3 ms·

Heat can't be converted losslessly to useful energy, but converting to heat to other forms of energy is extremely common. What do you think a steam turbine, boi

Heat can't be converted losslessly to useful energy, but converting to heat to other forms of energy is extremely common. What do you think a steam turbine, boiler, internal combustion engine, etc. do?

raphinou 2mo ago
But all the examples you give have a heat source of high temperature and dense energy, and a lot of energy is lost in it's transformation, as illustrated by the combustion engine. How can heat be a good energy storage if so much is lost when you want to use it?
- SoftTalker 2mo ago
  If it's free to collect (solar) then efficiency matters a lot less.
  - bluGill 2mo ago
    It isn't just the cost to collect, it is also the cost to store. Heat is easy and safe to store at just above room temperature in rocks - but you need a lot of rocks. Just to heat my house for a few days would need tons of rock at temperatures I'd allow in my house (I have kids and so I worry they will open the rock container and get burnt). Rocks are not free. You can also use water for storage, but again you have the issue needing a tank (and water is a drowning hazard, plus the worry about leaks...)
    - markvdb 2mo ago
      Square cube law will optimise heat storage. These Finnish sand based heat batteries are a prime small-scale example.
    - danans 2mo ago
      > Just to heat my house for a few days would need tons of rock at temperatures I'd allow in my house (I have kids and so I worry they will open the rock container and get burnt). Rocks are not free Sand/rock heat storage is for industrial and utility scale application, not your house. > You can also use water for storage, but again you have the issue needing a tank (and water is a drowning hazard, plus the worry about leaks...) Tank heat pump water heaters are available and can already do load shifting on a daily basis (using off peak cheaper and cleaner electricity). I have one and it works great, and I guarantee it isn't a drowning hazard. There are versions of these systems that can store enough heat to heat a house, not just provide hot water at the tap.
      - bluGill 2mo ago
        > Sand/rock heat storage is for industrial and utility scale application, not your house. I've seen them for a house as well. (when the utility offers a large discount for power used overnight - 40 years ago my local coop was trying to sell my parents on this, but since we had natural gas heat it wasn't worth it) To hold more than a days worth of heat you need a lot of rocks. Heap pump water heaters work, but you need a large one to hold a days worth of hot water (40 years ago my family had 200 gallons of hot water storage - the utility gave my parents a discount on water heating because we only heated water at night, we ran out of hot water once that I remember). If you want to store more water than a days worth of use then you need more storage. This can become a drowning hazard if we are talking about a couple weeks worth.
        danans 2mo ago
        > I've seen them for a house as well. (when the utility offers a large discount for power used overnight - 40 years ago my local coop was trying to sell my parents on this, but since we had natural gas heat it wasn't worth it) To hold more than a days worth of heat you need a lot of rocks. Can you point at a residential scale rock heat storage system that is available? I saw one startup in the UK trying this several years ago, but can't find them now, and haven't seen anything since. > Heap pump water heaters work, but you need a large one to hold a days worth of hot water Mine (for a household of 4) is 45 gallons - small by US standards - but we keep it at a high temperature (along with a mixing valve) to get higher "virtual" hot water capacity during peak usage hours. It serves us fine for daily usage, but we also have efficient flow taps and showers throughout the house so that helps too. > If you want to store more water than a days worth of use then you need more storage I don't think it makes sense to store hot water for days if the goal is to minimize energy costs. The standby losses would be pretty significant, even with a very well insulated tank. If the goal is resilience to a grid outage, it makes more sense to have a solar PV array + batteries that you can use to run the heat pump water heater, or have a backup gas-powered heater.

raphinou 2mo ago

But all the examples you give have a heat source of high temperature and dense energy, and a lot of energy is lost in it's transformation, as illustrated by the combustion engine. How can heat be a good energy storage if so much is lost when you want to use it?

SoftTalker 2mo ago
If it's free to collect (solar) then efficiency matters a lot less.
- bluGill 2mo ago
  It isn't just the cost to collect, it is also the cost to store. Heat is easy and safe to store at just above room temperature in rocks - but you need a lot of rocks. Just to heat my house for a few days would need tons of rock at temperatures I'd allow in my house (I have kids and so I worry they will open the rock container and get burnt). Rocks are not free. You can also use water for storage, but again you have the issue needing a tank (and water is a drowning hazard, plus the worry about leaks...)
  - markvdb 2mo ago
    Square cube law will optimise heat storage. These Finnish sand based heat batteries are a prime small-scale example.
  - danans 2mo ago
    > Just to heat my house for a few days would need tons of rock at temperatures I'd allow in my house (I have kids and so I worry they will open the rock container and get burnt). Rocks are not free Sand/rock heat storage is for industrial and utility scale application, not your house. > You can also use water for storage, but again you have the issue needing a tank (and water is a drowning hazard, plus the worry about leaks...) Tank heat pump water heaters are available and can already do load shifting on a daily basis (using off peak cheaper and cleaner electricity). I have one and it works great, and I guarantee it isn't a drowning hazard. There are versions of these systems that can store enough heat to heat a house, not just provide hot water at the tap.
    - bluGill 2mo ago
      > Sand/rock heat storage is for industrial and utility scale application, not your house. I've seen them for a house as well. (when the utility offers a large discount for power used overnight - 40 years ago my local coop was trying to sell my parents on this, but since we had natural gas heat it wasn't worth it) To hold more than a days worth of heat you need a lot of rocks. Heap pump water heaters work, but you need a large one to hold a days worth of hot water (40 years ago my family had 200 gallons of hot water storage - the utility gave my parents a discount on water heating because we only heated water at night, we ran out of hot water once that I remember). If you want to store more water than a days worth of use then you need more storage. This can become a drowning hazard if we are talking about a couple weeks worth.
      - danans 2mo ago
        > I've seen them for a house as well. (when the utility offers a large discount for power used overnight - 40 years ago my local coop was trying to sell my parents on this, but since we had natural gas heat it wasn't worth it) To hold more than a days worth of heat you need a lot of rocks. Can you point at a residential scale rock heat storage system that is available? I saw one startup in the UK trying this several years ago, but can't find them now, and haven't seen anything since. > Heap pump water heaters work, but you need a large one to hold a days worth of hot water Mine (for a household of 4) is 45 gallons - small by US standards - but we keep it at a high temperature (along with a mixing valve) to get higher "virtual" hot water capacity during peak usage hours. It serves us fine for daily usage, but we also have efficient flow taps and showers throughout the house so that helps too. > If you want to store more water than a days worth of use then you need more storage I don't think it makes sense to store hot water for days if the goal is to minimize energy costs. The standby losses would be pretty significant, even with a very well insulated tank. If the goal is resilience to a grid outage, it makes more sense to have a solar PV array + batteries that you can use to run the heat pump water heater, or have a backup gas-powered heater.