Pietruschka et al. (2006) experimentally and theoretically investigated a range of liquid desiccant based air conditioning cycles located on the return air side of a
Fig. 2.7 Schematic of the hybrid VCS addressing the sensible and latent loads separately (Dai et al. 2001)
residential building, including a cellulose fibre contactor and an internally evaporatively cooled cross flow contactor. The cellulose contactor achieved better contacting potential, reduced pressure drop and desiccant carry-over compared to the internally cooled design. However, it was found that the best dehumidification performance was obtained with the internally evaporatively cooled contactor. The cellulose fibre contactor cannot be sufficiently internally cooled, and thus its dehumidification potential is limited. Figure 2.8 shows a schematic diagram of the internally cooled contactor. Both LiCl and CaCl2 desiccant solutions were investigated, the LiCl gave 40-50 % higher dehumidification rates. For the summer design condition the system could achieve 886 W of cooling power, generating a supply air temperature of 18.8 °C and absolute humidity of 0.007 kgvapour/kgdryair. The authors reported that if volumetric air flow is tripled from 100 to 300 m3 h-1 (typical fresh air supply to a domestic dwelling), a modest reduction in absolute humidity of 0.001 kgvapour/kgdryair is achieved. Pietruschka et al. (2006) suggested that if desiccant wetting rates of the plate could be improved from 35 to 60 %, a
Fig. 2.8 Internally evaporatively cooled cross flow contactor (Pietruschka
et al. 2006)
15 % improvement in dehumidification capacity can be achieved at a lower solution flow rate, thus reducing the risk of desiccant carry-over. If wetting rates on the evaporative cooler can be improved from 70 to 76 %, supply air temperatures of 18 °C are possible.
A solar powered liquid desiccant evaporative cooling system for greenhouse applications in hot and humid climates has been investigated by Lychnos and Davies (2012). Figure 2.9 provides a system schematic. The system first dehumidi- fies air in a falling film contactor; the contactor is internally cooled using cooling water from an evaporative cooling tower. The dehumidified air is then cooled by an additional evaporative cooler. The system can lower the maximum summer air temperature by 15 °C compared to a greenhouse purely conditioned by simple fan ventilation, which is 5-7.5 °C lower than what the conventional evaporative cooler could achieve on its own, indicting the potential of stand-alone liquid desiccant- evaporative systems.
Wang et al. (2009) have experimentally and theoretically investigated a 40 kW liquid desiccant evaporative cooling system for building applications. Figure 2.10 shows a schematic of the system. The system employs a packed bed contactor and uses a LiCl solution. External solution cooling is provided from an evaporative cooling tower. The system is able to treat both sensible and latent loads, with no additional refrigeration required. Inlet ambient air at 35 °C is cooled to the supply requirement of 18 °C. The COPth of the system was found to be 0.8 when the heat source temperate is 70 °C. Because the system can be operated with any heat source above the regeneration temperature it is well suited to solar and waste heat applications i.e. SOFC.
Das et al. (2012) presents an experimental investigation of a solar driven membrane based stand-alone desiccant air conditioning system for hot and humid climates, employing a LiCl solution. The system consists of a dehumidifier, regenerator, solution cooling tower, supply air indirect evaporative cooler (IEC), several
Fig. 2.10 Schematic of the desiccant evaporative system (Wang et al. 2009)
Fig. 2.9 Liquid desiccant-evaporative cooling system for greenhouses (Lychnos and Davies 2012)
heat exchangers and a solar collector as shown Fig. 2.11. The performance evaluation of the system under a Delhi climate showed that an absolute humidity change of 0.002-0.008 kgvapour/kgdryair and a moisture removal rate of 0.2-1.6 gs-1 are
Fig. 2.11 Indirect membrane based stand-alone evaporative cooled desiccant air condition system (Das et al. 2012)
attainable with an inlet air absolute humidity range of 0.01-0.023 kgvapour/kgdrya;r. The membrane based contactor latent and enthalpy effectiveness is measured in the range of 30-60 %. Throughout all tests only a small amount of air sensible cooling is provided by the liquid desiccant solution in the membrane contactor, therefore the reported enthalpy effectiveness is only marginally higher than the latent. Further air sensible cooling is provided by the second stage IEC. The supply air IEC wet-bulb effectiveness was found to be around 90 %. The authors state that if fresh air is used to evaporate water in the IEC instead of the treated dehumidifier air, the cooling capacity and COPth would almost double. However the supply air condition would also change and the effectiveness of the IEC will be lower. This demonstrates the potential shortcomings of the use of a second stage IEC. The system’s cooling capacity ranged from 2.5 to 5.5 kW and the COPth is between 0.4 and 0.8. The performance of the system increases with an increase in the inlet air absolute humidity. Das et al. (2012) believe that the close control of desiccant solution concentration to regulate the supply air condition is an area in need of future research.
A collaborative project named DEHUMID (Bonke 2007) had the goal of developing and testing a low cost, compact, and energy efficient liquid desiccant air conditioning system in order to save energy by reducing compressor size and eliminating excess chiller capacity in building applications. The system employs a cellulose fibre packed bed contactor, operating with a LiCl solution. The data analysis from the experimental testing of the system in a real world application gives encouraging COPel results of around 2.5, with a maximum of 5.9. However, issues encountered included: desiccant leakage and corrosion and insufficient sensor placement creating a lack of data to sufficiently validate mathematical models. A conclusion from the work demonstrates that such systems are only viable in Southern European regions where it is impossible to reduce high indoor air humidity by increasing the supply air flow. Factors requiring further work include material of fabrication/mechanical construction as corrosion and durability issues were encountered, simplification of the control system and the use of hard water in the cooling unit.
Liu (2008) has carried out theoretical and experimental work focussed on a novel heat recovery/liquid desiccant air conditioning system employing a LiCl working solution. The system is composed of an air to air liquid desiccant filmed cellulose fibre heat and mass exchanger combined with a cellulose fibre packed bed contactor driven by solar energy. The complete system is shown in Fig. 2.12.
Conclusions from the experimental work presented by Liu (2008) are summarised with respect to the dehumidifier, regenerator and system performance. Dehumidifier performance:
- • Higher desiccant temperature causes lower heat and mass transfer, and also a higher supply air temperature, thus adequate desiccant pre-cooling is required.
- • As desiccant solution supply flow rate is increased, the moisture absorption capacity per litre solution decreases and heat recovery effectiveness increases. Increasing air flow speed results in higher moisture absorption ability, but lower heat recovery effectiveness.
- • The use of cellulose fibre packing meant some desiccant carry-over into the supply air stream.
- • A higher desiccant temperature results in higher regeneration capacity. A temperature of at least 60 °C was required for a LiCl solution.
- • A low desiccant solution flow rate results in higher regeneration capacity and condensed solution concentration.
Fig. 2.12 Schematic of the heat recovery/desiccant cooling system (Liu 2008)
- • Regeneration capacity and condensed solution mass concentration increases with increasing fresh air flow speed, but then begins to decline, thus there is an optimal air flow speed.
- • A dry air stream results in better regeneration capacity.
- • Cooling capacity and COPel increase with increasing fresh air temperature, therefore this form of system is suitable for hot/humid locations. A maximum COPel of 13 was achieved when utilising solar energy, thus offering the potential for significant energy savings in summer.
- • Based on a LiCl solution, when not using renewable/waste heat, the lowest possible concentration of desiccant is preferred as this improves COPth. However when renewable/waste heat is available, the higher the concentration of desiccant results in improved cooling capacity and system COPth.
Lowenstein (2008) and Conde (2007) have identified several issues regarding the use of conventional stand-alone liquid desiccant air conditioning systems, these include; duct corrosion, parasitic energy consumption from fans and pumps, large space requirement and additional requirement of equipment (cooling tower/ heat source). Work carried out by Woods and Kozubal (2013) at The National Renewable Energy Laboratory proposes to overcome some of these shortcomings through the design and manufacture of an integrated stand-alone internally cooled dew-point IEC membrane based liquid desiccant contactor (DEVap). The system overcomes the issues listed above by:
- • Replacing desiccant spray with a low flow desiccant stream contained behind a semi-permeable membrane, thus reducing pumping power and carry-over.
- • Using internal indirect evaporative cooling (IEC) to cool the desiccant and remove the latent heat of condensation, thus reducing the requirement of an external cooling tower and the energy requirement of the pumps to circulate the cooling liquid.
- • Using a second stage dew point IEC, to provide further supply air sensible cooling so that no further air conditioning is required.
A diagram of the DEVap concept is provided in Fig. 2.13a. No work has been found relating to the proposed system; combing an indirect membrane based liquid desiccant contactor and internal dew point IEC into a single core. Because of the two separate stages, the DEVap system can be operated as either; a dehumidifier for outside air, IEC cooling only or a combined dehumidification and cooling system. The system demonstrates operational flexibility; advantageous in energy efficient air conditioning applications. Figure 2.13b shows an example diagram of a residential installation of the DEVap system.
Experimental analysis has been carried out for the DEVap system. Zero desiccant solution carry-over has been reported, and it has been demonstrated that that the inclusion of the internal dew point IEC improved dehumidification performance, with an outlet absolute humidity of 0.0084 kgvapor/kgdryair achieved
Fig. 2.13 a Schematic of the DEVap system, and b an example diagram of a domestic installation of the DEVap system (Woods and Kozubal 2013)
compared to 0.0089 kgvapor/kgdryair for the adiabatic test at 40 °C. An effective integrated energy efficiency ratio (EER) of 23.2 was calculated for the combined DEVap system. In comparison, a traditional VCS has an EER of 15.5 (Kozubal et al. 2012). The DEVap concept not only improves the performance of the liquid desiccant dehumidifier, it also acts as an autonomous air conditioner that can adapt to a variety of sensible and latent loads, reduces peak electrical load by 80 % and total energy use by 40-80 % compared to an equivalent VCS (Kozubal et al.