Assessment of Renewable Energy Potential in Water Supply Systems: A Case Study of Incheon Metropolitan City, Republic of Korea

Abstract

Water supply systems (WSSs) are energy-intensive infrastructure that present significant opportunities for decarbonization through the integration of renewable energy (RE). This study evaluated the RE generation potential within the WSSs of Incheon Metropolitan City (IMC), Republic of Korea, using a site-specific, data-driven approach. Three RE technologies were considered: solar photovoltaic (PV) systems installed in water-treatment plants (WTPs), micro-hydropower (MHP) utilizing the residual head at the inlet chamber of a WTP, and in-pipe MHP recovery using the discharge from water supply tanks in water distribution networks. Actual facility data, hydraulic simulations, and spatial analyses were used to estimate an annual RE generation potential of 32,811 MWh in the WSSs of IMC, including 18,830 MWh from solar PV in WTPs, 4938 MWh from MHP in WTPs, and 9043 MWh from in-pipe MHP. This corresponds to an energy self-sufficiency rate of approximately 22.3%, relative to the IMC WSS total annual electricity consumption of 147,293 MWh in 2022. The results demonstrated that decentralized RE deployment within existing WSSs can significantly reduce grid dependency and carbon emissions. This study provides a rare empirical benchmark for RE integration in large-scale WSSs and offers practical insights for municipalities seeking energy-resilient and climate-aligned infrastructure transitions.

1. Introduction

In response to the 2015 Paris Agreement and global commitment to achieve carbon neutrality by 2050, countries worldwide, including the Republic of Korea, are accelerating decarbonization strategies across key infrastructure sectors [1]. As urban areas aim to become more sustainable and resilient, increasing attention is being paid to integrating renewable energy (RE) into public utility systems to reduce greenhouse gas (GHG) emissions and external energy dependence [2,3,4,5].

Among urban infrastructure systems, water supply systems (WSSs), which comprise water intake facilities, water treatment plants, pumps, tanks, valves, and pipelines designed to deliver potable water at adequate pressure and quality, stand out as one of the most energy-intensive sectors because of the substantial electricity required for water intake, treatment, pumping, and pressure regulation. As energy prices fluctuate and decarbonization mandates intensify, it is imperative to explore how WSSs can actively contribute to energy-transition strategies.

In this context, solar photovoltaic (PV) systems and micro-hydropower (MHP) have emerged as viable technologies for integration within various segments of WSSs. Solar PV systems can be deployed not only on the rooftops and ground areas of water treatment plants (WTPs) but also at intake facilities and water supply tank (WST) zones, where structural space is often available. Their generation potential can be estimated based on the solar irradiation, panel efficiency, and spatial characteristics of each site. Additionally, wind power may be feasible at certain intake locations, particularly when open terrain or elevations are available. MHP systems can be broadly categorized into two types: (1) inlet-based MHP systems, which utilize the residual head at WTPs, typically by installing turbines upstream of the inlet basin, and (2) in-pipe MHP systems, which capture the available flow and excess pressure head within pressurized transmission and distribution pipelines. Furthermore, water-source thermal energy can be harnessed in the transmission sections between intakes and WTPs, where water-temperature differentials offer a renewable source for heating and cooling through heat pump technologies.

Despite growing interest in these technologies, their implementation in real-world WSSs remains limited. The integration of solar PV and MHP systems into the existing water infrastructure requires not only technical feasibility but also location-specific assessments that consider spatial, hydraulic, and operational constraints. Therefore, a systematic site-based evaluation is essential to understand the realistic potential of RE deployment within water systems and support informed policy and investment decisions.

WSSs are increasingly recognized as a key infrastructure in the transition toward carbon neutrality. Zakariazadeh et al. [6] presented a comprehensive review emphasizing the potential of integrating RE into smart water systems and highlighting the need for energy management and intelligent technologies along with renewable generation. Similarly, Lam et al. [7] investigated GHG emission reduction efforts in cities around the globe and concluded that solar PV and MHP integration, biogas valorization, and sludge optimization contributed significantly to GHG reduction in water utilities. Englehardt et al. [8] proposed a net-zero water management framework, demonstrating that simultaneous pressure management and MHP installation can achieve both energy recovery and leakage reduction.

The feasibility of solar PV in WTPs is well-documented. Bukhary et al. [9] assessed the energy consumption of a small WTP in the U.S. and proposed a solar PV system with battery storage that demonstrated positive environmental and economic outcomes. Saavedra et al. [10] examined solar PV applications in Colombian WTPs across six climatic regions and found that these were economically feasible, with water storage replacing expensive batteries. Soshinskaya et al. [11] analyzed solar PV and wind-based microgrid applications for a large-scale WTP in the Netherlands and revealed that up to 96% of the electricity demand could be met with RE. Amoroso [12] further demonstrated that solar PV systems could meet operational requirements under various scenarios in Ecuador. Earlier, Bukhary et al. [13] evaluated the application of distributed solar PV systems at a WTP in Pakistan and found that land availability and plant design were the key factors for RE adoption.

Small hydropower and in-conduit energy-recovery systems offer promising opportunities for WSSs. Su and Karney [14] conducted a case study in Vancouver to evaluate the feasibility and economic impact of energy recovery using turbines in water distribution lines. Giugni et al. [15] proposed an optimization framework for pressure-reducing valve (PRV) and pump-as-turbine (PAT) placement, showing that energy production can be increased with marginal pressure tradeoffs. Morani et al. [16] extended this approach by formulating a mixed-integer nonlinear programming model for joint PRV/PAT optimization. Fecarotta et al. [17] introduced a regulation strategy for PAT plants under variable flow and pressure to enhance the system reliability. Sitzenfrei and von Leon [18] emphasized long-term modeling to account for the water age and flow variability when designing small hydropower systems in Alpine WSSs. Latifi et al. [19] presented an operational strategy that leverages the optimal placement and scheduling of PATs in conjunction with PRVs to manage surplus pressure and generate renewable energy in WDNs, demonstrating both enhanced reliability and significant lifecycle cost reductions. Complementarily, Pugliese and Giugni [20] proposed an operative framework for the preliminary selection of centrifugal PATs by estimating key design parameters such as head drop, power output, and impeller dimensions, supporting cost-effective deployment with reduced computational effort. Technological developments related to in-conduit hydropower were thoroughly reviewed by Sari et al. [21], who highlighted innovations in modular turbine design and assessed 16 turbine types. Berrada et al. [22] proposed a techno-economic optimization algorithm for MHP plants within Moroccan WSSs, confirming cost savings and GHG reductions. Vilanova and Balestieri [23] analyzed hydraulic energy recovery in Brazil and presented models and case studies that demonstrated significant reductions in operating costs. Giudicianni et al. [24] provided a recent literature synthesis on energy-recovery strategies, including PATs, turbines, and emerging devices such as Green Valves, calling for more field-based evaluations. Yao et al. [25] developed a compact in-pipe crossflow turbine that generated 470 Wh/day in urban pipelines, demonstrating stable power output with minimal head loss. Alawadhi et al. [26] analyzed an in-pipe hydropower project in the UAE, estimating 218 kW of potential power with a short payback period (1–6 years) and significant CO₂ savings.

Although these studies have advanced the understanding of RE integration in WSSs, the existing literature remains limited in both scope and depth, particularly in terms of comprehensive, site-specific assessments using real operational data. Zakariazadeh et al. [6] provided a broad review of RE integration in smart WSSs but emphasized the lack of empirical evaluations. Ani et al. [27] compared African and U.S. initiatives, identifying technical and policy challenges, yet did not address the Korean context. Similarly, Min et al. [28] proposed a water–energy–carbon nexus framework for Incheon Metropolitan City (IMC) but did not directly estimate the RE generation potential. In addition, Syahputra et al. [29] analyzed a case study of a hybrid MHP and solar PV system in rural Indonesia, but this approach has not been applied to urban WSSs. Rahman [30] examined the classification, components, and efficiency of micro-hydropower systems within the infrastructural context of Malaysia; however, this study also did not perform site-specific quantitative assessments based on real WSSs data. This scarcity can be attributed to the fact that the field is still emerging, with most prior research focusing on theoretical modeling, techno-economic simulations, or small-scale pilot projects. Therefore, there is an urgent need for large-scale, site-specific analyses that reflect spatial, hydraulic, and operational constraints in real-world systems.

These RE options offer viable pathways to improve the energy self-sufficiency of WSSs. However, most previous studies have focused on either solar PV or MHP systems independently and have been limited to theoretical modeling, techno-economic simulations, or small-scale pilot implementations. Comprehensive site-specific assessments using real-world data from operational WSSs are scarce.

To address this research gap, this study evaluated the RE generation potential of both solar PV and MHP technologies as applied to the WSSs of IMC, Republic of Korea. The analysis covered multiple locations, including WTPs and pressurized pipelines, and quantified the expected generation capacity to assess the overall energy self-sufficiency potential. In doing so, this study aimed to provide empirical evidence and practical insights to support the large-scale integration of decentralized RE within existing WSSs.

The remainder of this paper is organized as follows. Section 2 reviews RE options applicable to WSSs. Section 3 outlines the analytical framework for evaluating solar PV and MHP integration. Section 4 presents the case study results for IMC, including energy generation estimates and a self-sufficiency assessment. Section 5 provides discussions on the key findings and their implications. Finally, Section 6 concludes the paper.

2. Renewable-Energy Applications in Water-Supply Systems

WSSs involve multiple energy-intensive stages, from intake to treatment, transmission, and distribution. However, each step offers distinct spatial and operational opportunities for the application of RE technologies. Figure 1 provides a schematic overview of the potential integration points of RE throughout WSSs. At the intake stage, wind turbines can be installed on nearby elevated terrain, and solar PV systems can be deployed on facility rooftops and adjacent land parcels. Two key technologies can be applied to transmission pipelines: MHP recovery and water-sourced thermal energy. MHP systems capitalize on the residual head and flow conditions to generate electricity, while also helping regulate the pipeline pressure. Water-source thermal energy utilizes the thermal characteristics of natural water bodies, such as dams, lakes, and rivers, whose temperatures are lower than the ambient air in summer and higher in winter. This enables efficient cooling and heating of buildings via heat pump systems that extract energy from water. The stable temperature and availability of the intake water make this source particularly effective for energy-efficient heating and cooling, particularly in WTPs and nearby municipal buildings. Within WTPs, solar PV systems can be widely implemented across rooftops and underutilized spaces. Additional MHP systems may also be installed at plant inlets where sufficient head pressures are available. Finally, in the pressurized distribution network downstream of water supply tanks, in-pipe MHP systems can be installed to recover the energy dissipated by the PRVs. These systems not only improve the energy efficiency but also contribute to pressure management within the network. The integration of RE into WSSs offers substantial benefits beyond electricity savings. It strengthens energy autonomy, reduces GHG emissions, supports net-zero targets, and enhances the resilience of water infrastructure to climate change. Moreover, decentralized clean-energy production within existing facilities helps ensure uninterrupted water services during emergencies, thereby improving the sustainability and reliability of urban utilities.

3. Overview of Analytical Framework

This study evaluated the RE potential of WSSs by examining three key application points. The methodological process comprises four sequential stages. First, the target city was selected, and its WSS was characterized by compiling baseline information such as the number and type of facilities—including intake stations, WTPs, and WSTs—as well as operational metrics such as intake and treatment capacities. Second, site-specific datasets for each facility within the WSS were collected, covering available surface areas, hydraulic conditions, and operational characteristics from both WTPs and relevant pipeline segments. Third, the RE generation potential for each technology—solar photovoltaic at WTPs, inlet-based MHP, and in-pipe MHP—was estimated using the collected data in combination with standard performance assumptions. Finally, the estimated outputs from all applicable facilities were aggregated to quantify the overall RE potential of the entire WSS and to assess its contribution to system-level energy self-sufficiency. This structured and stepwise approach ensures methodological consistency while clearly defining the technical and spatial conditions under which each RE solution can be effectively applied. As illustrated in Figure 1, the analysis framework also identifies the specific stages within WTPs and distribution networks where each technology can be implemented, and the following subsections describe in detail the methodologies applied for estimating the potential of each RE option.

3.1. Solar Power Generation at WTPs

Solar PV systems generate electricity by directly converting solar radiation into electrical energy using PV panels. Owing to their modularity and compatibility with unused spaces, solar PV systems are increasingly being integrated into WTPs as decentralized and sustainable energy sources. In general, solar PV systems can be installed on large flat surfaces within WTP facilities, such as sedimentation basins, filtration units, and clean water reservoirs. In some cases, the rooftops of administrative or auxiliary buildings are considered suitable for installation. The selection of installation areas typically considers structural safety, sunlight availability, accessibility, and operational interference.

The estimation of solar PV generation potential follows a two-step calculation.

Installed Capacity (kW) = Power Density (kW/m²) × Available Area (m²)

(1)

Annual Power Generation (kWh) = Installed Capacity (kW) × Average Solar Irradiation (h/day) × 365

(2)

Figure 2 illustrates the integration of a solar PV system in a WTP, which considers the available surface area. In solar power generation, power density refers to the amount of electricity that can be generated per unit area, typically expressed in kilowatts per square meter (kW/m²). The power density depends on factors such as solar irradiation at the site, the efficiency and quality of PV panels, and installation conditions, and it generally corresponds to approximately 0.10–0.15 kW/m² under typical conditions. This methodology provides a baseline for evaluating the potential contribution of solar energy to improving the energy self-sufficiency of a WTP.

3.2. MHP Generation at WTPs

MHP generation at a WTP utilizes the remaining hydraulic head of the influent water supplied from the intake facilities. As raw water flows into the inlet chamber of a WTP, gravitational potential energy can be harnessed through turbines to generate electricity before treatment begins. This process is illustrated in Figure 3, which shows the general flow from the intake facility through the MHP system to the water treatment and supply stages. This method converts the hydraulic energy of the flowing water into mechanical energy using turbines, which is then converted into electricity through generators.

The key parameters for estimating the MHP potential are the flow rate (Q), effective head (H), and system efficiency. The assumed values for turbine efficiency, generator efficiency, and capacity factor may vary depending on the type and scale of the hydropower equipment used. In this study, representative values were adopted based on practices in other regions of South Korea and supported by the previous literature [31,32,33,34,35]. General calculation follows three steps.

• Turbine Power Output (Pt, in kW):

$$\begin{array}{l}\mathrm{P}\mathrm{t}=9.8\times \mathrm{Q}\times \mathrm{H}\times \mathrm{E}\mathrm{t}\\ \mathrm{Q}=\mathrm{Design}\mathrm{flow}\mathrm{rate}({\mathrm{m}}^3/\mathrm{s})\\ \mathrm{H}=\mathrm{Effective}\mathrm{head}\left(\mathrm{m}\right)\\ \mathrm{Et}=\mathrm{Turbine}\mathrm{efficiency}\left(\mathrm{typically}0.80\right)\end{array}$$

(3)

2.

Generator Power Output (Pg, in kW):

$$\begin{array}{l}\mathrm{P}\mathrm{g}=\mathrm{P}\mathrm{t}\times \mathrm{E}\mathrm{g}\\ \mathrm{E}\mathrm{g}=\mathrm{Generator}\mathrm{efficiency}\left(\mathrm{typically}0.94\right)\end{array}$$

(4)

3.

Annual Power Generation (kWh):

$$\begin{array}{l}\mathrm{Annual}\mathrm{Power}\mathrm{Generation}=\mathrm{P}\mathrm{g}\times 24\times 365\times \mathrm{C}\mathrm{F}\\ \mathrm{C}\mathrm{F}=\mathrm{Capacity}\mathrm{factor}\left(\mathrm{typically}0.84\right)\end{array}$$

(5)

These equations provide a practical framework for estimating the theoretical and annual energy production potentials of MHP systems at WTPs based on the hydraulic and operational parameters.

3.3. In-Pipe MHP Generation in Distribution Networks

In-pipe MHP generation involves the recovery of hydraulic energy from excess head pressure within water distribution pipelines. As illustrated in Figure 4, this method utilizes the residual head available between WSTs and network endpoints to generate electricity, typically by installing compact turbines in pressurized pipes.

The residual head (H) can be determined by subtracting the minimum required service pressure (15 m), head loss along the pipelines, and elevation head at the end nodes from the total head at the water supply tank (WST). The electricity generation potential is then calculated using the following equation.

$$\begin{array}{l}\mathrm{P}=9.8\times \mathrm{Q}\times \mathrm{H}\times \mathrm{E}\mathrm{t}\times \mathrm{E}\mathrm{g}\\ \mathrm{P}=\mathrm{Power}\mathrm{output}\left(\mathrm{k}\mathrm{W}\right)\\ \mathrm{Q}=\mathrm{Flow}\mathrm{rate}({\mathrm{m}}^3/\mathrm{s})\\ \mathrm{H}=\mathrm{Residual}\mathrm{head}\left(\mathrm{m}\right)\end{array}$$

(6)

This approach provides a practical framework for evaluating in-pipe MHP opportunities using available hydraulic gradients in WSSs without requiring structural modifications to existing facilities.

4. Application Results

4.1. Study Area: Incheon Metropolitan City (IMC)

IMC, which is one of the six metropolitan cities in the Republic of Korea, is located in the western region of the country. It borders the Yellow Sea to the west and the capital city of Seoul to the east, with adjacent cities including Bucheon, Siheung, and Gimpo. As a major industrial and logistics hub, IMC has developed around the Port of Incheon and Incheon International Airport. As of December 2023, the water service area of IMC covers 435 km², serving a population of approximately 3,051,646, which corresponds to a water supply coverage rate of 99.1%. The daily water supply per capita was estimated to be 347.6 L. Four major WTPs serve the city: Gongchon, Bupyeong, Namdong, and Susan. The primary water sources for these WTPs include the Paldang and Pungnap intake facilities located upstream of the Han River Basin.

Table 1. Overview of intake facilities, WTPs, and WSTs in IMC.

Intake Facilities	Water Treatment Plants	Water Supply Tanks
Pungnap (352,854 ${\mathrm{m}}^3$/day)	Gongchon (260,914 ${\mathrm{m}}^3$/day)	Geomdan, Airport New Town, Yeonhui, Cheongna, Songsan, Seoknam
Paldang 1 (137,910 ${\mathrm{m}}^3$/day)	Bupyeong (273,606 ${\mathrm{m}}^3$/day)	Cheonmasan, Huimangcheon, Wonjeoksan, Gajwa
Paldang 2 (253,991 ${\mathrm{m}}^3$/day)	Namdong (253,991 ${\mathrm{m}}^3$/day)	Subongsan, Songhyeon, Manwolsan, Jayu Park, Mansu, Jangsu
Paldang 3 (313,847 ${\mathrm{m}}^3$/day)	Susan (313,847 ${\mathrm{m}}^3$/day)	Munhak, Ssukgol, Pureun Songdo, Seochang

provides an overview of the intake facilities, WTPs, and associated WSTs, offering the infrastructure context for this study [36]. Figure 5 illustrates the spatial characteristics of the WSSs in IMC. Figure 5a presents the administrative boundaries along with the locations of four major WTPs situated within each district. Figure 5b illustrates the locations of WSTs, with colors indicating their supplying WTPs: green for WSTs supplied by the Gongchon WTP, navy for those supplied by the Bupyeong WTP, sky blue for those supplied by the Namdong WTP, and purple for those supplied by the Susan WTP. In 2021, the water services of IMC consumed approximately 147,293 MWh of electricity, representing approximately 15.6% of the total public sector electricity consumption in IMC (943,788 MWh) [28]. This high share highlights the energy-intensive nature of urban water systems and underscores the importance of improving energy self-sufficiency in this sector. This study evaluated the potential of integrating RE into the water infrastructure of IMC, focusing on three areas: (i) the solar PV potential at WTP sites, (ii) MHP potential at WTP inlets, and (iii) in-pipe MHP potential at WSTs. The infrastructure characteristics summarized in Table 1 and the spatial distribution shown in Figure 5 formed the foundation for site-specific RE potential assessments.

4.2. Assessment of Site-Specific RE Generation

4.2.1. Estimated Solar Power Generation at WTPs

This section presents the estimated solar PV generation potential at the WTPs in IMC. The analysis considered both existing and new PV system installations at WTP sites. To estimate the generation potential, a power density of 0.10 kW/m² was used with Equation (1), reflecting standard PV panel specifications. The average solar irradiation in Equation (2) was based on a national average sunshine duration of 3.6 h per day in Korea [37]. The analysis covered various surface types, including the rooftops of process and facility buildings, parking lots, and other auxiliary structures. Figure 6 illustrates the key unit processes and structural layout of the Gongchon WTP, highlighting the representative locations suitable for solar PV deployment.

Table 2. Estimated solar power generation potential at four WTPs in IMC.

Water Treatment Plants	Installation Type	Process Facility	Area $({\mathbf{m}}^2$)	Installed Capacity (kW)	Annual Power Generation (MWh)
Gongchon	Existing	Chemical Sedimentation	16,956	1470	1932
		Etc. *		40	53
	New	Rapid Sand Filter	3584	360	473
		Clear Water Reservoir	5407	540	710
		Activated Carbon Filter	1500	150	197
		Administration Building	1000	100	131
	Total				3495
Bupyeong	Existing	Rapid Sand Filter	2707	100	131
	New	Chemical Sedimentation	12,984	1300	1708
		Clear Water Reservoir	2443	244	321
		Etc. *	7000	700	920
	Total				3080
Namdong	Existing	Rapid Sand Filter	799	120	158
		Chemical Sedimentation and Clear Water Reservoir	20,014	2936	3858
		Behind the Administration Building	244	41	54
		Rooftop of the Water Promotion Center	334	30	39
	New	Rapid Sand Filter	2000	200	263
		Etc. *	5000	500	657
	Total				5029
Susan	Existing	Rapid Sand Filter, Clear Water Reservoir	6690	1000	1314
		Chemical Sedimentation	28,260	2600	3416
		Sludge Dewatering Facility, Effluent Basin		400	526
	New	Rapid Sand Filter	10,000	1000	1314
		Clear Water Reservoir
		Etc.*	5000	500	657
	Total				7227

* Note: “Etc.” refers to additional available areas such as the rooftops of facility buildings, parking lots, and other auxiliary structures.

summarizes the estimated solar PV generation potential at WTPs. In this table, the “Etc.” category refers to additional available areas such as the rooftops of facility buildings, parking lots, and other auxiliary structures, which were estimated assuming that 5% of the total plant area consisted of usable surfaces. In some cases, discrepancies arose between the physical dimensions of the process units and the calculated installable areas. These reflected practical constraints such as access paths, maintenance clearances, and rooftop obstructions, which reduced the effective area available for PV system installations. According to Table 2, the existing PV systems currently generate approximately 11,480 MWh of power per year. Additional newly available areas offer an estimated potential of approximately 7350 MWh per year. The combined solar energy potential from existing and new installations amounts to approximately 18,830 MWh annually. These results highlight the significant potential of solar PV systems as a practical strategy to enhance energy self-sufficiency within the urban water infrastructure, particularly through the utilization of underused surfaces at WTP sites.

4.2.2. Estimated MHP Generation at WTP Inlets

This section presents the estimated MHP generation at the inlets of the four WTPs in IMC based on the design flow rate and effective head at each facility. The analysis considered the hydraulic head available at the inlet chamber of the WTP and utilized known or assumed values depending on data availability.

The residual head values for the Gongchon and Bupyeong WTPs were obtained directly from Incheon Waterworks Headquarters. In Bupyeong, raw water is sourced from two independent intake points, Paldang and Pungnap, which have different flow and head characteristics. Thus, the generation was assessed separately for each intake source. The effective head at the Namdong WTP was referenced from an existing MHP facility currently in operation at the site, ensuring that the applied values reflected the actual system performance. However, no reference data were available for the effective head at the Susan WTP; therefore, three different head conditions (2, 5, and 10 m) were assumed to examine the sensitivity and potential variability in the generation output. The design flow rates for all of the facilities were obtained directly from Incheon Waterworks Headquarters. Using these inputs, the turbine and generator outputs were calculated, and the annual generation was estimated under the assumption of constant daily operation.

The results presented in

Table 3. Estimated MHP generation at WTP inlets.

Water Treatment Plants		Maximum Installed $\mathbf{Capacity}({\mathbf{m}}^3/\mathbf{d}\mathbf{a}\mathbf{y})$	Average Daily Water $\mathbf{Production}({\mathbf{m}}^3/\mathbf{d}\mathbf{a}\mathbf{y})$	$\mathbf{Design}\mathbf{Flow}\mathbf{Rate}({\mathbf{m}}^3/\mathbf{s})$	Effective Head (m)	Turbine Power Output (kW)	Generator Power Output (kW)	Annual Power Generation (MWh)
Gongchon		413,000	271,200	3.20	1.29	32.4	30.5	224
Bupyeong	Paldang	375,000	219,500	0.20	2.11	2.9	2.7	22
	Pungnap			2.40		40.1	37.7	280
Namdong		542,000	248,400	2.95	14.67	339.4	319.0	2347
Susan		623,000	308,900	3.81	2.0	59.7	56.1	413
					5.0	149.2	140.3	1032
					10.0	298.5	280.6	2065

reveal that the estimated MHP generation varied significantly across the four WTPs, primarily because of differences in the effective head and design flow rate values. Notably, the facilities with larger treatment capacities and higher head conditions (the Susan and Namdong WTPs) exhibited greater generation potential, reaching over 1000 and 2300 MWh per year, respectively. In contrast, the smaller facilities (Gongchon and Bupyeong) produced less than 300 MWh annually under the same operational assumptions. These findings highlight the positive correlation between the plant scale and MHP generation potential, suggesting that larger WTPs with stable and elevated head conditions are more suitable for energy recovery. Furthermore, the sensitivity analysis for the Susan WTP under the three different head scenarios illustrated how head variability can significantly influence the generational outcome. This underscores the importance of accurate and effective head assessment and site-specific design when planning in-plant MHP integration.

4.2.3. Estimated In-Pipe MHP Generation in Distribution Pipelines

This section presents the results of an evaluation of the potential for in-pipe MHP generation within the pressurized pipelines of the IMC WSSs. The analysis targeted the transmission sections between WSTs and terminal demand nodes in each WST service area, where the residual head and flow conditions were used to estimate the recoverable energy.

The demand node with the lowest pressure was identified in each block (each WST service zone), and the residual head was calculated by subtracting the minimum pressure requirement (15 m) from the difference between the tank head and terminal node pressure. Zones with sufficient residual head pressures are suitable for MHP applications. Based on the calculated capacity, both the daily and annual power generation values were estimated. To express the scale of potential energy recovery, the number of households that could be supplied was calculated using the average household electricity consumption of 249.2 kWh per month for IMC, or approximately 8.3 kWh per day, based on the 2023 data from the Korean Statistical Information Service (KOSIS) [38].

As summarized in

Table 4. In-pipe MHP output by water tank block in WTP service areas.

Water Treatment Plants	Water Supply Tanks	Residual Head (m)	Design Flow Rate $({\mathbf{m}}^3/\mathbf{s})$	Installed Generator Capacity (kW)	Daily Power Generation Potential (kWh)	Annual Power Generation (MWh)	Estimated Household Coverage
Gongchon	Geomdan	16.28	0.27	30.59	734	225	75
	Airport New Town	6.24	0.08	3.56	86	26	9
	Yeonhui	31.94	0.28	63.54	1525	468	156
	Cheongna	13.56	0.38	32.18	772	237	79
	Songsan	10.51	0.13	10.07	242	74	25
	Seoknam	22.17	0.72	102.36	2457	753	252
Bupyeong	Cheonmasan	14.31	0.85	70.46	1691	519	173
	Huimangcheon	12.79	0.41	32.58	782	240	80
	Wonjeoksan	17.24	1.17	116.90	2806	860	288
	Gajwa	27.43	0.30	58.75	1410	432	145
Namdong	Subongsan	28.66	0.38	78.98	1896	581	194
	Songhyeon	27.84	0.37	74.80	1795	550	184
	Manwolsan	8.83	0.50	28.65	688	211	71
	Jayu Park	9.02	0.04	2.70	65	20	7
	Mansu	22.39	0.15	22.98	552	169	57
	Jangsu	31.78	1.34	292.00	7008	2149	719
Susan	Munhak	27.38	0.25	49.25	1182	362	121
	Ssukgol	31.28	0.33	75.41	1810	555	186
	Pureun Songdo	10.31	0.83	53.54	1285	394	132
	Seochang	21.47	0.19	29.72	713	219	73

, 20 blocks were evaluated. A high generation potential was found in blocks such as Jangsu (2149 MWh/year), Wonjeoksan (860 MWh/year), and Seoknam (753 MWh/year), primarily because of the favorable residual head and moderate flow conditions. The total annual in-pipe MHP generation potential was estimated to be 9044 MWh, which corresponds to approximately 6.1% of the total electricity consumption of IMC WSSs in 2022 [28]. This amount of energy could supply electricity to approximately 3000 households annually.

To visualize the distribution of the MHP potential across all the service blocks, Figure 7 presents a bubble chart plotting the design flow rate (CMS) on the x-axis and residual head (m) on the y-axis, where the size of each bubble represents the installed capacity (kW). This multivariate plot allows for the intuitive identification of blocks with the most favorable combinations of flow and head conditions. Blocks such as Jangsu, Seoknam, and Wonjeoksan appear prominently in the upper-right region of the graph, indicating their strong suitability for MHP development. These service blocks benefit from a high residual head, a substantial flow rate, or both, resulting in relatively large potential capacities. Conversely, sites such as Airport New Town, Jayu Park, and Songsan have low heads and flows, indicating their limited suitability for energy recovery.

These results suggest that an in-pipe MHP is a technically feasible decentralized RE option that can recover energy and regulate pressure without requiring major infrastructure upgrades.

4.3. Energy Self-Sufficiency Evaluation in IMC WSSs

According to operational data from 2022, the IMC WSSs consumed 147,293 MWh of electricity across their full process chains. This included 68,145 MWh for raw water pumping, 20,009 MWh for treatment processes, 58,705 MWh for transmission pumping, and 434 MWh for distribution pumping. This energy usage represented approximately 15.6% of the total public sector electricity consumption for the city in 2021 (943,788 MWh), which underscored the energy-intensive nature of urban water services [28].

This study comprehensively assessed the RE integration potential within the water supply infrastructure of the city using real facility data and site-specific capacity assumptions. The analysis quantified the feasible annual energy outputs from the three decentralized RE sources (

Table 5. Summary of RE generation potential by method and facility in IMC WSSs.

Generation Method	Facility Type		Facility Capacity (kW)	Annual Power Generation (MWh)
Solar PV at water treatment plants	Gongchon		2660.0	3495
	Bupyeong		2344.0	3080
	Namdong		3827.0	5028
	Susan		5500.0	7227
	Subtotal		14,331.0	18,830
Micro-hydropower at water treatment plants	Gongchon		31.0	224
	Bupyeong		41.0	302
	Namdong		319.0	2347
	Susan (H = 10 m)		281.0	2065
	Subtotal		672.0	4938
In-pipe micro-hydropower at water supply tank zones	Gongchon WTP Zone	Geomdan	30.6	225
		Airport New Town	3.6	26
		Yeonhui	63.5	467
		Cheongna	32.2	236
		Songsan	10.1	74
		Seolnam	102.4	753
	Bupyeong WTP Zone	Cheonmasan	70.5	518
		Huimangcheon	32.6	239
		Wonjeoksan	116.9	860
		Gajwa	58.7	432
	Namdong WTP Zone	Subongsan	79.0	581
		Songhyeon	74.8	550
		Manwolsan	28.6	210
		Jayu Park	2.7	19
		Mansu	23.0	169
		Jangsu	292.0	2148
	Susan WTP Zone	Munhak	49.3	362
		Ssukgol	75.4	554
		Pureun Songdo	53.5	394
		Seochang	29.7	218
	Subtotal		1229.0	9043
Total			16,232.0	32,811

).

• 18,830 MWh from solar PV systems installed in available areas at WTPs.
• 4938 MWh from inlet-based MHP utilizing residual head prior to WTPs.
• 9043 MWh from in-pipe MHP recovery downstream of WSTs in the distribution network.

The combined RE generation potential amounts to 32,811 MWh per year, which corresponds to an energy self-sufficiency rate of approximately 22.3% for all of the WSSs. This means that over one-fifth of the electricity demand required to operate WSSs could be covered by internally produced clean-energy sources, significantly reducing the dependence on grid electricity and the associated GHG emissions.

To put this into perspective, the sectoral breakdown revealed an uneven self-sufficiency potential across the system, with treatment facilities and distribution pumping having relatively low potential RE contributions as a result of limited site or head availability. The raw water and transmission pumping segments showed higher offset potential, particularly where MHP or PV installations are technically feasible. This suggests that targeted investment in RE installations could yield the highest returns in specific subsectors, rather than uniform deployment.

Furthermore, the 22.3% self-sufficiency represents a baseline under the current site conditions and technological assumptions. With future policy support, technological improvements, or expanded facility retrofits (e.g., floating PV systems or pressure recovery turbines), the actual self-sufficiency could be further enhanced.

Importantly, this assessment was grounded in the real-world operational and spatial characteristics of IMC WSSs and offers a rare empirical benchmark in the literature on urban water–energy integration. By quantifying both the generation potential and energy offset at the system scale, this study demonstrated the practical feasibility and strategic relevance of deploying site-specific RE technologies within an existing infrastructure. These findings can inform policy initiatives aimed at decarbonizing public utilities and advancing municipal-level climate targets, while serving as a decision-support reference for other cities seeking to enhance the energy resilience and sustainability of their WSSs.

5. Discussions

The results of this study indicate that RE integration could supply approximately 22.3% of the electricity demand of the IMC WSSs, providing a rare empirical benchmark grounded in real operational and spatial data. Unlike many previous studies, which have primarily focused on the technical feasibility of RE integration or on estimating potential energy savings rather than self-sufficiency, this research explicitly quantified the share of electricity demand that could be internally supplied. For example, Bukhary et al. [9] analyzed a small drinking WTP and demonstrated that grid-connected solar PV systems could offset up to 60% of electricity demand, thereby reducing dependence on external supply. Similarly, Saavedra et al. [10] assessed Colombian WTPs and concluded that solar PV systems can lower operational energy costs and improve sustainability under favorable economic conditions. While other studies have discussed broader carbon neutrality strategies [7] or the potential for energy-positive utilities through advanced technologies [8], explicit calculations of energy self-sufficiency in WSSs remain scarce, making direct comparisons challenging. In this study, the 22.3% self-sufficiency observed in IMC WSSs provides one of the few quantitative references available and demonstrates that medium- to large-scale utilities embedded in urban contexts can realistically achieve substantial levels of energy independence.

The methodology used in this study has several important advantages. First, it is empirically grounded, drawing directly from operational data of actual WSS facilities rather than relying solely on theoretical assumptions, thereby increasing the reliability of the estimates. Second, it provides comprehensive coverage by simultaneously evaluating solar PV, inlet-based MHP, and in-pipe MHP, which represent complementary technologies capturing different types of residual energy across the system. Third, the approach offers practical applicability, as the quantified self-sufficiency benchmark can serve as a decision-support tool for municipal governments seeking to align infrastructure planning with climate and energy policies. Finally, the methodology emphasizes sector-specific resolution, identifying where the greatest RE offsets are possible (e.g., raw water pumping and transmission), thereby supporting more targeted and cost-effective investment strategies.

At the same time, several limitations must be acknowledged. Solar PV estimates remain constrained by spatial requirements, as meaningful generation requires extensive roof or land areas, and output is subject to seasonal variability in solar irradiance. Inlet-based MHP, while stable and predictable, is limited by the availability of sufficient head and flow and often requires costly retrofitting or pipeline modifications. In-pipe MHP in distribution networks is highly sensitive to local pressure conditions, usually producing modest outputs that require deployment of multiple small units, which in turn increases the maintenance burden and may introduce additional water quality monitoring needs. Furthermore, the analysis relied on static assumptions about technology efficiency, land availability, and hydraulic conditions; future innovations such as floating PV installations, pressure recovery turbines, or energy storage systems were not incorporated. Lastly, the methodology does not explicitly address economic feasibility or lifecycle costs, which are essential for large-scale implementation.

Despite these constraints, the findings confirm that targeted investments in subsectors with higher RE potential—particularly raw water and transmission pumping stations—can maximize energy gains and cost-effectiveness. These findings are consistent with prior studies emphasizing the efficiency of localized MHP deployment at pressure-reducing sites [14,23]. Overall, the IMC case contributes to the literature by empirically demonstrating that more than one-fifth of the electricity demand in urban water systems can be offset through site-specific RE integration, while also providing a realistic appraisal of both the opportunities and limitations of current methodologies.

6. Conclusions

This study evaluated the potential of integrating RE sources into the WSSs of IMC through three technical applications: solar PV installations at WTPs, inlet-based MHP systems using the residual head at WTPs, and in-pipe MHP recovery from WSTs. The analysis incorporated real-world operational data, including flow rates, residual heads, and site-specific infrastructure layouts, provided by the IMC Waterworks Division.

The findings indicate that up to 22.3% of the electricity required to operate the IMC WSSs could potentially be met by RE applications without large-scale infrastructural modifications. This demonstrates a viable strategy for enhancing the sustainability and energy resilience of urban water systems. The results provide actionable insights for utility managers and policymakers seeking to align public services with carbon neutrality goals and rising energy costs.

Despite these promising results, this study faces several limitations. First, detailed hydraulic data at individual facilities required extensive coordination with municipal agencies, revealing the need for improved data-sharing mechanisms and standardized monitoring frameworks. Second, modern water system designs often aim to minimize hydraulic losses, which in turn reduces recoverable residual energy. This raises practical questions regarding the cost-effectiveness of small-scale energy recovery under current conditions. Importantly, this study does not propose overdesigning water systems for energy harvesting but rather focuses on evaluating recovery potential within existing infrastructure. Third, while technical feasibility was assessed, a detailed cost-benefit analysis of each RE technology was beyond the scope of this study and remains a critical area for future study.

Future research should expand this foundation by incorporating dynamic simulations with real-time operational data, comprehensive economic evaluations, and institutional analyses to facilitate actual deployment. With improved data, regulatory support, and stakeholder engagement, the integration of RE technologies into urban water systems can transition from theoretical feasibility to practical and scalable implementation.