Iole Nardi

Quantitative thermography for the estimation of the U-value: state of the art and a case study

Energy consumption of buildings could be significantly reduced by improving the efficiency of the envelope. Currently, the estimation of the energy performance of existing buildings requires the knowledge of the overall heat transfer coefficient (U-value) of the walls. U-values can be calculated through a theoretical approach, knowing the thermal conductivity and thickness of each material that constitutes the wall stratigraphy, from project data or coring. Alternatively, U-values can be obtained experimentally, through the ISO recommended heat flow meter measurements. Although generally accepted, the heat flow meter method suffers from some disadvantages. Recently, an alternative approach based on infrared thermography (IRT) has been proposed for in situ measurements. Main advantages of this new approach are non invasivity and the possibility of inspecting relatively large areas in real time. In this paper, after a brief description of the state of the art in the field of U-value measurement by IRT, a case study is described. In particular, the results obtained by IRT on an existing building are compared with U-values given by the standard ISO calculation and heat flow meter measurements; advantages and limitations of the new method are outlined. Some suggestions for a successful exploiting of the IRT approach are also given.

Validation of quantitative IR thermography for estimating the U-value by a hot box apparatus

Energy saving plays a key role in the reduction of energy consumption and carbon emission, and therefore it is essential for reaching the goal of the 20-20-2020 policy.In particular, buildings are responsible of about 30% of the total amount of Europe energy consumption; the increase of their energy efficiency with the reduction of the thermal transmittance of the envelope is a point of strength with the actions and strategies of the policy makers.Currently, the study of energy performance of buildings is based on international standards, in particular the Italian one allows to calculate the U-value according the ISO 6946 or by in-situ measurements, using a heat flow meter (HFM), following recommendations provided in ISO 9869.In the last few years, a new technique, based on Infrared Thermography (IRT) (also referred to as Infrared Thermovision Technique - ITT), has been proposed for in situ determination of the thermal transmittance of opaque building elements. Some case studies have been reported.This method has already been applied on existing buildings, providing reliable results, but also revealing some weaknesses.In order to overcome such weak points and to assess a systematic procedure for the application of IRT, a validation of the method has been performed in a monitored environment.Infrared camera, the heat flow meter sensors and a nearby meteorological station have been used for thermal transmittance measurement. Comparison between the U-values measured in a hot box with IRT as well as values calculated following international standards and HFM results has been effected. Results give a good description of the advantages, as well as of the open problems, of IR Thermography for estimating the U-value.Further studies will help to refine the technique, and to identify the best operative conditions.

Development of a low-cost temperature data monitoring. An upgrade for hot box apparatus

The monitoring phase has gained a fundamental role in the energy efficiency evaluation of a system. Number and typology of the probes depend on the physical quantity to be monitored, and on the size and complexity of the system. Moreover, a measurement equipment should be designed to allow the employment of probes different for number and measured physical quantities. For this reason, a scalable equipment represents a good way for easily carrying out a system monitoring. Proprietary software and high costs characterize instruments of current use, thus limiting the possibilities to realize customized monitoring. In this paper, a temperature measuring instrument, conceived, designed, and realized for real time applications, is presented. The proposed system is based on digital thermometers and on open-source code. A remarkable feature of the instrument is the possibility of acquiring data from a high and variable number of probes (order of hundred), assuring flexibility of the software, since it can be programmed, and low-cost of the hardware components. The contemporary use of multiple temperature probes suggested to apply this instrument for a hot box apparatus, although the software can be set for recording different physical quantities. A hot box compliant with standard EN ISO 8990 should be equipped with several temperature probes to investigate heat exchanges of a specimen wall and thermal field of the chambers. In this work, preliminary tests have been carried out focusing only on the evaluation of the prototypal systems performance. The tests were realized by comparing different sensors, such as thermocouples and resistance thermometers, traditionally employed in hot box experiments. A preliminary test was realized imposing a dynamic condition with a thermoelectric Peltier cell. Data obtained by digital thermometers DS18B20, compared with the ones of Pt100 probes, show a good correlation. Based on these encouraging results, a further test was carried out in hot box, comparing the data measured by digital thermometers, Pt100 and T-type thermocouples. In this case also, the analyses show a good correlation between either digital thermometers and analog sensors. From these results, it is reasonable to foresee that this measuring instrument could help those willing to realize or refurbish a hot box apparatus, and those who want to undertake temperature monitoring.

Energetic performance analysis of a commercial water-based photovoltaic thermal system (PV/T) under summer conditions

In the last years, the importance of integrating the production of electricity with the production of sanitary hot water led to the development of new solutions, i.e. PV/T systems. It is well known that hybrid photovoltaic-thermal systems, able to produce electricity and thermal energy at the same time with better energetic performance in comparison with two separate systems, present many advantages for application in a residential building. A PV/T is constituted generally by a common PV panel with a metallic pipe, in which fluid flows. Pipe accomplishes two roles: it absorbs the heat from the PV panel, thus increasing, or at least maintaining its efficiency; furthermore, it stores the heat for sanitary uses. In this work, the thermal and electrical efficiencies of a commercial PV/T panel have been evaluated during the summer season in different days, to assess the effect of environmental conditions on the system total efficiency. Moreover, infrared thermographic diagnosis in real time has been effected during the operating mode in two conditions: with cooling and without cooling; cooling was obtained by natural flowing water. This analysis gave information about the impact of a non-uniform temperature distribution on the thermal and electrical performance. Furthermore, measurements have been performed in two different operating modes: 1) production of solely electrical energy and 2) simultaneous production of thermal and electrical energy. Finally, total efficiency is largely increased by using a simple solar concentrator nearby the panel.