Andreas Luzzi

Realizing the hydrogen future: the International Energy Agency's efforts to advance hydrogen energy technologies

Realizing the hydrogen future: the International Energy Agnency´s effort to advance hydrogen energy technologies

A solar-driven ammonia-based thermochemical energy storage system

During 1998, over 20 years of research at the Australian National University came to fruition with the successful operation of the world-first solar-driven ammonia-based thermochemical energy storage system. This paper presents the latest results obtained with this system which operates at a nominal power level of 1 kWchem and uses a solar reactor design which is an improved version of a prototype first tested in 1994. Progress made in scaling the system up to accept the full 15 kWsol input from the 20-m2 dish concentrator being used, is also presented. The experimental results indicate that ammonia dissociation receiver/reactors are ideally suited to operation through solar transients and that stable operation of ammonia synthesis heat recovery reactors can be achieved at temperatures well suited to the production of superheated steam for Rankine cycle power systems.

TECHNO-ECONOMIC ANALYSIS OF A 10 MWe SOLAR THERMAL POWER PLANT USING AMMONIA-BASED THERMOCHEMICAL ENERGY STORAGE

The production of solar thermal power on a continuous, 24-h basis is possible by applying thermochemical energy storage. An international group of industrial and academic partners is studying such a base-load solar power plant concept, where the reversible thermo-catalytic ammonia reaction serves as the energy vector between supply and demand. Early results confirm the technical soundness of the concept using conventional technology, equipment and materials, and indicate also the potential for economic viability. A first-of-a-kind, solar-only demonstration power plant with a net capacity of 10 MWe would require a capital investment of the order of AUD 180 million and operate with a net solar-to-electric conversion efficiency of 18% and a capacity factor of 80%. This would result in levelised electricity costs of less than AUD 0.25 per kWh.

EXERGY ANALYSIS OF AMMONIA-BASED SOLAR THERMOCHEMICAL POWER SYSTEMS

The reversible dissociation of ammonia is one of the candidate reactions for use in closed loop solar thermochemical energy storage systems. The major determinant of achievable performance for such a system is the degree of thermodynamic irreversibility associated with the heat recovery process. Exergy analysis of a semi realistic 30 MPa isobaric system has revealed that the major irreversibilities occur within the exothermic reactor and the counterflow heat exchanger between ingoing and outgoing reactants. In this study, optimum reactor control yielded exergetic efficiencies up to 71%, which should translate to overall solar to electric conversion efficiencies of around 20%.

Optical performance of spherical reflecting elements for use with paraboloidal dish concentrators

While paraboloidal dishes have traditionally been used for high flux/high power solar concentration devices, the manufacture of multi-facet collectors has been complicated somewhat by the need to produce reflecting elements having different curvatures for different regions of the paraboloidal surface. This complication could be minimised by using identical spherical reflector sub-components mounted with a paraboloidal orientation on a space frame dish structure. This paper compares the optical performance and manufacturing feasibility of collectors having such a combination of surfaces.

Endothermic reactors for an ammonia based thermochemical solar energy storage and transport system

The ammonia dissociation reaction is one of a number of reactions which has been investigated for use in closed loop solar thermochemical energy storage systems, over a period of nearly two decades. A recent series of experiments with an electrically heated high pressure ammonia dissociation reactor has validated a two dimensional pseudo-homogenous theoretical reactor model, established rate parameters for the catalyst used, paved the way for a closed loop demonstration and simulated operation of a receiver/reactor under solar operation. The model has subsequently been used to investigate full sized receiver/reactor options for a 20 m2 paraboloidal dish. Technically feasible designs based on: directly irradiated catalyst filled tubes, sodium reflux heat transfer to catalyst tubes and direct absorption of radiation using a windowed pressure vessel, have been identified.

A solar thermochemical power plant using ammonia as an attractive option for greenhouse-gas abatement

The lack of effective means for energy storage and transport is often put forward as one of the major obstacles to the mass utilisation of solar energy. ANUs development of large paraboloidal solar collectors and a thermochemical heat-pipe transport concept could well be combined to offer an attractive solution to this problem. A 4-MWe solar-assisted natural gas power plant is under consideration for Tennant Creek in Northern Australia. This base-load power plant will employ a steam Rankine cycle power conversion unit and incorporate an array of 28 direct-steam-generating dishes with 400-m2 aperture each. A preliminary investigation of replacing its water/steam heat transfer network with an ammonia-based heat transfer system indicates that 24-hour storage could be provided at an additional cost of only 12%. Furthermore, for alternative sites with no natural gas back-up available, it was found that a thermochemical ammonia system could demonstrate 24-hour base-load solar power generation for the same per-dish capital cost as a solar-only steam system without storage. This opens the market to megawatt-size, remote, off-grid applications. Pollution and greenhouse-gas emissions from such a closed-loop solar power generation system would be zero.

Maximizing thermal power output of an ammonia synthesis reactor for a solar thermochemical energy storage system

Solar energy storage using a closed loop thermochemical system based on the reversible dissociation of ammonia, has been investigated at the Australian National University for over two decades. Theoretical and system studies have indicated that large scale systems offer reasonable thermodynamic and economic performance. Experimental investigation has confirmed the technical viability of the concept. This investigation has looked at the effect of operating parameters on the thermal output achievable from the heat recovery process. Pressure, massflow and inlet gas composition were all found to have significant effects on the output achievable. Maximizing the thermal output via adjustment of reactor wall temperature profiles indicates that the average temperature of the reactor walls is more significant than the shape of the profile. This investigation has indicated the potential and provided the foundations for future exergo-economic optimizations of the system.

Photoelectrolytic Production of Hydrogen: Annex-14 of the IEA Hydrogen R&D Program

Nine research groups from four countries have been collaborating for 4.5 years under Annex-14 with the aim to advance the science underlying photoelectrolytic hydrogen production from water-splitting. Significant research progress has been made in the areas of material science (semiconductor photoelectrodes, light absorption, photocatalyst stability and charge transfer) and systems development (monolithic multijunction systems, two-photon tandem systems, dual-bed redox systems and monomaterial two-step systems), resulting in the synthesis as well as development of new photoactive materials and the demonstration of various photoelectrochemical (PEC) water-splitting prototype devices, limited to typical laboratory-size miniature scales.Copyright