Cooling enhancement using acacia gum based hydrogel in roof ponds: model room experimental analysis.

doi:10.21203/rs.3.rs-1560597/v1

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Cooling enhancement using acacia gum based hydrogel in roof ponds: model room experimental analysis.

https://doi.org/10.21203/rs.3.rs-1560597/v1

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posted 27 Apr, 2022

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In this paper we report the observations of outdoor experiments which were carried out to test the cooling effect inside a model room due to evaporation from an acacia gum based hydrogel placed in an aluminium sheet roof pond. Unlike the direct evaporation from roof ponds, the hydrogels resist the evaporation rate and hence the trapped water particles could still provide the cooling effect on the next consecutive day. The experiments had three model rooms with only difference in the roof component: (a) bare aluminium sheet roof – considered as reference, (b) roof pond which contained only water, and, (c) roof pond containing acacia gum based hydrogel which was watered periodically. Two different sets of experiments were carried out: (1) The roof ponds were watered each day around 12 noon and (2) The roof ponds were maintained dry (not watered). In the first set of experiments, no significant difference in the air temperature of the model rooms on the first day was observed. But on the next consecutive day, around 2°C – 4°C difference was observed during the morning hours until both the roof ponds were watered again. The acacia gum and water mixture was used as hydrogel in the present experiments. The dilute solution (around 1:10 ratio) was observed to be reusable for several days without any bio-degradation. The second set of experiments showed that the heat transfer from the edges of the dry roof pond can be significant. The dry roof pond could cause 0.7°C – 1.2°C drop in average room air temperature between 6pm – 6am as compared to that of bare sheet roof. On the other hand, the same was observed to be around 1.3°C to 1.7°C in the case of dry roof pond with dry acacia gum. However, during day time, the room air temperature in the case of dry roof ponds was observed to be higher. Based on the present set of experiments, we concluded that the acacia gum can be a cost effective material for building cooling with reduced evaporation rate and the heat transfer from the edges of the roof pond can be significant.

evaporative cooling

energy efficient buildings

building envelope

roof pond

roof cooling

Evaporation is an interesting phenomenon, and it is an endothermic process. The change from a liquid phase to a gaseous phase occurs with the absorbed heat, which is the latent heat of vaporization. Water evaporation from the surface of exposed water storage is due to absorbed heat at the top surface of the water¹. Evaporation of water results in cooling² and this fact is exploited in numerous cooling applications including building cooling³. Buildings contribute to major energy consumption, particularly cooling energy demand, which has considerably increased in the recent years, and it is causing the global scarcity of energy resources⁴. There are several methods available for building cooling, and among them, passive cooling strategy like roof pond kind of arrangement with water evaporation helps in the absorption of heat coming from hot weather or direct solar radiation^5–7.

Evaporative roof cooling with water ponds on building rooftops is evident to have potential to reduce building cooling load demand^8–10. Sharifi et al.¹¹ in their study for the review on roof ponds have listed several factors which affect the performance of the roof ponds. Meteorological conditions like low relative humidity locations are desirable to get efficient cooling from the roof ponds. Evaporation and convective cooling can be enhanced by the higher wind speed. However, increasing the water depth of the pond would not help in evaporative cooling¹². The authors have also concluded that roof deck material and its thickness, emissivity of the roof pond cover are the parameters which can affect the performance of the roof pond. Sabzi et al.¹³ calculated the cooling load of a building required to maintain constant temperature using numerical methods and experiments (model house tested in Shiraz, Iran). They used three passive cooling systems like water pond on the roof, water jacket on the roof and radiation shield on the roof system. The roof water pond was observed to provide maximum cooling benefits followed by radiation shield and roof water jacket system. Kruger et al.¹⁴ conducted test cell experiments with different configurations of roof pond systems compared with the same sized control test cell without roof pond system. A significant temperature drop in the indoor air temperature can be expected with shaded and ventilated roof pond system. Abuseif and Gou¹⁵ have presented a detailed review of the roof ponds experimented in different climatic regions. Their review suggests that roof ponds in arid and tropical regions can ensure better cooling performance than that in the Mediterranean region. The roof ponds are proven to be worst performing in mountain, polar and temperate climatic regions.

Though the evaporative cooling can be a promising cooling technique for buildings, it comes with a cost i.e., fresh water loss due to evaporation. Reduction of water loss by evaporation but with optimized cooling is an appreciable strategy in this regard. The novelty of the present work is attributed to this fact. Providing a layer of cloth or gunny bag above the water pond was theoretically and experimentally investigated by Spanaki et al.¹⁶. With their theoretical study they found that emissivity of the gunny bag or top cloth material when reduced by 50%, could result in a 2.6℃ temperature drop at the bottom. On the other hand, in their experimental study when a low emissivity material layer above water was provided, could result in around 5.4℃ drop of pond temperature at the bottom. The gunny bags essentially add resistance for evaporation as the surface tension characteristics change. Having added resistance for water evaporation can affect the cooling considerably. From the recent studies several hydrogels are found to have potential for building cooling applications¹⁷. Basically a hydrogel is a combination of any natural or chemical material which has the ability to absorb water and retain the same for a significant amount of time. Water molecules can escape from the hydrogel at a specific rate based on the rate at which the hydrogel is heated. The hydrogels can also dissolve in water forming a uniform solution mixture - gum kind, gel type or in liquid form. Different types of hydrogels are used nowadays in many other applications such as in pharmaceuticals transportation¹⁸, eye contact lenses¹⁹, wound dressing²⁰, tissue engineering²¹, food additives²², etc.

Feng et al.¹⁷ proposed a bilayer porous polymer film for the passive building cooling approach. The authors proposed a bilayer film which consisted a hydrogel for evaporative cooling, and a hydrophobic top polymer layer for radiative cooling. This composite film was tested in Wuhan, China resulted in a sub-ambient temperature drop by 5℃ under direct sunlight which delivers 114 W/m² cooling power. Cui et al.²³ experimentally tested a tough double network hydrogel compared with a single network hydrogel for building cooling application. The experiments were conducted on a scaled model houses built with oak wood and tested for maximum cycles and cooling performance under simulated solar radiation. Both the hydrogels were soaked for 8 hours in deionized (DI) water and fixed over the roof and normal incident irradiance of 800 W/m² was applied for both the roofs. It was observed that the double network hydrogel exhibiting better cooling performance with water absorption capacity for more than 50 cycles. Wang et al.²⁴ tested combining hydrogels like sodium alginate and polyacrylamide integrated with 67% Glaubers salt to form a stable phase change hydrogel. This composite hydrogel could dissipate considerable amount of latent heat thermal energy. Abdo et al.²⁵ used saturated hydrogel for solar panel cooling in their solar simulator experiment. They observed that the temperature of solar panels drop by 9 and 9.6 ℃ at 600 and 1000 W/m² radiation. Hydrogels are beneficial and some of them are naturally available which might be the cost effective option for passive roof cooling. Acacia gum or Arabic gum (AG) is an edible, natural hydrogel used as a food additive in various food industries, stabilizers for manufacture of paints, inks, and in cosmetic industry²⁶. AG is obtained as dried gum discharge from branches and trunk of Acacia Seyal and Acacia Senegal trees found in Asian and African regions²⁷. AG can dissolve easily when mixed with water in a short time and result in solutions with low viscosity and with ability to seize water within it in the mixture²⁸. Majority use of AG is like a food additive in the food industry with its viscous property and texture. AG is also used in dairy application, flavour preparation, beverages, in cosmetics and in pharmaceuticals²⁹. In this paper, performance of AG based hydrogel for building cooling applications with model room experiments is presented.

The thin film formation over the water surface can significantly reduce the evaporation rate³⁰. WaterSavr (bio-degradable), a patented combination of calcium hydroxide, food grade steryl and cetyl alcohols was proposed to reduce the water evaporation as it could form a thin film over the free surface³¹. The experiments essentially involved the use of four evaporation pans, two with WaterSavr and two without WaterSavr. WaterSavr was observed to quickly spread into an invisible thin film over the water surface resulting in around 30% reduction of water evaporation. WaterSavr is claimed to be fully biodegradable in water within 48–72 hours and hence needs to be added again. Similarly Martínez-Alvarez et al.³² experimentally tested three different monolayer products like stearyl alcohol (C18OH), ethylene glycol monooctadecyl ether (C18E1) and WaterSavr on water experimented under controlled wind speed conditions in a glasshouse at Spain. It was observed that C18E1 being the most efficient among the considered with evaporation reduction in 40–71% range. A similar study was carried out by Tran et al.³³ to further reduce the evaporation rate. A blend of C18OH and C18E1 (50:50 ratio) was recommended by the authors to observe a relatively thick viscous film formed over the free surface. A mixture of cetyl and stearyl alcohol (1:1 ratio) over the water surface was experimented by Panjabi et al.³⁴; around 19.3% reduction in evaporation was observed.

Film formation over the free surface of water can significantly reduce the evaporation rate. However, this fact is not tested in building cooling experiments using roof ponds as one can optimize to reduce the evaporation loss. On the other hand, use of natural hydrogels for building cooling is also not reported. In the present work, the authors intend the usability of acacia gum based hydrogel as a fluid in roof ponds for building cooling and is first of its kind. If the AG based hydrogel is used in the roof ponds, some amount of water can be held in the AG based hydrogel for a day or two depending on the weather conditions. In the present paper the cooling potential of water trapped (due to watering on the previous day) in the AG based hydrogel contained in a roof pond over a model room is discussed.

Experimental setup. As mentioned earlier, the main objective of the experiments was to analyse the potential of water trapped in the AG based hydrogel to provide cooling on the next consecutive day. For such an analysis, three insulated model rooms were planned:

1st model room with roof as bare aluminium sheet as shown in Fig. 1(a),
2nd model room with roof aluminium tray having pure water as shown in Fig. 1(b),
3rd model room with roof aluminium tray having 1:10 AG and water mixture as shown in Fig. 1(c).

The sidewalls and the floor of each model room were made of 100 mm thick thermocol with internal room dimensions of 300 mm x 300 mm x 300 mm. Thermocol³⁵ was chosen because of its excellent insulating qualities having thermal conductivity around 0.05 W/mK and density 20 kg/m³. The top covering (roof) for the first model was covered with 1mm thick aluminium sheet and the other two models with aluminium sheet tray of internal dimensions of 410 mm x 310 mm x 45 mm. Thermal conductivity of aluminium sheet is 222 W/mK ³⁶. Hence, the heat transfer was ensured to be dominatingly through the roof component. This was important for the present objective. The experiments were setup over the terrace of a building at Vellore Institute of Technology, Vellore campus, India. Similar model rooms were used in several past building heat transfer studies³⁷.

AG and water (1:10) mixture preparation. Mixture of 100 gram of AG with 1 litre of water was prepared for the present experiments. This was chosen to keep the cost lower. A precise calibrated digital weighing machine with 0.01gram least count was used to weigh the AG and a beaker was used to measure the water quantity. The preparation details are as shown in Fig. 2(a) and 2(b). The uniformly dissolved mixture in the tray is considered for the experiments as shown in Fig. 2(c).

Data logger. Mezarit THD-172 thermo hygrometer data logger with a built in sensor was used to measure the temperature and humidity inside the closed model rooms, which is capable of recording 32,700 values. The data logger is wireless and can measure a temperature within the range of -40°C to 70°C (± 0.5^oC) and humidity in between 0-100% (± 3%). Data logger measuring cycle can be adjusted from 2 sec to 24 hours for each reading intervals. Sample picture of the data logger is as shown in Fig. 3. One data logger was placed inside each model room for the indoor temperature recordings. The data was recorded for every 10 minutes.

Data availability statement. All data generated or analysed during this study are included and provided the raw data in the supplementary file section.

Two different sets of experiments were carried out:

The aluminium sheet tray roofs of 2nd and 3rd model rooms were watered for three consecutive days at around 12PM – from 30-03-2021 to 01-04-2021 and
The same tray roofs were maintained dry and were observed for three consecutive days – from 27-03-2021 to 29-03-2021.

During these sets of experiments the 1st model room with aluminium sheet roof was considered as reference. All these experiments were carried out under clear sky conditions. The solar radiation, ambient air temperature and relative humidity in the vicinity were recorded and were as shown in the Figs. 4 and 5.

First set of experiments. On the first day of the experiments, the roofs of the three set-ups were dry; the third model room had dry acacia gum in the roof pond. To make the analysis simple, the authors use the following abbreviations for the three model rooms (Table 1):

Table 1

Abbreviations for model rooms
Model room roof type	Abbreviation used
The reference model room with aluminium sheet roof (NO roof pond)	NO-RP
Model room with roof pond to contain water (W) only	W-RP
Model room with roof pond to contain AG based hydrogel (AGH) only	AGH-RP

The model room air temperatures were observed to be same on the first day as shown in Fig. 6. Water was poured in the roof ponds of W-RP and AGH-RP rooms at around 12 noon on the second day. The roof pond of W-RP room contained only water and that of the AGH-RP room contained AG based hydrogel (1:10 ratio as mentioned earlier). The water used was filtered mineral water. The acacia gum was observed to take around 30 to 45 minutes to dissolve into water. The pouring of water was cyclically repeated on 3rd and 4th days also at the same time i.e., around 12 noon. The air temperatures of the W-RP and AGH-RP rooms were observed to drop considerably as the water poured in both the roof ponds could absorb heat due to solar radiation. As the water and AGH were open to solar radiation and outside air, water could evaporate in response to solar radiation. Water evaporation due to surrounding unsaturated moist air was also part of the process. On the other hand, the roof ponds could also absorb heat from the inside air of the respective model rooms during early morning, evening and night hours.

Table 2

Average room air temperature for model rooms (First set of experiments)
Time (Date)	Average Room Air Temperature in ℃			Time of water addition to the roof ponds
Time (Date)	NO-RP	W-RP	AGH-RP	Time of water addition to the roof ponds
12:00 NOON (29-03-21) to 6:00 PM (29-03-21)	43.3	44.2	43.4	Not watered.
6:00 PM (29-03-21) to 6:00 AM (30-03-21)	28.2	27.5	27.0	Not watered.
6:00 AM (30-03-21) to 12:00 NOON (30-03-21)	31.3	31.8	31.4	Not watered.
12:00 NOON (30-03-21) to 6:00 PM (30-03-21)	46.4	36.5	36.5	Water added at 12 noon.
6:00 PM (30-03-21) to 6:00 AM (31-03-21)	28.5	25.9	24.7	Not watered.
6:00 AM (31-03-21) to 12:00 NOON (31-03-21)	33.2	32.5	29.7	Not watered.
12:00 NOON (31-03-21) to 6:00 PM (31-03-21)	47.7	38.2	38.4	Water added at 12 noon.
6:00 PM (31-03-21) to 6:00 AM (01-04-21)	30.2	27.0	26.4	Not watered.
6:00 AM (01-04-21) to 12:00 NOON (01-04-21)	34.9	33.6	31.7	Not watered.
12:00 NOON (01-04-21) to 6:00 PM (01-04-21)	49.3	38.2	38.4	Water added at 12 noon.
6:00 PM (01-04-21) to 6:00 AM (02-04-21)	31.5	27.5	27.1	Not watered.
6:00 AM (02-04-21) to 12:00 NOON (02-04-21)	35.4	34.7	33.9	Not watered.

The average room air temperature \({T}_{room}\) for different specified time durations are tabulated in the Table 2. Analysis of this data could substantiate the usefulness of AGH in building cooling. For this, one should observe the average room air temperature for morning hours i.e., from 6AM to 12 noon. As mentioned earlier, the water was added to the roof ponds each day (second day onwards) around 12 noon and the effect of water left out in the roof ponds is significant during the next consecutive day morning hours. Let us consider 2nd and 3rd day as shown in the Figure. 6. After watering at around 12 noon on 2nd day, the drop in room air temperature in the case of both W-RP and AGH-RP was observed during 12 noon – 6pm, 6pm – 6am, and, 6am – 12 noon (i.e., on 3rd day). The drop in both the cases was almost same for 12 noon – 6pm duration (i.e., around 9.5^oC to 11^oC) as compared to that of NO-RP. Thereafter between 6pm – 6am, the temperature drop in the case of AGH-RP is relatively more than that in the case of W-RP i.e., by around 3.8^oC to 4.4^oC. It should be noted that No-RP was assumed as reference. During these 12 hours, there was no solar radiation. On the next consecutive day, it is interesting to note drop in temperature in the case of both W-RP and AGH-RP as compared to that of NO-RP. But the drop was higher in the case of AGH-RP around 3.5^oC; whereas the same was around 0.7^oC in the case of W-RP. This is attributed to the fact that water absorbed by the acacia gum due to watering on the previous day could evaporate due to solar radiation which in turn resulted in the temperature drop. On the other hand, the carried over water in W-RP also could evaporate but the solar heat absorbed in the process was negligible. The rate of evaporation of water was lower in the case of AGH-RP as compared to that in the case of W-RP. This was observed to be repeating for any given two consecutive days and is further tabulated in the same Table 2.

The rate at which the air temperature drops in the case of W-RP room and AGH-RP room was observed to be not being the same. This difference is attributed to the adding of acacia gum (AGH-RP room) in water. The temperature variations are discussed in 4 spells (refer Fig. 7): (i) From the time water was poured till the first minimum was noted, (ii) From the first minimum till the next peak was noted, (iii) From this peak till the time when W-RP / AGH-RP model room air temperature crosses the air temperature of the NO-RP room i.e., the reference model room with bare aluminium sheet roof was noted, and, (iv) Thereafter till the time when the water was poured for the next cycle.

The first spell may be divided into two halves. During the first half of the duration (till evening), the air temperature of the W-RP room starts dropping faster than that of the AGH-RP room. This can be due to the fact that the acacia gum based hydrogel has higher solar absorptivity as compared to that of water. As the solar radiation drops during evening, the AGH could lose heat energy by sky radiation apart from the latent heat loss due to evaporation. This radiation heat loss is also possible from the water surface (W-RP room) but is relatively less, resulting in higher air temperature inside the room in the evening - second half of the first spell. At the end of the first spell, the air temperature attains to a value which is the lowest during that spell.

The 2nd spell is interesting and is not well understood in the present study. This starts from the lowest air temperature noted during the first spell. During this 2nd spell, the air temperature inside both W-RP and AGH-RP rooms were observed to increase by 3.5℃ and 4℃ respectively during late evening between 8PM and 11PM. With all the side walls and floor being adiabatic, there are two possibilities due to which this increase in temperature was observed: (a) difference in specific heat of water and AGH, and, (b) temporary arrest in evaporation due to the adverse outside air temperature and relative humidity conditions. However, further experiments are required to substantiate the above mentioned attribution.

In the third spell, the air temperature of all the three rooms keep reducing till early morning and start increasing as the solar radiation increases thereafter. It should be noted that the air temperature of the AGH-RP room was observed to attain lowest temperature followed by W-RP room and then NO-RP room. The rates at which the room temperatures rise are not the same. The W-RP and AGH-RP room air temperatures start rising faster and between 9AM and 10AM they cross the NO-RP room air temperature. This is again attributed to the higher solar absorptivity of aluminium surface of W-RP (due to mineral deposition after water dries off) and AGH-RP than that of bare aluminium sheet roof of NO-RP room.

During the fourth spell, the air temperatures of the three model rooms simply rise due to increase in solar radiation. It is important to note that the AGH-RP room air temperature was observed to be lower than that of both W-RP and NO-RP model rooms. This is attributed to the continued evaporative cooling in the case of AGH-RP room. The water present in the AGH could evaporate due to the solar radiation and the surrounding unsaturated air. This evaporative cooling is due to the water which was poured on the previous day. Around 2.5^oC difference between the AGH-RP and W-RP room air temperatures and around 1.5^oC difference between AGH-RP and NO-RP room air temperatures during this fourth spell were observed. This phenomenon of slow evaporation from AGH can be used to carryover the water for evaporation with time lag. This continues until water was poured again in the roof ponds of W-RP and AGH-RP rooms. Thereafter, these four spells repeat.

Slow evaporation from the AGH

Pictures of the roof ponds were taken using a simple 3MP mobile phone camera before and after water was added on two consecutive days and are as shown in Fig. 8. In the case of AGH formation of thin film layer was observed. This porosity in the thin film formed could allow the water to evaporate. While the W-RP was found almost dry, the finger impressions in the case of AGH-RP are evident that there was considerable amount of water trapped in the AGH below the film formed as shown in Fig. 9. Hence the authors attribute the formation of this thin film layer for the slow evaporation in the case of AGH. Similar observations were also reported in^30–34. However, some more rigorous experiments may be carried out to understand the formation of thin layer and is out of the scope of present work.

Second set of experiments. As mentioned earlier, all the roof ponds were maintained dry from 27th March 2021 to 29th March 2021. This second set of experiments was carried out to check the influence of dry acacia gum and roof pond edges on the room air temperature. The roof ponds of W-RP and AGH-RP had around 33.8% of the total surface area of the aluminium sheet was used as edges of the roof ponds. On the other hand, the same size aluminium sheet was used flat over the NO-RP model roof (Fig. 1). The model room air temperatures were observed to be similar on all the days as shown in Fig. 10.

Similar to Table 2, the average room air temperature was analysed for morning hours – 6am to 12 noon, afternoon hours – 12 noon to 6pm, evening/night hours – 6pm to 6am. The average temperature during the morning hours was observed to be higher for W-RP by around 1^oC (or less) as compared to that of NO-RP model room. Whereas in the case of AGH-RP, the average temperature was found to either remain same or lower by around 0.9^oC (or less). Similarly during afternoon hours, the dry roof pond of both AGH-RP and W-RP model rooms resulted in temperature increase by around 0.1^oC to 1^oC respectively and consistently as compared to that of NO-RP. This shows that the roof ponds with total edge surface area around 33.8% can increase the average room air temperature by around 1^oC. This could be due to two reasons. One is the drop in wind velocity near the horizontal flat surface due to the edges. This can significantly reduce the convection heat transfer coefficient near the horizontal base of the roof pond. The other reason could be reflection of solar radiation from the edge surfaces to the horizontal base of the roof pond. This can further add the solar heat which can in turn increase the room air temperature. The latter mentioned reason was found to be more substantial by analysing the average room air temperature during the evening/night hours i.e., 6pm – 6am (refer Table 3). It was interesting to note that the average room air temperature was lower in the case of W-RP and AGH-RP model rooms. In particular, AGH-RP model room exhibited lowest room air temperature (around 1^oC – 2^oC). It should be noted that the dry AGH being dark in colour with relatively higher emissivity can result in higher heat loss to the sky during night hours. Hence, the radiation heat exchange between the edges and the roof pond base is significant.

Table 3

Average room air temperature for model rooms (Second set of experiments). No watering was done from 27th to 29th March.
Time (Date)	Average Room Air Temperature in ℃
Time (Date)	NO-RP	W-RP	AGH-RP
6:00 AM (27-03-21) to 12:00 PM (27-03-21)	30.8	31.4	30.8
12:00 PM (27-03-21) to 6:00 PM (27-03-21)	42.9	43.8	43.3
6:00 PM (27-03-21) to 6:00 AM (28-03-21)	27.8	26.8	26.0
6:00 AM (28-03-21) to 12:00 PM (28-03-21)	31.8	31.7	30.9
12:00 PM (28-03-21) to 6:00 PM (28-03-21)	42.6	43.6	42.8
6:00 PM (28-03-21) to 6:00 AM (29-03-21)	27.8	27.1	26.5
6:00 AM (29-03-21) to 12:00 PM (29-03-21)	31.7	32.5	31.7
12:00 NOON (29-03-21) to 6:00 PM (29-03-21)	43.3	44.2	43.4
6:00 PM (29-03-21) to 6:00 AM (30-03-21)	28.2	27.5	27.0
6:00 AM (30-03-21) to 12:00 NOON (30-03-21)	31.3	31.8	31.4

Acacia gum based hydrogel is edible, non-toxic and natural. The main objective of the present experiments was to check the cooling potential of the AGH as a roof pond liquid. The hydrogel can resist the evaporation and an organized periodic watering of the roof pond can reduce the water loss. The periodic watering was done around 12 noon each day in the present experiments. This can cause the carryover of water to the next day morning and the evaporation of the same can provide the cooling effect by around 2^oC – 4^oC (average).

On the day of watering, the reduction in room air temperature due to evaporation in the case of either AGH or only water was found to be the same. However, during evening and night till early morning (i.e., from 6pm – 6am), the reduction in average room air temperature was more in the case of AGH-RP by around 3.7^oC-4.4^oC as compared to NO-RP; whereas the same was observed to be 2.6^oC-4^oC with W-RP. Hence it was concluded that periodic watering of the roof pond with AGH could lower the room air temperature better than the case of watering a roof pond with only water. However, the watering should be done at around 12 noon.

Apart from this, another set of experiments were carried out with roof ponds kept dry. These experiments could help in understanding the influence of (a) dry AGH on room air temperature and (b) the roof pond edges. The dry AGH over the roof was still found to result in lowering of room air temperature between 6pm-6am (by around 1.6^oC as compared to NO-RP case); however the reduction was also observed in the case of dry roof pond without AGH but was next to with AGH case (by around 1^oC as compared to NO-RP case). This was attributed to the higher emissivity of the dry acacia gum surface. Due to higher emissivity, the heat loss by sky radiation was significant. This conclusion was substantiated by noting that the room air temperature was lowered in the case of both the model rooms i.e., W-RP and AGH-RP in-spite of reduction in convection heat loss due to the roof pond edges.

Acknowledgements

Authors are thankful to Vellore Institute of Technology, Vellore, India for funding this research work.

Competing interests

The authors declare no competing interests

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Cooling enhancement using acacia gum based hydrogel in roof ponds: model room experimental analysis.

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