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

Abstract

A control method for optimizing a solar-to-power efficiency of a solar-aided coal-fired power system under off-design working conditions is provided. Through reading the relevant information of the solar collecting system, the coal-fired power generation system, the environmental conditions, and the working conditions of the solar-aided coal-fired power system, the water flow rate range able to be heated by the solar collecting unit and the solar-coal feedwater flow distribution ratio range of the solar-aided coal-fired power system are determined; through establishing the relationship between the solar-to-power efficiency and the solar-coal feedwater flow distribution ratio of the solar-aided coal-fired power system under the off-design working conditions, the solar-coal feedwater flow distribution ratio is regulated, so that a flow rate of water entering the solar collecting system to be heated is controlled, thereby maximizing the solar-to-power efficiency and improving the economy of the solar-aided coal-fired power system under the off-design working conditions. The present invention provides clear guidance to optimize the solar-aided coal-fired power system under the off-design working conditions, enable solar energy to fully play its role in the solar-aided coal-fired power system, improve the utilization rate of solar energy, facilitate the consumption of the renewable energy, and greatly increase the economy of the solar-aided coal-fired power system.

Claims

1. A control method for optimizing a solar-to-power efficiency of a solar-aided coal-fired power system under off-design working conditions, wherein: an operational control objective of the solar-aided coal-fired power system under the off-design working conditions is to optimize the solar-to-power efficiency; the solar-to-power efficiency means a conversion ratio of solar radiation energy received by the solar-aided coal-fired power system into electricity, namely a ratio of additional power generation of the solar-aided coal-fired power system to the solar radiation energy received by the solar-aided coal-fired power system when the solar-aided coal-fired power system has the same boiler heat absorption as a conventional coal-fired power generation system uncoupled with solar energy; the solar-to-power efficiency is calculated through steps of: firstly, through equation (1), calculating a specific enthalpy of mixing water heated by a solar collecting system and high-pressure heaters; then, combined with an off-design working condition calculation method of a thermodynamic system, through equation (2), calculating a generation power W.sub.SCPP of the solar-aided coal-fired power system; next, assuming that the solar-aided coal-fired power system and the conventional coal-fired power generation system uncoupled with solar energy have the same boiler heat absorption, and calculating the solar-to-power efficiency η.sub.SE according to equation (3) and equation (4); $\begin{array}{cc}{h}_{w\left(i-1\right),\mathrm{in}}={\alpha }_{TCS}\times h_{s,\mathrm{out}}+\left(1-{\alpha }_{TCS}\right)h_{\mathrm{wi},\mathrm{out}}& \left(1\right)\end{array}$ in the equation (1), h.sub.w(i-1),in is the specific enthalpy of mixing water heated by the solar collecting system and the high-pressure heaters, i=1,2, . . . , n, in unit of kJ/kg; n is a total number of regenerative heaters of the solar-aided coal-fired power system, wherein the regenerative heaters are numbered 1 to n consecutively from high pressure to low pressure; α.sub.TCS is a solar-coal feedwater flow distribution ratio; h.sub.s,out is a specific enthalpy of water heated by the solar collecting system, in unit of kJ/kg; and h.sub.wi,out is a specific enthalpy of water heated by the high-pressure heaters, in unit of kJ/kg; $\begin{array}{cc}{W}_{SCPP}=D_0{h}_0+D_{zr}{h}_{zr}-\underset{i=1}{\overset{n}{.Math.}}{D}_i{h}_{\mathrm{wi},\mathrm{in}}-D_c{h}_c-D_{sg1}{h}_{sg1}-D_{sg2}{h}_{sg2}& \left(2\right)\end{array}$ in the equation (2), W.sub.SCPP is the generation power of the solar-aided coal-fired power system, in unit of MW; D.sub.0 is a flow rate of main steam of the solar-aided coal-fired power system, in unit of kg/s; h.sub.0 is a specific enthalpy of main steam of the solar-aided coal-fired power system, in unit of kJ/kg; D.sub.zr is a flow rate of reheated steam of the solar-aided coal-fired power system, in unit of kg/s; h.sub.zr is a specific enthalpy of reheated steam of the solar-aided coal-fired power system, in unit of kJ/kg; D.sub.i is a flow rate of i.sup.th-stage extraction steam of a steam turbine for coal-fired power generation in the solar-aided coal-fired power system, i=1,2, . . . , n, in unit of kg/s; h.sub.wi,in is a specific enthalpy of i.sup.th-stage extraction steam of the steam turbine for coal-fired power generation in the solar-aided coal-fired power system, in unit of kJ/kg; D.sub.c is a flow rate of exhaust steam of the solar-aided coal-fired power system, in unit of kg/s; h.sub.c is a specific enthalpy of exhaust steam of the solar-aided coal-fired power system, in unit of kJ/kg; D.sub.sg1 and D.sub.sg2 are flow rates of front shaft seal steam and back shaft seal steam of the steam turbine for coal-fired power generation in the solar-aided coal-fired power system, in unit of kg/s; h.sub.sg1 and h.sub.sg2 are specific enthalpies of front shaft seal steam and back shaft seal steam of the steam turbine for coal-fired power generation in the solar-aided coal-fired power system, in unit of kJ/kg; $\begin{array}{cc}{W}_{add}=W_{SCPP}-W_{eq}& \left(3\right)\end{array}$ in the equation (3), W.sub.add is additional power of the solar-aided coal-fired power system under the same boiler heat absorption, in unit of MW; W.sub.eq is an equivalent generation power of the conventional coal-fired power generation system uncoupled with solar energy when having the same boiler heat absorption as the solar-aided coal-fired power system under the same power generation load requirement, in unit of MW; $\begin{array}{cc}{\eta }_{SE}=\frac{10^6{W}_{add}}{\mathrm{DNI}.Math.A_c}& \left(4\right)\end{array}$ in the equation (4), η.sub.SE is the solar-to-power efficiency of the solar-aided coal-fired power system; DNI is a solar direct normal irradiance, in unit of W/m.sup.2; A.sub.c is a solar collecting area, in unit of m.sup.2; the control method for optimizing the solar-to-power efficiency of the solar-aided coal-fired power system under the off-design working conditions comprises steps of: (1) reading relevant information of the solar collecting system in parallel with the high-pressure heaters, a coal-fired power generation system and environmental conditions in the solar-aided coal-fired power system; (2) reading working conditions of the solar-aided coal-fired power system; (3) according to a working temperature range of heat transfer oil of a solar collecting unit and a safely working range of devices of the solar collecting unit, determining a water flow rate range able to be heated by the solar collecting unit; 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 of the solar-aided coal-fired power system; (4) in the solar-coal feedwater flow distribution ratio range calculated through the step (3), calculating a solar-to-power efficiency η.sub.SE of the solar-aided coal-fired power system under current solar irradiance conditions and power load conditions, and establishing a relationship between the solar-to-power efficiency and the solar-coal feedwater flow distribution ratio of the solar-aided coal-fired power system; (5) selecting an optimized solar-coal feedwater flow distribution ratio, specifically comprising steps of: in the relationship between the solar-to-power efficiency and the solar-coal feedwater flow distribution ratio under required working conditions, which is established through the step (4), selecting a solar-coal feedwater flow distribution ratio corresponding to a maximum value of the solar-to-power efficiency as the optimized solar-coal feedwater flow distribution ratio; and (6) according to the optimized solar-coal feedwater flow distribution ratio obtained through the step (5), regulating a flow rate of water entering the solar collecting system to be heated to an optimized flow rate D*.sub.wTCS, wherein D*.sub.wTCS is calculated through: $D_{wTCS}^{*}={\alpha }_{TCS}^{*}.Math.D_w;$ in the above equation, D*.sub.wTCS is the optimized flow rate of water entering the solar collecting system to be heated, in unit of kg/s; α*.sub.TCS is the optimized solar-coal feedwater flow distribution ratio; and D.sub.w is a feedwater flow rate under current working conditions, in unit of kg/s; when the working conditions of the solar-aided coal-fired power system change or the solar irradiance changes, the steps (1)-(6) are repeated to achieve the control objective again.

2. The control method, as recited in claim 1, wherein: in the step (1), the read relevant information of the environmental conditions comprises a current solar irradiance and an environmental temperature; the read relevant information of the solar collecting system comprises relevant information of the solar collecting unit and relevant information of a heliostat field; the read relevant information of the coal-fired power generation system comprises main steam parameters, extraction steam parameters of the steam turbine for coal-fired power generation, and operation information of the high-pressure heaters and low-pressure heaters, which are required for calculation of the generation power.

3. The control method, as recited in claim 1, wherein: a design solar irradiance is an average solar direct normal irradiance of a typical meteorological year at an operation location of the solar-aided coal-fired power system; in the step (5), the solar-coal feedwater flow distribution ratio corresponding to the maximum value of the solar-to-power efficiency is: under 80%-100% power load, the solar-coal feedwater flow distribution ratio is controlled to operate at a lower limiting value; under 60%-80% power load, when the solar direct normal irradiance is not less than 85%-105% design solar irradiance, the solar-coal feedwater flow distribution ratio is controlled to operate at the lower limiting value, while when the solar direct normal irradiance is less than 85%-105% design solar irradiance, the solar-coal feedwater flow distribution ratio is controlled to operate between 0.2 and 0.3; under 40%-60% power load, when the solar direct normal irradiance is not less than 94%-110% design solar irradiance, the solar-coal feedwater flow distribution ratio is controlled to operate between 0.4 and 0.45, while when the solar direct normal irradiance is less than 94%-110% design solar irradiance, the solar-coal feedwater flow distribution ratio is controlled to operate between 0.25 and 0.3.

4. The control method, as recited in claim 1, wherein: the solar collecting system of the solar-aided coal-fired power system is connected in parallel with a second-stage high-pressure heater and a third-stage high-pressure heater.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1 is a flow chart of a control method for optimizing a solar-to-power efficiency of a solar-aided coal-fired power system under off-design working conditions according to a preferred embodiment of the present invention.

[0026] FIG. 2 is a connection sketch view of regenerative heaters of an implementation system for the control method according to the preferred embodiment of the present invention.

[0027] FIG. 3 is a sketch view of a correspondence relationship between the solar-to-power efficiency and a solar-coal feedwater flow distribution ratio under a condition of 100% THA (Turbine Heat Acceptance) according to the preferred embodiment of the present invention.

[0028] FIG. 4 is a sketch view of a correspondence relationship between the solar-to-power efficiency and the solar-coal feedwater flow distribution ratio under a condition of 75% THA according to the preferred embodiment of the present invention.

[0029] FIG. 5 is a sketch view of a correspondence relationship between the solar-to-power efficiency and the solar-coal feedwater flow distribution ratio under a condition of 50% THA according to the preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0030] The present invention is further illustrated in detail with the accompanying drawings and the preferred embodiment as follows.

[0031] As shown in FIG. 1, according to the preferred embodiment of the present invention, a control method for optimizing a solar-to-power efficiency of a solar-aided coal-fired power system under off-design working conditions is provided, wherein: an operational control objective of the solar-aided coal-fired power system under the off-design working conditions is to optimize the solar-to-power efficiency; the solar-to-power efficiency means a conversion ratio of solar radiation energy received by the solar-aided coal-fired power system into electricity, namely a ratio of additional power generation of the solar-aided coal-fired power system to the solar radiation energy received by the solar-aided coal-fired power system when the solar-aided coal-fired power system has the same boiler heat absorption as a conventional coal-fired power generation system uncoupled with solar energy;

[0032] the solar-to-power efficiency is calculated through steps of: firstly, through equation (1), calculating a specific enthalpy of mixing water heated by a solar collecting system and high-pressure heaters; then, combined with an off-design working condition calculation method of a thermodynamic system, through equation (2), calculating a generation power W.sub.SCPP of the solar-aided coal-fired power system; next, assuming that the solar-aided coal-fired power system and the conventional coal-fired power generation system uncoupled with solar energy have the same boiler heat absorption, and calculating the solar-to-power efficiency η.sub.SE according to equation (3) and equation (4);

[00006]$\begin{array}{cc}{h}_{w\left(i-1\right),\mathrm{in}}={\alpha }_{TCS}\times h_{s,\mathrm{out}}+\left(1-{\alpha }_{TCS}\right)h_{\mathrm{wi},\mathrm{out}}& \left(1\right)\end{array}$

[0033] in the equation (1), h.sub.w(i-1),in is the specific enthalpy of mixing water heated by the solar collecting system and the high-pressure heaters, i=1,2, . . . , n, in unit of kJ/kg; n is a total number of regenerative heaters of the solar-aided coal-fired power system, wherein the regenerative heaters are numbered 1 to n consecutively from high pressure to low pressure; α.sub.TCS is a solar-coal feedwater flow distribution ratio; h.sub.s,out is a specific enthalpy of water heated by the solar collecting system, in unit of kJ/kg; and h.sub.wi,out is a specific enthalpy of water heated by the high-pressure heaters, in unit of kJ/kg;

[00007]$\begin{array}{cc}{W}_{SCPP}=D_0{h}_0+D_{zr}{h}_{zr}-\underset{i=1}{\overset{n}{.Math.}}{D}_i{h}_{\mathrm{wi},\mathrm{in}}-D_c{h}_c-D_{sg1}{h}_{sg1}-D_{sg2}{h}_{sg2}& \left(2\right)\end{array}$

[0034] in the equation (2), W.sub.SCPP is the generation power of the solar-aided coal-fired power system, in unit of MW; D.sub.0 is a flow rate of main steam of the solar-aided coal-fired power system, in unit of kg/s; h.sub.0 is a specific enthalpy of main steam of the solar-aided coal-fired power system, in unit of kJ/kg; D.sub.zr is a flow rate of reheated steam of the solar-aided coal-fired power system, in unit of kg/s; h.sub.zr is a specific enthalpy of reheated steam of the solar-aided coal-fired power system, in unit of kJ/kg; D.sub.i is a flow rate of i.sup.th-stage extraction steam of a steam turbine for coal-fired power generation in the solar-aided coal-fired power system, i=1,2, . . . , n , in unit of kg/s; h.sub.wi,in is a specific enthalpy of i.sup.th-stage extraction steam of the steam turbine for coal-fired power generation in the solar-aided coal-fired power system, in unit of kJ/kg; D.sub.c is a flow rate of exhaust steam of the solar-aided coal-fired power system, in unit of kg/s; h.sub.c is a specific enthalpy of exhaust steam of the solar-aided coal-fired power system, in unit of kJ/kg; D.sub.sg1 and D.sub.sg2 are flow rates of front shaft seal steam and back shaft seal steam of the steam turbine for coal-fired power generation in the solar-aided coal-fired power system, in unit of kg/s; h.sub.sg1 and h.sub.sg2 are specific enthalpies of front shaft seal steam and back shaft seal steam of the steam turbine for coal-fired power generation in the solar-aided coal-fired power system, in unit of kJ/kg;

[00008]$\begin{array}{cc}{W}_{add}=W_{SCPP}-W_{eq}& \left(3\right)\end{array}$

[0035] in the equation (3), W.sub.add is additional power of the solar-aided coal-fired power system under the same boiler heat absorption, in unit of MW; W.sub.eq is an equivalent generation power of the conventional coal-fired power generation system uncoupled with solar energy when having the same boiler heat absorption as the solar-aided coal-fired power system under the same power generation load requirement, in unit of MW;

[00009]$\begin{array}{cc}{\eta }_{SE}=\frac{10^6{W}_{add}}{\mathrm{DNI}.Math.A_c}& \left(4\right)\end{array}$

[0036] in the equation (4), η.sub.SE is the solar-to-power efficiency of the solar-aided coal-fired power system; DNI is a solar direct normal irradiance, in unit of W/m.sup.2; A.sub.c is a solar collecting area, in unit of m.sup.2;

[0037] the control method for optimizing the solar-to-power efficiency of the solar-aided coal-fired power system under the off-design working conditions comprises steps of:

[0038] (1) reading relevant information of the solar collecting system in parallel with the high-pressure heaters, a coal-fired power generation system and environmental conditions in the solar-aided coal-fired power system;

[0039] (2) reading working conditions of the solar-aided coal-fired power system;

[0040] (3) according to a working temperature range of heat transfer oil of a solar collecting unit and a safely working range of devices of the solar collecting unit, determining a water flow rate range able to be heated by the solar collecting unit; 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 of the solar-aided coal-fired power system;

[0041] (4) in the solar-coal feedwater flow distribution ratio range calculated through the step (3), calculating a solar-to-power efficiency η.sub.SE of the solar-aided coal-fired power system under current solar irradiance conditions and power load conditions, and establishing a relationship between the solar-to-power efficiency and the solar-coal feedwater flow distribution ratio of the solar-aided coal-fired power system;

[0042] (5) selecting an optimized solar-coal feedwater flow distribution ratio, specifically comprising steps of: in the relationship between the solar-to-power efficiency and the solar-coal feedwater flow distribution ratio under required working conditions, which is established through the step (4), selecting a solar-coal feedwater flow distribution ratio corresponding to a maximum value of the solar-to-power efficiency as the optimized solar-coal feedwater flow distribution ratio; and (6) according to the optimized solar-coal feedwater flow distribution ratio obtained through the step (5), regulating a flow rate of water entering the solar collecting system to be heated to an optimized flow rate D*.sub.wTCS wherein D*.sub.wTCS is calculated through:

[00010]$D_{wTCS}^{*}={\alpha }_{TCS}^{*}.Math.D_w;$

[0043] in the above equation, D*.sub.wTCS is the optimized flow rate of water entering the solar collecting system to be heated, in unit of kg/s; α*.sub.TCS is the optimized solar-coal feedwater flow distribution ratio; and D.sub.w is a feedwater flow rate under current working conditions, in unit of kg/s;

[0044] when the working conditions of the solar-aided coal-fired power system change or the solar irradiance changes, the steps (1)-(6) are repeated to achieve the control objective again.

[0045] In the preferred embodiment, in the step (1), the read relevant information of the environmental conditions comprises a current solar irradiance and an environmental temperature; the read relevant information of the solar collecting system comprises relevant information of the solar collecting unit and relevant information of a heliostat field; the read relevant information of the coal-fired power generation system comprises main steam parameters, extraction steam parameters of the steam turbine for coal-fired power generation, and operation information of the high-pressure heaters and low-pressure heaters, which are required for calculation of the generation power.

[0046] In the preferred embodiment, a design solar irradiance is an average solar direct normal irradiance of a typical meteorological year at an operation location of the solar-aided coal-fired power system; in the step (5), the solar-coal feedwater flow distribution ratio corresponding to the maximum value of the solar-to-power efficiency is: under 80%-100% power load, the solar-coal feedwater flow distribution ratio is controlled to operate at a lower limiting value; under 60%-80% power load, when the solar direct normal irradiance is not less than 85%-105% design solar irradiance, the solar-coal feedwater flow distribution ratio is controlled to operate at the lower limiting value, while when the solar direct normal irradiance is less than 85%-105% design solar irradiance, the solar-coal feedwater flow distribution ratio is controlled to operate between 0.2 and 0.3; under 40%-60% power load, when the solar direct normal irradiance is not less than 94%-110% design solar irradiance, the solar-coal feedwater flow distribution ratio is controlled to operate between 0.4 and 0.45, while when the solar direct normal irradiance is less than 94%-110% design solar irradiance, the solar-coal feedwater flow distribution ratio is controlled to operate between 0.25 and 0.3.

[0047] Referring to FIG. 2, in the preferred embodiment, the solar collecting system of the solar-aided coal-fired power system is connected in parallel with a second-stage high-pressure heater and a third-stage high-pressure heater.

[0048] In the preferred embodiment, Table 1 lists major parameters and major environmental information of the solar-aided coal-fired power system.

TABLE-US-00001 TABLE 1 Major parameters and major environmental information of solar-aided coal-fired power system Parameter Value Unit Rated power of coal-fired power 600 MW generation system Main steam flow rate 469.81 kg/s Main steam temperature 566.0 ° C. Main steam pressure 24.2000 MPa Reheated steam flow rate 387.52 kg/s Reheated steam temperature 566.0 ° C. Reheated steam pressure 3.6110 MPa Outlet temperature after heating 350.0 ° C. heat transfer oil Solar collecting area 134138 m.sup.2 Design solar irradiance 638 W/m.sup.2 Environmental temperature 20 ° C.

[0049] The research shows that: the correspondence relationship between the solar-to-power efficiency and the solar-coal feedwater flow distribution ratio under different working conditions is different and related to the solar irradiance. When the solar-aided coal-fired power system operates under the 80%-100% power load, with an example of 100% power load shown in FIG. 3, the correspondence relationship between the solar-to-power efficiency and the solar-coal feedwater flow distribution ratio is monotonically decreasing. Thus, the solar-coal feedwater flow distribution ratio is controlled to operate at the lower limiting value. When the solar-aided coal-fired power system operates under the 60%-80% power load, with an example of 75% power load shown in FIG. 4, the correspondence relationship between the solar-to-power efficiency and the solar-coal feedwater flow distribution ratio is decreasing when the solar direct normal irradiance is not less than 85%-105% design solar irradiance; the correspondence relationship between the solar-to-power efficiency and the solar-coal feedwater flow distribution ratio is firstly increasing and then decreasing when the solar direct normal irradiance is less than 85%-105% design solar irradiance; and the maximum value occurs in a range of 0.2 to 0.3. Thus, when the solar direct normal irradiance is not less than 85%-105% design solar irradiance, the solar-coal feedwater flow distribution ratio is controlled to operate at the lower limiting value; when the solar direct normal irradiance is less than 85%-105% design solar irradiance, the solar-coal feedwater flow distribution ratio is controlled to operate between 0.2 and 0.3. When the solar-aided coal-fired power system operates under the 40%-60% power load, with an example of 50% power load shown in FIG. 5, the correspondence relationship between the solar-to-power efficiency and the solar-coal feedwater flow distribution ratio is decreasing when the solar direct normal irradiance is not less than 94%-110% design solar irradiance; the correspondence relationship between the solar-to-power efficiency and the solar-coal feedwater flow distribution ratio is firstly increasing and then decreasing when the solar direct normal irradiance is less than 94%-110% design solar irradiance. Thus, when the solar direct normal irradiance is not less than 94%-110% design solar irradiance, the solar-coal feedwater flow distribution ratio is controlled to operate between 0.4 and 0.45; when the solar direct normal irradiance is less than 94%-110% design solar irradiance, the solar-coal feedwater flow distribution ratio is controlled to operate between 0.25 and 0.3.

[0050] Through regulating the solar-coal feedwater flow distribution ratio and controlling the flow rate of feedwater entering the solar collecting system to be heated, the present invention ensures the solar-to-power efficiency to reach a maximum value, which provides clear guidance for optimization of the utilization rate of solar energy under the off-design working conditions, improves the energy utilization rate of the solar-aided coal-fired power system, and is simple to operate and easily implemented.