Research On Energy Saving Of Cooling Source Filed And Coupling Mechanism Of Combined Heat And Power System

Since the reform and opening-up, the cogeneration utilities in China has been rapidly developed with the increase of urbanization and the continuous improvement of living conditions. However, the study on this system is still imperfect. Comparing to those in 2007, the installed capacity of the combined heat and power in 2008 increased by 8.71%, while the heat supply decreased by 3.83%; and in 2009 the installed capacity of the combined heat and power increased by 24.87%, while the heat supply increased by only 3.83%, compared to those in 2008. These indicated that the heating ability of some heating units is not in full play. "Energy conservation and pollution reduction, Science development" is an important measure for China's economic development at present. Combined heat and power is not only an effective energy-saving measure recognized at home and abroad, but also is an important method to improve the urban environmental quality. Moreover, it is the only way to develop the low-carbon economy. These proposed new goals for the further study of the combined heat and power.In this paper, traditional overall thermal efficiency formula of the power plant is improved. The numerator is composed of 3 parts, which are power generation at condensing runtime of the units, heat load during the heat-supply and the loss of generation produced by the heat-supply. The improved formula demonstrates the running conditions of the combined heat and power units during both the heating period and the off-heating period; it also provides three methods for improving the fuel utilization factors of the heating units; and the difference between the heating units and the condensing units can be definite according to the formula. The advance of the heating units should not only be demonstrated in increasing the heating ability of the heating units but also in generating as much as possible when the heating load is not changed. The research direction of the combined heat and power is the cold source. The total energy system, which is composed by the heating units, the heat exchanger for heating network, the heating network system, the heat users and the outdoor environment, is established as the combined heat and power system. The characteristics of the subsystems and the coupling mechanism between them are analyzed and the full conditions of the CHP are researched. Therefore, different running schemes are proposed for the heating unites with different capacities.The cut-in point of the combining characteristic research of the CHP system is the hot water heater for heating network. The heat transfer coefficient K can be computed accurately by means of the iterative computations for the wall temperature of the heat exchanger tubes according to the basic theory of heat transfer and the influence of the wall thickness on the heat transfer performance of heat exchanger tubes in the hot water heater for heating network. The performance curve under different backwater temperatures can be obtained by establishing the mathematical model of the hot water heater for heating network, which is denoted by the amount of heating extraction and the saturated vapor pressure. Combined with the thermal-system performance of the heating network, performance curve of the hot water heater for heating network can be obtained. The flow and the pressure of the extraction steam needed by the hot water heater for heating network are increasing with the increase of the heat load. The combined performance curve of the CHP system can be obtained by combining the performance of the hot water heater for heating network with that of the low pressure casing, the "V" style variation occurs in the variation process of intermediate pressure casing exhaust pressure, it is obtained that equilibrium without throttling of heating units is the minimum exhaust pressure of intermediate pressure casing when the units are operating (the back pressure of intermediate pressure casing is equal to inlet pressure of the low pressure casing and equal to the extraction pressure of the hot water heater for heating network); The optimum cooling source hot water heater for heating network, which makes the pressure of the CHP system under the condition without throttling reach the minimum pressure allowed by the intermediate casing, is proposed according to the performance of equilibrium without throttling; and then its calculation method is introduced detailedly. The combined performance of CHP is proposed, which points out the development direction for the energy reasonable match and the system optimization of the CHP system.The off-design equilibrium performance calculation of the heating units is developed to calculate the electricity production of the units under the different heating conditions, that quantizes the loss of generated output caused by the extraction steam of units for external heat-supply; and then the off-design calculation of the CHP system is programmed through combining the combined performance of the CHP system and the off-design performance of the heating units. Provided with the variation of the outdoor temperature, the supply and return water temperature of the heating network, the heat load demand, the main parameters of the hot water heater for heating network and the parameters of each section of the turbine can be obtained accurately and the heat balance chart of the turbine is provided under the operating equilibrium. It provides the reliable data criterion to achieve the aim of energy saving and efficiency improvement for the cooling source field of the CHP.The new operating models of the heating system that are series arrangement of heating system are proposed according to the theory of stepwise use of energy and combined performance of the CHP. The asymmetric series arrangement of heating system is proposed by optimizing the scheme in the application process; two heating units are in parallel arrangement; the heating network water is sectionalized heated, and at the same time, the heat exchange area of the hot water heater for heating network at high-temperature section are increased and the extraction pressure of the heating units are decreased, so the influence of the heating extraction on the electricity production of the units is decreased. Take the 200MW heating units for example; the generating power of the heating units in asymmetric series arrangement can increase about 10,000 kW more than that in the traditional parallel arrangement under the rated condition. At present, in our country, the pressure used to distinguish the intermediate pressure casing and low pressure casing is too high for the 300MW heating units, the lithium bromide absorption pumps are used in the heating-supply process of the units to recover the waste heat discharged into the condenser by units. The new heating model that two units are in series arrangement can increase the heat efficiency and the heating capacity of the units.