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regenerator using liquid desiccant
Renewable Energy 34 (2009) 699–705
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Modeling and performance analysis of solar air pretreatment collector/regenerator using liquid desiccant Donggen Peng*, Xiaosong Zhang School of Energy and Environment, Southeast University, Nanjing 210096, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 November 2007 Accepted 2 May 2008 Available online 17 July 2008
A solar liquid regenerator that embodies energy saving effect is a key part in solar liquid cooling airconditioning system. Solar air pretreatment liquid collector/regenerator as a novel solar C/R (collector/ regenerator) can achieve liquid regeneration in lower temperature, which is suitable to be employed in the high humidity area. The heat and mass transfer process was simulated in the novel liquid regenerator and the conclusions show that the increment of solution outlet concentration increases 70%, regeneration efficiency hz augments 45.7% and storage capacity SC increases 44% as effective solution proportion ESP falls from 100% to 62%. For higher solution outlet concentration needed in the dehumidifier, both lower solution mass flow rate and higher solution inlet concentration all can be adopted in the novel C/R, in which the decrease of effective solution proportion ESP can increase the rate of evaporation G significantly. Along with the augment of air mass flow rate, the rate of evaporation G rises fast firstly and then falls slowly. The simulated results show that there is huge potential of improving and regulating solution regeneration performance by employing the novel C/R. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Collector/regenerator Air pretreatment Effective solution proportion ESP Effective storage capacity ESC
1. Introduction The traditional refrigeration cycles are driven by electricity or heat, which strongly increase the consumption of electricity and fossil energy. The International Institute of Refrigeration in Paris (IIF/IIR) has estimated that approximately 15% of all the electricity produced in the whole world is employed for refrigeration and airconditioning processes of various kinds, and the energy consumption for air-conditioning systems has recently been estimated to 45% of the whole households and commercial buildings [1,2]. Furthermore, the traditional commercial, non-natural working fluids, like the chlorofluorocarbures (CFCs), the hydrochlorofluorocarbures (HCFCs) and the hydrofluorocarbures (HFCs) result in both ozone depletion and global warming. Under the increasingly austere situation of energy sources and environment problems, solar energy employed in refrigeration systems has large application merit. Solar energy-driven liquid desiccant cooling systems (LDCS) that can reduce electrical energy consumption by utilizing solar energy and avoid ozone depletion by employing natural working fluids – air or water have been developed as an alternative to vapor compression cooling devices for air-conditioning applications. A liquid desiccant cooling system using solar energy was first studied by Lo¨f
* Corresponding author. Tel.: þ86 025 83795435; fax: þ86 025 83795445. E-mail address: [email protected] (D. Peng). 0960-1481/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2008.05.015
[3]. From then on, there have been significant interests in studying the application of solar energy-driven liquid desiccant cooling systems in major developed countries [4, 5]. Two important parts of the solar liquid desiccant airconditioning system are the regenerator, in which the weak solution is concentrated and the solar collector in which solar radiation is transformed into heat energy. The two processes of liquid regeneration and solar collection can be disposed, respectively [6]. However, by using direct solar regenerators where the desiccant solution is itself the heat collecting fluid, the regeneration process could be made more effective. Combining solar collection and liquid regeneration together, a solar collector/regenerator (C/R) can be established that has attracted great interest from some scholars. According to the difference of flow impetus, there occur natural and forced flow collector/regenerators (C/R). In natural flow C/R, the regeneration of desiccant solution takes place on an open surface, opposed to the forced C/R with the single (or double) glazing. On the study of natural flow C/R, Kakabayev et al. [7] described a solar liquid desiccant cooling system in which the solar collection and regeneration of lithium chloride solution take place on an open surface. An analytical procedure for calculating the mass of water evaporated from the natural C/R has been developed by Collier [8]. One of the efficient methods of direct solar regeneration is the forced flow collector/regenerator. Alizadeh and Saman [9,10] developed a computer model on the forced flow collector/regenerator to simulate the rate of evaporation of water as a function of the system variables and the climatic condition and built the
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Nomenclature Cp C dk ESC ESP G hc hm hz h Ia Ic m P q r0 SC T
specific heat (kJ/(kg K)) salt concentration of desiccant solution (kg/kg) hydraulic diameter (m) effective storage capacity (MJ/m3) effective solution proportion (%) water evaporation/condensation rate for unit area of absorber (kg/(m2 h)) heat transfer coefficient between air stream and solution (kW/(m2 K)) mass transfer coefficient between air stream and solution (kg/(m2 s)) overall heat transfer coefficient between air and outdoor environment (kW/(m2 K)) specific enthalpy (kJ/kg) effective solar radiation intensity (kW/m2) solar radiation intensity (kW/m2) mass flow rate (kg/s) partial pressure of water vapor (kPa) heat loss from air stream to surrounding (kW/m2) heat of water vaporization/condensation (kJ/kg) storage capacity (MJ/m3) temperature ( C)
UL V X x Y
overall heat loss coefficient (kW/(m2 K)) volume flow rate/volume (m3/s/m3) water content of the salt (kg/kg) distance traveled by solution along the regenerator (m) humidity ratio of the air (g/kg)
Greek letters hz regeneration efficiency v outdoor wind velocity (m/s) r density/reflectance of absorbent solution for solar radiation (kg/m3) effective absorptance–transmittance product (sa) Subscripts a air dil.sol diluted solution equ equilibrium situation in inlet of the labeled flow out outlet of the labeled flow salt salt s solution w surroundings d air pretreatment process
prototype of the forced flow collector/regenerator to test its performance. Ru Yang and Pai-Lu Wang [11,12] evaluated the performance of single-glazed and double-glazed collectors/regenerators for an open-cycle absorption solar cooling system and the result shows that the double-glazed forced convection C/R gives a better system performance. Kabeel [13] compared the regeneration efficiency between the natural C/R and the forced C/R and finds enhancement of regeneration efficiency for forced flow compared with the free flow regeneration. The forced flow C/R employs an inclined flat blackened surface over which the absorbent solution to be concentrated trickles down as a thin liquid film. The schematic of the C/R was shown in Fig. 1. The bottom of the C/R is well-insulated and hence the rear heat loss coefficient can be neglected. In order to reduce top losses and eliminate contamination of the solution with dust, the C/R is covered by a single or double glazing. Due to absorption of solar energy by the plate, water evaporates from the liquid surface and is
removed by a forced air stream. The air stream flows parallel or counter to the liquid film. The study results [14] show that the performance of solar collector/regenerator is influenced by air inlet temperature and humidity significantly. In South China, the days in 70–80% relative humidity ratio take up more than half in hot summer in which the regeneration performance is necessary to be impaired heavily. In order to conquer the adverse influence of high humidity ratio of air on regenerator performance, a new solar liquid collector/regenerator is designed d solar air pretreatment liquid collector/ regenerator and the analysis of its performance is studied in this paper.
Fig. 1. Schematic diagram of traditional solar liquid collector/regenerator.
Fig. 2. Schematic diagram of solar air pretreatment collector/regenerator.
2. The system of solar air pretreatment collector/regenerator The system of solar air pretreatment collector/regenerator shown in Fig. 2 is composed of air cycle and solution cycle. The air cycle consists of blower, air pretreatment unit and solar collector/regenerator. The air stream coming from outside is forced by blower
D. Peng, X. Zhang / Renewable Energy 34 (2009) 699–705
into air pretreatment unit where it is contact with low temperature strong solution and takes place isoenthalpy dehumidification process and then flows into solar collector/regenerator in which the air is heated and humidified through heat and mass transfer with high temperature diluted solution, finally is ejected to atmosphere. The air cycle is an open cycle. The solution cycle is consisted of antisepsis solution pump, air pretreatment unit, collector/regenerator and liquid heat exchanger. The diluted solution out of the air pretreatment unit and the dehumidifier enters liquid heat exchanger where it is firstly heated by the strong solution leaving from the collector/regenerator and then is delivered by antisepsis solution pump into the solar collector/regenerator where the water in the solution is removed by air stream and the solution is regenerated, later comes back to the liquid heat exchanger preheating the cold diluted solution, finally flows into the dehumidifier and air pretreatment unit, respectively, where the solution is diluted by absorbing water vapor in air stream. In this way, a close circulation of dilution, regeneration and once more dilution is constructed. According to whether the flow directions of solution and air stream in the C/R are the same, or not, the novel solution C/R is divided into two working modes of parallel current and countercurrent. As air stream in the solar air pretreatment liquid collector/regenerator (novel C/R) need to be dehumidified in the air pretreatment unit, there are more solution mass flow rate needed in novel C/R than in the traditional C/R that will result in the increase of collector area. However, the solution outlet concentration can be raised in the novel C/R that combines heating regenerated liquid temperature, decreasing air humidity and increasing air temperature, three kinds of factors of promoting liquid regeneration together: compared with the traditional C/R that only depends on heating liquid temperature, the advantage of the novel C/R can make the best of lower grade heat source to achieve better regeneration effect that is well employed in high humidity climate area.
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Fig. 3. Unit volume of the C/R.
ms Cps dTs þ ma dha þ UL ðTs T0 Þdx ¼ Ia dx
(1)
msalt dX þ ma dua ¼ 0
(2)
ma Cpa dTa ¼ hc ðTs Ta Þdx hz ðTa Tw Þdx
(3)
ma dua ¼ hm uequ ua dx
(4)
3. Model foundation
where
3.1. The collector/regenerator model
Ia ¼ Ic ð1 rÞðsaÞ
(5)
In the solar air pretreatment liquid collector/regenerator, a key component is the solar collector/regenerator. In order to study the performance of the C/R, a mathematic model must be established, before which the assumptions of physical model are in the following, for the sake of simplifying calculation: (1) Neglecting the back heat loss of the C/R and only considering the heat loss through glazing cover. (2) Neglecting heat conduction and mass diffusion of air stream and liquid film along their flow directions and only considering convective heat and mass transfer. (3) Air reaches equilibrium with liquid on vapor pressure of water and temperature at their interface and ignoring the diffusion resistance of heat and mass along the thickness of liquid film. (4) The convective heat transfer coefficients for the air stream and glass cover and for the air stream and liquid film are equal (5) Neglecting the effect of pressure drop on regeneration performance in the C/R. For the finite volume scheme method, the C/R length was divided into small segments shown in Fig. 3 in parallel current mode and the mass and heat transfer balance equations were solved for each segment from the top to bottom of the C/R. In order to simplify the calculation, the effect of solar radiation on the absorber was assumed as constant heat flux Ia from the absorber to heat liquid and air stream in the C/R. The following energy and mass balance equations can be written for a segment C/R of unit width and length dx.
The overall heat transfer coefficient hz between the air stream and surroundings was solved by:
1 1 1 ¼ þ hz hc hw
(6)
The wind-induced heat transfer coefficient hw was evaluated from the following relation [15].
hw ¼ 5:7 þ 3:8v
(7)
The heat transfer coefficient between the air stream and solution has been evaluated for two basic conditions, namely turbulent flow shown in the literature [16] and laminar flow given by the following.
Nu ¼
hc dh
la
¼ 8:23
(8)
The mass transfer coefficient was determined by using the analogy
hm ¼
hc Cpa Le2=3
(9)
3.2. The air pretreatment model Another key component in the air pretreatment liquid C/R is the air pretreatment unit that can be adopted by a packed bed
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dehumidifier. A simplified model for air dehumidification with liquid desiccant was put forward by Gandhidasan [17] and the dimensionless air humidity was introduced:
a ¼
pa;in pa;out 1 pa;in ps;in
(10)
According to the principle of water vapor mass balance, the water vapor condensation rate G in the air pretreatment can be calculated by:
G ¼ msalt ðX out X in Þ ¼ ma ðY in Y out Þ
(11)
Based on the match relation between the C/R and the air pretreatment unit, the following equation was given:
X out ¼ Xin
(12)
(5) The design flow rate: ma ¼ 100 kg/h, ms ¼ 10 kg/h. (6) Heat of evaporation of water r0 is taken as 2551 kJ/kg. (7) The CaCl2 solution is adopted and its physical property can be seen in Ref. [18]. (8) The flow mode between air stream and liquid is parallel current, Le ¼ 1 The simulation results by Alizadeh and Saman [10] were compared with that by utilizing the collector/regenerator model in this paper under the same conditions as Alizadeh had, shown in Fig. 4. The results of both on the parameter of evaporation rate G agree very well along with the length of C/R, which validates the C/ R model. At the same time, Fig. 4 represents that the evaporation rate G increases about 0.1 kg/(m2 h) by decreasing the outdoor air humidity from 25.5 to 16.8 g/kg that explains the humidity ratio of outdoor air has significant impact on the liquid regeneration performance and by decreasing air humidity ratio the solution regeneration performance can be improved remarkably.
3.3. The definition of other variables 5. Simulation results and discussions Storage capacity:
SC ¼
mair DY r0 ¼ DX r0 Cps rdil:sol =1000 Vdil:sol 106
(13)
Storage capacity denotes energy consumed for the concentration of unit volume diluted liquid. Effective solution proportion:
ESP ¼
ms 100 1 ms
(14)
Effective solution proportion ESP indicates the mass ratio of liquid entering into the dehumidifier among the total regenerated liquid. The less ESP indicates the more flow rate of liquid needed in air pretreatment unit. Effective storage capacity:
ESC ¼ SC ESP
(15)
Effective storage capacity ESC can be defined as the product of storage capacity SC and effective solution proportion ESP, which reflects truly the slice of energy storage of unit volume diluted liquid employed for air dehumidified in the dehumidifier. Regeneration efficiency:
hz ¼
G r0 3600 I
The C/R (1.0 m wide 5.0 m long) employed for the solar air pretreatment liquid C/R in this paper was covered by a single glazing leaving a gap of about 5 cm. Air humidity Ya of 29 g/kg, and temperature Ta of 35 C in Nanjing, China were used as reference surrounding conditions in analysis. Other independent simulation parameters were given in the following: solution inlet concentration Cin (0.3 kg/kg) and temperature Ts,in (60 C) in the collector/ regenerator; solution inlet temperature Ts,in (35 C) in the air pretreatment unit. The design air and solution mass flow rates were 100 kg/h and 10 kg/h, respectively. The flow mode between air stream and liquid film is counter current seen in Fig. 2. Other assumptions were seen in the model validation section. 5.1. Effect of effective solution proportion (ESP) on the regenerator performance By varying the effective solution proportion ESP, the inlet air humidity Yin and temperature Ta,in can be influenced, with the changes of various regeneration parameters in the air pretreatment C/R. The effect of ESP on air inlet humidity Yin, solution outlet concentration Cout and regeneration efficiency hz are shown in Fig. 5. By decreasing ESP from 100% to 62%, the air inlet humidity
(16)
Regeneration efficiency hz denotes the proportion of energy consumed by liquid regeneration occupying the whole solar radiation energy. Before employing model to simulate the air pretreatment collector/regenerator, the model validation issue must be carried out, see below. 4. The validation of collector/regenerator model Before validating the collector/regenerator model, the following assumptions are given: (1) The C/R is 10 m 1 m 5 cm in size. (2) The solar radiant intensity is 1 kW/m2 and the value of (1 r) (sa) is 0.75. (3) The outdoor wind speed of 3 m/s and surroundings temperature Tw of 33 C. (4) The solution inlet concentration Cs,in of 0.4 kg/kg and temperature Ts,in of 33 C.
Fig. 4. Evaporation rate vs. the solar C/R length.
D. Peng, X. Zhang / Renewable Energy 34 (2009) 699–705
efficiency, the absorbed solar energy not only make up the heat consumption in air pretreatment unit, but also has more heat used for the latent heat absorption in liquid regeneration process that results in a higher solution outlet concentration compared to the traditional C/R. Based on the above analysis, it can be seen that the solution outlet concentration and solar utilizing efficiency in the air pretreatment C/R combining decreasing air humidity, increasing air temperature and heating desiccant liquid together to promote liquid regeneration can be increased better than in the traditional C/R that only depends on heating liquid temperature to achieve liquid regeneration. At the same time, in solar air pretreatment C/R by altering effective solution proportion (EPS) solution outlet concentration Cout can be regulated that can conquer the impact of fluctuating solar radiant intensity on liquid regeneration performance and the traditional C/R cannot achieve such regulation. In a word, solar air pretreatment C/R do achieve positive effect toward liquid regeneration by increasing regenerated solution quality and the utilization efficiency of solar energy significantly, which is a novel and highly efficient solar liquid regeneration equipment. 5.2. Effect of solution mass flow rate and inlet concentration on the regenerator performance The effect of solution mass flow rate ms on the performance of the air pretreatment C/R is represented in Fig. 7 that indicates the variation of water evaporation rate G as a function of solution mass flow rate under three kinds of different ESP and constant air mass flow rate ma of 100 kg/h. The regeneration effect under ESP ¼ 100% that means there occurs no solution stream in the air pretreatment unit and the air stream is not pretreated is equivalent to that in the traditional solar liquid C/R under the same regeneration conditions. As seen in Fig. 7, by increasing the solution mass flow rate ms, water evaporation G increases remarkably that explains the augment of solution mass flow rate is preferable to liquid regeneration. The more solution mass flow rate yields the lower solution concentration and temperature and the higher vapor partial pressure of solution in the solar C/R that result in the augment of water evaporation rate G. Comparing the variation of water evaporation G vs. solution mass flow rate under ESP ¼ 85% and ESP ¼ 70%, when solution mass flow rate ms is less than 75 kg/h, the water evaporation rate G is more under ESP ¼ 70% than under ESP ¼ 85% and when solution mass flow rate is more than 75 kg/h, the two regeneration conditions have the same water evaporation rate that indicates after solution mass flow rate exceeds certain limit, the drop of ESP cannot boost regeneration performance. This is because
ηz (Pretreatment)
0.60
ηz (Direct)
0.55
Cout (Pretreatment)
0.50
ΔCout/Cout
0.52 0.50
Cout (Direct)
0.48
ηz
0.46
0.40 0.44
0.35
Cout (kg/kg)
0.45
0.30 0.25
0.40
0.20
0.38 50
0
5
10
15
20
25
30
35
40
45
100-ESP(%) Fig. 5. Effect of ESP on variables of the air pretreatment C/R.
Fig. 6. Comparison between pretreatment C/R and traditional C/R.
12 9 6 3 0
Cout/Cout
0.42
∇
drops from 29 to 15 g/kg in isoenthalpy process and the air inlet temperature can also ascend. As the results of such changes of air inlet conditions, the regeneration effect in the C/R can be improved significantly. Solution outlet concentration Cout rises from 0.4 to 0.47 kg/kg and the increment of solution concentration from inlet to outlet increases 70%. The regeneration efficiency hz changes from 0.35 to 0.51 achieving the amplitude of 45.7%. The storage capacity SC increases 44% from 820 to 1180 MJ/m3. In the light of above analysis, the pretreatment process of isoenthalpy dehumidification for air stream before entering into the C/R does increase the solution regeneration performance, but the performance improvement of liquid regeneration is at the sacrifice of effective solution proportion ESP. Effective storage capacity ESC used to measure the influence of ESP on regeneration performance of liquid drops from 820 to 720 MJ/m3 reaching reduction extent of 12%. Hence, solar air pretreatment C/R can achieve better quality liquid (higher concentration) because of sacrificing a slice of solar energy employed for the pretreatment of air stream. In air pretreatment C/R, only the solution of effective solution proportion (ESP) among the whole regenerated liquid is utilized to dehumidify air in the dehumidifier. A question will occur there. When the solution mass flow rate in the traditional C/R amounts to the product of ESP and whole regenerated liquid flow rate in the air pretreatment C/R which regeneration effect will be better for the traditional C/R and air pretreatment C/R. By numerical simulation with ESP varying from 100% to 52%, the changes of outlet liquid concentration Cout and regeneration efficiency hz of traditional C/R and air pretreatment C/R are shown in Fig. 6. The regeneration efficiency hz in air pretreatment C/R is much higher than that in traditional C/R and when ESP reaches 52%, the discrepancy of two kinds of C/R amounts to 0.3. With respect to solution concentration, the solution outlet concentration Cout of air pretreatment C/R is higher than that of traditional C/R and when ESP reaches 52%, the discrepancy of two kinds of C/R amounts to 11%. Seen from energy transfer, the absorbed energy in traditional C/R except heat loss and sensible heat increment of air and liquid become latent heat consumed by solution regeneration residually and in air pretreatment C/R among the residual absorbed solar energy except the same heat loss as in the traditional C/R, only solar energy of the ESP can be consumed by liquid regeneration, the other used for air pretreatment of isoenthalpy dehumidification. As the comparisons in Fig. 6, in the air pretreatment C/R because of its high regeneration
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D. Peng, X. Zhang / Renewable Energy 34 (2009) 699–705
G (kg/m2·h)
0.7 1.2 1.1
0.6
0.4
1.0
0.44 5
6
7
8
ms (kg/h)
9
0.42
10
0.40
0.9
0.38
0.8
0.34
0.6 0.5
0.32 0
50
100
150
200
250
0.30 300
0.16 0.14 0.12 0.10 0.08
ΔG (kg/m2·h)
0.36
0.7
Cout(kg/kg)
G (kg/m2·h)
0.46
0.5
ms (Kg/h) G (ESP=100%) G (ESP=70%) Cout (ESP=70%)
G (ESP=85%)
ΔG (ESP=70–100%)
Fig. 7. Effect of solution mass flow rate ms on regeneration performance (the insect diagram indicates the effect of ms at ms < 10 kg/h).
when solution mass flow rate is greater than 75 kg/h, the solution outlet concentration maintains about 0.31 kg/kg and the air reaches equilibrium with the inlet solution of 0.31 kg/kg and 35 C in air pretreatment unit. Fig. 7 also shows that when ms is less than 10 kg/ h, the change of ESP has great impact on the regenerator performance and the difference DG of evaporation rate G between ESP ¼ 70 and ESP ¼ 100% increases linearly. However, after solution mass flow rate exceeds 10 kg/h, DG drops and tends to steady value when solution mass flow rate is greater than 75 kg/h. Based on the above analysis, the increase of solution mass flow rate does improve regeneration performance being at cost of sacrificing solution quality (solution concentration) that does harm to dehumidification performance in the liquid desiccant cooling system. For maintaining higher solution outlet concentration, lower solution mass flow rate must be adopted, in which the adjusting of effective solution proportion ESP powerfully improves regeneration performance that is just the advantage of solar air pretreatment C/R. Besides effect of solution mass flow rate on regeneration performance, the solution inlet concentration Cin has large impact on solution regeneration shown in Fig. 8 with ma ¼ 100 kg/h and ms ¼ 10 kg/h that represents when the solution inlet concentration increases, the water evaporation G decreases linearly and when inlet concentration Cin rises from 0.30 to 0.4 kg/kg, the evaporation rate G drops about 30 percent. The decrease of ESP can increase water evaporation G and the increment of G at lower solution inlet concentration Cin is higher than that at higher Cin. Because the inlet concentration Cin in the solar C/R depends on dehumidification condition in liquid desiccant cooling system, the lower Cin is adverse to dehumidification condition in that system and the higher Cin can impair regeneration performance in the C/R. In the contradiction relationship by employing air pretreatment function and decreasing effective solution proportion ESP, the regeneration performance can be improved remarkably. Simultaneously, the drop of ESP can be compensated by adding solar collector area and solution mass flow rate. 5.3. Effect of air mass flow rate on the regenerator performance In order to analyze the effect of air mass flow rate on regeneration process, the solution mass flow rate is settled at 10 kg/ h and the range of air mass flow rate is considered about 10–800 kg/
Fig. 8. Effect of solution inlet concentration Cin on evaporation rate G.
h. The water evaporation rates under ESP ¼ 100%, 85% and 70% are shown in Fig. 9 that shows the evaporation rates G rise fast at first and then fall slowly. The evaporation rates reach the maximum when the air mass flow rate ma amounts to about 200 kg/h in that the heat and mass transfer coefficient augments when air mass flow rate increases and the augment of air mass flow rate can decrease air humidity ratio in the C/R that enhances vapor pressure difference between air stream and solution, which is in favor of liquid regeneration. However, the increase of air mass flow rate can also elevate heat loss from the regenerator and when air mass flow rate exceeds some value, the heat loss in regeneration process take up dominant status that can decrease water evaporation G slowly. Fig. 9 also indicates that the drop of ESP yields great improvement of evaporation rate G that explains under different air mass flow rates, the regeneration process can be regulated by varying ESP in air pretreatment C/R. When air mass flow rate is less than 30 kg/h the fluctuation lines of water evaporation rates G under ESP ¼ 85% and 70% are almost superposed showing that when air mass flow rate is less than 30 kg/h, the fall of ESP cannot improve regeneration performance. This is because when air mass flow rate is very small, though ESP descends from 85% to 70% that increases the solution mass flow rate employed for air pretreatment, the outlet air humidity ratio in the air pretreatment unit reaches equilibrium with solution and cannot be increased by decreasing ESP that results in the same evaporation rate G. That range of air mass flow rate ma is called blind area for effect of ESP. From the above analysis, the effect of air mass flow rate on liquid regeneration performance has a maximum and the ESP has significant influence on regeneration performance except for a blind area at small air flow rate. 6. Conclusions In order to analyze the steady regeneration performance of solar air pretreatment liquid C/R, the C/R model and air pretreatment model were established and by applying numerical simulation the regeneration performance of the air pretreatment C/R are obtained as functions of effective solution proportion ESP, solution inlet condition and air mass flow rate. With respect to the effect of ESP on regeneration performance, when ESP fall from 100% to 62%, the regeneration efficiency hz ascends 45.7 percent, storage capacity SC increases 44 percent and the increment of solution concentration between outlet and inlet of the regenerator increases by 70 percent. All these explain the lower ESP can be in favor of liquid regeneration. Compared with traditional C/R at the same solar radiant intensity, the air pretreatment
D. Peng, X. Zhang / Renewable Energy 34 (2009) 699–705
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Research Innovation Program of Jiangsu Province China (CX07B_095z) and the Key Grant Project of Chinese Ministry of Education (307013).
References
Fig. 9. Effect of air mass flow rate ma on evaporation rate G.
C/R combines decreasing air humidity, increasing air temperature and heating desiccant liquid d three kinds of promoting solution regeneration methods together that do improve solution regeneration performance and solar energy utilization efficiency opposed to traditional C/R only depending on heating regenerated solution. Concerning effect of solution mass flow rate and inlet concentration on regeneration performance, the increase of solution mass flow rate can effectively improve regeneration performance, though it sacrifices solution outlet concentration. Under ensuring higher solution outlet concentration, whether adopting small solution mass flow rate or increasing solution inlet concentration in the C/R are available and the fall of ESP can boost water evaporation rate G and improve regeneration effect. The effect of air mass flow rate on evaporation rate G has a maximum and when ma is on the small side, there is a blind area that ESP has no effect on regeneration performance. Acknowledgements This research was supported by the National Natural Science Foundation of China (50676018), the College Graduate Scientific
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Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Modeling and performance analysis of solar air pretreatment collector/regenerator using liquid desiccant Donggen Peng*, Xiaosong Zhang School of Energy and Environment, Southeast University, Nanjing 210096, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 November 2007 Accepted 2 May 2008 Available online 17 July 2008
A solar liquid regenerator that embodies energy saving effect is a key part in solar liquid cooling airconditioning system. Solar air pretreatment liquid collector/regenerator as a novel solar C/R (collector/ regenerator) can achieve liquid regeneration in lower temperature, which is suitable to be employed in the high humidity area. The heat and mass transfer process was simulated in the novel liquid regenerator and the conclusions show that the increment of solution outlet concentration increases 70%, regeneration efficiency hz augments 45.7% and storage capacity SC increases 44% as effective solution proportion ESP falls from 100% to 62%. For higher solution outlet concentration needed in the dehumidifier, both lower solution mass flow rate and higher solution inlet concentration all can be adopted in the novel C/R, in which the decrease of effective solution proportion ESP can increase the rate of evaporation G significantly. Along with the augment of air mass flow rate, the rate of evaporation G rises fast firstly and then falls slowly. The simulated results show that there is huge potential of improving and regulating solution regeneration performance by employing the novel C/R. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Collector/regenerator Air pretreatment Effective solution proportion ESP Effective storage capacity ESC
1. Introduction The traditional refrigeration cycles are driven by electricity or heat, which strongly increase the consumption of electricity and fossil energy. The International Institute of Refrigeration in Paris (IIF/IIR) has estimated that approximately 15% of all the electricity produced in the whole world is employed for refrigeration and airconditioning processes of various kinds, and the energy consumption for air-conditioning systems has recently been estimated to 45% of the whole households and commercial buildings [1,2]. Furthermore, the traditional commercial, non-natural working fluids, like the chlorofluorocarbures (CFCs), the hydrochlorofluorocarbures (HCFCs) and the hydrofluorocarbures (HFCs) result in both ozone depletion and global warming. Under the increasingly austere situation of energy sources and environment problems, solar energy employed in refrigeration systems has large application merit. Solar energy-driven liquid desiccant cooling systems (LDCS) that can reduce electrical energy consumption by utilizing solar energy and avoid ozone depletion by employing natural working fluids – air or water have been developed as an alternative to vapor compression cooling devices for air-conditioning applications. A liquid desiccant cooling system using solar energy was first studied by Lo¨f
* Corresponding author. Tel.: þ86 025 83795435; fax: þ86 025 83795445. E-mail address: [email protected] (D. Peng). 0960-1481/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2008.05.015
[3]. From then on, there have been significant interests in studying the application of solar energy-driven liquid desiccant cooling systems in major developed countries [4, 5]. Two important parts of the solar liquid desiccant airconditioning system are the regenerator, in which the weak solution is concentrated and the solar collector in which solar radiation is transformed into heat energy. The two processes of liquid regeneration and solar collection can be disposed, respectively [6]. However, by using direct solar regenerators where the desiccant solution is itself the heat collecting fluid, the regeneration process could be made more effective. Combining solar collection and liquid regeneration together, a solar collector/regenerator (C/R) can be established that has attracted great interest from some scholars. According to the difference of flow impetus, there occur natural and forced flow collector/regenerators (C/R). In natural flow C/R, the regeneration of desiccant solution takes place on an open surface, opposed to the forced C/R with the single (or double) glazing. On the study of natural flow C/R, Kakabayev et al. [7] described a solar liquid desiccant cooling system in which the solar collection and regeneration of lithium chloride solution take place on an open surface. An analytical procedure for calculating the mass of water evaporated from the natural C/R has been developed by Collier [8]. One of the efficient methods of direct solar regeneration is the forced flow collector/regenerator. Alizadeh and Saman [9,10] developed a computer model on the forced flow collector/regenerator to simulate the rate of evaporation of water as a function of the system variables and the climatic condition and built the
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Nomenclature Cp C dk ESC ESP G hc hm hz h Ia Ic m P q r0 SC T
specific heat (kJ/(kg K)) salt concentration of desiccant solution (kg/kg) hydraulic diameter (m) effective storage capacity (MJ/m3) effective solution proportion (%) water evaporation/condensation rate for unit area of absorber (kg/(m2 h)) heat transfer coefficient between air stream and solution (kW/(m2 K)) mass transfer coefficient between air stream and solution (kg/(m2 s)) overall heat transfer coefficient between air and outdoor environment (kW/(m2 K)) specific enthalpy (kJ/kg) effective solar radiation intensity (kW/m2) solar radiation intensity (kW/m2) mass flow rate (kg/s) partial pressure of water vapor (kPa) heat loss from air stream to surrounding (kW/m2) heat of water vaporization/condensation (kJ/kg) storage capacity (MJ/m3) temperature ( C)
UL V X x Y
overall heat loss coefficient (kW/(m2 K)) volume flow rate/volume (m3/s/m3) water content of the salt (kg/kg) distance traveled by solution along the regenerator (m) humidity ratio of the air (g/kg)
Greek letters hz regeneration efficiency v outdoor wind velocity (m/s) r density/reflectance of absorbent solution for solar radiation (kg/m3) effective absorptance–transmittance product (sa) Subscripts a air dil.sol diluted solution equ equilibrium situation in inlet of the labeled flow out outlet of the labeled flow salt salt s solution w surroundings d air pretreatment process
prototype of the forced flow collector/regenerator to test its performance. Ru Yang and Pai-Lu Wang [11,12] evaluated the performance of single-glazed and double-glazed collectors/regenerators for an open-cycle absorption solar cooling system and the result shows that the double-glazed forced convection C/R gives a better system performance. Kabeel [13] compared the regeneration efficiency between the natural C/R and the forced C/R and finds enhancement of regeneration efficiency for forced flow compared with the free flow regeneration. The forced flow C/R employs an inclined flat blackened surface over which the absorbent solution to be concentrated trickles down as a thin liquid film. The schematic of the C/R was shown in Fig. 1. The bottom of the C/R is well-insulated and hence the rear heat loss coefficient can be neglected. In order to reduce top losses and eliminate contamination of the solution with dust, the C/R is covered by a single or double glazing. Due to absorption of solar energy by the plate, water evaporates from the liquid surface and is
removed by a forced air stream. The air stream flows parallel or counter to the liquid film. The study results [14] show that the performance of solar collector/regenerator is influenced by air inlet temperature and humidity significantly. In South China, the days in 70–80% relative humidity ratio take up more than half in hot summer in which the regeneration performance is necessary to be impaired heavily. In order to conquer the adverse influence of high humidity ratio of air on regenerator performance, a new solar liquid collector/regenerator is designed d solar air pretreatment liquid collector/ regenerator and the analysis of its performance is studied in this paper.
Fig. 1. Schematic diagram of traditional solar liquid collector/regenerator.
Fig. 2. Schematic diagram of solar air pretreatment collector/regenerator.
2. The system of solar air pretreatment collector/regenerator The system of solar air pretreatment collector/regenerator shown in Fig. 2 is composed of air cycle and solution cycle. The air cycle consists of blower, air pretreatment unit and solar collector/regenerator. The air stream coming from outside is forced by blower
D. Peng, X. Zhang / Renewable Energy 34 (2009) 699–705
into air pretreatment unit where it is contact with low temperature strong solution and takes place isoenthalpy dehumidification process and then flows into solar collector/regenerator in which the air is heated and humidified through heat and mass transfer with high temperature diluted solution, finally is ejected to atmosphere. The air cycle is an open cycle. The solution cycle is consisted of antisepsis solution pump, air pretreatment unit, collector/regenerator and liquid heat exchanger. The diluted solution out of the air pretreatment unit and the dehumidifier enters liquid heat exchanger where it is firstly heated by the strong solution leaving from the collector/regenerator and then is delivered by antisepsis solution pump into the solar collector/regenerator where the water in the solution is removed by air stream and the solution is regenerated, later comes back to the liquid heat exchanger preheating the cold diluted solution, finally flows into the dehumidifier and air pretreatment unit, respectively, where the solution is diluted by absorbing water vapor in air stream. In this way, a close circulation of dilution, regeneration and once more dilution is constructed. According to whether the flow directions of solution and air stream in the C/R are the same, or not, the novel solution C/R is divided into two working modes of parallel current and countercurrent. As air stream in the solar air pretreatment liquid collector/regenerator (novel C/R) need to be dehumidified in the air pretreatment unit, there are more solution mass flow rate needed in novel C/R than in the traditional C/R that will result in the increase of collector area. However, the solution outlet concentration can be raised in the novel C/R that combines heating regenerated liquid temperature, decreasing air humidity and increasing air temperature, three kinds of factors of promoting liquid regeneration together: compared with the traditional C/R that only depends on heating liquid temperature, the advantage of the novel C/R can make the best of lower grade heat source to achieve better regeneration effect that is well employed in high humidity climate area.
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Fig. 3. Unit volume of the C/R.
ms Cps dTs þ ma dha þ UL ðTs T0 Þdx ¼ Ia dx
(1)
msalt dX þ ma dua ¼ 0
(2)
ma Cpa dTa ¼ hc ðTs Ta Þdx hz ðTa Tw Þdx
(3)
ma dua ¼ hm uequ ua dx
(4)
3. Model foundation
where
3.1. The collector/regenerator model
Ia ¼ Ic ð1 rÞðsaÞ
(5)
In the solar air pretreatment liquid collector/regenerator, a key component is the solar collector/regenerator. In order to study the performance of the C/R, a mathematic model must be established, before which the assumptions of physical model are in the following, for the sake of simplifying calculation: (1) Neglecting the back heat loss of the C/R and only considering the heat loss through glazing cover. (2) Neglecting heat conduction and mass diffusion of air stream and liquid film along their flow directions and only considering convective heat and mass transfer. (3) Air reaches equilibrium with liquid on vapor pressure of water and temperature at their interface and ignoring the diffusion resistance of heat and mass along the thickness of liquid film. (4) The convective heat transfer coefficients for the air stream and glass cover and for the air stream and liquid film are equal (5) Neglecting the effect of pressure drop on regeneration performance in the C/R. For the finite volume scheme method, the C/R length was divided into small segments shown in Fig. 3 in parallel current mode and the mass and heat transfer balance equations were solved for each segment from the top to bottom of the C/R. In order to simplify the calculation, the effect of solar radiation on the absorber was assumed as constant heat flux Ia from the absorber to heat liquid and air stream in the C/R. The following energy and mass balance equations can be written for a segment C/R of unit width and length dx.
The overall heat transfer coefficient hz between the air stream and surroundings was solved by:
1 1 1 ¼ þ hz hc hw
(6)
The wind-induced heat transfer coefficient hw was evaluated from the following relation [15].
hw ¼ 5:7 þ 3:8v
(7)
The heat transfer coefficient between the air stream and solution has been evaluated for two basic conditions, namely turbulent flow shown in the literature [16] and laminar flow given by the following.
Nu ¼
hc dh
la
¼ 8:23
(8)
The mass transfer coefficient was determined by using the analogy
hm ¼
hc Cpa Le2=3
(9)
3.2. The air pretreatment model Another key component in the air pretreatment liquid C/R is the air pretreatment unit that can be adopted by a packed bed
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dehumidifier. A simplified model for air dehumidification with liquid desiccant was put forward by Gandhidasan [17] and the dimensionless air humidity was introduced:
a ¼
pa;in pa;out 1 pa;in ps;in
(10)
According to the principle of water vapor mass balance, the water vapor condensation rate G in the air pretreatment can be calculated by:
G ¼ msalt ðX out X in Þ ¼ ma ðY in Y out Þ
(11)
Based on the match relation between the C/R and the air pretreatment unit, the following equation was given:
X out ¼ Xin
(12)
(5) The design flow rate: ma ¼ 100 kg/h, ms ¼ 10 kg/h. (6) Heat of evaporation of water r0 is taken as 2551 kJ/kg. (7) The CaCl2 solution is adopted and its physical property can be seen in Ref. [18]. (8) The flow mode between air stream and liquid is parallel current, Le ¼ 1 The simulation results by Alizadeh and Saman [10] were compared with that by utilizing the collector/regenerator model in this paper under the same conditions as Alizadeh had, shown in Fig. 4. The results of both on the parameter of evaporation rate G agree very well along with the length of C/R, which validates the C/ R model. At the same time, Fig. 4 represents that the evaporation rate G increases about 0.1 kg/(m2 h) by decreasing the outdoor air humidity from 25.5 to 16.8 g/kg that explains the humidity ratio of outdoor air has significant impact on the liquid regeneration performance and by decreasing air humidity ratio the solution regeneration performance can be improved remarkably.
3.3. The definition of other variables 5. Simulation results and discussions Storage capacity:
SC ¼
mair DY r0 ¼ DX r0 Cps rdil:sol =1000 Vdil:sol 106
(13)
Storage capacity denotes energy consumed for the concentration of unit volume diluted liquid. Effective solution proportion:
ESP ¼
ms 100 1 ms
(14)
Effective solution proportion ESP indicates the mass ratio of liquid entering into the dehumidifier among the total regenerated liquid. The less ESP indicates the more flow rate of liquid needed in air pretreatment unit. Effective storage capacity:
ESC ¼ SC ESP
(15)
Effective storage capacity ESC can be defined as the product of storage capacity SC and effective solution proportion ESP, which reflects truly the slice of energy storage of unit volume diluted liquid employed for air dehumidified in the dehumidifier. Regeneration efficiency:
hz ¼
G r0 3600 I
The C/R (1.0 m wide 5.0 m long) employed for the solar air pretreatment liquid C/R in this paper was covered by a single glazing leaving a gap of about 5 cm. Air humidity Ya of 29 g/kg, and temperature Ta of 35 C in Nanjing, China were used as reference surrounding conditions in analysis. Other independent simulation parameters were given in the following: solution inlet concentration Cin (0.3 kg/kg) and temperature Ts,in (60 C) in the collector/ regenerator; solution inlet temperature Ts,in (35 C) in the air pretreatment unit. The design air and solution mass flow rates were 100 kg/h and 10 kg/h, respectively. The flow mode between air stream and liquid film is counter current seen in Fig. 2. Other assumptions were seen in the model validation section. 5.1. Effect of effective solution proportion (ESP) on the regenerator performance By varying the effective solution proportion ESP, the inlet air humidity Yin and temperature Ta,in can be influenced, with the changes of various regeneration parameters in the air pretreatment C/R. The effect of ESP on air inlet humidity Yin, solution outlet concentration Cout and regeneration efficiency hz are shown in Fig. 5. By decreasing ESP from 100% to 62%, the air inlet humidity
(16)
Regeneration efficiency hz denotes the proportion of energy consumed by liquid regeneration occupying the whole solar radiation energy. Before employing model to simulate the air pretreatment collector/regenerator, the model validation issue must be carried out, see below. 4. The validation of collector/regenerator model Before validating the collector/regenerator model, the following assumptions are given: (1) The C/R is 10 m 1 m 5 cm in size. (2) The solar radiant intensity is 1 kW/m2 and the value of (1 r) (sa) is 0.75. (3) The outdoor wind speed of 3 m/s and surroundings temperature Tw of 33 C. (4) The solution inlet concentration Cs,in of 0.4 kg/kg and temperature Ts,in of 33 C.
Fig. 4. Evaporation rate vs. the solar C/R length.
D. Peng, X. Zhang / Renewable Energy 34 (2009) 699–705
efficiency, the absorbed solar energy not only make up the heat consumption in air pretreatment unit, but also has more heat used for the latent heat absorption in liquid regeneration process that results in a higher solution outlet concentration compared to the traditional C/R. Based on the above analysis, it can be seen that the solution outlet concentration and solar utilizing efficiency in the air pretreatment C/R combining decreasing air humidity, increasing air temperature and heating desiccant liquid together to promote liquid regeneration can be increased better than in the traditional C/R that only depends on heating liquid temperature to achieve liquid regeneration. At the same time, in solar air pretreatment C/R by altering effective solution proportion (EPS) solution outlet concentration Cout can be regulated that can conquer the impact of fluctuating solar radiant intensity on liquid regeneration performance and the traditional C/R cannot achieve such regulation. In a word, solar air pretreatment C/R do achieve positive effect toward liquid regeneration by increasing regenerated solution quality and the utilization efficiency of solar energy significantly, which is a novel and highly efficient solar liquid regeneration equipment. 5.2. Effect of solution mass flow rate and inlet concentration on the regenerator performance The effect of solution mass flow rate ms on the performance of the air pretreatment C/R is represented in Fig. 7 that indicates the variation of water evaporation rate G as a function of solution mass flow rate under three kinds of different ESP and constant air mass flow rate ma of 100 kg/h. The regeneration effect under ESP ¼ 100% that means there occurs no solution stream in the air pretreatment unit and the air stream is not pretreated is equivalent to that in the traditional solar liquid C/R under the same regeneration conditions. As seen in Fig. 7, by increasing the solution mass flow rate ms, water evaporation G increases remarkably that explains the augment of solution mass flow rate is preferable to liquid regeneration. The more solution mass flow rate yields the lower solution concentration and temperature and the higher vapor partial pressure of solution in the solar C/R that result in the augment of water evaporation rate G. Comparing the variation of water evaporation G vs. solution mass flow rate under ESP ¼ 85% and ESP ¼ 70%, when solution mass flow rate ms is less than 75 kg/h, the water evaporation rate G is more under ESP ¼ 70% than under ESP ¼ 85% and when solution mass flow rate is more than 75 kg/h, the two regeneration conditions have the same water evaporation rate that indicates after solution mass flow rate exceeds certain limit, the drop of ESP cannot boost regeneration performance. This is because
ηz (Pretreatment)
0.60
ηz (Direct)
0.55
Cout (Pretreatment)
0.50
ΔCout/Cout
0.52 0.50
Cout (Direct)
0.48
ηz
0.46
0.40 0.44
0.35
Cout (kg/kg)
0.45
0.30 0.25
0.40
0.20
0.38 50
0
5
10
15
20
25
30
35
40
45
100-ESP(%) Fig. 5. Effect of ESP on variables of the air pretreatment C/R.
Fig. 6. Comparison between pretreatment C/R and traditional C/R.
12 9 6 3 0
Cout/Cout
0.42
∇
drops from 29 to 15 g/kg in isoenthalpy process and the air inlet temperature can also ascend. As the results of such changes of air inlet conditions, the regeneration effect in the C/R can be improved significantly. Solution outlet concentration Cout rises from 0.4 to 0.47 kg/kg and the increment of solution concentration from inlet to outlet increases 70%. The regeneration efficiency hz changes from 0.35 to 0.51 achieving the amplitude of 45.7%. The storage capacity SC increases 44% from 820 to 1180 MJ/m3. In the light of above analysis, the pretreatment process of isoenthalpy dehumidification for air stream before entering into the C/R does increase the solution regeneration performance, but the performance improvement of liquid regeneration is at the sacrifice of effective solution proportion ESP. Effective storage capacity ESC used to measure the influence of ESP on regeneration performance of liquid drops from 820 to 720 MJ/m3 reaching reduction extent of 12%. Hence, solar air pretreatment C/R can achieve better quality liquid (higher concentration) because of sacrificing a slice of solar energy employed for the pretreatment of air stream. In air pretreatment C/R, only the solution of effective solution proportion (ESP) among the whole regenerated liquid is utilized to dehumidify air in the dehumidifier. A question will occur there. When the solution mass flow rate in the traditional C/R amounts to the product of ESP and whole regenerated liquid flow rate in the air pretreatment C/R which regeneration effect will be better for the traditional C/R and air pretreatment C/R. By numerical simulation with ESP varying from 100% to 52%, the changes of outlet liquid concentration Cout and regeneration efficiency hz of traditional C/R and air pretreatment C/R are shown in Fig. 6. The regeneration efficiency hz in air pretreatment C/R is much higher than that in traditional C/R and when ESP reaches 52%, the discrepancy of two kinds of C/R amounts to 0.3. With respect to solution concentration, the solution outlet concentration Cout of air pretreatment C/R is higher than that of traditional C/R and when ESP reaches 52%, the discrepancy of two kinds of C/R amounts to 11%. Seen from energy transfer, the absorbed energy in traditional C/R except heat loss and sensible heat increment of air and liquid become latent heat consumed by solution regeneration residually and in air pretreatment C/R among the residual absorbed solar energy except the same heat loss as in the traditional C/R, only solar energy of the ESP can be consumed by liquid regeneration, the other used for air pretreatment of isoenthalpy dehumidification. As the comparisons in Fig. 6, in the air pretreatment C/R because of its high regeneration
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G (kg/m2·h)
0.7 1.2 1.1
0.6
0.4
1.0
0.44 5
6
7
8
ms (kg/h)
9
0.42
10
0.40
0.9
0.38
0.8
0.34
0.6 0.5
0.32 0
50
100
150
200
250
0.30 300
0.16 0.14 0.12 0.10 0.08
ΔG (kg/m2·h)
0.36
0.7
Cout(kg/kg)
G (kg/m2·h)
0.46
0.5
ms (Kg/h) G (ESP=100%) G (ESP=70%) Cout (ESP=70%)
G (ESP=85%)
ΔG (ESP=70–100%)
Fig. 7. Effect of solution mass flow rate ms on regeneration performance (the insect diagram indicates the effect of ms at ms < 10 kg/h).
when solution mass flow rate is greater than 75 kg/h, the solution outlet concentration maintains about 0.31 kg/kg and the air reaches equilibrium with the inlet solution of 0.31 kg/kg and 35 C in air pretreatment unit. Fig. 7 also shows that when ms is less than 10 kg/ h, the change of ESP has great impact on the regenerator performance and the difference DG of evaporation rate G between ESP ¼ 70 and ESP ¼ 100% increases linearly. However, after solution mass flow rate exceeds 10 kg/h, DG drops and tends to steady value when solution mass flow rate is greater than 75 kg/h. Based on the above analysis, the increase of solution mass flow rate does improve regeneration performance being at cost of sacrificing solution quality (solution concentration) that does harm to dehumidification performance in the liquid desiccant cooling system. For maintaining higher solution outlet concentration, lower solution mass flow rate must be adopted, in which the adjusting of effective solution proportion ESP powerfully improves regeneration performance that is just the advantage of solar air pretreatment C/R. Besides effect of solution mass flow rate on regeneration performance, the solution inlet concentration Cin has large impact on solution regeneration shown in Fig. 8 with ma ¼ 100 kg/h and ms ¼ 10 kg/h that represents when the solution inlet concentration increases, the water evaporation G decreases linearly and when inlet concentration Cin rises from 0.30 to 0.4 kg/kg, the evaporation rate G drops about 30 percent. The decrease of ESP can increase water evaporation G and the increment of G at lower solution inlet concentration Cin is higher than that at higher Cin. Because the inlet concentration Cin in the solar C/R depends on dehumidification condition in liquid desiccant cooling system, the lower Cin is adverse to dehumidification condition in that system and the higher Cin can impair regeneration performance in the C/R. In the contradiction relationship by employing air pretreatment function and decreasing effective solution proportion ESP, the regeneration performance can be improved remarkably. Simultaneously, the drop of ESP can be compensated by adding solar collector area and solution mass flow rate. 5.3. Effect of air mass flow rate on the regenerator performance In order to analyze the effect of air mass flow rate on regeneration process, the solution mass flow rate is settled at 10 kg/ h and the range of air mass flow rate is considered about 10–800 kg/
Fig. 8. Effect of solution inlet concentration Cin on evaporation rate G.
h. The water evaporation rates under ESP ¼ 100%, 85% and 70% are shown in Fig. 9 that shows the evaporation rates G rise fast at first and then fall slowly. The evaporation rates reach the maximum when the air mass flow rate ma amounts to about 200 kg/h in that the heat and mass transfer coefficient augments when air mass flow rate increases and the augment of air mass flow rate can decrease air humidity ratio in the C/R that enhances vapor pressure difference between air stream and solution, which is in favor of liquid regeneration. However, the increase of air mass flow rate can also elevate heat loss from the regenerator and when air mass flow rate exceeds some value, the heat loss in regeneration process take up dominant status that can decrease water evaporation G slowly. Fig. 9 also indicates that the drop of ESP yields great improvement of evaporation rate G that explains under different air mass flow rates, the regeneration process can be regulated by varying ESP in air pretreatment C/R. When air mass flow rate is less than 30 kg/h the fluctuation lines of water evaporation rates G under ESP ¼ 85% and 70% are almost superposed showing that when air mass flow rate is less than 30 kg/h, the fall of ESP cannot improve regeneration performance. This is because when air mass flow rate is very small, though ESP descends from 85% to 70% that increases the solution mass flow rate employed for air pretreatment, the outlet air humidity ratio in the air pretreatment unit reaches equilibrium with solution and cannot be increased by decreasing ESP that results in the same evaporation rate G. That range of air mass flow rate ma is called blind area for effect of ESP. From the above analysis, the effect of air mass flow rate on liquid regeneration performance has a maximum and the ESP has significant influence on regeneration performance except for a blind area at small air flow rate. 6. Conclusions In order to analyze the steady regeneration performance of solar air pretreatment liquid C/R, the C/R model and air pretreatment model were established and by applying numerical simulation the regeneration performance of the air pretreatment C/R are obtained as functions of effective solution proportion ESP, solution inlet condition and air mass flow rate. With respect to the effect of ESP on regeneration performance, when ESP fall from 100% to 62%, the regeneration efficiency hz ascends 45.7 percent, storage capacity SC increases 44 percent and the increment of solution concentration between outlet and inlet of the regenerator increases by 70 percent. All these explain the lower ESP can be in favor of liquid regeneration. Compared with traditional C/R at the same solar radiant intensity, the air pretreatment
D. Peng, X. Zhang / Renewable Energy 34 (2009) 699–705
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Research Innovation Program of Jiangsu Province China (CX07B_095z) and the Key Grant Project of Chinese Ministry of Education (307013).
References
Fig. 9. Effect of air mass flow rate ma on evaporation rate G.
C/R combines decreasing air humidity, increasing air temperature and heating desiccant liquid d three kinds of promoting solution regeneration methods together that do improve solution regeneration performance and solar energy utilization efficiency opposed to traditional C/R only depending on heating regenerated solution. Concerning effect of solution mass flow rate and inlet concentration on regeneration performance, the increase of solution mass flow rate can effectively improve regeneration performance, though it sacrifices solution outlet concentration. Under ensuring higher solution outlet concentration, whether adopting small solution mass flow rate or increasing solution inlet concentration in the C/R are available and the fall of ESP can boost water evaporation rate G and improve regeneration effect. The effect of air mass flow rate on evaporation rate G has a maximum and when ma is on the small side, there is a blind area that ESP has no effect on regeneration performance. Acknowledgements This research was supported by the National Natural Science Foundation of China (50676018), the College Graduate Scientific
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