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Applied research on water loop heat pump system based on a novel mechanism of energy conversion
Applied Thermal Engineering 153 (2019) 575–582
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Research Paper
Applied research on water loop heat pump system based on a novel mechanism of energy conversion
T
Yu Qi dong School of Environmental Science and Engineering, Tianjin University, No. 92 Weijin Road, Nankai District, Tianjin 300072, PR China
H I GH L IG H T S
energy and energy-saving principle of a WLHP system. • Reverse of energy conversion and novel method on system operation. • Mechanism range of a WLHP system and its validation. • Energy-saving office building group and optimization for heat pump projects. • An • Analysis for impact factors of system operation and suggestion on energy balance.
A R T I C LE I N FO
A B S T R A C T
Keywords: Water loop heat pump Reverse energy Circulating water Office building group Air source heat pump Auxiliary heat source
Water loop heat pump (WLHP) has been widely used as a heat recovery system to realize building energy efficiency, but it still was a difficult problem to how to determine the effect of extracting and rejecting heat on water loop, because the unit converted its operation to reverse mode with load changing but the process would have a reverse effect on system operation. In this paper, a novel mechanism of energy conversion is introduced into the WLHP system to solve the problem and its view is presented as follows. First, the operation of WLHP system produces a “Reverse Energy” and it originates from an energy difference caused by the converted unit to water loop and can be used as a quantitative index to illustrate the energy-saving principle of system operation. Second, system operation is a combined effect caused by load change and reverse energy and yields to a nonlinear law due to the dynamic effect of reverse energy, so, it is characteristic of generality and individuality. Third, we can replace building load with circulating water to determine the energy-saving range of system operation and its law, and the idea that the energy carrier is used to solve the problem caused by operation will be of an important significance for a complex system. At last, the impact factors of system operation can be converted into a single function on circulating water by extracting and rejecting heat and the reverse energy is also determined by power equation of circulating water. To validate the previous conclusions and optimize heat pump systems, an office building group with different air-conditioning projects was tested in Tianjin and the results show that the change of system load determines the overall trend of operation, but the effect of reverse energy can reduce the fluctuation caused by load change and energy consumption of auxiliary power to improve operational efficiency, so, the WLHP system can achieve a higher efficiency compared to an air source heat pump (ASHP), in addition, the total energy consumption can further be reduced by building an energy balance between auxiliary heat source and units. These contributions contribute to the development of WLHP system and building energy efficiency.
1. Introduction Energy resource has become a driving force and its use slowly changed the development strategy for a country, in this regard, a new policy of “replacing coal with electricity” [1–3] has gotten a better environmental benefit in China, which stated that the use of energy
resource had a positive effect but its structure needed to continuously be improved to meet with the development of the economy, so, it would be of an important significance for human society to exploit new resources [4–6] and improve energy recovery system [7–9] in 21st century. Heat pump [10–13] belonged to an advanced energy-saving device and it could extract energy from air, water and soil to provide
E-mail address: [email protected]. https://doi.org/10.1016/j.applthermaleng.2019.03.065 Received 6 August 2018; Received in revised form 13 February 2019; Accepted 11 March 2019 Available online 12 March 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.
Applied Thermal Engineering 153 (2019) 575–582
Q.d. Yu
Nomenclature
qdh
P f To Tk g η Qc Qh ε µ N Ec F B qcw qd
φ φc φh qCW ICW ϑ qh qc ϑc ϑh ηc
qccw qdc
qhcw
operation power [kW] function on temperature evaporation temperature [K] condensation temperature [K] function on load rate load rate [%] cooling load [kW] heating load [kW] coefficient of performance, here 4.49 is taken heating coefficient of heat pump, here 4.79 is taken number of units reverse energy of units to water loop [kW] power function load rate corresponding to inflection point [%] operation power of circulating water [kW] operation power of circulating water at design condition [kW] operation power of circulating water in cooling mode [kW] operation power of circulating water at design condition in cooling mode [kW] operation power of circulating water in heating mode [kW]
ηh tw Tw E P Pm N pi ti
operation power of circulating water at design condition in heating mode [kW] coefficient of energy conversion coefficient of energy conversion in cooling mode coefficient of energy conversion in heating mode operation power of circulating water at partial load [kW] evaluation number of operation power number of reverse energy rejected energy of circulating water [kW] stored energy of circulating water [kW] number of reverse energy in cooling mode number of reverse energy in heating mode the upper limit of energy-saving range in cooling mode [%] the upper limit of energy-saving range in heating mode [%] test temperature in summer [°C] test temperature in winter [°C] relative power unit/system at partial load [kW] unit/system at full load [kW] energy consumption number operation power at partial load [kW] operation time at partial load [h]
boiler was used to heat the circulating water, but note that the method also showed up a certain limitation due to the effect of cooling and heating load on evaluation index, on the other hand, Buonomano [23] designed a new dynamic simulation method to investigate its operation in European countries and the results proved that system operation could realize the higher efficiency than other HVAC system but its economic and environmental benefit were more obvious. In addition, Yuan [24] analyzed the effect of water temperature on compressor and cooling tower and put forward a practical strategy to optimize system operation, but, Fuentes [25,26] used system load as a method to reveal the operation law of heat pump system, which provided some new clues for its research. We believed that the previous investigations has had a positive impact on the development of WLHP system, but note that the building load was still used as a general method to evaluate system operation. According to the operation principle of WLHP system, we found that the attribute of building load was continuously changed for different rooms and the unit also converted its operation to reverse mode, in other words, the original heating load of a room was converted into cooling load with system operating and the process made the overall trend of energy consumption to be changed by extracting and rejecting heat
heating or cooling for the HVAC system to realize building energy efficiency, so, its application technology has become the frontier of engineering science. Water loop heat pump (WLHP) [14] was a special kind of water source heat pump and it used a closed loop to link all units to simultaneously provide heating and cooling for a building. In the system, the circulating water was the energy carrier of operation and key link between building and units and its temperature was in the range of 16–32 °C according to the previous investigation [15]. Note that an electric boiler should be started to heat the water loop when the water temperature was to drop to 16 °C, similarly, a cooling tower was also used to remove the unwanted heat at the upper limit of temperature, and the schematic diagram of a WLHP system was shown in Fig. 1. At present, the issues on WLHP system focused on its service conditions and research method. Zaidi [16,17] selected four kinds of WLHP, variable-air-volume (VAV), four-pipe and reheat HVAC system to discuss their operational efficiency in various climatic conditions and building types, and the results showed that the energy efficiency of WLHP system was highest in these projects but note that the building structure and system load had a great impact on overall operation, which has had a very positive effect on the development of WLHP system in USA. Fernández [18] studied the operation of the WLHP system in Iberian Peninsula and calculated system load by using energy simulation software, and he emphasized that the potential of WLHP system was to recover and redistribute the afterheat to realize the energy balance of a building compared to a four-pipe HVAC system. In China, Li [15] used the geothermal energy as a heat input to improve system efficiency, but Lian [19,20] analyzed different effects of electric and coal-fired boiler on system operation to determine its energy-saving range and service areas, in addition, he also designed an integrated system composed of gas engine heat pump (GHP) and WLHP to further reduce total energy consumption. As for building types, Chao [21] discussed the energy efficiency of WLHP system in a residential building and gave its control strategy based on the analysis for the energy consumption of unit and auxiliary device. As far as the research methods were concerned, Ma [22] proposed a static method of primary energy to evaluate system operation and reported its energy-saving range, meanwhile, he found that the auxiliary heat source played an important role for system efficiency, especially, when the coal-fired
Fig. 1. Schematic diagram of WLHP system. 576
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“Reverse Energy”. The reverse energy is a special kind of energy and it originates from an energy difference produced by the converted unit to water loop and is of a dynamic attribute, and note that the “Reverse” reveals that its effect is opposite to system load for the following reasons. First, there is a close relationship between system load and water temperature in WLHP system, namely, the temperature continuously rises with load increasing in summer but it rapidly drops in winter, and note that system power will obviously be increased in the process, so, the load change has a negative effect on system operation, especially, in winter. Second, a part of the units convert their operation mode after system operation is at partial load and note that the process reduces the previous effect of units on water loop but also makes up for the energy loss of circulating water to some extent, which reveals the process of forming a reverse energy, therefore, the effect of reverse energy can reduce the energy consumption of auxiliary power and it should be used as a quantitative index to illustrate the energy-saving principle of system operation. At last, fewer and fewer units convert their operation to reverse mode with system load increasing or even disappear, so, the effect of reverse energy is in a dynamic process, which explains why system operation can be converted from providing heating and cooling simultaneously to only heating or cooling. As stated above, the operation of the WLHP system should be a combined effect caused by load change and reverse energy but note that it deviates from original law and forms an inflection point due to the dynamic effect of reverse energy, and its law is described in Eq. (2).
from and to water loop, so, the conventional approach has shown up the limitation due to an instability caused by building load. Based on this, we felt that its research could be further improved if the following problems were solved. How to determine its effect on the water loop when a unit converted into the opposite operation mode? How to explore a new method to solve the limitation caused by building load? How to convert the impact factors of system operation into a function on single variable? How to validate its potential compared to other heat pump system? In this paper, a new mechanism of energy conversion is proposed to reveal the energy-saving principle of WLHP system and determine the effect of the converted units on system operation, on the other hand, an office building group with different air-conditioning projects is also designed to validate its energy-saving range and optimize heat pump system in Tianjin. These findings can provide a certain theoretical references and data support for further research. 2. Mechanism of energy conversion 2.1. Reverse energy According to the principle of refrigeration [27], the change of system load has a great impact on evaporation and condensation temperature of an air source heat pump (ASHP) and directly affects its COP, meanwhile, we also find that its operation still yields to an approximately linear law because the ASHP system usually only provides heating or cooling for a building, which is in accordance with Lu’s report [28], so, the operation law of ASHP system can be written in Eq. (1).
(
(2)
where we should notice the following problems. First, the formula is built to describe system operation in winter, and the heat extracted by
P = f (To, Tk ) = g (η) ⎧ dP η ∈ [0%, 100%] ⎨ dη ≈ const , ⎩
(
(1)
all units is NQh 1 −
Eq. (1) shows that the load rate plays a key role for an ASHP system but system operation also yields to an approximately linear law due to the dominant effect of load rate. Note that the conclusion will be changed for a WLHP system because, generally, the system simultaneously provides cooling and heating for a building but also the unit continuously converts its operation to reverse mode. In cooling mode, 1 the units reject the heat of Qc 1 + ε to water loop and the process is that the circulating water stores its energy based on the rising tem-
(
)
1 ⎧ ⎪ P = ηNQh 1 − μ − Ec = F (η , Ec ) ⎨ ∂P ≠ const , η ∈ (0%, B%) ∪ [B%, 100%) ⎪ ∂η ⎩
(
ηNqh 1 −
(
1 μ
)
at full load but it can be reduced to
) after a part of the units operate the cooling mode with
load decreasing. Second, the auxiliary heat source is used to make up for energy loss of circulating water but note that the supply heat should further be reduced because the effect of reverse energy can make the temperature of circulating water to rise to some extent. Third, the change of system power should yield to a nonlinear law but its inflection point reveals a conversion process from energy saving to high energy. As mentioned above, a novel mechanism of energy conversion is introduced to evaluate system operation and it includes the following several points. First, we can use a piece of energy chain to describe the operation of WLHP system, and it is composed of building, circulating water and units and note that the circulating water is the energy carrier of operation and key link between building and units, and its structure
)
perature; similarly, they extract the heat of Qh 1 −
1 μ
1 μ
) from the loop in
heating mode and the circulating water also rejects its energy in the process, so, the extracting and rejecting heat have a reverse effect on water loop. Based on this, a new energy effect can be introduced into the WLHP system to illustrate its energy-saving principle after a unit converts its operation to reverse mode, and the effect is defined as
Fig. 2. Structural diagram of energy chain. 577
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is shown in Fig. 2. Second, the circulating water can be used as a new method to determine the energy-saving range and operation law instead of building load, so, the impact factors of system operation can converted into a single function on circulating water to simplify the research process. Third, the unit converts its operation to reverse mode and the process produces a “Reverse energy” but the effect can be determined by the power change of circulating water. As shown in Fig. 2, we also find that the new method has a very positive effect for the development of WLHP system because it has solved the instability caused by the change of building load, in addition, the idea that the energy carrier is used to solve the operation problem will be of an important significance for a complex system.
1
1− ⎧ μ ⎪ φc = 1 + 1 ⎪ ε 1 ⎨ 1+ ε ⎪ φh = 1 1− ⎪ μ ⎩
According to the mechanism, there is a close relationship between building load and circulating water and it shows two aspects of load rate and reverse energy. On the one hand, we find the double attribute of load rate based on the different requirements for heating and cooling and they are quality and quantity, respectively, where, the “quality” refers to the attribute of building load but “quantity” stands for its size. On the other hand, when a room demands the cooling instead of original heating, the unit converts its operation mode and produces the reverse energy by extracting and rejecting heat. Based on this, the power equation of circulating water is expressed in Eq. (3).
⎧ Δε < ε ⎨ ⎩ Δμ < μ
1 ε ⎨ 1 ≈ 1 μ ⎩ μ ∓ Δμ
⎧ ε ∓ Δε ≈ (3)
1
(
Ec = ΔQh 1 −
)
1 μ 1 1+ ε
qdc , but note that the units in cooling
ηqdc − φc (1 − η) qdc = qdc (1.647η − 0.647) qCW = ⎧ ⎨ ηqdh − φh (1 − η) qdh = qdh (2.545η − 1.545) ⎩
qdc , η = 100%
⎧ ⎪ 1 1− ⎨ ηqdc − (1 − η) 1μ qdc , 0% < η < 100% 1+ ⎪ ε ⎩
(10) (4)
1−
(
ΔQc 1 +
1 ε
) = (1 −
1 η) 1ε 1− μ
μ
qdh , respectively. By the Eq. (3), the power
equation of circulating water is written as follows.
qhcw
qdh , η = 100% ⎧ ⎪ 1 1+ = ⎨ ηqdh − (1 − η) 1ε qdh , 0% < η < 100% 1− ⎪ μ ⎩
(0% < η < 100%)
Eq. (10) shows that the power change of circulating water is a function on η and φ at partial load because its design value can be calculated by the cooling and heating load of a building, and note that the load rate has a general impact on system operation and it can reflect the generality of heat pump systems, but the coefficient of energy conversion shows the effect of operation mode on system efficiency and it reveals the individuality of the WLHP system. In addition, we also find that the effect of reverse energy is continuously decreased when a unit converts its operation from cooling to heating mode, whereas, its effect will be increased, which explains why more energy is consumed to maintain the water temperature in winter. To get more clues, we use theqdc andqdh as the base lines to analyze the relationship between load rate and power of circulating water, and they are shown in Eq. (11).
Similarly, a part of the units convert their operation mode when the WLHP system is at partial load, and the power change of circulating water is (1 − η) qdh , but note that the process makes a part of heating load to be converted into the equal cooling load and corresponding (1 − η) qdh cooling load and reverse energy are ΔQc = and 1 1+
(9)
Introducing the Eq. (9) into power equations of circulating water, they can be rewritten as follows.
mode still reject the heat to water loop and its size is ηqdc . According to the Eq. (3), the power equation of circulating water is described in Eq. (4).
qccw =
(8)
1
1− ⎧ μ ⎪ φc = 1 + 1 = 0.647 ⎪ ε 1 ⎨ 1+ ε ⎪ φh = 1 = 1.545 1− ⎪ μ ⎩
)
1−
= (1 − η)
(0% < η < 100%)
Eq. (8) states that the change of COP has less impact on its inversion value at partial load, so, we can use the design value of COP to evaluate the effect of conversion process on system operation instead of other values and the coefficient is also rewritten as follows.
to the relationship of Δq = ΔQc 1 + ε , the process converts the part of cooling load into equal heating load and its size is (1 − η) qdc ΔQh = ΔQc = 1 , and the reverse energy produced by these units is ε
(7)
According to Eqs. (6) and (7), we can build an approximate equivalence between COP and its change value as follows.
In order to determine Ec in Eq. (3), we use the power change of circulating water to derive corresponding building load, which is a back-stepping method and general application for Newton's third law of motion [29]. In summer, the operation of some units is converted from cooling to heating mode with cooling load decreasing and the power change of circulating water is Δq = (1 − η) qdc in the process. According
1 μ
(0% < η < 100%)
1
qd , η = 100% qcw = ⎧ ηq − ⎨ ⎩ d Ec , 0% < η < 100%
1+
(6)
Eq. (6) shows that the coefficient reflects a conversion process from cooling to heating (or vice versa), in fact, it is a function on performance coefficient of heat pump. Note that the COP of unit can be considered as a constant value to discuss the effect of conversion process on system operation for the following reasons. First, the use of auxiliary heat source can maintain the normal temperature of circulating water at partial load, so, the change of COP is not obvious when the water temperature is in the range of 16–32 °C according to previous investigation [15]. Second, a part of the units convert their operation to reverse mode at partial load and the process produces the “Reverse Energy”, therefore, the change of COP can be further reduced due to the positive effect of “Reverse Energy” on circulating water. Third, according to Eq. (6) and previous discussions, the relationship between COP of unit and its change value can be described in Eq. (7).
2.2. Modeling and motion law
(
1 1 →φ=f( , ) ε μ
η − φc (1 − η) = 1.647η − 0.647 ICW = ⎧ ⎨ ⎩ η − φh (1 − η) = 2.545η − 1.545
(5)
In this paper, we introduce a coefficient of energy conversion (φ ) to describe the process of operation conversion, and its definition is written as follows.
(11)
According to Eq. (11), the power change of circulating water can be further described in Fig. 3. As shown in Fig. 3, we get the following conclusions. First, its change yields to an approximately linear law but 578
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of WLHP system can realize a higher efficiency in summer whenϑc is in the range of 0.135–0.607, where, 0.607 corresponds to load rate of 0% but its upper limit can be determined as follows.
0.647(1 − η) = 0.135 η + 0.647(1 − η)
ηc = 80.57%
→
(14)
So, the energy-saving range of system operation is at load rate of 0–80.57% in summer but other load rate is the range of high consumption. Similarly, its range also can be calculated in winter ifϑh is in the range of 0.607–0.724, and we should notice that 0.607 still corresponds to load rate of 0% but its upper limit is built in Eq. (15). Fig. 3. Power change of circulating water with load changing.
η = 0.724 η + 1.546(1 − η)
note that system load has a great impact on circulating water in winter than summer. Second, system operation can reach a dynamic balance at load rate of 40% in summer but the balance point occurs at load rate of 60% for winter, so, the energy efficiency of system operation is higher in cooling mode than heating. As mentioned above, the operation law of the WLHP system is reported as follows. First, system operation is a combined effect caused by load change and reverse energy and yields to a nonlinear law and its inflection point shows the conversion process from energy saving to high energy. Second, the effect of reverse energy can reduce the fluctuation caused by load change and energy consumption of auxiliary power, so, it should be used as an energy-saving index to evaluate overall operation. Third, the heating load has a great impact on system operation, so, it is of an important significant for the development of WLHP system to how to reduce system consumption in winter.
3. Test and discussion 3.1. Project overview The project is located in Tianjin and it is an office building group, and its layout is shown in Fig. 4. In group, the area of each building is about 1000 m2 and design temperatures are 21 °C and 27 °C in winter and summer, and its heating and cooling load are 70 kW and 105.5 kW, respectively. We design three sets of WLHP system to provide heating and cooling for 1#—3# building, and 23 sets of SHR series unitary units are used in each system and their total heating and cooling capacity are 105.5 kW and 120.8 kW and corresponding input powers are 25.2 kW and 23.5 kW, in addition, an electric boiler is installed to heat the circulating water in winter and its capacity is 35 kW. On the other hand, we uses a screw chiller to provide cooling for 4# building in summer and its capacity and input power are 110.5 kW and 18.98 kW, respectively, but the heating is provided by municipal pipeline network based on local condition and building layout, meanwhile, 24 sets of fan coil units (FC) is installed as terminal device and total power is 2.72 kW. Based on the effect of a policy of “replacing coal with electricity”, we design a set of ASHP system to provide heating and cooling for 4# building instead of original HVAC system. 2 sets of ASHP and an electric boiler with 35 kW are operated in new project and the heating and cooling capacity of each unit are 67 kW and 63 kW and corresponding input powers are 23.75 kW and 23 kW, respectively, and note that the original FC is still used as system terminal device.
According to the close relationship between building load and circulating water, we use the circulating water to determine the energysaving range of system operation instead of building load, but the process should notice the following several points. First, the focus of research should be placed at heating mode, in the meantime, we should notice the positive effect of reverse energy on system operation. Second, the rejected and stored energy of circulating water correspond to heating and cooling load, respectively in new method, and note that the rejected energy originates from the reverse energy of the units to water loop in summer, similarly, the effect of reverse energy also forms the stored energy of circulating water in winter. Third, a number of reverse energy (ϑ) is defined as a new index to determine the energy-saving range and it is a ratio of rejected energy to total energy change of circulating water and its definition is expressed as follows.
qh qh = qh + qc qh + Ec
(12)
Introducing Eqs. (4) and (5) into above formula, the coefficient can be rewritten as follows.
⎧ ⎪ ⎪ϑ = ⎪ c ⎪ ⎨ ⎪ϑ = ⎪ h ⎪ ⎪ ⎩
1 μ (1 − η) qdc 1 1+ ε
(15)
Based on this, the energy-saving range of WLHP system is at load rate of 0–80.22% in winter, and note that the effect of “Reverse Energy” will disappear because all units extract the heat from water loop if the load rate is increased to 80.22–100%, and the process makes the water temperature to rapidly drop and leads to consume the more auxiliary energy, so, we must cut down its lasting time to improve overall efficiency.
2.3. Discussion on energy-saving range
ϑ=
ηh = 80.22%
→
Heating Pipeline
1−
1 μ ηqdc + (1 − η) qdc 1 1+ ε 1−
ηqdh 1 ε 1 1− μ 1+
ηqdh + (1 − η) qdh
=
CAC system before retrofit
0.647(1 − η) η + 0.647(1 − η)
4#
40.32m
ASHP system after retrofit =
η η + 1.545(1 − η)
12.48m 21.76m
(13)
Eq. (13) states that the evaluation index is a single function on load rate, so, we can easily determine system operation by calculating its load rate, which solves the limitation of static method of primary energy, in the meantime, the result also has a very positive impact on an engineering project. According to the previous investigation [22] and relationship between building load and circulating water, the operation
12.48m
WLHP system 40.52m
1#
WLHP system 9.75m
40.78m
2#
WLHP system 11.15m
Fig. 4. Layout of office building group. 579
40.56m
3#
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Q.d. Yu
energy on system operation and conversion process from energy saving to high consumption. On the other hand, we also find that the energy efficiency of WLHP system is higher than that of an ASHP system at load rate of 20–80%, so, system load has a general impact and determines the change trend of operation, but the effect of reverse energy can make the operation of WLHP system to further smooth. In addition, the use of auxiliary heat source usually has a great impact on total energy consumption when the heat pump systems are used in northern China, and its effects on heat pump systems are presented in Figs. 7 and 8. As shown in Fig. 7, the degree of power change is smaller at load rate of 20–60% in summer than winter for an ASHP system, because the more heat is used to prevent from frosting caused by high humidity at the beginning of heating. With outdoor temperature dropping, the supply heat is continuously increased to maintain system operation and avoid frosting caused by low temperature, but note that the use of auxiliary power doesn’t change its original law, which again validates previous conclusion. Fig. 8 shows that the power change of WLHP system makes a great difference in different load range for the following reasons. Firstly, the degree of power change is greater at load rate of 20–60% in winter than summer, because the use of auxiliary heat source can improve the COP of a unit by heating circulating water but the process also increase system power, and note that its effect is more obvious on system than unit in this case, so, it has become a hot topic to how to build an energy balance between auxiliary power and units. Secondly, the auxiliary heat source has a very negative effect on system efficiency when the load rate is increased to 80–100%, and note that the key point occurs at load rate of 80%, so, the operation of WLHP system will lose its advantage over a conventional two-pipe HVAC system at load rate of 80–100%, which also validates its energy-saving range in winter. Using the Eq. (18), the changes of energy consumption are described in Figs. 9 and 10. Fig. 9 shows that their energy consumptions yield to a similar change law in two systems but there are also some differences between systems. First, the change of system load has a smaller impact on the COP of a unit at the beginning of cooling and the range is at load rate of 20–40% but note that its lasting time is less than other ranges, so, the difference of energy consumption isn’t obvious between systems. Second, the power change of WLHP system is lower than that of ASHP system due to the positive effect of reverse energy on system operation when the load rate is increased to 60% from 40%, and note that its lasting time is longer at load rate 40–80%, so, the difference of energy consumption will become more obvious, especially, at load rate 40–60%. Third, the energy consumption of WLHP system is rapidly increased without reverse energy when the load rate is increased to 80–100%, but note that its lasting time is shorter in the range, so, their differences can be reduced to some extent. In Fig. 10, we see that the difference of energy consumption has obviously been enlarged at load rate of 40–80% due to the use of auxiliary heat source. On the one hand, the effect of reverse energy can reduce the energy consumption of auxiliary power to improve operational efficiency in the range, on the other hand, the use of auxiliary heat source has a great impact on system operation compared to other devices, so, the difference of energy consumption will be enlarged with operating time increasing. As mentioned above, we can get the following conclusions. First, the
3.2. Test and method In this case, a part-load method [30] is used to simplify the process of experiment, and note that the method divides system load into five sections of 100–80%, 80–60%, 60–40%, 40–20% and 20–0% and selects corresponding load rates of 100%, 80%, 60%, 40% and 20% as the typical condition points to test system operation. According to the product manual, the formula of testing temperature is written as follows.
⎧ tw = (0.064°C/%) × η + 27°C, insummer ⎨ ⎩Tw = (−0.315°C/%) × η + 21°C inwinter
(16)
In summer, their temperatures are t1 = 33.4 °C(100%), t2 = 32.1 °C(80%), t3 = 30.8 °C(60%), t4 = 29.6 °C(40%) and t5 = 28.3 °C(20%), respectively. Similarly, the corresponding temperatures are T1 = −10.5 °C(100%), T2 = −4.2 °C(80%), T3 = 2.1 °C(60%), T4 = 8.4 °C(40%) and T5 = 14.7 °C(20%) in winter. Using this method, the experimental data of heat pump systems are showed in Tables 1 and 2, and note that the operational efficiency of a compressor is 70% and it has been considered in the calculation. In the paper, we propose a relative power method to deal with experimental data, and note that the method uses the system power at full load as a base line to evaluate the effect of load rate on system operation and its definition is written in Eq. (17).
E=
P Pm
(17)
By Eq. (17), we can determine the change law of system operation and its inflection point at which the operation deviates from original law and energy-saving range, however, the method can’t evaluate the energy-saving potential of overall operation due to different effects of operating time on energy consumption. Based on this, we define an energy consumption number (N) as a comprehensive index to evaluate system operation and its definition is described as follows.
N=
Pi ti 5
∑i = 1 Pi ti
(18)
Note that we can know the change trend of energy consumption and its distribution by using Eq. (18), which contributes to further evaluate the effect of load rate on system operation and optimize the design project. 3.3. Result and discussion Using the relative power method, the power changes of heat pump systems are described in Figs. 5 and 6. As shown in above figures, we see that the operation of ASHP system is relatively smooth with load increasing and it yields to an approximately linear law, whether in summer or winter, which proves the previous conclusion and also points out that the change of load rate has an important impact on system operation. But the operation of WLHP system shows an obviously nonlinear characteristic due to an inflection point at load rate of 80% and the result proves the previous investigation on energy-saving range, in the meantime, it also reveals the dynamic effect of reverse Table 1 Energy consumption of heat pump systems in summer. Load rate/%
100 80 60 40 20
Unit power /kW ASHP
WLHP
32.31 24.03 18.13 13.01 9.11
16.56 9.31 7.01 4.69 2.33
Pump power/kW
2.20 2.20 2.20 2.20 2.20
Cool tower power/kW
FC power/kW
System power/kW
ASHP
WLHP
ASHP
WLHP
ASHP
WLHP
0 0 0 0 0
1.50 0.81 0.64 0.43 0.22
2.72 2.21 1.65 1.11 0.56
0 0 0 0 0
37.23 28.44 21.98 16.32 11.87
20.26 12.32 9.85 7.32 4.75
580
Time/h
75 375 600 300 150
Energy consumption/kWh ASHP
WLHP
2792 10,665 13,188 4896 1781
1520 4620 5910 2196 713
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Q.d. Yu
Table 2 Energy consumption of heat pump systems in winter. Load rate/%
Unit power /kW
100 80 60 40 20
1
Pump power /kW
ASHP
WLHP
11.76 7.71 5.39 3.41 1.59
7.63 4.82 4.08 2.77 1.41
E
2.20 2.20 2.20 2.20 2.20
Auxiliary power /kW
FC power/kW
System power /kW
ASHP
WLHP
ASHP
WLHP
ASHP
WLHP
35.00 27.28 20.01 12.98 5.87
35.00 23.62 15.01 9.86 4.71
2.72 2.21 1.67 1.11 0.56
0 0 0 0 0
51.68 39.40 29.36 19.70 10.22
44.83 30.64 21.29 14.83 8.32
1 E
WLHP ASHP
0.8
0.6
0.4
0.4
0.2
0.2
20%
40%
60%
80%
0 0%
100%
ASHP WLHP
0.8
WLHP
3721 14,184 16,911 5674 1472
3228 11,030 12,263 4271 1198
40%
60%
80%
100%
Fig. 8. Power change of WLHP system.
0.4
E
ASHP
Curve of power change in summer Curve of power change in winter
20%
Fig. 5. Power change of heat pump systems in summer.
1
72 360 576 288 144
Energy consumption /kWh
0.8
0.6
0 0%
Time/h
N
0.35 0.3
ASHP WLHP
0.25
0.6
0.2 0.15
0.4
0.1
0.2 0
0.05 0
0
20%
40%
60%
80%
100%
0
20%
40%
60%
80%
100%
Fig. 9. Change of energy consumption number in summer.
Fig. 6. Power change of heat pump systems in winter.
0.6
1 E
Curve of power change in winter Curve of power change in summer
ASHP WLHP
0.5
0.8
0.4
0.6
0.3 0.2
0.4
0.1
0.2
0
0
N
0
20%
40%
60%
80%
100%
0
20%
40%
60%
80%
100%
Fig. 10. Change of energy consumption number in winter.
Fig. 7. Power change of ASHP system.
WLHP system and its operation yields to a nonlinear law due to an inflection point at load rate of 80%. Second, the use of auxiliary power has an important impact on system operation but its effect doesn’t the overall trend of operation, in addition, it can improve overall efficiency of system operation to build an energy balance between auxiliary heat
operation of ASHP system shows an approximately linear characteristic based on the dominant effect of load rate, but the combined effect caused by load change and reverse energy determines the operation of
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source and units. Third, the energy efficiency of WLHP system is higher than that of an ASHP system due to the positive effect of reverse energy on system operation in this case.
(2018) 624–636. [8] Feng Zhou, Wei Duan, Guoyuan Ma, Thermal performance of a multi-loop pumpdriven heat pipe as an energy recovery ventilator for buildings, Appl. Therm. Eng. 138 (2018) 648–656. [9] O. Ribé, R. Ruiz, M. Quera, et al., Analysis of the sensible and total ventilation energy recovery potential in different climate conditions. Application to the Spanish case, Appl. Therm. Eng. 149 (2019) 854–861. [10] Matteo Dongellini, Claudia Naldi, Gian Luca Morini, Sizing effects on the energy performance of reversible air-source heat pumps for office buildings, Appl. Therm. Eng. 114 (2017) 1073–1081. [11] Qunli Zhang, Lin Zhang, Jinzhe Nie, et al., Techno-economic analysis of air source heat pump applied for space heating in northern China, Appl. Energy 207 (2017) 533–542. [12] Jiangyu Wang, Guannan Li, Huanxin Chen, et al., Energy consumption prediction for water-source heat pump system using pattern recognition-based algorithms, Appl. Therm. Eng. 136 (2018) 755–766. [13] X.Q. Zhai, X.W. Cheng, R.Z. Wang, Heating and cooling performance of a minitype ground source heat pump system, Appl. Therm. Eng. 111 (2017) 1366–1370. [14] J.A. Pietsch, Water-loop heat pump systems assessment, ASHRAE Trans. 96 (1990) 1029–1038. [15] Li Xinguo, Thermal performance and energy saving effect of water-loop heat pump systems with geothermal, Energy Convers. Manage. 39 (1998) 295–301. [16] J.H. Zaidi, R.H. Howell, Energy use and heat recovery in water-loop heat pump, variable-air-volume, four-pipe and reheat HVAC system: part I, ASHRAE Trans. 99 (1993) 13–28. [17] J.H. Zaidi, R.H. Howell, Energy use and heat recovery in water-loop heat pump, variable-air-volume, four-pipe and reheat HVAC system: part II, ASHRAE Trans. 99 (1993) 29–39. [18] F.J. Fernández, M.B. Folgueras, I. Suárez, Energy study in water loop heat pump systems for office buildings in the Iberian Peninsula, Energy Proc. 136 (2017) 91–96. [19] Zhiwei Lian, Seong-ryong Park, Henian Qi, Analysis on energy consumption of water-loop heat pump system in China, Appl. Therm. Eng. 25 (2005) 73–85. [20] Zhiwei Lian, Seong-ryong Park, Wei Huang, et al., Conception of combination of gas-engine-driven heat pump and water-loop heat pump system, Int. J. Refrig. 28 (2005) 810–819. [21] Chao Chen, Feng-ling Sun, Lei Feng, et al., Underground water-source loop heatpump air-conditioning system applied in a residential building in Beijing, Appl. Energy 82 (2005) 331–344. [22] Z.L. Ma, Y. Cao, Static state analyses on the operating energy consumption of waterloop heat pump air conditioning system, J. Harbin Inst. Technol. 6 (1997) 68–74 (in Chinese). [23] Annamaria Buonomano, Francesco Calise, Adolfo Palombo, Buildings dynamic simulation: water loop heat pump systems analysis for European climates, Appl. Energy 91 (2012) 222–234. [24] Shui Yuan, Michel Grabon, Optimizing energy consumption of a water-loop variable-speed heat pump system, Appl. Therm. Eng. 31 (2011) 894–901. [25] E. Fuentes, D.A. Waddicor, J. Salom, Improvements in the characterization of the efficiency degradation of water-to-water heat pumps under cyclic conditions, Appl. Energy 179 (2016) 778–789. [26] David A. Waddicor, Elena Fuentes, Marc Azar, et al., Partial load efficiency degradation of a water-to-water heat pump under fixed set-point control, Appl. Therm. Eng. 106 (2016) 275–285. [27] Shan H. Wang, Handbook of Air Conditioning and Refrigeration, McGraw-Hill Companies, Inc, 2001. [28] Lu. Yajun, Building Cooling and Heat Sources, China Building Industry Press, Beijing, 2009. [29] M. Chaichian, H. Perez Rojas, A Tureanu. Basic Concepts in Physics: From the Cosmos to Quarks, Springer Press, Berlin, 2014. [30] Q.D. Yu, Research for energy consumption of water-loop heat pump system based on circulating-water energy gap under nonstandard conditions, J. Refrig. 34 (2013) 35–42 (in Chinese).
4. Conclusions In the paper, we find that system operation forms a “Reverse Energy” and it originates from an energy difference produced by the converted unit to water loop and contributes to reduce the fluctuation caused by load change, so, the operation of WLHP system is a combined effect caused by load change and reverse energy and is characteristic of generality and individuality. According to the novel effect and relationship between building load and circulating water, we propose a mechanism of energy conversion to evaluate system operation and it states that the circulating water can be used as a new method to simplify the research process and determine the reverse energy of units to water loop instead of building load, in the meantime, it also reveals the energy-saving principle of system operation and reports that the energy-saving ranges of WLHP system are at load rate of 0–80.57% in summer and 0–80.22% for winter. On the other hand, we design an office building group to test the operational efficiency of WLHP and ASHP system in Tianjin and further find that the change of system load has a general impact and determines the overall trend of operation, but the effect of reverse energy can reduce the fluctuation caused by load change but also energy consumption of auxiliary power, meanwhile, the total energy consumption can further be decreased if system operation realizes an energy balance between auxiliary heat source and units. In addition, the operation of WLHP system can achieve the higher efficiency in this case and its energy-saving rates are 25.2% and12.7% in summer and winter, respectively, compared to an ASHP system. References [1] Jiping Zhang, Yangcui Ning, Chunlan Liu, et al., Monitoring and analysis of effect of project “Replacing Coal with Electricity” improving atmospheric environmental quality in Mentougou District, Beijing, J. Ecol. Rural Environ. 33 (2017) 898–906 (in Chinese). [2] Han Chen, Wenying Chen, Potential impact of shifting coal to gas and electricity for building sectors in 28 major northern cities of China, Appl. Energy 236 (2019) 1049–1061. [3] yifeng Xue, jing Yan, xiaoqiang Wei, Impact air quality of beijing city by controlling the consumption of coal-fired, Res. Environ. Sci. 27 (2014) 253–258 (in Chinese). [4] Sudi Apak, Erhan Atay, Güngör Tuncer, Renewable hydrogen energy and energy efficiency in Turkey in the 21st century, Int. J. Hydrogen Energ. 43 (2018) 14047–14058. [5] E. Toklu, Biomass energy potential and utilization in Turkey, Renew. Energ. 117 (2018) 235–244. [6] Mariana Bernardino, C. Liliana Rusu, Guedes Soares. Evaluation of the wave energy resources in the Cape Verde Islands, Renew. Energ. 101 (2017) 316–326. [7] Seyed Alireza Mostafavi, Mojtaba Mahmoudi, Modeling and fabricating a prototype of a thermoelectric generator system of heat energy recovery from hot exhaust gases and evaluating the effects of important system parameters, Appl. Therm. Eng. 132
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Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Applied research on water loop heat pump system based on a novel mechanism of energy conversion
T
Yu Qi dong School of Environmental Science and Engineering, Tianjin University, No. 92 Weijin Road, Nankai District, Tianjin 300072, PR China
H I GH L IG H T S
energy and energy-saving principle of a WLHP system. • Reverse of energy conversion and novel method on system operation. • Mechanism range of a WLHP system and its validation. • Energy-saving office building group and optimization for heat pump projects. • An • Analysis for impact factors of system operation and suggestion on energy balance.
A R T I C LE I N FO
A B S T R A C T
Keywords: Water loop heat pump Reverse energy Circulating water Office building group Air source heat pump Auxiliary heat source
Water loop heat pump (WLHP) has been widely used as a heat recovery system to realize building energy efficiency, but it still was a difficult problem to how to determine the effect of extracting and rejecting heat on water loop, because the unit converted its operation to reverse mode with load changing but the process would have a reverse effect on system operation. In this paper, a novel mechanism of energy conversion is introduced into the WLHP system to solve the problem and its view is presented as follows. First, the operation of WLHP system produces a “Reverse Energy” and it originates from an energy difference caused by the converted unit to water loop and can be used as a quantitative index to illustrate the energy-saving principle of system operation. Second, system operation is a combined effect caused by load change and reverse energy and yields to a nonlinear law due to the dynamic effect of reverse energy, so, it is characteristic of generality and individuality. Third, we can replace building load with circulating water to determine the energy-saving range of system operation and its law, and the idea that the energy carrier is used to solve the problem caused by operation will be of an important significance for a complex system. At last, the impact factors of system operation can be converted into a single function on circulating water by extracting and rejecting heat and the reverse energy is also determined by power equation of circulating water. To validate the previous conclusions and optimize heat pump systems, an office building group with different air-conditioning projects was tested in Tianjin and the results show that the change of system load determines the overall trend of operation, but the effect of reverse energy can reduce the fluctuation caused by load change and energy consumption of auxiliary power to improve operational efficiency, so, the WLHP system can achieve a higher efficiency compared to an air source heat pump (ASHP), in addition, the total energy consumption can further be reduced by building an energy balance between auxiliary heat source and units. These contributions contribute to the development of WLHP system and building energy efficiency.
1. Introduction Energy resource has become a driving force and its use slowly changed the development strategy for a country, in this regard, a new policy of “replacing coal with electricity” [1–3] has gotten a better environmental benefit in China, which stated that the use of energy
resource had a positive effect but its structure needed to continuously be improved to meet with the development of the economy, so, it would be of an important significance for human society to exploit new resources [4–6] and improve energy recovery system [7–9] in 21st century. Heat pump [10–13] belonged to an advanced energy-saving device and it could extract energy from air, water and soil to provide
E-mail address: [email protected]. https://doi.org/10.1016/j.applthermaleng.2019.03.065 Received 6 August 2018; Received in revised form 13 February 2019; Accepted 11 March 2019 Available online 12 March 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.
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Q.d. Yu
Nomenclature
qdh
P f To Tk g η Qc Qh ε µ N Ec F B qcw qd
φ φc φh qCW ICW ϑ qh qc ϑc ϑh ηc
qccw qdc
qhcw
operation power [kW] function on temperature evaporation temperature [K] condensation temperature [K] function on load rate load rate [%] cooling load [kW] heating load [kW] coefficient of performance, here 4.49 is taken heating coefficient of heat pump, here 4.79 is taken number of units reverse energy of units to water loop [kW] power function load rate corresponding to inflection point [%] operation power of circulating water [kW] operation power of circulating water at design condition [kW] operation power of circulating water in cooling mode [kW] operation power of circulating water at design condition in cooling mode [kW] operation power of circulating water in heating mode [kW]
ηh tw Tw E P Pm N pi ti
operation power of circulating water at design condition in heating mode [kW] coefficient of energy conversion coefficient of energy conversion in cooling mode coefficient of energy conversion in heating mode operation power of circulating water at partial load [kW] evaluation number of operation power number of reverse energy rejected energy of circulating water [kW] stored energy of circulating water [kW] number of reverse energy in cooling mode number of reverse energy in heating mode the upper limit of energy-saving range in cooling mode [%] the upper limit of energy-saving range in heating mode [%] test temperature in summer [°C] test temperature in winter [°C] relative power unit/system at partial load [kW] unit/system at full load [kW] energy consumption number operation power at partial load [kW] operation time at partial load [h]
boiler was used to heat the circulating water, but note that the method also showed up a certain limitation due to the effect of cooling and heating load on evaluation index, on the other hand, Buonomano [23] designed a new dynamic simulation method to investigate its operation in European countries and the results proved that system operation could realize the higher efficiency than other HVAC system but its economic and environmental benefit were more obvious. In addition, Yuan [24] analyzed the effect of water temperature on compressor and cooling tower and put forward a practical strategy to optimize system operation, but, Fuentes [25,26] used system load as a method to reveal the operation law of heat pump system, which provided some new clues for its research. We believed that the previous investigations has had a positive impact on the development of WLHP system, but note that the building load was still used as a general method to evaluate system operation. According to the operation principle of WLHP system, we found that the attribute of building load was continuously changed for different rooms and the unit also converted its operation to reverse mode, in other words, the original heating load of a room was converted into cooling load with system operating and the process made the overall trend of energy consumption to be changed by extracting and rejecting heat
heating or cooling for the HVAC system to realize building energy efficiency, so, its application technology has become the frontier of engineering science. Water loop heat pump (WLHP) [14] was a special kind of water source heat pump and it used a closed loop to link all units to simultaneously provide heating and cooling for a building. In the system, the circulating water was the energy carrier of operation and key link between building and units and its temperature was in the range of 16–32 °C according to the previous investigation [15]. Note that an electric boiler should be started to heat the water loop when the water temperature was to drop to 16 °C, similarly, a cooling tower was also used to remove the unwanted heat at the upper limit of temperature, and the schematic diagram of a WLHP system was shown in Fig. 1. At present, the issues on WLHP system focused on its service conditions and research method. Zaidi [16,17] selected four kinds of WLHP, variable-air-volume (VAV), four-pipe and reheat HVAC system to discuss their operational efficiency in various climatic conditions and building types, and the results showed that the energy efficiency of WLHP system was highest in these projects but note that the building structure and system load had a great impact on overall operation, which has had a very positive effect on the development of WLHP system in USA. Fernández [18] studied the operation of the WLHP system in Iberian Peninsula and calculated system load by using energy simulation software, and he emphasized that the potential of WLHP system was to recover and redistribute the afterheat to realize the energy balance of a building compared to a four-pipe HVAC system. In China, Li [15] used the geothermal energy as a heat input to improve system efficiency, but Lian [19,20] analyzed different effects of electric and coal-fired boiler on system operation to determine its energy-saving range and service areas, in addition, he also designed an integrated system composed of gas engine heat pump (GHP) and WLHP to further reduce total energy consumption. As for building types, Chao [21] discussed the energy efficiency of WLHP system in a residential building and gave its control strategy based on the analysis for the energy consumption of unit and auxiliary device. As far as the research methods were concerned, Ma [22] proposed a static method of primary energy to evaluate system operation and reported its energy-saving range, meanwhile, he found that the auxiliary heat source played an important role for system efficiency, especially, when the coal-fired
Fig. 1. Schematic diagram of WLHP system. 576
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Q.d. Yu
“Reverse Energy”. The reverse energy is a special kind of energy and it originates from an energy difference produced by the converted unit to water loop and is of a dynamic attribute, and note that the “Reverse” reveals that its effect is opposite to system load for the following reasons. First, there is a close relationship between system load and water temperature in WLHP system, namely, the temperature continuously rises with load increasing in summer but it rapidly drops in winter, and note that system power will obviously be increased in the process, so, the load change has a negative effect on system operation, especially, in winter. Second, a part of the units convert their operation mode after system operation is at partial load and note that the process reduces the previous effect of units on water loop but also makes up for the energy loss of circulating water to some extent, which reveals the process of forming a reverse energy, therefore, the effect of reverse energy can reduce the energy consumption of auxiliary power and it should be used as a quantitative index to illustrate the energy-saving principle of system operation. At last, fewer and fewer units convert their operation to reverse mode with system load increasing or even disappear, so, the effect of reverse energy is in a dynamic process, which explains why system operation can be converted from providing heating and cooling simultaneously to only heating or cooling. As stated above, the operation of the WLHP system should be a combined effect caused by load change and reverse energy but note that it deviates from original law and forms an inflection point due to the dynamic effect of reverse energy, and its law is described in Eq. (2).
from and to water loop, so, the conventional approach has shown up the limitation due to an instability caused by building load. Based on this, we felt that its research could be further improved if the following problems were solved. How to determine its effect on the water loop when a unit converted into the opposite operation mode? How to explore a new method to solve the limitation caused by building load? How to convert the impact factors of system operation into a function on single variable? How to validate its potential compared to other heat pump system? In this paper, a new mechanism of energy conversion is proposed to reveal the energy-saving principle of WLHP system and determine the effect of the converted units on system operation, on the other hand, an office building group with different air-conditioning projects is also designed to validate its energy-saving range and optimize heat pump system in Tianjin. These findings can provide a certain theoretical references and data support for further research. 2. Mechanism of energy conversion 2.1. Reverse energy According to the principle of refrigeration [27], the change of system load has a great impact on evaporation and condensation temperature of an air source heat pump (ASHP) and directly affects its COP, meanwhile, we also find that its operation still yields to an approximately linear law because the ASHP system usually only provides heating or cooling for a building, which is in accordance with Lu’s report [28], so, the operation law of ASHP system can be written in Eq. (1).
(
(2)
where we should notice the following problems. First, the formula is built to describe system operation in winter, and the heat extracted by
P = f (To, Tk ) = g (η) ⎧ dP η ∈ [0%, 100%] ⎨ dη ≈ const , ⎩
(
(1)
all units is NQh 1 −
Eq. (1) shows that the load rate plays a key role for an ASHP system but system operation also yields to an approximately linear law due to the dominant effect of load rate. Note that the conclusion will be changed for a WLHP system because, generally, the system simultaneously provides cooling and heating for a building but also the unit continuously converts its operation to reverse mode. In cooling mode, 1 the units reject the heat of Qc 1 + ε to water loop and the process is that the circulating water stores its energy based on the rising tem-
(
)
1 ⎧ ⎪ P = ηNQh 1 − μ − Ec = F (η , Ec ) ⎨ ∂P ≠ const , η ∈ (0%, B%) ∪ [B%, 100%) ⎪ ∂η ⎩
(
ηNqh 1 −
(
1 μ
)
at full load but it can be reduced to
) after a part of the units operate the cooling mode with
load decreasing. Second, the auxiliary heat source is used to make up for energy loss of circulating water but note that the supply heat should further be reduced because the effect of reverse energy can make the temperature of circulating water to rise to some extent. Third, the change of system power should yield to a nonlinear law but its inflection point reveals a conversion process from energy saving to high energy. As mentioned above, a novel mechanism of energy conversion is introduced to evaluate system operation and it includes the following several points. First, we can use a piece of energy chain to describe the operation of WLHP system, and it is composed of building, circulating water and units and note that the circulating water is the energy carrier of operation and key link between building and units, and its structure
)
perature; similarly, they extract the heat of Qh 1 −
1 μ
1 μ
) from the loop in
heating mode and the circulating water also rejects its energy in the process, so, the extracting and rejecting heat have a reverse effect on water loop. Based on this, a new energy effect can be introduced into the WLHP system to illustrate its energy-saving principle after a unit converts its operation to reverse mode, and the effect is defined as
Fig. 2. Structural diagram of energy chain. 577
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Q.d. Yu
is shown in Fig. 2. Second, the circulating water can be used as a new method to determine the energy-saving range and operation law instead of building load, so, the impact factors of system operation can converted into a single function on circulating water to simplify the research process. Third, the unit converts its operation to reverse mode and the process produces a “Reverse energy” but the effect can be determined by the power change of circulating water. As shown in Fig. 2, we also find that the new method has a very positive effect for the development of WLHP system because it has solved the instability caused by the change of building load, in addition, the idea that the energy carrier is used to solve the operation problem will be of an important significance for a complex system.
1
1− ⎧ μ ⎪ φc = 1 + 1 ⎪ ε 1 ⎨ 1+ ε ⎪ φh = 1 1− ⎪ μ ⎩
According to the mechanism, there is a close relationship between building load and circulating water and it shows two aspects of load rate and reverse energy. On the one hand, we find the double attribute of load rate based on the different requirements for heating and cooling and they are quality and quantity, respectively, where, the “quality” refers to the attribute of building load but “quantity” stands for its size. On the other hand, when a room demands the cooling instead of original heating, the unit converts its operation mode and produces the reverse energy by extracting and rejecting heat. Based on this, the power equation of circulating water is expressed in Eq. (3).
⎧ Δε < ε ⎨ ⎩ Δμ < μ
1 ε ⎨ 1 ≈ 1 μ ⎩ μ ∓ Δμ
⎧ ε ∓ Δε ≈ (3)
1
(
Ec = ΔQh 1 −
)
1 μ 1 1+ ε
qdc , but note that the units in cooling
ηqdc − φc (1 − η) qdc = qdc (1.647η − 0.647) qCW = ⎧ ⎨ ηqdh − φh (1 − η) qdh = qdh (2.545η − 1.545) ⎩
qdc , η = 100%
⎧ ⎪ 1 1− ⎨ ηqdc − (1 − η) 1μ qdc , 0% < η < 100% 1+ ⎪ ε ⎩
(10) (4)
1−
(
ΔQc 1 +
1 ε
) = (1 −
1 η) 1ε 1− μ
μ
qdh , respectively. By the Eq. (3), the power
equation of circulating water is written as follows.
qhcw
qdh , η = 100% ⎧ ⎪ 1 1+ = ⎨ ηqdh − (1 − η) 1ε qdh , 0% < η < 100% 1− ⎪ μ ⎩
(0% < η < 100%)
Eq. (10) shows that the power change of circulating water is a function on η and φ at partial load because its design value can be calculated by the cooling and heating load of a building, and note that the load rate has a general impact on system operation and it can reflect the generality of heat pump systems, but the coefficient of energy conversion shows the effect of operation mode on system efficiency and it reveals the individuality of the WLHP system. In addition, we also find that the effect of reverse energy is continuously decreased when a unit converts its operation from cooling to heating mode, whereas, its effect will be increased, which explains why more energy is consumed to maintain the water temperature in winter. To get more clues, we use theqdc andqdh as the base lines to analyze the relationship between load rate and power of circulating water, and they are shown in Eq. (11).
Similarly, a part of the units convert their operation mode when the WLHP system is at partial load, and the power change of circulating water is (1 − η) qdh , but note that the process makes a part of heating load to be converted into the equal cooling load and corresponding (1 − η) qdh cooling load and reverse energy are ΔQc = and 1 1+
(9)
Introducing the Eq. (9) into power equations of circulating water, they can be rewritten as follows.
mode still reject the heat to water loop and its size is ηqdc . According to the Eq. (3), the power equation of circulating water is described in Eq. (4).
qccw =
(8)
1
1− ⎧ μ ⎪ φc = 1 + 1 = 0.647 ⎪ ε 1 ⎨ 1+ ε ⎪ φh = 1 = 1.545 1− ⎪ μ ⎩
)
1−
= (1 − η)
(0% < η < 100%)
Eq. (8) states that the change of COP has less impact on its inversion value at partial load, so, we can use the design value of COP to evaluate the effect of conversion process on system operation instead of other values and the coefficient is also rewritten as follows.
to the relationship of Δq = ΔQc 1 + ε , the process converts the part of cooling load into equal heating load and its size is (1 − η) qdc ΔQh = ΔQc = 1 , and the reverse energy produced by these units is ε
(7)
According to Eqs. (6) and (7), we can build an approximate equivalence between COP and its change value as follows.
In order to determine Ec in Eq. (3), we use the power change of circulating water to derive corresponding building load, which is a back-stepping method and general application for Newton's third law of motion [29]. In summer, the operation of some units is converted from cooling to heating mode with cooling load decreasing and the power change of circulating water is Δq = (1 − η) qdc in the process. According
1 μ
(0% < η < 100%)
1
qd , η = 100% qcw = ⎧ ηq − ⎨ ⎩ d Ec , 0% < η < 100%
1+
(6)
Eq. (6) shows that the coefficient reflects a conversion process from cooling to heating (or vice versa), in fact, it is a function on performance coefficient of heat pump. Note that the COP of unit can be considered as a constant value to discuss the effect of conversion process on system operation for the following reasons. First, the use of auxiliary heat source can maintain the normal temperature of circulating water at partial load, so, the change of COP is not obvious when the water temperature is in the range of 16–32 °C according to previous investigation [15]. Second, a part of the units convert their operation to reverse mode at partial load and the process produces the “Reverse Energy”, therefore, the change of COP can be further reduced due to the positive effect of “Reverse Energy” on circulating water. Third, according to Eq. (6) and previous discussions, the relationship between COP of unit and its change value can be described in Eq. (7).
2.2. Modeling and motion law
(
1 1 →φ=f( , ) ε μ
η − φc (1 − η) = 1.647η − 0.647 ICW = ⎧ ⎨ ⎩ η − φh (1 − η) = 2.545η − 1.545
(5)
In this paper, we introduce a coefficient of energy conversion (φ ) to describe the process of operation conversion, and its definition is written as follows.
(11)
According to Eq. (11), the power change of circulating water can be further described in Fig. 3. As shown in Fig. 3, we get the following conclusions. First, its change yields to an approximately linear law but 578
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of WLHP system can realize a higher efficiency in summer whenϑc is in the range of 0.135–0.607, where, 0.607 corresponds to load rate of 0% but its upper limit can be determined as follows.
0.647(1 − η) = 0.135 η + 0.647(1 − η)
ηc = 80.57%
→
(14)
So, the energy-saving range of system operation is at load rate of 0–80.57% in summer but other load rate is the range of high consumption. Similarly, its range also can be calculated in winter ifϑh is in the range of 0.607–0.724, and we should notice that 0.607 still corresponds to load rate of 0% but its upper limit is built in Eq. (15). Fig. 3. Power change of circulating water with load changing.
η = 0.724 η + 1.546(1 − η)
note that system load has a great impact on circulating water in winter than summer. Second, system operation can reach a dynamic balance at load rate of 40% in summer but the balance point occurs at load rate of 60% for winter, so, the energy efficiency of system operation is higher in cooling mode than heating. As mentioned above, the operation law of the WLHP system is reported as follows. First, system operation is a combined effect caused by load change and reverse energy and yields to a nonlinear law and its inflection point shows the conversion process from energy saving to high energy. Second, the effect of reverse energy can reduce the fluctuation caused by load change and energy consumption of auxiliary power, so, it should be used as an energy-saving index to evaluate overall operation. Third, the heating load has a great impact on system operation, so, it is of an important significant for the development of WLHP system to how to reduce system consumption in winter.
3. Test and discussion 3.1. Project overview The project is located in Tianjin and it is an office building group, and its layout is shown in Fig. 4. In group, the area of each building is about 1000 m2 and design temperatures are 21 °C and 27 °C in winter and summer, and its heating and cooling load are 70 kW and 105.5 kW, respectively. We design three sets of WLHP system to provide heating and cooling for 1#—3# building, and 23 sets of SHR series unitary units are used in each system and their total heating and cooling capacity are 105.5 kW and 120.8 kW and corresponding input powers are 25.2 kW and 23.5 kW, in addition, an electric boiler is installed to heat the circulating water in winter and its capacity is 35 kW. On the other hand, we uses a screw chiller to provide cooling for 4# building in summer and its capacity and input power are 110.5 kW and 18.98 kW, respectively, but the heating is provided by municipal pipeline network based on local condition and building layout, meanwhile, 24 sets of fan coil units (FC) is installed as terminal device and total power is 2.72 kW. Based on the effect of a policy of “replacing coal with electricity”, we design a set of ASHP system to provide heating and cooling for 4# building instead of original HVAC system. 2 sets of ASHP and an electric boiler with 35 kW are operated in new project and the heating and cooling capacity of each unit are 67 kW and 63 kW and corresponding input powers are 23.75 kW and 23 kW, respectively, and note that the original FC is still used as system terminal device.
According to the close relationship between building load and circulating water, we use the circulating water to determine the energysaving range of system operation instead of building load, but the process should notice the following several points. First, the focus of research should be placed at heating mode, in the meantime, we should notice the positive effect of reverse energy on system operation. Second, the rejected and stored energy of circulating water correspond to heating and cooling load, respectively in new method, and note that the rejected energy originates from the reverse energy of the units to water loop in summer, similarly, the effect of reverse energy also forms the stored energy of circulating water in winter. Third, a number of reverse energy (ϑ) is defined as a new index to determine the energy-saving range and it is a ratio of rejected energy to total energy change of circulating water and its definition is expressed as follows.
qh qh = qh + qc qh + Ec
(12)
Introducing Eqs. (4) and (5) into above formula, the coefficient can be rewritten as follows.
⎧ ⎪ ⎪ϑ = ⎪ c ⎪ ⎨ ⎪ϑ = ⎪ h ⎪ ⎪ ⎩
1 μ (1 − η) qdc 1 1+ ε
(15)
Based on this, the energy-saving range of WLHP system is at load rate of 0–80.22% in winter, and note that the effect of “Reverse Energy” will disappear because all units extract the heat from water loop if the load rate is increased to 80.22–100%, and the process makes the water temperature to rapidly drop and leads to consume the more auxiliary energy, so, we must cut down its lasting time to improve overall efficiency.
2.3. Discussion on energy-saving range
ϑ=
ηh = 80.22%
→
Heating Pipeline
1−
1 μ ηqdc + (1 − η) qdc 1 1+ ε 1−
ηqdh 1 ε 1 1− μ 1+
ηqdh + (1 − η) qdh
=
CAC system before retrofit
0.647(1 − η) η + 0.647(1 − η)
4#
40.32m
ASHP system after retrofit =
η η + 1.545(1 − η)
12.48m 21.76m
(13)
Eq. (13) states that the evaluation index is a single function on load rate, so, we can easily determine system operation by calculating its load rate, which solves the limitation of static method of primary energy, in the meantime, the result also has a very positive impact on an engineering project. According to the previous investigation [22] and relationship between building load and circulating water, the operation
12.48m
WLHP system 40.52m
1#
WLHP system 9.75m
40.78m
2#
WLHP system 11.15m
Fig. 4. Layout of office building group. 579
40.56m
3#
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energy on system operation and conversion process from energy saving to high consumption. On the other hand, we also find that the energy efficiency of WLHP system is higher than that of an ASHP system at load rate of 20–80%, so, system load has a general impact and determines the change trend of operation, but the effect of reverse energy can make the operation of WLHP system to further smooth. In addition, the use of auxiliary heat source usually has a great impact on total energy consumption when the heat pump systems are used in northern China, and its effects on heat pump systems are presented in Figs. 7 and 8. As shown in Fig. 7, the degree of power change is smaller at load rate of 20–60% in summer than winter for an ASHP system, because the more heat is used to prevent from frosting caused by high humidity at the beginning of heating. With outdoor temperature dropping, the supply heat is continuously increased to maintain system operation and avoid frosting caused by low temperature, but note that the use of auxiliary power doesn’t change its original law, which again validates previous conclusion. Fig. 8 shows that the power change of WLHP system makes a great difference in different load range for the following reasons. Firstly, the degree of power change is greater at load rate of 20–60% in winter than summer, because the use of auxiliary heat source can improve the COP of a unit by heating circulating water but the process also increase system power, and note that its effect is more obvious on system than unit in this case, so, it has become a hot topic to how to build an energy balance between auxiliary power and units. Secondly, the auxiliary heat source has a very negative effect on system efficiency when the load rate is increased to 80–100%, and note that the key point occurs at load rate of 80%, so, the operation of WLHP system will lose its advantage over a conventional two-pipe HVAC system at load rate of 80–100%, which also validates its energy-saving range in winter. Using the Eq. (18), the changes of energy consumption are described in Figs. 9 and 10. Fig. 9 shows that their energy consumptions yield to a similar change law in two systems but there are also some differences between systems. First, the change of system load has a smaller impact on the COP of a unit at the beginning of cooling and the range is at load rate of 20–40% but note that its lasting time is less than other ranges, so, the difference of energy consumption isn’t obvious between systems. Second, the power change of WLHP system is lower than that of ASHP system due to the positive effect of reverse energy on system operation when the load rate is increased to 60% from 40%, and note that its lasting time is longer at load rate 40–80%, so, the difference of energy consumption will become more obvious, especially, at load rate 40–60%. Third, the energy consumption of WLHP system is rapidly increased without reverse energy when the load rate is increased to 80–100%, but note that its lasting time is shorter in the range, so, their differences can be reduced to some extent. In Fig. 10, we see that the difference of energy consumption has obviously been enlarged at load rate of 40–80% due to the use of auxiliary heat source. On the one hand, the effect of reverse energy can reduce the energy consumption of auxiliary power to improve operational efficiency in the range, on the other hand, the use of auxiliary heat source has a great impact on system operation compared to other devices, so, the difference of energy consumption will be enlarged with operating time increasing. As mentioned above, we can get the following conclusions. First, the
3.2. Test and method In this case, a part-load method [30] is used to simplify the process of experiment, and note that the method divides system load into five sections of 100–80%, 80–60%, 60–40%, 40–20% and 20–0% and selects corresponding load rates of 100%, 80%, 60%, 40% and 20% as the typical condition points to test system operation. According to the product manual, the formula of testing temperature is written as follows.
⎧ tw = (0.064°C/%) × η + 27°C, insummer ⎨ ⎩Tw = (−0.315°C/%) × η + 21°C inwinter
(16)
In summer, their temperatures are t1 = 33.4 °C(100%), t2 = 32.1 °C(80%), t3 = 30.8 °C(60%), t4 = 29.6 °C(40%) and t5 = 28.3 °C(20%), respectively. Similarly, the corresponding temperatures are T1 = −10.5 °C(100%), T2 = −4.2 °C(80%), T3 = 2.1 °C(60%), T4 = 8.4 °C(40%) and T5 = 14.7 °C(20%) in winter. Using this method, the experimental data of heat pump systems are showed in Tables 1 and 2, and note that the operational efficiency of a compressor is 70% and it has been considered in the calculation. In the paper, we propose a relative power method to deal with experimental data, and note that the method uses the system power at full load as a base line to evaluate the effect of load rate on system operation and its definition is written in Eq. (17).
E=
P Pm
(17)
By Eq. (17), we can determine the change law of system operation and its inflection point at which the operation deviates from original law and energy-saving range, however, the method can’t evaluate the energy-saving potential of overall operation due to different effects of operating time on energy consumption. Based on this, we define an energy consumption number (N) as a comprehensive index to evaluate system operation and its definition is described as follows.
N=
Pi ti 5
∑i = 1 Pi ti
(18)
Note that we can know the change trend of energy consumption and its distribution by using Eq. (18), which contributes to further evaluate the effect of load rate on system operation and optimize the design project. 3.3. Result and discussion Using the relative power method, the power changes of heat pump systems are described in Figs. 5 and 6. As shown in above figures, we see that the operation of ASHP system is relatively smooth with load increasing and it yields to an approximately linear law, whether in summer or winter, which proves the previous conclusion and also points out that the change of load rate has an important impact on system operation. But the operation of WLHP system shows an obviously nonlinear characteristic due to an inflection point at load rate of 80% and the result proves the previous investigation on energy-saving range, in the meantime, it also reveals the dynamic effect of reverse Table 1 Energy consumption of heat pump systems in summer. Load rate/%
100 80 60 40 20
Unit power /kW ASHP
WLHP
32.31 24.03 18.13 13.01 9.11
16.56 9.31 7.01 4.69 2.33
Pump power/kW
2.20 2.20 2.20 2.20 2.20
Cool tower power/kW
FC power/kW
System power/kW
ASHP
WLHP
ASHP
WLHP
ASHP
WLHP
0 0 0 0 0
1.50 0.81 0.64 0.43 0.22
2.72 2.21 1.65 1.11 0.56
0 0 0 0 0
37.23 28.44 21.98 16.32 11.87
20.26 12.32 9.85 7.32 4.75
580
Time/h
75 375 600 300 150
Energy consumption/kWh ASHP
WLHP
2792 10,665 13,188 4896 1781
1520 4620 5910 2196 713
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Table 2 Energy consumption of heat pump systems in winter. Load rate/%
Unit power /kW
100 80 60 40 20
1
Pump power /kW
ASHP
WLHP
11.76 7.71 5.39 3.41 1.59
7.63 4.82 4.08 2.77 1.41
E
2.20 2.20 2.20 2.20 2.20
Auxiliary power /kW
FC power/kW
System power /kW
ASHP
WLHP
ASHP
WLHP
ASHP
WLHP
35.00 27.28 20.01 12.98 5.87
35.00 23.62 15.01 9.86 4.71
2.72 2.21 1.67 1.11 0.56
0 0 0 0 0
51.68 39.40 29.36 19.70 10.22
44.83 30.64 21.29 14.83 8.32
1 E
WLHP ASHP
0.8
0.6
0.4
0.4
0.2
0.2
20%
40%
60%
80%
0 0%
100%
ASHP WLHP
0.8
WLHP
3721 14,184 16,911 5674 1472
3228 11,030 12,263 4271 1198
40%
60%
80%
100%
Fig. 8. Power change of WLHP system.
0.4
E
ASHP
Curve of power change in summer Curve of power change in winter
20%
Fig. 5. Power change of heat pump systems in summer.
1
72 360 576 288 144
Energy consumption /kWh
0.8
0.6
0 0%
Time/h
N
0.35 0.3
ASHP WLHP
0.25
0.6
0.2 0.15
0.4
0.1
0.2 0
0.05 0
0
20%
40%
60%
80%
100%
0
20%
40%
60%
80%
100%
Fig. 9. Change of energy consumption number in summer.
Fig. 6. Power change of heat pump systems in winter.
0.6
1 E
Curve of power change in winter Curve of power change in summer
ASHP WLHP
0.5
0.8
0.4
0.6
0.3 0.2
0.4
0.1
0.2
0
0
N
0
20%
40%
60%
80%
100%
0
20%
40%
60%
80%
100%
Fig. 10. Change of energy consumption number in winter.
Fig. 7. Power change of ASHP system.
WLHP system and its operation yields to a nonlinear law due to an inflection point at load rate of 80%. Second, the use of auxiliary power has an important impact on system operation but its effect doesn’t the overall trend of operation, in addition, it can improve overall efficiency of system operation to build an energy balance between auxiliary heat
operation of ASHP system shows an approximately linear characteristic based on the dominant effect of load rate, but the combined effect caused by load change and reverse energy determines the operation of
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source and units. Third, the energy efficiency of WLHP system is higher than that of an ASHP system due to the positive effect of reverse energy on system operation in this case.
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4. Conclusions In the paper, we find that system operation forms a “Reverse Energy” and it originates from an energy difference produced by the converted unit to water loop and contributes to reduce the fluctuation caused by load change, so, the operation of WLHP system is a combined effect caused by load change and reverse energy and is characteristic of generality and individuality. According to the novel effect and relationship between building load and circulating water, we propose a mechanism of energy conversion to evaluate system operation and it states that the circulating water can be used as a new method to simplify the research process and determine the reverse energy of units to water loop instead of building load, in the meantime, it also reveals the energy-saving principle of system operation and reports that the energy-saving ranges of WLHP system are at load rate of 0–80.57% in summer and 0–80.22% for winter. On the other hand, we design an office building group to test the operational efficiency of WLHP and ASHP system in Tianjin and further find that the change of system load has a general impact and determines the overall trend of operation, but the effect of reverse energy can reduce the fluctuation caused by load change but also energy consumption of auxiliary power, meanwhile, the total energy consumption can further be decreased if system operation realizes an energy balance between auxiliary heat source and units. In addition, the operation of WLHP system can achieve the higher efficiency in this case and its energy-saving rates are 25.2% and12.7% in summer and winter, respectively, compared to an ASHP system. References [1] Jiping Zhang, Yangcui Ning, Chunlan Liu, et al., Monitoring and analysis of effect of project “Replacing Coal with Electricity” improving atmospheric environmental quality in Mentougou District, Beijing, J. Ecol. Rural Environ. 33 (2017) 898–906 (in Chinese). [2] Han Chen, Wenying Chen, Potential impact of shifting coal to gas and electricity for building sectors in 28 major northern cities of China, Appl. Energy 236 (2019) 1049–1061. [3] yifeng Xue, jing Yan, xiaoqiang Wei, Impact air quality of beijing city by controlling the consumption of coal-fired, Res. Environ. Sci. 27 (2014) 253–258 (in Chinese). [4] Sudi Apak, Erhan Atay, Güngör Tuncer, Renewable hydrogen energy and energy efficiency in Turkey in the 21st century, Int. J. Hydrogen Energ. 43 (2018) 14047–14058. [5] E. Toklu, Biomass energy potential and utilization in Turkey, Renew. Energ. 117 (2018) 235–244. [6] Mariana Bernardino, C. Liliana Rusu, Guedes Soares. Evaluation of the wave energy resources in the Cape Verde Islands, Renew. Energ. 101 (2017) 316–326. [7] Seyed Alireza Mostafavi, Mojtaba Mahmoudi, Modeling and fabricating a prototype of a thermoelectric generator system of heat energy recovery from hot exhaust gases and evaluating the effects of important system parameters, Appl. Therm. Eng. 132
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