Towards Net Zero Energy for Existing Buildings

Figure 1: PG&E Company 77 Beale Tower, San Francisco, CA. Copyright John Sutton Photography 2019.

Project Details

The Pacific Gas & Electric (PG&E) Company is a national and global leader in the mission towards clean and reliable energy with its groundbreaking energy efficiency programs. As a committed sustainability forerunner, PG&E envisages net zero energy strategies throughout its real estate portfolios. AlfaTech partnered with PG&E to outline an execution plan for ambitious facility energy savings in its existing San Francisco offices.

“PG&E embodies the effort for clean, reliable, and affordable energy for customers across Northern California. Its programs are innovative, impactful, and assumes great leadership in a World that increasingly looks for net zero energy developments.”


STUDIOS Architecture collaborated with AlfaTech to prepare design-assist style, “bridging documents,” for phased construction. The endeavor encompassed life-cycle cost analysis (LCCA) and state-of-the-art, high-performance energy benchmarking. Leading to project completion, the AlfaTech team peer reviewed the permit sets produced by the design-build contractor and their engineers.

CLIENT:  Pacific Gas & Electric Company
LOCATION:  San Francisco, CA
SIZE:  1,000,000 sq. ft.
SERVICES:  MEP Design-Assist Services, Technology, AV, Lighting, Commissioning
MEP LEAD:  Erik Elmtoft (Project Manager), TG Davallou (Principal)
MEP TEAM:  Amber Welsh, Jaime Carrion, Jose Gonzalez, Mark Lopez, Rohini Pendyala
ARCHITECT:  STUDIOS Architecture (Bridging Architect), RIM Architects (Production Architect)
GENERAL CONTRACTOR:  Roebbelen Construction

Figure 2: Example master plan renovation, Floors 9+10 of the 77 Beale Tower. Image courtesy of AlfaTech.


A master plan covering 1,000,000 sq. ft. of Class A office space was established as the main vehicle to achieve the targets set forth by our client. The intent was to reach beyond immediate needs and establish a framework to permanently lower energy usage. The transition started with the initial build-out of two floors in PG&E’s 77 Beale Street Tower to inform the master plan and vice versa. Specifically, whole-building due diligence, energy modeling, facade optimization, and system comparisons were necessary steps to achieve building life-cycle benefits.

Due Diligence

The 77 Beale Street Tower was completed in 1969 with its eclipsing 32-stories of high-rise. With the existing building in mind, a targeted effort was initiated as part of the master plan to identify opportunities for increased energy efficiency in daily operations. Examples included replacing single-speed central plant pumps with variable speed pumps, removing legacy equipment parasitizing on energy use, and installing high-efficiency chillers and cooling towers with thermal storage capabilities. The due diligence concluded with a comprehensive list of recommendations across 23 functional/performance categories to comply with current code and achieve energy savings. This was a cornerstone in determining successful candidates for overall building energy use reduction.

Figure 3: Proposed retrofit facade treatment of the existing glazing units.

Energy Optimization

Building improvements focused on significantly reducing the largest energy usage categories. Facade, HVAC, and lighting systems were identified early in the process as key constituents. A comparative study evaluated the appeal of options proposed for each system type. Chiefly, the distilled solutions were:

  • Facade System: Retrofit existing windows with reflective window film on the bottom 2/3 of each glazing unit to reduce absorption of excess solar energy during high cooling conditions. This was coupled with daylight redirecting film on the upper 1/3 of each glazing unit to harness free, readily available daylighting energy.
  • HVAC Systems: Condition the load directly at the source by installing active chilled beams by the perimeters while supplying ventilation across open office spaces with a variable air volume (VAV) system.
  • Lighting System: Controls and energy efficient LED luminaires to reduce lighting power density (LPD) by 25% below current code. Lighting design took into consideration glare control, and brightness management for maximum occupancy comfort.
Figure 4: Susceptibility to solar radiation from the south-west and the south-east. The data set is an annual average represented by 52,560 layered computations (10-min basis). Rendering by AlfaTech.

Engineering Retrofit Approach

The master plan building optimization materialized with the complete renovations of the 9th and 10th floors tenant spaces. Original floor-mounted perimeter induction units (PIU) were replaced on the building’s 9th and 10th floors with high-performance, active chilled beams, mounted in cloud type ceilings. Ventilation was designed to induce air overhead by way of a 4-zones facade-oriented VAV system, with direct digital control, to the open office spaces. Dedicated demand/supply air side systems were extended to conference rooms. Thermafusers were used here to create adaptable ventilation microzones. Condensation concerns were addressed with a closed-loop, leakage proof heat exchange system that enabled adjustable hydronic setpoints per floor.

Figure 5: Sectional view of the low-profile chilled beams. Cold air (blue) emanates from the thermally active nozzles, effectively inducing the warm air (red).

The design resulted in an integrated, seamless HVAC system zoned with a balance between load and system. Usable area was regained at the perimeters owed to the removal of floor-mounted PIUs. The as-designed system saved energy for the first two floors in excess of 20%, or 50,000 kWh/year, with HVAC only as compared to the most recent high-performance industry baseline. The retrofitted lighting system with Enlighted zonal control added another 20,000 kWh/year in energy savings. A logical by-product of this design was lower system demand peaks, which acts as a natural demand/response peak reduction for the life of the building.

Life Cycle Benefits

The overall benefits were channeled by the floor-by-floor life-cycle cost analysis undertaken for the 9th and 10th floors. The LCCA metric established quantitative performance in the context of operational considerations. This way, the implementation of the master plan was bounded with concrete, cost-effective, phased renovations in an overall restacking effort delivered to the contractor team.

The proposed optimizations yielded 10-year paybacks for each renovated floor owed to palpable reductions in operational costs associated with HVAC and lighting energy uses. Additional benefits reaped were easier systems maintenance, reliability, and improved comfort for employees. Combined for all floors, the predicted building performance pointed to savings north of 2,000,000 kWh per year. When implemented globally, the proposed approach would measure a significant step towards the desired net-zero energy design. Future building renovations can benefit from integrated building photovoltaics and hydrogen fuel cells as the primary net-zero energy catalysts.

Figure 6: Example master plan renovation, Floors 9+10 of the Beale Tower. Image courtesy of AlfaTech.

Published By



Erik Elmtoft, PE, MS, LEED AP BD+C
E: P: +1.415.403.3073
Erik is an Associate of Mechanical Systems Design at AlfaTech’s San Francisco office.  Erik has contributed to innovative design solutions for several construction projects in San Francisco and Los Angeles. Erik is an inaugural member of the ASHRAE Standards Committee 228P: Standard Method of Evaluating Zero Energy Building Performance.