Thermal performance evaluation during charging/discharging cycles of sensible energy storage beds for biogas production process

Abstract

The breakdown of organic waste, through anaerobic digestion, for production of biogas is a temperature dependent process. The favorable temperature required for efficient production is 35 °C. Attaining the temperature required for this process, under normal conditions, is a challenging task. Hence, the development of an effective heating system is necessary. The warm water from the solar water heater is circulated through the wall of the digester wall using a heat exchanger. The intermittency in the supply of energy, as in the case of solar radiation, creates a mismatch between the demand and the supply of the energy. Hence, development of efficient energy storage system that can be used as stand-by in case of unavailability of the primary energy source.The present work modeled and predicted the thermal performance of sensible heat storage system having 10 MJ capacity which can deliver warm water at 35 °C for 4 hr. The sensible heat storage system unit is a regenerator type heat exchanger which stores/releases heat on passing hot/cold heat transfer fluid through the tubes embedded into it. A mathematical model of a cylindrical configuration with embedded multi-tube is developed employing pebbles and cast iron as storage mediums. The number of embedded charging/discharging tubes in the storage model is optimized based on the charging time using a finite element method based simulation tool (®COMSOL Multiphysics). A parametric study, including charging/discharging time, energy stored/recovered, exergy stored/recovered, and charging/discharging fraction of the system, have been used to evaluate the thermal performance. Further the effect of heat transfer fluid tube diameter and velocity, volumetric heat capacity and thermal conductivity of the storage media on performance of the storage is also investigated.