In-Orbit Performance of the GRACE Follow-on Laser Ranging Interferometer

The Laser Ranging Interferometer (LRI) instrument on the Gravity Recovery and Climate Experiment (GRACE) Follow-On mission has provided the first laser interferometric range measurements between remote spacecraft, separated by approximately 220 km. Autonomous controls that lock the laser frequency to a cavity reference and establish the 5 degrees of freedom two-way laser link between remote spacecraft succeeded on the first attempt. Active beam pointing based on differential wave front sensing compensates spacecraft attitude fluctuations. The LRI has operated continuously without breaks in phase tracking for more than 50 days, and has shown biased range measurements similar to the primary ranging instrument based on microwaves, but with much less noise at a level of 1 nm/Hz at Fourier frequencies above 100 mHz. © 2019 authors. Published by the American Physical Society

Optimal Operation of Energy Microgrids under Uncertainty

Large-scale electrification of energy demand and growing shares of renewables in the grid generation mix pose significant challenges to power systems. Microgrids offer a decentralised alternative to traditional grid designs, and can improve reliability, resilience, environmental and economic aspects for participants they serve or the power system they may interact with. To harvest their potential, value streams must be created from operational management strategies for microgrids, e.g. bidding in energy markets, utilising flexibility and advanced control of microgrid resources. This thesis presents novel models and solution approaches for optimal bidding of grid-connected microgrids using stochastic programming and mixed-integer linear programming (MILP). A strategy for bidding in the day-ahead electricity market is proposed for a microgrid with power generator, photovoltaic (PV) system, battery and electricity demand. A novel two-stage stochastic MILP model incorporates an optimisation-based selection of bid prices for hourly bidding curves, while the number of breakpoints per curve strictly complies with market rules. Monte Carlo simulation demonstrates benefits over deterministic and stochastic bidding models from literature assuming uncertain electricity prices and PV power. The problem is then converted to multi-objective, with conditional value-at-risk (CVaR) serving as additional objective to control financial risk from microgrid bidding commitments. A solution procedure applies ε-constraint method and fast forward scenario reduction to obtain bidding solutions for joint cost and risk minimization in reasonable CPU times. Also, a novel stopping criterion for scenario reduction is suggested. Finally, virtual bidding is integrated into the optimisation model for microgrid bidding. Additional scenarios for the real-time market electricity price are generated, and constraints for implicit and explicit virtual bidding derived. Benefits of virtual bidding to mitigate financial risk or reduce cost are presented in computational examples, and the impact of upper bounds on virtually traded quantities is examined

Techno-Economic Comparison of Flexibility Options in Chlorine Production

In order to allow demand side management in the chlorine industry, we suggest seven modifications to the conventional chlor-alkali process. The modifications include the oversizing of electrolyzer cells, replacement of electrodes, and integration with flexible auxiliary units. We optimize the operation of the processes for four scenarios with different electricity price profiles and hydrogen prices. We then rank the processes in a merit order of three economic metrics including investment costs, operating costs, and payout time. While reasonable payout times are achieved with many of the flexible processes, the best option with the shortest payout time highly depends on the prices of hydrogen and electricity. The results indicate that flexible chlor-alkali processes without auxiliary units outperform steady-state chlor-alkali processes with flexible auxiliary units. In particular, the combination of two electrodes or the implementation of a bifunctional electrode for operational mode switching seems to be the best compromise

Testing the GRACE follow-on triple mirror assembly

We report on the successful testing of the GRACE follow-on triple mirror assembly (TMA) prototype. This component serves to route the laser beam in a proposed follow-on mission to the Gravity Recovery and Climate Explorer (GRACE) mission, containing an optical instrument for space-based distance measurement between satellites. As part of this, the TMA has to meet a set of stringent requirements on both the optical and mechanical properties. The purpose of the TMA prototype testing is to establish the feasibility of the design, materials choice and fabrication techniques. Here we report on co-alignment testing of this device to the arc second (5 μrad) level and thermal alignment stability testing to 1 μrad K-1

The design and construction of a prototype lateral-transfer retro-reflector for inter-satellite laser ranging

The Gravity Recovery and Climate Experiment (GRACE) mission, launched in 2002, is nearing an end, and a continuation mission (GRACE Follow-on) is on a fast-tracked development. GRACE Follow-on will include a laser ranging interferometer technology demonstrator, which will perform the first laser interferometric ranging measurement between separate spacecraft. This necessitates the development of lightweight precision optics that can operate in this demanding environment. In particular, this beam routing system, called the triple mirror assembly, for the GRACE Follow-on mission presents a significant manufacturing challenge. Here we report on the design and construction of a prototype triple mirror assembly for the GRACE Follow-on mission. Our constructed prototype has a co-alignment error between the incoming and outgoing beams of 9 μrad, which meets the requirement that this error must be less than 10 μrad