Utilization of return water in district heating
networks
Oddgeir Gudmundsson
Application Specialist
Marek Brand
Application Specialist
Jan Eric Thorsen
Director
Danfoss District Heating Application Centre
DK-Nordborg
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Content

Introduction

Concept

Technology

Cases

Conclusion
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Introduction
1G
2G
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3G
4G
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Concept

District heating schemes are operating with various temperature levels depending on the original
design of the network and the connected building stock

In many cases existing buildings may require high supply temperatures and consequently have
high return temperatures, f.ex. supply of 90-100°C and return of 40-60°C

Sufficient temperature levels vary and depend on the heating installations


New energy efficient buildings with floor heating installation only need supply temperatures of 35-40°C to fulfill
their heating requirement

Domestic hot water (DHW) temperature of 45°C is considered sufficient for everyday use.
Having DH supply temperature of 50°C is sufficient for preparing DHW at 45°C via instantaneous heat
exchanger solution, without risk of Legionella.
This fact gives the opportunity to utilize the return flow from existing areas in new areas and
hence utilize the capacity of existing DH networks to a greater extend with minimum investment
costs.

Utilize further the capacity of the distribution network

Increased efficiency at the plant due to lower return temperature

Reduced heat losses in the return line
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Conceptual example of cascading energy usage in district heating
Non-renovated
older buildings
All energy
classes
Low-energy
buildings
Large scale
solar
multi-apartment
single-family
CHP natural
gas
Recently build and
renovated
Newly
constructed /
renovated
areas
High temperature
supply, 90°C
CHP waste
incineration
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High temperature
return, 50°C
Low temperature
return, 25°C
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Utilization of return water for existing buildings
Renovation of the existing network
 Low Temperature District Heating
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Low Temperature DH for existing buildings

Project supported by the Danish government

75 single-family buildings from 1997

Floor heating
Realisation

New low-temperature DH in-house substation

New DH network

Heat loss reduced from 41% -> 14%

80% of heat demand supplied
from main DH return line

Before transition average
Tsupply = 70-75°C

After transition average
Tsupply = 55°C
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Area substation

As the DH return temperature can vary it may become necessary to raise the
return temperature before it is supplied to the secondary network

This can be achieved by mixing the return water with hot water from the main DH
supply pipeline
Primary network
Sønderby
Area substation
Tsupply=90°C
Treturn=52°C
Treturn=26°C
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Sønderby
low temperature
DH
Sønderby
Last consumer
Tsupply=52°C
Substation at
the consumer
Thermostatic
bypass
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Micro booster – Reduced temperature levels
 Ultra-Low Temperature District Heating
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Objective: Maximum utilization of district heating return flow
Area substation
Ultra-Low Temperature DH for existing buildings

Project supported by the Danish government

4 single-family buildings from before 1960

Radiators, mix of 1 pipe and 2 pipe systems
Realization

Micro heat pump DH substation in each house to boost
the supply temperature for instantaneous preparation of
Domestic Hot Water

New U-LTDH network

Heat loss only 46% of the heat loss that would be
experienced in a traditionally designed network

Area heat exchanger substation connected to the main
district heating network regulates the supply temperature

Supply temperature kept as low as possible at all times
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Micro booster concept

For floor heating temperature levels of 30-40°C are
sufficient

Domestic hot water of 45°C is sufficient for all normal use


Micro booster installed
With instantaneous preparation of DHW there is no risk of
Legionella
Micro heat pump unit boost the supply temperature to
53-55°C and stores the water in a primary side located
storage tank until DHW tapping occurs
Condenser
Evaporator
DH side
storage
tank
Instantaneous DHW
preparation
No Legionella risk
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Heat exchanger area substation

The aim of the substation is to maintain constant secondary side
temperature of 40°C

As the DH return temperature can vary primary side supply is mixed
with the return to maintain constant 45°C supply to the heat exchanger
by means of a pump control
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How applicable can this be?
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Low-temperature DH for existing buildings

Supported by the Danish government

8 single-family houses from 1970

With traditional radiators: 70/40/20°C

How much could Tsupply be reduced without sacrificing comfort?
70°C
40°C
heat output
100 %
same radiators
ṁ = constant
50°C
33°C
heat output
55 %
Numerical simulations
Real measurements
 Many possibilities
 Should follow real conditions
 Various refurbishment stages
 Results are coming soon…
 Low-temperature radiators
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 Single-family-house from 1970 – results from simulation
Heating curves for radiators
Duration of Tsup over/equal
certain temperature
Tsupply [°C]
hours above certain
temperature [%/a]
25
20
15
10
5
0
>55
>60
>65
>70
21.4
8.1
2.9
1.3
0.2
66.1
new windows
20°C
6.8
1.8
0.2
0.0
0.0
91.1
extensive
renovation 20°C
0.4
0.0
0.0
0.0
0.0
99.6
new windows
22°C
16.5
4.8
1.6
0.1
0.0
77.0
new windows
22°C LT rad.
1.8
0.3
0.0
0.0
0.0
97.9
non-renovated
20°C
!
Low-temperature
substation for DHW
!
=50
>50
Tsupply [°C]
*Extensive renovation = low energy windows +
roof insulation
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LTDH for Existing Buildings
 Existing buildings can be
supplied with LTDH already
today if Tsupply is in cold
periods increased above 50°C
 Required Tsupply to radiators
depends on:
 desired indoor
temperature
 state of the building
 heating system

DH companies should be
more strict in reducing Tsupply
to:
 reduce heat losses from
DH network
 integrate more
renewables

DHW applicatoin should
always be changed to lowtemperature one
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Conclusions

There is a potential to further utilize the capacity of existing district
heating network and reduce the network return temperature significantly
by cascading the energy use

Studies show that with light renovation of buildings the requirements to
the DH supply temperature sinks significantly

As new areas are built or building areas renovated close to existing
district heating network it is possible to establish district heating network
with low investment costs utilizing the remaining heat in the return
pipeline from the existing DH grid
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Thank you for the attention
Contact information:
Oddgeir Gudmundsson
Application Specialist, Application Centre
Danfoss District Energy, DK-Nordborg
og@danfoss.com
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