Performance analyses of a novel finned parabolic trough receiver with inner tube for solar cascade heat collection

Designing highly-efficient parabolic trough receiver(PTR)contributes to promoting solar thermal utilization and alleviating energy crisis and environmental problems.A novel finned PTR with inner tube(FPTR-IT),which can provide different grades of thermal energy with two heat transfer fluids(oil and water),is designed to improve thermal efficiency.In this FPTR-IT,an inner tube and straight fins are employed to respectively lessen heat loss at upper and lower parts of the absorber.Based on the design,a numerical model is developed to investigate its performance.Comparisons with other PTRs indicate that the FPTR-IT can combine the advantages of PTR with inner tube and finned PTR and obtain the best performance.Moreover,performance evaluation under broad ranges of direct normal irradiances(300–1000 W/m^(2)),flow rates(50–250 L/min)and inlet temperatures(400–600 K)of oil as well as flow rates(3.6–10 L/min)and inlet temperatures(298.15–318.15 K)of water is investigated.Compared with conventional PTR,heat loss is reduced by 20.7%–63.2%and total efficiency is improved by 0.03%–4.27%.Furthermore,the proportions of heat gains for water and oil are located in 8.3%–73.9%and-12.0%–64.3%,while their temperature gains are located in 11.6–37.9 K and-1.2–19.6 K,respectively.Thus,the proposed FPTR-IT may have a promising application prospect in remote arid areas or islands to provide different grades of heat for electricity and freshwater production.

A Comprehensive Study of a Low-Grade Heat-Driven Cooling and Power System Based on Heat Current Method

Combined cooling and power(CCP)system driven by low-grade heat is promising for improving energy efficiency.This work proposes a CCP system that integrates a regenerative organic Rankine cycle(RORC)and an absorption chiller on both driving and cooling fluid sides.The system is modeled by using the heat current method to fully consider nonlinear heat transfer and heat-work conversion constraints and resolve its behavior accurately.The off-design system simulation is performed next,showing that the fluid inlet temperatures and flow rates of cooling water as well as RORC working fluid strongly affect system performance.The off-design operation even becomes infeasible when parameters deviate from nominal values largely due to limited heat transfer capability of components,highlighting the importance of considering heat transfer constraints via heat current method.Design optimization aiming to minimize the total thermal conductance is also conducted.RORC efficiency increases by 7.9%and decreases by 12.4%after optimization,with the hot fluid inlet temperature increase from 373.15 to 403.15 K and mass flow rate ranges from 10 to 30 kg/s,emphasizing the necessity of balancing system cost and performance.

Research trend of cascade heat pumps

Most commercial and industrial facilities require very low temperatures for refrigeration and high temperatures for space heating and hot water purposes. Single stage heat pumps have not been able to meet these temperature demands and have been characterized by low capacities and coefficient of performance(COP). Cascade heat pump has been developed to overcome the weaknesses of single stage heat pumps. This study reviews recent works done by researchers on cascade heat pumps for refrigeration, heating and hot water generation. Selection of suitable refrigerants to meet the pressure and temperature demands of each stage of the cascade heat pump has been discussed. Optimization of design parameters such as intermediate temperature(low stage condensing and high stage evaporating temperatures), and temperature difference in the cascade heat exchanger for optimum performance of the cascade heat pump has been reviewed. It was found that optimising each design parameter of the cascade heat pump is necessary for maximum system performance and this may improve the exergetic efficiency, especially of cascade refrigeration systems, by about 30.88%. Cascade heat pumps have wider range of application especially for artificial snow production, in the supermarkets,for natural gas liquefaction, in drying clothes and food and as heat recovery system. The performance of cascade heat pumps can be improved by 19% when coupled with other renewable energy sources for various real time applications. Also, there is the need for much research on refrigerant charge amount of cascade heat pumps, refrigerant-refrigerant heat exchangers to be used as cascade heat exchanger, cascade heat pumps for simultaneous cooling, heating and hot water generation and on the use of variable speed compressors and their control strategies in matching system capacity especially at part load conditions.