Large Solar Plants Research Articles

Thermal advantages for solar heating systems with a glass cover with antireflection surfaces

Investigations elucidate how a glass cover with antireflection surfaces can improve the efficiency of a solar collector and the thermal performance of solar heating systems. The transmittances for two glass covers for a flat-plate solar collector were measured for different incidence angles. The two glasses are identical, except for the fact that one of them is equipped with antireflection surfaces by the company SunArc A/S. The transmittance was increased by 5–9%-points due to the antireflection surfaces. The increase depends on the incidence angle. The efficiency at incidence angles of 0° and the incidence angle modifier were measured for a flat-plate solar collector with the two cover plates. The collector efficiency was increased by 4–6%-points due to the antireflection surfaces, depending on the incidence angle. The thermal advantage with using a glass cover with antireflection surfaces was determined for different solar heating systems. Three systems were investigated: solar domestic hot water systems, solar heating systems for combined space heating demand and domestic hot water supply, and large solar heating plants. The yearly thermal performance of the systems was calculated by detailed simulation models with collectors with a normal glass cover and with a glass cover with antireflection surfaces. The calculations were carried out for different solar fractions and temperature levels of the solar heating systems. These parameters influence greatly the thermal performance associated with the antireflection surfaces.

Advances in solar thermal electricity technology

Various advanced solar thermal electricity technologies are reviewed with an emphasis on new technology and new market approaches. In single-axis tracking technology, the conventional parabolic trough collector is the mainstream established technology and is under continued development but is soon to face competition from two linear Fresnel reflector (LFR) technologies, the CLFR and Solarmundo. A Solarmundo prototype has been built in Belgium, and a CLFR prototype is awaiting presale of electricity as a commercial plant before it can be constructed in Queensland. In two-axis tracking technologies, dish/Stirling technologies are faced with high Stirling engine costs and emphasism may shift to solarised gas micro-turbines, which are adapted from the small stationary gas turbine market and will be available shortly at a price in the US$1 ppW range. ANU dish technology, in which steam is collected across the field and run through large steam turbines, has not been commercialised. Emphasis in solar thermal electricity applications in two-axis tracking systems seems to be shifting to tower technology. Two central receiver towers are planned for Spain, and one for Israel. Our own multi-tower solar array (MTSA) technology has gained Australian Research Council funding for an initial single tower prototype in Australia of approximately 150 kW(e) and will use combined microturbine and PV receivers. Non-tracking systems are described of two diverse types, Chimney and evacuated tubes. Solar chimney technology is being proposed for Australia based upon German technology. Air is heated underneath a large glass structure of about 5 km in diameter, and passes up a large chimney through a wind turbine near the base as it rises. A company Enviromission Ltd. has been listed in Australia to commercialise the concept. Evacuated tubes are growing rapidly for domestic hot water heating in Europe and organic rankine cycle engines such as the Freepower 6 kW are being considered for operation with thermal energy developed by evacuated tube and trough systems. These may replace some PV in medium sized applications as they offer potential for inexpensive pressurised water storage for 24 h operation, and backup by fuels instead of generators. In the medium term there is a clear trend to creation of smaller sized systems which can operate on a retail electricity cost offset basis near urban and industrial installations. In the longer term large low cost plants will be necessary for large scale electricity and fuels production. Retrofit central generation solar plants offer a cost effective transition market which allows increased production rates and gradual cost reduction for large solar thermal plant. In the paper the author describes current funding systems in Europe, Australia, and the USA, and makes suggestions for more effective programmes of support.

New large solar photocatalytic plant: set-up and preliminary results

A European industrial consortium called SOLARDETOX has been created as the result of an EC-DGXII BRITE-EURAM-III-financed project on solar photocatalytic detoxification of water. The project objective was to develop a simple, efficient and commercially competitive water-treatment technology, based on compound parabolic collectors (CPCs) solar collectors and TiO 2 photocatalysis, to make possible easy design and installation. The design, set-up and preliminary results of the main project deliverable, the first European industrial solar detoxification treatment plant, is presented. This plant has been designed for the batch treatment of 2 m 3 of water with a 100 m 2 collector-aperture area and aqueous aerated suspensions of polycrystalline TiO 2 irradiated by sunlight. Fully automatic control reduces operation and maintenance manpower. Plant behaviour has been compared (using dichloroacetic acid and cyanide at 50 mg l −1 initial concentration as model compounds) with the small CPC pilot plants installed at the Plataforma Solar de Almerı́a several years ago. The first results with high-content cyanide (1 g l −1 ) waste water are presented and plant treatment capacity is calculated.

Heat transfer analysis of low thermal conductivity solar energy absorbers

Polymers have been proven to be high potential low-cost materials for the design and mass production not only for ordinary solar water heaters but also for very simple large size, modular solar collectors, suitable for easy erection of large solar heating plants. Their major drawback for solar–thermal conversion applications is their low thermal conductivity, which prohibits their use unless an appropriate absorber design is employed. The low thermal conductivity of polymers has imposed the need of a particular absorber design, which is basically composed of a pair of dark, closely spaced parallel plates at the top of which solar radiation is absorbed, forming a thin channel for the flow of the heat transfer fluid. The aim of the present work is to investigate the particular limitations of this polymer plate absorber design, for a wide range of collector loss and convective heat transfer coefficients between heat transfer fluid and absorber plate. The aim is also to calculate the particular collector efficiency factors and conditions under which the associated collector performance parameters should be modified to account for the finite absorber plate conductance. This conductance was proven to be another decisive absorber design parameter, improper selection of which may probably lead to strong deterioration of the collector efficiency.

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.

Modelling study for compact Fresnel reflector power plant

This paper describes 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 receiver towers that are close enough for individual reflectors to have the option of directing reflected solar radiation to two linear receivers on separate towers. This additional variable in reflector orientation provides the means for much more densely packed arrays. Patterns of alternating reflector orientation can be set up such that closely packed reflectors can be positioned so that shading and blocking are almost eliminated. The avoidance of large reflector spacings and tower 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 advanced all glass evacuated tubular absorbers with low emittance selective coatings.

Design, analysis and performance of low-cost plastic film large solar water heating systems

Although the use of solar energy is obvious for the energy intensive nature of low-grade heating processes, solar heating technology has not yet become commercially available even in favorable geographical locations with high solar potential. This is not surprising, taking into consideration the unreasonably high cost of solar plants relative to the level of technology of most of the commercially available components and systems. In the present work an improved low-cost large solar heating system design has been proposed, based on earlier design developments, suitable for applications in mild sunny climates where the prospects for the promotion of solar heating technology are very favorable. An analysis is developed which allows design and operational behaviour predictions of the physical system. Long-term efficiency and typical input-output performance is also investigated based on statistically processed long-term meteorological measurements for Athens, Greece. Derived results indicate that substantial performance would be expected for the proposed fractional cost system design. This may probably lead to market expansion of large solar heating plants through “commitment based” promotion schemes.

A dynamic model for predicting solar plant performance and optimum control

Planning over the intermediate and long terms for the operation of solar energy systems must be based on the use of measured data and performance models of system components. We present a dynamic model for predicting the thermal performance of a low-temperature solar power plant. We analyse the optimal control regime for energy management. Validation of the model was performed on a large solar plant located in the north of Tunisia. The results showed good agreement between measured and model values. During the cold season, the plant efficiency increases considerably when a greenhouse is connected to the power plant.

96/06639 Large solar plant photocatalytic water decontamination: Degradation of atrazine

96/06639 Large solar plant photocatalytic water decontamination: Degradation of atrazine

Large solar plant photocatalytic water decontamination: Effect of operational parameters

The effect of the concentration of the photocatalyst, the organic substrate and the light intensity is presented for the photocatalytic degradation of pentachlorophenol in the large solar plant built at the Plataforma Solar de Almeria. The degradation of pentachlorophenol is easily achieved in short residence times. The main factors affecting the total efficiency are discussed in view of a simple model for the primary events in the photocatalytic system. The derived approximated equations are able to fit the experimental data well. The optimization of the degradation and mineralization rates is strongly dependent on the organic substrate and predominantly depends on kinetic factors. The partition or competition of the substrate and its degradation products on the surface of the photocatalyst seems unimportant.

Large solar plant photocatalytic water decontamination: Degradation of atrazine

Photocatalytic degradation of atrazine was successfully and efficiently carried out in a large modular flow trough system under solar light. The degradation leads to complete mineralization of the alkyl substituents of s-triazine, giving trihydroxy-s-triazine as a final product. An efficient detoxification can be attained. The presence of 1 × 10 −3 m of peroxydisulphate, kept constant during the treatment, is sufficient to remove 98% of atrazine with very short residence times (less than 6 min), and to convert the parent triazine to more than 90% of trihydroxy-s-triazine in less than 2 h. The peroxydisulphate is particularly advantageous when large amounts of polluted ground water have to be treated. The factors affecting the total efficiency are discussed in view of a simple model for the primary events in the photocatalytic system.

Large solar plant photocatalytic water decontamination: Degradation of pentachlorophenol

The photocatalytic degradation of pentachlorophenol in the large solar plant built at the Plataforma Solar de Almeria is presented. Degradation of cubic meters per hour of pentachlorophenol at a concentration of mg L −1 is demonstrated and is easily achieved in short residence times. Details of the plant based on parabolic solar concentrators, together with the examination of the main factors affecting total efficiency are discussed in comparison with small scale laboratory experiments. Experiments carried out with addition of peroxydisulphate show a noticeable increment in the degradation efficiency and are suggested for the treatment of concentrated or refractory pollutants.

Measurement and Analysis of Heliostat Images

A method is proposed allowing the physical interpretation of the sun image generated by reflecting systems, e.g., heliostats, on a target plane. The instantaneous flux distributions reflected from the target are measured by a distant video-type camera system in the visible spectrum and analyzed in terms of their constituents (sunshape, astigmatic aberration, mirror glass waviness, canting, and curvature errors). The single contributions to the total image are parmetrized by appropriate physically-based model functions and composed to a theoretical image to be compared with the measured image. Within a fit procedure, the parameter set is determined for every experiment allowing to check the consistency and uniqueness of the approach. A series of measurements on GAST heliostats performed between 1984 and 1987 was analyzed. They demonstrate the good optical performance of these systems, thus justifying their use in large solar thermal central receiver tower plants.

A low cost design solar desalination unit

A novel low cost design solar still is described and performance data are presented. The proposed design eliminates the need for support or side-walls, as required in most conventional solar still designs. A comparison of the measured still productivity values between the proposed design and a large solar distillation plant constructed in India has shown that our design simplification has not affected still performances. An economic analysis, based upon the fraction of still construction cost dedicated to the still wall construction, has indicated that the proposed design should reduce the overall cost/m 2 of the solar still construction by at least 20%.

The Technical Evaluation of a Large Scale Solar Distillation Plant

A theoretical evaluation of the heat and mass transfer interchange in an air-inflated solar still has been studied. Experimental verification tests have been carried out and the results compared with theoretical predictions. The percentage of unaccounted heat losses on the overall balance was under three percent for most tests. Continued modification of the theory is necessary in order to account for variations in some of the internal balances.

Description of a large solar distillation plant in the West Indies

Description of a large solar distillation plant in the West Indies