One of the key advantages of the use of sand in the STEM system is the fact that Silica sand is cheaper than diathermic oil and molten salt. It is also chemically inert and suitable for use in systems operating over a large temperature range.

By Andrew Williams

Last month, CSP Today explored the work being done at Masdar Institute and US Solar Holdings through two separate projects investigating the potential of sand as a thermal energy storage for CSP systems.

A third project in Italy pursuing its own research study into this potentially ground-breaking technology has resulted in a 100kWth demonstration CSP plant. The project is a collaboration between the Magaldi Industrial Group, University of Naples and National Research Council of Italy.

The core technology developed as part of the project is known as STEM, short for Solar Thermoelectric Magaldi, which is based on an air/sand fluidized bed solar receiver that is optimised for solar energy collection, transfer and storage.

Led by Magaldi Industrial Group - the project’s R&D activity was carried out by Magaldi research group with the cooperation of Naples University and two institutes of the Italian National Research Council - the IRC-CNR in Naples and the INO-CNR in Florence.

Fluidized Bed

As the Project's Scientific Director, Gennaro de Michele, explains, the project began with four years of research and developmental work, carried out in order to choose the best technology and select the most promising material to use in the fluidized bed, as well as to define the process conditions, optical system and 'solar cavity geometry.'

"The objective of the STEM project was to develop a low-cost CSP technology [that is] simple and flexible [and] doesn’t use any polluting fluids," de Michele told CSP Today in an exclusive interview.

"A fluidized bed works as a liquid but has many characteristics of solids; as a liquid it has high thermal diffusivity and high heat transfer coefficients, and as a solid it has high thermal capacity," he adds.

Pilot facility

According to de Michele, a key output of the research work was the development of a 'dynamical model' of the planned system, which laid the foundation for the basic design of a 100kWth demonstration CSP plant at the Magaldi facility in the southern Italian town of Buccino.

"The theoretical and experimental studies carried out at laboratory level [provided] the background behind the 100kW pilot plant, whose objectives are the demonstration of the STEM concept in terms of flexibility, heat transfer and storage capacity," he says.

"The fluid bed is the key technology of STEM. It is a well-known technology used in the chemical energy industries. The thermal diffusivity in the bed, as well as the heat transfer between the bed and the in-bed heat exchanger allows the capture of solar energy, its rapid transfer to the exchangers [and] efficient steam production, while energy storage is assured by the storage capacity of sand."

De Michele also stresses that the fluidized bed is capable of burning both gas and liquid fuel, making the STEM technology 'a natural hybrid generation system in which sun and fuel can be used together or alternatively in the same apparatus.'

Key advantages

According to de Michele, one of the key advantages of the use of sand in the STEM system is the fact that Silica sand is cheaper than diathermic oil and molten salt. Moreover, he points out that it is chemically inert, 'completely eco-compatible' and suitable for use in systems operating over a large temperature range.

"STEM is a modular technology [and] the aggregation of a defined number of modules within a generation system enables the production of process steam, electricity, desalinated water and [all] their possible combination," he says.

"The capacity of each module is 500 kWe, and the construction and operation of the first industrial module is [currently] the main objective of the Magaldi group.

Following this, de Michele highlights the fact that the series production of such industrial plant modules would enable the consortium to achieve 'a strong reduction of cost.'

Looking ahead, he also believes that the prospects for the application of STEM technology are 'very promising’ and could extend to large stand-alone large power plants of 50-100 MWe, with a storage capacity up to 5-6 hours, that are capable of being integrated with existing fossil fuel power plants.

He also points out that a strong application of STEM could be imagined in isolated sunny regions where the electric grid is not available and the application of some STEM modules could replace diesel generation at very competitive costs.

"The first objective is the construction of first 500kWe industrial plant. Then we would like to make the first multi-module commercial plant and to continue commercialisation."

"At the same time we will not stop the research and development activity, there are many improvements we are already studying that, together with the experience we will gain from field applications, will permit us to obtain a solar generation system [that is] always more efficient and cheap."

Could a material so widely available like sand be the future medium for heat transfer and thermal energy storage in utility-scale CSP? Projects such as STEM, as well as Masdar’s SANDSTOCK and US Holdings’ SandShifter are on their way to proving this theory.

In Magaldi Industrial Group's pilot plant, effective collection of incident solar radiation in a beam-down CSP arrangement is targeted at limiting re-emission of the incident radiation, and at minimizing local overheating at the surface of the receiver exposed to densely concentrated incident radiation.

To comment on this article, please contact the author, Andrew Williams.