Applied Energy 88 (2011) 150–155

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Applied Energy

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Performance of a micro engine with heptane as working fluid

Yang Wang, Zhijun Zhou *, Junhu Zhou, Jianzhong Liu, Zhihua Wang, Kefa CenState Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, Zhejiang, China

a r t i c l e i n f o

Article history:Received 22 December 2009Received in revised form 2 June 2010Accepted 30 June 2010Available online 3 August 2010

Keywords:HeptaneMicro scaleNewcomen engineThermodynamic cycle

0306-2619/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.apenergy.2010.06.024

* Corresponding author. Tel./fax: +86 571 8795161E-mail address: zhouzj@cmee.zju.edu.cn (Z. Zhou)

a b s t r a c t

A micro Newcomen engine is proposed. To overcome the problems of friction and leakage of the micromechanical parts during engine’s operation, we use a flexible ripple tube to integrate piston and cylinderinto one monolithic part. Heptane is used as the working fluid, and it works in the two-phase conditionfor higher energy output density per thermodynamic cycle. In the experiment, the prototype engine istested under different operational conditions. It works continuously and generates the net mechanicalwork of 0.833 J per cycle with an efficiency of 2.77% in the maximum. The experimental results proveits feasibility. However, the prototype engine still requires further improvement and optimization forbetter performance.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Batteries supply power for portable electronic devices, such aslaptops, cellular phones. But the low energy density of batterieslimits the operation time [1]. Micro power generators convertchemical energy of hydrocarbon fuels to electricity directly, thusthey have higher energy density [2]. Theoretically, their perfor-mance will excel the conventional batteries, if only the efficiencyis higher than 1% [3–5].

The first micro gas turbine based on Micro Electro MechanicalSystem (MEMS) technology was suggested in Massachusetts Insti-tute of Technology (MIT) in 1996 [6]. Whereafter, Jan Peirs devel-oped a single-stage axial micro turbine [1], and Lee produced amicro Wankel engine [7]. But the micro turbines and Wankel en-gine were only driven by cold compressed air and CO2, respec-tively, because heat would cause thermal deformation of themechanical parts. Besides, micro internal-combustion engineswere also proposed [8]. Homogeneous Charge Compression Igni-tion (HCCI) was introduced to improve their performance [9–11].However, the friction losses and charge leakage become severe inmicro internal-combustion engines [12], and most of the proto-types cannot work continuously. Geng fabricated a micro pulsejetwith the overall length of 8 cm [13], but the micro rocket cannotfunction as power source for electronic device. Micro energy con-verters based on thermoelectric or thermophotovoltaic materialwere also proposed [2,14–16]. But their efficiency is far less than1%, limited by the development of material.

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6..

Technology of manufacture limits the micromation of engines.The high accuracy is difficult to achieve, which causes aggravationof friction and abrasion of rotors or pistons. Leakage also decreasesenergy output and efficiency. All these problems inhibit the devel-opment of micro engines [1].

The invention of micro integrated circuit is based on integratingelectronic components. The concept of integration may be the keyof realizing micro engine. Whalen has already developed a novelintegrated P3 engine, which was made of a piezoelectric thin-filmmembrane [17,18]. It had power output around 100 lW, but theefficiency and details of thermodynamic cycle were unknown. Inthis paper, we propose a new micro engine. It has a flexible rippletube integrate the piston and cylinder into one monolithic part,thus solves the problems of friction and leakage of traditional en-gine. The prototype of micro engine has the dimensions ofu46 � 41 mm, the similar scale as the micro turbine(u15 � 25 mm) developed by Jan Peirs [1]. In the experiment, theprototype engine is tested under different operational conditionsto obtain the thermodynamic cycle and analyze its performance.

2. Experimental system

2.1. Experimental apparatuses

The micro engine is designed based on the Newcomen engine,which has simple structure. A schematic diagram of the experi-mental apparatuses is shown in Fig. 1. It consists of two majorcomponents, a ripple tube and a boiler [19]. All the other compo-nents are apparatuses for testing. In the preliminary phase of re-search, the boiler is heated with electricity instead of fuel for

Nomenclature

g efficiencyThot high temperature of thermodynamic cyclep pressures entropyv00 specific volume of saturated vaporps saturation pressureCp constant pressure heat capacityV volumeE energy output density per cycleD difference

W net mechanical work outputT temperaturex displacementv0 specific volume of saturated liquidTs saturation temperatureq fluid densityDh latent heat for vaporizationH enthalpy

Fig. 2. Photograph and schematic diagram of the ripple tube.

Y. Wang et al. / Applied Energy 88 (2011) 150–155 151

high stability and easy control. The generator, combustor, and cool-er [20] are all left out for simplicity.

The boiler is a vessel with dimensions of 25 � 45 � 52 mm.There is a electrical-heating tube (160 W) with dimension ofu10 � 50 mm installed in it. A thermal couple and a diffuse-sili-con-piezoelectric pressure sensor monitor the internal tempera-ture and pressure, respectively. A loader plays the role ofcompressor. The loader consists of a heavy, an AC motor and a lin-ear sensor. The motor drops and lifts the heavy. Once the heavydrops and contacts the ripple tube’s top, its gravity force com-presses the fluid inside. Then the heavy lifts, the compression is re-leased. A linear sensor mounted on the ripple tube monitors itstop’s displacement.

In the experiment, the temperature, pressure of working fluidand displacement of ripple tube’s top are measured to obtain thethermodynamic cycle. The control and monitor system of the mi-cro engine is as follows. A digital data acquisition device (WSP-D806) collects the temperature signal. The pressure and displace-ment signals are transmitted to the analog input model (I7017).A digital output model (I7050D) controls the switch of loaderand heater. All the devices are connected to the computer forreal-time recording.

The ripple tube is made of a thin film of stainless steel. It pul-sates (expand and contract rhythmically) to do mechanical work.A photograph of the ripple tube is shown in Fig. 2. For the conve-nience of manufacture, assembling and measurement, we use theripple tube of centimeter scale in the experiment. It has dimen-sions of u46 � 41 mm and thickness around 0.2 mm. The rippletube has life time of 107 pulsations, working under 8 MPa and

Fig. 1. Experimental system of the micro engine. The dashed lines are the signalroutes.

450 �C [21]. Assuming the period of thermodynamic cycle is100 s, the micro engine could work continuously for about 32 yearstheoretically.

Unlike traditional Newcomen engine, the micro engine utilizestwo-phase heptane instead of water as working fluid. Heptanehas almost the similar boiling point (98.5 �C) as water, but lowerlatent heat for vaporization (31.77 kJ/mol) and specific volume ofsaturated vapor (0.224 L/g) than water (40.8 kJ/mol and 1.675 L/g). According to Clausius–Clapeyron equation and Carnot equation(Eqs. (1) and (2)), while the pressure difference (DP) is fixed, lowerlatent heat for vaporization (Dh) and specific volume of saturatedvapor (v00) induce higher temperature difference (DT) of thermody-namic cycle (effect of specific volume of saturated liquid is small),thus increase the efficiency (g) [22]. According to calculation, effi-ciency of heptane is 7.4 times of water under the similar pressuredifference.

Moreover, heptane working in two-phase condition has higherenergy output density per cycle than pure gaseous heptane.According to Eq. (3), once efficiency is fixed, the energy outputdensity (E) mainly depends on the density (q) and heat capacity(Cp) of fluid. The liquid heptane has density and heat capacity of680 kg/m3 and 2241 J/kg K [23], respectively, much higher than4.26 kg/m3 and 1646 J/kg K of gaseous heptane [24]. Correspond-ingly, the energy output density of liquid heptane is 217 times ofgaseous heptane. But pure liquid has slight thermal expansion,thus heptane works in two-phase condition.

dps

dTs¼ Dh

Tsðv 00 � v 0Þ ð1Þ

g ¼ DTThot

ð2Þ

Table 1

152 Y. Wang et al. / Applied Energy 88 (2011) 150–155

E ¼ g � DT � q � Cp ð3Þ

Operation parameters in different cases. x is marked in Fig. 3, which indicates thecritical displacement of tube top for switch loader or heater.

Case Mass of heavy (kg) x0 (mm) x1 (mm) x2 (mm)

1 0.5 0 5 22 0.5 0 10 23 0.5 0 15 24 1 0 5 25 1 0 10 26 1 0 15 2

2.2. Operational processes

The thermodynamic cycle composes of four steps theoretically:A isobaric heating, B isentropic expansion, C isobaric cooling, Disentropic compression. First, heptane is heated under constantpressure, and transforms from saturated liquid into two-phasefluid. Second, heating continues, and isentropic expansion hap-pens. The absorbed heat converts into mechanical work duringexpansion. Third, heating stops. Isobaric cooling turns heptane intosaturated liquid. Forth, external pressure is applied to heptane. Theisentropic compression returns heptane to the initial state. Thethermodynamic cycle completes.

The operational steps of engine are shown in Fig. 3. The controland monitor system switches the on/off of heater and loader, usingthe displacement (x) of the ripple tube top as a benchmark. The de-tails are described as follows. During heating, the boil of heptanemakes the ripple tube expand. Once its top’s displacement exceedscertain value (x1), the heavy lifts and heater turns off simulta-neously. The expansion continues after the external pressure is re-leased suddenly. The residual heat of heptane is lost intoenvironment through passive natural-convective cooling, and con-tract happens. When the displacement passes certain lower value(x2), the loader turns on. The heavy drops and applies pressure toheptane, and compression happens. Finally the displacement re-turn to its initial value (x0), and the thermodynamic cyclecompletes.

The prototype engine is tested under different operational con-ditions, listed in Table 1. The expansion volume and pressure arethe most important factor affecting its performance [25,26]. Thusthe operational conditions vary with expansion volume and pres-sure. The expansion volume is adjusted through changing the crit-ical displacement of ripple tube’s top (x). Two heavies of 0.5 kg and1 kg change the pressure.

2.3. Calculation

The parameters of entropy (s), volume (V), temperature (T) andpressure (p) are obtained to depict the thermodynamic cycle, andderive the mechanical power, absorbed heat (Q) and efficiency(g). But the data of mechanical power is invalid, and the net

Fig. 3. Operational steps of the micro engine. x

mechanical work output per cycle (W) is analyzed instead. Becausethe prototype engine has period about 10 min per cycle duringexperiment, which induces power output as low as 0.001 W. Thenet mechanical work output is the area enclosed by the thermody-namic curve [17], as Eq. (4).

W ¼Z

p dV ð4Þ

The non-adiabatic boiler has environmental heat loss, thus onlythe heat absorbed by heptane is analyzed during discussion. Theabsorbed heat equals to the enthalpy difference (DH) subtractingthe net mechanical work output, as Eq. (5).

Q ¼ DH �W ð5Þ

The efficiency is defined as Eq. (6). Only the working fluid isanalyzed during discussion. The thermal and mechanical lossesassociated with the other components are excluded.

g ¼WQ

ð6Þ

3. Results and discussion

Fig. 4 shows a typical volume–pressure curve corresponding toone cycle in case 6 (Table 1). The maximum pressure and volumeare 122 kPa and 13.0 � 10�6 m3, respectively. The curve shows thatboth the volume and pressure increase during heating, which issupposed to be isobaric heating theoretically. The ripple tube hasflexibility, and it obeys Hooke’s Law. As the volume of ripple tubeincreases during the process of thermal expansion, its tension in-creases accordingly, which applies higher pressure to the inner

is the displacement of the ripple tube’s top.

Fig. 4. Volume–pressure curve of the thermodynamic cycle in case 6.

Fig. 5. Entropy–temperature curve of the thermodynamic cycle in case 6.

Fig. 6. Volume–pressure curves of the thermodynamic cycles in cases 1–3.

Fig. 7. Entropy–temperature curves of the thermodynamic cycles in cases 1–3.

Fig. 8. Volume–pressure curves of the thermodynamic cycles in cases 4–6.

Fig. 9. Entropy–temperature curves of the thermodynamic cycles in cases 4–6.

Y. Wang et al. / Applied Energy 88 (2011) 150–155 153

working fluid. Therefore, varying of pressure changes the isobaricheating to polytropic heating. Fig. 5 shows corresponding entro-py–temperature curve. Accordingly, the temperature also increaseswith pressure during heating. The polytropic heating slims thethermodynamic cycle, and decreases the efficiency and mechanicalwork output.

The pressure–volume curves in cases 1–3 with the heavy of0.5 kg are shown in Fig. 6. Fig. 7 shows corresponding entropy–temperature curves. Take case 3 as an example, the maximum vol-ume reaches 11.7 � 10�6 m3, and the pressure is up to 117 kPa.Simultaneously, the fluid has the peak temperature increase to369 K, with the entropy of 0.0888 J/K.

Table 2Absorbed heat (DH), net mechanical work output (W) and efficiency (g) underdifferent operational conditions.

Case DH (J) W (J) g (%)

1 8.76 0.067 0.762 16.7 0.208 1.243 27.0 0.598 2.224 11.7 0.134 1.155 22.7 0.474 2.096 30.1 0.833 2.77

154 Y. Wang et al. / Applied Energy 88 (2011) 150–155

To improve the performance of prototype engine, it is tested un-der higher-pressure condition. Thermodynamic curves of cases 4–6with the heavy of 1 kg are shown in Figs. 8 and 9. All the thermo-dynamic parameters increase. For example, in case 6, the maxi-mum volume reaches 13.0 � 10�6 m3, and the pressure is up to122 kPa. Simultaneously, the fluid has the peak temperature in-crease to 372 K, with the entropy of 0.0964 J/K. All the parametersare higher than those in case 3 with the heavy of 0.5 kg.

The values of absorbed heat, net mechanical work output andderived efficiency in all the cases are summarized in Table 2. Com-paring cases 1–3 with the similar heavy of 0.5 kg, the efficienciesrange from 0.76% to 2.22%. The net mechanical work output in-creases to a maximum of 0.833 J in case 3, exceeding 0.134 J in case1. The maximum expansion volumes of cases 3 and 1 are12.2 � 10�6 m3 and 4.48 � 10�6 m3, respectively. Accordingly, thethermodynamic cycle in case 3 envelops a larger area than theone in case 1. Therefore, it produces higher net mechanical work.Conclusively, it is feasible to improve the micro engine’s perfor-mance through increasing the ripple tube’s expansion volume.

After the heavy increases to 1 kg, the prototype engine’s perfor-mance also improves. The efficiencies in cases 4–6 range from1.15% to 2.77%, which are higher than the last group’s. The largermass of heavy applies higher pressure to the working fluid insidethe ripple tube. According to Clausius–Clapeyron equation and Car-not equation (Eqs. (1) and (2)), the increase of pressure difference(DP) induces higher temperature difference (DT) of thermody-namic cycle (the other parameters of heptane’s properties arefixed), thus induces higher efficiency (g). The net mechanical workoutput also increases with efficiency. For example, the netmechanical work output is 0.833 J in case 6, higher than 0.598 Jin case 3.

4. Conclusion

A new micro engine is proposed and tested in this paper. Theengine generates a net mechanical work of 0.833 J per cycle andefficiency of 2.77% in the maximum. Considering the energy lossesof all the components in micro power system [27], only a fractionof the thermal energy can be converted to electricity finally. Elec-tricity generator has efficiencies higher than 80% [28]. Micro com-bustor using hydrogen has very high combustion efficiency close to100% [29]. Therefore, the end-to-end efficiency for a micro powersystem will be about 2.21%.

But the experimental evidence shows that there is a positiveoutlook for improving its performance. The efficiency and mechan-ical work output increase with peak pressure and expansion vol-ume of thermodynamic cycle. For example, comparing cases 1and 4, the net mechanical work output increases from 0.067 J to0.134 J, and the efficiency increases from 0.76% to 1.15%, underhigher peak pressure. The increase of expansion volume has thesimilar effect. Comparing cases 1 and 2, the net mechanical workoutput increases from 0.067 J to 0.208 J, and the efficiency in-creases from 0.76% to 1.24% accordingly.

The prototype engine has period about 10 min per cycle duringexperiment, which induces low power output around 0.001 W.Considering heating, expansion and compression only occupy100 s per cycle, the long period is attributed to its high thermalinertia and inefficient natural cooling. To solve this problem, theprototype engine would be miniaturized further, and have coolerinstalled [30] in the future.

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