Making surface parking lots into solar farms

On hot days, if one parks one’s car in the sun for any length of time, the temperature inside can rise to values much higher than the ambient temperature, making the interior extremely hot to the touch. This is another example of the greenhouse effect, similar to what is heating up the Earth.

It’s only early April, but we’ve already had our first report of an infant found in a hot car in Tucson.  We talk about this every Spring and Summer in Arizona: the dangers of cars heating up in the sun.

Afternoon temperatures are about 20 to 25 degrees lower now than they will be in June, but it’s still hot enough to raise the temperature up to 120 degrees in about 45 minutes.

The air inside the car heats up so fast because of the “greenhouse effect”.  Here’s what it means:
Incoming solar radiation, known as “shortwave radiation”, shines through the windows and is absorbed in the car’s interior.

The heat released from the interior is known as “longwave radiation”, and is much weaker than the shortwave radiation.  The heat becomes trapped inside the car.  Heat continues to enter the car, but struggles to exit.

It only takes 10 to 15 minutes for a car in direct sun light to heat up above 100 degrees when it’s 80 degrees outside, even if the window is cracked.  When the air temperature climbs above 100 degrees here in June, the temperature inside a car can soar above 150 degrees to 170 degrees in less than an hour.

These are dangerous temperature levels. To avoid getting back to a car whose interior is hot to the touch, most people try to find a shady spot to park but that is rarely possible.

But Michael Mechanic writes that we can turn this problem into a solution by building roofs covered with solar panels over the parking lots. This would provide shade for cars while at the same time using solar energy for generating electricity rather that wasting it as thermal energy.

He has done calculations for what might be achieved with the massive parking lots at just Walmart superstores. He says that there are over 3,500 such stores in the US covering an area of 1,400 square miles. If they were all covered with solar panels, they would generate about 1.5 terawatts of energy, sufficient to power 1.25 million households. A single 1,000 space parking lot could power 100 EV charging stations and 350 nearby homes.

The transition to clean power will require massive solar expansion—not just home rooftop arrays but photovoltaic installations near population centers. Where to mount all the panels? One answer seems obvious: over parking lots, which also bake during the summer, contributing to dangerous “heat island” effects. Shading Walmart Supercenter lots with PV canopies, for instance, would generate loads of power and allow stores to entice shoppers with free EV charging and cooler lots. And a cool customer is a happy customer.

I can see Walmart actually doing this because it looks like it might make good business sense. It would enable them to save money by powering their stores and selling energy to the power grid and also allow them to brag about being green and doing their part to combat global warming.


  1. billseymour says

    It seems unlikely that the Waltons would charge your electric car for free.  Still, it sounds like a good idea.  Unfortunately, Republicans will be against it because it’s “woke”.

  2. steve oberski says

    @1 billseymour

    Not a Walmart fan but Walmart already allows RVs to use their parking lots for overnight parking.

    Not because they are starry eyed do gooders but because it makes good business sense to allow potential customers to use something that costs them almost nothing.

    I see similar economic forces a work for EV charging, if it brings the car to Walmart then that’s a potential customer.

    And it’s really good PR.

  3. says

    Here on the outskirts of Columbus, I see signs near the farms wanting to stop solar farms. Their conservatism, while extending to denying climate change or anything “liberal”, does not seem to extend to people being able to use their property as they wish (i.e., putting a solar array on it).

    But parking lot solar farms makes a lot of sense. As do proposals I’ve seen to put solar panels above all the aqueducts that feed water to California (and other places). Generate electricity while reducing evaporation of an increasingly rare resource.

  4. Alan G. Humphrey says

    … because those godless symmetrical arrays cover up those god-fearing chaotic weeds.

  5. TGAP Dad says

    As for adding solar panels over parking lots, Michigan State University actually did that for a few large-area lots on campus. The results have been a resounding success, as people love the canopy as a shield from the sun, snow and rain, and the campus has saved an incredible amount in natural gas used in the power generating plant. Even the local public schools are getting in on the action.

  6. Dago Red says

    This idea is not new and I’ve seen it discussed for decades. PV over and around Freeways, or even as the road surfaces itself, have also been proposed. If something like this is ever going to happen, it will require government incentives (similar to those already sponsored in California and Hawaii), massive alterations to our power distribution infrastructure (which currently does not handle decentralized power generation very well), and/or, perhaps, the development of a public/socialized power generation sector that isn’t very concerned with profits and far more concerned about making society work better.

    Even with the precipitous drop in solar panel costs in recent years (due to China’s big push into green energy generation) making massive PV installs cheaper than ever before, modern Capitalist businesses are way too short-sighted to value this kind of very long-term investment into something that is not their core concern. They cannot justify investing so much in something they see as ancillary, especially when it takes decades to fully pay off. It is still far more profitable (even wise given how hard it is for any business to stick around for, say, decades) to simply build more Walmart stores, for example, than for Walmart to diversify into long-term power generation. This problem — which is inherent to capitalism itself — has been holding back wide-spread solar installations for as long as solar has been in existence. Its also why most expansion into creating a solar power infrastructure has, so far, focused on getting home-owners,schools, and other public institutions, rather than businesses, to provide the space and efforts.

  7. Deepak Shetty says

    The Sutter Health medical place we visit already has this setup (built during the pandemic!)
    Ha ha , yes thats the first thought that came to mind that these suggestions will be dismissed as woke. I do wish we could get more good/useful things branded as “woke”

  8. Dunc says

    Whilst I fully endorse the sentiment, if we want to be technically correct (the best kind of correct!) we should note that the main driver of increased temperatures in small, enclosed spaces like cars and greenhouses is actually the restriction of convection.

  9. moarscienceplz says

    “It seems unlikely that the Waltons would charge your electric car for free.”
    Since it takes a fair amount of time to charge a car, it is actually beneficial for a business to offer this, especially if they can generate their electric supply for essentially free. The longer people stay in your store, the more they spend (on average).
    I work in a bookstore, and not only do we allow people to charge their electronics from our sockets, we also provide free wi-fi for them.

  10. Allison says

    ahcuah @3:

    Here on the outskirts of Columbus, I see signs near the farms wanting to stop solar farms.

    If it’s anything like what is happening in New York, the objection is that solar farms are taking over farmland; this, for various reasons, ultimately makes it harder for farmers that want to farm to continue doing so. They wouldn’t care about solar farms over parking lots.

  11. Alan G. Humphrey says

    larpar @ 12
    Also, they should build hydroelectric plants using locally captured rainfall before using that water to cool those nuclear plants…

    … and the shade in those parking lots will be deep!

  12. flex says

    @8, Dago Red, who wrote,

    … which currently does not handle decentralized power generation very well….

    Just spit-balling here, but that seems to be a problem which the current level of AI might be able to manage.

    Balancing power loads is difficult in the time scales which humans operate (200ms is about as fast as we can operate), but a properly configured AI might be able to manage the load fluctuations at a time scale within that of the variation.

  13. prl says

    The quoted explanation for why the interior of cars gets hotter than the surrounding air confuses how horticultural greenhouses work with how the global greenhouse effect works.

    Cars get hot inside when they’re exposed to the sun for the same reason as horticultural greenhouses do:

    The warmer temperature in a greenhouse occurs because incident solar radiation passes through the transparent roof and walls and is absorbed by the floor, earth, and contents, which become warmer. These in turn warm up the surrounding air within the greenhouse. As the structure is not open to the atmosphere, the warmed air cannot escape via convection due to the presence of roof and walls, so the temperature inside the greenhouse rises.

    This differs from the earth-oriented theory known as the “greenhouse effect”, which is a reduction in a planet’s heat loss through radiation.

    [my emphasis]

    Shading parking lots with solar panels is still a good idea, though.

  14. says

    Locally (central NY state), there is a proposal for a 900 acre PV site, much of it on disused farm land. Many local conservatives are complaining about it and using the excuse you mentioned regarding farming the land. These are not good-faith arguments; it’s all BS. The soil in question is not very good (heavy clay) and that contributed to why the land is no longer farmed, as it’s not very productive (generally, a high percentage of local farm land is used to grow corn for dairy cows). In fact, one of the farmers who leased a portion of his land to the PV company said that the lease income is the only way he could afford to hire people to help work the remainder of his land.

    I have seen suburban homes far from the PV site with lawn signs stating “Stop big solar! Fields will be farmed!” right next to their Trump flags on their quarter acre lots. So no, this really has nothing to do with the farmers and everything to do with solar being identified as a “liberal” thing. I have no doubt that they would be up in arms over having solar panels sitting above their cars in parking lots. I can already hear them screaming about how the panels will come crashing down on their precious SUV or giant pickup, and how they’re now being forced to “live in a cave”, shaded from the sun.

    Also, they have, apparently, never heard of agrivoltaics.

  15. Rich Rutishauser says

    jimf @18, I live in CNY too. I am not sure which site you are talking about for the solar but there have been several local to me that are each on the 25-50 acre scale. All of them are on land that has been unused for decades but we have the same morons you speak of complaining about “farmland”. It’s really the same bunch that we complaining about windmills causing cancer a few years ago.

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