Sep 10 2020

Miscibility Gaps Alloy Thermal Storage

I was recently sent this article about a new miscibility gaps alloy (MGA) thermal storage material. The technology is, perhaps, an incremental advance and may be useful for grid storage, but the article itself represents, in my opinion, horrible science communication. It seems like what you get when a general reporter, not trained in science journalism, reports on a complicated science topic. It didn’t give me any of the information I wanted, didn’t put this new technology into meaningful or accurate context, and didn’t explain some basic concepts involved.

Here is the basic story – a University of Newcastle (in Australia) team has developed an MGA material that could potentially be useful in grid storage by serving as a medium for thermal energy storage.  They also describe what an MGA material is by using an analogy to a chocolate chip cookie, where the chocolate chips melt when heated, storing most of the energy, but the rest of the cookie remains solid. That is about all the information you get from this article, stated in two sentences. The chocolate chip cookie analogy is fair, but following up with a slightly more technical definition would have been nice. MGAs are mixed materials where there is a range of temperatures (more specifically a region of the phase diagram that includes both pressure and temperature) where the different materials are in two or more phases. The rest of the article just states over and over again in different ways, like this is a new idea, why grid storage would be useful.

Why are MGAs particularly useful for thermal energy storage? First, the particles that melt store a lot of energy in the phase change while the particles that don’t melt can maintain the solidity of the overall material. But further, because of the liquid components, these materials have great thermal conductivity, so they don’t need infrastructure just to conduct the heat through the material. The Newcastle MGA is supposed to be an innovation because it is made from readily available material that is non-toxic.

I came away from the article with lots of important questions, all unanswered, and had to research them for myself. I was able to find information about MGAs in general, but not the Newcastle MGA specifically.

My first question, which you should ask about any proposed grid storage option, is – what is the round-trip efficiency? There are lots of grid storage options (which I review here), none of which are perfect. We need to know about each – what is the cost, how scalable are they, are they location-specific, what are the environmental effects, what are the energy losses over time, and what is the round-trip efficiency (the loss of energy from converting grid electricity to storage and then back to grid electricity). The best round trip efficiency is from pumped hydro, about 80-90%, but this is very limited by location and has serious environmental implications. Battery storage is not bad, at 60-70% round trip efficiency, but this is still an expensive option with lots of material waste and a limited lifespan.

There is no one answer for MGAs, and the efficiency depends as much on the system as the material used for thermal storage. There are several ways stored thermal energy can be used. It can, for example, be used directly, as heat. A lot of the energy we use is for heating anyway, so why convert heat to electricity and then back to heat – just distribute the heat directly. For this application lower temperature MGAs can be used (<600 C). But you can also store heat either from heating elements powered by electricity or directly from solar mirrors (not photovoltaics). These higher temperature MGAs can then be part of either a traditional turbine system or a multijunction photovoltaic (MPV) heat engine. The round trip efficiency of using thermal energy to run a turbine and generate electricity is 30-40%, so much lower than batteries or pumped hydro. The MPV system uses the heat to heat an element which essentially glows, and the photons from the glow are then converted to electricity by high-end photovoltaics, with an efficiency of 25-33%.

Using the heat directly seems to be the best option. As grid storage, any thermal storage system suffers from inherent low round trip efficiency. So this is not likely to be the grid storage revolution the article implies. It is potentially one more option among many grid storage options, and clearly not the best. All grid storage systems have trade offs, and the trade off for this one is low round-trip efficiency.

This is interesting for another reason. Renewable energy sources, which are intermittent, can be used extensively in one of two basic ways. First, there’s grid storage – store the energy for use when the sun is not shining and the wind is not blowing. But the second is to have a large grid with overproduction capacity of energy. This means, for example, that you have wind and solar capable of producing 150% (to use a hypothetical figure) of demand, but averaged out across the grid you can meet 100% of demand everywhere. The idea is that the wind is always blowing somewhere. The point is – both options require overproduction. If you have a 50% round trip efficiency of storage, and half the energy produced is stored for later use, then you need to produce 150% of demand (again, using hypothetical figures). Either way, you need overproduction, which reduces the cost efficiency of renewables the greater the percent of total energy production they represent.

So – will MGA be the revolution the article implies? No. But they may be one more imperfect option among many imperfect options, and they may find their niche. I think using MGAs for heat storage and distribution, rather than grid storage, is the better option. But there may be some limited grid storage options if systems are designed and optimized with MGAs in mind. I can imagine, for example, a solar mirror array heating an MGA tower that runs a turbine or MPV to generate electricity.

But I would have loved a link to a technical article describing all the actual specific information related to the Newcastle MGA being discussed in the news report. It was painful to read such superficial and useless science reporting.

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