How to cool electronics when the air is always hot

Question

I'm in the process of planning an experiment that involves placing some electrical equipment in a mine for a year or more. Where the equipment will be placed in the mine (770m in depth,) the amient temperature will constantly be approximately 45C. I need to check this but I imagine it will also be fairly humid.

Specifically the equipment I will be using includes: a laptop, two high-voltage (up to 2.5kV, low DC current) photo-multiplier tubes (whose power supplies undoubtably generate a significant amount of their own heat), and a low-voltage DAQ board.

The trouble is that in the mine (or equivalently in a desert or some high-temperature industrial setting) the constant heat means the normal method of cooling components - by fanning the ambient air over them - simply wont work. Furthermore, I had considered putting the equipment in a 'mini-fridge' but the heat exchanger on the back also relies on cool ambient air so it would soon break.

The equipment will be housed in a sealed crate so it may be possible to exploit this fact so as to maintain the entire inner-volume at a more processor-friendly temperature. To be honest however, I don't know where to start.

Answer 1

Frankly, if you "don't know where to start", get someone that does. It sounds like the cost of failure is very high, so this isn't the time to experiment and learn from mistakes. Find someone that knows this stuff. You can look over their shoulder and try to do as much of the grunt work as possible so next time you're the expert. I think I underestand the basic principles pretty well and could get something working. However, for extra high reliability there is usually much lore and experience that you wouldn't guess from the physics alone. I wouldn't take this on myself without a expert to at least ask occasional questions of and to bless the ultimate design. So from a non-expert view, here are some thoughts:

First 45C isn't all that hot. It's only 113F, which is not out of line for summer shade air temperature in various parts of the world. Ordinary electronics gadgets, unless specifically for lab use or other environmental constraints, will have been designed to run at 45C ambient. Think about it. If you left your cell phone on the dashboard of your car parked in the summer sun in Phoenix, you probably wouldn't blame it for not working. Of course you probably couldn't hold it without hurting yourself either then. But, if you were standing outside under the shade of a tree mid afternoon in summer in Phoenix, you'd probably be pissed of if your cell phone didn't work. That's 113F.

Second, two other advantages you have is that this 45C is highly reliable, and essentially infinite. In other words, it's 45C all the time whether its summer or winter on the surface. Also, you can dump a few kW into a reasonable size "room" of this mine, and it will still be 45C.

So the best solution, as Russell also pointed out, is to spec things so that they run fine at 45C ambient. Space them out enough so each thing sees the 45C ambient, perferably without needing any forced air cooling. That way you don't have to worry about what happens when the coiling breaks down. Again, most off the shelf electronic gear should already be fine with that. Lifetime may go down, so derate. Get good quality stuff that should last 10s of years normally.

If you really need cooling, my first knee jerk reaction is to build a small room and put 2 or 3 air conditioners in it. Make sure any one air conditioner has the capacity to cool the room sufficiently. The multiple air conditioners are for redundancy. You seem to say it's only a few kW, which is small as airconditioners go. 1 kW is only 3400 BTU/hour. Also, it sounds like every last item doesn't need to be cooled, so some of the power you need can be dissipated outside the room and doesn't add to the air conditioner load. 113F isn't that hot and there are certainly off the shelf air conditioners intended to work under those conditions. Having 2 or 3 units always on cooling the room is for reliability. However, this is a area you really should consult an expert on.

Answer 2

The answer is easy but not necessarily acceptable :-).

• Cooling will be easiest.

• Air conditioner with different refrigerant may suffice - HVAC experts can advise.

• System reliability and response to failure must be planned. Does it need to be redundant-failsafe or should it shut down under cooling failure.

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• You must determine the maximum "local ambient" temperature that the equipment will work in.

• You must provide a "local environment that is never any hotter than the acceptable maximum environmental temperature.

• If the necessary temperature is below the local ambient of 45 C you MUST provide cooling to maintain the local temperature.

Having established this necessary but unavoidable requirement you can address it in two ways.

Assume:

Ambient temperature is never > 45 C.

Ambient = mine temperature = Ta.

Local temperature = equipment housing temp = Tl

Then either

1 You can make the equipment able to operate at Ta (45C )

or

2 You can provide cooling of an enclosure to Tl.

1 is superior if you can achieve it as it involves less long term care and management and reliability. If you can find equipment that will run at 50C (Ta+5) you will be able to provide cooling with blown air and very good design. Ability to operate at Ta+10 (55C would be better and TA + 15 or 20 better still). Heatsinks of 1C/Watt rating are getting fairly impressive so at Ta+5 you can only dissipate 5 Watt with a 1C/W heatsink or 20 Watt at Ta+20. If you need substantially more than about 10 Watt dissipation you can use "heat pipe" transfer away from the equipment which then allows as substantial blown heat sinks as required.

You mention "a laptop" but you will probably find that there are custom devices that meet you computing needs and that are aimed at low power and rugged environmental conditions. Ultimately such a purpose designed device may be cheaper than a standard laptop + much pain.

If eg your photo multiplier tubes MUST run at under Ta then you MUST cool them.

COOLING:

There are numerous means of cooling in that environment. The likely easiest and simplest and highest capacity in that environment, provided enough power is available, is a standard "Rankine cycle" airconditioner or refrigerator BUT with the refrigerant type and pressures/fill designed by an airconditioning expert. While standard off the shelf units are unlikely to have refrigerants that are optimum for Thot = Ta = 45C it is quite likely that little more than a refill with a more appropriate refrigerant would be required. This is such an attractive option that talking to an HVAC expert should be one of your first actions. See [1] at end for some likely starting points.

Stirling cooler:

High efficiency Stirling cycle equipment. Several suppliers. Sunpower et al.

The Coleman Stirling camping cooler may meet your need directly. NOT to be confused with cheaper Peltier type coolers. I can provide references to the manufacturers of this and similar Stirling cycle equipment.

Peltier:

As per electric chilly bins BUT done properly. Peltier devices can work entirely adequately in the environment you describe. 10C drop is easyish and 20C plus is doable. Needs proper design. A custom solution may be best.

Needs little more than suitable heatsinks, substantial Ta air flow, lower Tl airflow, Peltier devices + power. Peltier coolers of 50W - 100W energy input capacity are available. Each of these would cool perhaps 5W - 10W of equipment load in this environment. TBD by design. Multiple Peltier modules are used to meet need.

Evaporative. An evaporatove cooler using water ("Swamp cooler") MAY suffice and one using other fluids will be able to. This approaches an air conditioner when properly done in this context.

More if needed ...

[1] Likely useful refrigerants sites:

http://www.refrigerant-supply.com/

20 refrigerants for sale ! : http://r22.org/

https://refrigerantcompany.com/

http://www.refrigerants.com/

http://www.alpinehomeair.com/view.cfm?objID=F6F91969-85BD-4AFB-99C3-DA02AAF00074

Answer 3

Thermo-electric devices (or thermo-electric coolers, aka TECs, aka Peltier elements) pump heat from one side of the device to the other, often capable to create temperature differences between both sides of up to 60°C. If you let them cool one side to 30°C the other side will be much hotter than the 45°C ambient, so it will be able to drain heat to the environment. Forced cooling (fan) may improve performance.

You'll have to construct something like a (mini)fridge, with the TECs between heat sinks on inside and outside of the box, resp. Size and power consumption of the TECs will depend on the amount of energy they have to displace.

Answer 4

You have basically three "tools" to use:

1.) Lower the thermal resistance from your heat source to ambient;

2.) Lower the heat generated;

3.) Lower "ambient" temperature;

Temperature of electronics = Te

Ambient Temperature = Ta

Heat generated = P; typically given in Watts (W).

Thermal resistance from Te to Ta is R; typically given in (degrees C)/W.

Te = Ta + PR. Another way of writing that is: Te-Ta = Delta T = PR

You have an allowable Te you need to maintain. Let's say (for sake of discussion) it is 80C (176F). Your Ta is 45C. That means your delta T is 80C - 45C = 35C. ~IF~ you had a nice comfy office, Ta would equal 25C, and your delta T would be 80C - 25C = 55C.

For an office, delta T = PR = 55. For a hot mine, delta T = PR = 35.

So, you see, a hot mine only makes it harder (not impossible) to cool your electronics. It means you need to make R for the hot mine scenario about 2/3 of what it would be for the cool office. you can do this by increasing thermal conductivity and/or airflow.

Not what I would do, but you could also lower "ambient" within the box with a refrigeration unit. Yes, it will work if you can get power to it. It doesn't have to keep your beer cold: lowering the ambient from 45C to 20C should be plenty enough to keep your laptop from frying.

You CAN also reduce P, by carefully shopping for the laptop and/or underclocking it. If you are doing datalogging you likely do not need the highest output quad core with a GPU that allows playing the latest FPS video games. Shop for low power.