Overview of comprehensive energy utilization technology based on multi energy complementary

Introduction

With the rapid development of our society and the improvement of people's living standards, the demand for energy in all walks of life is increasing, and the quality of energy supply is also increasing. However, the pressure of environmental pollution and global warming brought by the large-scale use of traditional fossil energy is also increasing, and with the growing demand for global energy, fossil energy will also face the problem of shortage and depletion. In order to meet the goal of sustainable development of energy, the development and utilization of renewable energy has gradually become a research hotspot and an important development direction in the field of energy.

Renewable energy, such as solar energy, wind energy, geothermal energy and biomass energy, on the one hand has the advantages of environmental protection and inexhaustible, on the other hand has the disadvantages of large fluctuation and strong randomness. Under the condition of recent technology level, the single use of a certain renewable energy will meet many challenges in power generation and grid connection and meet user load demand. In the way of multi energy complementation, we can take the way of multi energy complementation according to the resource conditions on the supply side and the energy consumption characteristics on the demand side, adjust measures to local conditions, and make comprehensive utilization, so that all kinds of energy can form a benign interaction, learn from each other, make up for the shortage of single energy supply mode, and promote clean, efficient, safe and reliable production and supply of energy.

Multi energy complementation includes two meanings, that is, complementary utilization of different kinds of energy (such as fossil energy and renewable energy complementation), or complementary utilization of different forms of one kind of energy (such as coal and natural gas complementation of fossil energy, wind energy and solar energy complementation of renewable energy, etc.). In addition, according to the complementary effect, it can be divided into three categories: time complementary, thermal complementary and thermochemical complementary. Multi energy complementary is an important way of distributed energy development. Based on the classification of complementary effects, this paper summarizes the multi energy complementary technologies that are currently being applied and researched in order to provide reference for the development of distributed energy.

Multi energy complementary utilization technology

1 time complementary

(1) Complementary scenery

Renewable energy generally has the characteristics of periodic changes, such as for China's solar and wind energy resources, the wind in winter is large, the solar radiation intensity is small, the wind in summer is small, the solar radiation intensity is large, there is a natural complementarity between the two in time. Complementing wind energy and solar photovoltaic can not only improve the safety and reliability of power output, reduce the adverse impact on the power grid system, but also share transmission equipment, improve the utilization rate of power grid channel capacity and reduce the channel construction cost. Wind solar complementary power generation system is mainly composed of wind turbine, solar photovoltaic panel array, controller, battery, inverter and other parts. According to the local resource conditions, the key to further optimize the power output quality is to reasonably allocate the power generation ratio of photovoltaic cells and wind turbines. For the terminal type energy supply system, especially in rural or remote villages, when the wind and solar resources are good, the complementary operation of wind and solar energy is also an effective way to solve the power supply problem in this area. At present, the wind solar complementary power generation system has been widely used in micro and small power generation equipment, such as power supply for communication base station, street lamp, LED display screen and other equipment.

(2) Complementary scenery and water

Wind water complementation makes full use of the regulation capacity above the daily regulation of conventional hydropower, transforms the wind power or photovoltaic power generation or the capacity of both into the potential energy of reservoir water storage, and redistributes it in time. Wind water complementary power generation system does not need to be equipped with batteries, it can achieve stable power generation and supply. In addition to the complementary operation of wind and solar energy with hydropower stations, wind and solar energy can also be developed and utilized in the form of wind and solar complementary pumped storage power stations. Its main equipment includes wind turbine, photovoltaic cell, inverter, water pump, reservoir (up regulating pool), supplementary water source (down regulating pool), water turbine generator unit, etc. With its high efficiency of energy storage and rapid response, wind solar complementary pumped storage power station can effectively solve the problems of low energy density and unstable output of wind and solar energy, and the requirements for water resources are not too high. The construction of a certain scale of pumped storage power station in a large-scale wind power or photovoltaic base can not only improve the utilization rate of solar energy and wind energy, but also reduce the power transmission scale and save the investment of power transmission and transformation projects. For wind power, according to the calculation and analysis, it is economic and reasonable to allocate about 30% pumped storage power station according to the scale of wind power transmission.

(3) Wind energy natural gas complementation

The fluctuation of renewable energy over time can not only be weakened by the complementary operation of different renewable energy sources, but also be regulated by the complementary use of fossil fuels. Fossil fuel complementary power generation equipment suitable for renewable energy such as wind power and solar energy requires low power generation cost, appropriate single unit capacity and high part load efficiency. Under the existing technical conditions, the more feasible conventional power generation equipment is the natural gas fueled fuel turbine and internal combustion engine, the system can be called wind (or photovoltaic) - natural gas complementary energy supply system. For this kind of complementary energy supply system (taking wind energy natural gas as an example), first of all, it is necessary to study the local wind energy resources to determine the reasonable proportion and optimization of the installed capacity of wind farms and the capacity of complementary power generation equipment. In addition, how to improve the operation economy of the complementary system should be considered. The main key technologies include short-term wind speed prediction and nonlinear stochastic optimal scheduling.

2 Thermal complementarity

(1) Solar geothermal energy complementation

Although the total amount of solar radiation reaching the earth's surface is very large, but the energy flow density is low. In order to meet the power generation capacity of a certain scale, the collector system covers a large area with high cost; and the power generation of the system is not stable due to the change of solar radiation intensity. The continuous development of geothermal energy alone will also cause the quality of geothermal resources to decrease too fast, resulting in the continuous decline of thermal efficiency. The joint development of solar energy and geothermal energy can effectively improve the thermal performance of the power station, improve the efficiency of power generation, and reduce the cost of solar power generation. Moreover, in the western part of China, where the geothermal resources of dry hot rock are good, the solar radiation intensity is large, which has good complementary conditions. There are two main ways of solar geothermal energy complementary power generation. The first is to increase the solar energy collector in the existing geothermal power generation system, which is used to increase the steam output or steam temperature, and increase the power generation of the system; or to reduce the mass flow of geothermal water under the condition of constant power generation, so as to extend the service life of geothermal reservoir. The second is solar power generation system. Geothermal water enters the circulating working medium of solar collector through heat exchanger preheating. According to the difference of solar energy and geothermal resources, different complementary integration methods are flexibly selected based on the principle of "temperature matching and cascade utilization".

(2) Solar biomass thermal complementary

Compared with other kinds of renewable energy such as solar energy or wind energy, biomass energy is a rare kind of renewable energy in physical form, which is easy to store and transport. Up to now, biomass thermal power generation technology is a very mature technology in China. Under the current technical conditions, the independent solar thermal power generation technology has the problems of high investment cost and low annual power generation efficiency. It can not only effectively solve the problem of unstable solar energy utilization, reduce the technical and economic risks of the development and utilization of solar thermal power generation, but also save biomass fuel and guarantee the supply of biomass resources. The whole complementary system is mainly composed of biomass circulating fluidized bed boiler, solar collector and turbine generator. When the solar radiation is sufficient, the biomass circulating fluidized bed boiler and solar collector are used to operate jointly; when there is no solar radiation or deficiency, the biomass boiler alone provides steam to drive the turbine to generate electricity. It can be seen that solar energy and biomass energy not only have thermal complementary effect, but also have time complementary effect. There are three ways of solar thermal power generation system complementary to biomass heat: trough type, tower type and trough tower combination.

(3) Solar energy fossil fuel thermal complementary

The investment cost of solar thermal power generation can also be effectively reduced by complementing solar energy with conventional fossil energy and making use of thermal power conversion components of traditional power stations. Moreover, when the solar energy is insufficient, the complementary system can still meet the basic needs by consuming fossil fuel, thus reducing the technical and economic risks of large-scale use of solar thermal power generation. According to the different types of fossil fuels, solar thermal complementary power generation system is mainly divided into solar energy and natural gas complementary power generation system, solar energy and coal complementary power generation system. According to the difference of the integrated main thermal cycle, it can be divided into natural gas simple supplementary combustion solar steam Rankine cycle, solar air Brayton cycle and solar complementary gas steam combined cycle. Due to the characteristics of energy resources of "rich coal, poor oil and little gas", the development of complementary power generation system of solar energy and coal is more in line with China's national conditions. Light coal complementary power generation technology is mainly used in steam Rankine cycle. Trough or linear Fresnel concentrator is used to generate medium and low temperature solar thermal energy, which is used to replace part of extraction steam to heat boiler feed water or directly generate steam, so as to reduce steam turbine regenerative extraction, increase power output of the power station, or reduce coal consumption and pollutant emissions. The light coal complementary power generation system can improve the efficiency of solar thermal power generation with the help of high capacity and high parameter steam turbine. It can also change the complementary local structure to make the system in the mode of increasing output power and saving fuel according to the actual needs. Moreover, when the irradiation is insufficient, it can maintain the stable operation of the system by adjusting the extraction steam flow, which has a good application prospect.3 thermochemical complementation

The characteristic of thermochemical complementarity is that heat energy is converted into chemical energy through endothermic thermochemical reaction, and then it is used by combustion power generation or multi generation. Thermochemical complementary technology is developed to solve the problems of intermittent, unstable, high cost and low thermal efficiency of the system. By converting the accumulated solar energy into the chemical energy of fuel, the storage and efficient power generation of solar energy can be realized; at the same time, the generated solar fuel can also realize the long-distance transmission and regional transfer of solar energy resources.

(1) Solar thermal decomposition water

Solar thermal decomposition water is mainly divided into direct thermal decomposition water and thermochemical cycle decomposition water. Direct thermal decomposition requires low pressure and high temperature environment to improve the decomposition rate of water. When the temperature and pressure are 2225 ℃ and 0.05 bar respectively, about 25% of water will decompose. Although the principle of the technology is simple, but due to the limitations of high-temperature resistant materials and product separation, there are still many problems need to be solved. The thermochemical cycle decomposition water divides the water decomposition process into several steps, reduces the reaction temperature to 600 ~ 950 ℃, improves the technical feasibility, and realizes the natural separation of products. At present, the existing problems include high heat collection temperature, low hydrogen production efficiency, and difficult reactor processing. Therefore, experimental research is still the main one.

(2) Solar energy and natural gas thermochemistry complement each other

The complementary utilization of solar energy and natural gas is generally carried out through the thermochemical reactions of natural gas reforming and cracking. The thermochemical reforming of natural gas is mainly the catalytic reaction between methane and water vapor or CO2. The product is synthetic gas (i.e. the mixture of CO and H2). The reaction temperature is 850-1200 ℃. The tower solar collector is needed to provide the reaction heat. Compared with the methane cracking reaction, the reaction temperature of the concentrated solar energy driven methane reforming reaction is about 200 ℃ lower, and the scale of the reactor is larger. The syngas produced by methane reforming is stored with solar energy, which can be generated by gas turbine or gas steam combined cycle combustion to effectively reduce CO2 emissions; in addition, the syngas produced can also be converted into other types of liquid fuel or chemical raw materials, such as methanol and dimethyl ether.

(3) Thermochemical complementation of solar energy and biomass energy

Solar concentrating heat provides reaction heat for biomass gasification process, so as to realize the complementary utilization of the two. According to the requirements of biomass gasification reaction temperature and large-scale production, it is necessary to use tower solar collector field focusing to generate 800-1200 ℃ high temperature heat to drive the gasification reaction. According to the principle of "comprehensive cascade utilization of chemical energy and physical energy", the syngas produced is first used to produce methanol and other chemical raw materials, and the unreacted gas is used as the fuel of gas steam combined cycle for power generation, So as to realize the multi generation of chemical industry and power. At present, the difficulty of thermal chemistry complementarity between solar energy and biomass energy lies in the coupling of solar energy collection process and biomass gasification process in absorption / reactor, including direct radiation heating and indirect radiation heating.

(4) Thermochemical complementation of solar energy and alternative fuel

The three kinds of solar thermal chemical complementary technologies mentioned above all require high solar energy collection temperature, so tower type, dish type and other high-cost solar energy collection methods are needed. Some domestic researchers have carried out the research work of solar thermochemistry at medium and low temperature. The trough concentrator with mature technology and low cost is used to gather about 300 ℃ solar heat and drive the reforming and cracking reaction of alternative fuels such as methanol or dimethyl ether. Methanol fuel is driven by solar energy with low and medium temperature. In the integrated absorption / reactor, it is catalytic cracked into syngas composed of CO and H2. Syngas is used as fuel to enter the gas steam combined cycle for power generation. Some researches show that the efficiency of solar thermal power generation can reach 35% and the share of solar energy accounts for 18% in the thermochemical power generation system with the temperature of 220 ~ 250 ℃.

epilogue

According to the characteristics of different renewable energy and fossil energy, we can make comprehensive use of various energy sources and form complementarity, so as to improve the quality and efficiency of energy supply and achieve the effect of energy conservation and emission reduction and green low carbon. Time complementarity uses and redistributes the energy supply from the perspective of time according to the different fluctuation characteristics and regulation ability of energy; heat complementarity mainly improves the efficiency of energy conversion or time complementarity by injecting different heat energy into the proper position of thermal cycle according to the principle of "temperature matching, cascade utilization"; thermochemical complementarity is to select coordination Suitable endothermic chemical reaction can transform heat energy into chemical energy of fuel, thus improving energy conversion efficiency, and realizing the storage and transfer of renewable energy such as solar energy.

At present, the technology that can be applied in practical engineering is mainly time complementary and heat complementary. Due to the complex process of the coupling of collector / endothermic and thermochemical reaction, as well as the problems of high temperature resistant materials, the theoretical analysis and experimental research are still the main methods at this stage. The key technologies of multiple energy time complementary mainly include capacity optimization matching, unit off design characteristics, short-term prediction of renewable energy, and optimal scheduling and control. Thermal complementarity and thermochemical complementarity mainly involve system integration, heat transfer and thermal control technology based on "comprehensive cascade utilization of energy" and grade coupling.

The development and application of multi energy complementary comprehensive energy system can promote the consumption of renewable energy, optimize the energy structure and improve the comprehensive efficiency of energy system. It is of great practical significance and far-reaching strategic significance for building a clean, low-carbon, safe and efficient modern energy system to make clear the realization mode and characteristics of multi energy complementary technology, adapt measures to local conditions, customize according to needs, coordinate development and comprehensive utilization.

Author: Chen Yi

Source: This article is published in the 19th issue of distributed energy sponsored by distributed energy committee of China Urban Gas Association. Please indicate the source of the reprint.