Solar aided coalfired power plants utilize various types of solar thermal energy for coupling coalfired power plants by using the characteristics of various thermal needs of the plants. In this way, the costly thermal storage system and power generating system will be unnecessary while the intermittent and unsteady way of power generation will be avoided. Moreover, the largescale utilization of solar thermal power and the energysaving aim of power plants will be realized. The contribution evaluating system of solar thermal power needs to be explored. This paper deals with the evaluation method of solar contribution based on the second law of thermodynamics and the principle of thermoeconomics with a case of 600 MW solar aided coalfired power plant. In this study, the feasibility of the method has been carried out. The contribution of this paper is not only to determine the proportion of solar energy in overall electric power, but also to assign the individual cost components involving solar energy. Therefore, this study will supply the theoretical reference for the future research of evaluation methods and new energy resource subsidy.
Since the energy crisis in the 1970s, the major developed countries of the world started a series of projects involving solar thermal power generation for energy substitution. Among the developments in this field, the research of America, Israel, Spain, Germany, and Italy began first and are consequently the most mature [
Pai proposed the integration of a solar concentrator field to a 210 MWe coalfired power plant [
The previous research as mentioned above has investigated the integration of a solar concentrator field with a power plant. However the integrated system introduces solar energy into the individual components in the traditional power plants, the researches about how to evaluate the contribution of solar energy in the system are hardly found.
A thermoeconomic method of evaluating solar contribution in the integrated system is firstly proposed in this paper. A new built 600 MW solar aided coalfired power generation unit is considered as a reference power plant. According to the design parameters, the contribution proportion has been achieved, and the generation cost has also been explored by using sensitivity analysis method.
Figure
Layout of the solar aided coalfired power plant.
In a typical reheated steam coalfired power plant, the combustion of coal takes place in the boiler. The unsaturated boiler feedwater from the condenser enters the boiler after going through four lowpressure reheaters (HTR1, HTR2, HTR3, and HTR4), three highpressure reheaters (HTR5, HTR6, and HTR7), and a deaerator (Deaerator). The outlet superheated steam from the boiler is transported to the highpressure cylinder to produce power, then, after being reheated in the boiler, drives the intermediate and lower pressure cylinders. Finally, the exhaust is condensed in the condenser. It can be seen from Figure
It can be seen from Figure
The principle behind the solar aided coalfired power generation system is to supply thermal energy by substituting high pressure feedwater heaters with oilwater heat exchangers. During a series of circulation, the energy contributes to the system power output indirectly. Therefore, it is meaningful to discuss how to evaluate the solar energy contribution in the solar aided coalfired power plant.
According to the first and second laws of thermodynamics, evaluation models based on energy balance and exergy balance are the possible methods for evaluating energy contribution such as solar energy and fuel. The method based on energy balance just considers the magnitude of energy without taking energy grade into account. The evaluation based on exergy balance has added energy grade to the system, while the nonequivalence of the same value of exergy at different points in the system has never been considered. Therefore, the evaluation based on the index of technical economics and energy equivalence has been explored in this study. The approach proposed in this paper is called the thermoeconomic cost method, considering both the energy grade and the nonequivalence.
The thermoeconomic cost method includes three steps as shown in Figure
Schematic representation of the calculating process of the thermoeconomic cost evaluation.
This part is shown in Figure
Schematic representation of solar aided coalfired power generation system.
The exergy balance equation of each subsystem is as follows.
(1) The exergy loss of oilwater heat exchanging is given by
The proportion of solar exergy loss assumed to be
The exergy achieved by water will be written as follows:
It equals the exergy released by solar energy, which may be written as follows:
The proportion of solar energy at point 1 may become as follows:
(2) The total exergy loss of preheater, steam generator and superheater is given by
The proportion of coal exergy loss assumed to be
Then the exergy loss of water will be,
It equals the exergy released by coal, which may be written as follows:
The proportion of solar energy at point 2 may become as follows:
(3) The proportions of solar energy at the import and export of turbine are equal.
For the third subsystem, the equation will be given by
For the fifth subsystem, the equation will be given by
(4) The total exergy loss of reheater is given by
The proportion of coal exergy loss assumd to be
Then the exergy loss of coal will be
The exergy achieved by water will be written as follows:
It equals the exergy released by coal, which may be written as follows:
The proportion of solar energy may become as follows:
(5) The total exergy loss is given by
The proportions of exergy loss at point 7 and 8 are assumed to be
Then, the exergy loss at point 7 will be
The exergy loss at point 8 will be
The equation will be written as follows:
This part is shown in Figure
Then, for the first subsystem, the equation is given by the following:
For the second subsystem, the equation is given by the following:
For the third subsystem, the equation is given by the following:
For the fourth subsystem, the equation is given by the following:
For the fifth subsystem, the equation is given by the following:
For the fifth subsystem, the equation is given by the following:
The cost of solar exergy flow
The costs of coal exergy flows
The costs of
The costs of
According to (
This part is shown in Figure
The schematic of cost flows.
According to the data above, the solar energy parts in each exergy flow have just been considered alone, and the cost proportions of solar energy will be explored.
Equations of each subsystem have been given as follows.
(1) The cost equation of oilwater heat exchanging is given by the following:
(2) The cost equation of preheating, steam generating, and superheating is given by the following:
(3) The cost equation of turbine is given by the following:
(4) The cost equation of reheating is given by the following:
(5) The cost equation of turbine is given by the following:
(6) The cost equation of heat exchanging is given by the following:
Since the cost of solar energy achieving is free, we assume
For the second and third subsystems, the import exergy flows of 10 and 11 will be 0, which means the solar parts in 10 and 11 will be 0:
Since the costs of
Since the flow rates of
The cost proportions of solar energy
A coalfired power plant of 600 MW in China has been chosen as the base plant, with the main designed parameters shown in Table
Main designed parameters of coalfired power plant.
Parameters  Values  Units 

Capacity  600  MW 
Parameters of main steam  24.2/566/566  MPa/°C/°C 
Feedwater mass flow rate  1645.15  t/h 
Condenser pressure  4.9  kPa 
Feedwater temperature  272.3  °C 
Coal consumption rate  257.4  g/kWh 
The solar aided coalfired power plant is using the same coal as the base case, except for the solar driven oilwater heat exchanger and the steam cutoff, other structures are the same as the base case. The data of solar field is based on real data from the GEGSVI station in USA [
Main parameters of the collector field.
Parameters  Values  Units 

Solar irradiation  925  W/m^{2} 
Area of per collector  235  m^{2} 
Number of collector in each row  16  
Rows of collectors  30  Rows 
Inlet temperature of heat transfer oil  250  °C 
Outlet temperature of heat transfer oil  328  °C 
The main designed parameters of solar aided power plant are shown in Table
Main designed parameters of solar aided power plant.
Parameters  Values  Units 

Capacity  600  MW 
Feedwater temperature  281.65  °C 
Coal consumption rate  243.7  g/kWh 
Thermal efficiency  50.41  % 
Exergy efficiency  48.07  % 
Area of all collectors  112800  m^{2} 
For the solar aided coalfired power plant, the energy efficiency and exergy efficiency can be defined as follows:
It can be seen from Tables
The capital cost of the plant is shown in Table
The investment of the plant.
Items  Cost (dollars) 

Solar concentration field  37528560.00 
Oilwater heat exchangers  4192170.00 
Super heaters of the boiler  168235561.38 
Reheaters of the boiler  22941212.92 
Highpressure turbine  24723323.22 
Intermediatepressure turbine  26877786.50 
Lowpressure turbine  42734428.98 
Condenser  14550480.00 
Other heat exchangers  21149680.00 
Deaerator  3156810.00 
Pumps  673317.35 
Based on the methodology proposed in Section
Exergy value of each flow and the proportion of solar power.
Exergy flows  Exergy value (MJ/h)  Exergy Proportion of solar 
Percent 


555712.8 

48.39 

2447325 

6.55 

1563747 

6.55 

2065157 

4.09 

89036.6 

4.09 

422330.4 

4.72 

157822.2 

6.55 

362203.9 

4.09 

357271.6 

100 

3551430 

0 

941377.7 

0 

664758.2 

6.55 

1495293 

4.09 
Exergy cost of each flow and the proportion of solar power.
Cost flow  values  Cost proportion of solar 
Percent 


5903 

1.98 

22724 

0.70 

14520 

0.70 

18896 

0.56 

815 

0.56 

5745 

0.70 

1465 

0.70 

3314 

0.56 

0 

100 

16181 

0 

4289 

0 

6833 

0.78 

15032 

0.62 
The overall exergy proportion of solar power in the plant can be calculated using
It can be seen from Table
The overall cost proportion of solar power in the plant can be calculated using
It can be seen from Table
The change of cost of electricity with coal cost.
The change of the overall cost proportion of solar with coal cost.
The power generation cost is calculated as follows:
The exergy cost of electricity includes two aspects, namely, fixed costs and variable costs. The fixed costs include equipment costs, material costs, depreciation costs, operation, and maintenance fees, and the variable costs include fuel costs, environmental costs, water charges. In order to take advantage of the calculation method proposed in this paper, all of the costs (except the fuel costs) have been converted to the equipment costs. Therefore, in this case, the cost of power generation is calculated as follows:
It can be seen from Figure
It can be seen from Figure
The change of the cost of electricity with the solar facilities’ investment.
The change of the overall cost proportion of solar with the solar facilities’ investment.
Figure
It can be seen from Figures
This paper proposed thermoeconomic cost method for solar contribution study in solar aided coalfired power plant and analyzed the plant based on the newly constructed and reconstructed 600 MW power plant.
Based on the second law of thermodynamics and the principle of thermoeconomics, this paper proposed the evaluation of solar contribution in the solar aided coalfired power plant, which can be used to assign the individual cost components involving solar energy. Its feasibility has been proven by analyzing a newly constructed power plant. The result shows: that when operating at the rated capacity, the proportion of solar power in overall electric power is 4.84%, which is about 29.04 MW.
The exergy value of the
The ratio of solar exergy loss in the total exergy loss when running the
The ratio of solar exergy flow in the
The unit cost of the
The nonenergy cost of the
The exergy cost ratio of the solar contribution in the
The ratio of the solar side of the nonenergy costs of the
The research work is supported by China National Natural Science Foundation (no. 51106048), the Fundamental Research Funds for the Central Universities and the Program for 863 Project (2012AA050604). The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the paper apart from the one disclosed.