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Teaser, summary, work performed and final results

Periodic Reporting for period 2 - RAISELIFE (Raising the Lifetime of Functional Materials for Concentrated Solar Power Technology)

Teaser

The European Renewable Energy Roadmap for 2020 establishes that 20% of final energy will be produced from renewable sources. With an increasing share of fluctuating wind and photovoltaic power generation, Concentrating Solar Power (CSP) technologies with thermal storage become...

Summary

The European Renewable Energy Roadmap for 2020 establishes that 20% of final energy will be produced from renewable sources. With an increasing share of fluctuating wind and photovoltaic power generation, Concentrating Solar Power (CSP) technologies with thermal storage become more important due to the flexibility in dispatching power to the grid. In the Technology Roadmap published in 2014 by the International Energy Agency, the global electricity share of CSP systems is envisioned to reach 11% by 2050. The RAISELIFE project will enhance the lifetime and performance of key materials used in CSP technologies and will contribute to the compliance of these targets.
The following materials are investigated in RAISELIFE: 1) protective and anti-soiling coatings for glass reflectors, 2) thin-glass composite reflectors for heliostats, 3) high-temperature secondary reflectors, 4) absorber coatings for tubular solar tower receivers, 5) absorber coatings for non-evacuated line-focus collectors, 6) abrasion resistant anti-reflective coatings for glass envelope tubes, 7) corrosion resistant high-temperature metals and coatings for molten salt.
The testing activities involve outdoor exposure and accelerated testing in climate chambers and under concentrated solar radiation. Finally, a techno-economic analysis is conducted using system simulation tools. This has the aim to determine the economic benefit of the newly developed materials compared to the state of the art materials.
The project brings together a broad consortium formed of industry partners, SMEs and research institutes of the CSP and material science sector with the final goal of increasing durability and performance of materials and in consequence reducing electricity generation costs.

Work performed

In the first 36 months of project implementation the following main achievements, grouped by key material, can be remarked:

1) Protective and anti-soiling coatings for glass reflectors:
- 5 types of two-layer protective coatings for silvered-glass mirrors have been tested at 11 outdoor exposure sites. The low-cost 2-layer mirror coatings show negligible corrosion (except the RLF5 coating) after 1 year of exposure but they corrode about twice as fast in the accelerated CASS test.
- Anti-soiling coatings for reflectors from Flabeg suffer from abrasion and reflect less than non-coated mirrors after 1-2 years of outdoor exposure in Spain. Contact angle measurements indicate a degradation of the wetting properties after 1 year of exposure. Novel anti-soiling coatings from AGC and HUJI are currently being tested.

2) Thin-glass composite reflectors for heliostats
- The light-weight thin glass composite heliostat prototype of 40m² was manufactured in China and tested successfully in Israel. Up to 1.5%-p higher reflectance compared to traditional 4mm glass mirrors is possible with this concept, although slightly higher corrosion rates were measured. Laser scanning of the prototype heliostat showed acceptable slope deviations due to gravity but significant deformations after thermal stress testing.

3) High-temperature secondary reflectors:
- The first generation of high temperature secondary mirrors does not achieve durability and optical property (ρ>95%, ε=95%) targets yet. The reflectance of the 1st generation of secondaries was 89% and the mirror was not stable at 400°C.

4) Absorber coatings for tubular solar tower receivers
- The lifetime of receiver coatings for solar tower has been improved compared to the state of the art Pyromark coating. BSII, INTA and DFI coatings match the optical efficiency of Pyromark 2500 but are more durable.
- The selective coating from Fraunhofer achieves the highest optical efficiency (about 10% higher than Pyromark 2500) but the coating also shows a faster degradation rate than the state of the art.
- An automatic coating machine prototype has been manufactured to coat 2x2m² receiver panels
- Solar curing of coated samples conducted on Inc617 and Haynes230 resulted in samples with similar optical properties and durability than furnace cured samples.

5) Absorber coatings for non-evacuated line-focus collectors
-Absorber coating for non-evacuated line focusing systems achieves excellent optical performance and high durability (α=95.4%, ε=7.8% at 250°C)
- Coating is stable for >15 months in furnace at 400°C without degradation

6) Abrasion resistant anti-reflective coatings for glass envelope tubes
- Anti-reflective coating with 2.5 higher abrasion resistance was deposited on receiver tubes in a commercial receiver tube factory

7) Corrosion resistant high-temperature metals and coatings for steam and molten salts:
- VM12 and the developed corrosion protection coatings perform better in molten salt environments than T91. The coated samples show negligible corrosion rates after 10,000h of testing.
- The ceramic-based sensor to monitor material corrosion rates has been successfully tested in molten salt environment

Final results

The main progress beyond the state of the art can be summarized as follows:
- 1.5 percentage points higher reflectance achieved with ultra-thin glass mirrors compared to state of the art 4mm silvered-glass mirror technology.
- Durability of absorber coatings for molten salt towers (up to 750°C skin temperature) is estimated to be doubled compared to state of the art Pyromark 2500 coating, while maintaing the same optical efficiency.
- 2x2 m² receiver panels for solar towers can be coated automatically. A homogeneous coating application was achieved: the thickness variation is less than 10µm compared to 30µm for manual painting.
- Abrasion resistance of anti-reflective coatings for parabolic trough receivers was increased by factor 2.5 compared to the commercial coatings, without transmittance changes (0.972).
- Selective absorber properties for low temperature absorbers have been increased to α=0.955; ε=0.078 (250°C) compared to the state of the art α=0.92; ε=0.13 (250°C), without any degradation at 400°C after more than one year of testing.
- No significant mass loss was detected on aluminide diffusion coated samples in contact with solar salt at 580°C for 10,000h, whereas for non-coated T91 and VM12 the material loss is 193µm at 580°C and 68µm.

The system simulations carried out, suggest the following impact associated to RAISELIFE developments in a 150 MWel molten salt reference solar tower (Levelised Cost of Electricity (LCOE) 6.35 $Ct/kWh):
- The optimum recoating interval of the receiver is eight years (for degradation rates of Δα=0.5%p/a, e.g. BSII coating), leading to LCOE reductions of 2.1%.
- The selective coating (α=95%, ε=40%, e.g. Fraunhofer-INTA coating) has the potential to reduce LCOE by 2.7%.
- With the integration of a secondary mirror, LCOE reductions of approximately 1.9 % can be reached.

As a result of the testing activity in RAISELIFE, BSII will employ the developed receiver coating in the largest single-site CSP project in the world, the 700MW molten salt tower awarded by Dubai Electricity and Water Authority. The tower will be constructed in 2020, it will have the world’s tallest solar tower, measuring 260 metres, and the LCEO will be 7.3 $Ct/kWh. The work related to improvement of line focussing absorbers will help to trigger the solar process heat market with its huge potential (0.9 million GWht/year corresponding to more than 400\'000 European jobs).

Website & more info

More info: https://www.raiselife.eu/.