Wind turbines exploit the kinetic energy of an air flow, i.e. wind velocity, to rotate and generate energy. In recent times, wind turbines have also been developed to work in a water flow, like natural streams, tides, ocean currents and artificial water networks, requiring flow velocities generally higher than 1 m/s. When a water flow is used instead of wind, the turbines are called hydrokinetic turbines (HT). Since water density is almost 800 times higher than that of air, HTs can produce more energy than wind turbines for the same flow velocity and turbine dimensions. Different research projects are currently under development in order to make HTs a standard market product. Furthermore, HT potential in the USA is estimated to be 120 TWh/year in rivers, 210 TWh/year from waves and 250 TWh/year from tidals.

Types of hydrokinetic turbines:

Hydrokinetic turbines are categorized into Cross Flow and Axial Flow, depending on the orientation of their rotation axis. 

Cross Flow HTs

Cross Flow HTs have either a horizontal axis (Fig.1), or a vertical axis (Fig.2), perpendicular to the flow direction (Fig.1). 

Cross Flow HTs can be further classified into drag type (like the Savonius turbine, which exploits the impact of the water flow, or the floating water wheel) and lift type (like the Darrieus turbine, where the airfoil effect is used to push the blade). 

Axial Flow HTs

Axial Flow HTs (Fig.3) have an horizontal axis, parallel to the water flow, similarly to hydropower turbines like Bulb turbines. HTs are completely immersed in water. Nevertheless, the first HT to be used has been the floating water wheel, a drag type and cross flow HT.

Hydrokinetic turbine power generation:

The maximum power that a hydrokinetic turbine can generate is defined by the Betz limit. The Betz limit claims that the ratio of the maximum power P generated by the HT, to the kinetic power K of the fluid flow that passes through the turbine, exhibits a maximum limit: this ratio is called power coefficient C_p = P/K, and the maximum value established by the Betz limit is C_p,max = 16/27 = 59.3%, independent of design configurations. The kinetic power is defined as K=0.5ρAv³, where ρ =1000kg/mc is the water density, A is the frontal area of the turbine interacting with the flow and v is flow velocity.

A well optimized HT can exhibit a power coefficient equal to 50%, although classical HTs usually exhibit a power coefficient of 20-25%. However, optimized HTs enclosed inside a duct can exceed the Betz limit because the pressure energy is used in addition to the kinetic one. This is the case that was published in the journal Renewable Energy, where the power coefficient of a turbine inside a duct reached C_p = 86.5%.

Developments:

The International Electrotechnical Commission is currently developing standards for HT applications, with specifications and design requirements for river applications, to help the promotion of hydrokinetic turbine international market. 

The United States Department of Energy states: 

“Hydrokinetic energy from flowing water in open channels has the potential to support local electricity needs with lower regulatory or capital investment rather than impounding water with more conventional means”. 

Installation of these units supports sustainable development and stimulates innovative research which is an important step in generating a stronger self-sustaining economy. Most HT devices are at the R&D stage, while some are at the initial commercial stage, but the industry is growing rapidly with more than 100 conceptual designs. More than USD 50 million have been invested for HT development by the United States Department of Energy. The Roosevelt Island Tidal Energy project of Verdant Power was one of the first projects of HT technology. 

Since 2013, more than 300 projects have been developed from 280 different companies. Among the biggest companies, 194 companies are based in the USA, 45 in the UK, 19 in Australia and 19 in Canada. In the European Union, 8 are in Ireland, 6 in Denmark and most of the developed HTs are axial flow turbines. Some of them are Guinard Energies, Ocean Renewable Power Company, Verdant Power and Marine Current Turbines. 

Real hydrokinetic turbine installations:

The Seagen turbine is an axial HT, 18 m in diameter installed in Strangford (Ireland), with rated power of 1.2 MW and producing 6000 MWh/year of electricity. 

The Verdant Power turbine is an axial HT, 5 m in diameter, 35 kW of power and power coefficient between 0.38 and 0.44; the installed 6 HTs generate 70 MWh of electricity at East River, New York. 

HTs can also be used to power off-grid irrigation pumps, like the axial HT in Neiva (Colombia), that is 1 m in diameter and 1.1 kW of power (similarly to the spiral pump). 

One other example is the Darrieus turbine installed in the Roza Canal (USA), 3 m in diameter and 10.9 kW.

Engineering challenges and studies:

Hydrokinetic turbines are under investigation by research and commercial companies. The most important challenges are: 

• Cost optimization (Niebuhr et al., 2019, estimated a payback period of 6.5 years for the Groblershoop HT plant in South Africa, made of a system with 4 HTs) 
• Performance evaluation (power coefficient) at different geometric configuration
• Deflectors and shroud use to better direct the water flow into the turbine
• Hydraulic behavior when more HTs are used (array of HTs)
• Requirement for diffusers and channel modifications to control flow velocity and direction

Installation of HTs can generate some small environmental impacts, like blocking of navigation and fishing; turbine components, noise and vibration can affect the river habitat, although fish mortality through HTs is generally very low, because fish tend to avoid HTs. Although the structural requirements are minimal, cavitation and high installation costs per kW remain an open research topic. Since HT technology is in its infancy, the cost can vary a lot among different installations. For example, a 500 kW HT was estimated to cost between 950-1150 $/kW, with respect to 700 $/kW of an analogous wind turbine. The US Department of Energy has defined a levelized cost of energy (LCOE) calculation method to allow comparisons across HEC technologies, ranging between 25 to 80 cents/kWh. 

Therefore, the interest of the scientific and engineering communities on hydrokinetic turbines will continue to grow. HTs will have a key role both as a local source of energy for householders in rural areas, for remote localities and villages in emerging countries, and as a powerful technology to exploit wave and tidal energy. Several companies are emerging as HT manufacturers and different research and industrial projects are under development, making HTs a rapidly developing sector.

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