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1 Energy and Buildings 46 (12) 3 13 Contents lists available at SciVerse ScienceDirect Energy and Buildings j ourna l ho me p age: Determining operation schedules of heat recovery ventilators for optimum energy savings in high-rise residential buildings Sang-Min Kim a, Ji-Hyun Lee b, Sooyoung Kim c, Hyeun Jun Moon d,, Jinsoo Cho e a Institute of Technology and Quality Development, Hyundai Engineering and Construction Co., Ltd., Yongin , Republic of Korea b Graduate School of Culture Technology, Korea Advanced Institute of Science and Technology, Daejeon 35-71, Republic of Korea c Department of Housing and Interior Design, Yonsei University, Seoul 1-749, Republic of Korea d Department of Architectural Engineering, Dankook University, Yongin , Republic of Korea e Department of Computer Engineering, Kyungwon University, Seongnam , Republic of Korea a r t i c l e i n f o Keywords: Heat recovery ventilator Optimum operation schedule Energy savings Natural infiltration Heat exchange Residential building a b s t r a c t This study examines the influence of heat recovery ventilators (HRVs) on energy savings in high-rise residential buildings to determine optimum operation schedules. Field measurements were conducted in two actual residential buildings, and computer simulations were performed to predict energy savings by the HRVs. Measurement results showed that energy consumption in each building was reduced when the HRVs were operated in line with recommended ventilation rates and comfortable temperature ranges. The HRVs achieved greater savings of energy during winter than summer. Simulation results showed that the HRVs contributed to the annual savings of heating and cooling energy by 9.45% and 8.8%, respectively, when the ventilators were operated continuously for 24 h. More energy was saved as the operating hours of the HRVs increased. The continuous operation of HRVs was effective for the savings of energy and to maintain recommended ventilation rates. The HRVs achieved effective energy savings and maintained necessary ventilation rates in high-rise residential buildings where natural infiltration was minimal, due to tightly sealed building envelopes. This study suggests that the influence of HRVs on the improvement of indoor air quality needs to be examined in conjunction with energy savings by HRVs. 11 Elsevier B.V. All rights reserved. 1. Introduction A building envelope is the physical separator between the interior and the exterior environments of a building that helps to maintain the comfortable indoor environment and to facilitate the micro climate control of the building. The envelopes of high-rise buildings constructed in recent decades in Korea are made of materials with high thermal resistance. These buildings have strong air tightness in order to minimize heat loss and gain through the envelopes. This design contributes to the savings of heating and cooling energy in buildings, but it also causes important ventilation issues by cutting off natural infiltration rates through the envelopes. While the air tightness applied to building envelopes is effective for energy savings, it reduces infiltration rates, and consequently results in the deterioration of indoor air quality. Due to these problems, appropriate alternatives have been applied to solve the problems caused by the tightly sealed Corresponding author. address: hmoon@dankook.ac.kr (H.J. Moon). envelopes of buildings [1 4]. In particular, ventilation systems that assure necessary ventilation rates with energy savings effectively should be adapted to the high-rise buildings and operated properly, since insufficient ventilation rates are critical factors that cause severe dissatisfaction in indoor environments. It is commonly understood that heat recovery ventilators (HRVs) are effective for saving energy and maintaining necessary ventilation rates. The type of heat recovery ventilators that reuse the heat ejected from indoor spaces have been effectively utilized in high-rise buildings in countries in Asia and Europe [5,6]. A variety of studies have been conducted to examine the influence of heat recovery systems on building energy performance [7 12]. These studies have proved that the application of heat recovery ventilators conserves energy for heating, but that more energy for cooling is necessary to handle particular outdoor conditions in summer. Other studies have shown that heat recovery ventilators that are capable of exchanging latent and sensible heat have successfully reduced heating and cooling energy together [13]. However, the operation of heat recovery ventilators is ineffective when the outdoor enthalpy is lower than the enthalpy of indoor air, while outdoor humidity is higher than that of the air supplied to the conditioned space [9,14] /$ see front matter 11 Elsevier B.V. All rights reserved. doi:1.116/j.enbuild

2 4 S.-M. Kim et al. / Energy and Buildings 46 (12) 3 13 In general, residents control heat recovery ventilators individually, based on their preferences for thermal needs, which vary unpredictably. Continuous operation may be satisfactory most of the time, but heat recovery ventilators are operated only during select hours when residents are at home. Previous studies have focused primarily on energy savings by heat recovery ventilators. Less attention has been paid to the operation strategies that optimally utilize heat recovery ventilators to achieve effective energy savings as well as satisfy the thermal needs of residents. The contribution of heat recovery ventilators to energy savings should be studied according to variable schedules that control when heat recovery ventilators are used in high-rise residential buildings. Therefore, this study examines the influence of heat recovery ventilators on energy savings under various control schemes in high-rise residential buildings to propose optimum operation schedules for maximum energy savings. Field measurements were conducted under four settings of heat recovery ventilators in two high-rise residential buildings. Computer simulations were performed to validate the results of the field measurements and to determine optimum control schedules for the heat recovery ventilators. 2. Research methods 2.1. Field measurements The buildings used for field measurements in this study are constructed of steel reinforced concrete and located in Seoul, Korea (latitude: N, longitude: E). Field measurements performed during the summer period were taken in a building with 69 floors located in Mokdong, which is in the western part of Seoul (building A). The building used for field measurements performed during winter has 46 floors, and is located in Seochodong, which is in the southern part of Seoul (building B). Two identical heat ventilator units on the 39th and 4th floors (unit numbers 393 and 43) in building A were used for the measurements. For building B, two identical units on the 1th and 11th floors (unit numbers 13 and 113) were used for the measurements. Views and floor plans of each of the buildings used for field measurements in this study are shown in Figs. 1 and 2. The floor areas of buildings A and B are 7 and 217 m 2, respectively. The window to wall ratio of building A is 43%, and the heat transfer coefficient of wall and window is 2.74 and 3.4 W/m 2 K, respectively. For building B, the heat transfer coefficient of wall and window is 2.65 and 3.34 W/m 2 K, respectively. The ratio of window to wall on its envelope is 41%. No neighboring buildings exist close to the faç ades of the two residential buildings, and the buildings are free from the effects of shadows from nearby buildings or any other structures. No particular shading devices had been installed on the glazed areas of the building envelopes. For the purposes of this research, the heat ventilator units were prepared for residential use, and furniture such as cabinets and bookshelves were placed in the living rooms and kitchens. The floor was furnished with flooring, specifically, linoleum on top of the Ondol, which is a radiant floor heating system widely utilized in residential buildings in Korea. In order to keep temperatures comfortable for residential use in the summer, individual air-conditioning systems were installed on the ceilings of the residential units. The air supply to each room of the residential units was provided by air-conditioning systems. After the initial supply of air, the circulated air in the rooms was mechanically sent outdoors by a centralized ventilation system. For heating during winter, a district heating system was applied to the Ondol to keep indoor temperatures within comfortable ranges. Sensible and total heat exchange types of heat recovery ventilators were installed in each of the identical units in the buildings. The heat recovery ventilator installed in building A was capable of exchanging sensible and latent heat. The mean exchange efficiency of sensible and latent heat was 39.3% and 62.5%, respectively. For building B, a heat recovery ventilator that was able to exchange sensible heat was installed. The mean exchange efficiency of sensible heat was 55%. In order to supply air from the outdoors to each room using the ventilator, air supply diffusers were installed in the bedrooms and living rooms. Diffusers for returning air were placed in the living rooms, kitchens, and dining rooms of both residential units. The diffusers were connected to the heat recovery ventilators through ducts, so that heat exchange occurred between the air exhausted from inside and the air supplied from outside. The schematic layout of diffusers connected to the ventilators is shown in Figs. 1 and 2. The heat recovery ventilators were regulated to supply recommended ventilation rates for residential use. The control settings assigned to each of the ventilators installed in the units are summarized in Table 1. In order to investigate the effects of heat recovery ventilators on energy savings, and to determine optimum operation schedules in residential buildings, the heat recovery ventilators placed in each of the units in buildings A and B were operated according to the operation conditions summarized in Table 1. In Case 1, both supplied air indoors and returned air outdoors, while the indoor and outdoor air passed through the heat recovery ventilator and exchanged heat. When this study s field Fig. 1. Floor plan (Building A ).

3 S.-M. Kim et al. / Energy and Buildings 46 (12) Fig. 2. Floor plan (Building B ). measurements were performed, the ventilation rate was set at.5 air change rate per hour (ACH) in order for the heat recovery ventilators to meet the national building codes of Korea effective in 3 [15]. The set up of this study s ventilation rates generally met the mandatory requirements for residential buildings. The ventilation guidelines currently in effect require.7 ACH for residential buildings only by mechanical ventilation [16]. Since this study was performed in 3, the ventilation rate established for the field measurements was.5 ACH, based on the ventilation guidelines then in effect. For Case 2, the heat recovery ventilator was operated without the core part where heat exchange occurs. Accordingly, outdoor and indoor air passed through the heat recovery ventilator without exchanging heat. The ventilation rate was set at.5 ACH. In Case 3, the heat recovery ventilator was shut completely off for 24 h, and no indoor or outdoor air passed through the ventilator. Accordingly, infiltration through building envelopes was the only source of ventilation, and no heat exchange occurred in the heat recovery ventilator. For each of these three cases, the indoor temperature was kept at 26 C during the entire period of data monitoring. The heat recovery ventilators used for Cases 4 6 in building B were operated with the same control settings used for Cases 1 3 in building A. For Cases 4 6, however, the indoor temperature was kept at 23 C during the monitoring period. To determine ventilation rates for Cases 4 6, two methods were utilized. First, the ventilation rate was determined using the total air volume supplied to each space through the heat recovery ventilator when the heat recovery ventilator was in use. The Table 1 Control Settings for heat recovery ventilators (HRVs). Operation conditions Unit no. Bldg Fan Heat exchanger Case 1 On On 393 A (Mokdong building) Case 2 On Off 43 Case 3 Off Off 43 Case 4 On On 13 B (Seocho building) Case 5 On Off 113 Case 6 Off Off 113 determination of ventilation rates was based on the volume of air monitored at the diffusers for air supply. Second, the gas concentration decay method was used to determine ventilation rates by infiltration and mechanical systems in room 3 and the living rooms of each unit. The gas concentration decay method is known to be an effective method to determine ventilation rates by infiltration and mechanical systems in buildings [17]. To investigate the consumption of energy by cooling and heating when the heat recovery ventilators were applied to the buildings, the total amount of electricity consumed by controllers of the heat recovery ventilators, air-conditioning systems, and fans was monitored. The heating energy consumed by the Ondol was determined according to the input calories of district hot water used for district heating in each building. Data monitoring in building A was performed from June 1 to August 3, 3, and monitoring in building B was performed from January 1 to February 28, Computer simulation The data collected via field measurements is limited because the measurements were only performed in summer and winter. The field data do not cover the remainder of the year. Hence, this study employs computer simulations to validate the results of the field measurements, and to predict energy consumption by heat recovery ventilators during the remaining seasons when field measurements were not conducted. The software TRACE 7 was utilized as the primary simulation tool to determine energy consumption under a variety of operation schedules for heat recovery ventilators. The algorithm of the TRACE 7 analyzes dynamic load calculations to predict building energy loads under various design alternatives, equipments, and system components. TRACE 7 is pre-programmed with common design parameters for construction materials, equipments, base utilities, weather conditions, and scheduling [18]. TRACE 7 has been effectively applied to energy analysis for various buildings since building loads were calculated using the response factor method that explains the effects of heat storage on building envelopes [19,]. In addition, various building energy loads, such as natural infiltration rates, global and diffuse

4 6 S.-M. Kim et al. / Energy and Buildings 46 (12) 3 13 Table 2 Operation schedules for heat recovery ventilators (HRVs). Where O.S, operation schedule; O.H, operation hour; S, sensible heat exchange type of HRV; T, total heat exchange type of HRV. irradiance, and heat gain by lighting systems and occupants, are also considered in the computation algorithms. When this study utilized TRACE 7, the input data for simulations was identical to the boundary conditions for the two buildings that were used for the field measurements. As usual, the general dimensions of each unit, heat transfer coefficients, and thermal resistances of materials applied to the envelopes, as well as the effects of lighting on energy loads were taken into account for data computation. Standard weather data for Seoul, Korea was also used as input data to consider real world situations in simulations [21]. Setting conditions identical to the settings used for the heat recovery ventilators during the field data monitoring periods were applied to the boundary conditions for the simulations. Various operation schedules were established and used for the computer simulations in order to examine the contribution of heat recovery ventilators to energy savings, and to determine optimum operation schedules. The operation schedules established for this study are summarized in Table 2. For the first step, schedules of 6, 9, and 12 h of operation were set up based on the periods of time when heating and cooling were most necessary to maintain comfortable temperature ranges indoors (schedules A, B, C, D, E, and F). For the second step, schedules of 6, 9, and 12 h of operation were set up based on the preferences of residents for typical activities (schedules G, H, I, J, L, and K). These hours of operation were determined based on previous research examining the preferences people have for operation of heat recovery ventilators according to their residential activities [18]. For the third step, the study assumed that heat recovery ventilators were operated for 1 h and subsequently shut off for the next 1 h. This schedule was repeated for 24 h (schedules M and N). Fourth, continuous operation for 24 h was established to examine the effects of continuous operation of heat recovery ventilators (schedules O and P). Finally, the non-operation of heat recovery ventilators (schedule Q) was established as the base case in order to compare energy savings by the various operation schedules of heat recovery ventilators.

5 S.-M. Kim et al. / Energy and Buildings 46 (12) Fig. 3. Amount of supply air to space. Fig. 4. Air change rate per hour in space. 3. Results 3.1. Outdoor temperature, ventilation rates and efficiency of heat recovery ventilators The outdoor dry bulb temperature, which was measured during the data monitoring periods, changed showing typical ranges of temperature variation for summer and winter. Overall, solar altitude effectively impacted the temperature in both seasons. Temperatures remained high during daytime with the existence of solar irradiance, but began to decrease as the sun set every evening. The outdoor dry bulb temperature in summer ranged from 23.4 C to 31.5 C. In the absence of solar irradiance at night, temperatures still remained higher than 26 C, and in some cases, exceeded 29.6 C. This variation means that cooling systems should be operated in summer to cool down the outdoor air and supply the cooler air to indoor spaces for ventilation. The outdoor dry bulb temperature in winter ranged from 12.5 C to 4.4 C, and remained below C for the majority of time. This variation is typical for winter days in Korea. The range of change still indicates that any heating system should be operated for most of the day to keep indoor temperatures comfortable. The temperature difference between indoors and outdoors was greater in winter than the temperature difference in summer. This differential was relevant to the energy consumption required to keep indoor spaces comfortable for occupants in both seasons. When a heat recovery ventilator is installed and operated in a residential unit, the exchange of heat between the outdoor air and the air that is exhausted from indoors is a critical factor in energy consumption. In this study, indoor air temperatures controlled primarily by heat recovery ventilators were monitored at ranges that varied from 25.3 C to 26.4 C in summer, and from 19.6 C to 23.6 C in winter, respectively. The ranges of indoor temperatures met recommended temperature guidelines for summer and winter. When indoor temperatures are kept within comfortable ranges, the effects of energy savings by heat recovery ventilators depend primarily on outdoor temperatures, which vary by season. In this study, the outdoor temperature in winter was significantly lower than the outdoor temperature in summer. It follows that the contribution of heat recovery ventilators to energy savings was more effective in winter than in summer. Ventilation rates in each residential unit were measured using the amount of air supplied to the space per hour while heat recovery ventilators were in use. Fig. 3 shows the amount of air measured at the diffusers in each space. The amount of air ranged from 17.1 m 3 /h to 38.2 m 3 /h in each space. The range of variation in the amount of air depended on the volume of each space. In order to examine whether ventilation rates satisfied the recommended standards, the absolute amount of air supplied to the space per hour was converted into air change rate per hour (ACH) to represent practical ventilation rates. Fig. 4 shows the ventilation rates as measured, using the amount of air supplied to the space of each unit when the heat recovery ventilators were in use. Overall, ventilation rates for each room ranged from.42 to.73 ACH. The differences in ventilation rates of each unit ranged from.13 to.25 ACH, depending on the location of adjacent rooms. Specifically, the mean ventilation rate of room 4 in Cases 1, 2, 4, and 5 was.52,.512,.562, and.514 ACH, respectively. These rates generally satisfied the ventilation rates required by guidelines. In addition, the ventilation rates in the living room approximately satisfied required rates, although the rates in Cases 4 and 5 were slightly lower than required rates by.6 ACH. Ventilation rates were measured using the gas concentration decay method for all cases, in order to examine the ventilation rates by natural infiltration and the effects of heat recovery ventilators on ventilation rates. Fig. 5 shows the measured ventilation rates in room 3 and the living room when the heat recovery ventilators were shut off, and air was blocked from passing through the heat recovery ventilators. Data shows that the infiltration rates varied from.19 to.32 ACH for unit numbers 393, 43, 13, and 113. Differences in infiltration rates between room 3 and the living room varied from.1 to.4 ACH. This implies that recommended ventilation rates were not met by the measured infiltration rates, and that additional ventilation rates using heat recovery ventilators should be added to the space in order to meet the standards. Fig. 5. Natural infiltration rates (gas concentration decay method).

6 8 S.-M. Kim et al. / Energy and Buildings 46 (12) Temperature [ºC] OA dry bulb Sensible heat Sensible & Latent heat Exchange efficiency [%] - Time [h] Fig. 6. Ventilation rate by HRVs (gas concentration decay method). Fig. 8. Efficiency of heat exchange (January/21). The data further suggests that infiltration rates from outside were not equal for all of the units located on different floors. Differences in outdoor air pressure appear to have caused the slight differences in each unit. Fig. 6 shows the contributions to ventilation rates by heat recovery ventilators in room 3 and the living rooms in each unit. Differences in ventilation rates in each space ranged from.7 to.25 ACH. For unit numbers 393 and 43, the ventilation rates by heat recovery ventilators in the living rooms were greater than the rates in room 3 of each unit. For unit numbers 13 and 113, the rates by heat recovery ventilators in room 3 were greater than the rates in the living rooms. This means that the heat recovery ventilators did not maintain equal ventilation rates in each space. Although the rates were slightly different in each space, overall ventilation rates satisfied the rates recommended by Korean national guidelines in 3 [15]. When the heat recovery ventilators were operated continuously for 24 h, the rates changed from.44 to.58 ACH. Greater heat exchange efficiency of the heat recovery ventilators effectively influenced energy savings when the ventilators were operated according to operation schedules. Figs. 7 and 8 show the efficiencies for both types of sensible and total heat exchanges in the heat recovery ventilators on a particular day during the periods of field measurements in summer and winter. Overall, the efficiencies varied according to changes in outdoor temperatures. On the sunny summer day August 21, when the temperature difference between indoor and outdoor was great, the mean exchange efficiency for sensible and latent heat was 33.9% and 32.6%, respectively. A maximum efficiency of 41.4% occurred at noon due the greatest temperature difference at that point in the day. On a sunny winter day, as shown in Fig. 8, the mean exchange efficiency of sensible heat was 37.3%, and the efficiency for total heat, including sensible and latent heat, was 35.3%. Efficiencies of heat exchange in heat recovery ventilators varied according to changes of outdoor temperatures and humidity in each season. In particular, heat exchange efficiency shows clear variations in regions where the temperature profile shows clear differences season by season. Accordingly, operation schedules for heat recovery ventilators should be commensurately determined to increase the effects of heat recovery ventilators on energy savings in Korea, where there are four discrete seasons every year Measurement of energy consumption and validation for simulations The energy consumption in the high-rise residential units was reduced when the heat recovery ventilators were operated in accordance with recommended ventilation rates and comfortable temperatures. Figs. 9 and 1 show the variations of energy consumption for selected periods of three continuous days in summer and winter to maintain indoor temperatures at recommended ranges for thermal comfort. Temperature [ºC] OA drybulb Sensible heat Sensible & Latent heat Exchange efficiency [%] Time (h) Fig. 7. Efficiency of heat exchange (August/21). Fig. 9. Cooling energy consumption (August/2 4).

7 S.-M. Kim et al. / Energy and Buildings 46 (12) Cooling energy [kwh] Measured-Case 1 Measured-Case 3 Simulated-Case 1 Simulated-Case Time [h] Fig. 1. Heating energy consumption (January/21 24). Fig. 12. Measured and simulated energy consumption (August/3). Regardless of the use of heat recovery ventilators, the higher the difference in temperature between indoors and outdoors, the more energy was consumed in both summer and winter. Overall, less energy was consumed when heat recovery ventilators were operated continuously in both seasons. This is because the air exhausted from indoors and the supply air from outdoors effectively exchanged heat while passing through the heat recovery ventilators. The total energy consumption in Cases 1 and 3 in summer conditions was and kwh, respectively. In winter conditions, the energy consumption for Cases 4 and 6 was and 597 kwh, respectively. During the periods of continuous operation over three days, the continuous operation of heat recovery ventilators in summer and winter reduced cooling and heating energy by 13.6% and 21.8%, respectively. This study conducted data monitoring of energy consumption in two residential buildings for a total period of 5 months during summer and winter seasons. To examine the contribution of heat recovery ventilators on energy savings over an entire year, computer simulations were conducted using TRACE 7. The data collected from field measurements was put into the simulations to determine the energy consumption during the periods in which the data monitoring was conducted. Standard weather data for Korea was used for the computer simulations to establish exact boundary conditions. Figs. 11 and 12 show variations in measured and predicted energy consumption for a particular day representing typical conditions in summer and winter. Overall, less energy was consumed when heat recovery ventilators were operated in both summer and winter. The amount of simulated energy consumption was larger than the amount of measured energy consumption for all cases. In winter, energy for heating was mainly consumed for 12 h in the periods of time from 4: to 11: and from 19: to 23:, when the outdoor air temperatures were lowest. In contrast, the primary consumption of energy for cooling occurred from 1: to 24:, when outdoor air temperatures were higher. These results indicate that the heat recovery ventilators were most effectively utilized when the differences between indoor and outdoor air temperatures were greatest. Data about both the measured and simulated monthly energy consumption required to keep indoor air temperatures within target ranges is shown in Figs. 13 and 14. Overall, less energy was consumed in Cases 1 and 4 than in Cases 3 and 6, because in Cases 1 and 4 more effective heat exchange occurred between the air exhausted from indoors and the air supplied from outdoors while the indoor and outdoor air passed through the heat recovery ventilators. Results of the field data indicated that the amount of energy consumed in August was greater than the amount consumed in June by 4.54% and 39.96% in Cases 1 and 3, respectively. Likely this occurred in accordance with temperature profiles showing that outdoor temperatures and humidity are higher in August than June in Korea. In general, the heat exchange in heat recovery ventilators is meaningful for energy savings when the temperature difference between the air exhausted from inside and the air supplied from Heating energy [kwh] Measured-Case 4 Measured-Case 6 Simulated-Case 4 Simulated-Case Time [h] Fig. 11. Measured and simulated energy consumption (January/23). Fig. 13. Monthly energy consumption (Bldg. A).

8 1 S.-M. Kim et al. / Energy and Buildings 46 (12) 3 13 Simulated energy [kwh] 25 Simulated = * Measured R-Square = Fig. 14. Monthly energy consumption (Bldg. B). outside is large. The heat recovery ventilators achieved greater energy savings during the winter because the temperature difference between the exhausted and supplied air was greater than the temperature difference during summer conditions. The target indoor temperature was 26 C in summer, and the temperature difference between exhausted and supplied air was less than 7 C. Given the weather conditions in Korea, it does not appear that the effects of latent heat recovery were significant for energy savings in winter. These results are inconsistent with previous studies that have been performed to investigate the effects of heat recovery ventilators on energy savings in high-rise buildings in a variety of climatic conditions [7,9]. In order to increase the savings of energy with heat recovery ventilators, the focus should be on latent heat recovery by heat recovery ventilators. The method of linear regression analysis was utilized in this study to examine the results of the field experiments and computer simulations. The simulated energy consumption was validated against the field data to perform further computer simulations to determine the amounts of energy consumption under various operation schedules for heat recovery ventilators. The linear relationship between the data is described in Fig. 15. ANOVA tests were conducted to determine if the relationships were acceptable. A summary of the tests is shown in Table 3. The ANOVA results implied that the relationships from the linear prediction model were acceptable at a very low significance level of.1 (F(1,71) = , p <.1), and the coefficient of determination (r 2 ) was.994. This implies that the error variance in simulated energy consumption was reduced by 9.94% when field data was used to predict energy consumption. Based on acceptable validation, this study performed further computer simulations to predict energy consumption for the balance of the year under a variety of operation schedules for heat recovery ventilators Measured energy [kwh] Fig. 15. Relationship between measured and simulated energy consumption. Fig. 16. Monthly energy consumption under schedule F, C, Q (Bldg. A) Determination of operation schedules for energy savings The predicted monthly energy consumption under particular operation schedules is shown in Figs and Table 4. Positive values indicate energy consumption for heating, and negative Fig. 17. Monthly energy consumption under schedule F, C, Q (Bldg. B). Table 3 ANOVA test results for validation. Variable Unstandardized coefficients t Sig. ANOVA test B Std. error F test Sig. (Constant) F(1,71) = Slope

9 S.-M. Kim et al. / Energy and Buildings 46 (12) Fig. 18. Monthly energy consumption under schedule P, O, Q (Bldg. A). Fig.. Annual energy consumption under schedules (Bldg. A). Fig. 19. Monthly energy consumption under schedule P, O, Q (Bldg. B). values indicate energy consumption for cooling in the figures. In general, compared with the cases in which the heat recovery ventilators were in use, more monthly energy was consumed under schedule Q, in which the heat recovery ventilators were shut off and no air passed through them. When both types of sensible and total heat exchanges occurred in heat recovery ventilators that were operated continuously for 24 h, as in schedules P and O in this study, the annual energy consumption of building A was reduced by 9.16% and 9.5%, respectively. Annual energy savings in building B ranged from 7.13% to 7.29%. Total energy savings obtained in buildings A and B were 4.35% and 3.38%, respectively, under schedule F, in which the heat recovery ventilators were operated for 12 h per day and they exchanged sensible and latent heat simultaneously. Energy savings achieved by the sensible exchange type of heat recovery ventilator were 4.24% and 3.38%, respectively. These findings imply that the exchange of sensible heat is primarily accountable for the main portion of energy savings. Annual energy consumption by the heat recovery ventilators showed clear differences under various operation schedules. Amounts of energy consumption under each operation schedule are shown in Figs. and 21. Overall, more energy was saved as the hours of operation of the heat recovery ventilators increased. The total heat exchange type of heat recovery ventilators achieved more energy savings than the sensible heat exchange type of heat recovery ventilators. Under schedules A, B, and C, where the sensible heat exchange type of heat recovery ventilators were operated for 6, 9, and 12 h, respectively, from midnight until morning in building A, achieved energy savings were 1.53%, 2.5%, and 4.59%, respectively. When the total heat exchange type of heat recovery ventilators were Table 4 Monthly energy consumption under schedules (unit: kwh/m 2 ). Bldg. O.S for HRV Month and energy (H: heating energy, C: cooling energy), unit: [kwh/m 2 ] Total H H H H H C C C C H C H H H A F C L I N M P O Q B F C L I N M P O Q

10 12 S.-M. Kim et al. / Energy and Buildings 46 (12) 3 13 heat recovery ventilators for a more complete picture. The results of this study do not address the indoor air quality, but rather focus primarily on the effects of energy savings by heat recovery ventilators in residential buildings. It follows logically, however, that heat recovery ventilators improve indoor air quality given that recommended ventilation rates were proved to be maintained by the heat recovery ventilators. 4. Conclusions and future studies Fig. 21. Annual energy consumption under schedules (Bldg. B). operated under schedules D, E, and F, slightly less energy was consumed by schedule F than schedules A, B, and C. Very similar patterns of energy savings occurred in building B when the heat recovery ventilators were operated under the same schedules applied to building A. It appears that the savings of energy by heat recovery ventilators was influenced by changes in outside temperatures. The total operation hours of heat recovery ventilators for schedules G, H, and I were equal to the hours of operation for schedules A, B, and C. However, less energy savings was achieved by schedules G, H, and I since the particular periods of time during which the ventilators were in use were determined according to the preferences of residents [18]. Presumably, the time periods for the operation of heat recovery ventilators were preferred by residents for the sake of satisfying needs such as ventilation for cooking and dinning. During the periods of time for the operation of heat recovery ventilators that the residents preferred, the outside air temperatures were warmer in schedules G, H, and I than the outside air temperatures under schedules A, B, and C. This tendency led to less energy savings under schedules G, H, and I, even though the heat recovery ventilators were operated for the same total amount of hours under all the schedules. To regulate these matters of inconsistency, the heat recovery ventilators needed to be operated continuously for an entire day. This study established the continuous operation of heat recovery ventilators for periods of 12 and 24 h per day. Effective energy savings were achieved under these two operation schedules. In particular, 9.84% and 7.9% of total energy savings were achieved by schedule P in buildings A and B, respectively. These results imply that the continuous operation of heat recovery ventilators was effective for reducing energy consumption for heating and cooling, in comparison to the situation in which the heat recovery ventilators were not operated at all. The operation of heat recovery ventilators also maintained the recommended ventilation rates in high-rise residential buildings. In summary, the heat recovery ventilators effectively contributed to the savings of heating energy due to the effective heat exchange occurring between the air exhausted from indoors and the air supplied from outdoors while the inside and outside air passed through the heat recovery ventilators. The heat recovery ventilators accomplished meaningful energy savings, as well as maintained recommended ventilation rates in the high-rise residential buildings where natural infiltration rarely occurs due to the tightly sealed building envelopes. The influence of heat recovery ventilators on the overall improvement of indoor air quality needs to be examined simultaneously with energy savings from This study was conducted to investigate the contribution of heat recovery ventilators to energy savings in high-rise residential buildings. The summary of findings is as follows. First, total energy consumption was reduced in each unit of the high-rise residential buildings when the heat recovery ventilators were operated to maintain recommended ventilation rates and comfortable temperature ranges. One important finding is that the heat exchange by heat recovery ventilators is most meaningful for energy savings when the temperature differences between the air exhausted from indoors and the air supplied from outdoors is great. Accordingly, the heat recovery ventilators achieved higher energy savings during the winter, when the temperature difference between the exhausted and supplied air was greater, than in the summer. Second, the linear prediction models developed to compare the results of this study s simulations and field measurements were acceptable under a meaningful significance level. The prediction results showed that the heat recovery ventilators contributed to annual savings of heating and cooling energy by 9.45% and 8.8%, respectively, when the ventilators were operated continuously for 24 h in the high-rise residential buildings. Third, more overall energy was saved as the operation hours of heat recovery ventilators increased. Further, the total heat exchange type of heat recovery ventilators achieved more energy savings than the sensible heat exchange type of heat recovery ventilators. This finding implies that the continuous operation of heat recovery ventilators is effective for energy savings, as well as for maintaining necessary ventilation rates since the continuous operation of heat recovery ventilators met recommended standards for ventilation rates in high-rise residential buildings. Finally, the heat recovery ventilators achieved effective energy savings and maintained appropriate ventilation rates in the highrise residential buildings where natural infiltration rarely occurs because of tightly sealed building envelopes. The influence of heat recovery ventilators on improving the quality of indoor air needs to be examined in conjunction with energy savings by the heat recovery ventilators. Based on the current research, it is logically assumed that heat recovery ventilators improve indoor air quality given that recommended ventilation rates were regularly maintained by the heat recovery ventilators throughout this study. The results of this study are based on field measurements performed within limited periods of time. Long-term field measurements are necessary in future research to achieve more reliable analysis results. Although the simulated data was validated against field measurement data, the predicted data is limited because the simulations contain their own algorithms for computation. Further computer simulations, using various software, would be useful in future research. Acknowledgement This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No ).

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