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Why heat exchangers are a renewable energy

Ralph Berger, Technical Director, Menerga, elaborates on the complexity of the debate surrounding the renewable energy status of heat exchangers and why there needs to be a greater appreciation for heat exchangers’ role in the sustainability narrative.


In recent years, the discussion has revolved around the intricate classification of renewable energy, especially concerning the distinction between heat pumps and heat exchangers. This discourse has persisted, marked by varying perspectives, technical nuances, and legislative delineations across different regions.

Ralph Berger

In discussing this topic, Ralph Berger, Technical Director, Menerga, highlights that at its core, the crux of the debate lies in defining what qualifies as renewable energy.

Heat pumps, a renewable energy source, draw from ambient sources like outside air or geothermal heat, channelling it into a usable form for heating or cooling. However, when the conversation shifts to heat exchangers, questions arise.

An instrumental role in energy recovery

Whether rotary or plate, a heat exchanger functions within a framework of energy recovery. It is pivotal in transferring heat or cold, facilitating heating or cooling processes. However, the thermal energy transferred from an air-to-air heat recovery system to the supply air is not classified as regenerative heat. Heat pumps are defined as renewable despite the heat exchanger’s instrumental role in energy recovery, which is often up to three times more efficient than heat pumps. The seasonal performance factors of an air/air heat recovery system are between 12 and 25, while the seasonal performance factors of a heat pump are between 3 and 6. As a result, the narrative veers toward classifying it as non-renewable, contrary to its heat pump counterpart.

The intricacies emerge when considering the source of heat or cold for these systems. Drawing from naturally replenishing sources, a heat pump aligns with the renewable energy narrative. The issue is that it is not the source of the heat energy that determines whether heat energy is regenerative or not but the system that makes the heat energy usable. Moreover, when a heat pump utilises return air from a building’s ventilation system, the extracted heat transforms into a renewable resource capable of heating water and power boilers. This is the case, for example, in toilet exhaust air units. These units have only one exhaust air volume flow and use the waste heat via a heat pump to heat drinking water, which is classified as regenerative heat. However, the delineation blurs when this extracted heat from return air is employed in plate heat exchangers or regenerative air systems to heat supply air.

In some markets, the temperature level at which the heat is recovered influences its classification as renewable. Heat exchangers tend to operate at lower temperatures than the sources from which they harness energy. This divergence in temperature levels seems to play a defining role in the classification of renewable heating energy.

The evolving debate

According to the amended German Buildings Energy Act, waste heat can only be counted as renewable energy if made usable via a heat pump, which is not technically understandable. The function of heat recovery in ventilation systems is analogous to that of heat pumps and even works more efficiently than them. In an air-to-air heat recovery system, the direct heat flow from the high to the low temperature of the two air flows is used. In a heat pump, the heat is transferred via an intermediate medium, which is vaporised and condensed at different pressures.

The debate extends further when considering industrial processes' waste heat. There are questions about whether a heat exchanger harnessing waste heat from an industrial process, heating fresh water to a lower temperature than the source, qualifies as renewable energy. The problem persists, further complicated by varying applications and temperature thresholds.

A critical part of the renewable energy narrative

Although the classification of heat exchangers as non-renewable may be rooted in the differentiation of temperature levels, the discussion persists with the call to highlight that energy recovery should be considered renewable energy, given its practical implications in certification, government funding, and building regulations.

The quest to revisit this debate stems from the potential advantages it could unlock—be it in certifications for new constructions, altering mindsets to leverage heat recovery in meeting renewable energy quotas or providing an edge to planners and consultants.

The EU perspective

For example, in Germany, there are calls to ensure that new buildings must operate 65% on renewable energy, independent of its renewable energy on the electrical or thermal side. Classifying heat exchangers as renewable energy could support planners and consultants of new buildings in meeting the target, as the minimum value can be fulfilled with a heat pump and heat recovery from ventilation.

This would have broader implications across Europe. The plenum of the EU Parliament on 12 October 2023 adopted the amendments to the Renewable Energy Directive from Fit for 55 and Repower-EU, paving the way for a mandatory renewables target of 42% in 2030 and faster approval procedures, considering sources of renewable energy.

A need for action

As the discourse endures, one thing remains clear: the classification of heat exchangers as non-renewable energy sources must be amended. This is because not categorising heat recovery as renewable ultimately leads to a deterioration in building efficiency, as less efficient processes categorised as renewable energy are favoured over very efficient heat recovery.

Not doing so would also go against “technology neutrality” and a level playing field. In this case, you one-sidedly favour heat pumps over other technologies. It is also important to remember that energy recovery systems reduce the need for heating and cooling in the first place and can reduce the necessary heat pump capacities.

The discussion continues, beckoning experts, policymakers, and stakeholders to untangle the intricacies and chart a more straightforward path toward a unified understanding of renewable energy in the context of heat exchange mechanisms.