It's funny how useful water is for power generation.

There's heat storage as discussed here.

Or you can store cold water in a reservoir as a giant battery, pumping it up high when you've got excess power, and letting it back down to generate hydroelectricity from it later.

Or you can boil water to make steam that spins a turbine and use it to convert anything that can heat water (coal, oil, nuclear...) to electricity.

> It's funny how useful water is for power generation.

It's gravity that does the generation. Water is convenient because it's weight per unit of volume is very high. Higher than most things we can get our hands on and it's also exceptionally safe.

Since water isn't perfectly clean the main problem you face is corrosion. Which can take a great system and turn it into a nightmare of buried leaks and sudden problems.

As far as our options go it _is_ really convenient.

And maybe it's obvious, but the largest thing that makes water so useful is that on our planet it's usually liquid but has easy to reach boiling and freezing points. Most notably a boiling point that is easily withstood by most metals and trivially reached by most methods of heating. Chemically something like quartz would work just fine, but its heating and boiling point are way too high to be practical. Occasionally we do reach for molten salts like lithium chloride

Indeed.

In the UK there was a unfortunate trend of ripping out these energy storage devices and replacing hot water tanks with on demand electric hot water heating ( only heat the water you need ). And new builds often have no tanks ( as it saves space in the new tiny homes ).

Very short sighted in my view - a very simple way to store energy and everyone uses hot water directly.

They don't work well with heat pumps. Heat pumps lose efficiency as the differential increases, so if you try to store heat in a tank, you quickly drop capacity and efficiency.

Versus resistance, which is exactly as efficient at 0°C and 1000°C, and why those storage heaters used to make sense.

(And storage is directly proportional to temperature differential above interior ambient)

Every air-to-water heat pump install will have a hot water tank. So I'm not sure why "don't work well" is the term used.

It is true that heat pumps coefficient of performance drops as the output temperature increases. So you need a proportionally larger hot water tank to store the same amount of energy. So it is fair to say there are tradeoffs. But hot water storage is still a necessary part of most heat pump installs - because peak output of heat pumps tends to be below the heat demand of showers.

EDIT: I misread "ripping out these energy storage devices" as https://en.wikipedia.org/wiki/Storage_heater which are basically bricks with nichrome wire.

You're right, of course heat pump water heaters use tanks to smooth out DHW demand, but that same thing isn't feasible for space heating.

Home hot water heating in the UK with heat pumps is about 250-300% efficient (slightly lower than the efficiency of home heating but still much better than resistive).

No one is storing 1000C water at home.

It is true that the temperature deltas affects efficiency. You can use the thermocline to draw from the cooler lower portion of the storage tank to push this further. Or less technically, just a bigger tank, though this has some tradeoffs.

In warmer countries they are set up differently can act as free air conditioning by extracting heat from indoor air at the same time as heating water.

> No one is storing 1000C water at home.

Right, but UK has/had "storage heaters" which were bricks with nichrome wire. They would heat the bricks really hot during cheap electricity times, and use that heat the rest of the day.

EDIT: I misread "ripping out these energy storage devices" as https://en.wikipedia.org/wiki/Storage_heater

Of course heat pumps for DHW should all have a tank for smoothing demand across several hours.

Isn't it better to heat the water up to a point (let's say 40 or 50 degrees) and to heat it the rest of the way with resistance heating?

>Versus resistance, which is exactly as efficient at 0°C and 1000°C

It isn't. The difference is smaller than for a heatpump tho obviously.

1. No, absolutely not. Why would you settle for COP=1 when you can have COP>1?

2. The electrical to heat conversion efficiency is indeed 100% regardless of the temperature of the resistor. And if you're putting out 1000W, then all input losses are also identical. If you put a 1000W light bulb in the middle of your room, or 2 of them but run both at 500W, you'll get EXACTLY the same heat output in your room, but the single bulb is much hotter.

Older heat pumps had max temperature limits and did often have resistance heaters to get that last push above 60C. Modern household heat pumps will reach 75C while staying above 100% efficient and can skip the resistance heater.

This is partly due to a change in the refrigerant used.

> Modern household heat pumps will reach 75C while staying above 100% efficient and can skip the resistance heater.

Is this adequately maintained even as temperatures drop? I was recently considering getting a heatpump in addition to my gas installation but I assume I need to go for more than a bit better than resistance heating during winter for that investment to make sense.

> It isn't. The difference is smaller than for a heatpump tho obviously.

Where does the energy go then?

It mostly leaks and such. Limescale buildup is also a small issue for their efficiency and more so if they run hot. If we reduced it to a simple input output calculation that would never be an issue except for some speed of transfer.

My hot water tank once fell off the wall. On Christmas day. Expensive repair.

Hot water tank was in the basement, which was not insulated. So the mass of hot water contributed very little as a heat reserve for the house.

House was in a northern clime.

Those are great for buffering only a few hours though. That would help avoid the expensive electricity price peaks.

it also reduces peak load - you can heat water up slower with a lower powered heater. I have a 35 liter warm water tank in my garden shed that pulls about 3.5kw - an equivalent on demand heater would need 14kw or more.

I don't see why that matters. You use the same amount of energy and the demand is smoothed out at grid scale (yes I know about tea in ad breaks).

You can get things like cheaper overnight tariffs when the demand is lower - if you have some sort of storage system - like a hot water tank - in effect the electricity company is distributing some of that smoothing function to things like hot water tanks, storage heaters or batteries.

If you have your own solar ( either direct solar water heating, or solar electricity generation ), the hot water tank is a simple, cheap, reliable energy store.

Sure capacity isn't that great - but pretty much every house in the UK used to have one, so it adds up.

Houses in the UK typically have 100A supply and the whole local grid is sized assuming people use relatively small amounts of electricity. If everyone gets an electric car and a massive heat pump, lots of local transmission will need upgrading

Right but unless everyone is drawing large amounts of power at the same time it doesn't matter if you use 1kW for 10 hours or 10kW for 1 hour. To the grid they look the same.

One interesting case where "at the same time" actually does happen is overnight car charging. Some chargers are configured to start charging exactly when a cheaper tariff kicks in, which causes big transient issues for the grid. I think modern chargers have a random delay to help with that.

> Some chargers are configured to start charging exactly when a cheaper tariff kicks in, which causes big transient issues for the grid. I think modern chargers have a random delay to help with that.

Here in the UK some electricity providers offer 'smart' charging (e.g. Octopus Intelligent Go).

In that situation the energy provider controls when to charge the car - e.g. you say "I want the car at 80% by 7am tomorrow" and the energy provider controls the timing of charges.

That's how my EV charges - I plug it in, and Octopus control it.

Benefit for me is that whenever the car is charging my entire home's use gets the overnight rate (even if part of the schedule is charged during the day).

Benefit for Octopus is they can use my car to balance grid demand / schedule the charge when it is most financially effective for them.

I can - at any time - override that logic if I just want it to charge at a specific time for whatever reason.

(I presume this sort of arrangement is becoming more common in other countries too)

Yep but while that is true at the level of the overall grid, actually the nearest part of "the grid" might be a local transformer that only serves 10-100 houses.

At that scale, it's definitely possible that you all plug in your electric cars and turn on your heat pumps more or less at once on a cold evening after work and start cooking your local transformer. Not my day job but I think it is a potential issue when everything is sized assuming ~2kW average demand or something

I think the advantage is that hot water loses heat over time, depending on how good the insulation is. However with phase change materials, the heat is trappped in the phase change and is stable until you release it.