Solar power: bright forecast

Jon Lawson

SCADA helps prove viability of thermodynamic solar power

Solar panels have been a dependable energy source for decades.

But to date they have tended to be confined to smaller applications in Europe, typically supplying only their local area and usually backed up by another power source in case the sun does not shine.

Engineers and scientists have spent many years trying to develop solar technology to overcome this scaling restriction so that grid-scale quantities of power can be generated for wide distribution to a significant population of domestic and commercial users.

Now they are in the final stages of perfecting a reconfiguration of existing solar technology, known as thermodynamic solar power, that has the potential for large-scale energy production through the day and also long after the sun has gone down.

An energy plant in Seville, Spain has, with the help of a Movicon 11 control system, proved that this technology is practical and reliable.

In the semi-arid Seville region, the new solar facility is now supplying power to 25,000 homes, businesses, schools and other energy users.

At the plant, a massive array of over 2,600 large mirrors redirects the sun’s rays toward a 150m tall tower that is filled with salt.

The energy provided by all this sunlight melts the salt (a mixture of 60% potassium and 40% sodium nitrate) and superheats it to over 500°C. This is used to produce steam, which in turn drives a turbine that is connected to a generator producing 400MWh of electricity for distribution to homes and businesses.

The mirrors can move individually or en-masse to track the sun on its daily journey across the sky or for fine adjustment of the energy directed towards the salt tower.

Thermodynamic solar power has several advantages over other solar solutions, the most important being that this technology can continue to generate electricity for up to seven hours after sunset. This is possible because of the incredible heat build-up in the salt mixture. The thermodynamic system works in cloudy weather, so is able to produce power throughout the year in many locations around the world.

A key to the efficient operation of the Seville thermodynamic power plant is being able to closely regulate the temperature in the tower tank and in the turbine. The plant managers also wanted a control system that had simple-to-follow graphic read-outs. For these vital supervisory and control functions, they selected Movicon11 SCADA technology.

Accurate measurements

The first challenge for the control engineers was to figure out how to get highly accurate temperature measurements from multiple locations within the salt tower. Installing heat-resistant thermal imaging cameras on steel poles around the tower and linking them through a fibre optic Ethernet network to the SCADA proved to be the answer.

Each camera is configured to measure maximum, minimum and average temperatures.

These measurements are processed by Movicon data-logging software and displayed in the system’s Trend and Historical screen pages.

All thermal image data is collected in real time by the SCADA software in the control room, from where operating staff can monitor the readings and if necessary adjust the control system accordingly.

If there is a problem or need to change the temperature, this information is sent to a PLC, which relays it to the mirror control system. Then, the mirrors can be quickly moved to redirect the reflected sunlight, increasing or decreasing the energy delivered to the tower. The system also produces trending information from recorded temperatures and provides options for setting adjustments so that power production can be optimised.

The supervision and control provided by the Movicon system has helped make the Seville power plant a highly reliable and safe facility with a simple-to-use control room. It has also proved the concept of thermodynamic solar power production, which should hopefully lead to similar installations around the world.

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