Application Potential of Solar-assisted Desiccant Cooling System in Sub-tropical Hong Kong

Transcription

1 Application Potential of Solar-assisted esiccant Cooling System in Sub-tropical ong Kong Kwong Fai Fong and Tin Tai Chow Building Energy and Environmental Technology esearch Unit, ivision of Building Science and Technology, City University of ong Kong Corresponding SUMMAY Feasibility study of solar-assisted desiccant cooling system (SCS) in ong Kong has been conducted, by comparing the two major approaches the outdoor air () scheme and the mixed air (MA) scheme. In the former, desiccant cooling would fully make use of to satisfy the required cooling load. In the latter, MA from the pretreated and the return air would be mixed and supplied for cooling purpose. In principle, the latter may be better due to the enthalpy conservation from the return air. owever from the year-round simulation study by using TNSYS, it was found that the scheme would provide better cooling and energy performance under the typical meteorological year of ong Kong. The system design of SCS would depend on the local climatic conditions, and the extent of solar irradiation, air temperature and humidity should be considered thoroughly in the study. INTOUCTION ong Kong has the sub-tropical climate, featured with long hot and humid summer and temperate climatic conditions for half a year. Air-conditioning for comfortable indoor environment is therefore indispensable. Owing to the fact that air-conditioning and refrigeration becomes the biggest electricity consumer in ong Kong [1], different measures of energy conservation and management have been implemented, in order to reduce energy consumption without sacrificing thermal comfort. Electrically driven compression refrigeration systems have been applied for a century, commonly found at homes, work places, industrial facilities and transportation. ue 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 if feasible technology for air-conditioning and refrigeration is available. In the recent years, there is encouragement from the government to seek for potential applications of renewable energy in ong Kong, particularly to make use of solar energy [2]. Solar heating is a well-known technology, no matter using the flat plate collectors or evacuated tubes. owever solar cooling has little mention, since it is a blooming technology even in Europe, USA and Japan, where research and development works are still ongoing [3,4]. The prominent advantage to utilize solar energy for building air-conditioning is the coincidence of solar irradiation availability and building cooling demand. In this study, the application potential of solar cooling in ong Kong was perceived through suitable simulation methodology.

2 SOLA COOLING IN CONTEXT The basic principle of solar cooling is to apply the heat acquired from solar collectors for the thermally driven system, so that chilled water or conditioned air can be produced for airconditioning purpose. From the latest research works [5-7], active solar cooling can be categorized in the following ways: a. Solar-electric refrigeration: PV-operated compression cycle (compressor driven by a direct current motor) b. Solar-thermal refrigeration i. Solar mechanical compression cycle (using ankine cycle to drive a conventional compressor) ii. Absorption (note ab ) refrigeration iii. Adsorption (note ad ) refrigeration iv. Steam jet cycle (a novel cycle driven in 0 C) c. Solar-thermal air-conditioning i. Solid desiccant cooling ii. Liquid desiccant cooling In active solar cooling, the solar-electric refrigeration and solar-thermal refrigeration have the focus on the development of new types of chillers for refrigeration purpose. In solar-thermal air-conditioning, conditioned air with both design temperature and humidity is directly provided to indoor space. As incorporated with the desiccant adsorber, the humidity control can specifically handle the required latent load. Therefore its dehumidification performance would be commonly better than the conventional cooling coil. SOLA-ASSISTE ESICCANT COOLING SYSTEM For the solar-assisted desiccant cooling system (SCS), the core part is the sorbent component. Currently, 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 moisture by solid sorbent are stable, that is more suitable to directly apply upon the conditioned air. esiccant wheel is commonly used as the solid sorbent component. SCS also includes the heat recovery unit, direct evaporative coolers, solar collectors and auxiliary heater, as shown in Figure 1. A typical psychrometric cycle in summer is also presented. In principle, the adsorption or desorption process is adiabatic, but the wave propagation effect during the regeneration process of desiccant wheel would cause deviation from the theoretical process. Solar air collector is a common choice for SCS. Since it can be directly coupled to SCS for handling the regeneration air, and instantly make use of thermal gain without the need of thermal storage. Solar air collector is effective when the cooling load profile is in line with the availability of solar irradiation in day time. The heat recovery unit is used to conserve the sensible heat and pre-cool the outdoor air. The evaporative cooler would cool down the supply air up to the humidity level capable of tackling the space latent load.

3 5 2.5 Proceedings of Clima 07 ellbeing Indoors Another evaporative cooler is installed to cool down the return air, for furnishing better sensible heat extraction at the heat recovery unit. Abbreviation 9 : auxiliary heater B: bypass damper : desiccant wheel EAF: exhaust air fan EC: evaporative cooler : heat recovery unit sac : outdoor air : solar air collectors : supply air fan 11 EAF 8 6 Indoor 7 B ah ASAE PSYCOMETIC CAT NO.1 NOMAL TEMPEATUE BAOMETIC PESSUE: kpa Copyright 1992 AMEICAN SOCIETY OF EATING, EFIGEATING AN AI-CONITIONING ENGINEES, INC SEA LEVEL SENSIBLE EAT TOTAL EAT Qs Qt ET BULB TEMPEATUE - C ENTALPY UMIITY ATIO h sac ENTALPY - KJ PE KILOGAM OF Y AI SATUATION TEMPEATUE - C % 80% 70% 60% 50% 40% % % % ELATIVE UMIITY VOLUME - CUBIC METE PE KG Y AI Y BULB TEMPEATUE - C UMIITY ATIO - GAMS MOISTUE PE KILOGAM Y AI ENTALPY - KJ PE KILOGAM OF Y AI Figure 1. Schematic diagram and typical psychrometric cycle of solar-assisted desiccant cooling system in the summer. The European countries have practical experiences in the system design and installations of SCS [4]. Generally SCS has the outdoor air () scheme and mixed air (MA) scheme. In the former, desiccant cooling would fully make use of the to fulfill the required cooling load. In the latter, the MA from the pretreated outdoor air and the return air would be mixed for cooling purpose. Therefore the scheme would have larger sizes and higher capital costs of equipment components, since the amount of full is involved, while the MA approach would have smaller equipment sizes thus lower initial costs due to smaller amount of required. It seems that the latter can have the advantage of enthalpy conservation from

4 the return air, but the cooling performance of the former may be better due to larger amount of conditioned air being produced. This would depend heavily on the local climatic conditions, particularly the solar irradiation, air temperature and humidity. As a result, simulation model was built for in-depth study between these two schemes. Moreover in each scheme, the regeneration air for the desiccant wheel can be either from the return air leaving the heat recovery unit, or a separate outdoor air stream. In summary, there are altogether four alternatives for the system design of SCS as follows: A. scheme with return air for regeneration (Figure 2a); B. scheme with separate outdoor air for regeneration (Figure 2b); C. MA scheme with return air for regeneration (Figure 2c); and. MA scheme with separate outdoor air for regeneration (Figure 2d). EAF1 EAF2 Indoor EAF2 Indoor (a) Alternative A (b) Alternative B EAF1 EAF2 A Indoor EAF2 A Indoor (c) Alternative C (d) Alternative Figure 2. Four alternatives for system design of solar-assisted desiccant cooling system EVELOPMENT OF SIMULATION MOEL In order to have an in-depth study and comparison for these four alternatives, simulation model of SCS was built by using TNSYS [8], with the support of the component models from TESS [9]. The desiccant wheel model could determine the regeneration air inlet temperature based on the humidity ratio set point. The heat recovery unit model had the sensible effectiveness of 0.8. Each evaporative cooler model could vary its saturation efficiency between 0 0%. Each fan model was the variable speed type, and its power was adjusted from the rated power according to the flow change. The solar air collector model was an un-glazed type that passed the air behind the absorbing plate. It had the absorptance of and emissivity of The auxiliary heater model was electric type with efficiency of unity. In ong Kong, the energy cost of electricity and that of town gas are comparable, so the electric heater was designed for system and operation simplicity. The ong Kong Typical

5 Meteorological Year [], which was in the EnergyPlus / ESP-r format (*.epw), was adopted in this simulation model. In order to understand the effectiveness of the cooling performance of SCS, it was designed to provide air-conditioning for function area with commonly high cooling intensity, therefore a seminar room located on the top floor was selected for simulation purpose. It had floor area of 213 m 2 and 3.82 m high. There were four external walls and a flat roof. The wallfenestration ratio was 0.4 to 0.6. The internal and external shading factors of fenestration were 0.8 and 0.2 respectively. The internal heat gains included 1 persons seated at rest and 13 /m 2 artificial lighting with % convective part. The daily air-conditioning period was from 08:00 to 18:00 to suit the solar availability. According to the local design practice, the rated mass flow rate and rated power of the variable supply air fan were 3.47 m 3 /s and 4.63 k respectively, with the flow range from to 0%. The rated flow rate of the variable exhaust air fans would depend on the alternatives, ranged from 1.04 to 3.47 m 3 /s. The rated power of the exhaust air fans EAF1 and EAF2 were 2.89 k and 5.79 k respectively. The power of heat recovery unit and evaporative cooler were 0.19 k and k respectively. The capacity of the electric auxiliary heater was 0 k. The set point of the desiccant dehumidification was 0.01 kg/kg. 0 m 2 solar air collectors with tilt angle of 22 C were installed on the roof directly above the air-conditioned space. Control scheme was included for year-round operation in response to different climatic conditions and changing heat gains. There were free cooling mode and desiccant cooling mode. Free cooling with adjustable fan speed of supply and exhaust air fans was implemented when the outdoor temperature was higher than 5 C. hen the outdoor temperature was higher than the room temperature 24 C, desiccant cooling was called in and the supply air temperature was set at 13 C. Bypass control was applied whenever there was no thermal gain at the solar air collectors. The entire plant and energy simulation model was developed in a total energy approach, with the consideration at the equipment level, system level and operation level, in order to reflect the effectiveness of SCS in different seasons in ong Kong. The general cooling performance and year-round energy consumption of this simulation model was validated with a similar project in Freiberg of Germany [4]. ESULTS AN ISCUSSION For the four alternatives, the simulation results for comparison are summarized in Table 1 and introduced below. Table 1. Year-round simulation results of the four alternatives. Alt Annual electrical consumption (kh) Annual thermal gain (kh) Average solar fraction our percent out of comfortable temperature Average COP A 154,417 17, % 1.07 B 225,417 33, % 0.51 C 79,167 13, % ,599 28, % -0.41

6 The annual electrical consumption is the year-round electrical energy consumption of all the involved components of SCS, including the fans, evaporative coolers, heat recovery unit, desiccant wheel, and electric auxiliary air heater. The annual thermal gain is the year-round thermal energy acquired by the solar air collectors during the operation of SCS. The average solar fraction is to compare the solar fraction of the thermal gain from solar air collectors Q sac to the heat generated from the auxiliary heater Q ah. The monthly average solar fractions were determined first, then the year-round average value SF avg was found as follows: Qsac, i Q i= 1 ah, i SF avg = (1) j j = 1 The hour percent out of comfortable temperature is to consider the effect about thermal comfort. In this study, the upper limit of 27 C of the summer comfort zone from ASAE [11] is used to check against the room air temperature. The percentage was based on the annual air-conditioning hours. The hourly COP (coefficient of performance) of SCS is defined as follows: COP = m& h supply ( outdoor supply) Q in h m& = Q h h supply ( 1 5) sac + Q ah (2) m& supply where is the mass flow rate of supply air for the conditioned space, h outdoor or h 1 is the outdoor air enthalpy, h supply or h 5 is the supply air enthalpy, Q in is the total thermal energy input that is the sum of Q sac and Q ah. The monthly average COP was determined, then the year-round average COP. In addition to the results in Table 1, the room conditions during the air-conditioning hours are plotted in red spots on the psychrometric chart. The ASAE summer comfort zone is encompassed by blue solid lines.

7 (a) Alternative A (b) Alternative B (c) Alternative C (d) Alternative Figure 3. oom conditions during the air-conditioning hours throughout a year for the four alternatives. From Table 1, it seems that Alternative C had the lowest annual electrical consumption among different system designs. But with regard to the other results, particularly Figure 3(c) and (d), both Alternatives C and could not generally provide the required comfortable conditions for the air-conditioning area. It accounted to 79.4% and 78.8% of air-conditioning hours higher than the comfortable upper limit of 27 C. These two alternatives were the MA scheme, although they had relatively high average solar fraction due to better thermal gain from % passing the same area of solar air collectors. But such flow rates of process air and regeneration air were not enough to provide the necessary cooling effect for the space loads. Therefore only about % of the room temperature was satisfactory in a year. In addition, the average COP of both Alternatives C and were negative in Table 1, implying that h outdoor was lower than h supply from Eq(2). This shows that the enthalpy of MA was even worse than that of the, and the room condition was not comfortable enough in most of the time. As a result, only Alternatives A and B would be under consideration. Both of them were designed based on the scheme. From Table 1, it is obvious that Alternative A was advantageous over Alternative B due to lower annual electrical consumption, greater average solar fraction, smaller hour percent out of comfortable temperature and better average COP. The annual thermal gain of Alternative A was less than that of Alternative B, but it did not

8 have contradiction to the result that Alternative A had a better solar fraction. It was because Alternative A applied the return air with higher temperature for regeneration in desiccant wheel (Figure 2(a)), while Alternative B used the with lower temperature for the same purpose (Figure 2(b)). Therefore the thermal gain in Alternative A would be less hence the demand of auxiliary heating for regeneration was also less, and the solar fraction was eventually higher. Consequently, Alternative A, the scheme of SCS with return air for regeneration had the best overall performance in view of energy and cooling effect. Monthly Average Solar Fraction Temperature Tsa Tra Tsac Tregen Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov ec our Figure 4. Profile of monthly average solar fraction in different months. Figure 5. Temperature profiles of supply air T sa, room air T ra, solar air collector outlet T sac, and regeneration air T regen on a typical summer day. For Alternative A, an in-depth performance appraisal was made. Figure 4 shows the changing profile of monthly average solar fraction. It is noted that the solar fraction was high around March and November, which are the spring and autumn in ong Kong. The weather and cooling load of these two months were moderate, so the solar contribution could be more significant. For the local summer period from May to September, the solar fraction was low, between 0.05 to This indicates that auxiliary heating was necessary in summer in order to achieve the design humidity set point for desiccant cooling purpose. Figure 5 illustrates the temperature profiles of supply air, room air, solar air collector outlet and regeneration air on a typical summer day. For both the supply and room air, their temperatures are fairly constant throughout a day, around 22 C and 27 C respectively. The outlet temperature of solar air collectors and regeneration temperature were around 50 C and 60 C. The temperature difference was accomplished by the auxiliary heater, this shows again the need of auxiliary heating in the summer time. CONCLUSION Although there are many solar cooling applications in the European and western countries, the system design should be carefully examined for local use based on the climatic conditions. In this study, the outdoor air scheme was found more suitable than the mixed air scheme for the solar desiccant cooling system (SCS) in ong Kong, and the system configuration of return air for regeneration was more effective. The average monthly solar fraction and COP were 0.15 and 1.07 respectively. Owing to the sub-tropical climate featured with hot and humid summer in ong Kong, auxiliary heating was necessary in order to achieve the design performance of SCS. Although the application potential of solar air collectors in ong Kong was generally assured, there were 15% air-conditioning hours exceeding the comfortable temperature limit for the air-conditioning area with high cooling load intensity. This problem can be firstly handled by the other choices of solar collectors, like the flat plate

9 collectors or evacuated tubes, and it would be studied in the next stage. Total solar cooling solution would be pursued to prevent from using auxiliary cooling. ACKNOLEGEMENT This research work is supported by the ivisional esearch Grant G01/06-07 from the ivision of Building Science and Technology, City University of ong Kong. EFEENCES 1. ong Kong Energy End-use ata 06, Electrical & Mechanical Services epartment, Government of the ong Kong Special Administrative egion, September Study on the Potential Applications of enewable Energy in ong Kong, Stage 1 Study eport, Electrical & Mechanical Services epartment, Government of the ong Kong Special Administrative egion, ecember Guidelines for solar cooling feasibility studies & analysis of the feasibility studies. Altener Project, Climasol, May enning, -M. 04. Solar-Assisted Air-Conditioning in Buildings, A andbook for Planners. Springer-Verlag ien New York. 5. Chow, TT. 06. Solar energy for building applications in the warm Asia Pacific region. In: Trends in Solar Energy esearch, Chapter 4, Nova Science Publishers, pp Balaras, C A, enning, -M, iemken, E, et al. 06. Solar Cooling: An Overview of European Applications & esign Guidelines. ASAE Journal, June, pp Klein, S A, eindl, T. 05. Solar efrigeration. ASAE Journal, September, S TNSYS TNSYS 16 a TaNsient SYstem Simulation program. Solar Energy Laboratory, University of isconsin-madison. 9. TESS 04. T.E.S.S. Component Libraries v2.0 for TNSYS v16.x and the TNSYS Simulation Studio, Parameter / Input / Output eference Manual. Thermal Energy System Specialists, LLC.. Chan, A L S, Chow, T T, Fong S K F, Lin J Z. 06. Generation of a typical meteorological year for ong Kong. Energy Conversion and Management 47 pp ANSI/ASAE Standard