Explaining Inefficiencies in Buildings Providing Ancillary Services

In a power system, having a high level of renewable energy penetration requires a large amount of ancillary services to balance the supply and demand in real time. Commercial buildings have significant thermal capacity and so offer large potential for providing such services. Recent research has shown that the power consumption of the Heating, Ventilation, and Air Conditioning (HVAC) system can be varied to track regulation signals arising from frequency regulation services, while maintaining a comfortable indoor climate. However, experimental results from a 30,000 m2 office building suggest that tracking zero-mean power variation signals may cause an increase in the HVAC energy consumption. This energy loss translates to additional costs and environmental impacts for commercial building HVAC systems that provide ancillary services. This paper investigates potential causes for this efficiency loss, and in particular considers the impact of thermal dynamics and control design. In the former case, we analyze a variety of factors including nonlinearity in the supply air fan and the heat exchange process between rooms and the ambient environment. The control design incorporates a nonlinear mapping between room thermostat set-point deviations and the resulting changes in the fan power. We investigate this design to ascertain its impact on efficiency and also consider interactions with the building automation system. The analyses in this paper help us better understand the mechanisms underlying efficiency loss when commercial building HVAC systems are used for ancillary services. These investigations underpin the design of controls that offer improved efficiency.