Performance Analysis of An Energy Flow Solar Assisted Desiccant Cooling FittedwithHoneycomb Adsorption

 

 

K.N.Wagh¹, P.W. Bedarkar²

¹Asst. Professor, Mechanical Engineering Department,Guru Nanak Institute of Technology, Nagpur, India

²Lecturer, Mechanical Engineering Department, Shri Guru Gobindsinghji College of Engg. &Technology, Nanded, India

 

ABSTRACT:

A new and potentially clean technology that can be used to condition the internal environment of buildings without the use of harmful refrigerants. Unlike conventional air conditioning systems, which rely on electrical energy to drive the cooling cycle, desiccant cooling is an open heat driven cycle, which uses a desiccant wheel and thermal wheel in tandem to achieve both cooling and dehumidification. Because it is a heat driven cycle, there is the potential to use environmentally cleaner sources of energy such as gas, hot water, waste heat or any heat source, including solar thermal energy, able to elevate the air temperature to a level adequate for reactivation. Desiccant materials, which absorb moisture, can be dried, or regenerated, by adding heat supplied by natural gas, waste heat, or the sun. In most systems, a wheel that contains a desiccant turns slowly to pick up humidity from incoming air and discharge that humidity to the outdoors. Desiccant cooling can also be used in tandem with an conventional air conditioning system in which the desiccant removes humidity and the AC system provides cooling, and in energy recovery ventilators(ERV) to dehumidify incoming fresh air in the summer.

 

 

INTRODUCTION:

In a typical dry desiccant system, the desiccant is mounted on a rotating wheel. As the wheel turns, the desiccant passes alternately through the incoming process air where the moisture is adsorbed and through a “regenerating” zone where the desiccant is dried and the moisture expelled. The wheel continues to rotate and the adsorbent process is repeated. Typically, about three-fourths of the desiccant wheel is exposed to the incoming air throughout the process. During regeneration, the desiccant is heated by a direct-fired gas burner or indirect-fired water or steam coil.

 

Process Inlet—

Air to be dried.  May be outside air, inside air or, more commonly, a mixture of air with high humidity content.

 

Process Outlet—

Air is dried by desiccant wheel.  May be cooled, filtered or otherwise handled.  Relative humidity is substantially lower and temperature slightly raised.

 

 

Reactivation Inlet—

Air flow, usually outside air, that drives moisture off wheel.   Reactivation air is heated by direct-fired gas burner or indirect-fired water or steam coils.

 

Reactivation Outlet—

Hot, wet air from wheel is exhausted outside or passed through an air-to-air heat exchanger.  Using a heat exchanger to preheat incoming process air offers substantial savings in northern climates.

Desiccant cooling systems are basically open cycle systems, using water as refrigerant in direct contact with air. The thermally driven cooling cycle is a combination of evaporative cooling with air dehumidification by a desiccant, i.e. a hygroscopic material. For this purpose, liquid or solid materials can be employed. The term ‘open’ is used to indicate that the refrigerant is discarded from the system after providing the cooling effect and new refrigerant is supplied in its place in an open-ended loop. Therefore only water is possible as refrigerant since a direct contact to the atmosphere exists. The common technology applied today uses rotating desiccant wheels, equipped either with silica gel or Lithium chloride as adsorption material                                                                                                                                  

 

The main components of a solar assisted desiccant cooling system is shown in the figure below :

Schematic drawing of a desiccant cooling system

Warm and humid ambient air enters the slowly rotating desiccant wheel and is dehumidified by adsorption of water (1-2). Since the air is heated up by the adsorption heat, a heat recovery wheel is passed (2-3), resulting in a significant pre-cooling of the supply air stream. Subsequently, the air is humidified and further cooled by a controlled humidifier (3-4), according to the desired temperature and humidity of the supply air stream. The exhaust air stream of the rooms is humidified (6-7) close to the saturation point to exploit the full cooling potential in order to allow an effective heat recovery (7-8).

Finally, the sorption wheel has to be regenerated (9-10) by applying heat in a comparatively low temperature range from 50°C-75°C, to allow a continuous operation of the dehumidification process.

In periods with low heating demand, heat recovery from the exhaust air stream and enthalpy exchange by using a fast rotating mode of the desiccant wheel may be sufficient. In case of increasing heating demand, heat from the solar thermal collectors and, if necessary, from a backup heat source (4-5) is applied.

 

Flat-plate solar thermal collectors can be normally applied as heating system in solar assisted desiccant cooling systems. The solar system may consist of collectors using water as fluid and a hot water storage, to increase the utilization of the solar system. This configuration requires an additional water/air heat exchanger, to connect the solar system to the air system. An alternative solution, leading to lower investment cost, is the direct supply of regeneration heat by means of solar air collectors.

 

Special design of the desiccant cycle is needed in case of extreme outdoor conditions such as e.g. coastal areas of the Mediterranean region.

Here, due to the high humidity of ambient air, a standard configuration of the desiccant cooling cycle is not able to reduce the humidity down to a level that is low enough to employ direct evaporative cooling. More complex designs of the desiccant air handling unit employing for instance another enthalpy wheel or additional air coolers supplied by chilled water can overcome this problem.

 

FUTURE SCOPE AND DEVELOPMENT NEEDS:

While significant progress has been made in efficiency and cost effectiveness of desiccant cooling systems for air conditioning, commercial systems and designs are available only for special applications such as supermarkets and other low-humidity applications. Lowering costs and improvements in the efficiency, size, reliability, and life-expectancy of components and systems are necessary to advance penetration of the desiccant cooling technology into broader commercial air conditioning market.

 

ADVANTAGES:

·      Desiccant systems offer significant potential for energy savings (0.1-0.4 quads) and reduced consumption of fossil fuels. The electrical energy consumption is small, and the source of thermal energy can be diverse (i.e., solar, waste heat, natural gas).

·      With desiccant systems the use of CFCs is eliminated (if used in conjunction with evaporative coolers) or reduced (if integrated with vapor compression units). CFCs contribute to depletion of the earth's ozone layer and will be banned by the end of the century.

·      Indoor air quality is improved because of higher ventilation and fresh air rates associated with desiccant systems. Such systems also offer lower humidity levels and the capability to remove airborne pollutants.

·      With desiccant systems, air humidity and temperature are controlled separately, enabling better control of humidity

 

APPLICATIONS:

The best circumstances for use of desiccant cooling and dehumidification are: (1)need for humidity control, (2)economic benefits from using low humidity, (3)high latent load versus sensible load, (4)low thermal energy cost versus high electric energy cost, and (5)need for dry cooling coils and duct work to avoid microbial growth.

 

CONCLUSION:

Desiccant cooling and dehumidification can be applied to many types of buildings  like supermarkets, hotels and motels, office buildings, hospitals and nursing homes, restaurants, health clubs and swimming pools, and residences. The success of desiccant cooling is being realized in super markets, which use four times more energy per unit floor space than most commercial buildings. Use of desiccant technology to provide dry, cool air for hotels and motels in humid climates (to avoid mold and mildew damage as a result of excess moisture) is expected to be the next major application of the technology. As per experimentation herein are useful for the development of desiccant, cooling systems that are free from CFCs, which require much less electric power  consumption.

 

REFERENCES:

[1]   Ashrae. (2008). Ashrae Handbook- HVAC System and Equipment. Atlanta, GA, USA: American Society of Heating, Refrigerating and Air-Conditioning Engineers; [Chapter 23]

[2]   Henning, H.M.,. Solar assisted air conditioning of buildings -an overview. Applied Thermal Eng., 2007; 27: 1734 -1749.

[3]   Angrisani, G., Capozzoli, A., Minichiello, F., Roselli, C., and Sasso, M. Desiccant Wheel Regenerated by Thermal Energy From a Micro Generator: Experimental Assessment of the Performances. Appl. Energy,201: 88:1354-65.

[4]   Dai, Y. J. (2008). Solar Cooling: Research and Application. Retrieved from http://www. sjtuirc.sjtu.edu.cn/wangshangjiaoxue/xuekeqianyan/2008 resoue/daiyj.pdf.on 8thOctober 2014.

[5]   Eicker, Ursula. (2010). Operational Experiences with Solar Air Collector Driven Desiccant System. Applied Energy, 2010: 87: 3735-3747.

[6]   Ge, T. S., Ziegler, F., and Wang, R. Z. (2010). A Mathematical Model for Predicting the Performance of A Compound Desiccant Wheel (a model of compound desiccant wheel). Appl. Therm. Eng.,30: 1005-15.

[7]   Henning, H. M. (2007). Solar Assisted Air Conditioning of Buildings-An Overview. Applied Thermal Eng.,; 27: 1734-1749

 

Received on 22.10.2016            Accepted on 12.12.2016            © EnggResearch.net All Right Reserved Int. J. Tech. 2017; 7(1): 07-10 DOI:10.5958/2231-3915.2017.00002.5