Performance assessment of a zeolite heat store.

Abstract

Abstract: According to the Department of Mineral Resources and Energy, the industrial sector in South Africa is the largest consumer of energy. Waste heat recovery can significantly reduce costs and carbon emissions in industry. In addition, energy supply does not always match demand and it is therefore necessary to investigate efficient ways of storing thermal energy. Sorption based thermochemical heat storage provides high energy densities while minimizing losses when utilised for long-term heat storage. To commercialize sorption storage systems, additional research and experimentation is required to validate numerical models of heat and mass transfer within the system to allow for accurate design calculations. A lab-scale prototype was developed to analyse the thermal storage characteristics of zeolite 13X in an open (non-pressurized) sorption system. The test unit consists of a packed (pellet) bed reactor, heating system, blower, and humidifier. Thermocouples, humidity sensors, and mass-flow meters were used to determine the mass and energy balances in the system. The experiments that were conducted involved a reversible reaction between zeolite 13X and water vapour in air. During the charging (desorption) process, the zeolite pellets were dehydrated by hot air which was heated using an electric heater. During the discharging (adsorption) process, humidified ambient air was supplied to the reactor bed, which rehydrated the zeolite pellets, resulting in hot dry air exiting the reactor bed. The packed bed was charged at three different temperatures (130 oC, 160 oC and 200 oC). It was discharged at three different values of relative humidity (25%, 70% and 100%,) and mass-flow rates (90 kg/h, 126 kg/h and 177 kg/h). The maximum amount of energy absorbed by the bed was 13.64 kWh (at 200 oC) and the maximum amount of energy released was 11.56 kWh at 100% relative humidity during discharging. This equates to a storage efficiency of 85% and an overall efficiency of 57% for the process. The highest temperature lift achieved was 107 oC during adsorption and the maximum energy storage density was 148 kWh/m3. By decreasing the regeneration temperature, the energy storage capacity was decreased and the desorption time was increased. A lower inlet humidity (during discharging) lowered the temperature lift and energy efficiency and increased the adsorption time. A lower flow rate also led to a lower efficiency and increased the adsorption time. The potential of zeolite 13X for long term storage was confirmed as a relatively high energy storage efficiency of 72% was achieved, for a period of 5 months between charging and discharging, in an open reactor. The application of zeolite for drying processes in industry was demonstrated using ceramic casting moulds. An average drying rate (water removal) of 0.67 kg/h was achieved. The moisture adsorption capacity of zeolite 13X was also investigated and a maximum of 286 gwater/kgzeolite was adsorbed at a relative humidity of 85%. The performance of the system was analysed by comparing the experimental results to calculations and models from previous studies. The next step is to develop and test a pilot-scale rig in a small-scale ceramic factory.