By Archit Patnaik, PE, PMP | Senior Project Manager, Pure Power Engineering
For municipal water utilities, the math is often as challenging as the geography. Across the United States, water and wastewater treatment facilities represent some of the largest electrical loads for local governments. According to the U.S. Environmental Protection Agency (EPA), energy costs account for a staggering 25% to 30% of typical water utility operation and maintenance (O&M) budgets. In regions like the County of Maui, Hawai‘i, these costs are further exacerbated by limited land availability, high electricity prices, and the logistical nightmare of importing energy-intensive resources to an island chain.
When a water utility’s primary operational expense is the electricity required to move water through pumps, boosters, and treatment plants, finding a way to generate power on-site is not merely a sustainability goal—it is a fiscal imperative. A recent initiative, delivered in partnership with Johnson Controls, has turned the County of Maui’s circular concrete water reservoirs into high-efficiency, non-penetrating solar platforms. This project serves as a masterclass in overcoming complex structural, regulatory, and mechanical constraints to unlock the potential of existing municipal infrastructure.
The Core Challenge: Infrastructure as Energy Generation
The logic of mounting solar arrays on water reservoirs is sound: the facilities already exist, they require significant power, and they occupy land that would otherwise be unproductive. However, the engineering reality is far more complex than a standard rooftop installation. A reservoir is not a commercial building; it is a critical, pressurized containment vessel.
The project sought to install solar arrays ranging from 105 to 141 kW-DC on the circular roofs of aging concrete storage tanks. The engineering team faced four primary hurdles:
- Structural integrity: Protecting the tank from roof penetrations and seismic overloading.
- Geometric constraints: Mapping rectangular solar modules onto circular surfaces.
- Operational access: Ensuring that the solar installation did not impede essential maintenance, sanitary inspections, or ventilation.
- Wind-load dynamics: Managing the interaction between uplift forces and the structural load-bearing capacity of the tank.
Chronology: A Multi-Phased Engineering Approach
The success of the Maui portfolio was rooted in a methodical, phased execution that prioritized the longevity of the reservoir over the immediate gains of the solar installation.
Phase 1: Structural Audit and Feasibility
Before a single bracket was designed, the team initiated a comprehensive structural evaluation. Because these reservoirs were built decades ago, original construction drawings were cross-referenced with modern field assessments. The primary goal was to establish the “gravity budget” of the roof. Solar PV is relatively lightweight, but the addition of ballast—necessary to avoid penetrating the waterproof membrane—required a rigorous understanding of the tank’s seismic behavior.
Phase 2: Geometry and Layout Optimization
The transition from a circular roof to a rectangular solar array necessitated advanced layout modeling. The design team had to account for the "inner disc" of each reservoir while maintaining mandatory setbacks from the curved perimeter. Furthermore, the layout had to clear critical infrastructure such as access hatches, air vents, and overflow points. The result is a unique, "stepped" array design that maximizes energy density without sacrificing the functionality of the tank’s essential systems.
Phase 3: The Wind-Seismic Equilibrium
The most significant technical breakthrough in this project was the integrated approach to wind and seismic loading. In an open environment, high-profile tanks are subject to significant wind uplift. While typical installations use heavy ballast to counteract this, the project team had to avoid adding so much weight that the tank would fail during a seismic event. Because the water inside the reservoir adds its own lateral force during an earthquake, the “seismic budget” was already partially consumed. The engineers had to iteratively solve for wind and seismic loads simultaneously, opting for a low-tilt design that minimized wind drag while keeping the ballast mass within safe structural margins.
Supporting Data: Why Engineering Precision Matters
The technical specifications of the Maui project highlight the nuance required for public utility solar. Adhering to the 2018 International Fire Code (IFC), Section 1204.3, the project designers successfully utilized an exception for structures with a footprint of 250 feet or less, reducing the required perimeter fire-access pathway from six feet to four feet. This allowed for an optimized layout that recovered significant usable square footage.
Furthermore, the electrical balance-of-system (BOS) was intentionally decoupled from the tank structure. By placing inverters, disconnects, and panelboards on freestanding racks at grade, the project avoided the complications of mounting vibration-prone, heavy electrical equipment on the reservoir walls. The conduit runs were meticulously engineered to account for the thermal expansion and contraction of the concrete, ensuring the structural coating of the tank remained compromised.
Official Perspectives: Sanitary and Regulatory Compliance
The EPA emphasizes that finished-water storage systems are vulnerable to contamination if access points are not maintained. For the County of Maui, the solar array was viewed not just as a power plant, but as a potential impediment to sanitary safety if designed incorrectly.
"The reservoir is the priority," notes the project engineering team. "A solar array lasts 25 to 30 years, but these tanks are expected to last much longer. We designed the system to ensure that maintenance crews still have clear access to every hatch and vent."
By keeping the array low-profile and routing conduits to avoid ponding areas on the roof, the team ensured that the solar installation does not create a "sanitary risk" through water trapping or blocked ventilation. The design ensures that the tank can be fully inspected, cleaned, or recoated without needing to dismantle the entire solar array.
Broader Implications for Municipal Utilities
The Maui water portfolio project offers a scalable blueprint for other municipalities globally. As the energy transition accelerates, utilities are under mounting pressure to achieve net-zero targets while managing rising operational costs.
Key Takeaways for Utility Managers:
- Asset Optimization: Existing assets like reservoirs, parking lots, and land around treatment plants should be the first candidates for renewable energy deployment.
- Integrated Design: Projects of this nature require a multidisciplinary approach involving structural, electrical, and civil engineers, alongside public health specialists.
- Repeatability: By developing a standard methodology—structural assessment, non-penetrating ballast, decoupled electrical equipment, and clear maintenance access—utilities can significantly lower the engineering costs for subsequent installations across their portfolio.
The Economic Value Proposition
The financial implications extend beyond simple utility bill offsets. By generating power at the point of consumption, the utility reduces transmission losses and provides a hedge against the volatility of grid-based electricity prices. In remote locations like the Hawaiian islands, where energy security is a critical concern, the ability to offset 105 to 141 kW of load per reservoir significantly enhances the resilience of the local water network.
A New Standard for Infrastructure
The Maui reservoir project proves that we do not always need to clear new land to build the energy transition. Sometimes, the most valuable real estate is already beneath our feet—or, in this case, on top of our water supply.
By treating the reservoir as a complex, living infrastructure asset rather than just a flat surface, the project team successfully balanced the competing needs of power generation and water security. This methodology is no longer a one-off experiment; it is a replicable standard for the future of municipal utilities. As more regions grapple with the dual challenges of water scarcity and energy costs, the "Maui Model" provides a clear path forward for integrating renewable energy into the backbone of our essential public services.
About the Author
Archit Patnaik, PE, PMP is a Senior Project Manager at Pure Power Engineering. He specializes in the design and delivery of utility-scale and commercial solar PV and battery energy storage systems (BESS). With a focus on structural and electrical integration, Archit leads engineering teams to solve the most complex deployment challenges for asset owners and EPCs. He is a NABCEP-certified PV Installation Professional and is dedicated to advancing the technical standards of the renewable energy industry.
