From Jonathan Allen:

Re: Scientists work on sun-charged "heat battery" (Oct. 27): There are other systems for storing heat, such as the latent heat of a molten salt. It is my understanding that an advantage of this system is that the "charged" material is not hot and hence will not lose its "charge" over time by thermal leakage. For solar-powered engines which need to run at night but get their heat replenished the next day, leakage is not a big problem, but for longer storage times it would be, and (if I understand the article correctly) this is where the "heat battery" should have an advantage.

A couple of big questions, aside from cost, are the energy density and efficiency. That is, how many joules can one store per unit mass or volume of the storage medium compared with, for example electrical storage batteries, especially since the heat storage is upstream of the heat engine? And how would the overall system efficiency (sunlight to recovered work) compare with that of an electrical storage battery charged by photovoltaic panels? In a solar thermal system, raising the collector temperature improves the engine efficiency (Carnot's Equation) but it also reduces the solar collection efficiency due to re-radiation from the absorber (Planck Theory). Thus there is a temperature tradeoff or optimization. Another point is that to generate temperatures approaching 200 deg C requires focusing concentrators. Thus only specular (direct) sunlight will contribute. Scattered light from the sky will not, and when the sun goes behind a cloud, the system output is nil. A photovoltaic system, on the other hand harvests all incoming rays and continues to work, albeit at reduced output, even on a cloudy day.

I am all for exploring this heat battery, but remain skeptical as to its widespread application even if we find a cheaper medium without the rare element ruthenium.

- Jonathan Allen, Ph.D.