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How Water Heaters Have Evolved into Grid-Scale Energy-Storage Devices (2014) (esource.com)
123 points by camtarn 89 days ago | hide | past | web | favorite | 144 comments

This is not a battery, or a battery killer, and the author surely knows better. This is a distributed load bank, located in places the heat can be put to good use.

Interesting idea though. The paper seems like it’s pushing a product instead of pushing concepts, because the applicability of this concept goes much broader than the article goes into, for greater benefit to the consumer. A heat pump (a compressor and some fans) can be switched on or off just as fast as a heating element, at least as far as the article is concerned. The water heater switch is a dead simple solution though, credit due for that.

A bit of a rabbit trail here: A heat pump would not provide as much potential load, but it gives the flexibility of working for both heating and cooling, and also puts the electricity to better use. The key would be to put a cheap heat storage element in between the customer-premises heat pump and the final use, so that the utility can fire up the heat pump regardless of the customer’s need, and the customer has a huge bank of hot or chilled water at their disposal. Combined with a radiant floor heating system, and tied into a water-source HVAC system, this is a real energy efficiency boon by taking waste energy and getting it past the meter into the customer premises. Go ahead and throw a Steffes(tm) resistance heater in the water heater while we are at it. Have your cake and eat it too.

My father works in the heating world. He installed a large efficient storage water heater in the house which would basically warm the water in one go at 5am and provide it to the house all day.

What I don't understand from the article is why this is being controlled by the grid? The US is a market system, can't I buy a smart immersion heater which can then react to real time pricing information? So I warm my water up at times when the grid are paying people to use electricity?

That places the incentives correctly:

a) The entity making the investment in a new heater also gets the financial benefits thereof.

b) What's to stop your electricity supplier making your water 5% hotter every year to boost their revenues?

The grid has topological constraints, because links in the network have finite capacity. Encoding that in a "price" would require pricing to vary not just based on total grid usage but also on the location of each load on the grid.

Tell that to grid controlled devices. You just encode a signal onto the carrier wave when you want to turn things off and on.

> can't I buy a smart immersion heater which can then react to real time pricing information?

This is exactly how Nest makes money. About 40$ per thermostat per year from the utilities.

Wow - I had no idea thats who their real customer was. Brilliant.

I’m confused, the utility companies pay Nest?

I assume thats what was meant - but these two articles are interesting:

1) getting into big data to help utils

2) shutting down (as a google does) a data service for users ---




Disclaimer: I do not know if this means they are/was/will-be providing data to utils/ users or not...

"What I don't understand from the article is why this is being controlled by the grid? "

Hypothetically if you bought your own you would need a smart meter so you're electricity company knew exactly when you had used power, there also needs to be a system to communicate to your smart appliance, to tell it to turn on. Neither of these really exist so I suppose its sensible for the utility to take ownership to iron out kinks, feel out how in works in practise and stuff.

To clarify, smart meters do exist, I don't believe they have the accuracy of metering needed for this application though.

Electric vehicle chargers that can be controlled by the utility company exists in Norway. So in the event of electricity scarcity they can turn it off. I assume it's controlled by ripple or something, but the details are unknown to me.

The modern smart meters are quite accurate, much more accurate than the mechanical ones, usually to the ratepayer’s chagrin. The modern meters will pick up an induction spike from a noisy motor such as in an inefficient refrigerator or fan, where the older generation would not. How accurate does it need to be?

Its the time accuracy that matters, if its for frequency stabilisation then you need granularity in the order of seconds, rather than minutes as I believe smart meters are at the moment.

Oh no, they're way down in the milliseconds.

In most electric markets in the US, you can’t gain access as a consumer to electricity spot pricing. Consumers and regulators value stability and predictability in power pricing.

Additionally, the system is a lot more stable if a central utility can control these water heaters as a bank of shapeable demand, rather than having each individual device making independent decisions. It’s more efficient for them to give you a periodic rebate for participating in the program and have them manage the details.

They are the only ones that can know when it's best to turn it off an on.

The utility can signal this by varying the price from minute to minute. The heater can retrieve this signal from the internet.

Most utilities won't allow data like that over the internet.

If the electricity is down, the. Internet will be down, and without this feedback mechanism working they wouldn't be able to keep the grid stable.

No, but a growing number of utilities can inform customers that it is a 'peak usage' time and customer devices can adapt accordingly.

Nest (and I assume other internet-connected) thermostats are able to accept peak-usage notifications from the power company and dynamically reduce usage. In my case (Portland Electric), it's two-way communications, if Nest can prove to the power company that I let its device reduce usage during peak-load times, I get a credit at the end of the year. The Nest in my house has almost paid for itself.

I don't know, have you seen a price graph? Part of the point of financial markets is to predict the future. Since this isn't possible to do accurately, the result is a random walk of meaninglessly precise prices when really the best prediction you can do is a wide range of uncertainty.

The point of having smart water heaters is to stabilize the market, not make it more chaotic in pursuit of more efficient trades. So instead of going with something complicated like a financial market, a simple feedback system controlled by the power company will do.

Electricity prices fluctuate on a daily basis to an extremely predictable degree.

They do now. Add a bunch of noise traders, and maybe not.

> A heat pump (a compressor and some fans) can be switched on or off just as fast as a heating element,

This is not true, and the article specifically addresses this. A heating element can be switched on and off roughly five orders of magnitude faster (100 000× faster) than a heat pump.

This is not really an issue. Usually you have 4-5 seconds to react for the "hot reserve". Also for the known peaks you can preventively switch off the loads (it is amazing what happens at 7:02am when everybody switches on their kettle at the same time, even worse with water supply and flushing peaks).

In addition such systems sometimes measure frequency deviation and switch off/on the load when it goes out of bounds.

The result is that this works really well for fast balancing as soon as you have few hundred installed spots in one grid. And there is real money in it with ROI 1..3 years for the service provider.

Disclaimer: my electronics engineering company has worked with research, design and engineering of such load balancing hardware

You clipped out the qualifying phrase, “as far as the article is concerned”, dumping a few extra MW of electricity for 30 mins or a few hours. I don’t see a need presented in the article to rapidly switch a heating element on and off a few 1000 times a second.

Grid frequency stabilisation would require that.

I'm not actually sure that that would be the major problem. A heat pump is basically a motor, which can cause a lot of noise in electrical circuits. I'm not sure you would want 1000s of electric motors turning on in unison. I wouldn't like to say whether that is a solved/solvable problem though.

> Grid frequency stabilisation would require that.

The grid has a lot of inertia and a motor can go from full load to none, or from no load to extra-high startup load, in a fraction of a second.

So we're back to "just as fast" for this use case.

"The grid has a lot of inertia"

Where does the inertia come from? I understood inertia in the grid to come from turbines and motors spinning up and down, which seems to be different from what you're suggesting?

Capacitance in the wires?

The inertia largely comes from all the turbines and synchronous motors that stay connected.

With something like a heat pump compressor, you're connecting or disconnecting it completely with the flip of a switch. It's also an asynchronous motor and unable to feed power back into the grid at all, it's a pure load.

Ok I don't understand how that kind of inertia would help in this situation. If a turbine is running, and supply and demand are at equilibrium, if you then added another load, that inertia wouldn't help maintain equilibrium, it would slow down spinning up the turbine hampering that goal, wouldn't it?

To put it another way. Say if energy use jumps X%, inertia may make up Y% difference in the short term, you'd still have to put in more than X% in the slightly less short term, and it would take longer to make up the Y% shortfall. Is the Y% shortfall not big enough to be important here.

> Ok I don't understand how that kind of inertia would help in this situation. If a turbine is running, and supply and demand are at equilibrium, if you then added another load, that inertia wouldn't help maintain equilibrium, it would slow down spinning up the turbine hampering that goal, wouldn't it?

If you add more load, you start dragging down the frequency. The more inertia you have in the turbines, the less the frequency is impacted. I'm not sure what you mean by "spinning up a turbine" in this situation. Do you mean to produce more power? You don't spin turbines faster to do that.

> To put it another way. Say if energy use jumps X%, inertia may make up Y% difference in the short term, you'd still have to put in more than X% in the slightly less short term, and it would take longer to make up the Y% shortfall. Is the Y% shortfall not big enough to be important here.

Yes, you have to make up the deficit. But the alternative means having a brownout. In a choice between "drain some inertia to cover the increased load, then overproduce by .1% to refill the inertia" and "don't cover the increased load, voltage freefalls until load drops", I prefer the former.

Ok think I've got it, I thought the grid was more sensitive then it actually is.

Reading around, as the amount of renewables (solar) increases this might become an issue due to less turbines being in use.

"spinning up a turbine"

As in spinning up to speed. It can't go 0 to X000 rpm instantaneously. Inertia!

Didn't realise they were designed to spin at a constant speed though, thanks.

When you bring a new turbine online you get it up to speed before connecting it, and you can do this long before it's actually needed. So inertia hurts your response rate slightly but the main factor is how fast you can get your boiler or water pipe ramped up. That's why you see gas turbines as good peaker plants, they have very little to ramp up.

Solar is trickier but it's also more flexible in how it injects power. The real issue is not lack of inertia but the fact that it can lose production when a cloud comes by.

I don't think frequency stabilization would require that. Unless there is some complex approach I simply can't imagine.

You don't need it to switch on and off 1000s of times a second (is that what you're referring to?) you need it to switch fast enough that it could do that though. If it takes too long you would get brown outs and surges.

Maximum switching resolution is about 10ms in Europe (50Hz) and 8ms in the states (60Hz). As you usually switch at the zero crossing. In real life it is usually two-three orders of magnitude longer due to calculation and communicaton overhead.

I'm not an expert, but I'd think letting external factors control the switching on/off of your heat pump would have two downsides: (1) efficiency, and (2) wear and tear.

If you're talking about a heat pump used in air conditioning, at least, longer cycle times result in better dehumidification and higher efficiency. If you interrupt the cycle, you're effectively short cycling.

There are a couple downsides here. When sizing a climate control system of any sort, it’s nice to size it such that, at full output, it will be just barely sufficient for the worst case usage. If you use it for demand response and time-of-use optimization, you either need to size for a worst-case day such that it’s still powerful enough when it’s switched off at expensive times or you need to store heat or cold for multiple days. Second, while a simple-ish hydronic system is great for heat, it’s less ideal for cooling. You need to make sure that a cooling system can remove sufficient water vapor to keep a comfortable humidity or you need a separate dehumidifier. (Ideally every new building in a humid climate would have independent dehumidification anyway, so this is not a show stopper.). It’s also important to avoid condensation in bad places — if you cool your floor or ceiling much below the dew point, you have a problem.

I was trying to work out their attitude to heat pumps too. Its almost "they're too efficient, lets use something that uses more electricity" ?!

I don't share your concerns with calling it a battery though, it stores energy. Combined with a peltier it could generate electricity with that energy.

If it eases marketing, and gets this into peoples homes, I'm all for it.

I did the math on this once. For residential it is not realistic due to the amount of water you would need to store. For commercial I think you would need a small lake.

Correct, if you are trying to utilize it for a sole method of heat and cool. You will still need to dump excess / add additional through more conventional means, depending on local climate.

To your underlying point, none of the added complexity is worth the effort unless a) the utility is paying you to take the excess load and/or b) someone really innovates on a slick, simple package.

My uncle has implemented version 0.1 of this in his weekend house. Power is cheaper during the night, so he has installed a clock timer that only powers the water heater during the night. It holds enough hot water for two persons to take a shower, and to wash hands during the day. (There is a separate water heater in the kitchen).

Works great. If one should ever run out of hot water, it's simple to remove the clock timer... but we didn't need that when we spent a week there.

With a well-insulated water heater, the idea to use them as energy storage sounds like a great idea.

I have version 0.2 in my London apartment - the bottom element of the water heater is connected to a different electric supply, with a separate meter, with energy that's almost 3 times cheaper than the rest of the wiring energy. The catch? The circuit is powered only when the power company feels like it (i.e. They need to dump excessive power somewhere), but they guarantee 2h power on by day and 6h by night.

Since this is the bottom element, and the top element is controlled by a thermostat (guaranteeing hot water on demand), this is a great setup for both the consumer and the power company!

I had economy 9 heating in the flat I rented in London. It would come on at night and heat up large concrete blocks which would slowly release the heat while I was out at work during the day.

Had the drafty Victorian building been properly insulated it might have worked quite well.

What an unusual arrangement. Are you allowed to connect other things to that power source?

I am not sure, I also have a storage heater on the same circuit. Probably I need to speak to the power company if I want to connect a power bank of some sorts, like charging a car.

The tariff is called Economy9 for reference. It's not widespread outside London.

Is it not? I know at least a few people who use Economy9 in Newcastle. And yes, their storage heaters are hardwired to this secondary circuit.

I had Economy 7 when i was a student in Oxford twenty years ago. How things have come on - just imagine, by 2040 we may be up to Economy 11!

We're on economy9 in Warwick. As I understand it, the landlord added it fairly recently (within the past decade).

Please make sure the water heater comes back up to temp (140F) before using water from it. This sort of scenario can cause bacteria to grow in the tank due to the hot-cold cycles, and unlike a gas water heater where convection takes place (which heat shocks the bacteria), it's unlikely all of the bacteria is killed before it flows out to hot water taps.




Honestly I was a bit lost at what was the innovation in the article, but reading your comment I think it's just that the US (?) didn't know about that.

What you describe is how water heaters work in most households in France and Belgium, and - I assumed until now - mostly everywhere with a well developed electrical infrastructure.

Except instead on people manually setting up timers, the utility send a signal over the electrical wires when off-peak hours begin and end, and a device (owned and installed by the utility. It can be integrated into the counter like in France, or a separate Zellweger device like in Belgium) turns that into a third wire that power hungry appliances can use to turn on during off-peak hours.

Usually, the utility guarantees a minimum amount of off-peak hours per day, with min/max times that are convenient enough (you will never have two hours of off-peak at 9am, then one hour at 2pm and then five hours from 4pm to 9pm, is going to be between 10pm and 8am in any case).

Water heaters will rarely be turned on during the day in this configuration. It doesn't work that well will storage heaters though.

> the utility send a signal over the electrical wires when off-peak hours begin and end

Is that how they do it? In the UK, it was/is sometimes still done by piggybacking a signal on a BBC radio channel:


Yeah. I'm not sure of the technology used in France, but in Belgium we get this: https://en.wikipedia.org/wiki/Zellweger_off-peak

It actually signals both the strict off-peak times (from 22:00 to 07:00 as well as Saturday and Sunday) and separately, a "night programme" that is variable and used to manage to grid more efficiently. Apparently, there are many different signals that go over the wire, and houses are more or less randomly tuned to one of them.

The strict times are used by the counter itself to know how to count usage, and the night programme is used for the appliances.

A lot of houses in Ireland and the UK do something similar.

But it doesn't work for "house heat" (storage heaters) as well as for water heaters

Presumably you can still make a dent in that 1 hour peak energy demand spike in the evening though? But yeah overall not as convenient

By the time I got home from work in the early evening heat from my storage heater had long since dissipated. If you were at home during the day or had better insulation maybe it would have been more useful.

It sounds disingenuous to call one-way energy sinks 'heat storage'. Calling such a one-way sink a battery is even worse.

On another note, I wonder how resisitve heaters compare to heat-pumps. The article states that heat-pumps can react faster, but are more efficient. What about putting a battery in front of the heat-pump to accelerate the response?

Why, exactly? It's not electrical power storage, sure. But a water heater is by definition energy storage. You want hot water, it takes time to heat, so keep it in a buffer.

And in fact a well-insulated modern tank is a very efficient store for energy that you're going to use at some point anyway. It makes perfect sense to use it as a flexible load to buffer grid management.

> On another note, I wonder how resisitve heaters compare to heat-pumps.

Heat pumps are sort of an orthogonal trick. Ultimately they work by using a lower temperature state to "steal" energy from the cooler outside temperatures. They don't make "heating" any more "efficient" per se. But if you have such an outside environment (i.e. colder than whatever the operating temperature of the heater is, but still warm enough to provide energy to the coolant) they are worthwhile.

Heat pumps are no trick any more than solar panels or solar water heaters are. Resistive heat is 100% efficient while heat pumps are over unity usually 300-400% efficient although usually denoted as coefficient of performance (COP) as number ex 3 or 4. For every 1 watt put in 3-4 watts of heat come out. Real efficiency in terms of energy paid for vs energy provided to heat is what matters.

The reason they are over unity is they are taking heat from one place and moving it somewhere else. If they are pulling heat from outside air they are basically solar assisted. Technically anything above absolute zero has heat energy, even very cold outside air has plenty of heat to pump into your living space. Temperature is not heat, you can raise temperature by putting the same amount of heat in a smaller space, its like using a transmission to increase torque by lowering rpm.

Not sure you're disagreeing with me. That's indeed how they work, and it's... a pretty neat trick! So are phototransistors, for that matter. Not sure I'd put radiative solar water heaters in the same boat, but semiconductors and thermodynamics make for great tricks.

I am disagreeing with "They don't make "heating" any more "efficient" per se."

They definitely do make it more efficient at least in the ways that matter to people, less energy paid for the same heat.

I guess all technology is a trick, but normally that denotes its not really doing what is said, which you reinforced by saying it is not more efficient.

Air source heat pump water heaters and direct solar water heaters are similar in they both utilize solar energy to heat water. Then again if you really get down to it you could argue everything is indirectly solar powered, just some forms you must pay for the energy, while heat pumps and solar collectors get it directly from outside your home heat pumps just require some energy to collect it.

I think you just mean different things by "heating" - if you're talking about heating in a closed system, they're the same - 5 joules of energy are needed to increase the total heat energy by 5 joules, with either approach.

The advantage of the heat pump is that we are, for, say, 3 joules of energy, heating part of the system by 5 joules and heating the other part of the system by -2 joules.

This is great for home heating and the like because we don't care about making the outside slightly colder.

Good thing we don't live in a closed system and instead have a giant fusion reactor in the sky to heat the outside air.

Whether it seems like a trick or not depends on your background.

There's a limit to the 'energy' the system can store: the amount of heating that can be pre-buffered and used effectively later.

This comes with the downside of leaking energy a lot faster than batteries. Moreover, the more energy stored, the faster the leakage.

All that is not to say this is useless. Just to say the role played by heaters is different from batteries. Notably, neither are a good solo solution to the volatility of renewables.

I know that in the Netherlands, stuf like this would require infrastructure upgrades. There is simply not enough copper running to consumer houses to handle the amperages.

> It sounds disingenuous to call one-way energy sinks 'heat storage’

One-way conversation of energy to heat is a very common use-case: Warm water, heating are all things a lot of people use. Currently, most of that conversion happens on-demand, when someone turns on the tap. Heat is a form of energy that is fairly easy to store. Have a large bucket of water, well insulated and you’re mostly set. It’s dead cheap. There are similar concepts that use a generator to provide electricity for buildings on demand and store the waste heat for warm water and heating.

> Calling such a one-way sink a battery is even worse.

The article does not. They are a replacement for batteries in some use cases. The article may overstate how often that’s the case but it does not pretend you could regain the energy stored as electricity.

The article headline is “BATTERY KILLER”

Yeah. It replaces batteries. It’s not a battery.

It doesn’t though. It serves a different purpose than a grid-battery at my house. I would avoid using my household battery UPS to ever heat water at all costs. This system just tinkers with my water heating to help the utility. The Battery Killer theme of the article is misleading.

It's far less useful than a battery, though, because heat is only good for heating things, which means you also need to provide fuel to convert it into electrical or motive applications. like computers and cars.

It’s far less useful in the general case, but substantially cheaper and more efficient in a special case that consumes a substantial chunk of the primary energy used. It’s not as if battery storage is lossless. Currently, heat generation is often done using primary energy, but that too, needs to be reduced if we want to tackle CO2 production levels.

You can also use the same principle to store cold in summer: Cool down a vat of coolant (though water likely won’t do) at times when you have electricity surplus and use it later.

Certainly, that not solving all problems, but it can kill battery usage for some cases.

It may be more efficient to time shift heating loads than to time shift electrical availability with batteries.

If heating loads are a significant amount of the total load, then this would help reduce the need for peaker plants or battery energy storage. And, if done properly, it provides a cost savings to the utility user too.

This kind of tech was actually investigated several years prior, at PNNL, where they experimented with using electric water heaters to provide grid frequency regulation:


Soon after, I worked on an team that, inspired by PNNLs work, created a research demand-response system using a variety of real and simulated loads, including electric water heaters. IIRC we used the current Texas wind output as our input power signal.

At least at the time, our finding was that a collection of smart loads could replace natural gas peakers for frequency regulation on the demand side, so much that the service (therefore economic opportunity of) frequency regulation would be met with a small percentage of actual loads.

The real long term and large scale value of dispatchable loads was thought to be the ability to adapt to the intermittency of renewables. I think that is still the case.

France has been doing it for the past 40 years. France has nuclear power running through the night, so electricity is very cheap. We have a special overnight tariff. France is a big user of water heater and electric radiators.

I have one of these! A domestic Solar iBoost. If our solar panels are generating excess energy, it automatically switches on an immersion heater. So the electricity heats up water rather than flowing back into the grid.

And, sadly, it is kinda pointless when combined with a battery. The days we produce more power than we can use and store - are generally warm and sunny days where we don't need as much hot water.

It saves very little money compared to the cost of gas - but it was a hell of a lot cheaper to install than the 2kWh battery.

It would be great if excess electricity could be used for carbon scrubbing out of the air, or desalination or similar.

So which one would you go for if you did it again, battery or iboost?

Battery. Electricity is more useful than steam.

Do the financials agree?

Thanks for the writeup. I couldn't find a breakdown for the iboost but the Maslow was 2kwh for £2000.

Given that a water heater could absorb much more energy for less money, wouldn't that be the way to go, or at least go water heater first until you've achieved some maximum utility, then go for battery?

I think the iboost was under £100.

Sure the water can absorb more energy, but what can I do with it?

New Zealand has this for grid attached water heaters. They can be placed on a lower tariff, but will be turned off during times of peak load. It is implemented by putting the stored heat device (water heater, air heater) onto a separate meter with something called "ripple control" [2].

This gives the power company the ability to avoid spinning up peaking plants - which are very expensive compared to base load. Nest also offers this as a service to power companies - they allow the power company to shift cooling load the same way.

The power company then passes this cost savings on to the customer. It ends up being about 1c/kwh [1].

Personally, I had to turn it off, our ripple control kept going bad. Nothing worse than a cold shower in the morning. Instead, I went with a timer and switched to a night rate. The heater would come on after hours and warm up the next day's water.

[1] https://www.genesisenergy.co.nz/plans-and-pricing

Anytime vs Controlled.

[2] https://en.wikipedia.org/wiki/Load_management#Ripple_control

I know people would complain incessantly about it, but I wish we had real-time electricity pricing.

A smart dryer could react minute by minute to draw the cheapest electricity over 5 hours. Same with car charging

My takeaway from this article is a little different.

Thermal loss is a function of three things.

First, thermal difference: A hot water tank is at around 50C / 120F, but it could be higher. Let's assume a 10C / 50F basement temperature.

Second, R-value: Dividing the temperature difference to compare loss rates. For example, an R-value of 20 that insulates a 40 degree difference will be as effective as an R-value material that insulates a 20 degree difference, all other things being equal. R-values can go as high as R45, but typical wood+insulation walls are about R15. Pure wood is around R1 per inch thickness.

Third, surface area of exposure. This is pretty self-explanatory.

But if you put all these ideas together, then making a heat battery becomes basically a matter of raw scale.

Say we design a cube storage tank. It is governed by these equations, where L is the side-length of the cube.

R = InsulationR * InsulationLayers

IA = InsulationArea = 6 x L^2

TIC = TotalInsulationCost = IA x InsulationUnitAreaCost * InsulationLayers

VS = VolumeStorable = L^3

WVHC = WaterVolumetricHeatCapacity = 4.2 ( Joules / K ) / cm^3

MTD = MaxTemperatureDifference = MaxTankTemp - RoomTemp

TES = TotalEnergyStorable = MTD * WVHC * VS

EL = EnergyLost = Time * AverageTempDifference * IA / R

Cost = ( TES - EL ) / TIC

Basically I think this is a great idea for power storage if that hot water is need anywhere in the semi-near future. The larger the pools of water you have the cheaper it gets, economies of scale aside.

Our rented apartment here in London has storage heaters in each room which only turn on at night, when tariffs are off peak; they then release the heat during the day.

I think there is a separate power circuit for that. There are also two separate switches for the main water boiler, one for off peak power and one for peak power.

It's not ideal - it gets too warm at night - but I'm sure it saves quite a lot in the energy bill.

it gets too warm at night

Most storage heaters have a flap you can move that affects how quickly the heat is released. Ours are marked "Room Temp Boost". We leave them on the lowest setting because there is usually someone home during the day.

In general they aren't very good, they are more expensive than gas and difficult to control. Landlords like them because they don't have to pay the bill. I looked at converting our house to oil but the figures didn't look very good at the time.

Thank you for the suggestion. For us it's not a flap, it's a very cumbersome dial and, honestly, I gave up on moving it every day. We just sleep in lighter pyjamas :)

"Landlords like them because they don't have to pay the bill"

This. I'd really wish something would be done to align incentives in this area.

Most other countries have a cheaper rate at night which encourages such behavior, eg Economy7 in the UK. The other device is a Night Storage Heaters which warm up thermal stores like bricks that release heat during the day. https://www.which.co.uk/reviews/home-heating-systems/article...

The US is really behind on Real Time electricity pricing. Smart devices should run when electricity is cheap and avoid in peaks. eg don't run clothes dryer in peak times, heat room at cheaper times. This isn't complicated stuff, some states have started, but should be urgency on this.

I did see some other comment on here that comed provides this. In IL? Anywhere else?

I'm on real-time electricity pricing (https://hourlypricing.comed.com/live-prices/) and have been thinking about this for a while.

I have a gas-powered main water heater, and my idea involved installing a second electrically heated tank to use as a feed/preheater to the main water heater and would only kick on when the real-time price was near zero or, as it happens in the summer sometimes, slightly negative which indicates a load-shedding situation.

At the end the cost of installing this all meant the ROI was really long. But I like this concept.

Is it telling that your utility company is one letter away from comedy?

Perhaps. At least they’re not burning California to the ground.

Using electricity to heat water is a waste of a high quality (i.e. low entropy) energy source. Carefully tuning when to use electrical water heaters and calling this energy storage is not quite right.

There are much better ways to heat water for domestic use, depending on the climate and context, such as solar panels, heat pumps with ground drilling, industrial heat waste, etc. These are standard technologies being used in large parts of the world.

For more info I recommend the excellent book Sustainable Energy Without the Hot Air by MacKay (pdf freely available at withouthotair.com).

The intention is to use primarily waste energy from times when production exceeds demand. The side effect is that you have a set of consumers that can take or shed load quickly and on demand, having a stabilizing effect on the grid. Grid stabilization is an important problem when using energy sources with less predictable power output such as wind and solar.

Residential heating with electricity is obviously wasteful, at least in temperate and cold climates when they are regularly used.

But for he tap on your kitchen sink, it can in many cases be the most efficient solution. That’s because an electric heater is a far smaller investment than running a gas pipe installing a furnace. Or, with a central furnace, to run th gap until warm water gets to the tap, then letting warm water in the long pipes go to waste when you turn it off.

As a rule of thumb, when using fossil fuels to make electricity, grid electricity is 1/3 efficient, and cost three times as much as having an appliance that uses the fossil fuel directly.

Of course there's plenty of details that make that statement an over simplification! The point is that you can recuperate your investment in a fossil fuels based heater extremely quickly.

Another problem with using electric water heaters is that they pull a high amount of amperage. Assuming you're going tankless, a water heater necessary for the temperatures in the northeast states could easily use over 100 amps. If you fill the bathtub, turn on the oven, and charge the car at the same time, you could trip your main breaker.

Electricity transmission can be expensive, both in terms of initial and recurring costs (which include safety provisions, wire maintenance, surge handling, regulation compliance, etc.). If you have a solar 10m away generating unnecessary power, heating water might be an OK option.

It seems like a much simpler solution would be to have dynamic rates based on demand needs. There would be a near overnight rollout of dirt cheap gadgets designed to help you regulate your energy use based on the pricing. You could have a simple web interface that lets you set the rates you are willing to let your appliances run at. The end result would be exactly like this without having anything centralized and letting people determine their own demand. The average smart home can probably do this already with a little extra software.

The core issue is that customers don’t want to do this kind of work. People are gradually being moved to a 2- or 3- tier time-of-use strategy that customers can adapt to over time, but nobody wants to get their utility bill and find that the spot price for power spiked up to 100x the nominal rate while you were cooking your Thanksgiving turkey.

A chest freezer might also work well. Lower the temp to -5 or so when power is available and let it creep up to 20f or so when not.

I assume that’s not bad for the food as long as you’re staying below freezing?

Ice changes densities at different temperatures. I'm not exactly sure what that would do to food, but you can add water jugs as a thermal mass to reduce the temperature swing.

Great question.

There's some debate, apparentl, but the basic result is that temperature variation has side-effects, but superfreezing (keeping temp below < -20F, or below 0F, depending on the studied and what is studied) avoids the problems.


Electric cars will end up being a much better choice for this application. Even today, there are a number of companies selling connected EVSE’s (“chargers”) that can be grid-tied to modify the charging rate on demand. The J1772 charge port standard includes a carrier signal that tells the car the max current its allowed to draw, so it’s just a matter of modulating that signal as needed. If there are 100,000 EV’s each charging at 5kW across the state of California every night, that’s 500MW of demand that can be shaped pretty broadly - assume each car needs 4 hours of charging between 9 PM and 7 AM, you have plenty of room to adjust rate and timing.

In the future, it’s likely that EV’s will be designed with bidirectional charging capability, at which point they will be able to return power to the grid in times of high demand, acting as a giant distributed battery.

Was just discussing with my buddy the other day how having a large fleet of electric vehicles will change the grid. You essentially have a Tesla power wall with wheels.

If the charging circuits were modified to allow discharge as well, then your car could act like a "household capacitor", handling high loads like induction cooktops during peak demand, reducing the strain on the grid. The car would then recharge later during low demand.

Not sure how electricity is billed around the world but here in Norway they'll be introducing a power element, that is you'll have to pay a certain amount each month for the peak power you want to be able to draw. This is typical in the industry I understand, but until now consumers here have just paid per kWh.

Thus, treating your car like a power wall would allow you to reduce the peak demand you need to satisfy, which would be reflected in your bill.

This webpage is a pretty sweet highly obsessive guide to making water heater timer controls... Or a tome for groceries style web design.. However you want to look at it!


The author notes that this system doesn't work well with heat pump water heaters. I use a heat pump water heater because it uses significantly less electricity and is cheaper to operate. Overall, efficiency is important.

A system like this might work better with hot tubs, where it's less common to use a heat pump. Offering a subsidy, so the hot tub is 5 or 10% cheaper, would probably make them very popular!

Another alternative would be tankless electric water heaters paired to a battery, or even just having better integration of whole house batteries, like the powerwall, into the grid.

Another alternative is figuring out how to retrofit this system into existing resistance electric water heaters.

Pardon my ignorance, but wouldn't one way to solve the "store electricity for later" problem we have with renewable energy be to use the unused power generated at peak times to produce a stable, long term material that can be used to generate energy on demand? Example: using excess energy collected at peak times to convert large amounts of water into oxygen and hydrogen by way of electrolysis, store the hydrogen for later, and then depend on hydrogen fuel cells during periods of little to no energy collection?

The reason why stuff like this (electrolysis, desalination, etc.) is not widely done is that the capital cost of the facilities is too high. If you spend a bunch of money on an electrolysis plant and only run it 10% of the time, you'll never make your money back, even if your electricity is free.

That is exactly how Orkney plans to enable the next ferries to be hydrogen powered, for the huge energy density compared to batteries. They already generate more from renewables than they can use for electric, heat and vehicles. Surplus is already splitting water. More here: http://www.surfnturf.org.uk/page/transport

Germany is trialling it for energy storage, has a number of trial sites running I believe, and already have some hydrogen powered trains.

Unless you need the energy density of hydrogen the simpler solutions are probably better.

Electrolysis to hydrogen and back to electricity is notoriously inefficient. It makes a great place to dump extra energy that MUST be burned off, but it would be more efficient to just not have that much energy that needs to be burned off. It can make sense with solar and hydro but imagine what that means with dirty energy.

You are describing charging a battery: https://www.tesla.com/powerwall

Also, see Gravity battery: https://news.ycombinator.com/item?id=17789456

That is too inefficient and the problem of hydrogen embrittlement has not been solved.

Can anyone comment on why larger resistive water heaters are being phased out? Heat pumps are ok when its not too cold, you aren't going to be using one in an Alaskan winter though.

Are you just going to be expected to buy a heat pump water heater and rely on the resistive backup 6 months of the year? That may be preferable from an energy use point of view, but massively more expensive, to the point of pushing people back to gas.

Another safety tip someone told me about but I have never tried it: in an emergency if you have no drinking water: drain a hot water heater for water.

Isn't this a rather bad idea due to the potential for bacterial and parasite growth in warm, non-flowing water? AIUI this is the whole reason that it's considered important to avoid the possibility of warm water entering the cold-water pipes.

You might be right about that. Someone described this to me as a real emergency measure- not something that sounds good.

Bunk. A water heater is a purely dissipative device. You cannot regain the energy, and it dissipates or is wasted if not put to good economic use. At best, it can be turned off when demand peaks, as lack of hot water won’t kill anyone. At worst, it dissipates during excess supply. What inefficiency is that highlighting?

Load balancing. The grid cannot react quickly enough for a certain kinds of changes in load and demand.

This goes both ways, sometimes there isn't enough electricity, and sometimes there's too much!

So, turning on everyone's water heater to heat up the water an extra degree or two helps when there is too much electricity. It's considered storage, because now everybody's water heater is already hot if you need to turn them off temporarily when there is too much load.

I think the main point is that this particular system is cheaper than installing grid-scale batteries.

Looking from the grid side it looks the same, you just shift the timing of the consumption. This can be done to limited extent.

> In times of overgeneration, fleets of water heaters can be switched on to absorb excess power, and in times of undergeneration, they can be switched off to shed load and redistribute the existing electricity on the grid

I don't want my public utility deciding when I should and should not be using my water heater. They don't know me, and they don't know my wants or needs. A much better answer: provide consumers with high frequency data regarding current electricity prices and a common API for devices, like water heaters, to utilize that information.

The water heater can pay attention to it's owners needs (either learned or configured) as well as electricity prices and optimize to reduce costs while never running out of hot water. The same goes for freezers, electric car chargers, battery systems, really anything where power can be used at varying times.

This has the disadvantage that customers don't have easy-to-follow rules regarding what the price will be in the future. It has the advantage, however, that consumer demand will rise and fall to meet supply, leveling out (and lowering) prices for all. And I can avoid letting a public utility I don't trust controlling devices in my house.

I got the sense from the article that the controller makes the process invisible to the customer. I.e, water heater maintains temp just like before, but the utility can overheat the system or add heat to the striated capacity, and then (potentially) blend with cold water back down to the desired temp.

Big water heater stay at a decent temp for 24hr+. So if they switch it off here or there for an hour you don't notice.

So what does one do with all this hot water? Would it not be better to power a desalination system that produces a useful byproduct rather than just bleeding off excess joules into a water tank?

Running a desalination plant intermittently would only make sense if the capital cost of building a plant is low compared to the electrical costs of operating it. My guess is they want to run those things 24 hours a day and store the water.

Making smarter use of existing hot water heaters on the other hand is in some cases just a software upgrade or the installation of a smart controller.

I dunno, I prefer taking showers with hot rather than cold. Hot water helps with the dishes, too...

Desalination is useful in certain areas, not so in others.

Eg in the UK we have absolutely no need for desalination, but wind speeds tend to be higher in winter, when most energy is used, so this tech could be combined with electric heaters also, as well as water heaters.

Why are these illegal?

They aren't anymore. This article is from 2014; the rule was amended a year later to make an exception for grid-enabled electric water heaters.[0]

[0] https://www.phcppros.com/articles/5312-water-heater-regulati...

Great - and as I understand it the original ban was for efficiency reasons that don't apply here

They are illegal above a certain capacity. I assume that's because to large storage is wasteful: heat will leak without being used efficiently.

But that's not how heat works. A larger water tank has a higher volume-to-surface-area ratio. This makes larger tanks more efficient than smaller ones, not less.

This is why elephants need such large ears. Their bodies are otherwise incredibly efficient at retaining heat. Those massive, thin, flappy ears provide them with a way to expose their blood to a much larger surface area of skin.

Your average household uses somewhere between 30 and 60 gallons of hot water a day [0], and a bigger tank will just have water sitting in it, going cold. Any extra surface area costs you extra heat, even if the marginal loss decreases with growing tank volumes.

[0] http://www.fsec.ucf.edu/en/publications/pdf/FSEC-PF-464-15.p...

You can use warm water to power your heating in winter. So it might be less efficient in summer, but the amount of energy you’d save in winter would exceed the losses in summer.

In general, the bigger the tank, the better the 'skin to volume ratio'. Hence I'd expect less % per second of loss. The total wattage of loss would still be higher though.

Perhaps legionnaires' disease has something to do with it instead?

It’s cheaper to use a gas-powered water heater, right? I assume that’s because it’s also more efficient?

No, it's because gas is (if your property is plumbed for it, obviously) cheaper to produce and transport than electricity for the same amount of heat energy. It doesn't make sense to talk about "efficiency" for heating itself -- energy is energy. I don't know whether it's more "efficient" in the sense of transport losses or not, my guess is it's a wash.

Obviously gas inherently involves digging carbon out of the grown and throwing it into the atmosphere, though, so there's that to think about.

For a lot of people the choice is between heating with gas and heating with electricity produced by burning gas- obviously using the gas directly for heat is much more efficient in this case because you are skipping the generation and transmission losses.

Yeah, it is a lot more efficient, primarily due to conversion losses. A gas powered power plant loses about half of the energy fed into it as gas to waste heat, whereas a gas heater has essentially 100% efficiency.

Also, no, gas does not inherently involve digging carbon out of the ground, you can produce methane in bio reactors or via electrolysis from electricity (see https://en.wikipedia.org/wiki/Power_to_gas ), which is another method of storing surplus electric energy, either to be converted back later with a gas turbine, or to be injected into the natural gas grid.

Yes gas is cheaper measured in btu/$. An electric heat pump could be more efficient, but that's not normal for domestic hot water heating.

It can be big deal for the environment. Compare "green" solar to heat water is much better than fracking your groundwater and creating greenhouses gases. If the electricity comes from coal burning, gas could be less worse.

"it effectively turns the water heater into a battery" More like a capacitor.

How does this compare to the cement blocks (aka gravity) energy storage?

It uses capital equipment that already exists, with some minimal cost to add the control circuitry, so it’s extremely low cost to build, virtually free to operate, and takes up no extra space.

First of all, although water heaters may be lowest cost to install, they "leak" or bleed off energy in the form of lost heat over time. Obviously super insulating them can cut some of this loss.

2nd, the "bidirectional control" seems a bit misaligned, water heaters as storage devices cannot give back energy into the grid to lessen overall load, they can only be throttled down to lessen load. Perhaps a solution will be obtained from energy storage salts or thermal electric devices to allow for energy harvesting and conversion ?

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