March 11, 2004

Mechanical systems shape green designs

CDi Engineers McCaw Hall expects to save thousands of dollars a year by using an air displacement ventilation system to cool its auditorium.

You've got an idea for a new building. Maybe you've selected the location and have an architect in place. You want the building to incorporate sustainable energy systems to improve marketability. When should you hire a mechanical engineer?

Right now!

Energy consumption is one of the largest contributors to the long-term cost of a building. A smart mechanical design will save a significant portion of those costs, making it easier to attract tenants.

And a good design can often lower construction costs by qualifying for incentive programs from utilities. But it is increasingly difficult to shoehorn innovative mechanical solutions into a building that's already designed.

Changing times

Five years ago, this was not the case. Architects and engineers usually worked in their own corners, with engineers designing systems for a building that had already largely taken shape.

Today, however, sustainable engineering considerations go far beyond selecting one type of HVAC system over another. We have learned that everything from site selection to building positioning and envelope design can play a part in conserving resources. Hence the trend among owners, architects and engineers to collaborate from the get-go.

Window design

Take windows, for example. While sketching out a preliminary building envelope, it's a good idea for the architect to consult with the mechanical and lighting engineers on energy implications and daylighting schemes.

Perhaps the engineer will suggest the building can be oriented differently, letting in more natural light during winter months. Or the engineer might bring up a new type of glazing solution or a way to minimize effects from prevailing winds. Shading from adjacent structures and vegetation could play a role in energy consumption.

For the Seattle Justice Center, a double-skin facade was used to reduce energy consumption from heating, cooling and lighting. By acting as a thermal buffer, the double-skin facade reduces heat loss as well as heat gain. It also improves thermal comfort along the perimeter spaces.

Incorporated into the facade are internal light shelves. These light shelves look something like a horizontal fin, and can be placed inside the building or over a window. By enhancing the usability of natural light, they reduce lighting and cooling costs.

Utility rebates

Several years ago you could apply for financial incentives from Seattle City Light or Puget Sound Energy after a building was constructed. Not anymore. Utilities figure that if you've already bought into sustainability, you don't need a carrot. They want to see your green concepts incorporated during the initial design phases.

Energy rebates can be considerable, so it's worth making sure your design accounts for them. Utilities will typically refund a percentage of what you spent on energy-efficient installations.

If you opt for high-performance windows that reduce energy consumption 20 percent, utilities will give you a credit for a percentage of the additional cost. Not only do you get help on your investment, but the green features make your building more appealing to tenants and investors.

Look to Europe

There are two basic ways to save energy in a building, and both should be considered early for maximum effect.

The first is to make your mechanical systems more efficient. At the Seattle Justice Center, variable flow chillers save up to 67 percent of pumping costs, and incremental efficiencies in each of the building's mechanical systems combine to cut energy costs by 32 percent.

After a certain point, though, extra efficiencies become prohibitively expensive. It can cost as much to get from 25 to 30 percent savings as it did to go from zero to 25 — which explains why engineers are putting so much effort into finding other alternatives.

The second basic approach involves a paradigm shift, rethinking the role played by mechanical systems.

Europeans have been doing this for years because they face higher costs and more stringent regulations than we do. So they install windows that can open. They use natural ventilation much more widely to supplement and even replace the mechanical cooling systems that we consider conventional. They also permit a broader range of indoor temperatures when outdoor conditions change.

Among the concerns that people have about buildings using natural ventilation for cooling is the effectiveness and reliability of such designs. Fortunately, engineers have new tools to analyze air movement, temperature and relative humidity inside a building.

One of the most powerful of these is computational fluid dynamics (CFD), a computer simulation tool that can be used to predict space comfort.

CFD comfort parameters recognize that not all degrees are created equal. Standing on a tropical beach with a gentle breeze, we experience 80 degrees in comfort. But in a stuffy cubicle, 80 degrees can feel like an oven.

Engineers can employ CFD to ensure air-circulation designs will keep cubicles feeling more like the beach. When owners and occupants are open to varying the design parameters, more than 60 percent of conventional energy costs can be saved.

Displacement ventilation represents yet another example of modified energy thinking. Rather than treating a building as a homogenous unit for temperature purposes, air is delivered from the floor directly to the occupied zone. The temperature of the volume of space above the occupants is allowed to rise well above normal comfort conditions.

Marion Oliver McCaw Hall, at Seattle Center, expects to save thousands of dollars a year in energy through just such a system.

Commissioning

Mechanical engineers can provide other services besides system design, such as commissioning.

Commissioning is like an audit, ensuring that building systems are designed, installed, functionally tested and capable of being operated to conform not only with the design intent, but with the actual use of the building.

Schedules are so tight today that contractors don't have the time to thoroughly test their work. Commissioning authorities ensure that systems are tested thoroughly with impartiality to either the contractor's or the designer's interests. Involving commissioning authorities early allows them to provide input during the design phase.

Commissioning authorities not only fine-tune mechanical systems and optimize them for actual conditions, but they also ensure maintainability. They also review maintenance manuals for completeness so facility managers can spot problems in advance and fix issues quickly.

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