A Configuration of Storage System to Reduce Storage Capacity in DC Microgrids

Minghao Wang, Siew-Chong Tan, Shu-Yuen Ron Hui
The University of Hong Kong

Battery energy storage systems (BESS) are adopted to buffer the intermittent solar power generated from photovoltaics (PV) renewable power system. In such PV-battery configuration, a higher power rating of a PV power system requires a larger energy capacity of the BESS, which increases the cost of the overall infrastructure. In this project, a new combination of non-critical load (NCL), PV and series-type DC electric spring (series ES), which is called PV-embedded series-type DC electric spring (PVES), is proposed as a means for the storage capacity reduction in DC microgrids with PV installation. By shifting parts of the PV panels from the grid to the series-type ES, the PV-ES-NCL unit can be formed. The shifted part of the PV panels can help the series ES to output energy when it is operated to suppress the system’s voltage. Thus, the overall storage capacity can be significantly reduced. The time profiles of solar generation and load consumption are mathematically modeled to formulate the cost function required for minimizing the storage capacity requirement. The results show that the new formulation of PV-ES-NCL unit requires less storage capacity to buffer the intermittent solar power when compared with the traditional BESS (PV-battery unit). The findings of this project are included as follows.

1. We have investigated the operating modes of two kinds of storage configurations, namely (i) BESS and (ii) series-type ES. It is found that the series-type ES discharges a great amount of energy when suppressing the system voltage.
2. The mathematical models of solar power profile and load profile during a day have been built. The peak generation happens at noon, while the maximum load occurs at night. Such a mismatch between the load and generation will increase the required battery storage capacity.
3. An objective function used to minimize the storage capacity has been formulated for the PVES. It is found that the PVES can reduce the required battery storage capacity than the traditional BESS.