Excavating holes for cooling during summer nights

Question

This idea is motivated by the readings of this Hungarian weather station:

It sits within a ponor/doline which is basically a hole in the ground that allows the collection of the dense cold air. It can be seen that as soon as the sun stops directly shining into it, the temperature starts to decrease quickly, then during night the temperature is below freezing.

In other parts of the country there is a heat wave with 35°C during day and 22°C at night. But that hole freezes and also doesn't heat up much during the day.

During a heat wave the sky is clear and there is not much wind, so a hole can lose a lot of heat by radiating it out. The air within cools down, becomes denser but can't escape anywhere, so it stays there such that a temperature inversion occurs and this happens.

To replicate this artificially, excavate large a hole in the ground and put the excavated earth around it to form a large basin to contain cool air, then the coolest air sinks to the bottom so we can add a drain at the bottom where we can drain the cool air to cool the house, and this hole can also double as rainwater collector.

Question: would it work? Or did anyone attempted to use something like this for cooling?

I searched the web and found nothing. Absolutely nothing. There is a term "fagyzug" for these kinds of holes Hungarian. There is also a term "hideg légtó" to describe the cold air collecting there, but Google Translate fails to provide useful translation for either.

There are topics about radiative cooling systems which use expensive materials that are highly emissive in long-wave infrared. But my idea is just a hole in the ground nothing else.

Answer 1

You may wish to research "dew ponds" and "air wells" — they all rely on the same fundamental physics.

The phenomenon works primarily because the large diameter, relatively shallow depth, and lack of vegetation, reduce radiation influxes at 'night' and result in a net energy outflow from the middle of the depression — radiative cooling.

Although the amount of 'coolth' that is generated in this manner is measurable, it's actually quite small because air has a very low specific heat capacity. Devoting 1000m² or more to generate a few thousand cubic metres of sub-zero air is a horrendously inefficient use of land. For the amount of diesel it would take to excavate such a hole, you could buy a few hundred large solar panels, a few thousand thermoelectric coolers, and chill your house down to -50°C in a heartbeat.

If you flow/pump the cool air into your house at the rate of 100m³ per hour, you will exhaust the supply in a single day. The ground temperature will rise as warm ambient air flows in, and the performance will suffer more and more every consecutive day that it is used. In other words, frost pockets stay frosty only as long as they don't get used/pumped out. The moment you start removing air they quickly cease to be frost pockets.

Can a very large, naturally-occurring depression be exploited to cool a tiny home or a single room? Yes, I'm sure it can. Can an artificial depression be created that can cool the average house? No, not in any 'economic' way at least.

Soil has a much higher specific heat capacity, so is capable of 'banking up' much more coolth than air is. As James writes in his answer, a cheaper and more efficient way to go is with a ground-coupled heat exchanger (aka 'earth tubes').

If you happen to be in the position of having lots of land and a tractor (or other earth-moving equipment) that is all bought and paid for, and like digging large holes, then you might want to experiment with such an excavation. It would probably be a good idea to brush up on your grasp of Bernoulli's Principle before breaking ground, however — prevailing winds can make or break such an experiment.

PS: Last week I started the process of deliberately creating a frost pocket on my property in order to help keep the water level in my pond more stable. Over 300m³ of earth moved so far. Many thousands more to go. Not for the faint of heart.

Answer 2

What you are looking for is called Ground-coupled heat exchanger it can be used for both heating and cooling.

Earth-air heat exchangers have been used in agricultural facilities (animal buildings) and horticultural facilities (greenhouses) in the United States of America over the past several decades and have been used in conjunction with solar chimneys in hot arid areas for thousands of years, probably beginning in the Persian Empire. Implementation of these systems in Austria, Denmark, Germany, and India has become fairly common since the mid-1990s, and is slowly being adopted in North America.

For cooling the "holes" are fairly shallow, 1.5 to 3 m (4.9 to 9.8 ft)

In colder climates, where heat is the primary goal, the term Geothermal heat pump is usually applied to deeper "holes" 15–122 m (50 to 400 ft)

Answer 3

The drain itself will probably provide more cooling than the hole. Make the drain as long and wide as possible, and put it deep enough to avoid picking up heat from warmer areas near the surface. The weight of the cooler air in your hole will help feed the air to your house, but you might also want to suck the warmer air out from high on the opposite (downwind) end of your home using a solar chimney or an adjustable vent. I have found that providing dense shade from vegetation in and around the hole area reduces the starting temperature at the intake and also reduces the effects of warm breezes that might blow away your precious cooled air. If your cool zone is upwind, even the warm breezes will push it towards your house to cool it externally.