Collector with Linear Fresnel Reflectors

The collector installed at the SESL premises (see Figure below) comprises 6 catoptric subsets, aligned with a north-south axis, each of which can be independently rotated about its horizontal axis. Each of these subsets is equipped with 120 flat reflectors of dimensions 72 cm x 12 cm, firmly attached on the metallic structure of the system. The catoptric subsets rotate during the day in such a manner that the solar rays incident on them are reflected towards and focused on an absorbing surface (receiver) which is positioned horizontally, facing downwards and has a rectangular geometry of dimensions 1200 cm x 10 cm. Its location is fixed, at a height of approximately 250 cm above the surface of the catoptric subsets.

This system has been evaluated in previous studies, both theoretically and experimentally, the results of which has been presented in the following journal publications :

1. Mathioulakis, E., Papanicolaou, E., & Belessiotis, V., "Optical performance and instantaneous efficiency calculation of linear Fresnel solar collectors", International Journal of Energy Research, vol. 42(3), pp. 1247-1261, 2018.

2. Bellos E., Mathioulakis E., Tzivanidis C, Belessiotis V., Antonopoulos, K.A., “Experimental and numerical investigation of a linear Fresnel solar collector with flat plate receiver”, Energy Conversion and Management, vol. 130, pp. 44-59, 2016.

From the analysis of the findings of the above studies, it was concluded that a series of retrofit actions were necessary, which were recently undertaken, aiming mainly at :

• Improvement in the system output by replacement of the reflecting mirrors.
• Improvement of the kinematics of the system, by implementing a new mode of movement of the catoptric subsets (affecting the mechanical parts, automated control system and software).
• Replacement of the mechanical support of the receiver and improvement of the accessibility of its location.

For better specifying these actions, it was first necessary to carry out a theoretical analysis of the optical and thermal-hydraulic behavior of the collector. The most important results of the above study have been included in the following journal publications :

1. Vouros A., Mathioulakis E., Papanikolaou E., Belessiotis V., Performance evaluation of a linear Fresnel collector with catoptric subsets Renewable Energy, Renewable Energy, vol. 156, 2020, pp. 68-83, https://doi.org/10.1016/j.renene.2020.04.062.
2. Vouros A., Mathioulakis E., Papanikolaou E., Belessiotis V., On the optimal shape of secondary reflectors for linear Fresnel collectors, Renewable Energy, vol. 143, 2019, pp. 1454-1464, https://doi.org/10.1016/j.renene.2019.05.044.

Based on the results of the optical and thermal-hydarulic analysis of the Fresnel collector, two different types of receiver specimens were fabricated for the absorption of the incident solar rays, after reflection on the mirrors of the collector. The two receivers differ between them in the presence or not of vacuum and have the features described below.

Receiver R1: Receiver with evacuated tube, with or without a secondary reflector, as shown in the figure below.

Receiver R2: Flat receiver

For the flow of the heat transfer fluid along the receiver, two different internal configurations were designed and respective specimens were fabricated :

• Multiple parallel channels of rectangular cross section
• Three-pass, meandering-type flow path

In the course of the project «PROMETHEUS : A Research Infrastructure for the Integrated Energy Chain» and following an open, public call for tenders, the following actions were undertaken by the contractor :
I. Reflectors A total of 360 new mirrors of dimensions 148 m x 12 cm were purchased and mounted on the metallic structure of the collector. The reflecting surface was made of aluminum sheets, which were anodized on both sides.

II. Sun tracking system A new electro-mechanical tracking system was designed and installed, capable of calculating in real time the angle of rotation of each catoptric subset, so as to ensure that the incident radiation is always directed towards the focal area. This system automatically and in real time rotates the catoptric subsets and, besides automated operation, it also allows for manual rotation. It can ensure accurate focusing on the absorbing surface throughout the day, using the sun position as a criterion, regardless of the intensity of solar radiation available.

III. Receiver support and working platform A horizontal, movable beam was fabricated and attached to the existing vertical ones. At the bottom of this beam, suitable receivers of various types and dimensions can be mounted, at a beam length of 1200 cm and maximum overall weight of 100 kg. It also includes a mounting and rolling mechanism on which the receiver can be attached so that any necessary work can be carried out with safety.