This is confusing, why PV to heat the dirt? Black paint will do it from sunlight directly. Or circulating hot liquid. Way cheaper and more efficient than PV at collecting heat from the sun.
You don’t get much long-term storage with your black paint as it’s just going to heat the surface. The point here is to heat up a big enough pile of dirt that you can draw out power months later.
Above that temperature, you need molten salts or liquid metals that are extremely corrosive. Your other option is to use gaseous media instead.
They've made no progress on getting the energy out of their heated dirt. They want to make a hot dirt powered boiler - run pipes through the dirt, put in water, and get steam. Not hot water, superheated steam. That's the hard part, and it will have to be custom.
Small steam turbines are available. Siemens has a whole range.[1] They start around 750KW, at the high end of automotive scale. Siemens can sell you a matching generator. When these guys can power one of those, it's real.
Right now, it's three guys. They need VC funding and a really good boiler engineer. The goal should be a working prototype at about 500KW scale. That would be a big enough prototype to get some meaningful efficiency measurements.
(Maintenance will be tough. You can't turn the heat source off.)
[1] file:///home/john/Downloads/SE-Brochure-Dresser-Rand-Steam-Turbines-2021-pdf_Original_20file.pdf
[1] https://www.siemens-energy.com/global/en/home/products-servi...
High-grade heat can be easily turned into electricity with a turbine, or reused in an industrial process (the entire point of a nuclear reactor is to create heat in this temperature range!) Medium-grade heat can still be used for some processes or used to generate electricity, but the electricity generation will be less efficient. Low-grade heat is under 100C and is a lot harder to use. You cannot economically generate electricity from it, or use it for most industrial processes, so use cases often focus on district heating.
The problem with these low-grade-heat district heating schemes, or more broadly any use of low-grade heat, is the economics. Let's take your idea. The efficiency from sunlight to heat is indeed high (much higher than PV panel -> resistive element) but the heat generated is all low-grade heat.
So what's the root cause here, why is low grade heat usually not economic to use? It comes back to two main causes: 1) efficiency, and 2) storage. Most power is generated from turbines that use heat - a type of heat engine. Carnot efficiency is the maximum theoretical efficiency of a heat engine. Carnot efficiency is η = 1 – Tcold/Thot where temperatures are absolute temperatures (Kelvin/Rankine.) In other words, 7.7% for 50C->25C (298K->323K), 37% for 200C->25C, and 61% for 500C->25C. Note that this is _theoretical_ maximum efficiency; real world efficiency varies quite a bit - from ~1/2 to ~1/10th of Carnot efficiency at peak depending on your heat engine. The second, storage costs, are even more important. You need to insulate your warm object to keep it warm, and if your heat is low-grade, then it is spread out across a huge volume.
Back to your idea, your "paint the dirt black" idea will generate far more heat, but very low-grade and entirely non-economic to use. You will have a somewhat warm pile of dirt, but nobody really wants a somewhat warm pile of dirt. This is the same reason why you see people, logically, tearing down their high-efficiency solar water heaters to install low-efficiency solar panels.
"Use solar panels to resistively heat the dirt", on the other hand, is less efficient and generates far less heat - but it generates high-grade heat. This startup proposes to eventually sell that heat to power plants, to generate electricity directly; and if you're doing that, the temperature of the heat is critical. As you can see from the Carnot efficiency, a power plant couldn't economically do anything with a warm pile of dirt, a solar water heater, or other similar technologies. But they _can_ do something with a source of high-grade heat - namely, they can run the turbines that currently run on fossil fuels. In other words, you can solve the seasonal solar curve problem and have constant electricity production year-round, even in northerly climes.
[0] Above - very roughly - 500C, it is harder to use the waste heat efficiently, because the engineering gets a lot harder, but the theoretical maximum efficiency is higher. That's one reason why there are a lot of efforts to try to build nuclear reactors working at these higher temperatures (see: molten salt reactors.)
For example, Nuc typically don't cross 400C, when Gas/Coal could work on 450C or even more.
Plus, non-Nuc could use some more exotic technologies, like MHD (magnetohydrodynamic) generator, to provide much more efficiency of power generation, or CO2 turbine to make installation much smaller. (Theoretically, somebody could build Nuclear MHD device, but it will not pass safety restrictions).
If you want to do practical district heating, you should look to the Soviets. Unfortunately, most of this information is available in Russian, on physical paper.
[0] http://waterworkshistory.us/DH/1967USSRreport.pdf page 18, figure 2
The steel-man case for Denmark is that trees usually do grow back, and indeed, global _tree_ cover is substantially higher than it was 35 years ago. But global _forest_ cover is still shrinking, and much of the wood entering Denmark comes from forests. Forest cover is more important than tree cover - for the aforementioned reason of albedo changes, but also biodiversity.
Where things really get absurd is when you begin to calculate the area of land you have to turn into a tree farm to keep your homes warm. Or when you calculate the market effects of buying a huge amount of trees - even if all _your_ trees were good ethical trees from a farm, you have just consumed a large amount of the supply and thus financially incentivized cutting down old-growth forests elsewhere - for example look at the history of Ikea’s wood use. If there were more Denmarks, and it was not just a small country with the population of the Chicago area, there would be massive worldwide devastation.
Don’t get me wrong, I am not an absolute opponent of biomass. It is true that burning things is a great source of high-grade heat. It can play a small role in a sustainable energy mix. But it is not the free lunch Danes think it is, and certainly not at the scale they consume it.
Solid state solar panels should be more reliable than any hydronic system.
Though I suppose their heat recovery system is probably hydronic.