Elsevier

Energy and Buildings

Volume 50, July 2012, Pages 111-119
Energy and Buildings

Characterization and assessment of cool coloured solar protection devices for Mediterranean residential buildings application

https://doi.org/10.1016/j.enbuild.2012.03.031Get rights and content

Abstract

Cooling down buildings and cities is becoming a priority as a consequence of global increase of air temperatures. This effect determines the growing of energy consumption for cooling, as well as the spread of urban heat island effect. Cool materials are characterised by high solar reflectance and high infrared emissivity. These two properties allow the material to stay cool under the solar radiation. Cool materials are deserving attention to mitigate surface temperatures in buildings and urban materials. This paper presents the first results of the potential benefits of cool materials applied on solar protection devices. Shutters are used for security and for solar shading in Mediterranean countries. These components are generally characterised by high solar absorbance, due to the colours traditionally used. An experimental campaign was carried out to measure and compare the optical and thermal properties of conventional and cool coloured materials. The results were used as input to assess the impact of these products in Italian dwelling, taking into account several variables: climatic zone, insulation level, and operative conditions. The cooling energy demand and the thermal comfort conditions were calculated with an hourly heat balance building simulation tool.

Highlights

► Spectral and angular reflectances were measured. ► Solar reflectance increased up to 4 times for cool coloured samples. ► Near infrared reflectance increased up to 10 times for cool coloured samples. ► Energy savings up to 10% were calculated.

Introduction

The increase of the city air temperatures is a phenomenon associated to climate changes and emphasised by the urban sprawl, with consequent increase of the air temperature in the built environment. In this context, cool materials for building and urban applications are becoming an attractive technique. Several studies carried out during the past years highlighted the impact of the urban heat island phenomenon on energy, environment and quality of life [1], [2], [3], [4], [5], [6], [7]. An impressive increase of energy cooling demand for buildings is also experienced. This trend is monitored everywhere but the magnitude is latitude dependent, Italy and the whole Mediterranean basin in fact are hazardous areas [8]. The term cool material refers to a construction material characterised by two thermo-physical surface properties: (a) high solar reflectance (ρe), which expresses the ability of a material surface to reflect the incident solar radiation; (b) high thermal emissivity (ɛ), defined as the ability of a surface to release away the absorbed heat. While all the construction materials, except the metal based ones, are characterised by high emissivity, they are also characterised by high solar absorptance getting hot under the solar radiation. Cool materials remain cool under the sun at daytime and radiate away the stored heat during nigh time. Many examples of vernacular architecture give evidence of how light colours were used to reduce the thermal load in buildings during the hot season in the past. White villages and small town spread in the Mediterranean area provide some shining examples still nowadays.

Cool roofs use cool materials to reduce the solar heating loads in buildings. Several studies demonstrated through calculation the positive impact in terms of increased thermal comfort and reduced cooling loads in buildings, with different uses and under different climatic conditions [9], [10], [11], [12], [13], [14]. Other studies present similar results referring to real building applications [15], [16], [17], [18], [19]. Being high reflectance an advantage in summer but a penalty during the heating season, the above researches highlight potentialities and limits of this technology. Recent studies carried out in urban areas demonstrate the cool materials potentialities in mitigating the air and the surface temperatures, increasing comfort and health conditions for human beings. It has to be noted that cool roofs reduce the heat going through the building envelope but they also improve the efficiency of energy systems, as demonstrated in [20], [21] for ducts and air-conditioner condensers, placed on the building rooftop. Other applications of cool materials are roofs equipped with cylindrical PV modules, where the latter can benefit of the solar radiation coming from the sky as well as of the component reflected from the roof.

Cool coloured materials are a particular and innovative category of cool materials. They have a spectral response in the visible range able to reproduce the design colour but present a very high reflectance in the near infrared range (NIR). This solution gives dark construction materials a better performance under the solar radiation, with solar reflectance values higher than conventional materials. Several studies were carried out during the past years to develop these products, mainly based on near infrared reflective pigments [22], [23], [24], [25].

This study focuses on the characterization and assessment of cool coloured materials for application on window shutters. Shutters are widely used in Mediterranean countries as security and anti-intrusion system, as well as solar protection devices for residential buildings. Traditionally wooden made, the current trend is to manufacture them in painted aluminium. Reasons are: improved durability, costs reduction, tilt adjustment of the lamellae, if provided, to modulate light and solar radiation transmission in the room. One thing remains unchanged through the time, the application of dark colours for a suitable architectural integration: brown, dark green, red, black. The surface temperature of aluminium elements covered with dark colours can reach high values during summer with energy problems in cooled buildings and thermal discomfort hazards in not cooled buildings.

The attempt to develop cool coloured materials for such application is of interest in Italy, where unofficial statistics estimate that more than three millions of dwellings are used for tourism only, representing more than 10% of the total residential building stock. These buildings are mainly used in summer, thus they have negligible energy consumption for heating but equipped with domestic air-conditioners. If also other touristic non-residential buildings are taken into account, the optimisation of the energy performance of a consistent portion of the national building can be obtained with cooling efficient technologies without risks of heating penalties.

This paper presents a detailed spectral optical and thermal characterization of powder-based cool paints and the potentiality of the technology for energy efficient applications in residential buildings in different Italian climatic conditions.

Section snippets

Product characteristics

The energy flux globally transmitted by a glazing unit eventually equipped with solar protection devices is expressed by the g-value, which is the sum of two components:

  • (a)

    the direct solar transmission, τe;

  • (b)

    the secondary heat transfer, q′, which expresses the amount of solar radiation absorbed by the assembled component and re-emitted at longer infrared wavelengths towards the built environment.

Several international standards describe calculation procedures, with different levels of accuracy, to

Optical and thermal characterization

The solar reflectance and absorptance (being the latter complement to 1 of the former) are the relevant properties driving the solar gains through opaque building materials. These values are calculated starting from spectral measurements, carried out with a commercial dual-beam Perking Elmer Lambda 950 spectrophotometer with automatic detection. The instrument used for this experimental campaign is equipped with a 15 cm integrating sphere coated with Spectralon, a special high reflective

Numerical analysis

This section of the study is aimed at evaluating the impact of aluminium shutters in the windows of buildings after the thermo-physical conditions of the shutters were calculated and measured. The numerical analysis is performed as a function of several variables, all important for the energy performance of buildings. The calculations were carried out for the cooling season and performed according to the profiles of use of buildings:

  • Calculation of net energy demand for a cooled building. In

Conclusions

The study demonstrates that cool materials can be used for shutters and solar protection devices, increasing the solar reflectance values without penalizing chromatic solutions, typical of the Italian architectural tradition. The optical measurement campaign shows that significant results can be achieved for some products, while more improvements are needed for the wood-alike essences.

It is also important noting that, according to the spectral curves, it is possible optimising the effectiveness

Acknowledgement

The authors warmly thank Bruno Buono for his continuous support for the product specifications and sample preparation.

References (41)

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