1. Introduction
Solid desiccant based evaporative air conditioning system (SDEAC) combines the dehumidifying capability of a solid desiccant system with the cooling capability of an evaporative cooling system to provide thermal comfort to the occupants of a conditioned space. The system’s main appeal lies in the fact that, if properly designed, it consumes less electrical energy than the conventional refrigerative air conditioning systems. A recent optimisation study has indicated that an electrical coefficient of performance higher than 20 [
1] could be attained. Low electrical energy consumption is made possible using thermal energy sources such as solar energy or waste heat as the regeneration heat source.
Recently, there have been several research works on solid desiccant evaporative air conditioning which investigate various related technical and economic performances of the systems and components [
1,
2,
3,
4,
5,
6,
7,
8,
9,
10,
11,
12,
13,
14]. Earlier, Henning [
15] presented a review on various possible system configurations suitable for certain climatic conditions whilst Camargo and Ebinuma [
16] presented thermodynamic equations of the system with focus on the humid climate applications.
To be able to compete technically and economically with the conventional refrigerative systems, SDEAC must be properly designed to achieve the following: (1) to minimise the parasitic energy consumption, (2) to minimise the use of regeneration heat, (3) to be on par with the conventional refrigerative systems in delivering the thermal comfort. To achieve the above goals, several system configurations have been proposed that are expected to suit certain climatic conditions [
15]. Other configurations can be found in [
5,
7,
17]. The standard desiccant cooling cycle mentioned in [
15] was recently studied to investigate the system performance for the Brisbane climate zone in Australia [
18] with mixed results; i.e., a slightly better electrical COP compared with the refrigerative system but with thermal comfort conditions of the conditioned spaces not fully satisfied.
While relatively comprehensive analyses have been carried out to study the performance of the SDEAC [
1,
6,
7,
8], these previous studies have largely overlooked the impact of such a system in terms of delivering the thermal comfort. For instance, the impact of outdoor humidity was mentioned in [
8], but the air temperature and humidity level in the conditioned space was not discussed. The same observation is true for a very recent study on two-stage desiccant system [
19,
20].
The success of SDEAC largely depends of its dehumidification potential. This paper evaluates the dehumidification capability of a SDEAC equipped with an enthalpy exchanger. In the previous study [
18], the performance of the single stage SDEAC reported in the earlier studies [
5,
6,
7,
8] was evaluated. The findings [
15] echo the conclusions presented in [
5]. However, no research work has been carried out on the performance analysis of SDEAC system for residential applications in tropical and subtropical climates where latent cooling load is an important factor. To develop this system as an alternative to the energy intensive vapor compression system, it is important to assess the dehumidification potential which will help the design of systems for these regions. Therefore, the current study focusses on dehumidification and cooling characteristics of SDEAC system for a residential building in the climate conditions in the Australian cities of Brisbane (27.4698° S, 153.0251° E), Townsville (19.2590° S, 146.8169° E) and Darwin (12.4634° S, 130.8456° E), which represent three distinct subtropical and tropical humid regions, respectively.
The project investigates the effects of adding an enthalpy exchanger in the desiccant evaporative cooling system cycle for subtropical and tropical climates. This study targets an objective method of assessing the thermal performance of the SDEAC system with enthalpy exchanger, i.e., direct evaluation of its capability to the required air temperature and humidity levels in the conditioned space. The direct evaluation of the system capability to deliver acceptable thermal comfort condition is always essential to develop it as a potential alternative to energy intensive refrigerated air-conditioning systems. Therefore, the paper is HVAC application oriented and includes the numerical models of SDEAC system using TRNSYS, which is a well-established software in the HVAC industry with all its components validated with experimental results.
2. An Overview of SDEAC System Configurations
A standard SDEAC system consists of a desiccant wheel, an energy recovery wheel, evaporative coolers (at supply and exhaust streams) and a regeneration (reactivation) source [
15], as shown in
Figure 1. The desiccant wheel dehumidifies the air but increases its temperature significantly (process 1→2). The first cooling of this supply air takes place in the energy recovery wheel (2→3), a constant humidity process. Before being supplied to the conditioned space, the air is further cooled in a direct (or indirect) evaporative cooler (3→4) where the air humidity may be slightly raised but kept well below the comfort requirement. As the supply air enters the conditioned space, its temperature and humidity increase due to the rooms sensible and latent heat gains (4→5).
The air leaving the air-conditioned space is directed to the exhaust where it undergoes several psychrometric processes starting with cooling in an evaporative cooler (5→6). This brings the air close to the saturation line with or without humidification, depending on the evaporative cooling process (direct or indirect). In the energy recovery wheel, the air is preheated (6→7) before it passes through the heater (7→8) where its temperature is further raised so that it will be able to desorb the water from the desiccant in the regeneration side of the desiccant wheel (8→9). The regeneration heat source (E) can be either a solar thermal system or waste heat. A typical wheel has a 50:50 split between dehumidification and regeneration area.
According to Henning [
15], this configuration is suitable for the temperate climate only. For a more humid climate, the dehumidification capacity of such a system is not adequate to enable the evaporative cooling system to provide the cooling at an acceptable room humidity level. This observation is supported by a recent numerical study for the subtropical climate of Brisbane [
18] where for a significantly high number of hours the resulting humidity levels fall beyond the ASHRAE upper thermal comfort boundary. Thus, although according to [
6,
7,
8] such a system has a relatively high electrical and thermal coefficient of performance in Darwin and Brisbane, the system may not be able to provide the comfortable humidity levels in the conditioned spaces. In the previous studies [
6,
7,
8] the room humidity level was not discussed.
Typical air temperature and humidity levels at the inlet and outlet of each component of this configuration extracted from [
15] are given in
Table 1.
An alternative to the above configuration is shown in
Figure 2 where an enthalpy exchanger (EE) is introduced to pre-cool and pre-dehumidify the ambient air using the return air from the building [
15].
The enthalpy exchanger referred to in this paper is an air to air heat recovery device which allows heat and mass transfer between two air streams passing through the device as shown in
Figure 3. The mass transfer is made possible through a permeable membrane separating the two streams [
21]. Two of the important parameters of the enthalpy exchanger are the latent and sensible effectiveness.
Typical air temperature and humidity ratio of this configuration extracted from [
15] is given in
Table 2, where the figures on the left of the component’s name represent the inlet values and those on the right represent the outlet values.
3. Overview of Climates of Brisbane, Townsville and Darwin
Brisbane (and Queensland in general) has a climate with “relatively high temperatures and evenly distributed precipitation throughout the year” (i.e., subtype Cfa—humid subtropical according to Köppen Climate classification) [
22]. Townsville is in the north eastern coast of Queensland; it has a tropical climate with a hot and humid summer but with relatively low rainfall compared with other tropical cities [
23]. Darwin has a tropical wet and dry or savannah climate (Aw in the Köppen–Geiger climate classification) [
24,
25].
Figure 4 shows the plot of the humidity ratio against the dry bulb temperature of air in Brisbane, Townsville and Darwin during the cooling months. As shown, the number of points (hours) where humidity ratios are below ASHRAE’s [
26] upper recommended value of 12 g/kg within 20–26 °C range in the three cities varies. Brisbane has the highest number of hours, followed by Townsville whilst numbers for Darwin are very low. The significance of this observation lies in the fact that these numbers reflect the duration during which the natural ventilation may be sufficient to provide the thermal comfort of the occupants. The higher this number the shorter the time when an air-conditioned system is required to provide thermal comfort.
Another relevant quantity that emerges from
Figure 4 is the number of points (hours) within the 26–36 °C range in which the humidity ratio is below 8 g/kg, appearing in the first configuration discussed in [
15], presented in
Table 1 and implemented in the control strategy discussed in [
8]. That is, only very few points (hours) in Brisbane, fewer in Townsville and none in Darwin of the outside air humidity ratio below 8 g/kg mentioned in [
8] that enable the control strategy to operate the system that would have saved the energy and increased the COP.
The above observation clearly indicates that for the three cities, the challenge to the SDEAC system to satisfy both the temperature and humidity levels for occupant comfort is obvious, with Darwin climate posing the biggest challenge, in particular during the build-up period [
24].
4. Methodology
To evaluate the performance of the above configurations, TRNSYS [
27] models of the cooling systems installed in houses located in the cities of Brisbane, Townsville, Darwin were developed based on the models of the following components detailed in [
21]: enthalpy exchanger, desiccant wheel, energy recovery wheel, evaporative cooler and regenerator. In addition to these components, a supply fan, air mixers to model the mixing of return air and the fresh air that are conditioned before being supplied to the room form part of the system modelling. In the model, the system is installed in a three bed-room house whose specifications and thermal zoning was detailed and reported earlier in [
18].
Figure 5 shows the interconnections between the components in the TRNSYS model.
The three bedrooms were conditioned from 23:00 to 07:00 and the combined living and kitchen room was conditioned from 07:00 to 23:00. The indoor temperature of both bed and living rooms was set to 26 °C with temperature dead band of ±0.5 K. While the SDEAC system has evaporative cooler as one of its components, the system is treated equal to the normal refrigerative system in the model in terms of its thermal comfort delivery capability. That is, it is expected to cool and dehumidify the air and not rely on the high velocity air to deliver the cooling effect. As such, the desired room humidity level is set according to the ASHRAE/Fanger comfort chart [
24] adapted for Australian homes in which the acceptable indoor design temperature range of 22.5–26 °C with dew point at or below 13 °C [
26]. A simple ON/OFF control strategy with ±0.5 K deadband was applied to control the room temperature.
Since the outside air conditions in the three cities are different, it is expected that the system will respond accordingly; i.e., it is expected that the system operating in the least challenging ambient conditions—in this case Brisbane—will perform better in terms of attainment of the desired temperature and humidity ratio of the conditioned space.
Once the minimum specifications that enable the delivery of the desired temperature and humidity ratio in the least challenging environment have been established, the specification for the system operating in warmer and more humid cities—in this case Townsville and Darwin—are modified until they satisfy the thermal conditions of the conditioned spaces. The initial inputs to the TRNSYS models which satisfy the “baseline” Brisbane conditions are listed in
Table 3.
6. Conclusions and Recommendations
Based on the results of the SDAEC system model and subsequent discussion the following conclusions can be drawn.
The SDEAC system with enthalpy exchanger performs better than that without enthalpy exchanger in terms of dehumidification, and the impact depends on the climate where the system operates. Specifically, the following findings can be reported:
For Brisbane with less humid climate, the system can bring thermal comfort for around 93% to 94% of the operating hours.
For the more humid city of Townsville, the system satisfies occupants’ thermal comfort for 74% to 79% of the operating hours.
For the hot and humid city of Darwin, the system thermal comfort capability drops to around 54% to 63% of the operating hours.
For all the three cities, the impacts of latent and sensible effectiveness of the enthalpy exchanger are marginal.
Cooling and humidification of the direct evaporative cooler results in the existence of the optimum mass flow rate of the air supply. In a very humid location like Darwin, increasing flow rate further from the optimum results in increase in the humidity ratio of conditioned space.
The electrical and thermal COP of the system depends on the system conditioning load, i.e., the higher the conditioning load, the lower the COP. The values of electrical COP for the systems operating in Brisbane, Townsville and Darwin were 4.8, 3.8 and 2.9, respectively. Likewise, the values of the thermal COP were 0.36, 0.28 and 0.21, respectively.
Like previous investigations, it can be concluded that the main drawbacks of a SDEAC system are: (1) the existence of main components with conflicting thermal capabilities, i.e., desiccant wheel and evaporative cooler, and (2) the requirement of significant regeneration heat, and consequently (3) the extra components that are required to deliver the cooling and dehumidification.
This study has presented a straightforward and objective method of assessing the thermal performance of the SDEAC system, i.e., direct evaluation of its capability to deliver thermal comfort, namely, acceptable air temperature and humidity levels in the conditioned space. While performance evaluation of each of the SDEAC components is important, especially for optimisation of component thermal performance, direct evaluation of the system capability to deliver acceptable thermal comfort condition is always necessary, as this is the ultimate goal of any air conditioning system.