Joe Coventry

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.

Optics of solar central receiver systems: a review

This article reviews the state of the art in optical design, modeling and characterization of solar central receiver systems.

Development of a higher-efficiency tubular cavity receiver for direct steam generation on a dish concentrator

An integrated model for an axisymmetric helical-coil tubular cavity receiver is presented, incorporating optical ray-tracing for incident solar flux, radiosity analysis for thermal emissions, computational fluid dynamics for external convection, and a one-dimensional hydrodynamic model for internal flow-boiling of water. A receiver efficiency of 98.7% is calculated, for an inlet/outlet temperature range of 60–500 °C, which is the ratio of fluid heating to receiver incident irradiance. The high-efficiency design makes effective use of non-uniform flux in its non-isothermal layout, matching lower temperature regions to areas of lower flux. Full-scale testing of the design will occur in late 2015.

Experimental testing of a high-flux cavity receiver

A new tubular cavity receiver for direct steam generation, ‘SG4’, has been built and tested on-sun based on integrated optical and thermal modelling. The new receiver achieved an average thermal efficiency of 97.1±2.1% across several hours of testing, and reduced the losses by more than half, compared to the modelled performance of the previous SG3 receiver and dish. Near-steady-state outlet steam temperatures up to 560°C were achieved during the tests.

Development of ASTRI high-temperature solar receivers

Three high-temperature solar receiver design concepts are being evaluated as part of the Australian Solar Thermal Research Initiative (ASTRI): a flux-optimised sodium receiver, a falling particle receiver, and an expanding-vortex particle receiver. Preliminary results from performance modelling of each concept are presented. For the falling particle receiver, it is shown how particle size and flow rate have a significant influence on absorptance. For the vortex receiver, methods to reduce particle deposition on the window and increase particle residence time are discussed. For the sodium receiver, the methodology for geometry optimisation is discussed, as well as practical constraints relating to containment materials.

Optical and thermal performance of bladed receivers

Bladed receivers use conventional receiver tube-banks rearranged into bladed/finned structures, and offer better light trapping, reduced radiative and convective losses, and reduced tube mass, based on the presented optical and thermal analysis. Optimising for optical performance, deep blades emerge. Considering thermal losses leads to shallower blades. Horizontal blades perform better, in both windy and no-wind conditions, than vertical blades, at the scales considered so far. Air curtains offer options to further reduce convective losses; high flux on blade-tips is still a concern.

Improved Tubular Receivers for Point-focus Concentrators

Optics, thermal emissions, convection and internal flow are treated in a unified model for a tubular cavity receiver for a dish concentrator; a new design is presented that shows a 40% reduction in receiver losses.

Novel Parabolic Trough Collectors Driving a Small-Scale Organic Rankine Cycle System

This paper presents component performance results of a new parabolic trough collector array driving an organic Rankine cycle (ORC) power generation system. The system has been installed in the National Solar Energy Centre at CSIRO Energy Technology in Newcastle, NSW, Australia. It consists of four rows of 18 parabolic mirrors each in a 2×2 matrix with a total aperture area of approximately 132m2 . The absorber tube is a laterally aligned, 40mm copper tube coated with a semi-selective paint and enclosed in a 50mm non-evacuated glass tube to reduce convection losses. The mirror modules, which are light-weight and robust, are made from thin low iron back silvered glass bonded to a sheet steel substrate. They are supported by a box truss on semi circular hoops running on rollers for single axis tracking. The mirror design has been chosen to allow low-cost manufacturing as well as simple commissioning and operation. The ORC unit is a FP6 unit sourced from Freepower Ltd. with a net power output of 6kWel at 180°C inlet temperature and a total heat input of 70 kWth . It uses a two-stage expansion process with hydrofluoroether as the working fluid. A wet cooling tower is used to dissipate the reject heat from the ORC. The two key components of the envisioned system are the trough reflector/receiver and the ORC unit. The optical performance of the mirror elements was investigated with regard to the flux mapping onto the receiver tube. The ORC unit has been tested separately using an electrical oil heater as the heat source. This paper presents results for irradiation capture and intensity over the receiver width of a single trough mirror module. The complete system including trough collectors and ORC has not been in transient operation yet, thus experimental steady-state results of the ORC unit are presented.© 2007 ASME

Optical properties of Pyromark 2500 coatings of variable thicknesses on a range of materials for concentrating solar thermal applications

In this paper we present the results of solar absorptance measurements of four metallic substrate materials, either coated with Pyromark 2500 at various thicknesses, or uncoated and oxidised. Absorptance is measured prior to aging, and during and after aging at three elevated temperatures. In many cases, thin coatings perform as well, or better than thick coatings and do not appear to have a higher rate of failure. However, a thicker coating did show an advantage after aging at the highest temperature tested (850°C), and it is expected that with longer exposure, similar trends may emerge for the 600°C and 750°C aging cases. Another finding is that the two nickel-based alloys tested, Haynes 230 and Inconel 625, both formed an oxide with very good absorptance, although durability requires further testing.

Geometrical exploration of a flux-optimised sodium receiver through multi-objective optimisation

A stochastic multi-objective optimisation method is used to determine receiver geometries with maximum second law efficiency, minimal average temperature and minimal surface area. The method is able to identify a set of Pareto optimal candidates that show advantageous geometrical features, mainly in being able to maximise the intercepted flux within the geometrical boundaries set. Receivers with first law thermal efficiencies ranging from 87% to 91% are also evaluated using the second law of thermodynamics and found to have similar efficiencies of over 60%, highlighting the influence that the geometry can play in the maximisation of the work output of receivers by influencing the distribution of the flux from the concentrator.