#06 | The operation of a large solar thermal plant explained through LIFE Remine Water pilot

By Etienne Letournel (Newheat)

Large solar thermal plants can have up to several tens of thousands of square meters of solar collectors installed to produce several tens of MW of renewable heat. The world’s largest plant using Flat Plate Collectors (FPC) is installed in Silkeborg, Denmark with an area slightly higher than 150.000 m².

With its 2 large solar collectors in series (approximately 30 m² of collection area for a maximum power of around 20 kW), the LIFE Remine Water solar thermal plant is a small-scale pilot plant. Nevertheless, it has been engineered by Newheat in exactly the same way that a large plant would have been (in order to make the results’ extrapolation easier). So, this is the perfect setup to explain how a large solar thermal plant works.

LIFE Remine Water solar thermal pilot plant, © Newheat

The different hydraulic loops of a solar thermal plant

As illustrated in the diagram on the left, a solar thermal plant is composed of different hydraulic loops. The 1^st one is the solar loop. It is the one integrating the solar collectors. This circuit is usually filled with a non-toxic water-glycol mix. In fact, the solar collectors are composed of thin tubes that can be damaged in case of frost in winter when the outdoor temperatures are low and the sun less intense. A circulation pump is used to help the fluid flow through the solar collectors where its temperature rises and then through a heat exchanger to exchange heat to the 2^nd loop: the storage loop.

Heat storage (stratified water tank) is mandatory in solar thermal plants, to compensate for heat production and heat consumption offsets in such installations: i.e. solar heat production during daytime and consumption possible at night. It is filled with softened water and kept clear from oxygen to avoid the risks of scaling and corrosion of the circuits. The volume of heat storage in the LIFE Remine Water project is 5 m³.

Finally, in the 3^rd and last loop, the process loop makes the connection with the heat demand process. A circulation pump sucks hot water from the top of the tank whenever it is needed. An optimized sizing of such a plant ensures the highest coverage of process heat requirements by renewable solar heat.

Simplified design of LIFE Remine solar thermal

Solar thermal plant: a 100% automated process

A solar thermal plant is a 100% automated process. It is controlled by a Programmable Logic Controller (PLC) embedding a computer program that receives all the measurement data generated by the system and automatically defines the optimal operation of the actuators of the plant: pumps and motorized valves.

Among this data, the solar irradiation measured by a pyranometer is surely the most significant one. Depending on the intensity of the solar irradiation, the PLC will modulate the flow rate of the solar circulation pump to maintain the desired temperature at the outlet of the solar collectors.

A solar thermal plant will follow the sun cycles every day:

At dawn, after the circuits were cooled down during the night, the plant will start with a preheating phase where the heat exchanger is bypassed in the solar loop.
After a certain time, when a high enough temperature is reached at the output of the solar collectors, the position of the valves changes in the solar loop to let the fluid flow through the heat exchanger. This is the beginning of the production phase where solar heat is transferred to the storage loop through the heat exchanger and used whenever needed by the consumer.

On actual installations, there are also active systems installed to prevent overheating of the solar loop when the heat demand is low for several days (e.g. tracking systems, dry coolers). In the case of the LIFE Remine Water solar pilot plant, solar collectors’ covers are sufficient to handle such situations.