[Introduction] There are few completed cases of PV-energy-direct-flexiblebuildings, and the electrical systems of PV-energy-direct-flexible buildings are quite different from those of traditional buildings. This article intends to analyze the system design from four aspects: photovoltaic power generation system, energy storage system, low-voltage DC system and flexible control platform of PV-energy-direct-flexible buildings. Taking an office building as an example, this article proposes the key points of photovoltaic component selection, self-consumption analysis method, low-voltage DC system wiring and main equipment selection, etc., for reference and reference for peers when designing similar systems.

1.Basic concepts of PV-energy-direct-flexible building electrical system

The concept of PV-energy-direct-flexible was first proposed in China. "PV" refers to building photovoltaics, "energy" refers to energy storage within the building and utilization of battery resources of electric vehicles in the nearby parking lot, "direct" refers to the use of DC power supply inside the building, and "flexible" is the purpose of "PV-storage-direct-flexible", which is to achieve flexible electricity use and make it a flexible load or virtual flexible power source of the power grid. The ultimate goal of the PV-energy-direct-flexible building electrical system is to transform the building power system from the current rigid load to a flexible load, which can adjust the power consumption at any time according to the supply and demand relationship of the power system, and is not determined by the power consumption of each electrical equipment in the system at that time.

The typical “PV-energy-direct-flexible” building electrical system wiring is shown in Figure 1. The wiring diagram shows the logical topological relationship of the external AC power supply, photovoltaics, energy storage, charging piles and electrical equipment forming an organic physical whole through the DC distribution network.

Figure 1 Typical PV-energy-direct-flexible building electrical system wiring diagram

According to the data from the national standard atlas "Design and Installation of Diesel Generator Sets", this project requires a net air inlet area of 40㎡. It is generally difficult to set up a completely hole-type vent, and air inlet louvers need to be installed. The area of the air inlet louvers needs to be 80㎡. Due to the proximity to the municipal street, the architectural professional has high requirements for the facade, and it is difficult to meet the technical conditions of the vent; on the other hand, it generates a lot of noise during operation, which has a certain impact on the surrounding environment.

2 Design principle of  PV-energy-direct-flexible building electrical system

The design principle of the PV-energy-direct-flexible building electrical system is shown in Figure 2. The design is mainly divided into the following four parts.

 Figure 2 Typical PV-energy-direct-flexible building electrical system wiring diagram

(1) Photovoltaic power generation system design. First, the form of the building's photovoltaic panels is determined in combination with the building's appearance requirements, and the installed capacity of the photovoltaic power generation system is determined. Then, the photovoltaic power generation consumption analysis is performed to determine the photovoltaic system access plan.

(2) Energy storage system design. The energy storage system includes distributed energy storage devices and electric vehicles connected through charging piles.

(3) Design of low-voltage DC power distribution system. Analyze the characteristics of power load, determine the access method of power equipment, and comprehensively determine the internal voltage level and system wiring form of the system based on the photovoltaic, energy storage and power equipment connected to the system. The design principle is to minimize the number of conversions to achieve efficient, economical, reliable and safe operation of the system.

(4) Design of flexible control platform for  PV-energy-direct-flexible  building system. The system control platform mainly realizes the following functions: predicting photovoltaic power generation, predicting building load power consumption, formulating energy storage and charging and discharging strategies for charging piles (electric vehicles), ensuring the internal voltage stability of the DC microgrid and providing the system load flexible regulation margin, and accepting the demand-side response of the power grid.

The following article uses cases to illustrate the specific design method of the PV-energy-direct-flexiblebuilding electrical system.

3 Key points for designing the electrical system of PV-energy-direct-flexible buildings

The design points of the PV-energy-direct-flexiblebuilding electrical system are explained with a case study. The case study is an office building located in Huizhou City, Guangdong Province, with a total construction area of 5,000 ㎡, 5 floors, and an area of about 1,000 ㎡ per floor. The building faces north and south, with a width of 50 m, a depth of 20 m, and a building height of 20 m. The first to fifth floors are offices. A multi-split air conditioning system is used, with the outdoor unit installed on the roof, covering an area of about 100 ㎡, and a curtain wall structure on the facade.

3.1 Photovoltaic power generation system design

3.1.1 Determination of PV Modules

Photovoltaic technology is constantly being updated and developed, and the types of photovoltaic cell materials are also increasing. They can be roughly divided into the following three categories according to their material structure: (1) Silicon-based photovoltaic cells, such as single-crystal silicon and polycrystalline silicon photovoltaic cells. (2) Thin-film photovoltaic cells, such as gallium arsenide, cadmium telluride, copper indium gallium selenide thin-film photovoltaic cells, etc. (3) New concept cells with theoretical high conversion efficiency and low cost advantages, mainly new photovoltaic cells, such as dye-sensitized photovoltaic cells, perovskite photovoltaic cells, organic solar cells and quantum dot solar cells. The performance of different types of photovoltaic cells is shown in Table 1.

At present, monocrystalline silicon and polycrystalline silicon cells still occupy the main market position due to their high cost performance. Such materials are generally considered for use in places such as roofs and outdoor canopies. Thin-film photovoltaic cells can be combined with photovoltaic curtain walls to achieve good facade display effects. In the integrated design of building photovoltaics, thin-film cells of different materials are often used to achieve better facade effects. New photovoltaic cells have great development potential in the building photovoltaic market due to their high theoretical conversion efficiency and low preparation cost. It should be noted that when thin-film cells are installed on facades, the actual conversion rate of thin-film cells needs to be determined in accordance with the building facade effect and transmittance requirements. The higher the transmittance, the lower the conversion efficiency per unit area.

In this case, combined with the architectural effect, it is possible to consider using monocrystalline silicon photovoltaic modules on the roof and installing cadmium telluride thin-film battery photovoltaic glass on the south facade.

3.1.2 Determination of the installation scale of photovoltaic power generation components

Before local policy documents and relevant standards are clarified, this case considers installing monocrystalline silicon photovoltaic power generation modules on the roof and the south outdoor parking canopy, and the photovoltaic modules are fixed with brackets. At the same time, considering the integrated application of photovoltaic power generation and building, photovoltaic glass is installed on the south facade.

According to the "Solar Energy Resource Level Total Radiation" GB/T 31155-2014, the solar energy resource level of the case location is "rich"; the representative annual total solar radiation is 1389.9kWh/㎡. The installation angle of the photovoltaic array has a great influence on the efficiency of the photovoltaic power generation system. For fixed grid-connected photovoltaic power generation systems, the inclination angle should maximize the annual radiation received on the inclined surface of the photovoltaic array.

The total radiation on the inclined surface of the battery module is the sum of the direct radiation, scattered radiation and ground reflected radiation on the inclined surface. According to the "Design Specifications for Photovoltaic Power Stations" GB 50797-2012, the radiation loss comparison table shown in Table 2 is obtained using PVsyst software, and the optimal inclination angle is 21°. Due to the limited roof construction area, if the optimal inclination angle is considered, the area occupied is large. In order to save land area and increase installation capacity, and considering that the building in this case is located on the edge of the coast and the building height is high, in order to ensure the safety and capacity of the project, after analysis, the roof of this case adopts a 5° module inclination for installation, and the canopy of the outdoor parking area on the south is installed with a 21° module inclination.

The curtain wall is based on the building wall, and the curtain wall glass is embedded in the wall, with an overall inclination of 90°. The curtain wall used for power generation in the project faces due south. PVsyst is used for analysis and calculation. Based on the site-calculated light resource data, when considering the azimuth due south, the curtain wall radiation of this project is 841.5 kWh/㎡, which is a serious loss compared to the horizontal radiation.

The case uses double-sided monocrystalline silicon photovoltaic modules and cadmium telluride photovoltaic glass. Their specifications are detailed in Tables 3-4, and their layout is shown in Figures 3-5. The number of strings of monocrystalline silicon photovoltaic modules and cadmium telluride photovoltaic glass are 20 and 16 respectively. A total of 441 monocrystalline silicon photovoltaic modules are installed with an installed capacity of 238kWp, and 1,353 cadmium telluride photovoltaic glass photovoltaic modules are installed with an installed capacity of 142 kWp. The total installed capacity of photovoltaic power generation in the project is 380 kWp.

Figure 3 Outdoor photovoltaic power generation canopy and ground floor power distribution trunk line plan

Figure 4 Rooftop photovoltaic power generation and power distribution trunk line plan

Figure 5 Layout of photovoltaic glass on the south facade

3.1.3 Analysis of power generation of photovoltaic power generation system

The photovoltaic power generation load curve is shown in Figure 6. According to the normal distribution curve, it reaches its peak at 12 noon.

Figure 6 Schematic diagram of daily photovoltaic power generation time-sharing curve

The estimated photovoltaic power generation for this case is shown in Table 5.

3.1.4 Selection of photovoltaic power generation system

"Photovoltaic storage direct and flexible" building photovoltaic power generation systems generally adopt grid-connected photovoltaic power generation systems. In order to avoid the situation where large-scale photovoltaic power generation is connected to the power grid, which makes it difficult to ensure the stability of the power grid, it is recommended to adopt the self-consumption grid-connected mode of "self-generation and self-use, surplus power connected to the grid" to minimize the impact of renewable energy power generation on the power grid. The power generated by solar cells is given priority to internal loads, and the excess power that cannot be used by the loads is sent to the power grid. When the photovoltaic power generation is not enough to supply the load, the power grid and the photovoltaic power generation system will supply power to the load at the same time. Therefore, when determining the photovoltaic access point, it is necessary to conduct a self-consumption analysis based on the characteristics of the building's power distribution system, the characteristics of the power load and photovoltaic power generation.

This case also adopts the self-consumption and grid-connected mode of "self-generation and self-use, and surplus power connected to the grid".

3.2 Energy storage system design

3.2.1 Selection of energy storage battery types

Based on the technical parameters provided by mainstream energy storage manufacturers, the performance comparison of various energy storage batteries is summarized (Table 6). According to different energy storage application requirements, energy storage batteries can be divided into power type batteries and energy type batteries. Power type batteries are batteries that work at a rated power less than or equal to 1 hour rate (1P), suitable for short-term fast charging and discharging (such as occasions to achieve rapid response on the demand side), mainly represented by lithium titanate batteries (LTO). Energy type batteries are batteries that work at a rated power greater than 1 hour rate (1P), mainly represented by lithium iron phosphate batteries (LFP). Lithium iron phosphate raw materials are abundant in reserves, so the cost is low. At the same time, they also have good safety and cycle performance and are widely used in energy storage.

When the requirement of rapid response on the demand side is not met in a “solar-storage-direct-flexible” building, energy batteries can generally be used.

3.2.2 Selection of energy storage capacity

The selection of energy storage capacity is first used to maintain the stable operation of the electrical system of the PV-energy-direct-flexible building; on the other hand, it is to improve the economy of the PV-energy-direct-flexiblebuilding, such as combining it with electricity price policies to achieve "peak shaving and valley filling" through "shifting" of electricity time or participating in the demand-side response of the power grid to reduce energy costs; it can also serve as a backup power source for important loads.

The electrical system of a "photovoltaic storage direct and flexible" building is generally connected to the mains. At this stage, electricity is still dominated by thermal power, and the grid stability is relatively high. Without separate energy storage, the internal stability of the "photovoltaic storage direct and flexible" building DC microgrid can be maintained by the mains. However, under the "dual carbon" strategy, my country's power system is also developing towards a new power system with a high proportion of new energy as the main body. With the installation and access of new energy, the power system will also show the characteristics of large randomness and poor inertia of wind and solar power generation systems, and energy storage devices need to be set up to match. According to the "Carbon Peak Action Plan before 2030" issued by the State Council, "By 2025, the installed capacity of new energy storage will reach more than 30 million kilowatts." This part of energy storage, in addition to being installed centrally on the grid side, may also be gradually installed on the user side according to local policy requirements. Therefore, the distribution system of new buildings needs to consider reserving interfaces and installation space for energy storage.

3.2.3 Installation methods of energy storage system

Since fire safety of energy storage is still a key point and difficulty that needs to be paid attention to, energy storage is generally considered to be installed outdoor.

3.2.4 Charging pile settings

Under the premise that new energy is mainly reflected in electricity, electric vehicles will be an important way for the automotive industry to decarbonize.

Car charging piles are also a flexible energy storage device, so the future development direction of car charging piles will be two-way, that is, charging the car power battery during the low electricity consumption period, and using the car energy storage battery to reversely supply power to the power grid during the peak electricity consumption period, realizing the auxiliary peak and frequency regulation of the power grid. In order to realize the active regulation of the charging pile on the power grid, the charging pile must be equipped with an intelligent control system. At present, all regions have issued requirements for the construction of charging piles. Before starting the project, it is necessary to understand the local policies and determine the ratio of fast and slow charging.

In this case, the energy storage is planned to be installed at 30% of the photovoltaic installed capacity. It is planned to configure a 120kWh lithium iron phosphate battery outdoors, use a container installation method outdoors, and set up 5 30kW non-vehicle chargers outdoors.

3.3 Low voltage DC distribution system design

3.3.1 Power load analysis

The main load types in this case are multi-split air conditioners, lighting, sockets, charging piles, elevators, intelligent power consumption, emergency lighting power consumption, etc., all of which are level 3 loads. The total installed capacity of power equipment is 710 kW. Referring to the office energy consumption data in the document "Statistical Analysis of Energy Consumption Characteristics of Large Office Buildings", the estimated annual power consumption and typical working condition power consumption of the case are shown in Table 7.

3.3.2 Self-consumption analysis

However, it can be seen from the typical working conditions of time sharing that there is a mismatch between the power generation and power consumption of the case. For example, 12:00 noon on weekdays is the peak power generation period. At this time, the power consumption equipment in the case cannot consume all the power generated, and it is necessary to store the unused power generation in the charging pile and energy storage. At 15:00, the power consumption period is peak, and the power generation cannot meet the power demand of the case. Considering that this time is generally the peak electricity price period of the city electricity, energy storage devices and two-way charging piles are used to store electricity for electric vehicles; if the energy storage and charging piles cannot balance the power generation and consumption, the case can obtain insufficient or excess power through interaction with the power grid after internal load adjustment; the energy storage capacity and charging pile capacity configuration of the case can basically achieve the peak power consumption period (12:00-15:00) without the need for power supply from the power grid, but the power consumption on weekends is small, and the energy storage capacity and charging pile capacity configuration of the case cannot achieve the full storage of the power generated by the photovoltaic power generation system, so that there is a large amount of surplus power generation that needs to be fed back to the power grid.

Therefore, although it seems that the difference between the annual power generation and consumption of the project is only 29166 kWh, which is not a big difference, due to the inconsistency between the peak periods of power generation and power consumption, the two-way flow of electricity between the PV-energy-direct-flexiblebuilding electrical system and the power grid is still very large. The actual amount of electricity interacting with the grid is far more than 29166 kWh, especially on weekends when the photovoltaic power generation system sends a large amount of surplus electricity back to the grid, which will bring greater pressure to the power dispatching and power consumption of the grid.

3.3.3 Self-consumption analysis

Combined with the installation of photovoltaic modules and energy storage, the case wiring diagram is designed as shown in Figure 8. The photovoltaic power generation system, energy storage, city power, rooftop multi-split, and office trunks all use a DC 750V power supply voltage. Elevators, intelligent electricity, public lighting, emergency lighting, water boilers, printers, and floor office power horizontal trunks use a DC 375V power supply voltage, and the terminal office room uses a 48V ultra-low safety voltage. The system adopts a two-wire system. Considering the poor short-circuit fault tolerance of the current converter, the system adopts a variable grounding form. When the system is working normally, the IT system is used in conjunction with insulation monitoring. After the system is grounded at one point, it is converted to a negative-grounded TN system and a DC residual current protection device is used to automatically cut off the power supply through a circuit breaker.

Figure 8 Wiring diagram of the electrical system of the case building

In the case, all devices connected to the 750V DC power distribution system are connected through an isolated converter, and the converter adopts a modular design. The 750V DC power distribution system circuit breaker uses a DC circuit breaker with a rated voltage of 1000V, and the 375V DC power distribution system circuit breaker uses a DC circuit breaker with a rated voltage of 250V. The number of poles of the circuit breaker is 4, and the wiring method adopts the four-pole two-two series wiring method shown in Figure 9. The release adopts a thermal magnetic type or a special DC electronic release.

Figure 9 DC circuit breaker wiring diagram

The terminal control protection unit is set up in the terminal office, and a DC 48V safe extra-low voltage power distribution system is set up inside. Except for the water boiler and printer, the electricity used in the rest of the office is included in the safe extra-low voltage power supply range to improve the safety of electricity use by terminal personnel. High-power equipment such as water boilers and printers that cannot be connected to the DC 48V system are directly connected to the socket circuit of the DC 375V system. These socket circuits are equipped with DC-specific residual current action protectors with a rated residual action current not exceeding 80mA. The metering instruments in the case are all DC-specific.

3.4 Design of flexible control platform for “solar-storage-direct-flexible” building electrical system

3.4.1 System structure and functions

The system platform framework is shown in Figure 10.

Figure 10 System framework diagram of the flexible control platform for the PV-energy-direct-flexible building electrical system

(1) The system is based on a cloud-edge-end architecture and adopts the Internet of Things, cloud-edge-end collaboration, big data, and AI intelligent analysis technology. It can receive signals from insulation monitoring systems, electrical fire systems, etc., and read the status of equipment, allocate power, and manage operations; (2) It supports the connection of "source-grid-load-storage" equipment such as converters to achieve plug-and-play of energy equipment; (3) The system supports connection with subsystems such as photovoltaic power generation, energy storage, charging piles, smart power distribution, and multi-connected units; (4) It supports equipment status assessment and health management, system fault prediction and diagnosis, emergency response, business management, and asset management of system equipment.

By providing a smart park system and a grid demand-side response interface, the system can be linked with the smart park to achieve unified maintenance and operational management of various terminals in the park.

3.4.2 Operation mode of PV-energy-direct-flexiblebuilding electrical system

(1) Grid-connected operation mode

When connected to the grid, the AC/DC port of the power router is connected to the AC grid. The DC/AC adopts PQ constant power control mode to operate. The energy storage port stabilizes the DC bus voltage. The power supply priority is photovoltaic → energy storage → mains. When the actual total power of the load is greater than the actual total output power of photovoltaic and energy storage, power must be taken from the mains to ensure stable operation of the system; if power is no longer taken from the mains, some loads need to be shut down to make the total output power of photovoltaic and energy storage greater than the total input power of the load.

(2) Off-grid operation mode

When off-grid, the AC/DC port of the power router is disconnected from the AC grid, and can automatically adjust the power balance between the source and the load to ensure stable operation of the system. There is no energy exchange between the DC/AC and the grid, and the energy storage port stabilizes the DC bus voltage. This mode is only operated when the mains power fails, and the power supply priority is photovoltaic → energy storage, and no reverse power supply to the mains is provided.

4

Conclusion and Outlook

This article uses an office building as an example to illustrate the design principles of the PV-energy-direct-flexiblebuilding electrical system. It explores the photovoltaic power generation system, energy storage system, low-voltage DC distribution system and the flexible control platform architecture of the PV-energy-direct-flexiblebuilding electrical system. It proposes solutions to the case's photovoltaic component installation scheme, photovoltaic power generation system self-consumption analysis, energy storage selection, low-voltage DC system wiring method, control system framework, etc., providing an effective design reference for later designers.