Cool your home with zero electricity costs!

Solar cooling sounds like a contradiction in terms, but it could become a household phrase in a sustainable future, especially in off grid regions and developing nations because, if done properly, it could reduce your home temperature by almost 10 degrees Celsius (19 F) with no electrical input at all. Now a team at MIT has developed the most efficient system yet.

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Research links

IEA - The future of Cooling

MIT article summary

MIT Paper

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[human_isomer]

Sounds good on the first look, but then... sorry, I need to debunk this.
1) Cooling 9 °C below ambient temperature would not be sufficient to conserve food, when in a climate where ambient temperature is around 30 °C or higher (for useful refrigeration, temperature would have be at 8 °C or below, inside the conservation unit). A wet blanket sitting in the shade with a little bit of wind blowing over it would have the same effect, but with much lower technical effort, and magnitudes cheaper. (And that method is actually how I keep my drinks cool when going to the beach in the summer.)
2) It still works mostly by evaporation of water. As you said, that's the last thing you want in a humid environment, where it will be much less efficient, due to the higher saturation of the surrounding air with water steam (which, by the way, is the reason it needs much less frequent replenishment of water in these environments). And in many areas, water is too scarce to use it for this purpose. Besides that, you'd need quite clean water, so the pores in the aerogel would not get clogged over time. I'm pretty sure that you'd actually need distilled water or similar purity, because even crystallising salts would clog the pores. But water of this purity is a) expensive and b) not even widely available in the industrialised countries.
3) The reflected radiation will only partly go out into space. Another part of it will be absorbed by the dust, the clouds, the water vapour, and the CO2 (and other gases) in the atmosphere. Even in a clear sky, there is still water steam and those gases. And a good part of the heat will be contained as the mentioned latent heat in the evaporated water, and thus, end up in the atmosphere, too.
In conclusion, I consider this "game changer" just an expensive and complicated way of putting a wet towel around your water bottle and placing it under a white sunshade.

[richardgreen]

Many of the world's 'problems' are self-inflicted. Most have obvious and known solutions.
- My household cooling system includes the natural reflectance and evaporative cooling provided by trees. Since it takes a long time to grow a tree, the best short-term alternative solution is to grow vines on the available building surfaces. While growing vines may increase building maintenance costs, vines should pay-back in terms of reduced cooling expenses and improved urban landscapes. In dense urban environments, one might also need to build porticoes over adjacent sidewalks to shield them from bird poop. But those porticoes would also shade the ground-floor walls and provide cooling via vine or turf coverings.

[wymptsu]

Thatched roofs have a similar effect because the organic materials absorb humidity and water(when it rains)and like wise when hot they evaporate.This bring back vivid memories of my first trip abroad which was to New Guinea, one of the most humid countries in the world with an almost constant temperature of 30 C for any non native person this unbearable but the moment you enter one of the native huts which are heavily thatched you are instantly cool, it was amazing and a great relief.

[agsystems]

I don't think you really understood how this one worked tbh. To my eye it looks like the evaporative side of things is just a side hustle to the main way it works and shouldn't be focused on without a solid understanding of radiative cooling. The primary idea is that the aerogel layer forms an 'anti-greenhouse', that lets the IR radiation from the water gel layer below out, while also keeping the atmospheric glow out (sunlight is let through, but reflected back out). This 'primary mode' is cool but isn't that novel, which is why the paper doesn't dwell on it much. It is standard radiative cooling. Where this is novel is the construction. In particular it does away with any attempt at making a 'closed' system, which is where evaporation comes in. The construction is considerably simplified by the fact that water gel is both the radiator and the heat exchange fluid, while even a barrier to keep it in is unnecessary. As water evaporates it actually augments the cooling, so is a feature rather than a bug. Humidity tends to be higher at night than during the day, so this effect makes it function better when it wants to function best.
This is really cool and I am glad you showcased it, but I'm afraid I don't think you met your usual standard here (50% will be below average :P). Nighthawk in light has a good video on cooling paint, that also explains why pore size control is so critical to these sorts of things. Might be worth revisiting this, as it is super cool.

[GordieGii]

Of course you would have to use distilled water, or the hydrogel would quickly turn into a block of salt and/or lime and need to be replaced frequently. It doesn't sound like either of these costs were factored into the calculation.
On the other hand you could bathe the condenser coils of a conventional AC unit with ground water or grey water at room temperature and still get a significant improvement in efficiency. Or evaporatively pre-cool it with the same fan that cools the condenser coils.

[silvergreylion]

1. Take a black oven tray, usually used for something like pizza or lasagna, and put it outside in the sun, on a level surface.
2. Take a liter of rainwater, and pour it into the tray.
3. Measure the temperature of the water as the sun causes it to slowly evaporate.

The temperature will creep down to a few degrees Celsius, if you've done it correctly. This is because black objects absorb most of the sunlight and heat up, but water is extremely efficient at absorbing heat, and the heated water molecules will easily jump through the very thin layer of water, and evaporate at the surface, cooling the water much more than if there was a much thicker layer of water.

[steverichmond]

I have used aerogel blanket cut into strips to prevent thermal bridging in timber sole plates and ring beams when building with SIPs. It works well and is not expensive. I have built airtight buildings using passive ventilation with great success in a desert in Spain using this system.

[kevinmcdonough]

Looks very promising! There must be some maintenance keeping the aerogel surface clean & avoiding mineral buildup in the hydrogel. Hopefully clever designs will minimize this.

[FutureSystem]

Fascinating-thanks. 👍
And yes here in Oz, (Brisbane) we have serious problems with heat and humidity- yet half the new houses around here have dark and even black roofs. Complete and utter Madness! The trouble is those dark roofs that our neighbours have heats our environment up as well.
We have a bright white (steel) colorbond roof and it makes an absolutely huge difference to our A/C requirements. (We also have 6.5kW of solar, and export a lot of power, even with airconditioning use.)

[kenbellchambers]

I used an ancient kerosene fridge for a few years. They burn kerosene with a little cylindrical wick. The hot gasses flow up a tube/chimney which is surrounded by an ammonia filled outer cylinder. The heat causes the ammonia to expand, build up pressure, then spray as an aerosol in order to act as a heat absorber or refrigerant. A gas fridge does the same. Low grade heat from a water heating solar panel should be able to use very hot water to do the job of cooling by this sort of approach. Low grade heat, in general, is an immense and often overlooked resource.

[AttilaTheHun]

1:34 ...that's why I love your channel. You are asking the right questions. There are too many channels out there praising new technology and ideas which will never make it in the real practical world.

[davestagner]

The Carbon Almanac is great! I recommend keeping a copy in your bathroom, so you can have something to read in small bites, rather than mindlessly scrolling your phone.

[willm]

Thanks Dave - make sure you check out the advancements made on closed-loop geothermal - if they are right about what is possible (one company is named EAVOR) then I think it is a a solution that is comparable in its potential to wind or solar.

[oldtimefarmboy]

If you live in a house with a pier and beam foundation or a trailer house, you can use the underside of your house to make the condenser of your air conditioner work better.

I built a wooden box around my condenser that covered all but the top where the hot air comes out and the back side that was butted up against the house where I had opened up part of the foundation and installed a metal frame with a screen on it. That way when the condenser was running it would pull air out from under the house.

The box was painted white and had foil backed foam insulation installed on the inside to stop radiant heat from getting in.

The air coming in is from the shaded area under the house so it is cooler. Any moisture under the house cools the air off even more.

The benefit is that you have cooler air going through the condenser, the air circulating under the house helps dry it out so there is less chance of mold and fungus that could cause rot of the floor structure, the box helps protect the condenser unit from inclement weather better than standing out in the open, and if done properly the box can easily be moved out of the way if there is a need for servicing.

[rocklover]

I think large unglazed clay pots full of water with a central water proof container to contain food to chill it was used centuries ago .
Water gradually leaked and evaporated on the outside of the clay pot to keep the inside of the pot cool .
Early American cars used swamp coolers

[tristanschreiber]

Thanks Dave again for your beautiful presentation. I guess its most effective on resource intensive single family homes?
Nice that you included the reference to the new book - looks excellent!

[kaboozle]

I thought this video was gonna be about adiabatic cooling (cooling by water evaporation but requiring constant flow of water) but you managed to take us down a new road entirely! Well done!
A note about future proliferation of AC units: in the Netherlands, all new houses require a heat pump and at least some solar panels at the moment. A few years from now, I believe in 2025, gas heaters will be phased out and will be replaced by (hybrid) heat pumps. From living in a modern heat pump equipped house with floor heating and solar panels I’ve experienced that 1) my electricity bill is net negative (and that includes charging my EV!), and 2) the house is cooled very well even during heat strokes. This all but eliminates the need for AC. So innovative housing design and targeted clever legislation will not only curb the future need for AC but yield significant financial gains for the home owners of the future.

[paulw]

Placing solar panels over all my north facing roof (Australia) on both my house and shed dropped the temperature within the rooms directly under them. An unintentional BONUS!

[RustyWalker]

Solar "chimneys" are pretty simple. Passively heating air above roof level in the "chimney" can draw air through the duct from the building below. This moves hot air out but needs a shady side from which to draw cooler air in, or one of these to cool the intake air passively as well.

[stephenlabarre]

The answer is conservation through better house design.

It saddens me to see the current crop of residential housing developments. Most are designed for their asthetic appeal to consumers that are more intertested in granite countertops and three car garages than R-values, roof lines or site orientations. While residential solar is not necessarily the only option for clean energy production, most homes on the market do not include a solar option. Home builders should consider structural orientaion, roof lines, tree locations and other design features that would facilitate the installation of rooftop solar (PV or not) at the time of construction or with consideration of future additions. For many people, ground mounted solar is not an option.

What's frustrating is the fact that these structures will be around for generations. Our kids' kids will live in these houses, never acheiving an efficiency or comfort level they desire because of the initial inefficient design. Sure, a gigawatt solar array could be installed (not really) powering a 100-ton air conditioning system (another exageration), but... why?
The best method for saving energy is not needing it in the first place. Energy conservation through better design reduces the need to purchase and install large A/C equipment. This translates to less complexity and repair, lower initial purchase and ongoing costs and more consistent comfort (if designed properly).
While all construction methods and materials have issues, they can be worked out if the desire is there. Look at lighting where current LED bulbs use a tenth or their incandescent counterparts. A TENTH! This took only one generation to accomplish. Or how about batteries? They have becone lighter, smaller and can be charged 1000's of times. With renewables and EVs being the driving force, more advances in battery chemistry are just around the corner.

The lessons here? Money moves mountains. We need to reward developers pushing the envelope. Codes need to be updated or 'stretched' to allow for thinking 'on the other side' of the box. Housing construction is like a hot air balloon; there's a delay between firing up the burner and seeing a change of direction. Inefficient, energy guzzling houses will still be around long after we die. QUIT BUILDING THEM!

Conserve first! You don't have to live like a caveman to live efficiently. And, you don't have to break the bank either. While it may cost a little more up front, an efficient home will have better resale value. And if you plan on staying put, the cost savings will mean a lot when you're financially strapped.

Let's not forget about residential water collection and usage. (we'll save that discussion for another day)