Converting Waste Heat to Electricity

Tuesday, December 16, 2014 @ 05:12 PM gHale

It is a basic contemporary issue: Modern power generation methods need to squeeze the most power from the least amount of fuel.

Along those lines, 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.

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One way to achieve this goal is to create an ammonia-based battery that not only captures and converts waste heat economically and efficiently, but can do so at a greater capacity than other similar systems. That is just what engineers at Pennsylvania State University are working on.

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 simply lost to the atmosphere via cooling towers or engine exhausts.

This is the case in 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 can 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 convert roughly 15 to 20 percent of heat to energy, while other more efficient substances, such as fulvalene diruthenium promise greater returns, but are far too expensive and rare to be practical yet.

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, Penn State researchers said. Unlike other batteries, however, the ammonia electrolyte is an anolyte (electrolyte surrounding an anode) that reacts with the copper electrode as the ammonia heats, 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 play. 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 ends up re-deposited on the electrode in what is now the cathode (formerly the anode) chamber.

In other words, the ammonia switches 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.”

The researchers said they can get a power density of around 60 watts per square meter over numerous charge/discharge cycles, along with an assertion their battery system power density is six to 10 times higher than that created by other liquid-centered thermal-electric energy conversion systems. Researchers saw increases in power density as a result of increasing the number of batteries in the system, thereby indicating the prototype may scale up to make it commercially viable.

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

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