1.
J Schumann; B Schiebler; F Giovannetti
Performance Evaluation of an Evacuated Tube Collector with a Low-Cost Diffuse Reflector Artikel
In: Energies, Bd. 14, Nr. 24, 2021, ISSN: 1996-1073.
@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}
}
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%.
2.
B Schiebler; S Jack; H Dieckmann; F Giovannetti
In: Solar Energy, Bd. 171, S. 271-278, 2018, ISSN: 0038-092X.
@article{Schiebler2018d,
title = {Experimental and theoretical investigations on temperature limitation in solar thermal collectors with heat pipes: Effect of superheating on the maximum temperature},
author = {B Schiebler and S Jack and H Dieckmann and F Giovannetti},
doi = {10.1016/j.solener.2018.06.036},
issn = {0038-092X},
year = {2018},
date = {2018-09-01},
journal = {Solar Energy},
volume = {171},
pages = {271-278},
abstract = {Heat pipes in solar thermal collectors enable to reduce the temperature loads in the solar circuit during stagnation periods by exploiting their dry-out limit. With this approach vapour formation in the solar circuit can be completely avoided, which is essential to reduce costs of solar thermal systems by simplified and more reliable solar circuits. The design of “deactivating” collector heat pipes with a desired maximum temperature requires a comprehensive understanding of the heat transfer processes in the heat pipe, in particular when dry-out takes place. We developed a model, which allows calculating the maximum fluid temperature in the collector for various working fluids. Compared to existing approaches, the effect of superheated vapour in the heat pipe during stagnation is additionally considered. The paper describes the theoretical model in detail and its extensive experimental validation. The results show that the model is able to predict the maximum fluid temperature with an accuracy better than 5 K. Based on parametric studies with different working fluids, we analyse and discuss the temperature limitation and its effect on the collector performance.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Heat pipes in solar thermal collectors enable to reduce the temperature loads in the solar circuit during stagnation periods by exploiting their dry-out limit. With this approach vapour formation in the solar circuit can be completely avoided, which is essential to reduce costs of solar thermal systems by simplified and more reliable solar circuits. The design of “deactivating” collector heat pipes with a desired maximum temperature requires a comprehensive understanding of the heat transfer processes in the heat pipe, in particular when dry-out takes place. We developed a model, which allows calculating the maximum fluid temperature in the collector for various working fluids. Compared to existing approaches, the effect of superheated vapour in the heat pipe during stagnation is additionally considered. The paper describes the theoretical model in detail and its extensive experimental validation. The results show that the model is able to predict the maximum fluid temperature with an accuracy better than 5 K. Based on parametric studies with different working fluids, we analyse and discuss the temperature limitation and its effect on the collector performance.
3.
S Föste; B Schiebler; F Giovannetti; G Rockendorf; S Jack
Butane Heat Pipes for Stagnation Temperature Reduction of Solar Thermal Collectors Artikel
In: Energy Procedia, Bd. 91, S. 35-41, 2016, ISSN: 1876-6102, (Proceedings of the 4th International Conference on Solar Heating and Cooling for Buildings and Industry (SHC 2015)).
@article{Föste2016b,
title = {Butane Heat Pipes for Stagnation Temperature Reduction of Solar Thermal Collectors},
author = {S Föste and B Schiebler and F Giovannetti and G Rockendorf and S Jack},
doi = {10.1016/j.egypro.2016.06.168},
issn = {1876-6102},
year = {2016},
date = {2016-06-01},
journal = {Energy Procedia},
volume = {91},
pages = {35-41},
abstract = {Heat pipes in solar thermal collectors enable to reduce the temperature loads in the solar circuit during stagnation periods by exploiting their dry out limit. Typically water, pentane or acetone are used as heat transfer media in collector heat pipes. Butane is very suitable to reach a high temperature gradient of the dry out even if the maximum temperature in the fluid circuit should be designed to 120 ̊C or below. The paper presents experimental results with butane heat pipes that operate up to a maximum temperature of 120 ̊C with a high temperature gradient in the dry-out region. This ensures that the collector performance in the operating range (typically up to 100 ̊C) is not affected negatively by the dry-out. Different approaches to increase the thermal conductance of butane heat pipes by enhancing the inner surface of the condenser or of both, the condenser and the evaporator are experimentally assessed and discussed. Measurement results report an increase of the heat pipes’ thermal conductance from 3 W/K (standard geometry) to 23 W/K.},
note = {Proceedings of the 4th International Conference on Solar Heating and Cooling for Buildings and Industry (SHC 2015)},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Heat pipes in solar thermal collectors enable to reduce the temperature loads in the solar circuit during stagnation periods by exploiting their dry out limit. Typically water, pentane or acetone are used as heat transfer media in collector heat pipes. Butane is very suitable to reach a high temperature gradient of the dry out even if the maximum temperature in the fluid circuit should be designed to 120 ̊C or below. The paper presents experimental results with butane heat pipes that operate up to a maximum temperature of 120 ̊C with a high temperature gradient in the dry-out region. This ensures that the collector performance in the operating range (typically up to 100 ̊C) is not affected negatively by the dry-out. Different approaches to increase the thermal conductance of butane heat pipes by enhancing the inner surface of the condenser or of both, the condenser and the evaporator are experimentally assessed and discussed. Measurement results report an increase of the heat pipes’ thermal conductance from 3 W/K (standard geometry) to 23 W/K.