Greening the Future with Energy-Efficient HVAC Systems
The U.S. Green Building Council (USGBC) describes its Leadership in Energy and Environmental Design (LEED) green building rating system as a voluntary standard for developing high-performance, sustainable buildings.
But as green design becomes more mainstream, LEED becomes less voluntary. For example, a number of institutions and agencies, such as the University of California, the U.S. General Services Administration, eight state governments and a dozen cities, now require LEED certification for all new buildings.
Professional associations are also doing their share to promote LEED. ASHRAE, for instance, has published a manual that helps users better understanding the design, installation and operation requirements of its ANSI/ASHRAE Standard 62.1-2004, "Ventilation for Acceptable Indoor Air Quality." In addition, another of its sustainable manuals, The Advanced Energy Design Guide for Small Office Buildings, recently earned the organization a leadership award from the USGBC.
But it isn't just the growing popularity of the sustainable movement that is pressuring designers to create highly energy-efficient buildings. There's also the issue of rising fuel costs. What follows are some basic recommendations for consideration when designing HVAC systems that not only help meet the LEED requirements, but also significantly reduce energy costs over the long term.
It all begins with site selection
LEED for new commercial construction and major renovation projects currently awards credits for urban and brownfield redevelopment as well as for selecting sites with an eye toward reducing the building's environmental impact.
Site selection, however, can have a significant impact beyond these issues. Building orientation on the site can affect building performance with respect to ventilation, photovoltaics, daylighting, solar hot water systems and passive solar techniques. For example, strategic building orientation can limit solar heat gain, thereby reducing HVAC loads. Correctly positioning the building in relation to the prevailing breezes can enhance natural ventilation, reducing or even eliminating the need for mechanical cooling. These factors will have a major impact on the type and size of HVAC systems specified. Consequently, mechanical engineers must have a voice from the very beginning in site selection and building orientation to ensure that opportunities are not lost.
Maximizing energy performance
Variable-volume systems have been the industry standard for energy efficiency for the last 30 years. Now, as energy regulations continue to raise the bar for building performance and energy costs increase dramatically, other approaches and systems that enhance energy performance of buildings are gaining in popularity. One strategy that has been popular for sustainable building is natural ventilation.
An engineer once said, "The most efficient motor is one that's not running." The corollary might be, "The most efficient system is one that doesn't exist." In temperate climates especially, natural ventilation is a reasonable solution as the primary source of cooling for a number of building types. In particular, individual enclosed offices provide a perfect opportunity to employ operable windows, putting comfort control at the occupants' fingertips.
Whether natural ventilation is an option in modern building construction depends on several factors including building type, location, hours of use and even seasons of use. The engineer must determine the hours per year that a space can stay within the industry-accepted range without mechanical cooling, given the project location. Effective solar control, particularly for the glass areas of the building, is critical to the ultimate success of a naturally ventilated system. Installing ceiling fans can also boost natural ventilation's effectiveness considerably.
Even if a space requires mechanical air conditioning, natural ventilation can still play a role. Installing microswitches at the windows to shut down the HVAC system when a window is opened will save energy and give control back to the building's occupants.
With underfloor air distribution (UAD) and displacement ventilation, conditioned air enters the room at the level where the people are, instead of being distributed at the ceiling. As a result, these methods require less energy to cool the air, since the air is supplied to the room at a higher temperature than possible with conventional ventilation systems without reducing comfort levels. UAD systems can also run in economizer mode for more hours than conventional systems, reducing the mechanical cooling energy and improving indoor air quality. Moreover, these systems typically have lower static pressures for the system, reducing fan energy consumption. And, they reduce first costs by allowing simpler building controls than possible with a standard variable-air-volume system.
Radiant heating and cooling
Radiant systems are typically installed in ceiling panel systems or in the floor. While radiant heating is a more well-known option, radiant cooling systems are becoming more prevalent. Radiant panel systems are ideal for small enclosed spaces, allowing for individual control and providing flexibility for remodeling. Radiant floors are well suited for large open spaces, such as office lobbies and airport terminals. Radiant floors eliminate the need to try to force air down to the occupied zone, and by relying on the building's inherent thermal mass, they can use less energy.
Dedicated Outside Air Systems
Instead of providing all of the required cooling by means of an all-air system, DOAS provides only the minimum ventilation required for occupancy. Radiant panels, chilled beams or fan coil units can then supply any additional cooling required. This system uses less energy to power fans and condition the ventilation air. In addition, the smaller size of the equipment and distribution system significantly reduces first costs. And since the ventilation system is constant volume and the supplemental cooling is locally controlled, the control system's complexity and cost are reduced.
For example, a laboratory project currently in design in our office showed a first cost savings of $850,000 in the supply and exhaust air systems, made possible by reducing the air quantity moving through the building to a minimum ventilation rate. The chiller plant size remains the same as in a centralized supply system, so that in the future, fan coil units can be added as specific equipment areas and loads are identified.
Indirect/direct evaporative cooling
This packaged equipment combines relatively simple technologies—including direct evaporative cooling, indirect evaporative cooling and heat recovery—in a single packaged unit. Because these are custom units, components can include chilled water or hot water coils, DX coils, gas furnaces and different filter combinations in a variety of arrangements.
For a recent project in Sonoma, Calif., we specified indirect/direct evaporative cooling packages that were able to provide all of the cooling for a 100,000-sq.-ft. wet lab, while using no energy for mechanical cooling. Although not all projects will see such extreme results, similar benefits are possible in many locations.
Energy & atmosphere
To obtain LEED certification, building systems commissioning is required—with good reason. The commissioning process is an invaluable tool to ensure that the design in which the construction team has invested so much time and energy is operating at peak efficiency.
At the 2003 Labs21 conference in Denver, Cornell University participants presented the results of their recommissioning process of existing buildings. The Cornell team reported that, in recommissioning 15 buildings on campus, its initial investment, primarily the time for staff that the university was already employing, resulted in energy savings equal to that investment—a one-year payback. That savings was then used to fund the next year's budget for another series of building recommissionings.
The examples above are only a few of the system possibilities to consider. Others, such as ground source heat pumps, reduce energy use and conserve water, while night-radiant evaporative spray systems can provide enough cooling for an entire building while using almost no energy. The impact of the correct system choice is further improved by carefully specifying components to minimize pressure drop and maximize motor and equipment efficiency.
All these measures can earn a number of LEED points for energy conservation and other categories. But designers must carefully consider which is best for the project.