@article{Schumann2021,

title = {Performance Evaluation of an Evacuated Tube Collector with a Low-Cost Diffuse Reflector},

author = {J Schumann and B Schiebler and F Giovannetti},

doi = {10.3390/en14248209},

issn = {1996-1073},

year  = {2021},

date = {2021-12-01},

journal = {Energies},

volume = {14},

number = {24},

abstract = {In order to increase the overall solar energy gain of evacuated tube collectors, rear-side reflectors are used. In this way, the otherwise unused incident radiation between the tubes can be reflected back to the absorber, and the performance of the collector can be improved. In this paper, the use of a low-cost, diffusely reflecting, trapezoidal roof covering made from a galvanized metal sheet is investigated and compared to a high-quality, specularly reflecting plane reflector made of aluminum. For this purpose, ray-tracing analysis and TRNSYS simulations were carried out. In the ray-tracing analysis, the experimentally determined zero-loss collector efficiency η0 as well as the incident angle modifiers for each reflector can be reproduced with an error lower than 7.5%. Thermal system simulations show that the performance of both reflectors is comparable. The use of the low-cost reflector leads to an increase in annual collector output of around 30% compared to an increase with the specular reflector of around 33%. Considering a typical domestic hot water system, both reflectors enable an increase in the solar annual yield of approx. 11%.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Mercs2016b,

title = {Innovative Smart Selective Coating to Avoid Overheating in Highly Efficient Thermal Solar Collectors},

author = {D Mercs and A Didelot and F Capon and J -F Pierson and B Hafner and A Pazidis and S Föste and R Reineke-Koch},

doi = {10.1016/j.egypro.2016.06.177},

issn = {1876-6102},

year  = {2016},

date = {2016-06-01},

journal = {Energy Procedia},

volume = {91},

pages = {84-93},

abstract = {Highly efficient solar thermal systems generally undergo stagnation conditions with temperature inside the solar collectors as high as 190-200 °C, as soon as the domestic hot water demand is poor or when the system is off while the collectors are still submitted to a strong solar radiation (> 950 W/m2). These stagnation conditions are known to be one of the major problem of thermal solar systems and often lead to vaporization and glycol degradation, loss of performances, and the need for regular maintenance with associated costs for the end user. Thanks to a novel smart selective coating, characterized by a strong increase of its infrared emissivity (thermochromic effect) at a critical temperature, stagnation temperatures can be reduced to 150 °C for solar radiation and ambient temperature of 1000 W/m2 and 35 °C, respectively. As the novel smart selective coating presents a high solar absorption coefficient (>94%) and a low emissivity (∼6%) at low temperature, and because the thermochromic effect starts at a temperature around 70 °C, the high performance of the new thermochromic thermal solar systems is guaranteed for domestic hot water heating. The properties of this new generation of selective coatings, based on a mixture of vanadium and aluminum oxides (VO2/VnO2n±1/Al2O3/SiO2), are presented and discussed with regard to composition, structure and optical properties analysis. FTIR spectroscopy and infrared camera pictures clearly show the strong increase of emissivity for temperature higher than 70 °C. Aging performances (high temperature, humidity, thermal cycling) are also presented in order to guarantee a minimum life time of 25 years for the new generation of thermochromic solar collectors. Finally, stagnation temperatures recorded under the same natural sun radiation on scale one (2.3m2) standard and thermochromic collectors are compared.},

note = {Proceedings of the 4th International Conference on Solar Heating and Cooling for Buildings and Industry (SHC 2015)},

keywords = {},

pubstate = {published},

tppubtype = {article}

}