Brief Technical Description of the SKAL-ET Collector

Functional principle and daily tracking

The SKAL-ET parabolic trough collectors consist of a concentration system with parabolic geometry, equipped with a sun tracking mechanism that moves the parabolic reflector panels at any moment into the optimum angle towards the incident solar radiation fall on. The figure on the left shows the functional principle and tracking of the collectors: Tracking the sun from East to West, the SKAL-ET collectors reflect and concentrate the direct solar radiation about eighty times on receiver tubes located in the focal line of the reflecting surface. A receiver tube consists of a stainless steel tube with a selective coating and covered by a quartz envelope tube to reduce thermal losses. The annular space between absorber tube and quartz tube is evacuated. Through the absorber tubes circulates a heat transfer fluid, normally synthetic oil, which is heated by the concentrated solar radiation up to a temperature of almost 400°C.

The SKAL-ET parabolic trough collectors are made up of identical 12 m long sub modules so called Solar Collector Elements (SCE). Each module comprises 28 parabolic reflector panels (RP’s) - 7 along the horizontal axis between pylons and 4 in a vertical cross-section. Each RP is supported on the structure at four points on its backside. This permits the glass to bend within the range of its flexibility without effect on the focal point. The 100 m long SKAL-ET100 has 8 sub modules and an aperture area of 545 m², the 150 m long SKAL-ET150 has 12 sub modules and an aperture area of  817.5 m².

SKAL-ET Demonstration Loop in Kramer Junction / California

Detailed wind tunnel tests have been conducted for obtaining a reliable database for the expected wind loads at different locations in the collector field. Bending and torsion forces have been determined in these experiments. Horizontal forces and pitching moments have been evaluated for different wind speed and direction, different collector positions in the field and various elevations of the collector. Comprehensive FEM investigations on the structural behaviour under various load cases (dead load, wind loads for a range of pitching angles of the collector and wind directions) for alternate designs, complex computer modelling and ray tracing were performed to obtain the best possible relationship of optical accuracy and collector cost.

Lateral View of a SKAL-ET-150 collector unit with 150 m length

Based on these studies a so-called torque-box design has been selected for the SKAL-ET parabolic trough collectors, with less weight and less deformations of the collector structure due to dead weight and wind loading. This reduces torsion and bending of the structure during operation and results in increased optical performance and wind resistance.

Characteristic parameters
of the SKAL-ET 150 collector

The central element of the box design is a 12-m long steel space-frame structure having a squared cross section that holds the support arms for the parabolic mirror facets. The box is built out of only 4 different steel parts. This leads to easy manufacturing, and minimizes assembling and erection on site. Transportation requirements have been optimized for economic packing and shipping.The SKAL-ET design utilizes mirror supports that make use of the glass facets as static structural elements, but at the same time minimize the forces on the glass sheets. This promises minimum glass breakage with the highest wind speeds. Receiver tube supports were designed such to minimize the breakage risk and to ease easy mirror cleaning. The accuracy of the SKAL-ET parabolic trough collector is achieved by a combination of prefabrication and with jig mounting on site. The majority of the structural parts are produced with steel construction tolerances. The accuracy for the mirror supports is introduced with the glass brackets on each of the cantilever arms. This concept allows minimum assembly manpower and cost in series fabrication of solar fields.

The SKAL-ET100 and SKAL-ET150 parabolic trough collectors are tracked with the sun during operation along their long axis with two hydraulic cylinders. Their drive system consists out of two hydraulic cylinders mounted on the drive pylon. From a control box mounted on the drive pylon signal and power lines lead to the hydraulic unit, the rotational encoder, limit switches and temperature sensors.

Installation of SKAL-ET Collector at Kramer Junction / SEGS V