5 Part I - Optimal operation and performance of low energy technologies in the Arctic
5.4 Sustainable Village in Fairbanks, Alaska
The case study Sustainable Village described in this chapter is a joint project between University of Alaska Fairbanks and the Cold Climate Housing Research Center in Fairbanks. More details about this project can be read in [36]. This chapter summarizes the content of paper III and (likewise the paper) only deals with a subtask of the Sustainable Village, project which was about monitoring of the IAQ and performance of the ventilation units within the Sustainable Village.
5.4.1 Introduction
The sustainable Village was built in summer 2012 in Fairbanks, Alaska and comprises of four houses.
Similarly to Apisseq, it is an accommodation for university students and also serves as a living laboratory where new building technologies are tested. Each house accommodates four students.
Despite similar layouts, each house has a different technology in it. Two houses are heated by hydronic floor heating whereas the other two have a forced air system in combination with a standalone heater. All four houses have CAV ventilation systems with heat recovery units. However, the units differ in defrosting strategy, manufacturer and energy recovery type as shown in Figure 15.
Figure 15. Sustainable Village houses and systems [27,28]
During December 2012 a survey of IAQ was performed. Two weeks of continuous measurements of CO2 as an indicator of IAQ in all bedrooms were completed along with measurements of the ventilation units.
28 5.4.2 Methods
The total air flow rates were measured by means of The Energy Conservatory Exhaust Fan Flow Meter (TECEFM). Temperatures, RH and CO2 concentrations were measured in all bed rooms. Air temperature in all four connections to the ventilation units was measured to calculate the temperature effectiveness.
Table 8. Uncertainty of measured values in Sustainable Village
Variable Uncertainty
Room temperature ± 0.35 K at -20 °C – 70 °C
Room RH ± 2.5 % at RH 5 % - 95 %
CO2 concentration ± 2 % of range; ±2 % of reading
Air temperature in ventilation units ± 0.25 K at 0 °C – 50 °C; ± 0.75 K at -40 °C – 0 °C Methods are explained in more details in [III].
5.4.3 Results
The measurements showed that the houses, although mechanically ventilated, do not fulfill the local ventilation requirements. In the case of the “Birch house” not meeting ventilation requirements, it was due to low fan speed selected on the control panel by the occupants. The ventilation units in the other three houses fulfilled the ventilation requirements under normal conditions. However, the air change with the outdoor air was reduced significantly either by a) defrosting of the heat exchangers (when in defrosting mode, the unit blocks the fresh air supply and exhaust and recirculates the air inside the house) or b) by users selecting the recirculation mode manually on the control panel (one of the unit’s operation modes is “20 min/h” in which the unit supplies fresh air for 20 minutes and then recirculates for 40 minutes). Reduced air change led to increased concentrations of indoor pollutants (such as CO2). Figure 16 shows the correlation between the actual ventilation rate and amount of time during night hours when the CO2 concentration in bedrooms of each house was above 1100 ppm.
Figure 16. Night CO2 concentrations above 1100 ppm and ventilation rates (the diamonds show the measured air flow whereas the triangles show the actual fresh air flow reduced by re circulation)
Tamarack Birch
Spruce Willow
Tamarack Willow
Spruce R² = 0.8997
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0
Percentage of time the CO2 was above 1100 ppm during the night
Ventilation rate [l/(s·pers]
67 % reduction
41 % reduction
27 % reduction
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The defrosting mode in the Venmar units is activated or deactivated according to the schedule in Table 9. The outdoor temperature during the measurements and during the test reference year is shown in Figure 17. From there it is seen that during 95 % of the monitoring period the temperature was below -5 °C and hence the defrosting function was active for most of the time in all three houses.
Table 9. Defrosting schedules [28]
Outside Temperature Recirculation Normal Operation Heat and moisture recovery unit
-10 °C 14 °F 7 min 25 min
-27 °C -17 °F 10 min 22 min
Heat recovery unit
-5 °C 23 °F 7 min 25 min
-27 °C -17 °F 10 min 22 min
In Figure 17 it can also be seen that typically (according to TRY) the temperature in Fairbanks is below -5 °C for 40 % of the entire year (almost 5 months). Reduction of the air change by the defrosting mode during this period is quite significant and has an obvious effect on the IAQ.
Figure 17. Cumulative percentage distribution of the outside temperature in Fairbanks
The sensible heat recovery effectiveness of the energy/heat exchangers ranged from 70.7 % to 76.6 % (for example see the performance of the HE in Tamarck house in Figure 18.)
-60 -50 -40 -30 -20 -10 0 10 20 30
400% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Outside temperature[°C]
Monitoring period Fairbanks TRY
Fairbanks TRY Oct-Mar
30
Figure 18. Temperatures and effectiveness of the ventilation unit in Tamarack house
Measurements of RH confirm the problem with air dryness in well ventilated arctic homes also found in Apisseq [II]. The more the house is ventilated the drier the indoor air, and therefore the highest RH is in the Willow house which is only ventilated by less than 3 l/(s·pers) (see Figure 19).
Figure 19. Relative humidity in all occupied bedrooms within each house in the Sustainable Village (the X and values in the boxes are mean values)
However, there is a noticeable improvement in the case of the Tamarack house, which although ventilated by the highest rate [6.8 l/(s·pers)], has an average RH higher than the Spruce and Birch house (see Figure 20).
24.1
28.3 28.3 29.8
18.8 23.2
17.7 22.5
34.8 31.1
35.5 39.5
24.0 27.9
20.0
10 15 20 25 30 35 40 45 50 55 60
Relative Humidity [%]
Tamarack
North-East Birch
North-West Willow
South-East Spruce
South-West
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Figure 20. RH and ventilation rates in the homes
5.4.4 Discussion
Lower ventilation rate in the Birch house where the Zehnder unit is installed can be fixed by reprogramming the controller of the unit in a way that it runs at a higher speed. A possible solution for the other three homes, where the Venmar units are installed could be an increase in the ventilation rate during periods when defrosting is active so that the reduced air change would be high enough to meet the requirement. To avoid an unnecessary increase of heat consumption, the increase of ventilation rates in all homes should only take place during occupied hours.
The thermal effectiveness of the heat exchangers (when in a normal mode – exchanging the air with the outside) was in the range from 70.7 % to 76.6 %, which is slightly higher than the HE in LEH and much higher than the HE in Apisseq. Increasing the air flows to meet the required air change will however cause the efficiency to decrease.
The reason for the higher RH in the Tamarack house when compared to the Birch and Spruce house (which both have lower air change and thus should have higher RH since the moisture loads are similar), is most likely the use of an energy recovery unit instead of just a sensible heat recovery unit used in the other houses.
Willow
Spruce
Birch
Tamarack
R² = 0.9998
R² = 0.5726
15 20 25 30 35 40
2.0 3.0 4.0 5.0 6.0 7.0 8.0
Average RH [%]
Ventilation rate [l/(s·pers)]
32