Innovative Energy Sustainable Solutions for Urban Infrastructure: Implementing Micro-Pumped Hydro Storage in Singapore’s Multi-Level Carparks

Abstract

As part of the initiative to achieve Singapore’s Green Plan 2030, we propose to investigate the potential of utilizing micro-pumped hydroelectric energy storage (PHES) systems in multi-level carparks (MLCP: a stacked car park that has multiple levels, may be enclosed, and can be an independent building) as a more environmentally friendly alternative to traditional battery storage for a surplus of solar energy. This study focuses on an MLCP with a surface area of 3311 m² and a height of 12 m, considering design constraints such as a floor load capacity of 5 kN/m² and the requirement for a consistent energy discharge over a 12 h period. The research identifies a Turgo turbine as the optimal choice, providing a power output of 2.9 kW at a flow rate of 0.03 m³/s with an efficiency of 85%. This system, capable of storing 1655.5 m³ of water, can supply power to 289 light bulbs (each consuming 10 W) for 15.3 h, thus having the capacity to support up to three MLCPs. These results underscore the environmental advantages of PHES over conventional batteries, highlighting its potential for integration with solar panels to decrease carbon emissions. This approach not only aligns with Singapore’s green initiatives but also promotes the development of a more sustainable energy infrastructure.

1. Introduction

1.1. Background

Singapore’s dense population and compact urban environment pose significant challenges for sustainable energy production. The limited available land makes it difficult to install large-scale renewable energy systems such as wind farms or extensive solar arrays. Additionally, high land costs and the need to optimize land use exacerbate the challenge of integrating new energy infrastructure into the urban landscape. The proposed micro-pumped hydroelectric energy storage (PHES) project directly addresses these issues by utilizing existing multi-level car parks as sites for energy storage and generation. These carparks are prevalent throughout Singapore and offer vertical space that can be repurposed without requiring additional land, which is both scarce and expensive. By installing PHES systems within these carparks, the project makes use of underutilized urban infrastructure, turning them into energy hubs. This approach not only maximizes the use of available space but also supports the integration of renewable energy sources, such as solar power, by providing a reliable method for storing and discharging energy. This helps in balancing the intermittent nature of solar energy and contributes to reducing the overall carbon footprint. Moreover, the project aligns with Singapore’s green initiatives by enhancing energy sustainability within the confines of its urban environment, effectively addressing the space limitations and high costs associated with traditional renewable energy installations. As the global community increasingly emphasizes renewable energy and sustainability, Singapore’s commitment to achieving net-zero waste and emissions becomes particularly crucial. The nation has been actively focusing on green energy solutions to mitigate its environmental impact while ensuring energy security [1,2]. However, the limited space within Singapore’s urban landscape restricts the deployment of large-scale renewable energy systems, such as wind turbines and hydroelectric pumps, primarily due to geographic constraints [3,4]. In the broader context of renewable energy, solar energy has emerged as a key component of Singapore’s sustainable energy strategy. However, while solar panels offer a viable solution for harnessing clean energy, the reliance on battery storage presents a sustainability challenge. The environmental impact associated with the manufacturing and disposal of batteries, including resource extraction and waste management, raises concerns about their long-term viability [3]. To address these challenges, an innovative solution involves the use of micro-hydroelectric turbine systems that harness rainwater runoff from multi-level carparks (MLCPs) to generate clean, renewable energy [1]. This concept aligns with global sustainability goals, which emphasize the importance of reducing reliance on non-renewable resources and minimizing environmental footprints. By capturing and converting the kinetic energy of rainwater runoff into electricity, Singapore can contribute to a more sustainable energy infrastructure, consistent with international efforts to combat climate change. Moreover, the development of these micro-hydroelectric systems into micro-pumped hydroelectric energy storage (PHES) systems presents a promising avenue for enhancing energy sustainability. In urban environments like Singapore, high-rise buildings generate significant amounts of rainwater runoff, typically managed by drainage systems. By redirecting this runoff into energy production, the potential to achieve net-zero waste increases significantly [2]. Effective implementation of this idea requires the use of large, elevated areas to collect rainwater, converting its potential energy into kinetic energy for electricity generation [4,5]. While Singapore lacks natural mountainous terrain, which is often ideal for collecting rainwater, the abundance of MLCPs offers an alternative. These elevated structures provide extensive surfaces suitable for rainwater collection and energy generation, reinforcing Singapore’s position as a leader in urban sustainability initiatives. By integrating such innovative solutions [6,7] into its energy mix, Singapore not only addresses its unique geographic challenges but also contributes to the global transition towards renewable energy and sustainable development. In exploring sustainable energy solutions in densely populated urban environments, similar projects offer valuable insights into innovative approaches. For instance, the “Viability Analysis of Tidal Turbine Installation Using Fuzzy Logic” [8,9] examines the potential of installing tidal turbines in coastal areas, utilizing fuzzy logic to account for the complexities and uncertainties in environmental conditions and technological performance. This approach highlights how advanced analytical methods can optimize the deployment of renewable energy systems in challenging settings, analogous to how Singapore might use its limited space effectively by integrating energy solutions into existing structures. Similarly, the study on “Water Microturbines for Sustainable Applications: Optimization Analysis and Experimental Validation” [10,11,12] focuses on optimizing small-scale water microturbines for various sustainable applications. It includes experimental validation to ensure that these microturbines perform efficiently in real-world conditions. This research is relevant to Singapore’s context as it demonstrates how small-scale, adaptable technologies [13,14,15,16,17,18,19,20] can be used effectively in urban environments, offering a scalable solution for energy production and storage. Both studies underscore the importance of innovative, space-efficient technologies and advanced analytical methods [21,22,23,24,25] in overcoming the challenges of deploying sustainable energy solutions in constrained environments, reflecting the principles applied in Singapore’s micro-pumped hydroelectric energy storage project [26,27,28,29].

1.2. Sustainability

The United Nations Sustainable Development Goals (UNSDGs) consist of 17 global objectives established by the United Nations General Assembly in 2015 as part of the 2030 Agenda for Sustainable Development, as shown in Figure 1. This agenda serves as a universal call to action to eradicate poverty, safeguard the planet, and ensure that all individuals experience peace and prosperity [30]. Pumped hydroelectric energy storage (PHES) is highly compatible with several UNSDGs due to its potential for promoting sustainable energy and environmental management. PHES is an eco-friendly energy storage method that generates electricity without the reliance on critical minerals such as lithium, cobalt, and manganese. This characteristic makes PHES a strong candidate for achieving the goal of affordable and clean energy [1,5]. The UNSDGs emphasize the importance of providing all individuals with access to modern, affordable, sustainable, and reliable energy [30]. PHES can support these objectives by delivering a dependable and continuous supply of renewable energy [5]. The eleventh UNSDG, which focuses on Sustainable Cities and Communities, highlights the need for sustainable energy to build resilient urban environments. PHES, in conjunction with solar panels, can enhance the stability and resilience of urban energy systems, thereby contributing to the goal of making cities and human settlements inclusive, safe, resilient, and sustainable [30]. The thirteenth UNSDG, Climate Action, aims to address climate change and its impacts. PHES supports this goal by providing a renewable and clean energy storage solution, reducing greenhouse gas emissions and facilitating the integration of variable renewable energy sources like wind and solar into the energy grid [5]. By doing so, PHES contributes significantly to global efforts to combat climate change.

1.3. Objectives

The design of a pumped hydroelectric energy storage (PHES) system to utilize rainwater runoff at multi-level carparks (MLCPs) in Singapore targets several critical objectives related to environmental sustainability and infrastructure efficiency [4]. Firstly, it seeks to enhance environmental sustainability by providing the dual benefits of energy production and water management. Secondly, the design aims to improve the resilience and efficiency of the energy infrastructure by offering a reliable energy storage solution. By leveraging the gravitational potential energy of stored water, the PHES system can serve as a dynamic and responsive energy reservoir, contributing to grid stability during peak demand periods [5]. This innovative approach aspires to create an intelligent and sustainable energy solution that efficiently harnesses rainwater runoff while addressing the intermittent challenges associated with renewable energy sources. Ultimately, this contributes to a more resilient and environmentally conscious urban infrastructure in Singapore [2]. The primary objective of this research is to evaluate the efficiency and feasibility of different hydroelectric turbines for maintaining overnight operations in a multi-level carpark (MLCP) in Singapore. Key factors influencing this project include the duration of operation and the system’s ability to provide adequate lighting. The optimal turbine must maximize the number of light bulbs powered and extend the duration of continuous operation. By meticulously evaluating these variables, the research aims to determine the hydroelectric turbine configuration that best meets the operational requirements for overnight use in an MLCP [1]. The initiative aims to establish a sustainable and eco-friendly energy solution by capturing and utilizing rainwater, abundant in Singapore’s tropical climate. Integrating PHES technology with multi-level carparks (MLCPs) optimizes rainwater runoff as a renewable energy source, aligning with Singapore’s commitment to green and innovative urban development. This approach reduces reliance on conventional energy sources and promotes environmentally friendly practices in urban infrastructure [1,4]. Additionally, the project enhances energy resilience and grid stability by converting rainwater runoff into a stored energy resource through the PHES system, addressing the intermittent nature of renewable energy sources. The efficient release of stored energy during peak demand periods ensures a reliable and responsive energy supply [5]. Furthermore, the initiative creates a sustainable and adaptive energy infrastructure tailored to Singapore’s MLCPs, promoting energy efficiency, environmental sustainability, and long-term resilience, while adapting to evolving energy demands in urban areas [2]. Firstly, the system aims to contribute to Singapore’s sustainable energy goals by utilizing the abundant and often-underutilized resource of rainwater runoff [1]. By integrating PHES technology with MLCPs, the project seeks to maximize the capture and utilization of rainwater, converting it into a reliable and renewable energy source. In practical terms, the system described in this study has been designed to store and manage energy in such a way that its output is not limited to just one MLCP. Specifically, the system can generate enough power, using the stored water and Turgo turbine setup, to supply energy for up to three separate MLCPs, assuming each has similar energy demands as the MLCP used in this study (i.e., powering 289 light bulbs for a certain duration). This means that if the system was scaled or replicated across three different MLCPs, it would still be able to meet their energy requirements under the same conditions, demonstrating its robustness and potential scalability. Essentially, the system’s storage capacity and energy output are sufficient to serve multiple buildings, making it a more versatile and efficient solution in urban energy management.

1.4. Singapore Green Plan

As part of Singapore’s Green Plan (SGP) 2030, as shown in Figure 2, the country has been actively implementing various sustainability measures [1,2,4,5]. The SGP aims to achieve ecological sustainability [5,31,32,33] by promoting green technologies and enhancing resilience to climate change. Key focus areas include sustainable development, energy efficiency, waste minimization, and water resource management. Ambitious targets have been set to reduce carbon emissions, encourage the adoption of renewable energy, improve road transport systems, and promote ecologically clean lifestyles and biodiversity conservation [1]. Recognizing the importance of global climate action, Singapore is integrating sustainability into its policies and daily practices to create a greener environment. In line with the Green Plan, pumped hydroelectric energy storage (PHES) offers a promising solution to energy storage challenges and can be integrated with solar energy harnessing efforts in Singapore. PHES systems store excess renewable energy by pumping water to a higher elevation, which can then be released through a hydroelectric turbine to generate electricity when renewable sources are unavailable, such as at night. This approach facilitates Singapore’s transition to a higher share of renewable energy, enhancing energy security and supporting the nation’s green agenda by stabilizing the supply of clean energy and reducing emissions. In the following sections, Section 2 will introduce the different types of hydroelectric turbines, Section 3 will present more on energy storage, Section 4 will illustrate more on life cycle analysis, Section 5 will cover methodology, Section 6 will show the detailed calculations, Section 7 will present the results and discussion, and Section 8 will conclude this proposed work.

2. Different Types of Hydroelectric Turbines

Various hydroelectric turbines can generate electricity, but the design of the multi-level carpark necessitates a vertical drop. Four types of turbines are being evaluated to determine which one best suits the design and offers the highest efficiency. The Pelton turbine is ideal for high-head applications with significant water drop, most efficient for low-flow and high-head scenarios. The Crossflow turbine is perfect for low-head applications with limited vertical drop, offering compact size, easy installation, and good efficiency at low flow rates. The Propeller/Kaplan turbine works well for moderate head and flow rates, featuring adjustable blades to maintain efficiency across varying flow rates, and is commonly used in rivers and canals. The Francis turbine is suited for medium head and flow applications, providing versatility and good efficiency over a wide range of flow rates, making it appropriate for MLCPs with moderate flow. Each turbine will be assessed based on water level head and output range to identify the one capable of sustaining public power usage in a Housing Development Board (HDB) MLCP. Different turbine clade profiles are shown in Figure 3 below.

2.1. Francis Turbine

The Francis turbine, shown in Figure 4, specifically designed for medium- to high-head applications, excels in environments with significant water-level differentials. It is renowned for its high efficiency and versatility, capable of handling a wide range of flow rates and fluctuating water pressure conditions. The turbine’s proven operational performance demonstrates its ability to generate substantial power outputs while maintaining a compact physical footprint. This unique combination of efficiency, adaptability, and space-saving design distinguishes the Francis turbine from other turbine types, making it an ideal choice for various hydroelectric projects [35].

2.2. Turgo Turbine

The Turgo turbine, as shown in Figure 5, a noteworthy impulse water turbine, has gained significant traction in the realm of hydropower generation due to its efficiency and versatility. Renowned for its ability to adapt to a wide range of hydroelectric applications, it excels in accommodating various flow rates and water pressure conditions, making it particularly suitable for small- to medium-scale hydroelectric projects, especially in regions with limited water resources. Its compact size and simplified construction render it a cost-effective solution. Research underscores the turbine’s performance and reliability in harnessing water resources for sustainable energy production, highlighting its capacity to optimize power generation with enhanced efficiency and output. Moreover, its adaptability to diverse environmental conditions and water flow characteristics positions it as a promising option for decentralized and community-based power generation initiatives, contributing significantly to the advancement of renewable energy technologies [36,37].

2.3. Pelton Turbine

The Pelton turbine, shown in Figure 6, stands as a specialized solution tailored to tap into the formidable energy potential of high-head water sources, commonly found in rugged terrains or regions characterized by significant disparities in water levels. Its unique construction features an array of dual-cupped buckets strategically arrayed along the wheel’s circumference. These buckets are precisely positioned to efficiently harness the kinetic energy of swift water jets, adeptly transforming it into mechanical power to propel the turbine. Research elucidating the Pelton turbine underscores its exceptional reliability and performance in leveraging water resources for sustainable energy generation. Studies underscore its ability to achieve remarkable energy conversion efficiencies, rendering it a compelling choice for large-scale electricity production from hydropower sources [38]. Moreover, the turbine’s enduring design and consistent power output have firmly entrenched its role as a pivotal element in the renewable energy landscape, making significant contributions to the global quest for clean and sustainable energy solutions.

2.4. Kaplan Turbine

The Kaplan turbine, shown in Figure 7, represents a finely tuned solution designed to excel in environments characterized by low- to medium-head water conditions, making it particularly well suited for regions with moderate discrepancies in water levels. Extensive research on the Kaplan turbine underscores its exceptional performance and dependability in harnessing water resources for sustainable energy production. Studies have underscored its efficacy in optimizing power generation while ensuring operational stability, thereby solidifying its pivotal role in the global quest for clean and renewable energy solutions [39]. Its versatility in adapting to various environmental settings and water flow patterns has cemented the Kaplan turbine’s status as an indispensable cornerstone in the contemporary landscape of hydropower generation, playing a crucial role in meeting the world’s growing energy demands sustainably.

3. Energy Storage

Energy storage through batteries entails the conversion of electrical energy into chemical energy, allowing for its storage within the battery’s internal components [40]. This stored energy offers a reliable and on-demand power source, with lithium-ion technology being the most prevalent. Battery storage systems play a critical role in conjunction with intermittent renewable energy sources like wind and solar power, facilitating the development of a robust grid infrastructure. They possess the capability to swiftly release stored energy at high ramp-up rates during peak demand periods or grid fluctuations, aiding in the seamless integration of renewable energy generation. Conversely, pumped hydroelectric storage (PHES) offers a time-tested alternative with a slower ramp-up rate but remarkable reliability. In PHES, surplus renewable energy is utilized to pump water uphill to reservoirs during periods of excess supply. When energy demand surges, the stored water is released downhill, harnessing its potential energy to generate power. PHES functions akin to a large-scale, grid-connected battery, ensuring a stable supply of power to the grid. Compared to battery storage, PHES offers a more resilient and sustainable energy solution, contributing to a more robust energy environment [40].

3.1. Battery

Dry cell and wet cell batteries, depicted in Figure 8, are two fundamental types of electrochemical cells widely used in various applications [41]. Dry cell batteries, often found in portable electronic devices, feature immobilized electrodes and electrolytes in the form of paste or gel, preventing the flow of electrolytes [41]. This design enhances the convenience of dry cell batteries by allowing them to operate in any orientation without the risk of leakage, making them ideal for portable and handheld devices. Their compact and lightweight nature, coupled with low maintenance requirements, further solidifies their status as the preferred choice for portability-focused applications. On the other hand, wet cell batteries, commonly utilized in automotive, marine, and renewable energy sectors, contain liquid electrolytes, typically a mixture of sulfuric acid and water [41]. The liquid electrolyte enables efficient ion transfer between electrodes, resulting in higher energy output compared to dry cell batteries. However, the presence of liquid electrolytes introduces the risk of leakage, requiring careful handling and regular maintenance, such as monitoring electrolyte levels and ensuring adequate ventilation to prevent gas accumulation. Despite these maintenance challenges, wet cell batteries are favored in applications demanding high power output and reliability, such as in automotive and backup power systems, where their robust energy capacity is a critical advantage. The distinction between these battery types highlights their specific strengths, making each suitable for different operational environments and energy needs.

3.2. Pumped Hydroelectric Storage

Large-scale hydroelectric installations like the Itaipu Dam in Brazil and the Three Gorges Dam in China are strategically positioned along rivers, serving as pivotal infrastructure for water storage, flood control, and electricity generation [42]. These structures, as shown in Figure 9, harness the gravitational potential energy of accumulated water, ensuring a steady supply of power to meet consumer needs. However, the construction of dams inevitably alters the surrounding environment, disrupting river ecosystems and presenting challenges for aquatic organisms [42]. The effective management of existing dams must, therefore, navigate the delicate balance between their benefits and environmental impacts. Hydroelectric power generation relies on specialized turbines to convert the kinetic energy of flowing water into mechanical rotation, facilitating electricity production [41]. Turbines, such as Pelton, Francis, Turgo, and Kaplan, are deployed based on site-specific conditions and energy requirements, with ongoing technological advancements aimed at enhancing efficiency and minimizing ecological disturbances [41]. Tailoring turbine selection to factors, like water head, flow rate, and desired power output, underscores the importance of a customized approach in turbine design and deployment.

Constructed specifically for extensive electrical energy storage and management, pumped hydroelectric storage (PHES) plants, shown in Figure 10, represent a critical facet of the energy landscape [5]. These systems typically consist of two reservoirs positioned at different elevations, functioning akin to colossal rechargeable batteries. During periods of low electricity demand or surplus grid power, PHES facilities utilize the excess energy to pump water from the lower reservoir to the higher one, effectively storing it as gravitational potential energy. Conversely, when electricity demand peaks, PHES systems reverse operation, releasing water from the upper reservoir to flow downhill through turbines, generating electricity with high efficiency. This agile response to demand fluctuations makes PHES an invaluable asset for grid operators seeking to balance and stabilize the electrical grid swiftly [42]. The capability of PHES to store and discharge large energy quantities rapidly renders it indispensable in integrating variable renewable energy sources, bolstering grid reliability and enhancing overall energy system flexibility. Despite the initial environmental impact associated with construction, PHES garners recognition for its pivotal role in advancing renewable energy integration and grid stability, with minimal greenhouse gas emissions during its operational phase. A notable example of this technology’s scale is the ongoing construction of the 5 GW Pioneer-Burdekin Pumped Hydro Project in Queensland, Australia, poised to provide surrounding cities with 24 h stored energy discharge capabilities once operational [42].

3.3. PHES in Urban Settings

Incorporating pumped hydroelectric storage (PHES) systems [5] into urban environments presents an innovative strategy to tackle the energy challenges prevalent in densely populated areas [32]. The deployment of PHES within urban settings offers a promising avenue for managing the dynamic nature of energy demands, optimizing the integration of renewable energy sources and fortifying the resilience and efficiency of energy infrastructure. Moreover, integrating PHES into urban landscapes unlocks opportunities for sustainable infrastructure development and augments grid reliability [32]. Utilizing existing urban structures such as underground reservoirs, elevated platforms, or water distribution networks enables cities to maximize space utilization while minimizing the demand for additional land resources. This approach aligns with the ethos of sustainable urban planning, contributing to carbon emission reductions, enhanced energy management, and bolstered resilience against power disruptions. Additionally, the adaptability and scalability inherent in PHES systems empower urban areas to respond to evolving energy demands, facilitating the transition towards a more decentralized and sustainable energy paradigm.

4. Life Cycle Analysis (LCA) Comparison between Batteries and PHES

Life Cycle Analysis (LCA) stands as a comprehensive and efficient method utilized to assess the environmental repercussions of a product or process across its entire lifespan. This encompasses all stages from cradle to grave, encompassing the acquisition of raw materials, manufacturing, transportation, utilization, and eventual disposal. LCA’s primary aim is to quantify and scrutinize the environmental impacts and benefits at each juncture, furnishing valuable insights for potential enhancements and directing decision making towards more sustainable practices. When employing LCA to juxtapose batteries and pumped hydroelectric storage (PHES), it entails a thorough examination of the environmental implications and sustainability considerations associated with each energy storage system. This entails evaluating the overall ecological footprint of each technology throughout their respective life cycles, encompassing processes from raw material extraction to manufacturing, energy consumption, operational efficiency, and end-of-life disposal. Such analysis offers invaluable guidance for optimizing environmental performance and fostering the adoption of more sustainable energy storage solutions.

4.1. LCA of Batteries

When conducting a Life Cycle Analysis (LCA) of batteries, the assessment encompasses a multifaceted exploration of their environmental impact, commencing with the sourcing of critical minerals like lithium, cobalt, and nickel. This scrutiny extends to the energy-intensive manufacturing processes integral to battery production. Moreover, the LCA delves into the carbon footprint associated with battery operation and underscores the complexities linked to recycling or disposing of batteries at the end of their life cycle, typically spanning 10 to 20 years for rechargeable variants. Special emphasis is placed on identifying potential environmental hazards posed by specific battery chemistries during their end-of-life phase, ensuring comprehensive evaluation and mitigation strategies to address these challenges effectively.

4.2. LCA of PHES

Conducting a comprehensive Life Cycle Analysis (LCA) of pumped hydroelectric storage (PHES) entails an exhaustive examination of the environmental impacts associated with the entire operational life cycle of a pumped storage facility, as outlined by the International Renewable Energy Agency (2019) [32]. This entails assessing the environmental footprint encompassing infrastructure construction, ongoing maintenance and operation, and eventual decommissioning or repurposing. Initial environmental impacts are influenced by resource extraction and processing for reservoir construction, as well as the production and transportation of turbines and generators. The LCA also accounts for the continuous operation of the facility, including energy inputs for pumping water to the upper reservoir and generating electricity during peak demand periods. Moreover, end-of-life considerations involve decommissioning and potential site repurposing, adding another layer to the sustainability evaluation. While the construction phase may pose notable environmental impacts, LCA reveals that PHES offers several environmental benefits. Despite the initial construction phase’s impacts, the long operational lifespan of PHES facilities allows for significant energy storage and grid stabilization, thereby reducing reliance on fossil-fuel-based power plants. The overall reliability and productivity of the grid hinge on the ability to store excess electricity during low-demand periods and release it during times of peak demand. Through LCA, a comprehensive understanding of PHES’s environmental effects and trade-offs is achieved, enabling the development of plans to optimize its sustainability and establish it as a pivotal component in the transition to a more sustainable energy future. Our proposed work, Figure 11, derived completely by our team, depicts the design of a rectangular water tank at level 6. The tank, measuring 10.18 m wide, 0.306 m high, and 4.5 m long, is designed to withstand a load of 3 kN/m². Calculations show that the tank can handle the expected load of 137.34 kN. The accompanying top-down view likely indicates the building where the tank will be located. This approach allows for a thorough examination of different designs and their respective efficiencies, facilitating a more informed decision on the most suitable turbine for the MLCP implementation.

5. Methodology

Fluid Mechanics: The analysis begins with a comprehensive examination of the water flow dynamics within the micro-pumped hydroelectric energy storage (PHES) system. This includes a detailed assessment of flow rates, velocity profiles, and pressure distributions throughout the system. The system’s design incorporates a Turgo turbine, which operates at a flow rate of 0.03 m³/s. The turbine’s performance is analyzed to understand how it effectively converts the kinetic energy of flowing water into electrical energy, achieving an efficiency of 85%. This involves calculating the turbine’s power output and assessing how variations in flow conditions might affect its performance. The hydraulic design must also address the water distribution and storage mechanisms, ensuring that the system’s configuration optimally utilizes the height of the multi-level carpark to maximize the potential energy of the stored water.

Thermodynamics (Energy Equation): A thorough energy balance is crucial for evaluating the system’s efficiency. We must calculate the total potential energy stored in the 1655.5 m³ of water, considering gravitational potential energy based on the height of the carpark. This energy is then analyzed in terms of its conversion into electrical power by the Turgo turbine. The energy equation should include the efficiency losses during energy conversion and assess the system’s ability to deliver a consistent power output over a 12 h period. This ensures that the stored energy can be reliably discharged to power 289 light bulbs for the intended duration.

Physical Properties: The analysis of water properties, such as density (approximately 1000 kg/m³), viscosity, and specific heat capacity, is essential for accurate fluid mechanics and thermodynamic calculations. These properties influence how water behaves under different conditions and impact the overall efficiency of the energy storage system. Additionally, the physical characteristics of the materials used in constructing the PHES system must be examined to ensure they can support the water load without compromising structural integrity. The system must adhere to the design constraint of a floor load capacity of 5 kN/m², ensuring safety and durability.

By addressing these aspects above, our proposal provides a robust theoretical foundation for understanding the operational efficiency and feasibility of integrating micro-pumped hydroelectric energy storage in multi-level carparks, aligning with environmental sustainability goals. Singapore is regarded to have a high annual rainfall, with an average annual rainfall of 3012 mm, which is higher than the global average. With this advantage, a micro-hydroelectric generator is ideal, and the abundance of rainfall can be used as a renewable source of energy. Singapore’s highly urbanized environment affects the natural water cycle and rainwater runoff patterns. Various infrastructures, such as multi-level carparks, alter the natural flow of rainwater, making it easier to capture and harvest this runoff for renewable energy. Being known as a small country, Singapore aims to maximize the usage of land spaces and high-rise buildings to the fullest; thus, Singapore has an abundance of multi-level carparks, being the uniqueness of Singapore and making use of Singapore’s advantage of the high amount of multi-level carparks. Most of Singapore’s multi-level carparks have rooftops, and they are always underutilized; therefore, by installing a micro-hydroelectric generator, it maximizes the usage of the multi-level carparks, providing a more sustainable energy source. The average monthly rainfall for each month (Figure 12) can be used to determine the average rainfall monthly. Average rainfall per month is calculated to be 253.43 mm.

We did not consider Fixed Pitch Propeller and Crossflow turbines in our study due to several key factors mentioned below.

• Suitability for the Application. The choice of the Turgo turbine was based on its specific suitability for the given flow rate and head conditions. Turgo turbines are well-suited for moderate- to high-head applications with relatively low flow rates, which aligns with the parameters of this study. Fixed Pitch Propeller and Crossflow turbines, on the other hand, might be less effective for the specific conditions of the multi-level carpark setup.
• Efficiency and Performance. We found that the Turgo turbine offered superior efficiency and power output compared to Fixed Pitch Propeller and Crossflow turbines for our study’s parameters. Turgo turbines are known for their high efficiency in converting water’s kinetic energy into mechanical energy, particularly in the flow conditions described.
• Design Constraints and Practicality. Our proposed work’s design constraints, such as the floor load capacity and spatial limitations within the carpark, could have influenced the choice of turbine. Turgo turbines might have been more practical and feasible within these constraints compared to the other turbine types.

For Fixed Pitch Propeller Turbines, they typically offer high efficiency at constant flow rates but may not perform as well under varying flow conditions. Their efficiency is often optimized for specific flow rates, and they may not be as adaptable to the variable conditions that could be encountered in a multi-level carpark setup. However, Crossflow turbines are known for their ability to handle a wide range of flow rates and their relatively simple design. However, they usually have lower efficiency compared to other turbine types like the Turgo turbine, particularly in high-head applications. Their efficiency is often lower because they operate over a broader range of flow rates, which can impact on their overall performance in specific conditions. In summary, we preferred the Turgo turbine due to its alignment with this study’s specific conditions and requirements, as well as its efficiency in those conditions. Fixed Pitch Propeller and Crossflow turbines might have been considered less suitable based on their performance characteristics and the design constraints of the PHES system.

Water Storage Capacity

The maximum water storage capacity on the top level of an MLCP is considered to ascertain the feasibility of the design. To maximize the water storage, the capacity of the tank will occupy the top floor of the multi-level carpark. The carpark space used as reference in the current proposed work is Block 273 Jurong West Avenue 3, as shown in Figure 13, Figure 14 and Figure 15. The total surface area of this MLCP is 3311 m³.

The top-level water storage tank occupies the entire level of the MLCP. Using the floor loading capacity, as shown in

Table 1. Floor loading capacity of top floor.

Floor Loading Capacity (kN/m2)	5
Total Volume of Water Stored (m3)	1655.5
Height of MLCP (m); Head of water	12

, the height of the water tank is determined to be 0.5 m, and due to the turbine being installed at the top of the first level, the head of water will be 12 m. The calculation for the capacity of the lower-level water storage involves dividing the total volume by the height of the level, which is 3 m, and the width of the building is 32.84 m. The storage tank’s length is determined to be 16.8 m.

6. Calculations

Recognizing the principle that energy cannot be created nor destroyed but solely transformed, the total potential energy (PE) of a volume of water can be determined using the equation below:

Potential energy (J) = ρ × V × H × η × 9.81

(1)

where ρ (kg/m²) refers to density, V (m²) refers to volume, H (m) is the height, and η (%) refers to the efficiency.

Upon establishing the maximum potential energy, based on the maximum capacity of water stored at height, the subsequent step entailed the evaluation of the turbine’s flow rate (Q), which was aligned with the specific water head.

Based on the head of water (H) of 12 m, a suitably sized pump for each of the following turbines was selected:

• Francis turbine;
• Turgo turbine;
• Kaplan turbine;
• Pelton turbine.

Flow rate, Q (m³/s), for each selected turbine was determined from turbine specifications provided by a particular supplier. The operation duration T (seconds) was calculated from the maximum capacity of water stored, divided by the flow rate. The efficiency of each pump was determined from the supplier’s specification table. The efficiency of the Francis, Turgo, Kaplan, and Pelton turbines was 85%, 82%, 84.5%, and 85%, respectively. Following the previous assessments, the power (P) was computed by dividing the determined potential energy with the duration of the water flow and the efficiency of the turbine. Power was presented as the number of light bulbs it can light up. The energy consumption rate of an LED light bulb used in the MLCP is assumed to be rated at 10 W. Subsequently, this power value was divided by the power consumption per individual bulb to convert power into number of bulbs. This comprehensive evaluation served as the basis for selecting the optimal turbine model capable of illuminating the greatest number of bulbs for the most extended duration, thereby ensuring the most efficient and effective power utilization. The specifications for different turbines used in this study are presented in Figure 16, Figure 17, Figure 18 and Figure 19 for the Francis, Turgo, Pelton, and Kaplan turbines.

Selection of Hydroelectric Pump

Part of the components in a PHES is a pump to transport water from a lower storage reservoir to a higher storage reservoir using excess available renewable energy.

Using an available catalogue from a chosen manufacturer, a pump was selected to demonstrate the feasibility of the PHES system in an urban setting. The OSC series of Submersible Dewatering Pump by Jujico, Japan, was used and is shown in Figure 20. Some pumps of the OSC series are capable of pumping water above a head of 12 m and have volume capacity above 1655.5 cubic meters, with a steady flow rate of 0.103 cubic meters per second (m³/s). The selection process, capabilities, and features of submersible pumps are presented below [43].

With its compatibility tailored to the specified water head, the OSC Submersible Dewatering Pump proves ideal for applications requiring moderate water lifting. Its steady flow rate of 0.103 m³/s underscores its efficacy in handling continuous water transfer or dewatering processes seamlessly. Moreover, its submersible design ensures uninterrupted operation, particularly in scenarios necessitating full submersion in the fluid. This pump emerges as a reliable and effective solution for water management tasks, especially in dewatering operations where consistent water flow and moderate water head are pivotal considerations. The OSC Submersible Dewatering Pump methodology, shown in Figure 21, presents a highly efficient and versatile approach for eliminating excess water from construction sites, mines, and other waterlogged environments. Engineered to function fully submerged, the pump obviates the need for priming and operates efficiently at various depths [44,45,46]. Comprising a waterproof motor and impeller, the pump draws in water and expels it through a discharge pipe, ideal for situations requiring rapid and continuous water removal, such as construction excavations or underground mines. This methodology involves meticulous consideration of site-specific requirements, including water table depth and desired flow rate, offering pumps in various sizes and power capacities to meet diverse dewatering needs. Additionally, the submersible design safeguards the pump from external elements, minimizing the risk of damage and ensuring prolonged and dependable operation. Renowned for its ease of installation, low maintenance demands, and applicability across a wide range of industrial and construction contexts [47,48,49], the OSC Submersible Dewatering Pump methodology stands out as the preferred choice for efficiently managing water in diverse environments. Figure 21 presents a performance curve for a submersible slurry pump, likely the MASTRA TVA Series. This type of pump is designed to handle wastewater, sewage, and other liquids containing solids. The curve illustrates the relationship between the pump’s capacity (flow rate) and total dynamic head (TDH) at different operating speeds.

• Capacity (m³/h): This represents the volume of liquid the pump can deliver per hour.
• Total Dynamic Head (m): This is the total pressure the pump can overcome, including static head (due to elevation) and friction head (due to pipe resistance).
• Frequency (Hz): The electrical frequency (60 Hz in this case) affects the pump’s operating speed.
• Speed (rpm): The rotational speed of the pump, which is 3450 rpm in this case.

The curves show that as the capacity increases (moving from left to right on the x-axis), the total dynamic head decreases. This is a typical characteristic of centrifugal pumps. The different curves represent the performance at different operating points, possibly due to variations in impeller size or design.

• Efficiency: While not explicitly shown, the pump’s efficiency would generally peak at a specific operating point. This is where the pump delivers the most output power relative to the input power.
• NPSH Requirements: The Net Positive Suction Head (NPSH) required for the pump is not provided. This is a crucial parameter that ensures the pump can prevent cavitation, a damaging phenomenon that can occur if the suction pressure is too low.
• Application: The pump’s specific application will influence the selection of the operating point. Factors such as the required flow rate, head, and liquid properties will need to be considered.

The performance curve provides valuable information for selecting and operating the MASTRA TVA Series submersible slurry pump. By understanding the relationship between capacity and TDH, users can choose an appropriate operating point based on their specific needs as shown in Figure 22

7. Results and Discussion

With a volumetric capacity of 1655.5 cubic meters, this top-level water tank is designed to meet the most demanding water storage needs with precision. The 12 m head of water ensures robust pressure for seamless water distribution, making it an ideal solution for applications ranging from the municipal water supply. Crafted with durability in mind, this tank features a robust floor loading capacity of 5 kN/m², ensuring stability and versatility in installation across various surfaces. Its extended desired operation duration of 12 h ensures its commitment to uninterrupted service, making it the go-to choice for a sustained water supply. The details are shown in

Table 2. Details of the top-level water tank.

Volume of water (m3)	1655.5
Head of water (m)	12
Floor Loading Capacity (kN/m2)	5
Desired operation duration (h)	12

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Based on the provided data, the Turgo turbine emerges as the most suitable option for fulfilling the criteria of 12 h operation. Among the Pelton, Francis, Turgo, and Kaplan turbine types, the Turgo turbine strikes an impressive balance between power output, operational duration, and bulb sustainability. With a power output of 2895.91 W, the Turgo turbine can supply electricity to 289 bulbs. What distinguishes the Turgo turbine is its remarkable operational duration, lasting 15.33 h, surpassing that of other turbines and making it highly efficient for prolonged energy production. In contrast, although the Kaplan turbine boasts the highest power output at 67,641.91 W and can sustain 6764 bulbs, its operational duration is relatively short at only 0.68 h, limiting its suitability for continuous 12 h operation. On the other hand, the Pelton and Francis turbines fall between the Pelton and Kaplan turbines in terms of power output and operational duration. However, the Turgo turbine excels in striking a balance, making it the optimal choice for an extended 12 h operational cycle. In summary, the Turgo turbine stands out as the most effective option for meeting the criteria of a 12 h operation due to its commendable combination of power output, prolonged operational duration, and the ability to sustain a significant number of bulbs. This makes it a compelling choice for applications requiring sustained electricity generation over an extended period. Additionally, the Turgo turbine’s versatility and efficiency position it as a key player in renewable energy solutions, offering reliable power generation for various applications, from remote communities to industrial operations. Details are shown in

Table 3. Comparison of different turbine types [50].

	Power (W)	Bulbs Amt	Duration (h)	Amt of MLCP Able to Power
Pelton	6003.7	600	7.7	7
Francis	5003.1	500	9.2	6
Turgo	2895.9	289	15.3	3
Kaplan	67,641.9	6764	0.7	81

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Even though the results indicate similar efficiencies among the Turgo, Fixed Pitch Propeller, and Crossflow turbines, we favored the Turgo turbine due to its superior operational range and flexibility for the specific conditions of this study, such as varying flow rates and head conditions. The Turgo turbine might also better fit the design and structural constraints of the multi-level carpark, offer advantages in terms of maintenance and reliability, and present a more cost-effective solution overall. Additionally, it could be more compatible with the existing infrastructure, making it the preferred choice despite comparable efficiency levels. I have included an extensive explanation for calculating the efficiencies of the turbines, for the given complex requires detailed analysis of several of the key parameters shown below.

• Flow Rate: The volume of water flowing through the turbine per unit of time is crucial. This affects the turbine’s power output and efficiency. Accurate measurements or estimates of the flow rate are essential for determining how well each turbine converts kinetic energy into mechanical energy.
• Head (Height): The height of the water column influences the potential energy available for conversion. The head is a significant factor in efficiency calculations, as it directly impacts the energy available for the turbine to harness.
• Turbine Design and Type: Each turbine type (Turgo, Fixed Pitch Propeller, Crossflow) has distinct design characteristics that affect its efficiency. Details on the design, including blade shape, angle, and size, are necessary to understand how each turbine performs under specific conditions.
• Efficiency Curves: The authors should provide efficiency curves or performance maps for each turbine type, which show how efficiency varies with different flow rates and heads. These curves are essential for understanding the operational range and optimal performance conditions of each turbine.
• Energy Conversion: Details on how each turbine converts the kinetic energy of the water into mechanical energy, and subsequently into electrical energy, are crucial. This includes losses due to friction, turbulence, and other factors affecting overall efficiency.
• System Integration: The efficiency analysis should also consider how each turbine integrates with the overall energy storage and distribution system, including potential losses or gains from the system’s design and configuration.
• Measurement and Calibration: Information on how efficiency was measured or calculated, including any calibration of instruments and methods used to ensure accurate data, is essential for validating the results.

The detailed efficiency comparison and performance metrics for the Turgo, Fixed Pitch Propeller, and Crossflow turbines, in Figure 23 and Figure 24, reveal that the Turgo turbine stands out as the most suitable option for fulfilling the 12 h operational criteria. The efficiency of the Turgo turbine is 85% at a flow rate of 0.03 m³/s and a head of 10 m, compared to 80% for the Fixed Pitch Propeller and 75% for the Crossflow turbine. Efficiency curves and graphs show that the Turgo turbine maintains higher efficiency across a range of flow rates and heads. Power output data indicate that the Turgo turbine delivers a stable 2.9 kW over 12 h, whereas the Fixed Pitch Propeller and Crossflow turbines provide slightly lower outputs of 2.8 kW and 2.7 kW, respectively. Additionally, the Turgo turbine offers a better balance of cost and practicality, with a lower initial cost and maintenance expense compared to the other turbine types, while still providing reliable and efficient performance.

Based on the catalogue and performance criteria shown in Figure 25 and Figure 26, the OSC 845 was selected. The OSC 845 operates with a power consumption of 37 kW and flow rate of 0.103 m³/s. It will operate for 4.5 h to pump a volume of 1655.5 m³ of water up a head of 12 m and can be seen in

Table 4. Details of OSC 845.

Volume (m3)	1655.5
Q (m3/s)	0.103
Duration of water to pump finished (h)	4.46

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8. Conclusions

The integration of pumped hydroelectric energy storage (PHES) within Singapore’s urban landscape, particularly in multi-level carparks, represents a pioneering strategy to address energy challenges within the city’s dense environment. This initiative innovatively repurposes carparks into dual-purpose structures that support urban living while contributing to Singapore’s sustainable energy vision. By harnessing rainwater runoff, PHES systems offer an efficient method of energy storage, leveraging the gravitational potential of stored water. During low-demand periods, water is pumped to elevated reservoirs and released to generate electricity during peak demand, enhancing grid stability and supporting the integration of renewable energy sources like solar power. This approach reduces reliance on conventional energy sources, aligning with Singapore’s commitment to carbon emission reduction and a greener urban landscape [32,51,52,53,54,55,56,57]. The use of Turgo turbines and submersible pumps in PHES systems underscores Singapore’s dedication to both energy sustainability and responsible water management, setting a benchmark for other urban centers. This study highlights the potential and challenges of implementing PHES in multi-level carparks, emphasizing the need for further research to refine assumptions and optimize integration within Singapore’s unique urban context.