General > General Technical Chat
This doesn't make sense - NYT article about a solar installation in the US
Siwastaja:
NiHaoMike's ice storage has the problem of mechanical complexity, including how the ice is separated from the evaporator coil, then transferred into insulated storage, and so on.
But if you can accept lower storage energy density, forget about utilizing the latent heat and only use sensible heat, i.e., go for liquid water. It has a long and very successful track record of being used for thermal energy storage, and energy can be trivially transferred with water pumps. It's almost too easy.
But yeah, for cooling (or for high-COP heating), the dT can't be much (say 10K) which is an energy density sacrifice. Gravimetric energy density is close to that of lead acid batteries (some 20Wh/kg actual), and volumetric even worse. But whenever you have excess space in your basement, i.e., don't live in some big city with expensive land cost, why the heck not.
I do exactly that, store heating energy in water at quite low dT, but clearly my 1200liter tank is too small for what I really want to do. What's actually needed is in order of 5m^3. This translates into approx. 2-3m^2 of hopefully not too valuable area in your basement.
NiHaoMike:
--- Quote from: Siwastaja on October 31, 2021, 12:04:03 pm ---NiHaoMike's ice storage has the problem of mechanical complexity, including how the ice is separated from the evaporator coil, then transferred into insulated storage, and so on.
--- End quote ---
Just put a loop of copper tubing in the tank, problem solved.
Marco:
--- Quote from: Siwastaja on October 31, 2021, 12:04:03 pm ---NiHaoMike's ice storage has the problem of mechanical complexity, including how the ice is separated from the evaporator coil, then transferred into insulated storage, and so on.
--- End quote ---
For hydronic not all that much. Instead of directly pumping through your storage tank, the heat pump has its own propylene glycol loop with two heat exchangers in the tank, one for each loop. Need 1 extra pump.
Just make the surface area on the heat exchanger large enough and finely enough distributed you can freeze the water in place. Dealing with the expansion will take a bit of engineering.
Siwastaja:
Freezing water is well known to burst pipes so I guess it would be non-trivial dealing with that.
The glycol solution inside the pipe won't freeze, but the water in tank would need to freeze; result is the same. I'm suspicious how to handle that force. Strong thick walled pipes could do it. Then the tank walls must cope with the force as well. OK, make it out of 10mm thick steel and weld reinforcements.
But no matter how strong you make everything, the expanding ice has to go somewhere. I guess when the structures are strong enough, then the ice just crushes itself. OK, let's call that sorted out.
But then again ice has worse thermal conductivity than liquid water (seems to be about one quarter) and even more importantly, within ice, natural or forced convection becomes impossible. So the ice first forms on the tubing, but as the layer thickness increases, power transfer worsens, or the transfer liquid inside pipes must be made even colder to compensate, worsening the COP.
This necessitates that the whole storage tank must be full of that strong pipework, because ice can be only generated within short distance (maybe a centimeter / half an inch?) of the pipe with good efficiency or rate (power). You can see this effect in action by noticing how about an inch of ice that has formed on the evaporator of a refrigerator already decreases the efficiency of the unit to the point it's constantly operating and still temperature inside starts to rise.
You can't use traditional expansion tanks either.
I just can't see how this is easy or trivial. I'm not saying it's impossible, though. Great engineering challenge.
And the prize is high if you can do it, at 334 kJ/kg ~= 0.1kWh/kg = 90 kWh/1000kg it would allow quite nice energy density compared to just using sensible heat; with cooling you would be limited to just dT=10degC which means 42kJ/kg = 12kWh/1000kg.
And to give you an idea how much force freezing water can generate, that's how exposed bedrock erodes in cold climates. Water seeps in tiny gaps and this is strong enough to crack the rock.
rstofer:
--- Quote from: Someone on October 31, 2021, 11:32:33 am ---
--- Quote from: station240 on October 31, 2021, 04:30:13 am ---As most of these problems exist in other parts of the world, there are solutions:
1) Demand management, digitally broadcast to homes when there is an over/under supply of power in their area. Whatever automatic system is in place can offer to increase/reduce by X kW, requires say fiber optic internet for low latency.
--- End quote ---
I believe that one came up on the forum before, but will repost the original here:
https://arstechnica.com/tech-policy/2021/06/texans-regret-opting-into-power-plan-that-remotely-raises-thermostat-temps/
Consumer received (likely upfront one-off) payment to participate in minor load shedding, then goes mental when thermostat is raised to 78° F (26° C) from 74° F (24° C).
You'd be amazed how demanding of absolute/selfish convenience the general public are. "why should I slightly modify my behaviour if its for the benefit of others" while completely ignoring the non-linear cost/inconvenience of complete failure/blackout.
--- End quote ---
Our utility has a scheme for remotely shutting down HVAC units for brief periods when system demand is high. During these brief high demand periods, HVAC compressors are locked out for up to 15 minutes every half hour. The program is voluntary and has some rewards.
https://www.pge.com/en_US/residential/save-energy-money/savings-solutions-and-rebates/smart-ac/program-faq/smartac-program-faq.page
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