P. Gandhidasan

Simulation of a hybrid liquid desiccant based air-conditioning system

A hybrid open-cycle vapor absorption and liquid desiccant system using LiBr for the process of absorption and dehumidification has been simulated. The simulation model of the hybrid cycle is formulated with a partly closed-open solar regenerator for regenerating the weak desiccant and a packed tower dehumidifier for the dehumidification of ambient air. The air is first dehumidified in the dehumidifier and then sensibly cooled in the evaporator. Subroutines for each component of the cycle have been used in order to calculate the varying parameters at all points in the cycle. The hybrid model presented is found to be an excellent alternative to conventional vapor absorption machines, particularly in hot and humid climates. The COP so obtained is about 50% higher than that of a conventional vapor absorption machine and is likely to increase further for the conditions which have been discussed.

Calculation of Heat and Mass Transfer Coefficients in a Packed Tower Operating With a Desiccant-Air Contact System

Packed towers can be used for solar regeneration of aqueous solutions and dehumidification of air using aqueous solutions. These processes involve simultaneous heat and mass transfer with heat effects. In order to design a packed tower for aqueous solution-air contacting operations, heat and mass transfer coefficients for each phase are required. For the present study, aqueous calcium chloride solution is used; ceramic Raschig rings and Berl saddles are used as the packing materials. In this paper air phase transfer coefficients are correlated with flow rates of air and liquid and the temperature of air, whereas liquid phase coefficients are correlated with rates of air and liquid flow, and the temperature and concentration of the liquid.

Thermodynamic analysis of liquid desiccants

Liquid desiccants are widely used in many solar applications. In order to analyze the performance of the system using desiccant technology, the thermophysical properties of desiccants are essential. In particular, the vapor pressure of the liquid desiccant is one of the important properties in air dehumidification. In this paper, an attempt is made to predict this property based on a classical thermodynamics approach and it is found that the predicted values for lithium chloride agree very well with the experimental results. The desired sorption properties can also be obtained by mixing the desiccants, which is another method of developing a new cost-effective liquid desiccant. In this paper, simple mixing rules are used to predict the vapor pressure, density, and viscosity of the desiccant mixture, namely calcium chloride and lithium chloride. It is found that the interaction parameter need not be included in calculating the density and vapor pressure of the above mixture but must be included in predicting the viscosity.

Experimental investigation of SI engine performance using oxygenated fuel

The current experimental study aims to examine the effects of using oxygenates as a replacement of lead additives in gasoline on performance of a typical SI engine. The tested oxygenates are MTBE, methanol, and ethanol. These oxygenates were blended with a base unleaded fuel in three ratios (10, 15, and 20 vol.%). The engine maximum output and thermal efficiency were evaluated at a variety of engine operating conditions using an engine dynamometer setup. The results of the oxygenated blends were compared to those of the base fuel and of a leaded fuel prepared by adding TEL to the base. When compared to the base and leaded fuels, the oxygenated blends improved the engine brake thermal efficiency. The leaded fuel performed better than the oxygenated blends in terms of the maximum output of the engine except in the case of 20 vol.% methanol and 15 vol.% ethanol blends. Overall, the methanol blends performed better than the other oxygenated blends in terms of engine output and thermal efficiency.

Regeneration of liquid desiccants using membrane technology

Liquid desiccants used for many industrial and domestic applications have to be reconcentrated for reuse. Instead of using thermal energy, a method is proposed in this paper to use mechanical energy for regeneration of weak desiccants. The osmotic pressure required to regenerate two such desiccants, namely calcium chloride and lithium chloride, for given operating conditions is predicted, and correlations are developed. It is found that the pressure required for calcium chloride is much less than that of lithium chloride for the same operating conditions.

Effectiveness of Moisture Removal for an Adiabatic Counterflow Packed Tower Absorber Operating with CaCl2-Air Contact System

In the open cycle desiccant cooling system, the room air or the room air mixed with ambient air is drawn through the desiccant dehumidifier where water vapor is absorbed. Then the dry air is cooled by a sensible heat exchanger by passing the air through an evaporative cooler. The desiccant may be either solid or liquid. Liquid desiccant systems are not as well developed as solid desiccant systems. Any attempt to improve the performance of a liquid desiccant should include a performance analysis of the dehumidifier. One of the efficient dehumidifiers for the liquid desiccant system is the packed bed dehumidifier and shown schematically. Because of the abundance of variables involved with the packed bed dehumidifier, the analysis becomes increasingly complex and is achieved at very high computational cost. Hence there is a need to define the performance of a packed bed quantitatively. A definition of air moisture removal effectiveness is presented based on the rigorous heat and mass transfer calculations for an adiabatic counterflow packed bed dehumidifier. This note also studies the influence of relevant parameters on tower performance quantified by the moisture removal effectiveness.

Prediction of pressure drop in a packed bed dehumidifier operating with liquid desiccant

Abstract In this paper a more rigorous model, which is valid for both structured and random packing columns, is used for predicting the irrigated pressure drop in a desiccant–air contact system. Calcium chloride solution is considered as the desiccant. Four different random packing materials and three different structured packing materials are considered in the present study. The effects of random packing shape and the type of structured packing on the hydraulic performance are studied. The model has been validated for a wide range of operating values available in the literature. It is found that the structured packing has the lower pressure drop and higher capacity compared with random packings. Among the random packing materials considered in the present study, Intalox saddles can provide the least irrigated pressure drop and among the structured packing materials the sheet-type Mellapak 250 Y has the lowest pressure drop.

A method of obtaining fresh water from the humid atmosphere

Abstract This paper proposes to use a suitable liquid desiccant to extract fresh water from the humid atmosphere. The night-time moisture absorption and the day-time moisture desorption take place in the same unit. It consists of a flat, blackened, tilted surface and is covered with a single glazing. During the night, the strong absorbent flows down as a thin film over the glass cover in contact with the humid ambient air. Due to absorption of moisture from the ambient air at night, the absorbent becomes diluted. In order to recover the fresh water from the weak absorbent, it flows down as a thin film over the absorber during the day and is heated by solar energy. The water that evaporates from the absorbent rises to the glass cover by convection where it is condensed on the underside of the glass cover. The absorbent leaving the unit becomes strong and ready for moisture absorption at night. The performance of the unit was computed analytically for typical summer climatic data for the month of August in Dhahran, Saudi Arabia, by solving the energy balance equations. It is shown that for the given operating conditions it is possible to obtain about 1.92 kg of water per m2 of the unit. The influence of absorbent concentration and its flow rate on the performance of the system are briefly discussed in the paper.

Exergy analysis of a liquid-desiccant-based, hybrid air-conditioning system

We present the applicable exergy analysis and estimate irreversible losses that are generated during operation of a hybrid air-conditioning cycle with emphasis on a partly closed solar regenerator that is used to regenerate weak desiccant. The desiccant mass-flow rate has been chosen as the fundamental parameter for analysing the system. We find an optimum mass-flow rate for minimum irreversibility, i.e. maximum exergy. Large irreversibilities occur for high ambient vapor pressures, which tend to decrease the system exergy and overall performance of the regenerator.

Theoretical study of tilted solar still as a regenerator for liquid desiccants

Abstract A closed type (tilted solar still) solar regenerator has been studied in application to regeneration with liquid desiccants. Essentially, it consists of a flat blackened, tilted surface with a transparent glazing as a covering. The absorbent solution which is to be regenerated trickles down as a thin film over the absorber and is heated by solar energy. The water vapor that evaporates from the liquid film is condensed on the under side of the glass cover and the solution leaving the regenerator becomes strong. A simple expression is derived, in this paper, to estimate the mass of water evaporated from the weak absorbent solution as a function of climatic conditions and initial conditions of the absorbent solution. A comparison is also made with a forced flow air circulation regenerator, and it was found that the closed type regenerator may be used for regeneration only in hot, humid climates.