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Solar powered smart windows break 11% efficiency – enough to generate more
than 80% of US electricity
Nov. 29th 2017 John Fitzgerald Weaver
The National Renewable Energy Laboratory, (NREL), has demonstrated a
prototype of a solar powered smart window [
]. The smart window lowers building temperatures by shifting from clear to
opaque under strong sunlight. When the shift to opaque occurs, the solar
prototype begins electricity production.
The prototypes tested reached up to 11.3% efficiency. The solar cell is
based on the lab/headline favorite material perovskite.
One potential smart window feature is darkening of windows to minimize heat
coming into a structure. Heating, cooling, and ventilation of commercial
structures is up to 80% of their energy costs.
In the USA, up to 80% of residential units and 50% of commercial units [
], use some sort of ‘Low-E’ (low heat emission) glass. In this particular
hardware, the smart darkening process begins the electricity producing magic
when the glass becomes a solar cell via a heat driven chemical reaction.
The chemical reaction described:
Upon illumination, photothermal heating switches the absorber
layer—composed of a metal halide perovskite-methylamine complex—from a
transparent state (68% visible transmittance) to an absorbing, photovoltaic
colored state (less than 3% visible transmittance) due to dissociation of
methylamine. After cooling, the methylamine complex is re-formed, returning
the absorber layer to the transparent state in which the device acts as a
window to visible light.”
An image of the structural shift in the paragraph above is noted in image a.
In the image d. above, the unit was able to produce at high levels for only
the first cycle of clear to opaque. The unit quickly falls from 1 mA to 50%
by the 3rd shift. NREL notes this, obviously, need be fixed.
The paper [
], in the section titled ‘Mechanism of switchable device degradation’,
breaks down the observations of the chemical issues leading to the
degradation seen in the image above. Existing smart windows work for 50,000
cycles. A standard solar panel is expected to be above 80% efficiency for
9,125 full day cycles (25 years).
The ‘champion’ prototype peaked at 11.3% efficiency – while the average of
the five units was 10.3%.
Recently, a group released an analysis suggesting that 40% of US electricity
could be supplied via the windows of structures [
]. Their model projected that the 5 to 7 million square feet of glass would
need be about 5% efficiency in order to hit those numbers.
There’s a funny set of dynamics in solar glass materials. We very much want
the extra sunlight inside of structures to help with our mental well-being,
but we also want to take advantage of sunlight and turn it into electricity.
And of course, we want our structures to be more energy-efficient – and
sometimes a bit less bright, so we try to tweak the amount of sunlight that
does come through the windows to heat the structure.
How would you feel if you got that corner office, and it turned out you were
still in a cave? It probably wouldn’t matter that much – as the darkening
smart window industry already exists and we’ve shown that we’re ok with the
trade offs, but it’s enough of a dynamic that we’ve not covered all our
windows with solar panels just yet.
I wonder if a product could be engineered that would let us choose the
opacity of our windows – so we could choose what meets personal preferences.
However, if a building owner invests a healthy amount of money on new
windows specifically to get the solar electricity – they might not value
your comfort over their electricity bills.
One challenge of getting to 80% of electricity coming from smart solar
windows is that it simply takes a long time for us to shift out our huge
volume of buildings. It’s a pretty number to put in a headline and grab your
attention, but it seems real growth in this field will most likely be as the
nations building stock is upgraded. However, if efficiency levels keep going
up – maybe there will be a time when the electricity being produced is
enough to cover the cost of upgrades.
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