PERFORMANCE OF PACKED BED ENERGY STORAGE SYSTEM FOR SOLAR AIR HEATERS

Abstract

In view of the intermittent nature of solar energy, it is necessary to provide a storage system with solar collectors to store energy and to meet the demand in the absence of solar radiation. Packed bed is generally recommended for thermal energy storage in solar air heaters. Packed bed sensible heat energy storage consists of solid material of good heat capacity packed in a storage tank through which the heat transport fluid is circulated. Solar air heater supplies hot air, which flows through the bed to transfer the thermal energy to the solid particles. The stored thermal energy can be recovered by making cold ambient air flow through the bed. Generally, small size storage materials in the size range of 1 to 3 cm are used to store thermal energy. Small size storage material having large effective surface area can provide high rates of heat transfer, however, it is also accompanied by large pressure drop in the bed. High pressure drop results in substantially large energy consumption to propel air through the bed. This reduces the overall benefit of the solar energy utilization system. Pressure drop in the bed could be reduced with the use of large sized material elements. Reduction in the heat transfer rate to large size material elements due to smaller surface area per unit volume of storage is compensated by substantial reduction in the amount of energy consumption by fan due to low pressure drop in the packed bed. It can therefore be concluded that the large size material could be more beneficial for use as the storage material. Most of the correlations of heat transfer coefficient and friction factor reported in the literature are for small size materials, which can not be utilized for the design of packed bed systems using large sized elements because fluid flow and heat transfer characteristics of such systems are substantially different from those with small size materials. The shape of the packing material and void fraction of the bed determine the size and distribution of the channels through which fluid flows for heat transfer. The effect of these parameters is required to be taken into account along with the operating parameters for accurate prediction of heat transfer and pressure drop in the bed. Since, the generalized heat transfer coefficient and friction factor correlations are not available in the literature which could predict the thermal and hydrodynamic performance of the system for different shapes ofthe large size material elements at different void fractions of the bed, it was planned to investigate the heat transfer and pressure drop characteristics of the packed bed with the following objectives: i) To investigate the effect of system parameters on heat transfer and pressure drop characteristics of packed bed as function of the operating conditions. ii) To develop the correlations for heat transfer coefficient (Nusselt number) and friction factor as function of the system (shape of material elements and void fraction of the bed) and operating parameters (Reynolds number). iii) To investigate thermal and hydrodynamic performance of the packed bed solar energy storage system. iv) To determine the optimum values of system parameters that yield the best performance of the packed bed solar energy storage system. in order to meet the above-mentioned objectives, an extensive experimental investigation has been planned. The experimental se,-up has been planned to collect extensive data on heat transfer and fluid flow characteristics of packed bed. The data „„ air temperature and material temperature a. different locations inside the packed bed and pressure drop across the bed were collected tor six different mass velocities 11 of air ranging from 0.155 to 0.266 kg/s m2 for each set of different material shapes and void fraction of the bed. To investigate the effect of material shape on heat transfer coefficient and pressure drop in the packed bed. material elements of five different shapes have been used. Sphericity parameter (y) is used to represent the shape of material elements. Table 1 gives the range of system and operating parameters used in the present investigation. Table 1 : Range of system and operating parameters. Parameter Range Sphericity (i//) 0.55- 1.00 Void fraction (£ ) 0.306-0.630 Mass velocity of air (G) 0.155-0.266 kg/s m2 Nusselt number (Nu). friction factor (f) and Reynolds number (Re) have been used to represent heat transfer coefficient, pressure drop and operating conditions respectively in non-dimensional form. It has been observed that Nusselt number and friction factor are strong functions of sphericity of material elements, void fraction of the bed and Reynolds number. The changes in the values ofheat transfer and friction parameters with sphericity and void fraction are substantial which call for ajudicious selection ofthese parameters to result in maximum heat transfer gain for a minimum friction penalty. Using the experimental data, following correlations for Nusselt number and friction factor have been developed as function ofReynolds number, sphericity and void fraction. in Correlation for Nusseit number: M, =0.437(Re)075 frf" C^)"'62 {exp[29.03(kw)2 ]} (0 Correlation for friction factor: / =4.466(Re) °2 for')0*6(*)-""{exp[ll.85(log^)2 ]) (2) ^ It has been observed that experimental values and the values of Nusselt number and friction factor predicted by the above correlations are in good agreement. Validity of the correlations has been checked by comparing the values generated from the correlations developed by previous investigators reported in the literature and those generated from these correlations for given values of the sphericity and void fraction of the bed. Agood agreement has been observed. Nusselt number and friction factor correlations developed in this work have been used to investigate the thermal and hydrodynamic performance of a packed bed solar energy storage system using a mathematical simulation technique. The inlet temperature to the bed is kept constant during the charging process by varying the flow rate of air. The performance study ofthe packed bed solar energy, storage system has been carried out to determine the stratification of the bed, thermal and available energy stored in the bed, energy consumption by fan and thermal efficiency of the collectors during charging of the bed as function of sphericity and void fraction. Since the objective of the system designer is to have the maximum exergy stored , therefore design of the packed bed system should be based on Second Law efficiency of the storage-recovery cycle. For this purpose, optimization of the IV system parameters has been carried out on the basis of Second Law efficiency of the cycle. Design plots have been prepared that can yield the optimum value of system parameters namely, sphericity and void fraction of bed for given values of operating parameters i.e. temperature rise parameter 'AT/I' and average insolation (I).