Energy and Human Ambitions on a Finite Planet, 2021a

13 Solar <strong>Energy</strong> 212
A major impediment to solar power is its intermittency. 68 Figure 13.15
shows 31 days of solar capture, along with typical state-wide electricity
dem<strong>and</strong>. The two do not look very similar: not well matched. Dem<strong>and</strong>
is far more constant than the solar input, which is obviously zero at
night. Even the peaks do not line up well, since dem<strong>and</strong> is highest in
the evening, well after solar input has faded away.
68: Recall that wind has a similar problem
(Fig. 12.6; p. 190).
relative power
1.2
1.0
0.8
0.6
0.4
0.2
0.0
31 5 10 15 20 25 30
March/April date in 2020
Figure 13.15: Solar input (red) <strong>and</strong> electricity dem<strong>and</strong> (blue) look nothing alike. Solar data from the author’s home begins 31 March 2020,
while dem<strong>and</strong> is for California. Tick marks denote the start of each date, at midnight. April 22–27 are essentially perfect cloudless days,
while the earlier part of the month had rainy periods. Note that even a very rainy day (April 10) provides some solar power (15% as much as
a full-sun day). Intermittent clouds cause the “hair” seen on some days. The capacity factor for the month is 19%, while the perfect six days
near the end perform at 27% capacity. From this, we infer that weather caused the yield to be 70% what it would have been had every day
been cloudless.
Storage is required in order to mitigate the intermittency, allowing the
choppy solar input to satisfy the dem<strong>and</strong> curve of Figure 13.15. We
don’t have good solutions for seasonal storage, 69 so a complete reliance
on solar energy would necessitate over-building the system to h<strong>and</strong>le
months of low-sun conditions through winter (see the annual variation
in Table 13.2), making it cost all that much more.
Finally, all energy is not equivalent <strong>and</strong> substitutable. Solar PV cannot
power passenger airplanes or power our cars down the road real-time
(only via storage). 70 For all it’s potential, the hangups are serious enough
that more than 60 years after the first demonstration of a photovoltaic
cell, less than 1% of our energy derives from this ultra-abundant source.
Box 13.3: Why no Solar Planes?
Consider that full overhead sun delivers 1,000 W/m 2 . The top surface
area of a typical commercial airplane (Boeing 737) is about 450 m 2 .If
outfitted with the most expensive space-worthy multi-junction PV
cells getting 50% efficiency, the plane would capture about 500 W/m 2
<strong>and</strong> a total of 225 kW. Sounds like a lot! The problem is that a
Boeing 737 spends about 7 MW while cruising (<strong>and</strong> more during the
climb). We’re shy by a factor of about 25, even in optimal 71 conditions!
Any solar-powered airplane 72 would be very light <strong>and</strong> very slow
by air travel st<strong>and</strong>ards. See also Box 17.1 (p. 290) on the difficulty of
battery-powered planes.
69: It’s not a matter of how long the energy
is stored, but a matter of sufficient capacity
to store enough excess energy in summer for
use during the darker winter.
70: It is possible to build a solar-powered
aircraft or car, but not airplanes <strong>and</strong> cars
as we know them (see Box 13.3). We can
consider such things to be “cute” demonstrations,
rather than a viable path to substitution.
71: An airplane will not always have full
overhead sun!
72: If your niece or nephew draws a solar
plane in crayon, just smile, say it’s very nice,
put it on the refrigerator, <strong>and</strong> cry inside.
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.