European researchers are preparing to install automated dry-cleaning and sensor technology at operational Concentrated Solar Power plants that will cut water usage, labor costs and stabilize reflectance levels.

New technologies that reduce water consumption are set to cut costs and limit the environmental impact of CSP plants in the coming years.

Europe’s 12.6 million-euro SOLWARIS initiative will be a key driver of new solutions and project partners will soon bring advanced heliostat and mirror cleaning innovations, and power block cooling systems, to operational plants.

Led by Spain’s TSK and including 14 groups from seven countries, SOLWARIS (formerly SOLWATT) is one of three active European Union (EU)-funded initiatives developing water-saving solutions for CSP plants. SOLWARIS builds on R&D advancements made under the EU’s 6.0 million-euro Water Saving for Solar Concentrated Power (WASCOP) project, due to be completed this year.

By the end of 2020, SOLWARIS will begin operational tests at the 50 MW La Africana parabolic trough plant in Spain and the 121 MW Ashalim Plot B (Megalim) central tower plant in Israel.

One of the key SOLWARIS projects is an automated “dry-cleaning” process for tower system heliostats that requires minimal water and no labor. Many CSP developers are favoring tower technology in order to maximize energy storage capabilities. The value of energy storage is on the rise as growing PV and wind penetration creates intermittency and night time shortages.

Conventional methods for cleaning heliostats are labor-intensive, relatively high cost and waste large amounts of water. Up to 1 litre of water may be used per m2 of collector surface at each cleaning run, performed typically every one to 2 weeks. For a 50 MW plant with around 600,000 m2 of mirrors, this equates to 600 m3 of water.

The SOLWARIS solution aims to use just 0.02 m3/MWh of water, a fraction of the 0.29 m3/MWh consumed by the current leading technologies.

Combined with power block solutions, the new cleaning system could reduce operational costs for a 50 MW CSP tower plant by 500,000 euros/year.

“The automatic heliostat cleaning system could be the innovation with most commercial impact,” Eduardo Zarza Moya, Technical Coordinator at Spain’s CIEMAT-PSA research center, told New Energy Update.

CIEMAT-PSA is hosting validation testing at its facility in Almeria and the system could be commercially available “within the next four to five years,” he said.

Water challenge

CSP plants are relatively water-intensive compared with other technology types, mainly due to mirror cleaning and power block cooling requirements.

Water consumption also varies between types of CSP technology.

                Operational water consumption by generation type

(Click image to enlarge)

Source: NREL, 2015

Water usage is a particular challenge for CSP projects in the Middle East, a key growth area for the CSP industry. Dust and humidity in the region raises significant soiling challenges and poor water availability hikes water supply costs.

For example, water costs for the 50 MW Shagaya parabolic trough CSP plant in Kuwait are around 4 euros/m3, compared with only 0.03 euros/m3 at La Africana in Spain. The Shagaya plant is not connected to Kuwait’s water network, relying on tankers to deliver an estimated 40,000 m3/year of water to the site.

                              Natural water availability by country

(Click image to enlarge)

Source: Aquastat, 2014

Larger CSP plants will also require greater volumes of water. ACWA Power and Chinese partners are currently developing the world’s largest CSP plant in Dubai, United Arab Emirates (UAE). The 950 MW Noor Energy 1 project includes three 200 MW parabolic trough CSP systems, a 100 MW CSP tower plant, 250 MW of PV capacity and 15 hours of molten salt CSP storage capacity.

Stable performance

Developed by BrightSource Industries, the dry-cleaning technology for heliostats is currently being tested as a third generation prototype. The technology uses a small DC motor to apply a consistent level of dry pressure on the heliostat to remove soiling more regularly than current cleaning cycles.

Initial testing on three heliostats have indicated steady reflectance values of 90% when using the technology. Without the device, reflectance falls significantly between each manual cleaning.

                  Automatic cleaning performance by prototype version

Note: chart compares performance of SOLWARIS automatic dry-cleaning technology prototypes against manual cleaning process (no wiper).

Source: SOLWARIS.

The SOLWARIS researchers are developing a range of other cutting-edge technologies to aid cleaning of both tower and parabolic trough plant designs.

Under one of the projects, scientists will embed low-cost soiling sensors within mirrors, to allow targeted cleaning strategies while maintaining high reflectance accuracy, Itziar Azpitarte, Researcher at Spanish technological center IK4-Tekniker, SOLWARIS project partner, said. The sensors were developed as part of the WASCOP research.

“This allows for localized cleaning without the need to clean all the mirrors every seven or 10 days,” Azpitarte said.

Germany’s DLR and Spain’s Barcelona Supercomputing Center are developing a soiling rate forecast system to increase maintenance efficiency.

The forecast will enable solar field operators to foresee soiling events up to five days ahead, enabling them to make revisions to cleaning schedules such as skipping cleaning before a sandstorm.

Contactless cleaning

The research teams have also developed ultrasonic cleaning technology for parabolic trough mirrors which avoids brush scratches that compromise surface reflectivity. The system offers 99% reflectivity after cleaning and under SOLWARIS it will be integrated into an automated cleaning truck.

“Implementation of an automatic cleaning devices will reduce water consumption, reduce O&M cost and increase plant revenues by maintaining high [reflectance] values as well as provide immediate solution post dust storm events,” Zarza Moya said.

IK4-Tekniker are also developing an anti-soiling coating for mirrors which will help the research teams advance towards a fully-automated solution, Azpitarte said.

“In SOLWARIS, we will be able to coat large mirror samples with homogeneous anti-soiling coating,” he noted.

SOLWARIS partners will also look to provide greater control of site conditions in harsh weather regions. Currently, wind protection barriers around CSP plants prioritize wind speed reduction in order to avoid mirror breakage, rather than minimizing airborne particles.

Project partner Cranfield University is developing an aerodynamic porous dust barrier which will be installed around the parameter of the La Africana plant.

The new barrier will stop around “50% of dust and sand particles” from passing through the barriers,” Chris Sansom, Head of Centre – Renewable Energy Systems School of Water, Energy and Environment at Cranfield University, said.

By Kerry Chamberlain

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