CPC在太阳能集热器中的应用

CPC复合抛物聚光器是一种非成像低聚焦度的聚光器,它根据边缘光线原理设计,可将给定接收角范围内的入射光线按理想聚光比收集到接收器上。由于它有较大的接收角,故在运行时不需要连续跟踪太阳,只须根据接收角的大小和收集阳光的小时数,每年定期调整倾角若干次就可有效的工作。它可达到的聚光比在10以内,当聚光比在2以下时可做成固定式装置。它可接收直射太阳辐射和部分散射辐射,并能接收一般跟踪聚光器所不能接收的“太阳周围辐射”。此类集热器的结构比较简单,对聚光面型加工精度要求不严格,将其应用在太阳能集热器中,是一个值…     

Cost effective asymmetric CPC solar collectors

Low cost CPC solar collectors were designed, constructed and tested. The collectors consist of two separate absorbers, which are horizontally incorporated in a stationary asymmetric CPC mirror. The efficient operation of the proposed collectors is due to the direct absorption of a large part of the incoming solar radiation and to the thermal losses suppression by the inverted surface of both absorbers. Two collector types with the same basic design are presented. The first type has tubular absorbers which are used for direct water heating and the second has flat fin type absorbers with pipe. Test results showed that the proposed collectors operate efficiently and are suitable for hot water applications.     

Design considerations for solar collectors with cylindrical glass honeycombs

A properly designed cell structure placed between the solar absorber and outer cover glass can substantially reduce natural convection and infrared reradiation heat losses. Glass has merit for such a cellular structure or honeycomb because it is an inexpensive, abundant and stable material with low thermal conductivity and outstanding optical characteristics. To optimize the design of a honeycomb structure, i.e. to minimize the cost of the solar energy collected Z, requires the determination of the honeycomb solar transmission as a function of incidence angles of the sun, infrared effective emittance, cell Nusselt number, and cell wall conductance as well as an estimate of appropriate costs. For an array of circular tubes, the design parameters are wall thickness b, cell diameter d_i, and cell length L. It is difficult to make b less than about 0.2 mm. Typically, d_i must be no larger than 1.6 cm. Increasing decreases reradiation and conduction losses, but also decreases solar energy transmission. For d_i = 9.53 mm and b = 0.198 mm, optimum values ranged from 3 to 12 for collector temperatures (above ambient air temperature) between 22°C and 100°C. Since the Z vs curves have fairly broad minimums, values less than 9 can be used with less than a 3 per cent penalty in cost at the higher temperatures.A comparison of collector effiency characteristics indicated that cylindrical glass honeycomb collectors with nonselective-black absorbers were markedly superior to single-glazed selective-black and double-glazed nonselective-black collectors, especially at higher collector temperatures. Cost effectiveness studies also indicated honeycomb collector superiority at temperature differences between the working fluid and ambient air greater than about 35°C.     

Design considerations for residential solar heating and cooling systems utilizing evacuated tube solar collectors

As solar heating systems become a commercial reality, greater efforts are now being employed to incorporate solar cooling components in order to obtain a complete solar heating and cooling system and thus take advantage of the cost-effectiveness of year-round use of the solar equipment. Because of the exceptional performance and high efficiency of evacuated tube solar collectors, these advanced collectors are receiving considerable attention for use in solar heating and cooling systems. While improved performance is readily obtained with these sophisticated solar collectors, there are also numerous difficulties and problems associated with their use in a solar system. This paper addresses many of the design considerations which must be included in any realistic solar system design. Most of the considerations presented here are based on the experience gained in the design and performance of the solar heating and cooling systems for CSU Solar Houses I-IV.     

Relative cost-effectiveness of CPC reflector designs suitable for evacuated absorber tube solar collectors

Specular reflectors of the fixed CPC type are compared in terms of yearly energy collection and relative cost-effectiveness. The reflector designs used are designed for use with a circular-cylindrical evacuated tubular absorber, and a gap is allowed between reflector and absorber to accommodate the tubular glass envelope and evacuated space. Stainless steel, aluminum, and thin, back-silvered glass mirrors were modelled.The results show that the choice of acceptance angle of a reflector for use with a moderately priced evacuated tube at water heating temperatures is not critical; almost any reflector acceptance angle will do so long as the aperture is carefully chosen, and both North-South or East-West orientations have approximately similar performance. Under such conditions, other factors such as mirror self-cleaning and manufacturing ease may be decisive in the choice of design.At temperatures above 100°C or for high tube costs, an East-West reflector design of concentration >1.4 is strongly indicated.At the time of writing, polished stainless steel is as cost-effective a choice as any other for a mirror material, and is probably more durable and amenable to mass production.     

Detailed parametric analyses of heat transfer in CPC solar energy collectors

A detailed parametric analysis of heat transfer in compound parabolic concentrating solar energy collectors has been performed, using a unified model for their optical and thermophysical behaviour. The effects of angular inclination and collector acceptance angles on free convection within the cavity are presented. The circumferential variation of local Nusselt number about the absorber is determined. A convective heat transfer correlation is obtained for the average Nusselt number with respect to Grashof number that takes into account acceptance angle and angular inclination. The developed correlation is extended to truncated compound parabolic concentrators.     

Effect of inclination on the performance of CPC solar energy collectors

A theoretical numerical model of thermal transfer in a line-axis, symmetric, compound parabolic concentrating solar energy collector (CPC) is presented. The effect of the angle of axial inclination of an east-west aligned CPC and hence the effect of the latitudinal and tracking configuration of the CPC system on performance is investigated. The angle of inclination is taken into consideration in the determination of both internal and external convective heat transfer. The convective, radiative, conductive and overall heat transfer coefficients and system efficiency for various angles of inclination, concentration ratios and insolations was determined, and are presented as graphs of heat transfer variation and Hottel-Whillier-Bliss characteristic curves respectively.     

Truncation of CPC solar collectors and its effect on energy collection

Analytic expressions are derived for the angular acceptance function of two-dimensional compound parabolic concentrator solar collectors (CPC's) of arbitrary degree of truncation. Taking into account the effect of truncation on both optical and thermal losses in real collectors, we also evaluate the increase in monthly and yearly collectible energy. Prior analyses that have ignored the correct behavior of the angular acceptance function at large angles for truncated collectors are shown to be in error by 0–2% in calculations of yearly collectible energy for stationary collectors.     

Asymmetric CPC solar collectors with tubular receiver: Geometric characteristics and optimal configurations

We derive analytic expressions for the geometric characteristics of extremely asymmetric nonimaging solar concentrators (“asymmetric CPCs”) with tubular receivers, of fixed maximum concentration ratio C_max. We show that, at fixed C_max, there are nonideal concentrator configurations that minimize reflector area and average number of reflections, and determine configurations that are approximately optimal (maximizing yearly energy delivery at minimal material requirements), independent of climate and economics.     

Thermal analysis of CPC collectors

Mathematical formulations were developed to study thermal processes in a compound-parabolic-concentrator (CPC) collector. The system under investigation consists of a CPC cusp fitted with a concentric, evacuated double pipe to serve as a heat absorber. Heat is transmitted to the circulating fluid flowing inside a U-tube via the heat getter slipped inside the inner pipe. The collector has a cover for dust protection. Four nonlinear, simultaneous equations were derived to predict heat exchange among various components in the system. Collector efficiency equations were also developed following the Hottel-Whillier-Woertz-Bliss formalism. These equations were subsequently used in a computer program to test the collector performance under varied operating conditions. Test results indicate that, because of the high thermal resistance between the receiver jacket and the envelope, the collector performance is quite stable and is nearly independent of many parameters tested. The efficiency of the collector is high and shows only a very slight drop at high operating temperatures. The predicted results were also compared with experiments.     

Development of a flow distribution and design model for transpired solar collectors

Unglazed, transpired solar collectors offer a low-cost, high-efficiency means for preheating outside air for ventilation and crop drying applications. Although large building wall applications for these collectors have generally performed well in the field, many have exhibited poor flow distribution which can prevent maximum efficiency from being achieved. The objective of this work was to develop a computer model which would run quickly on a personal computer and allow designers of transpired collectors to easily adjust geometric parameters to achieve reasonable flow uniformities and to determine efficiencies. This paper describes how this model was developed and includes results from model runs. In order to allow the model to run quickly on the PC, pipe network methods were used to develop a set of simultaneous equations in the unknown flow rates. Previous research results on heat exchange effectiveness, pressure drop, and wind heat loss were incorporated.     

Mirror enclosures for double-exposure solar collectors

A conventional flat-plate collector panel can be employed in a double-exposure configuration when the panel is glazed on both sides and when mirrors are provided to reflect solar radiation onto the back side of the panel. In this paper, several flat-mirror configurations are evaluated and optimal configurations are determined for different solar energy applications at lat. 35, 40 and 45°. The various mirror configurations are evaluated theoretically by calculating direct-beam and diffuse solar radiation enhancement factors. The enhancement factors are defined as the ratio of the solar flux absorbed by both sides of a double-exposure panel to that absorbed by an identical single-exposure panel tilted at the latitude angle from the horizontal. The enhancement factors are calculated using the method of images and take account of the variation of glazing transmittance with incident angle. Optimal mirror configurations were determined for direct-beam solar radiation for both fixed-mirror configurations and adjustable-mirror configurations with semi-annual mirror rotations. Optimal fixed-mirror configurations were obtained for both winter space-heating and year-round applications. An adjustable-mirror configuration, however, was determined to be optimal for year-round solar collection and overall to be most adaptable to a variety of solar energy applications. The same adjustable-mirror configuration was determined to be optimal at all three latitudes and therefore a single design can be employed at diverse locations.     

Study of a novel solar adsorption cooling system and a solar absorption cooling system with new CPC collectors

One two-phase thermo-syphon silica gel-water solar adsorption chiller and LiBr-H₂O absorption chiller with new medium CPC (Compound Parabolic Concentrator) solar collectors were investigated. The reliability of adsorption chiller can be improved, because there is only one vacuum valve in this innovative design. Medium temperature evacuated-tube CPC solar collectors were firstly utilized in the LiBr-H₂O air conditioning system. The former system was applied in north of China at Latitude 37.45° (Dezhou city, China), the latter system was applied at Latitude 36.65° (Jinan city, China). Experimental results showed that the adsorption chiller can be powered by 55 °C of hot water. The adsorption chiller can provide 15 °C of chilled water from 9:30 to 17:00, the average solar COP (COPs) of the system is 0.16. In the absorption cooling system, the efficiency of the medium temperature evacuated-tube CPC solar collector can reach 0.5 when the hot water temperature is 125 °C. The absorption chiller can provide 15 °C of chilled water from 11:00 to 15:30, and the average solar COPs of absorption system is 0.19.     

Receiver design considerations for solar central receiver hydrogen production

The status of the Solar Central Receiver or Power Tower developments is briefly reviewed. with particular consideration given to the application of this technology to thermochemical hydrogen production. Emphasis is on the general considerations in the receiver-chemical reactor interface. High solar flux densities of 1.5–3 MW M⁻² must be transformed to 50–100 kW M⁻² for catalytic decomposition of sulfuric acid such as in the General Atomic (GA) water-splitting reactions. The different methods considered to accomplish such reactions include cavities, external heat pipe receivers, a falling curtain of particles, and other direct absorption media coupled to chemical reactions.     

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.     

Stationary mirror systems for solar collectors

An angle in solar geometry termed the EWV altitude is defined, and its variation with time and season is shown. This variation indicates the necessary acceptance angle of a stationary mirror system for solar collectors. It is shown that a completely stationary mirror cannot give any useful concentration, but that if the tilt is varied with the seasons, an eastwest cylindrical parabolic mirror without diurnal movement can yield a concentration of approximately three. This may be increased to about four with the aid of a small auxiliary (fixed) mirror to provide a second stage of optical concentration.