Entropic and Near-Field Improvements of Thermoradiative Cells

Hybrid Photovoltaic/Thermoelectric Systems for Round-the-Clock Energy Harvesting

Due to their emission-free operation and high efficiency, photovoltaic cells (PVCs) have been one of the candidates for next-generation “green” power generators. However, PVCs require prolonged exposure to sunlight to work, resulting in elevated temperatures and worsened performances. To overcome this shortcoming, photovoltaic–thermal collector (PVT) systems are used to cool down PVCs, leaving the waste heat unrecovered. Fortunately, the development of thermoelectric generators (TEGs) provides a way to directly convert temperature gradients into electricity. The PVC–TEG hybrid system not only solves the problem of overheated solar cells but also improves the overall power output. In this review, we first discuss the basic principle of PVCs and TEGs, as well as the principle and basic configuration of the hybrid system. Then, the optimization of the hybrid system, including internal and external aspects, is elaborated. Furthermore, we compare the economic evaluation and power output of PVC and hybrid systems. Finally, a further outlook on the hybrid system is offered.

The role of electrochemical potentials of solid-state energy emissive harvesters

We prove that semiconductor and metallic energy-emissive harvesters, thermoradiative cells and intermediate-band thermoradiative cells are obtained from the concept of core radiative material doing different approximations. We also show that using this concept it is possible to predict new outcomes on energy-emissive harvesters. Among the new results we highlight the possible failure of the particle conservation model if used on cold-carrier energy emissive harvesters, that intermediate band solar cells must produce small output powers and the reduction of the cell power of thermoradiative cells due to multiexcitonic processes. Moreover, we explain the physical principles of the differences found between results obtained using the detailed balance method and results that are obtained without using it.

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All energy-emissive harvesters can be described using a common physical model.

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The particle conservation model if used in energy-emissive converters can lead to wrong results.

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The incorporation of Intermediate Bands in thermoradiative cells will not improve their performance.

Near-field thermophotovoltaics for efficient heat to electricity conversion at high power density

Thermophotovoltaic approaches that take advantage of near-field evanescent modes are being actively explored due to their potential for high-power density and high-efficiency energy conversion. However, progress towards functional near-field thermophotovoltaic devices has been limited by challenges in creating thermally robust planar emitters and photovoltaic cells designed for near-field thermal radiation. Here, we demonstrate record power densities of ~5 kW/m² at an efficiency of 6.8%, where the efficiency of the system is defined as the ratio of the electrical power output of the PV cell to the radiative heat transfer from the emitter to the PV cell. This was accomplished by developing novel emitter devices that can sustain temperatures as high as 1270 K and positioning them into the near-field (<100 nm) of custom-fabricated InGaAs-based thin film photovoltaic cells. In addition to demonstrating efficient heat-to-electricity conversion at high power density, we report the performance of thermophotovoltaic devices across a range of emitter temperatures (~800 K–1270 K) and gap sizes (70 nm–7 µm). The methods and insights achieved in this work represent a critical step towards understanding the fundamental principles of harvesting thermal energy in the near-field.

Near-field thermophotovoltaic holds the potential for achieving high-power density and energy conversion efficiency by utilizing evanescent modes of heat transfer, yet the performance still lags behind the far-field counterpart. Here, the authors combine thermally robust planar emitter with InGaAs PV to push the limit of near-field device further.

Designing high-performance nighttime thermoradiative systems for harvesting energy from outer space

Energy harvesting using thermoradiative systems has been extensively explored in recent years as a novel strategy for further reducing our energy footprint. However, the nighttime application, thermodynamic limit, and optimal design of such a system remain largely unaddressed so far. Here we propose an improved nighttime thermoradiative system (NTS) for electrical power generation by optically coupling Earth's surface with outer space. Our theoretical model predicts that the NTS operating with Earth (deep space) at 300 K (3 K) yields a maximum power density of 12.3Wm^-2 with an efficiency limit of 18.5%, which is potentially more advantageous than previous nighttime energy harvesting systems, such as a nighttime thermoelectric generator. We find that optimizing the thickness of the active layer, enhancing thermal infrared emission, and employing a silver backreflector for photon recycling are crucially important in improving system performance. This Letter provides new insights for the optimal designs of NTSs and paves the way toward practical nighttime power generation.

Self-powered broadband photo-detection and persistent energy generation with junction-free strained Bi2Te3 thin films

We experimentally demonstrate efficient broadband self-powered photo-detection and power generation in thin films of polycrystalline bismuth telluride (Bi₂Te₃) semiconductors under inhomogeneous strain. The developed simple, junction-free, lightweight, and flexible photo-detectors are composed of a thin active layer and Ohmic contacts on a flexible plastic substrate, and can operate at room temperature and without application of an external bias voltage. We attribute the observed phenomena to the generation of an electric field due to a spontaneous polarization produced by strain gradient, which can separate both photo-generated and thermally-generated charge carriers in bulk of the semiconductor material, without a semiconductor junction. We show that the developed photo-detectors can generate electric power during both the daytime and the nighttime, by either harnessing solar and thermal radiation or by emitting thermal radiation into the cold sky. To the best of our knowledge, this is the first demonstration of the power generation in a simple junction-free device under negative illumination, which exhibits higher voltage than the previously used expensive commercial HgCdTe photo-diode. Significant improvements in the photo-detector performance are expected if the low-charge-mobility polycrystalline active layer is replaced with high-quality single-crystal material. The technology is not limited to Bi₂Te₃ as the active material, and offers many potential applications in night vision, wearable sensors, long-range LIDAR, and daytime/nighttime energy generation technologies.

Thermodynamic limits for simultaneous energy harvesting from the hot sun and cold outer space

The sun and outer space are two of the most important fundamental thermodynamic resources for renewable energy harvesting. A significant amount of work has focused on understanding the fundamental limit of energy harvesting from the sun. More recently, there have been several theoretical analyses of the fundamental limit of energy harvesting from outer space. However, far less is understood about the fundamental limits of simultaneous energy harvesting from both the sun and outer space. Here, we consider and introduce various schemes that are capable of simultaneous energy harvesting and elucidate the fundamental thermodynamic limits of these schemes. We show that the theoretical limits can far exceed the previously established limit associated with utilizing only one thermodynamic resource. Our results highlight the significant potential of simultaneous energy harvesting and indicate new fundamental opportunities for improving the efficiency of energy harvesting systems.

Design of an InSb thermoradiative system for harvesting low-grade waste heat

We propose a thin-film InSb-based thermoradiative system (TRS) and assess its performance characteristics by using a parametric design at low-grade waste heat. We consider the effects of several loss mechanisms on system performance, including optical, sub-gap radiation, and non-radiative losses. Our results predict that the 50 nm thick InSb TRS operating with a hot (cold) source at 500 K (300 K) may yield a power density of 113  Wm^-2 and an efficiency limit of 10.5%. To enhance the system performance, more efforts should be paid to optimize the layer thickness, enhance optical radiation, improve surface passivation, and fabricate an Ag back-reflective mirror and an optical filter for frequency-dependent photon recycling. This Letter provides new insights, to the best of our knowledge, for optimal designs and energy loss mechanisms, thus paving a route towards the development of practical TRS at a low temperature of around 500 K.

Performance enhancement of near-field thermoradiative devices using hyperbolic metamaterials

We analyze a near-field thermoradiative device that consists of an indium arsenide-based photodiode under negative illumination. We analyze a possible enhancement of conversion efficiency by use of hyperbolic metamaterial (HMM) in place of bulk metallic heat sink. A stack of alternating thin-films of metal [zirconium carbide (ZrC)] and dielectric [silicon dioxide (SiO₂)] is chosen to be the HMM under investigation. The presence of hyperbolic modes creates additional channels of near-field radiative transfer. An increased power density is predicted without a compromise in system efficiency.

Thermodynamic limits of energy harvesting from outgoing thermal radiation

We derive the thermodynamic limits of harvesting power from the outgoing thermal radiation from the ambient to the cold outer space. The derivations are based on a duality relation between thermal engines that harvest solar radiation and those that harvest outgoing thermal radiation. In particular, we derive the ultimate limit for harvesting outgoing thermal radiation, which is analogous to the Landsberg limit for solar energy harvesting, and show that the ultimate limit far exceeds what was previously thought to be possible. As an extension of our work, we also derive the ultimate limit of efficiency of thermophotovoltaic systems.

Negative illumination thermoradiative solar cell

The negative illumination thermoradiative solar cell (NITSC) consisting of a concentrator, an absorber, and a thermoradiative cell (TRC) is established, where the radiation and reflection losses from the absorber to the environment and the radiation loss from the TRC to the environment are taken into consideration. The power output and overall efficiency of the NITSC are analytically derived. The operating temperature of the TRC is determined through the thermal equilibrium equations, and the efficiency of the NITSC is calculated through the optimization of the output voltage of the TRC and the concentrating factor for a given value of the bandgap. Moreover, the maximum efficiencies of the NITSC at different conditions and the optimal values of the bandgap are determined, and consequently, the corresponding optimum operating conditions are obtained. The results obtained here will be helpful for the optimum design and operation of TRCs.