Exemplary Design = Elementary Success
Integration has been the buzzword for this Colorado district, even going back to the way funding was obtained. From a hydronics system designed with simplicity and smart sequencing in mind to right-sizing throughout the project, engineers made this school a study in simultaneously pursuing first-cost and long-term performance success.
In the 1990s, Colorado’s Poudre School District (PSD) took a unique and innovative approach to increasing school funding. While many school districts identify needs in a closed-door environment and then take a mill levy or bond issue vote to the public, PSD was successful in uniting community leaders with principals, teachers, parents, and more than 200 volunteers. After rounds of discussion and separating the wants from the needs, the group agreed upon a mill levy increase that would generate approximately $10 million a year and a $175 million bond ballot issue.
In mid-2000, prior to the November bond election, PSD decided to sponsor a three-stage, invitation-only design competition for a three-track prototype elementary school in an attempt to find the best design that could achieve the delicate balance between sustainability and functionality. Design teams that were successful in the first and second stages of the submittal process were invited to participate in a competition that spanned 5 weeks to propose their best ideas including design concept, partial schematic drawings, a building model, and an energy analysis model for the new PSD prototype.
In 2000, PSD was still utilizing prototype elementary schools that were designed in 1987. The 1987 prototype schools had served PSD well, but with a nationwide drive toward sustainability and energy conservation, it had become clear that it was time for change. In November 2000, PSD voters took initiative and approved the $10 million annual mill levy increase and $175 million bond issue, turning design into reality.
At the end of the design competition, our team, led by RB+B Architects, Inc. out of Fort Collins, CO, was chosen to design the new school. Our firm, Shaffer • Baucom Engineering & Consulting (SBEC), as part of the successful design team, was selected as the mechanical and electrical systems designer.
PSD has been very progressive in the area of sustainable design and notably in their utilization of an integrated design approach, involving all project stakeholders in the design process from start to finish. Each of the four prototype schools incorporates “microload” building design, which utilizes a high-efficiency building envelope, daylighting design, and reduces internal gains, allowing heating and cooling systems to be significantly downsized. The overall design cooling load is more than 50% below that of a conventional building construction and design, and the annual energy savings are greater than 50% when compared to conventional construction.
Rice Elementary SchoolOur firm was fortunate to work with an architect that believes in and applied the concepts of integrated design. The architectural features of Rice Elementary are the driving force behind the superior building energy performance, and allowed the HVAC systems to be significantly downsized. Some of the highlights of the architectural features of the 63,000-gross-sq-ft facility includes the following.
- Commitment to a high-performance building envelope;
- Attention to building orientation and minimizing the building footprint;
- Integrating day-lighting design with exterior shading and lighting controls;
- Understanding the contribution of building mass in the energy equation.
Mechanical measures taken to reduce the system capacity include the following.
- No cooling in the gymnasium - heating and ventilation only;
- A cooling space setpoint of 75°F;
- Demand-based ventilation control sequences in intermittently occupied spaces such as the gymnasium and the flex room/cafeteria;
- Heating system capacity assumes a portion of building heat will be provided from internal gains (i.e., lighting, computers, and occupants).
Weather and Building Behavior: Pushing the EnvelopeThe first two prototype schools are equipped with air cooled packaged chillers, and the electrical rates justified an ice storage system at one of the schools. With Rice, PSD pushed the design team to find an alternative to a traditional chilled water system. The cooling system design began with an evaluation of a local non-potable irrigation distribution system for possible use as a heat sink for building cooling. For a number of reasons, it was determined the system was not a feasible alternative. Our design team then turned its attention to a means of evaporative cooling. The District had no desire to apply direct evaporative cooling, so the team began looking into different types of indirect evaporative cooling. A centralized system was chosen in an effort to minimize maintenance, minimize air handling fan energy, and centralize water treatment.
The design team utilized the semi-arid climate along the Colorado Front Range to its fullest advantage. The high elevation and the relatively dry air results in a daily temperature range of approximately 24°, which aids in maintaining the building mass at a temperature that assists with indoor temperature stability during the cooling season. For the typical school day occupancy, and from the months of September through May, there are approximately 350 hrs that the building requires cooling. Of those hours, the coincident wetbulb temperature is below 55° for 300 of those hours (86%), which forms the basis of design for the indirect evaporative cooling. During more extreme cooling demand conditions, indoor temperatures will rise, but this occurs only during the the last hour or so of full building occupancy. After 3 p.m., the building cooling system is deactivated, and the building uses its own thermal inertia to “float” until the next day.
The indirect evaporative cooling system design uses the 95% design wetbulb temperature of 56° for the months of September through May, allowing the cooling tower to be right-sized for the traditional school year. The cooling tower is sized for a water temperature drop of approximately 5° to deliver the coldest practical evaporative-cooled water to the building.
Due to the nature of Colorado’s climate, the performance of the building, and the innovation of the design team, we were able to completely eliminate a chiller from the design equation, which met one of the school district’s primary goals.
System DescriptionsA significant design team goal, and one that leads to lower first cost and maintenance costs, is to maintain a level of simplicity in the design of all building systems. For the hydronic systems, several simple design ideas were added to gain efficiency. The heating system consists of two high-efficiency, gas-fired boilers with staged capacity control; two variable-flow, base-mounted distribution pumps; heating water distribution piping arranged in a reverse-return configuration; secondary pumped coils at each AHU; terminal reheat; and local entry heating. Coil pumps were included in the design to maximize coil heat transfer and minimize coil discharge air stratification.
The cooled water system consists of an open cooling tower with variable-speed fan, a below-grade cooling tower water sump, a vertical turbine-type cooling tower loop pump, a plate-and-frame heat exchanger, a base-mounted building cooling water distribution pump, and distribution piping arranged in a reverse-return configuration to the AHU-cooled water coils. AHU coils were selected with a minimum of six rows, and 12 fins-per-in., providing a maximum 3° approach between supply water and leaving air temperatures.
The temperature control system is electronic, with a communications link to the school district central facilities hub. Control programming sequences designed to manage the hydronic system operation and to get the most benefit from the right-sized system capacities include the following.
- An aggressive heating water temperature reset schedule, to maximize boiler efficiency;
- A morning warm-up sequence to direct heating capacity toward warming the building mass prior to occupancy;
- Staging air-handling systems on in a defined sequence so as to manage morning warm-up. For example, the kindergarten rooms may require earlier morning warm-up than the gymnasium, to accommodate small children seated on the floor.
Boiler capacity is 1,600 MBtuh input, or approximately 25.4 Btuh/sq ft. Construction from the 1980s and 1990s would require a boiler input of approximately 40 Btuh/sq ft for similar occupancy in a similar locale.
Cooling tower heat rejection is 66 tons, or approximately 950 sq ft/ton. Conventional school construction in this area typically sees chilled water system capacities of 450 to 500 sq ft/ton.
Progression of EfficiencyFrom PSD’s 1987 prototype to the 2007 prototype, energy consumption was reduced by approximately 50% within this one generation span. This can be accredited to the integrated design process undertaken by the entire design team. Where the hydronic HVAC equipment would normally consume most of the building energy, choices made by the architect for the selection of materials, envelope, and daylighting minimized this energy consumption. At the same time, PSD’s aggressive approach toward sustainable design affords the ability to downsize HVAC systems without sacrificing efficiency or functionality.
The performance of these schools has become a benchmark for other school districts in the region trying to achieve high-efficiency and sustainability. This drive toward sustainability led to the creation of the LEED for Schools Certification category in 2007. The USGBC recognized the increasing need for a separate category for K-12 education. By addressing the uniqueness of school spaces and children’s health issues, LEED for Schools provides a unique, comprehensive tool for schools that wish to build green, with measurable results.
According to a 2006 article written by Greg Kats, a former director of finance for energy efficiency and renewable energy at the DOE, “A new national report finds that building ‘green’ would save an average school $100,000 each year - enough to hire two new additional full-time teachers.”
Kats also cited another report sponsored by the American Federation of Teachers, the American Institute of Architects, the American Lung Association, the Federation of American Scientists, and the USGBC, that includes a detailed analysis of 30 green schools built in 10 states between 2001 and 2006. This analysis demonstrates that the total financial benefits of green schools are 20 times greater than the initial cost, and include energy and water savings, and improved student health and test scores. According to the report, if all new school construction and school renovations went green starting today, energy savings alone would total $20 billion over the next 10 years.
From the original elementary prototype to Rice, the design has significantly progressed and with Bethke slated for completion in 2008, PSD continues to lead by example. Stu Reeve reflected, “We keep learning from each project. It’s been a collaboration and a team effort to keep learning, growing and improving.” From the use of chillers in Zach and Bacon to the ability to completely eliminate chillers in Rice and Bethke, our team at Shaffer • Baucom Engineering & Consulting continues to prove our commitment to sustainable design. We’ll have to wait and see what’s next. ES
* A special thanks to Tammie Simpson and Rice Elementary School for picture usage; Stu Reeve and the rest of the PSD staff for their commitment to sustainable design; RB+B for their commitment to the integrated design process.