1. Efficiency definition of energy storage power station system
Comprehensive efficiency of power station
According to GBT 36549-2018 "Operation Indicators and Evaluation of Electrochemical Energy Storage Power Stations", the comprehensive efficiency of energy storage power stations should be the ratio of the on grid electricity and off grid electricity during the production and operation process of the energy storage power station during the evaluation period, that is, the total amount of electricity transmitted from the energy storage power station to the grid by the gateway meter between the energy storage power station and the grid during the evaluation period/the total amount of electricity received by the energy storage power station from the grid.
Efficiency of energy storage devices
According to GB/T 51437-2021 "Design Standards for Wind, Solar and Energy Storage Combined Power Stations":
The efficiency of energy storage devices should be calculated based on factors such as battery efficiency, power conversion system efficiency, power line efficiency, and transformer efficiency using the following formula:
Φ=Φ1×Φ2×Φ3×Φ4
Φ 1: Battery efficiency, the efficiency of energy storage batteries completing charge and discharge cycles, which is the ratio of the amount of electricity discharged by the battery body to the amount of electricity charged. According to the technical performance of energy storage batteries, the charge discharge conversion efficiency of the battery is not less than 92% (bidirectional) at 1C rate, and not less than 94% (bidirectional) at 0.5C rate;
Φ 2: Efficiency of power conversion system, including rectification efficiency and inverter efficiency; According to the market PCS production situation, 98.5% (one-way) is generally taken;
Φ 3: Efficiency of power lines, considering the efficiency after bidirectional transmission loss of AC/DC cables;
Φ 4: Transformer efficiency, taking into account the efficiency after considering the bidirectional transformation loss of the transformer.
2. Loss of auxiliary systems in energy storage power stations
As a whole that realizes certain functions, energy storage power stations rely on a large number of auxiliary equipment to ensure the safe and stable operation of the energy storage system during operation, such as integrated power systems, lighting systems, security systems, fire alarm systems, environmental systems, HVAC systems, automation systems, etc. These systems serve as auxiliary systems for energy storage power plants to ensure their reliable operation, therefore the power consumption of auxiliary equipment also accounts for a significant proportion of the total energy consumption of the energy storage power plant.
The energy storage system may be in operation or not in operation (standby state). For energy storage power stations participating in grid peak shaving and valley filling, if the operation strategy is to complete one charge and one discharge per day with a charge discharge rate of 0.5C, the energy storage system will be in operation during the charge discharge state (2h), and not in operation during the rest of the time. Regarding the operating status, the operating status of its auxiliary equipment is different from that in the non operating state. The main difference is that the HVAC system is turned on in the operating state and not turned on or occasionally turned on in the non operating state.
The main auxiliary equipment of the energy storage system consumes power in the battery prefabricated compartment, and the main power consuming equipment is industrial air conditioning. Industrial air conditioning, as a key thermal management equipment for battery prefabricated compartments, is an essential device during the operation of energy storage systems. It is mainly used to maintain the operating temperature of energy storage equipment and ensure the optimal performance of energy storage cells. The power consumption of auxiliary equipment is mainly related to operational strategies, seasons, and other factors. The air conditioning of the battery prefabricated compartment is mainly fully turned on when the energy storage system is in operation. When it is not in operation, the internal circulation air outlet is usually turned on, without cooling, and the power consumption is not high. Therefore, the daily work strategy has a significant impact on the power consumption of the air conditioner. With one charge and one discharge per day, the air conditioner runs for about 2 hours per day. With two charges and two discharges, the air conditioner runs for about 4 hours.
Different seasons also have a significant impact on the power consumption of air conditioning. The cooling capacity of an air conditioner is also related to the temperature of the outdoor environment. When the ambient temperature is high in summer, the cooling effect is poor, so the working hours will be extended. In winter, although the ambient temperature is low and the cooling effect is good, the cooling working time of the energy storage system is shorter than other seasons. However, when the energy storage is not running, the heating function still needs to be activated to ensure the working temperature of the energy storage battery cells. Therefore, the power consumption is relatively high in winter and summer.
3. Case analysis
System Overview and Losses
The configuration scale of a certain energy storage battery compartment is 2MW/2MWh, and the main power consuming equipment includes air conditioning, battery management system (BMS), fans, lighting, etc. The operation mode of the energy storage system is to participate in peak shaving and valley filling of the power grid, and the operating condition is 1C charging and discharging, with one cycle. Configure 2 air conditioning units, with a maximum cooling power of 17.5kW for each unit, totaling 35kW for 2 units. The maximum heating power for each unit is 15kW, totaling 30kW for 2 units. When the air conditioner operates in an internal circulation mode, the power consumption of a single air conditioner is 2kW, and the total power consumption of two air conditioners is 4kW. Other electrical devices include battery management systems (BMS), fans (installed in each battery module), lighting fixtures, etc., with a maximum power supply capacity of approximately 5kW.
(1) Loss of auxiliary system
According to the on-site test results, perform one complete charge and discharge cycle under 1C operating conditions. For summer scenarios, the air conditioner needs to operate in a cooling mode for about 3 hours, with a power consumption of 3 hours x 35 kW=105 kWh. The rest of the time is in an internal cycle mode, with a power consumption of 21 hours x 4 kW=84 kWh, totaling 189 kWh. Considering that other electrical equipment will not operate at full power at the same time for most of the time, if the simultaneous factor is considered as 0.5, the daily power consumption of other electrical equipment is approximately 5kW × 24h × 0.5=60kWh.
It can be seen that according to the on-site test results and the power consumption of other electrical equipment, in the summer scenario, assuming the operating mode and operating conditions (participating in grid peak shaving, 1C charging and discharging, and 1 charging and discharging cycle), the daily power consumption of the air conditioning and other electrical equipment in the energy storage battery compartment is about 249 kWh.
(2) Power line efficiency
When DC and AC cables pass current, they generate heat loss. The unidirectional efficiency of the DC side is about 99.83%, the unidirectional efficiency of the PCS AC side transformer low voltage side is about 99.95%, and the unidirectional efficiency of the high voltage AC side is about 99.89%. Considering unidirectional loss, the power line efficiency is 99.67%; Considering bidirectional losses, the power line efficiency is 99.34%.
(3) Transformer efficiency
The commonly used dry-type transformers in the project, according to GB/T 10228-2015 "Technical Parameters and Requirements for Dry type Power Transformers", have the following loss indicators for 35kV 2000kVA non excited voltage regulating power transformers:
No load loss: 4.23kW;
Load loss: 17.2kW (100 ℃);
At rated power operation, the transformer efficiency is (2000-4.23-17.2) ÷ 2000=98.93%, so the bidirectional efficiency of the transformer is 98.93% × 98.93%=97.87%.
Efficiency statistics
When calculating the efficiency of energy storage power stations, attention should be paid to the direction of energy flow, and the auxiliary system's electricity consumption should be considered as load loss during both charging and discharging processes. When calculating the efficiency of energy storage systems, it is necessary to combine standard definitions to determine whether the calculation application is bidirectional efficiency or unidirectional efficiency. The efficiency statistics of the above models are as follows:
| Number | Efficiency composition | Bidirectional efficiency | Unidirectional efficiency | Notes |
| 1 | Battery System | 92% | 95.92% | Assuming that the charging efficiency is consistent with the discharging efficiency |
| 2 | Energy storage inverter | 97.02% | 98.5% | |
| 3 | Power line efficiency | 99.34% | 99.67% | |
| 4 | Boosting efficiency | 97.87% | 98.93% |
Efficiency analysis
(1) Charging efficiency of energy storage system (considering only unidirectional efficiency during the charging process)
Assuming the SOC of the battery system is consistent and the charging and discharging depth is considered to be 90%, if a 2MWh energy storage system needs to be fully charged in 1 hour, the initial charging energy on its AC side is required to be:
Initial charging capacity on the communication side=(rated capacity of the system x depth of charge and discharge) ÷ charging efficiency of the battery system ÷ rectification efficiency of the energy storage converter ÷ transformer efficiency ÷ power line efficiency+power consumption of auxiliary equipment (considering full load operation of the auxiliary system within 1 hour of charging)=2000 × 0.9 ÷ 95.92% ÷ 98.5% ÷ 98.93% ÷ 99.67%+(35+5) × 1=1972.12kWhl,
The charging efficiency of the AC side of the energy storage system is (2000 × 0.9) ÷ 1972.12=91.27%.
(2) Discharge efficiency of energy storage system (considering only the unidirectional efficiency during the discharge process)
Initial discharge energy on the communication side=(rated capacity of the system x depth of charge and discharge) x charging efficiency of the battery system x inverter efficiency of the energy storage converter x transformer efficiency x power line efficiency - power consumption of auxiliary equipment (considering full load operation of the auxiliary system within 1 hour of charging)=2000 × 0.9 × 95.92% × 98.5% × 98.93% × 99.67% - (35+5) × 1=1636.91kWh,
The discharge efficiency of the AC side of the energy storage system is 1636.91 ÷ (2000 × 0.9)=90.94%.
(3) Energy storage device efficiency (according to the formula above, bidirectional efficiency should be utilized)
According to the definition of energy storage device efficiency, the efficiency of the energy storage device can be obtained as:
Φ=Φ 1 × Φ 2 × Φ 3 × Φ 4=92% × 97.02% × 99.34% × 97.87%=86.78%.
(4) Comprehensive efficiency of power station
Assuming the evaluation cycle is a full charge discharge, that is, charging for 1 hour and discharging for 1 hour, without considering standby conditions, the comprehensive efficiency of the power station in one cycle=discharging energy in one cycle ÷ charging amount in one cycle=1636.91 ÷ 1972.12=83.00%.
Assuming the evaluation cycle is 1 day, with 1 cycle per day, i.e. charging for 1 hour, discharging for 1 hour, and standby for 22 hours. The daily discharge capacity is 1 discharge capacity, which is calculated as 1972.12 kWh in the previous text. In addition to the 1 charge capacity of 1972.12 kWh, the daily charging capacity also needs to consider the power loss of the auxiliary system during standby period. (In the previous calculation, the auxiliary electricity consumption in the energy storage battery compartment was 249kWh per day. However, in the process of calculating the charging and discharging power, the auxiliary electricity consumption within 2 hours of charging and discharging has already been considered to be 40kWh per hour. This part cannot be counted repeatedly.)
Overall, the daily comprehensive efficiency of energy storage power stations is calculated as follows: daily discharge energy ÷ daily charge=1636.91 ÷ (1972.12+249-40 × 2)=76.45%.