The achievement of carbon neutrality requires the reduction of the impact of the building sector, the most energy consuming sector. Buildings, in fact, are responsible for almost 40% of the total CO2 emissions of European countries [1], whose building stock is strongly heterogeneous and changes at a very slow rate [2]. On average, only one building out of ten was built between 2000 and 2008, while almost one out of two has more than 50 years [3]. This consistent part of the actual building stock, mainly built before the introduction of any energy regulation, has poor energy performance, aggravated by old and obsolete equipment and appliances. The energy saving potential of these buildings is therefore huge. The IEA in 2017 estimated that deep energy renovations of existing building envelopes represent a saving potential greater than all the final energy consumed by the G20 countries in 2015, or around 330 EJ in cumulative energy savings to 2060 [4]. Interventions aimed at improving the energy performance of existing buildings are therefore mandatory in order to achieve not only the reduction of greenhouse gas emissions, but also the ambitious goals of reducing energy consumption for heating and cooling and improving indoor environmental quality. The main applied technologies include interventions on the building envelope, on the heating and cooling systems, on the lighting system, and the integration of renewable energy sources [5]. In order to achieve energy efficiency goals, multiple solutions can be combined. However, despite the consistent number of available technologies, several limitations hinder their application, such as design, complexity, integration, regulatory, economic or behavioural limitations.
In this context, phase change materials (PCMs) have been identified as one of the most interesting solution in building energy refurbishment. Being characterized by high energy storage capacity per unit volume at a nearly constant temperature [6], PCMs act as an almost isothermal reservoir of heat [7], making buildings more energy effective [8]. When integrated in the building envelope, PCM increases the thermal mass of the building envelope while undergoing the phase change [9], thus allowing the storage of significant amount of energy without the uncomfortable temperature swings and the large structural mass that is necessary with sensible heat storage [10]. The remarkable effect a building envelope has on the energy demand for heating and cooling [11] led to much research over the years focusing on the integration of PCM within various building components [12–20]. PCMs can be directly integrated in the building structure, such as in bricks or concrete, or in cladding layers, such as in wallboards or plasters. The addition of PCM into the structural parts is possible only in case of new constructions, while the addition into finishing layers is possible in both new constructions as well as building renovations. As regards the focus of this research, PCMs were mixed within plaster, more specifically lime-based ones, considered more compatible with existing historical buildings. In order to maximise the effect of the PCM, the knowledge of the materials’ properties is essential. In addition to this, when dealing with building materials the actual properties does not always agree with those declared [21]. This may be due to the fact that building materials are poorly characterized at laboratory scale, if considering sample sizing several centimetres [22].
The most used technique for the determination of the PCM thermophysical properties is the Differential Scanning Calorimeter (DSC) [23], which consists in the measure of the amount of heat required to warm up and cool down a sample of the material to be tested, which is estimated in comparison to that of a reference material. However, this technique is affected by some critical limitations due to the sample’s properties. In a DSC measure, in fact, the sample size is normally small, less than 15 mg, and has to be pure and homogeneous [24]. Nevertheless, in case of building envelope applications, PCMs are usually added as encapsulated materials, both micro-encapsulated, in the form of powder or granules, as well as macro-encapsulated, such as within plastic or metallic containers. The characterization of a building material enhanced with PCM, therefore, cannot be considered reliable if carried out through a DSC analysis. To overcome these limitations, an alternative set up was considered necessary in order to roughly estimate the new enhanced materials’ thermal properties. Several researchers had already tried in the past to overcome the main instruments’ limitations by developing their own set up.
Yesilata and Turgut [25], for instance, developed a dynamic measurement technique to measure the effective thermal properties of anisotropic materials consisting in a dynamic adiabatic-box where the outer and bottom walls were heavily insulated while the top wall was the test specimen, a thin layer whose thermal conductivity was much higher than that of the other walls. Inside the box, close to the bottom wall, there was a heater immersed into water and controlled by a thermostat that heats up water to a desired temperature. Once turned the heater off, the cooling rate of water could be considered a measure of the thermal transmittance of the specimen. This technique was found to be functional and robust for a preliminary estimation of thermal performances of anisotropic specimens, even though the testing time is quite long.
The challenge gets even harder when PCMs are integrated in the materials to be tested. As already mentioned, PCMs at lab scale are usually tested by means of DSC, which requires small and homogeneous samples, conditions that are hard to achieve. Several researchers worked for the development of new equipment able to use macroscopic samples, more similar to actual constructive systems. Among them, de Gracia et al. [26] developed a new equipment to test the steady and transient thermal response of building materials containing PCM. The set up consisted in a highly insulated wooden structure whose interior was divided into two cavities, which were meant to be used to simulate the inner and outer conditions of a building envelope by means of cooper coils connected to programmable water baths. These were separated by the sample to be tested, which was placed between the two cavities with insulated edges to ensure the mono dimensional heat transfer. Three different types of experiments were possible to evaluate the performance of the test sample. The thermal conductivity was estimated in steady-state conditions using the thermal gradient between surfaces, then the heat storage capacity and the dynamic thermal response were estimated with unsteady-state tests. The former was determined by measuring the heat through the sample that was subjected to a determined temperature interval, while the latter was estimated by analyzing the delay between the temperature peaks reached in the cavities.
Barreneche et al. [27] at the University of Barcelona developed two devices to characterize the effective thermal conductivity and to register the temperature-time response curves of materials including PCMs at macroscale. The first one was a conductimeter, which consisted in a sample positioned between a hot and a cold plate, connected to thermostatic baths and both insulated to avoid any influence from the ambient temperature, surrounded by an insulation frame made of refractory bricks that allowed a mono-dimensional heat transfer on the z-axis. The thermal conductivity of the sample was determined through the thermal gradient achieved between the two surfaces, with an estimated accuracy close to 2%. The other device, named T-t curves device, consisted in two samples, a reference and the test one, positioned on the sides of a heating device and framed by an insulation casting to avoid heat transfer on the edges. During the experiments, which occurred at the controlled temperature of 18°C, the samples were heated until a steady-state condition was reached and then, once the heating plate was disconnected, they were thermally compared during the cooling process by means of several temperature sensors.
Borreguero et al. [28] developed an experimental set up to evaluate the improvement of the thermal storage capacity of gypsum samples integrated with different percentages of PCMs. It consisted of a hollow aluminium box through which water, at a determined temperature, flowed continuously by means of a peristaltic pump, connected to a thermostatic bath. Each of the samples to be tested was placed on the surface of the aluminium box and then wrapped up in insulation. Two different studies were carried out, namely an absorption study, where the temperature of the thermostatic bath was changed from 21°C to 40°C, and a reversibility study, where the temperature of the thermostatic bath was cyclically changed from 21°C to 36°C.
Through the customised set up realised at the University of Ferrara [29], the experimental characterisation of two PCM-enhanced plasters was carried out, followed then by the thermo-physical properties validation against experimental data gathered under real outdoor conditions. The PCMs used have phase change temperatures of 27°C and 28°C, and are hereafter named TK27 and AS28, respectively.