J.S. Saini

Heat transfer and friction factor correlations for packed bed solar air heater for a low porosity system

An experimental investigation has been carried out on a low porosity packed bed solar air heater. This investigation covers a wide range of geometrical parameters of wire screen matrix, i.e. wire diameter 0.795 to 1.40 mm, pitch 2.50 to 3.19 mm and number of layers from 5 to 12. The correlations have been developed for the Colburn j factor and friction factor for a low range of porosities from 0.667 to 0.880 and packing Reynolds number range from 182 to 1168. It is observed that both the heat transfer coefficient and the friction factor are strong functions of geometrical parameters of the porous packed bed. A decrease in porosity increases the volumetric heat transfer coefficient.

Simulated Performance of Packed Bed Solar Energy Storage System having Storage Material Elements of Large Size - Part I

Packed bed or rock bed is very commonly known thermal energy storage component for solar air heaters. The major concern for design of packed bed energy storage system is to maximize the heat transfer with minimum pressure drop or pumping power. Under the given operating conditions, system parameters could affect heat transfer and pressure drop in the bed considerably. An attempt has been made to report the simulated performance of a packed bed solar energy storage system. Large size storage material elements of five different shapes have been used to carry out the present simulation study. Nusselt number and friction factor correlations reported by the authors under previous experimental study have been utilized to analyze the packed bed energy storage system for a range of system and operating parameters. Performance of the system has been evaluated based on temperature distribution, thermal energy stored and available energy stored in the bed, energy consumption by fan to propel air through the bed and thermal efficiency of the collector as a function of system and operating parameters. In present part of the paper, various component models of packed bed solar energy storage system and development of computer program have been reported. Results of simulated system performance have been reported and discussed in Part-II and III of this paper.

Experimental Investigation of Thermohydraulic Performance of a Rectangular Solar Air Heater Duct Equipped with V-Shaped Perforated Blocks

This paper presents the thermohydraulic performance of rectangular solar air heater duct equipped with V-shaped rectangular perforated blocks attached to the heated surface. The V-shaped perforated blocks are tested for downstream (V-down) to the air flow at Reynolds number from 2000 to 20000. The perforated blocks have relative pitch ratio (P/e) from 4 to 12, relative blockage height ratio (e/H) from 0.4 to 1.0, and open area ration from 5% to 25% at a fixed value of angle of attack of 60∘ in a rectangular duct having duct aspect ratio (W/H) of 12. Thermohydraulic performance is compared at different geometrical parameters of V-shaped perforated blocks for equal pumping power which shows that maximum performance is observed at a relative pitch of 8, relative rib height of 0.8, and open area ration of 20%. It is also observed that the performance of V-shaped perforated blocks was better than transverse-perforated blocks.

Models for Predicting Thermal Performance of Packed Bed Energy Storage System for Solar Air Heaters - A Review

Packed bed is generally recommended for attaching with solar air heater in order to store thermal energy of hot air. For designing such a system under the given system and operating parameters, it is required to predict performance of the system by using mathematical models. In the present paper an attempt has been made to discuss mathematical models reported in the literature for predicting thermal performance of packed bed energy storage system for solar air heaters. The designer may be benefited from the consolidated information reported in the present paper. The continuous use of fossil fuels resulted energy crisis and environmental threat. It is felt that renewable energy sources are quite capable of meeting energy demand of todays world. The use of renewable energy sources for meeting energy needs can conserve the conventional energy sources for more number of decades. Among renewable energy sources solar energy is considered to be one of the most dominating energy source. It has many advantages like large potential, free of cost, available everywhere, environment friendly etc. However time dependent nature is the major disadvantage of solar energy. In order to overcome this disadvantage it is required to attach an energy storage system with the solar energy utilization system. With such a provision the stored energy can be utilized in the absence of solar radiation or under peak load conditions. In case of solar air heaters it is required to store thermal energy of flowing hot air. Packed bed is generally recommended for such an application. Packed bed consists of a container in which solid material elements having good heat capacity remain packed. The hot air flows from top to bottom of the bed to transfer heat energy. The rise in temperature of solid material takes place and energy can be retained by having properly insulated packed bed. The stored energy can be retrieved by making flow of cold air from bottom to top of the bed. The schematic of packed bed energy storage system for solar air heaters is shown in Fig. (1). The working of such a system is described in detail by Duffie and Beckman (1).

Heat Transfer Enhancement due to V-Shaped Perforated Blocks in a Solar Air Heater Duct

An experimental study of enhancement of heat transfer due to V-shaped perforated blockages attached to the heated surface has been presented in this paper. The duct equipped with perforated V-blocks had an aspect ratio (W/H) of 12, relative blockage height ratio (e/H) of 0.8, angle of attack (α) of 60° and open area ratio (β) of 20%, while relative pitch ratio (P/e) was varied from 4 to 12. The values of Nusselt number and friction factor of the duct with blockages were compared with values of Nusselt number and friction factor of the smooth duct operating under similar experimental conditions. It was found that there was a significant effect on the Nusselt number ratio and friction factor ratio when the pitch ratio was changed and there was exist an optimum value of pitch ratio. Thermal hydraulic performance was found to be maximum corresponding to relative pitch value of 8.

Optimization of System Parameters of Packed Bed Solar Energy Storage System having Storage Material Elements of Large Size

Major concern for design of a packed bed solar energy storage system is to maximize heat transfer during charging and discharging phase by spending minimum amount of pumping power. Shape of the material elements and void fraction of the bed are the system parameters for a given material size, which could affect heat transfer and pressure drop in the bed considerably. In the present study, simulation has been carried out to evaluate optimum values of system parameters in order to have the best thermo hydraulic performance of packed bed solar energy storage system. Five different shapes of large size material elements have been analyzed to carry out the present optimization study. In order to select optimum values of system parameters under the given operating conditions, design plots have been prepared and presented in the paper.