Sun and sand. Sand and sun.
No, do not imagine a beach, the scent of the sea and the sound of waves crashing on the shoreline. Same ingredients: different combination, with the same winning level. The result is the invention of a mechanism that uses hot fine grains of sand, so thin as moving like a liquid, as a heat tank in a solar plant.
On June 30the first STEM plant in the world was inaugurated. To be clear: STEM stands for Solar Thermal Magaldi, which is the name of the company that, along with A2A, has supported the entire process of research and development, the installation has taken place at the Integrated Energy Area A2A San Filippo del Mela in the province of Messina and Italy proved to be a perfect cradle of this remarkable technological progress. The type of facility where the STEM technology finds application is the Concentrating Solar Power, a solar concentrator system. The name this time might evoke a correct image: a CSP uses a set of reflective surfaces, trivially a combination of lenses and mirrors, to concentrate on a receiver the sun's rays incident. A concentrating solar power, also known as solar thermal power plant, will be made up of three parts: a field of mirrors, called heliostats and supplied/furnished with articulated joints on the base that enables a continuous alignment in an optimal direction WITH the position of the sun, a secondary reflector, that will be a central tower for this particular technology, and a receiver, on which the reflected rays are focused by the first field, and then from the tower. The receiver will contain the so-called "process fluid": in the case of the solar thermodynamic STEM, this will consist in large volumes of quartz sand. The grains, less than 0.2 mm diameter, reach, after the double reflection of incident sunlight, temperatures around 650 Celsius degrees. The use of sand as the process's fluid is the biggest innovation of this technological development. The conventional systems use in the receiver tipically a diathermic mineral oil, that conducts heat, or a mixture of "molten salts": these are not environmentally friendly, the risk is to pollute (contaminate) the soil, and they require complex procedures for the "end of life practices". The STEM system instead, using only sand and air, is very environmentally friendly and sustainable, ensuring the same performance of a traditional solar plant.
Whatever is the fluid used in this process, it will have to keep the heat for a few hours after the sun has gone down: this ability allows the plant (system) to produce even during the night superheated steam that makes the turbines turning and producing electricity. With this heat-storage strategy is possible to minimize the bad effects on the network of the natural intermittence of the solar source. Its periodical discontinuity represents the biggest obstacle to the affirmation of this technology. Materials and solutions are still under development, but the chance to use sand in STEM systems is a decisive step forward the thermal storage: has been achieved the perspective of preserving in the grains fluid the energy collected during the solar hours and reuse it when the sun is not there.
The one in San Filippo del Mela is small plant but, producing 2 MW it can provide about 1,500 homes with power, saving the environment from the pollution of 890 tons of CO2 per year. To spread on a large-scale this technology has a significant potential: just try to imagine how many countries with high insolation exist and how many activities a CSP plant could also support in remote areas of the world. Water desalination, reclamation of polluted sites, agriculture and food processing; perhaps we will begin to look differently at the sun and sand combination.