Vernonia K-12 School: LEED Indoor Environmental Quality Credit 7.1 28 July 2011 Test zones in the context of simplified geometry: Southwest Level 2 Classroom, Level 2 Administrative Area, Science Laboratory, and Computer Laboratory Michael J. Hatten, Principal mikeh@solarc-ae.net Alexandra R. Rempel, Building Scientist alexandrar@solarc-ae.net EUGENE - 541-349-0966 223 West 12th Avenue, Eugene, Oregon 97401 www.solarc-ae.net PORTLAND - 503-223-5253 319 SW Washington, Suite 311, Portland, Oregon 97204
Table of Contents Vernonia School: 1.0 Summary Introduction Thermal Comfort Criteria Modeling Approach Results LEED Requirements 2.0 Thermal Comfort Design Criteria Acceptable Thermal Conditions in Naturally Conditioned Spaces Elevated Air Speed 3.0 Modeling Parameters Weather Conditions EnergyPlus Objects Cool Water Supply Temperatures 4.0 Results Passively Cooled Spaces Non-Mechanically Cooled Spaces 5.0 Narrative for LEED Submission Project Temperature and Humidity Design Criteria Narrative 2
1.0 Summary 1.1 Introduction The purpose of this report is to document cooling-season thermal comfort in zones of the Vernonia K-12 school categorized as passively cooled by the U.S. Green Building Council in response to a Credit Interpretation Request. These include all zones that receive nighttime pre-cooling of concrete floors followed by daytime use of operable windows and ceiling fans: classrooms, laboratories, and administrative offices. While some of these zones may also receive stored cool water during the daytime, this option was not modeled, in part because prioritization between radiant floors and air-handling unit coils is changeable and in part to ensure that comfort data reflect fully-passive daytime operation. The range of acceptable thermal conditions in naturally conditioned spaces (ASHRAE 55-2004 Section 5.3 and Figure 5.3.) were therefore adopted as the comfort criteria for these spaces, and allowances for ceiling fan use (Section 5.2.3) were claimed to the extent applicable. 1.2 Thermal Comfort Criteria The requirements of ASHRAE 55-2004 Sections 5.2.3 and 5.3 set a maximum operative temperature (% acceptability limit) of 82.30F for the naturally-conditioned spaces. No humidity or air speed limits are required when this option is used. 1.3 Modeling Approach The hottest period in the applicable TMY3 weather file occurs from July 17-19, during which late afternoon drybulb temperatures reach into the 90 s for several consecutive days. At that time, however, administrative offices, the science laboratory, and the computer laboratory are unoccupied, while the one summer class in session has the option of using a cool Level 1 north-side classroom. The days of September 13-14 are instead the most thermally challenging, with full daytime occupancy and a combination of high dry bulb and high wet-bulb temperatures that substantially limits evaporative cooling potential. For this reason, all test data are presented for this time period. Because of its ability to simulate radiant systems explicitly, as well as to simulate surface temperatures, EnergyPlus was chosen to model the four zones most likely to overheat: the Level 2 southwest classroom, the computer laboratory, the Level 2 administrative office suite, and the science laboratory. These zones were identified by output of the 100% CD Proposed Design equest / DOE2 model. The gyms and commons, though occupied in part during the summer, were not predicted to overheat. All geometry, envelope elements, internal loads, and schedules of the four test zones were represented exactly as in the 100% CD Proposed Design model, except for updated summer occupancy schedules. Radiant floors and mechanical ventilation were modeled explicitly, while the cooling system was modeled as a plant loop that included a simple district cooling object scheduled to deliver water at an hourly-varying temperature calculated externally. 1.4 Results All four test zones will satisfy the ASHRAE 55-2004 thermal comfort criteria for naturally-conditioned spaces, at times requiring ceiling fan operation and the accompanying credit for elevated air speed. In addition, the gyms, commons, and band areas will satisfy comfort criteria for typical spaces, according to DOE2 model output for air temperatures (in this case an acceptable proxy for operative temperatures) and humidity ratios. 1.5 LEED Requirements The LEED Indoor Environmental Quality Credit 7.1: Thermal Comfort, Design submittal will require two elements: specific thermal comfort design criteria and a narrative describing the way(s) in which the designed systems intend to meet these criteria. The results of this work are condensed into the necessary table and narrative, referencing figures in the Results section, at the end. 1
2.0 Thermal Comfort Design Criteria 2.1 Acceptable Thermal Conditions in Naturally-Conditioned Spaces A Credit Interpretation Request (CIR) was submitted in February, 2011 to determine whether zones experiencing nighttime slab cooling, followed by daytime use of ceiling fans and operable windows, should be evaluated according to the thermal comfort standards for conventional or for naturally-conditioned spaces. The ruling stated, in part: The applicable spaces, as described, appear to meet the definition of naturally conditioned spaces, occupant controlled, and per ASHRAE Standard 90.1-2004, evaporative cooling is not considered mechanical cooling, nor are ceiling fans. Therefore, the comfort criteria in the spaces described may be evaluated according to ASHRAE Standard 55-2004 Section 5.3, and modeling is an acceptable means of documenting credit compliance. Section 5.3 establishes acceptable indoor operative temperature ranges based on an area s mean monthly outdoor dry-bulb air temperature, where operative temperature describes the average of the mean radiant and ambient air temperatures, weighted by the respective convective heat transfer coefficients and the radiant heat transfer coefficient of the occupant. The TMY3 weather file applicable to Vernonia shows a mean outdoor air temperature of 590F in September (blue line), corresponding to an acceptable indoor operative temperature maximum of 78.F (red line). 2.2 Elevated Air Speed The upper boundary of the operative temperature range may be extended further by increasing air velocity, if occupants can control the velocity. According to ASHRAE 55-2004 Section 5.2.3 and its accompanying figure, for sedentary activity, acceptable indoor operative temperatures can be increased by up to 5.4 F through this method, which would involve both ceiling fans and cross ventilation in the School design. Since the mean radiant temperature is predicted to be approx. 90F lower than the air temperature, however, diminishing the effectiveness of increased air speed, a more conservative value of 3.50F should be used. Assuming that ceiling fans and cross ventilation are excluded from the model, therefore, simulated operative temperatures would be acceptable if they fall below an upper boundary of 82.30F (green line). 2
3.0 Modeling Parameters The EnergyPlus model was constructed to simulate cooling-season conditions in four passively-cooled test zones, exclusively. Neither heating nor mechanical cooling were simulated. 3.1 Weather Conditions Vernonia s cooling season encompasses the months of July, August, and September (Fig. 1, next page), within which the most challenging days for maintenance of thermal comfort in the applicable TMY3 weather file are September 13-14. During these days, afternoon dry-bulb temperatures exceed 900F and nighttime wet-bulb temperatures remain above 550F; the school is also fully occupied, imposing maximum typical internal loads on the four zones most likely to overheat: the second-floor southwest classrooms, the computer laboratory, the second-floor administrative area, and the science laboratory. 3.2 EnergyPlus Objects Each test zone was modeled identically to its analog in the 100% CD QUEST / DOE2 Proposed Design model, including geometry, envelope constructions, interior loads, shading, and schedules; the only differences occurred in the zone HVAC systems, which could not be represented explicitly in the DOE2 model. In EnergyPlus, radiant floors were modeled as low-temperature radiant hydronic tubing, embedded 2 deep in concrete, with 5/8 inner diameter and spaced 12 on center, as specified in the 100% CD mechanical drawings. The four radiant systems were collected in parallel to form the demand side of a plant loop; the supply side of the plant loop consisted of a District Cooling object with a scheduled supply water temperature, the value of which was calculated externally (Section 3.3). Table 3.1. Model Parameters Measure / Parameter DOE2 Model Value Source / Notes EnergyPlus Value Notes Weather File Toledo-Winlock TMY3 closest match of all nearby TMY2 same as DOE2 model and TMY3 weather files to local weather station data 2.1 Envelope Opaque Wall U=0.05 (R-20) 4 extruded polystyrene (tiltup) same as DOE2 model or 2 extr. poly. + 6 batt (stud) Interior Floor Roof Infiltration (Perimeter) Window Glazing and Assembly Skylight Glazing and Assembly External Shading Roof U=0.033 (R-30 types) or =0.02 (R-50 type) 0.04 cfm/sf of external wall or roof area Tvis=70, Ucog=0.27, SHGC=0.38, Uassembly=0.41 Unit skylights: Ucog=0.29, SHGC=0.65, Tvis=0.75; metal-framed skylights: Ucog=0.55; SHGC=0.33; Tvis=0.39 3 building shades on S classrooms; 5.5 building shade on reading room concrete slab insulated underneath with R-15 batt insulation Construction:Internal Source object 6in extr. poly. (R-30) or 10in same as DOE2 model extr. poly (R-50) over metal deck best-practice values provided by same as DOE2 model facade consultants S class, Admin, Sci Lab: clear same as DOE2 model glass, Solarban 70XL on 2, Computer Lab: same but Solarban 60 Unit skylights: acrylic inner same as DOE2 model dome, fiberglass outer dome, white; Metal skylights: insulating laminated glass, Guardian SN-54 on 2, butyral interlayer soft white same as DOE2 model source=2 deep, 5/8 ID, 12 o.c. windows specified in WINDOW6 and imported skylights specified in WINDOW6 and imported to preserve spectral values 3
7/27 7/29 7/31 8/02 8/04 8/06 8/08 8/10 8/12 8/14 8/16 8/18 8/20 8/22 8/24 8/26 8/28 8/30 9/01 9/03 9/05 9/07 9/09 9/11 9/13 9/15 9/17 9/19 9/21 9/23 9/25 9/27 9/29 3.0 Modeling Parameters 100 Vernonia Cooling Season 90 70 60 Temperature (F) 50 40 Hourly Dry Bulb Temperature 30 Hourly Wet Bulb Temperature Average Daily Dry Bulb Temperature Average Daily Wet Bulb Temperature 20 Administrative Office Occupancy Begins School Session Begins Modeled Period for Radiant Cooling 7/01 7/03 7/05 7/07 7/09 7/11 7/13 7/15 7/17 7/19 7/21 7/23 7/25 4
3.0 Modeling Parameters 2.2 Internal Loads Occupancy 8am-6pm weekdays + intermittent weekend / evening use of commons, gyms. July: gym camps + 1 class + breakfast / lunch; Aug: admin + 1 class + bf/ ln; Sept: school in session Lighting Power Density classrooms: 0.58 W/sf; offices: 0.66 W/sf Daylight Sensors Equipment Power Density 2.3 Cooling Plant Evaporative Cooling Tower, Cool Water Storage Tank, AHU Cooling Coils 2.4 HVAC Systems Ventilation Radiant Cooling DOE2 Model Values Source / Notes EnergyPlus Values Notes 30fc setpoint; continuous dimming to 10% minimum classroom: 0.5 W/sf; science lab: 1.0 W/sf; admin: 1.5 W/sf; computer lab: 2.0 W/sf. 40% phantom load. electric screw chiller with high COP to represent cooling tower efficiency; externally calculated cool water production; scheduled cool water supply for AHU coils modeled as heating / ventilating systems with summer night flush cooling early morning slab pre-cooling modeled as nighttime natural ventilation of mass diversity scheduled in gyms, classrooms, arts areas; student occupancy of classrooms ends at 4pm but continues in gyms and arts areas consistent with 0.8 W/sf in building overall, with allowance for occupancy sensors see Incentives report for details see Incentives report for details see Incentives report for details same as DOE2 model same as DOE2 model same as DOE2 model same as DOE2 model District Cooling object with externally-calculated and scheduled cool water temperature supplies PlantLoop Zone:Ventilation objects provide equivalent outside air without fan energy ZoneHVAC:LowTemperatureRadiant:Variable Flow objects form demand side of PlantLoop see section 3.3 for details 3.3 Cool Water Supply Temperatures The operation of the cooling plant was not simulated; instead, an external calculation was used to provide an hourly schedule of storage tank water temperatures. The current mechanical design directs the cooling tower to deliver water 30F greater than the ambient wet-bulb temperature, or 590F, whichever is higher (M507). These parameters were kept explicitly in the external calculation. The current design also directs the cooling tower to be enabled at 10pm each night and anytime the tank bottom temperature is greater than 620F, and to continue until the tank top and bottom reach temperatures within 0.50F of one another. These parameters, in contrast, were altered to prevent the tower from adding heat to the tank, which would have resulted on occasion due to high wet-bulb temperatures. During these hours, maximum flow rates to both the tower (500gpm) and to the heat exchanger (500gpm) were assumed, allowing each hour s average stored water temperature to be calculated from the volume-weighted average of the condenser water temperature (the greater of Twb+30F or 590F), the return water temperature (obtained as the model s hydronic loop return temperature), and the previous hour s tank water temperature. The resulting temperature was then increased by 20F, to account for losses through the heat exchanger, and input to the model as its scheduled hydronic loop cooling supply water temperature. Running the model then generated 5
3.0 Modeling Parameters return water temperatures, which allowed scheduled input temperatures to be revised; within several cycles, return water temperatures converged to within 0.01%. Because of uncertainties regarding daytime use of stored cool water in air-handling unit cooling coils, no daytime radiant cooling was modeled, and accordingly, no daytime increase of storage tank temperatures was included explicitly in the external calculation. Instead, the tank turnover time of 1h 40min (consistent with maximum pump operation) was incorporated as a delay between tower start-up and tank cooling at the beginning of each enabled period. Cool water supply and return temperatures, alongside an example slab temperature, are shown below (Fig. 2). 95 90 Cool Water Supply Temperatures Tower On Tower On Tower On 85 Night Cooling Night Cooling Temperature ( F) 75 70 65 60 55 50 45 40 Slab Temperature Cool Water Supply Temperature Cool Water Return Temperature Outdoor Dry Bulb Temperature Outdoor Wet Bulb Temperature September 13 September 14 Fig. 2. Externally-calculated temperatures of the cool water supplied to the radiant floors used in nighttime cooling of mass. All parameters were generated by the EnergyPlus model and Toledo-Winlock TMY3 weather file except for the cool water supply temperature (orange line), which was calculated from the wet-bulb temperature, cooling tower operation specifications, tank mixing and heat exchanger limitations, and modeled return-water temperatures. 6
4.0 Results 4.1 Passively Cooled Spaces Southwest Classroom, Level 2. The west-wing classrooms on the upper level, with southern exposure, present the greatest cooling challenge among the building spaces due to intermittently high occupancy, solar gain through skylights and windows, and minimal losses through the well-insulated envelope. Nevertheless, model-predicted indoor operative temperatures remained well within the ASHRAE 55-2004 adaptive comfort range, with credit taken for elevated air speed (ceiling fan operation), even on the hottest September days (Fig. 3). Computer Laboratory. Although this space has a northern exposure, high internal loads cause it to experience some warm hours. As for the other classroom, though, these remain within the adaptive comfort range. Administration, Level 2. Offices have the highest sustained internal loads in the building, causing the skylit upper administration area to be an area of concern. Here, too, operative temperatures remained within the adaptive comfort range. Science Laboratory. The second-floor laboratory, with scientific equipment loads and intermittent high occupancy, had the fourth-greatest cooling load and was therefore included in the test zones; however, its operative temperatures remained in the comfort range even without the elevated air speed credit. Temperature ( F) 95 90 85 75 70 65 60 55 50 45 40 Southwest Classroom, Level 2 September Comfort Zone Outdoor Dry Bulb Temperature Indoor Operative Temperature Slab Temperature Outdoor Wet Bulb Temperature September 13 September 14 Fig. 3 (above and following pages). Indoor operative and slab temperatures during the 48-hour test period, compared to outdoor dry-bulb and wet-bulb temperatures. The September comfort zone denotes the September adaptive comfort threshold (66-78.F) with the addition of the 3.50F allowance for elevated air speed. 7
4.0 Results 95 90 85 Computer Laboratory Temperature ( F) 75 70 65 60 September Comfort Zone 55 50 45 40 Outdoor Dry Bulb Temperature Indoor Operative Temperature Slab Temperature Outdoor Wet Bulb Temperature September 13 September 14 95 90 85 Administration, Level 2 Temperature ( F) 75 70 65 60 September Comfort Zone 55 50 45 40 Outdoor Dry Bulb Temperature Indoor Operative Temperature Slab Temperature Outdoor Wet Bulb Temperature September 13 September 14 8
4.0 Results Temperature ( F) 95 90 85 75 70 65 60 55 50 45 40 Science Laboratory September Comfort Zone Outdoor Dry Bulb Temperature Indoor Operative Temperature Slab Temperature Outdoor Wet Bulb Temperature September 13 September 14 4.2 Non-Mechanically Cooled Spaces The gyms, band areas, and commons are to be cooled by either outdoor air (nighttime purge and daytime economizer modes) or by air passed over cooling coils suppled by evaporatively-cooled storage tank water. These spaces were not addressed in the LEED Credit Interpretation Request, but it is clear that they must be evaluated according to the standard comfort criteria depicted in ASHRAE 55-2004 Fig. 5.2.1.1; although evaporatively-cooled water is regarded as a passive cooling component, the conditions of these spaces are not regulated primarily by the opening and closing of windows by occupants, nor do they accommodate primarily sedentary activities (ASHRAE 55-2004, Section 5.3). Because these spaces will have no radiant cooling, and surface temperatures are therefore not expected to differ substantially from air temperatures during the cooling season, the 100% CD Proposed Design DOE2 model was used for their evaluation. 9
4.0 Results For these spaces, the hottest hours similarly occurred on the afternoon of September 14 (Fig. 4), but did not exceed the standard comfort zone. Of note, however, is the tendency toward high humidity levels in the band and, to a lesser extent, stage areas. This results primarily from the higher-than-sedentary metabolic levels in these spaces, combined with their current scheduling for occupancy until 6pm on most afternoons. Fig. 4. Indoor dry-bulb temperatures, used as a proxy for operative temperatures according to ASHRAE 55-2004 Appendix C, and corresponding humidity ratios for the most extreme occupied hours of September 13 and 14 in the High School Gym ( ), Commons ( ), Band ( ), and Middle School Gym ( ). 10
5.0 Narrative for LEED Submission 5.1 Project Temperature and Humidity Design Criteria Season Spring Summer Fall Winter Classrooms: Offices: Gyms: Commons: Classrooms: Offices: Gyms: Commons: Classrooms: Offices: Gyms: Commons: Classrooms: Offices: Gyms: Commons: Maximum Indoor Space Design Temperature (0F) 82.3 82.3 82.3 82.3 82.3 82.3 Minimum Indoor Space Design Temperature (0F) 66.2 66.2 67 67 66.2 66.2 67 67 Maximum Indoor Space Design Humidity (Ratio) n/a n/a n/a n/a n/a n/a 5.2 Narrative Naturally Cooled Spaces. The classrooms, offices, and laboratories of the School are to be cooled at night by a radiant hydronic system supplied by evaporatively-cooled water; during the day, further cooling will be provided by operable windows and ceiling fans. A Credit Interpretation Request ruling has deemed this strategy natural conditioning, and the comfort of these spaces is therefore evaluated according to ASHRAE 55-2004 Section 5.3, Optional Method for Determining Acceptable Thermal Conditions in Naturally Conditioned Spaces, and Section 5.2.3, Elevated Air Speed. The most thermally challenging days were found to be September 13 and 14, in which afternoon temperatures in the applicable TMY3 weather file exceed 900F, nighttime wet-bulb temperatures remain above 550F, and the school is fully occupied. Vernonia s mean September monthly temperature is 590F, corresponding to a maximum allowable indoor temperature (% acceptability limit) of 78.F; addition of a 3.50F credit for elevated air speed in a space with a mean radiant temperature 90F below the air temperature raises this threshold to 82.30F. To document the comfort compliance of these spaces, the four warmest were identified from the Proposed Design equest / DOE2 model and modeled in EnergyPlus, allowing hourly operative temperatures to be reported and allowing the hydronic cooling process to be simulated more precisely. All other geometric parameters, envelope constructions, internal loads, and schedules were identical to those in the DOE2 Proposed Design model. Despite the afternoon heat on the reported days, the nighttime cooling of mass was effective, and operative temperatures remained below 82.30F in all four test zones (Figure 3, above). 11
5.0 Narrative for LEED Submission Non-Mechanically Cooled Spaces. The gyms, commons, and arts areas are served by air-handling units rather than radiant hydronic systems. These spaces are to be cooled through nighttime ventilation with unconditioned outdoor air and through daytime cool air, with cooling coils served by stored evaporatively-cooled water. Although they do not possess compressor-based cooling, they are not primarily sedentary spaces, and their comfort is therefore evaluated according to the boundaries of ASHRAE 55-2004 Section and Figure 5.2.1.1., Graphical Method for Typical Indoor Environments. These spaces meet all of the criteria listed in Informative Appendix C, Acceptable Approximation for Operative Temperature, allowing air temperatures to be substituted for operative temperatures, and DOE2 model outputs were therefore used to document their compliance (Figure 4, above). Mechanically Cooled Spaces. The 1st Floor Administrative Offices, the Library, and the Computer room A203 are mechanically cooled by a direct-expansion packaged rooftop air handling unit with zonal air volume controls. The rooftop unit and the zonal distribution and control system are designed to maintain operative temperatures of 750F and 60% relative humidity on a design cooling day in each zone. The Vernonia K-12 school s design criteria are the same as defined by ASHRAE 55-2004 as a typical indoor environment, and all zones served by this rooftop unit meet ASHRAE 55-2004 Graphical Method requirements for % occupant acceptability. Mechanically Heated Spaces. The building has a biomass-fueled hot water boiler, as well as a natural gas fueled condensing hot water boiler, which are each sized to provide heating for the entire building on a peak design day. Zonal heating temperature control is achieved by hydronic radiant floors for some spaces, and fan systems with hydronic air coils in others. Zonal temperature control for all occupied spaces within the building will meet a minimum space temperature of 6F at design heating conditions, which meets ASHRAE 55-2004 requirements. 12