Experimental Investigation on Cooling Performance of Water Spray Window

Abstract

The cooling performance of the energy-saving water spray window cooling system under summer conditions in Nanning city in southern China, is experimentally examined in this study. By constructing two identical test rooms for comparison, in the experiment we analyzed the effects of water temperature (22 °C, 26 °C, 30 °C), spray frequency (8, 10, 12, 15 min intervals), glass structure (single-layer, double-layer, triple-layer), air conditioning temperature (26 °C, 27 °C, 28 °C, 29 °C), and outer lamination thickness (30, 50, 100, 200 mm) on the cooling effect of windows, aiming to determine the optimal operating conditions. The experimental outcomes demonstrate that the appropriate operating mode for the water spray window system involves using the coldest water source (22 °C), with a spray interval of 8 min, a three-layer air cavity sprayed glass construction, an air cavity thickness set at 100 mm, and the best air conditioning temperature (26 °C). The study found that the water spray system can reduce the outer glass surface temperature of the window by 6 °C and the inner surface temperature by 2 °C. Moreover, the more glass layers and the thicker the lamination, the higher the energy saving rate; with a maximum energy saving rate of 35.19%. The water spray window has good adaptability and significant energy-saving effects in Southeast Asia. By scientifically selecting energy-saving glass types and fine-tuning operating modes, it is expected that efficient building energy conservation in hot climates can be achieved.

1. Introduction

With the growing severity of climate warming, the importance of effective indoor temperature control has become increasingly prominent. The windows of building facades were widely regarded as a core element affecting building energy conservation. In the southern China and ASEAN regions, where summers are long and characterized by high temperatures and humidity, the air trapped within window lamination was heated by solar radiation and the glass, becoming a potential factor for indoor temperature increase [1,2]. Therefore, effectively reducing the glass and lamination temperature has significant implications for building energy efficiency, especially in the context of deepened global carbon emission policy reforms [3,4]. Optimizing the cooling performance of the windows is a crucial step towards achieving this goal.

In recent years, with the rise of environmental awareness, green energy-saving glass cooling technology has garnered significant attention due to its substantial energy-saving benefits [5]. Especially in the field of window technology, innovative techniques such as phase-change material cooling technology [6,7], heat-reflective film technology [8,9], water spray evaporation technology [10,11], and high-pressure mist spray [12,13,14] have become the focus of current research. Among these techniques, heat-reflective film technology reduces heat absorption by reflecting solar radiation. However, although it effectively reduces heat conduction, it cannot lower the temperature of the glass itself, and may lead to light pollution problems. Phase-change material (PCM) cooling technology, on the other hand, relies on the heat absorption and release characteristics of PCM at specific temperature ranges, effectively regulating the surface temperature of the glass. Nevertheless, this technology still shows limitations in rapid cooling. Water spray evaporation technology utilizes the principle of water evaporation to reduce the temperature of the glass. This technology has gained attention due to its simple working principle and low maintenance cost; however, its evaporation efficiency is low and it consumes a significant amount of water, limiting its application scenarios. In contrast, high-pressure mist spray can produce fine water droplets, which significantly enhance evaporation efficiency and reduce water consumption, effectively lowering the temperature of the glass and its surrounding environment. This improvement also leads to a notable decrease in air conditioning cooling load, as shown in

Table 1. Comparing common green energy-saving window cooling methods.

Cooling Method	Working Principle and Advantages	Disadvantages
Thermal reflection film technology	Reduce heat absorption by reflecting the sunlight, improve indoor comfort, significant energy saving effect.	Leads to light pollution, is not friendly to the outdoor environment, and cannot solve the problem of high temperature.
Phase-change ma-terial cooling technology	The phase change material can absorb and release a large amount of heat to reducing the glass temperature, pollution-free, stable cooling.	High cost, technology maturity needs to be improved, and application scenarios are limited, which cannot quickly cool down.
Water spray and evaporation technology	Using the principle of water evaporation and heat absorption to reduce the glass temperature, its working principle is simple, low-maintenance cost.	Low efficiency of evaporation, high water consumption, prone to mold growth.
High-pressure spray evaporative cooling technique	Through the evaporation process of fine water mist, effectively absorb and take away the heat from the glass surface, so as to achieve rapid cooling.	High evaporation efficiency, large water consumption, and certain requirements for water quality.

. Therefore, the environmental cooling characteristics of high-pressure mist spray have demonstrated superiority in practical applications [15].

Abdulrazak Qahtani et al. have investigated the cooling performance of the exterior surfaces of the glass curtain wall in Malaysia [16,17], and have found that the formed fluid water films could effectively lower the surface temperature by 2 to 4 °C, thereby decreasing the indoor temperature by 2 to 4 °C. However, the influence of external wind on the glass surface was significant, leading to unstable effects and a high consumption of water, thus limiting its application. Hence, the application of inner spray in the air lamination has become a consensus. Ye et al. attempted to introduce spray measures in the glass layer [18], testing the application effects of spraying from different directions on the window layer. The results showed that moderate spraying could reduce window temperature and reduce air conditioning usage time. Contrary to the downward ventilation in the wind tower mist cooling system [19,20], the sun evaporated the water vapor in the glass laminate, which then ventilated upwards and carried away the heat from the laminate. The size of the droplet was a crucial parameter; reducing the size of the droplet could enhance the cooling efficiency. The water droplets produced by high-pressure spray were typically smaller, ranging from 10 to 50 μm in size. The temperature of the mist was also lower than the temperature of the water [21]. Researchers have tested the thermal performance of high-pressure mist cooling glass roof lights, revealing that the temperature of the surface of the glass was 8.0 °C lower than the ambient temperature, while the sunlight transmittance decreased by 14% [22].

In the southern China and southeast Asian regions, abundant rainfall and high humidity characterizes the summer months [23], providing the local population with a wealth of low-temperature renewable water resources primarily consisting of rainwater and air conditioner condensation water [24,25]. Firstly, abundant rainfall was one of the primary sources of renewable water. The construction of sponge cities could store a large amount of rainfall in underground reservoirs, which are currently used for irrigation, road washing, and other purposes [26,27] and have not been extensively utilized. By constructing an effective collection and storage system complemented with appropriate filtration and disinfection treatment, these rainwater resources could be utilized for urban mist cooling. When sprayed into the air, water vapor absorbed heat, effectively lowering the temperature of the surrounding air. Secondly, high humidity conditions in the air environment were favorable for the production of air conditioning condensate water. In daily life and work environments, frequent use of air conditioners generated a large amount of condensate water which, after treatment, could meet the quality standards for drinking water [28] and was a green, environmentally friendly, and high-quality source of low-temperature water.

In this context, this study proposes a high-pressure mist spray cooling system specifically designed for building windows, which aims to significantly reduce the temperature of the exterior window glass during the summer. This system utilizes the evaporation process of mist droplets to absorb heat from the air, coupled with the formation of running water on the glass surface that efficiently removes accumulated heat, effectively reducing the heat gain on the exterior and air lamination of the window. This technology not only substantially enhances the thermal performance of exterior windows but also reduces the cooling energy consumption in buildings, and has a positive impact on mitigating urban heat islands [29,30,31]. Currently, there is a noticeable lack of experimental research that verifies the cooling performance and energy-saving benefits of the high-pressure mist spray cooling system during the summer in this region. Hence, the adaptability of this system requires thorough evaluation.

This study undertakes a detailed analysis of the thermal performance of the cooling system and probes into its cooling mechanism, with the ultimate goal of optimizing the cooling effect. The research outcomes are expected to have considerable practical application value. The study leverages an outdoor test room in Nanning, located in the Guangxi region of China, as a case study. It compares various window forms, encompassing single-layer, double-layer, and triple-layer glass; employing a quantitative analysis method. Through rigorous evaluation of variables such as water temperature, spray frequency, window structure, air conditioning temperature, and air layer thickness, this study elucidates the optimal operation mode for water spray windows.

2. Materials and Methods

2.1. Experimental Setup

To evaluate the cooling and energy-saving effects of water spray windows, an experiment was conducted at the West Campus Experimental Base of Guangxi University. This area experiences a summer season from May to October, with a duration of over half a year [32], characterized by the hot and humid climates typical of southern China and southeast Asia. The experiment was conducted in the outdoor parking lot of the experimental base, with the northern side facing the structural test hall, and the southern side located nearby the campus road. The site, where two identical test rooms (2 m long × 2 m wide × 2 m high) were constructed, was relatively open. To ensure the accuracy of the test data, the distance between the two test rooms was set at 6 m, avoiding any overlapping effects between them. The walls and roofs of both rooms were constructed with metal sandwich panels, with 60 mm thick polystyrene foam in the middle. This significantly enhanced the thermal and sound insulation performance. The flooring was made of wooden material and, in order to prevent water ingress into the room, there was a 15 mm high airspace between the floor and the ground. The western room (R1), designated as the experimental group, was equipped with a water spray cooling system. Meanwhile, the eastern room (R2) was set up as the contrast group, with standard glass windows installed. All windows were placed on the south-facing walls, and were 1.2 m high and 1.0 m wide. Furthermore, a small meteorological station was set up on the eastern side of the test room, to monitor and record real-time meteorological data in the experimental area. Both rooms were equipped with a 2600 W air conditioning system, aiming to test the energy consumption of air conditioners under different configurations; as shown in Figure 1.

2.2. Spray System

The water spray window was composed of an ordinary glass window and spray devices. The water spray device included an atomizer, water pipe, atomization spray head, water tank, etc. It was installed outside the window or lamination to reduce the temperature of the glass surface. The water storage bucket was equipped with an automatic water replenishment system, without manual operation. The main source of water was municipal feed water, and air conditioning condensate was collected into the water storage bucket. The sprayer had the function of intelligently setting the spray time and frequency, and was connected to the timing socket to automatically start and close the system. The parameters of the pump and nozzle are listed in

Table 2. Main components of the spray system and their parameters.

Components	Model	Specifications
High-pressure pump	FOG MACHINE1.2 L	Power: 750 W
Spray nozzles	hongbaoshi01	Working pressure: >3 MP Droplet particle size: 10–30 μm

. After the system was started, the high-pressure sprayer delivered the water from the bucket to the atomized nozzle. The high-pressure water was atomized into fine water mist through the special nozzle, sprayed onto the glass surface, and evaporated or formed a water film downward, taking away the heat from the glass surface; as shown in Figure 2. The ejected water droplets descended from the bottom of the window. The water flowing down the glass was collected, filtered, and transported to the storage bucket, where it could be recycled. To compare the cooling efficiency of various structural glasses, single-layer, double-layer, and triple-layer glass windows were selected for experimental testing. The single-layer glass had a thickness of 6 mm. The double-layer glass window consisted of two 6 mm glass panes with a 12 mm laminate layer in between. The triple-layer glass window included two 12 mm laminate layers with three 6 mm glass panes. Moreover, to compare the cooling effect of the inner layer, the outer layer was also modified, including a sprayer and a water pipe, with a thickness of 30 mm, 50 mm, 100 mm, and 200 mm; as shown in Figure 3.

From the upper and lower nozzles, the water vapor released had smaller particles and a lower temperature. A large amount of mist obstructed solar radiation from entering the indoor space, with droplet diameters ranging from 50 to 100 microns. The contact of water vapor with the glass surface and its evaporation absorbed a substantial amount of heat, thereby reducing the temperature of the laminate. The water droplets sprayed onto the glass formed a downward-flowing water film that absorbed heat from the glass surface. Multiple factors contributed to the reduction in window temperature, improving the indoor and outdoor environment. This reduction in temperature led to a decrease in the cooling load required by air conditioning systems, which in turn ultimately resulted in lowered energy consumption within the indoor space, as shown in Figure 4a.

2.3. Data Measurement and Acquisition

The experiment utilized a T-type thermocouple to measure the temperature of the water body and glass surface. The measurement range of the thermocouple was between −50 °C and 200 °C, with a measurement accuracy of ±0.01 °C, as illustrated in

Table 3. Experimental instrument and specifications.

Instruments	Model	Measuring Range	Accuracy
Temperature sensors	Thermocouple	−50–200 °C	±0.01 °C
Date logger	THMA	-	±0.01 °C

. The layout of the nozzle is shown in Figure 4b. Since the temperature sensor had adhered to the glass surface and was directly exposed to solar radiation, its measurement accuracy may be affected. To reduce the interference of solar radiation on temperature measurement, silicon grease was applied to the sensor, which was then covered with reflective aluminum foil. All sensors were calibrated uniformly before the experiment began. Additionally, all critical parameters were recorded by data recorders, which stored data via a laptop connection or USB. The experiments were conducted between July and October of 2021 and 2023, with the climate conditions throughout the testing period remaining mostly clear. The daily solar radiation typically exceeded 20,000 W/m², with the climate being hot and humid. The average environmental temperature exceeded 30 °C. Under these conditions, the municipal water supply system provided water at a temperature of approximately 30 °C, with the temperature of the condensation water being maintained within the range of 16 to 20 °C. The insulation performance of the water supply pipeline was well insulated.

2.4. Thermal Performance Analysis

Upon application of a spray on the exterior surface of the glass, a layer of flowing water film was formed. When solar radiation struck this surface, part of the radiation was reflected, while the remaining portion penetrated through the glass into the interior. The remaining radiation was absorbed by the window glass and the flowing water layer. Under these conditions, the energy–water exchange equilibrium is shown in Figure 5. Furthermore, the energy exchange equation between the water film and the surface is given as [11]:

$${\alpha }_c\left(T_a-T_w\right)+\epsilon \left(R_L-\sigma T_w^4\right)-{\alpha }_w\left(T_w-T_s\right)-l\frac{{\alpha }_c}{C_m}\left(X_s-X_a\right)=C_w{W}_q\frac{\mathsf{\partial }{T}_w}{{\mathsf{\partial }}_Z}$$

(1)

The energy balance equation for the water film and the flowing water layer is as follows:

$$a{R}_{\mathrm{s}}+{\alpha }_{\mathrm{w}}\left(T_{\mathrm{w}}-T_{\mathrm{s}}\right)=-\lambda \frac{\mathsf{\partial }T}{\mathsf{\partial }\chi }$$

(2)

where $a$—solar absorptance;

${\alpha }_{\mathrm{c}}$ is considered to be a function of wind speed and is given by the Jurges equation. ${\alpha }_w$ is determined as follows:

$${\alpha }_{\mathrm{w}}=\frac{N_u{\lambda }_w}{Z}$$

(3)

where N_u is computed from the following equation (Johnson–Rubesin equation):

$$N_u=0.0296R_e^{0.8}{P}_r^{0.6}$$

(4)

$R_e$ is determined as $R_e=U_w\raisebox{1ex}{$z$}\!\left/ \!\raisebox{-1ex}{$\nu $}\right.$.

Upon ensuring that the specifications of the comparison room and the test room are consistent in terms of appearance, size, envelope structure, functional layout, setting parameters for air conditioning cooling equipment, meteorological parameters, etc., statistical data on the air conditioning system energy consumption of the comparison room and the test room over the same period were collected and calculated. The energy conservation rate was calculated based on the difference in air conditioning energy consumption between the comparison room and the test room divided by the air conditioning energy consumption of the comparison room. The current experiments primarily focus on assessing the energy-saving effects of spray cooling technology on glass windows, and the energy consumption of spray equipment such as water pumps has not yet been considered. Relevant research will be conducted in subsequent studies. The formula for calculating the energy conservation rate is as follows:

$$\eta =\frac{Q_c-Q_{wfw}}{Q_c}\times 100\%$$

(5)

In which $Q_c$ represents the comparison of the electricity consumption of the air conditioner, $Q_{wfw}$ represents the electricity consumption of the test air conditioner, and $\eta$ represents the relative energy efficiency between the two rooms.

3. Results and Discussion

3.1. Cooling Effects of Water Temperatures

To investigate the impact of different water temperatures on the surface temperature of single-layer glass, a series of spray cooling experiments were conducted from 10–17 October 2021. During the experiments, both external windows were equipped with single-layer glass, and their water temperatures were set to 22 °C, 26 °C, and 30 °C, respectively, and the air conditioning temperature was set to 26 °C. The total duration of temperature changes was recorded for 60 min, with the spray duration lasting for 10 min. During the test period, both glass panes were exposed to direct sunlight, with solar radiation values exceeding 600 w/m². The wind speed was less than 0.5 m/s, and the temperature of the glass exterior surface exceeded 35 °C. The corresponding experimental results are presented in Figure 6.

The first set of experiments conducted on 10 October 2021, at 11:20 a.m. used 22 °C water as the spray medium to treat the surface of the test glass. Observation records indicated that the surface temperature of the test glass decreased from 35 °C to 26.2 °C after 10 min of spray treatment, with a cooling rate of 8.8 °C. After 60 min, the surface temperature of the test glass was lower by 4 °C compared to the control glass. In another experiment group, 26 °C water was used as the spray medium, and spray treatment began at 1:30 p.m. on 17 October 2021. Results showed that the surface temperature of the test glass decreased from 35 °C to 26.4 °C after 10 min of spray treatment, with a cooling rate of 8.6 °C. After 60 min, the surface temperature of the test glass was the same as that of the control glass. In the third set of experiments, 30 °C water was used as the spray medium, and spray treatment began at 11:07 a.m. on 17 October 2021. After 10 min of spray treatment, the surface temperature of the test glass decreased from 35 °C to 27 °C, with a cooling rate of 8 °C. After approximately 40 min, the surface temperature of the treated glass increased to match that of the control glass.

The experimental results indicate that the cooling effect of spray on the glass surface was practically equivalent under three different water temperatures. However, the duration of cooling was positively correlated with water temperature. Specifically, a longer cooling duration could be achieved using water at a lower temperature, such as 22 °C, exceeding 60 min. In contrast, the cooling duration would be shortened to around 40 min using water at 30 °C. Considering this, prioritizing lower-temperature water for spray evaporation enhanced cooling efficiency.

3.2. Cooling Effect at Different Periods of a Typical Day

To investigate the specific impacts of water mist spraying on the outdoor surface temperature of windows at different times of the day, the period from 6:00 a.m. on 11 August to 6:00 a.m. on 12 August (a typical day in summer) was chosen to conduct a water mist spraying cooling experiment on the outdoor surface of single-pane glass windows. During the experimental process, the air conditioning temperature was set to 26 °C. The operation of the system was divided into seven selected time periods; specifically, from 9:00 a.m. to 11:00 a.m., 12:00 p.m. to 2:00 p.m., 3:00 p.m. to 5:00 p.m., 6:00 p.m. to 8:00 p.m., 9:00 p.m. to 11:00 p.m., 12:00 a.m. to 2:00 a.m., and 3:00 a.m. to 5:00 a.m. Each time period represents a single 2 min spray and a 12 min interval between sprays. Throughout this series of experiments, detailed records were kept of the meteorological data on the day of testing, the temperature of the water supply, and the temperature changes on the outdoor surface of the windows. These data are shown in Figure 7.

The highest temperature of the day occurred between 12:00 p.m. and 17:00 p.m., where the cooling effect of glass surfaces was most significant, with up to a 6 °C decrease. The average temperature of the cooled glass was maintained at 30.5 °C. In the morning, from 9:00 a.m. to 11:00 a.m., and in the afternoon, from 18:00 p.m. to 20:00 p.m., the cooling effect of glass surfaces was approximately 3.5 °C, with an average temperature of 29 °C. During these two time periods, the fluctuations in solar radiation intensity were significant, resulting in a temperature change exceeding 2 °C. This suggested a positive correlation between the cooling effect of glass surfaces and the intensity of solar radiation. At night, from 21:00 p.m. to 5:00 a.m., although there was no influence from solar radiation, the temperature of the glass surface remained above 30 °C due to factors such as the temperature of the air layer and other influencing factors. Windows treated with fogging had a cooling effect of approximately 3 °C, with the temperature of the glass surface maintained at around 28 °C. This temperature—lower than the outdoor temperature—fell within the human comfort range, making it beneficial to reduce the heat transfer efficiency of windows and the cooling demand of the air conditioning system; thereby reducing energy consumption, as illustrated in

Table 4. Glass surface temperature reduction and average temperature in each time period.

Time Period	6:00~8:00	9:00~11:00	12:00~14:00	15:00~17:00	18:00~20:00	21:00~23:00	0:00~2:00	3:00~5:00
Highest drop (°C)	2.4	2.8	3.5	3.8	3.5	3.2	3.1	3.1
Average drop (°C)	1.9	1.7	1.9	1.7	1.9	2.0	2.3	2.3
Temperature after cooling (°C)	28.5	31.3	33.2	34.9	32.8	30	29	28.4

.

The results indicated that during the three hottest periods of the day, the evaporation cooling effect did not persist once the spraying ended. The glass rapidly absorbed heat from the surrounding environment and solar radiation, causing its temperature to rise rapidly. During a certain period, the temperature of the glass even surpassed that of the unsprayed window surface. Therefore, to maintain the efficiency of spray cooling, a reasonable spraying interval should be set to avoid the negative effects caused by the sudden rise in temperature. Moreover, although the cooling effects of spraying at night may not be significant, it helps maintain the glass surface temperature close to 28 °C; thereby reducing the cooling demand on the air conditioning system. Particularly in areas with high nighttime temperatures, spraying at night significantly improves the indoor thermal environment of buildings.

3.3. Cooling Effect of Spray Frequency

From 23–26 July 2021, a consecutive four-day experimental study was conducted to investigate the cooling effects under various spray intervals. The focus was on a single-layer glass window, with each spray session lasting precisely 2 min and intervals strictly controlled at 8, 10, 12, and 15 min. This methodology ensured accurate and reliable data collection. The testing encompassed a full 24 h period, starting at 6:00 a.m. on the first day and ending at 6:00 a.m. on the second day. During the experimental process, the air conditioning temperature was set to 26 °C.

To gain a deeper understanding of the cooling effects throughout different times of the day, eight specific time slots were selected for detailed spray cooling tests: 6:00–8:00 a.m., 9:00–11:00 a.m., 12:00–2:00 p.m., 3:00–5:00 p.m., 6:00–8:00 p.m., 9:00–11:00 p.m., midnight-2:00 a.m., and 3:00–5:00 a.m. Additionally, meteorological data for these testing days were carefully documented, providing a comprehensive overview of the environmental conditions during the study. Refer to Figure 8 for detailed meteorological information.

Comparative analysis revealed a positive correlation between spray interval and cooling efficacy. Specifically, shortening the interval to 8 min yielded a maximum 4 °C cooling on the window surface, with more stable temperature fluctuation. In contrast, the cooling effect at 10- and 12-min intervals was around 3 °C, while 15-min intervals showed a limited effect, especially during the daytime, when surface temperature rose rapidly after spraying. Nighttime cooling effect was only about 1 °C, with limited efficacy. See Figure 9 for a detailed comparison.

Based on the analysis of experimental data, it was observed that, although reducing the spray interval enhanced cooling, the efficiency increase was not significant. Specifically, at 6 min intervals, numerous water droplets frequently persisted on the glass surface. This observation was a primary factor in determining the decision to set the spray interval start point at 8 min. Taking into account both cooling effectiveness and water conservation, an interval of 12 min appeared to be more suitable. Additionally, the experiment revealed that nighttime spraying exhibited significantly better cooling compared to daytime spraying, further validating the effectiveness of night spraying for optimal cooling. Detailed results can be found in Figure 9.

3.4. Cooling Effect of Glass Structure

To quantitatively evaluate the impact of water spray on the surface temperature of glass windows in buildings, three days in September 2022 were chosen for this study: the 12th, 20th, and 29th. The experiment was conducted on single-layer, double-layer, and triple-layer glass windows, comparing the temperature changes on the glass surfaces of windows with different layers. The experimental system ran continuously from 8:00 a.m. on the first day until 6:00 a.m. the following day, with a spray applied every 12 min (each spray lasting 2 min) to simulate actual operation conditions. The relevant meteorological data and water temperature are shown in Figure 10.

During the three-day experimental cycle, the single-layer glass exhibited the most noticeable cooling amplitude, thereby making it highly susceptible to external temperature fluctuations, resulting in significant temperature variations. On the other hand, the double-layer glass demonstrated a comparatively stable cooling trend accompanied by fewer temperature fluctuations. Moreover, it exhibited significantly better cooling performance at night in comparison to during the day. However, because of its low thermal conductivity, the triple-layer glass offered excellent insulation. The introduction of two air layers served as a thermal buffer, causing a slight reduction in the cooling effects at night when compared to daytime. It is important to mention that, apart from a significant temperature decrease during the 12 to 18 h period, there were minimal temperature differences between the triple-layer glass and the interior window surface during other times; as clearly depicted in Figure 11.

Experimental verification has confirmed that the temperature on a window’s inner surface was significantly influenced by the thermal transmittance coefficient. When this coefficient was low, the impacts of spray cooling on the indoor environment were comparatively minor. Among different types of glass, single-layer glass exhibited the most pronounced cooling effect; with double-layer glass following; and triple-layer glass showing the least effect. This was primarily determined by their respective thermal insulation properties. It is worth noting that double-layer glass demonstrated a comparatively stable cooling effect alongside considerable energy-saving potential. Considering that most newly constructed buildings in southern China are equipped with double-layer windows, spray evaporation cooling holds promising potential for application in that region. Furthermore, triple-layer glass demonstrated remarkable cooling abilities even under high-temperature spray conditions.

3.5. Effect of Air Conditioning Setting Temperature on Energy-Saving Rate

To investigate the potential impact of the water spray flow on the energy consumption of air conditioning systems, comprehensive experiments were conducted in October 2022. In these experiments, three different types of windows were chosen for testing: single-layer, double-layer, and triple-layer glass windows. Additionally, two air conditioning devices were installed in two test room, with temperature settings varying between 26 °C, 27 °C, 28 °C, and 29 °C. The operational timings were maintained consistently with previous tests.

Upon analyzing the experimental results, it became evident that, as the set temperature increased, the relative energy efficiency of the test room also rose proportionately. Specifically, the energy efficiency enhancement achieved through spray cooling on single-layer glass windows increased significantly from 18.16% to 22.89%. This finding underscores the high susceptibility of single-layer glass windows to external temperature fluctuations and highlights the effectiveness of spray technology in reducing the exterior surface temperature, thereby minimizing heat infiltration into the indoor environment.

There was also a moderate improvement in energy efficiency observed in spray cooling for double-layer glass windows, which rose from 6.23% to 13.19%. Although double-layer glass windows exhibited lower energy consumption compared to single-layer ones, the enhancement in energy efficiency was relatively modest, indicating a limited role of spray technology in boosting the efficiency of these windows.

A substantial increase in energy efficiency was noticed in the test room with triple-layer glass windows when spray technology was applied, jumping from 20.03% to 26.58%. Contrary to previous experimental data that suggested a minimal effect of spray action on glass surface temperatures, the spray’s exceptional thermal insulation properties significantly contributed to reducing indoor heat, thereby increasing the proportion of heat dissipated through the water spray. This remarkable impact on energy efficiency underscores the value of employing spray technology, particularly for triple-layer glass windows. A detailed analysis of these performance indicators is presented in

Table 5. Daily energy-saving rates of three different types of windows at four air conditioning temperatures.

Air Conditioning Temperature (°C)	Single-Layer Window (%)	Double-Layer Window (%)	Triple-Layer Window (%)
26	18.16	6.23	20.03
27	19.69	8.24	24.45
28	19.96	12.13	26.51
29	22.89	13.19	26.58
Average Value	20.17	9.95	24.40

.

When comparing energy efficiency data between nighttime and the entire day, significant disparities were noted. Precisely, the single-layer window demonstrated a 4% boost in energy efficiency during the night relative to daytime, whereas the double-layer window revealed a 1.85% enhancement during the night in comparison to daytime. In contrast, the triple-layer window experienced a 9.58% drop in energy efficiency during the night when juxtaposed with daytime, as illustrated in

Table 6. Night energy-saving rates of three different types of windows at four air conditioning temperatures.

Air Conditioning Temperature (°C)	Single-Layer Window (%)	Double-Layer Window (%)	Triple-Layer Window (%)
26	20.60	10.08	18.98
27	21.30	10.30	17.16
28	26.79	12.54	12.54
29	27.91	14.31	10.61
Average Value	24.15 (rise by 4%)	11.80 (rise by 1.85%)	14.82 (drop by 9.58%)

.

The augmented energy efficiency of single-layer and double-layer windows during nighttime hints that the cooling impact of the foggy water spray is more pronounced at night rather than during the day, thereby affirming their nighttime cooling capabilities. On the other hand, the diminished energy efficiency of the triple-layer window during the night mirrors the inherently low energy consumption of the test room during this period, thereby restricting the cooling effect of the foggy water spray. This decline in energy efficiency suggests that triple-layer windows predominantly aid in energy conservation during the sweltering daytime hours.

3.6. Effect of Spray on the Cooling and Energy Consumption of Different Lamination Thickness

To evaluate the impact of aerosol cooling on the surface temperatures of glass layers with varying thicknesses and spaces between them, a series of aerosol cooling experiments were conducted on specified dates in 29 September and 1, 4, and 6 October 2023. The samples used in these experiments were triple-glazed windows, with the outer glass layers varying in thickness: 30 mm, 50 mm, 100 mm, and 200 mm, while the inner glass layer remained a constant 12 mm thick. The air spaces between the glass layers varied accordingly. The spray cooling system was activated at the same time as in previous experiments. Relevant meteorological data and experimental results are presented in Figure 12.

The experimental findings revealed that, as the thickness of the outer glass layer increased from 30 mm to 50 mm, there was a notable decrease in the surface temperature of the sprayed window. However, no substantial difference in cooling effect was observed when the outer glass thickness was increased to 100 mm compared to the 50 mm thickness. It is worth mentioning that the lowest cooling efficiency was achieved with a 200 mm thick outer glass layer. Upon analyzing the average cooling range data, it became apparent that the sample with a 50 mm thick outer glass layer and its corresponding air space exhibited superior cooling performance compared to the thicker glass layers and their respective spaces. Surprisingly, the cooling effects were very similar for both the 100 mm and 200 mm glass thicknesses and their associated spaces. As depicted in Figure 13.

To evaluate the influence of different thicknesses of air layers on air conditioning energy consumption under spray conditions, experiments were conducted on 30 September, 2 October, 5 October, and 7 October 2022. The air conditioner temperature in both test rooms was set at 27 °C, and the system opening time was kept consistent with previous experiments. The relevant meteorological data and water temperature are shown in Figure 14.

Through experimental data analysis, it was observed that, as the thickness of the corrugated layer increased, the overall energy efficiency exhibited an ascending trend. When the thickness was increased from 30 mm to 50 mm, the improvement in energy efficiency was relatively small. However, when the thickness was increased from 50 mm to 100 mm, the energy efficiency rose by 3%. Furthermore, when the thickness was increased from 100 mm to 200 mm, the improvement in energy efficiency reached 5%. Overall, increasing the thickness of the air layer promoted the contact area between the spray and air, thereby enhancing cooling efficiency and boosting energy savings. These findings are shown in

Table 7. List of daily energy-saving rates of four lamination thickness.

Air Conditioning Temperature (°C)	30 mm (%)	50 mm (%)	100 mm (%)	200 mm (%)
27	26.38	27.10	30.25	35.19

. Notably, the energy efficiency at a thickness of 200 mm showed the best performance; indicating that, given suitable conditions, it is advisable to prioritize increasing the thickness of the air layer.

During the analysis of nighttime energy efficiency rates, it was observed that, as the thickness of the intermediate layer increased gradually from 50 mm, the energy efficiency rose gradually. However, upon increasing the thickness from 100 mm to 200 mm, the energy efficiency rate did not continue to ascend as anticipated; but rather, it exhibited a reversal, dropping even below the daytime energy efficiency rate. After further investigating this phenomenon, it was primarily attributed to the fact that the cooling potential of the nighttime itself is not high. As illustrated in

Table 8. List of night energy-saving rates of four lamination thickness.

Air Conditioning Temperature (°C)	30 mm (%)	50 mm (%)	100 mm (%)	200 mm (%)
27	31.95 (rise by 21%)	32.18 (rise by 18%)	35.19 (rise by 16%)	30.49 (drop by 13%)

, the natural decrease in outdoor temperature leads to a reduction in the indoor–outdoor temperature difference. Consequently, the thicker intermediate layer does not significantly contribute to nighttime energy efficiency; thereby resulting in a lack of notable cooling effects.

3.7. Comparison of Results, Error Analysis, and Measurement Uncertainty

In the conduct of this study, the importance of result comparing, error analysis, and the uncertainty of measuring parameters such as water temperature, pressure, and humidity were recognized in ensuring the reliability, accuracy, and validity of experimental data. Therefore, measures were implemented throughout the experimental design and data processing to validate results, analyze potential error sources, and assess measurement uncertainty.

3.7.1. Comparison of Results

After rigorous analysis, the water spray window cooling system has demonstrated outstanding performance. In terms of cooling effect, it has successfully lowered the temperatures of both the interior and exterior window surfaces through the utilization of high-pressure water spray technology, resulting in an improved indoor thermal environment. This cooling effect was comparable to that achieved by spray evaporation cooling techniques reported in the literature. For instance, Document [22] reported a surface cooling value of approximately 8 °C for skylights, which is closely aligned with the 6 °C reduction achieved by our water spray window. Additionally, Document [33] cites an optimal spray interval of 10/15 min, while the current study found a preferred frequency of 2/12 min. This discrepancy can be attributed to regional climate variations; yet both studies emphasized the importance of frequency control in enhancing cooling efficiency. Furthermore, while relevant literature seldom discusses comparisons of experimental water temperature and interlayer thickness, the current findings provide a foundation for utilizing condensate water as a water source and employing triple-layer glass interlayer spray windows. In regards to energy consumption and savings, the water spray window system significantly mitigates the cooling load on air conditioning systems, thus reducing the overall energy consumption of buildings. This is achieved by effectively blocking solar radiation and lowering window temperatures.

Extensive experimental testing and comparative analyses have revealed the system’s excellent stability and reliability, enabling it to consistently and effectively reduce window temperatures and enhance indoor thermal comfort. These observations align closely with the stability findings of spray evaporation cooling technologies, further validating the reliability and practicality of water spray cooling technique.

3.7.2. Error Analysis and Measurement Uncertainty

A thorough analysis was conducted to identify potential error sources. Attention was paid to the inherent error of temperature sensors, especially thermocouples used for glass surface temperature measurements. Aluminum foil was applied to reflect sunlight and minimize direct heating errors. Additionally, high-accuracy sensors from reputable manufacturers were selected. The uncertainty associated with measuring parameters such as temperature, pressure, and humidity was carefully assessed. Consideration was given to equipment limitations, environmental fluctuations, and random errors during operation. Through rigorous error propagation analysis and data interpretation, the impact of these uncertainties on conclusions was minimized. Multiple validation methods were employed to confirm the reliability and effectiveness of experimental results. Comparisons with previous studies, independent replicate experiments, and internal consistency checks were conducted.

While measures were implemented to minimize errors, validate results, and assess measurement uncertainty, certain limitations remain. These may include experimental conditions, sample size considerations, and measurement equipment precision. However, the comprehensive approach has provided reliable and valid results within the study scope.

In summary, while various measures were implemented to reduce potential error sources, some degree of uncertainty remains in the experimental results. However, the overall trends and conclusions provide valuable insights into the cooling effects of water spray window.

4. Conclusions

The present study has conducted a comprehensive investigation into the cooling performance of water spray windows under varying conditions, and their potential for energy savings. Through a rigorous experimental comparison analysis, the impacts of water temperature, spraying frequency, and glass structure on the cooling effectiveness of the system were examined. Additionally, the influence of air conditioning set temperatures and layer thickness on building energy efficiency was explored. Utilizing the experimental data, a multi-faceted analysis was conducted, encompassing thermal environment regulation and energy consumption reduction, leading to the following conclusions:

(1)

Water spray windows, in contrast to conventional glass windows, exhibit notable cooling effects and enhanced temperature stability. This technology is capable of achieving a cooling effect of up to 6 °C during the day and lowering the temperature by approximately 3 °C at night. This significantly mitigates the rise in window surface temperature and substantially reduces temperature fluctuations within the building, thereby elevating indoor comfort. The use of lower temperature water extends the cooling effects, while the utilization of condensate and rainwater further enhances renewable energy efficiency, resulting in reduced energy consumption costs.

(2)

The substantial benefits of energy savings have been affirmed by experimental data. By adjusting to optimal spraying frequencies and air conditioning set temperatures, the mist-spraying window system can effectively decrease energy consumption, leading to energy efficiency improvements ranging from 6% to 27%. Additionally, there exists a positive correlation between energy efficiency and air conditioning set temperature. It is worth noting that double-layer glass windows, when compared to single-layer ones, offer more significant energy savings during nighttime use, which especially highlights the tremendous energy-saving potential of mist technology.

(3)

The advantages of insulating spray in triple-layer windows: Research has revealed that the insulating spray within triple-layer windows possesses superior performance both in terms of energy saving and temperature reduction when compared to outdoor spray. The findings indicate that, up to a certain thickness, increasing the thickness of the insulating layer enhances energy efficiency; particularly within the 30–50 mm range. Significant energy savings are observable at night. However, a decrease in efficiency is noticed when the thickness reaches 200 mm, suggesting the need to consider an optimal thickness range during the design phase.

(4)

Applicability: Given the widespread use of large-scale exterior windows in commercial, office buildings, and modern industrial plants; coupled with prolonged air conditioning usage, the introduction of spray-on window technology presents an innovative solution for temperature regulation and energy savings. Spray-on window technology offers significant advantages in temperature control, humidity adjustment, and dust suppression, especially in environments such as modern factories, where precise environmental conditions are crucial for production processes. This is especially the case in the southern China and ASEAN regions, where the unique climatic conditions create a highly favorable environment for applying spray-evaporation cooling technology. By leveraging these conditions, more lightweight, energy-efficient window systems with improved fogging efficiency can be developed; to elevate living and working comfort, subsequently enhancing the quality of life for residents and the productivity of industrial operations.