Integrating and optimizing power and mechanical systems

Smart buildings, Smart Grid, intelligent buildings, and integrated systems are buzzwords in the architecture and engineering industry, but can we deliver integrated designs? Mechanical and electrical systems are closely integrated, and must be carefully managed to achieve optimum performance.

By Suzan X. Sun-Yuan, PE, LEED AP, CDT, and Mohsen Aghai, Environmental Systems March 20, 2014

Learning Objectives

  1. Understand which portions of a building’s engineered systems can be integrated.
  2. Learn that a building’s HVAC system can be adjusted per space load. Mechanical system operation and electrical demand also can be integrated to monitor final electrical usage.
  3. Understand how to provide building power management (such as a submetering system) to monitor electrical usage.

Electrical utility vaults, switchgear, UPS/battery, and telecommunication rooms all require ventilation and cooling systems, according to the International Building Code (IBC) and other local codes. Mechanical equipment, such as fans and pumps used to support these mechanical, electrical, plumbing (MEP), and fire protection systems rooms, require electrical power distributed by switchboards, transformers, and panelboards.

To integrate successfully, these systems need a platform on which they can communicate with each other. The BAS provides this platform. BAS are centralized, interlinked networks of hardware and software that monitor and control MEP and fire protection systems. For instance, the BAS may control and monitor chillers, boilers, air handling units, rooftop units, fan coil units, and variable air volume boxes in addition to lighting systems, power/electrical systems, security, the fire alarm system, elevator/escalators, and plumbing and water flow systems.

As the overall “brains” of the building, the BAS coordinates the work of all the building’s engineered systems, including the power management system. Overall building power demand requirements must be carefully evaluated by both mechanical and electrical engineers during design, with respect to the challenges of both disciplines. The first step to true integration is to understand the owner’s requirements and the building’s ultimate function. Once each discipline understands its role in meeting both of these objectives, project goals can be defined. Establishing effective communication and collaboration between design disciplines as early as the pre-design phase will lead to true systems integration and yield improved efficiency, while lowering construction costs.

Integration to optimize building operation

Optimizing daylighting will help reduce a building’s lighting power energy consumption. This can be achieved with well-designed building shading systems, improved glazing product building orientation, and integrated lighting controls. Building envelope performance, for example high-performance glazing system and glazing orientation, also impacts cooling capacity and therefore can decrease the facility’s overall electrical power demand. The synergy between mechanical and electrical systems can reduce mechanical equipment size, including ductwork, hydraulic piping, and chillers. Ultimately, electrical equipment such as distribution boards and transformers can be reduced in size because of decreases in motor horsepower.

Solar heating, cooling, or even geothermal systems implementation can also reduce electrical consumption. Occupancy sensors in offices, conference rooms, washrooms, and other areas of high occupancy fluctuation can be another effective approach to reducing mechanical cooling loads as well as electrical loads. The power consumption of a tenant-occupied office plan can be monitored through a tenant power distribution panel. For instance, with an occupancy sensor installed in the conference room, the terminal fan powered box operates only when the temperature falls above or below the control range, or when people present in the conference room. For many of these examples, metering systems can be employed not only to monitor the power utilization in real time, but also to provide the necessary information to adjust mechanical systems operation to improve overall building operation.

Actual energy use

Per the 2013 ASHRAE Handbook—Fundamentals, a building’s mechanical equipment is sized to meet up to 99.6% of the annual weather conditions in that location, with peak horsepower requirements and electrical equipment sized to meet the anticipated peak loads. But, because the majority of mechanical equipment is operating at partial load conditions most of the time, the peak power demand is used only a few days per year on average.

Measured peak electrical power load is often much less than designed peak capacity. Take for example a 45-story high-rise office building in Chicago with a conference center, fitness area, restaurant, kitchen, and multiple floors of office space. Each of these individual zones must be designed to meet peak load for the hottest day in July and the coldest day in January, as recommended by ASHRAE Standard 90.1.

Thorough analysis has determined that this building’s major HVAC equipment and elevator peak demand takes place in mid-July and requires a total connected power load of 6,560 kW. But, available utility records from this facility reveal that the real-time power consumption is actually less than 2,200 kW.

MEP engineers employ a variety of energy and cost-saving solutions to control the facility’s fans and pumps and take advantage of partial load conditions (see “Effective partial-load condition strategies”). While employing these energy-efficient technologies to meet real-time power demands is good design practice, today’s high-capacity buildings can also repurpose the difference between their peak power design and peak power usage by creating a second or even third category of MEP equipment to place on the emergency power system.

Enhancing emergency power

Per NFPA 70: National Electrical Code, Article 700, Emergency Systems are designed and installed to maintain illumination and provide power for essential equipment if normal power supply fails. These systems are essential for life safety (Category 1). Per NFPA 70 Article 701, Legally Required Systems are intended to provide electric power to aid in firefighting, rescue operations, control of health hazards, and similar operations (Category 2). Per NFPA 70 Article 702, Optional Standby Systems are typically installed to provide an alternate source of electric power to prevent physical discomfort or serious disruption to business (Category 3).

Building code may require a typical commercial building to have generator backup power for fire protection pumps, firefighter elevators, emergency lighting, stair pressurization fans, and smoke management systems subject to national and local building codes. However, a variety of building types have their own independent requirements as well.

After examining the applicable code and taking into account any additional requirements based on the building’s function, it becomes clear that additional standby power may be required to support the operation of the code-required systems. For instance, the firefighter elevator requires backup power per building code in most cases. To ensure proper elevator operation, the elevator machine room cooling system may also require generator backup power. Ventilation serving the machine room is required to be on emergency power if available, per IBC 3003.1.4.

Power management system

The power management system (PMS) provides integrated control of the mechanical and electrical systems to optimize the generator use. Critical systems requiring generator backup power are organized into prioritized tiers based on code and operational requirements as indicated above. The categories are then divided into load blocks that can be added or removed from generator support in a sequential manner depending on available generator capacity. The PMS controls generator load by controlling which systems are physically connected to the generator plant and then controlling the mechanical systems once connected. It optimizes generator use by controlling how many generators are running and matching the generator capacity to the load.

The PMS provides a dynamic balance between load and running generator capacity. It also acts as a safeguard to ensure that code-mandated life safety loads have the highest priority. For example, if a generator fails causing the generator plant to become overloaded, the PMS will shed lower priority loads and start generators until the overload condition is eliminated.

The first category of the emergency power system is the emergency loads as defined by Article 700 of NFPA 70. This branch is intended to automatically supply egress illumination and power to life safety equipment in the event normal power is lost.

The second category of emergency power systems includes legally required standby systems as defined under NFPA 70. This branch is typically installed to serve loads such as heating and refrigeration, communications systems, ventilation and smoke removal systems, elevators used as a path of egress, sewage disposal equipment, lighting systems, and industrial processes that, when stopped during any interruption of normal electrical supply, could create hazards or hamper rescue and firefighting operation.

In addition to the above-required loads, the third category of optional standby systems are typically installed to provide an alternative source of electric power to serve certain loads in event of any power outage could cause discomfort and serious interruption to process. For instance, firefighter elevator cooling or underground garage exhaust systems can be created to reflect the owner’s requirements and the building’s function, thus using more power capacity offered from the generator than the required systems. Furthermore, a hospital in a cold climate may want to put a patient room heating system onto a third category of generator power, and an office building in a hot climate may want to consider an additional cooling system.

Engineers should identify which systems should go into this third category of PMS, which requires mechanical and electrical systems integration as well as collaboration between the owner and the engineers through consideration of the building budget and the intended design.

The dynamic nature of power control systems offers a wide range of features that make building emergency power systems more effective and efficient. It also offers high visibility into power movement within the emergency systems. It calculates the real-time power consumption versus total power capacity based on hierarchy architecture. Within the same category, upon receiving signal, a block of power can be control and access as required.

Additional systems with dual power connection can increase the scope of the emergency distribution system as well as the size of the generator, resulting in higher initial capital costs. Sophisticated emergency power management requires a high-speed data transfer system to ensure emergency system activation and accurate time delay between systems, which will increase the cost of the building’s network systems.

Emergency power systems can be among the most critical investments for any large, complex facility. While creating tiers of MEP systems to use emergency power as it becomes available may increase a building’s first costs and will require additional HVAC, electrical, and BAS integration, these power systems will also champion safety and use the building’s existing MEP systems to their fullest potential, optimizing operations even during a critical outage.

Suzan X. Sun-Yuan is a senior associate and lead mechanical engineer with Environmental Systems Design. She has experience with super-tall buildings, food labs, and central plant designs and upgrades. Mohsen Aghai is a senior associate and lead electrical engineer with Environmental Systems Design. He has experience with super-tall, mixed-used, commercial, and governmental buildings as well as hospitals and central plant designs and upgrades.