A Zero Carryover Liquid-Desiccant Air Conditioner for Solar Applications

Abstract

A novel liquid-desiccant air conditioner that dries and cools building supply air has been successfully designed, built and tested. The new air conditioner will transform the use of direct-contact liquid-desiccant systems in HVAC applications, improving comfort and indoor air quality, as well as providing energy-efficient humidity control. Liquid-desiccant conditioners and regenerators are traditionally implemented as adiabatic beds of contact media that are highly flooded with desiccant. The possibility of droplet carryover into the supply air has limited the sale of these systems in most HVAC applications. The characteristic of the new conditioner and regenerator that distinguishes them from conventional ones is their very low flows of liquid desiccant. Whereas a conventional conditioner operates typically at between 10 and 15 gpm (630 and 946 ml/s) of desiccant per 1000 cfm (0.47 m3/s) of process air, the new conditioner operates at 0.5 gpm (32 ml/s) per 1000 cfm (0.47 m3/s). At these low flooding rates, the supply air will not entrain droplets of liquid desiccant. This brings performance and maintenance for the new liquid-desiccant technology in line with HVAC market expectations. Low flooding rates are practical only if the liquid desiccant is continually cooled in the conditioner or continually heated in the regenerator as the mass exchange of water occurs. This simultaneous heat and mass exchange is accomplished by using the walls of a parallel-plate plastic heat exchanger as the air/desiccant contact surface. Compared to existing solid and liquid desiccant systems, the low-flow technology is more compact, has significantly lower pressure drops and does not “dump” heat back onto the building’s central air conditioner. Tests confirm the high sensible and latent effectiveness of the conditioner, the high COP of the regenerator, and the operation of both components without carryover.

A review on the use of calcium chloride in applied thermal engineering

“…A machine at almost commercial level, using CaCl 2 aqueous solution as liquid desiccant has been reported by Lowenstein et al [87,95] who have been investigating the process since 1988. The proposed design of liquid desiccant air-conditioner, which has been tested in several locations under full scale operation, is given in Fig.…”

“…Indeed, the researchers consider that LiCl and CaCl 2 aqueous solutions are the most viable liquid desiccant for solar applications [87]. However, the operation of the proposed machine with CaCl 2 aqueous solution leads to a more cost effective machine than when LiCl aqueous solution is used, though LiCl aqueous solution provides higher dehumidification than CaCl 2 aqueous solution [95].…”

“…However, calcium chloride is usually selected because of its safety characteristics [4] and its low cost. Indeed, a desiccant cooling system that uses liquid desiccants can strongly reduce its cost for storage by replacing lithium chloride with calcium chloride [87] because the cost for calcium chloride is approximately one-twentieth that of lithium chloride [88].…”

Recent research developments in polymer heat exchangers – A review

“…The COP of the regenerator was found to be 0.93 when concentrating a solution of lithium chloride from 36% to 40% and supplied with hot fluid at 93.3•C. Figure 11 A 6,000 cfm low-flow liquid-desiccant conditioner 41 When using CFC or HCFC as refrigerants in liquid desiccant cooling systems, the generator temperature has to be above 150•C. If solar energy is used as main driving force for the system, the solar collector has to be of the evacuated tube type instead of a simple flat plate collector, this will definitely lead to higher costs.…”

“…The testing results demonstrated that at a heat exchanger angle of 45•, there was an optimum value of air mass flow rate at which the effectiveness and dehumidification efficiency of the plate heat exchanger were maximized. Figure 10 Schematic diagram of the cross-flow type plate heat within the absorber are the simultaneous steady-exchanger 63,64 Another low-flow liquid desiccant conditioner using a plastic-plate heat exchanger was presented by Lowenstein et al 41 . The cross section of the plate (as shown in Figure 11) was 2.5mm by 305mm, with 110 cooling passages running the length of the extrusion.…”