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The Design of Small Solar Thermal Dish Stirling 500 W Stand Alone in Thailand.

Eng’r. Suravut, Snidvongs*1 and Dr. Sirinuch, Chindaruksa*2

*1 Vice President, Asian Renewable Energy Development and Promotion Foundation, Bangkok, 10400, Thailand ,

airscan@cscoms.com, PhD Students, School of Renewable Energy Technology (SERT), Naresuan University, Pitsanulok, THAILAND.

*2 Physic Department, Faculty of Science, Naraesuan University, Pitsanulok, THAILAND. Abstract

Thailand has the average insolation of 550 W/m2 daily average which is quite low when compare to the existing system such as SES or Solo, which designed their system for 1000 W/m2, but do not require 1000 W/m2 to operate. Operating the Solar Dish Stirling with medium insolation the system must design with proper conditions, such as Increase Dish Diameter, Minimum Power to track the dish, Reduce Piston and Drive mechanism friction.

A purpose of this research is to design a Small Solar Thermal Dish Stirling 500 W Stand Alone System that can operate with lower insolation such as Thailand conditions. 1. Introduction

Insolation in Thailand is varying between in 450 to 550 W/m2 daily whole year round, and the average value is at 550 W/m2 daily over the year.

To operate the Solar Dish Stirling with medium insolation the system must design to operate with proper conditions, such as increasing dish diameter, reduce power to track dish structure, reduce piston and drive mechanism friction.

The rotary drive mechanism seems to be lower friction than other mechanism such as gear drive, and whisper tech swash plate system [1]. In this research the author will design a Small Solar Thermal Dish Stirling 500 W stand alone System that can operate with lower insolation such as Thailand conditions. 2. Solar Stirling Engine Design

The preliminary Stirling Engines designs usually use the Beale number concept (1). The power output of many Stirling Engines conformed approximately to the simple equation:

P = 0.015pfVo (1)

Where P = engine power (watts), p = mean cycle pressure (bar), f = cycle frequency or engine speed (hertz), Vo = displacement of power piston (cm3)

Copyright© 2007 by the Japan Society of Mechanical Engineers. All rights reserved.

This can be rearranged as

P/(pfVo) = constant (2) = 0.015 The equation was found by Bale to be

approximately true for all types and sizes of Stirling Engines for which data were available including free-piston machines and those with crank mechanism. In most instances the engines operated at temperatures of 650 ºC and cooler temperatures of 30 ºC [2].

The size of the engine cylinder required may then be computed from the Beale number. There is some evidence that the Beale number is conservative for large engine. Furthermore, in such a high capital cost application, a sophisticated design with adequate cooling might be expected. Therefore perhaps it would be reasonable to double the value of the constant in equation (2) from 0.015 to 0.03. Finally, it is advantageous at this stage to recall, from the aspect of seal, bearing, and piston-ring wear, the attractions of an ‘over square’ engine, i.e. a large bore and short stroke.

From Beale equation considers only the speed, volume, mean pressure, and constant. All those information can let us find the stroke and piston diameter.

For the design data we must give the expected power output, desire speed, and mean pressure, for the example 550 W, 20 hertz (1200 RPM), 1 MPa. For a square engine the piston and stroke ratio usually we use 1 from equation 1.

Stroke = D Vo = ¶ / 4 D2 x D (3) P/(pfVo) = 0.03 Vo = P/(0.03pf) (4) D = (4P/(0.03¶pf))1/3 (5) = (4*550/(0.03*¶*10*20)) 1/3

D = 4.89 cm Stroke = 4.89 cm Beale equation can give roughly diameter and

stroke of power and displacement piston. Beale equation cannot give more details on the difference size of power and displacement piston.

The Schmidt analysis equation was published by Gustav Schmidt of the German Polytechnic Institute of Praque in 1871 [3] in which he obtained closed form solutions of these equations for the special case of sinusoidal volume variations of the working spaces. Schmidt and Simple analysis can give us more accurate and give the difference size of power piston, displacement piston, heater area, cooler area, and regenerator size. The calculation used the math lab program from Dr. Urieli [4].

p = MR (Vc/Tk + Vk/Tk + (Vrln(Th/Tk))/(Th-Tk)

+ Vh/Th + Vc/Th)-1 (6)

Where p = Mean Pressure bar M = Mass R = Gas Constant Vc = Compression Space Volume Vr = Regenerator Space Volume Vh = Heater Space Volume Vk = Cooler Space Volume Th = Heater Temperature, K Tk = Cooler Temperature, K From Schmidt equations it can give the volume of

heater and compression volume by this we can get the size of the power and displacer piston. Schmidt equations give us more accurate piston size.

In this prototype engine the author decided to use 4 cylinders gamma type Stirling Engine with internal regenerator, Figure 1. This type of engine has the advantage of having separate cylinders out weight. Gamma type engine can also be compounded into a compact multiple cylinder configuration, as shown in Figure 2.

Figure 1 Diagram of a simple displacer type gamma engine.

Source: www.ent.ohio.edu/~urieli/stirling/engines

This engine is enabling an extremely high specific power output. The four cylinders are interconnected (daisy chain), so that the expansion space of one cylinder is connected to the compression space of the adjacent cylinder via a series connected heater, regenerator and cooler. The pistons are typically driven by a negative swash-plate (Rotary Drum), resulting in

a pure sinusoidal reciprocating motion having a 90 degree phase difference between the adjacent pistons.

Figure 2 Compound Gamma type Stirling engine

Source: By author The advantages of Rotary Drive Mechanism

Stirling engine are that it is easy staring, smooth running, and has good low end torque. Rotary Drive Mechanism works well for applying the generator at the top of a Stirling engine. It easy to installed at the focal point of a parabolic reflector.

This Solar Stirling engine was first tested with 700 W x 4 electric heaters 1 ф 220 V, adjustable power, and final tested with real solar insolation in Bangkok, Thailand at AREF.

This Solar Stirling engine was designed by author, namely “Siam Solar Stirling Engine III”. Don Bosco Technical School has meanwhile supported the fabrication work on the Dish Structure, Stirling Engine components, Tracking mechanism and assembly the engine. The author and his staff have then continued to do the testing. All tests were done at AREF, Bangkok in Thailand. 3. Solar Stirling Engine Specification

The Solar Stirling engine was designed as the following Table 1:

Table 1 Solar Stirling Engine Specification Type Gamma Acting Double Driver Mechanism Negative Rotary Working Gas Air Expansion space Temp 650 C (+/- 5 C) Compression space Temp 40 – 50 C (+/- 5 C) Ambient Temperature 40 C (+/-5 C) Thermal Efficiency 60 % Power Control Variable Pressure No. of Cylinders 4 Means Pressure 0.5 MPa Maximum Pressure 1 Mpa Power Piston Diameter, mm. 48 Stroke, mm. 40

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Power Displacement, cc. 72 x 4 Displacer Diameter, mm. 42 Displacer Length, mm. 150 Heater surface area, cm2 11.40 Cooler surface area, cm2 29.12 Regenerator surface area, cm2 161.60 Expansion swept, cm3 340.00 Compression swept, cm3 220.80 Speed, rpm 500 – 1200 Regenerator Type Tube Cooling type Ethyl glycol Electric heater 220 V 700 x 4 W Mechanical Output Power 215 W @ 20 psi Mechanical Output Power 550 W @ 80 psi

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Source : By Author 4. Rotary Drive Mechanisms

The four pistons gamma type Stirling engine, are daisy chained together with negative Swash Plate (Rotary), Figure 3. This is done by driving the displacer via linear drive rods which are attached to the top of power pistons. The rods pass through a sealed bulkhead and the displacer cylinders are ported to power piston cylinders that are phased behind them 90°. This allows an engine that only requires one crank through per piston where other Stirling engine requires two.

The bronze bushing sealed bulkhead was replaced with guide bearing with rubber seal, and replace the Mitter gear with Rotary Drum. This could call Negative Swash plate. It also added more bearing and universal joint. The rotation, Clock-wise or counter clock-wise, can be done by adjust a little phase angle, positive or negative, different.

This engine stands a height of 60 cm. The surface area of the heating side is 11.40 cm2. The total weight when mostly made of aluminum casting is around 20 kgs. The working piston diameter is 4.8 cm, the restrictor piston diameter is 5.00 cm, and the stroke is 4.00 cm. The engine was designed with pressured air as working fluid at an operating pressure of up to 1 MPa (147 Psi). The method of heating it arbitrary since it is a Striling engine, but the prototype was heated with 700 x 4 W electric heater, adjustable power, each cylinder the engine produces 130 W mechanical work.

The maximum heat input is 2800 W which is more than the estimated heat input from the concentrating dish, 2,000 W at solar insolation 550 W/m2, average daily Thailand insolation. The engine was circulating with Ethyl Glycol to cool the engine. The engine is estimated to have the thermal efficiency of 60 % and mechanical efficiency of 30 %.

The engine starts running at 200 °C and produces 550 W power output when near the max temperature of 650 °C. The ideal speed is approximately 1200 rpm and the torque output is 10.4 Nm.

Figure 3 Swash Plate Type 2 (Rotary) Source : By Author

Figure 4 Swash Plate Type 2 (Rotary)

Source : By Author

5. Calculation results

5.1 Engine data Comp clearance vols 4.0 cm3

Comp swept vols 72.0 cm3

Exp clearance vols 3.0 cm3

Exp swept vols 69.0 cm3

Expansion phase angle advance 90.0 deg

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5.2 Annular heat exchanger Cooler data Void vols 137.22 cm3

Free flow area 0.13 cm2

Wetted area 3.43 cm2

Hydraulic diameter 16.00 mm Cooler length 10.50 cm 5.3 Tubular regenerator housing with stacked wire mesh matrix Matrix porosity 0.160 Matrix wire dia 0.130 mm Hydraulic dia 0.024 mm Total wetted area 0.0542 m2

Regenerator length 148.0 mm Void vols 0.24 cm3

5.4 Annular gap heat exchanger heater data Void vols 21.44 cm3

Free flow area 0.02 cm2

Wetted area 2.86 cm2

Hydraulic dia 3.00 mm Heater length 10.00 cm 5.5 Operating parameters Gas Type Air Mean pressure 4,200 kPa Cold sink temperature 313.0 K Hot source temperature 923.0 K Effective regenerator temperature 546.1 K Operating frequency 20.0 Hz Pressure phase angle beta 18.0 deg Total mass of gas 0.991 gms

5.6 Schmidt Analysis Work 5.067 J Power 101.3 W Qexp 7.667 J Qcom -2.600 J Indicated efficiency 0.661 5.7 Ideal Adiabatic Analysis Heat transferred to the cooler -59.73 W Net heat transferred to the regenerator 0.00 W Heat transferred to the heater 160.67 W Total power output 101.34 W Thermal efficiency 0.631 5.8 Simple Analysis Heat transferred to the cooler -82.11 W Net heat transferred to the regenerator 0.00 W Heat transferred to the heater 179.48 W Total power output 97.85 W Thermal efficiency 0.545

5.9 Heater Simple analysis Average Reynolds number 1841.00 Maximum Reynolds number 3323.90 Heat transfer coefficient 202.79 W/m2*K Heater wall/gas temperatures Twh 923.00 K Tgh 892.00 K 5.10 Cooler Simple analysis Average Reynolds number 4110.70 Maximum Reynolds number 7135.40 Heat transfer coefficient 38.94 W/m2*K Heater wall/gas temperatures Twh 313.00 K Tgh 374.50 K 5.11 Converged heater and cooler mean temperatures heater wall/gas temperatures Twh 923.00 K Th 892.00 K cooler wall/gas temperatures Twk 313.00 K Tk 374.50 K 5.12 Regenerator Simple Analysis Average Reynolds number 18.100 Maximum Reynolds number 32.800 Stanton number (Average Re) 0.203 Number of transfer units 2,487.000 Regenerator effectiveness 1.000 Regenerator net enthalpy loss 0.500 W Regenerator wall heat leakage 13.900 W 5.13 Pressure Drop Simple Analysis Pressure drop available work loss 26,946.7 W Actual power from simple analysis -26,848.8 W Actual heat power in from simple analysis

193.9 W

Actual efficiency from simple analysis

-13,845.0 %

6. Dish specifications

Table 2. Dish Specification Dish Type Parabolic Structure Type Space Truss Dish Diameter, m 2.50 Dish Focus, m 1.56 Depth. m 0.25 Total height, m 3.20 Aperture area, m2 4.90 Reflective, % 90 Power at receiver, W 2,000 Tracking sensor H bridge LED Tracking power, W 10 x 2

Source : By Author

Figure 5 Parabolic Dish Structure at AREF, Bangkok, Thailand. Basic engineering and calculation for steel structure and foundation was created by the author. Steel fabrication work was built by the Don Bosco Technical School. Erection and Installation work was also created by the author and the AREF staffs as well as controllers system, Solar Tracker mechanism, the circuit design and assembly work. Source : By author

Delta Truss Support Structure made from steel. The Delta Ring attached to the top of Delta Truss Support Column. The dish structure made from GRP, 12 panels. The acrylic mirror was glued to the GRP dish. The reflector attach to the Delta Ring with adjustable screw. The Solar Stirling engine install at the center of focus point. The temperature at the focus is around 650 °C. 7. Performance

Table 3 Solar Stirling Engine Performance Cost USD 1,280.00 Mean Time Between Fail (hrs) 18,000 Maintenance Time (hrs) 3,500 Torque (N-m) 10.4 Thermal Eff. % 60 Mechanical Eff. % 31

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Source : By Author 8. Results

Rotary Drive Friction Test

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2

4

6

8

10

12

500 700 900 1100 1300 1500 1700 1900

V

A

W

Figure 6 Daisy Chain Rotary, Friction Test

Source : By Author

9. Conclusions The “Siam Solar Stirling Engine System III (SSES

III) prototype” was designed to meet Thailand’s weather environment (such as humidity, solar insolation, soft land, and wind load, etc.).

The Dish structure and Solar Stirling engine components were fabricated by the Don Bosco Technical School under the author’s supervision. The system is now under testing for reliability and endurance.

Figure 3 shows the SSESIII. This engine stands a height of 60 cm. The surface area of the heating side is 11.40 cm2. The total weight when mostly made of aluminum casting is around 20 kgs. The working piston diameter is 4.8 cm, the restrictor piston diameter is 5.00 cm, and the stroke is 4.00 cm. The engine was designed with pressured air as working fluid at an operating pressure of up to 0.5 MPa (72 Psi). The method of heating it arbitrary since it is a Striling engine, but the prototype was heated with 700 x 4 W electric heaters, adjustable power, each cylinder the engine produces 130 W mechanical work. The maximum heat input is 2,800 W which is more than the estimated heat input from the concentrating dish, 2,000 W at solar insolation 550 W/m2, daily average Thailand solar insolation. The engine was circulating with Ethyl Glycol to cool the engine. The engine is estimated to have the total efficiency of 31 %. The engine starts running at 200 °C and produces 550 W power output when near the max temperature of 650 °C. The ideal speed is approximately 1200 rpm and the torque output is 10.4 Nm.

SSESIII can be rotate Clock-wise or counter clock-wise by adjust a little phase angle, positive or negative, different.

The Negative Swash Plate Drive Mechanism (Rotary Drive), all piston rods install with guide bearing on both side to reduce the friction. This type of Mechanism needs very accurate workmanship to make. It is very quite during the operation. The Stirling engine can be rotate clockwise or counter clockwise by adjust a little phase angle, positive or negative, different. The maintenance time are high, 18,000 hrs. Figure 6 show the friction test for this rotary mechanism.

As the weight of Rotary drum is in balance so this type of mechanism create much lower vibration than other system. This type of Drive Mechanism has fewer parts than Daisy Chain Gear Drive but complicate to make. The cost of the system is medium high. The friction is about 6.1 W. The engine torque 10.4 N-m is quite good. Thermal efficiency is 60 %. and Mechanical Efficiency is 31 %. The Rotary Drive Mechanism can be improved mechanical efficiency up to 31 %. SSESIII required power to track only 10 x 2 W.

By increasing dish diameter, reduce power to track, enlarge displacer piston, reduce friction for drive

mechanism, and reduce power piston friction make the SSESIII operate with Thailand conditions.

Friction of Stirling engine may not have direct effect to the insolation, but if the Stirling engine has less friction, Stirling engine will require less power to overcome the friction. This will gain the performance of the Stirling engine up. Thus, with medium insolation as Thailand, average 550 W/m2 daily, Stirling engine could perform better.

ACKNOWLEDGMENTS This research was prepared by Mr. Suravut SNIDVONGS, Vice President, Asian Renewable Energy Development and Promotion Foundation, EIT member, a PhD Student, School of Renewable Energy Technology, Naraesuan University, Pitsanulok, Thailand. The author would like to acknowledge the assistance and guidance of Asian Renewable Energy Development and Promotion Foundation Dr. Sub.Lt. Prapas Limpabandhu Deputy Minister of Foreign Affair, Mr. Sutas AROONPAIROJ and staffs, the Engineering Institute of Thailand members who provided a critical review of this research through its various stages, including Asist. Prof. Dr. Sirinuch, Chindaruksa, Physic Department, Faculty of Science Naraesuan University, as Advisor, Dr. Vichit, Yamboonrung, Assoc. Prof. Dr. Wattanapong Rakwichien, Asist. Prof. Dr. Mathanee Sanugansermsri, as Co-Advisor and the Naraesuan University Staffs, Pitsanulok, Thailand. Especially the Don Bosco Technical School staffs for their fabrication and construction work on the prototype. Finally, the author would like to thank the numerous industries to provide information for this research.

References [1] Suravut, Snidvongs, The comparison for different type of drive mechanism for small solar stirling engine 500 w in Thailand, 2007. ISEC 2007, 24-26 September, 2007. Tokyo, JAPAN. [2] Ronald J. Steele, The Stirling Steele Engine drawing, 1994. U.S.A [3] G.Walker, Stirling Engines, Claarendon Press, Oxford, 1980, p.73. [4] Schmidt G 1871 The Theory of Lehmann’s Calorimetric Machine Z. ver. Dtsch. Ing. 15 part [5] http://www.ent.ohio.edu/~ureli/stirling/ [6] Chris Newton, Team Solaris, Final Design, Fall 2003.

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