Energy conservation benefit of water-side free cooling in a liquid desiccant and evaporative cooling-assisted 100% outdoor air system
Introduction
A liquid desiccant and indirect/direct evaporative cooling-assisted 100% outdoor air system (LD-IDECOAS) has been proposed as an energy-conserving air conditioning system [1], [2]. According to the literature [3], a liquid desiccant and evaporative cooling system has significant energy-saving potential among the non-vapor-compression heating, ventilation, and air conditioning (HVAC) systems.
The major components of the LD-IDECOAS are the liquid desiccant (LD) unit, the indirect evaporative cooler (IEC), and the direct evaporative cooler (DEC). In a hot and humid climate, outdoor air (OA) is dehumidified by the LD unit, and then the IEC and DEC cool the supply air (SA) to achieve the target condition [4], [5], [6]. The dehumidification performance of the LD unit plays a significant role in enhancing the cooling potential of the evaporative coolers and in the system's energy-saving potential.
Absorption of moisture from the process air in the LD unit is an exothermic process, so the desiccant solution in the absorber should be cooled to maintain dehumidification. The desiccant solution temperature at the absorber inlet is an important control parameter affecting the dehumidification rate [7]. The operating temperature range of the desiccant solution in the absorber of a conventional LD system is 20–30 °C, suitable for use with the water-side free cooling approach.
Water-side free cooling commonly uses a cooling tower to produce cooling water delivered directly to the load [8], [9]. The OA wet-bulb temperature (WBT) critically affects the cooling tower performance [10], [11], [12].
Theoretical and experimental studies have been conducted to assess water-side free cooling with an LD system. Katejanekarn and Kumar [13] conducted simulation research on OA pre-conditioning with a solar-regenerated LD system integrated with water-side free cooling. They assumed that the cooling tower effectiveness was 33%, and cooling water that passed through the LD system was re-used to cool the SA. Katejanekarn et al. [14] also conducted experimental research in which a 10-t cooling tower cooled the water to a temperature near the OA WBT. They oversized the cooling tower to sufficiently cool the SA. Gommed and Grossman [15] simulated an LD system with water-side free-cooling using the ABSIM [16] program and found that the water-side economizer maintained the desiccant solution around 29.5 °C. They conducted additional experimental research, and their results indicated that the desiccant solution in the absorber can reach 22–27 °C with water-side free cooling [17]. Jain et al. [18] conducted experimental research on an LD dehumidification system with a calcium chloride and lithium chloride solution. Their results showed that the desiccant solution temperature in the absorber can be maintained at 26.0–31.7 °C using water-side free cooling. Alizadeh [19] conducted small-scale experimental research on a solar-assisted LD air conditioner and found that the desiccant solution, which was heated to 70 °C for regeneration, could be effectively cooled to 23 °C by water-side free cooling. Several studies have been conducted on the use of a cooling tower in an LD system; however, there is insufficient research to determine the energy-saving potential of the proposed system integrated with a cooling tower over a conventional air-cooled chiller.
The main purpose of this research is to experimentally investigate the effect of water-side free cooling in LD-IDECOAS operation during the cooling season with hot and humid OA conditions. The energy-saving potential of water-side free cooling in LD-IDECOAS is also evaluated by comparing the proposed system's operating energy consumption with that of a conventional air-cooled chiller, and the initial cost breakdown of the LD-IDECOAS pilot system is provided.
Section snippets
System configuration
The LD-IDECOAS pilot system serves a 6 × .2 × 2.4 m3 office. The office is in an interior zone and does not have a window. Physical information about the conditioned space is presented in Table 1.
As shown in Fig. 1, the main components of the proposed system are an LD unit, an IEC, and a DEC on the upstream and a heating coil and a sensible heat exchanger are located downstream. Depending on the thermal load variation, the SA flow rate is modulated to achieve the indoor condition setpoint. Only OA
Results and discussion
The pilot system of the LD-IDECOAS was operated in the cooling season when the OA DBT varied within the 20–32 °C range and the OA WBT was 20–25 °C (Table 4), covering a wide range of hot and humid conditions.
The SA flow was modulated to produce a target indoor air temperature of 24 °C. To simulate internal heat gain, a 0.9 kW electric heater was operated inside the space. The LiCl solution was delivered at flow rate of 0.95 kg/s to the absorber, and the desiccant solution was in direct contact with
Conclusions
The feasibility and energy-saving potential of water-side free cooling applied to the liquid desiccant and evaporative cooling-assisted 100% outdoor air system during the hot and humid season was validated experimentally in this research. The operating data indicate that water-side free cooling can produce 22–27 °C cooling water for the LD unit under hot and humid OA conditions, although this relatively high cooling water temperature can limit the dehumidification performance of the LD unit.
Acknowledgement
This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (No. 2015R1A2A1A05001726).
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