Building thermal storage technology: Compensating renewable energy fluctuations

Highlights

• Evaluation of energy savings potential of a thermally activated building system.

• A procedure based on CFD simulations and a simplified demand assessment model was adopted.

• The thermal inertia of the system (PCM) offers a wide operating range adaptable to the limitations of renewable energies.

• Savings of over 40% in heating demand are possible even in the least favourable circumstances.

Abstract

Emerging technologies and new intelligent management systems will be needed to rise to the energy challenges posed by buildings today. Thermally activated building systems (TABS) are attracting growing interest on the back of their energy savings potential. The TABS studied in this article, a new prefabricated panel designed for installation in residential building façades, was characterised by the high thermal inertia afforded by the phase change materials in its composition. The design and assessment of the potential savings derived from TABS require specific characterisation methodologies to estimate the amount of useful energy available to control the indoor environment. A two-stage approach was adopted for the TABS studied here with ``ideal'' operating control (the building is assumed to be at a constant desired temperature). The first stage involved a simplified method for characterising system behaviour based on performance maps developed from CFD simulations. Such maps can be used to quickly assess changes in system energy performance following on variations in design and operating parameters. In the second, the TABS was integrated into a building with a simplified model to assess monthly energy demand to evaluate the system potential for energy savings in representative types of Spanish single-family housing in different climate zones. The first-stage findings showed that given the system significant inertia, it discharged for several days, even when charging occurred only on the first, ensuring a wide operating range adaptable to renewable resource limitations. The analysis of potential, in turn, revealed that savings of over 40% in heating demand are possible even under the least favourable circumstances.

Introduction

According to the International Energy Agency (IEA) [1], the worldwide demand for energy will be more than 25% higher in 2040 than at present and despite advances in renewable energy and electric power, the planet is still far from meeting the objectives to combat climate change.

In the European Union, building construction accounts for 40% of primary energy consumption and 36% of CO₂ emissions [2]. Approximately 70% of residential building consumption is used to control indoor environments [3], vastly more than goes to lighting and other uses. The construction industry consequently holds high potential for reducing energy use.

The ultimate aim is zero energy buildings or even buildings with net, positive energy [4], buildings that produce at least as much energy as they consume. Among others, that involves installing renewable energy systems in buildings. The reference standard for nearly zero energy buildings (NZEB) [5] defining steps to achieve this goal: first, reduce energy needs; second, reduce energy consumptions and third, increase renewable energies. So, designing NZEB is a complex task. It requires evaluating each plan and the different combination of interventions in terms of economic and environmental parameters. Therefore, high usability procedures are necessary to study the behaviour of the building, conventional & innovative systems, passive & active measures etc.

Against that drawback, thermal energy storage (TES) and thermally activated building systems (TABS) are generating great interest, in light of their energy savings potential [6]. The implementation of thermal energy storage techniques enhances building energy efficiency by lowering peak-time needs and decoupling building requirements from energy generation systems [7]. It also favours the use of renewable energies and energy efficiency management [8], [9], [10], while reaping the energy benefits of buildings high thermal inertia [11].

Thermally activated building systems (TABS) are a type of HVAC configuration built into a building structure. The main advantage of such systems, in which heat is exchanged primarily via radiation and stored in the building thermal mass, is the use of the building thermal inertia to store and large surface areas to transfer heat [12], [13].

Depending on their design, TABS may have very high thermal mass with which they can mitigate temperature variations in enclosed spaces by storing or releasing heat. The energy lost or absorbed lowers peak loads, relaying energy consumption by high efficiency or low-cost periods [14], [15]. Building thermal inertia is a complex property that can be harnessed with active systems to enhance indoor comfort and lower energy demand [11].

Since the large areas involved in such systems ensure the exchange of considerable amounts of heat across narrow temperature gradients, they are potentially compatible with energy sources at low temperature [16]. Most of the drawbacks to TABS designs, attributable to the low convection-driven heat exchange between the surface and the conditioned space, can be circumvented by integrating TABS into ventilation systems [17], [18].

TABS layouts vary with the type of operation planned, position and working fluid [12]. In their review of the state of the art, Romani et al. [12] discussed the non-uniformity of TABS nomenclature and proposed a general typological division covering all manner of thermally activated solutions (radiant floors, radiant ceilings, hollow core slabs, concrete cores, and embedded-pipe envelopes).

Radiant floors and ceilings use water as the working fluid. Whilst radiant floors are used primarily for heating [19], [20], they have also been successfully applied to cool large glazed indoor areas designed to receive direct sunlight during the day [21], [22]. Radiant ceilings are installed essentially to cool indoor spaces [23], [24], although they have also been used for heating [25].

Hollow core slabs, in which air is the exchange fluid, are used primarily for cooling spaces [26], [27], [28], such as in night-time ventilation [29]. Air circulating across openings can also be used to ventilate rooms. Water is also the working fluid in concrete core and embedded-pipe envelopes, in which pipes are installed deep into the concrete. Concrete core is the name generally given to horizontal (floor and ceiling) systems and embedded-pipe envelope to vertical (wall) systems and both can be applied to heat or cool indoor space [36].

Researchers have adopted new approaches to achieve energy savings with TABS by including phase change materials (PCMs) in system design. One such study showed that energy consumption can be lowered by 10% to 62% with the addition of such materials, depending on the climate zone. With suitable PCMs and in certain climate zones, indoor temperatures can be maintained within the comfort range even without HVAC [30], [31].

This study characterised the operation and design variables for a pipe-embedded building envelope to assess its potential energy savings. The solution proposed consists of a prefabricated panel for residential building façades. With concrete as its essential constituent, the panel ensures effective insulation and high thermal inertia and can accommodate phase change materials (PCMs). As this new panel is the result of ongoing development with industry companies, in addition to its thermal advantages, it carries a guarantee of commercial viability. As a sequel to that research, Navarro et al. [32] conducted an exhaustive empirical study to choose the optimal PCM, whilst Olivieri et al. [33] ran theoretical and experimental tests to characterise the mechanical and thermal properties of this new PCM mortar and develop the basic guidelines for its manufacture.

A study was subsequently in order to determine the energy impact of the system when implemented in a building and its dependence on the design variables. Such a study would need to be conducted to a conventional procedure for determining nearly zero energy building (NZEB) consumption to ensure comparability of the system proposed to other conventional energy alternatives and optimise system and building design.

The design and assessment of the potential savings derived from TAB systems require specific characterisation methodologies to estimate the amount of useful energy available to control the indoor environment. Most of the literature published to date show that such estimates involve complex transitional heat transfer calculations, however.

Numerical methods accurately solve TABS heat transfer and heat storage capacity. Domínguez et al. [34] ran CFD simulations to analyse the heat exchanged between TABS, building space and occupants, which they validated with full-scale measurements under laboratory conditions. Shen et al. [35], [36] adopted an integral numerical model to simulate dynamic heat transfer in a pipe-embedded building envelope, which they subsequently validated experimentally. Romaní et al. [10] used a numerical model validated against earlier experimental findings to study and compare different HVAC approaches proposed for a radiant wall that stored thermal energy to shift the peak load in a heat pump coupled to a photovoltaic generator.

As numerical methods involve complex, computationally costly procedures, researchers have developed sufficiently accurate simplified models to lower the computer power required in TABS assessment. The use of simplified characterisation models for systems installed in specific buildings supports TABS design and energy assessment and consequently the comparison of energy savings potential between TABS and other technologies.

Research on simplified models for characterising TABS has been conducted by several authors. Li et al. [37] developed and validated a simplified resistance and capacitance (RC) network model for radiant floors. Experimental validation showed that the mean relative errors for the RC model predictions of upper and lower surface heat flows were under 5.5%. Ma et al. [38] used a Matlab / Simulink RC model to study the system requirements for a building fitted with a TABS. Zhu et al. [14] described a semi-dynamic, building-integrated model to assess thermal performance for system design and indoor environmental control, the first part of which was a simplified dynamic RC model that readily predicted heat transfer across the width of the structure. The second part was a conventional NTU model to assess heat transfer along a pipe and total heat transfer on both sides of the structure. These authors also developed a CFD model as a virtual experimental test platform for simulating the thermal characteristics of the structure. Later, they [39] experimentally validated the simplified semi-dynamic model for pipe-embedded building envelope proposed. Lastly, Zhu et al. [40] introduced a simplified dynamic thermal model based on genetic algorithms and frequency analysis for a pipe-embedded building envelope.

On the other hand, recent studies show that passive thermal designs alone are not enough to fully exploit the potential for energy efficiency in buildings: in fact, the harmonization of active elements for interior thermal comfort with the passive design of the building It can lead to further improvements. both in energy efficiency and comfort. These improvements can be achieved by designing appropriate Building Optimization and Control (BOC) systems, a task that is more complex in high-inertia buildings than in conventional ones [41], [42]. Given the innovative nature of the study solution, the work carried out evaluates the theoretical potential of the system assuming an ideal control. The study of the optimization of the control of the system would be interesting to carry out in later studies. So, the study for the TABS proposed here included a two-stage theoretical analysis for the characterization and integration of the system into the building with ``ideal'' operating control (the building is assumed to be at a constant desired temperature). In the first, the system was modelled with CFD in the possible design and operation configurations. The second stage, carried out after the system performance was determined under certain design and operation conditions, consisted of integrating the TABS into the building with a simplified monthly energy demand assessment model.

Emerging technologies and new intelligent management systems will be needed to rise to the energy challenges posed by buildings today. Such systems deliver high performance at a moderate cost. In addition, they should encourage energy demand flexibility to decouple building demand and consumption, which will call for deploying renewable energy, lowering costs and integrating these systems in smart grids.

From that perspective, TABS are among the most promising alternatives, for they guarantee high building envelope performance and methods for capitalising on building thermal mass to store energy. The study aimed to characterise the thermal performance of one such system when the design and operating conditions were varied and subsequently analyse its energy savings potential in different climate zones and residential building types.

In this paper, a two-stage methodology was developed to characterise and assess the TABS proposed. In the first stage, the system was CFD-modelled to a number of possible design and operating configurations. The performance maps developed in that first stage constituted a simplified characterisation of system behaviour for speedy assessment of system energy performance when its design and operating parameters were varied within the established range. In the second stage, building-integrated TABS analysis was conducted with a simplified model to assess monthly energy demand with a view to evaluating the system potential for energy savings in representative types of Spanish housing in different climate zones. The content of the document is divided into the following sections: Section 3, which describes the thermally activated construction system studied. Section 4 describes the methodology performed in that study. As mentioned earlier, this methodology is based on a two-stage analysis, where the thermal characterization of the system is performed first and, secondly, the evaluation of the potential for improvement due to the integration of the system. Section 5 shows the results of the thermal characterization and in Section 6 the results obtained after the evaluation of the energy saving potential in different climatic zones and types of residential buildings.