Solar Assisted District Heating Research Articles

Modelling of solar assisted district heating system with seasonal storage tank by two mathematical methods and with two climatic data as input

The goal of the study is to present two independent solutions capable of modelling the thermal behaviour of the solar assisted district heating systems with seasonal thermal storage. The study uses a new approach in modelling the thermal behaviour of the solar assisted district heating systems, by using two calculation methods: the analytical method and TRNSYS to perform the simulations, and by using two climatic data as input: EnergyPlus and Meteonorm. The heat demand of the buildings, the shares of energy produced by the solar thermal system, and by the gas boiler used as backup, and a very large number of the system parameters: temperature variation in each layer of the seasonal heat storage tank, temperatures, and flow rates on all the circuits, etc. were calculated. The results were compared using three parameters: Mean Deviation, Mean Bias Error and Root Mean Square Error. All the results calculated with the two methods were found to be very close. The annual solar fraction was found to be in the range of (34.9–50.2) %, depending on the type of the considered solar thermal collector, the combination of calculation method and climatic data as input. The study proves that both analytical method and TRNSYS can be used successfully to simulate the thermal behaviour of solar assisted district heating systems with seasonal heat storage.

Solar-assisted district heating networks: Development and experimental validation of a novel simulation tool for the energy optimization

Due to the growing interest in 4th and 5th generation district heating systems, characterized by lower working fluid temperatures compared to the past, poligenerative solar-assisted networks are experiencing increasing attention from the research community. In this framework, the adoption of large solar-based thermal systems will become more common for such applications. Therefore, optimising the design and operation of each solar field servicing a specific network will be crucial to improving its overall performance. In this context, the objective of the current research is to develop a method to maximise the performance of solar-assisted poligenerative district heating networks by optimising each associated solar system separately. To this end, a novel dynamic simulation tool for solar thermal field design and optimization has been developed and experimentally calibrated in the Simulink/Simscape environment. The tool is capable of investigating innovative control logics, and accurately assessing both the thermal and hydraulic behaviour of the systems. The latter is one of its novelty, as ignoring the hydronic behaviour of the system can lead to the adoption of flawed design or control logic. The resulting tool will enable accurate optimization of the design and management of each solar field servicing a given district heating network, thereby improving the efficiency of the system as a whole. In order to demonstrate the viability of the proposed method and the capabilities of the developed simulation tool, a proof-of-concept analysis is conducted on an existing solar thermal field serving the Geneva district heating network. Diverse control logics (including Rule-Based and Model Predictive Control) are evaluated for the selected case study. The performed analysis demonstrates that the proposed method has the potential to accomplish significant primary energy reductions (up to 10.3%) and CO2 emissions avoidance (up to 10.9 tCO2/year).

Heat distribution concepts for small solar district heating systems – Techno-economic study for low line heat densities

• Two novel distribution concepts are compared to conventional district heating. • A novel distribution system with central domestic hot-water heating is presented. • Hybrid distribution outperforms a conventional steel pipe system. • A complete low-temperature system outperforms hybrid distribution. • Reduction in line heat density favours a low-temperature GRUDIS system. The high operating temperatures in today’s district heating networks combined with the low energy demand of new buildings lead to high relative network heat losses. New networks featuring lower operating temperatures have reduced relative heat losses while enabling an increase in the use of solar heat. The primary aim of this study was to determine if a particular district heating system can be made more effective with respect to heat losses and useful solar energy, by considering different distribution concepts and load densities. A small solar assisted district heating system with a novel hybrid distribution system has been modelled based on a real case study. This model serves as a basis for two other models where the distribution system and heating network operating temperature is changed. A secondary aim of the study was to determine the economic implications of making these changes, by using costs estimates to calculate the contribution of essential system components to total system cost. Results indicate that a novel distribution concept with lower network temperatures and central domestic hot water preparation is most energy efficient in a sparse network with a heat density of 0.2 MWh/m∙a and a performance ratio of 66%, while a conventional district heating system performs worst and has a performance ratio of less than 58% at the same heat density. In an extremely sparse network with heat density of 0.05 MWh/m∙a, the performance ratio is 41% and 30% for these systems, respectively. A simple economic analysis indicates that the novel distribution concept is also best from an economic point of view, reducing the initial investment cost by 1/3 compared to the conventional concept, which is the most costly. However, more detailed calculations are needed to conclude on this.

Economic Analysis of Heat Distribution Concepts for a Small Solar District Heating System

One challenge in today’s district heating systems is the relatively high distribution heat loss. Lowering distribution temperatures is one way to reduce operational costs resulting from high heat losses, while changing the distribution system from steel pipes to plastic pipes and changing the heat distribution concept can reduce investment costs. The result is that the overall life cycle cost of the district heating system is reduced, leading to the improved cost competitiveness of district heating versus individual heating options. The main aim of this study was to determine the most cost-efficient distribution system for a theoretical solar district heating system, by comparing the marginal life cycle cost of two different distribution systems. A secondary aim was to determine the influence of the employed pipe type and insulation level on the marginal life cycle cost by comparing detailed economic calculations, including differences in pipe installation costs and construction costs, among others. A small solar-assisted district heating system has been modeled in TRNSYS based on a real system, and this “hybrid” model is used as a basis for a second model where a novel distribution system is employed and the heating network operating temperature is changed. Results indicate that a novel distribution concept with lower network temperatures and central domestic hot water preparation is most efficient both from an energy and cost perspective. The total life cycle costs vary less than 2% for a given distribution concept when using different pipe types and insulation classes, indicating that the investment costs are more significant than operational costs in reducing life cycle costs. The largest difference in life cycle cost is observed by changing the distribution concept, the novel concept having approximately 24% lower marginal life cycle cost than the “hybrid” system.

Techno-economic analysis of control strategies for heat pumps integrated into solar district heating systems

This present work focuses on assessing the techno-economic benefits of different control strategies for a heat pump integrated into the solar assisted district heating system (SDHS). The system has been developed using dynamic simulation software (TRNSYS) and optimized based on a genetic algorithm. With an industrial-sized heat pump connected to thermal storage tanks for domestic hot water (DHW) and space heating (SH) for the requirements of the community, a SDHS is operated by applying two different control mechanisms for the heat pump based on its reference operating temperature. The application of the methodology is applied to a residential neighborhood community of 10 buildings located in Madrid to act as a proxy for the Mediterranean climates. The results showed a significant effect for the heat pump control in the techno-economic benefits where the proposed system is able to provide a solar fraction up to 99%. Furthermore, the total electricity consumption of the heating system varied by 10% between the best and the worst cases. Besides, the annual seasonal storage efficiency improved up to 90% with a life cycle expense up to 67.12 Euro/MWh, and a payback period of 29 years.

Flexible heat pump integration to improve sustainable transition toward 4th generation district heating

The movement toward the 4th generation district heating (4GDH) embraces a great opportunity to support the future smart energy development concept. However, its development calls for addressing technological and economic obstacles aligning with the need for a reformation of the energy market to ensure the quality of service. In this context, our paper presents a comprehensive analysis based on a multi-objective optimization framework incorporating an artificial neural network-based model for the possibility of integrating heat pump (HP) into solar assisted district heating system (SDHS) with seasonal thermal energy storage to support the sustainable transition toward 4GDH. The study evaluates the performance of the proposed system with the help of key performance indicators (KPI) related to the 4GDH characteristics and key stakeholders for possible market growth with consideration for the environmental benefits. The proposed analysis is applied to a small neighbourhood of 10 residential buildings located in Madrid (Spain) to investigate the optimal integration of HP under different control strategies into a SDHS. Inherent the SDHS operator perspective, the results reveal a significant improvement in the stabilization of the SDHS performance due to the HP integration where the solar field temperature never exceeds 80 °C, and the seasonal storage tank (SST) temperature stands at 85.4 °C. In addition, the share of solar energy stands above 86.1% with an efficiency of 73.9% for the SST, while the seasonal HP performance factor stands above 5.5 for all optimal scenarios. From the investor viewpoint, an energy price of 59.1 Euro/MWh can be achieved for the proposed system with a payback period of 26 years. Finally, from the policymaker perspective, along with the significant economic and sustainable improvement in the SDHS performance, a substantial environmental improvement of 82.5% is achieved when compared to the conventional boiler heating system. The proposed analysis reflects a great motivation for different stakeholders to propose this system as a path toward the 4GDH in the future district energy systems.

A framework for the optimal integration of solar assisted district heating in different urban sized communities: A robust machine learning approach incorporating global sensitivity analysis

A promising pathway towards sustainable transaction to clean energy production lies in the adoption of solar assisted district heating systems (SDHS). However, SDHS technical barriers during their design and operation phases, combined with their economic limitation, promote a high variation in quantifying SDHS benefits over their lifetime. This study proposes a complete multi-objective optimization framework using a robust machine learning approach to inherent sustainability principles in the design of SDHS. Moreover, the framework investigates the uncertainty in the context of SDHS design, in which the Global Sensitivity Analysis (GSA) is combined with the heuristics optimization approach. The framework application is illustrated through a case study for the optimal integration of SHDS at different urban community sizes (10, 25, 50, and 100 buildings) located in Madrid. The results reveal a substantial improvement in economic and environmental benefits for deploying SDHS, especially with including the seasonal storage tank (SST) construction properties in the optimization problem, and it can achieve a payback period up to 13.7 years. In addition, the solar fraction of the optimized SDHS never falls below 82.1% for the investigated community sizes with an efficiency above 69.5% for the SST. Finally, the GSA indicates the SST investment cost and its relevant construction materials, are primarily responsible for the variability in the optimal system feasibility. The proposed framework can provide a good starting point to solve the enormous computational expenses drawbacks associated with the heuristics optimization approach. Furthermore, it can function as a decision support tool to fulfill the European Union energy targets regarding clean energy production.

Comparison of distributed and centralised integration of solar heat in a district heating system

Ambitious greenhouse gas emission reduction targets set by EU pose a significant challenge for the member countries. District heating as an efficient solution for heat supply and distribution can play a major part in meeting these targets. One key issue concerning district heating is the integration of renewables. The current study focuses on solar assisted district heating systems.Centralised and distributed solar collectors in an existing district heating system are investigated. The effects of reducing supply temperature are studied in two scenarios. Outdoor dependent supply temperature level (115-80 °C) and low supply temperature level (65 °C) are considered. Both distributed and centralised systems are defined with same investment costs, but different capacities due to differences in unit costs (€/kW). The aim is to study cost-efficient solutions for integrating solar heating an existing district heating system.Heat demand consisting of space heating and domestic hot water consumption, solar collectors and district heating network itself are all modelled by dedicated models in order to provide realistic input for a techno-economic analysis.Investigation is based on a case study of a local district heating system in Finland with an annual heat consumption of 1.2 GWh and a heat demand density of 0.74 MWh/m.

Effective thermal conductivity of moistened insulation materials as a function of temperature

Seasonal heat stores for waste heat applications or solar assisted district heating systems require thermal insulation in order to minimize heat losses. The insulation of these thermal energy stores is exposed to environmental influences. Especially the insulation of buried tank or pit heat stores may degrade due to increased moisture content caused by penetrating water (vapour). The effective thermal resistance of insulation materials decreases with increasing temperature and moisture content. Hence, thermal losses may become higher than the design values. For the insulation of buried heat stores bulk materials are more suitable than sheets. However, the availability of measured data of the thermal conductivity of bulk insulation at higher temperatures and at elevated moisture contents is poor. Models for the thermal conductivity given in the VDI Heat Atlas and elsewhere have to be modified in order to obtain good agreement between calculated and measured data of the thermal conductivity of porous materials with partially closed and thus for water inaccessible pores. This paper gives a detailed description of modelling and measurement of the effective thermal conductivity of porous bulk materials at temperatures up to 80 °C and moisture contents below free water saturation.

Validation of a computer model for solar assisted district heating systems with seasonal hot water heat store

The TRNSYS XST-model for the calculation of the thermal behaviour of ground buried hot water heat stores was validated. For the validation procedure measured data of the seasonal hot water heat store in Hannover (Germany) were used. In contrast to previous investigations the temperatures of the surrounding ground were also taken into consideration. The determination of the heat store parameters was carried out using TRNSYS in combination with the parameter identification software DF. The deviation between measured and calculated temperatures is less than ±3%. The measured and calculated heat quantities are also in good agreement (annual deviation less than 2%). The validated XST-model was integrated into a TRNSYS model to calculate the thermal behaviour of the solar assisted district heating system in Hannover in 2002. The deviations between measured and calculated heat quantities do not exceed 5%.

Solar-assisted district heating plants: status of the German programme solarthermie-2000

Solar-assisted district heating plants: status of the German programme solarthermie-2000

Solar-Assisted District Heating Plants: Status of the German Programme Solarthermie-2000

Within the 10 year Programme “Solarthermie-2000” of the German Federal Ministry for Economics and Technology (BMWi), large-scale solar-assisted heating plants have been funded in Germany. Long-term monitoring programmes are carried out to prove the technical and economic feasibility of various solar system concepts with and without seasonal storage. The paper presents a summary and review of the present status and results of the activities in the Programme. Long-term monitoring programmes have shown that the design data of the solar plants can be achieved if realistic assumptions are made. The main obstacle for the implementation of the concepts on a broader scale without public subsidies is the high investment costs of the solar systems. The main goal of the Programme is the further improvement of the cost-effectiveness of the solar concepts.