Application of solar collectors for hot water supply, space heating, and cooling plays a significant role in reducing building energy consumption. For conventional solar collectors, solar radiation is absorbed by spectral selective coating on the collectors’ tube/plate wall. The poor durability of the coating can lead to an increased manufacturing cost and unreliability for a solar collector operated at a higher temperature. Therefore, a novel nanofluid-based direct absorption solar collector (NDASC) employing uncoated collector tubes has been proposed, and its operating characteristics for medium-temperature solar collection were theoretically and experimentally studied in this paper. CuO/oil nanofluid was prepared and used as working fluid of the NDASC. The heat-transfer mechanism of the NDASC with parabolic trough concentrator was theoretically evaluated and compared with a conventional indirect absorption solar collector (IASC). The theoretical analysis results suggested that the fluid’s temperature distribution in the NDASC was much more uniform than that in the IASC, and an enhanced collection efficiency could be achieved for the NDASC operated within a preferred working temperature range. To demonstrate the feasibility of the proposed NDASC, experimental performances of an NDASC and an IASC with the same parabolic trough concentrator were furthermore evaluated and comparatively discussed.

The solar gravity heat pipe has been widely used for solar thermal water heating because of its high efficient heat transfer and thermal diode characteristics. Operated on fluctuant and low intensity solar radiation conditions, a solar gravity heat pipe may frequently start up. This severely affects its solar collection performance. To enhance the thermal performance of the solar gravity heat pipe, this study proposes using graphene/water nanofluid as the working fluid instead of deionized water. The stability of the prepared graphene/water nanofluid added with PVP was firstly investigated to obtain the optimum mass ratios of the added dispersant. Thermophysical properties—including the thermal conductivity and viscosity—of nanofluid with various graphene nanoplatelets (GNPs) concentrations were measured at different temperatures for further analysis. Furthermore, based on the operational evaluation on a single heat pipe’s start-up process, the performance of nanofluid-enhanced solar gravity heat pipes using different concentrations of GNPs were compared by using water heating experiments. Results indicated that the use of 0.05 wt % graphene/water nanofluid instead of water could achieve a 15.1% and 10.7% reduction in start-up time under 30 and 60 W input heating conditions, respectively. Consequently, a higher thermal efficiency for solar collection could be expected.

Cryogenic CO₂ capture is considered as a promising CO₂ capture method due to its energy saving and environmental friendliness. The phase equilibrium analysis of CO₂-mixtures at low temperature is crucial for the design and operation of a cryogenic system because it plays an important role in analysis of recovery and purity of the captured CO₂. After removal of water and toxic gas, the main components in typical boiler gases are N₂/CO₂. Therefore, this paper evaluates the reliabilities of different cubic equations of state (EOS) and mixing rules for N₂/CO₂. The results show that Peng-Robinson (PR) and Soave-Redlich-Kwong (SRK) fit the experimental data well, PR combined with the van der Waals (vdW) mixing rule is more accurate than the other models. With temperature decrease, the accuracy of the model improves and the deviation of the N₂ vapor fraction is 0.43% at 220 K. Based on the selected calculation model, the thermodynamic properties of N₂/CO₂ at low temperature are analyzed. According to the results, a new liquefaction combined with a desublimation system is proposed. The total recovery and purity of CO₂ production of the new system are satisfactory enough for engineering applications. Additionally, the total energy required by the new system to capture the CO₂ is about 3.108 MJ·kg⁻¹ CO₂, which appears to be at least 9% lower than desublimation separation when the initial concentration of CO₂ is 40%.

This article presents a numerical study of a new packed bed using a stratified phase change material (PCM). The PCM is injected into polycarbonate spheres as phase change capsules. Water is used as heat transfer fluid (HTF) for charging or discharging thermal energy to the PCM. Three kinds of paraffin that have different melting points are used as the PCM and placed in the packed bed at different heights. The PCM at the higher melting temperature is placed nearer to the hot water inlet. The numerical calculation of HTF is based on the one-dimensional Schumann's model, while the temperature of the PCM is simulated using the apparent heat capacity method. Both charge and discharge processes of this new stratified packed bed have been studied. The effects of HTF average velocity and inlet temperature have also been investigated. Both the energy and exergy performance are compared with those of the packed bed using only one kind of PCM. The comparison results indicate that this new stratified packed bed has an advantage over traditional packed bed in terms of energy and exergy.