Evaluation of heat pump dryers from the perspective of energy efficiency and operational robustness

Vapor compression heat pump dryers (HPDs) are widely applied in food and industrial drying. Many efforts have been devoted to evaluating the HPD performance from the perspectives of energy, exergy, economy, etc. The current work offers a perspective of both energy efficiency and operational robustness to evaluate HPDs, intending to measure their design and off-design performance for various drying applications. Based on the proposed method, a case study on conveyor seaweed drying is presented. The preferable system(s) are found out among three candidates in general use, i.e., the basic heat pump dryer HPDbasic, the HPD with air bypass HPDbypass, and the HPD with air bypass and subcooler HPDsubcooler. The results show that, HPDbasic is not applicable for seaweed drying due to the required huge circulating air flow rate. HPDbypass and HPDsubcooler are more preferred with better energy performance and operational robustness. Moisture extraction rate (MER) of 30.4 kg/h and specific moisture extraction rate (SMER) of 2.20 kg/kWh are reached in HPDbypass at the design condition (Tdb = 45 C, RH = 0.4), and are further increased to 35.8 kg/h and 2.58 kg/kWh in HPDsubcooler. Meanwhile the off-design robustness indices are 0.14 and 0.22 for HPDbypass and HPDsubcooler, and can be enlarged to 0.19 and 0.33 after reselecting a compressor withstanding condensing temperature up to 58 ? (2 C higher). Besides, HPDbypass and HPDsubcooler can bear at least 30% air leakage in operation. Finally, operational robustness on other applications is also discussed for conveyor and batch dryers.