Microwave-powered home boiler | Hacker News Reader

Microwave-powered home boiler

theguardian.com

8 pointsfaichai28 comments

> The creators of the world’s first microwave-powered boiler have said it can provide a straightforward, zero-emissions replacement for the gas boilers that heat most homes in the UK.

> The boiler uses electricity to heat water which can then be pumped through existing radiators and to taps and showers and baths.

What am I missing here? The novelty microwave part? There have been electric boilers for ages but no one buys them because electric heating is costly in most parts of the world, never mind the NY/NE area I live in. Though I think it is competitive in the Niagara region like Buffalo, NY or Toronto, ON.

Though electric hot water heaters are not uncommon if the area does not have gas or apartments and so on.

Electricity rates are the same across Ontario, and natural gas prices are mostly the same. I live near Toronto, and when I did the math a couple of years ago, an electric heat pump water heater would cost about 4x more to run than natural gas (back-of-the-envelope math). A resistive water heater would be even worse.

> They say the boiler is 84% efficient in converting electricity into hot water, and another 12% of waste heat is recycled, giving a total efficiency of 96%.

An electric water heater is 100% efficient.

And electric heat pumps can be 350% efficient.

However, the USP of this product is that it's a drop-in replacement for existing gas boilers which are prevalent in some parts of the world. The article says the unit costs around £3,500 and installation of a replacement gas boiler costs around £500, so maybe ~£4,000 to switch from gas to electric. An equivalent air-source heat pump installation could easily cost £12,000, and often gas boilers are located in areas which are space-constrained.

Right, trying to see some reason why heating water with microwaves would be preferable to standard resistive heating elements other than for marketing purposes.

The article doesn’t say it explicitly, but I’m assuming it heats water on demand, rather than heating a tank which immersion heaters do. You can buy a resistive element electric shower that gives instant hot water and requires up to about 10kW (and costs £100-200 not £3500) but a gas central heating boiler might pump out 22-30kW equivalent of heat. You would need modifications to your electricity supply to achieve that with electric heaters. If the microwave technology was able to give better performance that would be in its favour, but the boiler cost is not competitive (like many new products). Air source heat pumps are more efficient but again the purchase/install costs make the return on investment long, it was going to be more than 40 years in my case (unless of course carbon pricing changed the calculation), which may be longer than the lifetime of the unit. You also have to be mindful of the refrigerant used in ASHPs because if it leaks the warming potential is several tonnes of CO2e, thankfully a lot of change is happening there too.

What will make the difference to all this is the combination of increased efficiency plus production at scale that makes retrofitting attractive to homeowners. Part of that scale could come from mandatory fitting of such equipment to all new builds.

The only reason to heat a water tank rather than on demand is if your electrical connection can't supply enough power for peak loads. This can be true for domestic hot water, like showers and sinks. Tankless hot water heaters exist (and are cheap), but take 3-4x as much as much instantaneous power, and can be expensive to install for that reason.

If this microwave tech has a COP of less than 1 (meaning it doesn't generate more thermal heating than you put in), then it doesn't solve this problem and better than resistive electric heating. You have the same electrical installation problems.

There may be some other advantage here (long-term maintenance?), but not power efficiency. Thermodynamics says so.

The only way to do better is to make something else colder while you heat your house, using some kind of heat pump.

Resistive heating is 100% efficient, they are saying 96% for microwave, meaning you will need 4% more power to achieve the same heating either in a tank or no tank.

Yes, I wasn’t suggesting that this solution was more efficient, but that solutions that are more efficient than what they replace are what we should be focusing on. It’s fine to say that resistive is 100% efficient, but my point was that a 10kW instant electric heater is not enough to meet the demands of many households (and falls well short of a gas boiler output) and you can’t just say install a 20kW electric heater because typically household supply is not built for that. And so if you heat a tank of water (a typical immersion heater is 3kW) then all of a sudden you have heat loss and you are nowhere near 100% efficient.

Again the discussion being resistive versus microwave, restive should be better even in an instant water heater, again microwave has no advantage that I see.

And when talking about efficiency you must specify context. In this case heating water via electricity, not losses of storing the heated water which is a separate issue based on tank vs instant regardless of heating method.

I am curious what form of energy you think the other 4% turns into?

Both systems are 100% efficient at turning electricity into heat. Neither system is 100% efficient at turning electricity into useful hot water. There are always losses. They're just being honest with their accounting.

The electronic components that create microwaves (magnetron, transformer etc.) are only 50-70% efficient at making microwaves the rest being lost to waste heat. In order for this device to be 96% efficient they must be capturing waste heat in the water through some sort of heat exchanger.

Whats the point then? A resistive element immersed in water converts nearly 100% (probably some light losses at the elements base) of the energy to heat in the water its immersed in.

So on one hand you have complicated expensive microwave system that still resistively heats the water in a much more complicated way losing 4% still to waste heat or you have a much more simple resistive heating system that probably does better.

Charitably, it's less prone to limescale insulating the elements, increasing service life and making the system more efficient as you don't need to run water as long waiting for it to heat up.

Why is it less prone to scale buildup? Scale comes from heating the water, which both do.

I have a gas instant hot water heater, with a water softener it basically never builds up scale, otherwise a yearly acid flush is recommended and it has flush port that make it a easy job.

Scale builds up on the heating elements because it precipitates out when the water turns briefly to gas. Once present, the scale will negatively impact the coefficient of heat transfer between the element and the water. A microwave system wouldn't have any heating elements, and any precipitates would fall harmlessly to the bottom, not hurting efficiency.

Resistive heaters easily get encrusted in limescale, often causing them to fail. My guess is this can be near zero maintenance.

I guess that already exists so can't launch as a new product. I think it's called 'immersion heater' in the UK.

Most homes have one already in fact, as a backup for when the boiler breaks.

Think of all the delightfully higher upfront costs vs standard immersion heating elements plus also the ongoing maintenance! It's a veritable gold-mine. My guess is that's why.

Also water heater using a heat pump is 200-500% efficient.

Not going to heat instantly using a heat pump, it would be tank based and it needs solar heated air (warm outside or warm garage).

I don't understand what you are saying. There are air to water heat pumps as well as ground source heat pumps. Both have typically tanks.

Not really. The boiler would need perfect insulation for that to be true, which is theoretically impossible.

Why would new houses be built without reversible heat pump/air conditioning installed? Isn't most of the cost in the retrofitting for existing houses? That's not the case for new houses, can put air pipes in as you design and build them

This (using an inefficient magnetron instead of 100% efficient resistive heat) is snake oil.

Also: Here in the US, where I live.. running a water heater off of natural gas produces less CO2 than doing it off of the fossil fueled power grid, when all of the efficiencies are considered.

Upgrading the grid to handle more current is going to eat up a lot of resources, but renewable power will help balance that out, so 20-30 years out, electric resistive heat won't be as awful a choice as it would be right now.

The article explicitly says it doesn't use a magnetron, but rather "precise solid-state microwave generators... which can be tuned specifically to heat water and better targeted [sic] than the magnetrons used in microwave ovens".

(I'd have thought magnetrons counted as "solid state" too, as they have no moving parts, but still - they're very clear that's not what this is)

My bad... magnetrons are still the way to go for reliable high power microwaves... there's nothing "precise" required to heat water.

This is as zero-emission as the grid, simply having someone else burn fossil fuels doesn't make it clean (it's probably better but still).

Also gas is a lot cheaper to heat with at current UK prices than electricity. One of the reasons heat pumps are used is that the increased efficiency offsets that.

electric boilers at 100% efficiency are also drop in replacements for gas boilers... They are just much more expensive to run then gas or a heat pump.

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