7 ms·

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

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.

buckle8017 1y ago
There's very few materials cheaper than dirt. Solid state solar panels should be more reliable than any hydronic system. Though I suppose their heat recovery system is probably hydronic.
hackernudes 1y ago
My understanding is this is for excess PV capacity. But I think they should address this! Their plan is to heat deep inside a big pile of dirt using resistive heating elements and then pump the heat out in pipes. Maybe it's hard or expensive to add solar heating into the system? But why couldn't they pump it in? Maybe they don't have extra space because the dirt mound is covered in PV panels.
theptip 1y ago
Water will be limited to 100C, and tanks are going to be way more expensive to maintain. 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.
- fabbbbb 1y ago
  Liquid doesn’t need to mean water? Traditional solar thermal systems use special liquids that can get way more hot than 100C.
  - cyberax 1y ago
    You can get to around 200C with oil-based media. But oil goes bad pretty quickly, and it leaches tons of crap out of everything. Above that temperature, you need molten salts or liquid metals that are extremely corrosive. Your other option is to use gaseous media instead.
    - jacquesm 1y ago
      And those have the problem of not having enough density so they'll be pretty inefficient.
  - theptip 1y ago
    Yeah good point. I think the point about cost is the real crux here, regardless of working temp. (CSP has used molten salt but it is quite expensive to store and maintain.)
owenversteeg 1y ago
The sibling comment by theptip explains this well for this specific case, but the other sibling comment still seems confused, so I will explain more broadly: productive uses of heat are all about the temperature. The boundaries vary, but "high-grade" heat is roughly 300-500C, medium-grade is 100-300C, and low-grade <100C. Up to a certain point [0], heat is easier to use the hotter it is. 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.)
Animats 1y ago
Because they're trying to go way up on temperature. The idea is to get dirt up to 600°C. It's easy to get warm water from solar, and solar water heaters have been around for decades, if not centuries. But that's well below 100°C. If you start from electricity, you can reach any temperature up to plasma. If you're just storing warm water, you can't get much of the energy back. E = (T2-T1)/T2, remember. 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
- Animats 1y ago
  Correct link: [1] [1] https://www.siemens-energy.com/global/en/home/products-services/product/D-r-steam-turbines.html https://www.siemens-energy.com/global/en/home/products-servi...

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.

buckle8017 1y ago

There's very few materials cheaper than dirt. Solid state solar panels should be more reliable than any hydronic system. Though I suppose their heat recovery system is probably hydronic.

hackernudes 1y ago

My understanding is this is for excess PV capacity. But I think they should address this! Their plan is to heat deep inside a big pile of dirt using resistive heating elements and then pump the heat out in pipes. Maybe it's hard or expensive to add solar heating into the system? But why couldn't they pump it in? Maybe they don't have extra space because the dirt mound is covered in PV panels.

theptip 1y ago

Water will be limited to 100C, and tanks are going to be way more expensive to maintain. 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.

fabbbbb 1y ago
Liquid doesn’t need to mean water? Traditional solar thermal systems use special liquids that can get way more hot than 100C.
- cyberax 1y ago
  You can get to around 200C with oil-based media. But oil goes bad pretty quickly, and it leaches tons of crap out of everything. Above that temperature, you need molten salts or liquid metals that are extremely corrosive. Your other option is to use gaseous media instead.
  - jacquesm 1y ago
    And those have the problem of not having enough density so they'll be pretty inefficient.
- theptip 1y ago
  Yeah good point. I think the point about cost is the real crux here, regardless of working temp. (CSP has used molten salt but it is quite expensive to store and maintain.)

owenversteeg 1y ago

The sibling comment by theptip explains this well for this specific case, but the other sibling comment still seems confused, so I will explain more broadly: productive uses of heat are all about the temperature. The boundaries vary, but "high-grade" heat is roughly 300-500C, medium-grade is 100-300C, and low-grade <100C. Up to a certain point [0], heat is easier to use the hotter it is. 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.)

Animats 1y ago

Because they're trying to go way up on temperature. The idea is to get dirt up to 600°C. It's easy to get warm water from solar, and solar water heaters have been around for decades, if not centuries. But that's well below 100°C. If you start from electricity, you can reach any temperature up to plasma. If you're just storing warm water, you can't get much of the energy back. E = (T2-T1)/T2, remember. 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

Animats 1y ago
Correct link: [1] [1] https://www.siemens-energy.com/global/en/home/products-services/product/D-r-steam-turbines.html https://www.siemens-energy.com/global/en/home/products-servi...