1. 1 topic background

China is a big country of energy production and consumption, and the rapid economic development leads to the rapid growth of energy demand [1]. According to the Statistical Bulletin on National Economic and Social Development of People's Republic of China (PRC) in 20 13 released by the National Bureau of Statistics on February 22nd, 20 14, the total energy consumption of China in 20 13 was 3.75 billion tons of standard coal, an increase of 3.7% over the previous year. Coal consumption increased by 3.7%; Crude oil consumption increased by 3.4%; Natural gas consumption growth13.0%; Electricity consumption increased by 7.5%. This shows that China has become the world's largest disposable coal energy consumer and the second largest energy consumer in the world. Therefore, the rational use of energy, energy conservation and emission reduction has become one of the strategic policies of China's sustainable development [2].

At present, one of the reasons for the low comprehensive efficiency of thermal power plants is that after the exhaust steam of the unit is discharged into the condenser, its heat is taken away by the circulating water, and then discharged into the atmosphere through the cooling tower or discharged into the river with the circulating water, resulting in a great loss of cold source [3], and the waste heat discharged with the low temperature water accounts for about 55%-60% of the total loss [4]. China's energy utilization rate is only 33%, and there is great energy-saving space and potential [5]. The low efficiency of energy utilization means that the rapid development of China's economy and society must be at the expense of consuming a lot of disposable energy, which makes the already severe petrochemical energy situation in China even more severe and does not meet the requirements of sustainable development strategy. Moreover, a large amount of energy consumption and inefficient energy utilization will inevitably produce huge heat emission and pollution. The emission of dust, sulfur oxides and nitrogen oxides will aggravate air pollution, and the emission of carbon dioxide will cause greenhouse effect. According to China's Twelfth Five-Year Plan, the newly started capacity of coal-fired thermal power units is expected to be 300 million kilowatts, and the total installed capacity of power generation will reach 65.438+43.6 million kilowatts in 2065.438+05, of which the installed capacity of thermal power will reach 933 million kilowatts. In these units, except for some water-deficient areas in the north, air cooling is used, and most units use circulating water to cool the exhaust steam. In the process of increasing the installed capacity of coal-fired thermal power units, the total carbon emissions will also increase, and the emissions of pollutants such as sulfur dioxide will also increase significantly. If the heat in circulating water can be used to improve the comprehensive utilization efficiency of energy, the use of petrochemical energy will be saved and a win-win situation of environment, economy and energy will be realized [6].

Because the temperature of circulating water is generally low (generally 20-35℃ in winter), it can not meet the requirements of direct heating. If you want to use it for heating, you must find a way to raise its temperature properly. Small and medium-sized condensing steam turbines can increase the temperature of circulating water (60-80℃) by reducing the vacuum of exhaust hood, that is, circulating water heating in low vacuum operation. This technology can achieve high energy efficiency in theory, and there are many examples of research and successful operation at home and abroad. The technology is very mature, especially in some northern cities in China. The traditional low vacuum operation unit is similar to the back pressure unit in a thermal power plant, and the amount of steam it passes depends on the user's thermal load, so the power generation is restricted by the user's thermal load and cannot be adjusted independently, that is, its operation is also' determined by heat', so it is only suitable for the heating system with stable user's thermal load. In addition, the low vacuum operation of the unit requires the corresponding transformation of the unit structure, which is only suitable for small units and a few medium-sized units, but is totally not allowed for modern large units. In a large cogeneration system with intermediate reheat steam turbine, too high condensation pressure will make the last steam temperature of the unit too high and the steam volume flow rate too small, which will cause strong vibration of the unit and endanger the operation safety. The inlet temperature of circulating cooling water for large steam turbine units is generally required not to exceed 33℃ (the corresponding outlet temperature is about 40℃). If the heating temperature is within this range, the structure of the unit does not need to be changed, and it is suitable for any capacity and type of unit. But at present, the only heating device suitable for this temperature range is floor low-temperature radiation heating, so its application scope is greatly limited [7].

Another method to increase the temperature of circulating water in power plant is to use heat pump technology, that is, circulating cooling water in power plant.

As a low-level heat source, heat pump technology is used to extract its heat and provide heat to users. Compared with the commonly used heat pump heat source, the circulating water in power plant has the advantages of huge heat, moderate and stable temperature, good water quality, safety and environmental protection, and is a high-quality heat pump heat source. Using the circulating water of power plant as the low-level heat source of heat pump for heating can match the quality and quantity between heating and user demand conveniently and flexibly, and will not have a great impact on the original thermal system of power plant [8]. The waste heat of circulating cooling water recovered by the heat pump device returns to the thermal system to heat the condensed water, which can reduce the steam extraction consumption of the corresponding low-pressure heaters, thereby increasing the power generation of the power plant, reducing the coal consumption value of power generation and improving the economy of power plant operation. Therefore, the circulating water source heat pump in power plant is an ideal way to recover the waste heat of circulating water in power plant for heating.

1.2 research purpose and significance

In order to utilize a large amount of low-temperature circulating cooling water with a temperature about 10 degrees higher than the ambient temperature, the first type of absorption heat pump is selected, and its circulation mechanism is analyzed from the perspective of improving the thermal perfection of the system. On this basis, taking a 300MW unit as an example, the thermal calculation is carried out and its economy is analyzed.

Using heat pump technology, part of the cooling system is used for circulating cooling water, extracting the residual heat of cooling water, reducing the temperature of cooling water, realizing the recycling of residual heat, transforming the residual heat energy into energy that can be effectively utilized, saving the consumption of steam energy in the process, and creating direct economic benefits for enterprises while realizing energy conservation, emission reduction and environmental protection [9].

Second, the research trends and development trends at home and abroad in this research field.

2. 1 foreign research trends and development trends

The research on waste heat recovery in Europe, America and Japan has a long history. Since the 1973 energy crisis, countries have attached great importance to energy issues.

1976 American B.C.L.(Battele Columber Labs) put forward the concept and made a market forecast, convinced that the technology of recovering waste heat by absorption heat pump has practical value [10]. In the suburb of Philadelphia, the crozet-Chester Medical Center, covering an area of 407 mu, has 25 buildings and installed energy conversion systems. Part of the system uses industrial heat pump to transfer the waste heat from the air-conditioned room of the medical center to the hot water used in the laundry room. This facility alone will save more than half a million dollars in ten years [1 1]. A telephone conversion center of Bell Telephone Company in Pennsylvania, USA, uses heat pump to absorb the waste heat accumulated by 270 tons of air conditioning system cooling devices, which will save 270 million US dollars every year during the analysis period of 10 [12]. Since 198 1 year ago, Sanyo Company of Japan has established more than 20 AHT units with the scale of 2000-5000 kw for Japan and other parts of the world, most of which are used to recover the heat of organic steam at the top of distillation columns in petrochemical enterprises [13]. So far, the pre-established device has been successfully operated for more than ten years. They used a single-stage heat pump to recover industrial waste heat, and increased the boiler feed water from 93℃ to 1 17℃, and successfully applied it in the industrial field, with the total number of application devices accounting for more than half of the world [14].

In recent years, the development of heat pump has made great progress. Vander Pal[ 15] and others developed a compression/absorption hybrid heat pump unit, which raised the industrial waste heat below 100℃, established a simulation calculation model of the hybrid heat pump, and verified it by actual measurement. The results show that when the compressor is located between the evaporator and the adsorption reactor, its influence on the energy efficiency of the unit is significantly greater than that when the compressor is located between the adsorption reactor and the condenser, and the energy efficiency of the latter is almost the same as that of the pure heat-driven unit, which fully proves the research. Miyazaki [16] and others proposed a double evaporator absorption refrigerator. This new refrigerator consists of two evaporators, 1 condenser and three absorbers. Evaporation and absorption are carried out at two different pressures at the same time, which can expand the concentration variation range of adsorbate during concentration and dilution. The experimental results show that the performance coefficient of double evaporator absorption unit is 3.4 times that of ordinary unit under given conditions. Christian Keil[ 17] and others studied the application of absorption heat pump in low-temperature central heating system.

2.2 Domestic research trends and development trends

The development of waste heat recovery in China is later than that in foreign countries. The recovered waste heat is mainly the sensible heat of flue gas and combustible gas discharged in the production process, and the utilization of low-temperature waste heat is still in its infancy. Moreover, in China, the recovery of waste heat (especially low-grade waste heat) mainly adopts compression heat pump. The application of absorption heat pump is still very backward. In recent years, many people have done a lot of research on absorption heat pump technology to recover waste heat.

Xiao [18] of Dalian Sanyo Refrigeration Co., Ltd. put forward the scheme of using lithium bromide absorption heat pump to recover the waste heat of geothermal tail water and provide heating water for oilfield operation area. After two heating seasons, the gas cost 12 10000 yuan was saved, and the energy saving rate reached 46% of the original system energy consumption.

Last week, Northeastern University [19] studied that using heat pump device to recover the waste heat of circulating cooling water and then heating the inlet air of boiler can reduce the consumption of auxiliary steam and extraction steam, thus improving the thermal economy of power plant.

Zhou Chongbo of Huadian Electric Power Research Institute and others [20] tested the performance of the waste heat recovery and utilization system of 125MW thermal power plant and 300MW thermal power plant, which used large absorption heat pumps to recover the waste heat of circulating water for central heating in cities. It is concluded that the adverse effects caused by the increase of backwater temperature and the decrease of driving steam pressure in heating network are greater than the benign effects caused by the reverse change of corresponding parameters, and the influence of driving steam on heating capacity and waste heat recovery is greater than that of heating network water and waste heat.

Guo [2 1] of Hebei Electric Power Research Institute uses the heat exchange coefficient of electric energy to discuss the economy of compression heat pump and absorption heat pump, which is of great significance to guide the selection of heat pump.

According to the actual situation of Datang No.3 Thermal Power Plant, Ms. Lu and [22] analyzed three situations in which industrial extraction, industrial extraction, heating extraction and heating extraction are the driving heat sources, and calculated the thermal economy.

Wu Xing [23] and others found that under the same heating load, circulating water heating needs more pipe network investment and water pump power consumption than urban heating network because of the small temperature difference between supply and return water (10- 15℃). Therefore, the application range of circulating water heating is 3-5 kilometers around the power plant.

Sun Zhixin of Xi Jiaotong University [24] established the mathematical model of circulating water source heat pump in power plant, analyzed the influence of condenser temperature on the main parameters of heat pump, such as evaporation temperature and heating coefficient, and calculated the critical parameters of heat pump heating superior to extraction heating.

Wang Baoyu of Huadian Electric Power Research Institute [25] analyzed the energy-saving performance of three units with rated loads of 200 MW, 300 MW and 600 MW according to the circulation mode of condenser replacing low-pressure heater in heat pump system, which can simplify the heating system of power plant and is an important way for system optimization and energy saving.

Tsinghua University's heating technology based on absorption heat pump to recover the waste heat of circulating water has been implemented successively in Chifeng, Inner Mongolia and Datong, Shanxi, which greatly improved its heating capacity [26]. Many power plants in Beijing, Shanxi and other places use absorption heat pump units to absorb the waste heat of circulating water for heating, which has achieved good corporate and social benefits and set a good example in energy saving and environmental protection. For example, a power plant in Datong has an annual water saving benefit of 3301.2 million yuan, an annual saving of 68,000 tons of standard coal, and an annual reduction of carbon dioxide 1.7 million tons [27].

Jin [28] of Liaohe Branch of China Petroleum Company compares the economy of boiler heating system and absorption heat pump heating system, and thinks that the economy of heat pump system is better than the former.

Ye [29] takes the supercritical 660WM unit as an example, and calculates and analyzes the economy of absorption heat pump by using the equivalent enthalpy drop method.

Liu Zhenyu [30] of Xishan Coal and Electricity Group discussed several different technical routes of waste heat recovery and heating in view of the low utilization rate of cogeneration central heating in coal-fired power plants.

Thirdly, the main research content, research scheme and technical route of this topic are put forward.

3. 1 main contents of the study

(1) According to the theoretical cycle process of absorption heat pump, find out the typical state points in the cycle process, and analyze the influencing factors in the actual cycle of heat pump by consulting the data;

(2) Taking the heat exchangers of the heat pump system as key components, an analysis and calculation model for recovering the waste heat of circulating water by absorption heat pump is established;

(3) Taking a 300MW heating unit as an example, the system energy efficiency of this unit is calculated and analyzed.

3.2 Research plan

The absorption heat pump can be divided into the first type absorption heat pump (heating type) whose output heat temperature is lower than the driving heat source and the second type absorption heat pump (heating type) whose output heat temperature is higher than the driving heat source. The first type of absorption heat pump is suitable for the utilization of circulating water waste heat in thermal power plants. In this paper, lithium bromide absorption heat pump is taken as the object, and the cycle process of absorption heat pump system is analyzed by understanding the properties of working fluid. Assuming that the whole system is in a state of thermal balance and stable flow, the working fluid at the outlet of evaporator and condenser is saturated, and the lithium bromide solution at the outlet of absorber generator is saturated, the heat exchange loss of heat exchanger, adiabatic throttling process in throttle valve and the change of water physical parameters of heating network are not considered. The heat exchange capacity of each heat exchanger and the thermodynamic coefficient of the system are obtained, and the economy of the heat pump system is discussed from two aspects: sufficient heating capacity and insufficient heating capacity of the unit under the heating condition.

3.3 Technical route

(1) According to the enthalpy-concentration diagram of lithium bromide solution or the specific enthalpy calculation equation of lithium bromide solution, determine the enthalpy of each typical state point of the heat pump system;

(2) Taking each heat exchanger of the heat pump system as the key component, the model of recovery of circulating water waste heat by absorption heat pump is established, and the heat load equation of each heat exchanger is listed according to the heat balance, and the heat load of each specific heat exchange component is obtained from the enthalpy value of each state point, and then the heat coefficient of the system is obtained from the heat balance equation of the whole system;

(3) Under the condition that the heating load and the initial and final parameters of steam are unchanged, the extraction amount of heating steam and the extraction amount of heat source driven by heat pump are calculated. In the case of insufficient heating, the economy is discussed directly with the waste heat of circulating water recovered by heat pump. Under the condition of sufficient heating of the unit, calculate the air extraction saved by installing the heat pump system, increase the power of the computer unit, calculate the coal saving, and get its energy-saving benefits.

Fourthly, the possible difficulties and problems in the research process of this topic are put forward, and the preliminary solutions are put forward.

Possible difficulties and problems: The actual operation of the heat pump will be affected by many factors, which makes the establishment and calculation of the model very difficult. When analyzing the energy-saving benefits, the results obtained from the perspective of heat alone may be too different from the actual benefits. Can you find a relatively accurate method to judge its economy?

The initial solution: first, we should be familiar with and understand the properties of lithium bromide solution and the working principle of lithium bromide absorption heat pump. When modeling the heat pump system, we should ignore some influencing factors and make some ideal assumptions. When analyzing its energy-saving benefits, the heating capacity or heating demand is discussed. When you encounter specific problems, you should carefully consult relevant materials and consult your predecessors and teachers.

V. Progress and expected goals of this study.

5. 1 research schedule

(1) 20xx.09-20xx.10 Understand the subject, consult the materials and write the opening report;

(2) 20xx.11-20xx.01complete the opening report and start to build the heat pump system model;

(3)20XX.03-20XX.05 calculate the model and make economic analysis to complete the small paper;

(4) 4) Temporary defense of 20xx.06-20xx.07;

(5)20XX.09-20XX.03 Write graduation thesis and prepare graduation defense.

5.2 Expected objectives

(1) Understand the principle of heat pump and its application in power plant through learning;

(2) Study the heat transfer of each component of the heat pump system, calculate its heat load, and complete the economic analysis;

(3) publishing 2-3 papers with high level;

(4) Successfully completed the master's thesis.

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