Parabolic Trough Solar Field Technology
A parabolic trough power plant's solar field consists of a large, modular array of single-axis-tracking parabolic trough solar collectors. Many parallel rows of these solar collectors span across the solar field, usually aligned on a north-south horizontal axis.
The basic component of a parabolic trough solar field is the solar collector assembly or SCA. A solar field consists of hundreds or potentially thousands of solar collector assemblies. Each solar collector assembly is an independently tracking, parabolic trough solar collector composed of the following key subsystems:
- Concentrator structure
- Mirrors or reflectors
- Linear receiver or heat collection element
- Collector balance of system
Also, each parabolic trough solar collector assembly consists of multiple, torque-tube or truss assemblies (often referred to as solar collector elements or modules).
The structural skeleton of the parabolic trough solar collector is the concentrator structure. The concentrator structure:
- Supports the mirrors and receivers, maintaining them in optical alignment
- Withstands external forces, such as wind
- Allows the collector to rotate, so the mirrors and receiver can track the sun.
Types of collectors include:
- Luz system
Luz System Collectors
Luz system collectors represent the standard by which all other collectors are compared. The industrial nature of these collectors—made from galvanized steel—makes them suitable for commercial power plant applications. And they have proven to be highly reliable. For example, most of the SEGS (solar electric generation system) power plants used Luz system collectors.
There are two types of Luz system collectors: LS-2 and LS-3.
The LS-2 collector features a very accurate design. Its torque-tube structure is simple to erect and provides torsional stiffness. It has six torque-tube collector modules, three on either side of the drive. And each torque tube has two 4-meter-long receivers. Unfortunately, the torque tube uses a lot of steel and requires precise manufacturing to build.
For reducing manufacturing costs, Luz designed the larger LS-3 to lower manufacturing tolerance and require less steel. It proved to be a very reliable design. The LS-3 uses a bridge truss structure in place of the torque-tube. Luz's LS-3 collector has truss assemblies on either side of the drive. Each LS-3 truss assembly has three, 4-meter-long receivers.
The LS-3 truss design didn't lower manufacturing costs as much as expected. It also suffered from insufficient torsional stiffness, which led to lower than expected optical and thermal performance.
Following the demise of Luz, a European consortium—EuroTrough—initiated the development of a new collector design intended to build on the advantages of the LS-2 and the LS-3. The EuroTrough collector utilized a torque-box design to integrate the torsional stiffness of a torque tube and the lower steel content of a truss design.
Under the U.S. Department of Energy's USA Trough Initiative, Solargenix Energy developed a new collector structure through a cost-shared, R&D contract with NREL.
The Solargenix collector is made from extruded aluminum. It uses a unique organic hubbing structure, which Gossamer Spaceframes initially developed for buildings and bridges. The new design:
- Weighs less than steel designs
- Requires very few fasteners
- Requires no welding or specialized manufacturing
- Assembles easily
- Requires no field alignment.
The 64-MWe Nevada Solar One parabolic trough project features the Solargenix SGX-1 collector.
Mirrors or Reflectors
The most obvious features of the parabolic trough solar collector are its parabolic-shaped mirrors or reflectors. The mirrors are curved in the shape of a parabola, which allows them to concentrate the sun's direct beam radiation on the linear receiver.
All current parabolic trough power plants use glass mirror panels manufactured by Flabeg. The mirrors are second-surface silvered glass mirrors (which means that the reflective silver layer is on the backside of the glass). The glass is a 4-milimeter-thick, special low iron or white glass with a high transmittance.
The mirrors have a solar-weighted specular reflectivity of about 93.5%. A special multilayer paint coating protects the silver on the back of the mirror. And each mirror panel is approximately 2 square meters in area.
The LS-3 collector features 224 mirror panels on each solar collector assembly. The 80-MWe SEGS IX power plant has 888 LS-3 solar collector assemblies and almost 200,000 mirror panels.
The glass mirror panels have performed very well during the operation of the SEGS (solar electric generating system) power plants. They've maintained high reflectivity and suffer low annual breakage rates. However, mirror breakage does occur and replacements have been relatively expensive. A number of alternative mirror concepts have been under development to reduce cost, improve reliability, or increase performance.
For more information, see our publications on parabolic trough mirrors.
Linear Receiver or Heat Collection Element
The parabolic trough linear receiver, also called a heat collection element (HCE), is one of the primary reasons for the high efficiency of the original Luz parabolic trough collector design.
The receiver is a 4-meter-long, 70-mm diameter stainless steel tube with a special solar-selective absorber surface, surrounded by an anti-reflective evacuated 115-mm diameter glass tube. Located at the mirror focal line of the parabola, the receiver heats a special heat transfer fluid as it circulates through the receiver tube.
The receiver has glass-to-metal seals and metal bellows to accommodate for differing thermal expansions between the steel tubing and the glass envelop. They also help achieve the necessary vacuum-tight enclosure.
The vacuum-tight enclosure primarily serves to significantly reduce heat losses at high-operating temperatures. It also protects the solar-selective absorber surface from oxidation.
The selective coating on the steel tube has good solar absorptance and a low thermal emittance for reducing thermal radiation losses. The glass cylinder features an anti-reflective coating to maximize the solar transmittance. Getters—metallic compounds designed to absorb gas molecules—are installed in the vacuum space to absorb hydrogen and other gases that permeate into the vacuum annulus over time.
The original Luz receiver design suffered from poor reliability of the glass-to-metal seal. Solel Solar Systems and Schott Glass have developed newer designs that have substantially improved:
- Receiver reliability
- Optical and thermal performance
- The lifetime of receivers.
Collector Balance of System
A number of other key components make up the balance of system in the parabolic trough solar field, including:
- Pylons and foundations
- Collector interconnect
Pylons and Foundations
The pylons support the collector structure. They allow the collector to rotate and track the sun. The pylon is mounted on a concrete foundation that can support the weight and wind loading on the collector. Pylons also support the drive and controls at the center of the collector, and the bearings between each solar collector element (truss or torque tube) and at the end of the collector.
Each solar collector assembly includes one drive. The drive positions the collector to track the sun during the day. The sun's beam radiation continuously reflects off the mirrors and onto the linear receiver.
The drive is located at the center of the collector. It can be either a standard motor and gear box configuration (LS-2) or can use a hydraulic drive system (LS-3, EuroTrough, Solargenix SGX-1). The drive must be able to accurately position the collector for tracking. It should also be able to handle the wind loads.
Each solar collector assembly has its own local controller (LOC) that controls its operation. The local controller controls the tracking of the collector. It also monitors for any alarm conditions, such as a high or low fluid temperature in the receiver.
The local controller communicates with a supervisory computer in the power plant control building. The supervisory computer sends commands to the local controller telling it when to start tracking the sun or when to stop tracking at the end of the day.
Each solar collector assembly operates independently from the adjacent collector. Luz installed insulated flexible hoses at the end of the collector for connecting the receiver to header piping and between two adjacent collectors.
The flex hose allows the collectors to rotate independently. The original flex hose designs proved to be inadequate for the service and suffered from high failure rates. KJC Operating Company developed a new ball joint assembly to replace the flex hose. The ball joint assemblies appear to be more reliable and have lower pumping losses.