# Key Challenges for High Temperature Thermal Energy Storage in Concrete—First Steps towards a Novel Storage Design

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Innovative Concrete Formulations for High Temperature TES

## 3. Conventional TES Concept

^{3}. The prototype (Figure 1) was tested at the Plataforma Solar de Almeria in Spain in 2003–2004 [9,12,17].

^{®}XA with reduced environmental impact. The objective of this system was to produce thermal energy for industry, not electricity, and the results for this design were presented in Ktiskis et al., 2021 [18].

## 4. Challenges

- (i)
- On-site construction.

- (ii)
- Different thermal expansion coefficient of steel and concrete.

- (iii)
- Poor thermal conductivity of concrete.

- (iv)
- HTF thermal oil or molten salts with limited operating temperature range.

- (v)
- HTF thermal oil or molten salts in direct contact with concrete: migration of oil/salt in concrete.

## 5. New Concept Proposal

^{3}/s (optimised for improved air transfer coefficient) until the concrete block was fully charged (Figure 5). The figure shows that the concrete block reached 450 °C homogeneously, where the difference between the inlet section and the outlet section of the module was about 50 °C in the middle of the charging period (Figure 5b). The total energy stored in the concrete block was 4266 kJ (1.185 kWh).

## 6. Conclusions and Future Work

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**DLR concrete storage concept. Reprinted/adapted with permission from Ref. [9]. 2009, American Society of Mechanical Engineers.

**Figure 2.**Concrete thermal element design for the HEATCRETE vp1 concept [20].

**Figure 4.**Concrete block concept: (

**a**) fitting connections, (

**b**) stacked distribution example, (

**c**) connection points.

**Figure 5.**Computational fluid dynamics of the proposed concrete block: (

**a**) initial charge, (

**b**) mid-charge, (

**c**) full charge.

Formulation | Cement | Water-Cement Ratio | Sand | Aggregate | Super- Plasticizer | Curing And Drying Protocol | Thermal Cycling | Compression Strength (MPa) | Porosity | Thermal Properties | Reference |
---|---|---|---|---|---|---|---|---|---|---|---|

Cement paste | OPC | 0.34 | --- | --- | --- | 28 days curing in water | 20–200 °C 20–400 °C 20–600 °C 20–800 °C | Loss of stability in thermal cycling above 400 °C | Open porosity decreases with thermal cycles | Decrease in the thermal conductivity from 1 W/m·K to around 0.5 W/m·K after thermal cycling | [10] |

Cement paste | CAC | 0.34 | --- | --- | --- | 28 days curing in water | 20–200 °C 20–400 °C 20–600 °C 20–800 °C | Decrease after first thermal cycle with stabilisation later on | Open porosity increased with temperature and thermal cycling | Lower thermal conductivity than OPC but higher heat capacity | [10] |

Mortar | 70% CAC + 30% blast furnace slag (BFS) | 0.44 | Standard siliceous | --- | 1% | 3 days @105 °C | 290–550 °C | 72.67 ± 1.97 (after 7 days curing) | --- | --- | [11] |

Concrete | Blast furnace cement | --- | --- | Iron oxides, flue ash, and other | --- | --- | --- | Medium material strength with several cracks | --- | 916 J/kg·K (@350 °C) 1.0 W/m·K (@350 °C) 9.3 ·10 ^{−6}/K (@350 °C) | [12] |

Concrete | 70% CAC + 30% blast furnace slag (BFS) | 0.5 | Standard siliceous | Natural from crash stone, silicon calcareous aggregate (SCA) | 0.8% | 3 days @105 °C | 290–550 °C | 50% decrease after first thermal cycle with stabilisation later on | 100% increase after thermal cycles | --- | [11] |

Concrete | 70% CAC + 30% blast furnace slag (BFS) | 0.57 | Standard siliceous | Natural SCA + industrial waste slag | 0.8% | 3 days @105 °C | 290–550 °C | 50% decrease after first thermal cycle with stabilisation later on | 100% increase after thermal cycles | --- | [11] |

Concrete | CAC | 0.43 | --- | Basalt 0–6 mm | 0.9% | Left @95% RH and @20 °C until testing | 300–600 °C | --- | --- | 1.2–2 W/m·K Decrease of 20–40% in the thermal conductivity after first thermal cycle | [13] |

Concrete | CAC | 0.43 | --- | CAT 0.25–4 mm | 0.9% | Left @95% RH and @20 °C until testing | 300–600 °C | --- | --- | 1.2–2 W/m·K Decrease of 20–40% in the thermal conductivity after first thermal cycle | [13] |

Concrete | CAC | 0.43 | --- | Slag 0.25–2 mm | 0.9% | Left @95% RH and @20 °C until testing | 300–600 °C | --- | --- | 1.2–2 W/m·K Decrease of 20–40% in the thermal conductivity after first thermal cycle | [13] |

Concrete | CAC | 0.43 | --- | Slat 3–7 mm | 0.9% | Left @95% RH and @20 °C until testing | 300–600 °C | --- | --- | 1.2–2 W/m·K Decrease of 20–40% in the thermal conductivity after first thermal cycle | [13] |

Concrete | CAC | 0.43 | --- | Calcareous 0–6 mm | 0.9% | Left @95% RH and @20 °C until testing | 300–600 °C | --- | --- | 1.2–2 W/m·K Decrease of 20–40% in the thermal conductivity after first thermal cycle | [13] |

Concrete | CAC | 0.43 | --- | Siliceous 0–3 mm | 0.9% | Left @95% RH and @20 °C until testing | 300–600 °C | --- | --- | Up to 5 W/m·K Decrease of 50% in the thermal conductivity after first thermal cycle | [13] |

Concrete | CAC | 0.43 | --- | Siliceous + polypropylene fibres | 1% | Left @95% RH and @20 °C until testing | 300–600 °C | Loss of 74% after one thermal cycle | --- | Decrease of 50% in the thermal conductivity after first thermal cycle | [14] |

Concrete | CAC | 0.43 | --- | Calcium aluminate (CAT) + polypropylene fibres | 1% | Left @95% RH and @20 °C until testing | 300–600 °C | Loss of 63% after one thermal cycle | 1.6–2 µm Increase to 24–27 µm after thermal cycling | Decrease of 50% in the thermal conductivity after first thermal cycle | [14] |

Concrete | CAC | 0.43 | --- | CAT + crushed basalt + polypropylene fibres | 1% | Left @95% RH and @20 °C until testing | 300–600 °C | Loss of 69% after one thermal cycle | 0.7 µm Increase to 24–27 µm after thermal cycling | Decrease of 50% in the thermal conductivity after first thermal cycle | [14] |

Concrete | CAC | 0.43 | --- | CAT + crushed basalt + 15% waste slag + polypropylene fibres | 1% | Left @95% RH and @20 °C until testing | 300–600 °C | Loss of 69% after one thermal cycle | 1.6–2 µm Increase to 24–27 µm after thermal cycling | Decrease of 50% in the thermal conductivity after first thermal cycle | [14] |

Concrete | CAC | 0.43 | --- | CAT + crushed basalt + 30% waste slag + polypropylene fibres | 1% | Left @95% RH and @20 °C until testing | 300–600 °C | Loss of 61% after 1 thermal cycle | 1.6–2 µm Increase to 24–27 µm after thermal cycling | Decrease of 50% in the thermal conductivity after first thermal cycle | [14] |

Concrete | Cement | 0.32 | 9% washed sand 0-4 mm | Aggregate + metal and synthetic fibres | 0.43% | 28 days in a tank @100% HR and @15-20 °C | 50–200 °C 50–300 °C 50–400 °C (1 cycle) | Higher values at low temperature | Density was characterised | Specific heat is constant with temperature treatment Thermal conductivity around 2 W/m·K | [15] |

Concrete/PCM | Cement | 0.37-0.41 | 9% washed sand 0-4 mm | Aggregate + metal and synthetic fibres + PCM impregnate in porous material | 0.43% | 28 days in a tank @100% HR and @15-20 °C | 50–200 °C 50–300 °C 50–400 °C (1 cycle) | PCM content helps in maintaining higher values after thermal treatment | Density was characterised | Specific heat increases with temperature treatment Thermal conductivity decreases strongly with PCM content | [15] |

Geopolymer concrete | 20% OPC + 80% inorganic geopolymer | 0.6 | --- | Steel slag | --- | 1 day @100% RH + 28 days @room temperature | --- | --- | --- | More stable thermal properties than OPC as temperature increases | [16] |

HTF | Melting Point (°C) | Stability Limit (°C) | Viscosity (Pa·s) | Thermal Conductivity (W/m·K) | Heat Capacity (kJ/kg·K) |
---|---|---|---|---|---|

Air | --- | --- | 0.00003 (@ 600 °C) | 0.06 (@ 600 °C) | 1.12 (@ 600 °C) |

Water/steam | 0 | --- | 0.00133 (@ 600 °C) | 0.08 (@ 600 °C) | 2.42 (@ 600 °C) |

Thermal oils | −20 | 300 | --- | ~0.1 | --- |

Mineral oil | −20 | 350 | --- | ~0.1 | --- |

Synthetic oil | −20 | 400 | --- | ~0.1 | --- |

Biphenyl/diphenyl oxide | 12 | 393 | 0.00059 (@ 300 °C) | ~0.01 (@ 300 °C) | 1.93 (@ 300 °C) |

Solar salt (60 wt.% NaNO_{3}-40 wt.% KNO_{3}) | 220 | 600 | 0.00326 (@ 300 °C) | 0.55 (@ 400 °C) | 1.1 (@ 600 °C) |

Property | Concrete |
---|---|

Thermal conductivity x, y, z direction [W/m·K] | 1.01 |

Density [kg/m^{3}] | 2306 |

Specific heat [kJ/kg·K] | 0.837 |

Emissivity [-] | 0.95 |

Transmissivity [-] | 0 |

Electrical resistivity [ohm·m] | 0 |

Wall roughness | 0 |

Property | Air |
---|---|

Density | Equation of state |

Viscosity [poise] | 0.0001817 |

Thermal conductivity [W/m·K] | 0.02563 |

Specific heat [kJ/kg·K] | 1.004 |

Compressibility [Cp/Cv] | 1.4 |

Emissivity | 1 |

Wall roughness | 0 |

Parameter | Value |
---|---|

Resolution factor | 1 |

Edge growth rate | 1.1 |

Minimum points on edge | 2 |

Points on longest edge | 10 |

Surface limiting aspect ratio | 20 |

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**MDPI and ACS Style**

Cabeza, L.F.; Vérez, D.; Zsembinszki, G.; Borri, E.; Prieto, C.
Key Challenges for High Temperature Thermal Energy Storage in Concrete—First Steps towards a Novel Storage Design. *Energies* **2022**, *15*, 4544.
https://doi.org/10.3390/en15134544

**AMA Style**

Cabeza LF, Vérez D, Zsembinszki G, Borri E, Prieto C.
Key Challenges for High Temperature Thermal Energy Storage in Concrete—First Steps towards a Novel Storage Design. *Energies*. 2022; 15(13):4544.
https://doi.org/10.3390/en15134544

**Chicago/Turabian Style**

Cabeza, Luisa F., David Vérez, Gabriel Zsembinszki, Emiliano Borri, and Cristina Prieto.
2022. "Key Challenges for High Temperature Thermal Energy Storage in Concrete—First Steps towards a Novel Storage Design" *Energies* 15, no. 13: 4544.
https://doi.org/10.3390/en15134544