Discuss distributed power in smart grids

The concept of a distributed energy system was promoted in the United States after the promulgation of the US Public Utility Management Policy Act in 1978, and was accepted by other developed countries. A distributed energy system is a clean and environmentally friendly power generation facility located at or near the power load center with power ranging from tens of kilowatts to tens of megawatts distributed modularly near the load, which can generate electricity economically, efficiently and reliably. Distributed power generation is a form of power generation that differs from traditional centralized power generation, long-distance transmission, and large Internet. The distributed energy system does not simply use traditional power generation technology, but is based on new technologies such as automatic control systems, advanced material technologies, and flexible manufacturing processes. It has low pollution emissions, flexibility and convenience, high reliability and High-efficiency new energy production system. The power generation system that constitutes the distributed energy system has the following characteristics: â‘  efficient use of waste energy generated by power generation to generate heat and electricity; â‘¡ renewable energy system on site; Use the system.

Distributed energy sources can use natural gas, coal bed methane and other fuels, and can also use waste gas resources such as biogas, coke oven gas, and even renewable resources such as wind energy, solar energy, and water energy. Currently, most distributed energy projects are built in cities, so most distributed energy fuels are natural gas or diesel. Specifically, the importance of developing distributed energy has the following aspects:

(1) Economical

Since distributed energy can use the waste heat of power generation for heating and cooling, the energy can be used in a reasonable cascade, which can improve the efficiency of energy use (up to 70% .90%). Due to the integration of distributed power sources, the construction of large power plants and high-voltage transmission grids has been reduced or delayed, and the construction of grids has been slowed down to save investment. At the same time, the power flow of the transmission and distribution network is reduced, and the network loss is reduced accordingly.

(2) Environmental protection

Because it uses natural gas as fuel or uses hydrogen, solar energy, and wind energy as energy sources, it can reduce the total amount of harmful substances and reduce the pressure on environmental protection. Not only reduces the electromagnetic pollution of high-voltage transmission lines, but also reduces the land acquisition area and line corridors of high-voltage transmission lines, and reduces the felling of trees under the line, which is beneficial to environmental protection.

(3) Diversity of energy use

Distributed power generation can use a variety of energy sources, such as clean energy (natural gas), new energy (hydrogen) and renewable energy (wind and solar energy), and at the same time provide users with a variety of energy applications such as cold, heat, electricity, etc. It is a good way to solve the energy crisis, improve energy efficiency and energy security.

(4) Peak shaving

Summer and winter are often the peak periods of load. At this time, the use of natural gas as the fuel gas turbine and other cold, heat and electricity triple supply systems can not only solve the winter and summer cooling and winter heating needs, but also provide a part Electricity can thus play a role in cutting peaks and filling valleys. In addition, it also partially solved the problem of excessive peak-valley difference in natural gas supply, and played a complementary role between natural gas and electricity.

(5) Safety and reliability

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

(6) Power market issues

Distributed power generation can meet the needs of the development of the electricity market, with multiple funds raised to run electricity, and give play to the competition mechanism in 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. Therefore, the investment risk brought by the construction period of similar large-scale power plants can be avoided.

(8) Power supply in remote areas Many remote and rural areas in China are far from large power grids, so it is difficult to supply power from large power grids. Independent power generation systems using solar photovoltaic power generation, wind power generation and biomass power generation are a preferred method.

From a global perspective, countries with higher energy utilization rates and better environmental protection are more enthusiastic about the development and application of distributed energy technologies, and the clearer the support policy. For example, Denmark, the Netherlands, and Japan have adopted a series of incentive policies for distributed power; after the "911 incident", developed countries have accelerated the pace of distributed power construction for the sake of power supply safety. Up to now, the United Kingdom has There are more than 1,000 distributed power stations; there are more than 6,000 distributed power stations in the United States, and more than 200 distributed power stations on university campuses alone. Among many countries, Denmark is recognized as a model for the organic integration of economic development, resource consumption and environmental protection in the world. It is a country that has achieved sustainable development. In the past 20 years, Denmark ’s gross national product has doubled but energy consumption has not increased, and environmental pollution has not increased. The secret lies in Denmark ’s active development of combined heat, cold, and electricity, the promotion of scientific energy use, and the support of distributed energy. Support the development of the national economy by improving energy efficiency. At present, there is no thermal power plant in Denmark that does not provide heat, and no heating boiler room does not generate electricity. It turns the generation of cold, heat and electricity products into high-tech combined generation of cold, heat and electricity, making scientific and technological progress true. Productivity.

According to literature reports, between 2010 and 2010, 25% to 30% of the cumulative new generation capacity worldwide will be distributed generation. The United States is the country with the most development of new energy and renewable distributed energy power generation in the world, and it is also the main provider of most commercial distributed power equipment in the world. In 2004, the total distributed generation capacity of the United States was 67 GW, accounting for about 7% of the total domestic generation. It reached the world average. According to the forecast of the Electric Power Research Institute, 25% of the new generation capacity in the United States will be used in 2010 Distributed power, and the National Natural Gas Foundation estimates that it is as high as 30%. By 2020, more than half of new commercial or office buildings will use distributed power, and by 2020 15% of existing buildings will use distributed power. The development of distributed power in Europe is at a leading level in the world; in 2000, the installed capacity of distributed power in the EU was 74 GW, while the total power generation of distributed power in Denmark, the Netherlands and Finland accounted for 52% of the total domestic power generation , 38% and 36%, 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. Development status and expectations of distributed power generation at home and abroad

In recent years, distributed power generation technology has attracted more and more attention because of its unique environmental protection and economy. Based on these understandings of distributed energy systems, many countries have already formulated magnificent distributed energy system research and development plans, and conducted many related researches. The University of Newcastle, United Kingdom, is developing a comprehensive distributed energy system evaluation software that can combine theory and engineering projects for the design, optimization and optimization of distributed energy systems driven by micro gas turbines, fuel cells and gas internal combustion engines. monitor. Institutions such as the University of California, ELCOM, etc. have formulated magnificent research plans for distributed energy systems. Australian research institutions are establishing an energy center in Newcastle, the purpose of which is to provide the latest research results and development facilities in the field of energy, provide technical support to more than 100 research groups, and demonstrate application cases of new energy technologies. The International Energy Agency (IEA) is carrying out an international energy technology research and development cooperation plan including 33 countries to carry out technological development and improvement in the fields of energy production and energy consumption. Currently, there are 40 research projects in progress, including fossil fuels Technologies, distributed energy systems, energy efficiency technologies for end users, etc. These projects involve more than 400 government or private research institutions at an annual cost of $ 120 million.

In our country, adequately ensuring power supply will play a decisive role in the sustainable development of the economy. On the basis of the established central power station and power grid, vigorously developing distributed power supply technology will be an inevitable trend for the future development of China's power system. At present, China's research on distributed energy systems has started in China. Some scientific research institutions and universities have invested manpower and financial resources to conduct research on distributed energy systems. Shanghai University of Science and Technology built a C60 micro gas turbine produced by Capstone as the core, combined with supplementary combustion waste heat boiler, supplementary combustion absorption chiller, cold storage, heat storage system, etc., to build a demonstration "energy island" for the research of distributed energy systems . With a 100kW micro gas turbine as the core, Xi'an Jiaotong University has established a distributed energy system with the hotel as the application object. 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 Laboratory of Energy Cleanliness and Key Utilization of North China Electric Power University has established an experimental platform of dual-source reversible heating (air conditioning) system.

In recent years, China has attached great importance to the development and utilization of renewable energy, and has accelerated the development of renewable energy such as wind energy, solar energy, and biomass energy as an important strategy for energy development during the "11th Five-Year Plan" period. The effective form of utilization of renewable energy is to vigorously develop distributed power sources in distributed energy systems. In the "Energy Conservation Law of the People's Republic of China", "Regulations on the Development of Combined Heat and Power", and the "Eleventh Five-Year Plan" for Energy Development, it is clearly stated that: vigorously promote combined heat and power, central heating; increase heat and power The utilization rate of the unit; develop the cascade utilization technology of heat energy, the combined heat, electricity and cold generation technology and the triple supply technology of heat, electricity and gas to improve the comprehensive utilization rate of heat energy.

In the rapid economic development of China in the past two decades, environmental pollution and ecological problems caused by coal-based energy structure have put tremendous pressure on China's sustainable development. The Chinese government has recently begun to consider major adjustments to its energy strategy. It has successively made the decision to transfer natural gas from west to east and from west to east to speed up 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 pressure on environmental and ecological protection. These have created a huge demand for the distributed energy system of combined heat, power and cooling. It is estimated that tens of thousands of distributed energy stations will need to be built in China in the next ten years, which is an unprecedented historical opportunity. At present, it is at the beginning of this historical stage. However, if from now on, distributed energy stations will develop at a rate of hundreds or even thousands per year, then immediately a series of questions arise: where does the capital come from, where does the technology depend, and how does the user market How to develop and how it will develop, whether the current government policies and regulations adapt 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 grid companies and natural gas companies. These problems are extremely realistic and severe challenges for the development of distributed energy stations. The distributed energy stations currently under construction in Beijing, Shanghai, Guangzhou and other places have encountered many obstacles and challenges in these areas. It is unrealistic to think that distributed energy will naturally develop smoothly, or that there is no way to overcome the current obstacles and challenges, so that it cannot be developed. We must seriously face the current opportunities and challenges in the development of distributed energy, and study and formulate its rapid development strategy in China.

At present, China's natural gas-based distributed energy system construction has gradually entered the stage of substantial development and implementation. In China's cities and towns, distributed power generation technology as an important supplement to the centralized power supply technology will become an important development direction. The first technology of combined heat, power and cooling is the first, because for residential, commercial buildings, hospitals, public buildings and factories in most parts of China, there are power supply and heating or cooling needs, and many are equipped with backup power generation equipment. A broad market for multi-target distributed energy supply systems for combined heat, power and cooling. In residential communities, shopping mall buildings, and university towns in major cities such as Beijing, Shanghai, and Guangzhou, a number of demonstration projects of combined heat, power, and cold generation have been put into operation. For example: Shanghai Pudong International Airport Energy Center 4000 kW gas turbine cogeneration project; Shanghai Huangpu District Central Hospital 1000 kW gas turbine cogeneration project; Beijing Zhongguancun Software Park Cogeneration Project. For the vast economically underdeveloped rural areas (agricultural and pastoral areas and remote mountain areas), the formation of a certain scale and strong centralized power supply and distribution network requires huge investment and a long time period. Energy supply seriously restricts these Regional economic development. Distributed power generation technology can just make up for these limitations of centralized power generation. For example, in the vast rural areas of northwestern China, the wind resources are very rich. For example, Inner Mongolia has formed an annual power generation of 100 million kWh. In addition to its own use, it can also be sent to Beijing. This non-polluting green energy can reduce the local environment. Pollution.

In general, the research of distributed energy systems has achieved fruitful research results in advanced Western countries. The research field has ranged from distributed energy systems operating alone to the connection of distributed energy systems and large power grids that have just emerged. Has extensive experience in the operation of distributed energy systems. However, domestic research on distributed energy systems is still in its infancy, and large-scale utilization of distributed energy systems still has a long way to go. There are many studies on distributed energy systems operating independently at home and abroad, mainly on the economic evaluation and evaluation standards of various distributed energy systems that are not connected to the grid. However, there are few domestic and foreign studies on the distributed energy system connected to the large power grid. There are mainly distributed generation and its impact on the power system in China, and the impact of distributed generation on the voltage distribution of the distribution network.

3. Classification of distributed power generation and selection of energy forms

â‘´ Distributed generation technology based on fossil fuels

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

â‘¡ Micro gas turbine technology: Micro gas turbine refers to the ultra-small gas turbine with power of less than several hundred kilowatts and using natural gas, methane, gasoline and diesel as fuel. 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% -15%, so the current method of cogeneration of domestic heat and power is used to utilize the waste heat 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 combined heat and power, low pollution, and simple operation and maintenance. It is currently one of the most mature and commercially competitive distributed power sources. Its technical key is mainly high-speed bearings, high-temperature materials, parts processing, etc.

â‘¢ Fuel cell technology: a fuel cell is an electrochemical device that directly converts chemical energy into direct current energy at an isothermal state. When the fuel cell is working, it does not need to burn, and at the same time does not pollute the environment, its electrical energy is obtained through an electrochemical process. A hydrogen-rich fuel is passed on the anode, air is passed on the cathode, and the two substances are separated by the electrolyte. In the process of obtaining electrical energy, some by-products are only heat, water and carbon dioxide. Hydrogen fuel can be produced from various sources of hydrocarbons, under pressure, through steam reforming processes or from oxidation reactions.

⑵ Hybrid distributed generation technology

An important direction is the multi-target distributed energy supply system mentioned above for combined heat, power and cooling, usually referred to as distributed energy supply system for short. While producing electricity, it can also provide thermal energy or meet the needs of heating and cooling. Compared with a simple power supply system, the distributed energy supply system can integrate the scientific energy consumption of cascades, thereby greatly improving energy utilization, reducing environmental pollution, and improving the thermal economy of the system. There are two types of cogeneration:

â‘  Mainly power generation, using large-capacity units, only a small part of the waste heat is used, and the power enters the high-voltage transmission grid for distribution.

â‘¡ Set electricity with heat, and on the premise of satisfying a certain heat load demand, build a small heating unit as much as possible. The electricity is digested by the user itself or in the local distribution network area.

⑶ Distributed generation technology based on renewable energy

â‘ 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 no fuel consumption, no geographical restrictions, flexible scale, no pollution, safe and reliable, and simple maintenance. 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 a wide range of applications.

â‘¡Wind power generation technology: The power generation technology that converts wind energy into electrical energy has become more and more popular because of its environmentally friendly, renewable, globally feasible, low cost, and significant scale benefits. It has become one of the fastest growing new energy sources. It can be divided into two types: independent and grid-connected operation. The former is a micro or small wind turbine with a capacity of 100W to 10kW, and the latter usually has a capacity of more than 150kW. There are usually multiple wind turbines with larger capacity to form a wind turbine group It is called a wind farm (also called wind farm or wind farm), which has the characteristics of large-scale generating units, centralized installation and control. In recent years, the technology of wind power generation has made rapid progress, and the technology with a unit capacity of less than 2MW has been very mature.

4. The 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 to complement each other. Combining the advantages of traditional power grid and distributed power generation, they are reasonably integrated. There are three specific ways of networking: (1) The output of distributed power is converted from direct rectification to AC output, and it is required to maintain absolute Synchronization; (2) After the distributed power is converted into AC output, it directly supplies certain loads to complement the large power grid. (3) Isolate the distributed power source from the large power grid and completely become an independent power generation system.

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