1. Introduction
According to the recent literature [
1], liquid desiccant and evaporative cooling-assisted air conditioning systems are attracting more interest for realizing energy conservative air-conditioning options. The liquid desiccant system, which dehumidifies the process air, plays an important role in enhancing the performance of evaporative coolers. In a liquid desiccant system, the desiccant solution should be heated for regeneration and cooled for enhancing the moisture removal efficiency, which accounts for over 40% of the total energy consumption in a liquid desiccant and evaporative cooling-assisted air conditioning system [
2]. However, if combined with low-grade heat sources, the liquid desiccant-integrated system may provide high energy saving potential over conventional air conditioning systems [
3]. There is research available that [
4,
5,
6,
7,
8] addresses the potential of deploying waste heat recovery or solar energy as a heat source, for regenerating the desiccant solution, and the water-side free cooling approach can be applied for cooling the desiccant solution.
Katejanekarn and Kumar [
4] conducted a simulation on a solar-regenerated liquid desiccant system for dehumidifying the outdoor air. They concluded that solar radiation, ventilation rate, and desiccant solution concentration were the most influential factors determining the performance of their proposed system. Gommed and Grossman [
5] proposed a liquid desiccant system which is assisted by a solar thermal system and a cooling tower. Their experimentally validated simulation confirmed that their proposed system could provide a high degree of dehumidification. Jain et al. [
6] conducted an experimental study on the performance of a liquid desiccant system with a water-side free cooling and an electric heater for solution cooling and heating. They used lithium chloride and calcium chloride solutions and found that lithium chloride showed a better dehumidification performance. Alizadeh [
7] conducted a theoretical and experimental study on a liquid desiccant air conditioning system assisted by a solar thermal system composed of flat plate solar collectors and an evaporative cooling system. It was found that the suggested system was an effective way of air-conditioning in hot and humid climates. Buker and Riffat [
8] studied the adsorption of a polyethylene heat exchanger installed under photovoltaic panels for liquid desiccant regeneration. They concluded that their proposed system could be beneficial in relation to energy and contributive to a sustainable environment.
On the other hand, the heat pump-driven liquid desiccant system (HPLD) has also been studied [
9,
10,
11,
12] by focusing on its dehumidification performance, energy consumption, and matching heat pump capacity to the required solution cooling and heating loads. Yamaguchi et al. [
9] conducted HPLD performance tests and discussed methods for improving the system efficiency. They concluded that the coefficient of performance (COP) of the HPLD could be increased by enhancing the compressor isentropic efficiency and the solution heat exchanger efficiency. Bergero and Chiari [
10] evaluated the system performance of an HPLD with a hygroscopic solution and hydrophobic membrane using a SIMULINK program (The Mathworks, Inc., Natick, MA, USA) by varying a few significant operating parameters. The simulation results showed that compared to a conventional system, their system could achieve savings exceeding 50%, under certain operating conditions. Zhang et al. [
11] focused on methods for effectively releasing the leftover heat from the heat pump condenser after regenerating the desiccant solution. They tested two different types of assistant condensers for exhausting the extra heat: air-cooled and water-cooled assistant condensers. They indicated that systems with air-cooled and water-cooled assistant condensers showed 18% and 35% higher COP, respectively, compared to the reference HPLD system with no assistant condensers. Niu et al. [
12] investigated methods for matching the heat pump capacity to the desiccant solution heating and cooling loads. They showed that the solution flow rate in the condenser, the revolutions of the compressor, and the air flow rate in the air-cooled assistant condenser should be simultaneously controlled to match the load in the HPLD system.
Recently, a liquid desiccant and indirect and direct evaporative cooling-assisted 100% outdoor air system (LD-IDECOAS) has been suggested, and its energy saving potential has also been addressed in the available studies [
2,
13,
14]. In the existing LD-IDECOAS, the strong desiccant solution is cooled by the water-side free cooling of the cooling tower before the absorber, and the weak desiccant solution is heated by the conventional gas-fired boiler or by renewable heat sources. However, an HPLD system has not been considered as an option for the dehumidification section in the existing LD-IDECOAS. Consequently, in this study, the liquid desiccant (LD) part of the existing LD-IDECOAS is replaced by an HPLD (i.e., HPLD-IDECOAS). Subsequently, a quantitative analysis and energy performance comparison of both the established LD-IDECOAS and the proposed HPLD-IDECOAS are conducted via a detailed energy simulation. The HPLD-IDECOAS suggested in this research uses lithium chloride (LiCl) solution as the liquid desiccant, and R-134a is chosen as the refrigerant for the heat pump for cooling and heating of the desiccant solution. The simulations of both systems are conducted by modeling each system using a commercial engineering equation solver (EES) program.
5. Conclusions
In this research, it was found that the HPLD-IDECOAS was able to provide a 33% reduction in annual primary energy consumption, compared with the LD-IDECOAS previously recommended in the literature. The significant energy saving potential expected in the proposed system is mainly obtained through the reduction in the gas consumption (i.e., 83%) required for regeneration of the weak desiccant solution, which is accomplished by means of the heat pump in the liquid desiccant section of the system. In fact, the LD-IDECOAS was intended to realize a non-vapor compression based air handling system. However, for successful deployment of the system in the relevant markets, the energy consumption for regeneration of the desiccant solution should be reduced as much as possible. As shown in this research, a heat pump applied in the liquid desiccant system is an adequate solution for the relatively high regeneration energy consumption observed in the LD-IDECOAS. By reclaiming sensible heat from the desiccant solution at the absorber side, and providing the recovered heat to the regenerator side using the heat pump, the HPLD-IDECOAS could enhance energy saving potential, and would be a successful renovation of the LD-IDECOAS.