Abraham Kribus

Summary: Temperature is a key control over biological activities from the cellular to the ecosystem scales. However, direct, high‐precision measurements of surface temperature of small objects, such as leaves, under field conditions with large variations in ambient conditions remain rare. Contact methods, such as thermocouples, are prone to large errors. The use of noncontact remote‐sensing methods, such as thermal infrared measurements, provides an ideal solution, but their accuracy has been low (c. 2°C) owing to the necessity for corrections for material emissivity and fluctuations in background radiation L_bg. A novel ‘dual‐reference’ method was developed to increase the accuracy of infrared needle‐leaf surface temperature measurements in the field. It accounts for variations in L_bg and corrects for the systematic camera offset using two reference plates. We accurately captured surface temperature and leaf‐to‐air temperature differences of needle‐leaves in a forest ecosystem with large diurnal and seasonal temperature fluctuations with an uncertainty of ± 0.23°C and ± 0.28°C, respectively. Routine high‐precision leaf temperature measurements even under harsh field conditions, such as demonstrated here, opens the way for investigating a wide range of leaf‐scale processes and their dynamics.

Temperature is a key control over biological activities from the cellular to the ecosystem scales. However, direct, high precision measurements of surface temperature of small objects such as leaves under field conditions with large variations in ambient conditions remain rare. Contact methods such as thermocouples are prone to large errors. The use of non-contact remote sensing methods such as thermal infrared measurements provides an ideal solution, but their accuracy has been low (in the order of ~2 °C) due to necessity for corrections for material emissivity and fluctuations in background radiation ( L _bg ). A novel ‘dual-reference’ method was developed to increase the accuracy of infrared needle-leaf surface temperature measurements in the field. It accounts for variations in L _bg and corrects for the systematic camera offset using two reference plates. We accurately captured surface temperature and leaf-to-air temperature differences of needle-leaves in a forest ecosystem with large diurnal and seasonal temperature fluctuations with an uncertainty of ±0.23 and ±0.25 °C, respectively. Routine high precision leaf temperature measurements even under harsh field conditions, such as demonstrated here, opens the way for investigating a wide range of leaf-scale processes and its dynamics.

Accurate determination of infrared (IR) emissivity is important for non-contact temperature measurement and for energy balance evaluation in systems that exchange radiation. A method for accurate measurement is proposed based on active modulation of the background radiation. The hemispherical directional reflectance is measured as a proxy for directional emissivity using an IR camera and an integrating sphere, while the background radiation is modulated using an IR emitter and a mechanical shutter. Measurement of the apparent temperature observed by the camera under two different illumination conditions allows the extraction of reflectance and emissivity. The accuracy of the measurement and its sensitivity to surface properties are analyzed, showing uncertainty values as low as 0.004 in some cases. Example measurements of natural and artificial surfaces are presented.

The thermionic‐thermoelectric solid‐state technology, characterized by solar‐to‐electric conversion efficiency feasibly >40%, is comprehensively proposed and discussed for conversion of concentrating solar power. For the first time, the related solar generator prototype is designed and fabricated by developing advanced materials functionalized for the specific application, such as thermally resistant hafnium carbide‐based radiation absorbers, surface‐textured at the nanoscale to obtain a solar absorptance >90%, and chemical vapor deposition diamond films, acting as low‐work‐function (2.06 eV) thermionic emitters. Commercial thermoelectric generators and encapsulation vacuum components complete the prototype. The conversion efficiency is here evaluated under outdoor concentrated sunlight, demonstrating thermionic stage output power of 130 mW at 756 °C, combined to the maximum thermoelectric output power of 290 mW. The related solar‐to‐electric conversion efficiency is found to be 0.4%, but, once the net thermal flux fed to the conversion stages is considered, a thermal‐to‐electric efficiency of 6% is revealed. Factors affecting the performance of the present prototype are analyzed and discussed, as well as a strategy to rapidly overcome limitations, in order to prepare an efficient and highly competitive solid‐state conversion alternative for future concentrating solar plants.

Photon-enhanced thermionic emission (PETE) converts sunlight to electricity with the combined photonic and thermal excitation of charge carriers in a semiconductor, leading to electron emission over a vacuum gap. Theoretical analyses predict conversion efficiency that can match, or even exceed, the efficiency of traditional solar thermal and photovoltaic converters. Several materials have been examined as candidates for radiation absorbers and electron emitters, with no conclusion yet on the best set of materials to achieve high efficiency. Analyses have shown the complexity of the energy conversion and transport processes, and the significance of several loss mechanisms, requiring careful control of material properties and optimization of the device structure. Here we survey current research on PETE modeling, materials, and device configurations, outline the advances made, and stress the open issues and future research needed. Based on the substantial progress already made in this young topic, and the potential of high conversion efficiency based on theoretical performance limits, continued research in this direction is very promising and may yield a competitive technology for solar electricity generation.