Renewable Energy Global Innovations features: Solar collector integrated with a pulsating heat pipe and a compound parabolic concentrator

Significance 

Solar energy is the most abundant renewable energy that has the capability to meet the world’s growing demand. However, it requires good solar concentrators to increase the trap efficiency. The efficient mean to utilize solar energy is to convert solar energy into heat stored in water by solar thermal collectors. Techniques such as high efficiency heat transfer absorber and solar radiation concentration are the main methods to improve the performance of solar thermal collector.

Pulsating heat pipe is one of the highly efficient absorber with simple stricture and low cost. The pulsating heat pipe has three working states namely: start-up, steady state and dry-out as the heat input increases.  Altogether, the pulsating heat pipe exhibits an excellent potential for application as heat collector credit due to its high heat transfer capacity. However, the heat flux of the of the evaporation section of pulsating heat pipe should be sufficiently high to meet the demand of its steady and high-efficiency work, which has a significant effect on the thermal performance of pulsating heat pipe. Therefore, a solar concentrator is necessary in order to increase the heat flux of the pulsating heat pipe absorber to ensure that efficient heat transfer capacity of pulsating heat pipe can be fully utilized.

Researchers led by Professor Rong Ji Xu from Beijing University of Civil Engineering and Architecture and in collaboration with Dr. Hua Sheng Wang at Queen Mary University of London proposed a study on a novel solar collector that integrates a closed-end pulsating heat pipe and a compound parabolic concentrator. Their main objective was to test the operating characteristics and thermal performance of the detailed designed collector, under different weather conditions. Their work is now published in the research journal, Energy Conversion and Management.

Briefly, the research team initiated their empirical procedure by developing a prototype of the solar collector. Secondly, they analyzed the operating characteristics of the pulsating heat pipe absorber. The team then assessed the thermal efficiency of the solar collector under different weather conditions.

The authors observed that the collector showed start-up, operational and shutdown stages at the starting and ending temperatures of 75 ⁰C. More so, they noted that the solar collector operated stably even in cloudy days. Additionally, the thermal resistance of the pulsating heat pipe absorber was seen to decrease with the increase in ambient temperature, solar intensity, and evaporation temperature which was found to be the main factor that affects the thermal efficiency of the collector.

Rong Ji Xu and colleagues successfully presented a novel solar collector that integrates a closed-end pulsating heat pipe and a compound parabolic concentrator. In their study, they have assessed the effects of operating parameters on the operating characteristics of the pulsating heat pipe and the performance of the solar collector under varying weather conditions. The experimental results suggest that the heat flux of the pulsating heat pipe absorber’s evaporation section concentrated by compound parabolic concentrator with a concentration ratio of 3.4 is appropriate and the use of compound parabolic concentrator is reasonable. Their proposed design offers a promising efficiency of 50% when compared with conventional solar collectors and pulsating heat pipe solar collectors.

According to Rong Ji Xu, the mathematical model of the solar collector has been built. The effects of the solar density, ambient temperature, weed speed, glass thickness and collecting temperature on the thermal performance were simulated. A theoretical efficiency of 70% can be realized which is more promising than experimental results.

About the author

Rongji Xu，PhD，Associate Professor
School of Environment and Energy Engineering, Beijing University of Civil Engineering and Architecture
1 Zhanlanguan Road, Beijing 100044, China
Email: xurongji@bucea.edu.cn

Research interests:

• Heat and mass transfer
• Development and utilization of solar energy
• Design and optimization of refrigeration & air-conditioning system
• Organic Rankine cycle (ORC)

Research grant applications:

• Failure mechanism study on pulsating heat pipe used in solar energy collector, Project principal, NSFC (No. 51506004)
• Mechanism study on pulsating heat pipe with mixture working fluid, Project principal, BNSF (No. 3162009)
• Development of PV air conditioner, researcher co-investigator, Project principal, University-Industrial Collaboration Project
• Design and optimization on fin-and-tube heat exchanger of air conditioner, Project principal, University- Industrial Collaboration Project

Reference

Rong Ji Xu, Xiao Hui Zhang, Rui Xiang Wang, Shu Hui Xu, Hua Sheng Wang. Experimental investigation of a solar collector integrated with a pulsating heat pipe and a compound parabolic concentrator. Energy Conversion and Management 148 (2017) 68–77

 

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Renewable Energy Global Innovations features: Dynamic filtration control performance of N2/liquid CO2 foam in porous media

Significance 

In the extraction of oil and gas, fracturing fluids are used to create and widen a fracture for easier production. The fluid should be compatible with the rocks. Liquid CO₂ has been used as a fracturing fluid because it is highly soluble in most oil extracts therefore less damaging to the oil extracted as compared to water-based fluids. The high solubility in oil helps in lowering the viscosity of oil and improves the extraction of oil from the ground.

Liquid CO₂ is, however, difficult to control in porous media due to its low viscosity and lack of filter cake formation properties for controlled filtration. This may affect the shape of the desired fracture. Researchers have tried to improve on the viscosity of liquid CO₂ by adding thickening agents, however, improvement of viscosity of liquid CO₂ by enhancement does not guarantee a change in its filtration performance and liquid CO₂ dissolves poorly with thickening agents making the insoluble residues potential pollutants.

Filtration control performance of liquid CO₂ was improved without pollution by mixing it with Nitrogen (N₂). Gupta et al. stabilized the N₂ and CO₂ mixture by introducing fluorochemical stabilizers into the liquid CO₂ and then bubbling N₂ into the liquid to form the N₂/liquid CO₂ foam. The liquid CO₂ was the external phase while the N₂ was the inner face and the fluorochemical stabilizers separated the two phases.

Qichao Lv, under the guidance of Professor Zhaomin Li, at China University of Petroleum investigated the dynamic filtration control performance of N₂/liquid CO₂ foam with a fluorochemical (HFE) as a stabilizer.

The filtration behavior of N₂/liquid CO₂ foam is uncertain as the external phase of the foam is unstable. Temperature and pressure can affect its density, viscosity and phase and changing the flow properties of the foam. Moreover, the behavior of waterless foam with the unique fluorochemical interface is uncertain. The interface may also be a potential pollutant. The experiment seeks to study the factors that may affect the dynamic filtration performance of N₂/liquid CO₂ foam including viscosity, foam quality, temperature, pressure, permeability and the damaging effects of the foam on porous media after filtration.

The setup for the experiment was done as shown in their paper where the preparation for the foaming solution was done and the viscosity measurement and dynamic filtration tests were done. The viscosity measurement results showed that the use of foam enhanced the viscosity of liquid CO₂. The apparent viscosity is related to temperature, pressure and foam quality. The viscosity increased as the foam quality was increased from 31 % to 71 %. The viscosity of high quality foams were 1 order of magnitude larger than that of liquid CO₂ at the same conditions. The apparent viscosity is at a maximum at a foam quality of about 80 % before it starts decreasing as it becomes fragile and sensitive to disturbances such as interactions and pressure fluctuations.

The filtration control performance of the N₂/liquid CO₂ foam was compared to that of liquid CO₂ and a N₂/liquid CO₂ mixture. The results showed that the filtration control properties of the N₂/liquid CO₂ foam was better than the others. The leak off coefficient lowered with an increase in foam quality up to 80 % where it increased. Foams of 50 – 80 % quality had a high filtration performance with permeability change. Low initial foam quality foams had better filtration performance at high pressure difference. As the foam enters the porous media, the liquid part would evaporate hence increasing the foam quality with depth. Damage by the foam on the porous media depends upon the pressure difference between the two sides of the porous media.  Damage is small under low pressure difference as the CO₂ turns to gas under a high pressure media damaging the porous media.

In their study the research team were able to prove that by mixing N₂, liquid CO₂ and HFE, properties such as viscosity and filtration control performance of the resultant N₂/liquid CO₂ foam, increased substantially without damage to porous media.

About the author

Qichao Lv is currently a doctoral candidate at China University of Petroleum, East China and a research scholar of Foam Fluid Enhanced Oil & Gas Production Engineering Research Center in Shandong province. He is also a member of Nano-Technology for Energy and Environment Group in University of Calgary. His primary areas of interest include foam technology for EOR and fracturing. In particular, he has done an excellent work in green and clean foam fracturing for unconventional oil and gas reservoirs such as shale gas, tight sand oil and gas, and CBM formations. He has published more than 20 articles in peer-reviewed scientific journals and applied for 15 patents of China and US. Because of his contribution to the development of unconventional reservoirs, he has won a first prize of provincial science and technology award as first investigator.

Contact: qichaolv@s.upc.edu.cn

About the author

Prof. Zhaomin Li is vice president of the China University of Petroleum, East China and director of Foam Fluid Enhanced Oil & Gas Production Engineering Research Center in Shandong province. His research emphasis is on the flow laws and equipment for foam fluid, new technologies of heavy oil recovery, CCUS theories and their application. In recent years, he has participated in more than 10 national programs as main contributor, and published more than 100 articles, of which 34 articles are indexed by SCI and 36 articles are indexed by EI.

He also holds more than 30 invention patents, and has established 3 standards for oil and gas industry as manager. In addition, he has won three first prizes of provincial science and technology award as first investigator. In particular, one of his invention as participant, HDCS enhanced oil recovery technology for ultra-heavy oil reservoirs, has increased crude oil production by several millions of tons, which was also selected as one of ten chemical technology highlights by China Chemical Industry News in 2010. A serials of foam stimulation techniques he invented are serving B&R countries, which has been reported by the Journal of International Innovation.

Contact: lizhm@upc.edu.cn

Reference

Lv Q, Li Z, Li B, Zhang C, Shi D, Zheng C, Zhou T. Experimental study on the dynamic filtration control performance of N2/liquid CO2 foam in porous media. Fuel. 2017 Aug 15; 202:435-45.

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