Elsevier

Applied Thermal Engineering

Volume 110, 5 January 2017, Pages 318-326
Applied Thermal Engineering

Research Paper
Performance estimation of Tesla turbine applied in small scale Organic Rankine Cycle (ORC) system

https://doi.org/10.1016/j.applthermaleng.2016.08.168Get rights and content

Highlights

  • One-dimensional model of the Tesla turbine is improved and applied in ORC system.

  • Working fluid properties and system operating conditions impact efficiency.

  • The influence of turbine efficiency on ORC system performance is evaluated.

  • Potential of using Tesla turbine in ORC systems is estimated.

Abstract

Organic Rankine Cycle (ORC) system has been proven to be an effective method for the low grade energy utilization. In small scale applications, the Tesla turbine offers an attractive option for the organic expander if an efficient design can be achieved. The Tesla turbine is simple in structure and is easy to be manufactured. This paper improves the one-dimensional model for the Tesla turbine, which adopts a non-dimensional formulation that identifies the dimensionless parameters that dictates the performance features of the turbine. The model is used to predict the efficiency of a Tesla turbine that is applied in a small scale ORC system. The influence of the working fluid properties and the operating conditions on the turbine performance is evaluated. Thermodynamic analysis of the ORC system with different organic working fluids and under various operating conditions is conducted. The simulation results reveal that the ORC system can generate a considerable net power output. Therefore, the Tesla turbine can be regarded as a potential choice to be applied in small scale ORC systems.

Introduction

Primary energy consumption is enlarging rapidly with the development of the human society. Energy shortage and environmental deterioration are two consequent crucial issues that the developing world has to face. In order to solve these problems, the utilization of low grade heat sources, such as the geothermal energy [1], [2], the solar energy [3], [4], the biomass energy [5], [6] and the waste heat [7], [8], is attracting broad attention in recent years. Among all of the existing technologies, the Organic Rankine Cycle (ORC) has been proven to be one of the most effective methods for the low grade energy conversion [9], [10], [11], [12], [13], [14]. The axial flow turbine and the radial in-flow turbine are typically selected as the expanders in the ORC system. However, in small scale applications, the traditional organic expanders are not suitable since the flow loss will be considerably large. In addition, the high rotation speed of the traditional turbines under small mass flow rate condition also limits their practical applications. In this case, the Tesla turbine allows a low-cost and reliable design for the organic expander that could be an attractive option for small scale ORC systems.

The Tesla turbine was invented by the famous scientist, Nikola Tesla, in 1913 [15]. It is a kind of turbo-machinery that combines a series of flat parallel discs rather than rotating blades. Thus, the Tesla turbine is called the bladeless turbine as well. The discs distribute co-axially along a shaft such that a small gap is formed between any two adjacent discs. This design makes use of the viscous effect of the working fluid which occurs in the boundary layer flow between the rotating discs. The working fluid flows spirally from the outer part to the inner part and transfers the kinetic energy to the discs. Then the working fluid flows out through the holes located between the inner part of the discs and the shaft. The combination of the discs and the shaft is placed inside a shell and a plenum chamber is formed, out of which several nozzles are distributed uniformly to supply the inflow working fluid.

In the subsequent years after the invention of the Tesla turbine, this novel concept has received enormous attention in both technical and industrial fields. Many analytical and experimental investigations have been conducted to explore the performance of the Tesla turbine. Rice [16] reviewed the principles of the Tesla-type turbomachinery and discussed the problems with nozzles and diffusers. In addition, the analytical methods that had been found useful in modeling and calculating the flow in the rotor and the experimental results obtained by some investigators were described. Couto et al. [17] presented a simple and straightforward technique, using basic fluid mechanics, to estimate the needed number of discs required for a Tesla turbine, compressor or pump. Lemma et al. [18] presented experimental and numerical study to explore the performance characteristics of viscous flow turbines and the results indicated that the adiabatic efficiency of this kind of turbomachinery was around 25%. Lampart et al. [19] presented results of the design analysis of a Tesla bladeless turbine intended for a co-generating micro-power plant of heat capacity 20 kW, which operated in an organic Rankine cycle with a low-boiling medium; the simulation results showed that the best obtained solutions can be competitive as compared with classical small bladed turbines. Enign et al. [20] researched the experimental and theoretical characterization of a multiple-disc fan based on the principle of conservation of angular momentum. The effect of gap width and rotational speed were numerically investigated for both design and off-design volume flow rates. Carey [21] developed a 1D model analysis for flow and momentum transport in the Tesla turbine and evaluated the turbine use in Rankine cycle solar thermal power generation systems. Guha and Sengupta [22] presented a simple theory that described the three-dimensional fields of velocity and pressure in the Tesla disc turbine, which gave the torque and power output that had been verified by comparing the theoretical predictions with recently published experimental results.

In this paper, the one-dimensional model for the Tesla turbine is used to predict its performance, which focuses on the flow characteristic and the momentum transfer in the Tesla turbine. As for a low grade heat source, a small scale ORC system is designed to utilize the energy and the Tesla turbine is applied to generate the power output. The one-dimensional model is used to predict the turbine efficiency. The influence of the working fluid properties and the ORC system operating conditions on the Tesla turbine performance is evaluated. Thermodynamic analysis of the ORC system is conducted to explore the potential of applying the Tesla turbine in such small scale systems.

Section snippets

Thermodynamic model of ORC system

Fig. 1 shows the schematic diagram of a basic ORC system, which consists of a working fluid pump, an evaporator, an organic expander and a condenser. The liquid organic working fluid from the condenser is firstly pumped into the evaporator, where it is converted into saturated or superheated vapor by the heat source. Next, the organic vapor expands in the expander to produce power. Afterwards, the exhaust organic vapor from the expander is condensed to liquid in the condenser by the cooling

Model analysis

Fig. 3 shows the schematic diagram of a Tesla turbine. The working fluid expands in the inlet nozzles and then flows spirally into the rotor. The viscous effect that occurs in the boundary layers drags the discs to rotate, within which the momentum of the working fluid transfers to kinetic energy of the rotating discs. Afterwards, the working fluid flows out through the hole near the inner part of the discs and the shaft.

A one-dimensional model [21] is presented to analysis the flow

Heat source condition and working fluid selection

The selected heat source is hot water, the mass flow rate and the initial temperature of which are 0.5 t/h and 393.15 K, respectively. According to the temperature level of the heat source, seven organic fluids are selected as the working fluid candidates. The properties of these working fluids are listed in Table 2, which are obtained from REFPROP 9.1.

Thermodynamic analysis of the ORC system with different working fluids is conducted. The simulation is carried out by a computer program written

Conclusions

The Tesla turbine offers an attractive option for the expander design in small scale ORC systems due to its simplicity and low capital cost. This paper improves the one-dimensional model for the Tesla turbine, which adopts a non-dimensional formulation that identifies the dimensionless parameters that dictate performance features of the turbine. Although the model embodies several simplifying assumptions, its predictions are found to agree reasonably well with available measured performance

Acknowledgement

This research study was supported by the cooperative scientific research project of energy conversion and emission reduction among China-Europe enterprises (No. SQ2013ZOC200005).

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