Optimised Solar PV-BESS Sizing for Maximum Demand Reduction
Written by Gopinath Subramani and Vigna K. Ramachandaramurthy
Maximum Demand (MD) charges were established to encourage the commercial and industrial users to alter their electricity use pattern to decrease the peak demand and lower the requirement for costly peaking plants. Many energy users are opting for solar photovoltaic (PV) system to be installed on their building’s rooftop for self-consumption purposes, thus reducing the electricity bill. However, atmospheric conditions such as temperature, and irradiance influence the solar PV power. Hence, it is likely that the peak solar PV output does not coincide with the instant of maximum demand. This is where batteries can play a crucial role. So, what should be the threshold above which the maximum demand will be shaved to ensure the highest Return-of-Investment (ROI)?
The optimal threshold for the maximum demand will allow commercial and industrial users to prudently invest on techno-economically sized solar PV and battery energy storage system (BESS). The determination of the optimal threshold and techno-economically sized solar PV-BESS can be complicated due to the varying load pattern and the intermittent behaviour of solar PV power output. Moreover, batteries are relatively expensive.
In this article, the Maximum Demand Reduction (MDRed) model is shown as an effective tool for the optimization of the MD threshold and solar PV-BESS sizing. The MD and net consumption reduction due to solar PV-BESS is shown in Figure 1. The optimized MDRed model is based on the mathematical formulation of the MD charges, energy consumed and the cost of solar PV modules, inverter, and batteries. Subsequently, the financial model focuses on the overall cost of solar PV-BESS in comparison to the energy/demand savings for fastest Return-On-Investment (ROI).
Figure 1: Conceptual graph of the MDRed model optimization
The amount of power that can be shaved depends on the battery capacity, the discharge rate, and other specifications. However, MD shaving may fail if the energy stored in batteries is inadequate for the desired period during peak demand. Thus, the size of the inverter must be considered together with the batteries capacity in obtaining the optimal system cost. The Depth of Discharge (DOD) substantially affects the battery service life. To accurately estimate the investment’s payback period, the DOD must also be considered in the cost estimation. Battery charging and discharging operation are based on the dynamic electricity pricing strategies for MD reduction and monetary expense reduction.
From an operational standpoint, the MDRed scheme will keep monitoring the building/facility’s MD limit and net load after load reduction from the solar PV. If the energy user load is about to exceed the MD limit, the battery discharging mode will be activated to offset the required load demand. Once the net load drops below the MD limit, the battery discharging mode will automatically stop. This process will take place throughout the day during MD charges period. Additionally, the battery charging mode will be activated during non-MD charges period. The battery charging mode will be automatically stopped once the battery is fully charged, as per the battery charging specifications.
For comparison purpose, the optimized results and ROI for solar PV-BESS was compared with different scenarios where only solar PV or BESS were used. Results indicated that the solar PV-BESS resulted in higher ROI compared to the other two scenarios. In summary, the determination of the optimized threshold and techno-economic sizing for solar PV-BESS can help commercial and industrial loads to reduce their monthly electricity bill.
This article edited by Jose Medina
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