Energy
M. Mahmoudi; H. Farzan; E. Hasan Zaim
Abstract
Asphalt materials commonly have high absorption coefficients, and their surface temperature reaches as high as 80 oC during daytime hours since their surfaces are exposed to solar radiation for long periods. Hence, asphalt pavements can easily be converted to solar air heaters (SAHs) to collect solar ...
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Asphalt materials commonly have high absorption coefficients, and their surface temperature reaches as high as 80 oC during daytime hours since their surfaces are exposed to solar radiation for long periods. Hence, asphalt pavements can easily be converted to solar air heaters (SAHs) to collect solar energy. Even though asphalt materials have low thermal conductivity, resulting in a weak convection heat exchange rate between the flowing air and asphalt surface. The current experimental study analyzes utilizing aluminum shavings as asphalt coating materials to improve SAHs’ thermal performance. To this aim, a serpentine SAH prototype was constructed, and several sensors were utilized to monitor its dynamic thermal response. Black-painted aluminum shavings were utilized as coating materials to improve the convective heat exchange rate and increase the roughness of the absorber surface. Two scenarios were considered, including the uncoated absorber plate and coated one with 0.2 kg aluminum shavings. The experiments were carried out for two air mass flow rates of 0.02 kg/s and 0.03 kg/s under field conditions. Based on the air mass flow rate, the coated absorber reaches higher temperatures, approximately 5 oC to 9 oC, than the uncoated one. The acquired results illustrate that the coated SAH has nearly 4 oC to 5 oC higher maximum exhaust air temperature; hence, the coating strategy improves the thermal efficiency by 24.75% and 44% in two air mass flow rates of 0.02 kg/s and 0.03 kg/s, respectively.
M. Ameri; H. Farzan; M. Nobari
Abstract
Glass plates have been commonly used as collectors’ covers due to technical feasibility, high transmissivity in shortwave solar irradiation, and low transmissivity in long-wavelengths. However, they are vulnerable to stones and hail. Plastic plates have high transmissivity in shortwaves but also ...
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Glass plates have been commonly used as collectors’ covers due to technical feasibility, high transmissivity in shortwave solar irradiation, and low transmissivity in long-wavelengths. However, they are vulnerable to stones and hail. Plastic plates have high transmissivity in shortwaves but also have transmission bands in the middle of the thermal radiation spectrum. The current study represents an experimental assessment of different covering strategies, including single acrylic-cover, single glass-cover, double glass- acrylic cover, and double glass-cover. Two solar air heaters (SAHs) prototypes were constructed for this study. The acquired experimental runs illustrated that the single glass-covered SAH represents higher thermal performance than the single acrylic-covered SAH due to the lower transmissivity of glass plates in long wavelengths. The double-covered SAHs have higher performance than the similar single-covered SAHs. In the double-covered SAHs, the convective-radiant heat loss is reduced. However, increasing the cover number improves the radiant resistance to solar irradiation and reduces the collector performance when solar irradiation is insufficient and the absorber temperature is low, especially at the beginning of daytime hours. The SAH using a double-glass cover is preferable; however, the heat dynamics of the double glass- acrylic-covered SAH are so close to the double glass-covered one, and the acrylic plate is more resistant to harsh ambient conditions.
H. Farzan; M. Ameri; S. M. Jaafarian
Abstract
Asphalt solar collectors (ASCs) offer a low-cost and reliable alternative to harvest energy from available infrastructures such as roads and pathways by employing the simple techniques. This paper represents an experimental study to evaluate the effectiveness of continuous and discontinuous-flow strategies ...
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Asphalt solar collectors (ASCs) offer a low-cost and reliable alternative to harvest energy from available infrastructures such as roads and pathways by employing the simple techniques. This paper represents an experimental study to evaluate the effectiveness of continuous and discontinuous-flow strategies in the dynamics and performance of a self-constructed ASC under field conditions. To this aim, an ON/OFF switching controller commands to run and stop the system at different time intervals. During the experimental simulations, all the crucial environmental and operational parameters were measured and monitored. This approach assesses the effects of numerous scenarios with different intervals of time on the dynamics of the constructed collector. Continuous and discontinuous-flow strategies were evaluated by comparing three different scenarios, including continuous-flow mode, 5 min OFF-mode and, 10 min OFF-mode. The results show that by extending the OFF-mode, the water is kept stagnant in the hot embedded pipes for more extended periods. Therefore, the temperature difference at the inlet and outlet of collector reduces, and the water leaves the collector at higher temperatures; however, the efficiency of the ASC decreases. Also, even though extending the OFF-mode results in heated water exits the collector at higher temperatures, but the mass of heated water decreases due to continuous interruption of current flow. The test results prove that in continuous-flow strategy, cumulative heat gain improves. Therefore, the continuous-flow strategy shows higher performance than introduced discontinuous-flow strategy. The exergy analysis illustrates that the available useful exergy has significantly affected by considering the pump consumed energy.