SENV 7006
Case Study
Qualitative analysis
Passive strategy : thermal insulation based on building Envelope
The building envelope and windows play an important role in energy saving, and a building envelop is made up of walls, roof , floor and fenestration in a building. Many efforts have done in designing and constructing the IUB/OCA’s thermally efficient envelope. Here we emphasize on window to wall, window glazing. The architects and engineers obsessively detailed about the envelope to measure thermal bridging.
Figure 9 Continuously Insulated Building Envelope[3]
Orientation
As mentioned before, each side of façade is determined by sun exposure, running the two-wing structure along an east-west axis with a shallow north-south depth provided optimum solar orientation. This orientation, along with a proper footprint depth, allowed the building to take advantage of natural ventilation opportunities. Besides, at the south elevation, the fenestration can maximize passive heat gain in winner time.
Window -wall-floor-roof
The envelope has a window-to-wall ratio of 34.6% to receive higher heat gain and visible light, Actually The ASHRAE 90.1 Appendix G base building has a window to wall ratio between 30 and 40%[3].Here the climate belongs to cold winter but hot summer, white precast concrete with continuous insulation and non-thermally conductivity plays a high-performance envelope, reducing traditional thermal bridging at roof interfaces, foundation walls, and windows. Continuous insulation wraps uninterrupted from the roof into the thermal wall panel and then down and around the foundation system and across the underside of the slab on grade[4].
Particular attention is paid to the interface of the ground floor slab into the vertical wall construction, one area proven to be a significant heat sink in other high-performance buildings. A common metric for evaluating the energy efficiency of an envelope is R-value to compare the thermal resistance and thermal transmittance. In IUB/OCA building, the thermal resistance such as wall, floor can be measured in experiment by using heat flux transfer and two temperature transducers. Shown in the figure 9 are measurement locations in terms of R-value.
The strategies focused on different wall and roof construction with increasing R- values
Figure 10 building layout and R-value measurement locations[2]
The measurements are performed on an east facing wall in the Loading Dock room within the link section, a south facing wall in the Hearing Room within the south wing, and a north facing wall in the Records Center Workroom within the north wing, seen in the Figure 9. Then referring to ASHRAE Standard 90.1 and following to ASTM Standards C1046-95 and C1155-95. ASTM C1046 – 95 to state the minimum thermal resistance and transmittance values for the envelope components based upon a building’s climate zone[2].
Window and glazing
Different from most commercial office building which use closed windows, the IUB/OCA building use operable windows to enhance staff comfort and satisfaction as well as improve indoor air quality. In order to ensure proper use, the staff are again notified to close the windows when the outside conditions deteriorate or at the end of the working day. It is notable that although it has not favorbale effect when associated with mechanical ventilation with operable windows, they are mostly a function to enrich satisfaction for people who work here.
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To better determine when natural ventilation is best, the design team investigates some general, probative and controllable guidelines. According to the construction documents, outside air conditions are considered optimal for opening windows when the temperature is ranging from 62°F to 79°F, the relative humidity is less than 50%, the wind speed is less than 10 mph and the conditions remain desirable for longer than 1 hour[4].
Figure 11 operable windows allow natural ventilation[1]
Shown in the figure 12 is when 25 operable windows opened or closed from an email sent on June 7, 2012 regarding the open/close signals. On that day, we can see, eleven of the 25 operable windows opened immediately or shortly after the initial open signal was sent. Eight of the 25 closed immediately or shortly after the close signal was sent. Three opened before the open signal was sent, and five closed long after the close signal was sent. Nine operable windows did not have any change in status that day. In general, about half of the windows were operated as intended.
Figure 12 an Example of Operable Windows Status vs. Optimal Opening Period[2]
Figure 13 window glazing[1]
As for glazing, the building employs high performance glass, specially selected for different orientation of function use. Glazing with a higher solar heat gain coefficient (SHGC) of 0.62 and a visible light transmittance of 74% was employed[1]. The higher SHGC at the south allows for the envelope to maximize passive heat gain in the winter time. Visible light transmittance is maximized at this area to work in concert with the same sunscreen and provide adorable daylight from the southern source. At the west and east direction, a lower SHGC of 0.38 to keep sun glare and a visible light transmittance of 44% were used to minimize heat gain.
Mechanical strategy : occupant comfort based on HVAC system along with GHP and ERV
Staff here spend more than 90% time in the room, so indoor thermal environment is quite important for planers and designers and provide people a pleasant and satisfied working atmosphere. Although the envelope capture passive energy and reduce energy, necessary efficient mechanical system can employ a high performance heating and cooling effects. Unlike traditional mechanical HVAC system consumes large proportion electricity and gives off los of exhaust gas, the IUB/OCA building brings to a geothermal heat pump and total energy recovery ventilator, which considered to green strategies to energy saving.
Geothermal Heat pump
The IUB/OCA building is designed to maintain a 72F for heating temperature and 74F for cooling set point with an maximum relative humidity 50%[3]. As a result, the team employs a high efficiency heating and cooling system introduced as a geothermal field tied to dual-stage water to air heat pumps and water to water heat pumps. As we know, Heat pump systems move thermal energy very efficiently to and from the building via a reservoir or heat sink. The basic heat pump unit utilizes a vapor compression cycle with a reversing valve to provide warm air during the heating season and cool air during the summer[6].
In terms of heat pump underground, the temperature below the ground in Des Moines remains relatively constant, ranging between 50 and 60°F[6], throughout the year. Variations are due primarily to climate and latitude, but soil composition also affects heat transfer characteristics. The energy exchanged by a geothermal system is mainly due to the solar energy absorbed by the ground. Besides, normally forced air steams can be transferred energy based on geothermal heat pump, while since water holds almost twice heat capacity than masonry and gives up heat quicker. So in the final stage, dual stage water-to-air heat pumps are chosen.
Figure 14 Simplified Site Plan with Geo-well Field[2]
Figure 15 simplified schematic of Geothermal Heat Pump System[2]
Energy Recovery Ventilation
To further provide fresh air for ventilation and reduce energy consumption, the design team use a total heat recovery ventilation, along with CO2 monitoring and control, an under floor air distribution system (UFAD),which varies the thermostatic setpoint and setback temperatures, ASHRAE Standard 62.1 provides the basis of determining the appropriate ventilation rates for time of the building being occupied. A building can be occupied while a certain room or space might only be utilized periodically. Providing ventilation air based on CO2 levels can make massive energy savings realized by operating the ventilation system intermittently instead of using it continuously [2].
A total energy recovery ventilator (ERV) is a device that can transfer both sensible and latent heat between air streams and allows for some moisture to be exchanged, potentially raising the humidity of the incoming outdoor air in the winter and lowering it during the summleer. The use of energy recovery ventilator can efficiently reduce the operational costs of the HVAC system.
Mechanical humidification is energy intensive machine which requires an additional 1,100 Btu/lb of energy to change phase to increase the relative humidity and generally requires overcooling the supply air to wring the excessive moisture out but may result in the need for reheating it.The ERV is a fair cost strategy of regulating the outdoor air and provide pleasant indoor air quality and environment but lower energy and effective prize..
Figure 16 total Energy Recovery Ventilator[2]
Quantitative analysis
Building Envelope
Figure 17 Heat Flux, Surface Temperatures and Accumulated R-value for East Wall[2]
As mentioned before(figure 10), there are 3 R-value measurement locations in this building. Actually, accurately measuring R-value seems to more challenging. From figure 17, we can see overall indoor surface temperature is stable no matter what the weather condition likes outside. The heat flux has the same trend as outside temperature, which means the building envelope is continuous concrete and it is believed this of construction leads to more stable, uniform temperature and heat flux profiles. Besides, compared to two other locations, the east wall with R-value of 23.0 corresponds well to the design.
HVAC system
From figure 18, we can see that monthly average Geo-loop temperature differences. The monthly averaged geo-loop temperature difference is between +1.8°F in winter and -5.4°F in summer. During transition seasons, March to April and October to November, there are periods when the average outside air temperature was between 40 ~ 50°F and the monthly averaged geo-loop supply-return temperature differences were near zero degrees; therefore the building exchanged minimal energy with the ground[1].
Figure 18 Monthly Geo-loop Temperature Differences vs. Outside Air Temperature[2]
Figure 19 Monthly Energy Consumption of Heat Pumps[2]
Figure 19 shows the monthly electrical energy consumption of the whole HVAC system and monthly average outdoor air temperatures. The figure indicates that in the winter months the total pump and heat pumps’ energy used is the highest because the water-to-water heat pumps are operating and providing additional heating through the radiant heating system. In summer, the water-to-water heat pumps are off and the water-to-air heat pumps provide cooling to the building spaces. Besides the summer months, the total water-to-air heat pump energy use is at a minimum during April and November since weather condition. In July, during peak summer conditions, the average daily outside air temperature approaches 80°F and energy consumed by the water-to-air heat pumps occurs[2].
Note:The overall energy performance of IUB/OCA office building was analyzed based on the data collected in one full operating year starting in April 2012 through March 2013.