Performance of Adsorptive Heat Storage Devices for Heat Supply

Abstract

The chapter is focused on modelling of performance of adsorptive heat storage devices and estimation of performance of heat storage devices. Two groups of models of adsorptive heat storage units suggested previous researchers are analyzed. The first one is focused on predicting the heat energy storage density, it being based on Dubinin-Polanyi theory. The second one is devoted to analyzing the kinetic of adsorption processes and performance of the adsorber or adsorptive-desorptive reactor filled with traditional adsorbent or salt which forms crystalline hydrates. The key drawback of both groups of models concerns with considering only one stage of exploitation of adsorptive heat storage device in spite of its operating in two-stage mode, that is, alternating discharge (adsorption) and charge (regeneration). It inhibits estimation of efficiency of adsorptive heat storage device basing on full operating cycle and its involving in heat supply system. Two algorithms for estimation of operating parameters are proposed by authors for closed-type and open-type heat storage devices. The algorithm for calculation of operating parameters of closed type adsorptive heat storage device is proposed: calculation of the mass transfer coefficient, adsorption, useful heat, that is, heat of adsorption, determination of the heat input, it being calculated as heat inputs for heating the adsorbent, device housing, water in the tank, evaporation of water in the tank, heating of the adsorbed water and desorption. Then efficiency factor is calculated. The operating characteristics of a closed-type heat energy storage device were studied when the composite adsorbent ‘silica gel – sodium sulphate' used. The effect of the humid airflow velocity on the efficiency factor is taken into account by introducing a coefficient equal to the value of the adsorption. An increase in the efficiency coefficient was stated when the velocity and relative humidity of the airflow. It is shown that the humid air flow temperature practically does not affect its value. Having been used the suggested algorithm, the optimal operating characteristics of an adsorptive heat storage device of a closed type based on a composite adsorbent ‘silica gel – sodium sulphate' for a private house heating system are revealed to be humid air velocities of 0.6 – 0.8 m/s and relative humidity 40 – 60%. When these operational data applied, the efficiency coefficient is shown to reach the maximum values (about 55%). Algorithm of calculation of operating parameter of open-type heat storage device includes computation of mass transfer coefficient, adsorption, useful heat (heat of adsorption), heat input for heating the adsorbent, device casing, water in the humidifier, evaporation of water, heating the adsorbed water, desorption, and calculating efficiency coefficient. Performance of open-type heat storage device based on the composite adsorbent ‘silica gel – sodium sulphate' is estimated. The optimal operating conditions of the heat accumulating device which allow operating with maximal magnitudes of efficiency coefficients 53 – 57% are stated to be humid airflow speed of 0.6 – 0.8 m/s and relative humidity of 40 – 60%. Correlation between efficiency factors obtained by experiments and calculated with suggested algorithm is confirmed. The possibility of reducing the power consumption when heat storage devices applied in 2,4 – 90 times versus decentralized heating systems on basis of solid fuel boiler, gas boiler and electric boiler is stated when open-type sorptive heat storage device used. Results of the study can be used to develop adsorptive storage devices in decentralized heat supply and ventilation systems and adsorption units for utilization of low-temperature waste heat.