one difference is that a phase change stores energy at constant temperature, which may be desirable given that heat pump efficiency is inversely proportional to temperature output temperature

I was recently wondering if any particular type of food contributes to mouth cancer prevalence and I concluded that at the top of that list would be be lasagna and cherry tomato on a pizza/in a soup.

And yet, if you make it like I do, there always manages to be one piece that is ice cold while the rest is shockingly hot. Even if you're start with warm ingredients.

Physics does not apply to lasagna.

Also I suck at making lasagna.

I went to Moscow in December several years ago and learned they've got one the world's largest district heating systems.

Ask This Old House had an episode literally yesterday where they installed a \ solar-assisted split heat pump water heater. There is a component that goes on the outside of the house but not on the roof which simplifies installation.

https://www.youtube.com/watch?v=uqyAWkXXt3A

https://www.neshw.com/residential/solar-heat-pump-water-heat...

I think a heat pump only for water isn't the right way to go. In the EU, new systems I see use a single heat pump for all heating and cooling in the house including heating water.

I do miss my natural gas on-demand water heater from when I lived in the states though. Unlimited hot water was nice, and it took up almost zero space.

You're about 20 years behind.

My heat pump is working great at 0F. It's 7 years old.

I had a heat pump installed in 2010. In a cold climate. Only used for heating. It paid for itself extremely quickly - less than three years. It's still going strong, in 2026. It's important to maintain it regularly, i.e. deep cleaning every two years or so. The first time I got a company to do it for me, and the technician taught me how to do it all by myself, so that's what I do. In any case having a professional doing it wasn't expensive either. And I clean the dust filters (very easy) every second week or so.

They use them a lot in Norway, it's hardly warm there.

I'm in the northern US and am very happy with mine. I self-installed with a county rebate, so the total cost was a Saturday and $700. My old electric unit was EPA rated for $450/year, and the new one has averaged $170/year over 4 years, so I've already broken even.

> government mandate will force enough demand that manufacturers can enjoy some economies of scale

So you want the government to pick winners and you want to do business with a monopoly? This is the opposite of what you would want.

If the product saves me money, and it's _actually_ better, I will buy it in a heartbeat. If you're involving the government it's because one of those things isn't true.

It also exists, as described in the headline, as a tank of heated water.

The phase change stuff has positives like taking up less physical space but it's also a much less mature tech than storing hot water.

I would guess that is intended for a daily cycle, perhaps using air source heat pumps at times of day when the air temperatures are higher and electricity prices are lower, then using it as required.

As it works on phase change (e.g. think of melting ice) heat is added (or removed) without changing the temperature of the store (which, I guess, might be hotter or colder than where the heat is extracted or used).

1. The device is just storage. It would be paired with an air or ground source heat pump.

2. With good insulation you can easily store heat for a day which is all you need. You're never going to get close to storing summer heat for the winter. That's not impossible but not feasible for something this scale (and not cost effective at any scale).

3. You just heat it up and cool it down. There are no fancy chemical processes happening other than the phase change. It's exactly like a phase change hot/cold pack you can buy on Amazon.

4. I'm pretty sure this is designed for domestic heating...

It's kind of an obvious idea tbh. I don't think they've done anything super innovative... They made an aluminium heat sink..

In this work the authors use a ceramic-coated extruded aluminum heat spreader to improve thermal conductivity through the bulk PCM, but I wonder if the graphite flake+powder additive demonstrated recently by Tech Ingredients[1] would be a viable alternative? It might need a stabilizer (thickener) to prevent the ingredients from separating.

[1] https://www.youtube.com/watch?v=s-41UF02vrU

I think this can work and instead of that the heatpump pumps the heat into your house (when "solar is plenty") it would pump it into the storage. (I have a similar setup, but heat the water but of course this is rather limited)

unrelated: a simple technical solution to your window problem would be home assistant and a few sensors to notify you when the windows are open too long or open when too cold inside.

Heat pump heat is just heat.

This is exactly the kind of thing government is for, even though it's missing the other half: subsidies. At the very least buying heat pumps for the next 5 years should be tax deductible. Even better: a $2000 or similar rebate.

    Energy property - Heat pumps and biomass stoves and boilers

    Heat pumps that meet or exceed the CEE highest efficiency tier, not including any advanced tier, in effect at the beginning of the year when the property is installed, and biomass stoves and boilers with a thermal efficiency rating of at least 75% qualify for a credit up to $2,000 per year. Costs may include labor for installation.

    Qualified property includes new:

    Electric or natural gas heat pumps
    Electric or natural gas heat pump water heaters
    Biomass stoves and boilers

If you're making plans 3 years out in the US, you're a fool.

I don't know about all heat pump systems, but mine at least requires the water tank to have a resistive immersion too. If the tank temperature gets below some threshold the heat pump refuses to work and turns the immersion on instead until it's warmed up enough.

That's a Biden-Harris administration action. What are the chances that Trump deletes it as a 'Democrat/WEF climate hoax con job' as soon as he's made aware of it?

The problem with heat pumps replacing electric heaters (in cold climates) is that the waste cold air gets dumped into the house and needs to be heated again. Generally, electric water heaters are expensive to run compared to gas ones, so people use them in places a gas heater is not possible to install (e.g. no way to vent the exhaust). This also means that the heat pump would have nowhere to vent cold air.

This kind of thing is why I don't like bans like this. The specifics matter a lot.

Is that 2-3x before or after the plumber marks it up?

What an exceptionally moronic thing to ban, the market solves this naturally. Resistance heaters are 100% efficient whatever fraction of the year is heating days. So if that's 1/2 the year and the water heater can't last 16yr because of water quality the heat pump heater will never pay you back.

This reminds me a lot of the time some jerks in west coast desert states convinced the feds to regulate plumbing fixtures so that eastern "we take from the river and put back in the river" municipalities that have more water than they know what to do with have to suffer through low flow everything.

This exists, in german it's called Eisspeicherheizung. You have a few cubic meters of water buried in a concrete bunker and you use a heat pump to pull energy out of the water until it freezes. The system not only uses the thermal mass of the water, but the thawing/freezing energy which is higher than the energy required to heat water by 1degree by a factor of 80 - meaning if you freeze 1kg of water, you need to pull out enough energy to heat one kg of water by 80 degrees.

You can then use a heat pump that's optimized for the expected temperature range and you don't even need to insulate your water storage tank - you actually want the cold in winter to seep out into the surrounding soil, free energy.

In summer you have cold storage for your AC.

I ran the numbers for this a while ago. I live where we have proper winters (currently -22c). I wanted something simple just with solar thermal and water pumps (no heat pump). Sand batteries work at an industrial level, but for domestic use you want something simple so that means just water.

A 100m3 (100,000 litres or 26,500 gallons) cylindrical water tank (approx 5x5m) buried and insulated with 50cm of XPS could provide around 4000kWh of deliverable heat throughout winter. Which would be more than enough for heating and domestic hot water for my house.

In the summer you'd use solar thermal to charge it to 85c. In the winter you'd run water through underfloor heating and discharge it to 35c (so you just need a mixer valve and pump).

The structural engineering part of it isn't actually that complicated (with a garden on top, not a house). You can buy plastic water tanks of that size, it just needs to be buried and have XPS foam placed around it.

Because it's volume, it scales up well. An extra one meter in each direction would increase the volume by around 60%, but you have a lower overall heat loss, so the heat capacity would more than double.

The important part of it is the XPS foam though, without this the loses are too great and you don't retain any heat. This is why insulating your foundation and slab is so effective.

  • The centigrade is capitalized when used after a number. There is also a singular glyph for the entire degree-centigrade convention: ℃.

  • There are also superscript numerical characters to use with volumes, without having to use formatting: m³.

So…geothermal? I wish this was possible too but I don’t see how it will work scientifically. Water is one of the chemicals that have one of the highest thermal mass/specific heat (maybe 1/3 of salt hydrates). Even then, you have to bury a crapton of water underground. This design mentioned in the article is more for short term, like 12 hours storage (since they’re accommodating for solar in nighttime)

I live in a climate where, for most of the year, the daily high-low temperature range includes 20C, so I'd like a whatever sized one is needed to average that out, and run most of the year without any active heating or cooling.

You seem to be describing ground sourced heat pumps. If you wanted, you could insulate a a chunk of foundation or earth to avoid heat loss. But just the ground under your building seems to work well enough.

A heat pump gets more heat from a given amount of electricity than if the electricity is use for resistive heating. So the ideal design is solar cell + sodium battery + heat pump.

Even LFP batteries can work out better.

I live in Switzerland where these are available. A Cowa 58 [0] costs CHF 4692 [1] and stores up to 13.5kWh. If you're heating the water with a heat pump, that's ~6kWh of electricity, so ~CHF 782/kWh.

I'm in the process of installing a 33kWh battery and the battery + inverter cost CHF 13600 in total for just the hardware, so ~CHF 482/kWh.

If you add solar panels, the inverter does double-duty producing AC from both the battery and the panels. The battery does double-duty producing both hot water and allowing you to use solar energy outside the times when the sun is shining.

That said, having ordered a heat pump recently and being in the process of having solar + batteries installed, the amount of electrical work needed for the solar/battery install is substantially higher than was needed for the heat pump and here, the labour costs quite a lot, pushing the upfront cost difference even higher.

I think that's where these heat storage things fit in: they have a much lower upfront cost. No matter how cheap the battery, for it to be useful in a Swiss residence, it needs to output a substantial amount of 3-phase power (3-phase is standard here, even in most apartments), which means you need to spend a couple thousand Francs on an inverter and electrical work. These heat storage devices are quite cheap and don't even need someone qualified to handle refrigerants, I imagine they could be installed by a normal plumber.

That reduced upfront cost makes them far more accessible than electrical batteries, at least for now.

[0]: https://www.cowa-ts.com/uploads/files/Dokumente/Datenblaette...

[1]: https://nettoheizungshop.ch/Cowa-COMPACT-Cell-58

Climates that need a hot tank of water to buffer for heat pumps, will not have meaningful solar panel output during winter. Or do you mean, just load the battery when electricity is cheap? A tank of water is 1k max, probably 10% of a sodium battery.

There’s also solar thermal panels that heat up a liquid circulating in the system and cut out the need for a battery - and can just store the heated liquid.

Stanford's cogen plant has an underground "ice cube" for campus/municipal chilled water infrastructure. Perhaps scaling something like that makes sense or perhaps to use an absorption heat pump (AHP) that can operate like and the reverse of an Einstein–Szilard refrigerator?

IIRC BMW used to have a form of this in their cars about 25-30 years back so that the hvac would be able to blow heat before the engine coolant was up to temp after sitting overnight.

Yes, it's the same tech. There's been products on the market for a while even though this press release tries to spin it like it's new and linked to heat pumps.

It's all about efficiency. You can store heat in anything, but the question is for how long and how much energy can you get back out later. The first part is easy and how we got ovens and stoves, the second part can be pretty tricky depending on your requirements. Large scale energy storage sometimes uses massive amounts of sand for example, but they heat it to hundreds of degrees which is not really feasible in most settings.