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Gas turbine waste heat driven multiple effect absorption system
Heat Recovery Systems & CHP Vol. 11, No. 5, pp. 407-413, 1991 Printed in Great Britain
0890-4332/91 S3.00+ .00 Pergamon Press plc
GAS TURBINE WASTE HEAT DRIVEN MULTIPLE EFFECT ABSORPTION SYSTEM S. E. ALY Mechanical Engineering Dcpt, King Alxtulaziz University, P.O. Box 9027, Jeddah 21413, Sandi Arabia (Received in revised form 15 November 1990) Abstract--In this article, two arrangements of the multiple effect absorption (MEA) type are presented. They are using LiBr-H20 and are powered by the exhaust of gas turbines. The first arrangement (MEA-I) is used as a cooling device and is coupled to an engine that drives a VC cooling unit. The other one (MEA-II) is used as a solution concentration machine and is coupled to an engine that drives a RO unit. Thermodynamic analysis for MEA-I showed a COP, of 1.31 and 2.18 for evaporation temperatures of 5°C and 14°C respectively. Relative to the VC, the MEA-I increased the cooling capacity by 65% and 77% with payback periods of 35 months and 29 months for evaporation temperatures of 5°C and 14°C respectively. The MEA-II is analyzed using sea water as an example for a water based solution. Relative to a gas turbine driven RO desalination unit the MEA-II increased the fresh water produced by 13.5%. As a solution concentration machine the MEA-II requires 28% of the heat of evaporation with a solution top temperature of 36°C which makes it equivalent to a machine with five evaporators, where the solution top temperature reaches 66°C. Based on selling the released solvent byproduct water alone, the MEA-II wouid have a payback period of 77 months.
INTRODUCTION
The thermal efficiency of gas turbine simple cycles is around 30% [1]. However, users continue to find it more economical to employ such engines because of the various possibilities in which they can be integrated into the varied energy requirements of industrial applications. Cogeneration is one example in which the engine produces shaft power while the exhaust is utilized for steam generation [2, 3]. Energy recovery from the engine exhaust could be realized by various arrangements such as firing a reversed absorption power cycle [4], powering a multiple effect distillation system [5], driving an absorption vapour compression distillation plant [6], generating shaft power for a sea water RO unit [7], or boosting the engine shaft power capacity [8]. In cooling applications, for instance, the exhaust energy could be utilized in firing thermally driven air conditioning units such as absorption machines and desiccant dehumidifiers. Knowing that the first of these demand higher firing temperatures than the second, then the heat recovery could be improved by firing the first by the exhaust gas and powering the second by the heat rejected from the first one [9]. Performance of the cooling systems is usually expressed in terms of the coefficient of performance (COP). The thermal COP (COPt) is defined as the ratio of the cooling effect generated to the thermal energy supplied to the system. In LiBr-H20 absorption units, for a chilled water of 7°C, the COPt is 0.7 and 1.25 for single effect and double effect units respectively [10]. The vapour compression (VC) cooling units, on the other hand, are more efficient in terms of the COPt. If a VC powered by an engine, with 28% thermal efficiency for example, the COPt would be 1.0 and 1.35 for refrigerant evaporation temperatures of 5°C and 15°C, respectively [11]. In the future, VC systems could face difficulties due to the pollution caused by the refrigerants they are using. Actually, absorption units can replace the VCs provided that they have a comparable COlt and their costs (capital and operating) are acceptable. Hence, an improvement in the absorption unit's COPt is appreciated, especially if it is done by utilizing the exhaust of a gas turbine in a total energy cooling system. This work presents data on a LiBr-H20 multiple effect absorption (MEA) system fired by the exhaust of a gas turbine. Two MEAs are analyzed for two different applications, the MEA-I which operates as a cooling system and the MEA-II that operates as a solution concentration system. In each application the MEA is analyzed for a single product gas turbine total energy system. ,RS 11/5---F
407
408
S.E. ALY
MEA-I COOLING SYSTEM This is shown schematically in Fig. 1, for an evaporator temperature of 14°C. It comprises three effects connected in series with an absorber, an end condenser and an evaporator. The analyzed total energy cooling system comprises a VC unit driven by a gas turbine whose exhaust (Qg) fires the generator of the first effect Gen-I. The distilled refrigerant (state 4) fires Gen-II and then goes to the flashing tank (FT) where the flashed vapour joins the vapour at state 7 and both fire Gen-III. Thus the condenser receives the FT effluent plus the vapour and the condensate downstream of Gen-III. From the condenser, via a throttling valve, the refrigerant reaches the evaporator. The strong solution leaving the generators (state 11) absorbs the vapour generated in the evaporator (state 12). Heat of absorption is carried away by the cooling water and the resulting weak solution of LiBr-H20 repeats the cycle. Each generator is connected to a circulation pump and a regenerative heat exchanger. The three generators are operating at similar solution concentrations. Thus the circulation mass ratios, in kg LiBr-H20 per kg vapour from the generator in each effect, are maintained uniform throughout the system. The MEA-I system is analyzed for two evaporation temperatures namely for 5°C and 14°C. The system is fired by the exhaust of the engine that drives the VC unit. The engine is a Makila T.I. type [12] with a rated power of 1200 kW, 28% efficiency and 5.5 kg/s exhaust at 525°C.
MEA -I performance The MEA-I arrangement for evaporation temperature of 14°C is shown on the equilibrium chart of the LiBr-H20 solution in Fig. 2. The solution concentrations, by weight, are maintained at 52.5% and 55.5% for the absorber and the generators, respectively. Throughout the analysis equilibrium conditions are assumed to prevail and state properties are obtained from the literature [13]. Although it is not shown on the equilibrium chart the released vapour in any generator is superheated at the generator temperature and its equilibrium pressure. The exhaust fires Gen-I at a pinch point of 20°C and a terminal temperature difference of 5°C is assumed for the heat exchangers. For the stated concentrations in Fig. 2 the mass circulation ratios are estimated as 18.5 and 17.5 for the absorber and the generators respectively. Energy conservation yields a heat demand (Qg) of 2866 kJ/kg refrigerant released in Gen-I. The overall mass and energy conservations for the MEA-I give the refrigerant mass flow rate to the evaporator i st effect
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as 107 kg/min. Thus the MEA-I cooling capacity would be 1207 ton refrigeration (TR) compared to 1568 TR capacity for the VC unit. The heat loads for the condenser (36°C) and the absorber (40°C) are estimated as 1575 kW and 4620 kW per unit refrigerant mass flow rate released in Gen-I. These loads are handled by the cooling water that enters the condenser at 30°C. The COPt of the MEA-I is estimated as 2.18 for the considered evaporation temperature of 14°C. As far as the cooling capacity is concerned MEA-I represents 77% of that for the VC unit. In other words, this is equivalent to increasing the cooling system overall thermal efficiency from that of the engine of 28% to 49.5%. When the MEA-I is analyzed for a reduced evaporation temperature of 5°C the number of generators will be two. In this case the COPt is 1.31 which differs slightly from the estimated 1.25 for the double effect absorption machine of ref. [10]. This difference could be attributed to the difference in the temperatures of the source and sink employed in the analysis of each case. The MEA-I cooling capacity, for 5°C evaporator, amounted to 771 TR while that of the engine driven VC unit is 1194 TR for similar evaporator conditions. Thus the MEA-I would increase the cooling capacity by 64.5% which means increasing the overall thermal efficiency of the cooling system from that of the engine at 28% to 46%. As far as the availability of the exhaust energy is concerned, the MEA-I utilizes 46% and 43 % of the exhaust energy for evaporation temperatures of 5°C and 14°C, respectively. Also the MEA-I recovers 67% and 66% of the maximum (reversible) exhaust energy for evaporation temperatures of 5°C and 14°C, respectively. Comparison of the MEA-I performance for the two evaporation temperatures considered in the present analysis is shown in Table 1. On the economy side, the additional capital costs required for the MEA-I are estimated as $360,000 and $260,000 for evaporation temperatures of 14°C and 5°C, respectively. Installation and sundries expenses are assumed as 50% and 10% of the capital cost, respectively. The selling price per kWh cooling is estimated as $0.09/kWh based on an electric energy cost of $0.06/kWh. The annual costs for operation and maintenance (O&M) are assumed as 10% of the capital cost each. The interest rate and inflation rate are 15% and 10%, respectively. The economical analysis showed the expected payback periods for the MEA-I as 29 months and 35 months for evaporation temperatures of 14°C and 5°C, respectively. Table 2 shows a summary for the economical analysis of the MEA-I employed for the cooling purposes.
410
S.E. ALv Table 1. System performance* MEA-I Evaporation temperature (°C) Exhaust gas (°C) Heating water (°C) No. effects Generator I (°C) Pinch point (°C) MEA capacity (TR) MEA C O P t MEA/VC (TR/TR) (%) Water capacity m3/d MEA/RO (%) Overall thermal efficiency (%)
5 525 -2 144 20 771 1.31 64.5 --46
MEA-II
5 [10] -140 2 ---1.25 -----
14 525 -3 167 20 1207 2.18 77 --49.5
30 525 -5 135 20 ---268 13.5 32
*Employing a gas turbine, 28% efficiency, 1200 kW rated power.
MEA-II SOLUTION C O N C E N T R A T I O N SYSTEM solution concentration process could be either an independent process or a step in the drying industry. Evaporation is the common technique used to concentrate the solutions where heat boils it to release the solvent and the concentrated brine is obtained as a solid suspension or a slurry [14, 15]. In processing the milk, for example, it is never dried in its original form due to its high water content of about 90%. As a first step, the milk is evaporated to a solid concentration of 45-52%, by weight, in a multiple effect evaporator before the final drying process takes place. Usually, as in most water based solutions, the moisture released during the concentration process is discharged to the atmosphere. Moreover, evaporation of a thermally sensitive solution may require to heat it up to a temperature level that could affect the value of the product. The concentration process could also be carried out using a hot air stream as a moisture carrier and the process is controlled by the humidity and the temperature of the air stream. Thus the process energy demands will include the heat of evaporation, the sensible heating for the released moisture as well as the air stream up to the discharge conditions, and losses. Using heated outside air for that purpose might be successful in dry climatic conditions where humidity ratio is low. However, this will be difficult in humid areas, such as Jeddah, Saudi Arabia, where the humidity ratio reaches 0.024 kg H20/kg dry air. The solution concentration system, MEA-II, is intended to overcome some of the difficulties encountered in concentrating the water based solutions particularly those thermally sensitive ones. This is shown schematically in Fig. 3 as a five effect LiBr-H20 absorption machine. It comprises five generators, five regenerative heat exchangers, five circulation pumps, three intermediate flashing tanks (FT), an end condenser, one absorber and one process solution flashing tank (PSFT). The general arrangement of MEA-II is similar to that of the MEA-I of Fig. 1 except that the evaporator is replaced by the PSFT which is connected to the absorber. The pressure inside the PSFT is dictated by the equilibrium conditions in the absorber. Moreover, the MEA-II operates in an open cycle whereas the MEA-I operates in a closed cycle. The FTs are used to increase the heat recovery from the condensed vapours and are acting as pressure reducing devices between the successive generators which are operating at similar LiBr-H20 concentrations. The condensate, from the condenser, is collected and is pumped out of the system as a byproduct fresh water. Table 2. System economy* MEA-I Evaporation temperature (°C) Capital cost ($) Installation ($) Sundries ($) Annual operation ($) Annual maintenance ($) Annual interest (%) Inflation (%) Payback (month)
*Employing a gas turbine, 28% ~
5 260,000 130,000 26,000 26,000 26,000 15 10 35 ,
14 360,000 180,000 36,000 36,000 36,000 15 10 29
MEA-II 30 530,000 265,000 53,000 53,000 53,000 15 10
77
1200kW rated power.
Multiple effect absorption system
411
MEA-II performance The MEA-II is used to concentrate sea water, as an example of water based solutions, to produce a concentrated brine and a fresh water byproduct. The system is fired by the exhaust of a gas turbine that drives an energy efficient fresh water producing facility such as a sea water reverse osmosis (RO) unit. Hence, the system shown in Fig. 3 is fired by the exhaust of a 28% efficiency engine similar to that used with the MEA-I at a 20°C pinch point. A terminal temperature difference of 5°C is allowed for the heat exchangers while the conditions in both the condenser and the absorber are maintained similar to these of the MEA-I in Fig. 1. The difference of the working fluid concentrations, between the absorber and the generators, is constant and equal to 3%. The conditions in the MEA-II are as shown on the LiBr-H20 equilibrium chart in Fig. 4. Care should be taken to minimize the potential for contamination of the water collected down-stream of the condenser, by the working fluid and the working fluid by the sea water in the PSFT. Theoretically speaking there should be no contamination of the vapour produced in the generators by the LiBr since it wouldn't evaporate at the generators conditions. However, there might be some carry over of the working fluid with the released water vapour. Such possible contamination of the collected water downstream of the condenser could be avoided by means of a proper design for the generators, using a high quality demister and passing the vapour through a drying bed. In extreme cases, however, the byproduct water could be used for applications other than direct human consumption. At the PSFT, sea water distillation is accomplished by flash evaporation. Thus contamination of the working fluid, in the absorber, by the sea water carry over could be minimized through a careful design of the flashing chamber and employing a high quality demister. The power consumption of the RO, with 25% allowance for auxilliaries, is estimated as 14.3 kWh/m 3 fresh water. With an engine power of 1200 kW the RO at a 30% recovery ratio would reject 55 kg/s of the pretreated pressurized brine. This is mixed with the sea water and is used as the cooling water (state 18, Fig. 3) which is preheated in the condenser. This is heated further in the absorber where its temperature reaches 35°C upstream of the PSFT where 75% of it is processed and the balance is rejected. The flashed vapour in the PSFT (saturation temperature of 30°C) goes to the absorber where it is absorbed by the strong LiBr-H~O solution (state 17, Fig. 3) and it is finally collected downstream of the condenser as a byproduct fresh water. It is worth mentioning that the PSFT acts as a cooling tower for the process sea water and the concentrated brine leaves the tank at 30°C. Thus the concentration ratio of the brine could be increased by recirculation provided that the boiling point elevation is accounted for. I |~ Effect
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The MEA-II is thermodynamically analyzed for a case study, Fig. 4, where the temperatures are considered as 135°C, 36°C, 40°C asnd 30°C in Gen-I, the condenser, the absorber and the PSFT respectively. The engine exhaust gas temperature reaches 155°C downstream of Gen-I which is comparable to a stack level for no low temperature corrosion conditions. The circulation mass ratios in the system are estimated as 13.6 and 12.6 for the absorber and the generators respectively. Energy conservation yields the heat input (Qg) as 2130 kJ/kg water vapour distilled in Gen-I. This is 47% of the exhaust availability and 66.2% of the maximum recoverable (reversible) exhaust energy. Overall mass and energy conservations showed that the collected byproduct fresh water could amount to 11.18 m3/h at a specific energy input of 685 kJ/kg water (about 28% of the heat of evaporation). In comparison with the engine driven RO unit the MEA-II increased the fresh water produced by 13.5% (see Table 1). Based on electricity price of $0.06/kWh, the energy cost for the water produced by the RO would be $0.86/m 3 which represents about 30% of the water selling price. Thus, selling the byproduct water of the MEA-II would yield an annual return of $254,000. The additional capital cost for the system is estimated as $530,000. Installation costs are assumed as 50% of the capital invested while the O&M costs are 10% of the capital each. The interest and inflation rates are 15% and 10°/0, respectively. Economical analysis based on selling the byproduct water showed a payback period of 77 months for the MEA-II (see Table 2). Actually the MEA-II is equivalent to a multiple effect evaporation unit where the process solution top temperature reaches 66°C compared with 35°C in the MEA-II. Heat demand per kg water produced in the MEA-II is higher than that of the conventional sea water distillation systems. However, the MEA-II requires no pretreatment, no chemical treatment for scales control and would create less corrosion problems. When the system is used to concentrate solutions, other than sea water, such as milk for example, the solution could be preheated in a separate pass in the condenser and/or the absorber. In this case the solvent separation in the PSFT takes place near the ambient temperature which keeps the value of the product.
Multiple effect absorption system
413
CONCLUSIONS
Two arrangements of the MEA type are investigated utilizing the exhaust energy of a gas turbine. The MEA-I is employed for cooling purposes whereas the MEA-II is used as a water based solution concentration machine. Both cases are analyzed using the exhaust of 1200 kW, 28% efficiency gas turbine at 525°C. The MEA-I is studied at two specific evaporation temperatures of 5°C and 14°C where it showed a COlt of 1.31 and 2.18, respectively. When the gas turbine is used to drive a VC cooling unit the percentage increase in the cooling capacity, relative to the VC, reached 65 and 77 for evaporation temperatures of 5°C and 14°C, respectively. Thus the overall thermal efficiency of the VC/MEA-I system would reach 46% and 49.5% for evaporation temperatures of 5°C and 14°C, respectively with corresponding payback periods for the MEAd of 35 months and 29 months respectively. The MEA-II, for solution concentration, has a PSFT where the solution is processed and the generated flashed vapour is finally collected as a byproduct fresh water. This arrangement could be useful for concentrating thermally sensitive materials. As a case study the MEA-II used the sea water as an example for the water based solutions. The system is fired by the exhaust of an engine that drives an RO unit. With the PSFT at 30°C the MEA-II produced 268 m3/day of fresh water. This is 13.5% of the RO capacity. By selling only the produced water from the MEA-II this would have a payback period of 77 months. One of the main advantages of the MEA-II is that the solution concentration process takes place at near ambient temperature which would keep the value of the concentrated product. REFERENCES 1. H. Cohen, G. Rogers and H. Saravanamuttoo, Gas Turbine Theory, 3rd edn. Longman Scientific and Technical U.K. (1987). 2. R. Allen and J. Kovacik, Gas turbine cogeneration-principles and practice. J. Engng Gas Turbines and Power 106, 725-730 (1984). 3. G. Cerri and A. Colage, Steam cycle regeneration influence on combined gas-steam power plant performance. J. Engng Gas Turbines and Power 107, 574-581 (1985). 4. A. Kallina, Combined cycle system with novel bottoming cycle, d. Engng Gas Turbines and Power 106, 737-742 (1984). 5. S. E. Aly, Gas turbine waste heat recovery distillation system. Heat Recovery Systems & Clip 7, 375-382 (1987). 6. S. E. Aly, Vapour compression distillation waste heat absorption systems. Desalination 68, 57--68 (1988). 7. S. E. Aly, Reverse osmosis desalination by waste heat absorption power cycle. J. Institute Energy, 33-37 (1988). 8. S. E. Aly, Efficient part load gas turbine arrangements. J. Institute Energy, 31-38 (1990). 9. S. E. Aly and K. Fathalah, Combined absorption-desiccant solar powered air conditioning system. Warme-und Stoffubertragung 23, 111-121 (1988). I0. J. Mitchel, State of the art of active solar cooling, Proc. 1986 Annual Meeting, American Solar Energy Society, Inc. Boulder Colorado, 59-71, 11-14 June (1986). 11. J. Howell, Active hybrid solar cooling systems, NATO, ASI Series, Applied Sciences, number 129, pp. 388--408. Martinus Nijhoff Publishers (1988). 12. V. deBiasi, Turbomeca introducing high efficiency Makila-T. I. for commerdial applications. Gas Turbine World 18, 22-26 (1988). 13. ASHRAE, Fundamentals Handbook. Atlanta, GA, U.S.A. (1981). 14. R. Keey, Introduction to Industrial Drying Operations. William Clows & Sons Limited (1978). 15. A. Williams-Gardner, Industrial Drying. Gulf Publishing Company (1977).
0890-4332/91 S3.00+ .00 Pergamon Press plc
GAS TURBINE WASTE HEAT DRIVEN MULTIPLE EFFECT ABSORPTION SYSTEM S. E. ALY Mechanical Engineering Dcpt, King Alxtulaziz University, P.O. Box 9027, Jeddah 21413, Sandi Arabia (Received in revised form 15 November 1990) Abstract--In this article, two arrangements of the multiple effect absorption (MEA) type are presented. They are using LiBr-H20 and are powered by the exhaust of gas turbines. The first arrangement (MEA-I) is used as a cooling device and is coupled to an engine that drives a VC cooling unit. The other one (MEA-II) is used as a solution concentration machine and is coupled to an engine that drives a RO unit. Thermodynamic analysis for MEA-I showed a COP, of 1.31 and 2.18 for evaporation temperatures of 5°C and 14°C respectively. Relative to the VC, the MEA-I increased the cooling capacity by 65% and 77% with payback periods of 35 months and 29 months for evaporation temperatures of 5°C and 14°C respectively. The MEA-II is analyzed using sea water as an example for a water based solution. Relative to a gas turbine driven RO desalination unit the MEA-II increased the fresh water produced by 13.5%. As a solution concentration machine the MEA-II requires 28% of the heat of evaporation with a solution top temperature of 36°C which makes it equivalent to a machine with five evaporators, where the solution top temperature reaches 66°C. Based on selling the released solvent byproduct water alone, the MEA-II wouid have a payback period of 77 months.
INTRODUCTION
The thermal efficiency of gas turbine simple cycles is around 30% [1]. However, users continue to find it more economical to employ such engines because of the various possibilities in which they can be integrated into the varied energy requirements of industrial applications. Cogeneration is one example in which the engine produces shaft power while the exhaust is utilized for steam generation [2, 3]. Energy recovery from the engine exhaust could be realized by various arrangements such as firing a reversed absorption power cycle [4], powering a multiple effect distillation system [5], driving an absorption vapour compression distillation plant [6], generating shaft power for a sea water RO unit [7], or boosting the engine shaft power capacity [8]. In cooling applications, for instance, the exhaust energy could be utilized in firing thermally driven air conditioning units such as absorption machines and desiccant dehumidifiers. Knowing that the first of these demand higher firing temperatures than the second, then the heat recovery could be improved by firing the first by the exhaust gas and powering the second by the heat rejected from the first one [9]. Performance of the cooling systems is usually expressed in terms of the coefficient of performance (COP). The thermal COP (COPt) is defined as the ratio of the cooling effect generated to the thermal energy supplied to the system. In LiBr-H20 absorption units, for a chilled water of 7°C, the COPt is 0.7 and 1.25 for single effect and double effect units respectively [10]. The vapour compression (VC) cooling units, on the other hand, are more efficient in terms of the COPt. If a VC powered by an engine, with 28% thermal efficiency for example, the COPt would be 1.0 and 1.35 for refrigerant evaporation temperatures of 5°C and 15°C, respectively [11]. In the future, VC systems could face difficulties due to the pollution caused by the refrigerants they are using. Actually, absorption units can replace the VCs provided that they have a comparable COlt and their costs (capital and operating) are acceptable. Hence, an improvement in the absorption unit's COPt is appreciated, especially if it is done by utilizing the exhaust of a gas turbine in a total energy cooling system. This work presents data on a LiBr-H20 multiple effect absorption (MEA) system fired by the exhaust of a gas turbine. Two MEAs are analyzed for two different applications, the MEA-I which operates as a cooling system and the MEA-II that operates as a solution concentration system. In each application the MEA is analyzed for a single product gas turbine total energy system. ,RS 11/5---F
407
408
S.E. ALY
MEA-I COOLING SYSTEM This is shown schematically in Fig. 1, for an evaporator temperature of 14°C. It comprises three effects connected in series with an absorber, an end condenser and an evaporator. The analyzed total energy cooling system comprises a VC unit driven by a gas turbine whose exhaust (Qg) fires the generator of the first effect Gen-I. The distilled refrigerant (state 4) fires Gen-II and then goes to the flashing tank (FT) where the flashed vapour joins the vapour at state 7 and both fire Gen-III. Thus the condenser receives the FT effluent plus the vapour and the condensate downstream of Gen-III. From the condenser, via a throttling valve, the refrigerant reaches the evaporator. The strong solution leaving the generators (state 11) absorbs the vapour generated in the evaporator (state 12). Heat of absorption is carried away by the cooling water and the resulting weak solution of LiBr-H20 repeats the cycle. Each generator is connected to a circulation pump and a regenerative heat exchanger. The three generators are operating at similar solution concentrations. Thus the circulation mass ratios, in kg LiBr-H20 per kg vapour from the generator in each effect, are maintained uniform throughout the system. The MEA-I system is analyzed for two evaporation temperatures namely for 5°C and 14°C. The system is fired by the exhaust of the engine that drives the VC unit. The engine is a Makila T.I. type [12] with a rated power of 1200 kW, 28% efficiency and 5.5 kg/s exhaust at 525°C.
MEA -I performance The MEA-I arrangement for evaporation temperature of 14°C is shown on the equilibrium chart of the LiBr-H20 solution in Fig. 2. The solution concentrations, by weight, are maintained at 52.5% and 55.5% for the absorber and the generators, respectively. Throughout the analysis equilibrium conditions are assumed to prevail and state properties are obtained from the literature [13]. Although it is not shown on the equilibrium chart the released vapour in any generator is superheated at the generator temperature and its equilibrium pressure. The exhaust fires Gen-I at a pinch point of 20°C and a terminal temperature difference of 5°C is assumed for the heat exchangers. For the stated concentrations in Fig. 2 the mass circulation ratios are estimated as 18.5 and 17.5 for the absorber and the generators respectively. Energy conservation yields a heat demand (Qg) of 2866 kJ/kg refrigerant released in Gen-I. The overall mass and energy conservations for the MEA-I give the refrigerant mass flow rate to the evaporator i st effect
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Fig. 1. Multiple effect absorption cooling scheme (MEA-I).
Multiple effect absorption system
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t 60
Fig. 2. Multiple effect absorption cooling diagram (MEA-I).
as 107 kg/min. Thus the MEA-I cooling capacity would be 1207 ton refrigeration (TR) compared to 1568 TR capacity for the VC unit. The heat loads for the condenser (36°C) and the absorber (40°C) are estimated as 1575 kW and 4620 kW per unit refrigerant mass flow rate released in Gen-I. These loads are handled by the cooling water that enters the condenser at 30°C. The COPt of the MEA-I is estimated as 2.18 for the considered evaporation temperature of 14°C. As far as the cooling capacity is concerned MEA-I represents 77% of that for the VC unit. In other words, this is equivalent to increasing the cooling system overall thermal efficiency from that of the engine of 28% to 49.5%. When the MEA-I is analyzed for a reduced evaporation temperature of 5°C the number of generators will be two. In this case the COPt is 1.31 which differs slightly from the estimated 1.25 for the double effect absorption machine of ref. [10]. This difference could be attributed to the difference in the temperatures of the source and sink employed in the analysis of each case. The MEA-I cooling capacity, for 5°C evaporator, amounted to 771 TR while that of the engine driven VC unit is 1194 TR for similar evaporator conditions. Thus the MEA-I would increase the cooling capacity by 64.5% which means increasing the overall thermal efficiency of the cooling system from that of the engine at 28% to 46%. As far as the availability of the exhaust energy is concerned, the MEA-I utilizes 46% and 43 % of the exhaust energy for evaporation temperatures of 5°C and 14°C, respectively. Also the MEA-I recovers 67% and 66% of the maximum (reversible) exhaust energy for evaporation temperatures of 5°C and 14°C, respectively. Comparison of the MEA-I performance for the two evaporation temperatures considered in the present analysis is shown in Table 1. On the economy side, the additional capital costs required for the MEA-I are estimated as $360,000 and $260,000 for evaporation temperatures of 14°C and 5°C, respectively. Installation and sundries expenses are assumed as 50% and 10% of the capital cost, respectively. The selling price per kWh cooling is estimated as $0.09/kWh based on an electric energy cost of $0.06/kWh. The annual costs for operation and maintenance (O&M) are assumed as 10% of the capital cost each. The interest rate and inflation rate are 15% and 10%, respectively. The economical analysis showed the expected payback periods for the MEA-I as 29 months and 35 months for evaporation temperatures of 14°C and 5°C, respectively. Table 2 shows a summary for the economical analysis of the MEA-I employed for the cooling purposes.
410
S.E. ALv Table 1. System performance* MEA-I Evaporation temperature (°C) Exhaust gas (°C) Heating water (°C) No. effects Generator I (°C) Pinch point (°C) MEA capacity (TR) MEA C O P t MEA/VC (TR/TR) (%) Water capacity m3/d MEA/RO (%) Overall thermal efficiency (%)
5 525 -2 144 20 771 1.31 64.5 --46
MEA-II
5 [10] -140 2 ---1.25 -----
14 525 -3 167 20 1207 2.18 77 --49.5
30 525 -5 135 20 ---268 13.5 32
*Employing a gas turbine, 28% efficiency, 1200 kW rated power.
MEA-II SOLUTION C O N C E N T R A T I O N SYSTEM solution concentration process could be either an independent process or a step in the drying industry. Evaporation is the common technique used to concentrate the solutions where heat boils it to release the solvent and the concentrated brine is obtained as a solid suspension or a slurry [14, 15]. In processing the milk, for example, it is never dried in its original form due to its high water content of about 90%. As a first step, the milk is evaporated to a solid concentration of 45-52%, by weight, in a multiple effect evaporator before the final drying process takes place. Usually, as in most water based solutions, the moisture released during the concentration process is discharged to the atmosphere. Moreover, evaporation of a thermally sensitive solution may require to heat it up to a temperature level that could affect the value of the product. The concentration process could also be carried out using a hot air stream as a moisture carrier and the process is controlled by the humidity and the temperature of the air stream. Thus the process energy demands will include the heat of evaporation, the sensible heating for the released moisture as well as the air stream up to the discharge conditions, and losses. Using heated outside air for that purpose might be successful in dry climatic conditions where humidity ratio is low. However, this will be difficult in humid areas, such as Jeddah, Saudi Arabia, where the humidity ratio reaches 0.024 kg H20/kg dry air. The solution concentration system, MEA-II, is intended to overcome some of the difficulties encountered in concentrating the water based solutions particularly those thermally sensitive ones. This is shown schematically in Fig. 3 as a five effect LiBr-H20 absorption machine. It comprises five generators, five regenerative heat exchangers, five circulation pumps, three intermediate flashing tanks (FT), an end condenser, one absorber and one process solution flashing tank (PSFT). The general arrangement of MEA-II is similar to that of the MEA-I of Fig. 1 except that the evaporator is replaced by the PSFT which is connected to the absorber. The pressure inside the PSFT is dictated by the equilibrium conditions in the absorber. Moreover, the MEA-II operates in an open cycle whereas the MEA-I operates in a closed cycle. The FTs are used to increase the heat recovery from the condensed vapours and are acting as pressure reducing devices between the successive generators which are operating at similar LiBr-H20 concentrations. The condensate, from the condenser, is collected and is pumped out of the system as a byproduct fresh water. Table 2. System economy* MEA-I Evaporation temperature (°C) Capital cost ($) Installation ($) Sundries ($) Annual operation ($) Annual maintenance ($) Annual interest (%) Inflation (%) Payback (month)
*Employing a gas turbine, 28% ~
5 260,000 130,000 26,000 26,000 26,000 15 10 35 ,
14 360,000 180,000 36,000 36,000 36,000 15 10 29
MEA-II 30 530,000 265,000 53,000 53,000 53,000 15 10
77
1200kW rated power.
Multiple effect absorption system
411
MEA-II performance The MEA-II is used to concentrate sea water, as an example of water based solutions, to produce a concentrated brine and a fresh water byproduct. The system is fired by the exhaust of a gas turbine that drives an energy efficient fresh water producing facility such as a sea water reverse osmosis (RO) unit. Hence, the system shown in Fig. 3 is fired by the exhaust of a 28% efficiency engine similar to that used with the MEA-I at a 20°C pinch point. A terminal temperature difference of 5°C is allowed for the heat exchangers while the conditions in both the condenser and the absorber are maintained similar to these of the MEA-I in Fig. 1. The difference of the working fluid concentrations, between the absorber and the generators, is constant and equal to 3%. The conditions in the MEA-II are as shown on the LiBr-H20 equilibrium chart in Fig. 4. Care should be taken to minimize the potential for contamination of the water collected down-stream of the condenser, by the working fluid and the working fluid by the sea water in the PSFT. Theoretically speaking there should be no contamination of the vapour produced in the generators by the LiBr since it wouldn't evaporate at the generators conditions. However, there might be some carry over of the working fluid with the released water vapour. Such possible contamination of the collected water downstream of the condenser could be avoided by means of a proper design for the generators, using a high quality demister and passing the vapour through a drying bed. In extreme cases, however, the byproduct water could be used for applications other than direct human consumption. At the PSFT, sea water distillation is accomplished by flash evaporation. Thus contamination of the working fluid, in the absorber, by the sea water carry over could be minimized through a careful design of the flashing chamber and employing a high quality demister. The power consumption of the RO, with 25% allowance for auxilliaries, is estimated as 14.3 kWh/m 3 fresh water. With an engine power of 1200 kW the RO at a 30% recovery ratio would reject 55 kg/s of the pretreated pressurized brine. This is mixed with the sea water and is used as the cooling water (state 18, Fig. 3) which is preheated in the condenser. This is heated further in the absorber where its temperature reaches 35°C upstream of the PSFT where 75% of it is processed and the balance is rejected. The flashed vapour in the PSFT (saturation temperature of 30°C) goes to the absorber where it is absorbed by the strong LiBr-H~O solution (state 17, Fig. 3) and it is finally collected downstream of the condenser as a byproduct fresh water. It is worth mentioning that the PSFT acts as a cooling tower for the process sea water and the concentrated brine leaves the tank at 30°C. Thus the concentration ratio of the brine could be increased by recirculation provided that the boiling point elevation is accounted for. I |~ Effect
2 nd Effect
3rd Effect
&th Effect
S'~h Effect Cooling
-.,®
Qg
;
~ C~ienser
Product
I
.,..°
Fig. 3. Multiple effect absorption for vapour distillation and collection (MEA-II).
412
S.E. ALV
~2o °c (~,
, st.eftect
200
,oo 2nd
i00
e t f ec~
v
ao "c
~ ~'~'~" 3 rd e f f e c t v
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/
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@
0 o
~0 ~.
~o
so
eo
ioo Solution
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Fig. 4. Multipleeffectabsorption for vapour distillationand collection(MEA-II).
The MEA-II is thermodynamically analyzed for a case study, Fig. 4, where the temperatures are considered as 135°C, 36°C, 40°C asnd 30°C in Gen-I, the condenser, the absorber and the PSFT respectively. The engine exhaust gas temperature reaches 155°C downstream of Gen-I which is comparable to a stack level for no low temperature corrosion conditions. The circulation mass ratios in the system are estimated as 13.6 and 12.6 for the absorber and the generators respectively. Energy conservation yields the heat input (Qg) as 2130 kJ/kg water vapour distilled in Gen-I. This is 47% of the exhaust availability and 66.2% of the maximum recoverable (reversible) exhaust energy. Overall mass and energy conservations showed that the collected byproduct fresh water could amount to 11.18 m3/h at a specific energy input of 685 kJ/kg water (about 28% of the heat of evaporation). In comparison with the engine driven RO unit the MEA-II increased the fresh water produced by 13.5% (see Table 1). Based on electricity price of $0.06/kWh, the energy cost for the water produced by the RO would be $0.86/m 3 which represents about 30% of the water selling price. Thus, selling the byproduct water of the MEA-II would yield an annual return of $254,000. The additional capital cost for the system is estimated as $530,000. Installation costs are assumed as 50% of the capital invested while the O&M costs are 10% of the capital each. The interest and inflation rates are 15% and 10°/0, respectively. Economical analysis based on selling the byproduct water showed a payback period of 77 months for the MEA-II (see Table 2). Actually the MEA-II is equivalent to a multiple effect evaporation unit where the process solution top temperature reaches 66°C compared with 35°C in the MEA-II. Heat demand per kg water produced in the MEA-II is higher than that of the conventional sea water distillation systems. However, the MEA-II requires no pretreatment, no chemical treatment for scales control and would create less corrosion problems. When the system is used to concentrate solutions, other than sea water, such as milk for example, the solution could be preheated in a separate pass in the condenser and/or the absorber. In this case the solvent separation in the PSFT takes place near the ambient temperature which keeps the value of the product.
Multiple effect absorption system
413
CONCLUSIONS
Two arrangements of the MEA type are investigated utilizing the exhaust energy of a gas turbine. The MEA-I is employed for cooling purposes whereas the MEA-II is used as a water based solution concentration machine. Both cases are analyzed using the exhaust of 1200 kW, 28% efficiency gas turbine at 525°C. The MEA-I is studied at two specific evaporation temperatures of 5°C and 14°C where it showed a COlt of 1.31 and 2.18, respectively. When the gas turbine is used to drive a VC cooling unit the percentage increase in the cooling capacity, relative to the VC, reached 65 and 77 for evaporation temperatures of 5°C and 14°C, respectively. Thus the overall thermal efficiency of the VC/MEA-I system would reach 46% and 49.5% for evaporation temperatures of 5°C and 14°C, respectively with corresponding payback periods for the MEAd of 35 months and 29 months respectively. The MEA-II, for solution concentration, has a PSFT where the solution is processed and the generated flashed vapour is finally collected as a byproduct fresh water. This arrangement could be useful for concentrating thermally sensitive materials. As a case study the MEA-II used the sea water as an example for the water based solutions. The system is fired by the exhaust of an engine that drives an RO unit. With the PSFT at 30°C the MEA-II produced 268 m3/day of fresh water. This is 13.5% of the RO capacity. By selling only the produced water from the MEA-II this would have a payback period of 77 months. One of the main advantages of the MEA-II is that the solution concentration process takes place at near ambient temperature which would keep the value of the concentrated product. REFERENCES 1. H. Cohen, G. Rogers and H. Saravanamuttoo, Gas Turbine Theory, 3rd edn. Longman Scientific and Technical U.K. (1987). 2. R. Allen and J. Kovacik, Gas turbine cogeneration-principles and practice. J. Engng Gas Turbines and Power 106, 725-730 (1984). 3. G. Cerri and A. Colage, Steam cycle regeneration influence on combined gas-steam power plant performance. J. Engng Gas Turbines and Power 107, 574-581 (1985). 4. A. Kallina, Combined cycle system with novel bottoming cycle, d. Engng Gas Turbines and Power 106, 737-742 (1984). 5. S. E. Aly, Gas turbine waste heat recovery distillation system. Heat Recovery Systems & Clip 7, 375-382 (1987). 6. S. E. Aly, Vapour compression distillation waste heat absorption systems. Desalination 68, 57--68 (1988). 7. S. E. Aly, Reverse osmosis desalination by waste heat absorption power cycle. J. Institute Energy, 33-37 (1988). 8. S. E. Aly, Efficient part load gas turbine arrangements. J. Institute Energy, 31-38 (1990). 9. S. E. Aly and K. Fathalah, Combined absorption-desiccant solar powered air conditioning system. Warme-und Stoffubertragung 23, 111-121 (1988). I0. J. Mitchel, State of the art of active solar cooling, Proc. 1986 Annual Meeting, American Solar Energy Society, Inc. Boulder Colorado, 59-71, 11-14 June (1986). 11. J. Howell, Active hybrid solar cooling systems, NATO, ASI Series, Applied Sciences, number 129, pp. 388--408. Martinus Nijhoff Publishers (1988). 12. V. deBiasi, Turbomeca introducing high efficiency Makila-T. I. for commerdial applications. Gas Turbine World 18, 22-26 (1988). 13. ASHRAE, Fundamentals Handbook. Atlanta, GA, U.S.A. (1981). 14. R. Keey, Introduction to Industrial Drying Operations. William Clows & Sons Limited (1978). 15. A. Williams-Gardner, Industrial Drying. Gulf Publishing Company (1977).