G.L. Morrison

Modelling of parabolic trough direct steam generation solar collectors

Solar electric generation systems (SEGS) currently in operation are based on parabolic trough solar collectors using synthetic oil heat transfer fluid in the collector loop to transfer thermal energy to a Rankine cycle turbine via a heat exchanger. To improve performance and reduce costs direct steam generation in the collector has been proposed. In this paper the efficiency of parabolic trough collectors is determined for operation with synthetic oil (current SEGS plants) and water (future proposal) as the working fluids. The thermal performance of a trough collector using Syltherm 800 oil as the working fluid has been measured at Sandia National Laboratory and is used in this study to develop a model of the thermal losses from the collector. The model is based on absorber wall temperature rather than fluid bulk temperature so it can be used to predict the performance of the collector with any working fluid. The effects of absorber emissivity and internal working fluid convection effects are evaluated. An efficiency equation for trough collectors is developed and used in a simulation model to evaluate the performance of direct steam generation collectors for different radiation conditions and different absorber tube sizes. Phase change in the direct steam generation collector is accounted for by separate analysis of the liquid, boiling and dry steam zones.

Compact Linear Fresnel Reflector solar thermal powerplants

This paper evaluates Compact Linear Fresnel Reflector (CLFR) concepts suitable for large scale solar thermal electricity generation plants. In the CLFR, it is assumed that there will be many parallel linear receivers elevated on tower structures that are close enough for individual mirror rows to have the option of directing reflected solar radiation to two alternative linear receivers on separate towers. This additional variable in reflector orientation provides the means for much more densely packed arrays. Patterns of alternating mirror inclination can be set up such that shading and blocking are almost eliminated while ground coverage is maximised. Preferred designs would also use secondary optics which will reduce tower height requirements. The avoidance of large mirror row spacings and receiver heights is an important cost issue in determining the cost of ground preparation, array substructure cost, tower structure cost, steam line thermal losses, and steam line cost. The improved ability to use the Fresnel approach delivers the traditional benefits of such a system, namely small reflector size, low structural cost, fixed receiver position without moving joints, and non-cylindrical receiver geometry. The modelled array also uses low emittance all-glass evacuated Dewar tubes as the receiver elements. Alternative versions of the basic CLFR concept that are evaluated include absorber orientation, absorber structure, the use of secondary reflectors adjacent to the absorbers, reflector field configurations, mirror packing densities, and receiver heights. A necessary requirement in this activity was the development of specific raytrace and thermal models to simulate the new concepts.

Desalination by solar heated membrane distillation

Abstract This paper examines the feasibility of a solar powered membrane distillation plant for the supply of domestic drinking water in the arid/rural regions of Australia. The plant differs from conventional solar powered devices in its capacity to recover large proportions of the latent heat of vapourisation using conventional heat-exchange devices. The plant has been designed and constructed using data obtained from a computer simulation of the process. Preliminary tests have shown the plant capable of achieving the required production capacity. An economic sensitivity analysis has been used to select the optimum heat recovery.

Thermosyphon circulation in solar collectors

Abstract Theoretical predictions of flow rate in thermosyphon solar collectors are compared with experimental measurements obtained using a laser doppler anemometer. Modifications to the usual method of analysis are proposed to improve the accuracy of the predictions, and the results are compared with flow rate predictions and measurements in other investigations.

Performance evaluation of solar thermal electric generation systems

Abstract A unified model of a solar electric generation system (SEGS) is developed using a thermo–hydrodynamic model of a direct steam collector combined with a model of a traditional steam power house. The model is used to study the performance of different collector field and power house arrangements under Australian conditions. To find the effect of collector inclination on the SEGS overall performance, the SEGS performance is studied with a horizontal and an inclined DSG collector field by considering phase distribution and pressure drop in the collector field piping network. The appropriate spacing of collector arrays in the field is calculated by considering the shading between collectors. The annual performance of the SEGS is evaluated using the hourly radiation data for different sites in Australia.

Transient response of thermosyphon solar collectors

Abstract The response of thermosyphon solar water heaters to step changes of insolation is investigated. Measurements of the transient flow development in a thermosyphon circuit were obtained using a laser doppler anemometer and a mathematical model was developed to simulate the transient performance. The results show that although there are long time delays associated with the development of the thermosyphon flow the energy collection capability is not affected by thermosyphon time delays.

Simulation of the long term performance of thermosyphon solar water heaters

Abstract A finite element simulation model for predicting the long term performance of thermosyphon solar water heaters is presented, and the simulation results are compared with the measured performance of six systems supplying typical domestic hot water loads. To obtain consistent simulation results the storage tank temperature stratification had to be accurately simulated using 20 nodes with 5 min time steps for vertical tanks and 30 nodes with 2 min time steps for horizontal tanks. A distributed return model for the mixing of the collector return flow in the tank was also found to be necessary to accurately model tank temperature stratification. The performance of single tank thermosyphon systems is shown to improve as the flow through the collector is reduced to approximately 1 tank volume per day and thermosyphon systems are shown to be slightly more efficient than equivalent pumped circulation systems.

Hydrodynamic Analysis of Direct Steam Generation Solar Collectors

Direct steam generation collectors are considered with the aim to improve the performance of a parabolic trough collector leading to a reduction of operating costs of solar electric generation systems. In this study a hydrodynamic steady state model is developed and linked with a thermal model to optimize the performance of once-through direct steam generation solar collectors. The hydrodynamic model includes flow pattern classification and a pressure drop model. Flow pattern maps for typical DSG collectors with horizontal and inclined absorber tubes are generated to investigate the variation of flow conditions with radiation level, tube diameter, tube length and flow rate. Two-phase flow frictional pressure drop correlations for the range of operating conditions in a DSG collector are selected from the wide range of published correlations by comparison with experimental data for typical steam-water flow conditions in a DSG collector. Pressure drop is calculated for different operating conditions for both horizontal and inclined solar absorber tubes. Alternative operational strategies are evaluated to achieve optimum performance of a direct steam generation collector at different radiation levels.

Long term performance of thermosyphon solar water heaters

Abstract The performance of 6 thermosyphon solar water heaters was measured while the systems were supplying typical domestic hot water loads. The effect of system configuration, daily load draw off times and off-peak vs continuous boosting was studied. In contrast to forced circulation systems the performance of thermosyphon systems was found to be best if a morning peak load pattern was used. Operation with off-peak boosting was found to improve the annual contribution by 14 per cent.

Characteristics of Vertical Mantle Heat Exchangers for Solar Water Heaters

The flow structure in vertical mantle heat exchangers was investigated using a full-scale tank designed to facilitate flow visualisation. The flow structure and velocities in the mantle were measured using a particle Image Velocimetry (PIV) system. A CFD simulation model of vertical mantle heat exchangers was also developed for detailed evaluation of the heat flux distribution over the mantle surface. Both the experimental and simulation results indicate that distribution of the flow around the mantle gap is governed by buoyancy driven recirculation in the mantle. The operation of the mantle was evaluated for both high and low temperature input flows.