EVAPORATIVE WATER AND AIR COOLERS FOR SOLAR COOLING SYSTEMS. ANALYSIS AND PERSPECTIVES

The concept of evaporative coolers of gases and fluids on the basis of monoblock multichannel polymeric structures is presented. Different schemes of indirect evaporative coolers, in which the natural cooling limit is the dew point of the ambient air are discussed. In such systems the cooling temperature is lower than the wet bulb temperature of the ambient air. Special attention is paid to the recondensation of water vapor for deep evaporative cooling. It is shown that for the solution of the recondensation problem it is necessary to vary the ratio of the contacting air and water flows, particularly in each stage of the multistage system. Recommendations for the deep cooling process implementation in the evaporative coolers of gases and liquids are given.


This work is licensed under the Creative Commons Attribution International License (CC BY).
http://creativecommons.org/licenses/by/4.0/NOMENCLATURE F area (m2 ) c p isobaric heat capacity (kJ kg -1 K -1 ) G mass flow rate (kg s -1 ) p pressure (Pa) Q heat flow (W) h enthalpy (kJ kg -1 ) r heat of vaporization (kJ kg -1 ) t temperature ( o C or K) Greek letters α heat-transfer coefficient (W m -2 K -1 ) β mass transfer coefficient (kg m -2 s -1 ) φ relative humidity (%) Subscripts a air g das P primary S secondary wb wet bulb dp dew point w water 1 entrance 2 exit

I. INTRODUCTION
The interest to the opportunities the evaporative cooling steadily increases, that is caused by their low energy consumption and environmental cleanness [1][2][7][8][9][10].Evaporative water and air coolers (EC) can be used in autonomous mode in refrigeration and air conditioning systems, as well as in dessicant-evaporative coolers, based on heat-driven absorption cycle, when preliminary drying of the air provides high efficiency of subsequent EC in refrigeration systems and heat and humidity treatment of the air in air conditioning systems.
Direct evaporative coolers of air (DEC), water cooling towers (CTW), as well as indirect evaporative coolers (IEC) found wide practical application in different areas.The opportunities of such coolers for possible cooling temperature level are limited by the wet bulb temperature of ambient air t wb , which is the natural limitation of the cooling.This considerably depends on the climatic conditions.Besides the value of t wb , the limitation of the cooling in EC is also determined by the ratio of gas and liquid in heat and mass transfer device.The real value of the temperature limitation of cooling is a little higher than t wb ; this should be considered in calculations and design of EC [1].
The area of practical application of the EC methods is determined by systems with cooling towers and air coolers, refrigeration systems with the cooling of condenser, air conditioning systems for temperature and humidity air handling.
The decreasing of the temperature level of cooling also provides the decreasing of the water quantity used in EC for the compensation of the evaporated water (up to 20-25%) [1][2][3]

II. SCHEMATIC DIAGRAMS OF SOLAR COOLING SYSTEMS ON THE BASIS OF HEAT-DRIVEN ABSORPTION CYCLE AND EVAPORATIVE COOLERS OF LIQUIDS AND GASES
The concept of solar liquid-desiccant cooling and air conditioning systems designing is shown in Figures 1  and 2. The principle of indirect regeneration of desiccant is used in dehumidifying part of such systems.The dehumidifying part consists of desorber-regenerator (DBR), absorber-dehumidifier (ABR), solar heating system with solar collectors SC and tank-accumulator with additional heating source of traditional type (gas or electric heater), heat exchanger of "weak cooled desiccant and strong hot desiccant" flows, and technological cooling tower for absorber cooling (CTWt).
In cooling part of the solar absorption system the following solutions are considered: -chiller air cooler (Ch-Rg) is designed; the cooled primary air flow is supplied to the cooling spacesuch design is shown in Figure 1A.For cooling of the air after absorber the cooled air from the "wet" part of the cooler (secondary flow "B") as it is shown in Fig. 1B.
-chiller water cooler (Ch-Rw) can be used; here cooled water is supplied to the cooling space (Figure 2).Part of the air flow cooled in heat exchanger (7) can be delivered directly to the cooling space (Figure 2B), or this can be ambient air cooled in heat exchanger (7*); cooled air from cooling tower CTW of the chiller Ch-Rw can be used for desiccant cooling before absorber (in heat exchanger on the line of cooled exhaust air flow and strong desiccant).
Before in [1, 2, 5 and 6] the comparative analysis of the possibilities of solar liquid-desiccant cooling and air conditioning systems with direct and indirect regeneration of the desiccant was made.Flat plate solar collector (SC) can be used in systems with indirect regeneration of the desiccant, and gas-liquid solar collector regenerator is used in systems with direct regeneration [1].Each solution has advantages and disadvantages.The results of the present research cover the study of the solar systems with direct regeneration of the desiccant.

III. DESIGN OF THE HEAT AND MASS EXCHANGERS FOR DEHUMIDIFYING AND COOLING CYCLES OF SOLAR COOLING SYSTEM
Chiller air cooler Ch-Rg is composed from evaporative water cooler and water-air heat exchanger, in which the air from EC is cooled at constant moisture content.This decreases the wet bulb temperature, the natural limit of cooling level is decreased, and it can reach the dew point temperature.Chiller water cooler Ch-Rw is composed from CTW and water-air heat exchanger.This provides cooling of water temperature lower than the wet bulb temperature of the ambient air.
Constructive execution of packing for all heat and mass transfer devices (HMTD) of the dehumidifying and cooling cycles is unified.A multichannel monoblock structures made from polymeric materials are used for the packing.This creates the series of channels in which the liquid film (water in EC and desiccant in absorber and desorber) flows down the walls.The mode of contact gas and liquid flows can be counterflow as well as crossflow.HMTD with high density of packing layers are used [1, 2, 5, 6, 9 and 10].
HMTD can be of direct type (CTWs and air coolers), as well as indirect type, when several processes are realized in one device.For example, the primary air flow (P) is cooled due to evaporative cooling of the liquid film in neighboring alternate channels, when the liquid film interacts with the secondary air flow (S).The separating thin wall can be made from polymeric material because its thermal resistance is comparable to the thermal resistance of liquid film.Such a way an absorber with inner heat exchanger (Figures 1 and 2), in which the heat, released during absorption of the water vapor by the film of desiccant, is removed by cooled water from technological CTWt.Desorber is made the similar way, where heat from the solar heating system is supplied through the channels.All the HMTDs of dehumidifying and cooling cycles can be incorporated in one cooling unit, which can be placed on the roof of the building as well as inside it.
Solar heating system is based on the application of flat plate solar collectors SC, in which all the elements are made from polymeric multichannel monoblock structures [1, 2, 5 and 6].

III. THEORY THE HEAT AND MASS TRANSFER IN EVAPORATIVE COOLERS
The process of coupled heat and mass transfer during evaporative cooling is discussed in this section on the example of the direct evaporative cooling of water in CTW.The decreasing of the water temperature is reached by the combined influence of the following processes: 1) heat transfer during contacting (heat transfer due to thermal conductivity and convection); 2) heat transfer due to radiation; 3) surface evaporation of water into the air flow (diffusion of water vapor in the air).The main role here plays the evaporation from the surface (70-80% of heat released from water).A total quantity of heat, released from water, can be found from: The assumption is made, that the temperature gradient along the depth of the liquid film is absent and its thermal resistance equals zero: R w = 0.In studies of [1,11] it was shown that in general for polytropic process R w ≠ 0 и R Σ = R g + R w .The velocity of the vapor molecules from adjoining steam and gas layer being transferred to the air is proportional to the difference (р* g -р g ), where р g is a partial pressure of steam in the air located at substantial distance from the water surface (in the core of the air flow).The quantity of the evaporated liquid can determined from: where β pis a mass transfer coefficient divided by full partial pressure difference of water steam (kg/(m 2 s)).
The heat consumed during evaporation can be found from: Total transferred heat is determined from the following equation: The partial pressure difference as a motive power of the mass transfer process can be substituted by the difference of moisture content In this case Eq. ( 5) will take the following form: Here the assumption is made, that F  = F  = F.This factor usually is ignored; in [1,2] it was shown that for packing with tight structure the influence is great. where: For the system of water-air the ratio of heat and mass transfer coefficients is constant.This is the expression of the similarity of the heat and mass transfer process, which take place in the dynamic field of tempera-tures and moisture contents.The existence of such similarity, which can be expressed by Lewis relation le, is dependant from the actuality of the undergoing in the system processes, from the ratio of heat and mass transfer surfaces.It cannot be applied if saturated wet air is used, when «recondensation process» takes place in the region near saturation curve.Neglecting the dependence of r from the temperature, the following equation can be obtained: where К his the total coefficient of heat and mass transfer (according to [11]), divided by enthalpy difference.
It shows the coupled heat and mass transfer process intensity, which is defined by joint mechanism of convection and diffusion.Eq. ( 9) is the main equation of the «the method of enthalpy potential».This equation helps to make it easier the calculation of the heat and mass transfer process, because only one driving force is used instead of two driving forces, which is enthalpy head.Only one coefficient of K h is used instead of two coefficients of transfer α g and β x. .When it is necessary to consider the thermal resistance of the liquid film (R w ≠ 0), Equation (10) will take on the following form: where h g + is the enthalpy for t g = t * and φ g = 100%.
The analysis of the coupled heat and mass transfer.when direct contacting of gas and liquid takes place, was carried out for the following assumptions: -the liquid flow rate is constant (ΔG w = 0); during evaporation or condensation this flow rate will be changed; -error from the assumption of substitution of dp by dx g and from the influence of the Stephen mass flow is not big (convective mass flow appearing from the impenetrability of the liquid surface for the air flow; the law of the one-sided diffusion of Stephen); -assumption, that empirical relation of Lewes equals one (le=1).This point is related to the question of assumption about equality of the exchange surfaces ( for polytropic process in system of water-air the thermal resistance of the system is dispersed uniformly between two phases [1,11]; -additional error can be take place when the driving force is averaged Approximate methods and the inaccuracy of the averaging are studied in [1]; the approximate methods are based on substitution of the equilibrium curve by straight-line, parabolic or exponential dependence; the values of r and c p are considered as constants for the design range of main parameters.
The main contribution to the summary error is brought by the assumption R w = 0 and G w = 0.The value of the error can be from 10% to 15 % [1].It is necessary to mention that without simplifying background it is impossible to get the Eq. 10.
The equation of enthalpy balance has the following view: This is the equation of the «working line» of the evaporating cooling process.The main Merkel's equation with consideration of Equation ( 13) can be written as follows: The right part of Equation ( 14) includes only thermodynamic parameters of the flows; the left part includes constructive and operational characteristics of EC.This makes the Equation ( 14) convenient for practical calculations.The value of K h F/G w =K v has a name of «evaporation criteria».
For two relevant cases ( l =  (R l = 0) and  l   (R l  0)) the main equation of the «method of the enthalpy potential» will be written as Equation (15): where m is the value accounting the saturation line cur- vature (tangent slope of the saturation line).
Equation ( 15) is the equation of the «additivity of phase resistance».It connects the total thermal resistance in the system (RΣ=1/FK h ) with thermal resistance of air and water phase R g =1/Fβ h and , respectively.The influence of the resistance of gas or liquid film is determined by the solubility of the gas in liquid.In [1,2] it is stated, that R w can be up to 50% from R  .In monograph [11] it is stated that R w can be 27-46% from the total resistance of the enthalpy transfer between phases.

IV. DEVELOPMENT OF SOLAR LIQUID COL-LECTOR SC W TAKING INTO ACCOUNT THE UNEVENNESS OF DISTRIBUTION OF LIQUID IN CHANNELS OF ABSORBER SCw
In the solar refrigeration systems absorbing opencycle is used in composition an absorber, desorber-regenerator and solar heater system providing the required temperature level of regeneration of absorbent.The liquid solar collectors of SCw enter in the complement of the solar system.By authors before [1] it was worked out SCw on the basis of polymeric materials.From polymer-ic multichannel flags an absorber and transparent coverage are executed in such collector.A key problem for liquid SCw is a problem of unevenness of distribution of liquid in the channels of absorber.This problem is aggravated in transition on polymeric materials, as an unevenness of distribution of liquid threatens to cause temperature shock (deformation of the structural elements of SCw, executed from polymeric materials), that, along with the decline of efficiency of transformation of solar energy in SCw, threatens destruction of basic elements of solar collector.Authors are execute theoretical and experimental researches of problem of unevenness.Thus,

Figure 1  2 .
Figure 1  Creation principle of the solar heat-driven absorption system with indirect desiccant regeneration on the basis of chiller air cooler Ch-Rg.

Figure 2 
Figure 2  Creation principle of the solar heat-driven absorption system with indirect desiccant regeneration on the basis of chiller air cooler Ch-Rw.

Figure 3 
Figure 3  Construction of polymeric solar collector of SC-P (A, B) with the basic variants of location of hydraulic collectors (C -D) Denotations: H* is a height of transparent coverage; h* is a height of channel of transparent coverage; b* is a width of channel of transparent coverage; H** is a height of absorber; h** is a height of channel of absorber; b** is a width of channel of absorber; H*** is a thickness of heat-insulation. 1 corps of SC-P; 2  absorber, 3  heat-insulation; 4  transparent coverage; 5 and 6  collectors.

Figure 4 
Figure 4  Distribution of liquid in channels at V=1.5 (m/s) on an entrance in U-type absorber; the internal (d*) diameter of hydraulic collectors was varied in calculations

Figure 5 Розділ 2 .Figure 6 Figure 7 
Figure 5  Distribution of liquid in channels at V=1.5 (m/s) on an entrance in S-type absorber; the internal (d*) diameter of hydraulic collectors was varied in calculations