Combat climate change, peak oil, nuclear risk reduction, energy independence… All these buzzwords push for a sustainable energy future. The main challenge is to balance all three dimensions of energy sustainability (security, equity, environment) at a reasonable cost. A full energy transition with current technologies is not possible, which calls for further development. Windthermal energy is an underestimated, but promising approach to generating renewable heat.

The Energy Trilemma

The European energy landscape is facing one of the most challenging decades of the last century. Ambitious climate goals combined with the current gas crisis call for an urgent transition to a secure, equitable and environmentally sustainable energy infrastructure. The World Energy Council (WEC) defined the three dimensions of energy sustainability as followed: Energy security means that energy supply has to be reliable and based on a resilient infrastructure; energy equity means that energy has to be accessible, abundant and affordable for everyone; and environmental sustainability of energy systems means that environmental harms and climate change impacts have to be mitigated and avoided. All these three dimensions need to be balanced together, which results in the trilemma to find the perfect trade-off of energy security, energy equity, and environmental sustainability.

The test facility makes it possible to collect valuable data for further development and to test the technology in practice. © DLR

An incomplete transition

Since the beginning of the new millennium, European leaders and policy makers have become aware of the necessity of a sustainable energy system. At the beginning, this energy transition was mainly focused on electricity. However, the electricity sector accounts for only a quarter, the mobility sector for another quarter, and the heating sector for half of the total energy consumption in Europe. To envision a 100% renewable and independent Europe we need to fundamentally rethink and widen our horizon: When we talk about energy transition, we should think about heat instead of electricity.

Every piece matters

The development of sustainable heating has been neglected in the past. Therefore, current solutions are insufficient to meet a climate neutral and independent heat generation in Europe. Hereafter, I want to point this out on the seven most promising heating technologies.

Energy conservation

The best energy is energy that we do not consume. For instance, renovating and insulating existing buildings can reduce space-heating demand. Nevertheless, the renovation rate needs to be doubled to generate a significant impact. This is not impossible, but it is challenging.

Solar thermal energy

The sun shines in summer and we need most heat in winter. This timely decorrelation of supply and demand calls for seasonal storage solutions that can store heat for several months. However, we are still waiting for a techno-economically feasible solution.

Biomass

Acreage is one of the most valuable currencies, and governments limit the land assigned for energy crops. According to the IPCC this food vs. fuel dilemma means that energy generated from biomass will probably barely double by 2050.

Geothermal energy

In most parts of Europe, geothermal wells are deep and the drilling costs are mainly linked to this depth. This results in unforeseeable project risks if the well happens to be unsuitable for geothermal exploration.

 

Power-to-heat

Lately, everybody is talking about heat pumps, but if every European household would heat with heat pumps, the electric energy demand would increase significantly, resulting in two questions:

  1. We are not even able to supply our current electricity demand with renewables. How do we generate that much additional (renewable) electricity?
  2. The intermittency of the two main renewable sources, wind and sun, is already overloading the electricity grids and entails high investments in additional grid infrastructure and storage solutions. How do we finance this?

Green hydrogen

Electrolysis is an energy-intensive process. According to Fraunhofer IEE, one needs six times more electricity to heat a house with green hydrogen than with an electrical heat pump. As it is quite expensive, green hydrogen should rather be used in applications where no other technology can replace it. This is the case for the mobility sector and for high-temperature processes.

The main part of the "thermal platform" is a hydrodynamic retarder. It generates and regulates the heat depending on the respective application. © DLR

District heating

This is not exactly a generating technology, but a valuable infrastructure. Currently, we use waste heat – mainly from fossil-fired processes – to supply our local district heating grids. Unfortunately, we still need to develop a more sustainable solution to take advantage of this infrastructure.

In summary, we need every piece of the puzzle to tackle green energy transition in Europe.

An unlocked potential

The seven above mentioned heating technologies represent the who is who of renewable energy sources, but one is missing: wind. Windheat is commonly only known as power-to-heat from excess wind generation (wind to power to heat), but there is still no commercially available technology that converts the wind directly into heat (wind to heat). Windthermal energy is a potentially simple, reliable, cost-effective and renewable energy source that could balance all three dimensions of energy sustainability:

  1. Security: Heat is easier and cheaper to store. Windthermal together with storage applications, so called windthermal energy systems (WTES), could stabilize the energy supply and relieve infrastructure capacity.
  2. Equity: Windthermal energy saves one conversion step compared to excess wind generation, resulting in a more lightweight engineering. This could potentially drastically reduce the levelized cost of energy compared to wind power.
  3. Environment: According to the German Environment Agency (UBA), wind power is currently one of the most sustainable, if not the most sustainable technology. Windthermal energy would save the electrical components, making it even more sustainable than wind power.

In other words: Windthermal energy has the potential to make a meaningful contribution to solve the energy trilemma and make our world more sustainable.

Underdeveloped state of the art

Despite its high potential, this promising technology is still underdeveloped, remaining on a low technology readiness level (TRL). Theoretically, there exist three different ways to convert wind directly into heat:

Compression

The wind turbine rotation moves a piston to compress a fluid, heating the fluid by compression.

Friction

The wind turbine is decelerated by a more or less sophisticated break, creating friction heat.

Induction

The wind turbine rotates a magnet, creating an alternating magnetic field. A resulting flow of eddy currents in a conducting material dissipates into heat.

To my best knowledge, none of the three mentioned windthermal technologies is currently commercially available. In the 1990s, the Danish brand Calorius offered a friction-based turbine. Despite its success, they had to shut down after selling more than 200 windmills due to high certification costs. Since 2010, Rotaheat, an English manufacturer, is working on an induction-based solution, which is still in the development phase. All other approaches haven’t left yet laboratory conditions.

The potential use of the technology are diverse and range from industrial processes to the desalination of seawater and the storage and re-conversion of heat. © DLR

The awakening

Its huge potential and low TRL inspired researchers at the German Aerospace Center to start the project “Aerothermie” and prove the technical and economic feasibility of windthermal energy on a pilot plant. The results were stunning: at the first attempt, the output temperatures reached 70°C, more than enough to provide space heating. Nevertheless, there are still a lot of challenges to overcome. The success depends on further technical development. The large distance transport of heat is expensive and lossy, which implies that windthermal turbines need to be as close as possible to the consumer. Conventional wind energy struggles with the so-called nimbyism (not in my backyard mentality), and a wind turbine remains a wind turbine, even if the conversion inside the nacelle differs. Last but not least, windthermal energy must compete with other heat generation technologies in the dimensions price, reliability, temperature level, and location.

The only way to prove the competitiveness of windthermal energy is levelling up its TRL in future projects. Let’s awaken this simple, reliable, cost-effective and renewable energy source.

At its site in Braunschweig, DLR conducts research in the fields of aeronautics, transport, space, and energy. © DLR

German Aerospace Center

The German Aerospace Centre (DLR) is the research centre for aeronautics and space in Germany and conducts research and development activities not only in the fields of aeronautics and space, but also in energy, transport, security and digitization. DLR researchers are developing the energy system of the future, investigating highly efficient technologies to produce and use sustainable energy. Regarding heat, besides windthermal energy, the DLR explores many different innovative alternatives to make heat generation and consumption more sustainable: A tool to analyse and evaluate the feasibility of renovating existing buildings, high temperature heat pumps for industrial processes, a limestone heat storage, to mention a few.

This Article is part of issue 04-2022 of the German Wind Power Magazine, the international magazine of the German Wind Energy Association (BWE) about innovations of the German wind industry. You can read the full magazine here, online and for free.


References

World Energy Council (WEC).
World Energy Trilemma Index. Retrieved 12 October 2022. URL: https://www.worldenergy.org/transition-toolkit/world-energy-trilemma-index

Intergovernmental Panel on Climate Change (IPCC)
IPCC Special Report on Renewable Energy Sources and Climate Change Mitigation. Retrieved 12 October 2022. URL: https://energy.lbl.gov/publications/ipcc-special-report-renewable-energy

Fraunhofer Institute for Energy Economics and Energy System Technology (Fraunhofer IEE)
Green hydrogen or green electricity for building heating? Retrieved 12 October 2022. URL: https://www.iee.fraunhofer.de/en/presse-infothek/press-media/overview/2020/Hydrogen-and-Heat-in-Buildings.html

German Environment Agency (UBA).
Best-Practice-Kostensätze für Luftschadstoffe, Verkehr, Strom- und Wärmeerzeugung. Anhang B der Methodenkonvention 2.0 zur Schätzung von Umweltkosten. Dessau-Roßlau, 2014. URL: https://www.umweltbundesamt.de/publikationen/methodenkonvention-20-zur-schaetzung-von-1

German Aerospace Center
Windthermal energy. Retrieved 12 October 2022. URL: https://windheat.dlr.de/en/

Neumeier M, Cöster M, Marques Pais RA, Levedag S (2022). State of the art of Windthermal Turbines: A Systematic Scoping Review of Direct Wind-to-Heat Conversion Technologies. ASME J. Energy Resour. Technol., 144(4): 040802. https://doi.org/10.1115/1.4052616.

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