• Energy Research

Abstract At present, the analysis of the performance of air handling units based on desiccant wheels is a complex task for a HVAC&R designer. It is difficult to access a simple model that can represent, with enough accuracy, the behaviour of commercialized desiccant wheels operating with variable inlet states and variable airflows. This work is devoted to investigate the feasibility of a simplified simulation method based on the effectiveness concept. Additionally, an analysis is made of the appropriateness of using a new pair of independent effectiveness parameters instead of the classical approach based on the combined potentials. The first running tests performed with two manufacturers’ software and with a detailed numerical model show that the airflow inlet states have a non negligible influence on the effectiveness parameters. As expected, it is also observed that the effectiveness parameters depend strongly on the airflows rates. So, the effectiveness method based on constant values is not recommended to be used in practise. A new approach using 2D linear interpolation on an irregular grid to take into account the effectiveness dependence on the inlet states of both airflows is presented. A correlation of exponential type is proposed for predicting the effectiveness parameters in case of variable balanced airflows rates, and it is subsequently tested either separately or combined with 2D linear interpolation approach. The proposed methodology requires a reduced amount of data to model a given desiccant wheel, and the predicted results evidence good accuracy. This approach can be used with great advantages in the generalised dynamic analysis of systems integrating a desiccant wheel including the effects of variable inlet states and of variable balanced airflows rates.

Abstract In the present work, a detailed model of simultaneous heat and mass transfer through a desiccant micro-porous media of an element of a channel wall of compact desiccant matrix was used to numerically evaluate the overall transfer coefficients of pseudo-gas-side-controlled models (PGSCMs). In the detailed modelling, the relevant phenomena considered within the porous medium are the surface diffusion of adsorbed water, Knudsen diffusion of water vapour, heat conduction and the sorption process. The convective mass and heat fluxes predicted by the detailed model are used to estimate the evolution of the pseudo-global resistances to heat and mass transfer during a sorption process, which are required by the PGSCM. The simplified formulation of the PGSCM is presented for an element of the channel wall of the desiccant matrix. Results of the evolutions of these pseudo-global resistances are presented for a wide range of values of the layer thickness of the porous medium, which is supposed to be silica gel RD. It is concluded that the pseudo-gas-side-controlled model with constant transfer coefficients is valid only for layer thicknesses lower than 0.1 mm, approximately.

Abstract The desiccant wheel is a key component in solid-desiccant systems. At present, it is difficult to access a component model that can represent the behaviour of commercialised desiccant wheels with enough accuracy. In this work, it is investigated the feasibility of a simplified simulation method based on the effectiveness concept. The pair of independent effectiveness parameters based on the changes of enthalpy and of the ratio of the partial to the saturation vapour pressures is adopted instead of the combined potentials. Extensive parametric studies are conducted using the software of a manufacturer and a detailed numerical model to generate reference data. Exponential-type correlations are investigated to account for the influences of the process and regeneration airflow rates, balanced or unbalanced, and of the rotation speed on the effectiveness parameters. The predicted results provided by the correlations are compared with reference data, separately or combined with the recently developed interpolation approach that accounts for the influence of variable inlet states of both airflows. Cases are identified with an acceptable agreement demonstrating that the correlations can be used with great advantages in the generalised dynamic analysis of systems integrating a desiccant wheel.

Performance parameters of solar cookers have conventionally been determined by assuming a linear trend between the cooker power and the difference between load temperature and ambient air temperature. This approach may not be convenient for some solar cooker designs. In the present work, the suitability of a non-linear regression derived from fitting the measured load temperature to a second order exponential polynomial was investigated and compared with the linear regression. Both regressions were compared with the corresponding experimental curves of a panel cooker and a box cooker. In the case of the panel cooker, the linear trend of the experimental plot was confirmed over a large period of the conducted test. Minor deviations from the experimental data were observed only at the beginning and at the end of the test. On the contrary, in the box solar cooker, significant deviations between the linear regression plot and the experimental points were observed, while smaller deviations were obtained using the non-linear regression. Thus, the proposed method can be seen as a promising approach that should be considered when updating the existing procedures for testing and reporting the performance of solar cookers. info:eu-repo/semantics/acceptedVersion

Abstract An empirical model for predicting the thermal capacity of a coil operating in dry mode is investigated. It is based on the well-known e-NTU method. Thermal resistances are estimated by empirical correlations calibrated with manufacturer data by using two operating cases. Large number of operating cases by ranging the inlet states, the flow rates of both fluids, the altitude and the propylene-glycol concentration is considered in tests. The validity of several approaches of the empirical model is investigated for heating and dry-cooling operating cases. When considering pure water as heat transfer fluid, acceptable results were achieved without any correction of properties. Similar tests were conducted considering propylene-glycol aqueous solution as heat transfer fluid. The error indicators are significantly reduced when the variation of the fluid properties is taken into account. According to the large number of tests conducted, the approach considering the correction of properties of both fluids is a promising procedure for the simulation of the coil, operating either as a heater or as a dry cooler, in TRNSYS, EnergyPlus or other dynamic simulation tools. The validity of the empirical component model is also investigated by using available experimental data published in the literature using aqueous solution of ethylene glycol as heat transfer fluid. Although the number of experiments used is not large and the accuracy of some measured cases is not as good as desirable, the tested approaches with and without correction of properties provide acceptable results.

Abstract A simplified methodology for predicting the overall behaviour of heating coils is presented in this paper. The coil performance is predicted by the e-NTU method. Usually manufacturers do not provide information about the overall thermal resistance or the geometric details that are required either for the device selection or to apply known empirical correlations for the estimation of the involved thermal resistances. In the present work, heating capacity tables from the manufacturer catalogue are used to calibrate simplified approaches based on the classical theory of heat exchangers, namely the effectiveness method. Only two reference operating cases are required to calibrate each approach. The validity of the simplified approaches is investigated for a relatively high number of operating cases, listed in the technical catalogue of a manufacturer. Four types of coils of three sizes of air handling units are considered. A comparison is conducted between the heating coil capacities provided by the methodology and the values given by the manufacturer catalogue. The results show that several of the proposed approaches are suitable component models to be integrated in dynamic simulation tools of air conditioning systems such as TRNSYS or EnergyPlus.

Five funnel solar cookers have been tested to investigate the influence of the aperture area on their performance. The largest cooker had an aperture area of 0.5 m(2) and it was tested side by side with two other two smaller cookers. Each cooker was tested with the same amount of glycerine. The linear performance curves relating the efficiency with the specific temperature difference was determined. Then, the determined regressions of the cooker opto-thermal ratio and the reference time on the aperture area were used to predict: i) the influence of the solar irradiance and the aperture area on the maximum temperature achieved by the load, ii) the time duration required for achieving load temperature from 65 to 140 degrees C, and iii) the power of the cooker. It was found that for a solar irradiance range of 600-1100 W m (2), the pasteurization temperature can be achieved even by the smallest cooker, and the efficiency of the largest cooker is close to the efficiency of a cooker with optimum aperture area. Moreover, when using the largest cooker, under an irradiance of 1100 W m (2) and ambient temperature 20 degrees C, the load can achieve 180 degrees C, implying that frying is possible. info:eu-repo/semantics/publishedVersion

Several solar cooker designs have been proposed over the last decades. The funnel cooker is a well-known model, representative of the category of panel-type solar cookers. From the empirical experience of real practice, it is known that the use of partial loads has an impact on the performance of the cooker. However, this effect has not yet been investigated rigorously. This work aims to fill this lack. Extensive experimental work was performed to determine the effect of partial loads on a funnel cooker thermal performance. Tests were conducted on two identical funnel cookers, in Malaga, Spain, with low sun elevation. Cookers were tracked only azimuthally. Experimental protocol was based on ASAE 580.1 Standard for better replicability. Results showed that the standardised power drops by about 15% of the original value when the water volumetric load fraction drops by 25%, for both cooker operations, with and without glass enclosure. This important reduction is explained on how the fill level of the cooking vessel affects its function as a thermal radiation receiver. Results from experimental tests were correlated into a simple formula of practical interest. Finally, a new cooking vessel design, that improves performance at partial loads, i.e., a 25.4% increase in cooker power, was proposed and tested Funding for open access charge: Universidad de Málaga / CBUA