Cosmic Background Refrigeration

[Breaking news: now baked and commercialised. See the two [link]s added Feb 25 2023]

A parabolic [*shape disputed, see annos] reflector pointed at a clear night sky will tend to bring the temperature of a body at its focal point into equilibrium with the cold vastness of space, at approximately 3 kelvin. You can easily prove this at home with some reflectivey stuff and a bucket of water. You may even get it to freeze. [see link 1]

Cloudless desert nights can be icy icy cold, which anyone who's been to central Australia can testify.

I propose a passive home cooling and refrigeration system for hot, cloudless environments i.e. deserts.

A rooftop parabolic or compound parabolic [link 2] trough system, as known from solar thermal heating systems, is exposed to the night sky, pointing straight up, and used as a thermal emitter (radiator) rather than a thermal concentrator.

Through black tubes at the foci runs a fluid with a high thermal capacity and a low freezing point, perhaps a salt/water/ antifreeze mixture. These tubes converge into a manifold that runs to a storage tank, situated lower than the emitter plates. This in turn is connected via controlled valves to "radiators" in the rooms requiring cooling during the day, and by a separate (and separately regulated) line to a cool-room or refrigerator.

I say "radiators" because these will look much like (and may even be) the baseboard radiators seen in buildings with hot-water central heating, but will in fact be acting as thermal collectors ("emitting" cold) and will need to be high in the room where the hot air gathers, rather than low.

The heat gathered here causes the coolant to thermosiphon [3] through the system.

During the day, as desired by the inhabitants or by thermostat and solenoid, the valves between the cold storage tank and the room collectors are opened. The heated fluid from the rooms rises, taking the heat with it (or, to put it another way, the chilled coolant sinks to the rooms below, and absorbs heat) tending to equalise the temperatures of the room and the tank (or to put it another way, keeping you and your food, medicines, morgue inhabitants, etc cool in summer).

At night, the valves between the tank and rooftop radiators are opened, and thermosiphoning begins anew. The slightly warmed coolant rises into the black tubes. From here, the excess heat is dumped into space by radiation.

Insulated shutters come down over the radiators during the day to stop the coolant from boiling. Should the coolant freeze solid in the tubes, the shutters can be opened awhile.

Alternatively, one could under-fill and under-pressure the system and create a phase change heat pump [4] or heatpipe [5], where the fluid boils at the hottest point and re-condenses at the emitter. This would allow much more rapid heat transfer at the expense of more critical materials tolerances and seals.

Attainable temperatures will depend mainly on the freezing and boiling points of your chosen refrigerant. Liquid nitrogen should work nicely :) The size of your storage tank will depend on the area you wish to keep cool, local ambient temperatures, and the ratio between cloudy and clear nights in your location (reserve capacity).

Using more common fluids or water, this system could be built with standard plumbing and A/C parts, built and certified by any qualified refrigeration engineer, or home-brewed by hobbyists, survivalists, low-impacters and villagers. It is as applicable to skyscrapers in Houston as is is to medical clinics in Africa. It consumes no power and has no moving parts other than the valves.

It mitigates global warming two ways: indirectly, by displacing power-hungry active refrigeration, and directly, by heatsinking the Earth.