José Gonzálvez-Maciá

Experimental study of a heat pump with high subcooling in the condenser for sanitary hot water production

The use of heat pumps in order to produce sanitary hot water have been demonstrated as a very efficient alternative to traditional boilers. Nevertheless, the high water temperature lift (usually from 10°C to 60°C) involved in this application has conditioned the type of used solutions. In order to overcome it, transcritical cycles have been considered as the most suitable solution. The current article analyzes a new heat pump prototype able to enhance the heat pump efficiency using a subcritical cycle. The proposed prototype is able to control the system subcooling and make it capable to work at different subcoolings in the condenser. That kind of mechanism has demonstrated its capability to increase the efficiency of the heat pump. The obtained results have shown that coefficient of performance depends strongly on subcooling. In nominal condition (inlet/outlet water temperature at evaporator is 20°C/15°C and the water inlet/outlet temperature in the heat sink is 10°C and 60°C), the optimal subcooling is 42 K with a heating coefficient of performance of 5.35, which is about 25% higher than the same cycle working without subcooling.

A comparative study between a two-dimensional numerical minichannel evaporator model and a classical effectiveness–NTU approach under different dehumidifying conditions

In this article, a two-dimensional numerical model for a minichannel evaporator is implemented. This model takes into account the variation of wall temperature and moist air properties in both longitudinal and transverse directions. The verification of the current model is done with an analytical effectiveness–number of transfer units (ϵ–NTU) approach, and the results of the two approaches show very good agreement and consistency. Different refrigeration and air-conditioning applications have been chosen that represent various inlet conditions to the evaporator. A range of tube temperatures has also been selected to allow different dehumidifying scenarios for the tube and fin. A comparative study is conducted between the current model results and the results of the traditional ϵ–NTU method based on two different models of simultaneous heat and mass transfer. Significant deviations in results between the current model and the traditional ϵ–NTU approach are found, especially in latent heat transfer. These deviations are mainly due to the assumptions that are normally adopted by the ϵ–NTU method and fin theory, such as no variation in moist air temperature and humidity ratio along the direction between tubes, no accounting for partially wet fin conditions, and finally, the assumption of a constant average saturation line slope within a specific evaporator segment.

Error estimation of single phase effectiveness and LMTD methodologies when applied to heat exchangers with phase change

Single phase formulas based on logarithmic mean temperature difference (LMTD) or Effectiveness (ɛ-NTU) are widely and wrongly used for the thermal analysis of evaporators and condensers. Those formulas do not take into account that temperature variation during phase change is due to pressure variation and/or concentration changes when using nonazeotropic refrigerant mixtures. This paper first presents the correct evaluation of the mean temperature difference and effectiveness, for parallel and counter flow arrangements, under the hypothesis of linear temperature variation, for both evaporator and condenser cases. Then, the analytical solution is employed to evaluate the error of applying the single phase formulas of LMTD and Effectiveness to the phase change part of evaporators and condensers.