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Energy saving potential of low-e coating based retrofit double glazing for tropical climate
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Energy Saving Potential of Low e-coating based Retrofit Double Glazing for Tropical Climate Sivanand Somasundaram , Alex Chong , Zhang Wei , Sundar Raj Thangavelu PII: DOI: Reference:
S0378-7788(19)31278-2 https://doi.org/10.1016/j.enbuild.2019.109570 ENB 109570
To appear in:
Energy & Buildings
Received date: Revised date: Accepted date:
30 April 2019 11 October 2019 1 November 2019
Please cite this article as: Sivanand Somasundaram , Alex Chong , Zhang Wei , Sundar Raj Thangavelu , Energy Saving Potential of Low e-coating based Retrofit Double Glazing for Tropical Climate, Energy & Buildings (2019), doi: https://doi.org/10.1016/j.enbuild.2019.109570
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Highlights
Real life test bedding of low-e coating based retrofit double glazing in tropical climate, was conducted Retrofit double glazing can lead up to 4% annual energy savings when installed on a clear glass for tropical climate and up to 7.5% for retrofit double glazing installed in grey tinted glass (under Singapore climatic conditions) Lux levels were reduced by 75% after installation of retrofit double glazing. Mean radiant temperatures at the centre of the room was 1.3 deg C lower after installation of retrofit double glazing.
Energy Saving Potential of Low e-coating based Retrofit Double Glazing for Tropical Climate Sivanand Somasundaram*, Alex Chong, Zhang Wei, Sundar Raj Thangavelu E-mail: [email protected], [email protected] EPGC, A*star, Singapore- 627590
Abstract Double glazed facades are a well established feature for buildings located in cold ambient climate. In tropical climate there has been some penetration of Double Glazing Units (DGU) to reduce air-conditioning load. However, for existing buildings with older glass (usually having lower thermal insulation and higher solar transmission) a simple retrofit solution is to attach a low-e (hard coat) coated glass internally in the building without an explicit air tight seal. To analyse the proposed retrofit double glazing solution, experiments were conducted in a fully instrumented living lab to observe the performance viz: energy saving potential, Lux levels, and mean radiant temperature. Simulation using an energy model was conducted to estimate the annual energy savings. It is observed that retrofit hard coat low-e double glazing saves up to about 9 % of daily energy consumption of Air conditioners (with annual average of 3% for Singapore weather conditions), when installed on all three sides (SE,NW,NW) , for clear glass with window to wall ratio of 20 % and air change rate of 1 ACH. For the same conditions the annual average energy savings can go up to 7.5 % when installed on an existing grey tinted glass.
Keywords Retrofit double glazing, test bedding, energy model
1. Introduction Every country has a strong plan to have a large number of green buildings within the next decade. The major challenge for this goal is to turn the large number of existing old buildings (about 80% of the building stock in Singapore) greener with minimal cost. As HVAC related energy consumption in commercial buildings for hot and humid climates is about 40-50% [1–3] , of the total building energy consumption, the first step has been to install and replace with more energy efficient modern chillers, which has been implemented as a regulation in many countries including Singapore. The second step is to reduce the total heat load by various measures, one of which is to reduce the heat gain through the envelope. It has been reported that the insulation
of the wall and roof, the air tightness and window replacement have the most impact on energy saving and allowed reducing 45% of the total annual energy consumed [4]. Most of the commercial buildings have multiple glass facades for various reasons like – aesthetics, to have natural view of outdoors to provide comfort for residents, to use natural day lighting and as well as to reduce the total building weight. Energy saving potential of double glazing in cold climates to reduce heating energy is well established and widely adopted in practice. However, for tropical hot climates the adoption has been low. So, there have been numerous studies[5–8] to understand the optimal type of double glazing and their energy and economic impact as well. Older buildings have normal clear or tinted glass (single glass facade). This in turn leads to increased glare in the building and hence occupants opt for window curtains/blinds and use of artificial lighting. Similarly, heat gain into the room in the form of direct solar transmission and convected/re-radiated thermal heat is increased with use of clear or tinted glass. Various retrofit studies [9–14] to improve the performance of existing buildings have also indicated that glazing is a critical part of the building envelope and need to be focused on to push the existing old building towards nZEB (net zero energy building) state. Puala et al. [13] discuss the commonly implemented building energy retrofit options. Among the options discussed upgrading window type and improving facade insulation is shown to be significant. Serghidehes et al.[12] studied the economic impact of various retrofit measures and their impact on carbon emissions as well the payback period of proposed retrofit measures. The results indicated that for Cyprus, replacing single glazed windows by double glazed windows can result in energy savings which can lead to an annual pay back of investment within 4-5 years. Mohamad et al. [8] investigated the impact of retrofit glazing in existing housing apartment and found that replacing a single clear glass by a double grey low-e glazing reduced cooling load by 14%, and by use of a triple low- e glazing , cooling loads can be reduced by up to 31% . However, most of these studies considered a complete upgradation of window by a double or triple glazing after replacing the existing single glazing. In such a solution, the (material and labour) cost, installation time and building downtime are all very high and it is difficult to convince the building owner. So, the proposed solution (retrofit double glazing) is to install a hard coat low e-coating based glass as the second layer behind the existing glass as shown in Fig. 1. This will retain the aesthetics, outside view, daylight and will simultaneously reduce the heat load that the air-conditioner needs to work against. Ariosto et al. [15] describe different possible window retrofit solutions which do not require complete replacement of the existing window, but involves using additional systems like curtains, drapes, blinds, films,
screens, shutters and storm windows. They analysed the performance of exterior storm windows in cold climates and concluded significant thermal performance gain without impacting the daylight. Smith et al.[16] report of secondary glazing (or internal storm windows) that can be used as a functional alternative to retrofitted insulated glazing units (IGUs) in existing domestic single-glazed window frames. They investigated the thermal transmittance of four secondary-glazing products – plastic film, magnetically attached plastic sheet, plain and low-E glass. This was in the context of winter climates and they found that the proposed secondary glazing increased the resistance values from the base value of 0.15 m2 K W-1, to a range of 0.34 to 0.57 m2 K W-1. It was a lab test to measure the thermal properties and not real test bedding. Moreover they have not reported the impact on SHGC etc. The current work aims to quantify the energy savings, thermal and visual comfort of the proposed retrofit double glazing under tropical climate in real life conditions. The proposed solution can be implemented in two ways.
Figure 1. Illustration of Retrofit Double glazing First method is by creating a local on-site double glazing unit with a sealed air gap as in conventional double glazing unit. This would involve creating a single glass piece of similar size to be manufactured and installed with a standard spacer. The sealed construction would allow high performance low- e coatings such as
soft coatings and would have more impact on energy savings however the cost and practical challenges in installation cost and time, especially for large and tall facades need to be taken in consideration. The second method is to install an inner window having a non-sealed air gap. In this method the whole facade is covered by multiple panels bridged by non-air tight metallic strips and placed on a custom frame. The room air flows in and out of this air cavity. So the low-e coating in this method would be typically hard-coat type to ensure durability of the coating. This leads to slightly lower impact in energy savings but this method is very easy, quick and cheap to do the retrofit installation. The current work investigates the retrofit double glazing installed by the second method wherein hard coat low-e coating at position 3 (facing the existing facade) serves as an inner window.
2. Description of test-bedding The test bedding was carried out in level 3 of a low rise office cum research facility building located in Singapore. The room had 3 existing window facades in the SE, SW and NW directions. The NE direction was covered by an opaque wall. The room orientation is indicated in Fig. 2. The room is approximately 10.65 m in length and 5.95 m in width. The total height from floor to roof is 4.95 m, with a 1.65 m high false ceiling formed by gypsum boards. There are fixed sunshades over the SW and NW facade.
Sensors Uncertainty in measurements Method of analysis Plots and discussion Simulation model description Parameters set
Shade
SW Facade
Figure 2. Simplified plan view of room used for test-bedding
The room is cooled by two (Daikin) fan coil units (FXFQ80P, 9.3kW cooling capacity and FXFQ80P, 14 kW cooling capacity) which are connected to two individual outdoor compressor/condenser units (RXMQ4P and RXMQ5P). The cooling capacity and power consumption of RXMQ4P was 11.2 kW and 2.95 kW and that of RXMQ5P was 14 kW and 3.97 kW. The room was cooled from 9 A.M. to 6:30 P.M. to a set point temperature of 25 °C. The fresh air flow measured at the fresh air duct was 5.7 m s-1 and it was equivalent to an air change rate of 1 ACH (Air Change per Hour). As per Singapore Standards 553: 2016 [17], for the size of this room, the minimum recommended ACH turns out as 0.78 ACH. The infiltration through the building envelope during day time when the fresh air and fan coil units are operating is almost zero as it a positive pressurised HVAC system, with room pressure slightly above the ambient. The room occupancy is nominal about 5 persons for 1 hour during 12 P.M. to 1 P.M. The NW façade has an additional solar control film on the existing glass facade (15 mm clear glass) installed at position 2. The properties of the solar film are stated in Table 1 below. The SE and SW facades have 10 mm thick clear glass windows, which are without solar control film. The properties of the existing glass facades and the retrofit glass (as per specification sheet) are tabulated in Table 2. Table 1. Properties of existing Solar Control Film in NW Note: The following are specs of the IR film (IR9010, 2 mil thickness) when applied on a 6 mm clear glass. Total transmission Solar energy - Reflection Solar energy - Absorbed Visible light – Transmission Visible light- Reflection Infra-red rejection UV rejection Shading coefficient Total solar energy rejection Heat transmission (W/m2K)
9% 20% 71% 15% 11% 75% 99% 0.25 82% 5.7
Table 2. Properties of glass facades Glass description
Visible light %
Solar energy %
U value
Clear glass
Transmittance
Internal. Reflection
External reflection
Transmittance
Reflectance
10 mm( SE and SW)
87
-
8
72
7
4.96
0.89
15 mm ( NW)
84
-
8
58
7
4.88
0.85
Shading coefficient
W m-2 K-1
Sunergy Gray, 8 mm, with hard coat low-e (Position 2)
26
4.9
8.3
23.2
5.7(int) 9.4(ext)
4.1
0.41
The retrofit glass (Sunergy grey, AGC) was a hard coat low-e glass (specifications listed in Table 2). The hard low-e coating was on position 3, facing the existing glass.
Figure 3. Details of installed retrofit glazing system The details of the final retrofit glazing installed in the test bedding room is shown in Figure 3.The existing thickness of frame in SE and SW windows, made it necessary to increase the air gap between the existing glass and the retrofit glass to about 30 mm, whereas in NW it was maintained at 17 mm. As described earlier the air gaps are not sealed and are exposed to room environment. SE glazing measured 5.9 m in width and 1.7 m in height, SW glazing was 4.95 m in width and 1.75 m in height, whereas NW glazing was 5.9 m in width and 2.34 m in height. The walls were made of 200 mm bricks with 10 mm gypsum plastering on the inner and outer sides of the brick. The roof construction had
multiple layers - 200 mm reinforced concrete slab with a 100 mm screed layer followed by water proofing and then with a 50 mm cast concrete layer. The floor construction was using 250 mm concrete slab. There were three sets of data measured as follows a)
Outdoor Environmental Conditions
The first group consisted of outdoor weather data which included – Global (Total) solar irradiance, diffuse solar irradiance, ambient temperature, relative humidity, rain status and outdoor illuminance. The sensors were located at the open roof area nearby. The Global solar irradiance and diffuse solar irradiance was measured by a pyranometer (class 1) with accuracy of ±5% ± 10 W m-2 for hourly averages and ±8% ±10 W m-2 for individual readings. Ambient dry bulb temperature and relative humidity was measured with accuracy of ±0.25 °C and ±1.5% respectively. The rain detector senses the presence of rain droplets on the sensor and logs the binary output, to help classify rainy periods. The outdoor illuminance was measured with accuracy of ±3%. b) Room Conditions The second group of sensors were for room internal condition monitoring which included - room temperature, mean radiant temperature and relative humidity (both at the centre and at the corner of the room), Lux meter at the centre of the room, room CO2 levels, heat flux sensors, Indoor normal solar irradiance, façade temperature using surface thermocouple. There were also two condensation sensors placed in NW and SE façade to detect any condensation in the air gap. The room temperature and humidity was measured with accuracy of ±0.2°C and ± 1.7 % respectively. The globe temperature sensor to obtain mean radiant temperature used a Pt-100 sensor with accuracy of ± 0.1 °C. The room Lux levels were measured using a sensor which had an accuracy of 1.5% ± 2 Lux. CO2 levels were measured with accuracy of ± 20 ppm ± 1 % of reading. The indoor normal solar irradiance transmitted through the facade was measured by placing a class 1 pyranometer, (tilted by 90° positioned inside the room facing the facade), which had an accuracy of 1.5 %. Heat flux sensors were placed both on surface position 2 and position 4 to measure the convected and radiated heat from the facade surface. Heat flux sensors have an accuracy of ± 3%. Facade surface temperatures of both existing panel and new panel, were measured using a surface mount T-type thermocouple with accuracy of 0.75%. The air temperature in the air gap between the panels was also measured using a bare junction T type thermocouple with 0.75% accuracy. The condensation sensor is used to detect the presence of condensation, frost, or ice using dielectric based capacitance sensing. It is located in the air gap between the glass panels.
The data acquisition card for 4-20 mA signals (indoor and outdoor temperature and RH, Indoor and outdoor Lux, CO2 level) had a measuring error of ± 0.3 %. The card for measuring voltage outputs (Heat flux sensors, condensation sensor, Total and diffuse irradiance measurement by pyranometer) had a measuring error of ± 0.05 %. The card used for measuring Pt-100 sensor for globe temperature in calculation of mean radiant temperature had an measuring error of ± 0.5 °C. For outdoor illuminance measurement – photometric probe (LP-02, Delta Ohm) was used it is a solid state sensor, whose spectral response is corrected by filters to fit the response of the human eye. For indoor illuminance Photodiode based sensor was used. The sensor used for indoor temperature measurement is a calibrated digital temperature sensor while for outdoor ambient measurement a Pt1000 RTD Class F0.1 is used, c)
Air-conditioning, Power and Energy Consumption
The third group of sensors were for monitoring the Air-con system parameters, this included two power meters to measure the individual energy consumption of both the FCU’s and also data from in-built sensors in the Daikin air-conditioning system like system status, air flow speed and direction, return air temperature and temperature set point. The digital power meters were able to record both instantaneous power consumption of air-conditioning (outdoor unit) as well as the cumulative energy consumption both with an accuracy of 0.2% All the three sets of data were collected at one second interval and subsequently sampled at 30 second interval for plotting comparative graphs. The data was averaged on hourly basis for hourly data comparisons. First, a baseline data was collected by conducting a baseline experiments wherein all the data were collected without installing the retrofit double glazing. This was carried out from 14 th July 2018 to 1st August 2018. Subsequently, same set of data was collected after installing the retrofit double glazing (inner window) from 6th August to 28th August 2018.
3. Details of energy model simulation The test bedding described above serves as a demonstration of the proposed retrofit double glazing in the tropics. The test bedding is useful to evaluate the thermal comfort, visual comfort, detection of condensation and also the energy savings. But the energy savings cannot be strictly quantified as there are seasonal variations and to have all variations accounted for in the one to one comparison of baseline and retrofit scenarios is practically infeasible. So in order to assess the yearly performance and quantify the annual energy savings a simulation model was necessary.
The room was modelled in Design builder interface, which uses Energy Plus as the background engine. The objective was to calculate the total cooling load in the room arising from various sources of heat gain during the baseline and as well as in retrofit double glazing configuration. Properties of walls, roofs and Glazing were assigned as per the as-built drawings. The walls were assigned as 200 mm bricks with gypsum plastering. The roof was modelled as a 200 mm layer of reinforced concrete (with multiple over layers consisting of screed, cast concrete and asphalt). The glass properties of the baseline glass, solar film and the retrofit inner window were set as per the specifications listed in Tables 1 to 2. The fresh air flow rate into the room was set to 1 ACH as per actual site measurement. This flows only during working hours (8 A.M. to 6:30 P.M.), which is the schedule for the air-conditioners in this room. The false ceiling was set as a void zone in the model, so the volume above false ceiling is not accounted for in ACH calculations. With this setting the air above the false ceiling is also not conditioned as in the room however, the heat gain from the side walls in the false ceiling to the side wall area(below false ceiling) does contribute to the heat gain of the room. The air conditioner was set to ―simple:HVAC‖ type with a supply air of 16 °C (99 % R.H). So, the partial dehumidification load was taken into account as well. The HVAC system was designed for the room to be in positive pressure, so infiltration from building envelope during the working hours (8 A.M. to 6:30 P.M.) was assumed to be zero. The night time infiltration (6:30 P.M. to 8 A.M.) was estimated to be 0.3 ACH. The plug loads were also estimated which arise mainly from control panel fans (24 hours operation) and as well as from the power consumption of indoor fan coil units (8 A.M. to 6:30 P.M.). For simulation of outdoor weather, when comparative results with respect to the measurements were required, a custom weather file based on measured outdoor weather parameters was used. However for the annual simulation the ASHRAE’s IWEC weather file for Singapore was used.
4. Data analysis of measured data from test-bedding The objective of the study was to compare the HVAC energy consumption, lux levels and thermal comfort before and after installation of retrofit double glazing, when the outdoor ambient weather conditions (Solar irradiance, temperature and humidity) are similar. It was very rare to have comparable two days which had exactly the same weather pattern from morning 8 A.M. to 6:30 P.M. So for comparison purposes, hourly averages of weather parameters and measured objectives (Lux, temperature, energy consumption) were computed and compared when the baseline and retrofit double glazing had similar hourly averages of all the three weather parameters (temperature, humidity and solar irradiance). So it was essential to fix a tolerance level (difference between two parameters) for assuming that the two hours in consideration are representative of
similar weather conditions. So the air conditioner’s power consumption was plotted as function of the following variables -∆Tar(Ambient DBT- Room DBT) , ∆Har(Ambient WBT- Room WBT) and Total solar irradiance. It was observed that the ∆Tar and ∆Har played the dominant roles in affecting the air-conditioners energy consumption, with almost a direct proportionality. This is probably because when the outside ambient temperature is high, the conduction heat gains into the room and as well the sensible heat load of fresh air increases and also the COP of the air conditioner decreases with increasing ambient temperature. Similarly when the ambient humidity is high the dehumidification load from fresh air on the evaporator coil is higher. Solar irradiance did have an effect on the air conditioner’s power consumption with the amount of solar radiation transmitted into the room. However, the effect was not as significant as the other two parameters, (for the current scenario having a particular geometry and fresh air flow rate). Another possible reason for this observation is that in the approach taken by comparing individual hourly weather zones independent of the previous hour zones, some effect of the solar radiation is carried on to subsequent hours and effect is not realized instantaneously as the walls and roofs will absorb the heat and radiate it slowly. This could be another reason why solar radiation is not having a clear correlation like ambient temperature and humidity. So, it was set that hourly averages which satisfy the following tolerance levels for all the three parameters can be considered as similar weather for the sake of one to one comparison of baseline and retrofit double glazing- 0.2 deg C for ∆Tar , 0.3 deg C for ∆Har, ±20% variation for Total solar irradiance.
5. Results and discussion from test-bedding 5.1 Air-conditioning energy consumption The section below analyses the measurement results for configuration 1 and 2. All three facades (SE, SW and NW) using the existing single glazing (baseline glass alone) is referred to as configuration 1/Baseline and all three sides with retrofit double glazing (Inner window) is referred to as configuration 2/Double glazing. While making comparisons between Baseline and Double glazing it was ensured that the time of the day is more or less within the same time period. So comparisons of data were made separately for the following time periods: 9 A.M. to 12 P.M., 12 P.M. to 3 P.M. and 3 P.M. to 6 P.M.
5.1.1 Comparison during morning period (9 A.M. to 12 P.M.) During this period of time in a day the ambient temperature is not very high and is slowly rising, also the main solar radiation transmission is through SE facade as the sun is mostly in the east during this period. The hourly variation of air-con electricity power consumed in (kW) is plotted with varying dry bulb temperature difference
∆Tar (between ambient and room) in Figure 4a, with Total solar irradiance in Figure 4b and with wet bulb temperature difference ∆Har (between ambient and room) in Figure 4c. Room temperature and room humidity refers to values measured at the centre of the room. The overall trend depicts that hourly air-con power consumption increases from 1 kW to 1.6 kW when ∆Tar increases from 1.2 to 4 °C. It is also observed that in general the retrofit Double glazing (configuration 2) consumes less power at most instances for the same ∆Tar. In Figure 4b, the increasing air-con power consumption with increasing solar radiation due to increase in transmitted direct solar radiation into the room is observed. The effect is not so strongly pronounced as with ∆Tar. In Figure 4c effect of wet bulb temperature (WBT) difference between ambient and room, ∆Har, which is a measure of the latent load (heat rejected to the evaporator coil when the water vapour in the return/fresh air condenses) is shown. As expected to maintain a lower WBT in the room a higher power consumption is required. The plots in Figure 4 give a general picture, but the individual isolated effect cannot be observed as the other two parameters in each plot is uncontrolled and not kept as a constant. So in order to quantify the actual savings in a similar weather condition, time periods between Baseline and Double glazing which are very closely matching were chosen, based on tolerance levels discussed in section 3, and analysed. It was observed that power savings for individual comparable hourly zones varied from 2 to 22 % and some instances have –ve effect varying form – 5 to -12 %. Installation of retrofit double glazing reduces transmission of direct solar radiation, but the blocked radiation is absorbed and re-radiated into the room. These are two competing effects which determine the net heat gain from the facade. One pair of comparable hourly zones from baseline and double glazing is illustrated in Figure 5 and 6. 31/7/2018 (Baseline) and 18/8/2018 (Double glazed) (11 A.M. to 12 P.M.) were chosen to depict the variation of air-con power and facade heat flux in both the cases. Figure 5 depicts that they are comparable periods with similar weather conditions.
Figure 4. a) Variation of Air-con power with dry bulb temperature difference between ambient and room for configuration 1 and 2, between 9 A.M. to 12 P.M. b) Variation of Air-con power with solar irradiance for configuration 1 and 2, between 9 A.M. to 12 P.M. c) Variation of Air-con Power with wet bulb temperature difference between ambient and room for configuration 1 and 2, between 9 A.M. to 12 P.M. From Figures 6a and 6b it is clear that the thermal heat flux entering into the room through the SE and SW facades in the Double glazed configuration is much higher than the Baseline cases (as the difference in heat flux (Baseline- Double glazed) is negative), especially through SE where there is direct sunshine, normal to the facade, during the current period (11 A.M.) of analysis. This is attributed to the direct solar radiation blocked by the low e-coating in the double glazing at position 3 and the subsequent heating of the air gap which is eventually emitted as thermal radiation in the room and partially to the ambient. In the NW facade since the solar film cuts the incoming solar radiation and reflects back, the heating up of air gap and re-radiation into the room does not happen here as seen in Figure 6c. From Figures 6d and 6e , it is observed that there are higher power spikes during Baseline days due to higher incoming heat and the double glazed scenario had 22% lower energy consumption for this hour.
Figure 5. a) Variation of ∆Tar b) Variation of ∆Har c) Variation of Total Solar irradiance.
Figure 6. a) Difference in heat flux (Baseline- Double glazed) through SE facade into room b) Difference in heat flux (Baseline- Double glazed) through SW facade into room c) Difference in heat flux (Baseline- Double glazed) through NW facade into room d) Variation of instantaneous electricity consumption by first AC unit. e) Variation of instantaneous electricity consumption by second AC unit
5.1.2 Analysis for afternoon periods (12 P.M. to 3 P.M.) During this period of time the ambient temperatures are higher than morning, also the main solar radiation transmission is through SW and NW facade as the sun is mostly in the west during this period. The hourly variation of air-con electricity power consumed in (kW) is plotted with varying dry bulb temperature difference (between ambient and room) in Figure 7a, with Total solar irradiance in Figure 7b and with wet bulb temperature difference (between ambient and room) in Figure 7c. The overall trend depicts that hourly air-con power consumption increases from 0.8 kW to 2.2 kW when the temperature difference increases from 1 to 5 ° C, in the Baseline case. However in the Double glazed cases the power consumption is more or less within a constant band of 1.3 to 1.8 kW, probably due to the heat trapped in the air gap and re-mitted into the room, which makes the total air-con power consumption less dependent on the actual ambient temperature outside. In Figure 7b, the increasing air-con power consumption with increasing solar radiation (due to increase in transmitted direct solar radiation into the room) is observed. In order to quantify the actual savings in a similar
weather condition and for similar thermal comfort, time periods between Baseline and Double glazing which are very closely matching were analyzed. It was observed that the power savings range varies from 8 to 26 % and some instances have –ve effect of -2 to -10 %. The two competing effects of reduced transmitted solar radiation and increased thermal re-radiation are applicable in this facade too, but this facade (SW) area is smaller and the facade has a shade as well. 19/7/2018 (Baseline) and 6/8/2018 (Double glazed) (2 P.M. to 3 P.M.) were chosen to depict the variation of air-con power, and facade heat flux in both the cases. Figure 8 depicts that they are very much similar and comparable weather periods. From Figures 9d and 9e it can be observed that there are similar power cycles in AC unit 1 but higher power spikes in unit 2 during Baseline days due to higher incoming heat and thus the retrofit Double glazing scenario led to 12.7% lower power consumption. From Figures 9 a and 9b it is clear that the thermal heat flux entering into the room through the SE and SW facades in the Double glazed configuration is much higher than the Baseline cases (as the difference between heat flux values in Baseline and Double glazing is negative). Even though the sun has moved away from the east the heat flux from SE facade in the Double glazed facade is significant because of the radiated heat from the heated air gap earlier in the morning. However, in the NW facade since the solar film cuts the incoming solar radiation and reflects back the heating up of air gap and re-radiation into the room does not happen here as seen in Figure 9c.
Figure 7. a) Variation of Air-con power with ∆Tar for configuration 1 and 2, between 12 P.M. to 3 P.M. b) Variation of Air-con power with solar irradiance for configuration 1 and 2, between 12 P.M. to 3 P.M. c) Variation of Air-con Power with ∆Har between 12 P.M. to 3 P.M.
Figure 8. a) Variation of ∆Tar b) Variation of ∆Har c) Variation of Total Solar irradiance
Figure 9 a) Difference in heat flux (Baseline- Double glazed) through SE facade into room b) Difference in heat flux (Baseline- Double glazed) through SW facade into room c) Difference in heat flux (Baseline- Double glazed) through NW facade into room d)Variation of instantaneous electricity consumption by first AC unit. e) Variation of instantaneous electricity consumption by second AC unit.
5.1.3 Analysis for evening periods (3 P.M. to 6 P.M.) The hourly variation of air-con electricity power consumed in (kW) is plotted with varying ∆Tar in Figure 10a, with Total solar irradiance in Figure 10b and with ∆Har in Figure 10c. The overall trend depicts that hourly air-
con power consumption increase from 0.8 kW to 2.5 kW when the temperature difference increases from 1 to 5 ° C, in the Baseline case. In Double glazed cases the power consumption varied between 1.3 to 2.3 kW for a temperature difference between 2 to 6 °C. Compared to mornings and afternoons, this period of 3 P.M. to 6 P.M. has very strong linear effect of humidity, probably because the solar transmission is reduced during this period as the NW façade which receives direct sunlight during this period has 2 layers of protection - solar film and retrofit double glazing. In order to quantify the actual savings in a similar weather condition and for similar thermal comfort, time periods between Baseline and Double glazing which are very closely matching were analysed. It was observed that power savings varied from 5 to 15 %. 23/7/2018 (Baseline) and 15/8/2018 (Double glazed) (4 P.M. to 5 P.M.) were chosen to depict the variation of air-con power, facade heat flux and direct transmission of solar radiation through the facade in both the cases.
Figure 10. a) Variation of Air-con power with ∆Tar for configuration 1 and 2, between 12 P.M. to 3 P.M. b) Variation of Air-con power with solar irradiance for configuration 1 and 2, between 3 P.M. to 6 P.M. c) Variation of Air-con Power with ∆Har for configuration 1 and 2, between 3 P.M. to 6 P.M.
Figure 11. a) Variation of ∆Tar b) Variation of ∆Har c) Variation of Total Solar irradiance
Figure 12. a) Difference in heat flux (Baseline- Double glazed) through SE facade into room b) Difference in heat flux (Baseline- Double glazed) through SW facade into room c) Difference in heat flux (Baseline- Double glazed) through NW facade into room d) Variation of instantaneous electricity consumption by first AC unit. e) Variation of instantaneous electricity consumption by second AC unit f) Variation of indoor normal irradiance near NW facade. Figure 11 depicts that they are very much similar and comparable weather periods. From Figures 12d and 12e we observe similar power cycle in AC unit 2 but higher power spikes in unit 1 during Baseline days due to higher incoming heat. From Figures 12a and 12b it is clear that the thermal heat flux entering into the room through the SE and SW facades in the Double glazed configuration is much higher than the Baseline cases. Even though the sun has moved away from the east the radiation from SE facade in the Double glazed facade is significant because of the radiated heat from the heated air gap earlier in the morning. However, in the NW facade since the solar film cuts the incoming solar radiation and reflects back, the heating up of air gap and reradiation into the room does not happen here as seen in Figure 12c. The actual direct solar radiation (in visible spectrum is reduced) coming into the room is reduced by 80 % from 40 W m-2 to 8 W m-2 after installation of Double glazing as shown in Figure 12f . Indoor normal irradiance in South east and South west are unavailable as the sensor was placed only towards NW facade. From the above analysis it is clear that there are two competing physical mechanisms operating after installation of retrofit low- e inner window. The direct solar
transmission is significantly cut down as seen by drop in indoor irradiance and Lux measurements (discussed in section 5). However the stopped radiation is absorbed by the low-e layer and re-radiated both into the room as well as to ambient. So the net difference between reduction in direct solar transmission and increase in reradiated/convected heat from the hot inner window determines the total heat gain reduction of the room. However, for NW facade with solar film on the baseline glass significant amount of solar radiation is rejected in the baseline glass and so the low-e layer does not become very hot unlike in a plain clear glass facade(as in SE/SW). But the low- e layer does cut down additional solar transmission over and above that of solar film. So the temperature of the retrofit inner window is not very high and the re-radiated or convected heat into the room is smaller (as seen by lower heat flux in NW facade after installation of retrofit inner window) and hence the total room heat gain is lower.
5.2 Indoor Lux levels Figure 13. shows the daily Lux values for configuration 1 and configuration 2 measured at the centre of the room at 1.2 m height, which corresponds to working level. It is very evident that Double glazing on all three sides of the room reduced the Lux levels to the comfortable range of 500, which is within the recommended Lux level as per Singapore Standards 531: part 1, 2006 [18] standards (lighting for standard office work is to be between 300- 500 Lux). The average reduction in Lux levels was about 70%.
Figure 13. Daily variation of Lux for configuration 1 vs 2
Figure 14a shows the daily variation of Lux profile during a sunny day for configuration 1 (Baseline) and configuration 2 (Double glazed). The Double glazed scenario has reduced the glare and maintains comfortable range of Lux levels except during the 1st working hour (8 A.M. to 9 A.M., when the Lux levels are low and would require supplementary lighting). Figure 14b shows the outdoor illuminance for the days compared in Figure 14a and outdoor illuminance is similar on both days, yet the indoor illuminance was well controlled and glare was prevented due to installation of retrofit double glazing. In the absence of this retrofit installation, blinds might have been used to protect from glare, but eventually would have required supplementary lights to be turned on.
Figure 14. a) Typical day variation of Lux for configuration 1 vs 2 b)Outdoor illuminance variation for typical days compared in Fig 7.2
5.3 Risk of condensation in air-gap The air gap in the current design of retrofit inner window is not sealed. It is also not ventilated, so under humid conditions and at low ambient temperatures there is a possibility of humid air condensing in the air gap. To monitor the condensation status, one condensation sensor was placed in the air gap in SE façade and one in NW façade. The outdoor ambient humidity levels reached maximum values around 98% during rainy periods. However, no condensation was observed throughout the entire testing period.
5.4 Thermal Comfort Mean radiant temperatures were measured at the centre of the room and also at the corner (intersection of SW and NW facade) of the room. The mean, maximum and minimum mean radiant temperatures (MRT) during the baseline measurement (during 8 A.M. to 6:30 P.M. at the centre of the room ) were 33.4 °C, 35.6 °C and 31.6 °C respectively , whereas after retrofit double glazing of low-e glass they were 32.1 °C, 35.1 °C and 31.0 °C.
Since thermal comfort of the occupants is influenced both by the air temperature and as well the surface temperature of the room, lower MRT would reflect as better thermal comfort for the occupants. Similarly, the average mean radiant temperature measured at the SW-NW corner of the room was 1.9 °C lower after installation of retrofit double glazing. This will lead to better thermal comfort for occupants seated nearer to the facade.
6. Results from energy model simulation The simulation models (for both baseline and Double-glazed configuration) was run using the actual measured weather data during the test bedding. Then the simulation predicted HVAC daily consumption was plotted along with the actual measured HVAC daily consumption (by power meters). The same is plotted in Figure. 15. (July data points represent baseline and August represents Double glazing). The average deviation between simulated power consumption and model predicted power consumption ( for the whole data set July – August) was about 10%, which confirmed that the simulation model is robust and reflects the actual real-life conditions.
Daily HVAC Power Consumption (kWh)
23
Measured daily HVAC power consumption vs Simulated daily HVAC power consumption
21 19 17 15 13 11 9
Simulation Model
7
Actual measurement
Date Figure 15. Comparison of simulated HVAC daily power consumption with the actual measured power consumption.
8/28/2018
8/26/2018
8/24/2018
8/22/2018
8/20/2018
8/18/2018
8/16/2018
8/14/2018
8/12/2018
8/10/2018
8/8/2018
8/6/2018
8/4/2018
8/2/2018
7/31/2018
7/29/2018
7/27/2018
7/25/2018
7/23/2018
7/21/2018
7/19/2018
7/17/2018
5
Figure 16. Typical hourly heat gain distribution in baseline scenario (clear glass on all three sides and one facade (NW) with a solar film) compared with typical hourly heat gain distribution after retrofit installation of Inner window over the existing configuration Both the actual configuration with existing baseline glass as well as after installation of retrofit inner glazing was modelled separately. Table 3. lists the average (averaged over 19 days) hourly cooling load for every individual hour. Retrofit double glazing has higher power consumption in the 1st hour due to higher room temperature at the start of the day as it cannot lose heat to the ambient (during night) easily unlike the single glass facade. Subsequently, it is seen that percentage energy savings are minimal in the 9 A.M.-12 P.M. period, moderate in the afternoon 12 P.M. to 3 P.M. and maximum in late afternoon 3 P.M. to 6 P.M. As explained in the earlier sections, it can be attributed to the fact that during afternoons and late afternoons amount of incident solar radiation on NW facade (installed with solar film) increases. The retrofit inner window installed in this facade is more effective as it is able to operate at lower operating temperatures. Figure 16 illustrates the drastic heat gain reduction via window solar transmission after installation of the double glazing, though there is a significant increase in glazing heat gain (conduction and convection). But the net effect is reduced heat gain after retrofit installation.
Table 3. Simulated hourly average cooling load
Time
Baseline
Retrofit Inner window
Cooling load (kW)
Cooling load (kW)
8-9
3.92
4.02
-2.70
9-10
3.31
3.25
1.72
10-11
3.38
3.34
1.33
11-12
3.46
3.37
2.73
12-13
3.81
3.71
2.64
13-14
3.74
3.62
3.17
14-15
3.84
3.62
5.64
15-16
4.00
3.70
7.54
16-17
4.04
3.73
7.80
17-18
3.97
3.66
7.64
18-18:30
1.85
1.72
6.91
Total day
39.31
37.74
4.00
% Energy savings(HVAC)
1.93
3.82
7.66
Annual simulation using Singapore’s typical weather was carried out for scenarios, before and after installation of retrofit inner window. The daily savings along with the average ambient temperature is plotted in Figure 17.
Figure 17. Annual energy savings of retrofit double glazing for the current room configuration (SE and SW clear glass and NW clear + solar film) It is evident that the daily energy savings increases on hotter days and can be negative on cloudy or rainy days, when the ambient temperatures are lower. The maximum energy savings was about 6% and the yearly average was about 3%. Also a different scenario or case was studied where the existing glass was assumed to be 10 mm thick grey glass (solar Transmittance- 0.383, VLT 0.483) (SE and SW – grey glass and NW – clear glass with solar film). The annual energy savings for this scenario is plotted in Figure 18.
Figure 18. Annual energy savings of retrofit double glazing for grey glass room configuration (SE and SW grey glass and NW clear + solar film) It is evident that installing retrofit double glazing on a grey glass has improved the annual performance significantly. The maximum daily savings can be as high as 12 % and the yearly average turns out as 7.5%.
7. Acknowledgements The project was carried out as part of A*STAR’s internally funded project ICES-17188A04. We acknowledge and thank AGC Asia Pacific Pte Ltd for supply and installation of hard coat low-e glass panels and for providing technical specifications of the glass panels.
8. Conclusions The test bedding of double glazing in tropical climates was carried out and its effect on air-con power consumption, mean radiant temperature and indoor illuminance levels were observed and reported. From the overall analysis of both measurement and simulation results it is evident that double glazing can provide significant energy savings in tropical climate (from 4 to 10% depending on existing glazing type and area and as well as other heat gains of the building). It is more effective when used along with additional protection from solar radiation as in following scenarios:
Installing retrofit double glazing when the original facade is a tinted glass (grey/green/blue etc)
Installing retrofit double glazing over clear glass after installation of solar film on the clear glass
Mean radiant temperatures were 1.3 °C lower after installation of retrofit double glazing, which will increase the thermal comfort of the occupants and can possibly allow a slightly higher set point for the room air temperature.
It is very clear by consistent measurement results that the current tested (ATTOCH Sunergy Grey) Low- e based double glazing reduces the visible light transmittance by about 75%. It is useful in scenarios with high level of day lighting to reduce glare and to bring down the Lux level to recommended levels. However, if double glazing is used along with solar film selective sides of a room alone must have solar films or a lighter tint version of solar film or Low –e coating should be chosen to help maintain the required minimum Lux levels. Alternatively, an optimization calculation between reduction in cooling load and lighting loads can be down to identify the optimal combination of solar film and Low-E based double glazing. The effect of installing a retrofit double glazing over a clear glass facade and over a solar film protected clear glass was inferred by analysis of measurement and simulation data. However, in future studies we expect to isolate two of the facades in the room at a time to study the individual effect of each type of facade. It would also be of interest to study the effectiveness of the proposed retrofit solution in each individual direction (north, south, west and east).
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Energy Saving Potential of Low e-coating based Retrofit Double Glazing for Tropical Climate Sivanand Somasundaram , Alex Chong , Zhang Wei , Sundar Raj Thangavelu PII: DOI: Reference:
S0378-7788(19)31278-2 https://doi.org/10.1016/j.enbuild.2019.109570 ENB 109570
To appear in:
Energy & Buildings
Received date: Revised date: Accepted date:
30 April 2019 11 October 2019 1 November 2019
Please cite this article as: Sivanand Somasundaram , Alex Chong , Zhang Wei , Sundar Raj Thangavelu , Energy Saving Potential of Low e-coating based Retrofit Double Glazing for Tropical Climate, Energy & Buildings (2019), doi: https://doi.org/10.1016/j.enbuild.2019.109570
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Highlights
Real life test bedding of low-e coating based retrofit double glazing in tropical climate, was conducted Retrofit double glazing can lead up to 4% annual energy savings when installed on a clear glass for tropical climate and up to 7.5% for retrofit double glazing installed in grey tinted glass (under Singapore climatic conditions) Lux levels were reduced by 75% after installation of retrofit double glazing. Mean radiant temperatures at the centre of the room was 1.3 deg C lower after installation of retrofit double glazing.
Energy Saving Potential of Low e-coating based Retrofit Double Glazing for Tropical Climate Sivanand Somasundaram*, Alex Chong, Zhang Wei, Sundar Raj Thangavelu E-mail: [email protected], [email protected] EPGC, A*star, Singapore- 627590
Abstract Double glazed facades are a well established feature for buildings located in cold ambient climate. In tropical climate there has been some penetration of Double Glazing Units (DGU) to reduce air-conditioning load. However, for existing buildings with older glass (usually having lower thermal insulation and higher solar transmission) a simple retrofit solution is to attach a low-e (hard coat) coated glass internally in the building without an explicit air tight seal. To analyse the proposed retrofit double glazing solution, experiments were conducted in a fully instrumented living lab to observe the performance viz: energy saving potential, Lux levels, and mean radiant temperature. Simulation using an energy model was conducted to estimate the annual energy savings. It is observed that retrofit hard coat low-e double glazing saves up to about 9 % of daily energy consumption of Air conditioners (with annual average of 3% for Singapore weather conditions), when installed on all three sides (SE,NW,NW) , for clear glass with window to wall ratio of 20 % and air change rate of 1 ACH. For the same conditions the annual average energy savings can go up to 7.5 % when installed on an existing grey tinted glass.
Keywords Retrofit double glazing, test bedding, energy model
1. Introduction Every country has a strong plan to have a large number of green buildings within the next decade. The major challenge for this goal is to turn the large number of existing old buildings (about 80% of the building stock in Singapore) greener with minimal cost. As HVAC related energy consumption in commercial buildings for hot and humid climates is about 40-50% [1–3] , of the total building energy consumption, the first step has been to install and replace with more energy efficient modern chillers, which has been implemented as a regulation in many countries including Singapore. The second step is to reduce the total heat load by various measures, one of which is to reduce the heat gain through the envelope. It has been reported that the insulation
of the wall and roof, the air tightness and window replacement have the most impact on energy saving and allowed reducing 45% of the total annual energy consumed [4]. Most of the commercial buildings have multiple glass facades for various reasons like – aesthetics, to have natural view of outdoors to provide comfort for residents, to use natural day lighting and as well as to reduce the total building weight. Energy saving potential of double glazing in cold climates to reduce heating energy is well established and widely adopted in practice. However, for tropical hot climates the adoption has been low. So, there have been numerous studies[5–8] to understand the optimal type of double glazing and their energy and economic impact as well. Older buildings have normal clear or tinted glass (single glass facade). This in turn leads to increased glare in the building and hence occupants opt for window curtains/blinds and use of artificial lighting. Similarly, heat gain into the room in the form of direct solar transmission and convected/re-radiated thermal heat is increased with use of clear or tinted glass. Various retrofit studies [9–14] to improve the performance of existing buildings have also indicated that glazing is a critical part of the building envelope and need to be focused on to push the existing old building towards nZEB (net zero energy building) state. Puala et al. [13] discuss the commonly implemented building energy retrofit options. Among the options discussed upgrading window type and improving facade insulation is shown to be significant. Serghidehes et al.[12] studied the economic impact of various retrofit measures and their impact on carbon emissions as well the payback period of proposed retrofit measures. The results indicated that for Cyprus, replacing single glazed windows by double glazed windows can result in energy savings which can lead to an annual pay back of investment within 4-5 years. Mohamad et al. [8] investigated the impact of retrofit glazing in existing housing apartment and found that replacing a single clear glass by a double grey low-e glazing reduced cooling load by 14%, and by use of a triple low- e glazing , cooling loads can be reduced by up to 31% . However, most of these studies considered a complete upgradation of window by a double or triple glazing after replacing the existing single glazing. In such a solution, the (material and labour) cost, installation time and building downtime are all very high and it is difficult to convince the building owner. So, the proposed solution (retrofit double glazing) is to install a hard coat low e-coating based glass as the second layer behind the existing glass as shown in Fig. 1. This will retain the aesthetics, outside view, daylight and will simultaneously reduce the heat load that the air-conditioner needs to work against. Ariosto et al. [15] describe different possible window retrofit solutions which do not require complete replacement of the existing window, but involves using additional systems like curtains, drapes, blinds, films,
screens, shutters and storm windows. They analysed the performance of exterior storm windows in cold climates and concluded significant thermal performance gain without impacting the daylight. Smith et al.[16] report of secondary glazing (or internal storm windows) that can be used as a functional alternative to retrofitted insulated glazing units (IGUs) in existing domestic single-glazed window frames. They investigated the thermal transmittance of four secondary-glazing products – plastic film, magnetically attached plastic sheet, plain and low-E glass. This was in the context of winter climates and they found that the proposed secondary glazing increased the resistance values from the base value of 0.15 m2 K W-1, to a range of 0.34 to 0.57 m2 K W-1. It was a lab test to measure the thermal properties and not real test bedding. Moreover they have not reported the impact on SHGC etc. The current work aims to quantify the energy savings, thermal and visual comfort of the proposed retrofit double glazing under tropical climate in real life conditions. The proposed solution can be implemented in two ways.
Figure 1. Illustration of Retrofit Double glazing First method is by creating a local on-site double glazing unit with a sealed air gap as in conventional double glazing unit. This would involve creating a single glass piece of similar size to be manufactured and installed with a standard spacer. The sealed construction would allow high performance low- e coatings such as
soft coatings and would have more impact on energy savings however the cost and practical challenges in installation cost and time, especially for large and tall facades need to be taken in consideration. The second method is to install an inner window having a non-sealed air gap. In this method the whole facade is covered by multiple panels bridged by non-air tight metallic strips and placed on a custom frame. The room air flows in and out of this air cavity. So the low-e coating in this method would be typically hard-coat type to ensure durability of the coating. This leads to slightly lower impact in energy savings but this method is very easy, quick and cheap to do the retrofit installation. The current work investigates the retrofit double glazing installed by the second method wherein hard coat low-e coating at position 3 (facing the existing facade) serves as an inner window.
2. Description of test-bedding The test bedding was carried out in level 3 of a low rise office cum research facility building located in Singapore. The room had 3 existing window facades in the SE, SW and NW directions. The NE direction was covered by an opaque wall. The room orientation is indicated in Fig. 2. The room is approximately 10.65 m in length and 5.95 m in width. The total height from floor to roof is 4.95 m, with a 1.65 m high false ceiling formed by gypsum boards. There are fixed sunshades over the SW and NW facade.
Sensors Uncertainty in measurements Method of analysis Plots and discussion Simulation model description Parameters set
Shade
SW Facade
Figure 2. Simplified plan view of room used for test-bedding
The room is cooled by two (Daikin) fan coil units (FXFQ80P, 9.3kW cooling capacity and FXFQ80P, 14 kW cooling capacity) which are connected to two individual outdoor compressor/condenser units (RXMQ4P and RXMQ5P). The cooling capacity and power consumption of RXMQ4P was 11.2 kW and 2.95 kW and that of RXMQ5P was 14 kW and 3.97 kW. The room was cooled from 9 A.M. to 6:30 P.M. to a set point temperature of 25 °C. The fresh air flow measured at the fresh air duct was 5.7 m s-1 and it was equivalent to an air change rate of 1 ACH (Air Change per Hour). As per Singapore Standards 553: 2016 [17], for the size of this room, the minimum recommended ACH turns out as 0.78 ACH. The infiltration through the building envelope during day time when the fresh air and fan coil units are operating is almost zero as it a positive pressurised HVAC system, with room pressure slightly above the ambient. The room occupancy is nominal about 5 persons for 1 hour during 12 P.M. to 1 P.M. The NW façade has an additional solar control film on the existing glass facade (15 mm clear glass) installed at position 2. The properties of the solar film are stated in Table 1 below. The SE and SW facades have 10 mm thick clear glass windows, which are without solar control film. The properties of the existing glass facades and the retrofit glass (as per specification sheet) are tabulated in Table 2. Table 1. Properties of existing Solar Control Film in NW Note: The following are specs of the IR film (IR9010, 2 mil thickness) when applied on a 6 mm clear glass. Total transmission Solar energy - Reflection Solar energy - Absorbed Visible light – Transmission Visible light- Reflection Infra-red rejection UV rejection Shading coefficient Total solar energy rejection Heat transmission (W/m2K)
9% 20% 71% 15% 11% 75% 99% 0.25 82% 5.7
Table 2. Properties of glass facades Glass description
Visible light %
Solar energy %
U value
Clear glass
Transmittance
Internal. Reflection
External reflection
Transmittance
Reflectance
10 mm( SE and SW)
87
-
8
72
7
4.96
0.89
15 mm ( NW)
84
-
8
58
7
4.88
0.85
Shading coefficient
W m-2 K-1
Sunergy Gray, 8 mm, with hard coat low-e (Position 2)
26
4.9
8.3
23.2
5.7(int) 9.4(ext)
4.1
0.41
The retrofit glass (Sunergy grey, AGC) was a hard coat low-e glass (specifications listed in Table 2). The hard low-e coating was on position 3, facing the existing glass.
Figure 3. Details of installed retrofit glazing system The details of the final retrofit glazing installed in the test bedding room is shown in Figure 3.The existing thickness of frame in SE and SW windows, made it necessary to increase the air gap between the existing glass and the retrofit glass to about 30 mm, whereas in NW it was maintained at 17 mm. As described earlier the air gaps are not sealed and are exposed to room environment. SE glazing measured 5.9 m in width and 1.7 m in height, SW glazing was 4.95 m in width and 1.75 m in height, whereas NW glazing was 5.9 m in width and 2.34 m in height. The walls were made of 200 mm bricks with 10 mm gypsum plastering on the inner and outer sides of the brick. The roof construction had
multiple layers - 200 mm reinforced concrete slab with a 100 mm screed layer followed by water proofing and then with a 50 mm cast concrete layer. The floor construction was using 250 mm concrete slab. There were three sets of data measured as follows a)
Outdoor Environmental Conditions
The first group consisted of outdoor weather data which included – Global (Total) solar irradiance, diffuse solar irradiance, ambient temperature, relative humidity, rain status and outdoor illuminance. The sensors were located at the open roof area nearby. The Global solar irradiance and diffuse solar irradiance was measured by a pyranometer (class 1) with accuracy of ±5% ± 10 W m-2 for hourly averages and ±8% ±10 W m-2 for individual readings. Ambient dry bulb temperature and relative humidity was measured with accuracy of ±0.25 °C and ±1.5% respectively. The rain detector senses the presence of rain droplets on the sensor and logs the binary output, to help classify rainy periods. The outdoor illuminance was measured with accuracy of ±3%. b) Room Conditions The second group of sensors were for room internal condition monitoring which included - room temperature, mean radiant temperature and relative humidity (both at the centre and at the corner of the room), Lux meter at the centre of the room, room CO2 levels, heat flux sensors, Indoor normal solar irradiance, façade temperature using surface thermocouple. There were also two condensation sensors placed in NW and SE façade to detect any condensation in the air gap. The room temperature and humidity was measured with accuracy of ±0.2°C and ± 1.7 % respectively. The globe temperature sensor to obtain mean radiant temperature used a Pt-100 sensor with accuracy of ± 0.1 °C. The room Lux levels were measured using a sensor which had an accuracy of 1.5% ± 2 Lux. CO2 levels were measured with accuracy of ± 20 ppm ± 1 % of reading. The indoor normal solar irradiance transmitted through the facade was measured by placing a class 1 pyranometer, (tilted by 90° positioned inside the room facing the facade), which had an accuracy of 1.5 %. Heat flux sensors were placed both on surface position 2 and position 4 to measure the convected and radiated heat from the facade surface. Heat flux sensors have an accuracy of ± 3%. Facade surface temperatures of both existing panel and new panel, were measured using a surface mount T-type thermocouple with accuracy of 0.75%. The air temperature in the air gap between the panels was also measured using a bare junction T type thermocouple with 0.75% accuracy. The condensation sensor is used to detect the presence of condensation, frost, or ice using dielectric based capacitance sensing. It is located in the air gap between the glass panels.
The data acquisition card for 4-20 mA signals (indoor and outdoor temperature and RH, Indoor and outdoor Lux, CO2 level) had a measuring error of ± 0.3 %. The card for measuring voltage outputs (Heat flux sensors, condensation sensor, Total and diffuse irradiance measurement by pyranometer) had a measuring error of ± 0.05 %. The card used for measuring Pt-100 sensor for globe temperature in calculation of mean radiant temperature had an measuring error of ± 0.5 °C. For outdoor illuminance measurement – photometric probe (LP-02, Delta Ohm) was used it is a solid state sensor, whose spectral response is corrected by filters to fit the response of the human eye. For indoor illuminance Photodiode based sensor was used. The sensor used for indoor temperature measurement is a calibrated digital temperature sensor while for outdoor ambient measurement a Pt1000 RTD Class F0.1 is used, c)
Air-conditioning, Power and Energy Consumption
The third group of sensors were for monitoring the Air-con system parameters, this included two power meters to measure the individual energy consumption of both the FCU’s and also data from in-built sensors in the Daikin air-conditioning system like system status, air flow speed and direction, return air temperature and temperature set point. The digital power meters were able to record both instantaneous power consumption of air-conditioning (outdoor unit) as well as the cumulative energy consumption both with an accuracy of 0.2% All the three sets of data were collected at one second interval and subsequently sampled at 30 second interval for plotting comparative graphs. The data was averaged on hourly basis for hourly data comparisons. First, a baseline data was collected by conducting a baseline experiments wherein all the data were collected without installing the retrofit double glazing. This was carried out from 14 th July 2018 to 1st August 2018. Subsequently, same set of data was collected after installing the retrofit double glazing (inner window) from 6th August to 28th August 2018.
3. Details of energy model simulation The test bedding described above serves as a demonstration of the proposed retrofit double glazing in the tropics. The test bedding is useful to evaluate the thermal comfort, visual comfort, detection of condensation and also the energy savings. But the energy savings cannot be strictly quantified as there are seasonal variations and to have all variations accounted for in the one to one comparison of baseline and retrofit scenarios is practically infeasible. So in order to assess the yearly performance and quantify the annual energy savings a simulation model was necessary.
The room was modelled in Design builder interface, which uses Energy Plus as the background engine. The objective was to calculate the total cooling load in the room arising from various sources of heat gain during the baseline and as well as in retrofit double glazing configuration. Properties of walls, roofs and Glazing were assigned as per the as-built drawings. The walls were assigned as 200 mm bricks with gypsum plastering. The roof was modelled as a 200 mm layer of reinforced concrete (with multiple over layers consisting of screed, cast concrete and asphalt). The glass properties of the baseline glass, solar film and the retrofit inner window were set as per the specifications listed in Tables 1 to 2. The fresh air flow rate into the room was set to 1 ACH as per actual site measurement. This flows only during working hours (8 A.M. to 6:30 P.M.), which is the schedule for the air-conditioners in this room. The false ceiling was set as a void zone in the model, so the volume above false ceiling is not accounted for in ACH calculations. With this setting the air above the false ceiling is also not conditioned as in the room however, the heat gain from the side walls in the false ceiling to the side wall area(below false ceiling) does contribute to the heat gain of the room. The air conditioner was set to ―simple:HVAC‖ type with a supply air of 16 °C (99 % R.H). So, the partial dehumidification load was taken into account as well. The HVAC system was designed for the room to be in positive pressure, so infiltration from building envelope during the working hours (8 A.M. to 6:30 P.M.) was assumed to be zero. The night time infiltration (6:30 P.M. to 8 A.M.) was estimated to be 0.3 ACH. The plug loads were also estimated which arise mainly from control panel fans (24 hours operation) and as well as from the power consumption of indoor fan coil units (8 A.M. to 6:30 P.M.). For simulation of outdoor weather, when comparative results with respect to the measurements were required, a custom weather file based on measured outdoor weather parameters was used. However for the annual simulation the ASHRAE’s IWEC weather file for Singapore was used.
4. Data analysis of measured data from test-bedding The objective of the study was to compare the HVAC energy consumption, lux levels and thermal comfort before and after installation of retrofit double glazing, when the outdoor ambient weather conditions (Solar irradiance, temperature and humidity) are similar. It was very rare to have comparable two days which had exactly the same weather pattern from morning 8 A.M. to 6:30 P.M. So for comparison purposes, hourly averages of weather parameters and measured objectives (Lux, temperature, energy consumption) were computed and compared when the baseline and retrofit double glazing had similar hourly averages of all the three weather parameters (temperature, humidity and solar irradiance). So it was essential to fix a tolerance level (difference between two parameters) for assuming that the two hours in consideration are representative of
similar weather conditions. So the air conditioner’s power consumption was plotted as function of the following variables -∆Tar(Ambient DBT- Room DBT) , ∆Har(Ambient WBT- Room WBT) and Total solar irradiance. It was observed that the ∆Tar and ∆Har played the dominant roles in affecting the air-conditioners energy consumption, with almost a direct proportionality. This is probably because when the outside ambient temperature is high, the conduction heat gains into the room and as well the sensible heat load of fresh air increases and also the COP of the air conditioner decreases with increasing ambient temperature. Similarly when the ambient humidity is high the dehumidification load from fresh air on the evaporator coil is higher. Solar irradiance did have an effect on the air conditioner’s power consumption with the amount of solar radiation transmitted into the room. However, the effect was not as significant as the other two parameters, (for the current scenario having a particular geometry and fresh air flow rate). Another possible reason for this observation is that in the approach taken by comparing individual hourly weather zones independent of the previous hour zones, some effect of the solar radiation is carried on to subsequent hours and effect is not realized instantaneously as the walls and roofs will absorb the heat and radiate it slowly. This could be another reason why solar radiation is not having a clear correlation like ambient temperature and humidity. So, it was set that hourly averages which satisfy the following tolerance levels for all the three parameters can be considered as similar weather for the sake of one to one comparison of baseline and retrofit double glazing- 0.2 deg C for ∆Tar , 0.3 deg C for ∆Har, ±20% variation for Total solar irradiance.
5. Results and discussion from test-bedding 5.1 Air-conditioning energy consumption The section below analyses the measurement results for configuration 1 and 2. All three facades (SE, SW and NW) using the existing single glazing (baseline glass alone) is referred to as configuration 1/Baseline and all three sides with retrofit double glazing (Inner window) is referred to as configuration 2/Double glazing. While making comparisons between Baseline and Double glazing it was ensured that the time of the day is more or less within the same time period. So comparisons of data were made separately for the following time periods: 9 A.M. to 12 P.M., 12 P.M. to 3 P.M. and 3 P.M. to 6 P.M.
5.1.1 Comparison during morning period (9 A.M. to 12 P.M.) During this period of time in a day the ambient temperature is not very high and is slowly rising, also the main solar radiation transmission is through SE facade as the sun is mostly in the east during this period. The hourly variation of air-con electricity power consumed in (kW) is plotted with varying dry bulb temperature difference
∆Tar (between ambient and room) in Figure 4a, with Total solar irradiance in Figure 4b and with wet bulb temperature difference ∆Har (between ambient and room) in Figure 4c. Room temperature and room humidity refers to values measured at the centre of the room. The overall trend depicts that hourly air-con power consumption increases from 1 kW to 1.6 kW when ∆Tar increases from 1.2 to 4 °C. It is also observed that in general the retrofit Double glazing (configuration 2) consumes less power at most instances for the same ∆Tar. In Figure 4b, the increasing air-con power consumption with increasing solar radiation due to increase in transmitted direct solar radiation into the room is observed. The effect is not so strongly pronounced as with ∆Tar. In Figure 4c effect of wet bulb temperature (WBT) difference between ambient and room, ∆Har, which is a measure of the latent load (heat rejected to the evaporator coil when the water vapour in the return/fresh air condenses) is shown. As expected to maintain a lower WBT in the room a higher power consumption is required. The plots in Figure 4 give a general picture, but the individual isolated effect cannot be observed as the other two parameters in each plot is uncontrolled and not kept as a constant. So in order to quantify the actual savings in a similar weather condition, time periods between Baseline and Double glazing which are very closely matching were chosen, based on tolerance levels discussed in section 3, and analysed. It was observed that power savings for individual comparable hourly zones varied from 2 to 22 % and some instances have –ve effect varying form – 5 to -12 %. Installation of retrofit double glazing reduces transmission of direct solar radiation, but the blocked radiation is absorbed and re-radiated into the room. These are two competing effects which determine the net heat gain from the facade. One pair of comparable hourly zones from baseline and double glazing is illustrated in Figure 5 and 6. 31/7/2018 (Baseline) and 18/8/2018 (Double glazed) (11 A.M. to 12 P.M.) were chosen to depict the variation of air-con power and facade heat flux in both the cases. Figure 5 depicts that they are comparable periods with similar weather conditions.
Figure 4. a) Variation of Air-con power with dry bulb temperature difference between ambient and room for configuration 1 and 2, between 9 A.M. to 12 P.M. b) Variation of Air-con power with solar irradiance for configuration 1 and 2, between 9 A.M. to 12 P.M. c) Variation of Air-con Power with wet bulb temperature difference between ambient and room for configuration 1 and 2, between 9 A.M. to 12 P.M. From Figures 6a and 6b it is clear that the thermal heat flux entering into the room through the SE and SW facades in the Double glazed configuration is much higher than the Baseline cases (as the difference in heat flux (Baseline- Double glazed) is negative), especially through SE where there is direct sunshine, normal to the facade, during the current period (11 A.M.) of analysis. This is attributed to the direct solar radiation blocked by the low e-coating in the double glazing at position 3 and the subsequent heating of the air gap which is eventually emitted as thermal radiation in the room and partially to the ambient. In the NW facade since the solar film cuts the incoming solar radiation and reflects back, the heating up of air gap and re-radiation into the room does not happen here as seen in Figure 6c. From Figures 6d and 6e , it is observed that there are higher power spikes during Baseline days due to higher incoming heat and the double glazed scenario had 22% lower energy consumption for this hour.
Figure 5. a) Variation of ∆Tar b) Variation of ∆Har c) Variation of Total Solar irradiance.
Figure 6. a) Difference in heat flux (Baseline- Double glazed) through SE facade into room b) Difference in heat flux (Baseline- Double glazed) through SW facade into room c) Difference in heat flux (Baseline- Double glazed) through NW facade into room d) Variation of instantaneous electricity consumption by first AC unit. e) Variation of instantaneous electricity consumption by second AC unit
5.1.2 Analysis for afternoon periods (12 P.M. to 3 P.M.) During this period of time the ambient temperatures are higher than morning, also the main solar radiation transmission is through SW and NW facade as the sun is mostly in the west during this period. The hourly variation of air-con electricity power consumed in (kW) is plotted with varying dry bulb temperature difference (between ambient and room) in Figure 7a, with Total solar irradiance in Figure 7b and with wet bulb temperature difference (between ambient and room) in Figure 7c. The overall trend depicts that hourly air-con power consumption increases from 0.8 kW to 2.2 kW when the temperature difference increases from 1 to 5 ° C, in the Baseline case. However in the Double glazed cases the power consumption is more or less within a constant band of 1.3 to 1.8 kW, probably due to the heat trapped in the air gap and re-mitted into the room, which makes the total air-con power consumption less dependent on the actual ambient temperature outside. In Figure 7b, the increasing air-con power consumption with increasing solar radiation (due to increase in transmitted direct solar radiation into the room) is observed. In order to quantify the actual savings in a similar
weather condition and for similar thermal comfort, time periods between Baseline and Double glazing which are very closely matching were analyzed. It was observed that the power savings range varies from 8 to 26 % and some instances have –ve effect of -2 to -10 %. The two competing effects of reduced transmitted solar radiation and increased thermal re-radiation are applicable in this facade too, but this facade (SW) area is smaller and the facade has a shade as well. 19/7/2018 (Baseline) and 6/8/2018 (Double glazed) (2 P.M. to 3 P.M.) were chosen to depict the variation of air-con power, and facade heat flux in both the cases. Figure 8 depicts that they are very much similar and comparable weather periods. From Figures 9d and 9e it can be observed that there are similar power cycles in AC unit 1 but higher power spikes in unit 2 during Baseline days due to higher incoming heat and thus the retrofit Double glazing scenario led to 12.7% lower power consumption. From Figures 9 a and 9b it is clear that the thermal heat flux entering into the room through the SE and SW facades in the Double glazed configuration is much higher than the Baseline cases (as the difference between heat flux values in Baseline and Double glazing is negative). Even though the sun has moved away from the east the heat flux from SE facade in the Double glazed facade is significant because of the radiated heat from the heated air gap earlier in the morning. However, in the NW facade since the solar film cuts the incoming solar radiation and reflects back the heating up of air gap and re-radiation into the room does not happen here as seen in Figure 9c.
Figure 7. a) Variation of Air-con power with ∆Tar for configuration 1 and 2, between 12 P.M. to 3 P.M. b) Variation of Air-con power with solar irradiance for configuration 1 and 2, between 12 P.M. to 3 P.M. c) Variation of Air-con Power with ∆Har between 12 P.M. to 3 P.M.
Figure 8. a) Variation of ∆Tar b) Variation of ∆Har c) Variation of Total Solar irradiance
Figure 9 a) Difference in heat flux (Baseline- Double glazed) through SE facade into room b) Difference in heat flux (Baseline- Double glazed) through SW facade into room c) Difference in heat flux (Baseline- Double glazed) through NW facade into room d)Variation of instantaneous electricity consumption by first AC unit. e) Variation of instantaneous electricity consumption by second AC unit.
5.1.3 Analysis for evening periods (3 P.M. to 6 P.M.) The hourly variation of air-con electricity power consumed in (kW) is plotted with varying ∆Tar in Figure 10a, with Total solar irradiance in Figure 10b and with ∆Har in Figure 10c. The overall trend depicts that hourly air-
con power consumption increase from 0.8 kW to 2.5 kW when the temperature difference increases from 1 to 5 ° C, in the Baseline case. In Double glazed cases the power consumption varied between 1.3 to 2.3 kW for a temperature difference between 2 to 6 °C. Compared to mornings and afternoons, this period of 3 P.M. to 6 P.M. has very strong linear effect of humidity, probably because the solar transmission is reduced during this period as the NW façade which receives direct sunlight during this period has 2 layers of protection - solar film and retrofit double glazing. In order to quantify the actual savings in a similar weather condition and for similar thermal comfort, time periods between Baseline and Double glazing which are very closely matching were analysed. It was observed that power savings varied from 5 to 15 %. 23/7/2018 (Baseline) and 15/8/2018 (Double glazed) (4 P.M. to 5 P.M.) were chosen to depict the variation of air-con power, facade heat flux and direct transmission of solar radiation through the facade in both the cases.
Figure 10. a) Variation of Air-con power with ∆Tar for configuration 1 and 2, between 12 P.M. to 3 P.M. b) Variation of Air-con power with solar irradiance for configuration 1 and 2, between 3 P.M. to 6 P.M. c) Variation of Air-con Power with ∆Har for configuration 1 and 2, between 3 P.M. to 6 P.M.
Figure 11. a) Variation of ∆Tar b) Variation of ∆Har c) Variation of Total Solar irradiance
Figure 12. a) Difference in heat flux (Baseline- Double glazed) through SE facade into room b) Difference in heat flux (Baseline- Double glazed) through SW facade into room c) Difference in heat flux (Baseline- Double glazed) through NW facade into room d) Variation of instantaneous electricity consumption by first AC unit. e) Variation of instantaneous electricity consumption by second AC unit f) Variation of indoor normal irradiance near NW facade. Figure 11 depicts that they are very much similar and comparable weather periods. From Figures 12d and 12e we observe similar power cycle in AC unit 2 but higher power spikes in unit 1 during Baseline days due to higher incoming heat. From Figures 12a and 12b it is clear that the thermal heat flux entering into the room through the SE and SW facades in the Double glazed configuration is much higher than the Baseline cases. Even though the sun has moved away from the east the radiation from SE facade in the Double glazed facade is significant because of the radiated heat from the heated air gap earlier in the morning. However, in the NW facade since the solar film cuts the incoming solar radiation and reflects back, the heating up of air gap and reradiation into the room does not happen here as seen in Figure 12c. The actual direct solar radiation (in visible spectrum is reduced) coming into the room is reduced by 80 % from 40 W m-2 to 8 W m-2 after installation of Double glazing as shown in Figure 12f . Indoor normal irradiance in South east and South west are unavailable as the sensor was placed only towards NW facade. From the above analysis it is clear that there are two competing physical mechanisms operating after installation of retrofit low- e inner window. The direct solar
transmission is significantly cut down as seen by drop in indoor irradiance and Lux measurements (discussed in section 5). However the stopped radiation is absorbed by the low-e layer and re-radiated both into the room as well as to ambient. So the net difference between reduction in direct solar transmission and increase in reradiated/convected heat from the hot inner window determines the total heat gain reduction of the room. However, for NW facade with solar film on the baseline glass significant amount of solar radiation is rejected in the baseline glass and so the low-e layer does not become very hot unlike in a plain clear glass facade(as in SE/SW). But the low- e layer does cut down additional solar transmission over and above that of solar film. So the temperature of the retrofit inner window is not very high and the re-radiated or convected heat into the room is smaller (as seen by lower heat flux in NW facade after installation of retrofit inner window) and hence the total room heat gain is lower.
5.2 Indoor Lux levels Figure 13. shows the daily Lux values for configuration 1 and configuration 2 measured at the centre of the room at 1.2 m height, which corresponds to working level. It is very evident that Double glazing on all three sides of the room reduced the Lux levels to the comfortable range of 500, which is within the recommended Lux level as per Singapore Standards 531: part 1, 2006 [18] standards (lighting for standard office work is to be between 300- 500 Lux). The average reduction in Lux levels was about 70%.
Figure 13. Daily variation of Lux for configuration 1 vs 2
Figure 14a shows the daily variation of Lux profile during a sunny day for configuration 1 (Baseline) and configuration 2 (Double glazed). The Double glazed scenario has reduced the glare and maintains comfortable range of Lux levels except during the 1st working hour (8 A.M. to 9 A.M., when the Lux levels are low and would require supplementary lighting). Figure 14b shows the outdoor illuminance for the days compared in Figure 14a and outdoor illuminance is similar on both days, yet the indoor illuminance was well controlled and glare was prevented due to installation of retrofit double glazing. In the absence of this retrofit installation, blinds might have been used to protect from glare, but eventually would have required supplementary lights to be turned on.
Figure 14. a) Typical day variation of Lux for configuration 1 vs 2 b)Outdoor illuminance variation for typical days compared in Fig 7.2
5.3 Risk of condensation in air-gap The air gap in the current design of retrofit inner window is not sealed. It is also not ventilated, so under humid conditions and at low ambient temperatures there is a possibility of humid air condensing in the air gap. To monitor the condensation status, one condensation sensor was placed in the air gap in SE façade and one in NW façade. The outdoor ambient humidity levels reached maximum values around 98% during rainy periods. However, no condensation was observed throughout the entire testing period.
5.4 Thermal Comfort Mean radiant temperatures were measured at the centre of the room and also at the corner (intersection of SW and NW facade) of the room. The mean, maximum and minimum mean radiant temperatures (MRT) during the baseline measurement (during 8 A.M. to 6:30 P.M. at the centre of the room ) were 33.4 °C, 35.6 °C and 31.6 °C respectively , whereas after retrofit double glazing of low-e glass they were 32.1 °C, 35.1 °C and 31.0 °C.
Since thermal comfort of the occupants is influenced both by the air temperature and as well the surface temperature of the room, lower MRT would reflect as better thermal comfort for the occupants. Similarly, the average mean radiant temperature measured at the SW-NW corner of the room was 1.9 °C lower after installation of retrofit double glazing. This will lead to better thermal comfort for occupants seated nearer to the facade.
6. Results from energy model simulation The simulation models (for both baseline and Double-glazed configuration) was run using the actual measured weather data during the test bedding. Then the simulation predicted HVAC daily consumption was plotted along with the actual measured HVAC daily consumption (by power meters). The same is plotted in Figure. 15. (July data points represent baseline and August represents Double glazing). The average deviation between simulated power consumption and model predicted power consumption ( for the whole data set July – August) was about 10%, which confirmed that the simulation model is robust and reflects the actual real-life conditions.
Daily HVAC Power Consumption (kWh)
23
Measured daily HVAC power consumption vs Simulated daily HVAC power consumption
21 19 17 15 13 11 9
Simulation Model
7
Actual measurement
Date Figure 15. Comparison of simulated HVAC daily power consumption with the actual measured power consumption.
8/28/2018
8/26/2018
8/24/2018
8/22/2018
8/20/2018
8/18/2018
8/16/2018
8/14/2018
8/12/2018
8/10/2018
8/8/2018
8/6/2018
8/4/2018
8/2/2018
7/31/2018
7/29/2018
7/27/2018
7/25/2018
7/23/2018
7/21/2018
7/19/2018
7/17/2018
5
Figure 16. Typical hourly heat gain distribution in baseline scenario (clear glass on all three sides and one facade (NW) with a solar film) compared with typical hourly heat gain distribution after retrofit installation of Inner window over the existing configuration Both the actual configuration with existing baseline glass as well as after installation of retrofit inner glazing was modelled separately. Table 3. lists the average (averaged over 19 days) hourly cooling load for every individual hour. Retrofit double glazing has higher power consumption in the 1st hour due to higher room temperature at the start of the day as it cannot lose heat to the ambient (during night) easily unlike the single glass facade. Subsequently, it is seen that percentage energy savings are minimal in the 9 A.M.-12 P.M. period, moderate in the afternoon 12 P.M. to 3 P.M. and maximum in late afternoon 3 P.M. to 6 P.M. As explained in the earlier sections, it can be attributed to the fact that during afternoons and late afternoons amount of incident solar radiation on NW facade (installed with solar film) increases. The retrofit inner window installed in this facade is more effective as it is able to operate at lower operating temperatures. Figure 16 illustrates the drastic heat gain reduction via window solar transmission after installation of the double glazing, though there is a significant increase in glazing heat gain (conduction and convection). But the net effect is reduced heat gain after retrofit installation.
Table 3. Simulated hourly average cooling load
Time
Baseline
Retrofit Inner window
Cooling load (kW)
Cooling load (kW)
8-9
3.92
4.02
-2.70
9-10
3.31
3.25
1.72
10-11
3.38
3.34
1.33
11-12
3.46
3.37
2.73
12-13
3.81
3.71
2.64
13-14
3.74
3.62
3.17
14-15
3.84
3.62
5.64
15-16
4.00
3.70
7.54
16-17
4.04
3.73
7.80
17-18
3.97
3.66
7.64
18-18:30
1.85
1.72
6.91
Total day
39.31
37.74
4.00
% Energy savings(HVAC)
1.93
3.82
7.66
Annual simulation using Singapore’s typical weather was carried out for scenarios, before and after installation of retrofit inner window. The daily savings along with the average ambient temperature is plotted in Figure 17.
Figure 17. Annual energy savings of retrofit double glazing for the current room configuration (SE and SW clear glass and NW clear + solar film) It is evident that the daily energy savings increases on hotter days and can be negative on cloudy or rainy days, when the ambient temperatures are lower. The maximum energy savings was about 6% and the yearly average was about 3%. Also a different scenario or case was studied where the existing glass was assumed to be 10 mm thick grey glass (solar Transmittance- 0.383, VLT 0.483) (SE and SW – grey glass and NW – clear glass with solar film). The annual energy savings for this scenario is plotted in Figure 18.
Figure 18. Annual energy savings of retrofit double glazing for grey glass room configuration (SE and SW grey glass and NW clear + solar film) It is evident that installing retrofit double glazing on a grey glass has improved the annual performance significantly. The maximum daily savings can be as high as 12 % and the yearly average turns out as 7.5%.
7. Acknowledgements The project was carried out as part of A*STAR’s internally funded project ICES-17188A04. We acknowledge and thank AGC Asia Pacific Pte Ltd for supply and installation of hard coat low-e glass panels and for providing technical specifications of the glass panels.
8. Conclusions The test bedding of double glazing in tropical climates was carried out and its effect on air-con power consumption, mean radiant temperature and indoor illuminance levels were observed and reported. From the overall analysis of both measurement and simulation results it is evident that double glazing can provide significant energy savings in tropical climate (from 4 to 10% depending on existing glazing type and area and as well as other heat gains of the building). It is more effective when used along with additional protection from solar radiation as in following scenarios:
Installing retrofit double glazing when the original facade is a tinted glass (grey/green/blue etc)
Installing retrofit double glazing over clear glass after installation of solar film on the clear glass
Mean radiant temperatures were 1.3 °C lower after installation of retrofit double glazing, which will increase the thermal comfort of the occupants and can possibly allow a slightly higher set point for the room air temperature.
It is very clear by consistent measurement results that the current tested (ATTOCH Sunergy Grey) Low- e based double glazing reduces the visible light transmittance by about 75%. It is useful in scenarios with high level of day lighting to reduce glare and to bring down the Lux level to recommended levels. However, if double glazing is used along with solar film selective sides of a room alone must have solar films or a lighter tint version of solar film or Low –e coating should be chosen to help maintain the required minimum Lux levels. Alternatively, an optimization calculation between reduction in cooling load and lighting loads can be down to identify the optimal combination of solar film and Low-E based double glazing. The effect of installing a retrofit double glazing over a clear glass facade and over a solar film protected clear glass was inferred by analysis of measurement and simulation data. However, in future studies we expect to isolate two of the facades in the room at a time to study the individual effect of each type of facade. It would also be of interest to study the effectiveness of the proposed retrofit solution in each individual direction (north, south, west and east).
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