ENERLYT STIRLING Engine

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Description and functional principle of a Micro-CHP system with Stirling engineThe Micro-CHP system with STIRLING engine is designed not only to supply heat to asingle-family house, but also to generate electricity. Decentralised CHP systems withSTIRLING engines feature high potential CO2 savings thanks to the generation of combinedheat and power and the avoidance of distribution losses for fuels, electricity and heat. Thesystem can be operated with both conventional and renewable fuels, and continuouscombustion ensures exhaust emission levels are low.At the heart of the new CHP system is a special type of engine which makes ideal use of thealmost 200 year-old STIRLING principle. A working gas enclosed in the engine isheated in one section and cooled in the other. During the cyclical process, the pistonmotions ensure that the pressure differences are reduced at high temperature andincreasing volume, and built up again at low temperature and decreasing volume. In this way,the gas can transfer mechanical energy to an external application via pistons and a crankshaft.

Cycle 1 (red) as per the attached sheet provides an example for the following explanation.

Figure 1 – Figure 2One part of the enclosed working gas is heated by a burner via a heating unit. However, asthe pistons are mobile, this does not result in a pressure increase, but in a piston motionwhich reduces the pressure and is directed in such a way that the total gas volume withinthe engine is increased during power output at the crankshaft. When thevolume is increased, the gas temperature as well as the pressure would normally fall;however, as a result of the heat supply, it remains constant.

Figure 2 – Figure 3After this so-called isothermal expansion process the expanded gas which leaves theheater is moved through the regenerator, which serves to store the heat contained in thegas internally. By transferring the heat to this short-term heat store, the gas’s temperatureand pressure both drop, providing the best pre-conditions for the subsequent compressionphase. Without the regenerator, compression would take place against a markedly higherpressure, and a large portion of the mechanical energy previously gained would bere-converted into less valuable heat.

Figure 3 – Figure 4After the internal heat transfer to the regenerator, the working gas is compressed by thepistons. In order to minimise the unwanted and energy consuming pressure increase, oneportion of the gas is pressed through the cooler by means of the pistons during compression.The heat produced during compression can thus be transferred at a consistently lowtemperature. The cooler is a heat transfer unit: while working gas flows into its inside,cooling water surrounds its outside. The water used as cooling water in a Micro-CHPsystem is also the heating return water of the building, which, having been appropriatelyheated, can be used to heat rooms and drinking water.

Figure 4 – Figure 1After compression the compressed gas from the cooler flows through the regenerator againin the opposite direction and re-absorbs the heat that has been previously stored. This isthe ideal way to prepare for a renewed expansion process, as the temperature and pressurerise after this internal heat supply, i.e. the gas’s parameters are improved. The cycle is nowcomplete.

The mechanical energy gained is the difference between energy surplus during expansionand the energy required due to compression and losses. The usable mechanical energy ofthis CHP system, whose crankshaft is at the same time the generator shaft, is convertedinto electrical energy and fed directly into the public grid.

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Basic engine configuration

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Yet the engine does not only feature one but four STIRLING processes. With a 90 degrees’phase shift, they perform a cycle as described above. This ensures a uniform torque at thecrankshaft. It has been possible to achieve the four cycles using two double pistons only,which makes the engine very compact. The gear is simple and benefits from a durabledesign as the engine has only two piston rods which have to transmit forces. Comparablewith the pedalling mechanism of a bicycle, the generator shaft is directly driven at both endsvia two connecting rods.

A familiar STIRLING engine design which also uses four cycles is the so-called SIEMENSengine. In the SIEMENS engine design the forces of the four pistons are transferred via acomplicated gear system such as a swash plate mechanism. In addition, the gear systemsof these engines have to transmit higher forces. The working gas cycles of the SIEMENSengine are arranged in series and connected hydraulically in such a way that if expansiontakes place, for example, at the first of the four pistons, compression is simultaneouslyperformed at the third. This means the compression forces have to be transmitted via thegear system.

ENERLYT’s engine design, however, ensures that most of the forces are transferred via thedouble pistons without stressing the crank shaft. This arrangement allows compression andexpansion to take place in the same piston unit. Each double piston generates an excessin torque which is maintained for about two thirds of the total time of a revolution. As thetorque of both double pistons is precisely shifted by half a turn, the total engine torque isconstant. The double pistons complement each other perfectly.

In the past, isothermal processes achieved by STIRLING engines were rather poor. Thetemperature dropped during expansion and increased during compression further than wasdesired. This meant that less mechanical energy was produced. As the ENERLYT enginefeatures clearly separated thermal sections, the isothermal conditions have markedlyimproved. The expansion cylinders glow at a temperature of 630 °C and ensure additionalheat is supplied alongside that from the heater. At the same time the heat conductionlosses between expansion and compression spaces are reduced, as, unlike SIEMENS andother STIRLING engines, the expansion and compression pistons run in separate cylinders.Thanks to the thermal separation of both expansion and compression spaces it has alsobeen possible to overcome the so-called shuttle losses which are the result of unwantedheat transport through the pistons.

Its compact and modular design makes the engine easy to install and very suitable forlarge-scale production.

Research and development partnersFraunhofer Institut für Angewandte Materialforschung in Dresden STASSKOL Kolbenstangendichtungen GmbH StaßfurtSteinbeis-Forschungszentrum Wärme- und Energietechnik, Reutlingen

Configuration of an ENERLYT engine(Patents: DE 10 2005 039 417, EP 1917434, US 7,891,184 B2, Japan 4638943)

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Fig.: Basic arrangement of a 4-cycle engine including reference numbers

1 Expansion piston rod of first double piston unit2 Compression piston of first double piston unit3 Compression piston of first double piston unit4 Piston rod extension of first double piston unit5 Cylinder housing6 Expansion piston rod of second double piston unit7 Compression piston of second double piston unit8 Piston rod of second double piston unit9 Piston rod extension of second double piston unit

10 4-cycle heater11 Regenerator Cycle 112 Regenerator Cycle 213 Regenerator Cycle 314 Regenerator Cycle 415 Cooler Cycle 116 Cooler Cycle 2

17 Cooler Cycle 318 Cooler Cycle 419 Piston rod rings for sealing20 Thermal insulation21 Piston rod seal22 Linear guide elements23 Connecting rod24 Crankshaft25 Generator26 Crankshaft housing27 Control valve, Cycle 1 with Cycle 228 Control valve, Cycle 3 with Cycle 4 Z1 Cycle 1Z2 Cycle 2Z3 Cycle 3Z4 Cycle 4

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Innovation and technical benefits of the engine (Summary)

1. In contrast to conventional STIRLING engines, this engine design aims to achieve athermo-dynamic cyclic process (ERICCSON) which produces fewer losses. For thisreason the expansion pistons are of a larger design than the compression rods. Inaddition, both expansion cylinders are allowed to glow during operation. Thesemeasures have the effect that the normally adiabatic processes will become moreisothermal and the isochoric processes almost isobaric. The bottom line is that the workyielded by the engine is much higher.

2. The engine is extremely compact. The design features a multi-level, easy-to-assembleset-up: 1) Gear system level 2) Cooler level with compression spaces 3) Regeneratorlevel with the expansion spaces which are thermally separated from the compressionspaces 4) Heater level with expansion spaces, burner and air preheater.

3. The gear system is straightforward. Only one connecting rod is needed for 2 pistons inthe form of one double piston unit. The crankshaft and linear guide elements employ acorrespondingly straightforward design.

4. The system configuration is such that generator and crankshaft are identical. This savesthe manufacture and storage of an extra crankshaft normally required.

5. The working cycles support each other. Each double piston unit provides largely net workwhich is transferred to the crankshaft. The expansion process causes compression totake place on the opposite piston side and does not impose a load on the gear system.Compared with a SIEMENS-STIRLING engine, this means less load on the crankshaftand lower lateral forces. Forces acting upon the crankshaft are very uniform.

6. Conventional STIRLING engines are fitted with complicated heaters. The 4-cycle enginefeatures a simple, single-pipe arrangement. The fact that the heater head can bereplaced means that the heater can be easily adjusted to the type of fuel utilised.

7. The engine features minimum shuttle and heat conduction losses as the double-actingexpansion cylinder including piston is thermally separated from the correspondingcompression cylinder and piston.

8. The gear system only requires 2 piston rod seals in order to be sealed to the working gasspaces. It is also possible to easily enclose and pressurise it.

Technical data of a STIRLING engine CHP system for single-family homes(As the product is still under development, the technical parameters specified are provisional and without any obligation.)

Engine 4-cycle, STIRLING T Heater wall 800 °CCylinder capacity 1,640 cm³ T Cooler wall 70 °CFuel Natural gas (biogas) Compression ratio 1.6Working gas Helium, Nitrogen Fuel output 3.5 kWMean pressure 6.5 bar Electrical power output 1.0 kWRotational speed 1,100 rpm Thermal power output 2.2 kWGenerator 3-phase, synchronous Electrical efficiency 28.6 %Weight 150 kg Noise level < 40 dB (A)Size (B xHxT) 700 x1.500x520 mm CO2 reduction 3,818 kg/a (biogas)

Differences compared with competitor products(Micro-CHP system in single-family homes)While conventional CHP systems are clearly too large to handle the basic load in single-family houses, the Dachs unit from SENERTEC is perfectly suited for use in multi-familyresidential applications, for example. As the generated heat cannot be used during the sum-mer and late spring/early autumn, the economic efficiency in single-family applications islimited and, consequently, the CO2 savings effect reduced. The Diesel and Otto enginesrequire a large amount of maintenance and, due to the internal combustion principle, arepoorly suited for use with renewable fuels.

The STIRLING technology is just starting to develop in this market segment. Reputablemanufacturers such as the Baxi Group, Viessmann, Bosch Thermotechnik GmbH/Enatecmicro-cogen B.V., Buderus, SenerTec GmbH and Vaillant GmbH are involved in thedevelopment and testing of STIRLING engines. WhisperGen, New Zealand, offers aSIEMENS STIRLING engine fired by natural gas, providing low engine output. The electricalefficiency of this low-noise engine which uses a special wobble yoke mechanism is under10 % according to information from field test partners. Three companies have meanwhilebrought products to market (Product overview: Stromerzeugende Heizung, 10/2011). Withthe exception of WisperGen, all companies use the Sunpower (USA) machine designprinciple: A free piston beta engine which has undergone development since 1975 (cf. thefirst edition (1996) of Steimle, Stirling-Maschinen-Technik).

ApplicationsIn basic load operation, the Micro-CHP system can provide the lion's share of electricity andheat production for a single-family house. Market surveys project a potential of 5 millionhouseholds for Germany alone whose heating systems need to be refurbished (one suchsurvey was presented by Andreas Prohl for the Committee for Economic and Environment-Friendly Energy Consumption (ASUE) at the ISH 2009 trade fair).

The fact that the ENERLYT engine can be adapted for use with different fuel types and hasa low engine output means that it is capable of catering for the entire single-family house

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segment. Auxiliary and emergency power generator units, boat drives, CFC-free heatpumps and cooling equipment (reverse process) present further applications.

CustomersThe Micro-CHP system’s customers are users/owners of single and multi-family houses andsmall-scale commercial businesses. In principle, every user of a heated building is apotential customer. The decisive aspect for economic operation is that the annual heatdemand of a building is higher or equal that of a typical German single-family house.

Benefits/advantages over conventional heating systemsExisting conventional heating systems which just produce heat rather than electricity andheat can be both energetically and ecologically upgraded with a CHP system. The heatingsystem of a building serves to provide comfortable heat to rooms and to heat drinking water.A Micro-CHP system not only allows buildings to be heated but also enables financial gainon the basis of the current feed-in remuneration acts, namely the KWK (Heat-PowerCogeneration) and the EEG (Renewable Energy Resources). Systems can pay forthemselves in less than 10 years. Thanks to the cogeneration of heat and power it ispossible to cut down on centralised power plant capacities with high CO2 emissions whichdo not harness the heat produced. This is beneficial to the environment.

Cost advantagesCompared with other STIRLING engines there is a variety of cost advantages which will beexplained here. Instead of four pistons, two double pistons are used. These thereforerequire two piston rods which connect to the gear, rather than four. The same applies to thepiston rod seals which ensure sealing to the gear system; again, instead of four, only twoare required. Two simple connecting rods drive the generator shaft directly, making onecrankshaft redundant. Thanks to the uniform torque, rectification and subsequent controlledinversion of the current to provide mains quality are not necessary.

A favourably priced commercial synchronous generator feeds high-quality alternatingcurrent directly into the low-voltage grid. Due to its complicated geometry and high-temperature resistance, the heater in a STIRLING engine is often one of the most expensivecomponents. With the ENERLYT engine we have succeeded in designing a cost-effectivesingle-pipe heater which has been manufactured using a straightforward pipe bendingmethod. The heater is subsequently provided with external fins which are fitted usinghigh-temperature soldering. This reduces the heater price to under 50 % of technicallycomparable designs.

Thanks to the engine’s long service life and maintenance-friendliness, operating costsamount to less than 50 % of that for comparable combustion engines. Oil change, valvesettings and the replacement of spark plugs and filters are completely unnecessary.

Technical comparison of CHP systems

STIRLING engines suitable for use in single-family houses have yet to face strong competition.

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Manufacturers of Viessmann Werke 1) WhisperGen 2) ENERLYTMicro-CHP systems Vitotwin 300-W

STIRLING engine 1 Cycle (Beta) 4 Cycles (Alpha) 4 Cycles (Alpha)‘free piston’

Fuel Natural gas Natural gas Natural gas (biogas)

Fuel power input 7.0 kW 9.1 kW 3.5 kW

Electrical power output 1.0 kW 1.0 kW 1.0 kW

Thermal power output 5.7 kW (Stirling) 7.5 kW 2.2 kW

Electrical efficiency 14.3 % 11.0 % 28.6 %1) Source: Planning instructions VITOTWIN 300-W, Type C3HA, from Viessmann2) Source: Whisper Gen – die Strom erzeugende Heizung für Ein- und Zweifamilienhäuser (the electricity-generating heating for single- or two-family houses)

(Leaflet from DSE Direkt-Service Energie GmbH)

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CHP unit from Naturfeuer AG with ENERLYT STIRLING engine

Contact

Am Buchhorst 35 A14478 Potsdam

Tel.: +49.(0)3 31.8 88 44-0Fax: +49.(0)3 31.8 88 44-44

E-Mail: info@enerlyt.deInternet: www.enerlyt.de

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2013