Heat pump evaporation: An alternative of membrane filtration system aiming pure water production

Abstract

The daily electrical power generation and consumption in Hungary were analyzed, and a 2-h-long interval was specified for the 5000 MW excess power generation, and 1,500,000 MJ energy was identified as an excess. Two pure-water-producing systems (membrane-based system and heat pump evaporator) were compared to utilize this excess electrical energy at industrial scale. Heat pump evaporation was tested as an alternative method for pure water production because its role in pure water production hasn't been discussed. Suspended-solid-free feed water was used for both systems. Lab-scale experiments proved that in the case of freshwater, the four-step membrane filtration technology can be replaced by a one-step heat pump evaporation, resulting in almost the same quality of pure water characterized by the conductivity; 2 and 4 μS/cm for membrane filtration and heat-pump evaporation, respectively. When using seawater at a lab-scale, the proposed membrane technology resulted in desalted water having a conductivity of 425 μS/cm; meanwhile, by heat pump evaporation, pure water could be attained, having a conductivity of 19 μS/cm. Considering the specific electric energy consumption at laboratory scale, the one-step heat pump evaporator surely exhibits an energy-saving method for pure water production; 6.12 MJ/L and 2.31 MJ/L were measured for membrane system and heat pump evaporator, respectively. Considering an industrial-scale make-up water demand, up-scaling calculations, including hydraulic calculations and pump selection producing the same amount of pure water, resulted in a reverse trend regarding specific electric energy consumption; 25.35 MJ/m³ and 14,850 MJ/m³ estimated for membrane system and heat pump evaporator, respectively. Both systems were analyzed in terms of environmental impacts. According to wastewater quality and their disposability, the wastewater of freshwater treatment can be drained into the sewage system, but in case of seawater, crystallization of concentrated NaCl solutions is possible. Based on our analysis, the theoretically producible water quantity is more than physically attainable by the treatment technologies, thus more options shall be considered for the utilization of excess energy generation.