I was reading the papers of James Prescott Joule (experimental physicist/chemist from the mid 1800’s). One paper of his discussed the economic possibilities of using zinc batteries instead of steam engines and generators to produce electricity.
I was seriosly thrown off by his statements. Batteries store electrcity that is produced by generators, right? What his he talking about batteries as an alternative to steam engines for?
An alternative to steam turbines
But this is not the case. There are two types of batteries: primary and secondary cells. Secondary cells are the rechargable type, for instance, lead/acid batteries. Primary cells are actually a source of electricity. In a primary cell, the energy is produced by reducing the metal (for instance, reducing zinc sulfide to metalic zinc) in a furnace, and then that thermal energy is released as electricity by being put into a cell with acid and copper oxide and all that.
Primary cells are recharged by taking out the spent zinc slufide or oxide and reducing it in a furnace. Thus electricity is produced without a turbine/generator.
Apparently, this is an ancient method of producing electricity, which far predates generators and steam engines. The builders of the pyramids were known to electroplate gold on some of their vessels, and the absence of evidence of burning torches in the inner depths of the pyramids means the egyptians either worked in the dark or used electric arc lights. I read about ancient electricty in this article.
But how effective is it?
Once I got over the fact that batteries can be a source of electricity and not just a method of storage, I was struck by something else in Joule’s paper. His analysis was very, very thin. One pound of coal produces 5 times the electricity through a turbine that a pound of zinc does in a cell.
That was the extent of his analysis. I guess even the world most famous proponent of the law of conservation of energy can be allowed to slip sometimes. Thermodynamics and the laws of entropy were still new, if even yet ennunciated at this time. A proper analysis would have addressed some more relevant facts.
One important fact is the theoretical limit to the efficency of a steam engine. Carnot says that the maximum possible efficency of any heat engine (internal or external combustion) is determined by the highest achievable temperature. For internal combustion and decentralized solar steam generators, this limit is about 40. Usually less than this is achieved. Centralized nuclear plants do better (~70).
A battery faces no such theoretical limit. If the furnace is well insulated and has good heat exchange between flue gasses and fuel, there is no reason high efficencies couldn’t be achieved on a decentralized scale.
(Actually, both processes face a second temperature dependant efficency, that of heat exchange between fuel and the fluid/metal worked on. But without corispondingly high pressures to contain at high temperatures, the primart cell process is much simpler and less dangerous.)
On a human scale
If electricity is what you want, producing primary cells is more direct and I think has more potential on a smaller scale. On a massive centralized level, scraping out the spent zinc and moving thousands of pounds of zinc metal powder in and out of acid or alkali baths is ridiculous, and a steam turbine makes more sense.
But on another scale, where solar heat and biogas/woodgas are the energy inputs and electricity consumption is not constant (and peak loads for welding and other tasks are massive), periodic smelting of a lot of zinc to be used over a period of time makes more sense. The same furnace may be used for steel the next day or not run at all depending on weather conditions, biomass stores and people’s desires.
Using primary cells solves the energy storage issue, as the energy is immediately put into a very stable, compact form. And although 1lb of coal produces 5 times the electricity through a turbine as a 1lb of zinc in a cell, I’d reckon that lb of coal could produce at least 5 lbs of reduced zinc (just because the efficiency of a chemical reaction over a heat engine). Also, the furnace could be run when conditions for utilizing waste heat and capturing CO2 with algae are favorable and shut down at other times, with a constant flow of electricity coming from the batteries the whole time.
The carbothermic reduction of zinc oxide happens at about 1200C, at which point the pressure of zinc vapor is 1 atm (it needs to be condensed after reduction). The same reduction for zinc sulfide is probably at a similar temperature. The Lalande cell, patented by Felix Lalande and Georges Chaperon a hundred years ago is a suitable battery, and was used by the telegraph industry for quite some time.