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Energy Procedia 61 (2014) 2168 – 2171
The 6th International Conference on Applied Energy – ICAE2014
Heat Pump Application for Efficient DH Systems
Girts Vigants, Gundars Galindoms, Edgars Vigants, Ivars Veidenbergs, Dagnija
Blumberga
Institute of Energy Systems and Environment of Riga Technical University, Kronvalda boulvard 1, Riga, LV1010, Latvia
Abstract
The energy efficiency of district heating (DH) systems can be increased by not only optimising the operational
parameters of existing systems but also by integrating new elements into such systems. The paper recommends the
installation of a high-temperature heat pump in the return water pipe system of a heating network in order to cover a
part of the consumer's heat load. The authors provide a diagram for calculating the integration of a heat pump into the
DH system and have also performed modelling in order to determine the changes in the network parameters over the
course of a heating season if the heat pump has been working at a nominal capacity. The calculation model contains a
system of equations that describes the mass and energy balances of the elements as well as changes within them over
the course of a heating season.
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is an openLtd.
access article under the CC BY-NC-ND license
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2014Published
The Authors.Published
by Elsevier
(http://creativecommons.org/licenses/by-nc-nd/3.0/).
Selection and/or peer-review under responsibility ofICAE
Peer-review under responsibility of the Organizing Committee of ICAE2014
Keywords: CO2 heat pumpsystem, district heating system, energy efficiency, modelling
1. Introduction
An efficient, environmentally friendly heat supply system that uses renewable energy resources is an
essential part of energy systems in the future. Increasing the effectiveness is a complex problem that
involves not only rational energy consumption on the consumerside but also efficient generation of
energy at the source [1]. One of the ways to increase the efficiency of district heating systems is to
integrate a heat pump to cover part or all heat load.
Nomenclature
M
flow: return water through gas cooler ms; consumer water mp; supply water mt, kg/s
Q
capacity: heat pump gas coolerQDz; consumerQp, kW
t
temperature:outdoor tag; supply water tt; return water tat; consumer water before tp1 and after tp2; after heat pump vaporiser
tiz, °C
1876-6102 © 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/3.0/).
Peer-review under responsibility of the Organizing Committee of ICAE2014
doi:10.1016/j.egypro.2014.12.101
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Girts Vigants et al. / Energy Procedia 61 (2014) 2168 – 2171
When using heat pumps in traditional systems, it becomes necessary to ensure a high supply
temperature for the system, which in turn produces a high refrigerant temperature in the condenser.
Research [2, 3] shows that under such conditions CO2 is the best choice as a refrigerant both from a
technical as well as environmental standpoint: its potential for ozone layer depletion is zero, and its
potential to contribute to global warming is insignificant [4]. Also it should be noted that, compared to the
usual refrigerants, CO2 can be combined with lubricants and standard machine construction materials, it is
non-toxic, fire-resistant and significantly reduces the degree of compression [5]. One of the most
significant disadvantages compared to standard refrigerants is its lower coefficient of performance (COP)
associated with greater loss due to work material expansion [6].
Start
Inputdata
tag, Qp, Gp, ttu,
tatΔtt
Tag=tag+1
Finish
x=tag
Y=Qp, Gp, ttu, tat, Δtt
Findy=f(x)
mp=Gp/3,6
Output
Qp=f(tag),
Gp=f(tag),
ttu=f(tag),
tat=f(tag),
Δtt=f(tag)
Yes
Chooseanother
tag?
No
Output
tiz, tp1, tp2,mt,
Input
QDz kW
Qiz kW
Calculate ms
Calculate
tiz, tp1, tp2,mt
Output ms
ts2˃90
No
Calculate ts2
Output ts2
Fig. 1. Flow chart for the simulation model
Analytic and experimental studies regarding the possibilities of improving the parameters of hightemperature heat pump systems have been conducted [7,8,9]. The studies have examined parallel cycles
with closed system water heaters and two-stage compression cycles. Test results show that as the
condensation temperature rises from 80° C to 90°C, the COP falls from 4.5 to 3. Various schemes are
used for the integration of a high temperature heat pump into a district heating system. The authors [10]
have performed a theoretic and experimental study of the installation of a heat pump to the return heat
network. As a result, the return temperature of the network is reduced from 70° C to 50° C. On the whole,
this technological solution increases the temperature difference between the network’s supply and return
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Girts Vigants et al. / Energy Procedia 61 (2014) 2168 – 2171
temperatures, decreases the diameter of the pipes and thereby the financial investment necessary and also
lowers the energy consumption of the circulation pumps. The authors have also studied the integration of
a heat pump into the heat network return [11]. In such a scheme, the heat pump vaporiser is switched on
in the system return and the gas coolant in the recirculation pipe, which mixes the return water with the
outgoing water before it reaches the consumer. In the heat supply system examined, the heat source is the
cogeneration station. In this case, the lowered return water temperature promotes the generation of
additional energy in the cogeneration station.
2. Methodology of calculation. Algorithm
The calculation of the scheme for the integration of a heat pump into a heat supply system creates a
model that includes the empirical and analytical correlations and algorithm necessary for the calculations.
The goal of the calculation is to determine the temperature of the diverted return network water after the
heat pump gas cooler ts2, the diverted water flow through the heat pump gas cooler ms, the return water
temperature after the heat pump vaporiser tiz, the return water temperature after the consumer and the
mixed-in outgoing water flow mt. The calculation assumes that, as the result of the heat pump operations,
the water temperature before the consumer tp1 is Δtt° C higher than the supply water temperature ttu. A
unique feature of the suggested calculation method is that it combines empirical equations that describe
heat supply systems that have been obtained by processing experimental data characterising the system's
operations and analytical expressions. The analytical correlations are obtained from mass and energy
balance equations of the scheme's elements and nodes.
The scheme's calculation model and algorithm, which consists of 16 blocks, is shown in Figure 1.
3. Testing of model. Results
The methodology is tested for Ludza district heating system. The modelling begins with the entering of
the parameters of the city district heating system branch in which the heat pump could be installed. A
correlation in the form of empirical equations as the function of the outdoor temperature has been
obtained for each of parameters used in model. Various charts of changes of DH system parameters
(temperatures and flows) before and after installation of heat pump are obtained.
70
65
Temperature, 0C
60
55
50
tp2
45
tiz
40
35
30
-30
-25
-20
-15
-10
-5
Outdoor temperature,
0
5
10
15
0C
Fig.2. Changes in heat carrier temperature after the consumer depending on the outdoor temperature
For example, changes in the heat carrier temperature after the consumer can be seen in Figure 2. The
analysis shows that the heat pump temperaturetiz within the outdoor temperature interval is lower than the
network return temperature. This means that the use of a heat pump in a heat supply system according to
the scheme presented lowers the network return water temperature. At low outdoor temperatures (below
16° C) the network water temperature after the consumer group corresponds to the temperature tp2.The
results of the performed analysis hold true if the initial assumptions of the modelling are observed. The
most important of these is the condition that the heat pump works with the nominal gas cooler QDz and
vaporiser Qiz capacities.
Girts Vigants et al. / Energy Procedia 61 (2014) 2168 – 2171
The authors' theoretical and experimental studies [10] in the use of heat pumps in heat supply systems
show that within the examined temperature range the heat pump's COP changes between 2.8 and 3.4.
4. Conclusions
The paper presents a scheme for the integration of a high-temperature heat pump into a city's heat supply
system to partially cover the consumer's load with the return network heat.
A model has been created and calculations have been performed for changes in the heat supply system
parameters as the outdoor temperature changes and a heat pump is used. The model includes equations of
the mass and energy balance of the scheme's nodes and elements as well as empirical correlations
describing the system.
As a result of the modelling, it has been determined that a heat pump may be used if the outdoor
temperature is higher than -16° C. As the outdoor temperature increases, an increasing proportion of the
consumer's heat is ensured with the heat pump.
As the result of the heat pump operations, the system's return water temperature decreases, which increase
the efficiency of the flue gas condenser installed in the boiler house.
References
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[2] Kim M.H., Pettersen J., Bullard C.W. Fundamental process and system design issues in CO 2vapor compression systems.
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Biography
Girts Vigants, M.sc.ing., Riga Technical University (RTU), third year PHD student. His research topic is connected with
energy efficiency improvement in DH systems.
Gundars Galindoms, M.sc.ing., (RTU), second year PHD student. His research topic is connected with heat and mass
transfer processes in flue gas condensing units.
Edgars Vigants, Dr.sc.ing., docent , RTU, Institute of Energy Systems and Environment (IESE). He has participated in
renewable energy projects as well as is author of more than 20 publications and 1 book.
Ivars Veidenbergs, Dr.hab.sc.ing., professor, RTU, IESE. He has participated in different biomass and energy efficiency
projects and is an author of more than 100 publications and 5 books.
Dagnija Blumberga, Dr.hab.sc.ing., professor, RTU, IESE. She has participated in different local and international
projects related to energy and environment as well as is author of more than 200 publications and 14 books.
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