Engineering a sustainable school
Alexandria City Public Schools (ACPS) engaged b2E Consulting Engineers P.C., Leesburg, Va., to design an energy-efficient HVAC replacement for its 134,000-sq-ft T.C. Williams High School Minnie Howard Campus, which serves ninth graders. When completed in 2009, the school’s existing building was 43 years old. The existing system consisted of a two-pipe through-wall unit ventilator system with a constant flow air-cooled chiller and two water tube boilers serving the two-story 32-classroom wing, gymnasiums, and locker room areas (~66,000 sq-ft). The remainder of the building (~68,000 sq-ft) was packaged rooftop HVAC units with direct expansion (DX) cooling and natural gas fired heating.
We hired Hayes Large Architects (HLA) from Leesburg, Va., to assist in developing a master plan of sustainable design initiatives for the facility. In the process of meeting with ACPS, we determined that the client wanted an HVAC system concept that could be incorporated throughout the school district. ACPS wanted to use the energy-saving features of the building as an educational showcase with a “Greenovation Lab” to teach students the fundamentals of energy savings. In addition, ACPS wanted the energy-saving features of the building to be visible to the general public.
A ductless variable refrigerant multiple zone (VRMZ) heat pump system was appropriate for this building because it has a low slab-to-slab height. Ductwork had to be greatly reduced. The VRMZ system delivers heating and cooling through refrigerant piping using ductless ceiling-mounted terminal units in lieu of hot water (HW) and chilled water (CHW) piping and ducted ceiling-mounted fan coil units. The engineers used the Mitsubishi City-Multi System as the basis of design.
We decided that double-plate heat exchanger energy recovery ventilation units would be used to deliver 14,000 cfm 100% outside air to meet IBC-2006 code required ventilation. These units could not be supported on the existing pre-stressed concrete plank roof structure, so they had to be fit into the basement.
b2E Consulting Engineers decided that the VRMZ system should be connected to a ground loop heat exchanger in lieu of an air-cooled or typical hydronic (boiler and cooling tower) solution. The bus loop was the only location available for the ground exchanger. The available space was so limited that a creative solution was necessary.
Local zoning required that if more than 2,500 sq ft of land is disturbed, a site grading plan would be required. The estimated time (~12 months) to complete the civil plans was unacceptable to ACPS because the existing system was failing. We met with the local zoning board and worked out a plan whereby work on the well field under the existing bus loop would not be considered in the disturbed area because the final product would be the same as before the construction. Zoning asked ACPS to add some Filteras (planters that collect stormwater runoff from the pavement to improve surface runoff water quality), and ACPS agreed.
We helped ACPS procure a conductivity test of the site. The test results indicated that the borings could be 310 ft deep with a conductivity of 0.9 kBTU/ft F hr. The available area could only support 65 wells at 300 ft deep. However, this fell short of the 80 tons of cooling needed for the two-story classroom wing and locker room area renovation.
The well field was calculated using a double loop high-density polyethylene (HDPE) PEXa piping system. Capacity of the 65-well field increased from 68 to 80 tons. This system delivers approximately 15% more capacity using the same number of borings.
The engineers wanted to eliminate redundant systems and decided a separate hot water heating boiler was not necessary. Domestic hot water boilers were already required. The domestic hot water heating system would be used to heat the energy recovery unit (ERU) ventilation air from plate heat exchanger-2 leaving air temperature (PHX-2 LAT) to neutral 72 F supply air temperature to the classrooms.
The locker room—which also needed to be renovated—had too many showers and no office or conference areas. HLA developed a new concept to reconfigure the space. A ground-source heat pump (GSHP) unit was designed to deliver 100% ventilation air to the space.
Because the school’s students do not use the showers often, the heat stored in the domestic hot water (DHW) storage tank is primarily used for the kitchen, building service closet mop sinks, hand sinks, and lavatories. Knowing this, the engineers designed the hot water storage tanks to accommodate auxiliary double-wall tube bundles for heating non-potable water.
The DHW heating system was designed using condensing boilers with thermal stratification DHW tanks in lieu of mixing-type tanks. This reduces the boiler entering water temperature, which increases DHW boiler efficiency. Two tanks were used and piped in series: Domestic hot water storage tank-1 (DHWST-1) is a preheat tank and DHWST-2 is a final DHW tank.
Solar hot water
To reduce natural gas consumption, a solar hot water array was used to generate solar-heated hot water year-round. The solar panel angle was set at 45 deg. to maximize hot water generation in winter.
ACPS wanted to showcase renewable energy systems, but the ground loop heat exchanger piping would be hidden under the bus loop. However, the solar array could be visible—the front of the building faces due south. HLA performed a study to set the panels in front of the single-story classroom wing to shade the fenestration in summer and allow daylight UV radiation to enter the classroom in winter. A self-supporting steel structure was designed to hold 42 38-sq-ft panels in a linear array in public view.
The concept of the geo-solar system is to have one or two DHW storage tanks designed to store the solar hot water continuously generated by the array. This array is designed to generate 500,000 BTUH. The heat is stored in the tanks using a solar double-wall tube bundle in the bottom half of each tank. The solar panels can make up to 180 F water in full sun and no cooler than 85 F water on cloudy days.
- When the temperature of the 50% polypropylene glycol solution leaving the solar array is greater than 140 F, the solar hot water is diverted to the final DHWST-2. When the temperature is less than 140 F, the solar hot water is diverted to the pre-heat DHWST-1.
- DHWST-2 is maintained at 140 F. A double-wall tube bundle is located in the top half of the tank. Water is circulated through this tube bundle to the ERU HW heating coils to heat the air, leaving PHX-2 to 72 F SAT for neutral supply ventilation to the classrooms.
- DHWST-1 (1,250 gal.) is maintained at as high a temperature as possible. When DHWST-2 (900 gal.) is fully charged, this tank is used to dissipate the solar hot water. This tank has a double-wall tube bundle with a connection to the ground loop heat exchanger. A portion of the GSHP condenser water entering the heat pumps from the well field is mixed through the tube bundle. This keeps the heat pump EWT from falling below 55 F. For every degree the EWT is increased in heating mode, the coefficient of performance (COP) is increased by approximately 1%.
The system uses two 500,000 BTUH DHW condensing boilers connected in a stratifying piping arrangement to charge the tanks from top to bottom when there is inadequate solar contribution. The dispersion tube in the top of the tank must be designed carefully not to break the stratification layer during charging.
The existing building HVAC system energy consumption was determined using the existing utility bills. A base case new four-pipe CHW and HW system was evaluated using fan coil units in classrooms with energy recovery units in the basement, an air-cooled chiller on the roof (due to the noise ordinance), and condensing hot water boilers with a primary piping arrangement for both CHW and HW plants. The HW plant has a variable speed pumping system.
The proposed new Geo-Solar HVAC System for the two-story classroom wing, including areas of the building renovated using rooftop DX cooling and natural gas fired heating units, was included in the calculations. This was done because the new HVAC system is not submetered. The renovated building was calculated to save approximately $32,500 per year over the base case CHW/HW system.
The National Institute of Standards and Technology (NIST) Building Life-Cycle Cost (BLCC) Program calculation procedures were used to evaluate the energy consumption measures in this analysis. A real discount rate of 3% was used excluding general inflation.
The initial capital investment cost to install the geo-solar system over the four-pipe CHW/HW system was $675,000. The energy cost savings for the actual geo-solar system as compared to pre-renovation energy cost is ~$640,000 in present-value dollars over the 20-year study period. The base case air conditioning chiller and HW boiler systems were assigned $10,000 higher annual maintenance cost due to the required service contract for the systems. The nonannual repair and replacement costs were estimated to be the same for both systems; the chiller was given a $10,000 rebuild overhaul in year 10.
A residual valve of 50% ($102,500 in present-value dollars) was given to the ground-loop heat exchanger because it should last 60 years or three system lifetimes. However, we only assigned two system lifetimes in the analysis.
A year’s worth of actual utility bills was used for comparing the renovation of the existing building to the installed geo-solar system. The results are economically, ecologically, and politically positive. The school system will pay off the initial incremental investment cost and provide a calculated net savings of ~$406,500 over the 20-year system lifecycle. The simple payback period is only five (5) years and the discounted payback is twelve (12) years which is less than the system lifetime (20 years). The system will save approximately 54,630 MBtu, 6,426 metric tons CO2, 27.1 metric tons SO2, and 11.7 metric tons NOX over its lifetime. See Table 2 for the BLCC details.
The project was designed and constructed in three phases.
- Phase IA: Rooftop HVAC unit replacement for the 1955 and 1966 building areas
- Phase IB: Rooftop HVAC unit replacement for the auxiliary gyms, main gym, and auditorium
- Phase IIA: Geo-Solar HVAC system upgrade for two-story classroom wing and locker rooms
- Phase IIB: Rooftop HVAC unit replacement of the administration offices.
Each phase of the work was competitively bid. ACPS bid and purchased the packaged rooftop HVAC equipment to ensure all equipment would be by the same manufacturer for Phase IA, IB, and IIB.
Phase IIA (geo-solar system) was bid as a complete package and included the new electrical service. Energy-efficient light fixtures were replaced in each phase in accordance with the HVAC area upgraded.
The building lifecycle cost analysis was performed based on preliminary construction cost budgets for all phases of construction. However, the actual geo-solar investment cost considers the actual cost for the project.
The phases of construction using packaged rooftop HVAC equipment were completed over the summer recess period from June 20 to Aug. 26, 2008, and opened before the school year began.
The geo-solar phase had to be initiated early to allow the well driller to begin its work prior to June to meet the completion schedule. The wells were drilled in the bus loop between drop-off and pick-up each day and capped off with a concrete plug with zero effect on the educational program. One well was drilled 310 ft deep and grouted each day. The well field was completed at the end of June and pavement marked and trenched to lay the horizontal piping.
The 1-in. RAUGEO PEXa double U-bend loops were combined into one 1 ¼-in. PEXa pipe underground using REHAU Everlock fittings. One set of pipes runs back individually from each well to the REHAU Pro-Balance XP manifolds mounted inside quazite boxes installed flush with grade. Five 6x4x4-ft quazite boxes with removable covers were installed around the bus loop. Each box has 13 well loops connected to the balancing manifolds. One set of 6-in. HDPE SDR-11 ASTM 3408 pipes connect the manifolds and run back into the basement mechanical room behind the building.
Two water pumps are installed in a standby arrangement each sized for the full 240 gpm flow rate.
The stand-alone solar HW heating array using 1-in. connections to Helio-Dyne Gobi 410 blue cobalt sputtered copper absorbers in six-way series arrays piped in parallel for temperature rise, pressure loss, and balancing was fabricated and installed early to allow time to connect everything together for substantial completion.
The stratifying-type domestic hot water storage tanks and double-wall tube bundles required several iterations of shop drawings to meet the specific arrangement and tube bundle capacity since the vendors were unfamiliar with this technology at the time.
The Mitsubishi Variable Refrigerant Multiple Zone (VRMZ) ground-source heat pump system was also new at the time; however, the certified installer had no problem meeting the completion schedule.
Basket style T-5 fluorescent two-step dimming light fixtures were installed with two switches. One switch illuminates to 50% (~23 foot candle) and the second switch to 100% (~54 foot candle). The teachers are using this manual dimming technique to save energy if daylighting is sufficient in classrooms. Occupancy sensors were installed in all areas in accordance with local energy code requirements.
The Mitsubishi VRMZ air handling units are extremely quiet. The teachers are very happy that the old unit ventilators have been removed.
The domestic water tanks were installed first and set in place. The energy recovery units were very large and each had to be shipped in three sections to fit through a basement wall opening.
The new electric service installation was complicated and proved to be the most challenging aspect of the project during construction due to the short construction duration, and the associated utility company coordination.
The asphalt pavement was patched, milled, bond coated, final surface asphalt laid, and striped. This proved to be tricky because the drilling process heaved the asphalt in a few places. Although the installation has not caused puddling, care must be taken during designs over existing parking lots to accurately survey and mark the grades to ensure the system is restored to its original condition.
The consulting engineers included a complete commissioning specification in the construction documents including the commissioning process, the functional checklists, and the performance verification checklists.
- Commissioning process: The commissioning specification required that the contractor self-commission system operation first then demonstrate system operation to the owner/engineer.
- Functional checklists: These lists, one for each specification section, are provided to verify that the physical installation complies with the construction documents’ details and the manufacturer’s installation manuals, and to confirm that the system start-ups are complete. The lists also require that final documentation such as balancing reports, operational and maintenance manuals, and as-built drawings are submitted for final review.
- Performance verification: This is a point-by-point checkout procedure written by the engineer of record for the contractor’s benefit. The energy management control system contractor ensures the system functions in accordance with the sequence of operation, then demonstrates the operation to the engineer of record. This process worked very well on this project because it is a rather new system application. However, we do it on all of our projects when authorized by the client. We find it is quicker and less costly to the owner than the traditional commissioning process.
Beddow is principal of b2E Consulting Engineers P.C. He has worked in Switzerland, Germany, and the United States as a consulting engineer. His expertise is the application of energy-efficient technologies in innovative ways to maximize energy and cost savings for clients.