Optimal Dispatch Model Considering Environmental Cost Based on Combined Heat and Power with Thermal Energy Storage and Demand Response

In order to reduce the pollution caused by coal-fired generating units during the heating season, and promote the wind power accommodation, an electrical and thermal system dispatch model based on combined heat and power (CHP) with thermal energy storage (TES) and demand response (DR) is proposed. In this model, the emission cost of CO2, SO2, NOx, and the operation cost of desulfurization and denitrification units is considered as environmental cost, which will increase the proportion of the fuel cost in an economic dispatch model. Meanwhile, the fuel cost of generating units, the operation cost and investment cost of thermal energy storage and electrical energy storage, the incentive cost of DR, and the cost of wind curtailment are comprehensively considered in this dispatch model. Then, on the promise of satisfying the load demand, taking the minimum total cost as an objective function, the power of each unit is optimized by a genetic algorithm. Compared with the traditional dispatch model, in which the environmental cost is not considered, the numerical results show that the daily average emissions CO2, SO2, NOx, are decreased by 14,354.35 kg, 55.5 kg, and 47.15 kg, respectively, and the wind power accommodation is increased by an average of 6.56% in a week.


Introduction
In recent years, clean energy is promoted all over the world to reduce environmental pollution.Although China is rich in wind energy resources, the main operation pattern "determining power by heat" for combined heat and power (CHP) and the "anti-peak regulation" character of wind power in heating season causes severe wind curtailment [1,2].In the heating season, the thermal load demand of consumers must be met, but the thermal power constrains the electrical power of CHP.This condition leads to low wind power accommodation when most of electrical load demand could be covered by the electrical power of CHP during the off-peak hours [3].Meanwhile, slather utilization of fossil fuel releases a large amount of CO 2 , SO 2 , and NO x , which causes the greenhouse effect and environmental pollution [4].
Many studies have been performed recently about the dispatch model of CHP with thermal energy storage (TES) to increase the wind power accommodation, such as in References [5][6][7][8], in which the minimum wind curtailment or minimum coal consumption cost was used as the fitness function Energies 2019, 12, 817 2 of 18 to reduce wind curtailment.In Reference [9], a dispatch model including CHP, conventional thermal power units, and renewable energy source was proposed, and the heating process by a three-stage heat transfer model of the extraction steam was described.In Reference [10], a multi-timescale rolling dispatch model based on real-time error compensation was proposed.The power of thermal power unit was revised in real-time to compensate for the error of wind power forecast.In addition, some scholars have made further study on the district heating system, and taken the heat storage capacity of pipelines and buildings into account.For example, in Reference [11], a dispatch model considering the TES capacity of pipelines, buildings and TES devices was proposed.The advantage of this model was that it could be simplified to any combination of the TES capacity of pipelines, buildings, and TES devices.In Reference [12], in order to coordinate the operation of the electric power system and the district heating system, a dispatch model including CHP and TES was proposed.In this dispatch model, the temperature dynamics of the district heating network were considered as energy storage to manage the variability of wind energy.In Reference [13], Dai et al. proposed a detailed model for the district heating network, building envelopes, and TES, and taken heat transfer constraints into account.In Reference [14], the conventional thermal units, wind turbines, and CHP with TES were included in an integrated electrical-thermal dispatch model.The authors proposed an iteration method to solve this nonlinear programming problem.However, these dispatch methods only considered the devices in generation side, and it is not very effective to promote the wind power accommodation, because the burden of the power generation side is enormous when the wind power in large scale grid-connected.Therefore, scholars in References [15,16] pointed out that demand response (DR) was a key approach to be responsive to grid connection when the wind power reached a large proportion of generation.In References [17,18], a DR model based on time-of-use pricing mechanism was employed, and the optimal results were applied in the game theoretic model for multiple virtual power plants dispatch.In Reference [19], the dynamic dispatch problem was optimally integrated with the incentive-based DR program, and the author pointed out that intelligent implementation of DR programs not only decreases electricity price in electricity markets, but also improves network reliability.Some scholars have conducted further study on DR.For example, in Reference [20], a fuzzy inference system for considering the customer load profile attributes in DR bids was presented, and the impact of load profiling attributes on the DR exchange mechanism was investigated.In References [21,22], a commitment model based on incentive-based considering different types of DR resources was proposed."Source-load-energy storage" coordination is an important method to improve the system operation economy and the accommodation rate of renewable energy.However, all of these references ignored the environmental problems caused by the electric power industry.In other words, the emissions of CO 2 , SO 2 , and NO x caused by coal-fired units were not taken into account.
Some scholars realized that environmental pollution needs to be considered in the electric power industry.For instance, in Reference [4], a unit commitment model with wind power that considers the dispersion of air pollutants was proposed.In Reference [23], Nguyen et al. proposed an environment-friendly distributed control approach, in which the pollutant emission costs were incorporated into the fitness function.However, in these dispatch models, the emission cost of CO 2 was not taken into account.In Reference [24], Deylamsalehi et al. proposed a new approach for the power grid to minimize the electricity costs and the pollutant emissions under various electricity markets.In this approach, the electric power price and the fuel price were used to find the lowest total electricity cost path.However, DR as an effective dispatch resource was not included in these approaches.
For the research of the above references, there were few studies concretely considering the environmental cost for electric and thermal system dispatch model, or ignoring the advantages of DR, which could effectively reduce the pressure of generating units.
In this paper, aiming at reducing the emissions of CO 2 , SO 2 , and NO x , and promoting the wind power accommodation, an electrical and thermal system dispatch model considering environmental cost based on CHP-DR is proposed.Then, a test system is studied to demonstrate the accuracy and effectiveness of the proposed model.On the premise that the electrical power balance and thermal power meet the user's comfort, the power of power generation units and the regulating variable of demand response are optimized by a genetic algorithm (GA) in MATLAB 2018a (MathWorks, Natick, MA, USA).Compared with the traditional dispatch model, not only the emissions of CO 2 , SO 2 , and NO x are reduced, but also the wind power accommodation is promoted, the test results demonstrate the reasonableness and validity of the proposed model.The contributions of this work are as follows:

•
We analyze the operation characteristics of CHP with TES and incentive-based DR, respectively.Then, the cost formula of incentive-based DR participating in power grid dispatching is given.

•
An economic power dispatch model based on CHP-DR is proposed.In this dispatch model, the emission cost of CO 2 , SO 2 , NO x , the operation cost of desulfurization units, and the operation cost of denitrification units are considered as environmental cost, which increases the cost of power generation.

•
Two dispatch models are set and the data in one week is utilized to verify the effectiveness.One dispatch model takes the environmental cost into account and the other does not.The results show that the daily average emissions of CO 2 , SO 2 , and NO x are decreased by 14,354.35kg, 55.5 kg, and 47.15 kg, respectively, and the wind power accommodation is increased by an average of 6.56%.• We analyze the operation characteristics of CHP with TES and incentive-based DR, respectively.Then, the cost formula of incentive-based DR participating in power grid dispatching is given.

•
An economic power dispatch model based on CHP-DR is proposed.In this dispatch model, the emission cost of CO2, SO2, NOx, the operation cost of desulfurization units, and the operation cost of denitrification units are considered as environmental cost, which increases the cost of power generation.

•
Two dispatch models are set and the data in one week is utilized to verify the effectiveness.One dispatch model takes the environmental cost into account and the other does not.The results show that the daily average emissions of CO2, SO2, and NOx are decreased by 14,354.35kg, 55.5 kg, and 47.15 kg, respectively, and the wind power accommodation is increased by an average of 6.56%.

Operation Characteristic and Modeling of CHP with TES
Combined heat and power (CHP) is an efficient device to generate electrical power and thermal power at the same time.CHP can be classified into two types: extraction-condensing turbine and non-condensing turbine [9].In this paper, the extraction-condensing unit is studied.Due to the operation pattern "determining power by heat" of the extraction-condensing unit, the electrical power, and thermal power are coupled.The operation characteristics of the extraction-condensing unit are shown in the blue curve in Figure 1 [25].
Various means to improve the flexibility of CHP have been performed, but thermal energy storage (TES) is the most attracted owing to its high efficiency, large capacity, and high efficiency.The addition of TES units on the traditional CHP units can break the thermo-electric coupling characteristic, as shown by the red curve in Figure 1 [26].Various means to improve the flexibility of CHP have been performed, but thermal energy storage (TES) is the most attracted owing to its high efficiency, large capacity, and high efficiency.The addition of TES units on the traditional CHP units can break the thermo-electric coupling characteristic, as shown by the red curve in Figure 1 [26].
In Figure 1, P h max is the maximum thermal power of CHP; P h med is the thermal power with the minimum electrical power; P e min and P e max are the minimum and maximum electrical power of CHP under the condition of pure condensation.
We assume that the heat storage of TES is sufficient, and set the maximum discharge power as P TES dmax and the maximum charge power as P TES cmax .For a certain power generation, the maximum thermal power P h max of the whole system (CHP with TES) will be increased by P TES dmax on the original basis.This is equivalent to the segment AB and the segment BC in Figure 1 is shifted to the right by P TES max .In Figure 1, P h J = P h max + P TES dmax , P h K = P h med − P TES dmax , P h H1 = P h − P TES dmax .In addition, there is a minimum thermal power, when the electrical power of CHP is between P e min and P e C (i.e., section CD in Figure 1).After the configuration of TES, the minimum thermal power will shift to the left by P TES cmax .Therefore, the feasible region of heat production and electricity production is AGIJKL.In other words, when the thermal power is P h , the feasible region of electrical power is from [P e F ,P e E ] to [P e M ,P e H ]. The regulation of CHP is significantly increased, and the coupling characteristic of CHP is reduced.
The cost of CHP units with TES can be described as [27]: where F CT is the total cost of CHP with TES, i = 1, . . ., N is the number of CHP units, and j = 1, . . ., M is the number of TES units.C c,i is the fuel cost of CHP i with TES.For CHP with TES, the operation cost should consider both thermal power and electric power.In general, the electric power and thermal power need to be converted into electric power under the condition of pure condensation.Therefore, the fuel cost of CHP units with TES can be described as in Equation ( 2).F TC,j is the average daily investment cost of TES j, which is calculated by Equation (3).F TW,j is the operation and maintenance cost of TES j, which is shown in Equation (4): where t = 1, . . ., T, T is the number of hours in operation.P e i,t is the electrical power of CHP i at time t, and P h i,t is the thermal power of CHP i at time t.P TES j,t is the thermal power of TES j at time t. a m , b m , and c m are the operation cost coefficients of CHP with TES.c v is the linear supply slopes of thermal power and electric power of CHP.P TES R,j is the rated power of TES j, E TES R,j is the rated capacity of TES j, C TP , and C TE are the cost coefficients of TES, T T,j is the service life of TES i. C U is the operation and maintenance cost coefficient of TES.
(1) Constraints of CHP: where P e min,i and P e max,i are the lower and upper limits of electrical power for CHP i respectively, P h min,i and P h max,i are the lower and upper limits of thermal power for CHP i respectively.Such as Equation ( 2), the electrical power and thermal power should be converted into electrical power, the ramp rate of CHP is given by: Energies 2019, 12, 817 5 of 18 where r C d,i and r C u,i are the lower and upper ramp rate limits of CHP unit i respectively.(2) Constraints of TES: S min,j ≤ S j,t ≤ S max,j where S j,t is the capacity of TES j at time t, S min,j and S max,j are lower and upper capacity limits of TES j, respectively, P TES min,j and P TES max,j are the lower and upper thermal power limits of TES j, respectively.

Incentive-Based DR
In this part, firstly, we analyze the mechanism of incentive-based demand response (DR) participating in power grid dispatch to improve wind power accommodation.Then, the cost formula of incentive-based DR participating in power grid dispatching is given.
Demand response (DR) refers to the behavior of users to change their short-term or long-term electricity consumption mode, through market price incentives or direct instructions from system operators.Usually, DR is divided into two main categories namely the incentive-based and time-based programs [28].This paper mainly focuses on incentive-based DR, which is implemented voluntarily or compulsorily.
The process of users participating in dispatch can be described as basing on the situation of power generation and consumption in each scheduling period.The load control center issue dispatching instructions to the industrial users who signed the agreement, and provision of compensation.Figure 2 shows the schematic diagram of the industrial load participation schedule.
Equation ( 2), the electrical power and thermal power should be converted into electrical power, the ramp rate of CHP is given by: where Sj,t is the capacity of TES j at time t, Smin,j and Smax,j are lower and upper capacity limits of TES j, respectively, TES min, j P and TES max, j P are the lower and upper thermal power limits of TES j, respectively.

Incentive-Based DR
In this part, firstly, we analyze the mechanism of incentive-based demand response (DR) participating in power grid dispatch to improve wind power accommodation.Then, the cost formula of incentive-based DR participating in power grid dispatching is given.
Demand response (DR) refers to the behavior of users to change their short-term or long-term electricity consumption mode, through market price incentives or direct instructions from system operators.Usually, DR is divided into two main categories namely the incentive-based and time-based programs [28].This paper mainly focuses on incentive-based DR, which is implemented voluntarily or compulsorily.
The process of users participating in dispatch can be described as basing on the situation of power generation and consumption in each scheduling period.The load control center issue dispatching instructions to the industrial users who signed the agreement, and provision of compensation.Figure 2 shows the schematic diagram of the industrial load participation schedule.It can be seen that the load demand in 11:00-13:00 is transferred to 00:00-3:00, when the wind power is greatly abundant.As the "anti-peak regulation" character of wind power, DR can effectively promote wind power accommodation, and reduce the emissions of CO2, SO2, and NOx.
The incentive cost of DR can be described as follow [29]: It can be seen that the load demand in 11:00-13:00 is transferred to 00:00-3:00, when the wind power is greatly abundant.As the "anti-peak regulation" character of wind power, DR can effectively promote wind power accommodation, and reduce the emissions of CO 2 , SO 2 , and NO x .
The incentive cost of DR can be described as follow [29]: where F DR is the total incentive cost of DR, ρ is the incentive cost coefficient, P ∆L,t is the value of dispatch load at time t, ∆t is the dispatch time interval.In order to ensure the production efficiency of the industries, P ∆L,t should be within limits:

Objective Function
As illustrated in Figure 3, thermal power units, CHP units, wind turbines, EES and TES are considered as generating units to support power and heat demands.The CO 2 produced by coal is directly discharged into the air, and SO 2 and NO x are processed by the desulfurization and denitrification device and then release in the air.
dispatch load at time t, Δt is the dispatch time interval.
In order to ensure the production efficiency of the industries, Δ , L t P should be within limits: where PLoad,t is the load demand at time t.ζ Lmin and ζ Lmax are the minimum and maximum values of load regulation rate, respectively.

Objective Function
As illustrated in Figure 3, thermal power units, CHP units, wind turbines, EES and TES are considered as generating units to support power and heat demands.The CO2 produced by coal is directly discharged into the air, and SO2 and NOx are processed by the desulfurization and denitrification device and then release in the air.In this paper, a combined dispatch model for reducing the emission of CO2, SO2, and NOx is established.Factors such as the fuel cost of conventional thermal power units, the cost of CHP units with TES, average daily investment and the operation and maintenance cost of EES units, the cost of load dispatch, the emission cost of CO2, SO2, and NOx, the operation cost of desulfurization and denitrification device and the cost of wind curtailment are comprehensively considered.
(1) Cost of thermal power units As a controllable power source, the thermal power unit can track the changes of grid load through reasonable dispatch.At this time, the generation cost mainly includes the fuel cost of the units: In this paper, a combined dispatch model for reducing the emission of CO 2 , SO 2 , and NO x is established.Factors such as the fuel cost of conventional thermal power units, the cost of CHP units with TES, average daily investment and the operation and maintenance cost of EES units, the cost of load dispatch, the emission cost of CO 2 , SO 2 , and NO x , the operation cost of desulfurization and denitrification device and the cost of wind curtailment are comprehensively considered.
(1) Cost of thermal power units As a controllable power source, the thermal power unit can track the changes of grid load through reasonable dispatch.At this time, the generation cost mainly includes the fuel cost of the units: where F 1 is the fuel cost of thermal power units.i = 1, . . ., n is the number of thermal power units.C h,i is the fuel cost of thermal power unit i, which is calculated by Equation ( 14): where a i , b i , and c i are the cost coefficients of thermal power unit i. P i,t is the power of thermal power unit i at time t.
(2) Cost of EES The storage capacity of electrical energy storage (EES) makes the wind power have the ability of space-time translation.But there was less use because of the high costs of EES, accordingly.The cost of EES including average daily investment and the operation and maintenance cost is given by: where F 2 is the total cost of EES.i = 1, . . ., m is the number of EES.F EC,i is the average daily investment cost of EES i, which is calculated by Equation (16).F EW,i is the operation and maintenance cost of EES i, which is calculated by Equation (17): where C EP and C EE are the power and capacity cost coefficients, respectively.P R EES,i is the rated power, E R EES,i is the rated capacity, and T E,i is the service life of EES i. C O is the operation and maintenance cost coefficient of EES, P EES i,t is the power of EES i at time t.(3) Carbon cost In order to maintain a long-term stable supply of energy while avoiding a further deterioration of the environment, the emission cost of CO 2 is given by: where F 3 is carbon cost, C CP is the environmental cost coefficient of CO 2 .W C is the emission of CO 2 , which is calculated by Equation ( 19): where β m is the unit-price of coal.δ C is the emission of CO 2 by a unit mass of coal combustion.(4) Operation cost of desulfurization and denitrification equipment and the emission cost of SO 2 and NO x Both conventional thermal power units and CHP units will produce pollutants and cause environmental pollution during operation.The use of desulfurization and denitrification equipment can effectively reduce environmental pollution, but at the same time increases the operation cost of the power plant.This cost can be described as a positive linear correlation with the emissions of SO 2 and NO x : where F 4 is the operation cost of desulfurization and denitrification equipment and the emission cost of SO 2 and NO x .C S and C N are the operating cost coefficients of desulfurization and denitrification device to remove a unit mass of SO 2 and NO x .W S is the emission of SO 2 , which is calculated by Equation ( 21), and W N is the emission of NO x , which is calculated by Equation (22).η is the efficiency of desulfurization and denitrification device.C SP and C NP are the environmental cost coefficients of SO 2 and NO x : Energies 2019, 12, 817 8 of 18 where δ S and δ N are the emissions of SO 2 and NO x by a unit mass of coal combustion, respectively.
(5) Cost of wind curtailment The cost of wind curtailment can be described as follows: where F 5 is the cost of wind curtailment.C Q is the unit-cost of wind curtailment.P ∆Wind,t is the wind curtailment at time t.Summing up Equations ( 1), ( 10), ( 13), ( 15), ( 18), ( 20), ( 23), the objective function is expressed as follows: minF = F CT (P e i,t , P h i,t , P TES j,t ) + F DR (P ∆L,t ) ) where F is the total cost of the dispatch model.

Constraints
(1) Power balance Total generations and electric loads should be balanced at each period: where P w i,t is the power of the wind turbine i at time t, P Load,t is load demand at time t.(2) Thermal power range Since the thermal inertia of the devices in user-side could maintain the temperatures, the unbalance between thermal power generation and thermal power demand can be accepted [30], but to ensure user comfort, the unbalance should be required within a limited range: where P Heat,t is the heat demand at time t.χ Hmin and χ Hmax are the lower and upper limited proportions of adjustment of thermal power demand, respectively.
(3) Constraints of conventional thermal power units: where P min,i and P max,i are the lower and upper power generation limits of unit i, respectively, r d,i and r u,i are the lower and upper ramp rate limits of thermal power unit i, respectively.(4) Constraints of EES SOC min,i ≤ SOC i,t ≤ SOC max,i where SOC min,i, and SOC max,i are lower and upper limits of the state of charge (SOC) for EES i, respectively, P EES min,i and P EES max,i are the lower and upper power limits for EES i, respectively.
The constraints of CHP with TES and the constraints of dispatch load are shown in Equations ( 5)-( 9).The constraints of demand response are shown in (11) and (12).

Analysis of Examples
In this section, the effectiveness of the dispatch model is verified.Firstly, the parameters of the simulation and the forecasts of load demand and wind power in a week are given in Table 1 and Figure 4, respectively.Then, we compare the emission of CO 2 , SO 2 , and NO x and the wind power accommodation with traditional dispatch model, in which the environmental cost is not considered.Finally, we select one day for concrete analysis, including the role of environmental costs in economic dispatch and the effect of environmental cost coefficient on the results.The constraints of CHP with TES and the constraints of dispatch load are shown in Equations ( 5)-( 9).The constraints of demand response are shown in (11) and (12).

Analysis of Examples
In this section, the effectiveness of the dispatch model is verified.Firstly, the parameters of the simulation and the forecasts of load demand and wind power in a week are given in Table 1 and Figure 5, respectively.Then, we compare the emission of CO2, SO2, and NOx and the wind power accommodation with traditional dispatch model, in which the environmental cost is not considered.Finally, we select one day for concrete analysis, including the role of environmental costs in economic dispatch and the effect of environmental cost coefficient on the results.

The Setup of Simulation
In this paper, the input parameters are the forecasts of load demand and wind power generation, which are given in Figure 4, and we assumed the forecasts are precise.To simplify the calculation, one CHP with TES, one thermal power unit, and one EES are performed in this test system.The parameters of the dispatch model are shown in Table 1.And the environmental cost coefficient of CO2, SO2, and NOx are 0.02 USD/kg, 6 USD/kg, and 28 USD/kg, respectively [31,32]; the operating cost coefficient of desulfurization and denitrification device are 2.99 USD/kg and 15 USD/kg, respectively [33]; the efficiency of desulfurization and denitrification device η is 85%; the initial heat storage of TES is 600 MWh; the initial SOC of EES is 0.5; the dispatch time interval is one hour.

The Setup of Simulation
In this paper, the input parameters are the forecasts of load demand and wind power generation, which are given in Figure 4, and we assumed the forecasts are precise.To simplify the calculation, one CHP with TES, one thermal power unit, and one EES are performed in this test system.The parameters of the dispatch model are shown in Table 1.And the environmental cost coefficient of CO 2 , SO 2 , and NO x are 0.02 USD/kg, 6 USD/kg, and 28 USD/kg, respectively [31,32]; the operating cost coefficient of desulfurization and denitrification device are 2.99 USD/kg and 15 USD/kg, respectively [33]; the efficiency of desulfurization and denitrification device η is 85%; the initial heat storage of TES is 600 MWh; the initial SOC of EES is 0.5; the dispatch time interval is one hour.
The GA-Toolbox in MATLAB 2018a (MathWorks, Natick, MA, USA) is used to solve this optimization, and the relevant parameters of GA are the default values.The steps to optimize the dispatch models are as follows: • Firstly, we confirm that the optimization variables are P e i,t , P h i,t , P TES i,t , P ∆L,t , P i,t , P EES i,t , and P ∆Wind,t .The objective of optimization is to minimize the total dispatching cost.

Comparison of Different Dispatch Models
In order to verify the effect of introducing environmental cost into dispatch model, the dispatch model proposed in this paper is compared with the traditional dispatch model, in which the environmental cost is not considered.Figure 8 shows the comparison of wind power accommodation.As shown, the wind power accommodation in dispatch model I is higher than that in dispatch model II.As shown in Table 2, the average wind power accommodation proportion in dispatch model I and dispatch model II are 76.74% and 70.18%, respectively.The wind power accommodation is increased by 6.56%.

Analysis of Dispatch Results of One Day
In this part, we select the first day for concrete analysis.For dispatch model I, the optimized electric power of each unit is shown in Figure 9, and the optimized thermal power of each unit is shown in Figure 10.
power and electric power have certain coupling characteristics.In order to reduce the power of the thermal power unit during the peak load period, it is necessary to increase the electric power of CHP.TES releases heat in the peak period of the electric load, so as to reduce the thermal power of the CHP unit, and then to increase its electrical power, to achieve the goal of reducing the comprehensive operation cost.Concretely, the optimized costs in dispatch model I and dispatch model II are shown in Table 3.As shown in Table 3, the optimized total dispatching costs in dispatch model I and dispatch model II are 235.94kUSD and 191.76 kUSD, respectively.The total dispatching cost of dispatch model I is higher than that in dispatch model II.However, the environmental cost is not taken into account in dispatch model II.The environmental cost of dispatch model I is 46.87 kUSD, accounting for 19.87% of the total cost.According to the optimized results, if environmental protection is considered in an economic dispatch model, the proportion of environmental cost cannot be ignored.
Figures 11 and 12 show the comparison of CHP and thermal power unit.It can be seen that the power of CHP and thermal power unit in dispatch model I is lower than that in dispatch model II, especially between 1 a.m. to 8 a.m., the contrast of electrical power is obvious.However, the comparison of thermal power of CHP is not obvious.There are two main reasons: (1) The addition of TES to CHP improves the flexibility of CHP.Therefore, the operation pattern "determining power by heat" of CHP is alleviated; and (2) The operating characteristics of the CHP itself.In Figure 1, we know that when the thermal power is P h , the feasible region of electrical power is from [ e F P , e E P ].In other words, the thermal power in both dispatch models, the thermal power of CHP is "P h " in these dispatch models, and the electrical power varies between e F P and e E P .Therefore, the electrical power of the CHP is more obvious than thermal power.
Figure 13 shows the comparison of DR.The positive value means increasing the load power and the negative value means decreasing the load power.Owning to both of the dispatch models are emphasized on reducing the electrical power of CHP and thermal power unit to reduce the emissions of CO2, SO2, and NOx and promote the wind power accommodation.It can be seen that the fluctuations of DR in dispatch model I are more pronounced than dispatch model II.Especially As shown in Figures 9 and 10, in this dispatch model, the electric power value of the units at each time is equal to the electric load demand, while the heat power value does not need to be equal to the thermal load demand, because of the thermal inertia of thermal generation units.Comparing the two figures, we could find that during the time interval from 1 a.m. to 6 a.m., the electrical power demand is increased and the heating power demand is decreased through the means of DR.The fuel cost of the CHP is significantly lower than that of the thermal power unit, and its thermal power and electric power have certain coupling characteristics.In order to reduce the power of the thermal power unit during the peak load period, it is necessary to increase the electric power of CHP.TES releases heat in the peak period of the electric load, so as to reduce the thermal power of the CHP unit, and then to increase its electrical power, to achieve the goal of reducing the comprehensive operation cost.Concretely, the optimized costs in dispatch model I and dispatch model II are shown in Table 3.As shown in Table 3, the optimized total dispatching costs in dispatch model I and dispatch model II are 235.94kUSD and 191.76 kUSD, respectively.The total dispatching cost of dispatch model I is higher than that in dispatch model II.However, the environmental cost is not taken into account in dispatch model II.The environmental cost of dispatch model I is 46.87 kUSD, accounting for 19.87% of the total cost.According to the optimized results, if environmental protection is considered in an economic dispatch model, the proportion of environmental cost cannot be ignored.
Figures 11 and 12 show the comparison of CHP and thermal power unit.It can be seen that the power of CHP and thermal power unit in dispatch model I is lower than that in dispatch model II, especially between 1 a.m. to 8 a.m., the contrast of electrical power is obvious.However, the comparison of thermal power of CHP is not obvious.There are two main reasons: (1) The addition of TES to CHP improves the flexibility of CHP.Therefore, the operation pattern "determining power by heat" of CHP is alleviated; and (2) The operating characteristics of the CHP itself.In Figure 1, we know that when the thermal power is P h , the feasible region of electrical power is from [P e F ,P e E ].In other words, the thermal power in both dispatch models, the thermal power of CHP is "P h " in these dispatch models, and the electrical power varies between P e F and P e E .Therefore, the electrical power of the CHP is more obvious than thermal power.Figure 13 shows the comparison of DR.The positive value means increasing the load power and the negative value means decreasing the load power.Owning to both of the dispatch models are emphasized on reducing the electrical power of CHP and thermal power unit to reduce the emissions of CO 2 , SO 2 , and NO x and promote the wind power accommodation.It can be seen that the fluctuations of DR in dispatch model I are more pronounced than dispatch model II.Especially between 1 a.m. to 5 a.m., when the wind power is sufficient, the electrical load demand based on DR is obviously increased.Between 7 a.m. to 9 a.m., when the electrical load demand is at a peak and the wind power is at a low level, the electrical load demand based on DR is obviously decreased    Thus, the lower output of coal-fired unit will decrease the utilization of coal, meanwhile, the emissions of CO 2 , SO 2 , and NO x will also be decreased.In addition, the reduced power of the generator and the increased load based on DR would provide the accommodation space for wind power.

Conclusions
An economic power dispatch model considering the environmental cost based on CHP-DR is proposed in this paper.The optimized results show that, compared with the traditional dispatch model, the daily average emissions of CO2, SO2, and NOx are decreased by 14,354.35kg, 55.5 kg, and 47.15 kg, respectively, and the wind power accommodation is increased by 6.56%.Simulation results demonstrate that the dispatch model is efficient to reduce the emissions of CO2, SO2, and NOx, and increase the wind power accommodation.The following conclusions can be drawn: (1) The emissions of CO2, SO2, and NOx could be reduced, and the wind power accommodation could be promoted, when considering the environmental cost in an economic dispatch model; (2) environmental costs can be counted as the cost of power generation, since the environmental cost is closely related to the amount of coal burned; and (3) environmental cost accounts for a certain

Conclusions
An economic power dispatch model considering the environmental cost based on CHP-DR is proposed in this paper.The optimized results show that, compared with the traditional dispatch model, the daily average emissions of CO 2 , SO 2 , and NO x are decreased by 14,354.35kg, 55.5 kg, and 47.15 kg, respectively, and the wind power accommodation is increased by 6.56%.Simulation results demonstrate that the dispatch model is efficient to reduce the emissions of CO 2 , SO 2 , and NO x , and increase the wind power accommodation.The following conclusions can be drawn: (1) The emissions of CO 2 , SO 2 , and NO x could be reduced, and the wind power accommodation could be promoted, when considering the environmental cost in an economic dispatch model; (2) environmental costs can be counted as the cost of power generation, since the environmental cost is closely related to the amount of coal burned; and (3) environmental cost accounts for a certain proportion of the total economic cost, so it is significative to consider environmental cost in an economic dispatch model.
Dispatch Model of CHP with TES and DR 2.1.Operation Characteristic and Modeling of CHP with TES Combined heat and power (CHP) is an efficient device to generate electrical power and thermal power at the same time.CHP can be classified into two types: extraction-condensing turbine and non-condensing turbine [9].In this paper, the extraction-condensing unit is studied.Due to the operation pattern "determining power by heat" of the extraction-condensing unit, the electrical power, and thermal power are coupled.The operation characteristics of the extraction-condensing unit are shown in the blue curve in Figure 1 [25].

Figure 1 .
Figure 1.Heat-electricity relationship of CHP unit with TES.Figure 1. Heat-electricity relationship of CHP unit with TES.

Figure 1 .
Figure 1.Heat-electricity relationship of CHP unit with TES.Figure 1. Heat-electricity relationship of CHP unit with TES.

Figure 2 .
Figure 2. The power curve of the industrial load before and after dispatch.

Figure 2 .
Figure 2. The power curve of the industrial load before and after dispatch.

Figure 3 .
Figure 3. Structural diagram of the entire system.

Figure 3 .
Figure 3. Structural diagram of the entire system.

Figure 8 .
Figure 8.Comparison of wind power accommodation.

Figure 7 .
Figure 7.Comparison of NO x emission.

Figure 8 .
Figure 8.Comparison of wind power accommodation.Figure 8. Comparison of wind power accommodation.

Figure 8 .
Figure 8.Comparison of wind power accommodation.Figure 8. Comparison of wind power accommodation.

Figure 9 .
Figure 9. Electric power dispatch result of each unit.Figure 9. Electric power dispatch result of each unit.

Figure 9 . 19 Figure 10 .Table 3 .
Figure 9. Electric power dispatch result of each unit.Figure 9. Electric power dispatch result of each unit.Energies 2019, 12, x FOR PEER REVIEW 13 of 19

Figure 10 .
Figure 10.Heat power dispatch results of combined heat and power (CHP) and thermal energy storage (TES).

Figure 12 .
Figure 12.Comparison of thermal power unit.

Figures 14 and 15
Figures 14 and 15 show the comparison of the emissions of CO2, SO2, and NOx, and Figure 16 shows the comparison of wind power accommodation.It can be seen that the emissions of CO2, SO2, and NOx in dispatch model I are much lower than dispatch model II.The wind power accommodation in dispatch model I is significantly improved.Therefore, taking environmental costs into account can significantly reduce the emissions of CO2, SO2, and NOx and promote wind

Figure 12 .
Figure 12.Comparison of thermal power unit.

Figures 14 and 15
Figures 14 and 15 show the comparison of the emissions of CO2, SO2, and NOx, and Figure 16 shows the comparison of wind power accommodation.It can be seen that the emissions of CO2, SO2, and NOx in dispatch model I are much lower than dispatch model II.The wind power accommodation in dispatch model I is significantly improved.Therefore, taking environmental costs into account can significantly reduce the emissions of CO2, SO2, and NOx and promote wind power accommodation.It meets the development requirements of promoting the development of

Figure 12 .
Figure 12.Comparison of thermal power unit.

Figure 12 .
Figure 12.Comparison of thermal power unit.

Figures 14 and 15
Figures 14 and 15 show the comparison of the emissions of CO2, SO2, and NOx, and Figure 16 shows the comparison of wind power accommodation.It can be seen that the emissions of CO2, SO2, and NOx in dispatch model I are much lower than dispatch model II.The wind power accommodation in dispatch model I is significantly improved.Therefore, taking environmental costs into account can significantly reduce the emissions of CO2, SO2, and NOx and promote wind power accommodation.It meets the development requirements of promoting the development of new energy power generation and energy conservation and emission reduction.
Figures 14 and 15  show the comparison of the emissions of CO 2 , SO 2 , and NO x , and Figure16shows the comparison of wind power accommodation.It can be seen that the emissions of CO 2 , SO 2, and NO x in dispatch model I are much lower than dispatch model II.The wind power accommodation in dispatch model I is significantly improved.Therefore, taking environmental costs into account can significantly reduce the emissions of CO 2 , SO 2 , and NO x and promote wind power accommodation.It meets the development requirements of promoting the development of new energy power generation and energy conservation and emission reduction.

Figure 15 .
Figure 15.Comparison of the emissions of SO2 and NOx.

Figure 15 .
Figure 15.Comparison of the emissions of SO2 and NOx.Figure 15.Comparison of the emissions of SO 2 and NO x .

Figure 15 .
Figure 15.Comparison of the emissions of SO2 and NOx.Figure 15.Comparison of the emissions of SO 2 and NO x .

Figure 15 .
Figure 15.Comparison of the emissions of SO2 and NOx.

Figure 16 .
Figure 16.Wind power forecast and accommodation power.

Figure 16 .
Figure 16.Wind power forecast and accommodation power.
Energies 2019, 12, x FOR PEER REVIEW 3 of 19 power meet the user's comfort, the power of power generation units and the regulating variable of demand response are optimized by a genetic algorithm (GA) in MATLAB 2018a (MathWorks, Natick, MA, USA).Compared with the traditional dispatch model, not only the emissions of CO2, SO2, and NOx are reduced, but also the wind power accommodation is promoted, the test results demonstrate the reasonableness and validity of the proposed model.The contributions of this work are as follows: Load,t is the load demand at time t.ζ Lmin and ζ Lmax are the minimum and maximum values of load regulation rate, respectively.

Table 1 .
The parameters of dispatch models.

Table 2 .
Dispatching results of different cases.

Table 3 .
Optimized costs of dispatch model I and dispatch model II.
Lower/upper ramp rate limits of CHP i c vThe linear supply slopes of thermal power and electric power of CHP PTESThe value of dispatch load at time t ∆t Dispatch time interval P Load,t /P Heat,t Electrical load demand/thermal load demand at time t ζ Lmin /ζ Lmax Lower/upper values of load regulation rate χ Hmin /χ Hmax Lower/upper limited proportions of adjustment of thermal power demand F 1 Fuel cost of thermal power units C h,i Fuel cost of thermal power unit i a i /b i /c i The cost coefficients of thermal power unit i P i,t Power of thermal power unit i at time t P min,i /P max,i Lower/upper power generation limits of thermal power unit i r d,i /r u,i Lower/upper ramp rate limits of thermal power unit i F 2 Total cost of EES F EC,i /F EW,i Average daily investment cost/the operation and maintenance cost of EES i C EP /C EE Power of EES i at time t T E,i Service life of EES i C O Operation and maintenance cost coefficient of EES SOC min,i /SOC max,i Lower/upper SOC limits of EES i SP /C NP Environmental cost coefficient of CO 2 /SO 2 /NO x W C /W S /W N Emission of CO 2 /SO 2 /NO x δ C /δ S /δ N Emission of CO 2 /SO 2 /NO x by a unit mass of coal combustion β m Unit-price of coal F 4 Operation cost of desulfurization and denitrification equipment and the emission cost of SO 2 and NO x C S /C N Operation cost of desulfurization and denitrification device to remove unit mass of SO 2 /NO x