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3 April 2018

Creating a Sustainable Future with Seawater

Over the last decade, the use of positive displacement pumps in SWRO has taken off. With proven efficiency rates up to 92% and energy savings of 20%, the benefits of the simple and flexible solution are tangible and deserve exploration.

Positive displacement pumps

Positive displacement pumps outperform centrifugal pumps in SWRO, returning energy savings of 20% or more and with proven efficiency rates up to 92%.

 

Introduction

The world’s freshwater resources are facing an uncertain future. Rapid urbanisation and population growth are creating an unprecedented level of demand for freshwater, at a time where arid areas are spreading across the globe. The call for solutions to the world’s water demands is critical and the UN has now made universal access to freshwater one of the 17 Sustainable Development Goals. Desalination plants, which convert seawater to freshwater are one of the most effective ways to secure continued water supplies. However, to achieve the best possible reliability and energy efficiency, one solution is emerging as the most sustainable: Seawater Reverse Osmosis (SWRO) with high pressure positive displacement pumps.

 

Facts about water

  • 70% of the Earth is covered with water
  • 97% of this water is seawater
  • Only 1% of the Earth’s water is drinkable
  • Worldwide consumption of drinking water doubles every 20 years
 

The call for solutions to water scarcity

According to the UN, more than one billion people suffer from lack of freshwater today, and over 300 million people in 150 countries rely wholly or partly on seawater desalination for their needs. Desalinated water is not only vital for drinking water in these countries, but also for irrigation, hygiene and sanitation as well as numerous industrial processes – onshore and offshore.

Desalinated water resources are becoming more and more commonplace. As some regions become increasingly threatened by water scarcity due to climate change or resource depletion, cities and industries in greater numbers are turning to desalination plants as a solution. Global Water Intelligence (GWI) forecasts that by 2022, global newly contracted desalination capacity will grow by 7.3%, fuelling the demand for sustainable and cost-effective technologies.

Tackling the energy costs of reverse osmosis

Among several desalination techniques, Seawater Reverse Osmosis (SWRO) has emerged as the preferred method. Compared to other processes, SWRO has proved to be a highly efficient way of producing clean, uncontaminated freshwater from seawater. The technology for SWRO is also simple and safe; high-pressure pumps force seawater through desalination membranes to create multi-purpose freshwater, removing over 99 percent of dissolved salts and impurities in the process.

However, the process of pumping large amounts of seawater through desalination membranes requires large amounts of energy. To save on the energy costs and CO2 emissions produced by the SWRO process, substantial investments have been directed towards making the technology smarter and more energy efficient, resulting in several recent innovations.

The new solution: positive displacement pumps vs centrifugal pumps

For decades, SWRO has typically relied on the use of centrifugal pumps to desalinate seawater, with this technology often considered the best choice of high-pressure pump for larger SWRO plants. In recent years, however, the increasing focus on energy savings has led to the invention of new and highly efficient pumping solutions, most notably the positive displacement pump.

A positive displacement pump delivers a nearly constant flow regardless of the pressure, unlike centrifugal pumps. If the system pressure goes up, the positive displacement pump delivers constant flow at the requested pressure without any need for regulation. When the flow increases, only the power consumption of the motor goes up, and vice versa when the flow is reduced. In several installations, the positive displacement pump has been seen to outperform the centrifugal pump on most operational parameters in the desalination process.

See how the positive displacement pump works

The key to energy efficiency: positive displacement pumps and variable frequency drives

There is a widespread myth that using a Variable Frequency Drive (VFD) in the desalination process is expensive and complicated to control. Therefore, throttling remains the preferred control method in many SWRO applications. However, throttling wastes a lot of energy in the daily operation; typically, 8-10 percent of the total energy consumption of the pump. By comparison, the energy loss in VFD is 75 percent lower than the energy loss of the throttle valve.

In a system with several parallel-couple positive displacement pumps, regulating the flow on one pump does not affect the flow on the other pumps. This means that flow regulation is required on only one of the pumps, and the VFD can be sized for one pump only, while the rest of the pumps run directly online.

This simple configuration with parallel-coupled positive displacement pumps saves money on the VFD and returns significant energy savings during operation.

Over the last ten years, the number of SWRO plants using positive displacement pumps has grown significantly. These plants have typically returned energy savings of 20 percent or more compared to centrifugal pumps and with proven efficiency rates of up to 92 percent.

 

Annual power consumption when producing 3,500 m3/day

 

Trending now: parallel-coupled positive displacement pumps

The trend towards modularity for mid-size SWRO plants, in order to cater to different demand scenarios, is fuelling the uptake of parallel-coupled positive displacement pumps for onshore and offshore desalination plants.

This is the case for a 3,500 m3/day SWRO plant at a shipbuilding and maintenance yard in Bahrain, where six Danfoss APP 30 pumps are mounted in parallel to achieve the required flow capacity. The multiple-pump train ensures that the capacity can be increased incrementally according to the need for water at any given time. During operation, the production of water can be adjusted simply by changing the speed of the pump to harvest additional energy savings.

The parallel-coupled pump solution has allowed the SWRO plant in Bahrain to operate 24/7 all year round. The robust and simple pump design, with few moving parts, also allows for easy service and maintenance. If one of the pumps is out for maintenance, the remaining pumps take over the full operation and the pumps can run for over 12 months between inspection and service.

Adding up the advantages, the conclusion at the Bahraini shipyard is clear. Despite the upfront cost of six APP pumps compared to one large centrifugal pump, the low cost of ownership returns has improved profit margins over the service life of the SWRO plant.

 

The benefits of using several parallel-coupled positive displacement pumps:

  • High uptime and uninterrupted operation when one pump is out for maintenance
  • One small, redundant pump is cheaper than one, large redundant pump
  • Simple and energy efficient flow control
 

The way to sustainable SWRO: energy recovery devices and large pumps

Previously, the flow capacity of a single positive displacement pump has been a concern for some SWRO applications. But recent technology developments in the field have produced larger pumps for SWRO operations. The larger pumps are particularly suited for parallel-coupled trains with a total capacity of 30,000 m3/day or even more. One model of large pump, the Danfoss APP 53-86, has turned out to be a first-rate choice for high output plants, with the company’s high-pressure pumps currently covering flow rates from 0.6 to 86 m3/hour.

Danfoss has also worked to increase the energy savings produced by SWRO with positive displacement pumps, through its iSave Energy Recovery Device (ERD) product.

The invention of ERDs has helped to further cut down on the energy requirements of SWRO and reduce the C02 emissions generated by the desalination process. The ERD captures hydraulic energy from the high pressure reject stream of the SWRO process and transfers the energy back into the system, reducing the energy consumption regardless of flow rate and size of the plant. The energy recovery process can reduce the energy consumption of the SWRO process by as much as 60 percent.

Danfoss’s iSave ERD can be used in any SWRO application, land-based or marine, new or retrofit. The smart energy recovery device comprises pressure exchanger, booster pump and motor as one compact unit for high efficiency and short payback time. The 3-in-1 design of the recovery unit also allows for fast commissioning, continuous monitoring of performance and easy regulation to accommodate seasonal changes and demand patterns.

The Danfoss APP pump has been proven to produce more fresh water per cubic metre than any other system in the market, and even more when fitted with the iSave, leading the way in sustainable seawater desalination for the world.

 

Danfoss iSaves are powerful ERDs for SWRO plants with a water capacity of 110 to 1,650 m3 per day. With integrated pressure exchanger, booster pump and motor in one compact unit, the iSave saves up to 60 percent on the energy bill.

 

Danfoss – your partner in pump solutions for SWRO

Danfoss is committed to supporting the desalination industry with energy efficient solutions that can help bring clean water supplies to regions suffering from water scarcity. The company’s constant research in new technologies and development of new positive displacement pumps will contribute to fulfilling the global development goal of safe and affordable drinking water for all by 2030.

Our range of high-pressure pumps optimized for SWRO applications provide the following benefits:
  • The most compact high-pressure pump on the market with more power per kilo than any other pump
  • Energy savings of more than 20% compared to other pump types
  • Efficiency rates of up to 92%
  • Oil-free to ensure contamination-free operation
  • Compact footprint and low weight
  • Few parts, long service intervals, easy maintenance
  • Easy to mount and control multiple pumps in parallel
  • Proven technology – references available worldwide

 

Danfoss High Pressure Pumps is part of the Danfoss Group, which is a leading global player within the development and production of mechanical and electronic products and controls. As a privately held company Danfoss has grown from a solo enterprise into a world-leader. Since 1933, the extensive know-how within Danfoss has made modern life easier and Danfoss continues to break new ground in the core business areas. Danfoss engineers the technologies that enable the world of tomorrow to do more with less. We meet the growing need for infrastructure, food supply, energy-efficiency and climate-friendly solutions. Every day, more than 250,000 items are produced at 70 factories in 25 countries by more than 24,000 employees.