Cooling Maxwell

After I've inserted miners (even if they're only robo-miners) into a shaft in a Venus highland, how do I keep them productive? It's hot down there - more so for humans. This term is Climatic Hazard.

My region of Colorado quit digging after the Second World War. There remain observable deposits, almost on the surface: silver at Georgetown (I've seen the dragon-tears for myself), molybdenum at Crested Butte. Silver is cheap and will likely always be cheap, and Coloradans prefer to ski on the Butte rather than dig in it. As one ventures into less-populated and uglier regions, and digs for more-valuable mineral, one tends to chase the veins deeper. South Africa and deep Canada sport some of the deeper mines here. These are the elder kratons - which, geologically, may well be the best analogue for Venus' highland continents.

Underground is chilly - when you're not that far underground, and the machines are silent. As one digs deeper, the pressure rises and the "virgin rock" temperature rises too. It also becomes more difficult to run the air-pumps. At some point the miner isn't at 10 Celsius, he's at 70. (Also about the temperature of Venus' clouds at one-bar-pressure at the equator, which is why I've relocated Landis' farms to the 50s latitude-bands slightly higher.) 70 C doesn't boil water but it does boil monkeys.

I don't know the virgin-rock temperature of the inside of a Venerean highland but basic thermodynamics tell us that it starts at the temperature of the outside air, 700s K at a highland. So: the Venus mine needs airconditioning. It also needs to deal with the 50-70 bar pressure (high up Ishtar Terra), the lack of water, and the carbon-dioxide atmosphere... which all, as we shall see, cease to be problems once the A/C is on.

I would start near the surface. For Earth analogues, I look to regions already hot on the surface: South Africa and Australia. They use Bulk Air Cooling on surface-side (pdf), running that down with refrigeration-shafts. Below 2.5 km, they make water ice on the surface and ship that down (pdf). Local air-conditioning units are also used. Either way, the now-cooler air on the bottom displaces hot air back up, allowing some energy recapture at the surface by turbine.

Amin Kamyar et al. "Current Developments" (U Wallongong, 2016) summarises MJ Mcpherson, Subsurface ventilation and environmental engineering (New Delhi: Springer Science & Business Media, 2012), thus:

The vapour compression cycle works via compressing the refrigerant vapour to a high pressure (and high temperature) before sending it to the condenser (a heat exchanger) where it reaches a liquid form. Condensation is done with the aid of cold water coming from cooling towers. High pressure liquid then flows into a receiver followed by an expansion valve. Upon passing through the valve, the liquid refrigerant experiences an abrupt drop in pressure (along with a dramatic drop in temperature) and sudden expansion (flash off) resulting in the evaporation of the liquid. The low-pressure liquid then flows to the surge drum which separates the liquid and gas phases to ensure only vapour is sent to the compressor. The liquid refrigerant passes through the evaporator (another heat exchanger) where it absorbs the heat from air or water and boils. The vaporized refrigerant then enters the compressor and the refrigeration cycle restarts.

If we were keeping pressures Earth-level, the compressor would just be "the outside", at Ishtar's 60s bar = 6 to 7 MPa. If.

The condensation portion should be done by some means other than water, since the mine is importing that from the clouds and can't afford to waste it. This applies to all hydrogen volatiles, like ammonia and Freon. UPDATE March 2020: Steep slopes like Maxwell can use katabasis, and Babbage-like steampunk architecture, for the very basics.