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Towards net zero energy design for low-rise residential buildings in subtropical Hong Kong
Applied Energy 93 (2012) 686–694
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Towards net zero energy design for low-rise residential buildings in subtropical Hong Kong K.F. Fong ⇑, C.K. Lee Building Energy and Environmental Technology Research Unit, Division of Building Science and Technology, City University of Hong Kong, Hong Kong, China
a r t i c l e
i n f o
Article history: Received 1 September 2011 Received in revised form 30 December 2011 Accepted 4 January 2012 Available online 29 January 2012 Keywords: Net zero energy building Residential building Renewable energy Dynamic simulation
a b s t r a c t Hong Kong is a typical metropolis in the subtropical South China, where high-rise buildings are all around the city. This generally implies that the density of energy demand is extremely high, even the renewable energy facilities are involved, they can just play as a minor energy provider at the current technology level. It seems only the low energy design for buildings can be made possible, not the zero energy. Nevertheless, one group of the feasible places for implementing the net zero energy (NZE) design is the lowrise residential buildings in Hong Kong. Typically they are three-storey village houses, in which the renewable energy provisions can be installed in the available space, like the flat roof and the external walls. However, the question is still there – can the NZE target be achieved in this type of building even all the possible space is used up for the renewable energy facilities? As such, a dynamic simulation study was carried out to evaluate the year-round energy performances and the related factors. The answer opens a way to both the new and retrofit projects, which would enhance the low carbon roadmap in the subtropical Hong Kong. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The role of various renewable energy sources in buildings has been getting full attention around the world, particularly responding to the climate change mitigation [1,2]. As the buildings account for the major energy consumers in the modern cities, renewable energy becomes an essential domain for the design of low energy or even zero energy buildings. In case of the distributed installation of various renewable energy facilities, like solar panels and wind turbines, one critical issue is the space available for their accommodation. In Hong Kong (22.3°N and 114.2°E), most of the buildings are multi-storey, to consider the low energy building design in such a densely populated city, there is no straightforward solution [3]. As such, it is very difficult to make the design of zero energy building become true, especially for the high-rise commercial and apartment buildings. From a variety of demonstration projects [4–8], the zero energy design happens in the low-rise buildings, in which the available space for renewable energy installations to the energy demand is comparatively high. In Hong Kong, there are low-rise residential buildings with two or three stories, they offer a potential for the zero energy design. Due to the intermittent nature of renewable energy and the relatively high energy demand, the net zero energy
(NZE) design would be more appropriate, so that the energy drawn from the power grid in case of deficit would be balanced with the energy fed to the grid during surplus. According to the latest energy end-use report in Hong Kong [9], residential buildings account for 25.2% of the total energy consumption in 2008. As the Hong Kong Government has proposed to increase the renewable energy contribution to 3–4% by 2020 for revamping fuel mix of electricity generation [10], successful development of NZE design for the lowrise residential buildings can help in reducing the energy demands and consequently the carbon emission. As a result, this study is expected to work out suitable solution for the hot and humid city where peak demand of electricity is commonly found in the summer. With the help of the education level of people in a metropolis, more possible measures for energy saving could be covered. This paper is structured in the following way. Section 2 describes the typical low-rise residential building and the appropriate renewable energy provisions for achieving NZE. Section 3 discusses the building and system details for the energy study of the village house around a year. Section 4 presents the methodology of study and the strategy of analysis. Section 5 discusses the results and investigates the factors involved. Section 6 is the conclusion and recommendation.
2. Design and provisions of NZE low-rise residential building ⇑ Corresponding author. Address: Tat Chee Avenue, Kowloon, Hong Kong, China. Tel.: +852 3442 8724; fax: +852 3442 9716. E-mail address: [email protected] (K.F. Fong). 0306-2619/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2012.01.006
The common type of low-rise residential buildings in Hong Kong is the village house, which can easily be found in the outskirt of the
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Nomenclature AC ACH BIPV BR LDR NED NZE PV RC
air-conditioning air change per hour building-integrated photovoltaic panels bedroom living/dining room net energy deficit (kW h) net zero energy photovoltaic panels room cooler
city. Each village house is three-storey high with a gross area of 65.3 m2 per floor and a floor-to-floor height of 3 m. Fig. 1 shows the typical floor layout of a common village house in this study. All dimensions are in mm. Each floor consisted of one living/dining room (LDR), three bedrooms (BR1–BR3), one toilet and one kitchen. A glass door of size 2500 mm (W) 1500 mm (H) was installed on the shorter side external wall of the LDR, with a balcony built on the outside. The orientation of the typical floor is shown in Fig. 1, where the LDR and its glass door was facing the south. Besides the glass door, a number of windows of size 600 mm (W) 1200 mm (H) were also installed as indicated in Fig. 1. To achieve the NZE design, the active low energy approach was adopted, since it can be directly applied to the existing buildings for retrofit purpose. As such, various renewable energy provisions were covered as far as practicable, including photovoltaic panels (PVs) installed on the roof and integrated on the external walls, small wind turbines erected at the four corners of the roof, and solar water heating system (SWHS) (solar–thermal collector integrated with storage tank) placed on the roof for water heating. These renewable energy facilities are being promoted by the government [11], since these are proven technologies and a number of demonstration projects have been set up in Hong Kong. A power
SWHS tEdem tEfg tEren tEtg h
solar water heating system year-round total building electricity demand (kW h) electricity from power grid (kW h) year-round total electricity generated from renewable sources (kW h) electricity to power grid (kW h) azimuth of glass door measured clockwise from the south direction (°)
regulator was used to collect all the electricity generated from the PV panels and the wind turbines. It would draw electricity from the utility grid in case the generated electricity could not meet the demand. Meanwhile, it could also supply any surplus electricity generated from the renewable sources back to the grid when the demand in the house was low. 3. Building and system details for year-round energy study 3.1. Air-conditioning Window type room coolers (RCs) were used in the LDR and the BR’s to provide air-conditioning. R410A was used as the refrigerant which is common for the RC with small cooling capacity. Figs. A1– A12 in Appendix summarizes the weekly operating schedules for different energy-consuming devices at various building zones. It was assumed that the living patterns of the occupants at the three floors were the same. The estimated design cooling load for each zone was based on the design indoor conditions of 24 °C/50% RH, and the design outdoor conditions of 32.8 °C/71% RH. The system loading depended on the infiltration rate which was taken as 0.5 air change per hour (ACH). All BR’s were equipped with RC’s of the same capacity. Air-conditioning would only be provided between April and October according to a weekly operating schedule as shown in Appendix. During the time when no air-conditioning was needed, the infiltration rate would be increased to 3 or 12 ACH depending on whether the zone temperature was below or above 25.5 °C. Moreover, oscillating fans were provided in the LDR and the BR’s which would operate when the zone was occupied with temperature above 27 °C. Table 1 summarizes the various internal heat gains used to determine the design zone cooling loads. Based on the above information, the design capacities of the RC’s were 5.2 and 2.8 kW for the LDR and the BR’s respectively, with the corresponding design coefficient of performance of 2.77 and 2.48. 3.2. Lighting, miscellaneous appliances and cooking Since the winter is generally mild in subtropical climate, space heating is not essential. All the energy demand from the house including cooking and auxiliary water heating was assumed to be electricity. Table 2 shows the miscellaneous equipment loads which consumed electricity. For the hair dryer, a daily operating period of half hour was assumed. Table 1 Summary of internal heat gains of different zones on typical floor.
Fig. 1. Typical floor plan of the village house.
Max. occupants Equipment (W) Lighting (W/m2)
LDR
BR1 (master)
BR2
BR3
4 400 14
2 200 17
1 200 17
1 200 17
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Table 2 Miscellaneous energy demands at various zones.
4. Methodology of analysis
Electric appliance
Loading (W)
Electric oven in kitchen Electric cooker in kitchen Microwave oven in kitchen Washing machine in kitchen Refrigerator in kitchen Exhaust hood in kitchen Hot water pot in kitchen (keeping warm) Lighting in kitchen Exhaust fan in toilet Lighting in toilet Oscillating fan in LDR Oscillating fan in BR’s Hair dryer in BR’s
2800 630 800 440 120 240 75 100 30 80 50 40 600
3.3. Hot water Hot water for bathing was provided throughout the whole year except in June, July and August. The daily hot water consumption was taken as 160 L/floor according to a weekly operating schedule as shown in Figs. A13–A15 in Appendix. Each bath was assumed to take 12 min within the middle of the operating hour except during the Saturday night when all the four baths were taken in the same hour. Auxiliary electric heaters were used to back up the solar– thermal heating system with a design set temperature of 60 °C. The temperature of the make-up water was assumed to rise linearly from 15 °C in January to 25 °C in July, and then got back linearly to 15 °C in January. 3.4. Renewable energy facilities As the roof was the best place to install the solar collectors for getting the highest output, it became a public space for the three floors of occupancy and the roof area should be fully utilized. Hence, PV panels of area 50 m2 were installed and three sets of the SHWS (each including a flat-plate collector of size 2 m2) were also placed on the roof with all the solar collectors facing the south at an inclination of 22°. Four small wind turbines were fixed at the four corners of the roof. For the surrounding external walls (except for those outside the toilets), building-integrated PV (BIPV) were used as much as possible. A step of 0.5 m was taken for the physical dimensions of the BIPV, which is commonly found in the available products in the current PV market. Table 3 summarizes the total area (for the three floors) of BIPV installed at the various zones and directions. Fig. 2 illustrates how the entire renewable energy system, including BIPV, PV, small wind turbines and SWHS serve the demands of air-conditioning, lighting, electric equipment and hot water of the NZE village house. Table 3 Summary of the installed BIPV outside various zones and directions of the entire village house. Zone
Direction
Area (m2)
LDR LDR BR1 BR1 BR2 BR2 BR3 BR3 Staircase Kitchen
West South North West North East East South West East
42.75 15 29.25 24.75 15 24 24 15 9 18
South (total) West (total) North (total) East (total)
30 76.5 44.25 66
Year-round dynamic simulation was adopted in this study, TRNSYS [12] and its component library TESS [13] were used for system modeling of the proposed NZE low-rise residential building. TRNSYS offered standard components for the other equipment including the PV panels (Type 94), the building-integrated PV panels (BIPV) (Type 568), the SWHS (Type 45), the small wind turbine (Type 90), the auxiliary water heater (Type 6), the power regulator (Type 48) and the building zone (Type 56). The performance of the small wind turbine was calculated based on a power curve from the manufacturer [14]. A nominal efficiency of 10% was taken for the PV panels and the BIPV. A new TRNSYS component was developed for the RC based on the model of Lee and Lam [15] and Jung et al. [16], the iteration of the vapor compression cycle calculation was carried out according to the energy, the refrigerant flow and the refrigerant mass balances. Fig. 3 indicates the connection of various components of the renewable energy system and the village house on the TRNSYS simulation platform. It also briefly shows the flow of input, simulation and output components: The schedule and weather inputs on the left; the renewable facilities and control provisions in the middle; while the building zone and output manipulation on the right. System simulations was made for 1 year based on the weather data of the typical meteorological year for Hong Kong [17] using a simulation time step of 6 min. No shading effect from adjacent buildings or other obstacles was assumed, since this study intended to identify the possible extent of renewable energy resources for the NZE design in the subtropical city. Before investigating the possibility of achieving NZE design, the year-round energy consumption of the conventional village house (without the installation of any renewable energy equipment) was first analyzed and compared with the survey data from Wan and Yik [18], in order to validate the appropriateness of the loading levels and schedules for the various energy-consuming devices in this study. Then, the performance of the NZE village house (with all the renewable energy sources installed) would be analyzed. The year-round specific outputs of the solar collectors (kW h per unit area) were compared. To achieve a NZE design, different nominal efficiencies for the BIPV and PV panels were tested. Finally, the effect of the building orientation on the year-round total energy demand tEdem was investigated to determine the best setting for the village house. In order to raise the energy-consciousness of the public, the NZE design can be counted on the appropriate involvement of the building occupants. As the human behavior would have impact on the building energy performance [19], various energy-saving enhancement strategies were considered as shown in Table 4, and their effects on the year-round energy consumption of the village house were studied. These energy-saving strategies were mainly related to the improvement of the occupants’ mindset and habit, and the reduction of energy could be realized without incurring any additional initial cost.
5. Results and discussion 5.1. Performance of conventional village house Table 5 summarizes the total year-round energy consumption of a conventional village house including all the three floors under different energy-saving strategies. Concerning the base case without any energy-saving strategy (S0), the air-conditioning (AC) load contributed the highest proportion of the total energy consumption. The corresponding specific energy consumption from AC, lighting and equipment were 44.8, 30.5 and 39.3 kW h/m2 of floor area respectively. They were very close to the benchmark figures
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Small Wind Turbines
PV Panels
Power Grid
Air Conditioning
Power Regulator
Lighting
BIPV
BIPV
BIPV
BIPV
(North)
(East)
(South)
(West)
Solar-thermal Collectors (with Storage Tank)
Electric Equipment
Hot Water
Fig. 2. Renewable energy system for NZE village house.
Fig. 3. Renewable energy system for NZE village house on TRNSYS simulation platform.
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Table 4 Energy-saving enhancement strategies for residential building. Strategy
Description
S0 S1 S1a S1b
No energy-saving strategy Increase of the air-conditioning set point from 24 °C to 25.5 °C Strategy S1 plus switching off bedroom RC when not occupied Strategy S1a plus switching off bedroom lighting when not occupied Strategy S1b plus switching off all equipment when not used
S1c
Table 5 Summary of annual energy consumption of a conventional village house under different energy-saving enhancement strategies. Category
Annual energy consumption at different energy-saving strategies (kW h) S0
Air-conditioning Lighting Miscell. appliances Cooking Hot water Total
8776 5979 7700 5302 6566 34,323 (baseline)
S1
S1a
7053 5979 7711 5302 6566
5787 5979 7712 5302 6566
32,611 (;5.0%)
31,346 (;8.7%)
S1b
S1c
5669 4342 7711 5302 6566
5651 4342 7286 5302 6566
29,590 (;13.8%)
29,147 (;15.1%)
given by [18] which yielded 45, 31 and 39 kW h/m2. The proposed building model was therefore validated. In the equipment energy consumption, there were slight variations of different energysaving strategies against the baseline, it was mainly due to the slight change of energy consumption of the oscillating fans in the corresponding cases. In order to have thorough energy study of the village house, the energy use of cooking and hot water were also included according to the data in Sections 3.2 and 3.3. Their proportions of total energy consumption were also validated with [9]. From Table 5, it was found that 19.6% reduction in the AC energy demand could be achieved simply by increasing the temperature set point from 24 °C to 25.5 °C in the strategy S1. By switching off the room coolers in the bedrooms when unoccupied in S1a, a further reduction of 17.9% could be reached for AC energy use. Substantial saving potential would therefore be found in AC energy consumption only by these two strategies, and total 8.7% energy reduction could be achieved for the whole building. By switching off the lighting in the bedrooms when unoccupied in S1b, a decrease of 27.4% in the lighting energy consumption could be made against S1a. The shut-off of all the miscellaneous appliances when unused in S1c contributed a 5.5% saving in the equipment energy consumption against S1b. The above results highlighted that good human behaviors were very effective and essential in achieving energy saving in a building, and the payback would be immediate with no additional initial cost. In fact, the adoption of S1c led to a reduction in the total building energy consumption by 15.1% when compared with the baseline S0. 5.2. Performance of NZE village house 5.2.1. Net energy deficit Upon verifying the building model and evaluating the human behaviors for the village house, the effect of implementing the various renewable energy sources on the reduction of the year-round energy consumption was then analyzed. Table 6 summarizes the year-round total energy generated from different renewable energy sources. With the installation of the BIPV, the heat transmission from the walls was reduced, which lowered the air-conditioning demand by 11.6%. Consequently, the total energy demand
was lower than that of the baseline shown in Table 5. The BIPV, PV and small wind turbines provided 62.1%, 26.5% and 11.4% of the total electricity generated from the renewable energy sources tEren respectively. Still, there was an overall net energy deficit NED of 10,188 kW h, which had to be input from the power grid. The existence of NED implies the proposed renewable energy system would not have surplus of electricity in general. In this regard, it is not worthwhile to include battery that commonly causes losses in the system. Generally the adoption of the energy-saving strategies had no effect on the power generated from the renewable energy sources, but they would reduce the NED. It was noted that the NED was higher than the difference between tEdem and tEren, since there was conversion loss of the power regulator involved. In this study, it was also intended to evaluate the extent of harnessing solar energy at a place like Hong Kong (22.3°N) where is slightly lower than the Tropic of Cancer (23.5°N), so the BIPV was included in the north side. From Table 6, the electricity generated from BIPV in the north, east, south and west are 15.4%, 30.6%, 15.6% and 38.4% of the total electricity generated from the BIPV respectively. This indicates that the BIPV installed on the north side could still have contribution from a year-round approach. 5.2.2. Effect on PV efficiency To reduce the NED and hence achieve a NZE design, the effect of efficiencies on the PV panels and the BIPV was studied. Fig. 4 shows the variation of the NED at various nominal efficiencies for the PV panels and BIPV under different energy-saving enhancement strategies. A negative value means that there was surplus of energy generated from the building, and the NZE could be basically met for the NED in zero. It was found that without any energy-saving strategy (S0), the nominal efficiencies of the PV panels and BIPV had to be over 16% in order to fulfill a NZE design. However, with the adoption of the strategy S1c, the nominal efficiencies only needed to be raised to 13.5%. Even if the nominal efficiencies remain unchanged, the NED still reduced by about 5000 kW h through switching the strategies from S0 to S1c. With the continual development of PV efficiency, the human behaviors could help in reducing the installed area of the solar panels, hence their initial cost. 5.2.3. Effect on building orientation So far in the above analysis, the orientation of the village house was based on the one shown in Fig. 1. In reality, the setting of a building depends on the location of the land plot. Hence, it is important to know the effect of the building orientation on the overall energy performance of the proposed NZE village house. Fig. 5 depicts the variations of tEdem, tEren and NED with the azimuth of the glass door at the LDR h measured clockwise from the due south. Hence, a value of 90° for h means that the glass door was facing to the west direction. For the solar collectors installed on the roof, their azimuth is taken as 0° (facing the south direction) when h equals 0° (S), 90° (W), 180° (N) and 270° (E); and 45° (facing the south-west direction) when h equals 45° (SW), 135° (NW), 215° (NE) and 315° (SE). The nominal efficiencies for the PV panels and the BIPV were assumed to be 10%, and the energy-saving strategy S0 was adopted. From Fig. 5, it was found that the NED was the lowest and the tEren was the highest when the glass door was at h of 180°, i.e. facing the north. To account for this, the specific year-round outputs of the solar collectors at different h had to be examined. Fig. 6 shows the specific output of the BIPV installed on the building walls facing the same direction as the glass door at different h. Clearly, the specific output was the highest when the BIPV was facing to the south-west direction and reached a minimum when facing the north, as agreed with a previous study for Hong Kong [20].
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Zone
Direction
Annual total (kW h)
Heat collected from SHWS Electricity from PV Electricity from wind turbines Electricity from BIPV Electricity from BIPV Electricity from BIPV Electricity from BIPV Electricity from BIPV Electricity from BIPV Electricity from BIPV Electricity from BIPV Electricity from BIPV Electricity from BIPV
Roof Roof Roof LDR LDR BR1 BR1 BR2 BR2 BR3 BR3 Staircase Kitchen
South (22° titled) South (22° titled) N/A West South North West North East East South West East
3558 6787 2924 3408 1242 1614 1973 828 1769 1769 1242 718 1326
Total electricity from renewable sources, tEren Total building electricity demand, tEdem Electricity from power grid, tEfg Electricity to power grid, tEtg Net energy deficit, NED (= tEfg tEtg)
25,600 30,150 23,369 13,181 10,188
Remark: N/A refers to ‘not applicable’.
15000
S0
Specific output (kWh/m2)
10000
NED (kWh)
90
S1 S1a S1b
5000
S1c
0 -5000 -10000
85 80 75 70 65 60 55 50 0 S
-15000 10
11
12
13
14
15
16
17
18
19
20
45 SW
90 W
135 NW
180 N
225 NE
270 E
315 SE
Azimuth of glass door at LDR (deg)
Nominal efficiencies of PV and BIPV (%) Fig. 4. Variation of the NED with the nominal efficiencies of PV panels under different energy-saving enhancement strategies.
Fig. 6. Variation of specific output of BIPV facing the same direction as the glass door at different azimuths of the glass door.
As the total area of BIPV installed on the side of glass door was much smaller than those on the other sides of the building, the total output from all the BIPV, which accounted for more than 60% of tEren, should then be the highest when that side was facing the
north. Meanwhile, the specific outputs for the PV panels installed on the roof facing the south and south-west directions were 135.8 and 134.5 kW h/m2 respectively which indicates a different trend from the BIPV installed vertically. For the SDWS, the specific thermal outputs facing the south and south-west directions were 593.1 and 587.4 kW h/m2 respectively, which followed the same pattern as the PV panels installed on the roof. This highlights the effect of the solar collector slope on the optimum direction for the solar collector, and the performance of the inclined one is different from that of the vertical one. Since the variations of tEren and
30,000 25,000
1
tE_dem
20,000
tE_ren 15,000
Occupancy
0.8
NED
Schedule
Year-round total energy (kWh)
35,000
10,000
Lighting Equipment
0.6
A/C
0.4 0.2
5,000 0 S
45 SW
90 W
135 NW
180 N
225 NE
270 E
315 SE
Azimuth of glass door at LDR (deg)
0 0
5
10
15
20
Time (Hour) Fig. 5. Variation of energy performance of the NZE village house with the azimuth of the glass door at LDR under the strategy S0.
Fig. A1. Loading schedule of LDR on weekdays.
25
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1
1 Occupancy
0.8
Lighting Equipment
0.6
Schedule
Schedule
0.8
A/C
0.4
Occupancy
0.6
Lighting Equipment
0.4
A/C
0.2
0.2
0 0
5
10
15
20
0
25
0
5
10
Time (Hour) Fig. A2. Loading schedule of LDR on Saturday.
Occupancy Lighting
25
Occupancy
Equipment
Lighting
A/C
1
Equipment
0.6
0.8
A/C
0.4
Schedule
Schedule
20
Fig. A6. Loading schedule of BR1 on Sunday.
1 0.8
15
Time (Hour)
0.2
0.6 0.4
0 0
5
10
15
20
25
0.2
Time (Hour) 0 0
Fig. A3. Loading schedule of LDR on Sunday.
5
10
15
20
25
Time (Hour) Fig. A7. Loading schedule of BR2 and 3 on weekdays.
Occupancy
1
Lighting
A/C
Equipment
Lighting
A/C
1
0.6
0.8
0.4
Schedule
Schedule
Occupancy
Equipment
0.8
0.2 0 0
5
10
15
20
0.6 0.4
25
0.2
Time (Hour)
0
Fig. A4. Loading schedule of BR1 on weekdays.
0
5
10
15
20
25
Time (Hour) Fig. A8. Loading schedule of BR2 and 3 on Saturday.
Occupancy
Equipment
Lighting
A/C
1
1
0.8
Schedule
Schedule
0.8 0.6 0.4
Occupancy Lighting
0.6
Equipment A/C
0.4 0.2
0.2
0
0 0
5
10
15
20
Time (Hour) Fig. A5. Loading schedule of BR1 on Saturday.
25
0
5
10
15
20
Time (Hour) Fig. A9. Loading schedule of BR2 and 3 on Sunday.
25
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Lighting
1
Equipment
1
Saturday
Schedule
0.8
Schedule
Weekdays
0.8
0.6 0.4
0.6
Sunday
0.4 0.2
0.2
0 0
5
0 0
5
10
15
20
10
15
20
25
20
25
Time (Hour)
25
Time (Hour)
Fig. A14. Weekly cooking schedule.
Fig. A10. Loading schedule of kitchen on weekdays.
1
1
Weekdays
0.8 Saturday
Lighting
Schedule
Schedule
0.8
Equipment
0.6 0.4
0.6
Sunday
0.4 0.2
0.2 0 0
0 0
5
10
15
20
25
5
10
15
Time (Hour)
Time (Hour) Fig. A15. Weekly hot water schedule. Fig. A11. Loading schedule of kitchen on Saturday.
the orientation of the village house had little effect on its energy performance.
1
Schedule
0.8
Lighting Equipment
0.6 0.4 0.2 0 0
5
10
15
20
25
Time (Hour) Fig. A12. Loading schedule of kitchen on Sunday.
1 Weekdays
Schedule
0.8
5.2.4. Economic consideration of NZE village house Unlike the places widely promoting renewable energy applications, such as Germany [21] and Japan [22], there is no incentive scheme for the involvement of the solar collectors and other renewable energy provisions in Hong Kong at present. In addition, there is no concrete scheme for the electricity feed-in tariff whenever surplus of electricity from the renewable energy facilities is collected. If the proposed renewable energy facilities for the NZE village house are evaluated according to the conventional economic analysis, it would take the payback period over 50 years, which is normally longer than the typical building life. As such, appropriate incentive strategies, including installation subsidies and feed-in tariff for the renewable energy facilities should be accounted for by the local government. 6. Conclusion
Saturday
0.6 Sunday
0.4 0.2 0 0
5
10
15
20
25
Time (Hour) Fig. A13. Weekly loading schedule of Toilet.
tEdem were just within ±1.8% and ±0.3% of their respective averages, the effect of h on energy performance was not apparent. As a result,
From this study, the proposed design of NZE village house in Hong Kong can be realized for the nominal efficiencies of the PV panels and the BIPV above 13%, together with good human behaviors. The required PV efficiency is in effect technically feasible at present. In the NZE design, solar energy is used as the primary source for both electricity and heat generation, through the BIPV on the walls, as well as the PV and SWHS on the roof. It is also found that the human behaviors can effectively contribute to the NZE target. If the information of NED is available all the time, it would be very useful to let the occupants keep the energy-saving consciousness. Based on the proposed NZE design, the building orientation has minimal effect to the year-round energy performance, so the target can be well maintained. In the current study, the shading effect was not taken into account since it is geographically
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dependent. Even the energy performance may be affected by such factor, there are still rooms of application of other low energy measures, like the utilization of energy-efficient light fittings and cooking utensils, the application of thermal insulation for building fabrics, and the involvement of more passive architectural designs. Since the major objective of this study is to apply the NZE design directly to the existing buildings for retrofit projects, the aforementioned low energy measures were therefore not included. On the other hand, it is not straightforward to evaluate the economic feasibility of the NZE village house at the time being. Since there is no subsidy or incentive scheme for the installation of the solar panels and other renewable energy facilities in Hong Kong. There is also no tangible electricity feed-in tariff scheme for the surplus electricity from the renewable energy facilities. Nevertheless, NZE implies no more running cost of building energy and minimum call for the grid power. As the Hong Kong Government has intended to increase the renewable energy contribution for revamping the fuel mix of electricity generation, the present study reveals a possible direction towards the low carbon society. Both the new and existing low-rise buildings can be involved, this would certainly facilitate the energy efficiency in the subtropical Hong Kong and the promotion of energy-saving consciousness to the public. The proposed NZE design is also useful in setting the necessary incentive and feed-in tariff schemes for long-term sustainable development. Acknowledgment
[2]
[3] [4] [5]
[6]
[7] [8]
[9]
[10] [11]
[12] [13] [14] [15] [16]
The work described in this paper was fully supported by a grant from City University of Hong Kong (Strategic Research Grant, Project No. 7008037).
[17]
Appendix A. Appendix
[19]
[18]
[20]
A.1. Weekly operating schedules See Figs. A1–A15
[21] [22]
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S, von Stechow C, editors. IPCC special report on renewable energy sources and climate change mitigation. Cambridge, United Kingdom and New York, NY, USA: Cambridge University Press; 2011. Verbruggen A, Fischedick M, Moomaw W, Weir T, Nadai A, Nilsson LJ, et al. Renewable energy costs, potentials, barriers: conceptual issues. Energy Policy 2010;38(2):850–61. Hui SCM. Low energy building design in high density urban cities. Renewable Energy 2001;24:627–40. Wang LP, Gwilliam J, Jones P. Case study of zero energy house design in UK. Energy Build 2009;41(11):1215–22. Zhu L, Hurt R, Correa D, Boehm R. Comprehensive energy and economic analyses on a zero energy house versus a conventional house. Energy 2009;34(9):1043–53. Deng S, Dalibard A, Martin M, Dai YJ, Eicker U, Wang RZ. Energy supply concepts for zero energy residential buildings in humid and dry climate. Energy Convers Manage 2011;52:2455–60. Bojic´ M, Nikolic´ N, Nikolic´ D, Skerlic´ J, Miletic´ I. Toward a positive-net-energy residential building in Serbian conditions. Appl Energy 2011;88:2407–19. Blengini GA, Di Carlo T. Energy-saving policies and low-energy residential buildings: an LCA case study to support decision makers in Piedmont (Italy). Int J Life Cycle Assess 2010;15:652–65. Hong Kong energy end-use data 2010. Energy Efficiency Office, Electrical and Mechanical Services Department, The Government of Hong Kong Special Administrative Region; September 2010. Hong Kong’s climate change strategy and action agenda – consultation document, Environment Bureau, HKSAR Government; October 2010. New and renewable energy. Electrical and Mechanical Services Department, The Government of the Hong Kong Special Administrative Region.. TRNSYS 16, a TRaNsient SYstem simulation program, The Solar Energy Laboratory, University of Wisconsin-Madison; 2006. TESS library documentation, Thermal energy system specialists; 2006. Electric energy, renewable energy systems. . Lee CK, Lam HN. Computer simulation of ground-coupled liquid desiccant air conditioner for sub-tropical regions. Int J Therm Sci 2009;48(12):2365–74. Jung D, Park C, Park B. Capillary tube selection for CFC22 alternatives. Int J Refrig 1999;22(7):604–14. Chan ALS, Chow TT, Fong SKF, Lin JZ. Generation of a typical meteorological year for Hong Kong. Energy Convers Manage 2006;47:87–96. Wan KSY and Yik FHW. Representative building design and internal load pattern for modelling energy use in residential buildings in Hong Kong. Appl Energy 2004;77(1):69–85. Gill ZM, Tierney MJ, Pegg IM, Allan N. Low-energy dwellings: the contribution of behaviours to actual performance. Build Res Inform 2010;38:491–508. Chow TT, Hand JW, Strachan PA. Building-integrated photovoltaic and thermal applications in a subtropical hotel building. Appl Therm Eng 2003;23:2035–49. Renewable energies, Germany Energy Agency. . Renewable energy. Ministry of Economics, Trade and Industry. .
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Towards net zero energy design for low-rise residential buildings in subtropical Hong Kong K.F. Fong ⇑, C.K. Lee Building Energy and Environmental Technology Research Unit, Division of Building Science and Technology, City University of Hong Kong, Hong Kong, China
a r t i c l e
i n f o
Article history: Received 1 September 2011 Received in revised form 30 December 2011 Accepted 4 January 2012 Available online 29 January 2012 Keywords: Net zero energy building Residential building Renewable energy Dynamic simulation
a b s t r a c t Hong Kong is a typical metropolis in the subtropical South China, where high-rise buildings are all around the city. This generally implies that the density of energy demand is extremely high, even the renewable energy facilities are involved, they can just play as a minor energy provider at the current technology level. It seems only the low energy design for buildings can be made possible, not the zero energy. Nevertheless, one group of the feasible places for implementing the net zero energy (NZE) design is the lowrise residential buildings in Hong Kong. Typically they are three-storey village houses, in which the renewable energy provisions can be installed in the available space, like the flat roof and the external walls. However, the question is still there – can the NZE target be achieved in this type of building even all the possible space is used up for the renewable energy facilities? As such, a dynamic simulation study was carried out to evaluate the year-round energy performances and the related factors. The answer opens a way to both the new and retrofit projects, which would enhance the low carbon roadmap in the subtropical Hong Kong. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The role of various renewable energy sources in buildings has been getting full attention around the world, particularly responding to the climate change mitigation [1,2]. As the buildings account for the major energy consumers in the modern cities, renewable energy becomes an essential domain for the design of low energy or even zero energy buildings. In case of the distributed installation of various renewable energy facilities, like solar panels and wind turbines, one critical issue is the space available for their accommodation. In Hong Kong (22.3°N and 114.2°E), most of the buildings are multi-storey, to consider the low energy building design in such a densely populated city, there is no straightforward solution [3]. As such, it is very difficult to make the design of zero energy building become true, especially for the high-rise commercial and apartment buildings. From a variety of demonstration projects [4–8], the zero energy design happens in the low-rise buildings, in which the available space for renewable energy installations to the energy demand is comparatively high. In Hong Kong, there are low-rise residential buildings with two or three stories, they offer a potential for the zero energy design. Due to the intermittent nature of renewable energy and the relatively high energy demand, the net zero energy
(NZE) design would be more appropriate, so that the energy drawn from the power grid in case of deficit would be balanced with the energy fed to the grid during surplus. According to the latest energy end-use report in Hong Kong [9], residential buildings account for 25.2% of the total energy consumption in 2008. As the Hong Kong Government has proposed to increase the renewable energy contribution to 3–4% by 2020 for revamping fuel mix of electricity generation [10], successful development of NZE design for the lowrise residential buildings can help in reducing the energy demands and consequently the carbon emission. As a result, this study is expected to work out suitable solution for the hot and humid city where peak demand of electricity is commonly found in the summer. With the help of the education level of people in a metropolis, more possible measures for energy saving could be covered. This paper is structured in the following way. Section 2 describes the typical low-rise residential building and the appropriate renewable energy provisions for achieving NZE. Section 3 discusses the building and system details for the energy study of the village house around a year. Section 4 presents the methodology of study and the strategy of analysis. Section 5 discusses the results and investigates the factors involved. Section 6 is the conclusion and recommendation.
2. Design and provisions of NZE low-rise residential building ⇑ Corresponding author. Address: Tat Chee Avenue, Kowloon, Hong Kong, China. Tel.: +852 3442 8724; fax: +852 3442 9716. E-mail address: [email protected] (K.F. Fong). 0306-2619/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2012.01.006
The common type of low-rise residential buildings in Hong Kong is the village house, which can easily be found in the outskirt of the
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Nomenclature AC ACH BIPV BR LDR NED NZE PV RC
air-conditioning air change per hour building-integrated photovoltaic panels bedroom living/dining room net energy deficit (kW h) net zero energy photovoltaic panels room cooler
city. Each village house is three-storey high with a gross area of 65.3 m2 per floor and a floor-to-floor height of 3 m. Fig. 1 shows the typical floor layout of a common village house in this study. All dimensions are in mm. Each floor consisted of one living/dining room (LDR), three bedrooms (BR1–BR3), one toilet and one kitchen. A glass door of size 2500 mm (W) 1500 mm (H) was installed on the shorter side external wall of the LDR, with a balcony built on the outside. The orientation of the typical floor is shown in Fig. 1, where the LDR and its glass door was facing the south. Besides the glass door, a number of windows of size 600 mm (W) 1200 mm (H) were also installed as indicated in Fig. 1. To achieve the NZE design, the active low energy approach was adopted, since it can be directly applied to the existing buildings for retrofit purpose. As such, various renewable energy provisions were covered as far as practicable, including photovoltaic panels (PVs) installed on the roof and integrated on the external walls, small wind turbines erected at the four corners of the roof, and solar water heating system (SWHS) (solar–thermal collector integrated with storage tank) placed on the roof for water heating. These renewable energy facilities are being promoted by the government [11], since these are proven technologies and a number of demonstration projects have been set up in Hong Kong. A power
SWHS tEdem tEfg tEren tEtg h
solar water heating system year-round total building electricity demand (kW h) electricity from power grid (kW h) year-round total electricity generated from renewable sources (kW h) electricity to power grid (kW h) azimuth of glass door measured clockwise from the south direction (°)
regulator was used to collect all the electricity generated from the PV panels and the wind turbines. It would draw electricity from the utility grid in case the generated electricity could not meet the demand. Meanwhile, it could also supply any surplus electricity generated from the renewable sources back to the grid when the demand in the house was low. 3. Building and system details for year-round energy study 3.1. Air-conditioning Window type room coolers (RCs) were used in the LDR and the BR’s to provide air-conditioning. R410A was used as the refrigerant which is common for the RC with small cooling capacity. Figs. A1– A12 in Appendix summarizes the weekly operating schedules for different energy-consuming devices at various building zones. It was assumed that the living patterns of the occupants at the three floors were the same. The estimated design cooling load for each zone was based on the design indoor conditions of 24 °C/50% RH, and the design outdoor conditions of 32.8 °C/71% RH. The system loading depended on the infiltration rate which was taken as 0.5 air change per hour (ACH). All BR’s were equipped with RC’s of the same capacity. Air-conditioning would only be provided between April and October according to a weekly operating schedule as shown in Appendix. During the time when no air-conditioning was needed, the infiltration rate would be increased to 3 or 12 ACH depending on whether the zone temperature was below or above 25.5 °C. Moreover, oscillating fans were provided in the LDR and the BR’s which would operate when the zone was occupied with temperature above 27 °C. Table 1 summarizes the various internal heat gains used to determine the design zone cooling loads. Based on the above information, the design capacities of the RC’s were 5.2 and 2.8 kW for the LDR and the BR’s respectively, with the corresponding design coefficient of performance of 2.77 and 2.48. 3.2. Lighting, miscellaneous appliances and cooking Since the winter is generally mild in subtropical climate, space heating is not essential. All the energy demand from the house including cooking and auxiliary water heating was assumed to be electricity. Table 2 shows the miscellaneous equipment loads which consumed electricity. For the hair dryer, a daily operating period of half hour was assumed. Table 1 Summary of internal heat gains of different zones on typical floor.
Fig. 1. Typical floor plan of the village house.
Max. occupants Equipment (W) Lighting (W/m2)
LDR
BR1 (master)
BR2
BR3
4 400 14
2 200 17
1 200 17
1 200 17
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Table 2 Miscellaneous energy demands at various zones.
4. Methodology of analysis
Electric appliance
Loading (W)
Electric oven in kitchen Electric cooker in kitchen Microwave oven in kitchen Washing machine in kitchen Refrigerator in kitchen Exhaust hood in kitchen Hot water pot in kitchen (keeping warm) Lighting in kitchen Exhaust fan in toilet Lighting in toilet Oscillating fan in LDR Oscillating fan in BR’s Hair dryer in BR’s
2800 630 800 440 120 240 75 100 30 80 50 40 600
3.3. Hot water Hot water for bathing was provided throughout the whole year except in June, July and August. The daily hot water consumption was taken as 160 L/floor according to a weekly operating schedule as shown in Figs. A13–A15 in Appendix. Each bath was assumed to take 12 min within the middle of the operating hour except during the Saturday night when all the four baths were taken in the same hour. Auxiliary electric heaters were used to back up the solar– thermal heating system with a design set temperature of 60 °C. The temperature of the make-up water was assumed to rise linearly from 15 °C in January to 25 °C in July, and then got back linearly to 15 °C in January. 3.4. Renewable energy facilities As the roof was the best place to install the solar collectors for getting the highest output, it became a public space for the three floors of occupancy and the roof area should be fully utilized. Hence, PV panels of area 50 m2 were installed and three sets of the SHWS (each including a flat-plate collector of size 2 m2) were also placed on the roof with all the solar collectors facing the south at an inclination of 22°. Four small wind turbines were fixed at the four corners of the roof. For the surrounding external walls (except for those outside the toilets), building-integrated PV (BIPV) were used as much as possible. A step of 0.5 m was taken for the physical dimensions of the BIPV, which is commonly found in the available products in the current PV market. Table 3 summarizes the total area (for the three floors) of BIPV installed at the various zones and directions. Fig. 2 illustrates how the entire renewable energy system, including BIPV, PV, small wind turbines and SWHS serve the demands of air-conditioning, lighting, electric equipment and hot water of the NZE village house. Table 3 Summary of the installed BIPV outside various zones and directions of the entire village house. Zone
Direction
Area (m2)
LDR LDR BR1 BR1 BR2 BR2 BR3 BR3 Staircase Kitchen
West South North West North East East South West East
42.75 15 29.25 24.75 15 24 24 15 9 18
South (total) West (total) North (total) East (total)
30 76.5 44.25 66
Year-round dynamic simulation was adopted in this study, TRNSYS [12] and its component library TESS [13] were used for system modeling of the proposed NZE low-rise residential building. TRNSYS offered standard components for the other equipment including the PV panels (Type 94), the building-integrated PV panels (BIPV) (Type 568), the SWHS (Type 45), the small wind turbine (Type 90), the auxiliary water heater (Type 6), the power regulator (Type 48) and the building zone (Type 56). The performance of the small wind turbine was calculated based on a power curve from the manufacturer [14]. A nominal efficiency of 10% was taken for the PV panels and the BIPV. A new TRNSYS component was developed for the RC based on the model of Lee and Lam [15] and Jung et al. [16], the iteration of the vapor compression cycle calculation was carried out according to the energy, the refrigerant flow and the refrigerant mass balances. Fig. 3 indicates the connection of various components of the renewable energy system and the village house on the TRNSYS simulation platform. It also briefly shows the flow of input, simulation and output components: The schedule and weather inputs on the left; the renewable facilities and control provisions in the middle; while the building zone and output manipulation on the right. System simulations was made for 1 year based on the weather data of the typical meteorological year for Hong Kong [17] using a simulation time step of 6 min. No shading effect from adjacent buildings or other obstacles was assumed, since this study intended to identify the possible extent of renewable energy resources for the NZE design in the subtropical city. Before investigating the possibility of achieving NZE design, the year-round energy consumption of the conventional village house (without the installation of any renewable energy equipment) was first analyzed and compared with the survey data from Wan and Yik [18], in order to validate the appropriateness of the loading levels and schedules for the various energy-consuming devices in this study. Then, the performance of the NZE village house (with all the renewable energy sources installed) would be analyzed. The year-round specific outputs of the solar collectors (kW h per unit area) were compared. To achieve a NZE design, different nominal efficiencies for the BIPV and PV panels were tested. Finally, the effect of the building orientation on the year-round total energy demand tEdem was investigated to determine the best setting for the village house. In order to raise the energy-consciousness of the public, the NZE design can be counted on the appropriate involvement of the building occupants. As the human behavior would have impact on the building energy performance [19], various energy-saving enhancement strategies were considered as shown in Table 4, and their effects on the year-round energy consumption of the village house were studied. These energy-saving strategies were mainly related to the improvement of the occupants’ mindset and habit, and the reduction of energy could be realized without incurring any additional initial cost.
5. Results and discussion 5.1. Performance of conventional village house Table 5 summarizes the total year-round energy consumption of a conventional village house including all the three floors under different energy-saving strategies. Concerning the base case without any energy-saving strategy (S0), the air-conditioning (AC) load contributed the highest proportion of the total energy consumption. The corresponding specific energy consumption from AC, lighting and equipment were 44.8, 30.5 and 39.3 kW h/m2 of floor area respectively. They were very close to the benchmark figures
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Small Wind Turbines
PV Panels
Power Grid
Air Conditioning
Power Regulator
Lighting
BIPV
BIPV
BIPV
BIPV
(North)
(East)
(South)
(West)
Solar-thermal Collectors (with Storage Tank)
Electric Equipment
Hot Water
Fig. 2. Renewable energy system for NZE village house.
Fig. 3. Renewable energy system for NZE village house on TRNSYS simulation platform.
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Table 4 Energy-saving enhancement strategies for residential building. Strategy
Description
S0 S1 S1a S1b
No energy-saving strategy Increase of the air-conditioning set point from 24 °C to 25.5 °C Strategy S1 plus switching off bedroom RC when not occupied Strategy S1a plus switching off bedroom lighting when not occupied Strategy S1b plus switching off all equipment when not used
S1c
Table 5 Summary of annual energy consumption of a conventional village house under different energy-saving enhancement strategies. Category
Annual energy consumption at different energy-saving strategies (kW h) S0
Air-conditioning Lighting Miscell. appliances Cooking Hot water Total
8776 5979 7700 5302 6566 34,323 (baseline)
S1
S1a
7053 5979 7711 5302 6566
5787 5979 7712 5302 6566
32,611 (;5.0%)
31,346 (;8.7%)
S1b
S1c
5669 4342 7711 5302 6566
5651 4342 7286 5302 6566
29,590 (;13.8%)
29,147 (;15.1%)
given by [18] which yielded 45, 31 and 39 kW h/m2. The proposed building model was therefore validated. In the equipment energy consumption, there were slight variations of different energysaving strategies against the baseline, it was mainly due to the slight change of energy consumption of the oscillating fans in the corresponding cases. In order to have thorough energy study of the village house, the energy use of cooking and hot water were also included according to the data in Sections 3.2 and 3.3. Their proportions of total energy consumption were also validated with [9]. From Table 5, it was found that 19.6% reduction in the AC energy demand could be achieved simply by increasing the temperature set point from 24 °C to 25.5 °C in the strategy S1. By switching off the room coolers in the bedrooms when unoccupied in S1a, a further reduction of 17.9% could be reached for AC energy use. Substantial saving potential would therefore be found in AC energy consumption only by these two strategies, and total 8.7% energy reduction could be achieved for the whole building. By switching off the lighting in the bedrooms when unoccupied in S1b, a decrease of 27.4% in the lighting energy consumption could be made against S1a. The shut-off of all the miscellaneous appliances when unused in S1c contributed a 5.5% saving in the equipment energy consumption against S1b. The above results highlighted that good human behaviors were very effective and essential in achieving energy saving in a building, and the payback would be immediate with no additional initial cost. In fact, the adoption of S1c led to a reduction in the total building energy consumption by 15.1% when compared with the baseline S0. 5.2. Performance of NZE village house 5.2.1. Net energy deficit Upon verifying the building model and evaluating the human behaviors for the village house, the effect of implementing the various renewable energy sources on the reduction of the year-round energy consumption was then analyzed. Table 6 summarizes the year-round total energy generated from different renewable energy sources. With the installation of the BIPV, the heat transmission from the walls was reduced, which lowered the air-conditioning demand by 11.6%. Consequently, the total energy demand
was lower than that of the baseline shown in Table 5. The BIPV, PV and small wind turbines provided 62.1%, 26.5% and 11.4% of the total electricity generated from the renewable energy sources tEren respectively. Still, there was an overall net energy deficit NED of 10,188 kW h, which had to be input from the power grid. The existence of NED implies the proposed renewable energy system would not have surplus of electricity in general. In this regard, it is not worthwhile to include battery that commonly causes losses in the system. Generally the adoption of the energy-saving strategies had no effect on the power generated from the renewable energy sources, but they would reduce the NED. It was noted that the NED was higher than the difference between tEdem and tEren, since there was conversion loss of the power regulator involved. In this study, it was also intended to evaluate the extent of harnessing solar energy at a place like Hong Kong (22.3°N) where is slightly lower than the Tropic of Cancer (23.5°N), so the BIPV was included in the north side. From Table 6, the electricity generated from BIPV in the north, east, south and west are 15.4%, 30.6%, 15.6% and 38.4% of the total electricity generated from the BIPV respectively. This indicates that the BIPV installed on the north side could still have contribution from a year-round approach. 5.2.2. Effect on PV efficiency To reduce the NED and hence achieve a NZE design, the effect of efficiencies on the PV panels and the BIPV was studied. Fig. 4 shows the variation of the NED at various nominal efficiencies for the PV panels and BIPV under different energy-saving enhancement strategies. A negative value means that there was surplus of energy generated from the building, and the NZE could be basically met for the NED in zero. It was found that without any energy-saving strategy (S0), the nominal efficiencies of the PV panels and BIPV had to be over 16% in order to fulfill a NZE design. However, with the adoption of the strategy S1c, the nominal efficiencies only needed to be raised to 13.5%. Even if the nominal efficiencies remain unchanged, the NED still reduced by about 5000 kW h through switching the strategies from S0 to S1c. With the continual development of PV efficiency, the human behaviors could help in reducing the installed area of the solar panels, hence their initial cost. 5.2.3. Effect on building orientation So far in the above analysis, the orientation of the village house was based on the one shown in Fig. 1. In reality, the setting of a building depends on the location of the land plot. Hence, it is important to know the effect of the building orientation on the overall energy performance of the proposed NZE village house. Fig. 5 depicts the variations of tEdem, tEren and NED with the azimuth of the glass door at the LDR h measured clockwise from the due south. Hence, a value of 90° for h means that the glass door was facing to the west direction. For the solar collectors installed on the roof, their azimuth is taken as 0° (facing the south direction) when h equals 0° (S), 90° (W), 180° (N) and 270° (E); and 45° (facing the south-west direction) when h equals 45° (SW), 135° (NW), 215° (NE) and 315° (SE). The nominal efficiencies for the PV panels and the BIPV were assumed to be 10%, and the energy-saving strategy S0 was adopted. From Fig. 5, it was found that the NED was the lowest and the tEren was the highest when the glass door was at h of 180°, i.e. facing the north. To account for this, the specific year-round outputs of the solar collectors at different h had to be examined. Fig. 6 shows the specific output of the BIPV installed on the building walls facing the same direction as the glass door at different h. Clearly, the specific output was the highest when the BIPV was facing to the south-west direction and reached a minimum when facing the north, as agreed with a previous study for Hong Kong [20].
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K.F. Fong, C.K. Lee / Applied Energy 93 (2012) 686–694 Table 6 Summary of annual total energy generated from different renewable energy sources. Type
Zone
Direction
Annual total (kW h)
Heat collected from SHWS Electricity from PV Electricity from wind turbines Electricity from BIPV Electricity from BIPV Electricity from BIPV Electricity from BIPV Electricity from BIPV Electricity from BIPV Electricity from BIPV Electricity from BIPV Electricity from BIPV Electricity from BIPV
Roof Roof Roof LDR LDR BR1 BR1 BR2 BR2 BR3 BR3 Staircase Kitchen
South (22° titled) South (22° titled) N/A West South North West North East East South West East
3558 6787 2924 3408 1242 1614 1973 828 1769 1769 1242 718 1326
Total electricity from renewable sources, tEren Total building electricity demand, tEdem Electricity from power grid, tEfg Electricity to power grid, tEtg Net energy deficit, NED (= tEfg tEtg)
25,600 30,150 23,369 13,181 10,188
Remark: N/A refers to ‘not applicable’.
15000
S0
Specific output (kWh/m2)
10000
NED (kWh)
90
S1 S1a S1b
5000
S1c
0 -5000 -10000
85 80 75 70 65 60 55 50 0 S
-15000 10
11
12
13
14
15
16
17
18
19
20
45 SW
90 W
135 NW
180 N
225 NE
270 E
315 SE
Azimuth of glass door at LDR (deg)
Nominal efficiencies of PV and BIPV (%) Fig. 4. Variation of the NED with the nominal efficiencies of PV panels under different energy-saving enhancement strategies.
Fig. 6. Variation of specific output of BIPV facing the same direction as the glass door at different azimuths of the glass door.
As the total area of BIPV installed on the side of glass door was much smaller than those on the other sides of the building, the total output from all the BIPV, which accounted for more than 60% of tEren, should then be the highest when that side was facing the
north. Meanwhile, the specific outputs for the PV panels installed on the roof facing the south and south-west directions were 135.8 and 134.5 kW h/m2 respectively which indicates a different trend from the BIPV installed vertically. For the SDWS, the specific thermal outputs facing the south and south-west directions were 593.1 and 587.4 kW h/m2 respectively, which followed the same pattern as the PV panels installed on the roof. This highlights the effect of the solar collector slope on the optimum direction for the solar collector, and the performance of the inclined one is different from that of the vertical one. Since the variations of tEren and
30,000 25,000
1
tE_dem
20,000
tE_ren 15,000
Occupancy
0.8
NED
Schedule
Year-round total energy (kWh)
35,000
10,000
Lighting Equipment
0.6
A/C
0.4 0.2
5,000 0 S
45 SW
90 W
135 NW
180 N
225 NE
270 E
315 SE
Azimuth of glass door at LDR (deg)
0 0
5
10
15
20
Time (Hour) Fig. 5. Variation of energy performance of the NZE village house with the azimuth of the glass door at LDR under the strategy S0.
Fig. A1. Loading schedule of LDR on weekdays.
25
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1
1 Occupancy
0.8
Lighting Equipment
0.6
Schedule
Schedule
0.8
A/C
0.4
Occupancy
0.6
Lighting Equipment
0.4
A/C
0.2
0.2
0 0
5
10
15
20
0
25
0
5
10
Time (Hour) Fig. A2. Loading schedule of LDR on Saturday.
Occupancy Lighting
25
Occupancy
Equipment
Lighting
A/C
1
Equipment
0.6
0.8
A/C
0.4
Schedule
Schedule
20
Fig. A6. Loading schedule of BR1 on Sunday.
1 0.8
15
Time (Hour)
0.2
0.6 0.4
0 0
5
10
15
20
25
0.2
Time (Hour) 0 0
Fig. A3. Loading schedule of LDR on Sunday.
5
10
15
20
25
Time (Hour) Fig. A7. Loading schedule of BR2 and 3 on weekdays.
Occupancy
1
Lighting
A/C
Equipment
Lighting
A/C
1
0.6
0.8
0.4
Schedule
Schedule
Occupancy
Equipment
0.8
0.2 0 0
5
10
15
20
0.6 0.4
25
0.2
Time (Hour)
0
Fig. A4. Loading schedule of BR1 on weekdays.
0
5
10
15
20
25
Time (Hour) Fig. A8. Loading schedule of BR2 and 3 on Saturday.
Occupancy
Equipment
Lighting
A/C
1
1
0.8
Schedule
Schedule
0.8 0.6 0.4
Occupancy Lighting
0.6
Equipment A/C
0.4 0.2
0.2
0
0 0
5
10
15
20
Time (Hour) Fig. A5. Loading schedule of BR1 on Saturday.
25
0
5
10
15
20
Time (Hour) Fig. A9. Loading schedule of BR2 and 3 on Sunday.
25
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Lighting
1
Equipment
1
Saturday
Schedule
0.8
Schedule
Weekdays
0.8
0.6 0.4
0.6
Sunday
0.4 0.2
0.2
0 0
5
0 0
5
10
15
20
10
15
20
25
20
25
Time (Hour)
25
Time (Hour)
Fig. A14. Weekly cooking schedule.
Fig. A10. Loading schedule of kitchen on weekdays.
1
1
Weekdays
0.8 Saturday
Lighting
Schedule
Schedule
0.8
Equipment
0.6 0.4
0.6
Sunday
0.4 0.2
0.2 0 0
0 0
5
10
15
20
25
5
10
15
Time (Hour)
Time (Hour) Fig. A15. Weekly hot water schedule. Fig. A11. Loading schedule of kitchen on Saturday.
the orientation of the village house had little effect on its energy performance.
1
Schedule
0.8
Lighting Equipment
0.6 0.4 0.2 0 0
5
10
15
20
25
Time (Hour) Fig. A12. Loading schedule of kitchen on Sunday.
1 Weekdays
Schedule
0.8
5.2.4. Economic consideration of NZE village house Unlike the places widely promoting renewable energy applications, such as Germany [21] and Japan [22], there is no incentive scheme for the involvement of the solar collectors and other renewable energy provisions in Hong Kong at present. In addition, there is no concrete scheme for the electricity feed-in tariff whenever surplus of electricity from the renewable energy facilities is collected. If the proposed renewable energy facilities for the NZE village house are evaluated according to the conventional economic analysis, it would take the payback period over 50 years, which is normally longer than the typical building life. As such, appropriate incentive strategies, including installation subsidies and feed-in tariff for the renewable energy facilities should be accounted for by the local government. 6. Conclusion
Saturday
0.6 Sunday
0.4 0.2 0 0
5
10
15
20
25
Time (Hour) Fig. A13. Weekly loading schedule of Toilet.
tEdem were just within ±1.8% and ±0.3% of their respective averages, the effect of h on energy performance was not apparent. As a result,
From this study, the proposed design of NZE village house in Hong Kong can be realized for the nominal efficiencies of the PV panels and the BIPV above 13%, together with good human behaviors. The required PV efficiency is in effect technically feasible at present. In the NZE design, solar energy is used as the primary source for both electricity and heat generation, through the BIPV on the walls, as well as the PV and SWHS on the roof. It is also found that the human behaviors can effectively contribute to the NZE target. If the information of NED is available all the time, it would be very useful to let the occupants keep the energy-saving consciousness. Based on the proposed NZE design, the building orientation has minimal effect to the year-round energy performance, so the target can be well maintained. In the current study, the shading effect was not taken into account since it is geographically
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dependent. Even the energy performance may be affected by such factor, there are still rooms of application of other low energy measures, like the utilization of energy-efficient light fittings and cooking utensils, the application of thermal insulation for building fabrics, and the involvement of more passive architectural designs. Since the major objective of this study is to apply the NZE design directly to the existing buildings for retrofit projects, the aforementioned low energy measures were therefore not included. On the other hand, it is not straightforward to evaluate the economic feasibility of the NZE village house at the time being. Since there is no subsidy or incentive scheme for the installation of the solar panels and other renewable energy facilities in Hong Kong. There is also no tangible electricity feed-in tariff scheme for the surplus electricity from the renewable energy facilities. Nevertheless, NZE implies no more running cost of building energy and minimum call for the grid power. As the Hong Kong Government has intended to increase the renewable energy contribution for revamping the fuel mix of electricity generation, the present study reveals a possible direction towards the low carbon society. Both the new and existing low-rise buildings can be involved, this would certainly facilitate the energy efficiency in the subtropical Hong Kong and the promotion of energy-saving consciousness to the public. The proposed NZE design is also useful in setting the necessary incentive and feed-in tariff schemes for long-term sustainable development. Acknowledgment
[2]
[3] [4] [5]
[6]
[7] [8]
[9]
[10] [11]
[12] [13] [14] [15] [16]
The work described in this paper was fully supported by a grant from City University of Hong Kong (Strategic Research Grant, Project No. 7008037).
[17]
Appendix A. Appendix
[19]
[18]
[20]
A.1. Weekly operating schedules See Figs. A1–A15
[21] [22]
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