Geothermal energy, Heat pump, Exergy analysis, District heating system
Anna NITKIEWICZ, Robert SEKRET*
EXERGY ANALYSIS OF LOW-TEMPERATURE
GEOTHERMAL DISTRICT HEATING SYSTEM
In this paper the exergy efficiencies of low temperature geothermal heating system was presented.
The heating system utilize geothermal water at 19,5°C using electrical heat pump working in
monovalent system. The exergy analysis was made for whole heating system and its components. The
research compares following types of heat networks temperature characteristics: 60/50°C, 55/45°C,
45/35°C and 35/28°C and was made for different dead state temperature: -20°C,-15°C, -10°C, -5°C,
0°C and 3°C (seasonal average) as well. The exergy destruction was quantified and illustrated using
exergy flow diagram. It was observed that the highest exergy loss takes place in geothermal water
transportation subsystem. The total exergy efficiencies of the system varies from 4-22% and depends
on the heating system supply water temperature and reference temperature as well.
1. INTRODUCTION
Geothermal energy has been widely used for decades for supply heating systems
and produce electricity. The high and medium temperature geothermal sources are
commonly used for district heating systems in many countries all over the world. In
Poland most of geothermal heating plants use high and medium temperature water
40-100°C. The first low-temperature heat-generating plant was build in Słomniki with
water temperature at 17°C. In this case the heat pump is integrated with a gas-fired
boiler.
Due to growing concern about energy conservation and environmental protection
the utilization of low temperature sources seems to be very attractive. However, to
evaluate environmental benefits from these sources, not only energy but also exergy
analysis should be carried out. Exergy analysis is useful to indentifying causes and
locations of process inefficiencies [1]. It is commonly used in assessment of heating
systems and geothermal energy sources [2-5].
* Częstochowa University of Technology, Department of Heating, Ventilation and Air Protection,
ul. J. H. Dabrowskiego 73, 42-201 Czestochowa, Poland.
2. LOW-TEMPERATURE GEOTHERMAL HEATING SYSTEM
For the specific location in Commune of Poczesna a heating supply system based
on low temperature geothermal source with water at 19,5°C and spontaneous outflow
24m3/h was proposed. In the considered case the geothermal water is drawn by the
well pump through the heat pump. After cooling geothermal water runs into water
supply system. The heat pump is driven by electricity and it works in monovalent
system as a single heat source. It covers all the heating demand of buildings located
near geothermal bore-hole. A schematic diagram of the system is shown in Fig. 1.
GW subsystem
HP
subsystem
HD subsystem
HE
subsystem
TGW,2=5°C
TGW,1=19,5°C
To
Heat pump
Ts
Ts=60/55/45/35°C
Tr=50/45/35/28°
C
Heat
exchanger
L=1240
m
Fig. 1. Schematic diagram of the heating plant system
3. SYSTEM EXERGY ANALYSIS
For analysis purpose, the heating system was divided into four components:
geothermal water transportation (GW), heat pump (HP), heat distribution (HD) and
heat exchange (HE) as illustrated in Fig.1.
The following assumption was made during the exergy analysis:
- design heat demand 𝑄̇ℎ𝑒𝑎𝑡 =330 kW
- minimal outdoor temperature -20°C
- mass flow of geothermal water is constant, ṁGW = 13,89 kg/s
- discharge of geothermal water temperature, TGW,2=5C
- the well pump and the circulating pump efficiencies are 75% and 74%,
respectively,
- the mass flow of heating network is constant (quality regulation), the supply and
return temperatures of heating distribution water depends on outdoor temperature
and it is determined from eq. 1 and 2, respectively.
Ts = Tindoor + T
Ts,max −Toutdoor
indoor −Toutdoor,min
Tr = Tindoor + T
(Tindoor − Toutdoor )
(1)
(Tindoor − Toutdoor )
(2)
Tr,max −Toutdoor
indoor −Toutdoor,min
The analysis was carried out for different circulation water characteristic: 60/50°C,
55/45°C, 45/35°C and 35/28°C and at different dead state temperature T0: -20°C,15°C, -10°C, -5°C, 0°C and 3°C (seasonal average) as well.
3.1. MODELING
The total exergy of the system can be divided into four components: physical
exergy ExPH, kinetic exergy ExKN, potential exergy ExPT and chemical exergy ExCH
Ex = Ex PH + Ex KN + Ex PT + Ex CH
(3)
In this study only physical exergy is taken into consideration, thus the general
exergy destruction in thermal system can be expressed as follow:
Eẋd = Eẋheat − Eẋwork + Eẋmass,in − Eẋmass,out
(4)
Exergy rate of geothermal water and heating network water is determined using
following equation:
EẋGW = ṁGW [(hGW − h0 ) − T0 (sGW − s0 )]
(5)
The exergy destruction in the geothermal water transportation, heat pump, heat
distribution, heat exchanger, and total system are presented in equations 6-10,
respectively:
Eẋd,gw = Ẇgp + Eẋin,gw − Eẋout,gw
(6)
Eẋd,hp = Ẇhp + Eẋin,hp − Eẋout,hp
(7)
Q
Eẋd,hd = Ẇpump + Eẋin,hp − Eẋout,hp − Eẋhd
(8)
Eẋd,he = Eẋin,he − Eẋout,he
(9)
̇ d,he
Eẋd,sys = Eẋd,gw + Eẋd,hp + Eẋd,hd +Ex
(10)
The exergy efficiency of subsystem i and total system is written as
εi =
Eẋi,out
Eẋi,in
(11)
3.2. RESULTS
The exergy flow diagram for heating system with water supply temperature
at 60/50°C and seasonal average temperature as a reference temperature is given
in Fig. 2. It shows that only 11,91% of the total exergy input is utilized, while the
88,19% is lost. The highest exergy loss is found to be 69,5% (corresponding to109,3
kW) from the geothermal water transportation. The heat pump loss is determined to be
20,4% (corresponding to 31,99 kW). The exergy destruction of heat distribution and
heat exchanger subsystems are relatively small and all together accounts for 2,53%.
Fig. 2. System exergy flow diagram for supply temperature 60/50°C and reference temperature 3°C
The efficiencies of subsystems for different heating network characteristics and
reference temperature equal to 3°C are shown in Fig.3. The lowest exergy efficiency
occurs from geothermal water transportation 19,91% (for all cases) and it is related to
high electric power demand of geothermal pump. Heat pump efficiency is changing
with supply water temperature, the highest 31,17% and lowest 17,05% exergy
efficiency were determined for 60/50°C and 35/28°C circulating water temperature,
respectively. Exergy efficiencies of heat distribution subsystem are about 80% and
only for very low temperature system (35/28°C) the efficiency is 75%. The heat
exchanger subsystem is characterized by the highest exergy efficiency, which is
similar for each circulating water characteristics.
100
o
60/50 C
o
55/45 C
o
45/35 C
o
35/28 C
90
o
Reference temperature ( C)
80
70
60
50
40
30
20
10
0
GW
HP
HD
HE
Subsystems
Fig. 3. Exergy efficiency of subsystems for various circulating water temperatures and reference
temperature T0=3°C.
24
22
o
60/50 C
o
55/45 C
o
45/35 C
o
35/28 C
o
Reference temperature ( C)
20
18
16
14
12
10
8
6
4
-20
-15
-10
-5
0
5
Exergy efficiency (%)
Fig. 4. Exergy efficiency of system for various circulating water temperatures and at different
reference temperatures.
The total exergy efficiency of heating system is changing along with reference
temperature and supply water temperature as well. As it is shown in Fig.4. by
dropping the supply temperature the total efficiency is decreasing due to constant
geothermal water exergy rate. The highest total efficiency was determined for water
supply temperature at 60/50°C and the change is linear with the reference temperature
change for all cases.
4. CONCLUSION
The presented results show that the exergy efficiency of the low-temperature
geothermal system decrease with supply water temperature drop. The difference
between geothermal water and supply water temperature plays a key role in terms of
exergy efficiency. To increase the total exergy efficiency, the district heating supply
temperature should be higher in order to increase exergy efficiency of heat pump and
whole system as well. It will cause an increase of exergy losses in distribution network
but nevertheless relatively small due to heat pump exergy destruction.
The highest exergy lost is determined for geothermal water transportation due to
electrical energy required for geothermal pump. In order to avoid exergy loss
utilization of geothermal water spontaneous outflow should be taken into account.
REFERENCES
[1] DINCER I., ROSEN W.A., Exergy. Energy, Environment and Sustainable Development,
Elsevier, 2007.
[2] LEE K. C., Classification of geothermal resources by exergy, Geothermics 30, Elselvier
2001, 431 – 442.
[3] OZGENER L., HEPBASLI A., DINCER I., Energy and exergy analysis of geothermal
district heating systems: an applitacion. Building and Environment 40, Elsevier, 2005,
1309-1322.
[4] OZGENER L., HEPBASLI A., DINCER I. Effect of reference state on the performanace of
energy and exergy evaluation of geothermal district heating systems: Balcova example.
Building and Environment 41, Elsevier, 2006, 699-709.
[5] SZARGUT J., ZIĘBIK A. Podstawy energetyki cieplnej, Warszawa, PWN, 2000, 343.
STRESZCZENIE
W niniejszym artykule zaprezentowano sprawność egzergetyczną systemu ciepłowniczego
wykorzystującego niskotemperaturowe geotermalne źródło energii. Źródło ciepła stanowi
sprężarkowa pompa ciepła pracująca w układzie monowalentnym. Analiza egzergetyczna została
wykonana dla całego systemu ciepłowniczego oraz każdego z jego elementów. Obliczenia zostały
przeprowadzone dla następujących charakterystyk wody sieciowej: 60/50°C, 55/45°C, 45/35°C i
35/28°C oraz dla różnych temperatur odniesienia. Największe straty egzergii występują w
podsystemie pozyskiwania wody geotermalnej i stanowiły one ponad 70% strumienia egzergii
doprowadzonej. Sprawność systemu wahała się od 4 do 22% w zależności o temperatury wody
zasilającej oraz temperatury odniesienia.
This paper is an effect of scientific research realized by financial means for
science in the years 2009-2012. Project No. N N513 359637.