PERFORMANCE STUDIES FOR AN EXPERIMENTAL SOLAR OPEN-CYCLE LIQUID DESICCANT AIR DEHUMIDIFICATION SYSTEM

Transcription

1 Solar Energy Vol. 44, No. 3, pp , 199 Printed in the U.S.A X/9 $3. +. Copyright 199 Pergamon Press pie PERFORMANCE STUDES FOR AN EXPERMENTAL SOLAR OPEN-CYCLE LQUD DESCCANT AR DEHUMDFCATON SYSTEM S. PATNAK, T. G. LENZ,* and G.. G. LOF* Solar Energy Appliations Laboratory, Colorado State University, Fort Collins, CO 8523, U.S.A. Abstrat-A nominal 1.5-kW (3-ton) open-yle liquid desiant d ehumidifiation system has been design_ed, installed, and suessfully operated at the Solar Energy Appliations Laboratory, Colorado State Umvers1ty. Paked bed units were used to dry the air in the dehumidifier and to onentrate the desiant _m the regenerator. Liquid distribution in the regenerator was studied for two systems: a gravity tray d1stnbutor, and a spray nozzle system. Higher apaities ( 4-5% mrease) and.lower pressure drop ( 3-4% redution ) for the air flow were observed with the spray system. Coohng apaities of kw ( refngerat1n tons) were ahieved for both the regenerator and dehumidifier. Funt1nal relat1onsh1ps orrelatmg.the independent variables to the rate of vaporization in the regenerator and rate of ondensation m the dehum1~fier were obtained by statistial analysis of the experimental data. These studies thus provj~e data and.orrelations useful for design guidane and performane analysis of similar open-yle hqu1d desiant oohng systems, partiularly for the liquid / vapor ontat units. 1. NTRODUCTON The required ooling load and available solar energy being in phase (in ontrast to heating loads), makes solar an attrative energy soure for ooling of buildings. Desiant ooling systems using solar as an energy soure have potentially favorable eonomis for humid limates where the solar heating equipment an provide useful energy for ooling, heating, and domesti hot water supply aross the entire year. The ability of desiant systems to use air at moderate temperatures as the working fluid makes them partiularly attrative. Moreover, air-based desiant systems do not have to be hermetially sealed, and are adaptable to appliations with high ventilation loads. n these systems, air is onditioned by dehumidifiation to redue the latent load whih an then be followed by a variety of heat exhange and evaporative ooling whih redues the sensible load. A liquid desiant ooling system using solar energy was first suggested and studied by LOf[ 1]. This early liquid-desiant researh used triethylene glyol as the desiant and solar heated air to regenerate the desiant. t was found to be eonomially attrative but had the problem of triethylene glyol migrating into building spae. A later study identified a system using dehumidifiation in onjuntion with sensible ooling to be promising for humid limates [ 2]. Johannsen [ 3] has reported a glyol system in whih the weak solution is regenerated in the solar olletor itself. Lodwig [ 4] tested a liquid desiant air onditioner using hot water from flat-plate solar olletors to power a Niagara " Hygrol" system. Robison [ 5] designed a system with a maximum apaity of 17.5 kw (5 tons), and used triethylene glyol as the sorbent. Beause of the high loss oftriethylene glyol, it was suggested that *!SES member. an aqueous solution of alium hloride be used as the desiant [ 6]. A system employing alium hloride and water as the desiant has been reported by Mullik and Gupta[?]. The alium hloride-water solution has been regenerated in a olletor-um-regenerator and the results analyzed by Gupta and Gandhidasan [ 8]. Turner proposed a solar-powered liquid desiant system in whih a portion of the dehumidified air is used to produe hilled water by evaporative ooling[ 9]. An absorber design was proposed and mathematial models of heat and mass transfer were developed by Peng and Howell [ 1]. Several other regenerator designs have also been proposed and models developed [ 11]. The onept of an "air moisture removal effetiveness" for an adiabati ounterflow paked tower dehumidifier has been disussed by Ullah et al. (12]. The regeneration of the desiant by solar energy imposes a onstraint on the maximum apaity that an be attained. Considerable researh has been onduted on the means of ahieving highest mass transfer rates in the regenerator alone. An open-yle absorption unit operated in the Soviet Union had the regenerator mounted on a sloping roof over whih dilute absorbent solution trikled, and was onentrated by solar radiation [ 13]. Studies have also been performed on the regeneration oflithium hloride brine in a solar still ( 14]. More reently, lithium hloride and lithium bromide have been onentrated suessfully in ounterurrent paked bed olumns [ 15, 16, 17]. There have been relatively few experimental studies ofliquid desiant systems using solar energy. An entire system using lithium hloride as the desiant has been built and used to provide air onditioning of a house ( 18]. A liquid desiant loop has been inorporated into the air onditioning system of the Siene Museum of Virginia ( 19]. A 1.5-kW ( 3-ton) nominal apaity air onditioning system using triethylene gly- 123

2 124 S. p A TNAK et al. T Solution Air Cooling Water T T - Temperature DP - Dew Point F - Flow C - Conentration T DP Steam DPT.F ~ ~ 1 Cooling T Tower F! F Ct k,...-~~~... ~ 1 : F ~ T ~ '4---~ DP T Colletors T --~~...--T T DP T F r!~t 5(' :, r -_, ~ F T T ~ 'T ~:~l~n~- ~-n~t _ J. ~..., Eonomizer T Heaters T Fig.. Experimental open-yle desiant system. ol has also been tested [ 2]. The effet of regeneration temperature and onentration on the oeffiient of performane has been determined experimentally by operating the above system in a deoupled mode [ 2]. For the present study, an experimental liquid desiant ooling system has been onstruted to obtain and analyze data under losely ontrolled onditions. Our primary objetives in arrying out this researh inluded investigating system design and operating onditions giving highest vapor transfer rates at minimum pressure drop for both the dehumidifier and regenerator units. These studies should thus prove useful in evaluating theoretial models of liquid desiant ooling systems. 2. EXPERMENTAL SYSTEM DETALS The experimental aqueous lithium bromide-based system employed in our studies is shown shematially on Fig.. A brief desription of this experimental system is given below, and a more detailed desription is given elsewhere [ 22]. The regenerator is a Fiberglas tower having a diameter of 81 m and a height of 2 m. Tripak No. /2 polyethylene spheres (an effiient, low ilp paking) are used as the paking material and the height of the paked bed were typially 4-28 m. Dilute salt solution from the dehumidifier, after passing through the eonomizer, flows into the paked bed regenerator at the top of the tower. The solution was distributed at the top of the paking by a tray in earlier runs, and by nozzles in later runs. Air from the olletor flows ounterurrent to the solution. Liquid entrainment is minimized by passing the air through a mist eliminator before exiting at the top of the tower. Hot air to the regenerator is provided by a flat-plate solar olletor array having a total area of 55.7 m 2. n the dehumidifier setion of the liquid desiant experimental system an eletri steam humidifier is used to simulate onditions of high humidity. Auxiliary heat is also supplied by three eletri strip heaters to maintain a onstant air temperature to the dehumidifier. The dehumidifier is similar in dimension and onstrution to the regenerator unit. Conentrated solution from the regenerator passes through the eonomizer and the ooling unit before entering the dehumidifier, as shown in Fig.. An eonomizer is used to reover heat from the hot solution exiting the regenerator, by exhanging heat with the ooler solution from the dehumidifier. After passing through the eonomizer, the warm solution is further ooled by water in a four pass, shell-and-tube heat exhanger. Water is supplied to the heat exhanger from a ooling tower whih has a rated apaity of 28 kw (8 tons). nstrumentation for the experimental liquid desiant ooling system is also shown in Fig.. Only marosopi measurements are available; hene, measurements of film thikness, onentrations, temperatures, and humidities are not made within the paked bed regenerator and dehumidifier themselves. Temperatures, voltages, and dew points are reorded diretly on tape using a Fluke model 224A data aquisition system. System omponents suh as pumps,..

3 Performane studies for an experimental solar open-yle liquid desiant air dehumidifiation system 125 Table. Regeneration of Li Br (tray distributor) Airflow Air inlet Air inlet Solution Solution rate Temp. humidity flow rate inlet temp. (kg/s) (QC) (g/kg) (kg/ s) (QC) Solution Evaporation Capaitiy Overall mass one. rate refrigeration trans. oeff. (wt% LiBr) (g/s) (kw) ( ) O l kg/(s-m 3 bed log mean driving fore) l l l l and blowers were hosen on the basis of availability, ost and flexibility, and are thus not optimum as regards power requirements for these evolving experimental studies. Air inlet absolute humidity: Air flow rate: Paking depth: g/ (kg dry air)* kg/s 4 m 3. RESULTS AND DSCUSSON Regenerator operation (deoupled mode) Liquid distribution onsiderations. n earlier studies, solutions of lithium hloride [ 15] and lithium bromide [ 16] have been onentrated in paked olumns having tray liquid distributors. A maximum apaity of6.7 kw ( 1.9 refrigeration tons) was obtained in these early regenerator studies. A prime objetive of the present study was therefore to improve heat and mass transfer in the paked bed by designing a more effiient liquid distribution system. Beause the parasiti power requirement is proportional to the air pressure drop through the paked bed, it was important to ensure low pressure loss with the new liquid distribution system. Sine the gas side pressure drop was to be kept to a minimum, a spray system was sought to provide higher mass and heat transfer at low blower power requirements. n the regenerator, a spray system was designed to distribute the solution over the paking. Due to the diffiulties in desribing mass transfer in the spray droplets in a generalized manner, no effort has been made to separate mass transfer ourring in the spray droplets from that ourring in the paked bed. Mass transfer oeffiients are therefore reported per unit volume of paking (negleting the mass transfer and surfae area provided by the spray droplets) Mass transfer studies. The performane of the regenerator, operated in a deoupled mode, was studied under the following ranges of operating onditions, for both the tray and spray distribution systems: Tables 1 and 2 summarize data for the tray liquid distributor and the spray nozzle system, respetively. These data show that the apaity of the regenerator inreased from a maximum of 6. 7 kw ( 1.9 tons) with the tray distributor system to 1.5 kw ( 3. tons) with the nozzle system. Similar inreases in the mass transfer oeffiients are observed with the spray system. t was not possible to isolate and study the effet of any single variable independently in these studies; this was primarily due to the diffiulty in maintaining some ambient-dependent variables onstant. To obtain a measure of the relative dependene of the evaporation rate on these variables, the data were thus subjeted to multiple regression analysis for variables known to have a potentially important effet on mass and heat transfer. Funtional relationships orrelating the independent variables to the evaporation rate were obtained for both the tray and spray onfigurations. Sine the solution is reyled from the sump of the regenerator, the solution temperature approahes its adiabati saturation value, and thus the solution temperature is not an independent variable. The air flow rate is not inluded in the statistial analysis beause it remained fairly onstant through all runs. The orrelations obtained for the evaporation rate in the regenerator are Spray nozzle system M, = (Ta)l.5 1 (Ha) - l. 9 l(m,) (X, ) Tray liquid distributor M, = (Ta)3.9(Ha)-.77(M, ).79 (X, )-5.69 Solution inlet onentration: Solution inlet temperature: Solution flow rate: 57-6 weight perent Li Br 4-56 C kg /s Our geographi loation and present equipment onfiguration did not permit operating at higher inlet humidities. However, using house air for regenerator inlet air would fall in our humidity range. We plan to study higher regenerat1n inlet air absolute humidities in the future.

4 126 S. PATNAK et al. Table 2. Regeneration of Li Br (spray nozzles) Airflow Air inlet Air inlet Solution rate Temp. humidity flow rate (kg/s) ( C) (g/kg) (kg/s) * kg/(s-m 3 bed log mean driving fore). Solution inlet temp. oq l Solution Evaporation Capaitiy Overall mass one. rate refrigeration trans. oeff. (wt% LiBr) (g/s) (kw) (*) where M. = evaporation rate of water, g/ s Ta = air inlet temperature, C Ha = absolute humidity of inlet air, g H 2 / kg dry a1r Ms= solution flow rate, kg/s Xs = inlet solution onentration, weight perent Li Br. This orrelation was obtained for the range of variables given above in this Setion The values of the evaporation rate predited by the orrelations are ± 15% of the observed value. nsight into the effet of individual parameters on the evaporation rate predited by the regression equations an be obtained by holding all but one variable onstant on plots. Evaporation rates plotted against eah of the four variables are shown in Figs When held on- stant, the variables were maintained at the following levels: Air inlet temperature: Air inlet humidity: Solution flow rate: Solution inlet onentration: 7 C 8. g H 2 / kg dry air.7 kg/s 58 weight perent LiBr For the tray liquid distributor, the orrelation shows somewhat greater dependene on the air inlet temperature and solution onentration than on the air inlet humidity and solution flow rate. The stronger dependene on the solution flow rate suggests that spray nozzles are more effiient when operated at onditions around the design point. The stronger dependene of the evaporation rate on the air inlet humidity for the spray system may be explained in the following manner. Due to the in- 4..,..._ Ol.: 3. ll'. : ~ L a. > w 1.. -t-~~~~~~~-.-~~~~~~~~~~~~~~~~~~~~~--l Spray nozzles Air nlet Temperature, C + Troy distributor Fig. 2. Evaporation rate vs. air inlet temperature for the regenerator.

5 Performane studies for an experimental solar open-yle liquid desiant air dehumidifiation system "' O 4. i 3. ::: : a. w > a Air nlet Humidity, o/kg dry air Spray nozzle + "Yray dlstrbutor Fig. 3. Evaporation rate vs. air inlet humidity for the regenerator. reased mass transfer ourring with the spray system, the absolute humidity ratio of the air is higher at any point within the paked bed when ompared to the tray distributor system. n other words, for the same inlet humidity ratio, the ondition of the air is loser to equilibrium within the paked bed with the spray system than with the tray distributor. This derease in driving fore for mass transfer in the spray system results in a greater dependene of evaporation rate on the air inlet humidity. From earlier studies on the performane of the regenerator, it is known that the relative resistane of the liquid phase is 7-8% of the total overall resistane [ 16 ]. For this reason, both orrelations exhibit a relatively high dependene on the solution onentration. n the spray nozzle system the liquid stream is broken into tiny droplets. When ompared to the tray distributor, where solution flows by gravity in thin streams, the fine droplets formed by the spray system offer less resistane to mass and heat transfer on the... "' O ti ::: :;; a. ~ ~~~~~~~...-~~~~~~~.--~~~~~~-,-~~~~~~~-; a Spray nozzles Solution flow rote, kg/s + Troy distributor Fig. 4. Evaporation rate vs. solution flow rate for the regenerator.

6 128 S. PATNAK el al , '-...! O> 3. :;; ~ 2. - a. ~ -r~==:::============~:;::==================~i==========-~~~~-=j 1. - J'. -+-~~~~~~~~~~-y-~~~~~~~~~~..-~~~~~~~~~ Solutlon Conentrotion, wt % UBr o Sproy nozzles + Tray distributor Fig. 5. Evaporation rate vs. solution onentration for the regenerator. liquid side. Consequently, the evaporation rate is likely less sensitive to solution onentration with the spray system. The effet of air inlet temperature on the apaity for the two distribution systems is not very well understood. As explained earlier, due to the reirulation of the solution, the solution inlet temperature approahes its adiabati saturation value. This solution inlet temperature is thus a funtion of the air inlet temperature. Also, due to higher evaporation rates at high air temperatures with the spray nozzle system, the inlet solution temperature is lower. The inrease in solution visosity at these lower temperatures affets the performane of the spray system (and the paked bed) more adversely than for the tray distributor system. The two opposite effets ourring due to high air inlet temperatures, that is, higher mass transfer, but lower inlet solution temperatures (higher visosity) likely result in a lower dependene of evaporation rate on the air inlet temperature, in the ase of the spray nozzle system Pressure drop studies. Pressure drop for air flow through the paked bed regenerator was studied and ompared for the tray liquid distributor and spray nozzle systems. Air pressure drop was measured by a Dwyer miromanometer. n the tray distributor system the total pressure drop was the loss over the paked bed, the tray distributor, and a thin demister. n addition to the paked bed, the muh larger mist eliminator ontributed to the total pressure drop in the spray system. Results of these pressure drop studies are presented in Tables 3 and 4. When the regenerator was operated in a dry mode (no solution flow), the total pressure drop was slightly higher for the spray system, due to the pressure loss aross the mist eliminator. The tray distributor did not offer as muh resistane to the air flow in the dry mode. But when the bed was irrigated, Table 3. Pressure drop results (tray distributor) Volumetri Pressure air flow Mass air Li Br LiBr mass Pressure drop in Total rate flow rate onentration flow rate drop in bed distributor pressure drop (m 3 /s) (kg/s) (wt%) (kg/s) (Pa) (Pa) (Pa) * * * * * * * * *Dry bed.

7 Performane studies for an experimental solar open-yle liquid desiant air dehumidifiation system 129 Table 4. Pressure drop results (spray nozzles) Volumetri Pressure air flow Mass air Li Br LiBr mass Pressure drop in Total rate flow rate onentration flow rate drop in bed demister pressure drop (m 3 /s) (kg/s) (wt%) (kg/s) (Pa) (Pa) (Pa).72.6 ** ** ** ** ** ** ** ** l l ** Dry bed. the total pressure drop was higher for the tray distributor system; this happens beause the pressure loss aross the mist eliminator remains fairly onstant for both dry and wet modes of operation. However, the pressure loss aross the tray distributor was muh higher in the wet mode, sine the solution impeded the flow of air through the orifie openings in the tray distributor. Our pressure drop data show very reasonable power requirements for the air side (about.1 HP for eah olumn), and thus that our system is a reasonable alternative to ompressor-based systems Dehumidifier operation (deoupled mode) The paked bed dehumidifier was operated in a deoupled mode.to study its performane under various onditions of solution and air temperatures, solution and air flow rates, solution onentrations, and air humidities. n a deoupled mode, dehumidifiation of the air stream results 'in a gradual derease in the solution onentration as a run proeeds. After dilution to a onentration of perent by weight LiBr, the solution was transferred to the regenerator, where the onentration was brought up to the desired level (58-6 perent by weight LiBr). The heat generated in the solution during dehumidifiation was removed by the ooling unit, thus maintaining a onstant inlet solution temperature. The following ranges of operating variables were used to study the performane of the dehumidifier: Solution inlet onentration: Solution inlet temperature: Solution flow rate: Air inlet humidity ratio: Air inlet temperature: Air flow rate: Paking depth: weight perent Li Br 24_ kg/s g H 2 / (kg dry air) 24_ kg/ s 28 m. Tables 5-7 summarize the performane of the dehumidifier for varying onditions. During the ourse of these runs the solution flow rate, onentration, temperature and air flow rates were set at the desired values, while the air humidity and temperature depended on the prevailing room onditions. Condensation rate was used as the riterion of performane. Dehumidifier performane at three solution inlet temperatures is shown in Table 5. At eah temperature Table 5. Deoupled dehumidifier performane (l) solution inlet temperature variation Air Air inlet Air inlet Solution Solution Solution Condensation *Overall flow rate temp. humidity flow rate inlet temp. one. rate mass trans. (kg/s) (CC) (g/kg) (kg/s) (CC) (wt%) (kg/s) oeff l l 12.l l l * kg/(s-m 3 bed log mean driving fore).

8 13 S. PATNAK el al. Table 6. Deoupled dehumidifier performane (2) solution flow rate variation Air Air inlet Air inlet Solution Solution Solution Condensation *Overall flow rate temp. humidity flow rate inlet temp. one. rate mass trans. (kg/s) (OC) (g/kg) (kg/s) (OC) (wt%) (g/s) oeff l * kg/(s-m 3 bed log mean driving fore). the performane was monitored at several onentra- humidifier. Therefore, the humidity and temperature tions; these runs show that dehumidifier apaity was of the air entering the dehumidifier ould not be set at not strongly affeted by the solution inlet temperature any desired value. Moreover, steady-state onditions over the range of onditions run. However, at low so- ould not be ahieved due to the ontinuous derease lution onentrations the ondensation rate dereased. in solution onentration. Hene, it was not possible Table 6 shows dehumidifiation results at three differ- to isolate and study the effet of any one variable inent solution flow rates. Capaity of the dehumidifier dependently in these initial deoupled dehumidifiais seen to derease with a lowering of solution flow tion studies. The dependene of ondensation rate on rate. Table 7 shows that ondensation rate in the de- six independent variables ompliates the situation humidifier is not affeted greatly by air flow rate. further. To obtain a measure of the relative dependene The steam humidifier and the dut heaters were of ondensation rate on these variables, the data were not operated during these initial runs. Water was then subjeted to multiple regression analysis. A power sprayed in the regenerator and the warm and humid law orrelating the independent variables was obtained air exiting was used as the inlet air supply to the de- by use of statistial software [ 2 1]. Table 7. Deoupled dehumidifier performane (3) air flow rate variation Air Air inlet Air inlet Solution Solution Solution Condensation *Overall flow rate temp. humidity flow rate inlet temp. one. rate mass trans. (kg/s) (OC) (g/kg) (kg/s) (OC) (wt%) (g/s) oeff l kg/(s-m 3 bed log mean driving fore).

9 Performane studies for an experimental solar open-yle liquid desiant air dehumidifiation system 13 1 The orrelation obtained on the performane of the dehumidifier is M = 8.53*1-4 ( T ) - (M ) (H 2 2 ) x ( T,)-o.JJ(M, )2.1s(X, ) J.2s. This orrelation was obtained for the range of variables given earlier in this Setion 3.2. The number of data points used in the regression was 252, and the model fitting results and analysis of variane for the full regression are available elsewhere [ 22]. The dependene of ondensation rate on a given variable was again obtained from the regression equation by holding all exept one variable onstant and plotting the results. Unless otherwise shown in the plot, the variables were maintained at the following onditions: Air flow rate: Air inlet temperature: Air inlet humidity: Solution flow rate: Solution inlet onentration: Solution inlet temperature:.75 kg/s 3 C 18. g H 2 / kg dry air.39 kg/s 55 weight perent LiBr 28 C. The effets of these variables on the ondensation rate are disussed in detail below l Air flow rate. The air flow rate was varied between kg/s. From earlier studies[ 15] it is known that the relative resistane of the liquid phase is 7-8% of the total overall resistane. For this reason, it is likely that the apaity of the dehumidifier would not be appreiably affeted by air flow rate, as is shown in Fig Solution flow rate. The solution flow rate in the dehumidifier was varied between.3-.5 kg/s. Figure 7 shows a derease in apaity at lower liquid flow rates. This an be explained in the following manner. First, a lower solution flow rate adversely affets the wetting of the paking, thus reduing the area available for mass transfer between the two phases. Seond, the heat effets ourring during dehumidifiation, namely, sensible heat transfer from the air stream and heat of ondensation and dilution, result in a higher liquid exiting temperature, and lower driving fore for mass transfer. Moreover, effetiveness of a spray system dereases when operated at onditions away from the design point (at lower flow rates) Air inlet temperature. The apaity of the dehumidifier was found to depend strongly on air inlet temperature, as shown on the plot of ondensation rate versus the air inlet temperature in Fig. 8. At higher air inlet temperatures, more sensible heat is transferred to the liquid stream. This results in higher liquid temperatures and rise in interfaial equilibrium humidity. The driving fore for mass transfer thus dereases and onsequently redues the apaity of the dehumidifier Air inlet humidity. Figure 9 shows a plot of the ondensation rate against air inlet humidity, predited by the orrelation. A higher inlet air humidity provides a greater driving fore for mass transfer whih results in the higher observed ondensation rates. The ondensation rates at two different air inlet temperatures are shown on the same plot. As explained earlier, at lower air inlet temperatures, the sensible heat transfer to the liquid stream is redued, and/or the sensible heat transfer from the liquid to the air stream is inreased, resulting in a higher apaity Solution inlet temperature. A plot of predited ondensation rate against the solution inlet tempera Ul ' O> 2 a:: :;::; Ul., "O 3. - u Air Flow Rote, kg/ s Fig. 6. Condensation rate vs. air flow rate for the dehumidifier

10 132 s. PATNAK el al "'7. Ci ~ 6... r: a: u Solution F low Rete. kg/s Fig. 7. Condensation rate vs. solution flow rate for the dehumidifier. tu re is shown in Fig.. Although a derease in solution onentration and an inrease in inlet solution temperature both tend to inrease the interfaial humidity, the dehumidifiation apaity does not show a high dependene on the solution inlet temperature. This result is likely beause heat generated in the dehumidifiation proess (heat of ondensation and dilution) affeted the temperature of the solution. A lower solution inlet temperature will result in higher ondensation rate in the upper part of the olumn, leading to a greater temperature inrease. Moreover, a lower so- lution inlet temperature provides a greater driving fore for sensible heat transfer from the air stream, again raising the temperature of the solution passing through the paking. Thus, a lower solution inlet temperature does not neessarily mean a lower solution temperature through the entire length of the tower. The relative insensitivity of ondensation rate on the solution inlet temperature may be related to the above heat transfer effets Solution onentration. The effet of solution inlet onentration on the ondensation rate at two J) ' o> 2 6. [l' J) ll u Air nlet Temperoture, C Fig. 8. Condensation rate vs. air inlet temperature for the dehumidifier.

11 Performane studies for an experimental solar open-yle liquid desiant air dehumidifiation system ' (Jl "' O'. 6. :g 5. "'., 1J u Air nlet Humidity, g / kg dry air o Air Temp.: 3 C + Air Temp.: 24 C Fig. 9. Condensation rate vs. air inlet humidity for the dehumidifier. solution inlet temperatures is shown on Fig. 11. At lower onentrations the interfaial humidity of the liquid is greater. Also, at lower onentration the interfaial equilibrium humidity inreases more rapidly with temperature (Fig. 12). The driving fore for mass transfer is thus redued substantially whih results in lower ondensation rates at lower solution onentrations. 4. CONCLUSONS We have designed, installed, and operated a nominal 1.5-kW open-yle liquid desiant dehumidifi- ation system whih utilizes aqueous lithium bromide as the absorbent. Performane of the regenerator has been evaluated for two types of liquid distribution systems-a gravity tray distributor and a spray nozzle system. Higher apaities ( 4-5% inrease) in the regenerator were obtained with the spray system. Statistial analyses of data suggest that due to higher mass transfer the regenerator apaity has a stronger dependene on the air inlet humidity ratio for spray as opposed to tray distribution of the liquid. Pressure drop studies indiated that lower pressure losses (3-4% derease) are enountered with a spray system (in "' '(Jl O' "' 4. -., 1J a 3. - u Solution nlet Temperature, C Fig. 1. Condensation rate vs. solution inlet temperature for the dehumidifier.

12 134 S. PATNAK et al '-.. "' 7. CJ> u 6. Cl:'. 5. :;:; "' u 4. "O u Solution Conentration, wt. 3 LiBr Temp. : 24 C + LiBr Temp. : 32 C Fig. 11. Condensation rate vs. solution onentration for the dehumidifier. eluding a mist eliminator) than for a tray liquid distributor. A funtional relationship orrelating the independent variables to the rate of ondensation for deoupled dehumidifier operation has been obtained by regression analysis of experimental data. This orrelation indiates a strong dependene of the dehumidifiation rate on solution onentration, flow rate, air inlet temperature, and its absolute humidity. Relative temperature of the air and liquid streams in the dehumidifier appear to be important in determining ahievable maximum a- paity. Temperature profiles of dehumidifier liquid and air streams indiate that sensible heat transfer an our in both diretions, to or from the liquid stream. Our experimental studies should prove useful to others in the design, analysis, and modeling of liquid desiant ooling systems. Although it is tempting to treat systems suh as that desribed in this paper by simplified methods, suh as assuming air-side ontrol of mass transfer (true for pure liquid phases), our results suggest that suh simplified methods ould lead to substantial error. With the highly onentrated, vis- A: 45 wt. 1. Li Br 8 : 5 wt. /o L Br C: '55 wt. /o L Br 28 er ;,.. 24 ~. a..., er :> "' " e 12 ~ DRY BULB TEMPERATURE C Fig. 12. LiBr-water-air equilibrium hart.

13 Performane studies for an experimental solar open-yle liquid desiant air dehumidifiation system 135 ous salt solutions employed in many liquid desiant ooling systems, only double-resistane film theory seems reasonable, and is ommuniated via appropriate mass transfer oeffiients. We are ontinuing our studies to better define these liquid desiant ooling systems, whih should lead to better understanding (first ost, parasiti power, et.) of their potential and limits relative to other approahes. Aknowledgment- The authors aknowledge with appreiation the Ative Heating and Cooling Division, Offie of Solar Heat Tehnologies, U.S. Department of Energy, for the support of the work on whih this paper is based. NOMENCLATURE M, evaporation rate of water, g/ s M ondensation rate of water, g/ s T. air inlet temperature, C M. air flow rate, kg/ s H. absolute humidity of air, g/kg dry air T, solution inlet temperature, C M, solution flow rate, kg/s x, solution onentration, weight perent LiBr REFERENCES. G.. G. Lof, Cooling with solar energy, 1955 Congress on Solar Energy, Tuson, AZ, pp (1955). 2. J. C. Kapur, A report on the utilization of solar energy for refrigeration and air onditioning appliations, Solar Energy 4, ( 1956). 3. A. Johannsen, Design and operation of a liquid-desiant type solar air onditioning system, Proeedings of the nternational Solar Energy Soiety, Atlanta, GA, pp (1979). 4. E. Lodwig, Jr., et al., A solar powered desiant air onditioning system, solar olletors heating and ooling systerns, Vol. l, Proeedings of the 1977 Annual Meeting, Amerian Setion ofses ( 1977). 5. H.. Robison, Liquid sorbent solar air onditioner-a/- ternative energy soures enylopedia, Hemisphere Publishers, Washington, D.C., ( 1978). 6. H.. Robison, Liquid desiant solar heat pump for deve/oping nations, paper presented in the nternational Symposium Workshop on Solar Energy, Cairo, Egypt, June 18-24( 1978 ). 7. S. C. Mullik and M. C. Gupta, Solar air onditioning using absorbents, Seond Workshop on the Use of Solar Energy for the Cooling of Buildings, Los Angeles, CA, August 4-6 ( 1975). 8. M. C. Gupta and P. Gandhidasan, Open yle 3-ton solar air onditioner: Conept, design and yle analysis, Proeedings of the nternational Solar Energy Soiety Congress, New Delhi, ndia ( 1978). 9. N. C. Turner, United States Patent no. 4, 171,62. O. C. S. P. Peng and J. R. Howell, Analysis and design of effiient absorbers for low-temperature desiant air onditioners, Journal of Solar Energy Engineering, 13, 67-74, Feb. ( 1981 ). 11. C. S. P. Peng and J. R. Howell, The performane of various types of regenerators for liquid desiants, J. Solar Energy Engineering, 16, , May ( 1984). 12. M. R. Ullah, C. F. Kettleborough, and P. Gandhidasan, Effetiveness of moisture removal for an adiabati ounterflow paked tower absorber operating with CaCii-air ontat system, J. Solar Energy Engg. 11, ( 1988). 13. A. Kakabayev,. K.lyshhhayeva, S. Tuiliev, and A. Khandurdyev, Experimental study of thermotehnial harateristis of glazed solution regenerator, Geliotekhnika 14, ( 1978). 14. K. G. T. Hollands, The regeneration of lithium hloride brine in a solar still, Solar Energy 7, ( 1963 ). 15. R. yer, Paked bed reonentration of lithium hloride solution in an open yle absorption ooling system, M.S. Thesis, Colorado State University, Fort Collins, CO ( 1984). 16. S. V. Kaushik, Mass transfer in paked bed reonentration of lithium bromide for open yle absorption ooling, M.S. Thesis, Colorado State University, Fort Collins, CO ( 1984 ). 17. T. G. Lenz, G.. G. LOf, S. Patnaik, and G. Sandell, Open yle absorption ooling studies, DOE Report no. DE-AC-383-SF 1927, May ( 1986). 18. J. W. Mithell, State of the art of ative solar ooling, Proeedings of the Amerian Solar Energy Soiety, n., June 11-14(1986). 19. Gershon Mekler Assoiates, P. C., Data olletion and model development of liquid desiant integrated H VAC system, final report prepared for the U.S. Department of Energy, Otober ( 1986). 2. H. Pareja and A. F. Orlando, Charateristis of a solar air onditioning system using a liquid dehumidifier, ASME Honolulu meeting ( 1987) STATGRAPHCS, Statistial Graphis System, Version 1., Serial no , STSC, n., Copyright ( 1985). 22. S. Patnaik, Performane studies for a solar open-yle liquid desiant ooling system, M.S. Thesis in Chemial Engineering, Colorado State University, Fort Collins, CO ( 1988).