INTRODUCTION

The 2015 Nepal earthquake was estimated to have damaged 262 micro-hydropower plants (MHPs, plants with power outputs < 100 kW) in the districts surrounding the earthquake’s epicentre, impacting over 37,000 households [1]. Small-scale natural disasters are also prevalent in Nepal, such as landslides, flooding and avalanches, and these can often affect MHPs due to their location in the base of valleys near water courses. Even without extreme events, commissioned sites often do not last as long as the 50 years that could be expected. Sites can fail for many reasons but generally due to poor maintenance [2]. The impact of a failed community MHP can be severe, impacting lifestyle, health and income generation [3]. These failures provide an opportunity to renovate, refurbish and upgrade hydropower sites, using updated designs and technology to provide an improved service to the community [4].

Around the world, there is significant activity in the renovation and refurbishment of existing hydro sites. In Europe, some 350,000 small and micro-hydropower sites (including historic mills) are estimated to be abandoned, with over 24,000 restored recently, some adopting new turbine technologies [5]. Upgrading existing watermills with a generator for electricity generation has been a successful strategy in some Asian countries [6]. In Japan, the potential is being reassessed to include less conventional sites from existing infrastructure facilities, such as dams, weirs, irrigation channels, water supply and sewerage systems, to avoid environmental impacts [7].

Whilst many guidelines exist for the development of small-scale hydropower plants [8,9], there are limited resources regarding the renovation of plants. Documents such as a European Union commissioned handbook that offers guidelines for the renovation process [5] and a World Bank report [10] that discusses important economic technical issues, are the exceptions here. Within academia, several studies have focused on various considerations related to renovation. It has been found that it is difficult and expensive to extend an existing system and often, the whole system has to be re‐built to improve power output [11]. In Croatia, Bojic et al. [12] considered that the refurbishment option for a small hydropower plant, was the most favourable option across a number of technical and economic indicators, such as improved reliability, and increased electricity production with the lowest investment cost. Quaranta et al. [13] presented a techno-economic analysis to aid users considering repowering vertical axis water mills. Across all these sources, community ownership, a key aspect present in Nepal, is not considered.

Previous work has explored some of the challenges associated with the development and operation of community owned micro-hydropower projects. As a site’s geographical features affect seasonal waterflow and frequency of landslides and flooding, design and construction of civil structures is important. In Nepal, poor design and low quality of construction have been reported [14], and whilst installation companies are responsible for directing communities in construction, but often the results are poor [15]. At the operational stage, training of maintenance staff and finances have been identified as important factors affecting economic sustainability of plants [16]. In particular, the availability of trained operators for plants is a universal problem and was identified 30 years ago by Faruqui [17]. The remote location of sites has an impact on many of these issues, reducing the availability of trained operators, affecting the provision of spare parts, and reducing the opportunity for income generation. In plant renovation, these pre-existing challenges should not be overlooked.

Regardless of ownership status, when considering sites for renovation, it is important to consider multiple technology options and determine the most viable, particularly for the turbine (and the complete electromechanical sub-system). A comparison of key indicators such as return on, and amount of, capital investment cost, and predicted power output (and efficiency) against the previous installation is useful for decision making [18]. Essentially, these indicators provide both economic and technical drivers for final selection. In general, despite some efforts [19], cost modelling of civil works is challenging. For electro-mechanical sub-systems, however, the cost of component replacement can be more accurately estimated meaning the benefits of higher efficiency and reliability can be evaluated. Therefore, an important choice is the turbine and generator set, as it not only represents a large cost commitment [20–22], but also is crucial in determining the potential performance at the existing site (see Figure 1). Costing approaches have been developed for many different turbine types: Pelton [18,21,23], Francis and Kaplan [21,22], Crossflow [18] and Turgo [13]. Other important considerations will also need to be made, such as reliability of flow, nearby utilisation of generated electricity, environmental factors and water quality [24].

FIGURE 1

Figure 1. Median cost contribution to micro-hydro power installations. Civil works include the penstock cost. Adapted from references [18,21,22,25].

The option of renovation and repowering a site can enable new turbine designs to be installed to improve the performance. In the Nepali context for micro-hydropower turbines, historically there have been only two designs that are locally manufactured, that of the Crossflow and the Pelton, both introduced by international development agencies [26]. However, over the past ten years, three additional designs have been developed that can be manufactured locally in Nepal: a propeller turbine [27], Francis turbine [28] and Turgo turbine [16]. The Turgo turbine is especially interesting, as it can easily be retrofitted into existing infrastructure with minimal adaptation for both Pelton and Crossflow sites. Furthermore, it can bridge between the typical operational ranges (e.g., head and flow rate limits) of the Pelton and Crossflow.

In this paper, the use of a Turgo turbine will be investigated for the renovation an existing micro hydropower installation site in Nepal. A methodology is developed for assessing the viability of the renovation, and the process of renovation and its outcomes are presented. The efficacy of the methodology and the potential for the implementation of the Turgo turbine elsewhere are also discussed.

TURGO TRUBINE

In Nepal and other countries in the Global South with significant hydropower resources, local companies produce hydro-turbines at the micro-scale. Typically, these companies manufacture Pelton turbines to serve sites with high head, and Crossflow turbines for sites with low head. Whilst these turbines can accommodate many different sites, an extended range of designs could provide advantageous features and cost benefits.

Using data from existing sites in Nepal from [29] and data collated by the authors, Figure 2 shows the flow rate and head for a selection of Crossflow and Pelton sites. The site characteristics for the renovated site are also included. The general trend is that Crossflow turbines are used at sites with head less than 50 m, and Pelton turbines for those with a higher head. By applying several constraints, it is possible to identify the region with greatest uncertainty in turbine selection. That is, a region where for the same site characteristics, either turbine could be selected. The region is generated between horizontal lines passing through points corresponding to the lowest head for a Pelton site and highest head for a Crossflow site, and between vertical lines corresponding to the lowest flow rate for a Pelton site and highest flow rate for a Crossflow site. Consequently, the region represents the combination of head and flow rate where, based on the site data, there is no substantial preference for either turbine type. It should be noted that the dataset is limited in size. From the literature, it is suggested that at the interface between the turbine types, Peltons are large (i.e., expensive), and Crossflows are narrow (i.e., difficult to manufacture) and inefficient [30]. For sites at the interface, the Turgo turbine could provide a viable alternative. The Turgo turbine operates similarly to a Pelton turbine, the change of momentum of a jet interacting with a blade is used to generate torque. The difference is that the jet enters and exits on opposites sides of a runner (as shown in Figure 3a). An isometric drawing of a Turgo turbine runner is shown in Figure 3b.

FIGURE 2

Figure 2. Flow rate vs. head for Pelton and Crossflow installations in Nepal. Adapted from reference [29].

A standardised design for a small-scale Turgo turbine has been the focus of research by the authors for the last decade [27]. The intention was the design would be scalable, open source and, to a large extent, manufacturable by local companies, and this was found to be the case as reported here. Other studies suggest that the manufacturing technology is available in Nepal to adequately produce Francis turbines too [28]. Increased local manufacture has the potential to promote sustainable supply chains and system reliability through access to skills and spare parts.

FIGURE 3

Figure 3. (a) Diagram showing Turgo turbine operation adapted from [31] and (b) isometric drawing of a Turgo runner including dimensions (in mm) for the installation described in this paper, PCD indicates the pitch circle diameter.

METHODOLOGY

In order to demonstrate that the Turgo turbine could be used to renovate a MHP, it was necessary to find a feasible site. In this case, the intention was to identify a site that would be appropriate for the Turgo turbine. Despite this, the methodology presented here can still be applied at any existing micro-hydropower plant to determine the feasibility of a site for renovation. Figure 4 summarises the methodology. As indicated in the literature, the development and operation of MHPs is complex, depending on social, technical, environmental, and economic factors alongside legal and governmental considerations. The methodology considered these five areas. An initial phase involved data collection to address key questions relating to the characteristics of the site, its present condition and information that would help determine feasibility. The information collected could then be used to complete a series of actions, shown in the green boxes, that would ultimately determine whether a site was feasible for renovation.

It involved a phase of data collection followed by preliminary design and costing. The yellow boxes indicate the key questions to be answered that determined the site’s feasibility and its characteristics. The information collected could then be used to complete the subsequent actions.

FIGURE 4

Figure 4. Methodology for site renovation feasibility.

At the final stage, if the project was deemed feasible by the community, typical steps of the project cycle could be followed, e.g., detailed design, manufacture, community mobilisation for construction, testing and commissioning, as described in [15,32]. In the following sections, the data collection, and design and costing phases for the project are described.

A potential site for renovation called Mangmaya Khola I MHP (referred to here as Mangmaya), was identified in Sangu, Taplejung district. The site was identified by Nepal Yantra Shala Energy (NYSE, a project partner and turbine manufacturer) who had installed a turbine in the local area. Table 1 lists important features of the MHP.

TABLE 1

Table 1. Key details of Mangmaya Khola I MHP.

Over time, the power output of the plant had reduced to approximately 18 kW. It was observed that the existing turbine was leaking which contributes to reduced power output. The turbine was not disassembled but it can be assumed that due to the length of time in operation, the Crossflow turbine blades will have been eroded. This contributes to decreased power output due to deviation of the blades from their original form. The inside of the powerhouse including key components is shown in Figure 5.

To establish site feasibility, staff from the People, Energy and Environment Development Association (PEEDA, a project partner and non-governmental organisation) were sent to conduct the data collection methods outlined in Table 2. Using these methods, data was collected that allowed the key questions to be answered. A summary of the collected data is shown in Table 3.

FIGURE 5

Figure 5. Inside of the powerhouse at Mangmaya Khola I including turbine (1), inlet valve (2), belt drive transmission (3), generator (4) and control panel (5).

TABLE 2

Table 2. Methods for collecting data.

TABLE 3

Table 3. Summary of collected data from Mangmaya Khola MHP.

As explained in [9], the primary considerations for turbine selection are head and flow rate. The performance requirement under part-flow conditions and the operational speed of the turbine are also factors that require consideration in the selection process, particularly at sites where more than one turbine could be implemented. As demonstrated in Figure 2, in the context of Nepal, particular combinations of head and flow rate may result in the selection of different turbines. As a result, as part of renovation, it should not be assumed that a turbine with the same dimensions should be installed. Furthermore, environmental (e.g., reduced rainfall) or social changes (e.g., alternative water use) may have changed the volume of water available.

As the objective of this project was to install a Turgo turbine, the selection was focused on assessing the feasibility of the Turgo turbine alone. In other cases, it is likely that turbine manufacturers will employ their own methods for determining what is the most appropriate turbine to install at any given site. Examples of selection methods can be found in [9] and, specific to Nepal in [34]. Previous work by the authors has focused on the design and optimisation of a Turgo turbine runner for manufacture in Nepal. This has included experimental testing of a small-scale 1 kW prototype [35], and subsequent optimisation using computational fluid dynamics (CFD) resulting in an improvement in efficiency from 69% to 81% [36]. The resulting optimised design could then be scaled using non-dimensional numbers derived from the outputs of computational modelling. Site parameters could then be used to determine whether a Turgo turbine could be installed. For hydropower turbines, commonly used dimensionless groups are the flow coefficient (C_Q), head coefficient (C_H), power coefficient (C_P), and the specific speed (C_ω), which is derived from the power and head coefficients.

where, Q is the flow rate in m³/s, ω is the rotational speed in rad/s, D is a length dimension in m (typically the pitch circle diameter (PCD), the tangential diameter at which the centre of the jet impacts the runner [37]), g is the acceleration due to gravity in m/s², and H is the head in m. Specific speed is a dimensionless parameter associated with the maximum efficiency of dimensionally similar turbines and is commonly used in scaling [38]. Using the head and flow rate of the site, it was possible to calculate that a Turgo turbine could be scaled from the existing size to a PCD of 0.225 m and expected rotational speed of 1270 rpm. Based on [37], it was decided that the chosen design could be used in a direct drive configuration operating at 15% less than a rated generator speed of 1500 rpm.

The design of all sub-systems impacts upon the plant’s performance; both efficiency and reliability are affected by the scheme’s design. In Nepal, there is evidence from multiple sources of poorly designed or constructed civil structures, e.g., de-silting bays and canals [15,27]. When defective, these sub-systems are likely to increase the volume of debris that passes through the civil works and the turbine, resulting in more erosion. In general, renovation can provide an opportunity to identify and address issues with the plant with the objective of improving overall energy production [11]. Even where considerable design changes (other than the turbine) are not required, the enforced system downtime can allow for maintenance activities that support improved energy generation.

Based on the site assessment, several site improvements were identified to improve the plant’s reliability. These improvements were:

In addition to these improvements, based on the site assessment and the interviews, it was decided that the powerhouse and penstock could be adapted to allow the new turbine to be installed alongside the existing turbine. The motivation for this was to allow the existing turbine to be used if the new turbine faced any issues during its testing and commissioning. Measurements were taken at the powerhouse to confirm that this adaptation would be possible.

For energy projects, in general, cost is a crucial driver when assessing project feasibility. When deciding to refurbish a project, accurate prediction allows the cost of refurbishment to be compared with the installation of alternative energy technologies. For hydropower, a number of different costing methods have been developed for different turbine types. In general, the cost of hydro-mechanical and electrical components is more predictable than for civil structures, which can vary significantly depending on local geography [22]. Consequently, where refurbishment focuses on hydro-mechanical or electrical components alone, cost estimation can be performed based on limited information.

Similar methods for costing for hydropower (at a range of sizes) have been demonstrated in [18,21,22]. For the Mangmaya site, where the renovation included the turbine, transmission system, generator, and butterfly valve, the costing could be estimated using the method and data from [18]. A regression analysis was used to predict the total cost of these sub-systems based on data from 28 quotations from micro-hydropower manufacturers in Nepal. The resulting expression related the site rated power to the expected cost. Unlike the expressions presented in [18] and [21], it was found that due to a p-value of greater than 0.05 for the head, the cost could be estimated with reasonable accuracy using the site rated power alone. The resulting expression (where P is site rated power in kW) was in United States Dollar (USD):

The expression is the estimated purchase cost for the turbine, transmission system, generator and butterfly valve. It was derived from quotations for Pelton and Crossflow turbines and thus provided a reasonable benchmark for the equipment cost in Nepal, rather than a direct estimation for a Turgo turbine. The data was collected in 2019 and the average exchange rate for 2019 of 1 NPR (Nepali Rupee) = 0.0088 USD was applied [39]. For the assumed rated power of 30 kW, the estimated cost was $9527.

The mechanism for approval of hydropower projects varies between countries, however, projects generally must meet certain legislative and governmental requirements. In Nepal, at the micro-hydropower scale, the process of subsidy application ensures that new projects meet with the expected requirements [15]. The process ensures that projects have a certificate of registration for water use from the District Water Resources Committee. The community must also register as a Micro-Hydropower Functional Group, cooperative or a private company. During the subsidy application process, local governmental approval is required, and to be awarded the subsidy funding depends on approval from the Alternative Energy Promotion Centre, the national government agency focused on renewable energy. For the refurbishment of micro-hydropower projects in Nepal, a subsidy is also available. For the site at Mangmaya, a subsidy was not required. Regardless, approval was sought from local government for the project. Prior to a final community decision, an agreement between the committee, PEEDA and NYSE was signed.

Community owned renewable energy projects have proven success in providing electricity and providing developmental benefits. It has also been emphasised that the sustainability of these projects depends on creating responsibility for them amongst the users, managers and local staff [40,41]. Whilst achieving this can be challenging, financial contributions, labour and participation in decision making by local residents have all been identified as supportive activities. In the case of Mangmaya, a committee for the MHP had been formed during the development of the existing project. PEEDA’s site visit had ascertained that there was evidence of reasonable engagement in the MHP: attendance of committee members at the meeting, payment of tariffs and the results of interviews were all informative. To achieve interest and develop ownership in the renovated project, the community was involved in the decision-making process. Following all other stages of data collection, design and costing, the complete proposal for the project renovation was presented to the community. Based on the information provided, the committee decided, as a collective, to proceed with the renovation.

RESULTS

The development of the design for the Mangmaya turbine took place collaboratively between the first author and NYSE. Based on an existing design that was optimised using computational fluid dynamics [29] and manufactured and tested in Nepal, the turbine runner was scaled up. A full package of engineering drawings was developed for the scaled runner and all other components of the turbine. The design for the turbine was developed such that it was appropriate for manufacture in Nepal. For example, the blades of the runner were designed for sand casting (see Figure 6a,b) with the mould developed from 3D printed parts. The individual blades were then fabricated together to develop the complete runner, as shown in Figure 6c,d. All components including the turbine casing, inlet pipework and spear valve assembly (see Figure 6e,f) were manufactured at NYSE. The cost of the installed components was NPR 930,000, approximately $7812 using an average exchange rate for the year of manufacture (in 2021, 1 NPR = $0.0084) [39].

FIGURE 6

Figure 6. Components of the Turgo turbine during manufacture, (a) leading edge of the sand cast runner blade, (b) trailing edge of the sand cast runner blade, (c) turbine runner during fabrication, (d) turbine runner after machining, (e) turbine casing, runner and inlet pipe, (f) turbine with cover on.

To prepare the powerhouse for the installation of the new turbine, it was necessary to dig a new tailrace channel. Subsequently, the turbine base frame could be anchored and concreted into position (see Figure 7a). The y-section of penstock pipe was installed allowing the flow to be directed to either of the turbines (see Figure 7b). This adaption was intended to minimise disruption to the local community; when the new turbine was not operational, the old turbine could be used to generate power. It required a new hole to be made in the wall of the powerhouse to accommodate the new pipework. A direct replacement of the turbine would have required less work. Figure 7c shows the Turgo turbine installed in the powerhouse. Site works were completed by NYSE with support from community members. During these works, NYSE staff provided clear directions to community members to ensure the quality of work completed. The installation took place during the COVID-19 pandemic. As a result, there were some delays as the project team responded to locally imposed restrictions.

FIGURE 7

Figure 7. Site works for the renovation, (a) installed y-section of penstock pipe, (b) cementing of new turbine frame, (c) installed Turgo turbine in the powerhouse.

Upon completion of the installation, representatives of the Turbine Testing Laboratory at the Kathmandu University conducted field testing at the site. During testing, the flow rate was varied using the turbine’s spear valve. The static head of the system remained approximately constant, however, there may have been some small variations in the forebay tank. The measurement of head was taken on the inlet pipe section directly preceding the nozzle. The rotational speed of the turbine was controlled by an electronic load controller (ELC). Using the electrical frequency, the ELC adjusts the load to maintain the rotational speed of the turbine at the generator’s rated speed [37], in this case 1500 rpm. Table 4 lists the measurands and the equipment used.

TABLE 4

Table 4. Measurands and equipment used in field testing.

The power and water-to-wire efficiency were calculated from the product of the measured current and voltage in each phase, and then compared to the power available at the nozzle of the turbine using the measured head at the turbine inlet. The measured current in one phase of the electric generator was found to be unreliable during post-processing of the data, so the average power in the other two phases was scaled to calculate the expected power output from the turbine. Using the measured data, the power input and power output (both with units of watts), and the efficiency were calculated using the following expressions:

where most terms and units are defined in Table 4, and ρ, the density of water is assumed to be 1000 kg/m³, and g, the acceleration due to gravity is assumed to be 9.81 m/s². It should be noted that the expression for POUT uses only two of the three phases of the generator; the third phase is assumed to be an average of the other two, resulting in the coefficient of 1.5 in Equation 7. The manufacturer’s stated accuracy for measuring equipment can be used as an approximation of the systematic uncertainty. Using the method described in [45], the total systematic uncertainty can be approximated as the square root of the sum of the squares of the individual uncertainties. Therefore, the total systematic uncertainty for the efficiency is equal to 3.8%. It should be noted that the uncertainty in the measurement of the power factor (read from the system’s control panel) is unknown. The measured output power and efficiency of the turbine are shown in Figure 8 across a range of flow rates. On the graph, 2 data points are considered to be anomalous due to current readings that are substantially higher than the trend, and so have not been included.

For increasing flow rate, the head loss at the turbine nozzle increases. This occurs due to increased friction losses caused by the increase in the velocity of the flow through the penstock [46]. The head loss at the turbine nozzle and the normalised flow rate can be calculated using the following expression:

where, H_L is the head loss, H_max is the measured head at the lowest flow rate and H is the head (all with units of m), and Q_max and Q are the maximum measured flow rate and the flow rate respectively (both with units of m³/s). The head loss at the turbine nozzle, compared to the maximum measured head in the system, can be seen in Figure 9. This loss in head in the penstock causes a reduction in the overall system efficiency, and increases quadratically with the flow rate, causing the efficiency of the overall system to drop off at higher flow rates, as is seen in Figure 8.

FIGURE 8

Figure 8. Expected output power and efficiency vs. flow rate.

FIGURE 9

Figure 9. Normalised flow rate vs. head loss at the turbine nozzle exit compared to the maximum head measured.

DISCUSSION

The discussion section will focus on three elements. Firstly, the outcomes of the implementation at Mangmaya and the extent to which the methodology was effective. Second, the lessons learnt from applying the methodology and its potential for application elsewhere. Third, the opportunity to use the Turgo turbine both in Nepal and elsewhere.

From a technical perspective, the project resulted in the installation and commissioning of the Turgo turbine at Mangmaya Khola I MHP. The installed turbine provided increased power output (32 kW) in comparison to the existing turbine’s recent power output (18 kW) and its original rated power (27 kW). The increase in power generated from the system is due to the increased efficiency of the Turgo system over the Crossflow at the rated conditions. The water-to-wire efficiency of the Turgo of approximately 70% for flow rates between approximately 25 and 90 l/s (22% and 80% of Q_max respectively) is comparable with typical micro-hydropower installations in Nepal [47]. Above 90 l/s, the efficiency dropped. Field based testing is inherently difficult and subject to variabilities and as mentioned in the results section, the current results for one phase of the generator were unreliable. Alongside this, the results suggest that the variation in rotational speed may have also affected the system’s efficiency. When performing turbine scaling, it was assumed that a small difference (~15%) between turbine operational speed and rotational speed of the generator would be acceptable (e.g., reducing overall efficiency by several percent). Due to instability in the ELC, there was some variation in the rotational speed of the turbine. As the flow rate was increased from the lowest to the highest value (as shown in Figure 8), the rotational speed increased from 1385 up to 1493 rpm. Therefore, as more water flowed through the turbine, the rotational speed increased linearly in relation to increasing power output. At higher output powers, as the control system brought the rotational speed closer to 1500 rpm, the turbine was operating further from its design speed at 1270 rpm. This difference will have contributed to reduced efficiency at higher flow rates, along with the increased loss in the penstock. Considering the turbine’s fluid mechanics, variation of the rotational speed means it will operate away from its ideal velocity ratio (the ratio between rotational speed and jet velocity). Also, when the turbine operates with the highest flow rates, there is greater potential for turbine performance to be affected by splashing and secondary flows. The runner design was scaled directly based on the optimised CFD design [38]. This direct scaling was intended to make use of all available head and flow rate, without altering the ratio between PCD and the blade’s dimensions. Further computational work is required to explore varying this ratio so that, where appropriate, the PCD could be directly matched to the rated speed of the generator.

From a social and economic perspective, the project was implemented using the funding from EnergizeNepal and with contributions of labour from beneficiaries. The provision of funding meant that a local financial contribution was not required. Consequently, the community were keen to proceed with the project. Without grant funding, the community would have been responsible for the complete cost, perhaps with support from the Alternative Energy Promotion Centre and local government. Ideally, income from plant tariffs can be saved for the costs of repair and maintenance. In practice, challenges associated with tariff collection means that this does not happen universally [32]. Whilst the focus of the project team was the installation of the turbine, as part of the project implementation, a number of suggestions were made to the management committee relating to financial management. It was emphasised to the community that the frequency of tariff collection should be increased, income and expenditure should be recorded, and regular meetings of the management committee should be held. The costing method predicted a cost for the electro-mechanical components of $9527. In practice, the actual cost was lower at $7812. Due to the relatively small dataset (n = 28) used to predict cost and the one-off nature of the Turgo turbine construction, no major conclusions regarding the cost of the Turgo turbine can be drawn. However, the relatively small difference (~20%) suggests that the costing method offered a reasonable estimate. The expression for estimating renovation cost (Equation 5) should be validated by comparison with other hydropower site renovations in Nepal.

The methodology was successful in identifying the information required regarding the renovation. It depended on a site visit to assess the existing system and interview key stakeholders. The presence on site is useful for developing a tacit understanding and acceptance of a project, particularly when it is community owned and operated. The information collected under the environmental heading was done by PEEDA’s employees. Whilst familiar with micro-hydropower and qualified engineers, they were not geological experts. If the renovation had required significant alterations to the existing civil structures, a redesign of the civil structures may have been required. The information collected was enough to allow the community to decide whether to proceed with the project. As noted previously, however, with the grant substantially covering the cost, the economics were less complex than for other renovation projects with more complex financing arrangements.

The lessons learnt from applying the methodology at Mangmaya can be used to inform applications of the methodology elsewhere. In general, the local knowledge and the experience of PEEDA and NYSE were important. If applied elsewhere, ideally similarly experienced local practitioners would be responsible for data collection. In relation to social, economic, and legal areas, there is greatest scope for variation in the local context and the impact that this might have upon renovation. For example, in a different country, legal requirements for the renovation of a hydropower plant may vary; locally based actors are best placed to support communities. In its current form, the presented methodology makes recommendations for key questions to be answered. In the case of Mangmaya, adapted versions of the methods explained in [34] were used to gather information. The methodology could be developed further to employ more prescriptive methods for collection of specific data. For example, a detailed analysis of the plant’s income and expenditure, or an environmental risk assessment. Such information could be used to conduct more detailed analyses. A cost-benefit analysis would enable comparisons between renovation, construction of a new hydropower plant, and other technology choices (e.g., solar photo-voltaic). An environmental assessment could aid identification of climate proofing measures that could be implemented [48].

The renovation of the Mangmaya was the first documented example of the implementation of a Nepali manufactured micro-hydropower Turgo turbine. As such, alongside an understanding of the applied methodology, the outcomes relating to the use of the Turgo turbine can also be evaluated. Based on the manufacture and testing of a prototype (rated to 1 kW) [35], the turbine design was optimized using CFD [36] and scaled up. The resulting package of engineering drawings was used to manufacture the turbine at NYSE. Implementation at Mangmaya was complicated by the decision to install the new turbine in parallel with the existing one. The requirement to construct a new tail race, and manufacture and install the ‘Y-section’ of pipe were additional steps in the renovation process. As discussed above, the scaling procedure resulted in a difference between the turbine’s rated speed and generator speed. In future, the impact of moving the pitch circle diameter of blades to match the turbine and generator rotational speeds should be explored. Analytical or computational methods could be used to explore the impact of this change. Engineering drawings and supporting tools for the present design are available online [49].

CONCLUSIONS

There are many challenges but also opportunities when considering renovating existing micro-hydropower sites, not just in Nepal, but around the World. These have been reviewed here, with literature sources and the experience of the authors used to develop a high-level methodology to assist in decision making. The potential benefits of renovating existing sites are many, but in terms of increased power outputs, higher efficiencies and plant reliability, the options around utilising modern turbine designs in particular are essential in order to achieve higher performance at investment and operating costs which are economically viable over the plant’s lifetime. The methodology was successful in collecting the information required to carry out the renovation. An open source and scalable Turgo turbine design has been successfully installed and operated in an existing MHP site in Nepal. The increased power from a site has the potential for providing more people in the community with a stable electricity supply. Furthermore, with the potential to generate additional capacity, the current and future demands of the community can be met with potential for further benefits of new income generation opportunities.

Despite its success, the implementation at Mangmaya is currently the only example of this particular Turgo turbine design. Experience, particularly relating to use in other country contexts can be developed through applications of the design. In further developments, it is important to consider where the Turgo turbine can be applied and whether its application region can be expanded. With the existing Turgo design, scaling allows adjustment for different combinations of head and flow rate. The application region identified here is not intended to be exclusively for the Turgo turbine. Turbine selection involves evaluation of key trade-offs (e.g., cost, efficiency, part-flow efficiency) and further work is required to explore these. This understanding could be used to both, identify the best application region for the Turgo turbine and to optimise its design further.

NOTE

After operating for 18 months, the plant was badly affected by a landslide in the local area in July 2023. The landslide severely damaged the penstock and the powerhouse. The Nepali project partners are exploring how the plant can be restored to operation. As mentioned in the paper, the incident highlights the risk that landslides pose to MHPs in Nepal.

DATA AVAILABILITY

The dataset of the study is available from the authors upon reasonable request.

AUTHOR CONTRIBUTIONS

JPB, SW, PK, SP, BP, BG designed the study and implemented the project. JPB, PK, SP, BP, and PS collected the data. JPB, SW, JDB and PS analysed the data. JPB, SW and JDB wrote the paper with input from all authors. Abbreviations: JPB—Joe Butchers, SW—Sam Williamson, JDB—Julian Booker, PK—Prem Bikram Karki, SP—Suman Raj Pradhan, BP—Bikram Raj Pradhan, BG—Biraj Gautam, PS—Prajwal Sapkota.

CONFLICTS OF INTEREST

The authors declare that there is no conflict of interest.

FUNDING

The study was funded by an EPSRC Impact Acceleration Account and through Energize Nepal, project reference number ENEP-RENP-17-01.

ACKNOWLEDGMENTS

This research has taken place due to the efforts of staff and researchers at the People, Energy & Environment Development Association; Nepal Yantra Shala Energy; and the Turbine Testing Laboratory at Kathmandu University. The Hydro Empowerment Network has supported the project throughout.