Advanced Energy Conversion最新文献

Measurements of Hall effect, conductivity, and thermoelectric figure of merit $\text{L}$ were made over the temperature range 77–300°K on Bi₂Te₃ samples which had been sintered for 1, 5, and 30 hr, and on a Bi₂Te₃ single crystal which had been prepared by the Bridgman method. Conductivity, magneto-resistance and $\text{L}$ were also measured over the same temperature range on Bi₉₅Sb₅ samples qhich had been sintered for 5 and $\text{10}\text{1}\text{4}$ hr. The latter results were compared with data reported by Smith and Wolfe [4] for a Bi₉₅Sb₅ single crystal. In all cases, $\text{L}$ was lower for the sintered samples than for the corresponding single crystals. Over the above temperature range, the $\text{L}$ of the best sintered specimen of Bi₂Te₃ varied from 0 · 5 × 10⁻³/°K to 1 × 10⁻³/°K, and the $\text{L}$ of the sintered damples of Bi₉₅Sb₅ ranged from 1 · 38 × 10⁻³/°K to 0 · 52 × 10⁻³/°K.

A theoretical evaluation of a heterojunction photovoltaic cell is presented. Short circuit current densities, collection efficiencies, open circuit voltages, and maximum power outputs are calculated and compared to those of equivalent homojunction cells made of the two semiconductor materials forming the heterojunction. The analysis considers the abrupt p-n heterojunction and treats only the case of uniform illumination. Open circuit voltages and short circuit currents, as derived, cannot exceed the values obtained in the equivalent homojunction cells. Cell efficiencies for heterojunctions are thus bounded by the efficiencies of the corresponding homojunctions and any advantages of the cells are limited to those arising out of the window effect (low energy photons pass through one semiconductor to generate carriers in the junction region of the other): i.e., reduced surface recombination losses, and lower sheet resistances.

Efficient design of thermoelectric devices has resulted in definite contact resistance criteria. These criteria, however, are based on room temperature data. This paper reports measurements of the contact resistance (up to 800°K) of iron-plated lead telluride. Two idealized theoretical models are prepared to explain this temperature dependence. The results agree that the temperature exponent is approximately 2·5. For typical thermoelectric materials this indicates that the temperature dependence of the contact resistance is essentially the same as that of bulk mobility.

Completely gas-tight high-temperature cells can be constructed by using electrolytes in so-called paste form. The paste is a blend of 70−50 weight per cent of an inert finely powdered material such as MgO, and 30−50 per cent of a lithium-sodium-potassium carbonate melt. It behave as a deformable solid with very low specific resistance.

In combination with nickel powder (or sieve) fuel electrodes and silver powder (or screen) air electrodes H₂ + CO₂ mixtures, as well as hydrocarbons, with added steam, can be utilized in such cells at 600–700°C. The cell life appears to be related to the current density drawn. At 25 mA/cm², practically constant voltages (0·85−0·90 V) can be maintained for periods of more than three months. Until recently, however, current densities of 100–150 mA/cm² caused severe deterioration of the fuel electrode within one or two weeks, at 700°C.

By improvement of the paste-electrolyte properties, the authors recently succeeded in avoiding the mentioned instability to a great extent. With essentially unmodified silver and nickel electrodes, continuous currents of 100 mA/cm² at about 700 mV, may be drawn for at least 3100 hr. †

The corresponding specific power output of 0·70 kW/m² reaches the level at which at least the cost price of the electrodes and the electrolyte remains below $75/kW.

At this power level the construction of a 5–10 kW battery becomes attractive, since it is felt that only experimental study on such a unit may reveal a realistic estimate of the true costs.

The recent discoveries of huge quantities of natural gas in the Dutch province of Groningen—at least gas—are a great impetus for studies of this kind.

A model is developed for analysis of porous gas electrodes in which an electrolyte film exists on the walls for part of the gas filled section of the pore. The influence of electrode kinetics and transport of current in the film is considered along with that of dissolved gas diffusion. It is found that the reaction occurs over lengths of film from the intrinsic meniscus equal to thousands of film thicknesses and that diffusion of dissolved gas is not controlling under most circumstances. An example based on the oxygen electrode in KOH solution is treated.

A rig has been developed for the testing of nuclear heated thermionic converters, which allows direct determination of the converter efficiency by measuring the waste heat flow through three thermal bridges. A 50 hr in-pile test of the device was performed in December, 1963. The converter consisted of a cylindrical molybdenum emitter and a niobium collector, with an electrode spacing of 0·5 mm. The emitter was heated by radiation from a (UZr)C fuel pin. The converter operated for 45 hr, with electrical power outputs between 30 and 40 W for most of the time, and with efficiencies of up to 11 per cent. At the end of this period a permanent partial short occurred in the converter.

Several high-power communications satellites have been proposed which were based on the availability of an electric power supply which could provide 60 kW for one year (minimum) at the cost of 3000 lb. These were the initial specifications of the dual SNAP-8 nuclear powerplant. Since that time, however, it has become clear that the SNAP-8 will be unable to meet these specifications. The purpose of this paper is to present a conceptual design study for a steam-cycle power supply which provides the required specific weight (around 50 lb/kWe) together with reasonable expactation of obtaining the required one-year minimum lifetime.

The proposed cycle is the conventional Rankine steam cycle utilizing superheated turbine-inlet steam at 1200 psi and 1200°F. After removal of residual superheat in a recuperator, the saturated vapor at the turbine exhaust is condensed in a radiator at approximately 400°F and returned by a pump to the energy source.

At first glance, it does not appear possible to reject heat at such a low temperature without enormous radiator weights. Indeed, the radiator surface area is considerably larger than that required for the much higher temperature liquid metal or gas cycles. However, the unique combination of using steam at low temperatures permits the utilization of two design features which provide remarkably low radiator weight per unit area in the conventional flat fin-and-tube configuration. First, the high latent heat of the condensing steam allows only small volumetric through-put per kilowatt output, resulting in very small tuve diameters. Since the tubes and headers are the only major parts of the radiator which require meteoroid armor, the total tube and header weight does not become excessive. Second, because of the low temperature, it is permissible to use aluminum as the radiator material. Its high thermal conductivity therefore permits the use of quite thin, large-area fins between the tubes without suffering the conduction loss necessitated by the higher-temperature materials of other systems. Thus the fraction of radiator are occupied by the heavy, aluminum-armored tubes becomes quite small. In the sample design (30 kW) presented in this paper, the combination of small tubes and large fins results in a weight per unit radiating area of less then 0·25 lb/ft² of radiating area.

The energy source may be either a nuclear reactor, the most favorable configuration of which would be a single-pass coiled-tube design (although more conventional boiler-superheater reactors may be used with little weight penalty), or a solar-powered boiler-superheater. Other cycle components are a conventional turbogenerator, recuperator and pump. The specific weight of the sample (30kW) design is 70lb/kW, but upon scaleup to higher powers, improved turbine efficiency, radiator segmentation, and reduced fractional weight of the energy source and turbine can provide estimated specific weights competitive wi