Storing Solar Thermal Energy

News from phys.org/:

""Storing Solar Thermal Energy in Chemical Form has the Potential to Make it Indefinitely Storable and Transportable.

David L. Chandler, MIT News Office

A molecule of fulvalene diruthenium, seen in diagram, changes its configuration when it absorbs heat, and later releases heat when it snaps back to its original shape.

Broadly speaking, there have been two approaches to capturing the sun’s energy: photovoltaics, which turn the sunlight into electricity, or solar-thermal systems, which concentrate the sun’s heat and use it to boil water to turn a turbine, or use the heat directly for hot water or home heating. But there is another approach whose potential was seen decades ago, but which was sidelined because nobody found a way to harness it in a practical and economical way.
This is the thermo-chemical approach, in which solar energy is captured in the configuration of certain molecules which can then release the energy on demand to produce usable heat. And unlike conventional solar-thermal systems, which require very effective insulation and even then gradually let the heat leak away, the heat-storing chemicals can remain stable for years.
Researchers explored this type of solar thermal fuel in the 1970s, but there were big challenges: Nobody could find a chemical that could reliably and reversibly switch between two states, absorbing sunlight to go into one state and then releasing heat when it reverted to the first state. Such a compound was discovered in 1996, but it included ruthenium, a rare and expensive element, so it was impractical for widespread energy storage. Moreover, no one understood how the compound worked, which hindered efforts to find a cheaper variant.
Now researchers at MIT have overcome that obstacle, with a combination of theoretical and experimental work that has revealed exactly how the molecule, called fulvalene diruthenium, accomplishes its energy storage and release. And this understanding, they said, should make it possible to find similar chemicals based on more abundant, less expensive materials than ruthenium.
Essentially, the molecule undergoes a structural transformation when it absorbs sunlight, putting the molecule into a higher-energy state where it can remain stable indefinitely. Then, triggered by a small addition of heat or a catalyst, it snaps back to its original shape, releasing heat in the process. But the team found that the process is a bit more complicated than that.
“It turns out there’s an intermediate step that plays a major role,” said Jeffrey Grossman, the Carl Richard Soderberg Associate Professor of Power Engineering in the Department of Materials Science and Engineering. In this intermediate step, the molecule forms a semistable configuration partway between the two previously known states. “That was unexpected,” he said. The two-step process helps explain why the molecule is so stable, why the process is easily reversible and also why substituting other elements for ruthenium has not worked so far..
In effect, explained Grossman, this makes it possible to produce a “rechargeable heat battery” that can repeatedly store and release heat gathered from sunlight or other sources. In principle, Grossman said, a fuel made from fulvalene diruthenium, when its stored heat is released, “can get as hot as 200 degrees C, plenty hot enough to heat your home, or even to run an engine to produce electricity.”
Compared to other approaches to solar energy, he said, “it takes many of the advantages of solar-thermal energy, but stores the heat in the form of a fuel. It’s reversible, and it’s stable over a long term. You can use it where you want, on demand. You could put the fuel in the sun, charge it up, then use the heat, and place the same fuel back in the sun to recharge.”
In addition to Grossman, the work was carried out by Yosuke Kanai of Lawrence Livermore National Laboratory, Varadharajan Srinivasan of MIT’s Department of Materials Science and Engineering, and Steven Meier and Peter Vollhardt of the University of California, Berkeley. Their report on the work, which was funded in part by the National Science Foundation and by an MIT Energy Initiative seed grant, was published on Oct. 20 in the journal Angewandte Chemie.
The problem of ruthenium’s rarity and cost still remains as “a dealbreaker,” Grossman said, but now that the fundamental mechanism of how the molecule works is understood, it should be easier to find other materials that exhibit the same behavior. This molecule “is the wrong material, but it shows it can be done,” he said.
Jeffrey Grossman explains how this material can be used to store and release energy in the form of heat.
Video: Jeffrey C. Grossman; additional editing: Melanie Gonick
The next step, he said, is to use a combination of simulation, chemical intuition, and databases of tens of millions of known molecules to look for other candidates that have structural similarities and might exhibit the same behavior. “It’s my firm belief that as we understand what makes this material tick, we’ll find that there will be other materials” that will work the same way, Grossman said.Roman Boulatov, assistant professor of chemistry at the University of Illinois at Urbana-Champaign, said of this research that “its greatest accomplishment is to overcome significant challenges in quantum-chemical modeling of the reaction,” thus enabling the design of new types of molecules that could be used for energy storage. But he adds that other challenges remain: “Two other critical questions would have to be solved by other means, however. One, how easy is it to synthesize the best candidates? Second, what is a possible catalyst to trigger the release of the stored energy?”
Grossman plans to collaborate with Daniel Nocera, the Henry Dreyfus Professor of Energy and Professor of Chemistry, to tackle such questions, applying the principles learned from this analysis in order to design new, inexpensive materials that exhibit this same reversible process. The tight coupling between computational materials design and experimental synthesis and validation, he said, should further accelerate the discovery of promising new candidate solar thermal fuels.""

Thermochromic glass (IV)

I would like to finish the article at the address http://www.hindawi.com/journals/jnm/2012/491051/ 

""The morphology of the undoped and W-doped VO₂ nanopowders is characterized by SEM as shown in Figure 1. It is observed in Figures 1(a) to 1(g) that the tungsten dopant concentration almost has no effect on the morphology of the nanoparticles, and the particle sizes are about 20–60 nm. The experimental results also indicate that particles will be congregated with the increase of annealing time. Especially, the particles with 2 at% W-doped are relatively uniform, and the size is about 25 nm, which is in favor of the practice application on thermochromic window coatings. As is known, small and uniform particles are relatively easy to disperse in solvent and obtain homogeneous coating. [..]
TEM images of the undoped VO₂ and 2 at % W-doped VO₂ nanopowders are shown in Figures 4(a) and 4(c). The morphologies and sizes of the as-obtained samples are consistent with those of SEM images in Figures 1(a) and 1(e). Figures 4(b) and 4(d) show the lattice-resolved HRTEM images.[..] ""

 -) What is the difference between TEM, SEM and HRTEM?

Here is the answer difference-between-tem-and-sem/
or here on Wikipedia: High-resolution_transmission_electron_microscopy 

""When the phase transition of VO₂ occurs, it exhibits a noticeable endothermal or exothermal profile in the DSC curve. Figure 5(a) shows the typical DSC curves of undoped and 2 at% W-doped VO₂ nanopowders. With 2 at% W-doped sample, Mott phase transition arises at around 44°C and 34.5°C for the heating and cooling cycles, compared to 71°C and 58°C for the undoped VO₂, respectively. The phase transition can be modified under the different factors such as defect density or lattice change . The appearance of endothermal and exothermal peaks during the heating and cooling process confirms the first-order transition between monoclinic VO2 (M) and tetragonal rutile VO₂ (R) . To be vital for the practical thermochromic effect applications, the phase transition temperature of W-doped must be approaching to room temperature. In this case, the phase transition temperature could be reduced to 35°C with 3 at% W-doped in Figure 5(b).""

For the full discussion please see the cited article.

Lihua Chen, Chunming Huang, Gang Xu, et al., “Synthesis of Thermochromic W-Doped VO2 (M/R) Nanopowders by a Simple Solution-Based Process,” Journal of Nanomaterials, vol. 2012, Article ID 491051, 8 pages, 2012. doi:10.1155/2012/491051

Comfort (I)

I would like to continue with the environmental comfort discussion, trying to answer the following question:

-) Which indices should be studied for the environmental comfort?

There are two index called PMV (Predicted Mean Vote) and PPD (Planned Percentage of Dissatisfied). Let us try to understand them.
From materials by E. Moretti , we can read:

     "" The PMV (Predicted Mean Vote) is a mathematical function that depends on: clothing, air temperature, activity, mean radiant temperature, air velocity, humidity.
     

It represents the average grade given by a large sample of people residing in the same environment, expressing their thermal sensation through a psychophysical scale that ranges from a value of +3 (very hot) to -3 (very cold) through intermediate situations in which the 0 corresponds to neutrality.
     The PMV is related experimentally to PPD (Planned Percentage of Dissatisfied) (%), a parameter that expresses the number of people who would be dissatisfied of climatic conditions.
     PPD = 100 - 95exp [- (0.03353 + 0.2179 PMV⁴ PMV²)] ""

I finish this post bringing the graph of relationship between PDD and VMP. From barcol-air.nl read:

""There is a fixed relation Between the PMV and PPD value. When the PMV value is Between the recommended -0.5 + 0.5 and the PPD will be lower than 10%. The percentage dissatisfied in this model will never be lower than 5%.""

Thermochromic glass (III)

I'm always afraid to publish the material I found on the net, especially if it's in articles.

I found at the site http://www.hindawi.com that, in his articles it writes:

    ""Copyright © 2012 This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

You are free:
to Share — to copy, distribute and transmit the work
to Remix — to adapt the work
to make commercial use of the work

Under the following conditions: Attribution — You must attribute the work in the manner specified by the author or licensor (but not in any way that suggests that they endorse you or your use of the work. ""

So I think I'm free to report some articles.
If someone know more about this License, please contact me.
Well, today I talk about the vanadium doping with tungsten (finally)!
To do this, I use the article http://www.hindawi.com/journals/jnm/2012/491051 [I].
Repeating the first concept already expressed before, as the basis of thermochromism:

""[..] At temperatures ranging from −147°C to 68°C VO₂ materials show the fully reversible phase transition between monoclinic VO₂ (M) and tetragonal rutile phase VO₂ (R) fascinatingly around 68°C. [..]

Furthermore, the phase transition temperature can be adjusted to near room temperature by doping, which is realized by the incorporation of metal ions into the VO₂ lattice. Tungsten, molybdenum, chromium, titanium, fluorine, and niobium, and so forth are frequently used for this purpose because they produce relatively larger TC shifts with less dopant concentrations. [..]

TC is the transition temperature.

    ""So far, as an intelligent window material, the study of W-doped VO₂ mainly focused on thin films and nanoparticles. It has been prepared by a variety of methods involving excimer-laser-assisted metal organic deposition (ELAMOD) , magnetron sputtering , chemical vapor deposition (CVD) , pulsed laser deposition (PLD) , and vacuum evaporation . However, all of these methods are not suitable for putting into practice because of complex control parameters, unstable technology, and the necessity of special and expensive equipment . Chemical solution deposition seems to be an alternative solution to the above problems due to its low cost and the option of metal doping. But this method usually requires specific raw materials or pretreatments which limit their practical applications . [..] In this paper, we report a simple solution-based process to prepare pure VO₂ and W-doped VO₂ nanopowders with cheap and nontoxic vanadium (V) precursors and short reaction times. ""

Here it is introduced ELAMOD.
-) What is the process?On http://staff.aist.go.jp/t-nakajima/research-e.html we read:

    ""The ELAMOD is the process which applies the laser energy instead of high temperature. At first, metal-organic solution is spin-coated onto a substrate, and then the film is pre-heated to decompose organic components, if needed. Finally, the film is directly irradiated by excimer laser. This method is very useful for low-temperature fabrication. [..] So this process is enable to fabricate thin films onto the substrate which are weak against high temperature such as Si and glass. Moreover, the patterning can be also easily realized since the film is crystallized at only laser irradiated region. Thus, the ELAMOD process is quite prospective for its application potentials.""

[I] : Lihua Chen, Chunming Huang, Gang Xu, et al., “Synthesis of Thermochromic W-Doped VO2 (M/R) Nanopowders by a Simple Solution-Based Process,” Journal of Nanomaterials, vol. 2012, Article ID 491051, 8 pages, 2012. doi:10.1155/2012/491051

Thermochromic glass (II)

As we have seen, thermochromic coatings have two problems:
-) High critical temperature,
-) Limited visible transmittance. Today we write about this.
Through experimental observations, Babulanam and Granqvist noted as an anti-reflection coating of SiO₂ contribute to increase the light transmittance of the glass.
In details, it is compared: the luminous transmittance between a device with only a glassy layer of 200 nm of VO₂ and one with VO₂ in turn covered with a film of SiO₂, having various thicknesses.
The results show that, with a thickness of 100 nm of SiO₂, the luminous transmittance at a wavelength of 650 nm increases from 42% to about 55% at 20 °C, justifying what has been said.
Many compositions have been analyzed to create such a protective film, including those based on SnO₂, In₂O₃ or CeO₂. I want to cite the use of  TiO₂ as important because of its possible applications in DSSC (dye-sensitized solar cells) called Grätzel cells (photoelectrochemical cells) which will be discussed shortly.

Photochromic glass

Today I write about photochromic glasses . In simple terms, the photochromism is a mechanism where the device darkens with the sun, and then returns to the initial state if the exposure vanishes. At the transparent state is characterized by a transmission in the visible of about 80-90%, which decreases progressively during the change up to 10-15%. [1]
In more technical terms, the photochromic materials have the ability to reversibly change their absorption characteristics in response to the wavelengths of radiation (especially ultraviolet light). This phenomenon is obtained using iron oxide with fluorides (or chlorides) of silver or copper.

An animation of the process is here.

The phenomenon was observed as early as 1960 by using a Corning silica glass doped with silver, but the difficulty in producing large areas and his "uncontrollability" has allowed the use of this phenomenon only to visual lenses, in devices for cars .
An interesting case is the anti-glare rearview mirror that some manufacturers offers, since it's an example of a user controllable photochromic device (UCPC defined = user controlled photo-Chromic). [2] It's able to change the color even if the radiation is low, or almost completely absent (as we have seen, the glass isn't transparent to UV) with a very good response times. In reality, a UCPC is very similar to a glass of electrochromic type, which will be analyzed in another post.

Here is an example graph of transmittance (I don't have real experimental data).

Red line = clear glass

Blue line = tinted glass 

The complete discussion with the data is in [2].

[1] www.corning.com

[2] Gimtong Teowee, Todd Gudgel, Kevin McCarthy, Anoop Agrawal, Pierre Allemand, John Cronin, "User controllable photochromic (UCPC) devices", Electrochimica Acta 44, (1999), 3017-3026.

Innovative prototype

As you can read in the history of PPG [1], the idea of coupling multiple panes spaced a few millimeters existed already in the 40s, initially using air in the chamber. Then the use of inert gases such as argon, Krypton (but only in the most important) has been implemented as well as creating a vacuum between the panes (optimal solution but hardly used).
There are innovative devices, one of them from BEET/project_page:

The system consists of a reversible window frame holding two glazing components: a transparent glazing that provides a weatherproof seal, and an absorptive glazing having top and bottom vent openings for airflow. While the airflow may be natural or mechanically-driven, the window frame can be rotated so that the absorptive glazing is either on the interior (for space heating in winter) or on the exterior (for reducing unwanted-heat in summer).

Taken from [2]

The development test was conducted in Hong Kong, where best solution is "summer mode", with a reduction of heat transfer equal to about 70%.

Taken from [2]

Since the heat extraction capability of flowing water is much better than flowing air, there is a version with water into the chamber.
In addition to reducing the transmission of the incoming heat in the building, this has other advantages, including the possibility of obtaining a pre-heating of the water. For this reason its possible use in a pool that has large windows, for example, fascinates me.

You can read the full discussion on [3]. It requires knowledge of fluid mechanics to analyze the motions of the water flow, but it's a very interesting article, also because of the tests and comparisons using different glass types in the assembly.

[1] http://www.ppg.com/en/Pages/home.aspx
[2]: BEETRU
[3] Chow Tin-Tai, Li Chunying, Lin Zhang. Innovative solar windows for cooling demand climate. Solar Energy Materials and Solar Cells, 94(2), 2010, 212-220.