Micro combined heat and power (mCHP)systems, providing electrical outputs from0.5 to 10kWe (3-30kW thermal), comprisean engine-driven generator – producing

electricity – a heat recovery system, control andexhaust systems, and acoustic enclosure. Heatfrom the engine can also heat water and supplyspace heating. The technology is available now butis not yet commercially viable.

Domestic mCHP users could cut their energybills by up to €300 a year, and theirCO2 emissions by up to 25%.

The economic and environmentalbenefits of mCHP are well recognisedand are part of the EU’s CO2 reductionplans, as well as meshing with distrib-uted power generation strategies.

In fact, if every suitable homeinstalled a mCHP unit, the UK couldreduce its carbon emissions by 16 mil-lion tonnes a year.

The Stirling engine, about 180 yearsold, is currently the leading power sup-ply for mCHP applications that willreplace traditional domestic heatingboilers. This engine, in an isothermalcycle, uses air in a closed circuitbetween a power piston and a heatabsorbing regenerator piston, or “dis-placer”. The latter comprises fine wiregauze, that first retains, then gives upthe heat in each cycle. One end stayshot, the other cold, and the air ismoved between the two by the displacer, cyclicallyexpanding and contracting, causing the power pis-ton to drive a crankshaft. Because the air is fullycontained, and because heat is reclaimed, theStirling engine is highly thermally efficient. Theseengines provided quiet, reliable power for manyapplications for nearly 100 years.

Modern Stirling engines transfer heat inputfrom the external combustion of fuel to the work-ing gas through a heat exchanger at a temperatureof between 950-1,000°K. The heat is rejected tothe cooling water in a heat exchanger at 300-350°K. Today’s engines run on a closed circuit ofpressurised argon, helium or nitrogen gas ratherthan air. Generated AC power can be connected togrid without an inverter.

Other technologiesOther feasible technologies include fuel cells,micro gas turbines and thermoelectric, thermionicand thermophotovoltaic generators. The fuel cellcertainly has promise but this and most of the oth-ers are far from being developed sufficiently.

Fuel cells, producing DC voltage, are environ-mentally friendly and have good potential formCHP. A fuel cell electrochemically convertshydrogen and oxygen into water, making electricityand heat in the process. It is like recharging a bat-tery, but instead of recharging using electricity, afuel cell uses hydrogen and oxygen.

Micro gas turbines – thermally efficient, verysmooth, relatively simple and requiring little main-tenance – will run on a range of fuels. However,their first costs are comparatively high and noisemight be a problem. Also, gas turbines are onlythermally efficient at or near full load and they pro-duce significant emissions. Turbines have alreadybeen tried with some success in public sector mini-CHP. Much further development would berequired for domestic use.

Thermoelectric generators (TPV) convert heatinto electrical power using joined electrical con-ductors of different materials. If the junctions arekept at different temperatures, a current flows. Bylinking together a large number of semiconductorthermocouples in series, a significant current canbe generated without moving parts. TPV have notyet reached commercial status.

Thermionic and thermoelectric generators alsohave no moving parts and use electron gas as theworking fluid, converting heat directly into electri-cal energy. They are simple, reliable and compact.Thermionic generators are highly efficient but thethermoelectric type is far less so. However, the lat-ter has the advantage of requiring a relatively lowtemperature heat source, whereas the thermionictype needs a temperature of typically 1,500°K.

Any power source for mCHP engines will fail if itcannot run 5,000-6,000 hours per year at full load.Service intervals must, therefore, be kept to a min-imum – just like conventional domestic boilers.

Stirling or internal combustion?Thermally efficient Stirling engines have manyadvantages. They can use any heat source, but formCHP the power source will almost certainly benatural gas, or landfill/biogas. Because combustionis external, burning is carefully controlled, resultingin very low emissions. Stirling engines are alsoquiet with low torque variation, and they boastlong, low-maintenance lives.

Stirling engines do have some disadvantages.Combustion ash seriously reduces efficiency –although this is unlikely when burning natural gas.Excellent sealing is essential any working fluid leakwill cause the engine to stop. Also, for real energy

8 MARCH 2004 EUROPEAN POWER NEWS

Pressing home the advantStirling engines are expected to take the lead role in bringing

combined heat and power technology into European homes, at least

until the commercialisation of fuel cells. JAMES HUNT examines the

advantages that will secure the Stirling engine’s position,

particularly against the internal combustion engine

Domestic CHP: Stirling engines

PRAGUE CONFERENCE: FOR SPONSORSHIP DETAILS CONTACT: m.skelton@highburybiz.com

A compact Enatec mini-CHP Stirling unit (left),and a cutaway showinga modern Stirlingengine by STM Powerof the United States

10 Stirling engines Mar 04 4/8/04 3:37 PM Page 2

EUROPEAN POWER NEWS MARCH 2004 9

savings, the hot parts need to work reliably to1,200°C. This means expensive materials.

Stirling-powered mCHP units will be up to 90%efficient, compared with about 60-80% for theaverage elderly British boiler.

Another venerable major contender is the inter-nal combustion (IC) engine. It is today almosttotally reliable and is far more powerful weight forweight than the Stirling engine (though weightmatters little in mCHP application. However, ICengines could produce the same power as Stirlingengines from a smaller package.

IC engines, though, are not particularly ther-mally efficient, at around 33-38% (overall systemefficiency about 80%). Therefore, they are not aseconomical as they should be – even when run un-throttled, as is necessary for best economy.However, all that wasted energy could be used inthe CHP cycle. IC engines are also comparativelycomplex, noisy and run less smoothly. Perhaps thebiggest difficulty is the comparatively dirty emis-sions when running on hydrocarbon fuels.Significant amounts of unburned hydrocarbons,NOx, CO and CO2 are emitted because the fuel isimproperly burned in the internal combustion sys-tem. Natural gas will avoid most difficulties of thistype, but perhaps the greatest potential is offeredby hydrogen gas. Developments may allow this tobe used, eliminating all emissions except for water.

All aspects of the Stirling engine are currentlybeing developed including hot/cold ends, regener-

ator materials and the combustion system, includ-ing special metals or ceramics. Wobble yoke mech-anisms are being evaluated to obtain the bestpiston phasing. The most efficient methods ofextracting waste heat are being examined. Thedesign of generators and their adaptation toStirling engines is also very important.

Current research involves looking at ultra-lowemission combustors. However, these may beunnecessary for engines burning hydrogen. Otherparameters, such as bore and stroke, mean pres-sure, rated speed and cooler design can all be opti-mised according to fuel and application.

Recent Stirling developmentsModern engines are hermetically sealed and usegas at around 4MPa mean pressure. Critical pistonand rod seals are typically made of low-friction

PTFE. Careful design can eliminate many dynamicseals, greatly reducing leaks and seal wear.

Good control will be critical to the success ofany power source for mCHP. However, the designwill depend upon the header system design, build-ing arrangements, the heat-related services, suchas air conditioning and occupancy pattern. Theability to sense temperature and demand, and tocontrol heat flows will be crucial to success.

All technologies are being developed in thedrive for the best and most efficient power plant formCHP. The fuel cell in particular has great poten-tial, but it is the more mature technologies that arealmost ready to go – the Stirling and IC engines.

Even so, for its economy, smoothness, quietness,reliability and complete combustion, the Stirlingengine is still the current leader in terms of modernmCHP applications.

“Internal

combustion engines,

though, are not

particularly

thermally efficient,

at around 33-38%

(overall system

efficiency about

80%). Therefore,

they are not as

economical as they

should be”

PRAGUE CONFERENCE: FOR DELEGATE INFORMATION CONTACT: s.linnell@highburybiz.com

vantages

Honda builds compact ICengine for mCHP testHonda Motor Company has built ICengines for mCHP applications. The tiny1.63m3 GE160V engine, the world’s small-est reciprocating natural gas engine, fea-tured a fairly conventional four-stroke,water-cooled, single-cylinder OHV layout.It has a compact, lightweight electrical gen-eration system using Honda’s own sinewave inverter. A three-way catalyst and oxygen feedbackcontrol reduced NOx emissions, resultingin cleaner exhaust emissions than conven-tional domestic boilers. The generatormotor doubles as starter motor, reducingstart-up noise and vibration. This, com-

bined with multi-chamber air intakesilencer and high-volume air cleaner,resulted in very low noise. Special attentionwas paid to avoiding wasted heat, even tothe extent of integrating catalyst and heatexchanger. The electrical output was 1kW (AC100/200 V); the thermal output was3.25kW, and this unit was sized at640x380x940mm.Designed to provide 10 hours daily use in adetached, single-family dwelling, the unitachieved an overall energy efficiency of85%, when used with hot water/heatingsystems, while the CO2 output for the aver-age Japanese home was reduced by 20%.The unit featured a 6,000-hour, or three-year, maintenance interval, and a 20,000-hour, or 10-year durability rating.

EPN

10 Stirling engines Mar 04 4/8/04 3:37 PM Page 3

The Stirling engine is an external combustionheat engine that works on a closed regenera-tive cycle and was invented at the beginning ofthe 19th century by the Scottish minister, the

Reverend R Stirling. The engine had two pistonsmoving in a cylinder, with the top of the cylinderwashed by hot flue gases leaving a coal burner.

The thermodynamic cycle consists of twostrokes – compression and expansion. Heat isintroduced by the combustion of the air-fuel mix-ture when the piston is approaching the vicinity ofits top dead centre. Combustion products with ahigh temperature and pressure force the pistondown and the power stroke takes place. Heat isrejected from the cycle as exhaust gases leave thecylinder at the concluding stage of the power(expansion) stroke.

Producing the power strokeIt is possible to produce the power stroke (usefulwork) in a similar design of the engine. In thisdesign the heat is introduced to the cycle from theoutside of the cylinder. If the heat is introducedinto the gas enclosed in the cylinder through thewalls of the cylinder from the moment when thepiston stops at its top dead centre then it will resultin an increase in the temperature and pressure ofthe gas. This high-pressure acting on the pistonwill become a power stroke. To obtain the usefulwork over the cycle, heat should be rejected fromthe gas in the cylinder during compression at thelower temperature.

It is impractical to heat the walls of the cylinderduring the expansion stroke and to cool it down

during the compression stroke. The next prudentaction would be to subdivide the single cylinderspace into space zones for the heat input and theheat rejection. For example, this can be achievedby using two cylinders with pistons. The walls ofthe “hot” cylinder are kept at a constant elevatedtemperature. Similarly, the walls of the “cold”cylinder are kept at a lower temperature. Themovements of the pistons are synchronised andarranged in such a way that the hot piston leads byapproximately 90-120° of the crankshaft angle.Such motions of the piston result in the gas beinglocated mainly in the hot cylinder during theexpansion stroke and in the cold cylinder in thecompression stroke.

To increase the power output from the engine itis necessary to increase an initial (charge) pressureof the gas inside the cylinders and the amount ofthe heat introduced and, consequently, rejectedfrom the cycle. Since the areas of external surfacesof the cylinders are not sufficiently large to providesuch heat input and rejection, special heatexchangers with developed heat transfer surfacesshould be introduced.

Yet these additional heat exchangers shouldhave as small volumes as possible. Their volumesare also called “dead” volumes and decrease thecompression ratio of the engine.

To obtain high power output and efficiency, it isnecessary to keep the compression ratio of theengine as high as possible.

The heater with the elevated temperature andthe cooler are in contact and this results in greatheat losses from the hot to the cold zone. Hence,the efficiency of the engine will be low. To prevent

heat losses, a special heat economiser – a regenera-tor – is introduced into the design. The regeneratoris a porous medium made from a material with ahigh heat capacity and should ideally have aninfinitive radial and a zero axial conductivity. Itacts as a heat sponge – heat is transferred to thematerial of the regenerator and stored when theworking fluid flows from the hot to the cold zone.The stored heat then returns to the working fluidwhen it flows in the opposite direction.

Heat capacityGases with a high heat capacity and low viscosity,such as hydrogen and helium, are used in machineswith high performance. The pressure inside of theengine may be up to 220 bar, hence special atten-tion should be paid to the sealing to prevent theleakage of these gases. To keep costs down, enginesmay be designed to run using air, nitrogen andother easily available gases. However, this results inan increase in dimensions of the engine for thegiven level of the maximum pressure in the cycleand power output.

Lubrication is another challenge when designingStirling engines. It is undesirable to have an oillubricant or its vapour inside cylinders and heatexchangers, since the lubricant may burn off andform a film deposit on the internal surfaces of theinternal gas circuit of the engine. This then willdramatically decrease the heat transfer betweenheat exchangers and the working fluid.

An additional negative effect will be an increasein the hydraulic resistance of the regenerator, sinceit will be blocked by particulate matter in the prod-ucts of the oil burning. Hence, in engines with oillubrication of the drive mechanism it is necessaryto use a special sealing technique to prevent lubri-cating oil from passing to the cylinders.

As far as the pistons are concerned, they haveguiding and sealing rings made from fluoroplasticmaterials, similar to those used in compressors.When using fossil fuels, a continuous combustionprocess takes place in the burners and using differ-ent techniques it is possible to achieve a very lowlevel of pollutants in flue gases. Since there is noexplosive type combustion of the fuel in the cylin-ders and a valve-train mechanism, then the noisein an operating Stirling engine is significantly lowerin comparison with internal combustion engines.

The hot cylinder, heater and the casing of theregenerator are made of a stainless steel-type mate-rial to withstand high temperatures and pressures

10 JULY/AUGUST 2003 EUROPEAN POWER NEWS

Stirlingconversions

Recent developments in power generation technology imply that

domestic combined heat and power may be feasible in the near

future. Dr KHAMID MAKHAMOV* describes the significant progress

made with the Stirling engine in the past decade towards this

Domestic CHP conference

Stirling engine withseparate “hot” and“cold” cylinders

The engine developedand manufactured by R Stirling in the 19th century

07 Makhamov Jul Aug 03 4/8/04 3:32 PM Page 2

EUROPEAN POWER NEWS JULY/AUGUST 2003 11

in the cycle (up to 1,000°C and 220 bar respect-ively), which makes Stirling engines more expen-sive in their production than internal combustionengines. Another problem is in the complexity of acontrol system of Stirling engines.

If necessary, the control of the machine isachieved by the combination of a reduction in theheat output from the combustion chamber, pres-sure in the cycle or alternatively by increasing thedead volume of the engine.

There are three main layouts of modern Stirlingengines – so-called alpha, beta and gamma config-urations. In an alpha single-acting engine, both thepistons are used to produce work from the engine.When using a conventional crank-drive mecha-nism the phase-angle in the motion of pistons isequal to the angle between the axes of cylinders,hence the cylinders are arranged with the anglebeing 90-120°. An alpha engine can be designed tobe a double acting machine, in which the compres-sion space of each cylinder is connected throughthe channels of the heat exchangers to the expan-sion space of the adjacent cylinder.

Pistons and cylindersIn the beta-type Stirling engine, two pistons areinstalled in a single cylinder. The top piston is notused to produce useful work and its only function isto control the flow of the working fluid from thecompression to the expansion space and vice-versa. This piston is called a displacer. The usefulwork in the cycle is produced by the second pistonwhich is called a power piston.

In a gamma engine in which the compressionspace is split into parts. The first part is located inthe hot cylinder under the displacer and the sec-ond part is located above the power piston in thecold cylinder. Both the parts are connected to each

other by channels made in the casing of the cylin-ders or by pipes. Since in Stirling engines the heatis introduced into the cycle from outside, then anyheat source can be used for its operation. Thismakes Stirling engines very attractive for the pro-duction of useful energy using such renewable heatenergy sources as solar insulation, biomass in thesolid or gas form, landfill gases, waste heat, etc.

CHP technology is a prospective market for theapplication of Stirling engines, especially micro ordCHP. MCHP installations can replace convenientboilers and produce AC or DC electricity and theprocess heat. Each of these can be a main productor a co-product, depending on the circumstancesof application.

Flue gases from the combustion chamber arefirst used to drive the engine with the built-in elec-trical alternator and them most of their remainingenergy is used in a boiler to heat up the water.

Application of such sophisticated methods ofmathematical modelling will allow developers ofmCHP installations to define optimal designdimensions of new or refine existing designs ofcomponents of Stirling engines and other elementsof CHP installations.* Dr Khamid Mahkamov is a lecturer in ther-mofluids at Durham University’s school of engi-neering. www.durham.ac.uk. This paper iswritten in conjunction with D Djumanov, a PhDstudent under Dr Mahkamov’s supervision.

Double acting and(above left) beta-type

Stirling engines

07 Makhamov Jul Aug 03 4/8/04 3:32 PM Page 3

Combined heat and power (CHP) works byusing waste heat produced by power genera-tion, rather than rejecting it into the atmos-phere. Typically for CHP, the proportion of

the used fuel’s energy rises from about 35% – in aconventional plant – to 85-90%. The economicand environmental benefits are well recognised, atleast in Europe, and the European Commission hasmade increased CHP capacity a key part of its CO2reduction strategy.

The main components of any CHP systeminclude the following: �an engine driving a generator�a generator to produce electricity�a heat recovery system�control and exhaust systems, acoustic enclosure.

The technology is already available for small-scale CHP applications but it needs developmentto be commercially viable. Typically, such applica-tions demand electrical outputs from 0.5-10kWe,or 3-30kWth.

Consumers could cut their home energy bills byup to €300 per year – and their CO2 emissions byup to 25% – with domestic CHP (dCHP). The UKCombined Heat and Power Association believesthat “domestic CHP puts consumers in the drivingseat” by giving them the choice to deliver theirown green power and consequently cut costs. TheEnergy Saving Trust believes that 700,000 dCHPunits could be in place by 2010.

Feasible technologies include fuel cells, microgas turbines, internal combustion (IC) engines,Stirling engines and thermoelectic, thermionic andthermophotovoltaic generators. Yet, with most ofthese being as much as nine years away, the onlysuitable contender is the venerable Stirling engine.

Stirling dCHP units will be up to 90% efficient,compared with an average 60% for the traditionalboiler. Modern condensing boilers are just as effi-cient as they recirculate flue gases to extract heat,but dCHP also cuts down on electricity generatedby power stations. If every suitable home installeda dCHP unit, the UK would reduce carbon emis-sions by a massive 16 million tonnes per year.

In the beginningHot air engines have been around for centuries.An example is the chimney “Smoke Jack” thatkept roasting meat turning nicely in grand countryhouse kitchens. Another is the hot air balloon.Steam engines had been producing power success-fully since the 18th century but were thermallyinefficient – typically between 2-15%. Reciprocat-ing hot air engines were developed to improve this.

The first working piston-powered hot air enginewas devised by George Caley at the turn of the19th century. Previously, he had experimentedwith an early IC engine using gunpowder, whichwas – inevitably – unsuccessful. Caley’s engineused power and displacer pistons connected to acrank via a beam. Heat was provided by an exter-nal coal-fired furnace and the displacer pistonpushed air through the furnace, heating it and pro-viding power for the main piston. The used air wasthen exhausted. Therefore, in any kind of hot air

engine, the power cylinder is “hot” while the dis-placer cylinder is “cold”. This engine had a lowthermal efficiency because new air had to be hea-ted continually. Heat distortion and rapid wearwere major problems.

The most successful hot air engine was theStirling, first proposed in 1816 by the ReverendRobert Stirling, who found a way to save energy byreclaiming heat. The mechanical design wasbroadly similar to Caley’s, but used air in a closedcircuit and a regenerator piston, comprising a finewire gauze, that first retained, then gave up theheat as required by each cycle. Because the air thatshuttled back and forth between the two was fullycontained – and because heat was reclaimed by theregenerator – Stirling’s engine was highly therm-ally efficient. However, the regenerator gave trou-ble and the cylinders’ heaters burned out.

Even so, many engines provided quiet, reliablepower for a huge range of small power applicationsfor nearly a century.

A notable hot air engine was Ericsson’s engine.Using a separate regenerator, this was commer-cially successful and was even applied – unsuccess-fully – to an ocean-going paddle steamer whichfeatured the largest diameter reciprocating pistonsever made – four of more than four metres each.

As with all early hot air engines, these sufferedfrom materials and lubricants that could not copewith the heat.

Pros and consMore recently, in the 1960s, a tiny free-piston hotair engine was developed by Martini to power anartificial heart. Its heat source was nuclear power!Engines working on the Stirling cycle do not haveto be rotative – free pistons can be used instead.

Stirling engines have many advantages. Mostusefully, they can use any heat source – even thesun. Also, as combustion is external, burning canbe carefully controlled, resulting in low or negligi-ble emissions. Stirling engines are also extremelyquiet with low torque variation and the products of

combustion are kept away from the moving parts –making for a long, low-maintenance life.Importantly, most of the heat losses go to coolantinstead of into the exhaust, making the Stirlingengine ideal for dCHP.

There are also disadvantages. The poor weight-to-power ratio matters little for dCHP, but any ashfrom combustion can clog the hot end and seri-ously reduce efficiency. Any working fluid leak willquickly cause the engine to stop, so good designand excellent sealing are essential.

More seriously still, expensive materials will beneeded for the hot parts. For example, tempera-ture-resistant nickel-bearing stainless steels have

APRIL 2003 EUROPEAN POWER NEWS

The Stirling engine, patented as long ago as 1816 – and never

developed to its full potential – is now the front-running power

supply to replace traditional heating boilers for new generation

domestic CHP applications. JAMES HUNT reports

Stirling engines

More than just a lot of ho

10

A cutaway showing amodern Stirling engineby STM Power of the US(top); a compactEnertec CHP unit(above); and a Solo uniton test (left)

04 Stirling engines Apr 03 4/8/04 3:28 PM Page 2

EUROPEAN POWER NEWS APRIL 2003 11

been developed for burners and heat exchangers,as have ceramics.

Such materials have allowed the hot end to riseto 1,200°C, raising efficiency enough to offer realenergy savings.

The modern Stirling engine works by transfer-ring the heat input from the external combustionof fuel to the working gas through a hot heatexchanger, or heater, at a high temperature – typi-cally 670-730°C. This heat is rejected to the cool-ing water in a cold heat exchanger at 27-77°C.

Most modern Stirling engines will run on aclosed circuit of pressurised argon, helium or nitro-gen working gas rather than air. The cycle isisothermal. AC power can be generated at Gridfrequency and voltage, allowing Grid connectionwithout a costly inverter.

All aspects of the Stirling engine are beingdeveloped – both hot and cold ends, the combus-tion system, regenerator and the mechanical side –such as the “wobble yoke mechanism”. The bestmethods of extracting waste heat for dCHP arealso being researched.

One group has been looking closely at the hotparts of the engine, specialising in ultra-low emis-sion combustors combined with internal combus-tion gas recirculation. A recent example of its workwas a 10kWe @1500rpm CHP plant. An ultra-lowemission, lean, low air-fuel ratio, premix preheatedair combustion system was devised for it.

Stirling engines can use any fuel, though someare more suitable than others. Gas is the most obvi-ous fuel for dCHP, and it will not foul the heaterpassages. Liquid fuels are almost as good.

Solid fuels, such as coal or biomass are muchmore likely to cause fouling problems and needextra space for combustion. Therefore, the tube/findistance in the heat exchanger has to be sized toaccept combustion particulate matter. Once the

fuel has been chosen, the heater can be designed tosuit. Other parameters, such as bore and stroke,mean pressure, rated speed and cooler design canthen be optimised.

Any CHP engine will typically be expected torun for 5,000-6,000 hours per year at full load. Thedesign must, therefore, be aimed at keeping serviceintervals to a minimum.

A recent engine, backed by Elkraft and theDanish Energy Agency, was a compact square fourparallel cylinder, hermetically-sealed design usinghelium at 4MPa mean pressure. Critical piston androd seals were of low-friction PTFE. The asynchro-nous alternator was incorporated in the oil-freepressurised crank casing, so that only static sealswere necessary, and leaks and seal wear were elim-inated. Air pre-heat gave the required high com-bustion temperatures.

Good control systems will be critical but theirdesign will depend upon strategies. Currently,there appear to be no rules. The best strategy willdepend upon header system configuration, theroom layouts, the requirements for different heat-related services, whether there is air conditioningand the occupancy pattern. The ability to sensetemperature and demand and to control heat flowswill be crucial.

Ready to goThe Stirling engine is, technologically speaking,almost ready for dCHP applications. Further devel-opment is necessary to improve control techniquesand also increase hot end temperatures for betterthermal efficiency. Special combustion processesaimed at still further reducing emissions are beingsought. Better packaging will also be necessary but,in essence, the Stirling engine for dCHP is prettywell ready to go.

f hot air“With other feasible

technologies up to

nine years away, the

only suitable

contender for

domestic CHP is the

Stirling engine.

Stirling dCHP units

will be 90%

efficient, compared

to 60% for

traditional boilers”

EPN

04 Stirling engines Apr 03 4/8/04 3:28 PM Page 3