1、4752 Improving Laboratory Building Energy Performance and Indoor Air Quality Using a Laboratory Air-Handling Unit System (LAHU) Yujie Cui Student Member ASHRAE ABSTRACT The laboratory air-handling unit (LAHU) system is designed to improve building energy performance and indoor air qualily (IAQ) in l
2、aboratory buildings. The LAHU system sends more (up to 100%) outside air to the ofice section and recirculates the ofice section air to the laboratory section. This theoretical study shows that the potential annual thermal energy savings varies from 20% up to 40% depending on the climate and ratio o
3、f ofice airflow to the laboratory section airflow. The LAHUprovides more outside air intake directly to the ofice section during both cold winter and hot summer months when the IAQ is critical for building occupants. When the ofice airflow ratio is less than 50%, the LAHUprovides close to 100% outsi
4、de air to the oflce section at all times. INTRODUCTION Modem research buildings often contain both laboratory and office sections. Conventionally dedicated AHU systems serve the two sections separately. The laboratory sections are designed to use 100% outside air, resulting in energy usage several t
5、imes higher than that in commercial buildings. To reduce energy costs, a number of energy conservation measures have been developed and implemented in laboratory buildings. These measures include exhaust air heat recovery, supply air reheat, variable air volume (VAV) fume hoods, usage-based control
6、devices (UBC), and integrated AHU systems. A significant amount of thermal energy can be recovered from the exhaust air by using “rotary wheel,” “fixed plate,” or “heat pipe” (thermosiphon) units (Bowlen 1974; Street and Setty 1983; Cames 1988). During winter, the outside air is warmed up by the exh
7、aust air to reduce preheat energy Mingsheng Liu, PhD, PE Member ASHRAE consumption. During summer, the hot outside air is pre-cooled by the exhaust air to reduce mechanical cooling. Exhaust air heat recovery has been thoroughly investigated (Moyer 1978; Barker 1994; Bard 1994) and has been installed
8、 in most of the facilities. Heat pipe with run-around coil is another energy-efficient measure to reduce thermal energy consumption (Hill and Jeter 1994; Scofield 1993). Two heat exchangers are separately installed in front of and behind the cooling coil. During summer, the heat exchanger in front o
9、f the cooling coil receives heat from the hot outside air, and the other coil behind the cooling coil discharges the heat into the supply airstream. During mild weather and winter, this system consumes exces- sive fan power without providing any thermal energy benefits. In addition, the heat pipe an
10、d run-around coils have relatively high cost. Its applications are limited. VAV and usage-based control (UBC) systems have been used to reduce heating and cooling energy consumption through reduced outside airflow. After ten years of VAV appli- cations in commercial buildings, VAV fume hoods and con
11、trol technologies were developed (Neuman and Rousseau 1986; Doley et al. 1993). The VAV fume hood maintains a constant sash face velocity. When the sash is partially closed, the exhaust air is proportionally reduced. The VAV fume hood reduces the exhaust airflow as much as 60% when the hood sash is
12、closed. When compared with the constant air volume fume hood, the outside air requirement from the AHU is significantly decreased. Consequently, both heating and cool- ing energy are reduced. The potential energy savings were investigated using a theoretical approach (Muny 1983; Davis and Benjamin 1
13、987; Moyer and Dungan 1987; Lentz and Seth 1989) as well as field experiments (Parker et al. 1993; Rabiah Yujie Cui is a PhD candidate and Mingsheng Liu is a professor and graduate study chair in the Department of Architectural Engineering, University of Nebraska-Lincoln. 02005 ASHRAE. 113 liil i- l
14、 ll Figure I Schematic diagram of the LAHU and Welkenbach 1993). A case study showed that VAV fume hoods used an average of 40% less outside air in a chemistry building (Bard 1995). UBC devices detect the presence of fume hood operators. When an operator is not present, the UBC reduces the fume hood
15、 face velocity to a lower value. When an operator is present, the UBC increases the face velocity to a normal oper- ation value. More and more new facilities are designed with the UBC although the cost is still significant. A single system that serves both the laboratory and office sections can sign
16、ificantly improve the buildings thermal energy and IAQ performance. The single system draws return air from the office section and provides supply air to both ofice and laboratory section at the same temperature. It reduces the total building outside air intake to the minimum and has a higher outsid
17、e air intake ratio to the office section. Charneux (2001) demonstrated that the single system increased the outside air intake ratio to 35% in one industry case and has lower initial cost as well. Since the laboratory section often requires a higher supply air temperature than the ofice section, the
18、 single system has to choose the lower value of the office and laboratory section supply air temperature. Consequently, a significant amount of thermal energy is wasted for reheat. The single system cannot take full advan- tages of the free cooling andior economizer opportunities. To further improve
19、 energy performance in laboratory buildings, an integrated AHU system is proposed. Because this system is specifically developed for laboratory buildings, it is called the “laboratory air-handling unit” (LAHU) system. This paper compares the energy performance and IAQ impacts of the LAHU system with
20、 a conventional separated AHU system. LAHU SYSTEM Figure 1 shows a schematic diagram of the LAHU system. The LAHU system serves both the office and labora- tory sections with two supply air fans (SF1 and SF2), one return air fan (RF), and fume hood exhaust fan(s). Supply air fan 1 (SF1) supplies air
21、 to the office section. Supply air fan 2 (SF2) supplies air to the laboratory section. The return fan circulates air from the office section to either supply fan 1 or 2, or both. In addition, return air can be sent either upstream or downstream of cooling coil 2 or in both locations. Supply air fan
22、2 must be a draw-through style when the return air needs to be sent downstream of cooling coil 2. The return air distribution to each supply air fan is modulated using return air dampers 1,2, and 3 and the relief air damper. The outside air intake to the LAHU system is modulated by outside air damp-
23、 ers 1 and 2. Outside air damper 1 is linked with return air damper 1. Heat recovery coils transfer heat between the labo- ratory exhaust airstream and outsideair intake to the LAW system. The LAHU systemresets the supply air temperature to the laboratory section according to the zone thermal load t
24、o mini- mize reheat. The required supply air temperature can be achieved by mixing the return air and the airstream from cool- ing coil 2. A constant discharge air temperature may be used for cooling coil 2 to maintain suitable humidity control when the outside air dew point is higher than the room
25、air required dew point. The LAHU system resets supply air temperature to the office section to minimize both reheat and fan power. If the outside air dew point is higher than the room air design dew point, the discharge air temperature is 12.8”C (55F). Other- wise, the supply air temperature can be
26、reset based on the sensible load. Economizer can be implemented in both the laboratory and office sections. The LAHU system reduces reheat and preheat significantly compared with conventional systems. The LAHU system sends more outside air to the office section. Since the LAHU system reuses the fres
27、h air to the office section, the minimum building outside air requirement equals the sum of the laboratory section exhaust and the office section common exhaust. The LAHU system uses less outside air than the conventional system requirement under both hot and cold weather conditions. Consequently, t
28、he LAHU system uses less cooling and heating energy and improves the office section IAQ. The LAHU system also has lower construction cost than conventional systems due to (1) smaller cooling coil capaci- ties, (2) smaller preheating coil capacities, and (3) smaller reheat coils in the laboratory sec
29、tion if the ofice section AHU has the same schedule as the laboratory section AHU. The LAHU system is unsuitable for laboratory buildings where the laboratory sections do not allow use of return air from the office section. THERMAL ENERGY MODELS Thermal energy savings is defined as the difference in
30、 thermal energy consumption between the base system, consisting of two dedicated AHUs, and the LAHU system. The thermal energy consumption includes cooling, heating (preheat and reheat), and humidification energy, presented as Equations 1 and 2 for the base and LAHU system, respec- tively. 114 ASHRA
31、E Transactions: Research Table 1. Cooling Energy Consumption and Outside Air Intake in the Base System Qc, 1 k1cptm, 1 - c, 119 01 + MiCp(tm, 1 -Tc, 1) Coil Mode Outside Air Condition Dry toll Tc, 1 toa 2 Tc,l Coil - m, min - (1 - min, i), + min, itoa 9 hm, min = ( - min, i )r + Pmin, i aa Qc, 2 Mi
32、n/l, Cf t, 2 - T, 21 0 1 + M2Cf fm, 2 - TC,21 01 + Mr+Me,1+M2 M, + M, 1 + M2 6 = Qc+ Qrh + Qfhu (2) The general thermal energy savings (AQ) can be deter- mined based on the difference of cooling energy consumption (0,-Q,) and the difference of total outside air intake (Mi - hf i ) by Equation 3 (see
33、 Appendix A for details of math- ematical deduction), which provides a simpler mathematical processing. AQ = 2(Qc-Qtc)+ (M;-hfi)(hr-hoa) (3) where Qc = Qc,1+Qc,2 Qc = Qc, 1 + Qc,2 The cooling energy models are developed first for both the base and the LAHU system. Table 1 summarizes the outside air
34、intake control schedule (pi, 2,), cooling energy ( Qc, 1 , Qc, 2 ) models, and the outside airflow (hi ) of the base system. The minimum outside air intake (min,i) is used during summer (when outside air temperature is higher than room air temperature or outside air enthalpy is higher than room air
35、enthalpy) for the office section. The outside air intake is deter- mined based on the economizer principle under other condi- tions. The laboratory uses 100% outside air at all times. The cooling energy is calculated using temperature differences when the outside air humidity ratio is lower than the
36、 designed room air humidity ratio. This is called dry coil mode. The cool- ing energy is calculatedusing the enthalpy difference when the outside air humidity ratio is higher than the design room air humidity ratio. This is called wet coil mode. Table 2 summarizes the equations for cooling energy co
37、nsumption (o, , Q, 2 ) and outside air intake calculations (hi ) for the LAHU system. Different formulas are used for the dry and wet coil modes. In order to satis total outside airflow requirements, the mixed air temperature may be lower than the cold deck temperature when the outside air temperatu
38、re is lower than the cold deck temperature. To prevent negative cooling energy value, a maximum function is used in the cool- ASHRAE Transactions: Research 115 ing energy calculation equation. When the mixed air temper- ature is lower than the cold deck setpoint, the cooling energy consumption is au
39、tomatically assigned to zero. Extra reheat energy is consumed. The outside air intake schedule is not defined here. It is one of the objectives of this research to investigate the impacts of the outside air control on thermal energy savings for the LAHU system. To eliminate the impact of building si
40、ze, the unit energy savings is defined as the ratio of the total energy savings ( AQ ) to the total building supply airflow rate ( Ml + if2 ). Acj = AQ/(h/r, +AlhaiB. Fan TI Equation A-7 and Equation A-8 can be repre- sented as Equation A-9 and Equation A-10 separately. The supply air temperature of
41、 the laboratory section is set at 18.3“C (65F) for the LAHU system. (A-9) The office and laboratory room air condition is assumed to be 23.9“C (75F) and 50% relative humidity. The actual (A-10) , + 0, + Qeh - Sri = &;(hr- boa) room relative humidity conditions in the laboratory section may be slight
42、ly higher (2% to 4%) depending on the return air ratio dufing: smer when the LAHU system is used. It is also Substituting Equation A-9 and Equation A-10 into Equa- is deduced tion A-6, the potential thermal energy savings u as Equation A- 1 1. assumed that common exhaust is 3% of the office supply a
43、irflow and the minimum outside air intake for IAQ require- AQ = 2(Qc-Qc)+(Mi-Itr;)(hr-h0,) (A-11) ment is 10%. ASHRAE Transactions: Research 121 DISCUSSION E.M. swan”, Vice president, MDC Systems, B, pa.: The system includes a bypass ofretum air at the suction side “bypass” pretreatment heating and
44、cooling coils for the and a possible shortcoming of air at the bypass, resulting in a “drift” of conditions. Yujie Cui: The links are required to ensure coil freeze protec- tion and humidity control during the summer. The supply air temperature from the cooling coil must be maintained at 55F control
45、 is the same as any other dampers in AHUs. The stabil- rience. of the LAHU supply fan. This bypass allows operation to to ensure humidity control dun% the smer. The damper iq primarily depends On the control loop based on ou expe- LAHU. Concern is raised over the stabiliq of airflow control 122 ASHFIAE Transactions: Research
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