Image copyright Other Image caption The Chinese character for "house"

Look closely at the Chinese character for "house".

Yes, it's obviously got a roof, but what's that under the roof? Rather stylised these days, it is, of course, a pig.

Some 3,000 years ago - when the character "jia" originated - every comfortable village house in China had a pig inside, eventually for food.

But during its life, the pig would provide at least part of the domestic heating.

European farmhouses put people and cattle together in the same way for the same reason until the end of the 19th Century. Cows warded off the chill of winter.

And now here is Frenchman Paul Benoit trying to do the same thing in the 21st Century. But instead of the pig, he's using computers.

'Digital heaters'

An engineer by training, 10 years ago Mr Benoit worked in the computer department of a French bank, where ever increasing computing power and capacity was seen as a competitive advantage. That meant more and more yoked-together computer servers.

And those computer arrays were generating ever more heat, which the bank then spent a lot of power cooling down.

Image copyright Qarnot Image caption Paul Benoit says that the demand for computer centres will only continue to grow

Mr Benoit started thinking about that. And came up with a simple but devilishly clever idea for a new company, Qarnot Computing.

It sells computing capacity to corporate clients, generates heat with the computers... and then finds ways of channelling off the heat to use as central heating.

To conventional computer engineers, this heat is a worrisome by-product of the way the machines work. To Mr Benoit, it's not a problem but a product he can find uses for.

He calls what he's created a "digital heater". Essentially it's a bank of computers designed so that the heat they generate is channelled around a room or building that needs heating.

His ingenious software ensures that when users turn up the thermostat, enough extra computation is rustled up from corporate clients to increase the emitted heat.

Image copyright Qarnot Image caption Qarnot has designed computer servers that are fitted like radiators

If there's not enough work for the computers to do when clients need heat, the company offers that spare capacity to university labs for free.

Qarnot uses banks of servers in data centres to meet the heat needs of corporate clients. But the system can also work in individual houses.

Four of the company's computers generate enough to heat a room, says Mr Benoit. Distributed computing, in other words.

The company reimburses people for the electricity their digital heaters use. In other words, they get their heating for free.

This is an idea cleverly meeting a need that has been apparent for some time.

Growing energy consumption

With internet search and corporate big data and cloud computing, the demand for number crunching has been doubling for years.

And the more it happens the hotter it gets in the vast rooms now housing thousands of computers, all yoked together for specific tasks.

The huge buildings housing server farms for companies such as Google and Amazon are often built in places such as Oregon where electricity is notably cheap.

Image copyright Qarnot Image caption Computer servers can give off a large amount of heat

Even though the power used by one individual machine may be tiny, it all adds up when that personal computer (or mobile phone or tablet) does search or video conferencing or plays games or movies.

But in many places the power usage is compounded by the need to cool down the computers once they have been exercised by carrying out the number crunching.

Really big server farms with hundreds of thousands of machines are being located in cold climates such as Finland, where the cooling might be done more naturally.

But instead of cooling it down, why not use the heat? It's an obvious question.

Paul Benoit says that while today's data centres consume about 2% of total world energy, in five years' time it will have risen to 4%, and to 8% in 10 years.

And as more and more companies find they have a need for data centres, Mr Benoit thinks they will start waking up to the environmental and cost-effective need to use their computers for heating as well as number crunching.

When you think as he does, there is certainly a lot of surplus wasted heat generated in the modern world: in lighting, in a host of transformers attached to gadgets in every home and office, in internal combustion engines.

It cries out to be used.