Cogeneration trigeneration and beyond

Generating the electricity that the modern world depends on by burning hydrocarbons can be a very inefficient process. This is particularly the case where the electricity is generated in large power stations then transmitted long distances to the consumer. National electricity grids may be very convenient, for producer and consumer alike, but as little as one-third of the chemical energy in the fuels burned actually arrives in the form of electrical power. This applies to both fossil fuels and renewable biomass. Some of the energy is lost in the transmission lines, but most is dumped into the atmosphere or convenient waterways in the form of waste heat. When power stations were smaller and located much closer to, or actually right in, the centre of urban areas it was common practice

Battersea Power Station Tube Extension
Hot water from London's Battersea Power Station was stored in this 'accumulator' - a large thermal store - on the other side of the Thames and used to heat houses in Pimlico (Reproduced with permission from Fin Fahey)

to use the hot cooling water to heat nearby buildings; in the case of the landmark Battersea power station in London, the hot water was actually pumped under the Thames to Pimlico on the far bank. This practice gradually died out as power stations were banished to remote country areas because they needed to be larger and larger in the search of greater efficiency and cost effectiveness. These district heating schemes, as they were first known, never completely died out, however, and there is now growing interest in the concept and its more recent variants.

Using one combustion process to generate both hot water and electricity is known as either combined heat and power (CHP) or cogeneration . Cogeneration is perhaps the most appropriate appellation, as it fits neatly with the use of the term trigeneration for installations where the heat generated also powers an absorption cooler (see Chapter 13). Some medium-sized trigeneration installations also produce steam, and the term quadgen-eration is coming into use for these. Any form of hydrocarbon fuel could in theory be used, but for the small- to medium-sized installation the most common are biomass, natural gas, syngas, biogas and producer gas (see Chapters 7 and 11); although biofuels such as rape-seed oil are an alternative. Basic biomass is burnt in a furnace, and the heat produced is used to generate electricity either by raising steam to power either a reciprocating steam engine or steam turbines or perhaps to directly heat a Stirling engine (see below).

Steam Burns Scale
Cogeneration on the larger scale is well established - this Danish plant burns more than 60,0001 of straw a year, and produces 8.3MW of electricity and 20.8MW of heat

Gas can also be used to raise steam, but in small- to medium-sized installations of less than 1 MW the most convenient options are to use it to provide heat for a Stirling engine or as fuel for some form of internal combustion (IC) engine. The final choice depends mainly on the cleanliness and calorific value of the gas to be used (see Chapter 11).

Historically, the first IC piston engines were large low-speed stationary units fuelled by town or wood gas, and used to provide electric and hydraulic power for factories and other industrial facilities. Engines powered by petroleum products came later, but these are now often adapted to run on various forms of gas from biomass. The advantages of this approach include low capital cost and proven reliability, but recovering waste heat from their cooling systems and exhaust gases is far from straightforward.

There are also a number of manufacturers who offer purpose built IC gas piston engines, usually turbocharged, designed to run on a wide range of gases and able to function efficiently even when the calorific value of the gas is relatively low. Usually these are specifically intended for cogeneration applications, so heat recovery is simpler and more reliable. A typical example is the modular SenerTec DACHs range, which starts with units producing 5.5 kW of electricity and 12.5 kW of heat.

Microturbines able to generate between 5kW and 1MW have undergone extensive research and development. The introduction of reliable foil bearings, a simpler form of air bearing which allows the turbine to operate at more than 100,000 rpm without lubrication, has transformed their economics. Waste heat is almost entirely confined to the exhaust, simplifying heat recovery, and noise levels are low. A number of manufacturers now offer packaged cogeneration microturbine units designed to run on natural gas. Syngas and producer gas would normally have to be carefully cleaned of tars and other condensates before they could be considered as fuels for microturbines, but good quality biogas with low levels of siloxanes would be acceptable (see Chapter 11). The US Capstone Turbine Corporation, for example, currently offers microturbine units with integral generators producing between 30 and 65 kW, which can run on either natural gas or good quality biogas.

Generator cooling fins

Exhaust outlet

Fuel injector

Air intake

Generator cooling fins

Exhaust outlet

Fuel injector

Air intake

Generator

Capstone Microturbine

Air bearings

Turbine

This Capstone MicroTurbine® C65 engine cutaway has foil bearings that allow it to operate at 100,000rpm (Reproduced with permission from Capstone MicroTurbines®)

Generator

Air bearings

Turbine

This Capstone MicroTurbine® C65 engine cutaway has foil bearings that allow it to operate at 100,000rpm (Reproduced with permission from Capstone MicroTurbines®)

Microturbine Energy
Capstone C65 MicroTurbine® Integraded Combined Heat and Power units ranging from 30-65kW (Reproduced with permission from Capstone MicroTurbines®)

Ever since it was invented in the early nineteenth century, the Stirling engine has fascinated and beguiled generations of engineers and technologists. Effectively an external combustion piston engine, its apparent simplicity and quiet operation are in stark contrast to the complexities and noise of the internal combustion engine. There are very good reasons, however, why the Stirling engine has, until now, been very much the poor relation of the ubiquitous internal combustion engine, not least that it is very hard to vary its operating speed. The main reason in practice is that the technology of the nineteenth and most of the twentieth century was simply not up to the task of manufacturing a Stirling engine that could develop anything like its theoretical efficiency - as much as 80%.

In principle, a Stirling engine extracts energy from an external heat source and uses it to heat a working fluid inside. This causes expansion, which drives a piston connected to a crank. The working fluid is then cooled, causing it to contract. A second piston then transfers the working fluid back to the hot section of the engine, ready for the cycle to begin again. There are several potential configurations, notably the Alpha, Beta and Gamma Stirling designs, each with their particular advantages and disadvantages. Originally the working fluid was air, but efficiency was low. The ideal fluid is hydrogen gas, which puts high demands on sealing technology as it has the unwelcome ability to leak out through solid metal. These days helium is the usual choice.

Heat exchangers are key to the concept. The larger the difference in temperature between the hot fluid and the cooled fluid the more power the engine will produce, although it will work with remarkably small differences. Thus the heat exchangers must be highly efficient, resistant to corrosion and easy to maintain. Usually this means that the cooling fluid has to be circulated through proportionally large radiators, another factor reducing the attractiveness of Stirling engines as automotive prime movers. Increasing internal pressures also increase the power at the cost of greater loads on the seals, thicker cylinder walls and so on.

Rhombic Stirling Engine

A rhombic drive Beta type Stirling engine (Reproduced with permission from Togo)

Used as stationary engines in a cogeneration or trigeneration installation these drawbacks are more than outweighed by the benefits. Continuous combustion produces fewer unwanted emissions than the intermittent combustion of the IC piston engine.

Stirling Engine 55kw

This Stirling engine generator set by STM Power produces 55kW and can run on biofuels and biogas (Reproduced with permission from Wtshymanski)

Proportionally greater weight and size for the same power output is less relevant, as is the need for a protracted warm-up before it starts to generate power. Constant speed operation is perfectly acceptable in practice. Recovery of waste heat is simpler.

In theory, a Stirling engine can be run in reverse to produce cold air or chilled water. Stirling cryocoolers, which produce very low temperatures, are commercially available in a range of sizes. A cogeneration installation could be upgraded by arranging for electric power to drive the generator in reverse as a motor - the power could come from the national grid or alternative energy sources such as wind or solar PV. This may be more practical in the smaller sizes than the use of surplus heat to power effective but complex absorption coolers.

Modern versions of the traditional reciprocating steam engine also have much to offer in the right applications. More than 150 years of development has produced units which have few of the practical disadvantages of their forbears, most notably that they rarely explode. They are more efficient than steam turbines below the 1MW power threshold, and perform well at part load. When biomass is used to raise steam its naturally variable calorific value results in variable steam output and parameters, which turbines struggle to cope with. Reciprocating steam engines are much more tolerant. Traditionally, maintenance costs were higher, despite the lower operating speeds, pressures and temperatures, and noise and vibration levels were worse. More modern designs offer much improved performance.

The Swedish Ranator system, for example, features compact and efficient steam generators - which can have catalytic coatings to further reduce emissions - and 'buffers', thermal stores on both the steam supply and the condenser which smooth out variations in load and speed. Great attention is paid to seals and piston rings, to eliminate the traditional loss of lubricant into the steam, producing a so-called oil-free design. Efficiency is said to approach 35%, double that of a traditional design.

Smooth, high-efficiency oil-free operation is also offered by rotary steam engines such as the Canadian Quasiturbine and the German 'Steam Cell'. These are sometimes known as screw-type steam engines, thanks to their resemblance to modern screw air compressors. Modular oil-free reciprocating steam engines are also available from Spilling of Germany with outputs ranging from 25-1,500 kW.

Another oil-free engine concept is the heat regenerative cyclone engine, currently (2007) in the final stages of development by Florida-based Cyclone Power Technologies. Claimed to offer all the theoretical advantages of the Stirling engine without the practical drawbacks, the cyclone engine features a cyclonic combustion chamber in which the fuel/ air mixture is spun to promote complete combustion. The working fluid is supercritical de-ionised water, power is extracted via a multi-cylinder radial engine, and the inclusion of heat exchangers and regenerators boosts thermal efficiency up to Diesel engine values. A wide range of fuels can be used, including natural gas, biogas, bio-oils, and even powdered coal. A Waste Heat Engine variant is also under development. This utilises the hot gases from internal combustion engine exhausts or gas turbines, or solar heat. It can also be intimately linked to a biomass boiler.

Reciprocating steam engines and steam turbines both operate in accordance with the thermodynamic Rankine cycle, first described in the nineteenth century. For use in turbines

Steam Engine Cyclone
The radial cylinder arrangement of the Cyclone Waste Heat Engine (Reproduced with permission from Cyclone Power Technologies Inc.)
Heat Regenerative Cyclone
Schematic of the Cyclone Waste Heat Engine powered by biomass combustion (Reproduced with permission from Cyclone Power Technologies Inc.)

the steam must be superheated - heated to well above 100°C - so that when it is expanded in the turbine there is little or no condensation, which would erode the turbine blades. An attractive alternative, especially for smaller installations, is to replace water with some organic compound such as liquid refrigerants, ammonia or silicon oil. Turbines powered by such fluids are said to be operating on the organic Rankine Cycle (ORC). There are many potential benefits. Superheating is not essential: in fact ORC installations can operate on waste heat at no more than 70°C. Due to the greater density of the organic fluid the size of the installation will be proportionally smaller. The unit is a closed loop system, so maintenance is simple and it can operate automatically and unsupervised. ORC units are often installed downstream of a large gas engine to recover energy from the gas engine's exhaust - this is particularly useful where the demands on a gas engine cogeneration or trigeneration installation are primarily electrical (see below.)

On the downside, lower operating pressures restrict efficiency, the technology is relatively expensive, and some of the working fluids are both potentially toxic and inflammable. A number of manufacturers offer ORC units in a range of sizes.

In strictly mechanical terms, the simplest way of converting the chemical energy of a hydrogen-rich gas into electrical power is to pass it through some form of fuel cell. This is an electrochemical device in which the gas is reacted with air or oxygen to produce water/ steam and electricity - effectively electrolysis in reverse. Many different forms of the fuel cell have been tried since the first practical example was demonstrated in 1959, but most require virtually pure hydrogen as fuel. Some types can use less pure fuels such as natural gas, biogas or producer gas, and are therefore particularly suitable for cogeneration. Of these there are three types currently available, although others are in the final stages of development.

All fuel cells contain anodes and cathodes separated by an electrolyte. In most forms the hydrogen in the fuel is split into protons and electrons at the anode, which may incorporate a catalyst. The positively charged protons can pass through the electrolyte to reach the cathode, but the negatively charged electrons have to flow through an external circuit to meet up with the protons at the cathode. There, both react with the available oxygen to form water. It is the flow of electrons through the external circuit that generates the

H Hydrogen fuel is channelled through field flow plates to the anode on one side of the fuel cell, while oxygen from the air is channelled to the cathode on the other side of the cell.

At the anode, a platinum catalyst causes the hydrogen to split into positive hydrogen ions (protons) and negatively charged electrons.

Hydrogen Backing Hydrogen gas a layers

Air (oxygen)

Hydrogen Backing Hydrogen gas a layers

Air (oxygen)

At the anode, a platinum catalyst causes the hydrogen to split into positive hydrogen ions (protons) and negatively charged electrons.

Fuel Cell Oxygen Flow Field

Oxygen flow field

The Polymer electrolyte membrane (PEM) allows only the positively charged ions to pass through it to the cathode. The negatively charged electrons must travel along an external circuit to the cathode, creating an electrical current.

At the cathode, the electrons and positively charged hydrogen ions combine with oxygen to form water, which flows out of the cell.

All fuel cells work on similar principles, although this type demands pure hydrogen

Oxygen flow field

The Polymer electrolyte membrane (PEM) allows only the positively charged ions to pass through it to the cathode. The negatively charged electrons must travel along an external circuit to the cathode, creating an electrical current.

At the cathode, the electrons and positively charged hydrogen ions combine with oxygen to form water, which flows out of the cell.

All fuel cells work on similar principles, although this type demands pure hydrogen power. Sometimes it is the oxygen that is broken down to electrons and protons: in both cases the electrical output is low voltage direct current, and individual cells are stacked together to produce a useful voltage.

Types of fuel cell differ basically in the materials used for anode, cathode and electrolyte. The longest established type capable of working on less than pure hydrogen is the phosphoric acid fuel cell (PAFC), which uses liquid phosphoric acid as the electrolyte and platinum coated carbon paper electrodes. Operating temperature is between 150°C and 200°C, but because phosphoric acid solidifies at 40°C start up is difficult and PAFCs are only really suitable for continuous operation. The cost of platinum is another drawback, as are its projected long-term shortages.

A solid oxide fuel cell (SOFC) differs from most other types of fuel cell in that it is made up entirely from solid-state materials, usually ceramics. Operating at temperatures up to 1,000°C, SOFCs have no need of expensive catalysts and can accept a wide range of gaseous fuels, up to and including paint fumes - provided all sulphur compounds are removed first. This latter step is becoming easier as new sorbent technologies based on rare earth oxides come on stream. SOFCs can be flat sandwiches or concentric tubes: tubular SOFCs are easier to seal but have slightly lower performance. The biggest drawback of current SOFCs is the long start-up period needed to minimise thermal shock to the system. Research continues to develop lower operating temperatures - which will reduce material costs - and to shorten start-up times.

High operating temperatures are the distinctive feature of molten carbonate fuel cells (MCFC). The electrolyte is a mixture of carbonate salts - usually potassium and lithium carbonate - suspended in a porous, insulating and chemically inert ceramic matrix. Anodes are powdered nickel/chromium alloy; cathodes are porous nickel oxide doped with lithium. Operating temperature is around 650°C, and for the reaction to work carbon dioxide must be present at the cathode. This is usually recycled from the exhaust gases. MCFCs are currently the most efficient type of fuel cell available, but durability is still lower than other types. The German company MTU CFC Solutions GmbH is currently offering the HotModule MCFC, which, when fuelled by natural gas, produces 245kW of electrical power and 180 kW of heat. It can also accept biogas, producer gas and similar hydrogen-containing products of pyrolisation and gasification (see Chapter 11).

The exhaust, mainly steam at around 400°C, can be used for both space and water heating and cooling and the generation of more electricity via steam turbines, Stirling engines or ORC units. Although most fuel cells have to incorporate start-up heaters, the reactions once initiated are exothermic, and useful heat can be recovered for cogeneration or trigen-eration purposes.

At first sight, therefore, a cogeneration or trigeneration installation can seem like the ideal solution for many projects, especially if the fuel used comes from renewable sources. Under optimum conditions this type of installation can recover 90% of the energy in the fuel and put it to use. But there is one key drawback, an Achilles' heel that in some circumstances can render the whole concept impractical. The production of heat and electrical power are inextricably linked. If more power is needed, then more heat will be automatically produced - and vice versa. In all types of building there will be times when the demand for heat and power are out of sync, and there will be surpluses or shortfalls of one or the other.

Surplus heat could be diverted to a thermal store (see Chapter 15) and used to make up shortfalls at other times. At worst it could be vented to atmosphere. A surplus of electricity is harder to deal with. Electrical energy stores tend to be expensive and complicated. One option is to use the surplus electricity to generate heat that goes to a thermal store, and in the near future unitised regenerative fuel cells might be a practical option, (see Chapter 15), but the preferred solution for many cogeneration and trigeneration installations is to sell surplus power back to the national grid.

On the domestic scale, the Honda-developed ECO-WILL microcogeneration unit is now in widespread use in both Japan and the US. Based on a natural gas or propane fuelled specially developed single cylinder IC engine, the unit produces 3kW of thermal energy and 1 kW of electricity - and, at 44dB (A), is no noisier than a conventional air conditioner. Its relatively modest heat output minimises summertime surpluses but a back-up furnace or boiler is needed for the depths of winter in most locations. Electrical output is also modest, meaning the household will often need top-up power from the grid. Any surplus can hopefully be sold back to the main electricity supplier.

Power Chp Buffer
Natural gas fuelled CHP, like this 15kW installation from EC Power, is a well-developed technology (Reproduced with permission from EC Power)

It is unlikely that any building project would deliberately shun the benefits of back-up grid connection (see Chapter 1). The use of net-metering to allow the building owner to be paid for the surplus electricity exported to the grid and to set those payments against the cost of taking power from the grid to make up shortfalls or keep the building functioning during any outages of the building's own generation facility is attractive in its evident simplicity. In practice, this may turn out to be a complicated and frustrating exercise, and anyone contemplating cogeneration and the like is advised to discover the local electricity supplier's attitude towards distributed generation and net-metering at an early stage in the project's development.

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Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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Responses

  • sebhat
    How to build homemade organic Rankine (waste heat recovery) cycle?
    5 years ago
  • Taneli
    How trigeneration works?
    5 years ago

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