Solar thermal power plants, with parabolic trough technology, capture the sun’s heat to drive traditional steam turbines, rather than producing electricity with photovoltaic cells. The heat is focused using mirrors and generates steam to drive the turbines via a tube filled with a special heat transfer liquid. The heat transfer liquid is a mixture of biphenyl/diphenyloxide, and has a similar appearance and viscosity to water, but with very different characteristics. It boils at 300°C, making it ideal to cope with the high temperatures needed to drive the turbines. It also has a freezing temperature of 12°C, and it must be kept above this temperature at all times.
The heat transfer fluid may contain dissolved moisture which presents a problem at night, when the fluid cools, as the dissolved moisture may reach saturation point and liquid water will condense within the fluid, forming a liquid-liquid mixture. If allowed to occur, such liquid water may evaporate as the fluid mixture heats up the next day to form pockets of high pressure steam – potentially causing physical damage to the plant. This risk can only be averted by continuous on-line monitoring of the dissolved moisture to enable effective drying processes within the heat transfer liquid loop.
Michell’s Liquidew moisture in liquid analyser was chosen because of its wide measurement range – from 0.0001 ppmW to saturation levels – as well as its proven reliability. The instrument uses a moisture sensor which is resilient in chemically aggressive media and reliable for continuous long-term operation in dense liquid phase sample flows. The sensor is part of Michell’s calibration exchange programme, which ensures continued accurate measurements with the minimum of disruption for maintenance.
The supplied sampling system was designed to meet Extresol’s specifications by Michell’s dedicated systems engineering department. Drawing on Michell’s 35 years experience of process moisture analysis, they produce both standard and customised sampling systems to cope with even the most demanding applications – such as the extremely high day time operating temperatures of the solar thermal power plant.
The Extremadura plant, which generates 50 megawatts, covers 195 hectares and contains 156 mirror arrays, with a total length of tubing of 92km.
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