Design and Implementation of Frequency-responsive Thermostat Control

A power system’s AC frequency is a system-wide indicator of the immediate power balance in the system. This is used by selected generators to adjust the generation to level out imbalances continuously, as well as in the case of disturbances in the system. However, previous theoretical and empirical studies indicate that this service might as well be provided by demand-side control. In this paper we investigate in detail how thermal energy storage capacity in thermostatically controlled electric loads can be utilised for frequency-responsive control. We use two specific applications as cases for our work: Refrigerators and electric space heating. These two cases clearly illustrate the vast diversity of critical parameters like heat capacity, switching cycles, and temperature tolerance. Based on these, we design appropriate control algorithms that bridge the gap, between on the one hand, the unique properties and needs of each application, and on the other hand the requirements of the system operator. The control algorithms are implemented on a microcontroller unit that is interfaced with existing thermostats for each application. To validate the control algorithms and overall system design, a series of experiments are conducted, where the controller is subject to the actual grid frequency as well as designed frequency inputs, such as step inputs. The results demonstrate that frequency-responsive thermostats can indeed provide a wide range of the frequency-responsive ancillary services requested by the system operators, with little negative impact on device operation and performance. The design analyses and the implementation experience also show that the retro-fitting approach to frequency response limits the ability for some devices to provide these services, compared with integrating the frequency response when designing the thermostat.