Configurations and exergy analysis of district heating substations based mainly on renewable energy

. The energy in the district heating system is transforming into a clean, low-carbon and integrated production and consumption mode to reduce greenhouse gas emissions and dependence on fossil energy. The district heating system has developed to the fifth generation currently, and the water temperature is continuously reduced, and thus the heating efficiency is improved. The proportion of renewable energy sources such as solar, wind, and geothermal energy in substations will continue to increase. In this paper, substation configurations of the fifth-generation district heating system are firstly reviewed, and the exergy analysis is applied to the thermodynamic simulation of the district substations. Thermal simulation models of several types of substations mainly based on solar water heating, water source-and air source-heat pumps or combined forms are developed, the conventional heat exchanger substation is also modeled to compare their performance. The optimization algorithm in Matlab/Simulink is used to solve the thermodynamic numerical model, and performance indicators such as exergy and efficiency are selected to evaluate the performance of these systems.


Introduction
With growing concerns about climate change, there is a strong global emphasis on reducing energy consumption and greenhouse gas emissions in buildings, and improving the energy efficiency of building heating systems contributes to global energy conservation and reduction of environmental pollution.Compared with other forms of heating, district heating can flexibly use various forms of energy, such as waste heat from industrial production, geothermal energy, solar energy, wind energy, and more and more surplus energy in the future.So in the future, if we want to achieve 100% renewable energy heating, district heating will play an important role [1].
In order to take advantage of district heating systems, we must efficiently design and operate district heating substations.District heating substations are designed and operated inefficiently, resulting in poor energy performance and the poor indoor environment.Also, the use of less or no low-grade renewable energy into existing district heating systems [2] is not conducive to global energy conservation and reduction of greenhouse gas emissions.
The current district heating network has developed into the fifth generation, and 5GDHC is defined as a DH network operating at near ground temperature, through seasonal storage, utilizing bidirectional heat and cold exchange between connected buildings.This network needs to be connected to the building's heat pump to achieve the proper domestic hot water temperature [3].
Not only can heat be absorbed from the network, but it can also be released to storage units in the network for storage.
In the literature, several researchers have done research and analysis on different aspects of district heating networks, in terms of piping in the network the reduction of the return water temperature of the primary network will improve the system efficiency [4].The concept of low-temperature heating network is used as the operation plan of the district heating network to meet the operation requirements of energy-saving buildings the variation in performance of the low temperature-based operating sub-network was evaluated [5].For mass flow [6], the entire network is analyzed and calculated using the matlab function, and the distribution network reduced models are verified with a detailed physical model, and the mass flow error is less than 5%.Using simulation tools developed in Matlab/Simulink to analyze the flow patterns of the network without changing the physical structure.In the use of renewable energy, the consumers connected to the district heating system are classified according to the degree of utilization of solar energy [7], and it is concluded that the cost can be greatly reduced by integrating the bidirectional thermal power station into the energy network simulation platform.To study the loss of the network and other aspects, the researchers used many simulation software and different simulation methods, The feasibility and operation characteristics of using energy storage as an auxiliary in the district heating network are analyzed, and a network simulation model is established in the Matlab/Simulink environment [8], by comparing with the traditional district heating network, the role of the heat storage device is revealed.Considering the characteristics of the pipe network [9], a numerical model of the heat flow and temperature change of the energy network system was developed using Simulink, and combined with its specific conditions, a small thermal network for studying heat loss was developed, and the temperature of each node was analyzed [10].The flow patterns of the network were analyzed using a simulation tool developed in Matlab/Simulink without changing the physical structure, and the pressure drop and temperature drop were reproduced and visualized through modular processing [11], For the district heating dynamic model, two simulation tools, EBSILON Professional and the Simscape physical modeling tool in Matlab/Simulink, were used to carry out technical feasibility studies, the study found that EBSILON only allows one-way flow, while Simscape can allow twoway flow, and it has better fitting and response to the model [12].A variety of modeling methods that can be applied to district heating networks are listed [13].The energy system component library developed in the Matlab/Simulink environment can assemble network models in different forms, taking into account the heat loss and pressure loss of the heat transfer fluid.

Systems Topologies
In order to analyze the performance of the thermal power station, the hydraulic schemes of different district heating thermal power stations are listed.Compare a heat plant dominated by renewable energy with a conventional heat plant.
Figure 1 shows a conventional district heating heat exchange station, Figure 2 shows a thermal station with solar energy as a supplementary energy, and Figure 3 shows the topology of a thermal station with renewable energy (low-grade energy such as solar energy and geothermal energy) as the main energy source.Figure 3 shows the use of renewable energy (solar energy, geothermal energy, etc.) as the energy source, and the secondary network is heated by the heat pump.When the heat of the renewable energy is sufficient, it can be stored, and when the heat is not enough, it can be used as a supplement.If the stored energy is still insufficient, the energy used in the central heating system continues to be replenished.

Model of Exergy Analysis
Exergy analysis combines the first law of thermodynamics and the concept of entropy to improve the energy conversion efficiency of the system, and the quality of different energy sources can be evaluated through exergy analysis to ensure that they can be used reasonably.In the LTDH system, exergy analysis can provide valuable insight in the system performance study [14].Matching between low-quality building heat demand and low-quality waste heat supply can be improved through exergy analysis, Defects in energy distribution and utilization can be identified through exergy analysis.Depending on the type, location, and magnitude of operating potential reduction, solutions can be given to reduce irreversible (exer) dissipation in individual components and achieve overall system performance optimization, Exergy analysis helps to assess location and potential to improve the quality match between energy demand and energy supply.This improvement can be achieved by utilizing commercially available heating technologies and identifying suitable renewable heat sources.
Any energy E consists of E x and A n : x n

E E A
(1) Compared with physical exergy, kinetic exergy and potential exergy are negligible.The physical composition of the flowing exergy is expressed as: ) When the flowing working medium undergoes a process (from 1 to 2), the change of its exergy is the difference between the transfer of net exergy in the process and the exergy loss caused by irreversibility, that is, the change of physical exergy is:

] E E E m h h T s s
(3) The exergy added to and consumed by the low temperature secondary grid flow comes from the primary side district heating supply flow and the electrical energy pumped on the secondary side of the thermal station.And for mass flow, only the energy/exergy used is considered an input, i.e. the physical exergy from the main DH supply and return minus the physical exergy of the water flow leaving the primary substation (see Figure 4).Therefore, for the DH thermal power station system, the functional exergy efficiency of its actual use can be expressed as [15]: Equation ( 4) has added the exergy loss in the heat exchange process into the expression (see Figure 5).In order to quantify the exergy loss generated in each process, the steady-state exergy balance of heat exchange is:  A rigorous exergy analysis in a DH network must take into account exergy losses at the pump.Electric pumps generate water flow by overcoming the pressure head of the distribution network.Exergy losses occur when pure exergy electricity is converted into mechanical energy at the pump and then dissipated into low-quality heat energy due to friction.The exergy loss of heat dissipation is ignored, so the exergy loss caused by the pump is:

Substation model
Selecting solar energy among renewable energy sources for simulation, the generated heat is transferred to water for household consumption.It uses blocks from the Simscape™ Foundation™, Simscape Electrical™, and Simscape Fluids™ libraries.To model the reflection, absorption and transmission of light in the glass cover, an optical model is embedded in a Matlab® Function block (see Figure 7).The sun rises at 6:00 and sets at 19:00.The irradiance follows a bell curve that peaks at 12:30.The incidence angle changes from π/3 to 0. The area of the solar panel is 0.03 m 2 , the initial surface temperature is 298K the same as the ambient temperature, the pipe length is 6m, and the cross-sectional area is 0.0009 m 2 .The energy needed for domestic water can be met by solar energy(see Figure 8).Through the establishment of the above model, we find that the total efficiency including thermal efficiency is more than twice that of solar power generation.It can be seen that solar heat is used for domestic water use, and the system efficiency will be improved.Simulink® Design Optimization™ or other optimization tools can be used to find optimal values of certain parameters suitable for control, thereby improving the overall efficiency.
Controllers can continue to be added to equipment such as pumps to drive the system to optimize performance.

Conclusion and Outlook
In order to simulate the substation mainly based on renewable energy, a model based on Matlab/Simulink is established.The model allows for reverse flow in the pipe network, and the flow direction can be switched in pipe blocks.
By establishing an exergy analysis model, it is concluded that by reducing the temperature of the supply and return water in the pipeline, the system efficiency can be improved, the exergy loss can be reduced, and the efficiency of the district heating network can be improved by adjusting the operation.Adding heat exchangers to renewable energy-based substations increases additional cost and operational complexity.In order to further improve the energy efficiency of district heating networks, it is necessary to replace fossil fuels with low-grade energy sources, and through model calculations, we know that the use of solar heat to generate heat will significantly improve energy efficiency.
In the following research, the simulation model of the second part of the topology will be realized, and the operation strategy will be improved.

4 Description of the Simulation Model 4 . 1
exergy, kJ/kg exergetic flow rate, kW exergetic efficiency specific enthalpy, kJ/kg mass flow rate, kg/s specific entropy, kJ/kg•K absolute temperature, K Pipe model To achieve reverse flow, the piping model consists of a supply and return pipe, combined in one block, Pipe block with Supply and Return pipe and Switches for change of the flow direction.Figure 6 shows the structure of the pipe block.

Fig. 8 .
Fig. 8.The internal flow of the pipeline corresponds to the demand.