Impact of Gas-Filled Annular Space on Thermal and Optical Performances of a Heat Pipe Parabolic Trough Solar Collector

In this work, we demonstrate the improvement of thermal performances of a heat pipe parabolic trough solar collector by optimizing the annulus space, between the evaporator and the glass envelope, and using an appropriate filling gas. We compared the system thermal effectiveness for nine filling gases (hydrogen, air, helium, neon, oxygen, nitrogen, argon, krypton, and xenon). The results showed that using xenon or krypton leads to the best thermal yield (70%). While krypton arises to be the most energetically efficient filling gas, argon, with a thermal yield of 62%, presents a best compromise as the cheapest inert gas. In addition, we showed that the annular space size should be less than a critical value to minimize heat losses and to reduce the material cost during manufacturing.

A Hybrid Ray-Tracing Optical Model for Compound Parabolic Concentrators

Compound parabolic concentrator (CPC), as a hybrid of the stationary and the tracking collectors, can collect both direct beam and diffuse radiation. CPCs are favorable choices for medium-temperature applications for their high thermal efficiency and their cost-effectiveness. Optical models are important tools to predict the solar concentrating capability of the CPC. Despite the numerous, optical models developed in the literature and used for parametric studies of the optical characteristics of CPCs, the angular optical properties of the glass envelope, reflector, and receiver are rarely included. Moreover, most existing optical modeling studies of CPCs did not consider or present the loss associated with the refraction in the glass envelope. This study aims to fill these gaps by developing a comprehensive CPC optical model with the capability of profile generation, hybrid ray-tracing (HRT), surface property simulation, and sky model. The HRT can achieve high accuracy using significantly fewer computation resources compared with Monte Carlo ray-tracing (MCRT) and was validated against tracepro. The new optical model incorporates angular and spectrum dependence of optical properties for refraction and reflection using multilayer thin-film theory. Finally, the proposed HRT model was used to analyze the error associated with neglecting geometric design parameters and angular dependency of optical properties in optical simulation. The results suggest that the gaps between the receiver, glass envelope, and the reflector, the refraction of the glass and angular dependence of transmittance, and absorptance should be included in simulation to avoid considerable errors.

Applicability of Heat Mirrors in Reducing Thermal Losses in Concentrating Solar Collectors

In the present work, a novel parabolic trough receiver design has been proposed. The proposed design is similar to the conventional receiver design except for the envelope and the annulus part. Here, a certain portion of the conventional glass envelope is coated with Sn-In2O3 and also Sn-In2O3 coated glass baffles are provided in the annulus part to reduce the radiative losses. The optical properties of the coated glass are such that it allows most of the solar irradiance to pass through, but reflects the emitted long wavelength radiations back to the absorber tube. Sn-In2O3 coated glass is referred to as “transparent heat mirror.” Thus, effectively reducing the heat loss area and improving the thermal efficiency of the solar collector. A detailed one-dimensional steady-state heat transfer model has been developed to predict the performance of the proposed receiver design. It was observed that while maintaining the same external conditions (such as ambient/initial temperatures, wind speed, solar insolation, flow rate, and concentration ratio), the heat mirror-based parabolic trough receiver design has about 3–5% higher thermal efficiency as compared to the conventional receiver design. Furthermore, the heat transfer analysis reveals that depending on the spatial incident solar flux distribution, there is an optimum circumferential angle (θ = θoptimum, where θ is the heat mirror circumferential angle) up to which the glass envelope should be coated with Sn-In2O3. For angles higher than the optimum angle, the collector efficiency tends to decrease owing to increase in optical losses.