Keith Lovegrove

Experimental Investigation of Natural Convection Heat Loss From a Model Solar Concentrator Cavity Receiver

Natural convection heat loss inevitably occurs in cavity-type receivers in high concentrating solar dishes, downward focusing systems and solar towers. In most applications, it can contribute a significant fraction of total energy loss, and hence it is an important determining factor in system performance. To investigate natural convection losses from cavity type receivers, an electrically heated model receiver, was tested at inclinations varying from -90 deg (cavity facing up) to 90 deg (cavity facing straight down), with test temperatures ranging from 450 to 650 deg C. Ratios of the aperture diameter to cavity diameter of 0.5, 0.6, 0.75, 0.85 and 1.0, were used. In addition to measurements of overall heat loss, the Synthetic Schlieren technique was used to visualize the flow pattern out of the cavity. Numerical modeling of the convection losses from the cavity was carried out for positive angles with the commercial computational fluid dynamics software package, Fluent 6.0. Good agreement was found between the numerical flow patterns at the aperture region with the schlieren images and between measured and predicted values for heat loss. Of the previously published work that has been reviewed, a model proposed by Clausing, A. M., 1981, An Analysis of Convective Losses from Cavity Solar Central Receivers, Sol. Energy 27 (4) pp. 295-300 shows the closest prediction to both numerical and experimental results for downward facing cavities despite its original use for bigger-scale central receivers.

Development of an approach to compare the 'value' of electrical and thermal output from a domestic PV/thermal system

When considering the design of a PV/Thermal system, it is essential to determine the ratio of the values of the electrical and thermal output from the system. Otherwise there is no rational approach for optimising such a system, as there will be no single output to optimise. This paper focuses on methods that can be employed to develop a ratio between electrical and thermal output from a domestic style PV/Thermal system. Methods discussed include thermodynamic analysis using exergy; market analysis for both an open market and a renewable energy market; and environmental analysis using avoided greenhouse gas emissions. Ratios are developed for each method based on real data. An example is given comparing a PV/Thermal system that uses amorphous silicon cells with one that uses crystalline silicon cells. Levelised energy cost is plotted against the energy value ratio to show that there is a critical electrical-to-thermal energy value ratio below which a collector with a-Si cells is more cost effective than one with c-Si cells.

A Review of Ammonia-Based Thermochemical Energy Storage for Concentrating Solar Power

The development of a thermochemical energy storage system based on ammonia, for use with concentrating solar power is discussed in this paper. This is one of a number of storage options for concentrating solar power, including molten-salt storage, which is already operating commercially. The ammonia storage development has involved prototype solar receiver/reactors operated in conjunction with a 20-m 2 dish concentrator, as well as closed-loop storage demonstrations. An ongoing computational study deals with the performance of an ammonia receiver for a 489-m 2 dish concentrator. The ammonia storage system could employ industry-standard ammonia synthesis converters for superheated steam production. A standard 1500 t/day ammonia synthesis reactor would suffice for a 10-MWe baseload plant with 330 large 489-m2 dishes. At this stage, an updated economic assessment of the system would be valuable.

Numerical Investigation of Natural Convection Loss From Cavity Receivers in Solar Dish Applications

In open cavity receivers employed in solar paraboloidal dish applications, natural convection occurs and contributes a significant fraction of energy loss. Its characteristics hence need to be clarified so that it can be effectively minimized in order to improve the system efficiency. The investigation of natural convection loss from cavity receivers was undertaken numerically and was validated using the published experimental results for four different receiver geometries. A good agreement between experimental and numerical results was obtained. Furthermore, the numerical results of all receivers were qualitatively comparable to the predictions by other available correlations hitherto, although it was found that each correlation has a limited range of applicability arising from the particular cavity geometry and experimental conditions used to derive it. To address this shortcoming, a new correlation based on the numerical results for three of the above four receivers has been proposed. The correlation employs a new concept of an ensemble cavity length scale, to take into account the combined effects of cavity geometry and inclination. Despite a wide variety of cavity geometries and operating conditions, the proposed correlation predicts approximately 50% of the data within ±20% and 90% of the data within ±50%. This is better than any of the other correlations published to date. The new correlation is also simpler to use than the most accurate of those previously published.

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.

Solar Thermal Energy Systems in Australia

Australia has developed world leading solar thermal technologies, with only very low national market penetration. Domestic solar water heating is the most common solar thermal instrument, with around 5% of homes using it and most of these systems are conventional flat plate thermosyphon systems. Other low temperature solar thermal research includes solar crop drying, solar ponds and solar air heating but all on a small scale. There is a worldwide resurgence in interest in high temperature solar thermal through solar concentrating systems. Australia has a number of these systems many of which are near commercial fulfilment; notably, Solar Heat and Power Pty Ltds Compact Linear Fresnel Array system currently being implemented at Liddell Power station and the ANU 400m2 Big Dish now being commercialized by Wizard Power Pty Ltd. CSIRO has recently opened a solar energy centre in Newcastle that features a solar central receiver tower system and a trough concentrator array.

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.