• Energy Research

Abstract This paper presents the analysis of the performance of a solar cooling facility along one summer season using a commercial single-effect water–lithium bromide absorption chiller aiming at domestic applications. The facility works only with solar energy using flat plate collectors and it is located at Universidad Carlos III de Madrid, Spain. The statistical analysis performed with the gathered data shows the influence of five daily operational variables on the system performance. These variables are solar energy received along the day (H) and the average values, along the operating period of the solar cooling facility (from sunrise to the end of the cold-water production), of the ambient temperature ( T ¯ ), the wind velocity magnitude (V), the wind direction (θ) and the relative humidity (RH). First order correlation functions are given. The analysis of the data allows concluding that the most influential variables on the daily cooling energy produced and the daily averaged solar COP are H, V and θ. The period length of cold-water production is determined mainly by H and T ¯ .

Abstract A facility for experimental evaluation of innovative components forming part of a single effect LiBr–H 2 O adiabatic absorption chiller is described. Plate heat exchangers have been incorporated in the design. Two adiabatic absorber configurations, droplets and liquid sheets, were designed and tested and its performances parameters were experimentally determined. Evaporator limitations have been identified and included in the analysis. Liquid accumulation leads to overflow of refrigerant out of the evaporators. Some general observations, regarding particular operating conditions, are reported. The absorber performance has been characterized in terms of heat and mass transfer through the heat conductance and the effectiveness. The influence of the recirculation ratio is presented for both cases. The liquid sheet configuration has shown better evaluation parameters than droplets configuration. A significant reduction in the absorber size (up to 50%) as compared to using droplets is possible.

The simulation of the heat and mass transfer in a H2O-LiBr microchannel desorber endowed with a microporous membrane is presented. The heat and mass transfer processes are modelled by means of selected correlations gathered from the open literature. The simulation provides the evolution along the channels of the parameters involved in the process: heat and mass transfer coefficients, solution concentration, temperatures of the working fluids and pressure potential. The calculated values of the desorption rate are compared to experimental data gathered from the open literature. For the case considered in this study, the maximum ratio between cooling capacity of the chiller and desorber volume is 2312 kW m(-3). This value is more than one order of magnitude higher than the ones found in conventional small-size absorption cooling chillers.

A microchannel absorber working adiabatically with the H2O-LiBr pair was tested experimentally using three different nanofibrous flat membranes separating the vapour from the solution. Pore diameters of the membranes were 1 and 0.45 mu m, and thicknesses vary from 25 to 175 mu m. The experimental absorption rates varied from 1.5.10(-3) to 2.6.10(-3) kg/m(2) s varying linearly with the solution mass flow rate circulating through the channels. The reduction in pore diameter from 1 mu m to 0.45 mu m induced the need for higher pressure potential or solution mass flow rate to obtain similar performance. Relationships between changes in diameter pore and membrane thickness from previous models were used to quantify the effect of these membranes characteristics on the absorption ratio. The analytical results compared well with our experiments. In the present design, the solution film thickness was 150 mu m and the solution mass transfer resistance dominated the process. The experimental overall resistances, compared with calculated values from correlations used in previous models, showed differences below 30%.

Abstract In the present work the use of low-temperature solar heat is studied to produce cooling at 5°C, using a double-stage LiBr–H2O air-cooled absorption cycle. A solar plant, consisting of flat plate collectors feeding the generators of the absorption machine, has been modeled. Operating conditions of the double-stage absorption machine, integrated in the solar plant without crystallization problems for condensation temperatures up to 53°C, are obtained. Results show that about 80°C of generation temperature are required in the absorption machine when condensation temperature reach 50°C, obtaining a COP equal to 0.38 in the theoretical cycle. A comparative study respect to single-stage absorption cycles is performed. Efficiency gain of the double-stage solar absorption system, over the single-stage one, will increase with higher condensation temperatures and lower solar radiation values. Single-stage cycles cannot operate for condensation temperatures higher than 40°C using heat from flat plate collectors. For higher condensation temperatures (45°C) the generation temperatures required (105°C) are very high and crystallization occurs. Condensation temperatures able to use in double-stage cycles may be increased until 53°C using heat from flat plate collectors without reaching crystallization.

A plate-and-frame microchannel H2O&-LiBr absorber using a microporous membrane as contactor between the vapour and the solution is simulated. The heat and mass transfer equations, describing the absorption of the vapour phase into the solution, are solved for different membrane properties and for variable design and operating conditions. The parametric study evaluates the sensitivity of the ratio between the cooling capacity of the chiller and the absorber volume (rqV) to changes in the following parameters: width and height of the solution and cooling water channels; concentration, temperature and mass flow rate of the solution; temperature and mass flow rate of the cooling water; porosity, pore diameter, thickness and thermal conductivity of the membrane; thickness and thermal conductivity of the interface wall between the solution and the cooling water; and temperature, pressure and mass flow rate of the vapour. At the design stage of the membrane absorber, the parameters that can be optimised to maximise rqV are porosity, pore diameter, solution channels depth and membrane thickness. The thickness of the interface wall between the solution and the cooling water, as well as the solution channels width should be also taken into account. For a good performance during the operation of the absorber, special care should be taken to select the adequate vapour pressure and solution inlet temperature and concentration. The financial support of this study by the Ministerio de Economía y Competitividad of Spain through the research grant ENE2013- 43131-R is greatly appreciated.

A microporous membrane is used in combination with rectangular microchannels in the absorber of an absorption chiller with the aim of reducing the size of this cooling technology. The simulation of the heat and mass transfer between the solution and the vapour phase in a H2O&-LiBr absorber using porous fibres is considered. Heat and mass transfer processes are modelled by means of selected correlations and data gathered from the open literature. This new model is applied for the simulation of the absorber under typical operating conditions of absorption cooling chillers. Absorption rate, heat and mass transfer coefficients, solution concentration, temperatures of the working fluids and pressure potential along the absorption channels are calculated. For the case considered in this study, the absorber channels are of 5 cm length, offering a maximum ratio between cooling capacity of the chiller and absorber volume of 1090 kW/m3. This ratio is higher than twice the usual values found in falling film absorbers using conventional circular tubes. The mean absolute error between the model results and the experimental data gathered from the open literature is 8.5%, showing the capability of the model to predict the performance of membrane-based absorbers. The financial support of this study by the Ministerio de Economía y Competitividad of Spain through the research grant ENE2013-43131-R is greatly appreciated.

Abstract This paper presents the experimental heat transfer evaluation during subcooled and saturated boiling of ammonia–lithium nitrate solution in a fusion plate heat exchanger, acting as a vapor generator under operating conditions representative of single-effect absorption machines. The solution flow rate and outlet temperature were modified in the ranges of 0.041–0.083 kg/s and 78–97 °C, respectively. The region where vapor bubbles begin to arise is estimated using a correlation for the wall superheat required for the onset of nucleate boiling. Results show that subcooled boiling is present in the generator. The initial boiling temperature is about 3.1 °C lower than the saturation temperature. The influence of the heat and mass fluxes on the boiling heat transfer coefficient is analyzed. The paper offers a correlation for the Nusselt number, including the subcooled and saturated boiling regions.