1、4680 Energy Savings Potential of Energy Recovery Ventilation in an Animal Housing Facility Sebastian Freund Sanford A. Klein, Ph.D. Douglas T. Reindl, Ph.D., P.E. Student Member ASHRAE Fellow ASHRAE ABSTRACT This paper summarizes eforts to identifi economically viable strategies to reduce HVAC-relat
2、ed energy use while improving the indoor air quality for a public zoo facility that houses primates and large cats. The primary focus of energy conservation strategies for the facility centered on use of air- to-air energy recovery devices. Computer simulations, vali- dated with experimental data fr
3、om the facility, were used to estimate energy savings for alternative energy conservation strategies. Thefindingssuggest that more than 80%ofthe heat- ing energy and 45% of the cooling energy can be saved by implementing air-to-air energy recovery equipment coupled with alternative temperature contr
4、ol settings. An extension of the energy analysis to environmental impacts suggests that up to 73 tons of CO, emissions can be saved annually by imple- menting the equipment and operating strategies identified in this study. INTRODUCTION Heating, ventilating, and air-conditioning of commercial buildi
5、ngs are relatively energy intensive processes, represent- ing about 6% of the total U.S. energy usage (ADL 2002). A large part of this energy use is a result of conditioning outside air to meet ventilation requirements for acceptable indoor air quality. Consequently, the amount of fresh air brought
6、into a building is of great importance in terms of energy usage and HVAC costs. Energy recovery ventilation (ERV) systems can save substantial amounts of HVAC energy by recovering otherwise wasted energy from the exhaust air to precondition intake air. Because the installation of ERV systems decreas
7、es cooling and heating coil loads, the capacity of the HVAC equipment Member ASHRAE can be downsized. Reduced size of HVAC equipment can help offset the additional costs of the ERV equipment, further improving the economics of ERV systems. This paper presents the results of a project aimed at under-
8、 standing the impacts of two energy recovery ventilation system alternatives on the energy use and indoor air quality of a building. The building is the primate house at the Henry Vilas Zoo, located in Madison, Wisc. Residents of this facility include primates, large cats, visitors, and staff. Histo
9、rically, the building experienced high utility bills and less than opti- mal indoor air quality (IAQ), as evident by strong odors. Figure 1 shows the relation of ventilation rate and energy consumption in the building considered in this paper. A goal of the project was to find options capable of low
10、ering the facil- 1 Actual data 1999 -4 -D- wing 1.5E+06 1.OE+06 5.OE+05 w O.OE+OO O 1 2 3 4 5 Ventilation Rate llh Figure1 Heating and cooling energy as a function of outdoor air ventilation rate (air changes per hour), simulated and measured data. Sebastian Freund is a graduate student in mechanica
11、l engineering, S.A. Klein is a professor of mechanical engineering, and D.T. Reindl is an associate professor and director of the HVAC OA outside aiK EA exhaust air. rotary heat exchangers can be considered counterflow heat exchange devices. The heat exchanger wheel consists of a matrix made of eith
12、er desiccant-coated aluminum foil or a polymer membrane containing a desiccant substance, such as silica gel or molecular sieves. A certain amount of exhaust air entrained in the matrix is transferred into the outside airstream, a process referred to as cross-contamination. The fraction of exhaust a
13、ir in the outside air depends on the pressure differential and the wheel type and rotation speed and is usually on the order of 1 % to 3% of the outside airflow rate. In critical use applications, cross-contam- ination can be minimized by implementing a “purge” section in the energy recovery wheel.
14、A computer model of the enthalpy exchanger was created as a module to be used in an existing building simulation program (Klein et al. 2002) in order to simulate its operation in a building HVAC system. The enthalpy exchanger model is based on a semi-empirical method that identified the NTU and an e
15、ffectiveness correction factor from manufacturers refer- ence data (Freund et al. 2003). The required reference data are the sensible and latent effectiveness as well as pressure drop for two different reference flow rates. Using only these data, the model is able to predict sensible and latent effe
16、ctiveness for any balanced or unbalanced flow configuration. The uncer- tainty of this method compared to experimental data is usually less than 2%. The model also calculates the pressure drop and additional fan power required to operate the rotary heat exchanger. Runaround Loops A runaround loop is
17、 an air-to-air heat recovery system that relies on the use of plate-finned coils along with a second- ary fluid and a circulating pump. A typical runaround loop has one coil located in the exhaust airstream with the second coil located in the outdoor air intake stream. The heat exchange between the
18、two airstreams is accomplished by circulating a heat transfer fluid between the two coils, as shown in Figure 3. Because runaround loops only recover sensible energy, their performance is lower compared to an enthalpy exchanger. However, an advantage of runaround loops over enthalpy exchangers is th
19、at intake and exhaust ducts do not ASHRAE Transactions: Research 121 Fluid LOOP uumae Air LOII 3 Pump i Figure 3 Schematic of a runaround loop. have to be located adjacent to each other. This feature makes runaround loops easier to install for retrofit applications. It also eliminates cross-contamin
20、ation between exhaust and fresh air intake streams that can occur with rotary energy recovery devices. The runaround loop is modeled as two liquid-coupled counterflow plate-finned heat exchangers using the E-NTU method (Kays and London 1964). Coil parameters, including a number of rows and circuits,
21、 lengths, tube diameters, and fin spacing, are used to calculate heat transfer coefficients based on Nusselt number correlations (Kakac and Shah 1987) and the heat transfer area. The model distinguishes between dry and wet coil performance, where the effectiveness is increased due to condensation on
22、 the coil located in the exhaust airstream. The wet coil effectiveness is calculated based on the method outlined in Braun et al. (1989) and Threlkeld (1962). The runaround loop includes a three-way valve to bypass warm fluid around the outdoor air coil for frost control and operation during economi
23、zer mode (Freund 2003). The model also calculates liquid- and air-side pressure drops as well as the additional fan and liquid pumping power required for its oper- ation. The model was validated by comparison with experi- mental results from Sauer et al. (1 98 1) and Forsyth and Besant (1988) and sh
24、owed agreement within 5%. Frost Control Energy recovery ventilation devices applied in cold climates are prone to conditions that result in frost formation on the heat transfer surfaces. Frosting can occur in the building exhaust air side of the heat exchanger whenever the heat exchanger operating t
25、emperatures drop below freezing point. Frost buildup will occur when the exhaust heat exchanger surface temperature is coincidentally below the freezing point of water and the dew point of the exhaust air. Frost buildup increases the air-side pressure drop through the heat exchanger and, if allowed
26、to continue, will eventually block airflow through the heat exchanger. Since the benefit of preconditioning ventilation air is greatest when the outdoor air temperature is lowest, the potential for frosting can signifi- cantly compromise ERV operation. Strategies for preventing frost formation and i
27、ts energy penalties have been considered in the present analysis at times when exhaust air conditions may result in frost formation in the heat exchanger matrix. Frost control for enthalpy exchang- ers can be achieved by preheating intake air, reheating exhaust air, bypassing outside air, or reducin
28、g the wheel rotation speed. Preheating outdoor air has proven to be the most efi- cient method of frost control for enthalpy exchangers (Freund et al. 2003). This strategy can be implemented with a controlled electric heating coil in the intake airstream. Preheating intake air can also be used for f
29、rost control in runaround systems; however, there are other options to protect the exhaust air coil of a runaround loop from frost buildup. Frost buildup can effectively be avoided by controlling the coil surface temperature to be above 0C (32F) via a three-way bypass valve (as shown in Figure 3), w
30、hen condensation occurs in the exhaust coil. This strategy results in more effi- cient operation than the alternative of reducing the heat trans- fer liquid flow rate in both heat exchangers (Freund 2003). Economizer Control Buildings with significant internal heat gains often require cooling even w
31、hen the outdoor air temperatures are low. To offset internal heat gains, the air-conditioning systems supply air temperature has to be lower than the room setpoint. Providing the building with ventilation air during times when the outdoor air temperature is low can often meet the cooling load withou
32、t operating mechanical refngeration equipment. This operational mode is often referred to as an “air-side econ- omizer.” The presence of an air-to-air energy recovery device effectively increases the outdoor air temperature due to heat exchange with the warmer exhaust airstream. This behavior works
33、against the conditions desired for the operation of an air-side economizer and can result in increased cooling energy use. Hence, the effectiveness of ERV systems has to be decreased when the outdoor temperature rises above the temperature point, requiring additional heating, and the energy recovery
34、 system has to be shut down whenever the outdoor temperature reaches the balance point temperature of the building. Controls for economizer operation with run-around loop exchangers are as follows. The system effectiveness is decreased proportionally to zero for outdoor temperatures that range betwe
35、en the zone heating point and the zone balance point. The runaround loop circulation pump remains off when the outside temperature is between the balance point and the exhaust air temperature. For outdoor temperatures above the building exhaust temperature, the system operates with maxi- mum effecti
36、veness to precool ventilation air. The effectiveness is controlled either by reducing the fluid flow rate with a vari- 122 ASHRAE Transactions: Research able-speed pump or by bypassing fluid around the outside air coil by the use of a three-way valve. To calculate the effective- ness, the outside ai
37、r inlet and outlet temperatures have to be measured, and the temperature difference compared to the temperature difference between outdoor and exhaust air temperature. If the effectiveness is higher than required, a feedback controller opens the bypass valve or decreases the pump speed. For enthalpy
38、 exchangers, the control strategy is based partially on enthalpy rather than temperature alone due to their ability to exchange moisture. Enthalpy exchangers operate in cooling and dehumidifying mode whenever the zone exhaust enthalpy is lower than the enthalpy of the outdoor air or when the exhaust
39、 temperature is lower than outdoor air temperature, thus minimizing combined sensible and latent cooling power. For the analysis considered in this paper, the effectiveness of the ERV system is proportionally decreased when the outdoor temperature rises above the zone setpoint. The effectiveness of
40、the enthalpy exchanger is varied by the use of an outside air bypass. Another option is wheel speed control, although here the latent and sensible effectiveness would be decreased at different rates due to effects of unsteady thermodynamic conditions when reducing the wheel speed (Freund et al. 2003
41、; Freund 2003). Building Model The building is a single-story animal housing and visitor facility. It has a total floor area of 13 16 m2 (1 6,307 fi2) and it is divided into two zones on the ground level and two zones in the basement. It houses primates in display cages and visitors on the first flo
42、or, as well as several primate species along with large cats (tigers and lions) in the basement. Each species has different thermal requirements; hence, the four zones are indi- vidually served by four air-handling units. Each zones air-handling unit is designed as a constant volume air delivery sys
43、tem operating with a fixed outside air fraction. The hot water heating coils in the air-handling units as weil as additional zone booster coils are served by three 132 kW (450 mBtu/h) boilers. Boiler efficiency is assumed to vary linearly from 80% to 88% at minimum and maximum load, respectively. Th
44、e cooling system consists of air-cooled R-22 condensing units with direct-expansion coils having capaci- ties of 26.5 kWT (7.5 tons), two units at 70 kW, (20 tons) and one at 140 kWT (40 tons). The COP is assumed to be 2.8 for all four units. A model of the building structure and its mechanical syst
45、em is constructed using a simulation program (Klein et al. 2002). The simulation program provides a standard compo- nent for multizone buildings. This building model includes all physical data of the building envelope and accounts for inter- nal and solar gains. The building internal gains from sour
46、ces such as lights, fans, pumps, and other equipment, as well as those from different animals and humans for their respective activity levels, are scheduled in accordance with building occupancy. Areas, orientations, and materials of all external and internal walls and windows are included in the mo
47、deling effort to account for thermal transmission, radiation, and ther- mal capacity of the structure. The annual simulations are driven using actual hourly measured weather data from 1999. In addition, short-term simulations to validate the model used detailed field-measured hourly data for the mon
48、ths of February and July 2002. The building model is validated in two ways. First, measured temperatures and energy use at hourly intervals for two-week periods in February and July 2002 were compared with the simulation results. Second, the monthly and annual energy use predicted by the simulation
49、was compared with utility bills for the 1999 calendar year. On an hourly basis, the results showed differences of up to 15% for heating and 5% for cooling. On an annual basis, the simulation-predicted facility energy consumption is within 3% for heating and 1 % for cool- ing. Simulation of the HVAC System Including Enthalpy Exchangers The impact of adding enthalpy exchangers to the HVAC systems serving the primate house is evaluated in this section. The analysis assumes that each ofthe four building zones air- handling units is equipped with a rotary enthalpy exchanger. The size of ea
