As modern power generation methods are designed to squeeze the most power from the least amount of fuel, engineers are constantly looking at techniques to improve efficiency. One way to achieve this is to scavenge waste energy left over from the production process to capture and convert low-grade heat into usable energy. In pursuit of this goal, engineers at Pennsylvania State University have produced an ammonia-based battery that not only captures and converts waste heat economically and efficiently, but is claimed to do so at a greater capacity than other similar systems.

Waste heat is a consequence of all powered mechanical work and energy generation and – depending upon the efficiency of energy conversion – may produce a large amount of heat that is simply lost to the atmosphere via cooling towers or engine exhausts. This is particularly true of coal and nuclear power plants that generate high temperatures in the production of electricity, and consequently large amounts of low-grade heat. The Thermally Regenerative Ammonia Battery (TRAB) created by the Penn State engineers is designed to capture this waste heat, wring out its remaining energy and store it for later use.

"The use of waste heat for power production would allow additional electricity generation without any added consumption of fossil fuels," said Bruce E. Logan, Evan Pugh Professor and Kappe Professor of Environmental Engineering. "Thermally regenerative batteries are a carbon-neutral way to store and convert waste heat into electricity with potentially lower cost than solid-state devices."

Other methods of converting waste heat to electrical energy often produce too small a charge relative to the amount of electrolyte or conversion material used; telluride based batteries, for example, convert roughly only 15 to 20 percent of heat to energy, whilst other more efficient substances, such as fulvalene diruthenium promise greater returns, but are far too expensive and rare to be practical yet.

According to the Penn State researchers, the new thermally regenerative battery system uses copper electrodes and plentiful ammonia as an electrolyte and converts around 29 percent of the chemical energy contained in the battery into electricity. Unlike other batteries, however, the ammonia electrolyte is only used as an anolyte (electrolyte surrounding an anode) that reacts with the copper electrode as the ammonia is heated, generating electricity. The reaction of the ammonia with heat on the copper electrode, however, can only last so long.

"The battery will run until the reaction uses up the ammonia needed for complex formation in the electrolyte near the anode or depletes the copper ions in the electrolyte near the cathode," said Fang Zhang, postdoctoral fellow in environmental engineering. "Then the reaction stops."

This is where this new type of battery comes into its own. Harnessing waste heat from an outside source, the researchers distil ammonia from the used fluid in the battery anolyte chamber and then recharge it into the battery’s cathode chamber. As a result, the chamber now containing the ammonia becomes the anode chamber and copper is re-deposited on the electrode in what is now the cathode (formerly the anode) chamber.

In other words, the ammonia is switched back and forth between the two holding chambers, thereby sustaining the amount of copper deposited on the electrodes.

"Here we present a highly efficient, inexpensive and scalable ammonia-based thermally regenerative battery where electrical current is produced from the formation of copper ammonia complex," said the researchers in their report. "When needed, the battery can be discharged so that the stored chemical energy is effectively converted to electrical power."

A power density of around 60 watts per square meter over numerous charge/discharge cycles is claimed by the researchers, along with an assertion that their battery system power density is six to 10 times higher than that being created by other liquid-centered thermal-electric energy conversion systems. Further, increases in power density were observed by the researchers as a result of increasing the number of batteries in the system, thereby indicating that the prototype may be scaled up to make it commercially viable.

Given that the prototype battery has been constructed using non-critical components in a laboratory, the engineers also noted that further optimizing of the battery’s components and construction could also yield even greater power, whilst using commercial construction techniques should reduce costs.

Supported in part by the King Abdullah University of Science and Technology, researchers have a preliminary patent filed for this work, which was published in the journal Energy & Environmental Science

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