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 inSłomniki 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].
2. LOW-TEMPERATURE GEOTHERMAL HEATING SYSTEM
For the specific locationin Commune of Poczesnaaheating supply system based on low temperature geothermal source with water at 19,5°C and spontaneous outflow 24m3/hwas proposed. In the considered case the geothermal water is drawn by the well pump through the heat pump. After cooling geothermal water runsinto 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.
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 =330kW
-minimal outdoor temperature -20°C
-mass flow of geothermal water is constant, kg/s
-discharge of geothermal water temperature, TGW,2=5C
-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.
(1)
(2)
The analysis was carried out for different circulationwater 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 exergyExPH, kinetic exergyExKN, potential exergyExPT and chemical exergyExCH
(3)
In this study only physical exergy is taken into consideration, thus the general exergy destruction in thermal system can be expressed as follow:
(4)
Exergy rate of geothermal water and heating network water is determined using following equation:
(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:
(6)
(7)
(8)
(9)
(10)
The exergy efficiency of subsystem i and total system is written as
(11)
3.2. RESULTS
The exergy flow diagram for heating system with water supply temperature at60/50°C and seasonal average temperature as a reference temperature is given inFig.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 temperature3°C
The efficiencies of subsystems for different heating network characteristicsand 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 relatedto 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.
Fig. 3. Exergy efficiency of subsystems for various circulating water temperatures and reference temperature T0=3°C.
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 exergyefficiency 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 geothermalpump.In order to avoid exergy loss utilization of geothermal waterspontaneous outflowshould 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. Podstawyenergetykicieplnej, Warszawa, PWN, 2000, 343.
STRESZCZENIE
W niniejszym artykule zaprezentowano sprawnośćegzergetyczną 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.
Thispaperis an effectofscientificresearchrealizedbyfinancialmeansforscience in theyears 2009-2012. Project No. N N513 359637.
[* ]Częstochowa University of Technology, Department of Heating, Ventilation and Air Protection,
ul. J. H. Dabrowskiego 73, 42-201 Czestochowa, Poland.