Performance of small-scale bladeless electromagnetic energy harvesters driven by water or air

For solar Rankine cycle combined heat and power systems for residential buildings and other small-scale applications (producing 1-10 kWe), a low manufacturing cost, robust, and durable expander is especially attractive. The Tesla-type turbine design has these desired features. This paper summarizes a theoretical exploration of the performance of a Tesla turbine as the expander in a small-scale Rankine cycle combined heat and power system. A one-dimensional idealized model of momentum transfer in the turbine rotor is presented, which can be used to predict the efficiency of the turbine for typical conditions in these systems. The model adopts a nondimensional formulation that identifies the dimensionless parameters that dictate performance features of the turbine. The model is shown to agree well with experimental performance data obtained in earlier tests of prototype Tesla turbine units. The model is used to explore the performance of this type of turbine for Rankine cycle applications using water as a working fluid. The model indicates that isentropic efficiencies above 0.75 can be achieved if the operating conditions are tailored in an optimal way. The scalability of the turbine design, and the impact of the theoretical model predictions on the development of solar combined heat and power systems are also discussed.

Assessment of Tesla Turbine Performance for Small Scale Rankine Combined Heat and Power Systems

The flow induced by rotating discs has attracted some of the greatest minds in fluid dynamics like von Karman and Batchelor, and still is a vigorously active research area. In comparison, the available analysis of the rotating flow in the narrow gaps among closely-spaced co-axial multiple discs of a Tesla turbine, which produces power, is limited. In this paper a simple theory has been presented that describes the three-dimensional fields of velocity and pressure in the Tesla disc turbine. The theory gives the torque and power output which have been verified by comparing the theoretical predictions with recently published experimental results. The governing conservation equations have been cast in a form that makes it possible to formulate analytical solutions and to develop clear physical interpretation for each term in the equations. Thus the roles of each of the centrifugal, Coriolis, inertial and viscous forces in generating torque and power, and in establishing the pressure field have been comprehensively investigated and explained here. This physical exposition of the rotating flow in a Tesla disc turbine has been achieved for the first time in the present paper. Several subtle flow physics and fluid dynamic behaviors have been elucidated. As an example, it is shown here that a Tesla disc turbine may generate net torque and power even when the tangential fluid speed at the disc periphery is less than the local tangential speed of the disc. The subtle role of the Coriolis acceleration in establishing such flow conditions, which involve flow reversal and complex pathlines, has been explained.

The fluid dynamics of the rotating flow in a Tesla disc turbine

Investigations of a new free piston expander engine cycle

In the present article, a mathematical theory for the flow field within a Tesla disc turbine has been formulated in the appropriate cylindrical co-ordinate system. The basis of the theory is the Navier–Stokes equations simplified by a systematic order of magnitude analysis. The presented theory can compute three-dimensional variation of the radial velocity, tangential velocity and pressure of the fluid in the flow passages within the rotating discs. Differential equations as well as closed-form analytical relations are derived. The present mathematical theory can predict torque, power output and efficiency over a wide range of rotational speed of the rotor, in good agreement with recently published experimental data. The performance of the turbine is characterized by conceptualizing the variation of load through the non-dimensional ratio of the absolute tangential velocity of the jet and the peripheral speed of the rotor. The mathematical model developed here is a simple but effective method of predicting...

A theory of Tesla disc turbines

This article discusses how to achieve maximum theoretical effectiveness when using a viscous flow turbine such as Tesla turbine.

On Scaling Down Turbines to Millimeter Size

Atmospheric transport and wet deposition have contributed to the worldwide distribution of per- and polyfluoroalkyl substances (PFAS) in terrestrial and aquatic ecosystems, even in remote areas far from known industrial sources. However, little is known regarding the impact of cloud and precipitation formation dynamics on PFAS transport and wet deposition, nor the range of variability in PFAS concentrations within a closely distributed monitoring network. Precipitation samples were collected from a network of 25 stations in a focused geographic region (the Commonwealth of Massachusetts, USA) from stratiform and convective storm systems to evaluate if the different cloud and precipitation formation mechanisms in these two fundamental types of storm systems influenced PFAS concentrations in precipitation, and to assess the range of variability in PFAS concentrations in precipitation at a regional scale. PFAS were detected in 11 of 50 discrete precipitation events. Of the 11 events from which PFAS were detected, 10 of the events were convective in nature. PFAS were detected during only one stratiform event at one station. This suggests that local and regional atmospheric PFAS sources entrained by convection events controls regional atmospheric PFAS flux, and that PFAS flux estimates should consider the type and magnitude of precipitation events. The PFAS detected were primarily perfluorocarboxylic acids, with relatively higher detection frequency for shorter-chained compounds. Compilation of PFAS data from precipitation across primarily the eastern United States, from urban, suburban, and rural areas, including those in industrial areas, indicates population density is a poor predictor of precipitation PFAS concentrations. While the total PFAS concentration in precipitation in some areas exceeds 100 ng/L, the median concentrations across all areas are generally less than about 10 ng/L.

Ralph D. Collins

Papers

On Scaling Down Turbines to Millimeter Size

Influence of convective and stratiform precipitation types on per- and polyfluoroalkyl substance concentrations in rain.