Archimedes Screw Turbines: Comparison
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Archimedes Screws Turbines (ASTs) are a new form of generators for small hydroelectric powerplants that could be applied even in low head sites. ASTs offer a clean and renewable source of energy. They are safer for wildlife and especially fish. The low rotation speed of ASTs reduces negative impacts on aquatic life and fish. Considering the flexibility and advantages of ASTs, they offer economic, social, and environmental advantages to support sustainable development to:

- Increase the number of suitable sites and maximize power generation.

- Retrofit old dams or upgrade current dams or mills to make them economically (power generation) and environmentally (renewable energy) reasonable.

- Reduce the hydroelectricity major operational and/or maintenance costs.

- Reduce the disturbance of natural erosion and sedimentation processes.

- Make hydropower generation safer for aquatic wildlife, especially for fish.

- Generate electricity for small communities, developing countries, and regions with limited access to the power grid or other infrastructures, or regions that are hard to access or connect to the power grid.

 

  • Archimedes screw turbine/generator
  • small/micro/pico hydropower plant
  • run of river powerplant
  • fish friendly turbine
  • low head hydropower
  • sustainable development

1. Sustainable Development

Sustainable development is described as “the organizing principle for the achievement of human development objectives while at the same time preserving the capacity of natural systems to provide the natural resources and ecological services on which the economy and community rely” [1]. Sustainable development often implies “a development that addresses current needs without influencing potential generations’ capacity to fulfill their own needs” [2,3][2][3].
In 1979 René Passet proposed a three-sphere framework for sustainable development projects [4] (Figure 1) According to this framework, development can be considered as sustainable only if it simultaneously has positive social, environmental, and economic impacts. If a project satisfies the economic and social aspects but fails to satisfy the environmental aspects, it is categorized as equitable. If a development project can satisfy the environmental aspects but fails one of the social or economic aspects, the development could still be bearable or viable, respectively. If a development cannot satisfy at least two of the three mentioned aspects, it cannot be categorized anywhere in this definition. Some authors consider a fourth sustainability pillar of culture, institutions, or governance [5], or reconfigure the four domains to be social-ecology, economics, politics, and culture [6]. Overall, the focus of the modern sustainable development concept is simultaneous economic development, social progress, and environmental protection for current and future generations [7].
 
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Figure 1. Sustainable development as the confluence of three constituent parts. Adapted from [8].
 

2. Renewable Energy

Renewable energy is defined as energy that is obtained from resources that are fully replenished on a human time scale [9]. According to REN21’s year 2019 report, renewable resources provided 2378 GW of power capacity, which is more than 33% of the world’s total electrical generating capacity. In this list, hydropower capacity (excluding pure pumped storage capacity) is 1132 GW, which is about half of total renewable energy. It is worth mentioning that in 2018, annual new investment in hydropower grew 11% in comparison to the previous year [10]. The majority of hydropower investment is in large dams and associated generating stations that inherently include complex networks of social, economic, and ecological effects, maybe more than any other large infrastructure project [11].
 

3. Hydropower

Hydropower is one of the most efficient and confident sources of renewable energy [12] and has considerable value for a sustainable future [13]. By the end of 1999, around 2650 Terawatt hours (TWh) (19%) of the world’s total electricity relied on hydropower [14]. It rose to about 3100 TWh until 2009, and it is estimated that it reaches to 3606 TWh in 2020 [15]. Dams are essential tools for controlling, storing, managing, and operating water for humankind. Large dams serve various specific purposes for our civilization, including water supply, flood control, navigation, sedimentation control, and hydropower [16]. However, they also come with disadvantages, including flooding large areas of land, impeding fish migration, and affecting the physical characteristics of the dam’s downstream river [17]. Construction of large dams needs significant capital, so many large dam projects are national (or even international) in scope. Currently, most new large dams are being constructed to provide combinations of energy, irrigation, and flood control in developing countries. At the same time, dam decommissioning is an increasing trend in developed countries because of environmental impacts and the economic costs of maintaining aging structures [11].
Hydropower plants can be classified based on their installed electrical generating capacity. Typical categories and associated capacities are: large hydro (>10 MW), small hydro (<10 MW), mini-hydro (<1 MW), micro-hydro (<100 kW), and pico-hydro (<5 kW) [18]. It is estimated that about 10% of global hydropower is generated from powerplants with less than 10 MW of capacity [18]. Micro-hydro plants often utilize the natural flow of water in a run-of-river (ROR) configuration [19]. ROR plants include little or no controlled water storage, meaning that ROR typically has small or no reservoirs. The lack of a large reservoir formed by a dam, or significant control of river flow, avoids or minimizes the disadvantages associated with large reservoirs, at a cost of having to accept more variable or poorly timed power generation.
Micro hydropower plants can often be considered as a sustainable development option for generating electricity in both developing and developed countries. There is often no need to build expensive dams and flood massive areas for the reservoir. This minimizes land and soil destruction, threats to wildlife, climate change effects, and other environmental impacts, especially on ecosystems [20] as well as the social impacts of ROR hydropower plants. New ROR hydropower technologies such as Archimedes Screws Turbines (ASTs) can be particularly advantageous in these regards.
 

4. Archimedes Screws

The Archimedes screw is considered to be one of the earliest hydraulic machines [21]. It is composed of a helical array of simple blades that are wrapped around a central cylinder, like a woodscrew [22]. This screw is supported within a surrounding fixed trough. There is a small gap between the trough and screw that allows the screw to rotate freely while allowing only a small amount of water to leak past the blade edges. It is believed that the Archimedes screw was invented by Archimedes of Syracuse (circa 287-212 BCE), the Greek physicist, mathematician, and inventor [23]. However, there is evidence suggesting the invention and use of the screw technology may date back to over three centuries before Archimedes under the reign of King Sennacherib (704-681 BCE) in the 7th century BCE in the Assyrian Empire [24].
 

4.1. Archimedes Screw Pump

Archimedes screws have been used as water pumps for irrigation and de-watering for a long time [23]. Some historical sources claim that Archimedes used the device to launch a ship [23]. Archimedes screws are commonly used today as high-volume pumps and are particularly adapted to wastewater treatment facilities since debris and obstructions in the water usually have minimal or no effect on the operating screw [25]. Figure 2 shows that the Archimedes screw can be configured as either a pump or a generator [26].
 
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Figure 2. Archimedes screw pump (left) and an Archimedes screw hydropower plant (right).
 

4.2. Archimedes Screw Turbine

Archimedes screws can be also used to produce power if they are driven by flowing fluid instead of lifting fluid. Water transiting the screw from high to low elevation generates a torque on the helical plane surfaces, causing the screw to rotate. This mechanical rotation can be used to produce electricity by attaching a generator [27]. In this way, the AST is a variation of the ancient Archimedes screw pump. However, ASTs have only been in use since the 1990s [28]. ASTs offer a clean and renewable source of energy and can be safer than other types of hydroelectric turbines for wildlife and especially fish [29]. The first AST was installed in the 1990s [25]. Since then, several hundred ASTs have been installed to generate electricity [28]. Almost all of these have been built in Europe. There are only two operational ASTs connected to the grid in North America, the first of which was installed near Waterford, Ontario, Canada, in 2013 [30].
Generally, there are two overall categories of modern hydropower turbines: impulse and reaction. However, work is done by ASTs due to pressure differences across the blades created by the weight of the water, so they do not categorize as using either an impulse or reaction mechanism. ASTs constitute a third category of hydropower converter that is driven by the weight of water, which would also include water wheels. These machines can be considered quasi-static pressure machines.
A water wheel is generally a circular rotor with some form of buckets around the circumference. Most turn about a horizontal axis, but there are several different configurations (Figure 3). Water at a higher elevation fills buckets, which empty at a lower point as the wheel turns [31]. Horizontal waterwheels have a vertical rotation axis and vertical ones have horizontal rotation axes [31].
 
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Figure 3. (a) Horizontally and (b) Vertically Oriented Water Wheels. (b-1) Undershot, (b-2) Overshot (b-3) Centershot. From [32].
The energy transfer mechanism in an Archimedes screw is similar to a water wheel, although the configuration is different. In an AST, a water bucket is defined as the volume of water entrapped between two adjacent helical plane surfaces.
 

5. Design Archimedes Screw Turbines/Generators

Studies show that the volume of flow that passes through Archimedes screws is a function of the inlet depth, diameter and rotation speed of the screw [33]. Therefore, the following analytical equation could be used to design Archimedes screws [34]:
Based on the common standards that the Archimedes screw designers use this analytical equation could be simplified as [35]:
The value of η could simply determinate using the η graph[34] or Θ graph[35]. The other design parameters of Archimedes screw Turbines could be easily calculated using an easy an step by step analytical method [34].

5. Design Archimedes Screw Turbine/Generator Hydropower plants

The easy guideline to design Archimedes screw turbine/generator hydropower plants is available which is based on the analytical method to design Archimedes screw turbines/generators.
 

57. Conclusions

Archimedes Screws Turbines (ASTs) are a new form of turbines for small hydroelectric powerplants that could be applied even in low head sites. ASTs offer a clean and renewable source of energy. They are safer for wildlife and especially fish. The low rotation speed of ASTs reduces negative impacts on aquatic life and fish.
It is important to note that ASTs are not a uniquely global solution for all energy generation needs. ASTs have their own drawbacks just like any other technologies: using Archimedes screws as generators is a relatively new technology, and in comparison with other hydropower technologies, there are many not well-known things about ASTs. Currently, there are no standards for the design of ASTs, and AST hydro powerplant designs are highly dependent on the experience of the engineer who designs them. For very high flow rates or water heads, a single screw may not take advantage of all available potential due to material, structural, technical, and physical limitations. However, the increasing interest in ASTs, new advancements, and ideas such as multi-AST powerplants offer some solutions to extend AST usability.
ASTs provide a range of practical advantages for generating electrical energy at suitable locations. For supporting sustainable development, ASTs offer economic, social, and environmental advantages. Considering the flexibility and advantages of ASTs, they could be considered as one of the most practical options for a more sustainable electricity generation:
  • To increase the number of suitable sites for power generation even in sites with very low flow rates and/or water heads. ASTs can be designed to operate in a wide range of flow rates (currently from 0.01–10 m3/s) and water heads (currently from 0.1–10 m), including at sites where other types of turbines may not be feasible. This increases the number of potentially suitable sites for hydropower.
  • To maximize hydropower generation even in rivers with high flow rate fluctuations. ASTs can handle flow rates even of up to 20% more than optimal filling without a significant loss in efficiency [63][36]. Even when the conditions are not perfect for a single screw, installing more than one screw, and utilizing variable-speed ASTs, allows developers to fully utilize available flow at a wider range of sites, including those with high seasonal variability.
  • To retrofit old dams or upgrade current dams or mills to make them economically (power generation) and environmentally (renewable energy) reasonable. Using ASTs as an upgrade for retrofitting old dams or upgrading operational dams makes it possible to add electrical generation with extremely low incremental environmental impact, at reasonable costs and with good potential for low social impacts while providing an incentive to maintain ageing dams and infrastructure. ASTs utilized in this manner could help to reduce fossil fuel usage and greenhouse gas emissions by displacing electricity generated by more polluting methods.
  • To reduce the hydroelectricity major operational and/or maintenance costs: In addition, to retrofit/upgrade current dams advantages, at appropriate sites, the capital costs of AST hydropower can be less than other hydropower technologies. The overall maintenance demands and costs of ASTs are often lower than other turbines. Major maintenance is required after the 20 to 30 years.
  • To reduce the disturbance of natural erosion and sedimentation processes which could lead to soil and land conservation.
  • To make hydropower generation safer for aquatic wildlife, especially for fish.
  • To generate electricity for small communities or regions that are hard to access or connect to the power grid, especially because of the low operation and maintenance demands and costs of ASTs. These characteristics make ASTs a potential candidate for providing electrical power in undeveloped, remote regions, and small communities that currently lack energy infrastructure.
  • To improve the welfare of the developing countries and regions with limited access to the power grid or other infrastructures. Despite many other technologies, ASTs do not require high manufacturing capabilities and hi-tech technologies to design, implement, operate, or maintain. Simplicity, low operational demands, and moderate costs make ASTs a practical environment-friendly and sustainable solution for supplying energy, especially in developing countries. At remote locations with a low head water supply, ASTs may provide a possible means of providing electricity that would otherwise be impractical in developing communities. Improving the economy and welfare of such communities is a win-win futuristic sustainable development approach that could be facilitated by using AST hydroelectric plants.

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