The circular cylindrical reflector: Application to a shallow solar pond electricity generating system

The circular cylindrical reflector is shown to be a means of effectively tilting a solar energy collecting plane. This is particularly useful for swimming pools and solar ponds with free water surfaces that cannot be tilted. Such reflectors are applied to an electricity generating system which is driven by shallow solar ponds at a 40° lat. Thereby the annual electrical energy production can be increased by 40 per cent, to a large extent power production can be leveled on an annual basis, and the turbine inlet temperature can be maintained at a constant level through the year.     

Experimental study of natural brine solar ponds in Tibet

A salinity gradient solar pond (SGSP) is a simple and effective way of capturing and storing solar energy. The Qinghai-Tibet Plateau has very good solar energy resources and very rich salt lake brine resources. It lacks energy for its mineral processes and is therefore an ideal location for the development and operation of solar ponds. In China, solar ponds have been widely applied for aquaculture, in the production of Glauber’s salt and in the production of lithium carbonate from salt lake. As part of an experimental study, a SGSP using the natural brine of Zabuye salt lake in the Tibet plateau has been constructed. The pond has an area of 2500 m² and is 1.9 m deep. The solar pond started operation in spring when the ambient temperature was very low and has operated steadily for 105 days, with the LCZ temperature varying between 20 and 40 °C. During the experimental study, the lower convective zone (LCZ) of the pond reached a maximum temperature of 39.1 °C. The results show that solar ponds can be operated successfully at the Qinghai-Tibet plateau and can be applied to the production of minerals.     

Solar ponds in hydrometallurgy and salt production

The possibilities of using solar ponds in the mining industry are explored. Their advantages are identified from an economic point of view and the main technical points for proper operation are discussed. A short account is given of the hydrometallurgical and salt production processes of interest from the point of view of solar ponds. Solar ponds can provide a working environment for many mineral processing systems, not only as a source of energy, but also as a large basin maintained at nearly constant temperature where different operations can be performed. Examples are described for applications in the production of sodium sulfate, boric acid, copper, potassium chloride, and sodium borate.     

Solar ponds

This report provides the background to, and the current status of, solar ponds as proven viable large-area collectors capable of providing both low-cost thermal energy and mechanical or electrical energy using state-of-the-art low-temperature turbo-generators.After a short background statement giving the history and motivation to create a viable large-area collector with built-in storage, the basic theory of salt-gradient solar ponds is sketched. (More detailed-theory is available from the given references, particularly two recently published handbooks.) NaCl and MgCl₂ are two common and low-cost salts suitable for solar ponds. A number of problems such as the adverse effect of wind, leakage, fouling—and their solutions—are indicated as are some fundamental constraints (Section 8) that limit the sites suitable for solar ponds. Practical details include how ponds are built and filled and how the heat is extracted. Section 7 presents a condensed account of solar pond experience in a number of countries.Practical operating temperatures of 90°C are obtained with collection efficiencies usually between 15 and 25 per cent: this permits a number of practical applications as discused in Section 10, i.e. heating and cooling, power production and desalination.Realistic pond cost figures indicate thermal energy costs equivalent to US$41 per ton of fuel for a sunny climate (using a conservative 11.7 per cent annual charage on capital): such low-cost calories permit thermodynamic conversion to power: although the conversion efficiency is low, the solar pond power station (SPPS) is viable in many cases. Bus-bar power costs, for a sunny climate, vary from a high of US13.5 cents/kWh—using present technology—to a low 5.3 cents in sizes of 20 GWhr(e) per annum or larger.A 150-kW SPPS has already been built and successfully operated in Israel since December 1979 and a 5000-kW unit is due for completion in the next 2 yr.The ability of a solar pond to store heat even from summer to winter greatly increases its usefulness in almost all applications: for power production, the SPPS can—like a hydro-electric plant, provide peaks of power, on demand—far in excess of the pond mean capacity. The estimate that SPPS costs flatten out at 20–40 MW is of interest to developing countries that could install generating capacity in relatively small steps as demand grows.     

Performance investigation of a salt gradient solar pond coupled with desalination facility near the Dead Sea

Solar ponds provide the most convenient and least expensive option for heat storage for daily and seasonal cycles. This is particularly important for a desalination facility, if steady and constant water production is required. If, in addition to high storage capacity, other favorable conditions exist, the salt gradient solar ponds (SGSPs) are expected to be able to carry the entire load of a large-scale flash desalination plants without dependence upon supplementary sources. This paper presents a performance investigation of a SGSP coupled with desalination plant under Jordanian climatic conditions. This is particularly convenient in the Dead Sea region characterized by high solar radiation intensities, high ambient temperature most of the year, and by the availability of high concentration brine. It was found that a 3000 m² solar pond installed near the Dead Sea is able to provide an annual average production rate of 4.3 L min⁻¹ distilled water compared with 3.3 L min⁻¹ that would be produced by El Paso solar pond, which has the same surface area. Based on this study, solar ponds appear to be a feasible and an appropriate technology for water desalination near the Dead Sea in Jordan.     

Constructal design of salt‐gradient solar pond fields

Salt‐gradient solar ponds (SGSPs) are water bodies that capture and accumulate large amounts of solar energy. The design of an SGSP field has never been analyzed in terms of studying the optimal number of solar ponds that must be built to maximize the useful energy that can be collected in the field, or the most convenient way to connect the ponds. In this paper, we use constructal design to find the optimal configuration of an SGSP field. A steady‐state thermal model was constructed to estimate the energy collected by each SGSP, and then a complementary model was developed to determine the final temperature of a defined mass flow rate of a fluid that will be heated by heat exchangers connected to the solar ponds. By applying constructal design, four configurations for the SGSP field, with different surface area distribution, were evaluated: series, parallel, mixed series‐parallel and tree‐shaped configurations. For the study site of this investigation, it was found that the optimal SGSP field consists of 30 solar ponds of increasing surface area connected in series. This SGSP field increases the final temperature of the fluid to be heated in 22.9%, compared to that obtained in a single SGSP. The results of this study show that is possible to use constructal theory to further optimize the heat transfer of an SGSP field. Experimental results of these configurations would be useful in future works to validate the methodology proposed in this study. Copyright © 2016 John Wiley & Sons, Ltd.     

Optimum thickness of the nonconvective zone in salt gradient solar ponds

The nonconvective gradient zone of a salt gradient solar pond tends to more effectively transmit incident solar energy to the storage brine below as its thickness is reduced. However, that same gradient zone tends to more effectively reduce heat loss from the warm brines as its thickness is increased. Therefore, there exists an optimum gradient zone thickness for which the net rate of energy collected and retained is a maximum. This report describes a technique for using a numerical simulation model to determine the optimum thickness of the gradient zone in ponds; provided other basic design, operating and climatic factors are specified. Significant improvements in pond efficiency may be obtained if the thickness of the gradient zone is adjusted monthly, seasonally or even if maintained at the annual average optimum thickness as compared with operating the pond with other than an optimum gradient zone thickness.     

A study on the transient behaviour of solar ponds

In this paper, the transient behaviour of solar ponds has been investigated using a finite difference formulation. The performance of solar ponds can be successfully analysed and the effects of various parameters studied. The thickness of the layer with density gradient has a profound effect on the performance of a solar pond and an increase of the thickness of this layer from 1 to 2m increases substantially the operating temperature for the same overall efficiency.

The effect of the pattern of heat extraction is discussed. Heat extraction at a constant rate will result in large temperature fluctuations from summer to winter. But, if the heat is extracted at a varying rate proportional to the monthly average solar radiation, then the temperature fluctuation is considerably decreased as compared to the case of constant heat removal for the same yearly efficiency. It is possible to operate a solar pond in the Melbourne area with a yearly efficiency of 15% having a high temperature of 67 °C in summer and a low of 40 °C in winter.

The performances of solar ponds in Alice Springs and Darwin have been studied. It is found that the maximum bottom temperature does not go beyond 80 °C for a pond efficiency of 20%. Since these two cities have the most favourable locations for solar ponds, the indicated maximum temperature casts doubt on the prevalent view that a solar pond can operate at a temperature above 90 °C with an efficiency of 20%.     

Status of thin film solar cells in research, production and the market

Photovoltaics is one of the fastest growing industries at present. Over the last five years, the production of photovoltaic solar cells has steadily increased at an annual average of 40%, driven not only by the progress in materials and processing technology, but by market introduction programmes in many countries around the world. This growth is mainly being attained by an increase in manufacturing capacities based on the technology of crystalline, single junction devices. Consistent with the time needed for any major change in energy infrastructure, another 20–30 years of sustained and aggressive growth will be required for photovoltaics to substitute a significant share of conventional energy sources. The question is whether a switch will be possible with the current technologies alone or whether this growth will only be possible with the continuous introduction of new technologies. It leads us to the search for new developments with respect to material use and consumption, device design and production technologies as well as new concepts to increase overall efficiency. This paper analyses the current status of thin film solar cells and their outlook for future developments.     

Numerical and experimental analysis of a salt gradient solar pond performance with or without reflective covered surface

An experimental salt gradient solar pond having a surface area of 3.5 × 3.5 m² and depth of 2 m has been built. Two covers, which are collapsible, have been used for reducing the thermal energy loses from the surface of the solar pond during the night and increasing the thermal efficiency of the pond solar energy harvesting during daytime. These covers having reflective properties can be rotated between 0° and 180° by an electric motor and they can be fixed at any angle automatically. A mathematical formulation which calculates the amount of the solar energy harvested by the covers has been developed and it is adapted into a mathematical model capable of giving the temporal temperature variation at any point inside or outside the pond at any time. From these calculations, hourly air and daily soil temperature values calculated from analytical functions are used. These analytic functions are derived by using the average hourly and daily temperature values for air and soil data obtained from the local meteorological station in Isparta region. The computational modeling has been carried out for the determination of the performance of insulated and uninsulated solar ponds having different sizes with or without covers and reflectors. Reflectors increase the performance of the solar ponds by about 25%. Finally, this model has been employed for the prediction of temperature variations of an experimental salt gradient solar pond. Numerical results are in good agreement with the experiments.     

Hydrogen production by coupling pressurized high temperature electrolyser with solar tower technology

Solar hydrogen production by coupling of pressurized high temperature electrolyser with concentrated solar tower technology is studied. As the high temperature electrolyser requires constant temperature conditions, the focus is made on a molten salt solar tower due to its high storage capacity. A flowsheet was developed and simulations were carried out with Aspen Plus 8.4 software for MW-scale hydrogen production plants. The solar part was laid out with HFLCAL software. Two different scenarios were considered: the first concerns the production of 400 kg/d hydrogen corresponding to mobility use (fuel station). The second scenario deals with the production of 4000 kg/d hydrogen for industrial use. The process was analyzed from a thermodynamic point of view by calculating the overall process efficiency and determining the annual production. It was assumed that a fixed hydrogen demand exists in the two cases and it was assessed to which extent this can be supplied by the solar high temperature electrolysis process including thermal storage as well as hydrogen storage. For time periods with a potential over supply of hydrogen, it was considered that the excess energy is sold as electricity to the grid. For time periods where the hydrogen demand cannot be fully supplied, electricity consumption from the grid was considered. It was assessed which solar multiple is appropriate to achieve low consumption of grid electricity and low excess energy. It is shown that the consumption of grid electricity is reduced for increasing solar multiple but the efficiency is also reduced. At a solar multiple of 3.0 an annual solar-to-H₂ efficiency greater than 14% is achieved at grid electricity production below 5% for the industrial case (4000 kg/d). In a sensitivity study the paramount importance of electrolyser performance, i.e. efficiency and conversion, is shown.     

Performance characteristics of solar ponds operating at different latitudes

The results of a comparative study of the performance of solar ponds operating at different latitudes are presented in this paper. A mathematical tool was developed to study the behaviour of solar ponds operating under different physical and operational conditions. The hourly meteorological data of Kew and Singapore were used. The pond operating at higher latitudes, i.e. Kew, where seasonal variations are significant, must be deeper than a pond operating near the equator. The deeper pond can also act as an interseasonal storage device. A pond operating at a location in Singapore attains a fairly high temperature but the temperature requirements are also high for the desired applications such as space cooling in conjunction with a refrigeration cycle. Although a pond at Kew attains a lower temperature compared with a pond at Singapore, it can supply considerable amounts of thermal energy to support a space heating load. Both the ponds require auxiliary heating, the magnitude being dependent upon the nature of the load.     

A water requirement model for salt gradient solar ponds

A model for predicting the salt gradient solar pond (SGSP) area that could be maintained with a given water supply is presented together with several specific applications. For example, based on 30-year average water flows, the model predicts that 1.93 × 10⁹ m² (477,000 acres) of solar ponds, 1.02 × 10⁹ m² (253,000 acres) of evaporation ponds to recycle salt, and 0.51 × 10⁹ m² (125,000 acres) of freshwater storage reservoirs could be maintained at the Great Salt Lake of Utah. Water use requirements per unit of electrical energy from solar ponds are calculated as 600,000 m³/MW·yr. This is roughly 30 times the water evaporated per unit of electrical energy from coal-fired generating plants using wet cooling towers, but substantially less than water evaporation losses per unit of electrical energy produced from typical hydropower dams and reservoirs. It is concluded that water use requirements for solar ponds, although not necessarily prohibitive, are substantial; and in many locations may be the physical factor that limits solar pond development.     

Large scale solar cooling design using salt gradient solar ponds

The future of large scale cooling is closely linked to the long term economically viable component development for collection and storage of solar energy at relatively high temperatures. As such, solar ponds at the present state of their development are undoubtedly considered as the only promising large scale solar collection and heat storage device for such applications. The present analysis, based on numerical calculations, allows a parametric investigation of solar pond design and operational characteristics to the capacity of a conventionally designed, commercially available, absorption chiller. The results can prove very useful for the rough design and pond size selection for operation of chillers of a substantial capacity, directly from solar ponds.     

Examining potential benefits of combining a chimney with a salinity gradient solar pond for production of power in salt affected areas

The concept of combining a salinity gradient solar pond with a chimney to produce power in salt affected areas is examined. Firstly the causes of salinity in salt affected areas of northern Victoria, Australia are discussed. Existing salinity mitigation schemes are introduced and the integration of solar ponds with those schemes is discussed. Later it is shown how a solar pond can be combined with a chimney incorporating an air turbine for the production of power. Following the introduction of this concept the preliminary design is presented for a demonstration power plant incorporating a solar pond of area 6 hectares and depth 3 m with a 200 m tall chimney of 10 m diameter. The performance, including output power and efficiency of the proposed plant operating in northern Victoria is analysed and the results are discussed. The paper also discusses the overall advantages of using a solar pond with a chimney for production of power including the use of the large thermal mass of a solar pond as a practical and efficient method of storing collected solar energy.     

Optimisation of minimum backup solar water heating system

Current flat plate solar water heaters overproduce slightly in summer and have poor performance in winter at the time of maximum load. They use an expensive absorber plate over the entire absorbing aperture of the collector and fail to use the backside of the absorber. They often have under insulated tanks and are not optimised as integrated systems. This paper describes a design approach taken to use existing commercial flat plate absorber and tank components in a new way to maximise solar contribution and minimise material usage in the construction of the system. The design criterion used is not maximum peak efficiency, but minimum annual backup energy supplied to the system to meet an annual load. This corresponds to meeting a minimum greenhouse emissions requirement in both invested pollution during manufacture and pollution from backup energy supplied. Two new designs are shown which allow the solar fraction of systems to be increased to approximately 80–90% in Sydney Australia using a standard model of domestic hot water usage specified in Australian Standard AS4234. Pollution from fuel use drops to as little as 40% of that of conventional flat plate solar water heaters. These new designs use one absorber plate instead of two and a smaller and better insulated tank. Comparisons of solar fraction are evaluated for a range of climatic conditions. An important insight is that with such a performance optimised system the ultimate solar fraction is limited by occasional long duration cloud cover at the site of installation and making the system larger only increases dumped energy, not utilisable energy. Technical efficiency improvements only reduce the required collector area. However, some additional backup fuel reductions can be made through manual control of backup energy use, because this allows finer control of backup relative to real demand. Pollution from backup fuel usage may be able to be reduced to 1/4 that of current flat plate solar water heaters.     

Sede boqer shallow pond project

The use of shallow solar ponds (convecting solar ponds) for the conversion of solar energy into low grade thermal energy has been a subject of intensive investigation in recent years. At the Institute for Desert Research at the Sede Boqer Campus we have been testing this concept with emphasis placed upon the utilization of locally manufactured components.The daily performance of four small module shallow solar ponds has been monitored almost continuously between August 1978 and May 1979. The ponds are each 2 × 1.3 m in size. They all have the same black PVC lower film, but differ either in the type of upper transparent film, glazing material or glazing angle. The daily performance is characterized by three factors, viz. the maximum daily water temperature achieved, the total daily thermal energy collected and the daily efficiency. Monthly average performance factors for the SSP modules have been determined.Based upon the experimental data, we conclude that the SSP system is capable of supplying ~ 3 GJ/m²-yr of thermal energy under climatic conditions similar to those prevailing at Sede Boqer, i.e. semi-arid zones. The economic feasibility of such a system has been analyzed in comparison with the following alternate energy sources: oil (heavy fraction), natural gas and electricity.     

Heliostat field layout optimization for high-temperature solar thermochemical processing

The layout of the heliostat field of solar tower systems is optimized for maximum annual solar-to-chemical energy conversion efficiency in high-temperature thermochemical processes for solar fuels production. The optimization algorithm is based on the performance function that includes heliostat characteristics, secondary optics, and chemical receiver-reactor characteristics at representative time steps for evaluating the annual fuel production rates. Two exemplary applications for solar fuels production are selected: the thermal reduction of zinc oxide as part of a two-step water-splitting cycle for hydrogen production, and the coal gasification for syngas production.     

A dynamic linear programming approach to market allocation of renewable resources in the italian energy system: The case of solar thermal and biogas technologies

In Italy solar thermal energy and energy from biogas are two possible means of reducing dependence on energy imports. Using a multiperiod LP model (MARKAL) the authors assessed the likely potential of both technologies under various circumstances. The study covered the period 1980–2005, in five segments of five years. It focused only on the subsystem of the energy end-uses which can be substituted for by solar thermal and biogas technologies. The overall non-renewable sources which can be saved in 20 years by these technologies total 450 PJ (1 PJ = 10^{1 5} J) if the fuel prices rise at 0 per cent average annual, 1450 PJ if the fuel prices rise at 4.2 per cent average annual, 1860 PJ if the fuel prices rise at 7.2 per cent average annual and 3780 PJ if the fuel prices rise at 15 per cent average annual. However the most competitive technologies appear to be solar water heaters used mainly in the private and commercial sectors and biogas systems used mainly in the agricultural sector. The study was carried out by APRE under ENEA (formerly CNEN) contract and was intended to serve as an analytical basis for establishing an overall development and demonstration strategy for end-use renewable technologies in Italy.     

Bifacial photovoltaic panels field

Bifacial solar cells may produce more output energy than mono-facial solar cells because both sides of the cell, front and rear, can absorb solar radiation. This occurs when the nearby ground or other artificial surfaces are highly reflective. A gain in output power of 5–20% has been reported in the literature for special applications. The present article deals with the calculation of the annual incident irradiation on a solar field comprising of bifacial photovoltaic panels deployed in multiple rows and separated by a distance between the rows. These types of fields are designed for large scale solar electricity production. The calculation of the annual incident irradiation is compared between two types of deployments: (a) bifacial photovoltaic panels installed with an optimal tilt angle facing south, (b) bifacial photovoltaic panels installed vertically and facing the east-west direction. The study shows that bifacial photovoltaic panels installed with an optimal tilt angle may produce 32% more energy than vertical bifacial photovoltaic panels, for the same environmental conditions. On the other hand, more vertical collectors can be installed in fields with the same field dimensions.