A need for more flexible energy systems
As well introduced by Jesus in his article of January, the electrical grids are complex systems that must be balanced at every moment, so that the energy being produced is consumed instantly. This task proved to be relatively easy when power grids were supplied only by conventional plants (nuclear or coal for example): their electrical output could easily be adjusted to our energy needs.
Nowadays, the need for decarbonizing our energy systems is becoming more and more evident every day (except maybe for the new US administration), therefore we have to integrate more renewable energy sources, such as wind and solar power. It is obviously good news that the EU amongst other countries pushes towards the development of such cleaner energies. However, we cannot tell the wind to blow or the sun to shine when we need, this kind of energy is there when it is there. Therefore if we rely on a large share of these renewables in our energy mix, we expose ourselves to possible discrepancies between the electricity production and demand. It challenges us researchers to design the smart grids of the future so that we improve this matching, and make use of as much renewable energy as possible.
To avoid imbalances in the grid, several solutions exist, such as curtailment of renewable sources or rotational load shedding for example, but none of them is ideal. We will here focus on a set of solutions grouped under the name “demand-side management”, or DSM. They stem from the simple observation that a paradigm shift is needed: if the energy production cannot anymore adapt to our demand, then our demand will have to adapt to the variable production from renewables. DSM covers a wide range of solutions, and can apply to various areas of electrical engineering or energy storage engineering. As a building engineer, my focus resides more in how buildings can become active elements in DSM programmes, and which services they can bring to the energy grids through their flexibility.
Energy flexibility in buildings
Buildings represent good candidates for DSM programmes for several reasons. First of all, they account for around 33% of the world’s final energy use . Let’s imagine that all the buildings would be used to provide energy flexibility: that would represent a great potential. Secondly, buildings possess a built-in thermal storage that is usually not exploited: their own thermal mass. The building envelope, walls, internal furniture etc. all constitute a certain amount of thermal capacity, which means they can retain heat – or cold – for a certain amount of time. By charging or discharging this already existing storage (or adding extra storage in the form of water tanks for instance), the flexibility can easily be activated in a building. We only consider in this introduction the flexibility potential of building thermal loads – heating and cooling, but other types of loads can be made flexible too, such as smart appliances, electric vehicle charging, or ventilation.
The International Energy Agency (IEA) and its programme for Energy in Buildings and Communities (EBC) has well understood the interest for further research in this field. Earlier, they had already created international-joint projects (Annex 52 and Task 40) aiming to study Net Zero Energy Buildings (NZEBs); the objective targeted by a NZEB being to reach a zero energy balance over the course of one year [2,3]. More recently, they opened another Annex project (number 67) to assess the potential and feasibility of energy flexibility in buildings [4,5], which is a logical continuation of this research. In this framework, the following definition was proposed: “the energy flexibility of a building is the ability to manage its demand and generation according to local climate conditions, user needs and grid requirements”. The user needs and local climate conditions have long been considered by building designers, but the grid requirements here constitute the new component. This evolution is only natural, in a world where all the systems are ever more interconnected. Buildings are thus meant to play a more important role as active stakeholders in the future smart grids.
In practice, how does it work to activate energy flexibility in a building?
Let’s take the case of space heating as an example. Figure 1 presents the general concepts. When for instance the sun is shining and the wind is blowing, a lot of wind and solar power (renewables) is generated, and we can call these conditions “favorable”. In this case, we will “charge the built-in thermal storage”, which basically means that we will heat the indoor space a little more than usual, storing this energy for later.
A few hours later, the night has dropped and the wind has stopped, reducing the amount of renewable energy available; let’s call these conditions “unfavorable”. In that case, we will rather “discharge the built-in thermal storage”. In other words, we will decrease the indoor temperature, preventing our heating system to use energy during this period. Instead, the building will rely on its own thermal inertia to provide a comfortable indoor environment, using the heat previously stored in the indoor space, until favorable conditions appear again.
In this way, we can shift part of the heating demand from “unfavorable” periods to more “favorable” ones. This is only one example of energy flexibility implementation, there exist many others. My Individual Research Project focuses more specifically on how to transform this concept into concrete control strategies for heat pumps, aiming at improving the flexibility of heating or cooling loads.
And what about the users in all that?
You might wonder at this point, which benefits the end-users might get out of this process. The utility company sure has something to gain out of this, but what about you or me, the consumer? Why would you let the systems in your house change the indoor temperature according to some random schedule depending on wind and solar power availability? You might say “I would rather be in control, and keep the indoor temperature that I want!”
You are probably right. As designers of such strategies, we take the users’ feeling into account, and we try to maintain comfort while playing with the indoor temperature. This is done through implementing comfort constraints: for example the control strategy can only play within the range 20-22°C in winter and 23-26°C in summer. We can also consider that the variations will be hardly noticeable if they have low amplitude (±1°C for instance) and are realized over a long period (a few hours). Another aspect is to take into account occupancy and enable larger variations when users are not present, so they are not bothered.
Monetary incentives are also a way to provide advantages to the users, and thus increase their acceptance of such control strategies. By using variable electricity tariffs, the electricity company INCITEs (ba dum tsss!) users to consume electricity at certain hours, and to refrain from doing so at other times. The construction of this price profile can recreate the “favorable/unfavorable” conditions by setting low/high prices on the energy use. Using prices varying hourly will become more and more common in smart grids’ contexts, and with the implementation of smart meters.
Energy flexibility probably represents the future of green buildings . The current trend has been to reduce more and more the energy use of our buildings, until reaching “zero-energy” or even “plus-energy” targets. The next step will consist in integrating this kind of buildings into the smart grids of the future. The buildings will need to become active players in the energy grids, enabling their thermal energy storage for supporting the grid operation. Aggregated at a larger scale, this process can eventually facilitate the integration of variable renewable energies, thus reducing the environmental impact of our electricity use.
References REN21 (2016). Renewables 2016 Global Status Report. Paris: REN21 Secretariat. ISBN 978-3-9818107-0-7. Available online: http://www.ren21.net/status-of-renewables/global-status-report/  IEA EBC Annex 52 website: http://www.iea-ebc.org/projects/completed-projects/ebc-annex-52/  IEA SHC Task 40 website: http://task40.iea-shc.org/  IEA EBC Annex 67 website: http://annex67.org/  Marszal-Pomaniowska A.J. and Østergaard Jensen S. (2016). IEA EBC Annex 67 Energy Flexible Buildings. CLIMA 2016 - proceedings of the 12th REHVA World Congress: volume 10. Aalborg: Aalborg University, Department of Civil Engineering.  D’Angiolella R., De Groote M. and Fabbri M. (2016). NZEB 2.0: interactive players in an evolving energy system. In REHVA Journal, May 2016. http://bpie.eu/news/nzeb-2-0-interactive-players-in-an-evolving-energy-system/