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.

03/20/2014


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.

This article has been peer-reviewed.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.

Figure 1: This illustrates a variable frequency drives motor speed at reduced system flow and head pressure. All graphics courtesy: Environmental Systems DesignIntegration 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.


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