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Conversion of solar radiation to electricity with photon-enhanced thermionic emission (PETE) holds the theoretical promise of high conversion efficiency. Basic questions of converter materials properties and conversion process thermodynamics were addressed in recent work. Here we investigate two configurations of front and back electrical contacts on a silicon cathode, and the dependence of device efficiency on the contact area, using a two-dimensional detailed simulation. The impact of contact area on efficiency is different at moderate vs. high temperature, due to a change from electron recombination to electron injection at the contact. When the contact is small enough, a local potential gradient develops, which forms an effective barrier against electron recombination. The back contacts lead to higher efficiency compared to front contacts at all temperatures, and allow much higher contact area that may serve to reduce Ohmic loss and to absorb IR radiation.

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.

Unused animal waste rendered fat is a potential feedstock for marine biofuels. In this work, bio-oil was generated using hydrothermal liquefaction (HTL) of nitrogen-free and low sulfur rendered bovine fat. Maximum bio-oil yield of 28 ± 1.5% and high heating value of 38.5 ± 0.16 MJ·kg-1 was obtained at 330 ℃ at 50% animal fat solid load and 20 min retention time. The nitrogen and sulfur content were negligible, making the produced bio-oil useful marine biofuel, taking into account current stringent regulations on NOx and SOx emissions. The economic analysis of the process, where part of the bovine fat waste is converted to the bio-oil and the semi-solid residues can be used to supply the heat demand of the HTL process and alternately generate electricity, showed that our process is likely to generate a positive profit margin on a large scale. We also showed the growing economic importance of electricity in the revenues as commercial production becomes more energy efficient. [ABSTRACT FROM AUTHOR]

The efficiency of both solar thermal and photovoltaic (PV) systems for power generation is usually in the range of 10-30%, meaning that more than two-thirds of the collected radiation energy is lost. Cogeneration, or more generally polygeneration, means capturing and using some of the wasted energy and therefore increasing the overall efficiency of the solar conversion. Several paths of solar polygeneration are investigated: both thermal and photovoltaic receivers, with use of the waste heat to generate additional electricity by a heat engine, a direct use as heat, and use of the waste heat to operate a thermal process (absorption cooling). Appropriate optical and thermal energy losses are taken into account in all cases. The receiver temperature and the sunlight concentration level serve as free parameters. It is shown that concentrating the solar radiation is essential to effective polygeneration, and that there is an optimal operating temperature for each system. Polygeneration leads to increased conversion efficiency in all cases, and the scenarios based on PV show better results than those based on thermal converters. Scenarios showing electricity replacement up to 43% (normalized to the incident radiation) are presented. [DOI: 10. l115/1.2804618] Keywords: concentrators, photovoltaics, cogeneration, polygeneration, absorption cooling

Tracking motions in current heliostats are usually performed in discrete steps, even though the motion of the sun is continuous. Aiming errors due to the discrete steps are often about 1 mrad or more. A significant reduction of tracking error by smooth continuous tracking is presented. The implementation uses an electronic speed control unit to modify the rotational speed of the two AC motors on an existing heliostat. The continuous tracking system was implemented and successfully operated at the Weizmann Institute heliostat field. Measurements of heliostat motion show that aiming error due to tracking intervals was practically eliminated. A comparison of heliostat motions and flux on the target in step-tracking and continuous tracking modes is reported. [DOI: 10.1115/1.1668026]

Measured physical and optical properties of a stable polydisperse carbon black particle cloud at 532 nm and 1064 nm are reported. The particle cloud consisted of 99.7% spheroid primary particles (45-570 nm diameter) and 0.3% large irregularly shaped agglomerates (1.2-7.25 [micro]m equivalent diameter). Although the numerical fraction of the agglomerates was only 0.2%, they contributed 60% to the cloud's scattering cross section. The extinction coefficient, scattering coefficient and the scattering phase function were measured for both parallel and perpendicular polarized radiation at linear extinction coeificients ranging from 0.6 to 4.1 [m.sup.-1]. The cloud exhibited strong forward scattering, with 62% of all scattered energy in a forward lobe of 15 [degrees] at 532 nm and 48% at 1064 nm. The scattering albedo was measured to 35% at 532 nm and 47% at 1064 nm. The dimensionless extinction coefficient was measured to 8.25 at 532 nm. The experimental data was compared to standard Mie theory by integrating the weighed contribution based on particle size, including agglomerates, according to the detailed measured population distribution. Neglecting the contribution of the agglomerates to the cloud's optical properties was shown to introduce discrepancies between Mie theory and measured results. The results indicate that the-Mie theory can be used for estimating the optical properties of a partially agglomerated carbon black particle cloud for simulation of a solar particle receiver. [DOI: 10.1115/1.1756924] Keywords: Optical Properties, Scattering Phase Function, Radiative Transfer Equation, Mie Theory, Carbon Particles

Accurate knowledge of surface emissivity is essential for applications in remote sensing (remote temperature measurement), radiative transport, and modeling of environmental energy balances. Direct measurements of surface emissivity are difficult when there is considerable background radiation at the same wavelength as the emitted radiation. This occurs, for example, when objects at temperatures near room temperature are measured in a terrestrial environment by use of the infrared 8-14-[micro]m band. This problem is usually treated by assumption of a perfectly diffuse surface or of diffuse background radiation. However, real surfaces and actual background radiation are not diffuse; therefore there will be a systematic measurement error. It is demonstrated that, in some cases, the deviations from a diffuse behavior lead to large errors in the measured emissivity. Past measurements made with simplifying assumptions should therefore be reevaluated and corrected. Recommendations are presented for improving experimental procedures in emissivity measurement. OCIS codes: 120.0280, 260.3060, 300.2140.

The conventional design of a parabolic dish for a small solar conversion system places the receiver along the line between the center of the dish and the sun. This forces the receiver to move in a large arc during tracking, and produces some shading of the dish. In some applications, such large movement of the receiver is not acceptable. A new concentrator design is proposed for small systems with a constraint of limited mobility of the receiver. This is accomplished by using a first polar axis and a second axis that is aligned with the normal to the ecliptic plane. The new design features limited motion of the receiver, with inclination changing only within [+ or -] 23.45[degrees]; off-axis reflector to eliminate shading; constant rotation speed in both axes; and constant flux distribution on the receiver.