Hacker News Reader

Comment by hinkley | Hacker News Reader

Heat batteries were tried in furnaces a couple of decades ago, but they used some sort of high heat capacity bricks and forced the house air through them. As GP asks, the matter of maintenance was forgotten.

Recently I heard someone is trying this again with water heaters. The working fluid has roughly four times the heat capacity of water, so your “water” heater is substantially a fluid heater with either no water tank or a very small one. Incoming water is heated with the fluid, and the fluid is heated on demand. Only the heat exchanger has maintenance concerns.

Driving the whole thing with a heat pump makes the math a little more fiddly, since peak pump efficiency and peak power prices overlap some of the same parts of the day. You’re calculating BTUs per dollar, not watt hours per dollar.

View on HN

It’s already cheap to just use a larger water heater as the thermal battery.

Few households use more than 80 gallons of hot water per day which is still within the normal range of hot water heaters. Just add a timer so it doesn’t kick on until powers cheap and you’ll be good to go. At least assuming your cheap electricity window is long enough to fully heat the water.

This is one of the reasons people overestimate the need for grid energy storage. People time shift electricity to the night because it’s cheap. If it was cheaper 10AM-4PM then significant demand would shift to that window.

> significant demand would shift to that window

Sounds like we would need “smart” water heaters that can query energy prices. That most likely means they would be online sharing all kinds of data most of us probably don’t want shared and would have a small monthly fee. The water heater might even be very inexpensive because of this.

I know it wouldn’t have to be implemented this way, but of course that’s how it would go.

Such things already exist: “ If applicable download the Time of Use schedule of your local electric utility to prioritize heating when the rates are most affordable” https://www.statewaterheaters.com/literature/spec-sheets/res...

Not necessarily smart. Here (France), most water heaters can be controlled with an extra "pilot" wire, or closing a circuit. It has been common practice since maybe the 60s to have multiple mettering rates, and a dry contact (relay) on the electricity meter to switch on/off some loads (mostly electric water heaters, though some electric space heaters can be connected as well).

These days, rates can be more flexible thanks to "smart" meters (communicating via PLC). Just after the russian invasion of Ukraine, there were plans in place to open the dry contacts of customers outside of the scheduled hours in case the electricity grid needed to shed extra load. Thankfully this didn't end up being necessary.

What I'm saying is that you don't really require smart end devices. Put the incentives in place (deep discounts) and the rest will follow. You probably need at least one smart device to control the others if a timer is not enough, and I think the electricity meter is a good fit for that role.

What liquid has 4x the specific heat of water? Water is 4.1 J/g°C

I dont think much else even comes close

so i put on my scientist hat and started to write a reply going through the mechanics of beating water's heat capacity, first pointing out that what you want for these types of systems is likely high volumetric heat capacity rather than gravimetric. then i went on to discuss the density of room temp ionic liquids (RTILs), only to find myself digging into their heat capacity numbers and man, you're right! I forgot how well water stores heat compared to other liquids. RTILs vol. heat capacity has trouble breaking 2 J/cm3-K![1]

[1] Table 4, https://www.researchgate.net/publication/231542196_Heat_Capa...

To beat water's specific heat, you have to cheat and use a phase change material (special wax) that melts near the desired water temp, or a bit higher if you use a cold water mixing value.

Wax

Paraffin is 2.1 Kj/whatever

https://www.researchgate.net/publication/336006577_Comparing...

That’s missing the phase change energy, which is in the zone we’re looking at here.

Not necessarily. If you're trying to heat water to 120º you may not want the paraffin to phase change at all. Because then you have a problem transferring heat from liquid->solid->pipe->liquid.

Phase change seems to work better for trying to maintain an ambient temperature. The input side is always going to be cold, and at some times of year far colder than others.

> some sort of high heat capacity bricks..., the matter of maintenance was forgotten.

Tangent: Startup Andora Energy claims to have solved the maintenance problem with some novel carbon bricks.

Further: Their next gen product plans to use thermophotovoltaics, converting heat into electricity, directly. That'd be spiffy.

Reply on news.ycombinator.com