Power electronics solutions in off-grid solar system design
The rapid decline in photovoltaic solar installation costs has made this technology a practical solution to support off-grid applications. The National Renewable Energy Laboratory (NREL) calculated in 2010 that the solar system installation cost was slightly higher than $ 7 per watt, and SolarBuzz said that system pricing is moving towards $ 15.50 per kWh. This is the pricing of the entire system, including solar cells, energy storage equipment, and charger and inverter power electronics.
From portable highway construction signs and flashing lights to remote pumping stations and communication networks, off-grid applications seem to have endless opportunities. Excessive costs are the main obstacles to the implementation of such projects, and the continuously decreasing prices are gradually breaking such barriers, making the implementation of the project practical.
This article will focus on power electronics systems and some key opportunities and trade-offs to keep in mind when designing off-grid solar systems. When solving most system-level problems, the easiest way is usually to start with the final application requirements and then go back to determine and define the entire system and determine the size of the system.
load
The load can be almost any device, but off-grid applications are off-grid for a reason. Applications may need to be portable, such as construction information signs or simple hazard warning flashers. Considering its portability, it is obviously not feasible to connect every load to the grid. Or, perhaps the application is located in a remote location, such as a cellular communication tower or a remote pumping station.
We should first identify some key factors that need to be considered when developing off-grid solar solutions. Figure 1 shows the high-level system block diagram. When using the energy balance method, the key is to understand the load, including its type and the characteristics exhibited over time.
Figure 1: High-level system block diagram
First determine the load type and any special requirements. Is it a constant load or a variable load? Is it used only during the day or only at night? Is it used intermittently or continuously? Understanding the load type and its characteristics helps determine the system's implementation. For example, construction hazard warning flashers are usually of constant pulsating load and may only need to be used at night. Therefore, we should charge these loads during the day and determine the battery capacity, and then run these loads at night. The information sign may be a pulsating load, but it needs to be operated during the day and night. In this case, the system needs to be determined to support constant load operation during the day, while charging the battery pack to support continuous operation throughout the night. Similarly, the pump load needs to work day and night, and is not necessarily a constant load. Here, the system needs to be able to cope with the worst case, or a backup system is needed to solve the worst case. For example, a pumping station used to drain rain may not be very suitable for off-grid solar applications, because there is not much sunlight to charge the battery when it rains. Obviously, the load types that can be used are almost endless. Figure 2 only lists some of the load types discussed above.
Figure 2: Common load types
Given the amplitude and frequency and operating characteristics, the key is to understand the average daily load. Once the load and operating conditions are fully understood, specifying energy storage requirements becomes straightforward.
Determine the scale of energy storage
Since the sun rises and falls every day, we can calculate the basic energy storage requirements by a simple 24-hour energy balance method. Note Figure 3, which shows the working conditions. Obviously, the situation on the left side of the diagram must be supported when determining the scale of energy storage.
Figure 3: Working status
The load may have a definite amount, such as several construction hazard warning flashers, or there may be a lot of changes, such as pump applications. When dealing with varying loads, it is best to consider the following two situations. The first case is the "normal" case, covering 95% of the work. The energy storage component should be charged during the day, and its capacity should be set to be sufficient to drive the load during the rest of the night. The second case is the worst normal working condition, covering more than 95% of the cases (ie, the pump starts working at dusk and then runs at full load overnight).
The following formula captures the worst-case energy storage requirements by combining the maximum load power per hour during the uncharged time period.
There is a worse situation, that is, when the load needs to work and there is no sunlight, the battery pack has not been fully charged. The focus of consideration is the cost of failure (ie no flashing or suction). If the pump does not pump, the cost can be considerable. The most obvious solution is to increase the capacity of the energy storage components. However, there will always be worse. When failures are not tolerated or when maximum power is rarely required, it is most reasonable to install a backup system (such as a diesel generator) and adjust the size of the rest of the system to only apply to normal conditions.
The left side of Figure 3 is used to determine the energy storage requirements, and the right side of Figure 3 is used to determine the size of the solar array.
Determine the size of the solar array
After better understanding the load requirements, we can determine the size of the solar array. According to Fig. 3, the scale of the solar array must be set so that the charging capacity meets the energy storage requirements within the prescribed charging time, while supporting the average load output during this time period. The following formula identifies this higher-order relationship.
Solar array output power = energy storage capacity / charging time + average load power
We can estimate the size of energy storage and solar array components through a simplified energy balance method. However, in order to make accurate adjustments to the estimated values, it is necessary to understand some internal and external factors. From the perspective of external factors, one of the most important factors is the location of off-grid applications, especially latitude. This alone can predict the peak solar exposure and its change over the year. For example, based on the location of the application location relative to the sun, it can be predicted that the sun exposure is the least in winter and the most in summer. Other external factors (including cloud cover and ambient temperature) may also adversely affect the amount of sunlight exposure and energy conversion efficiency that the system is expected to receive. It should be clear that these external factors will vary depending on the application and location.
In addition, internal factors such as system architecture (especially how the devices are connected) also affect the size of the components. Unfortunately, it is impossible to achieve 100% conversion efficiency, so the loss must be considered. In the above discussion, the size of the energy storage and solar array determines the energy and power that can be provided. To calculate the power that needs to be generated, you need to consider the power electronics.
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