INTERMEDIATE RESULTS OF THE ENATEC MICRO COGENERATION SYSTEM FIELD TRIALS

Transcription

1 February 2004 ECN-RX INTERMEDIATE RESULTS OF THE ENATEC MICRO COGENERATION SYSTEM FIELD TRIALS Ruud van der Woude Evert ten Haaken ** Sjaak Zutt Bauke Vriesema Ger Beckers ** ATAG Verwarming B.V. Paper presented at the International Stirling Forum 2004, Osnabrück, Germany, May 5-7, 2004 A B Revisions Made by: Approved by: ECN-Clean Fossil Fuels Stirling Technology Checked by: Issued by:

2 Acknowledgement The Enatec micro cogeneration system field trials are an integral part of the development programme, which is financed by Enatec and which is carried out by ECN for Enatec. Abstract In autumn 2002, Enatec started field trials with its Stirling based micro-cogeneration system, which have continued for a two season period until spring Some intermediate results after one season of testing are presented in this article. In general, the systems were doing well and reliability was proven. Noise and vibration levels in parts of the system were successfully addressed, thus increasing the viability of the system. However, the efficiency measurements were not yet conclusive, mainly because of a power limitation on the Stirling engine. In the summer of 2003, the engines were upgraded to 1 kw and initial results of the second trial season look promising. ECN-RX

3 Intermediate results of the Enatec micro-cogeneration system field trials Ruud van der Woude *, Evert ten Haaken **, Sjaak Zutt *, Bauke Vriesema *, Ger Beckers * Enatec micro-cogen B.V. Summary In autumn 2002, Enatec started field trials with its Stirling based micro-cogeneration system, which have continued for a two season period until spring Some intermediate results after one season of testing are presented in this article. In general, the systems were doing well and reliability was proven. Noise and vibration levels in parts of the system were successfully addressed, thus increasing the viability of the system. However, the efficiency measurements were not yet conclusive, mainly because of a power limitation on the Stirling engine. In the summer of 2003, the engines were upgraded to 1 kw and initial results of the second trial season look promising. Introduction The Enatec micro-cogeneration system is a combination of a power producing Stirling system and a conventional condensing boiler, as is shown in fig. 1. The cogeneration system is designed to let the Stirling generator convert some 10% of the available heat into power, while at the same time, the condensing boiler ensures that no heat is wasted, giving a total efficiency of 107%. 230V 50 Hz Gridbox FPSE Burner Air Gas Fluegas 60 L 3 Way valve Fig. 1. System design of the Enatec Stirling micro-cogeneration system The corner stones of the Enatec technology are the system design guidelines, the control strategy and the three main components of the Stirling system: the gridbox, the burner and the * Energy research Centre of the Netherlands ** ATAG Verwarming B.V. ECN-RX

4 Stirling engine. It is foreseen that after successful completion of the field trials licensees will build their own version of the cogeneration system based on these building blocks. The engine that was selected for the Stirling system, is the RG Free Piston Stirling Engine (FPSE) of Stirling Technology Company (STC). The engine has been adapted by the Energy research Centre of the Netherlands (ECN) to fit the specific demands of a cogeneration system and is now capable of delivering 600 to 1000 W of power in a system that is meanwhile producing 5 to 24 kw of useful heat. The gridbox serves as the interface between the engine and the grid, connecting the engine to the grid and overseeing proper operation on and off grid. Finally, the Stirling system comprises a dedicated burner, which has been optimised for low NO x emission and a high yield of useful radiation. The details of the burner have been presented earlier at the ESF 2000 conference (Ribberink et al., 2000). Field trial programme and assessment The field trials are an integral part of Enatec s development programme. The objectives of the trials are to gain experience with micro-cogeneration systems under real circumstances, to test modifications and to demonstrate the viability of the technology. The trials have begun in autumn 2002, span a two-year period and involve ten systems at different locations. This article will address the intermediate results and conclusions after one season of testing. In total, ten engines and systems were available for the field trials. Six were installed at field locations, i.e. with families across the Netherlands, three at the ATAG and ECN laboratories and one system was kept in reserve. Details of the six field locations are shown in table 1. Table 1 Details of field locations across the Netherlands A B C D E F House detached semi-detached detached terraced detached terraced Persons Insulation good good moderate moderate none moderate Radiators yes yes yes yes yes yes Floor heating yes - yes Convectors yes - The six field locations represent a cross-section of the market of larger houses in Northern Europe for which the micro-cogeneration system has been designed. During the first season of the trials, the Stirling engine was still under development and input power had to be restricted tot 8 kw (LHV), some 3 kw short of the system s full potential. As a result the Stirling engine s performance lagged considerably behind the target of 1 kw of electrical power at 10% efficiency. However, other aspects determining the system s performance, such as hours of operation, hours on grid, electricity production and noise and vibration levels could be monitored throughout, making the effort very worthwhile nevertheless. A summary of the measurements regarding these parameters is given in table 2. ECN-RX

5 Table 2 Results of systems at the ATAG laboratory and the at field locations across the Netherlands Lab 1 A B C D E F Time on grid 1 [h] Total operation time [h] Electricity production [kwh] Max. el. power 2 [W] Stirling efficiency 3 [%] Systems B,D and F were installed in the closing parts of the season 2 3 Operation on grid; the Lab 1 engine is a somewhat improved version, running almost continuously Average seasonal efficiency Table 2 shows superior performance for the system in the ATAG laboratory, that unlike the systems in the field, was fitted with a somewhat improved Stirling engine and more advanced software for both the gridbox and the cogeneration system as a whole. Furthermore, the monitoring system, which tended to cause a lot of shut downs due to tight safety margins was first updated in the laboratory. In particular a number of modifications of the gridbox software resulted in a considerable performance improvement. In the Enatec system the Stirling engine is started off grid and connected to the grid after proper synchronisation. This synchronisation procedure was not perfected until late in the season, giving a relatively high percentage of failed connection attempts and prolonged running off grid. Experiences and modifications of the system Although several problems surfaced during the season, most were solved quickly and the field trial systems have been running largely according to plan. In the end three problems remained which needed special attention: lack of flame ionisation, water vibrations and noise levels. The proper functioning of the burner is monitored using the flame ionisation mechanism. A pin in the gap between the burner and the engine s heater head is used to collect a small current which is only running if the gases are ionised, i.e. very hot and the burner is working properly. A lack of current is interpreted as a burner malfunction and the system is switched off automatically. However, during the trials the ionisation pins suffered extensive wear leading to early malfunctioning and systems had to be shut down regularly. The problem has been addressed with other pin designs and other choices of material, which resulted in a considerable improvement, but to date the problem has not been solved completely yet. The problems regarding water vibrations and noise levels are partly a consequence of some features of the field trial system which are shown in figure 2. As can be seen, the Stirling engine is suspended from two steel wires allowing free horizontal movement. The axial vibrations of the engine are that way, to a large extent, insulated from the frame. However, the burner is mounted fixed to the frame and in order not to restrict free movement of the engine, an extra large gap with the engine s heater head is left open, which is sealed with a silicon rubber flap. Finally, also to insulate engine vibrations, cooling water is supplied to the engine through hoses rather than tubing. ECN-RX

6 Fig. 2 Overview of the field trial system and a detailed view of the Stirling engine, the burner, the adapter and the condensing heat exchanger Unexpectedly the engine vibration caused vibrations in the cooling water, despite the use of flexible hoses. This phenomenon only showed at location E, where the water vibrations were transferred to the convectors, giving an annoying noise in the house. Homes with radiators were not affected, although the water vibrations were equally present at these locations as well. Fortunately, the water vibrations could be surpressed sufficiently by fitting vibration dampers, which are now standard in the system. Reducing noise levels proved more difficult. Although engine vibrations are insulated from the frame successfully, the engine itself tends to work as a bass, giving a 50 Hz noise, which is difficult to surpress. Still, by preventing panels from vibrating as result of this 50 Hz engine noise and further insulating the cogeneration system, outside noise has been reduced to a level of 50 dba in the laboratory. However, most engines in the field have not been fitted with these modifications and are still running at 54 dba. Engine modifications Parallel to the field trials, which is a test of the system design, the Stirling engine itself is continuously being developed at ECN. So far this programme has resulted in an improvement of the electrical efficiency to 11%, a result that can be largely attributed to the raise of the Stirling head temperature to the original design level of 620 C, which was achieved by a retuning of the engine. Basically, the capacity of the thermodynamic cycle while running at 50 Hz, was reduced to make it better fit to the rate of heat transfer between the burner and the heater head. This was done by minor changes to the moving mass and the motor constant of the linear alternator as well as the rod diameter and moving mass of the displacer. In the summer of 2003 all field trial engines were fitted with these modifications. Preliminary results, presented in table 3, show an increase of the Stirling efficiency. ECN-RX

7 Table 3 Preliminary results of system E halfway into the second season of the field trials E in lab 1 E E first season Time on grid [h] Total operation time [h] El. production on grid [kwh] Total el. production [kwh] Number of grid connections [-] El. power 2 [W] Stirling efficiency 3 [%] The system is running continuously 2 3 Average power Average efficiency Under real circumstances the power and efficiency of the system are considerably less impressive than in the laboratory, where the system is running continuously. This can be mainly attributed to the large number of starts and stops of the engine, on average 13 a day. In future systems the number of starts and stops should be limited by choosing an operation mode with low burner power thus extending gridtime and improving average Stirling efficiency. Concluding remarks In summary, the field trials have shown that the Enatec Stirling engine in its present state of development is a reliable machine, which can operate unattended in a micro cogeneration system providing a home with up to 1 kw of power at 10 % efficiency. On average these figures are somewhat lower due to a large number of starts and stops which is a consequence of the present control strategy. However, a new control strategy based on prolonged operation with low thermal load should decrease the importance of this phenomenon. Finally, future developments at Enatec will be aimed at further improvement of the engine performance, reduction of manufacturing cost, preparation for mass production and further optimisation of the Stirling system as a whole. Acknowledgement The development phase of Enatec, including the field trial, is being subsidised by the Dutch ministry of Economic Affairs through its Novem agency. Reference Ribberink, J.S., Zutt, J.G.M, Rabou, L.P.L.M., Beckers, G.J.J., Baijens, C.A.W., Luttikholt, J.J.M. (2000), Stirling Based Micro Co-Generation System for Single Households, Proceedings of the European Stirling Forum 2000, Osnabrück, Germany, February, 2000 ECN-RX