Pumps as Turbines (PaTs): Extracting Energy from Water
As the global demand for energy grows, the focus on reducing energy consumption is becoming high priority for governments, investors, and plants operators. Although small hydropower plants could play a significant role in generating energy, the high initial capital cost has slowed down the development of this energy potential.
This has raised a question; is there any other solution?
By Muhammad Abou Daoud, Regional Sales Manager, several well-know pump manufacturers
The simple answer to this question is yes. Pumps as Turbines (PaTs) can be a lower capital cost alternative.
The notion of using PaTs is not a new concept. The process was investigated following the First World War and has since been well recognized by many pump manufacturers. Due to increasing energy cost and energy conservation awareness, interest in this application has recently grown.
By running a centrifugal pump in reverse mode, to operate as a turbine, the impeller takes the hydraulic energy out of water and converts it into mechanical energy. The induction motor will act as generator and convert the mechanical energy into clean electricity, which helps to generate renewable energy, and recover energy. It also reduces CO2 emissions, and improves the overall plant efficiency.
The generated power can be consumed either at the plant itself, to assist other rotating equipment which results in reduced energy cost, or it can be fed back to the electrical grid network by using the feed-in tariff which helps generate revenue.
Why Use Pumps as Turbines?
There are a number of advantages to using a pump as a turbine:
• Considerable equipment cost savings: the capital cost of PaT may be less than 50% of comparable turbines. There are significant savings due to the fact that manufacturing turbines is more expensive than standard pumps of comparable size (especially for units below 50 kW), and there are significantly less turbines manufacturers. The turbines market is small compared to pumps market.
• Availability of pumps with spare parts is far better than turbines.
• They have low maintenance and operation costs.
• Short payback period: 2-3 years if the PaT is running efficiently (especially below 50 kW).
It is important to note that the cost advantage of PaTs will be reduced for output above 500 kW, as pumps will no longer be standard. By always selecting the right PaT system, however, this disadvantage can be reduced.
Moreover, as with all technologies there are challenges that should should be considered, such as, the PaT has no possibility to react on varying flow rates and pressures, which can cause a relatively quick drop of efficiency. This situation can be overcome by using a frequency inverter that allows a slight adjustment of the characteristic curves.
Type of Pump to be Used as Turbine
The different centrifugal pump designs available provide suitable PaTs for almost any application with head starts from 10m, where end suction pumps can be used. For larger flows axial split case pumps can be used, while multi-stage pumps can be used for higher heads (up to 300m).
Fields of Application
There are three primary fields that can benefit from PaTs.
Water Sector: PaTs are especially applicable for the water sector in areas where there is a big difference between the elevation of the source and treatment plant. Typically water should be slowed by using pressure reducing valve (PRV) to ensure that it flows into the first phase of treatment process with no pressure. By installing PaT instead of PRVs a significant amount of energy can be recovered without affecting the system operation.
Industry Plants: PaTs are also beneficial in processes where fluid should be kept at high pressure but must let down afterwards, such as: reverse osmosis in desalination plants, petrochemicals, or as energy recovery turbines in pulp & paper industry.
Mini-Hydro Plants: Up to 2 MW is possible with one PaT, but the most common PaT installation is for projects up to a max 500 kW per unit. A combination of more PaTs in parallel, will however open a wide range of opportunities, making the PaT a suitable option for small hydro plants.
Project developer need to understand PAT technology before starting any industry related journey. Detailed analyses should be conducted on any application prior to use, as it is important to be familiar with the requirements and limitations of the components, to avoid unnecessary delays and failures.
About the Author:
Mr. Abou Daoud has BSc in Mechanical Engineering. He has over 10 years of experience in project sales, management, and business development. He has also contributed to the development of water management, industrial, and building services sectors in the Middle East. With his extensive knowledge in rotodynamic centrifugal pumps, both standard and engineered, he has participated in the execution of several strategic projects in different countries.