Contextualizing Non-Powered Dam Site Selection for Archimedes Screw Turbines: A Methodology for Responsible Archimedes Screw Turbine Conversion at Existing Dams

Abstract

Non-powered dams represent 97% of dams in the United States and their energy generation potential has not been fully realized. The use of an Archimedes screw turbine to generate power at non-powered dams offers a dual benefit; producing electricity, and acting as downstream fish passage, helping to reconnect previously separated ecosystems. In this study, we assess the technical, environmental, social, and economic feasibility of generating power at non-powered U.S. dam sites using Archimedes screw turbines by integrating mechanical constraints, social impact metrics, proximity to infrastructure, and environmental sensitivity data. Results account for future precipitation predictions and show, between 2024 and 2050, the number of sites where Archimedes screw turbines are viable decreases by one site, but overall generation capacity increases due to increased flow rates across persisting locations. Our analysis identified 82 non-powered dam sites with a mean generation capacity of 49 kW that meet the mechanical requirements for Archimedes screw turbine technology in 2024. Our analysis presents a framework for considering social, environmental, and economic impacts of specific turbine technologies to convert non-powered dams to generate power.

1. Introduction

Hydropower is a major source of renewable energy, generating 6.2% of total electricity and 28.7% of all renewable generation in the United States (U.S.) [1]. Due to environmental impacts and high capital costs associated with construction, it is unlikely that a large number of new hydropower dams will be built in the U.S. [2,3,4]. This highlights the importance of leveraging existing infrastructure, such as non-powered dams (NPDs), to expand hydropower capacity sustainably and cost-effectively without the environmental and financial burdens of building new large-scale facilities.

Non-powered dams (NPDs) are best suited for providing baseload power because they often have smaller reservoirs compared to current conventional hydropower facilities. This limits their load-following capabilities, but enables run-of river operations, operations that mimic natural flow regimes by using the same inflow rate as the discharge rate for power generation, to provide predictable power generation.

In cases where an NPD still functions for flood control, irrigation, recreation, or other purposes, it may be able to be retrofitted for hydropower production. This study does not account for upstream water pulls outside of the U.S. Army Corps of Engineers-reported NPD flow rates. Retrofitting could provide an alternative to constructing new conventional hydropower dams, offering increased generation with fewer new environmental impacts since reduced river connectivity, the most striking impact of conventional hydropower, has already occurred. Moreover, in the United States, dams without hydropower are not subject to the same rigorous environmental requirements such as fish passage assessments and water quality mitigations. Adding generation to an NPD thus also provides environmental protection opportunities by bringing the dam under U.S. Federal Energy Regulatory Commission (FERC) jurisdiction. When under FERC jurisdiction, there could be environmental mitigation requirements to obtain and maintain the hydropower license. These requirements depend on each site’s location but could include fish passage and protection requirements, fish habitat improvements, hiking trails, boat ramps, and nesting boxes for birds [5].

Previous research on hydropower potential finds limited large hydropower development sites remaining but suggest low-head hydropower potential remains undeveloped [6,7,8]. However, the variety and volume of data and publications can make it difficult to combine and develop solutions [9]. The selection of a turbine for converting an NPD involves trade-offs between power density, capital costs, and environmental impact. Kaplan and Francis turbines offer higher power density and reduced capital costs for heads above the range of an Archimedes screw turbines (AST) but can be more expensive on low-head applications giving ASTs opportunities in this range. Archimedes screw turbines are more dependent on site specifics, but they are well suited for low-head applications (6 m or less) and are considered fish friendly (Figure 1) [10,11]. These low-head conditions lead to slower rotation, reduced turbine blade tip speeds, and decreased shear forces, all of which minimize harm to fish [12]. An additional consideration is that the AST can more easily pass fish and sediment from upstream to downstream increasing the connectivity of the river reach movement [11,13,14].

In this study we consider which of the 89,274 non-powered, low-head (10 m or less) dams in the continental U.S. (CONUS) fit the technical specification for AST deployment at NPD sites. We consider all potential NPD locales based on their proximity to protected areas, use by boaters, and mechanical practicalities and consider environmental, economic, and social factors when evaluating ASTs for NPD conversion sites. Specifically, this study (1) quantifies economic opportunity while considering precipitation trends; (2) evaluates the environmental impacts at each prospective NPD site; and (3) evaluates the social impact on surrounding communities.

2. Materials and Methods

2.1. Data Curation

The U.S. Non-Powered Dam Characteristics Inventory, developed by Oak Ridge National Laboratory (ORNL), was used to gather information on existing non-powered dam infrastructure and included dam identifiers, the number of whitewater access points, and the number of daily fishing trips per year used to quantify the social impact of each NPD facility in our analysis. The National Inventory of Dams (NID), developed by the Army Corps of Engineers (USACE), contains information on dam height, location, and flow rates and was used to calculate the number of mechanically feasible sites in the CONUS [16,17].

Percentage changes in precipitation from 1901 to 2000 across all defined climate regions within the CONUS was obtained from the National Atmospheric and Oceanic Administration’s (NOAA) published precipitation trends [18]. The average percentage change per year for each climate region was then calculated (Equation (1)) and applied to each facility’s flow rate within that region. The equation considers the precipitation percentage change from 1901 to 2000 (P) and divides that by 99 years.

$$\mathrm{A}\mathrm{n}\mathrm{n}\mathrm{u}\mathrm{a}\mathrm{l} \mathrm{P}\mathrm{r}\mathrm{e}\mathrm{c}\mathrm{i}\mathrm{p}\mathrm{i}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{C}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{g}\mathrm{e}={\displaystyle \frac{\mathrm{P}}{99}}.$$

(1)

The average daily flow at each facility was obtained from the NID dataset from 1985 to 2014. Projected flow rates for 2024, 2037, and 2050 (y) were determined using the average precipitation percentage change per year (P, %) and the known average flow rate between 1985 and 2014 (q) (Equation (2)). This helped determine candidate sites for NPD conversion across all three studied years: 2024, 2037, and 2050. All datasets used for our analysis are publicly available.

$${\mathrm{S}\mathrm{t}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{m}\mathrm{f}\mathrm{l}\mathrm{o}\mathrm{w}}_{\mathrm{y}\mathrm{e}\mathrm{a}\mathrm{r}}=\mathrm{q}+\mathrm{P}\left(\mathrm{y}-2000\right).$$

(2)

2.2. Analysis

2.2.1. AST Site Parameters

The NID database was used to determine NPDs that could be converted to powered facilities using an AST in the years 2024, 2037, and 2050 based on given head height and flow rates. As precipitation trends change, water availability in river reaches could vary leading to a site falling below the desirable AST operating range in future years. Considering not only the present but future potential of NPD conversions ensures installed infrastructure will continue to work as designed, even with the impacts of changing climate.

The mechanical parameters for an AST include hydraulic height between 1 m and 6 m, flow rates between 0.1 m³/s and 6 m³/s, and efficiencies between 69% and over 75% [11,19]. When considering flow rate, we bound the maximum daily flow rate and the minimum daily flow rate to these levels because we only consider one AST at all locations and the maximum flow one turbine can stand is 6 m³/s [19]. Other studies have explored enhancing efficiency by testing tilt angle, diameter, and flow rates, and have found that efficiencies range from 60 to 80% [20,21,22,23]. Here, we holistically consider the AST as a single turbine unit and do not vary these input parameters, while we acknowledge the possibility of installing multiple turbines at one site. To ensure a balanced representative analysis, we use an efficiency of 69% which falls near the midpoint of the reported range and apply that across NPDs to AST conversion sites (Equation (3)). When conducting site-specific research in future studies, there may be cases where the site efficiency is not 69%, which will affect site capacity and economic feasibility. The selection of a minimum (69%) for our calculations allows for a broader consideration of NPDs across the U.S., thereby mitigating the complexities associated with site-specific calculations.

$$\mathrm{S}\mathrm{i}\mathrm{t}\mathrm{e} \mathrm{C}\mathrm{a}\mathrm{p}\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{t}\mathrm{y}=\mathrm{n}\ast \mathrm{m}\mathrm{i}\mathrm{n}\mathrm{i}\mathrm{m}\mathrm{u}\mathrm{m}\left(\mathrm{Q},6\right)\mathrm{g}\ast \mathrm{h}$$

(3)

where n is the 69% efficiency rate used across all sites as a decimal, the flow rate is limited to the minimum between the stream flow for that given year (Q) and 6 m³/s, the maximum an AST installation can handle, 9.8 m/s² is the acceleration due to gravity (g), and the head height (h, m) is the net height of the fall of the water. It is assumed that the NPD head remains the same throughout the entire year; this assumption is accurate for NPDs that allow water to flow over the top of the dam; however, this may not apply to all NPDs. We also assume one turbine per site, but site-specific studies can be performed in future research to consider the flow rate-to-head relationship, and the benefits of adding more than one turbine.

Installed power and annual generation are typically calculated using maximum flow and average flow, respectively. This approach minimizes the influence of extreme low-flow events on generation estimates. Historical average daily flow data is used to project future performance, while site capacity is often based on the minimum annual flow. This method offers a conservative estimate of maximum capacity by accounting for worst-case conditions, seasonal fluctuations, or droughts, ensuring reliable flow operations during low-flow periods. Adopting this conservative approach that emphasizes extreme conditions, we provide a more cautious and robust estimate of AST performance.

2.2.2. Cost Evaluation

To evaluate the cost of installed ASTs, a cost adjustment factor was needed for pricing across regions. The U.S. Environmental Protection Agency uses the U.S. Army Corps of Engineers’ (USACE) published cost factors for each state when reporting cost estimates in feasibility studies. For our analysis we used the state price indices from the USACE 2022 report to estimate capital costs, operations, and maintenance costs [24,25]. These construction index values are reported in the Index Value column in Table 1.

Two case studies (one from Canyon Hydro commissioned by DOE, and the other from ORNL) were used for estimating AST installation costs and one survey across 71 AST sites was used to verify the USD/kW derived from the case studies. In the case studies, Canyon Hydro’s technical report, a reference site with a power capacity of 841 kW and a cost of USD 3.4 million (USD 4043/kW) located in Lodi, California, was identified and uses two turbines [26]. An ORNL report focusing on a cost analysis of power conversion options for NPDs identified a screw turbine as one option [10]. The case study for the screw turbine, located at Chouteau L&D in Oklahoma, uses two turbines with a generation a capacity of 4300 kW combined, costing USD 7995/kW for capital costs and USD 369/kW for development costs. Together, the Lodi and Chouteau case study sites are well-defined examples that illustrate a range of generation capacities, costs, and geographic contexts for AST installations. These factors are considered representative for evaluating the applicability of AST technology across a diverse array of NPD conversion sites. Capital costs are defined as the construction and material costs (Equation (4)), whereas development costs are licenses and permits. Table 1 gives a side-by-side comparison of these two cases studies and compares them using the USACE cost index.

Table 1. Cost comparison of example AST conversion studies where the index value is the cost factor applied within each state. The national average is 1.0, an index factor greater than 1.0 means the location is more expensive for construction than the national average and an index factor below 1.0 is less expensive than the national average.

Site Name	State	USD/kW (One Turbine)	Index Value *	Year Estimated	USD (2020)/kW (National Conversion) *
Lodi	California	4042.81	1.23	2017	3755.82
Chonteau L&D	Oklahoma	3997.5	0.83	2020	4816.27

* The national conversion uses Equation (4) below to account for the index value and national average inflation rate from 2017 to 2020 [27].

Table 1. Cost comparison of example AST conversion studies where the index value is the cost factor applied within each state. The national average is 1.0, an index factor greater than 1.0 means the location is more expensive for construction than the national average and an index factor below 1.0 is less expensive than the national average.

Site Name	State	USD/kW (One Turbine)	Index Value *	Year Estimated	USD (2020)/kW (National Conversion) *
Lodi	California	4042.81	1.23	2017	3755.82
Chonteau L&D	Oklahoma	3997.5	0.83	2020	4816.27

* The national conversion uses Equation (4) below to account for the index value and national average inflation rate from 2017 to 2020 [27].

$$\mathrm{C}\mathrm{a}\mathrm{p}\mathrm{i}\mathrm{t}\mathrm{a}\mathrm{l} \mathrm{C}\mathrm{o}\mathrm{s}\mathrm{t}=\mathrm{g}\ast {\mathrm{C}}_{\mathrm{a}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{g}\mathrm{e}}\left({\displaystyle \frac{{\mathrm{I}}_{\mathrm{d}}}{{\mathrm{I}}_{\mathrm{c}}}}\right).$$

(4)

Equation (4) calculates the adjusted cost from the current location (I_c) to the desired location (I_d) using the USACE index values, the initial cost of the location, and the site capacity (g). Equation (4) does not account for inflation rates. The national average cost is USD 4286.05/kW (C_average), calculated as the year-adjusted weighted average from Table 1, using the USD 2020/kW column. This national average is used to calculate the capital costs of potential NPD conversion sites (Equation (4)).

When calculating the return on investment of each NPD conversion location, the operations and maintenance (O&M) costs are estimated at 4% of installed costs [28]. The price hydropower was sold at varies across five regions: Northwest (NW), Southwest (SW), Midwest (MW), Southeast (SE), and Northeast (NE). The prices for each of these regions were gathered from the DOE 2023 Hydropower Market Report; the median energy prices were used and can be found listed by region in

Table 2. Median hydropower observed energy prices from 2006 to 2020.

Region	USD/MWh
Midwest	46.97
Northeast	61.36
Northwest	54.67
Southeast	58.43
Southwest	47.83
Midwest	46.97

[29].

Additionally, the relative closeness to other grid infrastructure is important and can impact costs; this is reported as distance to nearest substation in our analysis [30]. While infrastructure is not directly incorporated into our economic calculations of NPD conversions with ASTs, population density, which is considered in the social impact analysis, can serve as an approximation of grid infrastructure in each area. The assumption is that a larger population means more grid infrastructure and less expensive integration of new power generation.

The calculation of return on investment (ROI) is also included in our cost analysis (Equation (5)). In this analysis we consider ROI to be the number of years until the project has paid back the capital costs. This calculation considers the cost of installation in a given year (C_year), 8760 h per year, the estimated kW capacity of the facility (g), and the cost of hydropower within each region (H) (Table 2).

$$\mathrm{R}\mathrm{O}\mathrm{I}=\left({\displaystyle \frac{{\mathrm{C}}_{\mathrm{y}\mathrm{e}\mathrm{a}\mathrm{r}}}{8760}}\right)\mathrm{g}\ast \mathrm{H}.$$

(5)

We verified these rates with a survey that distinguished a difference between fixed speed and regulated speed turbines [19]. When considering the economics reported in this survey, the total cost of power plants increases linearly with installed electrical capacity. However, the specific cost of work to install electrical power shows a non-linear trend: as installed electrical capacity increases, the specific cost of work decreases at a decreasing rate. This survey offers operational costs for kWh of operation instead of by kW of capacity. To equate the range of 0.5–2 EUR/kwh, this analysis assumes an annual capacity factor of 48% and converts EUR to USD then accounts for inflation from 2010 to 2020 [19,23]. We compare the case studies and the results in this survey in

Table 3. Comparison of sources USD/kWh.

Source	USD/kWh
Lashofer et al. [19] (survey)	0.07 to 2.63
Canyon Hydro [26] (Lodi)	0.89
Oladosu et al. [10] (Chonteau L&D)	1.15

below and find that the Lodi and Chonteau L&D sites fall within the range given by the survey; therefore, this analysis uses the average of the Lodi and Chonteau L&D sites to evaluate site economics.

2.2.3. Environmental Evaluation

Several factors at potential NPDs, including wildlife and permit restrictions, could complicate or preclude the deployment of ASTs for hydroelectric generation [17]. While ASTs are considered a more fish-friendly alternatives compared to other turbine types, relative closeness to protected areas (areas designated and managed for the purpose of maintaining biological diversity, recreation, historical, and cultural significance) can impact the implementation of hydropower generation at NPD sites [31]. One metric for environmental evaluation is quantifying the presence of sensitive species, including inland/oceanic sturgeon, paddlefish, clupeid, salmonoid, eel, and lamprey, which could trigger fish passage requirements or tighter operating constraints on flow regimes at hydropower facilities. The biodiversity protections surrounding the dam also weigh heavily, as installations proposed for sites situated on public park or wildlife sanctuaries would likely face prohibition or strict regulation, limiting construction and operating conditions. Biodiversity designations of the site include five options: “Dam is not on protected land”, “Disturbance events may occur”, “Disturbance events suppressed”, “Managed for multiple uses- subject to extractive or off-highway vehicle use”, and “No biodiversity protection”. The designation of “No biodiversity protection” is the least restrictive whereas, “disturbance events” considers whether the management of natural disturbances is allowed [31]. Our environmental assessment considers the status of each mechanically viable NPD site. Sites with the above listed fish species were considered feasible for conversion but each fish species was listed with its associated NPD facility.

Dams with restrictions or protected fish species present are not precluded from an NPD conversion but should be the subject of future studies, including a site-specific review. This screening approach categorizes candidate sites based on readily accessible indicators of likely wildlife effects and permit/construction constraints. Sites showing no signs of elevated sensitivity can proceed more directly to AST development pathways, while others need further evaluation clarifying feasibility and conditions for responsible installation in future work. This method provides an efficient structure that matches information needs and analytical requirements for potential NPD to power production conversions.

2.2.4. Social Impact Evaluation

To evaluate the potential social impact of adding ASTs to NPDs for hydroelectric generation, we consider local population density, daily fishing trips per year, and whitewater access points from the NID dataset [17]. Only data reported from the NID dataset was used; these data points were not projected into the future. Local population density, measured by the total residents with the hydrologic unit code (HUC) 12 watershed, signifies the approximate size of the community situated adjacent to the candidate facility. Daily fishing trips per year, freshwater fishing recreation demand in day trips per year, whitewater access points, and the number of whitewater put-in or take-out sites in the HUC 12 are considered recreational usage metrics and quantify disruption to recreation and tourism economies reliant on the preservation of aquatic environments and water flows surrounding dams slated for retrofitting.

Together, these indicators characterize the local communities’ dependence on economies potentially impacted by repurposing NPDs. While our analysis reports on the data for these metrics and identifies trends, decisions on whether to move forward or not with implementing an AST at one of these sites can be the product of future work. Our evaluation of the number of residents served, the number of daily fishing trips, and number of whitewater pit-in/take-out sites can help determine a site’s development feasibility and social trade-offs. Outreach around AST proposals can also be customized based on population and recreational affiliations.

While not encapsulating all recreational and tourism intricacies, screening sites using publicly available population and recreation data enables a contextualization of AST decision-making processes with visibility into lifestyle and economic activities linked to local dams. By targeting installations projected to require less community disruption, the methodology provides a filter for identifying socially viable NPD conversion candidates.

3. Results

We identified an initial 89,274 NPDs across the contiguous U.S. that currently do not have hydroelectric generation capabilities. After applying mechanical restrictions on hydraulic height and flow rates, the dataset was reduced to 82 NPDs in 2024. We considered NPD sites in the 48 contiguous U.S. states when considering sites viable for AST conversion.

3.1. Precipitation Trends Impact

We assessed the potential of each NPD site (89,274) to meet future AST minimum discharge requirements in 2024, 2037, and 2050 based on changing precipitation trends. In 2024 82 NPDs were found to be viable for conversion with ASTs. This number stayed the same for 2037 and will decrease to 81 NPDs in 2050 (

Table 4. Viable NPDs for AST conversion and their associated mean capacity, mean cost of installation, and mean ROI.

Year	NPDs	Mean Capacity (kW)	Mean National Cost (USD)	Mean National ROI (USD)
2024	82	49	USD 221,324	10.5
2037	82	70	USD 313,382	10.5
2050	81	91	USD 409,824	10.5

). This decrease could be the result of the anticipated changing hydrologic patterns, which influence water flow availability and in turn conversion viability.

3.2. Capacity and Costs

The average potential hydropower capacity per NPD was 49 kW in 2024, 70 kW in 2037, and 91 kW in 2050 (Table 4). The average cost of installation was USD 221,324 in 2024, USD 313,382 in 2037, and USD 409,824 in 2050 (Table 4). The mean national ROI was 10.5 years, respectively, across all three studied years (Table 4). The unchanged ROI across all the years is likely from the increase in the simultaneous increase in build cost and capacity as seen in the ROI calculation (Equation (5)). These values do not include licensing or interconnection, which would likely increase the costs. These values were also evaluated on a state-by-state level and

Table 5. Top five states with the most viable NPD sites for conversion with ASTs. [29].

State #	NPDS	Mean Capacity 2024 (kW)	Mean Cost (USD)	Mean ROI (USD)	Mean USD/kW
Wisconsin	23	24.8013	121,181.5	11.9	USD 4886.097
Michigan	20	72.52006	307,716.4	10.3	USD 4243.19
Maine	11	68.89391	292,329.9	7.9	USD 4243.19
Montana	9	61.14715	293,529.3	10.0	USD 4800.376
Utah	6	28.62094	131,257.7	11.0	USD 4586.074

shows the five states with the most AST convertible NPDs located in them. Figure 2 shows the gradient of installation costs by year and the ROI for 2024, while Figure 3 presents a graph of the increasing overall generation capacity.

The proximity to substations was considered as an indicator for the ease of integration into the grid for each site. The proximity is not factored into the cost estimation metrics. Figure 4 below shows a heat map of how close each NPD is to a substation on a gradient and

Table 6. Five states within the contiguous United States with the lowest median distance to a substation showing the median distance and mean distance for each state.

State	Median Distance (Miles)	Mean Distance (Miles)
Rhode Island	0.88	0.88
Massachusetts	2.15	2.15
Utah	3.22	10.35
Wisconsin	4.07	3.69
Maine	4.11	5.57

presents the median and mean distance for the five states with the lowest median distance to a substation.

3.3. Ecological and Social Impacts

The analysis identified all potential NPD sites with mechanical viability for ASTs as having at least one protected species existing at that location (

Table 7. The number of NPDs eligible for AST conversion in 2024 for each species type. If a site has more than one species, they are counted once for each species.

Species	NPD Count
Sturgeon species	2
Ocean salmonid species	6
No fish recorded	10
Ocean clupeid species	12
Ocean eel/lamprey species	16
Inland sturgeon/paddlefish species	25
Inland salmonid species	63
Other inland species	53

). This means that all sites potentially require special considerations for NPD conversion due to the impact on sensitive ecological areas. The analysis of fish species revealed the presence of multiple species spread across potential NPD conversion sites (Table 7 and Figure 5).

For this analysis, biodiversity land status was considered, and it has two designations: on protected land, not on protected land. This was evaluated on a year-by-year basis, and most NPD sites were found to not be located on proetid lands (

Table 8. Biodiversity land status of viable NPD conversion sites using AST technology across the three studied years: 2024, 2037, and 2050.

Biodiversity Land Status	2024	2037	2050
Dam is on protected land	10	10	9
Dam is not on protected land	72	72	72

).

In addition to ecological impacts, the social benefits and impacts of these NPD conversions were assessed. The median number of daily fishing trips across all viable NPD sites in 2024 was 6924, while the average number of whitewater paddling sites was 0.49. Figure 6 shows population density by the size and color of the circles representing NPDs, along with a heat map for the number of daily fishing trips per year and the number of whitewater access points for viable NPD sites in 2024.

4. Discussion

After filtering over 80,000 NPDs based on mechanical constraints, we identified 82 dams that could feasibly support AST conversions. This approach provides a wholistic framework that can be applied beyond AST compatible sites when considering other NPD conversion technologies. Underscoring the necessity to account for multiple conditions when considering NPD conversions informs sustainable policy decisions. Previous studies of NPD conversion focused on the comparison of turbine types for use during conversions and considered the economic or environmental reasons to pursue each [10]. Our study differed from previous studies by considering only ASTs for NPD conversion, enabling us to consider a wider range of factors that could impact decisions to install a turbine at an NPD site. These factors included future hydrological conditions, aquatic species impacts, mechanical feasibility, economics, and social impacts. Our analysis indicated significant potential for converting existing NPDs across the contiguous U.S. into hydroelectric facilities using AST systems.

4.1. Climate Impacts on NPD Conversions

Conventional hydropower is a site-specific energy generation source because it must be developed on a river where favorable conditions are met. Our precipitation projections account for regional hydrological changes in the years 2024, 2037, and 2050, ensuring the selected potential NPD sites remain viable under future climate conditions. While these changing precipitation patterns reduced the number of quality sites by one from 2024 to 2050 over time, the potential generation capacity increases, reaching 3335 kWacross 81 sites in 2050. This represents a 141% capacity increase from the 2024 site capacity of 2369 kW across 82 sites. This decrease in number of sites is not substantial but indicates that the future viability of sites must be considered both in terms of their potential as hydropower plants and their capacity for expansion. The decrease in viable sites could be an indicator that climate change is narrowing the geographic range where hydropower generation remains feasible. Highlighting this shrinking availability of viable hydropower locations is crucial, as future research could investigate expanding transmission infrastructure to ensure efficient electricity transfer from high-output hubs to areas with lower production. Future work could use climate models in place of historical trends to predict precipitation changes under a gradient of model scenarios and would provide additional information on different climate scenarios.

4.2. Economics

Analyzing the regional data reveals significant variations in capacity, cost, and ROI for potential NPD conversion sites. Specifically, considering the five states with the most NPD sites viable for conversions with ASTs (Wisconsin, Michigan, Maine, Montana, and Utah; Table 5), Wisconsin has the smallest average capacity among the studied regions (24.8 kW) and the highest average ROI at 11.9 years. Conversely, Maine has the lowest average ROI at 7.39 years and the largest average capacity (72.52 kW). This implies factors of scale are important for driving down the ROI. Maine also has the lowest USD/kW construction cost along with Michigan (USD 4243) and the second highest mean capacity of these sites (68.89 kW). These observations about Maine’s viable AST development suggest that the cost of construction per kW and capacity could contribute to ROI.

A regional pattern in the Midwest emerges when observing Figure 2. States in the Midwest, including Wisconsin and Michigan, have lower installation costs than other areas of the country. These states are often characterized as having colder climates, and ice freezing on rivers could severely impact flow rates and seasonal operations that are not considered in this analysis. Future works could consider river ice freeze–thaw cycles and their impact on small hydropower generation. This could prove highly impactful for optimal hydropower operations in colder areas.

The findings emphasize the need for state-specific strategies in hydropower development and installation cost considerations alongside capacity generation potential because, as we observed, the states with the largest average generation do not always have the lowest average ROI. Additionally, other climate change-related factors, such as the impact of river ice, could have implications for the viability of the NPD sites considered in this analysis for regions in the U.S. like the Midwest and Northeast. Future research on the economics of NPD conversions with ASTs could investigate regulatory frameworks, ice formation, and the impact of climate change on seasonal operations to gain further knowledge of state- and region-specific development considerations.

4.3. Social Implications of NPD Conversions with ASTs

The social impact analysis considered impacts to local populations by quantifying the number of daily fishing trips per year, and number of whitewater access sites near each potential NPD site, offering a lens into the recreational value and usage patterns associated with these locations. The average of 0.49 whitewater access points and 6924 annual fishing tips per viable NPD conversion site in 2024 highlights the intensity of recreational engagement and the diversity of how communities interact with these natural resources. Consideration of the impact NPD development could have on recreation is crucial to the projects’ public acceptance and can weigh heavily if the project is approved for construction [32,33]. Sites with higher levels of recreational use might face strong public resistance to development due to perceived impacts to valued activities or economic benefits from outdoor recreation. Conversely, sites with fewer recreational activities may present a better opportunity for development because of the minimal disruption and greater public acceptance.

When comparing the maps in Figure 6, there appears to be a strong correlation between areas of high population density and the frequency of daily fishing trips compared to whitewater access points. Notable population centers, such as the New England area near Maine and Massachusetts, exhibit this pattern. These observations support the notion that daily fishing trips are a broadly accessible activity requiring only a water source, whereas whitewater rafting is inherently more site-specific, limited to locations with suitable river conditions. Future research could include identifying the specific types of fish being targeted by fishing in the area and assessing the impact that installing an AST would have on those populations. This could provide a greater understanding of the ecological, economic, and recreational impacts of an NPD conversion.

Given the dynamics of the analyzed recreational, social, and population density factors, our findings suggest strategically placing NPD conversions with ASTs can leverage proximity to population centers as an indicator of recreational fishing demand. Each site’s cultural significance and impact on recreational activities should also be considered. This approach aligns with the broader goal of working to ensure that projects are both socially and environmentally sustainable. Future work could prioritize sites in low-income communities, where residents often allocate up to 30% of their income to energy costs [34], to address historic energy injustices by providing localized, affordable energy generation. By integrating considerations of population density, this approach aligns with broader efforts to maximize social benefits when considering the mechanical feasibility of NPD conversion.

4.4. Environmental Implications of NPD Conversions with ASTs

Dam removal may be a preferred alternative for protecting and restoring aquatic biodiversity and riverine ecosystems; removal is not always possible, particularly in cases where a dam is providing a barrier to the spread of invasive species or is holding back contaminated sediment (e.g., Polychorinated biphenyl (PCB) contamination in Kalamazoo River, Michigan, although dams will be removed eventually when sediment abatement is complete) [35,36,37]. When considering sensitive wildlife habitats and the preservation of protected lands, ASTs are more fish friendly than other turbine types; however, their suitability varies depending on the size of the fish. For smaller fish species, ASTs could facilitate safe passage in previously severed river reaches, but larger species might face the challenge of not fitting between screw blades which could complicate permit processes and increase project timelines [38]. Archimedes screw turbines can be designed for their specific location by adjusting the distance between the screw blades and rotational speed. This suggests that screws can be fitted to allow the passage of fish of varying sizes; however, one study found no correlation between fish size and mortality rates [39]. The potential for fish strikes and injury from using ASTs exists, but it is generally lower than other technologies and fish passage studies are likely to be required in permit and licensing processes for hydropower NPD conversion sites.

When considering biodiversity land status (Table 8) across all years and potential NPD conversion sites, 87% of sites are not on protected land across all years. The designation of protected vs. non-protected land can indicate the challenges a development may face at each site. Designations are from the protected areas database of the U.S. which considers biological diversity, natural, cultural, recreation, and historical significance when giving an area a designation. This is important to note when considering conversion sites because suppressed disturbance events could mean no retrofitting of NPDs can occur at this location. Future work could consider these designations in more detail and incorporate other designations like impairments. Impairment designations for connected water bodies introduce further scrutiny; the assessment of factors causing degraded conditions, like nutrient loading, sedimentation, or thermal pollution, could inform the required impact-prevention measures.

There are potential benefits of additional FERC oversite at AST-converted sites. These benefits could include increased environmental monitoring and may lead to the installation of fish passages at previously severed river sections. This analysis considers both fish species presence and protected land status, but future research could conduct a trade-off analysis between the impacts of FERC jurisdiction, dam removal, and taking no action.

The presence of protected fish species and protected land status underscores the need for the early engagement of involved parties for ecological assessments, mitigation of risks, and ensuring compliance with environmental regulations. Early engagement with the parties involved has also been shown to increase public and political acceptance of the project [32]. Additionally, adding generation to these locations may bring them under FERC jurisdiction, which may include the need of fish passages and additional assessments, including an environmental impact assessment.

4.5. Comparison to Other Turbines

The development of small hydropower systems has several benefits over solar and wind systems such as higher efficiency and more consistent power supply [40]. Additionally, small hydropower projects (those below 10,000 kW in size) can apply for certain licensing requirement waivers [41]. While ASTs are often considered novel turbines when compared to other types such as Kaplan and Francis turbines, which allow for greater efficiency (up to 95%) and are widely used, ASTs excel in their environmental compatibility, particularly concerning fish passage [42].

ASTs often receive a higher fish-friendly ranking, with studies showing higher survival rates for species like juvenile salmon and eels compared to Kaplan turbines [43,44,45]. Specific studies considering the use of ASTs as a mode of fish passage found that less than 1% of fish had effects from using this mode of passage [46]. However, the trade-off of using an AST is its reduced ability to operate under conditions beyond low flow and low head, which can limit its energy-generation capacity compared to other turbines [11]. Additionally, the existing infrastructure and global expertise available for installing Kaplan and Francis turbines make them more accessible in certain contexts [42,47].

Ultimately, each turbine technology has trade-offs to consider. ASTs clearly offer environmental benefits and are suitable for low-head applications, while Kaplan and Francis turbines enable greater energy conversion efficiencies and their potential power output is greater, but they do not have the benefit of low-head application. Additionally, there is more existing infrastructure and expertise for installing the mainstream Kaplan and Francis turbines globally. Understanding each site’s conditions and regulations is paramount to determining the correct conversion type.

4.6. Broader Impacts

Our analysis identified 82 NPD sites in 2024 and 81 NPD sites in 2050 that met the mechanical criteria for AST deployment across the CONUS. This study demonstrated that AST systems can match well with existing dam infrastructure across an array of locations to unlock flexible, low-impact hydropower. Additionally, layering of precipitation forecasting provides added confidence in long-term generation potential amidst climate shifts. This approach reveals a decrease in viable sites but an increase in average capacity at the remaining sites [48,49]. These findings underscore the dynamic nature of hydropower potential in a changing climate and the importance of incorporating future projections to assess feasibility. Future efforts could implement similar layered filtering techniques at state and utility levels, incorporating factors such as drought tolerance, seasonal flow fluctuations, and flood control to prioritize sites capable of consistent power generation.

It is important to acknowledge the limitations inherent in this high-level resource assessment. One such limitation is that upstream fish passage requires the reversal of the AST turbine; in this study we do not account for downtime to allow for upstream fish passage. Additional site-specific ecological details that can significantly impact project feasibility, especially for original licenses, should be specifically denoted in future work by focusing on sites with no endangered species, fish passage requirements, and ultimately low environmental complexity. While this study considers the presence of endangered fish species and presence of protected lands, future work could also account for the impact on endangered terrestrial species and whether AST technology is suitable for fish passage at each site.

Additional considerations related to tribal and culturally significant lands introduce an additional analytical layer of complexity for NPD conversions. Non-powered dams located on tribal lands are subject to tribal regulations, which may be different from federal or state processes, as tribes are sovereign nations. Many areas of cultural importance are confidential to prevent desecration, and cultural importance is often difficult to discern on a map. For many tribes, natural resources such as fish, animals, plants, and water hold significance, creating the need for additional NPD conversion considerations [50,51,52,53].

Like tribal lands, dams located within National Parks or Forest Service land, where jurisdictional complexities could pose additional regulatory considerations, highlight the need for future studies to incorporate all the involved parties to ensure that the project considers environmental and cultural impacts on the same level as technical practicalities. These considerations have been central to FERC decision-making in recent years and are equally relevant for AST projects [54,55].

5. Conclusions

This research meets a critical need for responsible strategies to transition aging infrastructure into distributed, low-impact, clean energy resources. Frameworks and findings contribute to policy, planning, and project decisions that could shape sustainable electricity production for local communities and on a national scale. NPD conversions, utilizing AST installations, are shown to be achievable and have the potential to supply another generation asset for a decarbonized grid. This study considers viable sites in 2024, 2037, and 2050, and while the number of viable sites decreases by one during this time, the mean capacity of the remaining sites increases by 13 kW. The cost of installation was estimated to be USD 4286.05/kW; with the increase in site capacity, the mean cost of installation increased by USD 52,600 (these are estimated in 2020 USD).

All mechanically viable NPD sites had the presence of at least one protected fish species, signaling the need for carful environmental impact assessments and potential mitigation strategies, highlighting the critical importance of a holistic site selection and technical potential assessment. Focusing solely on technical feasibility can overlook crucial environmental and social considerations that may significantly impact project viability, local environments, and local economics. This study provides a layered and quantitative methodology for evaluating low-head, NPD sites across the CONUS for conversion to power production through the installation of ASTs. Integrating publicly available environmental, social, and engineering data initially across all NPDs limits the mechanically possible facilities to over 80 sites in 2024. This analysis reveals the potential to expand renewable energy production through the responsible retrofitting of existing dam infrastructure.

Careful site prioritization and selection means that the most sensitive ecological regions and habitats can be avoided. Similarly, focusing on lower-profile locations limits impacts on electricity rates or recreational access for local communities. With precipitation modeling decreasing the number of available sites only marginally as projected to 2050, ASTs align well with local dam contexts and changing regional climates to maximize renewable generation.