Svend Svendsen

• MSc in Civil Eng., PhD
• Professor (Full) at Technical University of Denmark

With regard to the green transition toward 4th generation district heating (4GDH), a critical element is ensuring low operating temperatures in networks. This can help leverage the technical and economic potential of connecting renewable energy sources and recovering excess local heat. Poorly controlled and operated heating systems in existing buil...

Storage tanks are commonly used for domestic hot water (DHW) preparation in large buildings supplied by district heating (DH), especially to cope with peak demand. The charging control of DHW tank systems is often suboptimal, increasing return temperatures and harming the overall DH operation efficiency. This paper presents two novel control concep...

The digitalization of the demand side increased significantly in the last few years. This was mainly due to the impulse of the European Energy Efficiency Directive (EED 2012/2018), binding member states to have all energy meters remotely readable by January 2027. In collaboration with the Danish industrial partners and the local district heating (D...

Future low-temperature district heating systems (LTDH) require supply and return temperatures as low as 55 °C and 25 °C, respectively. In this direction, the use of night setbacks in heating of office buildings is a problem. This article investigated in a typical office building with large distribution system by tests and simulations the energy and...

Low-temperature district heating (LTDH) networks can integrate sustainable energy sources and waste industrial heat towards decarbonisation goals by 2050. LTDH networks can be realised through the low-temperature operation of heating systems in buildings. However, the low-temperature operation of heating systems is obstructed by inefficient radiato...

Modernization of the existing district heating systems is a necessary activity to improve their energy efficiency in the field of generation, distribution and use of the heat. This paper presents the results of several years of field research carried out for 13 multi-family buildings supplied from the district heating network. The benefits associat...

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 grea...

Automated hydronic balancing in space heating systems is crucial for the fourth-generation district heating transition. The current manual balancing requires labor- and time-consuming activities. This article presents the field results of an innovative electronic radiator thermostat tested on two Danish multi-family buildings. The prototypes had an...

With international awareness of the need to decrease greenhouse gas emissions, the Danish government set a target to be fossil fuel-free by 2050. To achieve that, existing buildings will need to be retrofitted with energy-saving technologies such as improved thermal insulation. In Denmark, a larger mass of the building stock from around 1850 to the...

The study proposed and investigated a new concept for hydronic floor heating in dwellings with the aim of reducing hot water temperatures toward a more robust and energy efficient operation. Modern heating systems often rely on low return temperatures to improve operation efficiencies through reduced heat losses from return pipes, greater utilisati...

With increasing focus on the performance of district heating systems, a concept is developed to obtain low district heating return temperatures from domestic hot water systems with a high share of circulation loss. For these systems, it is challenging to realize a low district heating return temperature by direct heat exchange only, due to the high...

Edited by Kristina Lygnerud and Sven Werner This guidebook was written between 2018 and 2021 by seventeen authors that used approximately 15 000 hours of work within the IEA DHC TS2 annex. The content is based on more than 250 literature references and 165 inspiration initiatives to obtain lower temperatures in buildings and heat distribution netw...

The operation of typical domestic hot water (DHW) systems with a storage tank and circulation loop, according to the regulations for hygiene and comfort, results in a significant heat demand at high operating temperatures that leads to high return temperatures to the district heating system. This article presents the potential for the low-temperatu...

The low-temperature operation of existing radiator systems connected to district heating is hindered by various factors, including non-ideal local control by users. The use of only a few of the available radiators to provide the necessary thermal comfort in an apartment is a common example. This requires a higher supply temperature and results in a...

In order to reach targeted 4th generation district heating temperatures around 55 °C supply and 25 °C return, it is necessary to ensure that heating installations inside buildings are designed and operated properly. In this study we investigated the best-case of current design and operation of building installations with the aim of identifying whet...

Industrial exhausted heat can be used as the heat source of central heating for higher energy efficiency. To recover more industrial exhausted heat, a new low-temperature central heating system integrated with industrial exhausted heat using distributed electric compression heat pumps is put forward and analyzed from the aspect of thermodynamics an...

This study investigated a double loop network operated with ultra-low supply/return temperatures of 45/25 °C as a novel solution for low heat-density areas in Denmark and compared the proposed concept with a typical tree network and with individual heat pumps to each end-users rather than district networks. It is a pump-driven system, where the sep...

Thermal treatment for sterilising domestic hot-water systems requires a circulation temperature higher than 50 oC, 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 sy...

In the district heating (DH) systems, the thermal storage tank is widely used for domestic hot water (DHW) preparation to shave the peak load. Compared to the space heating, the domestic hot water demand exists for all seasons, and the variation of which is more dramatic within short time span. Therefore, the well-controlled storage tank is of grea...

The hot water tank is widely used for domestic hot water (DHW) preparation. When supplied by district heating (DH), the operation of DHW tanks directly influences the DH return temperature, thereby affecting the DH system efficiency, supply capacity and the realization of the low temperature district heating. However, the conventional charging meth...

The IEA EBC Annex 64 – LowEx Communities was a three year international research project which aims to demonstrate the potentials of low exergy thinking on a community level as energy and cost efficient solutions to achieve 100 % renewable and GHG emission-free energy systems. Central challenges are the identification of promising and efficient tec...

Low-temperature district heating will play an important role in a future free of fossil fuels. This will only be able to be realized through the low-temperature operation of heating systems in existing buildings. Existing radiator systems can operate with low temperatures for most of the year because they are designed for extremely cold days, but e...

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 sys...

Proper heating system operation in buildings is a vital area for the realization of low-temperature district heating with supply and return temperatures of 55 °C and 25 °C respectively. But the operating area of the district heating companies usually only extends to the substations at the entry of the buildings, and very little is known about the a...

This study analyses the heat load in a total of 11,584 rooms in 1645 Danish houses and the heat output of the radiators in them to evaluate whether typical radiators are over-dimensioned. The aim was to find out whether radiators in existing houses are suited for low-temperature district heating. We found that new houses are generally more likely t...

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 integ...

Low temperature district heating (DH) system gives easier access to the renewable energy as heat sources and improves the heat distribution efficiency. From the exergy point of view, low DH supply temperature also better matches the exergy demand of space heating and domestic hot water. It is more beneficial to operate district heating system under...

The efficiency of district heating systems is greatly affected by network supply and return temperatures. However, the opportunities to lower the temperatures and thereby increase network efficiency are restricted by customer installations. Very little is known about the customer installations, because heating system operation is only rarely monito...

This Excel add-in involving of user-defined-functions that calculate the friction pressure loss in circular pipes with full flow water, with options to select through different solver types ('Darcy-Weisbach' or 'Hazen-Williams') and various Darcy friction factor algorithms ('Moody', 'Colebrook-White', 'Clamond', 'Swamee-Jain', 'Zigrang-Sylvester',...

This Matlab tool calculates the friction pressure loss in circular pipes with full flow water, with options to select through different solver types ('Darcy-Weisbach' or 'Hazen-Williams') and various Darcy friction factor algorithms ('Moody', 'Colebrook-White', 'Clamond', 'Swamee-Jain', 'Zigrang-Sylvester', and 'Haaland'). Besides, there are othe...

The large price drop in solar PV and electrical batteries offer new opportunities for optimizing district energy plants, but requires a more complex daily operation of these plants. Solar PV production used locally by a ground source heat pump (GSHP) with a minimal use of the national grid is one opportunity. Even if, for the benefit of the GSHP, t...

The renovation of single-family houses in Denmark is progressing only slowly. Changes in current policy are needed if the political goal of a fossil-free building sector as part of a fossil-free society is to be achieved. Known barriers and motivators for energy renovation are identified, and arranged in a framework with three main fields (Informat...

This paper presents results of a research study into improving energy performance of small-scale district heat network through water supply and return temperature optimization technique. The case study involves establishing the baseline heat demand of the estate’s buildings, benchmarking the existing heat network operating parameters, and defining...

District heating is predicted to play a large role in the future fossil free energy system. Apart from providing energy savings by utilizing surplus heat, the district heating system also provides flexibility to fluctuating electricity generation by bridging the electricity and the heating sector. These benefits can be maximized if district heating...

The building sector is responsible for more than one third of the final energy consumption of societies and produces the largest amount of greenhouse gas emissions of all sectors. This is due to the utilisation of combustion processes of mainly fossil fuels to satisfy the heating demand of the building stock. Low temperature district heating (LTDH)...

District heating (DH) is a cost-effective method of heat supply, especially to area with high heat density. Ultra-low-temperature district heating (ULTDH) is defined with supply temperature at 35-45 °C. It aims at making utmost use of the available low-temperature energy sources. In order to achieve high efficiency of the ULTDH system, the return t...

Building renovation is too often carried out with only one objective: necessary maintenance, updating design and functions, or reducing energy consumption. But, if a necessary maintenance is exploited as an opportunity for renovation, energy improvements can be implemented, house functions can be updated, and indoor climate improved with minimal nu...

One way of reducing the use of fossil fuels in Denmark is to explore possible energy savings in the building stock, especially the large number of single-family houses built from 1960 through 1980. Energy renovation in this housing segment is progressing slowly. The aim of this project was to determine how a one-stop-shop (OSS) or full-service conc...

Roof windows are efficient and flexible daylight sources that are essential in certain types of houses if they are to achieve sufficient daylighting throughout. Previous studies have indicated that, for such buildings to meet nearly zero-energy targets in an easy and robust way without compromising on daylighting and thermal comfort, the thermal pr...

Densely occupied spaces such as classrooms can very often have problems with overheating. It can be difficult to cool such spaces by means of a ventilation system without creating draughts and causing discomfort for occupants. The use of a wall radiant cooling system is a suitable option for spaces with a high occupant density. Radiant systems can...

The development of DH (District Heating) is an environmentally friendly and energy-efficient strategy in China. Currently, the vast majority of DH systems are SH (Space Heating) only and do not provide DHW (Domestic Hot Water). DHW is mainly produced by individual water heaters due to the cost-effective issues of the centralized DHW systems. From t...

Dynamic solar shading is commonly suggested as a means of reducing the problem of overheating in well-insulated residential buildings, while at the same time letting daylight and solar irradiation in when needed. To critically investigate what dynamic shading can and cannot do compared to permanent alternatives in buildings with very low space-heat...

This study presents a method to adapt existing hydronic systems in buildings to take advantage of low temperature district heating (LTDH). Plate radiators connected to double string heating circuits were considered in an optimization procedure, based on supply and return temperatures, to obtain the required logarithmic mean temperature difference (...

Current research and development shows that it is technical feasible to change high/medium temperature district heating system to low temperature district heating system for both new and existing building areas. We collect and identify promising technologies in low-temperature district heating application to meet the goals of future renewable based...

District heating is predicted to play a large role in the future fossil free energy system. Apart from providing energy savings by utilizing surplus heat, the district heating system also provides flexibility to fluctuating electricity generation by bridging the electricity and the heating sector. These benefits can be maximized if district heating...

This study investigated the performances of five different substation configurations in single-family houses supplied with ULTDH (ultra-low-temperature district heating). The temperature at the heat plant is 46 °C and around 40 °C at the substations. To avoid the proliferation of Legionella in the DHW (domestic hot water) and assure the comfortable...

Hydraulic unbalance is a common problem in Chinese district heating (DH) systems. Hydraulic unbalance has resulted in poor flow distribution among heating branches and overheating of apartments. Studies show that nearly 30% of the total heat supply is being wasted in Chinese DH systems due to a lack of pressure and flow control. This study investig...

A common problem with Chinese district heating systems is that they supply more heat than the actual heat demand. The reason for this excess heat supply is the general failure to use control devices to adjust the indoor temperature and flow in the building heating systems in accordance with the actual heat demand. This results in 15–30% of the tota...

By introducing better interfaces between the demand and supply, district heating and cooling (DHC) systems can be transformed into a smart grid energy system on a district level. In the IEA DHC Annex TS1 project, the Subtask C collects and identifies promising models, concepts, and technologies to meet the goals of future renewable based community...

Low-temperature district heating is a promising technology for providing homes with energy-efficient heating in the future. However, it is of great importance to maintain thermal comfort in existing buildings when district heating temperatures are lowered. This case study evaluated the actual radiator sizes and heating demands in 4 existing Danish...

Low-temperature heating provides an efficient way of heating our buildings. To obtain a high efficiency it is important that the heating systems in the buildings are operated with both low supply and return temperatures. This study set out to investigate how typical assumptions in the modelling of heat emissions from existing hydraulic radiators af...

As existing buildings are renovated and energy-efficiency measures are implemented to meet requirements for reduced energy consumption, it becomes easier to heat our homes with low-temperature heating. This study set out to investigate how much the heating system supply temperature can be reduced in typical Danish single-family houses constructed i...

Windows are central for the development of liveable nearly zero-energy homes and require careful consideration. Various studies have indicated that the effect of windows on energy consumption may change significantly with improved building insulation levels. Current guidelines on windows may therefore not apply in very well-insulated buildings, and...

The use of internal insulation is investigated in a heritage building block with wooden beam construction and masonry brick walls as part of an energy renovation. Measurements were carried out and compared to results from a hygrothermal simulation model. The risk of mould growth in the wooden beams and in the interface between the insulation and th...

To improve energy efficiency and give more access to renewable energy sources, low-temperature district heating (LTDH) is a promising concept to be realized in the future. However, concern about Legionella proliferation restricts applying low-temperature district heating in conventional systems with domestic hot water (DHW) circulation. In this stu...

The investigation constructed and simulated moisture balance equations for single-room ventilation with a non-hygroscopic rotary heat exchanger. Based on literature, the study assumed that all condensed moisture in the exhaust subsequently evaporated into the supply. Simulations evaluated the potential for moisture issues and compared results with...

Low temperature district heating (LTDH) is a cost-efficient way of supplying space heating and domestic hot water (DHW) for buildings in urban areas. However, there is concern that the potential hygiene problems (Legionella) might occur if LTDH is implemented, especially for large buildings. In this study, electric heat tracing system was investiga...

District heating is a cost-effective way of providing heat to high heat density areas. Low-temperature district heating (LTDH) is a promising way to make district heating more energy-efficient and adaptable to well-insulated buildings with low heating demand in the future. However, one concern is the multiplication of Legionella due to insufficient...

Appropriate window solutions are decisive for the design of ‘nearly zero-energy’ buildings with healthy and comfortable indoor environment. This paper focuses on the relationship between size, orientation and glazing properties of façade windows for different side-lit room geometries in Danish ‘nearly zero-energy’ houses. The effect of these parame...

The chapter presents a method for determining the optimal capacity of a renewable-energy-based energy conversion systems for meeting the energy requirements of a given district as considered on a monthly basis, with use of a low-temperature district heating system operating at a low temperatures, as low as 55°C for supply and 25°C for return and wi...

Rooms with a high density of occupants inevitably have high internal heat gains. It is possible to remove large sensible internal heat gains solely by radiant cooling systems if large areas of internal surface can be activated for radiant cooling. The ventilation system then only has to supply the small amount of fresh air required by standards to...

The existing building stock will likely undergo widespread energy renovations to meet future emissions targets. Single-room ventilation may enable the process due to its simple installation, low fan power, and potential for local heat recovery. A short plastic rotary heat exchanger is developed for single-room ventilation based on thermal design th...

The hypothesis investigated in this article is: it is possible to carry out moisture safe energy renovations in the old existing multi-storey buildings with heritage value and still save 50% of the building's energy consumption by use of existing technologies. A holistic energy renovation on an old multi-storey building with heritage value was carr...

This paper presents a method for the dimensioning of the low-temperature District Heating (DH) piping networks operating with a control philosophy of supplying heat in low-temperature such as 55 °C in supply and 25 °C in return regularly while the supply temperature levels are being boosted in cold winter periods. The performance of the existing ra...

Regardless of where they are in the world, people depend on a reliable and sufficient supply of domestic hot water (DHW) for daily use. Some countries that have district heating (DH) infrastructure, such as Denmark and China, combine spacing heating (SH) and DHW together, with the aim of having a smart, energy efficient and environmentally friendly...

Regardless of where they are in the world, people depend on a reliable and sufficient supply of domestic hot water (DHW) for daily use. Some countries, which have district heating infrastructure, combine spacing heating (SH) and DHW together, with the aim of having a smart, energy efficient and environmentally friendly energy-consumption system, su...

In the design of very well-insulated homes, there is a need for a more nuanced design that takes into account winter and summer conditions. In this paper, we compare a traditional design for a typical Danish single-family house with large glazing areas oriented towards the south and smaller glazing areas towards the north, and a design with an even...

In the context of the EU project "Sinfonia", that aims to reduce Bolzano's primary energy consumption up to 40%, SEL AG, an Italian energy company is planning to extend the existing district heating and cooling (DHC) network and explore strategies to improve efficiency, environmental, and economic performance. This research aims to assess the poten...

The Danish government plans to make the Danish energy system to be completely free of fossil fuels by 2050 and that by 2035 the energy supply for buildings and electricity should be entirely based on renewable energy sources. To become independent from fossil fuels, it is necessary to reduce the energy consumption of the existing building stock, in...

This paper defines the concept of 4th Generation District Heating (4GDH) including the relations to District Cooling and the concepts of smart energy and smart thermal grids. The motive is to identify the future challenges of reaching a future renewable non-fossil heat supply as part of the implementation of overall sustainable energy systems. The...

Using a bypass to redirect a small flow through the in-house DH (district heating) substation directly to the return pipe is a commonly used but energy-inefficient solution to keep the DH network “warm” during non-heating seasons. Instead, this water can be redirected to the bathroom FH (floor heating) to cool down further and thus reduce the heat...

Aim This paper presents a two-fold evaluation method determining whether to renovate an existing building or to demolish it and thereafter erect a new building. Scope The method determines a combination of energy saving measures that have been optimised in regards to the future cost for energy. Subsequently, the method evaluates the cost of undert...

Denmark has a long-term objective of being free of fossil fuels by 2050, with the energy supply mix for buildings being fossil-free by 2035. Energy consumption for existing buildings needs to be decreased concurrent with the conversion from fossil-fuel supply to renewable-energy (RE) supply. When end-use savings are implemented in buildings concurr...

In Nordic countries significant primary energy saving potential exists in houses built before 1980. These old houses need to be renovated, which provides an opportunity for implementation of energy efficiency measures. However, there are several economic and market hindrances and the renovation markets are dominated by handicraft-based individual s...

Denmark is aiming for a fossil-free heating sector for buildings by 2035. Judging by the national heating plan, this will be achieved mainly by a further spread of DH (district heating) based on the renewable heat sources. To make the most cost-effective use of these sources, the DH supply temperature should be as low as possible. We used IDA-ICE s...

In this paper, the configuration of a district heating network which connects from the heating plant to the end users is optimized. Each end user in the network represents a building block. The connections between the heat generation plant and the end users are represented with mixed integer and the pipe friction and heat loss formulations are non-...

This paper describes an investigation of the effect on electric lighting demand of applying occupancy models of various resolution to climate-based daylight modelling. The lighting demand was evaluated for a building zone with the occupant always present, with occupancy corresponding to absence factors, based on an estimated annual mean occupancy,...

Low-temperature district heating systems, operating at low temperature of 55 °C as supply and 25 °C as return, can be the energy solution as being the prevailing heating infrastructure in urban areas, considering future energy schemes aiming at increased exploitation of renewable energy sources together with low-energy houses in focus with intensif...

This paper evaluates the daylighting performance of a prototype external dynamic shading and daylight-redirecting system, and the main focus is on the performance simulation. The demonstration project was carried out on a building with an open-plan office. Part of the original façade was replaced with the prototype façade. This layout allowed the u...

Climate-based daylight modelling is based on the available weather data, which means that the weather data used as input to the daylight simulations are of great importance. In this paper, the effect on the outcome of the daylight simulations of using one weather data file rather than another for the same location was investigated. Furthermore, the...

The thermal performance of radiant heating and cooling systems (RHCS) composed of capillary micro tubes (CMT) integrated into the inner plate of sandwich elements made of high performance concrete (HPC) was investigated in the article. Temperature distribution in HPC elements around integrated CMT was studied. Thermal heat flux on the inner surface...

In many locations mechanical ventilation has been the most widely used principle of ventilation over the last 50 years but the conventional system design must be revised to comply with future energy requirements. This paper examines the options and describes a concept for the design of mechanical ventilation systems with minimal pressure loss and m...

An experimental study is reported in this paper for a diffuse ceiling ventilation concept. The analyses were carried out with two different porous surfaces mounted in a suspended ceiling: perforated tiles of aluminium and of gypsum. Ventilation air was supplied above the suspended ceiling effectively creating a plenum for air distribution. The expe...

The thermal performance of radiant heating and cooling systems (RHCS) composed of capillary micro tubes (CMT) integrated into the inner plate of sandwich elements made of High Performance Concrete (HPC) was investigated in the article. Temperature distribution in HPC elements around integrated CMT was studied. Thermal heat flux on the inner surface...

Low-Temperature District Heating (DH) systems, providing great energy savings by means of very low operating temperatures of 55 °C and 25 °C for supply and return respectively, were considered to be the 4th generation of the DH systems for a low-energy future. Low-temperature operation is considered to be used in a low-temperature DH network to car...

The low-energy district heating concept has the potential of increasing the energy and exergy efficiencies of heat supply systems and of exploiting renewable energy, provided technical solutions for its wide application can be developed and implemented. This article investigates the dynamic behaviour of district heating branch pipes in low-temperat...

The preponderant number of multistorey buildings constructed in Denmark in the period between 1850 and 1930 were built with masonry walls incorporating wooden floor beams. Given the nature of this construction, it is supposed that significant energy savings could be achieved by simply insulating the facades of such buildings. To maintain the exteri...

In theory, there is huge potential for reducing the energy consumed by existing single-family houses by thoroughly renovating them. For the successful market development of highly energy-efficient integrated renovations, supply chain collaboration is very important, while at the same time customer demand for integrated renovations has to be stimula...

The study deals with low-temperature District Heating (DH) networks operating in low temperatures such as 55 °C in terms of supply and 25 °C in terms of return. The network layout, additional booster pumps, and different substation types such as storage tanks either equipped or not equipped in domestic hot water production site were examined. Effec...

The control strategy for mechanical ventilation systems has significant impact on the performance of the system, in terms of energy consumption and correct air distribution. This paper presents a static pressure reset control system employing a new type of flow damper with lower pressure loss for use in low pressure ventilation systems. The flow da...

Low-temperature operation in low-temperature District Heating (DH) systems is rewarding for increased exploitation of low-temperature renewable energy sources, heightened efficiency at heat extraction, and intensified energy efficiency at heat distribution. Success of heat delivery in low-temperature operation such as 55 °C in terms of supply and 2...

This study focused on investigation of non-fossil fuel heat sources to be supplied to low-temperature district heating systems operating in low temperature such as 55°C and 25°C in terms of, respectively, supply and return. Vast variety of heat sources classed in categories such as fossil fuel, renewable energy, and nuclear energy was evaluated in...

Low temperature district heating with reduced network supply and return temperature provides better match of the low quality building heating demand and the low quality heating supply from waste heat or renewable energy. In this paper, a hypothetical low temperature district heating network is designed to supply heating for 30 low energy detached r...

This paper discusses the opportunities and challenges of implementing District Heating (DH) in Canada, with focus on the network design and operation. We selected for case study an urban area in Ottawa. First, we proved that the medium-temperature district heating (MTDH) (70 °C ≤ Tsupply ≤ 90 °C) had better energy delivery performance than high-tem...

Low-Temperature District Heating (DH) systems with low-temperature operations, such as 55 °C in terms of supply and 25 °C in terms of return, were considered to be the 4th generation of the DH systems for the low-energy future with energy efficiencies focused to be achieved at newly built and existing buildings by the Danish building regulations. T...

This article evaluates the performance of an integrated micro structural perforated shading screen (MSPSS). Such a system maintains a visual connection with the outdoors while imitating the shading functionality of a venetian blind. Building energy consumption is strongly influenced by the solar gains and heat transfer through the transparent parts...