Chapter 2 — Yves Lenoir & Professor Suominen: a different way of thinking about heat pumps

The Finnish paradox

As France was gradually turning its back on heat pumps following the failures of the PERCHE programme, Yves Lenoir observed something intriguing: the Finns were achieving excellent results with the same technology.

The paradox was striking. If a heat pump works correctly in Finland — where winters are far harsher than in France — why should it perform poorly under the Norman or Bordeaux climate? The answer, which Lenoir would build drawing on the work of Professor Risto Suominen of the University of Jyväskylä, was both simple and unsettling: the problem is not the heat pump. It is its design.

What Suominen had understood

The French machines of the time were optimised for relatively mild climatic conditions. Their output dropped rapidly as soon as outdoor temperatures turned negative — precisely when heating demand was increasing. This was a design flaw, not a thermodynamic inevitability.

The Finnish teams, by contrast, were developing machines conceived from the outset for Nordic winters, capable of operating down to −20 °C and beyond, thanks to several deliberate technical choices:

• a better-sized evaporator, to maintain sufficient heat exchange even at low temperatures;
• more effective defrost management, an inevitable but energy-costly phenomenon if poorly handled;
• controls adapted to large temperature swings;
• an overall design coherent with the constraints of extreme cold.

What Suominen demonstrated — and what Lenoir reported from the early 1980s — was that the limits of heat pumps are not written into the laws of physics. It is an engineering problem, and therefore a solvable one.

Forty years on: Lenoir was right

In France, the hasty conclusion drawn from the PERCHE failures had been: “heat pumps don’t work in cold weather.” Lenoir showed even then that this claim was false — or rather, that it confused the technology with the shoddy products of a market launched in haste.

With forty years of hindsight, history has proved him right on every count. Modern air-to-air and air-to-water heat pumps now maintain high COP¹ values down to −15 °C, −20 °C, and even −25 °C for the most recent models. What was once a confidential Nordic speciality has become the global standard.

The technological evolution that made this possible deserves closer attention, as it illustrates exactly what Suominen had anticipated.

What has changed since the 1980s

Each of these developments corresponds to a problem that Suominen had identified. The variable-speed compressor — the keystone of the modern heat pump — is the direct answer to the flaw he pointed to: a fixed-output machine cannot adapt to the diversity of climatic conditions.

A marginal idea that became a global standard

What strikes one on re-reading Lenoir’s article is less Suominen’s technical foresight than the resistance these ideas encountered in France. They were not secret: they were published, demonstrated, and working in real conditions thousands of kilometres away. But the French market, traumatised by the PERCHE failures and reluctant to question a nascent industrial consensus, was not ready to hear them.

It took until the late 1990s and the arrival of affordable scroll compressors and inverter drives for these principles to become widespread — first in Japanese reversible air conditioners, then throughout the global heat pump industry.

Today, the heat pump has become what it should have been from the start: a robust technology, performing well in cold weather, and adaptable to all climates. The next chapter details the physical principles that make this possible.