Use of renewable solar energy can be a good source for regeneration heat provided in reactivating desiccant-based HVAC in comfort cooling applications. Desiccant dehumidification assisted vapour-compression-based hybrid air-conditioning systems can be successfully coupled with renewable solar thermal power to dampen electricity use and to conserve environment in place of traditional air-conditioning. The energy saving potential of solar powered desiccant cooling technologies is promising as compared to the vapour compression-based conventional cooling systems. It is found that the solid desiccant integrated hybrid cooling systems are more favourable than the conventional vapour compression cooling, because it is a better means for economical and cleaner air-conditioning. The present article can ameliorate the usefulness of solar energy as renewable regeneration heat source in heat driven solid desiccant based hybrid cooling system and helps in inspiring investigators to take the research opportunities in this area.
Requirement for ventilation air-flow rate during air-conditioning in building is growing day by day due to the ameliorated environmental standards prescribed by HVAC manufacturers. Furthermore, due to the diluted outdoor conditions in urban areas by exponential growth in industrialization leads to increase in demand for air-conditioning globally. This leads to substantial rise in space cooling applications at places – where control of temperature and humidity is required. The increasing demand for air-conditioning leads to substantial rise in requirement of high grade electrical energy. Demand for electricity use in space cooling reaches at its peak in hot summer season. The CO2 emissions by thermal power station for electricity generation are expected to increase exponentially leading to global warming. Urban areas having severe outdoor ambient conditions due to vehicular and industrial pollutions lead to wide use of traditional vapour compression air-conditioning that leads to prime use of electricity. With suitable modification and advancement in conventional cooling technology by integrating it to freely available renewable solar heat for cooling applications can alleviate the problem of excess energy demand in cooling.
Solid desiccant and Vapour Compression Refrigeration (VCR) integrated hybrid space cooling is an innovative approach – in the field of space cooling applications to overcome environmental and economic issues generated due to use of stand-alone conventional vapour-compression cooling systems. The hybrid air-conditioning system improves indoor room environment by integration of solar thermal energy to reduce the use of electrical power. In hybrid cooling, optimal air-conditioning would result as vapour-compression system performs only cooling operations while desiccant dehumidification system takes care of humidity control. Thus, in desiccant assisted hybrid cooling system both sensible and latent loads are handled separately and effectively. So, the desiccant-based hybrid cooling can control temperature and humidity of cooling air independently. This type of hybrid cooling neglects the requirement of low dew point temperature of evaporator cooling coil and subsequently post-reheating in VCR cooling unit. It also alleviates the condensation of air while cooling when outdoor humidity rises. Its operating costs saved substantially by the use of freely available solar energy for regenerating the desiccant wheel. The greatest cooling requirement in building during the summer season is also associated with availability of intense solar radiation – provides an excellent opportunity to use freely available renewable solar energy to integrate with desiccant-based hybrid air-conditioning. Thus, desiccant assisted space cooling can meet the demands of thermal comfort, economy, energy conservation and environmental protection.
Working of solar powered solid desiccant–vapour compression hybrid cooling
Solar powered solid desiccant–vapour compression hybrid air-conditioning system works on principle of adsorption of moisture. In its operation, moisture laden ventilated or recirculated air is first passed through channels of rotary dehumidifier, so that the water vapour present in it is adsorbed by a rotating desiccant dehumidifier due to the pressure difference between them. During the process of dehumidification, adsorption heat increases the temperature of process air at dehumidifier exit. The dehumidifier process air available at outlet of dehumidifier is cooled up to the desired room supply conditions – by combined effort of heat recovery wheel and vapour compression cooling coil. The cycle runs continuously by regenerating the desiccant wheel by driving off moisture with help of supplying hot reactivation air. This is achieved by heating the desiccant material to its temperature of reactivation, which mainly depends on the type of the desiccant used in matrix channel. Thermal energy needed for regenerating the rotary dehumidifier is made available by use of solar thermal collector. A desiccant-based hybrid air-conditioning system, consists mainly of four components, namely the reactivation air heater, the rotary dehumidifier, heat recovery wheel and the traditional vapour compression cooling unit (Fig. 1).
Use of different solar collectors in solar powered solid desiccant – vapour compression hybrid air-conditioning system
Solar energy harvesting techniques to supply regeneration heat in desiccant based hybrid cooling are divided in two main categories: passive solar techniques and active solar techniques. Examples of passive harvesting techniques are materials selections favourable for their thermal specifications, building designs with respect to natural air circulation and building oriented to the sun and sun light dispersing. Active harvesting techniques are solar collectors including electric photovoltaic panels and thermal collectors, which are used to convert solar radiation and heat into energy as shown in Fig. 2. Solar thermal collectors collect solar radiations and use that to produce heat. In other words, it is defined as the conversion of solar radiation into thermal energy. Solar thermal collectors are commonly used in hot water systems. It also can be used to heat different fluids, such as air, which can be used directly, for space heating, or to generate electricity. Solar photovoltaic (PV) modules convert solar radiations directly into electricity (Direct Current) using photovoltaic cells (PV) as shown in Fig. 2.
Fig. 2. Active solar energy techniques (a) photovoltaic cells (b) solar collectors…
Non-concentrated type solar thermal collectors, such as flat plate, can collect solar irradiance without using magnifying or concentration mediums like mirrors. It is also able to generate heat at low and medium temperature scale (40–251°C) by use of concentrating types of solar collectors like as concentrating evacuated tube collectors and compound parabolic concentrators as shown in Fig. 3.
Comparison of solar powered solid desiccant–vapour compression hybrid cooling system with other existing cooling systems
In the solar powered desiccant based hybrid air-conditioning system, the latent load is handled effectively by the rotary desiccant dehumidifier while the sensible load is mainly taken by the sensible cooling coil of evaporator in conventional vapour compression system. So, the electricity consumption of the VCR evaporators can be greatly reduced. Fig. 4 shows the monthly energy savings in case of solar powered desiccant-based hybrid cooling as compared with the conventional vapour compression space cooling system.
Solar powered solid desiccant – vapour compression hybrid space cooling is relatively an advanced space cooling technology – and is a good candidate to substitute for traditional vapour compression air conditioning system – especially in hot and humid climate as solar energy can lead to substantial energy saving.
In hybrid cooling systems solid desiccant dehumidification system used in conjunction with conventional vapour compression system have advantage of high efficiency over solid desiccant-based evaporative cooling systems in which moisture is added to the cooled air stream because the process air steams that come in direct contact with the cooling water becomes less efficient as air becomes saturated in humid conditions. Thus, if economical factors are taken into consideration, the application of freely available renewable solar energy for desiccant regeneration in hybrid space cooling technology would be more beneficial. Therefore, desiccant-based hybrid space cooling technique is sustainable, economically and environmentally more feasible for the climates with high outdoor humidity or hot-humid summer characteristics. By making the direction of future research on space cooling towards solar powered solid desiccant–vapour compression hybrid space cooling augmenting the contribution of solar assisted desiccant-based hybrid cooling that can bring to the amelioration of comfort, energy and cost savings.
Dr. (Prof.) D. B. Jani; an Associate Professor at GEC, Dahod under Gujarat Technological University (GTU), Ahmedabad; received his Ph.D. in Thermal Science (Mechanical Engineering) from Indian Institute of Technology (IIT) Roorkee. Currently, he is a recognized Ph.D. Supervisor at GTU. He published more than 150 Research Articles in reputed International Conferences and Journals along with 5 popular books. His areas of research include Desiccant cooling, ANN, TRNSYS and Exergy.