Boric Acid: a High Potential Candidate for Thermochemical Energy Storage

energies

Article Boric Acid: A High Potential Candidate for Thermochemical Energy Storage

Clemens Huber 1,* , Saman Setoodeh Jahromy 1 , Christian Jordan 1, Manfred Schreiner 2, Michael Harasek 1 , Andreas Werner 3 and Franz Winter 1 1 Institute of Chemical, Environmental and Bioscience Engineering, TU Wien, Getreidemarkt 9/166, 1060 Vienna, Austria; [email protected] (S.S.J.); [email protected] (C.J.); [email protected] (M.H.); [email protected] (F.W.) 2 Institute for Natural Sciences and Technology in the Arts, Academy of Fine Arts Vienna, Augasse 2–6, 1090 Vienna, Austria; [email protected] 3 Institute for Energy Systems and Thermodynamics, TU Wien, Getreidemarkt 9/302, 1060 Vienna, Austria; [email protected] * Correspondence: [email protected]

Received: 15 February 2019; Accepted: 18 March 2019; Published: 21 March 2019

Abstract: This paper aims to describe the capability of the system boric acid–boron oxide for thermochemical energy storage. As part of the systematic research and in-depth analysis of potential solid/gas reaction systems, performed during the last years, this reaction system appears to be highly promising for the future of worldwide sustainable energy supply. The analysis of the reaction heat, by means of thermogravimetric and macroscopic investigations, not only showed a signiﬁcantly higher energy density of 2.2 GJ/m3, compared to sensible- and latent energy storages, but the reaction kinetic further demonstrated the reactions’ suitability to store energy from renewable energy and waste heat sources. This paper, therefore, shows a new approach regarding the application of the boric acid–boron oxide reaction system and elaborates on the advantages and challenges for its use as energy storage.

Keywords: thermochemical energy storage; boric acid; boron oxide; thermogravimetric analysis; ICTAC

1. Introduction It lies within human nature to stick to accustomed patterns as long as they bring the desired result, despite knowing about the potential severe consequences for the future. Acting usually starts only when time is already close of running out. The steady rise of the global energy demand and the rise of the global air pollution are closely interlinked and one of the greatest challenges of the present. Emitting gases like carbon dioxide or nitrogen oxides are, therefore, rightly doomed for their contribution to environmental pollution and an increase of the global warming effect with severe consequences, such as rising sea levels, droughts, floods, or hurricanes. It took politics until the end of the 20th century to slowly recognise global warming and its harmful impact as a serious global threat. The Kyoto Protocol concluded in 1997 was one of the first international political confessions concerning environmental protection, with a legal obligation to reduce greenhouse gas (GHG) emissions [1]. It was followed by further regulations, national commitments, and activities, such as the Europe 2020 strategy or the 2030 climate energy framework [2,3]. Renewable energy sources and the promotion of efficiency measures are thereby seen as key tasks for a sustainable reduction of GHG emissions. It is expected, that the global demand for energy will rise by 30% until the year 2040. Additionally, there will be a slight increase of energy related CO2 emissions [4]. To meet the defined targets, many

Energies 2019, 12, 1086; doi:10.3390/en12061086 www.mdpi.com/journal/energies Energies 2019, 12, 1086 2 of 17

different strategies and schemes are necessary. Due to the fact, that a very large proportion of CO2 emissions originates from industrial processes the usage of industrial waste heat—a hitherto largely unused energy source—would be one possible step into the right direction. Waste heat definitely has the potential to at least satisfy the energy demand of private homes. Miro et al. [5] already described the large potential of industrial waste heat as significant and despite some slight data inaccuracies, he leaves no doubt about its significance for future energy supply. While already various powerful CO2 free energy sources can be found around the world, such as solar energy, wind, geothermal energy, and hydro- or tidal power, they are all limited by one major drawback: the energy release depends on geographic locations, seasons, or daytimes and does not take into account the actual energy demand. If it cannot be used in the moment of its release, or stored for further usage, this energy gets simply lost to the environment and converts to anergy (useless energy at the ambient level). Keeping this in mind, there is the urgent necessity to bridge the time gap between the provision of thermal energy (renewable and waste heat) and the varying energy demand. Thermal energy storages systems (TES) will, therefore, play a significant role in the future. At present, there are several methods for storing thermal energy such as sensible, latent or thermochemical heat storage systems (see inter alia [6–9]). Sensible heat storages use the high specific heat capacity for storing thermal energy, while the mechanism of latent heat storages is based on a phase change process (mainly solid–liquid or liquid–gas). Thermochemical energy storage systems are still at an experimental stage, whereas the other TES’ are relatively well studied and mostly market ready. Nevertheless, they all share the common disadvantage of a huge energy loss. Due to the laws of thermodynamic (e.g., Fourier’s law) storing energy at a higher temperature level, compared to the environment, leads to an unstoppable energy loss. Hence, storage time is limited and there is a strong need for proper insulation. Due to their much higher energy density compared to ordinary storage systems, thermochemical energy storage (TCES) recently experienced particular scientific attention. Within the TCES, one can distinguish between two different types of storage systems, based on their mechanism; physical sorption processes and chemical reaction processes. For the scope of this paper, TCES only comprises chemical reaction processes. TCES processes use the energy of reversible chemical reactions and the stored amount of energy is equal to the reaction enthalpy. During the endothermic charging process, thermal energy gets absorbed and decomposes a chemical compound A, into two products -B and C:

νA A + ∆HR νBB + νCC (1)

The products obtained through this reaction, contain the reaction enthalpy and are ready to be stored at ambient temperature. By turning back into A in the course of an exothermic reaction, B and C release the previously stored energy whenever needed and can then be charged again (Figure1). To facilitate the separation of the products, one of the products B or C is preferably a solid and the other one a gas. In case, the gas is easily available, the storage volume could be signiﬁcantly reduced by storing only the solid product. Energies 2019, 12, x FOR PEER REVIEW 3 of 17

Energiesand its2019 suitability, 12, 1086 as TCES system. Research results regarding the characteristics of the material3 ofwill 17 provide the basis for further investigations.

Figure 1. SchematicSchematic thermochemical thermochemical energy energy storage storage proces processs illustrated illustrated by the by boric the boric acid/boron acid/boron oxide oxidereaction. reaction.

2. MaterialsCharacteristics and Methods like reversibility, the rate of reaction, operation conditions, and the reaction kinetics are necessary information to determine whether a substance is suitable to serve as TCES [9]. A small molar2.1. Discovery volume and paired Mechanism with a large reaction enthalpy, further maximizes the thermal storage capacity. Additionally, no side reactions should affect the reaction turnover and a good cycle stability should The work of Deutsch et al. [10], who developed a new systematic search algorithm for preserve the energy storage density over a long period of time [6]. thermochemical storage materials, provided the basis for the discovery of this reaction. They applied Nevertheless, there is still no technology yet market ready to bridge the time gap, between the a mathematical search algorithm to a chemical database for identifying potential solid/gas reactions. potential energy deriving from industrial waste heat or sustainable energy sources, and the selective Due to practical reasons, they further narrowed the scope to reactions of solid inorganic substances energy demand. To reach this goal, new ways for a sustainable energy supply need to be found, and with a defined gaseous reactant, and thereby revealed more than a thousand unique reactions. in particular TCES storage technologies should become the focus of attention. Studies regarding the Limiting the equilibrium temperature of this reactions to 1000 °C, and taking further limitations like boric acid–boron oxide reaction system were primarily driven so far by the industrial sector (e.g., glass. heat storage capacity, availability, and the price into account, yielded a final list of potential reaction ceramics, fire protection). This paper wants to present a new promising approach of boric acid and its systems. To proof their real suitability as TCES, the listed reaction systems were briefly analysed— suitability as TCES system. Research results regarding the characteristics of the material will provide using different analytical techniques—and then ranked in respect to their characteristics. the basis for further investigations. As a result, the reaction system B2O3/H3BO3 was further investigated. Due to its high energy 2.density Materials of 2238 and MJ/m³ Methods and the apparent possibility to assure a complete conversion over an endless number of reaction cycles, strongly differed it from other reaction systems. Error! Reference source 2.1.not found. Discovery summarizes and Mechanism different thermal energy storage systems (TES). The work of Deutsch et al. [10], who developed a new systematic search algorithm for Table 1. Comparison of different thermal energy storage systems (TES). thermochemical storage materials, provided the basis for the discovery of this reaction. They applied a mathematicalType search algorithm Material to a chemical Energy database Storage forDensity identifying Temp-Range potential/Melting solid/gas Point reactions. Ref. DuePhysisorption to practical TES reasons, Zeolites they further 13X/H2 narrowedO the 601 scope MJ/m³ to reactions of solid 65–130 inorganic °C substances [11] with a definedChemisorption gaseous TES reactant, MgSO and4 × thereby 7H2O revealed1512 more MJ/m³ than a thousand unique150 reactions. °C Limiting [11] the equilibriumSensible temperatureTES HITEC of this solar reactions salt to 1000 2.9◦ C,MJ/m³ and taking further limitations 250–450 °Clike heat storage [12] Sensible TES Water at 50 °C 206 MJ/m³ 50 °C [13] capacity, availability, and the price into account, yielded a final list of potential reaction systems. Latent TES MgCl2 1048 MJ/m³ 714 °C [14] To proofLatent their TES real suitability NaF as TCES, the listed 2031 MJ/m³ reaction systems were briefly 996 °C analysed—using [6] differentTCES analytical techniques—andCu2O then ranked 3606 in MJ/m³ respect to their characteristics. 350–1100 °C [15] As aTCES result, the reactionH3 systemBO3 B2O3/H3BO 22383 wasMJ/m³ further investigated. 90–<200 Due °C to its high energy [16] density of 2238 MJ/m3 and the apparent possibility to assure a complete conversion over an endless numberThe ofreaction reaction mechanism cycles, strongly of boric differed acid it(H from3BO3)–boron other reaction oxide (B systems.2O3)—with Table the1 summarizesintermediate differentmetaboric thermal acid (HBO energy2)—is storage still systemsnot fully (TES). and clearly examined. On one hand, Zachariasen [17]

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Table 1. Comparison of different thermal energy storage systems (TES).

Type Material Energy Storage Density Temp-Range/Melting Point Ref. 3 ◦ Physisorption TES Zeolites 13X/H2O 601 MJ/m 65–130 C[11] 3 ◦ Chemisorption TES MgSO4 × 7H2O 1512 MJ/m 150 C[11] Sensible TES HITEC solar salt 2.9 MJ/m3 250–450 ◦C[12] Sensible TES Water at 50 ◦C 206 MJ/m3 50 ◦C[13] 3 ◦ Latent TES MgCl2 1048 MJ/m 714 C[14] Latent TES NaF 2031 MJ/m3 996 ◦C[6] 3 ◦ TCES Cu2O 3606 MJ/m 350–1100 C[15] 3 ◦ TCES H3BO3 2238 MJ/m 90–<200 C[16]

The reaction mechanism of boric acid (H3BO3)–boron oxide (B2O3)—with the intermediate metaboric acid (HBO2)—is still not fully and clearly examined. On one hand, Zachariasen [17] described three crystalline modiﬁcations of metaboric acid. According to this theory, that was also represented inter alia by Sevim et al. [18], Balcı et al. [19] or Zhang et al. [20], the thermal decomposition of boric acid by thermogravimetric analysis (TG) would reveal two distinguished steps:

2H3BO3 → 2HBO2+2H2O ↑ (2)

2HBO2 → B2O3 + H2O ↑ (3) On the other hand, while studying different crystalline structures of boric acid, Harabor et al. [21] observed three different thermal induced reactions. The mass loss was attributed to each of this reaction steps. Within this new approach different crystalline modifications of boric acid were analysed, however, in this case the crystalline modifications of metaboric acid appear to be insignificant for the reaction [22,23]: H3BO3 → HBO2 + H2O ↑ (4) 1 1 HBO → H B O + H O ↑ (5) 2 4 2 4 7 4 2 1 1 1 H B O → B O + H O ↑ (6) 4 2 4 7 2 2 3 4 2

2.2. Occurrence and Application of B2O3/H3BO3 Borate minerals occur in regions with volcanic activity and arid climate. The minerals mainly used by the industry are:

• Colemanite Ca[B3O4(OH)3] × H2O • Kernite Na2[B4O6(OH)2] × 3H2O • Tincal Na2[B4O5(OH)4] × 8H2O • Ulexite CaNa[B5O6(OH)6] × 5H2O In 2017, the annual global production volume of boron was about ten million metric tons (excluding the amount of the US production), mainly supplied from countries such as Turkey, Peru, Chile or Argentina. Global (explored) reserves of at least one billion tons can be found in North- and South America, as well as in regions in South- and Central Asia [24]. A future increase in demand for boron compounds would lead to further explorations of potential deposits all over the world (e.g., Serbia, Iran) [25]. Nearly 75% of the worldwide borate consumption concerns the production of ceramics, detergents, fertilizers, and (heat-resistant) glass [24]. In addition to this area of application, it is further used for mineral wool, sport equipment, insecticides, or medical treatments [26–28]. Furthermore, there were investigations regarding its usage as rocket propulsion or military use [29]. As already mentioned above, the material properties of a chemical compound are strongly affecting the performance of the thermal energy storage. Therefore, various inﬂuencing parameters for an energy storage process have to be taken into account, such as: melting point, density, latent heat of Energies 2019, 12, 1086 5 of 17 fusion, speciﬁc heat, thermal conductivity, cost, availability, thermal and chemical stability, volume change, toxicity, corrosiveness, as well as many others [7].

2.3. Material Properties

2.3.1. Material The analysed sample material was taken from:

• Boric Acid: Sigma Aldrich (St. Louis, MO, USA) (11606/CAS: 10043-35-3) • Boron Oxide: Alfa Aesar (Wardhill, MA, USA) (12290/CAS: 1303-86-2) • Metaboric Acid: Santa Cruz Biotechnology (Dallas, TX, USA) (sc-228460/CAS: 13460-50-9)

The purity of the samples—declared by the manufacturer—was above 99%. This was checked inter alia by means of inductive plasma mass spectroscopy (ICP), trace metal analyses and acidimetric titration. Since the standardized detection methods by titration does not distinguish between boron oxide, metaboric acid or boric acid, a possible degradation of boron oxide due to its hygroscopic behaviour—e.g., with humidity—could neither be detected nor excluded.

2.3.2. Particle Size The particle size of boric acid and boron oxide was analysed by a laser diffraction measurement device (Mastersizer 2000, Malvern Instruments, Malvern, UK). Equipped with a dry dispersion module Scirocco 2000), using air for particle-in-gas sizing, it provided an analysis of the particle size in the interval between 0.020 and 2000 µm. Boron oxide was analysed twice, before and after one hour of milling with a ball mill.

2.3.3. Scanning Electron Microscopy For a deeper insight into the sample structure, a scanning electron microscope device (SEM) (FEI Quanta 250 FEGSEM) was used for imaging the particle surface. To ensure pure samples, boron oxide was initially thermally pre-treated (300 ◦C for 10 h).

2.3.4. X-Ray Diffraction Analysis To analyse the sample material, powder X-ray diffraction measurements (XRD) were carried out on a PANalytical X’Pert Pro diffractometer in Bragg-Brentano geometry using a mirror for separating the Cu Kα1,2 radiation and an X’Celerator linear detector. For in situ monitoring of experiments an Anton Paar HTH1200N chamber was used.

2.3.5. Thermal Conductivity/Diffusivity Daniel Lager [30] further investigated the thermal conductivity of this reaction system. He examined the thermal conductivity of boron oxide and boric acid by using a laser ﬂash analysis (LFA) device (NETZSCH LFA 467), as well as a Transient Hot Bridge (THB) analysing device (Linseis THB 3 100). Bulk samples of H3BO3 and B2O3 (ρ = 951 and 1280 kg/m ) have been analysed via the THB 3 device, whereas LFA technique has been applied to pressed pellet samples of H3BO3 (ρ = 1436 kg/m ).

2.3.6. Reaction Heat Analysis All the experimental examinations of the thermochemical reaction were performed by a simultaneous thermal analysis (STA) device (Netzsch STA449 Jupiter), equipped with a TGA-DSC sample holder. The oven allowed experimental investigations with temperatures ranging from 25 ◦C to 1250 ◦C, regulated by an S-Type thermocouple. Mass ﬂow meters (Red-y smart, Voegtlin, Aesch, Switzerland) were used to control the gas ﬂow rate of nitrogen. Aluminium oxide crucibles without lids (Ø = 6 mm, 75 µL) were used for all experiments. Furthermore, nitrogen as inert gas with a Energies 2019, 12, 1086 6 of 17

Energies 2019, 12, x FOR PEER REVIEW 6 of 17 flow rate of 100 mL/min was adjusted for each experimental run. Equipped with a steam generator, the instrumentThe analysis allowed of the thermalhydration investigations reaction was underperformed defined under moist isothermal conditions. conditions while adding a predefinedThe analysis water of vapour the hydration mass flow reaction to the was gas performed flow. The underdehydration isothermal process conditions was examined while adding under anon predefined isothermal water conditions vapour (2, mass 4 an flowd 8 K/min). to the gas According flow. The to dehydration the recommendations process was of examinedthe International under nonConfederation isothermal for conditions Thermal (2, Analysis 4 and 8 K/min).and Calorime Accordingtry (ICTAC) to the recommendationsKinetics Committee, of thethe Internationalmass should Confederationbe kept as small for as Thermalpossible to Analysis ideally andobtain Calorimetry results not (ICTAC) influenced Kinetics by the Committee,sample mass the [31]. mass Therefore, should bea sample kept as mass small of as 2 possible mg was to used ideally for obtainall experiments. results not In influenced addition, by preliminary the sample studies mass [31 with]. Therefore, various asample sample masses mass ofwere 2 mg performed, was used to for estimate all experiments. the significance In addition, of the preliminarymass influence studies on the with result. various sample masses were performed, to estimate the significance of the mass influence on the result. 2.3.7. Macroscopic Reaction Heat Analysis 2.3.7. Macroscopic Reaction Heat Analysis A simple experimental setup (Dewar reactor) was realized for the analysis of the hydration A simple experimental setup (Dewar reactor) was realized for the analysis of the hydration reaction comprising: a Dewar vessel with a capacity of one litre (Agil 1, Air Liquide, Paris, France) a reaction comprising: a Dewar vessel with a capacity of one litre (Agil 1, Air Liquide, Paris, France) a stirrer and a thermocouple with a logging function (Figure 2). The process was started, by feeding stirrer and a thermocouple with a logging function (Figure2). The process was started, by feeding water from the vessel lid triggering the exothermal reaction. water from the vessel lid triggering the exothermal reaction.

Figure 2. Dewar reactor—Set up for macroscopic reaction heat analysis. Figure 2. Dewar reactor—Set up for macroscopic reaction heat analysis. 3. Results 3. Results 3.1. Particle Size 3.1. Particle Size Figure3 shows the measured particle size distribution (Frequency) and the cumulative frequency (Sum)Figure of the 3 analysed shows the samples. measured Due particle to the great size hardnessdistribution of B2(Frequency)O3—in the rangeand the of 1.5cumulative GPa for amorphousfrequency (Sum) and tenof the times analysed higher sample for thes. highDue to pressure the great conﬁguration hardness of B [322O]—no3—in the signiﬁcant range of effect 1.5 GPa of milling—whetherfor amorphous and on ten particle times higher size nor for onthedistribution—was high pressure configuration noticed. The[32]—no metaboric significant acid effect sample of apparentlymilling—whether consisted on of particle amorphous size fragments.nor on distribution—was Therefore, it was noticed. excluded The from metaboric particle sizeacid analyses. sample apparently consisted of amorphous fragments. Therefore, it was excluded from particle size analyses.

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Figure 3. Particle size distribution of B2O3 (before and after milling) and H3BO3 samples. FigureFigure 3. 3.Particle Particle sizesize distributiondistribution of of B B22OO33 (before and after milling) andand HH33BO3 samples.samples.

3.2. Scanning Electron Microscopy (SEM)(SEM) The images of boricboric acidacid revealedrevealed aa ratherrather roundround shapeshape andand aa cleanclean andand smooth,smooth, plate-shapedplate-shaped surface (Figure(Figure4 4).). BoronBoron oxide oxide and and metaboricmetaboric acid acid showed showed an an angularangular particle particle shape shape and and anan apparentapparent porousporous surface.surface. The angular particle shape is very li likelykely a result of grinding and therefore the samples have beenbeen assumed vitreous.vitreous. The images have been taken with an accelerating voltagevoltage ofof 2.52.5 kVkV atat aa workingworking distancedistance betweenbetween 10.210.2 andand 10.410.4 mm.mm.

Figure 4. SEM images of H 3BO 3 (ﬁrst column), HBO 2 (second column) and B 2O3 (third column) at Figure 4. SEM images of H33BO33 (first(first column),column), HBOHBO22 (second(second column)column) andand BB22O3 (third(third column)column) atat × × × × different resolutionsresolutions (lines)(lines) ofof 120120×,, 5000×, 5000 ,20,000×, 20,000 andand, and 40,000×40,000× 40,000 (accelerating(accelerating(accelerating voltage:voltage: voltage: 2.52.5 2.5 kV).kV). kV).

EnergiesEnergies 20192019,, 1212,, x 1086 FOR PEER REVIEW 8 8of of 17 17 Energies 2019, 12, x FOR PEER REVIEW 8 of 17 3.3. X-Ray Diffraction Analysis 3.3.3.3. X-Ray X-Ray Diffraction Diffraction Analysis Analysis The XRD analysis showed the purity of the used boric acid and identified the samples monoclinicTheThe XRDXRD crystal analysisanalysis system showed showed (Figure the 5).the purity However, purity of the of an used the analysis boricused acidofboric boron and acid identiﬁedoxide and was identified the not samples possible the monoclinic duesamples to its obviousmonocliniccrystal system amorphous crystal (Figure system structure.5). However,(Figure This 5). an However,would analysis corres an of analysis boronpond oxidewith of boron the was oxideSEM not possible resultswas not dueaspossible boron to its due obviousoxide to itsis producedobviousamorphous amorphous by structure.dehydration structure. This of wouldboric This acid. correspond would In this corres proces withpond thes, the SEMwith boric results the acid SEM asgets boron results heated oxide asup boron isto produced1000 oxide °C and byis ◦ causesproduceddehydration a melting by dehydration of boricof the acid.solid. of InTheboric this resulting acid. process, In vitreousthis the proces boric borons, acid the oxide boric gets gets, heatedacid after gets up cooled heated to 1000 down, up Cto andground,1000 causes °C and a sieved.causesmelting aThe ofmelting theamorphous solid. of the The solid. structure resulting The resultingwith vitreous randomly vitreous boron dist oxide boronributed gets, oxide atoms after gets, cooledscatter after cooleddown,X-rays ground,down,in many ground,and directions sieved. and andsieved.The therefore amorphous The amorphous causes structure only structure a with large randomly bumpwith randomly (Figure distributed 6). dist ributed atoms atoms scatter scatter X-rays X-rays in many in many directions directions and andtherefore therefore causes causes only only a large a large bump bump (Figure (Figure6). 6).

Figure 5. X-ray diffraction analysis of monoclinic boric acid. Figure 5. X-ray diffraction analysis of monoclinic boric acid. Figure 5. X-ray diffraction analysis of monoclinic boric acid.

FigureFigure 6. 6. X-rayX-ray diffraction diffraction analysis analysis of of amorphous amorphous boron boron oxide. oxide. Figure 6. X-ray diffraction analysis of amorphous boron oxide. 3.4. Thermal Conductivity/Diffusivity 3.4. Thermal Conductivity/Diffusivity 3.4. ThermalFigure7 Conductivity/Diffusivity shows the thermal diffusivity of boric acid. The thermal diffusivity decreases with rising Figure 7 shows the thermal diffusivity of boric acid. The thermal diffusivity decreases with rising temperatures until fifty degrees, as well as the thermal conductivity calculated from this data. The THB temperaturesFigure 7 showsuntil fifty the thermaldegrees, diffusivity as well as of the boric thermal acid. Theconductivity thermal diffusivity calculated decreases from this with data. rising The analysis shows a temperature influenced increase in thermal conductivity for boron oxide and boric THBtemperatures analysis showsuntil fifty a temperature degrees, as influencedwell as the increa thermalse in conductivity thermal conductivity calculated forfrom boron this oxidedata. andThe acid. The increase in the standard deviation of boric acid, at temperatures above 70 degrees, could be a boricTHB analysisacid. The shows increase a temperature in the standard influenced deviation increa ofse boric in thermal acid, at conductivity temperatures for above boron 70 oxide degrees, and result of boric acid decomposing to metaboric acid and vapour. Since the convective share of the heat couldboric acid.be a resultThe increase of boric inacid the decomposing standard deviation to metaboric of boric acid acid, and atvapour. temperatures Since the above convective 70 degrees, share transport is related to the gas flow velocity, the evolving water vapour causes a stochastic turbulent ofcould the beheat a resulttransport of boric is related acid decomposing to the gas flow to velo metaboriccity, the acid evolving and vapour. water vapourSince the causes convective a stochastic share gas flow in the bulk sample, hence, influencing the apparent thermal conductivity of the bulk material. turbulentof the heat gas transport flow in is the related bulk tosample, the gas hence, flow velo influencingcity, the evolvingthe apparent water thermal vapour conductivity causes a stochastic of the bulkturbulent material. gas flow in the bulk sample, hence, influencing the apparent thermal conductivity of the bulk material.

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6 Figure 7. ThermalThermal conductivityconductivity ((λλ)) andand thermal thermal diffusivity diffusivity (a (a× ×10 106)) forfor BB22OO33 andand HH33BO3 (pellet and powder) [[30].30]. 3.5. Reaction Heat Analyses 3.5. Reaction Heat Analyses 3.5.1. Hydration 3.5.1. Hydration Experiments to investigate the hydration reaction of boron oxide, revealed an apparent Experiments to investigate the hydration reaction of boron oxide, revealed an apparent evaporation of the resulting boric acid in the presence of water vapour. Thus, an accurate analysis of evaporation of the resulting boric acid in the presence of water vapour. Thus, an accurate analysis of the hydration process of boron oxide became impossible. Walter Dressler [33] analysed the volatility the hydration process of boron oxide became impossible. Walter Dressler [33] analysed the volatility of boric acid in relation to water content and temperature. He was one of the first who described the of boric acid in relation to water content and temperature. He was one of the first who described the influence of temperature and varied boric acid concentrations in water on the volatility inter alia by influence of temperature and varied boric acid concentrations in water on the volatility inter alia by steam distillation. Even low concentrations lead to an evaporation of boric acid and further increasing steam distillation. Even low concentrations lead to an evaporation of boric acid and further increasing linear with a rising concentration. linear with a rising concentration. To verify this theory, isothermal experiments with various temperatures and steam content were To verify this theory, isothermal experiments with various temperatures and steam content were performed. Hydration experiments of boron oxide showed a mass loss, influenced by temperature, performed. Hydration experiments of boron oxide showed a mass loss, influenced by temperature, initial mass and steam content. The first mass increase, due to formed boric acid, is then followed by a initial mass and steam content. The first mass increase, due to formed boric acid, is then followed by mass loss due to its vaporization. This mass loss is apparently influenced by temperature (Figure8) a mass loss due to its vaporization. This mass loss is apparently influenced by temperature (Figure and water vapour content (Figure9). The higher the temperature and/or steam content (including the 8) and water vapour content (Figure 9). The higher the temperature and/or steam content (including evolving water vapour from the boric acid decomposition), the faster the mass reduction of the reacted the evolving water vapour from the boric acid decomposition), the faster the mass reduction of the boric acid. reacted boric acid.

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Figure 8. Hydration of 9.8 and 1.3 mg B2O3 at constant water vapour mass flow (5 g/h H2O within 100 Figure 8. Hydration of of 9.8 9.8 and and 1.3 1.3 mg mg B B2O2O3 at3 at constant constant water water vapour vapour mass mass flow ﬂow (5 g/h (5 g/h H2O H within2O within 100 mL/min N2)—temperature influence on the volatility of the resulting H3BO3. 100mL/min mL/min N2)—temperature N2)—temperature influence inﬂuence on the on volatility the volatility of the of resulting the resulting H3BO H3.3 BO3.

◦ Figure 9. HydrationHydration of of 10 10 mg mg B2O B32 Oat3 constantat constant temperature temperature of 150 of °C—volatility 150 C—volatility of the resulting of the resulting H3BO3 Figure 9. Hydration of 10 mg B2O3 at constant temperature of 150 °C—volatility of the resulting H3BO3 H BO depending on various water vapour mass flows. depending3 3 on various water vapour mass flows. Experiments to quantify the volatility of boric acid without added water vapour have been Experiments to quantify the volatility of boric acid without added water vapour have been conducted. Therefore, boric acid was thermal decomposed (up to 200 ◦C) in a flask, under a defined conducted. Therefore, boric acid was thermal decomposed (up to 200 °C) in a flask, under a defined nitrogen gas flow (100 L/h) and defined pressure conditions. Analysing the deposited mass of nitrogen gas flow (100 L/h) and defined pressure conditions. Analysing the deposited mass of boric boric acid, by weighting each individual component of the experimental set up, before and after acid, by weighting each individual component of the experimental set up, before and after the the experiment, revealed that even the evolving vapour of the decomposition is sufficient to cause a experiment, revealed that even the evolving vapour of the decomposition is sufficient to cause a relative mass loss of around one percent during a period of 40 minutes (Figure 10). relative mass loss of around one percent during a period of 40 minutes (Figure 10).

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2.00

1.80

1.60

1.40

1.20

1.00 Massloss % / 40 min

0.80

0.60 100 200 300 400 500 600 700 800 900 1000 1100

abs. Pressure mbar

Figure 10. Volatility of 2.5 mg boric acid heated up to a temperature of 200 ◦C, as a function of absolute Figure 10. Volatility of 2.5 mg boric acid heated up to a temperature of 200 °C, as a function of absolute pressure [34]. pressure [34]. 3.5.2. Dehydration 3.5.2. Dehydration Comparing different experimental results with varying initial sample masses (2 mg, 13.1 mg, and 16.6 mg)Comparing revealed different a significant experimental influence results of the samplewith varying mass on initial the measurement sample masses results (2 mg, (Figure 13.1 mg,11). Forand that16.6 mg) reason, revealed it was a significant conclusive influence to conduct of the sample experiments mass on with the a measurement sample mass results of 2 mg. (Figure On average11). For that the resultsreason, forit was the decompositionconclusive to conduct reaction the showed experiments a specific with energy a sample content mass of of 1.7 2 mg. GJ/m On3 (Figureaverage 12 the). The results cause for for the the decomposition significant difference reaction between showed the a theoretical specific energy (2.2 GJm content3) and experimentalof 1.7 GJ/m3 (~1.7(Figure GJ/m 12).3 )The specific cause energy for contentthe significant of boric di acid,fference might between be the usagethe theoretical of an open (2.2 crucible GJm for3) and the thermalexperimental analysis (~1.7 (energy GJ/m3) lossesspecific to energy gas stream). content Contraryof boric acid, to the might hitherto be the published usage of an results, open crucible the use offor lower the thermal sample analysis masses (energy yielded losses a shift to of gas the stream reaction). Contrary to lower to temperatures. the hitherto published The results results, revealed the ause reaction of lower temperature sample masses range yielded (depending a shift of on the the re heataction rate), to lower from temperatures. 70 ◦C to 200 ◦ TheC. Additionally, results revealed no differencea reaction totemperature the classical range approach (depending of the intermediateon the heat rate), reaction from (two 70 °C or to three 200 steps°C. Additionally, with mass loss) no coulddifference be noticed to the (Figureclassical 13 approach). of the intermediate reaction (two or three steps with mass loss) could be noticed (Figure 13).

Energies 2019, 12, x FOR PEER REVIEW 12 of 17 Energies 2019, 12, 1086 12 of 17 Energies 2019, 12, x FOR PEER REVIEW 12 of 17

Figure 11. Dehydration (TG) of boric acid at a heating rate of 2 °C/min subject to different initial Figuremasses. 11.11. DehydrationDehydration (TG) (TG) of of boric boric acid acid at a at heating a heating rate ofrate 2 ◦ C/minof 2 °C/min subject subject to different to different initial masses. initial masses.

Figure 12. Dehydration (TG and DSC) of 2 mg HH33BO3 atat different heating rates of 2 (turquoise), 4 (violet), and 8 ◦C/min (red) with the speciﬁc energy content of the reaction (1.3, 1.2, and 1.1 KJ/g). (violet),Figure 12. and Dehydration 8 °C/min (red) (TG with and the DSC) specific of 2 energymg H3 BOcontent3 at different of the reaction heating (1 rates.3, 1.2, of and 2 (turquoise), 1.1 KJ/g). 4 (violet), and 8 °C/min (red) with the specific energy content of the reaction (1.3, 1.2, and 1.1 KJ/g).

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◦ Figure 13. Temperature-dependent dehydrationdehydration start start of of 2 2 mg mg H H33BOBO33 atat aa heatingheating raterate ofof 22 °C/minC/min (TG, (TG, DSC, DTG, DDSC). SpeciﬁcSpecific energyenergy contentcontent ofof thethe reaction:reaction: 1.31.3 KJ/g.KJ/g.

3.6. Macroscopic Reaction Heat Analysis Experiments using a stoichiometric ratio of waterwater caused a strong increase of the viscosity due to agglomeration (caking), (caking), as as well well as as a adecreasing decreasing mixing mixing quality. quality. This This led led to an to anoverload overload stop stop of the of themixer mixer before before achieving achieving a full a full conversion. conversion. The The agglomeration agglomeration further further caused caused a apoor poor quality quality of the measured thermalthermal signal.signal. Hence, Hence, to to achieve achieve a homogeneousa homogeneous reaction reaction progress progress through through the reactorthe reactor and aand full a conversion,full conversion, the strong the strong hygroscopic hygroscopic behaviour behavi ofour boron of trioxideboron trioxide called forcalled a water for a surplus water surplus ratio in theratio range in the between range between ﬁve to thirty five to (Equation thirty (Equation (7)). (7)). λ n n The water surplus is deﬁned by the molar amount of water ( nH2O) and boron oxide ( nB2O3 ) as: The water surplus λ is defined by the molar amount of water ( HO2 ) and boron oxide ( BO23) as:

nH2O 1 λ =nHO2 ∗ 1 (7) λ =*3 nB2O3 (7) 3nBO23 In comparison to a surplus of 10, a surplus of ﬁve led to a doubled temperature increase (more thanIn 40 comparison◦C), with a to nearly a surplus full conversion of 10, a surplus (Figure of five14). led Figure to a 15doubled shows temperature the inﬂuence increase of the initial(more temperaturethan 40 °C), andwith thea nearly water full surplus conversion on the resulting(Figure 14). temperature. Figure 15 shows Providing the ainfluence full conversion, of the initial with atemperature lower initial and temperature the water surplus enables on a higher the resulting rise of temperature,temperature. whileProviding at initial a full temperatures conversion, with above a 70lower◦C thereinitial was temperature no further enables measurable a higher temperature rise of temperature, rise. while at initial temperatures above 70 °C there was no further measurable temperature rise. A further decrease of the water surplus while simultaneously solving the agglomeration problem enables a further significant increase of the resulting temperature. Due to this strong influence on the reaction, appropriate storage conditions are necessary to avoid a degradation of the reactant and to guarantee the best reaction kinetic and conservation of the stored energy.

Energies 2019, 12, 1086 14 of 17 Energies 2019, 12, x FOR PEER REVIEW 14 of 17

Figure 14. Temperature rise of the hydration reaction de dependingpending on different water surplus ratios ( λ) with a common initialinitial temperaturetemperature ofof 2525 ◦°CC[ [35].35].

λ Figure 15. Total temperature rise of the hydration re reactionaction as a function of water surplus λ (20, 15, and 10) and initial temperature TTi [[35].35]. and 10) and initial temperature Tii [35]. A further decrease of the water surplus while simultaneously solving the agglomeration problem 4. Conclusions enables a further significant increase of the resulting temperature. Due to this strong influence on the The outcome of these investigations demonstrates the capability of the boric acid–boron oxide reaction,The appropriateoutcome of these storage investigations conditions are demonstrate necessarys to the avoid capability a degradation of the boric of the acid–boron reactant and oxide to reaction for the application as thermochemical storage system. Compared to other thermal energy guaranteereaction for the the best application reaction kinetic as thermochemical and conservation storage of the system. stored Compared energy. to other thermal energy 3 storage systems, the system shows a very high theoretical energy density of 2.2 GJ/m3 (measured with 3 4.losses: Conclusions 1.7 GJ/m3) without significant problems for a storing process. Therefore, it is a suitable candidate for thermal energy storage. Even if there is still a lot of work to do until then, it definitely candidateThe outcome for thermal of these energy investigations storage. Even demonstrates if there is still the a capabilitylot of work of to the doboric until acid–boronthen, it definitely oxide has a strong potential to meaningfully contribute to a global sustainable energy supply. In addition, reactionhas a strong for thepotential application to meaningfully as thermochemical contribute storage to a global system. sustainable Compared energy to other supply. thermal In addition, energy the analysis with reduced sample masses has shown a significant reduction of the measured reaction storagethe analysis systems, with thereduced system sample shows masses a very has high shown theoretical a significant energy reduction density of the 2.2 measured GJ/m3 (measured reaction temperatures. In accordance with the recommendations of the International Confederation for withtemperatures. losses: 1.7 In GJ/m accordance3) without with significant the recommend problemsations for a storingof the process.International Therefore, Confederation it is a suitable for Thermal Analysis and Calorimetry (ICTAC), the resulting temperature for the decomposition candidateThermal Analysis for thermal and energy Calorimetry storage. (ICTAC), Even if there the isresulting still a lot temperature of work to do for until the then, decomposition it definitely reaction ranges between 70 °C and 200 °C. Furthermore, no evidence regarding the presence of more hasreaction a strong ranges potential between to 70 meaningfully °C and 200 °C. contribute Furthermor to a globale, no evidence sustainable regarding energy the supply. presence In addition, of more than two thermally-induced steps of decomposition could be found. Unfortunately, the volatility of

Energies 2019, 12, 1086 15 of 17 the analysis with reduced sample masses has shown a signiﬁcant reduction of the measured reaction temperatures. In accordance with the recommendations of the International Confederation for Thermal Analysis and Calorimetry (ICTAC), the resulting temperature for the decomposition reaction ranges between 70 ◦C and 200 ◦C. Furthermore, no evidence regarding the presence of more than two thermally-induced steps of decomposition could be found. Unfortunately, the volatility of the material in the presence of water vapour impeded a thermogravimetric analysis of the hydration process, which is why the results did not show the full (theoretical) potential of the reaction. However, the experimental results using a Dewar reactor and a water surplus, to ensure a good mixing quality and a complete conversion, granted promising results for the future.

5. Patents As result of this research, the application of this reaction system as thermochemical storage system was patented [36].

Author Contributions: Conceptualization: C.H.; data curation: C.H.; formal analysis: C.H. and S.S.J.; investigation: C.H., S.S.J., and C.J.; methodology: C.H. and S.S.J.; project administration: M.H. and A.W.; resources: S.S.J., M.H., A.W., and F.W.; software: C.H. and S.S.J.; supervision: M.H., A.W., M.S., and F.W.; validation: S.S.J. and C.J.; visualization: C.H.; writing—original draft: C.H.; writing—review and editing: C.H., S.S.J., C.J., M.S., and F.W. Funding: Austrian Research Promotion Agency (FFG), SolidHeat Pressure (#853593), SolidHeat Kinetics (#848876), and SolidHeat Basic (#841150). Acknowledgments: The authors thank the Austrian Research Promotion Agency (FFG) for the financial support of the project SolidHeat Kinetics (848876) and SolidHeat Pressure (853593. The Universitäre Service-Einrichtung für Transmissions Elelektronenmikroskopie (USTEM) and the X-Ray Center (XRC) of TU Wien are acknowledged for providing access to the scanning electron microscopy (SEM) and powder X-ray diffractometers. Many thanks to the research group of Mechanical Engineering and Clean Air Technology of the institute for providing the Mastersizer 2000 for measuring the particle size distribution of samples. The authors acknowledge the TU Wien University Library for financial support through its Open Access Funding Program. Conflicts of Interest: The authors declare no conflict of interest.

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