Experimental Analysis on Thermal Efficiency and Heat Loss of the Evacuated Tube Collector with Variable Solar Radition and Wind Speed

Abstract

The experimental study was performed on a solar water heater- type evacuated tube collector and results were drawn for efficiency and overall heat transfer coefficient along with net heat loss. In this research, the paper calculation was done on the basis of varying solar radiation and wind speed, average values of five readings were taken to get a better output. Maximum and minimum values were analyzed for which the performance of the system would be optimum. The maximum efficiency was found to be 64% at maximum radiation of 750W/m2 and wind speed of 2m/s, also as solar radiation inversely varies with X factor so at minimum radiation that is 250W/m2 it was found to be maximum at different wind speed. It can be seen that the overall heat transfer coefficient and net heat loss increases with increasing solar radiation and wind speed so, overall heat transfer coefficient (Ue) was found minimum at radiation of 250 W/m2 is 0.5039 at 2m/s and maximum at radiation of 750W/m2 that is 0.55 at 4m/s and Net heat loss (Q) values were 20.65 minimum and 28.25 maximum for corresponding solar radiation and wind speed.

Introduction

I. INTRODUCTION

For a country like India having been blessed with immense sunlight, solar energy is one of the most important and rapidly growing sources of renewable energy in India as well as the world today.  It is a clean, sustainable, and cost-effective alternative to fossil fuels which have been the primary energy source for many decades. The potential of solar energy is ever-expanding and limitless and it has become an increasingly popular option for households, and industries. With the rapid advancements in technology solar energy is becoming more efficient, affordable, and accessible than ever before. Its application is in various fields such as the heating of buildings, solar distillation, and solar water heating. As mainly used for water heating the heat from sun is trapped using green house effect. The reflective surface transmits short wave and reflect long wave radiation. When the shortwave radiation hits a collectors absorber heat and infrared rays are produced and is trapped inside collector, which is then transffered to the liquid for heating purpose. One such apparatus used for water heating is the Evacuated Tube Collector system which uses solar energy to heat large quantities of water.

This system contains evacuated parallel glass tubes cylindrical in shape inducted with thermally conducted copper tubes in which sunrays hit perpendicularly. Cold water from the storage tank comes to the manifold (header) and then the water travels through evacuated tubes in which it gets heated with the help of solar energy and due to density difference hot water comes up to the manifold which can be stored and used for various purpose. Using evacuated tubes helps to maximize the temperature and improves the efficiency for longer duration. An evacuated tube collector system produces 25%-40% more efficiency as compared to flat plate collector and also due to its cylindrical shape it is able to absorb solar radiation from all directions [1] and its value can be depicted from Table1.Using an evacuated tube collector temperature of the hot water was around 71.66% efficient and efficiency was 60.11% with minimum relative error [2].

With increasing mass flow rate and solar radiation, the thermal efficiency of collector is higher at negative inlet temperature [3].  At higher value of flow rate, the efficiency of evacuated tube collector system varies from 0.12-0.5 and the maximum temperature of air was found to be 56.7? [4]. It can also be seen that with mass flow rate also affects the thermal performance and maximum useful energy is affected by parameters such as solar radiation and outlet temperature [5]. Also using the reflectors in the evacuated tube collector system, the efficiency increased by 16% and double the reflectors increases its performance [6]. The presence of air thermal resistance raises the temperature by 30? and its absence reduces the collector efficiency and outer temperature by 10% and 16% [7].  Yearly efficiency of the evacuated tube containing the heat pipe was 0.62 while in forced circulation it reached 0.516 [8].

Apart from the tubes the gain in energy and thermal performance of the system also increases as compared to the normal condition by using the bypass tubes in the storage tank [9].

It can be seen that in the evacuated tube collector system the tubes are connected in series with the manifold which provides high temperature as the Solar radiation increases [10]. Thermal radiation is responsible for the performance of the system and the selective coating helps to increase the performance of the system [11].  Increasing the solar radiation and mass flow rate along with the number of tubes raises the exit temperature but to a limit after which it would maintain a constant value [12]. Radiation directly varies with the water productivity and its maximum value was achieved at a tilt angle of 15 degrees which was 27.21% daily [13]. It can also be seen that the system efficiency is reduced at lower solar radiation and higher energy is released at starting of the day [14]. And because of the solar irradiation, the thermal efficiency of the collector is poorer with a high heat loss coefficient [15]. Solar Radiation highly affects the outlet air temperature as compared to the wind speed and ambient temperature which is of minor importance [16]. The effect of wind speed is that by increasing it by 0.86m/s the efficiency reduces by 67% [17]. And at different parameters for an effective tube length of 1.5m, improved efficiency is achieved for different diameters of tubes [18]. It can be seen that higher solar radiation leads to higher hot water temperature in which collector efficiency was found to be 72% for hot water temperature of 43? and ambient temperature of 21? [19].

 When the losses are considered from the evacuated tube heating system it is less in amount as compared to the storage tank [20]. These losses are raised to 2.7%   from the evacuated tube at 0.01-1 MPa and it increases with the increasing temperature [21]. Tubes of the collector in an evacuated tube collector have a higher overall heat transfer coefficient with respect to the varying gas inside the envelope [22]. And the heat loss coefficient from the tube with an aspect ratio of 32.9 is about 0.742W/m2K [23].  Within the daytime, it can be seen the losses are about 1.8 W/m2k and the efficiency can be seen at about 51% [24]. Considering the air gap heat transfer coefficient plays a vital role in facilitating the heat transfer which can be calculated as 9.992W/m2K with the temperature of the copper plate as 107.62? [25].

II. MATERIAL AND METHOD

The experimental apparatus of the evacuated tube collector system contains a transparent evacuated tube made up of borosilicate or soda lime glass connected parallel to the header. Reason for using such material of glass because it has optical clarity, corrosion resistance, inert behaviour and it is affordable. Each tube consists of a thicker outer tube and a thinner inner tube which helps in absorbing high solar radiation and impedes heat loss and the calculated result with less overall and net heat loss can be seen in Table 3 and 4. Due to its curved surface, it is able to absorb high amounts of radiation from all direction which helps in improving systems performance also the effect on efficiency with respect to inlet temperature considering ambient and solar radiation effects can be drawn from Table 2 .The trapped air is removed between both the tubes hence it is called vaccum or evacuated, which acts as an insulator and reduces the heat loss either by convection or radiation.

The insulation properties of vaccum are such good that while the inner tube temperature raises to its maximum the outer tube remains cold. Due to the vaccum created between the tubes, the overall efficiency of the system is higher and it improves the overall performance of the system. Inside the glass tube, a copper or aluminium fin is attached to the heat pipe which is placed inside the inner tube, fins are covered with a selective absorber coating that helps in transferring the energy to the fluid and raising the temperature of the water to its maximum value which is then used for various purpose such as in residential buildings, commercial applications and can also be used for space heating is the location of the building is off the grid. Nowadays solar water heating system of this type is preferred because they are able to extract heat from air in humid and overcast weather conditions which do not need direct sunlight to operate.

This experimental setup consists of multiple pressure and temperature sensors along with flow control valves and a monitoring screen called control unit that displays processed values which helps in getting accurate results while working with apparatus. The various other devices needed in the function of this experiment contain a radiation meter, IR temperature gun, and Anemometer which is used to measure the solar radiation coming form the halogen fixture, the temperature of the absorber plate, and the wind speed from tower fan, a regulator is provided through which the intensity of the light coming from the source can be varied. Various experiments are performed on the setup with the help of an artificial source of sunlight and its performance and parameters are calculated. The parameters calculated such as heat loss, efficiency and net heat loss, through which results can be verfied to the optimum in the working of setup. Readings can be taking considering the different parametes such as solar radiation, wind speed and ambient temperature and better performance for the given values can be measured. Hence in the given setup below all the different situations were taken into consideration and values were taken out which would give accurate result.

III. WORKING

In the experiment performed the artificial source of sunlight is provided with the help of halogen fixtures whose values can be altered with the help of a dimmer which is used to give varying solar radiation. The cold water is poured into the storage tank and with the help of pump 1 mentioned in Fig.1. the water is transferred to the manifold (header) and then it is distributed to the evacuated tubes, where a source of sunlight is provided with the help of halogen fixtures and water is heated in the evacuated tubes. The tubes contain a heat pipe that helps to heatup the water coming from the manifold and also it collects the solar radiation and converts solar energy in the form of hot water, after the water is heated according to the varying radiation, due to density difference the hot water comes up and it travels upwards to the header. This hot water passes to the storage tank which can be used for various purposes. The setup also contains the heat exchanger, with the help of which the hot water transfers its heat and cold water again from the storage tank repeats its cycle.  Some experiments also contain a chiller in another running cycle which helps to keep the water at constant inlet temperature. The operation performed in that cycle is that water from the heat exchanger goes to the chiller where the temperature of the water is lowered to maintain the water temperature at the storage tank. Also experiments can be done when the inlet and outlet temperature would be equal and complete amout of water will get heated at certain time which is calculated with minimum wind speed at higher radiaition at Table 5 and 6. With the help of the experiments performed in the above setup varying the operating values such as solar radiation, wind speed, etc. values can be calculated for which the performance of the system would give an optimum result. This system is preferred because it is reliable, has longevity, provides resistance to (environmental conditions, large variations in temperature, and leakage), is stable, is easy to install, and is effective in energy conservation

VII. NOMENCLATURE

Conclusion

1) The efficiency of the system increases with solar radiation and decreases with increasing wind speed that is it is considered maximum for 750W/m2 and 2 m/s velocity as 64%. 2) Efficiency inversely varies with X factor for as the radiation is maximum the value of X factor decreases and for minimum radiation its value increases and its value reached minimum as 0.014. 3) The overall heat loss coefficient of the evacuated tube increases with increasing wind speed and higher solar radiation, it should have lower value which was found to be 0.5039W/m2K at 250W/m2 radiation and 2m/s wind speed. 4) The Net heat loss from the evacuated tube is also raised with the high value of wind speed at a higher radiation level considering its minium value as 20.65 Watt. 5) The inlet and outlet temperature of the system would be equal at which increasing radiation level as less time would be taken to heat the maximum amount of water it was found at 750W/m2 radaiton and minimum 2m/s wind speed which took 6 hrs approx to reach the temperature of about 60?. 6) From the above results it can be concluded that for better performance of the system varying solar radiation from minimum to maximum would give better results with minimum effect of wind speed.

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Copyright

Copyright © 2023 Surbhi Kori, Yuvraj Shakya, Bhupendra Gupta, Jyoti Bhalavi. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.