The HiFlex project will develop and demonstrate a complete pre-commercial flexible CSP prototype plant featuring

    • cheap solid particles as storage and heat transfer medium, operating over a temperature span of 700°C
    • a thermal storage system with 2.5x higher storage density and 50% lower cost
    • dispatchable power supply with fast ramping capability through low-intertia steam generator
    • support of grid stability by acting as Power-to-Heat-to-Power system, capable to store excess power from the grid for later use
    • ensured plant capacity by renewable fuel backup


Infographic of Hiflex system

The HiFlex concept is based on the innovative solid particle technology. 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 technoeconomic optimization of lower and upper particle temperatures in the process. A study on the optimal temperature range has shown that a large temperature spread leads to a significant cost reduction. Using solid particles with a temperature range from 300°C to 1000°C results in the very high storage density of 544 kWh/m³, which is about 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).

Due to the higher temperature level more efficient power cycles with higher process temperatures and thus improved efficiency can be integrated.

Modern steam cycles with temperatures up to 620°C can be used. Also, high efficiency supercritical CO2 cycles that are currently under development can be used when they become available. A simplified scheme of the HiFlex system is shown above. The solar radiation is concentrated by heliostats onto the direct absorption particle receiver, where the particles are heated up (e.g. to 1000°C) while passing through the receiver. The hot particles are then stored in the hot storage container. For power production, particles are passing through the steam generator to generate steam for the steam cycle for power production. After passing through the steam generator, the cooled particles are collected in the ―cold storage (e.g. at 300°C). During sunshine hours, the particles are transported up to the tower and get heated again.

Hot and cold temperatures of the particles can be selected according to a techno-economic optimization, without strict technical limits.

The large temperature span of the particles is advantageous resulting in a significant increase of storage density, and a significant decrease of storage cost. As the driving temperature difference in the steam generator is high, the low heat transfer area and smaller units with reduced thermal inertia allow faster ramping of the power cycle, being mainly limited by the ramping capability of the turbine system.

Unlike with molten salt, the particle system does not experience the danger of fluid freezing and does not need any trace heating.

As the particles do not 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, the 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.

A simple burner for renewable fuel is foreseen to ensure the full production capacity for grid stability.

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 hydrogen) 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 during the year. Due to the limited usage of this feature the total amount of renewable fuel is expected to be quite small (< 5%).

The steam turbine will not be demonstrated in the project. Steam turbines are state-of-theart, and a steam turbine with the power level of the pre-commercial HiFlex system (800 kWth steam will produce about 200 kWe) is not representative for a commercial configuration. Such small steam turbines have also high specific cost so that the limited EC budget would have been exceeded. However, in order not to waste the energy produced in the planned continuous plant operation, the solar energy will be converted from 620°C steam to process heat (hot pressurized water at 135°C) that will be delivered to the pasta production facility of Barilla for continuous use.