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Hydrophilic and hydrophobic surfaces

PUBLISHABLE SUMMARY

PROJECT OBJECTIVES

Humanity is facing a dramatic challenge created by the progressive exhaustion of fossil fuels and the onset

of climate change most probably due to CO2 pollution. The utilization of renewable energies, and among

them, of photovoltaic technologies, as a major component of the global action, is then becoming an

absolute and urgent necessity. Among photovoltaic technologies, Concentrating photovoltaic (CPV) can

play a key role in the transition towards a sustainable global

energy system, owing to the possibility to reach very high

system efficiency values as well as low environmental impact.

Nowadays CPV technology is still in a developing phase and

therefore there are several scientific/technical issues still to

be faced, as well as there is the necessity to pursue initiatives

of knowledge dissemination in order to promote its wide

exploitation. One of the challenge to be faced for a full

deployment of the CPV technology concerns the soiling, that

is, the dust settlement which can accumulate on glass of the

CPV modules. Soiling inherently disrupts the intended

function at that first surface/light interface, which can significantly reduce the solar technology power

output. Since CPV modules use optical system (like lenses) to collect the direct sun light and the dust

accumulation on the module glass increases the amount of diffused light, the soiling effect is particularly

severe for such technology. Considering these challenges, the following objectives have been pursued in

the SUN on CLEAN project:

1. to investigate different strategies for module glass surface functionalization (hydrophilic and

hydrophobic surfaces) in order to reduce the dust accumulation (Soiling effect) on the module

surface and trough a reciprocal transfer of knowledge and a proper research plan, to arrive

proposing new self-cleaning surfaces,

2. to perform a joint comparative testing of the CPV modules, in the different Countries where the

Partners are located (Italy, Spain and Brazil) understanding the limit and advantages of the

proposed self-cleaning surfaces in different climate condition,

3. to potentiate the reciprocal competences and improve the research capacity of the partners on the

CPV technology throughout several cross actions, comprising training, seminar and joint research

activities,

4. to promote a better scientific comprehension on the CPV technology through educational courses.

WORK PERFORMED IN THE PROJECT

In order to achieve the objective of the projects, a research plan has been implemented along with a staff

exchange program which allowed pursuing a multidisciplinary approach and creating an innovative ambient

in which each partner could expand its research capacity and produce results that could not be achieved by

itself alone. The research plan concerned there main research activities:

1) Concentrating photovoltaic (CPV) module preparation with multi-junction (MJ) solar cells (WP1) 2) CPV module characterization (WP2)

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3) Glass surface modification of the CPV modules, that is, the development of anti-soiling coatings

(WP3)

RSE (Italy) realized InGaP/InGaAs/Ge MJ solar cells that have been assembled by IOFFE (Russia) in CPV

modules. Such modules have been delivered to RSE (Italy), TECNALIA (Spain) and CEFET (Brazil) for the

module performance inter-comparison activity and for the study of the soling affect. Different anti-soiling

coatings (super hydrophobic or super hydrophilic) have been developed by TECNALIA and CEFET and then

deposited on glass samples which have been installed beside the CPV modules. This has allowed to start

ascertaining the effect of the soling and the effectiveness of the anti-soling coatings in the different

locations. Several test and characterization techniques have been applied to study the anti-soiling thin film:

adhesion tests (scratch test), damp heat test, film thickness measurement, roughness and surface

topography by Atomic force microscope (AFM) and Scanning electron microscope (SEM), Transmittance

and Raman spectroscopy.

Figure 1. Geographical and schematic representation of the research activities carried out in the SUN On CLEAN

project.

In the Sun on Clean project N.19 staff the exchange secondment periods have been successfully carried out,

in order to support the research activities and promote the knowledge dissemination on the CPV

technology. In particular, the staff exchange secondment periods concerned:

• N.6 Trainings on MJ solar cells growth e processing, CPV test facility management, indoor

characterization on MJ solar cells and modules and data acquisition

• N.4 Courses on III-V MJ solar cells and CPV technology and surface functionalization by thin film

deposition technology

• N.1 Seminar on dip coatings.

• N.6 Joint research activities: on modelling the module PV performances in function of soiling,

hybrid process implementation, anti-soling coating,

• N.1 Workshop

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PROJECT RESULTS

The project has been successful in term scientific results

TECNALIA and CEFET proceed with complementary routes in order to produce super hydrophobic or super

hydrophilic films. TECNALIA developed porous SiO2 coatings by sol-gel technique and modifications to

obtain hydrophobic surfaces in order to provide ‘Easy to clean’ properties. Most promising formulations

and coating systems have been scaled-up and deposited onto glasses of relevant size for industry. CEFET

developed TiO2/SiO2 composite hydrophilic films, that showed high transmittance in visible range,

maintaining self-cleaning properties. Both coatings showed performance better than the stat of the-art-

coatings. The set up by RSE of the first CPV test facility in Minas Gerais has allowed performing a

comparative testing on the CPV modules in Italy and Brazil with similar instrumentation understanding how

the climate and soiling influences the CPV module performance. A new module characterization

methodology have been created and experimented in order to isolate the soling effect with respect to the

spectrum effect on the MJ solar cells based CPV modules performances.

The project has been successful in term of transfer of knowledge

• RSE have transferred knowledge to IOFFE on new MOPCD process and post growth technique.

• IOFFE has transferred to RSE knowledge on testing MJ cells and modules with flash solar simulator

for high concentration

• RSE has transferred to CEFET knowledge on the CPV technology

• CEFET has transferred to RSE knowledge on surface functionalization and sol gel process

• The interaction between CEFET and TECNAIA on the different strategies for surface

functionalization has allowed comparing them for a better evaluation of their potentiality and limit.

The project has been successful in term of dissemination

Dissemination of the results took place throughout several actions: courses, scientific publication

participation in several congresses, interconnection with other European project (B.BICE+), workshop and

by means of the Sun on Clean Web site: www.sunonclean.eu and other web platform. The dissemination

actions allowed promoting of a better scientific comprehension on the CPV technology status and

prospective in properly answering the energy needs.

The project has been successful in term of exploitation

The use of advanced functional materials able to modify the glass surface with the double purpose of

minimising Fresnel losses (thus increasing energy generation) and achieving super hydrophobic or super

hydrophilic surface, which substantially reduces the soling effect, has become of crucial importance to

reduce the capital expenditure and solar power plant maintenance. The coating developed in Sun on Clean

demonstrated enhanced performance vs state of the art glass coatings and have been realized by adopting

a deposition technology (sol-gel dip coating process) that can be applied at a relevant scale. Therefore such

coatings can be very successful in view of their potential exploitation for photovoltaic application.

Furthermore, the partners have acquired a unique experience, which allowed setting up new

characterization methodologies useful to analyse the impact of the soling on the PV performance,

distinguishing it from the impact of the change of the solar spectrum. This experience has brought the

partners to an outstanding position to evaluate the performance of the advanced functional materials

when applied to the solar technologies and allowed them to prepare a new research proposal answering

the call H2020-NMP-16-2015, in Horizon 2020’s Work Programme. The new characterization

methodologies to be applied to CPV modules based on MJ solar cells will be proposed as new standard in

the frame of ICE W7 group.

In conclusion the present multi-annual International Research staff exchange program has allow

strengthening t h e research partnerships a m o n g E u r o p e a n a n d T h i r d C o u n t r i e s in

the field of Concentrating photovoltaic (CPV) enlarging both the European and third Country expertise and

knowledge on this technology, promoting a wider exploitation of this renewable energy resource towards a

sustainable global energy system. The project has a strong relevance from a scientific and social point of

view, because has allowed implementing an insight analysis on the soiling effect on the performance of this

technology, developing as mitigating solutions new anti-soiling highly transparent coatings and has

contributed in increasing the awareness of the potentiality of this renewable energy technology towards an

environmental friendly energy production.