Distributed scheduling for efficient HVAC precooling operations

Distributed Scheduling for Eﬃcient HVAC Pre-cooling Operations ⋆ Yang Su ∗ Rong Su ∗ Kameshwar Poolla EXQUISITUS, Centre of E-City, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Email: suyang, rsu@ntu.edu. sg Mechanical Engineering Department, University of California, Berkeley, 5141 Etcheverry Hall, Mailstop 1740, Berkeley, CA 94720-1740, USA, Email: poolla@me.berkeley. edu Abstract: Supply fan takes up to 30% of Heating, Ventilation and Air Conditioning (HVAC) systems’ in-building energy consumption. Since the fan power consumption is a cube function of the mass ﬂow rate of supply air, it is thus possible to reduce the energy consumption signiﬁcantly by elongating and scheduling the pre-cooling processes. To handle the computational complexity raised for large scale buildings, two distributed consensus-like scheduling algorithms are proposed in this paper. Simulation studies indicate higher energy eﬃciency compared to uncoordinated pre-cooling processes; the computation is faster than centralized optimization, although with performance sacriﬁce. Keywords: HVAC; Pre-cooling; Distributed scheduling. 1. INTRODUCTION HVAC systems consume 40% of the building energy in USA as estimated by Department of Energy. The eﬃciency of HVAC system design, operation and control has received extensive research attention for the purposes of resource conservation, cost reduction and etc. Through energy disaggregation analysis for variable air volume (VAV) systems (Englander [1990]), it was found that a large portion of energy use in HVAC systems is attributed to air movement devices. Therefore, an accurate estimation of fan power model and eﬃcient operation of fans are critical for reducing energy consumptions. The problem of energy consumption saving of the supply fan is studied in this paper. As reported in the literature, this problem can be discussed from diﬀerent perspectives. Firstly, a good fan controller, regardless of the control structure and controlled variables, which produces more stable system performance is foundation for comfort guar- antee and fan’s routine operation (Roger [2005]). Secondly, it was noticed that energy is wasted at the terminal dampers or economizer dampers. The static pressure and correspondingly the fan power will not be unnecessar- ily high if these damper positions are chosen properly (Englander [1990]Nassif [2010]). Here another angle from temporal perspective is adopted. The cooling or heating tasks are properly scheduled along the time line such that the total energy is reduced. This approach is motivated by the fan’s power function, which is proportional to the ⋆ This research is funded by the Republic of Singapore’s National Research Foundation through a grant to the Berkeley Education Alliance for Research in Singapore (BEARS) for the Singapore-Berkeley Building Eﬃciency and Sustainability in the Tropics (Sin-BerBEST) Program. BEARS has been established by the University of California, Berkeley as a center for intellectual excellence in research and education in Singapore. cube of the mass ﬂow rate. Rooms have to be cooled down to predeﬁned temperature setpoints before occupied according to the room occupancy schedule. This process is called pre-cooling. By elongating pre-cooling with a lower supply air mass ﬂow rate, the energy consumption can be signiﬁcantly reduced. Pre-cooling strategies, in its various forms, have been proposed in the literature. The potential to reduce peak- period electrical demand by adjusting HVAC controls was validated in Xu [2004]. Morgan [2007] explored the eﬀectiveness of diﬀerent pre-cooling strategies in diﬀerent locations. Optimization based pre-cooling strategies were investigated in Yin [2010]. All these proposals achieve either energy savings or peak cost reduction via pre-cooling the thermal masses. However, it is common that groups of rooms require pre-cooling at similar times, for example in the morning around 08 : 00AM. If individual rooms operate inde- pendently without a central coordinator, an aggregated energy demand peak is often observed. Thus a pre-cooling scheduler, which coordinates the pre-cooling processes for all rooms supplied by a single supply fan, may pro- vide potential energy saving. In fact, optimization based scheduling for HVAC systems has shown great potential for energy conservation and cost reduction. An approach called ‘Green Scheduling’ was proposed in Nghiem [2011] to reduce the peak demand. The approach was further extended and successfully applied to chiller plants and radiant systems in buildings (Behl [2012]Nghiem [2013]). An optimal scheduling of the pre-cooling processes was proposed in previous study Nikitha [2014]. Diﬀerent with the Xu [2004]Yin [2010]Morgan [2007], which either con- sider assignment of temperature setpoint and chiller per- formance or develop uniform pre-cooling strategy for the