Energy Flow Function and Operational Strategy of CCHP System

The energy flow characteristics of Combined Cooling, Heating and Power (CCHP) system are analyzed in detail and the energy flow function is proposed for its two operational strategies: thermal load dominant mode and electrical load dominant mode. In order to improve its economic and environmental efficiency, an optimal economic and environmental dispatch model is built, which includes the fuel cost, power purchase cost and pollution emission penalty as environmental cost in its objective function and takes the energy flow function as its energy balance equality constraint. Case simulation shows that, the proposed energy flow function and model optimizes the dispatch of CCHP system, reduces the productive cost, improves the environmental efficiency and enhances the energy utilization level.


INTRODUCTION
CCHP system can provide cold energy, thermal energy, electrical energy and other forms of energy simultaneously to users.Owing to the advantages of energy cascade utilization and less polluting emissions, CCHP gets wide attention across the world [1]- [3].Different operational strategies determine the economical efficiency and environmental protection property of a CCHP system: thermal load dominant mode and electrical load dominant mode [4].The former mode preferentially meets heat load demand and the electricity shortage or surplus can be balanced by the connected main grid.On the contrary, the latter mode preferentially meets electricity load demand and the heat shortage can be provided by auxiliary boilers.As for the islanded MG where there is no dependence on the main grid, CCHPs should be in electricity-load-based mode because ac-electrical systems require a precise electricity balance at all times [5]- [8].
In order to give full play to the advantages of CCHP system energy-step-utilization and use renewable energy efficiently, PV panels is integrated into the traditional CCHP system driven by gas engines.By coordinated scheduling and optimization between solar system and CCHP system, the energy and environmental benefits could be improved greatly [9].On the basis of CCHP system, biomass power generation, fuel cell and various energy storage device, e.g.battery, storage tank and ice storage device are taken into consideration, which increases the operational cost of the system to some extent, whereas the energy supply reliability and renewable energy usage improve a lot [10].For different operating strategies of the system, with the annual load of cooling, heating and power as equality constraints, different objective functions [11]- [13], e.g., utilization rate of primary energy, operational cost and carbon dioxide emissions, are determined.These documents take the cumulative heat and electricity balance in a certain period into consideration, which cannot reflect the real-time balance of electricity accurately.In addition, hardly can previous studies elaborate the real-time energy flow function of the system operated under the above-mentioned two modes precisely.
Consequently, in order to tackle the issue, this paper take wind power and other renewable energy into account [14] and our research is focused on establishing the corresponding real-time energy flow function for the above two operational strategies and developing economic and environmentally friendly scheduling optimization model based on fuel cost, power purchase cost and pollution emission penalty as environmental cost.

CCHP SYSTEM AND ITS ENERGY FLOW
A typical CCHP system is made up of a cluster of components, e.g., gas turbine, auxiliary boilers, heat recovery steam generator(HRSG), wind Turbine and absorption chillers.As illustrated in Fig. (1), , and respectively mean the natural gas consumption for the whole CCHP system, gas turbine and auxiliary boilers.For the purpose of reflecting the characteristics of the energy flow accurately, this paper use energy flow function to describe the balance state of cooling, heating and power.

P pgu kW
H pug kW

P w kW
---the electric power provides for the system, ; ---the heating generated by auxiliary boilers, ; ---the drive power for absorption chillers, ; ---the generated cooling power.
Gas turbine, wind turbines and main grid operate coordinately so as to meet the electricity demand .Gas turbine can simultaneously provide heating and power output, which has a certain feasible region, as shown in Fig. (2).The feasible region of the output of gas turbine is considered as follows: (1) where, ---the amount of linear constraints of the feasible region; ---corresponding parameters of various linear constraints.
---function formalism of the feasible region.
Independent variables of the function change according to the operational strategies.

Energy Flow Function of Thermal Load Dominant Mode
CCHP system operates under thermal load dominant mode preferentially satisfy the balance of thermal load.To satisfy the constraints of heating and power output and realtime balance of electric power, there should be real-time electric power interaction between the CCHP system and main grid.When the difference between electric load and total electric power generated by each generator set is greater than 0, electric power flows from main grid to the system.Otherwise, electric power flows from the system to main grid.The strategy includes 2 operational situation: 1.
When the sum of thermal load and the power of absorption chillers is less than the maximum output of HRSG( ), thermal load of the system is provided by HRSG, and auxiliary boilers are out of operation. ( The output of each generator changes according to various circumstances: 1) If , so that and ; 2) If , so that and ; 3) If , so that , and the electric load shortage is supplemented by the main grid: (3)

2.
When the sum of thermal load and the power of absorption chillers is no less than the maximum output of HRSG( ), gas turbine operates at maximum thermal output and the thermal load shortage is supplemented by auxiliary boilers.This paper assumes that auxiliary boilers can meet the maximum thermal load demand. ( The output of each generator changes according to various circumstances: 1) If , so that and ; 2) If , so that and ; 3) If , so that , and the electric load shortage is supplemented by the main grid:

Objective Function
Objective function is composed of fuel cost( ), power purchase cost( ) and pollution emission penalty as environmental cost( ).
(6) (7) where is the total fuel cost of a certain gas turbine; and are the corresponding thermal power and electric power, respectively, , is the total fuel cost of a certain auxiliary boiler; is the corresponding thermal power, , T is the total dispatching times in a dispatching period.

Fuel cost
Fuel cost of gas turbine is as followings: (8) where, ---the number of gas turbine; ---fuel cost factors of gas turbine.
Fuel cost of auxiliary boiler is as followings: (9) where, ---the number of boilers; ---fuel cost factors of boilers.

Power Purchase Cost
The cogeneration system purchases power from main grid to supplement electricity shortage and sells power to main grid to improve energy efficiency.Taking peak-valley TOU pricing model into account, power purchase cost holds: (10) where and are the unit price of selling power and purchasing power respectively, means purchasing power ( ) or selling power ( ) to main grid.

Environmental Cost
Environmental cost is mainly the expense of processing emissions: (11) where, , are emission factors of polluting gases, ; is penalty factor of polluting gases, ; is the number of wind turbines.

Constraints of the thermal load dominant mode 1) Equality constraints
Equality constraints of the model include energy flow functions (2)-( 5) under the thermal load dominant mode and power balance constraints: (12) (13) (12) and ( 13) are electric power balance and thermal power balance, respectively.Energy loss of the system is neglected.

Energy Flow Function of Electrical Load Dominant Mode
CCHP system operates under electrical load dominant mode preferentially satisfy the real-time balance of electrical load.However, the inherent feasible region of output of gas turbine could lead to the situation that the thermal power output is greater than the thermal load demand.In this paper, thermal storage equipment is out of consideration.It is assumed that the excessive thermal energy is discharged directly into the atmosphere and the maximum thermal load demand of the system can be satisfied by gas fired boiler.
The strategy includes 3 operational situation: 1.
When the electrical load demand is less than the maximum output of gas turbine( ), electrical load of the system is provided by the output of gas turbine ( ).The output of wind turbine is 0: The output of each generator changes according to various circumstances: 2. If , the thermal load is satisfied by gas turbine: (18) 3. If , the thermal load is mainly satisfied by gas turbine( ), and thermal storage is supplemented by auxiliary boiler: When the electrical load demand is no less than the maximum output of gas turbine and is smaller than the sum of wind power and maximum output of gas turbine ( ), gas turbine operates at maximum electrical output and the electrical load shortage is supplemented by wind turbine: (20) The output of each generator changes according to various circumstances: When the electrical load demand is no less than the sum of wind power and maximum output of gas turbine ( ), gas turbine operates at maximum electrical output and the wind power is totally sent into the system: The electrical shortage is supplemented by main grid: (24) The output of each generator changes according to various circumstances: 1.
If , the thermal load demand is completely satisfied by gas turbine: (25) 2. If , so that the thermal load is mainly satisfied by gas turbine( ), and thermal storage is supplemented by auxiliary boiler: (26)

Objective function
Objective function under electrical load dominant mode is the as (6).

Inequality constraints
Inequality constraints include output constraints of auxiliary boilers (14) and gas turbine under electrical load dominant mode: (27) (28)

Case Study and Parameters
Taking typical days of a certain district as an example, the cool, heat and power load curve are presented in Fig.The time-of-use electricity price is as followings: from 6 a.m. to 9 p.m., the price of selling and purchasing power are 0.13, 0.10 , respectively.From 9 p.m. to 6 a.m. next morning, the price of selling and purchasing power are

Simulation Result
The total cost of a CCHP system have difference under the above 2 operational strategies, as illustrated in Fig. (4).The environmental cost curve is presented in Fig. (5).Table 1 lists different costs during a scheduling period.

Result Analysis
As illustrated in Fig. (4), operational strategy have a great impact on the economy of the system.From 7 a.m. to 10 p.m., total cost of the system operated under electrical load dominant mode is evidently higher than that of another mode.However, from 11 p.m. to 6 a.m. next morning, the total cost of the system operated under these 2 strategies are quite basic.It is because that both of these 2 strategies can provide enough cool, heat and power load to the system, and there is no thermal output or electrical output surplus during the process.
As can be seen from Fig. (5), from 9 a.m. to 9 p.m., environment cost of the system operated under thermal load dominant mode is greater.The reason is that thermal load demand and power purchase cost increase a lot during the time, which could increase environment cost of the system.Make a detailed analysis of Table 1, it can be concluded that total cost of the system operated under thermal load dominant mode is lower than that of another mode, which means the economy of the system is improved.However, since that the system operated under electrical load dominant mode preferentially meets electrical load demand and the power purchase from the connected main grid is limited, power purchase cost of this mode is lower.That means system operated under this mode has less dependence on the main grid, and consequently, the system has less impact on the main grid.Apparently, for the reason that the thermal load dominant mode relies heavily on the main grid, power purchase cost increases a lot.Moreover, the penalty factor of generating electricity far outweighs the penalty factor of burning natural gas .
Therefore, environment cost of thermal load dominant mode is greater under the circumstances.

CONCLUSION
Analyzing energy flow of cooling, heating and power in detail and taking the real-time balance of energy flow into account, the paper proposes an useful conception-energy flow function, which could reflect the real-time balance of electricity accurately.Furthermore, energy flow functions of the system operated under electrical load dominant mode and thermal load dominant mode are established, which could be the research foundation of optimization scheduling of CCHP systems.
Taking the energy flow function as its energy balance equality constraint, an optimal economic and environmental dispatch model is built, which includes the fuel cost, power purchase cost and pollution emission penalty as environmental cost in its objective function.From the results, it can be seen that the proposed energy flow function and model enhances the energy utilization level, reduces the productive cost and improves the environmental efficiency.
Simulation results show that the system operated under thermal load dominant mode has prominent economy, whereas the power purchase cost is higher and the impact on main grid is greater.In comparison, the system operated under electrical load dominant mode has significant environmental benefits.The CCHP system adjusts its optimal operation scheme according to its operational strategy.

CONFLICT OF INTEREST
The authors confirm that this article content has no conflict of interest.