Super interesting!

How does it work in hot summers? I'm assuming a heat pump? But if there's a need for cooling, I would assume the only option is electricity....

Although if it has solar panels, then it could generate the needed electricity itself?

That house looks like a fairly practical solution for its location. I checked climatological data and found the average high temperature for the warmest month, August, was 70.7°F. Thus, air conditioning should seldom be needed and outside air ventilation would suffice even on most warmer days, particularly with the heavy insulation to keep the inside temperature from fluctuating quickly. The heat pump and solar panels for electricity mentioned in the article shouldn't have much trouble keeping the house at a comfortable temperature.

The other major feature of the house and the obvious passive element is the large bank of south-facing windows. They should contribute significantly to the seasonal heating needs and the heat pump, apparently powered entirely or nearly so, by the solar panels is the back-up to solar heating. The ability of the house to remain comfortable on cold, dark nights then depends on the very heavy insulation to minimize interior temperature swings.

I looked at various passive solar heating schemes when I was considering a home addition some time ago. The ability to admit the heat of the sun during the day, absorb it inside the building or the heat storage space during the day and release it during the night all come into play. The ability to reject solar heat during warmer months may also become important as people living in areas such as Florida or Arizona well know.

Combining the passive elements with the active control via the heat pump helps and powering it with solar electricity pushes the whole house toward a neutral fuel stance. Based on what is reported in that story, the house in Maine has a net zero or less electrical consumption, paying only the connection charge for their electrical service. I suspect they have more than enough solar capacity to power the heat pump and supply their other electrical needs during most days and depend on the grid for nighttime. Otherwise, they would need some other nighttime electricity source such as batteries to store solar energy collected by their solar panels during the day.

Living completely off the grid is more challenging and a problem that needs a solution if we are to wean ourselves off fossil fuels. Some renewables such as hydropower are available and nuclear power is also not dependent on variables like wind or sunshine. Storage via batteries, pumped storage, and some more esoteric mechanisms is another possibility. Fusion is a more distant hope, but it's a complicated puzzle no matter how you put the pieces together.

Big Al

A number of passive solar houses like that were built in the 70s. The big problem with them was always overheating in the summer. Unless those folks figure out how to shade all that south- facing glass the house will be unlivable in the summer - even in Maine.

Al, keeping the house warm overnight was also a problem. The usual workaround was put massive amounts of rock/concrete in the sunroom for heat storage - that or drums full of water. Movable insulation over the south glass was also used. Works great until it gets cloudy and all that mass sucks up whatever back up heat you’re able to produce.

UC Riverside built a demonstration house that used many tons of rock under the house and a reversible fan, the idea being to store heat from the sun in the winter and blow cool air over the rocks at night during the summer to use for cooling during the day.

Alas, none of it was ever used. The house was 80% underground and remarkably efficient. One of the guys on the project told me that once the temp in the house settled they found that they could pretty much heat it with a toaster. A/C was never needed, even on when outdoor temps topped 110 degrees. The slick, wood burning, semi sealed combustion fireplace (adjacent to a grove of eucalyptus, placed there to provide the wood) was used only once.

The only time they tried it the interior hit 90 in a matter of minutes and was well over 100 degrees by the time they got the fire put out.