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Performance analysis of ground water-source heat pump system with improved control strategies for building retrofit
Renewable Energy 80 (2015) 324e330
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Performance analysis of ground water-source heat pump system with improved control strategies for building retrofit Na Zhu*, Pingfang Hu, Wei Wang, Jianming Yu, Fei Lei Department of Building Environment and Equipment Engineering, School of Environment of Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 May 2014 Accepted 1 February 2015 Available online 28 February 2015
This paper presented a case study on performance analysis of ground water-source heat pump system (GWHP) for a hotel building retrofit. Splitting air-conditioner was used for cooling and coal fired boiler was used for heating in original system. The GWHP system could save electricity energy and water source consumption under improved operation methods compared with the common operation methods. In this study, the experimental average cooling coefficient of performance (COP) of the units and system were 5.53 and 3.29, respectively (from Jun. to Sep.). The average heating COP of the units and system were 4.45 and 2.79, respectively (from Nov. to Feb.). The primary energy consumption of GWHP system was reduced by 42.9% when compare with the original cooling/heating system in a year. The indoor air temperature fluctuated in comfortable range (19.3 Ce24.4 C) during test day in winter. The results show that the GWHP system was particularly suitable for this hotel under our improved operation methods. This renewable and environment-friendly technology could promote energy conservation and sustainability obviously for the hotel building. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Ground water-source heat pump Retrofit technology Improved operation methods
1. Introduction As one of the largest developing country in the world, China's rapid economic growth is accompanied with serious environment challenges such as pollution, energy shortage and climate change [1]. Energy shortage is the most serious problem at present. The structure of energy consumption in China is dominated by coal consumption, while clean energies only represent a very small percentage. The GWHP's high energy efficiency and low environmental impact characteristics have already drawn a fair amount of attention as one of the clean energies in China. GWHP technology provides a new and clean way of heating/cooling for buildings in the world. It is estimated that, heating, ventilation and air-conditioning (HVAC) accounts for about 65% of the energy consumption in the Chinese building sector [2-3]. The HVAC system accounts for about 44% for the electricity consumption in the hotel in China [4]. Engineers and relevant researchers strongly desire to reduce energy
* Corresponding author. Tel.: þ86 027 87792164x415. E-mail address: [email protected] (N. Zhu). http://dx.doi.org/10.1016/j.renene.2015.02.021 0960-1481/© 2015 Elsevier Ltd. All rights reserved.
consumption in HVAC systems. Professionals and policymakers have been making great efforts in this aspect. The overall process of a building retrofit can be divided into five major phases [5]. The first phase is the project setup and preretrofit survey. The second phase comprises an energy audit and performance assessment (and diagnostics). The third phase is the identification of retrofit options. The fourth phase is site implementation and commissioning. The final phase is validation and verification of energy savings. This hotel retrofit follows these five phases step by step. The retrofit technologies can be categorized into three groups, including supply side management, demand side management [610], and change of energy consumption patterns, i.e. human factors. The retrofit technologies for supply side management include building electrical system retrofits and the use of renewable energy, such as solar hot water, solar photovoltaic, wind energy, geothermal energy et al., as alternative energy supply systems to provide electricity and/or thermal energy for buildings. In the last five years, it had an increasing interest in the use of renewable energy technologies as building retrofit solutions due to the increased awareness of environment issues. The use of renewable energy technologies may bring more benefits for commercial office buildings where a utility rate structure including time-of-use
N. Zhu et al. / Renewable Energy 80 (2015) 324e330
Nomenclature CE COP HVAC GWHP N P Q SEP T W
primary energy (tce) coefficient of performance heating, ventilation and air conditioning ground water-source heat pump power of system (kW) power of heat pump (kW) cooling/heating load (kW) saving energy percentage (%) temperature ( C) power consumption (W)
Subscripts c cooling comp compressor fan fan g ground water-source heat pump system h heating hp heat pump o original cooling/heating system s system
differentiated electricity prices and demand charge is applied. The use of ground water-source heat pumps falls into this category. GWHP technology is regarded as an effective technology for using renewable energy. Chen et al. [11] investigated an underground water source heat pump system installed in a tall apartment building in Beijing, China. By analyzing this system for two years, operation methods and a controlling algorithm of the system were developed. Nam et al. [12,13] studied the performance of a GWHP system. This system depended on the temperature and depth of the water, and its efficiency was much higher than that of the air source heat pump. Numerical methods of underground heat water transfer, energy balance method and thermal storage method were presented to calculate the capacity of unit area of shallow groundwater aquifer by Liang et al. [14]. The model results indicate that the numerical method, which was based on performance efficiency of GWHP, represented the behavior of groundwater pumping/recharging processes, and served better than energy balance method and thermal storage method, in addition it had the advantages of energy saving and environmental protection. Lei et al. [15] investigated the performance characteristics of a GWHP system on the actual operation, using the energy and exergy analysis method. The available researches focused on system experimental tests or model analyses. Because of the complexity of the GWHP systems compared with the original cooling/heating system, the actual operation performance and energy efficiency of the systems during long periods need to be validated further. However, the operation efficiency of GWHP is not satisfactory in some buildings due to many factors, including inefficient equipment, improper control schemes and many malfunctions happened in the building operation. In addition, the groundwater quality must be protected and the total quantity of ground water source must be kept balance. Groundwater recharging is an important factor for sustainable and stable operation of the GWHP system. This study aims at analyzing the performance of the GWHP system based on the site test results in a whole year and system design data. In this study, common operation methods are improved and presented. The electricity energy and water resource consumption could be reduced under
325
our improved methods compared with common ones. Indoor thermal comfortable is an additional factor to verify the feasibility of the retrofit technology.
2. System design 2.1. Overview This hotel is located in Wuhan (latitude 30.52 N, longitude 114.32 E), China. Wuhan is a cooling-dominant area with hot summer and cold winter. The total building area is 2200 m2 and total air-conditioning area is 1862 m2. There are eight floors in this hotel building. There are totally twenty-seven guest rooms in this hotel and fifty-four beds in the hotel. The hot water demand quality for each bed is about 0.2 m3 per day. Hot water in this hotel is available 24 h a day all year round. The cooling and heating load of this hotel are 208 kW and 170 kW, respectively. The cooling period for the hotel was from Jun. to Sep. and the heating period was from Nov. to Feb. The performance of this GWHP system was tested from Jun 1st to Aug 31st in 2012 (summer test) and from Dec 20th to Feb 18th in 2013 (winter test). The load ratio of the GWHP system was 65% during site tests. The weather profiles include outdoor air temperature and relative humidity during the test two periods in Wuhan, shown in Figs. 1 and 2. The set-point of indoor air temperature and relative humidity were 26 Cand 60% in summer. The set-point of air temperature and relative humidity were 20 Cand 40% in winter. The weather profiles show that the indoor air temperature and humidity were not in the comfort range both in summer and winter, so cooling and heating technologies were necessary to improve indoor thermal comfort with reasonable technology. The original cooling/heating system mainly included splitting air-conditioners, coal fired boilers, terminal equipments and pipes. Splitting air-conditioners were only used for cooling in summer. The rated cooling COP of the splitting air-conditioner system was 2.7. The coal fired boilers were used as heat source supplying airconditioner hot water and bath hot water. The terminal equipments in guest rooms were radiators hanging on the wall used for heating in winter and water heaters used for bathing over the year. The design heat efficiency of boiler was 68%. There were twentyseven splitting air conditioners for twenty-seven guest rooms and
Fig. 1. Weather profile in summer (Jun 1steug 31st).
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Fig. 2. Weather profile in winter (Dec 20theFeb 18th).
two coal-fired boilers in this hotel. More detail technical specifications of original cooling/heating system were listed in Table 1. The original heating system was not environment-friendly since too much coal cinders after burning for supplying room heat in winter and hot water over the year. The supply/return water temperature from/to the boiler varied with outdoor air temperature. So, the reliability of boiler was not good enough and the boiler did not operate in very good working conditions when the outdoor air temperature varied greatly. In addition, the maximal room air temperature difference was about 4 C because of heat imbalance during heating in winter. It made some people feel very uncomfortable in the guest rooms in winter. The original cooling system was scrapped after over ten years' service. In order to improve the energy efficiency of the cooling/heating system and reduce pollution, the original cooling/heating system will be replaced by new system using renewable and sustainable energy technology. The project targets and client's concern for the environment had a significant impact on the selection of retrofit technologies. The GWHP system was selected by hotel owners as retrofit technology for this project based on investigation finally. This project aimed at analyzing the energy performance and indoor thermal comfort of
Table 1 Technical specifications of original cooling/heating system. Type
Specification
Value
Number
Splitting air-conditioner
Cooling capacity Cooling input power Cooling COP Circulation Dehumidification Dimension (W D H)
27
Boiler
Input power Steam pressure Steam temperature
3500 W 1255 W 2.7 610 m3/h 1.2 L/H Indoor:844 mm 183 mm 300 mm Outdoor:848 mm 540 mm 320 mm 0.1 MW 0.4Mpa Outlet: 95 C Inlet: 70 C A Ⅱ soft coal 8.147 kWh/kg 90% 68%
Fuel Coal calorific value Pipe heat efficiency Boiler heat efficiency
2
the GWHP system. A general view of the GWHP system was shown in Fig. 3. This system mainly consisted of the GWHP system, an indoor air-conditioner system and a data acquisition system. The specifications of the equipments appeared in Fig. 3 were listed in Table 2. The GWHP system included two MWH-020 type GWHP units and two circulating pumps. GWHP unit could operate in two modes, cooling and heating. The hot water GWHP system included one MWH-020 type GWHP unit and one circulating pump. It could operate in three modes, supplying hot water, cooling and heating. Supplying hot water mode was prior to cooling/heating mode. The design inlet/outlet water temperatures of the heat pump units were listed in Table 3. The underground water source for this air-conditioner system came from three pumping wells around the building, 23.6 m depth and 450 mm diameter. The total water volume provided by three wells was 90 T/h and each pumping well was 30 T/h. The used water went back to recharge wells and the total volume was 90 T/h. The water pumping from wells was returned back to wells fully. The variability of underground water lever was less than 0.5 m according to professional program calculation and water lever isograms of pumping wells and recharging wells under same working condition. In this project, the grit capacity in the well-water was 1/ 10,000. The indoor air-conditioner system included four types of fan coil units (FP-51, FP-68, FP-102, FP-136) and fresh air handing units. Besides the ground temperature sensors, data acquisition system also included several calorimeters. Each calorimeter consisted of one flow meter with the minimum flow rate of 0.05 L/h and two Pt1000 temperature sensors with ±0.1 C accuracy. All power consumptions were recorded by the wattmeter.
2.2. Operation modes Operation modes were switched by different valves control. Based on the requirements of guests in this hotel, five operation modes were designed in the present GWHP system, including: a) cooling and supplying hot water mode; b) cooling mode; c) heating and supplying hot water mode; d) heating mode; e) supplying hot water mode. There were some deficiencies on common operation methods of the GWHP system. The pump always pumped ground water and then sent it back to the ground after exchange heat no matter the unit was on or off. When the cooling load was not very high, the water pump electricity and water resource consumption will be waste in this mode. The total heat recovery unit used in this system only supplied hot water but didn't supply heat in winter. The total heat recovery unit stopped working when the temperature was below set-point, but the unit could not transfer to heating mode. So, the unit was not fully used. When the heat of total heat recovery unit was not sufficient for heating water to the set-point, the water was not hot enough for bath, so the operation modes of the units were needed to be improved. In order to improve the common operation methods and operate in five operation modes successfully, new operation methods were designed. There were some characteristics of the new operation methods. First, when all the units stopped working, which meant the tank hot water temperature reached set-point and the terminal side circulating water temperature reached set-point, only one terminal side circulating water pump operated and the water heating circulating pump stopped working. In-take pump stopped working after cushion, and it could save electricity consumption of the intake pump and water resource consumption.
N. Zhu et al. / Renewable Energy 80 (2015) 324e330
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Fig. 3. Schematic of GWHP system. 1:GWHP unit; 2:hot water GWHP unit; 3:terminal circulating pump; 4:hot water circulating pump; 5:submerged pump; 6:desander; 7:plate heat exchanger; 8:circling pump; 9:hot water tank; 10:hot water supply pump.
Second, when the total heat recovery unit heated water to setpoint in winter, hot water flowed direction could be switched according to the on/off electromagnetic value, and water heating mode could be switched to heating mode automatically. It could complement insufficient heat demand. Third, when the summer cooling load was not very high, heat from the total heat recovery was not sufficient for heating tank water to set-point. Under this condition, the unit transfer
electromagnetic to take heat from well and make sure the tank water temperature was heated to set-point. In a word, the improved operation modes of this system could be switched successfully to satisfy the users' needs in five operation modes and save electricity and water resource consumption obviously. Five operation modes were achieved according to the control strategies. The water pumps control strategies in different seasons could be summarized as follow.
Table 2 Equipments list of GWHP system.
2.2.1. Operation modes in summer
No
Type
Specification
Number
1
GWHP unit
MWH-020, Qc ¼ 70 kW,Nc ¼ 13.5 kW, Qh ¼ 78 kW, Nh ¼ 19.5 kW MWH-020, Qc ¼ 70 kW, Nc ¼ 13.5 kW, Qh ¼ 78 kW, Nh ¼ 19.5 kW Q ¼ 15 m3/h, H ¼ 35 m, N ¼ 4 kW Q ¼ 12.5 m3/h, H ¼ 12.5 m, N ¼ 1.1 kW Q ¼ 30 m3/h, H ¼ 40 m, N ¼ 5.5 kW XLC-40B M15-EF G8 Q ¼ 30 m3/h, H ¼ 40 m, N ¼ 5.5 kW 4t 3.3 kW 1.0 m 1.0 m 1.0 m DN100 DN100
2
2
Hot water GWHP unit
3
Terminal circulating pump
4
Hot water circulating pump
5
Submerged pump
6 7 8
Desander Plate heat exchanger Circling pump
9 10 11 12 13
Hot water tank Hot water supply pump Expansion water tank Water collector Water distributor
1
a) Hot water heating circulating pump When the tank hot water temperature was equal or higher than 50 C, the electromagnetic valve No.1 was opened and the electromagnetic valve No.2 was closed. The hot water heating circulating pump was closed. The three units supply cooling normally.
4 2 2 1 1 1 1 2 1 1 1
Table 3 Design inlet/outlet water temperatures of heat pump units. Season
Side
Component
Inlet water temperature
Outlet water temperature
Summer
Terminal side Underground water side Underground water side
12 C 20 C 18 C
7 C 34 C 32 C
Winter
Terminal side Underground water side Underground water side
Evaporator condenser Plate heat exchanger Condenser Evaporator Plate heat exchanger
40 C 16 C 18 C
45 C 8 C 10 C
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When the tank hot water temperature was lower than 42 C, the electromagnetic valve No.2 was opened and the electromagnetic valve No.1 was closed. The hot water heating circulating pump was opened. The unit No.2 heated water and the other two units supplied cooling normally. b) Submerged pump in ground source side The submerged pump should be started up firstly before chiller operates. c) Circulating pump in user side The three water pumps could parallel operate simultaneously and operate separately. And there was one more water pump for standby.
GWHP system could be fully used and meet client's need under these improved operation methods. 3. Analysis of results The performance of the GWHP unit and system is evaluated by COP. Eqs. (1)e(2) are the definition of the unit and system COP. The unit COP is defined by power consumption of heat pump unit, which includes the compressor and fans for the heat exchangers. The power consumption for a heat pump coming from the compressor and the fans is shown in Eq. (3). The system COP is defined by the power consumption of the system, which includes power consumption for a heat pump and pumping power, is defined in Eq. (4).
COP ¼
Q P
(1)
2.2.2. Operation modes in winter a) Hot water heating circulating pump When the tank hot water temperature was equal or higher than 50 C, the hot water heating circulating pump was closed. The inside valve of the unit conversed to terminal side and stopped supplying hot water. The three units supplied heating normally. When the tank hot water temperature was lower than 42 C, the inside valve of the unit conversed to hot water heating side, then the hot water heating circulating pump was opened. The unit No.2 heated water and the other two units supplied heating normally. b) Submerged pump in ground source side The submerged pumps should be started up firstly before chiller operates. c) Circulating pump in user side The three water pumps could parallel operate simultaneously and operate separately. And there was one more water pump for standby.
Q Ns
(2)
P ¼ Wcomp þ Wfan
(3)
Ns ¼ Wcomp þ Wfan ¼ Wpump
(4)
COPS ¼
where, COP and COPs are COP of heat pump unit and system, respectively. Q is actual cooling/heating load. P is power input of unit. Ns is total power input of system. Wcomp and Wfan are power consumptions of the compressor and fan, respectively. Wpump is the pumping power consumption. Figs. 4 and 5 show the COP of GWHP units and system in cooling season and heating season, respectively. In the cooling season, it was found that the COP of the units varied from 2.17 to 9.45 and average value was 5.53. The COPs of the system varied from 1.38 to 5.52 and average value was 3.29 in the cooling season. In the heating season, the COP of the units varied from 2.24 to 7.66 and average value was 4.45. The COPs of the system varied from 1.33 to 5.69 and average value was 2.79 in the heating season. The energy efficiency was promoted compared with splitting air-conditioner units.
2.2.3. Operation modes in remaining seasons a) Hot water heating circulating pump When the tank hot water temperature was equal or higher than 50 C, the hot water heating circulating pump and submerged pump in ground source side were closed. When the tank hot water temperature was lower than 42 C, the hot water heating circulating pump and submerged pump in ground source side were opened. b) Submerged pump in ground source side The submerged pump and the hot water heating circulating pump were closed or opened simultaneously. c) Circulating pump in user side The circulating pumps in user side were closed all. A major objective of using the GWHP system was to obtain an improved integration between the available renewable sources and residential thermal requirements, while guaranteeing a satisfactory level of comfort and quality of energy use under all situations. The
Fig. 4. COP of units and system in summer (Jun 1steAug 31st).
N. Zhu et al. / Renewable Energy 80 (2015) 324e330
329
Fig. 7. Inlet/outlet water temperature of condenser/evaporator (Dec 20theFeb 18th).
Fig. 5. COP of units and system in winter (Dec 20theFeb 18th).
Three GWHP units operated during the test two periods. In the supply side, more detail data for this air-conditioner system were recorded for two periods, including inlet and outlet water temperature of the evaporator and condenser, the water flow mass of well, the water temperature difference between inlet and outlet of well. These test data could evaluate the heating/cooling performance of this retrofitted system. Figs. 6 and 7 show the inlet and outlet water temperature flowing through the condenser and evaporator during the two periods, respectively. We could find that the inlet and outlet average water temperatures of evaporator were 12.8 C and 9.22 C, and inlet and outlet average water temperatures of condenser were 25.7 C and 31.7 C during the cooling season. The outlet water of evaporator was higher than design value (7 C) might because of unusual operation of water pump or block of strainer in water cycling. In the heating season, inlet and outlet average water
temperatures of condenser were 40.9 C and 44.6 C, and inlet and outlet average water temperatures of evaporator were 13.8 C and 9.1 C. The inlet water of evaporator was lower than design value (16 C) might because of underground water temperature was low and on-way heat loss of tubes. This GWHP system operated well according to the cooling/ heating test. The saving energy percentage of the GWHP system compared with original cooling/heating system is calculated by Eq. (5).
SEP ¼
CEo CEg CEo
(5)
where SEP is saving energy percentage (%); CE0 is primary energy consumption of original cooling/heating system (tce); CEg is primary energy consumption of GWHP system (tce). The splitting air-conditioner used for cooling and coal fired boiler used for heating and hot water were considered as original cooling/heating system. The total energy consumptions were listed in Table 4. Table 4 showed that the primary energy saving percentage of the GWHP system was 42.9% compared with original cooling/ heating system. In Jan 12th, one of the guest rooms was selected randomly as example to test supply and return air temperature in the terminal side, shown in Fig. 8. Fig. 8 showed that the supply air temperature fluctuated around 31.0 C averagely with 28.3 C (4:00 am) in minimum and 36.3 C (15:00 pm) in maximum. The return air temperature which was equal to the indoor air temperature in the hotel room approximately indicated thermal comfort of the room. The return air temperature fluctuates around 21.3 C averagely with 19.3 C (4:00
Table 4 Saving energy percentage of GWHP system. Air-conditioner type
Fig. 6. Inlet/outlet water temperature of condenser/evaporator (Jun 1steAug 31st).
Total primary Saving energy Heating Cooling percentage season energy season energy energy consumption consumption consumption (%)
Original cooling/ 118,800 kWh 170.3t (coal) heating system (electricity) GWHP system 47,850 kWh 20,152 kWh (electricity) (electricity)
48.12tce
e
27.47tce
42.9%
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N. Zhu et al. / Renewable Energy 80 (2015) 324e330
temperature of the water flowing through heat pump. The total saving energy percentage of GWHP system was 42.9% compared with original cooling/heating system. (c) During the cooling season, the inlet/outlet water temperatures difference flowing through the condenser and evaporator were about 6.62 C and 2.86 C, respectively. During the heating season, these values were about 3.63 C and 4.42 C respectively. The design inlet/outlet water temperature difference of heat pumps was 5 C. It implied that the heat pumps could supply sufficient hot water and cooling/heating for this hotel all around year. (d) The supply and return air temperature in the test room indicated that the indoor air temperature was in comfortable range (19.3 Ce24.4 C) and could supply sufficient heat for users during heating season.
Fig. 8. Supply and return air temperature in terminal side (Jan 12th).
am) in minimum and 24.4 C (15:00 pm) in maximum. The supply and return air temperatures difference was around 10 C during the test day. It implied that the indoor air temperature reached design set-point and could supply sufficient heat for the guests during winter. During the heating mode, water temperature changes varied with time, which implied that changes in COP varied widely over time. This conclusion could be validated by the COP of units and system shown in Figs. 4 and 5. The water average temperature difference flowing through the condenser during two periods were about 6.62 C and 3.36 C, respectively. The water average temperature difference flowing through the evaporator during two periods were about 2.28 C and 4.42 C, respectively. The temperature fluctuation in the evaporator was lesser than that in the condenser. 4. Conclusions In this study, a GWHP system for cooling/heating purpose and sustaining hot water supply was designed for original cooling/ heating system retrofit in a hotel. An energy analysis was performed in order to evaluate the efficiency of the GWHP system under our improved operation methods. Energy analysis was performed by tests. The main research results are as follows. (a) This system could operate in five modes according to the actual needs in the hotel. Some improvements had been made compared with the common operation modes. Electricity energy and water source consumption could be reduced obviously under the improved operation modes. (b) The COP of the units and system varied from 2.17 to 9.45 and 1.38e5.52 respectively during the cooling season. The COP of the units and system varied from 2.24 to 7.66 and 1.33e5.69 respectively during the heating season. The average COP was higher than that of original heat pump system using ambient air heat source. Since the COP of the units varied widely, the system was needed to be re-configured to control the
Therefore it concluded that this system was particularly suitable for this hotel in Wuhan and the original cooling/heating system was retrofitted successfully. The GWHP system in this hotel could operate in five modes to meet guests' requirements on heating/ cooling and hot water needs for 24 h all the year round. The operating electricity and water resource consumption could be reduced under the improved operation modes. This study might be helpful for designers to apply GWHP technology in building retrofit. Acknowledgment This work presented in this paper is financially supported by a grant (No. 51078160) of National Science Foundation of China and The Central University Special Funding for Basic Scientific Research Business Expenses (No.2013NY007, No.2013 KXVQ006). References [1] Li ZS, Zhang GQ, Li DM, Zhou J, Li LJ, Li LX. Application and development of solar energy in building industry and its prospects in China. Energy Policy 2007;35(8):4121e7. [2] Liu Y, Joseph CL, Tsang CL. Energy performance of building envelopes in different climate zones in China. Appl Energy 2008;85(9):800e17. [3] Yao R, Li B, Steemers K. Energy policy and standard for built environment in China. Renew Energy 2005;30(13):1973e88. [4] Xue ZF. Diagnose and retrofit of energy saving for exiting building. China: China Building Industry Press; 2007 [in Chinese]. [5] Ma ZJ, Cooper P, Daly D, Ledo L. Existing building retrofits: methodology and state-of-the-art. Energy Build 2012;55(12):889e902. [6] Barlow S, Fiala D. Occupant comfort in UK offices-how adaptive comfort theories might influence future low energy office refurbishment strategies. Energy Build 2007;39(7):837e46. [7] Xing YG, Hewitt N, Griffiths P. Zero carbon buildings refurbishment-a hierarchical pathway. Renew Sustain Energy Rev 2011;15(6):3229e36. [8] Krarti M. Energy audit of building systems: an engineering approach. 2nd ed. Boca Raton, Florida, USA: CRC Press, Taylor & Francis Group; 2011. [9] Baker NV. The handbook of sustainable refurbishment: non-domestic buildings. London, UK: Earthscan; 2009. [10] Fusion. A review of retrofit technologies. UK: Salford centre for research & innovation in the built & human environment, University of Salford; 2010. [11] Chen C, Sun F, Feng L, Liu M. Underground water-source heat-pump airconditioning system applied in a residential building in Beijing. Appl Energy 2005;82(4):331e44. [12] Nam Y, Ooka R, Shiba Y. Development of dual-source hybrid heat pump system using groundwater and air. Energy Build 2010;42(6):909e16. [13] Nam YJ, Ooka R. Numerical simulation of ground heat and water transfer for groundwater heat pump system based on real-scale experiment. Energy Build 2010;42(1):69e75. [14] Liang J, Yang QC, Liu LC, Li XY. Modeling and performance evaluation of shallow ground water heat pumps in Beijing plain, China. Energy Build 2011;43(11):3131e8. [15] Lei F, Hu PF. Energy and exergy analysis of ground water heat pump system. Phys Procedia 2012;24:169e75.
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Performance analysis of ground water-source heat pump system with improved control strategies for building retrofit Na Zhu*, Pingfang Hu, Wei Wang, Jianming Yu, Fei Lei Department of Building Environment and Equipment Engineering, School of Environment of Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 May 2014 Accepted 1 February 2015 Available online 28 February 2015
This paper presented a case study on performance analysis of ground water-source heat pump system (GWHP) for a hotel building retrofit. Splitting air-conditioner was used for cooling and coal fired boiler was used for heating in original system. The GWHP system could save electricity energy and water source consumption under improved operation methods compared with the common operation methods. In this study, the experimental average cooling coefficient of performance (COP) of the units and system were 5.53 and 3.29, respectively (from Jun. to Sep.). The average heating COP of the units and system were 4.45 and 2.79, respectively (from Nov. to Feb.). The primary energy consumption of GWHP system was reduced by 42.9% when compare with the original cooling/heating system in a year. The indoor air temperature fluctuated in comfortable range (19.3 Ce24.4 C) during test day in winter. The results show that the GWHP system was particularly suitable for this hotel under our improved operation methods. This renewable and environment-friendly technology could promote energy conservation and sustainability obviously for the hotel building. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Ground water-source heat pump Retrofit technology Improved operation methods
1. Introduction As one of the largest developing country in the world, China's rapid economic growth is accompanied with serious environment challenges such as pollution, energy shortage and climate change [1]. Energy shortage is the most serious problem at present. The structure of energy consumption in China is dominated by coal consumption, while clean energies only represent a very small percentage. The GWHP's high energy efficiency and low environmental impact characteristics have already drawn a fair amount of attention as one of the clean energies in China. GWHP technology provides a new and clean way of heating/cooling for buildings in the world. It is estimated that, heating, ventilation and air-conditioning (HVAC) accounts for about 65% of the energy consumption in the Chinese building sector [2-3]. The HVAC system accounts for about 44% for the electricity consumption in the hotel in China [4]. Engineers and relevant researchers strongly desire to reduce energy
* Corresponding author. Tel.: þ86 027 87792164x415. E-mail address: [email protected] (N. Zhu). http://dx.doi.org/10.1016/j.renene.2015.02.021 0960-1481/© 2015 Elsevier Ltd. All rights reserved.
consumption in HVAC systems. Professionals and policymakers have been making great efforts in this aspect. The overall process of a building retrofit can be divided into five major phases [5]. The first phase is the project setup and preretrofit survey. The second phase comprises an energy audit and performance assessment (and diagnostics). The third phase is the identification of retrofit options. The fourth phase is site implementation and commissioning. The final phase is validation and verification of energy savings. This hotel retrofit follows these five phases step by step. The retrofit technologies can be categorized into three groups, including supply side management, demand side management [610], and change of energy consumption patterns, i.e. human factors. The retrofit technologies for supply side management include building electrical system retrofits and the use of renewable energy, such as solar hot water, solar photovoltaic, wind energy, geothermal energy et al., as alternative energy supply systems to provide electricity and/or thermal energy for buildings. In the last five years, it had an increasing interest in the use of renewable energy technologies as building retrofit solutions due to the increased awareness of environment issues. The use of renewable energy technologies may bring more benefits for commercial office buildings where a utility rate structure including time-of-use
N. Zhu et al. / Renewable Energy 80 (2015) 324e330
Nomenclature CE COP HVAC GWHP N P Q SEP T W
primary energy (tce) coefficient of performance heating, ventilation and air conditioning ground water-source heat pump power of system (kW) power of heat pump (kW) cooling/heating load (kW) saving energy percentage (%) temperature ( C) power consumption (W)
Subscripts c cooling comp compressor fan fan g ground water-source heat pump system h heating hp heat pump o original cooling/heating system s system
differentiated electricity prices and demand charge is applied. The use of ground water-source heat pumps falls into this category. GWHP technology is regarded as an effective technology for using renewable energy. Chen et al. [11] investigated an underground water source heat pump system installed in a tall apartment building in Beijing, China. By analyzing this system for two years, operation methods and a controlling algorithm of the system were developed. Nam et al. [12,13] studied the performance of a GWHP system. This system depended on the temperature and depth of the water, and its efficiency was much higher than that of the air source heat pump. Numerical methods of underground heat water transfer, energy balance method and thermal storage method were presented to calculate the capacity of unit area of shallow groundwater aquifer by Liang et al. [14]. The model results indicate that the numerical method, which was based on performance efficiency of GWHP, represented the behavior of groundwater pumping/recharging processes, and served better than energy balance method and thermal storage method, in addition it had the advantages of energy saving and environmental protection. Lei et al. [15] investigated the performance characteristics of a GWHP system on the actual operation, using the energy and exergy analysis method. The available researches focused on system experimental tests or model analyses. Because of the complexity of the GWHP systems compared with the original cooling/heating system, the actual operation performance and energy efficiency of the systems during long periods need to be validated further. However, the operation efficiency of GWHP is not satisfactory in some buildings due to many factors, including inefficient equipment, improper control schemes and many malfunctions happened in the building operation. In addition, the groundwater quality must be protected and the total quantity of ground water source must be kept balance. Groundwater recharging is an important factor for sustainable and stable operation of the GWHP system. This study aims at analyzing the performance of the GWHP system based on the site test results in a whole year and system design data. In this study, common operation methods are improved and presented. The electricity energy and water resource consumption could be reduced under
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our improved methods compared with common ones. Indoor thermal comfortable is an additional factor to verify the feasibility of the retrofit technology.
2. System design 2.1. Overview This hotel is located in Wuhan (latitude 30.52 N, longitude 114.32 E), China. Wuhan is a cooling-dominant area with hot summer and cold winter. The total building area is 2200 m2 and total air-conditioning area is 1862 m2. There are eight floors in this hotel building. There are totally twenty-seven guest rooms in this hotel and fifty-four beds in the hotel. The hot water demand quality for each bed is about 0.2 m3 per day. Hot water in this hotel is available 24 h a day all year round. The cooling and heating load of this hotel are 208 kW and 170 kW, respectively. The cooling period for the hotel was from Jun. to Sep. and the heating period was from Nov. to Feb. The performance of this GWHP system was tested from Jun 1st to Aug 31st in 2012 (summer test) and from Dec 20th to Feb 18th in 2013 (winter test). The load ratio of the GWHP system was 65% during site tests. The weather profiles include outdoor air temperature and relative humidity during the test two periods in Wuhan, shown in Figs. 1 and 2. The set-point of indoor air temperature and relative humidity were 26 Cand 60% in summer. The set-point of air temperature and relative humidity were 20 Cand 40% in winter. The weather profiles show that the indoor air temperature and humidity were not in the comfort range both in summer and winter, so cooling and heating technologies were necessary to improve indoor thermal comfort with reasonable technology. The original cooling/heating system mainly included splitting air-conditioners, coal fired boilers, terminal equipments and pipes. Splitting air-conditioners were only used for cooling in summer. The rated cooling COP of the splitting air-conditioner system was 2.7. The coal fired boilers were used as heat source supplying airconditioner hot water and bath hot water. The terminal equipments in guest rooms were radiators hanging on the wall used for heating in winter and water heaters used for bathing over the year. The design heat efficiency of boiler was 68%. There were twentyseven splitting air conditioners for twenty-seven guest rooms and
Fig. 1. Weather profile in summer (Jun 1steug 31st).
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Fig. 2. Weather profile in winter (Dec 20theFeb 18th).
two coal-fired boilers in this hotel. More detail technical specifications of original cooling/heating system were listed in Table 1. The original heating system was not environment-friendly since too much coal cinders after burning for supplying room heat in winter and hot water over the year. The supply/return water temperature from/to the boiler varied with outdoor air temperature. So, the reliability of boiler was not good enough and the boiler did not operate in very good working conditions when the outdoor air temperature varied greatly. In addition, the maximal room air temperature difference was about 4 C because of heat imbalance during heating in winter. It made some people feel very uncomfortable in the guest rooms in winter. The original cooling system was scrapped after over ten years' service. In order to improve the energy efficiency of the cooling/heating system and reduce pollution, the original cooling/heating system will be replaced by new system using renewable and sustainable energy technology. The project targets and client's concern for the environment had a significant impact on the selection of retrofit technologies. The GWHP system was selected by hotel owners as retrofit technology for this project based on investigation finally. This project aimed at analyzing the energy performance and indoor thermal comfort of
Table 1 Technical specifications of original cooling/heating system. Type
Specification
Value
Number
Splitting air-conditioner
Cooling capacity Cooling input power Cooling COP Circulation Dehumidification Dimension (W D H)
27
Boiler
Input power Steam pressure Steam temperature
3500 W 1255 W 2.7 610 m3/h 1.2 L/H Indoor:844 mm 183 mm 300 mm Outdoor:848 mm 540 mm 320 mm 0.1 MW 0.4Mpa Outlet: 95 C Inlet: 70 C A Ⅱ soft coal 8.147 kWh/kg 90% 68%
Fuel Coal calorific value Pipe heat efficiency Boiler heat efficiency
2
the GWHP system. A general view of the GWHP system was shown in Fig. 3. This system mainly consisted of the GWHP system, an indoor air-conditioner system and a data acquisition system. The specifications of the equipments appeared in Fig. 3 were listed in Table 2. The GWHP system included two MWH-020 type GWHP units and two circulating pumps. GWHP unit could operate in two modes, cooling and heating. The hot water GWHP system included one MWH-020 type GWHP unit and one circulating pump. It could operate in three modes, supplying hot water, cooling and heating. Supplying hot water mode was prior to cooling/heating mode. The design inlet/outlet water temperatures of the heat pump units were listed in Table 3. The underground water source for this air-conditioner system came from three pumping wells around the building, 23.6 m depth and 450 mm diameter. The total water volume provided by three wells was 90 T/h and each pumping well was 30 T/h. The used water went back to recharge wells and the total volume was 90 T/h. The water pumping from wells was returned back to wells fully. The variability of underground water lever was less than 0.5 m according to professional program calculation and water lever isograms of pumping wells and recharging wells under same working condition. In this project, the grit capacity in the well-water was 1/ 10,000. The indoor air-conditioner system included four types of fan coil units (FP-51, FP-68, FP-102, FP-136) and fresh air handing units. Besides the ground temperature sensors, data acquisition system also included several calorimeters. Each calorimeter consisted of one flow meter with the minimum flow rate of 0.05 L/h and two Pt1000 temperature sensors with ±0.1 C accuracy. All power consumptions were recorded by the wattmeter.
2.2. Operation modes Operation modes were switched by different valves control. Based on the requirements of guests in this hotel, five operation modes were designed in the present GWHP system, including: a) cooling and supplying hot water mode; b) cooling mode; c) heating and supplying hot water mode; d) heating mode; e) supplying hot water mode. There were some deficiencies on common operation methods of the GWHP system. The pump always pumped ground water and then sent it back to the ground after exchange heat no matter the unit was on or off. When the cooling load was not very high, the water pump electricity and water resource consumption will be waste in this mode. The total heat recovery unit used in this system only supplied hot water but didn't supply heat in winter. The total heat recovery unit stopped working when the temperature was below set-point, but the unit could not transfer to heating mode. So, the unit was not fully used. When the heat of total heat recovery unit was not sufficient for heating water to the set-point, the water was not hot enough for bath, so the operation modes of the units were needed to be improved. In order to improve the common operation methods and operate in five operation modes successfully, new operation methods were designed. There were some characteristics of the new operation methods. First, when all the units stopped working, which meant the tank hot water temperature reached set-point and the terminal side circulating water temperature reached set-point, only one terminal side circulating water pump operated and the water heating circulating pump stopped working. In-take pump stopped working after cushion, and it could save electricity consumption of the intake pump and water resource consumption.
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Fig. 3. Schematic of GWHP system. 1:GWHP unit; 2:hot water GWHP unit; 3:terminal circulating pump; 4:hot water circulating pump; 5:submerged pump; 6:desander; 7:plate heat exchanger; 8:circling pump; 9:hot water tank; 10:hot water supply pump.
Second, when the total heat recovery unit heated water to setpoint in winter, hot water flowed direction could be switched according to the on/off electromagnetic value, and water heating mode could be switched to heating mode automatically. It could complement insufficient heat demand. Third, when the summer cooling load was not very high, heat from the total heat recovery was not sufficient for heating tank water to set-point. Under this condition, the unit transfer
electromagnetic to take heat from well and make sure the tank water temperature was heated to set-point. In a word, the improved operation modes of this system could be switched successfully to satisfy the users' needs in five operation modes and save electricity and water resource consumption obviously. Five operation modes were achieved according to the control strategies. The water pumps control strategies in different seasons could be summarized as follow.
Table 2 Equipments list of GWHP system.
2.2.1. Operation modes in summer
No
Type
Specification
Number
1
GWHP unit
MWH-020, Qc ¼ 70 kW,Nc ¼ 13.5 kW, Qh ¼ 78 kW, Nh ¼ 19.5 kW MWH-020, Qc ¼ 70 kW, Nc ¼ 13.5 kW, Qh ¼ 78 kW, Nh ¼ 19.5 kW Q ¼ 15 m3/h, H ¼ 35 m, N ¼ 4 kW Q ¼ 12.5 m3/h, H ¼ 12.5 m, N ¼ 1.1 kW Q ¼ 30 m3/h, H ¼ 40 m, N ¼ 5.5 kW XLC-40B M15-EF G8 Q ¼ 30 m3/h, H ¼ 40 m, N ¼ 5.5 kW 4t 3.3 kW 1.0 m 1.0 m 1.0 m DN100 DN100
2
2
Hot water GWHP unit
3
Terminal circulating pump
4
Hot water circulating pump
5
Submerged pump
6 7 8
Desander Plate heat exchanger Circling pump
9 10 11 12 13
Hot water tank Hot water supply pump Expansion water tank Water collector Water distributor
1
a) Hot water heating circulating pump When the tank hot water temperature was equal or higher than 50 C, the electromagnetic valve No.1 was opened and the electromagnetic valve No.2 was closed. The hot water heating circulating pump was closed. The three units supply cooling normally.
4 2 2 1 1 1 1 2 1 1 1
Table 3 Design inlet/outlet water temperatures of heat pump units. Season
Side
Component
Inlet water temperature
Outlet water temperature
Summer
Terminal side Underground water side Underground water side
12 C 20 C 18 C
7 C 34 C 32 C
Winter
Terminal side Underground water side Underground water side
Evaporator condenser Plate heat exchanger Condenser Evaporator Plate heat exchanger
40 C 16 C 18 C
45 C 8 C 10 C
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When the tank hot water temperature was lower than 42 C, the electromagnetic valve No.2 was opened and the electromagnetic valve No.1 was closed. The hot water heating circulating pump was opened. The unit No.2 heated water and the other two units supplied cooling normally. b) Submerged pump in ground source side The submerged pump should be started up firstly before chiller operates. c) Circulating pump in user side The three water pumps could parallel operate simultaneously and operate separately. And there was one more water pump for standby.
GWHP system could be fully used and meet client's need under these improved operation methods. 3. Analysis of results The performance of the GWHP unit and system is evaluated by COP. Eqs. (1)e(2) are the definition of the unit and system COP. The unit COP is defined by power consumption of heat pump unit, which includes the compressor and fans for the heat exchangers. The power consumption for a heat pump coming from the compressor and the fans is shown in Eq. (3). The system COP is defined by the power consumption of the system, which includes power consumption for a heat pump and pumping power, is defined in Eq. (4).
COP ¼
Q P
(1)
2.2.2. Operation modes in winter a) Hot water heating circulating pump When the tank hot water temperature was equal or higher than 50 C, the hot water heating circulating pump was closed. The inside valve of the unit conversed to terminal side and stopped supplying hot water. The three units supplied heating normally. When the tank hot water temperature was lower than 42 C, the inside valve of the unit conversed to hot water heating side, then the hot water heating circulating pump was opened. The unit No.2 heated water and the other two units supplied heating normally. b) Submerged pump in ground source side The submerged pumps should be started up firstly before chiller operates. c) Circulating pump in user side The three water pumps could parallel operate simultaneously and operate separately. And there was one more water pump for standby.
Q Ns
(2)
P ¼ Wcomp þ Wfan
(3)
Ns ¼ Wcomp þ Wfan ¼ Wpump
(4)
COPS ¼
where, COP and COPs are COP of heat pump unit and system, respectively. Q is actual cooling/heating load. P is power input of unit. Ns is total power input of system. Wcomp and Wfan are power consumptions of the compressor and fan, respectively. Wpump is the pumping power consumption. Figs. 4 and 5 show the COP of GWHP units and system in cooling season and heating season, respectively. In the cooling season, it was found that the COP of the units varied from 2.17 to 9.45 and average value was 5.53. The COPs of the system varied from 1.38 to 5.52 and average value was 3.29 in the cooling season. In the heating season, the COP of the units varied from 2.24 to 7.66 and average value was 4.45. The COPs of the system varied from 1.33 to 5.69 and average value was 2.79 in the heating season. The energy efficiency was promoted compared with splitting air-conditioner units.
2.2.3. Operation modes in remaining seasons a) Hot water heating circulating pump When the tank hot water temperature was equal or higher than 50 C, the hot water heating circulating pump and submerged pump in ground source side were closed. When the tank hot water temperature was lower than 42 C, the hot water heating circulating pump and submerged pump in ground source side were opened. b) Submerged pump in ground source side The submerged pump and the hot water heating circulating pump were closed or opened simultaneously. c) Circulating pump in user side The circulating pumps in user side were closed all. A major objective of using the GWHP system was to obtain an improved integration between the available renewable sources and residential thermal requirements, while guaranteeing a satisfactory level of comfort and quality of energy use under all situations. The
Fig. 4. COP of units and system in summer (Jun 1steAug 31st).
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Fig. 7. Inlet/outlet water temperature of condenser/evaporator (Dec 20theFeb 18th).
Fig. 5. COP of units and system in winter (Dec 20theFeb 18th).
Three GWHP units operated during the test two periods. In the supply side, more detail data for this air-conditioner system were recorded for two periods, including inlet and outlet water temperature of the evaporator and condenser, the water flow mass of well, the water temperature difference between inlet and outlet of well. These test data could evaluate the heating/cooling performance of this retrofitted system. Figs. 6 and 7 show the inlet and outlet water temperature flowing through the condenser and evaporator during the two periods, respectively. We could find that the inlet and outlet average water temperatures of evaporator were 12.8 C and 9.22 C, and inlet and outlet average water temperatures of condenser were 25.7 C and 31.7 C during the cooling season. The outlet water of evaporator was higher than design value (7 C) might because of unusual operation of water pump or block of strainer in water cycling. In the heating season, inlet and outlet average water
temperatures of condenser were 40.9 C and 44.6 C, and inlet and outlet average water temperatures of evaporator were 13.8 C and 9.1 C. The inlet water of evaporator was lower than design value (16 C) might because of underground water temperature was low and on-way heat loss of tubes. This GWHP system operated well according to the cooling/ heating test. The saving energy percentage of the GWHP system compared with original cooling/heating system is calculated by Eq. (5).
SEP ¼
CEo CEg CEo
(5)
where SEP is saving energy percentage (%); CE0 is primary energy consumption of original cooling/heating system (tce); CEg is primary energy consumption of GWHP system (tce). The splitting air-conditioner used for cooling and coal fired boiler used for heating and hot water were considered as original cooling/heating system. The total energy consumptions were listed in Table 4. Table 4 showed that the primary energy saving percentage of the GWHP system was 42.9% compared with original cooling/ heating system. In Jan 12th, one of the guest rooms was selected randomly as example to test supply and return air temperature in the terminal side, shown in Fig. 8. Fig. 8 showed that the supply air temperature fluctuated around 31.0 C averagely with 28.3 C (4:00 am) in minimum and 36.3 C (15:00 pm) in maximum. The return air temperature which was equal to the indoor air temperature in the hotel room approximately indicated thermal comfort of the room. The return air temperature fluctuates around 21.3 C averagely with 19.3 C (4:00
Table 4 Saving energy percentage of GWHP system. Air-conditioner type
Fig. 6. Inlet/outlet water temperature of condenser/evaporator (Jun 1steAug 31st).
Total primary Saving energy Heating Cooling percentage season energy season energy energy consumption consumption consumption (%)
Original cooling/ 118,800 kWh 170.3t (coal) heating system (electricity) GWHP system 47,850 kWh 20,152 kWh (electricity) (electricity)
48.12tce
e
27.47tce
42.9%
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temperature of the water flowing through heat pump. The total saving energy percentage of GWHP system was 42.9% compared with original cooling/heating system. (c) During the cooling season, the inlet/outlet water temperatures difference flowing through the condenser and evaporator were about 6.62 C and 2.86 C, respectively. During the heating season, these values were about 3.63 C and 4.42 C respectively. The design inlet/outlet water temperature difference of heat pumps was 5 C. It implied that the heat pumps could supply sufficient hot water and cooling/heating for this hotel all around year. (d) The supply and return air temperature in the test room indicated that the indoor air temperature was in comfortable range (19.3 Ce24.4 C) and could supply sufficient heat for users during heating season.
Fig. 8. Supply and return air temperature in terminal side (Jan 12th).
am) in minimum and 24.4 C (15:00 pm) in maximum. The supply and return air temperatures difference was around 10 C during the test day. It implied that the indoor air temperature reached design set-point and could supply sufficient heat for the guests during winter. During the heating mode, water temperature changes varied with time, which implied that changes in COP varied widely over time. This conclusion could be validated by the COP of units and system shown in Figs. 4 and 5. The water average temperature difference flowing through the condenser during two periods were about 6.62 C and 3.36 C, respectively. The water average temperature difference flowing through the evaporator during two periods were about 2.28 C and 4.42 C, respectively. The temperature fluctuation in the evaporator was lesser than that in the condenser. 4. Conclusions In this study, a GWHP system for cooling/heating purpose and sustaining hot water supply was designed for original cooling/ heating system retrofit in a hotel. An energy analysis was performed in order to evaluate the efficiency of the GWHP system under our improved operation methods. Energy analysis was performed by tests. The main research results are as follows. (a) This system could operate in five modes according to the actual needs in the hotel. Some improvements had been made compared with the common operation modes. Electricity energy and water source consumption could be reduced obviously under the improved operation modes. (b) The COP of the units and system varied from 2.17 to 9.45 and 1.38e5.52 respectively during the cooling season. The COP of the units and system varied from 2.24 to 7.66 and 1.33e5.69 respectively during the heating season. The average COP was higher than that of original heat pump system using ambient air heat source. Since the COP of the units varied widely, the system was needed to be re-configured to control the
Therefore it concluded that this system was particularly suitable for this hotel in Wuhan and the original cooling/heating system was retrofitted successfully. The GWHP system in this hotel could operate in five modes to meet guests' requirements on heating/ cooling and hot water needs for 24 h all the year round. The operating electricity and water resource consumption could be reduced under the improved operation modes. This study might be helpful for designers to apply GWHP technology in building retrofit. Acknowledgment This work presented in this paper is financially supported by a grant (No. 51078160) of National Science Foundation of China and The Central University Special Funding for Basic Scientific Research Business Expenses (No.2013NY007, No.2013 KXVQ006). References [1] Li ZS, Zhang GQ, Li DM, Zhou J, Li LJ, Li LX. Application and development of solar energy in building industry and its prospects in China. Energy Policy 2007;35(8):4121e7. [2] Liu Y, Joseph CL, Tsang CL. Energy performance of building envelopes in different climate zones in China. Appl Energy 2008;85(9):800e17. [3] Yao R, Li B, Steemers K. Energy policy and standard for built environment in China. Renew Energy 2005;30(13):1973e88. [4] Xue ZF. Diagnose and retrofit of energy saving for exiting building. China: China Building Industry Press; 2007 [in Chinese]. [5] Ma ZJ, Cooper P, Daly D, Ledo L. Existing building retrofits: methodology and state-of-the-art. Energy Build 2012;55(12):889e902. [6] Barlow S, Fiala D. Occupant comfort in UK offices-how adaptive comfort theories might influence future low energy office refurbishment strategies. Energy Build 2007;39(7):837e46. [7] Xing YG, Hewitt N, Griffiths P. Zero carbon buildings refurbishment-a hierarchical pathway. Renew Sustain Energy Rev 2011;15(6):3229e36. [8] Krarti M. Energy audit of building systems: an engineering approach. 2nd ed. Boca Raton, Florida, USA: CRC Press, Taylor & Francis Group; 2011. [9] Baker NV. The handbook of sustainable refurbishment: non-domestic buildings. London, UK: Earthscan; 2009. [10] Fusion. A review of retrofit technologies. UK: Salford centre for research & innovation in the built & human environment, University of Salford; 2010. [11] Chen C, Sun F, Feng L, Liu M. Underground water-source heat-pump airconditioning system applied in a residential building in Beijing. Appl Energy 2005;82(4):331e44. [12] Nam Y, Ooka R, Shiba Y. Development of dual-source hybrid heat pump system using groundwater and air. Energy Build 2010;42(6):909e16. [13] Nam YJ, Ooka R. Numerical simulation of ground heat and water transfer for groundwater heat pump system based on real-scale experiment. Energy Build 2010;42(1):69e75. [14] Liang J, Yang QC, Liu LC, Li XY. Modeling and performance evaluation of shallow ground water heat pumps in Beijing plain, China. Energy Build 2011;43(11):3131e8. [15] Lei F, Hu PF. Energy and exergy analysis of ground water heat pump system. Phys Procedia 2012;24:169e75.