Session: 08-09-01: Thermal Energy Storage

Paper Number: 99292

99292 - Salt Hydrate Composites for Thermochemical Energy Storage in Buildings 

In the United States, a third of the primary energy consumption is attributed to the building sector. Of this, about 60% is used for space heating and hot water, which motivates the need to decarbonize these thermal loads. Promising solutions include solar heating and renewable electrification, but their intermittency necessitates the use of thermal energy storage (TES) to match demand and supply. Of the different TES categories, thermochemical materials (TCMs) exhibit average volumetric energy densities that are an order of magnitude higher than sensible heat storage and at least 2x higher than phase change materials (PCMs). This compactness attribute is especially important for adoption of TES in buildings where space is limited. Furthermore, TCMs exhibit negligible heat loss (i.e., no self-discharge) as they store and release energy via reversible chemical reactions. A TCM-based storage system can be implemented using salt hydrates that undergo an endothermic dehydration reaction (charging) and an exothermic hydration reaction (discharging) at a given temperature and relative humidity. Selection of the salt hydrate requires careful consideration of multiple thermodynamic and kinetic properties that influence both heat and mass transport such as the energy storage density, hydration/dehydration temperatures and vapor pressures, thermal conductivity, hygrothermal stability during cycling, porosity, etc.. Owing to this complexity, there is  a large discrepancy between theoretical and experimentally measured energy density values in the literature. This is also attributed to the poor hygrothermal stability of these materials under cycling – salt particles either agglomerate and form compact blocks that hinders water vapor diffusion (results in slower hydration kinetics), or they undergo deliquescence (results in a loss of their energy storage ability). To mitigate these stability challenges, we discuss two promising approaches to achieve form-stable thermal batteries based on TCMs: (i)  an inorganic-organic composite material that encapsulates the salt into a highly porous hydrogel matrix to form mm-size beads instead of individual µm-size particles that agglomerate, and (ii) a binary mixture of two salt hydrates with complementary phase diagrams and hydration kinetics that is more stable compared to the individual salts. The novelty in these approaches implements porous hydrogels with high salt loading, as well as constructing a standard approach in salt selection for binary mixtures. We demonstrate that both these approaches enhance water vapor diffusion (high permeability) and storage density (high salt loading), while maintaining structural stability under thermal cycling. Implementation of TCM-based energy storage  for space heating in the buildings could yield a primary energy savings potential of 25% , with the added advantage of lowering the carbon footprint significantly.

Presenting Author: Erik Barbosa Georgia Institute of Technology

Presenting Author Biography: Erik received his B.S. in Mechanical Engineering at Brigham Young University in 2021 where he conducted research on development of a sensor to measure the thermal conductivity of molten salts using the transient hot wire method. Erik also did a summer research internship at Los Alamos National Lab, where he used multi-sensor collaborative sampling schemes to reconstruct mechanical system signals. Currently, he is a Ph.D. student in Mechanical Engineering at Georgia Tech, where his research is focused on developing thermochemical energy storage to decarbonize heat for building applications. Outside of research, Erik enjoys outdoor activities and playing video games.

Authors:

Erik Barbosa Georgia Institute of Technology
Akanksha Menon Georgia Institute of Technology