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Temperature optimization in district heating systems
Temperature optimization in district heating systems
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P. Lauenburg Lund University, Lund, Sweden
11.1 Introduction This chapter deals with the temperature levels in district heating (DH) systems, including building heating systems. The different temperatures in the systems are of large importance for the systems' efficiency and consequently, for their economic and environmental performance. Generally, low temperatures are favourable, however several factors limit how low temperatures can actually be. The objective of this chapter is to provide the reader with knowledge about the implications of changed temperatures on the supply and distribution of district heat, and how temperatures in end-user heating systems affect the DH system. Furthermore, present and future development and trends are reviewed.
11.1.1 A historical overview of DH temperature levels In recent years, it has become common to talk about the future of DH in terms of a development of a fourth generation of DH technology (Frederiksen and Werner, 2013). The first generation of DH technology used steam as a heat carrier and was applied in new constructions, until approximately 1930. A few systems are still in operation, e.g. in New York, Paris and partly in Copenhagen. The second generation entailed a shift to pressurized water as a heat carrier, typically with temperatures well above 100 °C. Concrete ducts and shell and tube heat exchangers are typical features, which are still common in existing systems. The third generation has been dominating since the 1980s, with lower temperatures, pre-fabricated and pre-insulated pipes and material-lean components, as some of its distinctive features. Each generation has been state-of-the-art for 40–50 years. Facing ever harder competition from other technologies, such as heat pumps and natural gas, and decreasing specific heat demand in new and renovated buildings, there is a need for research and development in order to introduce the fourth generation over the coming years. Typical features of the fourth generation will be further lowered temperatures, along with assembly-oriented components and more flexible materials. This chapter focuses on the significance of temperatures on the different parts of the DH system. For more information about the four generations of DH, the paper by Lund et al. (2014) is recommended.
Advanced District Heating and Cooling (DHC) Systems. http://dx.doi.org/10.1016/B978-1-78242-374-4.00011-2 Copyright © 2016 Elsevier Ltd. All rights reserved.
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11.2 The connection between district heating temperatures and different parts of the district heating system In this section, some basic information is provided regarding what temperature levels are required in DH systems and how they relate to the performance of supply and distribution. To begin with, the temperature demands that arise as a result of end-use, mainly from building space heating and domestic hot water consumption, are dealt with. Then follows a section on the importance of temperature levels for distribution of DH, and finally, how network temperatures influence the heat supply. The temperatures discussed in this chapter are displayed in Figure 11.1. The DH supply temperature (TDH,supp) must be high enough (1) to satisfy the temperature demands in the building internal systems (space heat and domestic hot water) and (2) to keep the flow rate in the DH network sufficiently low in order to avoid unreasonably high flow velocities. Space heating temperatures are displayed in blue for a so-called 80/60 system (TSH,supp and TSH,ret). While the space heating temperatures are dependent on the outdoor temperature, the domestic hot water temperature (TDHW,supp) is constant. The return temperature from a building (TDH,ret) is the combination of the return temperatures from the space heating and hot water system.
11.2.1 Building-internal heat supply systems The two main services provided for by DH, particularly in the case of dwellings, are the provision of domestic hot water and space heating. Other purposes, such as washing and drying of clothes, refrigeration and industrial applications, are not addressed here. Although domestic hot water and space heating systems can be found TSH, supp
TSH, ret
TDHW, supp
TDH, ret
Tin
Temperatures
TDH, supp
Outdoor temperature
Figure 11.1 The different temperatures discussed in this chapter.
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in all types of buildings, regardless of the heating source, the design and operation of the systems have sometimes been affected in various ways depending on whether they are connected to DH. One example is the need for a heating system that provides a low return temperature of DH water. This also applies to buildings served by, for instance, heat pumps, while boilers, traditionally, have demanded relatively high return temperatures. Flue gas condensation in boilers is however common nowadays, also gaining from low return temperatures.
11.2.1.1 Domestic hot water Provision of domestic hot water is normally through either heaters with accumulation or heat exchangers (instantaneous water heaters). Practices have differed between countries. There are advantages and drawbacks for both methods. For example, accumulation involves the use of smaller dimensions for DH pipes and valves, while instantaneous heaters generally provide lower return temperatures. Regarding the required tap water temperature, conditions also vary between countries. Typically, 50 °C is required at the tap. Two factors determine the required temperature: comfort demand and Legionella prevention. If Legionella prevention cannot be handled separately, it will constitute the lower boundary. With ever lower temperatures required in space heating systems, as discussed in the next section, it is also likely that the domestic hot water system in the future will set the overall boundary for how low a supply temperature can be allowed in a DH system.
11.2.1.2 Space heating Space heating is often provided for by water-borne radiator systems, especially in countries with a cold climate or with a large share of DH. Underfloor heating has gained significant popularity in recent years. Ventilation systems are sometimes also used for space heating, especially in commercial premises, causing the heat to be air-borne. In the latter case, heat supplied by the air typically covers the heat losses from ventilation, while a radiator system covers the heat losses from transmission. Regarding radiator system temperatures, as well as for DH network temperatures, different levels prevail–both between countries but also depending on the age of the systems. The general ambition has been to lower the temperatures, since these are the single largest factors determining the temperature levels in the DH networks. Skagestad and Mildenstein (2002) give some examples of typical design radiator temperatures (supply/return temperatures) in various countries: Denmark and Finland—70/40 °C; Korea—70/50 °C; Romania and Russia—95/75 °C; the UK— 82/70 °C; Poland—85/71 °C; and Germany—80/60 °C. The development in Sweden clearly shows how lower radiator temperatures are strived for. Previously, higher temperatures, such as 90/70 and 80/60 °C have been used, partly because there were no incentives for low temperatures in systems with boilers, but also because smaller radiators could be employed. The advantages of low temperatures have led to lower temperatures being used today, e.g. 60/45, 60/40 or 55/45 °C. Since 1982, temperatures
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higher than 55 °C (60 °C in certain cases with DH) are not allowed in new heating systems, which should promote the use of low-temperature heating systems, e.g. solar heating (Frederiksen and Werner, 1993). When looking at these temperatures, it must be kept in mind that they express the design temperatures, i.e. the temperatures required at extremely low outdoor temperatures. For most of the heating season, significantly lower heating system temperatures are required. Another factor affecting the radiator temperatures in practice is the degree of oversizing of the heating system. There is a substantial oversizing of the radiator system in general and of the radiator surfaces in particular, as presented in both Swedish (Gummérus and Petersson, 1999; Trüschel, 2002) and international studies (Liao et al., 2005; Peeters et al., 2008; Skagestad and Mildenstein, 2002). This is due to an overestimation of a building's heat losses, which also often tend to decrease over time if energy-saving measures are implemented. Another reason is that, during the design stage, the components are generally selected in sizes larger than required to ensure safety margins. In order for oversizing not to cause overheating of a building, the supply temperature, or the circulation flow, or both, in the radiator system must be adjusted. As described in Lauenburg (2009), the control curves of the various substations in an area were found to differ to a surprisingly large degree, despite the fact that the houses were built at the same time and were of a similar structure. Similar experiences have been described by Lindkvist and Walletun (2005). The most common approach is to reduce the supply temperature in an oversized radiator system to avoid indoor temperatures becoming too high. However, the flow can also be reduced, and the system can be adjusted to work as a so-called low-flow system. By substantially reducing the flow, while maintaining a high supply temperature, a low return temperature can be achieved. Although the heat transfer in the heat exchanger is deteriorated, a lower DH return temperature is generally obtained. To intentionally install larger radiators in order to obtain a low-flow system is rarely economically viable (Bøhm, 1986). On the other hand, oversizing and the design of systems for a high flow and a small temperature difference often render it possible to adapt the low-flow method. Early studies on low-flow systems were made by Schelosky (1980) and Amberg (1980). Today, the method is used to some extent, although there still seems to be a division between those who advocate it and its opponents. Trüschel (2002), who conducted a comprehensive study on hydronic heating systems, showed that the return temperature is the lowest and thermostatic radiator valves are the most effective in a low-flow system. The low flow leads to very low pressure drops in the system, and all thermostatic radiator valves thus work at approximately the same differential pressure and with a high authority. The low DH return temperature of low-flow systems has also been demonstrated by, among others, Gummérus and Petersson (1999), Petersson (1998), and Petersson and Werner (2005). Moreover, all these authors stressed the importance of the systems being balanced, since this has the greatest impact on the DH return temperature. One drawback of the method may be that a reduction in the DH supply temperature can result in an increased DH return temperature, due to an increased flow if the difference between primary and secondary supply temperatures is small. This is most
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critical around the so-called supply temperature ‘breakpoint’, typically between 0 and 5 °C outside air temperature, where the supply temperature has not yet been raised, although the heat load is relatively high. This also could become a real problem if network supply temperature was significantly lowered. In a study of DH substations in operation, Råberger (1995) found that high return temperatures are mainly due to the return temperature of the radiator system. However, extremely high-return temperatures depend on malfunctions in the substation. Furthermore, it is important that the heating system is well balanced. This was also shown by Trüschel (2005), where the value of balancing of three heating systems was estimated to give a pay-off time between 1.5 and 5.5 years.
11.2.2 Heat distribution 11.2.2.1 Temperature and flow Theoretically, for any given heat load, a lowered supply temperature will result in a higher return temperature. The reason for this is that the required DH flow must be increased in order to supply the same amount of heat. This in turn leads to a shorter time of stay for the water in heat exchangers or radiators, and consequently an increased return temperature. In practice, however, intentional and unintentional short circuits in the distribution system may disturb this relation. Intentional short circuits are caused by by-pass connections used at peripheral locations in the network in order to avoid stagnant DH water during summer, especially during hours with very low domestic hot water consumption, which would lead to a very long waiting time once hot water is requested. A bigger problem is the unintentional short circuits caused by, for instance malfunctioning substations and other defects in the network, typically developed over a long time and poorly mapped out. In a recent study (Falkvall and Nilsson, 2014), the average return temperature from all substations in a network was significantly lower (between 5 and 10 °C) than the return temperature at the heat supply plants, implying there is substantial short circuits in the distribution network system, feeding water directly from the supply pipe to the return pipe and causing increased return temperatures. Consequently, the step response from a distinct change in supply temperature was that the return temperature followed the supply temperature. It might be tempting to look at a lowered supply temperature followed by a lowered return temperature as favourable, but in fact, it reveals a condition where substantial parts of the hot water distributed in the network returns to the heat supply without having passed any heat-transferring surfaces.
11.2.2.2 Capacity, heat loss and pump energy An increased temperature difference of the DH water, i.e. a lowered return temperature for any given supply temperature, means that the flow rate in the network can be reduced, which in turn, leads to less pump energy being required and to electricity being saved. Alternatively, the higher temperature difference increases the capacity of the network and enables more customers to be connected to the network, without having to increase the flow rate, or reduce problems with bottlenecks. On the other
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hand, the heat losses in an existing network can be reduced if the temperatures can be lowered. Approximately, one-third of the heat losses can be attributed to the return pipe and two-thirds to the supply pipe. In connection with the construction of new networks, a substantial decrease of the supply temperature can also lead the way to the use of plastic pipes, thereby reducing the installation costs.
11.2.2.3 Direct or indirect connection Practice varies among, and often within, countries regarding the choice between indirect and direct DH connection, i.e. with or without heat exchangers hydraulically separating building-internal heating systems from the network. There are advantages and disadvantages of both methods. Drawbacks with a direct connection are: the risk of a leak in the heating system having large consequences; and that it can be difficult to handle large pressure variations in networks with significant differences in height. In addition, there are risks involved from the fact that the supply temperature is generally high in the DH network in order to meet the needs of all customers in the system. Unless the DH network keeps a sufficiently low pressure and temperature levels, it must be reduced to match the internal systems of a building. The most common drawback with the indirect connection is that the use of heat exchangers entails a thermodynamic loss, as a result of the return temperature from a heat exchanger always being higher than the incoming return temperature, although the difference can be kept low with the use of modern plate heat exchangers. The use of heat exchangers also involves higher costs, although, it is not evident that the total cost of the installation always needs to be higher, considering that the primary temperature and differential pressure may need to be adjusted to the level of the secondary systems (Frederiksen and Werner, 1993).
11.2.2.4 High return temperatures The main cause for high return temperatures can be attributed to malfunctions in the DH substations. In a study from Werner (2004), an annual mean return temperature of 47 °C was found in Swedish DH systems; a decrease from 50 °C had occurred during 1993–2003. However, Petersson (1998) estimates the possible return temperature to be 32 °C with today's technology. Winberg and Werner (1987) found that the actual return temperatures during part load were higher than estimated. The study concluded that high return temperatures primarily depend on individual reasons, since neither age, user category, nor size can fully explain the high return temperatures. Common malfunctions include components not being properly designed, components not working properly, deviations from standard designs, high temperature levels of heating systems, faulty connections and incorrect control. Similar results were found by Råberger (1995) and in a report from the Swedish District Heating Association (2000). Another important reason for the high return temperatures is hydraulic shortcuts in the DH network. Zinko et al. (2005) reported that 60% of the discovered malfunctions can be ascribed to the heating system, 30% to the domestic hot water system and the remaining 10% to components in the substation, such as the heat exchanger, pump and control
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equipment. One-third of all malfunctions was related to comfort problems, while twothirds caused high return temperatures. The most important issue for the return temperature is the heating system's temperature level. Other factors that influence the return temperature include, as already mentioned, the choice between direct or indirect DH connections of the heating systems and the substation connection scheme. The influence on the return temperature from various connection schemes has been the subject of numerous studies. Simply put, one can say that connection schemes including cascading of heat exchangers, have traditionally been attributed to lower return temperature, which is linked to the generally high radiator temperatures that have prevailed. Thus, the two-stage substation is the most common connection scheme used in multi-dwelling buildings in Sweden. Examples of studies that have indicated a lower return temperature from the two-stage connection compared with the parallel connection scheme include those of Frederiksen et al. (1991), Snoek et al. (2002) and Gummérus (1989). The trend towards lower temperatures in radiator systems has reduced the benefits of cascading. Lindkvist and Walletun (2005) found that the connection scheme is of secondary importance in the selection of a new DH substation. Of higher significance, is the balancing of the secondary systems.
11.2.3 Heat supply Generally, the impact of network temperature on various sources of heat supply can be summarized for a lowered return and a lower supply temperature. For a lowered return temperature: ●
●
●
Increased efficiency in boilers with flue gas condensation, regardless of whether the boiler is heat-only or part of a combined heat and power (CHP) plant Increased electricity generation in CHP plants for a given heat load – if two or more condensers are used, which usually is the case Increased amount of utilized heat from low-grade heat sources, such as surplus heat from industrial processes and abundant renewable sources, such as solar and geothermal heat. This includes cases where two or more heat pumps connected in series are used to raise the temperature of a heat source.
For a lowered supply temperature: ●
●
Increased electricity generation in CHP plants for a given heat load Increased opportunities for utilization of low-grade heat sources, such as surplus heat from industrial processes and abundant renewable sources, such as solar and geothermal heat. This includes cases were heat pumps are used to raise the temperature of a heat source.
11.2.3.1 Flue gas condensation If district heat is supplied by a heat-only-boiler, network temperature is of little importance. The advancement of flue gas condensing boilers has offered a significant improvement in boiler efficiency. It has also helped to put focus on the pursuit of lowered return temperatures because they constitute the lower limit for how much heat can be extracted from the flue gas. Flue gas condensation increases the boiler heat output with
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10–30% depending on the fuel used. Boilers using fuels with higher moisture content, typically biofuels, gain more than, for instance boilers using natural gas. If the return temperature is lowered, the gain in heat output is also larger in the former case. The increase in heat output from a 5 °C lowered return temperature is estimated to be 1–5%.
11.2.3.2 Combined heat and power The previous section is valid also for CHP plants whenever flue gas condensation is employed, which often is the case for biomass-fired CHP plants. One thing should, however, be kept in mind: since the flue gas condenser increases the heat supply, less heat has to be supplied by the steam turbine to the condenser, which leads to less electricity being generated. The impact of network temperatures on the steam cycle is more complex. A lower supply temperature always will increase the power-to-heat ratio, i.e. more power can be produced from a given heat load. If the DH supply temperature is lowered, the turbine's condensing temperature, and consequently pressure, will be reduced, and the expansion ratio of the turbine will increase. Typical, modern, biomass-fired CHP plants employ two condensers, utilizing heat at two pressure levels. This way, more electricity can be generated. With this configuration, a reduced DH return temperature will lead to increased electricity generation in the low-pressure part of the steam turbine. The potential is, however, substantially bigger if the supply temperature can be lowered. The gain estimates are roughly a 2% increase of the power-to-heat ratio if the supply temperature is lowered by 5 °C (Falkvall and Nilsson, 2014; Johansson, 2011; Saarinen and Boman, 2012). Johansson (2011) gives a comprehensive review on distribution temperature influence on CHP plant performance.
11.2.3.3 Heat pumps Large heat pumps generally do not have a large share of the DH supply. However, there is reason to believe that they will be more common in the future. Not least in combination with large shares of intermittent power generation, heat pumps supplying DH can contribute to the development of smart grids. As for CHP plants, the DH supply temperature has a great impact on efficiency, while the return temperature has some influence in certain configurations. Generally, a lowered supply temperature is always beneficial. The coefficient of performance (COP) will increase by typically 5% or more if the supply temperature is lowered by 5 °C (Falkvall and Nilsson, 2014; Selinder and Walletun, 2009; Zinko (ed.) et al., 2005). In order to achieve higher COPs, two (or more) heat pumps are often connected in series. Each heat pump then can operate with a smaller temperature difference. In such a case, the DH return temperature will also influence the overall COP. A lowered return temperature results in a lower condensing temperature for the first heat pump in the series.
11.2.3.4 Low-grade heat sources There is growing interest in the DH industry for different kinds of low-grade heat sources. Without trying to define this somewhat vague concept, it is about utilizing,
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e.g. industrial surplus heat, solar heat, and geothermal heat, often with lower temperatures than used today in DH networks. It might also include customers that are sometimes generating heat, which is fed into the DH network (so-called prosumers), evoked by an interest among property owners to generate their own energy. In a truly smart energy system, not only will electricity be fed into the grid by end-users, but this will also be the case in thermal grids. It is quite evident that future temperature levels in DH systems are crucial for such a development. A study on the integration of prosumers is presented by Brand et al. (2014).
11.2.4 Economic value from optimized temperatures Several detailed analyses of the economic benefit of reduced temperatures are outlined in Zinko (ed.) et al. (2005). Among others, a comprehensive study by Rütschi (1997) and a calculation model called LAVA, from the Swedish District Heating Association, for the evaluation of changes in system temperatures, depending on the composition of heat production, are referred to. The value of a reduced return temperature varies greatly in different networks, as shown by Frederiksen and Werner (2013), but a typical value among 27 studied networks in Sweden is approximately 0.15 EUR/ MWh,°C. This means that if the return temperature could be lowered by 10 °C in a medium-sized network with an annual heat sale of 1 TWh, approximately 1.5 MEUR could be saved. An evaluation of the work towards increasing the cooling in the DH network in Gothenburg, gave similar results. A pay-off time of less than 3 years was obtained (Fransson, 2005). For the system in Lund in Sweden, Falkvall and Nilsson (2014) found that the average annual supply temperature could be lowered by approximately 5 °C just by an improved control method and produce a saving of approximately 0.3 MEUR. A complete evaluation of the distribution temperatures' influence on the heat supply plants must not only take into account increased efficiency and power-to-heat ratio, but it must also consider that if one heat supply plant can produce more heat, operational hours for other, more expensive and often less environmental-friendly, heat plants, can be reduced. The importance of individual analysis should also be pointed out. In some cases, it may be rational to increase the supply temperature in order to reduce the return temperature and increase the heat output of flue gas condensers and minimize pumping energy, or increase the network capacity. Therefore, there is no evidence to say which the optimal supply temperature is in a DH network. However, for any given supply temperature, the return temperature should be kept as low as possible, i.e. to maximize the temperature difference of DH water for the given supply temperature. In the long term, there is much to gain if supply temperatures can be lowered. Traditionally, there has been larger focus on return rather than supply temperatures, not least in Sweden, caused by a quite small share of CHP and as a way to handle expanding networks. At present, with the rapid expansion of CHP and a general awareness about the importance and potential of low network temperatures, the focus on lower supply temperatures is set to increase.
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11.3 Temperature optimization in district heating systems The main significance of the previous section is that no matter what part of a DH system one considers, higher temperatures than necessary inevitably lead to an increased use of primary energy resources, or vice-versa, generally lowered temperatures in the DH systems will increase energy efficiency. This section will look into existing and future ways to implement optimization of system temperatures.
11.3.1 Improving building-internal systems Werner and Sköldberg (2007) carried out an exposition of the knowledge and research situation for DH, in the world. Among other things, this exposition highlights the question of whether conventional radiator systems will be driven out of competition due to their static development in recent years. Such trends can be perceptible, since air-borne systems often are chosen in passive houses. In the present situation, however, the installation of underfloor heating systems is common, and these are especially favourable from a DH point of view, thanks to the very low temperature level. This section is intended to highlight a number of innovations in recent years aimed at increasing the efficiency and competitiveness of DH-connected hydronic heating systems. The development is important both for future competitiveness and in view of the large number of radiator systems that are in operation and will be so for a long time to come. The construction of new buildings typically amounts to less than 1% per year, relative to the existing building stock. In recent years, there has been a trend towards more ‘intelligent’ DH substations, where the aim is to use modern technology to ensure a proper operation and low return temperatures. In a report by Andersson and Werner (2005), an evaluation of the so-called function-integrated DH substation in operation was reported, and the return temperature reduction was estimated to be 10–11 °C. Typical for this substation is, among other things, that the DH flow is calculated and governed rather than regulated based on feedback control. By measuring temperatures and flows in the substation, the required flow is continuously computed. The result is a smoother control that can reduce the energy usage by avoiding overheating. The evaluation found that the benefits of a function- integrated substation exceed the cost of a conventional substation. Demand-side management involves the manipulation of the consumption of heat in order to optimize production and distribution of district heat and has been widely investigated (van der Meulen, 1988; Olsson Ingvarsson and Werner, 2008; Wigbels et al., 2005). In this process, the building's potential to be used as heat storage is considered to be significant. The idea is to even out the heat power, and flow needs of a DH network, depending mainly on the fluctuations in domestic hot water loads. In this way, the need for expensive and environmentally unsound peak production is reduced. This problem is also identified in connection with electric heating of buildings. So-called load shedding, when domestic hot water consumption is prioritized, is another type of demand-side management.
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Johansson (2014) achieved impressive results through a distributed load control system using load shedding in an entire neighbourhood where the substation that is currently best suited ‘lends’ flow to the DH network. As already mentioned, space heating temperatures have been lowered substantially over the years. Modern buildings can be designed for very low radiator supply temperatures without the need for extremely large radiators, thanks to the buildings' high energy performance. Moreover, there is an ongoing discussion whether underfloor heating should be employed in new buildings or if no hydronic heating systems at all are required. Some claim that underfloor heating (Sandberg, 2011; Sikander and Ruud, 2011) increases heat use, while others (Schneiders, 2005) emphasize that it actually facilitates lower use of primary energy because of the low operating temperatures. Underfloor heating can also provide a more even indoor temperature with a high degree of self-regulation (Karlsson and Hagentoft, 2012; Karlsson, 2010). From an economical point of view, radiator systems are generally cheaper. Even cheaper would be to completely omit a hydronic heating system and only supply heat via the ventilation system. This has become common in passive houses and other buildings with extremely high energy performance. In terms of temperature levels, air-borne heating also entails low DH temperatures. A drawback is that user-controlled ventilation often is employed in order to reduce ventilation heat losses, which may result in a more uneven heat load. In order to secure proper indoor comfort for the end-user, additional room heaters are often used. Due to the relatively few operational hours and low heat load for such heaters, electric resistance heaters are sometimes the chosen type of heater. In Lauenburg and Wollerstrand (2014), a radiator control method based on control of both the supply temperature and flow rate in the radiator system is reported. The study shows that the DH return temperature can be lowered. A strength of the proposed control method, which can be described as a combination of a low-flow system and a system with normal flow depending on the heat load, is that it automatically adapts to varying working conditions, such as long-term changes in the primary supply temperature. Therefore, it always strives to provide the lowest possible return temperature. Lowered network supply temperature or energy-saving measures, signifying an increase in the oversizing of a system, represent examples of changes that the control method can adapt to. Johansson (2011) has studied the overall system impact of employing so-called radiator fans, devices that induce heat transfer by forced convection and consequently lower the return temperature, and found substantial potential primary energy savings. Another study on ways to lower radiator system temperatures by increased convection was performed by Ploskic (2013). It is suggested that existing radiator systems can operate at substantially lower temperatures.
11.3.2 Lowering of network temperatures Initially, we discuss in this section, the prospects of lowering network temperatures in existing networks. Then we look into the development of the fourth generation of DH systems and how they can be incorporated into existing networks.
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11.3.2.1 Existing systems In many DH systems, there has been substantial improvement in terms of lowered network temperatures. However, there is still much to be done. Many systems are operated on a supply temperature based on tradition, older outer conditions, and use large safety margins. Traditionally, there has been a large focus on the return temperature, however, it becomes more and more relevant to lower the supply temperature to go along with increasing shares of CHP and other sources of surplus heat. Moreover, future energy savings by renovation of existing buildings must be translated into lowered network temperatures.
11.3.2.2 Low temperature DH Currently, we can see many small networks being built in many different countries employing substantially lower temperatures, either as solitary systems with their own heat supply, or as subsystems connected to an existing network (Wiltshire, 2013). Figure 11.2 shows characteristic temperatures in DH systems. Even if the best systems still have not reached the vision of the fourth generation of DH, the development has come a long way. The Danish low temperature DH (LTDH) project is probably the closest case that reaches the suggested level for network temperatures in the fourth generation of DH. There are several papers, reports and theses describing the project (see for
Figure 11.2 Examples of district heating supply and return temperatures. Based on Andersson (2014), Frederiksen and Werner (2013), and Werner (2013). LTDH is short for low temperature district heating and 4GDH for the fourth generation district heating. The reason for Västerås to be on a lower level than state-of-the-art is that the latter is based on design criteria that implies higher network temperatures.
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instance Brand, 2013; Dalla Rosa 2012, for a thorough review). The project includes many novelties. One is the handling of domestic hot water provision. From a comfort perspective, 45 °C was set as the design temperature. In order to prevent Legionella growth, which in most building standards’ demands at least 50 °C in all parts of a domestic hot water system, a German standard, DVGW 551 (Gerhardy, 2012), was referred to, which does not set any demand for the lowest allowed temperature in domestic hot water systems of a total volume (excluding water heater) of 3 l. The demand not to exceed 3 l is normally fulfilled if every dwelling has its own heat exchanger that limits the required number of pipes between the heat exchanger and the taps. Multi-dwelling buildings must therefore, in practice, adopt so-called flat substations, a technology common in some countries but not in others. Special substations were developed to be able to provide hot water at 47 °C with a supply temperature of 50 °C, resulting in a return temperature of 20 °C. There was a 2 °C margin in order to fulfill the comfort demand. The radiator systems were indirectly connected. Generally, the LTDH project has put the focus on minimizing the distribution heat losses by using low temperatures and also by using small pipe diameters. Many low temperature systems are worth mentioning (Wiltshire, 2013). In Figure 11.2 the example of the Swedish town Västerås was displayed. There, the aim was to find an economically viable DH connection for newly built houses with high energy performance. The focus was to reduce heat losses by using low temperatures; reduce installation cost by, among other things, improving the cooperation with real estate companies and substituting household electricity use for white goods, bathroom floor heating and towel dryers with district heat. Moreover, plastic pipes and cheap heating ventilation and air condition (HVAC) installations were prioritized.
11.3.2.3 Integration of fourth generation into third generation A matter of great importance is the integration of the fourth generation of DH into the third generation. One scenario is the development of complete new low-temperature systems with dedicated heat supply. In such a case, the heat supply can take full advantage of the low network temperatures. If a new low-temperature system is developed within an existing system, it is however not evident how the existing heat supply plants will benefit from the low network temperatures if the rest of the network still requires a high return temperature. A sub-network can for instance be connected to the main network via a shunt connection, to reduce the supply temperature, or via a heat exchanger, to reduce the supply temperature and the differential pressure. The sub-network will still benefit from lower heat losses. One way to increase the benefit for the main network is to use the return pipe to supply the sub-network. This method has been demonstrated, for instance, in Denmark in connection with the LTDH project (Christensen and Kaarup Olsen, 2011; Holm Christiansen, 2013). In this case, the main return pipe becomes the supply pipe in the LTDH-network. When needed, additional hot flow is supplied from the main supply pipe. In the future, it will be increasingly important to take advantage of the low-temperature networks.
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11.4 Future trends We have now seen the rise of low-temperature systems, especially in connection with newly built energy efficient housing. The future will tell which supply temperature level will prevail: 60 °C – which will make it easier to comply with existing regulation on Legionella prevention, to supply heat for dishwashers and laundry machines and to possibly supply heat-driven cooling; 50 °C – as the Danish LTDH project has demonstrated is quite possible, resulting in very low heat losses and the possibility of utilizing the return pipe in existing networks; or maybe even lower, e.g. 40 °C – with the aim to mainly supply space heat, while domestic hot water provision would need some additional heat supply, e.g. a small heat pump which is also being investigated in Denmark (Zvingilaite et al., 2012). The lower the supply temperature, the greater the potential to integrate low-grade heat sources with greater potential to save money and reduce environmental impact. As mentioned in the previous section, much attention will be directed to the integration of the fourth generation technology into existing systems. Although much focus is directed towards new buildings, the main part of the future building stock already exists. A lot of old buildings are in need of renovation, which opens up space for substantial improvement of energy performance. This can be viewed as a threat to future deliveries of district heat or to drastically reduce network temperatures, especially if existing space heating systems are maintained, offering larger relative heat transfer surfaces. Other matters of future interest concern if, or when, low-grade, distributed, heat supply will gain larger interest. One possible development that would facilitate such a development is increased prices on biofuels due to increasing demand, for example, from the transport sector. In a future with increasing shares of low-grade, often abundant, heat supply, such as solar and geothermal heat, the question arises whether there will be the same focus as today on minimizing distribution heat losses. Perhaps, there must intensified efforts to reduce installation costs in order to compete with less infrastructure-intensive technologies, such as heat pumps, resistance heating and, to some extent, natural gas and solar heat. In the buildings' HVAC systems, development can be towards either reducing temperature demands as much as possible, for example, by using underfloor or wall heating, or towards reducing installation costs, for example, by employing cheap radiator systems. On the heat supply side, distributed technology is yet to be explored. For conventional technologies, there is much to explore in relation to substantially lowered network temperatures. For instance, CHP plants can be much more efficiently designed if a DH supply temperature of, e.g. 60 °C is used (Genrup, 2014).
11.5 Sources of further information As mentioned before, and as the reader might recognize, much of the facts presented in this chapter can be found in the textbook by Frederiksen and Werner (2013), not least when it comes to heat supply plant technology. For extensive background on heat supply plants and district heat distribution economy related to network temperatures, the
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work of Rütschi (1997) and Zinko (ed.) et al. (2005) is recommended. For everything regarding the Danish LTDH project, the reader is referred to Dalla Rosa (2012) and Brand (2013). For future activities on the fourth generation of DH, the large Danish research center, 4DH will be a useful source of information.
References Amberg, H.F., 1980. Improved utilization of district heating systems by lowering the return flow temperature. Fernwärme Int. 9, 151–155. Andersson, K., 2014. Mälarenergi, Västerås, Sweden. Internal material. Andersson, S., Werner, S., 2005. Utvärdering av funktionsintegrerad fjärrvärmecentral [Evaluation of Function-Integrated District Heating Substation]. No. Report FoU 2005:125. Swedish District Heating Association, Stockholm. Bøhm, B., 1986. On the optimal temperature level in new district heating networks. Fernwärme Int. 15, 301–306. Brand, M., 2013. Heating and Domestic Hot Water Systems in Buildings Supplied by Low- Temperature District Heating (Doktorsavhandling). Technical University of Denmark. Brand, L., Calvén, A., Englund, J., Landersjö, H., Lauenburg, P., 2014. Smart district heating networks – a simulation study of prosumers’ impact on technical parameters in distribution networks. Appl. Energy 129, 39–48. http://dx.doi.org/10.1016/j.apenergy.2014.04.079. Christensen, S.K., Kaarup Olsen, P., 2011. New district heating concept: use the return water for supply in new areas/networks. Hot Cool 2011. Dalla Rosa, A., 2012. The Development of a new District Heating Concept – Network Design and Optimization for Integrating Energy Conservation and Renewable Energy Use (Doktorsavhandling). DTU, Danmark. Falkvall, M., Nilsson, V., 2014. Optimerad framledningstemperatur i Lunds fjärrrvärmenät [Optimized Supply Temperature in Lund District Heating Network]. Master Thesis. Lund University. Fransson, A., 2005. Avkylningsarbete på Göteborg Energi AB 1995–2004 – Praktisk tillämpning av FoU-kunskap [Optimizing of the District Heating System in Gothenburg 1995–2004 – Practical Application of R&D Knowledge]. No. Report FoU 2005:132. Swedish District Heating Association, Stockholm. Frederiksen, S., Werner, S., 1993. Fjärrvärme – Teori, teknik och funktion [District Heating – Theory, Technology and Function]. Studentlitteratur, Lund. Frederiksen, S., Werner, S., 2013. District Heating and Cooling. Studentlitteratur, Lund. Frederiksen, S., Nikolic, D., Wollerstrand, J., 1991. District heating house stations for optimum operation. In: Presented at the UNICHAL Congress, Budapest. Genrup, M., 2014. Associate Professor in Thermal Power Engineering. Department for Energy Sciences, Lund University. Personal communication. Gerhardy, K., 2012. Das DVGW-Arbeitsblatt W 551 und die 3-Liter-Regel. Downloaded February 1, 2014, from https://www.dvgw.de/fileadmin/dvgw/wasser/gesundheit/1202gerhardy.pdf. Gummérus, P., 1989. Analys av konventionella abonnentcentraler i fjärrvärmesystem [Analysis of Conventional District Heating Substations in District Heating Systems]. Doctoral Thesis. Department of Energy Conversion, Chalmers University of Technology, Gothenburg. Gummérus, P., Petersson, S., 1999. Robust fjärrvärmecentral [Robust District Heating Substation]. No. Report A 99-223. Department of Energy and Environment, Chalmers University of Technology, Gothenburg.
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Holm Christiansen, C., 2013. Low temperature networks: concept, demonstration and guideline. In: Presentation Held at DHC+ Conference, November 5, 2013. Link: http://www.euroheat. org/Admin/Public/Download.aspx?file=Files%2FFiler%2FPresentations%2F20131105_ DHCplusConfernce%2F131105_3B_1600_1730+Christiansen.pdf. Olsson Ingvarsson, L., Werner, S., 2008. Building mass used as short term heat storage. In: Presented at the 11th International Symposium on District Heating and Cooling, Reykjavik. Johansson, P.-O., 2011. Buildings and District Heating – Contributions to Development and Assessments of Efficient Technology. Doctoral Thesis. Lund University. Johansson, C., 2014. On Intelligent District Heating. Doctoral Thesis. Department of Computer Science and Engineering, Blekinge Institute of Technology, Karlskrona. Karlsson, H., 2010. Thermal Modelling of Water-Based Floor Heating Systems – Supply Temperature Optimisation and Self-Regulating Effects. Doctoral Thesis. Chalmers University of Technology, Gothenburg. Karlsson, H., Hagentoft, C.-E., 2012. Byggnadsintegrerad uppvärmning – Utveckling av simuleringsmodell med tillämpningar samt analys av alternativa, förenklade och självreglerande vattenburna golvvärmesystem i småhus [Building-Integrated Heating – Development of Simulation Model with Applications and Analysis of Alternative, Simplified and Self- Regulating Waterborne Underfloor Heating Systems in Single-Family Houses]. Chalmers University of Technology, Gothenburg. Lauenburg, P., 2009. Improved Supply of District Heat to Hydronic Space Heating Systems. Doctoral Thesis. Department of Energy Sciences, Faculty of Engineering, Lund University, Lund. Lauenburg, P., Wollerstrand, J., 2014. Adaptive control of radiator systems for a lowest possible district heating return temperature. Energy Build. 72, 132–140. http://dx.doi.org/10.1016/j. enbuild.2013.12.011. Liao, Z., Swainson, M., Dexter, A.L., 2005. On the control of heating systems in the UK. Build. Environ. 40, 343–351. http://dx.doi.org/10.1016/j.buildenv.2004.05.014. Lindkvist, H., Walletun, H., 2005. Teknisk utvärdering av gamla och nya fjärrvärmecentraler i Slagsta [Technical Evaluation of Old and New District Heating Substations in Slagsta]. No. Report FoU 2005:120. Svensk Fjärrvärme, Stockholm. Lund, H., Werner, S., Wiltshire, R., Svendsen, S., Thorsen, J.E., Hvelplund, F., Mathiesen, B.V., 2014. 4th Generation District Heating (4GDH): integrating smart thermal grids into future sustainable energy systems. Energy 68, 1–11. http://dx.doi.org/10.1016/j. energy.2014.02.089. Peeters, L., Van der Veken, J., Hens, H., Helsen, L., D’haeseleer, W., 2008. Control of heating systems in residential buildings: current practice. Energy Build. 40, 1446–1455. http:// dx.doi.org/10.1016/j.enbuild.2008.02.016. Petersson, S., 1998. Analys av konventionella radiatorsystem [Analysis of Conventional Radiator Systems]. Licentiate Thesis. Department of Energy and Environment, Chalmers University of Technology, Gothenburg. Petersson, S., Werner, S., 2005. Långtidsegenskaper hos lågflödesinjusterade radiatorsystem [Long-Term Properties for Low-Flow Systems]. No. Report FoU 2003:88. Swedish District Heating Association, Stockholm. Ploskic, A., 2013. Technical Solutions for Low-Temperature Heat Emission in Buildings. Doctoral Thesis. KTH Royal Institute of Technology, Stockholm. Råberger, L., 1995. Effektivisering av abonnentcentraler i fjärrvärmenät [Efficiency of Consumer Substations in District Heating Systems]. Licentiate Thesis. Department of Energy and Environment, Chalmers University of Technology, Gothenburg. Rütschi, M., 1997. The return temperature in district heating networks – a key factor for the economical operation of district heating. Fernwärme Int. 26, 498–508.
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Saarinen, L., Boman, K., 2012. Optimized District Heating Supply Temperature for Large Networks. No. P08–830. Värmeforsk, Stockholm. Sandberg, E., 2011. Energirelaterade godhetstal för flerbostadshus [Energy-Related Key Figures for Multi-Residential Buildings]. http://www.aton.se/img/userfiles/file/Nyproduktiongodhetstal.pdf. Schelosky, H.U., 1980. The possibilities of achieving lower return-flow temperatures in district-heating systems. Fernwärme Int. 9, 275–280. Schneiders, J., 2005. Dynamische Verhalten und Wärmeübergabeverluste von Flächenheizungen. Passivhausinstitut. Selinder, P., Walletun, H., 2009. Modell för värdering av ändrade driftförutsättningar i fjärrvärmenät [Model for Evaluation of Changed Operational Conditions in District Heating Networks]. Swedish District Heating Association. http://www.svenskfjarrvarme.se/ medlem/fokusomraden-/kundanlaggningar/berakningsprogram/. Sikander, E., Ruud, S., 2011. Teknik- och systemlösningar för lågenergihus. En översikt [Technical and System Solutions for Low-Energy Buildings. An Overview]. Report No. 2011:68. SP Technical Research Institute of Sweden. Skagestad, B., Mildenstein, P., 2002. District Heating and Cooling Handbook. International Energy Agency, IEA District Heating and Cooling, Programme of Research, Development and Demonstration on District Heating and Cooling, Sittard, the Netherlands. Snoek, C., Yang, L., Frederiksen, S., Korsman, H., 2002. Optimization of District Heating Systems by Maximizing Building Heating System Temperature Differences. No. Report 2002:S2. International Energy Agency, IEA District Heating and Cooling, Programme of Research, Development and Demonstration on District Heating and Cooling, Sittard, the Netherlands. Swedish District Heating Association, 2000. Avkylningen i ett fjärrvärmesystem [District Heating System Cooling]. No. Report FVF 2000:03. Stockholm. Trüschel, A., 2002. Hydronic Heating Systems – The Effect of design on System Sensitivity. Doctoral Thesis. Department of Building Services Engineering, Chalmers University of Technology, Gothenburg. Trüschel, A., 2005. Värdet av injustering [The Value of Balancing Procedure]. No. Report FoU 2005:134. Swedish District Heating Association, Stockholm. van der Meulen, S.F., 1988. Load management in district heating systems. Energy Build. 12, 179–189. Werner, S., 2004. Nytta med svensk fjärrvärmeforskning [The Benefit of Swedish District Heating Research]. No. Report FoU 2004:9. Swedish District Heating Association, Stockholm. Werner, S., 2013. Current and future temperature levels in district heating systems. In: 4DH, First PhD Student Seminar. Werner, S., Sköldberg, H., 2007. Fjärrvärmens omvärld – Beskrivning av kunskaps- och forskningsläge [The District Heating World – Current Knowledge and Possible Research Issues]. No. Report FoU 2007:4. Swedish District Heating Association, Stockholm. Wigbels, M., Bøhm, B., Sipilae, K., 2005. Dynamic Heat Storage Optimization and Demand Side Management. No. Report 2005:8. International Energy Agency IEA District Heating and Cooling, CHP Annex VII & NOVEM, Sittard, the Netherlands. Wiltshire, R., 2013. International Energy Agency (IEA) Research on District Heating. UK Energy Research Centre – Low Carbon Heat: Research Gaps & Opportunities. Winberg, A., Werner, S., 1987. Avkylning av fjärrvärmevatten i befintliga abonnentcentraler [Cooling of District Heating Water in Existing Substations]. No. Report Hetvattenteknik 272. Värmeforsk.
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Zinko (ed.), H., Hoon, L., Bong-Kyun, K., Youn-Hong, K., Lindkvist, H., Loewen, A., Seungkyu, H., Walletun, H., Wigbels, M., 2005. Improvement of Operational Temperature Differences in District Heating Systems. No. Annex VII 2005:8DHC-05.03. International Energy Agency, IEA District Heating and Cooling, Programme of Research, Development and Demonstration on District Heating and Cooling, Sittard, the Netherlands. Zvingilaite, E., Ommen, T., Elmegaard, B., Franck, M.L., 2012. Low temperature district heating consumer unit with micro heat pump for domestic hot water preparation. In: Proceedings of the 13th International Symposium on District Heating and Cooling, Copenhagen.
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P. Lauenburg Lund University, Lund, Sweden
11.1 Introduction This chapter deals with the temperature levels in district heating (DH) systems, including building heating systems. The different temperatures in the systems are of large importance for the systems' efficiency and consequently, for their economic and environmental performance. Generally, low temperatures are favourable, however several factors limit how low temperatures can actually be. The objective of this chapter is to provide the reader with knowledge about the implications of changed temperatures on the supply and distribution of district heat, and how temperatures in end-user heating systems affect the DH system. Furthermore, present and future development and trends are reviewed.
11.1.1 A historical overview of DH temperature levels In recent years, it has become common to talk about the future of DH in terms of a development of a fourth generation of DH technology (Frederiksen and Werner, 2013). The first generation of DH technology used steam as a heat carrier and was applied in new constructions, until approximately 1930. A few systems are still in operation, e.g. in New York, Paris and partly in Copenhagen. The second generation entailed a shift to pressurized water as a heat carrier, typically with temperatures well above 100 °C. Concrete ducts and shell and tube heat exchangers are typical features, which are still common in existing systems. The third generation has been dominating since the 1980s, with lower temperatures, pre-fabricated and pre-insulated pipes and material-lean components, as some of its distinctive features. Each generation has been state-of-the-art for 40–50 years. Facing ever harder competition from other technologies, such as heat pumps and natural gas, and decreasing specific heat demand in new and renovated buildings, there is a need for research and development in order to introduce the fourth generation over the coming years. Typical features of the fourth generation will be further lowered temperatures, along with assembly-oriented components and more flexible materials. This chapter focuses on the significance of temperatures on the different parts of the DH system. For more information about the four generations of DH, the paper by Lund et al. (2014) is recommended.
Advanced District Heating and Cooling (DHC) Systems. http://dx.doi.org/10.1016/B978-1-78242-374-4.00011-2 Copyright © 2016 Elsevier Ltd. All rights reserved.
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11.2 The connection between district heating temperatures and different parts of the district heating system In this section, some basic information is provided regarding what temperature levels are required in DH systems and how they relate to the performance of supply and distribution. To begin with, the temperature demands that arise as a result of end-use, mainly from building space heating and domestic hot water consumption, are dealt with. Then follows a section on the importance of temperature levels for distribution of DH, and finally, how network temperatures influence the heat supply. The temperatures discussed in this chapter are displayed in Figure 11.1. The DH supply temperature (TDH,supp) must be high enough (1) to satisfy the temperature demands in the building internal systems (space heat and domestic hot water) and (2) to keep the flow rate in the DH network sufficiently low in order to avoid unreasonably high flow velocities. Space heating temperatures are displayed in blue for a so-called 80/60 system (TSH,supp and TSH,ret). While the space heating temperatures are dependent on the outdoor temperature, the domestic hot water temperature (TDHW,supp) is constant. The return temperature from a building (TDH,ret) is the combination of the return temperatures from the space heating and hot water system.
11.2.1 Building-internal heat supply systems The two main services provided for by DH, particularly in the case of dwellings, are the provision of domestic hot water and space heating. Other purposes, such as washing and drying of clothes, refrigeration and industrial applications, are not addressed here. Although domestic hot water and space heating systems can be found TSH, supp
TSH, ret
TDHW, supp
TDH, ret
Tin
Temperatures
TDH, supp
Outdoor temperature
Figure 11.1 The different temperatures discussed in this chapter.
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in all types of buildings, regardless of the heating source, the design and operation of the systems have sometimes been affected in various ways depending on whether they are connected to DH. One example is the need for a heating system that provides a low return temperature of DH water. This also applies to buildings served by, for instance, heat pumps, while boilers, traditionally, have demanded relatively high return temperatures. Flue gas condensation in boilers is however common nowadays, also gaining from low return temperatures.
11.2.1.1 Domestic hot water Provision of domestic hot water is normally through either heaters with accumulation or heat exchangers (instantaneous water heaters). Practices have differed between countries. There are advantages and drawbacks for both methods. For example, accumulation involves the use of smaller dimensions for DH pipes and valves, while instantaneous heaters generally provide lower return temperatures. Regarding the required tap water temperature, conditions also vary between countries. Typically, 50 °C is required at the tap. Two factors determine the required temperature: comfort demand and Legionella prevention. If Legionella prevention cannot be handled separately, it will constitute the lower boundary. With ever lower temperatures required in space heating systems, as discussed in the next section, it is also likely that the domestic hot water system in the future will set the overall boundary for how low a supply temperature can be allowed in a DH system.
11.2.1.2 Space heating Space heating is often provided for by water-borne radiator systems, especially in countries with a cold climate or with a large share of DH. Underfloor heating has gained significant popularity in recent years. Ventilation systems are sometimes also used for space heating, especially in commercial premises, causing the heat to be air-borne. In the latter case, heat supplied by the air typically covers the heat losses from ventilation, while a radiator system covers the heat losses from transmission. Regarding radiator system temperatures, as well as for DH network temperatures, different levels prevail–both between countries but also depending on the age of the systems. The general ambition has been to lower the temperatures, since these are the single largest factors determining the temperature levels in the DH networks. Skagestad and Mildenstein (2002) give some examples of typical design radiator temperatures (supply/return temperatures) in various countries: Denmark and Finland—70/40 °C; Korea—70/50 °C; Romania and Russia—95/75 °C; the UK— 82/70 °C; Poland—85/71 °C; and Germany—80/60 °C. The development in Sweden clearly shows how lower radiator temperatures are strived for. Previously, higher temperatures, such as 90/70 and 80/60 °C have been used, partly because there were no incentives for low temperatures in systems with boilers, but also because smaller radiators could be employed. The advantages of low temperatures have led to lower temperatures being used today, e.g. 60/45, 60/40 or 55/45 °C. Since 1982, temperatures
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higher than 55 °C (60 °C in certain cases with DH) are not allowed in new heating systems, which should promote the use of low-temperature heating systems, e.g. solar heating (Frederiksen and Werner, 1993). When looking at these temperatures, it must be kept in mind that they express the design temperatures, i.e. the temperatures required at extremely low outdoor temperatures. For most of the heating season, significantly lower heating system temperatures are required. Another factor affecting the radiator temperatures in practice is the degree of oversizing of the heating system. There is a substantial oversizing of the radiator system in general and of the radiator surfaces in particular, as presented in both Swedish (Gummérus and Petersson, 1999; Trüschel, 2002) and international studies (Liao et al., 2005; Peeters et al., 2008; Skagestad and Mildenstein, 2002). This is due to an overestimation of a building's heat losses, which also often tend to decrease over time if energy-saving measures are implemented. Another reason is that, during the design stage, the components are generally selected in sizes larger than required to ensure safety margins. In order for oversizing not to cause overheating of a building, the supply temperature, or the circulation flow, or both, in the radiator system must be adjusted. As described in Lauenburg (2009), the control curves of the various substations in an area were found to differ to a surprisingly large degree, despite the fact that the houses were built at the same time and were of a similar structure. Similar experiences have been described by Lindkvist and Walletun (2005). The most common approach is to reduce the supply temperature in an oversized radiator system to avoid indoor temperatures becoming too high. However, the flow can also be reduced, and the system can be adjusted to work as a so-called low-flow system. By substantially reducing the flow, while maintaining a high supply temperature, a low return temperature can be achieved. Although the heat transfer in the heat exchanger is deteriorated, a lower DH return temperature is generally obtained. To intentionally install larger radiators in order to obtain a low-flow system is rarely economically viable (Bøhm, 1986). On the other hand, oversizing and the design of systems for a high flow and a small temperature difference often render it possible to adapt the low-flow method. Early studies on low-flow systems were made by Schelosky (1980) and Amberg (1980). Today, the method is used to some extent, although there still seems to be a division between those who advocate it and its opponents. Trüschel (2002), who conducted a comprehensive study on hydronic heating systems, showed that the return temperature is the lowest and thermostatic radiator valves are the most effective in a low-flow system. The low flow leads to very low pressure drops in the system, and all thermostatic radiator valves thus work at approximately the same differential pressure and with a high authority. The low DH return temperature of low-flow systems has also been demonstrated by, among others, Gummérus and Petersson (1999), Petersson (1998), and Petersson and Werner (2005). Moreover, all these authors stressed the importance of the systems being balanced, since this has the greatest impact on the DH return temperature. One drawback of the method may be that a reduction in the DH supply temperature can result in an increased DH return temperature, due to an increased flow if the difference between primary and secondary supply temperatures is small. This is most
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critical around the so-called supply temperature ‘breakpoint’, typically between 0 and 5 °C outside air temperature, where the supply temperature has not yet been raised, although the heat load is relatively high. This also could become a real problem if network supply temperature was significantly lowered. In a study of DH substations in operation, Råberger (1995) found that high return temperatures are mainly due to the return temperature of the radiator system. However, extremely high-return temperatures depend on malfunctions in the substation. Furthermore, it is important that the heating system is well balanced. This was also shown by Trüschel (2005), where the value of balancing of three heating systems was estimated to give a pay-off time between 1.5 and 5.5 years.
11.2.2 Heat distribution 11.2.2.1 Temperature and flow Theoretically, for any given heat load, a lowered supply temperature will result in a higher return temperature. The reason for this is that the required DH flow must be increased in order to supply the same amount of heat. This in turn leads to a shorter time of stay for the water in heat exchangers or radiators, and consequently an increased return temperature. In practice, however, intentional and unintentional short circuits in the distribution system may disturb this relation. Intentional short circuits are caused by by-pass connections used at peripheral locations in the network in order to avoid stagnant DH water during summer, especially during hours with very low domestic hot water consumption, which would lead to a very long waiting time once hot water is requested. A bigger problem is the unintentional short circuits caused by, for instance malfunctioning substations and other defects in the network, typically developed over a long time and poorly mapped out. In a recent study (Falkvall and Nilsson, 2014), the average return temperature from all substations in a network was significantly lower (between 5 and 10 °C) than the return temperature at the heat supply plants, implying there is substantial short circuits in the distribution network system, feeding water directly from the supply pipe to the return pipe and causing increased return temperatures. Consequently, the step response from a distinct change in supply temperature was that the return temperature followed the supply temperature. It might be tempting to look at a lowered supply temperature followed by a lowered return temperature as favourable, but in fact, it reveals a condition where substantial parts of the hot water distributed in the network returns to the heat supply without having passed any heat-transferring surfaces.
11.2.2.2 Capacity, heat loss and pump energy An increased temperature difference of the DH water, i.e. a lowered return temperature for any given supply temperature, means that the flow rate in the network can be reduced, which in turn, leads to less pump energy being required and to electricity being saved. Alternatively, the higher temperature difference increases the capacity of the network and enables more customers to be connected to the network, without having to increase the flow rate, or reduce problems with bottlenecks. On the other
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hand, the heat losses in an existing network can be reduced if the temperatures can be lowered. Approximately, one-third of the heat losses can be attributed to the return pipe and two-thirds to the supply pipe. In connection with the construction of new networks, a substantial decrease of the supply temperature can also lead the way to the use of plastic pipes, thereby reducing the installation costs.
11.2.2.3 Direct or indirect connection Practice varies among, and often within, countries regarding the choice between indirect and direct DH connection, i.e. with or without heat exchangers hydraulically separating building-internal heating systems from the network. There are advantages and disadvantages of both methods. Drawbacks with a direct connection are: the risk of a leak in the heating system having large consequences; and that it can be difficult to handle large pressure variations in networks with significant differences in height. In addition, there are risks involved from the fact that the supply temperature is generally high in the DH network in order to meet the needs of all customers in the system. Unless the DH network keeps a sufficiently low pressure and temperature levels, it must be reduced to match the internal systems of a building. The most common drawback with the indirect connection is that the use of heat exchangers entails a thermodynamic loss, as a result of the return temperature from a heat exchanger always being higher than the incoming return temperature, although the difference can be kept low with the use of modern plate heat exchangers. The use of heat exchangers also involves higher costs, although, it is not evident that the total cost of the installation always needs to be higher, considering that the primary temperature and differential pressure may need to be adjusted to the level of the secondary systems (Frederiksen and Werner, 1993).
11.2.2.4 High return temperatures The main cause for high return temperatures can be attributed to malfunctions in the DH substations. In a study from Werner (2004), an annual mean return temperature of 47 °C was found in Swedish DH systems; a decrease from 50 °C had occurred during 1993–2003. However, Petersson (1998) estimates the possible return temperature to be 32 °C with today's technology. Winberg and Werner (1987) found that the actual return temperatures during part load were higher than estimated. The study concluded that high return temperatures primarily depend on individual reasons, since neither age, user category, nor size can fully explain the high return temperatures. Common malfunctions include components not being properly designed, components not working properly, deviations from standard designs, high temperature levels of heating systems, faulty connections and incorrect control. Similar results were found by Råberger (1995) and in a report from the Swedish District Heating Association (2000). Another important reason for the high return temperatures is hydraulic shortcuts in the DH network. Zinko et al. (2005) reported that 60% of the discovered malfunctions can be ascribed to the heating system, 30% to the domestic hot water system and the remaining 10% to components in the substation, such as the heat exchanger, pump and control
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equipment. One-third of all malfunctions was related to comfort problems, while twothirds caused high return temperatures. The most important issue for the return temperature is the heating system's temperature level. Other factors that influence the return temperature include, as already mentioned, the choice between direct or indirect DH connections of the heating systems and the substation connection scheme. The influence on the return temperature from various connection schemes has been the subject of numerous studies. Simply put, one can say that connection schemes including cascading of heat exchangers, have traditionally been attributed to lower return temperature, which is linked to the generally high radiator temperatures that have prevailed. Thus, the two-stage substation is the most common connection scheme used in multi-dwelling buildings in Sweden. Examples of studies that have indicated a lower return temperature from the two-stage connection compared with the parallel connection scheme include those of Frederiksen et al. (1991), Snoek et al. (2002) and Gummérus (1989). The trend towards lower temperatures in radiator systems has reduced the benefits of cascading. Lindkvist and Walletun (2005) found that the connection scheme is of secondary importance in the selection of a new DH substation. Of higher significance, is the balancing of the secondary systems.
11.2.3 Heat supply Generally, the impact of network temperature on various sources of heat supply can be summarized for a lowered return and a lower supply temperature. For a lowered return temperature: ●
●
●
Increased efficiency in boilers with flue gas condensation, regardless of whether the boiler is heat-only or part of a combined heat and power (CHP) plant Increased electricity generation in CHP plants for a given heat load – if two or more condensers are used, which usually is the case Increased amount of utilized heat from low-grade heat sources, such as surplus heat from industrial processes and abundant renewable sources, such as solar and geothermal heat. This includes cases where two or more heat pumps connected in series are used to raise the temperature of a heat source.
For a lowered supply temperature: ●
●
Increased electricity generation in CHP plants for a given heat load Increased opportunities for utilization of low-grade heat sources, such as surplus heat from industrial processes and abundant renewable sources, such as solar and geothermal heat. This includes cases were heat pumps are used to raise the temperature of a heat source.
11.2.3.1 Flue gas condensation If district heat is supplied by a heat-only-boiler, network temperature is of little importance. The advancement of flue gas condensing boilers has offered a significant improvement in boiler efficiency. It has also helped to put focus on the pursuit of lowered return temperatures because they constitute the lower limit for how much heat can be extracted from the flue gas. Flue gas condensation increases the boiler heat output with
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10–30% depending on the fuel used. Boilers using fuels with higher moisture content, typically biofuels, gain more than, for instance boilers using natural gas. If the return temperature is lowered, the gain in heat output is also larger in the former case. The increase in heat output from a 5 °C lowered return temperature is estimated to be 1–5%.
11.2.3.2 Combined heat and power The previous section is valid also for CHP plants whenever flue gas condensation is employed, which often is the case for biomass-fired CHP plants. One thing should, however, be kept in mind: since the flue gas condenser increases the heat supply, less heat has to be supplied by the steam turbine to the condenser, which leads to less electricity being generated. The impact of network temperatures on the steam cycle is more complex. A lower supply temperature always will increase the power-to-heat ratio, i.e. more power can be produced from a given heat load. If the DH supply temperature is lowered, the turbine's condensing temperature, and consequently pressure, will be reduced, and the expansion ratio of the turbine will increase. Typical, modern, biomass-fired CHP plants employ two condensers, utilizing heat at two pressure levels. This way, more electricity can be generated. With this configuration, a reduced DH return temperature will lead to increased electricity generation in the low-pressure part of the steam turbine. The potential is, however, substantially bigger if the supply temperature can be lowered. The gain estimates are roughly a 2% increase of the power-to-heat ratio if the supply temperature is lowered by 5 °C (Falkvall and Nilsson, 2014; Johansson, 2011; Saarinen and Boman, 2012). Johansson (2011) gives a comprehensive review on distribution temperature influence on CHP plant performance.
11.2.3.3 Heat pumps Large heat pumps generally do not have a large share of the DH supply. However, there is reason to believe that they will be more common in the future. Not least in combination with large shares of intermittent power generation, heat pumps supplying DH can contribute to the development of smart grids. As for CHP plants, the DH supply temperature has a great impact on efficiency, while the return temperature has some influence in certain configurations. Generally, a lowered supply temperature is always beneficial. The coefficient of performance (COP) will increase by typically 5% or more if the supply temperature is lowered by 5 °C (Falkvall and Nilsson, 2014; Selinder and Walletun, 2009; Zinko (ed.) et al., 2005). In order to achieve higher COPs, two (or more) heat pumps are often connected in series. Each heat pump then can operate with a smaller temperature difference. In such a case, the DH return temperature will also influence the overall COP. A lowered return temperature results in a lower condensing temperature for the first heat pump in the series.
11.2.3.4 Low-grade heat sources There is growing interest in the DH industry for different kinds of low-grade heat sources. Without trying to define this somewhat vague concept, it is about utilizing,
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e.g. industrial surplus heat, solar heat, and geothermal heat, often with lower temperatures than used today in DH networks. It might also include customers that are sometimes generating heat, which is fed into the DH network (so-called prosumers), evoked by an interest among property owners to generate their own energy. In a truly smart energy system, not only will electricity be fed into the grid by end-users, but this will also be the case in thermal grids. It is quite evident that future temperature levels in DH systems are crucial for such a development. A study on the integration of prosumers is presented by Brand et al. (2014).
11.2.4 Economic value from optimized temperatures Several detailed analyses of the economic benefit of reduced temperatures are outlined in Zinko (ed.) et al. (2005). Among others, a comprehensive study by Rütschi (1997) and a calculation model called LAVA, from the Swedish District Heating Association, for the evaluation of changes in system temperatures, depending on the composition of heat production, are referred to. The value of a reduced return temperature varies greatly in different networks, as shown by Frederiksen and Werner (2013), but a typical value among 27 studied networks in Sweden is approximately 0.15 EUR/ MWh,°C. This means that if the return temperature could be lowered by 10 °C in a medium-sized network with an annual heat sale of 1 TWh, approximately 1.5 MEUR could be saved. An evaluation of the work towards increasing the cooling in the DH network in Gothenburg, gave similar results. A pay-off time of less than 3 years was obtained (Fransson, 2005). For the system in Lund in Sweden, Falkvall and Nilsson (2014) found that the average annual supply temperature could be lowered by approximately 5 °C just by an improved control method and produce a saving of approximately 0.3 MEUR. A complete evaluation of the distribution temperatures' influence on the heat supply plants must not only take into account increased efficiency and power-to-heat ratio, but it must also consider that if one heat supply plant can produce more heat, operational hours for other, more expensive and often less environmental-friendly, heat plants, can be reduced. The importance of individual analysis should also be pointed out. In some cases, it may be rational to increase the supply temperature in order to reduce the return temperature and increase the heat output of flue gas condensers and minimize pumping energy, or increase the network capacity. Therefore, there is no evidence to say which the optimal supply temperature is in a DH network. However, for any given supply temperature, the return temperature should be kept as low as possible, i.e. to maximize the temperature difference of DH water for the given supply temperature. In the long term, there is much to gain if supply temperatures can be lowered. Traditionally, there has been larger focus on return rather than supply temperatures, not least in Sweden, caused by a quite small share of CHP and as a way to handle expanding networks. At present, with the rapid expansion of CHP and a general awareness about the importance and potential of low network temperatures, the focus on lower supply temperatures is set to increase.
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11.3 Temperature optimization in district heating systems The main significance of the previous section is that no matter what part of a DH system one considers, higher temperatures than necessary inevitably lead to an increased use of primary energy resources, or vice-versa, generally lowered temperatures in the DH systems will increase energy efficiency. This section will look into existing and future ways to implement optimization of system temperatures.
11.3.1 Improving building-internal systems Werner and Sköldberg (2007) carried out an exposition of the knowledge and research situation for DH, in the world. Among other things, this exposition highlights the question of whether conventional radiator systems will be driven out of competition due to their static development in recent years. Such trends can be perceptible, since air-borne systems often are chosen in passive houses. In the present situation, however, the installation of underfloor heating systems is common, and these are especially favourable from a DH point of view, thanks to the very low temperature level. This section is intended to highlight a number of innovations in recent years aimed at increasing the efficiency and competitiveness of DH-connected hydronic heating systems. The development is important both for future competitiveness and in view of the large number of radiator systems that are in operation and will be so for a long time to come. The construction of new buildings typically amounts to less than 1% per year, relative to the existing building stock. In recent years, there has been a trend towards more ‘intelligent’ DH substations, where the aim is to use modern technology to ensure a proper operation and low return temperatures. In a report by Andersson and Werner (2005), an evaluation of the so-called function-integrated DH substation in operation was reported, and the return temperature reduction was estimated to be 10–11 °C. Typical for this substation is, among other things, that the DH flow is calculated and governed rather than regulated based on feedback control. By measuring temperatures and flows in the substation, the required flow is continuously computed. The result is a smoother control that can reduce the energy usage by avoiding overheating. The evaluation found that the benefits of a function- integrated substation exceed the cost of a conventional substation. Demand-side management involves the manipulation of the consumption of heat in order to optimize production and distribution of district heat and has been widely investigated (van der Meulen, 1988; Olsson Ingvarsson and Werner, 2008; Wigbels et al., 2005). In this process, the building's potential to be used as heat storage is considered to be significant. The idea is to even out the heat power, and flow needs of a DH network, depending mainly on the fluctuations in domestic hot water loads. In this way, the need for expensive and environmentally unsound peak production is reduced. This problem is also identified in connection with electric heating of buildings. So-called load shedding, when domestic hot water consumption is prioritized, is another type of demand-side management.
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Johansson (2014) achieved impressive results through a distributed load control system using load shedding in an entire neighbourhood where the substation that is currently best suited ‘lends’ flow to the DH network. As already mentioned, space heating temperatures have been lowered substantially over the years. Modern buildings can be designed for very low radiator supply temperatures without the need for extremely large radiators, thanks to the buildings' high energy performance. Moreover, there is an ongoing discussion whether underfloor heating should be employed in new buildings or if no hydronic heating systems at all are required. Some claim that underfloor heating (Sandberg, 2011; Sikander and Ruud, 2011) increases heat use, while others (Schneiders, 2005) emphasize that it actually facilitates lower use of primary energy because of the low operating temperatures. Underfloor heating can also provide a more even indoor temperature with a high degree of self-regulation (Karlsson and Hagentoft, 2012; Karlsson, 2010). From an economical point of view, radiator systems are generally cheaper. Even cheaper would be to completely omit a hydronic heating system and only supply heat via the ventilation system. This has become common in passive houses and other buildings with extremely high energy performance. In terms of temperature levels, air-borne heating also entails low DH temperatures. A drawback is that user-controlled ventilation often is employed in order to reduce ventilation heat losses, which may result in a more uneven heat load. In order to secure proper indoor comfort for the end-user, additional room heaters are often used. Due to the relatively few operational hours and low heat load for such heaters, electric resistance heaters are sometimes the chosen type of heater. In Lauenburg and Wollerstrand (2014), a radiator control method based on control of both the supply temperature and flow rate in the radiator system is reported. The study shows that the DH return temperature can be lowered. A strength of the proposed control method, which can be described as a combination of a low-flow system and a system with normal flow depending on the heat load, is that it automatically adapts to varying working conditions, such as long-term changes in the primary supply temperature. Therefore, it always strives to provide the lowest possible return temperature. Lowered network supply temperature or energy-saving measures, signifying an increase in the oversizing of a system, represent examples of changes that the control method can adapt to. Johansson (2011) has studied the overall system impact of employing so-called radiator fans, devices that induce heat transfer by forced convection and consequently lower the return temperature, and found substantial potential primary energy savings. Another study on ways to lower radiator system temperatures by increased convection was performed by Ploskic (2013). It is suggested that existing radiator systems can operate at substantially lower temperatures.
11.3.2 Lowering of network temperatures Initially, we discuss in this section, the prospects of lowering network temperatures in existing networks. Then we look into the development of the fourth generation of DH systems and how they can be incorporated into existing networks.
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11.3.2.1 Existing systems In many DH systems, there has been substantial improvement in terms of lowered network temperatures. However, there is still much to be done. Many systems are operated on a supply temperature based on tradition, older outer conditions, and use large safety margins. Traditionally, there has been a large focus on the return temperature, however, it becomes more and more relevant to lower the supply temperature to go along with increasing shares of CHP and other sources of surplus heat. Moreover, future energy savings by renovation of existing buildings must be translated into lowered network temperatures.
11.3.2.2 Low temperature DH Currently, we can see many small networks being built in many different countries employing substantially lower temperatures, either as solitary systems with their own heat supply, or as subsystems connected to an existing network (Wiltshire, 2013). Figure 11.2 shows characteristic temperatures in DH systems. Even if the best systems still have not reached the vision of the fourth generation of DH, the development has come a long way. The Danish low temperature DH (LTDH) project is probably the closest case that reaches the suggested level for network temperatures in the fourth generation of DH. There are several papers, reports and theses describing the project (see for
Figure 11.2 Examples of district heating supply and return temperatures. Based on Andersson (2014), Frederiksen and Werner (2013), and Werner (2013). LTDH is short for low temperature district heating and 4GDH for the fourth generation district heating. The reason for Västerås to be on a lower level than state-of-the-art is that the latter is based on design criteria that implies higher network temperatures.
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instance Brand, 2013; Dalla Rosa 2012, for a thorough review). The project includes many novelties. One is the handling of domestic hot water provision. From a comfort perspective, 45 °C was set as the design temperature. In order to prevent Legionella growth, which in most building standards’ demands at least 50 °C in all parts of a domestic hot water system, a German standard, DVGW 551 (Gerhardy, 2012), was referred to, which does not set any demand for the lowest allowed temperature in domestic hot water systems of a total volume (excluding water heater) of 3 l. The demand not to exceed 3 l is normally fulfilled if every dwelling has its own heat exchanger that limits the required number of pipes between the heat exchanger and the taps. Multi-dwelling buildings must therefore, in practice, adopt so-called flat substations, a technology common in some countries but not in others. Special substations were developed to be able to provide hot water at 47 °C with a supply temperature of 50 °C, resulting in a return temperature of 20 °C. There was a 2 °C margin in order to fulfill the comfort demand. The radiator systems were indirectly connected. Generally, the LTDH project has put the focus on minimizing the distribution heat losses by using low temperatures and also by using small pipe diameters. Many low temperature systems are worth mentioning (Wiltshire, 2013). In Figure 11.2 the example of the Swedish town Västerås was displayed. There, the aim was to find an economically viable DH connection for newly built houses with high energy performance. The focus was to reduce heat losses by using low temperatures; reduce installation cost by, among other things, improving the cooperation with real estate companies and substituting household electricity use for white goods, bathroom floor heating and towel dryers with district heat. Moreover, plastic pipes and cheap heating ventilation and air condition (HVAC) installations were prioritized.
11.3.2.3 Integration of fourth generation into third generation A matter of great importance is the integration of the fourth generation of DH into the third generation. One scenario is the development of complete new low-temperature systems with dedicated heat supply. In such a case, the heat supply can take full advantage of the low network temperatures. If a new low-temperature system is developed within an existing system, it is however not evident how the existing heat supply plants will benefit from the low network temperatures if the rest of the network still requires a high return temperature. A sub-network can for instance be connected to the main network via a shunt connection, to reduce the supply temperature, or via a heat exchanger, to reduce the supply temperature and the differential pressure. The sub-network will still benefit from lower heat losses. One way to increase the benefit for the main network is to use the return pipe to supply the sub-network. This method has been demonstrated, for instance, in Denmark in connection with the LTDH project (Christensen and Kaarup Olsen, 2011; Holm Christiansen, 2013). In this case, the main return pipe becomes the supply pipe in the LTDH-network. When needed, additional hot flow is supplied from the main supply pipe. In the future, it will be increasingly important to take advantage of the low-temperature networks.
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11.4 Future trends We have now seen the rise of low-temperature systems, especially in connection with newly built energy efficient housing. The future will tell which supply temperature level will prevail: 60 °C – which will make it easier to comply with existing regulation on Legionella prevention, to supply heat for dishwashers and laundry machines and to possibly supply heat-driven cooling; 50 °C – as the Danish LTDH project has demonstrated is quite possible, resulting in very low heat losses and the possibility of utilizing the return pipe in existing networks; or maybe even lower, e.g. 40 °C – with the aim to mainly supply space heat, while domestic hot water provision would need some additional heat supply, e.g. a small heat pump which is also being investigated in Denmark (Zvingilaite et al., 2012). The lower the supply temperature, the greater the potential to integrate low-grade heat sources with greater potential to save money and reduce environmental impact. As mentioned in the previous section, much attention will be directed to the integration of the fourth generation technology into existing systems. Although much focus is directed towards new buildings, the main part of the future building stock already exists. A lot of old buildings are in need of renovation, which opens up space for substantial improvement of energy performance. This can be viewed as a threat to future deliveries of district heat or to drastically reduce network temperatures, especially if existing space heating systems are maintained, offering larger relative heat transfer surfaces. Other matters of future interest concern if, or when, low-grade, distributed, heat supply will gain larger interest. One possible development that would facilitate such a development is increased prices on biofuels due to increasing demand, for example, from the transport sector. In a future with increasing shares of low-grade, often abundant, heat supply, such as solar and geothermal heat, the question arises whether there will be the same focus as today on minimizing distribution heat losses. Perhaps, there must intensified efforts to reduce installation costs in order to compete with less infrastructure-intensive technologies, such as heat pumps, resistance heating and, to some extent, natural gas and solar heat. In the buildings' HVAC systems, development can be towards either reducing temperature demands as much as possible, for example, by using underfloor or wall heating, or towards reducing installation costs, for example, by employing cheap radiator systems. On the heat supply side, distributed technology is yet to be explored. For conventional technologies, there is much to explore in relation to substantially lowered network temperatures. For instance, CHP plants can be much more efficiently designed if a DH supply temperature of, e.g. 60 °C is used (Genrup, 2014).
11.5 Sources of further information As mentioned before, and as the reader might recognize, much of the facts presented in this chapter can be found in the textbook by Frederiksen and Werner (2013), not least when it comes to heat supply plant technology. For extensive background on heat supply plants and district heat distribution economy related to network temperatures, the
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work of Rütschi (1997) and Zinko (ed.) et al. (2005) is recommended. For everything regarding the Danish LTDH project, the reader is referred to Dalla Rosa (2012) and Brand (2013). For future activities on the fourth generation of DH, the large Danish research center, 4DH will be a useful source of information.
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Zinko (ed.), H., Hoon, L., Bong-Kyun, K., Youn-Hong, K., Lindkvist, H., Loewen, A., Seungkyu, H., Walletun, H., Wigbels, M., 2005. Improvement of Operational Temperature Differences in District Heating Systems. No. Annex VII 2005:8DHC-05.03. International Energy Agency, IEA District Heating and Cooling, Programme of Research, Development and Demonstration on District Heating and Cooling, Sittard, the Netherlands. Zvingilaite, E., Ommen, T., Elmegaard, B., Franck, M.L., 2012. Low temperature district heating consumer unit with micro heat pump for domestic hot water preparation. In: Proceedings of the 13th International Symposium on District Heating and Cooling, Copenhagen.