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A new ejector heat exchanger based on an ejector heat pump and a water-to-water heat exchanger
Applied Energy 121 (2014) 245–251
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A new ejector heat exchanger based on an ejector heat pump and a water-to-water heat exchanger Fangtian Sun a,b, Lin Fu b,⇑, Jian Sun b, Shigang Zhang b a b
Beijing University of Civil Engineering and Architecture, Beijing 100044, China Tsinghua University, Beijing 100084, China
h i g h l i g h t s EHE is based on the reverse Carnot cycle and current heat transfer mechanisms. EHE can decrease the return water temperature in the PHN to 35 °C. EHE can increase the heating capacity of the existed PHN by approximately 43%. The return water temperature in the PHN is much lower than that in the SHN. EHE has a simpler structure, lower manufacture cost, and better regulation characteristics.
a r t i c l e
i n f o
Article history: Received 13 September 2013 Received in revised form 24 December 2013 Accepted 5 February 2014 Available online 4 March 2014 Keywords: Ejector heat exchanger Heating capacity Waste heat recover system Exergy distribution ratio Regulation characteristics
a b s t r a c t As urban construction has been developing rapidly in China, urban heating load has been increasing continually. Heating capacity of the existed primary heating network (PHN) cannot meet district heating requirements of most metropolises in northern China. A new type of ejector heat exchanger (EHE) based on an ejector heat pump and a water-to-water heat exchanger (WWHE) was presented to increase the heating capacity of the existed PHN, and the EHE was also analyzed in terms of laws of thermodynamics. A new parameter, the exergy distribution ratio (EDR), is introduced, which is adopted to analyze regulation characteristics of the EHE. We find that the EHE shows better performance when EDR ranges from 44% to 63%. EHE can decrease the temperature of return water in the PHN to 35 °C, therefore, this can increase the heating capacity of existed PHN by about 43%. The return water with lower temperature in the PHN could recover more low-grade waste heat in industrial systems. Because of its smaller volume and lower investment, EHEs could be applied more appropriately in district heating systems for long-distance heating and waste heat district heating systems. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction District heating is one of the most common heating systems in China because of two significant advantages: a higher efficiency and less pollution. In the past decades, urban construction has been developing quite rapidly in China. In particular, many lowrise buildings have been replaced by high-rise ones. Thus, urban floor area has been increasing sharply in most cities in China. Therefore, both the requirement and the density of urban heating loads have been increasing considerably in most metropolises in northern China, so the heating capacity of the primary heating network (PHN) has to be increased. However, the heating capacity of ⇑ Corresponding author. Tel.: +86 010 62773885; fax: +86 010 62770544. E-mail addresses: [email protected] (F. Sun), [email protected] (L. Fu). http://dx.doi.org/10.1016/j.apenergy.2014.02.018 0306-2619/Ó 2014 Elsevier Ltd. All rights reserved.
existed PHN has often approached its maximum capacity; therefore it cannot meet the rapidly growing heating load density requirement in most metropolises in northern China [1–3]. Meanwhile, the existed PHN cannot be reformed or rebuilt easily because of heavy urban traffic in these cities. Lower heating capacity of the existed PHN has obviously weakened the heating quality in several cities in northern China. Therefore, the issue of how to increase the heating capacity of the existed PHN is significant for most district heating systems. In general, there are two ways for increasing the heating capacity of an existed PHN. One way is to increase the circulating water flux. However, this would lead to a dramatic increase of the electricity consumption of circulation pumps. The other way is to increase the temperature difference between the supply water and the return water in the existed PHN without changing water flux. However, the upper limit temperature of the supply water is
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Nomenclature WWHE EHE EHP PHN SHN EDR AHE PEE m h u
e P A N e ms i s T
water-to-water heat exchanger ejector heat exchanger ejector heat pump primary heating network secondary heating network exergy distribution ratio absorption heat exchanger product exergetic efficiency mass flux, kg/s specific enthalpy, kJ/kg velocity, m/s entrainment ratio pressure, Pa sectional area, m2 power, W specific exergy, kJ/kg mixing section input specific entropy, kJ/kg K temperature, K
restricted by the heat insulator outside heating pipes, so it cannot be increased for a built pipe. Therefore, an available method is to decrease the return water temperature in the existed PHN. For current water-to-water heat exchangers (WWHEs) in heating substations, the return water temperature in the PHN is 5– 15 °C higher than that of the secondary heating network (SHN) [3]. For district heating systems with an indirect connection between the PHN and the SHN, there is a temperature difference between the return water in the PHN and that in the SHN, which is often more than 3 °C. This is necessary even for high-efficiency heat exchangers such as plate heat exchangers [4] and heat pipe heat exchangers [5,6]. For district heating systems with a direct connection between the PHN and the SHN, the return water temperature in the SHN is equal to that in the PHN [7,8]. At present, a direct connection between PHN and SHN is not widely utilized in district heating systems in China because of its disadvantages of a smaller heating area and greater water loss in the PHN. Not only because of the temperature difference between the return water and the supply water in the PHN restricted by the return water temperature in the SHN; the temperatures of the supply water and return water in the existed SHN are decided by heating users, so these temperatures cannot be changed freely, either. Consequently, the heating capacity of an existed PHN cannot be increased obviously when using current WWHEs [9–11]. It is found that there is a large thermodynamic irreversible loss during the heat transfer process in WWHEs in heating substations of district heating systems, and the supply water temperature in an existed PHN is often higher than 90 °C [12]. Basing on these results, Fu et al. [12–14] invented the absorption heat exchanger (AHE) and presented a new district heating system with Co-generation based on the absorption heat exchange cycle (Co-ah). In the Co-ah, AHEs replace WWHEs in heating substations, which is aimed to decrease the return water temperature in the PHN to 25 °C. Compared with existed district heating systems based on CHP, Co-ah can increase the heating capacity of the existed PHN by 75%. The lower return water temperature in the PHN could recover waste heat of exhausted steam from steam turbines in thermal power stations. Indeed, AHEs are crucial features in Co-ah systems. So far, AHEs have been applied in 198 heating substations in numerous cities in China, such as Beijing and Datong [12,15]. Generally, due to its complex structure, the shape of AHE is often over
Q Ex n
thermal energy, W exergy, kJ number
Sub indexes pf, sf primary fluid, secondary fluid cv control volume 1w circulating water of PHN 2w circulating water of SHN rp refrigerant pump he heat exchanger ej ejector c condenser g generator e evaporator r refrigerant o output 0 referred state point
three times larger than the WWHE for the same heat transfer capacity. Consequently, AHEs can be used instead of WWHEs in new heating substations or large space existed ones, but not available in heating substations with small space. Furthermore, AHEs are much more expensive than WWHEs. It is clear that AHEs cannot meet the requirements of all district heating systems because of these shortcomings. Therefore, it is necessary to develop a new compact heat exchanger for decreasing the return water temperature significantly in the PHN, which has smaller volume and lower cost. An ejector as a key component is currently used to improve performance of heat pump and recover waste energy [16–18]. The ejector is used to raise the absorber pressure, and improve performance of an absorption refrigeration system [19]. Ejectors are divided into three types: a liquid–liquid ejector, a liquid–gas ejector and a gas–gas ejector. The liquid–liquid ejector is used in ammonia absorption systems as a mixer between the feed solution and the high concentrated solution coming from the rectifying section [20]. The liquid–gas ejector which is used to facilitate pressure recovery and improve absorption process is introduced at the absorber inlet of the absorption refrigeration cycle system with R22-DME-TEG [21], or R125-organic absorbents [22,23], or ammonia-lithium nitrate solution [24]. However, the liquid–gas ejector is not suitable to operate with low-density vapor such as water. The gas–gas ejector is commonly used in an ejector refrigeration system to recovery waste heat, and it is also introduced at the generator outlet or absorber inlet in an absorption refrigeration system [25,26]. An injection valve is also used in two-stages rotary compressor of air-to-water heat pump (AWHP) to increase inlet pressure of the secondary stage compressor, and it can improve the heating capacity by 48% and the coefficient of performance (COP) of the AWHP by 36% [27]. Chen et al. [28] indicated that the COP of a hybrid ejector-CO2 vapor compression refrigeration system for vehicles is higher 45.2% than that of a conventional CO2 vapor compression refrigeration system. Obviously, the ejector heat pump is suitable for waste energy recovery because of its advantages such as lower cost and simpler structure. According to both technical characteristics of the ejector heat pump and thermodynamic characteristics of heat transfer process of the WWHE in heating substation, a new ejector heat exchanger (EHE) based on an ejector heat pump and a WWHE was given.
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cooled circulating water from the evaporator of the EHP is used as the return water in the PHN.
2. Description of the ejector heat exchanger (EHE) The EHE is a new heat transfer basing on the reverse Carnot cycle.
2.2.2. Operating principle of the circulating water in the SHN The return water in the SHN is divided into two parts (A and B) at point 1. Part A enters into the WWHE and is heated by the higher-temperature circulating water in the PHN from the generator of the EHP. Part B enters into the condenser of the EHP and is heated by the higher-temperature refrigerant vapor from the generator of the EHP. Both parts (A and B) converge at point 2, which is used as the supply water in the SHN.
2.1. Composition of the EHE The EHE is combined by a WWHE and an ejector heat pump (EHP) [29]. Specifically, the EHP is composed by a generator, a condenser, an ejector, an evaporator, a circulating refrigerant pump, and other attachments. And these components are connected together by refrigerant tubes. The EHP is used for refrigeration, and it can be driven by solar energy or waste heat because of its low-grade energy input requirement [30,31]. Current available working fluids for the EHP include R141b, R236fa, R152a, R134a, R123, and R718 [31,32], which could be chosen depending on the operating condition and actual requirements. In this study, R718 was selected because it is a natural refrigerant without environmental hazards. The structure sketch of the EHE is shown in Fig. 1. Compared to AHEs, EHE has no absorbers, so it does not need corrosion protection or anti-crystallization protection. Also, the generator of the EHE has greater heat transfer efficiency than that of the AHE. Compared to the AHE, the structure of the EHE is simpler and its volume is smaller obviously.
2.2.3. Operating principle of the working fluid in EHP The liquid refrigerant left the tank of the EHP is divided into two parts (C and D). Part C is pumped into the generator of the EHP by a refrigerant pump and is heated by the supply water in the PHN. The refrigerant liquid then turns to higher-pressure refrigerant vapor in the generator of the EHP. Part D flows through a throttling valve where its pressure is reduced, which could result in a gas–liquid two-phase flow. Part D then enters into the evaporator of the EHP where it is heated to become lower-pressure refrigerant vapor. The higher-pressure refrigerant vapor (part C) from the generator of the EHP, which is used as the primary fluid, enters into the motive nozzle of the ejector where its internal energy is converted into kinetic energy. Part C has supersonic velocity and lower pressure in the suction chamber of the ejector. The lower-pressure refrigerant vapor (part D) from the evaporator of the EHP, which is used as the secondary fluid, is entrained into the suction chamber of the ejector. Both parts (C and D) then enter into the mixing chamber of the ejector, where they exchange momentum, kinetic energy, and internal energy, and they become one stream with almost a uniform pressure and speed. Kinetic energy of refrigerant vapor is converted into internal energy in the diffuser of the ejector, which could reach a pressure higher than the inlet pressure of part D in the suction chamber of the ejector. The structure of the ejector is shown in Fig. 2. Refrigerant vapor of both parts (C and D) flows into the condenser of the EHP, which is cooled to become refrigerant liquid by one part of the return water in the SHN. The refrigerant liquid left the condenser of the EHP enters into the refrigerant tank, and a complete ejector heat pump cycle is finished. The thermal energy distribution of the circulating water in the PHN is shown in Fig. 3. The high-temperature thermal energy of
2.2. Operating principle of the EHE The operating principle of the EHE is combined by the circulating water in the PHN, the circulating water in the SHN, and the working fluid in the EHP. 2.2.1. Operating principle of the circulating water in the PHN Firstly, the high-temperature supply water in the PHN, which is used as the driving heat source, enters into the generator of the EHP to heat the refrigerant. Secondly, the cooled circulating water in the PHN from the generator, which is used as a heating source, enters into the WWHE to heat the return water in the SHN. Thirdly, the cooled circulating water in the PHN from the WWHE, which is used as a low-temperature heat source, enters into the evaporator of the EHP to heat the refrigerant, which is cooled again by the lower-temperature refrigerant. Therefore, the return water temperature in the PHN is much lower than that in the SHN. Finally, the
Ejector Heat Pump Supply water Generator
Condenser Ejector
C
Refrigerant tank Throttling valve
D
Evaporator A
Return water
The primary heating network
Water-to-water Heat exchanger
2
Refrigerant pump
The secondary heating network
Supply water
Return water 1 B Fig. 1. Structure sketch of the ejector heat exchanger.
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Diffuser chamber 3
Primary fluid
Suction Mixing chamber chamber 1
1
Motive nozzle
3
Secondary fluid
Thermal energy in supply water in the PHN
Fig. 2. Structure sketch of the ejector.
Driven heating source of EHP
Thermal energy is conserved in the EHE. Released thermal energy of the circulating water in the PHN is equivalent to the increased thermal energy of the circulating water in the SHN. Return water temperature in the PHN is much lower than that in the SHN in the EHE, which is impossible for a current WWHE. In order to analyze the performance and applicability of the EHE, thermodynamic analysis was presented. Following assumptions were given: (1) (2) (3) (4) (5)
PHN and SHN are two different closed cycles. There is no heat loss or water leakage. The environmental temperature is 0 °C. The flow inside the ejector is steady and one-dimensional. Normal shock occurs at the ends of the constant area mixing chamber. (6) Loss coefficients for the primary fluid nozzle, secondary fluid passage, and diffuser are 0.95, 0.95, and 0.85, respectively [33]. 3.1. Thermodynamic models of the EHE
Heating source of WWHE
The energy conservation equation for the generator is given as follows:
m1w ðh1w;i h1w;g;o Þ ¼ mr;g ðhr;g;o hr;g;i Þ
Low temperature heating source of EHP Evaporator of EHP
3. Thermodynamic analysis of the EHE
ð1Þ
The energy conservation equation for the WWHE is given as follows: WWHE of EHE
Generator of EHP
Temperature of supply water of PHN Fig. 3. Utilization method of the thermal energy of the supply water in the PHN.
m1w ðh1w;g;o h1w;he;o Þ ¼ m2w;he ðh2w;he;o h2w;i Þ
The energy conservation equation for the evaporator is given as follows:
m1w ðh1w;he;o h1w;o Þ ¼ mr;e ðhr;e;o hr;c;o Þ the supply water in the PHN, which is used as the driving heat source, is utilized by the generator of the EHP. The mid-temperature thermal energy of the supply water in the PHN, which is used as a heating source, is utilized by the WWHE. The low-temperature thermal energy of the supply water in the PHN, which is used as a low-temperature heat source, is utilized by the evaporator of the EHP. Thus, the thermal energy of the supply water in the PHN is utilized step by step by using an EHP and a WWHE according to different energy grade. The return water temperature in the PHN is lower than that in the SHN because the water is further cooled by the evaporation of refrigerant liquid in the EHP. The EHE is used to exchange more heat from the high-temperature supply water in the PHN using the EHP and decrease the return water temperature in the PHN. The EHP which is driven by the high-temperature supply water in the PHN is used to take in the low-grade heat from the circulating water in the PHN in the evaporator, and it could transmit this low-grade heat from the circulating water in the PHN to the one in the SHN. Thus the EHE could decrease return water temperature in the PHN obviously. This return water with lower temperature could increase temperature difference between return water and supply water in the PHN without changing the flowing rate. Therefore, the heat transmission capacity of the PHN is increased obviously. In the waste heat district heating systems based on heat pumps, the return water with lower temperature in the PHN could improve performance of heat pumps for heat recovery in the heating source center. Because of this, more lower-grade heat could be recovered and the primary energy resource is saved. The total energy efficiency of heat recovery system adopting the EHE is enlarged.
ð2Þ
ð3Þ
The formula for the entrainment ratio of the ejector is described as follows:
e¼
mr;e mr;g
ð4Þ
where entrainment ratio e is defined as the ratio of secondary fluid mr,e to primary fluid mr,g. The mass conservation equation for the ejector is given as follows:
mr;g ð1 þ eÞ ¼ mr;ms
ð5Þ
The energy conservation equation for the ejector is given as follows:
mr;g hr;g;o þ mr;e hr;e;o ¼ mr;ms hr;c;i
ð6Þ
The momentum equation for the mixing chamber of the ejector is given as follows:
mr;g upf þ mr;e usf mr;ms ums;o Z Ams;i ¼ Pms;o Ams;o þ PdA ðPpf ;ms;i Apf ;ms;i þ Psf ;ms;i Asf ;ms;i Þ
ð7Þ
Ams;o
The energy conservation equation for the condenser is given as follows:
mr;ms ðhr;ms;o hr;c;o Þ ¼ m2w;c ðh2w;c;o h2w;i Þ
ð8Þ
The mass conservation equation for the SHN is given as follows:
m2w;c þ m2w;he ¼ m2w
ð9Þ
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ð10Þ
The energy conservation equation for the refrigerant liquid left the condenser to the generator of the EHP is given as follows:
mr;g hr;c;o þ Nrp ¼ mr;g hr;g;i
ð11Þ
The energy conservation equation for the EHE is given as follows:
m1w ðh1w;i h1w;o Þ þ Nrp ¼ m2w ðh2w;o h2w;i Þ
ð12Þ
Specific exergy is calculated as follows:
e ¼ ðh h0 Þ T 0 ðS S0 Þ
ð13Þ
The mass conservation equation of the WWHE is given as follows:
ðm1w;i þ m2w;i Þ ðm1w;o þ m2w;o Þ ¼ 0
ð14Þ
The energy conservation equation of the WWHE is given as follows:
Q cv þ ðm1w;i h1w;i þ m2w;i h2w;i Þ ðm1w h1w;o þ m2w h2w;o Þ ¼ 0
ð15Þ
The exergy conservation equation of the WWHE is given as follows:
T0 þ ðm1w;i e1w;i þ m2w;i e2w;i Þ ðm1w;o e1w;o þ m2w;o e2w;o Þ ¼ 0 Q cv 1 T ð16Þ The mass conservation equation, energy conservation equation and exergy conservation equation for the EHP are the same as the WWHE. 3.2. Evaluation indexes The second law of thermodynamics is used to discuss irreversibility. And exergetic analysis is used to determine the location and magnitude of exergy loss, so exergetic analysis is often used to optimize the designs of energy supply systems. Donnellan et al. [34] discussed the most influential factors affecting the coefficient of performance of a triple absorption heat transformer (THAT) by the First and Secondary thermodynamics laws in order to optimize the THAT system. Modesto and Nebra [35] analyzed a power generation system using blast furnace oven gas by an exergoeconmic analysis method, and defined product exergetic efficiency. The product exergetic efficiency (PEE) is defined as follows:
PEE ¼
Exo Exi
44
ð17Þ
PEE is defined as the ratio of product exergy to input exergy [35]. Product exergy is defined as the exergy change between the supply water and return water in the SHN. Input exergy is the sum of the exergy change between the supply water and return water in the PHN and the electricity exergy of the refrigerant circulating pump. Exergy distribution ratio (EDR) is defined as follows:
EDR ¼
Exi;WWHE Exi;EHE
When 130 °C/50 °C for the PHN and 60 °C/45 °C for the SHN, PEE of the WWHE is about 0.657. However, when 130 °C/35 °C for the PHN and 60 °C/45 °C for the SHN, PEE of the EHE is about 0.707. Obviously, the irreversible loss of the EHE is less than that of the WWHE. This is just because of that different grades heat of the supply water in the PHN are utilized step by step. Therefore, the EHE could decrease exergy loss in the process of heat transfer between the PHN and the SHN in the heating substation. Fig. 4 shows the relationships among EDR, return water temperature in the PHN, and PEE of the SHN with the same temperatures (60 °C/45 °C) in the SHN. As EDR increases, the return water temperature in the PHN decreases firstly and then goes up. Conversely, PEE rises firstly and then decreases as EDR increases. Therefore, there is an optimum EDR. In addition, the lowest return water temperature in the PHN and the largest PEE are both related to the same EDR for the EHE. The greater utilization of exergy in the circulating water in the PHN will lead to higher performance of heat pump in the EHE. Besides, higher the condensing temperature could lead to worse performance of the heat pump. And a larger EDR will bring the lower condensing temperature and higher generating temperature of the EHP. EDR has a positive effect on the heat pump cycle performance of the EHP, but it has a negative effect on the refrigerating capacity of the EHP. Therefore, there is an optimum EDR, which is an important parameter for the design of the operation and control strategy of the EHE. Fig. 5 shows the relationships among the heating parameters of the PHN and SHN. In the EHE, the return water temperature in the PHN is affected by other heating parameters of PHN and SHN. As the supply water temperature in the PHN increases, the return water temperature in the PHN is reduced slowly under the same heating parameters of the SHN, which is led by that the quality of input thermal energy and the exergy utilization ratio of the EHP increase when the supply water temperature in the PHN increases. The return water temperature in the PHN decreases as the heating parameters of the SHN decrease under the same supply water temperature in the PHN. In general, the lower the temperature parameter of the SHN is, and the lower the condensing temperature of the EHP produces. This is because of that the heat pump cycle performance and refrigerating capacity of the EHP both rise when the condensing temperature goes down. It is found that the lower heating parameters of the SHN help to decrease the
ð18Þ
EDR is defined as the ratio of input exergy of the WWHE to input exergy of the EHE. The input exergy of the WWHE is the exergy change between the inlet and outlet of the circulating water in the PHN for the WWHE. The input exergy of the EHE is the exergy change between the supply water and return water in the PHN.
Temperature of return water / ć
m2w;c h2w;c;o þ m2w;he h2w;he;o ¼ m2w h2w;o
4. Results and discussion
0.660 Temperature of return water
42
0.655
Product exergetic efficiency
40
0.650
38
0.645
36
0.640
34
0.635
32
Product exergetic efficiency / W/W
The energy balance equation of node 2 (in Fig. 1) is given as follows:
0.630 0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Exergy distribution ratio of WWHE to EHE / W/W Fig. 4. Relationships among PEE, EDR, and the temperature of the return water in the PHN.
F. Sun et al. / Applied Energy 121 (2014) 245–251
Temperature of return water in the PHN / ć
250
of the SHN has little effect on EDR. It also can be found that the adjustable range of the operation condition is smaller for the EHE when the heating temperature of the SHN is designed to be 70 °C/55 °C from Figs. 5 and 6. In other words, EHEs can show better regulation characteristics in low-temperature district heating systems. When the supply water temperature in the PHN changes from 140 °C to 150 °C, EDR is barely affected by the heating temperature of the SHN. Therefore, the value of EDR is an extremely important parameter for the design of the operation and control strategy of the EHE when the supply water temperature in the PHN is below 140 °C.
55 50 45 40 35 30 60/45ć for SHN
25
65/50ć for SHN
20
70/55ć for SHN
15
5. Conclusions
10 80
90
100
110
120
130
140
150
160
Temperature of supply water of PHN / ć Fig. 5. Relationships among the heating parameters of the PHN and SHN.
return water temperature in the PHN and increase the heat transmission capacity of the existed PHN. Therefore, EHEs can play a more significant role in increasing the heating capacity of the existed PHN in low-temperature district heating systems without changing flowing rate. Furthermore, the return water with lower temperature in the PHN could recover more low-temperature waste heat in a heat recovery system. Besides, return water with lower temperature are available to recover different kinds of low grade heat in different industries. In other words, this device could be employed in many heat recovery systems, which could enlarge the total energy efficiency obviously. For the PHN, the larger temperature difference between the supply water and return water can significantly decrease the electricity consumption of circulation water pumps for the same heating capacity. Thus, EHEs are also quite suitable for district heating systems with long-distance heating system. Fig. 6 shows the relationships among EDR, supply water temperature in the PHN, and heating parameters of the SHN. It can found that EDR is affected by both the supply water temperature in the PHN and the heating parameters of the SHN. EDR decreases as the supply water temperature in the PHN increases under the same heating parameters of the SHN. And EDR decreases when the heating parameters of the SHN increase under the same supply water temperature in the PHN. However, the heating temperature
0.80
Acknowledgements The authors wish to acknowledge the support provided by the Natural Science and Technology Support Plan of the People’s Republic of China (No. 2012BAJ04B00), the Beijing Municipal Science and Technology Plant (No. D131100003813001) and the Beijing Key Laboratory of Heating, Gas Supply, Ventilating and Air Conditioning Engineering, Beijing University of Civil Engineering and Architecture.
References
0.70
EDR of WWHE to EHE / W/W
Thermal energy of the supply water in the PHN is utilized step by step by using an EHE. Compared to a WWHE, an EHE can decrease the return water temperature in the PHN (which is lower than that in the SHN), and increase the heating capacity of the existed PHN. An EHE can decrease the exergy loss of the heat transfer process by decreasing the return water temperature in the PHN through using an EHP. Its thermal energy and exergy distribution between the EHP and the WWHE in the EHE can be regulated by the EHP. EHEs will show better regulation characteristics in low-temperature district heating systems. They can yield a lower return water temperature in the PHN, which not only increases the heating capacity of the PHN, but also makes it possible to recover much more low-grade waste heat for waste heat district heating systems. Because of its simple structure and better heat transfer performance, EHEs have a smaller volume and lower manufacture cost when compared with AHEs. As a result, it is much easier to instead of WWHEs in existed small space heating substations. Thus, EHEs could be widely applied in district heating systems with long-distance heating and waste heat district heating systems.
0.60 0.50 60/45ć for SHN
0.40
65/50ć for SHN
0.30
70/55ć for SHN
0.20 0.10 0.00 80
90
100
110
120
130
140
150
160
Temperature of supply water of PHN / ć Fig. 6. Relationships among EDR and the heating parameters of the SHN and PHN.
[1] Heating office of Beijing Municipal Committee. Challenge and thinking in the development of Beijing Heating. Urban Manage Sci Technol 2008;6:20–2 (in Chinese). [2] Building Energy Research Center of Tsinghua University. 2011 Annual report of china building energy conservation. Beijing: China Architecture and Building Press; 2011 (in Chinese). [3] Sun FT. Development of key equipments and research on modes of waste heat heating of circulating water in power plant, Post-doctoral Research report of Tsinghua University 2010; 6 (in Chinese). [4] Abu-khader MM. Plate heat exchangers: recent advances. Renew Sust Energy Rev 2012;16(4):1883–91. [5] Srimuang W, Amatachaya P. A review of the applications of heat pipe heat exchangers for heat recovery. Renew Sust Energy Rev 2012;16(6):4303–15. [6] Vasiliev LL. Heat pipes in modern heat exchangers. Appl Therm Eng 2005;25(1):1–19. [7] Jacimovic B, Zoivkovic B, Genic S, Zekonja P. Supply water temperature regulation problems in district heating network with both direct and indirect connection. Energy Buildings 1998;28(3):317–22. [8] Zsebik A. Heat exchanger connection in substations – a tool of decreasing return temperature in district heat networks. Energy Eng 2001;98(5):20–8. [9] Bergles AE. ExHFT for fourth generation heat transfer technology. Exp Therm Fluid Sci 2002;26(2–4):335–44.
F. Sun et al. / Applied Energy 121 (2014) 245–251 [10] Guo ZY, Cheng XG, Xia ZZ. Least dissipation principle of heat transport potential capacity and its application in heat conduction optimization. Sci Bull 2003;48(4):406–10 (in Chinese). [11] Goldstein RJ, Ibele WE, Patankar SV, Simon TW, Kuehn TH, Strykowski PJ, et al. Heat transfer – a review of 2005 literature. Int J Heat Mass Transfer 2010;53(21–22):4397–447. [12] Sun FT, Fu L, Zhang SG, Sun J. New waste heat district heating system with combined heat and power based on absorption heat exchange cycle in China. Appl Therm Eng 2012;37(6–7):136–44. [13] Fu L, Jiang Y, Zhang SG. One kind of heat exchanger unit based on heat pump technology, China Patent No. 200810101064.5, 2008-12-24 (in Chinese). [14] Fu L, Sun FT, Zhang SG et al. A method of reduction return water temperature of heating tube, China Patent No. 200910091337.7, 2011-6-29 (in Chinese). [15] Fu L, Li Y, Zhang SG, Jiang Y, Zhao XL. Application and conception of heat transfer based on absorption cycle. Building Sci 2010;26(10):136–40 (in Chinese). [16] Chua KJ, Chou SK, Yang WM. Advances in heat pump systems: a review. Appl Energy 2010;87(12):3611–24. [17] Lakew AA, Bolland O, Ladam Y. Theoretical thermodynamic analysis of Rankine power cycle with thermal driven pump. Appl Energy 2011;88(9):3005–11. [18] Sözen A, Özalp M. Solar-driven ejector-absorption cooling system. Appl Energy 2005;80(1):97–113. [19] Srikhirin P, Aphornratana S, Chungpaibulpatana S. A review of absorption refrigeration technologies. Renew Sust Energy Rev 2001;5(4):343–72. [20] Bogart M. Ammonia absorption refrigeration in industrial processes. Houston (TX, USA): Gulf Pub. Co.; 1981. [21] Chen Li-Ting. A new ejector-absorber cycle to improve the COP of an absorption refrigeration system. Appl Energy 1988;30(1):37–51. [22] Levy A, Jelinek M, Borde I. Numerical study on the design parameters of a jet ejector for absorption systems. Appl Energy 2002;72(2):467–78. [23] Jelinek M, Levy A, Borde I. Performance of a triple-pressure-level absorption cycle with R125-N, N0 -dimethylethylurea. Appl Energy 2002;71(3):171–89.
251
[24] Vereda C, Ventas R, Lecuona A, Venegas M. Study of an ejector-absorption refrigeration cycle with an adaptable ejector nozzle for different working conditions. Appl Energy 2012;97:305–12. [25] Kuhlenschmidt D. Absorption refrigeration system with multiple generator stages, US Patent No. 3717007, 1973. [26] Aphornratana S, Eames IW. Experimental investigation of a combined ejectorabsorption refrigerator. Int J Energy Resour 1998;22(3):195–207. [27] Ko Yh, Park S, Jin S, Kim B, Jeong JH. The selection of volume ratio of two-stage rotary compressor and its effects on air-to-water heat pump with flash tank cycle. Appl Energy 2013;104:187–96. [28] Chen XJ, Worall M, Omer S, Su YH, Riffat S. Theoretical studies of a hybrid ejector CO2 compression cooling system for vehicles and preliminary experimental investigations of an ejector cycle. Appl Energy 2013;102:931–42. [29] Sun FT, Wang N, Li DY et al. New heat exchanger based on ejector heat pump, China Patent No. 201120186592.2, 2011-12-28 (in Chinese). [30] Little AB, Garimella S. A review of ejector technology for refrigeration applications. Int J Air-Cond Refrig 2011;19(1):1–15. [31] Abdulateef JM, Sopian K, Alghoul MA, Sulaiman MY. Review on solar-driven ejector refrigeration technologies. Renew Sust Energy Rev 2009;13(6– 7):1338–48. [32] Selvaraju A, Mani A. Analysis of a vapor ejector refrigeration system with environment friendly refrigerants. Int J Therm Sci 2004;43(9):915–21. [33] Henzler JJ. Design of ejectors for single-phase material systems. Ger Chem Eng 1983;6(5):292–300. [34] Donnellan P, Byrne E, Oliveira J, Cronin K. First and second law multidimensional analysis of a triple absorption heat transformer (TAHT). Appl Energy 2014;113:141–51. [35] Modesto M, Nebra SA. Exergoeconomic analysis of the power generation system using blast furnace and coke oven gas in a Brazilian steel mill. Appl Therm Eng 2009;29(15–17):2127–36.
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
A new ejector heat exchanger based on an ejector heat pump and a water-to-water heat exchanger Fangtian Sun a,b, Lin Fu b,⇑, Jian Sun b, Shigang Zhang b a b
Beijing University of Civil Engineering and Architecture, Beijing 100044, China Tsinghua University, Beijing 100084, China
h i g h l i g h t s EHE is based on the reverse Carnot cycle and current heat transfer mechanisms. EHE can decrease the return water temperature in the PHN to 35 °C. EHE can increase the heating capacity of the existed PHN by approximately 43%. The return water temperature in the PHN is much lower than that in the SHN. EHE has a simpler structure, lower manufacture cost, and better regulation characteristics.
a r t i c l e
i n f o
Article history: Received 13 September 2013 Received in revised form 24 December 2013 Accepted 5 February 2014 Available online 4 March 2014 Keywords: Ejector heat exchanger Heating capacity Waste heat recover system Exergy distribution ratio Regulation characteristics
a b s t r a c t As urban construction has been developing rapidly in China, urban heating load has been increasing continually. Heating capacity of the existed primary heating network (PHN) cannot meet district heating requirements of most metropolises in northern China. A new type of ejector heat exchanger (EHE) based on an ejector heat pump and a water-to-water heat exchanger (WWHE) was presented to increase the heating capacity of the existed PHN, and the EHE was also analyzed in terms of laws of thermodynamics. A new parameter, the exergy distribution ratio (EDR), is introduced, which is adopted to analyze regulation characteristics of the EHE. We find that the EHE shows better performance when EDR ranges from 44% to 63%. EHE can decrease the temperature of return water in the PHN to 35 °C, therefore, this can increase the heating capacity of existed PHN by about 43%. The return water with lower temperature in the PHN could recover more low-grade waste heat in industrial systems. Because of its smaller volume and lower investment, EHEs could be applied more appropriately in district heating systems for long-distance heating and waste heat district heating systems. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction District heating is one of the most common heating systems in China because of two significant advantages: a higher efficiency and less pollution. In the past decades, urban construction has been developing quite rapidly in China. In particular, many lowrise buildings have been replaced by high-rise ones. Thus, urban floor area has been increasing sharply in most cities in China. Therefore, both the requirement and the density of urban heating loads have been increasing considerably in most metropolises in northern China, so the heating capacity of the primary heating network (PHN) has to be increased. However, the heating capacity of ⇑ Corresponding author. Tel.: +86 010 62773885; fax: +86 010 62770544. E-mail addresses: [email protected] (F. Sun), [email protected] (L. Fu). http://dx.doi.org/10.1016/j.apenergy.2014.02.018 0306-2619/Ó 2014 Elsevier Ltd. All rights reserved.
existed PHN has often approached its maximum capacity; therefore it cannot meet the rapidly growing heating load density requirement in most metropolises in northern China [1–3]. Meanwhile, the existed PHN cannot be reformed or rebuilt easily because of heavy urban traffic in these cities. Lower heating capacity of the existed PHN has obviously weakened the heating quality in several cities in northern China. Therefore, the issue of how to increase the heating capacity of the existed PHN is significant for most district heating systems. In general, there are two ways for increasing the heating capacity of an existed PHN. One way is to increase the circulating water flux. However, this would lead to a dramatic increase of the electricity consumption of circulation pumps. The other way is to increase the temperature difference between the supply water and the return water in the existed PHN without changing water flux. However, the upper limit temperature of the supply water is
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Nomenclature WWHE EHE EHP PHN SHN EDR AHE PEE m h u
e P A N e ms i s T
water-to-water heat exchanger ejector heat exchanger ejector heat pump primary heating network secondary heating network exergy distribution ratio absorption heat exchanger product exergetic efficiency mass flux, kg/s specific enthalpy, kJ/kg velocity, m/s entrainment ratio pressure, Pa sectional area, m2 power, W specific exergy, kJ/kg mixing section input specific entropy, kJ/kg K temperature, K
restricted by the heat insulator outside heating pipes, so it cannot be increased for a built pipe. Therefore, an available method is to decrease the return water temperature in the existed PHN. For current water-to-water heat exchangers (WWHEs) in heating substations, the return water temperature in the PHN is 5– 15 °C higher than that of the secondary heating network (SHN) [3]. For district heating systems with an indirect connection between the PHN and the SHN, there is a temperature difference between the return water in the PHN and that in the SHN, which is often more than 3 °C. This is necessary even for high-efficiency heat exchangers such as plate heat exchangers [4] and heat pipe heat exchangers [5,6]. For district heating systems with a direct connection between the PHN and the SHN, the return water temperature in the SHN is equal to that in the PHN [7,8]. At present, a direct connection between PHN and SHN is not widely utilized in district heating systems in China because of its disadvantages of a smaller heating area and greater water loss in the PHN. Not only because of the temperature difference between the return water and the supply water in the PHN restricted by the return water temperature in the SHN; the temperatures of the supply water and return water in the existed SHN are decided by heating users, so these temperatures cannot be changed freely, either. Consequently, the heating capacity of an existed PHN cannot be increased obviously when using current WWHEs [9–11]. It is found that there is a large thermodynamic irreversible loss during the heat transfer process in WWHEs in heating substations of district heating systems, and the supply water temperature in an existed PHN is often higher than 90 °C [12]. Basing on these results, Fu et al. [12–14] invented the absorption heat exchanger (AHE) and presented a new district heating system with Co-generation based on the absorption heat exchange cycle (Co-ah). In the Co-ah, AHEs replace WWHEs in heating substations, which is aimed to decrease the return water temperature in the PHN to 25 °C. Compared with existed district heating systems based on CHP, Co-ah can increase the heating capacity of the existed PHN by 75%. The lower return water temperature in the PHN could recover waste heat of exhausted steam from steam turbines in thermal power stations. Indeed, AHEs are crucial features in Co-ah systems. So far, AHEs have been applied in 198 heating substations in numerous cities in China, such as Beijing and Datong [12,15]. Generally, due to its complex structure, the shape of AHE is often over
Q Ex n
thermal energy, W exergy, kJ number
Sub indexes pf, sf primary fluid, secondary fluid cv control volume 1w circulating water of PHN 2w circulating water of SHN rp refrigerant pump he heat exchanger ej ejector c condenser g generator e evaporator r refrigerant o output 0 referred state point
three times larger than the WWHE for the same heat transfer capacity. Consequently, AHEs can be used instead of WWHEs in new heating substations or large space existed ones, but not available in heating substations with small space. Furthermore, AHEs are much more expensive than WWHEs. It is clear that AHEs cannot meet the requirements of all district heating systems because of these shortcomings. Therefore, it is necessary to develop a new compact heat exchanger for decreasing the return water temperature significantly in the PHN, which has smaller volume and lower cost. An ejector as a key component is currently used to improve performance of heat pump and recover waste energy [16–18]. The ejector is used to raise the absorber pressure, and improve performance of an absorption refrigeration system [19]. Ejectors are divided into three types: a liquid–liquid ejector, a liquid–gas ejector and a gas–gas ejector. The liquid–liquid ejector is used in ammonia absorption systems as a mixer between the feed solution and the high concentrated solution coming from the rectifying section [20]. The liquid–gas ejector which is used to facilitate pressure recovery and improve absorption process is introduced at the absorber inlet of the absorption refrigeration cycle system with R22-DME-TEG [21], or R125-organic absorbents [22,23], or ammonia-lithium nitrate solution [24]. However, the liquid–gas ejector is not suitable to operate with low-density vapor such as water. The gas–gas ejector is commonly used in an ejector refrigeration system to recovery waste heat, and it is also introduced at the generator outlet or absorber inlet in an absorption refrigeration system [25,26]. An injection valve is also used in two-stages rotary compressor of air-to-water heat pump (AWHP) to increase inlet pressure of the secondary stage compressor, and it can improve the heating capacity by 48% and the coefficient of performance (COP) of the AWHP by 36% [27]. Chen et al. [28] indicated that the COP of a hybrid ejector-CO2 vapor compression refrigeration system for vehicles is higher 45.2% than that of a conventional CO2 vapor compression refrigeration system. Obviously, the ejector heat pump is suitable for waste energy recovery because of its advantages such as lower cost and simpler structure. According to both technical characteristics of the ejector heat pump and thermodynamic characteristics of heat transfer process of the WWHE in heating substation, a new ejector heat exchanger (EHE) based on an ejector heat pump and a WWHE was given.
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cooled circulating water from the evaporator of the EHP is used as the return water in the PHN.
2. Description of the ejector heat exchanger (EHE) The EHE is a new heat transfer basing on the reverse Carnot cycle.
2.2.2. Operating principle of the circulating water in the SHN The return water in the SHN is divided into two parts (A and B) at point 1. Part A enters into the WWHE and is heated by the higher-temperature circulating water in the PHN from the generator of the EHP. Part B enters into the condenser of the EHP and is heated by the higher-temperature refrigerant vapor from the generator of the EHP. Both parts (A and B) converge at point 2, which is used as the supply water in the SHN.
2.1. Composition of the EHE The EHE is combined by a WWHE and an ejector heat pump (EHP) [29]. Specifically, the EHP is composed by a generator, a condenser, an ejector, an evaporator, a circulating refrigerant pump, and other attachments. And these components are connected together by refrigerant tubes. The EHP is used for refrigeration, and it can be driven by solar energy or waste heat because of its low-grade energy input requirement [30,31]. Current available working fluids for the EHP include R141b, R236fa, R152a, R134a, R123, and R718 [31,32], which could be chosen depending on the operating condition and actual requirements. In this study, R718 was selected because it is a natural refrigerant without environmental hazards. The structure sketch of the EHE is shown in Fig. 1. Compared to AHEs, EHE has no absorbers, so it does not need corrosion protection or anti-crystallization protection. Also, the generator of the EHE has greater heat transfer efficiency than that of the AHE. Compared to the AHE, the structure of the EHE is simpler and its volume is smaller obviously.
2.2.3. Operating principle of the working fluid in EHP The liquid refrigerant left the tank of the EHP is divided into two parts (C and D). Part C is pumped into the generator of the EHP by a refrigerant pump and is heated by the supply water in the PHN. The refrigerant liquid then turns to higher-pressure refrigerant vapor in the generator of the EHP. Part D flows through a throttling valve where its pressure is reduced, which could result in a gas–liquid two-phase flow. Part D then enters into the evaporator of the EHP where it is heated to become lower-pressure refrigerant vapor. The higher-pressure refrigerant vapor (part C) from the generator of the EHP, which is used as the primary fluid, enters into the motive nozzle of the ejector where its internal energy is converted into kinetic energy. Part C has supersonic velocity and lower pressure in the suction chamber of the ejector. The lower-pressure refrigerant vapor (part D) from the evaporator of the EHP, which is used as the secondary fluid, is entrained into the suction chamber of the ejector. Both parts (C and D) then enter into the mixing chamber of the ejector, where they exchange momentum, kinetic energy, and internal energy, and they become one stream with almost a uniform pressure and speed. Kinetic energy of refrigerant vapor is converted into internal energy in the diffuser of the ejector, which could reach a pressure higher than the inlet pressure of part D in the suction chamber of the ejector. The structure of the ejector is shown in Fig. 2. Refrigerant vapor of both parts (C and D) flows into the condenser of the EHP, which is cooled to become refrigerant liquid by one part of the return water in the SHN. The refrigerant liquid left the condenser of the EHP enters into the refrigerant tank, and a complete ejector heat pump cycle is finished. The thermal energy distribution of the circulating water in the PHN is shown in Fig. 3. The high-temperature thermal energy of
2.2. Operating principle of the EHE The operating principle of the EHE is combined by the circulating water in the PHN, the circulating water in the SHN, and the working fluid in the EHP. 2.2.1. Operating principle of the circulating water in the PHN Firstly, the high-temperature supply water in the PHN, which is used as the driving heat source, enters into the generator of the EHP to heat the refrigerant. Secondly, the cooled circulating water in the PHN from the generator, which is used as a heating source, enters into the WWHE to heat the return water in the SHN. Thirdly, the cooled circulating water in the PHN from the WWHE, which is used as a low-temperature heat source, enters into the evaporator of the EHP to heat the refrigerant, which is cooled again by the lower-temperature refrigerant. Therefore, the return water temperature in the PHN is much lower than that in the SHN. Finally, the
Ejector Heat Pump Supply water Generator
Condenser Ejector
C
Refrigerant tank Throttling valve
D
Evaporator A
Return water
The primary heating network
Water-to-water Heat exchanger
2
Refrigerant pump
The secondary heating network
Supply water
Return water 1 B Fig. 1. Structure sketch of the ejector heat exchanger.
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Diffuser chamber 3
Primary fluid
Suction Mixing chamber chamber 1
1
Motive nozzle
3
Secondary fluid
Thermal energy in supply water in the PHN
Fig. 2. Structure sketch of the ejector.
Driven heating source of EHP
Thermal energy is conserved in the EHE. Released thermal energy of the circulating water in the PHN is equivalent to the increased thermal energy of the circulating water in the SHN. Return water temperature in the PHN is much lower than that in the SHN in the EHE, which is impossible for a current WWHE. In order to analyze the performance and applicability of the EHE, thermodynamic analysis was presented. Following assumptions were given: (1) (2) (3) (4) (5)
PHN and SHN are two different closed cycles. There is no heat loss or water leakage. The environmental temperature is 0 °C. The flow inside the ejector is steady and one-dimensional. Normal shock occurs at the ends of the constant area mixing chamber. (6) Loss coefficients for the primary fluid nozzle, secondary fluid passage, and diffuser are 0.95, 0.95, and 0.85, respectively [33]. 3.1. Thermodynamic models of the EHE
Heating source of WWHE
The energy conservation equation for the generator is given as follows:
m1w ðh1w;i h1w;g;o Þ ¼ mr;g ðhr;g;o hr;g;i Þ
Low temperature heating source of EHP Evaporator of EHP
3. Thermodynamic analysis of the EHE
ð1Þ
The energy conservation equation for the WWHE is given as follows: WWHE of EHE
Generator of EHP
Temperature of supply water of PHN Fig. 3. Utilization method of the thermal energy of the supply water in the PHN.
m1w ðh1w;g;o h1w;he;o Þ ¼ m2w;he ðh2w;he;o h2w;i Þ
The energy conservation equation for the evaporator is given as follows:
m1w ðh1w;he;o h1w;o Þ ¼ mr;e ðhr;e;o hr;c;o Þ the supply water in the PHN, which is used as the driving heat source, is utilized by the generator of the EHP. The mid-temperature thermal energy of the supply water in the PHN, which is used as a heating source, is utilized by the WWHE. The low-temperature thermal energy of the supply water in the PHN, which is used as a low-temperature heat source, is utilized by the evaporator of the EHP. Thus, the thermal energy of the supply water in the PHN is utilized step by step by using an EHP and a WWHE according to different energy grade. The return water temperature in the PHN is lower than that in the SHN because the water is further cooled by the evaporation of refrigerant liquid in the EHP. The EHE is used to exchange more heat from the high-temperature supply water in the PHN using the EHP and decrease the return water temperature in the PHN. The EHP which is driven by the high-temperature supply water in the PHN is used to take in the low-grade heat from the circulating water in the PHN in the evaporator, and it could transmit this low-grade heat from the circulating water in the PHN to the one in the SHN. Thus the EHE could decrease return water temperature in the PHN obviously. This return water with lower temperature could increase temperature difference between return water and supply water in the PHN without changing the flowing rate. Therefore, the heat transmission capacity of the PHN is increased obviously. In the waste heat district heating systems based on heat pumps, the return water with lower temperature in the PHN could improve performance of heat pumps for heat recovery in the heating source center. Because of this, more lower-grade heat could be recovered and the primary energy resource is saved. The total energy efficiency of heat recovery system adopting the EHE is enlarged.
ð2Þ
ð3Þ
The formula for the entrainment ratio of the ejector is described as follows:
e¼
mr;e mr;g
ð4Þ
where entrainment ratio e is defined as the ratio of secondary fluid mr,e to primary fluid mr,g. The mass conservation equation for the ejector is given as follows:
mr;g ð1 þ eÞ ¼ mr;ms
ð5Þ
The energy conservation equation for the ejector is given as follows:
mr;g hr;g;o þ mr;e hr;e;o ¼ mr;ms hr;c;i
ð6Þ
The momentum equation for the mixing chamber of the ejector is given as follows:
mr;g upf þ mr;e usf mr;ms ums;o Z Ams;i ¼ Pms;o Ams;o þ PdA ðPpf ;ms;i Apf ;ms;i þ Psf ;ms;i Asf ;ms;i Þ
ð7Þ
Ams;o
The energy conservation equation for the condenser is given as follows:
mr;ms ðhr;ms;o hr;c;o Þ ¼ m2w;c ðh2w;c;o h2w;i Þ
ð8Þ
The mass conservation equation for the SHN is given as follows:
m2w;c þ m2w;he ¼ m2w
ð9Þ
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ð10Þ
The energy conservation equation for the refrigerant liquid left the condenser to the generator of the EHP is given as follows:
mr;g hr;c;o þ Nrp ¼ mr;g hr;g;i
ð11Þ
The energy conservation equation for the EHE is given as follows:
m1w ðh1w;i h1w;o Þ þ Nrp ¼ m2w ðh2w;o h2w;i Þ
ð12Þ
Specific exergy is calculated as follows:
e ¼ ðh h0 Þ T 0 ðS S0 Þ
ð13Þ
The mass conservation equation of the WWHE is given as follows:
ðm1w;i þ m2w;i Þ ðm1w;o þ m2w;o Þ ¼ 0
ð14Þ
The energy conservation equation of the WWHE is given as follows:
Q cv þ ðm1w;i h1w;i þ m2w;i h2w;i Þ ðm1w h1w;o þ m2w h2w;o Þ ¼ 0
ð15Þ
The exergy conservation equation of the WWHE is given as follows:
T0 þ ðm1w;i e1w;i þ m2w;i e2w;i Þ ðm1w;o e1w;o þ m2w;o e2w;o Þ ¼ 0 Q cv 1 T ð16Þ The mass conservation equation, energy conservation equation and exergy conservation equation for the EHP are the same as the WWHE. 3.2. Evaluation indexes The second law of thermodynamics is used to discuss irreversibility. And exergetic analysis is used to determine the location and magnitude of exergy loss, so exergetic analysis is often used to optimize the designs of energy supply systems. Donnellan et al. [34] discussed the most influential factors affecting the coefficient of performance of a triple absorption heat transformer (THAT) by the First and Secondary thermodynamics laws in order to optimize the THAT system. Modesto and Nebra [35] analyzed a power generation system using blast furnace oven gas by an exergoeconmic analysis method, and defined product exergetic efficiency. The product exergetic efficiency (PEE) is defined as follows:
PEE ¼
Exo Exi
44
ð17Þ
PEE is defined as the ratio of product exergy to input exergy [35]. Product exergy is defined as the exergy change between the supply water and return water in the SHN. Input exergy is the sum of the exergy change between the supply water and return water in the PHN and the electricity exergy of the refrigerant circulating pump. Exergy distribution ratio (EDR) is defined as follows:
EDR ¼
Exi;WWHE Exi;EHE
When 130 °C/50 °C for the PHN and 60 °C/45 °C for the SHN, PEE of the WWHE is about 0.657. However, when 130 °C/35 °C for the PHN and 60 °C/45 °C for the SHN, PEE of the EHE is about 0.707. Obviously, the irreversible loss of the EHE is less than that of the WWHE. This is just because of that different grades heat of the supply water in the PHN are utilized step by step. Therefore, the EHE could decrease exergy loss in the process of heat transfer between the PHN and the SHN in the heating substation. Fig. 4 shows the relationships among EDR, return water temperature in the PHN, and PEE of the SHN with the same temperatures (60 °C/45 °C) in the SHN. As EDR increases, the return water temperature in the PHN decreases firstly and then goes up. Conversely, PEE rises firstly and then decreases as EDR increases. Therefore, there is an optimum EDR. In addition, the lowest return water temperature in the PHN and the largest PEE are both related to the same EDR for the EHE. The greater utilization of exergy in the circulating water in the PHN will lead to higher performance of heat pump in the EHE. Besides, higher the condensing temperature could lead to worse performance of the heat pump. And a larger EDR will bring the lower condensing temperature and higher generating temperature of the EHP. EDR has a positive effect on the heat pump cycle performance of the EHP, but it has a negative effect on the refrigerating capacity of the EHP. Therefore, there is an optimum EDR, which is an important parameter for the design of the operation and control strategy of the EHE. Fig. 5 shows the relationships among the heating parameters of the PHN and SHN. In the EHE, the return water temperature in the PHN is affected by other heating parameters of PHN and SHN. As the supply water temperature in the PHN increases, the return water temperature in the PHN is reduced slowly under the same heating parameters of the SHN, which is led by that the quality of input thermal energy and the exergy utilization ratio of the EHP increase when the supply water temperature in the PHN increases. The return water temperature in the PHN decreases as the heating parameters of the SHN decrease under the same supply water temperature in the PHN. In general, the lower the temperature parameter of the SHN is, and the lower the condensing temperature of the EHP produces. This is because of that the heat pump cycle performance and refrigerating capacity of the EHP both rise when the condensing temperature goes down. It is found that the lower heating parameters of the SHN help to decrease the
ð18Þ
EDR is defined as the ratio of input exergy of the WWHE to input exergy of the EHE. The input exergy of the WWHE is the exergy change between the inlet and outlet of the circulating water in the PHN for the WWHE. The input exergy of the EHE is the exergy change between the supply water and return water in the PHN.
Temperature of return water / ć
m2w;c h2w;c;o þ m2w;he h2w;he;o ¼ m2w h2w;o
4. Results and discussion
0.660 Temperature of return water
42
0.655
Product exergetic efficiency
40
0.650
38
0.645
36
0.640
34
0.635
32
Product exergetic efficiency / W/W
The energy balance equation of node 2 (in Fig. 1) is given as follows:
0.630 0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Exergy distribution ratio of WWHE to EHE / W/W Fig. 4. Relationships among PEE, EDR, and the temperature of the return water in the PHN.
F. Sun et al. / Applied Energy 121 (2014) 245–251
Temperature of return water in the PHN / ć
250
of the SHN has little effect on EDR. It also can be found that the adjustable range of the operation condition is smaller for the EHE when the heating temperature of the SHN is designed to be 70 °C/55 °C from Figs. 5 and 6. In other words, EHEs can show better regulation characteristics in low-temperature district heating systems. When the supply water temperature in the PHN changes from 140 °C to 150 °C, EDR is barely affected by the heating temperature of the SHN. Therefore, the value of EDR is an extremely important parameter for the design of the operation and control strategy of the EHE when the supply water temperature in the PHN is below 140 °C.
55 50 45 40 35 30 60/45ć for SHN
25
65/50ć for SHN
20
70/55ć for SHN
15
5. Conclusions
10 80
90
100
110
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140
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Temperature of supply water of PHN / ć Fig. 5. Relationships among the heating parameters of the PHN and SHN.
return water temperature in the PHN and increase the heat transmission capacity of the existed PHN. Therefore, EHEs can play a more significant role in increasing the heating capacity of the existed PHN in low-temperature district heating systems without changing flowing rate. Furthermore, the return water with lower temperature in the PHN could recover more low-temperature waste heat in a heat recovery system. Besides, return water with lower temperature are available to recover different kinds of low grade heat in different industries. In other words, this device could be employed in many heat recovery systems, which could enlarge the total energy efficiency obviously. For the PHN, the larger temperature difference between the supply water and return water can significantly decrease the electricity consumption of circulation water pumps for the same heating capacity. Thus, EHEs are also quite suitable for district heating systems with long-distance heating system. Fig. 6 shows the relationships among EDR, supply water temperature in the PHN, and heating parameters of the SHN. It can found that EDR is affected by both the supply water temperature in the PHN and the heating parameters of the SHN. EDR decreases as the supply water temperature in the PHN increases under the same heating parameters of the SHN. And EDR decreases when the heating parameters of the SHN increase under the same supply water temperature in the PHN. However, the heating temperature
0.80
Acknowledgements The authors wish to acknowledge the support provided by the Natural Science and Technology Support Plan of the People’s Republic of China (No. 2012BAJ04B00), the Beijing Municipal Science and Technology Plant (No. D131100003813001) and the Beijing Key Laboratory of Heating, Gas Supply, Ventilating and Air Conditioning Engineering, Beijing University of Civil Engineering and Architecture.
References
0.70
EDR of WWHE to EHE / W/W
Thermal energy of the supply water in the PHN is utilized step by step by using an EHE. Compared to a WWHE, an EHE can decrease the return water temperature in the PHN (which is lower than that in the SHN), and increase the heating capacity of the existed PHN. An EHE can decrease the exergy loss of the heat transfer process by decreasing the return water temperature in the PHN through using an EHP. Its thermal energy and exergy distribution between the EHP and the WWHE in the EHE can be regulated by the EHP. EHEs will show better regulation characteristics in low-temperature district heating systems. They can yield a lower return water temperature in the PHN, which not only increases the heating capacity of the PHN, but also makes it possible to recover much more low-grade waste heat for waste heat district heating systems. Because of its simple structure and better heat transfer performance, EHEs have a smaller volume and lower manufacture cost when compared with AHEs. As a result, it is much easier to instead of WWHEs in existed small space heating substations. Thus, EHEs could be widely applied in district heating systems with long-distance heating and waste heat district heating systems.
0.60 0.50 60/45ć for SHN
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65/50ć for SHN
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70/55ć for SHN
0.20 0.10 0.00 80
90
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130
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Temperature of supply water of PHN / ć Fig. 6. Relationships among EDR and the heating parameters of the SHN and PHN.
[1] Heating office of Beijing Municipal Committee. Challenge and thinking in the development of Beijing Heating. Urban Manage Sci Technol 2008;6:20–2 (in Chinese). [2] Building Energy Research Center of Tsinghua University. 2011 Annual report of china building energy conservation. Beijing: China Architecture and Building Press; 2011 (in Chinese). [3] Sun FT. Development of key equipments and research on modes of waste heat heating of circulating water in power plant, Post-doctoral Research report of Tsinghua University 2010; 6 (in Chinese). [4] Abu-khader MM. Plate heat exchangers: recent advances. Renew Sust Energy Rev 2012;16(4):1883–91. [5] Srimuang W, Amatachaya P. A review of the applications of heat pipe heat exchangers for heat recovery. Renew Sust Energy Rev 2012;16(6):4303–15. [6] Vasiliev LL. Heat pipes in modern heat exchangers. Appl Therm Eng 2005;25(1):1–19. [7] Jacimovic B, Zoivkovic B, Genic S, Zekonja P. Supply water temperature regulation problems in district heating network with both direct and indirect connection. Energy Buildings 1998;28(3):317–22. [8] Zsebik A. Heat exchanger connection in substations – a tool of decreasing return temperature in district heat networks. Energy Eng 2001;98(5):20–8. [9] Bergles AE. ExHFT for fourth generation heat transfer technology. Exp Therm Fluid Sci 2002;26(2–4):335–44.
F. Sun et al. / Applied Energy 121 (2014) 245–251 [10] Guo ZY, Cheng XG, Xia ZZ. Least dissipation principle of heat transport potential capacity and its application in heat conduction optimization. Sci Bull 2003;48(4):406–10 (in Chinese). [11] Goldstein RJ, Ibele WE, Patankar SV, Simon TW, Kuehn TH, Strykowski PJ, et al. Heat transfer – a review of 2005 literature. Int J Heat Mass Transfer 2010;53(21–22):4397–447. [12] Sun FT, Fu L, Zhang SG, Sun J. New waste heat district heating system with combined heat and power based on absorption heat exchange cycle in China. Appl Therm Eng 2012;37(6–7):136–44. [13] Fu L, Jiang Y, Zhang SG. One kind of heat exchanger unit based on heat pump technology, China Patent No. 200810101064.5, 2008-12-24 (in Chinese). [14] Fu L, Sun FT, Zhang SG et al. A method of reduction return water temperature of heating tube, China Patent No. 200910091337.7, 2011-6-29 (in Chinese). [15] Fu L, Li Y, Zhang SG, Jiang Y, Zhao XL. Application and conception of heat transfer based on absorption cycle. Building Sci 2010;26(10):136–40 (in Chinese). [16] Chua KJ, Chou SK, Yang WM. Advances in heat pump systems: a review. Appl Energy 2010;87(12):3611–24. [17] Lakew AA, Bolland O, Ladam Y. Theoretical thermodynamic analysis of Rankine power cycle with thermal driven pump. Appl Energy 2011;88(9):3005–11. [18] Sözen A, Özalp M. Solar-driven ejector-absorption cooling system. Appl Energy 2005;80(1):97–113. [19] Srikhirin P, Aphornratana S, Chungpaibulpatana S. A review of absorption refrigeration technologies. Renew Sust Energy Rev 2001;5(4):343–72. [20] Bogart M. Ammonia absorption refrigeration in industrial processes. Houston (TX, USA): Gulf Pub. Co.; 1981. [21] Chen Li-Ting. A new ejector-absorber cycle to improve the COP of an absorption refrigeration system. Appl Energy 1988;30(1):37–51. [22] Levy A, Jelinek M, Borde I. Numerical study on the design parameters of a jet ejector for absorption systems. Appl Energy 2002;72(2):467–78. [23] Jelinek M, Levy A, Borde I. Performance of a triple-pressure-level absorption cycle with R125-N, N0 -dimethylethylurea. Appl Energy 2002;71(3):171–89.
251
[24] Vereda C, Ventas R, Lecuona A, Venegas M. Study of an ejector-absorption refrigeration cycle with an adaptable ejector nozzle for different working conditions. Appl Energy 2012;97:305–12. [25] Kuhlenschmidt D. Absorption refrigeration system with multiple generator stages, US Patent No. 3717007, 1973. [26] Aphornratana S, Eames IW. Experimental investigation of a combined ejectorabsorption refrigerator. Int J Energy Resour 1998;22(3):195–207. [27] Ko Yh, Park S, Jin S, Kim B, Jeong JH. The selection of volume ratio of two-stage rotary compressor and its effects on air-to-water heat pump with flash tank cycle. Appl Energy 2013;104:187–96. [28] Chen XJ, Worall M, Omer S, Su YH, Riffat S. Theoretical studies of a hybrid ejector CO2 compression cooling system for vehicles and preliminary experimental investigations of an ejector cycle. Appl Energy 2013;102:931–42. [29] Sun FT, Wang N, Li DY et al. New heat exchanger based on ejector heat pump, China Patent No. 201120186592.2, 2011-12-28 (in Chinese). [30] Little AB, Garimella S. A review of ejector technology for refrigeration applications. Int J Air-Cond Refrig 2011;19(1):1–15. [31] Abdulateef JM, Sopian K, Alghoul MA, Sulaiman MY. Review on solar-driven ejector refrigeration technologies. Renew Sust Energy Rev 2009;13(6– 7):1338–48. [32] Selvaraju A, Mani A. Analysis of a vapor ejector refrigeration system with environment friendly refrigerants. Int J Therm Sci 2004;43(9):915–21. [33] Henzler JJ. Design of ejectors for single-phase material systems. Ger Chem Eng 1983;6(5):292–300. [34] Donnellan P, Byrne E, Oliveira J, Cronin K. First and second law multidimensional analysis of a triple absorption heat transformer (TAHT). Appl Energy 2014;113:141–51. [35] Modesto M, Nebra SA. Exergoeconomic analysis of the power generation system using blast furnace and coke oven gas in a Brazilian steel mill. Appl Therm Eng 2009;29(15–17):2127–36.