Comparison of energy parameters in various dryers

This study aimed at investigating the energy output, thermal output, drying efficiency and specific energy in various drying methods for drying of chamomile. These methods included convective, infrared, convective–infrared, microwave, microwave–convective, microwave–vacuum, vacuum, and hybrid photovoltaic–thermal solar (with/without heat pump). Results of data analysis showed that the highest energy output of 49.99% belonged to the microwave dryer, while the lowest 1.41% belonged to the vacuum dryer. Moreover, the maximum and minimum thermal outputs being 78.22% and 2.68% were associated with the vacuum–microwave and vacuum dryers, respectively. In the hot-air-related dryers, drying efficiency increased with temperature. In microwave-related dryers, however, drying efficiency first increased (up to a microwave power of 300 W) and then decreased with further increase in microwave power. Assessment of the specific energy requirement in various dryers showed that the highest and lowest values belonged to the vacuum and microwave methods with 318.42 and 4.32 MJ/kgw, respectively. Additionally, results of analysis indicated that adding a heat pump to the photovoltaic solar dryer increases drying efficiency, energy output and thermal output, while it reduces the required specific energy.