Thermal Energy Storage

Thermal Energy Storage Materials & Systems

Many people do not realize that the majority of the energy that we use as a country is consumed in the form of heat, not electricity. A full 63% of the energy we use is heat to power industrial manufacturing processes, transportation, or to regulate the temperature of residential and commercial buildings. Right now, almost all of this heat comes from burning fossil fuels. However, by developing new technologies and materials designed from the ground up to store and convert heat, our group is helping to increase the share of thermal energy that can come from renewable sources, and the efficiency with which we use it.


Tunable Thermal Energy Storage for Smart Building Envelopes

New approaches to energy storage that can provide flexibility are essential for increasing the reliability and resiliency of our energy systems. To meet this challenge, we are developing dynamically tunable, and solid-state thermal energy storage materials integrated with thermal switches for building envelope application. This new technology has the potential to enable optimal thermal routing in both space and time.  Combining the new thermal switches with dynamically tunable thermal storage should enable: thermal micro-grids within a building envelope; time shifting thermal loads, for example to exploit nighttime cooling the following afternoon; and intelligent thermal isolation among local hot/cold zones within a single building.Material embedded within a building wall acts like a thermal energy storage battery, and can control the room temperature by absorbing or releasing heat.

The dynamically tunable thermal storage materials developed here can modify their switching temperature or characteristics to operate optimally in both summer and winter. This will significantly reduce the cost of thermal storage when compared to that of existing thermal storage materials that can perform optimally in either summer or winter. To enable flexibility we are developing thermal switches, which could control directional heat transfer. The thermal switches will be integrated with dynamically tunable thermal storage to provide control over the timing of charging or discharging. This project is funded by the DOE’s Building Technology Office, and is in collaboration with Gao Liu in ESDR, Prof. Chris Dames in UC Berkeley and NREL

Thermochemical Storage for Buildings

Our team is developing thermochemical material (TCM)-based thermal energy storage. In a TCM, energy is stored in reversibly forming and breaking chemical bonds. TCMs have the fundamental advantage of significantly higher theoretical energy densities (200 to 600 kWh/m3) than phase change materials (PCMs; 50 to 150 kWh/m3). They also exhibit negligible self-discharge because the energy is stably locked up in chemical bonds, making them uniquely suited as compact, stand-alone solutions for daily-seasonal energy storage in buildings. TCMs can be used as a thermal battery, charging with solar energy or excess grid electricity and discharging to supply thermal end-uses in buildings such as space and water heating.

A thermochemical storage tank connected to a building is charged using sunlight or grid electricity for diurnal or seasonal storage, and discharged at night or in winter for various thermal end-uses. Charging triggers an endothermic dehydration reaction below 100 C. Discharging triggers an exothermic hydration reaction.

Although promising, the available research shows TCM-based storage suffers from instabilities both at the material and reactor level resulting in poor multi-cycling efficiency and a high levelized cost of thermal energy storage. Our aim is to fundamentally investigate TCMs to overcome these challenges by developing new design rules at both the material and reactor level. This project seeks to bridge the gap between the high theoretical storage potential of thermochemical salt hydrates (>600 kWh/m3) and their sub-par performance when integrated into real thermochemical reactors for energy storage with repeated cycling (<70 kWh/m3, and fewer than 20 cycles). This project is funded by the DOE’s Building Technology Office, and is in collaboration with Prof. Chris Dames in UC Berkeley, NREL and NET Energy Inc. 

Composite Materials for High Temperature Thermal Energy Storage

Large-scale inexpensive energy storage could smooth out the timing disparity between renewable energy over-production and grid demand, enabling the switch to a 100% renewables-powered grid and reducing global greenhouse gas emissions by ~25%. Most existing energy storage technologies are either too expensive to use at these scales (e.g. electrochemical batteries), or have other limitations such as geographic constraints (e.g. pumped hydro).

To solve this problem, we are developing a composite material capable of converting excess electricity into heat and storing it at temperatures in excess of 2,000 C, from which it can be dispatched up to 100 hours later when demand peaks. The high-grade thermal energy can then be converted back into electricity for the grid, or supplied directly as process heat to industrial manufacturing processes (e.g. for making steel or cement, which require T > 1,500 C) that otherwise rely on burning fossil fuels to supply steady thermal energy. This project is funded by the Laboratory Directed Research and Development Program (LDRD) and ARPA-E. Project collaborators include Antora Energy.

An optimized composite material stores heat at temperatures over two thousand C. The thermal energy storage can be charged using electricity from wind, nuclear, solar, or other low-priced grid electricity. Later, the thermal energy can be dispatched as heat to high-temperature industrial applications, or converted back into electricity.