An analysis of potential applications of wide-scale solar energy in Hong Kong

Abstract

Renewable energy plays a crucial role in replacing major part of fossil fuels to generate sustainable, inexhaustible, clean, and safe energy. In Hong Kong, solar energy has been identified suitable for wide-scale applications. Photovoltaic (PV) and solar water heating (SWH) facilities are the two promising solar-based conversion technologies. Electricity and hot water generations via solar energy means fossil fuel saved together with the likely pollutants and greenhouse gases reductions. However, there are a number of barriers including high initial cost and large installation space required. This paper studies the cost, energy, and environmental issues when PV and SWH systems are widely used in Hong Kong. The energy expenditures in the forms of electricity, gas, and hot water, and the global solar radiation in Hong Kong were analyzed. The total required land areas, the financial implications, and the environmental benefits for such solar energy applications were estimated and reported.

Practical application: Solar energy plays an important role in replacing fossil fuels to generate electricity without emitting pollutants and requiring no fuel. This paper analyzes the energy, financial, and environmental aspects for solar thermal and solar electric installations. The findings in this study provide the information when active solar energy systems are widely applied in Hong Kong.

Keywords

Photovoltaic solar water heating solar radiation life cycle cost greenhouse gases

Introduction

As the world's economy continues to grow, the demand for finite energy sources such as coal, oil, and natural gas also increases. Hong Kong has no indigenous fuels and most of the imported fossil fuels are mainly for energy generation. There are many immediate adverse effects on the environment including large amount of emissions of greenhouse gases and pollutants.¹ Greenhouse gases cause the formation of global warming and climate change, and pollutants bring out acid rain and air pollution. Such environmental hazards have become strong driving forces for the use of inexhaustible, safe, sustainable, and pollutant-free emission energy.^2,3 Renewable energy (RE) can play an important role in meeting the ultimate goal of replacing parts of the fossil fuels.⁴ There has been a considerable improvement in energy generation from renewable sources.⁵ Recently, solar energy in the forms of solar electric and solar thermal has been identified as having the potential for wide-scale utilization in Hong Kong.⁶ Locally, solar energy applications are not popular and mainly of small-scale systems.⁷ Open field or stand-alone solar energy applications have been used in rural and remote areas.⁸ In modern urban cities, most buildings are high-rise and the roof area would be very limited for stand-alone solar energy system installations. In this building envelope type, the concept of building integrated photovoltaic (BIPV) which consists of PV panels mounted on the opaque parts of the external walls for generating electricity would be an appropriate alternative form.⁹ The introduction of semi-transparent BIPV facades can generate electricity and allow daylight to penetrate into the buildings, enhancing the active solar energy applications.^10,11 This paper studies the potential energy performance, financial issues, and environmental implications when PV and SWH systems are widely used in Hong Kong. The energy demand for Hong Kong is reviewed. Energy, cost, and the environment aspects are examined and discussed. Although the work only relates to Hong Kong, most of the local building projects are high-rise located in densely-built zones which are typical in most modern cities. The findings can therefore be employed for places with similar climatic conditions and architectural and urban designs.

Energy use in Hong Kong

In Hong Kong, most energy is consumed by building stocks.¹² Figure 1 shows the annual electricity use of three main end-users, namely the commercial, domestic, and industrial sectors between 1970 and 2012 (43 years).¹³ With population growth and improvements in living standard, there has been significant rise in electricity expenditures in domestic buildings. Annual electricity consumption in housing stocks rose from 3340 TJ in 1970 to 41,190 TJ in 2012, accounting for an average increase rate of about 6% per year. During the past few decades, the local economy shifted from manufacturing-oriented to a more service-based such as financial services and tourism, and the industrial sector has been replaced by commercial sector which became the largest electricity end-user since 1980. The electricity consumed by commercial sector increased from 6020 TJ in 1970 to 102,050 TJ in 2012. This represents to an annual average increase rate of 6.8%. For industrial sector, there was a steady declining trend in the electricity consumption starting in 1990 due mainly to relocation of the manufacturing activities to Mainland China where the land and labor costs were cheaper than those in Hong Kong. The electricity use increased from 6590 TJ in 1970 to 25,180 TJ in 1989 and quite steady between 1989 and 1991. However, it dropped gradually from 25,050 TJ in 1991 to 11,140 TJ in 2009. Between 2009 and 2012, the electricity consumption was almost no change. Overall speaking, commercial and residential sectors were the two major electricity end-users,¹⁴ denoting around 66% and 27% of the total electricity consumed by building stocks in 2012, respectively. These established trends suggest that the electricity use will increase continuously.

Figure 1.

Annual electricity consumption by sectors (1970–2012).

Gas (town gas and liquefied petroleum gas) is another major form of energy consumed in buildings. In subtropical Hong Kong, gas is largely used for cooking and water heating. Likewise, the annual gas used by the three building sectors from 1970 to 2012 is displayed in Figure 2. The annual increase rates for individual building types are quite similar. Overall gas consumption rose from 958 TJ in 1970 to 28,360 TJ in 2012, standing for an average increase rate of 8.2% per year. Commercial and residential sectors expended most of gas consumption. In 2012, over 95% of gas was consumed by these two building types and less than 5% was used by industrial buildings. Hot water is required in large quantities in commercial sectors including hospitals, hostels, and hotels, and in domestic buildings for cooking, bathing, and washing clothes and utensils. Electricity and gas are the main forms of energy used to produce hot water and Figure 3 depicts the annual consumption. The total energy consumed for hot water increased from 12,230 TJ in 2000 to 15,740 TJ in 2010, an increase of 28.7% during these 11 years. In 2000, electricity use was only around 40% as much as gas consumption. However, electricity overtook the latter in 2008. In general, the energy required for hot water producing is not large, contributing less than 10% of the total building energy expenditure.

Figure 2.

Annual town gas consumption by sectors (1970–2012).

Figure 3.

Annual hot water consumption by electricity and gas (2000–2010).

Solar radiation in Hong Kong

The energy generated by the active solar facilities is strongly affected by the amount of solar radiation falling on the panels. Information of the availability of solar radiation would be invaluable to the determination of performance of such solar energy systems. Reliable databases should always be based on long-term data measurements. In Hong Kong, daily global solar radiation on a horizontal plane was first measured in June 1958 and the hourly global radiation measurements (also on a horizontal plane) were started in December 1978 by the Hong Kong Observatory (HKO). The 34-year global radiation data recorded between 1979 and 2012 were obtained from the HKO for the study and Figure 4 presents the annual results. The annual global radiation ranged from 4310 MJ/m² in 1997 to 5310 MJ/m² in 2011 with a mean value of 4750 MJ/m². Variability defined here is the ratio of the range to the average value. The annual variability from year to another is not very large and the mean variability during the 34-year period is 21% of the yearly average. Typical weather year data representing the long-term climatic conditions are often used for analysis of solar energy applications.¹⁵ Weather data based on not less than 30-year period are conservatively stable for such studies. Previously, the year 2000 was identified as the Test Reference Year using weather data recorded from 1979 to 2008.¹⁶ Figure 5 shows the mean daily global radiation for different months recorded by the HKO in 2000. The global values varied from 9.3 MJ/m²/day in February to 17.7 MJ/m²/day in July. The high solar altitude and long day-length are the main reasons of the peak and high global radiation occurring in summer. The smaller value observed between November and April was due mainly to the low solar altitude and short day-length in winter as well as the unstable weather days in spring. Generally, the global radiation in Hong Kong is quite appropriate for active solar energy applications and larger than a number of places.¹⁷

Figure 4.

Annual global solar radiation on a horizontal surface (1979–2012).

Figure 5.

Monthly average daily global solar radiation on a horizontal surface in 2000.

Solar energy applications

Solar-based energy conversion systems can be classified into two categories, namely solar thermal and solar electric. SWH is one of the main typical examples of solar thermal system. Normally, over 50% of the solar radiation falling on a solar panel is converted into thermal energy.¹⁸ It means that solar thermal enjoys quite high efficiency and consequently a reasonable payback time for places with year-round hot water demands. It is particularly appropriate for low-rise building blocks such as guesthouses and bungalows located in low-density areas where the solar collectors can be installed on rooftops with no to slightly obstructed external environments. Solar electric makes use of PV effect to produce electricity energy from solar radiation without using any thermodynamic cycles and mechanical generators. There are three common types called mono-crystalline, multi-crystalline, and amorphous silicon cells. Pure mono-crystalline silicon is used to manufacture mono-crystalline silicon cells which have high conversion efficacy of a typical value around 15%. Since the manufacturing process is quite sophisticated, the capital cost is the highest among the three cell types. The multi-crystalline silicon cells are produced from molten silicon. Generally, the efficiency is slightly less than the mono-crystalline silicon cells with a usual value of around 12%. The amorphous silicon is created by deposition onto a substrate and the cells can be thinner than that from wafer sawing with a typical thickness of 0.4 mm. Due to the thin layer characteristic, the amorphous silicon cells are also called “thin film”. The conversion efficiency is relatively low and is typically below 10%.

The outcomes in terms of cost, energy, and the environment for wide-scale solar energy applications can be illustrated using case studies. Hong Kong is a quite typical case in most densely populated modern cities. Building integrated solar energy systems would be the appropriate approach for wide-scale solar energy applications. The data, methods, and findings in this study could be applicable in other urban contexts with minor modifications. Hong Kong has a total area of about 1100 km². To widely implement the active solar energy applications, a number of large-scale SWH and PV systems should be installed. The amount of output power generated by solar energy facilities is directly proportional to the collector areas and the required land spaces are very important. Table 1 summarizes the approximate area of the land utilization in Hong Kong in 2011.¹⁹ The urban and built-up land including residential, commercial and industrial areas, institution/open space, and transportation contribute 265 km² corresponding to a quarter of the total Hong Kong area. The remaining land areas of 843 km² are mainly woodlands, shrublands, and grasslands. As it is the land development policy in Hong Kong, the original land utilizations should be persevered. It is proposed to use only the urban and built-up lands which are largely residential, commercial, and industrial areas for installing PV systems. The natural country parks regions such as woodlands, shrublands, and grasslands are not considered. In subtropical Hong Kong, the water heating demand is not very large compared with electricity consumption. Thus, SWH can be designed to fulfill the whole Hong Kong demands. The flat plate solar collectors were assumed to be used for hot water production and the average energy output of 2100 MJ/m² based on local weather conditions was adopted.²⁰ Referring to the annual energy consumed for hot water of 15740 TJ in 2010, the required solar panel area was computed to be 7.5 km². It is planned that such solar panels can be evenly installed in the urban and built lands (i.e. residential, commercial, and industrial lands). For solar electric, it is impossible and inflexible using solar energy only to meet all the electricity expenditures at present. There are many other forms of RE such as wind turbines and energy-efficient buildings designs including daylight-linked lighting controls, natural ventilation, and use of high energy efficiency electrical appliances.^21–23 It is considered that 30% of urban and built-up lands were used for installing PV systems.⁶ The urban and built-up lands are mainly residential, commercial and industrial areas containing most of the building developments. The total required solar panel areas were computed to be 80 km². BIPV systems that make use of vertical façades and roof would be the appropriate methods to generate the output electricity. As such, 30% of urban and built-up lands could be used for PV panel installation. There would be a number of uncertainties and a sensitivity study to address these issues will be undertaken as further investigation. The data for analysis are mainly averaged based on long-term periods which should be more representative to evaluate the long-term performance. The results can be used to compare with those obtained from similar procedures and models. The input global solar radiation measured in 2000 and the mean conversion efficiency of 12%²⁴ were employed to determine the output electricity. Accordingly, the annual output energy was estimated to be 46,270 TJ which corresponds to about 30% of the total electricity consumptions by the three typical building sectors.

Table 1.

Land utilization in Hong Kong.

Class	Approximate area (km²)
Residential	76
Commercial	4
Industrial	26
Institution/open space	50
Transportation	56
Other urban or built-up land	53
Agriculture	68
Woodland/shrubland/grassland/wetland	738
Barren land	7
Water bodies	30
Total	1108

Environment issue

The global environmental hazard posed by emissions from burning fossil fuels has also become an additional driving force for solar energy schemes. Hong Kong is currently infamous for its poor air quality and polluted conditions. Our previous work revealed that the CIE Standard Sky 13 (cloudless polluted with a broader solar corona) was categorized as the main clear sky type in Hong Kong.^25–27 In Hong Kong, electricity is mainly generated based on fossil fuels by two power companies. Greenhouse gases and pollutants produced from electricity generation are considered as one of the main causes of the local air pollution. Electricity is high-grade energy which is less efficient than other commonly used fuels. Generally, three units of primary energy inputs are required to generate one unit of electricity output, with two units being wasted as heat. It implies that one unit of electricity saved means about three units of non-renewable fossil fuels conserved together with the likely pollutants and the greenhouse gas reductions. Table 2 summaries the emissions of greenhouse gas and major emitters namely SO₂, NO_x, and Respirable Suspended Particulates (RSP) by electricity generation including town gas production in Hong Kong for the 14-year period from 1997 to 2010.²⁸ The greenhouse gas emission varied from 20,000 to 29,600 kton. The increase rate is not significant which is in line with the electricity consumption (Figure 1). For the pollutants, the emissions were far less than greenhouse gas, ranging from 87 to 18 kton for SO₂ and varying between 56 and 27 kton for NO_x. For RSP, the amount of emission was just between 2.4 and 1 kton. Since 2005, the power plants statutory emission caps have been imposed and the power companies increased the use of natural gas and low emission coal in electricity generation and prioritized the use of coal-fired generation units equipped with emission control devices including flue gas desulphurization units and NO_x abatement devices. As a result, the emissions were substantially reduced between 2005 and 2011. If the proposed wide-scale SWH and PV installations were adopted, the annual average emissions of greenhouse gas, SO₂, NO_x, and RSP could be further reduced by 10,960, 7.2, 10.8, and 0.4 kton, respectively based on emissions in 2010. Overall speaking, the environmental benefits would be very significant.

Table 2.

Greenhouse gas and air pollutants emissions (kilotonnes) in Hong Kong.

Year	CO₂	SO₂	NO_x	RSP
1997	20,000	55	56	2.3
1998	22,100	61	56	2.4
1999	20,100	50	40	1.8
2000	21,200	57	42	2.3
2001	21,600	59	36	1.5
2002	23,400	59	37	1.6
2003	26,500	84	48	2
2004	26,400	87	43	2.4
2005	28,600	76	44	2.2
2006	28,500	67	40	1.8
2007	29,600	58	42	1.6
2008	28,000	50	39	1.5
2009	29,100	46	38	1.5
2010	27,400	18	27	1

Cost analysis

The monetary payable period (MPBP), the life cycle cost (LCC), the internal rate of return (IRR), and the net present value (NPV) are important indicators for financial justification. The MPBP of the proposed solar thermal and solar electric installations can be evaluated by comparing the saved tariffs (electricity and gas) to the capital cost of the applications. The capital costs of the SWH system include material and installation cost of centralized pipework and auxiliary heaters. By making reference to the recent SWH projects in Hong Kong,²⁹ the capital cost of a SWH system was assumed to be HK$ 3130/m² (US$ 400/m²). Electricity and town gas are commonly used to operate water heaters. In Hong Kong, electricity is mainly generated by two local power companies, namely the China Light and Power (CLP) and Hong Kong Electric (HKE). Currently, the electric tariffs are HK$ 0.941/kWh (US$ 0.12/kWh) for CLP and HK$ 1.231 kWh (US$ 0.16) for HKE. For town gas, the tariff is HK$ 0.823/kWh (US$ 0.106/kWh). Referring to these data and the annual energy output per collector area of 2100 MJ/m², the simple MPBPs were computed to be 4.4, 5.7, and 6.5 years based on the tariffs for HKE, CLP, and town gas, respectively. Recently, trading of CO₂ has been executed to promote green environment. Using the records between 2005 and 2009,³⁰ the rate of CO₂ emission per unit energy (i.e. electricity and town gas) generated was 0.642 kg/kWh. According to the market rate for the carbon credit of US$ 0.01465/kg in September 2011,³¹ the MPBP could be further reduced by 0.3 to 0.5 years. For PV systems, the capital costs include the solar modules, inverters, accessories, and the installations. The market price for a medium size BIPV system of HK$ 8450/m² (US$ 1083/m²) was used.³¹ The efficiency of solar electric is far less than that of solar thermal. Accordingly, the MPBPs were estimated to be 55.5 years and 42.5 years based on CLP and HKE tariffs, respectively. When carbon credit was considered, the payback year could be subsequently lowered to 51.5 years for CLP tariff and 40 years for HKE tariff.

To have a better understanding of the financial implications of the wide-scale solar thermal and solar electric applications, LCC analysis was also conducted for these two systems. Based on the parameters such as initial cost (IC), operating and maintenance cost (OMC), dismantling cost (DC), and salvage value (SV), the LCC approach converts and considers all money terms and benefits to present values by setting a discounting factor over a certain time period. The concept of the NPV of the energy cost over a life cycle (in terms of years) is provided as follow

NPV = \sum_{n = 1}^{m} (EC)_{n} = (EC)_{1} \sum_{n = 1}^{m} (\frac{1 + i}{1 + d}) n - 1

(1)

where NPV is the net present value of energy cost over a life cycle period in years (HK$), (EC)₁ is the energy cost during the first year (HK$), i is the rate of fuel cost increase, d is the market discount rate, n, m is the number of years, and

(\frac{1 + i}{1 + d}) n - 1

is the discounting factor for the nth year.

In accordance with yield information of the exchange fund notes (representing the market discount rate) issued by the local monetary authority for the past 10 years between 2003 and 2012, the average discount rate of 1.86% was adopted in the energy cost calculation.³² The OMC, DC, and SV for solar thermal applications were respectively considered 1%, 10%, and 20% of the IC³. For solar electric installation, due to the relative high IC, the OMC, DC, and SV were assumed to be 0.17%, 4%, and 2% of the IC.³³ An average lifetime of 25 years was considered for solar collectors. Accordingly, the LCCs were determined and Tables 3 and 4 summarize the results for solar thermal and solar electric systems, respectively. The present value factor and OMC for each year are displayed in the two tables. With a 25-year life cycle, the resulting total LCC (i.e. capital plus running costs) for solar thermal was HK$ 3555/m² (US$ 456/m²) and for solar electric was HK$ 8781/m² (US$ 456/m²). It should be pointed out that the significant cost and environmental impacts associated with PV manufacture, deployment, and maintenance of power quality are important and huge topics which should be examined separately. They are not the scopes for the present study.

Table 3.

Life cycle cost analysis for SWH system.

Year	Present value factor (fraction)	IC (HK$/m²)	O&M (HK$/m²)	D (HK$/m²)	S (HK$/m²)
0	1	3131		197.5	−388.6
1	0.9817		30.43
2	0.9638		29.87
3	0.9462		29.33
4	0.9289		28.79
5	0.9119		28.27
6	0.8953		27.75
7	0.8789		27.24
8	0.8629		26.75
9	0.8471		26.26
10	0.8316		25.78
11	0.8165		25.31
12	0.8015		24.84
13	0.7869		24.39
14	0.7725		23.95
15	0.7584		23.51
16	0.7446		23.08
17	0.7310		22.66
18	0.7176		22.24
19	0.7045		21.84
20	0.6917		21.44
21	0.6790		21.05
22	0.6666		20.66
23	0.6545		20.28
24	0.6425		19.91
25	0.6308		19.55

Total Sum		3131 (US$ 401/m²)	615.3 (US$ 78.9/m²)	197.5 (US$ 25.3/m²)	−388.6 (US$ 49.8/m²)

Life cycle cost(HK$/m²)					3555.2 (US$ 455.8/m²)

SWH: solar water heating.

Table 4.

Life cycle cost for solar electric system.

Year	Present value factor (fraction)	IC (HK$/m²)	O&M (HK$/m²)	D (HK$/m²)	S (HK$/m²)
0	1	8445.9		183.5	−112.
1	0.9817		13.1
2	0.9638		12.86
3	0.9462		12.63
4	0.9289		12.4
5	0.9119		12.17
6	0.8953		11.95
7	0.8789		11.73
8	0.8629		11.52
9	0.8471		11.31
10	0.8316		11.1
11	0.8165		10.9
12	0.8015		10.7
13	0.7869		10.5
14	0.7725		10.31
15	0.7584		10.12
16	0.7446		9.94
17	0.7310		9.76
18	0.7176		9.58
19	0.7045		9.4
20	0.6917		9.23
21	0.6790		9.06
22	0.6666		8.9
23	0.6545		8.73
24	0.6425		8.58
25	0.6308		8.42

Total Sum		8445.9 (US$ 1083/m²)	265 (US$ 34/m²)	183.5 (US$ 23.5/m²)	−112.6 (US$ −14.4/m²)

Life cycle cost (HKD$/m²)					8781.9 (US$ 1126/m²)

SWH: solar water heating.

The IRR is the discount rate that makes the cash inflows in a project equal to the investment costs both in present values (i.e. the sum of capital and saving costs during life-cycle period is zero). The higher IRR means more profit for the shareholder and it is also an important factor being used to determine the ranking on investment proposals. Based on the energy prices¹³ over a 5-year period from 2007 to 2011, the inflation rate of the fuel cost for CLP, HKE, and town gas were computed to be 1.94%, 2.26%, and 0.47%, respectively. Accordingly, the NPV and IRR for various cases were estimated and Table 5 shows the findings. The NPV ranges from HK$ 6957 (US$ 892/m²) to HK$ 15767 (US$ 2021/m²) and IRR varies between 15.4% and 25.6% for SWH. With such good IRRs, SWH appears to be a lucrative investment with limited business risks. It is the main reason why SHW is quite popular in many countries. For solar electric, both NPV and IRR are negative. The results show that solar electric is not a lucrative investment and business risks would be very high. It should be pointed out that the benefits due to the reduction of the pollutants emission were not considered for the cost analysis. It means that the government should subsidize and implement more effective incentive schemes with exhaustive justification and include the pollutant reduction aspects for financial evaluation.

Table 5.

NPV and IRR for the SWH and PV systems.

	SWH			PV
	CLP	HKE	Town gas	CLP	HKE
NPV (HK$/m²)	10733 (US$ 1376/m²)	15767 (US$ 2021/m²)	6957 (US$ 892/m²)	−4605 (US$ −590/m²)	−3210 (US$ −412/m²)
IRR (%)	19.5	25.6	15.4	−3.6	−1.5

NPV: net present value; IRR: internal rate of return; SWH: solar water heating; CLP: China Light and Power; HKE: Hong Kong Electric; PV: photovoltaic.

Conclusions

The energy use in the forms of electricity, gas, and hot water for Hong Kong in previous years was studied. Commercial and residential sectors are the two major energy-users and the established trends show that the electricity use will increase continuously. The solar radiation data measured by the HKO from 1979 to 2012 were analyzed. The mean annual global radiation was found to be 4710 MJ/m² which is quite good for active solar applications. Wide-scale applications of PV and solar hot water systems in Hong Kong were considered to illustrate their cost, energy, and environmental aspects. To meet the annual energy demand for hot water of 15740 TJ, the required solar panel area was computed to be 7.5 km². For solar electric applications, 30% of the urban and built-up areas were proposed for installing PV systems. The annual output energy was estimated to be 46,270 TJ which corresponds to about 30% of the total building electricity consumptions in Hong Kong. In the environmental point of view, the annual emissions of greenhouse gas, SO2, NOx, and RSP could be reduced, respectively, by 10,960, 7.2, 10.8, and 0.4 kton if the proposed wide-scale SWH and PV installations were adopted. In view of financial aspect, the LCC, MPBP, IRR, and NPV for solar thermal applications are of reasonable figures. The latter three indicators also echo to recent findings about the financial feasibility of applying solar water panels for hotel in nearby city-Shenzhen.³⁴ For solar electric installations, owing to the large IC and low efficiency, the MPBP is far longer than the lifetime and both NPV and IRR are negative. However, these adverse outcomes should not abort the wide-scale PV applications. Instead, the government should subsidize and implement more effective incentive schemes thorough justification and include the benefits due to the reduction of the pollutants emission for cost assessments. Further research studies on such important issues including sensitivity studies on the parameters for large-scale solar energy applications are required.

Footnotes

Funding

The work described in this paper was fully supported by a General Research Fund from the Grant Council of the Hong Kong Special Administrative Region, China [Project no. 9041777 (CityU 116312)].

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