- Email: [email protected]
chilling cogeneration cycle with low-grade waste heat
Applied Thermal Engineering 64 (2014) 483e490
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Thermodynamic analysis of ammoniaewater power/chilling cogeneration cycle with low-grade waste heat Junye Hua a, b, *, Yaping Chen a, Yaodong Wang b, A.P. Roskilly b a Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China b Sir Joseph Swan Centre for Energy Research, Newcastle University, Newcastle NE1 7RU, UK
h i g h l i g h t s A modified Kalina cycle is proposed for cogeneration from low-grade waste heat. A subcooler, throttle and an evaporator are set to complete cooling sub-process. The adjustable concentrations make the system with higher efficiency. The chilling fraction can be set to fulfill various demands for power or refrigeration. Thermal and exergy efficiency of combined cycle can reach up to 16.4% and 48.3%.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 June 2013 Accepted 18 December 2013 Available online 28 December 2013
An ammoniaewater absorption cycle for power and chilling output cogeneration from mid/low-grade waste heat was analyzed and optimized, which is a modified Kalina cycle adding an evaporator and a subcooler to realize the chilling effect. The cycle achieves higher efficiency by generating chilling output from proper internal recuperation process without consumption of additional heat resource and by realizing heat transfer with suitable ammonia concentrations for variable phase change processes to match both heat source and cooling water. Analysis of the impact of key parameters for the system on the thermal and exergy efficiencies was carried out. The results show that there are matching basic and work concentration pairs for a higher efficiency. The smaller circulation multiple and greater chilling fraction are favorable to the efficiencies but restricted respectively by heat transfer constraint of recuperator and the demand. The calculation example with the turbine inlet parameters set at 195 C/2.736 MPa and the cooling water inlet temperature set at 25 C with chilling fraction of 0.5 shows that the thermal efficiency and exergy efficiency reach up to 16.4% and 48.3%, about 24.24% and 8.16% higher than those of an ammoniaewater power cycle under identical condition. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Ammoniaewater Cogeneration cycle Circulation multiple Solution concentration Thermal performance
1. Introduction The increasing demand for electric power and air-condition chilling load but short supply of energy has become an urgent issue for industries, environment and daily life. It is essential to utilize energy more efficiently with fewer harmful emissions and explore energy resources such as geothermal or mid/low-grade industrial waste heat for power generation and air-condition chilling.
* Corresponding author. Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China. E-mail address: [email protected] (J. Hua). 1359-4311/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2013.12.043
Kalina cycles [1,2], a bottoming cycle in which ammoniaewater is used as the working fluid, has the potential to reclaim power from mid/low-grade waste heat efficiently. Ammoniaewater has different characteristics from pure water and other organic working fluids. The most dramatic characteristic is its variable temperature in phase change, which matches the exothermic curve of the heat source as well as the endoergic curve of the cooling water to reduce exergy losses in both boiling and condensing processes by evaporating with richer ammonia composition and condensing with leaner composition through the absorption process. The thermal performances of some different types of modified Kalina cycles for specific applications were studied, simulated, and compared by many researchers [3e12]. In recent years, some modified Kalina cycles, including the cogeneration cycles, have been proposed [13e17]. Goswami et al. [13e15] proposed a power/
484
J. Hua et al. / Applied Thermal Engineering 64 (2014) 483e490
Nomenclature
hex hth
exergy efficiency thermal efficiency
Symbols COP e f g G h n p q s t T w x
Subscript 0 a B b c d E h l m number T w
benchmark state chilling water heat resource basic (concentration) cooling water dilute (concentration) evaporator, chilling part high low mid state points of the cycle turbine based work (concentration)
refrigeration coefficient exergy (kJ/kg) circulation multiple relative steam quality mass flow (kg/s) enthalpy (kJ/kg) chilling fraction pressure (MPa) specific heat (kJ/kg) entropy (kJ/(kg K)) temperature ( C) temperature (K) specific work (kJ/kg) concentration (ammonia fraction)
chilling cogeneration cycle that uses exhaust fluid from a turbine to realize chilling effect. However, its chilling effect should be very limited as the turbine exhaust is vapor or wet vapor instead of liquid. Zheng et al. [16] and Zhang et al. [17] proposed respectively two similar ammonia absorption power/refrigeration cogeneration cycles. Nevertheless, these cycles use rich ammonia as refrigerant and adopt the generator-rectifier column which consumes external heat and might be detrimental to the performance and efficiency of the cogeneration cycles. This study presents a novel ammoniaewater absorption power/ chilling cogeneration cycle. It uses low-grade industrial waste heat to vaporize the working fluid in the boiler. The chilling effect is actuated by partial evaporation of some work solution, the same concentration as in the turbine, instead of pure ammonia, thus no rectifier is needed. The chilling effect is like the by-product of the power generation cycle and it consumes very little additional heat. A numerical model was built to analyze the thermodynamic performance of the proposed cycle, and a parametric analysis was conducted. Optimum and suitable values for the key parameters (i.e., the circulation multiple f, the work concentration xw, the basic concentration xb, the thermal efficiency, and the exergy efficiency) were sensitively studied to evaluate the thermal performance of the system. 2. The ammoniaewater power/chilling cogeneration cycle 2.1. Description of the cycle The power/chilling cogeneration cycle is based on the Kalina cycle, and following modifications were adopted. 1) A preheater (PH) is set for heating the work solution before flowing into the boiler (B), and a water cooler (WC) is added at the liquid outlet of the separator (S) for cooling the dilute solution before entering to the low-p-absorber (A1). And the output water with higher temperature from the water cooler can be used as sanitary hot water as another by-product of the cycle. 2) The work solution at the outlet of the high-pressure pump split into two streams, one for power generation and the other for chilling. The chilling loop consists of an evaporator (E), a throttle valve (V3) and a subcooler (SC).
Fig. 1 shows the proposed ammoniaewater power/chilling cogeneration cycle, with following four sub-processes: 1) The power sub-process: The work solution (11) from high pressure pump is preheated (12) by rich ammonia vapor (400 ) from separator (S) in the preheater (PH) and flows into the boiler (B). The high-pressure and high-temperature ammoniaewater vapor (15) from the boiler (B) expands in the turbine (T) to generate power. 2) The absorption sub-processes: The low-p-absorber produces the saturated basic solution (1), and then it is pumped to the mid pressure (2), while the mid-p-absorber produce the saturated work solution (9), and then certain part of the work solution (10) is pumped to the boiler with high pressure (11). To simplify the calculation, the state point 8 or 19 is assumed to illustrate the mixture state before absorption in the corresponding absorber. 3) The desorption or separation sub-process: Heated by the turbine exhaust vapor (16), part of the basic solution (2a) is heated to the two-phase-flow state (4) and then is separated in the separator (S) into two streams: the rich ammonia vapor (400 ) and the dilute solution (40 ). The stream (400 ) is cooled in preheater by work solution and then enters the mid-p-absorber, while the stream (40 ) is cooled in the water cooler (WC) and then throttled before enters low-p-absorber and sprays on the tube bundle. The heat transfer process in recuperator can be divided into sub-cooling part and partial evaporating part, and the state point 3 just stands for the saturation point of cold stream, while the state point 17 stands for the corresponding point of hot stream. 4) The chilling sub-process: The stream 9 is divided into two parts: one (10) goes into the boiler (B) and turbine (T) as the power generating fluids after being pumped to high-pressure (11); the other (20) goes through the subcooler (SC) and then is throttled in V3 (22) and enters the evaporator (E) to absorb the airconditioning chilling output. The partial vaporized two-phase flow from evaporator goes through subcooler to the low-pabsorber. The valve V1 is used for throttling dilute solution from midpressure to low-pressure. The valve V2 is used for controlling the basic solution flow percentage in recuperator (R) and mid-p-absorber
J. Hua et al. / Applied Thermal Engineering 64 (2014) 483e490
485
Fig. 1. The schematic description of the ammoniaewater power/chilling cogeneration cycle.
(A2). The valve V3 is used to complete the throttling process from mid-pressure to low-pressure before evaporator (E) for chilling effect. 2.2. tep and tex diagrams The proposed cycle can use 200 Ce450 C industrial waste heat as the heat source. To illustrate the cycle more clearly, The Fig. 2 shows the temperatureepressure (tep) and temperatureeconcentration (tex) diagrams, and the numbers correspond with the state points shown in Fig. 1. As shown in Fig. 2, the system has three stages of pressure: the low pressure (pl), determined by the basic concentration (xb) and the saturation temperature (t1); the mid pressure (pm), determined by the work concentration (xw) and the saturation temperature (t9); and the high pressure (ph), which is not influenced by the
desorption process but depends on the work concentration and the temperature of saturated vapor in boiler. In order to complete the desorption process, the mid-pressure should meet the heat transfer constraint of the recuperator as shown in Fig. 2(b). Otherwise, the heat capacity of the turbine exhaust vapor might be not sufficient to heat the basic solution (2a) into the designed desorption two-phase-flow state (4). What gives the cycle one more degree of freedom is the concentration of ammoniaewater mixture. There are three main ammonia concentrations realized by the separator and the absorber: the work concentration (xw), the basic concentration (xb) formed in the mid-p-absorber and the low-p-absorber respectively, and the dilute concentration (xd) from the liquid outlet of the separator. These solution concentrations are interconnected with a mathematical equation with circulation multiple f (shown in Table 1(5)).
Fig. 2. pet and tex diagrams for the proposed cogeneration cycle.
486
J. Hua et al. / Applied Thermal Engineering 64 (2014) 483e490
Table 1 Basic formula of mathematical model Components
Calculation formula
Boiler
GB cpB ðtB1 tB4 Þ ¼ GT ðh15 h12 Þ
(1)
Turbine inlet
h15 ðor s15 Þ ¼ f ðph ; xw ; t15 Þ
(2)
Isentropic turbine outlet h16s ¼ f ðpl ; xw ; s15 Þ
(3)
Actual turbine outlet
h16 ¼ h16s þ ð1 hT Þðh15 h16s Þ
(4)
Circulation multiple f
f ¼ ðxw xd Þðxb xd Þ
(5)
Recuperator
ð1 nÞðh16 h18 Þ ¼ G4 ðh4 h2a Þ
(6)
Low-p-absorber
Gc1 cpc ðtc1 tc2 Þ ¼ fGðh19 h1 Þ
(7)
h19 ¼ ð1 nÞg18 hg18 þ ng24 hg24
þ ðf 1Þh6 f
. þ G40 ðh40 h5 Þ n
(8)
Preheater
h12 ¼
Separator
xd ðor x40 ; h40 ; h40 Þ ¼ fðpm ; t4 Þ
(10)
G4 ¼ G3 ¼ Gðf 1Þðh40 h40 Þ=ðh4 h40 Þ
(11)
Mid-p-absorber
Gc3 cpc ðtc3 tc4 Þ ¼ Gðh8 h9 Þ
(12)
h9 ¼ G40 h5 þ ðf G3 Þh7
(13)
Water cooler
h6 ¼ fðpm ; xd ; T6 Þ
(14)
Throttle valve
h6 ¼ h6a ; h22 ¼ h21
(15)
Pumps
h2 ¼ h1 ; h10 ¼ h9
(16)
Evaporator
qE ¼ nðh23 h22 Þ; n ¼ G20 =G9
(17)
Subcooler
h20 h21 ¼ h24 h23
(18)
ð1 nÞh11
(9)
According to the mass and energy conversion law, the mathematical calculation model with ammonia concentration, flow rate, and the related enthalpy has been established (shown in Table 1). The relative flow rate in the mid-p-absorber is set at 1. This is also the total amount of fluids for power generation and chilling. The chilling fraction n is defined as the stream 20 for chilling generation over the total work solution stream 9; and the stream 10 for power generation is (1 n). Besides, the flow ratio between the solutions at outlets of low and mid-p-absorbers is defined as circulation multiple f. 3. Modeling and calculation The thermal performance analysis of the cycle requires accurate thermal property data of the ammoniaewater solution. The Schulz state equation [18], which has been widely applied in ammoniae water absorption refrigeration [19], is selected for the study. One of the advantages for the Schulz state equation is that it is a mathematical formula related to concentration, pressure, and temperature that simplifies the iterative solution process. Unlike the pure working fluid, the ammoniaewater needs at least two parameters from concentration, pressure, or temperature to determine the thermal properties of the mixture. The Schulz state equation is a set of free enthalpy equations built with two free variables: temperature and pressure. In terms of the liquid phase equilibrium equation and the vapor phase equilibrium equation, a simultaneous equation is established according to the principle that the chemical potential of each phase should be balanced. During calculation, what should be done first is to judge which region the working fluid is in: the saturated state, the super-heated state, or the sub-cooled state. Subroutine has been established for each state region and is called in system static thermodynamic calculation programming.
For the solution in the sub-cooled region, the value of enthalpy should be the same as the saturated liquid enthalpy under identical temperature. If it is in the two-phase region, the value of enthalpy can be calculated by adding the liquid and vapor entropy with the rule of lever. Adopting the computer language Visual Basic, correcting to the same data as the ammoniaewater phase diagram (i.e., with enthalpy set to 0 kJ/kg, pure water set to 0 C, and pure ammonia set to 77 C), the ammoniaewater thermal property calculation programming has been completed. Adapting right computer sentence to describe the state equation, with reasonable iteration method (Newton iteration method, bisection method, etc.), calculation programming has been realized. According to some studies [20], the Schulz equation seems to overestimate the ammonia concentration near the critical vapor phase as well as the mixture critical pressure. However, the Schulz equation agrees well with the existing ammoniaewater phase diagram. In addition to the ammoniaewater property calculation program, the static model of the whole cycle were built, following the conservation laws of thermodynamics and heat transfer, neglecting the thermal and pressure losses along the pipes and equipment. Table 1 shows the basic formula for the model and Table 2 shows the evaluation indexes for the cycle. Thermal efficiency for power generation cycle is the index to measure the generated power over the input energy. But for the cogeneration cycle, due to the different grades of power and chilling output, the COP (¼3.5) [21] of average chillers is introduced. The main basic parameters and assumptions are shown in Table 3. For the simulation of the cogeneration system, the main assumptions and conditions are showed in Table 3 and listed as follows: 1) The heat exchangers are all of countercurrent type, and the pinch point temperature differences are set as more than 5 K except for the evaporator and subcooler which are set as 3 K. 2) The ammoniaewater expansion in the throttle valves is considered isenthalpic. 3) The superheat degree of the ammoniaewater vapor at turbine inlet is set as no less than 5 K. The restraint of turbine exhaust vapor no less than 88% dryness is also set. Like water, the ammoniaewater mixture is “wet” fluid. Consequently, the
Table 2 Main efficiency indexes of cycle Index Thermal efficiency hth
Formula hth ¼ w wp1 wp2 þ qE =COP qB
(19)
¼ w wp1 wp2 þ eE eB
(20)
Exergy efficiency hex
hex
Input heat qB
qB ¼ ð1 nÞðh15 h12 Þ
(21)
Turbine work solution flow based input heat qBT Work output w
qBT ¼ h15 h12
(22)
w ¼ ð1 nÞðh16 h15 Þ
(23)
Turbine work solution flow based work output wT Cooling capacity qE
wT ¼ h16 h15
(24)
qE ¼ nðh23 h22 Þ
(25)
Turbine work solution flow based cooling capacity qET Input heat exergy eB
qET ¼ nðh23 h22 Þ=ð1 nÞ
(26)
eB ¼ ð1 nÞ½ðh15 h12 Þ T0 ðs15 s12 Þ; T0 ¼ 273:15 K
Cold exergy eE
(27)
eE ¼ n½ðh23 h22 Þ T0 ðs23 s22 Þ; T0 ¼ 273:15 K
(28)
J. Hua et al. / Applied Thermal Engineering 64 (2014) 483e490 Table 3 Basic parameters and main assumptions for operation and constraint conditions. Medium or equipment Cooling water Turbine
Heat exchangers Pump Heat source Evaporator
Parameters
Units
Inlet temperature Temperature difference Inlet temperature Isentropic efficiency Backpressure Dryness of the exhaust vapor Pinch point temperature difference Efficiency Medium of heat source Inlet temperature Chilling water inlet/ outlet temperature (a1/a2)
Values
C K C % Pa %
25(c1,c3)/33(c4) 8 195 70 >101325 88
K
3e15
% / C C
60 Thermal oil 200 15/7
turbine exhaust vapor might fall into the less than 88% dryness two-phase region, which leads blade corrosion and is harmful to the turbine operation [22,23].
4. Results and discussion 4.1. The thermal performance of the system Table 4 shows the state points for the system under the following conditions: the concentration multiple is set as 4, the chilling fraction is set as 0.5, and the working and basic concentrations are set as 0.55 and 0.35, respectively. The thermal performance results are shown in Table 5. The computer code for the cycle was operated intensively to evaluate the performance of the cogeneration cycle. The influence to the performance of the cycle of following parameters were
Table 4 State points for the system. State no.
Pressure/MPa
Temperature/ C
Concentration/kg kg1
Relative flow rate/d
1 2 2a 2b 3 4 40 40 ’ 5 6 6a 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
0.16 0.49 0.49 0.49 0.49 0.49 0.49 0.49 0.49 0.49 0.16 0.49 0.49 0.49 0.49 2.736 2.736 2.736 2.736 2.736 0.16 0.16 0.16 0.16 0.49 0.49 0.16 0.16 0.16
30 30 30 30 64.1 78.417 78.417 78.417 67.54 40 38.5 30 64.8 30 30 30 73.417 99 190 195 88.31 79.6 35.4 34.2 30 15 2.05 12 15.65
0.35 0.35 0.35 0.35 0.35 0.35 0.283 0.935 0.283 0.283 0.283 0.35 0.55 0.55 0.55 0.55 0.55 0.55 0.55 0.55 0.55 0.55 0.55 0.35 0.55 0.55 0.55 0.55 0.55
4 4 3.342 0.658 3.342 3.342 3 0.342 3 3 3 0.658 1 1 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 4 0.5 0.5 0.5 0.5 0.5
487
Table 5 Performance properties. Item
Unit
Value
Circulation multiple f Inlet temperature of heat source Specific work output Specific cooling capacity Specific uptake heat Specific power of pump 1 Specific power of pump 2 Vapor quality at turbine exit Hot water from water cooler (c5) Total thermal efficiency hth Exergy efficiency hex
e C kJ/kg kJ/kg kJ/kg kJ/kg kJ/kg e C % %
4 200 139.1 119.53 1024.65 2.49 2.32 0.912 48.86 16.4 48.3
sensitively studied, such as concentration, chilling fraction, and circulation multiple. The thermal performance of the cycle includes power output w, chilling output qE, thermal efficiency hth and the exergy efficiency hex. 4.2. Ammonia concentration The ammoniaewater working fluid changes temperature during evaporation or condensation processes. A richer ammonia concentration in the boiler and a leaner concentration in the absorber make it possible to match the exothermic curve of the heat source and the endoergic curve of the cooling water to reduce exergy losses. The work concentrations selected are 0.45, 0.5, and 0.55, the basic concentrations range from 0.3 to 0.41, and the chilling fraction n is 0.5. The chilling fraction n is defined as the flow rate ratio of stream 20 and stream 9. The results are showed in Fig. 3. The subfigures (a)e(f) show respectively how the ammonia concentration xw and xb pairs influence the input heat qB, the power output w, the chilling output qE, the thermal efficiency hth, the exergy efficiency hex and the cycle pressures ph, pm, pl. Under a certain work concentration, the input heat increases with the increase of basic concentration (Fig. 3a). The turbine output and the chilling output reveal the opposite tendency (Fig. 3b and c). Consequently, both the thermal efficiency hth and the exergy efficiency hex decrease with the increase of basic concentration xb (Fig. 3def). The Fig. 3g shows the reason for this lies in the variation of the cycle pressures that as the xb increases with other parameters kept steady, the low pressure pl also increases; thus the pressure drop in turbine and the power output become smaller. However, the value of xb has lower limit, and the nature of better performance of the cycle with optimal pair of work concentration xw and basic concentration xb can be illustrated as the effect of heat transfer constraint in the recuperator. Moreover, as shown in Fig. 3d and e, the higher work concentration xw is beneficial for higher efficiency of the cycle with the optimal xb and xw pair. There is also a limitation to keep the xb not too low that to maintain the turbine backpressure higher than the atmosphere to prevent the system from inward air leakage. Since the pressure determines also the value of specific volume of working medium directly, and a positive turbine backpressure could minimize turbine size and free from the non-condensable air, it has great impact on the design and operation of the turbine. Therefore the main consideration for selection of work concentration xw is the working pressure. 4.3. Circulation multiple The index of the circulation multiple is defined as the flow ratio of the basic solution at the low-p-absorber outlet and the absorbed work solution, thus, for a pure power generation ammoniaewater
488
J. Hua et al. / Applied Thermal Engineering 64 (2014) 483e490
Fig. 3. Impact of ammonia concentration on thermal performance (f ¼ 4, n ¼ 0.5, tB ¼ 200 C).
cycle the absorbed work solution is solely from the turbine [11,12], while for the proposed power cogeneration cycle, the absorbed work solution is also from the evaporator. Therefore the circulation multiple is defined as the flow ratio of the basic solution from lowp-absorber and the work solution entering the low-p-absorber which equals to the work solution from mid-p-absorber. As the work solution is split to two streams for power and chilling generations, the index of the chilling fraction n is defined to describe the flow rate fraction for chilling generation. The circulation multiple discussed is from 3 to 4.5, the chilling fraction n is 0.5 and the xw and xb are fixed at 0.55 and 0.35 respectively. Fig. 4 shows the impact of circulation multiple f on thermal performances of the cycle. It is showed that the power generation w and chilling output qE keep unchanged, while the input heat qB increases with the increase of the circulation multiple f (Fig. 4a). This is why the thermal efficiency and exergy efficiency decrease as the circulation multiple increases (Fig. 4b). As the heat transfer process of basic solution is divided into sub-cooling and evaporation in the recuperator, the temperatures of saturation point and the inlet/outlet points of the basic solution as well as their hot fluid side counterparts are showed in the Fig. 4c. When the f increases, the temperatures at turbine exit (16), the inlet (2a) and saturation point (3) of basic solution are kept unchanged, while the outlet temperatures of both basic solution (4) and work solution (18) decrease and the temperature of point 17 increases. Fig. 4d shows the temperature differences at the three points vary with the circulation multiple. It can be seen that the pinch point is at either end of the recuperator that only in the range of from 3.3 to 4.15 the circulation multiple meet the heat transfer constraint of minimum pinch point temperature difference of 5 K. When the circulation multiple f is less than 3.3, the turbine exhaust vapor cannot supply enough heat for the desorption process and the cycle cannot be completed. In fact, the circulation multiple f and the desorption
temperature t4 have a mutual relationship once the work and basic concentrations are fixed. 4.4. Chilling fraction The calculated value of chilling fraction n ranges from 0 to 0.6, as shown in Fig. 5. If the chilling fraction n is set at 0, the cogeneration cycle will become a power generation cycle. As n increases, the flow rate fraction for the power generation decreases, which results in a decrease of the specific values of power output and the input heat, based on the flow rate of total work solution flow at outlet of mid-p-absorber, but an increase of the chilling output (Fig. 5a). When based on the turbine work solution flow, as n increases, the specific values of the power output wT and the input heat qBT keep almost unchanged while the chilling output qET increases (Fig. 5b). In general, the expense of the chilling output is very small and both the thermal efficiency hth and the exergy efficiency hex favors a higher chilling fraction n (Fig. 5c). As previously discussed, an ammoniaewater power cycle is formed if the chilling fraction is set at 0. The thermal efficiency and exergy efficiency of the proposed cogeneration cycle at chilling fraction n equals 0.5 is 24.24% and 8.16% higher than those of the power cycle under the same condition. To more clearly illustrate the superiority of the cycle, the power cycle can be compared with the Kalina cycle under the same conditions, as details shown in Table 6. The most important criterion with which to assess a system is net power output. In other words, the most desirable outcome is to convert more energy to electricity. According to engineering thermodynamics, under the same values of inlet/outlet pressure as well as ammonia concentration, the enthalpy drop in turbine of each cycle should be the same; however, the amount of energy devoted to the Kalina cycle should be more.
J. Hua et al. / Applied Thermal Engineering 64 (2014) 483e490
489
Fig. 4. Impact of circulation multiple on thermal performances (xw ¼ 0.5, xb ¼ 0.35, n ¼ 0.5, tB ¼ 200 C).
With a preheater before the boiler, the input heat can be reduced thus thermal efficiency can be raised; and with a water cooler in the dilute solution loop, the sanitary hot water is provided and the discarded heat to the cooling water sink decrease. According to the results, the net power efficiency of proposed cycle is 11.97% higher than the Kalina cycle.
5. Conclusion 1) The use of ammonia water mixtures allows efficient heat recovery with variable temperature in phase change processes to match both heat source and cooling water with adjustable
solution concentration in addition to the cheap working medium cost. 2) The work concentration xw of ammoniaewater solution is selected mainly for matching volumetric flow of turbine and for positive turbine backpressure. Under the certain work concentration xw, there is an optimum basic concentration xb forming a coupled solution concentration pair to yield higher efficiency and power/chilling outputs. 3) There is applicable range of circulation multiple of the cogeneration cycle limited by heat transfer constraint. However, a lower circulation multiple is good for higher thermal efficiency. 4) The chilling fraction can be adjusted for balance of the power and chilling output. The total thermal efficiency of the proposed
Fig. 5. Impact of chilling fraction on thermal performance (xw ¼ 0.5, xb ¼ 0.35, f ¼ 4, tB ¼ 200 C).
490
J. Hua et al. / Applied Thermal Engineering 64 (2014) 483e490
Table 6 Comparison of performances of different cycles. Item
Unit
System discussed in this paper (n ¼ 0.5)
Kalina System cycle discussed in this paper (n ¼ 0)
Inlet pressure of turbine ph Inlet temperature of turbine t15 Inlet temperature of heat resource th1 Evaporation temperature in boiler Condensation (absorption) pressure pl Inlet temperature of cooling water tc1 Temperature rise of cooling water Dtc Enthalpy drop in turbine Work output Specific uptake heat Specific power of pump 1 Specific power of pump 2 Net power efficiency
MPa C C
2.736 195 200
2.736 195 200
2.736 195 200
C MPa
99/190 0.16
99/190 0.16
99/190 0.16
C
25
25
25
C
8
8
8
278.2 139.1 1024.65 2.49 2.32 13.1
278.2 278.2 2049.3 2.49 4.64 13.2
278.2 278.2 2256.78 2.49 4.64 11.7
kJ/kg kw kw kw kw %
cogeneration cycle at chilling fraction equals 0.5 is 24.2% higher than that of the power cycle under the same condition.
Acknowledgements This work is supported dation Programs of China Science and Technology Achievements of Special (BY2011155).
by the National Nature Science Foun(50976022, 51276035) and Provincial Innovation and Transformation of Fund Project of Jiangsu Province
References [1] A.I. Kalina, Combined-cycle system with novel bottoming cycle, J. Eng. Gas. Turbines Power 106 (1984) 737e742. [2] H.A. Mlcak, An introduction to the Kalina cycle, in: Proceedings of the International Joint Power Generation Conference, Houston, Texas, 1996. [3] X.X. Zhang, M.G. He, Y.A. Zhang, Review of research on the Kalina cycle, Renewable Sustainable Energy Rev. 16 (2012) 5309e5318.
[4] U.R. Nasruddin, M. Rifaldi, A. Noor, Energy and exergy analysis of Kalina cycle system(KCS)34 with mass fraction ammoniaewater mixture variation, J. Mech. Sci. Technol. 23 (2012) 1871e1876. [5] O. Arslan, Exergoeconomic evaluation of electricity generation by the medium temperature geothermal resources, using a Kalina cycle: Simav case study, Int. J. Therm. Sci. 49 (2010) 1866e1873. [6] C. Zamfirescu, I. Dincer, Thermodynamic analysis of a novel ammoniaewater trilateral Rankine cycle, Thermochim. Acta 477 (2008) 7e15. [7] W.R. Wagar, C. Zamfirescu, I. Dincer, Thermodynamic performance assessment of an ammoniaewater Rankine cycle for power and heat production, Energy Convers. Manage. 51 (2010) 2501e2509. [8] O. Sirko, Integration of Kalina cycle in a combined heat and power plant, a case study, Appl. Therm. Eng. 29 (2009) 2843e2848. [9] R.S. Murugan, P.M.V. Subbarao, Thermodynamic analysis of RankineeKalina combined cycle, Int. J. Thermodyn. 11 (2008) 133e141. [10] Y.P. Chen, Thermodynamic analysis of a simplified Kalina cycle, Therm. Power Gener. 47 (1997) 20e23 (in Chinese). [11] J.Y. Hua, Y.P. Chen, H.J. Liu, J.F. Wu, Thermodynamic analysis of a simplified dual-pressure ammonia-water absorption power cycle, J. Cent. South Univ. Technol. 19 (2012) 797e802. [12] Y.P. Chen, Thermodynamic analysis of a modified Kalina cycle, J. Southeast Univ. 19 (1989) 52e58 (in Chinese). [13] D.Y. Goswami, F. Xu, Analysis of a new thermodynamic cycle for combined power and cooling using low and mid temperature solar collectors, J. Sol. Energy Eng. 121 (1999) 91e97. [14] H.A. Ingley, R. Reed, D.Y. Goswami, Optimization of a scroll expander applied to an ammonia/water combined cycle system for hydrogen production, in: Proceedings of the Solar World Congress, Orlando, FL, August 2e12, 2005. [15] R.V. Padilla, G. Demirkaya, D.Y. Goswami, Analysis of power and cooling cogeneration using ammonia-water mixture, Energy 35 (2010) 1e9. [16] D.X. Zheng, B. Chen, Y. Qi, A Thermodynamic analysis of a novel absorption power/refrigeration combined cycle, J. Eng. Thermophys. 23 (2002) 201e204 (in Chinese). [17] N. Zhang, N. Lior, Development of a novel combined absorption cycle for power generation and refrigeration, J. Energy Res. Technol. 129 (2007) 254e 265. [18] S. Shulz, Equations on state for the system ammoniaewater for use with computers, in: International Congress of Refrigeration Proceedings, Meeting Commission II, Washington, 1971. [19] Y.P. Chen, C.M. Lin, Y. Tian, NH3eH2O solution cooling absorption refrigeration driven by fishing boat diesel exhaust heat, in: Proceedings of the 6th International Symposium on Heating, Ventilation and Air Conditioning, Nanjing, China, Nov. 6e9, 2009. [20] K. Mejbri, A. Bellagi, Modelling of thermodynamic properties of the watere ammonia mixture by three different approaches, Int. J. Refrig. 29 (2006) 211e 218. [21] C.L. Joseph, Energy analysis of commercial buildings in subtropical climates building and Environment, Build. Environ. 35 (2000) 19e26. [22] A. Franco, M. Villani, Optimal design of binary cycle power plants for waterdominated, Geothermics 38 (2009) 379e391. [23] W. Fu, J. Zhu, W. Zhang, Z. Lu, Performance evaluation of Kalina cycle subsystem on geothermal power generation in the oilfield, Appl. Therm. Eng. 54 (2013) 497e506.
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Thermodynamic analysis of ammoniaewater power/chilling cogeneration cycle with low-grade waste heat Junye Hua a, b, *, Yaping Chen a, Yaodong Wang b, A.P. Roskilly b a Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China b Sir Joseph Swan Centre for Energy Research, Newcastle University, Newcastle NE1 7RU, UK
h i g h l i g h t s A modified Kalina cycle is proposed for cogeneration from low-grade waste heat. A subcooler, throttle and an evaporator are set to complete cooling sub-process. The adjustable concentrations make the system with higher efficiency. The chilling fraction can be set to fulfill various demands for power or refrigeration. Thermal and exergy efficiency of combined cycle can reach up to 16.4% and 48.3%.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 June 2013 Accepted 18 December 2013 Available online 28 December 2013
An ammoniaewater absorption cycle for power and chilling output cogeneration from mid/low-grade waste heat was analyzed and optimized, which is a modified Kalina cycle adding an evaporator and a subcooler to realize the chilling effect. The cycle achieves higher efficiency by generating chilling output from proper internal recuperation process without consumption of additional heat resource and by realizing heat transfer with suitable ammonia concentrations for variable phase change processes to match both heat source and cooling water. Analysis of the impact of key parameters for the system on the thermal and exergy efficiencies was carried out. The results show that there are matching basic and work concentration pairs for a higher efficiency. The smaller circulation multiple and greater chilling fraction are favorable to the efficiencies but restricted respectively by heat transfer constraint of recuperator and the demand. The calculation example with the turbine inlet parameters set at 195 C/2.736 MPa and the cooling water inlet temperature set at 25 C with chilling fraction of 0.5 shows that the thermal efficiency and exergy efficiency reach up to 16.4% and 48.3%, about 24.24% and 8.16% higher than those of an ammoniaewater power cycle under identical condition. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Ammoniaewater Cogeneration cycle Circulation multiple Solution concentration Thermal performance
1. Introduction The increasing demand for electric power and air-condition chilling load but short supply of energy has become an urgent issue for industries, environment and daily life. It is essential to utilize energy more efficiently with fewer harmful emissions and explore energy resources such as geothermal or mid/low-grade industrial waste heat for power generation and air-condition chilling.
* Corresponding author. Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China. E-mail address: [email protected] (J. Hua). 1359-4311/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2013.12.043
Kalina cycles [1,2], a bottoming cycle in which ammoniaewater is used as the working fluid, has the potential to reclaim power from mid/low-grade waste heat efficiently. Ammoniaewater has different characteristics from pure water and other organic working fluids. The most dramatic characteristic is its variable temperature in phase change, which matches the exothermic curve of the heat source as well as the endoergic curve of the cooling water to reduce exergy losses in both boiling and condensing processes by evaporating with richer ammonia composition and condensing with leaner composition through the absorption process. The thermal performances of some different types of modified Kalina cycles for specific applications were studied, simulated, and compared by many researchers [3e12]. In recent years, some modified Kalina cycles, including the cogeneration cycles, have been proposed [13e17]. Goswami et al. [13e15] proposed a power/
484
J. Hua et al. / Applied Thermal Engineering 64 (2014) 483e490
Nomenclature
hex hth
exergy efficiency thermal efficiency
Symbols COP e f g G h n p q s t T w x
Subscript 0 a B b c d E h l m number T w
benchmark state chilling water heat resource basic (concentration) cooling water dilute (concentration) evaporator, chilling part high low mid state points of the cycle turbine based work (concentration)
refrigeration coefficient exergy (kJ/kg) circulation multiple relative steam quality mass flow (kg/s) enthalpy (kJ/kg) chilling fraction pressure (MPa) specific heat (kJ/kg) entropy (kJ/(kg K)) temperature ( C) temperature (K) specific work (kJ/kg) concentration (ammonia fraction)
chilling cogeneration cycle that uses exhaust fluid from a turbine to realize chilling effect. However, its chilling effect should be very limited as the turbine exhaust is vapor or wet vapor instead of liquid. Zheng et al. [16] and Zhang et al. [17] proposed respectively two similar ammonia absorption power/refrigeration cogeneration cycles. Nevertheless, these cycles use rich ammonia as refrigerant and adopt the generator-rectifier column which consumes external heat and might be detrimental to the performance and efficiency of the cogeneration cycles. This study presents a novel ammoniaewater absorption power/ chilling cogeneration cycle. It uses low-grade industrial waste heat to vaporize the working fluid in the boiler. The chilling effect is actuated by partial evaporation of some work solution, the same concentration as in the turbine, instead of pure ammonia, thus no rectifier is needed. The chilling effect is like the by-product of the power generation cycle and it consumes very little additional heat. A numerical model was built to analyze the thermodynamic performance of the proposed cycle, and a parametric analysis was conducted. Optimum and suitable values for the key parameters (i.e., the circulation multiple f, the work concentration xw, the basic concentration xb, the thermal efficiency, and the exergy efficiency) were sensitively studied to evaluate the thermal performance of the system. 2. The ammoniaewater power/chilling cogeneration cycle 2.1. Description of the cycle The power/chilling cogeneration cycle is based on the Kalina cycle, and following modifications were adopted. 1) A preheater (PH) is set for heating the work solution before flowing into the boiler (B), and a water cooler (WC) is added at the liquid outlet of the separator (S) for cooling the dilute solution before entering to the low-p-absorber (A1). And the output water with higher temperature from the water cooler can be used as sanitary hot water as another by-product of the cycle. 2) The work solution at the outlet of the high-pressure pump split into two streams, one for power generation and the other for chilling. The chilling loop consists of an evaporator (E), a throttle valve (V3) and a subcooler (SC).
Fig. 1 shows the proposed ammoniaewater power/chilling cogeneration cycle, with following four sub-processes: 1) The power sub-process: The work solution (11) from high pressure pump is preheated (12) by rich ammonia vapor (400 ) from separator (S) in the preheater (PH) and flows into the boiler (B). The high-pressure and high-temperature ammoniaewater vapor (15) from the boiler (B) expands in the turbine (T) to generate power. 2) The absorption sub-processes: The low-p-absorber produces the saturated basic solution (1), and then it is pumped to the mid pressure (2), while the mid-p-absorber produce the saturated work solution (9), and then certain part of the work solution (10) is pumped to the boiler with high pressure (11). To simplify the calculation, the state point 8 or 19 is assumed to illustrate the mixture state before absorption in the corresponding absorber. 3) The desorption or separation sub-process: Heated by the turbine exhaust vapor (16), part of the basic solution (2a) is heated to the two-phase-flow state (4) and then is separated in the separator (S) into two streams: the rich ammonia vapor (400 ) and the dilute solution (40 ). The stream (400 ) is cooled in preheater by work solution and then enters the mid-p-absorber, while the stream (40 ) is cooled in the water cooler (WC) and then throttled before enters low-p-absorber and sprays on the tube bundle. The heat transfer process in recuperator can be divided into sub-cooling part and partial evaporating part, and the state point 3 just stands for the saturation point of cold stream, while the state point 17 stands for the corresponding point of hot stream. 4) The chilling sub-process: The stream 9 is divided into two parts: one (10) goes into the boiler (B) and turbine (T) as the power generating fluids after being pumped to high-pressure (11); the other (20) goes through the subcooler (SC) and then is throttled in V3 (22) and enters the evaporator (E) to absorb the airconditioning chilling output. The partial vaporized two-phase flow from evaporator goes through subcooler to the low-pabsorber. The valve V1 is used for throttling dilute solution from midpressure to low-pressure. The valve V2 is used for controlling the basic solution flow percentage in recuperator (R) and mid-p-absorber
J. Hua et al. / Applied Thermal Engineering 64 (2014) 483e490
485
Fig. 1. The schematic description of the ammoniaewater power/chilling cogeneration cycle.
(A2). The valve V3 is used to complete the throttling process from mid-pressure to low-pressure before evaporator (E) for chilling effect. 2.2. tep and tex diagrams The proposed cycle can use 200 Ce450 C industrial waste heat as the heat source. To illustrate the cycle more clearly, The Fig. 2 shows the temperatureepressure (tep) and temperatureeconcentration (tex) diagrams, and the numbers correspond with the state points shown in Fig. 1. As shown in Fig. 2, the system has three stages of pressure: the low pressure (pl), determined by the basic concentration (xb) and the saturation temperature (t1); the mid pressure (pm), determined by the work concentration (xw) and the saturation temperature (t9); and the high pressure (ph), which is not influenced by the
desorption process but depends on the work concentration and the temperature of saturated vapor in boiler. In order to complete the desorption process, the mid-pressure should meet the heat transfer constraint of the recuperator as shown in Fig. 2(b). Otherwise, the heat capacity of the turbine exhaust vapor might be not sufficient to heat the basic solution (2a) into the designed desorption two-phase-flow state (4). What gives the cycle one more degree of freedom is the concentration of ammoniaewater mixture. There are three main ammonia concentrations realized by the separator and the absorber: the work concentration (xw), the basic concentration (xb) formed in the mid-p-absorber and the low-p-absorber respectively, and the dilute concentration (xd) from the liquid outlet of the separator. These solution concentrations are interconnected with a mathematical equation with circulation multiple f (shown in Table 1(5)).
Fig. 2. pet and tex diagrams for the proposed cogeneration cycle.
486
J. Hua et al. / Applied Thermal Engineering 64 (2014) 483e490
Table 1 Basic formula of mathematical model Components
Calculation formula
Boiler
GB cpB ðtB1 tB4 Þ ¼ GT ðh15 h12 Þ
(1)
Turbine inlet
h15 ðor s15 Þ ¼ f ðph ; xw ; t15 Þ
(2)
Isentropic turbine outlet h16s ¼ f ðpl ; xw ; s15 Þ
(3)
Actual turbine outlet
h16 ¼ h16s þ ð1 hT Þðh15 h16s Þ
(4)
Circulation multiple f
f ¼ ðxw xd Þðxb xd Þ
(5)
Recuperator
ð1 nÞðh16 h18 Þ ¼ G4 ðh4 h2a Þ
(6)
Low-p-absorber
Gc1 cpc ðtc1 tc2 Þ ¼ fGðh19 h1 Þ
(7)
h19 ¼ ð1 nÞg18 hg18 þ ng24 hg24
þ ðf 1Þh6 f
. þ G40 ðh40 h5 Þ n
(8)
Preheater
h12 ¼
Separator
xd ðor x40 ; h40 ; h40 Þ ¼ fðpm ; t4 Þ
(10)
G4 ¼ G3 ¼ Gðf 1Þðh40 h40 Þ=ðh4 h40 Þ
(11)
Mid-p-absorber
Gc3 cpc ðtc3 tc4 Þ ¼ Gðh8 h9 Þ
(12)
h9 ¼ G40 h5 þ ðf G3 Þh7
(13)
Water cooler
h6 ¼ fðpm ; xd ; T6 Þ
(14)
Throttle valve
h6 ¼ h6a ; h22 ¼ h21
(15)
Pumps
h2 ¼ h1 ; h10 ¼ h9
(16)
Evaporator
qE ¼ nðh23 h22 Þ; n ¼ G20 =G9
(17)
Subcooler
h20 h21 ¼ h24 h23
(18)
ð1 nÞh11
(9)
According to the mass and energy conversion law, the mathematical calculation model with ammonia concentration, flow rate, and the related enthalpy has been established (shown in Table 1). The relative flow rate in the mid-p-absorber is set at 1. This is also the total amount of fluids for power generation and chilling. The chilling fraction n is defined as the stream 20 for chilling generation over the total work solution stream 9; and the stream 10 for power generation is (1 n). Besides, the flow ratio between the solutions at outlets of low and mid-p-absorbers is defined as circulation multiple f. 3. Modeling and calculation The thermal performance analysis of the cycle requires accurate thermal property data of the ammoniaewater solution. The Schulz state equation [18], which has been widely applied in ammoniae water absorption refrigeration [19], is selected for the study. One of the advantages for the Schulz state equation is that it is a mathematical formula related to concentration, pressure, and temperature that simplifies the iterative solution process. Unlike the pure working fluid, the ammoniaewater needs at least two parameters from concentration, pressure, or temperature to determine the thermal properties of the mixture. The Schulz state equation is a set of free enthalpy equations built with two free variables: temperature and pressure. In terms of the liquid phase equilibrium equation and the vapor phase equilibrium equation, a simultaneous equation is established according to the principle that the chemical potential of each phase should be balanced. During calculation, what should be done first is to judge which region the working fluid is in: the saturated state, the super-heated state, or the sub-cooled state. Subroutine has been established for each state region and is called in system static thermodynamic calculation programming.
For the solution in the sub-cooled region, the value of enthalpy should be the same as the saturated liquid enthalpy under identical temperature. If it is in the two-phase region, the value of enthalpy can be calculated by adding the liquid and vapor entropy with the rule of lever. Adopting the computer language Visual Basic, correcting to the same data as the ammoniaewater phase diagram (i.e., with enthalpy set to 0 kJ/kg, pure water set to 0 C, and pure ammonia set to 77 C), the ammoniaewater thermal property calculation programming has been completed. Adapting right computer sentence to describe the state equation, with reasonable iteration method (Newton iteration method, bisection method, etc.), calculation programming has been realized. According to some studies [20], the Schulz equation seems to overestimate the ammonia concentration near the critical vapor phase as well as the mixture critical pressure. However, the Schulz equation agrees well with the existing ammoniaewater phase diagram. In addition to the ammoniaewater property calculation program, the static model of the whole cycle were built, following the conservation laws of thermodynamics and heat transfer, neglecting the thermal and pressure losses along the pipes and equipment. Table 1 shows the basic formula for the model and Table 2 shows the evaluation indexes for the cycle. Thermal efficiency for power generation cycle is the index to measure the generated power over the input energy. But for the cogeneration cycle, due to the different grades of power and chilling output, the COP (¼3.5) [21] of average chillers is introduced. The main basic parameters and assumptions are shown in Table 3. For the simulation of the cogeneration system, the main assumptions and conditions are showed in Table 3 and listed as follows: 1) The heat exchangers are all of countercurrent type, and the pinch point temperature differences are set as more than 5 K except for the evaporator and subcooler which are set as 3 K. 2) The ammoniaewater expansion in the throttle valves is considered isenthalpic. 3) The superheat degree of the ammoniaewater vapor at turbine inlet is set as no less than 5 K. The restraint of turbine exhaust vapor no less than 88% dryness is also set. Like water, the ammoniaewater mixture is “wet” fluid. Consequently, the
Table 2 Main efficiency indexes of cycle Index Thermal efficiency hth
Formula hth ¼ w wp1 wp2 þ qE =COP qB
(19)
¼ w wp1 wp2 þ eE eB
(20)
Exergy efficiency hex
hex
Input heat qB
qB ¼ ð1 nÞðh15 h12 Þ
(21)
Turbine work solution flow based input heat qBT Work output w
qBT ¼ h15 h12
(22)
w ¼ ð1 nÞðh16 h15 Þ
(23)
Turbine work solution flow based work output wT Cooling capacity qE
wT ¼ h16 h15
(24)
qE ¼ nðh23 h22 Þ
(25)
Turbine work solution flow based cooling capacity qET Input heat exergy eB
qET ¼ nðh23 h22 Þ=ð1 nÞ
(26)
eB ¼ ð1 nÞ½ðh15 h12 Þ T0 ðs15 s12 Þ; T0 ¼ 273:15 K
Cold exergy eE
(27)
eE ¼ n½ðh23 h22 Þ T0 ðs23 s22 Þ; T0 ¼ 273:15 K
(28)
J. Hua et al. / Applied Thermal Engineering 64 (2014) 483e490 Table 3 Basic parameters and main assumptions for operation and constraint conditions. Medium or equipment Cooling water Turbine
Heat exchangers Pump Heat source Evaporator
Parameters
Units
Inlet temperature Temperature difference Inlet temperature Isentropic efficiency Backpressure Dryness of the exhaust vapor Pinch point temperature difference Efficiency Medium of heat source Inlet temperature Chilling water inlet/ outlet temperature (a1/a2)
Values
C K C % Pa %
25(c1,c3)/33(c4) 8 195 70 >101325 88
K
3e15
% / C C
60 Thermal oil 200 15/7
turbine exhaust vapor might fall into the less than 88% dryness two-phase region, which leads blade corrosion and is harmful to the turbine operation [22,23].
4. Results and discussion 4.1. The thermal performance of the system Table 4 shows the state points for the system under the following conditions: the concentration multiple is set as 4, the chilling fraction is set as 0.5, and the working and basic concentrations are set as 0.55 and 0.35, respectively. The thermal performance results are shown in Table 5. The computer code for the cycle was operated intensively to evaluate the performance of the cogeneration cycle. The influence to the performance of the cycle of following parameters were
Table 4 State points for the system. State no.
Pressure/MPa
Temperature/ C
Concentration/kg kg1
Relative flow rate/d
1 2 2a 2b 3 4 40 40 ’ 5 6 6a 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
0.16 0.49 0.49 0.49 0.49 0.49 0.49 0.49 0.49 0.49 0.16 0.49 0.49 0.49 0.49 2.736 2.736 2.736 2.736 2.736 0.16 0.16 0.16 0.16 0.49 0.49 0.16 0.16 0.16
30 30 30 30 64.1 78.417 78.417 78.417 67.54 40 38.5 30 64.8 30 30 30 73.417 99 190 195 88.31 79.6 35.4 34.2 30 15 2.05 12 15.65
0.35 0.35 0.35 0.35 0.35 0.35 0.283 0.935 0.283 0.283 0.283 0.35 0.55 0.55 0.55 0.55 0.55 0.55 0.55 0.55 0.55 0.55 0.55 0.35 0.55 0.55 0.55 0.55 0.55
4 4 3.342 0.658 3.342 3.342 3 0.342 3 3 3 0.658 1 1 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 4 0.5 0.5 0.5 0.5 0.5
487
Table 5 Performance properties. Item
Unit
Value
Circulation multiple f Inlet temperature of heat source Specific work output Specific cooling capacity Specific uptake heat Specific power of pump 1 Specific power of pump 2 Vapor quality at turbine exit Hot water from water cooler (c5) Total thermal efficiency hth Exergy efficiency hex
e C kJ/kg kJ/kg kJ/kg kJ/kg kJ/kg e C % %
4 200 139.1 119.53 1024.65 2.49 2.32 0.912 48.86 16.4 48.3
sensitively studied, such as concentration, chilling fraction, and circulation multiple. The thermal performance of the cycle includes power output w, chilling output qE, thermal efficiency hth and the exergy efficiency hex. 4.2. Ammonia concentration The ammoniaewater working fluid changes temperature during evaporation or condensation processes. A richer ammonia concentration in the boiler and a leaner concentration in the absorber make it possible to match the exothermic curve of the heat source and the endoergic curve of the cooling water to reduce exergy losses. The work concentrations selected are 0.45, 0.5, and 0.55, the basic concentrations range from 0.3 to 0.41, and the chilling fraction n is 0.5. The chilling fraction n is defined as the flow rate ratio of stream 20 and stream 9. The results are showed in Fig. 3. The subfigures (a)e(f) show respectively how the ammonia concentration xw and xb pairs influence the input heat qB, the power output w, the chilling output qE, the thermal efficiency hth, the exergy efficiency hex and the cycle pressures ph, pm, pl. Under a certain work concentration, the input heat increases with the increase of basic concentration (Fig. 3a). The turbine output and the chilling output reveal the opposite tendency (Fig. 3b and c). Consequently, both the thermal efficiency hth and the exergy efficiency hex decrease with the increase of basic concentration xb (Fig. 3def). The Fig. 3g shows the reason for this lies in the variation of the cycle pressures that as the xb increases with other parameters kept steady, the low pressure pl also increases; thus the pressure drop in turbine and the power output become smaller. However, the value of xb has lower limit, and the nature of better performance of the cycle with optimal pair of work concentration xw and basic concentration xb can be illustrated as the effect of heat transfer constraint in the recuperator. Moreover, as shown in Fig. 3d and e, the higher work concentration xw is beneficial for higher efficiency of the cycle with the optimal xb and xw pair. There is also a limitation to keep the xb not too low that to maintain the turbine backpressure higher than the atmosphere to prevent the system from inward air leakage. Since the pressure determines also the value of specific volume of working medium directly, and a positive turbine backpressure could minimize turbine size and free from the non-condensable air, it has great impact on the design and operation of the turbine. Therefore the main consideration for selection of work concentration xw is the working pressure. 4.3. Circulation multiple The index of the circulation multiple is defined as the flow ratio of the basic solution at the low-p-absorber outlet and the absorbed work solution, thus, for a pure power generation ammoniaewater
488
J. Hua et al. / Applied Thermal Engineering 64 (2014) 483e490
Fig. 3. Impact of ammonia concentration on thermal performance (f ¼ 4, n ¼ 0.5, tB ¼ 200 C).
cycle the absorbed work solution is solely from the turbine [11,12], while for the proposed power cogeneration cycle, the absorbed work solution is also from the evaporator. Therefore the circulation multiple is defined as the flow ratio of the basic solution from lowp-absorber and the work solution entering the low-p-absorber which equals to the work solution from mid-p-absorber. As the work solution is split to two streams for power and chilling generations, the index of the chilling fraction n is defined to describe the flow rate fraction for chilling generation. The circulation multiple discussed is from 3 to 4.5, the chilling fraction n is 0.5 and the xw and xb are fixed at 0.55 and 0.35 respectively. Fig. 4 shows the impact of circulation multiple f on thermal performances of the cycle. It is showed that the power generation w and chilling output qE keep unchanged, while the input heat qB increases with the increase of the circulation multiple f (Fig. 4a). This is why the thermal efficiency and exergy efficiency decrease as the circulation multiple increases (Fig. 4b). As the heat transfer process of basic solution is divided into sub-cooling and evaporation in the recuperator, the temperatures of saturation point and the inlet/outlet points of the basic solution as well as their hot fluid side counterparts are showed in the Fig. 4c. When the f increases, the temperatures at turbine exit (16), the inlet (2a) and saturation point (3) of basic solution are kept unchanged, while the outlet temperatures of both basic solution (4) and work solution (18) decrease and the temperature of point 17 increases. Fig. 4d shows the temperature differences at the three points vary with the circulation multiple. It can be seen that the pinch point is at either end of the recuperator that only in the range of from 3.3 to 4.15 the circulation multiple meet the heat transfer constraint of minimum pinch point temperature difference of 5 K. When the circulation multiple f is less than 3.3, the turbine exhaust vapor cannot supply enough heat for the desorption process and the cycle cannot be completed. In fact, the circulation multiple f and the desorption
temperature t4 have a mutual relationship once the work and basic concentrations are fixed. 4.4. Chilling fraction The calculated value of chilling fraction n ranges from 0 to 0.6, as shown in Fig. 5. If the chilling fraction n is set at 0, the cogeneration cycle will become a power generation cycle. As n increases, the flow rate fraction for the power generation decreases, which results in a decrease of the specific values of power output and the input heat, based on the flow rate of total work solution flow at outlet of mid-p-absorber, but an increase of the chilling output (Fig. 5a). When based on the turbine work solution flow, as n increases, the specific values of the power output wT and the input heat qBT keep almost unchanged while the chilling output qET increases (Fig. 5b). In general, the expense of the chilling output is very small and both the thermal efficiency hth and the exergy efficiency hex favors a higher chilling fraction n (Fig. 5c). As previously discussed, an ammoniaewater power cycle is formed if the chilling fraction is set at 0. The thermal efficiency and exergy efficiency of the proposed cogeneration cycle at chilling fraction n equals 0.5 is 24.24% and 8.16% higher than those of the power cycle under the same condition. To more clearly illustrate the superiority of the cycle, the power cycle can be compared with the Kalina cycle under the same conditions, as details shown in Table 6. The most important criterion with which to assess a system is net power output. In other words, the most desirable outcome is to convert more energy to electricity. According to engineering thermodynamics, under the same values of inlet/outlet pressure as well as ammonia concentration, the enthalpy drop in turbine of each cycle should be the same; however, the amount of energy devoted to the Kalina cycle should be more.
J. Hua et al. / Applied Thermal Engineering 64 (2014) 483e490
489
Fig. 4. Impact of circulation multiple on thermal performances (xw ¼ 0.5, xb ¼ 0.35, n ¼ 0.5, tB ¼ 200 C).
With a preheater before the boiler, the input heat can be reduced thus thermal efficiency can be raised; and with a water cooler in the dilute solution loop, the sanitary hot water is provided and the discarded heat to the cooling water sink decrease. According to the results, the net power efficiency of proposed cycle is 11.97% higher than the Kalina cycle.
5. Conclusion 1) The use of ammonia water mixtures allows efficient heat recovery with variable temperature in phase change processes to match both heat source and cooling water with adjustable
solution concentration in addition to the cheap working medium cost. 2) The work concentration xw of ammoniaewater solution is selected mainly for matching volumetric flow of turbine and for positive turbine backpressure. Under the certain work concentration xw, there is an optimum basic concentration xb forming a coupled solution concentration pair to yield higher efficiency and power/chilling outputs. 3) There is applicable range of circulation multiple of the cogeneration cycle limited by heat transfer constraint. However, a lower circulation multiple is good for higher thermal efficiency. 4) The chilling fraction can be adjusted for balance of the power and chilling output. The total thermal efficiency of the proposed
Fig. 5. Impact of chilling fraction on thermal performance (xw ¼ 0.5, xb ¼ 0.35, f ¼ 4, tB ¼ 200 C).
490
J. Hua et al. / Applied Thermal Engineering 64 (2014) 483e490
Table 6 Comparison of performances of different cycles. Item
Unit
System discussed in this paper (n ¼ 0.5)
Kalina System cycle discussed in this paper (n ¼ 0)
Inlet pressure of turbine ph Inlet temperature of turbine t15 Inlet temperature of heat resource th1 Evaporation temperature in boiler Condensation (absorption) pressure pl Inlet temperature of cooling water tc1 Temperature rise of cooling water Dtc Enthalpy drop in turbine Work output Specific uptake heat Specific power of pump 1 Specific power of pump 2 Net power efficiency
MPa C C
2.736 195 200
2.736 195 200
2.736 195 200
C MPa
99/190 0.16
99/190 0.16
99/190 0.16
C
25
25
25
C
8
8
8
278.2 139.1 1024.65 2.49 2.32 13.1
278.2 278.2 2049.3 2.49 4.64 13.2
278.2 278.2 2256.78 2.49 4.64 11.7
kJ/kg kw kw kw kw %
cogeneration cycle at chilling fraction equals 0.5 is 24.2% higher than that of the power cycle under the same condition.
Acknowledgements This work is supported dation Programs of China Science and Technology Achievements of Special (BY2011155).
by the National Nature Science Foun(50976022, 51276035) and Provincial Innovation and Transformation of Fund Project of Jiangsu Province
References [1] A.I. Kalina, Combined-cycle system with novel bottoming cycle, J. Eng. Gas. Turbines Power 106 (1984) 737e742. [2] H.A. Mlcak, An introduction to the Kalina cycle, in: Proceedings of the International Joint Power Generation Conference, Houston, Texas, 1996. [3] X.X. Zhang, M.G. He, Y.A. Zhang, Review of research on the Kalina cycle, Renewable Sustainable Energy Rev. 16 (2012) 5309e5318.
[4] U.R. Nasruddin, M. Rifaldi, A. Noor, Energy and exergy analysis of Kalina cycle system(KCS)34 with mass fraction ammoniaewater mixture variation, J. Mech. Sci. Technol. 23 (2012) 1871e1876. [5] O. Arslan, Exergoeconomic evaluation of electricity generation by the medium temperature geothermal resources, using a Kalina cycle: Simav case study, Int. J. Therm. Sci. 49 (2010) 1866e1873. [6] C. Zamfirescu, I. Dincer, Thermodynamic analysis of a novel ammoniaewater trilateral Rankine cycle, Thermochim. Acta 477 (2008) 7e15. [7] W.R. Wagar, C. Zamfirescu, I. Dincer, Thermodynamic performance assessment of an ammoniaewater Rankine cycle for power and heat production, Energy Convers. Manage. 51 (2010) 2501e2509. [8] O. Sirko, Integration of Kalina cycle in a combined heat and power plant, a case study, Appl. Therm. Eng. 29 (2009) 2843e2848. [9] R.S. Murugan, P.M.V. Subbarao, Thermodynamic analysis of RankineeKalina combined cycle, Int. J. Thermodyn. 11 (2008) 133e141. [10] Y.P. Chen, Thermodynamic analysis of a simplified Kalina cycle, Therm. Power Gener. 47 (1997) 20e23 (in Chinese). [11] J.Y. Hua, Y.P. Chen, H.J. Liu, J.F. Wu, Thermodynamic analysis of a simplified dual-pressure ammonia-water absorption power cycle, J. Cent. South Univ. Technol. 19 (2012) 797e802. [12] Y.P. Chen, Thermodynamic analysis of a modified Kalina cycle, J. Southeast Univ. 19 (1989) 52e58 (in Chinese). [13] D.Y. Goswami, F. Xu, Analysis of a new thermodynamic cycle for combined power and cooling using low and mid temperature solar collectors, J. Sol. Energy Eng. 121 (1999) 91e97. [14] H.A. Ingley, R. Reed, D.Y. Goswami, Optimization of a scroll expander applied to an ammonia/water combined cycle system for hydrogen production, in: Proceedings of the Solar World Congress, Orlando, FL, August 2e12, 2005. [15] R.V. Padilla, G. Demirkaya, D.Y. Goswami, Analysis of power and cooling cogeneration using ammonia-water mixture, Energy 35 (2010) 1e9. [16] D.X. Zheng, B. Chen, Y. Qi, A Thermodynamic analysis of a novel absorption power/refrigeration combined cycle, J. Eng. Thermophys. 23 (2002) 201e204 (in Chinese). [17] N. Zhang, N. Lior, Development of a novel combined absorption cycle for power generation and refrigeration, J. Energy Res. Technol. 129 (2007) 254e 265. [18] S. Shulz, Equations on state for the system ammoniaewater for use with computers, in: International Congress of Refrigeration Proceedings, Meeting Commission II, Washington, 1971. [19] Y.P. Chen, C.M. Lin, Y. Tian, NH3eH2O solution cooling absorption refrigeration driven by fishing boat diesel exhaust heat, in: Proceedings of the 6th International Symposium on Heating, Ventilation and Air Conditioning, Nanjing, China, Nov. 6e9, 2009. [20] K. Mejbri, A. Bellagi, Modelling of thermodynamic properties of the watere ammonia mixture by three different approaches, Int. J. Refrig. 29 (2006) 211e 218. [21] C.L. Joseph, Energy analysis of commercial buildings in subtropical climates building and Environment, Build. Environ. 35 (2000) 19e26. [22] A. Franco, M. Villani, Optimal design of binary cycle power plants for waterdominated, Geothermics 38 (2009) 379e391. [23] W. Fu, J. Zhu, W. Zhang, Z. Lu, Performance evaluation of Kalina cycle subsystem on geothermal power generation in the oilfield, Appl. Therm. Eng. 54 (2013) 497e506.