THERMOHYDRAULIC PERFORMANCE AND OPTIMIZATION OF PACKED BED SOLAR AIR HEATER

Abstract

A solar air heater is a simple device to heat air by utilizing solar energy. The efficiency of a conventional solar air heater has been found to be low because of low convective heat transfer coefficient between the absorber plate and the energy extracting and carrier fluid, namely air. Several designs of a solar air heater have been proposed to enhance the thermal performance. Use of porous packings inside the solar air heater duct is one of the important methods that has been proposed to achieve the objective of high thermal efficiency. Several experimental studies in the past have attempted to obtain the detailed information on the heat transfer mechanism in the porous packings in a duct. Their results revealed that the volumetric heat transfer coefficient is generally higher for beds with lower porosities. With this important point in mind, a critical review of literature in the area of heat transfer in packed beds was carried out with particular emphasis on the effect of bed porosity on the volumetric heat transfer coefficient. These studies refer to woven wire screen matrices inside the circular tubes in which air flows normal to the plane of wire screen matrices. Use of wire screen matrices in the solar air heater duct results in an entirely different flow arrangement because the air flows direction is parallel to the plane of screens. There are no correlations available in the literature for a system using a low porosity system. It is observed that there is a substantial rise in the volumetric heat transfer coefficient with a decrease in porosity and hence low porosity system would be a better choice when one looks for enhancement of performance of a solar air heater utilizing packed bed in its duct. In view of the above, the present investigation has been carried out with the following objectives: i) Collection of experimental data of heat transfer and friction in a packed bed solar air heater using low porosity wire screen matrices system under actual outdoor conditions. ii) Development of correlations for heat transfer coefficient and friction factor on the basis of experimental data in terms of geometrical parameters of the bed. iii) Investigation of the enhancement of thermal performance of a packed bed solar air heater compared to that of a conventional solar air heater. iv) Development of an analytical model based on heat transfer and fluid flow in a packed bed solar air heater for predicting the thermal performance. v) Optimization on the basis of the thermohydraulic performance of a packed bed solar air heater. Optimum bed parameters are proposed to be obtained for a given set of operating conditions. Therefore, an experimental setup was designed, fabricated and used for data collection for the packed bed and conventional collector ducts. Experimental data on heat transfer and friction has been used to determine the values of heat transfer factor, jh, and friction factor, fp as function of bed and operating parameters. The analysis of experimental data on heat transfer and friction characteristics revealed that the heat transfer coefficient and friction factor are strong functions of bed and operating parameters. It is observed that the heat transfer coefficient increases with a decrease in porosity. This appears to be due to higher level of turbulence created in the flow as the porosity decreases and the flow passage becomes more tortuous and narrower with higher solidity of such a ii 1 ) 0.47 ) 0.23 rt (nP d w jh = 0.42 Friction factor (fp) 1.46 ReP -0.61 2.75 1 \0.47 ( p )0.23 ° 71 t ] dw (nP,Re-0.425 system. An increase in the number of layers in given bed depth for the same value of wire diameter and pitch, affects the heat transfer coefficient. Besides porosity and number of layers, the pitch to diameter ratio also affects the flow behaviour; at lower value of pitch to diameter ratio a standing vortex may be formed with relatively weak circulation of fluid and low heat transfer coefficient is observed, while in the case of higher value of pitch to diameter ratio a higher value of heat transfer coefficient is obtained. A similar functional dependence of friction factor has also been observed. Based on these observations, the functional relationships of Colburn j factor and friction factor, fp, with the system and operating parameters are sought and the following correlations have been developed. Colburn j factor (jh) A comparison of the experimental and values predicted by these correlations shows that there is a good matching between them. The thermal efficiency of a packed bed solar air heater has been found to improve appreciably in comparison to that of a conventional collector as a result of packing the collector duct with wire screen matrices. The enhancement in thermal performance has been found between 57% to 83% in the range of iii system and operating parameters investigated. This enhancement in the thermal efficiency appears to be due to absorption of energy in depth resulting in better heat transfer. The thermal efficiency of a packed bed collector is higher for lower porosity matrices. The thermal efficiency of a packed bed solar air heater increases with an increase in mass flow rate because of an increase in heat transfer coefficient and lower amount of thermal losses. In the range of lower flow rate, the rate of increase in the thermal efficiency with an increase in flow is higher whereas in the higher range of flow rate, the rate of increase in efficiency decreases. This is due to a corresponding decrease in the rate of increase in heat transfer coefficient as the mass flow rate increases. The temperature rise parameter, (t0 — ti) / I, is higher at low mass flow rate. As the value of mass flow rate increases, the temperature rise parameter decreases. This trend has been seen for all the packed bed solar air heaters investigated as well as for the conventional collector. The efficiency index, Ili (= Oh I Jhc) / (fp / fc)) considering the heat transfer and friction both for the packed bed and the conventional solar air heater as a result of using low porosity has been found between 1.14 to 1.58. The enhancement of heat transfer rate in terms of the ratio of j factor, in/ Jhc is found to lie between 1.25 to 1.83. Using energy balance, the governing equations have been developed for steady state conditions for a packed bed solar air heater. These equations are converted to dimensionless form and then are solved with the finite difference iv method. A mathematical model has been developed for predicting the thermal efficiency. The correlation developed in this work has been used for calculating the convective heat transfer coefficient between wire screen matrices and air. The thermal efficiency predicted by this analytical model has been compared with the experimental thermal efficiency. The average absolute deviation between the two has been found to be 9.6%. This shows that the analytical model is satisfactory. This analytical model has been used for the prediction of the effective efficiency that takes into account the thermal performance along with the pressure drop across a collector. It has been found that the values of the effective efficiency and the thermal efficiency are nearly the same- at higher values of temperature rise parameter whereas at lower values of temperature rise parameter a substantial difference between them is observed; the effective efficiency is much lower compared to the thermal efficiency at low temperature rise parameter. A clear maxima in effective efficiency is seen with respect to temperature rise parameter for all bed parameters, the reason for this occurrence being the much sharper increase in the pumping power as the mass flow rate increases (and consequently as the temperature rise parameter decreases). The value of temperature rise parameter corresponding to the maximum effective efficiency is different for different bed parameters. The maximum effective efficiency is higher at low porosity corresponding to higher temperature rise parameter, but at low temperature rise parameter the maximum effective efficiency occurs in higher porosity systems. It has been observed that the gap between effective efficiency of a packed bed and an unpacked collector decreases as the temperature rise parameter decreases. The curves cross over each other beyond a certain value of the temperature rise parameter; a conventional collector shows better performance thereafter. The plots of thermal efficiency reveal that the gap between the packed bed and the unpacked collector are same as that observed between the plots of the effective efficiency. At lower values of the temperature rise parameter, however, the gap is considerably wider in the case of thermal efficiency plots. The values of the effective efficiency under optimum conditions of a packed bed collector are higher at lower values of insolation corresponding to higher values of temperature rise parameter. But the trend reverses in the case of lower values of the temperature rise parameter; the value of effective efficiency under optimum condition is higher at higher insolation. The reason for this is that at higher intensity top losses are more at higher values of the temperature rise parameter and vice versa. The crossing over of the curves of the packed bed and conventional collectors shows that beyond a certain value of the temperature rise parameter, the performance of a conventional collector is better. A maxima exists in the effective efficiency for a given intensity of solar radiation and this maxima shifts to lower values of the temperature rise parameter as the intensity of solar radiation increases. The maximum effective efficiency occurs corresponding to the maximum intensity of solar radiation. The maximum enhancement in the maximum effective efficiency as a result of using packed bed is 56% at insolation of 1000 W/m2. The bed parameters corresponding to the maximum effective efficiency are P = 0.80, Pt/dw= 2.5, n = 8. The values of the enhancement in the effective efficiency at different insolation values has been found to lie between 38% to 56%. vi The data pertaining to the values of effective efficiency as a function of bed parameters has been utilized to determine the set of optimum values of bed parameters that yields the maximum effective efficiency for given set of values of temperature rise parameter and insolation. Design plots have been prepared to represent the optimum values of bed parameters as a function of operating parameters (temperature rise parameter and insolation). These plots can be used for the selection of optimum bed parameters to yield a maximum effective efficiency under a given set of system and operating conditions. A procedure for obtaining the optimum conditions for a packed bed solar air heater has been proposed, such that if the desired temperature rise parameter and the average insolation is known, the design plot can be used to obtain the optimal bed parameters and for this set of optimum parameters the thermal efficiency of the system can be obtained. Based on this thermal efficiency and known energy requirements, the collector area can then be estimated. The outcome of the present study can be summarized as: (i) considerable amount of enhancement in thermal efficiency can be obtained by using a low porosity packed bed solar air heater. (ii) the design plots for a low porosity packed bed solar air heater have been prepared; which would be a useful design tool for a practicing engineer for developing a packed bed solar air heater with optimum geometric parameters yielding a maximum effective efficiency for given operating parameters. VII