1、OR-05-1 1-2 Eva1 u at ion of Dem an d-Con t ro I led Ventilation and Enthalpy Exchangers in Small Commercial Buildings Kevin B. Mercer Associate Member ASHRAE ABSTRACT The ventilation airstream required to maintain acceptable indoor air quality is typically a significant contributor to the total hea
2、ting and cooling load in commercial buildings. Demand-controlled ventilation (DCV) and enthalpy exchang- ers are two technologies that can be used to reduce ventilation loads and overall operating costs. This paper evaluates the operating cost savings and overall economics associated with these two
3、different ventilation load reduction technologies for a variety of small commercial building types and climates. A specialized simulation tool was developed and utilized to perform the evaluations. Both demand-controlled ventilation and enthalpy exchangers were shown to result in signijcant operatio
4、nal cost savings as compared with Jixed ventilation and no energy recovery in a number ofsmall commercial build- ings and locations. For both technologies, the savings poten- tial and economicpaybackare better in more extreme climates. The opportunities are particularly great in cold climates becaus
5、e the ventilation load is a largerportion of the overall load. For retrojt applications, demand-controlled ventilation (DCV) with an economizer was found to give the greatest savings in utility costs for all the buildings andclimates consid- ered. Air-conditioning utility cost savings for DCV of up
6、to 52% were found for ofJices and restaurants. Payback ranged from about one to six years for most locations. Although utility cost savings were found to be less for enthalpy exchangers, they have the advantages of allowing smallerprimary cooling and heating equipment for new applications and provid
7、ing better indoor air quality. The operating cost savings for both DCV and enthalpy exchangers are dependent on assumptions concerning occupancy. The impact of occupancy schedules on James E. Braun, PhD, PE Member ASHRAE cost savings for DCV and enthalpy exchanger systems should be considered in mor
8、e detail in future studies. INTRODUCTION Indoor air quality (IAQ) in commercial buildings is main- tained through ventilation with outdoor air as specified in ASHRAE Standard 62-2001. The loads due to ventilation typically account for about 20% to 40% of the annual heating and cooling loads (ASHRAE
9、1993). A common ventilation control strategy employed for air conditioning is an econo- mizer. An economizer uses outside air to reduce or eliminate mechanical cooling requirements when outdoor air conditions are favorable. Demand-controlled ventilation (DCV) involves controlling outdoor ventilation
10、 air based on indoor carbon dioxide (CO,) levels and is usually combined with an econo- mizer (EC). CO, levels are assumed to be an indicator of occu- pancy. A heat recovery enthalpy exchanger (HXHR) operates between the ventilation and exhaust streams of the ventilation system. The typical implemen
11、tation involves the use of a rotary device containing a permeable medium with a large surface area. The medium can be made of polymer or mineral material or fabricated from metal. The exhaust and ventilation airstreams exchange heat and mass across the medium in a counterflow geometry. Much research
12、 has been conducted in order to understand the energy savings associated with DCV and enthalpy exchangers. Several researchers performed field tests of DCV to address implementation issues and evaluate cost savings (Donnini et al. 1991; Gabel et al. 1986; Janssen et al. 1982; Zamboni et al. 1991). T
13、hese field studies generally showed that there is significant savings potential for DCV and it is feasible for a wide variety of applications. Many of the previ- Kevin B. Mercer is a product development engineer for Modine Manufacturing Company, Racine, Wisconsin. James E. Braun is a professor of me
14、chanical engineering at Ray W. Herrick Laboratories, Purdue University, West Lafayette, Indiana. 02005 ASHRAE. a73 ous studies used simulation tools to address control method- ologies, pollutant transport, and energy savings for large to mid-sized commercial buildings (Knoespel et al. 199 1 ; Haghig
15、hat et al. 1993; Carpenter 1996; Enermodal 1995). Overall, the simulation studies also demonstrated very signif- icant energy savings for DCV. Energy savings varied depend- ing on control methods, climate, economizer integration, design occupancy assumptions, and other parameters. Bran- demuehl and
16、Braun (1999) conducted a simulation study to examine energy impacts of various economizeriDCV combi- nations for small commercial buildings that employ packaged air-conditioning and heating equipment. Four prototypical building types were considered in 20 different U.S. climate zones. The savings as
17、sociated with DCV were found to be very significant, but highly dependent on the occupancy rela- tive to the design occupancy. In most climates, it was found that an economizer should be incorporated with DCV to real- ize cooling season savings. Enthalpy exchangers were developed in the 1970s and ha
18、ve been commercially available for many years. Early research focused on the development of enhanced materials and improved designs with laminar flow geometries to mini- mize pressure drop (Pesaran et al. 1992). Regerajan et al. (1996) and Shirey and Rengarajan (1 996) used simulations to conclude t
19、hat the use of enthalpy exchangers offset additional energy use associated with meeting the new ventilation requirements of ASHRAE Standard 62-1989 for all areas of Florida. Kavanaugh and Xie (2000) studied seven different ventilation air treatment options for ground-source heat pump systems in an o
20、ffice building and school for three different climates. One of their results was that heat recovery units, which were effective at reducing energy usage in cold climates, consumed as much or more energy than conven- tional systems in warm climates because of increased auxil- iary power requirements
21、associated with fans and other equipment. The powers that they assumed were based upon manufacturers data for the different technologies. For rotary enthalpy exchangers, an additional power requirement of 1.4 W/ch of ventilation of flow was included to account for a ventilation fan, an exhaust fan,
22、and the motor used to rotate the wheel. Nuernberger and Law (2004) expanded on the study of Kavanaugh and Xie for enthalpy exchangers and considered more climates but found similar results. Freund et al. (2004) studied the impact of enthalpy exchangers in an animal hous- ing facility in Madison, Wis
23、consin. The facility was ventilated 24 hours a day at an average of 2.14 air changes per hour. It was found that annual energy savings of up to 76% for heating and 23% for cooling could be realized. The enthalpy exchanger system utilized an outside air bypass to allow economizer savings and was size
24、d for a new design, thus allowing for primary unit downsizing compared to the base case. The base case for this study assumed a fixed damper ventilation rate with a boiler efficiency calculated between 80% and 88% and an air conditioner EER rating of 9.5. Nyman and Simonson (2004) have also shown th
25、at enthalpy exchangers can be very An-bient Conditions Ambient Condions Building Sch Reiurn Air Conditions Operating hi and Payback U Co2 and Latent Gains Figure 1 Simulation jlow diagram. environmentally friendly in addition to saving energy. For modeling enthalpy exchanger performance, Stiesch et
26、al. (1995) developed a component model and incorporated it into the transient simulation program, TRNSYS, to estimate system annual performance. The greatest cooling savings potential was found for cooling in warm and humid climates. There are very limited comparisons of DCV and enthalpy exchangers
27、in the literature. The primary objective of the study described in this paper was to compare energy cost savings and economic payback for demand-controlled ventilation and enthalpy exchangers in small commercial applications that utilize packaged equipment. A number of different prototypical buildin
28、gs were simulated in a range of different climates. APPROACH The simulation study was performed for small commer- cial buildings serviced by packaged, constant air volume equipment. A transient systems simulation tool was developed to determine energy usage and operating costs for the HVAC equipment
29、. This tool, termed the ventilation strategy assess- ment tool (VSAT), was built from a pre-existing simulation tool that considered DCV and economizer control (Brande- muehl and Braun 1999). VSAT incorporates the same struc- ture and calculation engines as the original tool, however, with the added
30、 capability to simulate other ventilation strategies including enthalpy exchangers and heat pump heat recovery (see Mercer and Braun 2005). VSAT is structured according to the flow diagram shown in Figure 1. The building model outputs hourly building loads for heating and cooling season in order to
31、maintain zone temperature setpoints. Ambient conditions, a physical build- ing description, equipment and occupancy schedules, and a thermostat control are input to the building model. The build- ing model is fairly detailed and considers transient heat conduction through walls, floors, and roofs us
32、ing simplified transfer function representations. Predictions of the building model compare well with other detailed models from the liter- ature with substantially faster calculation speeds (see Braun and Mercer 2003). The space conditioning model uses quasi- steady energy and mass (COz and moistur
33、e) balances within 874 ASHRAE Transactions: Symposia the zone and air distribution system. A fixed ventilation effec- tiveness is employed for the zone to consider short-circuiting of supply air to the return duct. The mixed air conditions, calculated depending upon the employed ventilation strategy
34、, are output to the equipment model. The equipment model calculates average supply air conditions to the space from entering mixed air conditions, the sensible equipment capac- ity, and a bypass-factory model. Capacity and COP are calcu- lated by applying correction factors to a rated capaciy and CO
35、P. This methodology is discussed in more detail by Bran- demuehl et al. (2000). Moisture storage in building materials and re-evaporation of moisture off the coil during compressor off-cycles are not considered. A nonlinear equation solver is implemented to iteratively solve for the inlet and outlet
36、 air conditions at each time-step in the simulation. The economic model predicts operating costs for each system employing a different ventilation strategy based on utility rate structures. Percent savings are calculated from annual results with respect to an assumed base case. The DCV and economize
37、r controls are assumed to be ideal. At any hour, the model determines the minimum venti- lation air necessary to maintain a specified CO, setpoint for the zone. The minimum ventilation flow is utilized unless the economizer is enabled. The enthalpy economizer operates whenever the enthalpy of the am
38、bient air is less than the return air enthalpy and a cooling requirement exists in the zone. When the economizer is engaged, the ventilation and return air dampers are modulated between the minimum and maximum positions to maintain a mixed air temperature setpoint of 55F (12.78“C) supplied to the RT
39、U. The enthalpy exchanger is modeled using an approach developed by Stiesch et al. (1995). This component model predicts temperature, humidity, and enthalpy effectiveness based on a dimensionless wheel speed and media NTU. The enthalpy exchanger is assumed sized appropriately to achieve an air flow
40、velocity of 650 fpm. According to manufacturers literature at this velocity, the enthalpy exchanger has a constant effectiveness for heat and mass transfer when oper- ating at recommended wheel speeds. VSAT assumes an NTU between the media and ventilation air of 6.0, which corre- sponds to an effect
41、iveness of 0.75. A correlation is incorpo- rated for the effect of wheel speed on effectiveness. Enthalpy exchangers typically incorporate additional exhaust and venti- lation fans to overcome the pressure drop associated with heat transfer media. The power associated with the exhaust and ventilatio
42、n fans was modeled using a specified power per cfm of air flow rate. The enthalpy exchanger control strategy incor- porated in the simulation was identified from manufacturers information. The enthalpy exchanger operates when the primary fan is on and the ambient temperature is less than 55F (12.78“
43、C) or greater than the return air temperature. When the ambient temperature is between 55F (12.78“C) and the return air temperature, it is assumed that a cooling requirement exists and it is better to bring in cooler ambient air. When the ambient California CA Climate temperature is below 55F (12.78
44、“C), then a feedback control- ler adjusts the speed to maintain a ventilation supply air temperature of 55F (12.78“C). When the ambient tempera- ture is above the return air temperature, then the wheel oper- ates at maximum speed. Feedback control of wheel speed is also initiated under conditions wh
45、ere water vapor in the exhaust stream would condense and freeze. A frost setpoint is specified based on winter ambient and zone design conditions as discussed by Stiesch et al. (1995). A complete description ofthe simulation tool and compo- nent models is provided by Braun and Mercer (2003). VSAT wa
46、s validated by comparing annual equipment loads and power consumption for similar case studies in Energy-10 and TRNSYS. Energy-10 is a design tool developed for the U.S. Department of Energy (DOE) to analyze residential and small commercial buildings. TRNSYS is a complex transient system simulation
47、program that incorporates a detailed build- ing load model (Type-56 multi-zone building component). Neither of these tools incorporates the ventilation strategies considered in this study. Therefore, VSAT was validated for a base case employing fixed ventilation rates with an econo- mizer. In genera
48、l, the VSAT predictions were within about 5% of the hourly, monthly, and annual predictions from TRNSYS and Energy-10 (see Braun and Mercer, 2003). Hot and Hot and Dry Humid Cold Las Veaas, NV Norfolk, VA Farpo. ND CASE STUDY DESCRIPTIONS Zones: to CACZ 16 Simulations were performed for combinations
49、 of seven building types in twenty-two locations. The complete set of case studies is given by Mercer (2003) and Mercer et al. (2003). The focus of the original study was on California climates. California contains hot and dry climates and moder- ate coastal climates. Additional climates from cold and hot and humid portions of the U.S. were added. Due to space Iimi- tations, only a subset of the original study results is presented in this paper. Table 1 shows the U.S. cities considered for this study. Figure 2 shows the location of climate zones considered in California. The climate
