Selectively coated high efficiency glazing for solar-thermal flat-plate collectors

In order to increase the efficiency of solar-thermal flat-plate collectors at temperatures above 100 °C or with low solar irradiation, we implement a double glazing with a low-emitting (low-e) coating on the inner pane to improve the insulation of the transparent cover. Since commercially available low-e glazing provides only insufficient solar transmittance for the application in thermal flat-plate collectors we are developing a sputter-deposited low e-coating system based on transparent conductive oxides which provides a high solar transmittance of 85% due to additional antireflective coatings and the use of low-iron glass substrates. Durability tests of the developed coating system show that our low e-coating system is well suitable even at high temperatures, humidity and condensation.     

An experimental study on energy generation with a photovoltaic (PV)–solar thermal hybrid system

A hybrid system, composed of a photovoltaic (PV) module and a solar thermal collector is constructed and tested for energy collection at a geographic location of Cyprus. Normally, it is required to install a PV system occupying an area of about 10 m² in order to produce electrical energy; 7 kWh/day, required by a typical household. In this experimental study, we used only two PV modules of area approximately 0.6 m² (i.e., 1.3×0.47 m²) each. PV modules absorb a considerable amount of solar radiation that generate undesirable heat. This thermal energy, however, may be utilized in water pre-heating applications. The proposed hybrid system produces about 2.8 kWh thermal energy daily. Various attachments that are placed over the hybrid modules lead to a total of 11.5% loss in electrical energy generation. This loss, however, represents only 1% of the 7 kWh energy that is consumed by a typical household in northern Cyprus. The pay-back period for the modification is less than 2 years. The low investment cost and the relatively short pay-back period make this hybrid system economically attractive.     

Optimal time-invariant operation of a power and water cogeneration solar-thermal plant

Conceptual design, system-level models, and optimization of operation are presented for a cogeneration solar-thermal plant. The solar-thermal energy collected and concentrated in a salt pond is used in a regenerative Rankine steam cycle with an extraction turbine to produce electricity and process steam. The desalination system is based on reverse osmosis (RO) and multi-effect distillation (MED). An equation-oriented modeling environment is used for the development of time-dependent system-level models required for optimization of the plant. A meteorological radiation model is used to estimate the hourly distribution of beam radiation as a function of time (day and hour), location, and local weather (mainly visibility and humidity). A recently developed model is used to estimate the field efficiency, including projection losses and shading/blocking for a given heliostat layout. Time-invariant optimal operating conditions are presented for a summer day, considering Cyprus as a case study. Seawater desalination processes, RO and MED, are modeled by adapting and extending models from the literature. A control-volume model is developed for the steam cycle based on the first and second law, with given isentropic efficiencies, turbine leaks, and a detailed model for thermodynamic properties of steam/water. This model is validated and allows for optimization over a wide range of operating conditions, e.g., various extraction pressures. The optimization problem is formulated as a nonlinear program (NLP) with dynamics embedded and a heuristic global optimization approach is used. The sequential method of optimization is used, decoupling the simulation from the optimization. The results show that for the plant size considered (4 MW_e equivalent nominal capacity) and the MED design chosen based on the literature and industry practice, RO is preferred over MED from an energy point of view. In addition, under the current feed-in tariff (FiT) and water prices in Cyprus, extracting steam for MED is not recommended. In contrast, if current market prices for electricity and water in Cyprus are used, i.e., FiT is neglected, with a typical steam cycle design, extracting steam for MED at low pressures yields maximum income. A new process configuration is presented based on the findings from the case studies, resulting in significantly higher income and exergetic efficiencies.     

Second-law analysis of solar-thermal processes

The second law of thermodynamics provides an analytic framework for the assessment of the potential displacement of fossil fuels by solar energy. the most promising areas are those which have entropy levels corresponding to the entropy level of the solar resource as converted to heat in various types of solar collectors. Since the entropy of solar heat can be partitioned by the means of collection—d.g., by the collector concentration ratio—solar can be matched much more precisely to many tasks at temperatures up to 300°C than can fossil fuels which are low entropy sources now widely misused for high entropy tasks.     

Reduced graphene oxide modified melamine sponges filling with paraffin for efficient solar-thermal conversion and heat management

A novel type of reduced graphene oxide(rGO) modified melamine sponges(rGS) filling with paraffin(rGS-pf) is developed for efficient solar-thermal conversion and heat management.The micro structures,filling and holding capacity of paraffin in porous rGS,solar-thermal energy conversion and energy harvesting efficiency of the prepared rGS-pf have been investigated systematically.The content of rGO nanosheets coated on the skeletons of rGS-pf is only 0.11%,while the loading content of paraffin in th...     

Optimization of AlxOy/Pt/AlxOy multilayer spectrally selective coatings for solar-thermal applications

Spectrally selective Al_xO_y/Pt/Al_xO_y multilayer absorber coatings were deposited onto corning 1737 glass, Si (111) and copper substrates using electron beam (e-beam) vacuum evaporator at room temperature. The employment of ellipsometric measurements and optical simulation was proposed as an effective method to optimize and deposit multilayer solar absorber coatings. The optical constants (n and k) measured using spectroscopic ellipsometry, showed that both Al_xO_y layers, which used in the coatings, were dielectric in nature and the Pt layer was semi-transparent. The optimized multilayer coatings exhibited high solar absorptance α ∼ 0.94 ± 0.01 and low thermal emittance ? ∼ 0.06 ± 0.01 at 82 °C. The Rutherford backscattering spectroscopy (RBS) data of Al_xO_y/Pt/Al_xO_y multilayer absorber indicated the Al_xO_y layers present in the coating were nearly stoichiometry. The scanning electron microscope analysis (SEM) result indicated that the average diameter and inter-particles distance of Pt grains were statistically about 146 ± 0.17 nm and 6-10 ± 0.2 nm respectively.