This article presents technical aspects of concentrated solar power (CSP). CSP plants are seen as an attractive option to reduce pollutants and the emission of greenhouses gases not only for the United States regions of Sun Belt, but also for European Union, North Africa, and Middle East. CSP is achieving a growing penetration into global electricity markets. CSP technologies are based on the concept of concentrating solar radiation to be used for electricity generation within conventional power cycles using steam turbines. CSPs can achieve high operating temperatures of over 1000 ̊C, enabling them to produce hot air for gas turbine operation. It has long been recognized that the possibility for integration of Thermal Energy Storage (TES) is one of the key advantages of CSP over other forms of renewable energy. TES technologies are important to accelerate market penetration of CSP plants, overcoming the limitation due to the intermittence of the solar source.
Among renewable technologies, Concentrated Solar Power (CSP) plants are seen as an attractive option to reduce pollutants and the emission of greenhouses gases (e.g. CO2) not only for the United States regions of the Sun Belt (where CSP plants are in commercial use for more than 20 years ), but also for European Union, North Africa and Middle East.
CSP technologies are based on the concept of concentrating solar radiation to be used for electricity generation within conventional power cycles using steam turbines (most mature and common technology, gas turbines or Stirling engines).
CSP Solar tower plants can achieve high operating temperatures of over 1000 °C, enabling them to produce hot air for gas turbine operation. Solarized hybrid Gas Turbines can be used in combined cycles, composing a Solar Hybrid Combined Cycle yielding conversion efficiencies of more than 50 % (fig.1), this being a leap in performance for solar energy conversion.
The Importance of Storage
Concentrating Solar Power (CSP) is achieving a growing penetration into global electricity markets. It has long been recognized that the possibility for integration of Thermal Energy Storage (TES) is one of the key advantages of CSP over other forms of renewable energy. In fact, TES allows excess solar energy to be harnessed from the Central Receiver System (CRS) during the daytime and stored, as thermal energy, for periods of insufficient solar supply, such as in cloudy hours or at night. In this way, the output of a CSP plant becomes dispatchable, allowing it to supply controllable power on demand to consumers.
TES technologies are important to accelerate market penetration of CSP plants, overcoming the limitation due to the intermittence of the solar source.
The Test RIG
Brayton cycle integrated with CSP preheating and TES system is under study by various research institutes (e.g. DOE and DLR ,,), but a power block over 1 MW has not yet arrived at an experimental stage.
The TPG (Thermochemical Power Group –www.tpg.unige.it) of the University of Genoa, in collaboration with D'Appolonia SpA, is developing an innovative layout and control scheme to avoid the need for any high temperature valves, thus featuring lower costs and higher reliability, demonstrating it at laboratory scale.
The TES is made by five ceramic honeycomb modules (fig.1), and it is being integrated with a slip stream from the 100 kW mGT already present on site (fig.2), while the solar input will be physically simulated with electric heaters.
The resulting layout requires no high-temperature valve (800-900°C). Moreover, only one regulating three-way valve (2) and one on/off three-way valve (3) are needed. This result is achieved using the high-T orifice, which is a calibrated flange with a desired pressure drop.
Indeed, the three-way valves are subject to the compressor outlet temperature, at which conventional materials can be employed. In this respect, the bottom segment between the compressor and the combustor is called the "cold side" of the plant, while the top segment is called the “hot side” (800-900°C).
Such a concept promises low cost and high reliability, despite introducing permanent pressure losses due to high-T orifice.
Charging and discharging phases of the TES are regulated thanks to three-way valves (valve 2 and 3) and a control based on temperature and mass flow thresholds. This system is capable of storing a portion of the solar heat collected by the CRS, so it can be used to preheat the air between the compressor outlet and the combustor inlet, like a recuperative cycle.
Along the storage medium inside the regenerator it is desirable to have a steep temperature gradient between the hot side and the cold side of the plant. So, two threshold temperatures, a low and a high one, can be defined to manage the switch between the charging and discharging phases.
Before installation, the new testrig and the layout concept was modelled in the TRANSEO  simulation tool developed at TPG for dynamic and control analysis of gas turbine based energy systems (simulation results are being presented at ASME TurboExpo 2014  for a 12MW class gas turbine).
In the test rig, as design specification, the temperature in the low layer of the regenerator (cold side), near the threeway valves, should not exceed 450°C. The temperature in top layer of the regenerator (hot side) should be higher than 650°C before starting the discharging of the storage.
The European project RESILIENT is greatly acknowledged for partially financing this activity.
The Authors wish to thank Matteo Campodonico and Alessandro Spoladore, graduate students of University of Genoa, for their contribution to this research activity.
The authors wish to acknowledge TENOVA SpA for their contribution in selecting the high temperature storage technology and supplier.