> The system features several attractive properties, such as a long energy storage half-life (40 h) at room temperature,
> Concerning the energy storage efficiency, 0.88% of the solar energy could theoretically be stored in a neat sample (S5, ESI†). The highest efficiency that can be expected for a 5104M solution is 0.02%, and for a 2104M solution is 0.01%
> However, the reaction rate was still too low to be used for macroscopic heat release purposes
This isn't a commercialisable solution. What this is, is fundamental research. Which often looks completely terrible because it's a first attempt at something. Once the relevant quantum effects are more understood, better candidate chemicals can be developed.
So this work seems to be about demonstrating the engineering aspects of molecular solar thermal energy storage. They put all this work into describing the microfluidic chip they use, pumps, and other details. They also use an azobenzene derivative for energy storage, which was probably chosen because the synthesis of it was trivial(2 steps) meaning it was easy for them to make enough for the test set up. Azobenzene has been considered for solar thermal energy storage since about 1976, and I suspect even earlier[0]. But there are better molecules out there. In this paper here[1] the same authors develop a molecule based around norbornadiene that has an estimated energy storage efficiency of 3.8%, an energy density of 0.48 MJ/kg, and a half life of 10 months. They even have a molecule with a half life of 18 years in one of their tables. So this publication is probably more in line with the storing solar power for decades from the headline.
Good point, but I don't understand how to square that with the part where the article specifically says the researcher is working on commercializing it:
> Moth-Poulsen plans to spin off a company that would advance the technology and says he’s in talks with venture capital investors. The storage unit could be commercially available in as little as six years and the coating in three, pending the $5 million of additional funding he estimates will be needed to bring the coating to market.
If the inefficiency is that bad and is a real problem, why are VCs talking to him?
Though in recent decades sourced from naturally-occurring reserves, a practice which raises both suffiency and unintended consequence concerns, these are also produced within the biosphere (though at levels insufficient to present or projected future levels of human consumption), and may be synthesized by artificial means from surplus electricity generation, or possibly, other forms of energy.
Those require a large deal of work to be reformed into an usable form after storage. Even an year is enough to damage them to the point that they are unfit for many mainstream applications.
If directly synthesized in to desired forms, absent an antifungicidal treatment, storage for months to decades is well within reason. Particularly for direct thermal use.
The problem with the really old stuff is that the synthesis process was a bit slipshod and the Designer(s) failed to anticipate the strict quality controls required of present consumers.
Articles like this remind me of this house, which was built in Iowa City, IA by a Physics professor (he was my professor for several classes). At the time this house was being built, ISU had an "active solar" research facility, i.e., using all kinds of mechanical means to turn solar energy into heat for homes. Once this house was built, ISU shut down the "active solar" research because, as this house showed, passive solar was just so, so much cheaper and efficient: https://lhodges.public.iastate.edu/house.htm
It's actually remarkable how much heat a simple "soda can solar panel" or "downspout solar panel" can inject into a room. Add a photovoltaic panel to run a 12v brushless 100CFM fan to stir the air and inject the hot air into the room, and you have a "free" source of 120F air to heat a few hundred square feet. Add a south-facing window shining sunlight onto a dark grey slate tile floor (which acts like heat sink) and the floor will release that heat well into the night. So with two very simple passive elements, one can, to a large degree, supplement a traditional heat source (heat pump, gas furnace, etc.). I've seen this approach work with vacation cabins in the mountains, and RVs, a well as the traditional house-on-the-cul-de-sac.
These heat collectors are cheap and simple to make (<$500), and should be quite reliable, giving a +50 degree F delta with almost zero maintenance and no utility consumption. The only moving part is the inexpensive and reliable fan.
These are particularly suited to cold climates, where thermal capture and storage are possible. I'd first run across this concept via Thorsten Chlupp's Reina, LLC, in Fairbanks, Alaska, where he builds (and lives in) net-zero energy homes based largely on passive designs. The key consideration is thermal energy, though his designs incorporate some PV electrical generation as well.
Passive cooling and dehumidification, both critical to interior comfort in the American midwest, are more challenging. I'd like to see how Hodges accomplished those aspects.
Rocky Mountain Institute (Amory Lovins) is another example of passive / conservation-based energy design, including the firm's landmark facilities in Snowmass, Colorado. I've seen the banana "orchard" within the structure's foyer, as well as other design elements.
Yes. Successful passive solar homes typically aren't all that passive, as they often need fans to transfer heat between the solar space and the storage medium (e.g., Hodges's house had pipes in the cement floor with fans; it was also common in other passive solar houses to have a massive rock bed beneath the foundation with air ducted through it). The traditional passive solar house where a mass floor is heated really only works in climates where it's sunny on a regular basis in winter (basically, the Plains, Rockies, and Southwest). If it can get cloudy, the house will need far more thermal storage and a means of heat transfer and control. The capital cost of the storage and system is considerable, and then operation requires maintenance. Also, the glazing typically needs to be covered overnight to keep from radiating all the heat out. It's not very passive. (I own a so-called passive solar home which is woefully lacking in storage; on a sunny day it'll heat up to 78F inside, but it will lose all the heat out the windows overnight).
Contrast with the one-two punch that is heat pumps and solar panels, each benefiting from mass manufacturing economies of scale. The heat pump can pull in 3KW of heat for every 1KW of electricity applied, and the solar panel can provide the electricity. The total cost is probably less than $20K, works just about anywhere, and provides air conditioning in the summer.
But there's something special about having a low-tech solution which isn't as likely to break, and doesn't require a genius to troubleshoot. As an engineer myself, I prefer a calmer, low-tech approach when possible.
Solar tempering is great! But the point is to get to 100% heating from "passive" solar, you'll need a lot of expensive storage and an active means of controlling heat transfer to/from the storage. It's cheaper to use heat pumps, and, when coupled with solar panels, it lets you get to net-zero, and in existing houses that weren't designed for passive solar.
About 10 years back, I didn't understand why people didn't put up solar thermal panels on their roof, because it was such an efficient way to convert solar energy into hot water.
But since that time, the price of solar PV has dropped so much that panels + wires + an electric water heater is probably the cheapest most maintenance free thing you could do.
That said, I suspect a hydrionic floor system with thermal panels outside for heat and a groundwater loop for cooling might still be the best solution for heating/cooling because of the energy requirements.
I think the domestic applications are partly covered by phase change materials: waxes selected for a melting point at the desired temperature. So, for example, you can embed a 23C phase change material in the plaster of a room - giving the room a high apparent thermal mass at the temperature you want it to stay at. A phase change material formulated to melt at 60C can make a hot water tank much "bigger" for coupling with a solar thermal system. These days houses can and should be built with enough insulation that they need very little heating (in most climes).
On the other hand there probably are valuable industrial applications for this sort of material, particularly now that we really want high efficiency. There might even be uses in neighbourhood or utility scale power project.
> A big unknown is whether the system can produce electricity.
So what! In the winter, demand for energy goes up for heating. With current solar technology, it's impossible for someone to stockpile energy from rooftop solar for winter heating. (Net metering is a numbers game.)
If this works, and I could somehow put panels on my roof or exterior walls that allow me to stockpile energy throughout the year to use for winter heating, it would be "the solution" for me.
Edit: I should add, anyone who sells a "magic thing" that charges up throughout the year, and then provides free heat through the winter, will be extremely rich.
Disappointingly this article doesn't provide a link to the research. I think this paper might be it[0], but I am not sure. So in the paper they develop a material that has an estimated storage efficiency of 3.8%(this might be solar conversion efficiency) and energy density of 0.48 MJ/kg. This material is what's called a photochromic meaning it changes color when exposed to sunlight. This is just like those glasses which darken when you go outside. Well it turns out that many photochromic molecules accomplish this by changing or reconfiguring their molecular structure, with azobenzene being the classic example of this[1].
Now as you may know color changing glasses eventually change back to their original color, this is because the color changed state is at a little bit higher energy than the clear state. There might be a bit of a hill it has to overcome before it can go all the way downhill to the lower energy state, but eventually random thermal motion will push it over. So because the color changed state is at higher energy it can release energy as it falls down.
So the interesting thing they show here is that their molecule has a half life or rate at which it converts back to its original form of 10 months, which is a pretty long time. The energy density is also pretty good too, 0.48 MJ/kg is about the energy density of alkaline batteries. And while 3.8% energy conversion efficiency seems pretty bad it's pretty good for photochromic materials of which efficiencies less than 1% are typical. Of course they want to put this in windows and since you can't make the whole window out of this material the energy density of the material+window plastic ends up being ~2.7-3.8 kJ/kg. Of course there's another problem here too. The material they develop is yellow in its uncharged state only converting to clear when exposed to sufficient sunlight, which may not be very attractive to consumers.
table 1 of the paper you reference [0], also lists "N5d" with a half-life of 18 years
I am confused with the different efficiency metrics, one kind of metric hovers around the single digit percentages, but Table 1 lists Φ/photo conversion [%] between 46% and 77% ...
The single digit efficiency metric is probably overall conversion efficiency of sunlight. The two efficiencies are probably quantum yield, or the efficiency at converting one wavelength of light. We have two because the molecule's tristable. Converting from state 0 to 1 has one efficiency while converting from state 1 to 2 has another. State 0 and 1 also absorb different wavelengths of light. Because sunlight's a mix of different wavelengths not just the two our molecule's active at, the efficiency at sunlight conversion will be different than the quantum efficiency.
The claims are reduced somewhat:
> The system features several attractive properties, such as a long energy storage half-life (40 h) at room temperature,
> Concerning the energy storage efficiency, 0.88% of the solar energy could theoretically be stored in a neat sample (S5, ESI†). The highest efficiency that can be expected for a 5104M solution is 0.02%, and for a 2104M solution is 0.01%
> However, the reaction rate was still too low to be used for macroscopic heat release purposes
This isn't a commercialisable solution. What this is, is fundamental research. Which often looks completely terrible because it's a first attempt at something. Once the relevant quantum effects are more understood, better candidate chemicals can be developed.