Series I Simulations

Results for the first series of simulations are summarized in Figure 13, which presents the number of hours over the five-month simulation period when the indoor temperature falls into specified temperature bins (a total of 3,648 hours). Results for each case will be discussed next.

In addition to building-construction details, the base case is defined by the air-change rate and the schedule and magnitude of internal heat gains, both of which strongly affect indoor temperatures. We estimated a wind-driven airflow of 7.5 ACH, deriving this value from nodal airflow equations and an estimate of both open-window area and wind speed at the building façade. We used a southerly wind of 0.5 m/s, which was reduced from the annual mean meteorological value of 1.9 m/s (Chen and Cai 1994) by the screening effect of neighboring buildings (Jiang et al 1999).

Internal gains include lights, equipment, and occupants. We modeled lighting with a peak power of 8 W/m2 and 7 hours of operation, with 2 hours in the morning and the remainder in the evening. We assigned an average of 3 W/m2 for equipment, peaking in the

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 5

Hour

Figure 14 Study 2 -hourly indoor and outdoor dry-bulb temperatures for the base case (7.5 ACH, 16 July ofTMY weather data)

Figure 15 Study 2 - hourly indoor and outdoor dry-bulb temperatures for mechanical ventilation with 15 and 30 ACH (16 July of TMY weather data)

Hour

Figure 15 Study 2 - hourly indoor and outdoor dry-bulb temperatures for mechanical ventilation with 15 and 30 ACH (16 July of TMY weather data)

morning and evening, and simulated an occupancy of four people, not at home during the day. The average lighting and equipment load per unit was 470 W, appropriate for a single-family residence (ASHRAE 2001). ASHRAE notes that a lower figure of 350 W is appropriate for a unit, but that computers or other equipment may increase that value. We investigated the impact of ir|t lowering the internal loads as part of our study. The ext- sensible heat gain from occupants was 67 W/person, appropriate for sedentary occupants (ASHRAE 2001).

Indoor temperatures are uncomfortable (> 29.4°C) 28 percent of the summer hours for the base case. In practice, the number of hot hours may be even larger, because occupants often enclose their balconies to make a sunspace, thereby eliminating the benefit of the balcony floor as a shade for windows and balcony doors below it. Figure 14 shows the diurnal variation of indoor and outdoor dry-bulb temperatures for a single July day. The indoor temperature always exceeds the outdoor temperature, peaking at 35°C later than the outdoor peak of 33°C.

Subsequentsimulationsshown in Figure 13 explored methods to reduce the number of uncomfortable hours. Simulation 2 placed insulation on the exterior rather than interior surface of exterior walls, to promote the flow of 16ach heat into and out of building mass. This may reduce indoor temperatures during very hot weather but will also reduce the benefit of adjusting the thermostat to lower values in winter during unoccupied or nighttime hours. Thermostat adjustments would also make sense in summer if the occupants of such a building found it necessary to use mechanical cooling. Our simulations focused only on summer months and indoor temperatures were allowed to float, so effective building thermal mass should in principle be of some benefit. Figure 13 shows that the benefit of external insulation, with no other measures, is extremely modest. For the

30 ach ext.

base-case building, therefore, the location of insulation should be made on the basis of other factors, including ease of construction and the elimination of thermal bridges, which can occur where interior and exterior walls join and which we have not simulated. Exposed thermal mass can have a more beneficial impact when other changes to the building are made.

Simulation 3 focused on mechanical ventilation. Jiang et al (1999) showed that wind-driven natural ventilation was constrained at the building location in question because the demonstration building was very close to a tall building to the north and low-rise buildings to the south, both of which blocked ambient wind. To enhance low levels of natural ventilation, we proposed the use of one or more central fans to exhaust air from the units in the building. Around-the-clock ventilation at 15 ACH reduced the uncomfortable hours to 16 percent of the total. This air-change rate corresponded to a volumetric flow of about 2 m3/s for each of the 4 units in the building and an average flow velocity of about 1 m/s through open windows. Such a ventilation rate appeared very feasible; we measured an average air speed of 2.8 m/s and a volumetric flow of about 0.7 m3/s for a 0.25 m2 fan that could be placed in a window. An improvement over such fans would be a single exhaust fan for each unit.

Simulation 4 increased the air-change rate to 30 ACH, reducing the uncomfortable hours to 9 percent of the cooling season. Figure 15 shows the impact of mechanical ventilation on indoor temperatures over a diurnal cycle. Note that with 15 ACH, the indoor peak relative to the base case (shown in Figure 14) has been reduced by 2°K, and that an additional doubling of the airflow rate further reduces the peak by a smaller amount, 1°K.

Simulations 5 and 6 quantified the impact of adding shading to south-facing windows that did not already

35 30

a 25

15 10

have it in the base case. Most of the openings in the south façade were sliding doors that opened to balconies and were shaded by balconies on the floor above. Adding shading to the remaining south windows reduced the uncomfortable hours to 27 percent, only one percent better than the base case. Shading south-facing windows had an equally modest impact when added to a case that included mechanical ventilation: 15 percent uncomfortable hours, compared to 16 percent for simulation 3.

Simulation 7 combined exterior insulation with around-the-clock mechanical ventilation set at 15 ACH. With increased airflow to take away heat stored in exposed building mass, exterior insulation had a bigger impact than in the absence of increased ventilation, reducing the uncomfortable hours from 15 percent in simulation 3 to 13 percent.

---Simulations-15ACH

+ exterior insulation + shading southern windows

night cooling + 1.5 ACH day ventilation + exterior insulation + shading southern windows

- exterior

Simulation 8 combined 15 ACH of ventilation with exterior insulation and shading of southern windows, a reasonable combination of strategies that reduced the uncomfortable hours to 12 percent. Simulation 9 reduced airflow during the day (8 AM-9 PM) to 1.5 ACH and retained 15 ACH at night. This strategy led to higher indoor temperatures because there were both solar and internal heat gains during the day. The number of uncomfortable hours increased to 21 percent. For the same day as used previously, diurnal temperatures for these two cases are plotted in Figure 16. Indoor peaks are below the outdoor peak, improving thermal comfort relative to the base case.

35 30

a 25

15 10

---Simulations-15ACH

+ exterior insulation + shading southern windows

night cooling + 1.5 ACH day ventilation + exterior insulation + shading southern windows

- exterior

Hour

Figure 16 Study 2 -hourly indoor and outdoor dry-bulb temperatures for two cases: continuous and night-only mechanical ventilation, each with shaded southern windows and external insulation

Hour

Figure 16 Study 2 -hourly indoor and outdoor dry-bulb temperatures for two cases: continuous and night-only mechanical ventilation, each with shaded southern windows and external insulation

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Figure 17 Study 2 - percentage of discomfort hours for variations in building construction, shading, and ventilation for Series 2 simulations

Figure 17 Study 2 - percentage of discomfort hours for variations in building construction, shading, and ventilation for Series 2 simulations

- Exterior

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Hour

Figure 18 Study 2 - diurnal indoor and outdoor temperatures when the building is sealed during the day and ventilated at night, with reduced internal heat gains (16 July of TMY weather data)

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Renewable Energy 101

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