Now-a-days an innovative approach in the field of space cooling application has been introduced as a hybrid Solid desiccant and vapour compression refrigeration system to overcome environmental and economic issues due to maximum use of standalone vapour compression air condition systems. Solar thermal energy is feasible to reduce electrical power in hybrid air-conditioning systems that can improve indoor room environments. 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 vapour compression refrigeration 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 providing 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.
Especially in the hot and humid area, major part of conventionally produced electrical energy is consumed by air conditioning. Also, in the rapidly growing world, scarcity of clean water is a dilemma as equal as green-house and ozone layer depletion. Thus, there is an emergence of dehumidified air for human comfort conditions with having potable clean water same time.
In summer with largely available solar radiation, use of hybrid solar system is a viable option to overcome problems of dehumidification of desiccant as well as of potable water. Up till now the conventional system judiciously integrated to desiccant dehumidification to form hybrid cooling that has been replaced over conventional systems.
Novel features of solar powered desiccant dehumidification and cooling system
Novel features of the desiccant cooling system as compared to conventional air conditioning system are as follows:
- Low cost and weight: As the indoor contact device, regenerator is made of plastic, the cost and weight are low.
- Desiccant cleans: The air desiccant used in the indoor contacting device cleans the ambient air before it enters the conditioned space.
- No blower or fan required for regenerator: A chimney utilising the density difference between moist air and ambient air is sufficient to ensure adequate circulation of air through the regenerator.
- No additional support for regenerator: The collector is mounted on the roof of the building.
- No corrosion of the parts: Use of plastics eliminate the problem of corrosion which is commonly encountered with metals.
Use of renewable solar energy for desiccant dehumidifiers
Regenerating the solid desiccant dehumidifiers with help of renewable heat sources like solar thermal energy not only reduces the electricity consumption but also achieves the substantial fossil energy saving. The solar air heating for regeneration is an interesting option to converge the cooling demands of conditioned space as the available intensity of solar energy and demand for cooling are greatest during the same period. The construction of solar assisted solid desiccant evaporative cooling system includes replacement of conventional HVAC by means of the downsized auxiliary cooling coil powered by a vapour compression refrigerator or a direct evaporative cooler.
Working of the system
The integrated form of the solar powered solid desiccant – vapour compression hybrid air-conditioning commonly known as ventilation system while it makes use of fresh ventilated (outdoor) air as a process air at the dehumidifier inlet. In ventilation mode, outdoor fresh air is used as process air at dehumidifier inlet. In the process air side of the system configuration shown in Fig. 1, the fresh ambient air stream (state 6) passes through various channels of rotary dehumidifier. Its humidity is substantially lowered by the desiccant material owing to pressure difference between it and the vapour in air can be said and the heat of adsorption increases its temperature so that a dehumidified warm air stream exiting the dehumidifier (state 2). Then it is cooled successively in the heat recovery wheel (2–3), and later in vapour compression cooling coil (3–4) up to the room supply designed comfort conditions.
Existing condition (state 5) within conditioned space is also shown. In regeneration side, room return air (state 1) is sensibly heated by passing through heat recovery wheel, simultaneously its pre-cools passing process air on the other side. This is necessary to reduce the regeneration heat consumption. So, the temperature of reactivation air is elevated while coming out from the heat recovery wheel while the humidity ratio is constant (state 7). This heated air is finally reached its reactivation temperature (state 8) by passing through liquid to air heating coil for reactivating the desiccant material used in the dehumidifier. The hot and humidified air available after the regeneration process at dehumidifier exit (state 9) is exhausted to the ambient.
In case of recirculation mode as shown in Fig. 2, return room air which is generally available at lower temperature and humidity as compared to the outdoor condition is used as process air at dehumidifier inlet. In particular cases, air is not 100% recirculated all the time due to needs of fresh air intake to satisfy indoor air quality is called mixed mode, a fraction is fresh air mixed with recirculated air.
The most important component of hybrid solid desiccant vapour compression refrigeration system is desiccant wheel. It is a driving system having a motor powered by electricity, wheel disc and belt to rotate wheel.
Recent advancements in the field of desiccant cooling systems
Recently, a solar powered desiccant-based innovative cooling system has been associated with photovoltaic thermal collectors, a gas-powered CHP tri-generation system (Fig. 3) and a reversible heat pump and is connected to the electrical grid of the building. It works on the idea of the removal of waste heat from PV cells, and the parallel utilization of that heat through the convective air flow behind photovoltaic panels. These PV cells produce the electricity required for the heat pump and for domestic use, whereas the convective air flow under the PV panels cools the PV cells and recuperates the captured solar energy, which is used for the conditioning of a indoor space.
Conclusions
Hybrid solid desiccant – vapour compression air conditioning systems powered by solar energy are relatively feasible for advanced air conditioning systems, which can be used as an alternative of conventional vapour compression air conditioning systems in hot and humid weather conditions as solar energy can lead to substantial energy saving.
It is found that the solar assisted hybrid solid air-conditioning has a strong potential for significant primary energy savings as regeneration of desiccant wheel can be done. Thus, if economic factors taken into consideration, the application of freely available renewable solar energy for desiccant regeneration in hybrid air conditioning or space cooling technology would be more feasible and beneficial.
Desiccant-based hybrid space cooling techniques are economically sustainable, and environmentally more feasible for high outdoor humidity or hot and humid summer weather conditions. 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 lead to the amelioration of comfort, energy and cost savings.
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 240 Research Articles in reputed International Conferences and Journals. He has also published 10 reputed books and book chapters in the 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.
