Parametric study, Space cooling with outdoor air, Simulation, Centralized HVAC system, Weather conditions

Transcription

1 A FEASIBILITY STUDY ON SPACE COOLING WITH OUTDOOR AIR USING A DYNAMIC ENERGY SIMULATION PROGRAM PART 1: PARAMETRIC STUDY ON FEASIBILITY OF SPACE COOLING WITH OUTDOOR AIR ON SEVERAL WEATHER CONDITIONS Shin-ichi Matsumoto 1, Motonori Futamura 2, Hiroshi Kobayashi 3, Kahori Genjo 1, Yasuo Utsumi 3, and Ken-ichi Hasegawa 1 1 Department of Architecture & Environment System, Akita Prefectural University, Yurihonjo, Japan 2 Yamatake Corporation, Fujisawa, Japan 3 Department of Architecture, Miyagi National College of Technology, Natori, Japan ABSTRACT As a part of our research on energy conservation and thermal comfort of detached houses having centralized HVAC systems, study with numerical simulation techniques was started. The model is the house that has mechanical supply and exhaust ventilation systems with a centralized air duct equipped with a sensible heat exchanger and has large return air (4 times an hour). The parameters in the simulation are the set point temperature and outdoor air flow rate. The authors have already showed energy calculation results for Tokyo, and it has been discussed that the utilization of fresh outdoor air as a cooling source during the intermediate season save a few percent of the annual heating/cooling loads (Matsumoto et al. 23). However, whether this finding is sure except for Tokyo is not clear. This paper discusses the generality of this finding by showing the results of the similar energy calculations using several weather data differing from that of Tokyo. As a result, it would be rash to conclude that the effect of space cooling with outdoor air is small without considering of building conditions, for example, in case of the building with high heat storage performance, large solar heat gain and large internal heat load during intermediate season. KEYWORDS Parametric study, Space cooling with outdoor air, Simulation, Centralized HVAC system, Weather conditions INTRODUCTION The authors have examined the energy saving application method of the centralized HVAC system with a centralized air duct for the detached houses with the numerical simulation. Outline of the simulation program and the results of the feasibility study on the outdoor air cooling with the weather data of Tokyo have already been reported. According to the results, the effect of the outdoor air cooling during intermediate season was found to reduce the annual thermal load by only 2 or 3 %, and found to be small compared to the energy saving effect by control of the setting temperature level for each season. However, it would be rash to conclude that the effect of the outdoor air cooling is small. The reasons of this are as follows: 1) we have just examined the weather data of Tokyo, and we have not examined the possibility that the quantitative effect will differ by weather condition of each city. 2) it is considered that the effect of outdoor air cooling will be relatively larger depending on the building conditions with different thermal performance, internal heat load and solar shading. Then, this paper discusses and confirms the generality of this finding by showing the results of the similar energy calculations using several weather data differing from that of Tokyo. Corresponding Author: Tel: , Fax: address: matsu@akita-pu.ac.jp

2 ANALYSIS OF OUTDOOR AIR COOLING HOUR POTENTIAL BASED ON WEATHER DATA Method of the analysis The outdoor air cooling hour potential is defined as shown in the Figure 1. Average value for each point is calculated from the data over 15 years with the expanded AMeDAS weather data (Akasaka et al. 23), and the distribution for the each average value is examined. Results and discussions The average of outdoor air cooling potential was 35.% and ranged from 23.3% to 53.1%. Figure 1 shows the results of the analysis as the map. Generally, the outdoor air cooling potential was found to be large around the coast and small around the inland, in addition, it was found to be large at the point located over low latitude. In this study, based on the climatic zones of the Japanese residential standard, Sendai and Nagano were selected from Zone III, Nagoya, Osaka, Fukuoka were selected from Zone IV in addition to Tokyo. This index suggests that the energy saving effect of outdoor air cooling for each city is smaller than that for Tokyo. SIMULATION OF ANNUAL HEAT LOAD ON SEVERAL WEATHER CONDITIONS Purpose of the simulation The main purpose of the simulation is to clarify the energy saving effect of outdoor air cooling during the intermediate season under various weather conditions. The energy saving effect of the setting temperature level that differs with each season as well as the weather conditions is investigated. The simulation results of annual heat load, cooling load during intermediate season are mainly compared. Methods and conditions of simulation The shape and wall structure of the model are almost same as the wooden detached house model of AIJ standard problem (Udagawa 15). The heat insulation of wall, floor and ceiling and the kind and the thickness of window are set as Table 1 according to the weather conditions because of assumption of thermal performance as to the purpose of the simulation. Airtightness performance is constant in this simulation, and the simulation is conducted for the model with the distribution of the cracks that are 55 Outdoor air cooling hour potential 5 outdoor air cooling possible hours 45 (annual hours complete with conditions 1)-3)) = annual hours (87h) 4 1)14 <daily average temperature< ) <hourly average temperature<26 3 3)hourly average humidity ratio<12g/kg' 25 2 (%) Fukuoka (42.5%) Nagano (34.3%) Nagoya (38.6%) Osaka (41.9%) Sendai (38.%) Tokyo (43.7%) Figure 1. Distribution of the outdoor air cooling hour potential and the calculation point

3 Table 1 Setting of the building model for each weather condition Weather conditions Heat insulation level Details of window Heat loss (Climatic zone) Ceiling Wall & Floor coefficient Sendai (Zone III) Low-E double glazing Nagano (Zone III) (G3-A6-G3) 1.82 W/m 2 K Glass wool Glass wool K=3.49, τ=.44*, a=.14 Tokyo (Zone IV) K 16K Aluminum double glazing Nagoya (Zone IV) 2 mm mm (G3-A6-G3) Osaka (Zone IV) K=4.65, τ=.49*, a= W/m 2 K Fukuoka (Zone IV) K: heat transfer coefficient (W/m 2 K), τ: solar transmittance (-), a: solar absorptance (-) Equivalent leakage area 2 cm 2 /m 2 Table 2 Calculation conditions excepting weather conditions Parameters Level Winter Intermediate season Summer Setting temperature level Energy saving Normal Rich Outdoor air cooling Off (outdoor airflow rate) On - 2 ach at space cooling with outdoor air - displaced with the point openings. The living schedule is identical to the one of the standard problem. The weather conditions of six points shown in Figure 1 are used. The calculation conditions excepting weather conditions are displayed in Table 2. Winter and summer are defined as the days that the daily averaged temperature is under 14 and over 26, respectively. Intermediate season is defined as the days except for winter and summer. The outdoor air cooling mode is operated during intermediate season when the conditions 2) and 3) given in Figure 1 are complete. In this paper, the heat load means the building load. The simulation program and the setting of the air conditioning system model are same as the previous paper. RESULTS OF THE SIMULATION The effect of the setting temperature level on the annual heat load Figure 2 shows the results of the annual heat loads. The annual heat loads were found to vary widely with the setting temperature level. The annual heat loads for rich and energy saving ranged -134% and 72-77% of those for normal, respectively. These ratios vary small, unexpectedly. The effect of outdoor air cooling operation on the annual heat load By comparison of the simulation results of the same weather condition and setting temperature level in the case on with those in the case off, the outdoor air cooling operation was found to reduce the annual heat load by only 1-3 % and the energy saving effect was smaller than the setting temperature level. This finding was in accordance with the simulation result of Tokyo in the previous paper, the same tendency was seen for the other five points except for Tokyo. The saving effect of the annual heat load is expected to be the smallest in Nagano of the six points from Figure 1. In this point, Figure 2 presented the results as expected, but the difference with each point was small. Simulation under the extreme weather conditions was not conducted this time, therefore the saving effect of the annual heat load by the outdoor air cooling method can be said to be several percentage, and it can not be widely different by the weather conditions. However, it is needed to confirm the saving effect under the different building conditions such as thermal performance, internal heat load and solar shading.

4 % Cooling (GJ/yr) Off On Off On Off On Off On Off On Off On Outdoor air cooling Sendai Nagano Tokyo Nagoya Osaka Fukuoka Weather condition (4.6) (47.2) (38.4) (44.3) (41.1) (42.2) the value within round bracket: annual thermal load in case of normal SP and outdoor air cooling off (GJ/yr) the value at the side of a bar: relative value in case of a value within round bracket as Heating Annual thermal load Rich Normal Energy saving Setting temperature level Figure 2. Comparison among annual heat loads GJ GJ GJ GJ (GJ/yr) % GJ 9.1 Off On Off On Off On Off On Off On Off On Sendai Nagano Tokyo Nagoya Osaka Fukuoka the value at the top of a bar: cooling load during intermediate season in case of outdoor air cooling off (GJ/yr) the value at the side of a bar: relative value in case of outdoor air cooling on against that in case of outdoor air cooling off as.1.2gj Rich Normal Energy saving Cooling loads during intermediate season Setting temperature level Outdoor air cooling Weather condition Figure 3. Comparison of the cooling loads during intermediate season The effect of outdoor air cooling operation on the cooling load during intermediate season Figure 3 presents the comparison of the cooling loads during intermediate season. From Figure 3, the energy saving effect of outdoor air cooling method on the cooling loads during intermediate season was clearly found to be 7-11% in the setting temperature level of rich and normal, and -16% in that of energy saving, respectively. Therefore, another way of saying this would be that the definite energy saving effect of the outdoor air cooling method shown in Figure 3 is behind with the large heat and cooling loads of summer and winter when the annual heat load is used as the index of the comparison. It can be said that the outdoor air cooling is the effective method in the following cases: the case that the solar heat gain during intermediate season is more than that of summer due to the relation between shapes of the eaves and solar altitude in the building with high heat storage performance, or the case that the space cooling is the main air-conditioning operation for the removal of relatively large internal heat load during intermediate season.

5 The effect of outdoor air cooling operation on the outdoor air cooling hours The percentage of the operation hours when outdoor air cooling is conducted to the annual 87 h was calculated as the outdoor air cooling hour rate in the case of the setting temperature level normal, and then it was plotted with outdoor air cooling hour potential mentioned above as the horizontal axes, illustrated in Figure 4. There was found to be positive linear correlation between the outdoor air cooling hour potential and the outdoor air cooling hour rate, and the outdoor air cooling hour rate was expected to be -17% (-14 h: about two months) if the extrapolation of the regression line permits. The ratio within round bracket in Figure 4 means that how long the cooling hours during intermediate season in case of outdoor air cooling off are reduced. As seen in Figure 4, the cooling hours were able to reduce by 4% in Tokyo, Osaka and Fukuoka, and % in Sendai and Nagano. The effect of outdoor air cooling operation on the peak cooling load during intermediate season Figure 5 presents the examples of the cumulative relative frequency distribution for the cooling load in Nagano and Tokyo. The outdoor air cooling is set to be operated during intermediate season, therefore the hourly cooling load in summer (cooling season) is not effected by its operation as seen in Figure 5. It was found that the cooling load during intermediate season differ by the outdoor air cooling operation on or off and the outdoor air cooling operation have the effect to reduce hourly cooling load. However, the saving effect of the hourly cooling load was found to be large only when it is small and there would be not so much difference near the peaks of it. On the other hand, the saving effect of the outdoor air cooling on the cooling load was about 3 % in both Nagano and Tokyo in terms of the cooling load of excessive hazard rate 2.5 %. This is probably because that outdoor air available for space cooling is generated mainly at night. Validity to this supposition was able to be confirmed in Outdoor air cooling possible hours (h) the value within round bracket: the ratio of the cooling hours in case of outdoor air cooling on to the cooling hours during Fukuoka (39.7%) intermediate season in case of Osaka outdoor air cooling off (36.7%) Nagoya Tokyo (45.4%) (39.8%) y =.52 x (R 2 =.897) Nagano (58.2%) Sendai (57.2%) Outdoor air cooling hour potential, x (%) Figure 4. Relationship between the weather condition and the outdoor air cooling hours Outdoor air cooling hour rate, y (%) Outdoor air cooling hours (h) Cumulative relative frequency (%) 5 (a) Nagano (Normal setting temperature level) Intermediate season broken line: outdoor air cooling off Summer (Cooling season) outdoor air cooling on : kW outdoor air cooling off :5.273kW Summer(Cooling season): 5.919kW the line for excessive hazard rate 2.5% (b) Tokyo (Normal setting temperature level ) Intermediate season broken line: outdoor air cooling off Summer (Cooling season) 5 5 Hourly cooling load (kw) Hourly cooling load (kw) Figure 5. Examples of the cumulative relative frequency distribution for the cooling load outdoor air cooling on : 4.963kW outdoor air cooling o ff :5.137kW Summer(Cooling season): 5.1kW

6 Cumulative frequency(%) Hourly heat load (kw) 8 7 (a) Tokyo, Normal setting temperature level, outdoor air cooling off. 7.5 Cumulative frequency(%) Hourly heat load (kw) (b) Tokyo, Normal setting temperature level, outdoor air cooling on Hour (h) Hour(h) Figure 6. Examples of the cumulative relative frequency distribution over % for the cooling load per hour during intermediate season Figure 6, that is to say, the main difference of Figure 6(a) and Figure 6(b) was the wideness of the bottom of a ravine seen after 19: and it was narrow in the case of the outdoor air cooling on. CONCLUSIONS This paper has investigated the energy saving effect of the outdoor air cooling method as the energy saving method of the centralized HVAC system for the detached houses with numerical simulation as changing of the parameters such as the weather conditions. The conclusions are as followed. 1) Building thermal load show a dynamic change with the setting temperature level. 2) The space cooling with outdoor air saves only three percent of the annual air-conditioning loads, similarly the peak air-conditioning loads during intermediate season. 3) It is difficult to consider that the above-mentioned two findings occur an exception because of different weather conditions. 4) The space cooling loads during intermediate season are decreased largely using space cooling with outdoor air. 5) It would be rash to conclude that the effect of space cooling with outdoor air is small without any considering of building conditions, for example, in case of the building with high heat storage performance, large solar heat gain and large internal heat load during intermediate season. Systematic parametric study based on the above finding 5) would be developed in the future. REFERENCES 1. S. Matsumoto et al. (23) Analysis of Air-Conditioning Loads for Detached Houses with Centralized HVAC Systems Using d Dynamic Simulation Program TASP++ Part1: Description of the Program and a Feasibility Study on Space Cooling by Fresh Air, Technical Papers pf Annual Meeting The Society of Heating, Air-Conditioning and Sanitary Engineers of Japan, Vol. I, (in Japanese). 2. H. Akasaka et al. (23) Expanded AMEDAS Weather Data, published by Maruzen. 3. M. Udagawa (15) The proposal of standard problem-residential standard problem, The 15th thermal symposium, Architectural Institute of Japan, (in Japanese).