1 | 23 Fraunhofer magazine T he German federal government aims to drive the energy transition by means of a massive expansion in climate-friend- ly heating systems. At a virtual summit on heat pumps held in June 2022, Rob- ert Habeck, the German minister for climate action, revealed his plans to install six million heat pumps by 2030. The Green party politician has set a no less ambitious target for the operation of heating systems in general: By 2024, all such systems must run on a minimum of 65 percent renewable energy. “That alone singles heat pumps out as the systems of the future — because there is hardly any other option that can meet this requirement,” says Dr. Marek Miara, who has been driving progress in this technology at the Freiburg-based Fraunhofer Institute for Solar Energy Systems ISE for 20 years. Heat pumps use natural heat from the surround- ing environment — i.e., the thermal energy from the air, water and earth — to keep buildings warm. Unlike wood, oil and gas heating, there is no energy carrier combustion and so no waste gas emissions to damage the climate. Instead, heat pumps operate based on a cyclical process that uses suitable cooling agents and alternating pressure, taking warmth from the sur- rounding environment and raising its temperature level before emitting the resulting heat directly into the building’s heating and water systems. “Just 15 years ago, you constantly had to explain to people that yes, using cold air to heat a house actually works. Later on, people questioned whether heat pumps are efficient enough. And now, there are more than 5,000 different devices available on the market in Germany alone,” says Dr. Miara. Dr. Habeck’s campaign is following the exact strate- gy for achieving climate targets that many scientific studies have been advocating for years. For the heat- ing industry, the six million pump plan was a shock at first. Now, after many workshops and another heat pump summit, the stakeholders are rising to the challenge. Leading manufacturers are planning to join forces to invest several billion euros in new production capacity. “In the past, investments of this volume would not have been thought possible,” emphasizes Dr. Miara, stressing how dramatic the re- cent developments have been. At present, around 1.6 million of these systems are operating in Germany. A good 200,000 new systems were installed in 2022, and another 350,000 are set to be produced, sold and installed this year. If this pace is maintained, then the annual minimum target of 500,000 units that has been set for 2024 onward would actually start to sound realistic. All the same, it’s an extremely am- bitious goal, and reaching it will not be a question of increasing heat pump efficiency. Instead, the focus is on how to quickly install these systems in as many houses as possible. To solve this riddle, it is helpful to look back on what has already been achieved. Fraunhofer ISE is known for its extensive monitoring projects, which now cover more than 350 buildings that use heat pumps. In a study completed in 2020, Dr. Miara and his team conducted a metrological analysis of 56 models from different manufacturers, including models that use external air, indoor air and downhole heat exchangers as their energy sources. These systems were used to heat single- and multi-family homes (for up to four families) that were all at least 15 years old, and in most cases, more than 40 years old. The results disproved some widely held misconceptions, points out Dr. Miara, who headed up the study. “First of all, we showed that all the systems work well and provide sufficient heat, even in the existing building stock. This is something that is constantly being called into question. Second, we demonstrated that they also work efficiently in houses that have undergone little or no restoration work, and consequently have a high en- ergy demand for heating. Our third finding is that heat pumps can function well with ordinary radiators. Naturally, underfloor heating and other forms of panel heating are better as a rule, because they can get by with a lower supply temperature. But radiators do not automatically disqualify a building from using a heat pump.” The bottom line is that even for existing buildings, a correctly scaled pump can guarantee sufficient heating. Heat pumps are also proving themselves in other contexts, even — and especially — in multi-family houses. This has been demonstrated in analyses from eight European countries, which were commissioned by the International Energy Agency. “But we are also seeing that there are too many different solutions,” reveals Dr. Miara, who also led the IEA project. “To put it in slightly exaggerated terms, if in every one of the many thousands of models available on the mar- ket, the pipe connection openings in the device’s housing are in a different place, then the technicians have to learn the new method from scratch every time they install a system. We urgently need more stan- dardization here,” insists the Fraunhofer researcher. “And that doesn’t just apply to the products, but also to the models and system schematics.” This is because connecting the heat pump to the house’s existing 33 Can we turn up the heating with a clear conscience as regards the environment? It’s easier with a heat pump.