings show μDEs as highly promising for thermoregulating buildings.

physics.app-ph↗

arXiv2021

Accurately Quantifying Radiative Cooling Potentials: A Temperature-correction to the Transmittance-based approximation

Theoretical calculations of the cooling potential of radiative cooling materials are crucial for determining their cooling capability under different meteorological conditions and evaluating their performance. To enable these calculations, accurate models of long-wave infrared downwelling atmospheric irradiance are needed, However, the transmittance-based cosine approximation, which is widely used to determine radiative cooling potentials, does not account for the cooling potential arising from heat loss to the colder reaches of the atmosphere itself. Here, we show that use of the approximation can lead to > 10% underestimation of the cooling potential relative to MODTRAN 6 outputs. We propose a temperature correction to the transmittance-based approximation which accounts for heat loss to the cold upper atmosphere, and significantly reduces this underestimation, while retaining the advantages of the original model. In light of the widespread and continued use of the transmittance-based model, our results highlight an important source of potential errors and a means to correct for them.

physics.ao-ph↗

arXiv2021

Nanostructured Plasmonic Metal Surfaces as Optical Components for Infrared Imaging and Sensing

Thermal imaging and sensing technologies offer critical information about our thermally radiant world, and in recent years, have seen dramatic increases in usage for a range of applications. However, the cost and technical finesse of manufacturing infrared optical components remain a major barrier towards the democratization of these technologies. In this report, we present a solution processed plasmonic reflective filter or PRF as a scalable and inexpensive thermal infrared optic. The PRF selectively absorbs sunlight and specularly reflects thermal infrared TIR wavelengths with performance comparable to state-of-the-art TIR optics made of materials like Germanium. Unlike traditional infrared optical components, however, the PRF can be conveniently fabricated using inexpensive materials and a dip and dry chemical synthesis technique, and crucially, has manufacturing costs that are orders of magnitude lower. We experimentally demonstrate the core optical functionality of the PRF, as well as its integration into infrared imaging and sensing systems without compromising their thermographic or radiometric capabilities. From a practical standpoint, the inexpensive and convenient fabricability of the PRF represent a significant advance towards making the benefits of thermal imaging and sensing systems more affordable and accessible. Scientifically, our work demonstrates a previously unexplored optical functionality and a new direction for versatile chemical synthesis in designing optical components.

physics.optics↗

arXiv2021

Radiative Cooling and Thermoregulation in the Earth's Glow

Passive radiative cooling involves a net radiative heat loss into the cold outer space through the atmospheric transmission windows. Due to its passive nature and net cooling effect, it is a promising alternative or complement to electrical cooling. For efficient radiative cooling of objects, an unimpeded view of the sky is ideal. However, the view of the sky is usually limited - for instance, the walls of buildings have >50% of their field of view subtended by the earth. Moreover, objects on earth become sources of heat under sunlight. Therefore, building walls with hot terrestrial objects in view experience reduced cooling or heating, even with materials optimized for heat loss into the sky. We show that by using materials with selective long-wavelength infrared (LWIR) emittances, vertical building facades experience higher cooling than achievable by using broadband thermal emitters like typical building envelopes. Intriguingly, this effect is pronounced in the summer and diminishes or even reverses during the winter, indicating a thermoregulation effect. The findings highlight a major opportunity to harness untapped energy savings in buildings.

physics.app-ph↗

arXiv2020

Broadband directional control of thermal emission

Controlling the directionality of emitted far-field thermal radiation is a fundamental challenge in contemporary photonics and materials research. While photonic strategies have enabled angular selectivity of thermal emission over narrow sets of bandwidths, thermal radiation is inherently a broadband phenomenon. We currently lack the ability to constrain emitted thermal radiation to arbitrary angular ranges over broad bandwidths. Here, we introduce and experimentally realize gradient epsilon-near-zero (ENZ) material structures that enable broad spectrum directional control of thermal emission by supporting leaky electromagnetic modes that couple to free space at fixed angles over a broad bandwidth. We demonstrate two emitter structures consisting of multiple semiconductor oxides in a photonic configuration that enable gradient ENZ behavior over long-wave infrared wavelengths. The structures exhibit high average emissivity (greater than 0.6 and 0.7) in the p polarization between 7.7 and 11.5 micron over an angular range of 70 deg - 85 deg, and between 10.0 to 14.3 micron over an angular range of 60 deg-75 deg, respectively. Outside these angular ranges, the emissivity dramatically drops to 0.4 at 50 deg and 40 deg. The structures broadband thermal beaming capability enables strong radiative heat transfer only at particular angles and is experimentally verified through direct measurements of thermal emission. By decoupling conventional limitations on angular and spectral response, our approach opens new possibilities for radiative heat transfer in applications such as thermal camouflaging, solar heating, radiative cooling and waste heat recovery.

physics.optics↗

arXiv2019

Li4Ti5O12: A Visible-to-Infrared Broadband Electrochromic Material for Optical and Thermal Management

Broadband electrochromism from visible to infrared wavelengths is attractive for applications like smart windows, thermal-camouflage, and temperature control. In this work, the broadband electrochromic properties of Li4Ti5O12 (LTO) and its suitability for infrared-camouflage and thermoregulation are investigated. Upon Li+ intercalation, LTO changes from a wide band-gap semiconductor to a metal, causing LTO nanoparticles on metal to transition from a super-broadband optical reflector to a solar absorber and thermal emitter. Large tunabilities of 0.74, 0.68 and 0.30 are observed for the solar reflectance, mid-wave infrared (MWIR) emittance and long-wave infrared (LWIR) emittance respectively. The values exceed, or are comparable to notable performances in the literature. A promising cycling stability is also observed. MWIR and LWIR thermography reveal that the emittance of LTO-based electrodes can be electrochemically tuned to conceal them amidst their environment. Moreover, under different sky conditions, LTO shows promising solar heating and sub-ambient radiative cooling capabilities depending on the degree of lithiation and device design. The demonstrated capabilities of LTO make LTO-based electrochromic devices highly promising for infrared-camouflage applications in the defense sector, and for thermoregulation in space and terrestrial environments.

physics.optics↗

arXiv2018

Scalable, "Dip-and-dry" Fabrication of a Wide-Angle Plasmonic Selective Absorber for High-efficiency Solar-Thermal Energy Conversion

A galvanic displacement reaction-based, room-temperature "dip-and-dry" technique is demonstrated for fabricating selectively solar-absorbing plasmonic nanostructure-coated foils (PNFs). The technique, which allows for facile tuning of the PNFs' spectral reflectance to suit different radiative and thermal environments, yields PNFs which exhibit excellent, wide-angle solar absorptance (0.96 at 15°, to 0.97 at 35°, to 0.79 at 80°) and low hemispherical thermal emittance (0.10) without the aid of antireflection coatings. The thermal emittance is on par with those of notable selective solar absorbers (SSAs) in the literature, while the wide-angle solar absorptance surpasses those of previously reported SSAs with comparable optical selectivities. In addition, the PNFs show promising mechanical and thermal stabilities at temperatures of up to 200°C. Along with the performance of the PNFs, the simplicity, inexpensiveness and environment-friendliness of the "dip-and-dry" technique makes it an appealing alternative to current methods for fabricating selective solar absorbers.

physics.app-ph↗