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PhD theses







Author: Christian Milan
See full thesis (preliminary)

Academic supervisor: Mads Pagh Nielsen, Carsten Bojesen

Industrial Advisor: Ivan Katic, Danish Technological Institute


  • Paper 1: An optimization methodology for the design of renewable energy systems for residential net zero energy buildings with on-site heat production. Christian Milan; Carsten Bojesen; Mads Pagh Nielsen. Proceedings of the 6th Dubrovnik Conference on Sustainable Development of Energy Water and Environmental Systems.


  • Paper 2: A cost optimization model for 100% renewable residential energy supply systems. Christian Milan; Carsten Bojesen; Mads Pagh Nielsen. Energy, Vol. 48, Nr.1, 2012, p.118-127
  • Paper 3: Choosing the right technology – Optimized design of renewable supply systems for residential houses. Christian Milan; Carsten Bojesen; Mads Pagh Nielsen. Proceedings of the 11th International Conference on Sustainable Energy Technologies.


  • Paper 4: Local versus National – Designing Supply Systems for Individual Net Zero Energy Buildings with flexible electricity prices. Christian Milan; Carsten Bojesen; Mads Pagh Nielsen. Paper presented at International Conference on Energy and Sustainability.


  • Paper 5: Energy management strategy based on short-term generation scheduling for a renewable micro-grid using a hydrogen storage system. Mario Petrollese; Giorgio Cau; Daniele Cocco; Søren Knudsen Kær; Christian Milan. Journal: Energy Conversion and Management
  • Paper 6: Modelling of non-linear CHP efficiency curves in distributed energy systems. Christian Milan; Michael Stadler; Gonçalo Cardoso. Draft ready for submission at IEEE.

This work presents a methodology to identify and investigate the cost optimal design of supply systems for Low and Net Zero Energy Buildings with the focus on residential single family houses. A preliminary analysis investigating relevant literature and existing computer tools resulted in the conclusion that for this specific scope a lack of adequate approaches existed. It is briefly discussed, why linear programming and the software platform GAMS have been chosen to define the optimization problem. In the following the concept and structure of the methodology is explained with the main required input data and resulting outcomes. The theoretical part describes the general equations of the approach and summarizes the parameter set, which is applied to model individual technologies.
The next chapter describes each considered supply option in detail with relevant cost and technical data. Further, individual performance models are defined. For small scale residential systems the hot water tank is one of the main components, connecting supply and demand side and acting as a buffer during mismatch periods. For this reason, the developed hot water tank model is rather detailed accounting for three different temperature layers, two different supply and demand loops as well as individual heat losses. It is presented at the end of the technology chapter. Subsequently, the methodology is validated by investigating the output with one single technology at a time and thus the individual performance models in a case study. It is found that resulting energy generation rates are in reasonable ranges and according to the performance models defined throughout this work.
As an alternative option to validate the proposed methodology an experimental setup has been designed as a part of this work, which features most of the investigated technologies and would allow the comparison with performance under real conditions and different consumption patterns. However, the setup could not be taken into operation within the timeframe of this thesis but is briefly described in Chapter 5. This is followed by a summary of the main findings obtained during different case studies and which have been published and presented as a part of this thesis. It was concluded that lowering the 100% Net ZEB demand would lead to a reduction of overall system costs and that local optimal system solutions do not necessarily contradict with public grid interests, when the control is adapted. Further, results show that the assumption of constant efficiencies for CHP technologies does not lead to large changes in investment decisions. However, considering flexible efficiencies might be important when optimizing operational schedules of an existing system involving fuel cells as their part load operation played a moderate role in the optimal solution of the conducted case studies. Additionally, a method is presented, which accounts for uncertainties in user behavior and weather profiles.

A Model for Enquiry of Sustainable Homes – of Model Home 2020

Author: Gitte Gylling Hammershøj Olesen, VELUX a/s, VKR Holding, Aalborg University
See full thesis.

Academic supervisor:
Professor, Architect MAA Mary-Ann Knudstrup, Department of Architecture and Media Technology, Aalborg University
Professor Per Heiselberg, Department of Civil Engineering, Aalborg University

Industrial Advisor:
Engineer Jens Christoffersen, Department of Daylight, Energy and Indoor Climate, VELUX a/s
Architect MAA Ellen Kathrine Hansen, VKR Holding


  • G.G.H. Olesen, M.A. Knudstrup, P. Heiselberg, E.K. Hansen, Measuring Sustainable Homes – A Mixed Methods Approach, Conference proceedings: Considering Research: Reflecting upon current themes in architectural research, Architectural Research Centres Consortium, Detroit, USA, 2011
  • G.G.H. Olesen, M.A. Knudstrup, P. Heiselberg, E.K. Hansen, Holistic Evaluation of Sustainable Buildings through a Symbiosis of Quantitative and Qualitative Assessment Methods, Conference Proceeding of 27th Conference on Passive and Low Energy Architecture PLEA 2011, Conference proceedings: Architecture and Sustainable Development, vol.2, Belgium, 2011

From this question the article ‘Enquiring Perceived Quality in Sustainable Architecture: A More Tangible Approach’ (Olesen & Knudstrup, 2013b) forms a scheme for collection, treatment and dissemination. Conclusions are that focus on daylight, fresh air, every-day functionalism and natural resources add to the perceived quality of the houses.
Acquired knowledge is compiled in a holistic model of enquiry which is applied through empirical testing of indoor environment in three Model Home 2020 homes:
IV. How can indoor environment in sustainable homes be enquired through respectively occupant perspectives, perceptual quality and technical ability; so the approaches supplement each other and establish a holistic illustration of the sustainability unfolded?

The article ‘Exploring a Model for Enquiry of Sustainable Homes through Indoor Environmental Aspects’ (Olesen et al. 2014) suggests and empirically enquire a holistic model that compiles data from four knowledge fields respectively, architectural field studies, occupant blogs, occupant questionnaires and technical measurements on perceived indoor environment. Conclusions are that the houses generally have high quality indoor environments and that the variety of methods are appropriate for capturing dissimilar aspects of sustainable life-form and thereby supplement each other in creating a more holistic illustration sustainable homes in-use.
Future sustainable buildings are not merely optimized mechanical constructions with intelligent adjustment systems but houses that imply and require quality in their environments to support and embrace life displayed in and around them. Therefore, it is becoming increasingly central to develop more holistic approach to enquiry and thereby the understanding of sustainable environments is viewed in balance between perceptual qualities as well as technical abilities. Results of this thesis are thus:

i. A model for enquiry of sustainable homes that include occupant perspectives, perceptual quality and technical ability.

ii. Occupant perspectives of perceived everyday encounters with automated homes through a combination of questionnaire and blog enquiry.

iii. Enclosure of perceived quality in enquiry of sustainable architecture and a more tangible approach to collect, treat and explore aspects of perceptual nature.

iv. Focus on aspects of variability in enquiry of sustainable architecture.

This Industrial PhD project is formed in collaboration between holding and investment company VKR Holding A/S, International roof window producer and thought leader VELUX A/S, the Danish Board of Research and Innovation and Department of Architecture, Design and Media Technology, Aalborg University. The project revolves around the Active House vision and three demonstration houses in-use developed within the frame of VELUX based Model Home 2020 project.
The objective of this thesis is to develop a Model for Enquiry of Sustainable Homes. The purpose is to establish multi-perspective enquiry of inhabited sustainable homes based on intention to create a more complete illustration of sustainable life but the technical measurable ones development of sustainable architecture is mainly driven by today.
Thus, the main research question is:

I. How can a model for enquiry of sustainable homes based on a mixed methods approach include occupant perspectives, perceptual quality and technical ability; so the approaches supplement each other and establish a holistic illustration of the sustainability unfolded?

To explore how such a model can be compiled the research enquire how sustainable homes in-use can be evaluated from different fields of knowledge and methods. As a first step to explore this, aspects of occupant perspectives are enquired:

II. How are everyday encounters with sustainable functionalism perceived by occupants, and what aspects does this bring to an automated, sustainable life-form which is probably a circumstance of the future?

Based on the research question the article ‘Encountering Sustainable Functionalism: Feedback as a Method to Raise Awareness on Energy Use and Indoor Environment in Automated Homes’ (Olesen et al., 2013) explores how a multiple method approach including questionnaire survey and blog posts can provide information on experiences of life in sustainable homes. The article concludes that providing occupants’ information on energy and comfort support and motivate their ability to pursue sustainable life-form.
Secondly, aspects of perceived quality are enquired:

III. Aspects of perceived quality are central to create value for human beings in the built environment of the future, but how can perceived quality in sustainable architecture be registered, analysed, weighed up and conveyed without losing their qualitative nature? 

Energy flow and thermal comfort in buildings - Comparison of radiant and air-based heating & cooling systems

Author: Jérôme Le Dréau
See full thesis.

Academic supervisors:
Professor Per Heiselberg, Department of Civil Engineering, Aalborg University
Associate Professor Rasmus Lund Jensen, Department of Civil Engineering, Aalborg University


  • J. Le Dréau, P. Heiselberg, Potential use of radiant walls to transfer energy between two building zones. Proceedings of ISHVAC. Vol. 1 Tsinghua University Press (2011) p. 120-126.
  • J. Le Dréau, P. Heiselberg, Comparison of the thermal performances of radiative and convective terminals: A conceptual approach. PLEA 2012 Lima Peru - Opportunities, Limits & Needs: Towards an environmentally responsible architecture (2012).
  • J. Le Dréau, P. Heiselberg, Sensitivity analysis of the thermal performance of radiative and convective terminals. Submitted to Energy and Buildings.
  • J. Le Dréau, P. Heiselberg, R.L. Jensen, A full-scale experimental set-up for assessing the energy performance of radiant wall and active chilled beam for cooling buildings. Submitted to Building Simulation: An International Journal.
  • J. Le Dréau, P. Heiselberg, R.L. Jensen, Experimental investigation of convective heat transfer during night cooling with different ventilation systems and surface emissivities. Energy and Buildings 61 (2013) p. 308-317.
  • J. Le Dréau, P. Heiselberg, R.L. Jensen, Experimental investigation of the influence of the air jet trajectory on convective heat transfer with air-based and radiant cooling systems. Submitted to Journal of Building Performance Simulation.

Heating and cooling terminals can be classified in two main categories: convective terminals (e.g air conditioning, active chilled beam, fan coil) and radiant terminals. The two terminals have different modes of heat transfer: the first one is mainly based on convection, whereas the second one is based on both radiation and convection. Radiant terminals have the advantage of making use of low grade sources (i.e. low temperature heating and high temperature cooling), thus decreasing the primary energy use of buildings. But there is a lack of knowledge on the heat transfer from the terminal towards the space and on the parameters influencing the effectiveness of terminals. Therefore, the comfort conditions and energy need of four types of terminals (active chilled beam, radiant floor, wall and ceiling) have been compared for a typical office room, both numerically and experimentally. This thesis addressed mainly the cooling case.

From the steady-state numerical analysis and the full-scale experiments, it has been observed that the difference between the two types of terminals is mainly due to changes in the ventilation losses (or gains). At low air-change rates (below 0.5 ACH), radiant and air-based terminals have similar energy needs. For higher air change rate, the energy need of radiant terminals is lower than that of air-based terminals due to the higher air temperature. At 2 ACH, the energy savings of a radiant wall can be estimated to around 10 % compared to the active chilled beam (in terms of delivered energy). The asymmetry between air and radiant temperature, the air temperature gradient and the possible short-circuit between inlet and outlet all play a role equally important in decreasing the cooling need of the radiant wall compared to the active chilled beam. The higher the air change rate and the warmer the outdoor air, the larger the savings achieved with a radiant cooling terminals. Therefore, radiant terminals have a large potential of energy savings for buildings with high ventilation rates (e.g. shop, train station, industrial storage). Among radiant terminals, only small differences have been observed for the geometry considered. Only if the occupants are assumed to be sitting, the large view factor with the floor can lead to a reduction of the energy need for floor cooling systems.
These conclusions are valid for multi-storey and/or highly insulated buildings (R > 5 m2.K/W). In case of single-storey building with a low level of insulation, the effectiveness of radiant terminals is lower due to the larger back losses, and an air-based terminal might be more energy-efficient than a radiant terminal (in terms of delivered energy).

Regarding comfort, a similar global level has been observed for the radiant and air-based terminals in both numerical and experimental investigations. But the different terminals did not achieve the same uniformity in space. The active chilled beam theoretically achieves the most uniform comfort conditions (when disregarding the risk of draught), followed by the radiant ceiling. The least uniform conditions were obtained with the cooled floor due to large differences between the sitting and standing positions. Local comfort conditions (radiant asymmetry, vertical air temperature gradient, risk of draught) have also been evaluated both theoretically and numerically, and no discomfort has been observed for normal cooling needs. 

Spatial Heat Planning and Heat Demand Reductions in

Author: Steffen Nielsen, Aalborg University
See full thesis.

Academic supervisors:
Bernd Möller, Department of Planning, Aalborg University
Henrik Lund, Department of Planning, Aalborg University

Industrial Advisor: AffaldVarme Aarhus


  • A Geographic Method for High Resolution Spatial Heat Planning. / Nielsen, Steffen. In: Energy, Vol. 67, 2014, p. 351-362.
  • GIS based analysis of future district heating potential in Denmark. / Nielsen, Steffen; Möller, Bernd. In: Energy, Vol. 57, 2013, p. 458-468.
  • Excess heat production of future net zero energy buildings within district heating areas in Denmark. / Nielsen, Steffen; Möller, Bernd. In: Energy, Vol. 48, No. 1, 2012, p. 23-31.


  • Heat Mismatch of future Net Zero Energy Buildings within district heating areas in Denmark. / Nielsen, Steffen; Möller, Bernd. 2011. Paper presented at 6th Dubrovnik Conference on Sustainable Development of Energy, Water and Environment Systems, Dubrovnik, Croatia.
  • GIS Based Analysis of future district heating potential in Denmark. / Nielsen, Steffen; Möller, Bernd. 13th international symposium on district heating and cooling: 3rd of September - 4th of September, Copenhagen, Denmark. District Energy Development Center, 2012. p. 252-259.
  • Long-term impacts of heat demand reductions within the Aarhus district heating area. / Nielsen, Steffen; Möller, Bernd. 2013. Paper presented at 8th Conference on Sustainable Development of Energy, Water and Environment Systems, Dubrovnik, Croatia.

In recent years, great attention has been paid to renewable energy systems both at a local and national level. This interest on renewable energy systems is mainly due to an increased acceptance that climate change is anthropogenic and directly related to the use of fossil resources. Changing over to an energy system based 100% on renewable energy is not just a fi-ne-tuning of the existing system, but is a fundamental change of the entire energy system. However, similar to the use of fossil fuels, biomass re-sources, which account of a large share of the renewable energy sources, are limited in relation to their sustainability. In order to use biomass re-sources in a sustainable manner, their use must be minimised by increasing the efficiency in the energy system and reducing the end-use consumption. Other types of renewable energy, such as wind and solar, are sustain-able but geographic limitations and their fluctuating energy production gives other challenges. To be able to use such renewable resources it is necessary to increase the flexibility of both the energy system and the end-use consumption.

This PhD thesis involves topics related to the transition towards 100% renewable energy with a main focus on the heat supply in Denmark. Even though the focus is on Denmark, the methods and results are applicable to other places as well, as they build on assumptions that are also present in other countries. In Denmark approximately half of the heat demand is sup-plied by district heating, and half of the district heating already uses renew-able energy. Within densely built-up areas district heating systems are re-source effective compared to individual solutions as district heating systems can use excess heat from power plants, industries, and waste incineration as well as renewable energy, like geothermal and large solar collectors, resources which are inaccessible to individual households. District heating systems also help integrate renewable energy production fluctuations through large heat pumps and heat storages. Therefore, district heating is often seen as an important part of 100% renewable energy systems.
The goal of the thesis is to examine the following questions: 1) Is renewable energy production from buildings useful in district heating areas? 2) What benefits are related to heat savings in buildings within district heating are-as? 3) How do geographic differences in heat consumption and supply influence the expansion possibilities of district heating? These questions are examined by establishing different models that focus on both temporal and spatial differences. The primary focus of the first part of the thesis is on the use of solar heating from buildings in general types of district heating are-as. This is followed in the second part by a case study which examines heat savings in buildings. The primary focus of the third part of the thesis is on the geographic aspects of district heating and develops geographic models for district heating expansion potentials and uses them in analyses.

In general geographic placement of buildings, and hereby their heat demands, is an important factor when choosing the best heat supply option. The heat supply decision should be based on larger coherent areas of buildings because the benefits of district heating are only found when whole systems are examined instead of individual buildings. Another challenge dis-cussed in this thesis is how to reduce the heat demand in buildings within district heating areas as the consumer price often does not reflect the long-term costs of heat production. The analyses show that heat savings results in considerable long-term savings in investments in production capacity and fuel costs. Through a case study, the amount of these long-term savings is compared to the costs of implementing heat savings. The case study shows that heat reductions of roughly 50% are feasible if the long-term costs are included. Savings of 75% are less feasible. In general the conclusions on rates of savings are based on which heat supply exists in each district heating area; in areas with expensive heat production the heat savings will be more feasible than in less expensive areas.

New geographic models have also been developed in this thesis with the aim of finding the costs of district heating expansion compared to the costs of individual solutions. With the help of these models, it is found that district heating expansion is still possible in Denmark. The potential is reduced if significant heat savings are introduced in the building stock. This conclusion is based on analysing the existing systems, and therefore, it is important to improve the district heating systems by increasing the efficiency of the systems as well as minimising the heat loss on the grids. In the geo-graphic modelling of district heating system expansions, it is important to distinguish between production, distribution and transmission costs. Differentiating between these factors ensures that geographic differences in heat density and distance between areas are included. In the models of distribution networks, it is important to include the geographic placement of buildings within each area. When calculating the transmission costs, it is important to consider that more than one area can use the same connection, minimising both the costs and the heat loss in the network.

It is clear that both temporal and spatial tools are required in the future when 100% renewable energy plans is developed. It is essential that both dimensions are includes, as this is the basis for finding the best solutions in a sustainable way. Both methods contribute with important knowledge in regards to the usage and integration of renewable energy.

Intelligent Glazed Facades – An Experimental Study.

Author: Frederik Vilbrad Winther, Aalborg University
See full thesis

Academic supervisors:
Professor Per Heiselberg, Department of Civil Engineering, Aalborg University
Associate Professor Rasmus Lund Jensen, Department of Civil Engineering, Aalborg University


  • Winther, F., Heiselberg, P., Jensen, R. L., Intelligent Glazed Facades for Fulfillment of Future Energy Regulations. TS Larsen & S Pedersen (red), i: Towards 2020 - Sustainable Cities and Buildings: 3rd Nordic Passive House Conference 7-8 October 2010. Aalborg Universitetsforlag, Aalborg.

Glazed facades are becoming an increasingly dominant choice of facades for office buildings. Glass has always been attractive and intriguing and has been a symbol of power, money and prestige. The façade is a means of communicating an image of prestige and power. However the glazed façade suffers tremendously when analyzing the energy usage of a building. Glazed façade contribute to higher energy demand for heating, cooling, etc. The glazed façade therefore has significant challenges to overcome in order to remain as an attractive envelope solution in future zero-energy-building concepts.

Through the use of energy calculation software and thermal building simulation software the initial analysis in this thesis strives towards defining the potential of controlling the energy transport across the façade for a typical glazed office building located in Denmark and fulfilling the energy regulation from 2008. The simple analysis defines the key technologies to focus on as a result. The focus on each technology is to perform fullscale experiments on each technology showing the performance of the technology under varying boundary conditions. The results from the demonstration are then followed by a numerical model development of the technologies applied for use in thermal building simulation software as well as in practical façade control systems

The results from the initial analysis show the performance of a dynamic façade which enables the variation and control of the heat transfer, transmitted irradiance, transmitted natural light, mass transport and thermal energy storage. The potential from this analysis on the typical glazed office building in Denmark, show that the dynamic façade opposed to the static façade can decrease energy demand for building services by approximately 75 %. The focus is therefore on analyzing the used technologies which concern dynamic Uvalue, dynamic g-value, thermal energy storage and control strategies for dynamic facades. The full-scale experiments are used as demonstration and for validation of the performance of the developed numerical models. The developed numerical models show a good description of the applied technologies and show that the variation of the technologies function and characteristics can both be done through use of thermal building simulation software as well as full-scale experiments. The façade control system, which uses the results from the developed numerical models of the dynamic facade technologies, can lower the energy demand
for heating, cooling and lighting as a results of the heat transfer across the façade by between 50 % and 88 %.

The obtained results from the initial analysis through the developed numerical models to the final analysis show that there are significant energy savings in the design of the dynamic façade. The energy savings need to be further investigated under laboratory conditions generating more precise data and description of the used technologies. The further development of dynamic façade technologies which are able to control the energy balance across the façade is also necessary to analyze the behavior in practice and to gain knowledge of the human interaction with the adaptive glazed façade.

Heating and Domestic Hot Water Systems in Buildings Supplied by Low‐Temperature District Heating.

Author: Marek Brand, Technical University of Denmark
See full thesis

Academic supervisors:
Professor Svend Svendsen, Department of Civil Engineering, Technical University of Denmark
Professor Bjarne W Olesen, Department of Civil Engineering, Technical University of Denmark

Industrial Advisor:
Jan Eric Thorsen, Danfoss A/S


  • Brand M, Thorsen JE, Svendsen S. Numerical modelling and experimental measurements for a low‐temperature district heating substation for instantaneous preparation of DHW with respect to service pipes. Energy 2012, vol. 41(1), p. 392‐400.
  • Brand M, Svendsen S. Renewable‐based low‐temperature district heating for existing buildings in various stages of refurbishment, ENERGY, Volume: 62, pp 311-319, 2013

District heating (DH) systems supplied by renewable energy sources are one of the main solutions for achieving a fossil‐free heating sector in Denmark by 2035. To reach this goal, the medium temperature DH used until now needs to transform to a new concept reflecting the requirement for lower heat loss from DH networks required by the reduced heating demand of low‐energy and refurbished buildings combined with the lower supply temperatures required by using renewable heat sources. Both these needs meet in the recently developed concept of low‐temperature DH designed with supply/return temperatures as low as 50°C/25°C and highly insulated pipes with reduced inner diameter. With this design, the heat loss from the DH networks can be reduced to one quarter of the value for traditional DH designed and operated for temperatures of 80°C/40°C. However, such low temperatures bring challenges for domestic hot water (DHW) and space heating (SH) systems, from the perspective of both DH customers and the DH company.

The aim of this work was therefore to identify, evaluate and suggest solutions. The first part of the research focused on the feasibility of supplying DHW with no increased risk of Legionella and on the performance of low‐temperature DH substations. The Danish Standard DS 439 for DHW requires that DHW should be delivered in reasonable time, without unwanted changes in desired temperatures (comfort) and without increased risk of bacterial growth (hygiene). While the comfort requirements set the minimum DHW temperature to 45°C, the hygiene requirements set it to 60°C, which is simply not reachable for low‐temperature DH. However, the German DHW standard DVGW 551 makes no requirement about minimum DHW temperature if the overall DHW volume is below 3L. This rule was adopted as a cornerstone for the research and for the whole lowtemperature DH concept in general, so the minimum DHW temperature is defined by a requirement for 45°C at the kitchen tap.

The performance of a low‐temperature DH substation with instantaneous DHW preparation was evaluated based on the results from laboratory measurements supplemented with results from the verified numerical model developed in MATLAB‐Simulink. The laboratory measurements showed that the low‐temperature substation can heat the required flow of DHW to 47°C with 50°C DH water while keeping the return temperature as low as 20°C. The results of numerical simulations considering the influence of the DH network, represented by a 10 m long service pipe connection for the substation equipped with an external bypass with a set‐point temperature of 35°C, showed that the time needed to produce 40°C DHW was 11 s with and 15 s without the external bypass, respectively. DS 439 suggests 10 s as the reasonable waiting time for DHW, so a low‐temperature DH substation based on the instantaneous principle of DHW preparation should be equipped with bypass solution keeping the service pipe warm and reducing the waiting time.
Traditional bypass solutions simply redirect the bypassed water back to the DH network without additional cooling, but bypassed water can instead be redirected to floor heating in the bathroom to be further cooled and thus reduce heat loss from the DH network while improving comfort for occupants and still ensure fast DHW preparation. Various solutions for the redirection and control of bypass flow were developed and their detailed performance tested on the example of a low‐energy single‐family house modelled in building energy performance simulation tool IDA‐ICE 4.22. The effect on the DH network was simulated with the commercial program Termis on a case study of 40 single‐family houses supplied by low‐temperature DH. In comparison to the reference case with a traditional external bypass, the proposed solution resulted in average cooling of bypassed water by 7.5°C, reducing the heat loss from DH network during non‐heating period by 13% and increasing the average floor temperature by 0.6‐2.2°C without causing overheating. The price for heating the bathroom floor during the non‐heating period depends on the location of the house and was between 98 and 371 DKK/house, but it seems reasonable to bill all customers with an even and discounted price, reflecting the fact that 40% of the heat delivered to the bathroom floor is covered by reduced heat loss from the DH network.

It can be concluded that low‐temperature DH with a supply temperature low as 50°C can be used for the delivery of DHW with the desired temperature and without increased risk of Legionella if the DH substation and DHW system are designed for the low‐temperature supply conditions. To ensure the fast provision of DHW during non‐heating periods, the supply service pipe should be kept warm, preferably with the bypass solution redirecting the bypass flow to bathroom floor heating and thus at least partly exploiting the additional heat loss caused by keeping the DH network ready to use.

The second part of the work focused on SH systems in low‐energy and existing buildings supplied by lowtemperature DH. The feasibility of supplying existing buildings with low‐temperature DH was investigated using the IDA‐ICE program by modelling the example of single‐family house from the 1970s, representing a typical example of Danish building stock. The results show that, to maintain the desired indoor temperature and not exceed the originally designed flow rate from the DH network, the DH supply temperature would need to be increased above 50°C in cold periods. In its original state, the house would need to be supplied with a DH temperature above 50°C for 21% of the year and above 60°C for 3% of time, with the highest temperature being 73°C. But if the windows are replaced, which can be expected because their lifetime is coming to an end, the maximum supply temperature is reduced to 62°C and the periods are reduced to 7% and 0.2% respectively. Further improvements, such as the addition of ceiling insulation or the installation of lowenergy windows and low‐temperature radiators, will allow DH water supply at 50°C the whole year around.

The results show that supplying existing buildings with low‐temperature DH is not a serious problem and
that DH companies should be stricter in reducing the supply temperature, which is very often kept high just because of the malfunctioning of the in‐house systems of customers. Moreover DH companies should
require that all newly installed and refurbished DH substations should be designed for low‐temperature DH to ensure the gradual transition to a temperature level of 50°C in the shortest possible period.

The IDA‐ICE program was also used to model the performance of a space heating system with radiators in the low‐energy single‐family house. The space heating system was investigated from the perspective of the customer, represented by thermal comfort, and the DH company, represented by a smooth heat demand and low return temperature. To accord with the literature, the modelling of internal heat gains reflected the improved efficiency of equipment by reduction of value from 5W/m2 to 4.2W/m2, also modelled as intermittent heat gains based on a realistic week schedule. Furthermore, the indoor set‐point temperature was increased from 20°C to 22°C to reflect a temperature level preferred by occupants. The results showed that an SH system with radiators can provide the desired indoor temperature while ensuring a smooth heat demand from the DH network and proper cooling. However, using input values suggested by the literature leads to up to 56% greater heat demand than values suggested in the Danish national calculation tool Be10, and in 40% lower connection power than for an SH system dimensioned in accordance with DS 418. Use of be10 input data in cost‐effectiveness analyses for DH networks therefore means worse results, because less heat is sold to customers and there is higher heat loss in the network. Similarly, higher connection power than needed means bigger pipe diameters are needed, resulting in higher heat losses as well. Using realistic values is therefore very important for feasibility calculations of DH. 

Life Cycle Cost Optimization of a BOLIG+ Zero Energy Building.

Author: Anna Joanna Marszal, Aalborg University
See full thesis

Academic supervisor:
Professor Per Heiselberg, Department of Civil Engineering, Aalborg University
Professor Henrik Lund, Department of Planning, Aalborg University


  • A.J. Marszal, P. Heiselberg, J.S. Bourrelle, E. Musall, K. Voss, I. Sartori, A. Napolitano, Zero Energy Building - A Review of definitions and calculation methodologies, Energy and Buildings 43 (2011) 971–979
  • A.J. Marszal, P. Heiselberg, Life Cycle Cost analysis of a multi-storey residential Net Zero Energy Building in Denmark, Energy 36 (2011) 5600-5609
  • A.J. Marszal, P. Heiselberg, R.L. Jensen, J. Nørgaard, On-site or off-site renewable energy supply options? Life cycle cost analysis of a Net Zero Energy Building in Denmark, Renewable Energy 44: p. 154-165, 2012.
  • Lund, H., Marszal, A., Heiselberg, P. Zero Energy Buildings and Mismatch Compensation Factors. Energy and Buildings 43 (2011) 1646–1654
  • A.J. Marszal, J.S. Bourrelle, J. Nieminen, B. Berggren, A.Gustavsen, P. Heiselberg, M. Wall “North European Understanding of Zero Energy/Emission Buildings” In Proceedings of Renewable Energy Research Conference, Trondheim, Norway,
  • 2010, pp. 167-178
  • A.J. Marszal, J.S. Bourrelle, E. Musall, P. Heiselberg, A. Gustavsen, K. Voss, Net Zero Energy Buildings - Calculation Methodologies versus National Building Codes, In Proceedings of EuroSun 2010: International Conference on Solar Heating, Cooling and Buildings, Graz, Austria, 2010
  • Marszal, A., Nørgaard, J., Heiselberg, P., Jensen, R.L. Investigation of a cost-optimal zero energy balance - A study case of a multifamily NetZEB in Denmark. PLEA2012 - 28th Conference, Opportunities, Limits & Needs Towards an environmentally responsible architecture Lima, Perú 7-9 November 2012

Buildings consume approximately 40% of the world’s primary energy use. Considering the total energy consumption throughout the whole life cycle of a building, the energy performance and supply is an important issue in the context of climate change, scarcity of energy resources and reduction of global energy consumption. An energy consuming as well as producing building, labelled as the Zero Energy Building (ZEB) concept, is seen as one of the solutions that could change the picture of energy
consumption in the building sector, and thus contribute to the reduction of the global energy use. However, before being fully implemented in the national building codes and international standards, the ZEB concept requires a clear understanding and a uniform definition.

The ZEB concept is an energy-conservation solution, whose successful adaptation in real life depends significantly on private building owners’ approach to it. For this particular target group, the cost is often an obstacle when investing money in environmental or climate friendly products. Therefore, this PhD project took the perspective of a future private ZEB owner to investigate the cost-optimal Net ZEB
definition applicable in the Danish context.

The review of the various ZEB approaches indicated a general concept of a Zero Energy Building as a building with significantly reduced energy demand that is balanced by an equivalent energy generation from renewable sources. And, with this as a general framework, each ZEB definition should further specify: (1) the connection or the lack of it to the energy infrastructure, (2) the unit of the balance, (3) the period of the balance, (4) the types of energy use included in the balance, (5) the minimum energy performance requirements (6) the renewable energy supply options, and if applicable (7) the requirements of the building-grid interaction. Moreover, the study revealed that the future ZEB definitions applied in the Denmark should mostly be focused on grid-connected ZEBs – Net ZEBs, and the annual primary energy balance.

The Life Cycle Cost (LCC) analysis conducted with a study case of a multi-storey residential Net ZEB aimed to determine the cost-optimal “zero” energy balance, minimum energy performance requirements and options of supplying renewable energy. The calculation encompassed three levels of energy frames, which mirrored the Danish low-energy building classes included in the current building code, and ten
renewable energy supply systems including both on-site and off-site options. The results indicated that although the off-site options have lower life cycle costs than the on-site alternatives, their application would promote renewable technologies over energy efficiency measures. Thus, they oppose the Danish plans to gradually make the energy performance requirements stricter. Moreover, the results showed that district heating is a less cost-attractive solution than a ground source heat pump for a private building owner. Finally, with 2010-level of energy prices, cost-optimal “zero” energy balance accounts only for the building related energy use.

An Integrated Control System for Heating and Indoor Climate Applications.

Author: Fatemeh Tahersima, Aalborg University
See full thesis

Academic supervisors:
Professor Jakob Stoustrup, Department of Electronic Systems, Aalborg University
Associate Professor Henrik Rasmussen, Department of Electronic Systems, Aalborg University

Industrial advisor:
Peter C. Andersen, Klaus L. Nielsen, and Peter G. Nielsen, Danfoss A/S


  • Tahersima, F, Stoustrup, J & Rasmussen, H 2013, 'An analytical solution for stability-performance dilemma of hydronic radiators' Energy and Buildings, vol 64, s. 439-446., http://dx.doi.org/10.1016/j.enbuild.2013.05.023
  • Tahersima, F, Stoustrup, J, Rasmussen, H & Meybodi, SA 2012, 'Economic COP Optimization of a Heat Pump with Hierarchical Model Predictive Control'. i Decision and Control (CDC), 2012 IEEE 51st Annual Conference on. IEEE Press, s. 7583 - 7588 . I E E E Conference on Decision and Control. Proceedings, , http://dx.doi.org/10.1109/CDC.2012.6425810
  • Tahersima, F, Stoustrup, J, Meybodi, SA & Rasmussen, H 2011, 'Contribution of domestic heating systems to smart grid control' I E E E Conference on Decision and Control. Proceedings., http://dx.doi.org/10.1109/CDC.2011.6160913
  • Tahersima, F, Stoustrup, J & Rasmussen, H 2011, 'Eliminating oscillations in TRV controlled hydronic radiators' I E E E Conference on Decision and Control. Proceedings., http://dx.doi.org/10.1109/CDC.2011.6161216
  • Tahersima, F, Stoustrup, J & Rasmussen, H 2011, 'Optimal Power Consumption in a Central Heating System with Geothermal Heat Pump' Proceedings of the 18th IFAC World Congress, 2011.
  • Tahersima, F, Stoustrup, J & Rasmussen, H 2011, 'Stability Performance Dilemma in Hydronic Radiators with TRV' I E E E Conference on Control Applications. Proceedings., http://dx.doi.org/10.1109/CCA.2011.6044404
  • Tahersima, F, Stoustrup, J, Rasmussen, H & Gammeljord, P 2010, 'Thermal analysis of an HVAC system with TRV controlled hydronic radiator'. i Proceedings of the 6th annual IEEE Conference on Automation Science and Engineering, Toronto, Ontario, Canada, August 2010. IEEE Press., http://dx.doi.org/10.1109/COASE.2010.5584535

Low temperature hydronic heating and cooling systems connected to renewable energy sources have gained more attention in the recent decades. This is due to the growing public awareness of the adverse environmental impacts of energy generation using fossil fuel. Radiant hydronic sub-floor heating pipes and radiator panels are two examples
of such systems that have reputation of improving the quality of indoor thermal comfort compared to forced-air heating or cooling units. Specifically, a radiant water-based sub-floor heating system is usually combined with low temperature heat sources, among which geothermal heat pump, solar driven heat pumps and the other types are categorized
as renewable or renewable energy sources.

In the present study, we investigated modeling and control of hydronic heat emitters integrated with a ground-source heat pump. Optimization of the system performance in terms of energy efficiency, associated energy cost and occupants’ thermal comfort is the main objective to be fulfilled via design of an integrated controller. We also proposed
control strategies to manage energy consumption of the building to turn domestic heat demands into a flexible load in the smart electricity grid. We developed a simulation infrastructure for computer-based testing of the developed
control methodologies. As the basis for components modeling, dynamical modeling of hydronic radiators controlled by thermostatic radiator valves is studied thoroughly. We have shown via analytical studies that a simply designed gain scheduling controller will overcome the well know instability problem of radiators which usually occurs in low heat demand conditions. We dealt with the problem as a dilemma between stability and performance. Since, controller parameters can be chosen such that the radiator works stable in the entire operation region, as a result the performance will become deteriorated during the cold season. To overcome the dilemma, an adaptive controller is designed analytically which satisfies both performance and stability at all operating points. The studied radiator model is further adapted to the modeling of the sub-floor heating system.

In order to minimize the electric power consumption of the integrated heating system, a novel hypothesis is proposed and further investigated via experimental and simulation studies. The idea is to minimize the forward temperature of hot water in order to maximize the heat pump’s efficiency and by this means reduce the power consumption of the heat pump. The hypothesis is that such an optimal point coincides with saturation of at least one of the subsystems control valves. The idea is implemented experimentally using simple PI and on/off controllers on a real test setup i.e. a multiple room detached house in Copenhagen; the hypothesis is further investigated by designing a hierarchical control structure which uses model predictive controller (MPC) at the top level orchestrating single control loops at the lower level of control hierarchy. MPC is specifically chosen in order to embed measured exogenous disturbances e.g. comfort profile, weather forecast and electricity price signals. Incorporation of the latter knowledge in the decision making enables the domestic energy consumer to act as a flexible load in the smart electrical grid to regain balance.