Energy and exergy analysis of the transient performance of a qanat-source heat pump using TRNSYS-MATLAB co-simulator

In this study, the transient performance of a qanat source heat pump is investigated using a TRNSYS-MATLAB co-simulator. The water/ethylene glycol-to-air compression heat pump and the helical coil heat exchanger, which is used to inject heat to or to extract heat from the qanat water, are mathematically modeled in matrix laboratory (MATLAB), and then, coupled to transient systems simulation (TRNSYS) model to evaluate the system transient performance and calculate the heating and cooling loads of the case study building. Comparison of the performance of the qanat source heat pump with an air source heat pump showed that the coefficient of performance of the qanat source heat pump is at least 5% and at most 34% higher than that of the air source heat pump. By increasing the flow rate of the working fluid in the helical coil heat exchanger from 2 L/min to 8 L/min, the coefficient of performance of the qanat source heat pump increases at least 12% and at most 34.1%. The maximum increase in energy efficiency ratio and free energy ratio of the system by the similar increase in the flow rate is 46.4% and 24.8%, respectively. The exergy analysis of the qanat source heat pump reveals that the minimum and maximum exergy efficiency of the system is 32% and 85.5%, respectively. The findings also indicate that the most exergy destruction occurs in the condenser in heating mode and in the evaporator in cooling mode.

Diagnostics of a Domestic Hot Water Storage Tank under Operating Conditions

This paper presents a new method for the diagnostics of a hot water storage tank under operating conditions. Depending on the operating point of the tank, the method enables determination of thermal conductivity coefficients of the coil heat exchanger, which allows us to determine the intensity of heat transfer between the transfer medium and water in the tank as well as of tank walls, which consequently enables determination of heat losses to the environment. Furthermore, the dynamic properties of the tank may also be determined by applying this method. The advantage of this method is possibility of analyzing changes in the material constants of the coil heat exchanger, tank walls, and dynamic properties of the tank as a function of mass flow of the medium supplying the coil heat exchanger. The possibility of determining coefficients of thermal conductivity as well as the inertia of tank and exchanger, based on temperature measurements acquired in operating conditions is a novelty in this paper. Knowing the variability of material constants and of dynamic properties of the tank as a function of medium flow rate allows multicriteria optimization to be performed which, with a conventional design of the tank, results in a reduction of up to 10% in the time taken to prepare domestic hot water.

Modeling the Temperature Field in the Ground with an Installed Slinky-Coil Heat Exchanger

In order to find the temperature field in the ground with a heat exchanger, it is necessary to determine temperature responses of the ground caused by heat sources and the influence of the environment. To determine the latter, a new model of heat transfer in the ground under natural conditions was developed. The heat flux of the evaporation of moisture from the ground was described by the relationship taking into account the annual amount of rainfall. The analytical solution for the equations of this model is presented. Under the conditions for which the calculations were performed, the following data were obtained: the average ground surface temperature Tsm = 10.67 °C, the ground surface temperature amplitude As = 13.88 K, and the phase angle Ps = 0.202 rad. This method makes it possible to easily determine the undisturbed ground temperature at any depth and at any time. This solution was used to find the temperature field in the ground with an installed slinky-coil heat exchanger that consisted of 63 coils. The results of calculations according to the presented model were compared with the results of measurements from the literature. The 3D model for the ground with an installed heat exchanger enables the analysis of the influence of miscellaneous parameters of the process of extracting or supplying heat from/to the ground on its temperature field.