1、4840 Study on the Applicability of Combining a Desiccant Cooling System with a Heat Pump in a Hot and Humid Climate Yaw-Shyan Tsay Makoto Koganei Shinsuke Kato, PhD Member ASHRAE Ryozo Ooka Member ASHRAE Norio Shoda ABSTRACT In hot and humid climates, dampness in buildings leads to significant respi
2、ratory symptoms and damage to buildings. A damp house can be recognized by one or more of the following characteristics: damp stains, mold growth, condensation, a musty smell orstufi atmosphere, and insects such as silverjsh and sow bugs. To improve this problem, a desiccant cooling system has been
3、suggested as a suitable system to improve indoor air quality (IAQ) by its superiorperformance of humid- ity control. However, this system has a defect in its lower energy eficiency, so combining it with a power generation system, such as a cogeneration system, is a common solution to improve energy
4、eficiency. In this papel; the authors study the applicability of combining a desiccant cooling system with a heat pump through experiments in hot and humid climatic conditions andpropose a new HVACsystem for the nextgener- ation. INTRODUCTION Many indoor air pollutants have been extensively studied
5、during the past decade. In recent years, issues about the health problems related to dampness in buildings were widely reported in Europe and America. Dampness in buildings leads to significant respiratory symptoms and damage of buildings (Andrade et al. 1999; Haverinen et al. 2001). A damp house ca
6、n be recognized by one or more of the following character- istics: damp stains, mold growth, condensation, a musty smell or stufsr atmosphere, and insects such as silverfish and sow bugs (van Wageningen et al. 1992). Using a desiccant air-cooling system can realize a zero condensate environment (no
7、condensation occurs indoors or inside the air cooling system) to improve IAQ. But the coef- ficient of performance (COP) of a common desiccant cooling system is almost lower than 1. In this paper, a new desiccant air-cooling system is proposed to improve IAQ and reduce energy consumption. In this sy
8、stem, exhaust heat from the condenser of a heat pump is proposed to be the regeneration heat source of the desiccant dehumidifier to improve energy efficiency. DEVELOPMENT OF DESICCANT COOLING SYSTEM A conventional air-conditioning system dehumidifies air by condensation of water vapor on the evapor
9、ator coil. However, this kind of dehumidification method is often related to the growth of fungi on the drain (Abe 1996) and with dampness problems that get worse. Since Pennington (1955) obtained the first patent on a desiccant cooling system in 1955 and Dunkle (1 965) constructed a solid desiccant
10、 dehumidifi- cation and cooling rotary wheel in 1965, a large number of studies on desiccant dehumidifiers have been accomplished. Such systems were used widely in certain facilities and build- ings. EXPERIMENTAL FACILITY AND SYSTEM SETUP Figure 1 shows a photo of the test system, and the sche- mati
11、c diagram is shown in Figure 2. The experiments were carried out in an experimental building located in Ciba City, Japan. The test room has a floor area of 16.7 m2 and volume of 38.4 m3 and is well insulated toward the ground. The desic- cant cooling system is located near the room and connected to
12、the room by ducts. Exhaust heat produced by the condenser of a heat pump was proposed as the heat source of the regenera- tion side. However, at this early stage, an electric heater is used Y.S. Tsay is a PhD student, S. Kat0 is a professor, and R. Ooka is an associate professor at The University of
13、 Tokyo, Japan. M. Koganei is chief researcher and N. Shoda is a researcher in the Research and Development Center, AsahiKogyosha Co. Ltd., Chiba, Japan. 02006 ASHRAE. 189 Sensible Heat Exchanger Desiccant Rotor Q Humidity Sensor Figure 1 System picture. Desiccant Rotor n Cooling Cod n O Wind Velocit
14、y Sensor Figure 3 System flow and measurement points of case 2. for simulation. Thermocouples, humidity sensors, and wind velocity sensors, as shown in Figure 2, are used to measure the temperature, humidity, and wind velocity of different posi- tions of the desiccant air-cooling system. Thermocoupl
15、es having a radius of O. 1 mm are used for the measurement, and the inference of radiation emitted by the electric heater is considered to be very small because the surface area of ther- mocouples is extremely small. Case 1 was carried out in early summer (May 2004) and Case 2 was carried out in the
16、 middle of summer (July 2004); the outdoor condition and system setting are shown in Table 1. A heat load of 1200 W and moisture load of 1340 g/h were set inside the room to simulate the indoor load. The purpose of case 1 was to study the system perfor- mance under early summer climatic conditions a
17、nd to study the effect of changing the regeneration temperature from about 40C to 70C to satisfy the zero condensate premise. (The regeneration temperature is the average temperature measured by nine thermocouples set after the heater of the regeneration side.) Figure 2 System flow and measurement p
18、oints of case 1. Test Room Sensible Heat Exchanger Desiccant Rotor -1 Air Cooler wl Figure 4 Systemflow of an ideal system that combines heat pump and desiccant air-cooling system. The purpose of case 2 was to study the system perfor- mance under mid-summer climate and to simulate an ideal system of
19、 combined heat pump and desiccant cooling system. In this case, a constant regeneration temperature was supplied to simulate the exhaust heat of the condenser of the heat pump, and sensible heat recovery was not used in this case, as shown in Figure 3. This is because when a heat pump is coupled wit
20、h a desiccant dehumidification system, as shown in Figure 4, sensible heat recovery would reduce the heat pumps COP. RESULT OF EXPERIMENT Figures 5 and 6 show the variation of temperature and absolute humidity of cases 1 and 2, respectively. In case 1, indoor temperature varies in the range of 24“C-
21、26“C, and indoor absolute humidity changes from 10 to 14 gikg in differ- ent regeneration temperature situations. A psychrometric chart showing the conditions of air at the position inside the desiccant cooling system and test room is shown in Figure 7, and the values of temperature, relative humidi
22、ty, and mass 190 ASHRAE Transactions: Research Table 1. Outdoor Conditions and System Setting o- v 3 20 9 C: 15 Condition Y - 17 i c I m c 0.011 0.010 0.m9 0.008 0.007 0.006 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59 61 63 65 67 69 71 Dry Bulb Temperature (“c) F
23、igure 7 Flow chart andpsychrometric chart of case 1 (RT = regeneration temperature). flow at each point of the system are listed in Table 2. In this experiment, the highest relative humidity appeared at the air outlet of the cooling coil (point 4) and is under 70% when the regeneration temperature i
24、ncreases to 66.9“C, as shown in Table 2. However, condensation occurs when the regeneration ttemperature decreases to 42.9“C (Figure 7, humidity differ- ence between points 3 and 4). In case 2, indoor temperature changes with outdoor temperature in the range of 25“C-27“C, and the absolute humidity d
25、ifference between the outdoor and indoor atmo- sphere remains at about 7 gllcg during the experiment when a constant regeneration temperature is provided. Table 3 shows the cumulative time and ratio at point 4 of Figure 7 (the air outlet of the cooling coil) in different relative humidities. The cum
26、ulative ratio of relative humidity over 70% is under 30%, which keeps the risk of mold growth at a low level (Yanagi et al. 2004). Therefore, using arelatively low regeneration temperature (6O0C-70“C) could satis6 the zero condensate premise and provide a good IAQ condition without mold pollution. E
27、VALUATION OF SYSTEM PERFORMANCE The thermal COP (heat source based) is calculated to evaluate the system performance. The energy consumption of 192 ASHRAE Transactions: Research Table 2. Temperature, Humidity, and Mass Flow of Case 1 Temperature (above, OC) and Relative Humidity (below, YO) of the A
28、ir at the Position in Figure 7 Regeneration WS WR Temperature (kglh) (kg/h) 1 2 3 4 5 6 7 8 9 10 28.6 36.1 29.5 19.4 19.5 24.4 25.2 32.2 42.5 34.1 64.5 34.4 49.4 87.6 87.0 66.4 63.4 42.2 24.1 47.8 29.2 39.2 29.5 19.3 19.3 24.2 25.8 34.9 51.5 38.3 62.4 26.3 44.4 82.0 82.0 64.3 58.3 34.6 14.6 40.8 42.
29、9“C 5 1.3“C 1288 1220 59.8“C 28.9 43.3 30.2 18.8 18.9 23.9 25.8 37.5 59.7 42.1 63.4 18.2 36.9 72.8 75.0 57.5 51.3 26.5 8.8 34.3 28.8 45.8 31.4 19.0 19.1 23.5 24.4 40.1 67.3 46.5 63.7 14.6 30.7 63.9 63.0 53.7 50.7 20.9 5.8 27.8 Table 3. Cumulative Time of Relative Humidity at Point 4 of Case 2 Relati
30、ve Humidity at Cumulative Time Cumulative Ratio Point 4 h) WO) Over 80% 4.8 Over 75% 12.2 Over 70% 20.3 6.5 16.8 27.9 the transportation system, such as that used by fans and pumps, is ignored in the calculations below. Heat removal from the test room (internal load) can be calculated by Wdh6 - h5),
31、 and heat removal from outside air (cooling load due to ventilation) can be calculated by Wdh 1 - h6), where W, (g/s) and WR (g/s) are the mass airflow rates of the supply side and return side, h (J/g) is the enthalpy of the air, and the subscript number is the place of the desiccant cooling system
32、(Figure 7). Therefore, system cooling output can be calculated by Wdh 1 - h5), the sum of heat removal from the test room and ventilation. The system heat input can be calculated by the regenera- tion heating power of the electric heater WR(h9 - h8) and cool- ing power of the cooling coil WR(h3 - h4
33、) to evaluate the system performance. Therefore, the COP based on heat and cooling input (copheat) can be calculated by Equation 1. ws x (h 1 - h 5) Opheat = W,(h9 - h8) + Ws(h3 - h4) (1) However, heating power and cooling power are provided by the electric heater and heat pump, so the heating and c
34、ool- ing energy input should be considered to evaluate the system performance. Figure 8 shows the variation of cooling energy consumption of the heat pump and heating energy consump- tion ofthe electric heater in case 2. Heating power is almost the same as heating energy input, and the difference be
35、tween heat- E l4 I , Heating energy input , Heating power of heater w o I Jul-30 11:OO Jul-31 11 100 Aw-O1 11 100 Ag-02 11 100 Figure 8 Energy input and heating and cooling power of the system. ing power and heating energy input-about 2%-5% of heating energy input-is considered to be the heat loss o
36、f the electric heater. On the other hand, cooling power is about 3.1-3.3 times the cooling energy input and shows a normal perfor- mance of an air-cooling-type chiller. The heating power is about 1.2-1.7 times the cooling power, which shows the appli- cability of the system from the viewpoint of hea
37、ting and cool- ing characteristics of a heat pump. The electricity consumption (energy input) of the heater Eheater (w) and cooling coiiEhp (w) is used to calculate system performance, COPelectric (the COP based on heat and cooling energy input) in Equation 2. However, if the heat supplied from the
38、condenser of the heat pump can be used to completely substitute for the electric heater for regeneration, the energy consumption of the heater can be saved. According to this assumption, system perfor- ASHRAE Transactions: Research 193 mance of this ideal system (COPdesiccant), as shown in Figure 4,
39、 can be calculated by Equation 3. (3) In case 1, the values of Copheat are between 0.67 and 0.8, as shown in Table 4, which are reasonable values when compared with a usual two-rotor (desiccant rotor and sensible heat exchange rotor) type desiccant cooling system. copheat increases when regeneration
40、 temperature decreases, with decreasing heat loss of the desiccant rotor. However, the lowest limit of the regeneration temperature (5 1.3“C) should be ensured to prevent condensation in the system. In case 2, the COP based on cooling and heating power (copheat) shows a value of 0.43 (Table 5), and
41、the COP based on energy consumption (COPe,ect,.ic) shows a value of 0.6. The system performance is a little lower than a normal two-rotor- type desiccant cooling system, but when the heat supplied from the condenser of the heat pump can be used to completely substitute for the electric heater for re
42、generation, the COP (COPdesiccant) can be calculated as 2.32, projecting a very substantial increase in energy efficiency. 3. At the air outlet of the cooling coil, the cumulative ratio of relative humidity over 70% is under 30%, which keeps the risk of mold growth at a low level. Therefore, using t
43、he rela- tively low regeneration temperature (6O“C-7O0C) could satis the zero condensate premise and provide a good IAQ condition without mold pollution. 4. The relatively low regeneration temperature provided by exhaust heat of the condenser of a heat pump can improve the energy efficiency of a sys
44、tem. On typical summer days (case 2) in Tokyo, the system COP (C0Pdesi,J is esti- mated to be 2.32 when assuming that the COP of the heat pump is about 3.1-3.3. ACKNOWLEDGMENTS A part of this study was supported by the Tostem Foun- dation for construction materiais industry promotion, and the author
45、s would like to thank Mr. Koichi Kawamoto for his detailed information about the desiccant cooling system, Dr. Kosaku Nishida for his advice regarding heat pumps, and Ms. Naomi Nakamura and Mr. Kazuhiro Ishimaru for their coop- eration during the experiments. REFERENCES CONCLUSIONS The condition of
46、a zero condensate system is studied through experiments under hot and humid climatic condi- tions. Increasing regeneration temperature could effectively reduce the relative humidity of both the air outlet of the cooling coil and the indoor environment. Using a relatively low regeneration temperature
47、 under 70C could satis the thermal comfort of the indoor environment and zero condensate conditions under the early summer climate. Table 4. System Performance of Case 1 Regeneration Temperature COP, of Case1 42.9“C 51.3“C 593C 66 9oc 0.80 0.79 0.75 n 67 Table 5. System Performance of Case 2 0.43 0.
48、60 2.32 Andrade, C., et al. 1999. Relative humidity in the interior of concrete exposed to natural and artificial weathering. Cement and Concrete Research 29( 1999-8): 1249-1259. Abe, K. 1996. Evaluation of fungal growth at the air outlet and inlet of air conditioners. Indoor Air 96 3:185-190. Penni
49、ngton, N.A. 1955. Humidity changer for air condition, van Wageningen, N., et ai. 1992. Health complaints and indoor molds in relation to moisture problems in homes. Desiccant Cooling and Dehumidijication, pp. 16-20. Atlanta: American Society of Heating, Refrigerating and Air-conditioning, Inc. Dunkle, R.V. 1965. A method of solar air condition. Mech. Haverinen, U., et al. 2001. Modeling moisture damage and its association with occupant health symptoms. Pro- ceedings of IAQ 2001, Moisture, Microbes, and Heath Efects: indoor Air Quality and Moisture in Buildings Conference. Atl
