Industrial and commercial energy storage systems bring significant economic benefits to enterprises through various business models such as peak and valley arbitrage, demand management, and self-generation and self-consumption. The efficiency of the energy storage system directly affects its economy and market competitiveness. Therefore, accurate calculation and in-depth analysis of the efficiency of commercial and industrial energy storage systems are of great significance for optimizing system design and improving energy utilization efficiency.
Commercial and industrial energy storage systems are usually composed of the following parts:
Battery system: This is the heart of the energy storage system, responsible for storing and releasing electrical energy. Which is like a large rechargeable treasure, capable of storing energy when power is sufficient and releasing it when needed.
Battery Management System (BMS): This system is like the battery's “intelligent housekeeper,” responsible for monitoring the battery's status to ensure safe and healthy operation. It checks the battery's temperature, voltage, and current in real time to prevent overcharging or overdischarging and prolong the battery's service life.
AC/DC inverter system (PCS): This system is like a “power converter”, capable of converting DC to AC or AC to DC. This allows power to flow in both directions and ensures efficient transmission between the storage system and the grid.
Transformer: If the storage system needs to be connected to the high-voltage grid, a transformer comes in handy. It can adjust the voltage of the energy storage system to a level that matches the voltage of the grid, ensuring the smooth transmission of electricity.
Energy Management System (EMS): This system is like the “intelligent brain” of the energy storage system, responsible for monitoring the operating status of the entire system. It intelligently optimizes charging and discharging strategies according to power demand and grid conditions to ensure the most efficient use of power. At the same time, it can also interact with the grid to achieve more flexible power management.
Air conditioning system: used to regulate the temperature of the energy storage system to ensure that the batteries work at the right temperature and extend battery life.
Fire protection system: to ensure the safety of the energy storage system and prevent fire and other accidents.
Monitoring and alarm system: real-time monitoring of the operational status of the energy storage system, once abnormalities are found, an alarm will be issued immediately to ensure timely handling.
The efficiency of industrial and commercial energy storage systems is affected by a variety of factors, mainly including the following aspects:
Battery efficiency: the charging and discharging efficiency of the battery itself is a key factor affecting the efficiency of the energy storage system. Different types of batteries (such as lithium-ion batteries, lead-acid batteries, etc.) have different charging and discharging efficiencies. In addition, factors such as battery aging, temperature, and charge/discharge multiplier also affect battery efficiency.
Power conversion efficiency: PCS will produce certain losses in the process of power conversion, and its efficiency directly affects the overall efficiency of the energy storage system; with the development of technology, the efficiency of PCS is constantly improving, but there is still room for improvement.
Electrical connection and line loss: the current through the cable and switchgear will produce resistance loss, this part of the loss is relatively small, but in the large-scale energy storage system should not be ignored.
Auxiliary equipment energy consumption: air conditioning, cooling systems, lighting , and other auxiliary equipment in the operation process will consume electricity, thus reducing the overall efficiency of the energy storage system. Especially in high-temperature environments, the energy consumption of air-conditioning systems will increase significantly.
System design and control strategy: Reasonable system design and optimized control strategy can minimize energy loss and improve the efficiency of the energy storage system. For example, charging and discharging strategies can be optimized by accurately predicting changes in electricity prices and loads; battery temperature can be reduced and battery efficiency can be improved through reasonable heat dissipation design.
“Operation Indicators and Evaluation of Electrochemical Energy Storage Power Stations”, the comprehensive efficiency of an energy storage power station is defined as the ratio of the amount of electricity connected to the grid to the amount of electricity taken off the grid during the production and operation of the storage power station within the evaluation cycle, i.e.: Comprehensive Efficiency = Total amount of electricity delivered to the grid by the storage power station within the evaluation cycle ÷ Total amount of electricity received by the storage power station from the grid.
Initial charging quantity of AC side = (rated capacity of system × charging and discharging depth) ÷ charging efficiency of battery system ÷ rectification efficiency of energy storage converter ÷ efficiency of power line ÷ efficiency of transformer + power consumption of auxiliary equipment.
Charging efficiency = (rated system capacity × charging and discharging depth) ÷ initial charging amount on the AC side.
Discharge efficiency
AC side initial discharge = (system rated capacity × charging and discharging depth) × battery system charging efficiency × storage converter inverter efficiency × power line efficiency × transformer efficiency - auxiliary equipment power consumption.
Discharge efficiency = AC side initial discharge amount ÷ (system-rated capacity × charging and discharging depth).
Battery efficiency is one of the most critical factors in the energy storage system. According to the performance requirements of battery clusters in Lithium-ion Battery for Electric Energy Storage, it is known that the initial energy efficiency of battery clusters should not be less than 92% under the conditions of (25±5)℃ and rated power, while according to the performance requirements of battery clusters in the latest Lithium-ion Battery for Electric Energy Storage, it is known that the initial energy efficiency of battery clusters should not be less than 92% under the conditions of (25±5)℃ and rated power. Initial energy efficiency should not be less than 95% under the condition of (25±5℃) and rated power; considering the above efficiency requirements for the initial efficiency, comprehensive energy storage system operation , and market product development, the temporary battery system efficiency is 93% (bidirectional).
The power conversion system efficiency includes rectifier efficiency and inverter efficiency. According to the market PCS production situation, generally takes 98.5% (one-way).
Power lines generate heat loss when transmitting current. Due to the high integration of industrial and commercial energy storage cabinets, the DC side line loss is negligible, PCS AC side-transformer AC side due… to the consideration of the actual situation at the site is different, the loss is also different, specific to the actual loss calculation; this time temporarily according to the unidirectional efficiency of about 99%, taking into account the bidirectional loss, the efficiency of the power line is about 98.01%.
At present, the industrial and commercial energy storage cabinet is mainly applied to low-voltage access program, the integrated cabinet PCS outlets connected to the plant have a transformer low-voltage bus, not to consider the efficiency of independent high-voltage transformer losses.
Energy storage power plant in operation requires certain auxiliary equipment, such as security systems, fire alarm systems, air conditioning systems, etc. The power consumption of these equipment accounts for a larger proportion of the total energy consumption of the energy storage power station. Especially in specific environments, due to changes in ambient temperature, the power consumption of the air-conditioning system will increase accordingly.
Taking an industrial and commercial energy storage project as an example, its configuration size is 1MW/2MWh, the depth of discharge is designed according to 90%, and the main power-consuming equipment includes the security system and air-conditioning system. The energy storage system utilizes the electricity price difference to realize peak and valley arbitrage, two charging and two discharging cycles, 0.5C charging , and discharging, full power two hours charging and discharging is completed; the single system auxiliary power consumption average operating power is about 1.5kW/h.
AC side initial charging = (system rated capacity × charging and discharging depth) ÷ battery system charging efficiency ÷ energy storage converter rectifier efficiency ÷ AC line efficiency + auxiliary equipment power consumption (auxiliary system power consumption in the process of charging for 2 hours)
= 2000 × 0.9 ÷ 96.44% ÷ 98.5% ÷ 99% + (1.5 × 10) × 2 = 1944.01kWh.
Charging efficiency of the AC side of the energy storage system = (2000 × 0.9) ÷ 1944.01 = 92.59%.
Discharge efficiency of energy storage system (considering single discharge)
AC side initial discharge = (system rated capacity × charging and discharging depth) × battery system charging efficiency × energy storage converter rectifier efficiency × AC line efficiency - auxiliary equipment power consumption (auxiliary system power consumption during charging for 2 hours)
=2000×0.9×96.44%×98.5%×99%- (1.5×10) × 2=1662.78kWh.
Charging efficiency of AC side of energy storage system = 1662.78 ÷ (2000 × 0.9) = 92.38%.
In the case that the evaluation cycle is 1 day and the cycle is 2 times per day (4h of charging and 4h of discharging, without considering standby); the daily comprehensive efficiency of the energy storage power plant is calculated as follows:
Daily integrated efficiency = daily discharge volume \ daily charge volume
= {2×(2000×0.9×96.44%×98.5%×99%-(1.5×10)×2)}÷
{2×(2000*0.9÷96.44%÷98.5%÷99%+(1.5×10)×2)} =85.53%.
In practice, there will be partial deviation of data in the evaluation cycle under different application scenarios and working modes, and different working environment conditions and energy storage devices need to be taken into account in the project measurement for reasonable analysis.
We can adjust the charging and discharging strategy of the battery to reduce self-discharge loss and thus improve the overall efficiency. In addition, it is also key to choose efficient energy storage devices and control strategies, which can make the energy storage system run more smoothly.
Introducing smart technology is a great way to improve the efficiency of energy storage. Through real-time monitoring and scheduling management, we can accurately control the operating time and power output of the energy storage equipment, so that the efficiency of energy utilization will be greatly improved. At the same time, having the energy storage system intelligently integrated with the power system can enhance overall efficiency and stability.
Regular maintenance and inspections are important as the system ages over time, environmental conditions (e.g., temperature) change or the equipment may malfunction. This ensures that the equipment is functioning properly, thereby increasing the efficiency of the energy storage system.
In summary, the efficiency calculation of industrial and commercial energy storage systems is a complex but important process that involves multiple factors and aspects. By understanding the system efficiency calculation and optimizing the design, the efficiency of the energy storage system can be improved and the operating cost can be reduced.
