Optical cavity for improved performance of solar receivers in solar-thermal systems

A principal loss mechanism for solar receivers in solar-thermal systems is radiation from the absorbing surface. This loss can be reduced by using the concept of directional selectivity in which radiation is suppressed at angles larger than the incident angle of the sunlight striking the absorber. Directional selectivity can achieve efficiencies similar to high solar concentration, without the drawbacks associated with large heat fluxes. A specularly reflective hemispherical cavity placed over the absorber can reflect emitted radiation back to the absorber, effectively suppressing emission losses. An aperture in the cavity will still allow sunlight to reach the absorber surface when used with point focus concentrating systems. In this paper the reduction in radiative losses through the use of a hemispherical cavity is predicted using ray tracing simulations, and the effects of cavity size and absorber alignment are investigated. Simulated results are validated with proof of concept experiments that show reductions in radiative losses of more than 75% from a near blackbody absorber surface. The demonstrated cavity system is shown to be capable of achieving receiver efficiencies comparable to idealized spectrally selective absorbers across a wide range of operating temperatures.United States. Dept. of Energy (“Concentrated Solar Thermoelectric Power”, a DOE SunShot CSP Grant, under award number DE-EE0005806

Thermal Emission Shaping and Radiative Cooling with Thermal Wells, Wires and Dots

We discuss radiative heat extraction and spectral shaping via engineering of the density of confined photon states in low-dimensional potential traps. Applications include thermophotovoltaics, radiative cooling, energy up- and down-conversion.United States. Department of Energy. Office of Basic Energy Science. Division of Materials Sciences and Engineering (Award No. DE - FG02 - 02ER45977United States. Department of Energy. Solid State Solor-Thermal Energy Conversion Center (Award No. DE-SC0001299/DE-FG02-09ER46577

Concentrating solar thermoelectric generators with a peak efficiency of 7.4%

Concentrating solar power normally employs mechanical heat engines and is thus only used in large-scale power plants; however, it is compatible with inexpensive thermal storage, enabling electricity dispatchability. Concentrating solar thermoelectric generators (STEGs) have the advantage of replacing the mechanical power block with a solid-state heat engine based on the Seebeck effect, simplifying the system. The highest reported efficiency of STEGs so far is 5.2%. Here, we report experimental measurements of STEGs with a peak efficiency of 9.6% at an optically concentrated normal solar irradiance of 211 kW m⁻², and a system efficiency of 7.4% after considering optical concentration losses. The performance improvement is achieved by the use of segmented thermoelectric legs, a high-temperature spectrally selective solar absorber enabling stable vacuum operation with absorber temperatures up to 600 °C, and combining optical and thermal concentration. Our work suggests that concentrating STEGs have the potential to become a promising alternative solar energy technology.United States. Department of Energy (DE-EE0005806)Solid-State Solar-Thermal Energy Conversion Center (DE-SC0001299)Solid-State Solar-Thermal Energy Conversion Center (DE-FG02-09ER46577

Improvements to solar TEGs through device design

Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (pages 145-150).A solar thermoelectric generator (STEG) is a device which converts sunlight into electricity through the thermoelectric effect. A STEG is nominally formed when a thermoelectric generator (TEG), a type of solid state heat engine, is placed between a solar absorber and a heat sink. When the solar absorber is illuminated by sunlight, it heats up and the TEG is subjected to a temperature gradient. Heat flows through the TEG, some of which is converted to electricity. Recent advancements have improved STEG efficiency considerably, however more work is required before STEGs will be able to compete commercially with other solar to electricity conversion technologies. This thesis explores two device level improvements to STEG systems. First, thin-film STEGs are explored as a method to potentially reduce the manufacturing costs of STEG systems. It is shown through modeling that thin-film STEGs have only a slight degradation in performance compared to bulk STEGs when identical materials properties are used. Two parameters are found which can guide device design for thin-film STEGs regardless of system size. Second, an optical cavity is investigated which can improve opto-thermal efficiency for STEGs or any other solar-thermal system. The cavity improves performance by specularly reflecting radiation from the absorber back to itself, reducing radiative losses. It is shown through modeling and with some preliminary experimental results that such a cavity has the potential to significantly improve the opto-thermal efficiency of solar-thermal systems and operate efficiently at high absorber temperatures without the use of extremely high optical concentration ratios.by Lee A. Weinstein.S.M

Improving solar thermal receiver performance via spectral and directional selectivity

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2017.Cataloged from PDF version of thesis.Includes bibliographical references (pages 142-158).Adoption of renewable energy technologies has accelerated rapidly in recent years due to growing energy demand and concerns over climate change. Among renewable energy sources, solar energy conversion systems are particularly promising due to the abundance of solar energy reaching Earth. Despite its abundance, the solar resource is dilute, so solar energy must be collected efficiently in order for it to meet an appreciable portion of demand. The efficiency of solar energy conversion systems can be improved by taking advantage of the spectral and directional properties of sunlight. Spectral properties refer to the distribution of wavelengths associated with solar photons, with most solar energy arriving as photons with wavelengths from 300 - 2500 nm. Spectral selectivity entails absorbing these solar photons while suppressing losses associated with infrared photons at longer wavelengths. Directional properties refer to the incident vector of sunlight, which spans a small solid angle due to the sun's distance from Earth. Directional selectivity entails absorbing radiation from the direction of the sun while suppressing losses to other directions. This thesis explores the theoretical limits of performance enhancement via spectral and directional selectivity, as well as practical devices designed to take advantage of those effects. Limits to spectral selectivity are investigated by applying the Kramers-Kronig relations to spectrally selective absorbers. Limits to directional selectivity are studied via geometrical limits, and are compared to the limits of concentrating sunlight. Two silica aerogel based solar receivers are presented as practical devices utilizing spectral selectivity. A solar thermal aerogel receiver is predicted to achieve similar performance to state of the art vacuum tube receivers, and a hybrid aerogel receiver that collects electricity from photovoltaic cells and heat is shown to potentially achieve higher efficiency than photovoltaics or a thermal receiver alone. A macroscale reflective cavity is demonstrated as a method for achieving directional selectivity in solar absorbers, and can be used to improve the performance of both solar thermal systems and photovoltaic cells.by Lee A. Weinstein.Ph. D

Modeling of thin-film solar thermoelectric generators

Recent advances in solar thermoelectric generator (STEG) performance have raised their prospect as a potential technology to convert solar energy into electricity. This paper presents an analysis of thin-film STEGs. Properties and geometries of the devices are lumped into two parameters which are optimized to guide device design. The predicted efficiencies of thin-film STEGs are comparable to those of existing STEG configurations built on bulk materials.United States. Dept. of Energy. Office of Basic Energy Sciences (Solid-State Solar-Thermal Energy Conversion Center Award DE-FG02-09ER46577

Aerogel-based solar thermal receivers

In any solar thermal application, such as solar space heating, solar hot water for domestic or industrial use, concentrating solar power, or solar air conditioning, a solar receiver converts incident sunlight into heat. In order to be efficient, the receiver must ideally absorb the entire solar spectrum while losing relatively little heat. Currently, state-of-the-art receivers utilize a vacuum gap above an absorbing surface to minimize the convection losses, and selective surfaces to reduce radiative losses. Here we investigate a receiver design that utilizes aerogels to suppress radiation losses, boosting the efficiency of solar thermal conversion. We predict that receivers using aerogels could be more efficient than vacuum-gap receivers over a wide range of operating temperatures and optical concentrations. Aerogel-based receivers also make possible new geometries that cannot be achieved with vacuum-gap receivers. Keywords: Solar receiver; Solar thermal; AerogelUnited States. Defense Advanced Research Projects Agency (Grant DE-AR0000471)United States. Department of Energy (Grant DE-EE0005806

Diverging polygon-based modeling (DPBM) of concentrated solar flux distributions

This paper presents an efficient and robust methodology for modeling concentrated solar flux distributions. Compared to ray tracing methods, which provide high accuracy but can be computationally intensive, this approach makes a number of simplifying assumptions in order to reduce complexity by modeling incident and reflected flux as a series of simple geometric diverging polygons, then applying shading and blocking effects. A reduction in processing time (as compared to ray tracing) allows for evaluating and visualizing numerous combinations of engineering and operational variables (easily exceeding 106 unique iterations) to ascertain instantaneous, transient, and annual system performance. The method is demonstrated on a linear Fresnel reflector array and a number of variable iteration examples presented. While some precision is sacrificed for computational speed, flux distributions were compared to ray tracing (SolTrace) and average concentration ratio generally found to agree within ∼3%. This method presents a quick and very flexible coarse adjust method for concentrated solar power (CSP) field design, and can be used to both rapidly gain an understanding of system performance as well as to narrow variable constraint windows for follow-on high accuracy system optimization.United States. Defense Advanced Research Projects Agency (Award DE-AR0000471

Hybrid Optoplasmonic Structures and Materials: from New Physics to New Functionalities

We develop hybrid optoplasmonic architectures to tailor resonant energy transfer between trapped photons, plasmons, quantum emitters and elementary heat carriers for emission manipulation, radiative cooling, imaging, and ultrasensitive detection.United States. Department of Energy. Office of Basic Energy Science. Division of Materials Sciences and Engineering (Award No. DE - FG02 - 02ER45977)United States. Department of Energy. Office of Basic Energy Science. Division of Materials Sciences and Engineering (Award No. DE - SC0010679