Performance improvement of a pump running as turbine for energy recovery considering the effects of polymer additives: An experimental study

Employing pumps as turbines (PAT) has proven to be a cost-effective and efficient method for producing clean electricity in various engineering applications. PATs are now being considered for use in pumped hydro storage plants (PHS) instead of using a series of separate pumps and turbines and also in oil, petrochemical, and refinery industries as pressure energy recovery. Therefore, it is highly desirable to improve the hydraulic and mechanical characteristics of PATs in such applications. In this study, the effects of polymer additives as drag reducing agents on the performance of a centrifugal pump running as turbine mode is experimentally investigated. Reducing the friction drag within a turbulent flow using additives such as long-chain flexible polymers has attracted considerable attention from an energy-saving perspective. For this purpose, a test rig including the feed pump and the main pump has been set up where aqueous solutions of polyacrylamide with concentrations of 150, 300, and 450 ppm are used as working fluid. In order to compare the turbine mode with the pump mode, the pump performance in direct operation is also evaluated. The findings reveal that compared to tap water, using polymer solutions increases the efficiency and output power of the PAT, which even further improves with increasing polymer concentration. In the turbine mode, the highest increase in efficiency occurred at 450 ppm, which represented an increase of 7.61%. On the other hand, in the pump mode, the maximum improvement in pump efficiency (8.30%) has been recorded when the 150 ppm solution is used instead of tap water. By increasing the flow rate of polymeric solutions, the percentage of increase in the pump's efficiency, compared to tap water, is ascending. Conversely, the percentage of increase in the PAT's efficiency declines as the flow rate of polymeric working fluid passing the turbine is increased. The degradation of the utilized polymer in solutions leads to a decrease in output power and efficiency of the PAT compared to its fresh state. Nevertheless, the PAT efficiency and generated power in the degraded condition of the solution are still higher than what can be achieved in presence of tap water as the working fluid. The results of this study generally show that the use of working fluids with viscoelastic properties, which contain additive polymers, significantly improves the performance of reverse and direct pumps. As such, it can be used as an alternative working fluid in the industries mentioned above.