The energetic characterization of solar-control environments

Highlights Abstract The energy consumption associated with maintaining thermal comfort in buildings remains a significant and partially unjustified share, accounting for 30-40% of total energy consumption. The windowed components, often with thermal insulation performances even higher than the minimum requirements, still lack design and characterization in terms of the incident solar radiation control, a particularly delicate topic for achieving indoor wellness conditions and more generally for a higher energy efficiency in buildings. Despite the increasingly pressing regulations in the energy sector (EPBD recast, law 90/2013 and the consequent decree 162 of 2015), which push towards policies and financial measures to promote buildings with almost zero energy, the quality of the internal environment remains one of the critical parameters on which to pay more attention in view of a consistent evaluation of the Nzeb building. The authors show, in the present paper, the results related to the energetic characterization of two test rooms, identical for surface and opaque envelope, but with different typologies of windows on the South-East and South-West wall.

Applying a sufficient electric voltage to the polymer layer, via the transparent conducting films, makes the particles align and become parallel to the electric field thereby yielding a higher transmittance. In the ''off'' state, when no voltage is applied, the particles are randomly dispersed and therefore absorb light and create a dark appearance; conversely, in the ''on'' state the particles align and the character of the glass changes from dark to clear. Nowadays energy efficiency and lifetime not yet proven. Liquid Crystal Windows are switchable glass panes with a liquid crystal layer changing light transmission properties in order to control light and heat intake. The use of switchable dyes, hosted in liquid crystals, allow more or less radiation to permeate, depending on their alignment, making the window appear darker or brighter. Nowadays mainly used for privacy control, but not yet recognized as an energy savings device or proven to have wide-angle, low glare properties. Chromic materials are classified into four types: gasochromic, photochromic, thermochromic (TC) and electrochromic (EC). In gasochromic devices a hydrogen gas (H2) is applied to switch a thin layer of tungsten oxide (WO3), coveredby a very thin layer of platinum between colored and bleached states. This process can be reversed by introducing diluted oxygen. The hydrogen and oxygen are produced by an electrolyser. These are generally cheaper and simpler devices than the other chromic ones, not requiring the ion conductor and storage layers. Although, gasochromic devices exhibit some merits such as better transmittance modulation, lower required voltage, staying lucid in the swap period, and adjustability of any middle state between transparent and entirely opaque; only a few numbers of EC materials can be darkened by hydrogen.
Furthermore, strict control of the gas exchange process is another issue. The best transmittance values obtained for a coated double-glazed unit, with a moderate film thickness, and hydrogen concentrations below the combustion limit are 76% and 77% for solar and visual transmittance, respectively, in the bleached state and 5% and 6% for solar and visual transmittance, respectively, in the coloured state. Darker states can be obtained by applying thicker films associati all'utilizzo di vetrate elettrocromiche su edifici in clima mediterraneo.
Photochromic materials change their transparency in response to light intensity.
They found success in eyeglass that change from clear in the dim indoor light to dark in the bright outdoors. Since photochromic materials are responsive to light intensity, but remain unchanged with temperature changes, windows made from these materials darken when exposed to light irrespective of the temperature level outside. The promising approach is the development of hybrid systems that integrate some type of active smart windows technology with photochromic materials to address the problem of automatic darkening during cold, sunny days [13]. Thermochromic (TC) materials change color in response to temperature variations. The TC interlayer thin film (within 0,3-1 mm) is extruded through the draw plate onto unadhereable underlying material or directly on the glass. As the temperature becomes higher than the transition point, the TC material changes its nature from monoclinic (behave as semiconductors, less reflective especially in near-IR radiation) to rutile state, behaving like a semi-metal and reflecting a wide range of solar radiation.
Most of heat gain in solar spectrum takes place at NIR range (800-1200 nm), therefore the more direct and intense the sunlight is on the glass the darker it will become. When the polymeric interlayer is doped with complexes of transition metals (Fe, Cu, Cr, Co etc.), a reversible change of light transmission (LT) and/ or colour occurs. Since the emissivity of the coatings are high in both monoclinic and rutile states, this technology does not work well in cooler climates currently [2]. The central part of an electrochromic device is a five layers coating applied to the glass pane: an electron accumulation layer, an ion conductor layer (usually LiAlF4), an electrode layer (usually tungsten trioxide WO3), and two outer layers made of transparent conductive oxides.
When voltage is applied, Li + ions pass from the accumulation layer to the electrode determining a change in color from transparent (SHG and LT about 0,49 and 69%) to dark (SHG and LT about 0,09 and 1%) in the electrode layer (cathodic coloration), or in the accumulation layer (anodic coloration) or in both according to the electrochromic materials employed. The process is reversible by turning off the electrical stimulus that triggers the return of ions from the electrode to the accumulation layer. Energy required to switch between the different control states is not greater than 3 Wp/m 2 and even less (<0,4 W/m 2 ) is the one needed to maintain a desired tinted state (energy is required only for transition) [5]. About, EC windows require less energy for lighting than TC ones and both demand the lowest cooling energy, if compared to clear, tinted or reflective glass [13]. However, the necessity of wiring in EC glazing and the better ability of TC windows to maintain the visible transmission (when doped properly), besides their simple structure, make TC windows economically more competitive.
In [11,14] [15,16]. As the switching time is several minutes, a high time constant should be used to control the EC glass, in order to avoid frequent changes.

METHODOLOGY
The two test environments, identical in size and type of envelope, are located on the terrace of the DiCAAR Department (figure 1a) at 20 meters from each other, with gross dimensions 4,15x4,14x3,19 m. They are two wooden buildings realized with a balloon frame system, pillars and joists with reduced section, arranged at close intervals and with continuous uprights from the floor to the roof. The SE wall includes the entrance door with glazed area equal to 0,67x1,15 m; the SO wall (figura 1c) includes six windows, two with dimensions 1,42x0,94 m located in the centre, and four, of 0,60x0,95 m, on the sides. The only difference between the two buildings is precisely in these glazed elements: traditional high thermal performance window frames (7 mm external plate, 12 mm argon, 4 mm internal plate with low emission treatment) in the building further south (from here on we will call it "LE" building) and the electrochromic glass, above described, in the building further north (from here on we will call it "EC" building).  [5]. A riguardo si osservi che le finestre EC richiedono meno energia per l'illuminazione rispetto a quelle TC, mentre entrambe richiedono una minore energia di raffreddamento, se confrontate con un vetro trasparente, colorato o riflettente [13]. Per contro, la necessità di cablaggio nella vetratura EC e la migliore capacità delle finestre TC di mantenere la trasmissione visibile (se opportunamente drogata), oltre alla loro struttura semplice, rendono le finestre TC economicamente più competitive. In [11,14]  The electrochromic glazing, used in this research, consist of two glass plates separated by a 12 mm full space of argon. The outer plate is composed of a 4 mm tempered panel, a 0,9 mm ionoplastic interlayer and a 2,1 mm annealed glass sheet on which inner surface the electrochromic layers are deposited. The internal plate consists of a simple 6 mm tempered panel. The manufacturer provides the following performance characteristics, respectively for the "clear" and "fully tinted" states: solar factor g = 48% and 10%; light transmittance factor TL = 62% and 2%; thermal transmittance of the whole frame U = 0,28 W/m 2 K. This is not a last-generation type, which today have solar factors below 30% with a selective ratio TL/g greater than 2 (highperformance selective glass).
The stratigraphy of the vertical opaque walls and their transmittance, calculated in accordance with UNI EN ISO 6946 [17], is reported in figure 2.
The two test environments were instrumented (figure 3) for the detection of the main parameters for assessing indoor comfort, with sensors having metrological characteristics in accordance with ISO 7726 [18].
Furthermore, in order to measure surface temperatures of window elements, K-type thermocouples, suitably shielded by solar radiation, have been placed on the internal and external surfaces of the two larger windows on the SO side and of the window on the SE side. The uncertainty of measuring the surface temperature is better than ±1°C.
It should be noted that the two rooms are not equipped either with air conditioning systems, or controlled ventilation systems, therefore the measurements were finalized to compare the indoor conditions in the two di occupazione, ora del giorno e stagione [15,16] rooms, for different outdoor climatic conditions. The sampling period was equal to 15 min. The electro chroming glass are equipped with a remotecontrol system able to manage four different chromatic spectrums, ranging from light green on 4 variants (fully clear) to dark blue (fully tinted), according to the solar path. In order to verify the shielding effectiveness on the internal solar radiative load, the authors set a hourly programme to switch on (fully coloured) and off (fully clear) the SW windows and the SE ones, as shown in table 1.

RESULTS
It is evident in figure 4 that the greater internal load, from solar gain, occurs in both buildings in the afternoon, due to the glazed surface on the SW wall,    gains in the evening hours. As regard the dynamic simulation the results in figure 6 show that the use of EC glass in winter condition could cause a partial nullification of the advantages achieved during the summer. In winter, not being able to count on solar inputs in the central hours of the day corresponds to an increase of more than 80% of the relative energy need, from 15 kWh/m 3 with LE glazing to about 27 kWh/m 3 with EC glazing.
On the other hand, in the summer, the benefic effects in a Mediterranean climate are evident with a reduction in energy conditioning needs near to 90%, from about 71 kWh/m 3 with LE glazing to about 8 kWh/m 3 with EC windows.

CONCLUSIONS
With the national decree of 11th of January in 2017 the new Minimum Environmental Criteria for public buildings have been published. They provide, throughout the national territory, for renovation and/or energy requalification projects concerning the building envelope, for compliance with the minimum values of thermal transmittance. In this regard, for redevelopment of public buildings (with degree days from 900 to 1400), involving the replacement of window frames with orientation from East to West, the value of the total solar transmission factor must be not greater than 35%.
The technology to overcome the impasse in the use of shielding on historic buildings is available today, albeit at a still high cost, with the use of windows able to switch between different shielding capabilities, as widely demonstrated in this paper for electrochromic technologies. Significant progress has been made in recent years, above all on the activation times of the active layer.
Surely, the window of the future should firstly remain "a window", ensuring visibility outside and visual comfort inside, but it should be able to convey heat and light separately, through a dynamic switch between the visible and infrared spectra, possibly including shielding elements for privacy. It should remain a durable and reliable element but should become multi-functional: source of natural light (in OFF mode) or artificial (in ON mode) through built-in micro accumulation systems; a decorative element or a projection surface. The next step is therefore its transformation from a simple transparent element of the envelope, into an active element, equipped with sensors for the detection of the main internal comfort parameters, varying the transparency range (or duration) according to the seasons or weather conditions. It will probably be the moment when it will finally stop to be "transparent".