Thermodynamics Performance Analysis of Solar Stirling Engines

Research Article
Thermodynamics Performance Analysis of Solar Stirling Engines
A. Asnaghi, S. M. Ladjevardi, P. Saleh Izadkhast, and A. H. Kashani

Renewable Energy Department, Energy and Environment Research Center, Niroo Research Institute, Ministry of Energy, P.O. Box 14665 517, Tehran, Iran

Received 3 April 2012; Accepted 2 May 2012

Academic Editors: W. He and K. Sumathy

Copyright © 2012 A. Asnaghi et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Abstract

This paper provides numerical simulation and thermodynamic analysis of SOLO 161 Solar Stirling engine. Some imperfect working conditions, pistons' dead volumes, and work losses are considered in the simulation process. Considering an imperfect regeneration, an isothermal model is developed to calculate heat transfer. Hot and cold pistons dead volumes are accounted in the work diagram calculations. Regenerator effectiveness, heater and cooler temperatures, working gas, phase difference, average engine pressure, and dead volumes are considered as effective parameters. By variations in the effective parameters, Stirling engine performance is estimated. Results of this study indicate that the increase in the heater and cooler temperature difference and the decrease in the dead volumes will lead to an increase in thermal efficiency. Moreover, net work has its maximum value when the angle between two pistons shaft equal to 90 degrees while efficiency is maximum in 110 degrees.
1. Introduction

The urgent need to preserve fossil fuels and use renewable energies has led to the use of Stirling engines, which have excellent theoretical efficiency, equivalent to the related Carnot cycle. They can consume any source of thermal energy (combustion energy, solar energy, etc.) and they make less pollution than the traditional engines, [1].

A Stirling cycle machine is a device, which operates in a closed regenerative thermodynamic cycle. In the ideal cycle of the Stirling engine, the working fluid is compressed at constant temperature, heated at constant volume, expanded at constant temperature, and cooled at constant volume. The flow is regulated by volume changes so there is a net conversion of heat to work or vice versa.

The Stirling engines are frequently called by other names, including hot-air or hot-gas engines, or one of a number of designations reserved for particular engine arrangement.

Numerous applications of Stirling engines were raised during the 19th century and at the beginning of the 20th century. Stirling engine in the 19th century was confined largely by the metallurgical possibilities and problems of the time. By these reasons, the engine was finally pushed back by newly developed internal combustion engines. The Stirling engine was almost forgotten until the 1920s.

It was only in 1938, a small Stirling engine with an output of 200 W by Mr. N. V. Philips which stimulated interest in this engine type again. Development in material production technologies that took place in the 1950s opened new perspectives as well for the Stirling engine.

In the course of 1969-1970, Philips developed a drive unit with a rhombic mechanism for a municipal bus. A detailed calculation finally showed that, with a batch of 10000 pieces annually, the price would still be 2.5 times higher than that of a compression ignition engine of the same output due to the substantial complexity of the engine.

In the 1970s United Stirling worked hard on the development of a drive unit for passenger cars. One of the following V4X35 types was fitted in the Ford Taurus car in 1974. Despite the satisfactory results of a driving test covering 10,000 km, series production was never commenced due to the price of the drive unit.

High heat efficiency, low-noise operation, and ability of Stirling engines to use many fuels meet the demand of the effective use of energy and environmental security. Stirling engine-based units are considered best among the most effective low-power range solar thermal conversion units [2].

On the other side, the main disadvantages of Stirling engines are their large volume and weight, low compression ratio, and leakage of working fluid from the engine inner volume. To increase the specific power of Stirling engines, a number of methods have been developed, such as using hydrogen or helium as working fluid at high charge pressure, increasing the temperature difference between hot and cold sources, increasing the internal heat transfer coefficient and heat transfer surface and using simple mechanical arrangements, for example, a free piston Stirling engine [3].

The Stirling engine performance depends on geometrical and physical characteristics of the engine and on the working fluid gas properties such as regenerator efficiency and porosity, dead volume, swept volume, temperature of sources, pressure drop losses, and shuttle losses.

A machine that utilizes the Stirling cycle could function as an engine that converts heat energy from an appropriate heat source into kinetic energy, or, by employing the reverse cycle, as a refrigerator that can achieve low temperatures or provide a heat-absorbing effect by the injection of kinetic energy from an electric motor. The Stirling engine has also been proposed as a driver for electricity generators and heat pumps, and its practical applications have been realized. Conventionally, fossil fuels and solar energy have been considered as potential heat sources; more recently, however, the practical applications of engines that utilize biomass or the waste heat generated from diesel engines as fuel have been discovered [4].

The thermal limit for the operation of a Stirling engine depends on the material used for its construction. In most instances, the engines operate with a heater and cooler temperature of 923 and 338 K, respectively. Engine efficiency ranges from about 30 to 40% resulting from a typical temperature range of 923–1073 K, and normal operating speed range from 2000 to 4000 rpm [5].

Numerical investigations of Stirling engines have been performed by many researches. Thermodynamics analysis is the basic of almost all of these researches. Some of these researches focus on a specific part of engine, for example, regenerator [6]. Some of the others focus on a specific type of Stirling engine, for example, free piston Stirling engine [7]. Considering specific operating conditions, for example, thermoacoustic conditions, are objective in many of these researches [8]. In all of these researches, the main objective is to calculate the optimum operating condition. In the current study, maximum output power and maximum performance efficiency are considered as main objectives. Numerical model is developed using thermodynamics analysis to generate performance variations comparing different parts’ characteristics and conditions.
2. Stirling Engine Cycle
2.1. Description

Stirling engines are mechanical devices working theoretically on the Stirling cycle in which compressible fluids, such as air, hydrogen, helium, nitrogen, or even water vapor, are used as working fluids. The Stirling engine offers possibility for having high-efficiency engine with less exhaust emissions in comparison with the internal combustion engine. The Stirling engine is an external combustion engine. Therefore, most sources of heat can power it, including combustion of any combustive material, field waste, rice husk, or the like, biomass methane and solar energy. In principle, the Stirling engine is simple in design and construction and can be operated without difficulty [9].
2.2. Application

The Stirling engine could be used in many applications and is suitable where(1)multifueled characteristic is required;(2)a very good cooling source is available; (3)quiet operation is required; (4)relatively low speed operation is permitted; (5)constant power output operation is permitted; (6)slow changing of engine power output is permitted; (7)A long warmup period is permitted.
2.3. Ideal Stirling Cycle

The engine cycle is represented on PV and TS diagrams in Figure 1. The ideal cycle of the Stirling engine is formed by two isochoric processes and two isothermal processes. Consider a cylinder containing two opposed pistons with a regenerator between the pistons as shown in Figure 2. The regenerator is like a thermal sponge alternatively absorbing and releasing heat which is a matrix of finely divided metal in the form of wires or strips. The volume between regenerator and the right side piston is expansion volume and between regenerator and left side piston is compression volume. Expansion volume is maintained at heater temperature which is called hot temperature, and compression volume is maintained at cooler temperature which is called cold temperature [2].

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