1. Definition of Energy Storage Power Station System Efficiency

Comprehensive Efficiency of Energy Storage Power Station
According to the standard GBT 36549-2018 "Performance Index and Evaluation of Electrochemical Energy Storage Power Stations," the comprehensive efficiency of an energy storage power station is defined as the ratio of the electricity delivered to the grid to the electricity received from the grid during the evaluation period. This is measured at the metering point between the energy storage power station and the grid, calculated as the total energy delivered to the grid divided by the total energy received from the grid during the evaluation period.

Energy Storage Device Efficiency

According to GB/T 51437-2021 "Design Standards for Wind-Solar-Storage Combined Power Stations," the efficiency of an energy storage device should be calculated based on battery efficiency, power conversion system (PCS) efficiency, power line efficiency, and transformer efficiency, as shown in the following formula:
Φ = Φ₁ × Φ₂ × Φ₃ × Φ₄

· Φ₁: Battery efficiency, which is the ratio of the energy discharged by the battery to the energy charged into the battery during a charge-discharge cycle. For storage batteries with a 1C rate, the round-trip efficiency is no less than 92%, and for a 0.5C rate, the round-trip efficiency is no less than 94%.

· Φ₂: Power conversion system efficiency, including rectifier efficiency and inverter efficiency; typically, it is around 98.5% (one-way).

· Φ₃: Power line efficiency, considering the bidirectional transmission losses in AC and DC cables.

· Φ₄: Transformer efficiency, considering the bidirectional loss in transformer conversions.

2. Auxiliary System Losses in Energy Storage Power Stations

As a complete system, energy storage power stations rely on numerous auxiliary devices to ensure safe and stable operation. These auxiliary systems include integrated power supply systems, lighting systems, security systems, fire alarm systems, environmental systems, HVAC systems, and automation systems, among others. These systems consume electricity and contribute significantly to the total energy consumption of the station.

Energy storage systems may be in operation or in standby mode. For storage stations participating in peak-shaving and valley-filling, if the operating strategy is one charge-discharge cycle per day, with a charge-discharge rate of 0.5C, the system is in operation for 2 hours, and the rest of the time it remains in standby mode. In the operational state, auxiliary systems, especially the HVAC system, are active, while in standby mode, they are either off or occasionally active.

The main auxiliary equipment consuming energy is typically housed in the battery container, and the key energy-consuming device is the industrial air conditioner. The air conditioner is crucial for thermal management during system operation, maintaining the battery's optimal working temperature. The power consumption of the auxiliary equipment is influenced by operational strategies and seasonal variations. For example, in summer, air conditioners work harder due to higher outdoor temperatures, while in winter, although cooling is more efficient, heating is required during system downtime to maintain battery temperature.

3. Case Study

System Overview and Losses
A battery container with a configuration of 2MW/2MWh is used, with the main power-consuming devices being air conditioners, battery management systems (BMS), fans, and lighting. The system participates in grid peak-shaving and valley-filling with a 1C charge-discharge cycle. The container is equipped with two air conditioners, each with a maximum cooling power of 17.5 kW (35 kW in total) and a maximum heating power of 15 kW (30 kW in total). In recirculation mode, the power consumption of a single air conditioner is 2 kW, with both consuming 4 kW. Other power-consuming equipment, such as the BMS and fans, has a maximum total power consumption of about 5 kW.

(1) Auxiliary System Losses
Based on field tests under a 1C operating condition for one complete charge-discharge cycle in summer, the air conditioners run in cooling mode for about 3 hours, consuming 3h × 35kW = 105kWh. For the remaining time, the air conditioners run in recirculation mode, consuming 21h × 4kW = 84kWh, totaling 189kWh. The other power-consuming devices operate at less than full capacity, with a simultaneous coefficient of 0.5, consuming about 5kW × 24h × 0.5 = 60kWh per day.
Thus, the total power consumption of the auxiliary equipment in the battery container during summer is approximately 249kWh per day.

(2) Power Line Efficiency

There is energy loss due to heat in both AC and DC cables when current passes through. The DC-side efficiency is approximately 99.83%, the PCS AC-side efficiency (transformer low-voltage side) is about 99.95%, and the high-voltage AC-side efficiency is around 99.89%. Considering one-way losses, the overall power line efficiency is 99.67%; considering two-way losses, the power line efficiency is 99.34%.

(3) Transformer Efficiency

For common dry-type transformers, according to GB/T 10228-2015 "Technical Parameters and Requirements for Dry-Type Power Transformers," the losses of a 35kV 2000kVA transformer are as follows:

· No-load loss: 4.23kW

· Load loss: 17.2kW (at 100℃)
At rated power, the transformer efficiency is:
(2000 - 4.23 - 17.2) / 2000 = 98.93%,
so the two-way transformer efficiency is:
98.93% × 98.93% = 97.87%.

4. Efficiency Calculation

When calculating the various efficiencies of an energy storage station, the direction of energy flow must be considered, as auxiliary system consumption is treated as a load loss during both charging and discharging. The efficiency calculation should be based on the appropriate application of one-way or two-way efficiency, as defined in the standards.

(1) Charging Efficiency
Assuming the battery system's state of charge (SOC) is consistent, and the depth of discharge is 90%, the required initial energy on the AC side to fully charge a 2MWh system in one hour is:
Initial AC-side charging energy = (Rated capacity × Depth of discharge) ÷ Battery system charging efficiency ÷ PCS rectifier efficiency ÷ Transformer efficiency ÷ Power line efficiency + Auxiliary power consumption (assuming full load operation for 1 hour during charging)
= 2000 × 0.9 ÷ 95.92% ÷ 98.5% ÷ 98.93% ÷ 99.67% + (35 + 5) × 1 = 1972.12kWh.
Therefore, the AC-side charging efficiency of the energy storage system is:
Charging efficiency = (2000 × 0.9) ÷ 1972.12 = 91.27%.

(2) Discharging Efficiency

Initial AC-side discharging energy = (Rated capacity × Depth of discharge) × Battery system discharging efficiency × PCS inverter efficiency × Transformer efficiency × Power line efficiency - Auxiliary power consumption (assuming full load operation for 1 hour during discharging)
= 2000 × 0.9 × 95.92% × 98.5% × 98.93% × 99.67% - (35 + 5) × 1 = 1636.91kWh.
Therefore, the AC-side discharging efficiency of the energy storage system is:
Discharging efficiency = 1636.91 ÷ (2000 × 0.9) = 90.94%.

(3) Energy Storage Device Efficiency
Based on the previous formula for energy storage device efficiency, the device efficiency is:
Φ = Φ₁ × Φ₂ × Φ₃ × Φ₄ = 92% × 97.02% × 99.34% × 97.87% = 86.78%.

(4) Comprehensive Efficiency of the Power Station

Assuming the evaluation period is one full charge-discharge cycle (charging for 1 hour and discharging for 1 hour), the comprehensive efficiency for one cycle is:
Comprehensive efficiency = Discharged energy per cycle ÷ Charged energy per cycle
= 1636.91 ÷ 1972.12 = 83.00%.

If the evaluation period is one day, with one charge-discharge cycle per day (charging for 1 hour, discharging for 1 hour, and 22 hours in standby), the daily comprehensive efficiency of the station is:
Daily comprehensive efficiency = Daily discharged energy ÷ Daily charged energy
= 1636.91 ÷ (1972.12 + 249 - 40 × 2) = 76.45%.