  This paper studies the optimal control of a commercial building's
thermostatic load during off-peak hours as an ancillary service to the
transmission system operator of a power grid. It provides an algorithmic
framework which commercial buildings can implement to cost-effectively increase
their electricity demand at night while they are unoccupied, instead of using
standard inflexible setpoint control. Consequently, there is minimal or no
impact on user comfort, while the building manager gains an additional income
stream from providing the ancillary service, and can benefit further by
pre-conditioning the building for later periods. The framework helps determine
the amount of flexibility that should be offered for the service, and cost
optimized profiles for electricity usage when delivering the service. Numerical
results show that there can be an economic incentive to participate even if the
payment rate for the ancillary service is less than the price of electricity.
