RICERCA ITALIANA     

Thermo-fluidodynamic analysis of adaptative systems for comfort and energy saving

Università degli Studi di Salerno

Abstract

The respect of the Kyoto protocol related to the reduction of both the greenhouse gases emissions and the consumption of fossil fuels, and the awareness that the HVAC plants dramatically contribute to both the atmospheric and the indoor pollution, result in a notable development of the research in the building energy saving in the last decade. Particularly, great efforts have been made concerning the performances improvement of both plants and the building components.
This kind of research has produced valuable important results and other studies may be performed in such direction. However a significant development in the next future should be obtained in studying, proposing and optimising the use of the active building envelope components and their integration with the HVAC systems.
The adaptive components are those building components which, closely connected with the HVAC devices, can contribute in reaching a good compromise between the energy performances and the indoor microclimate comfort conditions, reacting in a dynamic way to the boundary conditions and taking advantage of the climatic resources.
In these way the building components themselves (structures, floors, internal walls, etc.) are logically and rationally integrated with the building services coupled with the HVAC system in order to contribute to heating, cooling and ventilation functions.
The adaptability of the component, depending on the case, can be automatically checked by the HVAC system management according to the indoor/outdoor environmental parameters, or it can manually managed by the user. Such last question is very interesting since the autonomous managing of the HVAC system results very pleasant to the users.
This research deals with the behaviour of the window-system integrated with the plant, the dynamically insulated multiple skin façade systems, the double skin façades (DSF) characterised by two transparent surfaces separated by a cavity in which an air natural or forced flow and the solar screen are present.
The targets of the work are many. Among them we highlight the definition of procedures allowing the assessment of the real performances and potentiality of some adaptive components in terms of environmental control and obtainable energy saving; the optimisation of existing adaptive components and the proposal of novel solutions; the development of experimental and numerical analysis methods able to predict the both the behaviour and the performances of the adaptive building components and the integrated systems; the compiling of new guidelines for the designers allowing an optimal choice of the integration ways between each component and setting up of new regulation and control strategies.
This research involves 5 operational unities (O.U.): by the Universities of Cassino and Salerno, the Polytechnics of Milan and Turin and the IUAV of Venice. Each structure is characterised by an "ancient" and documented experience in its research area, dealing with the building envelope thermal fluid dynamics, thermal fluid dynamics measurements and with the microclimate of the indoor environment. <<<

Principal Investigator

Francesca Romana D'AMBROSIO Università degli Studi di SALERNO

Research Objectives

The use of adaptive components as well as the integrated systems is not currently wide spread apart of few building and , usually, on demonstrative projects. As far as the three components object of this research concerns, this it is absolutely true for the green roofs and for the window-plant integrated systems. The situation is a little bit it more favourable for the transparent double skin ventilated façades often used for their aesthetical aspect rather than for their performances in the last years, wrongly criticized.
It is certain that some aspect of these components has to be improved: their thermal-fluid dynamic behaviour; their influence on the acoustic, thermal and visual comfort; the difficulties for the designer induced on the ventilation matter. Nevertheless the use of the adaptive components represents a reasonable compromise between opposite need in terms of performances. As a matter of fact, the maximization of the free contribution of energy with a high thermal insulation of the building during the heating season should be simultaneously obtained with the minimisation of the solar contributions and the possibility to dissipate to the outside the endogenous loads during the summer.
According to such situations the aims pursued by this research work will be:
1. developing of both experimental a numerical analysis methods for the behaviour exhibited by adaptive building components as the double skin façades, the window-plant systems and the green roofs;
2. using the procedures quoted at the above point for the assessment of the real performances exhibited by some adaptive component in terms of environmental control and energy saving obtainable (such aspect through both numerical simulations and laboratory measures and/or monitoring in situ will be studied);
3. optimising the current adaptive components and/or proposing novel solutions;
4. recognising the optimal strategies for the integration and the coordinate management of the adaptive components and microclimatic control plants;
5. editing of the new guidelines for the designers allowing an optimal choice of the integration ways between each component and setting up of new regulation and control strategies.
6. the definition of synthetic performance and comfort parameters allowing a right pre-planning or an adequate design by means of traditional calculation tools.
7. the definition of usability and potentiality charts for components and systems with reference to the restrictions imposed by the respect of the conditions of comfort and energy saving. <<<

Timescale

24 months

National and international background

INTRODUCTION
In the last years the demand of a better life quality in terms of built environment microclimate is notably increased. Such request substantially disagrees with the preservation of the external environment in observance of the Kyoto protocol. As a matter of fact, the microclimate improvement involves the use of HVAC systems for long periods of the year, dramatically increasing both the issue of greenhouse gases and the electric consumptions. With regard to such question, we have to keep in mind what happened during the past summer season, when the electrical power peaks, mainly related to a very high number of working HVAC systems, caused the crisis of the national electric network.
Therefore, if we take into account that around a third of the total consumptions of primary energy in the industrialized countries is related to the building HVAC systems, it is easy to conclude that the research of solutions and technologies allowing a high indoor microclimate quality level without any traditional HVAC system, is the only way to follow for improving both working and life conditions in the indoor environment protecting the external one.

ADAPTIVE COMPONENTS
In the last decade an important development of the research in the field of building physics, especially concerning the performances increasing of both building components and HVAC system elements was carried out.
This kind of research has produced valuable important results and other studies may be performed in such direction. However a significant development in the next future should be obtained in studying, proposing and optimising the use of the active building envelope components and their integration with the HVAC systems.
The adaptive components are those building components which, closely connected with the HVAC devices, can contribute in reaching a good compromise between the energy performances and the indoor microclimate comfort conditions, reacting in a dynamic way to the boundary conditions and taking advantage of the climatic resources.
In these way the building components themselves (structures, floors, external walls, internal walls, etc.) are logically and rationally coupled with the HVAC system in order to contribute to heating, cooling and ventilation functions.
The adaptability of the component, depending on the case, can be automatically checked by the HVAC system management according to the indoor/outdoor environmental parameters, or it can manually managed by the user. Such last question is very interesting since the autonomous managing of the HVAC system results very pleasant to the users.
The coupling between adaptive components and HVAC systems may be:
• functional - in such cases the adaptive component is not physically connected to the HVAC system but, thanks to its action, the thermal and pollution loads may be reduced and/or the indoor thermal comfort and IAQ can be increased. Examples of such components are green roofs, active thermal mass elements, Phase Change Material (PCM), ...
• Integrated (hardware) - in such cases the adaptive component is itself an element of the HVAC system. Examples of such components are ventilated facades, cooling ceilings, air ventilation systems embedded in the building structures.
The importance of such question is clearly expressed by the IEA (International Energy Agency) document of strategic planning for the triennium 2003-2006 which suggests as R&D strategies for Building Products: "Improve thermal performance of building envelopes to minimise cooling and heating loads, and integrated solutions with other building components" and "Improve the performance and energy-efficiency of HVAC and lighting systems, and optimise integrated solutions". Moreover in the IEA area a special international research group was established.
The advantages achievable from studies on the use of active components and on the integrated systems would be notable for our Country, where the need of a good climatic control and the reduction of the energetic consumption during the winter, often resulted in both the dissatisfaction of the energetic optimisation criteria and in the thermal discomfort during the summer.

THE "WHOLE BUILDING APPROACH"
As clearly proved, the right design of the building-plant system results rather complex, as it requires a reasonable compromise between opposite needs. As a matter of fact, the maximization of the free contribution of energy with a high thermal insulation of the building during the heating season should be simultaneously obtained with the minimisation of the solar contributions and the possibility to dissipate to the outside the endogenous loads during the summer. It is also evident that such results could be reached through a "whole building" approach by means of the use of building adaptive elements allowing the passive control of the thermal stress (partial at least), reducing therefore the use of the HVAC plant and allowing an energy saving keeping the indoor environmental quality.

DOUBLE SKIN FACADES
With the aim to find energy efficient, comfortable and visual attractive facades, dynamically insulated multiple skin façade systems are currently under investigation. Among them today the most popular with architects and investors are the glassed double façade system, so called "double skin façade".
In the early 1990s, much of the literature dealing with such typology, was extremely positive with no quantitative proof given for performance claims. Only in a second phase, more critical reviews have been published revealing increased dissatisfaction with DSFs and countering the starting euphoric descriptions. Several problems has been observed. During significant portions of the year, one cannot achieve a comfortable indoor climate with natural ventilation through DSFs, and DSF buildings without active cooling fails with an increase of the costs. Moreover, the DSFs numerical simulations available in the literature are often useless because of the boundary conditions. The fluid dynamics is also not clear because not always the airflow is strictly upward/downward.
As far as the comfort concerns, the presence of ventilation gaps, as the communication between the floors related to the airy cavity, leads to acoustic problems; the air temperature in the gap can create significant thermal discomfort and force closure of interior windows designed to allow natural ventilation.
On the other hand, the advantages related to the employment of such façades in terms of energetic saving, acoustic performances with respect to the urban noise, the enhancement of the thermal comfort and the utilisation of the natural light, are not enough to justify the use of the DSFs up to today designed: a big effort of the research on the optimization of this type of component from both the technological and the design point of view is therefore required.
Another very important side is that the DFSs characterisation should not to be based on the traditional performance parameters (thermal transmittance, solar factor) and on simplified calculation models (as technical regulations suggest). Moreover, if we take into account that the two skin façade is often integrated with the HVAC system, the building energy efficiency has to be assessed for the whole system. Consequently, in order to make deeper the question, both experimental and numerical research activity are required. By this way, reliable protocols for the measure and the evaluation of the real performances of the system, efficient calculation tools for simulating the system behaviour in different climatic situations and the promulgation of technical regulations will be pursued.

EXPERIMENTAL EVALUATION AND NUMERICAL MODELLING OF THE THERMOPHYSICAL PROPERTIES
The main problem related to the experimental study of the thermophysical properties and the fluid-dynamic behaviour of the adaptive components, is mainly due to the strong variability of the characteristics and the operational conditions. This occurrence several times needs to carry out the analysis in transient conditions. Moreover the physic phenomena occurring in the adaptive components and especially in the integrated systems, by the simultaneous presence of the three mechanisms of heat exchange and sometimes changing of phase are characterised. As a consequence, the experimental techniques and several numerical approach traditionally used, result inadequate to the specific demands requiring for alternative solutions.
Particularly, the thermal analysis trough contact temperature technique can be carried out. This is a real heat transfer problem, since the presence of the probe affects the heat transfer mechanisms often resulting in coarse measurement mistakes. Another way very useful for in-situ analyses may be the use of thermo-graphic techniques, however requiring an especial care since the significant surface emissivity variations could affect the results. Moreover, the implementation of thermophysical properties measurement in the presence of dynamic boundary conditions, needs novel processing and data analysis techniques whose reliability and precision have to be assessed and validated.
Finally it should be not forget the need to continuously measure for long time intervals the latent thermal flows and mass exchanges between the component and the plant or the component and the outdoor environment.

MICROCLIMATIC ASPECTS OF THE PROBLEM
The use of adaptive components as double skin facades and green roofs can also affect the microclimate, as the thermal environment and the indoor air quality ensemble. Such question is related to the life quality enhancement and it affects the productivity of the occupants also. At the right time we have to remind that a poor quality of the microclimate in the building is often revealed by the "Sick Building Syndrome" (SBS).
From this point of view, the verification of the microclimatic conditions in the environments provided with adaptive components, undoubtedly represents a good functionality check tool with respect to the reaching of the optimal microclimatic conditions.
Since 70's the research in the field of the thermal environment was focused on the environments provided with HVAC systems, and for such situations the well known PMV index was validated with the introduction of the local discomfort indexes inspiring the current regulations.
From some year people is realising that the PMV index is not representative of the thermo-hygro-metrical conditions in the environments not air-conditioned, especially in the not ventilated ones. Therefore the research effort is mainly focused in identifying new correction factors usable in such conditions. Moreover in these environments a larger percentage of dissatisfies (PPD) could be accepted since the psychological aspects related to the possibility of the occupants of "managing the thermal environment" (i.e. opening or closing the windows) strongly affects the thermal comfort. We have to keep in mind that ASHRAE adopted a similar kind of approach, but such model shows a series of restriction such as it is applicable only to the case where the main way to adjust the thermal conditions is the opening and the closing of the windows. It is also clear that further researches should be carried out in order to obtain a model with a larger scale of applicability and, above all, effective in such situations that the thermo-hygrometrical conditions are brought by the presence of building components integrated with the plant.
Another side of the matter concerns the indoor air quality (IAQ), which is strictly brought with the need to assure an adequate change of air in the absence of a mechanical ventilation plant.
If we think that, up to now, an index of air quality accepted by everybody and an IAQ measurement protocol internationally acknowledged do not exist, it is clear how much essential is the research development in this area, especially for novel situations as the application of adaptive components. <<<