Thermodynamic Performance Analysis of Hybrid Air Conditioning System

Transcription

1 Thermodynamic Performance Analysis of Hybrid Air Conditioning System Dr. S.P.S Rajput and Shankar Kumar Abstract - This paper deals with the concept of hybrid air conditioning system. The hybrid air conditioning combines the dehumidification process of outside air with the help of desiccant materials and sensible cooling process by cooling coil of conventional vapour compression air-conditioning system. This proposed thermodynamic system may save about 40-50% of total power consumption especially in hot and wet summer season. The heat energy required for regeneration of desiccant material can be achieved from the heat rejected from the condenser or solar air preheater. Here complete thermodynamic analysis of the proposed system and also various comparisons of high grade energy (electricity) used in proposed system and conventional vapour compression air conditioning system for different values of room sensible heat factors, ratios of recalculated and fresh air and outside weather conditions have been presented. Keywords - Vapour Compression Refrigeration system (VCRS), Desiccant system, Hybrid Air Conditioning System (HACS), Room sensible heat factor (RSHF) I I. INTRODUCTION N tropical countries like India, there are two types of summer season one is hot and dry and another is hot and wet. Conventional air conditioning is applicable for both seasons. This requires high quantity of electricity for the compression process of refrigerant in the compressor results in a high operating cost. This operating cost increases at a very fast rate if the outside air humidity level increases. This is due to additional dehumidification load. Desiccant based cooling system which requires desiccant dehumidification and evaporating cooling process unable to provide complete comfort for the human beings due to insufficient magnitude of the evaporative cooling process in hot and humid weather [1] [2]. But if we achieve dehumidification from desiccant material and only sensible cooling from cooling coils of conventional vapour compression air conditioning system, than saving in electricity consumption in compressor and complete comfort inside the conditioned space may be achieved. Typically, the summer air conditioning load includes two elements-latent cooling (dehumidification) and sensible cooling (temperature control) [3]. Conventional electric driven air conditioning cools and dehumidifies simultaneously in a highly energy intensive process: the air is chilled deeply to condense out adequate moisture, then reheated to a comfort level (or mixed with warmer air) prior to distribution. Latent cooling/dehumidification represents as much as 40% of the total air conditioning load in some areas [4]. Desiccant cooling removes this portion of load from the electric powered refrigeration system and dries the air with a moisture absorbing desiccant. The desiccant dehumidification process requires heat to regenerate the desiccant for reuse. Thus desiccant cooling reduces the requirement for refrigeration energy. In light of this fact, well integrated desiccant cooling application has become cost effective in many parts of the country due to the following factors [5, 6, 7, 8]: 1. High electric utility demands charges, which shifts from conventional air conditioning system. 2. Lower off-peak electric utility rates, as well as cash incentive for many utilities for thermal storage equipment which can store night-generated heat for daytime desiccant regeneration. II. THE CONCEPT OF HYBRID AIR CONDITIONING SYSTEM The supply state for the air to the conditioned space may be achieved in three ways. Fig. 1 shows all the processes of three system i.e. Vapour Compression air conditioning ( ), desiccant cooling (1-2b-3b-5-6) and hybrid system (1-2a-3a-5-6) on psychometric chart. In hybrid air conditioning system the dehumidification process (1-2a) is achieved by the application of liquid desiccant in the dehumidifier. The sensible cooling of dry and heated air from state point 2a to 3a is achieved by the application of cold water in the indirect evaporative cooler [9]. The further sensible cooling from 3a to supply state 5 is achieved in cooling coil of conventional VCRS. The power saving in the hybrid system mainly depends on following two factors: 1. The temperature of cooling water applied in process 2a-3a. The lower cooling water temperature decreases the temperature of air at state point 3, which in turn decreases the power consumption over the cooling coil. 2. The inclination of room sensible heat factor line i.e. the magnitude of RSHF. Lower values of RSHF increases the power consumption in VCRS, but decrease the power consumption in hybrid system. Dr. S.P.S Rajput is Associate Professor, Department of Mechanical Engineering, MANIT, Bhopal (India), - spsrajput@gmail.com Shankar Kumar is PhD Scholar, Dept. of Mechanical Engineering, MANIT, Bhopal (India) - shankarlnct@yahoo.com 324

2 III. DESCRIPTION OF THE HYBRID AIR CONDITIONING SYSTEM The schematic of proposed air conditioning system is shown in fig 2. The fresh air from the atmosphere mixes with the recirculated from the air conditioned space in the desired portion before being inlet air is passed through a liquid desiccant spray. The dehumidification is achieved because of the lower water vapour pressure at the surface of the desiccant. Water vapour condenses out of air and the latent heat of condensation liberated, heats the air sensibly. Hence due to condensation, specific humidity falls and this heat of condensation heats air, increasing its dry bulb temperature (DBT). The hot and dehumidified air from dehumidifier is sensibly cooled in the tubes. The chilled water from the cooling tower is circulated across the cotton wounded tubes. Some water will get evaporated from the surface of cotton and will absorb its latent heat circulated through blower at the inlet of the dehumidifier. The ratio of mixing of fresh air to recirculated air depends on ventilation level required in the conditioned space. The of vaporization from the surface, which results in further decreases in temperature of air inside the tubes. Now the desired supply temperature of air to the conditioned space is achieved by passing this air over the cooling coils of conventional vapour compression refrigeration system. The heated water from the air tubes is collected at the bottom of the tubes in a tank and recirculates to the cooling tower through pump. To maintain constant mass flow rate of water over the air tubes make up water is added in the tank of cooling tower. IV. ANALYSIS OF HYBRID AIR CONDITIONING SYSTEM (HACS) The following assumptions have been taken in the analysis of comparison of conventional VCRS and HACS [10, 11, 12, 13]. 1. By pass factor of 0.1 for the cooling coils have been taken constant throughout the analysis. 2. The heat transfer losses from or to the dehumidifier are neglected. 3. The cooling tower used in the circuit may provide much lower temperature of water, but here the inlet water temperature over the air tubes has been assumed as 5 C lower than the atmospheric temperature. 4. The indoor conditions are maintained in the conditioned space as 25 C DBT and 50% RH. 5. A constant value of 3.5 has been assumed for the COP of VCRS [14]. 6. To operate the two pumps, one is the desiccant circuit and another is in the water circuit, it is assumed that they would consume 10% more power compared to theoretical values. The supply point C is achieved by mixing the fresh air from point A and recirculated air from point B in the desired ratio (R) (Fig. 3.) Latent heat removed in KW 325

3 Heat supplied during sensible heating Sensible heat removed in KW. P ower consumption in hybrid system to get the same supply state E for the given atmospheric condition and mixing ratio R. Q L = 50 (Cmm) ΔW KW The room sensible heat factor: For the different values of RSHFs and different outside states, the power consumption in VCRs, power consumption in hybrid system and percentage saving in power consumption is calculated. A: Outside state of Air B: Inside Design C: Supply state of air to the cooling Coil D: Outlet state of air from cooling coil E: Supply state of air to the conditioned space F: Dehumidified air G: Sensibly Cooled air Fig: 3 Psychometric Representations V. RESULT AND DISCUSSION THE performance of the conventional vapour compression cycle and the hybrid cycle for typical hot and humid climate in India, has been carried out for three typical summer/monsoon outdoor air conditions viz. 30 C DBT, 80% RH; 37.5 C DBT, 75% RH and 40 C DBT, 70% RH. The power consumption in the proposed hybrid air conditioning system and the conventional VCRS is compared on the basis of three sets of outside condition of air, room sensible heat factor and the mixing ratio of fresh air and recirculated air from the conditioned space. RSHF is the function of room sensible heat and room latent heat; both have to be absorbed by the supply air at the inlet of the conditioned space. Four values of RSHF viz. 0.6, 0.5, 0.4, and 0.3 have been considered for the analysis. Ventilation quantity of supply air i.e. ratio of recirculated air to fresh air also effects the power consumption in air condition system. Here three values of this ratio viz. 4:1, 3:2, and 1:1, for the common air conditioning applications, have been selected for the study. The results for the three sets are calculated and variations of power consumptions of VCRS and hybrid system are shown in the figure 4 to 12. The following conclusion has been drawn from these figures: 1. Power consumption in VCRs increases by decreasing the value of RSHF (Fig 4, 7 and 10) whereas power consumption in hybrid air conditioning system decreases by decreasing the value of RSHF (Fig 5, 8 and 11). For the given values of mixing ratio R and mass flow rate of air ma, decreasing RSHF increases heating load on the repeater. Because in this case the supply air DBT will get increased and RH decreased. 2. For a constant value of RSHF, the power consumption in VCRS increases with decreasing value of mixing ratio R (Fig. 4, 7, and 10) i.e. mixing ratio 1:1 consumed more power compared to the power consumed in mixing ratio 4:1, whereas power consumption in hybrid a, and r conditioning system increases with increasing mixing ratio R (Fig. 5, 8 and 11) higher mixing ratio decreases the dehumidification load on the cooling coil and also the heating load on the repeater. This is because of the fact that for a constant value of RSHF increasing mixing ratio decreases the DBT and increases RH of supplying air to the conditioned space. On the other hand decreasing DBT of supply air by increasing mixing ratio, for a constant value of RSHF, increasing the sensible cooling required on the cooling coil of hybrid air conditioning system, which results in more power consumption for higher values of mixing ratio R. the constant value of RSHF for different mixing ratios indicates the same rate of increasing of sensible load and latent load in the conditioned space. 326

4 3. The percentage saving in the power consumption by hybrid air conditioning system increases with decreasing values of RSHF for different mixing ratios (Fig. 6, 9 and 12). This is because of the 1 and 2 above. The percentage saving in the power consumption increases more rapidly with respect to RSHF for the mixing ratio 4:1 compared to mixing ratio 3:2 and 1:1. But for any value of RSHF, the percentage saving in power consumption increases with the lower values of RSHF. 4. Outside condition of air i.e. the state of fresh air highly affects the performance of hybrid air conditioning system. The percentage of power saving is highest in case of 35 C DBT, 80% RH for any value of RSHF and mixing ratio compared to 37.5 C, 75% RH and 40 C, 70% RH). This fact justifies the suitability of hybrid air conditioning system for humid climates rather than hot climates. 5. The power saving in the hybrid air conditioning system is also the function of cooling water temperature from cooling tower to the air tubes. 6. The capital cost of hybrid air conditioning system is approximately 25% higher than conventional VCRS for the same cooling load. 327

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6 Air Dehumidification System, Solar Energy, Vol. 44, No.3, pp [13] Rix, J., Nevrala, D., Chauvet, L., and Probert, K.D., 1993, Gas Powered Liquid Desiccant Dehumidifier, Applied Energy, Vol. 45, No. 2, pp [14] Sharma, J.K., 1985, Solar Desiccant Cooling, Reviews of Renewable Energy Resources, Vol. 2, Wiley Eastern Ltd., New Delhi, India. REFERENCES [1] Mecker, G., 1990, Use of Desiccant to produce Cold Air in Gas Energised Cold air HVAC System, ASHRAE Trans., Vol. 96, pt. I, pp [2] Arora, C.P., 2001, Refrigeretaion and Air Conditioning, 2 nd Edition McGraw Hill Pub. Co. Ltd, New delhi. [3] Chung, T.W, 1994, Prediction of the Moisture Removal Efficiencies for Packed Bed Dehumidification Systems, Gas Separation and Purification, 8(4), pp [4] Ertas, A., Gandhidasan, P., Kiris, I. and Anderson, E.E., 1994, Experimental study on the Performance of a Refrigeration Tower for Various climatic Conditions, Solar Energy, 53(1), pp [5] Gandhidason, P., 1994, Performance Analysis of an Open Cycle Liquid Desiccant Cooling System Using Solar Energy for Regenaration, International Journal of Refrigeration, 17(7), pp [6] Grossman, G., and Johanssen, A., 1981, Solar Cooling and Air Conditioning, Progress in Energy Combustion Science, Vol. 7, pp [7] Hausen, H., 1935, Air Conditioning System, U.S.Patent, 1994, 515. [8] Ji, L.J., and Wood, B.D., 1993, Performance Enhancement Study of Solar Collector/Rgenerator for Open Cycle Liquid Desiccant Regeneration, Proc. Of 1993 Annual Conference of the American Solar Energy Society; Washington, D.C., pp [9] Kern, D.Q., 1970, Process Heat Transfer, McGraw Hill, Japan. [10] Khan, A.Y., and Ball. H.D., 1992, Development of Mathematical Model and Computer Simulation to predict the Annual Energy Consumption of Coil Type Liquid Desiccant System, ASHRAE Trans., 98, pp [11] Maclaine Cross, I.L., 1987, Hybrid Desiccant Cooling in Australia, AIRAH Journal, Vol. 41, No. 5, May, pp [12] Patnaik, S., Lenz, T.G., and Lof, G.O.G., 1990, Performance Studies for an Experimental Solar Open Cycle Liquid Desiccant 329