Demand-Controlled Ventilation

Transcription

1 Demand-Controlled Ventilation When a building is occupied, it is unlikely that every room is at occupant capacity at any given point in time. As the number of people in a zone varies, the quantity of outdoor air required to properly ventilate that zone also varies. However, most buildings are ventilated at design occupancy levels in all spaces at all times, resulting in over-ventilated buildings. Demand-controlled ventilation (DCV) is a strategy that attempts to dynamically reset the outdoor airflow delivered to a zone based on the changing population within that zone. DCV strategies include time-ofday schedules, occupancy sensors, and CO 2 sensors. This article discusses strategies for implementing CO 2 based DCV and how to model these strategies in TRACE 7. Minimum Breathing Zone Requirements ASHRAE Standard 62.1 states that amount of outside air required to properly ventilate a space is the sum of the outside air required to dilute volatile organic compounds (VOCs) given off by paints, wall paper, furniture, etc., and the outside air required to dilute bioeffluents from the occupants in a space. In addition, the standard also permits dynamic reset of intake (outdoor) airflow as operating conditions change, as long as the system provides at least the required breathing zone outdoor airflow whenever a zone is occupied. The minimum breathing zone outdoor airflow (V bz ), is determined using Equation 6-1: Where: V bz = (R p P z ) + (R a A z ) V bz = breathing zone outdoor airflow A z = zone floor area: the net occupiable floor area of the ventilation zone ft 2 (m 2 ) R a = outdoor airflow rate required per unit area as determined from Table 6-1 P z = zone population: the number of people in the ventilation zone during typical usage. R p = outdoor airflow rate required per person as determined from Table 6-1

2 TRACE Scenario Let s take a look at how equation 6-1 comes into play in TRACE, using an example break room that is 2 ft. by 14 ft. In TRACE, the Apply ASHRAE Std /27 field must be set to Yes on the Create Rooms-Rooms tab. The values for R a and R p are set on the Create Rooms - Airflows tab. In this example, R p = 5 cfm per person and R a =.6 cfm per square foot. Area can be calculated from the room dimensions: A z = 2 14 = 28 sq ft Knowing the population density, the design population can also be calculated: P z = 28 sq feet 2 sq feet = 1.4 people The design ventilation for the break room can then be calculated by solving equation 6-1: V bz = (5 cfm per person 1.4 people) + (.6 cfm per sq ft 28 sq ft) = 23.8 cfm

3 The result is a linear relationship between the outdoor air requirement and the zone population. Outside Air vs Population Vot - Vbz Figure 1 TRACE will recalculate the zone ventilation requirement on an hourly basis during the simulation phase when the system ventilation flag on the Create Systems Advanced screen is set to ASHRAE Std /27 w/ Vent Reset. Because the values for R a, R p and the square footage are fixed for a given room, the only variable in equation 6-1 is the zone population (P z ). In this example, the population varies according to the People Office schedule.

4 The hourly ventilation airflow is calculated by multiplying the design ventilation airflow by the percentage from the people schedule. For this example, the hourly weekday ventilation when DCV is enabled is shown in Table 1: Time Percentage Design Ventilation (cfm) Hourly Ventilation (cfm) Mid - 7 am am - 8 am am - 5 pm pm - 6 pm pm - 7 pm pm - Mid 23.8 Table 1 CO 2 Based DCV Just as our surroundings outgas volatile organic compounds (VOCs), humans are also a source of pollution. This is especially apparent when large numbers of people live or work in enclosed, poorly ventilated areas for extended periods of time. The substances we emit into the air through normal biological processes are called bioeffluents. Studies have found that bioeffluents emitted from each person into the air can contain as many as 15 volatile substances. Principal among these are acetone, human skin, ethyl alcohol, methyl alcohol, and ethyl acetate. CO 2 based DCV assumes that people produce both bioeffluents and CO 2 in proportion to their activity level and as a result, the concentration of CO 2 in a space accurately represents the concentration of human bioeffluents. It is also assumed that CO 2 based steady-state equations can be used to estimate the people component of the ventilation load. Equation C-1 in Appendix C of ASHRAE 62.1 provides the following mass balance equation to calculate the outdoor airflow rate needed to maintain the steady-state CO 2 concentration below a given limit. Where: V o = outdoor airflow rate per person N = CO 2 generation rate per person C s = CO 2 concentration in the space C o = CO 2 concentration in the outdoor air N V o = C s C o

5 CO 2 based DCV makes one additional assumption: occupant activity level in a zone (in terms of metabolic rate, m) can be estimated with reasonable accuracy. Then the CO 2 generation rate per person (N) can be expressed in terms of metabolic rate (m) and the per person CO 2 generation rate (k): N = m x k Where: N = CO 2 generation rate per person m = activity level of occupants, met k =.84 cfm per met per person for healthy people The outdoor airflow rate per person (V o ) can be expressed in terms of V bz and P z : V o = V bz Pz Where: V bz = the minimum breathing zone outdoor airflow P z = the zone population Rewriting equation C-1 and substituting for N and V o yields equation 1: C s = k m + C V o bz Pz Proportional Control DCV This control strategy varies the intake airflow in direct proportion to the actual differential CO 2 level. Two control points (min CO 2 concentration, min ventilation airflow) and (max CO 2 concentration, design ventilation airflow) establish a linear relationship between the CO 2 concentration in the space and the ventilation airflow. A CO 2 sensor in the space measures the concentration and reports this value to a controller programmed with this relationship which resets the outdoor requirement in the space. (See Figure 2) (max CO 2, design OA) Outside Air (cfm) (min CO 2, min OA) CO2 Concentration Figure 2 The minimum CO 2 concentration is generally accepted to be that of the outdoor air (35 ppm - 4 ppm). The minimum ventilation airflow and maximum ventilation airflow are

6 calculated using equation 6-1, selecting Pz values of zero and the design occupancy respectively. The maximum CO 2 concentration is calculated using equation 1 as shown in the following example. Applying the proportional control strategy to the break room example the calculations yield the following results: Minimum CO 2 concentration Minimum ventilation airflow (V bz min ) CO 2 min = 4 ppm (Assumed) V bz min = (5 cfm per person people) + (.6 cfm per sq ft 4 sq ft) = 24 cfm Maximum ventilation airflow (V bz max ) V bz max = (5 cfm per person 2 people) + (.6 cfm per sq ft 4 sq ft) = 124 cfm To calculate the maximum CO 2 concentration it is first necessary to determine the metabolic rate (m) using Figure C-2 from Appendix G in ASHRAE Std Choosing an activity level of office work for the break room the metabolic rate is determined to be 1.2 met units. (See Figure C.2)

7 The maximum CO 2 concentration can now be calculated for the design occupancy using equation 1. C max =.84 cfm per met per person 1.2 met 124 cfm per person 2 people ppm = ppm The resulting ventilation is plotted along with the ASHRAE Std minimum ventilation requirements in Figure 3. Outside Air vs Population Vot - Vbz Vot - CO2 Figure 3 The difference between the two lines is an excess amount of outside air the space will receive for any given population. The reason for the over ventilation is that the controller models the change in CO 2 as a linear function. A plot of the total CO 2 in the break room versus population shows that it is not. (Refer to Figure 4)

8 Total CO2 vs Population Total CO Co2 Linear Figure 4 To understand how the controller determines the ventilation airflow, choose a discrete value for the number of people currently in the space. For this example it will be assumed that there are four people in the break room. The resulting CO 2 level in the space will be 1316 ppm. (See Figure 5) Total CO2 vs Population Total CO ppm Co2 Figure 5 A controller employing proportional control DCV will sense 1316 ppm of CO 2 in the space but has no knowledge of the actual population. The only thing it can do is control based on where the CO 2 level intersects linear function shown in Figure 4. In this example, the CO 2 level of 1316 ppm corresponds to 11.3 people in the space. (Refer to Figure 6)

9 Total CO 2 vs Population 25 2 Total CO ppm 5 Figure 6 Plotting the calculated population of 11.3 people against the minimum ventilation requirements calculated using equation 6-1 results in an outside air requirement (V ot ) of 8.3 cfm. (Refer to Figure 7) Outside Air vs Population Vot - Vbz Figure 7 The outside air requirement (8.3 cfm) can now be plotted against the break room population of four people. (See Figure 8)

10 Outside Air vs Population Vot - Vbz 4 People Figure 8 Plotting the outside air requirements for the entire population results in the curve shown in Figure 9 along with ASHRAE Std minimum ventilation requirements. Outside Air vs Population Vot - Vbz Vot - CO2 Figure 9

11 Single Setpoint CO 2 DCV This single setpoint strategy requires the user to enter only one value, the zone population for which both the fixed CO 2 setpoint and minimum ventilation airflow limit are to be calculated. The controller adjusts the outdoor airflow intake to maintain a fixed CO 2 level in the space, thereby maintaining a fixed outdoor airflow rate per person. A minimum ventilation airflow limit is enforced if CO 2 levels in the space fall below the setpoint. Figure 1 illustrates how CO 2 levels will change in response to varying populations in the break room example with a setpoint population of 1 people. Total CO 2 vs Population Total CO Co2 Figure 1 The fixed CO 2 setpoint is calculated for the setpoint occupancy using equation 1. C max =.84 cfm per met per person 1.2 met 74 cfm per person 1 people ppm = ppm Where the minimum ventilation airflow (V bz min ) V bz min = (5 cfm per person 1 people) + (.6 cfm per sq ft 4 sq ft) = 74 cfm The resulting ventilation is plotted along with the ASHRAE Std minimum ventilation requirements in Figure 11.

12 1 People Single Setpoint Min 1 Vot Figure 11 The difference between the two lines is the excess amount of outside air the space will receive whenever the space population is above or below the setpoint population. Figures 12 and 13 illustrate how different setpoint populations will affect the amount of excess ventilation for different populations People Single Setpoint Min 5 Vot Figure 12

13 15 People Single Setpoint Min 15 Vot 2 Figure 13 Modeling CO 2 Based DCV in TRACE 7 On the Rooms tab of Create Rooms (or on the thermostat template), select a CO2 Sensor location (either Room or None) for the rooms that will have demand-control ventilation control. On the Airflows tab of Create Rooms, select Yes in the Apply ASHRAE Std /27 field.

14 The DCV Min OA Intake field must now be edited to calculate the appropriate setpoint according to the CO2 based DCV control strategy that is going to be implemented. DCV Min OA Intake options % Room Population Best practice is to select % Room Population for both proportional control and single setpoint DCV. When % Room Population is selected, the percentage is multiplied times the design occupancy of the space to determine the number of people at which to calculate the minimum ventilation airflow using equation 6.1. Where: V bz = (R p P z ) + (R a A z ) V bz = breathing zone outdoor airflow A z = zone floor area: the net occupiable floor area of the ventilation zone ft2 (m2) R a = outdoor airflow rate required per unit area as determined from Table 6-1 P z = zone population: the number of people in the ventilation zone during typical usage. R p = outdoor airflow rate required per person as determined from Table 6-1 When using Single Setpoint control, the number of people is also used to calculate the fixed CO 2 setpoint. (Refer to the Single Setpoint CO 2 DCV section of this document.) % Dsn OA Airflow Note: % Dsn OA Airflow is only applicable with proportional control DCV. The reason is that population is required to calculate the fixed CO 2 setpoint for single setpoint control. % Dsn OA Airflow allows the user to calculate a minimum ventilation airflow based on the design outside airflow rather than using equation C-1.

15 Default Values CO2 based DCV method: None or Proportional Control If the field is left blank, the program will default to % CO2 based DCV method: Single Setpoint If the field is left blank or set to %, the program defaults to 4%. If the value field is left blank the default of zero will be used. Selecting Proportional Control or Single Setpoint On the Selection tab of Create Systems, click the Advanced button. Select the option ASHRAE Std /27 w/ Vent Reset in the System Ventilation Flag field. Note: System-level ventilation reset is required for the system to be able to handle zone-level demand-control ventilation. At the bottom of the screen, select the desired C2-based DCV option (None, Proportional Control, or Single Setpoint).