Elsevier

Energy

Volume 38, Issue 1, February 2012, Pages 276-290
Energy

Improving the dimensioning of piping networks and network layouts in low-energy district heating systems connected to low-energy buildings: A case study in Roskilde, Denmark

https://doi.org/10.1016/j.energy.2011.12.002Get rights and content

Abstract

The paper presents a method for the design of a low-energy district heating (DH) system, concerning the studies of different pipe dimensioning methods, substation types and network layouts. Computations were carried out separately on each of the pipe segments of which the DH network consisted. A simultaneity factor was taken account in connection with each of these pipe segments. The applicability of the developed optimization method was investigated with outcomes of its being highly useful in the pipe dimensioning and of its being superior in respect to traditional dimensioning methods. It was shown that an appreciable reduction in heat loss from the DH network could be achieved. The optimal pipe dimensions found were evaluated by use of the commercial software Termis with input of several randomly generated heat demand scenarios involving peak winter conditions. The effects of the network type on the pipe dimensions were investigated for substations of different types containing buffer tanks and heat exchangers and for booster pumps installed at the DH network. Two types of network layouts were compared in terms of satisfaction of customers concerning the supply temperatures and heat loss within the DH network to prevent excessive drops in supply temperature during the summer months.

Highlights

► Simultaneity factor was used for defining heat load at each pipe segment separately. ► Proposed optimization method reduced significant heat loss from the network. ► Optimal pipe diameters were evaluated in Termis with generated heat demand scenarios. ► Network layouts were examined to prevent low supply temperature at summer months. ► The effect of substation type and booster pump on pipe dimensions were analysed.

Introduction

Efforts to reduce energy consumption in European buildings, together with intensified energy efficiency measures that are being undertaken, and the increasing exploitation of renewable energy sources for providing heat have led to the search for a more adequate conception and better network design of new-generation District Heating (DH) systems for low-energy buildings [1], [2], [3], [4], [5], [6]. Both the integration of new low-energy buildings and the low-energy renovation of existing buildings increase the percentage of heat loss from the piping network of a traditional DH system. Heat loss from the network has a significant impact on the cost-effectiveness and energy efficiency of a DH system [7], [8], [9]. In one project in this area [10] it was found that a low-energy DH system operating at very low temperatures, 55 °C in the case of supply and 25 °C in the case of return, can satisfy the heating demand of consumers through adequate control of the substations [9], [11], [12], [13]. There are also studies [12], [14], [15] which have shown that existing indoor heating systems in already existing buildings can continue satisfying the heat demand at low supply temperatures since the existing indoor heating systems were formerly over-dimensioned in their design stage. In addition, certain heat loss can be avoided through operation at low temperatures [11], [16], providing savings in heat production as well [9], [14], [17], [18], [19], [20], [21]. The heat loss from a DH network is affected by the diameter of the pipes and the insulation material employed, as well as by the temperature of the supply and by the return heat carrier medium. Accordingly, special attention needs to be directed at the dimensions of the DH piping network so as to take advantage of DH in the best possible way [2], [5], [22], [23], [24], [25], [26]. Traditional methods of DH pipe dimensioning involve use of a size-searching algorithm in which the lowest pipe diameter possible is defined in accordance with the maximum velocity and/or with the maximum pressure gradient, so as to avoid the installation of an over-dimensioned and unnecessarily costly DH network [4], [16], [23], [27]. The risk of obtaining an over-dimensioned piping network can be prevented by optimal design of the DH network [28], [29].

It is not expected that each consumer will consume heat at a full demand level or at exactly the same time. This is the basic idea behind the use of simultaneity factor [30]. Special attention was thus directed at determining the heat load in each pipe segment, consideration being given to the consumer load to which each pipe segment is subjected. Three methods for the dimensioning of piping networks, two of them based on use of maximum pressure gradient criteria [31] and the other on optimization [11], [28], [29], [32], [33], were investigated, their being compared in terms of heat loss from the DH network. Also, DH networks connected to two different substations each containing a buffer tank or a heat exchanger, used for domestic hot water (DHW) production were investigated. In addition, further opportunities for reducing the dimensions involved were studied by installing additional booster pumps in the DH network together with the substations containing heat exchangers for DHW production. The reliability of the DH network with optimal pipe dimensions was evaluated by use of the hydraulic and thermal simulation software Termis, in which peak winter scenarios representing different heat consumption profiles of consumers were compared, these being based on the degree of simultaneity of the heat demands of the different consumers [34]. Supply temperature in the DH network is lowered, in particular through the heat consumption being reduced when there is no need for space heating (SH) and through consumers being absent during holidays and vacation periods. Two types of network layouts were investigated – branched networks with bypasses at leaf nodes and looped networks without bypasses – with the aim of determining how best to prevent marked drops in the supply temperature and at the same time keep heat loss from the DH network at a minimum. The heat consumption profiles of consumers have been found to affect the operation of DH networks [11], [36]. Accordingly, different DH network layouts were investigated in terms of energy performance under conditions of low heat demand in the summer, on the basis of time series simulations involving use of the Termis software and of different scenarios.

The objective of this study is to design low-energy DH networks operating in low temperature of 55 °C as supply and 25 °C as return for a new settlement, in which low-energy houses are planned to be built, with focus given on network dimensioning method, substation type, and network layout.

Section snippets

Description of the site

A case study was carried out concerned with a suburban area of Trekroner in the municipality of Roskilde in Denmark, in which extensive building construction is planned (Fig. 1), the DH system there supplying heat to 165 low-energy houses. The piping network is to have a total length of about 1.2 km in the layout of the branched type and 1.4 km in the layout of the looped type, the length of the end-user connections not being figured in here. Future network extension was assumed to not be

Results and discussion

The pipe-node list for the Trekroner DH network model is shown in Table 2, together with the length of each pipe segment.

Summary

The paper has presented a new method for designing low-energy district heating systems, pipe dimensioning methods, network layout and types of substations being taken up in particular. It was shown that a considerable reduction in heat load in such systems can be achieved through taking account of simultaneity factor in planning of each pipe segment. An optimization method aimed at reducing heat loss in a DH network, also when pressure changes in the various routes through the system are at a

Conclusions

A number of general conclusions not yet taken up can be drawn. One is that a district heating system should always be designed in accordance with what works best within the district itself. Another conclusion is that it is highly important to take into consideration, for each pipe segment separately, the degree of simultaneity of the heat consumers involved. In addition, it appears that significant savings can be achieved by use of the proposed optimization method, which makes use of the

References (60)

  • J. Rosada

    Characteristics of district heating - advantages and disadvantages

    Energy and Buildings

    (1988)
  • L. Dobos et al.

    Controller tuning of district heating networks using experiment design techniques

    Energy

    (2011)
  • N. Yıldırım et al.

    Piping network design of geothermal district heating systems: case study for a university campus

    Energy

    (2010)
  • K.C.B. Steer et al.

    Control period selection for improved operating performance in district heating networks

    Energy and Buildings

    (2011)
  • A. Dalla Rosa et al.

    Method for optimal design of pipes for low-energy district heating, with focus on heat losses

    Energy

    (2011)
  • I. Gabrielaitiene et al.

    Modelling temperature dynamics of a district heating system in Naestved, Denmark – a case study

    Energy Conversion and Management

    (2007)
  • R. Langendries

    Low return temperature (LRT) in district heating

    Energy and Buildings

    (1988)
  • A. Durmayaz

    An application of the degree-hours method to estimate the residential heating energy requirement and fuel consumption in Istanbul

    Energy

    (2000)
  • European Parliament legislative resolution of 23 April 2009 on the proposal for a directive of the European Parliament...
  • P.A. Østergaard et al.

    A renewable energy scenario for Aalborg municipality based on low-temperature geothermal heat, wind power and biomass

    Energy

    (2010)
  • P.K. Olsen et al.

    A new low-temperature district heating system for low-energy buildings

  • Christiansen CH, Paulsen O, Bøhm B, Thorsen JE, Ting Larsen C, Jepsen BK et al. Development and demonstration of...
  • Foged M. Low temperature operation [News]. DBDH [electronic mail on the Internet]. 1999 [cited 2011 Jan];1:pp....
  • O. Paulsen et al.

    Consumer unit for low energy district heating net

  • O. Mertoğlu et al.

    Alternatif enerji kaynakları ve düşük sıcaklıklı jeotermal bölgesel ısıtma

    Tesisat Mühendisliği

    (2000)
  • M. Rämä et al.

    Challanges on low heat density district heating network design

  • H. Lund

    Renewable energy systems: the choice and modeling of 100% renewable solutions

    (2010)
  • M. Ala-Juusela et al.

    Summary report – heating and cooling with focus on increased energy efficiency and improved comfort

    VTT

    (2004)
  • D. Schmidt

    New ways for energy systems in sustainable buildings - increased energy efficiency and indoor comfort through the utilisation of low exergy systems for the heating and cooling of buildings

  • Danfoss, 8 steps – control of heating system. Available from:...
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