Angelo Zarrella

Short time-step performances of coaxial and double U-tube borehole heat exchangers: Modeling and measurements

Several authors have investigated the long time-step performance of borehole heat exchangers. In that time scale, the borehole thermal capacitance is generally neglected, since the time span of interest is on the order of months or years. The borehole thermal capacitance consists both of grouting material and heat carrier fluid, and it mostly affects the short time-step behavior, when hourly or shorter time intervals are considered. Some models are available in the literature for short time-step simulations of borehole heat exchangers. In this article, short time-step analyses of coaxial and double U-tube heat exchangers are performed by means of numerical capacity resistance model. The capacity resistance model is compared with field measurements, analytical solutions available in the literature, and commercial software based on the finite-element method. Further comparisons between coaxial and common double U-tube heat exchangers are carried out by means of measurements performed in a suitable plant system. Furthermore, the series and parallel arrangements for double U-tube heat exchangers are investigated considering the borehole thermal capacitance.

A comparison of numerical simulation methods analyzing the performance of a ground-coupled heat pump system

Ground-coupled heat pumps are increasingly being utilized to heat and cool buildings. Although it is difficult to size and to predict their behavior and performance, their design can be optimized via simulations. EnergyPlus is a popular energy simulation program for modeling building heating and other energy flows and, since it is organized to consider borehole heat exchangers via the well-known g-functions approach, it can be used advantageously for that purpose. The Capacity Resistance Model is another recent numerical simulation tool devoted to ground and borehole heat exchangers. In this work, two methods to calculate the g-fucntions were analyzed, using as case-study a real office building, whose imbalance between the heat extracted and injected into the ground was found to be appreciable. The energy imbalance involves a ground temperature drift affecting the system efficiency. The results of the EnergyPlus g-functions and the Capacity Resistance Model model approaches were compared. The capacity of the two methodologies to accurately simulate this phenomenon were analysed also with reference to the available buildings long-term monitoring data. The analysis showed the importance of using g-functions suitable to reflect the layout of the borehole field, in order to correctly evaluate the energy performance of the entire ground source heat pump system.

ulti-Source Heat Pump Coupled with a Photovoltaic Thermal (PVT) Hybrid Solar Collectors Technology: a Case Study in Residential Application

Nowadays the heat pump technology is an efficient solution in the residential sector since it allows to reduce CO2 emissions and to exploit renewable energy sources. At the same time, the solar energy is the renewable source for excellence. In this study, different combinations of a photovoltaic thermal hybrid solar collector (PVT) and a multi-source heat pump for space heating and domestic hot water (DHW) production have been investigated. The air, solar and ground heat sources for the heat pump were analysed. The case study regards the heat demand of a single-family house building located in north-east of Italy. The considered system configurations have been investigated by means of the simulation tool TRNSYS coupled to a dedicated mathematical model for the estimation of the electrical and thermal performances of solar panels. This model is based on the equivalent electrical circuit. As a result, all the investigated multi-source systems reported an increase of energy efficiency between 14% and 26% compared to a standard air to water heat pump system. The system with air and solar sources has an energy efficiency equal to 3.64, slightly lower than that obtained for the more complex systems.