Subject matter

1 Main Objective
Energy consumption by heating/cooling of the existing building stock accounts for almost 40 percent of the total European energy demand. Energy efficiency, by implementing novel technologies, is considered the key approach for cutting back this high consumption for existing but also for new buildings. For this, three main visions have been identified viz. (i) energetically enhancing the building envelope (heat insulation for sealing, windows and facades), (ii) employing advanced technical building equipment (lighting, heating and ventilation systems) and (iii) using sustainable building systems (i.e., sustainable energies, automatization operations and efficiency-centred planning).
In this context, storing thermal-energy in cementitious-composites (i.e., solar and/or environmental heat/cooling) in components of residential or non-residential buildings is a promising way to level-out daily temperature differences, and to significantly reduce energy demands/consumptions.
The main aim of this new TC is, therefore, to create a State of the ARt (STAR) report, possibly along with recommendations and guidelines, on how to (i) experimentally characterize, (ii) analytically analyse, (iii) numerically describe, and (iv) provide recipes for these novel cementitious-composites capable of Thermal Energy Storage (TES), and which can be employed as energy efficient applications in buildings and/or civil engineering structures.
The TC activities will cover three main TES phenomena in cementitious composites, such as:

1. Sensible Heat Storage (SHS): by using the material heat capacity and temperature changes during heating and cooling;
2. Latent-Heat Storage (LHS): associated with processes of phase changes: solid-liquid (melting) and vice versa (crystallizations), solid-solid, etc. These materials are classical known as Phase Change Materials (PCMs).
3. Thermo-Chemical Storage (TCS): with the thermal energy storage deriving from the conversation of chemical potentials and vice versa.

The proposed TC will review actual experimental test procedures, numerical solution strategies and other relevant results available in literature, on components, composites, constructive elements (like walls, roofs, floors, building envelopes) and buildings made of TES cementitious composites. Moreover, the TC will aim at giving further directions on the reviewed results and to the employability of available scientifically-based test methods and numerical models for TES applications. Several numerical tools and simulation programs, such as TRNSYS, EnergyPlus, ESP-r, IDA ICE or WUFI, are nowadays already available for predicting and analyzing the energy efficiency of buildings. Mostly, these models are used to determine the energy consumption, to evaluate the cost of energy usage in buildings, and/or to determine the most appropriate choice of Heating, Ventilation and Air Conditioning (HVAC) systems, with the aim to control the targeted indoor thermal comfort. The available models very often consider only one aspect of a building energy balance, such as the heat transfer and thermal comfort analysis, while sometimes couple multiple transport phenomena like heat, air, moisture, light, in a unique modelling tool. In this TC, however, emphasis will be only on the various physical-chemical-mechanical properties that determine the efficiency of TES in construction applications. Available models will be reviewed, systematically discussed and explained on their employability for academics, industrial representatives, standardisation bodies, practitioners, engineers, architects and other interested users in general. The possible RILEM recommendation will therefore provide a framework for bringing TES-based solutions via energy and building guidelines/recipes into practice, whereas it will also serve to identify eventually existing omissions and knowledge gaps that may be used by scientists for future research directions.

2 Approach
The TC will consider the following structure (based on a 4 years duration):

• Experimental characterization methods (year 1 and 2): Identifying and selecting the currently available experimental methods available for investigating the thermal energy storage properties of cementitious composites (storage of sensible heat - SHS, of latent heat - LHS and of chemical heat - TCS). Four scale levels will be identified: (i) components (e.g. binders, aggregates, PCM, etc.); (ii) composites (pastes, mortars, concretes); constructive systems (walls, envelope layers, etc.) and full-scale buildings. A possible set-up of a database for components, composites and constructive systems for cementitious materials and buildings will be considered as well.
• Theoretical models and numerical tools (year 2 and 3): Reviewing TES constitutive models at different levels of observations: nano-, micro-, meso-, macro and building scale level. Scientifically-based codes and commercially available models will be reviewed.
• Codes, standards and guidelines (year 3 and 4): Developing a workable and practical framework for employing TES-based composites in cement-based applications for structures and/or buildings. Three complementary documents are foreseen to be provided: i) a STAR: reporting the actual state of the art on TES-based cementitious composites; ii) Recommendations on experimental testing: standardized procedures for experimental characterization of TES-based cementitious composites; iii) Technical guidelines on how to design a TES-based cementitious composites with energy-based simulation software and/analysis.

Terms of reference

The estimated lifetime of the new TC is 4 years. During this period, objectives will be pursued by combining literature and member-based information. In this framework, members of the new TC will be actively approached via the RILEM network, conferences and/or invitation for participation. Aim is to select members from academia and industry (i.e., scientists, designers, contractors, consultants, owners, and maintainers) in such a way that the full chain of activities related to energy-saving in construction will be covered.

- Will the work imply bibliographical research? Or round-robin testing? Or development of new equipment
The work will imply bibliographical research and a targeted round-robin test. Particularly, DSC measurements for investigating the thermal-energy storage properties of material components, characterized by both sensitive and latent heat storage capacity, will be carried out within the framework of a round-robin test, among the participating institutes. The proposed methodology and approach has the purpose to identify sample and instrument-specific influences, which turned out to largely affect DSC characterization measurements (especially during latent - phase change - processes), and hence, to make DSC results more objective and comparable.
Two components/composites will be considered for this purposes:

• Paraffinic waxes (PCM having a melting ranges between 23-26 °C);
• Cement pastes enhanced with the above PCM (microencapsulated, i.e. MPCM);

TU-Darmstadt will prepare the following samples to send to each the participant:

• Paraffinic waxes: 3 samples of 10 mg and 3 samples of 20 mg;
• PCM-cement paste samples (28 days cured and fully dried – 10% of amount of MPCM): 3 samples of 20 mg;
• PCM-cement paste samples (28 days cured and fully dried – 20% of amount of MPCM): 3 samples of 20 mg.

- Clarify in which respect the work of the TC will have direct industrial relevance.
The work of the TC will have direct industrial impact since new energy saving products are demanded in order to comply with the European carbon neutrality in 2050 (https://ec.europa.eu/clima/policies/strategies/2050_en). The guidelines and/or recommendations will directly support the industry on this.

Detailed working programme

The new TC is aiming at organizing two regular meetings per year to adjust progress and to (re)schedule activities. These two meetings will be scheduled during the yearly RILEM week (classically done in autumn season) and RILEM spring convention (classically done in spring season). Two additional internet-based meeting may be also scheduled based on request. Potential events combined with the meetings are planned as follows:
2020
Kick-off meeting: introduction of members, suggestion of new members, overview of members’ competences and experiences, overview of the activities, organisation of initial workshop.
Meeting will possibly be done during the RILEM Week 2020.
2021
½ -year meeting: Listing initial results, presenting progress and figuring out the structure of the STAR.
1st-year meeting: Results, presenting progress, draft of chapters and discussing the overall content of the STAR.
The date and location of the two meetings will be decided during the kick-off meeting and the ½ -year meeting.
2022
1+1/2-year meeting: Preparing first draft of the STAR and discussing the structure of the “Recommendations on experimental testing”.
2nd-year meeting: Completing the STAR draft of chapters and discussing the overall content of the “Recommendations on experimental testing”.
The date and location of the meetings will be decided during the 1st-year meeting and the 1+1/2 -year meeting.
2023
2+1/2-year meeting: Preparing first draft of the “Recommendations on experimental testing” and discussing the structure of the “RILEM technical guidelines”.
3rd-year meeting: Completing the “Recommendations on experimental testing” draft of chapters and discussing the overall content of the “RILEM technical guidelines”.
The date and location of the meetings will be decided during the 2nd-year meeting and the 2+1/2 -year meeting.
2024
3+1/2 -year meeting: Preparing first draft of the “RILEM technical guidelines”.
4th end of the committee: Finishing the committee work by summarizing the TC findings, presenting a special issue of an International symposium held in Sofia. One or more journal publications, STAR and RILEM Test Methods and Guidelines/Recommendation will be published.
The date and location of the meetings will be decided during the 3rd-year meeting and the 3+1/2-year meeting.