Research on Park Energy System Based on Grid Connection of Renewable Energy Power Generation

With the maximum consumption of renewable energy, the minimum emission of pollutants and the minimum comprehensive investment cost as the optimization objectives, the operation model of the park’s energy system considering the thermal power balance constraint, natural gas balance constraint and power grid power constraint was proposed, and the scenic power generation and electricity equipment of the park were studied. By comparing different operation scenarios, the influence of adding different equipment on the operation optimization of the whole park under the thermoelectric coupling is comprehensively analyzed. The balance curve of equipment output and related energy use is analyzed on a yearly cycle, indicating that the power supply grid of the park based on the thermoelectric coupling has great reliability and effectiveness in the consumption of renewable energy, and its economy is also significantly improved.


Introduction
The trend to incorporate more renewable energy into the energy structure is due to the rise in global population and economic development [1][2].Hydropower, solar energy, and wind power account for the majority of renewable energy usage, reducing the centralized power supply grid's regional load [3].Thermoelectric coupling and other multi-energy complementary technologies have advanced significantly, but the volatility, unpredictability, and intermittency of renewable energy generation present a challenge to the operation and planning of the power supply and distribution network.It is important to plan the capacity configuration of devices connected to energy storage in accordance with power generation and load side demand.In order to assess the proper installation capacity, literature [4][5] creates a distributed power supply model from the perspectives of power supply dependability, economy, and environmental protection.Analysis of the energy system model's economy and grid stability is covered in depth in the literature [6][7][8].Using thermal inertia to translate or convert peak load to lower operation costs, literature [9] proposes a day-ahead operation optimization method for residential buildings' integrated energy systems.Literature [10][11] took the cooling and heat load demand of energy network into consideration, and optimized the capacity of each equipment in the energy network This study builds a planning model of a park thermal power grid that comprises a fan, photovoltaic system, gas storage tank, battery, micro gas turbine, and electric boiler.The relevant components are mathematically modeled, operational restrictions are examined, features of thermal and cold power loads and the energy conversion process are investigated, and a dispatching model with the lowest total cost and highest rate of absorption of renewable energy is developed.The comprehensive operation scheduling model is shown to be accurate, efficient, and reasonable.

Electrothermically coupled integrated energy system model
Figure 1 depicts the overall block diagram of the comprehensive energy system of the park's optimization operation.Electric boilers, P2G, and other electric load equipment are powered by wind, photovoltaic, and micro gas turbine units to convert electric energy into natural gas.Micro gas turbine and air conditioning refrigerating machine output cold power to supply cooling load, while heat load is mainly provided by electric boiler and cogeneration unit.

Annual comprehensive cost of comprehensive energy system
The total investment cost of an integrated energy system is determined by recurring expenses such as initial investment, equipment connected to its operation and maintenance, and annual energy supply: is the total investment cost of comprehensive system in the park;  is the average annual cost of the total initial investment cost of the system. is the operation and maintenance cost of related equipment;  is the cost of purchasing power from the large power grid. is the system comprehensive gas purchase cost;  is demand side response compensation;  is the penalty cost of carbon emission.  is the initial investment cost of the ith device; γ is the discount rate, which is 5%;  is the expected service life, take 15 years.

Constraints (1) Operation constraints of CHP units
The cogeneration operation considered in this paper adopts a fixed thermoelectric ratio under the comprehensive park, and its relevant operation constraints are shown below.
, ℎ .represent the upper and lower limits of electromechanical output and the upper and lower limits of thermal output, respectively.
(2) P2G equipment constraints P2G equipment can convert excess electric energy into natural gas for heat generation or storage:  .  . .
and  .represent the lower limit and upper limit of the relevant gas generation power of P2G in the time period t.
(3) Energy balance constraint In the system, constraints on thermoelectric balance, natural gas balance and demand response of the park should be comprehensively considered.
i) Power balance P P P P P P represents the electrical load after flexible regulation response in the integrated park; P is the output of cogeneration unit in time t; P is the output of wind power in time t, P is the output of photovoltaic in time t, ii) Thermodynamic equilibrium h h h h represents the heat load at time t.

Model analysis and example solving
Mixed integer linear programming model must be solved using CPLEX solver and MATLAB toolbox.The example in this paper is based on the electricity and heat load data of a school park in a northeast city and the wind power and photovoltaic output data of literature to optimize and improve.Figure 2 shows the schematic diagram of scenic output and related electrical and thermal loads of a typical day park.

Figure. 2. Curves of wind-power output and electrical and thermal load
CHP units can couple electric heating energy and generate electricity using wind and photovoltaic energy.Plan 1 includes conventional elements, Plan 2 adds a 1MW electric boiler, Plan 3 adds a 1MWh gas storage tank and P2G equipment for supply-demand side response regulation.Based on the real local power purchase price, as stated in Plan 1 involves a significant amount of wind discard volume due to CHP units being used for heating and nighttime heat load demand.Plan 2 involves raising the thermal energy produced by electric boiler equipment to supply electric demand, while Plan 3 involves building additional gas storage equipment to absorb more renewable energy and prevent wastage of power generated during off-peak time.Gas is produced using photoelectric energy produced by enough daylight and electric energy produced by extra wind energy at night, and the excess is transferred to the gas storage tank for storage.The electrical load following the demand response can also be taken into account, and applicable guarantee agreements can be executed.The peak-valley difference in the response side can be reduced, the power supply curve is smooth, and the stability of the system improved by reducing the use of reduceable load during peak or high electricity prices and shifting the transferable load to low electricity demand at night.The addition and use of electric boilers, gas storage tanks, and P2G equipment has a positive effect on the consumption of renewable energy and the reduction of comprehensive investment cost.The output of thermoelectric units for plan 1, plan 2 and plan 3 is shown in the figure 3. The heat load required by plan 1 is all generated by cogeneration units, which cannot absorb renewable energy, resulting in serious wind and light abandonment.Plan 2 adds electric boiler equipment, which can absorb the excess light and wind energy and convert it into heat supply load.Plan 3 can convert more renewable energy into electric energy and make it into gas for storage, which can significantly reduce the total energy purchase cost.

Figure 3. Output analysis of CHP unit
The thermal power produced by the electric boilers in plan 2 and plan 3 is shown in figure 4. The wind speed is usually high from 00:00 to 06:00 in the morning and from 18:00 to 24:00 in the night, so there will be excess wind power generated.The electric boilers can convert electric energy into heat energy for storage, thus reducing the extra output of the CHP units.During 10:00-15:00 in the daytime, when the light is abundant, the photovoltaic panel has a higher electric conversion efficiency and a lower electricity consumption.Thermal energy conversion can also be carried out through the electric boiler equipment.However, this puts forward a large requirement for the capacity configuration of the heat storage tank, and will lead to a large heat loss.The excess electric energy is converted into gas energy for storage.When the price of electricity is high, the gas in the gas storage tank is released to generate electricity and heat.The total cost of the system under the three schemes is shown in Table 2.The calculation period is year by year, and the initial investment cost is evenly distributed to each year.It can be seen that under the three schemes, although the annual investment cost slightly increases due to the addition of equipment, the operation cost is significantly reduced, and the annual comprehensive cost is also successively reduced.In addition, the amount of wind and light discarded in plans 2 and 3 has been significantly improved, and the consumption of renewable energy has been effectively increased.4. Summarize 1.The installation of electric boilers, P2G, and energy storage devices significantly boosts the park system's use of renewable energy, increases the grid-connected power supply's dependability, lowers overall costs, and improves the efficiency of the energy systems.
2. This study only takes into account two renewable energy sources-wind and light-and does not extensively examine the detrimental effects of their instability on the overall energy system, which should be taken into account in future studies of the entire park.Also, the development of the carbon trading model is not enough advanced, and the processing of the boundary conditions of carbon trading is not sufficiently excellent, which still requires in-depth analysis and processing in future study.

Figure 4 .
Figure 4. Operation analysis of electric boiler

Table 1 ,
the time-of-use electricity pricing used in this work: Table1TOU electricity price

Table 2
Cost and amount of wind and light discarded