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Cake day: June 11th, 2023

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  • There is another major advantage…

    There is a major problem with solar and wind. Daily and seasonal variation in solar flux and wind speed forces us to size our renewable generators based on their minimum expected output. We have to install enough solar panels that we can supply our needs with only low-angle sunlight on short, winter days. But we won’t do that, because that many solar panels are about four times what we need to supply our needs on long summer days. With that much oversupply on the grid, generators won’t be able to command sufficient revenue to justify that number of panels. But we need that number of panels to supply our winter demands.

    We can match a large percentage of daily variation with sufficient grid-scale storage. We fill up reservoirs with our excess mid-day production, and run that water through hydropower plants overnight. But it is simply not possible to expand storage sufficiently to match seasonal variation.

    If we build out sufficient solar generation capacity to meet winter demand, we don’t need seasonal storage. The problem we have becomes one of seasonal oversupply. The solution to that problem is an increase in demand. We need energy-intensive products that can be brought online in daylight hours from spring to autumn, then shut down for winter.

    Producing net carbon-zero fuels could very well create part of the demand needed to justify massive expansion of our renewable power grid.


  • Liquid fuels have a couple advantages in certain scenarios. Aircraft, for example. The energy density of liquid fuels is considerably higher than batteries. Aircraft only take on as much fuel as they need to safely reach their destination. They takeoff with more weight than they can safely land, burning off fuel weight throughout their flight until they are light enough to land. Dumping fuel overboard to get down to landing weight in an emergency.

    Switch these aircraft over to batteries, and their landing weight is the same as their takeoff weight. They carry the same “fuel” weight for a regional flight as they do for a maxinum-range flight.
















  • You have identified the purpose of these questions. They are determining your mindset when dealing with novel circumstances. Do you make an effort to explore and understand the actual constraints, or do you impose your own, preconceived notions on the scenario? Do you limit yourself needlessly?

    The worst you can do is to treat it as a riddle and immediately give the “correct” answer. An interview isn’t a knowledge test. They aren’t trying to determine if you’ve seen and retained the accepted solution. They ask this sort of question to gain some insight into your problem solving skills.

    A better answer is to step in to the question, and treat it like a real world scenario. Acknowledge the stated constraints, then explore them.

    How much effort should we put into this problem? How much time and treasure are we going to spend on this? Why are we even determining which switch controls the light in the first place? What are the consequences of a wrong answer? If we’re going to get fired for a wrong answer, we should take our time and get it right. If the consequences are “go try again”, let’s just start flipping switches.

    Do we have other resources available? Is there someone in the room? Can we put someone in the room? Is there someone else available who uses the switch regularly? Can we ask their assistance? (If the room isn’t being used often enough for anybody to know how the switches work, should it be repurposed to something more useful?)

    Do we know that these are normal, simple switches? If they are three-way switches, or installed upside down, we can’t trust their position.

    Is it safe to assume the bulb is functional? The “riddle” answer fails on this.

    Is it safe to assume the bulb starts cold? Did they run this test with another candidate a minute earlier? Did they leave it in a “hot” state for us already?

    Is the light accessible when we get into the room, or is it inside a ceiling fixture, 12-feet over our heads?

    What are the other switches connected to? If they control fans or lights or other appliances that can be sensed outside the room, we don’t even need to leave the first room.

    What is the necessity of the specific, given constraints? If this is a real-world scenario, we’re probably not going to have a limitation on entering the room only once. If we can eliminate that constraint, the problem is a lot easier to solve.

    Get feedback from the interviewer: Have we adequately explored this scenario to their satisfaction? Is there some other aspect we need to address?