(240c) Dynamics and Control of Energy Utilization in Residential Homes Featuring Phase-Change Thermal Storage

Dynamics and Control of Energy Utilization in Residential Homes Featuring Phase-Change Thermal Storage

Cara Touretzky and Michael Baldea

McKetta Department of Chemical Engineering

University of Texas at Austin

The need to maintain occupant comfort in buildings accounts for about 60% of the electricity consumption in the United States [1]. Commercial and residential building consumption is also an important source of fluctuations in the daily load on the electric grid. Ongoing efforts concerned with developing and implementing the smart grid aim to improve the electricity production and transmission through the use of two-way communications between electricity users and producers, variable prices, and alternative energy sources and storage options, with the ultimate goal of reducing energy consumption and limiting the variability of demand during the day [2].  The optimal use of all of the elements of the smart grid necessitates the use of advanced control, both at the network level and at the level of individual users. In this presentation, we will focus on the use of energy storage for mitigating fluctuations in heating, ventilation and air conditioning (HVAC)-related energy consumption at the residential user level.

Specifically, we consider a dual-temperature thermal energy storage system based on phase-change materials (PCMs). The system comprises two components: i) a set of PCM elements incorporated in the exterior walls, and, ii), a cold-water storage tank filled with encapsulated PCM elements [3].The wall PCM acts as a buffer in the heat flow between the building and its environment [4]; in the particular case of a hot climate location (cooling environment), the PCM elements melt during the day and mitigate wall heating. On the other hand, the PCM elements in the water tank increase its thermal mass and its capacity to store refrigeration (chilled water) generated during off-peak hours [3].

Intuitively, these additions allow us to alter the energy utilization pattern of the building, e.g., by using the stored chilled water during the late afternoon, when temperatures and electricity prices are highest. However, in order to achieve substantial energy savings and the desired load-leveling effect, the charging and discharging of the storage system must be appropriately controlled using a model-based control system. However, buildings have a complex dynamic behavior that exhibits multiple time scales [5]. The presence of storage elements adds further complications, in the form of extended time constants and supplementary nonlinearity associated with melting/solidification phenomena. Thus, the development of model based control strategies for buildings with energy storage is particularly challenging.

In this paper, we focus on the development of such a model-based approach within the framework of nonlinear model predictive control (NMPC). Using model-reduction techniques introduced in [5], we develop a reduced-order model of the building dynamics that is suitable for controller design and is amenable to online implementation. We present a case study based on the Thermal Façade Lab, a single zone test building located on the UT Austin campus. We compare the energy consumption and cost savings achieved when using the proposed NMPC and a set of heuristics devised to schedule the charging and discharging times in a cooling season. The tradeoffs between the two control schemes are discussed, and the benefits of the proposed dual-temperature storage system are presented along with a set of guidelines for selecting the capacity of the storage system for optimal dynamic performance.

[1]       U.S. Energy Information Administration. (2012). Annual energy review 2011.

[2]       Siirola, J. J., & Edgar, T. F. (2012). Process energy systems: Control, economic, and sustainability objectives. Computers & Chemical Engineering, 47, 134–144.

[3]       Zhu, N., Ma, Z., & Wang, S. (2009). Dynamic characteristics and energy performance of buildings using phase change materials: A review. Energy Conversion and Management, 50(12), 3169–3181.

[4]       Kuznik, F., David, D., Johannes, K., & Roux, J. J. (2011). A review on phase change materials integrated in building walls. Renewable and Sustainable Energy Reviews, 15(1), 379–391.

[5]       Touretzky, C., & Baldea, M. (2013). Dynamics and Nonlinear MPC of Residential Buildings with Energy Recovery. J. Proc. Contr., submitted