Thermal comfort improvement of naturally ventilated patient wards in Singapore

Energy and Buildings 154 (2017) 499–512

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Thermal comfort improvement of naturally ventilated patient wards in Singapore Lan Lan a,∗ , Wayes Tushar a , Kevin Otto b , Chau Yuen a , Kristin L. Wood a a b

Singapore University of Technology and Design (SUTD), 8 Somapah Road, Singapore 487372, Singapore Aalto University, Finland

a r t i c l e

i n f o

Article history: Received 4 June 2017 Received in revised form 26 July 2017 Accepted 27 July 2017 Available online 1 September 2017 Keywords: Sustainable building solutions Natural ventilation Thermal chimney Night air purge Thermal comfort Tropical climate

a b s t r a c t Located near the equator, Singapore has a tropical rainforest climate with high temperature and high humidity. In hospitals of Singapore, the subsidized patient wards are designed to be naturally ventilated, considering the affordability for patients. However, due to the high occupant density of the patient wards and the hot humid climate, occupants may feel discomfort, especially in the older hospital wards which were not well designed for natural ventilation. In this paper, the thermal comfort level of occupants at Singapore’s Changi General Hospital (CGH) is evaluated based on both in-situ measurements and modeling analysis. Against this backdrop, several low energy solution concepts that potentially improve the thermal comfort level of occupants in patient wards are analyzed and simulated using detailed building thermodynamic and airflow simulation. We found that this approach of combining thermodynamics, computational fluid dynamics, and thermal comfort level models was effective for analyzing and comparing the thermal comfort impact of alternative, low-energy building retrofit concepts. We also found that passive solutions to ventilation could be used effectively for a patient hospital ward, even in the tropical warm climate of Singapore. © 2017 Elsevier B.V. All rights reserved.

1. Introduction There has been a significant increase in the global energy demand in the past few years, and buildings are one of the major contributors to this huge demand. It is shown in [1] that 48% of the total energy in the United States is consumed by buildings, mostly due to the extensive use of air conditioners (ACs). As such, considerable research efforts have been devoted to devising solutions to help reduce the use of ACs in buildings. For example, by designing naturally ventilated building space to improve the thermal comfort of building occupants [2]. However, most such studies have been conducted under temperate climates in European countries and other developed regions of the world, which give outcomes that cannot be transferred to a tropical country like Singapore. Singapore is 1-degree north of the equator. It has a tropical rainforest climate which is characterized by relatively uniform temperature and pressure, high humidity, and abundant rainfall. The average monthly temperature, relative humidity and sunshine hours are shown in Fig. 1. The mean monthly temperature varies from 26 ◦ C in December and January to 27.8 ◦ C in May and

∗ Corresponding author. E-mail address: lan [email protected] (L. Lan). http://dx.doi.org/10.1016/j.enbuild.2017.07.080 0378-7788/© 2017 Elsevier B.V. All rights reserved.

June. The humidity is high, ranging from 82.6% to 86.7%. Monthly sunshine hours (with direct irradiation from the sun of at least 120 W/m2 ) range from 129.6 h in November to 192.7 h in March. In Singapore, wind directions are mainly northerly to north-easterly during the Northeast Monsoon season (December to March) and southerly to south-easterly during the Southwest Monsoon season (June to September). Wind strength is greater during the Northeast Monsoon. The inter-monsoon months (April, May, October and November) are transition periods among the monsoons and show weaker and more variable winds. To achieve thermal comfort of occupants, mechanical cooling has been widely installed in all types of buildings including residential, commercial, and industrial buildings. It was reported that electricity consumption for cooling accounts for 60% of total building electricity usage [4]. To build a sustainable environment, the Building and Construction Authority (BCA) of Singapore launched the Green Mark Scheme in 2005 to promote environmental awareness in the construction and real estate sectors. Natural ventilation is one of the promoted approaches in Green Mark to improve the energy efficiency of buildings. In a well-designed naturally ventilated building, no or very little energy is needed for cooling and/or ventilation. With this approach, thermal comfort is achieved through passive designs. While the potential demand for passive natural ventilation is high, the design of such systems is diffi-

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Nomenclature and Abbreviations AMV BCA CFD CFFT CGH CIBSE DSFs ECM EPI HDB IDC IES VE WHO WSN ISO IWEC NAP PMV PPD SUTD UHI

actual mean vote Building and Construction Authority computational fluid dynamics Complex Fast Fourier Transform Changi General Hospital Chartered Institution of Building Services Engineers Double Skin Facades energy conservation measure energy performance index Housing and Development Board SUTD-MIT International Design Centre Integrated Environmental Solutions virtual environment World Health Organization Wireless Sensor Network International Organization for Standardization International Weather for Energy Calculations night air purge predicted mean vote predicted percentage of dissatisfied Singapore University of Technology and Design Urban Heat Islands

cult, primarily due to the need to ensure the thermal comfort of occupants. This is particularly true for existing buildings, where existing structures may inhibit the application of many passive design strategies. In this context, simulation and analysis of potential concepts become crucial to virtually simulate, test, and explore. In this paper, we develop and explore this approach of combining thermal, airflow and comfort level simulation into a combined analysis of naturally ventilated retrofit design concepts for hospital patient wards. We find the approach efficient and effective for screening and comparing alternative retrofit concepts. The most commonly used passive design strategies for cooling in Singapore include building massing to allow for good ventilation, orientation to minimize heat gain, well-insulated building envelope systems, sun-shading devices, and vegetation, etc. However, those strategies are most applicable in the design phase of new building constructions. It is more difficult to improve thermal performance of existing

buildings with fixed physical conditions using retrofitted passive cooling strategies. However, there are passive cooling strategies that are potentially suitable for existing buildings as retrofits. These include thermal chimneys, night air purge, windcatchers, wind walls, active window shades, and reflective surfaces. These are technologies which can be installed onto and within existing structures. Their applicability to any given project varies due to the existing building layout, orientation, construction, and use. Due to the special features of hospital wards, the design of passive cooling strategies can be different from normal building types. The special features of hospital wards include: (1) High occupant density so the cooling load is high and the thermal conditions can be severe. (2) Large shared open spaces with few obstacles, implying potential alignment for good natural ventilation design. (3) Different thermal comfort levels of patients and healthcare staff due to their different activities. Patients are lying down or seated most of the time so they have lower metabolic rate, while healthcare staff are walking around to take care of patients so they have higher metabolic rate. For Singapore’s Changi General Hospital (CGH) patient ward, three low energy strategies were considered. These are thermal chimneys, night air purge (NAP), and wing walls. Thermal chimneys enhance the ventilation of a building through a stack effect, using the principle that hot air rises because it is at a lower density. When the building exhaust air is further heated in a solar heated dark chimney, it can thereby draw air from the building. The design of such systems is difficult, requiring careful zoning. In [5], a thermal chimney system was constructed to enhance the air ventilation within the interior spaces using a series of solar assisted ducts that linked the lower floor classrooms and upper floor hall. Results showed that the thermal chimney system operates well in hot and humid tropics, including on cooler days. However, [6] shows that an attempt at a passive stack retrofit, incorporating the principle of airflow due to buoyancy, did not sufficiently enhance air velocity in a Housing and Development Board (HDB) residential apartment building in Singapore. However, a modified active stack was shown to be effective, operating an airflow boost when needed based on the suction effect induced by a fan fixed at the top of the

Fig. 1. Average monthly (a) temperature, (b) humidity, and (c) sunshine hours in Singapore [3].

L. Lan et al. / Energy and Buildings 154 (2017) 499–512

stack. This lead to a substantial increase in the air velocity in the room and thereby affected the thermal comfort conditions. NAP is another concept whereby windows are opened at night to let in outdoor fresh air to cool the interior thermal mass. The windows are then closed during the day to prevent outdoor heat from entering . [7] shows that in the hot humid climate of Israel it is possible to achieve a reduction of 3–6 ◦ C in a heavy constructed building without operating an air conditioning unit. [8] observed that, in the tropical climate of Malaysia, the night ventilation technique lowered the peak indoor air temperature by 2.5 ◦ C and reduced nocturnal air temperature by 2.0 ◦ C on average, compared with the current window opening patterns, i.e. daytime ventilation. Finally, wing walls are a concept whereby, similar to a wing, a small wall projects outward next to a window so that even a slight breeze against the wall creates a high-pressure zone on one side and low pressure on the other, thereby amplifying air flow into and out of openings. [9] indicated that wing walls can promote natural ventilation by increasing the air change per hour and the mean indoor air speed relative to wind speed at various wind speeds and wind directions. In this paper, several practical passive cooling solutions are modeled for a naturally ventilated patient ward at the main building of CGH in Singapore. The CGH main building was built in 1997, without consideration for natural ventilation, other than a fraction of the windows being operable and the orientation taking advantage of the prevailing wind. The naturally ventilated wards are densely occupied, and so it has become challenging to achieve thermal comfort satisfaction of patients, visitors, and health care staff. We stress that a number of studies in the literature has been devoted to investigating passive cooling strategies for buildings. Examples of such studies include [10,2,11–14]. However, the considered building is a hospital ward, whose occupant density and sensitivity to temperature change is different from other residential and commercial buildings (e.g. [14]), and so this contribution is unique. Further, most of the existing studies have focused on either environment and climate conditions that are significantly different than Singapore (such as in [2]) or have considered strategies (e.g. [10–13]) that did not cover as many choices as we have considered in this paper. As such, the focus of this study is unique as it analyzes the performances of number of passive cooling strategies in existing naturally ventilated hospital wards in Singapore, which is a tropical and warm country. Further, the considered space is naturally ventilated hospital wards in Singapore, which is different from other residential and office buildings in terms of a number of aspect as mentioned previously. The remainder of the paper is organized as follows. A review of related works is discussed in Section 2. We study the set of the proposed strategies in Section 3, followed by the related results in Section 4. Finally, we provide some conclusions in Section 5.

2. Related work Recently, significant attention has been devoted to various aspects of thermal comfort improvement and green building designs worldwide. To provide an overview of these efforts, we divide the existing studies into four categories. The first category of studies in [15–19] focuses on the thermal comfort of hospital patients. A thermal comfort study in Belgium [15], which involves 99 patients with different types of illness, showed that there was no significant difference between PMV obtained from objective measurements and AMV for all the different wards except for neurology. Also, it showed that the difference between the PPD obtained by the application of the PPD-formula in ISO 7730 as function of AMV, and the PPD obtained from personal acceptability votes is not signifi-

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cant. Thus, it concluded that PMV and PPD indices may be used to adequately predict mean thermal responses for these wards except for neurology. In [16], a field survey that was conducted in a university hospital in Taiwan with 927 sets of collected data showed that physical strength had a highly significant effect on thermal sensation, but gender, age, and acclimatization had not. The frail population wanted a warmer condition. The effective thermal-neutral temperature for the frail population is about 0.3 ◦ C higher than that of the vigorous population. A field study [17] on the environmental condition and patients’ thermal comfort carried out in Hospital University Kebangsaan Malaysia (HUKM) revealed that during the daytime patient ward is “slightly cool” to “cool” and during nighttime it is “cold”. It shows that the neutral temperature in daytime is around 26 ◦ C, comfort range is between 25 ◦ C and 27.7 ◦ C, and the acceptable range would be as wide as 23.8 ◦ C to 29 ◦ C. In [18], the authors indicated that there have been various studies investigating which population groups may be more susceptible to temperatures at or beyond the thresholds of the World Health Organization (WHO) thermal comfort range. For example, people with certain chronic medical conditions are more susceptible to heat, such as cardiovascular and cerebrovascular diseases, diabetes, respiratory and renal diseases, Parkinson’s disease, Alzheimer’s disease, and epilepsy. [19] established that patients expect a warmer indoor environment than neutrality. The authors of studies that fall within the second category discussed the thermal comfort aspect of not only the patients, but also the staff of the hospital. In [20], the authors studied the thermal comfort of 114 medical staff in hospitals in Malaysia and found that a higher comfort temperature was required for Malaysians in hospital environments compared with the temperature criteria specified in ASHRAE Standard (2003). The field survey analysis revealed that the neutral temperature for Malaysian hospitals was 26.4 ◦ C. The comfort temperature range that satisfied 90% of the occupants in the space was in the range of 25.3 ◦ C to 28.2 ◦ C. A field survey conducted in Japan and based on 36 patients and 45 staff members in [21] showed that with relatively low temperature (20–23 ◦ C) and low humidity (less than 40%), the percentage of staff who felt “warm”, “hot” or “very hot” was 64.5% in total, which was much higher than that of patients which was only 22.3%. Thus, healthcare staff experience more warmth than patients. [22] studied the thermal comfort of patients and staff in four Iranian hospitals. It showed that, with air temperature from 20 ◦ C to 28 ◦ C, air velocity from 0.1 m/s to 0.5 m/s, and relative humidity from 30% to 60%, the hospital staff generally experience uncomfortably hot thermal conditions, patients with blankets are generally the most thermally comfortable group in the hospital, though with a tendency towards overly warm conditions in the afternoons. For patients without a blanket for any reason the thermal conditions will generally be very cool. AMV survey of 160 staff members and 65 patients in [23] indicated that under same thermal environment, patient found the thermal environment acceptable while staff had thermal comfort issue. A field study conducted in air-conditioned hospitals in Thailand indicated that the acceptable temperature range for patient, and medical staff was at 21.8–27.9, and 24.1–25.6 ◦ C respectively [24]. It means that patients have wider acceptable temperature range than staff do. In the third category, the discussion is mainly on designing low energy building in the tropics, as shown in [25–29]. In [25], a low energy office building in Malaysia was investigated to examine the efficiency of different passive and hybrid cooling strategies thermal stack flue, cross ventilation and water walls improved the internal thermal conditions of the atrium and lobby despite the hot and humid climate. The work highlighted the importance of cross ventilation in improving the atrium indoor thermal conditions and enhancing the performance of the other strategies. [26]

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presents a study on potential application of Double Skin Facades (DSFs) in naturally ventilated office buildings under tropical climates. The simulations in this study were based on a hypothetical building comprises of 11 floors of open plan office, with longest side facing north/south. The results showed that with optimized models the acceptable comfort levels could be achieved for about 70% of occupied hours. [27] investigated on the energy saving potential of underground buildings. The results showed that underground buildings have the potential to reduce the energy demand in comparison to a conventional above ground building, by using beneficial soil temperatures and large amounts of earth cover as insulation. In [28], several passive roofing technologies including cool paint, radiant barriers, etc. were investigated using an analytical Complex Fast Fourier Transform (CFFT) method. A field experiment was carried out on two naturally ventilated units located on the top floor of a 12-storied residential building in Singapore to validate the CFFT model. In [29], the authors have shown that in cities with a tropical climate, roofs with a thermal insulation layer and selective coatings can reduce the thermal load of buildings and weaken the increased temperature effects of Urban Heat Islands (UHI). However, although the passive roof technologies are effective for the top floors of high-rise buildings, heat gains of the other floors cannot be reduced effectively because those floors are not directly connected to the roof. [30] indicates that several simple passive retrofit strategies including reflective coating, selective glass film and roof insulation could significantly improve thermal comfort of occupants in naturally ventilated residential buildings in Kolkata, India, based on simulations via EnergyPlus [31]. [32] compared four passive cooling techniques (shading, natural ventilation, cool painting and thickness of interior gypsum plaster) for a pre-fabricated building made of a sandwiched-structured composite. The results showed that through combining all the four techniques, the maximum indoor temperature can be reduced from 40 ◦ C to 34.5 ◦ C in Mumbai, India. [33] monitored several patient wards in a Nucleus type hospital in UK and found that the major discomfort was from night-time overheat. [34] shows that through applying different ECMs in hospitals in Jaipur city, India, there could be 113.54 kWh/m2 /y of energy performance index (EPI) reduction, which includes HVAC, lighting and electric motors. Finally, the fourth category of the existing literature discusses the energy performance of healthcare facilities. It is shown in [35–37] that healthcare buildings have the highest building energy consumption intensity among all the building types. The high space heating, cooling and ventilation loads, continuous 24-h operation for the majority of the facilities [38], and the high number of medical equipment [39] result in high energy consumption in healthcare buildings. Many challenges and efforts to achieve energy efficient healthcare facilities were shown in [39], including operational missions, organizational and cultural constraints, issues specifically related to the legacy of existing facilities, and codes and standards. As such, as shown in [40], technical regulations and directives have been developed to provide guidelines and promoting measures for the reduction of the energy consumption of hospitals. These measures must be adopted and applied to the hospital design, construction, retrofit, operation and maintenance, also by integrating advanced energy-efficient technologies and renewable energy sources.

Table 1 Description of the various aspects of the building considered in this study. Information

Changi General Hospital

Building completion date Gross floor area (m2 ) Number of floors Floor level of naturally ventilated wards Natural ventilation design features Floor height (m) Ceiling height (m) Number of beds per cubicle Type of windows

1997 107,000 9 aboveground, 2 underground 4, 5, 6 Operable windows; ceiling fans; site orientation 3.8 2.6 6 or 8 Operable awning window

Fig. 2. The front and top views of the CGH building in Singapore.

as shown in Table 1. The building is multifunctional. The basements to the 3rd floor are carparks, food courts, emergencies services, outpatient treatment facilities, meeting rooms, etc. The 4th–6th floors are subsidized patient wards designed with no air-conditioning, which are the focus of this study. However, portable air-conditioners are used in these wards due to the uncomfortable thermal condition provided by natural ventilation only. The 7th–9th floors are patient wards with air-conditioning. The building is H-shaped, as shown in Fig. 2, and its orientation takes advantage of prevailing winds for natural ventilation. Unfortunately, it does not have any other features to enhance natural airflow. The building height is restricted because it is near the airport. The ceiling height of the patient wards is 2.6 m which is relatively low compared to typical hospital wards. Operable windows cover about 70% of north and south surfaces of the naturally ventilated wards. Rotatable ceiling fans are installed above each patient bed and nurse station.

3. Study setup 3.2. Methodology 3.1. Building description The CGH main building is located in the east side of Singapore. It has nine (9) stories above ground and two (2) stories underground

In the first part of the study, a baseline model was built representing the current thermal condition of the naturally ventilated wards. The model was calibrated based on the data collected from

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Fig. 3. Floor plan of the chosen wards with measured airspeed.

the building manager, a Wireless Sensor Network (WSN), and a portable anemometer deployed into the space. Because the baseline conditions had become intolerable to the patients and staff in the naturally ventilated wards, portable plug-in air-conditioners had been installed by the staff. Therefore, to measure the thermal conditions provided by naturally ventilation alone, an empty ward was chosen to be monitored (no occupants or internal heating sources). The additional internal heat gain was then determined through detailed surveys of the occupied wards and added into the energy models. The chosen wards were located on the 5th floor with 36 patient beds and one nurse station. The monitored area was about 415 m2 as shown in Fig. 3. In the second part of the study, the thermal comfort level of occupants was analyzed based on measured data and simulation results from the calibrated baseline model. Thermal comfort was then evaluated from these results using Fanger’s PMV model and an extension of PMV model developed for non-air-conditioned buildings in warm climates. Finally, in the last part of the study, several low energy measures and their impact on thermal comfort were analyzed. The low energy measures include both individual concepts (e.g. NAP) and combined concepts (e.g. NAP with Wing wall).

3.3. Data collection The indoor and outdoor thermal conditions were monitored for 10 days in April 2015. During the first five (5) days, all of the windows were kept open, while during the second five (5) days, all windows were closed. Outdoor weather information was obtained from a weather station at SUTD, which is 1.48 km from CGH. Twenty (20) multi-purpose wireless sensor nodes were deployed into the wards to monitor indoor temperature and humidity, as shown in Fig. 4. An air flow meter was used to periodically measure the air flow in the wards manually. The measurement locations are shown in Fig. 3. The specifications of sensors are shown in Table 2. The communication protocol of the WSN is Zwave. The network uses star topology in that all the sensor data is transferred directly to the gateway with transmission interval of 15 min. A local server collects all the data from the gateway and pushed it to the cloud in real time. The data can then be accessed through a web application. The average indoor airflow provided by natural ventilation (except areas near windows) was measured to be near 0 m/s (as shown in Fig. 3). With the ceiling fans turned on, the air velocity at

Fig. 4. Data collection via WSN.

Table 2 Specification of different types of sensors used in the study. Multipurpose wireless sensors

Resolution Measurement range Accuracy Time steps (min)

Temperature

Humidity

0.1 ◦ C −10–50 ◦ C ±1 ◦ C 15

1% 0–100% ±5%RH 15

Anemometer

0.01 m/s 0.01–30 m/s ±3% F.S. (≤20 m/s) 15

the center of a patient bed was 0.48 m/s, and the average indoor air velocity was 0.29 m/s. As can be seen from Fig. 5, the measured indoor temperature is lower than the outdoor temperature at most of the time. And the indoor temperature fluctuation is less than outdoor. This is mainly due to the following reasons: • The adjacent space of the ward is air-conditioned. Thus, the heat conduction and convection from the adjacent air-conditioned space makes the ward cooler than outdoor. • The air exchange rate of the naturally ventilated ward is very low. Thus, the outdoor weather condition has limited effects on the indoor condition. • The building envelope provides shading and insulation.

In this space, the window status was found to have little influence on the indoor thermal condition, due to the low effectiveness of natural ventilation from window openings.

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Fig. 5. Variation of temperature and humidity over time of an empty ward for (a) windows open and (b) windows closed.

3.4. Baseline model Given these measured data, a baseline model of the space was developed through the simulation software called Integrated Environmental Solutions Virtual Environment (IES VE) [41]. Essentially, IES VE is a whole-building simulation software which provides a virtual environment to model and evaluate the performances of buildings and systems. It is validated under ASHRAE Standard 140. Of the various tools in IES VE, the tool used for dynamic thermal simulation of heat transfer processes of buildings is ApacheSim. MacroFlo, which runs as an adjunct to ApacheSim, simulates the flow of air through openings in the building envelope. There is also a CFD module (called MicroFlo) in IES VE. The boundary condition applied to the CFD model in MicroFlo is from the results of ApacheSim. ApacheSim uses a stirred tank model of the air in a room. In a stirred tank model, the air temperature and humidity are assumed to be uniform within the room. A finer spatial resolution of these variables can be achieved by subdividing the room. In this study, the investigated space is divided into 7 zones including five wards, one corridor and one nurse station to get a finer resolution. The task of determining thermal conditions throughout the building proceeds by balancing sensible and latent heat flows entering and leaving each air mass and each building surface. MacroFlo is used to calculate the natural ventilation air flow rates so the balancing process also includes inter-dependence between MacroFlo variables and those calculated within ApacheSim. The simulation time-step is set to be 10 min. The baseline model was calibrated based on the data collected by sensors. The calibration process is as follows:

(1) Develop the building thermal simulation model using floor plan and construction data from the building manager. (2) Compare the outdoor temperature from the weather station with the modeled weather data file from International Weather for Energy Calculations (IWEC). The measured outdoor temper-

ature was found to be 0.5 ◦ C higher than the weather file for the same period, which is within acceptable limits. (3) The measured mean indoor temperature is 2 ◦ C lower than outdoor. It is found that this is due to heat transfer between the wards and adjacent air-conditioned spaces. Given this fact, the adjacent space conditions were set and the air tightness of the interior doors was adjusted in the model until the mean indoor temperature was 2 ◦ C lower than the outdoor temperature. (4) To match the range of indoor temperature variation between the model and measured data, the effective ventilation area of exterior windows was adjusted. This made the fluctuation amplitude of indoor temperature the same as measured data, but 0.5 ◦ C lower, matching the measured condition. (5) Step 4 impacts the result of Step 3 and vice versa. Iteratively repeat 3 and 4 until the two targets are simultaneously achieved.

Next, because the thermal conditions were measured in empty space with no internal heat gains, the actual thermal conditions in an occupied ward need to be added to the model. A survey for internal heat gain was carried out, and the internal heat gain was added to calculate the thermal condition of the space when occupants and equipment exist. The list of internal heat sources is shown in Table 3. The heat gain values of different heat sources are obtained from the CIBSE Guide: Internal heat gains [42]. The simulation results show that the average indoor temperature is 28.8 ◦ C when windows are open, an uncomfortable baseline condition. CFD simulations were performed using MicroFlo. MicroFlo is based on a Finite Volume Method of discretization of the partial differential equations that describe the fluid flow. The boundary conditions are conditions exported from the results of Apache Simulation and imported into MicroFlo. The grid spacing is 0.1 m in this study. The turbulence model is k-e. The k-e model calculates turbulent viscosity for each grid cell throughout the calculation domain by solving two additional partial differential equations, one for turbulence kinetic energy and the other for its rate of dissipa-

Table 3 The list of internal heat sources within the selected space. ID

Zone type

Area (m2 )

No. of people

18 W

36 W

1 2 3 4 5 6

Cubicle 1 Cubicle 2 Cubicle 3 Cubicle 4 Cubicle 5 Nurse station

52 52 69 69 69 34

7 8 8 9 8 8

6 6 8 8 8 15

6 6 8 8 8 0

7

Corridor

70

8

0

0

No. of fluorescent lamps

Equipment

6 ceiling fans, 40 W each 6 ceiling fans, 40 W each 8 ceiling fans, 40 W each 8 ceiling fans, 40 W each 8 ceiling fans, 40 W each • 3 PCs, 320 W each; • 3 monitors, 70 W each • 3 ceiling fans, 40 W each • 1 television, 200 W • 1 printer, 1144 W each • 3 printers, 820 W each • 6 laptops, 100 W each • 2 pagewriters, 65 W each • 1 HeartStart XL Defibrillator/Monitor, 150 W

L. Lan et al. / Energy and Buildings 154 (2017) 499–512 Table 4 Parameters for calculating PMV.

Air velocity w/o fan (m/s) Air velocity w fan (m/s) Basic clothing insulation (clo) Metabolic energy production (W/m2 )

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Table 5 Current thermal condition for healthcare staff. Patient

Healthcare staff

0.07 0.48 0.3 58

0.07 0.29 0.3 100

tion. Convergence is achieved when the residuals falls to zero. Due to the numerical approximations involved, the residual will never become zero but fall to a value as close to zero as possible. In IES VE, the ways of judging whether the run has completed are as follows: (1) Check the residual history printed on screen. The sum of the normalized absolute residuals should diminish steadily. (2) Check the monitoring cell location for the dependent variables at the user set location within the fluid domain. For calibrating the baseline of this study, we design the process to ensure that the temperature and air flow in the model can fairly represent the real building condition by comparing the data of weather file and data collected by weather station and sensors. This is mainly due to the fact that the calibration of baseline in this study is for natural ventilation. As a consequence, the purpose of the calibration is to make the performance of the baseline model as close to the real building performance as possible. 3.5. Thermal comfort level To quantify thermal comfort in a repeatable manner, ISO 7730 [43] was used to estimate the thermal comfort of occupants in the wards. Thermal comfort level is calculated with the PMV/PPD index. The PMV/PPD model uses heat balance equations and empirical studies on skin temperature to define comfort. PMV uses a thermal scale from cold (−3) to hot (+3). The comfort zone is defined as when PMV is within the recommended limits of (−0.5 < PMV <+0.5). The parameters for calculating PMV are shown in Table 4. We note that the occupants in tropical climates may sense the warmth as being less severe than the PMV predicts in non-air-conditioned buildings in warm climates. Therefore, we consider that the upper bound of accepted PMV value in naturally ventilated buildings in Singapore is 0.7 instead of 0.5, as shown in [44]. In [44], the authors used highquality field experiment data of 583 samples in naturally ventilated building in Singapore reported in [45] to develop the extended PMV model. Despite the high temperature, the PMV value indicated patients feel comfortable to slightly warm when the fans are turned on, while patients feel slightly warm to uncomfortable if fans are off. Also, as indicated in the literatures in Section II, patients who are sedentary are more comfortable with the elevated temperature compared to predicted neutral temperature. Overall for patients, thermal comfort can be achieved through using fans and control of clothing. However, the healthcare staff feel slightly warm to uncomfortably hot even though the fans are turned on. This is because that the staff are not sedentary and quite active moving about the wards. The baseline thermal condition for healthcare staff is with fans running and windows open, as shown in Table 5.

Air speed (m/s) Clothing insolation (clo) Metabolic energy production (W/m2 ) Average temperature (◦ C) Average humidity (%)

0.29 0.3 100 28.8 83

As shown in Table 6, to achieve recommended comfort levels of healthcare staff, one approach is to decrease the temperature to 26.3 ◦ C using solutions such as NAP, or alternatively increasing the air velocity to 0.8 m/s using solutions such as louver window, thermal chimney and wing walls. Although the appropriate air speed for indoor environment is up to 0.2 m/s, it is possible to increase the air velocity to 0.8 m/s for increasing thermal comfort as suggested in [46,47]. Because the indoor and outdoor thermal conditions are different, change of one parameter (e.g. airflow rate) may affect other parameters (e.g. temperature, and humidity). For example, in cases when the space is highly occupied, the occupants generate a lot of moisture causing the indoor humidity higher than outdoor. If it is possible to increase the air exchange rate between indoor and outdoor, the indoor humidity will decrease. To show an overview of the thermal comfort level, the average temperature and humidity from simulation results is used to calculate the average thermal comfort level over a year. 3.6. Low energy measures In this section, we explore alternative retrofit natural ventilation concepts using simulation. The IES VE tool set was used to assess the thermal condition. 3.6.1. Change of window type Simulation results of IES VE and measured data showed that airflow in the wards is not sufficient. Because the CGH main building is configured with an orientation that makes use of the prevailing wind, increasing the effective ventilation area of window openings may lead to significant improvement of airflow through the wards. The current windows are awning windows as shown in Fig. 6a. The maximum opening angle of the awning window is 30◦ , which provides very little ventilation in relation to the overall window size. Breezway, a manufacturer of facades, indicates that for awnings, the ventilation area compared to the total window area is typically 1% to 10% [48]. In the baseline model, the ratio was set to be 8% based on calibration with the measured data. If the window type is changed to louvered window (as shown in Fig. 6b), the ratio can be 75–80% which is 9–10 times that of an awning window [48]. According to the simulation results, the airflow will increase by 9.2 times when changing to louvered windows with the same operable window area. As shown in Fig. 7a and b, for a particular point in time (12pm, 15 January), if the window type is changed from projected windows to louvered windows, the airflow through the southern windows of the ward will increase from 2120 l/s to 19,052 l/s. Fig. 6c and d show the CFD simulation results of one ward (marked in Fig. 6e) for baseline and louvre window, respectively. Because the current awning windows are not effective for natural ventilation, the remaining solutions discussed will all incorporate louvered windows.

Table 6 Thermal comfort level from simulation.

Current Approach 1 Approach 2

Average temperature (◦ C)

Average relative humidity

Average air velocity (m/s)

28.8 26.3 27.4 (current outdoor T)

83 83 83

0.29 (with fan) 0.29 0.8

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Fig. 6. Change of window type.

Fig. 7. Air flow through southern opening.

3.6.2. Thermal chimney A vertical shaft painted black can be installed on one side of a building to enhance ventilation. This structure absorbs solar energy and heats up the air in the shaft which will then move up and draw outdoor fresh air into the building. To implement a thermal chimney properly, the local climate should be taken into consideration. Considering the wind direction, outdoor temperature and sun path, two thermal chimneys are placed at the north of the CGH main building to ensure higher airflow rate during the warmest months. To protect the wards from overheating, a thermal insulation layer is added to the chimney walls which are in contact with the wards in the thermal chimney model. As shown in Fig. 8, the thermal chimneys are located at the north side of the building and is at building height. Unfortunately, the thermal chimneys cannot exceed the current building height due to airport height restrictions in the area. The material for the thermal

chimney is chosen to be aluminium, which is cost effective. The effective ventilation opening is set to be the same size as the louvre windows of the wards. Fig. 9 shows CFD results of the ward with a thermal chimney. However, the thermal chimney may impede natural daylight and the outward views of the patients and staff. It may thus negatively affect the overall healing environment within the ward. This issue may need to be addressed with a creative, new chimney layout and shape that provides a chimney’s functionality while achieving natural daylight conditions and views. 3.6.3. Night air purge NAP is a concept to open windows at night to draw in cool outdoor fresh air to cool the internal thermal mass, and to close windows during the day to prevent hot outdoor heat from entering. NAP can be applied with many other concepts. Therefore, NAP was applied to different concepts including louvered windows, and

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Fig. 8. Thermal chimney.

Table 7 Average annual indoor temperature for various measures.

Fig. 9. CFD results.

Fig. 10. Comparison between different measures.

both louvered windows and shades.1 The shades are assumed to have a transmission factor of 0.23. A sample of 6-day data is shown in Fig. 10. The baseline model has the least temperature variation because the ventilation rate is the lowest due to small ventilation openings. The louvered window offers the higher ventilation rate so the indoor temperature fluctuates with the outdoor temperature. This result makes the indoor temperature higher than baseline during hot periods while lower in cool periods. Through the use of NAP, the peak indoor temperature will decrease by about 2 ◦ C from the louvered windows concept alone. If shades are installed, the peak temperature will drop further by about 0.5 ◦ C.

1 Although the performance for using NAP in Singapore may not as good as places with larger diurnal temperature difference, it still has the potential to contribute to passive cooling in Singapore, e.g. a combination of night cooling and other passive cooling strategies can be integrated to achieve better cooling performances.

Measures

Annual average temperature (◦ C)

Baseline (window open) Baseline (window closed) Louvre window NAP w Louvre window (fixed schedule) NAP w Louvre window (auto damper) NAP w Louvre window and shades (fixed schedule) NAP w Louvre window and shades (auto damper) Outdoor

28.8 29.2 27.9 28.2 27.3 27.9 27 27.5

The annual average temperature for various measures is shown in Table 7. The temperature of NAP with louvered windows is 0.3 ◦ C higher than the measure with louvred windows only. This is due to the fact that during some time of the day, the outdoor temperature can be lower than the indoor temperature, but because the schedule of window operation is fixed, it only opens between 7pm to 7am each day. In this case, windows keep the cool air out and indoor temperature will be higher than outdoor temperature. Overall, NAP can decrease daily peak temperature during most time of the year. Because sometimes the outdoor temperature can be lower than the indoor temperature, a fixed schedule of window opening is found to be ineffective. Automatic dampers are recommended to further enhance the thermal performance of NAP [49]. The performance of NAP with automatic dampers is shown in Table 7. In addition, the high humidity issue in NAP should be taken into consideration. High humidity issue in NAP is caused by two factors: the humid outdoor air which is drawn into the wards at night; and the latent heat generated by occupants which is trapped inside the wards during the daytime when windows are closed. It is difficult to control humidity using passive solutions because both indoor and outdoor humidity levels are high. Nevertheless, one potential solution with relatively low energy consumption to reduce humidity could be the use of dehumidifiers. 3.6.4. Wing wall Wing walls project outward next to a window so that even a slight breeze against a building’s walls creates a high-pressure zone on one side of the wing and a low-pressure zone on the other. The pressure differential draws outdoor air in through one open window and out the adjacent one. Thus, wing walls catch wind parallel with windows. As shown in Fig. 11b, with wind direction parallel to the windows, a high-pressure zone is created in Zone A, and a low pressure zone is created in Zone B. Fig. 11c and d shows indoor air velocity of the baseline and Wing Wall. The air velocity increases from 0.16 m/s to 0.3 m/s. The positions of the designed wing wall are shown in Fig. 12a. The size of Wing Wall is set to be 2 m × 2.6 m. To make wing walls work effectively, the windows need to be controlled such that the windows of Zone A and Zone B are opened

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Fig. 11. Baseline vs. wing wall.

Fig. 12. Wing wall design.

and those of Zone C and Zone D closed in the case of a west wind as shown in Fig. 12a. In the case of an east wind, the windows need to be controlled in the opposite way. This is the most efficient strategy of positioning wing walls for ventilation; unfortunately, it is not practical to change which windows are open and closed as the wind shifts. Hence, a more practical solution is proposed as shown in Fig. 12b. With two wing walls (size of 2 m × 2.6 m; 4 m away from each other and with windows closed in between) on each exterior wall, the internal air velocity of areas near windows will increase from 0.15 m/s to 0.24 m/s with 0.6 m/s outdoor wind that is parallel to the window, as shown in Fig. 13. The result is a more comfortable space, albeit with an uneven distribution of that comfort. There is larger airflow near the windows. It is therefore suggested to let patients who prefer weaker airflow stay in the middle area of the space because airflow near windows is stronger. It should be noted that the windows between the two wing walls should be kept closed only during inter-monsoon months (April, May, October and November) to make wing walls work effectively with the west wind and east wind but not reduce the effective ventilation area for north and south wind during the North and South monsoons respectively.

4. Results Table 8 shows the sources of data used for thermal comfort calculation. PMV and PPD for both healthcare staff and patients were calculated based on ISO 7730. Besides the air velocity, temperature, and relative humidity shown in Tables 9 and 10, other input parameters are: Metabolic Rate of 100 W/m2 for healthcare staff and 58 W/m2 for patients; and Clothing Insulation of 0.3 clo for both. 4.1. Effect of low energy measures A summary of environmental qualities and PMV2 of the different low energy measures considered is shown in Table 9. As seen from the table, the thermal comfort level of patients is mostly within the comfortable range due to their low metabolic rate. At the existing

2 The real sensation of occupancy may not be as severe as the PMV value appears for non-air-conditioned buildings in warm climates. As shown in [44], the highest comfortable PMV (i.e. when PPD equals to 10%) for occupants in Singapore is 0.7 instead of 0.5.

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509

Fig. 13. Baseline vs. wing wall (2).

Table 8 Sources of data used for thermal comfort calculation. Low energy measures

Means of data collection

Baseline Baseline with fans Enlarged louvred window Thermal chimney, enlarged louvred window NAP, enlarged louvred window

NAP, shade, enlarged louvred window Wing wall, enlarged louvred window NAP, shade, wing wall, enlarged louvred window

Indoor air velocity

Temperature

Relative humidity

Field measurement

ApacheSim output

ApacheSim output

CFD simulation Field measurement (when window closed, fan on) CFD simulation (when window open) CFD simulation Field measurement (when window closed, fan on) CFD simulation (when window open)

Table 9 Summary of different low energy measures with existing window frame size. PMVs calculated in this table are based on the PMV/PPD model of ISO 7730. Case

Low energy measures

Indoor air velocity (m/s)

Temperature (◦ C)

Relative humidity

PMV (healthcare staff)

PPD (healthcare staff)

PMV (patients)

PPD (patients)

#0-0 #0-1 #1-0 #2-0

Baseline Baseline, fans on Louvred window Thermal chimney, louvred window NAP, louvred window

0.07 0.29 (staff)/0.48 (patients) 0.34 0.45

28.8 28.8 27.9 27.8

84 84 81 82

1.67 1.48 1.14 1.03

60.3 49.9 32.2 27.4

1 0.28 −0.07 −0.3

26.1 6.7 5.1 6.9

0.29(staff)/0.48 (patients) (window closed, fan on); 0.34(window open) 0.29 (staff)/0.48 atients) (window closed, fan on); 0.34(window open) 0.29 (staff)/0.48 (patients) (window closed, fan on); 0.34(window open) 0.29 (staff)/0.48 (patients) (window closed, fan on); 0.34(window open) 0.37 0.29 (staff)/0.48 (patients) (window closed, fan on); 0.37(window open)

28.2 (fixed schedule)

90

1.33

41.9

−0.02

5

27.3 (auto damper)

90

1.29 1.05

39.9 28.2

0.19 −0.57

5.7 11.8

27.9 (fixed schedule)

91

1 1.24

26.1 37.4

−0.33 −0.19

7.2 5.8

27 (auto damper)

91

1.2 0.96

35.3 24.5

0.03 −0.74

5 16.5

27.9 27.0 (auto damper)

82 91

0.91 1.12 0.96

22.5 31.4 24.5

−0.49 −0.12 −0.74

10 5.3 16.5

0.88

21.4

−0.55

11.3

#30

#40

#5-0 #60

NAP, shade, louvred window

Wing wall, louvred window NAP, shade, wing wall, louvred window

state, healthcare staff feel slightly warm to warm with PMV of 1.48 (fans on) and 1.67 (fans off). This is due to the fact that natural ventilation is not effective, which caused the internal heat gains from the large number of occupants and equipment to be accumulated in the wards, and the air to be still in most areas of the ward. Measure 1-0 is to change existing awning windows to louvred windows. The increase of effective ventilation area makes indoor air velocity improve by about 10 times over the baseline. Furthermore, temperature decreases by 0.7 ◦ C because stronger air exchange between indoor and outdoor removes more internal heat gains. With thermal chimneys (Case 2-0) attached to the ward, indoor air velocity increases to 0.45 m/s which is 32% improvement compared to Case 1-0. NAP with louvred window (Case 3-0) is able to decrease average indoor temperature to 27.3 ◦ C with automated damper controlling status of windows. However, with fixed schedules, NAP fails to reduce average indoor temperature as explained in Section 3. Therefore, automated dampers need to be used for NAP

in hospital wards which are with dense occupancy that generates lots of heat resulting in higher indoor temperature than outdoor sometimes during daytime. If shades (transmission factor of 0.23) are applied (Case 4-0), indoor temperature will drop by 0.3 ◦ C due to reduction of radiant heat. Wing wall with louvred windows (Case 5-0) is able to increase indoor air velocity to 0.37 m/s and works effectively during inter-monsoon periods. A combination of NAP, shade, wing wall and louver window (Case 6-0) offers the best PMV (0.88–0.96) among all the low energy measures. To further improve thermal comfort, enlarged louvred window measures are proposed as shown in Table 10. Enlarged louvred window means extending the current window frames which are 0.6 m from floor level to the floor. A maximum PMV improvement of 0.13 can be achieved through enlarging the window frame size. Therefore, larger window opening for natural ventilation in high occupant density patient wards in the tropical climates is beneficial to thermal comfort improvement. Note that some PMV

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Table 10 Summary of different low energy measures with enlarged window frame size. PMVs calculated in this table are based on the PMV/PPD model of ISO 7730. Case

Low energy measures

Indoor air velocity (m/s)

Temperature (◦ C)

Relative humidity

PMV (healthcare staff)

PPD (healthcare staff)

PMV (patients)

PPD (patients)

#0-0 #0-1 #1-1 #2-1

Baseline Baseline w fans Enlarged louvred window Thermal chimney, enlarged louvred window NAP, enlarged louvred window

0.07 0.29(staff)/0.48 (patients) 0.46 0.6

28.8 28.8 27.8 27.7

84 84 82 82

1.67 1.48 1.02 0.91

60.3 49.9 27.1 22.3

1 0.28 −0.31 −0.56

26.1 6.7 7.1 11.6

0.29 (staff)/0.48 (patients) (window closed, fan on);

28.1 (fixed schedule);

90

1.3

40.3

−0.08

5.1

0.29 (staff)/0.48 (patients) (window closed, fan on); 0.46 (window open) 0.29 (staff)/0.48 (patients) (window closed, fan on); 0.46(window open) 0.29 (staff)/0.48 (patients) (window closed, fan on); 0.46(window open) 0.51

27.2 (auto damper)

90

34.2 1.02

−0.05 26.8

5.1 −0.63

13.3

27.8 (fixed schedule)

91

0.87 1.21

21 35.8

−0.6 −0.25

12.5 6.3

26.9 (auto damper)

91

1.08 0.93

29.7 23.2

−0.23 −0.8

6.1 18.5

27.8

82

0.77 0.99

17.6 25.8

−0.77 −0.38

17.4 8.1

0.29 (staff)/0.48 (patients) (window closed, fan on); 0.51(window open)

26.9 (auto damper)

91

0.93

23.2

−0.8

18.5

0.74

16.4

−0.85

20.2

#31

#41

#5-1 #61

NAP, shade, enlarged louvred window

Wing wall, enlarged louvred window NAP, shade, wing wall, enlarged louvred window

Fig. 14. Recommended measures.

values in Tables 9 and 10 have demonstrated cold sensation of patients, which is mainly due to the lower metabolic rate of patients (58 W/m2 [43]) and light clothing (0.3 clo). Nevertheless, in real life, patients have control of the type of clothing they wear and therefore can prevent the cold sensation. It is important to note that air draft risks, which could be caused by louvred windows, thermal chimneys and wing walls should be assessed before implementing such solutions. Although as indicated in [47] that high speed air movement (0.8 m/s) can be quite acceptable for natural ventilation in hot and humid climates, patients who are sensitive to draft should be located to areas with lower air velocity as indicated in CFD simulation results. For example, in the louvered window solution, the patients can be shifted to the area that is away from the window. In addition, light weight objects should be kept in safe areas of the building where the air velocity is lower. 4.2. Recommendations Advantages and disadvantages of certain measures are observed. For example, the thermal chimney may impede natural daylight and outward views of patients which may affect the overall healing environment within the ward. Also enlarging window frames may not be practical. And in NAP, dehumidifier may

be needed due to high humidity issue. Considering PMV improvement and practicability, recommendations for CGH patient wards are shown in Fig. 14 in ascending order of thermal comfort improvement.

5. Conclusion The primary contribution of this paper is the development of an approach of combining thermodynamics, computational fluid dynamics and thermal comfort level models to effectively analyze and compare the thermal comfort impact of alternative, lowenergy building retrofit concepts in a hospital patient wards. We have found that this approach is effective and provides a systematic method for simultaneously considering low-energy building designs and user comfort. As an application of the approach, the current thermal condition inside naturally ventilated wards in Singapore’s CGH was determined using a Wireless Sensor Network and a portable anemometer. Based on the data collected, the target thermal conditions could be established using PMV model and an extended PMV model for non-air-conditioned buildings in warm climates. To improve thermal comfort, several passive building solutions were modeled and analyzed including a change of the window type,

L. Lan et al. / Energy and Buildings 154 (2017) 499–512

thermal chimneys, night air purge, and wing walls. The louvered window alone offers a 32% improvement of thermal comfort. It is incorporated into all other solutions since the current awning windows have low effective ventilation area. The orientation of the CGH main building effectively makes use of the prevailing wind, but wing walls enhance this effect, particularly the indoor airflow during inter-monsoon months (April, May, October and November). Overall, these recommended measures are very effective and may provide sufficient thermal comfort to the patients. For further improvement on the PMV value, window shades and NAP through automated window dampers can be used. Overall, we find passive solutions to ventilation can be used effectively for a patient hospital ward, even in the tropical warm climate of Singapore.

Acknowledgement This work was supported in part by CGH-SUTD HealthTech Innovation Fund CGH-SUTD-HTIF-2013-008, in part by the project NRF2015ENC-GBICRD001-028 funded by National Research Foundation (NRF) via the Green Buildings Innovation Cluster (GBIC), which is administered by Building and Construction Authority (BCA) – Green Building Innovation Cluster (GBIC) Program Office, and in part by the SUTD-MIT International Design Centre (IDC; idc.sutd.edu.sg). Findings, conclusions, or opinions expressed in this document are those of the authors and do not necessarily reflect the views of the sponsors.

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