Primary Heating Network Research Articles

Economic optimization and thermodynamic analysis of a novel nuclear district cooling system

“Yanlong” pool-type low-temperature heating reactor (DHR-400) stands at a highly promising nuclear heating technology. However, its annual utilization duration faces constraints when exclusively employed for winter heating. To cope with this challenge, a novel nuclear district cooling system driven by DHR-400 is proposed for the first time in this paper. As the core component of the proposed nuclear district cooling system, single-effect double-lift LiBr-H2O absorption refrigeration system (SDARS) can significantly increase the temperature difference between the supply and return of the primary heating network (PHN), thereby reducing the transmission energy consumption. Despite the significant potential, the optimization design of SDARS has received limited attention. An optimization model for SDARS with the annual cost (AC) as the objective function is developed. Focusing on a real district cooling scenario in Xi'an High-tech Zone, the heat transfer areas of all process units and the operating parameters of all streams in SDARS are optimized. Results demonstrate that the optimal annual average coefficient of performance (AACOP) is improved by 10.63 % and AC is reduced by 5.44 % compared with the Benchmark case. The dynamic payback period for the combined heating and cooling system driven by DHR-400 is reduced by 5.53 years compared to the heating-only system.

Control strategies and performance analysis for a novel nuclear heating system with heat storage

A nuclear heating system integrated with a heat storage tank (HST-NHS) is a promising technical solution for countries with large-scale heating demand. HST-NHS can greatly curtail carbon dioxide emissions and contribute to sustainable development. Appropriate control strategies are not only crucial to realize the real-time matching between HST-NHS heat supply and heat demand, but also ensure the stable and safe operation of HST-NHS. Until now, control strategies for HST-NHS have received limited attention. A refined dynamic simulation model for HST-NHS is established based on APROS software. The verification of the built model in steady state and transient state is completed. Two basic control strategies for the primary heating network (PHN) of HST-NHS, constant supply water temperature and variable flow rate (CT-VF) and variable supply water temperature and constant flow rate (VT-CF) are presented in detail. Based on various typical working conditions of a central heating region in Liaoyuan City, the control effects of two control strategies are investigated. Additionally, a comprehensive performance comparison between the two strategies is explored. Simulation results indicated that under the two control strategies, the indoor temperature can be effectively controlled at a high comfort level. During the same typical day, the total energy consumption cost under the VT-CF control strategy is 0.5 × 105 yuan lower than that under the CT-VF control strategy.

Techno-economic and environmental performance of a novel thermal station characterized by electric power generation recovery as by-product

As the quality of life continues to improve, the requirement for district heating is gradually growing, contributing to problems of climate change and environmental degradation. The electricity consumption and heat losses in thermal stations are significant. To address these issues, a novel self-sufficient thermal station in electric power is proposed in this paper. The techno-economic and environmental performances of the novel electricity self-sufficient thermal station under operating conditions is analyzed employing a mathematical model. The results show that the higher the temperature of the primary heating network supply water, the better the overall performance of the system. Additionally, a lower point pinch temperature difference is beneficial for improving the performance of the system. At the primary heating network supply water temperature of 100 °C and the evaporating temperature of 69 °C, the PBP obtains the minimal value of 6.53 years. The annual saving coal can reach 73.21 t and reducing CO2 emissions can reach 199.55 t. Therefore, the novel electricity self-sufficient thermal station can meet certain technical requirements for combined heating and power generation while demonstrating superior economic capability.

Combined heating and power generation performance of thermal station for electricity self-sufficient characterized by two-stage heat transfer

As the living standard improves, the demand for space heating has been increasing, leading to global warming and environmental problems. The thermal station consumes a large amount of electric power. A novel electricity self-sufficient thermal station based on the organic Rankine cycle (ORC) is proposed using the primary heating network supply water as the heat source and the secondary heating network return water as the heat sink. The mathematical model is established based on the laws of thermodynamics, and the performance of combine heating and power is numerically analyzed under the design condition. The results show that the power generation performance is directly proportional to the primary heating network supply water temperature. However, the power output of ORC is at the expense of reducing the same amount of heating capacity of secondary heating network. The novel self-sufficient thermal station in electric power lowers the irreversible loss of heat transfer between the primary and secondary heating networks to construct cascade heat transfer, therefore recovering the potential power generation being ignored and realizing the self-sufficient supply of electric power.

Thermodynamic performance evaluation of a novel thermal station for self-augmented heating capacity during operating variations

The increasing heating demand calls for a correspondingly augmented heating capacity of the primary heating network. However, due to the restriction of existing pipes, the heating capacity can be finitely increased. Therefore, based on the organic Rankine cycle (ORC) and vapor compression cycle (VCC), a novel thermal station is proposed to realize self-augmented heating capacity. Based on laws of thermodynamics, the mathematical model is constructed, and the dynamic performances of power generation and self-augmented heating capacity are analyzed. Results show that the supply water temperature of the primary heating network greatly influences the power generation performance. The self-sufficient electricity can achieve approximately 2000 h, accounting for 61.70% of the heating period. Additionally, the proposed thermal station can flexibly reduce the return water temperature of the primary heating network to 20 °C, thereby achieving self-augmented heating capacity. The self-augmented heating capacity rate can reach 53.56%. Therefore, the proposed thermal station demonstrates excellent self-augmented heating capacity, which provides a new path for satisfying the ever-increasing heating demand.

Integration of compressed air energy storage into combined heat and power plants: A solution to flexibility and economy

To achieve carbon neutrality, conventional coal-fired combined heat and power (CHP) plants require higher operation flexibility to improve the grid's accommodation for renewable energy. Based on the promising converging interests between compressed air energy storage (CAES) and CHP, a novel CHP-CAES system with higher operation flexibility, energy efficiency, and round-trip efficiency (RTE) is proposed in this paper. The novelty of this system is that the released compression heat of CAES during the charge process can be recovered by the CHP, and the heat consumed during the discharge of CAES comes from the primary district heating network (DHN) instead of CHP. Firstly, the theoretical feasibility of the proposed CHP-CAES system is verified. Then, the operation flexibility, energy efficiency, and RTE of the CHP-CAES system are investigated. Results show that when the output heat of the CHP-CAES system is the design value of 150 MW, the power overload capacity can be increased by 3.04 %, and the feasible minimum load is decreased by 2.00 % of the design power. The maximum energy efficiency of the CHP-CAES system is increased from 68.38 % to 69.57 %, compared with the separate CHP system. Besides, the RTE of the CAES component is increased from 64.35 % to 77.05 %. Further, optimized scheduling strategies of this CHP-CAES system are proposed for the renewables accommodation scenario. Results show that the imbalance between power provision and demand can be notably reduced from 416.68 MWh to 1.01 MWh after the integration of the CAES system. Moreover, the profitability of the CHP-CAES system also would be improved by 14,449CHY by adopting the proposed scheduling strategies on the reference day, benefiting from the time-of-use (TOU) electricity price.

Optimal operation of integrated power and thermal systems for flexibility improvement based on evaluation and utilization of heat storage in district heating systems

Heat storage in district heating system is promising for renewable energy accommodation because it effectively improves system flexibility. However, its proper evaluation remains unclear, preventing its full utilization. This work distinguishes heat storage mechanisms of heating network (HN) and buildings rigorously by combining a unified multi-energy flow model and energy conservation analysis. Heat storage of HN is attributed to heat migration processes inside, while that of buildings originates from heat capacities of envelopes. On this basis, water flow rate and supply water temperature in primary HN (PHN) and set values of indoor air temperature are optimized to improve the system flexibility, namely the wind power accommodation. Different combinations of the three optimizations are also studied to improve system flexibility to the greatest extent. Results show that optimizing water flow rates and supply water temperature in PHN and optimizing water flow rates in PHN and set values of indoor air temperature yield 14.3% and 15.3% additional increase of wind power accommodation, respectively, compared with their ideal superposition. On the contrary, optimizing supply water temperature in PHN and set values of indoor air temperature gives a 30.2% reduction of wind power accommodation increase compared with the sum of two standalone optimizations.

Research on predictive control of energy saving for central heating based on echo state network

It is very important for energy conservation and environmental protection to realize energy-saving control of central heating system by prediction method. In this paper, the system model of central heating is built according to the general principle of secondary heating network, and an energy-saving control scheme based on the combination of dual networks and the regulation of secondary heating network is developed. In order to achieve better energy-saving control, the echo state network method is used to predict the heating water temperature in the heat exchange station controller. In the configuration process of echo state network, the water supply circuit temperature of the secondary heating network is taken as the output, and the 8 parameters of the primary heating network and the secondary heating network are taken as the input. After the stable echo state network is obtained through iterative training, the prediction is completed by using iterative multi-step method. Through the experimental results of three groups of sample data, it can be seen that the echo state network and prediction method constructed in this paper are superior to other methods, and are suitable for energy-saving prediction of central heating system.

Dynamic modeling and intelligent heating strategies of district heating system based on the standardized thermal resistance

The intelligent heating system can improve heating efficiency and promote renewable energy accommodation that positively affects the realization of the dual-carbon strategy. This paper dynamically modeled the key components based on the standardized thermal resistance defined by the inlet temperature difference and constructed the overall dynamic heat current model of the heating system from the heat source to the user. The dynamic model simultaneously and comprehensively reflects the heat transfer, storage, and delay characteristics of the heat exchanger, piping, and building. On this basis, an iterative method is proposed and used to develop various heating strategies for the hourly quality regulation of the primary heating network. The proposed user-following heating strategy has the greatest energy-saving potential of 25.27% compared with the all-day heating strategy. Moreover, based on the user-following heating strategy, we proposed five different heating strategies, namely all-day constant heating type, inverted triangle type, stair-step type, trapezoid type, and parabolic type, to achieve energy-saving operation of the heating system for the actual application. The trapezoid type has the highest energy-saving ratio of 9.41% compared to the all-day constant heating type. In all, this standardized thermal resistance-based dynamic modeling method and the proposed heating strategy provide a new theoretical basis and solutions for the intelligent heating system.

Optimized Dispatching for Integrated Energy System With Wind Power

The intermittency and unpredictability are the obstacles to absorb all wind power. The combined heat and power (CHP) system has been widely utilized as a heat and electricity source, and the heating load limits the regulatory capacity of CHP to reduce curtailed wind power. This study proposes an optimization model of an integrated power and heating network system to fully utilize curtailed wind power. The thermal inertia in the primary heating network, thermal comfort of end-users, and electric boiler in the secondary heating network are included in multiple scenarios to analyze different operation strategies. The simulation results show that considering the heating network, the inertia and thermal comfort of end-users could reduce the curtailed wind power ratio from 15% to 9%. Adding a peak-shaving electric boiler on the user side can increase the system electric load and reduce the curtailed wind power, while reducing the heat output of the CHP unit during the peak and low periods of the heat load at night. Moreover, there is less than 1% of the curtailed wind power ratio if the peak-shaving capacity ratio is higher than 10%, which means the application recommendations of installed electric boiler capacity are obtained to increase enough flexibility of the power and heating network system.

New medium-low temperature hydrothermal geothermal district heating system based on distributed electric compression heat pumps and a centralized absorption heat transformer

Development of geothermal heat for district heating has attracted considerable attention in China. However, transporting geothermal heat for long distance has become the bottleneck for developing large-scale medium-low temperature hydrothermal geothermal fields that located far away from heat load areas. To solve this problem, a new medium-low temperature hydrothermal geothermal district heating system based on distributed electric compression heat pumps and a centralized absorption heat transformer is proposed and evaluated both from the aspects of thermodynamic performance and economic benefit. Analysis of the results may lead to following main conclusions: (i) for the proposed system, its cost-effective main line length of the primary heating network is about 10 km. (ii) The annual coefficient of performance, annual product exergy efficiency, and heating cost of the proposed system are found to be 24.5, 61.4% and 55.62 ¥/GJ, respectively. The centralized absorption heat transformer can reduce irreversible loss of the heating station and improve performance of distributed electric compression heat pumps. (ⅲ) Unlike the conventional medium-low temperature hydrothermal geothermal district heating system with longer distance of transporting geothermal heat, the annual coefficient of performance and annual product exergy efficiency of the proposed one can be improved by about 4.34 and 7.4%, respectively.

Evaluation and optimization of insulation status test for primary heating network

Abstract To accurately understand the insulation status of primary heating pipe network in the Chengyin District of Daqing City, its present insulation status has been tested through the enthalpy drop method. The field test results, with the data processing, are compared and analyzed, which reveals that there are weak links in the insulation structure of the primary heating pipe network in the Chengyin area. Before the insulation structure is optimized according to the field conditions, which provides a basis and reference for testing the insulation status of the heating pipe network, and has a guiding significance for evaluating and optimizing the insulation structure of pipe network.

Local Heating Networks with Waste Heat Utilization: Low or Medium Temperature Supply?

District heating enables an economical use of energy sources that would otherwise be wasted to cover the heating demands of buildings in urban areas. For efficient utilization of local waste heat and renewable heat sources, low distribution temperatures are of crucial importance. This study evaluates a local heating network being planned for a new building area in Trondheim, Norway, with waste heat available from a nearby ice skating rink. Two alternative supply temperature levels have been evaluated with dynamic simulations: low temperature (40 °C), with direct utilization of waste heat and decentralized domestic hot water (DHW) production using heat pumps; and medium temperature (70 °C), applying a centralized heat pump to lift the temperature of the waste heat. The local network will be connected to the primary district heating network to cover the remaining heat demand. The simulation results show that with a medium temperature supply, the peak power demand is up to three times higher than with a low temperature supply. This results from the fact that the centralized heat pump lifts the temperature for the entire network, including space and DHW heating demands. With a low temperature supply, heat pumps are applied only for DHW production, which enables a low and even electricity demand. On the other hand, with a low temperature supply, the district heating demand is high in the wintertime, in particular if the waste heat temperature is low. The choice of a suitable supply temperature level for a local heating network is hence strongly dependent on the temperature of the available waste heat, but also on the costs and emissions related to the production of district heating and electricity in the different seasons.

Thermodynamic and economic evaluation of a novel heat supply design using low-pressure feedwater in a cogeneration plant

An innovative heat supply design for cogeneration systems has been proposed. Instead of the conventional heat source (extraction steam of the turbine), the low-pressure feedwater of the heat regeneration system is employed to provide district heat in the new scheme, through warming the supply-water of the primary heating network in a plate heat exchanger. A detailed thermodynamic analysis was conducted to evaluate the proposed design compared with the traditional one, based on a large-scale coal-fired power plant that needed to be retrofitted into cogeneration. The results show that the net power output of the cogeneration system is boosted by 4.68 MW owing to the novel concept. Furthermore, the total system efficiency and effective electric efficiency of the cogeneration system are promoted by 0.43 and 0.46 percentage points. This is because the exergy efficiency of the supply-water heating process is raised by 29.65 percentage points, and the exergy efficiency of the cogeneration system is improved by 0.42 percentage points. Finally, the new design was economically analyzed, and the results indicate that the net present value reaches 16543.2 thousand USD (8381.5 thousand USD more than that of the conventional design). Therefore, the proposed concept is feasible and profitable in engineering.

Thermo-economic analysis of composite district heating substation with absorption heat pump

A novel composite district heating substation (CDHS) with absorption heat pump was proposed and analyzed in this paper. The CDHS was composed of a water-LiBr absorption heat pump and two plate heat exchanges, which could improve the utilization efficiency of geothermal water and the primary supply water of the primary district heating network. The effect of geothermal water temperature and mass flow rate, and primary supply water mass flow rate variation on system performance were investigated and analyzed by case study. The objective was to maximize net profit and minimize payback period by the cascade utilization of the geothermal water and primary supply water. In addition, economic analysis and multi-objective optimization were conducted to find the optimal mass flow rate of the primary supply water based on the TOPSIS decision making method. Exergy loss analysis was applied under the optimal condition to discover which components had the largest exergy loss. Results indicated that, the proposed system had a net profit of 16.22 M$ in the life time and the minimum payback period was 2.2 years at the optimal primary supply water mass flow rate of 46.16 kg/s where the COP and exergy efficiency of the system were 1.85 and 59.81%, respectively.

New configurations of district heating system based on natural gas and deep geothermal energy for higher energy efficiency in northern China

To decrease irreversible loss and reduce natural gas consumption of conventional district heating systems based on natural gas fired boilers, new configurations of the district heating system based on natural gas and deep geothermal energy are proposed and analyzed from the perspective of thermodynamics and financial benefit. The results show that the proposed district heating system based on natural gas and deep geothermal energy with absorption heat exchangers and a gas-fired absorption heat pump (DH-AHE-DAHP) not only has higher thermodynamic performance, but also enables us to gain better financial benefit, and its configuration is optimal. Compared with conventional district heating systems based on natural gas, the DH-AHE-DAHP could increase product exergy efficiency by about 12%, reduce natural gas consumption by about 54% and heating cost by about 25%, and its cost-effective heat transmission distance of the primary heating network could approach about 28 km, which contributes to efficient exploiting deep geothermal fields far away from urban districts for space heating.

Improving wind power integration by a novel short-term dispatch model based on free heat storage and exhaust heat recycling

Many wind farms in northern China have faced a worrying phenomenon “wind curtailment” during the heating seasons. The limited operating flexibility of combined heat and power (CHP) units is one of the major barriers to integrating wind energy. In this study, wind power integration is enhanced by using the free heat storage of the primary district heating network (DHN) and introducing extra heat pumps (HPs). A novel short-term dispatch model is proposed to minimize the total primary energy consumption (TPEC) of the district energy system (DES). After necessary linearization processes, the unit commitment problem can be solved efficiently and analytically by the interior point method. A test DES that contains a real large-scale DHN is utilized to verify the feasibility of the proposed dispatch model. In case studies, comparisons are made to evaluate the improvement of wind power integration and TPEC reduction under different operating scenarios. The results show that both heat storage utilization and electric HP introduction can reduce wind curtailment considerably, however, exhaust heat recycling has a better performance. In addition, using the existing heat storage of the primary DHN could reduce the TPEC of DES further even if electric HPs have already been used.

Study of new cascade heating system with multi-heat sources based on exhausted steam waste heat utilization in power plant

Utilizing waste heat of exhausted steam in power plant as a new type of heat source is a key direction to achieve both energy-saving and clean district heating. This paper firstly reviews current mature waste heat utilization technologies, including “absorption heat pump”, “increasing back pressure of turbine” and “decreasing return water temperature of heat network”. Using only one method is not applicable to recovery all the exhausted steam waste heat of multiple turbine units due to its respective limitations. In view of this technical difficulty, this paper proposes a new cascade heating system with multi-heat sources based on waste heat utilization. Extraction steam and exhausted steam are involved to achieve the cascade temperature-rise of heat network water. Corresponding evaluation methods are presented, including the last-stage safety of the large unit and the comprehensive energy efficiency of the new cogeneration heating system. Taking 2 × 300 MW water-cooled units as the example to actualize the new system, the effects of key parameters on energy efficiency and economy are analyzed, and then further researches about integration optimization of the new system are developed. In this study, first keep constant return water temperature of primary heat network, the optimal condition is gained. At this point, all the waste heat of exhausted steam is recovered, the energy consumption of power plant is reduced by 33% and the total cost per heat output ct is reduced by 22.3%, compared with conventional cogeneration system. Then reduce the return water temperature in different level, both the energy consumption and the total cost per heat output ct can be further lowered, the energy efficiency and the economy are better improved.

Simulation research on a variable-lift absorption cycle and its application in waste heat recovery of combined heat and power system

To solve the dilemma between temperature lift capability and coefficient of performance (COP) when absorption heat pump is applied in low grade heat utilization, a variable-lift absorption cycle and an improved one based on it are introduced here. The variable-lift absorption cycles have relatively simple systems and contains enough number of absorbers and condensers to develop plenty of gradually increasing temperature levels to match heating process. For a given heating demand and waste heat temperature level, the results of application of variable-lift absorption cycles in waste heat recovery of combined heat and power(CHP) system show that, the variable-lift cycles can always work efficiently when return water temperature of primary heating network (PHN) varies from 30 °C to 50 °C. The needed heat source temperatures of variable-lift cycles are always over 12 °C lower than single effect cycle. The higher return water temperature of PHN yields more obvious advantage of the improved cycle. When the return water temperature of PHN is 50 °C, variable lift-2C cycle and variable lift-3C cycle can reduce the equivalent power consumption for space heating by 8.5% and 15% respectively when comparing to single effect cycle. The essence of the variable-lift cycles is further clarified from the heat transfer analysis in T-Q diagram.

New low temperature industrial waste heat district heating system based on natural gas fired boilers with absorption heat exchangers

To efficiently recover low temperature industrial waste heat, a new low temperature industrial waste heat district heating system based on natural gas fired boilers with absorption heat exchangers was proposed, and also analyzed from the perspective of thermodynamics and economics. In heating substations, absorption heat exchangers are used to greatly decrease the return water temperature of the primary heating network. The lower temperature return water helps to efficiently recover waste heat in heating station and greatly increase economic heat transmission distance. Annual primary energy efficiency and product exergy efficiency of the new district heating system are 215.0% and 42.5% respectively. When waste heat price and heat transmission heat distance are 25¥/GJ and 60km respectively, its heating cost and investment recovery period are 6¥/GJ and 0.7years respectively lower than that of current district heating systems based on natural gas fired boilers. Characterized by better thermodynamic performance and economic benefit, the new district heating system would be promising for low-temperature industrial waste heat recovery.