“Wet-type” solar cells with semiconductor electrodes

The photovoltages and photocurrents arising from “wet” solar cells of the type, semiconductor electrode¦aqueous solution of electrolyte¦platinum electrode, have been measured and discussed. Earlier work by the present authors and others on such “wet” solar cells using a semiconductor electrode is briefly and critically reviewed. The effect of dyes, either dissolved in the solution or coated on the electrode surface, was studied in detail including the effect of reducing agents, which turned out to act as electron suppliers. The photovoltaic effects in nonaqueous solutions were also studied. The photocurrent efficiencies of the dye-sensitized cells were improved by increasing the quantities of dyes absorbed on the surface and reached 19 per cent for monochromatic illumination at the wavelengths of the absorption peaks of the dyes and under sufficient anodic biases.     

A “once-through” solar water heating system

In China, solar water heater is being popularized and most existing solar water heaters are the natural circulation system. Due to some shortcomings of the natural circulation system, a “once-through” system is proposed. In a once-through system, the storage tank can be placed below the collector, therefore, the load on the roof will be cut down significantly. This system also has the advantages of no mixing of hot and cold water, no reverse flow and being able to provide hot water earlier, etc. Both theoretical and experimental investigations have been conducted to compare the collector efficiences between the once-through and natural circulation systems and they coincide with each other very well. The once-through system performs worse in the morning but better in the afternoon than the natural circulation system and the difference of daily collector efficiency between these two systems is negligible.     

Performance of a “black” liquid flat-plate solar collector

A cost-effect, “black” liquid, flat-plate solar collector has been designed, and prototypes have been built and tested. In these collectors a highly absorbent “black” liquid flows in transparent channels and directly absorbs solar energy. The liquid is the hottest substance in the collector, and no metals are required anywhere in the design. The collector differs in the following ways from conventional flat-plate collectors:

1. 1. Solar radiation is absorbed directly by the black liquid without the need to heat any other structures within the collector.

2. 2. Lower heat losses are possible since energy is absorbed directly by the working fluid, and the flow pattern can be arranged so that the hottest spot is in the center of the collector away from all edges. As the fluid moves progressively inward toward the exit, which is located at the center of the collector, it will pick up some of the heat loss along the radial direction.

3. 3. Lower cost may be possible since no metal is required in construction and only glass and/or plastic need be used in addition to the insulation and frame. The absence of metal should eliminate all corrosion problems.

4. 4. New avenues of research are opened up by the use of black liquids: an entirely new class of materials are available which may aid in finding inexpensive, durable absorbers.

5. 5. New configurational arrangements are possible with the absence of metal absorbers.

Experimental performance data for the black liquid collector is presented which compares favorably with other conventional flat-plate collectors.     

Performance of the “shower” cooling tower in different climates

The paper summarizes experimental data from testing a new type of a passive cooling system developed by the author, the “Shower” tower, and compares its performance under three different climatic conditions: Los Angeles in California, USA, Riyadh (Saudi Arabia) and Yokohama (Japan).     

Stable selective coating “black nickel” for solar collector surfaces

Electrodeposited black nickel coatings plated from chloride baths on metallic substrates have high solar absorptance coefficients (> 0,92) and low thermal emittances (< 0,15). The black nickel selective coating consisted of two layers of different porosity with non-overlapping pores. This coating is very stable under thermal implication and humid conditions.     

Towards high-efficiency thin-film silicon solar cells with the “micromorph” concept

Tandem solar cells with a microcrystalline silicon bottom cell (1 eV gap) and an amorphous-silicon top cell (1.7 eV gap) have recently been introduced by the authors; they were designated as “micromorph” tandem cells. As of now, stabilised efficiencies of 11.2% have been achieved for micromorph tandem cells, whereas a 10.7% cell is confirmed by ISE Freiburg. Micromorph cells show a rather low relative temperature coefficient of 0.27%/K. Applying the grain-boundary trapping model so far developed for CVD polysilicon to hydrogenated microcrystalline silicon deposited by VHF plasma, an upper limit for the average defect density of around 2 × 10¹⁶/cm³ could be deduced; this fact suggests a rather effective hydrogen passivation of the grain-boundaries. First TEM investigations on μc-Si : H p-i-n cells support earlier findings of a pronounced columnar grain structure. Using Ar dilution, deposition rates of up to 9 Å/s for microcrystalline silicon could be achieved.     

Apparent “gettering” of the Staebler-Wronski effect in amorphous silicon solar cells

The stability behaviour of intrinsic amorphous silicon materials incorporated in a p⁺-i-n⁺ solar cell structure is considerably different from that observed by electrical characterization methods in individual thin films. This is due to the fundamental difference in Fermi-level position in a single layer compared to the situation occuring in devices. We have employed the differences in the re-equilibration behaviour that have been observed in various intrinsic materials when the Fermi-level is shifted towards the valence band edge, in order to design a cell with a new profiled i-layer which would possess an improved electric field distribution after light soaking compared to cells with a constant i-layer. The contribution of the interface region to the stabilized conversion efficiency is greatly improved, whereas the first 50 nm of the cell structure remain unchanged. Thus, it appears that the Staebler-Wronski effect is gettered away from the junction, much like the impurity gettering concept in crystalline solar cells.     

A note on the formation of “salt-finger” and “diffusive” interfaces in three-component systems

Thin density interfaces determine the fluxes of heat or solute through doubly-diffusive convection. Vertical transports are achieved by either “salt finger” convection or molecular diffusion. The influence of a third diffusing property upon the type of interface formed at an initial density discontinuity is explored here. There may be a strong dependence upon molecular diffusivities, and some interfaces are observed to have a complicated structure.     

The derivations of several “Plate-efficiency factors” useful in the design of flat-plate solar heat collectors

All variables influencing the efficiency of a flat-plate solar heat collector as a heat exchanger can be combined into a single “efficiency factor.” These efficiency factors are more or less design constants of the particular collector design, and are only slightly influenced by operating conditions. Consequently they are extremely convenient for use in accurate design and performance calculations. The full mathematical derivations are presented for several of these efficiency factors for various types of collectors, together with graphical data and examples of their use.     

The “shell” dryer—modelling and experimentation

The “shell” dryer is an African solar food dryer which uses natural convection. Its design has been defined by users, in terms of local working conditions and ease of use. However, it is necessary to optimize the design of the dryer's air flow section for best performance. We present here a theoretical study of a simple process using mathematical modelling tools.     

European solar standards : Final approval of CEN/TC312 “Thermal solar systems and components”

A powerful tool for the promotion of the use of the solar thermal systems is the existence of technical standards and regulations for these industrial products. These technical specifications permit the quality improvement of the products and, consequently, the increase of the consumers' confidence to this technology. Eva Kotsaki, CEN/TC 312 Secretary, CRES — Energy Policy and Planning Division, Greece provides an update on the current position regarding European solar thermal standards.     

Thermal evaluation of a “Winter House”

In this paper, the transient thermal model of a “Winter House” has been presented for a cold climatic region of Srinagar in India. In order to reduce the fluctuations in living room temperature, the effect of an isothermal mass has been incorporated in the thermal model. An analytical expression for room temperature has been derived to evaluate the performance of the “Winter House” by incorporating the effect of movable insulation during the night. Numerical calculations were done in terms of system as well as climatical parameters. Some isothermal mass, like water, has been considered to stabilize the room temperature throughout the day and right. For further heating, the effect of the roof as an air collector has also been taken into account.     

Mesoscale modeling of a “Dunkelflaute” event

In the near future, wind and solar generation are projected to play an increasingly important role in Europe's energy sector. With such fast‐growing renewable energy development, the presence of simultaneous calm wind and overcast conditions could cause significant shortfalls in production with potentially serious consequences for system operators. Such events are sometimes dubbed “Dunkelflaute” events and have occurred several times in recent history. The capabilities of contemporary mesoscale models to reliably simulate and/or forecast a Dunkelflaute event are not known in the literature. In this paper, a Dunkelflaute event near the coast of Belgium is simulated utilizing the Weather Research and Forecasting (WRF) model. Comprehensive validation using measured power production data and diverse sets of meteorological data (e.g., floating lidars, radiosondes, and weather stations) indicates the potential of WRF to reproduce and forecast the boundary layer evolution during the event. Extensive sensitivity experiments with respect to grid‐size, wind farm parameterization, and forcing datasets provide further insights on the reliability of the WRF model in capturing the Dunkelflaute event.     

Salt gradient solar pond with reflective bottom: Application to the “saturated” pond

The “saturated” salt gradient solar pond operates near the solubility limit. Consequently, temperature fluctuations may cause precipitation of the salt, which can increase the reflectivity of the bottom. It is shown that this can reduce the width of the nonconvective zone and seriously degrade the performance of the pond. The temperature distribution, efficiency and optimum operating conditions are calculated, taking account of diffusely reflected light from the bottom of the pond. The mechanism for narrowing the nonconvective zone is described. A semiquantitative analysis is made of a known case of simultaneous salt precipitation and nonconvective zone destruction. It is argued that the boundary between the nonconvective zone and the lower convective zone will move to its maximum temperature position if the solubility is a sufficiently strongly increasing function of temperature.