Off-Peak Building Control Considerations Utilizing CO 2 Based Demand-Controlled Ventilation (DCV) with Large Packaged Rooftop Units

Transcription

1 Off-Peak Building Control Considerations Utilizing CO 2 Based Demand-Controlled Ventilation (DCV) with Large Packaged Rooftop Units By: Bill Timmons Mark Tozzi Carrier Corporation Syracuse, New York September, 2000

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3 Introduction This white paper will highlight operational challenges associated with part-load control of packaged rooftop equipment and discuss innovative solutions to maintain comfort and save substantial amounts of energy while simultaneously meeting the intent of ASHRAE s ventilation standard, Std In order to set the tone for the premise of this topic, the following observations are made. While air-conditioning equipment is always sized to maintain desired indoor room conditions at peak design conditions, the majority of actual operating hours occur at off-peak conditions. For a packaged rooftop product, for instance, this peak design condition usually occurs at the summer design dry bulb temperature. Building envelope loads, that is load components associated with the outdoor conditions (such as transmission and solar), are only one component of the total cooling load. Internal loads such as lights, people and equipment are transient in nature and fluctuate based on occupant usage patterns rather than due to external weather conditions. Following ASHRAE s Standard Ventilation Rate Procedure, design ventilation rates are based on the total number of people in the space as well as their associated activity levels. However, due to the transient nature of people in the building, that is people move from space to space during the day, the ventilation requirements for a given space will vary throughout the day as the number of people and their associated activity levels change. For instance, a conference room designed for 12 people may contain only 10 people at certain times and be packed with 15 people on other occasions. These same people may later move to other areas in the building, as indicated in Figure 1. The total number of people in the building may not change, in this case, but the occupancy level in each zone varies dramatically throughout the day. How would you design a ventilation system to handle this scenario? Figure 1 Without special controls you would have to assume that the design occupancy exists for all zones at all times during the day. Why? Because With a control system that cannot re-set the outdoor air damper position in response to the zone occupancy levels, you must assume the maximum occupancy levels for each zone and pre-program these occupancy schedules ahead of time into the control system. This means that since you are guessing at the occupancy levels, rather than actually measuring occupancy, you must err on the side of conservancy and overestimate the number of people in any given space. This results in over-ventilating and wastes energy. In our conference room example, we would be supplying five times as much ventilation as required if there were only two people in the room (10/2 = 5). A well-engineered HVAC system must be able to modulate its capacity to closely match the continually changing loads in the building. Variable air volume (VAV) systems were created such that as the building cooling loads change, the corresponding equipment capacity may be modulated to closely match the actual building cooling load. The end result was improved comfort and substantial energy savings. Given the fact that the cooling loads in a building change throughout the day, or from season to season, would we ever design a system with a constant cooling capacity? Of course not! With the exception of small systems with single 3

4 steps of capacity, most larger commercial equipment has two or more cooling capacity stages. Hypothetically, how well would a single 50-ton system, with one step of capacity, control a building? Not very well! The system would constantly cycle on and off attempting to maintain setpoint. Continuing with the VAV system analogy, would it be prudent to introduce design quantities of ventilation air into a building that is not fully occupied? Of course not! But that s exactly what many HVAC systems deliver. design ventilation rates during all occupied times. Why? Because unless you know how many people are in each zone of the building at all times you must provide design ventilation rates to cover the worst case scenario, that is 100 percent occupancy. Of course you must design and select the equipment at peak occupancy and peak design temperatures.but this doesn t mean that that you have to operate the system that way all the time. Design vs. Operating Conditions As mentioned previously, a dichotomy exists because the equipment must be sized at design conditions but rarely operates at those conditions. Controlling the quantity of ventilation air introduced into a building based on assumed maximum occupancy levels for each zone is not very accurate, nor is it the most costeffective method of controlling ventilation air. Sizing the ventilation system for design occupancy without a feedback mechanism to tell you how you are doing is a risky design strategy. The good news is, achieving acceptable IAQ, and at the same time saving lots of energy, no longer requires a complex, sophisticated and expensive HVAC system. Direct digital (DDC) controls are now available factory-installed on relatively inexpensive, packaged rooftop unit systems that provide excellent control of IAQ at the individual zone level, as well as save substantial amounts of energy, especially when compared to more complex, built-up type HVAC systems. A control strategy known as Demand Controlled Ventilation, or DCV, has been developed, which meets ASHRAE Std requirements. This control strategy uses zone level measurement of local CO 2 conditions to determine occupancy levels as people enter and leave the zones throughout the day. This allows for a reduction in the amount of ventilation air that needs to be introduced into the building and yields significant energy savings over other ventilation control strategies. A more detailed description of DCV is included later in this discussion. Smoke & Mirrors One particular HVAC manufacturer offers an elaborate and proprietary control algorithm and complicated outdoor air damper device that claims to perform continuous, real-time calculations of the ASHRAE recommended ventilation rates. This calculation is based on the ventilation requirements of the critical zone, that is the zone with the highest required percentage of ventilation air. This manufacturer claims that their controls continuously and automatically adjust the ventilation quantity to ensure compliance with ASHRAE Std. 62. This sounds good at first glance. Using ASHRAE s own Equation 6.1 from Standard 62 to calculate the ventilation air quantities would seem to meet the intent of Standard 62 wouldn t it? Sure it would, but what if the actual occupancy levels in the space are greater than or less than the assumed design occupancy levels? If the actual occupancy levels are less than design levels, (which is the case over 90% of the time) then you waste energy by over-ventilating the space. On the other hand, if the actual occupancy levels are greater than design levels (which may happen occasionally) then you under-ventilate the space and violate ASHRAE Std. 62 requirements. Sure, the control system that calculates Eq. 6-1 may sometimes deliver the proper amount of ventilation.but only when the actual space occupancy levels exactly match the assumed design values. CO 2 Sensors - People Meters People exhale carbon dioxide (CO 2 ) in quantities proportional to their activity level. The higher their activity levels, the higher the CO 2 production. As a matter of fact, unless you re talking about a brewery, people are the predominant contributor of CO 2 in a building. Another fact: humans exhale CO 2 at concentrations of approximately 40,000 ppm. 4

5 The average concentration of CO 2 in outdoor air is approximately 350 ppm. The latest revision of ASHRAE Standard , Appendix D, contains a detailed description of how to calculate differential CO 2 levels between indoor and outdoor air and how these values may be used to predict the level of occupancy in the space. Using ASHRAE s Ventilation Rate Procedure, designers must supply a certain ventilation air rate (CFM) per person based on a maximum occupancy level. Seems simple enough doesn t it? Count the maximum number of people in the building, then size the ventilation system to meet the worst-case occupancy levels. This is where the problems begin. Some manufacturers have added CO 2 measurement in the return air duct as a "failsafe" mechanism so that they may override outdoor damper position if CO 2 levels exceed a certain setpoint. This is analogous to using a return air temperature sensor and trying to control individual zone temperatures. It cannot be done! Any sensor mounted in the return air duct measures "average" values. In order to control ventilation at the zone level, you must measure CO 2 at the zone level. DCV Advantages A Demand Controlled Ventilation (DCV) strategy offers the best of both worlds: precise control of ventilation rates plus the added benefit of dramatic energy savings over constant ventilation or ASHRAE Equation 6.1 based control systems. What is DCV and why is DCV better? Since people are generally the only producers of CO 2 in a building, it is very easy to measure the CO 2 level in each zone of the building as well as changes in these CO 2 levels over time. The concept of correlating the number of people in the building with the indoor CO 2 levels is not new and has been demonstrated for several years now with a high degree of accuracy. Since ASHRAE states that we must deliver a prescribed quantity of ventilation air per person, and we know the number of people in the space (from the CO 2 level), we can continually adjust the outdoor air damper position to maintain the required quantity of ventilation air for each individual zone in the building. Part-Load Profile A representative 100-ton building cooling load profile is shown in Figure 2. A simplified method of representing a building load profile is to construct a straight line between the cooling and heating design points. Note that the maximum cooling load occurs at the summer design temperature and that the minimum Capacity (tons) Ambient Temp. Bldg. Load Equip. Capacity Figure 2 - Unit Capacity & Load vs. Ambient Temp. 5

6 cooling load occurs where the load profile line meets the horizontal (temperature) axis. Loads below this point are heating loads and are not shown on this graph for simplification. Another interesting phenomenon occurs. As the outdoor ambient temperature drops, equipment capacity actually increases as the outdoor (condensing) temperature drops, as represented by the dashed line in Figure 2. This means that as the building load is dropping, the equipment capacity is increasing. This is exactly the opposite of what we would like to see happen. Ideally, we would like the equipment capacity to drop in direct proportion to the building load. To achieve a reduction in equipment capacity, as the building load drops, manufacturers offer multiple steps of capacity control. This is very critical as outside air loads are highly variable due to changes in ambient temperature and fluctuating quantities of ventilation air required. Figure 3 illustrates the part-load operation of a typical, large tonnage rooftop unit. does this mean? Cycling! Cycling of compressors. When a compressor starts, there is a significant in-rush of electrical current to the compressor motor windings. This places thermal and mechanical stresses on the compressor components, primarily the motor windings. The end result is premature compressor wear. What other effects does this excessive cycling cause? With only 4-steps of capacity control, you are effectively banging on and off 25-tons of cooling capacity at a time. This results in a significant change in the leaving coil conditions every time a compressor or a bank of compressors turns on and off. The end result? Wide fluctuations in room temperature and humidity levels. Not exactly what you want for a VAV system that is controlling based on leaving air temperature (LAT). The solution? Add more steps of capacity control to more closely match the unit s capacity to the building load. Figure 4 illustrates the same condition with 9-steps of capacity control % 100 Capacity (tons) % Oversized (cycles) 50% Undersized (cycles) 75% Ambient Temp. 4-Step Capacity Load Figure 3 - Capacity Stages vs. Building Load for 4-step RTU Notice that there are four discrete capacity steps as represented by the stair-step shape of the capacity line. Also notice that the unit capacity exceeds the actual building cooling load a significant amount of time. Finally, notice that the equipment capacity is below the actual cooling load a significant amount of time. What Notice how much closer the unit with 9 steps of capacity matches the actual building load profile. How can you get additional steps of capacity? Either by adding more compressors or by using compressors with capacity control. The most common capacity control device is called a cylinder unloader. A cylinder unloader basically 6

7 cuts-off the flow of refrigerant to some of the compressor's cylinders, thereby reducing the capacity of the compressor. This is analogous to making an 8-cylinder car engine operate more fuel efficiently by using only 4 cylinders, when needed, instead of continuously braking and then pressing the accelerator to maintain desired speed. The end result of having these additional capacity steps is much closer tolerances for the LAT, room conditions and relative humidity and a significant reduction in wear to the compressors. one theater may be showing a very popular movie and be completely full, while the theater next door is showing a less popular movie and has a 50% or less occupancy level. Other likely candidates for DCV would be applications such as conference rooms, gymnasiums, school cafeterias and any area that has widely fluctuating occupancy patterns. For the movie theater analysis, there were 10 screens served by a total of 180-tons of variable air volume (VAV) packaged rooftop units. This analysis considered the movie theaters only and 120 Capacity (tons) % 30% 40% 50% 60% 70% 80% 90% 100% Ambient Temp. 9-Step Capacity Load Figure 4 - Capacity Stages vs. Building Load for 9-step RTU Potential Energy Savings The primary advantages to providing more steps of capacity control is enhanced comfort, reduced compressor wear due to less cycling and more precise control of the space temperature and humidity levels, however there are significant energy saving opportunities also available when combined with a Demand Controlled Ventilation (DCV) strategy. To quantify the costs & benefits of DCV, a 10- screen multiplex cinema located in Dallas, Texas was chosen for this analysis. This application is a prime candidate for applying a DCV strategy as people loads fluctuate drastically throughout the day as well as from individual theater to theater. The afternoon matinee may only have a 30-40% occupancy level while the Friday evening premier of the new blockbuster movie will have a packed house (100 percent occupancy). Also, not the open common areas such as concessions, hallways and front lobby areas. For this analysis three different scenarios were analyzed. In the Baseline Case, each theater is ventilated based on worst case (100 percent) maximum occupancy levels at all occupied times (constant ventilation strategy) and served by VAV packaged rooftop units with only 4-steps of capacity control. Although this assumption is an unlikely scenario, it was deliberately chosen to establish a benchmark for maximum energy consumption. The second scenario, Alternate #1, was exactly like the first except a slight diversity was taken for the people, in anticipation that the theaters would not be filled to 100 percent maximum occupancy levels except at night. In other words, since we don't know, and can't measure, the number of people in the theater we must assume 7

8 maximum predicted occupancy levels that would be expected during a typical day. This is the way a control system based on the ASHRAE Eq. 6-1 equation operates. The third scenario, Alternate #2, simulated a DCV control strategy utilizing CO 2 sensors to measure anticipated changes in occupancy levels throughout the day, in conjunction with VAV packaged rooftop units with 9-steps of capacity control. The weekend people schedule is shown in Figure 5. The weekend occupancy schedules were assumed to be slightly higher than those during the week, as would be expected. For simplification, and since occupancy varies dramatically at different times, all theaters were assumed to have identical occupancy schedules throughout the day. higher, worst-case (100 percent) design values rather than the values that would be measured by a CO 2 sensor located in each theater. The difference in people occupancy levels is the primary component of the annual energy savings achieved by using the DCV ventilation strategy over the other ventilation strategies. Energy Simulation An hourly computer simulation program was used to calculate the design cooling and heating loads and also to simulate the annual operation of all three scenarios and to calculate annual operating costs. It is important to understand that with a DCV control system we are still sizing our equipment and ventilation system for the worst-case, 100 percent design occupancy levels. However, by % Hour % People (weekend, w/dcv) % People (weekend, max) % People (weekend, 100%) Figure 5 - Weekend occupancy schedules Notice the difference in the people schedules between the max values and the DCV values. The DCV values are lower because the outside air quantity will exactly match the number of people in the theater. The max values are higher because we must provide higher quantities of ventilation air (essentially over-ventilate) since we don t know the number of people. In other words, with the max values we are supplying ventilation rates closer to the actually measuring the number of people, and the changes in occupancy over the course of the day (by measuring the changing CO 2 levels), we are able to throttle our ventilation air damper and reduce the amount of outside air brought into the building. In addition, since all theaters are on a common duct system, and assuming there are times where some theaters are nearly empty (low CO 2 levels) while others are nearly full (higher CO 2 levels), the resulting mixed air CO 2 level is 8

9 reduced. By utilizing fresh air from adjoining spaces with lower CO 2 levels, often we do not have to open the outside air damper to satisfy one zone that requires additional ventilation. This results in significant energy savings as indicated in the next section. The cooling load components for the theater on a design day are indicated in Figure 6. Ventilation 35% Other 2% People 28% Wall/Roof 12% Lights 23% Figure 6 - Cooling Load Components As you can see from the chart, the design cooling load is comprised of approximately 1/4 lights, 1/4 people, 1/3 ventilation, and the remainder is the roof and wall transmission and other miscellaneous loads. As indicated, the ventilation air is the single largest cooling load component and thus offers the most opportunity for potential energy savings. At design conditions you are seeing the worst-case scenario with all internal loads (lights, people, equipment, etc.) and all system loads, such as ventilation air, at their highest values. We will focus the remainder of our discussion on the ventilation air component of the cooling load because this is where we have the greatest potential for savings using the DCV strategy. This analysis assumes that the occupants are the primary source of indoor air contaminants in the building. These people-generated contaminants are also referred to as bioeffluents. There are actually two components that make-up the total required ventilation air quantity. The first component is referred to as the occupant ventilation rate (e.g. 15 CFM/person), and is listed in Table 2 of ASHRAE Std The second component, sometimes referred to as the base ventilation rate, is a minimum recommended value to dilute any buildinggenerated contaminants such as cleaning products, fumes from construction materials or building furnishings (carpet, wallpaper and furniture). In other words, if there were no people in the building you would still need to supply the base ventilation rate to dilute any building-generated contaminants. The base ventilation rate is generally assumed to be between percent of design, depending on the age of the building. A new building may have higher amounts of fumes due to construction and an older building will have little off-gassing of fumes from building materials. A base ventilation rate of 20 percent of the design ventilation rate is a reasonable assumption for an existing building and was used in this particular analysis. Case Name Baseline Case Alternate #1 Alternate #2 Description Annual HVAC Total Energy Cost Annual Operating Cost Savings over Baseline Case Constant Ventilation (100%) with 4-steps of capacity $53,833 $0 ASHRAE Eq. 6-1 control system with 4-steps of $45,889 $7,944 (15%) capacity Demand Controlled Ventilation (DCV) with 9-steps of capacity $40,101 $13,732 (25%) Table 1 - Annual Operating Cost Results 9

10 Operating Cost Savings Annual operating costs for each scenario are indicated in Table 1. To summarize the economic results: Alternate #2 (ASHRAE Eq. 6-1 control ventilation strategy) did save approximately 15 percent over the Baseline Case, however Alternate #3, the DCV strategy, resulted in an additional 10 percent energy savings, for a total savings of 25 percent. To reiterate, this additional savings in annual operating costs was attributed to two things. First, the reduction in ventilation air quantity due to the DCV strategy. Second, the higher amount of cycling of compressors for the system with only 4-steps of capacity control resulted in a reduction in efficiency at part load conditions. This reduction in efficiency occurs due to the lower saturated suction temperature (SST) that occurs when a system is oversized. As illustrated previously in Figure 6, with only 4-steps of capacity the system operates in an oversized condition much of the time. The reduced SST requires the compressor to work harder since the compressor "lift" is greater during these times, resulting in a reduced part-load energy efficiency ratio (EER). Analysis Assumptions External static pressure on the RTUs was 2.0 in. w.g. Ventilation air quantities were selected per ASHRAE Std for a theater at 7-1/2 CFM/person. This is one-half of the ASHRAE Std Table 2 value (15 CFM/person) as we are taking credit for intermittent occupancy and the fact that theaters are continuously occupied for periods of less than three hours (per Std , paragraph ). Internal lighting levels were assumed to be 3.0 Watts/sq. ft. Miscellaneous electric was assumed to be 1.0 Watt/sq. ft. to account for equipment, projectors, etc. The electric power rate was assumed to be a flat rate of $0.10/kWh. The cost for natural gas was assumed to be $0.70/therm. Summary & Conclusions For this particular building, simulated in Dallas, Texas, the DCV system with nine steps of capacity control yielded an annual operating cost savings of approximately 25 percent over the constant ventilation system. In addition, the DCV system yielded 10 percent savings over the ASHRAE Eq. 6-1 based control system, which had only four steps of capacity control. Let s reiterate the advantages of using DCV combined with additional steps of cooling capacity: DCV is an ASHRAE Std approved method Results in significant energy savings when compared to constant ventilation or ASHRAE Eq. 6-1 type systems First cost savings; since controls are available with relatively inexpensive packaged rooftop unit equipment Zone CO 2 levels may be monitored and data-logged for trending as well as for documenting compliance with ventilation codes Does not require expensive and complex flow measuring stations and dampers Reduces over or under ventilation of the zones during varying occupancy times Tighter control of LAT, (especially in VAV systems), when compared to systems with fewer steps of capacity control Improved comfort levels in the space Less cycling of compressors, thereby improving the long-term reliability and life of the equipment By unloading compressors, rather than cycling them, the RTU operates at an optimum saturated suction temperature (SST) and compression ratio. Without these additional capacity steps the RTU will operate at lower SST and higher compression ratios, and as a result compressor efficiency decreases dramatically at part-load. 10

11 References 1. Hourly Analysis Program, v. 3.22; Carrier Corporation; Syracuse, NY 2. ASHRAE Standard 62-99; Ventilation for Acceptable Indoor Air Quality; American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.; Atlanta, GA; Schell, Mike B, Turner, Stephen C., P.E. and Shim, R. Omar. CO 2 -Based Demand-Controlled Ventilation Using ASHRAE Standard 62: Optimizing Energy Use and Ventilation ; ASHRAE Transactions Symposia; TO ; American Society of Heating, Refrigerating and Air- Conditioning Engineers, Inc.; Atlanta, GA;

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