PhD Anna Krammer's coatings prevent thermal collector overheating
In her PhD thesis defended with brio on 18 June 2020, Anna Krammer of the Solar Energy and Building Physic lab's Nanotechnology group led by Dr Andreas Schueler developed thermochromic absorber coatings that prevent overheating in solar panels, which tend to undergo prolonged stagnation periods. The multi-layered coatings solve a longstanding problem and have a very positive impact on maintenance costs.
Anna Krammer's thesis was supervised by Dr Andreas Schueler and Prof. Jean-Louis Scartezzini.
Due to their simple design and operation, solar thermal collectors for domestic hot water genera- tion and space heating are one of the most common solar energy harvesting systems in use today. During cold periods, all the absorbed energy is useful. During hot periods, however, when solar radiation is abun- dant and demand is low, stagnation occurs. Storage is limited and excess heat cannot be diverted. The heat transfer fluid evaporates and the temperature of the solar absorber can exceed 200°C even in central Euro- pean latitudes. Glycols in the heat transfer fluid degrade, and the frame, thermal insulation and selective absorber coating deteriorate and become less efficient.
A new generation of solar collectors is envisaged that can absorb and repel heat in a controlled manner by changing their optical properties in the infrared spectral region. Thermochromic VO2 based absorber coat- ings change their optical properties according to temperature. Through the perfectly reversible thermo- chromic transition, at a critical temperature TC = 68°C, the thermal emittance of the absorber changes markedly, from ≈ 0.05 below TC to ≈ 0.35 - 0.4 above TC. In this thesis, increased transition temperatures, beneficial for solar absorber applications, are achieved by Ge doping of VO2 films. For 5.9 at.% Ge doping, a Tc ≈ 96°C is observed. Ge doping also leads to the increase of thermal emittance modulation Δε, especially due to increased thermal emittance in the high-temperature state. Moreover, crystalline switching films are sputtered at substrate temperature as low as ≈ 310°C.
Advanced thermochromic concepts involving Fabry-Pérot inspired multilayers with greatly enhanced ther- mal emittance modulation, absorbers with light-trapping black spinel nanoneedles or plasmonic W nano- particles obtained by means of nanoimprint lithography are investigated.
For the main selective solar absorber application, the thermochromic function is successfully integrated into the multilayered coating design. First, absorbers based on VO2 or VO2:Ge and nanocrystalline Cu- CoMnOx black spinel layers are proposed. However, a large thermal emittance modulation of Δε > 0.3, is accompanied by an unwelcome increase of the solar absorptance with increasing temperature. Simulations indicate that a relatively high n and k material inserted between the substrate and the thermochromic layer can revert the change in solar absorptance. The concept is adapted to industrial absorber designs and im- proved absorbers, based on Al//TiAlSiN//VO2:Ge//SiO2, with decreasing αsol and increasing εth over the thermochromic phase transition are reported for the first time.
The positive emittance and negative absorptance modulation of the absorbers limit the collector stagnation temperature by nearly 20°C, to a maximum Tstagnation ≈ 159°C. This leads to a shorter duration of stagnation conditions and an overall reduction of thermal charges on the system. Glycol degradation is hindered, lead- ing to important reductions in maintenance costs. At Tstagnation ≈ 150°C and at 3 bar pressure typically pre- sent in such systems, evaporation of the heat transfer fluid in the collector loops is avoided.
Accelerated aging tests in dry and humid conditions reveal the environmental stability of the thermo- chromic absorber coatings and a minimum service lifetime of 25 years is guaranteed for multilayers with antireflective and oxidation barrier coatings.