CSP Today examines some of the leading strategies for maximising solar field output.
By Andrew Williams
In the ongoing quest to boost solar field output, CSP developers must focus on several key technological areas. With this in mind, what are the best control strategies to reduce thermal losses and stress? Which set points and parameters deliver the optimal output? And what techniques and component improvements can help to reduce the incidence of optical errors and maximise plant output?
Félix Téllez, Head of R&D on High Solar Concentration Technologies at CIEMAT-PSA, explains that for central receivers, strategies to minimise thermal loss and stress are best focused on the receiver itself.
"In both cases, the aiming strategies for the heliostat field are key points to obtain optimal balance between achieving homogeneous concentrated solar flux maps on receivers and not increasing spillage losses," he says.
Shmuel Fledel, CEO of Siemens Solar Thermal Energy Business Unit agrees, highlighting that the solar receiver is where much of the thermal heat can escape. His view is that 'excellent coating,' such as the Cermet selective coating in the company's UVAC 2010 solar receiver can help to keep emissivity to a minimum.
"In addition, solar receivers require excellent glass to metal seals to maintain their integrity and to withstand thermal stress," he says.
Miguel Angel Gómez, Area Manager, CSP at OPEX Energy also points out that operators are also able to control the amount of heat transfer fluid (HTF) that flows through the solar field, in an effort to reach the maximum HTF outlet solar field temperature.
However, when it comes to managing set-points and parameters to deliver the optimal output, Gómez stresses that CSP developers are sometimes a little more restricted in their options. This is largely because factors related to solar field output are often defined in the manufacturer's distributed control system (DCS).
"As with other technologies, the owner or client is in manufacturer´s hands," he says.
"In operation, for example, solar algorithms and parameters related to temperature and minimum flow to regulate solar field output are defined in the DCS by the manufacturer. For this reason, it is very difficult to make alterations," he adds.
Another key area in which operators can move towards maximizing plant output is through the use of techniques and component improvements to reduce the incidence of optical errors. According to Fledel, optical errors can result from the 'less than perfect alignment' of the mirror and solar receivers on the solar collectors. His view is that the parabolic framework must create 'absolute parts synchronisation' - something that it must continue to do throughout the life of the power plant.
"Our collectors achieve optical precision by using industrialised flow manufacturing in on-site parabola assembly buildings, using high tolerance metal parts. It can also help operators to achieve simple, fast and accurate assembly with a relatively low skilled labour requirement," he says.
"High torsion stiffness, vibration damping and corrosion resistance, together with a torque tube based frame, ensure that this precision will be maintained for many years. Lastly, an advanced field control system, using specialised sun position sensors, enables precise tracking and focus," he adds.
Meanwhile, Téllez explains that CIEMAT-PSA's expertise on optical and tracking errors mainly relates to the characterisation campaigns on heliostat prototypes (more than 20 different prototypes have been characterised at PSA).
Solar field design
However, in general terms, he believes that the reduction of optical errors may be afforded by selecting an appropriate heliostat design and ensuring that it is positioned, or 'canted' correctly during on-site installation. He also points out that operators should ensure they have an adequate strategy in place for mirror washing and tracking error diagnostics and correction.
For Gómez, any problems relating to optical errors begin during the construction and installation of components. His view is that it is very important for operators to ensure that the main equipment parts, such as the solar collector assembly, local controller, steam turbine, and main HTP pumps, are installed properly. He says it is also vital to avoid interference problems between the control systems of different manufacturers.
He also stresses that, at present, there are also a range of other problems that reduce plant output, including those related to the leakage and breakdown of ball joints, leakages and seal problems in HTF pumps and unplanned turbine stoppages.
To respond to these issues, he recommends operators make use of solar field services, such as Opex's own 'Loop System Test' (LST), a mobile unit for testing performance in operational conditions. He also mentions 'CSP Turboefficiency' - a patent system designed to reduce electricity consumption inside the plant and maximise solar field output.
By adopting some of the strategies outlined above, CSP operators can go at least some way to ensuring that plants under their control are operated at levels close to their full potential.
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