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Reducing CO2 emissions in the individual hot water circulation piping system
Energy and Buildings 84 (2014) 475–482
Contents lists available at ScienceDirect
Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Reducing CO2 emissions in the individual hot water circulation piping system M.C. Lee ∗ Department of Interior Design, National Taichung University of Science and Technology, Taichung, Taiwan
a r t i c l e
i n f o
Article history: Received 26 September 2013 Received in revised form 28 June 2014 Accepted 18 July 2014 Available online 1 September 2014 Keywords: Reducing CO2 emission Hot water circulation piping system Energy consumption Water demand Water and energy saving
a b s t r a c t Central hot water piping systems are used in large lodging buildings (e.g. hotels, dormitories, etc.) for comfort and convenience. However, the big problem to this type of system is that once the hot water leaves the storage tank and travels through the system piping, the water temperature drops due to the travel distance and ambient air temperature. Because of this drop in temperature, users need to wait for the undesirable cool water to flow out of the system before the desired hot water starts flowing out of the system. The ideal solution to this problem is to reheat the undesirable (cooled) water in the system by the use of an individual hot water circulating system. This study focuses on water saving and energy consumption in the individual hot water circulating system. The results show energy consumption is 50% less than the non-circulating central hot water system. Three parameters in wasted water demand, electric consumption, and heating source consumption were considered to transfer them with CO2 transfer coefficient to estimate the reducing CO2 emission. The hot water circulation system presents the benefits of water and energy saving, and reduction of CO2 emission, but the piping connection should be improved for easy installation in the existed buildings in the future. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Central hot water piping systems are used in large lodging buildings (e.g. hotels, dormitories, etc.) for comfort and convenience, as shown in Fig. 1. However, the big problem to this type of system is that once the hot water leaves the storage tank and travels through the system piping, the water temperature drops due to the travel distance and ambient air temperature. Large amounts of energy are required for continuous heating and circulation. To evaluate energy consumption and hot water temperature drop in piping, Lee et al. [1–3] utilized simplified empirical equations and figured out the hot water temperature drop in various lengths of stainless steel piping (13A) as shown in Fig. 2. The previous studies only discussed the individual hot water non-circulating system temperature drop and energy consumption. Cheng and Lee [4] figured out that energy consumption used during heating almost equaled the heat lost during transmission. Research of the energy consumption in a central hot water circulation system indicates that the amount of energy required for circulation and usage is approximately the same as the amount required for generating hot water.
∗ Tel.: +886 422196701. E-mail address: [email protected] http://dx.doi.org/10.1016/j.enbuild.2014.07.094 0378-7788/© 2014 Elsevier B.V. All rights reserved.
Numerous studies proposed various conservation methods such as the Building Energy Conservation Regulations in Japan (Association of Building Environment Energy Conservation [5]), which focused on energy consumption of the hot water supply system and included heat loss due to circulation. Kamata et al. [6] and Sakaue et al. [7] proposed a standard hot water temperature for a supply system based on energy conservation in Japan. Balaras et al. [8] determined that the variation in pipe heat loss was caused by vari´ et al. [9] proposed that ations in energy consumption. Jacimovic the heat loss from heated objects is a linear function of the outdoor temperature. And Morida [10] presented heat loss calculation equations for distribution pipes. Toyosada [11] figured out the economic benefit of carbon tax via water saving [12], Cheng focused on the inter-relationship between water use and energy conservation and proposed an evaluation model of CO2 emission for a water saving strategy [13]. Most residential buildings utilize a central hot water supply system because of the varying usage time periods, and to save energy. However, the problem with this type of system is once usage (circulation) stops, the hot water remaining in the pipes cools due to the ambient air around the piping. Therefore, before use, users need to wait for the undesirable cool water to flow out of the system before the desired hot water starts flowing out of the system. This process may waste a lot of water, depending on the length of the piping system. The ideal solution to this problem is to reheat the undesirable
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M.C. Lee / Energy and Buildings 84 (2014) 475–482
Nomenclature A B CW CE CS EH ER L N P Q1 Q2 S t
v
cross section of pipe (m2 ) numbers of household in residential building (household) CO2 emission of water demand (kg) CO2 emission of electic consumption (kg) CO2 emission of heating source consumption (kg) energy consumption of heater (kWh) energy consumption of circulator (kWh) pipe length (m) using times (time) persons in one household (persons/household) water volume in heater outlet (m3 ) water volume in faucet inlet (m3 ) heating source (kg in LPG, or in different heating source) period time (s) water velocity (l/s)
Greek letters W CO2 emissions coefficient = 0.1002 kg/m3 in water CO2 emissions coefficient = 0.62 kg/kWh in electric E S CO2 emissions coefficient = 2.09 kg/m3 in natural gas; 1.75 kg/kg in LPG; 2.92 kg/kg in coal; 2.73 kg/l in diesel oil; electric is same as E . * CO2 emissions coefficients are referred by Taiwan Environmental Protection Administration (EPA), greenhouse Gas Registry system, 2013. These coefficients relate to the heat capacity in different material producing process in different countries, please check them in the right data while using. Subscripts c circulation system pipe character (cross section area and length) i n non-circulation system p pipe
cool water inside the piping system by the use of an individual hot water circulation piping system. This study discusses water saving and energy consumption in the individual hot water circulation system.
Fig. 2. Hot water temperature dropping time in different length stainless pipe (13A).
2. Individual hot water circulator An individual hot water circulator with water reheat piping can save energy by reducing temperature drops while providing hot water at suitable temperatures. The circulation system consists of a check valve (C1 ) installed before ball valves (faucets) to connect with transmission pipes for recycling and reheating the cooled water in the independent hot water circulation system, as shown in Fig. 3. Two inlet pipes connect with a circulator, one pipe is for recycling the cooled water, and the other pipe is for cool water from urban water. A controller measures the recycled water temperature and sends an electronic signal to mechanically switch the water inlet between the recycled water and urban water. A check valve (C2 ) is installed to prevent urban water from flowing into the circulation pipe. The water flows out from the circulator into the heater (e.g. gas, electrical, solar, etc.) or hot water tank to heat the recycled water or urban water. The individual hot water circulator recycles the cooled water, provides hot water, and saves a considerable amount of energy compared with the central circulated hot water system. There are two additional parts needed in the individual hot water circulation system; the circulator, and the piping that connects from the supply pipe to the circulator and heater. Based on investigation of hot water pipe lengths in Taiwan [4], the average length is around 6.5 m (the shortest is 0.5 m, the longest is 20 m) in the residential buildings (apartments and houses). The controller for the circulator can be set by timer and remote panel to reduce the amount of electrical wiring. The added system installation costs is under US$500 with 10 m (13A) circulation pipes combined with the original hot water supply system via self-installation by the user in the newly constructed building or existing building.
Fig. 1. Typical central circulated hot water system.
M.C. Lee / Energy and Buildings 84 (2014) 475–482
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Fig. 3. Independent hot water circulator design concept.
3. Experiment An experiment was held to measure water recycling amounts and energy consumption. The experiment adopts the popular heater type in Taiwan (80%) [4]; gas heater with 52.3 kW heat and controllers, to heat and reheat water, and also adopts pipe material and length referred by Cheng and Lee [3], stainless steel pipe with insulator, up to 20 m in length. Due to limited experiment space and to reduce thermal radiation interference, 1 m length pipes (including 15 cm curves) are arranged in parallel (15 cm apart) to reach the desired design lengths. The optimal hot water circulation time should be under 1 min to reheat the water to the suitable temperature for comfort and energy conservation. The efficiency of the
pump and heater are also considered for the design. In this experiment, air temperature, water temperature (T), flow velocity (V), pressure (P), circulator energy consumption (including electronic control and pumping) (E2 ), heater (E1 ), and gas consumption (S) were measured in various pipe lengths (1 m, 2 m, 4 m, 8 m, 16 m, and 20 m); check valves (C) were also setup for the flow direction. When the heated water temperature reaches the stable flowing temperature of 54 ◦ C (heater set on 55 ◦ C), the water demand, electric consumption, and gas consumption can be compared between the circulation system and the non-circulation system. The experimental system as designed, contains, six ball valves, six sensor sets with temperature sensors, flow meters, and pressure sensors that are installed on the ball valve (faucets) inlet (T2–7,
Fig. 4. Whole experiment instruments setup with measure sensors.
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Fig. 5. Experiment instrument.
V2–7, P2–7) at 1 m, 2 m, 4 m, 8 m, 16 m, and 20 m from the heater and also on the heater outlet (T1, V1, P1). Circulation pipes with check valves are installed between ball valves and sensor sets, as shown in Fig. 4. The experiment was conducted indoors to minimize wind impacts, as shown in Fig. 5. Each ball valve is opened to allow hot water to flow into a water tank with a level gauge to check the amount of wasted water in the non-circulation system. The installation of the circulation system in the experiment is similar to the non-circulation system, the difference being, the circulation pipes, circulator, one sensor set with temperature sensor, flow meter, and pressure sensor installed before the circulator inlet (T8, V8, P8). All sensor sets measure temperature, velocity, pressure, electric consumption, and gas consumption while the ball valves are open. Once the water reaches the desired temperature, the ball valves close. Once testing of the non-circulating system is finished, cool water is introduced to cool the pipes to the ambient room
Table 1 Experiment records. System
Non-circulation
Factors Unit
T ◦ C
V l/s
P kPa
Circulation E1 W
S g
T ◦ C
V l/s
P kPa
E1 W
E2 W
S g
1, 2, 4, 8, 16, 20 m.
temperature, then the circulation system is tested. The test data is recorded in Table 1 during the test period. The test process was captured by thermal infrared cameras and showed the hot water flowing into the pipes and heating the pipes. Once the water flow was stopped, the water and pipes were cooled down by the ambient air temperature, as shown in Fig. 6. The captured photos presented the temperature variations from cool pipes to hot pipes and back to cool pipes.
Fig. 6. Temperature variations captured by thermal infrared camera.
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Fig. 7. Water volume and flowing period while water temperature over 54 ◦ C in various lengths of piping in non-circulation systems. Red lines present in Chian lines, and Blue lines present in Dot lines.
4. Results The results of the experiment focus on the temperature variations and water demand between the heater outlet (T1 , Q1 ) and
faucet inlet (T2 , Q2 ) in the non-circulation system, as shown in Fig. 7. The diagram in Fig. 7 shows the relationship between water volume and flowing period for the suitable temperature in various lengths of piping. The red line (chain line) shows the relationship between
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Table 2 Water demand and energy consumption in the non-circulation system. Pipe length (m)
Temperature (◦ C)
Period time (s)
Wasted water (Q2 ) (l)
Heated water (Q1 ) (l)
Gas consumption (g)
Heater power (W)
1 2 4 8 16 20
54.8 54.1 54.4 54.1 54.0 54.1
20 25 35 50 80 90
3.4 3.6 5.8 7.7 12.3 14.3
3.5 3.8 6.3 8.8 14.5 17.0
9.1 11.2 15.3 21.4 34.0 39.8
1.8 2.2 3.1 5.8 7.1 8.0
Table 3 Energy consumption in the circulation system. Pipe length (m)
Circulation period time (s)
Gas consumption (g)
Heater power (W)
Circulator power (W)
Total power (W)
1 2 4 8 16 20
13.0 15.0 20.0 30.0 55.0 60.0
8.8 10.2 11.9 16.6 25.8 31.0
1.2 1.3 1.8 2.7 4.9 5.3
1.3 1.5 2.0 3.0 5.5 6.0
2.5 2.8 3.8 5.7 10.4 11.3
period time and flowing out water temperature (T2 ) over 54 ◦ C. The red lines cross the two water demand volumes (calculated by flowing velocity and duration time) in heated water volume (Q1 ) and flowing out water volume (Q2 ), the volume difference between Q1 and Q2 is the remaining water in the transmission pipe. Therefore, the flowing out water is the wasted water in this system, as shown on the blue lines (dot lines) in Fig. 7. The water demand estimation is organized as (1) and (2). All related results with wasted water and energy consumption in the non-circulation system are summarized in Table 2. Q1i = (tn × vn )i
(1)
Q2i = Q1i − (Ap × Lp )i
(2)
Recycling and reheating the cooled water by the hot water circulator saves the water in the transmission pipe. The circulation system has additional power requirements due to the circulator; therefore, the total power consumption combines the power consumption of heater and circulator, as shown in Table 3. 5. Discussion
Fig. 8. Consumption of power and gas between non-circulation and circulation.
Based on the results of the experiment, heating resource (such as gas) consumption, circulator pumping power consumption, and water saving are main factors for CO2 emission estimation. The gas consumption in the circulation system is less than the noncirculation system because of the shorter circulation time period which improves the heating performance and decreases the heater operation time to save gas consumption, as shown in Fig. 8. The circulator pumping power consumption of the circulation system with short pipe lengths is similar to the non-circulation system, however, the circulator consumes more energy as pipe lengths increase due to friction and mixing with the larger volume of cool water inside the piping system. Cheng [12] figures out that “1 m3 water consumes about 1 kWh equivalent power”. The non-circulation system can waste large amounts of water depending on pipe lengths as cool water needs to be discharged from the pipes before the suitable water temperature flowing out. The volume of wasted water is measured and converted to electric equivalent power, then added to the heater power consumption, the results show the total energy consumption of the non-circulation system is 50% higher than the energy consumption of the circulation system in Fig. 9. The circulation system not only saves water, but also saves the energy within the system. The reducing CO2 emission in different pipe length is listed in Table 4.
To estimate CO2 emission in the hot water circulation system and in the hot water non-circulation system under real usage conditions in the residential building, three parameters need to be considered; wasted water demand (CW ), electric consumption (CE , such as circulator, heater controller, etc.), and heating source consumption (CS , such as gas, electric, coal and oil, etc.). To estimate whole CO2 emission in the building, the following parameters need to be considered: using times for each person per day (N), persons in one household (P), numbers of household in residential building (B), energy consumption of heater (EH ) and circulator (ER ), and heating source (S). Estimation equations are: CW = (Q1i − Q2i ) × N × P × W
(4)
CE = [(EH )n − (EH + ER )c ] × N × P × B × E
(5)
CS = (Sni − Sci ) × N × P × B × S
(6)
Below is an example to estimate the CO2 reduction between the circulation system with independent circulator and non-circulation system. Based on the results of Table 4 to assume the hot water pipe diameter is 13 mm and pipe length is 20 m, each person uses 40 l of water per shower once a day and other hot water demand in 40 l [4], 4 people in each household and 7.5 million households in Taiwan. The total estimated CO2 reduction is approx. 400,000 tons of CO2 emissions every year, the estimation is shown in Table 5.
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Fig. 9. Water waste converted into equivalent power to compare the total power consumption between the non-circulation and circulation system.
Table 4 Reducing CO2 emission between non-circulation and circulation system. Pipe length (m)
1 2 4 8 16 20
Non-circulation system
Circulation system
Reducing CO2 emission (g)
Wasted water (l)
Gas consumption (g)
Heater power (W)
CO2 emission (g)
Gas consumption (g)
Heater power (W)
Circulator power (W)
CO2 emission (g)
3.4 3.6 5.8 7.7 12.3 14.3
9.1 11.2 15.3 21.4 34.0 39.8
1.8 2.2 3.1 5.8 7.1 8.0
20.5 25.1 34.5 49.1 76.7 89.6
8.8 10.2 11.9 16.6 25.8 31.0
1.2 1.3 1.8 2.7 4.9 5.3
1.3 1.5 2.0 3.0 5.5 6.0
19.9 23.1 27.2 38.2 60.4 71.8
0.5 2.1 7.3 10.9 16.3 17.8
Table 5 Total estimated CO2 reduction based on 7.5 million residents in Taiwan. Items
Reduction (per time)
Household saving per year
Water Electric Gas
14.3 −3.3 8.8
41.8 9.1 26.3
l Wh g
m3 kWh kg
Total saving in Taiwan 313,389k 67,890k 197,100k
m3 kWh kg
Total estimated CO2 reduction
The benefit of the hot water circulation system is that it saves water and energy, but also reduces the CO2 emission within the system (as referred in Table 4), these benefits make the hot water system worth the installation. 6. Conclusion In the past, most residential buildings utilized a central hot water piping system because of the different using periods and energy saving. However, the big problem to this type of system is that once the hot water leaves the storage tank and travels through the system piping, the water temperature drops due to the travel distance and ambient air temperature. The ideal solution to this problem is to reheat the undesirable (cooled) water in the system by the use of an individual hot water circulating system. But installation may be difficult in existing buildings due to building limitations. This study focuses on water saving and energy consumption in the individual hot water circulating system. The experiment results show that the circulation system reduces the water reheating time, improves the heating efficiency and decreases the heater energy consumption. The energy consumption in circulation system is around 50% less than the noncirculation system while the wasted water is converted into electric equivalent power. The three parameters that need to be considered to estimate the energy consumption and reduced CO2 emissions are
CO2 transfer coefficient () 0.10 0.62 2.09
kg/m3 kg/kWh kg/kg
CO2 reduction 31,402 −42,092 411,939
T T T
401,249
T
wasted water demand, electric consumption, and heating source consumption. In our example, equations were used to estimate a reduction of approximately 400,000 tons of CO2 emissions in 7.5 million households in Taiwan every year by replacing the popular gas heaters (non-circulation) with the hot water circulation system. The hot water circulation system has the following benefits: saves water, reduces energy consumption and reduces CO2 emissions. However, to make installations in existing buildings easier in the future, the system piping connections should be improved. Acknowledgement The author would like to thank the National Science Council of Taiwan (NSC 101-2221-E-025-016, Ministry of Science and Technology of Taiwan (MOST 103-2221-E-025-005-), and Taiwan Taung-Liang Industries Co., Ltd. for their support, technical direction, and financial contributions for the research of the independent hot water circulation system and estimation of reduced CO2 emissions by the hot water circulation system. References [1] M.C. Lee, C.L. Cheng, Y.H. Lin, Hot water plumbing system and temperature drop mechanism in residential building in Taiwan, in: CIB-W62 30th International Symposium, September 9, 16–17, France, Paris, 2004.
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[2] M.C. Lee, C.L. Cheng, Y.H. Lin, Empirical approach to transmit energy for hot water plumbing system, in: CIB-W62 31st International Symposium, September 9, 14–16, Brussels, Belgium, 2005. [3] C.L. Cheng, M.C. Lee, Y.H. Lin, Empirical prediction method of transmission heat loss in hot water plumbing, Energy and Buildings 38 (2006) 1220–1229. [4] C.L. Cheng, M.C. Lee, Research of residential building hot water issues in subtropical Taiwan, Journal of Asia Architecture and Building and Environment 4 (2005) 259–264. [5] Association of Building Environment Energy Conservation, Building Environment Energy Conservation Regular and Calculation Guidebook, 3rd ed., Japan, 1996. [6] M. Kamata, K. Sakaue, M. Nabeshima, K. Emura, H. Nimiya, Estimation of water temperature for design of hot water supply system based on energy conservation, in: CIB-W62 23rd International Symposium, 1997, pp. 10–12, November, Yokohama, Japan. [7] K. Sakaue, M. Kamata, S. Iwamoto, H. Nimiya, The prediction method of water temperature in distribution pipes, in: CIB-W62 26th International Symposium, September 9, 18–20, Rio de Janeiro, Brazil, 2000.
[8] C.A. Balaras, K. Droutsa, A.A. Argiriou, D.N. Asimakopoulos, Potential for energy conservation in apartment building, Energy and Buildings 31 (2000) 143– 154. ´ ´ S. Genic, ´ P. Zekonja, Supply water temperature reg[9] B. Jacimovic, B. Zˇ ivkovic, ulation problems in district heating network with both direct and indirect connection, Energy and Buildings 28 (1998) 317–322. [10] T. Morida, Basis of Distribution Technical, Tokyo Denki University Press, Tokyo, Japan, 1980. [11] K. Toyosada, Y. Shimizu, S. Dejima, M. Yoshitaka, K. Sakaue, Evaluation of the potential of CO2 emission reduction achieved by using water-efficient housing equipment in Dalian, China, in: 38th International Symposium on Water Supply and Drainage for Buildings, 2012, pp. 28–30, August, Edinburgh, UK. [12] C.L. Cheng, Study of the inter-relationship between water use and energy conservation for a building, Energy and Buildings 34 (2002) 261–266. [13] C.L. Cheng, Evaluation model of CO2 emission for saving water strategy, in: 38th International Symposium on Water Supply and Drainage for Buildings, 2012, pp. 28–30, August, Edinburgh, UK.
Contents lists available at ScienceDirect
Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Reducing CO2 emissions in the individual hot water circulation piping system M.C. Lee ∗ Department of Interior Design, National Taichung University of Science and Technology, Taichung, Taiwan
a r t i c l e
i n f o
Article history: Received 26 September 2013 Received in revised form 28 June 2014 Accepted 18 July 2014 Available online 1 September 2014 Keywords: Reducing CO2 emission Hot water circulation piping system Energy consumption Water demand Water and energy saving
a b s t r a c t Central hot water piping systems are used in large lodging buildings (e.g. hotels, dormitories, etc.) for comfort and convenience. However, the big problem to this type of system is that once the hot water leaves the storage tank and travels through the system piping, the water temperature drops due to the travel distance and ambient air temperature. Because of this drop in temperature, users need to wait for the undesirable cool water to flow out of the system before the desired hot water starts flowing out of the system. The ideal solution to this problem is to reheat the undesirable (cooled) water in the system by the use of an individual hot water circulating system. This study focuses on water saving and energy consumption in the individual hot water circulating system. The results show energy consumption is 50% less than the non-circulating central hot water system. Three parameters in wasted water demand, electric consumption, and heating source consumption were considered to transfer them with CO2 transfer coefficient to estimate the reducing CO2 emission. The hot water circulation system presents the benefits of water and energy saving, and reduction of CO2 emission, but the piping connection should be improved for easy installation in the existed buildings in the future. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Central hot water piping systems are used in large lodging buildings (e.g. hotels, dormitories, etc.) for comfort and convenience, as shown in Fig. 1. However, the big problem to this type of system is that once the hot water leaves the storage tank and travels through the system piping, the water temperature drops due to the travel distance and ambient air temperature. Large amounts of energy are required for continuous heating and circulation. To evaluate energy consumption and hot water temperature drop in piping, Lee et al. [1–3] utilized simplified empirical equations and figured out the hot water temperature drop in various lengths of stainless steel piping (13A) as shown in Fig. 2. The previous studies only discussed the individual hot water non-circulating system temperature drop and energy consumption. Cheng and Lee [4] figured out that energy consumption used during heating almost equaled the heat lost during transmission. Research of the energy consumption in a central hot water circulation system indicates that the amount of energy required for circulation and usage is approximately the same as the amount required for generating hot water.
∗ Tel.: +886 422196701. E-mail address: [email protected] http://dx.doi.org/10.1016/j.enbuild.2014.07.094 0378-7788/© 2014 Elsevier B.V. All rights reserved.
Numerous studies proposed various conservation methods such as the Building Energy Conservation Regulations in Japan (Association of Building Environment Energy Conservation [5]), which focused on energy consumption of the hot water supply system and included heat loss due to circulation. Kamata et al. [6] and Sakaue et al. [7] proposed a standard hot water temperature for a supply system based on energy conservation in Japan. Balaras et al. [8] determined that the variation in pipe heat loss was caused by vari´ et al. [9] proposed that ations in energy consumption. Jacimovic the heat loss from heated objects is a linear function of the outdoor temperature. And Morida [10] presented heat loss calculation equations for distribution pipes. Toyosada [11] figured out the economic benefit of carbon tax via water saving [12], Cheng focused on the inter-relationship between water use and energy conservation and proposed an evaluation model of CO2 emission for a water saving strategy [13]. Most residential buildings utilize a central hot water supply system because of the varying usage time periods, and to save energy. However, the problem with this type of system is once usage (circulation) stops, the hot water remaining in the pipes cools due to the ambient air around the piping. Therefore, before use, users need to wait for the undesirable cool water to flow out of the system before the desired hot water starts flowing out of the system. This process may waste a lot of water, depending on the length of the piping system. The ideal solution to this problem is to reheat the undesirable
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M.C. Lee / Energy and Buildings 84 (2014) 475–482
Nomenclature A B CW CE CS EH ER L N P Q1 Q2 S t
v
cross section of pipe (m2 ) numbers of household in residential building (household) CO2 emission of water demand (kg) CO2 emission of electic consumption (kg) CO2 emission of heating source consumption (kg) energy consumption of heater (kWh) energy consumption of circulator (kWh) pipe length (m) using times (time) persons in one household (persons/household) water volume in heater outlet (m3 ) water volume in faucet inlet (m3 ) heating source (kg in LPG, or in different heating source) period time (s) water velocity (l/s)
Greek letters W CO2 emissions coefficient = 0.1002 kg/m3 in water CO2 emissions coefficient = 0.62 kg/kWh in electric E S CO2 emissions coefficient = 2.09 kg/m3 in natural gas; 1.75 kg/kg in LPG; 2.92 kg/kg in coal; 2.73 kg/l in diesel oil; electric is same as E . * CO2 emissions coefficients are referred by Taiwan Environmental Protection Administration (EPA), greenhouse Gas Registry system, 2013. These coefficients relate to the heat capacity in different material producing process in different countries, please check them in the right data while using. Subscripts c circulation system pipe character (cross section area and length) i n non-circulation system p pipe
cool water inside the piping system by the use of an individual hot water circulation piping system. This study discusses water saving and energy consumption in the individual hot water circulation system.
Fig. 2. Hot water temperature dropping time in different length stainless pipe (13A).
2. Individual hot water circulator An individual hot water circulator with water reheat piping can save energy by reducing temperature drops while providing hot water at suitable temperatures. The circulation system consists of a check valve (C1 ) installed before ball valves (faucets) to connect with transmission pipes for recycling and reheating the cooled water in the independent hot water circulation system, as shown in Fig. 3. Two inlet pipes connect with a circulator, one pipe is for recycling the cooled water, and the other pipe is for cool water from urban water. A controller measures the recycled water temperature and sends an electronic signal to mechanically switch the water inlet between the recycled water and urban water. A check valve (C2 ) is installed to prevent urban water from flowing into the circulation pipe. The water flows out from the circulator into the heater (e.g. gas, electrical, solar, etc.) or hot water tank to heat the recycled water or urban water. The individual hot water circulator recycles the cooled water, provides hot water, and saves a considerable amount of energy compared with the central circulated hot water system. There are two additional parts needed in the individual hot water circulation system; the circulator, and the piping that connects from the supply pipe to the circulator and heater. Based on investigation of hot water pipe lengths in Taiwan [4], the average length is around 6.5 m (the shortest is 0.5 m, the longest is 20 m) in the residential buildings (apartments and houses). The controller for the circulator can be set by timer and remote panel to reduce the amount of electrical wiring. The added system installation costs is under US$500 with 10 m (13A) circulation pipes combined with the original hot water supply system via self-installation by the user in the newly constructed building or existing building.
Fig. 1. Typical central circulated hot water system.
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477
Fig. 3. Independent hot water circulator design concept.
3. Experiment An experiment was held to measure water recycling amounts and energy consumption. The experiment adopts the popular heater type in Taiwan (80%) [4]; gas heater with 52.3 kW heat and controllers, to heat and reheat water, and also adopts pipe material and length referred by Cheng and Lee [3], stainless steel pipe with insulator, up to 20 m in length. Due to limited experiment space and to reduce thermal radiation interference, 1 m length pipes (including 15 cm curves) are arranged in parallel (15 cm apart) to reach the desired design lengths. The optimal hot water circulation time should be under 1 min to reheat the water to the suitable temperature for comfort and energy conservation. The efficiency of the
pump and heater are also considered for the design. In this experiment, air temperature, water temperature (T), flow velocity (V), pressure (P), circulator energy consumption (including electronic control and pumping) (E2 ), heater (E1 ), and gas consumption (S) were measured in various pipe lengths (1 m, 2 m, 4 m, 8 m, 16 m, and 20 m); check valves (C) were also setup for the flow direction. When the heated water temperature reaches the stable flowing temperature of 54 ◦ C (heater set on 55 ◦ C), the water demand, electric consumption, and gas consumption can be compared between the circulation system and the non-circulation system. The experimental system as designed, contains, six ball valves, six sensor sets with temperature sensors, flow meters, and pressure sensors that are installed on the ball valve (faucets) inlet (T2–7,
Fig. 4. Whole experiment instruments setup with measure sensors.
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Fig. 5. Experiment instrument.
V2–7, P2–7) at 1 m, 2 m, 4 m, 8 m, 16 m, and 20 m from the heater and also on the heater outlet (T1, V1, P1). Circulation pipes with check valves are installed between ball valves and sensor sets, as shown in Fig. 4. The experiment was conducted indoors to minimize wind impacts, as shown in Fig. 5. Each ball valve is opened to allow hot water to flow into a water tank with a level gauge to check the amount of wasted water in the non-circulation system. The installation of the circulation system in the experiment is similar to the non-circulation system, the difference being, the circulation pipes, circulator, one sensor set with temperature sensor, flow meter, and pressure sensor installed before the circulator inlet (T8, V8, P8). All sensor sets measure temperature, velocity, pressure, electric consumption, and gas consumption while the ball valves are open. Once the water reaches the desired temperature, the ball valves close. Once testing of the non-circulating system is finished, cool water is introduced to cool the pipes to the ambient room
Table 1 Experiment records. System
Non-circulation
Factors Unit
T ◦ C
V l/s
P kPa
Circulation E1 W
S g
T ◦ C
V l/s
P kPa
E1 W
E2 W
S g
1, 2, 4, 8, 16, 20 m.
temperature, then the circulation system is tested. The test data is recorded in Table 1 during the test period. The test process was captured by thermal infrared cameras and showed the hot water flowing into the pipes and heating the pipes. Once the water flow was stopped, the water and pipes were cooled down by the ambient air temperature, as shown in Fig. 6. The captured photos presented the temperature variations from cool pipes to hot pipes and back to cool pipes.
Fig. 6. Temperature variations captured by thermal infrared camera.
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Fig. 7. Water volume and flowing period while water temperature over 54 ◦ C in various lengths of piping in non-circulation systems. Red lines present in Chian lines, and Blue lines present in Dot lines.
4. Results The results of the experiment focus on the temperature variations and water demand between the heater outlet (T1 , Q1 ) and
faucet inlet (T2 , Q2 ) in the non-circulation system, as shown in Fig. 7. The diagram in Fig. 7 shows the relationship between water volume and flowing period for the suitable temperature in various lengths of piping. The red line (chain line) shows the relationship between
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Table 2 Water demand and energy consumption in the non-circulation system. Pipe length (m)
Temperature (◦ C)
Period time (s)
Wasted water (Q2 ) (l)
Heated water (Q1 ) (l)
Gas consumption (g)
Heater power (W)
1 2 4 8 16 20
54.8 54.1 54.4 54.1 54.0 54.1
20 25 35 50 80 90
3.4 3.6 5.8 7.7 12.3 14.3
3.5 3.8 6.3 8.8 14.5 17.0
9.1 11.2 15.3 21.4 34.0 39.8
1.8 2.2 3.1 5.8 7.1 8.0
Table 3 Energy consumption in the circulation system. Pipe length (m)
Circulation period time (s)
Gas consumption (g)
Heater power (W)
Circulator power (W)
Total power (W)
1 2 4 8 16 20
13.0 15.0 20.0 30.0 55.0 60.0
8.8 10.2 11.9 16.6 25.8 31.0
1.2 1.3 1.8 2.7 4.9 5.3
1.3 1.5 2.0 3.0 5.5 6.0
2.5 2.8 3.8 5.7 10.4 11.3
period time and flowing out water temperature (T2 ) over 54 ◦ C. The red lines cross the two water demand volumes (calculated by flowing velocity and duration time) in heated water volume (Q1 ) and flowing out water volume (Q2 ), the volume difference between Q1 and Q2 is the remaining water in the transmission pipe. Therefore, the flowing out water is the wasted water in this system, as shown on the blue lines (dot lines) in Fig. 7. The water demand estimation is organized as (1) and (2). All related results with wasted water and energy consumption in the non-circulation system are summarized in Table 2. Q1i = (tn × vn )i
(1)
Q2i = Q1i − (Ap × Lp )i
(2)
Recycling and reheating the cooled water by the hot water circulator saves the water in the transmission pipe. The circulation system has additional power requirements due to the circulator; therefore, the total power consumption combines the power consumption of heater and circulator, as shown in Table 3. 5. Discussion
Fig. 8. Consumption of power and gas between non-circulation and circulation.
Based on the results of the experiment, heating resource (such as gas) consumption, circulator pumping power consumption, and water saving are main factors for CO2 emission estimation. The gas consumption in the circulation system is less than the noncirculation system because of the shorter circulation time period which improves the heating performance and decreases the heater operation time to save gas consumption, as shown in Fig. 8. The circulator pumping power consumption of the circulation system with short pipe lengths is similar to the non-circulation system, however, the circulator consumes more energy as pipe lengths increase due to friction and mixing with the larger volume of cool water inside the piping system. Cheng [12] figures out that “1 m3 water consumes about 1 kWh equivalent power”. The non-circulation system can waste large amounts of water depending on pipe lengths as cool water needs to be discharged from the pipes before the suitable water temperature flowing out. The volume of wasted water is measured and converted to electric equivalent power, then added to the heater power consumption, the results show the total energy consumption of the non-circulation system is 50% higher than the energy consumption of the circulation system in Fig. 9. The circulation system not only saves water, but also saves the energy within the system. The reducing CO2 emission in different pipe length is listed in Table 4.
To estimate CO2 emission in the hot water circulation system and in the hot water non-circulation system under real usage conditions in the residential building, three parameters need to be considered; wasted water demand (CW ), electric consumption (CE , such as circulator, heater controller, etc.), and heating source consumption (CS , such as gas, electric, coal and oil, etc.). To estimate whole CO2 emission in the building, the following parameters need to be considered: using times for each person per day (N), persons in one household (P), numbers of household in residential building (B), energy consumption of heater (EH ) and circulator (ER ), and heating source (S). Estimation equations are: CW = (Q1i − Q2i ) × N × P × W
(4)
CE = [(EH )n − (EH + ER )c ] × N × P × B × E
(5)
CS = (Sni − Sci ) × N × P × B × S
(6)
Below is an example to estimate the CO2 reduction between the circulation system with independent circulator and non-circulation system. Based on the results of Table 4 to assume the hot water pipe diameter is 13 mm and pipe length is 20 m, each person uses 40 l of water per shower once a day and other hot water demand in 40 l [4], 4 people in each household and 7.5 million households in Taiwan. The total estimated CO2 reduction is approx. 400,000 tons of CO2 emissions every year, the estimation is shown in Table 5.
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Fig. 9. Water waste converted into equivalent power to compare the total power consumption between the non-circulation and circulation system.
Table 4 Reducing CO2 emission between non-circulation and circulation system. Pipe length (m)
1 2 4 8 16 20
Non-circulation system
Circulation system
Reducing CO2 emission (g)
Wasted water (l)
Gas consumption (g)
Heater power (W)
CO2 emission (g)
Gas consumption (g)
Heater power (W)
Circulator power (W)
CO2 emission (g)
3.4 3.6 5.8 7.7 12.3 14.3
9.1 11.2 15.3 21.4 34.0 39.8
1.8 2.2 3.1 5.8 7.1 8.0
20.5 25.1 34.5 49.1 76.7 89.6
8.8 10.2 11.9 16.6 25.8 31.0
1.2 1.3 1.8 2.7 4.9 5.3
1.3 1.5 2.0 3.0 5.5 6.0
19.9 23.1 27.2 38.2 60.4 71.8
0.5 2.1 7.3 10.9 16.3 17.8
Table 5 Total estimated CO2 reduction based on 7.5 million residents in Taiwan. Items
Reduction (per time)
Household saving per year
Water Electric Gas
14.3 −3.3 8.8
41.8 9.1 26.3
l Wh g
m3 kWh kg
Total saving in Taiwan 313,389k 67,890k 197,100k
m3 kWh kg
Total estimated CO2 reduction
The benefit of the hot water circulation system is that it saves water and energy, but also reduces the CO2 emission within the system (as referred in Table 4), these benefits make the hot water system worth the installation. 6. Conclusion In the past, most residential buildings utilized a central hot water piping system because of the different using periods and energy saving. However, the big problem to this type of system is that once the hot water leaves the storage tank and travels through the system piping, the water temperature drops due to the travel distance and ambient air temperature. The ideal solution to this problem is to reheat the undesirable (cooled) water in the system by the use of an individual hot water circulating system. But installation may be difficult in existing buildings due to building limitations. This study focuses on water saving and energy consumption in the individual hot water circulating system. The experiment results show that the circulation system reduces the water reheating time, improves the heating efficiency and decreases the heater energy consumption. The energy consumption in circulation system is around 50% less than the noncirculation system while the wasted water is converted into electric equivalent power. The three parameters that need to be considered to estimate the energy consumption and reduced CO2 emissions are
CO2 transfer coefficient () 0.10 0.62 2.09
kg/m3 kg/kWh kg/kg
CO2 reduction 31,402 −42,092 411,939
T T T
401,249
T
wasted water demand, electric consumption, and heating source consumption. In our example, equations were used to estimate a reduction of approximately 400,000 tons of CO2 emissions in 7.5 million households in Taiwan every year by replacing the popular gas heaters (non-circulation) with the hot water circulation system. The hot water circulation system has the following benefits: saves water, reduces energy consumption and reduces CO2 emissions. However, to make installations in existing buildings easier in the future, the system piping connections should be improved. Acknowledgement The author would like to thank the National Science Council of Taiwan (NSC 101-2221-E-025-016, Ministry of Science and Technology of Taiwan (MOST 103-2221-E-025-005-), and Taiwan Taung-Liang Industries Co., Ltd. for their support, technical direction, and financial contributions for the research of the independent hot water circulation system and estimation of reduced CO2 emissions by the hot water circulation system. References [1] M.C. Lee, C.L. Cheng, Y.H. Lin, Hot water plumbing system and temperature drop mechanism in residential building in Taiwan, in: CIB-W62 30th International Symposium, September 9, 16–17, France, Paris, 2004.
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