Thermal performance of a new CPC solar air collector with flat micro-heat pipe arrays

Significance Statement

Scientists from Beijing University of Technology presented a new kind of solar air heater flat micro-heat pipe array Compound Parabolic Concentrator (FMHPA-CPC SAC) thereby investigating its thermal performance, heat transfer and flow friction characteristics. The study is reported in Applied Thermal Engineering.

Flat-plate solar collectors are classified into water and air collectors based on their heat transfer fluid. A Compound Parabolic Concentrators works on the principle of solar energy concentrated on its receiver and absorbed by its body. Apart from its easy operation, heat pipe-type vacuum tube compound parabolic concentrator has a higher heat-collecting efficiency than those of heat pipe-type vacuum tube collector and heat compound parabolic concentrator collector under the same radiation intensity.

Various research have been done on solar air heater and compound parabolic concentrator with different absorbers and reflectors but little knowledge on thermal performance of solar air collector (SAC) and compound parabolic concentrator have been reported. This led to further investigations on thermal performance of CPC-SAC.

Flat micro-heat pipe arrays (FMPHA) technology was used as heat transfer system. It has two operating sections; evaporator section and condenser section. When the evaporator section evaporates saturated vapor from the working fluid, it flows upward to the condenser section and vapor condenses releasing heat. The condensing liquid then returns to the evaporator by gravity or capillary force.

Cylindrical absorber was constructed by inserting these FMPHA into an evacuated glass tube to transport heat during the working process. The new FMPHA-CPC SAC has a reflector half angle of 300 and concentration ratio (CR) of 1.8. The basic working principle of FMPHA-CPC SAC is that some of the incident solar radiation is directly absorbed by outer layer of the vacuum glass and subsequent absorbed by the inner layer with selective absorbing coating while the remaining solar radiation is absorbed by surface of the CPC reflector to the vacuum tube through one or two reflections.

In order to achieve this experiment, Zhu et al. (2016), installed FMPHA-CPC SAC at doors of Beijing University Technology with tilt collectors facing horizontal plane 450 southwards. Thermoresistors and TRT-2 pyranometer were used to measure inlet-outlet air flow temperatures and global solar irradiation in the plane respectively. Testo 512 differential manometer was used to measure the pressure drop across the collector while air flow rates was tested using air volume cover. For analysis of this experiment, optical efficiency of FMHPA-CPC SAC of 0.656 was calculated, thermal conduction between fins and FMPHA at condenser section was 0.083K/W. Thermal convection resistance between fins and air in duct was calculated to be 4.594KW. Thermal resistance of air duct installation of heat conduction and convection was 1.04m2.K/W and 0.0585m2.K/W.

On a certain weather condition, the variation of outlet temperature and efficiency with flow rates of 320m3/h, environmental temperature between 24.9 and 32.70C coupled with relative humidity and wind speed fluctuation between 30.5% to 46.9% and 0.2 to 2.1m/s respectively showed the outlet temperature of the collector to be between 26.3 and 51.10C. Thermal efficiency reached approximately 67.7% during test period. Thermal efficiency and outlet temperature of the collector was seen to vary with solar radiation intensity as efficiency decreased sharply with decrease in solar radiation of the compound parabolic concentrator plate.

Temperature behavior of FMHPA showed differences of 200C between the evaporator section temperature Te and condenser section temperature Tc when solar radiation is more than 600W/m2. However, fluctuations of solar radiation value was seen to influence temperature distribution of the FMPHA as well as outlet temperature. As solar radiation decreases, temperatures of evaporator and condenser sections of FMPHA decreases immediately.

Similar effects of solar radiation on evaporator and condenser of FMPHA was noticed. Evaporator and Condenser temperature reached a peak of 126.20C and 115.80C respectively. However, decrease in solar radiation intensity led to decrease of incident light absorbed by CPC which led to decrease in temperature of vacuum tube, evaporator and condenser.

In order to compare air volume flow rates, two different days was set and results showed two different thermal properties which was caused by different convection heat transfer conditions. This results show that high volume flow rate results to high heat transfer.

Pressure loss values of air duct under flow rates of 320, 260, 180,100 and 60m3/h were read to be 36.8, 24.6, 12.1, 4.6 and 2.0Pa leading to the fact that the collector has an higher advantage in pressure drop.

This report showed various advantages for FMPHA-CPC SAC in terms of optical efficiency, thermal efficiency and pressure drop which proves its overall performance.

 

Thermal performance of a new CPC solar air collector with flat micro-heat pipe arrays. Advances in Engineering

 

 

Thermal performance of a new CPC solar air collector with flat micro-heat pipe arrays. Advances in Engineering

Journal Reference

Ting-Ting Zhu,Yan-Hua Diao,, Yao-Hua Zhao,Feng-Fei Li. Thermal Performance of New CPC Solar Air Collector with Flat Micro-Heat pipe Arrays. Applied Thermal EngineeringVolume 98, 2016, Pages 1201–1213.

The Department of Building Environment and Facility Engineering, The College of Architecture and Civil Engineering, Beijing University of Technology, No.100 Pingleyuan, Chaoyang District, Beijing 100124, China

 

 

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