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= Evacuated Tube Solar Collector =

Abstract:
The most available renewable energy on earth is solar energy which is coming from sun in the form of radiation ,everyday millions of watts of energy coming from solar radiation only two third part of solar energy is absorbed by the earth and the rest is reflected by the land into space. Effect of solar energy on the environment for a variety of applications is minimal as it produces no harmful pollutants .So in all the energy ,solar energy is the most efficient ,eco-friendly ,renewable and growing in demand. It is very necessary to collect solar energy and utilize it efficiently to generate electric power .Evacuated solar collector is the best way to collect the solar energy in the various type of collectors.

Introduction:
There are particular challenges in the effective collection and storage of solar energy though it is free for taking. As solar radiation is only available during daytime, the energy must be collected in an efficient manner to make use of most of the daylight hours and then must be stored .Collectors are divided into two parts ,first is ‘stationary’ and the second is ‘tracking’.Different collector configurations can help to obtain a large range of temperature for example,20–80 degree C is the operating temperature range of a flat plate collector and 50–200 degree C is for an evacuated tube solar collector[ETSC]. The most productive and mostly used solar collectors are Flat plate collector[FPCs] but these collectors have comparatively low efficiency and outlet temperatures .Flat plate collector is popular due to its low maintenance cost and simple design. However ,FPS has two major drawbacks:-

1) Convection heat loss through glass cover from collector plate.

2) Absence of sun tracking.

ETSCs have less cost and heat loss from FPCs ,due to the presence of vaccum between two concentric glass tube of ETSC heat loss is minimum and it is designed for all type of weather but FPCs are mainly designed for sunny and warm climates. Its performance decreases during cold and cloudy days and they are influenced by the weather as moisture and condensation cause erosion.ETSC have more efficiency than the other ,especially at low temperature and isolation. An ETC is made of parallel evacuated glass pipes. Each evacuated pipe consists of two tubes ,one is inner and the other is outer tube. The inner tube is coated with selective coating while the outer tube is transparent .Light rays pass through the transparent outer tube and are absorbed by the inner tube. Both the inner and outer tubes have minimal reflection properties .The inner tube get sheated while the sun light passes through the outer tube and to keep the heat inside the inner tube, a vacuum is create which al lows the solar radiation to go through but does not allow the heat to transfer .In order to create the vacuum, the two tubes are fused together on top and the existing air is pumped out. Thus the heat stays inside the inner pipe and collects solar radiation efficiently .Therefore ,an ETSC is the most efficient solar thermal collector.Applications of evacuated solar collectors in water heating, heat engines, air conditioning, swimming pool heating, solar cooker, steam generation and solar drying .According to many researchers ETSCs have much more higher efficiencies than FPCs .ETSCs can collect both direct and diffuse radiations .Besides excellent thermal performances, ETSCs have convenient installation and easy transportability. Peak energy output is provided by an FPC only at mid-day when the sun is perpendicular to the surface of the collector whereas the evacuated solar tubes are able to track sun passively throughout the day.

There are some method by which we can improve the performance of Evacuated tube:-

During the past decades, the technology to make particles in nanometer dimensions has been improved and a new kind of solid–liquid mixture, which is called a nano-fluid. Nanofluids are an advanced kind of fluid containing a small quantity of nanoparticles. Nanofluids have been applied to enhance the thermal performance of many engineering systems. Recently, nanofluids have been used as heat transfer fluids to enhance the performance of solar collector device.We can improve the thermal conductivity of fluid which is used in evacuated tube due to whice heat transfer rate increases so the efficiency will also increases .But there is major problem that nanofluid is very coastly so we can not use it more.[1]

In this method we use LiBr(lithium Bromide) as a working fluid because lithium bromide solution in an evacuated tube can be heated up rapidly and generate vapour continuously so the total energy absorbed by the solution ,the thermal collecting efficiency and the solar coefficient of performance increases by using lithium bromide.[2]

In this method we use a set of evacuated tube solar high temperature air heaters with simplified CPC (compound parabolic concentrator) and concentric tube heat exchanger is designed to provide flow air with a temperature of 150–230C for industrial production. The solar air heater system consists of 30 linked collecting units. Each unit includes a simplified CPC and all-glass evacuated tube absorber with a concentric copper tube heat exchanger installed inside. A stainless steel mesh layer with high thermal conductivity is filled between the evacuated tube and the concentric copper tube .Due to this arrangement collector shows excellent high temperature collecting performance even in  winter. The air temperature at the collector outlet could exceed 160c at noon time and thermal efficiency also increases with this type of arrangement.[3]

In this method we use continuous carbon nano- tubes for improving thermal efficiency of evacuated tube. It was found that 10 layers of carbon nanotube sheets with medium sheet areal density produced the best solar selective coating .carbon nanotube sheet densification with organic solvents slightly increased the reflectance and significantly decreased the emissivity, improving the thermal efficiency of tubes. By taking advantage of the Joule heating effect, carbon nanotube sheets provide fast and efficient heating rates on demand, which can be useful in solar water heating systems when solar radiation is insufficient. Carbon nanotube sheets were found to have higher absorption and lower reflection in the visible and near UV region of the solar spectrum.[4]

The performance of a solar water heater with evacuated tubes integrated with phase change materials. The results show that phase change material integrated inside the inner tubes of evacuated tube solar collectors can effectively store energy (in the form of latent heat) next to the heat pipes and enable a delayed cooling after sunset or late evening. The solar collector utilizes two distinct phase change materials (dual-PCM), namely Tritriacontane and Erythritol .Due to delay in emission of heat ,thermal efficiency of evacuated tube increases.[5]

In this method U-shaped evacuated tubular collector integrated with compound parabolic concentrator. Such type of system is very useful in the preheating process for very high temperature industrial applications. The energy matrices like Energy pay back time, Energy production factor and Life cycle conversion efficiency of evacuated tubular collectors , gradualy increased .Here copper tube absorbe the heat and use this heat in preheating of water due to this efficiency of tube will increased.[6]

In this method Graphene nano platelets nano fluids were introduced as a thermal efficiency improvement of evacuated tube solar collector in solar-water heater systems. The GNP nanofluids were prepared by two step method with ultrasonication probe, which was stable for a long period. The GNP nanofluids have high thermal conductivity with Newtonian fluid behavior. The admirable heat-transfer property of GNP nanofluid could enhance the working fluid efficiency in solar collector field and other applications in solar energy.[7]

In this method solar system equipped with thermoelectric modules and evacuated tube collectors. The advantage of the use of thermoelectric modules in recycling the energy from condensation of vapor and the use of evacuated tubes for increasing the water temperature and as a result increasing the evaporation rate. Thermoelectric modules are used to utilize the heat from the vapor condensation in order to produce electricity. The hot side of the thermoelectric module has been heated up by the vapor and the cold side is in contact with the ambient air and this temperature difference leads to the generation of electricity. The results reveal that the use of the electricity generated by the thermoelectric modules, which is the result of the recycling of the latent heat of condensation.[8]

Reference:
[1]Das, Sarit Kumar, et al. "Temperature dependence of thermal conductivity enhancement for nanofluids." Journal of heat transfer125.4 (2003): 567-574.

[2]González-Gil, A., et al. "Experimental evaluation of a direct air-cooled lithium bromide–water absorption  prototype for solar air conditioning." Applied Thermal Engineering 31.16 (2011): 3358-3368.

[3]Wang, Ping-Yang, Shuang -Fei Li, and Zhen-Hua Liu."Collecting performance of an evacuated tubular solar high-temperature  air heater with concentric tube heat exchanger." Energy Conversion and Management 106 (2015): 1166-1173.

[4]Itkis, Mikhail E., et al. "Thermal conductivity measurements of semitransparent single-walled carbon nanotube films by a bolometric technique." Nano letters 7.4 (2007): 900-904.[5]Papadimitratos, Alexios, et al. "Evacuated tube solar collectors integrated with phase change materials." Solar Energy 129 (2016): 10-19.

[6]Mishra, R. K., Vihang Garg, and G. N. Tiwari. "Energy matrices of U-shaped evacuated tubular collector (ETC) integrated with compound parabolic concentrator (CPC)." Solar Energy 153 (2017): 531-539.

[7]Ahmadi, Alireza, DavoodDomiriGanji, and FarzadJafarkazemi."Analysis of utilizing Graphene nanoplatelets to enhance thermal performance of flat plate solar collectors." Energy Conversion and Management 126 (2016): 1-11.

[8]Shafii, Mohammad Behshad, et al. "Examination of a novel solar still equipped with evacuated tube collectors and thermoelectric modules." Desalination 382 (2016): 21-27.