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Innovative high temperature thermal energy storage concept for CSP plants exceeding 50% efficiency

Periodic Reporting for period 2 - GLASUNTES (Innovative high temperature thermal energy storage concept for CSP plants exceeding 50% efficiency)

Reporting period: 2018-05-01 to 2019-04-30

Energy provision is a big challenge for our Society, being the present production/consumption paradigm not sustainable. To change current trends, a large increase in the share of Renewable Energy Sources (RESs) is crucial.
The evolution towards a society not based on fossil fuels has become a matter of the greatest interest, and solar energy has the potential of providing 27% of global electricity in 2050 – above all others RESs – of which around 11% from Concentrated Solar Power (CSP).

CSP systems have the distinctive ability of providing dispatchable power: State-of-the-Art (SoA) CSP plants featuring Thermal Energy Storage (TES) run overnight or with cloudy sky, providing renewable base–load generation and ancillary services aiding the penetration of intermittent sources such as wind and solar PV. Still, deployment lags behind expectations and technology breakthroughs are needed in order to significantly reduce costs.
Most notably, TES options working at temperatures exceeding 700 C still need to be developed.

GLASUNTES aims at bridging this gap by achieving three main objectives, i.e. to prove the feasibility and assess the potential of

1. an innovative CSP concept whereby (i) the receiver is co-located with the TES vessel, and (ii) the solar radiation is directly absorbed by the liquid storage medium;
2. the adoption of common glass-forming compounds as novel TES materials. These are nontoxic and inexpensive (mainly sand), and the related know-how is already available from the glass manufacturing field;
3. the CSP systems resulting from the integration between receiver–TES and recently proposed high-performance power conversion units based on supercritical CO2 thermodynamic cycles.

The project successfully demonstrated that common glass in molten state can be effectively use to directly capture and store concentrated solar energy at temperature above 1000 C. During the experimental campaign glass temperatures as high as 1300 C were reached.
"An innovative experimental setup was designed which allowed to test the key process of direct absorption of concentrated sunlight into molten glass.
The setup was designed and constructed at ETH Zurich, and tested at the ETH High Flux Solar Simulator (HFSS) with radiative input as high as 1200 suns (or 1.2 MW/m2).
The ad-hoc designed system to measure the temperature field within the glass melt operated as expected and glass temperatures as high as 1300 C were measured. Different types and colors of common container glass samples were tested.
A numerical model of the system was also developed, which couples the ETH in-house Monte Carlo ray tracing software with the CFD solver Ansys Fluent. This numerical tool was validated against the measured glass temperatures.

These experimental and numerical results were disseminated to the scientific public through:
-publication in a peer-reviewed journal,
-presentation at the reference international conference in the field of concentrated solar power systems, and
-publication within the conference proceedings.

Further dissemination activities dedicated to the general public were conducted presenting the project results during during ""open-lab"" events and during lectures to high-school students.

The pioneering experiments conducted during the project demonstrated that molten glass can be used as a stable direct absorber of concentrated solar radiation at temperatures as high as 1300 C, without any particular containment issue. This results meets and exceeds the project target of 1000 ℃.
Avoiding the presence of heat transfer surfaces (e.g. tubes) removes the limitations of maximum temperatures and heat fluxes hampering the performance of current solar receivers.
The achieved temperatures would allow for the coupling of the proposed receiver-TES system with all the most promising thermo-electric conversion technologies (e.g. supercritical CO2 systems and gas turbines), getting closer to the high-efficiency CSP systems envisaged by all the international development roadmaps for this technology.