## 4 Thermochemical Storage in Power-to-Heat Applications
### Discussion and Outlook  
The articles reviewed show emerging power-to-heat/thermochemical applications as flexible coupling systems to address both integration of renewable energies and additional grid flexibility. High efficiency in balancing the excess of renewable generation is the key aspect that could led these applications towards an increasing development in the next future. Investigating the demand flexibility of power-to-heat conversion with thermochemical systems was a common aim of all authors. All three dimensions of flexibility were investigated: size (energy), time (power) and costs. A number of indicators were proposed to quantify the energy flexibility in terms of available storage capacity and/or efficiency. The usage of a non-common quantification method to estimate the energy flexibility makes difficult a straightforward comparison among the reviewed studies. Despite this limit, important considerations can be argued as follows. According to the thermodynamic and numerical analyses, the overall efficiency of the coupled system range from 39% to 56%. The highest value is obtained in power-to-heat/thermochemical applications coupled to power cycle [317], overcoming typical efficiencies of conventional power cycles. The reason lies in the use of raw thermochemical materials requiring higher operating temperatures, which increase the upper limit of the achievable thermodynamic efficiency according to [PERSON] \begin{table}
\begin{tabular}{c c c c} \hline \hline
**References** & **Application** & **Storage Material** & **Performance Indicators** \\ \hline [PERSON] et al. [139] & Power-to-heat (household application) & SrBr\({}_{2}\)/H\({}_{2}\)O & Energy density: 500 kJ/kg \\  & Power-to-heat & & \\ [PERSON] et al. [173] & (integrated into electric driven cooling system) & BaCl\({}_{2}\)/NH\({}_{3}\) & Energy density: 200 kJ/kg \\  & Power-to-heat & & \\ [PERSON] et al. [175] & (integrated into electric driven cooling system) & Zeolite 13X/H\({}_{2}\)O & Capacity: 5.6 kWh \\  & & & Efficiency: 0.96 \\ [PERSON] et al. [245] & Power-to-heat (to power) & Co\({}_{3}\)O\({}_{4}\)/CoO & Energy density: 3.9 kWh/m\({}^{3}\) \\ [PERSON] et al. [235] & Power-to-heat (to power) & CaCO\({}_{3}\)/CoO & Efficiency: 56.4\% \\ [PERSON] et al. [316] & Power-to-heat (to power) & CaCO\({}_{3}\)/CoO & Overall plant Efficiency: 39\% \\ [PERSON] et al. [316] & Power-to-heat (to power) & CuO/Cu\({}_{2}\)O & Energy density: 1600 kJ/kg \\  & & & Efficiency: 50\% \\ [PERSON] et al. [318] & Power-to-heat (to power) & NH\({}_{3}\)/LiNO\({}_{3}\) & Capacity: 0.36 MWh \\  & & & Efficiency: 44.3\% \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Thermochemical storage in PH and PH/HtP applications. Figure 11: PH/TCTES system developed by [PERSON] et al. [318]. * Heat is exchanged between the two tanks in order to compensate the ammonia expansion/compression cycle. principles. This suggests that more efforts should be paid to the design and test of thermochemical materials and related physical-chemical reactions, in order to boost further the process efficiency in view of the development of optimized systems. The studies reported in [246; 317] suggest that the high efficiency and flexibility of these innovative applications could be able to facilitate the integration in the power system not only of the photovoltaic but also of the wind power. A development in the wind energy integration could be crucial in energy systems characterized by a large share of wind power. High storage density, low heat loss, long storage period, highly compact energy storage are the main advantages common to all the power-to-heat/thermochemical technologies. Despite this, a series of limits, such as the high costs of the materials and the complexity of the equipment, makes these applications still not mature for large scale/market adoption as shown by the few prototypes developed and tested so far. Costs abatement and process simplification in optimized systems require further efforts for the development of techno-economically competitive applications.