The STEM-CSP (Solar Thermo-Electric Magaldi) is a patented Concentrated Solar Power technology developed by Magaldi, that produces renewable energy starting from solar radiation. This system is capable of providing both electrical and thermal energy with a useful life of 30+ years playing an important role in the global decarbonization goal.
Magaldi has been engineering, building, and integrating heavy duty equipment and systems in the high temperature bulk material handling arena for nearly 100 years. Magaldi Green Energy was developed in recent years out of the need to provide a longer term, small scaled CSP system for renewable energy generation.
The STEM Concentrated Solar Power system integrates a solid particles TES (Thermal Energy Storage) system within the solar receiver that is used to generate energy during cloudy periods and also even when solar radiation is no longer available (e.g. hours after sunset or before sunrise). The solar receiver consists of insulated tanks that contain a granular storage medium, such as silica sand, that is fluidized and heated to temperatures above 1000°C. When energy is needed, the thermal energy is released through high pressure steam, CO2 or even hot air systems.
This feature makes STEM-CSP a flexible source of renewable energy that can be dispatched on-demand.
The Magaldi’s concentrated solar system can be used mainly for two applications:
- Power application: Generation of renewable electric energy starting from solar radiation. The produced steam is used to drive a steam turbine or, thanks to the high temperature that the sand bed can reach, it is possible to use a supercritical CO2 Brayton cycle instead of the classic steam cycle.
- Thermal application: Generation of renewable thermal energy starting from solar radiation. The heat collected from solar radiation is used to produce the thermal fluid (steam or hot air) to be used directly in industrial thermal processes, replacing the classic fossil fuels and achieving a high level of decarbonization.
The generation of thermal and electrical energy does not have to take place in separate phases but the two applications can be covered simultaneously. STEM-CSP is equivalent to a CHP (Combined Heat and Power) system, where the fuel used is no longer fossil and CO2 emissions are zero.
Optical transmission system
The system’s main source of energy input is solar radiation that is stored as thermal energy in the sand bed. To ensure the best ratio of constructive simplicity and the maximum efficiency along the conversion process of the electromagnetic energy of the sun’s rays into thermal energy, Magaldi has developed an optical transmission system made up of two reflector components: the heliostats field and the beam-down system.
The heliostats field (1) represents the first reflector system which collects the direct component of the incident solar radiation and transmits it to the secondary reflector system, the beam-down (2), which collects the solar radiation directly into the solar receiver (3).
The solar receiver is the tank located below the beam down system, which contains the sand and provides an opening on the roof (shutter) that allows the introduction of solar radiation during the heating phase of the sand bed. When the solar radiation is no longer available, the shutter is closed to minimize heat losses to the outside.
The fluidization system
A key element to increasing thermal energy transfer during the solar radiation absorption phase and energy generation phase, is the fluidization system. When the solar radiation has favourable conditions and it is thus possible to heat up the receiver, the shutter is open and the solar radiation comes into direct contact with the sand. During this time, until the solar radiation is no longer available, the fluidization of the sand bed is activated. A certain quantity of air is blown into the receiver from the bottom, passes though the sand bed moving the solid particles.
Internal view of the receiver when the fluidization is active.
When fluidized, a bed of solid particles will behave as a fluid
This fluidization of sand particles dramatically increases in the heat exchange coefficient and the response time of the entire system.
Since solar radiation is an intermittent energy source, it is possible that there be a sudden change in the thermal load to be absorbed inside the receiver, causing thermomechanical problems due to the thermal shock suffered. To overcome this problem, the continuous movement of the fluidized sand bed of STEM-CSP technology allows the temperature field inside the receiver to homogenize, ensuring a constant and reliable heat exchange throughout the solar charge cycle, absorbing the abrupt variations of the incident solar radiation.
To maintain the heat capacity stored in the sand for as long as possible, the fluidization is stopped and the sand bed becomes packed. In these operating conditions, the heat losses are minimized and estimated to be less than 1% per day.
During the energy generation phase, a sand-immersed heat exchanger system is used to discharge the TES system and to provide the heat transfer fluid to the customer. To improve the heat exchange coefficient, the fluidization is reactivated.
Due to its characteristics, the fluidizable solid particles system is more suitable for CSP plants than other TES storage technologies. The most used technology in CSP plant is Molten Salts, which has some limits that STEM-CSP is able to overcome, for two main reasons in particular:
- High temperature reachable: the melting temperature of the silica sand is approximately 1700 °C and, up to this point, maintains its thermo-physical characteristics without a limited number of cycles. Having a very wide range of temperatures, with no minimum value limit, it is potentially able to treat both steam through the classic Rankine cycle but also supercritical CO2 by exploiting the innovative turbines capable of guaranteeing efficiencies in the order of 50% with significantly smaller volume occupation.
- Compactness of the system: thanks to the fluidizable bed of the solid particles system, the solar receiver, the heat exchanger and the Thermal Energy Storage system are all integrated in one device. This results in an easy to use and small-scale system with a reduced footprint, allowing the STEM-CSP system to be installed even in industrial complexes.