As a research focus, energy conservationhas attracted a great deal of attention in recent years due to theenergy crisis and environmental pollution concerns. Many countriesmade great efforts on both research and implementing energy conservationtechnologies. Currently more than 50% of the total world’s deliveredenergy is consumed by industry and about one sixth of the total energyconsumed is wasted as low-grade heat, for example through radiationloss, exhaust gas flows, and cooling fluid circuits. Therefore, therecovery and reuse of waste heat is an effective way to significantlyimprove energy utilization. In addition, solar energy can providelow grade heat and is a clean and renewable form of energy. The efficientuse of low grade heat from these sources can play an important rolefor a large number of applications.
Thermal driven cooling, heating andpower generation technology provides a means to practically and efficientlyuse low grade heat. Performance improvement, flexible operation ofhybrid systems, and low capital and operating costs are necessaryto ensure low grade heat recovery and reuse technologies are attractiveto end users. In this thematic special issue, seven invited paperscontribute new research and knowledge on optimized cycles of heating,cooling and power generation, as well as the feasibility for wasteheat reuse. Four papers focus on the development of solar systems.
A high temperature heat pump is anefficient technology to upgrade waste heat or reuse by the industrialprocesses. Z.Y. Xu and his coworkersproduce a comprehensive review of thermally driven absorption heatpumps for different applications, including heat amplification andtemperature lift cycles, working pairs and current projects. Theyconclude that large temperature lift heat pumps and open-cycle absorptionheat transformers, with direct contact with exhaust gas, should befurther studied and developed. Organic working pairs with good stability,no corrosion and good thermophysical properties are necessary to bestudied. H.S. Bao and her coworkerspropose a hybrid absorption-compression, high temperature heat pumpfor the recovery of industrial waste heat of 60°C–120°C.The cycle allows a flexible operation with energy input from wasteheat and electrical power. The study gives a numerical analysis andseveral features such as recirculation flow ratio are presented. B. Hu and his colleagues present an exergyanalysis for a multi-stage compression heat pump used for high temperatureheating applications with R1234ze(Z) as the refrigerant, which hasa low GWP value. The analysis is conducted on the basis of producingpressurized water at 120°C under different waste heat conditions.Single, double and three stage compression heat pumps are comparedand this indicates a multi-stage compression heat pump has less powerconsumption. A three-stage compression heat pump system has obviousadvantage of higher exergy efficiency. In addition to absorption andcompression heat pumps, a chemical heat pump provides another wayof upgrading heat with a large temperature lift. M. Xu and his coworkers carry out an exergy analysis onan Isopropanol-Acetone-Hydrogen chemical heat pump which can upgradewaste heat of 80°C–110°C to 200°C. The highest irreversibilityis in the distillation column and reactive distillation is an effectivealternative. The operating conditions could be optimized to obtaingreater thermodynamic performance.
Besides using heat pump technology,waste heat can be converted to produce cooling and electrical power. Y. Lu and his coworkers present a simulationstudy for a 1 kW organic Rankine cycle using a scroll expander torecover waste heat from the cooling system and exhaust gas of an engine.They pointed out a potential solution to fully recover engine coolantenergy with the penalty of reduced ORC thermal efficiency. The proposedsolution has the advantages of higher power output and less rejectedheat from the engine radiator compared with that of the engine ORCsystem only using the coolant energy as preheat source. A recuperatoris suggested to be added to the ORC system when the working fluidsare isentropic or dry types. When the engine is operated under ratedpower condition, the integrated waste heat recovery system can potentiallyimprove the overall system performance by 9.3%.
J. Cho and his coworkers develop a small-scale, multi-purpose experimentalS-CO2 system using a high-speed radial-typeturbo-generator. A preliminary experiment test is conducted usingR134a as a working fluid to determine the operational characteristicsof the closed Rankine cycle. A turbine power of 400 W is successfullygenerated. P. Gao and his colleaguespropose a solid sorption combined cooling and power system using thewaste heat from the exhaust gas of a vehicle. The cogeneration systemuses MnCl2/CaCl2-NH3 working pairs and its feasibility is demonstrated experimentally.
For solar systems, heat collectionand heat transfer issues are important to improve system performanceand to make system operation reasonable and stable. S. Tang and his colleagues review different approachesfor solar fuel production from spectrum-selective photo-thermal synergeticcatalysis. The meaning of synergetic effects, the mechanisms of spectrum-selectivityand photo-thermal catalysis are introduced and a number of experimentalor theoretical works are sorted by the chemical reactions and thesacrificial reagents applied. The studies are summarized based onthe operating conditions, spectrum-selectivity, materials and productivity.They suggest that the optimizations on materials and structure ofcatalysts and deeper understanding of the reaction mechanism, especiallythe photo-thermal synergy effects are necessary to be given more academicattentions. On the performance of direct steam generation solar powertower plant, Y. Luo and his coworkersstudy the impacts of solar multiple and thermal storage capacity,which are two key design parameters for the sensitivity analysis ofthe annual plant performance and the economic assessment. The analysisgives the effects of site, solar field equivalent electricity sizeand investment costs on the minimum levelized cost of electricity,optimal solar multiple and thermal storage capacity, which is basedon the reference cases. T.J. Wang and his colleagues present an experimental study on a quartz tubefalling particle receiver. Silicon carbide particles are proposedto be the heat transfer fluid when the temperature is higher than600°C in which case molten nitrate salt becomes chemically unstable.Particle receivers have the potential to increase the maximum temperatureof the heat-transfer media to over 1000°C. Experiments are carriedout to test the dynamic thermal performance of the receiver. The experimentalstudy focuses on the effect of particle diameter, particle inlet temperature,particle flow rate and type of the quartz tube on outlet particletemperature. It provides a novel strategy for the development of ahigh temperature heat transfer fluid. B.J. Lougou and his coworkers conduct a numerical analysis of radiation heattransfer and temperature distributions for a solar thermochemicalreactor used for syngas production. Finite volume discrete ordinatemethod and P1 approximation for radiation heat transfer are employed.The study reveals that the temperature drop due to the boundary radiationheat loss could not be neglected for the thermal performance analysisof the solar thermochemical reactor.
We hope these papers will provideyou with a good overview of the technologies which can be utilizedfor the efficient use of waste heat and solar energy. The coverageis limited but we hope that it inspires more research explorationin this extremely important field.
Higher Education Press and Springer-Verlag GmbHGermany 2017
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