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Study of the performance of an urban original source heat pump system
Energy Conversion and Management 51 (2010) 765–770
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Study of the performance of an urban original source heat pump system X.L. Zhao *, L. Fu, S.G. Zhang, Y. Jiang, Z.L. Lai Department of Building Science, School of Architecture, Tsinghua University, Beijing, PR China
a r t i c l e
i n f o
Article history: Received 27 March 2009 Accepted 30 October 2009 Available online 14 December 2009 Key words: Urban original sewage source Heat pump Engineering practice
a b s t r a c t As an energy-saving and environmentally friendly technology, the urban sewage source heat pump (USSHP) had been widely applied in the field of heating and air conditioning. Chinese researchers recently designed an urban sewage source heat pump system composed of a filth block device, a wastewater heat exchanger, a heat pump, and other assistant facilities, such as pumps, fans, and end user devices. The system was built in 2008, and has been in operation since then. We tested the parameters of the system on the heating and cooling status from the wastewater source to the heat pump, including the temperature and flux of sewage, the inlet and outlet parameters of different facilities, and the performance of different facilities for a typical operation status. Based on the test results, the overall COP of the system in the heating and cooling mode was computed, and the energy efficiency level was analyzed. Then a method was proposed to improve the system’s performance. After the improvements, the characteristic curves of the typical operation status were investigated. The results indicated that the heating COP is about 4.3, and the cooling COP is about 3.5 in the actual operating conditions. These results could serve as a reference for designing or evaluating urban sewage source heat pump systems. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Heat pump systems are energy-efficient devices for heating and cooling of buildings. The thermal characteristics of the heat source–sink used affect the technical and economic performance of heat pumps directly [1]. Water-source heat pump systems, which are now considered a viable alternative to conventional cooling and heating systems, have attractive performance characteristics when designed and installed properly [5]. The urban sewage source heat pump (USSHP) has gradually come into use. In the winter, USSHPs use urban sewage as a heat source-sink and convert it to high-grade heat supplied to the end users by an electrically-driven compressor. In the summer, USSHPs use urban sewage as a cooling source and convert it supplied to cold water to the end users by an electrically-driven compressor, so one USSHP machine can serve two functions [6]. Recently, many researchers have carried out a great deal of research about urban sewage heat pump systems. Baek studied the design problems about the sewage system in the early years [2,3], and Funamizu introduced a USSHP system engineering in Japan [4]. Wu and Sun studied the soft-dirt characteristic of the heat-exchanging pipe in a USSHP system, and studied the technical and economic analysis of the increase in heat pump temperature in the sewage disposal process [8,9]. Yao simulated the sewage source heat pump for low temperature wastewater treatment [10]. But until now, there * Corresponding author. Tel.: +86 1062773885; fax: +86 1062770544. E-mail address: [email protected] (X.L. Zhao). 0196-8904/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2009.10.033
have been no reports of any actual engineering applications using sewage source heat pump systems in China. In this paper, an original sewage source heat pump system is introduced; the system was commissioned and put into operation in 2008. The characteristic parameters were tested from the original sewage source to the heat pump in the typical operation conditions. Based on the test results, the system performance was studied, and a method to improve the system performance was proposed, which could serve as a reference for actual engineering practice.
2. Description of system The original sewage source heat pump system is shown in Fig. 1. The system is composed of a filth block device, a wastewater heat exchanger, a sewage source heat pump, and other pumps and valves. The original sewage is sent to the filth block device by the first-stage sewage pump, where the bigger aperture feculence is blocked and sent back to the original sewage, and the sewage that does not contain the bigger aperture feculence is sent to the wastewater heat exchanger by the second-stage sewage pump. In the wastewater heat exchanger, the sewage is changed into heat with clear water and is sent back to the filth block device. In the winter, valves 1–4 are open, and valves 5–8 are closed. The wastewater heat exchanger takes the heat from the original sewage, and sends it to the evaporator side of the heat pump as a low grade heat source. The heat pump releases the heat at the condenser side through the heat cycle, and sends the heat to the heat-supplying
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Nomenclature USSHP DAS E k G t e
g
urban sewage source heat pump data acquisition system energy-saving rate impedance volumetric flow rate (m3/s) temperature (°C) first energy consumption rate during cooling efficiency
system. In the summer, valves 1–4 are closed, and valves 5–8 are open. The wastewater heat exchanger sends the heat to the original sewage, at the condenser side of the heat pump. The combination of the wastewater heat exchanger and filth block device is equivalent to the cooling tower, and releases the heat to the original sewage.
3. Experiment conditions The system was tested from September 2008 to January 2009. In this paper, the performance of two typical operations was analyzed based on the results in a typical heating season and cooling season. On the inlet and outlet of the main equipment, the relative measurement devices were installed to measure all relevant temperatures, pressures and flow rates. The main parts of the system such as the filth block device and heat pump have their own data acquisition systems (DAS). The DAS is completely separate from the control system, and the readings that the DAS provides can be completely customized. The types of data the DAS can provide are as follows: temperature, pressure, liquid flow rate, electricity, current, and equipment status. Most of the sensors output are 4– 20 mA current, which has the advantage of avoiding signal attenuation over long wires. The current signal is converted into 0.88– 4.4 VDC voltage, with which the required data value is calculated through the DAS program and then stored in the computer. The test instrumentation and their measurement accuracies are given in Table 1.
4. Results and discussion 4.1. Performance of the filth block device
Subscripts c cooling con conventional e electrical h heating in inlet out outlet
device, and which is not advantageous in the summer operation mode. Furthermore, the reason for the rising temperature could be analyzed by the structure of the devices. The pressure and flow rate balance analysis of the filth block device is shown in Fig. 3 and the equivalent pipeline is shown in Fig. 4. The filth block device is composed of a fixed shell on the outside, and a columned grid and baffles inside. The interior is divided into the water-supply region and water-return region, and the water-supply region and water-return region are both divided into the inner cylinder area and outer cylinder area by a columned grid. The columned grid is run by the center of the circle as the axes, and the mesh is changed between the water-supply region and water-return region. When the device is running, the sewage is sent to the outer water-supply region by the first-stage sewage pump, and is filtrated by the meshes of the grid and is sent to the inner water-supply region. The bigger aperture feculence is blocked on the outside of the grid, and the sewage is sent to the wastewater heat exchanger by the second-stage sewage pump. Then the sewage is sent back to the inner water-return region, and enters into the outer water-return region through the columned grid and removes the feculence, which constitutes the back-wash filter process [7]. The area of the water-supply region is bigger than the water-return region, which results in a higher velocity of flow, and makes the back-wash process more effective. As illustrated in Figs. 3 and 4, the original sewage enters into the duct from the sewage trench (atmospheric pressure P0), and is sent Table 1 Parameters of the measurement instruments. Parameters
Instruments
Measurement precision
Electrical power
Electrical power meter Ultrasonic flow meter T thermocouple/resistance temperature device (RTD) Bourdon tube pressure gauge
±1%
Water flow rate
Fig. 2 shows the inlet and outlet temperatures of the first side and second side of the filth block device in the summer, when the system is first run. As illustrated in Fig. 2, the outlet temperature on the second side is increased by 2 °C, which denotes that the temperature of the original sewage is increased by the filth block
Water temperature Pressure
Fig. 1. Urban original sewage source heat pump system.
±0.5% ±0.5 °C/±0.1 °C ±0.5%
X.L. Zhao et al. / Energy Conversion and Management 51 (2010) 765–770
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Fig. 4. Equivalent pipelines of filth block device.
Fig. 2. Inlet and outlet temperature of filth block device.
to the outer water-supply region of the filth block device by the first-stage sewage pump, where the pressure is Pn1, and then the sewage is filtrated by the meshes of the grid and the pressure is changed to Pn2. Pn1 > Pn2 because of the pressure loss of the grid (impedance k2). The sewage in the inner water-supply region is pressurized by the second-stage sewage pump (pumping head P2), and the pressure is changed to Pn2 + P2. After this, it is sent back to the inner water-return region through the wastewater heat exchanger and the relative pipes (impedance k4), and the pressure is changed to Pn4. At this time, if the pressure of the inner water-return region Pn4 is bigger than the pressure of the inner water-supply region Pn2, the bypass of the return flow could occur between the gap of the grid and baffle. This kind of bypass causes the temperature of the sewage to rise. The sewage that is not bypassed enters into the outer water-return region through the grid (impedance k5); the pressure is changed to Pn3, and the sewage is sent back to the sewage trench. At this time, if the pressure of the outer water-supply region Pn1 is higher than the pressure of the outer water-return region Pn3, the sewage that comes from the inlet on the first side could be bypassed from the outer water-supply region to the outer water-return region directly,
and it would then be discharged to the outlet on the first side. This kind of bypass is defined as a bypass of the spur track, and could increase the power consumption of the first-stage sewage pump, but it would not cause the sewage temperature to rise. There are two kinds of bypass problems in the filth block device: the bypass of the return flow as shown in Fig. 4 from the n4 to n2 loop, and the bypass of the spur track as shown in Fig. 4 from the n1–n3 loop. Since the outlet temperature on the second side is raised, there could be a bypass problem. We measured the flow rate as follows: the flow rate of the second-stage sewage pump was 800 m3/h, and the flow rate of the first-stage sewage pump was 1003 m3/h. It can be concluded that the bypass of the spur track was 420 m3/h, and the bypass of the return flow was 216 m3/h according to the energy balance analysis, so the system consumes more power in the firststage sewage pump, and there is also a problem with the temperature rising, which worsens the working conditions of the wastewater heat exchanger. How can these two problems be solved? They could be analyzed by the system equivalent model. The mass and pressure balance equations could be given as:
8 2 G1 k1 ¼ G22 k2 þ G25 k5 G23 k3 > > > > 2 2 > > < P2 G4 k4 ¼ G3 k3 G0 ¼ G1 þ G2 > > > > G2 ¼ G5 > > : G2 þ G3 ¼ G4
Fig. 3. Pressure and flow rate balance analysis of filth block device.
ð1Þ
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where Gi represents the flow rate corresponding to the pipelines in Fig. 4, m3/s, and ki is the impedance of the pipeline, s2/m5. Eq. (1) could be simplified as
(
G23 k3 ¼ G22 ðk2 þ k5 Þ ðG0 G2 Þ2 k1 P2 ¼ G23 k3 þ ðG2 þ G3 Þ2 k4
ð2Þ
First, in order to avoid the bypass of the return flow, Pn2 = Pn4,
G2 ¼ G0 P2 ¼
G22
pffiffiffiffiffi pffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi k1 =ð k1 þ k2 þ k5 Þ
ð3Þ
k4
ð4Þ
Eq. (3) describes the condition of avoiding the bypass of the return flow. Furthermore, in order to avoid the bypass of the spur track, G0 = G2, we have,
k2 ¼ k5 ¼ 0
ð5Þ
Obviously, k2 and k5 represent the impedance of the grid, so k2 = k5 – 0; that is, in the conditions of avoiding the bypass of the return flow, the bypass of the spur track is unavoidable, and the bypass of the spur track flow rate could be given as
G1 ¼ G0
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi k2 þ k5 =ð k1 þ k2 þ k5 Þ
ð6Þ
Secondly, in order to avoid the bypass of the spur track, G1 should equal zero. Then the bypass of the return flow must exist, and the bypass flow could be given as
G3 ¼ G0
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffi k2 þ k5 = k3
ð7Þ
The bypass of the return flow causes the temperature to rise on the second side of the filth block device, so it is most important to avoid the bypass of the return flow, and the bypass of the spur track is unavoidable in this condition. Above all, we can adopt two methods: one is to increase the resistance (k3) from the water-supply region to the water-return region, that is, reducing the gap of grid and baffle in the process techniques; the other is to regulate the pumping head of the second-stage sewage pump to control the bypass flow, and reduce the supply and return of the second side of the filth block device. 4.2. Performance of wastewater heat exchanger In the system, the wastewater heat exchanger is the shell-tube heat exchanger. The sewage is in the tube, and the cooling water is in the shell, which is shown in Fig. 5. The sewage is sent to the shell underside, goes through four pathways and is discharged from the shell upper side. At the same time, the cooling water enters into the shell upper side, and also goes through four pathways and is discharged from the shell underside, which can be described as a heat exchange counter-flow. According to the test results, the temperature difference of the heat exchanger was too small, and the heat transfer coefficient was just about 368 W/(m2K), which was lower than the designed value, 500–700 W/(m2K). What was the reason for this? To find out, we put 20 temperature test points on the walls of two shells, which is shown in Fig. 5, and the test results in Fig. 6 show that the tem-
Fig. 5. Configuration of wastewater heat exchanger.
X.L. Zhao et al. / Energy Conversion and Management 51 (2010) 765–770
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Fig. 6. Temperature distribution of wastewater heat exchanger. Fig. 7. Performance of the system in typical cooling season.
perature of the cooling water increased on the second and fourth pathways, which indicates that the bypass occurred in the gap of the clapboard and the shell on the first and second pathways; it also occurred on the third and fourth pathways. According to the temperature data, it can be calculated that about 36–60% of the flow rate is bypassed. In order to block the bypass of the flow rates, based on the actual installation conditions, it is difficult to close the gap between the clapboard and the shell. We proposed a feasible method. First, the first and second pathways are connected on the exterior, and the third and fourth pathways are connected on the exterior too, and the part connecting the cooling water outlet on the upper shell and the cooling water inlet on the underside shell is blocked, and then, the two shells are connected on the other side, which is shown as dashed lines in Fig. 5. This method is the same as eliminating the clapboard and changing the cooling water side to two pathways. The heat transfer coefficient is improved to 510 W/ (m2K) by this design improvement. It was concluded that the bypass of the flow rate must be blocked when the system is designed and processed. If the bypass cannot be blocked in the process, we should not design two or more shell pathways in one shell. 4.3. Performance of sewage source heat pump After the improvement of the filth block device and the wastewater heat exchanger, the performance was tested. In this paper, we analyzed the system performance for two typical operation conditions. One was a typical cooling season, as shown in Fig. 7. In the typical cooling condition (with the temperature of cold water ranging from 7.7 °C to 10.7 °C), the inlet temperature of the cooling water was about 33 °C, and the outlet temperature of cooling water was about 37 °C; the temperature difference was about 4 °C, which allowed the heat to be released to the sewage source effectively. The coefficient of performance (COP) could be 3.5, which was lower than the design value; this was because the actual temperature of the original sewage was higher than the designed value. So when designing the USSHP system, the status of the upriver sewage had to be reviewed in detail. For example, we considered the temperature changes of the sewage, whether or not there was a sewage pumping station, and whether or not the operation strategy was intermittent discharge of the sewage. Any reason could lead to the change of temperature and flow rate, and would affect the overall performance of the system directly. In the design of a USSHP system, the most disadvantageous conditions should be considered. Fig. 8 shows the performance of the
Fig. 8. Performance of the system in typical heating season.
USSHP under typical heating conditions (with the temperature of the heat-supplying water ranging from 45 °C to 40 °C). At this time, the inlet temperature of the cold water was about 10 °C, and the outlet temperature of the cold water was about 7 °C. The average COP was about 4.3, which could remove the heat from the sewage source effectively. 4.4. Comparison of the energy consumption The USSHP system was compared with a conventional energy supply system, which uses a boiler to heat and a chiller to cool. The efficiency of the boiler was 75%, and the COP of the chiller was 3.5 because the temperature of the cooling tower was almost the same as the USSHP system’s sewage. The electrical efficiency of the power generation and power grid was 30%. The energy-saving rate in the winter and summer could be given as:
Eh ¼ 1
gh ge COPh
ð8Þ
Ec ¼ 1
COP con COP c
ð9Þ
where Eh and Ec are the energy-saving rate in the heating and cooling season,gh is the heating efficiency of the conventional energy
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Table 2 Energy consumption analysis. Scenario
Heating
Cooling
Boiler + Electrical chillers USSHP Energy-saving rate
75% 30% 4.3 42
30% 3.5 30% 3.5 0
supply, ge is the electrical efficiency, COPh and COPc are the heating COP and cooling COP of the USSHP system, and COPcon is the COP of the chiller in the conventional system. Table 2 shows a comparison of the USSHP system and conventional energy system operated for the same outcome to satisfy the needs of the end users. In the heating season, the energy-saving rate is (1–75%/4.3/ 30%) = 42%, which is because the conventional heating method uses boiler combustion directly, making the energy utilization low according to the second law of thermodynamics. In the cooling season, the COP is almost the same in the two energy utilization systems. The USSHP does not have the advantage of saving energy, but it can save much water compared with the cooling tower in the conventional system, and also, it reduces the local heat emission. 5. Conclusions In this paper, an original sewage source heat pump system is introduced. The study results led to several conclusions. (1) It is important to consider the effect of temperature differences on system performance. When we designed the USSHP system, the status of the upriver sewage had to be reviewed in detail; we considered temperature changes, whether there was a sewage pumping station, and whether the operation strategy was intermittent discharge of sewage. Any factor could lead to a change of the temperature and flow rate, and would affect the overall performance of the system directly. In the design of a USSHP system, the most disadvantageous condition should be considered. (2) In the study of the filth block device, it was found that there are two kinds of bypass problems in the filth block device: the bypass of the return flow and the bypass of the spur track. The key problem is to avoid the bypass of the return flow. The condition of the bypass of return flow was investigated by a system model, and the bypass of the spur track was unavoidable. First, we should increase the resistance (k3) from the water-supply region to the water-return
region, that is, the gap of the grid and baffle should be reduced. Second, when the system is designed, the balance equation should be used to design the pumping head of the pumps, and the supply and return of the second side of the filth block device should be reduced. (3) In the study of the wastewater heat exchanger, it was concluded that the bypass of the flow rate must be blocked when it is designed and operated. If the bypass cannot be blocked in the process, we should not design two or more shell pathways in one shell. (4) The system performance was tested, and improvements were proposed. After the improvement, the results indicated that the heating COP was about 4.3, and the cooling COP was about 3.5 under the actual conditions. These results could be a reference for designing or evaluating urban wastewater source heat pump systems.
Acknowledgement This study was conducted under the auspices of the 863 HiTech Program, which was financed by the Foundation for Advanced Energy Technology Research, Ministry of Science and Technology (2006 AA05Z252). References [1] Buyukalaca O, Ekinci F, Yılmaz T. Experimental investigation of Seyhan River and Dam Lake as heat source–sink for a heat pump. Energy 2003;28:157–69. [2] Baek NC. Study on the heat pump system using wastewater as a heat source. Energy R&D 1994;16(1):56–63. [3] Baek NC, Shin UC, Yoon JH. A study on the design and analysis of a heat pump heating system using wastewater as a heat source. Solar Energy 2005;78(3):427–40. [4] Funamizu N, Iida M, Sakakura Y, et al. Reuse of heat energy in wastewater implementation examples in Japan. Water Sci Technol 2001;43(10):277–86. [5] Kavanaugh SP. Design considerations for ground and water source heat pumps in southern climates. ASHRAE Trans 1989;95(1):1139–49. [6] Ma ZL, Yao Y. Application prospects of sewage source water heat pump system. J China Water Wastewater 2003;19(7):41–3. [7] Sun DX, Wu RH. Urban sewage hydraulic clearance with roller and grid bar. Patent No. 200410043654. [8] Wu RH, Sun DX. Research on the soft-dirt characteristic of heat-exchanging pipe in using urban original wastewater as cooling and heating source. J Fluid Mach 2005;33(12):58–61. [9] Wu RH, Xu Y, Sun DX, et al. Technology economic analysis of heat pump temperature rising in sewage disposal process in cold climate area [J]. Act energiae solaris sinica 2008;29(3):267–71. [10] Yao Y, Song Y. Simulation and analysis of sewage source heat pump for low temperature wastewater treatment. J China Water Wastewater 2006;22(13): 70–4.
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Study of the performance of an urban original source heat pump system X.L. Zhao *, L. Fu, S.G. Zhang, Y. Jiang, Z.L. Lai Department of Building Science, School of Architecture, Tsinghua University, Beijing, PR China
a r t i c l e
i n f o
Article history: Received 27 March 2009 Accepted 30 October 2009 Available online 14 December 2009 Key words: Urban original sewage source Heat pump Engineering practice
a b s t r a c t As an energy-saving and environmentally friendly technology, the urban sewage source heat pump (USSHP) had been widely applied in the field of heating and air conditioning. Chinese researchers recently designed an urban sewage source heat pump system composed of a filth block device, a wastewater heat exchanger, a heat pump, and other assistant facilities, such as pumps, fans, and end user devices. The system was built in 2008, and has been in operation since then. We tested the parameters of the system on the heating and cooling status from the wastewater source to the heat pump, including the temperature and flux of sewage, the inlet and outlet parameters of different facilities, and the performance of different facilities for a typical operation status. Based on the test results, the overall COP of the system in the heating and cooling mode was computed, and the energy efficiency level was analyzed. Then a method was proposed to improve the system’s performance. After the improvements, the characteristic curves of the typical operation status were investigated. The results indicated that the heating COP is about 4.3, and the cooling COP is about 3.5 in the actual operating conditions. These results could serve as a reference for designing or evaluating urban sewage source heat pump systems. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Heat pump systems are energy-efficient devices for heating and cooling of buildings. The thermal characteristics of the heat source–sink used affect the technical and economic performance of heat pumps directly [1]. Water-source heat pump systems, which are now considered a viable alternative to conventional cooling and heating systems, have attractive performance characteristics when designed and installed properly [5]. The urban sewage source heat pump (USSHP) has gradually come into use. In the winter, USSHPs use urban sewage as a heat source-sink and convert it to high-grade heat supplied to the end users by an electrically-driven compressor. In the summer, USSHPs use urban sewage as a cooling source and convert it supplied to cold water to the end users by an electrically-driven compressor, so one USSHP machine can serve two functions [6]. Recently, many researchers have carried out a great deal of research about urban sewage heat pump systems. Baek studied the design problems about the sewage system in the early years [2,3], and Funamizu introduced a USSHP system engineering in Japan [4]. Wu and Sun studied the soft-dirt characteristic of the heat-exchanging pipe in a USSHP system, and studied the technical and economic analysis of the increase in heat pump temperature in the sewage disposal process [8,9]. Yao simulated the sewage source heat pump for low temperature wastewater treatment [10]. But until now, there * Corresponding author. Tel.: +86 1062773885; fax: +86 1062770544. E-mail address: [email protected] (X.L. Zhao). 0196-8904/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2009.10.033
have been no reports of any actual engineering applications using sewage source heat pump systems in China. In this paper, an original sewage source heat pump system is introduced; the system was commissioned and put into operation in 2008. The characteristic parameters were tested from the original sewage source to the heat pump in the typical operation conditions. Based on the test results, the system performance was studied, and a method to improve the system performance was proposed, which could serve as a reference for actual engineering practice.
2. Description of system The original sewage source heat pump system is shown in Fig. 1. The system is composed of a filth block device, a wastewater heat exchanger, a sewage source heat pump, and other pumps and valves. The original sewage is sent to the filth block device by the first-stage sewage pump, where the bigger aperture feculence is blocked and sent back to the original sewage, and the sewage that does not contain the bigger aperture feculence is sent to the wastewater heat exchanger by the second-stage sewage pump. In the wastewater heat exchanger, the sewage is changed into heat with clear water and is sent back to the filth block device. In the winter, valves 1–4 are open, and valves 5–8 are closed. The wastewater heat exchanger takes the heat from the original sewage, and sends it to the evaporator side of the heat pump as a low grade heat source. The heat pump releases the heat at the condenser side through the heat cycle, and sends the heat to the heat-supplying
766
X.L. Zhao et al. / Energy Conversion and Management 51 (2010) 765–770
Nomenclature USSHP DAS E k G t e
g
urban sewage source heat pump data acquisition system energy-saving rate impedance volumetric flow rate (m3/s) temperature (°C) first energy consumption rate during cooling efficiency
system. In the summer, valves 1–4 are closed, and valves 5–8 are open. The wastewater heat exchanger sends the heat to the original sewage, at the condenser side of the heat pump. The combination of the wastewater heat exchanger and filth block device is equivalent to the cooling tower, and releases the heat to the original sewage.
3. Experiment conditions The system was tested from September 2008 to January 2009. In this paper, the performance of two typical operations was analyzed based on the results in a typical heating season and cooling season. On the inlet and outlet of the main equipment, the relative measurement devices were installed to measure all relevant temperatures, pressures and flow rates. The main parts of the system such as the filth block device and heat pump have their own data acquisition systems (DAS). The DAS is completely separate from the control system, and the readings that the DAS provides can be completely customized. The types of data the DAS can provide are as follows: temperature, pressure, liquid flow rate, electricity, current, and equipment status. Most of the sensors output are 4– 20 mA current, which has the advantage of avoiding signal attenuation over long wires. The current signal is converted into 0.88– 4.4 VDC voltage, with which the required data value is calculated through the DAS program and then stored in the computer. The test instrumentation and their measurement accuracies are given in Table 1.
4. Results and discussion 4.1. Performance of the filth block device
Subscripts c cooling con conventional e electrical h heating in inlet out outlet
device, and which is not advantageous in the summer operation mode. Furthermore, the reason for the rising temperature could be analyzed by the structure of the devices. The pressure and flow rate balance analysis of the filth block device is shown in Fig. 3 and the equivalent pipeline is shown in Fig. 4. The filth block device is composed of a fixed shell on the outside, and a columned grid and baffles inside. The interior is divided into the water-supply region and water-return region, and the water-supply region and water-return region are both divided into the inner cylinder area and outer cylinder area by a columned grid. The columned grid is run by the center of the circle as the axes, and the mesh is changed between the water-supply region and water-return region. When the device is running, the sewage is sent to the outer water-supply region by the first-stage sewage pump, and is filtrated by the meshes of the grid and is sent to the inner water-supply region. The bigger aperture feculence is blocked on the outside of the grid, and the sewage is sent to the wastewater heat exchanger by the second-stage sewage pump. Then the sewage is sent back to the inner water-return region, and enters into the outer water-return region through the columned grid and removes the feculence, which constitutes the back-wash filter process [7]. The area of the water-supply region is bigger than the water-return region, which results in a higher velocity of flow, and makes the back-wash process more effective. As illustrated in Figs. 3 and 4, the original sewage enters into the duct from the sewage trench (atmospheric pressure P0), and is sent Table 1 Parameters of the measurement instruments. Parameters
Instruments
Measurement precision
Electrical power
Electrical power meter Ultrasonic flow meter T thermocouple/resistance temperature device (RTD) Bourdon tube pressure gauge
±1%
Water flow rate
Fig. 2 shows the inlet and outlet temperatures of the first side and second side of the filth block device in the summer, when the system is first run. As illustrated in Fig. 2, the outlet temperature on the second side is increased by 2 °C, which denotes that the temperature of the original sewage is increased by the filth block
Water temperature Pressure
Fig. 1. Urban original sewage source heat pump system.
±0.5% ±0.5 °C/±0.1 °C ±0.5%
X.L. Zhao et al. / Energy Conversion and Management 51 (2010) 765–770
767
Fig. 4. Equivalent pipelines of filth block device.
Fig. 2. Inlet and outlet temperature of filth block device.
to the outer water-supply region of the filth block device by the first-stage sewage pump, where the pressure is Pn1, and then the sewage is filtrated by the meshes of the grid and the pressure is changed to Pn2. Pn1 > Pn2 because of the pressure loss of the grid (impedance k2). The sewage in the inner water-supply region is pressurized by the second-stage sewage pump (pumping head P2), and the pressure is changed to Pn2 + P2. After this, it is sent back to the inner water-return region through the wastewater heat exchanger and the relative pipes (impedance k4), and the pressure is changed to Pn4. At this time, if the pressure of the inner water-return region Pn4 is bigger than the pressure of the inner water-supply region Pn2, the bypass of the return flow could occur between the gap of the grid and baffle. This kind of bypass causes the temperature of the sewage to rise. The sewage that is not bypassed enters into the outer water-return region through the grid (impedance k5); the pressure is changed to Pn3, and the sewage is sent back to the sewage trench. At this time, if the pressure of the outer water-supply region Pn1 is higher than the pressure of the outer water-return region Pn3, the sewage that comes from the inlet on the first side could be bypassed from the outer water-supply region to the outer water-return region directly,
and it would then be discharged to the outlet on the first side. This kind of bypass is defined as a bypass of the spur track, and could increase the power consumption of the first-stage sewage pump, but it would not cause the sewage temperature to rise. There are two kinds of bypass problems in the filth block device: the bypass of the return flow as shown in Fig. 4 from the n4 to n2 loop, and the bypass of the spur track as shown in Fig. 4 from the n1–n3 loop. Since the outlet temperature on the second side is raised, there could be a bypass problem. We measured the flow rate as follows: the flow rate of the second-stage sewage pump was 800 m3/h, and the flow rate of the first-stage sewage pump was 1003 m3/h. It can be concluded that the bypass of the spur track was 420 m3/h, and the bypass of the return flow was 216 m3/h according to the energy balance analysis, so the system consumes more power in the firststage sewage pump, and there is also a problem with the temperature rising, which worsens the working conditions of the wastewater heat exchanger. How can these two problems be solved? They could be analyzed by the system equivalent model. The mass and pressure balance equations could be given as:
8 2 G1 k1 ¼ G22 k2 þ G25 k5 G23 k3 > > > > 2 2 > > < P2 G4 k4 ¼ G3 k3 G0 ¼ G1 þ G2 > > > > G2 ¼ G5 > > : G2 þ G3 ¼ G4
Fig. 3. Pressure and flow rate balance analysis of filth block device.
ð1Þ
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where Gi represents the flow rate corresponding to the pipelines in Fig. 4, m3/s, and ki is the impedance of the pipeline, s2/m5. Eq. (1) could be simplified as
(
G23 k3 ¼ G22 ðk2 þ k5 Þ ðG0 G2 Þ2 k1 P2 ¼ G23 k3 þ ðG2 þ G3 Þ2 k4
ð2Þ
First, in order to avoid the bypass of the return flow, Pn2 = Pn4,
G2 ¼ G0 P2 ¼
G22
pffiffiffiffiffi pffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi k1 =ð k1 þ k2 þ k5 Þ
ð3Þ
k4
ð4Þ
Eq. (3) describes the condition of avoiding the bypass of the return flow. Furthermore, in order to avoid the bypass of the spur track, G0 = G2, we have,
k2 ¼ k5 ¼ 0
ð5Þ
Obviously, k2 and k5 represent the impedance of the grid, so k2 = k5 – 0; that is, in the conditions of avoiding the bypass of the return flow, the bypass of the spur track is unavoidable, and the bypass of the spur track flow rate could be given as
G1 ¼ G0
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi k2 þ k5 =ð k1 þ k2 þ k5 Þ
ð6Þ
Secondly, in order to avoid the bypass of the spur track, G1 should equal zero. Then the bypass of the return flow must exist, and the bypass flow could be given as
G3 ¼ G0
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffi k2 þ k5 = k3
ð7Þ
The bypass of the return flow causes the temperature to rise on the second side of the filth block device, so it is most important to avoid the bypass of the return flow, and the bypass of the spur track is unavoidable in this condition. Above all, we can adopt two methods: one is to increase the resistance (k3) from the water-supply region to the water-return region, that is, reducing the gap of grid and baffle in the process techniques; the other is to regulate the pumping head of the second-stage sewage pump to control the bypass flow, and reduce the supply and return of the second side of the filth block device. 4.2. Performance of wastewater heat exchanger In the system, the wastewater heat exchanger is the shell-tube heat exchanger. The sewage is in the tube, and the cooling water is in the shell, which is shown in Fig. 5. The sewage is sent to the shell underside, goes through four pathways and is discharged from the shell upper side. At the same time, the cooling water enters into the shell upper side, and also goes through four pathways and is discharged from the shell underside, which can be described as a heat exchange counter-flow. According to the test results, the temperature difference of the heat exchanger was too small, and the heat transfer coefficient was just about 368 W/(m2K), which was lower than the designed value, 500–700 W/(m2K). What was the reason for this? To find out, we put 20 temperature test points on the walls of two shells, which is shown in Fig. 5, and the test results in Fig. 6 show that the tem-
Fig. 5. Configuration of wastewater heat exchanger.
X.L. Zhao et al. / Energy Conversion and Management 51 (2010) 765–770
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Fig. 6. Temperature distribution of wastewater heat exchanger. Fig. 7. Performance of the system in typical cooling season.
perature of the cooling water increased on the second and fourth pathways, which indicates that the bypass occurred in the gap of the clapboard and the shell on the first and second pathways; it also occurred on the third and fourth pathways. According to the temperature data, it can be calculated that about 36–60% of the flow rate is bypassed. In order to block the bypass of the flow rates, based on the actual installation conditions, it is difficult to close the gap between the clapboard and the shell. We proposed a feasible method. First, the first and second pathways are connected on the exterior, and the third and fourth pathways are connected on the exterior too, and the part connecting the cooling water outlet on the upper shell and the cooling water inlet on the underside shell is blocked, and then, the two shells are connected on the other side, which is shown as dashed lines in Fig. 5. This method is the same as eliminating the clapboard and changing the cooling water side to two pathways. The heat transfer coefficient is improved to 510 W/ (m2K) by this design improvement. It was concluded that the bypass of the flow rate must be blocked when the system is designed and processed. If the bypass cannot be blocked in the process, we should not design two or more shell pathways in one shell. 4.3. Performance of sewage source heat pump After the improvement of the filth block device and the wastewater heat exchanger, the performance was tested. In this paper, we analyzed the system performance for two typical operation conditions. One was a typical cooling season, as shown in Fig. 7. In the typical cooling condition (with the temperature of cold water ranging from 7.7 °C to 10.7 °C), the inlet temperature of the cooling water was about 33 °C, and the outlet temperature of cooling water was about 37 °C; the temperature difference was about 4 °C, which allowed the heat to be released to the sewage source effectively. The coefficient of performance (COP) could be 3.5, which was lower than the design value; this was because the actual temperature of the original sewage was higher than the designed value. So when designing the USSHP system, the status of the upriver sewage had to be reviewed in detail. For example, we considered the temperature changes of the sewage, whether or not there was a sewage pumping station, and whether or not the operation strategy was intermittent discharge of the sewage. Any reason could lead to the change of temperature and flow rate, and would affect the overall performance of the system directly. In the design of a USSHP system, the most disadvantageous conditions should be considered. Fig. 8 shows the performance of the
Fig. 8. Performance of the system in typical heating season.
USSHP under typical heating conditions (with the temperature of the heat-supplying water ranging from 45 °C to 40 °C). At this time, the inlet temperature of the cold water was about 10 °C, and the outlet temperature of the cold water was about 7 °C. The average COP was about 4.3, which could remove the heat from the sewage source effectively. 4.4. Comparison of the energy consumption The USSHP system was compared with a conventional energy supply system, which uses a boiler to heat and a chiller to cool. The efficiency of the boiler was 75%, and the COP of the chiller was 3.5 because the temperature of the cooling tower was almost the same as the USSHP system’s sewage. The electrical efficiency of the power generation and power grid was 30%. The energy-saving rate in the winter and summer could be given as:
Eh ¼ 1
gh ge COPh
ð8Þ
Ec ¼ 1
COP con COP c
ð9Þ
where Eh and Ec are the energy-saving rate in the heating and cooling season,gh is the heating efficiency of the conventional energy
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Table 2 Energy consumption analysis. Scenario
Heating
Cooling
Boiler + Electrical chillers USSHP Energy-saving rate
75% 30% 4.3 42
30% 3.5 30% 3.5 0
supply, ge is the electrical efficiency, COPh and COPc are the heating COP and cooling COP of the USSHP system, and COPcon is the COP of the chiller in the conventional system. Table 2 shows a comparison of the USSHP system and conventional energy system operated for the same outcome to satisfy the needs of the end users. In the heating season, the energy-saving rate is (1–75%/4.3/ 30%) = 42%, which is because the conventional heating method uses boiler combustion directly, making the energy utilization low according to the second law of thermodynamics. In the cooling season, the COP is almost the same in the two energy utilization systems. The USSHP does not have the advantage of saving energy, but it can save much water compared with the cooling tower in the conventional system, and also, it reduces the local heat emission. 5. Conclusions In this paper, an original sewage source heat pump system is introduced. The study results led to several conclusions. (1) It is important to consider the effect of temperature differences on system performance. When we designed the USSHP system, the status of the upriver sewage had to be reviewed in detail; we considered temperature changes, whether there was a sewage pumping station, and whether the operation strategy was intermittent discharge of sewage. Any factor could lead to a change of the temperature and flow rate, and would affect the overall performance of the system directly. In the design of a USSHP system, the most disadvantageous condition should be considered. (2) In the study of the filth block device, it was found that there are two kinds of bypass problems in the filth block device: the bypass of the return flow and the bypass of the spur track. The key problem is to avoid the bypass of the return flow. The condition of the bypass of return flow was investigated by a system model, and the bypass of the spur track was unavoidable. First, we should increase the resistance (k3) from the water-supply region to the water-return
region, that is, the gap of the grid and baffle should be reduced. Second, when the system is designed, the balance equation should be used to design the pumping head of the pumps, and the supply and return of the second side of the filth block device should be reduced. (3) In the study of the wastewater heat exchanger, it was concluded that the bypass of the flow rate must be blocked when it is designed and operated. If the bypass cannot be blocked in the process, we should not design two or more shell pathways in one shell. (4) The system performance was tested, and improvements were proposed. After the improvement, the results indicated that the heating COP was about 4.3, and the cooling COP was about 3.5 under the actual conditions. These results could be a reference for designing or evaluating urban wastewater source heat pump systems.
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