COOLING LOADS AND OCCUPANT COMFORT IN HIGHLY GLAZED BUILDINGS AND THE EFFECTIVE EVALUATION OF WINDOW RETROFITS

Transcription

1 COOLING LOADS AND OCCUPANT COMFORT IN HIGHLY GLAZED BUILDINGS AND THE EFFECTIVE EVALUATION OF WINDOW RETROFITS S. Armstrong ABSTRACT Urban centres contain thousands of mid to late 20th century commercial, institutional, and multi-unit residential buildings. In order to improve the efficiency of these buildings and reduce an ever-increasing infrastructure demand, our modern cities must face the retrofit imperative. Of course, it s not as simple as just installing insulation, window films, or extra sealant there are many challenges when evaluating building envelope retrofits. These challenges include obtaining reliable performance improvement information and determining realistic savings. This cannot be stressed enough: determining realistic savings is essential. We are often asked to evaluate various envelope upgrade measures, either as a standalone project or as part of a more holistic existing building commissioning (or re-commissioning) project. It doesn t take long to realize that finding accurate information on which improvements provide the best value is tough but accurate information is critical to the project s success. Equally critical is the need to understand the impetus for the upgrades: are occupants to hot? too cold? is there air leakage? water infiltration? The reason alters the approach and solution considerably. A recent example which highlights some of the challenges of evaluating envelope retrofits involved assessing the anticipated cooling load reduction with a post-applied solar heat gain control system for an office building in downtown Toronto. The project included an analysis of building energy use, in-situ monitoring of preand post-retrofit temperatures, in-situ heat gain monitoring, and energy cost savings calculations. The motivating factors: occupant comfort and energy savings. OBJECTIVE To determine an effective means for improving occupant comfort and achieving energy savings through post-applied solar control measures using in-situ monitoring of temperatures and solar energy gains and performing energy savings calculations. BACKGROUND One recent example where we encountered some of the challenges of evaluating envelope retrofits involved determining whether a reasonable retrofit solution existed for reducing solar load on an existing office building in downtown Toronto. The motivating factors: occupant comfort and energy savings. Our evaluation initially comprised a high-level analysis of possible retrofit options for controlling solar loads. The options considered included: active and passive exterior and interior shades and interior and exterior-applied window films. Our evaluation did not include solar heat gain through non-vision areas and did not consider the thermal resistance of vision or non-vision elements. 341

2 Existing Glazing Configuration The solar control for the existing vision lites comprises heat absorbing green-tinted float glass (heat strengthened) on the outer lite and a low-emissivity coating on the outer surface of the inner lite (surface #3). Based on the building s age, the low-e coating on the existing vision lites likely is a pyrolitic (hard coat) treatment, which has relatively low performance compared to modern sputter (soft coat) low-e treatments. For example, pyrolitic coatings have higher solar heat gain and u-values than sputter coatings. Pyrolitic coatings also have lower light-to-solar gain (LSG) values than sputter coatings, which means that for a given visible light transmittance a pyrolitic coating has a higher solar heat gain. While some buildings may benefit from higher solar heat gain, highly-glazed commercial office buildings rarely are in this category. The building has also had several units replaced over its service life, which comprise a green-tinted outer lite and sputter-coat low-e treatment similar in appearance to the original vision lite (i.e. green-tinted outer lite with high visible light transmittance and low outward reflectivity) but with a higher performance. The replacement units also comprise two lites of heat strengthened glass for improved durability. We calculated the solar heat gain co-efficient (SHGC) for the existing and replacement units by constructing the glazing unit in Optic5 (simulation software). Based on this simulation, we estimate the SHGC for the original glazing units to be approximately 0.70 and the SHGC for the replacement units to be approximately We estimate that approximately 10% of the original windows have been replaced as part of ongoing life-cycle replacement requirements. We calculated the vision-to-spandrel area for the building on each elevation and determined that the vision area (glass) comprises approximately 75-80% of the total wall area on occupied floors. The glazing is also quite clear in its appearance: the green tinted outer lite and low-e coatings on the inner lite do not greatly reduce the clarity of the glass or increase its reflectivity. This combination of a high ratio of vision to opaque (spandrel) area and visually clear glass results in high levels of daylighting and solar heat gain while providing views into the building from the exterior. Good daylighting of tenant spaces and clear views in and out of buildings are desirable characteristics in commercial office buildings; however, excess daylighting and solar heat gain can be the source of tenant discomfort. In the subject building, the owner had received numerous complaints of overheating (or undercooling) at the perimeter spaces over a prolonged period of time. These complaints were the impetus for the investigative program discussed herein. Based on the parameters of maintaining clear views, retaining good daylighting, and reducing solar heat gain, we investigated several retrofit strategies: exterior shades, interior shades, and retrofit window film. Exterior shades The most effective means of controlling solar heat gain is by blocking it before it strikes the façade. This can be achieved with exterior-applied shading systems designed to completely block incident solar radiation or designed to block a significant portion of incident solar radiation while maintaining desirable daylighting and views. Exterior shade systems can be simple, fixed blade elements or complex, fully integrated, solar tracking mechanisms that optimize shading effectiveness based on climatic conditions. All buildings must be evaluated on an individual basis and, for some, exterior-mounted shades may be part of a larger re-visioning or re-branding strategy; for others, they may be seen as detrimental to the aesthetic merits of an existing building. The architectural prominence of the subject property eliminated the possibility of deploying an exterior shading solution and, as such, we did not conduct an analysis of the performance 342

3 of this measure. However, the data presented in this paper still provides valuable insight into the possible savings from such systems. Interior shades The subject property has varying existing shade systems. At a minimum, the building is outfitted with horizontal mini blinds on all windows; additionally, some spaces have light-reducing fabric shades. Retrofitting the existing system to include automatic blind control devices and optional links to photosensitive lighting systems would provide the most significant improvement over the current strategy. Automatic blind and lighting control systems can react quickly and efficiently to the amount of incoming sunlight and these systems have the potential to reduce the energy consumption for lighting in office buildings by up to 30-80% (Galasiu, A. D., 2003). By casual observation, one can identify how manual shades provide sub-optimal performance: occupants may not be present in the room or may react too late to the high solar gain, resulting in sub-optimal performance; sun angle, temporary cloud cover, and time of day make the manual adjustment of shading to the ideal position difficult, and; solar heat gain is permitted to entering the occupied zone and is trapped near the glass. Look at any mid- or high-rise building in your neighbourhood no two shades are adjusted alike and many buildings look like a patchwork quilt. Upgrading the existing building and installing an integrated system to benefit from daylight harvesting would have required replacing all of the existing blinds and upgrading the lighting system to include dimmable ballasts, which would be very costly. Thus, further evaluation of this approach was abandoned. Window Films Window films can reduce solar heat gain by a significant amount, which, in highly glazed buildings, can improve occupant comfort by reducing the amount of direct gain experienced by occupants in perimeter zones. For example, ¼ clear glazing with a window film featuring ceramic technology can have a SHGC in the range of 0.41 to 0.53, compared to an original SHGC of 0.86 (Source: 3M, Ceramic Product Family Literature). Unfortunately, the effect of this solar heat gain reduction on cooling load is not as straightforward cooling loads in buildings with deep floor plates typically are driven by high internal loads (occupants, computers, equipment, etc.) and less by perimeter solar heat gain loads whereas cooling loads in buildings with shallow floor plates tend to be influenced more by solar heat gain. Without understanding how much solar heat gain impacts a building s cooling loads, the potential savings can be overstated. With approximately 80% glazing, a significant Architectural presence, and existing interior shades, the subject building seemed an ideal candidate for window film. Films can be visually clear or provide a range of options for modifying the reflective or tint qualities of glass. Thus, a window film retrofit was analysed in detail and is the subject of this paper. PROGRAM To determine the effectiveness of window film for addressing occupant comfort and reducing cooling costs, we conducted space temperature and thermal energy (BTU) transmission monitoring in two locations for a total of 16 days. The monitoring was performed during September 20 through October 5. The ambient outdoor temperature during this time ranged from daytime highs of 10.2 to 24.7 deg C and overnight lows of 5 to 18.4 deg C (Source: Environment Canada). Space temperature data was collected for 10 days prior to the film installation to develop a baseline and for 6 days after the film installation. Two simultaneous 343

4 temperature collection points were recorded using a HOBO U temperature relative humidity data logger with a single exterior temperature probe: one sensor was placed approximately 1 from the interior surface of the glass (perimeter zone) and one sensor was placed approximately 10 from the interior surface of the glass (interior zone). The sensor in the perimeter zone was approximately 8 above the floor and the sensor in the interior zone was approximately 5 above the floor (PHOTO 1). The sensors were placed at the south west corner of the building and both the horizontal mini blinds and the fabric shades were left in an open position for the duration of the test. The thermal energy transmission monitoring was conducted over a three day period using TES 1333R Solar Power Meter data loggers. Three data loggers were used simultaneously: one placed directly behind an untreated glazing unit and one placed behind each of the two film mock-up areas within the study area. In all cases, the thermal energy loggers were placed facing south and angled approximately 10 degrees above horizontal. The thermal energy monitoring was performed during the final three (3) days of the 16-day temperature monitoring period and after the retrofit film was installed. The monitored days coincided with high solar heat gain days (i.e. clear, sunny sky conditions), which may slightly overestimate the energy savings from reduced solar heat gain since cloudy days were not included in the test period. PHOTO 1: VIEW OF SPACE TEMPERATURE AND THERMAL ENERGY TRANSMISSION MONITORING SET-UP. Window Films We selected two window films for the study: 3M CM40 and CM50. These films were chosen based on their high solar energy rejection, visual clarity, and minimal interior reflection. The CM40 has a slightly lower visible light transmittance (VLT) compared to the CM50, which was selected as one of the options to help reduce glare. In addition to tenant concerns regarding overheating, glare was identified by the owner as an ongoing concern. While interior-applied windows films reject heat from the occupied space, this heat is absorbed both by the inner lite and the air space. Thus, it is important to consider glazing size and durability when selecting an interior-applied window film. Too aggressive a film (i.e. too much heat rejection) could result in breakage if the units are a certain size or if they comprise regular float glass. Solar Condition We developed a solar condition index (FIGURE 1) to identify whether a correlation exists between weather, sky condition, and solar heat gain. The solar condition index was developed using Environment Canada daily weather data and assigning a graduated scale to various sky conditions as follows: 344

5 We found a strong correlation between the solar condition index and space temperature data. For example, when the solar condition rating was 5 (or high), the interior space temperature at the glass and was high. Conversely, when the solar condition rating was lower, the interior space temperature at the glass (perimeter zone) was lower. This correlation helps support our understanding of solar heat gain on space temperatures in the perimeter zone. FIGURE 1: SOLAR CONDITION INDEX INDICATING THE RANGE OF ANTICIPATED SOLAR RADIATION BASED ON SKY CONDITION FOR THE DURATION OF THE STUDY. 345

6 Limitations The methodology was developed to obtain data which would assist in developing a meaningful conclusion pertaining to the impact of solar control film on space conditions. The study was restricted to three windows in a single area of an occupied open office area at the south west corner of the building on the 8 th floor. We were unable to measure directly the amount of cooling demand for the mock-up areas. The HVAC system and BAS were also incapable of measuring the amount of air delivered to the specific mock-up areas. It is our opinion that this type of direct monitoring of space cooling demand may provide additional data indicating whether a reduction in space cooling demand resulted in the mock-up areas. RESULTS Space Temperature Monitoring The space temperature monitoring data developed demonstrates that the temperature differential between the perimeter and interior zones decreased by between 0.8 (CM40) and 1 deg C (CM50) after the film was installed (FIGURE 2). This narrowing of space temperature differential occurred despite similar solar conditions (high solar heat gain) before and after the film installation. This narrowing of the space temperature differential between perimeter and interior zones could result in improved occupant comfort since cooling is delivered in response to interior zone space temperatures (i.e. the perimeter zone will be closer in temperature to the interior zone). 346

7 Thermal Energy Transmission The thermal energy transmission data indicates a reduction in energy transmission ranging between 38% and 45% depending on the film deployed (FIGURE 3). The fluctuations in thermal energy transmission correlated well with the fluctuations in temperature data and the solar condition index. 347

8 FIGURE 3: BTU ENERGY MONITORING SHOWING A COMPARISON BETWEEN THE EXISTING GLAZING AND TWO SAMPLES WITH RETROFIT WINDOW FILM INSTALLED. Energy Analysis The determination of potential savings as a result of installing solar control film is based on the above data collection and analysis as well as the conclusions resulting from an energy audit and existing building commissioning exercise conducted by our firm at the same building. Based on this energy audit, we obtained the following pertinent information: 1. The building is operating on a district energy cooling system, which means that approximately 50% of the total cooling costs are fixed and unrelated to actual consumption due to weather. 2. The consumption-related cooling costs are approximately $113,000 per year; the fixed cooling costs (unrelated to consumption) are approximately $95,000 per year; 3. Approximately 20% of the building s cooling load is attributable to weather, or $22,000 per year (based on consumption-related cooling costs of $113,000 per year) with the remaining load attributable to internal sources (people, lighting, computers, etc.); 4. The cooling costs are not significantly impacted by peak cooling demand, and; 5. Electrical consumption generally is unaffected by weather. A separate analysis was conducted of the district energy system billing method; however, that has been excluded for simplicity. Based on our research, solar heat gain is responsible for approximately 16% of typical large commercial office building cooling loads (Huang, J. 1999, pg 39) for buildings in climate zones similar to the subject building. The buildings included in the cited study had vision glazing in the range of 40-50% compared to 348

9 the approximately 80% vision glazing in the subject building. Due to the high percentage of glazing on the subject building, we conservatively estimate that solar heat gain contributes to approximately 40% of the total weather-related cooling loads, as summarized in TABLE 1. TABLE 1: SOLAR HEAT GAIN RELATED COOLING ENERGY CALCULATION. Based on the above assumptions, we prepared an estimate of the savings to be obtained by installing the solar control film using three methods. Note that calculations are based on the assumption that uniform savings or load reductions can be anticipated across all solar exposures. In reality, the effect will be more pronounced on areas with the highest solar exposure, which suggest that a targeted retrofit program may be most economical. Method 1: SHGC improvement The solar heat gain coefficient (SHGC) of the existing window units was compared to the SHGC of the improved windows with the solar control film installed. In order to determine the SHGC for the existing windows, we assumed that 10% of the glazing units have been replaced with newer units. To determine a baseline SHGC for the existing windows, we created a weighted average between the original and replacement units, resulting in a baseline SHGC of This baseline was compared against the improved windows with a SHGC of to calculate the potential savings (TABLE 2). TABLE 2: ESTIMATED COOLING ENERGY SAVINGS ($) AS A FUNCTION OF SOLAR HEAT GAIN MITIGATION. Method 2: BTU energy reduction The BTU energy transmission reduction obtained from the on-site monitoring was used to estimate the potential energy cost savings. The BTU reduction is measured using the most effective of the three films tested (TABLE 3). TABLE 3: ESTIMATED COOLING ENERGY SAVINGS ($) AS A FUNCTION OF BTU REDUCTION. 349

10 Method 3: Building Energy Modeling We created a representative building energy model based on the geometry of the subject building using EE-4 to identify potential cooling energy savings and heating energy cost increases. The window-to-wall ratio of the subject building was used as was building orientation and floor area. Based on this simulation, the peak cooling load decrease was insignificant and the total cooling load was offset completely by the heating consumption increase due to reduced winter free-heating. The following is a summary of the modeling results: Peak cooling reduction: 1 kw Cooling consumption reduction: 2,980 kwh/floor or $298/floor based on current building energy costs Heating consumption increase: 1,060 m3 of natural gas/floor or $371/floor based on current building energy costs Net energy savings: -$72/floor/year (i.e. net energy cost increase) Based on our assessment, the window film installation could not be justified on a simple payback resulting from cooling energy savings. The simple payback for a whole-building window film retrofit is in excess of 65 years based on the most optimistic energy savings. CONCLUSION Based on research, observations, data collection, and analysis, it appears that occupant comfort will be positively affected by installing a solar control window film on the interior surface of the glazing even if significant cooling energy savings aren t realized. The temperature differential between the interior and perimeter zone was reduced by installing the film in both test areas. The moderating of space temperature at the building perimeter should result in more consistent temperature control across the floor, leading to greater occupant comfort. The solar control film also reduced total solar heat entering the building, which would reduce the same radiant heat gain experienced by occupants sitting close to the exterior walls. The window film provides these benefits regardless of how occupants adjust their existing blinds or install additional interior shade systems. The window film also has minimal impact on the building s aesthetics, unlike the installation of an exterior shade structure. Although glare was not identified as a primary concern by occupants, glare was visibly reduced after installing the solar control film, particularly with the CM40. This observation was supported by anecdotal feedback from tenants in the immediate area of the test installation. It is conceivable that the solar control film and resultant solar heat gain and glare reduction will result in occupants keeping the base building and tenant shade systems in an open position during a greater portion of the day. At present, the lighting system is not capable of dimming in response to effective daylighting; however, effective glare control can facilitate this type of upgrade at a later date when the lighting system is due for a major upgrade. Based on our assessment of the potential energy savings from the window film installation, it was apparent that the film installation cannot be justified on a simple payback (in excess of 65 years) based on the most optimistic energy savings. Lastly, as with all solar heat gain reduction strategies, there will be a corresponding increase in heating energy consumption to account for the lost solar heat gain. In cooling-dominated buildings, the heating energy consumption increase typically is lower than the cooling energy consumption decrease, resulting in overall lower energy costs. 350

11 Based on the lessons learned during this study, we recommended that the decision to install a solar control film should be justified by its ability to improve occupant comfort resulting from reduced heat gain and glare. There will be some energy savings; however, this will is not the driving factor. Of course, all buildings are unique and those with shallower floor plates, less internal heat load, or higher solar heat gain may present a different scenario. REFERENCES Galasiu, A. D. Energy performance of daylight-linked automatic lighting control systems in large atrium spaces: report on two field-monitored case studies. Energy and Buildings, vol 35, M. Ceramic Product Family < evtsessssss &fn=ceramic%20family%20card_ > Environment Canada. Daily Data Report for 2011 < &dlyrange= &Year=2011&Month=9&cmdB1=Go#> Huang, Joe and Francioni, Ellen Commercial Heating and Cooling Loads Component Analysis. Lawrence Berkeley National Laboratory,