So we have some pretty cool energy systems here on “Mars”. We get the majority of our electricity from a 10 kW photovoltaic array, and store the energy on Sony lithium-iron-sulfide batteries for use at night. If we get a few dark days, then we do have some back-up energy in the form of hydrogen fuel cell generation. If we run out of hydrogen, we have a back-up gasoline generator. And if we somehow run the Li-Fe-sulfide batteries dead, and need to cold-start the system, we also have a back-up lead-acid battery to get things rolling again.
We had been noticing that under normal use, we were getting very close to the point where we needed the back-up H2 to get us through the night. A couple days of sloppy cooking (using energy later in the day than usual), we actually did need the H2 back-up, and then Second Tier Support started to notice as well. This was already on our radar, but we stepped up our effort at understanding then, and did a full energy audit of the Hab.
Each crew member took one or two areas, and measured the the power (kW) and estimated the amount of time we used every single item that was plugged in across the whole Hab. For items like fridges and pumps that cycle on and off, we measured the kWh use by either plugging the appliance in and measuring it for about 10 hours, or in the case of the water pump, reading off the name-plate data (volts and amps), and estimating the number of cycles it does during the day.
The battery capacity we have is about 19.1 kWh. We can use 90% of that before the H2 fuel cells kick in, giving us an actual capacity of 17.2 kWh. After our energy audit, we had accounted for 16.7 kWh of use under normal circumstances through the night, meaning we only had 0.5 kWh of energy to play with. So if our cooking went over by about 30 min, or if we watched an extra hour of movie on the overhead projector, we were screwed. I looked at each load to see if it was an unavoidable base load (like the fans for ventilation on the toilets – critical!) or an area for potential savings (like the standby power drain from the coffee maker or microwave). Of all the loads in the Hab, about 70% were unavoidable base loads. However, 20% offered potential savings that we were just throwing away, and would enable us to cook, longer, do research longer, or watch longer movies (‘cause you know the big marathons – LOTR, Star Wars, etc. – will happen at some point).
So a few more graphs for you. Over-all energy use in the Hab, broken down by area:
Of the over-all energy use, the washrooms and telemetry take up a huge chunk. And these are for the most part unavoidable loads. Of the avoidable loads, this is what shook out:
So the “Sea Can” is our storage area and workshop. We don’t use it a whole lot, and it was sucking up a lot of our spare energy. Sophie found that some of the chargers that were plugged in were still drawing power even when we weren’t charging anything. That totaled about 800 Wh of our energy. The kitchen, when you added up the stand-by draw of the hot plates, microwave, and coffee maker, were another 600 Wh. So those were immediately eliminated. It was actually pretty easy to cut down the kitchen loads, as Neil had already put up a bunch of power strips for these items when we were sorting out the Hab to our liking, so you can just switch off all the appliances at the wall with the flip of one switch.
These improvements bought us another 1.4 kWh of energy in the evening. To put that in perspective, that’s another hour of cooking time in the evening, or nearly another 6 hours of Sherlock viewing.
So I’ve probably bored everyone with this update, but I had a lot of fun doing the audit. Made good use of the power meter I brought with me, and everyone on the crew learned a lot about our energy use, and got to put certain loads in perspective of our total over-night battery capacity. So that was great!
It turned out we did the energy audit just in time – we had no sun for about 2 days straight, which put some serious constraints on our living situation. For two days there was no cooking, reduced research (since we ran all our laptops off batteries through the night to reduce the night-time load), and no tv or movie watching. Lots of games nights under low light (which is cool ‘cause we all like games). With the energy efficiency measures in place, we made it through the dark days without running out of hydrogen (which wasn’t at capacity when we started). And the day we made it back to 100% on the batteries, we cooked dinner late (pasta – we thought it was an easy, low energy meal, until we remembered we had to do two of everything to cater to gluten-free and non-gluten free), watched a movie, and still made it through the night.