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The world energy needs are increasing in all fields. Electrical energy is one of the most diffuse types of energy and it is often used to satisfy the residential and industrial  requests of heating and cooling. For these applications it can be more efficient to use the solar heating and solar cooling approach.

In such scheme the sun provide the thermal energy that can be used directly for heating or converted to cooling using an absorption chiller.

An important part of the energy conversion system is the solar thermal panel. To improve the panel efficiency the solar radiation is captured by highly selective absorbing coatings deposited of a low emissivity conductive substrate.

To optimize the efficiency of the system the absorbance should be maximized and the thermal losses minimized. Between the thermal losses, the  convection and the conductivity due to air are the most severe and vacuum is used to eliminate the convection losses, whereas high vacuum is required to reduce to gas conduction losses to a negligible level.   Once the high vacuum is obtained the next limiting factor is the quality of the absorber in term of high absorbance in the sun spectrum region and low emittance in the infrared region to reduce the thermal radiation loss at operational temperature.  

The R&D efforts are mainly devoted to:

a)Study solution to obtain and to keep the high vacuum during the 10 years life of the panels (studying baking cycle, analyse the gas desorbed from the panel under different condition, properties of getter pump materials…)
b) Characterize and optimize the solar selective coating to improve the panel performances.

The activities are done in a strong collaboration with an industrial partner that produces the high vacuum solar flat panels.

Evolution of pressure and temperature during the baking process.
The external parts of the panels are heated at more the 200C while the absorbers are heated at tempertures higher then 300C.

The Gases originated from the panels are pumped out trough a turbomolecular pump and a residual gas analyzer perform the gas composition analysis.

 

The gas partial pressures are followed during the baking and analyzed taking in to account the different ionization efficiency, the cracking factors, and the different pumping speed of the different mass species.

The main gas components of the vacuum are hydrogen and water, followed by carbon oxyde. They form more than 95% of the vacuum pressure.

 

 

The comparison of outgassing under different conditions allows to determine the best conditions for panels productions.

The getter material have to be able to pump the gas desorbed during the operation to keep the vacuum level lower than 1x10E-3mbar in order to neglect the thermal gas conduction.

 

 

The absorber should be highly selective to maximize the absorbance in the solar spectrum and minimized the thermal emission at the working temperature. Ideal cut-off wavelegth is a function of emission spectrum of the black body at the working temperature and it is around 2 micron for high temperature applications.

 

 

Contact: Roberto Russo

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