Outdoor climate

The outdoor dry bulb air temperature (TA10) by month is shown in Figure 6.1(a). The uncertainty in outdoor temperature based on calibration data is 0.6 °C. The temperature averages a minimum during the winter month of July and a maximum during the summer month of January. August has the least variation in temperature while January has the greatest. When considering the year from March 2011 through February 2012 the average was 13.2 °C. The temperature reached above 30 °C on 11 days: two days in December, four days in January and 5 days in February. It reached a maximum of 35.0 °C at the end of January. The temperature fell below 0 °C on 30 days: 12 days in May, 7 days in June, 8 days in July, 2 days in September and 1 day in October. It reaches a low of - 3.4 °C in mid July.

The outdoor temperature hourly profile is provided in Figure 6.1(b). The lowest average hourly temperature occurs at 5am and the highest occurs at 1 to 2pm.

(a) By month

(b) By hour

Figure 6.1: Outdoor air temperature, TP1-3

The outdoor relative humidity (RH10) by month is shown in Figure 6.2(a). The uncertainty in outdoor relative humidity based on calibration data is 0.8%. As expected, the outdoor relative humidity is lowest in the warm months when the temperature is highest, and highest in the cool months when the temperature is lowest. The hourly profile is shown in Figure 6.2(b). The relative humidity is highest in the morning hours and then starts to drop at about 7am, just as the air temperature starts to rise. The humidity is lowest just after noon and then slowly increases throughout the evening.

(a) By month

(b) By hour

Figure 6.2: Outdoor relative humidity, TP1-3

The outdoor specific humidity by month is shown in Figure 6.3(a). The uncertainty is calculated to be 0.2 g/kg, based on the uncertainty of temperature of 0.6 °C and the uncertainty in relative humidity of 0.8%. In general, the specific humidity is higher in the warmer months and lower in the cooler months, though August 2011 went against this trend. The hourly profile is shown in Figure 6.3(b). The hourly variation in specific humidity is quite small compared to the range of the data, indicating that the amount of moisture in the outdoor air changes very slowly over time.

(a) By month

(b) By hour

Figure 6.3: Outdoor specific humidity, TP1-3

The monthly wind speed (AS10) profile is shown in Figure 6.4(a). The uncertainty in wind speed is estimated at 1 m/s, which is twice the manufacturer’s stated accuracy. There is no apparent correlation between wind speed and month. The hourly wind speed (AS10) profile is provided in Figure 6.4(b). The wind is generally calmer in the early hours of the morning and then it peaks at approximately 3pm.

(B) By month

(b) By hour

Figure 6.4: Wind speed at test cell roof, TP1-3

The predominant wind directions as measured from the test cell roof are from the northwest and east, with nearly all winds above 4 m/s coming from the northwest. This is shown in the wind rose of Figure 6.5. The uncertainty in wind direction, based on on-site calibration, is 4°. This profile is consistent across all seasons.

N

N

Figure 6.5: Wind rose

The precipitation by month is shown in Figure 6.6. These data were purchased from BOM and represent the amount of rainfall since 9am local time. The value is reset just after 9am each day. March 2011 and May 2012 received more precipitation than other months.

Figure 6.6: Precipitation

The monthly profile of global irradiation is shown in Figure 6.7(a). The uncertainty is

approximately 0.05 kW/m2_{, based on twice the manufacturer’s stated accuracy. As expected, the}

maximum daily global radiation is highest during the summer months and lowest during the winter months, and the minimum daily global radiation is always 0. The average maximum daily radiation during the summer of December 2011 through February 2012 was 883 W/m2_{. The hourly profile}

(a) By month

(b) By hour

Figure 6.7: Global irradiation

The ground heat flux just outside the east wall of the test cell is shown in Figure 6.8. A positive value indicates a downward flow of heat. The monthly profile is provided in Figure 6.8(a). The maximum daily heat flux into the ground varies with season as expected, with the highest value in the summer months when the global radiation is also highest. On seven days throughout the March 2011 – February 2012 year, the maximum daily heat flux value was negative, indicating that the heat did not flow downward on those days. This occurred on two days in March, two days in April, one day in June, one day in August and one day in October. The minimum daily heat flux has very little seasonal variation. A negative value of heat flux occurred every day, indicating that at some point on every day heat was flowing upward from the ground. The outside heat flux shows a clear hourly trend, as shown in Figure 6.8(b), with the peak values occurring just after the peak global

irradiation, at 1pm. Generally heat flows downward into the ground between the hours of 10 am and 6 pm, and upward at other times.

(a) By month

(b) By hour

Figure 6.8: Outdoor ground heat flux

The linear relationship between outdoor ground heat flux (HF20) and global irradiation (RA14) is shown in Figure 6.9. 64% of the variation in heat flux (R2_{) can be attributed to the radiation. The}

slope is positive, indicating that as radiation increases, heat flux into the ground does also. The value of the slope indicates that of all the incident radiation on the ground surface, only 8.3% of that energy enters the ground.

Figure 6.9: Outdoor ground heat flux versus global radiation

The ground temperature outside the east wall of the test cell is shown in Figure 6.10. The ground temperature uncertainty was within 0.3 °C, as described in Appendix A.2.1. The average outside air temperature from March 2011 through Feb 2012 of 13.2 °C is indicated on the graph. This is less than the annual average ground temperatures of 14.4 °C at 150 mm deep and 14.2 °C at 600 mm deep. It is common that the average annual ground surface temperature and air temperature are similar (McInnes 2005). There is greater range in the temperature at 150 mm than at 600 mm deep, as expected. The seasonal trend of ground temperature is also as expected with the lowest

temperature occurring in winter in July and the highest temperature occurring in summer in January. The average monthly 150 mm temperature drops below the average monthly 600 mm temperature at the beginning of autumn and it rises above in mid-winter.

Figure 6.10: Outside ground temperature

Fourier’s law of steady-state heat conduction relates the rate of heat transfer through a medium, the temperature change with depth and the medium’s thermal conductivity (Cengel and Boles 2006). This law can be applied to estimate the thermal conductivity of the soil near the ground surface. Ground surface temperature is needed to calculate the temperature change with depth, but ground

surface temperature is not available. The average daily air temperature is similar to the average daily ground surface temperature (McInnes 2005), and ground temperature prediction models based on ground surface temperature have shown to be successful when substituting air temperature for ground surface temperature (Wu and Nofziger 1999). Thus, if the air temperature is substituted for ground surface temperature to calculate the temperature change with depth into the soil, then the heat flux is related to the temperature change as shown in Figure 6.11, where the temperature change is between the air temperature and ground temperature at 150 mm deep. The correlation has an R2 _{of 0.64.}

Considering the slope evident in Figure 6.11 and knowing that the vertical distance between the two soil temperature measurement locations is 150 mm, the soil thermal conductivity is estimated to be 0.63 +/- 0.09 W/mK. The uncertainty stems from an assumed uncertainty in heat flux of 10%, and the uncertainty in the temperature difference of 0.6 °C. This is within the typical range of 0.06 to 2.18 W/mK for soils (Hillel 2004).

Figure 6.11: Outdoor heat flux versus difference between air and ground temperature