1、AN-04-4-2 Life-Cycle Assessment (LCA) of Air-Handling Units with and without Air-to-Air Energy Exchangers Mikko Nyman ABSTRACT The life-cycle assessment (LCA) methodology is used in this paper to assess the environmental eflects of air-handling units HU) over a 20-year life cycle. This assessment is
2、 based on quantzJling the consumption of resources (energy and mate- rials), the harmful emissions into the environment (air, water, and soil), and the potential changes in the environment (climate change, acid$cation, and ozone production). A normal AHU, with a face velocity of 3 m/s (600 fpm), and
3、 a small AHU, with a face velocity of 4 mis (SOOfPm), are inves- tigated with and without two types of air-to-air energy exchangers late and rotating wheel). The research demon- strates the following benejis of air-to-air energy exchangers: reduced energy consumption, reduced emissions to the envi-
4、ronment, and reduced potential harmful changes in the envi- ronment. For both of the AHUs studied, these benejts are several timesgreater than the burdens arising from theproduc- tion and operation of the AHU, where the function of the AHU is to provide 2000 L/s (4200 cjin) of outdoor air to the bui
5、lding space for 2500 h/yeal; but not to condition this air: A larger AHU with an air-to-air energy exchanger of higher eficiency has the smallest harmful eflect on the environment. INTRODUCTION Energy use in buildings has been of considerable interest for several decades, but only recently has inter
6、est in ecological aspects of buildings arisen among building owners and clients (Adalberth 1997a, 1997b). The energy and materials used and waste produced during the construction, operation, mainte- nance, and demolition of buildings have significant financial and environmental implications. Since d
7、elivering outdoor ventilation air to building spaces has one of the greatest Carey J. Simonson, Ph.D., P.Eng. Associate Member ASHRAE impacts on indoor air quality in many buildings and is also responsible for 30% to 50% of the energy consumed in build- ings (Liddament and Orme 1998), it is the focu
8、s of this study. This paper describes a study undertaken in Finland to assess the ecological impact of ventilation units using the life-cycle assessment (LCA) methodology (Hkkinen et al. 1999). Life-cycle assessment (LCA) deals with the impact that a product has on the environment during its entire
9、life cycle, from production to disposal (Figure 1). This includes the extraction of basic raw materials and energy raw materials, production processes of materials and products, transporta- tion, use, and recycle. LCA is similar to life-cycle cost (LCC) analysis in that both address issues over the
10、life of the product or system rather than basing a decision on the first capital cost, clhnaIe* Addrndrn ? LCC uses money as the compar- ison scale, while LCA uses environmental indicators such as CO, emissions (indicating climate change), SO, emissions (indicating acidification potential), and othe
11、rs. There are three basic steps in the LCA methodology (IS0 1997). The first is to set the goals and scope of the study. This is an important step because it sets the stage for the inventory analysis (second step) and the impact analysis (third step). An additional step is the interpretations of the
12、 results (IEN ECBCS 2001), which compares the LCA results of different products. (This step was previously termed an improvement analysis and aimed to reduce the environmental impacts of a certain product or system as described by Liu et al. 19971). In this paper, the three steps of IS0 14040 (IS0 1
13、997) are presented for air-handling units (AHUs) with and without air- to-air energy recovery. In the first section (goals and scope), the functional unit and the system boundaries for the AHU are defined. In the second section (inventory analysis), the resources (raw materials and energy) consumed
14、and the emis- sions produced during production and operation of the AHUs are documented. Finally, an impact analysis is presented, which links the resource consumption in the inventory analy- sis to the potential harmful impacts on the environment. Here, emissions affecting climate change (equivalen
15、t emissions of CO,), acidification (equivalent emissions of SO,), and ozone formation (equivalent emissions of ethane) are considered. GOALS AND SCOPE It is important that HVAC designers be aware of the consequences of their choices during all phases of the design. Designing and dimensioning air-han
16、dling (ventilation) units have a significant effect on the use of energy in the building and, thus, both the costs and environmental impacts. The research project described in this paper and in more detail by Hkkinen et al. (1 999) investigated the effect of the size of the air-handling unit (AHU) a
17、nd heat recovery on the environ- mental impacts of AHUs. A comparison is carried out between AHUs of different sizes equipped with different types of energy recovery exchangers and one without energy recovery. The main goal is to assess the environmental impacts of energy recovery for ventilation ai
18、r. Functional Unit According to the IS0 14040 standard (IS0 1997), the functional unit is generally defined as a quantified performance of a product or system. In this paper, the functional unit is defined as providing an outdoor ventilation airflow of 2 m3/s (4200 cfm). This airflow is the outdoor
19、ventilation rate required for 200 people at 10 L/(s.person) in an office building (ASHRAE 2001a) and is provided by the AHU. The heating and cooling requirements of the ventilation air and building are separated from the functional unit. However, the materials used and pressure drop across the heati
20、ng coil are included in the analysis, as depicted in Figure 2. It is important to emphasize that the function of the AHLJ as defined in this study is to provide outdoor ventilation air, not to condition the ventilation air. In the case with energy recovery, the energy recovered from the exhaust air
21、is considered as a benefit of the AHU to the heat- ing and cooling systems of the building. However, the heating and cooling needs of the building are not included in the anal- ysis. Since the function of the AHU in this analysis is to provide a certain amount of outdoor ventilation air, neither the
22、 size of the AHU nor the presence of an air-to-air energy exchanger affect the function. The same ventilation function can be performed with different sizes of AHUs, which may be equipped with some sort of energy recovery, provided other functional requirements are constants. The other functional re
23、quirements of the AHU, such as total pressure increase across the AH (250 Pa or 1 in. of water to meet the pressure drop across the building ducting network), the filter class, and the noise level, were same for the different AHUs (Table 1). The manufacturers involved in this study sized the differe
24、nt AHUs such that the “normal AHU” is the size of AHU that the designer would normally select, while the “small AHU” is selected as one size smaller than the normally selected Am. The nominal face velocity of the normal AHU is 3 ms (600 fpm) and the nominal face velocity of the small AHU is 4 ds (80
25、0 fpm). It is assumed that, in all cases, the pressure drop across the supply and exhaust ducts is constant at 250 Pa (1 in. of water). Therefore, in addition to overcoming the internal pressure losses, both the supply and exhaust fans must provide COOLER A II however, this is expected to have a mar
26、ginal effect on the conclusions from this study because the environmental impacts associated with the use of the AHUs are generally 1 O to 40 times greater than those asso- ciated with production as will be presented in the impact anal- ysis section (Figures 4 to 6). Tables 2 and 3 show that, compar
27、ed to the normal AHU, the small AHU requires about 20% less material, which results in about a 20% reduction in energy consumption during production and distribution and about 20% lower emissions. The plate energy exchanger increases the mass, energy consumption, and emissions by about 50%, while a
28、wheel energy exchanger does not have a large impact on these vari- ables. Table 3. Energy (GJ) Required and Emissions to the Atmosphere that Result from the Production of the Normal and Small AHUs with and without a Plate or Wheel Air-to-Air Energy Exchanger 402 ASHRAE Transactions: Symposia 7000 60
29、00 5000 4000 3000 2000 1 O00 O Normal AHU Small AHU Normal AHU Small AHU Normal AHU Small AHU (plate) (plate) (wheel) (wheel) Figure 3 Fan energy consumption and energy recovered by the energy exchanger for one year of operation and over the 20-year life cycle. The energy exchanger recovers between
30、two and four times as much energy as required by the fan, even after the electricity consumption has been adjusted for its higher heating value. Energy Use During Operation As mentioned previously, the function ofthe AHU for this comparative study is to provide outdoor ventilation air, but not to co
31、ndition it. Therefore, the energy required to perform this function is the energy required to run the supply and exhaust fans in the AH. The energy required to perform this function depends on the pressure drop across the unit and the supply and exhaust ductwork. Since the pressure drop and fan size
32、 (Table 1) are different for the different AHUs, the energy consumption (Figure 3) is different as well. The energy consumption of the fans has been converted to equivalent heat- ing energy using the electricity production efficiency in Finland in 1995, which was 0.48. This information was obtained
33、from the Ilmatran Voima power company. The energy required when acquiring the raw materials for electric- ity production and the energy losses during distribution are also taken into account, and this reduces the overall efficiency to 0.46. The energy recovered by the air-to-air energy exchanger has
34、 also been converted to the higher heating value using the district heating efficiency of 0.83. This value was obtained from the Ilmatran Voima power company, and it includes the distribution losses. The emissions that result from the production of district heat are based on the calculations of the
35、Ilmatran Voima power company and the acquiring of raw material for energy production (Leino and Rissanen 1993). Even though the energy recovered from the exhaust air does not enhance the defined function of the AHU, it does provide benefits to other components in the HVAC system. The ventilation air
36、 will require less auxiliary heating because of the air-to-air energy exchanger. The energy recovered from the exhaust air (22C) by the energy exchanger was calculated, assuming a constant effectiveness (Table 1) when the temper- ature of the air leaving the exchanger was less than 17C. When the out
37、door temperature was warmer, it was assumed that the energy exchanger was controlled to prevent overheat- ing (Simonson et al. 2000a, 2000b). The maximum tempera- ture of the air leaving the air exchanger was chosen to be 17C. Although these assumptions assume ideal control of the sensi- ble heat ex
38、changer, they represent practice quite well because the outlet temperature of a sensible heat exchanger can be controlled reasonably well using a temperature sensor connected to a bypass or wheel speed control module. Further- more, the effectiveness of sensible heat exchangers is essen- tially cons
39、tant over a wide range of outdoor temperatures. These assumptions would be less accurate, however, for a device that simultaneously transfers heat and moisture because effectiveness is a function of the outdoor temperature and humidity for these devices, and it is significantly more difficult to sim
40、ultaneously control the outlet temperature and humidity (Simonson et al. 2000a, 2000b). Figure 3 shows that the energy consumed by the fan is significantly higher in the small AHU than in the normal AHU (43% when there is no energy exchanger, 46% for the plate exchanger, and 19% for the wheel). The
41、energy recovered by the energyexchangeris 3.1,2.0,4.3, and3.4 times greaterthan the energy required by the fan in the normal AHU (plate), small AHU (plate), normal AHU (wheel), and small AHU (wheel), respectively. An even more remarkable finding from the results in Figure 3 is revealed when the ener
42、gy saved by the energy exchanger is compared to the additional fan energy required to overcome the pressure drop across the energy exchanger. This analysis shows that the energy exchanger recovers 21, 12, and 39 times more energy than the additional energy consumed by the fan to overcome the pressur
43、e drop across the energy exchanger for the normal AHU (plate), small AHLJ (plate), and normal AHU (wheel), respectively. (It should be noted that these ratios would be nearly twice as large ASH RAE Transactions: Symposia 403 Table 4. Emissions to the Atmosphere that Result from the Energy Used by th
44、e Fans in the AHUs Over the 20-Year Life Cycle* Emission CO? 0) Normal AHU Small AHU 42 60 Normal AHU (plate) 50 (-325) - Small AHU Normal AHU Small AHU (plate) (wheel) (wheel) 73 (-289) 48 (-458) 57 (-414) CO (kg) NO, (kg) SO? (kg) The values in parentheses are the net emissions considering the emi
45、ssion reductions due to the air-to-air energy exchanger. A negative value means that the emission reduc- 63 90 74 (-80) 108 (-41) 71 (-137) 84 (-110) 99 142 117 (-668) 171 (-586) 112 (-947) 133 (-853) 71 102 84 (-703) 123 (-637) 81 (-981) 96 (-893) tions exceed the actual emissions. if the efficienc
46、y of electricity production and district heat distribution were not included.) This comparison is not rele- vant for the small AHU with a wheel because it has a smaller fan and lower energy consumption than the small AHU with- out a wheel as discussed previously. Table 4 lists the emissions that res
47、ult from the energy used by the fans over the life cycle of the AHU and the net emissions of the AHU considering the emission reductions due to the air- to-air energy exchanger. The reduction of emissions due to the energy recovery from the exhaust air was assessed on the basis of district heating,
48、which would normally be needed to heat the required outdoor ventilation airflow to a supply air tempera- ture of 17C. In all the AHUs with an air-to-air energy exchanger, the reductions in emission due to the energy recov- ered by the exchanger far exceed the emissions due to the elec- tricity used
49、by the fans. Comparing Table 3 with Figure 3 and Table 4 shows that the emissions and energy used during production are signifi- cantly smaller than the emissions produced and energy used during the 20-year operation of the AHUs. The energy consumed to operate the AHUs ranges from 40 to 80 times greater than the energy consumed to produce the AHUs. Simi- larly, the emissions resulting from operation are typically 20 to 40 times greater than those that result from production and transportation. voctot (kg) CH4 (kg) Particles (kg) IMPACT ANALYSIS The impact analysis (or as
