Subcooling control method for the adjustable ejector in the direct expansion solar assisted ejector-compression heat pump water heater

A subcooling control method is proposed in this paper, aiming to regulate the adjustable ejector in the presented direct expansion solar assisted ejector-compression heat pump water heater. The proposed subcooling control method adopts the PID (Proportion Integration Differentiation) algorithm and aims to help the presented system track the given subcooling degree through automatically adjusting the nozzle throat area of the adjustable ejector. The proposed subcooling control method is firstly examined by dynamic simulation method. The simulation results show that the proposed subcooling control method can meet the purpose of controlling sub cooling degree and reduce the power consumption by 7.95% under the considered condition. The experimental setup with and without the subcooling controller are tested for comparison, and the test results confirm the simulation conclusions. The experimental system with the subcooling controller maintains the subccoling degree in a range of 4-7 degrees C, and reduces the power consumption by 6.24% and 6.99% compared with those with two different ejector throat areas. Further analysis shows that the subcooling controller brings the highest ejector entrainment ratio but not evident improvement in the pressure lift ratio aspect. In brief, the proposed subcooling control method achieves satisfactory subcooling degree control precision and further improves the system performance.