This review article presents a description of contemporary developments and findings related to the different elements needed in future 4th generation district heating systems (4GDH). Unlike the ...first three generations of district heating, the development of 4GDH involves meeting the challenge of more energy efficient buildings as well as the integration of district heating into a future smart energy system based on renewable energy sources. Following a review of recent 4GDH research, the article quantifies the costs and benefits of 4GDH in future sustainable energy systems. Costs involve an upgrade of heating systems and of the operation of the distribution grids, while benefits are lower grid losses, a better utilization of low-temperature heat sources and improved efficiency in the production compared to previous district heating systems. It is quantified how benefits exceed costs by a safe margin with the benefits of systems integration being the most important.
•Provides a review of 4th Generation District Heating (4GDH) in scientific papers.•Shows how 4GDH is an important integrated part of future sustainable energy systems.•Quantifies costs and benefits of 4GDH in a future sustainable energy system.•Shows how benefits exceed costs by a safe margin.•Shows the significant benefits of systems integration.
District heating networks increasingly rely on heat pumps, condensing biomass boilers, and excess heat in the transition to sustainable energy systems. Accordingly, district heating operators seek to ...reduce their networks’ supply and return temperatures to maximise production efficiencies, minimise heat losses from distribution pipes and allow greater utilisation of renewable heat sources and excess heat. Experts have predicted that investing in solutions that reduce heating temperatures in buildings will yield a return on investment of 300% for district heating operators. Therefore, expecting incentives, building operators should identify methods to reduce supply and return temperatures to enable a rapid, widespread transition to low-temperature district heating. Ample research has investigated and documented the feasibility of low-temperature heating in buildings, and this paper presents the first comprehensive review. It synthesises available literature and adds new perspectives to help guide future implementation, research and development of low-temperature heating. The energy and temperature demands of various heating systems provides a background, leading to a review of typical malfunctions and their impacts. The article subsequently reviews the obtainable supply and return temperatures before and after renovating the building envelope and heating systems. It further identifies and summarises vital measures for decreasing heating system temperatures. Ultimately, the authors recommend minimising heating system temperatures using automatic balancing of space heating and ventilation systems, novel solutions for safe domestic hot water supply, and digitally-enabled performance monitoring and optimal control.
•Review of the energy and temperature demands for various heating systems.•Review of typical heating malfunctions in buildings served by district heating.•Review of the obtainable supply and return temperatures pre- and post-renovation.•Review of technical improvements to achieve low-temperature operation in buildings.•Key prospects and research areas for improving low-temperature heating systems.
This paper aims to provide an overview of the costs and benefits of preparing existing space heating systems for low-temperature district heating. Necessary heating system improvements were assessed ...based on previous work carried out on the topic. This included evaluating the necessity of installing larger heating elements and improving heating system control. The costs of the identified heating system improvements were then estimated and compared with the savings achieved if district heating temperatures were lowered, from the perspective of both the total Danish energy system and the individual district heating customers. The investigations resulted in simple payback times of 1.2–4.3 years from an energy system perspective and 0.3–18.7 years for an individual district heating customer. The current study thus indicates that it is economically feasible to invest in an improved heating system control to enable a reduction in the district heating return temperatures.
•Low-temperature heating can be implemented at low costs from a long-term perspective.•Investments in heating systems are attractive from an energy system perspective.•Customer investments in heating systems should be motivated economically.
This study presents a field study of the performance of three multi-zone gas-fired comboheating systems that provide space heating and domestic hot water in three new single-family houses in the ...Greater Toronto Area in the Province of Ontario, Canada. Two systems are composed of a gas-fired condensing tankless water heater integrated with a smart air handling unit, while the third system is composed of a gas-fired condensing combi boiler integrated with a smart air handling unit. The performance of the three combo heating systems was monitored, analyzed and evaluated over one heating season. The gas consumption of the three systems were compared with traditional gas-fired heating systems (composed of forced-air gas furnace and gas storage tank water heater). Results showed that combo heating systems tested in this study are reliable, met the peak space heating load, improved the indoor thermal comfort by providing two independently controlled temperature zones and achieved significant gas savings compared to traditional natural gas heating systems. The annual gas consumption and greenhouse gas emissions of the combo heating systems tested in this study are less than the traditional gas-fired heating system (forced-air gas furnace and gas storage tank water heater) by up to 29%.
•The largest solar district heating market worldwide has been presented.•Characters of solar district heating plants in Denmark were summarized.•Possibility of solar district heating in smart ...renewable energy system was shown.•Potential trend of solar district heating plants in Denmark was discussed.
Large solar collector fields are very popular in district heating system in Denmark, even though the solar radiation source is not favorable at high latitudes compared to many other regions. Business models for large solar heating plants in Denmark has attracted much attention worldwide. Denmark is not only the biggest country in both total installed capacities and numbers of large solar district heating plants, but also is the first and only country with commercial market-driven solar district heating plants. By the end of 2017, more than 1.3 million m2 solar district heating plants are in operation in Denmark. Furthermore, more than 70% of the large solar district heating plants worldwide are constructed in Denmark. Based on the case of Denmark, this study reviews the development of large solar district heating plants in Denmark since 2006. Success factors for Danish experiences was summarized and discussed. Novel design concepts of large solar district heating plants are also addressed to clarify the future development trend. Potential integration of large solar district heating plants with other renewable energy technologies are discussed. This paper can provide references to potential countries that want to exploit the market for solar district heating plants. Policy-makers can evaluate the advantages and disadvantages of solar district heating systems in the national energy planning level based on the know-how and experiences from Denmark.
Central solar heating plants contribute to the reduction of CO
2-emissions and global warming. The combination of central solar heating plants with seasonal heat storage enables high solar fractions ...of 50% and more. Several pilot central solar heating plants with seasonal heat storage (CSHPSS) built in Germany since 1996 have proven the appropriate operation of these systems and confirmed the high solar fractions.
Four different types of seasonal thermal energy stores have been developed, tested and monitored under realistic operation conditions: Hot-water thermal energy store (e.g. in Friedrichshafen), gravel-water thermal energy store (e.g. in Steinfurt–Borghorst), borehole thermal energy store (in Neckarsulm) and aquifer thermal energy store (in Rostock). In this paper, measured heat balances of several German CSHPSS are presented. The different types of thermal energy stores and the affiliated central solar heating plants and district heating systems are described. Their operational characteristics are compared using measured data gained from an extensive monitoring program. Thus long-term operational experiences such as the influence of net return temperatures are shown.
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.
•Solar assisted district heating was studied for an existing district heating system.•Performance of distributed and centralised solar collector systems was evaluated.•Centralised collector systems can provide cost savings from 7 to 21%.•Pay-back times between 10 and 11 years for centralised systems were recorded.•Low temperature distribution gave a boost in performance for centralised collectors.
•A generic multivariable thermal-economic optimization approach was developed.•The influences of different aim functions on the varied optimal results were shown.•Sensitivity analysis of the key ...design parameters was carried out.•The optimal heat price could provide practical reference for the end-use consumers.•The system levelized cost of heat can be reduced by 5–9% in this study.
Large-scale solar heating plants for district heating networks have gained great success in Europe, particularly in Denmark. A hybrid solar district heating plant with 5960 m2 flat plate collectors and 4039 m2 parabolic trough collectors in series was built in Taars, Denmark in 2015. The solar heating plant was used as a reference case in this study. A validated TRNSYS-GenOpt model was set up to optimize the key design parameters of the plant, including areas of both collector types, storage size, orientation of the parabolic trough collectors and so on. This study introduces a generic method to optimize the hybrid solar district heating systems based on levelized cost of heat. It is found that the lowest net levelized cost of heat of hybrid solar heating plants could reach about 0.36 DKK/kWh. The system levelized cost of heat can be reduced by 5–9% by use of solar collectors in the district heating network in this study. The results also show that parabolic trough collectors are economically feasible for district heating networks in Denmark. The generic and multivariable levelized cost of heat method can guide engineers and designers on the design, construction and control of large-scale solar heating plants.
Milk protein plays a crucial role in human diets. Thermal modification is one of the most widely used food processing techniques. Traditional heating methods used in food processing, especially for ...heat-sensitive products, may lead to unintended loss of quality. Therefore, it is urgent to preserve the nutritional value, taste, and appearance of milk as much as possible and to improve the utilization efficiency of thermal modifying heat energy during the thermal processing of dairy products.
This review provides an up-to-date overview of studies focusing on the fundamental principles, features, and impacts on the structure and functional characteristics of dairy proteins. Furthermore, it provides insights into the current application status of five emerging thermal technologies: ohmic heating, microwave heating, radio frequency heating, infrared heating, and extrusion heating. Additionally, this review highlights the existing problems of these novel thermal technologies and presents potential solutions.
These technologies modify protein structures, enhancing their functional and biological characteristics. At the same time, microwave heating is studied by most countries because of its simple operation and remarkable modification effect. Although microwave heating has the potential to be applied to large-scale industrial dairy processing, it is crucial to explore the characteristics of other technologies to broaden the thermal modifying methods.
•The essence of most emerging thermal technology is electric heating.•The emerging technology directionally changes the secondary structure of protein.•Microwave treatment has great potential in reducing milk protein allergy.•There is the closest cooperation in microwave heating research among countries.
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