Draft:Venturi-Enhanced Turbine Technology

Venturi-Enhanced Turbine Technology  

Venturi-Enhanced Turbine Technology (VETT) is a modern hydropower technology specifically designed for low-head locations with up to 5 metres head.  

The technology purpose of VETT is to reduce costs per kW and provide shorter payback times for low-head hydropower projects. VETT allows for cost efficient power production from low-head locations which is not possible with conventional low-head hydropower turbines, such as Kaplan or Francis turbines. These are typically very large, slow rotating and too expensive for low-head projects.  

Venturi Enhanced Turbine Technology uses the venturi effect to achieve a pressure amplification for the turbine so that smaller, faster, no gearbox turbines can be deployed in low head hydro settings. This makes the hydropower installations less expensive.

VETT turbines can be used at low-heads (1.5–5 metres) and medium to high flows (1m³/s–20 m³/s). Multiple turbines can be installed in parallel for larger projects. Applications include run-of-river and tidal settings, as well as other low-pressure sites such as outfalls at existing high head dams, water treatment plants, wastewater outfalls and pressure reduction valves. The technology is in operation in Eaton Socon., Cambridgeshire, UK.

Conventional Hydropower

Typically, in order to produce cost-effective energy, hydropower stations require large flow rates of water across large pressure drops. As power is a function of head and flow, generating significant power from low-head locations requires large volumes of water. When conventional hydropower technologies are used with low-head pressures, large volumes of water are required which produces low rotational speeds. Gearboxes are therefore required to efficiently drive generators. This results in large, heavy, expensive equipment and civil infrastructure.

Although there are many low-head sites worldwide, including numerous river weirs and potential tidal sites, the pursuit of low-head hydropower is often avoided because it is uneconomic. Thus, relatively flat countries such as the UK miss out on hydropower due to the lack of sufficient elevated water sources

Theory of Operation

VETT operating principle. The turbine discharges into a very low-pressure region created by the venturi flow. The pressure differential across the turbine therefore increases as the turbine not only experiences the site head pressure but also the considerably lower venturi pressure downstream of the turbine.

VETT increases the pressure differential across a standard axial flow propeller turbine. To achieve the necessary pressure amplification, 70-80% of the total design flow passes through a constriction in a pipe known as a venturi, where, as per Bernoulli’s theorem, the static pressure drops as the flow velocity increases. The remaining 20-30% of the flow passes through the turbine, discharging into the area of low pressure created within the venturi. The venturi effectively sucks flow through the turbine.

An energy exchange occurs between the two flows as they mix within the venturi, before the total mixed flow passes through a diffuser, which decelerates the flow, recovering the static pressure. The hydropower efficiency depends on how the turbine wake mixes together with the main pipe flow.

While the upstream pressure of the turbine is fixed, the downstream pressure is reduced and hence the pressure across the turbine (head) is amplified by 2.5x. This increases, for example, a 2 metre head into a 5 metre head. As a result, with a VETT system turbines can be up to 83% smaller and up to 15x faster. In addition they do not require a gearbox.  

Turbine Tank  

For larger flow projects a turbine tank is required to guide flow from upstream into VETT with minimum hydraulic loss whilst housing the turbine and fish screens. The design of the tank and the submersion depth of the turbine must be sufficient to prevent free-surface vortices or cavitation in the turbine, both of which can have a negative impact on efficiency.      

A rendered image of the VETT Turbine Tank complete with turbine and fish screens

Vortex formation is a function of diameter whereas cavitation is a function of diameter and speed. A larger diameter turbine therefore needs greater excavations to satisfy the water depth requirements which can be prohibitively expensive for a low head scheme. Since VETT promotes the use of smaller turbines, deep excavations can be avoided.    

The turbine tank is manufactured in concrete and can positioned into the riverbank or directly into the watercourse.  For run-of-river projects, each tank can be designed for up to 15m3/s and tanks can be installed in parallel for projects with very large flow rates. 

Fish and Environmental Safety

VETT systems provide a safe downstream migration route, which was verified by third party test data and the UK’s Environment Agency. 80% of the flow travels through the venturi part of a VETT unit which has no moving parts, allowing fish to migrate through the device and significantly reducing the risk of blade strike. Only a small part, typically 20%, of the total flow passes through the turbine, therefore only the turbine itself needs to be screened. This reduces the size of required fish screens by 5-9x compared to conventional propeller turbines.

Because fish will experience a sudden pressure drop while travelling through a VETT, extensive testing with live fish in an operating VETT was undertaken. Multiple full scale test programmes took place covering a range of physostomic and physoclist fish. Testing included juvenile and mature fish, such as: Atlantic salmon, rainbow trout, round goby, bream, lamprey, perch and European eel. An environmental acceptance criterion for installations was developed with the Environment Agency, and in 2017 the Environment Agency approved VETTs for installation.

Applications

VETT can be installed in a range of low-head settings:

• Run-of-river, including streams and larger rivers  
• Tidal barrages and lagoons  
• Outfalls at existing high head dams
• Water treatment plants
• Wastewater outfalls  
• Pressure reduction valves.

Variations

• Standard VETT, for 2-15m3/s per unit  
• VETT-in-a-Box, for 0.5-2.7m3/s per unit
• Bi-Directional Tidal VETT, up to 2,000 l/s per unit.  
  

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