Buildings: A sector particularly well suited to the Energy Transition

  ⁽¹⁾Assistant Professor, Faculty of Engineering of the University of Porto & Researcher, LAETEA & Affiliated Researcher, INESC TEC

An important sector

Buildings account for around 27% of the consumption of final energy in Portugal (IEA, 2022), and 42% in the European Union (EUROSTAT, 2022). In the European Union, they are the first consumer sector, while in Portugal they are the second, along with Industry and behind transportation. The milder climate in winter, less tradition in the use of generalised heating at home, and a particularly high car ownership are the likely explanations for these Portugal vs EU differences.

In terms of electricity, the current share of the buildings sector in Portugal is 62% of the total. Since 2000, the consumption grew by 32% in the residential buildings subsector, and 46% in the services buildings subsector (Figure 1).

Hence, it is a sector that is already very relevant, due to the weight it represents in consumption - whether of final energy or electricity. But it also seems a sector particularly suited to the energy transition, for the reasons explained below. Let's start with a summary of the main features of the energy system⁽²⁾ that are expected to be necessary for the transition to a more sustainable paradigm:

• 1.Efficiency in energy use;


• 2. Electrification of energy uses, whenever this is compatible with the intended services;


• 3. Production of electricity from renewable sources, preferably close to the place of use;


• 4. Storage capacity and time flexibility when using energy.


• 5. Use of green hydrogen and/or synthetic fuels (SF) when it is not possible to obtain the services required only through properties 1-4

Feature 5 should be perceived as a last resource solution, and not as an alternative of equal merit compared to the 2-3-4 set, since it is an option that will always be much less efficient, in a well to wheel analysis. This aspect has often been overlooked, but it becomes clear when one considers that the efficiencies in the production and use of hydrogen / SF are much lower than those associated with the direct use of electricity.

Let us analyse the positioning of the buildings sector in relation to each of the above key features.

Efficient use of energy

The main uses of energy in buildings are for the control of indoor temperature (with a clear predominance of heating in OECD countries), heating water for sanitary uses, domestic appliances, lighting (in services buildings) and electronic equipment. With regard to heating and cooling, there is ample scope for reducing needs, through improved insulation and solar protection, and more adequate control of ventilation; and as far as lighting is concerned, there is still a very large margin for better controls, switching off where and when not needed (the first rule of energy efficiency!).

Electrification of uses, and production of electricity from renewable sources

Practically all the energy services required in buildings can be provided through electrical energy, so there are no significant obstacles here. Traditionally, heating by Joule effect (air and water) was not recommended, as it is less efficient in terms of primary energy than the use of gas; however, the evolution in heat pump technologies and the greater availability of electricity from renewable sources make it possible to obviate this inconvenience. Also, the local production of electricity often finds advantageous conditions in buildings– at least, and since batteries are expensive, in the service buildings sector, where there is greater synchronisation between production and demand. A (re)evolution is taking place with the installation of photovoltaic panels on rooftops and facades. Solar thermal panels for water heating, if correctly installed and maintained, are also a mature and still sensible technology.

Energy storage and time of use flexibility

While storage of electrical energy is “difficult” (or, at least for now, expensive), heat storage is relatively easy and inexpensive. Combining the existence of thermal mass and a well thermally insulated envelope, it is easy to store heat for several hours or days. Many of us already have this experience in heating water in electric cylinders with a bi-hourly rate, in which the resistance only works during the night.

There is a big potential for temporal displacement of loads also in heating and cooling (load shifting, in the language of demand-side management). However, this requires a considerable interior thermal mass at buildings, , in addition to proper insulation. Unfortunately, the trend of the last few decades towards fast and cheap construction has led to the adoption of low mass solutions with little thermal inertia. However, there are still many older buildings with significant thermal mass. And retrofitting solutions for low inertia could be developed, using phase change materials (PCM) to increase the thermal mass of newer buildings (Leal and Almeida, 2021). Moreover, in refrigerators and freezers, solutions with PCM could be developed to create load shedding capacity for a few hours.

Use of green hydrogen and/or synthetic fuels

Unlike in Industry and transportation, in the case of buildings, nearly all energy services can be supplied through electricity. Nevertheless, the production and storage of hydrogen may be an appealing solution to use it for surplus electricity production during the day, and/or for mid-term storage. The economic viability of this solution is not yet clear; one may, however, observe that there are no significant technical obstacles to its adoption in some types of buildings (the larger ones).

Need for Research and Development

In the buildings sector, there are already many solutions, technically mature and with increasing economic attractiveness, which can be applied and produce very significant advances towards decarbonisation. The main challenge in this field is the training of the professionals in the design and construction, as well as the creation, of regulatory instruments necessary for the transformation of the market.

There are, however, areas where progress could be greater and/or faster using innovative solutions derived from R&D. Some areas ate particularly in need: solutions to increase the thermal mass of the compartments; refrigeration equipment with load shifting capability; integrated lighting and solar protection solutions.

In addition to the technologies “per se”, the creation of tools for analysing solutions and supporting the identification of the most technically and economically appropriate solutions is also of the utmost importance. Too often architects and designers quickly focus on a solution when it would be advantageous to consider multiple options before focusing on a specific solution. Tools to assist properties identification and improvement in buildings are equally important.

In the buildings sector, there are already many solutions, technically mature and with increasing economic attractiveness, which can be applied and produce very significant advances towards decarbonisation. The main challenge in this field is the training of the professionals in the design and construction, as well as the creation, of regulatory instruments necessary for the transformation of the market.

Portuguese specificities

It is known that, in Portugal, indoor temperatures during winter are below the 18-20ºC generally recognised as the recommendation for thermal comfort. A monitoring of about 160 households of high school students in four municipalities in the northern region of showed that less than 1/3 were in the comfort range, and that parts of the population even register temperatures around 12ºC (figure 2). Energy rehabilitation in these cases tends to cause smaller energy savings than those calculated by theoretical models, because of the effect known as rebound effect. This does not mean that it is not necessary: it is a question of quality of life.

Moreover, the fact that we have a significant daily temperature amplitude, where in winter the daily maximum temperature is usually above 10ºC, creates particularly favourable conditions for air-air heat pumps, in conditions of efficient and lower cost than air-to-water pumps that are being adopted in many countries in central and northern Europe.