Comparison Simulation between Ventilation and Recirculation of Solar Desiccant Cooling System by TRNSYS in Hot and Humid Area MMS DEZFOULI, SOHIF MAT, K.SOPIAN Solar Energy Research Institute (SERI, Universiti Kebangsaan Malaysia 43600 UKM Bangi, Selangor, MALAYSIA. Email: salehi.solar@yahoo.com; k_sopian@yahoo.com Abstract: Two modeling of solar desiccant cooling system in different modes of ventilation and recirculation were designed based on hot and humid weather of Malaysia. The latent load and sensible load of the test room were 12283.71 BTU/hr, 37533.56 BTU/hr respectively. The mass flow rates of process air and regeneration air were 0.86 kg/s. The effectiveness of components is selected based on high efficent system because the main objective of this study is comparison between ventilation and recirculation modes. By investigation of simulation results of ventilation and recirculation modes, it was found that, amount of ventilation and recirculation COP were 0.8, and 1.6 respectively. Therefore it was achieved that the recirculation solar desiccant cooling system in hot and humid area is higher efficent than the ventilation solar desiccant cooling system.. Keywords: desiccant cooling, ventilation, recirculation, solar energy 1. Introduction Due to high electrical consumption of conventional vapor compression systems, solar desiccant cooling system is one of the promising alternatives to cooling air where sensible and latent heats of air are being removed separately [1]. The first desiccant cooling system was recorded by Pennington in 1955 [2]. Generally, depending on using dehumidification material, there is two kind of desiccant cooling system: solid and liquid. Many Scientifics and researchers have studied in both kinds of desiccant cooling by using renewable energy [3]. The common materials used in solid and liquid desiccant cooling system were seliicagel, and, liquid water-lithium chloride. Desiccant cooling system is including of three main units such as desiccant unit, heat source unit, and cooling unit [4]. Depending on weather conditions of each kind of weather data, elements of each unit become designed. The components of basic cycle were consisted of solid desiccant wheel as dehumidifier, evaporative cooler as cooling unit, heat recovery wheel as per-heater and per-cooler in regeneration and process air side, respectively. In recent years, according to Pennington cycle, the different types of solid desiccant cooling cycles were designed by various researchers on the world. COP of desiccant cooling systems, regeneration temperature, mass flow rate, fresh air, relative humidity of supply air are the important parameters which were considered by researchers. Jain et al. [5] evaluated ventilation, and recirculation cycles based on India weather data, to find effect of the effectiveness of evaporative coolers on COP of cooling system. Haddad, K et al [6] have studied about simulation of a desiccant-evaporative cooling system for residential buildings. They found that the use of solar energy for regeneration of the desiccant wheel can provide a significant portion of the auxiliary thermal energy needed. Fong et al.[7] have designed a simulation model (TRNSYS of an integrated radiant cooling by absorption refrigeration and desiccant dehumidification. Dezfouli M.M.S. et.al [8] investigated solar hybrid desiccant cooling System in Hot and Humid Weather of Malaysia. They found that solar hybrid solid desiccant cooling system provided considerable energy savings in comparison with conventional vapor compression in hot and humid area. ISBN: 978-1-61804-175-3 89
This paper presents a comparison simulation study on ventilation and recirculation of solar desiccant cooling system in hot humid weather of Malaysia. 2. Methodology 2.1 case study description One solar desiccant cooling system with two modes (ventilation and recirculation was considered to provide supply air for one test room in technology park (UKM of Malaysia. The latent load and sensible load of the test room were 12283.71 BTU/hr, 37533.56 BTU/hr respectively. The cooling capacity was 14.6 Kw. According to ASHRAE comfort condition, the Indoor condition designs are consist of temperature at 25 C, relative humidity at 50%, and humidity ratio at 0.0098 kg/kg. According to Malaysia whether, the Outdoor condition design are temperature at 35 C, and relative humidity at 85%. The cooling system was including four main parts such as: 1- desiccant wheel as dehumidifier, 2- Heat recovery wheel, 3- evaporative cooling as humidifier, and 4- solar evacuated tube collector as heat source. Simulation designing with the TRNSYS software was carried out to investigation of solar desiccant cooling in different modes to find best operation of components and cooling system based on hot and humid weather of Malaysia. In this study, two modes such as ventilation mode and recirculation mode were simulated with same conditions and same amount of effectiveness of components. Solar desiccant cooling effectiveness of components is selected based on high efficent system because the main objective of this study is comparison between ventilation and recirculation modes. Effectiveness of dehumidifier is including two parameters F1 and F2 that were 0.05, and 0.95 respectively. The mass flow rates of process air and regeneration air are 0.86 kg/s. The effectiveness of heat recovery wheel and was 1. The saturation efficiency of evaporative cooling was 1. Also, efficiency of others components such as pump, fan, heat exchanger was 1. Solar desiccant cooling system description is divided to two parts that explained in section 2.2, and 2.3. 2.2 ventilation mode of solar desiccant cooling system As shown in figure 1, ventilation mode of solar desiccant cooling is open cycle that provides supply air to room from ambient air. The return air from room after few processes, releases to ambient ISBN: 978-1-61804-175-3 90 as exhaust air. So, there are two sides of air, process side that produces supply air, and regeneration side that releases return air from room to ambient. In the process side, at the first step, ambient air becomes dry by desiccant wheel. Then heat recovery wheel acts as per cooling in process air side. In the next step, air become cold by evaporative cooler and then air goes to room as supply air. Figure 1: schematic of solar desiccant cooling in ventilation mode In the regeneration side, return air that taken latent and sensible load from room, becomes cold by evaporative cooler. Heat recovery wheel acts as per heater in regeneration air side. Then heat from heat exchanger and heater transfers to air. So, in the last step of regeneration side, hot air takes humidity of desiccant wheel and releases to ambient as exhaust air. Figure 2 shows modeling of solar desiccant cooling system in ventilation mode that was designed by TRNSYS software. Figure 2: Studio TRNSYS simulation for solar 2.3 recirculation mode of solar desiccant cooling system Figure 3 shows recirculation solar desiccant cooling system. Generally, this system is not open cycle. Process air side in recirculation mode is close loop while regeneration air side is one open
cycle. In process air side, room air that including sensible load and latent load goes to desiccant wheel to removing latent load. Heat recovery wheel acts as per cooling in process air side. In the next step, air become cold by evaporative cooler and then air goes to room as supply air. Figure 3: schematic of solar desiccant cooling in recirculation mode In the regeneration side, ambient air becomes cold by evaporative cooler. In the next step, Heat recovery wheel acts as per heater in regeneration air side. Then heat from heat exchanger and heater transfers to air. So, in the last step of regeneration side, hot air takes humidity of desiccant wheel and releases to ambient as exhaust air. Figure 4 shows simulation modeling of recirculation mode solar desiccant cooling system that was designed by TRNSYS software. 2.4 determination of COP desiccant cooling system The Coefficient of Performance (COP of the solar desiccant cooling system can be calculated by rate of heat extracted share on rate of heat regeneration. Rate of heat extracted is cooling capacity of this system that supplied cooling air to room. Rate of heat regeneration is consisting regeneration heat input by heater and solar thermal. Therefore, the COP of the system is obtained by following relation[9] (1: QCOOL Q Re generation According to figure 1, the COP of ventilation mode can be written as: ms ( h5 h4 mr ( h9 h7 According to figure 3, the COP of recirculation mode can be written as: ms ( h5 h4 mr ( h10 h8 Where m s (g/s is mass flow rate of supply air, m r (g/s is mass flow rate of regeneration air, and h(j/g is enthalpy of air. 3. Results and Discussion In this section, results of ventilation and recirculation of solar desiccant cooling system are explained and then compared. These results are including temperature and humidity ratio of ventilation and recirculation modes against of time. Figure 5 shows important temperatures ( O C of ventilation solar desiccant cooling system versus time (h. Figure 4: Studio TRNSYS simulation for solar In this simulation, type 683 is desiccant wheel, type 506c is evaporative cooler, and type 760b is heat recovery wheel.the type 91 is heat exchanger, type 71 is evacuated tube solar collector, type 3b is pump, type 112a is fan, type 690 is zone load (room, and type 109-TMY2 is weather data of Kuala Lumpur (Malaysia. ISBN: 978-1-61804-175-3 91 Figure 5: temperatures of different components in Temperature of ambient air, process air after desiccant wheel, supply air, room, and regeneration temperature are shown in this figure. Regeneration temperature is one of the important temperatures that has main role in changes of COP desiccant cooling system. Regeneration temperature of
ventilation mode is 70 o C almost. Figure 6 shows important temperatures ( O C of recirculation solar desiccant cooling system versus time (h. Temperature of ambient air, process air after desiccant wheel, supply air, room, and regeneration temperature are shown in this figure. Regeneration temperature of recirculation mode is 50 o C almost. 0.0108 kg/kg, while amount of humidity ratio of room is 0.0124kg/kg. Figure 8 shows humidity ratio of cooling system components in recirculation mode versus time. Humidity ration of ambient air, process air after desiccant wheel, room, and supply air are shown in this figure. The amount of humidity ratio of supply air in recirculation mode is 0.011 kg/kg, while amount of humidity ratio of room is 0.0128 kg/kg. Figure 6: temperatures of different components in By comparison temperature results of ventilation and recirculation, it can be considerable that regeneration temperature of ventilation mode is higher than the regeneration temperature of recirculation mode. Temperatures of supply air for both modes are almost same. Also temperatures of the room for both modes are almost same. Therefore, based one equation 1, the COP of desiccant cooling system in ventilation mode is less than COP of desiccant cooling system in recirculation mode. Figure 7 shows humidity ratio of cooling system components in ventilation mode versus time. Figure 8: humidity ratio of different components in By comparison humidity ratio results of ventilation and recirculation, it can be considerable that humidity ratio of room and supply air in recirculation mode is a little more than humidity ratio of room and supply air in ventilation mode. Humidity ratios of process air after desiccant wheel in both modes are same (0.008 kg/kg. According to temperature and humidity ratio results of simulation models of solar desiccant cooling in ventilation and recirculation modes, the air properties such as enthalpy for all of important points in both modes were detected that shown in table 1 and table 2. Table 1: air properties of solar desiccant cooling system in ventilation mode Temperature ( o C Humidity ratio (gr/kg Enthalpy (kj/kg T 4 = 15 10.08 h 4 = 40.4 T 5 = 28 12.4 h 5 = 59.8 T 7 = 49 15.2 h 7 = 88.7 T 9 = 72 15.2 h 9 = 112.5 Figure 7: humidity ratio of different components in Humidity ratio of ambient air, process air after desiccant wheel, room, and supply air are shown in this graph. Humidity ratio of supply air is one of the important parameters that has main role in amount of removing latent load from room by desiccant cooling system especially in hot and humid weather same as Malaysia. The amount of humidity ratio of supply air in ventilation mode is ISBN: 978-1-61804-175-3 92 Table 2: air properties of solar desiccant cooling system in recirculation mode Temperature ( o C Humidity ratio (gr/kg Enthalpy (kj/kg T 4 = 17 11 h 4 = 44.9 T 5 = 29 12.8 h 5 = 61.9 T 8 = 42 20 H 8 = 93.8 T 10 = 52 20 H 10 = 104.2
The COPs of ventilation and recirculation modes were calculated by results data and equation 1. The amount of COP of ventilation and recirculation are 0.8, and 1.6 respectively. 4. Conclusion This paper presents a comparison study between two modes of solar desiccant cooling system in hot and humid weather of Malaysia. Ventilation and recirculation modes were simulated by TRNSYS software. Temperature and humidity ratio of different points of solar desiccant cooling system were results of both simulation modes. Results show that COP recirculation mode is 2 times more than COP ventilation mode. Therefore, it can be concluded that recirculation solar desiccant cooling system is higher efficent than ventilation solar desiccant cooling in hot and humid weather of Malaysia. Acknowledgements The authors would like to thank the Solar Energy Research Institute (SERI, University Kebangsaan Malaysia for providing the laboratory facilities and technical support. References [1] Dezfouli, M.M.S., et al., Experimental Investigate of Draying Chili by Solar Assisted Heat Pump Dryer with Multifunctional Solar Thermal Collector. [2] Davanagere, B., S. Sherif, and D. Goswami, A feasibility study of a solar desiccant air conditioning system Part I: psychrometrics and analysis of the conditioned zone. International journal of energy research, 1999. 23(1: p. 7-21. [3] Niu, J., L. Zhang, and H. Zuo, Energy savings potential of chilled-ceiling combined with desiccant cooling in hot and humid climates. Energy and Buildings, 2002. 34(5: p. 487-495. [4] Beccali, M., P. Finocchiaro, and B. Nocke, Energy and economic assessment of desiccant cooling systems coupled with single glazed air and hybrid PV/thermal solar collectors for applications in hot and humid climate. Solar energy, 2009. 83(10: p. 1828-1846. [5] Jain S, Dhar PL. Evaluation of solid desiccant-based evaporative cooling cycles for typical hot and humid climates. Int J Refrig 1995;18(5:287 96. [6] Haddad, K., B. Ouazia, and H. Barhoun. Simulation of a desiccant-evaporative cooling system for residential buildings. in Report#: NRCC-50591, 3rd Canadian Solar Buildings Conference, Fredericton, NB. 2008. [7] Fong, K., et al., Investigation on solar hybrid desiccant cooling system for commercial premises with high latent cooling load in subtropical Hong Kong. Applied Thermal Engineering, 2011. [8] Dezfouli, M.M.S., et al., Experimental Investigation of Solar Hybrid Desiccant Cooling System in Hot and Humid Weather of Malaysia. [9] Daou, K., R. Wang, and Z. Xia, Desiccant cooling air conditioning: a review. Renewable and Sustainable Energy Reviews, 2006. 10(2: p. 55-77. ISBN: 978-1-61804-175-3 93