A parametric design study of solar ponds

The salt stratified solar pond is found to be a reliable solar collector and storage system. This paper discusses the effect of varying certain design parameters on pond steady-state temperatures. These significant parameters are sizing parameters—pond surface area and depth of the pond; operating parameters—storage volume and the heat extraction fraction; and geo-climatic parameters3s?olar radiation, water table depth and upper convective zone thickness. Studies indicate that there is an optimum depth and storage volume of the pond for each application in terms of temperature and heat load desired.     

Application of the genetic algorithm for optimisation of large solar hot water systems

An implementation of the genetic algorithm in a design support tool for (large) solar hot water systems is described. The tool calculates the yield and the costs of solar hot water systems based on technical and financial data of the system components. The genetic algorithm allows for optimisation of separate variables such as the collector type, the number of collectors, the heat storage mass and the collector heat exchanger area. Optimisation can be focussed on, for example, payback time and CO₂ emission reduction. Constraints such as maximum initial costs and installation space are taken into account. The applicability of the genetic algorithm was tested for optimisation of large solar hot water systems. Among others, the sensitivity of the optimum system design to the tap water draw-off and the draw-off pattern has been determined using the optimisation algorithm. As the genetic algorithm is a discrete optimisation tool and is implemented in the design tool through the use of databases, the number of variables in principle is free of choice.     

Component exergy analysis of solar powered transcritical CO<Subscript>2</Subscript> rankine cycle system

In this paper,exergy analysis method is developed to assess a Rankine cycle system,by using supercritical CO2 as working fluid and powered by solar energy.The proposed system consists of evacuated solar collectors,throttling valve,high-temperature heat exchanger,low-temperature heat exchanger,and feed pump.The system is designed for utilize evacuated solar collectors to convert solar energy into mechanical energy and hence electricity.In order to investigate and estimate exergy performance of this system,the energy,entropy,exergy balances are developed for the components.The exergy destructions and exergy efficiency values of the system components are also determined.The results indicate that solar collector and high temperature heat exchanger which have low exergy efficiencies contribute the largest share to system irreversibility and should be the optimization design focus to improve system exergy effectiveness.Further,exergy analysis is a useful tool in this regard as it permits the performance of each process to be assessed and losses to be quantified.Exergy analysis results can be used in design,optimization,and improvement efforts.     

Design and optimization of solar industrial hot water systems with storage

This paper presents a new method for the design and optimization of solar industrial process hot water systems with storage. The single-pass open-loop design thermally “decouples” collectors from storage, hence insuring that collectors always heat the coldest fluid possible and that stored heat can be completely depleted by the nighttime load. So the single-pass open-loop design, in spite of the relatively low flow rates entailed, operates at higher system efficiency than conventional system designs. One solved example for an an industrial hot water application shows that the single-pass open-loop design delivers about 30 per cent more useful energy with roughly 30 per cent less storage than the conventional design. Moreover, storage tanks do not have to stand high pressures and can thus be significantly cheaper than in conventional systems. The effects of collector operating time, heat exchangers, and secondary system losses are also treated. The new method is extended to cover systems that require weekend storage. The introduction of weekend storage may be cost effective because it enables the designer to reduce collector area without reducing the yearly useful energy delivered by the system.     

Design and analysis of a novel low-temperature solar thermal electric system with two-stage collectors and heat storage units

The proposed low-temperature solar thermal electric generation is based on the compound parabolic concentrator (CPC) of small concentration ratio and Organic Rankine Cycle (ORC). The technologies of CPC and ORC are analyzed, and feasibility of the system is demonstrated. In particular, two-stage collectors and heat storage units are adopted to improve heat collection efficiency. Organic fluid is preheated by flat plate collectors (FPCs) prior to entering a higher temperature heat exchanger connected with the CPC. The two-stage heat storage units are composed of two types of phase change material (PCM) with diverse melting temperatures. The novel configuration is carefully designed to react to different operating conditions. The fundamentals are illustrated for both simultaneous and separate processes of heat collection and power conversion. Mathematic models are built for heat transfer and thermodynamics of the innovative system. Coupling relationship among the proportion of FPC to CPC, the melting temperature of the first-stage PCM and the overall collector efficiency is established. The benefits of the preheating concept and cascaded heat storages are investigated in detail in comparison with the single-stage system. The results indicate that the increase in collector efficiency of the two-stage system is appreciable.     

Experimental studies of a new solar water heater system using a solar water pump

The research goal was to develop a new solar water heater system (SWHS) that used a solar water pump instead of an electric pump. The pump was powered by the steam produced from a flat plate collector. Therefore, heat could be transferred downward from the collector to a hot water storage tank. The designed system consisted of four panels of flat plate solar collectors, an overhead tank installed at an upper level and a large water storage tank with a heat exchanger at a lower level. Discharge heads of 1, 1.5 and 2 m were tested. The pump could operate at the collector temperature of about 70–90 °C and vapor gage pressure of 7–14 kPa. It was found that water circulation within the SWHS ranged between 12 and 59 l/d depending on the incident solar intensity and system discharge head. The average daily pump efficiency was about 0.0014–0.0019%. Moreover, the SWHS could have a daily thermal efficiency of about 7–13%, whereas a conventional system had 30–60% efficiency. The present system was economically comparable to a conventional one.     

The daily performance of a solar pond integrated with solar collectors

The present study deals with heat storage performance investigation of integrated solar pond and collector system. In the experimental work, a cylindrical solar pond system (CSPS) with a radius of 0.80 m and a depth of 2.0 m and four flat plate collectors dimensions of 1.90 m × 0.90 m was built in Cukurova University in Adana, Turkey. The CSPS was filled with salty water of various densities to form three salty water zones (Upper Convective Zone, Non-Convective Zone and Heat Storage Zone). Heat energy collected by collectors was transferred to the solar pond storage zone by using a heat exchanger system which is connected to the solar collectors. Several temperature sensors connected to a data acquisition system were placed vertically inside the CSPS and at the inlet and outlet of the heat exchanger. Experimental studies were performed using 1, 2, 3 and 4 collectors integrated with the CSPS under approximately the same condition. The integrated solar pond efficiencies were calculated experimentally and theoretically according to the number of collectors. As a result, the experimental efficiencies are found to be 21.30%, 23.60%, 24.28% and 26.52%; the theoretical efficiencies to be 23.42%, 25.48%, 26.55% and 27.70% for 1, 2, 3 and 4 collectors, respectively. Theoretical efficiencies were compared with the experimental results and hence a good agreement is found between experimental and theoretical efficiency profiles.     

Review of solar dryers for agricultural and marine products

Drying for agricultural and marine products are one of the most attractive and cost-effective application of solar energy. Numerous types of solar dryers have been designed and developed in various parts of the world, yielding varying degrees of technical performance. Basically, there are four types of solar dryers; (1) direct solar dryers, (2) indirect solar dryers, (3) mixed-mode dryers and (4) hybrid solar dryers. This paper is a review of these types of solar dryers with aspect to the product being dried, technical and economical aspects. The technical directions in the development of solar-assisted drying systems for agricultural produce are compact collector design, high efficiency, integrated storage, and long-life drying system. Air-based solar collectors are not the only available systems. Water-based collectors can also be used whereby water to air heat exchanger can be used. The hot air for drying of agricultural produce can be forced to flow in the water to air heat exchanger. The hot water tank acts as heat storage of the solar drying system.     

Performance investigation of salt gradient cylindrical solar pond integrated and nonintegrated with evacuated tube solar collectors

In this study, the energetic and exergetic efficiencies of a salt gradient cylindrical solar pond (SGCSP) that integrated and nonintegrated evacuated tube solar collectors (ETSCs) are investigated to improve daily heat preservation performance of the heat storage zone (HSZ). The integrated system is consisted of an SGCSP and four ETSCs. The SGCSP has a surface area of 2 m², a depth of 1.65 m, salty water layers at different densities, and HSZ in which the cylindrical serpentine type heat exchanger (CSHE) is located. Thus, the daily effects of the heat storage performance of both the ETSCs and the SGCSP in the winter season was determined experimentally. The analysis of the data regarding the efficiencies of the system is investigated separately by means of experimental studies where the SGCSP is integrated and nonintegrated with the ETSCs. The number of ETSCs integrated with SGCSP is increased to 1, 2, 3, and 4, respectively, and each of the five different experimental systems is performed separately. The temperature distributions of the integrated system are measured by a data acquisition system on 11 different points per hour. The efficiencies are calculated using the data obtained from these studies. Consequently, the energetic and exergetic efficiencies of the SGCSP are obtained without collectors as 10.4% and 4.3% and with one collector as 12.83% and 6.15%, with two collectors 14.88% and 8.25%, with three collectors 16% and 9.35%, and finally with four collectors 16.94% and 10.3%, respectively. Furthermore, the theoretical efficiencies are found to be consistent with the experimental results obtained by increasing the collector numbers.     

Mathematical modeling and thermal performance analysis of unglazed transpired solar collectors

Unglazed transpired collectors or UTC (also known as perforated collectors) are a relatively new development in solar collector technology, introduced in the early nineties for ventilation air heating. These collectors are used in several large buildings in Canada, USA and Europe, effecting considerable savings in energy and heating costs. Transpired collectors are a potential replacement for glazed flat plate collectors. This paper presents the details of a mathematical model for UTC using heat transfer expressions for the collector components, and empirical relations for estimating the various heat transfer coefficients. It predicts the thermal performance of unglazed transpired solar collectors over a wide range of design and operating conditions. Results of the model were analysed to predict the effects of key parameters on the performance of a UTC for a delivery air temperature of 45–55 °C for drying applications. The parametric studies were carried out by varying the porosity, airflow rate, solar radiation, and solar absorptivity/thermal emissivity, and finding their influence on collector efficiency, heat exchange effectiveness, air temperature rise and useful heat delivered. Results indicate promising thermal performance of UTC in this temperature band, offering itself as an attractive alternate to glazed solar collectors for drying of food products.The results of the model have been used to develop nomograms, which can be a valuable tool for a collector designer in optimising the design and thermal performance of UTC. It also enables the prediction of the absolute thermal performance of a UTC under a given set of conditions.     

Optimization of parabolic trough solar collector system

Process heat produced by solar collectors can contribute significantly in the conservation of conventional energy resources, reducing CO₂ emission, and delaying global warming. One of the major problems associated with solar process heat application is fluctuation in system temperature during unsteady state radiation conditions which may cause significant thermal and operation problems. In this paper a transient simulation model is developed for analysing the performance of industrial water heating systems using parabolic trough solar collectors. The results showed that to prevent dramatic change and instability in process heat during transient radiation periods thermal storage tank size should not be lower than 14.5 l m^?2 of collector area. Small periods of radiation instability lower than 30 min do not have significant effect on system operation. During these periods when water flow rate of collector loop is doubled the time required to restore system normal operating condition increased by a ratio of 1.5. Copyright © 2005 John Wiley & Sons, Ltd.     

Solar heating using common building elements as passive systems

The paper reviews the first steps of a study on use of windows as passive solar air collectors, offsetting naturally the excess of heat in the thermal mass of the building itself, and of vertical solar collectors, with air as working fluid, and with storage systems designed as intergral parts of the building, incorporated in the concrete elements. Some examples of architectural solutions combining these principles in a modular design are suggested. Incorporated storage examples using ceilings and partitions with appropriate air transfer through them are proposed. The relevant thermal analysis on the use of windows as passive solar collectors is based on the Total Thermal Time Constant (TTTC) Method, developed by two of the authors. An analysis is also presented for a vertical flat-plate solar collector using air as working fluid and capable of integration in a blank (windowless) part of an external wall. Design and dimensioning of the fin surface are proposed for heat transfer from collector surface to fluid. The final section deals with experimental model based on the above principles and combining a vertical collector and heat storage for use in daytime and at night, respectively. The model was so dimensioned as to represent a 1:1 unit in modular building design. An overall thermal efficiency of about 18 per cent was obtained.     

DESIGN AND PERFORMANCE OF AN ORIGINAL FRENCH SOLAR HEATING SYSTEM: DIRECT SOLAR FLOOR HEATING

Direct Solar Floor Heating ( DSFH) is an original solar heating system made of a set of water circulation pipes embedded in a concrete floor which is directly connected to the solar collectors without heat exchanger or any other storage. The concrete slab is thick enough to be used as storage. Performance of many installations in France have been measured and give satisfaction. Furthermore, a simplified sizing method has been proposed. We set forth simulations with EMGP2, in order to improve both of these results and to optimize the economic and technical performances of the system. An optimum size which the method did not anticipate is indeed found if we correctly took into account the function of the floor in the system. Two points have to be supervised: comfort and regulation for which some preliminary informations are given. A methodology is proposed in order to analyse bare results from measured data.     

Yearly simulation of a solar-aided R22-DEGDME absorption heat pump system

The performance of a solar-aided R22-DEGDME absorption heat pump system designed for 100 kW cooling capacity is investigated by a computer simulation using hourly data for Ankara. In summer the generator, and in winter the evaporator, receives solar energy while the remaining demands are met by auxiliary heaters. When needed, these boost the temperature of the water from the storage tank to the minimum allowable levels which are determined as 20°C in winter and over 80°C in summer. The system performance, judged by the fraction of the load supplied from solar energy, is affected mostly from the climate, source temperature limit, collector type and area but little from storage tank size, for the sizes and configuration under investigation. With 400 m² of high efficiency collectors, the solar energy supplied 38% of the demand in winter and 91% of the demand in summer.     

The potential applications and advantages of powering solar air-conditioning systems using concentrator augmented solar collectors

Non-concentrated evacuated tube heat pipe solar collectors have been reported to show higher fluid temperatures with improved thermal performance in the low to medium temperature range (?60 °C) due to low heat losses but suffer higher heat losses at the medium to higher temperature range (?80 °C) which reduces their efficiency compared to concentrated evacuated tube heat pipe solar collectors. To operate as stand-alone systems capable of attaining temperatures in the range of 70-120 °C, an innovative concentrator augmented solar collector can be an attractive option. The performance of a combined low-concentrator augmented solar collector in an array of evacuated tube heat pipe solar collectors defined as concentrator augmented evacuated tube heat pipe array (CAETHPA) and an array of evacuated tube heat pipe collectors (ETHPC) were tested and compared and results presented in this paper. The analysis of the experimental data allows concluding that the use of a CAETHPA is a more efficient alternative for integrating renewable energy into buildings with higher fluid temperature response, energy collection and lower heat loss coefficient compared to the use of evacuated tube heat pipe collector array (ETHPA).     

A numerical model for seasonal storage of solar heat in the ground by vertical pipes

We present a three-dimensional numerical model for seasonal heat storage in the ground using vertical heat exchanger pipes. The model also accounts for convective heat flows in the ground. The storage is employed in a district solar heating system with a heat pump. The effects of storage volume, storage medium, collector area, and collector type on system performances are studied for the Helsinki (60°N) climate. Economic optimization of the storage and collector installation is also briefly discussed. For a 500-house community, a collector area of 35 m² per house and a rock storage volume of 550 m³ per house would provide a solar fraction of 70%.     

Seasonal performance of an active space heating system

This study discusses the results of an experimental analysis of an active space heating system. The solar installation mainly includes two flat-plate solar collectors, a liquid-to-liquid heat exchanger, a storage tank, a liquid-to-air heat exchanger, two circulation pumps, an expansion tank, and several valves. By the aid of an instrumentation system, mainly consisting of two pyranometers, 20 gauge copper-Constantan thermocouples, a digital multimeter, and a recorder, total solar radiation incident on the collectors, temperature fluctuations of the working fluids at the intersections of the major system components, and the room space and ambient temperatures were recorded throughout the heating period. The system was designed to operate in direct and indirect modes. Several conclusions are drawn to improve the system overall efficiency, and some inconveniences observed in the operation are discussed.     

Hybrid photovoltaic and thermal solar-collector designed for natural circulation of water

The electricity conversion-efficiency of a solar cell for commercial application is about 6–15%. More than 85% of the incoming solar energy is either reflected or absorbed as heat energy. Consequently, the working temperature of the solar cells increases considerably after prolonged operations and the cell’s efficiency drops significantly. The hybrid photovoltaic and thermal (PVT) collector technology using water as the coolant has been seen as a solution for improving the energy performance. Through good thermal-contact between the thermal absorber and the PV module, both the electrical efficiency and the thermal efficiency can be raised. Fin performance of the heat exchanger is one crucial factor in achieving a high overall energy yield. In this paper, the design developments of the PVT collectors are briefly reviewed. Our observation is that very few studies have been done on the PVT system adopting a flat-box absorber design. Accordingly, an aluminum-alloy flat-box type hybrid solar collector functioned as a thermosyphon system was constructed. While the system efficiencies did vary with the operating conditions, the test results indicated that the daily thermal efficiency could reach around 40% when the initial water-temperature in the system is the same as the daily mean ambient temperature.     

Determination of design space and optimization of solar water heating systems

In this paper, a methodology is proposed to determine the design space for synthesis, analysis, and optimization of solar water heating systems. The proposed methodology incorporates different design constraints to identify all possible designs or a design space on a collector area vs. storage volume diagram. The design space is represented by tracing constant solar fraction lines on a collector area vs. storage volume diagram. It has been observed that there exists a minimum as well as a maximum storage volume for a given solar fraction and collector area. Similarly existence of a minimum and a maximum collector area is also observed for a fixed solar fraction and storage volume. For multi-objective optimization, a Pareto optimal region is also identified. Based on the identified design space, the solar water heating system is optimized by minimizing annual life cycle cost. Due to uncertainty in solar insolation, system parameters and cost data, global optimization may not be utilized to represent a meaningful design. To overcome this, a region of possible design configurations is also identified in this paper.