8 ms·
Heat storage has an aspect that was counterintuitive to me, but follows from basic geometry. It benefits greatly from large scale, since the ratio of the volum
by retrac 3y ago
Heat storage has an aspect that was counterintuitive to me, but follows from basic geometry. It benefits greatly from large scale, since the ratio of the volume to surface area [1] decreases the larger you make a container. Accordingly, if a heat tank is large enough, the surface area becomes negligible relative to its volume, and it, in effect, becomes well-insulated by its own mass. For really big tanks, like might be used for an entire town to heat itself over a winter, practical self-discharge rates can be just a few % per month, which is better than most rechargeable battery technologies.
[1] https://commons.wikimedia.org/wiki/File:Comparison_of_surface_area_vs_volume_of_shapes.svg https://commons.wikimedia.org/wiki/File:Comparison_of_surfac...
- chii 3y ago> better than most rechargeable battery technologies you can't compare heat storage to electricity, because you can't directly use heat for anything else other than for heating, where as electricity can be used to perform motion. If you used heat storage as a battery, there's an additional loss when converting to electricity. However, if the heat is cheap/free during summer, storing it for winter is a no brainer.
- datameta 3y agoThe sand is gravity-fed into a heat exchanger which transfers the heat to a fluid that drives a combined cycle turbine, in this case. I'm curious what the conversion loss is here. To add to that, what is the energy expenditure of building the battery compared to sand containment plus heat exchanger and turbine - i.e. mining, refining, transport, manufacture, delivery?
- robviren 3y agoGas turbine efficiency targets for combined cycle are targeting 65%, so loss is at least 35%. To me I the loss sucks, but if it can be cheap to build and maintain it matters less. If solar is the energy source and that gets driven cheap enough then that loss could be acceptable.
- NavinF 3y agoNo way you'll get 65%. This sand won't be stored at the same temperature as a natgas generator's combustion chamber. Efficiency depends on delta T. I'd guess 20%
- regularfry 3y agoThey're estimating over 50%, apparently. The sand is stored at over 1000 degrees. Not combustion temperatures, true, but also nothing to sniff at.
- adrian_b 3y agoNope. The report says that the sand will be heated up to 1200 Celsius degrees. This is much higher than the maximum temperature for steam turbines and equal to the temperature of the gas in the best gas turbines. Therefore they will use a combined cycle, first a gas turbine will use hot air passed through the sand and the exhaust from the gas turbine will produce steam for a chain of steam turbines with decreasing working temperatures. There should have been no problem in reaching a 65% efficiency for the conversion from heat to electricity, except that between hot sand and a gas burner there is the same difference as between an electric capacitor and a battery, while heat is extracted from the sand, it cools down. Presumably, when the sand becomes too cold, the gas turbine is bypassed and the hot air just produces steam. When it becomes even colder, I suppose that the first steam turbine is also bypassed and only the low-temperature steam turbines are used. This will lower the average efficiency, probably to around 50%. If the residual heat (after the steam turbines) had been used for heating or for cooling (i.e. heat-powered air conditioning), the efficiency could have been higher, e.g. over 80% in the beginning, while the sand is still very hot.
- credit_guy 3y agoI did not read the report, but one way you can improve the round trip efficiency is to not let the sand cool below, let's say 1000 °C. You pay a one-time penalty of heating the sand to this temperature and then you operate the storage only between 1000 and 1200 °C. This reduces the capacity of the "sand battery", roughly by a factor of 6. But sand is (reasonably) cheap. I have seen another proposal where instead of sand the material would be graphite, which can be heated to more than 2000 °C. Graphite also has a huge heat conductivity. But it's more expensive.
- foreverobama 3y ago[dead]
- Scoundreller 3y agoDoesn’t even need to be seasonal. Plenty of times+places where you only need heating at night and still have some net electrical draw (because the sun isn’t shining at night). Also, cold fronts move in with a lot of wind, but then it can stay cold a few days with calm winds until a warm-front moves in.
- politician 3y agoNit: Rechargeable batteries have a loss when converting the chemical energy stored inside to electricity. There’s no free lunch.
- cogman10 3y agoYes, but for Lithium and sodium batteries you are looking at 90+ efficiency.
- 2OEH8eoCRo0 3y agoIs a battery electricity or is it a chemical reaction?
- westmeal 3y agoyou kinda can with sterling generators I think
- inetknght 3y ago> you can't directly use heat for anything else other than for heating, where as electricity can be used to perform motion. Maybe today. But NASA has some interesting metal tires [0][1] which might change your mind for the future. [0]: NASA info https://technology.nasa.gov/patent/LEW-TOPS-99 https://technology.nasa.gov/patent/LEW-TOPS-99 [1]: Neat youtube vid: https://www.youtube.com/watch?v=vSNtifE0Z2Q https://www.youtube.com/watch?v=vSNtifE0Z2Q
- quickthrowman 3y ago> you can't compare heat storage to electricity, because you can't directly use heat for anything else other than for heating You can use heat to generate steam that can spin turbines to perform work. But, I am not sure if it’s practical to generate steam from a sand battery, my background is in electrical construction. I’m guessing the sand battery isn’t nearly as hot as a natural gas steam boiler’s combustion chamber. https://www.johnsoncontrols.com/en_sg/hvac-equipment/chillers/yst-steam-turbine-centrifugal-chiller https://www.johnsoncontrols.com/en_sg/hvac-equipment/chiller...
- smallmancontrov 3y agoIs absorption refrigeration bad enough to be permanently non-viable here? Or is it "just" a matter of scaling a niche technology?
- bumby 3y agoDefine "bad enough" in more specific terms. I believe it largely depends on the application. If you have a lot of waste heat, it's potentially a way to get "free" refrigeration. (e.g., a paper plant that uses a lot of steam can use absorption chillers to make use of waste heat.) If fuel is much cheaper than electricity, it can be economically viable. Peak shaving can save lots of money. etc. But it's probably not competitive purely in terms of energy efficiency or GHG emissions.
- smallmancontrov 3y agoIf I had exact figures, I wouldn't need to ask for anyone's intuition. Thankfully, ChatGPT was willing to work with me, and I will summarize the results here. Typical refrigeration COP (Coefficients of Performance): Absorption refrigerator: 0.6-1.2 Compressor refrigerator: 1.5-4.0 Estimated TES economic advantage: 1.1-2.5x Conclusion: yes, absorption refrigeration is probably inefficient enough to make it a long shot in this application. The only way I can see it becoming viable is if extremely hot TES can completely change the efficiency game, and then only just.
- deleted 3y ago[deleted]
- bumby 3y agoThe temperature in TFA are outside the range of most commercial absorption chillers, so this is more about making cheap electricity. I would imagine absorption would be more applicable if the same tech was used to generate lower-quality heat that's not suitable for a combined-cycle generator.
- smallmancontrov 3y ago
- stcredzero 3y agobecause you can't directly use heat for anything else other than for heating There's Stirling Engines. If the solar collection is pure thermal, and if that collection and the storage can be made dirt cheap, then the 37% or so efficiency of conversion to electricity stops being a problem. But what are currently problems with Stirling Engines -- Hardly any of the industrial optimization has been applied to them Re: Wright's Law. So they are quite costly! Heat pipe solar thermal could be made dirt cheap through economies of scale, and it works very well, even in climates like England's. I could envision house construction changing to include sub-basements which are just polystyrene insulated boxes filled with sand. By over-provisioning storage by 4X, houses in cold climates could have huge electrical power stores, especially in summer. (Especially if the house uses heat exchangers which can draw directly from the thermal store.)
- usrusr 3y agoBut that doesn't really matter before all heating demand is served from storage. Sure, heat will never be the be all end all of energy storage, but there's a lot of demand in places that have winter. On top of this, when the conversion to heat is done by heat pumps, you not only get the benefit of a warmer baseline during the conversion, you also get some free cooling while charging.
- adrian_b 3y agoHeat can be used directly not only for heating, but also for cooling. There are air conditioning systems that are powered by heat, not by electricity. There are places where the power plants use the residual heat from the generation of electricity not only for heating during the winter, but also for cooling during the summer, by producing chilled water.
- autoexecbat 3y ago> There are air conditioning systems that are powered by heat, not by electricity. How does that work?
- quickthrowman 3y agoEither absorption refrigeration or a steam turbine that compresses the working fluid.
- xkcd-sucks 3y agohttps://en.wikipedia.org/wiki/Absorption_refrigerator https://en.wikipedia.org/wiki/Absorption_refrigerator
- jaggederest 3y agoHalf of all primary energy usage is for heating purposes. Heat is the best format to displace, as well, since a significant chunk of all that primary energy use is direct fossil fuel combustion right now. Especially since you can use heat to drive a heat pump, which ends up actually achieving above unity efficiency effectively since the heat input is used to move additional heat from the environment. Even if it's inefficient, if the heat would otherwise have been wasted, it's a net benefit.
- Tagbert 3y agoThe Pernell with heat compared to electricity is portability. It is easy and efficient to transmit electricity but heat is not
- specialist 3y agoThermovoltiacs, converting heat dirctly to electricity. IIRC, At least one of the "box of rocks" thermal battery startups (interviewed on the Volts podcast) intends to have both a two way heat exchanger and electricity play.
- stingraycharles 3y agoGravity seems much more natural to me than heat; eg pumping water up a dam and releasing it again when you need the energy. It strikes me that heat has the problem that it always loses energy in its “stable state” because the surrounding environment absorbs the heat, and gravity doesn’t have this problem.
- scotty79 3y agoWith water you lose some due to evaporation.
- sgc 3y agoThat was my first thought too. But lakes lose ~20% / year to evaporation, and with the use of shade balls that is cut by ~90%, so we are at 2% / year - which is about the same as very efficient daily loss from heat storage.
- pfdietz 3y agoIf the heat is being turned back to electricity, a heat sink is needed and if this is done by evaporation the water loss will greatly exceed that of natural evaporation from a PHES facility.
- sgc 3y agoIt's not the evaporation per se that matters in the pumped hydro, it is the evaporation loss of water you already invested in pumping, so it is just an efficiency loss. The vapor loss for the heat sink is indirect, you usually just calculate the turbine efficiency. I don't know if the (generally much smaller) pumping requirements for turbines is already included in their efficiency calculations, but it would need to be of course. Either way, it just goes to show further that pumped hydro would be more efficient, when and where it is feasible.
- usrusr 3y agoWith a mineshaft gravity storage you could actually try to spin up a convective loop powered by geothermal that dries the sand over time, allowing you to lift dry sand when charging and abseil heavier wet sand when discharging.
- bumby 3y agoAn extrapolation directly to heat transfer is the Biot number https://en.m.wikipedia.org/wiki/Biot_number#:~:text=The%20Biot%20number%20(Bi)%20is,the%20surface%20of%20the%20body https://en.m.wikipedia.org/wiki/Biot_number#:~:text=The%20Bi....
- today20201014 3y ago> the ratio of the volume to surface area decreases the larger you make a container Did you mean to write the reverse? i.e the ratio of the surface area to volume decreases the larger you make a container.
- hardlianotion 3y agoYes.
- deleted 3y ago[deleted]
- adverbly 3y agoI also think this is a very interesting approach to take. This might be a pipe dream but it would be really cool if we could just turn a big chunk of desert into a battery by plumbing some heat exchangers through it. Or maybe just putting a gigantic Fresnel lens in the desert and pointing it at the ground. I almost wish someone would try this just to see what would happen.
- kaliszad 3y agoWhen you can store electricity more directly instead of the (inefficient) conversion to and from heat it is just crazy to not do so. With electricity, thanks to heat pumps you can "move" heat instead of producing it, which makes heating a lot less expensive since the "efficiency" can often reach 300-400%. For that reason you want to keep using electricity for as long as possible in most cases. Of course, if you have a huge source of cheap heat the equation might change. In such cases you might still want to use some of the heat to get higher temperature heat. (https://en.wikipedia.org/wiki/Absorption_heat_pump https://en.wikipedia.org/wiki/Absorption_heat_pump) And no, it will not help too much if you use a heat pump before storing the energy in this facility. You still have huge conversion losses, the volume for low-temperature heat storage will be huge. If you want high temperature storage your heat pumps will need more stages and run less efficient.