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ASHRAE 62.1 (2007), Appendix C, allows the use of CO₂ sensors to regulate the fresh air intake in buildings. More specifically, it says that keeping the CO₂ level in the room no higher than 700ppm above the outdoor condition will yield satisfaction for a majority of visitors entering the building. The outdoor CO₂ level varies depending on the site location. In locations surrounded by lush greenery, the CO₂ level may be as low as 350ppm during the daytime, while in a city space, the CO₂ level may be has high as 450ppm due to exhausts from vehicles.

The IES simulation software used for this study has a default outdoor CO₂ level at the at world average of 380ppm (latest measurements have shown the world average CO₂ level to exceed 400ppm2_{). The simulation study with a CO}

2 level set

point of 900, 700, 600 and 500ppm was conducted, providing the following set points above the outdoor level: 1. 900 – 380 = 520 ppm above outdoor condition

2. 700 – 380 = 320 ppm above outdoor condition 3. 600 – 380 = 220 ppm above outdoor condition 4. 500 – 380 = 120 ppm above outdoor condition

In the worst case scenario, the simulated CO₂ set point is 520ppm above the outdoor condition, to account for a likely preference for a higher amount of fresh air by Malaysian occupants due to the many years of living in leaky buildings, instead of the 700ppm limit as specified by ASHRAE.

In this simulation study, the fresh air intake by the AHU was modelled to be controlled by a CO₂ sensor measuring the CO₂ level of the return air. It was basically modelled the way it is commonly practiced in Malaysian buildings (It should be

highlighted that after this simulation study was completed, the author found a paper that described this method as unreliable and is recommended not to be used anymore.3 _{Instead, it is proposed that the location of CO}

2 sensor should be placed in the breathing

zone, typically adjacent to the temperature sensor, because it was mentioned in the paper that return air sensing of the CO₂ level does not work in an actual building scenario).

In addition, the demand control mechanism was simulated to always provide a minimum amount of fresh air equivalent to the ventilation rate requirements for the floor area during air-conditioning hours as per the requirements in ASHRAE 62.1 (2007), even when the measured CO₂ level is below the set point.

The results of the simulation case study showed that the use of demand controlled fresh air intake (via a CO₂ sensor) reduces the peak cooling load in the simulated building by 2.3 W/m2_{. Upon investigation, it was found that the peak}

cooling load on the test building occurs during the morning hours when the air-conditioning system is switched on. During this time, the CO₂ level in the building is low due to the infiltration of fresh air from the night before, therefore, the demand controlled fresh air intake only needs to provide the minimum fresh air requirement, reducing the peak cooling load significantly.

158 | Building Energy Efficiency Technical Guideline For Active Design

In terms of energy efficiency, the benefit of using a demand controlled fresh air intake becomes more significant when the actual building occupant density is below the design occupant density. The estimated BEI savings due to the use of a CO₂ sensor at different percentages of occupancy is shown in Figure 7.15 below and Equation 7.9 is provided for a quick estimation of the BEI reduction due to the use of a CO₂ sensor for the tested case building scenario.

EqUATION 7.9

ΔBEI = 5.26 x ΔO

Where: ∆BEI ∆Oc

= Change of Building Energy Index (kWh/m2_.year)

= Change in Occupant Density from Design Scenario (10 m2_{/person) (%)}

The following rule-of-thumb is derived from this study on the use of a CO₂ sensor; for every 10% reduction of occupant density from the design scenario of 14 m2_{/person, the BEI reduces by 0.5 kWh/m}2_.year.

fIGURE 7.15 | RELATIONSHIP BETWEEN BEI AND PERCENTAGE Of ACTUAL OCCUPANCY WITH THE USE Of CO2 CONTROLLED fRESH AIR vENTILATION STRATEGY

y = 5.2557x + 151.96 R2 = 0.99737 153.5 154.0 154.5 155.0 155.5 156.0 156.5 157.0 157.5 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% BEI (kWh/m 2.year)

Percentage of Actual Occupancy to Design Occupancy

Finally, the energy saved due to the use demand controlled fresh air intake system is also closely linked to the CO₂ set point level. Figure 7.16 on the next page shows the results of different CO₂ set points in the case where the actual building occupancy is only at 40% of the design occupancy (i.e. 25 m2_{/person instead of 10 m}2_{/person). The sparsely}

occupied building does not increase the energy consumption at all by reducing the CO₂ level from 900ppm to 700ppm because the low number of people in the building has kept the CO₂ level below 700ppm from the minimum fresh air supplied (based on ASHRAE area requirements). However, reducing the CO₂ set point below 700ppm increases the BEI significantly. It can be seen from Figure 7.16 on the next page that in a sparsely occupied building, that below the 700ppm set point, every additional 100ppm reduction of the CO₂ set point increases the BEI by 3.9 kWh/m2_.year.

Building Energy Efficiency Technical Guideline for Active Design | 159

fIGURE 7.16 | IMPACT ON BEI BASED ON CO2 SET POINT IN A SPARSELY OCCUPIED

BUILDING (40% DESIGN OCCUPANCY)

Description With CO2 Sensor No CO2 Sensor

Design Condition Design Condition add 50% fa add 100% fa

Full Occupancy 157.3 158.6 164.1 169.8

80% Occupancy 156.1 157.6 162.8 168.5

60% Occupancy 155.1 156.6 161.7 167.4

40% Occupancy 154.1 155.6 160.7 166.1

TABLE 7.6 | BEI (KWH/M2_{.YEAR) WITH AND WITHOUT CO}

2 SENSOR AT vARIOUS BUILDING

OCCUPANCY LEvELS 153 154 155 156 157 158 159 160 161 162 163 400 500 600 700 800 900 1000 BEI (kWh/m 2.year) CO2 Set Point (ppm)

Table 7.6 shows that if the fixed fresh air supply system is supplying 50% more fresh air than is specified by the ASHRAE 62.1 (2007) requirement, the savings from the use of a demand controlled fresh air intake increases to 6.7 kWh/m2_{.year. If the fixed fresh air supply system is supplying 100%}

more fresh air than specified by the ASHRAE 62.1 (2007) requirement, the savings from the use of a demand controlled fresh air intake increases to 12.3 kWh/m2_{.year. This result indicates that it is}

very important to commission a building to ensure that the fixed fresh air intake system is operating according to the design assumptions. Over-provision of the fresh air intake is shown to be very expensive. Unfortunately, it is not a common practice to measure the fresh air intake during the commissioning of buildings in Malaysia. Based on this result, it is strongly recommended that all buildings in Malaysia should at the very least, measure and tune the fresh air intake into the building during the commissioning stage or implement CO₂ sensors to regulate the fresh air intake.

160 | Building Energy Efficiency Technical Guideline For Active Design

fIGURE 7.17 | BEI REDUCTION BASED ON THE BOTH SENSIBLE AND LATENT EffICIENCY Of HEAT RECOvERY