How to understand a company’s approach to efficiency, modularity and resiliency

Energy efficiency insights

• BlackRock’s new headquarters at 50 Hudson Yards in New York City achieves exceptional levels of energy efficiency, modularity, and resiliency through innovative mechanical strategies including a “building within a building” approach and a dual-temperature chiller plant.
• The project incorporates a Dedicated Outdoor Air System (DOAS) with energy recovery and an underfloor air distribution (UFAD) system to enhance energy efficiency, comfort, and air quality.
• BlackRock and Syska Hennessy’s mechanical design for the headquarters, which includes separating sensible and latent cooling loads and implementing a dual-temperature chiller plant, is setting new standards for commercial office spaces globally.

When BlackRock embarked on a journey in 2016 to find a new headquarters building, the firm established some tough criteria: The new headquarters had to meet the highest standards of quality, employee comfort and functionality while achieving exceptional levels of energy efficiency, modularity and resiliency. It also had to allow for the creation of a “building within a building,” which would provide BlackRock with an autonomous plant and operational control.

After several intense months of touring New York City’s building stock and development sites, the team found the right fit — a new development by the Related Companies at Hudson Yards. The building was not yet constructed, and the owner would allow input from the BlackRock team on how the infrastructure serving the office would be designed and constructed. BlackRock welcomed this opportunity, knowing that there was a key design challenge to surmount. Although the modern all-glass structure, with its large clear-spans and floor-to-ceiling windows, was architecturally stunning and filled with natural light, this type of building also comes with potential drawbacks, including significant solar gain and heat loss.

To overcome these challenges, the company strategized with several other groups to achieve peak energy efficiency within the strictures of the building typology, while also promoting flexibility and resiliency.

The end-result was an EUI of just under 40 kbtu/SF/year. This is below the carbon thresholds specified in Local Law 97 and top in class for energy efficiency. Furthermore, the project’s mechanical design that was pioneered by the team is being replicated in many skyscrapers across the world and has become a new standard for peak energy efficiency, decarbonization and comfort in the design of commercial office space.

Building within a building

BlackRock occupies the lower third of the 2.8-million-square-foot, 58-floor building. The interior fit-out encompasses a 400-person auditorium, full cooking cafeteria, client conferencing center, two trading floors and 11 floors of office space for over 4,000 employees. The design of the multi-floor space incorporates the planned “building within a building,” with a dedicated chiller plant, generator power and domestic water feeds to all floors. In addition, the building within a building features separate and independent electrical and mechanical infrastructure, which supports both flexibility and resiliency.

Separation of loads through a dedicated outside air system

The first major design decision was to separate the latent cooling loads from the sensible loads to the greatest extent possible. Benefits of separating the sensible and latent loads include greater control and the ability to match the total cooling, eliminating costly overcooling for dehumidification. To deliver fresh air to each floor in support of wellness and cognitive function and to exceed code requirements, BlackRock and Syska chose a dedicated outside air system (DOAS) with energy recovery.

The DOAS system supplies air at a low temperature to provide dehumidification. Since the internal and skin loads do not require the same low-temperature supply air, the team decided to separate the DOAS system from the air-handling units serving the spaces. Initially the intention was to make these DOAS units air-cooled through direct expansion, but the size of the needs made this method impractical. When the team looked at alternatives, it settled on an unusual solution — a dual temperature chilled water plant, which would allow the DOAS to be fed by low-temperature chilled water. This strategy, while deployed in laboratory and industrial type facilities, had never been implemented in a commercial office space.

The DOAS systems feed the conditioned dehumidified outside air to air towers located around the building’s core at each floor. These air towers mix return air with fresh air, then cool it through a sensible-only cooling coil using high-temperature chilled water (see figure 1).

Figure 1: The DOAS systems feed the conditioned dehumidified outside air to air towers located around the building’s core at each floor. Courtesy Syska Hennessy.

The DOAS is served by a seasonal low-temperature chilled water system at 42°F. Its dew point range for year-round discharge is 24°F to 48°F. Heat-recovery wheels recover 70% of the energy of the exhaust system to either preheat or pre-cool the outside air. This energy recovery allowed the team to use exhaust air volumes well above code minimums without sacrificing efficiency.

Furthermore, the system has provisions for activating charcoal filters to treat outside air. Should an external event create quality problems with outside air, the system will employ an emergency recirculation mode to provide latent cooling with zero outside air. To optimize energy efficiency, the low-temperature chillers only operate when the outside air has a dew point above 48°F and the building is in occupied mode.

Underfloor air distribution system

The underfloor air distribution (UFAD) system was chosen for many reasons. It offers high efficiency and comfort and affords future flexibility. In addition, it provides low-draft cooling with individual occupant control.

The UFAD was optimized for efficiency and comfort. The efficiency is achieved through the higher discharge temperature (65°F versus 55°F for overhead) and lower air system fan losses. Air towers discharge supply air to an underfloor air plenum serving both interior and exterior loads. At the perimeter, chilled/heating beams heat or cool the air to compensate for the additional skin loads when necessary. These chilled beams are fed by the high temperature chilled water system and have an active damper and control valve to maintain setpoint.

Occupant comfort was achieved by maximizing the supply air temperature serving the interior loads. By providing the ability to re-cool the air at the perimeter chilled beams, the higher internal temperature is maintained during all conditions. This eliminates the most typical complaint of UFAD systems: that the interior space must be kept uncomfortably cold to maintain proper temperature in the perimeter space during the peak cooling times.

The UFAD system puts the cooling near the occupants, which means that there is no need to cool the warmer stratified air region above the occupied zone (starting at 6’ above the floor). This represents a dramatic reduction in the conditioned volume of the high-ceilinged office space. Another benefit is that the underfloor void houses the modular power wiring system.

The use of underfloor air allows for future changes in the demising walls without an overhead ductwork change. It also enabled the team to use demountable partitions for most office spaces and rapidly reconfigure them as needed.

Dual temperature chiller plant

Figure 2: The dual-temperature chiller plant contributes to energy savings. Courtesy Syska Hennessy.

The use of a dual temperature chiller plant contributed to the energy savings: 28% below those offered by a traditional 42°F only chilled water plant (see figure 2). The plant serving BlackRock’s headquarters comprises (2) 500-ton low-temperature (42°F) chillers, (2) 800-ton high temperature (55°F) chillers, (1) 500/800-ton spare chiller to back up either system, (2) 125-ton heat recovery chillers and (3) 1000-ton cooling towers on the roof. (See Figure 3.)

Both the high-temperature and low-temperature chillers are oil-less magnetic-bearing machines. These chillers are exceptionally quiet and can duty cycle very easily. The main innovation of these machines is that they can operate “inverted” with condenser water that is colder than chilled water. When operating in this inverted mode, the machines are more efficient than plate-frame heat exchangers (at a system level). This allows for the elimination of the “free cooling” sequence, costly plate and frame heat exchanges and the associated operating complications. In this mode, the system efficiency is not impacted by climate change-induced higher enthalpy conditions that hobble free cooling operation.

Figure 3: The dual-temperature chiller plant. Courtesy Syska Hennessy.

The high-temperature chilled water loop (HTCHW) is the only water loop circulating outside of the chiller plant on the 19th floor and main air-handling-unit plant on the sixth floor. All process cooling is fed from this loop. In areas requiring supplemental cooling, the HTCHW cools variable refrigerant flows.

The building was provisioned for phase-change thermal storage modules for the HTCHW system to allow low-load operation without chiller operation. This phase change material will store thermal energy at 50°F and discharge at 55°F, enabling the chillers and cooling towers to shut down periodically during low-load operations.

Heat recovery chillers generate low-temperature hot water for perimeter heating during the shoulder heating season. When the cooling load can be satisfied by these chillers, the cooling towers can be shut down and makeup water can be conserved. An additional heat recovery chiller provides domestic hot water for the office and cafeteria.

The BMS system enhances the energy-savings: It includes a plant optimization panel that provides continuous optimization of the various system setpoints (CHW temp, pump pressure, CND temp, etc.) to achieve maximum efficiency.

Central air handling equipment

The central air handling equipment on the amenities floors feeds the large public assembly areas of the building. These air systems depend on outside air economizers. The large air handler in the auditorium has a bypass system to allow the decoupling of sensible and latent cooling. This is critical to controlling humidity during low-occupancy events. Furthermore, the space was furnished with supplemental VRF cooling units to handle unoccupied HVAC demands. The cafeteria makeup air system is served by both high-temperature hot water for preheat and low-temperature hot water to maximize heat recovery operations. In a few locations where reheat is required, a local water source heat-pump (sourced from the HTCHW) provides the hot water.

BlackRock headquarters. Courtesy: Syska Hennessy

These mechanical strategies contributed significantly to energy-efficiency, modularity and resiliency. But to reach BlackRock’s ambitious goals, the team also had to incorporate several electrical strategies. Part II of this article, which will be published soon, describes them in detail.

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