Low-temperature District Heating Research Articles

A new type of two-supply, one-return, triple pipe-structured heat loss model based on a low temperature district heating system

Based on the design concept of a fourth-generation smart pipe network system, this paper innovatively proposes a new TOTS (Two-supply/One-return, triple pipe structure) arrangement method for district heating systems. Moreover, to accurately predict the heat loss due to the pipeline operation of the multi-pipe system, based on the multipole calculation method, a new heat loss theoretical analytical model for the TOTS was created; additionally, a corresponding three-dimensional numerical simulation model was established, which was analyzed and numerically solved. The results showed that in comparison with thermal loss data measured by Danfoss et al., the above analytical and numerical models have a high accuracy, and the deviation is within 2%. Additionally, through calculations, it was found that the distance between the heating pipes is an important factor that affects the total heat loss from the new multi-control heating system and the actual heat exchange between pipes.

Investment Planning Methodology for Complex Urban Energy Systems Applied to a Hospital Site

Industrial process integration based on mixed integer linear programming has been used for decades to design and improve industrial processes. The technique has later been extended to solve multi-period and multi-scale problems for the design of urban energy systems. Assistance is indeed required for the elaboration of coordinated investment scheduling strategies to promote renewable and efficient urban energy infrastructure shaping the future energy context for the next decades. Major energy consumers, such as hospital complexes, airports, or educational campuses can act as a driving force for the development of renewable energy cities by attracting profitable large-scale energy networks and infrastructure. The proposed methodology generates optimal alternatives for the replacement, in a long-term perspective, of the various energy supply units and systems considering the evolution of the energy demand and the availability of the energy resources. Energy integration techniques are coupled to a parametric multi-objective optimization routine to select and size the energy equipment with both financial profitability and CO2 emission reduction as objectives. The originality of the developed method lies in the integration of a multi-period mixed integer linear programming formulation to generate long-term investment planning scenarios. The method has been demonstrated on a complex of eight hospitals totaling 466,000 m2 and an operating budget of 1.85 billion USD per year. The energy integration of new centralized and decentralized equipment has been evaluated on a monthly basis over four periods until the year 2035. The results show that among the four scenarios identified, the most optimistic alternative allows to decrease the final energy consumption of about 36%, cut the CO2 emissions by a half, multiply the renewable energy share by a factor 3.5 while reducing the annual total cost by 24%. This scenario considers mainly the integration of a very low temperature district heating with decentralized heat pumps to satisfy the heat requirements below 75°C, as well as heat recovery systems and the refurbishment of about 33% of the building stock.

Waste heat recovery from a data centre and 5G smart poles for low-temperature district heating network

The city of Espoo, Finland is planning to develop Kera as a green suburb with high level of energy efficiency and low CO2 emissions, using a high share of renewable energy and recycled or reused energy. For reaching this target, in this study, renewable energy resources such as solar, wind and waste heat are investigated for the study region. Two different technologies comprising heat pump (HP) and heat-only boiler (HOB) are investigated to retrieve waste heat from a data centre and LuxTurrim5G smart poles to use in a low-temperature district heating network. We investigate various scenarios to supply the required energy for the HP (which receives electricity from the electricity market, photovoltaic (PV) system, wind turbine (WT) and hybrid PV/WT; 4 scenarios) and HOB (which works with electricity, forest fuel wood, biogas, ammonia, wood pellets and industry wood residue; 6 scenarios). We found that the heat pump scenario is an efficient and cost-effective way to retrieve waste heat from the data centre and 5G smart poles with an LCOE of 3.192 ¢/kWh if electricity is produced by the PV system, and 3.516 ¢/kWh when the heat pump receives its electricity only from the electricity market.

Roadblocks to Low Temperature District Heating

Energy usage in buildings is coming increasingly under the spotlight as carbon policy focus shifts towards the utilization of thermal energy. In the UK, heating and hot water accounts for around 40% of energy consumption and 20% of greenhouse gas emissions. Heating is typically produced onsite, making widescale carbon or energetic improvements challenging. District heating networks (DHNs) can offer significant carbon reduction for many users but can only be implemented if the end user buildings have good thermal energy efficiency. This greatly limits the ability to implement advancing 4th and 5th generation DHNs, which are the most advanced systems available. We elucidate the current state of thermal efficiency in buildings in the UK and provide recommendations for necessary building requirements and modifications in order to accommodate 4th and 5th generation district heating. We conclude that key sectors must be addressed including creating a skilled workforce, producing relevant metrics and benchmarks, and providing financial support for early stage design exploration.

Low return temperature from domestic hot-water system based on instantaneous heat exchanger with chemical-based disinfection solution

Thermal treatment for sterilising domestic hot-water systems requires a circulation temperature higher than 50 °C, which is not suitable for low-temperature district heating systems. Chemical treatment is a temperature-independent sterilisation method for hot-water systems. This study demonstrates that the heat exchanger-based domestic hot-water system of a multi-family building can operate without the risk of Legionella through the use of a chemical sterilisation method. This solution enables the reduction of the circulation temperature to 45 °C to satisfy the comfort requirements. Three temperature operation strategies were tested in the circulation loop. According to the results, the Legionella concentration was minimised. The operation of several thermostatic control valves in the risers was found to be problematic as this resulted in high return temperatures to the heat exchanger. The use of the chemical sterilisation method allows the reduction of the supply temperature below 50 °C and the exclusion of most of the vertical pipelines from the circulation loop . This results in a low return temperature of 40 °C and a reduction in the circulation heat loss up to 66%.

Life Cycle Assessment of Low Temperature District Heating System in Gulbene Region

Abstract New district heating system technologies have arisen in the last years to deliver economic and environmental benefits to residential and commercial buildings. The extensive ranges of equipment, energy sources, temperature profile configurations, size of the network, energy demand, and many other intrinsic variables, make it difficult to identify if a determined district heating option is potentially better than another in environmental terms. As for the economic evaluation, there are several tools decision-makers can rely on to assess environmental performance. The main challenge is to provide a holistic point of view for which lifespan and complexity of implementable, new technological systems can be an obstacle. For this reason, in this paper, a Life Cycle Assessment is performed upon a technical evaluation of several district heating configuration options for the Gulbene region in Latvia, where DH systems in most of the assessed parishes are already operating under medium temperature regimes, also known as third-generation district heating. The goal of the study is to understand the environmental impact of moving from the current DH system to a low temperature one. Results show a considerable environmental benefit if low-temperature profiles, combined with the use of renewable energy sources are adopted in the current DH systems. A hotspot analysis is also performed showing the use stage is the one carrying most of the burden across the project’s lifetime, followed by infrastructure construction; also showing that the refurbishment of buildings does not play a major role in the total environmental impact contribution.

Optimized Layouts of Borehole Thermal Energy Storage Systems in 4th Generation Grids

Borehole thermal energy storage (BTES) systems are a viable option to meet the increasing cooling demand and to increase the sustainability of low-temperature district heating and cooling (DHC) grids. They are able to store the rejected heat of cooling cycles on a seasonal basis and deliver this heat during the heating season. However, their efficient practical implementation requires a thorough analysis from technical, economic and environmental points of view. In this comparative study, a dynamic exergoeconomic assessment is adopted to evaluate various options for integrating such a storage system into 4th generation DHC grids in heating dominated regions. For this purpose, different layouts are modeled and parameterized. Multi-objective optimization is conducted, varying the most important design variables in order to maximize exergetic efficiency and to minimize levelized cost of energy (LCOE). A comparison of the optimal designs of the different layouts reveals that passive cooling together with maximizing the heating temperature shift, accomplished by a heat pump, lead to optimal designs. Component-wise exergy and cost analysis of the most efficient designs highlights that heat pumps are responsible for the highest share in inefficiency while the installation of BTES has a high impact in the LCOE. BTES and buffer storage tanks have the lowest exergy destruction for all layouts and increasing the BTES volume results in more efficient DHC grids.

Control assessment in coupled local district heating and electrical distribution grids: Model predictive control of electric booster heaters

Intelligent control schemes are essential for the implementation of smart energy systems, where district heating and electric networks are tightly interconnected. The increasing complexity of such networked infrastructure has resulted in the need to test and assess control algorithms before field deployment. This work presents a method to assess advanced control schemes for thermal-electric appliances with explicit consideration of coupled heat and power networks. It is based on closed loop simulation of high-fidelity physical system models, using dynamic thermal-hydraulic district heating and electric distribution network models, and low-fidelity time-discrete advanced control models. Co-simulation is used to perform coupled simulations of the different involved domains and tools. A test case is presented where a model predictive control scheme for grid friendly operation of domestic hot water electric booster heaters is implemented in a low-temperature district heating and low-voltage electric distribution network. Test case results show that the control is able to reduce peaks in district heating and electric networks compared to a simple reference controller. A comparison between using perfect and naive forecasts shows that control performance highly depends on the availability of accurate predictions. The results underline the versatility of the method to assess different control schemes in integrated networks.

Assessment and optimisation of energy consumption in building communities using an innovative co-simulation tool

Energy efficiency in building sector is attracting an increasing interest in the scientific community, due to its strong impact in terms of greenhouse gas emissions. In this context, the REMOURBAN H2020 project has carried out a pilot deep refurbishing work on a small cluster of 10 homes, implementing energy saving measures and a hybrid energy-supply system to satisfy the heating and domestic hot water demand. The system aims to achieve near-zero-energy homes level of performance at reasonable cost by offsetting the energy consumption with local energy microgeneration. It is designed as a local low temperature district heating system and includes ground source heat pumps, photovoltaic panels, electric and thermal energy storage devices. The management of the complex hybrid system requires a suitable control strategy to optimise the energy consumption and consequently running cost. With this purpose a co-simulation tool has been developed, coupling a model of the energy system built using Dymola-Modelica and the EnergyPlus model of the buildings. This allows to develop different control strategies aiming to reduce the energy consumption from the grid, maximize the self-consumption of photovoltaic energy and ultimately move away from fossil fuel to sustainable energy resources.

Carbon dioxide-enhanced geothermal systems for heat and electricity production: Energy and economic analyses for central Poland

The pursuit of economically feasible and socially acceptable carbon capture technologies is often focused on the permanent storage of carbon dioxide (CO2). Among well-established CO2 capture, utilisation, and storage technologies such as enhanced oil recovery, some have recently gained interest for application in the energy sector. One such technology is the use of CO2 as a working fluid in enhanced geothermal systems. The potential benefits of this technology will strongly depend on the parameters of the geological reservoir. The use of supercritical CO2 as a working fluid can provide additional benefits because of the occurrence of partial sequestration of CO2. The main goal of this study is to investigate which of the geological and wellbore parameters of an enhanced geothermal system have the most significant impact on the energy and economic performances of the investigated power generation systems. This paper presents the results of energy assessment of the analysed power generation systems together with the economic effects of their operation. The presented results suggest that CO2-enhanced geothermal systems can be a feasible candidate for the utilisation of the captured CO2 and can simultaneously be a valid source of heat and/or electricity. The analysed process configurations prove that the optimal design of the surface part of the CO2-enhanced geothermal system strongly depends on the parameters of the geological reservoir as well as on the availability of heat sinks (e.g. a low-temperature district heating system), and that this design provides a payback time of 10–12 years in the most optimistic scenario. The analysed enhanced geothermal systems, when compared with conventional geothermal energy systems, are characterised by a higher cost of electricity generation (average-weighted levelized cost of electricity of about 110–170 EUR/MWh as compared with 60 EUR/MWh for conventional systems), mainly because of the significantly higher (1.5 times) total installation costs of the former.

APPLICATION OF THE THERMOELECTRIC HEAT PUMPS FOR THE DISTRICT HEATING SYSTEMS

The article presents a preliminary analysis of the possibility of applying thermoelectric heat pumps (THP) in low-temperature district heating systems (in the 4–5 generation DH systems). The advantages of THP are the high reliability, the lack of moving parts and freons in their composition, the compactness, the independence of the scale factor. Unlike the compression heat pump, THP provides effective circuit solutions, giving them additional tangible benefits. These features make it possible to reproduce the ideal Lorentz cycle, which can be realized with series-connected thermoelectric modules. The total operating temperature difference is divided into a large number of small intervals, which ensures the high efficiency of each module, and the required temperature difference and performance are achieved by the set of a corresponding number of series-connected micro-THP. It is clear that such a scheme cannot be implemented using compression heat pumps because of a number of technical and economic constraints. There are no such restrictions for thermoelectric heat pumps since each thermoelement is a micro-HPs that can act as a separate THPs. On the example of THP with a rated power of 1kW, the characteristics of the proposed scheme were analyzed. Unlike the classic HP, it has great flexibility. Under given temperature conditions, the power of the considered THP can vary in the range from 0.5 kW to 5 kW only due to a change in the supply current. At the same time, its effectiveness varies in the range from COP = 14 to COP = 2. This opens up new opportunities for the creation of «smart» heating systems that allow optimization of heat consumption modes, adapting them to current needs by purely programmatic methods. The estimated capital cost of THP G » 300 EUR/kW is based on actual data on the current level of component cost. Ref. 6, Fig. 5, Tab. 1.

Energy & economic assessment of façade-integrated solar thermal systems combined with ultra-low temperature district-heating

Abstract This paper conducts an energy and economic assessment of District Heating (DH) integrated Solar Thermal (ST) systems. An implementation with building-integrated ST collectors coupled to a Low Temperature District Heating (LTDH) system is studied, with special focus on unglazed collectors. ST heat is exploited in the building through direct use, while excess heat is delivered to the network. A novel control strategy for heat flows in the system is proposed. A meta-analysis of several DH configurations, interconnection schemes and installed ST capacity is performed in three different climates: Sevilla (Spain), Bordeaux (France) & Copenhagen (Denmark). Heat loads corresponding to buildings with various insulation levels and domestic hot water loads are assessed in hourly simulations. The proposed interconnection concept provides a variety of connection modes to the DH network, allowing up to a 50% increase in the provision of solar heat compared to an isolated ST system. Positive Return of Investment (ROI) for such a setup is achieved in 22% of the studied cases. The DH network is found to be a suitable heat sink in up to 25% of the buildings with ST systems installed.

A framework for techno-economic assessment of demand-side power-to-heat solutions in low-temperature district heating

In this paper, technical and economic assessment of power-to-heat (P2H) solutions installed at the building side is presented. A framework solution including different control algorithms, network and operational parameters, is proposed to conduct techno-economic assessment for P2H technologies in low-temperature district heating (LTDH) from an integrated energy system perspective. The framework facilitates the transition from conventional DH to LTDH solution with demand-side P2H technologies. An electric heat booster (EHB), as a typical demand side P2H solution, is modelled with different control algorithms and tank sizes and applied to a 23 single-family test energy system. Steady state combined heat and power flow is studied to investigate the impact on networks. Moreover, minimum differential pressure and supply temperature control of critical nodes are applied to ensure reliable DH operation. The results show the effectiveness of fuel shift control, where the ratio of DH loss is decreased by lowering supply temperature to 50°C. Furthermore, the minimum voltage magnitude is improved to be closer to 1 p.u., meanwhile, the power loss on electricity networks is increased. For the test energy system, the 51L EHBs contribute to lower energy cost of the system compared to the 92L ones, and can provide the lowest levelized cost solution with the supply temperature of 50°C.

Energy cascade connection of a low-temperature district heating network to the return line of a high-temperature district heating network

Heat supply from sustainable low-temperature district heating networks (LTDHN) can be considered as one of the most favourable heat supply option for urban buildings. If buildings are located in an area where heat is supplied by a well-established high-temperature district heating network (HTDHN) then it can be a problem to switch to low-temperature district heating. It can be possible in the case of a new planed buildings or buildings which will be fully renovated. There is an option to design them for LTDHN parameters and utilise the heat from the network return line. If those buildings are located closely, than it can be possible to establish a LTDHN branch connected to existing HTDHN. The goal of the study is to evaluate the technical and economic feasibility of integrating the energy cascade LTDHN into the existing large-scale HTDHN. Additional energy is necessary to raise the HTDHN return heating media temperature to the acceptable low-temperature district heating level because the return water temperature of HTDHN return line is, on average, 40–50 °C depending on specific network, when LTDHN maximum supply temperature can reach up to 65 °C. Two direct connection options are compared: a mixing shunt and 3-pipe connection shunt. Technical and economic aspects of this solution are analysed, including heat and electricity consumption, water flow rate and pressure, as well as additional investments and heating costs. The most feasible connecting option is compared with reference option applying exergy analysis. Particular emphasis is placed on assessing the decrease in the HTDHN return temperature caused by the integration of the LTDHN. The impact of the temperature decrease on the HTDHN is evaluated, including reduction of heat transfer losses, increase in combined heat and power plants (CHP’s) heat and electricity generation efficiency, and flue gas condenser efficiency. Other parameters, such as fuel savings are calculated as well.

Determining the useful life of district heating pipes: Correlation between natural and accelerated ageing

District heating (DH) systems constitute a smart and environmentally friendly solution for energy distribution in the heat sector in Europe. This technique is still expanding but already faces some issues such as status assessment of the current DH networks and the development of new generation networks for low-temperature DH. Therefore, it is essential to understand the ageing behaviour of pipes under operating conditions and to find the relevant parameters that control the degradation processes. Many factors affect the deterioration of DH pipes, especially the polyurethane foam, which makes it very complex to find a reliable prediction model. Models based on a linear Arrhenius relationship using results from high ageing temperatures seem to be incorrect. For this study, 10 pipes that have been in service for many years in Sweden and Norway were evaluated. The aim was to study the impact of natural ageing on the mechanical adhesion and chemical structure of the polyurethane foam, which affects the pipe's performance. A test method developed at the Research Institutes of Sweden (RISE), called RISE plug method, was used to study the mechanical adhesion strength. In addition, Fourier transform infrared spectroscopy was used to observe any change in the chemical structure. The results were compared with previous analyses of DH pipes exposed to accelerated ageing. This information helps to provide a better comprehension of the deterioration of the current generation of pre-insulated DH pipes and to improve the accelerated ageing methods used nowadays to predict the technical lifetime of DH pipes. Our results suggest that the lifetime of DH pipes has been underestimated when using artificial ageing at relatively high temperatures. The data collected from naturally aged pipes gave confirmatory information about their physical status compared with our laboratory tests. This study also suggests that infrared analyses could be used as an early indication of the degradation of the polyurethane foam at the interface with a steel pipe.

Low Temperature District Heating: An Expert Opinion Survey

Among the available solutions for building heating and cooling, district heating (DH) and district cooling (DC) systems are considered some of the best options since they can ensure a better control of pollutant emissions and greater efficiency than individual systems. Nevertheless, improvements are needed to increase their sustainability and reliability. The so-called “low temperature district heating” (LTDH) concept has been introduced in recent years in an attempt (i) to reduce the distribution heat losses through a temperature decrease in the DH network, (ii) to favor the integration with renewable energy sources, and (iii) to create the conditions required for the development of future smart energy systems. However, many concerns have been raised about its implementation in both existing and new systems. For this reason, this paper aims to identify the stakeholders’ ranking of the barriers against LTDH system development and implementation over the next few years. Aiming to this, a questionnaire was designed, including an analysis of current gaps and strengths, and then submitted to more than 50 Italian and international experts in the field of DH. An in-depth analysis of the received answers was performed, focusing in particular on the Italian experts’ answers. Comments and suggestions on how to promote the transition to the new LTDH approach are reported.

A temperature control strategy to achieve low-temperature district heating in North China

The data includes historical actual operating data of a District Heating System (DHS) in Tianjin and weather forecast data. The DHS operating data, which includes supply and return temperature from February 16, 2018, to March 12, 2018, is collected from a heat exchange station in Tianjin, China. The exchange station supplies heat for nearly 40 residential buildings in winter. The weather forecast data consists of outdoor temperatures, relative humidity, wind speed and air quality index (AQI).

Thermodynamic analysis of ultralow-temperature district heating system with shared power heat pumps and triple-pipes

An ultralow-temperature district heating system that uses neighborhood-scale heat pumps was recently proposed and investigated. Although the system was found more efficient than any existing solution, there is still potential in this concept for achieving better efficiency and cost-effectiveness. One of the main origins of losses here is through the enlarged pipes. In this work, triple-pipes are proposed to be used instead of twin-pipes to have individual domestic hot water and space heating pipes, reducing the size of heat pumps and decreasing thermal loss rates. The proposed concept is designed, sized and thermodynamically analyzed for a case study in Denmark. The performance is compared with the previous version of this concept as well as the low-temperature district heating system, which is another important competitor to the proposed solution. The results prove that the proposed system outperforms both of the competitors in terms of energy efficiency. For three consecutive days in typical warm/moderate/cold weather in the case study, the proposed solution results in the rates of heat loss of 4.5/10.9/16.5%, respectively. The heat loss rates, for the conventional configuration and the low-temperature design, are 4.9/13.8/19.6% and 5.6/17.8/33.1%, respectively.

Comparing strategies for improving thermal performance of an existing district heating (DH) network: low temperature DH in Omsk, Russia

In Russia district heating (DH) systems are characterized by the high wear-out rate of key facilities, particularly energy distribution networks and plants, the inadequate reliability of operation, significant heat losses, and the polluting impact on the environment. The objective of this paper is to indicate the feasibility and advantage of the renovation on a traditional high-temperature DH network. All the calculations are performed in Zulu©, Russian commercial software for simulation of a district energy system’s behavior. Reference operational data for the DH system was obtained from database established by the local heat supply company in the form of Microsoft Office Excel spreadsheet file. The second idea (Case-2) envisages installing a heat pump and increasing the supply temperature in peak load periods during the heating season to limit a size of a heat pump. Case-3 and -4 are related to installing in-room terminal units and operating the system ‘as is’ respectively. Terminal units include consoles, fan-coils, blower coils, furnaces, chimneys, and radiant panels. The fifth option introduces low temperature district heating (LTDH) concept representing a paradigm shift in DH development and features a low supply temperature and smart control. When designing all errors should be less than local regulations allowable for each object. Presented research revealed that the issue of modernisation in Omsk, Russia cannot be easily and clear resolved. The future state-determination of results of DH network retrofitting is innovatively proposed in this paper.

Study on the modernisation of an extra-worn district heating (DH) system in Russia: low temperature DH and 4 more options processing

Modeling was performed on the base of the DH system located in Omsk, Russia, where the DH network temperature requirements are not met and design outdoor temperature of extreme -37°C is. Surveyed investment in a transmission line to avoid penalties on disturbances is projected to have an original supply temperature of 150°C and is denoted as Case-1. The second idea (Case-2) envisages installing a heat pump and increasing the supply temperature in peak load periods during the heating season. The third option is to use of in-room terminal systems to provide heating to individual zones. Case-4 assumes maintaining an ordinary DH network without using any energy-efficient alternative and significant repair which means that the system continuous working ‘as is’. The fifth option introduces low temperature district heating (LTDH) concept featuring a low supply temperature and smart control. To sum up, this research indicates location of a heat pump and also shows how the piping system will be offset to allow the normal operation. This study presents a framework to represent, aggregate, dynamic thermal model and modernize a DH system based on a high-level equation-based simulation software and a five-option feasibility study.