Review of humidity control technologies in buildings

Author’s Accepted Manuscript Review of Humidity Control Technologies in Buildings Maher Shehadi

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S2352-7102(18)30436-4 https://doi.org/10.1016/j.jobe.2018.06.009 JOBE516

To appear in: Journal of Building Engineering Received date: 16 April 2018 Revised date: 12 June 2018 Accepted date: 15 June 2018 Cite this article as: Maher Shehadi, Review of Humidity Control Technologies in B u i l d i n g s , Journal of Building Engineering, https://doi.org/10.1016/j.jobe.2018.06.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Review of Humidity Control Technologies in Buildings Maher Shehadi, Ph.D. Assistant Professor of Mechanical Engineering Technology, School of Engineering Technology, Purdue Polytechnic, Purdue University, IN, USA Tel.: +1 (765) 455-9219; email: [email protected]

Abstract This paper reviews the current research and advances in humidity control for residential and commercial buildings. Desiccant and hygroscopic buffering zones are summarized. System types, performances and challenges are presented to help the reader select the best cooling dehumidification system for his application or project. More emphasis is put on liquid and hybrid systems along with advances in desiccant membranes and energy conservation. The paper concludes that liquid and hybrid desiccant cooling dehumidification systems offer higher flexibility and control for moisture removal, lower heating and cooling requirements for the regenerator and absorber, respectively, compared with the same for solid desiccant systems. Mixtures of multiple liquid desiccants offer better dehumidification results compared with single desiccant solutions. Hygroscopic humidity buffer zones play a significant role in predicting and meeting the comfort levels for building occupants. Keywords: Humidity control, solid liquid desiccants, membrane technologies, desiccant performance, hygroscopic buffer zone

1. Introduction The world demand for equipment needed for heating, ventilation and air-conditioning (HVAC) has increased from 50 billion US dollars in 2004 to more than 90 billion US dollars in 2014. This rate has increased in the US from almost 11 to 19 billion US dollars over the same period (Mujahid et al., 2015). Humidity control is an essential and vital parameter for the human comfort in indoor environments. Cooling and heating load requirements for any space air-conditioning is not exclusive with the sensible cooling and heating terms only, but rather both sensible and latent loads including the control of humidity (Mujahid et al., 2015). Energy consumption of a building can significantly increase if the humidity level is offset from the design set point. In high performance buildings, the percentage of dehumidification energy consumption from the building total energy consumption can rise from 1.5-2.7% to as high as 12.6-22.4% if the relative humidity (RH) is dropped from design value of 60% to 50% (Fang et al., 2011). Conventional vapor compression cycles dehumidify the supplied air through cool-reheat processes. Desiccant systems provide efficient methods for controlling moisture content in the supplied air. In a conventional compression cycle, the latent load of moisture content is reduced by reducing the temperature of the air. The air temperature might drop below the set value to achieve the desired humidity level, thus a reheat coil is used to increase the sensible temperature of the air back to its set value. In addition to that, (Mujahid et al., 2015) showed that thermal comfort condition can be met with conventional cooling systems only when the sensible heat ratio is above 0.75. Desiccant cooling can reduce energy consumption by reducing the thermal condensation load done by refrigerants inside the air handling units. Reducing the load on the refrigerant would make positive effects on global warming and ozone depletion accordingly, especially if the desiccant system is operated with renewable energy sources, such as, solar systems. (Fang et al., 2011) studied different 1

dehumidification parameters and concluded that desiccant dehumidifier systems delivering 50% RH is almost equivalent in energy consumption to a conventional cooling dehumidifier system delivering 60% RH, thus, providing 16.7% improvement in RH. The effectiveness of liquid desiccant dehumidification systems compared to conventional vapor compression system is best shown in Table 1 where desiccant systems are shown to be more cost effective on the long run due to lower operating costs, better indoor air quality, more accurate humidity levels, and good energy storage capacities. The system installment is more complicated than conventional compression system, but the cost savings and being more environment friendly, by utilizing renewable and sustainable energy resources, would justify the complexity and higher initial costs. Table 1. Comparisons between conventional vapor compression systems and liquid desiccant cooling systems (Mujahid et al., 2016)

Parameter

Conventional air conditioning system (CACS)

Desiccant dehumidification system (LDDS)

Cost of operation Driving source of energy

High Electricity, Natural gas, Vapor

Humidity control Quality of indoor air System installment Capacity for storage of energy

Average Average Average Average

Save about 40% Low grade energy e.g. solar energy, waste heat etc. Accurate Good Slightly complicate Good

This paper summarizes desiccant cooling systems including solid and liquid systems. More emphasis is put on liquid desiccant systems as they are shown to be more efficient than solid desiccants in many applications. System types, performances and challenges are presented to help the reader select the best cooling dehumidification system. Hybrid and other dehumidification systems, such as hygroscopic humidity buffer zones and recent advances in desiccant membranes are visited and summarized, as well.

2. Desiccant dehumidification systems Desiccant cooling principles were introduced more than 80 years back in 1930s by Hausen (Pesaran et al., 1992). Desiccants are chemicals that can absorb or release moisture with the surrounding air. The driving parameter for this exchange is the difference in vapor pressure between the desiccant surface and the surrounding air (Sherif et al., 1999). As long as the desiccant surface vapor pressure is lower than that for the surrounding air, moisture would transfer from air to desiccant material. Once the desiccant material is saturated and reaches equilibrium in moist content with surrounding air, the moist exchange process stops. In order to reuse the desiccant, it has to be regenerated by extracting the moist content out of it. The regeneration of the desiccant material can be done through an absorption process that includes chemical and physical transformation or through an adsorption process that does not include any chemical or physical exchanges. Desiccant materials can be of two types: solid or liquid desiccants. Solid desiccants work with adsorption processes, whereas liquid desiccants absorb the moisture through chemical and physical processes (Sherif et al., 1999). 2

2.1 Solid desiccants There are different types of solid desiccants such as silica gels, lithium chloride (LiCl), and molecular sieves (Sherif et al., 1999). Figure 1 summarizes some common solid and liquid desiccant materials used for humidity control purposes. Solid desiccants are usually embedded in a wheel or other means that allow the desiccant material to be in contact with two air streams, as shown in Figure 2.

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Figure 1. Common solid and liquid desiccant materials

Figure 2 represents a typical rotating wheel dehumidifier where a solid desiccant is contained inside the packing of the wheel. As the humid-process air crosses the rotary wheel, the desiccant absorbs the moist content and the air leaves with drier state, but hotter. The sensible temperature can be retained to its desired value by the use of another heat exchanger, evaporative coolers, or cooling coils. To reuse the desiccant repeatedly as the wheel rotates, another stream of warmer air than the desiccant is passed through the wheel. This warmer air absorbs the moist content from the desiccant material in the wheel. Then the desiccant material repeats the cycle of absorbing and releasing the moisture content with the process and regenerated air, respectively, as the wheel continues turning. The regenerated air can be a return exhaust from the conditioned space or can be a direct line from outdoor environment. This process was commercially introduced by Pennington in 1955 (Pesaran et al., 1992) although it was invented in 1933 by Miller and Fonda. After the invention by Miller and Fonda in 1935, the system was introduced in 1935 by Hausen, and was developed by others such as Shipman in 1936, Fleisher in 1939, Larriva in 1941, and Altenkirch in 1941 and 1944, but none of them was able

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Figure 2. Typical rotating wheel dehumidifier (Munters, 2015)

to come up with a commercial product to reduce the energy for AC until Pennington patented what was called “the ventilation cycle”, utilizing a solid silica desiccant inside a rotating wheel as shown in Figure 2. This was later improved by Munters in 1968 (Pesaran et al., 1992). Table 2 shows a list of experimental and numerical studies related to various solid desiccant materials and the associated refrigerants used in the study. Table 1. Solid desiccant related studies (Pang et al., 2013)

Year Published Authors 2003 2003 2004 2006 2008 2009 2011 2011 2012 2013

Jiangzhou et al. Saha et al. Wang et al. Wang et al. Wang et al. Hu et al. Habib et al. Zhong et al. Vasta et al. Jibri et al.

Solid desiccant

Refrigerant

Zeolite Silica gel Activated Carbon Zeolite Silica Gel Zeolite -Foam Aluminum Activated Carbon Zeolite Zeolite Activated Carbon

Water Water Methanol Water Water Water R1234a and R507 Water Water R1234ze ( E )

Solid desiccant systems can be installed in open or closed cycle configurations. Figure 3 shows a typical open cycle which is similar to Pennington regenerative simple cycle. The points shown in Figure 3 are plotted on the psychometric chart on Figure 4.

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Figure 3. Open dehumidification cycle

The performance of the cycle can differ based upon the position of the rotating wheel with respect to the cooling coil. Figure 5 compares the performance of two open cycles with different locations of the desiccant rotating wheels (Trane Inc., 2005). When dehumidification process takes place upstream of the cooling coil (Figure 5a), a higher capacity cooling coil is needed than when it is downstream of the cooling coil. The overall energy exchange might end up the same, but the cooling coil capacity, size, and cost would be higher.

Figure 4. Psychometric chart for the open cycle shown in Figure 3

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Figure 5. Open cycle desiccant dehumidification wheel (a) upstream (b) downstream of cooling coil (Trane Inc., 2015) (OA: outdoor air, CA: cooled air, RG: regenerative air, C: cooling coil, H: heating coil)

A double-stage adsorption desiccant system was implemented by (Ando et al., 2005). A schematic for the system and the corresponding psychometric process are shown in Figure 6(a & b). The system has four rotors: two dehumidifiers and two heat exchangers. The incoming supplied air is dehumidified twice and precooled twice, as well. The air is directed to an evaporative cooler after leaving the second heat exchanger and before being supplied to the conditioned space. The break down in both heat and dehumidification allows better control for the supply air temperature and humidity. The system provided sufficient dehumidification at high ambient humidity. Although the dehumidification performance might drop at high humidity levels, the large reductions in the sensible heat needed for the heat exchangers justifies such systems and results in a high system COP.

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Figure 6. (a) Double stage adsorption desiccant system and (b) its process on psychometric chart (OA: outdoor air, EA: exhaust air, SA: supply air, RA: return air) (Ando et al., 2005)

Closed cycles operating with solid desiccants were inefficient when first invented as they had a fractional COP. However, after the invention of heat pumps that operate with zeolite and water, a COP of 1.2 was achieved by Tchernev and Emerson in 1986 (Sherif et al., 1999). (Fu et al., 2016) modeled a dual-scale desiccant wheel where micro-scale molecular dynamics submodel for adsorbent material was used along with a macro-scale sub-model for heat and mass transfer. The novel organic–inorganic hybrid adsorbent material combined high adsorption and good mechanical durability. The two sub-models were linked together and the moisture adsorption capacity of the material was double compared to that of silica gel B. It was found that sensible and latent load removal were improved by 12% and 30%, respectively. A one-dimensional gas side resistance simulation for desiccant wheels was performed to optimize the performance of desiccant wheels. The simulated model results were compared to data available from literature and the model was used to analyze the effect of working conditions on desiccant wheels after finding good agreement between simulated and literature experimental results. Working conditions included regeneration air temperature, inlet regeneration air humidity ratio, mass flow rate, process air temperature, velocity, and humidity ratio (Antonellis et. Al., 2010). Another model was conducted by Nobrega and Brum in 2009 on enthalpy and heat wheels to analyze the heat and mass transfer in rotary exchangers. Similarly, this model was validated against previously obtained experimental results. Depending on atmospheric conditions, enthalpy recovery wheel, which are heat wheels impregnated with a hygroscopic material, are way more efficient than heat wheels (Nobrega and Brum, 2009). 2.2 Liquid desiccants 7

Dehumidification using liquid absorption is more common in household and industry applications, while solid adsorption systems are more common in automotive. The energy sources to operate the desiccant cooling and dehumidification systems control the system efficiency, capacity, and overall cost. Liquid desiccant systems are capable of saving energy while providing sufficient humidity independent of the supply air temperature. Properties for a good liquid desiccant as defined my Mujahid (2016) are high moisture absorption capacity, lower temperature for regeneration, noncorrosive, non-volatile in nature, low viscosity, non-toxic, non-flammable, and inexpensive (Mujahid et al., 2016). Similar to the components of a solid desiccant system shown in Figures 2 through 6, liquid desiccant system is composed of a dehumidifier, regenerator, heat exchanger, heater and cooler. In addition to that a pump is needed to circulate the liquid desiccant solution. Figure 7 shows a simple liquid desiccant system. The rotary wheel used to store the solid desiccant is replaced with a storage tank to store the liquid desiccant. Additionally, an absorber and regenerator are needed to absorb and release the moist from the liquid desiccant. A pump is needed to circulate the liquid desiccant through the system. Moister is removed from the process air in the dehumidifier or absorbed by the return air in the regenerator due to the difference in vapor pressure between the liquid desiccant surface and the air. Subsequently, heat is liberated during condensation of water and heat exchange due to mixing. In the dehumidifier section, the cold liquid desiccant, which is weak in moist content, is sprayed over the incoming hot and humid air stream. The desiccant will absorb moisture content from the air and the diluted solution is stored in the bottom of dehumidifier. The diluted solution is passed to a heat exchanger, a heater, and to a regenerator. In the heat exchanger, heat is transferred from the feed solution, leaving the regenerator on its way to absorber, to the moist rich solution leaving the bottom of the absorber and being sprayed in the regenerator. No mass transfer occurs in the heat exchanger, but only heat is being transferred. In the regenerator unit, the moist rich solution is sprayed to an inlet air stream which would absorb the moist in its way to the atmosphere. This process is illustrated in Figure 8 (Abdel-Salam et al., 2016) (the solution status numbering were added to Figure 7b to relate Figure 8 with the processes shown in Figure 7). Some liquid desiccant materials, as shown in Figure 1, include lithium and chloride based solutions such as LiCl, LiBr, CaCl2, NaCl, tri-ethylene glycol and mixtures of the pre-mentioned liquid desiccants. The performance of liquid desiccant systems is controlled by the effectiveness and operation of the dehumidifier and the regenerator. Both units are controlled by the airflow rates, air temperature and humidity level or moist content, liquid desiccant flow rate, desiccant temperature and concentration, cooling process for the desiccant in the dehumidifier, and heating process for the solution used in the regenerator. (Gao et al., 2012) investigated experimentally the effect of mass transfer in a cross-flow dehumidifier and regenerator. The results showed that air and desiccant inlet temperatures play a significant role in determining the dehumidifier and regenerator performances and, thus, the system performance. Performance of a cross-flow membrane liquid desiccant dehumidification system was experimentally investigated by (Bai et al., 2017). Calcium chloride was used as the desiccant. Solution to air mass flow ratio and Nusselt number were investigated under various ranges and conditions. It was concluded that the overall effectiveness can be improved by decreasing the solution inlet temperature.

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(a)

(b)

Figure 7. Simple liquid desiccant system: (a) (Mohammad et al., 2016); (b) (Mujahid et al., 2016)

Figure 8. Schematic diagram of desiccant vapor pressure to moisture content (Abdel-Salam et al., 2016)

(Zhao et al., 2016) investigated experimentally and numerically the effectiveness of mixed liquid desiccants as compared to a single desiccant solution such as those shown in Figure 1. Previous studies have shown that mixtures of various liquid desiccants result in better dehumidification efficiency, less energy consumption and lower material costs. However, mixtures were accompanied with difficulties in measuring the thermal properties. The study by Zhao et al. considered six absorbent solutions: LiBr/LiCl, CaCl2/MgCl2, and water/methanol. The various mixtures considered were 1) pure LiBr, 2) 43% LiBr and 4.8% CaCl2, 3) 41% LiBr and 9% CaCl2, 4) 39% LiBr and 13% CaCl2, 5) 37.5% LiBr and 16.7% CaCl2. The numerical analysis used the NRTL (non-random twoliquid) equation which was shown in literature as a very powerful tool for analyzing the vapor pressure of mixed liquid desiccants. The study showed that dehumidification process effectiveness for mixed solutions was higher than that for a single solution. The solution mixture made of 37.5% LiBr and 16.7% CaCl2 gave 18.5% better dehumidification compared to pure LiBr and resulted in the highest COP compared to other systems and solutions working under the same ambient conditions (temperature and humidity).

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(Koronaki et al., 2012) investigated the usage of counter flow dehumidifiers. Air was allowed to flow in between parallel plates, whereas desiccant liquid solution was circulated in the plates. For the same inlet conditions, LiCl was compared to LiBr rich desiccants. The results showed that humidity ratio is reduced by 31% when the absorber desiccant solution was rich with LiCl whereas this reduction was only 12% when using LiBr. This result shows that LiCl is more efficient than LiBr for dehumidification with parallel plate internal dehumidification processes. (Park et al., 2016) developed an empirical model to predict the effectiveness of LiCl liquid desiccant based dehumidifier. The following parameters were considered: process air mass flowrate, air temperature and humidity ratio, LiCl liquid desiccant mass flowrate, and desiccant temperature and concentration. Experimental data were collected to improve the proposed model from a typical liquid desiccant unit. A linear relation was found to exist between dehumidifier effectiveness and the above parameters with 10% error bounds. The final proposed model was verified by comparing it to various existing models. An extensive review on liquid desiccant materials and dehumidifier configurations was done by (Mujahid et al., 2016). LiCl was found to provide the lowest vapor pressure characteristics but it was expensive. When LiCl is combined with CaCl2, the mixtures provided very promising and cost effective solution. To avoid carry over problem, inner cooled dehumidifier was first proposed by (Khan and Martinez, 1998). (Mujahid et al., 2016) added an inner cooled dehumidifier using a liquid– air membrane in a counter flow exchanger. 2.2.1 Liquid desiccant system (LDS) types There are mostly two types of liquid desiccant dehumidifiers: adiabatic and inner cooled dehumidifiers (Kessling et al., 1998). Adiabatic dehumidifiers require high flow rates to allow better wettability for the surface which would result in high absorption of moisture content by the desiccant material. Inner cooled dehumidifiers presented by (Khan and Martinez, 1998) can avoid droplet carry over problem. Since evaporative cooling in hot and humid environments is not very efficient, desiccant systems integrated with evaporative cooling were presented along with various system configurations, operational modes, and different desiccant materials (Mujahid et al., 2015). Conventional vapor compression refrigeration cooling systems provide good efficiencies only when sensible heat ratio is greater than 0.75. Direct and indirect evaporative cooling can improve sensible and latent heat load removal. A schematic for the indirect evaporative cooler (IEC) is shown in Figure 9. (Shariaty and Gilani, 2009) showed by numerical analysis that IEC can provide good indoor conditions with severe outdoor conditions as high as 70% RH and 50˚C.

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Figure 9. Schematic for indirect evaporative coolers (IEC) (Shariaty and Gilani, 2009)

In the direct contact dehumidifier-regenerator system, heat and water vapor transfer takes place between air and desiccant solution stream. Several designs for direct contact dehumidifier-regenerator systems are presented in this paper including packed bed, spray tower and falling film types. In the packed bed dehumidifier, shown in Figure 10, desiccant solution is sprayed from top of the packed bed and flows over the packing where it comes in a direct contact with air stream. The packed bed dehumidifier-regenerator can be either adiabatic (Figure10a) or internally cooled/heated (Figure10b) (Abdel-Salam et al., 2016).

Figure 10. (a) Adiabatic packed bed (b) isothermal packed bed (Abdel-Salam et al., 2016)

In an adiabatic packed bed, shown in Figure 10(a), the temperature of the desiccant solution changes as it flows through the dehumidifier-regenerator due to the heat transfer with the air stream and due to phase change which accompanies the moist transfer. In an internally cooled/heated “isothermal” packed bed, shown in Figure 10(b), the solution is continuously cooled/heated by a third fluid. This fluid could be water. As a result, the solution’s temperature in the isothermal bed remains almost constant as it passes through the packed bed and consequently will have better mass transfer compared to an adiabatic packed bed dehumidifier. The spray tower dehumidifiers are similar to the packed bed configuration expect with no packing beds. There are two types of spray tower dehumidifiers: isothermal and adiabatic. The adiabatic spray tower has an improved carry over avoidance as compared to the packed bed type or the isothermal spray tower. The third type is falling film that allows liquid desiccants to fall in the vertical direction. There are three configurations associated with falling film dehumidifiers: cross flow, parallel flow, and counter flow. These configurations are shown in Figure 11 (Abdel-Salam et al., 2016).

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Figure 11. Schematics for the different types of falling film liquid desiccant dehumidifiers (Abdel-Salam et al., 2016)

2.2.2 Liquid desiccant systems performance, challenges, and advances Liquid desiccant system performance is affected by droplet carry over. This issue can be reduced and the dehumidification effectiveness could be improved using indirect contact design as presented in Figure 9. Using liquid-to-air membrane energy exchanger will be the best possible solution as suggested by (Mujahid et al., 2016). Combination of dew point evaporative coolers and liquid desiccant dehumidifiers gives better performance (Abdel-Salam et al., 2016). (Yin et al., 2016) investigated internally heated regenerators as a method to avoid carry over problem completely. Internally heated regenerator was found to be more promising than adiabatic regenerators with low flow rates. (Desai, 2012) has studied basic processes of liquid desiccant air cooling system. The research investigated dehumidifier piping material, carryover, and compatibility. The results showed that for liquid desiccant air-cooling system, copper and aluminum are the most preferable materials. Experimental analysis for liquid desiccant systems including both latent and sensible heat loads were conducted and compared to a developed mathematical model by (Xie et al., 2008). The variables used for the mathematical models matched the experimental measurements: outside air temperature 29.1˚C to 33.6 ˚C with humidity ratio 13.7 to 19.7 g/kg; supply air temperature 23.6 ˚C to 24.2 ˚C with humidity ratio of 7.4 to 8.6 g/kg . The COP of the liquid desiccant system was shown to save energy up to 30% compared to conventional vapor compression cycles. In 2014, Bassuoni performed an experimental analysis for a hybrid desiccant air conditioning system. Calcium chloride (CaCl2) solution was used as the working desiccant material with R134a refrigerant in the vapor compression cycle. The system performance was evaluated by analyzing the COP and the energy savings. An increase of 54% in the COP and energy savings up to 46% were achieved (Bassuoni, 2014). (Qi et al., 2014) showed that precooling of the liquid desiccant prior to its entrance to the dehumidifier can result in energy savings up to 22-47%. Upon the addition of a heat exchanger between the flows entering and leaving the regenerator, which would increase the temperature of the solution entering the regenerator, the energy savings increased to 49% along with a reduction in required solar collector area up to 50%. (Liu et al., 2010) conducted an experimental study to compare desiccant cooling with conventional air cooling. The experimental results showed that liquid desiccant cooling is a better choice compared to air cooling for getting good dehumidification levels. It was proposed to install an additional heat exchanger and a spray section. This would allow more and improved heat recovery. When water 12

driven processor was used, the COP was approximately 0.93-1.19. The system COP was significantly improved to as high as 5.0 in winter and summer by using an air processor run by a heat pump or was power driven. (Yinglin et al., 2016) analyzed the performance of liquid desiccant systems experimentally and numerically by developing a mathematical model. The results showed a cut down of 10% in cooling capacity for the evaporator could be achieved with a 1.5% difference of desiccant concentration. Further reduction in the desiccant solution temperature entering the evaporator and reduction in the heating load requirement for the regenerator were achieved by adding an auxiliary regenerator and increasing the desiccant concentration by 2.65% only. Spatial differentials of fluid properties for liquid desiccant systems were approximated using discretization to develop a dynamic model. Static experimental data through the Levenberg-Marquardt algorithm were used for estimate unknown parameters and then were refined through dynamic experimental data (Li et al., 2016). Another dynamic model for simulating liquid desiccant dehumidifiers was developed by (Wang et al., 2017) assuming one-dimensional conditions. It was shown that dynamic models agree better than static models with experimental data. Many advances in liquid desiccant dehumidification systems driven by heat pumps have been studied experimentally and analytically. A study modeled and analyzed hollow fiber membrane based twostage liquid desiccant air dehumidification system driven by a heat pump (Zhang et al., 2016). Droplets carry over was overcome using semi-permeable membranes. Compared with a single-stage dehumidification system, the two-stage system provided many advantages such as lower solution concentration exiting from the dehumidifier, lower condensing temperature and better thermodynamic performance. The COP was increased by 20% for two-stage system over one-stage (Zhang et al., 2016). At the same time, (Xie et al., 2016) experimentally and analytically investigated the performance of a counter flow liquid desiccant system driven by a heat pump. Key component performances were analyzed and simulated and system COP was investigated. It was concluded that adding a heat exchanger for the solution can significantly improve the system performance. The authors also recommended multi-stage heat pump cycle which can improve the solution and refrigerant properties and improve the energy efficiency of the counter-flow system (Xie et al., 2016). The performance of counter-flow liquid desiccant dehumidifier developed new empirical correlations for moisture effectiveness and enthalpy effectiveness along with critical inlet parameters (Wang et al., 2016). Empirical correlations were validated with experimental data of the same study and with other reported studies. Air and desiccant inlet conditions effects on the system performance along with the packing height were investigated (Wang et al., 2016). Internally cooled liquid desiccant dehumidifiers have been investigated as means to reduce or eliminate liquid carryover. An experimental study for internally cooled dehumidifier operating with single channel internally cooled dehumidifier was conducted by (Luo et al., 2015). Calculations of mass transfer coefficient were easier to obtain due to easy observance of the contact area with the testing rig. CFD simulation for an adiabatic internally cooled dehumidifier was done by (Luo et al., 2017). Heat and mass transfer were simulated and system performance was investigated. The study analyzed the interior conditions of the dehumidifier and pointed the significance of considering desiccant solution properties during CFD simulations. (Liu et al., 2016) quantitatively compared three internally cooled liquid desiccant dehumidifiers each with a different internal structure: parallel plate, fin coil, and packed tower. Key operating parameters were compared including specific surface area and heat and mass transfer coefficients. Fin-coiled internally cooled dehumidifier was shown to have

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the best performance among the three considered structures and packed tower was the worse (Liu et al., 2016). 2.2.3 Advantages of liquid over solid desiccant systems Liquid desiccant cooling systems have some merits compared to solid desiccant cooling systems: 1. Solid desiccant systems can cause overheating for the dry air stream, whereas cooling and dehumidification using a liquid desiccant air-conditioning system can be achieved simultaneously. 2. Moisture carrying capacity for liquid desiccants is higher than for solid desiccants. 3. The temperatures required for the regeneration of dilute liquid desiccant solutions are lower. 4. Energy storage is possible for liquid desiccant AC systems by storing concentrated desiccant solutions in storage tanks, while this is not possible with solid desiccants. Fumo and Goswami showed that when regeneration is considered, liquid desiccant provides better efficiencies than solid desiccants. Humidification and regeneration rates depend upon desiccant temperature, desiccant flow rate, air temperature and air flowrate. To achieve acceptable experimental results, Oberg model was adopted (Fumo and Gosami, 2002).

3. Hybrid desiccant systems with renewable energy sources Dehumidification cooling systems that combine desiccant and conventional vapor compression systems in one unit are called hybrid systems. In some applications, dehumidification cooling systems may not be enough to provide the required system load and, thus, an auxiliary system should be added. In addition to that, desiccant dehumidifiers offer promising results when combined with renewable energy sources such as solar power. However, these sources may not be always available when the system is running and, thus, an additional auxiliary back energy source is needed. Methods for converting solar radiation into useful energy for usage in the air-conditioning or cooling processes are summarized in Figure 12. Solar regeneration systems are very effective over the long

Figure 12. Methods for conversion of solar energy into useful energy for usage in AC or cooling processes (Mujahid et al., 2016) 14

term as they provide sustainable and renewable energy sources. Solar processes are generally characterized by high initial setup cost, but the return on investment is usually justifiable with the low operating costs compared to other conventional cooling systems. Figure 13 summarizes energy consumption for residential and commercial buildings (Song et al., 2015). HVAC shares the highest percentage in both sectors. Thus, reduction of energy bill used for cooling and dehumidification through solar and hybrid system is a very attractive technology that can benefit buildings’ owners, reduce carbon emissions, and would be more environmentally friendly. This section will present various works done to advance the technologies of hybrid cooling and dehumidification processes.

Figure 13. Energy consumption in (a) residential buildings (b) commercial buildings (Song et al., 2015)

Lithium based desiccants are considered very strong desiccants, whereas calcium based desiccants, such as calcium chloride, are considered weak desiccants. Examples of lithium based desiccants are lithium-bromide (LiBr) and lithium-chloride (LiCl) which can achieve a very low RH levels. LiBrwater pairs provide very promising results for absorption systems when combined with solar applications as it yields higher COP with temperatures obtainable by solar flat plate collectors. Two big drawbacks for LiBr are crystallization at low temperatures with high concentrations, higher than saturation, and the high corrosive chances on metals. Glycol is considered as a mild desiccant. Organic desiccants will get evaporated during the regeneration process, so there is a loss of desiccant. To overcome this desiccant loss, desiccant condensation is required (Grossman and Johannsen, 1981). In 1993, Waugaman et al. reviewed the state-of-art of desiccant cooling and dehumidification systems including hybrid systems operating with low-grade thermal energy. The study covered improvements done over a wide range of systems from simple cycle systems to complex hybrid-cogeneration designs. Regeneration process run by solar energy during daytime and by other alternative energy sources during night were very promising. Desiccant systems are very efficient in humid environments when latent loads are high (Waugaman et al., 1993). In 2007, Henning presented a solar driven solid desiccant system as shown in Figure 14. The solid desiccant adsorption was utilized for both adsorption cooling and adsorption dehumidification. As shown in Figure 14, as the exhaust air is released out to the atmosphere, it exchanges energy and absorbs the moist from the incoming supplied air. The incoming fresh air is dehumidified in the rotating dehumidifying wheel, precooled in the heat recovery exchanger, then humidified before being cooled further down, during summer time, to the desired temperature using a cooling chiller. During 15

winter times, the air is heated after leaving the humidifier using solar driven heating system. Thus, this system could regenerate the energy leaving the conditioned space and allowing dehumidification, cooling and/or heating using solar energy system (Henning, 2007). Similar work was conducted by (Dai et al., 2002) where an adsorption evaporator was used as a cooling chiller driven by a solar system. In their study, the return air was directed to the evaporative cooler, heated by a heater, and then directed to the rotating wheel to regenerate the desiccant. Dai et al. concluded that the COP of the system would increase with an increase in the ambient conditions including outdoor temperatures and humidity ratio. The authors also observed that the COP of the system had a parabolic relation with the effectiveness of the heat exchanger. They also concluded that the mass flow rate of air can improve the COP when increased, but this will limit its outlet temperature and, thus, recommended a balance between the system COP and the leaving temperature of air (Dai et al., 2002).

Figure 14. Solar driven solid desiccant system (Henning, 2007)

(Ge et al., 2010) simulated a solar driven two-stage rotary desiccant cooling system. The system was used to provide cooling for one floor in a commercial office building in two cities with different climates in Berlin and Shanghai. The thermal COP was 0.9 to 1.28 for regeneration temperatures of 85°C and 55°C, respectively. The thermal COP would be higher than 1 if the regeneration temperature does not cross 70°C. (Mandegari and Pahlavanzadeh, 2010) investigated the performance of a hybrid desiccant cooling system experimentally under different climates by changing the inlet air temperature and humidity. Electrical and thermal COPs were evaluated and compared to a simple vapor compression refrigeration cycle running with the same conditions. The thermal COP was decreased by 28% and 36% for hot-humid and hot-dry climates, but the electrical COP was shown to increase due to savings in electrical energy used to drive the hybrid desiccant cooling system. Also, it was noticed that the thermal COP is improved when the environment was more humid. (Liu et al., 2010) investigated liquid desiccant systems that control humidity independent of the supply air temperature similar to the system shown in Figure 15. The results showed that the required chilled water temperature by the system was increased from 7 ˚C to 17 ˚C due to the removal of the latent load from the chiller. Instead the chiller would control the sensible or dry bulb temperature only and the liquid desiccant dehumidifier would control the humidity. Such systems can save up to 2030% of the operating cost compared to conventional AC systems. A mathematical model for a liquid desiccant dehumidifier that dehumidifies a space independent of the air cooling systems was developed by (Keniar et al., 2015). The liquid desiccant regeneration was 16

integrated with a solar energy source as shown in Figure 15. Membrane technology was not investigated as part of this research, but rather the membrane was assumed to be permeable for water vapor only and not for liquid desiccant. The developed mathematical model was validated by conducting experimental testing. The developed model was used to compare the performance of the system shown in Figure 15 against a conventional AC system. An improvement of 10% in the dehumidification effectiveness was achieved. The initial cost of the system needed a payback period of 7 years and 8 months compared to conventional AC systems operating under the same conditions.

Figure 15. Schematic of the separate dehumidification system integrated with renewable solar energy (Keniar et al., 2015)

(Angrisani et al., 2015) investigated the dependence of a hybrid liquid desiccant system, operating with a silica gel desiccant rotating wheel and a small regenerator, on several parameters such as outdoor air and regeneration temperatures, outdoor humidity ratio, desiccant wheel rotational speed, and air and liquid desiccant flowrates. The study also investigated various energy sources such as electrical, thermal and primary energy sources. When regeneration temperature and liquid desiccant flowrate increased, the electric and primary energy sources increased leading to a reduction in the thermal COP of the system. The COP was best for wheel rotational speed at approximately 13-47 rotation per hour (0.22-0.78 rpm). It was concluded that outdoor air and humidity ratio do not play a significant role when optimizing the COP of the system. Liquid desiccant cooling system which can run on low grade heat was experimentally analyzed when running on solar energy to supply a small office. Lithium chloride was used as the liquid desiccant. To eliminate desiccant carryover problem, an indirect contact heat and mass exchanger was used. The system performance, effectiveness, and operating conditions were analyzed and optimum values were concluded (Das and Jain, 2017). A conceptual study examined four hybrid liquid desiccant airconditioning systems and compared their performance to conventional air-conditioning system under different climatic design conditions. The considered hybrid systems consisted of open absorption cycles with either a vapor compression cycles or an indirect evaporative cooling system. Different combinations between the absorber, the compression unit, the regenerator or the evaporative cooling unit were considered including series connection, partly combined, or integrated in one unit. Depending on the type of the hybrid liquid desiccant system used and the outdoor conditions, energy savings of 30-60% could be achieved (Mucke et al., 2016). Another study considering a hybrid liquid desiccant system consisting of a conventional liquid desiccant unit and a vapor compression heat pump was studied by (Yamaguchi et al., 2011). The liquid used was aqueous solution of LiCl with 17

R407C used as a refrigerant in the vapor compression heat pump. The absorber was combined with the evaporator, whereas the regenerator was combined with the condenser. The system was optimized for conditions in Japan and the COP was improved by respective improvement of the compressor isentropic efficiency and the solution temperature efficiency (Yamaguchi et al., 2011). An advanced hybrid air-conditioning system driven by a heat pump was analyzed considering various parameters of the inlet air and desiccant solution. LiCl solution was used as the liquid desiccant while developing a new method for determining the coupled mass and heat transfer coefficients. Experimental data were used to validate the new model which included the NTU-Le model. The study analyzed the thermophysical properties of air and desiccant solution and real time performance of the system (Chen et al., 2016). 4. Other advancements in humidity control technologies This section reviews two topics related to other humidity control techniques. The first one is advancements in desiccant membranes, whereas the second one summarized the significance of humidity buffer zones in buildings. Thermal condensation humidity control has been well understood in the literature. Desiccant cooling and dehumidification are highly dependable on the membrane technology that separates the desiccant solution from the system. The membrane can be permeable or semi-permeable allowing the passage of water vapor only with no mass transfer. Figure 16 shows a liquid to air membrane technology that is a semi-permeable membrane. It allows the transfer of water vapor but not any liquid. Usually, the flat plate “liquid-to-air membrane energy exchange” (LAMEE) consists of several air and solution channels that are separated by semi permeable membranes as shown in Figure 16b. Figure 16a shows a microscopic view of the membrane. This membrane is characterized by: 1) low vapor diffusion resistance, as low as 97 seconds per meter, that improves the dehumidification process, 2) high liquid penetration pressure that prevents the transfer of any desiccant droplets from the desiccant solution to the air channels, and 3) high modulus of elasticity which decreases the deflection of the membrane and, thus, reduces the flow mal-distribution in the air and desiccant channels. The pressure drop across the flat plate LAMEE is lower than 250 Pascals under normal operating conditions (AbdelSalam et al., 2014).

Figure 16. Liquid-to-air membrane energy exchange (a) microscopic view (b) flat bed (Abdel-Salam et al., 2014)

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(Huang et al., 2016) conducted experimental and numerical analysis for an internally cooled membrane-based liquid desiccant arranged in cross-flow with air. Water was allowed to flow vertically and formed falling films, similar to those shown in Figure 11. Air and the desiccant solution were separated by selective permeable membrane. Hydro-dynamic conditions were developed but thermal and mass concentrations were still developing. For the numerical approach, finite volume approach was used. Boundary conditions for the membrane surfaces, plastic plate surface and interface were numerically obtained. After validating significant controlling non-dimensional numbers, such as Sherwood and Nusselt local and mean numbers which were 2-3% less than for adiabatic conditions, it was found that the surface and interface temperatures met isothermal conditions. (ADMA, 2015) developed foil-like membranes for dehumidification processes as shown in Figure 17. This metal foil-like membrane consists of a paper-thin, porous metal sheet coated with a layer of water-loving molecules. This membrane technology is permeable to water vapor with high fluxes (high flow rates) while blocking air penetration resulting in high selectivity. The blocking of air penetration results in less energy usage, while the high permeation fluxes result in a more compact device. The new materials and the foil-like nature of the membrane facilitate the mass production of a low-cost, compact device. An example of the metal foil is porous titanium that has excellent filtration capabilities as shown in Figure17. The plate thickness ranges between 254 micro-m to 101.6 mm (0.01 to 4 inches) and its surface area can be as large as 0.258 m2 (400 in2). The cylinderical type is self-supporting. Some of the properties, such as average porosity and smallest particle filtered by different titanium grades are listed in the Table 3.

Figure 17. Titanium porous membranes

(ADMA, 2015)

Table 3. Titanium foil like membrane properties (ADMA, 2015)

Titanium Grade

Average porosity

mean pore size (microns)

Smallest particle filtered out (microne)

Tensile Strength (Psi)

Thickness (inches)

2003

20%

3

1

31,300

0.01 - 0.125

4010 5015 6025

40% 50% 60%

10 15 25

3 5 8

10,000 7,000

0.01 - 0.125 0.015 - 0.25 0.03 - 4

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(Mansourizadeh and Ismail, 2009) conducted a review for hollow fiber gas liquid membranes. Hollow fiber membrane contactor can offer a much larger contact area per unit volume than other conventional absorbers. Membrane contactors can become more efficient for gas absorption than conventional equipment and may reduce the size of gas absorber and stripper units by 63–65%. This effectiveness is possible by using a membrane with a highly porous structure. However, the gas absorption application in membrane contactor requires a high level of safety to prevent absorbent penetration into the membrane, especially in severe operation situations. Innovative three-fluid membrane contactor, which is able to exchange both sensible and latent heat with the process air, was investigated and tested. A condensing/evaporating refrigerant (first fluid) exchanges heat with a second fluid (water/liquid desiccant) flowing over the outer surface of hydrophobic membrane capillaries, inside which the process air flows (third fluid). This new component raises very interesting opportunities. Though fairly compact, it can reduce energy consumption and improve the refrigeration and air-handling unit efficiency. Membrane contactors with liquid desiccant based air dehumidification was investigated considering triple-bore hollow fiber membrane by Bettahalli et al. in 2016. The performance of poly-vinylidene fluoride based triple-bore hollow fiber membranes were investigated as liquid desiccant contactors (Bettahalli et al., 2016). Humidity buffer zones might be formed by the building walls, windows and furniture. Humidity buffering zones are critical for comfort level in buildings. Le et al. (2016) showed that neglecting humidity transfer through buffer zones created in building envelopes, carpets, furniture, and other structures can significantly increase the predicted percentage of dissatisfied indices. This would, inturn reduce the acceptability of indoor air quality during occupied periods. Hygroscopic curtains have been investigated analytically and experimentally as a mean to create a moisture buffer zone (Ghali et al., 2011). The hygroscopic curtains, through absorption and desorption, helped in reducing moisture content in a typical office by 7% compared to no-curtain case (Ghali et al., 2011) Effect of various solid desiccant wall materials on rotating wheels’ performance was investigated using a mathematical model. The model that was proposed by (Zhang et al., 2014) was validated against a Silica gel B wall desiccant wheel that was built by the same author. The model was used to predict the behavior of cyclic wheels by looking into effects of regeneration air temperature, process air temperature, and humidity. Ten commonly used materials were considered in the simulation and it was found that the best performance was achieved with silica gel 3A and gel RD (Zhang et al., 2014). Skewed flow over hollow fiber membrane bank for liquid desiccant air dehumidification were investigated considering skew angles between 0-90 degrees. Finite volume method was used to solve for the mass, energy and momentum equations and was experimentally validated. Correlations for predicting the convective transfer coefficients and friction factors at different skewed angles were proposed (Ouyang and Zhang, 2016). A liquid desiccant dehumidification cooling system was experimentally investigated independent of temperature and humidity control as a mean of improving the air-conditioning energy usage. The system consisted of the basic components of a liquid based dehumidifier including a dehumidifier, a regenerator, an evaporative cooler in addition to an air-to-air heat exchanger. The experimental results indicated that the air inlet conditions and solution concentration play a significant role towards system performance and effectiveness (Chen et al., 2017(b)).

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5. Conclusions This article reviewed the latest research and advances related to solid and liquid desiccant cooling systems, membrane technologies and humidity buffer zones. Various systems were introduced focusing on the advantages and limitations accompanied with the design and operation of each system. Desiccant cooling is capable of operating with low-grade thermal energy. Liquid desiccant systems are more promising for residential and commercial buildings than solid desiccant systems as they provide more advantages and control flexibility over solid desiccants. Other conclusions related to system COP and energy conservation include: -

-

As the regeneration temperature demand is decreased, the system COP is improved. Effectiveness of dehumidification and regeneration process with liquid desiccant depends on air and desiccant solution flowrates, air temperature and humidity, desiccant temperature and concentration. As the strong solution concentration increases, COP and specific moisture removal rate will increase. The COP of mixed desiccant LiBr-CaCl2-water system provides better performance than other solution mixtures or than pure LiBr desiccant solution.

Thermal COP calculations are not the same in all studies and researches due to differences in energy sources used. For that COP can be a misleading parameter when comparing the performance of different systems without considering other parameters such as energy sources. The energy source cost, the system initial cost, operating cost and the payback period or the return on investment (ROI) are vital parameters in parallel with the COP. Thermal and electrical COPs, when presented together would be an acceptable comparison as they include the electrical savings and the thermal improvements. However, the payback period and the ROI are still to be considered as part of the comparison. Desiccant cooling would provide very efficient energy solution when combined with renewable and sustainable energy sources. Hybrid desiccant cooling systems are currently an expensive technology, therefore a reduction of the investment cost is desirable, to exploit its energy and environmental advantages. Finally, humidity buffer zones are important and ignoring them can provide bias comfort level indices. This research did not receive any specific grant from funding agencies in the public, commercial, or notfor-profit sectors.

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Highlights: -

Overview of liquid and solid desiccant dehumidification and cooling system characteristics and challenges. Heat pump driven dehumidification systems are very effective Hybrid systems offer promising solutions specially when combined with sustainable energy resources. Hygroscopic humidity buffer zones play a significant role in predicting and meeting the comfort levels for building occupants.

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