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Energy saving potential of fresh air pre-handling system using shallow geothermal energy
Energy & Buildings 185 (2019) 39–48
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Energy saving potential of fresh air pre-handling system using shallow geothermal energy Weihua Lyu, Xianting Li∗, Baolong Wang, Wenxing Shi Department of Building Science, School of Architecture, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 11 August 2018 Revised 19 December 2018 Accepted 24 December 2018 Available online 5 January 2019 Keywords: Natural energy Ground heat exchanger Fresh air Air handling unit Building energy efficiency
a b s t r a c t To reduce the energy consumption for fresh air handling, a novel fresh air pre-handling system that fully exploits the shallow energy to precool and preheat fresh air is proposed in this study. The model of a typical all-air system with a pre-handling system is built on the TRNSYS platform. The energy saving potential of this fresh air pre-handling system is simulated based on the diverse climate of China. The results show that the ratios of the cumulated heat transfer capacity of the fresh air pre-handling system to the annual total of cooling and heating load range from 35% to 45% in the temperate zone and the subtropical zone, such as Shenyang, Beijing, and Shanghai, while the ratio in the tropical zone is approximately 10%. The energy saving rates are approximately 30% in most climate zones, but less than 5% in Guangzhou, the tropical climate zone. Moreover, the payback periods are within 4 years in the temperate and subtropical zones. However, the system is not economical in the tropical zone. All these results indicate that the fresh air pre-handling system with shallow geothermal energy is quite promising in most climate zones except in the hot summer and warm winter region with a tropical climate. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Currently, indoor air quality problems, such as volatile organic compounds (VOCs), smoke, odors, dust, and bacteria have led to a high demand in fresh air flow rates [1,2]. In fact, the energy consumed by fresh air handling accounts for approximately 30% of the total energy consumption of air conditioning [3]. The energy conservation of fresh air handling is critical in HVAC systems. Traditional exhaust heat recovery ventilators are used to reduce the energy consumption of fresh air handling if the exhaust air system is available in buildings [4–7]. Actually, the energy saving effect of exhaust heat recovery technique is remarkable only in certain situations [8–10]. Moreover, if the climate is suitable, e.g. in the dry and hot climate regions, the evaporative cooling technique is always used to produce cooling fresh air for buildings [11–16]. However, the performance of evaporative cooling is limited by the climate conditions predominantly. Little cooling capacity can be gained in humid regions. Another drawback of evaporative cooling is that it is not applicable for space heating, which further limits the application in cold regions. Generally, the earth-to-air heat exchanger system is applied to cool fresh air in summer, and heat it in winter efficiently. The earth-to-air heat exchanger system directly utilizes shallow
∗
Corresponding author. E-mail address: [email protected] (X. Li).
https://doi.org/10.1016/j.enbuild.2018.12.037 0378-7788/© 2019 Elsevier B.V. All rights reserved.
geothermal energy [17]. An underground tunnel is a typical earthto-air heat exchanger system [18]. Many cities have underground tunnels that offer the opportunity to fully utilize them to supply conditioned fresh air for the surrounding buildings [19]. Another type of earth-to-air heat exchanger system is the tubes buried in the ground as a heat exchanger [20]. Shallow geothermal energy can be transferred to the fresh air through the tubes. Utilizing the temperature difference, the fresh air can be efficiently cooled in the summer and heated in the winter [21,22]. However, the earthto-air heat exchanger system requires a high initial investment because of the complex tunnels and air tubes layout. Even worse, when the fresh air humidity is high, condensation would occur on the tunnel or the tubes inner surface, thus leading to molds growth. It is difficult to prevent and kill the molds as the heat transfer process is throughout the whole tunnel. Consequently, the fresh air would have been polluted before being supplied into the building. Meanwhile, the ground source heat pump (GSHP) has been widely used. As an important component of the GSHP, the ground heat exchanger (GHX) is always integrated with a heat pump to generate high-grade chilled water or hot water for heating and air conditioning [23,24]. In fact, for most of the cooling and heating season, the outlet water temperature of the GHX can be used directly for fresh air treatment. For example, in the typical hot summer and cold winter regions in China, the undisturbed underground soil temperature is approximately 15 °C, and the outlet
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Nomenclature Ca Cw EBC EBH EERBC EERBH ENC ENH Fmax Fmin HL hN ma mw Q QC1 QC2 Q_CL QH Q_HL tai tao tN tW WF WP Y
Specific heat capacity of air, kJ/(kg· °C) Specific heat capacity of water, kJ/(kg· °C) Energy consumption of the baseline system in the cooling season, kW·h Energy consumption of the baseline system in the heating season, kW·h Cooling energy efficiency ratio of the baseline system Heating energy efficiency ratio of the baseline system Energy consumption of the proposed system in the cooling season, kW·h Energy consumption of the proposed system in the heating season, kW·h Maximum fresh air volume, m3 /h Minimum fresh air volume, m3 /h Water circulating resistance Specific enthalpy of the return air, kJ/kg Air flow rate, kg/s Water flow rate, kg/s Heat exchange capacity of the AWHX, kW Precooling capacity of the AWHX in the minimum fresh air mode, kW Precooling capacity of the AWHX in the maximum fresh air mode, kW Fresh air cooling load, kW Preheating capacity of the AWHX, kW Fresh air heating load, kW Fresh air inlet temperature of the AWHX, °C Fresh air outlet temperature of the AWHX, °C Indoor design temperature, °C Outdoor temperature, °C Additional fan power, W Pump power Static payback period, year
Greek symbol ηw Pump efficiency ηf Fan efficiency λ Ratio of the annual total cumulated heat transfer capacity to the total cumulated fresh air load ε Heat transfer efficiency of the AWHX ξ Energy saving rate I Additional initial investment, CNY C Annual operation cost savings, CNY P Fan pressure drop caused by the heat exchanger coil of the AWHX, Pa Abbreviation AHU Air handling unit AWHX Air-water heat exchanger COP Coefficient of performance GHX Ground heat exchanger GSHP Ground source heat pump NTU The number of heat transfer units of the AWHX
water temperature of GHX is around 10 °C in the winter and 20 °C in the summer. Therefore, for the majority of the time, an adequate temperature difference will occur between the outlet water temperature and the outdoor fresh air temperature, both in the winter and summer, as the outdoor fresh air temperature is always below 0 °C in the winter and above 30 °C in the summer. Hence, the fresh air load can be efficiently handled by grading, and the low-
Fig. 1. Schematic of the proposed fresh air handling system.
grade fresh air load would be eliminated with the natural shallow geothermal energy. To develop an efficient fresh air handling method in more zones of different climates, and eliminate the drawbacks of fresh air handling with the earth-to-air heat exchanger system, a novel fresh air pre-handling system that use shallow geothermal energy directly for fresh air precooling or preheating is proposed in this study. The models of a typical all-air system with a fresh air pre-handling system is built on the TRNSYS platform [25]. The energy saving potential of this fresh air pre-handling system is simulated in diverse climate regions of China. It is hoped to fully exploit the low-grade natural energy to handle the low-grade fresh air load and reduce the demand of high-grade energy.
2. Principle of fresh air pre-handling system The novel fresh air pre-handling system uses a GHX to extract geothermal energy from the ground, and subsequently transfers the energy to the fresh air by utilizing an air-water heat exchanger (AWHX). The schematic of this system is illustrated in Fig. 1. In the cooling season, the cool water generated by the GHX is pumped to the AWHX. The high-temperature fresh air entering the air handing unit (AHU) will be cooled by the cool water when it flows through the AWHX. Subsequently, the temperature of the cool water rises, returns to the GHX, and rejects the heat to the ground. Hence, the fresh air can be precooled efficiently. Compared with the earth-toair system, the condensation would occur on the coil surface of AWHX instead of the whole inner surface of tunnel. Therefore, the method to deal with molds in traditional air handling unit can be used in the proposed system. Similarly, in the heating season, for the majority of the time, as the underground soil temperature is much higher than the cold fresh air temperature, especially in cold regions, the cold fresh air will be preheated similarly as that in the cooling season. Therefore, this novel fresh air pre-handling system can fully utilize the lowgrade shallow geothermal energy to handle the low-grade fresh air load [26]. Thus, the demand for high-grade energy would be reduced and substantial energy savings can be obtained.
3. Simulation method and evaluation index This study aims to propose an alternative for fresh air prehandling and investigate the energy saving potential of the fresh air pre-handling system in diverse climate zones. Therefore, the research is conducted based on a typical all-air system with a fresh air pre-handling system in a building. As a comparison, a same scale conventional all-air system with a traditional fresh air handling system is utilized as the baseline. Only the fresh air load as well as the energy consumption of fresh air handling are compared between the proposed system and the baseline system. The simulation method as well as the evaluation index are described in this section.
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Table 1 Climates of different cities in China. City
Shenyang
Beijing
Shanghai
Guangzhou
Climate region Global climate zone Cooling season Heating season
Sever cold Mid-temperate Jul. 1–Aug. 31 Nov. 1–Mar. 31
Cold Warm-temperate Jun. 11–Aug. 31 Nov. 15–Mar. 15
Hot summer and cold winter Subtropical Jun. 11–Sep. 10 Dec. 1–Feb. 28
Hot summer and warm winter Tropical May. 11–Oct. 20 No heating demand
Table 2 The primary specific parameters of GHX.
Fig. 2. Temperature distribution frequency of outdoor air.
3.1. Descriptions of the typical all-air system The air volume of this typical all-air system is 10,0 0 0 m3 /h. We assumed that the minimum fresh air ratio to meet the indoor health requirements is 20%. The air conditioning system serves an office building, and the occupied time is set as 8:00 am to 18:00 pm in the daytime. In the cooling season, to fully utilize the free cooling energy of fresh air, the all-air system would operate in the full fresh air mode when the enthalpy of outdoor air is lower than that of the return air. Similarly, when the enthalpy of outdoor air is higher than that of the return air, the all-air system would operate in the minimum fresh air mode. In the heating season, the all-air system would also operate in the minimum fresh air mode to reduce the heating energy consumption. The baseline conventional all-air system is the same with the typical all-air system except the fresh air pre-handling system. China has different climate zones, mainly including the severe cold climate region, cold climate region, hot summer and cold winter climate region, as well as the hot summer and warm winter climate region. A typical city is selected as the representative of each climate region in this study. Shenyang is a representative of severe cold climate region, and it has a short cooling season and a long heating season; Beijing is in the cold climate region, and it is cold in winter and hot in summer; Shanghai belongs to the hot summer and cold winter climate region, and it is cold in winter and very hot in summer; Guangzhou is located in the hot summer and warm winter climate region, and it has a hot climate and little heating demand exists. The climates of these four cities in China are shown in Table 1. In terms of the global climate zones, Shenyang is located in the mid-temperate zone; Beijing is located in the warm-temperate zone; Shanghai has a subtropical climate, while the climate in Guangzhou is similar to that of tropical zones. The temperature distribution frequency of the outdoor air in different cities is shown in Fig. 2. 3.2. Simulation models To investigate the energy saving potential of the fresh air prehandling system in diverse climate zones, the fresh air treatment process and energy simulation of a typical all-air system with the
Soil thermal conductivity
1.4 W/m·K
Soil thermal diffusivity Soil density Soil specific heat capacity Radius of U–tube pipe
0.00143 m2 /h 1800 kg/m3 1.61 kJ/(kg·K) 16 mm
fresh air pre-handling system are conducted using the dynamic simulation software, TRNSYS. The type 557a, a model of Duct Ground Heat Storage in TRNSYS is used to simulate the ground heat exchanger. The accuracy of this dynamic model has been validated in a detailed study. The primary specific parameters of the GHX assumed in this simulation are listed in Table 2. According to reference [27], the undisturbed average soil temperature is about 2 °C higher than the local annual average temperature. The average air temperature and soil temperature of the selected four cities are listed in Table 3. Considering the low soil temperature and low fresh air temperature in the winter in Shenyang, the antifreezing ethylene glycol solution is used to replace water, and circulate between the GHX and AWHX. The specific heat capacity of the antifreezing solution is 3.85 kJ/(kgK). Water is used in other cities. It should be noted that the antifreezing ethylene glycol solution is not distinguished from water in the following statement. Water is used to represent of both water and antifreezing ethylene glycol solution for statement convenience. The AWHX is a cross flow air-water heat exchanger and it uses the ε –NTU model to simulate the heat transfer process. The definition of ε and NTU are as follows:
C · m C ·m w w a a ε = 1 − exp NT U 0.22 exp − NT U 0.78 − 1 Ca · ma
Cw · mw
(1) NT U =
kA Ca · ma
(2)
where ɛ is the heat transfer efficiency of the AWHX; NTU is the number of heat transfer units of the AWHX; Ca is the specific heat capacity of air, kJ/(kg• °C); ma is the air flow rate, kg/s; Cw is the specific heat capacity of water, kJ/(kg• °C); mw is the water flow rate, kg/s. The air volume of the fresh air pre-handling system is designed based on the minimum fresh air volume, which is set as 20 0 0 m3 /h. The design air temperature difference is set as 10 °C. According to the pre-handling demand, the design parameters are shown in Table 4. As the average heat transfer rate of the GHX is set as 30–50 W per linear meter of buried pipe, approximately two boreholes of depth 100 m of a single U–tube pipe are required for the 20 0 0 m3 /h fresh air treatment. To conduct the simulation, the fresh air pre-handling system model is built on the TRNSYS platform. The circulating water driven by a pump flowed between the AWHX and the GHX. The heat transfer process of the GHX is integrated with that of the AWHX on the simulation platform. This study hypothesizes that the water temperature increase caused by the motor heat loss of water pump can be neglected. In addition, because of the small
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W. Lyu, X. Li and B. Wang et al. / Energy & Buildings 185 (2019) 39–48 Table 3 The annual average air temperature and soil temperature in different cities. City
Shenyang
Beijing
Shanghai
Chengdu
Guangzhou
Annual average temperature (°C) Soil initial temperature (°C)
8.56 10.56
12.65 14.65
16.69 18.69
16.62 18.62
22.23 24.23
Table 4 The design parameters of AWHX. Rated air flow rate
20 0 0 m3 /h
Rated water flow rate Flow resistance in water side Fan pressure drop Heat transfer area Heat transfer rate
1.92 m3 /h 5 m H2 O 180 Pa 50 m2 kA = 2240 W/K
tai , tao are the fresh air inlet temperature and outlet temperature of the AWHX, respectively, °C. When the enthalpy of outdoor air is lower than that of the return air in the cooling season, the all-air system would run in the full fresh air mode. Fresh air cooling load does not exist in this scenario. The hourly precooling capacity QC2 of the AWHX is expressed as formula (5), which can be used to eliminate the indoor cooling load.
Table 5 Operation strategies of the fresh air pre-handling system. No. of strategy
Cooling season
Heating season
Strategy Strategy Strategy Strategy
Without control ta −tsoil > = 3 °C ta −tsoil > = 5 °C ta −tsoil > = 8 °C
Without control tsoil −ta > = 3 °C tsoil −ta > = 5 °C tsoil −ta > = 8 °C
1 2 3 4
QC2 = Ca ·
Fmax · ρ · (tai − tao ) 3600
(5)
where Fmax is the maximum fresh air volume in the full fresh air mode, m3 /h. In the winter, the all-air system would also operate in the minimum fresh air mode to reduce the heating load of fresh air. The hourly fresh air heating load Q_HL and the hourly preheating capacity QH of the AWHX are respectively expressed as follows:
Fmin · ρ · (tN − tW ) 3600
(6)
Fmin · ρ · (tao − tai ) 3600
(7)
temperature difference between the outlet water of GHX and the ambient, the pipe insulation, and the limit pipe length from GHX to AWHX, the heat losses along the pipe have little effect on the proposed system and they are neglected in the following calculation.
QHL = Ca ·
3.3. Simulation methods
where tN is the indoor design temperature in the winter, which is set as 20 °C; tW is the outdoor temperature, °C. Next, the energy consumption of the fresh air system with different operation strategies is calculated. The energy consumption includes the energy consumption of the water pump, the additional fan power of the novel fresh air pre-handling system, and the further conventional dehumidification and cooling of fresh air. The power of the pump, Wp , is calculated as:
Four operation strategies are set to control the operation of the fresh air pre-handling system, i.e. to control the operation of the water pump of the fresh air pre-handling system. Strategy 1 means that the water pump always operates during the occupied period without control. Strategy 2 ∼ strategy 4 means that the water pump starts when the air temperature (ta ) is higher than the soil temperature (tsoil ) in the cooling season, and lower in the heating season. As shown in Table 5, the absolute temperature difference of strategy 2, strategy 3, and strategy 4 are set as 3 °C, 5 °C, and 8 °C, respectively. By the way, for many applications in practice, the control system may not work for a variety of reasons. Therefore, the strategy 1 is set to see whether there is any energy saving effect in the whole year even if there is no adjustment of the control system. First, the cooling and heating fresh air loads in different climate conditions are calculated. Then the cooling and heating capacities of the fresh air pre-handling system are obtained through the simulations. In the summer, the indoor design point is set as 26 °C, 60%, which is assumed as the same with the return air parameter. When the enthalpy of outdoor air is higher than that of the return air, the all-air system would operate in the minimum fresh air mode. The hourly fresh air cooling load Q_CL can be calculated using formula (3). The hourly precooling capacity QC1 of the AWHX is expressed as formula (4).
Q_CL
Fmin · ρ = · (hW − hN ) 3600
QC1 = Ca ·
Fmin · ρ · (tai − tao ) 3600
(3) (4)
where Fmin is the minimum fresh air volume, m3 /h; ρ is the fresh air density, assumed as 1.2 kg/m3 ; hW is the specific enthalpy of fresh air, kJ/kg; hN is the specific enthalpy of the return air, kJ/kg;
QH = Ca ·
WP =
mw · g · HL
(8)
ηw
where ηw is the pump efficiency, assumed to be 0.65; HL is the water circulating resistance, assumed to be a 15–m water column. The additional fan power, WF , caused by the heat exchange coil of the AWHX is calculated as:
WF =
F · P 3600η f
(9)
where F is the fresh air volume, m3 /h; P is the fan pressure decrease caused by the heat exchanger coil of the AWHX, assumed as 180 Pa; ηf is the fan efficiency, assumed to be 0.65. The energy consumption of the proposed fresh air handling system in the cooling season, ENC , is calculated as: cooling,e
ENC =
(QCL − QC1 − QC2 )
cooling,s
E E RBC
cooling,e
+
cooling,s
cooling,e
WP +
WF
(10)
cooling,s
where EERBC is the cooling energy efficiency ratio of the conventional fresh air handling system, assumed to be 5.0. cooling, s and cooling, eare the start and end times for cooling, respectively. The energy consumption of the novel fresh air handling system in the heating season, ENH , is calculated as:
W. Lyu, X. Li and B. Wang et al. / Energy & Buildings 185 (2019) 39–48 8760
(
QHL +
heating,e 1
heating,s
ENH =
8760
QHL ) − (
QH +
heating,e 1
heating,s
QH )
E E RBH 8760
+
heating,e
(WP + WF ) +
(WP + WF )
(11)
1
heating,s
where EERBH is the heating energy efficiency ratio of the conventional fresh air handling system, assumed to be 4.0. heating, s and heating, e are the start and end times for heating, respectively. The energy consumption of the baseline fresh air handling system in the cooling season, EBC , is calculated as cooling,e
EBC =
QCL
cooling,s
(12)
E E RBC
The energy consumption of the baseline fresh air handling system in the heating season, EBH , is calculated as 8760
EBH =
QHL +
heating,e
QHL
1
heating,s
(13)
E E RBH
3.4. Evaluation index The ratio of the annual total cumulated heat transfer capacity of the fresh air pre-handling system to the total cumulated fresh air cooling and heating load is calculated as cooling,e
λ=
Q C1 +
cooling,s
cooling,e
QC2 +
cooling,s
cooling,e
QCL +
cooling,s
8760
QH +
1
heating,s 8760
QHL +
heating,s
heating,e
heating,e
(14)
QHL
1
Q Wp + WF
(15)
where Q is the hourly heat exchange capacity of the AWHX that refers to the hourly precooling capacity and/or heating capacity, kW. Compared with the energy consumption of the baseline fresh air handling system, the energy saving ratio of the proposed fresh air handling system is defined as
ξ=
EBC + EBH −ENC −ENH × 100% EBC + EBH
(16)
where ξ is the annual energy saving ratio of the proposed fresh air handling system. The static payback period is evaluated as:
Y =
I C
heat exchange process of the fresh air pre-handling system is investigated in detail. The soil temperature profile, the average heat exchange temperature difference, as well as the air and water temperature variation are analyzed. Subsequently, the annual performances, i.e. the average COP and the energy saving potential, are calculated based on the specific heat exchange process. Finally, the applicability is studied according to the economical evaluations. 4.1. The operation periods of the fresh air pre-handling system The operation periods of the fresh air pre-handling system in different scenarios are demonstrated in Fig. 3. By the comparison of Fig. 3(a) and (b), it is obvious that the fresh air pre-handling systems operate in the minimum fresh air mode at most of the time in the cooling season, especially in Shanghai and Guangzhou, which have a subtropical climate and tropical climate, respectively. When the operation scenarios are changed from strategy 1 to strategy 4, the running periods of the fresh air pre-handling systems in the minimum fresh air mode decrease rapidly in Shanghai and Guangzhou. The fresh air pre-handling system hardly operates in full fresh air mode when strategy 2 to strategy 4 are applied. As is illustrated in Fig. 3(c), the strategies have little influence on the heating operation hours of the fresh air pre-handling system in the subtropical climate zone, e.g. Shanghai. However, the influences in the temperate climate zones, e.g. Shenyang and Beijing, are obvious because of the low soil temperature and the outdoor air temperature distribution in winter, as shown in Fig. 2. 4.2. The heat exchange profile of the fresh air pre-handling system
QH
The average coefficient performance of the fresh air prehandling system is defined as
COP =
43
(17)
where I and C are the additional investment and annual operation cost savings, respectively, as compared with the baseline fresh air system. 4. Results and discussion The energy saving potential as well as the applicability of the proposed system are mainly investigated from the following four aspects. First, the operation hours of the fresh air pre-handling system in different scenarios are calculated according to the meteorological data in different climate zones, and this facilitates in understanding the operation mode of the fresh air system. Next, the
Fig. 4 shows the soil temperature profile of the GHX operating in strategy 2. In Guangzhou, the fresh air pre-handling system is only used for precooling. The soil temperature increases slightly because the heat transfer capacity and the average temperature difference of the fresh air are small, as demonstrated in Fig. 5. As for the fresh air pre-handling system utilized in Shenyang, Beijing, and Shanghai, the soil temperature decreases in the heating season and increases in the cooling season, which is a dynamic performance process. In the cooling season, the average precooling temperature difference of the fresh air in different scenarios is illustrated in Fig. 5(a) and (b). It is apparent that the average precooling temperature difference in the minimum fresh air mode is generally larger than that in full fresh air mode because of the larger heat transfer temperature difference between the outdoor air temperature and the soil temperature as well as the smaller air volume in the minimum fresh air mode. The average fresh air temperature decrease ranges from 2.2 °C to 8.2 °C in different climate zones when the fresh air pre-handling system operates in the minimum fresh air mode. In the subtropical and tropical climate zones, e.g. Shanghai, and Guangzhou, when the operation scenarios are changed from strategy 1 to strategy 4, the average precooling temperature difference of the fresh air increase quickly in both the minimum fresh air mode and the full fresh air mode. However, the temperature difference in the full fresh air mode is still less than 2 °C. Especially for the full fresh air mode in Guangzhou with the tropical climate, the average precooling temperature difference is below zero when the fresh air pre-handling system operates according to strategy 1. This means that the general heat transfer in the cooling season is from the soil to the fresh air due to the high soil temperature in Guangzhou. In the heating season, the fresh air pre-handling system operates in the minimum fresh air mode and the average preheating temperature difference of the fresh air in different scenarios is illustrated in Fig. 5(c). The average fresh air temperature increase
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Fig. 4. Soil temperature of the ground heat exchangers in different regions.
inlet fresh air. On Jul. 30, the fresh air average precooling temperature decrease are 8.7 °C, 7.0 °C, 4.6 °C and 3.5 °C in Shenyang, Beijing, Shanghai, and Guangzhou, respectively. Meanwhile, the average fresh air preheating temperature increase on Jan. 15 are 10.1 °C, 5.06 °C, and 6.3 °C in Shenyang, Beijing, Chengdu, and Shanghai, respectively.
4.3. Annual performance of the fresh air pre-handling system
Fig. 3. The operation periods of the fresh air pre-handling system in different scenarios.
ranges from 5.7 °C to 8.3 °C in different regions. The large average temperature difference contributes to the good heating performance of the fresh air pre-handling system. To analyze the heat transfer profile of the fresh air pre-handling system, two typical days, i.e. Jul. 30 and Jan. 15, are selected as the representatives of the cooling season and heating season, respectively. The hourly air and water temperature at the inlet and outlet of the AWHX in these two days are shown in Fig. 6. In the occupied time, i.e. from 9:00 am to 18:00 pm, the soil temperature changes slightly. The temperature changes of the supply water, return water, as well as the outlet fresh air are consistent with that of the
The average COP of the fresh air pre-handling system in different scenarios is demonstrated in Fig. 7. The variation tendency of the COP with different strategies agree well with the temperature difference trend, as shown in Fig. 5. As shown in Fig. 7(a), the average cooling COP in the minimum fresh air mode is satisfying in all four cities. Even with the worst strategy, i.e. strategy 1, the minimum COP in the tropical climate zone, e.g. Guangzhou is about 7.9, which is larger than most conventional fresh air handling systems. However, the cooling performance becomes worse in the full fresh air mode. Fig. 7(b) indicates that the average cooling COP in the temperate climate zones, e.g. Shenyang and Beijing are barely accepted. In the tropical climate, e.g. Guangzhou, the average COP is negative with strategy 1. Fortunately, the operation hours of the fresh air pre-handling system in the full fresh air mode account for only a few hours, as shown in Fig. 3. According to Fig. 7(c), the average heating COP ranges from 20 to 30 in different climate regions. The annual energy saving ratios of the fresh air pre-handling system operating with different control strategy is indicated in Fig. 8. Compared with the baseline fresh air handling system, the maximum energy saving ratios are about 29%, 34%, 27%, and 4% in the mid-temperate zone, warm-temperate zone, subtropical zone and the tropical zone, such as Shenyang, Beijing, Shanghai, and Guangzhou, respectively. When the fresh air handling system operates with strategy 2, the maximum energy saving ratios are obtained in Beijing, Shanghai and Guangzhou on behalf of the warm-temperate zone, subtropical zone and the tropical zone, respectively. However, in Shenyang with the mid-temperate climate, the fresh air pre-handling system is advised to operate with strategy 1. Compared with Shenyang, Beijing, and Shanghai, the energy saving ratio is small in Guangzhou because of the relatively high soil temperature. When strategy 4 is applied in Guangzhou, the energy saving ratio is below zero. One reason is that the operation period of the fresh air pre-handling system with strategy 4 is short and the cooling capacity is small. The other reason is that the additional fan power caused by the heat exchange coil of the AWHX is always consuming energy during the occupied time.
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Fig. 5. The average pre-handling temperature difference of fresh air in different scenarios.
When the fresh air pre-handling system operates with the strategy that corresponds to the maximum energy saving ratio, the heat transfer capacity ratio and the average COP profile are illustrated in Fig. 9. The ratio of the cumulated cooling capacity of the fresh air pre-handling system to the total cumulated fresh air cooling load in Shenyang, with mid-temperate climate, is above 100%, implying that the fresh air can accommodate part of the indoor sensible cooling load after being processed by the fresh air pre-handling system. Regarding the annual performance, the ratios of the cumu-
Fig. 6. The hourly air and water temperatures at the inlet and outlet of the AWHX.
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Fig. 8. The energy saving ratio of the fresh air pre-handling system.
Fig. 9. The heat transfer capacity ratio and annual COP profiles.
Mediterranean climate. For example, Washington is located in temperate zone and the climate there is similar to Beijing. Atlanta has a similar climate with Shanghai. The climate in parts of South America has the hot climate, which is similar to the climate in Guangzhou. Therefore, according to the performance analysis in these four cities, it can be deduced that the proposed system would have energy saving potential around the world, such as some areas in Europe, Japan, Korea and America with the temperate climate and subtropical climate. However, even in these climate zones, the performance of the proposed system also should be investigated carefully based on the specific climate before being put into utilization. Fig. 7. The average COP of the fresh air pre-handling system in different scenarios.
4.4. Economic evaluations
lated heat transfer capacity of the fresh air pre-handling system to the annual total of cooling and heating load are about 38%, 45%, 36%, and 11% in Shenyang, Beijing, Shanghai, and Guangzhou, respectively. The average COP ranges from 11.4 to 22 in different climate regions. These four selected cities of China represent not only most climate regions in China, but also most of the global climate zones in the word. For example, most regions of Europe are located in the temperate and Mediterranean climate zones. The climate there is similar to that of Shenyang and Beijing. Korea and Japan have a temperate and subtropical climate. Most regions of Korea and Japan have the similar climate types with that of Beijing and Shanghai. America has a diverse climate including the temperate climate, subtropical climate, tropical climate as well as the
For the baseline fresh air system, the peak fresh air cooling load (PCL) and the peak fresh air heating load (PHL) are shown in Fig. 10(a). In the mid-temperate zone, e.g. Shenyang, the PHL is higher than the PCL, while the opposite is true in the other three regions. Therefore, the fresh air handling system size should be designed according to the PHL in Shenyang, and the PCL in Beijing, Shanghai, and Guangzhou. When the conventional GSHP is applied as the cold and heat source for fresh air handling, the heat pump unit size as well as the borehole numbers can be designed according to the peak load and the average cooling and heating COP. For example, the average cooling COP in Beijing is set as five, and the design heat flux through the borehole is set as 40 W/m in this study. Therefore, the design borehole number required in Beijing is 7 (22.1 × 10 0 0 × (1 + 1/5) / (40 × 100) = 6.63 ≈ 7). The design borehole numbers in the four cities are listed in Table 6.
W. Lyu, X. Li and B. Wang et al. / Energy & Buildings 185 (2019) 39–48
47
Table 6 Key items comparison between the baseline and proposed system. Systems
Items
SY
BJ
SH
GZ
Baseline fresh air handling system
PCL (kW) PHL (kW) Number of bore holes PCL (kW) PHL (kW) Number of bore holes Additional area of AWHX(m2 )
22.06 28.01 6 15.62 16.16 6 50.0
22.10 21.61 7 16.42 11.90 7 50.0
23.82 15.89 8 19.17 7.61 8 50.0
27.00 \ 9 24.20 \ 10 50.0
Proposed fresh air handling system
Table 7 The analysis of additional investment and profit. Items Price of GHX (CNY per borehole) AWHX price (CNY per m2 ) Electricity price (CNY per kW·h) Additional AWHX area (m2 ) Additional borehole number Additional cost (CNY) Annual electricity savings (kW·h) Annual cost savings (CNY) Payback periods (year)
SY
BJ
SH
GZ
0 2500 1920 1536 1.63
10 0 0 0 50 0.8 50 0 2500 1645 1315 1.90
0 2500 1087 869 2.88
1 12500 164 130 172
(Current exchange rate: 1 USD = 6.868 CNY)
the proposed system is only the AWHX coil in the fresh air prehandling system in these four cities. However, in Guangzhou, one additional borehole is also required for the proposed system under the adopted design condition in this study. According to the energy saving ratios in Section 4.3, the annual electricity savings are 1920 kW·h, 1645 kW·h, 1087 kW·h, and 164 kW·h in Shenyang, Beijing, Shanghai, and Guangzhou, respectively. Because of the high soil temperature, the electricity savings in tropical zone, e.g. Guangzhou, is limited. Neglecting the benefits from the decrease in the GSHP size caused by the reduction of the peak load, the analysis of the additional investments and profits are listed in Table 7. The maximum annual cost saving is 1536 CNY in Shenyang, followed by Beijing, Shanghai, and Guangzhou. Further, the payback periods are acceptable in Shenyang, Beijing, and Shanghai. However, the payback period in Guangzhou is poor and the proposed system is hardly applied in practice. Generally, the payback period is affected by both the electricity savings and the local price. Therefore, the economy of the proposed system in different climate zones should be evaluated separately. Fig. 10. Peak fresh air loads and reduction rates compared with the baseline system.
For the proposed fresh air system, because part of the fresh air load is eliminated by the fresh air pre-handling system, the PCL as well as the PHL are lower than those of the baseline system. As demonstrated in Fig. 10(b), the PCL reductions are 29%, 26%, 20%, and 10%, and the PHL reductions are 42%, 44%, and 52% in Shenyang, Beijing, Shanghai, and Guangzhou, respectively. The GSHP size decreases with the reduction of the peak load. The borehole numbers required in the proposed fresh air system include two parts: One part is fixed directly for the fresh air prehandling, which is set as two, as mentioned in Section 3.2. The other part is connected with the ground source heat pump unit. For instance, the design borehole number required in Beijing is also 7 (16.42 × 10 0 0 × (1 + 1/5) / (40 × 100) + 2 = 6.93 ≈ 7). The total borehole numbers of the proposed system in the four cities are listed in Table 6. The total design borehole numbers in the proposed system is approximately the same with that of the baseline system in Shenyang, Beijing, and Shanghai. Therefore, compared with the baseline fresh air system, the additional items of
5. Conclusion A novel fresh air pre-handling system with shallow geothermal energy is proposed for energy saving. The energy saving potentials of the proposed system are investigated based on a typical all-air system in typical climate zones of China. According to the study results, the conclusions are as follows: (1) The fresh air pre-handling system has different energy saving potentials in different climate zones. The energy saving rates are around 30% both in the temperate zone and the subtropical zone, e.g. Shenyang, Beijing, and Shanghai. However, the energy saving potential in the hot summer and warm winter region with tropical climate is limited. (2) The ratios of the cumulated heat transfer capacity of the fresh air pre-handling system to the annual total of cooling and heating load range from 35% to 45% in the temperate zone and the subtropical zone, such as Shenyang, Beijing, and Shanghai, while the ratio in the tropical zone, e.g. Guangzhou, is just 11%. (3) The fresh air pre-handling system is feasible in most climate zones. The payback periods are acceptable except for
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Guangzhou, which is located in the hot summer and warm winter region and similar to the tropical climate. Acknowledgements This study was supported by the China National Key R&D Program “Solutions to heating and cooling of buildings in the Yangtze river region” (Grant No. 2016YFC0700302) and the National Natural Science Foundation of China (Grant No. 51638010). Reference [1] C.Q. Liu, Y.D. Xu, Energy consumption of fresh air in hot and cold areas, Refrig. Air Cond. 5 (2016) 54–57 (in Chinese). [2] J. Cai, W. Liu, C. Huang, X. Wang, L. Shen, Z. Zou, Y. Hu, C. Sun, X. Wei, J. Chang, Z. Zhao, Y. Sun, J. Sundell, Validity of subjective questionnaire in evaluating dwelling characteristics, home dampness, and indoor odors in Shanghai, China: cross-sectional survey and on-site inspection, Energy Build. 127 (2016) 1019–1027. [3] J.L. Niu, L.Z. Zhang, Energy requirements for conditioning fresh air and the longterm savings with a membrane-based energy recovery ventilator in Hong Kong, Energy 26 (2001) 119–135. [4] L.-Z. Zhang, Progress on heat and moisture recovery with membranes: From fundamentals to engineering applications, Energy Convers. Manage. 63 (2012) 173–195. [5] K. Zhong, Y. Kang, Applicability of air-to-air heat recovery ventilators in China, Appl. Therm. Eng. 29 (5-6) (2009) 830–840. [6] M. Rasouli, C.J. Simonson, R.W. Besant, Applicability and optimum control strategy of energy recovery ventilators in different climatic conditions, Energy Build. 42 (9) (2010) 1376–1385. [7] S. Tafelmeier, G. Pernigotto, A. Gasparella, Annual performance of sensible and total heat recovery in ventilation systems: humidity control constraints for European climates, Buildings 7 (4) (2017) 28. [8] M.S. Nasif, R. Al-Waked, M. Behnia, G. Morrison, Air to air fixed plate enthalpy heat exchanger, performance variation and energy analysis, J. Mech. Sci. Technol. 27 (11) (2013) 3541–3551. [9] J. Liu, W. Li, J. Liu, B. Wang, Efficiency of energy recovery ventilator with various weathers and its energy saving performance in a residential apartment, Energy Build. 42 (1) (2010) 43–49. [10] W.M. El-Maghlany, A.A. ElHefni, M. ElHelw, A. Attia, Novel air conditioning system configuration combining sensible and desiccant enthalpy wheels, Appl. Therm. Eng. 127 (2017) 1–15.
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Energy & Buildings journal homepage: www.elsevier.com/locate/enbuild
Energy saving potential of fresh air pre-handling system using shallow geothermal energy Weihua Lyu, Xianting Li∗, Baolong Wang, Wenxing Shi Department of Building Science, School of Architecture, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 11 August 2018 Revised 19 December 2018 Accepted 24 December 2018 Available online 5 January 2019 Keywords: Natural energy Ground heat exchanger Fresh air Air handling unit Building energy efficiency
a b s t r a c t To reduce the energy consumption for fresh air handling, a novel fresh air pre-handling system that fully exploits the shallow energy to precool and preheat fresh air is proposed in this study. The model of a typical all-air system with a pre-handling system is built on the TRNSYS platform. The energy saving potential of this fresh air pre-handling system is simulated based on the diverse climate of China. The results show that the ratios of the cumulated heat transfer capacity of the fresh air pre-handling system to the annual total of cooling and heating load range from 35% to 45% in the temperate zone and the subtropical zone, such as Shenyang, Beijing, and Shanghai, while the ratio in the tropical zone is approximately 10%. The energy saving rates are approximately 30% in most climate zones, but less than 5% in Guangzhou, the tropical climate zone. Moreover, the payback periods are within 4 years in the temperate and subtropical zones. However, the system is not economical in the tropical zone. All these results indicate that the fresh air pre-handling system with shallow geothermal energy is quite promising in most climate zones except in the hot summer and warm winter region with a tropical climate. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Currently, indoor air quality problems, such as volatile organic compounds (VOCs), smoke, odors, dust, and bacteria have led to a high demand in fresh air flow rates [1,2]. In fact, the energy consumed by fresh air handling accounts for approximately 30% of the total energy consumption of air conditioning [3]. The energy conservation of fresh air handling is critical in HVAC systems. Traditional exhaust heat recovery ventilators are used to reduce the energy consumption of fresh air handling if the exhaust air system is available in buildings [4–7]. Actually, the energy saving effect of exhaust heat recovery technique is remarkable only in certain situations [8–10]. Moreover, if the climate is suitable, e.g. in the dry and hot climate regions, the evaporative cooling technique is always used to produce cooling fresh air for buildings [11–16]. However, the performance of evaporative cooling is limited by the climate conditions predominantly. Little cooling capacity can be gained in humid regions. Another drawback of evaporative cooling is that it is not applicable for space heating, which further limits the application in cold regions. Generally, the earth-to-air heat exchanger system is applied to cool fresh air in summer, and heat it in winter efficiently. The earth-to-air heat exchanger system directly utilizes shallow
∗
Corresponding author. E-mail address: [email protected] (X. Li).
https://doi.org/10.1016/j.enbuild.2018.12.037 0378-7788/© 2019 Elsevier B.V. All rights reserved.
geothermal energy [17]. An underground tunnel is a typical earthto-air heat exchanger system [18]. Many cities have underground tunnels that offer the opportunity to fully utilize them to supply conditioned fresh air for the surrounding buildings [19]. Another type of earth-to-air heat exchanger system is the tubes buried in the ground as a heat exchanger [20]. Shallow geothermal energy can be transferred to the fresh air through the tubes. Utilizing the temperature difference, the fresh air can be efficiently cooled in the summer and heated in the winter [21,22]. However, the earthto-air heat exchanger system requires a high initial investment because of the complex tunnels and air tubes layout. Even worse, when the fresh air humidity is high, condensation would occur on the tunnel or the tubes inner surface, thus leading to molds growth. It is difficult to prevent and kill the molds as the heat transfer process is throughout the whole tunnel. Consequently, the fresh air would have been polluted before being supplied into the building. Meanwhile, the ground source heat pump (GSHP) has been widely used. As an important component of the GSHP, the ground heat exchanger (GHX) is always integrated with a heat pump to generate high-grade chilled water or hot water for heating and air conditioning [23,24]. In fact, for most of the cooling and heating season, the outlet water temperature of the GHX can be used directly for fresh air treatment. For example, in the typical hot summer and cold winter regions in China, the undisturbed underground soil temperature is approximately 15 °C, and the outlet
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Nomenclature Ca Cw EBC EBH EERBC EERBH ENC ENH Fmax Fmin HL hN ma mw Q QC1 QC2 Q_CL QH Q_HL tai tao tN tW WF WP Y
Specific heat capacity of air, kJ/(kg· °C) Specific heat capacity of water, kJ/(kg· °C) Energy consumption of the baseline system in the cooling season, kW·h Energy consumption of the baseline system in the heating season, kW·h Cooling energy efficiency ratio of the baseline system Heating energy efficiency ratio of the baseline system Energy consumption of the proposed system in the cooling season, kW·h Energy consumption of the proposed system in the heating season, kW·h Maximum fresh air volume, m3 /h Minimum fresh air volume, m3 /h Water circulating resistance Specific enthalpy of the return air, kJ/kg Air flow rate, kg/s Water flow rate, kg/s Heat exchange capacity of the AWHX, kW Precooling capacity of the AWHX in the minimum fresh air mode, kW Precooling capacity of the AWHX in the maximum fresh air mode, kW Fresh air cooling load, kW Preheating capacity of the AWHX, kW Fresh air heating load, kW Fresh air inlet temperature of the AWHX, °C Fresh air outlet temperature of the AWHX, °C Indoor design temperature, °C Outdoor temperature, °C Additional fan power, W Pump power Static payback period, year
Greek symbol ηw Pump efficiency ηf Fan efficiency λ Ratio of the annual total cumulated heat transfer capacity to the total cumulated fresh air load ε Heat transfer efficiency of the AWHX ξ Energy saving rate I Additional initial investment, CNY C Annual operation cost savings, CNY P Fan pressure drop caused by the heat exchanger coil of the AWHX, Pa Abbreviation AHU Air handling unit AWHX Air-water heat exchanger COP Coefficient of performance GHX Ground heat exchanger GSHP Ground source heat pump NTU The number of heat transfer units of the AWHX
water temperature of GHX is around 10 °C in the winter and 20 °C in the summer. Therefore, for the majority of the time, an adequate temperature difference will occur between the outlet water temperature and the outdoor fresh air temperature, both in the winter and summer, as the outdoor fresh air temperature is always below 0 °C in the winter and above 30 °C in the summer. Hence, the fresh air load can be efficiently handled by grading, and the low-
Fig. 1. Schematic of the proposed fresh air handling system.
grade fresh air load would be eliminated with the natural shallow geothermal energy. To develop an efficient fresh air handling method in more zones of different climates, and eliminate the drawbacks of fresh air handling with the earth-to-air heat exchanger system, a novel fresh air pre-handling system that use shallow geothermal energy directly for fresh air precooling or preheating is proposed in this study. The models of a typical all-air system with a fresh air pre-handling system is built on the TRNSYS platform [25]. The energy saving potential of this fresh air pre-handling system is simulated in diverse climate regions of China. It is hoped to fully exploit the low-grade natural energy to handle the low-grade fresh air load and reduce the demand of high-grade energy.
2. Principle of fresh air pre-handling system The novel fresh air pre-handling system uses a GHX to extract geothermal energy from the ground, and subsequently transfers the energy to the fresh air by utilizing an air-water heat exchanger (AWHX). The schematic of this system is illustrated in Fig. 1. In the cooling season, the cool water generated by the GHX is pumped to the AWHX. The high-temperature fresh air entering the air handing unit (AHU) will be cooled by the cool water when it flows through the AWHX. Subsequently, the temperature of the cool water rises, returns to the GHX, and rejects the heat to the ground. Hence, the fresh air can be precooled efficiently. Compared with the earth-toair system, the condensation would occur on the coil surface of AWHX instead of the whole inner surface of tunnel. Therefore, the method to deal with molds in traditional air handling unit can be used in the proposed system. Similarly, in the heating season, for the majority of the time, as the underground soil temperature is much higher than the cold fresh air temperature, especially in cold regions, the cold fresh air will be preheated similarly as that in the cooling season. Therefore, this novel fresh air pre-handling system can fully utilize the lowgrade shallow geothermal energy to handle the low-grade fresh air load [26]. Thus, the demand for high-grade energy would be reduced and substantial energy savings can be obtained.
3. Simulation method and evaluation index This study aims to propose an alternative for fresh air prehandling and investigate the energy saving potential of the fresh air pre-handling system in diverse climate zones. Therefore, the research is conducted based on a typical all-air system with a fresh air pre-handling system in a building. As a comparison, a same scale conventional all-air system with a traditional fresh air handling system is utilized as the baseline. Only the fresh air load as well as the energy consumption of fresh air handling are compared between the proposed system and the baseline system. The simulation method as well as the evaluation index are described in this section.
W. Lyu, X. Li and B. Wang et al. / Energy & Buildings 185 (2019) 39–48
41
Table 1 Climates of different cities in China. City
Shenyang
Beijing
Shanghai
Guangzhou
Climate region Global climate zone Cooling season Heating season
Sever cold Mid-temperate Jul. 1–Aug. 31 Nov. 1–Mar. 31
Cold Warm-temperate Jun. 11–Aug. 31 Nov. 15–Mar. 15
Hot summer and cold winter Subtropical Jun. 11–Sep. 10 Dec. 1–Feb. 28
Hot summer and warm winter Tropical May. 11–Oct. 20 No heating demand
Table 2 The primary specific parameters of GHX.
Fig. 2. Temperature distribution frequency of outdoor air.
3.1. Descriptions of the typical all-air system The air volume of this typical all-air system is 10,0 0 0 m3 /h. We assumed that the minimum fresh air ratio to meet the indoor health requirements is 20%. The air conditioning system serves an office building, and the occupied time is set as 8:00 am to 18:00 pm in the daytime. In the cooling season, to fully utilize the free cooling energy of fresh air, the all-air system would operate in the full fresh air mode when the enthalpy of outdoor air is lower than that of the return air. Similarly, when the enthalpy of outdoor air is higher than that of the return air, the all-air system would operate in the minimum fresh air mode. In the heating season, the all-air system would also operate in the minimum fresh air mode to reduce the heating energy consumption. The baseline conventional all-air system is the same with the typical all-air system except the fresh air pre-handling system. China has different climate zones, mainly including the severe cold climate region, cold climate region, hot summer and cold winter climate region, as well as the hot summer and warm winter climate region. A typical city is selected as the representative of each climate region in this study. Shenyang is a representative of severe cold climate region, and it has a short cooling season and a long heating season; Beijing is in the cold climate region, and it is cold in winter and hot in summer; Shanghai belongs to the hot summer and cold winter climate region, and it is cold in winter and very hot in summer; Guangzhou is located in the hot summer and warm winter climate region, and it has a hot climate and little heating demand exists. The climates of these four cities in China are shown in Table 1. In terms of the global climate zones, Shenyang is located in the mid-temperate zone; Beijing is located in the warm-temperate zone; Shanghai has a subtropical climate, while the climate in Guangzhou is similar to that of tropical zones. The temperature distribution frequency of the outdoor air in different cities is shown in Fig. 2. 3.2. Simulation models To investigate the energy saving potential of the fresh air prehandling system in diverse climate zones, the fresh air treatment process and energy simulation of a typical all-air system with the
Soil thermal conductivity
1.4 W/m·K
Soil thermal diffusivity Soil density Soil specific heat capacity Radius of U–tube pipe
0.00143 m2 /h 1800 kg/m3 1.61 kJ/(kg·K) 16 mm
fresh air pre-handling system are conducted using the dynamic simulation software, TRNSYS. The type 557a, a model of Duct Ground Heat Storage in TRNSYS is used to simulate the ground heat exchanger. The accuracy of this dynamic model has been validated in a detailed study. The primary specific parameters of the GHX assumed in this simulation are listed in Table 2. According to reference [27], the undisturbed average soil temperature is about 2 °C higher than the local annual average temperature. The average air temperature and soil temperature of the selected four cities are listed in Table 3. Considering the low soil temperature and low fresh air temperature in the winter in Shenyang, the antifreezing ethylene glycol solution is used to replace water, and circulate between the GHX and AWHX. The specific heat capacity of the antifreezing solution is 3.85 kJ/(kgK). Water is used in other cities. It should be noted that the antifreezing ethylene glycol solution is not distinguished from water in the following statement. Water is used to represent of both water and antifreezing ethylene glycol solution for statement convenience. The AWHX is a cross flow air-water heat exchanger and it uses the ε –NTU model to simulate the heat transfer process. The definition of ε and NTU are as follows:
C · m C ·m w w a a ε = 1 − exp NT U 0.22 exp − NT U 0.78 − 1 Ca · ma
Cw · mw
(1) NT U =
kA Ca · ma
(2)
where ɛ is the heat transfer efficiency of the AWHX; NTU is the number of heat transfer units of the AWHX; Ca is the specific heat capacity of air, kJ/(kg• °C); ma is the air flow rate, kg/s; Cw is the specific heat capacity of water, kJ/(kg• °C); mw is the water flow rate, kg/s. The air volume of the fresh air pre-handling system is designed based on the minimum fresh air volume, which is set as 20 0 0 m3 /h. The design air temperature difference is set as 10 °C. According to the pre-handling demand, the design parameters are shown in Table 4. As the average heat transfer rate of the GHX is set as 30–50 W per linear meter of buried pipe, approximately two boreholes of depth 100 m of a single U–tube pipe are required for the 20 0 0 m3 /h fresh air treatment. To conduct the simulation, the fresh air pre-handling system model is built on the TRNSYS platform. The circulating water driven by a pump flowed between the AWHX and the GHX. The heat transfer process of the GHX is integrated with that of the AWHX on the simulation platform. This study hypothesizes that the water temperature increase caused by the motor heat loss of water pump can be neglected. In addition, because of the small
42
W. Lyu, X. Li and B. Wang et al. / Energy & Buildings 185 (2019) 39–48 Table 3 The annual average air temperature and soil temperature in different cities. City
Shenyang
Beijing
Shanghai
Chengdu
Guangzhou
Annual average temperature (°C) Soil initial temperature (°C)
8.56 10.56
12.65 14.65
16.69 18.69
16.62 18.62
22.23 24.23
Table 4 The design parameters of AWHX. Rated air flow rate
20 0 0 m3 /h
Rated water flow rate Flow resistance in water side Fan pressure drop Heat transfer area Heat transfer rate
1.92 m3 /h 5 m H2 O 180 Pa 50 m2 kA = 2240 W/K
tai , tao are the fresh air inlet temperature and outlet temperature of the AWHX, respectively, °C. When the enthalpy of outdoor air is lower than that of the return air in the cooling season, the all-air system would run in the full fresh air mode. Fresh air cooling load does not exist in this scenario. The hourly precooling capacity QC2 of the AWHX is expressed as formula (5), which can be used to eliminate the indoor cooling load.
Table 5 Operation strategies of the fresh air pre-handling system. No. of strategy
Cooling season
Heating season
Strategy Strategy Strategy Strategy
Without control ta −tsoil > = 3 °C ta −tsoil > = 5 °C ta −tsoil > = 8 °C
Without control tsoil −ta > = 3 °C tsoil −ta > = 5 °C tsoil −ta > = 8 °C
1 2 3 4
QC2 = Ca ·
Fmax · ρ · (tai − tao ) 3600
(5)
where Fmax is the maximum fresh air volume in the full fresh air mode, m3 /h. In the winter, the all-air system would also operate in the minimum fresh air mode to reduce the heating load of fresh air. The hourly fresh air heating load Q_HL and the hourly preheating capacity QH of the AWHX are respectively expressed as follows:
Fmin · ρ · (tN − tW ) 3600
(6)
Fmin · ρ · (tao − tai ) 3600
(7)
temperature difference between the outlet water of GHX and the ambient, the pipe insulation, and the limit pipe length from GHX to AWHX, the heat losses along the pipe have little effect on the proposed system and they are neglected in the following calculation.
QHL = Ca ·
3.3. Simulation methods
where tN is the indoor design temperature in the winter, which is set as 20 °C; tW is the outdoor temperature, °C. Next, the energy consumption of the fresh air system with different operation strategies is calculated. The energy consumption includes the energy consumption of the water pump, the additional fan power of the novel fresh air pre-handling system, and the further conventional dehumidification and cooling of fresh air. The power of the pump, Wp , is calculated as:
Four operation strategies are set to control the operation of the fresh air pre-handling system, i.e. to control the operation of the water pump of the fresh air pre-handling system. Strategy 1 means that the water pump always operates during the occupied period without control. Strategy 2 ∼ strategy 4 means that the water pump starts when the air temperature (ta ) is higher than the soil temperature (tsoil ) in the cooling season, and lower in the heating season. As shown in Table 5, the absolute temperature difference of strategy 2, strategy 3, and strategy 4 are set as 3 °C, 5 °C, and 8 °C, respectively. By the way, for many applications in practice, the control system may not work for a variety of reasons. Therefore, the strategy 1 is set to see whether there is any energy saving effect in the whole year even if there is no adjustment of the control system. First, the cooling and heating fresh air loads in different climate conditions are calculated. Then the cooling and heating capacities of the fresh air pre-handling system are obtained through the simulations. In the summer, the indoor design point is set as 26 °C, 60%, which is assumed as the same with the return air parameter. When the enthalpy of outdoor air is higher than that of the return air, the all-air system would operate in the minimum fresh air mode. The hourly fresh air cooling load Q_CL can be calculated using formula (3). The hourly precooling capacity QC1 of the AWHX is expressed as formula (4).
Q_CL
Fmin · ρ = · (hW − hN ) 3600
QC1 = Ca ·
Fmin · ρ · (tai − tao ) 3600
(3) (4)
where Fmin is the minimum fresh air volume, m3 /h; ρ is the fresh air density, assumed as 1.2 kg/m3 ; hW is the specific enthalpy of fresh air, kJ/kg; hN is the specific enthalpy of the return air, kJ/kg;
QH = Ca ·
WP =
mw · g · HL
(8)
ηw
where ηw is the pump efficiency, assumed to be 0.65; HL is the water circulating resistance, assumed to be a 15–m water column. The additional fan power, WF , caused by the heat exchange coil of the AWHX is calculated as:
WF =
F · P 3600η f
(9)
where F is the fresh air volume, m3 /h; P is the fan pressure decrease caused by the heat exchanger coil of the AWHX, assumed as 180 Pa; ηf is the fan efficiency, assumed to be 0.65. The energy consumption of the proposed fresh air handling system in the cooling season, ENC , is calculated as: cooling,e
ENC =
(QCL − QC1 − QC2 )
cooling,s
E E RBC
cooling,e
+
cooling,s
cooling,e
WP +
WF
(10)
cooling,s
where EERBC is the cooling energy efficiency ratio of the conventional fresh air handling system, assumed to be 5.0. cooling, s and cooling, eare the start and end times for cooling, respectively. The energy consumption of the novel fresh air handling system in the heating season, ENH , is calculated as:
W. Lyu, X. Li and B. Wang et al. / Energy & Buildings 185 (2019) 39–48 8760
(
QHL +
heating,e 1
heating,s
ENH =
8760
QHL ) − (
QH +
heating,e 1
heating,s
QH )
E E RBH 8760
+
heating,e
(WP + WF ) +
(WP + WF )
(11)
1
heating,s
where EERBH is the heating energy efficiency ratio of the conventional fresh air handling system, assumed to be 4.0. heating, s and heating, e are the start and end times for heating, respectively. The energy consumption of the baseline fresh air handling system in the cooling season, EBC , is calculated as cooling,e
EBC =
QCL
cooling,s
(12)
E E RBC
The energy consumption of the baseline fresh air handling system in the heating season, EBH , is calculated as 8760
EBH =
QHL +
heating,e
QHL
1
heating,s
(13)
E E RBH
3.4. Evaluation index The ratio of the annual total cumulated heat transfer capacity of the fresh air pre-handling system to the total cumulated fresh air cooling and heating load is calculated as cooling,e
λ=
Q C1 +
cooling,s
cooling,e
QC2 +
cooling,s
cooling,e
QCL +
cooling,s
8760
QH +
1
heating,s 8760
QHL +
heating,s
heating,e
heating,e
(14)
QHL
1
Q Wp + WF
(15)
where Q is the hourly heat exchange capacity of the AWHX that refers to the hourly precooling capacity and/or heating capacity, kW. Compared with the energy consumption of the baseline fresh air handling system, the energy saving ratio of the proposed fresh air handling system is defined as
ξ=
EBC + EBH −ENC −ENH × 100% EBC + EBH
(16)
where ξ is the annual energy saving ratio of the proposed fresh air handling system. The static payback period is evaluated as:
Y =
I C
heat exchange process of the fresh air pre-handling system is investigated in detail. The soil temperature profile, the average heat exchange temperature difference, as well as the air and water temperature variation are analyzed. Subsequently, the annual performances, i.e. the average COP and the energy saving potential, are calculated based on the specific heat exchange process. Finally, the applicability is studied according to the economical evaluations. 4.1. The operation periods of the fresh air pre-handling system The operation periods of the fresh air pre-handling system in different scenarios are demonstrated in Fig. 3. By the comparison of Fig. 3(a) and (b), it is obvious that the fresh air pre-handling systems operate in the minimum fresh air mode at most of the time in the cooling season, especially in Shanghai and Guangzhou, which have a subtropical climate and tropical climate, respectively. When the operation scenarios are changed from strategy 1 to strategy 4, the running periods of the fresh air pre-handling systems in the minimum fresh air mode decrease rapidly in Shanghai and Guangzhou. The fresh air pre-handling system hardly operates in full fresh air mode when strategy 2 to strategy 4 are applied. As is illustrated in Fig. 3(c), the strategies have little influence on the heating operation hours of the fresh air pre-handling system in the subtropical climate zone, e.g. Shanghai. However, the influences in the temperate climate zones, e.g. Shenyang and Beijing, are obvious because of the low soil temperature and the outdoor air temperature distribution in winter, as shown in Fig. 2. 4.2. The heat exchange profile of the fresh air pre-handling system
QH
The average coefficient performance of the fresh air prehandling system is defined as
COP =
43
(17)
where I and C are the additional investment and annual operation cost savings, respectively, as compared with the baseline fresh air system. 4. Results and discussion The energy saving potential as well as the applicability of the proposed system are mainly investigated from the following four aspects. First, the operation hours of the fresh air pre-handling system in different scenarios are calculated according to the meteorological data in different climate zones, and this facilitates in understanding the operation mode of the fresh air system. Next, the
Fig. 4 shows the soil temperature profile of the GHX operating in strategy 2. In Guangzhou, the fresh air pre-handling system is only used for precooling. The soil temperature increases slightly because the heat transfer capacity and the average temperature difference of the fresh air are small, as demonstrated in Fig. 5. As for the fresh air pre-handling system utilized in Shenyang, Beijing, and Shanghai, the soil temperature decreases in the heating season and increases in the cooling season, which is a dynamic performance process. In the cooling season, the average precooling temperature difference of the fresh air in different scenarios is illustrated in Fig. 5(a) and (b). It is apparent that the average precooling temperature difference in the minimum fresh air mode is generally larger than that in full fresh air mode because of the larger heat transfer temperature difference between the outdoor air temperature and the soil temperature as well as the smaller air volume in the minimum fresh air mode. The average fresh air temperature decrease ranges from 2.2 °C to 8.2 °C in different climate zones when the fresh air pre-handling system operates in the minimum fresh air mode. In the subtropical and tropical climate zones, e.g. Shanghai, and Guangzhou, when the operation scenarios are changed from strategy 1 to strategy 4, the average precooling temperature difference of the fresh air increase quickly in both the minimum fresh air mode and the full fresh air mode. However, the temperature difference in the full fresh air mode is still less than 2 °C. Especially for the full fresh air mode in Guangzhou with the tropical climate, the average precooling temperature difference is below zero when the fresh air pre-handling system operates according to strategy 1. This means that the general heat transfer in the cooling season is from the soil to the fresh air due to the high soil temperature in Guangzhou. In the heating season, the fresh air pre-handling system operates in the minimum fresh air mode and the average preheating temperature difference of the fresh air in different scenarios is illustrated in Fig. 5(c). The average fresh air temperature increase
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Fig. 4. Soil temperature of the ground heat exchangers in different regions.
inlet fresh air. On Jul. 30, the fresh air average precooling temperature decrease are 8.7 °C, 7.0 °C, 4.6 °C and 3.5 °C in Shenyang, Beijing, Shanghai, and Guangzhou, respectively. Meanwhile, the average fresh air preheating temperature increase on Jan. 15 are 10.1 °C, 5.06 °C, and 6.3 °C in Shenyang, Beijing, Chengdu, and Shanghai, respectively.
4.3. Annual performance of the fresh air pre-handling system
Fig. 3. The operation periods of the fresh air pre-handling system in different scenarios.
ranges from 5.7 °C to 8.3 °C in different regions. The large average temperature difference contributes to the good heating performance of the fresh air pre-handling system. To analyze the heat transfer profile of the fresh air pre-handling system, two typical days, i.e. Jul. 30 and Jan. 15, are selected as the representatives of the cooling season and heating season, respectively. The hourly air and water temperature at the inlet and outlet of the AWHX in these two days are shown in Fig. 6. In the occupied time, i.e. from 9:00 am to 18:00 pm, the soil temperature changes slightly. The temperature changes of the supply water, return water, as well as the outlet fresh air are consistent with that of the
The average COP of the fresh air pre-handling system in different scenarios is demonstrated in Fig. 7. The variation tendency of the COP with different strategies agree well with the temperature difference trend, as shown in Fig. 5. As shown in Fig. 7(a), the average cooling COP in the minimum fresh air mode is satisfying in all four cities. Even with the worst strategy, i.e. strategy 1, the minimum COP in the tropical climate zone, e.g. Guangzhou is about 7.9, which is larger than most conventional fresh air handling systems. However, the cooling performance becomes worse in the full fresh air mode. Fig. 7(b) indicates that the average cooling COP in the temperate climate zones, e.g. Shenyang and Beijing are barely accepted. In the tropical climate, e.g. Guangzhou, the average COP is negative with strategy 1. Fortunately, the operation hours of the fresh air pre-handling system in the full fresh air mode account for only a few hours, as shown in Fig. 3. According to Fig. 7(c), the average heating COP ranges from 20 to 30 in different climate regions. The annual energy saving ratios of the fresh air pre-handling system operating with different control strategy is indicated in Fig. 8. Compared with the baseline fresh air handling system, the maximum energy saving ratios are about 29%, 34%, 27%, and 4% in the mid-temperate zone, warm-temperate zone, subtropical zone and the tropical zone, such as Shenyang, Beijing, Shanghai, and Guangzhou, respectively. When the fresh air handling system operates with strategy 2, the maximum energy saving ratios are obtained in Beijing, Shanghai and Guangzhou on behalf of the warm-temperate zone, subtropical zone and the tropical zone, respectively. However, in Shenyang with the mid-temperate climate, the fresh air pre-handling system is advised to operate with strategy 1. Compared with Shenyang, Beijing, and Shanghai, the energy saving ratio is small in Guangzhou because of the relatively high soil temperature. When strategy 4 is applied in Guangzhou, the energy saving ratio is below zero. One reason is that the operation period of the fresh air pre-handling system with strategy 4 is short and the cooling capacity is small. The other reason is that the additional fan power caused by the heat exchange coil of the AWHX is always consuming energy during the occupied time.
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45
Fig. 5. The average pre-handling temperature difference of fresh air in different scenarios.
When the fresh air pre-handling system operates with the strategy that corresponds to the maximum energy saving ratio, the heat transfer capacity ratio and the average COP profile are illustrated in Fig. 9. The ratio of the cumulated cooling capacity of the fresh air pre-handling system to the total cumulated fresh air cooling load in Shenyang, with mid-temperate climate, is above 100%, implying that the fresh air can accommodate part of the indoor sensible cooling load after being processed by the fresh air pre-handling system. Regarding the annual performance, the ratios of the cumu-
Fig. 6. The hourly air and water temperatures at the inlet and outlet of the AWHX.
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Fig. 8. The energy saving ratio of the fresh air pre-handling system.
Fig. 9. The heat transfer capacity ratio and annual COP profiles.
Mediterranean climate. For example, Washington is located in temperate zone and the climate there is similar to Beijing. Atlanta has a similar climate with Shanghai. The climate in parts of South America has the hot climate, which is similar to the climate in Guangzhou. Therefore, according to the performance analysis in these four cities, it can be deduced that the proposed system would have energy saving potential around the world, such as some areas in Europe, Japan, Korea and America with the temperate climate and subtropical climate. However, even in these climate zones, the performance of the proposed system also should be investigated carefully based on the specific climate before being put into utilization. Fig. 7. The average COP of the fresh air pre-handling system in different scenarios.
4.4. Economic evaluations
lated heat transfer capacity of the fresh air pre-handling system to the annual total of cooling and heating load are about 38%, 45%, 36%, and 11% in Shenyang, Beijing, Shanghai, and Guangzhou, respectively. The average COP ranges from 11.4 to 22 in different climate regions. These four selected cities of China represent not only most climate regions in China, but also most of the global climate zones in the word. For example, most regions of Europe are located in the temperate and Mediterranean climate zones. The climate there is similar to that of Shenyang and Beijing. Korea and Japan have a temperate and subtropical climate. Most regions of Korea and Japan have the similar climate types with that of Beijing and Shanghai. America has a diverse climate including the temperate climate, subtropical climate, tropical climate as well as the
For the baseline fresh air system, the peak fresh air cooling load (PCL) and the peak fresh air heating load (PHL) are shown in Fig. 10(a). In the mid-temperate zone, e.g. Shenyang, the PHL is higher than the PCL, while the opposite is true in the other three regions. Therefore, the fresh air handling system size should be designed according to the PHL in Shenyang, and the PCL in Beijing, Shanghai, and Guangzhou. When the conventional GSHP is applied as the cold and heat source for fresh air handling, the heat pump unit size as well as the borehole numbers can be designed according to the peak load and the average cooling and heating COP. For example, the average cooling COP in Beijing is set as five, and the design heat flux through the borehole is set as 40 W/m in this study. Therefore, the design borehole number required in Beijing is 7 (22.1 × 10 0 0 × (1 + 1/5) / (40 × 100) = 6.63 ≈ 7). The design borehole numbers in the four cities are listed in Table 6.
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47
Table 6 Key items comparison between the baseline and proposed system. Systems
Items
SY
BJ
SH
GZ
Baseline fresh air handling system
PCL (kW) PHL (kW) Number of bore holes PCL (kW) PHL (kW) Number of bore holes Additional area of AWHX(m2 )
22.06 28.01 6 15.62 16.16 6 50.0
22.10 21.61 7 16.42 11.90 7 50.0
23.82 15.89 8 19.17 7.61 8 50.0
27.00 \ 9 24.20 \ 10 50.0
Proposed fresh air handling system
Table 7 The analysis of additional investment and profit. Items Price of GHX (CNY per borehole) AWHX price (CNY per m2 ) Electricity price (CNY per kW·h) Additional AWHX area (m2 ) Additional borehole number Additional cost (CNY) Annual electricity savings (kW·h) Annual cost savings (CNY) Payback periods (year)
SY
BJ
SH
GZ
0 2500 1920 1536 1.63
10 0 0 0 50 0.8 50 0 2500 1645 1315 1.90
0 2500 1087 869 2.88
1 12500 164 130 172
(Current exchange rate: 1 USD = 6.868 CNY)
the proposed system is only the AWHX coil in the fresh air prehandling system in these four cities. However, in Guangzhou, one additional borehole is also required for the proposed system under the adopted design condition in this study. According to the energy saving ratios in Section 4.3, the annual electricity savings are 1920 kW·h, 1645 kW·h, 1087 kW·h, and 164 kW·h in Shenyang, Beijing, Shanghai, and Guangzhou, respectively. Because of the high soil temperature, the electricity savings in tropical zone, e.g. Guangzhou, is limited. Neglecting the benefits from the decrease in the GSHP size caused by the reduction of the peak load, the analysis of the additional investments and profits are listed in Table 7. The maximum annual cost saving is 1536 CNY in Shenyang, followed by Beijing, Shanghai, and Guangzhou. Further, the payback periods are acceptable in Shenyang, Beijing, and Shanghai. However, the payback period in Guangzhou is poor and the proposed system is hardly applied in practice. Generally, the payback period is affected by both the electricity savings and the local price. Therefore, the economy of the proposed system in different climate zones should be evaluated separately. Fig. 10. Peak fresh air loads and reduction rates compared with the baseline system.
For the proposed fresh air system, because part of the fresh air load is eliminated by the fresh air pre-handling system, the PCL as well as the PHL are lower than those of the baseline system. As demonstrated in Fig. 10(b), the PCL reductions are 29%, 26%, 20%, and 10%, and the PHL reductions are 42%, 44%, and 52% in Shenyang, Beijing, Shanghai, and Guangzhou, respectively. The GSHP size decreases with the reduction of the peak load. The borehole numbers required in the proposed fresh air system include two parts: One part is fixed directly for the fresh air prehandling, which is set as two, as mentioned in Section 3.2. The other part is connected with the ground source heat pump unit. For instance, the design borehole number required in Beijing is also 7 (16.42 × 10 0 0 × (1 + 1/5) / (40 × 100) + 2 = 6.93 ≈ 7). The total borehole numbers of the proposed system in the four cities are listed in Table 6. The total design borehole numbers in the proposed system is approximately the same with that of the baseline system in Shenyang, Beijing, and Shanghai. Therefore, compared with the baseline fresh air system, the additional items of
5. Conclusion A novel fresh air pre-handling system with shallow geothermal energy is proposed for energy saving. The energy saving potentials of the proposed system are investigated based on a typical all-air system in typical climate zones of China. According to the study results, the conclusions are as follows: (1) The fresh air pre-handling system has different energy saving potentials in different climate zones. The energy saving rates are around 30% both in the temperate zone and the subtropical zone, e.g. Shenyang, Beijing, and Shanghai. However, the energy saving potential in the hot summer and warm winter region with tropical climate is limited. (2) The ratios of the cumulated heat transfer capacity of the fresh air pre-handling system to the annual total of cooling and heating load range from 35% to 45% in the temperate zone and the subtropical zone, such as Shenyang, Beijing, and Shanghai, while the ratio in the tropical zone, e.g. Guangzhou, is just 11%. (3) The fresh air pre-handling system is feasible in most climate zones. The payback periods are acceptable except for
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Guangzhou, which is located in the hot summer and warm winter region and similar to the tropical climate. Acknowledgements This study was supported by the China National Key R&D Program “Solutions to heating and cooling of buildings in the Yangtze river region” (Grant No. 2016YFC0700302) and the National Natural Science Foundation of China (Grant No. 51638010). Reference [1] C.Q. Liu, Y.D. Xu, Energy consumption of fresh air in hot and cold areas, Refrig. Air Cond. 5 (2016) 54–57 (in Chinese). [2] J. Cai, W. Liu, C. Huang, X. Wang, L. Shen, Z. Zou, Y. Hu, C. Sun, X. Wei, J. Chang, Z. Zhao, Y. Sun, J. Sundell, Validity of subjective questionnaire in evaluating dwelling characteristics, home dampness, and indoor odors in Shanghai, China: cross-sectional survey and on-site inspection, Energy Build. 127 (2016) 1019–1027. [3] J.L. Niu, L.Z. Zhang, Energy requirements for conditioning fresh air and the longterm savings with a membrane-based energy recovery ventilator in Hong Kong, Energy 26 (2001) 119–135. [4] L.-Z. Zhang, Progress on heat and moisture recovery with membranes: From fundamentals to engineering applications, Energy Convers. Manage. 63 (2012) 173–195. [5] K. Zhong, Y. Kang, Applicability of air-to-air heat recovery ventilators in China, Appl. Therm. Eng. 29 (5-6) (2009) 830–840. [6] M. Rasouli, C.J. Simonson, R.W. Besant, Applicability and optimum control strategy of energy recovery ventilators in different climatic conditions, Energy Build. 42 (9) (2010) 1376–1385. [7] S. Tafelmeier, G. Pernigotto, A. Gasparella, Annual performance of sensible and total heat recovery in ventilation systems: humidity control constraints for European climates, Buildings 7 (4) (2017) 28. [8] M.S. Nasif, R. Al-Waked, M. Behnia, G. Morrison, Air to air fixed plate enthalpy heat exchanger, performance variation and energy analysis, J. Mech. Sci. Technol. 27 (11) (2013) 3541–3551. [9] J. Liu, W. Li, J. Liu, B. Wang, Efficiency of energy recovery ventilator with various weathers and its energy saving performance in a residential apartment, Energy Build. 42 (1) (2010) 43–49. [10] W.M. El-Maghlany, A.A. ElHefni, M. ElHelw, A. Attia, Novel air conditioning system configuration combining sensible and desiccant enthalpy wheels, Appl. Therm. Eng. 127 (2017) 1–15.
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