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APPLICATION POTENTIAL OF SOLAR THERMAL TECHNOLOGIES FOR BUILDINGS IN HONG KONG SQUARE K. F. FONG, T. T. CHOW, JOHN Z. LIN, APPLE L.S. CHAN Building Energy and Environmental Technology Research Unit Division of Building Science and Technology, City University of Hong Kong Hong Kong, China

Among a variety of renewable energy, solar energy is the most feasible option for wide applications in Hong Kong. In the context of sustainable building design, there are growing interests in the photovoltaic installation and advancement in the recent years. The photovoltaic/thermal solar collector is also proven effective in building use. Solar heating exists in Hong Kong, but mainly limited to sparse domestic use and government projects. In the light of the building integrated approach, it is also techno-economically viable for wide application of solar water heating for the high-rise residential buildings in Hong Kong. In addition, the application potential of solar cooling such as the desiccant cooling, absorption or adsorption refrigeration is feasible based on the local sub-tropical conditions. Simulation-optimization is a useful mean in determining the design parameters of these solar thermal systems, so that optimal thermal gain and energy saving can be achieved for sustainable building design.

1.

Application potential of solar energy in Hong Kong

[4-6]. The capital costs of solar thermal and PV installations have substantially decreased in the 25 years from 1980 to 2005, dropped around 85% [7] as shown in Figure 1.

Hong Kong is located at 22.32°N and 114.17°E. Hong Kong has a sub-tropical climate, featured with temperate climatic conditions for half a year. With the low latitude effect, the daily sunshine period is plenty and relatively stable. Owing to the environmental impact of burning fossil fuel and non-stopping climbing of oil price, alternative energy sources other than fossil fuel generated electricity are being explored. Solar energy is definitely welcome to reduce the primary energy consumption for any related technologies in HVAC and building services systems. The Hong Kong government has actively considered the potential applications of different sources of renewable energy, particularly to make use of solar energy [1, 2]. An international scheme, called Solar Heating and Cooling Program has been advocated by the International Energy Agency since 1976, and a variety of publications for design, installations, operation and maintenance of the solar energy systems to heat, cool, power and light buildings are currently available to the building practitioners [3]. The contributions of solar energy for sustainable building design encompasses solar photovoltaic (PV), solar heating, solar lighting and solar cooling. For large scale or district application, concentrating solar power technologies would also be considered. Solar cooling is still a blooming technology in Europe, USA and Japan, where research and development works are still ongoing

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Figure 1. 1980-2025 records and projection of capital cost of solar energy installations.

From the Hong Kong Typical Meteorological Year [8], Hong Kong receives an annual global solar irradiation of 4.70 GJ/m2. The yearly profile of solar irradiation is shown in Figure 2, with the range of total irradiation from 9.14 MJ/(m2⋅day) to 17.73 MJ/(m2⋅day) (in January and July respectively), and the year-round average is 12.88 MJ/(m2⋅day). The direct solar 2 irradiation contributes 2.71 GJ/m annually, which accounts for 42.4% of the total solar radiation. The percentage profile of direct solar radiation in different months is shown in Figure 3. 1

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Figure 2. Monthly solar irradiation profile in Hong Kong.

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Figure 3. Percentage profile of direct solar irradiation in Hong Kong.

Since PV technology can directly produce the “clean” and convenient form of energy – electricity, effort of research and development has been driven in its equipment advancement. The first generation PV is now featured with lower silicon feedstock prices and thinner silicon wafer technology, and different kinds of improved versions in the areas of processing, integration and processing are being emerged. At this moment, the efficiency of the first generation PV is commonly in the range of 10 to 15%. The second and third generation PVs are being explored, making use of the technologies of thin film concentrators and quantum dots respectively, however there is still a decade to go. Technology advancement has been made in different kinds of solar thermal collectors, like the flat plate collectors, evacuated tubes, solar concentrators and solar air collectors. The direction is to enhance absorbance and reduce re-emittance, by improving selective coatings of absorbers using sputter technology, enhancing heat transfer from the absorber to tubes through ultrasonic or plasma welding, minimizing reflection loss through anti-reflective layers, optimizing geometry of vacuum collectors, and developing lowlevel concentrations of irradiation. The efficiency of

solar thermal system can be in the range of 40% to 50%, which is comparatively effective and attractive. This would make the application potential of solar thermal energy in both solar heating and solar cooling systems more economically viable. In Hong Kong, focus has been placed on the photovoltaic and solar water heating applications for buildings. For the government projects up to 2006, the installed capacity of photovoltaic panels is 776 kW, while the installed collector area for solar water heating is more than 2,200 m2. There are also a number of nongovernment projects applying PV and solar water heating, mainly in the educational institutions, hotels and dormitories. Although the SAR government has initiated a number of projects applying solar energy for more than 20 years, it does not have any tangible incentive scheme or policy to support wider application of solar energy at this moment. However the two local power companies have established the renewable energy funds to raise awareness of solar energy and other renewable energy, and to promote better understanding and practical applications in Hong Kong. Due to the limited land use and high population density in Hong Kong, there are high-rise buildings everywhere. The application of solar energy is rare, not just because of the relatively high initial cost, but also due to the very limited open space for accommodating the required number of solar collectors within the premises of the building development. The available space of building development is usually on the roof or podium, and the available area for solar installation is quite limited. The building-integrated approach would be a feasible solution for a high-rise building, by installing the solar collectors onto vertical façades or external shading devices.

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Solar heating

In the scope of solar heating in sustainable building design, it broadly covers solar water heating and solar space heating. The choice depends of the purpose, heat demand and medium of usage. 2.1. Solar water heating In Hong Kong, the demand of water heating is mainly for daily bathing and cooking purposes in the residential buildings, hotels and recreational facilities. In Greece, less life-cycle environmental impact of a domestic solar

3 water heating system is found as compared to the conventional electrical or gas water heating [9]. In Cyprus, solar water heating is widely installed for domestic use, and the payback period is about five years even with electricity or diesel-oil backup [10]. In order to provide the required load of hot water, electric or gas heating is commonly used in Hong Kong. With the thorough consideration of the local features and constraints, building integrated design of a centralized solar water heating system for high-rise apartment building has been proposed in our previous studies, and a solar fraction of 53% and payback period of 9 years were determined [11]. It is also found that a thermosyphon system without the use of circulation pump is viable for those locations with high solar irradiation throughout the year like Hong Kong.

been recommended, in order to reduce energy consumption without sacrificing thermal comfort. Electric-driven compression refrigeration systems have been applied for a century, commonly found at homes, work places, industrial facilities and transportation. Solar energy is definitely welcome if feasible technology for air-conditioning and refrigeration is available. From the latest research works [14-16], solar cooling can be categorized in the way shown in Figure 4.

2.2. Solar space heating Owing to the temperate climatic nature in Hong Kong, space heating is not essential for most of the commercial buildings. The demand of space heating may be necessary for the perimeter zone of major function areas. Solar space heating can be designed with either indirect water heating or direct air heating. By using flat plate collectors or evacuated tubes, the thermal gain would be stored in water tank, the hot water would be supplied to the heating coil for space heating purpose. On the other hand, by using solar air collectors, air can be directly heated and supplied to the space concerned. In general, the solar air collectors have lower efficiency than the water-based collectors because of lower heat transmission rate from the absorber to the medium. An example of solar air heating is demonstrated in a corporation headquarters in Tokyo, the system is designed for a single storey staff canteen to provide air heating during the cold season [12].

3.

Solar cooling

As the climate of Hong Kong is sub-tropical, with long hot and humid summer, the provision of airconditioning for comfortable indoor environment is therefore indispensable. Owing to the fact that airconditioning and refrigeration becomes the biggest electricity “consumer” in Hong Kong [13], different measures of energy conservation and management have

Figure 4. Categorization of solar cooling.

Passive solar cooling is to apply the principles of protection from sun, thermal mass control and natural ventilation to minimize the unwanted solar thermal gain into the indoor spaces. It is common to design and install the external/internal shading devices, low emittance glazing, double façade, and high thermal inertia in order to achieve this purpose. Active solar cooling, on the other hand, is to make use of the energy acquired from the solar collectors for the electric- or heat-driven system, so that chilled water or conditioned air can be produced for air-conditioning purpose. The focus of solar-electric refrigeration and solar-thermal refrigeration is to develop new types of chillers for refrigeration purpose. In solar-thermal airconditioning, conditioned air with suitable supply temperature and humidity is directly provided to the indoor space. So the solar-electric or solar-thermal refrigeration is classified to be closed-cycle system, while solar-thermal air-conditioning open-cycle system. The prominent advantage to utilize solar energy for

4 building air-conditioning is the coincidence of solar irradiation availability and building cooling demand.

compression refrigeration system would be more economical for large refrigeration system over 1000 TR.

3.1. Solar-electric refrigeration One of the developed solar-electric refrigeration is to adopt PV-operated compression cycle. The compressor is coupled with and driven by a direct current (dc) motor, which is powered by PV array as shown in Figure 5. The key consideration in designing the PVoperated compression cycle is to match the electrical characteristics of dc motor with current-voltage performance of the PV array. In general, the rated condition of the PV performance (commonly at 1000 W/m2) would be provided by manufacturers. However the changing solar irradiation would result different current-voltage characteristics, as well as the power output. In order to maximize the efficiency of the solarelectric refrigeration system, it is essential to determine the optimal voltage and apply it to the PV array in order to achieve the maximum power output. Abbreviation CD: condenser CM: compressor DC: direct current motor ED: expansion valve EV: evaporator PV: photovoltaic array

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Figure 6. Schematic diagram of solar mechanical compression refrigeration system.

3.2.2. Solar absorption refrigeration Typical absorption refrigeration cycle is driven by heat at generator to give out the refrigerant to the condenser. The common working pair of refrigerant/absorbent is water/lithium bromide or ammonia/water. The refrigerant from condenser is stepped down at the expansion device, and produces refrigeration effect at the evaporator. The refrigerant would then be absorbed in the absorber, compressed and delivered to the generator, and the cycle continues. Although a number of electrical pumps would be involved for compression and circulation purposes, the electrical energy is minimal as compared to the heat input. As a result, the solar thermal gain can be used for the absorption refrigeration as shown in Figure 7.

Chilled water supply and return for air side equipment

Figure 5. Schematic diagram of solar-electric refrigeration system.

3.2. Solar-thermal refrigeration 3.2.1. Solar mechanical compression refrigeration Solar mechanical compression refrigeration is driven by heat engine powered by solar energy, then the turbine or piston expander produces mechanical power to drive a compressor of a conventional vapour compression refrigeration system, as shown in Figure 6. The required driving temperature for the heat engine commonly ranges 100 – 200°C, so concentrating solar collectors should be used, even installed with sun tracking system. Due to the competing efficiency and large area for installation, the solar mechanical

Figure 7. Schematic diagram of a single-stage solar absorption refrigeration system.

Sumathy et al. have demonstrated a solar absorption installation in Shenzhen [17]. It was an

5 integrated solar cooling system using two-stage adsorption chiller run by driving temperature of 60 – 75°C. Li and Sumathy [18, 19] found that suitable design of the storage tank incorporated with internal partitioning, thermal stratification and water inlet would facilitate the performance of solar adsorption system. The absorption refrigeration technology is being continually developed, by using another choice of refrigerant/absorbent pair of ammonia/water, it can be adapted by low driving temperature at the generator. Such relatively low temperature can allow the solar thermal system more technically feasible. In the recent development, the evaporator temperature can be 5°C and COP higher than 0.6 [20].

3.2.3. Solar adsorption refrigeration Adsorption is a chemical endothermic process that gas is adsorbed by solid surface. A reverse process is called desorption that heat is input and gas is desorbed from the surface. The principle of adsorption can be applied to vapour refrigerant after producing refrigeration effect for the chilled water. The choices of working pairs of refrigerant/sorbent include water/silica gel, water/zeolite, ammonia/activated carbon and methanol/activated carbon. Usually the sorbent is a highly porous solid and the water/silica gel pair is the most common. In order to have smooth and continual operation of adsorption chiller, there are two identical compartments of sorbents alternatively heated and cooled in order to let the vapour refrigerant desorb and absorb respectively. Therefore the process is cyclic, and one compartment is charged (adsorption) while the other is regenerated (desorption). The schematic diagram of a typical solar adsorption refrigeration system is shown in Figure 8. Since the driving temperature is usually in the range of 55 – 90°C, so the economical solar collectors such as flat plate collectors can readily produce the refrigeration effect. The electricity input is minimal, only for the vacuum and unloading pumps operated occasionally.

Figure 8. Schematic diagram of solar adsorption refrigeration system.

A hybrid air-conditioning system has been installed in the green building demonstration project in Shanghai, in which solar adsorption chillers was designed [21]. The adsorption technology is even applied in refrigeration for ice making purpose [22]. Yong and Sumathy [23] have found that the adsorbent mass and lumped capacitance have significant effects on the performance of adsorption system, and the overall heat transfer coefficient would be less important if an optimal period of adsorption cycle is set.

3.3. Solar-thermal air-conditioning The current solar-thermal air-conditioning is typically a solar desiccant cooling system. The core part of this system is the sorbent component. Both solid and liquid sorbents are available, like silica gel and lithium chloride respectively. Although the liquid desiccant cooling has feature of thermal storage in the regenerated liquid sorbent, the choice of the hygroscopic sorbent is limited, since the sorbent would be carried over into indoor space by the conditioned air. For the solid desiccant cooling, the processes of adsorption and desorption of water vapour by solid sorbent are stable, so it is more suitable to directly apply upon the conditioned air. Desiccant wheel is commonly used as the solid sorbent component. Solar desiccant cooling system also includes the heat recovery unit, direct evaporative coolers, solar collectors and auxiliary heater, as shown in Figure 9. Solar air collector is a common choice for this system if the availability of solar irradiation in day time is in line with the space cooling load profile. The heat recovery unit is used to conserve the sensible heat and pre-cool the outdoor air after the desiccant wheel. The evaporative cooler EC1 would cool down the supply air up to the humidity ratio capable of tackling the space latent load. Another evaporative cooler EC2 is installed to cool down the

6 return air to furnish better sensible heat extraction at the heat recovery unit.

Figure 9. Schematic diagram of solar desiccant cooling system using solar air collectors.

There is continual technology development of desiccant rotary wheel and advancement of desiccant material. Experimental results show the corrugated paper with silica gel and calcium chloride can attain equilibrium within a very short period, and the hygroscopic capacity is also higher [24]. For detailed modeling of desiccant wheel, Nia et al. [25] have developed its numerical model that had direct correlation of outlet temperature or outlet humidity ratio with a number of physical and operating parameters. This model would be useful and efficient when yearround hourly energy simulation is conducted. A preliminary study has been made about the application potential of solar desiccant cooling system using solar air collectors in Hong Kong [26]. It was found that the outdoor air scheme was more suitable than the mixed air scheme, and the system design by using total return air for regeneration was more effective. Auxiliary heating was needed to acquire the design performance of desiccant cooling. The application potential of solar desiccant cooling system by using solar air collectors in Hong Kong was generally assured. Other choice of solar collectors like flat plat collectors or evacuated tubes would provide even better system performance.

3.4. New technologies in solar cooling There are continually emerging technologies in applying solar thermal energy to provide cooling and refrigeration. Pilot projects have been launched to design a refrigeration machine by using steam jet cycle,

which can satisfy the needs of high cooling power and continuous operation [27]. The advantage of steam jet cycle is its COP potentially exceeding the value of 1.0, which is higher than the other solar chillers like the absorption or adsorption types. Only water and no other fluid is used in the steam jet cycle, this makes the system simple. In addition, no moving parts are involved, that facilitates the reliability, maintenance and durability. However the driving temperature of the current steam jet cycle is 200°C, therefore it should be powered with the concentrating collectors installed with solar tracking system. As a result, economic viability of steam jet cycle chiller is still a hurdle ahead.

4.

Simulation-optimization of solar thermal system design

In order to achieve the wider application of solar energy in Hong Kong, it is important to pursue an appropriate engineering design of the solar thermal system, so that the energy saving and cost effectiveness can offset the relatively high initial cost. For the typical installation of solar collectors in the region of the northern hemisphere, there is a norm for the orientation to be due south and the tilt angle same as the local latitude. Therefore in Hong Kong, the solar collectors would be commonly designed to have surface azimuth to the south and tilt angle around 22°. However there is more understanding about the best orientation of the solar collectors to acquire the most beam and diffuse solar radiation in Hong Kong, and a previous study has shown that it is within the southwest quadrant [28], instead of exactly due south. In addition, the other supporting equipment of the solar thermal system, such as the storage capacity of the hot water calorifier and the duty of circulation pump set, should be designed to suit the year-round characteristics of the incident solar irradiation in Hong Kong. Through simulation-optimization approach, an indepth study of the major design parameters was carried out in order to achieve an optimal performance of a centralized solar water heating system for high-rise apartment building in Hong Kong [29]. The design of the centralized solar water heating system is shown in Figure 10, the array of 840 m2 flat plate collectors was installed to one single orientation of the building. The hypothetical apartment building had 28 stories. The thermal gain from the solar collectors was stored in a 36 m3 hot water calorifier with thermal stratification, in which the domestic hot water was directly drawn out.

7 An in-line auxiliary heater would be operated whenever the domestic hot water supply temperature was below 60°C, in order to comply with the local code for the prevention of Legionnaires’ disease [30]. The basic circulation pump flow rate was 47000 kg/hr according to the local design practice.

Figure 11. Modeling development of centralized solar water heating system in TRNSYS. Figure 10. Schematic diagram of solar water heating system for highrise apartment building.

System simulation is usually developed by using a system of mathematical expressions, in which the inputs and outputs of all the involved system components are linked up. If the component is modeled by the equation-fitted approach, the computation is straightforward. However the detailed models are usually established with cyclic form, or developed according to the solutions of a set of simultaneous differential equations, the computation would be complex. In this regard, suitable component-based simulation tool is required. In the simulation of centralized solar water heating system, it was developed by the simulation platform TRNSYS [31], with the support of the component models from TESS [32], as shown in Figure 11. In this simulation model, the Powel algorithm in TRNSYS was used to solve a set of non-linear equations for the simulation components, such as the thermal solar collector and hot water calorifier. In this simulation, it was based on the solar irradiation and weather data of the Hong Kong Typical Meteorological Year [8, 33]. The Reindl model [34] was used to resolve the beam and diffuse irradiation for different surface azimuths and tilt angles.

Four design parameters – tilt angle and surface azimuth of solar collectors, storage capacity of calorifier, and circulation pump flow rate – are involved in this optimization problem of the centralized solar water heating system. Due to the nonlinear and multidimensional nature of the problem, typical analytical or numerical optimization methods are difficult to solve it. Evolutionary algorithm was therefore adopted, and it is proven to be effective and efficient to search the optimal or near-optimal solution in a rugged and complex problem landscape [35-37]. By comparing with the conventional electric water heating installation for domestic use, the year-round energy saving can be maximized with the following optimal design values: • Optimal tilt angle: 22.3 degree • Optimal surface azimuth: southwest 7.7 degree • Optimal storage capacity: 40.4 m3 • Optimal flow rate of circulation pump: 22501.7 kg/hr From these optimization results, it is found that the tilt angle of solar collectors is same as the latitude of Hong Kong, while the surface azimuth is not exactly due south, but falling into the southwest quadrant as observed before [28]. A slightly increase of storage capacity of calorifier can accommodate the decrease of heat transfer rate due to the drop of pump flow rate, which in turn would save electrical energy consumption. On the other hand, the optimization performances of the three major paradigms – genetic algorithm (GA),

8 evolutionary programming (EP) and evolution strategy (ES) of evolutionary algorithm were compared [38]. The year-round energy saving of the centralized solar water heating was determined by each paradigm. ES and EP had better results than GA because they had mutation that involved a robust deterministic strategy parameter to provide the necessary search step length throughout the multidimensional and rugged landscape [29]. This shows that it is not suitable to blindly use GA for evolutionary optimization due to its popularity. Other paradigm of evolutionary algorithm like evolution strategy would be more reliable in searching the optimal solution of engineering optimization problems.

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Conclusion

While attention is made in the technological breakthrough of photovoltaic cells and panels, the potentials of solar thermal technologies have not been fully realized and utilized. Centralized solar water heating can be applied to the high-rise residential development, and the energy consumption for space cooling can be reduced if these solar collectors are installed in a building-integrated approach. On the other hand, the application potential of solar cooling is assured in Hong Kong no matter the absorption, adsorption or desiccant cooling type. Typical flat plate collectors can capture solar thermal energy to a relatively low driving temperature enough for desiccant cooling system and adsorption system. Further studies of a variety of system configuration and part-load control through simulation-optimization approach would provide practical guidelines in system design and operation for Hong Kong and South China regions. It is technologically feasible to have wider application of solar thermal technologies for sustainable building design in Hong Kong, and the solar thermal technologies are still major streams of renewable energy development. The econ-techno analysis of sustainable building design would be more attractive if tangible incentive scheme or policy can be acquired from the SAR government.

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