Exergy and economic analysis of a pyramid-shaped solar water purification system: Active and passive cases

Exergy and economic analysis of a pyramid-shaped solar water purification system: Active and passive cases

Energy 38 (2012) 31e36 Contents lists available at SciVerse ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy Exergy and econom...

846KB Sizes 0 Downloads 5 Views

Energy 38 (2012) 31e36

Contents lists available at SciVerse ScienceDirect

Energy journal homepage: www.elsevier.com/locate/energy

Exergy and economic analysis of a pyramid-shaped solar water purification system: Active and passive cases Ali Kianifar b, Saeed Zeinali Heris c, Omid Mahian a, * a

Young Researchers Club, Mashhad Branch, Islamic Azad University, Mashhad, Iran Department of Mechanical Engineering, Engineering Faculty, Ferdowsi University of Mashhad, Mashhad, Iran c Department of Chemical Engineering, Engineering Faculty, Ferdowsi University of Mashhad, Mashhad, Iran b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 August 2011 Received in revised form 28 December 2011 Accepted 29 December 2011 Available online 24 January 2012

An exergy analysis has been conducted to show the effect of a small fan on the exergy efficiency in a pyramid-shaped solar still. The tests were carried out in Mashhad (36 360 N), for two solar still systems. One of them was equipped with a small fan (active system), to enhance the evaporation rate while the other one was tested in passive condition (no fan). To examine the effects of radiation and water depth on exergy efficiency, experiments in two seasons and two different depths of water in the solar still basin were performed. The results show that during summer, active unit has higher exergy efficiency than passive one while in winter there is no considerable difference between the exergy efficiency of the units. Results also reveal that the exergy efficiency is higher when the water depth in the basin is lower. Finally, the economic analysis shows a considerable reduction in production cost of the water (8e9%) when the active system is used. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Solar still Fresh water Active system Passive system Exergy Cost analysis

1. Introduction Water is one of the most abundant resources covering threequarter of the planet’s surface, about 97% of this amount is saline, and only 3% is fresh water suitable for humans, plants, and animals needs, so that the fresh water shortage is going to be a profound social crisis after the oil crisis in the world [1,2]. Consequently, provision of fresh water is still one of the main problems in arid remote areas in the world. Solar desalination systems can solve part of the problem in the areas where solar energy is available [3]. Solar stills can be used to prevent the greenhouse gas emissions produce from the production of fresh water [4]. In Iran with abundant saline-water resources, solar distillation can be one of the best solutions and a simple way for distilling water [5]. During the last decades, many studies have been made to find the solutions of improving the performance of the conventional solar still. For instance, it has been found that the productivity of solar stills can be improved by utilizing; fin, sponges, pebbles, black rubber and sand in solar still, coupling the solar still with parabolic

* Corresponding author. Tel./fax: þ98 511 8816840. E-mail address: [email protected] (O. Mahian). 0360-5442/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2011.12.046

concentrator, solar ponds or solar collector, using solar still with wicked concave surface, tube-type solar still, weir-type inclined solar still, and energy storage media [6e15]. Recently the status of different types of solar stills and the history of previous researches in this field have been reviewed [16]. Tiwari and Tiwari [17] have collected a helpful summary consist of required information concerning different types of solar stills including design methods and modeling. The use of exergy analysis in actual desalination processes from a thermodynamic point of view is of growing significance to recognize the sites of maximum losses and improve the performance of the processes [18]. The literature of exergy analysis of solar stills can be found in references [19e24]. Kwatra [19] carried out an exergy analysis for describing the performance of multiple-effect solar stills. Nunez et al. [20] with an exergy analysis on a single slope passive solar still showed that attempts should be directed towards better collector design, still cover materials, evaporation-condensation modifications and the reduction of collector-brine temperature gap. Kumar and Tiwari [21] compared the exergy efficiency of a single slope solar still in passive case with an active one where the solar still was coupled with a photovoltaic thermal system. They concluded that the exergy efficiency of the active solar still was nearly 5 times higher than the passive one. Tiwari et al. [22] made

32

A. Kianifar et al. / Energy 38 (2012) 31e36

an theoretically exergy study of a single slope solar still coupled with a flat collector and investigated the effects of the number of collectors and water depth on exergy efficiency. It was found that by increasing of the water depth, the exergy efficiency decreases. Recently, Gaur and Tiwari [23] theoretically optimized the number of collectors for a PV/T hybrid active solar still on the basis of energy and exergy analysis. Dwivedi and Tiwari [24] evaluated the exergy efficiency of a double slope solar still coupled with a flat collector in a constant water depth. They observed that the exergy efficiency of double slope active solar still is higher than the exergy efficiency of double slope passive solar still. A review in the literature reveals the exergy analysis of a pyramid-shaped active solar still has not been studied yet. The present work concentrates on the testing and exergy analysis of a pyramid-shaped solar still for both passive and active systems. Also, an economic analysis has been conducted in order to obtain the cost of water produced by means of the active pyramid-shaped solar still. 2. Experimental set up Two units of a pyramid-shaped solar still have been built to examine the effects of different parameters under the same weather conditions. The two were identical, but only one of them was equipped with a small fan with a negligible power consumption (w2 Watts) to enhance the productivity of fresh water. The

schematic diagram of the pyramid-shaped solar still for the active system is shown in Fig. 1. The solar still has an effective basin area of 0.9 m2 and 0.25 m height. The solar still construction has been made of polyethylene. The thicknesses of side walls and the base are 8 mm and25 mm, respectively. A glass cover with the glass inclination of 36 has been fixed with respect to the horizontal axis. The glass inclination was the same as the latitude of the test location in order to receive the maximum annual solar radiation [25]. The solar radiation incident passes through the glass with a thickness of 4 mm and subsequently the solar heat is absorbed by a black plate in the bottom of the basin. Thus, the saline water is heated, and evaporation takes place. The distillate water is collected by a channel which is of galvanized steel sheet and connected to outlet (drain pipe). A hole in the still sidewall allows inserting two K-type thermocouples with accuracy of 0.5  C for measuring the basin water temperature as well as the inside air temperature. The fan has been installed in the middle of one of the side walls in the active solar still. The experiments were conducted during the summer and winter seasons to examine the solar still performance. The results are presented only for two typical days in June (summer) and two typical days in January (winter). Also, to study the water depth effect on the performance of solar still; the tests were conducted for two different water depths. The water depth of 4 cm was considered as the reference depth and 8 cm was selected to investigate the effect of increasing the water mass in the basin up to 2 times of its reference value.

Fig. 1. Schematic of the active solar still.

A. Kianifar et al. / Energy 38 (2012) 31e36

33

The meteorological data have been taken from a local meteorological station in Mashhad, North-East of Iran. The experimental data have been recorded every 1 h from 9 am to 6 pm local time. 3. Error analysis The minimum error (uncertainty) occurred in an instrument is equal to the ratio between its least count and minimum value of the output measured [6,7]. Using this definition, the amount of error due to using K-type thermocouple (with accuracy of 0.5  C) for the measurement of temperature inside the solar still becomes 4.5%. Also, the error for the collection tank, having 2 L capacity and accuracy of 10 ml, is almost 6.7%. 4. Exergy analysis The exergy efficiency is defined as [26]:

hEX ¼

_ evap Ex Exergy output of solar still ¼ _ Exergy input to solar still Exinput

(1)

Fig. 2. Variations of solar intensity and ambient temperature for both the summer and winter with local time (9 ame6 pm).

In a solar still; exergy output is as a result of evaporation and subsequently the condensation of saline water. In practice, some of the evaporated water after condensation on the glass cover, falls back into the basin, hence the evaluated exergy output from the experimental results would be less than the theoretical one. The hourly exergy output of a solar still can be defined as [22]:

To determine the exergy input; the effective parameters in producing the fresh water from saline water should be considered. These parameters may be listed as follows:

_ output ¼ Ex _ evap ¼ Ex

  Ta þ 273  1   Tw þ 273 3600 s:h1 _ ew L m

(2)

_ ew is hourly yield of solar still (kg/h), L is the latent heat of Where m vaporization (J/kg), Ta is the ambient temperature ( C) and Tw is the water temperature ( C).

1 Solar radiation received by the solar still. 2 The power consumed by devices such as pump, fan, heater. In the present study, only in the active solar still system a small fan with the power consumption of 2 Watts is used. Therefore, the exergy input can be determined as:

_ _ _ Ex input ¼ Exsun ðsolar stillÞ þ ExP:C ðfanÞ

Fig. 3. Daily productivity for different conditions.

(3)

34

A. Kianifar et al. / Energy 38 (2012) 31e36

Fig. 4. Variations of exergy efficiency with local time for a) Summer and b) Winter.

_ sun ðsolar stillÞ is the exergy input to solar still through where Ex radiation and can be obtained from the following equation [24,27]:

   _ sun ðsolar stillÞ ¼ A  IðtÞ  1  4  Ta þ 273 þ 1 Ex S S 3 TS 3    Ta þ 273 4  TS

(4)

where As is the area of basin in solar still(m2), IðtÞS is solar radiation on the inclined glass surface of solar still (W/m2) and TS is the sun temperature, 6000 K. _ Also, Ex P:C ðfanÞ is related to power consumption by the fan (2 Watts) in the active solar still system.

5. Results and discussion Fig. 2 shows the variations of the solar intensity (W/m2) and the ambient temperature ( C) respect to local time for two typical days in summer and winter. The maximum solar intensities are 960 W/ m2 and 560 W/m2 for summer and winter, respectively. Based on the latitude of Mashhad (36 360 N), the amount of solar radiation in summer is considerably higher than winter because the sun during the summer is nearly overhead while the sun’s rays are far more slanted during the winter months. Also, climate factors such as clouds, water vapor, dust, etc., would affect the amount of solar radiation received by the earth surface day by day. From Fig. (2), the maximum solar radiation occurs at around 2pm in summer while the maximum solar radiation in winter is around 1pm. Since, in the northern hemisphere which Mashhad is located the days are longer in summer compared to winter due to tilting the earth’s axis of rotation. Also, the maximum temperatures are 34  C and 18  C for summer and winter, respectively. Daily water productivity is illustrated in Fig. 3. It can be observed that daily productivity in active case is on average, 15%e 20% higher than the passive one. The fan increases the evaporation rate (hence increasing the water productivity); because in the case of air flowing over the water surface (by fan), the concentration of the vapor in the air is less likely to go up with time, so the relative humidity stays unsaturated near the water surface. This encourages faster evaporation. This is the result of the boundary layer at

the evaporation surface which decreases with flow velocity, and reduces the diffusion distances in the stagnant layer. It is observed that at 8 cm water depth, the productivity is higher for all cases. The experiments carried out for 8 cm of water depth under a higher solar intensity, thus higher water production is due to a higher solar radiation and not higher water depth. If 4 cm and 8 cm of water depth receive the same amount of radiation the productivity of water for the lower depth will be higher, because the lower water depth implies lower heat capacity of water which results in higher water temperature in the basin and hence more productivity [28]. Hourly variations of exergy efficiency have been presented for passive and active cases in both seasons in Fig. 4. Fig. 4 (a) shows the hourly exergy efficiency is maximized for the active solar still having 4 cm water depth. Also, the effect of water depth on exergy efficiency is more considerable during afternoon (3e6 pm) in

Fig. 5. Daily exergy efficiency for different conditions.

A. Kianifar et al. / Energy 38 (2012) 31e36

35

Table 1 The results of cost analysis for present work. Type of solar still

Interest rate (%)

M (Lit/m2)

CRF

FAC

SFF

ASV

AMC

AC

CPL ($/Lit/m2)

Passive (4 cm depth) Active (4 cm depth) Passive (8 cm depth) Active (8 cm depth)

12 12 12 12

604.28 697.56 665.11 762.44

0.177 0.177 0.177 0.177

25.5 27.2 25.5 27.2

0.057 0.057 0.057 0.057

1.71 1.82 1.71 1.82

3.83 4.08 3.83 4.08

27.62 29.46 27.62 29.46

0.046 0.042 0.042 0.039

summer. Note that in winter (Fig. 4(b)) at sunset the exergy efficiency tends to infinity because the solar radiation and hence the exergy input is nearly zero. Fig. 5 shows that during summer, the fan causes the active system to generate higher exergy efficiency than the passive one while in winter the exergy efficiency is nearly equal in both cases. Fig. 5 also indicates that for both summer and winter seasons the exergy efficiency is maximum where the water depth is minimum. Also, it is found that the exergy efficiency in summer is nearly 0.6%e0.9% higher than winter, it should be noted that the maximum exergy efficiency is nearly 3.3%. Therefore, from the viewpoint of the second law of thermodynamics (exergy analysis) it can be suggested to use a fan only in summer.

6. Economic analysis Using economic analysis, it would be possible to estimate the cost of 1 L of distillated water. In addition to the capital cost of the solar still, other parameters such as sinking fund factor, annual salvage value, annual maintenance cost, and interest rate per year should be considered. At this stage, the Capital recovery factor (CRF) is defined in terms of the interest per year i and also the number of life years of the system n [29,30]:

for the passive unit, and160 $ (by considering 10$ for the fan price) for the active one. By taking the salvage value of system S equal to 20% of capital cost:

S ¼ 0:2 P

Sinking fund factor (SFF) and annual salvage value (ASV) can be expressed respectively as [29]:

SFF ¼

ið1 þ iÞ ð1 þ iÞn 1

(5)

The interest per year i and the number of life years of the system n are assumed 12% and 10, respectively. Fixed annual cost (FAC) becomes:

FAC ¼ PðCRFÞ

(6)

where P is the capital cost of solar still. The capital cost includes the cost of the structure made of polyethylene as well as the costs of labor, glass cover, piping, galvanized steel sheet, paint and a DC fan( for the active system). In this work the capital cost P becomes 150 $

i ð1 þ iÞn 1

(8)

ASV ¼ ðSFFÞ S

(9)

The AMC which is annual maintenance operational cost of the system consists of collecting the fresh water, cleaning the glass cover, washing inside the unit to remove the deposited salt, and maintenance of DC fan. Here, 15% of fixed annual cost is considered as maintenance cost:

AMC ¼ 0:15 ðFACÞ

(10)

Therefore the annual cost (AC) is:

AC ¼ FAC þ AMC  ASV

(11)

Finally the cost of fresh water per liter can be calculated as:

n

CRF ¼

(7)

CPL ¼

AC M

(12)

where M is the average annual yield of the solar still. In this study the amount of M has been calculated based on daily water productivity of two typical days in winter for both water depths 4 and 8 cm. Table 1 provides the results of cost analysis for the present work. It is found that using a fan with a negligible cost results in a cost reduction by nearly 9%. In Fig. 6, the cost of fresh water per liter, CPL, for solar stills given in Ref. [29] and the passive and active solar stills in the present work is compared. The results show that both passive and active systems have lower productive cost in comparison with other solar stills, except the single slope type system.

7. Conclusion The main conclusions of the study are:

Fig. 6. Comparison between different solar stills and the present work.

➢ Using a small fan in a pyramid-shape solar still may result in 15e20% increase in daily productivity of fresh water. ➢ The exergy efficiency shows a higher value at lower water depth (comparing 4 cm and 8 cm depth during the tests). ➢ In summer, the exergy efficiency is higher for active system in comparison with the passive one while in winter the exergy efficiency is nearly the same for both systems. ➢ The exergy efficiency during summer time is higher than winter time.

36

A. Kianifar et al. / Energy 38 (2012) 31e36

➢ The cost of fresh water per liter for an active solar system is roughly 8e9% lower in comparison with the passive unit. Acknowledgment The authors wish to express their sincere appreciation to Prof. David Naylor at Department of Mechanical and Industrial Engineering, Ryerson University, Canada, for his valuable guidance and scientific discussion. References [1] Gude VG, Nirmalakhandan N, Deng S. Desalination using solar energy: towards sustainability. Energy 2011;36:78e85. [2] Deng R, Xie L, Lin H, Liu J, Han W. Integration of thermal energy and seawater desalination. Energy 2011;35:4368e74. [3] El-Sebaii AA. On effect of wind speed on passive solar still performance based on inner/outer surface temperatures of the glass cover. Energy 2011;36: 4943e9. [4] Palenzuela P, Zaragoza G, Alarcón-Padilla DC, Guillén E, Ibarra M, Blanco J. Assessment of different configurations for combined parabolic-trough (PT) solar power and desalination plants in arid regions. Energy 2011;36:4950e8. [5] Mahian O, Kianifar A. Mathematical modeling and experimental study of a solar distillation system. Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science 2011;225: 1203e12. [6] Velmurugan V, Deenadayalan CK, Vinod H, Srithar K. Desalination of effluent using fin type solar still. Energy 2008;33:1719e27. [7] Velmurugan V, Naveen Kumar KJ, Noorul Haq T, Srithar K. Performance analysis in stepped solar still for effluent desalination. Energy 2009;34: 1179e86. [8] Chaouchi B, Zrelli A, Gabsi S. Desalination of brackish water by means of a parabolic solar concentrator. Desalination 2007;217:118e26. [9] El-Sebaii AA, Ramadan MRI, Aboul-Enein S, Salem N. Thermal performance of a single basin solar still integrated with a shallow solar pond. Energy Conversion and Management 2008;49:2839e48. [10] Saleh A, Qudeiri JA, Al-Nimir MA. Performance investigation of a salt gradient solar pond coupled with desalination facility near the red sea. Energy 2011; 36:922e31. [11] Abdel-Rehim ZS. Lasheen Ashraf. Experimental and theoretical study of a solar desalination system located in Cairo, Egypt. Desalination 2007;217:52e64. [12] Kabeel AE. Performance of solar still with a concave wick evaporation surface. Energy 2009;34:1504e9. [13] Murase K, Yamagishi Y, Iwashita Y. Sugino k, Development of a tube-type solar still equipped with heat accumulation for irrigation. Energy 2008;33: 1711e8. [14] Sadineni SB, Hurt R, Halford CK, Boehm RF. Theory and experimental investigation of a weir-type inclined solar still. Energy 2008;33:71e80. [15] El-Sebaii AA, El-Ghamdi AA, Al-Hazmi FS, Faidah AS. Thermal performance of a single basin solar still with PCM as a storage medium. Applied Energy 2009; 86:1187e95. [16] Kabeel AE, El-Agouz SA. Review of researches and developments on solar stills. Desalination 2011;276:1e12.

[17] Tiwari GN, Tiwari AK. Solar distillation practice in water desalination systems. New Delhi: Anamaya Pub. Ltd; 2007. [18] Kalogirou SA. Solar energy engineering: processes and systems. Elsevier; 2009. [19] Kwatra HS. Performance of a solar still: predicted effect of enhanced evaporation area in yield and evaporation temperature. Jounal of Solar Energy 1996; 56(3):261. [20] Torchia-Nunez JC, Porta-Gundara MA, Cervantes-de Gortari JG. Exergy analysis of a passive solar still. Renewable Energy 2008;33:608e16. [21] Kumar S, Tiwari A. An experimental study of hybrid photovoltaic thermal (PV/ T)-active solar still. International Journal of Energy Research 2008;32:847e58. [22] Tiwari GN, Dimri V, Chel A. Parametric study of an active and passive solar distillation system: energy and exergy analysis. Desalination 2009;242:1e18. [23] Gaur MK, Tiwari GN. Optimization of number of collectors for integrated PV/T hybrid active solar still. Applied Energy 2010;87:1763e72. [24] Dwivedi VK, Tiwari GN. Experimental validation of thermal model of a double slope active solar still under natural circulation mode. Desalination 2010;250: 49e55. [25] Duffie JA, Beckman WA. Solar engineering of thermal processes. New York: Wiley; 1991. [26] Hepbalsi A. A key review on exegetic analysis and assessment of renewable energy sources for a sustainable future. Renewable and Sustainable Energy Reviews 2008;12(3):593e661. [27] Petela R. Exergy of undiluted thermal radiation. Solar Energy 2003;74(6): 469e88. [28] Nafey AS, Abdelkader M, Abdelmotalip A, Mabrouk AA. Parameters affecting solar still productivity. Energy Conversion & Management 2000;41: 1797e809. [29] Esfahani JA, Rahbar N, Lavvaf M. Utilization of thermoelectric cooling in a portable active solar still - An experimental study on winter days. Desalination 2011;269:198e205. [30] Kabeel AE, Hamed AM, El-Agouz SA. Cost analysis of different solar still configurations. Energy 2010;35:2901e8.

Nomenclature As: area of basin in solar still(m2) AC: annual cost AMC: annual maintenance operational cost of the system ASV: annual salvage value CPL: cost of fresh water($/lit) CRF: capital recovery factor _ Ex input : exergy input in solar still (W) _ evap : exergy output of solar still (W) Ex _ sun ðsolar stillÞ: exergy input from the Sun on solar still (W) Ex _ Ex P:C ðfanÞ: exergy related to power consumption by the fan (W) FAC: fixed annual cost i: interest per year IðtÞS : solar radiation on the inclined glass surface of solar still (W/m2) L: latent heat of vaporization( J/kg) _ ew : hourly yield of solar still (kg/h) m M: average annual yield of solar still (lit) n: number of life years of the system P: capital cost of solar still Ta : ambient temperature( C) Tw: temperature of water( C) TS : temperature of the sun ¼ 6000 K SFF: sinking fund factor