Sustainable HVAC Strategies
By now, the words “sustainable design” have crossed engineers’ desks many times, with potential clients frequently requesting it for their projects. Most engineers would probably respond to such a query by saying, “Of course, that’s what good engineering is!” The reality, however, is a bit grayer.
Sustainability should involve more than the individual engineer, engineering department or consultant. In fact, effective sustainable design requires participation from the entire building team, including the owner and government agencies.
Public interest in sustainability has expanded from its base as a small offshoot of the 1970s solar movement to building-team professional organizations and now to governmental procurement incentives throughout the country. The private sector has also begun to show interest as companies-from Fortune 500 to entrepreneurial start-ups-have started using sustainability as a criterion in selecting their building team.
Defining sustainable design
Sustainable design in building construction is the use of techniques that, if carried to their ultimate, will result in a building that, over its lifetime, will be a “net producer” of energy, food, clean water and air, and promote healthy humans and communities. While this is not necessarily achievable with a typical client, many facility owners would be interested in buildings that consumed less energy, offered a healthier environment and increased worker satisfaction without costing more to build. This is the promise of sustainable design-a building that requires fewer resources over its lifetime.
There are two good reasons to practice sustainable design: the environment and the bottom line. The impact of sustainability on operating and personnel costs is many times the construction cost. For example, a mechanical engineer may specify $3 million of conventional heating, ventilation and air-conditioning (HVAC) equipment in a typical year, which requires a megawatt of energy. However, by applying sustainable technology, a 20-percent to 50-percent reduction can be made in peak demand. Over a building’s lifetime, an engineer’s designs could save $36 billion, enough to cover the annual salaries of 65,000 office workers.
Research on individual temperature controls, daylighting and indoor-air quality has also shown measurable increases in worker productivity. While such research favors sustainable design, two entrenched facets of conventional design-building-team compartmentalization and short-term economic goals-are impediments.
Compartmentalization reduces the ability of team members to work together to optimize the project because each team member’s decisions impact the others. The building itself is a whole system and the design should take this into account. For example, fenestration and glazing impact the visual aspects of the building, applicability of light-level controls, heat gain/loss, landscaping, routing of plumbing and furniture/room layout. These items then affect the next level of design elements. Discussion and communication between team members allow each of the systems to work together. Trade-offs between building systems can increase overall building performance and reduce cost.
As for economics, sustainable design elements can be integrated within a conventional design budget, but a much greater impact on sustainability can be achieved through a long-term view of owning and operating costs, as well as budgeting by looking at the project as a whole rather than system by system.
Not only do the budgets of conventional projects emphasize the costs of individual systems, but the design also follows that pattern. Conversely, sustainable design looks at the building as a whole. For instance, if air intakes are analyzed according to their relationship with other building systems, they can be positioned to minimize site-generated heat gain and maximize the quality of outside air.
A more difficult issue with the conventional design process is the lack of incentive to innovate and “right-size.” It is far too easy to oversize and underdesign because there are few economic incentives, seemingly shorter and shorter project timelines and no liability for inefficiency. In fact, those projects that base fees on project cost often reward overdesign. Understandably, owners and occupants do not want insufficiency, but having a clear understanding of the design intent can provide a measurement of sufficiency that can be agreed to by all.
The design of systems that provide the most efficient way to achieve project goals is the bread and butter of building-team engineering. But how is efficiency defined? Is it kilowatt/ton? First cost? Operating cost? Life-cycle cost? Design cost? Completion time? Environmental impact?
The creation of a design-intent document categorizes and prioritizes goals of the design. There is no right answer as each project has different goals, but sustainable designs concentrate on long-term values over short-term gains. Better design is more sustainable because better design optimizes resources.
There are a number of guidelines and tools that can be used to create sustainable building designs. Because an integrated design process is important in creating sustainable buildings, the guidelines and tools involve the whole building team. Only portions of these tools are directly applicable to engineering, but it is important to consider the project-scale issues in order to view the building as a system.
For example, the Minnesota Sustainable Design Guide and the U.S. Green Building Council’s Leadership in Energy & Environmental Design guidelines help engineers to prioritize and identify possible sustainable strategies. Both of these manuals identify goals, categories, possible strategies and measurement tools. Points are awarded based on these goals, but achieving the design is more important than the raw score. Categories that involve the M/E consultant to a high degree include interior environment, energy performance and water strategies.
Sustainable HVAC design falls under both interior environment and energy-performance categories. The interior environment factors are closely linked to American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE) 62-1999 and ASHRAE 55-1992, with additional emphasis on individual controls and HVAC-system monitoring. Many building codes also base the design of mechanically-ventilated occupancies on ASHRAE documents, so those buildings with compliant design receive basic credit.
It is important to note that some codes allow operable windows instead of a mechanical ventilation system in certain occupancies, which, in extreme climates, reduces indoor-air quality to an unacceptable level. Other sustainability goals promote the use of operable windows and the possible elimination of major mechanical systems. However, assuring acceptable air quality requires more engineering. It is in the energy-performance category that the most dramatic gains in efficiency, and thus sustainability, can be realized.
Basic system selection and equipment sizing affect energy consumption to a great degree. According to a recent study, oversized HVAC equipment and a lack of lighting controls are among the top three missed opportunities to save energy in commercial buildings.
Good design does not mean overdesign. The majority of mechanical/electrical devices and systems perform at their best when selected near their maximum capacity. If future capacity is desired, it is wise to plan for this, especially to take advantage of improvements in technology and performance to the greatest extent possible. Computerized design tools, in place of the conventional steady-state hand-calculation methods, can assist in selecting properly sized equipment by allowing a more accurate dynamic thermal model of the building to be created. When selecting equipment such as chillers, it is better to look at the integrated part-load value efficiency, which models the performance of a chiller based on load profiles for a given location instead of the standard part-load efficiency.
Finding the best possible HVAC system for a given project can be based on office standards, cost, performance, existing systems, visual impact and/or available space. There are computer-based schematic design-analysis tools that allow basic HVAC performance comparisons to be made along with cost and environmental impact. This sort of tool can be useful in determining what general class of system should be considered.
Sustainability guidelines reward those systems that perform better than conventional systems. For HVAC systems, this can be measured using steady-state efficiency ratings, but to achieve higher levels of performance, an iterative process using energy modeling and several proposed systems is necessary. The complexity of buildings and the interrelation between building systems means that no one HVAC system is best for a given project. Comparing several systems against each other not only provides a method for selecting HVAC systems, but also allows follow-up performance testing to be performed.
A quick search on the Internet will turn up many computer programs and specialist consultants that perform energy modeling. Some of these programs are suited for preliminary design studies of site, structure, fenestration, daylighting and HVAC systems. The major tools for complete energy modeling are based on the Department of Energy’s DOE-2.1. Most programs provide a simple user interface and utilities to make it easy to use. Even with these user interfaces, the rule of “garbage in is garbage out” still applies, so it’s important to have a specialty consultant to assist in creating an accurate model and providing output interpretation. Maximum utility of this sort of modeling involves input and feedback from the architect and mechanical/electrical engineers.
Designing the building as a system process maximizes the interaction of systems within the building and suggest possible sustainable strategies. For instance, the selection of higher floor-to-ceiling heights can increase the feasibility of displacement ventilation, daylighting and indirect lighting, but it can decrease the duct space available over the rooms. If no acoustical ceiling is provided, individual ceiling-mounted geothermal heat pumps are probably not a good idea because radiated sound and the location of any roof-mounted equipment has to be carefully considered. Site issues such as the location of parking lots and roadways affects possible air intakes and geothermal loop fields.
No one design or engineering technique can be called sustainable, but the following is a list of common strategies that can help reduce consumption over time.
Displacement ventilation uses the natural effect of rising warm air to move comfortable low-velocity air upward after it is heated by people and equipment in the space. The air is transported to a high return or exhaust. In terms of sustainability, this type of system has high ventilation effectiveness and low power consumption, but will probably require outside-air preconditioning. Additional advantages can accrue if the system is paired with an access-floor air-distribution system.
Geothermal heat pumps use the internal heat of the earth or a body of water to improve the performance of a conventional heat-pump system. Controlling the heat-pump loop temperature to a more narrow range and optimizing the refrigeration system achieves this performance increase. Design and selection of the loop field or pond is highly dependent on site and subsurface conditions and may limit future growth. Loop-field design and system efficiency are contingent on the heat pumps selected, so these issues must be carefully considered.
Photovoltaic systems are becoming more cost effective. A major cost, both in efficiency and capital, is conversion and connection to the utility grid. If possible, stand-alone dedicated uses will leverage the investment, along with building- integrated systems and solar-cell cooling systems. Finding systems that can benefit, such as outdoor-air solar preheaters or solar water-heating and circulation systems, may determine whether a project is suitable for photovoltaics.
Integrated photovoltaic systems use roofing materials such as metal roofing panels and roofing tiles, or glazing materials such as skylights, as substrates for the solar cells. This eliminates the need for conventional framing systems and increases the cost effectiveness of the solar cells. Solar-cell cooling arranges the cells to use either natural convective or conditioned building relief to reduce the temperature of the cells and improve conversion efficiency. Photovoltaic-system efficiency is highly affected by integration with the building, site and other systems within the building.
Integrated outdoor-air preheaters rely on coordination, consultation and collaborative design to maximize the benefits. This technology uses a perforated metal plate to form a nonstagnating Trombe wall that preheats outdoor air or shades an exterior wall. A large southern-facing wall without windows on an industrial building is optimum for this product, but any building with good southern exposure and architectural form that is sympathetic to metal exterior panels is a good candidate.
Tunneling through the cost barrier
Dual-duct, dual-fan systems. Although more equipment is required to complete the system, the opportunity to reuse more of the plenum heat and reheat with simple gas-fired equipment, instead of a boiler and reheat-coil system, can improve performance and lower the operating cost.
Energy-recovery systems have become more varied and more common. At one time such systems were recommended for cold climates, then for hot humid climates and now for just about any project with significant outdoor-air requirements. Although this equipment has a cost, the reduction in heating- and cooling-plant equipment-and the ability to eliminate dedicated makeup-air systems-can offset the added investment.
There are still many HVAC-pumping systems that provide 100-percent standby pumps with lead/lag control, even though there has been much written about the benefits of parallel pumping. In a building with many pump systems, the cumulative electrically-connected horsepower can require larger electrical panels, feeders, service entrances and an emergency generator.
On the horizon
Sustainability means many things to many people. Today, in the area of HVAC and water systems, much of the current work is centered on energy efficiency and individual control. In Europe, zero-energy-usage projects, ventilation-induction towers and subsurface outdoor-air preconditioners are already being designed and built.
To be sustainable, a building team should look at the big picture and think in terms of building integration. Incremental steps in performance improvements are better than no change, but integrated building-system-centered design can have a much larger impact.