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Comment by leoedin | Hacker News Reader

To compare to an air conditioner. This device has about 40W of cooling per m2.

Apparently in Australia you should size between 80 and 120 W/m2 of air conditioning (I think this is cooling watts rather than power usage watts) - https://www.google.com/amp/s/www.crownpower.com.au/blog/choo...

So that means every square meter of living space needs 2 square meters of radiative cooling (assuming no other passive cooling infrastructure). I suspect you'd see further inefficiencies getting the heat to the passive cooler.

So it's within the same order of magnitude of an ac, but not powerful enough that it would be straightforward to retrofit.

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The weakness it has it that it needs a clear sky to work. For overcast muggy days or even a high coverage of cumulus clouds, performance will be absent or degraded.

It works at night, however, and for best results you could maximize insulation and "thermal mass" inside the building and minimize radiative transport through the windows.

The best thing about air conditioning, however, is de-humidification and that is a matter of cooling the air more than you have to and then re-heating it. I live in an 1850 farmhouse and the reason I want a ground source heat pump is that the humidity destroys books and other printed matter. I have inkjet prints curling off the walls and detailed logs of how 3M's best products only work 90% of the time in my applications.

Adhesives are not built for high-humidity environments. My parents live in the tropical rainforest of Panama. One of the challenges is that anything stuck together with adhesive usually comes apart over time. The glass panel on the door of my mom's oven fell off.

As an engineer I don't accept that things have to suck.

If other people think failure is OK I can't do anything about it, but if I have the problem that "Adhesive X does not work in Environment Y" I am going to change the adhesive, change the environment, or not use an adhesive.

I'm having trouble understanding why a clear sky is important. Surfaces radiate based on their temperature and emissivity only, right? So why would it matter what the surface is emitting toward?

Perhaps what I'm missing is that clouds emit some radiative heat back to the surface, whereas a clear sky emits very little, so the net heat loss from the surface under a cloudy sky would be lower.

> Perhaps what I'm missing is that clouds emit some radiative heat back to the surface, whereas a clear sky emits very little, so the net heat loss from the surface under a cloudy sky would be lower.

This is exactly it, yes.

Water vapour re absolves the IR akin too blowing on your sails however it's affect is reduced when it's cloudy can still function just not as well

I am trying to figure out whether one should take into account the cooling power of the surfaces being replaced. The figure of 80-120 W/m2 for air conditioning is presumably based on conventional building materials, which have negative cooling power.

In the paper, the figure of 40 W/m2 seems to be the net cooling power, which is defined in equation 1 as being the power radiated away minus various inflows of heat: radiatively, from the atmosphere; radiatively, from the sun; and by conduction and convection. As far as I can see, these corrections are all for this particular surface, not the surface it might be replacing. These will not, in general, be the same, and, given that this new surface is both highly reflective and vacuum-insulated, I would guess that its values for these properties are lower than the conventional building materials on which the a/c rule-of-thumb is based.

Nevertheless, I doubt that replacing the entire roof with this material would be sufficient cooling, on its own, in the Australian case, and I agree that this would not likely be a straightforward retrofit, to say the least!

This was my thought as well. I have an attic fan and monitor the temperature inside to control it. It regularly gets 120+ F in there on an 80 F day. I have to think the majority of the heating of my house is coming from the attic.

A typical AC cycles its power input. Its not constantly On. One benifit of using this system could be to continuously remove heat from the house and then use an AC on top of it.

There is always a consumption value to free beer.

>> A typical AC cycles its power input.

If it has reached the requested temperature. Like basically all consumer thermostats, it is a bang-bang controller. There is no set on-off cycle. If the AC unit is running at capacity, ie it is properly sized for requirements, it will just be on all the time.

But the temperature goes up and down all the time so there is no "fixed requirement".

If the AC is on all the time it is most likely undersized for the requirement at that time and can't maintain the desired temperature.

In a better-than-consumer setup you will have multiple chillers. Most will just stay on, with one going on-off to handle the variable bit of the load. Starting and stopping electric motors is less efficient tha just keeping them running as much as possible.

Check out interter ACs. Good ones should scale from 20-100% in power or better, so should be able to stay on for most of the time.

Yeah, most central heat pumps installed now are inverter models (also referred to as variable speed). More efficient and more comfortable since you've got a continuous flow of cool air, rather than blasts of cold interspersed with nothing.

There's a lot of potential applications outside of habitat AC in Australia.

For example cooling down stand-alone hardware in the field.

The technology has advanced since then, now white paints with high emissivity in the infrared window are being researched. So if you cover the entire building with that you would get some free, always-on cooling that way.

Opening the windows at night and running fans to equalize the temperature, then putting space blankets over the windows for the day works wonders on hot days in upstate NY.

It's very good as long as two conditions apply:

- the humidity has to be comfortably low

- the outside temperature has to be low enough

Historically, this is usually the case.

In the last few weeks, I've had one or both of those fail to apply on the majority of nights. Dropping to 65F doesn't help when the outside air is also at 99% humidity. If the overnight low is 75F, we're not getting much cooling out of it.

Any system has to deal with time-variable conditions.

I dream of getting a geothermal heat pump for my 1850s farm house which is normally heated with two wood stoves but has a propane backup. (e.g. the kind of compact heater that you see all the time in people's apartments in anime)

At points south the capacity of that kind of system is set by cooling demand but where I live it is set by heating demand. The woodstove could pick up the slack on the coldest days, but that defeats the main selling point of the heat pump which is extreme comfort (e.g. it switches seamlessly from heating to cooling)

- the outside air quality has to be okay

Between pollen count and pollution this is not always the case, and keeping the windows closed and running both the AC and an air filter unfortunately has health benefits for some individuals.

One question that I always wonder when hot days strike:

Given that I only have one portable fan, what is the best setup at night if it is colder outside than inside:

1. Open the windows and put fan so that it blows air out of one window

2. Open the windows and have fan mix the air inside the room

3. Open the windows and put fan on balcony to blow air from outside in.

4. Buy additional fan for ~$15.

Seriously though, it depends. If you have no other way to intake or exhaust air, probably 3, since fans are more effective at blowing than sucking. (IE: 1. would spend some of its power recirculating inside and outside air rather than just pulling inside air out.)

Most likely your bathroom and hopefully stove have exhaust fans, so even better would be to turn one or both of those on, and have the fan blow in a window on the opposite side of the house. It may not even be ideal to open all the windows. You want cool air flowing through the whole house. In an extreme example, if you have the fan blowing in the balcony and an open window right beside the balcony, it could just circulate air there, rather than reaching the rest. Likewise with exhaust, if you have a fan in the bathroom and the bathroom window open. So you'd need to experiment a bit to see what flows air best through the house.

Things also change if you have a central blower.

I had a similar problem in my previous house which didn't have central AC. The insulation in the house was great for about one day. But in an extended heatwave, outside 100F meant eventually inside 90F.

I got decent cooling results with two separate window fans, opposite sides of house, one blowing in one blowing out. Of course, all other windows are closed.

Depending on your window frame style, these types of dual fans fit inside a window frame and don't leak air. They can be set to: both in, 1 in 1 out, both out. So quite flexible. https://www.amazon.com/Bionaire-BW2300-N-Reversible-Airflow-...

A single window fan of the above style can cool a single room quite well, blowing cool air in and blowing hot air out.

As others have mentioned, depending on breeze and temperature, opening all the windows can sometimes work better than multiple fans.

Fans are just no substitute for a real AC unit, which can lower indoor air temp to below outside air temp and can also extract moisture from the inside air.

Another suggestion from your 3 - use an exhaust fan on one end of your home, blowing air from your ceiling out a window(you want to blow the air near the ceiling, it is warmest). Use an intake fan at the lowest elevation possible.

The premise is...cooler air falls, warmer air rises. You want to blow in the low(cool) elevation air, and exhaust the high(warm) air.

I wonder the same thing. If you get a chance, try to test the various configurations!

I would suspect you will also see different results depending if you have multiple windows or just 1. For example, if you have more than 1 window and can seal the opening except the fan, then the fan will move x amount of air in or out and will have similar results.

Simply mixing the air inside the room seems like it's probably the least effective because it will result in very little heat exchange at the windows themselves. However, making the temperature within your house more even may make it more comfortable on the average inside your house.

Matthias tries a few options and evaluates them– https://www.youtube.com/watch?v=1L2ef1CP-yw

> Experiments and anemometer measurements to figure out where to best place a fan to optimally air out the house to cool it down at night.

Nice! His take:

- Blowing out is better than blowing in.

- Fan should be some distance to window otherwise it is ineffective.

Open two windows. Only if needed, put the fan somewhere in between the two windows to facilitate airflow.

That's true, but I don't see why the two couldn't be used in tandem, thus reducing AC power consumption substantially.

Might as well just get solar PV then, that's around 150W per m2.

150W electricity, so you need to run a refrigerator cycle to pump that heat out of your house, which comes at a massive efficiency penalty.

Refrigerators actually have greater than 100% efficiency, often like 300% or so. Because you're just moving the heat, not creating it. Sounds like magic but it's not.

ah true. though that number is coefficient of performance, not efficiency

And an efficient heat pump can move four times as much energy as it consumes. But if some special paint/tile/insulation can reduce your needed cooling capacity by 30% you'll save a lot of money on equipment.

Keep in mind there is nothing preventing you from just angling the device (in its extreme, vertically) and just get an arbitrary amount of radiative surface with a given flat footprint.

(other than of course, it looking unsightly and construction costs)

edit: it would probably help a lot of you angle it such that it is normal to the sun rays, like where i live the sun sweeps from the east to the west, so if you angle the device north or south it would probably work even better.

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