Remember those old math questions you had in algebra class? Where water is entering a container at a certain rate and leaving at a different rate and you need to figure out when it’ll be empty? Well, that concept is critical to the “Mark Watney doesn’t die” project I’m working on.
– Mark Watney, from the book “The Martian”
In the book “The Martian” (and, I’m assuming, the movie too, we haven’t seen it yet), Mark Watney performs a lot of calculations. He has very limited resources to work with to keep himself alive. For example, he calculates the amount of calories available to eat from the potatoes he grows, and, from there, calculates how many days of food he has left. He also calculates the amount of water he needs to grow his potatoes and keep himself healthy.
Here on simulated Mars, our situation isn’t anywhere near as dire as Mark Watney’s. However, we still have limited resources that we have to manage carefully. As the crew’s chief engineer, just like Mark Watney, I perform calculations to understand how we’re using those resources.
I haven’t done any calculations involving potatoes or protein bars yet – our plants tend to be handled by Cyprien and Carmel. My tracking is instead focused on our system resources: that is, power and water. I perform these calculations on a daily basis, looking for patterns in usage and consumption, in a process known as “trending”. Although it sounds remarkably tedious – and it usually is, when everything is working correctly – trending gives the ability to find the subtle hints that something may be about to go wrong, before they have a chance to wreak havoc on the hab. For example, an unexplained uptick in the amount of water used per day can indicate that a leak has sprung somewhere, slowly draining away the habitat’s limited supply.
Of all the habitat systems, our power system most resembles the real systems that a crew would use on Mars. Our electrical power is chiefly provided by solar panels, which generate electricity and charge a pair of large storage batteries when the sun is shining. We use the power from these batteries at night and during cloudy weather. If the batteries run low and solar power isn’t available, then a set of backup hydrogen fuel cells turn on to provide power until the solar panels start charging again, usually around 7 or 8am. Unlike the solar panels, the hydrogen that powers the fuel cells is a limited resource. Unless the astronauts on Mars have a way to harvest usable hydrogen from the environment around them, they’ll have to wait roughly two years between top-ups.
Of all the numbers I look at every day, the number that interests me the most with this system is how much we power we use. This turns out to be fairly easy to calculate. The habitat’s computers report the amount of power being drawn by the hab. I simply take the recorded numbers, drop them into Excel, and integrate numerically over time using the trapezoid rule – for those that don’t speak calculus, that just means I’m adding it all up over time. This gives the total power we used in kilowatt-hours-per-day. Or, as Mark Watney called them, “pirate-ninjas”.
Here’s a chart of our pirate-ninjas…ahem, sorry, kilowatt-hours-per-day. The days that led us to run low on battery overnight and spend hydrogen are colored red. Looking at the chart, there appears to be a rising trend, especially recently. I can explain that: as we’ve gotten settled in, we’ve started up some of our opportunistic research experiments, including some that involve plant growth. Grow lights and water pumps use a fair amount of power, and it all adds up.
I’ve also tried calculating the amount of power we could be generating every day. I found this to be a little bit more difficult. Our solar radiation meter, which is wired to the habitat’s weather station, measures sunshine in Watts per square meter. I multiply that by the area of the solar array, multiply by the panel efficiency from the spec sheet, and integrate over time, which gives an estimate of how much power we theoretically have available each day. I say “theoretically”, because I’ve found my results are a bit optimistic. I’m missing several effects, such as aging and degradation of the panels. I’m also not including variations in power generation with temperature.
It turns out that this calculation doesn’t matter a whole lot, though. The nights that cause us to spend hydrogen tend to come after very cloudy days that don’t allow us to charge our batteries completely – no big surprise there. But, on most days, our batteries are fully charged by around noon. At that point, any power that isn’t being used immediately has nowhere to go – it’s radiated away as heat, and essentially wasted. Our big limit on most days is our battery capacity; we simply don’t have a way to use all the power we’re generating in the middle of the day, even if we plug in everything we have!
This observation has led us to change our approach to energy use a bit: we have a few 12-volt marine batteries that we can plug in and charge during the day to capture some of that wasted energy. Later in the day, we can use these batteries to power our plant-growth experiments. We’re considering ordering more for a future resupply to capture even more energy.
I’ll also talk about our water usage. Water reclaimers like the one Mark Watney uses are a real, but very expensive technology – NASA uses one on the ISS to process urine and evaporated sweat back into pure, drinkable water. For that reason, we don’t have one here at HI-SEAS. Instead, we simulate water generation and reclamation by receiving a delivery of water to top off our pair of 500 gallon storage tanks every few weeks. In between deliveries, it’s up to us to use our supply carefully.
Water usage is even more easy to measure and track than power. I simply take the tank level measurement each day at midnight. The change from the previous day’s measurement gives the total amount of water used that day.
This chart shows our daily usage, as well as the amount remaining at the end of each day. There’s quite a bit of variation – our daily usage is heavily influenced by whether or not we use the washing machine, which drinks a thirsty 20 gallons of water per load of laundry – but, on average, we use around 30 gallons or so per day, about 5 gallons for each of us. That’s pretty good…in comparison, the average family of four in the US uses over 400 gallons of water per day! We work hard to keep that number down: we have composting toilets that don’t flush. We limit ourselves to a couple of minutes of water per shower, and only a couple of showers per week. We capture shower and sink water, and reuse it to mop floors. It also helps that we don’t have a yard to water, a swimming pool to fill, or cars to wash.
There are a couple of days in mid-September that stand out, though. What happened there? Another easy explanation: we had been notified that a water delivery was coming in a couple of days. The tanks are topped off regardless of what’s left in the tank when the delivery arrives, so we decided to make as much use of the remaining supply as we could before the delivery. On the first high day, Carmel took 80 gallons to fill up her aquaponic plant-growth experiments. On the second day, we put as many loads of laundry through the washing machine as we could.
Just like Mark Watney, each day brings more numbers and more calculations. But, as the mission continues and I gather more data, perhaps more patterns will emerge, allowing me to gain more insight into our systems. For now, we’re heading into winter. We tend to base our power usage on how well we’re charging the battery, so I’m interested to see how the changing seasons will affect our power consumption and hydrogen usage.
…in the meantime, if you happen to know anyone over at 20th Century Fox, please let ’em know we’d love to watch “The Martian” here on simulated Mars!
I have a big group of friends I play board games with regularly back home in Denver. Thanks in large part to the culinary stylings of our friend Ryan, our game night group is usually well-fed. Justin, who is a frequent commenter on my blog posts here, occasionally whips up biscuits and gravy. For our brunch last Sunday, we were able to whip up some biscuits and gravy from our supply of shelf-stable ingredients: pre-cooked bacon, dehydrated sausage crumbles, buttermilk powder, and some of the sourdough starter that Shey talked about in her Mission Day 14 blog post. We haven’t figured out how to replicate chili dogs just yet, but this was a nice reminder of home.