Context

The balance of production and consumption

In the current technological landscape, storing massive amounts of electricity is expensive and complex. However, we expect a safe supply of electricity at all times. Consequently, the transmission system operators in Europe have to maintain the balance between the power produced and the power consumed at all time across the interconnected European Power grid.

This balance is achieved through a sophisticated planning and control infrastructure. Electricity is traded on power markets one day in advance to devise a daily production plan. This plan determines the planned power production on time intervals of 15 minutes or one hour for the following day. After the day-ahead planning, the production plan is updated as a result of transactions between producers and consumers of electricity, up to one hour before delivery (currently one hour in Switzerland, but differs depending on countries). For example, energy for the time slot 2pm to 3pm can be traded until 1pm the same day. Like that, the market participants can adjust their plans based on new forecasts. As a result, most of the energy production and consumption is agreed upon on the energy markets by large consumers and producers.

However, fluctuations and contingencies happening either faster than 15 minutes or too close to delivery time (less than one hour before delivery) are handled through a direct control mechanism. To this effect, the transmission operator pays providers to maintain part of their production capacity available for direct control. This service is called frequency control and is subdivided into different categories, namely primary, secondary and tertiary frequency control [2]. The power capacity reserved for frequency control is sometimes referred to as power reserves. The cost of power reserves in Switzerland reaches 170 million CHF per year, a cost that represents 1.5% of the final price of electricity.

As most European countries, Switzerland has set ambitious goals in terms of renewable energy production. The total share of the energy production coming from renewable energy sources (excluding hydro dams) is set to increase from 4% to 16% in 2035 [1]. Due to the intermittent nature of the solar and wind production, this faces the transmission system operator with an increasingly difficult task of ensuring the balance between production and consumption, and leads most specialists to believe an increase in power reserves will be necessary to tackle this challenge [3].

Load-side management and Demand Response

The current operation of the power grid is such that the demand is practically incontrollable, or at least very inelastic. Load-side management are mechanisms that aim at increasing the responsivity of loads, so that the overall operation of the grid becomes smoother and more economically efficient. If the controllability of loads could be guaranteed, they could even be used as power reserves, offering a new alternative to the use of conventionnal power plants as reserve providers.

Definition of Demand Response according to FERC

Changes in electric usage by demand-side resources from their normal consumption patterns in response to changes in the price of electricity over time, or to incentive payments designed to induce lower electricity use at times of high wholesale market prices or when system reliability is jeopardized.

Another alternative is the use of storage systems, for example electric batteries as providers of power reserves. There are two main issues with that:

• Despite prices shrinking quickly, storage systems and electric batteries remain expensive.
• Depending on the type of reserve provided, very large storage capacities may be required. Storage systems cannot produce or consume net amounts of energy by nature, so the use of storage systems requires energy neutrality over time, that is no significant bias towards production or consumption of energy. This has been explicitly taken into account in the re-design of PJM's reserve market (see this link for details)

The concept of building battery

Our research focuses on control techniques that help scheduling loads in order to provide Demand Response or reserve power. One of the research direction of our laboratory is the exploration the potential of using buildings as electric storage systems.

It is easy to see that the thermal inertia of a building can be used to store a release thermal energy (See Fig. 1 below). If the heating/coolin system is powered with electricity, this directly translates into storing and releasing electric power.

To characterize how much energy can be stored in the building, it is useful to compare it to an electric battery. Our research studies methods to characterize storage capacity available in a building, in order for example to be able to compare a building and a battery.

References

[1] Switzerland Bundesamt fur Energie. La Stratégie énergétique 2050 après la votation finale au Parlement, 2015. Link

[2] Y. G. Rebours, D. S. Kirschen, M. Trotignon and S. Rossignol, A Survey of Frequency and Voltage Control Ancillary Services—Part I and II in IEEE Transactions on Power Systems, vol. 22, no. 1, pp. 358-366, Feb. 2007.

[3]Erik Ela, Michael Milligan, and Brendan Kirby. Operating reserves and variable generation. Technical report, 2011.