WindFuels – Competitive Transportation Fuels From Wind Energy and Waste CO 2
It is essential for the market to help drive the dramatic cut needed in CO2 emissions to prevent a climate disaster in this century. We show that the economics for producing clean liquid hydrocarbons and alcohols from water and waste CO2 on wind farms improved by an order of magnitude between 2002 and 2008, and we present the scientific and technological basis for another factor-of-two improvement in these economics. Wind energy is by far the most competitive renewable energy resource in many regions. The perceived challenge is getting wind energy from good sites to where and when it is needed, both for the transportation sector and for the power grid. Efficient conversion of wind energy and waste CO2 into clean, stable, liquid fuels – also called WindFuels – solves these problems. Annual WindFuels production per land area in good wind regions should exceed biofuels production density in fertile farming areas by a factor of five. First, water and renewable electrical power are fed into an electrolyzer, which produces the required hydrogen. Then waste CO2 (from coal power plants) and the renewable hydrogen are piped into a novel Reverse Water Gas Shift (RWGS) process, which permits practical reduction of CO2 to the CO needed at efficiencies approaching theoretical limits. The renewable CO and H2 are then fed into a novel Renewable Fischer-Tropsch Synthesis (RFTS) process. There they are converted into mostly mid-alcohols (ethanol, propanol, and butanol) and other valuable chemicals. These fuels may then be readily stored and distributed by conventional means – pipelines and tanker trucks. The electrolyzer also produces a huge amount of oxygen, which may be sold if market conditions warrant, or it may be utilized in novel processes to improve the plant efficiency. From basic thermodynamics, one readily calculates that the theoretical maximum chemical efficiency of synthesis of ethanol from H2 and CO2 is 80.1% without utilization of the excess heat released from the Fischer Tropsch Synthesis (FTS) reactor. If this excess heat is also converted in an ideal heat engine, the theoretical combined-cycle efficiency limit is ~90%. One advantage of ethanol may be appreciated by noting that its synthesis from H2+CO2 results in 1.5 molecules of water per carbon atom in the fuel, while the synthesis of alkanes or alkenes results in 2 molecules of waste water per carbon atom in the fuel. Previously, fossil-based FTS efficiencies for environmentally attractive fuels, such as ethanol and propanol, have usually been under 40%. The need for very efficient recycling of the large amounts of H2, CO, and CO2 in the byproducts from mid-alcohols FTS has possibly been the strongest argument against mid-alcohols FTS compared to gasoline, lubricants, and diesel. A novel plant design is presented, validated by many simulations, that permits order-of-magnitude reduction in energy penalties associated with the major separations in a fully recycled mid-alcohols plant compared to previously published designs. The example wind-driven RFTS plant size chosen for illustration and simulated in detail assumes 250 MW mean input electrical power. It achieves 72% FTS-plant higher heating value (HHV) efficiency in production of mid-alcohols and other products from H2 and recovered (waste) CO2, or about 60% net HHV efficiency when including the electrolyzer at expected near-term performance. At least eight separate, substantial innovations in the system design combine to permit this major advance in RFTS efficiency and cost effectiveness. It is worth pointing out that as recycle efficiency and byproduct utilization both approach 100%, which is the primary thrust in the RFTS optimization, catalyst selectivity and activity both become much less important. Catalysts never affect the thermodynamics – only the kinetics of the various pathways. Hence, further development of catalysts will have limited effect on system efficiency in a plant with 95% recycle efficiency, though of course it will influence profitability by changing the product mix.
Practical Application of a Compact, High-Effectiveness, Gas-To-Gas, Compound Recuperator With Liquid Intermediary (CRLI)
Gas-to-gas recuperation with both high thermal effectiveness and order-of-magnitude improvement in cost effectiveness is critical to addressing global energy needs, especially via fuel synthesis from waste CO2 and renewable H2 . An uncommon compound recuperator with liquid intermediary (CRLI ) has been simulated for high-e heat exchange between a first shell-side gas stream and a second shell-side gas stream of similar thermal capacity rates (W/K). The compound recuperator uses a first Gas-to-Liquid (GL ) recuperator for a nearly complete transfer of available energy from a shell-side gas to an intermediary tube-side heat transfer liquid (HTL ), followed by a second GL recuperator to transfer the heat from the liquid intermediary to the second gas stream. Each GL recuperator resembles an arrangement of thermally isolated, serially connected, adjacent, cross-flow, finned-tube cores, such as used in AC condensers. They are arranged so to effectively achieve counterflow exchange between the HTL and the shell-side gas stream. The HTL may be water, an organic liquid, a molten alloy, or a molten salt. Minimization of exergy destruction for the case where there is a substantial temperature difference between the hot and cold sources requires (1) a fairly large number of series connected, thermally isolated cores, (2) similar thermal capacity rates in all three streams (the two gases and the liquid intermediary), (3) a relatively large value for the number of transfer units (NTU ), and (4) no phase change. The simulations show that the optimized CRLI recuperator can achieve effectiveness above 97% at very low pumping losses and has the potential for order-of-magnitude reduction in manufacturing costs compared to current technologies for clean gases at pressures above 0.3 MPa at heat transfer rates above ∼200 kW.Copyright © 2009 by ASME
Fully Integrated Bidirectional CMOS-MEMS Flow Sensor With Low Power Pulse Operation
Leveraging More-than-Moore technology, we demonstrate an integrated CMOS MEMS flow sensor via a very compact system on chip (SoC) that can sense the bidirectional N2 gas flow. Our SoC features a very low-noise instrumentation amplifier that is implemented on a CMOS wafer and serves as a readout integrated circuit for a thermoresistive micro-calorimetric flow sensor which is fabricated on a MEMS wafer. A compact heterogeneous integration is achieved by combining the two wafers via the proprietary InvenSense AlN process. The measured sensitivity of our CMOS MEMS flow sensor is 98 mV/sccm which is three times better than the state-of-the-art counterpart. The flow range measured by our SoC is from −26 to 26 m/s (from −50 to 50 sccm). Moreover, the pulsed heater operation makes this proposed SoC flow sensor a very low power (<9 mW) and, thus, a promising candidate for the Internet-of-Things applications in smart home/green buildings.