Demonstration of a Solar Absorption Air-Conditioning System Powered by Heat-Pipe Evacuated Tubular Collectors

DEMONSTRATION OF A SOLAR ABSORPTION AIR-CONDITIONING SYSTEM POWERED BY HEAT-PIPE EVACUATED TUBULAR COLLECTORS

HE Zinian Solar Thermal Division, Beijing Solar Energy Research Institute No.3 Huayuan Road, Beijing100083, China Tel: 86 (10) 62351066, Fax: 86 (10) 62012880, E-mail: [email protected]

Abstract A solar-powered absorption air-conditioning demonstration system with 100 kW cooling capacity has been successfully designed and constructed in Shandong Province, China. The system consists of heat-pipe evacuated tubular collector array, lithium-bromide absorption chiller, cooling tower, water storage tanks, circulating pumps, fan-coil units, auxiliary oil-burned boiler and control device. The solar collector array using 2160 heat-pipe evacuated tubular tubes, has a total aperture area of 540 m2. This paper introduces design characteristics and measuring performances of the system, which has a multifunction of space cooling in summer, space heating in winter and domestic water heating in other seasons. Thermal efficiencies of the collector array respectively are over 40% for space cooling, near 35% for space heating and above 50% for domestic water heating. Total cooling efficiency for the entire system is around 20%.

1. INTRODUCTION inlet temperature. In view of thermal performance of the system, a higher inlet temperature will result in a As we have known, electric power required for higher coefficient of performance (COP) value. providing air-conditioning takes a very large portion of Fortunately, heat-pipe evacuated tubular collectors, total electric power consumption in the world. For this developed by Beijing Solar Energy Research Institute reason, various solar-powered air-conditioning (BSERI) and recently manufactured by Beijing systems have been investigated. [1-4] Among them, SUNDA Solar Energy Technology Co., Ltd. in China, absorption air-conditioning systems were commonly can meet this requirement. [5] These heat-pipe utilized. Solar cooling has an obvious advantage that evacuated tubular collectors have been used for a solar the most cooling demand is matched with the strongest absorption air-conditioning system incorporated with a sunshine in summer. In addition, solar absorption lithium-bromide absorption chiller at an operating cooling can be combined with solar space heating and temperature about 88 . solar water heating so that this comprehensive system This paper describes design characteristics of the will obviously enhance economic benefit of the solar completed solar air-conditioning system, as well as absorption air-conditioning system. measuring results under conditions of space cooling, LiBr-H2O absorption chillers have been widely space heating and domestic water heating respectively. commercialized. The chiller requires moderately high

Fig. 1. Full view of the solar air-conditioning system in Rushan, Shandong Province, China 2. SITE OF THE SYSTEM the Solar Energy Hall of the Chinese Renewable Energy Popular Science Park in the Tourist & Holiday- The solar absorption air-conditioning system has Spending Zone. The Hall is a two-story building with been successfully designed and constructed by BSERI a construction area over 1000m2 and has been in Yintan Tourist & Holiday-Spending Zone in architecturally designed to meet requirements of solar Rushan, Shandong Province, China. collector array placement, as shown in Figure 1. Rushan is located at the southeast end of Shandong Peninsula, which is 36.7°north latitude and 3. SYSTEM DESIGN 121.5°east longitude. It is 100 km from Weihai in the east and 150 km from Qingdao in the west, bordering 3.1 Layout on the Huanghai Sea in the south. In this area, annual The solar air-conditioning system consists of average daily solar radiation is around 17.3 MJ/m2 and evacuated tubular solar collector array, lithium-water annual average air temperature is around 12.3 . The absorption chiller, cooling tower, water storage tanks, maximum air temperature in summer is 33.1 and the circulating pumps, fan-coil units, auxiliary oil-burned minimum air temperature in winter is – 7.8 . Under boiler and control device. Figure 2 shows a layout of the local climate, both space cooling and space heating the system. are required for comfort in summer and winter.

The solar air-conditioning system was installed in

Fig.2. Layout of the solar absorption air-conditioning system

1. Heat-pipe condenser section 2. Metal sealing cover 3. Glass envelop tube 4. Absorber plate 5. Heat-pipe evaporator section 6. Getter material

Fig.3. Configuration of the heat-pipe evacuated tube 3.2 Solar collector array between ambient temperature and chilled-water In order to supply the absorption chiller with higher temperature is much smaller than that between hot inlet temperature, heat-pipe evacuated tubular collectors water temperature and ambient temperature. Circulating have been used. pump P4 drives the chilled-water between the tank 3 and Each evacuated tube has an outer diameter of 100 mm fan-coil units. and a length of 2000 mm. The evacuated tube mainly These three tanks are also applied to store hot water consists of heat-pipe, absorber plate, glass envelope tube, for space heating in winter. metal sealing cover, getter material and others, as shown The largest tank 4 is called as domestic hot water in Figure 3. The heat-pipe is composed of an evaporator storage tank. It supplies hot water by means of a heat section and a condenser section. exchanger inside the storage. It also can be used to To obtain more solar irradiation over a day, evacuated rescue the system from overheating when necessary in tubes with a semicylindric absorber plate are selected. summertime. Theoretical calculation and measuring data show that the semicylindric absorber plate increases energy gain 10 - 3.5 Auxiliary boiler 14% more than the flat absorber plate.[6] To ensure an all-weather operation for the system, the As heat-pipe technology is applied to evacuated tubes auxiliary oil-burned boiler was installed. The boiler has as well as “dry connection” between evacuated tubes and a rated thermal power of 350 kW with a rated outlet manifolds is utilized to modules, the heat-pipe evacuated water temperature of 95 . A designed thermal tubular collector has many advantages, such as freeze efficiency of the boiler is approximately 88%. resistance, fast start-up, high pressure bearing, thermal shock endurance, etc. 3.6 Control device The solar collector array using 2160 heat-pipe The control device consists of temperature sensors, evacuated tubes, has a total aperture area of 540 m2 and a electro-actuating valves, network control modules and 2 total absorber area of 364 m. The collectors were operator working station. There are totally 9 temperature arranged in 9 rows, in which 7 rows were installed on a sensors, 2 three-way valves and 14 two-way valves in large inclined south-facing roof and 2 rows were installed the system. on a small flat roof with a tilt angle 35°. To reduce flow Positions indicated by the temperature sensors are as resistance within the system, front 4 rows and back 5 follows: rows were respectively connected in parallel. Then these T1 outlet of the collector array two parts were connected in series. T2 outlet of the tank 1 Circulating pump P1 drives the solar system. T3 inlet of the collector array T4 water in the tank 2 3.3 Cooling chiller T5 water in the tank 1 The air-conditioning system applies a Model LCC-03 T6 inlet of the chiller lithium-bromide absorption chiller that is made by Dalian T7 outlet of the tank 3 SANYO Refrigeration Co., Ltd in China. The chiller has T8 inlet of the tank 3 a maximum cooling capacity of 176 kW (50 USRT). The T9 water in the tank 4 solar collector array supplies hot water at about 88 to The control device has three main functions: an inlet of the chiller and the water leaves the chiller at Firstly, it serves to open/close valves and switch about 83 . The chiller produces chilled-water at about on/off circulating pumps to operate the system for space 8 and the water returns to the chiller at about 13 . The cooling or space heating or domestic water heating as cooling water temperature through the chiller is well as to arrange storage tanks for minimizing heat successively 31 and 37 . losses. Circulating pumps P2, P3 and P5 respectively drive Secondly, it serves to light up/off the auxiliary boiler the hot water, chilled water and cooling water through to compensate thermal power provided by the solar the chiller. collector array when water temperature is below a specified level. 3.4 Storage tanks Thirdly, it serves to prevent the system from There are totally 4 storage tanks in the system. overheating when necessary in summertime and also Volumes of tank 1, tank 2, tank 3 and tank 4 are from freezing when required in wintertime. respectively 8 m3, 4 m3, 6 m3 and 10 m3. The tank 1 and tank 2 are called as hot water storage 4. MEASUREMENTS AND INSTRUMENS tanks and used to store the hot water produced by the solar collector array. The smaller tank 2 aims to reach a In order to determine performances of the system, specified temperature value for the chiller in he early following parameters must be directly measured: solar morning. irradiance, inlet/outlet temperatures and water flow rates The tank 3 is called as chilled-water storage tank and respectively for the solar collector array, cooling chiller is used to store the chilled-water to reduce heat losses of and fan-coil units, water temperatures within storage the storage tank because temperature difference tanks and ambient temperature. Solar irradiance on a plane of the solar collector array is measured by an EPPLEY Model PSP temperatures in storage tanks and ambient temperature pyranometer which has a linearity of ±0.5% and a are conducted by a DATATAKER Model DT 600 data temperature dependence of ±1 %. acquisition device. Inlet /outlet water temperatures respectively for the solar collector array, cooling chiller and fan-coil units 5. RESULTS AND DISCUSSION are measured by Pt 100 resistance thermometers, which has an accuracy of ±0.1 . The solar absorption air-conditioning system has Water flow rates respectively for the solar been operated since December 1998. Performances of collector array, cooling chiller and fan-coil units are the system for different purposes were measured: measured by vortex flow-meters. The flow-meter has space heating from January to March 1999, domestic an accuracy of ±1%. water heating in May 1999 and space cooling from A computer program has been developed for data June to August 1999. acquisition and processing so that not only instantaneous flow rate but also the accumulated flow 5.1 Space cooling performance rate can be read out simultaneously. Furthermore, In summer, the measuring data indicated that the instantaneous heat flux and accumulated heat flux can outlet temperature was generally over 88 on a sunny be immediately calculated, by incorporating the day, which is necessary for operation of the chiller. instantaneous flow rate with the corresponding Figure 4 shows the variation of solar irradiance with temperature difference between inlet and outlet of each time on 16 August 1999. The solar irradiance ranged facility. from 320W/m2 to 985W/m2 during the day. The solar Data acquisition for solar irradiance, water isolation from 8:30 to 16:30 was 19.45 MJ/m2.

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0 8:30 9:30 10:30 11:30 12:30 13:30 14:30 15:30 16:30 Time Fig. 4. Variation of solar irradiance with time (16.08.1999)

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Solar power gain/ irradiation power£¨kW£© 50

0 8:30 9:30 10:30 11:30 12:30 13:30 14:30 15:30 16:30 Time

Fig. 5. Variation of solar power gain with time (16.08.1999) The variation of solar power gain with time on the Figure 6 shows the variation of cooling power with same day is shown in Figure 5. The total solar energy time in the whole day. It ranged from 55kW to 90kW. gain was 2832.5 MJ and the total incident solar energy This implies that cooling power was relatively stable was 7081.3 MJ. although solar irradiance varied over the day. As a definition, thermal efficiency of the solar Figure 7 indicates the variation of cooling power collector array is the total solar energy gain divided by with time on different days. It proves that the system the total incident solar energy. Using all above can provide cooling power of around 100 kW without measuring data, thermal efficiency of the collector any auxiliary heating supply. array was 40.0 %.

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Cooling power 200 Solar irradiation power

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0 9:15 10:15 11:15 12:15 13:15 14:15 15:15 Time Fig. 6. Variation of cooling power with time (16.08.1999)

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140 Cooling power 17.08 Cooling power 18.08 120

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60 Cooling power£¨kW£©

0 9:58 10:48 11:30 12:23 13:20 14:10 15:10 Time Fig. 7. Variation of cooling power with time on different days

It has been found that COP of the chiller changed The fact that total cooling efficiency of the entire with the variation of both inlet temperature and flow system is higher than that published in previous rate of the chiller. Under condition of utilizing only literatures, is owe to higher thermal efficiency of heat- solar energy, COP ranged from 0.50 to 0.71, and pipe evacuated tubular collectors and higher COP of average COP over a day was 0.57. the chiller at higher operating temperature. Total cooling efficiency for the entire system could Figure 8 shows temperature variation in the tank be defined as the total cooling energy divided by the 2 on 16 August. It can be seen that hot water total incident solar energy. On 16 August, the total temperature in the take 2 rose very fast and reached cooling energy was 1777.4MJ, and the total incident 88 which met requirements from the chiller at 9:15 solar energy was 7081.3MJ, therefore the total without supply of the auxiliary boiler. Furthermore, efficiency of the entire system was 25.1%. after starting the chiller at 9:20, water temperature at Table 1 gives the incident solar energy, solar upper part still kept unchanged although water energy gain, cooling energy, collector efficiency and temperature at lower part decreased 3 due to the total cooling efficiency on several days. It indicates returned water. that total cooling efficiency is from 22.0% to 25.2%. Table 1 Incident solar energy, solar energy gain, cooling energy, collector efficiency and total cooling efficiency

Date Incident solar Solar energy Cooling Collector Total cooling energy (MJ) gain (MJ) energy (MJ) efficiency (%) efficiency (%) 25.06.1999 7389.2 3264.9 1625.6 44.2 22.0 14.08.1999 3019.2 1242.8 695.9 41.2 23.0 15.08.1999 6489.4 2620.1 1440.5 40.4 22.2 16.08.1999 7081.3 2832.5 1777.4 40.0 25.1 24.08.1999 4521.9 1853.6 1141.6 41.0 25.2

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70 7:30 7:40 7:50 8:00 8:10 8:20 8:30 8:40 8:50 9:00 9:10 9:20 Ê ± ¼ä Fig. 8. Temperature increase in tank 2 (16.08.1999)

5.2 Space heating performance Measurements of solar irradiance, solar power gain 5.3 Domestic water heating performance and temperature distribution in the tank 2 for space Similar measurements and calculations have been heating were conducted from January to March 1999, done for domestic water heating purpose. with the same methods as that for space cooling in As shown in Table 3, water temperature inside the summer. Similarly, thermal efficiency of the collector tank 4 changed from 18.2 at 8:00 to 44.2 at 10:30, array has been calculated. which was suitable for domestic use. Table 2 gives the incident solar energy, solar energy Table 3 shows the water temperature increment in gain, collector efficiency and the lowest ambient the tank 4 along with the solar isolation on 25 May. At temperature during daytime on several days. It 10:30, the solar isolation was 5.83 MJ/m2, the solar indicates that average collector efficiency for space energy gain was 1088.6 MJ, and thus the collector heating was approximately 33% and the maximum efficiency was 51.3%. value could reach 35%. According to these data, it is evident that the system Figure 9 shows the temperature increase in the tank can supply approximately 32 m3 of domestic hot water 2 on Feb. 27. Hot water temperature in the tank 2 also around 45 per day. increased quickly without supply of the auxiliary boiler. Table 2 Incident solar energy, solar energy gain, collector efficiency and lowest ambient temperature during daytime

Date Incident solar Solar energy Collector Lowest ambient energy (MJ) gain (MJ) efficiency (%) temperature (°C) 15.01.1999 5973.2 1659.9 31.1 –2.9 16.01.1999 5798.5 1840.7 31.7 –2.1 25.02.1999 2857.4 953.3 33.4 4.8 26.02.1999 3549.0 1251.7 35.3 7.2 27.02.1999 8899.8 2966.6 33.3 0.5 01.03.1999 6235.3 2175.3 34.9 6.9

Temperature(¡ æ) Upper temperature Middle temperature 40 Lower temperature

30 8:30 9:30 10:30 11:30 12:30 13:30 14:30 15:30 Time Fig. 9. Temperature increase in tank 2 (27.02.1999)

Table 3 Water temperature in tank 4 and solar isolation (25.05.1999 ) Water temperature (°C) Isolation 2 Time Upper Middle Lower Average (MJ/ m ) 8:00 19.8 18.5 16.4 18.2 0.00 8:30 25.6 24.3 21.8 23.9 0.94 9:00 30.3 29.3 26.6 28.7 1.94 9:30 35.1 34.2 31.7 33.7 3.02 10:00 40.1 39.2 36.8 38.7 4.31 10:30 45.7 44.7 42.2 44.2 5.83

6. CONCLUSION early morning in summer, as well as to 55 in the early morning in winter. l The solar absorption air-conditioning system has l The chilled-water storage tank is obviously been used for space cooling, space heating and useful for reducing heat losses because domestic water heating. This multifunctional temperature difference between ambient and system enhances economic benefit of the system. chilled-water is much smaller than that between l Measured thermal efficiencies of the solar hot water and ambient. collector array are over 40% for space cooling, l Measured COP of the chiller is approximately near 35% for space heating and above 50% for 0.70. Total cooling efficiency of the entire domestic water heating. It indicates that heat- system is around 20% under local circumstance. pipe evacuated tubular collectors are suitable for applications at higher operating temperature and lower ambient temperature. l As the smaller storage tank is specially adopted, hot water temperature can be raised to 88 in the REFERENCES

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6. Z.N. He, H.C. Ge, F.L. Jiang and W. Li (1997) A Comparison of Optical Performance between Evacuated 6. Collector Tubes with Flat and Semicylindric Absorbers, Solar Energy 60, 109-117.