Enhanced solar energy collection using reflector-solar thermal collector combinations

The amount of direct light gathered by a combination of reflector plus flat-plate collector has been analyzed. The calculations were done allowing variable reflector and collector orientation angles, variables latitude, and arbitrary sun hour angle away from solar noon. The effects of reflection and transmission losses and of polarization of the incident light were included. Correction was also made for the finite size of the reflector. It was found that the optimum orientation has the collector plane almost perpendicular to the plane of the reflector. This optimum orientation is approximately independent of the sun's azimuthal dependence. The optimum reflector angle is found to be between 0° and 10° above the horizon for winter solar conditions. For typical winter operating conditions the enhancement in light gathering power for direct solar radiation is about a factor of 1·4–1·7. This results in an effective increase of 100% in the useful winter heat output from a practical reflector-collector combination with a reflector angle of 0°, over the useful heat output obtained with an optimally oriented simple flat-plate collector. An approximate calculation was also made of the overall enhancement in useful heat output for diffuse solar radiation; an increase by a factor of about 1·5 is predicted. Comparison with the preliminary analysis of the performance of the Coos Bay, Oregon solar house shows substantial agreement with the predictions of the present analysis.     

Planar concentrators for flat-plate solar collectors

A systematic study has been made of the effectiveness of planar specular reflectors for solar energy collectors. Two daily averaged indices of performance were used. One, the area ratio, indicates the amount by which the reflector extends the effective receiver area. The other is the enhancement factor, which is used to compare the energy received by an augmented collector with that by a reference collector at optimum tilt.

A reflector can be mounted either above or below a flat-plate collector. Both combinations are evaluated fully, by varying separately the angular position and dimensions of the reflector and of the collector. The principal parameters are identified and the main characteristics summarised as a series of performance curves. These curves provide an easy method for determining optimum reflector geometries.

Use of the performance curves may be extended to obtain the configuration of the two reflectors in a trough concentrator. This also allows the single-reflector system to be compared directly with the trough concentrator. Evidence is presented which shows the advantages of an asymmetrical trough configuration over a symmetrical concentrator.     

采用不同集热器的太阳能吸收式制冷系统经济性分析

以能源平均成本和动态投资回收期为经济性指标,对采用平板集热器、真空管集热器、复合抛物面集热器和槽式集热器驱动的太阳能单效溴化锂吸收式制冷系统进行了对比分析,同时以?效率和动态投资回收期为目标对优选的太阳能制冷系统进行了多目标优化。结果表明：采用真空管集热器的太阳能制冷系统的能源平均成本最低及动态投资回收期最短;发生器热水进口温度存在最优值使得系统?效率最高,能源平均成本最低;增加系统装机容量可有效降低系统的能源平均成本并且缩短投资回收期;太阳辐照强度越大,太阳能制冷系统的能源平均成本越低及投资回收期越短。此外,多目标优化结果表明发生器热水进口温度存在最优值可使得综合目标函数取得最小值。     

Stationary reflector-augmented flat-plate collectors

A general procedure for determining the optimum geometry of a reflector-augmented solar collector which produces a desired pattern of flux-augmentation is described. The example used for illustration is a stationary collector whose winter performance is to be improved. Consideration both a flat-plate collector with a bottom reflector and one with a top reflector led to distinct differences in their optimum configuration and performance being identified. Since either systems can be used to augment winter flux, a criterion for selecting the appropriate system is given. This criterion is based on the displacement in collector tilt from latitude inclination.     

Size optimization of conventional solar collectors

An expression for the optimum length of a flat-plate solar collector that maximizes the life-cycle savings of the collector is derived. An expression has been obtained also for the optimal distribution of a finite amount of thermal insulation that minimizes the energy loss from the back side of a flat-plate solar collector.     

Combined collector-reflector systems

The use of flat-plate reflectors can substantially improve the performance of flat-plate collectors. We present the fundamental characteristics of a simulation program to estimate the optimum configuration of reflectors, which should be added to a solar plant, under a variety of design parameters, in order to yield the greatest economic benefit. The performance results for these collector-reflector plants are compared with traditional, single-cover collector plants, as well as with plants made up of collectors with different numbers of cover plates. The flat-plate reflector is incorporated in large-scale solar collector plants; the reflector-enhancement factor and its cost are taken into account. A sensitivity analysis is performed to visualize the behaviour of these novel systems under different cost and operating conditions.     

Optimal control of flow in solar collectors for maximum exergy extraction

The best operation strategies for open loop flat-plate solar collector systems are considered. A direct optimal control method (the TOMP algorithm) is implemented. A detailed collector model and realistic meteorological data from both cold and warm seasons are used in applications. The maximum exergetic efficiency is low (usually less than 3%), in good agreement with experimental measurements reported in literature. The optimum mass-flow rate increases near sunrise and sunset and by increasing the fluid inlet temperature. The optimum mass-flow rate is well correlated with global solar irradiance during the warm season. Also, operation at a properly defined constant mass-flow rate may be close to the optimal operation.     

Exergy optimization of flow rates in flat-plate solar collectors

The optimal flat-plate collector mass flow rate is determined by maximizing the exergy (available energy) delivery of the collector as the objective function. Collector and storage dynamics are neglected. Although the case where the pumping power loss is ignored results in bang-bang control, the case where this loss is included in the exergy equation results, after some assumptions, in an optimal mass flow rate that is a function of collector parameters, inlet and ambient temperatures and solar heat gain. Daily performance of a typical flat-plate solar collector with optimum mass flow rate is compared with the performance of the same collector using the mass flow rate obtained by maximizing the difference between the collected thermal energy and the required pumping power.     

Estimation of incident radiation on a novel spherical solar collector

Most solar collectors commonly used are of the flat-plate type. In the present work, a novel type of solar collector, namely, spherical collector, is proposed. It consists of a stationary spherical body with a cover and an absorbing surface. The receiving hemisphere, normal to the incident beam radiation, keeps on shifting with the apparent position of the sun. The main advantage of this type of collector is its ability to effectively track the sun, without any actual mechanical movement. Both daily and hourly variations of incident radiation on such a spherical solar collector are calculated on the basis of available data. The results are compared with that on an equivalent flat-plate collector for different angles of tilt and latitude. From the results, the spherical collectors are found to be more effective in receiving solar radiation over equivalent flat-plate counterparts throughout the year.     

基于TRNSYS仿真数据的平板型太阳能集热器净得热非参数核密度估计概率模型

平板型太阳能集热器净得热量受天气等外部因素影响而表现出一段时间内的间歇性、随机性和周期性,采用概率建模方法能更准确地反映上述变化特点。目前概率建模中常用的参数分析法需假设先验概率分布,不具有普适性,为此提出一种采用非参数核密度估计对平板型太阳能集热器净得热概率建模的方法。通过TRNSYS动态仿真得到天津地区平板型太阳能集热器净得热样本数据,采用黄金分割算法求解非参数核密度估计的最优带宽,并通过拟合优度检验和误差检验,验证了该概率模型的准确性和在不同时间范围下的适应性。     

Collector performance enhancement with flat reflectors

The use of diffuse and specular flat reflectors to enhance the performance of flat-plate solar collectors has been explored by means of Fortran routines which optimize the size, shape, and placement of reflector and collector. Configuration factors for systems with a diffuse reflector and a collector whose absorptance varies with incidence angle are presented. Specular reflectors are more effective than diffuse reflectors, and, if south-facing, should be used with collectors which are elongated in the east-west direction. Design curves for the specific system of horizontal collector and south-facing reflector are presented. In this system, a moderately sized reflector can increase the midwinter yield per unit collector area by several times.     

Optimal design of orientation of PV/T collector with reflectors

Hybrid conversion of solar radiation implies simultaneous solar radiation conversion into thermal and electrical energy in the PV/Thermal collector. In order to get more thermal and electrical energy, flat solar radiation reflectors have been mounted on PV/T collector. To obtain higher solar radiation intensity on PV/T collector, position of reflectors has been changed and optimal position of reflectors has been determined by both experimental measurements and numerical calculation so as to obtain maximal concentration of solar radiation intensity. The calculated values have been found to be in good agreement with the measured ones, both yielding the optimal position of the flat reflector to be the lowest (5°) in December and the highest (38°) in June. In this paper, the thermal and electrical efficiency of PV/T collector without reflectors and with reflectors in optimal position have been calculated. Using these results, the total efficiency and energy-saving efficiency of PV/T collector have been determined. Energy-saving efficiency for PV/T collector without reflectors is 60.1%, which is above the conventional solar thermal collector, whereas the energy-saving efficiency for PV/T collector with reflectors in optimal position is 46.7%, which is almost equal to the values for conventional solar thermal collector. Though the energy-saving efficiency of PV/T collector decreases slightly with the solar radiation intensity concentration factor, i.e. the thermal and electrical efficiency of PV/T collector with reflectors are lower than those of PV/T collector without reflectors, the total thermal and electrical energy generated by PV/T collector with reflectors in optimal position are significantly higher than total thermal and electrical energy generated by PV/T collector without reflectors.     

Solar thermal collector augmented by flat plate booster reflector: Optimum inclination of collector and reflector

In this report we present a theoretical analysis of a solar thermal collector with a flat plate top reflector. The top reflector extends from the upper edge of the collector, and can be inclined forwards or backwards from vertical according to the seasons. We theoretically predicted the daily solar radiation absorbed on an absorbing plate of the collector throughout the year, which varies considerably with the inclination of both the collector and reflector, and is slightly affected by the ratio of the reflector and collector length. We found the optimum inclination of the collector and reflector for each month at 30°N latitude. An increase in the daily solar radiation absorbed on the absorbing plate over a conventional solar thermal collector would average about 19%, 26% and 33% throughout the year by using the flat plate reflector when the ratio of reflector and collector length is 0.5, 1.0 and 2.0 and both the collector and reflector are adjusted to the proper inclination.     

Comparison of long-term flat-plate solar collector performance calculations based on averaged meteorological data

The use of averaged meterological data for collector performance calculations is studied. To this end, a steady state, two-dimensional, nodal, heat transfer analysis is developed for a flat-plate solar collector. The analysis accounts for the temperature gradients in the fluid flow and vertical directions in the collector, the physical and thermodynamic properties of the materials in the collector, the collector location, the orientation and dimensions of the collector, the number of cover plates and any thin film selective coatings on the cover plates or absorber. Also accounted for are the time dependent variations in the meteorological conditions, insolation, and collimated and diffuse solar irradiation. The spectral nature of radiation heat transfer in the collector is modeled by two spectral bands, solar and thermal, with 3.0 μm as the cutoff frequency between the solar and thermal bands of radiation. The results indicate that long term collector performance calculations based on averaged meteorological data will not correlate with calculations based on hourly data if the weather is highly variable. When the weather variations are mild, averaged data can give results very close to those based on hourly data.     

Solar energy availability at Tehran: Performance of different types of solar collectors

In this report, the optimum tilt of the solar collectors in Tehran is computed through a computer program. The annual optimum tilt of the collector is found to be 30°. The summer and winter optimum tilts are found to be 0° and 50°, respectively. Based on these optimum tilts, the annual performance of different types of solar collectors which have experimental efficiency curves is predicted. The variation of the performance of the collectors with the optimum tilts is found to be small. The difference between the outlet temperatures of these collectors is also found to be small. The effect of changing the incident angle from zero degrees is found to increase the efficiency of the flat-plate collectors and to reduce the efficiency of the evacuated collectors.In the coldest month of the year, the flat-plate collector can collect heat up to 70°C, while the tubular one can collect heat with temperature more than 90°C. In the other months, all the collectors collect heat with temperatures more than 90°C. The performance of a well-mixed storage tank used with one of the flat-plate collectors is annually predicted. For the initial storage tank temperature assumed at 20°C, the minimum and maximum temperatures are found approximately to equal 42°C and 93°C, respectively. The results show the high potential of the solar collectors in Tehran.     

半圆形漫反射板对储热式真空管集热性能的影响

从理论上推导出单根储热式玻璃-金属真空管在有、无漫反射板两种不同情况下的晴天模型。利用该模型计算出理论的瞬时热效率及平均热效率,并确定半圆形漫反射板与真空管的最佳距离。同时通过实验对该真空管在有、无半圆形漫反射板两种不同的情况下的热效率进行对比。理论和实验表明,加置漫反射板以后,真空管的热效率提高了6.8%~24.2%,半圆形漫反射板与真空管的最佳距离约为1倍管径。     

The use of planar reflectors for increasing the energy yield of flat-plate collectors

A mathematical model to simulate the performance of flat-plate collector-reflector systems is presented. First the collector energy balance is modified to account for the reflected energy. Then the exchange area for a diffuse reflector is obtained by integrating over both reflector and collector surfaces. For the specular reflector, the collector area exposed to reflected radiation is calculated from geometrical relations. Shading effects are also found from the system geometry. Fair agreement is obtained between the model and some experiments on a water heating collector in Brisbane, Australia. Finally, the model is used to predict the annual performance of a water heating system with several values of the reflector angle.     

Optimum exergy efficiency of solar collectors

An optimum exergy efficiency is derived for flat-plate solar collectors as a ratio of exergy delivery of the collector to the maximum output exergy obtainable. It is a function of the optimum mass flow rate through the collector, which itself is obtained through an optimization of the exergy delivery of the collector.     

Discrete two-positional tracking of solar collectors

Northern European regions, such as Estonia at 60° latitude, receive about 980 kWh m⁻² y⁻¹ of solar radiation. These low insolation levels motivate solar collectors to be equipped with a tracking mechanism for an increased yield. Classical active tracking is complicated and energy intensive, negating tracking benefits for PV modules and thermal flat-plate collectors. This paper describes mainly the performance of PV modules with daily two-positional tracking. The symmetrical and asymmetrical positions about the north–south axis are analyzed, corresponding to the positions of sun in the morning and in the afternoon. The tracking drive is simple and requires a minimum energy input during the brief daily triggering of movement. Results indicate that the seasonal energy yield is increased by 10–20% over the yield from a fixed south-facing collector tilted at an optimal angle. The results are based on long-term solar data measured at the Tartu-Tõravere Meteorological Station in Estonia, confirmed by experimental testing during summer 2004 and 2005 in Tallinn University of Technology.