Te LOC is responsible for detecting and controlling the
angular position of the mirror system by means of sensors based on an incremental encoder or inclinometer. Te LOC PLC is provided with the type of memory and
capacity to store data and parameters permanently. Te angu- lar position of the collector should be stored in a permanent memory, even if an electrical blackout occurs. LOC internal soſtware must be able to calculate the ex-
pected position of the sun and the corresponding solar vector to which the mirror must be pointed. It should calculate the theoretical position of the sun, not
just “see” the position of the sun. Te reason for this is to avoid misreading the sun’s position due to the presence of clouds or other disturbances. Tese calculations should follow the NREL system and have a control algorithm of ±0.0003º, which is extremely demanding for an industrial process. Te soſtware architecture should be flexible to permit
future modifications, so that dedicated hardware solutions will not be a problem for the maintainability of the overall system. Te LOC communicates with the plant DCS and must be
able to deliver the following information to the DCS: • Mirror identification • Date and time • Configuration parameters • Calculated solar vector • Alarm status • Operation mode • Mirror position set-point angle • Fluid temperature • Internal temperature of the LOC.
Extreme Environmental Conditions Te real challenges for the local automation of the solar
field are heat, ultraviolet radiation and dust. It is easy to imagine what the outdoor LOC equipment in
a CSP plant in a desert location is subjected to. Te heat from the sun’s irradiation is intense and equipment exposed to ultraviolet rays ages quickly. Te electronics have to live with their worst enemies—the heat and dust. And, because custom- ers require completely sealed solutions, ventilation holes and movable parts like fans are not permitted. That is why the ABB Group (Zurich, Switzerland) has
designed a cubicle enclosure with double walls to withstand desert conditions and direct ultraviolet radiation. The external enclosure of the LOC is IP65/ NEMA 4X plastic molded. It is attached to the metallic structure of the drive pylon (the central pylon of the collector) without any pro- tective shadow. Te enclosures are designed for 25 years’ durability. Te
mechanical design can withstand extreme temperatures from -5ºC to 55ºC and relative humidity of 25–95%, including con- densation and corrosive vapors.
For each CSP project ABB engineers design the optimal alloca-
tion of components in the LOC to ensure that the thermal behav- ior of the heat generated in the LOC does not affect the function- ality, taking into account that the heat has to be dissipated only by means of passive solutions, without holes or fans in the cubicle. Each LOC prototype is tested in the expected operating
conditions and with the dimensions for the maximum ambi- ent temperature of the site (typically 40–45ºC) and 1000 W/ m² solar radiation, and with internal thermal dissipation. Te internal temperature has to be at least 5ºC below the least heat-resistant component. Every design is verified and certi- fied in an independent laboratory.
Communications Te LOC is provided with its own Ethernet communica-
tion port to the DCS by way of TCP/IP Modbus. Multimode fiber optic or copper RJ45 links connect each LOC to the next LOC within each network ring. Communication with any LOC does not inhibit, delay or
disturb communication with other LOCs, the DCS or other devices in the network. Te configuration program and configuration updates for each LOC are downloaded over the communication network. A serial communication port is provided for local configu-
ration and troubleshooting via the Ethernet communication port. For maintenance purposes, the LOC system includes a soſtware tool to link a PC or handheld device with any LOC in the solar field from any point in the communication network. It includes control and communication soſtware and human interface screens for any LOC or group of LOCs. Te LOC system has accurate time synchronization with
the plant DCS. Real-time clocks in the control equipment are synchronized to within 1 ms by a GPS clock installed at the plant. A network time protocol (NTP) server synchronizes the clocks of all on-site LOC systems at regular intervals.
ABB’s 1-GW Installed Base ABB has many years’ experience in automating CSP solar
fields in Spain, the US and Egypt, as well as active involvement in many CSP and ISCC (integrated solar and combined cycle) power plant projects in Morocco, South Africa, Australia, China, India, Chile, and other countries. Te company’s large installed base in these projects now amounts to more than 20 installations with a combined generating capacity in excess of 1 GW.
This article first appeared in ABB In Control, a publication of ABB Power Generation, a provider of integrated power and automation solutions for conventional and renewable-based power generation plants and water applications. ABB Power Generation is part of the ABB Group (Zurich, Switzerland). The Society of Manufacturing Engineers is grateful to ABB for granting permission to use this article.
Energy Manufacturing 2013 43
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