Development of a domestic combined heat and power unit based on a Free Piston Stirling Engine and Powered by ENATEC

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1 Development of a domestic combined heat and power unit based on a Free Piston Stirling Engine and Powered by ENATEC Abstract The Dutch consortium ENATEC was formed in 1997 with the objective to develop domestic combined heat and power technology based on condensing central heating technology and a Stirling generator of the free piston type. The driving force behind the development has been the awareness that the caloric efficiency of some 98% in state of the art central heating technology can not be improved significantly, whereas the electricity that is consumed in houses is often generated remotely with low overall fuel efficiency. Stirling Technology Company Inc developed the Stirling generator used in the ENATEC technology. This generator is intrinsically maintenance free, and designed for long life and low-cost manufacturability. One of the partners in ENATEC (the Energyresearch Center ECN) has adapted STC s design to allow stable operation when the generator is connected to the grid. ENATEC has built a number of prototype dchp units, integrating standard boiler components of ATAG Heating (the second partner) with the Stirling generator. An electric power level of 1000 Watt was choosen for the units. Eneco, the third partner in ENATEC, has been instrumental in the development by a third party of the electric interface between the generator and the public electricity grid. The units are successfully being tested in houses since late The paper introduces the concept of a Dutch reference house in terms of gas and electricity consumption. When replacing a modern condensing boiler in this reference house with an ENATEC unit, the overall gas consumption necessary to satisfy the demand for all of the heat and part of the electricity in that house, is reduced by some 14%. This of course not only benefits the end user of the unit, but also the environment. Tokyo, pagina 1 van de versie WS6_1_Woude_Fullpaper.doc

2 Introduction People living in the moderate and cold climate zones of the world all need comfort in their homes. Those who have the good fortune to live in the developed world, consider the availability of cold and warm running water and some form of central heating to be a basic requirement in their house. Also the un-interrupted supply of electrical power provided by some remote power generating facility is considered normal. This paper deals with the relatively new phenomena of so-called domestic combined heat and power: the combined generation in domestic appliances of heat for comfort and electrical power, both generated where they are needed: in a house. Why did dchp emerge? To answer that question let us first consider the evolution of domestic heating appliances. (In this paper a level of energy consumption per year per residence is used of m 3 natural gas for heating and kwh of electricity as reference. 75% of the exisiting Dutch houses consume these levels of energy or more). The caloric efficiency of central heating appliances in Europe has improved over the last 4 decades from some 60% to over 95 %. Restricting ourselves to natural gas as the choice combustible, the reference house now consumes about one third less natural gas for heating then in The ensuing reduction in CO2 emission is also about 33%. The emission of NOx is reduced even more because of steadily improving burner technology. Evolution of central heating technology in Europe IEB CB Caloric efficiency, % Gas consumption of Reference house m 3 / y CO2 emission, ton / y 5,6 4,8 4,2 3,7 NOx emission in flue gas, ppm <150 <90 <40 <40 IEB: Improved Efficiency CB: Condensing Boiler Fossil fuel fired power generating facilities of the present generation have caloric efficiencies of the order of 55%. Where economically viable, the waste heat is being utilized for domestic heating. In most facilities however the waste heat is lost. Also not all fossil fuel power generating facilities are of the latest design. In this paper an average efficiency of 41% is used for remote power generation. This includes transmission losses, running at about 6%, even in a small country like Holland. So what can be done to improve the efficiency of the overall domestic energy requirement, in order to reduce the environmental impact that is inevitable when burning fossil fuel? Naturally, the solution to this question must reflect that the end-user will want to maintain the high standard of comfort that he is used to. His house is comfortably warm all year round. He only needs to concern himself with getting his central heating appliance serviced every 18 months or so. When he opens the hot water tap, he gets hot water, often he does not even know what appliance is responsible for the heating of the water. When he flicks the switch, there is light. All he needs to do is pay the invoice that is sent by the utility company. Domestic combined heat and power is emerging because the end user can continue to be as comfortable as he is, at the same time paying his utility company less for that comfort, while causing less impact on the environment through the efficient use of the waste heat in the generation of electricity in his home. All this is made possible by the integration of in some cases new technologies and in some cases - like ENATEC - existing technologies. These technologies have in common that the waste heat from on-site electricity generation is used for heating the house. In 1997 Eneco, the third largest Dutch utility company, teamed up with ATAG Heating, an innovative boiler manufacturer in Holland, and ECN, the Energyresearch Center of the Netherlands to form the consortium ENATEC micro-cogen BV. Tokyo, pagina 2 van de versie WS6_1_Woude_Fullpaper.doc

3 The objective that was given to ENATEC was to develop a marketable dchp technology, and to license this technology to boiler manufacturers. It was soon determined that the Free Piston Stirling engine would be best suited as prime mover for an electricity generator in dchp appliances because of its simple and maintenance free design and external heating requirement. History of ENATEC In 1999 a Purchase and License Agreement was signed between ENATEC and Stirling Technology Company Inc. of Kennewick, WA (STC). ENATEC purchased three Stirling generators of 1000 Watt electrical output, and obtained a license for the use of STC s technology in dchp in Europe. The largest generator STC had available at the time was 350 W electrical, so a scale-up engineering effort was initiated. At the same time ATAG Heating started work on the integration of the generator in a central heating condensing boiler, ECN started building up Stirling know-how and the design of a burner for the Stirling, while Eneco commenced work on the electrical connection of the generator with the public grid. Basis for the development work in ENATEC has been that the developers were required to always have the end user in mind. It had to be a market driven development, not a technology driven development. More specific: the requirements for the appliance were that it should be as easy to install as the the current type of condensing boilers, it should not need more maintenance, it should produce a level of sound comparable to current boilers, and the enduser should earn his extra investment (for the generator etc.) back in an acceptable period of time. It is a tribute to all involved in ENATEC during the last four years that a field test is currently being conducted, and has been showing continued success since late last year. The remainder of the paper will provide some insight as to how and why the technology was developed as it was, and describes the field test units. Choice for 1000 W electrical output Very early in the development program a choice had to be made for the level of electrical output of the Stirling. The procedure that was followed here started with the determination of the heat demand in a standard (Dutch) one family house as function of time in the year. For this pupose a computer simulation was used, where the surface area and degree of insulation of the house was used as input on the one hand, and on the other hand the average weather conditions over the last ten years. In this way the heat demand was determined for houses with a range of annual gas consumptions on a five-minute interval. It was assumed that this heat demand was satisfied by micro cogen units with varying electrical capacities. It was further assumed that the electrical efficiency of the units was 10% and the thermal efficiency 90%. In the case where the thermal capacity was too small to satisfy the heat demand, a booster burner would be included in the unit. The result of this exercise was the amount of electricity produced over the year in five-minute intervals as a function of electrical capacity of the unit and yearly gas consumption of the house. These data were compared with electricity consumption patterns that were actually measured and available on the same five-minute basis. In this way the amount of electricity that would be consumed in the house was determined in relation to the amount to be delivered back to the grid. The price one would receive from the utility company for the amount of electricity delivered back to the grid was uncertain. Therefore a high and a low scenario were calculated, finally leading to an annual benefit for the house owner. On the other hand of course the cost differential of a dchp unit compared to a conventional heating appliance needed to be considered. Since the linear generator accounts for 80% of the mass in a Stirling engine, it was assumed that the cost of the Stirling in large-scale production would be dominated by this part. Tokyo, pagina 3 van de versie WS6_1_Woude_Fullpaper.doc

4 It was then assumed that the relationship between power output and price of the Stirling would be similar to that of commercially available electric motors. In this way a high, a medium and a low price level were calculated for the Stirling. With a simple pay-out time of five years the investment cost was compared to the annual income (or rather: benefit from lower utility bills) from producing electricity in-house. Relative financial benefit dchp after 5 years as function of power output 100 Relative benefit Power output Stirling [Watt] All calculations result in an optimal generating capacity between 800 and 1200 W. Furthermore it is noted that the optimum is fairly flat and that the absolute level of the maximum is mainly determined by the consumer price of the Stirling and the assumed electricity price. A generating capacity of 1000 W was choosen. Any other power level between 800 and 1500 W would lead to an equally viable system. Description of the Stirling engine Below the operation of a Stirling engine is described, see the schematic drawing. Also the feature that makes the Free Piston type of Stirling engine truly maintenance free is explained. (source: NASA) The functioning of a Stirling engine can be explained as follows. 1. The cycle starts when the lower piston, the power piston, moves up and compresses the working gas at low temperature. 2. In the second step the upper piston, called the displacer, moves down and displaces the gas from the cold space to the hot space. 3. In the next step the power piston comes down and the gas expands at high temperature. 4. Finally the displacer goes up again and the gas is pushed into the cold space. Tokyo, pagina 4 van de versie WS6_1_Woude_Fullpaper.doc

5 Heat is fed externally into the cycle at high temperature and extracted at low temperature. The cold and hot space are kept apart by the use of a so-called regenerator which absorbs the heat when the hot gas is flowing down and gives it back to the cold gas when it is flowing up. The cycle produces mechanical work. In the schematic drawing a thin rod is shown that is attached to the displacer, and that runs through the length of the power piston and piston rod. This displacer rod is removed and replaced by a spring in a free piston machine. The spring keeps the displacer in place. The power piston is suspended in a spring also. What we have now is an arrangement of two mass-spring systems, which by proper design can be made to oscillate at an eigenfrequency of 50 Hz. The huge advantage of this design is that the shaft-crank mechanism, that characterises the original kinematic Stirling design, is eliminated and that lubrication and maintenance are not required. In the STC Free Piston Stirling design a linear generator is incorporated. The mover of the linear generator is connected to the power piston rod. The total engine is contained in a pressurised vessel. Only electrical leads are fed through the vessel, allowing an optimal pressure of the working gas to be maintained. A disadvantage of the Free Piston Stirling design is the fact that the phase angle between the two moving pistons is not optimal, which leads to a small loss of efficiency. An important feature in the STC design is the use of special springs called flexures, which make a truly linear motion of the pistons possible. This allows the design of a very narrow gap between the piston and its cylinder, thus avoiding seals while keeping the power loss acceptable. Maintenance, associated with seals, is eliminated. By design this engine is maintenance free. Development of 1000 W Free Piston Stirling Engine The basic development of the 1000 W Free Piston Stirling Engine was performed by STC in the United States based on specifications drawn up by ENATEC. The main specifications that were given to STC included: Operating mode: Grid connected Power: > 1000 W Voltage: 230 V Frequency: 50 Hz Coolant: water at 45 C Life, constant: hours (15 years) Life, intermittent: cycles (15 years) The first engine was delivered to ENATEC in late 2000, delivering over 1000 W at around 230 V and around 50 Hz with, according to calculations, the desired life-time. However, this performance was reached when connected to an engine controller, i.e. in an operating mode where the current that is generated is used to charge batteries. In this mode the engine is free to run at the voltage and frequency that it wants. Since the stroke of the machine determines the voltage, in the grid connected mode the stroke and frequency are forced on the engine by the grid. Due to the absence of a stable and infinatively large 50 Hz, 230 V grid it was impossible to test the engine in the grid connected mode at STC in the USA. Therefore this testing and the fine-tuning of the engine were done by ECN. A second problem was encountered. The running times of the Stirling in the application in dchp units are relatively short, typically between 15 minutes and a few hours. In practice this means that the Stirling would hardly ever be in thermal equilibrium. It was found that the average temperature of the cold side of the engine normally ranges from 20 to 90 degree C. In free piston machines the pressure and thus the temperature influence the eigenfrequency of the engine through the spring rate of the working gas contained in the engine. In an engine that is designed to have an eigenfrequency of 50 Hz the frequency normally shifts from below 50 Hz at cold conditions to over 50 Hz at high temperatures. Tokyo, pagina 5 van de versie WS6_1_Woude_Fullpaper.doc

6 To achieve a successful and fast connection to the grid, the eigenfrequency of the machine has to be close to the 50 Hz of the public grid. This can be accomplished by choosing a relatively high filling pressure in the machine. However, if the eigenfrequency through a rise in temperature grows too much, the engine becomes unstable and grid connection is lost. This can be compensated by a relatively low filling pressure: contradicting requirements! By adjusting the mass ofsome of the moving parts, the number and spring rate of the flexures and the filling pressure in the engine ECN found an operating window with acceptable performance. The following performance now characterizes the STC engines, and the engines that were built at ECN: Time to grid connection: < 3 minutes Instability when grid connected: none Power: > 1000 W Power factor: > 0.9 System electrical efficiency: > 10% Development of the Gridbox Two electrical modes of operation exist for the Stirling: the grid control mode and the resistor control mode. The Stirling delivers its electrical power to the grid in the first mode. This is of course the desired mode. Whenever this mode is not possible the Stirling runs in the resistor control mode. The electrical energy produced by the Stirling is dumped in the hot water tank. There are a number of situations preventing the Stirling to be grid connected. First of all, during start up the Stirling needs some time to reach a frequency of 50 Hz.. Also there can be a situation where the grid is out of spec and delivery to the grid is not allowed. Thirdly there can be a situation where it is dangerous to deliver to the grid. This last situation occurs for example in case of a short in the mains. Finally the Stirling has to be protected from abnormalities on the grid, like voltage spikes. A so called gridbox was designed and developed that forces the Stirling to operate in either of the above mentioned two modes. The gridbox has the following functions: starting the engine in resistor control mode synchronizing the engine with the grid in resistor control mode connecting the Stirling to the grid: grid control mode switching to the resistor control mode whenever necessary. In the grid control mode there is nothing between the Stirling and the grid, so there are no losses. In the resistor control mode the current is dumped into a capacitor. Whenever the voltage over the capacitor exceeds a certain value a fast electronic switch (igbt) is opened and the capacitor is partly emptied in the dump resistor, which is located in the hot water storage tank. The Stirling can always dissipate the electric power that it produces, independent of the power level, when it is forced in this control mode by the gridbox. The voltage over the capacitor is kept constant when the gridbox is in the resistor control mode. However by modulating this voltage the movement in the Stirling can be influenced. The gridbox uses this phenomenon to synchronise the Stirling to the grid. The hardware of the gridbox was designed to be suitable for all countries with a 220 V, 50 Hz grid. The regulations that govern the feed-back of power into the public grid vary from country to country. Relatively simple software modifications will satisfy these different regulations. Some hardware midifications are necessary for the application in areas where the power is delivered at 110 V and 60 Hz. The basic design is the same however. It is important to note that the gridbox was designed with not only safety in mind, but also cost of manufacturing. The simplicity of design has led to a projected price for the gridbox of less than 15% of a possible alternative: an AC-DC-AC converter of the same power rating. Tokyo, pagina 6 van de versie WS6_1_Woude_Fullpaper.doc

7 Development of prototype dchp unit ENATEC has developed her technology for application in houses which consume at least m3 natural gas per year. (As noted before, 75% of the houses that have been built in Holland have this level of gas consumption). Therefore it was decided that the prototype dchp units have a heating capacity of some 28 kw. It was further decided that a hot water tank be integrated in the unit, as hot water comfort is considered important in the class of houses mentioned above. Furthermore the electricity that is produced by the Stirling can be dissipated in this tank, when it is not connected to the grid (e.g. during start up). ECN has designed a circular Stirling burner. The capacity for the burner follows from the previously mentioned electrical overall efficiency of the Stirling of 10%, and is 10 kw. The burner can modulate between 6 and 10 kw. The efficiency of the Stirling is almost independent of the heat input. Therefore the output of the Stirling is between 600 and Watt. ATAG Heating has integrated the Stirling generator and ECN burner with standard components of her succesfull Blue Angel II line of condensing central heating boilers as part of the ENATEC technology development program. The main components of the prototype unit are shown in the drawing above. They are from top left: Stirling engine with on the left the linear generator and on the right the heater head, with in between the external cooler using medium from the central heating system Stirling burner, fitting around the heater head of the Stirling Adaptor duct transferring the Stirling burner flue gas to the heat exchanger Heat exchanger of ATAG Heating s Blue Angel II line of products. The energy contained in the flue gasses of the Stirling burner needed to be captured and transferred efficiently to the house and hot water tank. A practical solution was choosen. The burner of the ATAG Heating Blue Angel II is situated on top of the heat exchanger and has a modular design, containing 3, 5 or 8 so called burner tiles. For the prototype dchp units the 5 tile burner was modified: 2 of the 5 tiles were taken out, allowing the flue gas from the Stirling generator to enter the heat exchanger via the adaptor duct. The remaining 3 tiles were kept functional to serve as peak burner. In the heat exchanger the energy contained in the flue gas of the Stirling burner is transferred to the medium that heats the house and hot water tank with the same condensing efficiency as the original ATAG Heating boiler. The integration of the controls of the Stirling burner with the controls of the conventional central heating parts of the dchp unit can not be visualised as easily. Naturally the strategy has been to maximize the time that the Stirling burner is in operation. Only when this burner can not satisfy heat demand, the 3 burner tiles kick in as peak burner, delivering some 18 kw in addition to the 10 kw of the Stirling burner. The photo on the next page shows one of the 10 floor standing field test units without covers. The ten Stirlings were built by ECN, the units were assembled by ATAG Heating. A 60 liter hot water tank was incorporated in the units. The controls are situated above the Stirling. The Stirling is suspended from a frame by cables, in order to minimize the transfer of vibrations from the Stirling to the unit. Not shown is the gridbox, it is situated outside the unit. Tokyo, pagina 7 van de versie WS6_1_Woude_Fullpaper.doc

8 One of the objectives of ENATEC was stated in the Introduction as being: license the ENATEC technology to third party boiler manufacturers. ENATEC has integrated the Stirling generator and Gridbox with a generic heat exchanger that is used by several European boiler manufacturers. Laboratory tests with this unit were successful, proving that the term Powered by ENATEC is feasable: the ENATEC technology can be used in conjunction with conventional boiler technology that is currently on the market, potentially extending the life cycle of the basic central heating system components in use by most European boiler manufacturers. Market The ENATEC dchp prototypes consume about 240 m3 gas per year more to satisfy the heat demand of the reference Dutch house. This extra gas is used by the Stirling generator to produce electricity. In return the end users receives 2265 kwh per year from his dchp unit. Possible future of central heating technology in Europe Caloric efficiency, % Gas consumption for reference house for heating, m3 / y for 2265 kwh electricity, m3 / y CO2 emission, ton / y NOx emission in flue gas, ppm Electricity, kwh / y IEB CB ENATEC dchp ,2 < ,7 < ,2 < IEB: Improved Efficiency Boiler CB: Condensing Boiler It can be calculated that a power generating facility with fuel efficiency as indicated in the introduction would consume 627 m3 gas to deliver the same amount of electricity to the end user. This signifies an improvement of ( ) / ( ) = 14% in terms of gas consumption - and therefore CO2 emission. Dchp can make a significant contribution towards the reduction of emissions, which are inevitably involved in burning fossil fuels for the energy requirements (heat and power) of households. Indeed the advance in efficiency and reduction in CO2 emission of 14% is very similar to the jump that was made when condensing boilers were introduced. For that reason some of the larger European gas and electricity utilities and boiler manufacturers in Europe regard ENATEC s estimate of the market potential as indicated below as conservative. They consider the number of units sold per year of 180,000 by 2020 small in comparison to the total European domestic boiler market of 6 million at present, as it only represents 3% of that market. Tokyo, pagina 8 van de versie WS6_1_Woude_Fullpaper.doc

9 Potential annual sales volume dchp units Europe % 3% The shape of the curves above is derived from the market penetration of condensing boilers in the Dutch central heating market. ENATEC deems it safe to assume that dchp will penetrate the present day market in the same way, as the deal in going from a condensing boiler to a dchp unit is equally good as in going from improved efficiency to condensing technology. Concluding remarks The free piston Stirling engine is a novel type of prime mover that is not being produced in large numbers yet. However it is succesfully being tested in an application with a huge market potential. Two requirements must be met in order to achieve an economically acceptable price in the mass-manufacturing of the Stirling. The first is simplicity of design. STC has designed, engineered and built three robust Stirling engines of 1000 Watt electrical output for laboratory testing. Basic choices were made and implemented in their design as regards cost of manufacturing. ENATEC has recently started a major effort to re-engineer the Stirling with mass manufacturability in mind. Potential manufacturers of the Stirling engine are currently being identified. The manufacturer of choice will be offered a license on this new design. Secondly, the price of the Stirling can only be acceptable when it is produced in large numbers: the mass market is also a prerequisite. End users buy appliances, not techologies. Ease of operation, plug and play installation, ecology and economics have been the basis of the development of the ENATEC technology for the central heating boiler of the future. It is therefore considered safe to assume that this mass market exists. ENATEC is offering European boiler manufacturers a license to use her technology, with which to serve that market of the future. Tokyo, pagina 9 van de versie WS6_1_Woude_Fullpaper.doc