Energy Recovery and Seawater Desalination

Conventional Design

Previously, the standard Hydropro design for SWRO with energy recovery incorporated a single multistage centrifugal pump (or positive displacement) with a Hydraulic Turbo Booster. This design is fairly simple and generally does not require a significant increase in system controls or instrumentation and is for the most part a sound, and energy efficient SWRO design.

The hydraulic turbo booster converts the hydraulic energy of the concentrate stream to mechanical energy and then applies this mechanical energy to the full flow of the feed stream in the form of a considerable pressure boost. In a single stage SWRO system, the energy benefit associated with this type of energy recovery device is realized solely in the form of lower pressure (and thus lower horsepower) requirements for the high pressure feed pump. Because the equations used to predict the pressure boost produced by a HTB are usually specific to the manufacturer and dependent upon the system parameters, they will not be explicitly discussed here. In this case, a reasonable assumption would be a 300 psi (693 feet H2O) pressure boost from the HTB operating in a system as described in Example 1 below. The following example is used to demonstrate the reduction in high pressure feed pump horsepower requirements:

This HTB energy recovery device provides a substantial reduction in specific energy consumption, which, depending on the duty cycle and cost of power could pay for itself in a relatively short amount of time.

New Technology

The concept of a work exchanger energy recovery device was certainly not new, and several variations of these devices have come and gone. However, at the time of this proposal, there seemed to be a new approach to the design of these positive displacement devices that eliminated many of the problems associated with previous versions. The PE from Energy Recovery, Inc. (ERI) is an example of a novel work exchanger device that was in a position to profoundly affect the design of SWRO and the energy recovery industry.

The main idea of the Pressure Exchanger is its ability to directly transfer most of the hydraulic energy in the concentrate stream to an equal amount of feed water. The result is a side feed stream equal in flow to the concentrate stream (minus bearing leakage) that is boosted to near membrane feed pressure by the Pressure Exchanger. A small high pressure booster pump is then required to boost the high pressure feed exiting the PE so that it equals the discharge pressure of the high pressure feed pump and the two feed streams can be combined. This pressure boost accounts for pressure losses associated with inefficiencies of the pressure exchanger, losses across the membranes, and piping and fitting losses throughout the system. By significantly reducing the size of the high pressure feed pump to approximate the flow of permeate, the horsepower of the high pressure pump can be reduced by approximately two thirds of the total pumping power required. This substantial reduction in horsepower is, for the most part, specific to the high pressure, low recovery nature of the SWRO system. To illustrate the effect of this reduction in pumping power required, the following example is used:

Although there are other energy considerations besides just pumping power when comparing a system with no energy recovery and a system with a PE, this simple analysis shows a significant reduction in energy consumption when using a Pressure Exchanger.