Selective Solar Absorber Research Articles

Multiscale Plasmonic Refractory Nanocomposites for High-Temperature Solar Photothermal Conversion.

Development of a refractory selective solar absorber (RSSA) is the key to unlock high-temperature solar thermal and thermochemical conversion. The fundamental challenge of RSSA is the lack of design and fabrication guidelines to simultaneously achieve omnidirectional, broadband solar absorption and sharp spectral selectivity at the desired cutoff wavelength. Here, we realize a ruthenium-carbon nanotube (Ru-CNT) nanocomposite RSSA with multiscale nanoparticle-on-nanocavity plasmonic modes. Ru conformally coated on the sidewalls of CNTs enables a spoof surface plasmon polariton mode for spectra selectivity; Ru nanoparticles formed at the tips of CNTs enable a localized surface plasmon resonance mode and plasmon hybridization for omnidirectional broadband solar absorption. The fabricated Ru-CNT RSSA has a total solar absorption (TSA) of 96.1% with sharp spectral cutoff at 2.21 μm. The TSA is maintained at over 90% for an incident angle of 56°. Our findings therefore guide full-spectrum optical and thermal control from visible to the far-infrared.

Enhancing selectivity of solar absorber using reduced graphene oxide modified nickel oxide nanocomposite thin films

Commercial solar selective absorbers prepared through several techniques have encountered drawbacks of necessitating complicated and expensive equipment, substantial materials requirement, and polluting environment. In this context, carbon incorporated metal oxide thin films have shown promise of low-cost, non-toxic, and efficient solar selective absorbers. In particular, association of reduced graphene oxide (rGO) and nickel oxide (NiO) could facilitate highly ascertained optical properties, environment friendliness, and low-cost. Present study focuses on preparing a novel, high performance, and low-cost rGO-NiO solar selective absorber thin films. Thin films comprised of reduced graphene oxide modified nickel oxide nanocomposites (rGO-NiO NCs) were prepared through solvothermal route and coated on aluminum (Al) substrates by dip-coating technique. The rGO-NiO NCs examined using X-ray diffractometer revealed formation of face centered cubic crystallite structure. The scanning electron microscope images disclosed spherical morphology of NiO decorated on the rGO sheets. The spectra derived from energy-dispersive X-ray spectroscopy ascertained absence of peaks corresponding to impurities. The Raman investigation exhibited high intensity ratio (0.97) of the rGO-NiO NCs that described higher disorder and binding sites. The optical property analysis of the rGO-NiO NC (0.2 wt% rGO) thin film performed in UV–vis-NIR reflectance spectroscopy showed high absorptance (α = 88.03 %) and low thermal emittance (ε = 4.5 %), which eventually delivered high solar selectivity (ξ) of 19.56. These findings suggest that the prepared 0.2 % rGO-NiO NC thin film can be utilized as an appropriate selective absorber for solar to thermal energy conversion.

A simple structured solar selective absorber for application in thermoelectric energy harvesters

This study focuses on green energy harvesting that aims to supply electricity from a different energy source: solar thermoelectric, which is present in the environment without grid connection or utilization of batteries. The concept of using solar selective absorbers and heat sinks is introduced to create temperature differences, which are then converted into electrical energy via the use of a thermoelectric power generator. The solar selective absorber used in this study is a light-absorbing material, and its voltage generation is then compared to a blackbody.

Self-doped Ti–TiOx nanomultilayer films as low-emissivity absorber by one-step sputtering

Solar absorbers are the heart of sustainable photothermal utilization. To obtain higher photothermal conversion efficiency, nanomultilayer absorbers with low emissivity are undoubtedly the best choice. However, the strategy of co-sputtering with at least two targets is usually employed, which makes the preparation process complex and costly, so it is necessary to develop a one-step sputtering preparation through a target. Herein, we propose preparation of Ti–TiOx nanomultilayer solar selective absorption coatings (SSACs) with a single metal Ti target by one-step sputtering. The self-doped Ti nanoparticles as well as the Ti2+, Ti3+ and Ti4+ oxides in TiOx matrix can easily customize the band gap from ultraviolet to visible light, resulting in a high absorption of 0.92 and a low thermal emissivity of 0.07 (298.15K). The obtained Ti–TiOx SSACs exhibit excellent thermal stability after heat treatment at 250 °C for 300 h in air. More significantly, the measured surface temperature reached 114.5 °C under 1.5 suns simulated illumination, indicating its outstanding solar-thermal conversion ability. This work extends the application range of the titanium oxide, developing a simple method for designing low-emissivity solar absorbers.

In-Situ Resistance Optimization for ALD Nanocomposite Resistive Materials

Atomic layer deposition (ALD) made resistive nanocomposite materials can be used in many emerging applications such as charge draining layers for electron optic and MEMS devices, functional resistive layers in electron/ion amplifying microchannel plates (MCPs) which are used in photodetectors and mass spectrometry, HF resistance with tunable electrical conductivity coatings for stabilizing Li-ion battery cathode materials, selective solar absorbing coatings for concentrated solar power (CSP), and barrier and encapsulation coatings. Over the last decade, we have worked broadly on the research and development of resistive nanocomposite coatings. In this context, we have developed many nanocomposite materials with tunable resistance based on mixing conducting ALD metal and insulating ALD metal oxide or metal fluoride processes. These ALD nanocomposite materials show robust material performance in many of the applications listed above. Here we are particularly targeting a functional resistive coating that now commercially used for manufacturing large area ALD-MCPs. For this, there are two key topics that we are addressing a) Minimum thickness requirement for resistive coating which provides a stable targeted resistance behavior for MCPs, and b) Can we tune the temperature coefficient of resistance (TCR) of the ALD nanocomposite materials so that the ALD MCPs can operate over a wide temperature range without affecting their performance. To address these two topics, we have utilized in-situ resistance monitoring during the ALD nanocomposite process. With this method we have optimized resistive nanocomposite material ALD processes e.g. (M:Al2O3 where M= W and Mo) for MCPs and also targeted the desired TCR of the materials and achieved ultra-low TCR values. We believe these optimized coatings will reduce the ALD-MCP processing time substantially and enable ALD-MCPs and photodetectors to operate over a broader temperature range compared to existing ALD-MCPs.

Janus Helical Ribbon Structure of Ordered Nanowire Films for Flexible Solar Thermoelectric Devices.

Solar thermoelectric devices play a significant role in addressing the problem of global warming, owing to their unique features of converting both waste heat and solar energy directly into electricity. Herein, a flexible 3D Janus helical ribbon architecture is designed, starting from well-aligned tellurium (Te) nanowire film, using an in situ redox process reacting with Ag+ and Cu2+ resulting in n-type, p-type, and photothermal sides in one film. Remarkably, the device shows all-day electricity generation and large temperature gradient by coupling the cold side with a passive radiative cooling technique and the hot side with a selective solar absorption technique, showing a temperature gradient of 29.5 K, which is much higher than previously reported devices under a low solar radiation of only 614 W m-2 . Especially, the device can still generate electricity even at night. The present strategy offers a new way for heat management by efficiently utilizing solar energy and the cold of the universe.

A Feasible and Promising Strategy for Improving the Solar Selectivity and Thermal Stability of Cermet-Based Photothermal Conversion Coatings.

A novel multilayer, solar selective absorbing coating that contains lamellar-distributed nanoparticles in its cermet-absorbing sublayers has been fabricated using ion-source-assisted cathodic arc plating. The multilayer coating shows an outstanding selectivity, i.e., a high solar absorptance (0.909), yet it has a low thermal emittance (0.163). More importantly, the long-term thermal stability tests demonstrate that the lamellar-structured absorbers can remain stable, even when annealed at 500 °C for 1000 h in ambient air. The coating’s enhanced selectivity and thermal stability were attributed to the formation of lamellar-distributed nanoparticles in the absorbing sublayer, which form many asymmetric Fabry–Pérot cavities. In this case, the light would be held in the Fabry–Pérot cavities and thus boost the absorptivity due to the increase in interaction time. Meanwhile, the unique distribution of the nanoparticles is also beneficial for enhancing the surface plasmon resonance absorption, and thus promoting the increase in solar selectivity. Furthermore, the excellent thermal stability is ascribed to the existence of amorphous matrices, which separate and seal the nanoparticles into honeycomb shells. In this case, the atomic diffusion in the nanoparticles would be significantly retarded as the amorphous matrices can remain stable below the crystallization temperatures, which can effectively slow down the growth and agglomeration of the nanoparticles.

Effects of Multilayer Structure on the Microstructure and Optical Properties of the DC Sputtered Chromium Thin Films for Selective Solar Absorber Applications

Chromium thin films exhibit several properties that make them potential for solar thermal applications. In this work the effect of film thickness and sputtering power based multilayer structure on the optical properties of DC sputtered chromium thin films is reported. The structural, topological, and optical properties of these films were determined by X-ray diffractometer, Atomic Force Microscopy, and Ultraviolet-Visible-Near Infrared spectrophotometer, respectively. XRD spectra revealed a single peak with preferential orientation of (200) and (210) for single layer and multilayer chromium films, respectively. The grain size and roughness were relatively higher for the multilayer compared to single layer Chromium films. Spectral transmittance showed very high sensitivity to film thickness with average peak of 69% and 5% at a wavelength range of 250-2500 nm for the film thickness of 12 nm and 94 nm, respectively. Spectral transmittance was also found to be higher in single layered films than multi-layered films. Findings from this study suggest that multilayer structure have the potential of tuning the microstructure and optical properties of low thickness Cr films, hence, extending its potential applications in selective solar absorber applications.
 Keywords: Multilayer, Chromium, Sputtering, Optical constants

Investigation of the Thermal Stability of a Solar Absorber Processed through a Hydrothermal Technique

In this work, we study the thermal stability of a hydrothermally treated stainless steel (SS) selective solar absorber by annealing in air in a temperature range between 300 °C and 700 °C for a soaking time of 2 h. Thermal stability testing in the presence of air is critical if the vacuum is breached. Therefore, the SS was characterized by X-ray diffraction (XRD), mechanical, and optical techniques. The XRD analysis shows that the grain size of the as-treated absorber is 67 nm, whereas those of the annealed absorbers were found to be in the range between 66 and 38 nm. The phase of the as-treated and annealed SS was further identified by XRD as Fe2O3. The EDS result shows that the elemental components of the SS were C, Cr, Fe, and O. The strain (ε) and stress (σ) calculated for the as-treated absorber are 1.2 × 10−1 and −2.9 GPa, whereas the annealed absorbers are found in the range of 4.4 × 10−1 to 5.2 × 10−1 and −121.6 to −103.2 GPa, respectively, at 300–700 °C. The as-treated SS absorbers exhibit a good spectra selectivity of 0.938/0.431 = 2.176, which compares with 0.941/0.403 = 2.335 after being annealed at 300 °C and 0.884/0.179 = 4.939 after being annealed at 700 °C. These results indicate a small improvement in absorptivity (0.941) and emissivity (0.403) after annealing at 300 °C, followed by a significant decrease after annealing at 700 °C. The obtained analysis confirms that the annealed SS absorber exhibits excellent selectivity and is suitable to withstand any thermal condition (≤700 °C) in air. Thus, using a cost-effective approach as demonstrated in this study, the as-treated and annealed SS absorber could be used for photo-thermal conversion applications.

Medium-Entropy Nitride ZrNbMo-W-N Nanofilm-Based Substrate-Independent Selective Solar Absorber by a Cosputtering Method

Harvesting solar energy across the broad spectrum yet suppressing radiative heat loss in the infrared region has essential implications for solar-thermal applications. Multicomponent alloy nitride nanofilms are a promising class of materials that boost the solar-thermal conversion efficiency. Here, we build up a medium-entropy nitride ZrNbMo-W-N nanofilm-based selective solar absorber (SSA) by a cosputtering method. By fully studying the optical properties of single-layer ZrNbMo-W-N films, the optimized absorber with a trilayer structure achieves a significant boost in the optical performance, that is, an exceptional solar absorptance of 95.2% and a low emittance of 18.7% (400 °C), as well as angle-insensitive absorption. Moreover, the medium-entropy nitride-based absorber also enables remarkable structural and optical robustness, which retains a sound absorptance of 93.8% and a suppressed thermal emittance of 10.3% after annealing at 400 °C for 168 h, promising great potential for medium- and low-temperature applications. More significantly, our designed SSA can be easily fabricated on various substrates using optimized deposition parameters. In particular, the absorber is well deposited on a flexible cotton substrate, showing excellent solar absorption. It is believed that the combination of a multicomponent strategy and a cosputtering method, which can further modulate optical properties by suitably tailoring the elemental content and species, paves an effective way to investigate this kind of state-of-the-art solar-thermal materials.

Recent trends and challenges in developing boride and carbide-based solar absorbers for concentrated solar power

Concentrating solar power technology has gained increased attention as a source for clean energy. However, because of low power density, it requires large areas and materials with high photothermal conversion efficiency, to harness solar energy. The solar receiver coated with solar selective material plays a critical role in efficiently converting solar energy to useful power. Because of the moderate solar selective nature and excellent thermal properties, significant efforts are currently being invested on the use of ultra-high temperature ceramics as solar selective absorbers. This literature survey presents a critical review of the research and development of early transition metal boride and carbide-based solar selective absorbers, both in bulk and tandem coating formats. In particular, we have emphasized the use of transition metal borides and carbides as intrinsic absorbers as bulk or even exploiting a multilayer design approach to improve the absorptance in the solar spectrum region. One of the key aspects is to introduce a gradient in optical constants (refractive index, extinction coefficient) to tailor high absorptance and low emissivity in tandem absorber coatings, with insightful understanding into microstructure-optical property correlation. The challenges in developing tandem solar absorbers with long-term thermal stability at mid to high temperature window in different working atmosphere are emphasized to spur the development of next generation spectrally-selective solar absorbers.

Enhancing thermal robustness of a high-entropy nitride based solar selective absorber by the incorporation of Al element

Engineering a high-performance spectrally selective absorber (SSA) has been a long-standing effort to address the challenges of inferior spectral selectivity and thermal instability at high temperatures. High-entropy nitrides have shown distinct merits in solar absorption and thermal robustness simultaneously. It is highly desirable to elaborately choose element compositions and optimize absorber structure to enable outstanding solar harvesting ability even at extremely high temperatures above 700 °C. Herein, by modulating the Al element content through a co-sputtering system, a high-entropy nitride MoTaTiCr–Al–N based SSA with a simple double-layer structure is successfully fabricated. As-deposited SSA shows a superior solar absorptance of 92.3% and a suppressed thermal emittance of 7%, together with satisfactory absorption over incident angles up to 60°, ideal for solar-thermal applications. Furthermore, the absorber also exhibits attractive thermal robustness, retaining decent optical performance even after annealing at 700 °C for 168 h. More significantly, the solar-thermal efficiency could reach up to 80.5% at a working temperature of 700 °C under 100 suns, which equals or even outperforms those of state-of-the-art solar absorbers. The competitive performance of the MoTaTiCr–Al–N based SSA is accompanied by a favorable cost reduction compared with those of multilayer thin film structures. All these merits enable it with enormous potentiality for practical solar-thermal applications.

Solar selective coatings for evacuated flat plate collectors: Optimisation and efficiency robustness analysis

Three Cr 2 O 3 /Cr based multilayer coatings were designed, optimised, and sputter-deposited. The selective solar absorbers presented in this study were specifically considered for evacuated flat plate solar collectors (EFPCs) at working temperatures of 100, 200, and 300 °C. The film matrix method and genetic optimisation algorithm were used to simulate and optimise the coatings. Further, the robustness of the performance of the coatings related to the unpreventable errors in layer thickness during the manufacturing stage was incorporated as a binding parameter of the genetic algorithm. This solution allowed the optimisation to converge to the thickness package with the highest performance, thereby ensuring proper robustness for given errors in layer thickness. Thereafter, the robustness of the optimised coatings was analysed, consequently confirming an efficiency loss of less than 2% for layer thickness variation of up to ±20% from the optimal thickness value. The obtained values of solar absorptance as high as 0.97, and thermal emittance lower than 0.05 up to 300 °C, validated through measurements on the fabricated samples, are expected to provide unparalleled performances for the emerging technology of EFPCs. • Selective solar absorbers were optimised for evacuated flat plate collectors. • Optimisation ensures robustness of performances for errors on layer thickness. • Importance of each layer in the robustness of performances is highlighted. • Coating efficiencies higher than 0.8 (0.7) have been obtained at 200 °C (300 °C). • Experimental measurements are in agreement with the simulated results.

Nanoparticles based single and tandem stable solar selective absorber coatings with wide angular solar absorptance

Solar selective absorber coatings with wide angular solar absorptance aids in attaining high photo-thermal conversion efficiency for solar thermal systems. In this regard, nanoparticles-based absorber coatings were developed on stainless steel grade 304 by combining the impregnation method, solvothermal process, and dip-coating technique. Developed nanocomposite (SiO 2 nanoparticles in transition metal oxide matrix) based single layer absorber coating with nano void textured surface, exhibits solar absorptance of 0.92 and spectral emittance of 0.12. MgF 2 nanoparticles based anti-reflective layer on single-layer absorber coating improves the solar absorptance to 0.94 by destructive interference mechanism. Both nanoparticles based single and tandem absorber coatings show wide angular solar absorptance of 0.88 and 0.89, respectively, at an incidence angle of 50°. Besides, developed absorber coatings show lower thermal emissivity of 0.17 and good photo-thermal conversion efficiencies at high operating temperatures (400–500 °C). These developed absorber coatings offer excellent thermal stability at an open atmospheric condition till operating temperature, such as 400 °C for 100 h. Selective nature with wide angular solar absorptance and low heat loss behaviour of stable absorber coating show the photo-thermal conversion efficiency of 91% can improve the performance of receiver tubes in solar thermal systems. • Cost-effective and novel nanoparticles based single and tandem absorber coatings. • Nanocomposite based absorber coating exhibits wide angular solar absorptance. • Tandem layer (MgF 2 AR/nanocomposite) absorber shows solar absorptance of 0.94 • Absorber coatings shows excellent photo-thermal conversion efficiencies (>90%). • Absorber coatings are thermally stable at 400 °C for 100 h with good adhesion.

DC Sputtering Deposition of Copper Oxide Thin Films Doped with Carbon for Efficient Solar Selective Absorbers

In this work, carbon-doped copper oxide thin films were deposited by the reactive DC sputtering method for use as selective absorbents. The properties of the DC discharge plasma were studied, using the emission spectrum, in the presence of pure argon and by mixing it with oxygen once and carbon dioxide again to know the effect of adding these gases on the properties of the resulting plasma used in the deposition of films. The structural properties of the deposited thin films prepared with different flow ratio of carbon dioxide gas were studied using x-ray diffraction. To examine the selective absorber coatings, the reflectance within the UV-Vis spectrum was measured to calculate the percentage of energy absorbed by solar radiation using numerical integration. The reflectance was also measured in the range from 2.5 to 25 μm to calculate the thermal emission of the solar heater within a temperature of 373 K. Measurements showed good efficiency as a selective absorption layer. The best absorptance of the solar spectrum (α) was 0.8750, and the lowest emittance (ε) in the infrared region was 0.254 at 25% percentage of carbon dioxide in the reactive gas of the sputtering system. So, this ratio has the highest efficiency as a selective absorber.

Spinel Cu-Mn-Cr Oxide Nanoparticle-Pigmented Solar Selective Coatings Maintaining >94% Efficiency at 750 °C.

High-temperature concentrating solar power (CSP) system is capable of harvesting and storing solar energy as heat toward cost-effective dispatchable solar electricity. Solar selective coating is a critical component to boost its efficiency by maximizing solar absorptance and minimizing thermal emittance losses. However, maintaining a high solar-thermal conversion efficiency >90% for long-term operation at ≥750 °C remains a significant challenge. Herein, we report spray-coated spinel Cu-Mn-Cr oxide nanoparticle-pigmented solar selective coatings on Inconel tube sections maintaining ≥94% efficiency at 750 °C and ≥92.5% at 800 °C under 1000× solar concentration after 60 simulated day-night thermal cycles in air, each cycle comprising 12 h at 750 °C/800 °C and 12 h cooling to 25 °C. The solar spectral selectivity is intrinsic to the band-to-band and d-d transitions of nonstoichiometric spinel Cu-Mn-Cr oxide nanoparticles. This feature offers a large fabrication tolerance in nanoparticle volume fraction and coating thickness, facilitating low-cost and scalable spray-coated high-efficiency solar selective absorbers for high-temperature CSP systems.

Lowered infrared emittance and enhanced thermal stability of solar selective absorption properties of anodic aluminum oxide photonic crystal coatings

Solar selective absorption coatings based on anodic aluminum oxides (AAO) were prepared via an anodization technique and a subsequent alternating current electrodeposition of Cu-Ni nanoparticles at the bottom. The phase structure, microstructure and solar selective absorption properties of the AAO coatings were investigated with respect to their preparation conditions. Moreover, the dependence of the solar selective absorption properties on thermal annealing history was examined. One-dimensional AAO photonic crystal coatings were produced by anodization under period pulse voltages. The spectral positions of photonic bandgaps were estimated for the AAO photonic crystal coatings. The first-order photonic bandgap was verified to be located in the infrared region. The construction of the photonic crystal structure appreciably lowered the infrared emittance and as a result improved the overall properties of the AAO photonic crystal coatings. A small amount of CuAl2O4 was formed in the electrodeposition layer of AAO photonic crystal coatings by controlling the amplitude of applied electrodeposition voltage at 14 V. The formation of the CuAl2O4 minimized the oxidization of the Cu-Ni nanoparticles at elevated temperatures and thereby enhanced the thermal stability of the solar selective absorption properties. The AAO photonic crystal coating prepared under the electrodeposition voltage showed a solar absorptance of 0.90, an infrared emittance of 0.11 and a spectral selectivity of 8.12 at room temperature, while retaining a spectral selectivity of 6.47 after being annealed at 300 °C for 24 h.

Characteristics of Al & Ti oxide-thin films for thermal solar energy selective absorption applications

Controlled oxidation of sputtered layers of Ti and Al results oxide layers with nano-sized thickness comparable to thermal solar selective absorber layers. In this work, RF magnetron sputtering was used to obtain thin layers of Al and Ti on substrates of stainless steels (St. St). Deposited thin films were oxidized at 400 °C and 800 °C for 4 hours and 1 hour, respectively. The as-deposited Ti, and Al layers were air–oxidized and their optical absorbance and emittance were studied. Microstructure, surface topography, structure and new phases formed after oxidation were characterized by scanning electron microscope (SEM), atomic force microscope (AFM), and X-ray diffraction (XRD). Thin films optical properties were investigated by spectrophotometer & Fourier Transform Infrared (FTIR) Spectroscopy. The new microstructures exhibited a dual metallic and oxide nature. Deposited thin film of Ti had maximum absorbance and lowest emittance about 90% and 1.8% respectively; while after oxidation, the deposited layers changed their selective absorbance pattern. The absorbance values increased, and the emittance values decreased for all wave lengths in the infrared (IR) range. The effect of the oxidation process of both Ti thin film and Al thin film was compared. The Al thin film showed lower absorbance in as-deposited and oxidized states.

Angular absorptance and thermal degradation analysis of TiB2/Ti(B,N)/SiON/SiO2 multilayer solar absorber coating

AbstractMultilayer solar selective absorber coatings have been developed in the last few decades. The thermal stability in terms of microstructure gives an insightful understanding of the optical properties of such coatings. In this context, we extensively utilized transmission electron microscopy (TEM) analysis to establish the thermal stability of TiB2/Ti(B,N)/SiON/SiO2 coating, under thermal cycling/continuous heating to 500°C in vacuum for 250 h. In particular, this work reports the variation in the solar absorptance of TiB2/Ti(B,N)/SiON/SiO2 coating with different angles of incidence of the solar radiation. Extensive analysis using the TEM technique reveals the presence of oxide interlayers that act as diffusion barrier layers to enhance the thermal stability of the coating. Computational simulation using SCOUT software validates the measured reflectance spectrum of the developed multilayer coating. The minor changes in absorptance and emissivity after heat treatment in vacuum at 500°C, together with high solar absorptance over a broad angular variation, establish the potential application of TiB2/Ti(B,N)/SiON/SiO2 as a selective coating in concentrated solar power systems.

All-day continuous electrical power generator by solar heating and radiative cooling from the sky

Thermoelectrical power generator (TEG) proves a promising way that utilizes ambient energy. However, all-day continuous power generation without an artificial heat source by the TEG remains a challenge. In this work, TEG is integrated with a selective solar absorber (SSA) to absorb heat from the heat source (i.e., the sun) and a passive daytime radiative cooling (PDRC) coating to release the heat towards the cold source (i.e., the outer space), providing all-day electrical power generation continuously. A theoretical model is proposed to investigate the all-day performance and design the device parameters (such as load ratio, area ratio, solar intensity, and convective heat transfer rate). It is found that the output power density peaks when the load ratio γ = 1.1 at the area ratio α = 1. Indoor experiments show that the output power in the nighttime and daytime can be 4 mW m−2 and 489 mW m−2 respectively while the outdoor results drop to 0.8 mW m−2 and 91 mW m−2 respectively without any thermal management. In addition, the thermodynamic limit in the nighttime and daytime can be 65 mW m−2 and 145 W m−2 respectively by replacing the TEG with a Carnot engine. These results provide a novel approach to unitizing the solar heating and out space cooling through the selective absorber/emitter, generating 24-h continuous electrical power for unsupervised small devices and remote areas.