A Survey of Small-Scale Cogeneration Technologies for ...cradpdf.drdc-rddc.gc.ca/PDFS/unc90/p532258.pdf · These include reciprocating engines, Stirling ... Scale Cogeneration Technologies

Defence R&D Canada – Atlantic

DEFENCE DÉFENSE&

A Survey of Small-Scale Cogeneration

Technologies for Military Applications

Gisele Amow

Technical Memorandum

DRDC Atlantic TM 2009-072

July 2009

Copy No. _____

Defence Research andDevelopment Canada

Recherche et développementpour la défense Canada

This page intentionally left blank.

A Survey of Small-Scale Cogeneration Technologies for Military Applications

Gisele Amow

Defence R&D Canada – Atlantic Technical Memorandum DRDC Atlantic TM 2009-072 July 2009

Principal Author

Original signed by Gisele Amow

Gisele Amow

Defence Scientist/Air Vehicle Research Section

Approved by

Original signed by Ken McRae

Ken McRae

Section Head/Air Vehicle Research Section

Approved for release by

Original signed by Ron Kuwahara for

Calvin Hyatt

Chair Document Review Panel / DRDC Atlantic

Sustain Thrust 12sn04

© Her Majesty the Queen in Right of Canada, as represented by the Minister of National Defence, 2009

© Sa Majesté la Reine (en droit du Canada), telle que représentée par le ministre de la Défense nationale, 2009

Abstract

Cogeneration, also known as combined heat and power, is an efficient, clean and reliable approach to the simultaneous production and utilization of electricity and thermal energy from a single fuel source. It provides benefits of higher efficiencies of fuel utilization and cost savings where there is a need for both electrical power and heat. This report is meant to provide sufficient background information on the benefits of cogeneration and to describe various cogeneration technologies that could be considered for military use when considering energy upgrades or replacement in deployment installations. The review also identifies prime mover technologies suitable for meeting the near- and long-term requirements of electric power and heat generation in the 1 kW to 500 kW range. These include reciprocating engines, Stirling engines, microturbines and fuel cell-base cogeneration systems. The report presents the principles of operation for each of these technologies, performance characteristics (efficiencies, part-load performances, fuels, emissions, and maintenance issues), as well as the strengths and weaknesses of each system.

Résumé

La cogénération, aussi appelée « production combinée de chaleur et d’électricité », est un moyen efficace, propre et fiable de produire et d’utiliser simultanément de l’électricité et de l’énergie thermique à partir d’une seule source de combustible. Elle est avantageuse, car elle permet une utilisation plus efficace des combustibles et une réduction des coûts lorsqu’il faut utiliser de l’électricité et de la chaleur. Le présent rapport vise à fournir autant de renseignements généraux que possible sur les avantages de la cogénération et à décrire les diverses techniques de cogénération envisageables à des fins militaires pour améliorer l’utilisation d’énergie ou effectuer des substitutions dans les installations de déploiement. On y présente aussi des appareils moteurs qui permettent de satisfaire à des besoins à court et à long termes allant de 1 à 500 kW d’électricité et d’énergie thermique, y compris des moteurs alternatifs, des moteurs Stirling, des microturbines et des systèmes de cogénération à base de piles à combustible. Il porte également sur les principes d’utilisation de chaque technique, ainsi que sur les caractéristiques de performance (rendement, fonctionnement à charges partielles, combustibles, émissions et entretien), les avantages et les désavantages de chaque système.

DRDC Atlantic TM 2009-072 i


ii DRDC Atlantic TM 2009-072

Executive summary


Gisele Amow; DRDC Atlantic TM 2009-072; Defence R&D Canada – Atlantic; July 2009.

Introduction: The primary issues affecting energy options for the Canadian Forces are those of availability, affordability, sustainability, and security. Cogeneration refers to the simultaneous production of electricity and heat from a single energy source such as natural gas or diesel fuel. By making use of the heat rejected from one process in the production of the other, substantial gains in energy efficiency and, thus, lowered costs, can be realized when compared to the independent production of both quantities when needed. Also, many cogeneration systems are designed to be fuel flexible, which from an energy security perspective, make them more reliable sources of electricity by diminishing issues associated with fluctuations in fuel prices and availability. The aim of this review is to explore cogeneration technologies based on prime mover technologies that have the capability of meeting the near- and long-term requirements of the Canadian Forces in deployed locations. Results: As a consequence of this review, four prime mover technologies were identified as being suitable of meeting the electrical and thermal demands in the 1 kW to 500 kW range. These are reciprocating engines, Stirling engines, microturbines and fuel cells. It is concluded that each technology provides its own unique advantages, and as with many other power source technologies, there is no one size fits all technology. This study has led to the follow-on activity, “The Evaluation of a 1kW PEMFC Cogeneration System” within the on-going Sustain ARP Project 12sn04 to increase knowledge and experience in cogeneration. Significance: Military facilities frequently have heat and electrical requirements that are in a building or cluster of buildings, which makes these locations well suited for energy cogeneration. The use of cogeneration systems to supply simultaneous heat and power can result in higher operating efficiencies and decreased operating costs. Practical areas where some these technologies may be employed include forward operating bases, field kitchens, and other military installations in deployed locations. This report serves as a scoping study for plans going forward within the Sustain Activity 12sn04 (Cogeneration Systems for Bivouac Requirements) and is of relevance to modernization programs such as within DLR-6 e.g. “Arctic capability, Energy and Bivouac Equipment”, “Relocatable Temporary Camp” and “LF Modern Power Sources”.

Future Plans: This report serves as a scoping study for plans going forward within the Sustain Activity 12sn04 (Cogeneration Systems for Bivouac Requirements) and as a result of this review, it is clear that further work is merited in this area. In addition to the evaluation of a 1kW PEMFC cogeneration system mentioned above, this report will lead to the natural consideration of hybrid systems and non-fuel fed cogeneration technologies e.g. solar driven Stirling engines.

DRDC Atlantic TM 2009-072 iii

Sommaire


Gisele Amow; DRDC Atlantic TM 2009-072; R & D pour la défense Canada – Atlantique; Juillet 2009.

Introduction : En matière de sources d’énergie, les principales préoccupations des Forces canadiennes concernent la disponibilité, les moyens financiers, la durabilité et la sécurité. La cogénération consiste en la production simultanée d’électricité et de chaleur à partir d’une même source d’énergie, comme le gaz naturel ou un combustible diesel. Il est possible d’accroître l’efficacité énergétique et d’ainsi réduire les coûts en effectuant un procédé grâce à la chaleur issue d’un procédé précédent plutôt qu’en produisant séparément l’électricité nécessaire aux deux opérations. Nombre de systèmes de cogénération sont conçus pour être alimentés avec divers combustibles. Du point de vue de la sécurité énergétique, ils constituent de ce fait des sources d’électricité plus fiables, car ils résolvent partiellement les problèmes posés par la disponibilité et la fluctuation du prix des combustibles. La présente étude porte sur des techniques de cogénération fondées sur des appareils moteurs qui permettent de satisfaire aux besoins à court et à long termes des Forces canadiennes lors de déploiements.

Résultats : La présente étude a mené à l’identification de quatre appareils moteurs qui permettent de répondre à des besoins en électricité et en énergie thermique allant de 1 à 500 kW, soit les moteurs alternatifs, les moteurs Stirling, les microturbines et les piles à combustible. On y conclut qu’ils présentent tous des avantages propres et qu’à l’instar de nombreuses autres sources d’énergie, ils ne s’appliquent pas à toutes les situations. La présente étude a aussi conduit à l’activité de suivi visant l’évaluation d’un système de cogénération à PCMEP d’une capacité de 1 kW dans le cadre du projet de soutien de la recherche appliquée 12sn04, dont l’objet est d’approfondir les connaissances et d’accroître le savoir-faire en matière de cogénération.

Importance : Les besoins en chaleur et en électricité des installations militaires sont souvent liés à un bâtiment ou à un ensemble de bâtiments, ce qui se prête bien à la cogénération. Le recours à des systèmes de cogénération pour fournir simultanément de la chaleur et de l’électricité peut accroître l’efficacité de l’exploitation et en réduire les coûts. Parmi les installations qui pourraient se prêter à certaines techniques de cogénération, on trouve notamment les bases d’opérations avancées, les cuisines de campagne et d’autres installations militaires en déploiement.

Le présent rapport constitue une étude d’évaluation de plans liés au projet de soutien de la recherche appliquée 12sn04 (systèmes de cogénération visant à répondre aux besoins des bivouacs). Il est aussi rattaché à des programmes de modernisation comme ceux de la DBRT 6 relatifs, entre autres, à la capacité dans l’Arctique, à l’énergie et au matériel des bivouacs, aux camps temporaires mobiles, ainsi qu’aux sources d’énergie modernes des forces terrestres.

Perspectives : Le présent rapport constitue une étude d’évaluation de plans liés au projet de soutien de la recherche appliquée 12sn04 (systèmes de cogénération visant à répondre aux

iv DRDC Atlantic TM 2009-072

besoins des bivouacs). À la lumière de celui-ci, il est évident que d’autres travaux doivent être exécutés dans le domaine. Il a conduit à l’activité de suivi visant l’évaluation du système de cogénération à PCMEP d’une capacité de 1 kW susmentionné et mènera aussi naturellement à l’étude de systèmes hybrides et de techniques de cogénération non fondées sur des combustibles, comme l’utilisation de moteurs Stirling à énergie solaire.

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Table of contents

... ........................................................................................................................................ i Abstract.... ........................................................................................................................................ i Résumé

....................................................................................................................... iii Executive summary ...................................................................................................................................... iv Sommaire.

.......................................................................................................................... vii Table of contents ................................................................................................................................ ix List of figures

................................................................................................................................... x List of tables ....................................................................................................................... xi Acknowledgements

.............................................................................................................................. 1 1 Introduction1.1 ................................................................................................................... 1 Background1.2 .............................................................................................................. 2 Aim of Review

......................................................................... 3 2 Cogeneration for Military Use - A Case Study2.1 ................................................................... 3 Case Study: Hospital Space Heating Loop

......................................................................................... 5 3 What is Small-Scale Cogeneration? ............................................................................................................................ 6 4 Prime Movers .............................................................................................................. 7 5 Reciprocating Engines

5.1 ................................................................................................... 7 Principle of Operation5.2 .......................................................................................... 9 Performance Characteristics

5.2.1 ................................................................................................. 9 Heat Recovery5.2.2 ..................................................................................... 9 Part-load Performance5.2.3 ................................................................................................................ 9 Fuels5.2.4 ...................................................................................................... 10 Emissions5.2.5 .................................................................................................. 11 Maintenance

...................................................................................................................... 12 6 Stirling Engines6.1 ................................................................................................. 12 Principle of Operation6.2 ........................................................................................ 15 Performance Characteristics

6.2.1 ...................................................................................................... 15 Efficiency6.2.2 ................................................................................... 15 Part-load Performance6.2.3 ............................................................................................... 15 Heat Recovery6.2.4 .............................................................................................................. 15 Fuels6.2.5 ...................................................................................................... 16 Emissions6.2.6 .................................................................................................. 16 Maintenance

.......................................................................................................................... 17 7 Microturbines7.1 ................................................................................................. 17 Principle of Operation7.2 ........................................................................................ 18 Performance Characteristics

7.2.1 ...................................................................................................... 18 Efficiency

DRDC Atlantic TM 2009-072 vii

7.2.2 ............................................................................................... 18 Heat Recovery7.2.3 ................................................................................... 18 Part-load Performance7.2.4 .............................................................................................................. 19 Fuels7.2.5 ...................................................................................................... 19 Emissions7.2.6 .................................................................................................. 20 Maintenance

................................................................................................................................ 21 8 Fuel Cells8.1 ................................................................................................. 22 Principle of Operation8.2 ........................................................................................ 23 Performance Characteristics

8.2.1 ...................................................................................................... 23 Efficiency8.2.2 ............................................................................................... 24 Heat Recovery8.2.3 ................................................................................... 24 Part-load performance8.2.4 .............................................................................................................. 25 Fuels8.2.5 ...................................................................................................... 26 Emissions8.2.6 .................................................................................................. 26 Maintenance

................................................................................................. 27 9 Commericial State-of-the-Art9.1 ................................................................................................. 27 Reciprocating Engines

9.1.1 .......................................................................................... 27 Honda Motor Co.9.1.2 ......................................................................................................... 28 Senertec9.1.3 .............................................................................. 29 Power Plus Technologies

9.2 ........................................................................................................... 30 Stirling Engines9.2.1 ............................................................................................ 30 Whispergen Ltd.9.2.2 ................................................................................ 30 Stirling Systems GmBH9.2.3 ................................................................................................... 30 Sunmachine

9.3 .............................................................................................................. 31 Microturbines9.3.1 ...................................................................... 31 Capstone Turbine Corporation9.3.2 ......................................................................................................... 31 Calnetix9.3.3 ..................................................................................................... 32 Turbec AB

9.4 ..................................................................................................................... 32 Fuel Cells9.4.1 ................................................................................................. 32 Ebara Ballard9.4.2 .................................................................................................... 32 Plug Power

10 ..................................................................................................... 35 Summary and Conclusions11 ............................................................................................................................... 38 References12 ........................................................................................................................... 41 Bibliography

............................................................................................................................. 43 Distribution list

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List of figures

............................................................ 1 Figure 1. Illustration of energy savings with cogeneration.

...................................................................... 4 Figure 2. Illustration of fuel cell space heating loop.

..................................... 7 Figure 3. Illustration of basic four-stroke internal combustion engine[16]

............................ 8 Figure 4. Illustration of Reciprocating Engine Based Cogeneration System[18]

........................................................... 10 Figure 5. Efficiency of a spark ignition engine vs load[4].

......................... 13 Figure 6. Three fundamental mechanical configurations of Stirling engines[21]

.......................................... 14 Figure 7. Illustration of a free-piston Stirling engine (Sunpower)[21]

................................... 14 Figure 8. Illustration of a Stirling engine based cogeneration system [22]

................................................. 18 Figure 9. Illustration of a microturbine cogeneration system[18]

.......................................................... 19 Figure 10. Part-load Performance of a 30kW microturbine

.......................................................................... 23 Figure 11. Fuel Cell based cogeneration system.

....................................................... 23 Figure 12. Illustration of fuel cell operation for PEMFC[29]

............................................................. 25 Figure 13. Part-load performance of a 200kW PAFC [4]

......... 27 Figure 14. Market Status Development of Small-Scale Cogeneration (adapted from ref.2)

Figure 15. Illustration of heat flow in the system during the normal combined heat and power mode of operation. ...................................................................................................... 28

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List of tables

...................................................................... 6 Table 1. Examples of Traditional Prime Movers[15]

................................................................................ 21 Table 2. Fuel Cell Types and Characteristics

........................ 29 Table 3. Summary of Cogeneration Systems with internal reciprocating engines

............................................... 31 Table 4. Summary of Cogeneration Systems with Stirling engines

.................................................. 33 Table 5. Summary of Cogeneration Systems with microturbines

......................................................... 34 Table 6. Summary of Cogeneration Systems with fuel cells

.... 36 Table 7. Summary of Advantages and Disadvantages Cogeneration Technologies Reviewed

..... 37 Table 8. Summary of Performance Characteristics of Cogeneration Technologies Reviewed

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Acknowledgements

This work was supported by the funds provided by the Sustain Thrust under the ARP Project 12sn04 (Cogeneration Systems for Bivouac Requirements).

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1 Introduction

1.1 Background The Canadian Forces often operate in geopolitical regions where energy supplies are

increasingly uncertain, unpredictable and costly. To ensure an effective and viable path for meeting its energy demands when deployed (and arguably, at home), the Canadian Forces must consider these issues to ensure solutions in the short- and long-term period. This will require a shift in the way power and energy is delivered to meet demands, through the use of modern, secure, energy-efficient systems that are also environmentally safe and cost-effective to operate.

Cogeneration, also known as combined heat and power (CHP), is an efficient, clean and reliable approach to the simultaneous production and utilization of electricity and thermal energy from a single fuel source.[1] The principle of cogeneration is based on the recognition that conventional power generation, by itself, is ~ 35% efficient with up to 65% of the energy being lost as waste heat. By harnessing the waste heat from electricity production into useful means can result in higher system efficiencies for fuel utilization of up to 85% or more (see Figure 1).

Figure 1. Illustration of energy savings with cogeneration. Thus, cogeneration is economically attractive, from both energy and cost standpoints,

where there is a simultaneous demand for heating (or cooling) and electricity production as the costs are lower when compared to the separate production of heat and electricity. The waste heat produced can be used in a number of ways, typically for space and hot water heating. However, by using thermally-fed systems such as absorption or engine-driven chillers, the exhaust heat could be used for cooling and refrigeration. In some cases also, the exhaust heat can be re-used into further producing electrical power as with, for example, recuperators on microturbines.

Cogeneration is often best suited when the system is located near the user so that electricity transmission losses (~3% to 6%), is minimized. This also has the inherent benefit of reducing the loads on the transmission system if connected to a grid, which can lend stability, reliability, and makes them less vulnerable to various disasters or blackouts. Cogeneration also offers the possibility of fuel flexibility as many cogeneration technologies can burn a variety of fuels such as natural gas, liquefied petroleum gas, coal-derived liquids and gases, diesel, and bio-mass. Furthermore, with the higher operating efficiencies and reduced fuel consumption afforded by cogeneration systems, there is the added benefit of reduced emissions of pollutants

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and greenhouse gases while producing the same amount of useful energy as in conventional systems (for example, by eliminating the need for a separate steam boiler fired with natural gas or other fuel, the emissions of nitrogen oxides, sulphur oxides, and particulates can be significantly reduced).

1.2 Aim of Review The primary purpose of this report is to provide an overview of fuel-fed cogeneration technologies for 1kW to 500 kW applications to improve system efficiencies and reduce costs where both electricity and thermal energy demands are required for Canadian Forces operations for example, bivouacs. With the advancements made in prime mover technologies, the report discusses reciprocating engines, Stirling engines, microturbines, and fuel cells. As such, this report excludes traditional prime mover cogeneration systems suited for large-scale applications (> 500 kW), such as gas turbines and steam turbines, and purposely excludes those technologies, which are in the very early stages of development such as Organic Rankine cycles[1] and thermophotovoltaics.[2] Much of the information provided here is derived from cogeneration systems developed for residential and small building applications, however, is aptly suited to the present discussion.[2-4] Each cogeneration technology surveyed is described in terms of the principles of operation of the prime mover and performance characteristics based on efficiencies, heat recovery, part-load performance, fuels, emissions and maintenance issues. Finally, examples of current market offerings are given for each technology; this is not meant to be an exhaustive compendium of manufacturer products given the dynamic nature of investments in this area, however, serves to illustrate the state of the art.

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2 Cogeneration for Military Use - A Case Study

Cogeneration has been used successfully on large industrial scales (megawatts) since the early 20th century, where most electricity production was done by coal-fired boilers, gas and steam turbine generators with the waste exhaust heat being re-used for heating applications. However, with the decreased energy costs due to competitive centralized electric power plants and reliable electric grids, cogeneration capacity declined significantly throughout the 1950’s until the mid-1970’s where fuel-supply crises became the norm, and pushed cogeneration back to the fore. In more recent years, resurgent interest in cogeneration is echoed with today’s global concerns for climate change threats, peak oil, energy security and sustainability; issues that resonate with the Canadian Forces and many examples exist of cogeneration being used globally.[5-7]

To a lesser extent, there are fewer examples where cogeneration has been used in Defence applications, for example in army installations and mobile kitchens[8, 9]. These include installations at U.S. Department of Defence (DoD) facilities in the United States[10], CFB Petawawa and CFB Valcartier.[6] In the CFB installations, gas turbines powered by natural gas have been employed as recently as 2000 (start date). A large number of residential-scale PEM fuel cell installations have been made for the DoD Fuel Cell Demonstration Program.[10, 11] A case study is presented here to demonstrate the significant cost savings that can be realized where there is a simultaneous demand for heat and electricity.

2.1 Case Study: Hospital Space Heating Loop A UTC PC25C fuel cell power plant was installed at the Edwards Air Force Base in July

1997. This unit has a rated generating capacity of nominal 200 kW at 480 volts. The electrical efficiency of the PC25C averages 35 to 40 percent, but total system efficiency can rise to over 80 percent if the waste heat is reused in a cogeneration system. The fuel cell was fed by natural gas and was located at the base hospital and is thermally connected to a steam heat exchanger serving the building's space heating loop. The unit was grid-connected to the site transformer. Space heating is required year round due to the cold nights and HVAC (heating, ventilation and cooling) reheat requirements. Fluid from the space heating return loop was passed through the fuel cell's high-grade heat exchanger before re-entering the steam heat exchanger, see Figure 2. This configuration allowed the steam system to operate when either the fuel cell was unavailable or unable to generate high enough temperatures to meet the needs of the space heating loop. It was estimated that the site would utilize about half of the total output capacity of the high-grade heat exchanger. Through this configuration, a net savings of $96,000 was realized based on the initial projected energy bill savings for this site: Electrical Savings: $122,000, Thermal Savings: $ 3,000, Natural Gas Costs: ($29,000) to yield a net savings of $ 96,000


Figure 2. Illustration of fuel cell space heating loop.


3 What is Small-Scale Cogeneration?

Cogeneration systems are often described as large-, small-, mini- or micro-scale based on loosely defined power ranges. As the name implies, small-scale cogeneration refers to a scaled down version of the larger cogeneration systems (typically several MW or more) used in industries and cater to a range of users from residential to larger apartment blocks and small commercial buildings (kW). For the purposes of this report, small-scale is defined as spanning the range of 1 kW to 500 kW and as such comprises micro-cogeneration, which has had varying definitions as being <15 kW[2], <50kW[12], or <1 MW[13].

Small-scale cogeneration is part of the distributed energy resource (DER) movement, which advocates parallel and stand-alone electric generation units located within an electrical distribution system at or near the end user. More specifically, DER refers to the decentralized generation of electric power in small- to medium-sized facilities near sites of power demand, in contrast to large centralized electrical generating plants.This has the advantage of minimizing transmission and distribution losses when connected to a centralized grid and promotes increased reliability of supply to the end user in the event of fluctuations and disruptions.

Small-scale cogeneration systems can be operated in two different modes, either electricity-driven or heat-driven. In the first case, the unit is designed to satisfy the electricity needs and the heat is used to contribute to water and space heating. In this case, a supplemental peak boiler may be required to meet total heat demand. In the second case, the cogeneration unit is sized to meet the heat demand while electricity is either used internally or exported to a public grid.[14]


4 Prime Movers

The technological roots of cogeneration date back to the early development of steam and Stirling engines in the 18th and 19th century, respectively. Cogeneration systems typically consist of a prime mover, generator, heat recovery equipment and electrical interconnections configured into a single integrated unit. Prime movers are defined as those devices used to convert fuel energy into rotating-shaft power to drive electrical generators, see Table 1. Each is distinguished from each other by fuel-type, their combustion processes, overall thermal efficiencies, and the type and amount of rejected thermal energy produced.

Table 1. Examples of Traditional Prime Movers[15]

Prime Mover Size Range (MW)

Electrical Generating Efficiency

Typical Overall Efficiency

Heat Quality

Back Pressure Steam

Turbine

0.5 to 500

UP TO 80%

Steam At 2 Press Or More 7 - 20%

Combined Cycle Gas Turbine

Medium Grade Steam High Temperature Hot

Water

3 to 300+ 35 – 55% 73 - 90%

High Grade Steam High Temperature E

Hot Water

Open Cycle Gas Turbine

0.25 to 50+ 25 – 42% 65 – 87%

Compress. Ignition Engine

Low Pressure Steam Low And Medium

Temperature Hot Water

0.2 to 20 35 – 45% 65 - 90%

Spark Ignition Engine

Low And Medium Temperature Hot

Water

0.003 to 6 25 - 43% 70 - 92%

The conversion process of the traditional prime movers in Table 1 are combustion-based, although the conversion process can also be based on direct electrochemical conversion from chemical energy stored in a fuel into electrical energy such as in fuel cells, while other processes can include photovoltaic conversion of radiation (e.g. photovoltaic devices) or thermoelectric systems. For small-scale cogeneration applications, the larger gas and steam turbines are often uneconomical to be operated at < 1 MW electrical output due to their comparatively low electrical efficiency and consequent high cost per kW electrical output. Hence, for fuel-fed cogeneration, reciprocating engines, Stirling engines, microturbines and fuel cells have been identified to meet the 1kW to 500 kW range and are discussed in further detail in the following sections.


5 Reciprocating Engines

Reciprocating engines are a mature technology with widespread use in automobiles, marine propulsion, lawn mowers etc. ranging in size from a few kilowatts to > 60 MW. When used in power generation applications, reciprocating engines are typically called ‘gensets’. In power only mode, reciprocating engines can be used for standby power, peak shaving, remote power and grid support. For cogeneration applications, reciprocating engines are the most prevalently-used due to their well-proven technology, robust nature, and reliability.[15] Engines are available from small sizes, 1 kW, to large 7 MW generators. The electrical efficiency of a reciprocating generally increases as engine size becomes larger, and range from 30% in smaller units (spark-engine) to 40% in larger units (diesel). The thermal efficiency is typically 40% to 50%, so the total efficiency approaches 90%. Of course, as electrical efficiency increases, the absolute quantity of thermal energy available to produce useful thermal energy decreases per unit of power output, and the ratio of power to heat for the cogeneration system generally increases.

Reciprocating engines have high demonstrated use and reliability with long lifetimes. They offer quick start-up times (seconds), good part-load performance, and lowest first cost of all cogeneration systems. Disadvantages include noisy operation with relatively high vibrations, and require frequent maintenance.

5.1 Principle of Operation

Reciprocating engines fall into one of two categories distinguished by their method for igniting the fuel; these are the spark ignition (Otto-Cycle) and compression ignition (Diesel-cycle) engines. In the spark ignition engine, a spark plug is used to ignite a premixed air-fuel mixture after it is introduced in the cylinder. By contrast, the diesel engine compresses the air introduced into the cylinder to a high pressure (compressed) thus causing a temperature rise above the auto-ignition temperature of the fuel, which is then injected into the cylinder under

Figure 3. Illustration of basic four-stroke internal combustion engine[16]


high pressure.[16] Reciprocating engines are even further categorized by crankshaft speed, operating cycle (2- or 4-stroke), and whether turbocharging is used.

The vast majority of reciprocating engines are four stroke. In this type of engine, power is generated through reciprocating movements of a piston in a cylinder attached to a crankshaft, in a sequence of four strokes: intake, compression, power (or expansion) and exhaust, see Figure 3. In the intake stroke, the piston moves downward in the cylinder and in doing so, creates a partial vacuum, which draws in air or a fuel-air mixture through an intake valve and into the cylinder. When the piston returns upward in the compression stroke, ignition takes place; in the case of the diesel engine, the fuel is injected near the end of the compression stroke and ignited by the high temperature of the compressed air in the cylinder, whereas, in spark-ignition engines, the compressed fuel-air mixture is ignited by an ignition source such as a spark plug. In the power stroke, acceleration of the piston occurs due to the expansion of the hot, high-pressure combustion gases. In the final stage of the process (exhaust stroke), the combustion products are expelled from the cylinder through an exhaust valve.

While diesel engines are widely and successfully used for cogeneration, it is the spark ignition engine that are used for small-scale applications of < 30 kW[2], which have been modified from derated automotive diesel engines are used[4, 17]. In these smaller-scale applications, natural-gas open chamber[15] spark ignition engines are typically used although they can be made to run on a variety of fuels (see Section 4.2.3). Natural gas engines are typically less efficient than diesel engines because of their lower compression ratios. However, large, high performance lean burn engine efficiencies may approach those of diesel engines.

A typical reciprocating engine-based cogeneration system is shown in Figure 4, which consists of an engine, generator, heat recovery system, exhaust system, controls and acoustic enclosure. Both automotive and industrial-type engines can be used. The generator is driven by the engine, and the useful heat is recovered from the engine exhaust and cooling systems. For cogeneration and power generation applications, reciprocating engines typically drive synchronous generators at constant speed to produce a steady alternating current.

Figure 4. Illustration of Reciprocating Engine Based Cogeneration System[18]


http://en.wikipedia.org/wiki/Piston

http://en.wikipedia.org/wiki/Cylinder_(engine)

5.2 Performance Characteristics

5.2.1 Heat Recovery There are potentially four sources where usable heat can be derived from a reciprocating

engine. These are predominantly from the exhaust gas, engine jacket cooling water, and to a lesser extent lube oil water and turbocharger cooling. Heat in the jacket water or coolant accounts for up to 30% of the energy input and is capable of producing 90oC to 100oC hot water. Higher water jacket temperatures (~130oC) can be achieved with engines that have high pressure or ebullient cooling systems. Engine exhaust heat represents 30% to 50% of the available waste heat with temperatures of ~450o oC to 650 C being typical. Thus, by recovering heat from the cooling systems and exhaust, ~70% to 80% of the fuel's energy can be effectively utilized to produce both power and useful cogeneration applications.[4]

Heat can generally be recovered from reciprocating engines in the form of hot water or low pressure steam. There are many different possible configurations for heat recovery, and all have their advantages and disadvantages. Standard heat exchangers are typically used to produce hot water and steam. Sometimes, however, ebullient cooling systems are used to produce steam and cool the engine in the process. With ebullient systems, a boiling coolant is circulated through the engine jacket and fed through an air-to-water heat exchanger along with the engine’s exhaust. Forced circulation systems, which utilize higher temperature and pressure water in the engine jacket, are sometimes used to produce pressurized steam.

5.2.2 Part-load Performance For cogeneration, the heat to power ratio is important. The high efficiencies at part-load performance, which describes system efficiency when not operating at full load, make reciprocating engines very attractive for practical use. As the load of an engine decreases, the efficiency does not significantly decrease until very low loads are reached as shown in Figure 5. For example, the efficiency at 50% load is approximately 8% to 10% less than full-load efficiency, which then decreases significantly at lower loads. Practically, this means that more fuel is required to produce per kWh of electricity at lower partial loadings and, hence, lower efficiencies are observed.

5.2.3 Fuels Reciprocating engines can utilize a variety of fuels, which include gaseous hydrocarbons

(propane, butane etc.), natural gas, coal gas, raw crude oil, standard fuel oil, diesel, gasoline of various grades etc. Spark ignition engines, which are suited for small-scale cogeneration systems are primarily run with natural gas, although they can be made to run on other fuels such as liquefied petroleum gas (propane and butane gas mixtures), sour-gas, biogas (landfill gas), gasoline and industrial waste gas. Diesel engines used for large scale cogeneration applications can employ fuels such as diesel or heavy oil, or can operate in dual fuel mode burning primarily natural gas with a small amount of diesel pilot fuel.


5.2.4 Emissions Exhaust emissions are the primary environmental concern with reciprocating engines.

The primary pollutants are nitrous oxides (NOx), carbon monoxide (CO), and volatile organic compounds (VOCs - unburned or partially burned non-methane hydrocarbons). If uncontrolled, there may also be other pollutants, such as sulphur oxides (SOx) and particulate matter depending on the engine and the type of fuel used. Emissions of sulphur compounds, primarily SO2, are directly related to the sulphur content of the fuel. Engines operating on desulphurized natural gas

Figure 5. Efficiency of a spark ignition engine vs load[4].

or distillate oil emit insignificant levels of SOx (this is only an issue only in larger, lower-speed diesel engines firing heavy oils). In addition, particulate matter (PM) can be an important pollutant for engines that use liquid fuels. Ash and metallic additives in the fuel and lubricating oil contribute to particulate concentrations in the exhaust.[4] Several emission control options exist to help mitigate the amount of pollutants produced. These may include the utilization of low sulphur fuels, combustion temperature control, and post-combustion emissions control via catalytic exhaust gas treatment. [3, 4] In addition, lean burn engines may be used, which use lean fuel-air mixtures. This approach lowers the peak temperatures within the cylinders resulting in low NOx emissions; any excess oxygen in the exhaust gases may be re-used for supplementary firing. With this approach, a trade-off is expected as with decreased temperatures, lower heat recovery from the exhaust is possible.


5.2.5 Maintenance The maintenance problems associated with reciprocating engines are due to increased

wear and tear with the costs differing with the type, speed, size, number of cylinders of the engine used. They often require routine short-interval inspections/adjustments, periodic changing of engine oil and filter, coolant, and spark plugs (typically at 500 hrs to 2,000 hrs).

Manufacturers often recommend a time interval for overhaul, which may vary from 12,000 to 15,000 hrs of operation for a top-end overhaul and 24,000 to 30,000 hrs of operation for a major overhaul. A top-end overhaul involves a cylinder head and a turbocharger rebuild, while a major overhaul involves piston/ring replacement as well as replacement of crankshaft bearings and seals. With proper maintenance, long performance times can be achieved for cogeneration systems.[3]


6 Stirling Engines

The Stirling engine was patented in 1816 and was developed as coal-burning, low pressure engines to compete with saturated steam engines for auxiliary power for manufacturing and mining. However, due to the emergence and widespread adoption of the internal combustion engine coupled with design issues, Stirling engine technology never reached commercial production. This has changed recently, however, with resurgent interest due to development of a free-piston design (described in Section 6.1) to enable solar electric power generation, hybrid automotive and cogeneration applications.

Unlike internal reciprocating engines, Stirling engines are external reciprocating combustion devices, which utilize heat supplied externally via heaters or heat exchangers to drive them. This, thus, allows a large degree of fuel flexibility for oil and gas, and renewable energy sources such as solar and biomass. In theory, the Stirling engine is the most efficient device for converting heat into mechanical energy. With regeneration, the efficiency of the Stirling engine can equal that of the Carnot cycle, which is the most efficient of all ideal thermodynamic cycles. In power-only mode, Stirling engines can be used for stationary power and distributed generation to provide peak shaving and base load power, remote power, backup or standby power and resource recovery (e.g. landfill gas). Much attention has also been placed on the development of Stirling engines for solar-dish concentrators.[16]

Stirling engine based cogeneration systems have some benefits with respect to reciprocating engines, which are high efficiency, good performance at partial load, fuel flexibility, low emission, vibration and noise levels, quiet operation, and long maintenance intervals[15]. Also, the fact that no internal combustion is needed means that a well controlled low-emission continuous combustion process can be used for heat supply. With fewer moving parts, there is limited wear on components and reduced vibration levels. Disadvantages are relative low electrical efficiency, long startup times, and working gas leakage. They have a theoretical efficiency of converting thermal energy into mechanical work of 40%[19] and are generally found in small sizes of 1-25kW and are used in specialized applications.


Stirling engines operate by the continuous heating and cooling of a fully enclosed working gas e.g. helium or hydrogen. The alternate compressing and expanding of a fixed amount of high pressure working gas is transformed into rotational movement to which an electric generator is connected. This process is described more succinctly by the ideal Stirling cycle, which consists of four gas processes: an isothermal expansion (heat addition from an external source), a constant-volume regeneration (internal heat transfer from the working fluid to the regenerator), an isothermal compression (heat rejection to an external sink), and a constant-volume regeneration (internal heat transfer from the regenerator back to the working fluid).[20]

Stirling engines are classified by their drive types, which fall in either of two classes: kinematic or free piston. In the kinematic version, aka crank-driven, the piston(s) are connected by means of connecting rods and a crankshaft with various designs for mechanical linkage such as the alpha, beta and gamma-configurations see Figure 6.


Alpha-type engines (which can be single acting or double acting) consist of two independent cylinders, each with a piston, on either side of the heater, regenerator and cooler. The pistons move uniformly in the same direction providing the constant-volume processes to heat or cool the working gas. When all of the gas has been transferred into one cylinder, one piston remains fixed while the other moves to compress or expand the gas. In the beta configuration, a displacer is introduced and is incorporated into the same cylinder with a power piston. The displacer moves the gas between the hot and cold ends of the cylinder through the heater, regenerator and cooler. The power piston, located at the cold end of the cylinder, then compresses the working gas when the gas is in the cool end and expands the gas when the gas has been moved to the hot end. The gamma configuration also uses a displacer and power piston; however, they are housed in different cylinders, which are pneumatically connected together. Due to the use of mechanical linkages, kinematic Stirling designs require special sealing to prevent leakages associated with the high pressure working gas and passing of the lubricated oil from the crankcase to inside the cylinder.

Figure 6. Three fundamental mechanical configurations of Stirling engines[21]

The free-piston drive, based on the beta configuration, was developed to mitigate the

technical barrier posed by leaking issues in the crank Stirling. Unlike the crank design, there are no mechanical linkages between the piston and an output shaft, see Figure 7. Instead, the piston can move alternately between the space containing the working gas and a spring, which causes the compression- expansion process. A displacer is used to move the working gas between hot and cold exchangers in order to reach the heat flows required for the cycle. To produce power, a linear alternator is attached to the piston, which can be hermetically sealed to prevent leaking of the working gas for substantial periods of time. Furthermore, the working gas serves as the lubricant. With the limitations imposed by linear alternators, the power output of free piston engines are generally limited to small sizes that are < 12.5 kW[4], which is suitable for small-scale applications.


Figure 7. Illustration of a free-piston Stirling engine (Sunpower)[21]

For a Stirling engine-based cogeneration system, exhaust gas resulting from an external combustion process enters a boiler-heat exchanger and releases heat to the active gas in the engine, for example, see Figure 8. Residual heat from the exhaust gas can be used for supplying heat with the help of an additional heat exchanger. Cooling in the cooler-heat exchanger happens with the help of the return pipe of the heat supply network. Thus, the heat discharged in the engine can be further utilized for heating purposes.

Figure 8. Illustration of a Stirling engine based cogeneration system [22]



6.2.1 Efficiency

While Stirling engines are theoretically capable of reaching high thermal efficiencies by reaching the Carnot efficiency in the ideal cycle, they exhibit much lower efficiencies in practice due to material limitations (friction) and inefficient heat transfer (conduction). The efficiency of a Stirling engine is also affected by a number of variables including fuel type, operating temperatures, and engine design. Currently available Stirling engines have generator efficiencies, which is the ratio of engine heat energy input to electrical power output, ranging from nearly 30% for systems as small as 50 W to around 40% for 3 to 5 kW capacity generators. The generator efficiency is largely determined by the efficiency of the alternator and the effectiveness of the regenerator used for the Stirling cycle. The total system efficiency is largely affected by the system employed to supply heat to the engine. In most cases, gaseous fuel such as propane is combusted in a burner, and the resulting heat is transferred to the engine heater head via convection or radiation. With appropriate heat recovery techniques, Stirling-based cogeneration systems can approach 98% efficiency since nearly all of the heat energy is used in some way.[23]

6.2.2 Part-load Performance Stirling engines also have good capability to operate under part-load conditions. It is

expected that while the full load efficiency can be 35-50%, the efficiency at 50% load can be expected to be in the 34-39% range.[15]

6.2.3 Heat Recovery

In a typical Stirling engine, about 30%-40% of the heat input is converted to electric power, and the rest is rejected to the cooling system and exhaust gases, which can be used for space and water heating or other low-temperature heating. In a natural gas fuelled Stirling engine, the sources of heat for heat recovery are the gas cooler, exhaust gas heat exchanger, and to a lesser extent, the cylinder walls and the lubricating oil.[3] Because Stirling engines are liquid-cooled, it is relatively easy to capture heat for cogeneration applications through a simple liquid-to-liquid heat exchanger.

6.2.4 Fuels

Stirling engines, being external combustion devices, can use heat supplied from a variety of external sources allowing the use of a wide range of fossil fuels, biomass, process heat, and solar energy. Stirling engines generally require a heat source between 815oC and 982oC. This heat source can be derived from any noncorrosive fuel that is relatively free of particulates. Rejected heat from other processes that meet these requirements also can be used. Candidate fuels include liquid fuels (distillate oil), liquefied petroleum gas (propane and butane mixtures), sour gas, biogas/biodiesel, industrial waste gases.[4] Contaminants are a concern with some waste fuels, specifically acid gas components (H S, halogen acids (HCl), hydrogen cyanide, ammonia, salts 2


and metal-containing compounds, organic halogen-, sulphur-, nitrogen-, and silicon-containing compounds); and oils.

6.2.5 Emissions

Emissions from current Stirling engine burners can be ten times lower than those of internal combustion engines without the use of catalytic converters[20]. This is due to the fact that Stirling engine burners feature continuous combustion, which considerably lowers the emission levels. The prominent emissions in Stirling engines are NOx and CO. Unburned hydrocarbon and particulate emissions in Stirling burners are negligible compared to those in reciprocating engines. This makes emissions generated from Stirling engines comparable to those of modern gas burner technology. Stirling burners utilize air preheating to achieve high combustion efficiency with low emissions e.g. 80-120 mg/m3 NO and 40-60 mg/m3

x CO, and traceable hydrocarbon and soot emissions[3].

6.2.6 Maintenance

With the sealed operating compartments in free piston Stirling engines, fewer moving parts and lower speeds than reciprocating engines, low wear with long maintenance intervals can be expected. For example, service intervals of 5,000 to 8,000 hrs have been noted for < 20kW systems, which are quite long compared with spark-ignition engines of the same range and resulting in lower maintenance costs[15]. Also, an important maintenance item is the working gas makeup. Since the engine operates with a relatively high working pressure, the working gas may escape over time, which would require periodic top up.


7 Microturbines

Microturbines are miniaturized versions of combustion gas turbines; both being mass flow devices and are thermodynamically the same. They were initially developed in response to the need for light-weight, compact, high-powered generators in the military and aerospace industry. Much of today’s microturbine technology was derived from automotive and truck turbocharger technologies, small jet engines, and auxiliary power units used for ground power for aircraft.

Microturbines produce both heat and electricity on a relatively small scale, which typically range between 25kW and 500kW and efforts are being made to produce smaller power outputs of a few kilowatts. In power only mode, microturbines can provide peak shaving, certain base load applications such as premium and remote power, and grid support where applicable.

Most microturbines are single-stage radial flow devices with high rotational speeds of ~100,000 rpm, however, other designs have been developed based on multiple stages and/or lower rotation speeds. Single-shaft designed microturbines are more common as they are simpler and less expensive to build. In this design, a single expansion turbine turns both the compressor and generator, whereas in a two-shaft design, one turbine is used to drive the compressor and another is used to drive the generator. Microturbines are further classified as being a). simple cycle where no heat is recovered from the exhaust for preheating of the combustion gases and b). recuperated where heat recovered from the exhaust gas is used to heat the combustion gases leading to less fuel consumption and thus, improve the overall efficiency. The majority of microturbines produced today are recuperated, where the use of a recuperator can double the electrical efficiency of the unit while reducing the amount of recoverable heat, which may or may not be desirable.

There are several advantages that make microturbines appealing. Microturbines have fewer moving parts than reciprocating engines, which translates into longer lifetimes (40,000 hrs or more), lower maintenance requirements and cheaper manufacturing costs. They are compact, easy to manufacture and install, produce low NOx emissions, are fuel flexible, have quiet operation with little vibration and can provide base load/ peak shaving and reliable electricity both for stand-alone and grid-connected systems. They also have quick start-up times (~ten minutes) and are modular, i.e. can be connected in parallel to provide larger loads and reliability. A significant disadvantage of microturbines is the limited number of times they can be started and shutdown, which makes them better-suited for continuous-use applications.[24]


The following description is provided for a single-shaft recuperated microturbine system. These microturbines are comprised usually of a single-stage radial compressor, combustor, single-stage radial turbine, recuperator and generator as shown in Figure 9. Microturbine operation follows the Brayton thermodynamic cycle where atmospheric air is compressed, heated, and then expanded.[25] Incoming air is compressed before passing through a recuperator, which transfers the heat from the turbine exhaust to the combustor inlet. The heated air enters the combustor, which is mixed with compressed natural gas introduced at high pressure. Ignition of this mixture creates the combustion gases that exhausts through the turbine making it rotate and, thus, extracts energy to drive the compressor and shaft-mounted alternator for electricity


oproduction. After this stage, the exhaust gas leaves the turbine at ~600 C and returns to the recuperator and into a boiler or absorption chiller for use in CHP or cooling applications. The electricity produced is high-frequency AC, which is usually converted into DC by electronic power conditioners.

Figure 9. Illustration of a microturbine cogeneration system[18]


7.2.1 Efficiency The cogeneration system efficiency is a function of the exhaust gas temperatures from the

recuperator and from the heat recovery unit. Typical electrical efficiencies are 25% to 40% for recuperated microturbines and, in a microturbine-based cogeneration system, efficiencies as high as 85% can be achieved. Manufacturers have developed substantially different designs for recuperators with varying parameters to balance microturbine electrical generating efficiency, CHP efficiency, and equipment cost.

7.2.2 Heat Recovery Heat is generally recovered in the form of hot water or low pressure steam (< 30 psi)

from the exhaust stream, which accounts for close to 70% of the inlet fuel energy. Heat recovery can be done with a heat exchanger and can be used in a variety of ways, including water heating, space heating, and driving thermally activated equipment such as absorption chillers or desiccant dehumidifiers[3]. It is also to be noted that recuperation necessary for achieving high electrical efficiencies limits the quality of thermal output (lowers the temperature of the microturbine exhaust).

7.2.3 Part-load Performance When less than full power is required from a microturbine, the output is reduced by

reducing rotational speed, which reduces temperature rise and pressure ratio through the compressor and temperature drop through the turbine, and by reducing turbine inlet temperature


so that the recuperator inlet temperature does not rise. In addition to reducing power, this change in operating conditions also reduces efficiency. The efficiency decrease is minimized by the reduction in mass flow (through speed reduction) at the same time as the turbine inlet temperature is reduced. Figure 10 shows a typical part-load efficiency curve based on a 30 kW microturbine[4, 26].

Figure 10. Part-load Performance of a 30kW microturbine

It is also to be notes that ambient conditions can have a significant effect on the power output and the efficiency of microturbine systems at elevated temperatures; both power and efficiency decreases.[4]

7.2.4 Fuels While most microturbines run on natural gas, they also lend themselves to fuel flexibility

including diesel, propane, methane, kerosene, sour gas, and biogas. Since each individual microturbine produces anywhere from 25 to 500 kilowatts of energy, they are often grouped to produce the required energy for a given application.

7.2.5 Emissions

The primary pollutants from microturbines are nitrous oxides (NOx), carbon monoxide (CO), and unburned hydrocarbons. Microturbines may also emit a negligible amount of sulphur dioxide (SO2) depending on the amount of sulphur in the fuel. Generally, the emissions are lower than their larger counterparts (steam and combustion turbines) and reciprocating engines. NOx levels have been reported at < 9 ppm for a Capstone microturbine (30 kW) running on natural gas without the use of any emission control technologies. Achieving less than 9 ppm is also the goal for microturbine projects using landfill gas and anaerobic digester gas, but this can be difficult to


obtain if the methane percentage falls below 40 percent. Still, NOx emissions < 9 ppm can almost always be achieved as long as a 15% oxygen mix is used. Some field tests show that when operating at part-load, NO emissions for microturbines are significantly > 9 ppm.[27] x

7.2.6 Maintenance Microturbines are different from normal steam and combustion turbines in that they

contain only one rotating part, and do not require liquids for cooling or lubrication. Due to their simpler design, their maintenance requirements are lower. Still, it is necessary to do perform periodic inspections on the combustor and associated hot section parts. Air and oil filters must be replaced periodically, and oil bearings are inspected. Microturbines operating in environments with extremely dusty air require more frequent air-filter changes. A microturbine overhaul is needed every 20,000 to 40,000 hrs, depending on manufacturer, fuel type, and operating environment. A typical overhaul consists of replacing the main shaft with the compressor and turbine attached, general inspection and, if necessary, replacing the combustor. At the time of the overhaul, other components are examined to determine if wear has occurred, with replacements made as required.

Microturbines in peak-shaving service are usually operated with at least one on-off cycle per day. There are general concerns about the effects of this type of operation on component durability due to thermal cycling, which causes material fatigue.

There is no known difference in maintenance for microturbine operation with fuels other than natural gas. However, experience with liquid fuels in industrial gas turbines suggests that liquid-fueled combustors should be inspected more frequently than natural gas-fueled combustors and may require more frequent maintenance.[4]


8 Fuel Cells

Although the principle on which fuel cells operate have been known for more than 150 years, their commercial success and widespread adoption has been hindered by cost and durability issues. Nevertheless, practical applications of fuel cells have been demonstrated in niche applications such as space flight and cogeneration. Unlike the other prime movers described in previous sections, which are combustion based, fuel cells use electrochemical reactions to produce the chemical energy stored in a fuel into electricity and thermal energy. They are similar to batteries in that they contain electrodes and an electrolyte to enable the electrochemical production of electricity. However, they differ from each other in that they are not storage devices but can only produce electricity continuously so long as fuel and an oxidant (usually air) is supplied.

There are several types of fuel cells, which have been under development, each distinguished by the type of electrolyte used and their operating temperatures see Table 1. Each electrolyte offers its own unique advantages and disadvantages, based on materials and manufacturing costs, operating temperature, achievable efficiency, power to volume (or weight) ratio, and other operational considerations. Currently, only phosphoric acid fuel cells (PAFCs) are being produced commercially for power generation. Other types, such as solid oxide fuel cells (SOFC), proton exchange membrane fuel cells (PEMFC), and molten carbonate fuel cells (MCFC), have entered the testing and demonstration phases.

Table 2. Fuel Cell Types and Characteristics

Polymer Electrolyte Membrane

Alkaline Phosphoric Acid

Molten Carbonate

Solid Oxide

Electrolyte Polymer Potassium hydroxide

Phosphoric Acid

Molten carbonate salt

Ceramic (e.g.Y-ZrO2)

Operating Temperature (

60-100 90-100 175-200 600-1000 600-1000

oC) Internal Reforming

No No No Yes Yes

Fuels Natural gas, hydrogen, propane,

Natural gas, hydrogen, propane,

Natural gas, landfill gas, digester gas, propane

Natural gas, hydrogen

Natural gas, hydrogen, landfill gas, fuel oil

Air Air Air or oxidant Air or O2 Purified Air

or Oxygen- Enriched Air O2

Size Rangea (kW)

1-250 100-200 100-200 250 1-1000

aFor stationary applications


Common to all fuel cells is the use of a fuel such as hydrogen, which combine with oxygen in air to produce either hot water or steam and an electrical current. Typically, fuel cells produce small voltages of ~ 1V; however, when combined serially into stacks, they can produce enough power for larger-scale power requirements with the hot-water or steam produced for thermal applications.

In power-only mode, fuel cells can be used to offer premium power, remote power, and grid support. For micro-cogeneration applications, fuel-cell-based cogeneration units are typically either PEMFCs and SOFCs operating at ~ 80oC and ~800oC respectively[2]. PEMFC-based units have found the most success in the residential market, particularly in Japan and Europe with large-scale national initiatives dedicated on the 10kW range fed with natural gas. Recent efforts have targeted the development of high-temperature MCFCs and SOFCs for this lower-power segment.

With no moving parts, the fuel cell itself offers quiet operation, although consideration must be made for balance of plant components. The electrochemical conversion is relatively clean when compared to combustion-driven prime movers, which produces very low emissions. Not constrained by Carnot efficiencies, fuel cells achieve high efficiencies when used in cogeneration applications. The high temperature variants, MCFCs and SOFCs, are often exempt from the use of expensive catalysts, and can produce high temperature exit gases and cooling fluids to drive combined gas turbine cycles e.g. SOFC-microturbines, however, these are usually in the MW range. Disadvantages of fuel cells include long start-up times for high-temperature fuel cell systems and their high costs.


The fuel cell system itself is comprised of three main systems: fuel processor, fuel cell stack and power conditioner, see Figure 11. The fuel processor is where the fuel is converted into a hydrogen-rich gas with the use of a steam reformer and a shift converter or reactor. For micro-cogeneration units, natural gas is the fuel of choice as it is readily available and consists mainly of the hydrogen-containing methane. The natural gas is converted into hydrogen via a reforming reaction for e.g. steam reforming or partial oxidation. Reformation takes place either in a separate device, called a reformer, or, as in the case of high-temperature fuel cells, within the stack (internal reforming)[28]. As an example, the steam reformer can convert methane to hydrogen and carbon monoxide over a nickel catalyst in the following reaction:

)()()( 24 gCOgHgCH +→

In some fuel cell systems, the CO is reacted with steam in the reactor over an iron-chrome catalyst at a lower temperature to produce more hydrogen, which reduces the CO content to < 1 ppm. A variety of gaseous and liquid fuels, including gas from coal, can be used depending on the design of the fuel processing section. The fuel cell stack is where the electrochemical conversion of the energy stored in the fuel is converted into useful electricity.

The basic principle of operation within a fuel cell is illustrated, for example with a PEMFC, in Figure 12. The fuel cell consists of two porous electrodes (anode and cathode), with an ion-conducting electrolyte between them. At the anode, the hydrogen gives up electrons to the electrode via an oxidation reaction, and enters the electrolyte as a positive ion (H+), while at the


cathode, the oxygen takes electrons and enters the electrolyte as a negative ion (O2- - or OH ). The respective ions (H+ 2- and O ) combine in the electrolyte and form water, while the electrons move through the external circuit to produce electric current.

Figure 11. Fuel Cell based cogeneration system.

Figure 12. Illustration of fuel cell operation for PEMFC[29]


8.2.1 Efficiency

Fuel cell systems do not rely on thermal energy conversion and are, thus, not bounded by Carnot efficiency limitations. As such, they operate in the 40% to 50% fuel efficiency range compared to the 35% for most internal combustion engines and 29% for most microturbines for


http://www.energysolutionscenter.org/distgen/AppGuide/Chapters/Chap4/Images/Figure_4-8_FuelCell_Flow.jpg

electricity production. Therefore, less fuel is need to achieve the same electric power and since efficiency is a factor in heat production, they produce less heat than the other technologies, but still enough to make cogeneration systems very effective.[30] PEMFCs operate at lower temperatures have high power density, and can vary their output quickly to meet shifts in power demand and are best suited for applications where quick start-ups are required.

.2.2 Heat Recovery

additional electricity with the use of a steam turbine, making them suitable for hybrid systems.

.2.3 Part-load performance

of rated power and does not fall off until loads to < 25% to 40% of full load, see Figure 13.[31]

8

There are four primary potential sources of usable by-product heat from a fuel cell system: exhaust gas, including water condensation, stack cooling, anode-off gases, and reformer heat. Heat can generally be recovered in the form of hot water or low-pressure steam, however, the amount and quality of heat is very dependent on the fuel cell operating temperature. PEMFCs operate at low temperatures ~ 80oC – 100oC. About 25% of the inlet fuel energy is recoverable as higher quality (higher temperature) heat from the stack and reformer subsystems, and another 25% is contained in the cathode exhaust gases that include the latent heat of the product water generated in the fuel cell. The most common use of this heat is to generate hot water or low-pressure steam (< 30 psig) for space heating, process needs, or domestic hot water. SOFCs, on the other hand, can generate medium-pressure steam (up to about 150 psig) from the fuel cell’s high-temperature cathode exhaust gas. The primary use of the exhaust-gas heat is in a recuperative heat exchanger with the inlet process gases. Like engine and turbine systems, fuel cell exhaust gas often can be used directly in drying processes. Also, SOFCs generate heat at much higher temperatures sufficient to produce

8

In both power generation and cogeneration applications, fuel cell systems have excellent load following characteristics. However, they also demonstrate higher part-load performance as fuel cell stack efficiency improves at lower loads, which results in a total system electrical efficiency that are relatively constant down to one-third to one-quarter of rated capacity. When compared to combustion driven devices, which operate most efficiently at full load and less so at part-load, the overall efficiencies of a fuel cell system with a fuel processor tends to be between 50% and 100%


Figure 13. Part-load performance of a 200kW PAFC [4]

8.2.4 Fuels

The primary fuels used in fuel cell systems include natural gas, hydrogen, and methanol and is specific to the type of fuel cell used. As mentioned earlier, the most common fuel used in fuel cell systems developed for cogeneration is natural gas. Typically, a natural gas fuel processor is integrated into the system design for a fuel cell power plant. This integration requires the supply of heat to the fuel processor to overcome the endothermicity associated with reformation chemistry. This heat can be supplied by the fuel cell (e.g., exhaust flow into the fuel processor) or by a combustor (auxiliary or anode gas reactor). The most common strategy uses steam reformation over a catalyst, but many other reformation technologies are available, including partial oxidation and autothermal reformation. When steam reformation is used, steam must be supplied to the fuel processor; the steam can be provided through the fuel cell exhaust stream or a separate steam generator.

Other fuels that can be used in fuel cell systems include hydrogen, which can be used directly in all fuel cell types, and methanol, which can be used directly only in a direct methanol fuel cell but can easily be reformed for use in other fuel cell systems.[23] Other sources may include: landfill gas, digester gas, gasoline, diesel, JP-8 (military fuels), dimethyl ether, ethanol, new petroleum distillates, coal gasification products, naptha, bio-gas. With alternative gaseous fuels, care must be taken to minimize contaminants. Contaminants such as sulphur and other components (e.g., chlorides) can have a detrimental impact of fuel cell operation and, may be removed prior to entering the fuel-processing catalyst.


8.2.5 Emissions

Since fuel cell systems produce electricity via electrochemical means, they have the potential to produce fewer emissions. The major source of emissions is the fuel processing subsystem because the heat required for the reforming process is usually derived from the anode-off gas, which consists of about 8-15% hydrogen, combusted in a catalytic or surface burner element. The temperature of this lean combustion process, if maintained below 1,000°C, prevents the formation of oxides of nitrogen (NOx). In addition, the catalytic reactions, level of temperature and the air excess will guarantee the oxidation of carbon monoxide (CO) and unburnt hydrocarbons. As sulphur is a poison for the catalysts used in the fuel processing and the fuel, it is critical to remove this by a catalytic reaction before entering the fuel processing section.

8.2.6 Maintenance

Maintenance costs for fuel cell systems will vary with type of fuel cell, size, and maturity of the equipment. They have the potential for very low maintenance costs because they have fewer moving parts when compared to reciprocating engines and microturbines. However, maintenance of balance of plant systems (BOP) such as pumps and fans needed for operating fuel cell systems can increase maintenance costs. In addition, these BOP systems can cause an increase in both scheduled and unscheduled downtime. Routine maintenance includes replacement of ancillary parts such as fuel filters, reformer igniter or spark plug, water treatment beds, flange gaskets, valves, electronic components, sulphur absorbent bed catalysts and nitrogen for shutdown purging. Major overhaul of fuel cell systems involves shift catalyzer replacement, reformer catalyzer replacement, and stack replacement. Stack replacement is expected between every 4 and 8 years. [3, 32]


9 Commericial State-of-the-Art

The various technological small-scale cogeneration systems are at different development

stages today, see Figure 14.

Figure 14. Market Status Development of Small-Scale Cogeneration (adapted from ref.2)

The examples given below serve to demonstrate the state of the art for each prime mover-based cogeneration technology and are by no means a comprehensive list of manufacturer’s offerings.

9.1 Reciprocating Engines

9.1.1 Honda Motor Co. American Honda Motor Co., Inc. and Climate Energy, LLC announced in April 2007 the

official start of retail sales of freewatt, their collaborative Micro-sized Combined Heat and Power (Micro-CHP) cogeneration system for homes in North America. The freewatt Micro-CHP system has four primary parts: a furnace module with a high efficiency auxiliary burner and ECM blower motor, a Honda MCHP module, a hybrid integration module (HI-module), and a microprocessor based system controller. Whenever heat is demanded by the room thermostat the Honda MCHP unit turns on and begins to generate 3.5 kW of heat and 1.0 kW of electricity. In addition, the system produces 30% less carbon dioxide emissions than a conventional heating system with electricity provided from the grid. Currently, a similar version of an MCHP system is retailed in Japan (Ecowill), with over 60,000 units sold since its introduction in 2006. The MCHP unit is a


natural gas driven, liquid-cooled, internal combustion spark engine-generator set specifically developed by Honda for the home cogeneration application. The heat that is produced by the Honda MCHP is captured and delivered to a heat exchanger in the HI -Module. The heat is transferred into the return air stream from the building and then delivered into the home by the furnace module blower operating in low air flow mode. The Freewatt System runs in this mode for many thousands of hours per year, maximizing the benefits of combined heat and power as well as improving the comfort of the home by maintaining a more constant temperature. If more heat is required than can be provided by the MCHP unit alone, the auxiliary burners in the furnace module are automatically operated.

Figure 15. Illustration of heat flow in the system during the normal combined heat and power mode of operation.

The above illustration shows the heat flow in the system during the normal combined heat and power mode of operation. Heat from the Honda MCHP is transferred to the HI Module via a liquid coolant circulating loop. Return air from the home is heated by engine coolant in the heat exchanger in the HI Module and the heated air is supplied to the home by means of the furnace blower. [33]

9.1.2 Senertec

Senertec has developed the Dachs, which is a micro-CHP unit based on a natural gas, LPG, fuel oil or bio diesel fuelled internal combustion engine. The Dachs has a continuous output of 5.5 kWe and 12.5 kW heating and achieves an overall efficiency of approx. 90% with a fuel input of 20.6 kW. An additional external exhaust heat exchanger can provide a further 2.5 kW of thermal energy, raising the efficiency up to approx. 98%. In combination with a Senertec SE 750 buffer vessel, domestic hot water module (capacity 30 l at 45°C) and an integrated peak load


boiler the so-called Dachs SE solution can meet a heat demand of up to 35 kW. SenerTec and its partner network have installed over 13,000 units. [33]

9.1.3 Power Plus Technologies PowerPlus Technologies GmbH (A company of the Vaillant Group) was founded as a

fully owned Vaillant subsidiary at the beginning of 2004. The technical data of the Ecopower micro CHPU with a specially designed gas fired combustion engine are: electrical output 1.3 to 4.7 kW modulating, thermal output 4 to 12.5 kW modulating, overall efficiency >90%, fuel natural gas, liquefied gas (Propane), flue gas temperature <900C, weight 395 kg, easy installation, low maintenance costs. The unit dimensions do not exceed the size of conventional heating boiler. The Ecopower CHPU is used to cover the basic or medium load while a heating boiler is directly activated to cover peak loads. The Ecopower micro CHPU has been developed especially for the use in small-up to medium –scale buildings and offers a vast range of applications thanks to its output modulation.[33]

Table 3. Summary of Cogeneration Systems with internal reciprocating engines

Honda Motor Senertec PowerPlus Technologies GmbH

Model freewatt Dachs Ecopower Power Pel (kW) 1-1.2 5-5.5 1.3-4.7 Pth (kW) 3.5 12.3-12.5 4.0-12.5 Efficiencies (%) > 90 88-89 > 90 ηel (%) n/a 26-30 25

aηth (%) n/a 59-63 65 Physical Dimensions

W (kg) 81.2 530 395 L (cm) 38.1 72 74 H (cm) 88.9 100 108 D (cm) 58.4 107 137

NG or LPG NG, LPG, Fuel oil or Biodiesel

NG or LPG Fuel Noise (dB) 47 @ 1m 52-58 @ 1m 56 @ 2m

a the efficiencies listed in these tables are based on the low heating value (LHV), which is net heat of combustion less the energy required to vaporize the water produced during combustion.


9.2 Stirling Engines

9.2.1 Whispergen Ltd. WhisperGen Ltd formed in 1995, has its head office, research and development and

manufacturing facilities in Christchurch, New Zealand. The AC WhisperGen microCHP system is grid-connected and ideal for the home or small business. It is gas- or LPG fired and designed to replace central heating boilers for water and space heating while simultaneously generating electricity. Similar in size and shape to a domestic dishwasher, the WhisperGen is quiet and requires minimal maintenance. The overall performance is more than 90% efficient. The electricity generated can be fed back into the electricity grid or used in the home, reducing electricity costs even further.

TMA DC WhisperGen microCHP (WhisperGen PPS16) is also offered for off-grid applications for remote power with either 12V or 24V output and can be made to run in conjunction with solar and wind power. The WhisperGen is being evaluated internationally with systems operating in several countries, including the UK, the Netherlands, Germany and France. Powergen has been working on the market introduction of the AC WhisperGen microCHP system in the UK with an initial focus to complete the installation of the technology in 400 homes throughout Great Britain.[33]

9.2.2 Stirling Systems GmBH Stirling Systems GmbH (previously known as SOLO STIRLING GmbH) has been

concentrating on Stirling technology since 1990. Initially, three 9 kW engines were designed and manufactured in solar version for the test station "Distal" by Schlaich, Bergermann and Partner (SBP), Civil Engineers in Stuttgart. The test site is located at Plataforma Solar in Almeria, Spain. SOLO acquired the license from SBP for the most advanced Stirling Engine SPS V160 with regard to life expectancy and reliability. The SOLO Stirling 161 cogeneration unit can be modified for electrical power output between 2-9 kW and for thermal energy output between 8-26 kW. This feature allows suitable application for medium to large living areas, factories or semi-government facilities. Solar energy, natural gas, biogas/solid biomass, waste heat can all be utilized with a Stirling Engine to provide the thermal input required for operation. Cooling can also be achieved with Stirling engine equipped with absorption chillers, which will be of benefit particularly in warmer climates.[33]

9.2.3 Sunmachine Sunmachine, based in Germany, develops Stirling heat and electricity generators for

domestic heat and power. The SUNMACHINE-Pellet transforms wood into gas, gas into heat, heat into electricity. A modern wood gasifier, burning pellets with a vertical flame, combined with the latest generation of Stirling engine. The electrical efficiency amounts approx. 20% and a condensing boiler overall efficiency up to 90% with the possibility of using different gaseous. liquid, solid, fossil and regenerative fuels.[33]


Table 4. Summary of Cogeneration Systems with Stirling engines.

WhisperGen Stirling Systems GmBH Sunmachine

Model WhisperGenTM PPS16 SOLO Stirling 161 Sunmachine-Pellet 1-cylinder Stirling engine pressurized to 40 bar with nitrogen

4-cylinder Stirling Stirling engine with a 2 V-cylinder motor and engine pressurized to 28

bar with nitrogen helium as operating gas @ 150 bar max. Prime Mover

Power Pel (kW) 0.8 2-9.5 3 Pth (kW) 5.5-6.5 6-26 10.5

90 92-96 90 Efficiencies (%) ηel (%) 12[20] 22-24.5 20 ηth (%) 78[20] 65-75 70 Physical Dimensions

W (kg) 90 460 410 L (cm) 45 128 116 H (cm) 65 98 159 D (cm) 50 70 76 Fuel Diesel NG, liquid gas Wood pellets Noise (dB) 50 dBA @ 7m 65 dBA @ 1m 49 dBA

9.3 Microturbines

9.3.1 Capstone Turbine Corporation

Capstone (headquartered in California, USA) is the pioneer in microturbine development with efforts beginning in 1988. It has the most mature commercial product on the market with its model 330 boasting 3 million hours of documented operations for all of its microturbines at the end of 2002. For cogeneration applications, Capstone offers its C65-ICHP model run on natural gas with electrical and thermal outputs of 65kW and 120kW for a total system efficiency of 82%. Other fuels can also be used such as diesel (including JP-8), biodiesel, and kerosene.[34]

9.3.2 Calnetix

The Calnetix Power Solutions TA100 Microturbine is offered either in recuperated or simple cycle configuration. In the recuperated configuration, combined heat and power (CHP) may be added. The rated electrical output of the TA100-CHP unit is 100kW and the thermal output at 172kW with total system efficiency at > 75%. The modular design of the TA100 provides flexibility for configuring in more complex applications to maximize the balance of


thermal and electrical needs. Additional flexibility is provided with full parallel capabilities whether grid connected, stand alone or in multiple unit arrays. Several hundred Calnetix Microturbines are installed around the world in a broad range of applications operating on various types of fuels (natural gas, coal bed methane, landfill gas, digester gas, bio-diesel, LPG, diesel and kerosene-based) while accumulating nearly 2 million operating hours to date.[35]

9.3.3 Turbec AB Turbec AB, headquartered in Sweden, was founded in 1998 as a joint venture between

Volvo AERO and ABB to develop microturbines for small-scale power generation. Its first commercial product is the T100 CHP system. It is rated at 100kW electrically with a thermal output of 155 kW at 77% overall efficiency. The unit can be run on natural gas or a variety of other fuels such as Natural gas, other fuels as biogas, diesel, kerosene, or methanol.[36]

9.4 Fuel Cells

9.4.1 Ebara Ballard Tokyo Gas has commercialized 1kW-class residential Proton-Exchange Membrane Fuel

Cells (PEMFC) cogeneration systems named LIFUEL into Japanese market in February 2005. By

the end of December 2007, about 450 PEMFC cogeneration systems have been already used in actual households by Tokyo Gas’s FC partnership customers. A system of LIFUEL

consists of a

fuel cell (FC) unit and hot water storage (HWS) unit. The former unit consists of mainly four parts; a fuel processor to reform natural gas into hydrogen, a PEMFC stack producing DC electricity as well as heat, an inverter converting DC electricity to AC electricity, and other balance of plants. The latter unit has a 200-litter tank where the heat recovered in the FC unit is stored as hot water, and a gas-fired instantaneous water heater for backup.

9.4.2 Plug Power

Plug Power Inc. (New York, USA) develops, manufactures, integrates and services proprietary fuel cell solutions. Since their inception in 1997, Plug Power has developed platform-based systems architecture using Proton Exchange Membrane (PEM) and related fuel-processing and system-management technologies. They have developed the GENSYS E-series for off-grid, prime power and grid parallel combined heat and power applications. There are currently three models in the E-series offering rated electrical outputs at 3.0, 4.6, and 8.0 kW and variable thermal outputs of 7 and 35 kW for overall efficiency of 85%.


Table 5. Summary of Cogeneration Systems with microturbines.

Capstone Turbine Corporation

Calnetix Power Solutions

Turbec Model C65-ICHP TA100-CHP T100 CHP Prime Mover Microturbine Microturbine Microturbine Power Pel (kW) 65 100 100 Pth (kW) 120 172 155

82 >75 77 Efficiencies (%) ηel (%) 29 29 30 ηth (%) 53 46 47 Physical Dimensions

W (kg) 1,121 1,860 2,770 L (cm) 76 300 277 H (cm) 211 211 181 D (cm) 196 84 90

Natural gas, other fuels as biogas, diesel,

Fuel NG, diesel, biodiesel, kerosene

NG, Coal Bed Methane, Landfill Gas, Digester Gas, Bio-diesel, LPG, Diesel and

kerosene, or methanol

Kerosene Noise (dB) 65 dBA @ 10m 62 dBA @ 10m 70 dBA @ 1 m


Table 6. Summary of Cogeneration Systems with fuel cells.

Ebara-Ballard Plug Power Model LIFUEL E-Series Prime Mover PEMFC High-T PEMFC Power Pel (kW) 1 3-8 Pth (kW) 1.4 7-25

>87 85 Efficiencies (%) ηel (%) 37 30 ηth (%) 50 55 Physical Dimensions

W (kg) 175 250 L (cm) 85 71 H (cm) 100 101 D (cm) 51 122 Fuel NG NG Noise (dB) n/a n/a


10 Summary and Conclusions

To meet near- and long- term energy requirements, the Canadian Forces must consider alternative ways in which it is used and delivered. This report summarizes the benefits of cogeneration for the simultaneous production of useful electrical power and heat and is of relevance to energy supply modernization programs within the CF, for example, within DLR-6 (“Arctic capability, Energy and Bivouac Equipment”, “Relocatable Temporary Camp” and “LF Modern Power Sources”).

Cogeneration results in higher system efficiencies with lowered operating costs where both heat and electrical power are needed with the added benefits of security and reliability of supply through fuel flexibility. Furthermore, this report summarizes the current state of the art in cogeneration technologies with reciprocating engines, Stirling engines, microturbines and fuel cells, see also Tables 7 and 8. From this review it is clear that no one single prime mover exists that is superior in all areas. Reciprocating engine-based cogeneration systems are the most mature technology, while microturbines, Stirling engines and fuel cells are considered emerging technologies, which, while having been demonstrated, are on the cusp of commercialization. They are challenged by their high cost per kW along with associated technical issues, not to mention the often demanding requirements placed on portability, low noise and thermal signatures, durability and size and weight issues for military use. Still, however, these three technologies remain quite attractive given their fuel flexibility, low environmental emissions and lower maintenance costs. However, on-going development is required to resolve some of the technical challenges they face and further work is needed to obtain more operational hours and longevity studies of these units to validate manufacturer’s specifications.

Thus, the following recommendations are made: a). that the market be monitored to determine if suitable advances are being made in these technologies, b). undertake an investigation of hybrid systems and non-fuel fed technologies using these prime movers, e.g. SOFC-microturbines and Stirling-Solar concentrators respectively, and, c). procure and evaluate commercial units to validate their performance against manufacturer’s specifications. Indeed, the latter activity will serve to enhance knowledge and provide direct experience with cogeneration technologies. As a result of this review, a follow-on activity for “The Evaluation of a 1kW PEMFC Cogeneration System” is already underway within the on-going ARP Project 12sn04.


Table 7. Summary of Advantages and Disadvantages Cogeneration Technologies Reviewed

Cogeneration

system Advantages Disadvantages

Reciprocating engines

• Limited to lower temperature cogeneration applications

• Lowest first cost of all cogeneration systems

• High maintenance costs • High efficiencies at part load operation • Must be cooled even if

recovered heat is not used. • Short start-up times to full loads (10-15 seconds), suitable for backup power systems and peak shaving applications;

• High levels of vibrations and low frequency noise

• Requires frequent maintenance intervals • High reliability

• Fuel versatility • Relatively high air emissions

Stirling engines • High cost • Relatively few moving parts, mechanically simple • Low electrical efficiency

• Low noise and vibration-free operation

• Low maintenance, and high reliability • Long life • Fuel versatility including solar power • Low emissions of NOx and unburned

fuel

Microturbines • High cost • Small number of moving parts • Relatively low electrical

efficiencies • No cooling required

• Long maintenance intervals • Loss of power output and

efficiency with higher ambient temperatures and elevation

• Can utilize waste fuels • Low emissions • Modular designs

Fuel Cells • No moving parts, except fans • High cost • Quiet operation • Fuels requiring processing

unless pure hydrogen is used • High electrical efficiencies under • No existing infrastructure for varying loads

large-scale supply of • Low emissions hydrogen • Modular designs


Table 8. Summary of Performance Characteristics of Cogeneration Technologies Reviewed

Reciprocating Engine

Stirling Engine Microturbine Fuel Cellsa

Size Range 1kW -7MW 1 kW -25 MW 25 -500 kW 1 kW -10 MWe e e e e e e

Electrical Efficiency (%)(HHV

25-40 30-40 25-40 (Recuperated)

25-60

b) Overall efficiency (%)(HHV)

70-80 60-90 80-95 60-95

Part-Load Performance

Good Better Good Best

Emissions (NOx, SO

Moderate Lower Low Lowest ) x

Fuels • hydrogen • liquefied petroleum gas (propane, butane etc.)

• liquefied petroleum gas (propane, butane etc.)

• liquefied petroleum gas (propane, butane etc.)

• natural gas • landfill gas • digester gas

• diesel • liquid fuels (distillate oil)

• natural gas • liquefied petroleum gas (propane, butane etc.)

• kerosene • coal gas • sour gas • sour gas • raw crude oil • biogas • biogas • standard fuel

oil • biodiesel • methanol • industrial waste

gases • diesel • gasoline

a Illustrative values for typically available systems; absolute values will vary for different fuel cell types bHigh heating value


11 References

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[7] World Alliance For Decentralized Energy. www.localpower.org. Last Accessed March

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[10] Holcomb, F. H., Binder, M.J. and Taylor, W.R. (2000). Cogeneration Case Studies of the

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[11] DOD Fuel Cell Program. www.dodfuelcell.com. Last Accessed March 2009. [12] European-Union (2004). Directive 2004/8/EC of the European Parliament and of the

Council of 11 February 2004 on the Promotion of Cogeneration Based on a Useful Heat Demand in the Internal Energy Market and Amending Directive (92/42/EE). Official Journal of the European Union. pp. 50-60

[13] Rule 024 Micro-Generation. www.auc.ab.ca/rule-development/micro-

generation/Pages/default.aspx. Last Accessed March 2009.


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[14] Praetorius, B., Bauknecht, D., Cames, M., Fischer, C., Pehnt, M., Schumacher, K., Voß,

J. (2008). Innovation for Sustainable Electricity Systems: Exploring the Dynamics of Energy Transitions, Heidelberg: Physica-Verlag.

[15] Frangopoulos, C. (2001). EDUCOGEN: A Guide to Cogeneration. The European

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[18] Resource Dynamic Corporation. (1999). Industrial Application for Micropower: A

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[19] Giampaolo, T. (2006). Gas Turbine Handbook: Principles and Practice. 3rd ed: Fairmont

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J. (2009). Efficiency and Emissions Measurement of a Stirling-Engine-Based Residential Microcogeneration System Run on Diesel and Biodiesel. Energy & Fuels 2009, 23, 1032–1039, 23 (1032-1039).

[21] Kreith, F., Goswami, D.Y. (2005). The CRC Handbook of Mechanical Engineering

Mechanical Engineering: CRC Press. [22] www.energytech.at/(en)/kwk/portrait_kapitel-2_6.html#h2. Last Accessed March 2009. [23] Boberly, A. a. K., J.F. (2001). Distributed Generation: The Power Paradigm for the New

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12 Bibliography

[1] Boyce, M. P., Handbook for Cogeneration and Combined Cycle Power Plants American Society of Mechanical Engineers: 2001. [2] Brandon, N. and Thompsett, D., Fuel Cells Compendium. Elsevier Science: 2005. [3] Hamilton, S. L., Microturbine Generator Handbook Pennwell Books Oklahoma, 2003. [4] Hooger, G., Fuel Cell Technology Handbook. Society of Automotive Engineers 2002. [5] Kolanowski, B. F., Guide to Microturbines. Fairmont Press: 2004. [6] Petchers, N., Combined Heating, Cooling & Power Handbook: Technologies & Applications: An Integrated Approach to Energy Resource Optimization. Fairmont Press: 2002. [7] Willis, H. L. and Scott, W.G., Distributed Power Generation: Planning and Evaluation. Marcel Dekker Inc.: New York, 2000.




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This report is meant to provide sufficient background information on the benefits ofcogeneration and to describe various cogeneration technologies that could be considered formilitary use when considering energy upgrades or replacement in deployment installations. Thereview also identifies various prime mover technologies suitable for meeting the near- andlong-term requirements of electric power and heat generation in the 1kW to <1MW range.These include reciprocating engines, Stirling engines, microturbines and fuel cell-basecogeneration systems. The report presents the scientific basis for each of these technologies,performance characteristics (efficiencies, part-load performances, fuels, emissions, andmaintenance issues), as well as the strengths and weaknesses of each system.

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Cogeneration; Reciprocating Engines; Stirling Engines; Microturbines; Fuel cells


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