JANUARY ISSUE: Sustainability–Hot, Cold & Stingy
- Jan 23, 2015
What do Federal Center South Building 1202 in Seattle, the SIERR Building in Spokane and the ASHRAE headquarters in Atlanta have in common? For starters, all three host plenty of top-notch engineers: the U.S. Army Corps of Engineers, energy-efficiency consultancy McKinstry and the professional association of HVAC and refrigeration engineers, respectively.
But the buildings share something else, too—a new twist on technology that is being adopted by some of the most energy-efficient properties in the Western Hemisphere. These structures use ground-source heat pump (GSHP) systems to substantially reduce the amount of power they need to buy from utilities to heat and cool their interior spaces. Also known as geothermal heat pumps and geoexchange systems, they tap energy absorbed from the sun by the earth’s crust. If deployed effectively, GSHP systems can produce substantial savings by drawing on a sustainable resource.
The Seattle, Spokane and Atlanta buildings employ the increasingly popular “closed-loop” GSHP system, which pumps a mixture of water and antifreeze through an enclosed network of pipes. Unlike generally more costly and higher-maintenance open-loop systems, closed-loop systems do not draw water from lakes, aquifers or other sources.
These systems use heat-exchange equipment linked with the building’s HVAC system to extract and deploy thermal energy from underlying (or nearby) earth or water. Because the temperature of that soil, aquifer or lake is relatively constant, it can serve as either a source of energy for heating or a “heat sink” cooling down uncomfortably warm air.
While installation of GSHP systems requires a sizeable upfront investment, their operations and maintenance costs are a pittance compared with conventional HVAC equipment. Though it is difficult to predict how much a system will reduce utility bills, a building equipped with a GSHP system can often get by with smaller chillers and boilers, while also reducing greenhouse gas emissions.
A case in point is the Spokane & Inland Empire Railroad Building, built in 1907 as a repair depot for electric streetcars. Twenty-five bores extending as far as 185 feet below the surface carry water pumped to and from the building’s three heat pumps, which in turn connect to piping coils embedded in radiant floor slabs. Each pump serves an area of the SIERR building with specific needs for heating and cooling, such as perimeter spaces next to large windows.
“The system can reject all the heat it needs to, even on the hottest day of the year,” noted Tony Marino, McKinstry’s lead engineer for the project. As a result, the GSHP system eliminates the need for some conventional HVAC equipment. Occasionally, the SIERR building needs supplemental heating from high-efficiency condensing boilers, since the GSHP system supplies only about 60 percent of the heating load on Spokane’s coldest days.
Since its renovation was completed in 2011, SIERR has averaged an energy-use intensity level of 50,000 BTUs per square foot (EUI is an indicator of consumption as a function of size or other characteristics). That lines up closely with initial projections for the building, and is little more than half of the ASHRAE baseline of 96,000-plus BTUs for comparable properties. Factoring in the entire alternative HVAC approach, the projected simple payback period comes to roughly 11 years.
Radiant-slab floors can be an exceptionally efficient means of drawing heat away from occupied spaces, Marino noted. Chilled beams suspended from ceilings can also do the trick, as long as ventilation systems distribute enough air to occupied spaces, he added.
Other noteworthy new buildings combining closed-loop GSHP systems with hydronic radiant heating/cooling loops include:
■ Federal Center South Building 1202 in Atlanta, which garnered the highest possible Energy Star score of 100—teaming its GSHP network with a custom-designed radiant-wave chilled sail and perimeter radiant beams to achieve “zero-net capable” status;
■ Cebula Hall, a laboratory facility at Saint Martin’s University in Lacey, Wash., which trains engineers and earned a hemisphere-leading LEED score of 97; and
■ Seattle’s Bullitt Center, which opened in 2013 and claims the title of the world’s greenest office building. During its first year of occupancy, the Bullitt Center registered an EUI of 9,000 BTUs. At stabilized full occupancy, it is expected to achieve a hard-to-beat score of 12,000 BTUs per square foot. Tenants include such green-minded occupants as PAE Consulting Engineers and the University of Washington’s Integrated Design Lab.
As this growing roster suggests, GSHP systems can reduce the energy a building needs from utilities to run the heating and cooling units associated with conventional forced-air HVAC systems. Such is the case with the twin 150,000-square-foot buildings that serve as Endo Health Solutions’ two-year-old headquarters complex in Malvern, Pa. The project shows that GSHP systems can effectively use bodies of water as thermal energy sources.
This relatively simple yet innovative closed-loop system taps cold water at the bottom of an adjacent quarry lake. Water is pumped from the lake bottom to pre-cooling coils, which feed intake air into the conventional direct-expansion A/C units. That makes the intake air much cooler than it would be otherwise when it reaches the evaporating coils of the A/C units. As a result, the building needs less electricity from the grid for mechanical cooling.
“We all saw the tremendous resource this deep-water quarry could represent for the project,” recalled Barry Henry, senior vice president with Trammell Crow Co., the project’s development manager.
Before construction, Vanderweil projected that it would take about 10,000 kilowatt-hours to run Endo’s GSHP each year, about the same as a typical American home. The expected payoff: annual savings averaging 440,000 kilowatt-hours of usage and $50,000 in utility costs.
The development team considered designing the system to meet some of the complex’s heating needs as well, but the amount of heating energy that could be extracted from water at lake-bottom temperatures would still need to be supplemented with mechanical heating, Henry and Polo noted.
The closed-loop system was selected in part because the tenant’s occupancy requirements demanded a fast-track effort—along with LEED certification—from the team, which also included L2 Partridge Architects, IMC Construction, MacIntosh Engineering and Worth Mechanical. Additional study and permitting would have been needed in order to install an open-loop system that taps lake water directly, Henry added.
As the experience of the Endo headquarters project team shows, it is no small task to decide whether to invest in a GSHP system and then select the most suitable design. Besides the characteristics of the building itself, many other issues come into play: the size and shape of the site, soil conditions, local climate, utility costs, environmental regulations and permitting. The cost-benefit analysis aside, however, the importance of environmental issues to the developer or occupants might be enough to greenlight this green initiative.
Having decided to move forward, project sponsors and their advisors must choose between vertical and horizontal bores and between the closed-loop and open-loop approaches. Generally speaking, the horizontal vs. vertical issue tends to be the tougher call.
If a developer has lots of unobstructed land to work with, and ground temperatures five or 10 feet below the surface are relatively constant, a horizontal field configuration might offer savings over deep-bore vertical drilling. The hitch is that such opportunities are limited, particularly for urban sites. Oak Ridge National Laboratory estimates that a horizontal field typically requires 1,500 to 3,000 square feet of land area per ton of cooling capacity, compared with just 250 to 300 square feet per ton for vertical borings.
And as McKinstry’s Marino related, “the deeper you go, the more constant the temperature.” Even though the compact SIERR building needs a modest 85 tons of cooling capacity, the site’s size and uses are incompatible with a horizontal system, he added.
Closed-loop systems appear to be gaining in popularity because they pencil out reasonably well with relatively small projects and tight sites. They are generally cheaper to maintain than their open-loop counterparts and require far less open land than vertical configurations. Open-loop systems tend to offer their biggest payoff with larger developments.
Another reason that open-loop systems often win the nod for bigger projects is well size. As Marino explained, bores for open-loop networks need larger wells than closed-loop systems, but fewer of them, to achieve the same energy savings. Also in the mix: the cost of rights to use groundwater or lake water, plus maintenance costs tied to higher sediment levels.
Ground conditions are an important consideration, too. GSHP systems perform better year-round in gravelly soil with some near-surface water flow than they do in bedrock, Marino related. Relatively loose soil is a more effective heat sink because it does a better job of dissipating rejected heat, he explained. By contrast, bedrock retains more of the heat that is being carried away from the building.