Tasks:Surviving a lunar night
From TeamFrednetWiki
|
2010 2 9 |
| Task title: | Tasks:Surviving a lunar night |
| Contact person: | Marcel Belmans |
| Blueprint: | (None) |
| Persons working on this task: | (No one) |
| Start: | (ASAP) |
| End: | (ASAP) |
| Group: | Category:Tasks_(Open)-Survey |
| Product: | (Undefined) |
| Dependencies: | (Undefined) |
| Impact: | (Undefined) |
| Percent complete: | (0%) |
| Man-hours Remaining: | (999 hours) |
| Relate page: |
Category Page_of_Tasks:Surviving a lunar night not found
|
Description:
Contents |
Introduction
To survive the lunar night, we need to heat certain parts of our spacecraft with electrical heaters. Of course, the amount of electrical energy available for this task is not unlimited. During the night, solar panels receive no light, and thus are the heaters depleting our batteries. How long can we survive?
As much systems as possible will have to go offline, to keep power consumption at an absolute minimum. Only a small set of heaters and a controlling piece of electronics do get power. Thus, no photographs, videos, earth comms, ...
Over here, we try to calculate actual values for thermal properties of insulation materials to keep our key parts warm enough during the complete lunar night.
Parts that need our attention
During the lunar night, the lunar environment (regolith) cools down to around 100K (-173°C) during ~350 hours. Keeping a complete lander/rover sufficiently isolated and heated will be impossible, batteries just don't have enough energy for this. We have to select the lander- and rover parts that need to be kept warm. All other parts will cool down to cryogenic temperatures. They'll have to be designed for it.
- Batteries, minimal temperature dependent on specific type. In any case > -50°C (see Power Battery)
- Onboard computer, minimal temperature?
- Electronics, minimal temperature?
- Camera : how about CCD-matrix? Lens? Minimal temperature?
- Moving parts : Roller bearings of wheels etc.... . Materials shrinking in cold circumstances can induce thermal stresses. Moving parts might get blocked by these. Minimal temperature ?
Heat transport through an isolation panel
What properties of an isolation panel define a certain heat flux through it?
- Q : heat transfer [ W ]
- ΔT : the temperature difference between inside and outside [K]
- A : the surface of the panel [m²]
- u (old US: h and old european: k ) : the heat transfer coefficient [ W/m²K ]
- Lambda (old US: k): a material-dependant value, called the thermal conduction coefficient, (its included in the u.) [W/m K]
- s : the thickness of a layer of the panel [ m ] (Note: in the picture its "d", but officially its "s" )
- Rse, Rsi : exterior and interior heat transfer resistance, [ m²K/W ] (on earth usually 0,04 and 0,13 )
- Q = A u ΔT
- with u = 1 / (Rse + s1/Lambda1 + s2/Lambda2 + ... + Rsi)
Battery or electronics Box
Let's take a box in mind that contains for example the batteries. We want to keep the minimal temperature inside at minimum 223K (-50°C). The temperature at the outside of the box is 100K (-173°C). We isolate this box with aerogel plates.
Data:
- length, width and heigth inside are 30cm = 0,3m
- thickness of the aerogel panels is 5cm=0,05m=d
- outside temperature 100K, inside 223K, thus ΔT = 123K
- aerogel has a thermal conduction coefficient Lambda=0,02 W/mK
- lunar night duration time : ~350h
The complete surface of the box inside equals 6*0,3m*0,3m=0,54m²=A
So, Q = k ΔT A / d = 0,22W
We can put some heater elements (which are basically electrical resistances, spread out over a certain surface) that consume 0,22W inside the battery box. They keep the temperature difference ΔT present. We need 0,22W*350h=76Wh of battery power to keep this box at -50°C.
This means that it is achievable to keep some stuff sufficiently heated during the entire night, but it does use A LOT of our battery reserves! A box of 30cm*30cm*30cm doesn't contain that much stuff.
---
Hi, I just calculated that box (I do this every day in my job/study with buildings. (Sorry, I wanted to put all that text into that frame, but somehow it didnt work..) A box, like you described, (I considerd it as a mini-house ;) ) has an acutal loose of 44,28 W to its -173 C° surrounding. If these temperatures stay constant for 350 hours, than we have a total need of thermal energy of 350h x 44,28 W = 15,498 kWh.
Calculation:
- Q = A u ΔT
- A = (0,3+0,05+0,05)[m] * (0,3+0,05+0,05)[m] * 6 = 0,96 m²
- u = 1 / (0,04[m²K/W] + 0,05[m] / 0,02[W/mK] + 0,13[m²K/W] ) = 0,375 [W]
- ΔT = 123 K
- Q = 0,96 [m²] * 0,375[W/m²K] * 123 [K] = 44,28 W
So, now we need to find a heat element, wich provides 44,28 W at the moment. If we chooese those,
you already mentioned, (0,22W) we would need 201 of those ;) (if 100% is beeing transformed into heat)
So, we should now:
- look for better heating elements. E.g. here is one with about 60 W (mini heat elements )
- use, for exemple 10 cm, not 5 cm insulation ( wich would bring us down to 22,8 W)
- make the box smaller,
- get better aerogel, ( Aspen Aerogel has some 0,014 ones ! )
- hope that it wont get down till -173°C ;)
- etc ..
But, I think, we should know first, if we really need to warm our box up to -50. In other words, what parts with what condition do we have inside, AND what ammount of heat energy do these parts allready give into that box ? (How much Watt do they use, most of that will end as heat anyway allready.) And what is the real geometry of our box ..
---
Efficiency
Heater elements do have an efficiency of 100%. All the energy it receives is transformed to heat.
Overheating problems
Putting powerful electronics inside a completely isolated box to keep it warm brings us a new problem : Overheating. A completely isolated device that has no possibility of losing heat (no radiation or convection possibilities) will very soon get too hot.
We will need to :
- include coolant lines inside this box to remove heat. More information on heat transport mechanism for the lunar lander can be found here.
- include a thermal capacity
.. Under Construction ... --Sel(B) 15:02, 26 February 2009 (EST)
Discussion on this problem Talk:Tasks:Surviving a lunar night.
