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Under review

new power system terminology

darker99 2 years ago updated by Tyler Owen (Lead Developer) 2 years ago 6

I've started my first playthrough since your overhaul of the electric system, and while it so far seems to work OK, there is one thing that as an Electrical Engineering student makes my heart cringe: the use of the term "Power Units" on the systems terminals.


It just so happens that power does have a unit, and that unit is called the Watt (W). Would you sleep for 6 "time units", eat lentils worth 150 "nutrition units" or have 6.6 "volume units" of oxygen in storage? Probably not, so why use "power units" as a measure of power?


You might argue you want to keep the game comprehensible for everyone rather than 100% realistic, but then I'd counterargue that you do use the term "sol" rather than "day", and that most people know that Watt has something to do with electricity consumption/production. Just for the sake of immersiveness, I urge you to change "Power Units" into "W" or "kW". Similarly, in the habitat status screen you could mention the charge as (kilo)watt-hour (Wh or kWh) rather than just as a percentage, similar to how you display the oxygen and water. Also, when talking about battery charge, you should use the term Energy rather than Power (as an analogy, energy is the amount of fuel in your car's tank, while power is l/km (gallon/mile for Americans)). In short, what now shows up as "Reserve Power: 39.9%" in the habitat panel should IMO be changed into "Reserve Energy: 123.4 kWh [39.9%]".


The good thing is that such a change would purely be textual. "Real" electricity works with "Power Units" as well (except that they have the name Watt), so nothing would have to be changed to the power system itself. Perhaps you'd just want to scale the number to get a more realistic result; as a reference, you could probably assume a 30%-ish solar panel efficiency when the game takes place, while the Viking 1 lander measured around 500 W/m2 (source: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19890018252.pdf), resulting in around 150 W (0.15 kW)  per square metre solar panel.

Under review

I'll just run you through my general thought process on this. The main reason for "power units" is so that it's not a directly convertible metric for comparing the real world power costs of the equipment used in the game. The annoyance you feel at using the term "power units" would get multiplied 10x by all the people saying "hey, why does the water reclaimer use such and such kW/h? That's completely unrealistic." There just has to be a certain level of abstraction somewhere to balance for fun and interesting gameplay and I'd opted to put the abstraction right at the power storage itself rather than the power costs and charging rates. So short answer... it's easier this way. But that doesn't mean it's better. I'm open to hearing your additional thoughts. I understand the comparison to the other stats in the game like liters of water and oxygen, so maybe you are on to something, but it would take quite a bit of work to make it feel sensible with the electricity I think.

You do have a point here. However, I feel that this particular solution obscures rather than solves the problem. Someone wanting to address the realism of LP would still be able to say "hey, the water reclaimer takes 10x as much energy than the heater, that's not right!" To me it seems you try to improve realism by removing the link with the actual world, which IMO is a bit counterproductive. People know that it's a game, so I think most people wouldn't have a huge problem with the figures being not 100% realistic. The ones that are put off by unrealistic numbers, I think, would be equally put off by a non-realistic quantity such as "power units".


Anyway, let's see if we can come up with a scientifically justified estimate for the power consumption.


Water reclaimer:

From the following source I found the estimate that it takes 2.4 kWh to extract .4L of water from Martian soil. Source: https://www.lpi.usra.edu/publications/reports/CB-1106/csm01.pdf


Oxygen generator:
I just saw an experimental technology mentioned with 95% efficiency, although I can't seem to find it anymore. You could assume such a tech to be in use when the game takes place. 95% is close enough to 100% to not take efficiency into account.
From what I could find on the internet, I've come up with the following estimations for the amount of energy required to get oxygen from water.
~4 kWh per L of water converted (~0.78L liquid O2)
~5 kWh per L of liquid O2 (=622L gas)
~6.2 Wh per L of gas O2

I assume you use the second measure (liquid O2) in the game?


Heater:
Heating 1 m3 of air by one deg C costs 1.006*1.225=1.232350 MJ = 342 Wh.
I think you could safely assume a very well above 100% efficiency as any such system would most likely use residual heat from the other systems rather than produce heat itself, so let's say a 1000% efficiency. A quick estimate of the habitat volume as 150m3 would result in some 1.5MWh or so to get the habitat to reasonable temperature, an increase of 30C or so. HOWEVER, once the habitat is at temperature, almost no energy would be required to keep it there. So IMO you could average it with, let's say, a 5% duty cycle so combined with the 1000% efficiency that would be around 7.5 kWh. You could for the sake of easiness assume the heater has an internal battery to heat the habitat from atmospheric temperature.


How it all comes together:
Let's first assume the various systems of the habitat itself (lighting, 3d printer etc. takes 3.5kWh of energy.
Now a human consumes about 550L of O2 a day, which happens to more or less correspond with 1 L of water. Add 1.5L water for consumption, and you end up with (1+1.5)*2.4/0.4+4=19kWh of energy to maintain H2O and O2 reserves. Add this to the heating requirement and habitat and you end up with some 30kWh of total energy requirement per day. Divide this by the 12 hours of sun, multiply by sqrt(2) or so to compensate for the low solar power at dawn and dusk and you end up with 3.5 kW of power required on average. That means that you'd need a little over 20 m^2 of solar panels using the above estimate of 150W/m^2 to keep the battery energy at a steady level and survive. Which is not too far from what you've got now, I think.


So here you are, a (very, very quick) scientific justification of your power system. You'd perhaps only have to change the relative power consumption of the three different systems (H2O: 19kWh, O2: 4kWh, heating: 7.5kWh, habitat: 5kWh), and I think you have an excellent approximation of the energy requirements of surviving on Mars, without making any substantial changes to your new energy system.

It's been quite a while since I tried to calculate the actual energy costs that might be required, but those values seem much closer to each other for each exterior module than I recall. I believe in-game I have them at 50pu, 50pu, 50pu, and 25pu which has been purely driven by mechanical game balance. For simplicity sake I think I would want to keep those same ratios of power consumption, but how would you feel about 10kWh, 10kWh, 10kWh, and 5 kWh as the displayed requirements instead of power units? I will admit that you are starting to convince me, but I'm still hesitant to change the actual power requirements. Since the values you cite are relatively close to each other though I think we could get away with displaying them as uniform kWh requirements per exterior module.

Well, you could easily assume the water reclaimer (partly) recycles water from e.g. the toilet (it probably would) and therefore is more efficient. Alternatively, the habitats are probably located on those specific spots for a reason, and that reason could well be an underground ice deposit which would also make water production much more efficient.


As for the other systems, it's always easy to assume something is less efficient, so I think 10, 10, 10 and 5 kWh would be a plausible estimate, especially since the values I mentioned above are those necessary for survival, while you'd generally want some surplus to survive long dust storms etc.


The values are indeed close to each other, and especially for the water reclaimer and oxygen generator, I feel reasonably confident. I did make an awful lot of assumptions for the heater, but heating physics are very dependent on things like isolation, external temperature etc.

I dug up some of my old notes, and as it appears, without any actual calculations, I ended up with a similar ratio of power requirements for the systems just based on what "feels" right for their functions and likely technical implementation.


I may have also suggested back when the major power rework was made that a less uniform power requirement could add another layer to power management in shortage situations. 15/10/10/5 for a simple "offset" ratio with a total of 40, but even 20/5/10/5 would work and that would be really close to the technical estimates presented above.

I'm open to changing the required power amounts down the road, but for right now I'll see how it feels just to change the displayed requirements to kWh. The current total power draw required in terms of kWh would be 35, so if I eventually changed to varying power requirements per module I would still want them to add up to 35 to keep the game balance of the charge rate to drain rate ratio.