COMPARATIVE ENVIRONMENTAL IMPACT AND SUSTAINABILITY ASSESSMENTS OF HYDROGEN AND COOLING PRODUCTION SYSTEMS

Global Conference on Global Warming (GCGW-2012) 8 12, 2012 Istanbul,Turkey COMPARATIVE ENVIRONMENTAL IMPACT AND SUSTAINABILITY ASSESSMENTS OF HYDROGEN AND COOLING PRODUCTION SYSTEMS T.A.H. Ratlamwala 1,*, I. Dincer 1, M.A. Gadalla 2 1 Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ontario L1H 7K4, Canada 2 American University of Sharjah, Sharjah, U.A.E Emails: tahir.ratlamwala@uoit.ca, ibrahim.dincer@uoit.ca, and mgadalla@aus.edu ABSTRACT In this paper, we study two integrated systems for hydrogen and cooling productions. The first system is a combination of solar PV/T, quadruple effect absorption cooling system and an electrolyzer; while the other is a combination of solar PV/T, quadruple effect absorption cooling system and a steam methane reformer. Detailed exergetic, environmental impact and sustainability assessments are conducted to investigate to find which one of these integrated systems is more environmentally benign. It is noted that the month of in the United Arab Emirates (UAE) is most beneficial from both exergetic and environmental impact point of views for both systems. For the month of, the environmental impact factor, environmental impact coefficient, environmental impact index, environmental impact improvement, exergetic stability factor, and exergetic sustainability factor for the first system are obtained to be 0.78, 4.65, 3.65, 0.27, 0.21, and 0.058, respectively. However for the second system for the month of environmental impact factor, environmental impact coefficient, environmental impact index, environmental impact improvement, exergetic stability factor, and exergetic sustainability factor are found to be 0.93, 14.96, 13.96, 0.07, 0.06, and 0.004, respectively. The results show that the first system becomes much better than the second one from both exergetic and environmental impact perspectives. Keywords: Hydrogen, cooling, environmental impact, energy, exergy, efficiency. INTRODUCTION Since the industrial revolution, humankind s demand for better life style and comfort has considerably increased. In order to meet such their demands more and more energy consuming devices have been introduced. In order to cope up with such an increase in energy demand, a large numbers of power plants running on fossil fuels are commissioned every week throughout the world. This huge energy consumption of fossil fuels is blamed for changes in climate patterns and the depletion of stratospheric ozone layer. In addition, systems running on fossil fuel release harmful gasses such as CO 2, NOx etc., which are not only harmful to living things but also to the environment. Recently environmental friendly fuels are introduced to cater the need of future energy demands. One such very attractive and completely environmental neutral fuel which is looked at is hydrogen. After the harmful effect of using fossil fuels, renewable energy resources are becoming essential options for replacing fossil fuels due to their clean and renewable nature (Sarhaddi et al., 2010). Photo Voltaic/Thermal (PV/T) systems have received greater attention due to their capability of producing both power and heat which makes them more effective as compared to stand alone photo voltaic or thermal system. A major benefit of using solar PV/T system is that they have no operating cost a part from rarely occurring cleaning of panels and duct costs because they use solar intensity as an energy source. Erdil et al. (2008) stated that solar PV/T systems have become very popular due to their capability of producing both heat and power. Ibrahim et al. (2011) mentioned that sustainable energy sources such as solar energy in a form of solar radiation has been identified as one of the promising resources of energy to replace the dependency on other energy carriers such as fossil fuels. Solar PV/T hybrid collectors producing electricity and thermal energy simultaneously have been reported earlier as cost effective collectors (Davidsson et al., 2010). Apart from cost effectiveness, the other major benefit of using solar PV/T system is that it has no by-products and therefore zero emission of green house gasses. Combined power and heat production capabilities of solar PV/T system make them a great contender for multi-generation purposes and have received attention from many researchers such as (Ratlamwala et al., 2011; Beccali et al., 2009). The heat produced by concentrated solar PV/T system can be used to produce cooling using an environmental benign system such as absorption system. Two types of absorption systems are available such as lithium bromide/ water and ammonia/water system. Lithium bromide/water chillers are suited for space cooling applications while ammonia/water systems provide industrial cooling to as low as 50 C (Zhai et al., 2011). Most amount of work is being done on single and double effect absorption systems by researchers [e.g., Tozer et al., 2005; Mathews and Oliviera, 2009]. Some researchers such as (Ratlamwala et al., 2011; Gadalla et al., 2010; Gomri, 2008) have investigated triple effect absorption systems (TEAS). The research conducted by (Ratlamwala et al., 2012a, 2012b) showed that the quadruple effect absorption systems (QEAS) have higher coefficient of performance as compared to TEAS. Kanoglu et al. (2010) stated that the cogeneration option which uses absorption cooling system appeared to provide significant savings in energy requirements. The common method of producing hydrogen using power is the water electrolysis or steam methane reforming (SMR). These technologies are well developed and have reached a mature level for dissociation of 351

water molecules into hydrogen and oxygen molecules. Hydrogen offers capabilities as an alternative fuel which is environmental friendly and sustainable at the same time. As suggested by many researchers ( Barelli et al., 2010; Saeed et al., 2010; Midilli and Dincer, 2009) hydrogen is expected to play a key role in the near future as an energy carrier for sustainable development. In this paper, we study concentrated solar PV/T system integrated with QEAS and electrolyzer or SMR for cooling and hydrogen production. A comparative study is conducted to compare the amount of hydrogen produced, energy and exergy efficiencies, and impact of these systems on the environment and to see how sustainable these systems are from exergy perspective. The effect of monthly weather data of United Arab Emirates (UAE) on the performance of the system from energy, exergy, environment, and sustainable perspective is studied. SYSTEM DESCRIPTION In this paper two integrated systems namely concentrated solar PV/T integrated with multi-stage ammonia cycle and electrolyzer and concentrated solar PV/T integrated with multi-stage ammonia cycle and SMR are studied. The systems studied in this paper are shown schematically in Fig.1. The process starts when solar radiation falls on top of concentrated PV/T system which is made of PV modules and a duct. Solar radiations falling on PV modules make the molecules inside the module vibrate, hence producing power. The air passes through the duct underneath the PV/T system is heated up and leaves the duct to enter the QEAS. In the absorption system heat is supplied to the generator where strong ammonia-water solution is converted to concentrated ammonia vapor and weak solution of ammonia-water as shown in Fig. 2. The weak solution of ammonia-water passes through heat exchanger network where it releases heat to the strong solution going to the generator. The concentrated ammonia vapor passes through several generators, a heat exchanger and a condenser where it releases heat to the incoming fluid in order to drop its temperature. This comparably lowers the temperature of ammonia vapor then enters the evaporator where it provides cooling to the incoming fluid before entering the absorber. In the absorber heat is released by the concentrated ammonia coming from the evaporator and weak ammonia-water solution coming from the generator to leave as strong ammonia-water solution in order to enter a pump. Detailed description of QEAS can be found in (Ratlamwala et al., 2012a). The power produced by the solar PV/T system is supplied to the electrolyzer and SMR for hydrogen production. In the electrolyzer, power is used to break the bond of water molecule in order to produce hydrogen. In the SMR, methane is compressed to increase the pressure and water is pumped to match the pressure of the methane. Methane and steam are then passed through the reformer, shift reactor, and hydrogen separation system as showing in Fig. 3. The hydrogen produced in these systems can later be used to produce heat and power using a Proton Exchange Membrane Fuel Cell (PEMFC). The exhaust gasses produced by SMR are released to the environment. Fig. 1. Schematic of the two systems. 352

Fig. 2. Schematic of SMR. Fig. 3. Schematic of QEAS Solar PV/T system The equations which are used to solve the mathematical model of solar PV/T system are derived from (Joshi et al. 2009a, 2009b). The equation used to calculate power produced by PV module is W η I β τ A (1) The rate of heat produced by solar PV/T system is calculated using equation 2. Q h z I U T T 1 exp (2) where z α τ 1 β h τ β α η (2a) The rate of exergy of solar energy is calculated by E 1. I A (3) The electrical and thermal efficiencies are defined as η η 1 0.0045 T 25 x100 (4) where 353

T τ β α η T T T 1 T (4a) (4b) (4c) η 100 (5) QEAS Unit The rate of heat to the V.HTG of an absorption system is provided using solar PV/T system. The rate of heat transfer obtained from geothermal water source is calculated using Q VHTG = Q (6) The mass balance equations of VHTG are given as follows m 36 x 36 = m 37 x 37 + m 38 x 38 (7) m 36 = m 37 + m 38 (8) In order to obtain the outlet conditions of the VHTG, the following equation is used m 36 h 36 + Q V.HTG = m 37 h 37 + m 38 h 38 (9) The exergy destruction in V.HTG becomes E x E x E x E x (10) where E x m h h T s s And the same relationship is employed for other states. The energy balance equations for VHHX are given below m 35 h 35 + Q VHHX = m 36 h 36 (11) m 38 h 38 = Q VHHX + m 40 h 40 (12) The mass and energy balance equations for the condenser are given below m 9 = m 7 + m 8 (13) m 9 h 9 = m 7 h 7 + m 8 h 8 + Q con (14) The equations for mass and energy balances of the evaporator are m 10 = m 11 (15) m 10 h 10 + Q eva = m 11 h 11 (16) The following energy balance equation is used to calculate the heat rejected from the absorber m 11 h 11 + m 16 h 16 = m 1 h 1 + Q abs (17) The work done by the pump is calculated using the equation given below W p m 1 h 2 h 1 (18) Electrolyzer The rate of hydrogen produced by electrolyzer is calculated using 354

η electrolyzer m H 2 elec LHV H2 W electrolyzer (19) where η electrolyzer = 56% and W electrolyzer = W Steam methane reforming solar. The rate of hydrogen produced by SMR is calculated using η SMR m H 2 SMR LHV H2 W SMR m CH 4 LHV CH4 (20) where η SMR = 56% and W SMR =W solar. The exergetic content of hydrogen is obtained using Ex H 2 m H 2 ex H2,ch ex H2,ph (21) where ex H2,ch 235.153 1000 and ex MW H2,ph h H2 h 0 T 0 s H2 s 0. H2 The exergetic content of supplied methane is obtained using Ex CH 4 m CH 4 ex CH4,ch ex CH4,ph (22) where ex CH4,ch 830.212 1000 and ex MW CH4,ph h H2 h 0 T 0 s H2 s 0. CH4 The overall energy and exergy efficiency of system 1 is found using η en,sys,elec m H 2 elec LHV H2 Q eva 100 (23) I A η ex,sys,elec Ex H 2 elec Ex eva 100 (24) E x solar The overall energy and exergy efficiency of system 2 is found using η en,sys,smr m H 2 SMR LHV H2 Q eva I A m CH 4 LHV CH4 100 (25) η ex,sys,smr Ex H 2 SMR Ex eva E x solar Ex CH 4 100 (26) The environmental impact factor is the positive effect of the system on exergy-based sustainability. By positive effect we mean to supply more desired exergy output and decrease the irreversibilities and minimize the waste exergy outputs during the systems operation. The reference value for this factor should be zero for better exergy based sustainability and is defined as,,, E x solar (27),,, E x solar Ex CH 4 (28) The environmental impact coefficient is related to the exergetic efficiency of the system. In ideal case its value should be one indicating that the system is working under ideal condition with no exergy destruction. This coefficienct is defined as, 1 η ex,sys,elec 100 (29) 355

1 η ex,sys,smr 100 (30) The environmental impact index is an important parameter to indicate whether or not the system damages the environment due to its unusable waste exergy output and exergy destruction. The smaller the value the better the system performance is. It is defined as,,, (31),,, (32) Environmental impact improvement indicates the environmental appropriateness of the system. In order to improve the environmental appropriateness of the system, its environmental impact index should be minimized to be closer to the best reference value. The higher value of environmental impact improvement means system is more usefull for the environment and it is defined as, 1,, 1, (33) (34) The exergetic stability factor is a function of the desired output, exergy destruction, and exergies by unused fuel. In this study it is assumed that all the fuel is utilized in the system. The best value of this factor should be close to one. This factor is defined as,,,,,,,,,,,,,, (35) (36) The exergetic sustainability index is defined as multiplication of environmental benign index and exergetic stability factor of the system. The higher the vallue of this index means better is the performance of the system from exergetic sustianability perspective. This index is defined as,,, (37),,, (38) RESULTS AND DISCUSSION In this paper, a comparative study of concentrated solar PV/T integrated with QEAS and electrolyzer or SMR is carried out. Effect of monthly solar data on the performance of the system from energy, exergy, environmental, and sustainable perspective is studied. Results obtained are compared to see which of the two systems are beneficial from environment and sustainability point of view. Figures 4 and 5 show the average amount of solar radiation and outside air temperature which is available every month in Abu Dhabi in order to analyze the integrated system under different operating conditions. These average values are calculated based on the data available in ASHRAE directory for 2009. Figure 6 illustrates how much hydrogen is produced by system 1 and system 2. Results show that the maximum amount of hydrogen produced by system 1 and system 2 is 364 kl/day and 17037 kl/day, respectively. The highest hydrogen production rate by both systems is obtained in the month of. This is observed because in solar radiation is on the higher side as well as the time for which it is available is highest. The high solar radiation and time for which it is available results in considerably higher amount of power production and that also for a longer time frame as compared to other months. The higher power production rate and longer time results in longer operation of the electrolyzer or SMR. It is also noticed that SMR produces lot more hydrogen as compared to electrolyzer and this is the reason that SMR technology is vastly used for large scale hydrogen production. Figure 7 helps us see the cooling production rate of the QEAS for each month. It is observed that highest amount of cooling is produced in the month of and its value is 210 kw. This is observed because in the month of the outside air temperature is high and is not capable of carrying huge amount of heat rejected by the back surface of the PV/T system and as a result the rate of heat supplied to the QEAS is considerably lower than other months. As the rate of heat supplied to the QEAS for same condenser load decrease its cooling capacity increases because of lower temperature of the concentrated ammonia entering the evaporator. Contradictory to the 356

hydrogen production rate the energy and exergy efficiencies of the electrolyzer system are higher than that of SMR as seen in Figs. 8 and 9. The results show that maximum energy and exergy efficiencies of both electrolyzer and SMR are obtained in the month of and there values are 88.3% and 21.5% and 58.4% and 6.7%, respectively. This shows that although the integrated SMR system produces greater amount of hydrogen but the process is not as effective as integrated electrolyzer system. It is also seen that energy efficiency value is greater than exergy efficiency value indicating that system has losses which are not considered in energy efficiency. The study helps reflecting the importance of exergy analysis. 650 35 Solar Radiation [W/m 2 ] 610 570 530 490 t = 12 h t = 12 h t = 13 h t = 13 h t = 10 h t = 10 h Air Inlet Temperature [ C] 31.5 28 24.5 21 t = 12 h t = 12 h t = 13 h t = 13 h t = 10 h t = 10 h 450 Fig. 4. Average solar radiation per month 17.5 Fig. 5. Average air inlet temperature per month 180 18000 215 150 Left scale Right scale 17000 210 V H,2,electrolyzer [kl/day] 120 90 60 30 16000 15000 14000 13000 VH,2,SMR [kl/day] Qeva [kw] 205 200 195 190 185 0 Fig. 6. Variation in amount of hydrogen produced per month 12000 180 Fig. 7. Variation in amount of cooling produced per month Figure 10 shows how both the systems perform from environmental impact factor perspective. The objective of this factor is to show much exergy of the total exergy is destructed by the system. The results show that minimum environmental impact factor for both electrolyzer system and SMR system is obtained in the month of and its value is 0.7852 and 0.9332, respectively. For SMR system the environmental impact factor hardly changes but for electrolyzer system this factor is seen to be varying. The results show that month of is best for the environment as in this month most amount of exergy supplied to the system is utilized as compared to other months. The environmental impact coefficient shows how much environmentally benign the system is by taking the inverse of its exergy efficiency. The results show that minimum environmental impact coefficient for both electrolyzer system and SMR system is obtained in the month of and its value is 4.655 and 14.96, respectively as shown in Fig. 11. This shows that the electrolyzer system is closer to the ideal value of 1 of environmental impact coefficient as compared to SMR system hence indicating that SMR system performs worst from environment point of view. 357

100 100 25 6.7 90 80 Left scale Right scale 90 80 20 Left scale Right scale 6.69 en,sys,elec [%] 70 60 50 40 30 70 60 50 40 30 en,sys,smr [%] ex,sys,elec [%] 15 10 6.68 6.67 ex,sys,smr [% ] 20 20 5 6.66 10 0 Fig. 8. Variation in energy efficiency per month 10 0 0 Fig. 9. Variation in exergy efficiency per month 6.65 The third environmental parameter studied is environmental impact index as shown in Fig. 12. This index helps us visualize weather any of the system damages the environment due to their waste exergy output and exergy destruction and in ideal case this index should approach 0. The results show that minimum environmental impact index for electrolyzer system and SMR system is obtained for the month of and its value is 3.655 and 13.96, respectively. This shows that electrolyzer system has lower amount of waster exergy output and exergy destruction as compared to SMR system. This makes sense because SMR system has harmful by-products such as carbon monoxide and carbon dioxide as compared to zero by-products of electrolyzer system. The comparison of both the systems from the environmental impact improvement perspective is shown in Fig. 13. The environmental impact improvement indicates the environmental appropriateness of the system and it is desirable to get this factor as high as possible. The study reveals that maximum environmental impact improvement for electrolyzer system and SMR system is obtained in the month of and its value is 0.2736 and 0.07163, respectively. The environmental impact improvement values show that electrolyzer system is more appropriate to the environment as compared to SMR system. The exergetic stability factor helps us realize how much stable the system is from exergy perspective. The comparison done on monthly basis as shown in Fig. 14 reveals that the highest value of exergy stability factor for electrolyzer system and SMR system is obtained in the month of and it value is 0.2148 and 0.06684, respectively. The desired value of exergetic stability factor is 1 which indicates that system is 100% stable from exergy perspective. The results show that electrolyzer system is far more stable from exergy perspective than SMR system because of lower overall exergy destruction and better utilization of input exergy. The last parameter studied is exergetic sustainability index as shown in Fig. 15. The purpose of this index is to show how much sustainable the system is from exergy perspective and it is desired to have high value of this index. Analysis conducted show that maximum value of exergetic sustainability index for electrolyzer system and SMR system is obtained in the month of and its value is 0.05878 and 0.004787, respectively. This index reveals that electrolyzer system is far more sustainable than SMR system. Environmental impact factor 0.96 0.92 0.88 0.84 0.8 0.76 Fig. 10. Variation in environmental impact factor per month Environmental impact coefficient 16 14 12 10 8 6 4 Fig. 11. Variation in environmental impact coefficient per month 358

Enivronmental impact index 16 14 12 10 8 6 4 2 Fig. 12. Variation in environmental impact index per month Environmental impact improvement 0.3 0.25 0.2 0.15 0.1 0.05 Fig. 13. Variation in environmental impact improvement per month Exergetic stability factor 0.24 0.2 0.16 0.12 0.08 Exergetic sustainability index 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.04 Fig. 14. Variation in exergetic stability factor per month 0 Fig. 15. Variation in exergetic sustainability index per month CONCLUSIONS In this paper, a comparative study of concentrated solar PV/T, QEAS and electrolyzer system and concentrated solar PV/T, QEAS and SMR system is presented. The comparison between these two systems is performed, based on the basis of energy, exergy, environment and sustainability performance criteria. The results show that although the SMR system produces greater amount of hydrogen as compared to electrolyzer system but electrolyzer system has higher energy and exergy efficiency. The environmental impact assessment shows that the electrolyzer system is more environmentally benign than SMR system and it perform best in the month of. The sustainability study also shows that electrolyzer system is more sustainable than SMR system from exergy perspective and its best value is obtained in the month of. The study concludes that although SMR system produces far more hydrogen than electrolyzer system but it fails to perform better than electrolyzer system from efficiencies, environmental and sustainable perspective. NOMENCLATURE A Area of PV module, m 2 b Breadth of PV module, m E Energy rate, kw E x Exergy rate, kw h Specific enthalpy, kj/kg h Heat transfer coefficient from black surface to air, W/m 2 K h Heat transfer coefficient from back surface to air through glass, W/m 2 K h Penalty factor due to presence of solar cell material, glass and EVA for glass to glass PV/T system, W/m 2 K 359

h Penalty factor due to presence of interface between glass and working fluid through absorber plate for glass to glass PV/T system, W/m 2 K Incident solar intensity, W/m 2 Length of the PV module, m LHV Lower heating value m Mass flow rate, kg/s 1 MW Molecular weight, kg/kmol P Power produced by PV/T Q Heat transfer rate, kw T Temperature, K U Overall heat transfer coefficient from bottom to ambient, W/m 2 K U Overall heat transfer coefficient from solar cell to ambient through top and back surface of insulation, W/m 2 K U Overall heat transfer coefficient from solar cell to ambient through glass cover, Wm 2 K U Overall heat transfer coefficient from glass to black surface through solar cell, W m 2 K W Work rate, kw x Concentration of ammonia-water Greek letters Absorptivity of solar cell Absorptivity of black surface Packing factor of solar cell η Efficiency Transitivity of glass θ Index Subscripts a Air ai Air inlet abs Absorber bs Back surface of PV/T c Solar cell C coefficient ch Chemical CHX Condenser heat exchanger con Condenser f Factor elec Electrolyzer ei Exergetic impact eii Exergetic impact improvement en Energy es Exergetic stability est Exergetic sustainability ex Exergy eva Evaporator G Subscript for glass to glass PV/T system geo Geothermal HTG High temperature generator HHX High temperature heat exchanger H Hydrogen input LHX Low temperature heat exchanger LTG Low temperature generator MTG Medium temperature generator MHX Medium temperature heat exchanger ph Physical sys System V.HTG Very high temperature generator V.HHX Very high temperature heat exchanger 1...33 State numbers 0 Ambient or reference condition 360

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