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To aid in the integration of renewable and residual energy sources in the energy system, energy flexibility is required. By charging and discharging energy storage, energy flexibility can be created and heat demand and heat generation can be matched in time. One possible source of energy flexibility is the thermal capacity of the water in district heating network pipes. Effective use of this thermal energy storage requires efficient techniques to determine the available flexibility. The goal of this paper is to determine the required level of detail of a substation model to characterise network flexibility through simulation. The substation models differ in the assumptions that are made and range from a detailed, non-linear model to a simple, linear model. To analyse the results, we identify different phases occurring during a network flexibility activation. By determining if reduced models are as effective in reproducing important flexibility characteristics as more detailed and computationally expensive models, network flexibility characterisation can be simplified and sped up. Results show that the network flexibility can be adequately characterised even with very simple models, provided correct assumptions are made.

We herein discuss a smart district heating (DH) grid whereby smooth operation is achieved, and individual heating solutions, and passive or energy+ building-specific systems are implemented by means of flexible heat demands, thermal energy storages, and transmission resources. For future energy systems, low-temperature district heating lines combined with heat savings and reduced environmental impact represent a promising alternative to integrating fluctuating renewable sources. What makes this study relevant is scientific need of having a low-temperature distribution branch within an existing exclusively high-temperature district heating system. Non-renewable heat production is partially replaced with heat pumps and thermal energy storage facilities. Heat pumps use renewable electricity to produce completely renewable heat. Fundamental technical and consumer-oriented aspects of the power system will have to be adapted to using heat pumps and thermal energy storages. We created a large database of validated heating uptime values for specific buildings and outdoor conditions. We herein dwell upon an alternative heating design. The new concept is based on using transformation units as well as branch lines going from street mainlines to consumers’ substations, whereby a branch line consists of two single pipes, a dual pipe, or a triple pipe. Such branching creates a subnetwork with different heating parameters. The temperature for such a subnetwork is specifically optimized. Optimization relies on such as data as use of thermal energy, heat loss rates, and parental-system data. As a result, we modeled a network whose primary nodes had a low operating temperature. This reduces heat loss as well as fuel consumption while enabling less environmentally harmful heat production. Besides, such use of thermal energy combined with self-production can be important for creating renewable energy systems in the future. Therefore, future research and development shall be targeted at creating low-enthalpy facilities that use renewable energy to produce and store thermal energy as simulated when modeling such a modified subsystem (subnetwork).

Suvremeni trendovi razvoja i korištenja sustava područnog grijanja usmjereni su k snižavanju temperaturne razine primarnog kruga. Niže temperature smanjuju transmisijske gubitke, povećavaju učinkovitost proizvodnje i omogućavaju integraciju novih niskotemperaturnih izvora energije kao što su dizalice topline i otpadna toplina. Osim smanjivanja temperature polaza teži se i povećanju temperaturne razlike između polaza i povrata. Povećanjem razlike može se smanjiti protok u toplovodu, a da se korisnike opskrbi istom količinom toplinske energije. Na temperaturnu razliku utječe način vođenja parametara medija u toplinskoj podstanici i stanje u sustavu grijanja potrošača. U ovom radu izveden je model termohidrauličkih procesa u sustavu grijanja referentne višestambene zgrade. Sustav grijanja je putem indirektne toplinske podstanice spojen na mrežu centralnog toplinskog sustava. Osim sustava grijanja zgrada također ima sustav pripreme potrošne tople vode koji je u podstanici spojen paralelno uz sustav grijanja. Kako bi se postigla najniža temperatura primarnog kruga na izlasku iz podstanice određena je optimalna radijatorska krivulja korištenjem niskog protoka u radijatorskom sustavu. Također, zbog mogućih varijacija u temperaturi polaza daljinskog grijanja na pragu potrošača, radijatorska se krivulja određuje u ovisnosti o temperaturi polaza, a ne u odnosu na vanjsku temperaturu. Konačno, napravljena je simulacija rada sustava grijanja u jesenskom i zimskom periodu. Modern trends in use and development of district hearing systems are directed towards the reduction of temperature levels in primary circuit. Lower temperatures reduce transmission losses, increase production efficiency and allow for integration of new, low-temperature sources such as heat pumps and waste heat. Besides lowering supply temperatures efforts are made to increase temperature differences between supply and return pipes. By increasing the temperature difference the flow in pipes can be reduced while supplying the consumer with the same amount of heat. The temperature difference is affected by the way heating substation is run and on the specifics of the heating system of the consumer. In this thesis model of thermohydraulic processes in an apartment building and its heating system has been made. The heating system is connected via indirect heating substation to the district heating network. Besides the heating system, the building also has a domestic hot water system connected in parallel with the heating system. To achieve the lowest return temperature optimal radiator control curve has been determined using a low flow of water in the radiator system. Also, because of possible variations in supply temperatures at the entrance to the substation, the radiator control curve is determined in relation to the supply temperature, as opposed to the outdoor temperature. Finally, the simulation of the whole system has been made for winter and fall periods.

MODEL FOR PLANNING LOADS IN DISTRICT HEATING OPERATION UNDER THE MODERN HEAT DEMAND SUMMARY. AIM. Nowadays a large number of district heat utilities are faced with excessive domestic hot water (DHW) load, at the same time its impact is not clear. METHODS. As well as reviewing the existing heat demands and densities we looked at development plan of 2016–2017. RESULTS AND THEIR DISCUSSION. There are several types of heat exchanger connections used for domestic hot water (DHW) and space heating (SH) water generation in the substations; it affects a water flow and return temperature. Larger amounts of water need to be circulated, resulting in higher pumping capacities, limited capacity of the network and poorer heat generation efficiency in case of co-generation. The disadvantage of serial connection compared to the other solutions is related to the control of DHW generation. The parallel connection of the DHW and SH heat exchangers compared to the serial connection allows lower primary supply temperature. An accurate evaluation of the heating loads requires a complete understanding and accounting of the building components that make up the thermal enclosure along with the outdoor/indoor contributions to the load. The load calculation for a house with controls and where the building enclosure has been enhanced with added air tightness strategies, better windows, and additional insulation will be more sensitive to manipulation of the inputs. Moreover, the results confirm that the forecast is strongly accurate in comparison to the real thing; the difference between the two configurations is less than 2% for the ongoing case considered. MAIN CONCLUSIONS. To summarize, the model itself and the new knowledge of opportunities for improving the design of load structures can be used as tools for developing the district heating system.

International audience; District Heating (DH) technology is an efficient and cost-effective solution to provide heating services to the built environment. However, the existing DH technology may not be technically and economically effective to service buildings with low energy demands. Here, low-temperature based district heating (LTDH) can provide a better match between supply and demand in terms of energy quality and quantity. This paper deals with the energy and exergy analyses of a LTDH substation supplying a secondary LTDH network as a subnet of the existing DH system. In order to improve the temperature match, a mix of supply and return streams from the primary DH network are used to supply the substation. Based on modelling and simulation, an energy and exergy analysis is employed to compare the performance of two proposed substation configurations to that of a conventional DH substation operating at low temperatures. The results show that the proposed LTDH substation reduced the share of the yearly energy demand of the LTDH network that is covered by the primary DH supply flow by 20% to 25%; the rest being supplied by the primary DH return flow. Likewise, by using the flow from the main DH return pipe, the final exergy efficiency of the overall system increased by 5% on average. The exergy destruction occurring at the system components was also identified and compared: during high heat demand periods the substation heat exchanger is responsible for the largest share of exergy destruction, whereas for low heat demands, it is due to the pumping effort. Based on these results, the proposed system is seen as an effective approach to increase the quality and quantity match between the low-temperature network and the conventional supply.

Denne ph.d.-afhandling er en sammenfatning af det treårige ph.d.-projektet vedrørende tre casestudier, som hver er unik med henvisning til en typisk regional dansk energiplanlægning med hensyn til omfattende brug af lavenergifjernvarmesystemer, der opererer med lave temperaturer såsom som 55 °C i fremløbet og 25 °C i returløbet og med en intensiv udnyttelse af de vedvarende energikilder.Hypotesen er at en detaljeret analyse af lavenergifjernvarmesystemers energimæssige egenskaber samt anlægs- og driftsudgifter kan benyttes som et rationelt grundlag for planlægningen af brugen af fjernvarme i områder med lavenergibygninger I det første case study, blev fokus rettet mod valg af designmetode til lavenergifjernvarmesystemer for nye bebyggelser af lavenergihuse. Designmetoden indeholdt hovedsageligt fastlæggelsen af en ny rør-dimensioneringsmetode baseret på optimering af diametre i stedet for at bruge tommelfingerregler, der er baseret på den maksimale trykgradient, maksimal hastighed og/eller samtidig behandling af begge. Et andet designelement omhandlede de vurderinger af (i) typer af fjernvarmeunits i bygningerne, der kunne benyttes i lavenergihusene sammen med ideen om udnyttelse af boosterpumper i fjernvarmenettet og (ii) udformningen af fjervarmenettet enten som forgrenede (træ- lignende) eller som ringe. De udviklede metoder blev afprøvet i et casestudie baseret på data leveret af Roskilde Kommune. I det andet case study blev fokus rettet mod løsning af en anden regional energiplanlægningsopgave, der vedrører eksisterende huse, der er tilsluttet et naturgasnet eller et konventionelt højtemperatur fjernvarmenet. Den forskningsmæssige idé til løsning af dette problem var ved anvendelsen af et lavtemperatur fjernvarmenet med en styringsstrategi baseret på en forøgelse af fremløbstemperaturen i kolde vinter-spidslastperioder af kort varighed sammenlignet med hele året. Det blev overvejet at anvende denne løsning i to situationer, den første (i) som led i ændringen af den kommunale varmeplan fra de nuværende naturgasnet til lavenergifjernvarmesystemer og den anden (ii) i forbindelse med driften af lavenergifjernvarmesystemer. Den eksisterende bebyggelse i Gladsaxe kommune blev valgt til casestudiet, fordi den repræsenterer bygninger med behov for energirenovering og ændring i forsyningsløsning fra det nuværende naturgasnet. I det tredje case study var fokus på bestemmelse af lokalt tilgængelige vedvarende energikilder til brug i energiforsyningssystemer. En undersøgelse af dette skete under det eksterne ophold på University of Ontario Institute of Technology (UOIT), Oshawa, ON, Canada under vejledning af professor Ibrahim Dincer. I denne fælles undersøgelse blev en ny metode udviklet til at være grundlaget for et beslutningsstøtteværktøj vedrørende en optimal udnyttelse af vedvarende energikilder med omfattende analyser med hensyn til: (i) den månedlige opfyldelse af forskellige typer af energibehov dvs. opvarmning, herunder krav til rumopvarmning og varmt vand, el og køling med henblik på at inddrage effektivisering af multi-generationssystemer, (ii) forskellige typer af energikonverteringssystemer såsom ’single-generation’, ’cogeneration’, og ’multi-generationssystemer’, (iii) den langsigtede lagring af varme til at klare den manglende overensstemmelse mellem produktion af varme fra vedvarende energikilder og behovet for varme, som begge følger forskellige variationer f.eks. kan den overproduktion af varme ved hjælp af sol-baserede systemer ikke anvendes, men kan opbevares i borehulslagersystemer og anvendes i den kolde vinterperiode, (iv) en omfattende økonomisk vurdering af teknologierne ved at tage hensyn til en række parametre for hver teknologi, såsom de specifikke udgifter til investering, drift og vedligeholdelse, levetid, kapacitetsfaktoren, og restværdi af energiforsyningssystemet, (v) den sæsonmæssige variation af energiproduktionen afhængigt af den vedvarende energikildes ydelse, (vi) og, i begrænset omfang de miljømæssige hensyn, der er mulige at opfylde under driften af hvert konverteringssystem samt forsyningssikkerheden for de optimale løsninger. Metoderne, der er opnået ved case-studierne, har samlet set givet de tekniske rammer for etablering af et integreret energiforsyningssystem baseret på anvendelse af vedvarende energikilder til forsyning af lavenergihuse ved hjælp af lavenergifjernvarmeanlæg. This PhD thesis presents a summary of a three-year PhD project involving three case studies, each pertaining to a typical regional Danish energy planning scheme with regard to the extensive use of low-energy district heating systems, operating at temperatures as low as 55°C for supply and 25°C for return, and with the aim of intensive exploitation of renewable energy sources. The hypothesis is that a detailed analysis of energy performance and cost of construction and operation of low energy district heating systems can be used as a rational basis for planning use of district heating in areas with low energy houses. The first case study focus was concerned with developing a method for the designing of low-energy district heating systems for new settlements in which low-energy houses were to be built. The method involved primarily the development of a novel pipe dimensioning method based on optimization of the pipe diameters rather than use of rule-of-thumb methods, through consideration of a certain value of a maximum pressure gradient or a maximum velocity, or both. In addition, attention was directed at the assessment of (i) substation types considered for use in connection with the low-energy houses involved, together with the idea of utilizing booster pumps in the district heating network and (ii) use of network layouts of either a branched (tree-like) or a looped type. The methods developed were applied in a case study, the data of which was provided by the municipality of Roskilde in Denmark. The second case study was aimed at solving another regional energy planning scheme, one concerned with already existing houses, the heat requirements of which were currently being met by use of a natural gas grid or a conventional high-temperature district heating network. The idea considered for employing a low-energy district heating system here involved use of an operational control approach of boosting the supply temperature during the peak winter months due to their shorter durations when compared to a year period. This approach can be considered in two different respects: (i) in the municipal infrastructure, transforming the current heating systems into lowenergy district heating systems and (ii) in the operation of low-energy district heating systems. The building settlement in question, one located in the municipality of Gladsaxe, was chosen for the case study carried out, due to the existing houses there being considered for renovation to houses of a low-energy class, and due to the existing heat-supply energy infrastructure there being a natural gas grid. The third case study carried out aimed at developing energy conversion systems based on use of renewable energy sources that were available locally. This was carried out in an external stay at the University of Ontario Institute of Technology (UOIT) in Oshawa, ON, Canada under the supervision of Prof. Ibrahim Dincer. In this colleborative study, a novel method was developed to serve as the basis of a decision support tool in investigating the optimal use of renewable energy sources, particular consideration being given to the following: (i) the monthly satisfaction of energy requirements of various types: heating (including the demands of space heating and of domestic hot water production), electricity, and cooling, in order to study the improvement in efficiency achieved by use of multi-generation systems, (ii) various types of energy conversion systems, such as single-generation, cogeneration, and multi-generation systems, (iii) the long-term storage of heat energy to cope with the mismatch between the energy production from renewable energy sources and the heat energy requirements, both in terms of the variations involved, such through the excessive production of heat by means of solar based systems, heat that cannot be used immediately but can be stored in borehole storage systems, to be used then in the cold winter period, (iv) an extensive economic assessment of the technologies involved, taking several different parameters into account, each unique for the technology in question, such as the specific investmet costs based on an economy-of-scale, operation and maintenance costs, the lifetime of the technology, the capacity factor, and the salvage value of the energy conversion system at the end of its lifetime, (v) seasonal variation in the generation of energy, in line with the availability of the renewable source in question, (vi) on a limited scale, aimed at gaining as much insight as possible into the complexities of the questions involved, examining the environmental concerns possible to encounter during the operations of each conversion system, the security of supply being figured on the basis of the optimal solutions obtained. In summary, the methods developed in the case studies concern the technical framework for establishing an integrated energy supply scheme involving the use of renewable energy sources for meeting the energy needs of low-energy houses by means of a city-wide low-energy district heating system.

For a district heating company it is of importance to have an efficient and well performing system. A central part in the work to lower temperature levels in district heating networks is to acknowledge and improve cooling capability in substations. The aim of this thesis is to analyse substations in Polish district heating systems in order to identify reasons of poor cooling and to present suggestions of implementable measures. Furthermore, the economical saving potential from an improved cooling is evaluated. The analysis was carried out for two of the five Polish companies included in this report; Sydkraft EC Slupsk and MEC Koszalin. It was followed by two scenarios created for evaluation of improvement possibilities based on calculated financial savings from reduction of distribution heat losses and distribution pumping. The results show that there are significant improvement possibilities. From the carried out scenarios a saving potential between 15 000 to 20 000 PLN/substation is possible to achieve for selected worst substations, if their individual annual average cooling is set to 30°C. The analysis further shows that causes of poor cooling in substations are highly individual but points out that customer owned substations are represented in the majority of worst substations.

Thermal grids will play a key role in the development of local energy communities and the achievement of 100% renewable societies. However, challenges are related to the capability of dealing with the increasing number of producers which could supply heat to thermal grids. Indeed, as in the case of the electricity grid, heat could be available for some hours in a day from RES plants installed at user level and distributed in the network (also referred to as “prosumers”). To achieve more reliable results when performing energy analyses of these innovative applications, there is a need to rely on accurate models of each component. For instance, substations are of paramount importance in the interaction of prosumers with thermal grids. These systems must properly exchange heat with the network according to the prosumer demand and production while complying with grid constraints. In this respect, there is a need to develop models of prosumers’ substation, which accounts for technical details (e.g., connections, heat exchangers, valves, etc.) and controls. Starting from the layout of a bidirectional substation for a thermal energy network proposed in the literature, this paper proposes a dynamic model that replicates the experimental setup in the TRNSYS environment. Simulations are performed to validate the model. The results show a good matching between experimental and simulated results in terms of dynamic behavior and energy balance. To show the capabilities of the proposed model, a prosumer with heat available from solar thermal collectors installed onsite is considered as a case study. Results show that almost 13.8% of the heat produced by the prosumers is exchanged with the grid. It is worth noting that the validated TRNSYS model will support studies on the feasibility of Thermal Energy Communities in existing systems, to develop strategies for energy-efficient and cost-effective operation and monitoring the performance.