Air conditioning systems are often used to provide comfortable indoor air conditions in general. Removing latent and sensible loads from outside air is usually required during summer to provide the desired indoor air conditions. Especially moisture removal accounts for peak loads of conventional air conditioning systems since it requires cooling process air below dew point temperature.
Cooling and dehumidification are coupled necessarily due to the process itself. Required cooling capacities are often provided by electrical driven vapour compression cycles. In contrast, removal of sensible and latent loads is separated within a desiccant assisted air conditioning process. A desiccant material is used to remove latent loads from the process air stream. Thus, required cooling capacities are reduced, especially at high outside air humidity ratios. Shallow geothermal energy can be utilized to remove sensible loads from the process air stream. Utilizing the soil for cooling, an equalized energy balance of the soil is essential regarding long-term efficiency of the geothermal system. This can be improved by using a ground-coupled heat pump for heat supply during winter.
Increasing the moisture level of supply air is a sensitive but often little noticed comfort aspect during winter. Dry indoor air conditions can adversely affect occupants’ comfort, especially in modern buildings relying on mechanical ventilation without additional humidification systems during winter. Conventional air conditioning systems require additional components to achieve sufficient supply air humidity ratios. This is an advantage of desiccant assisted systems, because moisture recovery by means of the existing hygroscopic material is possible. A further hygienic advantage of desiccant assisted moisture recovery against conventional air conditioning relying on adiabatic or isothermal air humidification is the fact that no liquid or vapourous water is sprayed into the process air stream. Thus, emission of bacteria caused by air humidifiers is avoided.
The working of desiccant assisted HVAC systems
Desiccants are a group of adsorbent (absorbent for liquid) materials that have a great affinity for water vapour. Due to this merit, desiccant materials can produce hot and dry air that can be used for the drying process as well as in air-conditioning applications to minimize the latent heat load. A good desiccant must have better moisture absorption (adsoption for solid) capability as well as lower regeneration temperature.
The moisture absorption (or adsoption) capability basically depends upon the desiccant characteristics like pore volume, apertures size and void fraction. Depending upon normal physical state, desiccants can be either solids or liquids. Solid desiccant materials such as silica gel, natural zeolites, molecular sieves, activated alumina, synthetic polymers are highly porous in nature and adsorb water by using mechanisms of chemical adsorption. Liquid desiccants such as Calcium chloride, tri-ethylene glycol, and lithium chloride are generally very strong solutions of ionic salts and their behaviour is controlled by changing its temperature and concentration.
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.
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. During working of the rotary wheel, the outdoor air flows through a process section, the moisture is transported from air to sorbents which are distributed in the flow channels, this transfer is to the vapor 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 airstream, which will increase the outlet airstream 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.
A brief description of system operation in summer and winter mode is given for the sake of completeness. Summer and winter operations are considered separately. Considering dehumidification mode during summer, outside air is dehumidified within a desiccant wheel (1→2) and pre-cooled by a sensible rotating heat exchanger (2→3). Water vapour is accumulated at the hygroscopic coating of the desiccant wheel; lithium chloride is used as desiccant. Afterwards, process air is finally cooled or heated to the desired supply air temperature within sensible water to air heat exchanger (3→4). Extract air from the reference room is preheated (5→6) by the heat recovery wheel and further heated to the required regeneration air temperature within another sensible water to air heat exchanger (6→7). Finally, extract air is used to regenerate the desiccant material (7→8), before it is emitted to the environment in form of exhaust air. To achieve efficient operation, different components of the air handling unit can be bypassed as shown in Figure 1. Thus, electricity demand of the fans is reduced for demand-oriented air conditioning.
Regarding winter operation, the desiccant wheel is operated as enthalpy wheel at higher rotational speed for coupled heat and mass transfer (1→2) relying on passive air humidification. Outdoor air is remoistened and reheated within this process using the extract air stream. If outdoor air humidity is within comfort limits regarding humidity ratio, it is preheated using the regenerative heat exchanger (2→3); the desiccant wheel is bypassed in this case. Otherwise, the heat recovery wheel is not utilized. The re-heater (3→4) is used to adjust process air to the desired sup temperature. Extract air is either used for sensible heat recovery (5→6) or coupled heat and moisture recovery (7→8). The heater (6→7) is not operated in winter operation mode. The reference room is connected to the air handling unit on the supply and extracts air side for air exchange. Furthermore, to cover sensible heat and cooling loads directly, it is equipped with under-floor heating and cooling ceilings.
Recommendation and future scope
Based on the previous investigations, here presented some discussions on issues and recommendations for future work that can help focus the necessary efforts to find solutions to urgent and pending problems, which lead to further improvements in the overall performance of desiccant air conditioners as follows:
- The discontinuity of solar energy represents one of the main challenges facing the incorporated solar energy with desiccant air conditioning. To treat intermittent solar energy, energy storage materials are used, where part of the thermal energy is stored in periods of high intensity of solar energy, and this energy is recovered again in periods of low solar energy intensity and the period after sunset.
- Solar concentrators that contain energy storage materials must be integrated with the desiccant air conditioning system to achieve the highest benefit rates from solar energy in improving the overall system performance.
- More research is needed to develop strategies of optimization control for solar-desiccant air conditioners, considering the dynamic performance of energy storage materials as well as the uncertainties of the optimal control strategy.
- More research is needed to develop innovative new types of composite desiccant materials that have high thermal properties.
- Innovating a hybrid system that combines desiccant air conditioning, PVT panels, solar concentrator, and HDH desalination unit, suitable for remote regions to achieve thermal comfort conditions, electricity generation, and produce fresh water.
- Innovating a hybrid system that combines desiccant air conditioning, PVT panels, solar concentrator, and drying unit, suitable for remote regions to achieve thermal comfort conditions, electricity generation, and drying plants.
Conclusion
A desiccant assisted HVAC is used for air dehumidification and cooling in modern building. Compared to conventional cooling, the desiccant cooling system is beneficial during summer mode. Savings in electrical energy demand of up to 35-40% are achieved compared to a conventional air conditioning process. In contrast, a reference system relying on adiabatic air humidification shows slight benefits for the investigated heating period.
Nevertheless, desiccant assisted humidification is advantageous compared to other humidification processes regarding hygienic aspects. If full year operation of an air conditioning system requires air dehumidification during summer, it is essential to make use of the pre-existing desiccant material for remoistening of process air during winter.
Additionally, the amount of mode specific equipment is reduced for desiccant assisted air conditioning with respect to full year operation if renewable solar energy is used for desiccant reactivation. Thus, the amount of renewable energy sources is increased significantly to ensure environmental friendly air conditioning.
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). He has published more than 180 Research Articles in reputed International Conferences and Journals. He has also published 5 reputed books in the area of thermal engineering. 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.
