Study on optimal energy dispatch of electricity-heat-cool-gas agricultural industrial Park with renewable energy

Since the release of China’s rural revitalization strategy, rural areas have experienced rapid development, but at the same time, the imbalance between rural energy demand and infrastructure has become more prominent. In this regard, this paper considers the use of abundant renewable energy in rural areas to participate in energy generation and build an integrated energy system for agricultural parks containing electricity, heat, cold and gas loads. Based on this idea, it is necessary to establish mathematical models for various equipment in agricultural parks. Then, the maximum output and daily load curves of the agricultural park were predicted, and the TOU price was added at the same time. Finally, the objective function is to minimize the cost of energy use, and the integrated energy system of the electricity-heat-cooling-gas agricultural park using renewable energy was optimized a few days ago. In addition, through simulation examples, it has been proven that this method can increase the utilization of renewable energy, improve energy utilization efficiency, and have dual economic and environmental benefits.


Introduction
With the completion of a comprehensive well-off society in China, the national economy has achieved rapid development, and a new goal of rural revitalization has been established [1].However, the energy infrastructure in rural areas is not yet sufficient to meet the energy needs of the population.The utilization rate of renewable energy is still at a low level, and the high carbon emissions are prominent problems [2][3].In 2020, our country came up with the "double carbon" goals [4].At the same time, China's rural areas cover a large area, which includes a variety of rural life and agricultural production energy, as well as abundant renewable energy capacity.The reasonable benefit of this part of rural energy, namely, the establishment of an integrated agricultural energy system, will have great potential for achieving carbon neutrality [5].Therefore, rural energy adjustment and transformation are imperative.
Renewable resources are abundant in vast rural areas, as well as straw and biogas resources produced by agricultural cultivation [6][7][8].Feng et al. [8] take the integrated electric and thermal energy system in rural areas as the research object and construct an energy system model containing biomass biogas for power generation.Li [9] elaborated on the reasonable utilization of biomass energy in different situations, and summarized its application advantages and development prospects.For the feasibility of constructing distributed biomass combustion furnaces in rural areas, Wang and Du [10] conducted a concrete analysis from the perspectives of cost, economy, and environment.Zhou et al. [11] proposed to establish a solar biomass combined complementary heating system in rural areas of northern China, which mainly generates heat from solar energy and is supplemented by biomass when solar energy is insufficient.It is obvious that rural areas have inherent conditions for developing electric heating, cooling, and gas multi-energy systems in agricultural parks containing renewable resources [12].
Considering the load-side energy consumption in agricultural parks, we had many known types of research on dual energy loads and triple energy loads, such as electric-thermal load [13], electric-gas load [14], electric-heat-gas load [15], electric-heat-cooling load [16].However, there are relatively few research results by researchers on the multi-energy of rural agricultural parks considering the four loads of electricity, heat, cooling, and gas.With the rapid development of rural areas, rural energy use is no longer dual energy loads or triple energy loads, comprehensive consideration of four kinds of loads can better meet the diverse needs of rural areas.
Therefore, the development of electricity, heat, cold and gas agricultural parks suitable for rural areas based on local conditions combined with local renewable resources will play a positive role in the goal of rural revitalization.The system can increase the energy utilization efficiency of agricultural parks through energy conversion equipment and increase the absorption of local renewable energy, both of which can reduce energy costs.On the other hand, the system also has a certain positive impact on stabilizing grid balance.The energy supply side is composed of the power grid and gas network, as well as local renewable resources.The system includes energy conversion and storage equipment to jointly meet the electricity, heat, and cooling load demands of the agricultural park user side.The energy flow diagram of the specific system is shown in Figure 1.

Structure of integrated energy system in an agricultural park
The optimization method and steps for the energy system of agricultural parks in this article are shown in Figure 2. First, it is necessary to establish mathematical models of all kinds of equipment for the park's integrated energy system.Then, according to the Monte Carlo method, the daily load curve of different energy types and the maximum wind power output are predicted.It also connects the external gas grid to participate in dispatching and adopts the TOU price for the power grid.Finally, under the conditions of meeting various equilibrium and imbalanced constraints, optimize the comprehensive energy system of agricultural parks with the lowest energy cost.By using the Cplex solver to calculate the lowest energy consumption cost and the operating conditions of various devices, The costs of various scenarios were compared and analyzed, and the output cycles and reasons for each device were analyzed.

Capacity model
The energy-generating equipment includes the wind turbine generator, solar energy generator, and biogas boiler.And the energy generation also includes the way of directly purchasing electricity and selling electricity with an external power grid, and purchasing natural gas from an external gas grid to meet the energy demand of the park.

Wind turbine generator model
Wind power is the main factor affecting its power generation.The power generation can be divided into three sections according to the wind speed.The mathematical model of its power output is: (1)

Solar energy generator model
The mathematical model of its photovoltaic power output is: 20 1 800 (2)

Biogas boiler model
As a rural area rich in biomass energy, the full utilization of biogas resources in the park can meet part of the heat demand.Moreover, the total amount of biogas consumption is only limited by the daily biogas production and does not participate in the balance of gas efficiency.The biogas pool model formula to be established is:

Conversion model
In addition to production capacity equipment and direct energy purchase, energy conversion equipment plays an indispensable role in regulating energy flow and energy complementarity.

Gas turbine model
The gas turbines can achieve the conversion of gas energy to electrical energy, while also obtaining a small amount of thermal energy.It is often used as the basic equipment in the system and has a complete mathematical model: (1 )

Heat recovery boiler model
Usually used as the next stage equipment connected to a gas turbine to absorb the remaining heat generated by the gas turbine and generate thermal energy, its mathematical model is as follows:

Air source heat pump model
As a power consumption and heat generation equipment, it is more environmentally friendly than burning straw to meet the rural heat load.Its mathematical model is as follows:

Regenerative heat boiler model
The equipment that consumes electric energy to generate heat energy, like the heat pump, accounts for a certain proportion of the heat supply.Its mathematical model is as follows:

P2G equipment model
The mathematical model is:

Refrigeration equipment model
Only by meeting the cooling load demand in rural areas can we further improve the satisfaction of rural residents, so they also need refrigeration equipment.The mathematical model is:

Energy storage model
The increase in energy storage equipment can alleviate some problems, which are energy surplus and energy shortage at different periods.The energy storage model of the integrated energy system includes the storage battery model, heat storage unit model, and Gasholder model.Because of their similarity, only Storage battery models were listed.
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constraint condition
The power balance constraints are: .

Example simulation and result analysis
Taking the facility agricultural park in northwest China as an example, the park mainly includes a modern planting area, breeding area, scientific research service area, daily living area, processing trade area, etc.The agricultural park of the facility is powered by distributed solar, wind turbine, biogas, local power grid and gas network, among which the raw materials for biogas fermentation generated in the process of livestock breeding, agricultural production and daily life of employees in the park can be collected and treated in the park, and biogas can be produced continuously through fermentation in the biogas fermentation tank.Energy conversion equipment in the park include a gas turbine, heat recovery boiler, biogas boiler, regenerative heat boiler, electric refrigeration equipment, heat pump, and P2G equipment.The storage devices include a storage battery, heat storage unit and gasholder.A typical summer day in the agricultural park is selected as the simulation object to optimize the energy integration of electricity, heat, cold and gas in one day, and the minimum unit of time is selected as 1 hour.Firstly, the Monte Carlo simulation method was used to predict the maximum electricity production of wind and solar energy, as well as the energy usage of electricity, thermal energy, cold energy, and natural gas in a typical summer in an agricultural park.A time-of-use tariff was adopted, with electricity purchase and sale prices shown in Table 1 and the gas purchase price of 1.2 yuan/m 3 .
Using the Cplex solver to solve the optimal scheduling model of the comprehensive energy system in agricultural parks in typical summers, the minimum energy cost in typical summers is 6907.0yuan.At the same time, the output of each piece of equipment is obtained and displayed by the electric, heating, cooling and gas power balance diagram, as shown in Figures 3-6.
As shown in Figure 3, the electric power balance chart.First, wind and solar energy, as clean energy, have their own advantages as low energy cost, low environmental pollution, and renewable energy, Therefore, without considering weather effects and demand on the load side, production capacity should be at its maximum value.Secondly, affected by the regulation of electricity prices by the local power grid, purchasing electricity from the grid is mainly during periods of low electricity prices.For the power shortage of part of the power load during the high electricity period, gas turbines are used to generate electricity by burning gas, and at the same time, the remaining electricity in the agricultural park can be transmitted back to the power grid for sale, in order to further reduce energy consumption costs.Then, heat pumps participate in heat generation during the first 6 hours of the day and P2G equipment that consumes electricity and generates gas, the main working areas also operate during the low electricity price period when electricity consumption is low.Finally, the battery is charged in the low-price and flat-price periods, and discharged in the high-price periods, so as to further reduce the cost of electricity.Figure 4 shows the results of the heat power balance.In the heat load balance, the heat generation consists of a regenerative heat boiler, heat pump, heat recovery boiler and biogas boiler.The waste heat boiler receives the waste heat from the gas turbine for power generation, so it works synchronously with the gas turbine and is the main heating equipment during high electricity prices.Secondly, biogas boiler consumes biomass energy.As a cleaner production equipment, it has low cost and low pollution, so it is the second heat generation equipment besides the heat recovery boiler and complementary heat generation with the heat recovery boiler.However, the amount of biogas consumed by biogas boilers is limited by the daily output of the park, and air source heat pumps need to be added to make up for the lack of heat energy demand.At the same time, the regenerative heat boiler is also added, which accounts for a small proportion of heat generation in each period.Finally, thermal storage equipment stores thermal energy during periods of excess or low-cost thermal energy, and releases thermal energy during the remaining time to further reduce thermal costs and balance heat production and consumption.Finally, as shown in Figure 6, for the gas load balance results, the sources of natural gas consumption include purchasing natural gas and producing natural gas from P2G equipment.Specifically, P2G equipment consumes electricity and produces gas during low electricity price periods.When the electricity price is high, it directly purchases gas to reduce the expenditure on gas consumption.Secondly, gas turbines in the park mainly consume natural gas to generate electric energy and heat energy during periods of high electricity prices.Finally, the gasholder stores natural gas when there is excess gas energy, and deflates gas to supplement gas demand in some periods to reduce gas purchase and balance gas production and consumption.

Conclusion
From the energy cost results of the selected simulation example and the analysis of the results of different equipment output periods, the rationality and complementarity of each equipment output period and the energy flexibility of the system are reflected.Compared with the single purchase of electricity from the grid, the system also uses local renewable energy, which can effectively reduce the energy cost of agricultural parks.It improves the consumption and utilization rate of renewable energy, while reducing the waste of agricultural production and living resources (such as biogas), and has significance for peak shaving and valley filling in rural power systems.
Therefore, the optimization and scheduling method for the comprehensive energy system of agricultural parks proposed in this article has the following characteristics:  This method has relatively comprehensive multi-energy complementarities, covering all types of energy used in the four types of parks: electricity, heat, cold and gas, and the same energy is supplied by multiple capacity equipment complementarities.At the same time, there are energy conversion equipment between different energy sources, and energy storage equipment of different energy sources, all of which play a role in stabilizing the system. This method uses renewable energy sources simultaneously, requiring only investment in equipment and a small amount of maintenance equipment to participate in energy supply.In agricultural industrial parks in rural areas, these renewable energies are abundant but have low utilization rates.Therefore, utilizing renewable wind and solar resources, reducing the purchase of electricity from the agricultural park grid, and increasing the consumption of wind and solar energy will play a positive role in reducing the energy costs of the park. Biogas boilers use biomass energy to produce heat, reducing electricity consumption and heat production, which is conducive to alleviating the problem of the large total amount of waste biomass energy resources in rural areas but generally low utilization rate and the serious environmental pollution and safety issues caused by direct incineration of residual biomass energy.

Figure 1 .
Figure 1.Energy flow diagram of the system in agricultural park

Figure 2 .
Figure 2. Steps for optimizing the system of agricultural parks

Figure 3 .
Figure 3. Electric power balance diagram Figure 4. Thermal power balance diagram

Figure 5 .
Figure 5. Cooling power balance diagram Figure 6.Gas power balance diagram Figure 5 is the cooling power balance diagram.In this case, only one kind of equipment is involved, which consumes electric energy to generate cold energy to meet the cooling demand of the load.Finally, as shown in Figure6, for the gas load balance results, the sources of natural gas consumption include purchasing natural gas and producing natural gas from P2G equipment.Specifically, P2G equipment consumes electricity and produces gas during low electricity price periods.When the electricity price is high, it directly purchases gas to reduce the expenditure on gas consumption.Secondly, gas turbines in the park mainly consume natural gas to generate electric energy and heat energy during periods of high electricity prices.Finally, the gasholder stores natural gas when there is excess gas energy, and deflates gas to supplement gas demand in some periods to reduce gas purchase and balance gas production and consumption.