Control method for optimizing generated power of solar-aided coal-fired power system under off-design working conditions

A control method for optimizing generated power of a solar-aided coal-fired power system under off-design working conditions, comprising steps of:

• setting maximizing the generated power as an optimal control goal;
• and
• maximizing the generated power referring that under conditions of keeping a main steam flow rate, a main steam temperature and a reheat steam temperature of a steam turbine of the solar-aided coal-fired power system unchanged, adjusting a solar-coal feedwater flow distribution ratio of a solar-coal complementary loop to adjust a water flow rate heated in a solar heat collection system, thereby maximizing an output power of the steam turbine;
• wherein the generated power is obtained by using a formula (1) to calculate a specific enthalpy of water heated by the solar heat collection system and water heated by a high-pressure heater after being mixed, and then by using formula (2) and combining a calculation method for a thermal system under off-design conditions to calculate the generated power;
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• wherein hw(i−1),in is the specific enthalpy of the water heated by the solar heat collection system and heated by the high-pressure heater after being mixed, i=1, 2, . . . , n, kJ/kg;
• n is a total number of regenerative heaters in the solar-aided coal-fired power system, and the regenerative heaters are numbered from 1 to n in an order of high pressure to low pressure;
• αTCS is the solar-coal feedwater flow distribution ratio;
• hs,out is a specific enthalpy of the water heated by the solar heat collection system, kJ/kg;
• hwi,out is a specific enthalpy of the water heated by the high-pressure heater, kJ/kg;
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• wherein D0 is the main steam flow rate of the solar-aided coal-fired power system, kg/s;
• h0 is a specific enthalpy of main steam of the solar-aided coal-fired power system, kJ/kg;
• Dzr is a reheat steam flow rate of the solar-aided coal-fired power system, kg/s;
• hzr is a specific enthalpy of reheat steam of the solar-aided coal-fired power system, kJ/kg;
• Di is an i-th stage extraction steam flow rate of the steam turbine of a coal-fired power generation system in the solar-aided coal-fired power system, kg/s;
• hwi,in is a specific enthalpy of an i-th stage extraction steam of the steam turbine of the coal-fired power generation system in the solar-aided coal-fired power system, kJ/kg;
• Dc is an exhaust steam flow rate of the solar-aided coal-fired power system, kg/s;
• hc is a specific enthalpy of exhaust steam of the solar-aided coal-fired power system, kJ/kg;
• Dsg1 and Dsg2 are a front shaft seal steam flow rate and a rear shaft seal steam flow rate of the coal-fired power generation system in the solar-aided coal-fired power system, kg/s;
• hsg1 and hsg2 are specific enthalpies of front shaft seal steam and rear shaft seal steam of the coal-fired power generation system in the solar-aided coal-fired power system, kJ/kg;
• the control method for optimizing the generated power comprises specific steps of:
• Step 1:
• obtaining operating conditions required by the solar-aided coal-fired power system, and obtaining relevant information of the solar heat collection system connected in parallel with high-pressure heaters, the coal-fired power generation system, and environmental conditions;
• Step 2:
• determining a water flow rate range heated by a solar heat collection unit according to a heat transfer oil working temperature range and an equipment safe working range of the solar heat collection unit;
• and then, according to a ratio of the water flow rate range to a feedwater flow rate of the coal-fired power generation system, determining a solar-coal feedwater flow distribution ratio range applicable by the solar-aided coal-fired power system;
• Step 3:
• under a condition of the set power load and a condition which keeps the main steam flow rate, the main steam temperature and the reheat steam temperature of the steam turbine of the solar-aided coal-fired power system unchanged, within the solar-coal feedwater flow distribution ratio range calculated in the Step 2, establishing a correspondence between the generated power and the solar-coal feedwater flow distribution ratio of the solar-aided coal-fired power system under current solar irradiance condition and the power load;
• Step 4:
• selecting a solar-coal feedwater flow distribution ratio corresponding to a maximum generated power under the required operating conditions according to the correspondence obtained in the Step 3, wherein a design solar irradiance is an average direct normal solar irradiance in a typical meteorological year at the operating location of the solar-aided coal-fired power system;
• when operating at 80% to 100% power load, controlling the solar-coal feedwater flow distribution ratio at a lower limit;
• when operating at 60% to 80% power load, and when the direct normal solar irradiance is greater than 78% of the design solar irradiance, controlling the solar-coal feedwater flow distribution ratio between 0.4 and 0.7 to obtain a maximum value of the generated power, and when the solar direct normal irradiance is lower than 78% of the design solar irradiance, controlling the solar-coal feedwater flow distribution ratio between the lower limit and 0.6 to obtain the maximum value of the generated power;
• when operating at 40% to 60% power load, and when the direct normal solar irradiance is greater than 94% of the design solar irradiance, controlling the solar-coal feedwater flow distribution ratio at an upper limit to obtain the maximum value of the generated power, and when the direct normal solar irradiance is between 47% and 94% of the design solar irradiance, controlling the solar-coal feedwater flow distribution ratio between 0.4 and 0.7 to obtain the maximum value of the generated power, and when the direct normal solar irradiance is lower than 47% of the design solar irradiance, controlling the solar-coal feedwater flow distribution ratio between 0.25 and 0.3 to obtain the maximum value of the generated power, wherein the generated power is close to the maximum value when the solar-coal feedwater flow distribution ratio is controlled between 0.3 and 0.8;
• and
• Step 5:
• according to an optimized solar-coal feedwater flow distribution ratio obtained in the Step 4, calculating an optimized value D*wTCS of the water flow heated by the solar heat collection system with a formula (3), and adjusting the water flow rate heated by the solar heat collection system to the optimized value;
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• wherein D*wTCS is the optimized value of the water flow rate entering the solar heat collection system for heating, kg/s;
• α*TCS is the optimized solar-coal feedwater flow distribution ratio;
• Dw is a feedwater flow under current working conditions, kg/s;
• when the operating conditions of the solar-aided coal-fired power system or solar irradiance changes, repeating Steps 1 to 5 again to reach the control goal.