Microwave weirdness

IN MARCH AND APRIL this year several national newspapers, including the normally sober Economist, carried articles promoting the concept of a microwave central heating boiler which could be fitted in place of a gas-fired one. The same story appeared in H&V News, a trade journal.

The originators’ objective is to promote the idea of decarbonising your heating by substituting (renewable) electricity for gas. The articles compare two options: electrifying your existing central heating system versus installing a heat pump. The latter, quite reasonably, they consider costly and not always feasible. But somewhat disingenuously they fail to mention another electrification option which I’ll come back to at the end.

Now suppose for some reason you did want your radiators fed by an electric central boiler. Why opt for the complexity of a microwave one rather than one based on simple resistive immersion heaters? It makes no sense because either way you can only get as much heat out as you put electrical energy in. Microwave sources confer no thermodynamic advantages. The promoters justify the more complex technology because (according to the account published in the Economist) immersion heaters “…must run continuously to deliver water at a suitable temperature. That often warms water which is never used.” Well, er, no. Immersion heaters would be thermostatically controlled, and wouldn’t consume energy that is never delivered as output heat.

Event alert: “Decarbonising heat: practical realities” on 8 July. Details at https://vesma.com/z200

In the version of the story on the Guardian web site, the company claims that the electrical load “will be about the same as an electric oven”. This also makes no sense. A built-in electric oven might be rated at 3 kW while a central heating boiler could quite typically have an output of around 30 kW and an electric version would need input power to match. Indeed according to the piece in H&V News  these microwave boilers can be up to 60 kW in capacity. And therein lies a problem. Even a 30 kW appliance would need a 120-amp mains supply at 250V, and that’s on the assumption that it is a simple resistive load. To put that in context, our whole house is fed through a 100-amp supply. Microwave units, however, have notoriously poor power factor so will inevitably draw much more than 120A for the same 30kW duty, all of which makes claims of quick and easy installation ring somewhat hollow.

The coverage in H&V News is rich in whizzy-sounding technology and buzzwords. For example it says that the heart of the system is termed a “technology stack”, which they say is a “solid-state, robust RF framework that uses configurable and controllable high performance amplifiers to generate energy”. Elevating the merely meaningless to Olympian heights of drivel.  The version of their story in the Economist includes the bizarre assertion that “the pipes that carry the water are also made of microwave-sensitive materials, as is the insulation that lags them”. Microwave-sensitive? Wait… like lasagne? And if the pipes heat up under microwave radiation what’s the benefit of the insulation doing likewise?

The article in H&V News quotes a director of the company claiming that their electric boiler “would cost the same to run as a gas boiler”. At least the Guardian had the wit to expunge that remark from the on-line version of the article. The truth is, if you want to use electricity for heating and a heat pump is not an option, individual room heaters would be the obvious way to go. Apart from being cheaper to fit than a new electric boiler, they would enjoy the advantage of room-by-room controllability.