Researchers from the University of California, Berkeley, have shed light on a significant factor affecting the efficiency of commercial building HVAC systems when used for grid flexibility, often referred to as “virtual batteries.” Their findings, published in the journal Applied Energy, highlight the impact of incomplete air mixing on the performance of these systems.
Commercial buildings equipped with HVAC systems can provide valuable flexibility to the electricity grid by adjusting their energy consumption based on grid demands. This concept is often modeled as a “virtual battery,” aligning with the terminology used by grid planners and operators. However, real-world experiments have shown that shifting HVAC loads can be inefficient, and current virtual battery models do not adequately capture this inefficiency.
The researchers developed a new analytical building model to explore how incomplete mixing of supply air into a conditioned space leads to inefficiencies in HVAC load shifting. Unlike previous models that use the average room temperature as the system’s “state of charge,” this new model considers factors such as airflow and mixing. The model qualitatively matches experimental results better than previous models, demonstrating that as air mixing worsens, the virtual battery becomes less efficient.
One of the challenges identified by the researchers is that air mixing is often unmeasured or unmeasurable. However, they found a solution by implementing a feedback loop around measurements of fan power. This approach improved the virtual battery’s performance without the need for direct air mixing measurements. In one case, they demonstrated a roundtrip efficiency improvement from 0.75 to 0.99.
For the energy sector, this research underscores the importance of considering air mixing dynamics in HVAC systems when planning for grid flexibility. By improving the accuracy of virtual battery models, grid operators can better integrate HVAC systems into the grid, enhancing overall efficiency and reliability. Practical applications include more effective demand response strategies and improved energy management in commercial buildings.
Source: Applied Energy, Volume 333, Part B, February 1, 2023
This article is based on research available at arXiv.