The 2 MWh Rondo heat battery at the Calgren ethanol plant in California.
courtesy Rondo Energy The 2 MWh Rondo heat battery at the Calgren ethanol plant in California.

It has been suggested that we live in the Age of the Battery. The proliferation of renewable power and the inherent need for energy storage, in addition to the increasing use of mobile electronics and electric vehicles, have motivated advances in battery technologies. The current standard is lithium-ion technology, the widespread energy storage medium powering most mobile phones and hybrid cars. Although Li-ion has impressive storage and recharging capacities, increasing demand for lithium has led to environmental concerns and geopolitical tensions. As a result, industries are looking for alternative substances that can replace or augment Li-ion technology. A surprising area of focus is building materials.

Inexpensive, mineral-based substances often used in construction have recently gained attention as thermal energy storage solutions. As the name implies, TES systems store heat and release it over time, helping to offset energy supply and demand. An established approach seen in solar thermal technologies and district heating systems, TES systems are increasing in popularity because they are cheaper than electricity storage, lack the problems associated with Li-ion, and have the potential to play a critical role in realizing a clean energy economy. From an architectural perspective, TES is essentially thermal mass. The same capacity of materials like brick and stone to store and delay heat dissipation, thus offsetting peak temperatures, enables such materials to function as thermal batteries.

Several companies have developed new technologies based on this approach. For example, Rondo Energy, based in Alameda, Calif., offers a heat battery made from bricks. The battery is a self-contained unit consisting of a stack of bricks and electrical heating elements that warm the modules via renewable power sources. The heaters raise the temperature of the bricks to 1,500 degrees Celsius, converting 100% of the renewable electricity into thermal energy. The bricks’ stored energy may be tapped through the delivery of forced air, which heats up to 1,000 degrees Celsius as it passes over the stack. This heated air is optimal for use in industrial processes ranging from cement and steel production to municipal infrastructure such as district heating or grid energy storage. As renewable power technologies become more capable and accessible, the heat battery represents a scalable solution to stabilize intermittent sources of renewable power and decarbonize many existing processes.

Brenmiller's bGen unit.
courtesy Brenmiller Brenmiller's bGen unit.

Israel-based Brenmiller offers a similar approach, using crushed rock as its TES technology. Brenmiller’s so-called bGen unit is activated via renewable sources that are either electrical or thermal, such as flue-gas or biomass-based heat. The temperature of the volcanic rock increases to 750 degrees Celsius, and the heat is stored for current or delayed use. Compared with Rondo’s air-based strategy, bGen employs water as its heat-delivery vehicle, utilizing a system of pipes to circulate liquid around the hot rock—and eventually transform the water into steam. The unit is designed to maintain a constant steam supply for on-demand utility and industrial applications similar to those of the heat battery. In addition, Brenmiller’s technology can be deployed as a thermal power plant, as seen in existing projects for Italian energy company Enel, the New York Power Authority, and other partners.

Another capable thermal storage material is sand. Finland-based Polar Night Energy has developed a sand battery as an energy reservoir for excess renewable power. The company claims to have installed the world’s first commercial sand battery in Kankaanpää, a town in Western Finland, as an integral part of a district heating network that includes commercial and residential uses. PNE’s first prototype is a 23-foot-tall enclosure containing 100 tons of sand, serving a community of 10,000 residents. The unit is sufficiently effective to mitigate seasonal differences in renewable power, as heat derived from solar cells during the summer may be stored for use during Finland's long winter nights. PNE currently offers two products, a 300 MWh capacity unit and a 1000 MWh capacity unit, with the potential to develop larger capacity systems.

Polar Night Energy’s sand battery.
courtesy Polar Night Energy Polar Night Energy’s sand battery.

Given the ubiquity of Li-ion technology, employing bricks, rock, and sand to store massive quantities of renewable energy for industry and utility use is eye-opening. These materials are not only much more widely available and less geopolitically divisive than lithium but also omnipresent in architecture and landscape architecture. This mineral-battery trend invites speculation about the potential benefits of merging buildings and infrastructure even more closely. For example, what if building enclosures could double as heat batteries? Such a hybrid application, if safely executed, could effectively combine thermal mass–based climate control with energy storage for heating, hot water, and electricity. Furthermore, the distribution of renewable power and heat batteries among individual edifices would decrease the demand on centralized utilities.

Powered enclosures would require rethinking existing building systems, transforming mono-functional elements like brick veneer walls into multifunctional masonry systems where energy capture is a prominent feature. As we look to a zero-carbon future, the architecture of energy represents a compelling design opportunity.

The views and conclusions from this author are not necessarily those of ARCHITECT magazine or of The American Institute of Architects.

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