Mercer Island council hears options for solar, stormwater and storage at public safety/maintenance facility

City officials and consultants on May 6 presented Mercer Island City Council with a set of schematic-design options for the city’s planned public safety and maintenance (PSM) facility, including three solar-array configurations, rainwater harvesting for nonpotable reuse, on-site potable water storage for emergencies and a hybrid approach to structural systems.

The presentation, led by the city manager’s office and consultants from Northwest Studio and PAE Engineers, outlined capital costs, 25‑year energy and water savings estimates and resilience benefits. Consultants said an expanded solar array sized to meet 100% of projected facility energy needs would increase capital cost but also increase long‑term energy savings; rainwater harvesting could offset a majority of nonpotable demand; and a modest potable storage tank would improve the building’s ability to operate during water outages.

Why it matters: the PSM facility will consolidate public safety and operations functions and is being designed both to meet everyday operating needs and to improve resilience during emergencies (power or water outages) on the island. Council direction tonight will shape the schematic shown to voters and contractors as the project proceeds toward cost estimating and, if approved, construction.

Most important details - Solar: Consultants presented three options: a limited array on the south‑facing roof, an expanded array that adds panels on the north roof to meet a larger share of annual energy needs, and no onsite solar. The limited southern array was estimated to cover roughly 60–63% of the facility’s projected annual energy use with an estimated 25‑year energy‑cost savings of about $975,000 and a system capital cost of about $1,400,000; the expanded array (sized to approach full estimated demand) had a 25‑year savings estimate of about $1,445,000 and an estimated capital cost just under $2,200,000. Consultants said the calculations assume a 25‑year performance period and a 0.45% annual panel degradation and used available utility rate schedules for forecasts.

- Emergency power: the schematic assumes a diesel generator sized to run the facility for about seven days; on‑site solar was estimated to potentially extend that duration by roughly 30% (an approximate extension to nine–10 days under typical usage), with longer operation possible if facility loads are reduced during an emergency.

- Stormwater and potable storage: rainwater harvesting sized with a 10,000‑gallon cistern was modeled to provide roughly 75% of projected nonpotable demand and, in the consultants’ analysis, a 20‑year net operational savings relative to capital cost. A potable water storage option (5,000 gallons for seven days’ potable storage) was presented as a resiliency measure; combined approaches (rainwater harvesting plus a smaller potable tank) produced greater aggregate savings in the consultants’ tables than potable storage alone. Consultants noted irrigation demands for landscape could require substantially larger storage to be fully supplied by harvested rainfall.

- Structural systems: staff proposed a hybrid approach. Concrete would be used for foundations, retaining walls and portions of the operations building; mass timber was proposed for the primary PSM building and upper floors (cited for speed of erection and alignment with climate goals); and steel or hybrid steel/timber systems for flexible canopy/yard areas.

Council discussion and direction Councilors asked detailed questions about the financial assumptions (utility rates, net‑metering limitations, maintenance costs and warranty assumptions), the interaction between solar, battery storage and EV fleet charging, and the operational maintenance burden of filtration and pumps for rainwater reuse. The city’s sustainability manager noted the city has a green power contract through 2038 that locks some rates for the near term but makes multi‑decadal projections uncertain.

By informal consensus (thumbs up), the council directed staff to carry forward solar readiness so the building can be solarized in the future and indicated support for including provisions for expanded solar capacity (with some councilors preferring the expanded option and others preferring to wait for improved economics). The council also expressed support for including rainwater harvesting and for potable‑storage capacity to improve water resiliency; staff said the items can be carried into the 60–90% design and that some elements could be included as bid alternatives to preserve flexibility.

Quotes (from meeting presenters) Aaron Young, partner with Northwest Studio: “This project is a good candidate for roof mounted solar power generation.”

Tony Marino, associate principal, PAE Engineers: “For the PSM Building, we’re using similar building types that PAE has worked on in similar climates.”

City Manager (presenter): “Water resiliency is one of our highest priorities for this facility to operate.”

Next steps Staff will update schematic designs and cost estimates and return to the council in June with more detailed estimates; several elements may be packaged as bid alternatives so the city can adjust scope if market or funding conditions change. If the ballot measure funding proceeds, the chosen schematic will inform final design and procurement.

Ending Councilors and staff emphasized resilience—especially water storage—when weighing cost versus long‑term benefits. Staff said they will refine assumptions on utility rates, net‑metering, EV fleet charging and battery storage and will present revised cost estimates with the June schematic update.