Heat loss through the piping of a large solar collector field

The effect of the heat loss from the piping system of a solar collector field of a large size is measured and the effect of this loss on the effective collector efficiency is evaluated. The data are used to develop a model that can be used to evaluate the effect of piping loss on collector efficiency for solar field of different areas. The collector field used in this study consists of 1064 evacuated tube collectors; each one has an absorber area of 1.75 m². They are connected in a series–parallel arrangement and designed in a U-shape. Measurements of the heat loss in the piping system indicate that a drop in the collector efficiency occurs due to this heat loss. The effect of the heat loss on the collector efficiency was found to depend on the collector x-parameter with higher x corresponding to larger efficiency drop. The standard Hottel–Whillier–Bliss (HWB) equation for the collector efficiency was modified so that the form of the equation is maintained while the parameters change due to the inclusion of piping loss in the equation. The modified parameters were estimated for the collector field based on measurement and the data were used in the model to generalize the results for field with different piping to collector area ratios.     

Efficiency bound of a solar-driven stirling heat engine system

A solar-driven Stirling engine is modelled as a combined system which consists of a solar collector and a Stirling engine. The performance of the system is investigated, based on the linearized heat loss model of the solar collector and the irreverisible cycle model of the Stirling engine affected by finite-rate heat transfer and regenerative losses. The maximum efficiency of the system and the optimal operating temperature of the solar collector are determined. Moreover, it is pointed out that the investigation method in the present paper is valid for other heat loss models of the solar collector as well, and the results obtained are also valid for a solar-driven Ericsson engine system using an ideal gas as its engine work substance. © 1998 John Wiley & Sons, Ltd.     

Performance analysis of conventional and sloped solar chimney power plants in China

The solar chimney power plant (SCPP) has been accepted as one of the most promising approaches for future large-scale solar energy applications. This paper reports on a heat transfer model that is used to compare the performance of a conventional solar chimney power plant (CSCPP) and two sloped solar chimney power plants (SSCPPs) with the collector oriented at 30° and 60°, respectively. The power generation from SCPPs at different latitudes in China is also analyzed. Results indicate that the larger solar collector angle leads to improved performance in winter but results in lower performance in summer. It is found that the optimal collector angle to achieve the maximum power in Lanzhou, China, is around 60°. Main factors that influence the performance of SCPPs also include the system height and the air thermophysical characteristics. The ground energy loss, reflected solar radiation, and kinetic loss at the chimney outlet are the main energy losses in SCPPs. The studies also show SSCPPs are more suitable for high latitude regions in Northwest China, but CSCPPs are suggested to be built in southeastern and eastern parts of China with the combination to the local agriculture.     

A detailed nonuniform thermal model of a parabolic trough solar receiver with two halves and two inactive ends

In this paper a detailed one dimensional nonuniform thermal model of a parabolic trough solar collector/receiver is presented. The entire receiver is divided into two linear halves and two inactive ends for the nonuniform solar radiation, heat transfers and fluid dynamics. Different solar radiation and heat transfer modes can be taken into consideration for these four different regions respectively. This enables the study of different design parameters, material properties, operating conditions, fluid flow and heat transfer performance for the corresponding regions or the whole receiver. Then the nonuniform model and the corresponding uniform thermal model are validated with known performance of an existing parabolic trough solar collector/receiver. For applications, the uniform thermal model can be used to quickly compute the integral heat transfer performance of the whole PTC system while the nonuniform thermal model can be used to analyze the local nonuniform solar radiation and heat transfer performance characteristics and nonuniform heat transfer enhancements or optimizations. Later, it could also be effectively used with an intelligent optimization, such as the genetic algorithm or the particle swarm optimization, to quickly evaluate and optimize the characteristics and performance of PTCs under series of nonuniform conditions in detail.     

Indirect thermosiphon flat-plate solar collector performance based on twisted tube design heat exchanger filled with nanofluid

Stationary solar collector such as flat-plate collector is a thermal device, which traps solar energy and converts it into heat that can be used in industrial and domestic applications such as water heating. Flat-plate collector thermal performance enhancement is investigated in this research paper. Two cross-sectional geometries of the tube in the heat exchanger were investigated; a normal circular tube and a twisted tube were used in the experiment. The aim of the twisted tube exchanger is to enhance the performance of heat transfer of the tubes and to reduce the shell pressure drop; flat-plate solar collector is the used application to study the heat exchanger performance. Both twisted tubes heat exchanger and normal circular tubes heat exchanger were examined in the same location and conditions with the same solar collector, both were used in the heat exchanger to study their effect, with two different working fluids, which are distilled water and multiwalled carbon nanotube (MWCNT)/water nanofluid. The system shows an increase in the performance when twisted tubes were used in the system compared with the circular tubes in both distilled water and MWCNT/water nanofluid by 12.8% and 12.5%, respectively, with an improvement by 34% for twisted tubes with MWCNT compared with normal circular tubes with distilled water.     

Effects of phase-change energy storage on the performance of air-based and liquid-based solar heating systems

Models describing the transient behavior of phase-change energy storage (PCES) units are presented. Simulation techniques are used in conjunction with these models to determine the performance of solar heating systems utilizing PCES. Both air-based and liquid-based systems are investigated. The effects of storage capacity, storage unit heat transfer characteristics, collector area and location on the system performance are investigated for systems utilizing sodium sulfate decahydrate and paraffin wax as storage media. Optimum ranges of storage sizes are recommended on the basis of systems' thermal performance. Comparison is made between systems utilizing PCES and those using sensible heat storage, viz. rock beds in air-based systems and water tanks in liquid-based systems. The variation of the solar supplied fraction of load with storage size and collector area is given for systems utilizing both types of storage. The effects of location and collector energy loss coefficient on the relative performance of PCES and sensible heat storage are also investigated.     

Performance characteristics of an irreversible solar-driven Braysson heat engine at maximum efficiency

A novel model of the solar-driven thermodynamic cycle system which consists of a solar collector and a Braysson heat engine is established. The performance characteristics of the system are optimized on the basis of the linear heat-loss model of a solar collector and the irreversible cycle model of a Braysson heat engine. The maximum efficiency of the system and the optimally operating temperature of the solar collector are determined and other relevant performance characteristics of the system are discussed. The results obtained here may provide some theoretical guidance for the optimal design and operation of solar-driven Braysson and Carnot heat engines.     

Thermodynamic analysis of the performance of a solar absorption heat transformer at maximum coefficient of performance

It is proven that a solar absorption heat transformer affected by the irreversibility of finite-rate heat transfer may be modelled as an equivalent combined system consisting of a solar collector and an endoreversible absorption heat transformer, the latter being further treated as a combined cycle having an endoreversible heat pump driven by an endoreversible heat engine. The maximum coefficient of performance of the system is determined, based on the linear heat loss model for solar collectors and the general optimum relation for endoreversible absorption heat transformers. The optimality problems concerning the primary performance parameters of the system are discussed. The results obtained here may serve as a good guide for the evaluation of existing real solar absorption heat transformers or provide some theoretical bases for the optimal design of future solar absorption heat transformers. © 1997 by John Wiley & Sons, Ltd.     

Performance of combined solar-heat pump systems

A comparative study of the performance of combined solar heat pump systems for residential space and domestic hot water heating has been undertaken. Simulations have been made with TRNSYS[1] of three basic combined configurations, as well as conventional solar and conventional heat pump systems, in two different climates, Madison, Wisconsin, and Albuquerque, New Mexico.The three combined systems are the series system in which the solar storage is used as the source for the heat pump, the parallel system in which ambient air is used as the source for the heat pump, and the dual source system in which the storage or ambient is used as the source depending on which source yields the lowest work input. The influence of collector area, number of glazings, main storage volume to collector area ratio, and heat pump coefficient of performance were determined.The results indicate that the parallel combined system is probably the most practical solar-heat pump configuration. The thermal performance at a given collector area is consistently superior to both the series or the dual source systems over the heating season. Costs and the extent to which summer cooling is a requirement determine the relative merit of the conventional heat pump, conventional heat pump, conventional solar, and parallel systems.     

Theoretical and experimental study of efficiency factor,heat transfer and thermal heat loss coefficients in solar air collectors with selective and nonselective absorbers

The thermal heat performance of a solar air collector depends strongly on the thermal heat loss and the efficiency factor. In order to increase these performances, it is necessary to use a solar air collector which is well insulated and where the fluid flow is fully developed turbulent flow. It needs a high heat transfer between the absorber plate and the fluid to decrease the absorber‐plate temperature and hence the heat loss by radiation from the absorber to the ambient. This increases the efficiency factor. In the present paper, the heat loss and efficiency factor are treated for solar air collectors with selective and nonselective absorber plate. It is shown that the selectivity of the absorber plate cannot play an important role in a well‐insulated solar collector with a fanned system which permits a fully developed turbulent flow and, in consequence, high heat transfer. Copyright © 1999 John Wiley & Sons, Ltd.     

太阳能热泵供暖系统的热经济性分析

基于有限时间热力学理论和集热器线性热损失模型,建立了太阳能热泵供暖系统的热力学模型,并对该系统进行了热经济分析.研究在给定供热率和初投资的约束条件下,以系统的供热系数COHF,作为热经济性目标函数,得出了在目标函数取最大时系统最佳的运行性能系数和设计参数.同时还研究了初投资对系统运行以及设计参数的影响,得出了对应给定供热率系统的最佳初投资及其相应的设计参数.     

线性菲涅尔反射式太阳能集热系统的设计与试验研究

本文研究了线性菲涅尔反射式太阳能集热系统,基于几何光学原理计算模拟了线性菲涅尔反射镜镜场和复合抛物面的接收系统,在减少末端损失的基础上设计了系统的机械结构,制作了线性菲涅尔反射式太阳能集热系统的装置,并进行了集热性能的实验测试。测试结果表明,该系统在9倍聚焦倍率下,导热油的最高温度可达176.2℃,系统的平均瞬时热效率约为53%,很好地实现了其集热性能。     

Yearly average performance of the principal solar collector types

The results of hour-by-hour simulations for 26 meteorological stations are used to derive universal correlations for the yearly total energy that can be delivered by the principal solar collector types: flat plate, evacuated tubes, CPC, collectors that track about one axis, collectors that track about two axes, and central receiver.The correlations are polynomials of first and second order in yearly average insolation, latitute, and threshold (= ratio of heat loss and optical efficiency). With these correlations the yearly collectible energy can be found by reading a single graph and multiplying the coordinates by the collector parameters. This simple method reproduces the results of hour-by-hour computer calculations with an accuracy (rms error) of 2 per cent for flat plates and 2–4 per cent for concentrators.This method can be applied to any system where the collectors operate year-round in such a way that no collected energy is discarded. This includes photovoltaic systems; solar-augmented industrial process heat systems; and solar thermal power systems. In addition, the method is recommended for rating collectors of different types or different manufacturers on the basis of yearly average performance. The method is also useful for evaluating the effects of collector degradation, the benefits of collector cleaning, and the gains from collector improvements (due to enhanced optical efficiency or decreased heat loss per absorber surface). For most of these applications, the method is accurate enough to replace a system simulation.     

Fabrication and testing of a non-glass vacuum-tube collector for solar energy utilization

An evacuated tubular solar collector was fabricated from acrylics for improved resistance to shattering. A plasmatron was employed to apply a thin gas-barrier coating to the surfaces of the plastic tube to prevent/alleviate gas infiltration. Experiments were conducted to investigate the effect of vacuum level on the performance of the non-glass vacuum-tube solar collector. Inserted in the evacuated tube was a finned heat pipe for solar energy collection and heat transfer to a water tank. Time variations of temperatures on the heat pipe surface and in the water tank were recorded and analyzed for different degrees of vacuum in the collector. The steady-state temperature of the non-glass collector was compared to that of a commercial glass vacuum-tube collector to assess the feasibility of the use of evacuated plastic tubes for solar energy collection. A simple analytical model was also developed to assist in understanding and analyzing the transient behavior and heat losses of the vacuum-tube solar collector.     

Cost of house heating with solar energy

There has long been a need for a practical method of predicting the true cost of heating a house with solar energy and designing the heating system (solar and auxiliary) to achieve the minimum total annual heating cost possible under the particular climatic, geographic, and residential characteristics involved. Rough approximations based on various types of averaged values of weather and seasonal variables have previously been developed, but the reliability of such methods and results is open to question. The authors have therefore made a rigorous analysis of projected solar heating costs in eight U.S. cities and have optimized the heating system design in each location.The analysis involved the use of a high speed computer and approximately 400,000 hourly observations in eight cities of radiation, temperature, wind, solar altitude, cloud cover, and humidity. Equations for performance of flat plate solar collectors and sensible heat storage systems were developed and programmed with the above weather variables and with eight design parameters comprising house size, collector size, storage size, collector tilt, number of transparent surfaces in collector, hot water demand, insulation on storage unit, and thermal capacity of collector. Capital and operating costs were quantitatively related to heating system design parameters, and the values of all design variables which yielded lowest annual heating cost in each city were then selected.The findings are presented in the form of two tables and ten graphs, showing heating costs as functions of various design and location factors. The relative importance of each factor is discussed, and the overall costs of solar heating are compared with the costs of conventional heat supply in each location. The method for designing the least-cost combination of solar and conventional heat supplies is also shown, and an example of the use of the method is presented.     

直膨式太阳能热泵运行特性的实验研究

对直接膨胀式太阳能热泵热水系统进行了实验研究,实验期间,太阳能辐照度变化范围为143.12~664.6 W/m2,分别采用三种不同结构的集热器和蒸发器,得出系统COP为2.49~3.47,表明该系统在各种天气情况下均能够可靠地生产45℃的生活热水,热性能稳定,可以全天候地提供生活热水且具有节能效果;同时选取双集热器的两组数据,分析了太阳辐照度对热泵系统运行的影响。     

Energy analysis and modeling of a solar assisted house heating system with a heat pump and an underground energy storage tank

An analytical model is presented and analyzed to predict the long term performance of a solar assisted house heating system with a heat pump and an underground spherical thermal energy storage tank. The system under investigation consists of a house, a heat pump, solar collectors and a storage tank. The present analytical model is based on a proper coupling of the individual energy models for the house, the heat pump, useful solar energy gain, and the transient heat transfer problem for the thermal energy storage tank. The transient heat transfer problem outside the energy storage tank is solved using a similarity transformation and Duhamel’s superposition principle. A computer code based on the present model is used to compute the performance parameters for the system under investigation. Results from the present study indicate that an operational time span of 5–7 years will be necessary before the system under investigation can attain an annually periodic operating condition. Results also indicate a decrease in the annually minimum value of the storage tank temperature with a decrease in the energy storage tank size and/or solar collector area.     

The energy saving obtainable with solar heating and heat pumps in a northern climate

The energy saving obtainable with active solar heating and heat pumps has been studied for several years in the Northern climate of Finland. The studies deal mainly with small houses. A computer program is developed which calculates hour by hour the annual energy balance of different heating systems. The performance, of the heating systems are also measured in inhabited houses. The calculations show that the useful solar energy obtainable from the collector is 50–400 kWh/m² annually depending on the system and the collector size. A heat pump in the system is very advantageous, because it keeps the heat losses low and the collector efficiency high. It approximately doubles the energy obtainable. The measurement results have not been as good as expected. The solar energy obtained from the collector has been 120–160 kWh/m² annually. The main reasons for the low solar energy are design and equipment faults and the shading effects. The best energy saving device is the earth heat pump. It is also therefore very advantageous that the peak power demand decreases markedly. When the area of the earth pipes is large enough, energy may be extracted from earth through the whole year. The annual coefficient of performance is 2–3. Also a heat pump which extracts heat from exhaust air in dwelling houses has been very promising.     

An evaluation on thermal performance of CPC solar collector

The main objective of this work is the investigation and improvement of thermal performance of evacuated CPC (Compound Parabolic Concentrator) solar collector with a cylindrical absorber. Modified types of this solar collector are always combined with the evacuated glass envelop or tracking system. The conventional stationary CPC solar collector has been compared with the single axis tracking CPC solar collector in outlet temperature, net heat flux onto the absorber and thermal efficiency. Numerical model has been analyzed based on the irradiation determined actually and the results have been calculated to predict the thermal efficiency. Based on the comparison of the measured and calculated results, it is concluded that the numerical model can accurately estimate the performance of solar collectors. The result shows the thermal efficiency of the tracking CPC solar collector is more stable and about 14.9% higher than that of the stationary CPC solar collector.     

Simulation model of a CPC collector with temperature-dependent heat loss coefficient

We describe a mathematical model for the optical and thermal performance of non-evacuated CPC solar collectors with a cylindrical absorber, when the heat loss coefficient is temperature-dependent. Detailed energy balance at the absorber, reflector and cover of the CPC cavity yields heat losses as a function of absorber temperature and solar radiation level. Using a polynomial approximation of those heat losses, we calculate the thermal efficiency of the CPC collector. Numerical results show that the performance of the solar collector (η vs. ΔT_f(0)/I_coll) is given by a set of curves, one for each radiation level. Based on the solution obtained to express the collector performance, we propose to plot efficiency against the relation of heat transfer coefficients at absorber input and under stagnation conditions. The set of characteristic curves merge, then, into a single curve that is not dependent on the solar radiation level. More conveniently, linearized single plots are obtained by expressing efficiency against the square of the difference between the inlet fluid temperature and the ambient temperature divided by the solar radiation level. The new way of plotting solar thermal collector efficiency, such that measurements for a broad range of solar radiation levels can be unified into a single curve, enables us to represent the performance of a large class of solar collectors, e.g. flat plate, CPC and parabolic troughs, whose heat loss functions are well represented by second degree polynomials.