Ultra-low-head hydroelectric technology: A review
Introduction
Although hydropower is considered to be a renewable energy resource, its sustainability is sometimes questioned because of the impacts of dams on the environment, which is a major barrier for the deployment of large- or mid-sized hydropower projects [1]. Interest in using small hydropower resources is increasing, and the technology is being developed worldwide because of its advantages in terms of scale (i.e., small), deployment time (i.e., short), and impact on the environment (i.e., low) [2]. To date, most published literature focuses mainly on small hydropower technologies that use low hydraulic heads between 2 m and 30 m [3], [4], [5], [6] or on hydro-kinetic energy conversion technology [7], [8], [9]. Nevertheless, not enough attention has been paid to water-energy development in situations where the hydraulic head is between 0 m and 3 m (i.e., ultra-low head [ULH]) because of the poor economic benefits of these resources [10].
ULH hydropower will become an attractive, renewable, and sustainable resource through advances in hydraulic turbines, simplified civil works, and reduced project costs. In addition, this type of water-energy technology is advantageous in that it can be distributed widely and implemented near human activities, and it is generally regarded as environmentally benign. Specifically, the low environmental impact of ULH hydropower is reflected in two main points: 1) the wide blade passages and low rotating speed can significantly reduce collision damage for fish; and 2) because no dam or a very low dam is involved, barriers for fish migration and navigation are avoided and water flow downstream are ensured. Although generally considered to be environmentally benign, inappropriate applications of the technology can result in harmful impacts to the environment [7], [8]. In this review, ULH water energy refers to situations where the hydraulic head is less than 3 m or the flow velocity is more than 0.5 m/s with zero head. Based on the classifications defined by Singh and Kasal [11], ULH hydropower can be pico-hydro (less than 5 kW), micro-hydro (5 kW~100 kW), mini-hydro (100 kW~1 MW), small-hydro (1–15 MW), or medium-hydro (15–100 MW) depending on the output. Many low-head projects seek to minimize infrastructure and costs, and as a result, low-head hydropower projects are almost always “run-of-the-river” installations (i.e., water-storage capabilities are small to nonexistent) [12].
This paper focuses on ULH water-energy deployment and provides an overview on ULH hydropower technology. It begins with the introduction of existing ULH water-energy sites, followed by discussions of turbine and generator selection for ULH hydropower sites. Then, we discuss ways to simplify implementation of the technology and provide a breakdown of project costs. Finally, we summarize future development objectives for ULH hydropower projects.
Section snippets
Sites of ULH water-energy resources
A comprehensive assessment based on the temporal and spatial flow properties of water-energy resources should be performed to confirm the economic value of candidate sites. However, it would be difficult to conduct a survey for reliable statistics because ULH water-energy resources are widely distributed geographically throughout the world. Thus, existing ULH water-energy sites are introduced within the following categorized examples: 1) Rivers and Streams; 2) Canals, Locks, and Pumping
Turbine selection for ULH applications
It is important to select a suitable hydro-turbine for ULH water-energy exploitation. Conventional water turbines can be classified as impulse turbines or reaction turbines. The main types of conventional hydropower turbine types include Pelton turbines, Turgo turbines, cross-flow turbines, Francis turbines, Kaplan turbines, and tubular turbines [4]. In addition, there are more than 20 types of emerging hydro-kinetic turbines for current energy conversion [52]. These emerging hydro-kinetic
Generator matched with turbine
It is very important to select the right generator for efficient ULH water-energy conversion under different head and flow velocity conditions. Usually, the characteristics of ULH hydropower generation are similar to those of wind power generation, such as low output power and slow fluctuating rotation speeds. Therefore, for generator selection, much can be learned from the wind power industry [78]. Two common types of generators are reported in the literature [79], [80], [81], [82]: 1)
Structural requirements
Structures in conventional hydropower facilities mainly include water-storage structures, water-diversion facilities, a powerhouse, and a tailrace passage. Costs for these structures often account for more than half of the total project cost. Therefore, it is very important to find some ways to reduce the costs of structures for ULH power projects.
Project costs
The costs of a ULH hydropower project are a major concern for a developer or an investor. The costs include not only the expected costs and benefits, but also the sensitivity (i.e., the corresponding economical risk and uncertainty) [97]. Because of the complexity of an economic assessment, our review focused only on project cost. At this time, the cost of ULH hydropower is nearly equal to the cost of a wind energy plant but less than that of a solar energy plant [54], [98].
Conclusions and prospects
Although ULH hydro-resources are abundant in many countries, the survey task is difficult because no comprehensive database has been established to collect relevant information from wide-ranging sources. Because of different site conditions and deployment methods for each ULH hydropower project, a more accurate cost-assessment model of ULH hydropower projects need to be established in the future. For ULH turbines, future development objectives are summarized below:
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First, high-efficiency
Acknowledgments
The study was funded by the Laboratory Directed Research and Development Program at Pacific Northwest National Laboratory (PNNL) and the U.S. Department of Energy Wind and Water Power Technologies Office. The study was conducted at PNNL, which is operated by Battelle for the U.S. Department of Energy.
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