Laser Sintering of Tungsten Nanoparticles for Solar Thermal Receivers

Solar thermal energy is a growing sector in the field of alternative energy. The advantage of solar thermal energy is that in addition to high scalability for utility power plants, it is also able to generate electricity at a later time by making use of thermal storage. Solar thermal technology consists of many components but one of the most important is the solar receiver. The purpose of the receiver is to absorb the concentrated sunlight and retain that heat as fluid carries away the energy that is used by the turbine to generate electricity. The higher the temperature of the fluid, the higher is the efficiency of the thermal conversion system. In order for the receiver to be efficient, it has to be coated with a layer called spectral selective coating. The goal of this coating is to have high solar absorptance and low thermal emittance. Therefore, it should have low reflectance for wavelength < 2 µm and high reflectance for > 2 µm. This will enable absorbed solar energy to be transferred in the form of heat to the fluid with low radiative losses.
Spectral selective coatings that are currently used for solar receivers are expensive to manufacture since they require vacuum fabrication techniques. A further drawback is that the current coatings are only stable up to 500 – 600 C in air. The driving factors for long-term performance degradation are oxidation, delamination of coatings, and cracking. Thus there is a need for receiver coatings that have high solar absorptance, low thermal emittance, and can operate at high temperatures in air for tens of years.
In this research, we demonstrate laser sintering of tungsten nano particles to achieve spectral selectivity. Laser sintered tungsten coating have microscale surface roughness that has been used to achieve high solar absorptance and low thermal emittance. Theoretical modeling was carried out to study the interaction of electromagnetic wave with rough surfaces in the visible and infra-red spectrum and in turn determine solar absorptance and thermal emittance. Morphological, structural, and compositional properties of laser sintered tungsten films were investigated to understand its optical properties and high temperature stability. We have demonstrated very high solar absorptance ~ 90.3% at room temperature and ~ 87% after heat treatment at 650 C. This coating was able to maintain its performance in air when heated at 650 C for at least 36 hours making it ideal for solar tower applications. The solar absorptance can be further increased with the use of an anti-reflection film on sintered coatings.