Stirling Engines in Generating Heat and Electricity for micro - CHP ...

Stirling Engines in Generating Heat and Electricity formicro - CHP Systems

DAN SCARPETE, KRISZTINA UZUNEANUDepartment of Thermal Systems and Environmental Engineering

University “Dunarea de Jos” of GalatiStr. Domneasca no. 47, 800008 Galati

[email protected] http://www.ugal.ro/

Abstract: - In this paper, an analysis of different generating heat and electricity systems with Stirling engine ismade from the point of view of benefits and limitations, both operational and economic and environmental.Stirling engine has the ability to work at low temperatures, and can also use all fossil fuels and biomass, torealize an environmentally friendly energy production. Stirling engines are more appropriate for the micro -CHP systems having the best value for overall system efficiency and they are regarded as the most promisingfor further development in generating energy systems for local use.

Key-Words: - Stirling engine, Generating heat and electricity system, m-CHP, Fossil fuels, Biomass

1 IntroductionThe Stirling engine was patented in 1816 by RobertStirling [1,2], and the first solar application ofrecord was by John Ericsson in 1872 [2]. Since itsinvention, prototype Stirling engines have beendeveloped for automotive purposes; they have alsobeen designed and tested for service in trucks, buses,and boats [2]. The Stirling engine has been proposedas a propulsion engine in passenger ships and roadvehicles such as city buses [2,3]. The Stirling enginehas also been developed as an underwater powerunit for submarines, and the feasibility of using theStirling engine for high-power systems has beenexplored by NASA. However, the Stirling cycleengine is well suited for stationary power anddomestic use [6].Stirling engines can be operated on a wide variety offuels, including all fossil fuels, biomass, solar,geothermal, and nuclear energy [9], with externalcombustion that facilitates the control of thecombustion process and results in low air emissions,low noise and more efficient process [10]. The mostoutstanding feature of the Stirling engine is itsability to work at low temperatures, namely belowthe temperature of boiling water [11]. Moreprecisely, even the temperature of the human body issufficient to put the engine into motion. Such a kindof an engine can use low temperature energy sourcesthat are widespread in nature: the hot water fromsolar collectors, geothermal water, hot industrialwastes.The Stirling engines are often used in the electricity-generating condensing boilers [12]. The Stirling

engines are 15-30% efficient in converting heatenergy to electricity, with many reporting a range of25 to 30% [13]. Since these engines show highthermal efficiencies they are most suitable forapplications where thermal requirements aresignificant [14], e.g. for CCHP (Combined Cooling,Heating and Power) or CHP (Combined Heating andPower) systems (fig. 1).Development of Stirling engines is proceedingworld-wide in spite of their admittedly higher costbecause of their high efficiency, particularly at partload, their ability to use any source of heat, theirquiet operation, their long life and their non-polluting character [11].

Fig. 1 - A schematic representation of a CCHP/CHPsystem with Stirling engine (adapted from [12]).

CCHP system

Fuel or heat Stirlingengine

Heatexchanger

Absorptionchiller

Power (P)

Heating (H)

Cooling (C)

CHP system

Recent Researches in Multimedia Systems, Signal Processing, Robotics, Control and Manufacturing Technology

ISBN: 978-960-474-283-7 149

In this paper, an analysis of different generatingelectricity systems with Stirling engine is made fromthe point of view of benefits and limitations, bothoperational and economic and environmental.In reviews of energy conversion, the Stirling engineis regarded as the most promising for furtherdevelopment of systems for generating heat andelectricity for local and domestic use.

2 The Stirling EngineThe ideal Stirling cycle combines four processes,two constant-temperature processes and two cons-tant-volume processes [2]. Because more work isdone by expanding high-pressure, high-tempera-turegas than is required to compress low-pressure, low-temperature gas, the Stirling cycle produces network, which can drive an electric alternator.In the ideal Stirling engine cycle [2], a working gasis alternately heated and cooled as it is compressedand expanded. The working fluid is contained in themotor and the mass of the fluid remains constant[14]. Gases such as helium and hydrogen, whichpermit rapid heat transfer and do not change phase,are typically used in the high-performance Stirlingengines [2,14]. Also, air is used as working fluid[11,13]. Hydrogen, thermodynamically a betterchoice [2], is more conductive and has a lowerviscosity and therefore lower flow losses thanhelium [8]; generally results in more efficientengines that does helium [2,7]. However, hydrogenis more hazardous, is more difficult to contain, andprobably causes hydrogen embrittlement [4].Helium, on the other hand, has fewer materialcompatibility problems and is safer to work with [2].Helium is an environmentally benign gas having anODP and GWP of zero [2].All Stirling engines fall into one of the followingtwo basic categories [2,4,7,13]:▪ Kinematic Stirling engines have a crankarrangement to convert the reciprocal piston motionto a rotational output, say to drive a generator. Thedisplacer is actuated through some form ofmechanical linkage.▪ Free-piston Stirling engines have norotating parts. In the majority of cases, output poweris taken from a linear (usually permanent magnet)alternator attached to the piston, while the displaceris actuated by the pressure variation in the spacebeneath the piston.In theory, the Stirling engine is the most efficientdevice for converting heat into mechanical work [2].The efficiency of thermal conversion cycle/engine islimited by the Carnot cycle (ideal engine) efficiency

derived from the second law of thermodynamics:the higher the temperature of thermal energy input,the higher the engine efficiency.As it is expected, the nature and pressure of theworking fluid influence the power performance ofthe Stirling engine.Results obtained at various temperatures of heatsource (700-1000 0C) using air and helium (pressurerange of 1-4.5 bar), for a gamma type Stirling enginewith 276 cc swept volume, are shown in Figure 2[4]. It is seen that an increase in the heatertemperature results in an increase in power out-put.Comparison of curves for helium and air at the sameconditions shows that for helium the engine poweroutput is about twice that of air.Several firms are mass-producing Stirling enginesthat outperform internal combustion engines andgas-turbine engines, with an efficiency of 40% evenat 600 - 700 0C [4]. In the best designs, themass/power ratio is 1.2-3 kg/kW, while theefficiency is 40-45%. Since Stirling engines showhigh thermal efficiencies they are most suitable forapplications where thermal requirements are signi-ficant, for example in geographical regions with ahigh annual heating demand [15].

Fig. 2 - Variation of brake power with heat sourcetemperature [4].

3 Stirling ConvertersStirling devices are usually developed for cogene-ration and power generation units [5,8].Kinematic Stirling engines rely on a separate electricgenerator or alternator to convert the mechanicalpower into gas electricity, while free-piston Stirlingengines integrate the alternator into the engine [12].The resulting engine/alternator with its ancillaryequipment is often called a converter or a powerconversion unit.Electric capacities for kinematic Stirling units are

0

Pow

er o

utpu

t (W

)

20

40

60

80

100

120

140

700 750 800 850 900 1000950

1.0 bar air1.5 bar air3.0 bar air1.5 bar helium3.0 bar helium4.0 bar helium

Hot source temperature ( C)

Recent Researches in Multimedia Systems, Signal Processing, Robotics, Control and Manufacturing Technology

ISBN: 978-960-474-283-7 150

between 5-500 kW [13], while the capacities forfree-piston units are between 0.01 and 25 kW [13].The last can immediately produce grid compatibleAC electricity [1,2].Larger sizes of free-piston Stirling units are feasiblebut unlikely to be commercially viable, as alternatorvolumes become excessive [4]. A solution longrecognized but not as yet put into practice is thecoupling of the free-piston engine to a pump andturbine [7]. The Stirling driver is comprised of twoconventional, displacer type, free-piston enginesconfigured as a dynamically balanced opposed pair.Each engine drives a simple single-acting blower (orlow pressure ratio gas pump), using the outer end of

Fig. 3 - System schematic (a) and single engine andblower (b) [13].

its power piston. The single turbine/generator isseparate from the engines and connected by

ductwork. The engines and turbines utilize the samehelium working fluid.This arrangement is shown schematically in Figure 3(a). Both the engine/blowers and the turbine/generator are hermetically sealed within pressurevessels. The net output of the system is 7 kWe. Aschematic of one half of the Stirling driver is shownin Figure 3 (b). Power control is accom-plished byvariation in the size of the turbine nozzle.The electrical efficiency of generating electricitysystems with Stirling engine is about 12-20%, withthe target of higher efficiencies than 30% [4]. Inhigher value micro-CHP applications, electricefficiencies of more than 40% and system overallefficiencies of more than 95% have been achieved[13]. Table 1 reports the main parameters of micro-CHP systems with Stirling engines. The highelectrical efficiency of the large Stirling engine leadsto the lowest GHG emissions [6].

Table 1 Characteristics of the various typesof micro-CHP devices [6]

Conversion effici-ency range (%)

Energyconversiondevice

Energy sourceElectric Thermal

Internal com-bustion engine

Liquid fuel,natural gas 30-38 45-50

Fuel cell Hydrogen,hydrocarbon 30-40 40

Stirling engineAny type offuel, solarradiation

10-35 60-90

Rankine cycleengine

Any type of fuel,solar radiation 10-20 70-85

4 Stirling Systems on BiomassThe Stirling cycle engine can use different types ofrenewable sources of energy including biomass,solar and geothermal energy [1,11].Biomass needs to undergo several processes so thatit can be widely used as a source of energy [11].These processes will transform its accumulatedenergy (carbon and hydrogen) into solid, liquid andgaseous fuels or into electricity.The problems concerning utilisation of biomassfuels in connection with a Stirling engine areconcentrated on transferring the heat from thecombustion of the fuel into the working gas. Thetemperature must be high in order to obtain anacceptable specific power output and efficiency, andthe heat exchanger must be designed so thatproblems with fouling are minimised.Possible fuels include peat, ground coal, shale,agricultural wastes, and wood pellets and chips.Stirling engines fueled by wood pellets are already

Freepiston

Stirlingengine

Freepiston

Stirlingengine

Singleactingblower

Singleactingblower

Engine-Pump

Natural gasburner

High pressuresupply to turbine

Turbine Rotarygenerator

Turbine-Generatorpressure vessel

Lowpressuresupplyfrom

turbine

a

Displacer

Gas spring

RegeneratorBlower outlet

Blower

Blowerinlet

Cooler Piston

Heater head

b

Recent Researches in Multimedia Systems, Signal Processing, Robotics, Control and Manufacturing Technology

ISBN: 978-960-474-283-7 151

in production. Another option that may have merit isto consider fuel switching between biogas andnatural gas with a Stirling engine which is a goodconcept applicable in the waste water treatmentplants [15]. The biogas can be also obtained fromthe dairy facilities [11].

5 Stirling Engine as a prime moverfor micro CHP SystemA prime mover in a micro-CHP system generateselectricity and the waste heat of the prime mover isused for heating and to drive the thermally activatedequipment [8].Cogeneration technologies for residential,commercial and institutional applications can be

classified according to their prime mover, as follows▪ Reciprocating engines▪ Stirling engines▪ Micro steam and gas turbines▪ Fuel cell systemsThis section presents an analysis of micro-CHPsystems based on the prime movers mentionedabove, used for electricity generation.A comparison of residential micro CHPtechnologies related on prime mover can be madeversus separate heat and power (SHP) [9]. Table 3shows by which factor micro-CHP efficienciesexceed those of separate heat and power. Theneeded SHP was calculated to match 1 unit of fuelinto each of the four micro-CHP technologies. Thusthe seventh column of Table 3 is the SHP fuelneeded to produce the same amount of electricityand thermal energy as the CHP unit. The data fromTable 3 indicate that the overall system efficiencyhas the best value for Stirling micro-CHPtechnology as well as for thermal/electric ratio.Some micro-CHP systems (<5 kW) are evaluatedfor use in residential applications by taking theviewpoint of a detached single family house(reference case). The calculated energy use profileswere then used to dimension the CHP and to assessthe building performances when CHP is used.The studied CHP technologies are (Table 4): Two gas engines running on natural gas:

Dachs HKA F 5.5 (Senertec)Mini-BHKW (Ecopower)

Two Stirling engines, commercial small scalesystems:

Stirling 161 microKWK module (Solo)Whispergen (Whispertech)

An Idatech fuel cell running on hydrogen gaswith reforming of natural gas

The data from Table 4 indicate that the electricefficiency is better for micro-CHP systems withreciprocating engines, and Stirling engines are in thesecond place. The thermal efficiency is better formicro-CHP systems with Stirling engines followedby reciprocating engines.For most CHP technologies, the annual savings inthe actual situation turn out to be low, which isprimarily due to the fact that the largest part (85–

90%) of the produced energy is sold to the grid atvery low prices.

In fact, the family can be seen as a small electricityproducer, which is very poorly paid. This makes theannual savings too low to return the investment costin a reasonable amount of time.

5.1 The energy demand of the residentialconsumer

The residential consumers energy demand is madeof the following:

- the heat demand for heating the household - the hot water household demand - the electrical demand for the home utilities

The residences energy demand has hourly, daily,monthly and seasonal variability.If we know the hourly variations of heat demand wecan determine the daily, weekly, monthly andannually energy consumptions.

Table 4 Power and efficiency of micro-CHPtechnologies [5].

CHP Pe(kW)

ηe(%)

Pt(kW)

ηt (%)

Senertec 5.5 27 12.5 61Ecopower 4.7 25 12.5 65Solo 2–9.5 24 8–26 72Whispertech 1 12 4.9–8 80Idatech 4 25 9 55

Table 3 Comparison of the main residential micro-CHP technologies to SHP [6].1 kW unit Electric

η, %Thermal

η, % Temperature Range Systemη, % T/E SHP/CHP|

fuelPEM Fuel Cell 29 46 80 - 100°C hot water 76 1.59 1.59SOFC Fuel Cell 27 45 80 - 1000°C hot water-high quality steam 82 1.67 1.51IC Engine 25 56 90 - 120°C hot water, low-grade steam 81 2.24 1.59Stirling 14 75 80 - 700°C hot water-med. quality steam 89 5.36 1.48

Recent Researches in Multimedia Systems, Signal Processing, Robotics, Control and Manufacturing Technology

ISBN: 978-960-474-283-7 152

5.1.1 The thermal energy demand of theconsumerThe coldest the local climate is and the higherinterior temperatures are, the higher the demand forthermal energy to heat the space. Fueling the heatingspace has to compensate for the losses of heattransmitted through walls and the roof, and also thelosses given by the heated air from the mechanicalor natural ventilation systems. The externaltemperature is the most important variable, forexplaining the daily influence and the year-to-yearvariations, the general demand for thermal energy.The specific heat consumptions differ from countryto country (climate), but also depend on theresidential consumers’ comfort level (figure 4).

Fig. 4 - Room temperature

The annual/monthly consumption of heat of aresidential building can be annually determined byusing the degree-day method as following:

hQ U A degree days (1)In which Qh - Thermal energy (heat) added oreliminated from the building in a certain time unit;U - Thermal transfer coefficient of the building,taking into account its components such as:windows, interior walls finishing, the insulation, theexterior wall etc.A – The exterior household area.Similarly we can determine the annual consumptionof cold demand from the residence,

C U A cooling degree days (2)

5.1.2 The hot water consumptionPreparing the hot water in household purposes is thesecond demand of thermal energy as amplitude,after the demand of heat for heating place. Thisdemand for thermal energy is more amplified in theresidential sector, comparatively to the industrialsector. A recent informative document or a hot watermedium consumption report in the Europeancountries does not exist yet. The last informativenewsletter available and used is the report (Eurostat,1999) about the energy consumed in EU15households and some CEE countries. The medium

hot water consumption is estimated to50/liters/day/person. Assuming a temperaturedifference of 50degrees between how water and coldwater we can determine by reporting to the numberof persons the monthly heat quantity needed by thehousehold hot water:

hw pQ N m c (3)where:Qhw – energy required to produce hot water ;Np – number of persons;m – water mass;c – caloric capacity of water;Δθ – temperature difference.This hot water household consumption has a dailyvariation according to figure 5

Fig. 5 - Hot water demand [15]

5.1.3 The estimate electric consumption demandThe electrical demand of the residential consumer isdependent on the endowment of electrical devices inthe house. If the consumed imposts power curve isknown we can determine the daily, monthly, yearlyelectrical consumption

Fig. 6 - Electricity demand [15]

The standard monthly consumption of a Romanianresidence is (100-300) kWh and can be consideredconstant for a residence. The input measurements inthis system are the required energies from theresidential consumer.

6 ConclusionsStirling engines can be used for primary powergeneration and as a bottoming cycle utilizing wasteheat for power generation and can also use all fossilfuels and biomass, to realize an environmentallyfriendly thermal and electrical energy production.The two types of Stirling engines, kinematic Stirlingand free-piston Stirling, show potential forgenerating heating and electricity systems. Electriccapacities for kinematic Stirling units are between 5-

Recent Researches in Multimedia Systems, Signal Processing, Robotics, Control and Manufacturing Technology

ISBN: 978-960-474-283-7 153

500 kW, while the capacities for free-piston unitsare between 0.01 and 25 kW. The Stirling enginesare 15-30% efficient in converting heat energy toelectricity, with many reporting a range of 25 to30%. The goal is to increase the performance to themid-30% range or even more than 40%.Autonomous power units with Stirling generatorsare irreplaceable in the oil and gas industry, wherepower is required for prospecting, drilling, welding,and other uses. In these conditions, possible fuelsare unpurified natural gas, byproduct gas extractedtogether with the oil, and gas condensate.Stirling engines have been identified also as apromising technology for the conversion ofconcentrated solar energy into usable electricalpower due to their high efficiency.The major disadvantages of the Stirling enginesinclude: the high cost, the engine needs a fewminutes to warm up and durability of certain parts isstill an issue.An evaluation of five micro-CHP systems (<5 kW)for use in residential applications was made onpower and efficiency basis. The electric efficiency isbetter for micro-CHP systems with reciprocatingengines, and Stirling engines are in the second place.The thermal efficiency is better for micro-CHPsystems with Stirling engines followed byreciprocating engines. From the view point ofprimary energy saving, the mCCHP systemimplemented using renewable energy (biomass,wood pellets) imposed the use of Stirling engine as aprime mover [15].

References:

[1] Aboumahboub, T., Schaber, K., Tzscheutschler,P., Hamacher, T. Optimization of the Utilizationof Renewable Energy Sources in the ElectricitySector, Proceedings of the 5th IASME / WSEASInternational Conference on ENERGY &ENVIRONMENT (EE '10), University ofCambridge, UK February 23-25, 2010, pp 196 –204.

[2] Brandhorst Jr., H. W. Free-Piston StirlingConvertor Technology for Military and SpaceApplications, Workshop on Power & Energy,New Delhi 2007.

[3] Chicco, G., Mancarella, P. PerformanceEvaluation of Cogeneration Systems: anApproach Based on Incremental Indicators,Proceedings of the 6th WSEAS InternationalConference on Power Systems, Lisbon,Portugal, September 22-24, 2006, pp 34 - 39.

[4] Corria, M. E., Cobas, V. M., and Lora, E. S.,Perspectives of Stirling Engines Use forDistributed Generation in Brazil, Energy Policy34, 2006, pp. 3402-3408.

[5] De Paepe, M., D’Herdt, P., Mertens, D., 2006,Micro-CHP Systems for ResidentialApplications, Energy Conversion andManagement 47, pp. 3435-3446.

[6] Kaarsberg, T., Combined Heat and Power forSaving Energy and Carbon in ResidentialBuildings, Building Industry Trends-10, pp.149-159.

[7] Kirillov, N. G. Power Units Based on StirlingEngines: New Technologies Based onAlternative Fuels, Russian EngineeringResearch 28(2), 2008, pp. 104-110.

[8] Li, H., et al., Energy Utilization Evaluation ofCCHP Systems, Energy and Buildings 38, 2006,pp. 253-257.

[9] Monteiro, E., Moreira, N. A., and Ferreira, S.Planning of micro-combined heat and powersystems in the Portuguese scenario, AppliedEnergy 86, 2009, pp. 290-298.

[10] Onovwiona, H.I., Residential CogenerationSystems: Review of the Current Technology,Renewable and Sustainable Energy Reviews 10,2006, pp. 389-431.

[11] Patrascu, R. Comparative analysis of differentcombined heat and power generation: fuel cells,gas turbine, internal combustion engine, 4thIASME/WSEAS International Conference onENERGY, ENVIRONMENT, ECOSYSTEMS andSUSTAINABLE DEVELOPMENT (EEESD'08),Algarve, Portugal, June 11-13, 2008, pp 27– 31.

[12] Pivec, G., Eisner, l., Kralj, D. OptimizationSupplying of Electricity and Heat Energy – AnAspect of Sustainability in the Hospital MariborProceedings of the WSEAS Int. Conference onEnergy Planning, Energy Saving,Environmental Education, Arcachon, France,October 14 - 16, 2007, pp 111 - 115

[13] Scollo, L., Valdez, P., and Baron, J. Design andconstruction of a Stirling engine prototype,International Journal of Hydrogen Energy 33,2008, pp. 3506-3510.

[14] Wu, D. W., and Wang, R. Z., CombinedCooling, Heating and Power: A review,Progress in Energy and Combustion Science 32,2006, pp. 459-495.

[15] Project RO–0054/2009 – Integrated microCCHP - Stirling Engine based on renewableenergy sources for the isolated residentialconsumers from South-East region of Romania

Recent Researches in Multimedia Systems, Signal Processing, Robotics, Control and Manufacturing Technology

ISBN: 978-960-474-283-7 154