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Utilizing shallow geothermal energy to develop an energy efficient HVAC system
Renewable Energy 147 (2020) 672e682
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Utilizing shallow geothermal energy to develop an energy efficient HVAC system Weihua Lyu, Xianting Li*, Shuai Yan, Sihang Jiang a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 February 2019 Received in revised form 4 September 2019 Accepted 10 September 2019 Available online 12 September 2019
Traditionally, shallow geothermal energy is utilized by a ground source heat pump. In fact, the temperature of shallow geothermal energy is typically quite suitable for the cooling/heating of building envelopes and fresh air. To utilize shallow geothermal energy more efficiently, an integrated system is proposed that combines pipe-embedded walls, pipe-embedded windows, and fresh air pre-handling system with the conventional ground source heat pump system. This proposal is based on the temperature comparison among indoor air, envelopes, fresh air and undisturbed soil. A simulation model of the integrated system is developed on the platform of TRNSYS. The free-running temperature and energy consumption of the integrated system applied in an office building in Beijing are investigated. The results show that the free-running temperature of the integrated system is always below 28 C throughout the year, and the non-air conditioning period is extended by 34% compared with the conventional GSHP system. Moreover, the integrated system can greatly reduce the peak load, and the heat pump system capacity can be reduced by as much as 30%. The annual cumulated load for the building is reduced by approximately 43%. Consequently, the energy saving rate is approximately 29% compared with that of the conventional GSHP system. The emission reduction of CO2 is more than 7 kg per square meter. Therefore, the integrated system can fully utilize shallow geothermal energy to build an energy efficient HVAC system. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Shallow geothermal energy Free-running temperature Pipe-embedded envelope Fresh air pre-handling Building energy efficiency
1. Introduction The issue of energy consumption in buildings has gained increasing concern worldwide due to the increase in living standards and rapid urbanization [1,2]. Because of the important roles of HVAC systems in ensuring indoor thermal comfort, the energy demand for HVAC systems in buildings will continue to increase in the future. The development of energy efficient HVAC systems that do not rely on fossil fuels will play a key role in energy saving [3]. Shallow geothermal energy, as a renewable energy, is popular in the HVAC field. At present, the ground source heat pump (GSHP) system has been regarded as an energy efficient way of utilizing shallow geothermal energy [4,5]. In its conventional application, the ground heat exchanger (GHX) is always integrated with a heat pump unit to generate high-grade hot water or chilled water for space heating and cooling [6e8]. Though the GSHP system has been
* Corresponding author. Department of Building Science, School of Architecture, Beijing Key Laboratory of Indoor Air Quality Evaluation and Control, Tsinghua University, Beijing, 100084, PR China. E-mail address: [email protected] (X. Li). https://doi.org/10.1016/j.renene.2019.09.032 0960-1481/© 2019 Elsevier Ltd. All rights reserved.
widely used to build energy efficient HVAC systems [9e12], the energy consumption of the GSHP system is still at a relatively high level. The primary benefit of shallow geothermal energy, i.e., warm in winter and cold in summer, has not yet fully been utilized by the GSHP system. Recently, the borehole free cooling (BFC) technique was proposed for indoor space cooling to further reduce the energy consumption of the HVAC system. Borehole free cooling involves the outlet water of the GHX being directly connected to the cooling terminal for space cooling. Yuan et al. [13] pointed out that the outlet water of GHX matches the indoor air parameter, and the borehole free cooling can significantly improve the annual cooling efficiency of the GSHP system. Wu et al. [14] investigated a ground source absorption heat pump (GSAHP) system that is integrated with borehole free cooling in cold regions. The borehole free cooling in summer can provide additional cooling and indoor comfort while decreasing the deterioration of the GSAHP system. The borehole free cooling technique just takes full advantage of the temperature difference between indoor air and the soil. On the other hand, many researchers have proposed that natural energy (e.g., the cooling water produced by cooling tower or the cooling/heating water produced by GHX) can be used to intercept
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Nomenclature A C CL E h F I HL U L P q Q m M SHGC t W
Surface area, m2 Specific heat capacity, kJ/(kg$ C) Cooling load, kW Hourly electricity consumption, kW Convection heat transfer coefficient, W/(m2$ C) Fresh air flow rate, m3/h Solar radiation intensity on the glass surface, W/m2 Heating load, kW Heat transfer coefficient, W/(m2$ C) Hourly building load, kW Power of heat pump unit, kW Heat gains in per square meter area, W/m2 Heat transfer capacity, kW Water flow rate, kg/s Annual emission reduction of CO2, kg Solar heat gain coefficient Temperature, C Hourly power consumption, kW
Greek symbol Influence coefficients of ambient temperature Influence coefficients of average water temperature Fresh air density, kg/m3 ε Energy saving rate of the integrated system j Long wave radiation coefficient h Building load reduction rate g Non-air conditioning ratio
a b r
AWHX BFC COP EER GHX GSHP HVAC NTU
Air-water heat exchanger Borehole free cooling Coefficient of performance Energy efficiency ratio Ground heat exchanger Ground source heat pump Heating, ventilation, and air conditioning The number of heat transfer units
Subscripts a base C ci conv e ei H i inte o PCL PHL pwall pwin rad sur trad w win
Fresh air Baseline system Cooling Condenser inlet Convection heat transfer Equivalent Evaporator inlet Heating Inlet Integrated system Outlet Peak cooling load Peak heating load Pipe-embedded wall Pipe-embedded window Radiation Inner surface Traditional Water Window
673
Abbreviation AHU Air handling unit
the heat transfer through building envelopes by embedding water pipes in walls and windows. The thermal performance of a pipeembedded wall was investigated by Xie et al. [15] and Shen and Li [16]. Researchers built a comprehensive numerical model to analyze the dynamic heat transfer process of this special building structure. Li et al. [17] utilized the active layer model of TRNSYS to study the energy saving potential of pipe-embedded walls integrated with GHX in five typical climate regions of China. Shen and Li [18] proposed a pipe-embedded window to reduce solar heat gains in summer. The energy efficiency of the pipe-embedded window has been proved both in the cooling season and heating season in the previous study [16,18e22]. All the referenced studies proved that the combination of natural energy and pipe-embedded envelopes is an energy efficient way to reduce the building load and energy consumption of the HVAC system. In fact, the temperature of shallow geothermal energy is quite suitable for the cooling and heating of envelopes and fresh air during most of the year. Taking the temperature ranges in Beijing as an example, the schematic diagram of temperature relationship of the soil with indoor and outdoor temperature is clearly described in Fig. 1. The difference between indoor temperature and the soil temperature is approximately 10 C in the cooling season and the cooling performance of directly utilizing the available temperature difference for indoor cooling has been investigated in the previously mentioned studies [13,14]. However, the differences between the soil temperature and outdoor fresh air temperature as well as the envelope temperature are much greater than that between soil and indoor temperature. According to Manz [23], the temperature of the blinds in a traditional double glass window can be heated to
60 C by absorbing solar radiation. The greater temperature difference should benefit a more efficient utilization of shallow geothermal energy. Unfortunately, the greater temperature differences have not yet been fully exploited. Moreover, the large differences between the soil temperature and outdoor fresh air temperature as well as the envelope temperature can also be utilized for heating. As shown in Fig. 1, the temperature difference between soil and fresh air would be larger than 20 C during most of the heating season. This indicates that shallow geothermal energy also has great potential for direct utilization in winter. Therefore, shallow geothermal energy should be preferred to eliminate the building load caused by building envelopes and fresh air. In previous studies, shallow geothermal energy was applied to the pre-handling of fresh air by using an earth-to-air heat exchanger system [24e26]. Shallow geothermal energy can be transferred to fresh air through buried air tubes. Utilizing the temperature difference, the fresh air can be efficiently cooled in the summer and heated in the winter [27,28]. However, the earth-to-air heat exchanger system requires a high initial investment. Moreover, when fresh air humidity is high, condensation may appear on the tunnel or the inner surfaces of tubes, which may lead to molds growth, and the fresh air would be polluted before being supplied to the building. Therefore, in this study, the GHX is used to extract shallow geothermal energy from soil and then transfers energy to the fresh air. This method can mitigate the drawbacks of the earthto-air heat exchanger system. Based on analyzing the temperature relationship among indoor air, building envelopes, fresh air and the stable soil, an integrated
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Fig. 1. Schematic diagram of temperature relationship between soil and ambient conditions based on the climate of Beijing.
subsystem of pipe-embedded windows, a subsystem of fresh air pre-handling, and a conventional GSHP system. The air-water heat exchanger (AWHX) is fixed in the air handling unit (AHU) for the pre-handling of fresh air. The water inlets of pipe-embedded walls, pipe-embedded windows, the AWHX, and the condenser/evaporator of the heat pump unit are connected with the distributor. The corresponding water outlets are connected with the collector. Therefore, the outlet water of the GHX can be delivered to these four parts based on the control strategies. In the cooling condition (the cooling season and part of the transition season), the outlet water of the GHX can be pumped to pipe-embedded walls, pipe-embedded windows, the AWHX, and the condenser of the heat pump unit as required. The cooling water circulating in pipe-embedded walls takes away the heat that is
system that can utilize shallow geothermal energy more efficiently is proposed in this study. The integrated system combines pipeembedded walls, pipe-embedded windows, and the fresh air prehandling system with the conventional GSHP system. To demonstrate the capability of the integrated system, a simulation model of the integrated system is built on the platform of TRNSYS. The freerunning temperature, building load reduction rates, and the energy saving potential of the integrated system are investigated by comparing the integrated system with a conventional GSHP system. 2. Principle of the integrated system
Pipe-embedded windows
Pipe-embedded walls
The schematic of the integrated system is shown in Fig. 2. The integrated system includes a subsystem of pipe-embedded walls, a
Fresh air AWHX inlet
Return air inlet AHU
Heat Pump
GHX
Distributor
P
Collector
Fig. 2. Schematic of the integrated system.
Air supply
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stored in the walls. The heat transfer between the external walls and indoor space is consequently intercepted. Part of the cooling water precools the high temperature fresh air in the AWHX. Thus, the fresh air load that has to be removed by the heat pump unit can be reduced. In the pipe-embedded window, the solar heat gains that are absorbed by the blinds and pipes can be removed by the flow water in time. Thus, the cavity temperature of the pipeembedded window is much lower than that of the double window with traditional blinds. The heat fluxes via windows to indoor space are reduced significantly. Therefore, with the effects of pipeembedded walls, pipe-embedded windows and the fresh air prehandling system, the free-running temperature of the room space may be maintained within a comfortable zone without turning on the heat pump system. When the free-running temperature cannot meet thermal comfort, the heat pump system will be turned on to remove the rest of the building load. In the heating condition (the heating season and part of the transition season), the outlet water of the GHX is delivered to pipeembedded walls, the AWHX and the evaporator of the heat pump unit as required. The heat transfer process is similar to that of the cooling season. The outlet water of the GHX can be used to reduce the heat dissipation of external walls and preheat fresh air. As a result, the non-air conditioning period in the heating season can be extended and the heating load that has to be removed by the heat pump system can be reduced.
3. Methods To investigate the performance of the integrated system, the system model is built based on a case study of an office building in Beijing where the GSHP system has been widely used in practice. The models of the key components and the evaluation indexes are interpreted in this section.
3.1. Building outlines A three-story office building in Beijing is chosen as the serviced object of the integrated system. The building is 40 m long from north to south, 10 m wide from west to east, and 10.5 m high. The occupant density is 8 m2/person. Each person has a printer and a computer with a total power of 140 W. The lighting power is 13 W/ m2. All the heat gains inside the selected room spaces are on/off controlled according to the regular occupancy schedule, which is set from 8:00 a.m. to 6:00 p.m. The cooling season is set from June 1st to August 31st and the heating season is set from November 15th to March 15th. The office building is simulated with good insulation and airtightness. The infiltration coefficient is set as 0.2. The heat transfer coefficient of the external walls and roof are respectively 0.522 W/(m2$ C) and 0.402 W/(m2$ C), which meet the Design standard for energy efficiency of public building GB 50189-2015 [29]. Pipes are embedded in the concrete layer of external walls and the roof in a serpentine pattern. Windows are fixed in the south and north walls and the window-wall ratio is 0.3. The windows are designed with double glass and pipes are embedded in the cavity of the double glass windows. The heat transfer coefficient of the double glass is 1.5 W/(m2$ C). A traditional office building with the same settings and same building envelopes except for the pipes, is utilized as the benchmark. The conventional GSHP system is applied for the traditional office building, which is analyzed as the baseline case. The configurations of the traditional double glass window and the pipe-embedded window [18] are shown in Fig. 3, and the main difference is the pipes.
675
Fig. 3. Configurations of the traditional double glass window and the pipe-embedded window.
3.2. Model of the pipe-embedded wall To analyze the heat transfer process of the pipe-embedded wall [30], two parts should be considered: the two-dimensional heat transfer of the cross-section of the pipe-embedded wall and the heat transfer along the water loop. The first part aims to calculate the temperature field in the plane of the cross-section. The RC (resistance and capacitance) model is utilized for this analysis. The second part takes the water temperature change into account. The finite element method (FEM) is used to solve this problem. Therefore, the calculated region needs to be divided into many small three-dimensional grid cells. The required variables can be calculated step by step for each cell and for each time step. In this study, the existing model of thermal active layer inside the building model, type 56 [30] of TRNSYS is used to simulate the heat transfer of pipe-embedded wall. The accuracy of the thermal active layer model in TRNSYS has been validated by the literature [31]. To achieve a high level of precision, the time step is 4 min for this simulation process. By referencing Li et al. [17], the parameters of the pipe-embedded walls are listed in Table 1. The water pump of the pipe-embedded wall system is on-off controlled. In the cooling condition, when the indoor temperature is higher than 25 C and the inlet water temperature of the pipe is lower than the inside surface temperature of the wall, the water pump is turned on. Similarly, in the heating condition, the water pump is activated when the room temperature is lower than 21 C and the inlet water temperature is higher than the inside surface temperature of the wall. According to the simulation, the internal surface temperature of each wall can be obtained. The hourly heat flux, Q wall , through the wall to indoor space can be calculated by equation (1).
Q wall ¼ hsur , Awall ,ðtsur troom Þ
(1)
where hsur is the convection heat transfer coefficient on the inner surface of the external wall, kW/(m2$ C); Awall is the internal surface area of the external wall, m2; tsur is the internal surface temperature of the external wall, C; troom is the indoor room temperature. Compared with the traditional wall, the hourly building load reductions caused by the pipe-embedded wall are calculated as:
DQwall ¼ Qwall;trad Qwall;pipe
(2)
where Qwall;trad and Qwall;pipe represent the hourly heat flux through the traditional external walls and the pipe-embedded walls to indoor spaces, respectively; The hourly heat capacity of the pipe-embedded wall, Qpwall , can be calculated as:
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W. Lyu et al. / Renewable Energy 147 (2020) 672e682 Table 1 Parameters of the pipe-embedded walls. Items
Pipe spacing (mm)
Pipe inner diameter (mm)
Water flow rate (m/s)
Values
150
16
0.5
Qpwall ¼ Cw ,mw , twall;o twall;i
(3)
where Cw is the specific heat capacity of water, 4.19 kJ/(kg$ C); mw is the total water flow rate, kg/s. twall;i and twall;o are the water pipe inlet and outlet temperatures, respectively, C.
3.3. Model of the pipe-embedded window The structure of the pipe-embedded window [18] that is used in this study is shown in Fig. 4. Referencing the literatures [18,19], the window materials are listed in Table 2. Compared with the traditional double glass window, the water pipes are fixed on the blinds. Because of absorbing solar radiation, the original temperature of the blinds is so high that relatively warm cooling water can be used to directly remove the heat gains. The heat transfer of a pipe-embedded window has been investigated completely in our previous study [20], in which the analytic method of a pipe-embedded window was developed based on the thermal network of a pipe-embedded window. According to the results of reference [20], a pipe-embedded window can be treated as an equivalent traditional window by using the equivalent heat transfer coefficient Ue, the equivalent solar heat gain coefficient SHGCe, and equivalent ambient temperature tout . All the short-wave solar heat, conductive, convective and long-wave radiative heat transfers have been considered in the analytic method. Based on the analytic method, the heat transfer of the pipe-embedded window used in the integrated system can be expressed as equations (4)e(6):
SHGCe represents the equivalent solar heat gain coefficient of a pipe-embedded window; tout represents the equivalent ambient temperature outside the window, which is affected by the water temperature and outdoor temperature, C; je is the ratio of absorption-conduction heat gains of a pipe-embedded window; t is the short-wave solar transmittance; I means the solar radiation intensity on the glass surface, W/m2; tw is the average water temperature in the pipes, C; a and b are the influence coefficients of ambient temperature and the average water temperature. For the specific pipe-embedded window that used in the integrated system, the values of Ue, je, t, a and b are 2.5, 0.05, 0.1, 0.2, and 0.8, respectively, which are referred to our previous study [20]. The solar heat gains of a pipe-embedded window include two parts, i.e, the absorption-conduction heat gains and the short-wave solar transmitted heat gains. As the influences of these two parts of heat gains on indoor thermal environment are different, the two parts solar heat gains of a pipe-embedded window should be considered separately when adding the model of a pipe-embedded window to the building model (type 56 in TRNSYS). Therefore, equation (4) is divided into equations (7) and (8).
qnv
pipe
qrad
¼ Ue , tout;e troom þ je ,I
pipe
¼ t,I
(7) (8)
where qnv pipe represents the hourly heat gains caused by conduction and convection in per square meter of window, W/m2; qrad represents the short-wave solar transmitted heat gains, W/m2. The hourly heat capacity of the pipe-embedded window, Qpwin , is calculated by equation (9) and equation (10).
qpwin ¼ Ue , tout;e troom þ SHGCe ,I
(4)
Qpwin ¼ 8 , tout;e tw þ 0:5 , I ,Awin ¼ Cw ,mw , twin;o twin;i
SHGCe ¼ je þ t
(5)
(9)
tout;e ¼ a,tout þ b,tw
(6)
where qpwin is the indoor heat gains caused by per square meter of a pipe-embedded window, W/m2; Ue represents the equivalent heat transfer coefficient of a pipe-embedded window, W/(m2$ C);
tw ¼ twin;i þ twin;o 2
(10)
where Awin is the window area, m2; twin;i is the inlet water temperature of the pipe-embedded window, C; twin;o is the outlet water temperature of the pipe-embedded window, C. For the traditional double glass window, the hourly heat gains caused by conduction and convection in per square meter of window, qnv trad , can be expressed as equation (11).
qconv
trad
¼ U,ðtout troom Þ þ j,I
(11)
where U represents the heat transfer coefficient of the window, W/ (m2$ C); tout represents the ambient temperature outside the window, C; j is the ratio of absorption-conduction heat gains of a traditional double window. For the specific traditional double glass window used in the baseline system, the values of U and j are 1.5 and 0.25, respectively. The short-wave solar transmitted heat gains of the traditional double glass window are the same as that of the pipe-embedded window because of the same arrangement of blinds.
qrad Fig. 4. Structures and dimensions of the pipe-embedded window used in this study.
trad
¼ qrad
pipe
(12)
Compared with the traditional double glass windows, the
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Table 2 Window materials. Items
External
Internal
Blinds
Material Thickness (mm) Specific heat (J/kg$ C) Density (kg/m3) Heat conductivity coefficient (W/(m$ C))
Double glazing 6 þ 12airþ6 100 1000 0.09
Stalinite 9 840 2200 0.28
Aluminum alloy 2 880 2700 180
hourly building load reductions caused by the pipe-embedded window are calculated utilizing equation (13).
DQwin ¼ qconv;trad qconv;pipe ,Awin
(13)
As for the control strategy of the pipe-embedded windows system, the water pump of the pipe-embedded windows can be activated when the solar radiation is greater than 200 W/m2 in the cooling condition. In the heating condition, the pipe-embedded windows system does not work, which means that no water is pumped to the pipe-embedded windows and the heat transfer performance of the pipe-embedded window is regarded as the same with the traditional double glass window with conventional blinds. 3.4. Model of the AWHX The fresh air volume is set as 30 m3/(h$person) for the air conditioning system. As described in section 2, the fresh air is prehandled by the AWHX in the AHU. The design air pre-handling temperature difference is 8 C and design water temperature difference is 3 C. The design parameters are listed in Table 3. The AWHX is a cross flow air-water heat exchanger and the εeNTU model is used to simulate its heat transfer process. The type 5e in TRNSYS is used to model the AWHX. The fresh air pre-handling system can only be activated when the inlet water temperature of the AWHX is 3 C less in cooling conditions and more in heating conditions than the inlet temperature of fresh air. The hourly heat exchange capacity QAWHX of the AWHX is expressed as equation (14).
DQAWHX ¼ abs Ca ,
F,r , ðtao tai Þ 3600
(14)
where F is the fresh air volume, m3/h; r is the fresh air density, assumed as 1.2 kg/m3; tai , tao are the fresh air inlet and outlet temperatures of the AWHX, respectively, C. The hourly building load reductions caused by the fresh air prehandling system compared with those caused by the baseline system are equal to the heat exchange capacity of the AWHX. 3.5. System design and evaluation index According to the calculation model of the pipe-embedded window in section 3.3, the convective heat gains and the radiative heat gains can be calculated separately. Thus the heat gains
caused by windows can be added to the building model in forms of indoor convective heat source and radiative heat source, respectively. In this way, the heat transfer model of the pipe-embedded window is implanted in the building model (i.e., Type 56) on TRNSYS. Thus, the new building envelopes can be built with the model of the pipe-embedded window and the existing model of thermal active layer inside of Type 56 [30]. The complete simulation model of the integrated system can be built on the platform of TRNSYS according to section 2. The ground heat exchanger is used as the heating and cooling source and the type 557a, a model of Duct Ground Heat Storage, in TRNSYS is used to simulate its heat transfer process. The type 557a has considered the effect of the rise/drop of soil temperature on the heat transfer of ground heat exchangers. Therefore, the effects of pipe-embedded walls, pipe-embedded windows, the fresh air prehandling system and GSHP unit on the change of soil temperature have been considered in the simulation. The accuracy of this dynamic model has been validated in a detailed study [32]. The single U-tube vertical GHX is utilized and the primary design parameters are listed in Table 4. The heat pump produces 45 C hot water for space heating and 7 C chilled water for space cooling. The inlet water temperature of the source side is 25 C in rated cooling condition and 10 C in rated heating condition. The performance is fitted according to the manufacturer's catalogue. The correction coefficients of the heat pump capacity and power are fitted as follows:
QC ¼ 0:0091,tci þ 1:2269; R2 ¼ 0:9976
(15)
PC ¼ 0:0177,tci þ 0:5572; R2 ¼ 0:9968
(16)
QH ¼ 0:0395,tei þ 0:5871; R2 ¼ 0:9985
(17)
PH ¼ 0:0038,tei þ 0:8927; R2 ¼ 0:9951
(18)
where QC , PC , QH and PH are the correction coefficients of the cooling capacity, cooling power consumption, heating capacity, and heating power consumption, respectively; tci and tei are the inlet water temperatures of the source side in cooling conditions and heating conditions, respectively. The simulation process includes three steps. First, the freerunning temperature is simulated by keeping the heat pump
Table 4 The primary design parameters of the GHX. Table 3 The design parameters of AWHX. 3
Design fresh air flow rate
4500 m /h
Design water flow rate Flow resistance in water side Fan pressure drop caused by AWHX Heat transfer area Heat transfer rate
3 m3/h 5 m H2O 180 Pa 48 m2 1920 W/ C
Items
values
Borehole depth Initial soil temperature Soil thermal conductivity Soil heat capacity Soil density Ground thermal conductivity Radius of Uetube pipe
100 m 14.56 C 1.4 W/(m$ C) 2016 kJ/(m3$ C) 1857 kg/m3 1.4 W/(m$ C) 16 mm
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system off and turning on the pipe-embedded walls, pipeembedded windows, and the fresh air pre-handling system. Second, the cooling and heating loads are simulated by turning on the heat pump system. The heat pump system capacity can be designed according to the peak load. Finally, the system performance is simulated based on the cooling and heating load profile and the selected heat pump system. Regarding the baseline, the peak cooling and peak heating loads are 141.2 kW (i.e., 118 W/m2) and 96.1 kW (i.e., 80 W/m2), respectively. The cumulated cooling load is 74454 kWh (62 kWh) while the cumulated heating load is 49512 kWh (42 kWh). The heat pump system capacity is designed based on the peak cooling load. According to engineering experience, the design heat flux of the GHX is 50 W per linear meter in cooling condition in Beijing. Consequently, thirty-six boreholes are required and the rated cooling capacity of the heat pump unit at the baseline is 143.4 kW. The number of boreholes for the integrated system is designed in keeping with the baseline system. To evaluate the energy performance of the integrated system, the available heat transfer temperature difference is estimated first, and the non-air conditioning ratio is defined based on the free-running temperature. Then, the building load reduction rates are provided. Furthermore, the COPs of the pipe-embedded walls, windows, and the fresh air pre-handling system are defined. Finally, the energy saving and emission reduction rates are evaluated. The available heat transfer temperature difference is defined as:
Dt i ¼ ti tsoil
(19)
where i refers to “wall”, “win”, “a”, and “room”. Thus, the Dt wall, Dt win, Dt a, and Dt room are the available heat transfer temperature differences between the wall, the window, the fresh air, the indoor air and the stable soil, respectively; twall is the wall temperature, represented by the average temperature of the inside and outside surfaces of the walls; twin is the window temperature, and it is represented by the average temperature of the cavity of the double glass windows; ta and tsoil are the fresh air and the soil temperatures, respectively. The non-air conditioning ratio is defined as:
g¼
TnonAC 100% Toccupied
hPHL ¼ hL ¼
ð
HLmax;base HLmax;inte 100% HLmax;base
P
CL þ
P P HLÞ ð CL þ HLÞinte P base P 100% ð CL þ HLÞbase
COPwall ¼
Qpwall Wwall
(24)
COPwin ¼
Qpwin Wwin
(25)
COPAWHX ¼
(20)
QAWHX WAWHX þ Wfan
(26)
where Wwall , Wwin , WAWHX are the hourly energy consumptions of water pumps used for the pipe-embedded walls, pipe-embedded windows, and the fresh air pre-handling system; Wfan is the hourly additional energy consumption of fans caused by the resistance of the AWHX. The energy consumption of the baseline system and the integrated system are mainly expressed as:
Ebase ¼ WGHX
base
Einte ¼ WGHX
inte
þ WHP
þ WHP
base
inte
þ
þ
Lbase EERindoor
(27)
Linte þ Wwall þ Wwin EERindoor
þ WAWHX þ Wfan
(28)
where E is the hourly electricity consumption; WGHX is the hourly power consumption of the water pump in the source side of the heat pump unit; WHP is the hourly energy consumption of the heat pump unit; L is the hourly building load; EERindoor is the energy efficiency ratio of the indoor air conditioning terminals, which is set as 30 in this study [33]. Compared with the baseline system, the energy saving rate of the integrated system is expressed as:
ε¼
where TnonAC is the non-air conditioning period, which means that the free-running temperature is between 20 and 26 C without the operation of the heat pump system; Toccupied is the period during which the office room is occupied. The hourly cooling and heating loads can be obtained directly from the simulation. The building load reduction rates are defined as follows:
CL CLmax;inte hPCL ¼ max;base 100% CLmax;base
baseline system and the integrated system. The average COPs of the pipe-embedded wall, pipe-embedded window, and the fresh air pre-handling system are defined respectively as follows:
P ðEbase Einte Þ P 100% Ebase
(29)
According to reference [34], the conversion relationship between electricity and standard coal is 1 kWh ¼ 0.404 kg standard coal. The CO2 discharge coefficient is 2.493 kg/kg standard coal. The emission reduction of CO2, MCO2 , can be calculated as:
MCO2 ¼ 0:404 2:493,ðEbase Einte Þ ¼ 1:007,ðEbase Einte Þ (30) 4. Results and discussion
(21)
(22)
P
(23)
where hPCL , hPHL , hL are the reduction rates of peak cooling load, peak heating load, and annual cumulated load, respectively; CL and HL are the hourly cooling load and the hourly heating load, respectively; and the subscripts “base” and “inte” indicate the
The energy efficiency of the integrated system is investigated as follows: (ⅰ) analyzing the available temperature difference in different heat transfer processes; (ⅱ) calculating the free-running temperature and the non-air conditioning ratio; (ⅲ) calculating the building load and system capacity reductions of the heat pump system; and (ⅳ) indicating the energy saving and emission reduction potential. 4.1. Available heat transfer temperature difference To better understand the design purpose of the integrated system, the available temperature differences in different heat transfer processes are analyzed to demonstrate the advantage of the
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integrated system. The temperature distribution of the baseline system in the occupied time is shown in Fig. 5. For the traditional GSHP system, the soil temperature changes minimally throughout the year. The cavity temperature of the traditional double glass windows can be over 60 C in the cooling season, which is caused by much solar radiation absorbed by conventional blinds. In the heating season, the fresh air temperature is much lower than the soil temperature while the indoor design temperature is always higher than the soil temperature. This indicates that shallow geothermal energy can be used to heat the fresh air but not to heat the indoor air. The available heat transfer temperature difference between the soil and the indoor air, the walls, the windows, as well as the fresh air is demonstrated in Fig. 6. The temperature difference between the soil and the indoor air is always 10e15 C in the cooling season and there is no available temperature difference in the heating season, because the indoor design temperature is 26 C and 20 C in the occupied time of the cooling season and heating season, respectively. The temperature difference between the soil and the wall ranges from in 10e15 C in the cooling season and 0e10 C in the heating season. Temperature differences between the soil and the window that are greater than 20 C account for approximately 80% of the cooling season. The available temperature difference between the soil and the fresh air is mostly between 10 and 20 C during both the cooling and heating seasons. The available temperature difference distribution frequency indicates that shallow geothermal energy would perform well to directly cool the windows, the walls, and the fresh air during the cooling season. Moreover, the integrated system also has the effect on preheating the fresh air and intercepting the heat transfer through the wall.
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Fig. 6. The available heat transfer temperature difference between the soil and the indoor air, the wall, the window, as well as the fresh air.
Fig. 7. Free-running temperature distribution frequency.
4.2. Increase in the non-air conditioning ratio To investigate the increase in the non-air conditioning ratio, the free-running temperature of both the integrated system and the baseline system are simulated. The free-running temperature of the corner room in the top floor is taken as an example, as is shown in Fig. 7. For the baseline office room, the period that freerunning temperature is lower than 15 C or higher than 30 C accounts for more than 70% of a year. This indicates that the room temperature is terribly uncomfortable during most of the year without the operation of the conventional heat pump system. For the office room with the integrated system, the free-running temperature will no longer exceed 30 C and the period of time that it is lower than 15 C is reduced apparently. The ratio of the free-running temperature in the comfortable zone (i.e., when the
Fig. 5. The temperature distribution of the baseline system in the occupied time.
temperature is between 20 and 26 C) is more than 40% in the integrated system but less than 10% in the traditional baseline system. The free-running temperature distribution of the baseline and the integrated systems are illustrated in Fig. 8. In the baseline system, the free-running temperature would be over 40 C. With the effect of the pipe-embedded walls, pipe-embedded windows, and the fresh air pre-handling system, the free-running temperature of the integrated system is always below 28 C throughout the cooling season. It is obvious that if the integrated system is applied for a residential building with small indoor heat gains, the period with acceptable thermal comfort will be much longer.
Fig. 8. The free-running temperature distribution of the baseline and the integrated system.
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4.3. Reduction of building load and capacity of heat pump system According to the simulation, the peak loads and heat pump system capacities of both the baseline system and the integrated system are listed in Table 5. In the cooling season, on the one hand, the high temperature fresh air is precooled by the shallow geothermal energy directly before being processed by the heat pump system; on the other hand, the embedded pipes in the building envelopes are adequate to intercept the heat transfer through the external walls and remove the solar radiation heat gains through the windows. Therefore, the cooling load decreases significantly. In the heating season, the available temperature differences are less than those in the cooling season, as demonstrated in Fig. 6. Therefore, the reduction rates of both the peak heating load and cumulated heating load are less than those of the peak cooling load and cumulated cooling load. The reduction rates of peak cooling load, peak heating load, and heat pump system capacity are 29%, 22%, and 28% respectively. Because the capability declines when the working condition turns worse at the peak cooling load, the rated heat pump capacities are larger than the peak cooling load. The hourly building load in the cooling season and heating season are shown in Fig. 9. Compared with the baseline system, the building load of the integrated system is reduced significantly, especially in the cooling season. The reason for the building load decrease is investigated by analyzing the inside surface temperature change of the south wall and south window. As presented in Fig. 10, in the baseline system, the inside surface temperatures of the wall and window are higher than the room design temperature, which is 26 C in the cooling season. When there are high levels of solar radiation, the inside surface temperature of the traditional double glass window can be 40 C. In the integrated system, the outlet water temperature has the most influence on the heat transfer performance of the pipe-embedded window and the pipeembedded wall. The inside surface temperatures of both the pipeembedded wall and the pipe-embedded window are much lower than those of the traditional wall and window in the baseline system. Most of the time, the inside surface temperature of the pipe-embedded wall is lower than the room design temperature, which means that the pipe-embedded wall also can eliminate part of the indoor cooling load. Therefore, the building load that has to be removed by the heat pump system can be reduced significantly. The reduction rates of the cumulated cooling load and the cumulated heating load are 56% and 22%, respectively. In terms of the whole year, the cumulated load reduction rate is approximately 43%. The cumulated load reduction consists of three parts, which are shown in Fig. 11: reduction by pipe-embedded walls, reduction by pipe-embedded windows, and reduction by fresh air pre-handling system. Because of the low outlet water temperature of GHX and the large heat exchange area, the reduction of the calculated load contributed to by the pipe-embedded walls is much greater than that contributed to by the pipe-embedded windows and fresh air pre-handling system. The contribution proportions of the three parts are 45%, 26%, and 29%, respectively.
Table 5 The rated heat pump capacity. Items
Baseline system
The integrated system
The number of boreholes Peak cooling load (kW) Peak heating load (kW) Rated heat pump capacity (kW)
36 141.2 96.1 143.4
36 98.9 75.1 103.2
Fig. 9. Hourly building load profile.
Fig. 10. The inside surface temperature of the building envelopes in the cooling season.
Fig. 11. Cumulated load reduction rates by pipe-embedded wall, pipe-embedded window, and fresh air pre-handling system.
4.4. Energy saving and emission reduction The outlet water of the GHX is integrated with pipe-embedded walls, pipe-embedded windows, and the fresh air pre-handling coils for direct utilization in the integrated system. The annual average COPs of the direct utilization of shallow geothermal energy are quite high, as demonstrated in Fig. 12. The annual average COP of pipe-embedded windows is approximately 50, which is much higher than that of pipe-embedded walls and the fresh air prehandling system. The minimum annual average COP of the three components is above 20 and it is about five times that of the conventional heat pump system. The energy saving and emission reduction are calculated
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Fig. 12. The annual average COPs of direct utilization of shallow geothermal energy.
Table 6 Energy saving and emission reduction.
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(1) The available temperature differences between the undisturbed soil and the building envelopes as well as the fresh air can exceed 20 C most of the time, which provides a way to utilize the shallow geothermal energy efficiently. (2) The free-running temperature of the integrated system is always below 28 C throughout the year, and the non-air conditioning rate increases by 34% compared with that of the conventional GSHP system. (3) The heat pump capacity of the integrated system can be reduced by as much as 30% compared to the conventional GSHP system. Moreover, the building annual cumulated load is reduced by approximately 43%. The contribution proportions of the pipe-embedded walls, pipe-embedded windows, and fresh air pre-handling system to the cumulated load reduction are approximately 46%, 25%, and 29%, respectively. (4) The energy saving rate of the integrated system compared to that of the conventional GSHP system is approximately 29%, and the reduction of CO2 emission is more than 7 kg per square meter of floor area.
Items
Energy savings (kWh/m2)
Emission reductions (kg/m2)
Energy saving rate
Acknowledgements
value
7.7
7.8
28.5%
This study was supported by the National Natural Science Foundation of China (Grant No. 51638010 and Grant No. 51578306).
compared with the baseline system, and the results are listed in Table 6. The energy saving rate is approximately 28.5%. The electricity savings are approximately 7.7 kWh per unit of floor area. This benefits the emission reduction of CO2, which is more than 7 kg per square meter of floor area. In addition, for the cost of pipe-embedded windows, the previous study [18] indicates that although cheap glass is adopted for the pipe-embedded windows, the heat flux can be reduced significantly, which implies that the investment would increase a little by using pipe-embedded windows. For the cost of pipe-embedded walls, the previous study [17] has been discussed the application issue in detail. The results show that the energy savings of pipeembedded wall system range from 10% to 45% in different climates. The cost of the GHXs is the main investment of the pipeembedded wall system. The cost of the fresh air pre-handling system has been discussed in our previous study [35]. The results show that the pay-back periods are one to three years in different regions. As for the integrated system in this study, the energy saving rate is approximately 29% in the case study of this paper. Therefore, it can be deduced that this integrated system has a great potential of economic feasibility compared with a traditional GSHP system. 5. Conclusions The shallow geothermal energy is not fully utilized by the conventional GSHP system. To improve the usage efficiency of shallow geothermal energy, an integrated system, in which pipeembedded walls, pipe-embedded windows, and a fresh air prehandling system are combined with the conventional GSHP system is proposed based on the analysis of the temperature relationship between the envelopes, fresh air, indoor air and undisturbed soil. The performance of the integrated system is investigated in terms of the increase in the non-air conditioning rate, the decrease in building load and the capacity of the heat pump system as well as the energy saving and emission reduction. The conclusions based on the case study of an office building in Beijing are as follows:
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Utilizing shallow geothermal energy to develop an energy efficient HVAC system Weihua Lyu, Xianting Li*, Shuai Yan, Sihang Jiang a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 February 2019 Received in revised form 4 September 2019 Accepted 10 September 2019 Available online 12 September 2019
Traditionally, shallow geothermal energy is utilized by a ground source heat pump. In fact, the temperature of shallow geothermal energy is typically quite suitable for the cooling/heating of building envelopes and fresh air. To utilize shallow geothermal energy more efficiently, an integrated system is proposed that combines pipe-embedded walls, pipe-embedded windows, and fresh air pre-handling system with the conventional ground source heat pump system. This proposal is based on the temperature comparison among indoor air, envelopes, fresh air and undisturbed soil. A simulation model of the integrated system is developed on the platform of TRNSYS. The free-running temperature and energy consumption of the integrated system applied in an office building in Beijing are investigated. The results show that the free-running temperature of the integrated system is always below 28 C throughout the year, and the non-air conditioning period is extended by 34% compared with the conventional GSHP system. Moreover, the integrated system can greatly reduce the peak load, and the heat pump system capacity can be reduced by as much as 30%. The annual cumulated load for the building is reduced by approximately 43%. Consequently, the energy saving rate is approximately 29% compared with that of the conventional GSHP system. The emission reduction of CO2 is more than 7 kg per square meter. Therefore, the integrated system can fully utilize shallow geothermal energy to build an energy efficient HVAC system. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Shallow geothermal energy Free-running temperature Pipe-embedded envelope Fresh air pre-handling Building energy efficiency
1. Introduction The issue of energy consumption in buildings has gained increasing concern worldwide due to the increase in living standards and rapid urbanization [1,2]. Because of the important roles of HVAC systems in ensuring indoor thermal comfort, the energy demand for HVAC systems in buildings will continue to increase in the future. The development of energy efficient HVAC systems that do not rely on fossil fuels will play a key role in energy saving [3]. Shallow geothermal energy, as a renewable energy, is popular in the HVAC field. At present, the ground source heat pump (GSHP) system has been regarded as an energy efficient way of utilizing shallow geothermal energy [4,5]. In its conventional application, the ground heat exchanger (GHX) is always integrated with a heat pump unit to generate high-grade hot water or chilled water for space heating and cooling [6e8]. Though the GSHP system has been
* Corresponding author. Department of Building Science, School of Architecture, Beijing Key Laboratory of Indoor Air Quality Evaluation and Control, Tsinghua University, Beijing, 100084, PR China. E-mail address: [email protected] (X. Li). https://doi.org/10.1016/j.renene.2019.09.032 0960-1481/© 2019 Elsevier Ltd. All rights reserved.
widely used to build energy efficient HVAC systems [9e12], the energy consumption of the GSHP system is still at a relatively high level. The primary benefit of shallow geothermal energy, i.e., warm in winter and cold in summer, has not yet fully been utilized by the GSHP system. Recently, the borehole free cooling (BFC) technique was proposed for indoor space cooling to further reduce the energy consumption of the HVAC system. Borehole free cooling involves the outlet water of the GHX being directly connected to the cooling terminal for space cooling. Yuan et al. [13] pointed out that the outlet water of GHX matches the indoor air parameter, and the borehole free cooling can significantly improve the annual cooling efficiency of the GSHP system. Wu et al. [14] investigated a ground source absorption heat pump (GSAHP) system that is integrated with borehole free cooling in cold regions. The borehole free cooling in summer can provide additional cooling and indoor comfort while decreasing the deterioration of the GSAHP system. The borehole free cooling technique just takes full advantage of the temperature difference between indoor air and the soil. On the other hand, many researchers have proposed that natural energy (e.g., the cooling water produced by cooling tower or the cooling/heating water produced by GHX) can be used to intercept
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Nomenclature A C CL E h F I HL U L P q Q m M SHGC t W
Surface area, m2 Specific heat capacity, kJ/(kg$ C) Cooling load, kW Hourly electricity consumption, kW Convection heat transfer coefficient, W/(m2$ C) Fresh air flow rate, m3/h Solar radiation intensity on the glass surface, W/m2 Heating load, kW Heat transfer coefficient, W/(m2$ C) Hourly building load, kW Power of heat pump unit, kW Heat gains in per square meter area, W/m2 Heat transfer capacity, kW Water flow rate, kg/s Annual emission reduction of CO2, kg Solar heat gain coefficient Temperature, C Hourly power consumption, kW
Greek symbol Influence coefficients of ambient temperature Influence coefficients of average water temperature Fresh air density, kg/m3 ε Energy saving rate of the integrated system j Long wave radiation coefficient h Building load reduction rate g Non-air conditioning ratio
a b r
AWHX BFC COP EER GHX GSHP HVAC NTU
Air-water heat exchanger Borehole free cooling Coefficient of performance Energy efficiency ratio Ground heat exchanger Ground source heat pump Heating, ventilation, and air conditioning The number of heat transfer units
Subscripts a base C ci conv e ei H i inte o PCL PHL pwall pwin rad sur trad w win
Fresh air Baseline system Cooling Condenser inlet Convection heat transfer Equivalent Evaporator inlet Heating Inlet Integrated system Outlet Peak cooling load Peak heating load Pipe-embedded wall Pipe-embedded window Radiation Inner surface Traditional Water Window
673
Abbreviation AHU Air handling unit
the heat transfer through building envelopes by embedding water pipes in walls and windows. The thermal performance of a pipeembedded wall was investigated by Xie et al. [15] and Shen and Li [16]. Researchers built a comprehensive numerical model to analyze the dynamic heat transfer process of this special building structure. Li et al. [17] utilized the active layer model of TRNSYS to study the energy saving potential of pipe-embedded walls integrated with GHX in five typical climate regions of China. Shen and Li [18] proposed a pipe-embedded window to reduce solar heat gains in summer. The energy efficiency of the pipe-embedded window has been proved both in the cooling season and heating season in the previous study [16,18e22]. All the referenced studies proved that the combination of natural energy and pipe-embedded envelopes is an energy efficient way to reduce the building load and energy consumption of the HVAC system. In fact, the temperature of shallow geothermal energy is quite suitable for the cooling and heating of envelopes and fresh air during most of the year. Taking the temperature ranges in Beijing as an example, the schematic diagram of temperature relationship of the soil with indoor and outdoor temperature is clearly described in Fig. 1. The difference between indoor temperature and the soil temperature is approximately 10 C in the cooling season and the cooling performance of directly utilizing the available temperature difference for indoor cooling has been investigated in the previously mentioned studies [13,14]. However, the differences between the soil temperature and outdoor fresh air temperature as well as the envelope temperature are much greater than that between soil and indoor temperature. According to Manz [23], the temperature of the blinds in a traditional double glass window can be heated to
60 C by absorbing solar radiation. The greater temperature difference should benefit a more efficient utilization of shallow geothermal energy. Unfortunately, the greater temperature differences have not yet been fully exploited. Moreover, the large differences between the soil temperature and outdoor fresh air temperature as well as the envelope temperature can also be utilized for heating. As shown in Fig. 1, the temperature difference between soil and fresh air would be larger than 20 C during most of the heating season. This indicates that shallow geothermal energy also has great potential for direct utilization in winter. Therefore, shallow geothermal energy should be preferred to eliminate the building load caused by building envelopes and fresh air. In previous studies, shallow geothermal energy was applied to the pre-handling of fresh air by using an earth-to-air heat exchanger system [24e26]. Shallow geothermal energy can be transferred to fresh air through buried air tubes. Utilizing the temperature difference, the fresh air can be efficiently cooled in the summer and heated in the winter [27,28]. However, the earth-to-air heat exchanger system requires a high initial investment. Moreover, when fresh air humidity is high, condensation may appear on the tunnel or the inner surfaces of tubes, which may lead to molds growth, and the fresh air would be polluted before being supplied to the building. Therefore, in this study, the GHX is used to extract shallow geothermal energy from soil and then transfers energy to the fresh air. This method can mitigate the drawbacks of the earthto-air heat exchanger system. Based on analyzing the temperature relationship among indoor air, building envelopes, fresh air and the stable soil, an integrated
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Fig. 1. Schematic diagram of temperature relationship between soil and ambient conditions based on the climate of Beijing.
subsystem of pipe-embedded windows, a subsystem of fresh air pre-handling, and a conventional GSHP system. The air-water heat exchanger (AWHX) is fixed in the air handling unit (AHU) for the pre-handling of fresh air. The water inlets of pipe-embedded walls, pipe-embedded windows, the AWHX, and the condenser/evaporator of the heat pump unit are connected with the distributor. The corresponding water outlets are connected with the collector. Therefore, the outlet water of the GHX can be delivered to these four parts based on the control strategies. In the cooling condition (the cooling season and part of the transition season), the outlet water of the GHX can be pumped to pipe-embedded walls, pipe-embedded windows, the AWHX, and the condenser of the heat pump unit as required. The cooling water circulating in pipe-embedded walls takes away the heat that is
system that can utilize shallow geothermal energy more efficiently is proposed in this study. The integrated system combines pipeembedded walls, pipe-embedded windows, and the fresh air prehandling system with the conventional GSHP system. To demonstrate the capability of the integrated system, a simulation model of the integrated system is built on the platform of TRNSYS. The freerunning temperature, building load reduction rates, and the energy saving potential of the integrated system are investigated by comparing the integrated system with a conventional GSHP system. 2. Principle of the integrated system
Pipe-embedded windows
Pipe-embedded walls
The schematic of the integrated system is shown in Fig. 2. The integrated system includes a subsystem of pipe-embedded walls, a
Fresh air AWHX inlet
Return air inlet AHU
Heat Pump
GHX
Distributor
P
Collector
Fig. 2. Schematic of the integrated system.
Air supply
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stored in the walls. The heat transfer between the external walls and indoor space is consequently intercepted. Part of the cooling water precools the high temperature fresh air in the AWHX. Thus, the fresh air load that has to be removed by the heat pump unit can be reduced. In the pipe-embedded window, the solar heat gains that are absorbed by the blinds and pipes can be removed by the flow water in time. Thus, the cavity temperature of the pipeembedded window is much lower than that of the double window with traditional blinds. The heat fluxes via windows to indoor space are reduced significantly. Therefore, with the effects of pipeembedded walls, pipe-embedded windows and the fresh air prehandling system, the free-running temperature of the room space may be maintained within a comfortable zone without turning on the heat pump system. When the free-running temperature cannot meet thermal comfort, the heat pump system will be turned on to remove the rest of the building load. In the heating condition (the heating season and part of the transition season), the outlet water of the GHX is delivered to pipeembedded walls, the AWHX and the evaporator of the heat pump unit as required. The heat transfer process is similar to that of the cooling season. The outlet water of the GHX can be used to reduce the heat dissipation of external walls and preheat fresh air. As a result, the non-air conditioning period in the heating season can be extended and the heating load that has to be removed by the heat pump system can be reduced.
3. Methods To investigate the performance of the integrated system, the system model is built based on a case study of an office building in Beijing where the GSHP system has been widely used in practice. The models of the key components and the evaluation indexes are interpreted in this section.
3.1. Building outlines A three-story office building in Beijing is chosen as the serviced object of the integrated system. The building is 40 m long from north to south, 10 m wide from west to east, and 10.5 m high. The occupant density is 8 m2/person. Each person has a printer and a computer with a total power of 140 W. The lighting power is 13 W/ m2. All the heat gains inside the selected room spaces are on/off controlled according to the regular occupancy schedule, which is set from 8:00 a.m. to 6:00 p.m. The cooling season is set from June 1st to August 31st and the heating season is set from November 15th to March 15th. The office building is simulated with good insulation and airtightness. The infiltration coefficient is set as 0.2. The heat transfer coefficient of the external walls and roof are respectively 0.522 W/(m2$ C) and 0.402 W/(m2$ C), which meet the Design standard for energy efficiency of public building GB 50189-2015 [29]. Pipes are embedded in the concrete layer of external walls and the roof in a serpentine pattern. Windows are fixed in the south and north walls and the window-wall ratio is 0.3. The windows are designed with double glass and pipes are embedded in the cavity of the double glass windows. The heat transfer coefficient of the double glass is 1.5 W/(m2$ C). A traditional office building with the same settings and same building envelopes except for the pipes, is utilized as the benchmark. The conventional GSHP system is applied for the traditional office building, which is analyzed as the baseline case. The configurations of the traditional double glass window and the pipe-embedded window [18] are shown in Fig. 3, and the main difference is the pipes.
675
Fig. 3. Configurations of the traditional double glass window and the pipe-embedded window.
3.2. Model of the pipe-embedded wall To analyze the heat transfer process of the pipe-embedded wall [30], two parts should be considered: the two-dimensional heat transfer of the cross-section of the pipe-embedded wall and the heat transfer along the water loop. The first part aims to calculate the temperature field in the plane of the cross-section. The RC (resistance and capacitance) model is utilized for this analysis. The second part takes the water temperature change into account. The finite element method (FEM) is used to solve this problem. Therefore, the calculated region needs to be divided into many small three-dimensional grid cells. The required variables can be calculated step by step for each cell and for each time step. In this study, the existing model of thermal active layer inside the building model, type 56 [30] of TRNSYS is used to simulate the heat transfer of pipe-embedded wall. The accuracy of the thermal active layer model in TRNSYS has been validated by the literature [31]. To achieve a high level of precision, the time step is 4 min for this simulation process. By referencing Li et al. [17], the parameters of the pipe-embedded walls are listed in Table 1. The water pump of the pipe-embedded wall system is on-off controlled. In the cooling condition, when the indoor temperature is higher than 25 C and the inlet water temperature of the pipe is lower than the inside surface temperature of the wall, the water pump is turned on. Similarly, in the heating condition, the water pump is activated when the room temperature is lower than 21 C and the inlet water temperature is higher than the inside surface temperature of the wall. According to the simulation, the internal surface temperature of each wall can be obtained. The hourly heat flux, Q wall , through the wall to indoor space can be calculated by equation (1).
Q wall ¼ hsur , Awall ,ðtsur troom Þ
(1)
where hsur is the convection heat transfer coefficient on the inner surface of the external wall, kW/(m2$ C); Awall is the internal surface area of the external wall, m2; tsur is the internal surface temperature of the external wall, C; troom is the indoor room temperature. Compared with the traditional wall, the hourly building load reductions caused by the pipe-embedded wall are calculated as:
DQwall ¼ Qwall;trad Qwall;pipe
(2)
where Qwall;trad and Qwall;pipe represent the hourly heat flux through the traditional external walls and the pipe-embedded walls to indoor spaces, respectively; The hourly heat capacity of the pipe-embedded wall, Qpwall , can be calculated as:
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W. Lyu et al. / Renewable Energy 147 (2020) 672e682 Table 1 Parameters of the pipe-embedded walls. Items
Pipe spacing (mm)
Pipe inner diameter (mm)
Water flow rate (m/s)
Values
150
16
0.5
Qpwall ¼ Cw ,mw , twall;o twall;i
(3)
where Cw is the specific heat capacity of water, 4.19 kJ/(kg$ C); mw is the total water flow rate, kg/s. twall;i and twall;o are the water pipe inlet and outlet temperatures, respectively, C.
3.3. Model of the pipe-embedded window The structure of the pipe-embedded window [18] that is used in this study is shown in Fig. 4. Referencing the literatures [18,19], the window materials are listed in Table 2. Compared with the traditional double glass window, the water pipes are fixed on the blinds. Because of absorbing solar radiation, the original temperature of the blinds is so high that relatively warm cooling water can be used to directly remove the heat gains. The heat transfer of a pipe-embedded window has been investigated completely in our previous study [20], in which the analytic method of a pipe-embedded window was developed based on the thermal network of a pipe-embedded window. According to the results of reference [20], a pipe-embedded window can be treated as an equivalent traditional window by using the equivalent heat transfer coefficient Ue, the equivalent solar heat gain coefficient SHGCe, and equivalent ambient temperature tout . All the short-wave solar heat, conductive, convective and long-wave radiative heat transfers have been considered in the analytic method. Based on the analytic method, the heat transfer of the pipe-embedded window used in the integrated system can be expressed as equations (4)e(6):
SHGCe represents the equivalent solar heat gain coefficient of a pipe-embedded window; tout represents the equivalent ambient temperature outside the window, which is affected by the water temperature and outdoor temperature, C; je is the ratio of absorption-conduction heat gains of a pipe-embedded window; t is the short-wave solar transmittance; I means the solar radiation intensity on the glass surface, W/m2; tw is the average water temperature in the pipes, C; a and b are the influence coefficients of ambient temperature and the average water temperature. For the specific pipe-embedded window that used in the integrated system, the values of Ue, je, t, a and b are 2.5, 0.05, 0.1, 0.2, and 0.8, respectively, which are referred to our previous study [20]. The solar heat gains of a pipe-embedded window include two parts, i.e, the absorption-conduction heat gains and the short-wave solar transmitted heat gains. As the influences of these two parts of heat gains on indoor thermal environment are different, the two parts solar heat gains of a pipe-embedded window should be considered separately when adding the model of a pipe-embedded window to the building model (type 56 in TRNSYS). Therefore, equation (4) is divided into equations (7) and (8).
qnv
pipe
qrad
¼ Ue , tout;e troom þ je ,I
pipe
¼ t,I
(7) (8)
where qnv pipe represents the hourly heat gains caused by conduction and convection in per square meter of window, W/m2; qrad represents the short-wave solar transmitted heat gains, W/m2. The hourly heat capacity of the pipe-embedded window, Qpwin , is calculated by equation (9) and equation (10).
qpwin ¼ Ue , tout;e troom þ SHGCe ,I
(4)
Qpwin ¼ 8 , tout;e tw þ 0:5 , I ,Awin ¼ Cw ,mw , twin;o twin;i
SHGCe ¼ je þ t
(5)
(9)
tout;e ¼ a,tout þ b,tw
(6)
where qpwin is the indoor heat gains caused by per square meter of a pipe-embedded window, W/m2; Ue represents the equivalent heat transfer coefficient of a pipe-embedded window, W/(m2$ C);
tw ¼ twin;i þ twin;o 2
(10)
where Awin is the window area, m2; twin;i is the inlet water temperature of the pipe-embedded window, C; twin;o is the outlet water temperature of the pipe-embedded window, C. For the traditional double glass window, the hourly heat gains caused by conduction and convection in per square meter of window, qnv trad , can be expressed as equation (11).
qconv
trad
¼ U,ðtout troom Þ þ j,I
(11)
where U represents the heat transfer coefficient of the window, W/ (m2$ C); tout represents the ambient temperature outside the window, C; j is the ratio of absorption-conduction heat gains of a traditional double window. For the specific traditional double glass window used in the baseline system, the values of U and j are 1.5 and 0.25, respectively. The short-wave solar transmitted heat gains of the traditional double glass window are the same as that of the pipe-embedded window because of the same arrangement of blinds.
qrad Fig. 4. Structures and dimensions of the pipe-embedded window used in this study.
trad
¼ qrad
pipe
(12)
Compared with the traditional double glass windows, the
W. Lyu et al. / Renewable Energy 147 (2020) 672e682
677
Table 2 Window materials. Items
External
Internal
Blinds
Material Thickness (mm) Specific heat (J/kg$ C) Density (kg/m3) Heat conductivity coefficient (W/(m$ C))
Double glazing 6 þ 12airþ6 100 1000 0.09
Stalinite 9 840 2200 0.28
Aluminum alloy 2 880 2700 180
hourly building load reductions caused by the pipe-embedded window are calculated utilizing equation (13).
DQwin ¼ qconv;trad qconv;pipe ,Awin
(13)
As for the control strategy of the pipe-embedded windows system, the water pump of the pipe-embedded windows can be activated when the solar radiation is greater than 200 W/m2 in the cooling condition. In the heating condition, the pipe-embedded windows system does not work, which means that no water is pumped to the pipe-embedded windows and the heat transfer performance of the pipe-embedded window is regarded as the same with the traditional double glass window with conventional blinds. 3.4. Model of the AWHX The fresh air volume is set as 30 m3/(h$person) for the air conditioning system. As described in section 2, the fresh air is prehandled by the AWHX in the AHU. The design air pre-handling temperature difference is 8 C and design water temperature difference is 3 C. The design parameters are listed in Table 3. The AWHX is a cross flow air-water heat exchanger and the εeNTU model is used to simulate its heat transfer process. The type 5e in TRNSYS is used to model the AWHX. The fresh air pre-handling system can only be activated when the inlet water temperature of the AWHX is 3 C less in cooling conditions and more in heating conditions than the inlet temperature of fresh air. The hourly heat exchange capacity QAWHX of the AWHX is expressed as equation (14).
DQAWHX ¼ abs Ca ,
F,r , ðtao tai Þ 3600
(14)
where F is the fresh air volume, m3/h; r is the fresh air density, assumed as 1.2 kg/m3; tai , tao are the fresh air inlet and outlet temperatures of the AWHX, respectively, C. The hourly building load reductions caused by the fresh air prehandling system compared with those caused by the baseline system are equal to the heat exchange capacity of the AWHX. 3.5. System design and evaluation index According to the calculation model of the pipe-embedded window in section 3.3, the convective heat gains and the radiative heat gains can be calculated separately. Thus the heat gains
caused by windows can be added to the building model in forms of indoor convective heat source and radiative heat source, respectively. In this way, the heat transfer model of the pipe-embedded window is implanted in the building model (i.e., Type 56) on TRNSYS. Thus, the new building envelopes can be built with the model of the pipe-embedded window and the existing model of thermal active layer inside of Type 56 [30]. The complete simulation model of the integrated system can be built on the platform of TRNSYS according to section 2. The ground heat exchanger is used as the heating and cooling source and the type 557a, a model of Duct Ground Heat Storage, in TRNSYS is used to simulate its heat transfer process. The type 557a has considered the effect of the rise/drop of soil temperature on the heat transfer of ground heat exchangers. Therefore, the effects of pipe-embedded walls, pipe-embedded windows, the fresh air prehandling system and GSHP unit on the change of soil temperature have been considered in the simulation. The accuracy of this dynamic model has been validated in a detailed study [32]. The single U-tube vertical GHX is utilized and the primary design parameters are listed in Table 4. The heat pump produces 45 C hot water for space heating and 7 C chilled water for space cooling. The inlet water temperature of the source side is 25 C in rated cooling condition and 10 C in rated heating condition. The performance is fitted according to the manufacturer's catalogue. The correction coefficients of the heat pump capacity and power are fitted as follows:
QC ¼ 0:0091,tci þ 1:2269; R2 ¼ 0:9976
(15)
PC ¼ 0:0177,tci þ 0:5572; R2 ¼ 0:9968
(16)
QH ¼ 0:0395,tei þ 0:5871; R2 ¼ 0:9985
(17)
PH ¼ 0:0038,tei þ 0:8927; R2 ¼ 0:9951
(18)
where QC , PC , QH and PH are the correction coefficients of the cooling capacity, cooling power consumption, heating capacity, and heating power consumption, respectively; tci and tei are the inlet water temperatures of the source side in cooling conditions and heating conditions, respectively. The simulation process includes three steps. First, the freerunning temperature is simulated by keeping the heat pump
Table 4 The primary design parameters of the GHX. Table 3 The design parameters of AWHX. 3
Design fresh air flow rate
4500 m /h
Design water flow rate Flow resistance in water side Fan pressure drop caused by AWHX Heat transfer area Heat transfer rate
3 m3/h 5 m H2O 180 Pa 48 m2 1920 W/ C
Items
values
Borehole depth Initial soil temperature Soil thermal conductivity Soil heat capacity Soil density Ground thermal conductivity Radius of Uetube pipe
100 m 14.56 C 1.4 W/(m$ C) 2016 kJ/(m3$ C) 1857 kg/m3 1.4 W/(m$ C) 16 mm
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W. Lyu et al. / Renewable Energy 147 (2020) 672e682
system off and turning on the pipe-embedded walls, pipeembedded windows, and the fresh air pre-handling system. Second, the cooling and heating loads are simulated by turning on the heat pump system. The heat pump system capacity can be designed according to the peak load. Finally, the system performance is simulated based on the cooling and heating load profile and the selected heat pump system. Regarding the baseline, the peak cooling and peak heating loads are 141.2 kW (i.e., 118 W/m2) and 96.1 kW (i.e., 80 W/m2), respectively. The cumulated cooling load is 74454 kWh (62 kWh) while the cumulated heating load is 49512 kWh (42 kWh). The heat pump system capacity is designed based on the peak cooling load. According to engineering experience, the design heat flux of the GHX is 50 W per linear meter in cooling condition in Beijing. Consequently, thirty-six boreholes are required and the rated cooling capacity of the heat pump unit at the baseline is 143.4 kW. The number of boreholes for the integrated system is designed in keeping with the baseline system. To evaluate the energy performance of the integrated system, the available heat transfer temperature difference is estimated first, and the non-air conditioning ratio is defined based on the free-running temperature. Then, the building load reduction rates are provided. Furthermore, the COPs of the pipe-embedded walls, windows, and the fresh air pre-handling system are defined. Finally, the energy saving and emission reduction rates are evaluated. The available heat transfer temperature difference is defined as:
Dt i ¼ ti tsoil
(19)
where i refers to “wall”, “win”, “a”, and “room”. Thus, the Dt wall, Dt win, Dt a, and Dt room are the available heat transfer temperature differences between the wall, the window, the fresh air, the indoor air and the stable soil, respectively; twall is the wall temperature, represented by the average temperature of the inside and outside surfaces of the walls; twin is the window temperature, and it is represented by the average temperature of the cavity of the double glass windows; ta and tsoil are the fresh air and the soil temperatures, respectively. The non-air conditioning ratio is defined as:
g¼
TnonAC 100% Toccupied
hPHL ¼ hL ¼
ð
HLmax;base HLmax;inte 100% HLmax;base
P
CL þ
P P HLÞ ð CL þ HLÞinte P base P 100% ð CL þ HLÞbase
COPwall ¼
Qpwall Wwall
(24)
COPwin ¼
Qpwin Wwin
(25)
COPAWHX ¼
(20)
QAWHX WAWHX þ Wfan
(26)
where Wwall , Wwin , WAWHX are the hourly energy consumptions of water pumps used for the pipe-embedded walls, pipe-embedded windows, and the fresh air pre-handling system; Wfan is the hourly additional energy consumption of fans caused by the resistance of the AWHX. The energy consumption of the baseline system and the integrated system are mainly expressed as:
Ebase ¼ WGHX
base
Einte ¼ WGHX
inte
þ WHP
þ WHP
base
inte
þ
þ
Lbase EERindoor
(27)
Linte þ Wwall þ Wwin EERindoor
þ WAWHX þ Wfan
(28)
where E is the hourly electricity consumption; WGHX is the hourly power consumption of the water pump in the source side of the heat pump unit; WHP is the hourly energy consumption of the heat pump unit; L is the hourly building load; EERindoor is the energy efficiency ratio of the indoor air conditioning terminals, which is set as 30 in this study [33]. Compared with the baseline system, the energy saving rate of the integrated system is expressed as:
ε¼
where TnonAC is the non-air conditioning period, which means that the free-running temperature is between 20 and 26 C without the operation of the heat pump system; Toccupied is the period during which the office room is occupied. The hourly cooling and heating loads can be obtained directly from the simulation. The building load reduction rates are defined as follows:
CL CLmax;inte hPCL ¼ max;base 100% CLmax;base
baseline system and the integrated system. The average COPs of the pipe-embedded wall, pipe-embedded window, and the fresh air pre-handling system are defined respectively as follows:
P ðEbase Einte Þ P 100% Ebase
(29)
According to reference [34], the conversion relationship between electricity and standard coal is 1 kWh ¼ 0.404 kg standard coal. The CO2 discharge coefficient is 2.493 kg/kg standard coal. The emission reduction of CO2, MCO2 , can be calculated as:
MCO2 ¼ 0:404 2:493,ðEbase Einte Þ ¼ 1:007,ðEbase Einte Þ (30) 4. Results and discussion
(21)
(22)
P
(23)
where hPCL , hPHL , hL are the reduction rates of peak cooling load, peak heating load, and annual cumulated load, respectively; CL and HL are the hourly cooling load and the hourly heating load, respectively; and the subscripts “base” and “inte” indicate the
The energy efficiency of the integrated system is investigated as follows: (ⅰ) analyzing the available temperature difference in different heat transfer processes; (ⅱ) calculating the free-running temperature and the non-air conditioning ratio; (ⅲ) calculating the building load and system capacity reductions of the heat pump system; and (ⅳ) indicating the energy saving and emission reduction potential. 4.1. Available heat transfer temperature difference To better understand the design purpose of the integrated system, the available temperature differences in different heat transfer processes are analyzed to demonstrate the advantage of the
W. Lyu et al. / Renewable Energy 147 (2020) 672e682
integrated system. The temperature distribution of the baseline system in the occupied time is shown in Fig. 5. For the traditional GSHP system, the soil temperature changes minimally throughout the year. The cavity temperature of the traditional double glass windows can be over 60 C in the cooling season, which is caused by much solar radiation absorbed by conventional blinds. In the heating season, the fresh air temperature is much lower than the soil temperature while the indoor design temperature is always higher than the soil temperature. This indicates that shallow geothermal energy can be used to heat the fresh air but not to heat the indoor air. The available heat transfer temperature difference between the soil and the indoor air, the walls, the windows, as well as the fresh air is demonstrated in Fig. 6. The temperature difference between the soil and the indoor air is always 10e15 C in the cooling season and there is no available temperature difference in the heating season, because the indoor design temperature is 26 C and 20 C in the occupied time of the cooling season and heating season, respectively. The temperature difference between the soil and the wall ranges from in 10e15 C in the cooling season and 0e10 C in the heating season. Temperature differences between the soil and the window that are greater than 20 C account for approximately 80% of the cooling season. The available temperature difference between the soil and the fresh air is mostly between 10 and 20 C during both the cooling and heating seasons. The available temperature difference distribution frequency indicates that shallow geothermal energy would perform well to directly cool the windows, the walls, and the fresh air during the cooling season. Moreover, the integrated system also has the effect on preheating the fresh air and intercepting the heat transfer through the wall.
679
Fig. 6. The available heat transfer temperature difference between the soil and the indoor air, the wall, the window, as well as the fresh air.
Fig. 7. Free-running temperature distribution frequency.
4.2. Increase in the non-air conditioning ratio To investigate the increase in the non-air conditioning ratio, the free-running temperature of both the integrated system and the baseline system are simulated. The free-running temperature of the corner room in the top floor is taken as an example, as is shown in Fig. 7. For the baseline office room, the period that freerunning temperature is lower than 15 C or higher than 30 C accounts for more than 70% of a year. This indicates that the room temperature is terribly uncomfortable during most of the year without the operation of the conventional heat pump system. For the office room with the integrated system, the free-running temperature will no longer exceed 30 C and the period of time that it is lower than 15 C is reduced apparently. The ratio of the free-running temperature in the comfortable zone (i.e., when the
Fig. 5. The temperature distribution of the baseline system in the occupied time.
temperature is between 20 and 26 C) is more than 40% in the integrated system but less than 10% in the traditional baseline system. The free-running temperature distribution of the baseline and the integrated systems are illustrated in Fig. 8. In the baseline system, the free-running temperature would be over 40 C. With the effect of the pipe-embedded walls, pipe-embedded windows, and the fresh air pre-handling system, the free-running temperature of the integrated system is always below 28 C throughout the cooling season. It is obvious that if the integrated system is applied for a residential building with small indoor heat gains, the period with acceptable thermal comfort will be much longer.
Fig. 8. The free-running temperature distribution of the baseline and the integrated system.
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W. Lyu et al. / Renewable Energy 147 (2020) 672e682
4.3. Reduction of building load and capacity of heat pump system According to the simulation, the peak loads and heat pump system capacities of both the baseline system and the integrated system are listed in Table 5. In the cooling season, on the one hand, the high temperature fresh air is precooled by the shallow geothermal energy directly before being processed by the heat pump system; on the other hand, the embedded pipes in the building envelopes are adequate to intercept the heat transfer through the external walls and remove the solar radiation heat gains through the windows. Therefore, the cooling load decreases significantly. In the heating season, the available temperature differences are less than those in the cooling season, as demonstrated in Fig. 6. Therefore, the reduction rates of both the peak heating load and cumulated heating load are less than those of the peak cooling load and cumulated cooling load. The reduction rates of peak cooling load, peak heating load, and heat pump system capacity are 29%, 22%, and 28% respectively. Because the capability declines when the working condition turns worse at the peak cooling load, the rated heat pump capacities are larger than the peak cooling load. The hourly building load in the cooling season and heating season are shown in Fig. 9. Compared with the baseline system, the building load of the integrated system is reduced significantly, especially in the cooling season. The reason for the building load decrease is investigated by analyzing the inside surface temperature change of the south wall and south window. As presented in Fig. 10, in the baseline system, the inside surface temperatures of the wall and window are higher than the room design temperature, which is 26 C in the cooling season. When there are high levels of solar radiation, the inside surface temperature of the traditional double glass window can be 40 C. In the integrated system, the outlet water temperature has the most influence on the heat transfer performance of the pipe-embedded window and the pipeembedded wall. The inside surface temperatures of both the pipeembedded wall and the pipe-embedded window are much lower than those of the traditional wall and window in the baseline system. Most of the time, the inside surface temperature of the pipe-embedded wall is lower than the room design temperature, which means that the pipe-embedded wall also can eliminate part of the indoor cooling load. Therefore, the building load that has to be removed by the heat pump system can be reduced significantly. The reduction rates of the cumulated cooling load and the cumulated heating load are 56% and 22%, respectively. In terms of the whole year, the cumulated load reduction rate is approximately 43%. The cumulated load reduction consists of three parts, which are shown in Fig. 11: reduction by pipe-embedded walls, reduction by pipe-embedded windows, and reduction by fresh air pre-handling system. Because of the low outlet water temperature of GHX and the large heat exchange area, the reduction of the calculated load contributed to by the pipe-embedded walls is much greater than that contributed to by the pipe-embedded windows and fresh air pre-handling system. The contribution proportions of the three parts are 45%, 26%, and 29%, respectively.
Table 5 The rated heat pump capacity. Items
Baseline system
The integrated system
The number of boreholes Peak cooling load (kW) Peak heating load (kW) Rated heat pump capacity (kW)
36 141.2 96.1 143.4
36 98.9 75.1 103.2
Fig. 9. Hourly building load profile.
Fig. 10. The inside surface temperature of the building envelopes in the cooling season.
Fig. 11. Cumulated load reduction rates by pipe-embedded wall, pipe-embedded window, and fresh air pre-handling system.
4.4. Energy saving and emission reduction The outlet water of the GHX is integrated with pipe-embedded walls, pipe-embedded windows, and the fresh air pre-handling coils for direct utilization in the integrated system. The annual average COPs of the direct utilization of shallow geothermal energy are quite high, as demonstrated in Fig. 12. The annual average COP of pipe-embedded windows is approximately 50, which is much higher than that of pipe-embedded walls and the fresh air prehandling system. The minimum annual average COP of the three components is above 20 and it is about five times that of the conventional heat pump system. The energy saving and emission reduction are calculated
W. Lyu et al. / Renewable Energy 147 (2020) 672e682
Fig. 12. The annual average COPs of direct utilization of shallow geothermal energy.
Table 6 Energy saving and emission reduction.
681
(1) The available temperature differences between the undisturbed soil and the building envelopes as well as the fresh air can exceed 20 C most of the time, which provides a way to utilize the shallow geothermal energy efficiently. (2) The free-running temperature of the integrated system is always below 28 C throughout the year, and the non-air conditioning rate increases by 34% compared with that of the conventional GSHP system. (3) The heat pump capacity of the integrated system can be reduced by as much as 30% compared to the conventional GSHP system. Moreover, the building annual cumulated load is reduced by approximately 43%. The contribution proportions of the pipe-embedded walls, pipe-embedded windows, and fresh air pre-handling system to the cumulated load reduction are approximately 46%, 25%, and 29%, respectively. (4) The energy saving rate of the integrated system compared to that of the conventional GSHP system is approximately 29%, and the reduction of CO2 emission is more than 7 kg per square meter of floor area.
Items
Energy savings (kWh/m2)
Emission reductions (kg/m2)
Energy saving rate
Acknowledgements
value
7.7
7.8
28.5%
This study was supported by the National Natural Science Foundation of China (Grant No. 51638010 and Grant No. 51578306).
compared with the baseline system, and the results are listed in Table 6. The energy saving rate is approximately 28.5%. The electricity savings are approximately 7.7 kWh per unit of floor area. This benefits the emission reduction of CO2, which is more than 7 kg per square meter of floor area. In addition, for the cost of pipe-embedded windows, the previous study [18] indicates that although cheap glass is adopted for the pipe-embedded windows, the heat flux can be reduced significantly, which implies that the investment would increase a little by using pipe-embedded windows. For the cost of pipe-embedded walls, the previous study [17] has been discussed the application issue in detail. The results show that the energy savings of pipeembedded wall system range from 10% to 45% in different climates. The cost of the GHXs is the main investment of the pipeembedded wall system. The cost of the fresh air pre-handling system has been discussed in our previous study [35]. The results show that the pay-back periods are one to three years in different regions. As for the integrated system in this study, the energy saving rate is approximately 29% in the case study of this paper. Therefore, it can be deduced that this integrated system has a great potential of economic feasibility compared with a traditional GSHP system. 5. Conclusions The shallow geothermal energy is not fully utilized by the conventional GSHP system. To improve the usage efficiency of shallow geothermal energy, an integrated system, in which pipeembedded walls, pipe-embedded windows, and a fresh air prehandling system are combined with the conventional GSHP system is proposed based on the analysis of the temperature relationship between the envelopes, fresh air, indoor air and undisturbed soil. The performance of the integrated system is investigated in terms of the increase in the non-air conditioning rate, the decrease in building load and the capacity of the heat pump system as well as the energy saving and emission reduction. The conclusions based on the case study of an office building in Beijing are as follows:
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