Seawater reverse osmosis solution achieves optimum energy efficiency

Low-energy seawater reverse osmosis (SWRO) plants are being integrated into hybrid plants to optimize the energy balance of power-water production facilities. Laurent Guey reports on Group Suez companies' work on developing new approaches to decrease energy consumption and make SWRO desalination plants even more competitive.

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Low-energy seawater reverse osmosis (SWRO) plants are being integrated into hybrid plants to optimize the energy balance of power-water production facilities. Laurent Guey reports on Group Suez companies’ work on developing new approaches to decrease energy consumption and make SWRO desalination plants even more competitive.

Laurent Guey

The Group Suez companies – Suez Energy International, Tractebel Engineering, and Degrémont – are working together to reduce energy consumption of seawater desalination plants since energy accounts for the largest expenditures associated with freshwater production through desalination. Specifically, they are promoting a hybrid design that uses a seawater reverse osmosis and distillation unit to create an association that allows flexibility, which is needed to improve energy efficiency.

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Reverse osmosis room at Fujairah, UAE

Desalination is now considered one of the main solutions for water scarcity. Most Middle Eastern nations face serious shortages of freshwater, a strategic resource essential for national development. Contrary to the lack of freshwater resources in the region, an unlimited source of ocean water can be desalinated for use in any water application, including drinking, agriculture, and industry.

Thermal evaporation and membrane separation are two well-proven processes that have been widely used to remove (separate) salts from seawater. Thermal evaporation, distillation of seawater, is still the desalination technology most preferred in the Middle East region due to low-cost fuel and high-energy demand per habitant. Thermal evaporation technologies include multi-stage flash (MSF) and multi-effect distillation (MED). Most desalination plants are coupled with power plants that generate steam used to evaporate water in the distillation process. Membrane separation includes reverse osmosis (RO).

Energy recovery

Thermal-powered water production plants can be designed with advanced energy recovery technology to increase the system’s overall power (fuel) efficiency. In the Middle Eastern region, the significant parameter used to qualify the efficiency between the capacity of power and water production in dual-purpose power and desalination plants is the Power to Water Ratio. It is calculated as follows:

P/W = Net output P at plant boundary1

Net output W at plant boundary2

Heat Rate = Gross energy supplied by fuel or gas Net output P at plant boundary1

The example refers to a power plant associated with distillation water production (MSF or MED); it does not consider RO.

Since the primary energy used to produce distillated water in low pressure steam is extracted in the steam path of a steam turbine, it is logical to assume when less steam is extracted, less fuel or gas will be used to produce electricity. In other words, the better the Heat Rate3 of the power plant , the higher the P/W, and the lower the Heat Rate. The best Heat Rate is achieved when there is zero water production, which is not the purpose.

Practically, the P/W Ratio is selected to optimize investment and fuel consumption. The optimum should vary depending on the cost of fuel and investment for each case. In the Gulf region, both parameters are stable and some have attempted to define a rough optimum for each type of thermal power plant configuration.

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Reducing the system’s overall power (fuel) efficiency of thermal-powered seawater desalination plant can be achieved by implementing Heat Recovery Steam Gas (HRSGs) technology or HRSGs combined with steam turbines (Combined Cycle). Such technologies improves P/W ratio. Most powered-power plants in Abu Dhabi were designed with back pressure turbines to achieve the typical P/W in the tabulation of 18 MW/MiGD.

The power to water relationship for Taweelah A1, designed by Tractebel Engineering in the United Arab Emirates (UAE), is explained in Table 1. Adding power only (A110P) increases the P/W to 18.8 and improves the heat rate by about seven percent.

In the Fujairah, UAE plant, designed and built by the Korean company Doosan, the improvement was more significant since it represents about 12 percent because the initial P/W was as low as 9.1, which implied the intensive use of duct firing to produce steam. The addition of two gas turbines with HRSGs increased the P/W to 12.7 (Table 2).

One should avoid generalizing. In the A1 Taweelah extension scheme with power only, similar to Fujairah, additional steam is produced in HRSGs, but no steam turbine is added. Hence there is a point where adding more power and steam will not further improve the Heat Rate. Because of the reference dispatch defined in percentages of the maximum outputs and not in absolute values, enough steam will be produced by the HRSGs to feed the desalination units. Excess steam cannot be passed through a steam turbine.

SWRO solution

At this point, a new approach can further increase the energy efficiency of the plant – a hybrid plant that couples a seawater reverse osmosis unit with a distillation unit. This association facilitates flexibility, which improves energy efficiency.

To explain further, the power demand (or electricity demand) should always cover or exceed the water demand, expressed as steam energy extracted from the power plant to supply the thermal evaporation technology, to keep the heat rate at its optimum value. Water is produced through the distillation unit when power demand exceeds water demand. The RO plant is activated only when power demand remains below water demand to prevent the power plant from functioning only to produce water. This hybrid design saves energy.

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The combination of both desalination processes, thermal evaporation (MSF or MED) and membrane separation (reverse osmosis), provides high flexibility and allows operators to choose between power productivity and cost. This hybrid plant can run at its optimum energy use. Fujairah was the first hybrid plant to combine MSF and RO membrane in order to lower the heat rate. Degremont designed and erected the 37.5-MiGD RO plant of Fujairah to produce water at low cost whatever the power demand is.

Such a solution could not be justified 20 years ago when energy consumption of SWRO desalination plant reached eight kWh/m3, but SWRO technology has improved over the past 20 years. From 1980 to 2000, the energy consumption of SWRO plants decreased by 100 percent, from eight kWh/m3 in 1980 to approximately four kWh/m3 in 2000. RO membrane suppliers have invested great effort into developing low-energy seawater elements with higher salt rejection rates, and they continue to work hard. Remarkably, a new energy recovery approach entered the SWRO market and has made a significant impact on the energy consumption and design of membrane desalination systems.

Two different energy recovery approaches are available in the market:

  • Technologies that convert the hydraulic energy found in brine into rotational energy. This rotational mechanical energy must be transferred into another pumping device that pressurizes feedwater (Pelton Wheel system, Turbocharger pumps).
  • Technologies that use the principle of positive displacement to transfer the energy contained in brine directly into feed water (Inc’s Pressure Exchanger PX, DWEER system, Kinetic’s system).

Degrémont has used the Pelton Wheel system in large desalination plants constructed in the last 10 years: Curaçao (Caribbean), Carboneras (Spain), Fujairah (UAE), Minera Escondida (Chile), Bahia de Palma (Spain) and Cartagena (Spain).

The PX Pressure Exchanger, developed by Energy Recovery Inc (ERI), uses a rotating ceramic multi-vessel concept. The incoming raw seawater is pressurized by direct contact with the reject brine of seawater membrane. The system’s overall efficiency reaches as high as 95 percent. ERI’s technology typically yields 15 percent to 17 percent higher energy recovery saving than the Pelton Wheel systems. Such a saving could achieve 0.5 to 0.65 kW/m3 of treated water.

Degrémont will install this technology in the 140,000-m3/day desalination plant of the City of Perth (Australia). The plant is designed with 12 ERI work exchanger units, with a capacity of 16,200 m3/day each. One work exchanger unit is made from 15 single elements of 50 m3/day each. The energy consumption of the first pass RO train with its dedicated ERI work exchanger is expected to be lower than 2.30 kWh/m3. This is the first large plant designed with this ERI technology.

Graph Power Water

At the nominal capacity and with 42 percent conversion, the plant will have a remarkably low total energy consumption value of four kWh/m3 (excluding treated water pumps and including raw water pumps).

In terms of energy, the SWRO plant could be cost-effectively operated at variable flow because of the high efficiency of pressure exchanger without regard to the flow variation of the input (reject brine). For three years, the Energy Recovery Work Exchanger has been available to the large SWRO market; the first plants are operating. Technical operators and engineers need to focus on the engineering of these technologies in order to optimize operations at variable flow in large SWRO plants.

Given their low energy consumption, seawater reverse osmosis plants are being integrated in hybrid plants to optimize the energy balance of power-water production complex. Degrémont is working with its partners (membranes, energy recovery systems, etc.) to decrease energy consumption in order to make its seawater reverse osmosis desalination plants even more competitive.

Author’s Note

Laurent Guey is the membrane marketing manager for Degrémont, Suez Group, based in Rueil-Malmaison, France. For more information, visit the website: www.degremont.com.

Footnotes:

1 Design attached.capacity (energy/electricity delivered by Power Plant) after deduction of the auxiliaries for the desalination plant until the storage tank. The auxiliary necessary for pumping the potable water into the network are included in the net P output.

2 Water produced (or delivered) by thermal process (MSF or MED) fed by thermal energy of power plant at plant boundary

3 Heat Rate : fuel/gas consumption to produce 1 kWh electricity output at plant boundary

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