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54 Int. J. Exergy, Vol. 25, No. 1, 2018

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Exergy analysis of evacuated tube solar collectors: a review

Gaurav Saxena* and Manoj Kumar Gaur Department of Mechanical and Automobile Engineering, Madhav Institute of Technology and Science, Gole ka Mandir, Gwalior, MP, 474005, India Email: [email protected] Email: [email protected] *Corresponding author

Abstract: The most recent studies show that different methodologies have been adopted to understand the concept of Exergy in general and exergy efficiency, exergy destruction, exergy losses, etc. in particular. The present paper reviews the concept of evacuated tube solar water heating systems (SWHSs) followed by fundamental laws of thermodynamics necessary to provide the concept of exergy analysis in detail. Mathematical modelling and experimental data provide the effect of mass flow rate, temperature gradient, inlet temperature, outlet temperature, collector efficiency, etc. on exergy. Finally, the exergy analysis and exergetic efficiencies along with exergy destruction sources for the evacuated tube collectors are presented.

Keywords: solar energy; ETC; evacuated tube collector; exergy analysis; exergy efficiency; SWHS; solar water heating system.

Reference to this paper should be made as follows: Saxena, G. and Gaur, M.K. (2018) ‘Exergy analysis of evacuated tube solar collectors: a review’, Int. J. Exergy, Vol. 25, No. 1, pp.54–74.

Biographical notes: Gaurav Saxena is an Assistant Professor at RJIT, Border Security Force Academy, Tekanpur (M.P.), India. He obtained his BE (Mechanical) and MTech (Production) from M.I.T.S., Gwalior. Currently, he is pursuing his PhD in Solar Energy under the guidance of Dr. M.K. Gaur from M.I.T.S., Gwalior. His area of research interest is the integration of PV and evacuated tube technologies in solar dryers. During his 10 years of teaching, he has published several research papers in the journals and conferences of international repute and guided several post graduation research.

Manoj Kumar Gaur is working as an Associate Professor and Head of the Department of Mechanical and Automobile Engineering at Madhav Institute of Technology and Science Gwalior (MITS) (an Autonomous Government aided Institute, Madhya Pradesh, India). He has done his PhD from Indian Institute of Technology Delhi on “Development of Heat and Mass Transfer Coefficients/ Correlations for High-Performance Solar Distillation Systems”. He possesses 15 years of teaching experience of Post Graduate and Under Graduate students. His broad area of interest includes thermodynamics, heat transfer, solar energy and solar distillation.

Exergy analysis of evacuated tube solar collectors: a review 55

1 Introduction

The solar energy is the most promising source of energy. It is an easily available source of renewable energy on earth as the earth receives millions of watts of energy every day coming from solar radiation. As a fact, only a fraction of it in the form of daylighting and photosynthesis is used by the natural world, one-third is reflected back into space and the rest absorbed by land, oceans and clouds. Solar energy as an available, cheap, and environmentally friendly alternative source has been the subject of many theoretical and experimental studies (Saidur et al., 2012). Hence, there are particular challenges in the effective tracking of solar energy by sun through the direct medium. So, solar collectors have to be used for the efficient and novel use of solar energy and storage of solar energy. Solar collectors are the existing element for tracking sun rays which are then turned into thermal energy and uproot to a primary heat transfer fluid consequently. Therefore solar collectors are termed as the fundamental element of any solar system (Singh et al., 2013). There are mainly two types of solar collectors stationary (non-concentrating collectors) and tracking (concentrating collectors) (Kalogirou, 2004). Figure 1 shows the classification of solar collectors.

Figure 1 Classification of solar collectors (see online version for colours)

The present paper gives emphasis to the method and outcomes of exergy analysis of Evacuated tube collector (ETC), in fact, presently several researchers are using this analysis in almost all applications of solar energy system for analysing the performance characteristic. Some recent and significant application of exergy analysis as a photovoltaic system, solar combi-systems, solar air and water heating systems etc are briefly discussed here. Acar and Dincer (2016) investigated Exergetic performance assessment of an integrated solar energy system with five outputs, namely electricity, heat, hot water, cooling, and air conditioning (humidifying/dehumidifying) for residential use. The findings showed the maximum exergy efficiency for the overall system. Joshi et al. (2014) worked on solar exergy maps for photovoltaic/thermal systems. The experimentation work was carried out at five different climatic conditions of India and nine different climatic conditions of USA. The comparative analysis of average exergy was used to determine the climatic conditions and month in which optimum results are obtained. Kaçan and Ulgen (2014) performed Energy and exergy analysis

56 G. Saxena and M.K. Gaur

of solar combi-systems in Turkey during January. As a result, assessments showed that solar combi-systems were applicable and smart solution to save energy besides this saving in energy and maximum instantaneous efficiency of the collector was determined. Karakilcik et al. (2013) carried out an experimental investigation of the exergetic performance of a solar pond integrated with solar collectors. They used density and temperature to determine energy efficiency and compared it with exergy efficiency of the integrated solar pond (ISP). The solar pond was analysed for three heat zones viz. upper convective zone (UCZ), nonconvective zone (NCZ) and heat storage zone (HSZ). The design integrated with four flat plate collectors (FPCs) for storing energy in HSZ. Performance of ISP was evaluated with respect to a number of the integrated collector. Kaze and Tchinda (2012) evaluated an exergetic analysis of a downward FPC, an unglazed selective absorber collectors and an artificial rough surface collector. They conduct the computational analysis. The results showed that under the turbulent regime, the increase in Reynolds no. result in a decrease of outlet fluid temperature. The highest exergy efficiency occurred in rough surface absorber whereas lower exergy efficiency occurs for the smooth duct. Ozturk et al. (2012) investigated thermodynamic and life cycle assessment of FPC, a photovoltaic system and photovoltaic thermal collector. The results of the analysis were used to determine maximum daily energy efficiency, major exergy efficiency of the systems under investigation. Kaymak and Sahin (2011) assessed exergy analysis of solar irradiation of different layers in the atmosphere through temperature profile. Database of monthly average global solar irradiation and temperature measurements of the specific place was used for estimation of exergy values at different levels of the atmosphere. The findings show the altitude height under which the variations of solar irradiation exergy values is maximum and suggested the end of mesopause or beginning of the thermosphere the most convenient level of atmosphere for construction of solar PV or the thermal power stations. Luminosu and Fara (2005) experimentally conducted a Thermodynamic analysis of an air solar collector to determine the optimum air flow rate. The relative errors related to exergy efficiency found to be bigger than those related to energy efficiency but considering the complexity of the relations and the low value of the quantities, they considered as acceptable.

The review of the discussed researches shows the broader area of application of exergy analysis in the field of energy systems. The present paper is specifically focused on relative advantages and exergy analysis of different types of ETCs. Various solar collectors configurations can help to obtain a large range of temperatures for example 20–80°C (Sharma and Diaz, 2011) is the operating temperature range for FPCs and properly designed ETCs can help in obtaining higher temperature range, for example, the temperature range is in between 50–200°C. FPCs are simple in construction, utilise the beam as well as diffuse radiation and do not require tracking (Kalogirou, 2013; Tyagi et al., 2012).These types of solar collectors are the most popular types of solar devices for low-temperature applications, but in comparison with ETCs, they have comparatively low efficiency and outlet temperatures. Major drawbacks of FPCs over ETCs are as follows:

• convection heat loss through glass cover from collector plate

• absence of sun tracking etc.

Hence, ETCs are considered more advantageous than FPCs.


2 Evacuated tube solar collector (ETSC) system

2.1 Construction and working principle

Evacuated tube collectors are made up of rows of parallel, transparent glass tubes. Each tube consists of a glass outer tube and an inner tube, or absorber, covered with a selective coating that absorbs solar energy well but inhibits radiative heat loss (Johari et al., 2012; Patel et al., 2012). The air is withdrawn (‘evacuated’) from the space between the tubes to form a vacuum, which eliminates conductive and convective heat loss. They are most suited to extremely cold ambient temperatures or in situations of consistently low-light. ETCs is suitable for industrial applications, where high water temperatures or steam need to be generated where they become more cost-effective (Yadav and Tripathi, 2016). Figure 2 shows the sectional view of ETC (Apricus Evacuated Tube Solar Water Heater System, 2017).

Figure 2 Sectional view of ETC (see online version for colours)

The system consists of evacuated vacuum tubes solar collector (ETSC) and is attached to one highly insulated water tank. The Evacuated tubes are designed such that the cold water enters in and when sunshine falls on it hot water moves up. Figure 3 shows the working principle of direct flow Evacuated tube solar collector (Apricus Evacuated Tube Solar Water Heater System, 2017). The heated water moves up and stored in the insulated water tank due to the decrease of density and the fresh water reaches the solar collector by natural thermo siphon (natural flow). This process carries automatically till the sunshine is available. The hot water in the tank will have a heat retain for two days (Sahabjisolar, 2016).

2.2 Classification of ETSC

Available types of evacuated tube solar collectors can be categorised into two groups as:

• single-walled glass evacuated tube

• Dewar tube.


There are many variations of the two basic types; for instance, heat extraction can be through a U-pipe, heat pipe or direct liquid contact (Gao et al., 2013). Figure 4 shows the types of Evacuated tube solar collectors. Kim and Seo (2007) investigated the thermal performance of an ETSC with four different shaped absorbers both experimentally and numerically. Four different shapes are finned tube (Model I), tube welded inside a circular fin (Model II), U tube welded on a copper plate (Model III) and U tube welded inside a rectangular duct (Model IV) as shown in Figure 5.

Figure 3 Working principle of ETC (see online version for colours)

Figure 4 Types of ETSCs (see online version for colours)

Dewar tube is another popular design of ETSC which is made of two thin borosilicate glass walls that form the inner and outer tubes. A selective absorbance coating is deposited on the outside wall of the inner tube to collect solar energy, and the layer between the inner and outer tubes is evacuated to reduce heat loss. Water in glass evacuated tube solar collectors (WGETSC) is currently most widely used for solar


hot-water systems than the Dewar tube with a U-pipe or heat pipe inserted (UPETSC) because of its lower price. UPETSC was developed based on improving the WGETSC. A U-pipe (generally copper piping with diameter 8–10 mm) and aluminium fins are inserted into the interior cavity of the tube. The key difference is that each evacuated-tube of a WGETSC is filled with working fluid and fluid is only contained in the U-pipe of a UPETSC. Fluid flows in the U-pipe to absorb and carry away the useful energy as shown in Figure 6 (Gao et al., 2013).

Figure 5 Schematic cross-sectional views of (a) Models-I; (b) Model-II; (c) Model-III and (d) Model-IV

2.3 Advantages of ETSC

Figure 7 shows the comparison between FPC and ETC (Sahabjisolar, 2016). There are two types of solar collector: ETC and FPC. The solar radiation incident on the FPC is not always perpendicular to the sun. Whereas a greater amount of solar radiations is tracked by the ETCs due to their cylindrical cross-section. However, when the sun moves in an arc through the sky, the FPC’s effective area become smaller and as the ETCs are cylindrical, the area presented toward the sun remains same.


Figure 6 Schematic cross-sectional views of Dewar Tube ETSCs: (a) water-in-glass evacuated-tube (WGET) and (b) U-pipe evacuated-tube (UPET)

Figure 7 Comparison between ETC and FPC (see online version for colours)

The leading advantages of ETCs are as follows:

• ETCs can gather more energy from Sun (Mangal et al., 2010).

• The efficiency of an evacuated tube solar water heater is better than FPCs during cold ambient temperatures.

• Evacuated tube technology is cost-effective.

• Under challenging climatic conditions ETCs shall provide more even output throughout the year.


• Evacuated tubes are strong and long-lasting. Owing to mishandling or minor accident, if a tube of an ETC gets broken then it requires just the replacement of the single broken tube which is cheaper whereas in case of FPC complete system need a replacement, making it costlier (Sabiha et al., 2015).

• As ETSCs have natural frost protection, without any damage ETSCs can be used in sub-zero temperatures where as antifreeze systems need to be installed for flat plate panels under same operating temperatures which make it complicated and expensive. Regular replacement is required for the glycol used in flat plate systems as it can cause damage by freezing. The glycol needs to be replaced every few years (generally three years) and adds to the recurring cost. In addition, leaking might happen while replacing glycol or due to damage of flat panel by storms which is an added risk.

2.4 Limitations of ETSC

The limitations of ETCs are as follows;

• Evacuated tubes are made of borosilicate glass due to this ETSCs are fragile in nature. Therefore, extra care must be taken while transporting or handling ETSCs.

• ETCs produce very high temperatures so care must be taken while its use for domestic applications. However, for commercial applications, ETCs are more versatile as it provides water at a relatively high temperature essential for industrial processes.

• ETCs do not shed snow as the collector surface is not always warm, the tubes are the insulator in nature and the collector surface is irregular which lets the snow stick on tubes for a long time. As the glass tubes are fragile, it is not possible to scrape off the accumulated snow which might make the system ineffective. So, care must be taken while using ETCs in extreme cold conditions. (Sabiha et al., 2015).

Exergy analysis of ETC is carried out with solar water heating systems (SWHSs), in which SWHS is one of the major component analysed for exergy analysis, therefore Exergy analysis of ETC, their applications, and suitability in solar thermal engineering systems are discussed in the present paper. The previous studies on exergy analysis of ETC are mainly related to their suitability and performance in various applications. Therefore, this review mainly investigates an exergy analysis of ETC research carried out at educational and scientific institutes by researchers for domestic and industrial applications.

3 Mathematical modelling for exergy analysis

Exergy analysis of ETC is carried out to provide most suitable and significant outcomes in the field of research work. The word exergy was introduced by Rant in 1956 and in terms of thermodynamics “Exergy is the maximum amount of useful work possible during a process that brings the system into equilibrium with a heat reservoir or surrounding” (Ersöz, 2016; Farahat et al., 2009; Kotas, 2013 and Tadese and Tesema, 2014). Exergy has the characteristic that it is conserved only when all processes occurring


in a system and the environment are reversible. Exergy is destroyed whenever an irreversible process occurs (Dincer and Rosen, 2012).

The analyses based on two methodologies are used to investigate the performance of ETC systems: the first law of thermodynamic (energy efficiency), and the second law of thermodynamic (exergy efficiency). Based on the first law of thermodynamic, the energy efficiency for all-glass evacuated solar collectors is defined as:

eff

,uth

p

QA S

η = (1)

where Seff is the effective insolation coefficient on a single tube (Ataee and Ameri, 2015). The useful heat gain (Qu) by the working fluid for the presented systems is calculated as follows:

out( ).u p inQ mC T T= − (2)

In this equation, Tin, Tout, Cp, ,m are the fluid inlet temperature, fluid outlet temperature, heat capacity and mass flow rate of the agent fluid, respectively.

In order to do exergy analysis, a general exergy balance equation is written as

out dest ,inE E E− =∑ ∑ ∑ (3)

where ,inE∑ out ,E∑ destE∑ are the total exergy input, total exergy output and total exergy destruction rates respectively.

The exergy rate can be determined as,

0 0 0[( ) ( )].fE m h h T S S= − − − (4)

In order to do an exergy analysis. The exergy balance equation is written as follows (Gholampour and Ameri, 2014)

solar fan out .inE E E E IR+ + = + (5)

This equation can also be written as

solar out fan( ) ,inE E E E IR− − − = (6)

where

useful out fan( ).inE E E E= − − (7)

Therefore,

useful solar .E E IR= − (8)

The solar radiation exergy rate, According to Petela theorem, is calculated by Petela (1964),

4

solar effsun sun

4 11 .3 3

a ap

T TE S A

T T

= − +

(9)


IR calculated by (Padilla et al., 2014)

loss,optical loss, dest, des, .p t s tIR E E E E∆ − ∆= + + + (10)

The exergy loss rate considered by (Padilla et al., 2014) 4

loss,optical effsun sun

4 1(1 ) 1 .3 3

a ap

T TE S A

T Tατ

= − − − +

(11)

Based on the second law of thermodynamics, the exergy efficiency can be calculated as,

useful,net4

effsun

.4 113 3

E

a ap

sun

E

T TS AT T

η = − +

(12)

The efficiency of the collector which was modelled experimentally by Budihardjo as a function of ambient temperature (Ta), average film temperature of inlet and outlet water temperature of the tube. (G) Global solar irradiance at the collector plane as a second order equation (Budihardjo and Morrison, 2009).

col

( ) ( )0.58 0.9271 0.0067 .f a f aT T T T

G Gη

− −= − − (13)

The results obtained from the mentioned formulation of equations (1)–(13) are foremost parameters used for energy and exergy analysis of evacuated tube solar collector (ETSC).

4 Evaluations of energy and exergy efficiency for ETCs

Exergy analysis of solar collector systems using closed and open loop systems are used for the performance evaluation of direct as well as indirect both types of SWHSs. The concept of exergy applied over the SWHSs, solar air heating systems, solar desalination, solar drying systems etc. through various approaches for the evaluation of thermal efficiency, energy efficiency and exergy efficiency. The researchers had adopted different methodologies to evaluate exergical parameters through mass flow rate, fluid temperature, water inlet and outlet temperature, solar radiation intensity, volume flow rate of water, yield and heat transfer coefficient. The energy and exergy analysis of a hot water preparation system, which is a boiler assisted vacuum tube solar collector, had been conducted by Yildizhan and Sivrioğlu (2016) using 40% Antifreeze-water mixture and for direct flow SWHS by Pandey et al. (2015). Hot water consumption pattern (Daghigh and Shafieian, 2016) using solar water heating-drying system. Pei et al. (2012) obtained the results for thermal and exergy efficiency of the system using evacuated tube solar water heater systems with and without a mini-compound parabolic concentrating (CPC) Reflector (C < 1). Modified design of solar still proposed for integrated ETC under forced mode was analysed by Kumar et al. (2014) to obtain the results for thermal


analysis and mass flow rate. Further Mishra et al. (2015) studied, considering four different types of weather conditions. In this regard, Exergy analysis of ETCs has potential application in the field of scientific researches, in industries and can also be used for domestic purposes. It is the extensive research where many researchers are still working on it to make the correct use of energy resources.

Ataee and Ameri (2015) performed Energy and Exergy analysis of all-glass evacuated tube solar collector tubes with the coaxial fluid conduit. The research work was carried on T-type and H-type models with forced convection flow. The result obtained by H-type model shows that the outlet flow temperature and exergy efficiency for both air and CO2 as working fluid is greater than the T-type model. Figure 8 shows the effect of changes in the mass flow rate on the outlet working fluid temperature and the exergy efficiencies respectively, for the T-type model with air and carbon dioxide working fluid.

Figure 8 Variation of exergy efficiencies with mass flow rate for T-Type model (see online version for colours)

Figure 9 shows the effect of increasing collector length with respect to the outlet water temperature results in increase of the exergy efficiency of the system.

Further Ersöz (2016) experimentally investigated Effects of different working fluid on the energy and exergy performance for evacuated tube solar collector with thermosyphon heat pipe. The velocities of air are determined as 2, 3, and 4 m/s. Among six working fluid chloroform and acetone show the best results in terms of energy and exergy performance of thermosyphon heat pipe evacuated tube collector (THPETC). As shown in Figure 10, the lowest exergy efficiency occur in the THPETC- hexane but for 2 m/s air velocity, the highest exergy efficiency in the THPETC-Acetone.


Figure 9 Variations of outlet air temperature, exergy efficiencies with length air temperature are shown for H-Type model (see online version for colours)

Figure 10 Exergy efficiencies VS [Tf,in – Ta]I–1 of the THPETCs for air velocity 2 m/s] (see online version for colours)

Later Pei et al. (2012) experimentally performed a comparative analysis of the thermal performance of evacuated tube SWHS with and without a mini-compound parabolic concentrating (CPC) Reflector (C < 1). The water in the tank was heated from 26.9 to 55,


65, 75, 85, and 95°C. The ETC solar water system without a mini- CPC reflector has higher thermal and exergy efficiencies as compared to the system with a mini CPC reflector. But, On the other hand, when attaining high-temperature water, the system with a mini CPC reflector has higher thermal and exergy efficiency than the other one. Figure 11 shows the average exergy efficiency of the five groups of comparison experiments.

Figure 11 Average exergy efficiency of the five groups of comparison experiments as observed by Pei et al. (2012) (see online version for colours)

According to Jafarkazemi et al. (2016) increasing water inlet temperature besides decreasing water mass flow rate results in a better exergetic performance. They performed energy and exergy efficiency of heat pipe evacuated tube solar collectors. The analysis presents a detailed theoretical and experimental method carried out showing that increase in water inlet temperature in heat pipe evacuated tube solar collectors leads to a decrease in heat transfer rate between the heat pipe’s condenser and water.

Yildizhan and Sivrioğlu (2016) carried out the experimental evaluation of exergy analysis of a Vacuum tube solar collector system having the indirect working principle with 40% anti freeze water- mixture. The results so obtained for the average energy and exergy efficiencies were found to be 13.6% and 1.3%, energy and exergy efficiencies for indirect operating principle, are lower than direct system working with solar collectors.

Another experimental evaluation is done by Pandey et al. (2015) for the Thermal performance of direct flow SWHS using exergetic approach with water as a working fluid. The experiments were carried out for different volume flow rates of water such as 10, 15, 20, 25 and 30 litres per hour (LPH) The energetic and the exergetic efficiencies were found to be 66.57% and 13.38% resp. at 15 LPH flow rate. Figure 12 shows the variation of solar radiation and efficiencies against time for 15 LPH volume flow rate.


Figure 12 Time vs. solar radiation for 15LPH (see online version for colours)

Kumar et al. (2014) conducted an experimental analysis of a solar still augmented with an ETC in forced mode. The optimum daily yield obtained was 3.9 kg with energy and exergy efficiencies as 33.8% and 2.6% respectively during a typical summer day. The integration of ETC with solar still increases the water temperature as well as yield. The daily yield obtained was 3.47 kg/m2 for 0.01 m basin water depth at 0.006 kg/s mass flow rate.

At weather conditions, Mishra et al. (2015) considered Thermal modelling and development of characteristics equation of evacuated tubular collector. The maximum outlet temperature increases from 53.91°C to 129.23°C and the useful daily thermal gain increases from a mass flow rate ( ) 0.002 kg/s.fm = As the number of ETC connected in series increases instantaneous thermal efficiency decreases and for two to six number of ETC, its value decreases from 53.5% to 34.4%. Similarly, Kalogirou et al. (2016) conducted an experimental evaluation of an exergy analysis of solar thermal collectors and processes.

Singh et al. (2013) investigated the performance of a solar still integrated with ETC in natural mode. The obtained results for the variation of instant overall energy and exergy efficiencies have been found to be at the rate of 5.1–54.4% and 0.15–8.25% resp. during the sunshine hours for 0.03 m water depth the respective daily energy and exergy efficiencies calculated as 33.0% and 2.5% and maximum along with daily yield of 3.8 kg/m2. Figure 10 shows the variation of daily energy and exergy efficiency for set combinations. To increase in water depth from 0.03 m to 0.05 m for the same number (Nc = 10) of tube in ETC. Further, it can be noticed that with the increase in the size of integrated ETC, the daily yield does not increase proportionate possibly due to higher thermal losses. Figure 13 shows the variation of energy and exergy efficiency for different set combinations.


Figure 13 Variation of daily energy and exergy efficiency for different set combinations

Further Daghigh and Shafieian (2016) carried out an experimental evaluation of energy and exergy analysis of a multipurpose evacuated tube heat pipe solar water heating drying system. They designed, manufactured and examined a solar water heating drying system. The results of exergetic efficiency and time plot of Figure 14 shows that at the end of the day the efficiency reached its maximum level about 4.5%.

Figure 14 Exergetic efficiency (see online version for colours)

Table 1 gives the summary of reviewed researches discussed in present paper for exergy analysis of ETCs.


Table 1 Summary of evaluation of exergy analysis of evacuated tube collectors

S. No. Author Type of investigation Working fluid Type of ETSC Main findings

1 Ataee and Ameri (2015)

Experimental Air and CO2 All-glass evacuated solar collector tubes with coaxial fluid conduit for T-type and H-type models

a Increases in the mass flow rate results in decreases of exergy efficiency

b The increase of outlet air temperature and the energy efficiency with solar radiation intensity follows the increasing of the Exergy efficiency.

2 Ersöz (2016)

Experimental Hexane, Petroleum, Ether, Chloroform, Acetone, Methanol and Ethanol

Thermosyphon Heat Pipe Evacuated Tube Collector

Highest exergy efficiency obtained by THPETC Chloroform

3 Jafarkazemi et al. (2016)

Theoretical and experimental

Water Heat pipe evacuated solar collector

Increasing the difference between water inlet and ambient temperature leads to a decrease in energy efficiency, it leads to an increase in exergy efficiency

4 Pei et al. (2012)

Experimental Water Evacuated tube solar water heater systems with and without a mini-CPC reflector

At low temperature ETC without mini-compound reflectors has higher thermal and exergetic efficiencies

5 Yildizhan and Sivrioğlu (2016)

Experimental 40% antifreeze water mixture

Vacuum tube solar collector

Average energy and exergy efficiencies of the experimental system was compared with that of vacuum tube solar collector

6 Pandey et al. (2015)

Thermal and experimental

Water Direct flow evacuated tube collector

Obtained better exergy and energy efficiency for new design of SWH system at flow rate of 15LPH

7 Kumar et al. (2014)

Experimental Water Water-in-glass evacuated tube

a The integration of ETC with solar still increases the water temperatures as well as yield

b The maximum daily energy and exergy efficiencies at optimum flow rates have been found to be as 33.8% and 2.6% respectively


Table 1 Summary of evaluation of exergy analysis of evacuated tube collectors (continued)

S. No. Author Type of investigation Working fluid Type of ETSC Main findings

8 Mishra et al. (2015)

Thermal and Experimental

Weather conditions

Evacuated tubular collector

a Maximum outlet temperature increases as the number of collector tubes increases

b The useful daily thermal gain increases from 3.29 kW h to 6.34 k h as the number of collectors increased from two to six

9 Singh et al. (2013)

Experimental Water Water-in-glass evacuated tube solar collector

a Natural circulation rate increases up to 44 kg/h

b The maximum daily energy and exergy efficiencies are found to be as 33.0% and 2.5% respectively

c The integration of ETC with solar still increases the water temperatures as well as yield

d The yield decreases further with the increase in water depth

10 Daghigh and Shafieian (2016)

Experimental Hot water Evacuated tube heat pipe collector

a The distribution pattern of temperature increases as the number of pipes reaches to optimum value

b Exergetic efficiency was ascending over time

5 Conclusion

This paper provides an overview of recent studies carried out on applications of exergy analysis of different types of ETC. The review shows that experimentation work carried out through exergy and energy analysis gives more compatible results. This shows significance of presented methodology as most efficient way used by researchers for analysing the performance of ETC. The conclusion drawn out from present paper is summarised as follows:

• Exergy and energy analysis is used in almost all the applications of solar energy for evaluation of performance.

• ETCs are an important component of any solar system, where exergy analysis gives a more representative performance evaluation, it is a valuable method to evaluate possible configurations of these systems.


• Exergy efficiency of solar systems is highly dependent on the daily solar radiation and radiation intensity. So, ETC is highly recommended for higher temperature applications as it can gain higher temperatures easily and also able to preserve heat even when the outside weather is cold.

• The exergetic efficiency of the ETC seems to be steady with temperature difference especially at higher values while the thermal efficiency decreases with increasing temperature difference.

• It is noted that the exergetic and energy efficiencies of evacuated tube SWHS can be increased by the use of nano fluids prepared from high conductivity nanoparticles.

• A significant difference in exergy efficiency is obtained by researcher, by variation in parameters such as mass flow rate, inlet and outlet temperature and change in design.

• For lower solar radiation intensity level, by increasing the mass flow rate of the fluid and air, the value of the exergy efficiency is negative.

• The techniques used by researchers for representing outcomes of exergy analysis are thermal exergy maps, simulated solutions, regression analysis, grassmann diagrams, mathematical modelling etc.

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Nomenclature

Qu Useful heat gain (J) Ap Aperture area (m2) Seff Effective insolation coefficient defined by equation (1)

m Mass flow rate (kg/s)

Tin Fluid inlet temperature (K) Tout Fluid outlet temperature (K) Cp Heat capacity (KJ/Kg-K) Ėin Exergy input (W) Ėout Exergy output (W) Ėdest Exergy destruction rates (W)

fm Mass flow rate of the fluid (kg/s)

T Temperature (K) h Enthalpy of the fluid in given condition (J/Kg) h0 Enthalpy of the fluid in dead state (J/Kg) S Entropy of the fluid in given condition (J/Kg-K)


S0 Entropy of the fluid in dead state (J/Kg-K) T0 Temperature at dead state (K) Ė Exergy (W) Ėsolar Exergy of incident solar radiation (W) Ėfan Exergy of working fluid (W) IR Exergy loss & destruction (W) Ėuseful Useful exergy gain (W) Ta Ambient temperature (K) Tsun Temperature of solar intensity (K) Tf Average film temperature K) G Global solar irradiation (W/m2) Ė Exergy (W) Greek symbols α Absorptance defined by equation (11) τ Transmittance defined by equation (11) (ατ) Optical efficiency defined by equation (11)

ηE Second law efficiency (%) defined by equation (12)

ηth First law efficiency (%) defined by equation (1)

ηcol Collector efficiency (%) defined by equation (13) Subscripts ETC Evacuated tube collector FPC Flat plate collector THPETC Thermosyphon heat pipe Evacuated tube collector a Ambient temperature dest Exergy destruction loss Exergy loss optical Optical efficiency P Absorber tube useful Useful exergy gain useful,net Actual useful exergy gain

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