Vassilios Belessiotis

This work concerns thermal energy storage in the medium-temperature range (100 – 250 °C), which is typical of parabolic-trough concentrating solar power (CSP) systems. Within this context, the use of phase change materials (PCMs) has been drawing attention in recent years as a promising technology. A class of PCMs suitable for the specific temperature range are sugar-alcohols and, among those, meso-erythritol (melting point: 118 °C) has been selected here as a such representative compound. Both experimental and computational methods are being used, whereby the experimental part of the work is carried out in a pilot facility, which is a hydraulic loop integrating a 100 L cylindrical storage tank. This is loaded with 200 hollow spherical containers (capsules) of 7.5 cm diameter made of steel, containing the PCM and forming a packed-bed arrangement. This offers a large surface area allowing for good heat transfer between the storage medium and the circulating heat transfer fluid, which is here a thermal oil (Therminol). For the computational work, a packed-bed model is used, considering the tank as a porous medium consisting of two phases (fluid and PCM) thermally interacting with each other and the continuous solid phase approach is used for the PCM, with suitable treatment of the phase-change in the energy equation. The model is validated and used for comparisons with preliminary experiments for the charging of the tank showing good agreement. Subsequently, a parametric investigation is carried out based on the model, by considering additional values of the main thermal-hydraulic parameters, such as oil flow rate and inlet temperature, and their effect on the melting rate and the charging efficiency are investigated. For a period of 8 hours, typical of a daily cycle of operation of a solar energy system, the evolution of the phase change process is evaluated from the numerical results for all parameters considered. It is shown for which values of these parameters the melting process can not be completed within the chosen time period and, thus, careful selection is needed when designing a storage system based on the given storage concept for optimum utilization of the harvested solar energy.

The energy efficiency of a small-scale solar concentrating thermal device is investigated, based on Monte-Carlo Ray-Tracing (MCRT) and Computational Fluid Dynamics (CFD) modeling. The device consists of a Fresnel lens collector—engraved on a 1 m rectangular plate—and a 10 cm sized plate receiver, with drilled cylindrical channels with a diameter of 10 mm. Inlet velocities and heat transfer fluid (HTF) temperatures lie within the range of 0.25–1 m/s and 100–200 °C, respectively. The configurations examined involve the utilization of a selective coating on the absorbing surface of the receiver, increasing the channel diameter to 15 mm and the receiver size to 20 cm, and insertion of a glass envelope in front of the absorbing surface. Energy efficiency increases with increasing fluid velocity up to 80%, a level beyond which no further improvement is observed. The coating contributes to a reduction in heat losses; it brings substantial benefits for the lower velocities examined. The increase in channels diameter also contributes to an increase in the energy efficiency, while the increase in receiver dimensions leads to the opposite effect. The glass cover does not improve the performance of the collector, due to substantial optical losses.

The turbulent flow (Re = 5124) and conjugate heat transfer in heat-sink designs of the tube-on-plate type are numerically investigated. The cooling configurations employ a serpentine tube partially (or fully) embedded inside the plate. Two-and four-pass configurations are investigated. A constant heat flux is applied at the bottom surface of the heat-sink plate. The SST k-ω model is used for the prediction of the turbulent flow and heat transfer. Two pairs of longitudinal vortices, as well as secondary flow separation, have been found to set in at the tube curved section. The combined secondary flow pattern enhances heat transfer at the tube sections over a considerable distance downstream of the 180° bends. In the last part of the analysis, the overall performance of the two configurations is compared using a number of evaluation criteria suitable for heat exchanging devices. The four-pass configuration with fully embedded tubing exhibits the best thermal (energetic) and exergetic performance.