CONVERSION OF WIND POWER TO HYDROGEN

Transcription

1 Proceedings of the 2004/2005 Spring Multi-Disciplinary Engineering Design Conference Kate Gleason College of Engineering Rochester Institute of Technology Rochester, New York May 13, 2005 Project Number: CONVERSION OF WIND POWER TO HYDROGEN Michael Miller, ME Stephen Raymond, ME Sarah Braymiller, IE Patrick Griffin, ME Quoc Khanh Ngo, IE Team Leader Justin Szratter, ME ABSTRACT Wind is an intermittent renewable energy source and cannot guarantee constant power supply, in other words it is not always there when needed. The generated power is also difficult to store. A local wind turbine s electricity, generated during the weekend, is currently sent back to the grid at no charge. The project goal is to determine the technical, economic, and environmental feasibility of a sustainable system design that will store excess energy as hydrogen. The hydrogen will then be used to generate electricity on demand. INTRODUCTION Harbec Plastics is a thermal injection manufacturer that wishes to set a precedent as the energy efficient facility of the future. Harbec s current power system utilizes a 250kW Fuhrländer wind turbine and an array of twentyfive 30kW natural gas powered microturbines. The microturbines are part of a CHP system that captures and utilizes waste heat in the HVAC system. During the week the wind turbine helps power the manufacturing facility, but over the weekend the power generated is transmitted back to the public power grid. Harbec seeks to capture this energy by converting it into hydrogen gas. Hydrogen is an attractive fuel because it has the highest energy to weight ratio of any molecule [1] and it is easily stored. The hydrogen will then be used during the workweek in conjunction with the wind turbine to create alternative energy for production. There are several different ways to utilize hydrogen for electricity production, such as: fuel cells, hythane, and internal combustion engines. Harbec in particular wanted to focus on fuel cells. Ultimately, four sustainable systems were developed, each with various energy production levels, scale, and capacity. A sustainable design is one that works in harmony with nature s resources, causing no lasting damage. Three of the systems involved fuel cells (denoted as Basic, Mid, and High), while the fourth was developed using hythane for use with Harbec s Capstone microturbines. Each system is designed to operate over a 48-hour weekend. Integrating a system of components for this type of industrial application requires the optimization of component characteristics limited by physical and monetary constraints. These characteristics included but were not limited to; the amount of power produced by the wind turbine, power consumption of the electrolyser, distiller, compressor, pumps, mass-flow of hydrogen produced, volume of hydrogen storage, target energy production, system footprint, and 2005 Rochester Institute of Technology

2 Proceedings of KGCOE 2005 Multi-Disciplinary Engineering Design Conference Page 2 the financial and environmental impacts of the system. This project is unique as there is no physical prototype constructed, but is rather a comprehensive feasibility study. This paper will primarily focus on the technical aspects of the study. electrolysers that could adjust to changing power supply, but costs are not in favor of such a system. NOMENCLATURE CHP Combined Heat and Power h enthalpy hr hour hythane Hydrogen/Methane Mixture kw kilowatt m& Mass flow rate m Mass Nm 3 Normalized Cubic Meter P Pressure PEM Proton Exchange Membrane psig Pounds per Square Inch (gauge) R Gas Constant T Temperature V Volume V & Volumetric flow rate VAC Alternating current voltage W & Total Work Rate SOLUTIONS t The limiting factor for maximizing hydrogen production was found to be the power generated by the wind turbine. This determined the overall size of the electrolyser, which dictates the amount of hydrogen storage, and ultimately, the amount of energy production. There are various techniques to hydrogen generation. One approach is to utilize large-scale electrolysers, but these units often consume a lot of energy. During power production troughs these systems may not function at all. The goal is that the high hydrogen production rates associated with large-scale electrolysers will offset the shorter operation times. Another approach is to utilize small electrolysers, these units have low electric loads and will likely operate during troughs supplying a near constant flow of hydrogen, however low system energy profiles means that during power production peaks energy will be wasted. The systems proposed are the latter that are designed to operate below the average turbine power output. The best system would incorporate many small Figure 1: Generalized Systems Schematic Both the fuel cell and hythane system share several main components, as shown in Fig. 1. The hydrogen generation and capture procedure is the same for each system. The basic principle is that the wind turbine will provide power to the electrolyser, which will in turn generate hydrogen by the electrolysis of water. The hydrogen is then compressed and stored in a pressure vessel. This is where the system similarities end. From there the fuel cell system will use the hydrogen to produce clean/green power, while the hythane system will blend the hydrogen with natural gas to be used to run the microturbines. Obviously this is a very generalized procedure; many other ancillary components are needed for proper system integration and function, however further technical analysis will focus on these main components: distiller, electrolyser, compressor, fuel cell, and microturbines. Distiller Electrolysis requires two fuels, water and electricity. The feed water usually needs to be purified prior to electrolysis either through conventional distillation, reverse osmosis, or by other means. Table 1. shows that over time distillers can consume a fair amount of energy. Energy savings are possible by routing the water by-product of the fuel cells back to the Paper Number 05305

3 Proceedings of the Winter KGCOE Multi-Disciplinary Engineering Design Conference Page 3 electrolyser. This would create a closed loop system that would reduce overall water requirements. This would improve system efficiencies by reducing dependency on power hungry distillers. It should be noted that this feature is not applicable to the hythane system, which will still depend on the distiller for feed water. Model Onsite model Precision Precision Load (kw) Production (gal/hr) Table 1: Distiller Specifications [2] Electrolyser The electrolyser is the fundamental component of the system, this is the unit that will take energy from the wind turbine and convert it into hydrogen. Although relatively new to the market, electrolysers come in many sizes, from small lab units to large scale industrial ones. The electrolyser uses the same principle as the fuel cell, only in reverse. Energy, in the form of electricity, is added to water to break it down into its constituent diatomic molecules (2H 2 O + energy 2H 2 + O 2 + heat). This process creates twice as many molecules of hydrogen as oxygen. Sequestration of oxygen was considered, but rejected because of cost and spatial limitations. Consequently all systems will vent oxygen to the atmosphere. Additionally, the units are all water cooled so the heat by-product can easily be added to the existing CHP system, further improving the efficiency. The wind turbine produces an annual average of 300,000 kilowatt-hours, so the average constant power supply is 34 kw. It was essential that the electrolyser load be well below this point because any additional components, such as the distiller and compressor, will need to be concurrently powered by the turbine. From Table 2. it can be seen that the highest electrolyser load is only 18kW, which allocates plenty of power for auxiliary components. The mid and highlevel systems both use moderately sized electrolysers with hydrogen production rates around 2Nm 3 /hr. The basic and hythane system use a smaller unit with about a quarter of the production rate. Energy consumption figures in Table 2. are based on 48-hour operation. Model HOGEN 20 HOGEN H-2 HM-50 Load (kw) Efficiency (kw hr/nm 3 ) Production (Nm 3 /hr) Energy Consumption (kw hr) Table 2: Electrolyser Specifications [3,4] Compression/Storage Storage was one of the biggest issues during design analysis; After considering four days of hydrogen storage, the spatial requirement acted as a limitation and it was reduced to just two days. Today s commercial electrolysers produce hydrogen at psig, but at this pressure 48- hour productions were reaching 130Nm 3. Compressors are required to pressurize it to around 2500psig for storage to be practical. At this high pressure 130Nm 3 was reduced to less than 1m 3. Storage volume was determined by the ideal gas law. A system summary can be found in Table 3. mrt V = (1) P Compressor C /300LX C /140LX Energy Consumption (kw) Volume (m 3 ) Table 3: Compressor Specifications [5] Microturbines Harbec currently has twenty-five Capstone C30 microturbines installed, each one capable of producing 30kW. They currently run on natural gas, however the fuel can be enriched with up to 5% hydrogen, this however is not a 1:1 relationship. Hydrogen has a much higher energy content than natural gas, and improper mixing can cause serious damage to the microturbines. One kilogram of hydrogen will replace approximately six kilograms of natural gas. In addition to reducing natural gas consumption the added hydrogen also burns cleaner, reducing CO 2 pollution. Copyright 2005 by Rochester Institute of Technology

4 Proceedings of KGCOE 2005 Multi-Disciplinary Engineering Design Conference Page 4 As mentioned before the fuel cell is the opposite reaction as the electrolyser. Hydrogen is combined with oxygen, usually supplied from the air, to produce water and electricity (2H 2 + O 2 2H 2 O + energy). PEM fuel cells use a special electrolyte that only allows protons (hydrogen ions) to pass through while electrons flow from the anode to cathode to produce an electric current (see Fig. 3.). Electricity produced is direct current and needs to be converted to alternating current to accommodate the standard 120VAC. Figure 2: Capstone C30 Microturbine Fuel flow rate was determined through a modified ideal gas law, Eq. (2), and verified with published figures [5]. Knowing the initial natural gas flow rate and total work rate, the hydrogen flow rate was found with Eq. (3). This rate was crucial for determining running time and natural gas savings. PV& m& = RT (2) W& t m& = h h (3) in out Note in Table 4. the 1.05MW hr energy production of this system dwarfs the energy consumption of the distiller, electrolyser, and compressor. This is because natural gas, a new energy source, is added to the hydrogen in the final step. This makes the system s efficiency difficult to determine, but if 5% of power production was allocated to hydrogen then the efficiency would be comparable to the basic fuel cell system. C30 microturbines Running Time (hr) 35 Energy Production (kw hr) 1050 System Efficiency 28% Table 4: Hythane System Outputs Fuel Cell Figure 3: PEM Fuel Cell [7] The same fuel cell, a 1.4kW/14V stack, was chosen for each system. This may not seem like a lot of energy but it provides power for a long time. A 5kW fuel cell, while providing more power, would have a higher fuel flow rate and deplete the hydrogen reservoir very quickly. Fuel Cell Stack Power (kw) 1.4 H 2 Consumption Rate (Nm 3 /hr) 0.72 Table 5: Fuel Cell Specifications [5] System Basic Mid High Running Time (hr) Energy Production (kw hr) System Efficiency 27% 15.3% 24.3% Table 6: Fuel Cell System Outputs System efficiencies were determined by dividing system energy production by system Paper Number 05305

5 Proceedings of the Winter KGCOE Multi-Disciplinary Engineering Design Conference Page 5 energy consumption. The low efficiency for the mid-level system is due to the low efficiency of the HOGEN H-2 electrolyser (9kW hr/nm 3 ). The high-level system s fuel cell will run for 170 hours, this is slightly greater than 7 days and may require occasional venting. CONCLUSIONS These systems are technically feasible; all of the system components are commercially available today, however, because it is feasible it does not warrant implementation. Hydrogen is still an emerging commercial energy source. Over the next few years there are bound to be major improvements; new generation electrolysers promise lower energy needs and high-pressure hydrogen without the need of a compressor [8]. This new technology is also still very expensive and produces unacceptable return on investment times. System efficiencies are fairly low, only around 25%. This is primarily due to energy losses from phase changes and waste heat. CHP integration will improve overall efficiency, but it can only do so much. It would be sensible to wait a few years to allow technology to improve before system implementation. [4] TITAN Hydrogen/Oxygen Generators. Teledyne Energy Systems, Inc. 12 Jan < Titan_HM_june02.pdf > [5] High Pressure Hydrogen Compressors. Hydro-Pac, Inc. < html> [6] Capstone. "C30.pdf." microturbine.com. Capstone. 24 Apr < 30.pdf> [7] Types of Fuel Cells. Fuel Cells. US Department of Energy < elcells/fuelcells/fc_types.html> [8] Green Hydrogen Production Opens Up New Possibilities. Mitsubishi Corporation < nage/e_bus0401.html> ACKNOWLEDGMENTS The conversion of wind power to hydrogen team would like to thank Harbec Plastics, especially the president, Bob Bechtold, for generously donating time and information. Paul Williams for helping us get an early handle on the electrical aspects of the project. And finally, Dr. Brian Thorn for his guidance and patience. REFERENCES [1] Zittel, Werner, Dr. Hydrogen in the Energy Sector. HyWeb. 7 Aug Feb < iew-eng.html> [2] Hydrogen Compressor Capacity Charts Hydro-Pac, Inc. < Path: capacity html;capacity html. [3] HOGEN Technical Specifications. Proton Energy Systems. 10 Jan < ml/gasproducts/generators/files/hogen_spe c_sheet04-04.pdf> Copyright 2005 by Rochester Institute of Technology