ET24SWE0028 - Electrifying Large Commercial Central Plants: Demonstration of TIER and Program Delivery Solutions
Standard designs for large commercial buildings rely on separate gas-fired hydronic heating and chilled water loops. This technology has slowly been refined with incremental energy efficiency improvements over several decades. The HVAC industry has not yet had enough time to establish how to serve large commercial building heating loads effectively and energy efficiently with all-electric equipment. Many engineers lack the expertise and time to keep up with new and rapidly evolving equipment needed in order to overcome the novel design challenges to combine the plant equipment into functional systems. To meet the urgent need to rapidly decarbonize buildings, the HVAC industry needs targeted support to overcome these new design challenges and barriers around system complexity and integration.
The first generation of all-electric large buildings has relied upon air-to-water heat pumps (AWHP), an approach that has had difficulty meeting design objectives and is cost prohibitive for most building owners. AWHPs are inherently inefficient during cold weather, have limited operating ranges, and have a large footprint (making that approach infeasible for existing buildings with limited space). Heat recovery chillers can operate at higher efficiencies to generate both hot and chilled water but only when simultaneous heating and cooling loads are available. Many early projects have struggled with the misapplication of new equipment and critical design oversights.
Time independent energy recovery (TIER) is a revolutionary all-electric heating and cooling plant concept that integrates heat recovery chillers, thermal energy storage (TES), and AWHPs to overcome the shortcomings of alternative all-electric plant configurations. Traditional TES is used to shift or reduce peak cooling loads, whereas TIER leverages TES for heat recovery. The result is a cascading all-electric system that maximizes heat recovery and smartly deploys the plant equipment to maintain the highest system efficiencies. TIER is estimated to provide energy savings of 40 percent compared to the current state-of-the-art all-electric central plants.
Though TES can have large space requirements, when sized appropriately in a TIER plant, the heat recovered in storage allows for dramatic reductions in required AWHP capacity, significantly reducing plant footprint and first cost. In California’s mild climates, the energy recovered from cooling loads alone can satisfy heating loads for most of the year. The AWHPs only operate when needed and to an intermediate temperature, which allows for higher efficiencies. Overall, the TIER design saves space, improves energy efficiency, supports grid-interactive efficient building initiatives, and reduces first costs and operating costs compared to alternative all-electric plant approaches.
Each of the equipment types in a TIER plant are commercially available, however, the proper design and control of the built-up central plants is complex and challenging to implement. The purpose of this project is to unravel the physics and operational characteristics of a TIER system. This project will leverage an in-depth performance review of the first-of-its-kind TIER plant for a new 300,000 ft2 building and expected to begin operation in early 2024. The Project Team will demonstrate the benefits of the concept and develop resources for supporting broader application and technology transfer. This will include a case study, design guide, and recommendations for utility program design.
Commercial HVAC systems have long relied on gas‑fired heating paired with separate chilled‑water, but the industry has yet to establish a clear, effective pathway for fully electrifying large central plants. Engineers face steep learning curves with new equipment, and the complexity of integrating all‑electric components has slowed progress at a time when rapid building decarbonization is urgently needed.
Early all‑electric designs have typically relied on air‑to‑water heat pumps (AWHPs), but these systems often struggle to meet performance and cost objectives. AWHPs have limited cold‑weather efficiency, narrow operating ranges, and large space requirements that make them difficult to retrofit into existing buildings. Heat recovery chillers (HRCs) can achieve higher efficiencies—but only when simultaneous heating and cooling loads are available—leading to common design oversights and misapplications on first‑generation projects.
Time Independent Energy Recovery (TIER) is an innovative approach that integrates heat‑recovery chillers, thermal energy storage (TES), and AWHPs into a coordinated, all‑electric plant. Unlike traditional TES applications, which focus on shifting peak cooling loads, TIER uses TES to store and deploy heat recovery, enabling a cascading system that maximizes overall efficiency. TIER is estimated to reduce energy use by up to 40 percent compared with standard AWHP plants, while also reducing space requirements, first costs, and operating costs and supporting grid‑interactive building strategies.
This project evaluated the first operational TIER installation, which serves a 1.2 million square foot office building in Sunnyvale, California (Climate Zone 4). The evaluation confirmed that the system delivers higher performance than both conventional AWHP plants and its own modeled predictions. The installed TIER system achieved an average combined heating and cooling COP of 5.5, compared to 4.0 for a comparable baseline AWHP plant. These results reinforce TIER’s advantages in energy efficiency, compactness, cost-effectiveness, and grid support.
Because current building‑simulation platforms cannot model complex central plants like TIER, the project team developed simplified spreadsheet tools to analyze TIER configurations using either hot‑water or condenser‑water storage. Modeling across three California climates and two building load profiles showed that TIER consistently reduces energy costs relative to chiller/boiler baselines, overcoming typical all‑electric “spark gap” cost penalties. One of the project goals was to provide generalized guidance on TIER plant design for different applications. However, a life-cycle cost analysis of different TES media and TES sizing did not indicate any strongly preferred approaches for the different building loads or climate zones evaluated; optimal solutions depend on site‑specific factors such as load profiles, part‑load equipment performance, utility rates, and opportunities to repurpose existing firewater tanks for storage.
While TIER has clear performance advantages, barriers to widespread adoption include system complexity, modeling software limitations, lack of standard design approaches, and lack of understanding and awareness. To address this, the project produced a design guide, consisting of example schematics, sequences of operation, and modeling tools to support engineers considering TIER systems. Lessons learned from early installations and from the analytical work included in this report can further improve implementation success.
Finally, the report outlines a coordinated market transformation strategy to accelerate adoption. Interventions include workforce training, improved modeling tools, manufacturer engagement, design guidance, incentives, and supportive policy pathways. These actions focus on reducing perceived risk, simplifying design, and increasing industry familiarity—creating a roadmap for evolving TIER from an emerging solution to a mainstream, standardized approach for large all‑electric HVAC plants.
