Network Types - Low Temperature District Heating

A short introduction about distribution network types with photos from nature. Please give a focus how the veins of these leafs in the photo are structured. Some branched, some having a loop formation and some a mixture of both! A question rises that highlights the necessity of distribution systems in our lives, especially district heating/cooling networks: why such natural elements have a structure of distribution instead of individual production? Please think about various forms of distribution in the nature: you can find lots of examples i.e. the blood vessels, neural system, etc.! The network layout (network topology) is briefly described after this inspiring-from-nature photo below. 

Low Temperature District Heating Systems, Individual Heat Production, Network Topology, Network Layout, PhD Thesis, DTU, Low-Energy District Heating
Network Types with Different Layouts as Observed in the Leafs: Inspiration to the Centralized Energy Networks
Obvious that nature at most uses centralized distribution to satisfy the local needs. So do the district heating systems! Although in my publications the network layout was took part in the observations found in the field of low temperature district heating systems, the mature technology district heating itself has already involve of these various layouts of distribution networks. 

Branched (Tree-Like) Network Layout

Due to its form like branches of a tree, branched networks are also named with the term of "Tree-Like". Its reside in a district heating system is with supply of hot water from the production site only in one direction from production to consumers. 

Nature in Science, District Heating in Areas with Low Energy Houses
Branched Type Network Layout as used in District Heating Systems

Looped Network Layout

Looped layout allows the circulated medium in the distribution piping network to reach the heat consumers from different directions. The direction of the flow is affected according to the dynamics of the heat consumption by the heat consumers.  

Looped Type Network Layout as used in District Heating Systems

Further information about low temperature district heating systems can be found in my publications: 


Substation Types - Low Temperature District Heating

A brief introduction can be rewarding about the substation types while talking about Low Temperature District Heating Systems. 

Direct Connection, Indirect Connection, Low Energy Buildings, Heat Storage Tank, Heat Accumulator, Buffer Tank, Thermostatic Control Valve, Radiator System, Shower, Domestic Hot Water, Space Heating, Heat Consumer, Peak Demand, Stock Photo, Legionella, UV Light, German Standard, Domestic Hot Water Standard, Substation Type, Trinkwassererwärmungs- und Trinkwasserleitungsanlagen, DVGW W551
Source: Tol, Hakan İbrahim, 2012, District Heating in Areas with Low Energy Houses (PhD Thesis)
First thing that comes to mind, in Low Temperature District Heating Systems, is if the low temperature supply at a temperature of 55°C can cause legionella growth or not while producing the domestic hot water. One of the solutions to overcome the legionella growth can be by equipping storage tank before the heat exchanger in the substations with indirect connection. Hence the storage tank (also termed as Heat Accumulator and Buffer Tank) will store the medium circulated in the distribution network not the domestic hot water that is used directly from the tap. According to the German Standard, if the volume storing the domestic hot water medium is less than 3 liter there is no minimal limit for the supply temperature [1]. Hence, keeping the volume of the domestic hot water in the pipeline less than 3 liters can restrain the growth of legionella. Besides, using UV light at the tap points (locations before the usage) can be another solution to kill the legionella [2]. 

Another point can be directed to the reduction effect on the heat demand by using the heat storage tank in the substation. This reduction is due to the low charging rate (from the district heating medium) in the off-peak periods. In detail, the storage tank gets charged in the night time and in the morning time peak-demanding moment comes with the usage of domestic hot water (this time from the heat storage tank) i.e. by using shower. In case of direct connection type for the substation, the heat demand for domestic hot water can be as high as 32 kW (that is directly taken from the district heating network medium). For the case of indirect connection, the charge of heat storage tank is with the rate of 3 kW. Of course, when the consumer opens her/his hot tap, the discharge rate is 32 kW but thanks to the heat storage tank since this high discharge rate of 32 kW is taken from the heat storage tank which was formerly charged at a rate of 3 kW in the long-lasting off-peak period (for example whole night) [3].

There is also connection differences for the space heating connection. Equipping heat storage tank in between district heating network and radiator system is also common. However direct connection is rewarding to avoid heat loss from the heat exchanger. 

[1] DVGW. W551 Trinkwassererwärmungs- und Trinkwasserleitungsanlagen [in Deutch]. - Link
[2] Liu Z., 1995, Efficacy of ultraviolet light on the disinfection of Legionella in a hospital water distribution system - Link.
[3] Tol, Hakan İbrahim, 2012, Phd Thesis, District Heating in Areas with Low Energy Houses - Link (Click!)

Glossary - Low Temperature District Heating

Low Energy Buildings, 4G, 4th Generation District Heating, Energy Planning, Futurism, Low Temperature Operation, Low Grade, Distribution Network, Branched Network, Piping Systems, Termis Screen Shot, 7T, Danish Cases, Danish Heat Supply Act, Radiator Performance, Bölgesel Isıtma Sistemleri, Renewable Energy Sources, Kentsel Dönüşüm, Lystrup, Danish Cases
Low Temperature District Heating Systems, the 4th generation (4G) district heating technology, are superior with their low degrees of temperature that is commonly with 55°C of supply and 25°C return. In this blog post, the terminology commonly used in the field of low temperature district heating systems is published to keep the consistency/standardization of the terms. Some being newly famous with the low temperature upgrade, though most are already common in the field of District Heating technology. Links for the original document which this blog post based on are provided at the bottom of the page.

Description of the Terms 

Low Temperature: Excessively low temperature supply of about 50°C to the heated space
Low Energy: Used for district heating systems operating at low temperatures.
Low grade: Surplus heat at low temperatures which it is hard to exploit.
Heat carrier medium: A medium consisting of a fluid used to transport heat in such a way that a change in enthalpy occurs through an endothermic reaction that takes place, for example at the heat source, and an exothermic reaction, for example one of positive effect for consumers.

Pipe Network: A closed circuit of several pipes connected to each other hydraulically for the purpose of circulating a heat carrier medium from a heat source to consumers.
Node: A junction of several pipes or a sign for heat consumers.
Leaf node: A node without any successor node (also used as end-node/end consumer).
Root node: A node without any predecessor node, in reference to a heat source.
Pipe segment: A short segment of pipe that connects a node to a succeeding node, in the order from root node to leaf node.
Route: A sequence of pipe segments extending from a root node to a respective leaf node.
End-user connection: A service pipe that provides the means of circulation taking place within a district heating network, used for in-house installation.
Network layout: The shape of a pipe network with respect to the interconnections between the pipes.
Branched network: The tree-like formation of a network providing a unidirectional flow from the root node to the leaf nodes.
Looped network: The looped formation of a network shaped in the form of closed paths composed of pipe segments, in which each heat-demanding node has a number of alternative paths for the flow to be supplied by one of the neighboring nodes.
Supply line: A pipe line employed to deliver the heat carrier medium after its enthalpy has increased at the heat source (Also known as feed line -especially in Denmark-).
Return line: A pipe line employed to transfer the medium back to the heat carrier medium after its enthalpy has been released through heat consumption by the in-house installations of consumers.
Single pipe: An insulated type of pipe used in district heating networks, the pipe being protected or shielded by an insulating casing.
Twin pipe: An insulated type of pipe used especially in district heating networks, involving two pipes, both of the same diameter, protected overshielded by an insulating casing.

Pump Station: A major facility in a district heating network, one that provides the difference in pressure difference required to circulate a heat carrier medium.
Pressure drop: A loss in pressure during circulation of the heat carrier medium through the pipe network involved, one due to frictional forces (also termed as pressure loss).
Head lift: The maximum amount of pressure the pump can provide, the measure of it being given as the vertical lift of the medium.
Booster pump: A small-scale facility a network is equipped with, one located close to the consumers, for the purpose of providing an increase in the pressure of the supply line, in addition to the residual pressure capacity from the main pump station.
Holding pressure: A certain minimum amount of pressure maintained in the heat carrier medium during its circulation, in order to prevent the risk of cavitation.
Pressure gradient: A physical quantity representing the rate of change in pressure with respect to the length of the pipe segment or segments to which it is applied.

Substation: An in-house installation at a consumer site that conveys the heat content to be used in the space heating of a house and in the production of domestic hot water
Buffer tank: A small-scale tank used for the storage of heat in a substation, one that stores heat content from the district heating network during off-peak times of day for it to be used for in-house heat needs at peak times (also termed as storage tank).
Thermostatic valve: A self-regulating valve used in connection with in-house heating systems, one that regulates the flow of an in-house heat carrier medium in accordance with the heat demand rate.
Bypass valve: A self-regulating valve used in connection with a district heating network, its transmitting the supply-heat carrier medium, when its temperature be degraded, to the return line, to then be circulated to the heat source.

Heat Demand: The heat energy requirement of a consumer site.
Heat load: The heat energy that needs to be conducted by a district heating pipe network.
Simultaneity factor: A factor reducing the effect on the estimated heat load through taking advantage of the asynchronous heat use by multiple consumers, since use of heat by different consumers involved occurs neither all at the same time nor at the same rate.
Heat load duration: A decrescent way of showing heat loads together with their duration of occurrence during the period of a year with respect to a given district.
Heat load factor: The ratio of any particular heat load to the peak heat load rate.
Heat density: A physical quantity representing the unit of overall heat load per area of the land on which the district heating network is employed.

Please cite to: Tol, Hakan İbrahim, PhD Thesis, 2015, District heating in areas with low energy houses - Detailed analysis of district heating systems based on low temperature operation and use of renewable energy.

This PhD thesis and other publication of the author can be reached from:
ResearchGate (Click!)
DTU Employer Page (Click!)
Google Scholar (Click!)
LinkedIn (Click!)
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