4 FM quick reads on HVAC
1. A VFD Offers Energy Savings, Other Benefits from Part-Load HVAC Operation
Today's tip comes from James Piper, contributing editor for Building Operating Management: Variable frequency drives, or VFDs, offer multiple benefits for HVAC systems.
For more than 20 years, VFDs have successfully been installed on fan and pump motors in wide range of variable load applications. The most significant benefit of the use of a VFD is energy savings. By matching system capacity to the actual load throughout the entire year, major savings in system motor energy use is achieved.
Another benefit of the units is reduced wear and tear on the motors. When an induction motor is started, it draws a much higher current than during normal operation. This inrush current can be three to ten times the full-load operating current for the motor, generating both heat and stress in the motor's windings and other components. For motors that start and stop frequently, the heat and other stresses produced contribute to early motor failures.
In contrast, when a motor connected to a VFD is started, the VFD applies a very low frequency and low voltage to the motor. Both are gradually ramped up at a controlled rate to normal operating conditions. With no significant inrush current, heating and stresses are practically eliminated, extending motor life.
VFDs also provide more precise levels of control of applications. For example, high rise buildings use a booster pump system on the domestic water supply to maintain adequate water pressure at all levels within the building. Conventional pump controls in this type of application can maintain the pressure within a certain range, but a VFD based system can maintain more precise control over a wider range of flow rates, while reducing energy requirements and pump wear.
Basic Ways That Building Control Systems Can Help Save Energy
Today's tip from Building Operating Management: Building control systems offer a variety of basic energy saving capabilities.
There are a variety of energy saving strategies built into the energy management function of the current generation of controls. The energy savings from these functions can help justify the cost of new or upgraded energy management system.
One basic function is automatic stop-start. While this saves energy by turning equipment off at scheduled times, a more powerful strategy can be more effective. Known as stop-start optimization, this approach goes beyond a schedule by considering indoor and outdoor temperature to decide when a piece of HVAC equipment should be started and stopped.
Another important function is the system's ability to change set points automatically in response to changing conditions inside or outside of the building. A simple example is the air-side economizer cycle. When the temperature and humidity of outdoor air are appropriate, that outdoor air can be brought into a building without being heated or cooled.
A control system can also optimize the operation of chillers, boilers, cooling towers and pumps, adjusting equipment operation on the basis of loads.
A sophisticated strategy is called load shedding. That strategy adjusts HVAC equipment operation to reduce energy use. This may be done when a building is in danger of setting a new demand peak load, or it may be initiated in response to a signal from a utility.
As useful as these and other control strategies are, they can't be taken for granted. Over time, for example, start-stop schedules may cease to reflect actual building operations, possibly because of changes to the occupancy of a building. What's more, control strategies are all too often overridden by maintenance and operations staff. Those overrides are frequently intended to solve a problem, but the long term effect is often energy waste.
Energy Model Can Improve HVAC System Energy Efficiency
Today's tip from Building Operating Management: Energy models are valuable in achieving high performance HVAC designs.
How efficient can a building's HVAC system be? To a very large extent, the answer depends on other factors in the building. The type of windows, the amount of insulation, the lighting system, the reflectivity of the roof — these factors and others like them can constrain the performance of the HVAC system by requiring it to work harder to heat and cool the building.
Today, it is possible to evaluate the HVAC impact of these other elements while the building is being designed. Powerful energy modeling software, available from a range of sources, enables the design team to estimate just how efficient a given set of design choices is, and then to compare other designs to identify the one that best meets the building owner's requirements.
For example, Option A may involve code-minimum insulation, ordinary insulated glass windows and a non-reflective roof. Option B, with more insulation, low-emissivity windows and a reflective roof, may initially cost more, but pay for itself in energy savings. What's more, savings associated with a smaller HVAC system can free up funds to cover the cost of those added efficiency measures. In some cases, the energy model may identify options that actually reduce the first cost of the project.
The use of an energy model is required to obtain federal tax deductions under Section 179D of the Internal Revenue Code. These are also known as EPAct tax deductions for the Energy Policy Act of 1995. To qualify for a deduction, an HVAC project must reduce energy costs at least 16.67 percent below the costs for a building designed to meet ASHRAE 90.1-2001. Energy modeling has to show the energy cost savings.
It's important to keep in mind that the energy model, as important as it can be at the design stage, is only an estimate. The actual energy efficiency of a building will depend on how the building systems are operated. A well-designed building can't overcome poor operation.
High-Performance HVAC System Requires an Integrated Design
Today's tip from Building Operating Management comes from Jeffrey Heiken, engineering design principal with Kling Stubbins: A high-performance HVAC system requires an integrated design.
Designing a high-performance HVAC system starts with understanding its end goals. It effectively serves the functional needs of the building and its occupants. It minimizes the use of resources by accurately "right-sizing" components and configurations. It is flexible in response to changes in use. And it employs design elements to capture waste heat, reduce material or energy use, or reuse materials wherever possible.
Across the spectrum of building types, high-performance HVAC systems provide more pleasant and satisfying work environments, and efficiencies which translate into lower owning and operating costs.
But a high-performance HVAC system is impossible without a truly integrated design team fully engaged and focused on project goals from the start. That's because so many aspects of design are interrelated and have to be considered simultaneously to achieve the goal of a high-performance HVAC system. For example, the HVAC design is affected by sustainability elements like daylight projection into the building, which helps drive building footprint and building aspect ratio (length to width dimensions) as well as solar orientation on a site. Daylighting also brings consideration of external shading devices and internal reflective light shelves to prevent glare and heat gain while bringing natural light to the occupied spaces. Internal shading (often mechanized and automated) in concert with computerized lighting control systems are also common energy conservation measures. With all of those factors affecting the HVAC design — not to mention the exterior wall construction, glazing, occupancy and utilization — it's clear that an integrated design approach is essential. From building siting to building envelope composition, the performance of all elements is enhanced.
HVAC can't be an afterthought once the shell of the building has been designed. Compared to the overall building life, the design process is brief and often fast-paced. Attention to HVAC performance at the earliest steps will be felt for decades. So will a lack of attention.