Cost reduction of heat pump assisted membrane distillation by using variable electricity prices

Abstract

Heat pump assisted membrane distillation (MD) has recently been proposed as a new method for simultaneous heating and cooling of membrane distillation. In this work, the cost of water (COW) of such a system is analysed by simulations. The COW can be reduced by using variability in electricity prices and an optimal control method e.g., electricity consumption can be reduced at high price moments and vice versa. Here, practical nonlinear model predictive control (PNMPC) was chosen as an optimal control method, and a validated model was used for the simulations of the MD system. The results indicate that using the variability in electricity prices reduced the COW of the heat pump driven system by 0.05 USD/m3, to a value of 4.93 USD/m3. In addition, simulations with resistive heating were done as it has a lower capital cost. Results show that the COW is higher with resistive heating due to the higher electricity consumption. The COW of resistive heating was found to be 10.49 USD/m3. The use of local cooling instead of evaporative cooling can reduce the COW by 0.05 USD/m3. Overall, the use of heat pumps is a promising method.