Discussing Distributed Power Generation in Smart Grid

The concept of distributed energy system was promoted in the United States after the promulgation of the U.S. Public Utility Management Policy Act in 1978, and then accepted by other developed countries. Distributed energy system is a clean and environmentally friendly power generation facility located in or near the power load center, with a power of tens of kilowatts to tens of megawatts and distributed near the load in a modular manner, which can generate electricity economically, efficiently and reliably. Distributed power generation is a form of power generation that is different from traditional centralized power generation, long-distance transmission, and the big Internet. Distributed energy system does not simply adopt traditional power generation technology, but is based on new technologies such as automatic control systems, advanced material technology, and flexible manufacturing processes. It has low pollution emissions, flexibility, convenience, high reliability and high efficiency. The power generation system that constitutes the distributed energy system has the following characteristics: ① Efficient use of waste energy generated by power generation to generate heat and electricity; ② Renewable energy system at the site; ③ Energy recycling system that uses on-site waste gas, waste heat and excess pressure difference to generate electricity.

　　Distributed energy can use fuels such as natural gas and coalbed methane, as well as waste gas resources such as biogas and coke oven gas, and even renewable resources such as wind energy, solar energy, and hydropower. Since most distributed energy projects are currently built in cities, most distributed energy fuels are natural gas or diesel. Specifically, the importance of developing distributed energy is as follows:

　　(1) Economical

　　Since distributed energy can use the waste heat of power generation for heating and cooling, energy can be reasonably utilized in a tiered manner, thereby improving energy utilization efficiency (up to 70% to 90%). Due to the grid connection of distributed power sources , the construction of large power plants and high-voltage transmission networks is reduced or delayed, and the construction of the power grid is delayed, saving investment. At the same time, the flow of the transmission and distribution network is reduced, which correspondingly reduces network losses.

　　(2) Environmental protection

　　Because it uses natural gas as fuel or hydrogen, solar energy, and wind energy as energy, it can reduce the total amount of harmful emissions and alleviate environmental pressure: a large amount of local power supply reduces the construction of large-capacity, long-distance, high-voltage transmission lines, thereby not only reducing the electromagnetic pollution of high-voltage transmission lines, but also reducing the land acquisition area and line corridors of high-voltage transmission lines, and reducing the felling of trees under the lines, which is beneficial to environmental protection.

　　(3) Diversity of energy use

　　Distributed power generation can utilize a variety of energy sources, such as clean energy (natural gas), new energy (hydrogen) and renewable energy (wind energy and solar energy, etc.), and at the same time provide users with multiple energy application methods such as cooling, heating, and electricity. Therefore, it is a good way to solve the energy crisis, improve energy utilization efficiency and energy security issues.

　　(4) Peak load regulation

　　Summer and winter are often peak load periods. At this time, if a gas turbine using natural gas as fuel is used, the combined cooling, heating and electricity system can not only meet the needs of cooling in summer and winter and heating in winter, but also provide a part of electricity, which can play a role in reducing peaks and filling valleys in the power grid. In addition, it also partially solves the problem of excessive peak-to-valley differences in natural gas supply, and plays a complementary role between natural gas and electricity.

　　(5) Security and reliability

　　When a large-scale power outage occurs in the main power grid, the specially designed distributed power generation system can still maintain normal operation, thereby improving the safety and reliability of power supply.

　　(6) Electricity market issues

　　Distributed power generation can meet the needs of the development of the electricity market, and multiple companies can pool funds to run electricity, giving play to the competitive mechanism of the electricity construction market and the electricity supply market.

　　(7) Investment Risk The installed capacity of distributed power generation is generally small and the construction period is short, thus avoiding the investment risks brought about by the construction period of similar large-scale power stations.

　　(8) Power supply issues in remote areas Many remote and rural areas in China are far away from the main power grid, so it is difficult to supply them with power from the main power grid. An independent power generation system using solar photovoltaic power generation, wind power generation and biomass power generation is an optimal method.

　　From a global perspective, countries with higher energy utilization and better environmental protection are more enthusiastic about the development and promotion of distributed energy technology, and their supporting policies are clearer. For example, Denmark, the Netherlands, and Japan have adopted a series of encouraging policies for distributed power sources ; after the "911 incident", developed countries have accelerated the pace of distributed power construction for the sake of power supply safety. So far, there are more than 1,000 distributed power stations in the UK; there are more than 6,000 distributed power stations in the United States, and there are more than 200 distributed power stations on university campuses alone. Among many countries, Denmark is recognized as a model of organic combination of economic development, resource consumption and environmental protection in the world, and is a country that has achieved sustainable development. Over the past 20 years, Denmark's gross national product has doubled, but energy consumption has not increased, and environmental pollution has not intensified. The secret lies in Denmark's active development of combined cooling, heating and electricity, advocating scientific energy use, supporting distributed energy, and supporting the development of the national economy by improving energy utilization. At present, there is no thermal power plant in Denmark that does not provide heat, and no heating boiler room that does not generate electricity. The separate generation of cold, heat and electricity products has been transformed into high-tech cogeneration of cold, heat and electricity, turning scientific and technological progress into real productivity.

　　According to literature reports, 25% to 30% of the world's cumulative new power generation capacity before 2010 was distributed power generation. The United States is the country with the most development of new energy and renewable distributed energy generation in the world, and is also the main provider of the vast majority of commercial distributed power equipment in the world. In 2004, the total capacity of distributed power generation in the United States was 67 GW, accounting for about 7% of the total domestic power generation in the United States, reaching the world average. According to the forecast of the Electric Power Research Institute of the United States, 25% of the new power generation capacity in the United States in 2010 will use distributed power sources, while the National Natural Gas Foundation estimates that it will be as high as 30%. By 2020, more than half of the new commercial or office buildings will use distributed power sources, and by 2020, 15% of existing buildings will use distributed power sources. The development of distributed power sources in Europe is at the leading level in the world; in 2000, the installed capacity of distributed power sources in the European Union was 74 GW, and in 2004, the total power generation of distributed power sources in Denmark, the Netherlands, and Finland accounted for 52%, 38%, and 36% of the total domestic power generation, respectively. The European Union predicts that it will reach 195 GW in 2020, and the power generation will reach 22% of the total power generation.

　　2. Current status and prospects of distributed power generation development at home and abroad

　　In recent years, distributed power generation technology has attracted more and more attention with its unique environmental protection and economical characteristics. Based on these understandings of distributed energy systems, many countries have formulated grand distributed energy system research and development plans and conducted relevant research in many aspects. Newcastle University in the UK is developing a comprehensive distributed energy system evaluation software that can combine theory with engineering projects for the design, optimization and monitoring of distributed energy systems driven by micro gas turbines, fuel cells and gas internal combustion engines. Institutions such as the University of California, ELCOM, etc. have formulated grand research plans for distributed energy systems. Australian research institutions are establishing an energy center in Newcastle, with the aim of providing the latest research results and development facilities in energy, providing technical support to more than 100 research groups, and demonstrating application cases of new energy technologies. The International Energy Agency (IEA) is conducting an international energy technology research and development cooperation program involving 33 countries to develop and improve technologies in the fields of energy production and energy consumption. Currently, there are 40 research projects underway, including fossil fuel technology, distributed energy systems, energy efficiency technologies for end users, etc. These projects involve more than 400 government or private research institutions, with an annual cost of $120,000,000.

　　In my country, sufficient power supply will play a decisive role in the sustainable development of the economy. On the basis of the built central power station and power grid, vigorously developing distributed power technology will be an inevitable trend for the future development of my country's power system. At present, the research on distributed energy systems in my country has begun in China, and some scientific research institutions and universities have invested manpower and financial resources in the research of distributed energy systems. Shanghai University of Technology built a demonstration "energy island" with the C60 micro gas turbine produced by Capstone as the core, combined with supplementary combustion waste heat boilers, supplementary combustion absorption chillers, cold storage, and heat storage systems for the research of distributed energy systems. Xi'an Jiaotong University established a distributed energy system with hotels as the application object with a 100kW micro gas turbine as the core. The Institute of Engineering Thermophysics of the Chinese Academy of Sciences has also conducted many studies on advanced system methods and related evaluations of distributed energy systems. The Clean and Key Utilization Laboratory of Energy of North China Electric Power University has established a dual-source reversible heating (air conditioning) system experimental platform.

　　In recent years, my country has attached great importance to the development and utilization of renewable energy, and has made accelerating the development of renewable energy such as wind energy, solar energy, and biomass energy an important strategy for energy development during the "Eleventh Five-Year Plan" period. The effective form of utilizing renewable energy is to vigorously develop distributed energy systems and distributed power sources. In laws and regulations such as the "Energy Conservation Law of the People's Republic of China", "Regulations on the Development of Cogeneration", and "The 11th Five-Year Plan for Energy Development", it is clearly stated that: vigorously promote cogeneration and centralized heating; improve the utilization rate of thermal power units; develop thermal energy cascade utilization technology, heat, electricity, and cooling cogeneration technology, and heat, electricity, and gas trigeneration technology to improve the comprehensive utilization rate of thermal energy.

　　In the past two decades of rapid economic development in my country, environmental pollution and ecological problems caused by the coal-based energy structure have put tremendous pressure on my country's sustainable development. The Chinese government has recently begun to consider major adjustments to its energy strategy, and has made decisions to transmit gas from west to east, transmit electricity from west to east, and accelerate the introduction of natural gas. The sustained and rapid development of the national economy requires the guarantee of energy supply and the advanced development of electricity, coupled with the need to improve energy efficiency and the pressure of environmental and ecological protection, which have created a huge demand for distributed energy systems for combined heat, power and cooling. It is estimated that in the next decade or so, tens of thousands of distributed energy stations will need to be built in China, which is an unprecedented historical opportunity. At present, it is at the beginning of this historical stage. However, if it is said that from now on, distributed energy stations will develop at a rate of hundreds or even thousands per year, then a series of problems will immediately arise: where will the funds come from, what is the technical support, how to open up the user-end market and how it will develop, whether the current government policies and regulations are adapted to this development trend, who will be the main body of distributed energy construction and development, and how to coordinate the relationship between distributed energy stations and power grid companies and natural gas companies. These problems are extremely realistic and severe challenges to the development of distributed energy stations. Distributed energy stations currently being planned and built in Beijing, Shanghai, Guangzhou and other places have encountered many obstacles and challenges in these aspects. It is unrealistic to think that distributed energy will develop smoothly and naturally, or that there is no way to overcome the current obstacles and challenges and therefore it is impossible to develop. We must seriously face the opportunities and challenges in the current development of distributed energy, and study and formulate a strategy for its rapid development in China.

　　At present, the construction of distributed energy systems fueled by natural gas in China has gradually entered the stage of substantial development and implementation. In China's towns, distributed power generation technology, as an important supplement to centralized power supply technology, will become an important development direction. The first choice is the trigeneration technology of heat, power and cooling, because for most parts of China's residential, commercial buildings, hospitals, public buildings, and factories, there are power supply and heating or cooling needs, and many are equipped with backup power generation equipment. These are the vast markets for multi-objective distributed energy supply systems of trigeneration of heat, power and cooling. In residential areas, shopping malls, and university towns in large cities such as Beijing, Shanghai, and Guangzhou, a number of demonstration projects for combined heat, electricity, and cooling have been put into operation. For example: 4000 kW gas turbine combined heat and power project of Shanghai Pudong International Airport Energy Center; 1000 kW gas turbine combined heat and power project of Shanghai Huangpu District Central Hospital; Beijing Zhongguancun Software Park combined heat, electricity, and cooling project, etc. For the vast rural areas with underdeveloped economy (agricultural and pastoral areas and remote mountainous areas), it takes huge investment and a long time to form a certain scale and strong centralized power supply and distribution network. The energy supply seriously restricts the economic development of these areas. Distributed power generation technology can just make up for these limitations of centralized power generation. For example, in the vast rural areas in northwest China, wind resources are very rich. For example, Inner Mongolia has formed an annual power generation of 100 million kWh of electricity. In addition to self-use, it can also be sent to Beijing. This pollution-free green energy can reduce local environmental pollution.

　　In general, the research on distributed energy systems has achieved fruitful results in advanced Western countries. The research fields range from independently operated distributed energy systems to the connection of the newly emerging distributed energy systems with large power grids. There is already rich experience in the operation of distributed energy systems. However, the research on distributed energy systems in China is still in its early stages, and there is still a long way to go for the large-scale utilization of distributed energy systems. There are many studies on independently operated distributed energy systems at home and abroad, mainly on the economic evaluation and evaluation standards of various non-grid-connected distributed energy systems. As for the research on distributed energy systems connected to large power grids, there are fewer studies at home and abroad. The main domestic studies include distributed generation and its impact on the power system, and the impact of distributed generation on the voltage distribution of the distribution network.

　　3. Classification of distributed generation and suggestions for energy form selection

　　⑴ Distributed power generation technology based on fossil fuels

　　① Reciprocating engine technology: The reciprocating engine used for distributed power generation adopts a four-stroke ignition or compression ignition type, and uses gasoline or diesel as fuel. It is currently the most widely used distributed power generation method. However, this method will cause an impact on the environment. Recently, through the improvement of its technology, the noise and exhaust gas emission pollution have been greatly reduced.

　　② Micro gas turbine technology: Micro gas turbine refers to an ultra-small gas turbine with a power of less than hundreds of kilowatts and fueled by natural gas, methane, gasoline, and diesel. However, compared with other existing power generation technologies, micro gas turbines have lower efficiency. The efficiency of full load operation is only 30%, and at half load, its efficiency is only 10% to 15%. Therefore, the current method of household cogeneration is mostly used to utilize the waste heat energy of the equipment to improve its efficiency (up to 75% or even higher). The characteristics of micro gas turbines are small size, light weight, high power generation efficiency when implementing cogeneration, low pollution, and simple operation and maintenance. It is one of the most mature and commercially competitive distributed power sources . Its technical key is mainly high-speed bearings, high-temperature materials, component processing, etc.

　　③ Fuel cell technology: A fuel cell is an electrochemical device that directly converts chemical energy into direct current electrical energy under isothermal conditions. When a fuel cell is working, it does not require combustion and does not pollute the environment. Its electrical energy is obtained through an electrochemical process. A hydrogen-rich fuel passes through the anode, and air passes through the cathode, and the two substances are separated by an electrolyte. In the process of obtaining electrical energy, some by-products are only heat, water, and carbon dioxide. Hydrogen fuel can be generated from various carbon and hydrogen sources through a steam reforming process or an oxidation reaction under pressure.

　　⑵ Hybrid distributed generation technology

　　An important direction is the multi-objective distributed energy supply system of the above-mentioned trigeneration of heat, power and cooling, usually referred to as a distributed energy supply system. While producing electricity, it can also provide heat energy or meet the needs of heating, cooling and other aspects at the same time. Compared with a simple power supply system, a distributed energy supply system can integrate the scientific use of energy at different levels, thereby greatly improving energy utilization, reducing environmental pollution, and improving the thermal economy of the system. There are two situations for cogeneration of heat and power:

　　① Mainly for power generation, using large-capacity units, only a small part of the waste heat is used, and the electricity enters the high-voltage transmission network for distribution. This type of cogeneration should be classified as centralized power generation;

　　② Determine electricity based on heat. On the premise of meeting a certain heat load demand, try to build small heating units, and the electricity will be consumed by the users themselves or within the local distribution network area.

　　⑶ Distributed power generation technology based on renewable energy

　　① Solar photovoltaic power generation technology: Solar photovoltaic power generation technology uses the photoelectric effect of semiconductor materials to directly convert solar energy into electrical energy. Photovoltaic power generation has the advantages of not consuming fuel, not being restricted by geographical location, flexible in scale, pollution-free, safe and reliable, and simple to maintain. However, the cost of this distributed power generation technology is very high, so at this stage, solar power generation technology still needs to be improved to reduce costs and be suitable for widespread application.

　　②风力发电技术：将风能转化为电能的发电技术，由于风力发电环保可再生、全球可行、成本低且规模效益显著，已受到越来越广泛的欢迎，成为发展最快的新型能源之一。可分为独立与并网运行两类，前者为微型或小型风力发电机组，容量为100W～10kW，后者的容量通常超过150kW，通常有多台容量较大的风力发电机组构成风力发电机群，称其为风电场（也称风力田、风田），具有机组大型化，集中安装和控制的特点。近年来，风力发电技术进步很快，单机容量在 2MW以下的技术已很成熟。

　　4. Combination of distributed generation and traditional power grid

　　The traditional large power grid will coexist with the distributed power generation model for a period of time and complement each other. Combining the advantages of the traditional power grid and distributed power generation, they are reasonably integrated together. There are three specific ways to connect the network: (1) Rectify the output of the distributed power source from DC to AC output, and at the same time require it to maintain absolute synchronization with the AC large power grid; (2) After the distributed power source is converted into AC output, it directly supplies certain specific loads to complement the large power grid. (3) Isolate the distributed power source from the large power grid and make it a completely independent power generation system.