1. Introduction
Hybrid air conditioning systems are increasingly becoming popular in different parts of the world [
1,
2,
3]. They are being employed in various capacities and applications, and usually offer a higher performance in comparison to utilizing one component individually. In a hybrid air conditioning system, there are a variety of selections for the components, from direct evaporative coolers (DEC) and indirect evaporative coolers (IEC) to desiccant dehumidifiers, rotary heat exchangers (RHE), direct expansion (DX) cooling coils, and so on [
4,
5,
6,
7,
8,
9,
10,
11].
In
Table 1, the recent investigations that have been performed in the field of hybrid air conditioning systems are listed. This table shows that either the conventional types of IEC, including common types of wet-bulb and dew-point coolers, or DEC have been employed in case the evaporative cooling was going to be utilized. Those are the studies listed with “Yes” regarding the question “Were any types of evaporative cooler considered?”. The researchers indicated the potential of both direct and indirect evaporative cooling systems to become a cheaper and environmentally sustainable source of cooling energy. Although using evaporative cooling leads to a lower initial purchase price compared to the other alternatives, and has better efficiency, it is accompanied by the consumption of a relatively huge amount of water [
12]. Considering the water scarcity, supplying water for the evaporative cooling system is taken into account as one of the serious challenges of using it, especially in arid climatic conditions [
13].
Another point that should be mentioned is, as
Table 1 reveals, the price of a number of components in the proposed hybrid air conditioning systems are high in most cases. It not only imposes an additional initial cost but could also lead to a reduction in the system reliability. This is due to the fact that the system reliability depends on the reliability of the individual elements and their number.
As it was provided by answering the question “Was a GAHP-based IEC examined as one part to enhance the system performance?”, to the best of the authors’ knowledge, a gravity-assisted heat-pipe-based indirect evaporative cooler (GAHP-based IEC) has not yet been employed in hybrid air conditioning systems. In this context, this can be considered as the main novelty of our study. This highlights the need for conducting further studies to develop more reliable and efficient hybrid air conditioning systems.
By answering the question “Were the analysis carried out from technical and energy efficiency points of view?”, the research gap that needs to be filled is also indicated. The proposed hybrid system is analyzed in terms of potential cooling capacity as a technical aspect in the conducted comparison. Moreover, the analysis is carried out on the basis of the electricity consumption and COP profiles, which describe the energy and efficiency aspects, respectively. In view of the above, this work intends to discuss the potential of implementation of a GAHP-based IEC.
In spite of the fact that a heat-pipe-based wet-bulb cooler has not been studied in hybrid air conditioning systems, a few studies have been performed on it as a stand-alone system. Riffat and Zhu disclosed their work on the use of heat pipes for indirect evaporative cooling [
29]. By conducting an extensive literature review, the authors claimed that no previous study of the principle of indirect evaporative cooling based on the application of heat pipes and a ceramic container had been revealed. After that, they evaluated the performance of the proposed heat-pipe-based heat and mass exchanger for building cooling. The characteristics of the indirect evaporative cooler were analyzed. A mathematical model to simulate its performance was developed, and system properties, such as cooling capacity and temperature drop under steady-state operating conditions, were also studied. Moreover, in order to increase the convective heat transfer in the evaporator section of the heat pipe, fins were used, while a porous ceramic water container was adopted around the condenser section. It provided uniform water distribution and optimal contact of the working air with water film.
A more recent study in the field of employing heat pipes in indirect evaporative cooling systems was conducted by Boukhanouf et al. [
30]. The main novelty was the combination of finned heat pipes and porous ceramic tube modules in the wet channel of the heat and mass exchanger. The results obtained by numerical simulations were consistent with the empirical data collected under controlled laboratory test conditions. Alharbi et al. [
31] introduced a prototype made of finned heat pipes and water-filled hollow porous ceramic cuboids. The construction, numerical simulation, and experimental measurements of an indirect evaporative cooler were presented. The wet-bulb effectiveness of up to 1.05 was achieved due to the high thermal conductivity of heat pipes and the good wettability properties of porous ceramic material. Rajski et al. [
32] assessed the cooling performance indices of the novel GAHP-based IEC. The most favorable ranges of operating conditions were determined by means of numerical simulations.
These studies lead to the conclusion that the implementation of heat pipes as an alternative to the conventional plate-type evaporative coolers offers low maintenance cost, longer life of the main components, and reduces the use of plastics.
Putting all the mentioned issues together, here, the potential of utilizing GAHP to enhance the performance of a stand-alone DX cooling coil, which is employed to meet the thermal comfort for an office building, is examined. The proposed hybrid air conditioning system is investigated from different aspects, including technical, energy, and efficiency perspectives. Moreover, the results are compared with the integration of a RHE for heat recovery with a DX cooling coil, as a routine way to improve the performance of the DX cooling coil. The investigation was performed for the climatic condition of Poland, and knowing that DX cooling coils are one of the most common methods for building cooling in Europe, the result will help to achieve a reliable and practical method to implement a hybrid air conditioning system which has a better condition from technical, energy efficiency, economic, and techno-economic perspectives. It can contribute to diminishing the peak electricity demand as well.
2. Working Principle of a GAHP-Based IEC
A heat pipe where the working fluid cycle happens inside it due to gravity is known as a gravity-assisted heat pipe [
33,
34,
35]. This special kind of heat pipe is also called a two-phase closed thermosiphon or a wickless heat pipe. The schematic of a GAHP is depicted in
Figure 1a. Based on the mentioned point, it can be concluded that the condenser of a GAHP always has to be placed above the evaporator to flow the fluid from the condenser to the evaporator based on the gravity. Generally, a GAHP consists of three characteristic regions, including the evaporator, adiabatic section, and condenser. The working principle is based on the phase change of the working fluid. After supplying heat flux through the evaporator wall to the working fluid, the working fluid starts to boil and to evaporate. As a result, the working fluid vapor moves to the condenser, where it condenses and releases the absorbed heat. Finally, the condensate returns to the pool inside the evaporator section to complete the GAHP thermal cycle.
The regenerative-counter flow arrangement in the proposed IEC is established as shown in
Figure 1b. In order to achieve better cooling performance, part of the outlet product airflow is branched to the wet channel and flows in countercurrent arrangement to the dry channel. The working fluid (moist air) absorbs sensible heat from the product airflow. The collected heat is then dissipated due to the water evaporation on the condenser’s outer surface. As a result, the supply airflow is cooled without changing the moisture content.
The wet section should be made of specially designed materials with high water retention and wettability (e.g., porous ceramic, porous plastic), which provides an even water distribution around the condensation part of the GAHP. Deionized water is used as the working fluid inside of a GAHP. Continuous plain fins for enhancement heat transfer are implemented only in the dry section of an IEC. The finned condensation section would result in difficulties with the water distribution on the outer surface of a GAHP.
5. Conclusions
In the present work, a novel hybrid air conditioning system in which gravity-assisted heat-pipe-based indirect evaporative cooling was used in combination with a direct expansion cooling coil was proposed and investigated. The possibility of enhancing energy efficiency in the system to bring the thermal comfort zone in an air-conditioned office space in Poland was discussed and compared with the rotary heat exchanger and DX cooling coil combination as the conventional hybrid system. A mathematical model to describe the novel air cooler was also developed. The results showed that, on the cooling design-day, the proposed system is able to perform better in terms of reducing electricity consumption and increasing the coefficient of performance compared to the conventional one.
The following findings according to the design-day of August 24 can be listed from this investigation. It was found that for the considered case study, the peak values of cooling capacity for System 1 and System 2 are 6.0 kW and 5.6 kW, respectively. For the design-day, System 1 delivers 53.5 kWh of cooling energy to the air-conditioned space, 28.9% of which was covered by the heat recovery system. On the other hand, System 2 supplies 49.0 kWh of cooling energy to the air-conditioned space, 65.7% of which was covered by the indirect evaporative cooler.
In addition, a significant reduction of 55.1% in the use of the DX cooling coil can be observed in terms of System 2. Hence, 55.0% of original electricity consumption was required by the proposed System 2. The proposed hybrid HVAC system improved the COP by 39.2% compared to System 1. Moreover, total operating cost was reduced by 51.7%.