End of the ice age for Stirling engines. Jean-Pierre Budliger Eckhart Weber

Transcription

1 End of the ice age for Stirling engines Jean-Pierre Budliger Eckhart Weber

2 End of the ice age for Stirling engines Jean-Pierre Budliger Eckhart Weber «Today s heating systems all share the same feature of allowing the user to quickly create a comfortable indoor climate by adjusting a thermostat. However, from an energy standpoint this feature comes with a drawback: owing to a relatively low energy price it tempts the user into a thoughtless use of heating energy. (Source: Volker Leiste: Standpunkt newsletter of November 1992). For decades it has mainly been the existing combustion process that has been improved in our heating units, at the same time significantly reducing their emissions levels. Thanks to condensing technology, combustion efficiency has been raised by around 10% and, as can be seen in the promotional material of well-known heating firms, overall efficiency has been raised to over 100%. So what more could we ask for? However, this approach completely ignores the fact that combustion heat in the flame of a boiler is released at around 1500 C, while the water circulation system heats radiators up to barely 50 C. In other words, the quality of the released thermal heat is very poorly exploited, being used only for humble domestic heating; this is the case for nearly a third of our national energy consumption. The example of combined heat and power systems (CHP) shows how the energy released can be far less wastefully used. In this system, the combustion heat is first converted in an engine (in the case of larger systems, in a turbine) mainly for the purposes of electricity generation, and only the surplus heat from this process is used for heating purposes. Image 1: The SUNMACHINE Stirling engine is specially developed for use as a power-generating domestic heating device. At 42% it features high efficiency, is maintenance-free and designed for a long service life of least 80,000 hours. The unit shown here is a kinematic machine with 2 pistons and 2 cylinders with oil-free driving gear and an electrical generator integrated in the engine. This combined effect has already been used for a long time in district heating systems, for example, from which houses are supplied with heat energy via long, insulated pipes. This technology has proven its worth most in densely populated areas of our cities if there are suitable power plants nearby. However, for low connection densities, the investment costs of this kind of distribution network are high, meaning that for many residential districts centralized heat supply is not economical. What is therefore needed is ecofriendly, mini power plants for the decentral supply of heat and power, which can be directly installed 12 Solarzeitalter 1/2003

3 in the basements of detached family houses and small residential buildings. For such applications, thanks to its continuous and low-emission operation, its ability to comply with the most stringent air content regulations for residential districts, and its external combustion, the Stirling engine has again become the focus of considerable interest. THE LONG HISTORY OF THE STIRLING IN BRIEF Since its invention in 1816 by Robert Stirling, this engine design, originally conceived as a hot air engine, has had a long and eventful history. It is often forgotten that in the 19th century thousands of these machines powered lathes, saws, fans, forging bellows and many other industrial tools and devices. However, with the advent of electrification and the invention of the diesel and spark-ignition engines, the hot air engine lost its practical importance in the early years of the 20th century. Around 1935 Philips took up the principle of this engine again when developing a small, quiet and portable generator unit for the emerging army radio communications systems. After the Second World War the development work continued, and it was soon found that with an external power source this unit was among other things well-suited to running low-temperature refrigeration devices. For many decades numerous leading physics, chemistry and biology laboratories were equipped with these increasingly advanced refrigeration devices, which have remained very popular down to the present day. Parallel to this, Philips continued the development of Stirling drive engines and in the early 1970s sold licences to United Stirling in Sweden, and later to M.T.I. in the US, as well as directly to the automobile industry. It was hoped that it would be possible to develop more efficient and especially lower emission engines for vehicles compared to the internal combustion engines of the time. However, Image 2: The SUNMACHINE Solar/Gas is a hybrid machine powered by a solar-dish-system with a reflector diameter of only 4 metres and which can provide as much energy as a 25 m² photovoltaic installation, 10 m² vacuum tube collectors, a 3 kw power unit and condensing boiler heating system combined. The hot water obtained can be used for home heating, heating of domestic water or room cooling by means of an integrated adsorption chilling machine. the Stirling was not able to make any inroads against the ongoing improvement of the conventional engine, especially after the introduction of the catalytic converter for the cleaning of exhaust gases. As a result, the Stirling remained successful only in the field of refrigeration technology; in the meantime, countless numbers of mini cryoplants were produced, mainly in the form of Gifford-McMahon or pulse tube cryocoolers, without which it is now difficult to imagine many areas of application. THE KEY NEW APPLICATIONS OF THE STIRLING AND THEIR DEMANDS Efforts now being made to further develop the Stirling units are first and foremost concerned with the CHP systems already mentioned. However, a wide range of other applications are also being worked on, of which we will mention only the most important below. For many other potential applications, there are as yet no clearly defined specifications for the Stirling Solarzeitalter 1/

4 units. The essence of the Stirling engine can be reduced to the temperature difference via a porous structure, the regenerator and an oscillating volume of gas everything else is customizable. This flexibility offers major development potential for new applications. For example, we have shown how it is possible to supply energy by means of a sound wave if it passes through the temperature difference of the regenerator (acoustic Stirling engine) or that tornados are driven by a temperature difference, and in the laboratory it has been possible to generate controlled mini-tornados research objects for solar updraft towers. MINI COMBINED HEAT AND PO- WER SYSTEMS (CHP) Image 3: Functional principle The largest market segment for the Stirling systems is probably that of the mini CHP plants, which have a capacity range of around 1-4 kw electrical power. This corresponds to a thermal power input (combustion capacity) of around kw. Such plants can be used solely to heat detached family houses, for example, without additional heating devices, depending on the heating requirements, or in dual mode to heat and provide some of the electricity for smaller apartment blocks or commercial premises in-house. In such cases, the peak heating power required on a small number of days per year can be provided by burning extra fuel in a practically unchanged Stirling heater head. The recommended heat-based operation of such CHP plants offers clear energy savings compared to the separate generation of heat and power. The output can be adjusted via the rate of revolutions, the gas pressure or, in the case of free piston operation, via the piston amplitude, and optimally matched with the actual heating requirements. In estimating the realizable gain it can be assumed that the electricity generated in addition to the heat output can be produced from the fuel with near 100% efficiency. Furthermore, locally-produced electricity will mean additional losses from transmission are avoided. This type of operation will require a conection with the public grid, in order that a possible surplus of generated electrical energy can be fed in or additional demand met by the grid. Using an intelligent tariff policy, agreements on the operating times of such plants and appropriate water management systems (provision of sanitary hot water for the household), both homeowners and utilities could derive optimal profit from this cooperation. Depending on the place of installation, the operation of such plants fully independently of the electrical distribution network is absolutely possible. However, in developed countries this type of operation may be the exception. In small commercial premises electrically powered operation or a combined operation for rationally handling peak periods is also conceivable. Image 4: The SUNMACHINE Wood Pellet is a mini block-type thermal power plant specially designed for modern wood pellet fuel. The pellet feed from a buried tank or storage area is fully automatic using pneumatic conveyors, as is the ash removal. Regardless of the type of operation, such a CHP unit must be flexible and easy to operate, provide a long service life and only require a minimum of maintenance. In the ideal scenario such units will be installed like a refrigerator and run almost servicing-free. Maintenance should be limited to the servicing required for safety reasons and to the necessary periodic emissions checks of the burner and flue. The investment costs for such Stirling units are acceptable provided the units can be manufactured in series. In the case of Stirling-only operation, there is no need to purchase a boiler, so that only the investment costs must be offset against the gain from the additional power generation. The unit must be as cost-efficient as possible to produce, require few components and be rigorously designed for longterm and maintenance-free operation. The machine should be sufficiently efficient in operation, even if this in no way provides the main criterion for the choice of appropriate unit. These units will be installed and connected in a similar way to a boiler, meaning they can also be easily fitted in existing buildings. RENEWABLE ENERGIES Prompted by the ongoing debate on climate change, renewable energies will certainly assume an increasingly important role in the future, and at the latest with the foreseeable rise in fossil fuel prices. The externally heated Stirling engine can be designed to run off both solar power and biomass fuel. A cost-efficient, series-produced Stirling 14 Solarzeitalter 1/2003

5 engine such as that planned for small CHP plants will form an important foundation stone of such systems. Its development therefore allows us to use our fossil fuel resources more rationally, promotes the use of renewable energies and also helps us significantly to reduce our CO2 emissions. Solar-powered installations in tropical or subtropical regions have shown a 2-3 times higher net energy gain than in our latitudes. The extremely flexible application possibilities offered by these units, which can run with or without connection to the public grid and under varying loads, thus provide ideal conditions for their widespread, decentral and local use, both here at home and in many developing countries. Such systems can also cool rooms (combined heat, power and cooling systems with adsorption chillers) or run conventional air-conditioning systems with the electricity they generate. It is particularly wellsuited to providing cooling capacity, as it runs according to sunshine or practically according to the prevailing daytime temperatures. Stirling units could open up interesting niche markets. STIRLING ENGINE DESIGNS It is virtually impossible to describe the many existing Stirling concepts here. We will limit ourselves to the designs which are of primary interest in connection with the markets sketched above. For these applications, a complete break must be made from the compact automobile engines previously developed at great expense with high power density but relatively limited service life (5000 hours approx). For CHP generation with small units (the primary market is in the 1-4 kw electrical power range), a virtually maintenance-free service life of 50-80,000 hours must be achievable. Such demands can only be met with drastically reduced power densities. Instead of the Siemens design (4 double-action pistons each with a 90o phase differential), the V-design of two piston-fitted cylinders (a hot expansion cylinder and a cold compression cylinder) or the single-cylinder design (with a power piston and a displacer piston) are preferred. With low capacities the reduced power density is of only marginal importance. In kinematic machines these pistons are moved via a connecting rod and crankshaft in a fixed phase position and amplitude. In most cases, these machines are fitted with greasedfor-life rolling bearings. In this way the use of oil (and thus the possible contamination of the working gas by this oil) is avoided. These solutions allow the engine to be installed and operated in the orientation of the user s choice, something which can be of significance especially in the cases of the solar dish systems and the new cryocoolers. For over two decades, free-piston Stirling engines have also been constructed in which the precise axial movement of the two pistons arranged in a single cylinder is ensured by means of flexible, mechanical plate springs. The pistons, which are fitted with simple gap rings, function almost without friction, thus making the entire system of mechanical gearing unnecessary. In CRYOTECHNOLOGY As we have already seen, Stirling-powered cryo-refrigeration technology has been successfully used for decades in a wide range of applications. New technical developments, such as the use of high-temperature superconductors, semiconductor technologies and new biomedical devices will increasingly require such low-temperature coolers and invigorate these markets. In the process, these products will have to be constantly and flexibly adapted to new technical demands and application conditions. The anticipated cost pressure calls for new concepts and will require the series production of the basic units, also putting greater demands on efficiency, reliability and maintenance-free operation. These prospects show that tried and tested Image 5: The SUNMACHINE Gas is a mini block-type thermal power plant using condensing technology for all gaseous fuels: bio-gas, natural gas etc. It generates approx. 30% power and approx. 75% heat and so has as is customary with condensing technology an overall efficiency greater than 100%. Solarzeitalter 1/

6 contrast, the pistons are acted upon by the gas pressure produced during the cycle and kept in cyclic motion by means of additional pneumatic spring capacity. As the piston dynamics and thermal circuit process are interconnected, the main problem of these machines consists in precisely controlling this process through the various spring and gas forces, thus allowing the required phase angles and amplitudes to adjust. A further solution is the so-called acoustic Stirling engine which features a single piston in a cylinder separating the hot expansion volume from the cold compression volume. By means of an additionally installed resonance tube, the phase of the pressure change of the working gas adverse to the piston movement (or the pressure change caused by the piston movement) is shifted, thus producing work output. (The resonance tube thus functions like an impedance in an oscillating circuit). This concept makes the lowest mechanical demands; however, it requires a relatively long resonance tube and has thus far been subject to less testing than the other concepts. DEVELOPMENT STATUS The concepts described above currently exist in the form of prototypes. In the great majority of the existing machines, the expected theoretical overall electrical efficiency mainly in the range of 25-30% is supported by experiments. This indicates that the basis of these machines has been understood and that the technology is available. A targeted development of a commercial product is possible within some years; here, the main focus will have to be on field testing these machines under their precise intended future application conditions. Equally, the expected long running times require extensive and continuous testing. Great efforts have also to be made regarding the simple design of these machines, in order that they can be affordably produced in series. A number of components, such as the heater head or regenerator must be produced in as standardized a form as possible, in order to ensure an inexpensive, reliable end product. An improvement in the existing machines is also expected from the appropriate choice of materials. Kinematic 2 pistons, 2 cylinders Kinematic 2 pistons, 1 cylinder SUMMARY Free piston 2 pistons, 1 cylinder Image 6: Stirling engine designs for use as mini block-type thermal plants. Acoustic 1 piston, 1 cylinder + resonance tube The calls for conscious energy savings are giving new impetus to and opportunities for the wider use of Stirling systems. So long as some of our electrical energy continues to be derived from fossil fuel sources, the decentral CHP system allows without loss of comfort heating oil and gas to be used much more rationally, thus saving large amounts of primary energy. In Germany alone, over 10 million small heating plants are in operation which could be replaced by decentral mini CHP plants. Assuming a market penetration of only about 20%, this would give an installed electrical capacity of over 4 million kw, corresponding to the replacement of 4 large nuclear power stations. 16 Solarzeitalter 1/2003

7 To address this market there are various machine concepts, all of which have advantages and shortcomings. This range of options on the one hand provides the assurance that for any required application a solution will be possible. However, at the current stage of development it can still hardly be decided which concepts can be successfully realized in the future. The necessary technology already exists and so the basic risk is low. What is now required is a maturation process in order to develop a marketable, commercial product from the current prototypes and ensure that this ecofriendly technology enters the market. Series production of these units will create new jobs in a technologically advanced field, which is ideally suited to the existing know-how in German industry. Furthermore, in addition to the domestic market mentioned above, there is also the potential to build up a significant export market. The construction of reliable, lowmaintenance units also facilitates the widespread use of renewable energies whether in the form of solar power or biomass. Affordable solutions can only be expected from series-produced products appropriate for a decentral use and which pave the way of these technologies in the future. STIRLING ENGINES AS POWER-GENERATING HOME HEATING DEVICES Stirling engines as mini block-type thermal power plants are very simple machines which offer specific advantages over conventional home heating, fuel cell heating devices and block-type plants powered by internal combustion engines: Benefits compared to conventional home heating (and public grid electricity): As well as generating heat, it also provides elctrical power for the home (CHP) also as an isolated system Generation of electricity reduces requirements from the public grid Generation of electricity reduces transfer loss via the public grid Reduction in primary energy consumption of up to 47% achieved even with fossil fuel energy sources Reduction in carbon dioxide emission of up to 62% achieved even with fossil fuel energy sources Can be powered by direct sunlight Reduction in harmful emissions of up to 94% Benefits compared to fuel cell heating devices Technology already available Can work with all types of fuel: gas, liquid, solid, fossil, regenerative Can be powered by direct sunlight Cheap to produce Greater overall efficiency Not dependent on the hydrogen energy industry (decentral) Maintenance-free operation possible Long service life Faster load-adjusting speed Benefits compared to block-type plants powered by internal combustion engines: A great many more fuels possible Can be powered by direct sunlight Combustion is times lower in emissions Maintenance-free operation possible Considerably quieter, even without sound insulation More efficient (with small output) Greater overall efficiency Jean-Pierre Budliger, Centre de Technologies Nouvelles, Plan-les-Ouates (Switzerland) Eckhart Weber, SUNMACHINE Gesellschaft für Stirlingtechnologien mbh, Nuremburg (Germany), Solarzeitalter 1/