Heat transfer characteristics of nocturnal cooling system for clear sky climate of Western Maharashtra, India

Objectives: To investigate heat transfer characteristics of nocturnal cooling system in the Indian climate with suitable material for radiator and proper coating to enhance the heat transfer. Methodology: A nighttime radiative cooling system with aluminum radiator in a dry area, is assessed both experimentally and theoretically. A theoretical model has been proposed to evaluate the radiative cooling potential for the dry type climate of Western Maharashtra, India. However, the end result can be improved by a nighttime cooling system based on nocturnal longwave radiation, which cools the air below ambient temperature. Findings: The experimental room temperature could be maintained at about 24.5◦C less than the outer room temperature. It is found that there is a huge scope in reducing the energy requirements for cooling purposes. Average radiative cooling rate of 55.84 71.88 W/m2 is achieved for without coating and 72.30 80.99 W/m2 for coating on the radiator. Novelty: In this study, we have designed and assessed the radiative cooling systemwhich has a possibility of reducing the power using up for space cooling purposes. For achieving the building healthier and more comfortable, the building design is a key factor for saving energy and reducing emissions by considering nocturnal cooling design principles with suitable design and materials. The design of energy-efficient and sustainable buildings is important for the future.


Introduction
The phenomena of worldwide warming and the decrease of non-renewable energy sources have encouraged long-drawn-out consideration towards energy use. The home structure region represents a notable 40-50 % amount (1) of the EU's entire power utilization, which reflects on the significance of using an energy-aware conventional system in the house store. There are available records that reveal about the system studied for different climates of different countries but there is no record of a systematic study specifically for Indian climate. However, India has a large scope to use nocturnal https://www.indjst.org/ radiation system for space cooling in different cities. Very few researches are conducted to analyse the effects of the combined active and passive cooling methods on the enhancement of heat transfer rate, and hence it is an appropriate time to carry out work on this new approach. These facts motivate us to do research with greater emphasis on the different materials for radiators with a suitable coating to improve the heat transfer. It is required to heat the building during cold days and cool during hot days for comfortable living. As a large amount of primary energy is required for cooling, compared heating up, it is a matter of concern which depends upon the nation primary energy conversion factor. As far as energy efficiency is concerned natural cooling systems are useful but various circumstances make an effect on performance.For natural cooling, there is availability of three natural heat sinks. More heat is carried into these three heat sinks. Night-time convective cooling is the process of ventilating the building by using the atmosphere as a heat sink. When the temperature of night is cold then building structure gets cooled either by natural or conventional way causing the formation of colder thermal mass for the next day. This method has been investigated by various studies. Many factors make an influence on the effectiveness of this method. The building design is a key factor for saving energy and reducing emissions through passive solar heating and cooling design principles and using the correct materials and suitable design tools. Conventional cooling methods, that utilize an enormous amount of energy with a major carbon footprint in pollution, is replaced by daytime radiative cooling which is emerging as a passive and environmentally friendly cooling approach. (2) The principal reasons for exploring options for proficient cooling systems are universal warming due to the emissions of carbon dioxide and hydro fluorocarbons used in refrigeration & air conditioning, exhaustion of non-renewable energy resources and demand for less polluting cooling systems. There is an urgent need of developing cooling systems which consumes less or non conventional energy source. However, we cannot fully replace conventional energy with non conventional or alternative form of energy but efforts should be made to reduce its consumption. In this regard, Radiative cooling can be the cost efficient technique to reduce the energy expence of cooling systems. It is a passive technique to exchange heat with large sink such as sky. These cooling techniques are mainly based on the principle of heat transfer by release of long-wave radiation from a more temperature body to a less body. (3) This type of radiative heat transfer takes place considerably between earth and sky during night. This occurs due to the transmission of earths radiation to the sky during nights along with the emission of atmospheric radiation due to the water vapor, carbon dioxide and ozone layer there by reducing our reliance on fossil fuels required for conventional cooling methods.
A considerable research work in the area of nocturnal cooling system has been carried out of which a few are reviewed. The previous studies were concerned with non-conventional cooling, dissimilar radiator materials with experimental and numerical assessment. In (4) studied the possibility of free cooling of house by nocturnal ventilation was examined by analyzing climatically information, irrespective of construction-particular parameters. Associate move toward for calculative degreehours supported a variable house temperature inside an identical vary of thermal comfort was presented and functional to climatically information. In (5) studied the current situation of continuous growth in energy price globally. Insulation of the exterior walls of the building is suggested as a strategy to increase the energy efficiency of constructions. In (6) they declared that the suitableness of free ventilation during night for cooling offices in modest climates like that of the United Kingdom was checked by describing plots of hot days climate information on the climatology chart for 3 locations inside the nation. In (7) they reported that the house nocturnal ventilation may not provide effective cooling performance with intense solar radiation, conventional systems become essential in western France. A nighttime radiator is cool inside air below ambient temperature under nighttime radiation of the sky to increase the cooling performance of the system. In (8) they explained a theoretical assessment of the probability of the cooling method to considerably decrease cooling requirements in houses in the North American atmosphere. The information represents the novel built-up cooling unit which uses nighttime radiation from a builtin heat exchange unit. They concluded that the performance of the system, system air temperature, radiator temperature and system airflow rate depend upon climate parameters such as outside temperature, cloudiness and wind speed. In (9) conducted experiments on an aluminium nocturnal radiator. A radiator is colored with a suitable white shade. The metallic radiator was used for cooling the outdoor air below its primary temperature. The energetic thermal performance of the system throughout the days during hot months was calculated using numerical model, on the basis of heat transferred from the air moving inside the aluminum radiator to the ambient air. In study (10) has examined that the sun radiates heat to earth in the day and at night the reverse happen warmer earth radiates heat to the cold night sky. Building roofs radiate heat at day and nighttime a rate of up to 75 W/m 2 . During the day, this is offset by solar radiation absorbed by the roofs of building, but at night, this heat loss has the ability to cool air as roofs can experience a temperature drop of 6 • C to 20 • C below ambient. (11) studied the mathematical modeling of a night sky radiation cooling system appropriate for a room located in the Namib Desert at Gobabeb, Namibia. The component of the system are radiator panels, a single water storage tank, room air-to-water natural convection heat exchangers or convectors, circulating pumps, interconnecting pipe work and temperature sensors and controls. In (12) Longwave radiance of the sky during the hot season of 1979 at five cities in USA were measured. They monitored longwave radiations which are more than 3 microns by a pyrgeometer. They found this radiation to be distributed in many spectral bands with five different zenith angles. They used a spectral radiometer for this purpose. In another study, they obtained various data related to the thermal radiance of clear skies in three U.S. cities and then they presented it in the terms of relation between sky emissivity and surface dew point temperature. In (13) studied for clear and cloudy sky conditions the experimental search of the aluminum material and their surface color on nocturnal cooling was presented. MPCM slurry has been cited to be a good medium for the combined application of natural cooling technique in an HVAC system (14) . The outcomes showed the energy saving potential can reach 77% and 62% for low-rise buildings in Lanzhou and Urumqi, which show evidence of strong attractions for building energy conservation and emission decline.
The literature review shows that numerous investigations has been performed for heat transfer characteristics of a night cool nocturnal radiation system for space cooling at different climates and focused on the consumption of energy for building the environment and its effects on the surrounding. Also, there is no record of a systematic study of thermal analysis of the night cooling system in an Indian climate. The experimentation and mathematical models are presented in this research to expect the possible cooling for a clear sky climate of Western Maharashtra. The cooling requirements for the Western Indian climatic conditions are from month March to June. So, in this paper, we evaluate the radiative cooling potential in the clear sky condition in summer days. Also, the detailed study of the basic principle of nocturnal cooling system and the effect of wind speed, a velocity of mass flow rate on a rate of heat transfer and efficiency of the system is discussed.

Experimental setup of nocturnal cooling system
While conducting an experimental investigation on nocturnal cooling radiator, proper development of system and design of set up is of prime importance. The various research papers by different authors on night cooling systems are studied and with their reference CAD models are developed and the real experimental model is also built. Some considerations are taken into account while erecting the model. Necessary data for the construction is collected from journal papers and references. Detail modeling of all the parts is done by using PRO-E software.  As previously mentioned, it was decided that a radiator or heat exchanger unit dimensions should be 2 m x 2 m. (15) But for easy handling and mounting purpose system two units of dimension 2 m x 1 m were fabricated as shown in Figure 2 (b). As the radiator (heat exchanger) has two aluminum sheet plates of 0.00044 m thickness separated by a gap of 0.1m. Nocturnal cooling is run by mostly the radiation of all surfaces towards the sky at night via thermal radiation change. The radiation between a radiating surface and the night sky can be described by approximating the surface as a black body. The sky longwave radiation https://www.indjst.org/ exchange is mainly a function of the sky effective temperature. In particular, radiative cooling is a result of heat loss by longwave radiation emission towards the sky, where the sky can be used as a heat sink for exterior surfaces of buildings. Radiative cooling is the largest at night darkness when the sky is clear. On a clear night, a building's external surface temperatures typically drop below the ambient temperature due to heat loss to the sky. The heat transfer takes place in the nocturnal cooling system due to all modes of heat transfer i.e. conduction, convection and radiation. Initially, air supplied by the exhaust fan enters in the radiator from the top inlet port and then it is distributed in the radiator uniformly. This hot air comes in contact with the surface of a radiator and gives its heat to the plate by the convection mode of heat transfer. Thus air is cooled on the loss of its heat energy. The temperature difference between the plate and air will create the driving force to take place heat transfer. To maintain the steady condition, the plate has to give transfer its heat to the surrounding air and sky. Therefore in plate heat is drawn out by conduction mode of heat transfer from all sides of a radiator. After this, heat is liberated to the surrounding flowing wind by the convection and the sky through electromagnetic waves form to the sky. Thus temperature difference takes the heat from circulating air to the atmosphere and cooled it which will be supplied back to the room. The entire process leads to maintain the required lower temperature in the room with the help of nocturnal cooling. Results show the correlation between the climate conditions, wind speed, velocity of the working fluid flow through the radiator and the getting temperature fall across the radiator and performance of the cooling rate.

Mathematical model of system
By using numerical equations the active thermal performance of the radiator has been determined. Entire heat extracted from radiator flowing air through the model is calculated by eq. (3.1), The change in internal energy of the circulating air in a radiator is useful the heat the airflow is calculated by eq. (3.2) An Efficiency of the radiator system is the performance of the radiator unit is efficiency calculated by eq.   (16) by eq. (3.11), https://www.indjst.org/

Sky temperature calculations,
• The effect of Cloudiness is denoted by 'CC' . The value of CC is varying from 0 to 1, according to the climate conditions. Take 0 for the clear climate and 1 for the fully cloudy climate. • Air temperature T air in (K).
• Dew point temperature i in Celsius is depending on the relative humidity in % and temperature of air ini. And it is calculated by eq. (3.12).
Model for clear sky temperature: At first, calculate the temperature of the clear sky (17,18) using eq. (3.13) Where, ε clear =Emissivity of clear sky (12,19) s calculated by the following eq. (3.14), Now, the temperature of the sky is calculated by using the following eq. Where, CC= Effect of cloudiness.

The R top resistance is the combination of resistances offered from radiation sky and convective wind is given by eq. (3.17)
Take values of h rad. sky &h conv. top are taken from eq. (3.10) & (3.11)

The R cond. top resistance due to the material of the radiator. Thermal resistance offered by a plate to conduction mode of heat transfer is calculated by eq. (3.18),
Where, t p = thickness of radiator plate in (m) Kp = Thermal conductivity of radiator plate material in (W/mK).

The rate of heat transfer between a radiator plate and air being circulated is calculated
using the below procedure. Figure 1 (b) shows the radiator which is used for experimental work. The heat exchanger unit having dimensions are length L, width W and height H of radiator in meter. The inside air of a room is let in through the top end of a radiator and let out of the radiator again to the room. As this air passing near the radiator surface offered some resistance i.e. R conv.air . The coefficient of convective heat transfer of circulating air is calculated using the below steps, https://www.indjst.org/

The airflow through the rectangular duct, hence the bulk mean temp is calculated by the following eq. (3.19),
Where, T in = Temperature of air at the inlet of a radiator system. T out = Temperature of air at the outlet of a radiator system.

The bulk mean temperature is required to select the properties of the air.
After calculating the bulk mean temperature, different properties of working fluid such as density, specific heat, thermal conductivity & dynamic viscosity are taken from a property table of air. (20) is required for calculate the Reynolds and Prandtl number calculated by the following eq. Where,V air = Velocity of air at the inlet of a system in (m /s). Cross-sectional area of pipe = π x D 2 / 4. D= Diameter of PVC pipe in (m).

Night time evaluation of system with investigation and results
Depending upon the mathematical equations given in the above part, the nocturnal radiative cooling possible has been defined. The air temperature reduction depends on the intensity of radiation in the form of longwave, wind speed and the airflow through the aluminum radiator. It is observed that the reduction in temperature of the air increases step by step with the increase of the radiation's concentration i.e. clear sky condition. On the other hand, to changing airflow, a maximum value of air temperature falls can be found. For the high mass airflow rate, the cooling outcome can only just be realized on the air temperature drop. After installation of the nocturnal cooling system on the building roof, by using experimental set-up readings were taken in clear https://www.indjst.org/ sky hot day i. e. summer climate condition. The performance of nocturnal cooling should be checked in the clear sky climate condition of western Maharashtra in India. The performance of a nocturnal cooling system was determined by comparing the results obtained under the above basic conditions of climate. The analysis of a nighttime cooling system has been done in month of May. A radiator with a total area of 4 (m 2 )of nocturnal cooling system was investigated. Domestic cooling load is dependent on individuals, electrical devices and independent from the climate. Comfort cooling depends on ambient temperature. The data contains parameters of each day such as ambient temperature, relative humidity, and wind speed, to estimate the radiative cooling possible. Using these factors sky temperature is calculated which will be used to calculate the cooling power. The performance of the system was tested by varying the mass flow rate of air which passes through the radiator. The mass flow rate was controlled by the control valve fixed between the duct and exchanger unit. The velocity of air at the inlet of the unit was kept as 3.6 (m/s) and 4.9 (m/s). The humidity and cloudiness are required for dew point and sky temperatures were taken from the metrological website of Government of India (23) . The dew point and sky temperature are depending upon relative humidity and presence of the cloud in the sky. After assessing different parameters as declared in Table 1 , the study is to estimate the radiative cooling possible. The sky emissivity is calculated using eq. (3.14). It is found that the emissivity of the sky varies from 0.887 to 0.869 on the dated of 23 May. The sky temperature and ambient temperature difference is 9.26 to 10.89 ( • C). Figure 4 shows the graph of time Vs variations in the temperature of a radiator, sky temperature and ambient temperature. The model's radiation heat loss coefficient is a depending emissivity and ambient temperature of the radiator; according to eq., (3.11), is an important factor for dissipating the excess heat from the top surface. In the cooling application, the radiator's emissivity is expected to be between 0.80 and 0.94. Figure 5 illustrates the slight effect of this parameter on the radiative https://www.indjst.org/ coefficient in different ambient temperatures, as determined throughout numerical calculations. It has been seen that the radiative heat transfer coefficient can be varied in the range 5.03 to 4.50 (W/m 2 ºK) for without coating of mass flow is 0.03 (kg/s). Also it can be varied in the range 5.20 to 4.74 (W/m 2 ºK) for without coating of mass flow is 0.05 (kg/s). The above results indicated that if the mass flow rate is 0.05 (kg/s) then a radiator provides desirable value for the radiation sky heat transfer coefficient than that of the mass flow rate is 0.03 (kg/s) a radiator without coating. It is because, the higher mass flow rate increases the velocity of flow in the device and hence it increases the Reynolds number and consequently the rate heat transfer between the surface and air. Therefore, it can be concluded that the mass flow rate is 0.05 (kg/s) of air to a radiator without coating is more desirable to enhance heat transfer of the system.
The gap of ambient temperature, sky temperature and radiator temperature in the clear sky condition was bigger than that in the cloudy sky condition due to the effect of the protection of the infrared radiation. As per the working principle, as increases in mass flow rate reduces the temperature difference between the radiator inlet and outlet which results in increases in the mean surface temperature of the radiator. Therefore theoretically higher value of total heat loss should be recorded for clear sky condition with coating and higher mass flow rate through the system. It is found valid with observations made during the experimental work. In the present study, the values of net cooling ranged for without coating and coating on aluminium radiator is from 55.84 to 71.88 (W/m 2 )and 72.30 to 80.99(W/m 2 ) as the mass of airflow increases from 0.03 to 0.05 (kg/s) respectively for clear sky condition. It is shown in the Figure 6 . It follows a similar type of variation as that of all four different conditions i.e. initially it goes on increasing up to 7 pm and reaches its higher values because of higher temperature difference, wind speed, lower temperature of the sky and surface https://www.indjst.org/ temperature of the radiator. Then it starts decreasing, majorly due to the reduced gap between sky temperature and surface temperature. In the present study, similar types of results are found as cooling rates are between 70 -80 (W/m 2 ) and 40 -55 (W/m 2 ) for clear sky and cloudy sky condition respectively. The net cooling rate obtained in the experiments coincides with that reported by Dobson, (11) who recorded a 60.8 (W/m 2 ) of net cooling rate. Martin and Berdahl (19) have recorded similar observation as this. In a clear sky night condition with surface temperature at 27 o C achieved cooling rate is about 75 (W/m 2 ). In a cloudy sky humid climate with larger atmospheric moisture content, the rate of cooling falls to about 60 (W/m 2 ). Okoronkwo C. A et al (24) have obtained cooling power during the dry season was greater than that of the raining season. A maximum net power of about 52.5 (W/m 2 ) was obtained during the dry season while for the wet season the cooling power dropped to 37 (W/m 2 ).
The effect of increased wind convective heat exchange and the overall heat loss coefficient (U Top) on the efficiency factor are expressed in eq. (3.10) and (3.6). The efficiency of the heat exchanger unit is determined by measuring parameters by using eq.(3.3). It observed that in Figure 7 the values of efficiency ranged for without coating aluminium radiator from 0.41 to 0.53 as the mass flow rate is 0.05 (kg/s) and 0.34 to 0.47 as the mass flow rate is 0.03 (kg/s). It was observed that increases in mass flow rate result in increases in the overall loss heat transfer coefficient, heat removal rate & efficiency.

Conclusion
In conclusion, the accomplishment of the radiative cooling system for buildings in the tropical climate, especially in Maharashtra, is relatively encouraging. The following evidences were found during research work, 1. The active thermal performance of a light-weight metallic radiator covered by an aluminium sheet was calculated using the mathematical model. Finally, the thermal behavior at above mentioned location was prepared with a lightweight nocturnal cooling system and was analyzed using the empirical mathematical model. A review of nocturnal cooling system has been undertaken. From this study, the following are the observations in the clear sky conditions,