Micro-tubular SOFC CHP Component
The original, intended BlueGEN SOFC CHP system is of a planar cell type structure. The newly acquired SOFC CHP unit is of a micro-tubular cell type structure. An introduction to micro-tubular SOFC technology is provided below.
Unlike other fuel cells, SOFCs can have multiple cell geometries. There are three SOFC geometries: planar, co-planar and micro-tubular. The planar type cell is currently the most commonly used geometry, seen in units such as the BlueGEN, and was described in detail in Chap. 4.
The micro-tubular type SOFC was invented in the early 1990s by Kevin Kendall (Howe et al. 2011). Most commonly, the materials used (Ni-YSZ/LSM/YSZ) and operating temperatures (600-1000 °C) are similar to those of the planar type cell however the cell structure is that of a hollow extruded tube of 1-5 mm in diameter, sealed at one end. The structure of this design is shown in Fig. 7.6a and can either
Fig. 7.6 a Basic SOFC micro-tubular designs (Howe et al. 2011), and b a 100 tube SOFC stack for CHP demonstration be electrolyte or anode supported. Depending on the support design, air or fuel is passed through the inside of the tube and the other gas is passed along the outside of the tube. The support tube is longer than the active cell length. The first tube segment provides a gas inlet tube and the outlet section can be used as a combustor tube, where the fuel (hydrogen or short chain hydrocarbons) and oxidant (oxygen or air) combine. Nickel and silver current collection wires are wound around the outside of the tubes, attached to the anode and cathode tubes respectively.
Because each tube only produces 0.5-5 W of electrical power, the single tubes are bundled together in various configurations to form stacks to meet application demands. Figure 7.6b shows a 100 tube stack designed by IVF-Swerea for a CHP demonstration project. A scale-up issue does exist for large power demand applications. This is due to the challenges of a large number of electrical connections and gas inlets, which require further equipment compared to the planar type design. In a CHP system, an integrated afterburner/recuperator is employed to combust unconsumed fuel leaving the stack and pre-heat the inlet cathode air- stream. The hot gases exiting the afterburner/recuperator are then passed through a second recuperator heat exchanger, transferring thermal energy to a working fluid such as water, to be used in a useful process.
Micro-tubular SOFCs show desirable operational characteristics such as high volumetric power density, good endurance against thermal cycling, and flexible sealing between fuel and oxidant streams (Panthi and Tsutsumi 2014). Furthermore, rapid start-up is possible with the micro-tubular design, one minute has been demonstrated, contrasting to the 1-24 h start-up times more conventionally observed with planar type stacks. As a result, micro-tubular SOFCs are suitable for the requirement of frequent restarting if necessary. However, the performance, namely fuel utilisation and thus electrical efficiency of the micro-tubular design is low. This is because of high cell resistance.
Because of micro-tubular SOFCs favourable power densities and rapid start-up times, they are being developed for small portable chargers and unmanned aerial vehicle applications as well as the stationary market. The planar type stack design still holds the largest market share for built environment CHP applications; which is primarily due to higher electrical efficiency and more favourable heat to power ratios. Currently, lower operating temperatures and higher power densities are the two main research targets for both planar and micro-tubular SOFC technology.
The micro-tubular SOFC CHP system has been acquired from Adelan Ltd. and installed in the laboratories at The University of Nottingham. The micro-tubular SOFC CHP test rig consists of the 250 We micro-tubular SOFC unit, a propane cylinder, gas regulator valve, sulphur trap, 12 V DC battery pack, a 12 V 250 W variable electrical load and a WHR circuit. A labelled photograph of the SOFC CHP experimental set-up is shown in Fig. 7.7.
The micro-tubular SOFC unit is made up of 48 anode supported co-extruded Ni-YSZ/LSM/YSZ tubes grouped together in bundles of four. These bundles are connected in series to form the stack. Each tube is 10 cm long, with a 20 cm2 surface area, and can produce 5-10 W of DC electrical power. The start-up time is 10-20 min, and can endure 3000 h of operation with 100 on/off cycles.
Fig. 7.7 Experimental test set-up for the micro-tubular SOFC CHP unit, with propane bottle, sulphur trap, electrical discharger and WHR circuit
A 50 L 2.5 bar propane tank is connected to the rear of the SOFC unit via a 1 bar gas regulator valve and sulphur trap. An advantage of SOFCs is there resilience to fuel poisoning, however they are still vulnerable to sulphur poisoning, and thus a sulphur trap is necessary. The propane regulator is set to 1 bar i.e. the fuel is supplied at atmospheric pressure. The micro-tubular SOFC unit uses approximately 100 g of propane per hour (1288 W fuel input). Ambient air is drawn in through the top of the unit via two 12 V DC axial flow fans. One airstream is delivered to the front of the unit, to be mixed with the propane fuel, at an air to fuel ratio of 3:1. The other airstream is passed through the unit’s recuperator/ afterburner to be pre-heated and then delivered to the cathode side of the cell to facilitate the electrochemical reactions. The propane air fuel mixture is introduced to the inside of the tubular cells where it is first heated to ~600 °C and catalyti- cally converted to hydrogen and carbon monoxide. The fuel is then delivered to the anode, where the electrochemical reactions take place, generating electricity, heat and water vapour. The stack has a fuel utilisation factor of 40-50 %, typical of a micro-tubular type stack. The parasitic power consumption of the fans is 35 W. The micro-tubular SOFC systems 12 V DC electricity output is connected to two parallel connected 12 V 65 Ah battery packs, which in turn are connected to an array of five 50 W 12 V lights.
Following the electrochemical reactions, the hot gases (hydrogen, propane, carbon monoxide, carbon dioxide and water vapour) leaving the micro-tubular SOFC stack are combusted in the afterburner. An integrated recuperator serves to pre-heat the cathode air inlet. The system is thermally managed by adjusting the cathode air flow to the stack. The hot gases leaving the afterburner are at approximately 350 °C. These hot gases are directed into a stainless steel flue, where they are then passed through a gas to liquid recuperator heat exchanger. Here the thermal energy is transferred to a water working fluid in the WHR circuit. The WHR circuit is made of 22 mm copper pipe lagged with 19 mm Climaflex Pipe Insulation. A Wilo-Smart A-rated 230 V AC pump is used to circulate the hot water in the heating circuit. A 22 mm gate valve is fitted to the WHR circuit to control the water volumetric flow. The WHR loop is connected to a vented 30 L insulated water tank. During CHP tests, a by-pass loop is used to circulate water from the recupertaor heat exchanger to the tank. During tri-generation testing the flow valves are set so that the hot water is directed to PX1 to heat the desiccant solution in the SDCS. Following waste heat recovery, the SOFC exhaust gases are rejected to the environment. Based on a 250 We output and the LHV of propane, the net electrical efficiency of the micro-tubular SOFC is approximately 19.4 %.
Section 7.3.1 has described the micro-tubular SOFC CHP component. Next, Sect. 7.3.2 details the liquid desiccant air conditioning component.