Human activities are the main influencing factor of climate change, and most of this impact comes from fossil fuels in the power industry. In 2012, 32% of greenhouse gas emissions in the United States came from the electricity industry, which is the main source of greenhouse gases. Traditional fossil fuel power generation equipment meets the majority of global energy requirements in the past. However, the impact of environmental and climate change poses serious challenges to social and environmental issues for fossil fuel power generation. Distributed generation (DG), especially photovoltaic (PV) power generation systems, has alleviated the challenges posed by climate change. Unlike off grid solar power generation systems, grid connected solar power generation systems (GCPVS) operate in parallel with the power grid and do not require energy storage systems. If GCPVS produces excess electricity (such as when the generated electricity exceeds the local load demand), the remaining electricity will be integrated into the large power grid. In addition, GCPVS can reduce transmission and distribution (T&D) losses. In 2010, although the average power loss in the United States reached 5.7%, the power loss was even smaller during peak periods. For example, in 2010, Southern California’s Edison and Pacific gas and electricity losses are expected to exceed 10%. DG can reduce the power loss caused by local load power supply.
The main focus is on the development and technical challenges of solar grid connected power generation systems in terms of overall performance and grid reliability. Standard management of GCPVS safety installation, operation, and maintenance, as well as improving the efficiency of solar systems using existing methods. The role of inverters in solar systems cannot be underestimated. The design of the inverter is to support and assist the power grid, especially considering the increasing number of small-scale GCPVS integrated into the power grid. Recently, HECO announced support for intelligent inverters that can instantly change low (high) voltage and frequency drops (or peaks).
1.Development status and trends of solar grid connected systems
Due to the decrease in prices of silicon and solar modules, technological progress in large-scale production, government encouragement of mature and good diffusion interconnection protocols, and continuous improvement of power converter technology, the solar energy industry will continue to maintain rapid development.
According to the 2013 summary review by the Solar Energy Industry Association (SEIA), the average price of solar systems was $2.59/W, and the average price of solar panels decreased by 60%. Following Swanson, the average global solar production capacity doubles every time, while the cost of solar cells decreases by 20%, further reducing the overall system cost of solar systems. In 2013, at least 3 GW of solar energy systems were installed globally, bringing in 139 GW of global solar power generation systems. From the initial stage of the development of the solar energy industry in the 1970s to 2012, the number of solar system installations worldwide accounted for 30% of the total installed capacity in 2013 alone. It is estimated that by 2018, the total installation of new solar energy may exceed 68 GW, and the total installation of global solar systems will be three times that of 2013. The rapid growth rate of GCPVS, especially in terms of utility of public facilities, is mainly due to the promotion and expansion of DG by the government and regulatory agencies. The clean energy plan proposed by the US Environmental Protection Agency in June 2014 will help reduce emissions by 30% over a period of 15 years. The EPA hopes to achieve better development of GCPVS through the public industry and promote the utilization of renewable energy. In addition, the government has responded to climate change through tax incentives and legislative policies, which have helped increase the financial feasibility of solar energy systems. The program initiative developed by the US Department of Energy Sunshot in 2011 aims to reduce the cost of solar systems to 75% on a large scale through the use of scientific research and advanced technology, narrowing the gap with fossil fuel power generation costs, before the government no longer subsidizes new energy development in 2020.
At the local and national government levels, the Renewable Energy Investment Portfolio Standard (RPS) seeks to increase mixed public industries by ensuring a fixed quantity of renewable energy within the power generation investment portfolio service area. Since 2014, both the District of Columbia and the second largest region of the United States have passed RPS legislation, specifying which public utilities belong to RPS and the percentage of new energy demand in the total power generation portfolio within the specified time frame in the future.
The RPS policy has become a major driving force in the new energy market, and the US energy market may increase by 3-5 GW/a until 2020. If fully compliant with the policy, a total of 94GW of new energy will be put into the market by 2035. NREL analyzed the demand for renewable energy from 2004 to 2015 in the report. According to this report, by the end of 2015, following the RPS policy will increase the demand for renewable energy by nearly twice compared to voluntary demand. There is a strong correlation between the introduction of the RPS policy and the general demand for renewable energy.
One of the strictest areas for RPS implementation is in California, USA, which requires investors’ utility and power service providers to increase their renewable energy procurement to 30% of the total procurement volume by 2020. The RPS of New Jersey requires 22.5% of electricity to be generated from renewable energy by the end of 2021.
In fact, in 2005, New Jersey was the first city in the United States to establish a Solar Renewable Energy Certificate (SREC), with the entire project generating 76 MW of solar energy in the first two years. Seven other states and the District of Columbia have also begun implementing RPS policies.
2. Challenges faced by solar grid connected systems
With the reduction of GCPVS costs, the application of solar power generation technology in residential, commercial, and public utilities is becoming increasingly widespread. Although the photovoltaic grid connected power generation mode has advantages such as long working life (20-30 years), low operating and maintenance costs, and better environmental benefits compared to fossil fuel combustion power generation, GCPVS still faces significant challenges. Some studies have shown that the investment in a large and extensive solar grid connected power generation facility can cause significant pressure on the large power grid. If there is no technical guarantee for public facilities, PVS will affect the power generation quality of the large power grid. Nowadays, how to reasonably schedule GCPVS will be the main strategy to solve the negative impact on the power grid. Due to the high permeability of GCPVS and the unpredictability of its output electricity, in the near future, some public utilities and independent system operators will force GCPVS to implement more rigorous management with the large power grid.
These requirements are inherent in GCPVS. Initially, it was because natural laws would affect the output of solar systems, and as demand increased, GCPVS could not fully provide electricity to the grid. Although during this period, power companies would improve traditional power equipment to meet the large demand for electricity. The California Independent Systems Operator (CAISO) used a “duck curve” to illustrate the impact of GCPVS on the power grid based on actual time analysis and grid demand forecasting from 2012 to 2020. The power grid requires the load to represent the number of traditional power generation equipment (including renewable energy generation) that are online at different times of the day. The preferred solution is to first couple the GCPVS energy storage system that cannot be scheduled, while extending the time for solar power generation.
In 2010, California passed Congressional Act (AB) 2514, authorizing the California Public Utilities Commission to set targets for large-scale investment in power facilities. Subsequently, in October 2013, the California Public Utilities Commission (CPUC) adopted energy storage procurement, authorizing utility investors and power companies to develop 1325 MW of electricity storage and 1% of the 2020 peak load under AB 2514. In addition, the storage requirements of new energy will promote the response of management needs, peak shaving of power plants, reducing conventional power generation costs, improving the stability of the power grid, and providing backup power in the event of power outages.
The development of energy storage systems faces enormous challenges and uncertainties, especially in the power generation industry where energy storage is still a relatively new technology. Large scale storage application facilities often face technological and market barriers, resulting in costs far greater than traditional power generation models. Because the electricity market price is largely based on the balance between load demand and power plant supply, there is an inevitable risk of updating the power generation model. In this case, oversupply will increase costs. In addition, the power generation mode of energy storage will face higher balancing costs for power utility suppliers, leading to the choice of transferring customers.
3. Optimal efficiency of solar grid connected systems
Due to the limitations of natural conditions in the operation of solar power generation systems, it is difficult to predict the amount of electricity transmitted to the load. However, a more efficient GCPVS system will not cause interference to the large power grid and loads it serves. By improving the performance of the system, the output power is maximized when the radiation intensity is low, and the predictability of the output electrical energy is improved. Here are three ways to improve work efficiency.
3.1 Maximum Power Point Tracking Method (MPPT)
The I-U and P-U curve characteristics of solar panels are highly non-linear, but they reach their maximum power point at a certain time under light intensity, which is influenced by the surrounding temperature and solar radiation intensity. The randomness of external temperature and changes in solar radiation intensity are a major factor in the instability of the efficiency of PV systems. The MPP tracker continuously calculates and analyzes the output electrical energy of the PV system without considering the load, ambient temperature, and light intensity. The MPP tracker controls DC/DC conversion and adjusts pulse width at different cycles.
MPPT plays an indispensable role in solar grid connected power generation systems and other grid connected power generation equipment, especially in controlling off grid solar power generation equipment to ensure that the load can obtain more reliable and stable electricity. In fact, the voltage of the common point is clamped by the public power grid, and MPPT utilizes voltage regulation and control to maximize the output power of GCPVS. When the external temperature and solar radiation intensity change, the output current of the solar module will also change. The MPPT system needs to respond quickly and ensure the maximum power output of the solar panel.
3.2 Solar tracking
Unlike inverters based maximum power tracking converters, solar trackers obtain the maximum amount of solar energy by monitoring and controlling the radiation angle of the sun through mechanical rotors. The positioning plate limits the surface area of direct contact with the sun. There are currently many devices that can detect the position of the sun.
For many solar applications, GCPVS can reliably predict changes in solar position and the amount of solar energy available, and is also an important parameter for designing an effective solar tracking system. However, in fact, even without considering the position of the sun and natural conditions, the cost of using a solar tracking system will far outweigh the benefits it brings. Firstly, adding components and more complex shelves will increase operational and maintenance costs, taking into account more frequent repairs, shutdowns, and downtime, which will increase the cost of using solar trackers (especially for large-scale PV systems). Secondly, because the solar tracker uses LDRS or photodiodes, external humidity and temperature can have an impact on the system, requiring more complex control algorithms. The complexity of hardware and software determines the increase in cost.
NREL research compared fixed tilt angle and single axis tracking polycrystalline silicon photovoltaic arrays. In fact, the annual operating and maintenance costs of single axis tracking PV systems exceed 200% of fixed tilt angle tracking systems, and the investment payback period is about 9 years. Research has shown that PV output DC power requires a solar tracking device, and the cost is much higher than a fixed tilt angle solar system.
3.3 Inverters without transformers
An inverter is a device that converts the direct current output of a PV system into the alternating current required by the power system. In many inverters, transformers serve as insulation components between direct current and alternating current, ensuring that sensitive electronic components on the DC side of the inverter are not damaged. Electrical isolation ensures that there is no physical connection between the primary and secondary windings. In other cases, the transformer needs to accelerate (increase) the output DC voltage of the solar module to match the AC voltage of the power grid. In addition, transformers can also filter unwanted high-frequency signals and noise signals generated in inverters.
The removal of transformers reduces the size and weight of the inverter, as well as energy loss (improving system efficiency), reducing complexity while also reducing the cost of the inverter. The benefits created by inverters without transformers exceed their costs, and some literature has shown that the efficiency of inverters without transformers can reach 97% in terms of economy and operability. For single-phase inverters without transformers, the European Union (EU) and the California Energy Commission (CEC) have found that the utilization efficiency of inverters in the market reaches 96% to 98%.
SMA solar technology is the world’s largest inverter manufacturer, and commercial transformers without inverters have higher peak power in the megawatt range than grid connected power generation systems. The benefits created by MPPT and transformer free inverters can reach up to 98.7%, meeting the Arc Fault Requirements (NEC) Article 690 of UL1741 and NFPA 70.
4. Conclusion
In recent years, although the solar photovoltaic market has experienced rapid development and significant cost reduction, it still faces technological challenges and the coordination between the actual economic situation and DG energy, keeping costs at a comparable level to traditional power generation models. To successfully adopt GCPVS on a large scale, it is necessary to develop new technology inverters that not only provide conversion functions between DC and AC. Within a certain cost, modern grid interactive inverters need to provide reactive power and voltage optimization control (power factor and voltage stability), frequency regulation, expanded storage, and fully utilize modern communication protocols. The new generation of inverters will be “smart inverters”.
Future GCPVS design requires inverter monitoring, based on instantaneous feedback response and adjustment of grid output. The inverter can also save and share data on trends, predictions, prevention, and maintenance in the facility management system. The new generation of intelligent inverters can record some data, such as fully utilizing battery storage time and capacity information, providing external alarms, and providing daily energy management information. Rethinking the role and functionality of inverters can expand GCPVS and create and support a more reliable power grid.