Heating, ventilating and air conditioning (HVAC) systems are designed to maintain specific indoor conditions, which vary depending on the application. The main factors that influence the thermal comfort of occupants are metabolic rate, clothing insulation, air temperature, mean radiant temperature, air velocity and relative humidity [1
]. The main purpose of the HVAC system is to provide good indoor air quality that meets the criteria for hygienic air conditions and satisfies the thermal comfort of occupants or products in a building or space. Moreover, the working environment can influence the productivity of workers, which is an economic reason for installing HVAC systems in buildings [2
Generally, HVAC systems require a large amount of energy, especially in large capacity applications. HVAC systems make a significant contribution to carbon-based energy consumption and greenhouse gases emissions [4
]. In the USA, nearly 50% of the energy consumption of buildings is used for HVAC systems [5
]. Furthermore, conventional HVAC systems cause pollution, not only due to consuming a large amount of energy, but also due to the use of hydro-chlorofluorocarbon (HCFC), hydrofluorocarbon (HFC) and other refrigerants that produce greenhouse gases [6
Cooling-based dehumidification systems are the most popular systems of recent decades [7
]. Such systems provide cooling and dehumidification by utilizing a single vapor-compression unit. Cooling coils are used to cool the air below its dew point so moisture can be removed from the air. Therefore, low humidity and low temperature air can be generated and a reheat coil is required to avoid overcooling, which consumes a large amount of energy and is difficult to control. Current studies are developing new approaches that are more energy efficient and more environmentally friendly.
Recently, novel modern air conditioning systems have been proposed, most notably utilizing split processes of cooling and dehumidification. Instead of the cooling coil unit used in conventional cooling dehumidification systems, a single unit that can handle latent heat is applied, so unnecessary energy use can be avoided. One developed method is a solid desiccant cooling system (SDCS) in which refrigerants of HCFCs are unnecessary and low-grade thermal energy can be used for regeneration [9
]. In an SDCS system, solid desiccants, such as silica gel, activated carbon, molecular sieves, alumina gel and other materials with strong hygroscopic ability, are used to dehumidify the process air. The solid desiccant itself has many pores, and the inner surface of each pore is concave. When the process air passes through the solid desiccant, because of the lower partial pressure of water vapor on the concave surface with a small radius of curvature, the vapor may migrate from the air to the concave surface of the pores. Then, the vapor condenses on the concave surface and releases adsorption heat to the desiccant. The most widely used solid desiccant dehumidification equipment is the rotary dehumidification wheel, in which solid desiccant is coated on the surface of wheel. The rotary dehumidification wheel can realize continuous dehumidification and regeneration through periodical rotation. The high-humidity process air passes through a portion of the desiccant wheel, and the vapor in the process air is absorbed by the solid desiccant of the wheel due to the vapor pressure difference between the air and the desiccant. Then, the temperature of the process air rises due to adsorption heat, and humidity is reduced after passing through the wheel. When the desiccant in the process air stream absorbs enough vapor from the process air, the vapor adsorption portion of the desiccant wheel is rotated to the high temperature regeneration air stream. While the regeneration air passes through the portion of the desiccant wheel containing a greater amount of vapor in the desiccant, the heat from the high temperature air stream leads to higher vapor pressure in the desiccant in comparison to the vapor pressure in the air stream. Then, vapor in the desiccant is ejected into the regenerated air stream until the lower vapor pressure condition in the desiccant is attained. The desiccant wheel periodically and dynamically rotates between the process and regeneration air streams. Dehumidification and regeneration processes are conducted in the corresponding air streams, and a low-humidity process air stream can be continuously obtained using the rotary wheel. Narayanan et al. [11
] analyzed the performance characteristics of a solid-desiccant evaporative cooling system with TRNSYS software. The results show that the system could provide thermal comfort, however, the capability of the system to provide suitable air temperature and humidity depends on the performance of the evaporative cooling, energy-recovery, and heat-generation systems. Therefore, the availability of a cheap or waste-heat source is essential in making this system economically viable. Narayanan et al. [12
] numerically investigated the dehumidification potential of a solid desiccant-based evaporative cooling system with an enthalpy exchanger operating in subtropical and tropical climates with TRNSYS software. The results show that in hot and humid conditions, the system thermal comfort capability drops to around 54% to 63%.
Solar energy utilization is a new approach developed in recent years specifically for space heating applications. The solar energy source is limitless and safer for the environment. Recently, heat from solar energy was developed for desiccant cooling systems. The heat generated by a solar thermal collector can be used for the regeneration process of the dehumidification wheel. Guidara et al. [13
] proposed the use of evaporative coolers for pre-cooling and re-cooling of the process air before and after the dehumidification wheel in order to satisfy the load demand of an air-conditioned space. From their numerical analysis, they indicated that the pre-cooling design is suitable to be applied in drier ambient conditions. Enteria et al. [14
] investigated the performance of a solar-desiccant cooling system with a silica gel and titanium dioxide desiccant wheel in East Asia. From numerical simulations, it was found that using solar desiccant cooling systems has great potential for East Asian countries. The study pointed out that, in the tropical region, a larger area and capacity of the solar thermal collector and greater regeneration air flow rate are required, and the operational performance of the system is in the range of 1.5 to 3. The major operational energy loss of the proposed system comes from solar collectors, water pipes, electric heaters and thermal storage tanks. Speerforck et al. [15
] numerically investigated the performance of a solar-desiccant cooling system incorporating borehole heat exchangers for direct cooling and solar energy for desiccant regeneration. They indicated that the proposed system allows electricity saving of 50% and reduces CO2
equivalent emissions by 91%. White et al. [16
] proposed a solar-assisted SDCS incorporating direct and indirect evaporative coolers in a serial arrangement to cool process air after the dehumidification wheel. From numerical analysis, it was indicated that the sensible heat removal capability of the process air can be enhanced by a two-stage evaporative cooling design, which is suitable for cities with low-humidity climatic conditions.
Environmental conditions in different regions may affect the performance of a desiccant cooling system (DCS). In hot and humid regions, the use of a hybrid SDCS that is integrated with a heat pump is suggested, instead of a conventional SDCS that utilizes an evaporative cooler and can generate a more humid air supply. Jani et al. [17
] proposed a hybrid cooling system integrated with a heat pump (hybrid SDCS), and indicated the proposed system has good performance in hot and humid climate conditions. The dehumidification performance of the system is also highly sensitive to the outdoor ambient conditions. The performance of the hybrid SDCS is better for cases where ambient humidity is high [18
]. It was found that the use of a hybrid SDCS can decrease the total power consumption by 20–30% and increase the cooling capacity by 40–60% [19
]. Jani et al. [20
] used the numerical software TRNSYS to simulate the performance of a solid desiccant-assisted hybrid space cooling system. The results show that the system achieves a good performance in hot humid climates. The humidity ratio of a room’s process air was substantially lowered, from 0.014 kg of water/kg of dry air to 0.006 kg of water/kg of dry air, by use of a solid desiccant-based rotary dehumidifier.
In addition to environmental conditions, regeneration temperature is one of the main parameters used to determine the performance of a SDCS. The performance of an SDCS is sensitive to changes in humidity and regeneration temperature [21
]. However, if the regeneration temperature in the hybrid SDCS is too high, the system performance may be reduced due to high condensation pressure and an increased amount of work performed by the compressor. Several heat sources can be used for the regeneration process, such as an electric heater, gas, a low-grade thermal energy such as solar energy, and waste heat. A further cogeneration system can also be considered as the heat source for the regeneration process.
] developed a new solar-assisted hybrid SDCS in which the condensing heat from the condenser of the incorporated heat pump is recovered to preheat the regeneration air stream before solar heating. Long-term measurements of the developed system were conducted in southern Italy. Five control modes based on the temperature and humidity of the outdoor air conditions were designed. From their performance analysis, it was pointed out that preheating of the recovered heat for the regeneration process can reduce the required heat from the solar collector by approximately 30%. In other words, the required area of the solar collector can also be moderately reduced. Fong et al. [25
] developed a solar-assisted SDCS incorporating an adsorption chiller and analyzed the performance of the system by numerical analysis. The outdoor air was dehumidified by a rotary dehumidification wheel and passed through a radiant cooling coil in an air-conditioned space. The chilled water from the adsorption chiller was provided to the radiant cooling coil for handling the sensible heat of the air-conditioned space. Both required regeneration heating for the rotary dehumidification wheel and the adsorption chiller supplied by the solar collector. Compared with the traditional centralized air conditioning system, the energy saving potential of the integrated system could reach 36.5%. As alternatives to silica gel, Bareschino et al. [27
] proposed other hygroscopic materials for desiccant wheels, [email protected]
(MILGO) and Campanian ignimbrite, in conjunction with an air-conditioning system driven by evacuated tube solar collectors equipped with a desiccant wheel. The numerical simulations were carried out by means of TRNSYS 17®
(version 17, Thermal Energy System Specialists, Madison, WI, USA) to dynamically assess the energy flows in the considered plants and compared with that of a conventional system. The results demonstrate that primary energy savings of approximately 20%, 29%, and 15% can be reached with silica-gel, MILGO and zeolite-rich tuff desiccant wheel-based air handling units, respectively. Li et al. [28
] investigated a two-stage rotary desiccant cooling/heating system driven by evacuated glass tube solar air collectors. The results show that the major advantage of the two-stage desiccant cooling system was that moisture removal reached 6.68–14.43 g/kg in hot and humid climate conditions. Solar heating with desiccant humidification can improve indoor comfort significantly. A solar hybrid SDCS is a good substitute for traditional vapor compression air conditioning systems, especially in hot and humid climates, since solar energy can result in energy savings in the range of 40–45% [29
]. Rambhad [30
] indicated regeneration temperatures of hot water in the range of 54.3 °C to 68.3 °C can be achieved in solar SDCS systems by simulation.
In previous studies, the solar-assisted hybrid SDCS system was considered a replacement of the refrigerant vapor compression air conditioning system due to its higher energy efficiency. However, most studies of the solar-assisted hybrid SDCS have been conducted using numerical analysis. Research into the system’s long-term practical operations and analysis of its performance under the effects of different ambient temperatures and humidity ratios is less common, especially in hot and humid environments. The performance of the solar-assisted hybrid SDCS has not been investigated experimentally in detail. In this study, the effect of ambient humidity and temperature on the performance of a solar-assisted hybrid SDCS was investigated under the high humidity and high temperature ambient conditions of Taichung, Taiwan. The performance analysis and comparisons of solar-assisted hybrid SDCS, hybrid SDCS and solar SDCS systems were conducted through long-term experiments in order to understand their characteristics and operational ranges in terms of environmental conditions.