Michele De Carli

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.

Building leakage analysis and infiltration modelling for an Italian multi-family building

The presented paper aims at detailing the results of an investigation that was recently conducted in Italy to evaluate the contribution infiltration makes to meeting ventilation needs in a recently renovated apartment building and the corresponding energy costs. It is years that increasing importance has been placed on the energy efficiency in residential buildings as about 70% of the existing Italian residential building stock was built before 1976 (i.e. before any measure related to the energy efficiency in buildings). As existing dwellings have been traditionally considered ‘leaky’, the actions for improving their energy efficiency have often determined tighter buildings, raising concerns about whether the amount of infiltration is sufficient to provide occupants with acceptable indoor air quality (IAQ). The current state of knowledge on infiltration in multi-family buildings in terms of measuring procedures, corresponding air change rates and airflow patterns was reviewed. The air tightness of a three-storey, six-unit, multi-family building which can be considered representative of the existing recently renovated Italian building stock was characterized by means of a series of fan pressurization tests. The performed blower door tests are documented and results of the data analysis are reported and discussed. A simulation model was developed; simulations were performed to analyse in detail the winter magnitude of air infiltration. Winter is usually detrimental to IAQ, as severe outdoor weather prompts occupants to keep closed any opening that could allow cold drafts into their homes. Modelling results were validated on the basis of a 3-week monitoring campaign. The developed model enabled to estimate the variation with time of infiltration rates and therefore the influence of infiltrating air on the resulting heat loss and IAQ. Numerical predictions were derived using the EnergyPlus simulation tool which allowed to combine whole building thermal simulation and detailed multi-zone airflow modelling. Results show that, during the considered heating season (October–April), the average air change rate due to infiltration was approximately 0.1 h−1. It was concluded that infiltration cannot be relied upon to provide adequate ventilation air and, if not assisted by other means of ventilation, IAQ deterioration is likely to occur.

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.

A Dynamic Model for the Thermal-Hygrometric Simulation of Buildings

Abstract Dynamic models for the energy simulation of building-plant systems are becoming useful tools in the process of building design by defining operating conditions and finding appropriate control strategies. Therefore simple softwares able to correctly predict the thermal behaviour of rooms, and thus allowing to get comfort conditions and loads, are needed. In this paper a dynamic simulation model (THESIS, “THermal SImulation Software”) is presented. The building structures equations are described by means of a LTI (Linear Time Invariant) state space model. The heat conduction equations for the walls are solved through an explicit finite difference technique. The model is implemented in the MATLAB/SIMULINK environment.