Greenhouse gas emissions and rising consumption of power resulting from extended use of the HVAC systems for producing indoor thermal comfort can be one of the most important challenges of the next decade, where energy consumption rates in HVAC systems reached 40-45% of energy consumption in commercial and residential buildings. Therefore, performance improvement of building cooling and heating systems is an urgent and necessary task to minimize greenhouse gas emissions and power consumption.
In recent decades, various technologies for heating and cooling systems in buildings have been invented and developed, for example, heat/enthalpy recovery, desiccant dehumidification, adsorption system, and absorption system to mitigate greenhouse gas emissions and to reduce energy consumption.
A desiccant air conditioner is considered a promising alternative to a traditional vapor-compression system, as well as takes advantage of solar energy and waste heat in the operation of the desiccant air conditioners. In contrast to a vapour-compression system, a desiccant air conditioner is therefore environmentally friendly. The desiccant integrated innovative air conditioning system uses sorbent to remove air moisture. This method differs from the traditional method utilized in traditional vapour-compression refrigeration systems, which works to condense moisture by cooling below dew point temperature, and then the air is reheated to the required temperature. In a desiccant air conditioner, sensible cooling was achieved through various methods, for example, cooling towers, evaporative coolers, and absorption chillers.
The use of desiccant air conditioners has proven to be highly effective in terms of economic, carbon emissions, and energy performance compared to the vapour compression systems. The use of solar-assisted liquid desiccant air-conditioners can reduce energy consumption rates by up to 35% and 47% for hotel buildings and office buildings, respectively, compared to traditional vapour-compression refrigeration systems.
The use of renewable solar energy powered rotary desiccant wheel air conditioners for building cooling in which annual power consumption rates were substantially reduced as compared to a vapour-compression refrigeration system. A comparison between the behaviour of liquid desiccant air conditioners and traditional vapour-compression refrigeration systems show favourable results in case of desiccant integrated cooling. The outcomes of the comparison showed that the use of liquid desiccant air conditioners reduces annual power consumption by 68% compared to the traditional vapour-compression refrigeration systems.
A novel desiccant air conditioner developed by incorporating it with solar collectors, which reduced electricity consumption rates by up to 70% compared to a vapour-compression system. An integration of the liquid desiccant air conditioning with a vapour-compression system; the coefficient of performance of the hybrid system was achieved ranging from 6 to 7. The desiccant air conditioner has also been combined with thermoelectric coolers, humidification-dehumidification desalination and refrigerant sub-cooling to improve configurations of desiccant air conditioners and thus enhance overall system performance.
Many investigators theoretically studied the influences of the mixing ratio of exhaust air and desired building air temperature on the behaviour of rotary desiccant air conditioners integrated with heat recovery units and dew point indirect evaporative coolers, and direct evaporative coolers. They concluded that the average monthly thermal COP can be substantially improved by use of desiccant based innovative cooling principle.
The effect of incorporating photovoltaic-thermal solar collectors with rotary desiccant air conditioners integrated with heat recovery units and dew-point indirect evaporative coolers, and direct evaporative coolers were also studied by many researchers. The influences of incorporating the solid rotary desiccant wheel with the liquid desiccant dehumidifier on the behaviour of novel air-conditioners were also investigated by many researchers. The influences of incorporating rotary desiccant dehumidifiers with the heat pump for building air conditioning systems were also found useful in different modern HVAC systems.
A comparative study among the liquid desiccant air conditioner and the solid desiccant air conditioner under different climatic conditions zones were conducted and it is examined that the influence of incorporating the solar energy-based solid desiccant with the Variable Refrigerant Flow (VRF) air-conditioning system on the performance of VRF air condition systems were found better.
The solar energy-based desiccant-assisted VRF air condition systems could be saving power at a rate reached 23.9% compared to conventional VRF air condition systems. A solid desiccant dehumidification system incorporated with the pre-cooling and re-circulating regenerative desiccant wheel was found suitable in hot and humidity regions as compared to traditional HVAC.
It is found that the utilization of vacuum-assisted regeneration reduces the regeneration temperature and enables the utilization of low-grade energy or renewable energy sources.
Desiccant materials
Desiccant material is a hygroscopic material that attracts moisture due to a difference in vapour pressure. It can be classified into liquid or solid, synthetic or natural, rock or bio, chemisorption or physisorption, etc. The term chemisorption or physisorption refers to the bond strength between the adsorbent and adsorbate. Moisture removed from the air is usually considered to be decomposition adsorption due to the reduced strength of the bond between the absorbent and the adsorbent. The bond strength is preferred to be low in desiccant air conditioners for a perfect reactivation process. Due to the ability of desiccant material to adsorb water vapour, it is widely utilized in many applications such as hospital, pharmaceutical, industrial applications, drying food, and storage.
Desiccant materials are one of the main factors in the development of a desiccant air conditioner. The behaviour of desiccant air conditioners is affected by the characteristics of the desiccant material being utilized, there are three main factors for selecting a suitable desiccant material:
- The ability of desiccant materials to adsorb water vapour.
- The desiccant material should be reactivated at low temperatures.
- Desiccant materials play a crucial role in developing desiccant air conditioning systems.
The characteristics of the selected desiccant material have an impact on the desiccant air conditioner’s performance, where the desiccant materials include activated alumina, activated carbon, silica gel, molecular sieve, calcium chloride, lithium chloride.
Working of desiccant integrated dehumidification and cooling
In a rotary desiccant wheel, mass and heat transfer takes place between moist air and sorbent material at a low rotation speed (8–10 revolutions per hour). The rotary wheel is made from a honeycomb with a thin layer of sorbents, and it is divided into two sections, one for reactivation and the other for a process.
The operation of the rotary wheel is summarized as presented in Fig. 1 as the outdoor air flows through a process section, the moisture is transported from air to sorbents that are distributed in the flow channels, this transfer is to the vapour pressure difference between the sorbents and air streams. During this process, due to the adsorption of latent heat, the sorbent’s temperature increases, as well as the heat will be transferred by convection to the air stream that will increase the outlet air stream temperature.
Reactivation process, when absorbent particles get saturated with water, they need to be reactivated. This is done by using a heat source (electric heater or solar/wasted heat) to heat the sorbents by passing the reactivation air through it; the temperature of reactivation air depends on the type of sorbent used. In the adsorption process, the latent heat is converted to sensible heat and does not produce useful cooling. So, to obtain a cooling effect, the rotating wheel is combined with the auxiliary coolant (evaporative cooler) to remove sensible heat.
As illustrated in Fig. 1, outdoor air at point 1 passes to the rotary wheel, as a result, it is dehumidified and its temperature increases due to adsorption heat impact. Through the process from state points 2 to 3, the hot dry air is sensibly cooled in the heat exchanger. Then, the process air flows to the evaporative cooler where it is evaporatively cooled, humidified, and supplied to the room at state 4.
On the reactivation air side, return air at point 5 passes to another evaporative cooler where it is cooled and humidified. Through the process from state points 6 to 7, the humidified cooled air will exchange the heat with process air in the heat exchanger, as a result, it is preheated and process air is pre-cooled. Then, warm air is heated from points 7 to 8 by the heat source and passes to a wheel to reactivate the sorbent material, as a result, it is humidified, and its temperature is decreased and exhausted at state point 9.
Hybrid solid desiccant powered air conditioners
Different configurations of hybrid desiccant air conditioners are presented to improve a system’s overall performance, for example, the integration of the desiccant air conditioner with a source of renewable energy such as water solar collectors, air solar collectors, and photovoltaic thermal collectors (Fig. 2) are utilized to heat the regeneration air to reactivate a desiccant material, as a result, the energy consumption decreases and system performance increases.
Also, the desiccant air conditioner can be coupled with a vapour-compression or absorption system when a high sensible load exists and in this case, the waste heat from the condenser can be utilized to heat a reactivation air required for the reactivation process. Moreover, the desiccant system can be coupled with a drying unit to preserve different products in supermarkets and stored cereals.
Conclusions
Recently, due to the many advantages of desiccant assisted air conditioners like limited use of ozone-depleting refrigerants, highly efficient moisture control etc., as compared to the traditional vapour-compression refrigeration based HVAC system, desiccant air conditioners are attracting increasing interest from HVAC designers and architects.
Still research and investigations may focus on improving the overall performance of desiccant air conditioners by innovating new composite desiccant materials, innovating new system configurations and improving modern HVAC system designs and controls.
Dr. (Prof.) D.B. Jani received Ph.D. in Thermal Science (Mechanical Engineering) from Indian Institute of Technology (IIT) Roorkee. Currently he is a recognized Ph.D. Supervisor at Gujarat Technological University (GTU). Published more than 240 Research Articles in reputed International Conferences and Journals. He has also published 10 reputed books and book chapters in area of thermal engineering. Working as Academic Editor for the Journal of Materials Science Research and Reviews. Presently, he is an Associate Professor at GEC, Dahod, Gujarat Technological University, GTU, Ahmedabad (Education Department, State of Gujarat, India). His area of research is Desiccant cooling, ANN, TRNSYS, and Exergy.
