An analytic model for the long-term performance of solar air heating systems

An analytic model is presented for the prediction of the monthly and yearly thermal performance of solar air heating systems. The effects of stratification in packed rock bed storage are explicitly taken into account. An expression for monthly solar fraction that depends on the important system and climatic variables is derived, which affords accurate predictions compared to corresponding f-chart calculations. The value of our analytic model, vis-a-vis calculational tools such as f-chart, is discussed. The method is applicable to all solar collector types as well as to load distributions of other than 24 hr/day.     

Collector and storage efficiencies in solar heating systems

A general definition of the effective efficiency of solar collector operating in a solar energy system is presented which gives a fair method of comparison of different collectors operating in that particular application. Based on comparison between the area required for the actual collector and that of a perfect collector-both giving the same fraction solar—the definition permits the definition of the effective average value of the collector input parameter, P = (T_fi − T_a)/S. The concept of the perfect collector also leads to a fair definition for the efficiency of the storage component in a solar heating system. These parameters are evaluated for the special case of residential space heating and service hot water systems of the standardized f-chart type operating in a number of Canadian cities. Simple methods for collector comparisons result from the study and indications are that a simple solar system design method will follow.     

A graphical method of measuring the performance characteristics of solar collectors

A graphical method to measure average and instantaneous efficiencies of a solar concentrator used for heating and boiling liquids and a flat plate collector is presented. The overall heat loss coefficient for the collectors and the optical loss factors: γ(τa)_b—the product for a concentrator and (τa)—the product for a flat plate collector, are also obtained. The method involves measuring the temperature of stagnated liquid in the absorber/collector as a function of time at noon. The efficiencies obtained are correct to within 5% of the efficiencies obtained from accurate measurements involving solar radiation data, the design parameters of collectors and the physical characteristics of the materials used in the fabrication of collectors.     

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.     

Flat-plate solar-collector performance evaluation with a solar simulator as a basis for collector selection and performance prediction

The use of a solar simulator for performance determination permits collector testing under standard conditions of wind, ambient temperature, flow rate and “Sun”. The performance results determined with the simulator have been found to be in good agreement with outdoor performance results.This paper reports the measured thermal efficiency and evaluation of 23 collectors which differ according to absorber material (copper, aluminum, steel), absorber coating (nonselective black paint, selective copper oxide, selective black nickel, selective black chrome), type of glazing material (glass, Tedlar, Lexan, anti-reflection glass), the use of honeycomb material and the use of vacuum to reduce thermal convection losses. The collectors are given performance rankings based on noon-hour solar conditions and all-day solar conditions. The determination with the simulator of an all-day collector performance is made possible by tests at different incident angles. The solar performance rankings are made based on whether the collector is to be used for pool heating, hot water, absorption air conditioning, heating, or for a solar Rankine machine.Another test which aids in selecting collectors is a collector heat capacity test. This test permits a ranking of collectors according to their heat capacity (and time constant), which is a measure of the rapidity of a collector's response to transient solar conditions. Results are presented for such tests.Final considerations for collector selection would of course be made on the basis of cost and the reliability of performance over the required life of a collector. Results of a cost-effectiveness study is given for conditions corresponding to those required for absorption or heating. These results indicate that the additional cost involved in the upgrading of collector performance (selective surfaces, anti-reflection glass, etc.) appears to be cost effective and therefore justified. Some data are also presented to illustrate a method for the determination of outdoor performance degradation by use of simulator tests carried out before and after a period of outdoor operation.     

Thermal performance of combined solar systems with different collector efficiencies

The performance of two kinds of solar systems for space- and domestic hot water heating has been compared by computer simulations. One system is a conventional radiator-based heating system with collectors of ‘ideal’ collector coefficients. The second system is a low temperature heating system with solar collectors of moderate efficiency. The investigation shows that the difference in performance of the two systems is in the order of 1–6%.     

Parametric sensitivity studies using paraffin wax storage sub-systems

Performance estimates are compared for a solar assisted domestic hot water ststem with a yearly reference load of 13.7 GJ(3811 kWhryr⁻¹) for storage subsystems of pure water and hybrid water and wax systems. Three models of the hybrid system are used ranging from the infinite heat transfer model, the detailed heat transfer model and the simplified constant Nusselt number model.Sensitivity studies are made for changes in collector type (loss coefficient), collector area, mass of water stored, mass of wax stored, heat exchanger gap spacing, and thermal properties of wax.The optimal heat exchanger gap spacing is established for a particular prototype wax store on hand and its performance figure is given.Although the conclusion is drawn that a paraffin wax store in this particular application would not replace water as a storage medium, other conclusions are discussed which would apply to the design and modelling of wax storage subsystems in other applications.     

Calculation of flat-plate collector utilizability

The “utilizability” or φ-curve method developed by Whillier [1] and later generalized by Liu and Jordan[2, 3], can be a very useful design tool for some types of solar energy systems which utilize conventional flat-plate solar collectors. The generalized φ-curve method in its original form, however, has several drawbacks. The calculation effort required to assess long-term collector performance is significant. The calculations can not be completely automated on a computer or hand calculator because many graphs are needed for which analytical representations are not available.In this paper, the φ-curve method is reviewed and situations for which the method is useful are described. Then, an easier method, using daily -charts, rather than the original hourly φ-curves, is presented. The -charts greatly reduce the calculations required to determine flat-plate collector utilizability. -charts can be easily implemented on a programmable hand calculator. A comparison of the original hourly and the new daily calculation methods is presented. An example demonstrating the use and utility of the -charts is also included.     

Thermal performance study of two-pass solar air heaters

The operation of a conventional solar air heater with two covers in a two-pass mode offers an inexpensive method of improving the collector efficiency by about 10–15 per cent. Heat transfer models are developed for two such two-pass flow arrangements and are compared with the performance of the single pass design. The computer models are validated by comparing with the experimental data from a series of collector testing experiments. The collector performance is examined over a wide range of design and operating conditions and the two-pass designs are found to perform better than the single pass system. For closed-loop systems with air recirculation the two-pass designs have some limitations in performance. Design curves for two-pass systems over a range of variables are presented.     

Intergration of evacuated tubular solar collectors with lithium bromide absorption cooling systems

By surrounding the absorber-heat exchanger component of a solar collector with a glass-enclosed evacuated space and by providing the absorber with a selective surface, solar collectors can operate at efficiencies exceeding 50 per cent under conditions of ΔT/H_T = 75°C m²/kW (ΔT = collector fluid inlet temperature minus ambient temperature, H_T = incident solar radiation on a tilted surface). The high performance of these evacuated tubular collectors thus provides the required high temperature inputs (70–88°C) of lithium bromide absorption cooling units, while maintaining high collector efficiency. This paper deals with the performance and analysis of two types of evacuated tubular solar collectors intergrated with the two distinct solar heating and cooling systems installed on CSU Solar Houses I and III.     

Improvement of basin type solar still performance : Use of various absorber materials and solar collector integration

Experimental measurements to determine conditions necessary for efficient solar desalination are given. The effects on performance of using various different absorber materials together with the integration of flat-plate collectors with storage systems in basin type solar stills are investigated. Correlations between daily yield (Y, in litres) and solar insolation (I, in MJ/m²day) are found to be, Y = 0.152I − 0.706 (for black-paint absorbers), Y = 0.180I − 0.987 (for charcoal absorbers), Y = 0.225I − 0.467 (for integrated solar collector and storage system, with black-paint absorbers). The calculated daily-average monthly yields for each case are also given.     

Advanced 3‐D CPC solar collector for thermal electric system

In this paper, the authors propose an innovative non‐tracking three‐dimensional compound parabolic concentrator (3‐D CPC) solar collector, which has excellent thermal efficiency for a high‐temperature range (100–200°C). In the past studies, in order to improve the thermal efficiency of the solar collector in a high‐temperature range, very high concentration ratios and tracking systems have been adopted. However, conventional high concentration solar collectors are not cost‐effective and are inappropriate for small‐rating thermal electric generation systems for residential use. The proposed 3‐D CPC collector has a moderate concentration ratio and does not need tracking. Initially, the tentative 3‐D CPC collector was fabricated and its thermal performance was tested. Next, numerical simulations of the optical characteristics of the 3‐D CPC collector were carried out via the ray‐tracing method. Finally, the specification of the optimal 3‐D CPC collector was clarified. Applications of the thermal electric system will also be mentioned. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(5): 323–335, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20121     

Thermal advantages for solar heating systems with a glass cover with antireflection surfaces

Investigations elucidate how a glass cover with antireflection surfaces can improve the efficiency of a solar collector and the thermal performance of solar heating systems. The transmittances for two glass covers for a flat-plate solar collector were measured for different incidence angles. The two glasses are identical, except for the fact that one of them is equipped with antireflection surfaces by the company SunArc A/S. The transmittance was increased by 5–9%-points due to the antireflection surfaces. The increase depends on the incidence angle. The efficiency at incidence angles of 0° and the incidence angle modifier were measured for a flat-plate solar collector with the two cover plates. The collector efficiency was increased by 4–6%-points due to the antireflection surfaces, depending on the incidence angle. The thermal advantage with using a glass cover with antireflection surfaces was determined for different solar heating systems. Three systems were investigated: solar domestic hot water systems, solar heating systems for combined space heating demand and domestic hot water supply, and large solar heating plants. The yearly thermal performance of the systems was calculated by detailed simulation models with collectors with a normal glass cover and with a glass cover with antireflection surfaces. The calculations were carried out for different solar fractions and temperature levels of the solar heating systems. These parameters influence greatly the thermal performance associated with the antireflection surfaces.     

太阳能空气集热建筑模块冬季热性能优化实验研究

以建筑集成的太阳能空气集热建筑模块为研究对象,利用与实际住宅等大的实验平台,针对不同的集热板形式和空气流动方式对集热建筑模块热性能的影响进行了实验研究。重点讨论了开孔平板型、开孔孔径大小以及设置空气间层分隔板等对改善集热建筑模块热性能的影响。实验结果表明,开孔平板型较实心平板型的热性能得到明显的改善,孔径优化后(孔径为5mm、孔间距为15mm)的热效率可提高30%以上;另外,设置空气间层分隔板相对于小孔径(d=5mm)而言,采用大孔径(d=10mm)时出风口升温效果更明显。     

A method of testing for rating solar collectors based on thermal performance

This paper describes a proposed test method for determining the efficiency of solar collectors under specified outdoor “steady-state conditions. The prescribed series of tests should provide useful data for the rating of solar collectors based on thermal performance. A study was made of existing theory, measurement practices and a number of collector test procedures in use prior to the publication of the proposed method.The test apparatuses and major components have been prescribed so a liquid or air can be used as the transfer fluid. The energy of the fluid entering and leaving the collector is determined by making appropriate measurements and these quantities are than compared to the energy incident upon the collector (also determined by measurement) in order to calculate the collector efficiency.The series of tests to be conducted consists of determining the average efficiency for 15 min periods (integrating the energy quantities) over a range of temperature differences between the average fluid temperature (average of inlet and outlet) and the ambient air. The test apparatuses have been designed so that the temperature of the fluid entering the collector can be controlled to a selected value. This feature is used to obtain the data over the temperature range desired. At least sixteen “data points” are required for a complete test series and they must be taken symmetrical with respect to solar noon (to prevent biased results due to possible transient effects).     

Simulation and experimental investigation of two hybrid solar domestic water heaters with drain water heat recovery

In this paper, the performance of two solar domestic hot waters (SDHW) with drain water heat recovery (DWHR) units is investigated. Both SDHW systems are recently installed at the Archetype Sustainable Twin Houses at Kortright Center, Vaughan, Ontario. The first SDWH system in House A consists of a flat plate solar thermal collector in combination with a gas boiler and a DWHR unit. The second SDHW system in House B includes an evacuated tube solar collector, an electric tank, and a DWHR unit. Both systems are modeled in TRNSYS, and the models are validated by experimental data. The addition of the DWHR and the flat‐plate solar thermal collector would result in 1831 kWh of annual energy saving in House A. While the addition of the DWHR and the evacuated tube collector in House B would result in an annual energy saving of 1771 kWh. Subsequently, the models are used to investigate the performance of similar systems for five major Canadian cities of Halifax, Montreal, Toronto, Edmonton, and Vancouver. The conjunctions of solar thermal collectors with DWHR units are found most beneficial in Edmonton. It is also noted from experimental and simulated results that flat‐plate solar collector‐based water heater produced more thermal energy than the system based on the evacuated tube solar collector for all major Canadian cities. Copyright © 2015 John Wiley & Sons, Ltd.     

Maximum performance of omnicolor photothermal and photovoltaic converters in our planetary system

This paper deals with the performance of photothermal and photovoltaic omnicolor (or “fully selective”) converters in the Solar System. Both interplanetary power stations and power systems placed on the surface of different planets are analyzed. The influence of the radiation concentration on system performance is outlined. The effect of the Sun's zenith angle is also discussed. Spectral distributions of the thermal collector optimum temperature and solar cell optimum voltage are shown.     

A parametric study of open loop solar heating systems—I

The performance of open loop solar heating systems is studied using a single non-dimensional equation. For any location the long term solar fraction depends primarily on three non-dimensional groups M, K and R. M is the ratio of energy available on the collector aperture to energy demand and is hence a collector sizing parameter. K is the ratio of a reference rate of collector heat loss to a reference rate of energy available and is a collector performance parameter. R is the number of days storage available. A correlation relating the solar fraction, E, to M, K and R is given and the range of validity of this correlation is examined. For open loop systems with short term storage, the correlation can be used as a design method for a large range of demand temperatures, demand patterns, collector types and orientations. Comparisons are made between the solar fraction calculated by an hour by hour computer model and that predicted by the design method. Agreement is good. Further comparisons are made between measured and predicted performance figures for a large 20 m² domestic hot water system. It is concluded that the correlation can be used as a reliable design method and will allow simple selection of optimum system designs.     

Vertical evacuated tubular-collectors utilizing solar radiation from all directions

A prototype collector with parallel-connected evacuated double glass tubes is investigated theoretically and experimentally. The collector has a tubular absorber and can utilize solar radiation coming from all directions. The collector performance is measured in an outdoor test facility. Further, a theoretical model for calculating the thermal performance is developed. In the model, flat-plate collector's performance equations are integrated over the whole absorber circumference and the model determines the shading on the tubes as a function of the solar azimuth. Results from calculations with the model are compared with measured results and there is a good degree of similarity between the measured and calculated results. The model is used for theoretical investigations on vertically-placed pipes at a location in Denmark (Copenhagen, lat. 56°N) and at a location in Greenland (Uummannaq, lat. 71°N). For both locations, the results show that to achieve the highest thermal performance, the tube centre distance must be about 0.2 m and the collector azimuth must be about 45–60° towards the west. Further, the thermal performance of the evacuated solar-collector is compared to the thermal performance of the Arcon HT flat-plate solar-collector with an optimum tilt and orientation. The Arcon collector is the best performing collector under Copenhagen conditions, whereas the performance of the evacuated tubular collector is highest under the Uummannaq conditions. The reason is that the tubular collector is not optimally tilted in Copenhagen but also that there is much more solar radiation “from all directions” in Uummannaq and this radiation can be utilized with the tubular collector. It is concluded that the collector design is very promising—especially for high latitudes.     

Exergy analysis of a passive solar still

This paper presents a steady-state and transient theoretical exergy analysis of a solar still, focused on the exergy destruction in the components of the still: collector plate, brine and glass cover. The analytical approach states an energy balance for each component resulting in three coupled equations where three parameters—solar irradiance, ambient temperature and insulation thickness—are studied. The energy balances are solved to find temperatures of each component; these temperatures are used to compute energy and exergy flows. Results in the steady-state regime show that the irreversibilities produced in the collector account for the largest exergy destruction, up to 615 W/m² for a 935 W/m² solar exergy input, whereas irreversibility rates in the brine and in the glass cover can be neglected. For the same exergy input a collector, brine and solar still exergy efficiency of 12.9%, 6% and 5% are obtained, respectively. The most influential parameter is solar irradiance. During the transient regime, irreversibility rates and still temperatures find a maximum 6 h after dawn when solar irradiance has a maximum value. However, maximum exergy brine efficiency, close to 93%, is found once T_col<T_w (dusk) and the heat capacity of the brine plays an important role in the modeling of collector–brine interaction. Nocturnal distillation is characterized by very low irreversibility rates due to reduced temperature difference between collector and an increase in exergy efficiency towards dawn due to ambient temperature decrease.