Manel Vallès

Performance of double effect absorption compression cycles for air-conditioning using methanol–TEGDME and TFE–TEGDME systems as working pairs

Abstract The organic working pairs trifluoroethanol (TFE)–tetraethyleneglycol dimethyether (TEGDME or E181) and methanol–TEGDME have some advantages over classical water–LiBr and ammonia water working pairs in absorption cycles. One of the most important features is the wide working range caused by the absence of crystallization, the low freezing temperatures of the refrigerants and the thermal stability of the mixtures at high temperatures. The performance of a double effect absorption cycle for these organic mixtures can be improved if a compression stage is introduced between the evaporator and the absorber. The coefficient of performance (COP) and primary energy ratio (PER) values in the cooling mode are significantly increased over a wide working range: the cycle can work with temperature lifts of 50oC at 5oC in the evaporator or it can also be powered by low grade heat. For these conditions COP and PER values are higher than 1.0 and 0.7 respectively, and the power supplied to the compressor represents up to 15% of the thermal energy supplied to the generator. As it is possible to work at high temperatures lifts, the absorber and condenser can be air cooled.

Absorption heat pump with the TFE-TEGDME and TFE-H2O-TEGDME systems

Abstract The binary mixture trifluoroethanol (TFE)-tetraethylene glycol dimethyl ether (TEGDME) can be more advantageous for absorption cycles at high temperature levels than classical working systems such as H 2 OLiBr and NH 3 H 2 O. This system is non-corrosive, completely miscible over a wide temperature range, thermally stable up to 250°C and has low working pressures. The low thermal conductivity and enthalpy of evaporation of TFE can be improved by using the TFEH 2 O mixture as a refrigerant, instead of pure TFE, with TEGDME as an absorbent. The effect of adding water on the performance of a single-effect absorption heat pump has been analysed. The simulation of such a cycle with a partial evaporator has been carried out for binary and ternary systems, in order to recover thermal wastes at 80°C and upgrade them to 120°C. The maximum COP of about 1.6 obtained for the TFE-TEGDME system in these conditions is hardly affected by the addition of water, although the solution and reflux flow ratios increase with the water content. The results show that about 15 mass% of water gives optimum performance in terms of the COP and flow ratios. The thermodynamic properties of the pure compounds (TFE and TEGDME) and the binary and ternary mixtures needed to study the performance of the absorption cycles are also presented.

Absorption of organic fluid mixtures in plate heat exchangers

It is well known that the absorber is the key component in energy conversion systems that are based on absorption cycles. This paper describes an experimental investigation into the absorption process of organic fluid mixtures in an absorption system which has a spray and a plate heat exchanger. The absorber consists of an adiabatic mixing chamber with a spray, where the solution that is weak in refrigerant is sprayed into the refrigerant vapour. A two-phase mixture is formed and enters a plate heat exchanger, where the solution is cooled to complete the absorption process. We carried out experiments with different types of spray nozzles using the organic fluid mixtures methanol–tetraethyleneglycol dimethylether (TEGDME) and trifluoroethanol (TFE)–TEGDME. We analyse how the solution mass flow rate, absorber pressure and cooling water temperature affected the absorber performance and we discuss the results in terms of the absorber load, absorbed mass flux, degree of subcooling of the solution at the absorber outlet, solution film heat and mass transfer coefficients. The results indicate that the absorption system proposed is suitable for relatively low pressures. For water temperatures of 30 °C and absorber pressures between 2 and 6 kPa, the absorption rates for TFE–TEGDME were 1 to 2.5 g·s−1·m−2. The corresponding values for methanol–TEGDME with absorber pressures between 10 and 15 kPa were 0.4 to 1.2 g·s−1·m−2.

Performance of air-cooled absorption air-conditioning systems working with water-(LiBr + Lil + LiNO3 + LiCl)

Abstract The potential of the fluid mixture water-(LiBr + Lil+ LiNO3 +LiCl) (5:1:1:2 molar) is studied for air-cooled absorption air-conditioning systems. This multicomponent system shows a considerably higher solubility than that of water-LiBr and is also less corrosive. It is, therefore, one of the potential alternatives to replace water-LiBr in future air-cooled absorption chillers. A comparative study based on thermodynamic simulation of the single-and double-effect cycles with water-LiBr and the new fluid mixture is first reported. Once the operating conditions were established in terms of temperature and concentration in the generator and the absorber, the absorption process of a falling film flowing on the inner surface of a vertical tube at typical air cooling thermal operating conditions of the absorber was modelled in order to compare the absorption rates of the multicomponent salt mixture with those of water-LiBr. This study was completed by an experimental characterization of the absorption process in a vertical falling film tube. The results show that the multicomponent salt solution is more suitable than water-LiBr if the temperatures in the generator and absorber/condenser are low and high, respectively. Therefore this new working fluid can be recommended for air-cooled absorption air-conditioning systems driven by low temperature heat sources.

Experiments on the characteristics of saturated boiling heat transfer in a plate heat exchanger for ammonia/lithium nitrate and ammonia/(lithium nitrate+water)

The new developments in absorption systems for air-conditioning applications respond to the new energetic and environmental situation that requires systems more efficient, use of renewable energy sources, such as thermal solar energy, and integration in polygeneration systems for the energy supply of buildings. Among these new developments, its of great interest to mention absorption systems designed specifically for solar applications and direct heat recovery from exhaust gases of micro-generation systems. Some of these new systems are already air-cooled. Ammonia-water mixture is one of the most used working pairs in absorption refrigeration systems; nevertheless, several authors have proposed the use of lithium nitrate as an absorbent instead of water. Ammonia/lithium nitrate refrigeration systems do not require a rectifier to remove the absorbent from the vapor stream leaving the desorber, and the driving temperature is lower than that required for ammonia-water systems. The main disadvantage of this working fluid is related with its high viscosity, which penalizes heat and mass transfer processes in the absorber and generator. To overcome this limitation a double solution is proposed. First, the use of plate heat exchangers in the design of the main components of the cycle and, second, the addition of a small amount of water to the ammonia/lithium nitrate mixture in order to reduce the viscosity but avoiding the need of rectification. In this work, we experimentally investigated saturated flow boiling heat transfer of the ammonia/lithium nitrate and ammonia/ (lithium nitrate + water) mixtures with water content in the absorbent of 20 % by weight, flowing in a vertical plate heat exchanger. The test section consists of four commercial plates with a chevron angle of 60°, referred to the vertical axis of the plate, forming three channels. The effects of heat flux ranging from 5 to 20 kW/m2, mass fluxes from 50 to 100 kg/s.m2 and mean vapor quality from 0 to 0.2 on the boiling coefficient and total pressure drop were analyzed. The results show that, at the considered operating conditions, boiling heat transfer coefficient increases with increasing the heat flux and mass flux, whereas the vapor quality slightly influences the boiling heat transfer coefficient. The addition of water increases the boiling heat transfer coefficient more than 30 % when compared with the binary mixture.

Thermophysical Properties and Heat and Mass Transfer of New Working Fluids in Plate Heat Exchangers for Absorption Refrigeration Systems

Ammonia/lithium nitrate and ammonia/(lithium nitrate + water) have been proposed as suitable working pairs for absorption refrigeration systems driven by low-temperature heat sources. The use of water in the absorbent of the ternary mixture reduces the viscosity of the binary mixture and increases the affinity between the refrigerant (ammonia) and the absorbent, which should have a positive effect on the absorption process. In this paper, a brief review on the thermodynamic and transport properties available in the open literature for NH3/LiNO3 and NH3/(LiNO3 + H2O) mixtures is presented, and the most significant results of an experimental characterization of absorption and desorption processes taking place in plate heat exchangers are discussed.