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Energy, exergy, environmental, enviroeconomic, exergoenvironmental (EXEN) and exergoenviroeconomic (EXENEC) analyses of solar collectors
Renewable and Sustainable Energy Reviews 69 (2017) 488–492
Contents lists available at ScienceDirect
Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser
crossmark
Energy, exergy, environmental, enviroeconomic, exergoenvironmental (EXEN) and exergoenviroeconomic (EXENEC) analyses of solar collectors Hakan Caliskan Department of Mechanical Engineering, Faculty of Engineering, Usak University, 64200 Usak, Turkey
A R T I C L E I N F O
A BS T RAC T
Keywords: Energy Exergy Environment Renewable energy Solar energy Sustainability
In this study, the energy, exergy, environmental, enviroeconomic, exergoenvironmental (EXEN), Exergoenviroeconomic (EXENEC) analyses are performed to a solar collector. The enviroeconomic (energy based environmental analysis), EXEN (exergy based environmental analysis) and EXENEC (exergy based environmental and economic analysis) analyses are firstly conducted in this kind of system in the literature. It is found that most of the energy and exergy are lost by the radiation. The major reason is the big temperature difference between sky and glass surface of the collector. Furthermore, the energy efficiency (25.40%) of the system is higher than the corresponding exergy efficiency (0.732%). Also, the solar exergy of the system is the maximum exergy input rate, and most of it is destructed in the system due to the irreversibility. It shows the major disadvantages of the solar collector system. The EXEN result (0.0727 kg CO2/day) is lower than the corresponding environmental one (0.0777 kg CO2/day). The enviroeconomic result (0.00112 $/day) is higher than the EXENEC result (0.00105 $/day). So, exergy based EXENEC method is more reliable. It can be generally concluded that the solar collector systems can be assessed more effectively by using the exergy and economy based EXEN and EXENEC methods, respectively due to the consideration of the environmental condition and useful energy into calculation.
1. Introduction Most of the needs of modern societies are supplied by energy. So, the economic growths of countries largely depend on it. Environmental threats and the depletion of fossil fuel reserves expose the importance of the renewable energy sources; because they are clean, safe and sustainable. Especially, solar energy is a promising energy source among the renewable energies. It is limitless free energy source which has capable of meeting the necessary energy demand of the world [1,2]. Solar collectors are used to convert solar energy to thermal for heating applications. Their performance can be determined by the exergy which is the maximum theoretical work obtained from the interaction between the system and the environment in the equilibrium state. Exergy is also defined as potential or quality of energy which is also known as useful energy (availability). Thermodynamic systems can be assessed effectively with exergy method. Also, it is possible to determine the sustainability assessment with the help of exergy [3]. The exergy analysis is generally used to optimize thermodynamic and energy systems more than fifty years [4]. It is also used to determine the monetary values and values of the environmental impact for the transport of energy. So, it is connected with the environmental and ecological assessments through Life Cycle Assessment and industrial
ecology [5–7]. Caliskan [8,9] concluded that the exergy can also be used for assessing the environmental impact of processes with economic point of view. Because, the exergy analysis is feasible to production/consumption chain which is from resource intake down to emission. These exergy related assessments are named as Exergoenvironmental (EXEN) and Exergoenviroeconomic (EXENEC) analyses which are developed by Caliskan [8], and used in the studies of Caliskan [8,9]. The EXEN analysis has three important components: exergy analysis, environmental parameter and working time. In the EXENEC analysis exergy, economy and environmental parameters are taken into account. In the open literature, there are not any studies about the EXEN and EXENEC analyses of solar collectors, however there are some studies related to energy (thermal), exergy, economy or environmental analyses. Faizal et al. [2] applied the energy, exergy, economic, and environmental analysis on a flat-plate solar collector operated with SiO2 nanofluid. It was found that energy and exergy efficiencies of nanofluids were higher than base fluids. Also CO2 emissions and payback periods of nanofluids are better. Tiwari et al. [10] studied on exergoeconomic and enviroeconomic analyses of partially covered photovoltaic thermal flat plate collector integrated solar distillation system. The energetic (thermal) and exergetic efficiencies were found
E-mail address: [email protected]. http://dx.doi.org/10.1016/j.rser.2016.11.203 Received 30 March 2016; Received in revised form 20 October 2016; Accepted 14 November 2016 1364-0321/ © 2016 Elsevier Ltd. All rights reserved.
Renewable and Sustainable Energy Reviews 69 (2017) 488–492
H. Caliskan
and compared with the previously studies. It was concluded that the solar collector system could meet the water and electrical power in sunshine hours. Chamoli [1] performed exergy analysis and optimization on a flat plate solar collector using MATLAB Simulink and optimal parameters. As a result, optimum design points were determined, optimum values of collector inlet temperature, mass flow rate, absorber plate area, and fluid outlet temperature for maximum exergy inflow were investigated. Battisti and Corrado [11] applied environmental analysis and optimization to the water storage coupled solar thermal collector. Simapro software program were used to obtain environmental indicators. It was found that the reduction of the impacts could be up to 40% and the environmental pay back times were 5–19 months. The aim of the study is to assess solar collectors along with the combination of the energetic, exergetic, environmental and economic point of view. So, this study differs from the previously conducted ones as follows: (i) The enviroeconomic, EXEN and EXENEC analyses, developed by the author, are first applied to a solar collector (This is the first time in the literature), (ii) The sustainability index and entropy generation rate of the solar collector are determined.
Table 1 Data of the system.
2. System description As a case study, the flat plate solar collector is chosen to assess a solar system by using the environmental, enviroeconomic, EXEN and EXENEC analyses. The schematic layout (a) and control volume (b) of the system are shown in Fig. 1. The solar radiation is incident on the glass cover of the solar collector at the rate of 650 W/m2. The glass transmits 88% of the incident radiation and has an emissivity of 0.90. The necessary hot water for a house in summer season can be supplied by a collector of 1.5 m high and 2 m wide, and tilted 40° from the horizontal. The temperature of the glass cover is measured to be 40 °C on a calm day when the surrounding air temperature is 20 °C. The effective sky temperature for radiation exchange between the glass cover and the open sky is −40 °C. The water enters the tubes attached to the absorber plate at a rate of 1 kg/min. It is assumed that the back surface of the absorber plate is insulated and the only heat loss occurs through the glass cover. The data of the system description are created with the best of author's knowledge about the solar collectors and systems in Turkey. The more detailed data of the system is given in Table 1 which includes the assumptions.
Symbol
Rate
Glass surface temperature Air temperature Average temperature Sky temperature Sun temperature Thermal conductivity Kinematic viscosity Prandtl number β=1/Tavg Area Perimeter Characteristic length Collector angle cos(θ) Acceleration of gravity Emissivity Stefan boltzmann constant Permeability Solar radiation Mass flow rate of water Specific heat Water input temperature Water output temperature CO2 emission value for the solar collector Working hours of the solar collector Carbon price
Tsurf Tair=To Tavg Tsky Tsun k ν Pr β A P L=A/P θ cos(40) g ε σ α q m cp Tin Tout yCO2 tworking cCO2
40 °C 20 °C 30 °C −40 °C 6000 K 0.02588 W/m °C 0.00001608 m2/s 0.7282 0.0032986971 K 3 m2 7m 0.428571429 m 40° 0.766044 9.81 m/s2 0.9 (5.67) 10−8 W/m2 K4 0.88 650 W/m2 0.016666667 kg/s 4180 J/kg °C 25 °C 31.25643842 °C 0.00000647 kg CO2/Wh 7 h/day 0.0145 $/kg CO2
where “Eṅ w, in ”, “Eṅ solar , in ”, “Eṅ w, out ” and “Eṅ loss ” are the energy input rate of the water, solar energy input rate, energy output rate of the water and energy loss rate of the system, respectively. The net energy rate of the water ( ΔEṅ w ) can be written as;
ΔEṅ w=Eṅ w, out −Eṅ w, in=ṁ w cp (Tw, out −Tw, in )
(2)
where “ṁ w ”, “cp ”, “Tw, out ” and “Tw, in ” are the mass flow rate, specific heat, collector output temperature and collector input temperature of the water, respectively. The solar energy input rate (Eṅ solar , in ) is found by
Eṅ solar , in=αqA
(3)
where “α ”, “q ” and “ A” are the permeability of the system, solar radiation rate and collector area of the system, respectively. The energy loss rate of the system (Eṅ loss ) consists of the convection heat loss rate (Q̇loss, conv ) and radiation heat loss rate (Q̇loss, rad ) as follows:
3. Analysis 3.1. Energy analysis
̇ , conv +Qloss ̇ , rad Eṅ loss=Qloss
(4)
The radiation heat loss rate is determined to be
The steady energy balance of the system is written as follows:
Eṅ w, in +Eṅ solar , in=Eṅ w, out +Eṅ loss
Parameter
4 4 ̇ , rad =εσA (Tsurf Qloss −Tair )
(1)
Fig. 1. Schematic layout and control volume of the system.
489
(5)
Renewable and Sustainable Energy Reviews 69 (2017) 488–492
H. Caliskan
⎛ ⎞ ̇ , conv ⎜1 − To ⎟ Eẋ loss, conv=Qloss Tsurf ⎠ ⎝
where “ε ”, “σ ”, “Tsurf ” and “Tair ” are the emissivity of the system, StefanBoltzmann constant, glass surface temperature and air temperature, respectively. The convection heat loss rate is calculated from
̇ , conv=hA Qloss
(Tsurf −Tair )
The exergy destruction rate of the system (Eẋ dest ) is determined as follows:
(6)
Eẋ dest =Eẋ w, in+Eẋ solar , in−Eẋ w, out −Eẋ loss
where “h ” is the convection heat transfer coefficient.
h=
kNu L
(7)
A P
̇ = Sgen
Nu=(0.
ψ=
(9)
gcosθ (Tsurf − Tair
ΔEẋ w Eẋ solar , in
SI =
) L3Pr
1 1−ψ
3.3. Environmental analysis
ΔEṅ w η= Eṅ solar , in
xCO2=yCO2 Eṅ solar , in tworking
The environmental analysis can be explained as follows [12–15]:
(11)
The steady exergy balance of the system can be given by
Eẋ w, in+Eẋ solar , in=Eẋ w, out +Eẋ loss+Eẋ dest
(12)
where “Eẋ w, in ”, “Eẋ solar , in ”, “Eẋ w, out ”, “Eẋ loss ” and “Eẋ dest ” are the exergy input rate of the water, solar exergy input rate, exergy output rate of the water, exergy loss rate and exergy destruction rate of the system, respectively. The net exergy rate of the water (ΔEẋ w ) is written as;
ΔEẋ w=Eẋ w, out −Eẋ w, in
[(Tw, in−To )−To
(13)
ln (Tw, out / To )] ln (Tw, in / To )]
(14) (15)
3.4. Enviroeconomic analysis
where “To ” is the dead state temperature. Exergy is considered with a reference environment. If a thermodynamic system is in equilibrium with the environment, the state of the system is called the “dead state” and the temperature is named as dead state temperature. It is generally equal to environment temperature. So, dead state can be considered as reference state [5]. The solar exergy input rate (Eẋ solar , in ) is determined by 4 ⎡ 1 ⎛ T ⎞ 4 ⎛ T ⎞⎤ Eẋ solar , in=Eṅ solar , in ⎢1 + ⎜ o ⎟ − ⎜ o ⎟ ⎥ ⎢⎣ 3 ⎝ Tsun ⎠ 3 ⎝ Tsun ⎠ ⎥⎦
The enviroeconomic analysis is defined by [13]:
CCO2=xCO2 cCO2
(16)
3.5. Exergoenvironmental (EXEN) analysis
(17) The EXEN analysis is calculated from [8,9]:
The radiation exergy loss rate is found by
⎛ ⎞ ̇ , rad ⎜1 − To ⎟ Eẋ loss, rad =Qloss T ⎝ surf ⎠
(25)
where “CCO2 ” is the enviroeconomic parameter and ‘‘cCO2 ’’ carbon dioxide emission price. Here, the LCA based environmental analysis is taken into account and carbon pricing methodology is also considered to assess the solar collector for enviroeconomic perspective. To prevent the climate change, carbon monoxide emissions need to be reduced or cut. So, pricing the carbon is considered as the best effective method. In this regard, carbon is priced. As a result, the enviroeconomic analysis is developed as a tool to assess the carbon pricing of the systems.
where “Tsun ” is the sun temperature. The exergy loss rate of the system (Eẋ loss ) includes the convection exergy loss rate (Eẋ loss, conv ) and radiation exergy loss rate (Eẋ loss, rad ).
Eẋ loss=Eẋ loss, conv+Eẋ loss, rad
(24)
where ‘‘xCO2 ’’ is the released carbon dioxide in a considered time, ‘‘yCO2 ’’ is the carbon dioxide value for the energy option defined by life cycle assessment methodology, ‘‘Eṅ solar , in ’’ is the energy rate of the solar energy option and ‘‘tworking ’’ is the working time of the system. The LCA provides a material balance and energy balance over the entire life of a product such as solar collector. In the LCA, goals, objectives and investigation boundaries of the framework are very important to assess the system correctly. They effect the results of the LCA parameters. There are four steps for the LCA: planning, inventory analysis, impact assessment, improvement analysis. If energy consumption/production is considered, it can be different for countries. But, it helps to determine the connection with the ambient by assessing its impact on environment. So, the environmental impact of the solar collector with its carbon dioxide value and energy production are considered for environmental analysis by taking into account the working period of time.
3.2. Exergy analysis
Eẋ w, in=ṁ w cp
(23)
(10)
ν2
[(Tw, out −To )−To
(22)
The sustainability index (SI) can be found using exergy efficiency
where “θ ” is the angle of the solar collector, “Pr ” is the Prandtl number, and “ν ” is the kinematic viscosity. The energy efficiency of the system (η ) is determined by
Eẋ w, out =ṁ w cp
(21)
as;
where “ A” and “P ” are the area and perimeter of the solar collector, respectively. Also, “Ra ” is the Rayleigh number as follows:
Ra=
Eẋ dest To
The exergy efficiency of the system (ψ) is calculated as follows:
(8)
15) Ra1/3
(20)
̇ ) is found from The entropy generation rate (Sgen
where “k ” is the thermal conductivity, “Nu ” is the Nusselt number, and “L ” is the characteristic length of the system.
L=
(19)
xex, CO2=yCO2 Eẋ solar , in tworking
(26)
where ‘‘xex, CO2 ’’ is the released carbon dioxide in a considered time considering exergetic values and ‘‘Eẋ solar , in ’’ is the exergy rate of the solar energy option.
(18)
The convection exergy loss rate is computed by 490
Renewable and Sustainable Energy Reviews 69 (2017) 488–492
H. Caliskan 1800
The EXEN analysis considers three important points: (i) related exergy rate of energy option, (ii) CO2 emission value generally calculated from Life Cycle Assessment method, and (iii) working hours of the system. So, it gives information about exergy (enhanced thermodynamic) analysis based CO2 value in considered period of time [8]. The difference between environmental and EXEN is that; the environmental analysis uses the energy analysis results, while EXEN analysis considers exergetic results.
1600
Energy rate (W)
1400 1200 1000 800 600 400 200
3.6. Exergoenviroeconomic (EXENEC) analysis
0 Net energy changing rate of the water
The EXENEC analysis is determined by exergoenviroeconomic parameter (Cex, CO2 ) as follows [8,9]:
Cex, CO2=xex, CO2 cCO2
Energy loss rate
Solar energy rate
Fig. 2. Energy analysis results.
(27) 1650
The EXENEC analysis is applied with a parameter “CexCO2 ” which gives information about exergetic price of CO2 emission. It also includes the EXEN parameters [8]. The EXENEC analysis is a new tool to assess the carbon dioxide emission considering the exergetic results of the system such as EXEN analysis that connected with exergetic results. So, it is important to obtain this value considering not only first law of thermodynamics but also first and second laws of the thermodynamics. The detailed information for the EXEN and EXENEC analyses methodologies can be seen from Refs.[8,9].
1500
Exergy rate (W)
1350 1200 1050 900 750 600 450 300 150 0 Net exergy changing rate of the water
4. Results and discussion
Exergy loss rate
Solar exergy rate
Exergy destruction rate
Fig. 3. Exergy analysis results.
The energy, exergy, environmental, enviroeconomic, EXEN, and EXENEC analyses are applied to the solar collector system. The results of the analyses are generally tabulated in Table 2. The energy analysis results are given in Fig. 2. The solar energy input rate, energy loss rate and net energy changing rate of the water are found to be 1716.000 W, 1280.135 W and 435.865 W, respectively. The most of the total energy input is lost due to the convection and radiation heat losses. Among the energy loss, the radiation heat loss has the maximum rate as 1019.801 W, while the convection heat loss rate is 260.334 W. Because, solar energy systems mostly depend on radiation heat transfer phenomena. So, it is normal that radiation heat transfer has higher than the convection. The exergy analysis results are shown in Fig. 3. The solar exergy input rate, exergy loss rate, exergy destruction rate and net exergy changing rate of the water are calculated as 1604.215 W, 50.267 W, 1542.204 W and 11.244 W, respectively. The net exergy input rate of
the water is computed to be 2.937 W, while the net exergy output rate of the water is determined as 14.681 W. It is normal to increase the exergy rate of water due to the solar exergy gain by solar radiation. Also, the radiation and convection exergy loss rates are found to be 33.640 W and 16.627 W, respectively. The radiation and convection comparison is similar to energetic comparison; radiative heat transfer is higher for solar collectors. Furthermore, the sustainability index is determined as 1.007 and the entropy generation of the system is calculated to be 5.261 W/K. The sustainability index is mainly connected with exergy efficiency, while the entropy generation is related with exergy destruction and dead state temperature. Both of these parameters are required the exergy calculation. So, they can change by only exergetic values changing. In this study, the dead state (reference) temperature is stable and also there is no parametric changing in exergetic values. The efficiencies of the system are illustrated in Fig. 4. The energy efficiency of the system is calculated as 25.40%, while the exergy efficiency is determined to be 0.732%. This shows the importance of the reference (dead) state condition to take into account in the calculation. Because, exergy is available energy that is connected with reference environment condition of the system. So, this shows the real useful part
Table 2 Results of the analyses. Rate
Unit
Net energy changing rate of the water Solar energy rate Energy loss rate Radiation heat loss rate Convection heat loss rate Energy efficiency Net exergy changing of water Exergy loss Solar exergy input Exergy destruction Exergy efficiency Exergy input rate Exergy output rate Sustainability index Entropy generation rate Environmental parameter Exergoenvironmental parameter Enviroeconomic parameter Exergoenvironmental parameter
435.865 1716.000 1280.135 1019.801 260.334 25.400 11.744 50.267 1604.215 1542.204 0.732 2.937 14.681 1.0073 5.261 0.077718 0.072655 0.001127 0.001053
W W W W W % W W W W % W W – W/K kg-CO2/day kg-CO2/day $/day $/day
28 24 20
Efficiency (%)
Results of the analyses
16 12 8 4 0 Energy efficiency
Exergy efficiency
Fig. 4. Efficiencies of the system.
491
Renewable and Sustainable Energy Reviews 69 (2017) 488–492
H. Caliskan 0.08
and most of it is destructed in the system due to the irreversibility. It shows the major disadvantages of the solar collector system. (iv) The EXEN result (0.0727 kg CO2/day) is lower than the corresponding environmental one (0.0777 kg CO2/day). Both of them very close rates, but even the small difference is important to make the environmental assessment effectively. So, the exergetic based environmental (EXENEC) analysis is more reliable. (v) The enviroeconomic result (0.00112 $/day) is higher than the EXENEC result (0.00105 $/day). So, the exergy based EXENEC method is more reliable due to the dead state and useful energy considerations. If it is accepted that the collector is used 4500 h in a year for hot climates, its Environmental and EXENEC results are calculated as 5.040 $/year and 4.725 $/year, respectively. It should be noted that, these are the values of the 3 m2 solar collector which can be considered very small. So, utilization of them with serial or parallel connections increase these prices.
0.07
Rate (kg CO2/day)
0.06 0.05 0.04 0.03 0.02 0.01 0 Environmental
Exergoenvironmental (EXEN)
Fig. 5. Environmental and exergoenvironmental (EXEN) results. 0.0012 0.001
As a final remark, the solar collector systems can be assessed more effectively by using the exergy and economy based environmental (EXEN) and enviroeconomic (EXENEC) methods, respectively due to the consideration of the environmental (dead state) condition into calculation. So, considering not only environmental and enviroeconomic analyses but also EXEN and EXENEC analyses for solar collectors can be promising methodology to assess the carbon pricing effectively with the help of life cycle and enhanced thermodynamics.
Rate ($/day)
0.0008 0.0006 0.0004 0.0002 0 Enviroeconomic
References
Exergoenviroeconomic (EXENEC)
Fig. 6. Enviroeconomic and exergoenviroeconomic (EXENEC) results.
[1] Chamoli S. Exergy analysis of a flat plate solar collector. Journal of Energy in Southern Africa 2013;24(3):8–13. [2] Faizal M, Saidur R, Mekhilef S, Hepbasli A, Mahbubul IM. Energy, economic, and environmental analysis of a flat-plate solar collector operated with SiO2 nanofluid. Clean Technol Environ Policy 2014. http://dx.doi.org/10.1007/s10098-014-08700. [3] Moran MJ, Shapiro HN. Fundamentals of engineering thermodynamics, 3rd Edition. New York, USA: John Wiley & $2 Sons; 1998. [4] Rant Z. Thermodynamische bewertung der verluste bei technischen energieumwandlungen. Brennst Wärme-Kraft 1964;16(9):453–7. [5] Dincer I, Rosen MA. Exergy: energy environment and sustainable development. UK: Elsevier; 2007, [ISBN: 0080445292, EAN: 9780080445298]. [6] Meyer L, Tsatsaronis G, Buchgeister J, Schebek L. Exergoenvironmental analysis for evaluation of the environmental impact of energy conversion systems. Energy 2009;34:75–89. [7] Tsatsaronis G, Morosuk T. Understanding and improving energy conversion systems with the aid of exergy-based methods. Int J Exergy 2012;11(4):518–42. [8] Caliskan H. Novel approaches to exergy and economy based enhanced environmental analyses for energy systems. Energy Conversion and Management 2015;89:156–61. [9] Caliskan H. Thermodynamic and environmental analyses of biomass, solar and electrical energy options based building heating applications. Renewable & $2 Sustainable Energy Reviews 2015;43:1016–34. [10] Tiwari GN, Yadav JK, Singh DB, Al-Helal IM, Abdel-Ghany AM. Exergoeconomic and enviroeconomic analyses of partially covered photovoltaic flat plate collector active solar distillation system. Desalination 2015;367:186–96. [11] Battisti R, Corrado A. Environmental assessment of solar thermal collectors with integrated water storage. Journal of Cleaner Production 2005;13(1314):1295–300. [12] Caliskan H, Dincer I, Hepbasli A. A comparative study on energetic, exergetic and environmental performance assessments of novel M-cycle based air coolers for buildings. Energy Convers Manage 2012;56:69–79. [13] Caliskan H, Dincer I, Hepbasli A. Exergoeconomic, enviroeconomic and sustainability analyses of a novel air cooler. Energy Build 2012;55:747–56. [14] Caliskan H, Dincer I, Hepbasli A. Thermodynamic analyses and assessments of various thermal energy storage systems for buildings. Energy Convers Manage 2012;62:109–22. [15] Caliskan H, Dincer I, Hepbasli A. Exergoeconomic and environmental impact analyses of a renewable energy based hydrogen production system. Int J Hydrogen Energy 2013;38:6104–11.
of the energy. The environmental and exergoenvironmental (EXEN) results are given in Fig. 5. According to the environmental result, 0.0777 kg CO2 is released in a day, while the corresponding (EXEN) rate is 0.0727 kg CO2/day. These carbon dioxide rates are low and the CO2 releasing happens due to the production of the solar collector. Also, taking the useful part of the energy is more reliable due to its consideration of dead state condition. The enviroeconomic and exergoenviroeconomic (EXENEC) results are shown in Fig. 6. The enviroeconomic and EXENEC rates are determined to be 0.00112 $/day and 0.00105 $/day, respectively. These economic results bases on the carbon dioxide emission price that was explained in the analysis section. As is seen that these values are also very low for the solar collector. 5. Conclusion The solar collector was assessed by the energy, exergy, environmental, enviroeconomic, EXEN and EXENEC analyses. As a result, the following main conclusions can be extracted from the study: (i) The most of the energy and exergy is lost by the radiation. The major reason is the big temperature difference between sky (−40 °C) and glass surface of the collector (40 °C). If the glass material of the collector is chosen more effectively (related with emissivity), the radiation loss can be decreased. (ii) The energy efficiency (25.40%) of the system is higher than the corresponding exergy efficiency (0.732%). So, the effect of the environment (dead state) temperature on the efficiency (exergetic) cannot be underestimated. (iii) The solar exergy of the system is the maximum exergy input rate,
492
Contents lists available at ScienceDirect
Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser
crossmark
Energy, exergy, environmental, enviroeconomic, exergoenvironmental (EXEN) and exergoenviroeconomic (EXENEC) analyses of solar collectors Hakan Caliskan Department of Mechanical Engineering, Faculty of Engineering, Usak University, 64200 Usak, Turkey
A R T I C L E I N F O
A BS T RAC T
Keywords: Energy Exergy Environment Renewable energy Solar energy Sustainability
In this study, the energy, exergy, environmental, enviroeconomic, exergoenvironmental (EXEN), Exergoenviroeconomic (EXENEC) analyses are performed to a solar collector. The enviroeconomic (energy based environmental analysis), EXEN (exergy based environmental analysis) and EXENEC (exergy based environmental and economic analysis) analyses are firstly conducted in this kind of system in the literature. It is found that most of the energy and exergy are lost by the radiation. The major reason is the big temperature difference between sky and glass surface of the collector. Furthermore, the energy efficiency (25.40%) of the system is higher than the corresponding exergy efficiency (0.732%). Also, the solar exergy of the system is the maximum exergy input rate, and most of it is destructed in the system due to the irreversibility. It shows the major disadvantages of the solar collector system. The EXEN result (0.0727 kg CO2/day) is lower than the corresponding environmental one (0.0777 kg CO2/day). The enviroeconomic result (0.00112 $/day) is higher than the EXENEC result (0.00105 $/day). So, exergy based EXENEC method is more reliable. It can be generally concluded that the solar collector systems can be assessed more effectively by using the exergy and economy based EXEN and EXENEC methods, respectively due to the consideration of the environmental condition and useful energy into calculation.
1. Introduction Most of the needs of modern societies are supplied by energy. So, the economic growths of countries largely depend on it. Environmental threats and the depletion of fossil fuel reserves expose the importance of the renewable energy sources; because they are clean, safe and sustainable. Especially, solar energy is a promising energy source among the renewable energies. It is limitless free energy source which has capable of meeting the necessary energy demand of the world [1,2]. Solar collectors are used to convert solar energy to thermal for heating applications. Their performance can be determined by the exergy which is the maximum theoretical work obtained from the interaction between the system and the environment in the equilibrium state. Exergy is also defined as potential or quality of energy which is also known as useful energy (availability). Thermodynamic systems can be assessed effectively with exergy method. Also, it is possible to determine the sustainability assessment with the help of exergy [3]. The exergy analysis is generally used to optimize thermodynamic and energy systems more than fifty years [4]. It is also used to determine the monetary values and values of the environmental impact for the transport of energy. So, it is connected with the environmental and ecological assessments through Life Cycle Assessment and industrial
ecology [5–7]. Caliskan [8,9] concluded that the exergy can also be used for assessing the environmental impact of processes with economic point of view. Because, the exergy analysis is feasible to production/consumption chain which is from resource intake down to emission. These exergy related assessments are named as Exergoenvironmental (EXEN) and Exergoenviroeconomic (EXENEC) analyses which are developed by Caliskan [8], and used in the studies of Caliskan [8,9]. The EXEN analysis has three important components: exergy analysis, environmental parameter and working time. In the EXENEC analysis exergy, economy and environmental parameters are taken into account. In the open literature, there are not any studies about the EXEN and EXENEC analyses of solar collectors, however there are some studies related to energy (thermal), exergy, economy or environmental analyses. Faizal et al. [2] applied the energy, exergy, economic, and environmental analysis on a flat-plate solar collector operated with SiO2 nanofluid. It was found that energy and exergy efficiencies of nanofluids were higher than base fluids. Also CO2 emissions and payback periods of nanofluids are better. Tiwari et al. [10] studied on exergoeconomic and enviroeconomic analyses of partially covered photovoltaic thermal flat plate collector integrated solar distillation system. The energetic (thermal) and exergetic efficiencies were found
E-mail address: [email protected]. http://dx.doi.org/10.1016/j.rser.2016.11.203 Received 30 March 2016; Received in revised form 20 October 2016; Accepted 14 November 2016 1364-0321/ © 2016 Elsevier Ltd. All rights reserved.
Renewable and Sustainable Energy Reviews 69 (2017) 488–492
H. Caliskan
and compared with the previously studies. It was concluded that the solar collector system could meet the water and electrical power in sunshine hours. Chamoli [1] performed exergy analysis and optimization on a flat plate solar collector using MATLAB Simulink and optimal parameters. As a result, optimum design points were determined, optimum values of collector inlet temperature, mass flow rate, absorber plate area, and fluid outlet temperature for maximum exergy inflow were investigated. Battisti and Corrado [11] applied environmental analysis and optimization to the water storage coupled solar thermal collector. Simapro software program were used to obtain environmental indicators. It was found that the reduction of the impacts could be up to 40% and the environmental pay back times were 5–19 months. The aim of the study is to assess solar collectors along with the combination of the energetic, exergetic, environmental and economic point of view. So, this study differs from the previously conducted ones as follows: (i) The enviroeconomic, EXEN and EXENEC analyses, developed by the author, are first applied to a solar collector (This is the first time in the literature), (ii) The sustainability index and entropy generation rate of the solar collector are determined.
Table 1 Data of the system.
2. System description As a case study, the flat plate solar collector is chosen to assess a solar system by using the environmental, enviroeconomic, EXEN and EXENEC analyses. The schematic layout (a) and control volume (b) of the system are shown in Fig. 1. The solar radiation is incident on the glass cover of the solar collector at the rate of 650 W/m2. The glass transmits 88% of the incident radiation and has an emissivity of 0.90. The necessary hot water for a house in summer season can be supplied by a collector of 1.5 m high and 2 m wide, and tilted 40° from the horizontal. The temperature of the glass cover is measured to be 40 °C on a calm day when the surrounding air temperature is 20 °C. The effective sky temperature for radiation exchange between the glass cover and the open sky is −40 °C. The water enters the tubes attached to the absorber plate at a rate of 1 kg/min. It is assumed that the back surface of the absorber plate is insulated and the only heat loss occurs through the glass cover. The data of the system description are created with the best of author's knowledge about the solar collectors and systems in Turkey. The more detailed data of the system is given in Table 1 which includes the assumptions.
Symbol
Rate
Glass surface temperature Air temperature Average temperature Sky temperature Sun temperature Thermal conductivity Kinematic viscosity Prandtl number β=1/Tavg Area Perimeter Characteristic length Collector angle cos(θ) Acceleration of gravity Emissivity Stefan boltzmann constant Permeability Solar radiation Mass flow rate of water Specific heat Water input temperature Water output temperature CO2 emission value for the solar collector Working hours of the solar collector Carbon price
Tsurf Tair=To Tavg Tsky Tsun k ν Pr β A P L=A/P θ cos(40) g ε σ α q m cp Tin Tout yCO2 tworking cCO2
40 °C 20 °C 30 °C −40 °C 6000 K 0.02588 W/m °C 0.00001608 m2/s 0.7282 0.0032986971 K 3 m2 7m 0.428571429 m 40° 0.766044 9.81 m/s2 0.9 (5.67) 10−8 W/m2 K4 0.88 650 W/m2 0.016666667 kg/s 4180 J/kg °C 25 °C 31.25643842 °C 0.00000647 kg CO2/Wh 7 h/day 0.0145 $/kg CO2
where “Eṅ w, in ”, “Eṅ solar , in ”, “Eṅ w, out ” and “Eṅ loss ” are the energy input rate of the water, solar energy input rate, energy output rate of the water and energy loss rate of the system, respectively. The net energy rate of the water ( ΔEṅ w ) can be written as;
ΔEṅ w=Eṅ w, out −Eṅ w, in=ṁ w cp (Tw, out −Tw, in )
(2)
where “ṁ w ”, “cp ”, “Tw, out ” and “Tw, in ” are the mass flow rate, specific heat, collector output temperature and collector input temperature of the water, respectively. The solar energy input rate (Eṅ solar , in ) is found by
Eṅ solar , in=αqA
(3)
where “α ”, “q ” and “ A” are the permeability of the system, solar radiation rate and collector area of the system, respectively. The energy loss rate of the system (Eṅ loss ) consists of the convection heat loss rate (Q̇loss, conv ) and radiation heat loss rate (Q̇loss, rad ) as follows:
3. Analysis 3.1. Energy analysis
̇ , conv +Qloss ̇ , rad Eṅ loss=Qloss
(4)
The radiation heat loss rate is determined to be
The steady energy balance of the system is written as follows:
Eṅ w, in +Eṅ solar , in=Eṅ w, out +Eṅ loss
Parameter
4 4 ̇ , rad =εσA (Tsurf Qloss −Tair )
(1)
Fig. 1. Schematic layout and control volume of the system.
489
(5)
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⎛ ⎞ ̇ , conv ⎜1 − To ⎟ Eẋ loss, conv=Qloss Tsurf ⎠ ⎝
where “ε ”, “σ ”, “Tsurf ” and “Tair ” are the emissivity of the system, StefanBoltzmann constant, glass surface temperature and air temperature, respectively. The convection heat loss rate is calculated from
̇ , conv=hA Qloss
(Tsurf −Tair )
The exergy destruction rate of the system (Eẋ dest ) is determined as follows:
(6)
Eẋ dest =Eẋ w, in+Eẋ solar , in−Eẋ w, out −Eẋ loss
where “h ” is the convection heat transfer coefficient.
h=
kNu L
(7)
A P
̇ = Sgen
Nu=(0.
ψ=
(9)
gcosθ (Tsurf − Tair
ΔEẋ w Eẋ solar , in
SI =
) L3Pr
1 1−ψ
3.3. Environmental analysis
ΔEṅ w η= Eṅ solar , in
xCO2=yCO2 Eṅ solar , in tworking
The environmental analysis can be explained as follows [12–15]:
(11)
The steady exergy balance of the system can be given by
Eẋ w, in+Eẋ solar , in=Eẋ w, out +Eẋ loss+Eẋ dest
(12)
where “Eẋ w, in ”, “Eẋ solar , in ”, “Eẋ w, out ”, “Eẋ loss ” and “Eẋ dest ” are the exergy input rate of the water, solar exergy input rate, exergy output rate of the water, exergy loss rate and exergy destruction rate of the system, respectively. The net exergy rate of the water (ΔEẋ w ) is written as;
ΔEẋ w=Eẋ w, out −Eẋ w, in
[(Tw, in−To )−To
(13)
ln (Tw, out / To )] ln (Tw, in / To )]
(14) (15)
3.4. Enviroeconomic analysis
where “To ” is the dead state temperature. Exergy is considered with a reference environment. If a thermodynamic system is in equilibrium with the environment, the state of the system is called the “dead state” and the temperature is named as dead state temperature. It is generally equal to environment temperature. So, dead state can be considered as reference state [5]. The solar exergy input rate (Eẋ solar , in ) is determined by 4 ⎡ 1 ⎛ T ⎞ 4 ⎛ T ⎞⎤ Eẋ solar , in=Eṅ solar , in ⎢1 + ⎜ o ⎟ − ⎜ o ⎟ ⎥ ⎢⎣ 3 ⎝ Tsun ⎠ 3 ⎝ Tsun ⎠ ⎥⎦
The enviroeconomic analysis is defined by [13]:
CCO2=xCO2 cCO2
(16)
3.5. Exergoenvironmental (EXEN) analysis
(17) The EXEN analysis is calculated from [8,9]:
The radiation exergy loss rate is found by
⎛ ⎞ ̇ , rad ⎜1 − To ⎟ Eẋ loss, rad =Qloss T ⎝ surf ⎠
(25)
where “CCO2 ” is the enviroeconomic parameter and ‘‘cCO2 ’’ carbon dioxide emission price. Here, the LCA based environmental analysis is taken into account and carbon pricing methodology is also considered to assess the solar collector for enviroeconomic perspective. To prevent the climate change, carbon monoxide emissions need to be reduced or cut. So, pricing the carbon is considered as the best effective method. In this regard, carbon is priced. As a result, the enviroeconomic analysis is developed as a tool to assess the carbon pricing of the systems.
where “Tsun ” is the sun temperature. The exergy loss rate of the system (Eẋ loss ) includes the convection exergy loss rate (Eẋ loss, conv ) and radiation exergy loss rate (Eẋ loss, rad ).
Eẋ loss=Eẋ loss, conv+Eẋ loss, rad
(24)
where ‘‘xCO2 ’’ is the released carbon dioxide in a considered time, ‘‘yCO2 ’’ is the carbon dioxide value for the energy option defined by life cycle assessment methodology, ‘‘Eṅ solar , in ’’ is the energy rate of the solar energy option and ‘‘tworking ’’ is the working time of the system. The LCA provides a material balance and energy balance over the entire life of a product such as solar collector. In the LCA, goals, objectives and investigation boundaries of the framework are very important to assess the system correctly. They effect the results of the LCA parameters. There are four steps for the LCA: planning, inventory analysis, impact assessment, improvement analysis. If energy consumption/production is considered, it can be different for countries. But, it helps to determine the connection with the ambient by assessing its impact on environment. So, the environmental impact of the solar collector with its carbon dioxide value and energy production are considered for environmental analysis by taking into account the working period of time.
3.2. Exergy analysis
Eẋ w, in=ṁ w cp
(23)
(10)
ν2
[(Tw, out −To )−To
(22)
The sustainability index (SI) can be found using exergy efficiency
where “θ ” is the angle of the solar collector, “Pr ” is the Prandtl number, and “ν ” is the kinematic viscosity. The energy efficiency of the system (η ) is determined by
Eẋ w, out =ṁ w cp
(21)
as;
where “ A” and “P ” are the area and perimeter of the solar collector, respectively. Also, “Ra ” is the Rayleigh number as follows:
Ra=
Eẋ dest To
The exergy efficiency of the system (ψ) is calculated as follows:
(8)
15) Ra1/3
(20)
̇ ) is found from The entropy generation rate (Sgen
where “k ” is the thermal conductivity, “Nu ” is the Nusselt number, and “L ” is the characteristic length of the system.
L=
(19)
xex, CO2=yCO2 Eẋ solar , in tworking
(26)
where ‘‘xex, CO2 ’’ is the released carbon dioxide in a considered time considering exergetic values and ‘‘Eẋ solar , in ’’ is the exergy rate of the solar energy option.
(18)
The convection exergy loss rate is computed by 490
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The EXEN analysis considers three important points: (i) related exergy rate of energy option, (ii) CO2 emission value generally calculated from Life Cycle Assessment method, and (iii) working hours of the system. So, it gives information about exergy (enhanced thermodynamic) analysis based CO2 value in considered period of time [8]. The difference between environmental and EXEN is that; the environmental analysis uses the energy analysis results, while EXEN analysis considers exergetic results.
1600
Energy rate (W)
1400 1200 1000 800 600 400 200
3.6. Exergoenviroeconomic (EXENEC) analysis
0 Net energy changing rate of the water
The EXENEC analysis is determined by exergoenviroeconomic parameter (Cex, CO2 ) as follows [8,9]:
Cex, CO2=xex, CO2 cCO2
Energy loss rate
Solar energy rate
Fig. 2. Energy analysis results.
(27) 1650
The EXENEC analysis is applied with a parameter “CexCO2 ” which gives information about exergetic price of CO2 emission. It also includes the EXEN parameters [8]. The EXENEC analysis is a new tool to assess the carbon dioxide emission considering the exergetic results of the system such as EXEN analysis that connected with exergetic results. So, it is important to obtain this value considering not only first law of thermodynamics but also first and second laws of the thermodynamics. The detailed information for the EXEN and EXENEC analyses methodologies can be seen from Refs.[8,9].
1500
Exergy rate (W)
1350 1200 1050 900 750 600 450 300 150 0 Net exergy changing rate of the water
4. Results and discussion
Exergy loss rate
Solar exergy rate
Exergy destruction rate
Fig. 3. Exergy analysis results.
The energy, exergy, environmental, enviroeconomic, EXEN, and EXENEC analyses are applied to the solar collector system. The results of the analyses are generally tabulated in Table 2. The energy analysis results are given in Fig. 2. The solar energy input rate, energy loss rate and net energy changing rate of the water are found to be 1716.000 W, 1280.135 W and 435.865 W, respectively. The most of the total energy input is lost due to the convection and radiation heat losses. Among the energy loss, the radiation heat loss has the maximum rate as 1019.801 W, while the convection heat loss rate is 260.334 W. Because, solar energy systems mostly depend on radiation heat transfer phenomena. So, it is normal that radiation heat transfer has higher than the convection. The exergy analysis results are shown in Fig. 3. The solar exergy input rate, exergy loss rate, exergy destruction rate and net exergy changing rate of the water are calculated as 1604.215 W, 50.267 W, 1542.204 W and 11.244 W, respectively. The net exergy input rate of
the water is computed to be 2.937 W, while the net exergy output rate of the water is determined as 14.681 W. It is normal to increase the exergy rate of water due to the solar exergy gain by solar radiation. Also, the radiation and convection exergy loss rates are found to be 33.640 W and 16.627 W, respectively. The radiation and convection comparison is similar to energetic comparison; radiative heat transfer is higher for solar collectors. Furthermore, the sustainability index is determined as 1.007 and the entropy generation of the system is calculated to be 5.261 W/K. The sustainability index is mainly connected with exergy efficiency, while the entropy generation is related with exergy destruction and dead state temperature. Both of these parameters are required the exergy calculation. So, they can change by only exergetic values changing. In this study, the dead state (reference) temperature is stable and also there is no parametric changing in exergetic values. The efficiencies of the system are illustrated in Fig. 4. The energy efficiency of the system is calculated as 25.40%, while the exergy efficiency is determined to be 0.732%. This shows the importance of the reference (dead) state condition to take into account in the calculation. Because, exergy is available energy that is connected with reference environment condition of the system. So, this shows the real useful part
Table 2 Results of the analyses. Rate
Unit
Net energy changing rate of the water Solar energy rate Energy loss rate Radiation heat loss rate Convection heat loss rate Energy efficiency Net exergy changing of water Exergy loss Solar exergy input Exergy destruction Exergy efficiency Exergy input rate Exergy output rate Sustainability index Entropy generation rate Environmental parameter Exergoenvironmental parameter Enviroeconomic parameter Exergoenvironmental parameter
435.865 1716.000 1280.135 1019.801 260.334 25.400 11.744 50.267 1604.215 1542.204 0.732 2.937 14.681 1.0073 5.261 0.077718 0.072655 0.001127 0.001053
W W W W W % W W W W % W W – W/K kg-CO2/day kg-CO2/day $/day $/day
28 24 20
Efficiency (%)
Results of the analyses
16 12 8 4 0 Energy efficiency
Exergy efficiency
Fig. 4. Efficiencies of the system.
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and most of it is destructed in the system due to the irreversibility. It shows the major disadvantages of the solar collector system. (iv) The EXEN result (0.0727 kg CO2/day) is lower than the corresponding environmental one (0.0777 kg CO2/day). Both of them very close rates, but even the small difference is important to make the environmental assessment effectively. So, the exergetic based environmental (EXENEC) analysis is more reliable. (v) The enviroeconomic result (0.00112 $/day) is higher than the EXENEC result (0.00105 $/day). So, the exergy based EXENEC method is more reliable due to the dead state and useful energy considerations. If it is accepted that the collector is used 4500 h in a year for hot climates, its Environmental and EXENEC results are calculated as 5.040 $/year and 4.725 $/year, respectively. It should be noted that, these are the values of the 3 m2 solar collector which can be considered very small. So, utilization of them with serial or parallel connections increase these prices.
0.07
Rate (kg CO2/day)
0.06 0.05 0.04 0.03 0.02 0.01 0 Environmental
Exergoenvironmental (EXEN)
Fig. 5. Environmental and exergoenvironmental (EXEN) results. 0.0012 0.001
As a final remark, the solar collector systems can be assessed more effectively by using the exergy and economy based environmental (EXEN) and enviroeconomic (EXENEC) methods, respectively due to the consideration of the environmental (dead state) condition into calculation. So, considering not only environmental and enviroeconomic analyses but also EXEN and EXENEC analyses for solar collectors can be promising methodology to assess the carbon pricing effectively with the help of life cycle and enhanced thermodynamics.
Rate ($/day)
0.0008 0.0006 0.0004 0.0002 0 Enviroeconomic
References
Exergoenviroeconomic (EXENEC)
Fig. 6. Enviroeconomic and exergoenviroeconomic (EXENEC) results.
[1] Chamoli S. Exergy analysis of a flat plate solar collector. Journal of Energy in Southern Africa 2013;24(3):8–13. [2] Faizal M, Saidur R, Mekhilef S, Hepbasli A, Mahbubul IM. Energy, economic, and environmental analysis of a flat-plate solar collector operated with SiO2 nanofluid. Clean Technol Environ Policy 2014. http://dx.doi.org/10.1007/s10098-014-08700. [3] Moran MJ, Shapiro HN. Fundamentals of engineering thermodynamics, 3rd Edition. New York, USA: John Wiley & $2 Sons; 1998. [4] Rant Z. Thermodynamische bewertung der verluste bei technischen energieumwandlungen. Brennst Wärme-Kraft 1964;16(9):453–7. [5] Dincer I, Rosen MA. Exergy: energy environment and sustainable development. UK: Elsevier; 2007, [ISBN: 0080445292, EAN: 9780080445298]. [6] Meyer L, Tsatsaronis G, Buchgeister J, Schebek L. Exergoenvironmental analysis for evaluation of the environmental impact of energy conversion systems. Energy 2009;34:75–89. [7] Tsatsaronis G, Morosuk T. Understanding and improving energy conversion systems with the aid of exergy-based methods. Int J Exergy 2012;11(4):518–42. [8] Caliskan H. Novel approaches to exergy and economy based enhanced environmental analyses for energy systems. Energy Conversion and Management 2015;89:156–61. [9] Caliskan H. Thermodynamic and environmental analyses of biomass, solar and electrical energy options based building heating applications. Renewable & $2 Sustainable Energy Reviews 2015;43:1016–34. [10] Tiwari GN, Yadav JK, Singh DB, Al-Helal IM, Abdel-Ghany AM. Exergoeconomic and enviroeconomic analyses of partially covered photovoltaic flat plate collector active solar distillation system. Desalination 2015;367:186–96. [11] Battisti R, Corrado A. Environmental assessment of solar thermal collectors with integrated water storage. Journal of Cleaner Production 2005;13(1314):1295–300. [12] Caliskan H, Dincer I, Hepbasli A. A comparative study on energetic, exergetic and environmental performance assessments of novel M-cycle based air coolers for buildings. Energy Convers Manage 2012;56:69–79. [13] Caliskan H, Dincer I, Hepbasli A. Exergoeconomic, enviroeconomic and sustainability analyses of a novel air cooler. Energy Build 2012;55:747–56. [14] Caliskan H, Dincer I, Hepbasli A. Thermodynamic analyses and assessments of various thermal energy storage systems for buildings. Energy Convers Manage 2012;62:109–22. [15] Caliskan H, Dincer I, Hepbasli A. Exergoeconomic and environmental impact analyses of a renewable energy based hydrogen production system. Int J Hydrogen Energy 2013;38:6104–11.
of the energy. The environmental and exergoenvironmental (EXEN) results are given in Fig. 5. According to the environmental result, 0.0777 kg CO2 is released in a day, while the corresponding (EXEN) rate is 0.0727 kg CO2/day. These carbon dioxide rates are low and the CO2 releasing happens due to the production of the solar collector. Also, taking the useful part of the energy is more reliable due to its consideration of dead state condition. The enviroeconomic and exergoenviroeconomic (EXENEC) results are shown in Fig. 6. The enviroeconomic and EXENEC rates are determined to be 0.00112 $/day and 0.00105 $/day, respectively. These economic results bases on the carbon dioxide emission price that was explained in the analysis section. As is seen that these values are also very low for the solar collector. 5. Conclusion The solar collector was assessed by the energy, exergy, environmental, enviroeconomic, EXEN and EXENEC analyses. As a result, the following main conclusions can be extracted from the study: (i) The most of the energy and exergy is lost by the radiation. The major reason is the big temperature difference between sky (−40 °C) and glass surface of the collector (40 °C). If the glass material of the collector is chosen more effectively (related with emissivity), the radiation loss can be decreased. (ii) The energy efficiency (25.40%) of the system is higher than the corresponding exergy efficiency (0.732%). So, the effect of the environment (dead state) temperature on the efficiency (exergetic) cannot be underestimated. (iii) The solar exergy of the system is the maximum exergy input rate,
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