A review of research and development work on solar dryers with heat storage

Drying of agricultural food products is one of the most attractive and cost-effective applications of solar energy. The solar dryer is less reliable due to the intermittent nature of solar energy. This shortcoming can be overcome to some extent by storing solar energy. Information on sensible and latent heat storage materials and systems is spread widely in the literature. In this paper, we try to gather information about the previous and current research works in the field of thermal energy storage technology for solar air heater and dryer. The relative studies are classified on the basis of the type of storage material used in solar dryers, i.e. phase change material (PCM), rock, water, etc. Several designs of solar dryers with different heat storage materials were proposed by researchers. Recent studies focused on PCMs such as Paraffin and salt hydrate, due to their high heat storage capacity per unit volume.     

A model for optimal sizing of photovoltaic irrigation water pumping systems

The previous methods for optimal sizing of photovoltaic (PV) irrigation water pumping systems separately considered the demand for hydraulic energy and possibilities of its production from available solar energy with the PV pumping system. Unlike such methods, this work approaches the subject problem systematically, meaning that all relevant system elements and their characteristics have been analyzed: PV water pumping system, local climate, boreholes, soil, crops and method of irrigation; therefore, the objective function has been defined in an entirely new manner. The result of such approach is the new mathematical hybrid simulation optimization model for optimal sizing of PV irrigation water pumping systems, that uses dynamic programming for optimizing, while the constraints were defined by the simulation model. The model was tested on two areas in Croatia, and it has been established that this model successfully takes into consideration all characteristic values and their relations in the integrated system. The optimal nominal electric power of PV generator, obtained in the manner presented, are relatively smaller than when the usual method of sizing is used. The presented method for solving the problem has paved the way towards the general model for optimal sizing of all stand-alone PV systems that have some type of energy storage, as well as optimal sizing of PV power plant that functions together with the storage hydroelectric power plant.     

Present status of solar distillation

In this communication an attempt has been made to review, in brief, work on solar distillation, its present status in the world today and its future perspective. The review also includes water sources, water demand, availability of potable water and purification methods including the state of art and historical background. The classification of distillation units has been done on the basis of literature survey till today. The basic heat and mass transfer relation responsible for developing, testing procedure for various designs of solar stills have also been discussed. The present status of solar distillation units in India, economics of single and double slope fibre re-inforced plastic on the basis of long-term performance and recommendations for future have been discussed in brief.     

Electrical and engine driven heat pumps for effective utilisation of renewable energy resources

Much of the energy used for domestic, commercial and industrial purposes is to provide efficient and effective heating of conditioned spaces and for specialist niche applications in process heat systems. Vapour compression heat pumps driven by electric motors or engines provide the real capability of upgrading low temperature sources of ambient and waste heat to match the desired load temperatures in such heating applications. Major source of ambient heat stem from the storage of solar energy in the ground, in lakes and rivers, and in atmospheric air. Heat pumps can therefore be used to effectively harness indirectly the daily solar radiation input. In addition many industries have major sources of waste low grade heat in the form of air or water discharged from the industrial process heat stream. Heat pumps are generally formally classified therefore as air source, ground source or water source units although there has also been considerable interest recently in hybrid units combining the attributes of two or more of these specific types mentioned above.     

Renewable energy in the outback of Australia

Many stations and small communities, particularly Aboriginal Communities, have switched from diesel generators to sensible Remote Area Power Supply (RAPS) systems consisting of wind and/or solar input, battery bank, inverter and generator back-up.The introduction of Synergy Power Corporation's low-wind regime turbines that can hover/reef rather than furl has allowed wind powered RAPS to penetrate markets in the desert communities that were previously considered unsuitable for wind systems. The unique reefing system is described and some interesting case studies given.Solar water pumping and solar powered microwave telecommunications have been common for the past ten years and have proved extremely reliable and have been well accepted.     

Performance of commercially available solar and heat pump water heaters

Many countries are using policy incentives to encourage the adoption of energy-efficient hot water heating as a means of reducing greenhouse gas emissions. Such policies rely heavily on assumed performance factors for such systems. In-situ performance data for solar and heat pump hot water systems, however, are not copious in the literature. Otago University has been testing some systems available in New Zealand for a number of years. The results obtained are compared to international studies of in-situ performance of solar hot water systems and heat pump hot water systems, by converting the results from the international studies into a single index suitable for both solar and heat pump systems (COP). Variability in the international data is investigated as well as comparisons to model results. The conclusions suggest that there is not too much difference in performance between solar systems that have a permanently connected electric boost backup and heat pump systems over a wide range of environmental temperatures. The energy payback time was also calculated for electric boost solar flat plate systems as a function of both COP and hot water usage for a given value of embodied energy. The calculations generally bode well for solar systems but ensuring adequate system performance is paramount. In addition, such systems generally favour high usage rates to obtain good energy payback times.     

太阳能热机的有限时间热力学优化准则

本文导出两种太阳能收集器损失关系情况下太阳能驱动热机参数选择的有限时间热力学优化准则,所得结果包含了前人工作的一些结论。     

Capillary pumping systems for solar heating application

Capillary pumping two-phase loops have been continuously investigated for electronic cooling systems, satellite thermal control and other space applications. Most tests were performed in capillary evaporators using plastic or metallic porous wick as capillary structure and anhydrous ammonia as the working fluid. In this work, capillary pumping system assisting flat solar collectors is proposed as an alternative to residential and commercial water heating systems, using fine circumferential grooves as capillary structure and acetone as the working fluid. The experimental results are obtained for a small-scale solar heating system, using one capillary pump attached to a flat copper plate of 46 cm in length and 6 cm in width. The capillary pump consists of a 19 mm OD and 500 mm long aluminium tube, with fine internal circumferential groves as the capillary structure. The working fluid is pumped from a condenser designed to delivery heat to the water storage. Heat inputs up to 14 W (507 W/m²) were estimated for heating purpose. The system presented reliable start-ups and good performance in continuous operation. The measurements were found to be in good agreement with theoretical results for the temperature field and solar power absorbed by the system.     

Application of photovoltaic array for pumping water as an alternative to diesel engines in Jordan Badia,Tall Hassan station: Case study

The availability of water and the ability to access are the key questions arising in developing countries including Jordan, which is the fourth poorest country in the world regarding water resources. Renewable energy, especially solar energy, can potentially play a role in the supply of safe water in Jordan Badia, where nearly 80% of the total area of Jordan is Badia, and in most cases these deep wells are far away from the national grid electricity, and in some of these areas there is an important quantity of groundwater at shallow depths. This paper introduces and compares the cost-effectiveness and the Present Value Cost (PVC) for the economic evaluation of power supply for pumping systems in remote areas in Northern Badia of Jordan by two different energy supply systems, photovoltaic systems and diesel engines.Many variables are taken into account such as the fuel prices, and the required investments. The comparison is made for a wide range of variable values, total head, tank capacity, photovoltaic array peak power and pumping requirements. A case study in Tall Hassan station is conducted to analyze the two power supply pumping systems, which are designed to supply drinking water.The results obtained are useful for choosing the best alternative for the power supply of pumping systems in wells in Northern Badia of Jordan.     

The use of waste heat in a solar still

There are instances in remote areas where heat is being wasted, e.g., in internal combustion, engines, etc. Some of this heat can be recovered to produce distilled water in solar stills.

The solar still replaces the cooling tower, ponds, or radiators normally used to control the engine temperature. The diesel cooling water in such a system remains separate from the saline water in the solar still.

The advantages of using such a system compared with a conventional solar still are:

1. (a) water costs are very much reduced

2. (b) the area occupied is much less, i.e., about 1/5th

3. (c) production has much less seasonal variation

4. (d) the efficiency of the solar still is improved due to the higher operating temperatures.

From experiments conducted at Highett using a Mk VI solar still fitted with a simple heat exchanger and a separate electrically-heated source of hot water to simulate the waste heat, design data are not available for application to working systems. The information required to match a solar still to a diesel's cooling requirement is:

1. (a) engine efficiency

2. (b) hourly fuel consumption

3. (c) hourly solar radiation

4. (d) hourly ambient temperatures.

A by-product of this work has been the production of a “solar water heater” which costs less than that of the cheapest conventional system. This “solar” hot water system uses a heat exchanger similar to what is used to transfer the waste heat to the saline water. It is envisaged to have hot water productions approximately the same as the distilled water productions. The influence of hot water production on the output of the waste heat solar still is discussed.     

Recent advances on nanofluids for low to medium temperature solar collectors: energy,exergy, economic analysis and environmental impact

The efficient exploitation of solar irradiation is one of the most encouraging ways of handling numerous environmental concerns. Solar collectors are suitable devices that capture solar irradiation and convert it into thermal energy and electricity. In the last years, the nanofluids used in solar thermal systems have been studied as a useful technique for enhancing the solar collectors’ performance and establishing them as viable and highly efficient systems. The present review paper aims to summarize and discuss the most important numerical and experimental studies in nanofluid-based solar systems for application at low and medium temperature levels, while the emphasis on the fundamental physical phenomena that occur. In the first part, numerous numerical models and the principal physical phenomena affecting the heat transfer rate in the nanofluid have been analyzed. More specifically, the importance of different forces in nanofluid flows that exist in particulate flows such as drag, lift (Magnus and Saffman), Brownian, thermophoretic, Van der Waals, electrostatic double-layer forces are considered. Moreover, an overview of the thermophysical properties, physical models, heat transfer models, and evaluation criteria of nanofluids are included in this work. In the second part, which is the main part of this work, a comprehensive review is performed to gather and discuss the new advantages in the nanofluid-based solar collectors that operate at low and medium temperatures. More specifically, the examined solar systems are the flat plate collectors, the evacuated tube collectors, the direct absorption collectors, and the thermal photovoltaic systems, while the investigated applications are space-heating, space-cooling, household hot water production, desalination, industrial activities, and power generation. The aforementioned collectors and applications are the most usual in the real systems, indicating the importance of the present work. Moreover, the emphasis is given in the thermal, exergy, economic, and environmental evaluation of the studied systems, as well as in the discussion of the possible limitations of the use of nanofluids like the lack of long-term stability, the agglomeration of nanoparticles, and the increased pumping work due to the increased pressure drop. Finally, it is found that the nanofluid utilization usually enhances the collector efficiency up to 5%, while higher enhancements can be found in thermal photovoltaics. Moreover, it is concluded that there is a need to emphasize issues such as stability and the use of eco-friendly solar systems. Lastly, the field's future trends are highlighted, and a clear image of the present situation and the next steps in the field are given.     

Small-scale solar pumping: The technology

The present work is concerned with the direct use of solar energy in water pumping. The available technologies of photovoltaic, thermal and other solar pumping systems are examined. It appears that the most suitable system for solar pumping is the one that satisfies several requirements such as: no movable mechanical parts, reasonable thermal efficiency, minimum manufacturing cost and compact in size.     

Heat transfer model for small-scale spark-ignition engines

A heat transfer model for small-scale spark-ignition engines has been proposed by authors in previous study. However, that model was developed based on experimental data of one engine, it may not be so accurate for others. In order to improve the accuracy of predicted heat transfer rate for different engines, a modified heat transfer model using Stanton number based on two engines is proposed. Prediction results of instantaneous heat flux, global heat transfer, heat release rate, and cylinder pressure based on the proposed model are compared with the experimental results of three engines and prediction results of previous model. It is found that the proposed model has prediction results closer to the measured data than the previous models at the most engine operation conditions.     

Evaluation of exergy and energy efficiencies of photothermal solar radiation conversion

In this paper, a new thermodynamic model for photothermal solar radiation conversion into mechanical through a heat engines is proposed. The developed equations allow for the energy and exergy contents of solar radiation to be found, as well as the energy and exergy efficiencies corresponding to concentration type solar-thermal heat engines operating under a range of conditions. The calculation method remains accurate to other published models when their assumed conditions are imposed to the newly developed model. The heat flux absorbed by the receiver (which is assumed to be a grey body and is placed in the focal point of the solar concentrator) depends on the hemispherical absorptivity and emissivity, concentration ratio and receiver temperature. The model is used to conduct a parametric study regarding the energy and exergy efficiencies of the system for assessing its performance. The use of a selective grey body receiver (having a reduced emissivity and a high absorptivity) for enhancing the conversion efficiency is also studied. If the absorptivity approaches one and the emissivity is low enough the photothermal conversion efficiency becomes superior to the known black body receiver limit of 0.853. It is found that in the limit of receiver emissivity tending to zero and absorptivity lending to one, the present model gives the exergy content of solar radiation because the work generated reaches its maximum. In this situation the energy efficiency approaches the exergy efficiency at 1-ITTIN0/TINS where T_S and T₀ are the sun and ambient temperatures, respectively. The influence of the ambient temperature on the exergy and energy efficiencies becomes apparent, with effects of up to 15%, particularly for high absorptivity and low emissivity. The heat transfer conductances at sink and source of the heat engine have a considerable impact on the efficiency of solar energy conversion. The present model is developed in line with actual power system operations for better practical acceptance. In addition, some irreversibility parameters (absorptivity, emissivity, heat transfer conductivity, etc.) are studied and discussed to evaluate the possible photothermal solar radiation conversion systems and assess their energy and exergy efficiencies.     

Adsorption refrigeration—An efficient way to make good use of waste heat and solar energy

This paper presents the achievements gained in solid sorption refrigeration prototypes since the end of the l970s, when interest in sorption systems was renewed. The applications included are ice making and air conditioning. The latter includes not only cooling and heating, but also dehumidification by desiccant systems. The prototypes presented were designed to use waste heat or solar energy as the main heat source. The waste heat could be from diesel engines or from power plants, in combined cooling, heating and power systems (CCHP). The current technology of adsorption solar-powered icemakers allows a daily ice production of between 4 and 7 kg m⁻² of solar collector, with a solar coefficient of performance (COP) between 0.10 and 0.16. The silica gel–water chillers studied can be powered by hot water warmer than 55 °C. The COP is usually around 0.2–0.6, and in some commercially produced machines, it can be up to 0.7. The utilization of such chillers in CCHP systems, hospitals, buildings and grain depots are discussed. Despite their advantages, solid sorption systems still present some drawbacks such as low specific cooling power (SCP) and COP. Thus, some techniques to overcome these problems are also contemplated, together with the perspectives for their broad commercialisation. Among these techniques, a special attention was devoted to innovative adsorbent materials, to advanced cycles and to heat pipes, which are suitable devices not only to improve the heat transfer but also can help to avoid corrosion in the adsorbers. Recent experiments performed by the research group of the authors with machines that employ composite adsorbent material and heat pipes showed that it is possible to achieve a SCP of 770 W kg⁻¹ of salt and COP of 0.39 at evaporation temperatures of −20 °C and generation temperature of 115 °C.     

Optimum design of a photovoltaic powered pumping system

Photovoltaic (PV) powered pumping systems are relatively simple and reliable, hence they are applied worldwide. Two conventional techniques are curently in use; the first is the directly coupled technique where a PV array is directly coupled to a d.c. motor-pump group, and the second is the battery buffered PV pumping system where a battery is connected across the array to feed the d.c. motor driving a pump. Recently, a third system is proposed to make use of the advantages of the previously mentioned conventional systems. It is the switched mode PV powered pumping system.

The switched mode PV powered pumping system couples the pumping system to the PV array directly when the storage battery is fully charged as explained in Ref. [5]. The objective of such a system is the maximum utilization of available solar radiation to minimize the cost per pumped cubic meter from a given water depth. For a given location, four main parameters affect the design of this system; (1) d.c. motor-pump group parameters, (2) PV array size, (3) battery storage size and (4) water storage tank size. The system designer has to determine the previously mentioned four parameters so that the minimum cost per pumped cubic meter is achieved. It is found that some factors are more effective in reducing the cost than others. The PV array size is the predominant factor, while the battery storage and water tank sizes have relatively less effect. The system installation cost is considered in the detailed economic analysis discussed in this work.     

Thermohydraulic performance of solar air heaters with roughened absorber plates

Artificial roughness has been found to enhance the heat transfer from the absorber plate to the air in a solar air heater duct. However, this improvement is invariably accompanied by increased pumping power. In this work, the effect of roughness and operating parameters on the thermal as well as the hydraulic performance of roughened solar air heaters is discussed and the thermohydraulic performance of roughened solar air heaters is compared with that of conventional smooth solar air heaters. The optimum design and operating conditions have been determined. On the basis of thermohydraulic considerations it has been found that the systems operating in a specified range of Reynolds number show better thermohydraulic performance depending upon the insolation. A relationship between the system and operating parameters that combine to yield optimum performance has been developed.     

PCM storage for solar DHW: An unfulfilled promise?

Phase change materials (PCM) have been repeatedly proposed for use in solar domestic hot water (DHW) systems. PCM storage designs have been proposed, but no detailed evaluation has been made of the actual contribution of the PCM to the total heat storage under typical end-use conditions. In this work annual simulations were done to compare the performance of a storage tank with PCM to a standard tank without PCM. A model was constructed to describe the heat storage tank with and without PCM, the collector, pump, controller and auxiliary heater. Realistic environmental conditions and typical end-user requirements were imposed. Annual simulations were carried out for different sites, load profiles, different PCM volume fractions, and different kinds of PCM. The results of all simulation scenarios indicate that, contrary to expectations, the use of PCM in the storage tank does not yield a significant benefit in energy provided to the end-user. The main reason for this undesirable effect is found to be increased heat losses during nighttime due to reheating of the water by the PCM.     

Energy conservation options for residential water heaters

Current designs of gas-fired and electric water heaters present a substantial opportunity for energy conservation through reductions in jacket losses, pilot losses and flue-gas losses. A systematic analysis and comparison of alternative energy-conserving designs has been carried out. Promising options for gas-fired water heaters include increased insulation, thermostat setback, forced-draft burners with intermittent ignition and flue closure, and instantaneous (low storage) designs. High efficiency electric water heaters incorporate increased insulation, thermostat setback, solar preheat, and heat pump operation. The evolution and market penetration of these alternative designs, some of which are commercially available at this writing, can reduce the energy usage for residential water heating in the U.S. by approx. 0.3 quads in the near-term, and up to 1.0 quads in the long-term as advanced designs achieve widespread saturation.     

Simulation and experimental validation of heat transfer in a novel hybrid solar panel

A hybrid solar panel has been invented to integrate photovoltaic (PV) cells onto a substrate through a functionally graded material (FGM) with water tubes cast inside, through which water serves as both heat sink and solar heat collector. Therefore, the PV cells can work at a relatively low temperature while the heat conduction to the substrate can be minimized. Solar panel prototypes have been fabricated and tested at different water flow rates and solar irradiation intensities. The temperature distribution in the solar panel is measured and simulated to evaluate the performance of the solar panel. The finite element simulation results are very consistent with the experimental data. The understanding of heat transfer in the hybrid solar panel prototypes will provide a foundation for future solar panel design and optimization. The finite element model is general and can be extended for different material design and other size of panels.