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HIgh storage density solar power plant for FLEXible energy systems

Periodic Reporting for period 1 - HIFLEX (HIgh storage density solar power plant for FLEXible energy systems)

Reporting period: 2019-09-01 to 2021-02-28

The HIFLEX concept is based on the innovative solid particle technology for solar energy exploitation where cheap ceramic solid particles (bauxite or similar) are used as heat transfer and storage medium. The wide applicable temperature range of such particles (up to 1000°C, no freezing) enables the techno-economic optimization of lower and upper particle temperatures in the process with a significant cost reduction. Using solid particles with a temperature range from 300°C to 1000°C results in the very high storage density up to 2.5x the storage density of molten salt in solar tower plants and even 7x the storage density of molten salt in parabolic trough plants.
A further advantage is that due to the higher temperature level more efficient power cycles with higher process temperatures and thus improved efficiency can be integrated.
The large temperature span of the particles is advantageous in several ways:
• it results in a significant increase of storage density, and a significant decrease of storage cost
• the driving temperature difference in the steam generator is high, resulting in low heat transfer area and smaller units with reduced thermal inertia. This will allow faster ramping of the power cycle, being mainly limited by the ramping capability of the turbine system.
As the particles don’t freeze they can also be used to keep the steam generator pre-heated at moderate temperature while the turbine is not in operation. This allows for faster start-up than with molten salt systems, adding additional flexibility to the system.
The storage system can be also charged when the particles are heated by other energy sources. When the increasing share of PV and wind leads to excess power in the grid, an electrical air heater can use this excess renewable power to heat particles for storage charging. This avoids curtailment of renewable power systems and enables further increase of capacity of PV and wind. In this way, the HIFLEX system can act in a controlled manner both as power supply as well as power sink, and thus contribute to grid balancing with positive and negative power. This feature will become more important in the future when the share of cheap intermittent renewables will further increase. Whether an additional oversizing of the storage is beneficial for the excess power storage, depends on the specific energy mix and resources.
In some periods when the solar subsystem cannot deliver enough power and cheap electricity from other renewables is not available, a simple burner for renewable fuel (e.g. biogas, cellulosic ethanol or renewable H2) is foreseen to ensure the full production capacity for grid stability. This feature avoids the need to keep other conventional power blocks in stand-by that would only be used for a limited number of hours along the year.
Both alternative options to heat the particles require cheap systems to perform this heating. As both heating options supply hot air or exhaust gas, a low-cost direct contact heat exchanger is foreseen to efficiently transfer the energy from the hot gases to the particles. Since both alternative options benefit from the equipment already existing in the CSP plant (storage, power block), significant flexibility can be added to the grid at low cost (only additional heaters).
The first 18 months of the project were mainly focused on the basic design engineering activities for the complete plan and to the achievement of all required authorization by local authorities for plant construction.

To proper carry out the design of the precommercial plant solar tower particle-based plant a clear identification of single plant component with relevant definition of the battery limits has been performed. The basic engineering design has been carried out around the overall system including the heliostat solar field, the receiver, the back up system (gas heater and electrical heater) the hot and cold storage, the steam generator, the tower layout, the solid particle transport system as well as the balance of the plant properly design to deliver solar heat to Barilla facility. All basic engineering design has been completed with definition of process conditions along the overall process scheme and preparation of the Basis of Design, Process Flow Diagram, Heat & Material Balance, equipment summary, data sheet and duty spec of main item and plant layout and elevation. Detailed engineering design is in progress.
The HIFLEX concept has the following innovative features:
- the use of small ceramic particles to be used as heat transfer and storage medium; these particles consist mainly of alumina, are chemically inert, environmentally benign and stable up to > 1000°C. Due to commercial mass production, (e.g. for fracking or casting), the particles are readily available at low cost .
- storage of hot particles allows dispatchable power generation. The large temperature span between hot and “cold” state leads to a volumetric storage density that is 2.5x higher than current solutions. Together with the low cost of particles this results in reduced specific storage cost (about 50% of current solutions).
- centrifugal particle receiver concept CentRec®: The basic concept of the receiver is a rotating cylinder with an inclined rotation axis. Particles enter into the receiver in the upper part and are accelerated by a feeding cone to the circumferential speed of the receiver. The particles then form a thin, but optically dense particle film on the inner surface of the cylindrical absorber cavity. They move along the inner wall towards the outlet governed by gravity and centrifugal forces. Concentrated sunlight enters through the aperture and heats up the dark particles through direct absorption of the solar radiation. Particles leave the rotating receiver part at the lower end where a collector ring catches the particles and conducts them to an outlet piping into the hot storage. Due to direct absorption, no high temperature metallic alloys are required as absorbing surface.
- an innovative particle-based steam generator with fast ramping capability, improving response to grid demand fluctuations
- an innovative particle system enables integration with advanced high efficiency power cycles (high temperature/pressure steam or supercritical CO2); the higher conversion efficiency results in smaller solar components (heliostat field, receiver, storage) at a given power level.
- electric particle heater system for storage charging when excess power is available in the grid thus allowing to avoid excess power curtailment.
The demonstration project will be located in the Italian commune Foggia in the southern region Apulia with a low per capita GDP (gross domestic product) compared to the national average. Therefore the demonstration plant will also have a socio-economic impact, as skilled and unskilled personnel for construction and for O&M (operation, cleaning and maintaining the plant) will be hired. Boosting a lighthouse project would not only bring environmental and technological advantages but also make the less developed region more attractive for industries in general. The social impact of the technology development is to make renewable energy competitive to fossil fuels, thus making a sustainable future affordable. Application of the developed technology in developing countries in North and South Africa, Middle East, India, China and South America will contribute to their development.
Overview Hiflex concept