Article
Fact or Fiction: Heat and Decarbonization
January 2025
Energy
Energy
Article
Fact or Fiction: Heat and Decarbonization
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When it comes to energy and climate, we’re faced with a wide range of common misconceptions that often lead to conflicting responses. With this FAQ focused on energy, Carbone 4 aims to shed light on the debate and separate fact from fiction by offering a scientific, data-driven analysis of each common misconception.
To learn everything there is to know about energy and climate, check out the other FAQs available
Whenever a debate on energy begins in France, The discussion often revolves around electricity. The perception that electricity is at the heart of the matter stems from the prominent role of nuclear power in the French debate, as well as from the everyday presence of numerous electrical applications such as lighting, household appliances, and computers.
In fact, electricity accounts for 25% of France's energy consumption. It can be used for the purposes mentioned above, as well as for heating, air conditioning, domestic hot water, cooking, etc.
25% is significant, but our transportation, for example—which relies almost exclusively on oil—accounts for 30% of our energy consumption.
Our heating needs are even higher. They account for 45% of France's energy consumption. Admittedly, some of it is covered by electricity, but natural gas and heating oil are still widely used.
The generation of this heat accounts for 2/3 of the energy needs of residential and commercial buildings (heating, domestic hot water, cooking) and half of the energy needs of industries (cooking, drying, smelting metals, etc.).
Global warming is causing an average rise in global temperatures. This is true throughout the year, but Milder winters paradoxically create a “positive” feedback loop[1] for the climate. Indeed, when it is less cold, there is less need to heat buildings. As a result, energy consumption and the associated greenhouse gas emissions decrease.
To measure how much or how little heating was needed over the course of a year, we use the number of heating degree-days. If the average temperature for a day is greater than or equal to 15°C, the number of degree-days for that day is zero. Otherwise, it is equal to the difference between 15°C and the average temperature for the day.
Around 1980, there were about 2,900 heating degree-days, whereas today the figure is around 2,000. This annual decrease of 1% per year in heating needs is expected to accelerate in the coming decades. By around 2050, it is likely that France will come very close to 1,500 heating degree-days. By that time (the lifespan of a heating appliance), We'll heat our homes less, but we'll still keep them nice and warm.
A quick note on the opposite need: cooling off in the summer when it's very hot. This time, the feedback loop[2] is “negative” for the climate. The hotter it gets, the more greenhouse gases are generated by the use of air conditioning.A similar unit is used—cooling degree-days—which measures the difference between 24°C and the average temperature on very hot days.
Even though the need for cooling has risen sharply, recently reaching 100 cooling degree-days, there will be “only” a few hundred degree-days in 2050. If everyone had an air-conditioning system (which is not the case, unlike with heating), then energy demand would be roughly 10 times lower than it is now for heating.
That said, with regard to energy consumption, This prevalence of heating does not hold true in all countries. Furthermore, the health impact of heat waves is much greater than that of cold waves. The key challenge, then, is adapting buildings. Insulation The use of bio-based materials provides thermal comfort in the summer that is unmatched by mineral or fossil-based materials.
Prioritize passive shading systems, and habits ventilation during the coolest hours of the day during the day make it possible to avoid using air conditioners. Air conditioners pose a problem by releasing hot air outside, especially in cities where they exacerbate the effect urban heat island effect.
The temperature levels can be classified into three categories:
Low temperature: less than 100°C
Average temperature: between 100°C and 400°C
High temperature: above 400°C
Since buildings are the largest consumers of heat, Low temperatures are the norm.It covers 75% of the needs.
However, to produce At this low temperature, fossil fuels and wood account for the vast majority (75%) of the energy used. However, in the context of decarbonization, fossil fuels are bound to disappear. Wood, on the other hand, is a scarce resource. It must therefore be used sparingly and for high-temperature applications for which there are no carbon-free alternatives.
To meet these low-temperature heating needs, there are economically competitive renewable thermal energy sources that also contribute to local employment: heat pumps, solar thermal energy, and geothermal energy.
Low-temperature heat lost in a diffuse manner is the least valuable form of energy, the ultimate—and rather pointless—outcome to which all energy is ultimately destined (thermal dissipation in an electrical cable, in car brakes, in a light bulb, etc.). Conversely, High-temperature heat has many more potential applications.
However, Not all types of heat can reach the same temperature ranges. Thus, combustion—particularly of fossil fuels—can produce high-temperature heat (although it is widely used for low-temperature applications), whereas air-to-air heat pumps can only provide low-temperature heat. Furthermore, each method of heat production has limited potential, particularly due to physical constraints (sunlight, biomass renewal, land availability, etc.).
It is therefore essential to match the various production methods with the different uses based on temperature levels. In other words, for For low-temperature heat, make the most of low-carbon, non-combustion-based production methods; for high-temperature heat, reserve low-carbon, combustion-based production methods.
Solar panels convert the energy from sunlight into another form of energy for various applications. There are two types of solar panels: photovoltaic solar panels, which generate electricity, and solar thermal panels that generate heat.
This is the first type of panel that has been accused of containing rare earth elements, a group of metallic elements that have similar properties. Contrary to what their name suggests, Rare earth elements are not “rare” within the Earth’s crust. On the other hand, China has a virtual monopoly on their production, raising concerns about dependence.
In fact, “The solar photovoltaic technologies currently on the market do not use rare earth elements.”[3]” but their production requires critical minerals such as copper or “metallic” silicon[4]. And above all, 89% of the solar panels installed in Europe come from China
By comparison, solar thermal panels fare much better. They contain no critical materials, and virtually all of them are manufactured within the European Union; however, they cannot be used for the same end uses (heat rather than electricity).
Solar energy, often associated with electricity generation, is also a source of heat. Solar thermal energy can indeed be used by both households and industrial facilities to meet a relatively large portion of their heating needs, whether for domestic hot water, building heating, or the supply of low-temperature heat for certain industrial processes.
Historically, solar thermal panels have been installed in hot, sunny regions, such as California as early as the beginning of the 20th century, or in France—in the southern part of mainland France and in the overseas territories—where many simple, inexpensive solar water heaters were used.
Today, these systems have become significantly more efficient, enabling them to operate effectively even in the northernmost regions. They often cannot meet 100% of energy needs and are therefore used in combination with another energy source and/or a heat storage solution.
In Europe, Denmark, Germany, and Austria have per capita solar thermal energy production that is significantly higher than that of sunnier countries such as France or Italy. One of the world's largest solar thermal power plants is located in Denmark. It has been in operation since 2016 and meets more than 20% of the demand of the 22,000 households connected to the district heating network.[5]. In Upper Austria, solar thermal energy is supported at the provincial level and accounts for 0.7 m² of collectors per capita (10 times the European average). It should be noted that in the latter case, since roof space is limited, solar thermal and photovoltaic systems must be integrated to maximize decarbonization while minimizing resource use.
In France, the rollout of the Solar thermal energy is lagging behind compared to the objectives of the 2018–2023 Multi-Year Energy Plan (PPE), which called for an increase in production in this sector of at least 50%.[6] Given the recent volatility in gas and electricity prices, solar thermal energy is more than ever a solution for the future.
Heat does not travel well over long distances (> 10 km) and must therefore be generated close to where it is consumed. Just as with electricity, production must be timed to match demand. This challenge is all the more significant when using generation sources that are difficult to control. For example, solar thermal production peaks in the middle of the day and is higher in summer than in winter. In contrast, heat demand for buildings is higher in winter, in the morning or evening, and for industry, it is often constant.
To address this balancing challenge, it is possible to use both variable and controllable generation sources as well as storage systems.
As with electricity, there are heat storage technologies that different time scales:
Technology The most widespread and mature is sensitive storage, based on the thermal inertia of a heated and insulated material (e.g., water stored in a tank or a pit dug into the ground). This material can then release the heat when it is needed. From summer to winter, it is possible to store several dozen GWh of thermal energy, with a loss of about 25%.[7]
Other, higher-density storage methods are being studied, such as latent heat storage (based on the energy absorbed by a material during a phase change, which can be released when the material returns to its initial state, or thermochemical storage, in which heat drives a reversible physicochemical reaction).
Geothermal energy encompasses two main types of technologies: deep geothermal energy (which extracts heat from the subsurface at depths between 200 and 2,500 meters) and shallow geothermal energy, which harnesses the stable temperature of the soil at shallow depths (<200 meters).
With regard to the deep geothermal energy, France has large basins with significant potential, notably the Paris and Aquitaine basins. More specifically (with higher temperatures at shallower depths), collapse ditches (located in the Rhine, Bresse, and Rhône river basins and in the Limagne region) as well as volcanic regions (in Guadeloupe, for example) are also areas of interest[8]. Operating facilities generally supply the district heating networks However, for such facilities to expand, there must be sufficient demand. This is precisely the case in Île-de-France (which accounts for more than 80% of France’s geothermal heat supplied to its district heating networks)[9]. Projects could be further developed elsewhere, given the extent of the potentially exploitable areas.
The shallow geothermal energy (used, for example, in heat pumps for residential homes) can be utilized in more than 90% of the territory because it does not require a subfloor. It harnesses the heat naturally present in the ground. Its deployment depends more on economic factors (funding and subsidies to reduce the high cost of investment) than on geographic or technical factors. There are significant disparities in deployment from one region to another: for example, Brittany and the Pays de la Loire region account for more than 50% of France's surface geothermal installations (more than 4,000 installations)[10]. Surface geothermal energy has significant potential. It accounts for only about France currently produces 5 TWh of heat annually, but could produce 100 TWh .[11]
With support from the French government for the production of more than 1 million units, expected to begin in 2027,[12] Heat pumps are widely favored as a way to decarbonize residential heating and reindustrialize France. Although growth was initially in line with the recommendations of the 2018–2023 Multi-Year Energy Plan (PPE),[13] Sales of heat pumps showed a slight slowdown between 2022 and 2023 (+4.6%)[14]. The factors at play include an inflationary environment, falling wholesale gas prices, and less transparent subsidies.[15]
One advantage of the CAP is to produce more energy than it consumes in electricity by harnessing “free” heat from its immediate surroundings: outdoor air for air-source heat pumps, a water source for water-source heat pumps, and the ground via a borehole for ground-source heat pumps. They then release the heat via blown air (reversible air conditioner) or water pipes (water-filled radiators, underfloor heating).
However, when temperatures drop, heat pumps are less efficient. The performance of heat pumps is measured by the coefficient of performance (COP), which is the ratio of useful thermal energy to the energy required to operate the unit. Under standard temperature conditions, a heat pump’s COP is therefore greater than 1 (it produces more energy than it consumes in electricity). By comparison, an electric radiator always has a COP of at most 1, since it converts electrical energy into thermal energy with a maximum efficiency of 100%.
The COP is intrinsically linked to the temperature difference between the cold source (from which it draws heat) and the hot source (to which it releases heat). It is inversely proportional to the temperature difference between the heat supplied by the heat pump and the energy drawn from the environment. When it is cold outside, with the setpoint temperature remaining unchanged, this difference widens and the performance of an air-source heat pump decreases. Note that this effect is very slight when the cold source is the ground, as its temperature remains around 13°C throughout the year.
The outdoor temperature limit is often -7°C, ensuring normal operation year-round across most of France. Some specialized heat pumps even operate at temperatures as low as -15/-20°C.[16]
1.
The feedback loop is technically negative because the two effects are opposite and therefore do not reinforce each other. In contrast, a feedback loop is said to be positive when the two effects reinforce each other. For example, the melting of permafrost releases methane, which accelerates global warming, which in turn accelerates the melting of permafrost.
2.
Technically, the feedback loop is positive because the warming effect increases the air conditioning's energy consumption
3.
Rare Earths, Renewable Energy, and Energy Storage, 2020, ADEME
4.
https://www.consilium.europa.eu/en/infographics/critical-raw-materials/ for silicon, Quartz (SiO₂) is a very abundant resource, but for technical and economic reasons, silicon “metal” is produced from very high-purity quartz, which determines the reserves. Furthermore, its criticality stems in particular from the concentration of producers, with China dominating the market with more than 60% of the share.
5.
Case Study on a Solar Heating Plant in Silkeborg, Denmark, European Commission, 2019, https://publications.europa.eu/resource/cellar/981d585d-c492-11e9-9d01-01aa75ed71a1.0001.01/DOC_1
6.
Renewable Heat: The Great Omitted Element of France’s Energy Strategy?, Carbone 4, 2022
7.
Example of the Neubrandenburg aquifer thermal storage project. December 2020 report by the Academy of Technologies on inter-seasonal storage.
8.
ADEME/BRGM: geothermies.fr - Deep Geothermal Technologies
9.
Cerema, FEDENE
10.
based on SYBASE - ADEME/BRGM data
11.
AFPG
13.
EurObservEr 2021 Heat Pump Barometer, pages 5 and 6
14.
EurObservEr 2024 Heat Pump Barometer, page 10
15.
The eligibility requirements for MaPrimeRénov’ were revised twice in 2024.
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