Household Energy Storage System’s Power Solution

Abstract: The intelligent information age greatly increases electricity demand, which, in return, pressures people into seeking for green power generation due to the severe environmental pollution and energy consumption. Solar PV power generation is expected to alleviate the pressure. With government’s policy support and less cost of power generation, energy storage systems are brought in tens of thousands of households. For the entire household energy storage system, MORNSUN provides a complete power solution to simplify customer’s design and increase system’s reliability.


Key words: solar PV power generation, energy storage system, power solution, MORNSUN




Introduction of household energy storage system

With the ongoing development of The Million Solar Roofs bill (the United States) and Energiewende (known as “energy transition”, Germany), household energy storage system is widely introduced in over 50 countries worldwide, especially when the governments give high subsidies to families whoever apply solar PV power generation. As a result, most families not only achieve self-sufficiency of household electricity but also store excess electricity. The market demand for household energy storage system is growing.



The household energy storage system is similar to a miniature energy storage power station, while its operation is free from the pressure of the utility. Battery pack in the system is self-charged during the trough period of using electricity, and discharges it during the peak period of using or powering off electricity. In addition to using as an emergency power supply, the system is able to balance the electricity load, thereby saving household electricity cost. 


Current market demand for household energy storage system more relies on the need for emergency power supply. However, in the eyes of professionals in the industry, the household energy storage system market is bright and promising. It combines new energy power generation system, e.g. solar, and promotes the new energy on the other hand, contributing to building intelligent power grid.  

Structure and components of household energy storage system

Household energy storage system is currently divided into two kinds, grid-connected and off-grid.


Grid-connected household energy storage system is mixed-powered by solar and the energy storage system, including five parts: solar array, grid-connected inverter, BMS management system, battery pack and AC load. When the utility works normally, the solar grid-connected system and the utility together power the load. When the utility powers off, the energy storage system and the solar grid-connected system together power the load. The grid-connected household energy storage system is divided into three working modes. Model I, solar provides energy storage and powers the utility. Mode II, solar provides energy storage and powers part of residential electricity. Mode III, solar only provides energy storage. Grid-connected household energy storage system is as shown in Diagram 1:

Diagram 1 Grid-connected household energy storage system (source: www.fortunenergy.com)


Off-grid household energy storage system is independent, without any electrical connection to the grid. Therefore the whole system does not need grid-connected inverter except PV inverter. The off-grid household energy storage system is also divided into three working modes. Model I, solar provides energy storage and powers residential electricity in sunny days. Mode II, solar and energy storage battery power residential electricity in cloudy days. Mode III, energy storage battery powers residential electricity at dusk and in rainy days. Off-grid household energy storage system is as shown in Diagram 2:


Diagram 2 Off-grid household energy storage system (source: www.fortunenergy.com)


In summary, current demands for energy storage equipment mainly are BMS management system, PV grid-connected inverter and energy storage inverter. Combined the demands with the safety isolation requirement of PV system’s unit circuits, MORNSUN puts forward a complete power solution of control unit.


Power Solution for BMS management system
Battery is the core energy storage device of the system and needs to be monitored online status in real time, so the importance of BMS is self-evident. In the BMS management system, BCU real-time communicates with CAN bus and BMU to get monomer voltages, cabinet temperature, insulation resistance and others, with current sensor to collect charge and discharge current and dynamic calculation SOC and with touch screen to display relevant data. The BCU calculates and analyzes all the information of the battery pack and then intelligently manages the system, communicating with the independent CAN bus and achieving the secondary protection of charge and discharge through a relay. The latter ensures an effective isolation for strong electricity and weak electricity, meets customer’s demand for diverse security control and guarantees stable and efficient operation of the system.

 Diagram 3 Power Solution for BMS management system


As shown in Diagram 3, isolation voltage conversion is the main core of power solution for BMS management system in the energy storage system. Main control unit is mainly based on 24VDC system, and the power of the whole main control unit is less than 5W. Therefore, URB2405YMD-10WR3 offering 5VDC output voltage is recommended to power F0505XT-1WR2 and LDO. The LDO converts 5VDC into 3.3V output voltage to power MCU. 6PCS F0505XT-1WR2 in the system are used to power four CAN modules, voltage detection circuit, insulation detection circuit and current detection circuit, and isolate power circuit, signal circuit and communication module at the same time to reduce interference and improve the stability and reliability. In addition, MORNSUN automotive-grade power supply CF0505XT-1WR2 is widely used in automotive BMS system.



Power Solution for solar PV inverter monitoring unit

Solar PV components converge energy and inverter converts DC into AC. The process of the inverter needs to be monitored, controlled and communicated so as to ensure its voltage to meet the requirement. This monitoring unit can directly get power from the utility.


As shown in Diagram 4, LH series directly gets power from the utility and converts it into 24V output voltage, which is converted into 5VDC output voltage by non-isolated switching regulator K7805-500R3. B0503XT-2WR2 outputs 3.3V to power the MCU with isolation function. Isolated high speed TD501D485H can inhibit electromagnetic interference and rise the ground loop’s resistance to protect the system circuit from external network. In addition, MORNSUN develops an isolated driver QP12W08S-37 integrating DC/DC power supply, specialized for full-bridge IGBT, to simplify the design of customer’s driven-control circuit and improve the reliability.

Diagram 4 Power Solution for solar PV inverter monitoring unit





Power Solution for solar PV grid-connected inverter monitoring unit
Solar PV grid-connected inverter, also known as bi-directional energy storage inverter and controller and inverter integrated machine, consists of AC-DC unit, DC-DC unit, control unit, communication unit and unit of transfer relay. Bi-directional energy storage inverter is powered from two kinds of power supply equipment. One is from the utility, and the other from the solar panel. 


Diagram 5 Power Solution for solar PV grid - connected inverter monitoring unit


As shown in Diagram 5, the power selection of AC/DC Converter should mainly take the power of main control board and fan into account. The equipment power in above system is 15KW. AC/DC Converter LH40-10B24 offering 24V output voltage is recommended to power the fan and the main control board. For the main control board, it’s recommended to use R3 DC/DC Converters URB2415YMD-10WR3 to power IGBT driver and URB2405YMD-6WR3 to power MCU and communication module. URB_YMD-6/10WR3 series apply fully automated production process, greatly reducing the labor costs. With superior performance ahead of the peer level and high cost-effective, they fully meet the overall cost demand of solar PV industry. 




MORNSUN power supplies 
Wide input voltage isolated DC/DC Converters
Features:
Wide input voltage range (4:1)
Operating temperature range: -40℃ to +85℃
Isolation: 1500VDC
Light-load efficiency high up to 78%, ripple & noise as low as 50mV                                                                 
No-load power consumption as low as 0.12W
Input under-voltage, output short-circuit, over-current and over-voltage protections
Meet CISPR22/EN55022 CLASS A, without external components
UL60950,EN60950 and IEC60950 approval




Fixed input voltage isolated DC/DC Converters
Features:
Continuous short-circuit protection
Operating temperature range: -40℃ to +105℃
Isolation: 3000VDC 
Ripple & noise as low as 50mV
High efficiency up to 80%



Isolated AC/DC Converters
Features:
Universal input voltage range: 85-264VAC/100-370VDC
Regulated output, low ripple & noise
Isolation: 3000VAC
Output short-circuit, over-current and over–voltage protections
IEC60950, UL60950, EN60950 approval
Plastic case, meeting UL94V-0



Gradually large-scale and universal household energy storage is a reasonable developmental trend for the world's energy demand. Actively participating in the development of green energy technology and combining own technology platform with independent innovation capability, MORNSUN provides a compete power solution for the system to simplify its circuit design, reduce costs of household construction and maintenance and improve the system’s stability, security and reliability.

References:
1. THE GERMAN ENERGIEWENDE BOOK. Craig Morris, Martin Pehnt. 12/2016.
2. The Energy Storage Market in Germany - Issue 2017/2018. Germany Trade & invest. 2017.
3. THE CALIFORNIA MILLION SOLAR ROOFS INITIATIVE. SCCGOV.  
4. Lithium battery is expected to dominate the household energy storage market. www.kuyibu.com. 2014. 

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