Wenfeng Gao

A parametric study on the thermal performance of a solar air collector with a v-groove absorber

In this paper, a parametric study on the thermal performance of a solar air collector with a v-groove absorber has been investigated. In this single-cover collector, the air flowing in the channel formed by the v-groove absorber and the bottom plate—which is flat and insulated—is along the groove, aiming at enhanced heat transfer rate between the air and the absorber by increasing the heat transfer surface area, which is crucial to the improvement of thermal performance of a solar air collector. To quantify the achievable improvements with the v-groove absorber, a flat-plate solar air collector where both the absorbing plate and the bottom plate are flat, is also considered. The thermal performance of these two types of solar air collectors is analyzed and compared under various configurations and operating conditions. The results show that the v-groove collector has considerably superior thermal performance to the flat-plate collector. It is also found that to achieve better thermal performance, it is essential to; use a small size of the v-groove absorber for the v-groove absorber collector and to maintain a small gap between the absorber and the bottom plate for the flat-plate collector; to use selected coatings that have a very high absorptivity of solar radiation but a very small emissivity of thermal radiation on the absorber and glass cover; to maintain an air mass flow rate above 0.1kg/m2s; and to operate the collectors with the inlet fluid temperature close to that of the ambient fluid.

A Comparative Study of the Thermal Performances of Cross-Corrugated and V-Groove Solar Air Collectors

In this paper, the thermal performance of cross-corrugated and v-groove solar air collectors has been compared under a wide range of configuration and operating conditions. For cross-corrugated collectors, the air flow channel is formed by the wavelike absorbing plate and the wavelike bottom plate, which are transversely positioned, whereas for v-groove collectors it is formed by the v-groove absorbing plate and the flat bottom plate. The use of the transversely positioned wavelike bottom plate in cross-corrugated collectors is to achieve better thermal performance by enhancing the heat transfer rate inside the air flow channel via augmented turbulence. It is found that cross-corrugated collectors are superior to v‐groove ones for all the configurations and operating conditions considered, confirming that the use of the transversely positioned wavelike bottom plate, in cross-corrugated collectors improves the thermal performance. The results also show that a slender configuration along the air flow direction, a small gap between the absorbing plate and the bottom plate, a selected coating on the absorbing plate and the glass cover that has a very high absorptivity of solar radiation but quite a small emissivity of thermal radiation, an air mass flow rate per unit area of collector above 0.1 kg/m2·s, and maintaining the inlet fluid temperature close to that of the ambient fluid, will lead to improved thermal performance for both collectors. However, it is found that the mean gap between the cover and the absorbing plates, the emissivity of thermal radiation of the bottom plate, the solar insolation, and the inclination of the collectors have negligible effects on the efficiencies of both collectors.

An Experimental Study on the Heat Storage Performances of Polyalcohols NPG, TAM, PE, and AMPD and their Mixtures as Solid-Solid Phase-Change Materials for Solar Energy Applications

The heat storage performance of polyalcohols NPG (Neopentyl glycol), TAM (Tris(hydroxymethyl)aminomethane), PE (Pentaerythritol), and AMPD (Aminoglycol) and their mixtures as solid-solid phase-change materials for solar energy applications are experimentally investigated by the DSC (differential scanning calorimetry) technique. It is found that pure AMPD is not suitable for practical applications due to its wide ranges of charge and discharge temperatures and its high hygroscopic property. The specific sample preparation process has significant impact on the heat storage performance of the mixtures of polyalcohols. If the experimental samples are prepared with the proper processes, there will usually be only one peak presented in the DSC curves instead of two peaks, as reported by some studies. Due to the poor heat storage performance and high hygroscopic property of AMPD, the binary mixtures of AMPD/NPG and AMPD/TAM do not have practical application values. However, the binary mixtures of AMPD/PE with a high or low weight ratio of AMP still are found to have potential practical application values. The experimental results also show that a mixture of more than two polyalcohols do not have better performances than a binary mixture.

The operational thermal performance of a simple passive solar house in winter: a case study in Kunming, China

In this article, the operational thermal performance of a simple passive solar house in Kunming in winter is studied analytically and experimentally. Such a solar house is ideal for low-income families in rural areas. The analysis shows that the solar house has an energy-saving rate of 70.2% in Kunming in winter. The comparative experiments reveal that on sunny days the average indoor air temperature in the solar house is 4.4°C higher than that of a comparative experimental house, with the largest temperature difference of 7.3°C and the smallest of 2.4°C; on cloudy days, the average indoor air temperature in the solar house is 4.1°C higher than that of the comparative house, with the largest temperature difference of 4.7°C and the smallest of 3.8°C. The analysis of the calculated Grashof number in the air channels of the heat-collecting components of the solar house with experimental data shows that there is strong natural convection in the air channels for most of the time on sunny days, whereas on cloudy days the natural convection is very weak.

An experimental study on the impacts of key parameters of all-glass evacuated tubes on the thermal performances of all-glass evacuated tube solar water heaters

In this paper, the effects of the stagnation parameter Y, the solar irradiance under stagnation H, and the average heat loss coefficient ULT of individual all-glass evacuated solar collector tubes, which were identified as the major parameters to characterize the thermal performance of the tubes, on the thermal performance of all-glass evacuated tube solar water heaters consisted of such evacuated tubes were investigated by experiments. The experiments were conducted to obtain the heat gain Qs produced by six all-glass evacuated tube solar water heaters with the similar specifications but manufactured by different producers. Each heater consisted of 20 all-glass evacuated solar collector tubes and one tube from each heater was selected randomly to measure its Y, H, and ULT . The experimental results show that both H and ULT have effects on Qs , with Qs decreasing when H or ULT increases, but H has more significant effects on Qs than ULT does. However, Y does not have significant effects on Qs . It is apparent that one effective way to increase the heat gain of an evacuated tube solar water heater is to reduce H and ULT of individual all-glass evacuated solar collector tubes. An error analysis was conducted which clearly shows that the errors caused by the uncertainties associated with the measured Qs , Y, H, and ULT have negligible effects on these results.

An experimental study on the application of polyalcohol solid-solid phase change materials in solar drying with cross-corrugated solar air collectors

In this paper, two identical solar driers with the same cross-corrugated solar air collectors and drying chamber were developed, one with phase-change materials (PCMs) and the other without PCMs. These two solar drying systems were tested in typical sunny and cloudy days in Kunming and their thermal performances were analyzed. The experimental results show that the temperature changing is smoother in the collector with the PCMs, which is beneficial for the drying as the useful drying time was prolonged. The same trend was also found in the chamber with the PCMs. The PCMs in solar drying system was found to play a role in temperature regulating. There were several cycles of heat charging-discharging in a cloudy testing day while the temperatures on collectors and in chambers with the polyalcohol PCMs is higher than each phase-change temperature. Nevertheless, there was only one cycle of heat charging-discharging in a sunny testing day. The collector with PCMs has higher daily useful heat gain than the collector without PCMs.

Natural convection boundary-layer flow in a Trombe wall channel

A scaling analysis has been carried out to obtain the scaling laws which relates the parameters of characteristic flow behavior of the natural convection boundary-layer flow of air in a Trombe wall channel with the operation parameters and the structure parameters. These scaling laws have been validated and quantified by a series of direct numerical simulation.