Thermal performance testing of flat-plate collectors

Existing standards for testing the performance of flat-plate solar collectors are documented in ASHRAE 93 [ANSI/ASHRAE Standard 93-2003, 2003. Methods of Testing to Determine Thermal Performance of Solar Collectors, ISSN: 1041-2336, ASHRAE, Inc., 1791 Tullie Circle, Ne, Atlanta, GA30329], ISO 9806-1 [ISO Standard 9806-1:1994(E), 1994. Test Methods for Solar Collectors – Part 1: Thermal Performance of Glazed Liquid Heating Collectors Including Pressure Drop, ISO, Case Postale 56, CH-1211 Geneve 20, Switzerland] and EN12975-2 [European Standard EN12975-2:2001, 2001. Thermal Solar Systems and Components – Solar Collectors – Part 2: Test Methods, CEN, Rue de Stasart, 36, B-1050, Brussels]. The ASHRAE 93 standard requires an experimental determination of the steady-state collector efficiency under prescribed environmental conditions for a range of collector fluid temperatures. Each test requires a minimum of 20 min and 22 tests are required to fully characterize a collector’s thermal performance. The ASHRAE 93 testing procedure is further complicated by the fact that the prescribed weather conditions do not often occur in some locations, which prolongs the time required to conduct the performance tests for a given collector. The EN12975-2 collector test procedure provides an alternative transient test method that can be conducted over a larger range of environmental conditions. This paper compares the results obtained by applying the EN12975-2 standard with results obtained from the ASHRAE 93 steady-state tests for a well-designed single-glazed selective surface flat-plate collector. The collector thermal parameters, F_R(τα)_e and F_RU_L obtained by the two test methods show good agreement. The incident angle modifier coefficient determined in the ASHRAE method, which uses a separate test for this purpose, was found to be more accurate than that determined in the transient method according to the EN12975-2 standard, which obtains this value and all other collector parameters in the same step. This transient method, however, uses a refined collector model that includes specific terms for the wind speed dependence and the collector thermal capacitance, which are absent in the ASHRAE model. The long-term collector thermal performance as a part of a water heating system was simulated using the efficiency curves derived from each of the test methods. The solar fractions obtained by simulation are within 7% for both cases.     

Calorimetric measurement of absorptance and emittance of the Sydney University evacuated collector

Simple calorimetric techniques have been developed for determining the absorptance and emittance of individual evacuated tubular collectors incorporating a selective surface, and the efficiency, η_o, of evacuated collectors in various mirror systems. The absorptance and efficiency measurements are made in natural sunlight without the use of a solarimeter by establishing an absorptance standard based on Nextel black paint. Calibration of solarimeters using the established absorptance standard is discussed. Emittance measurements are made by measuring radiative heat losses from the absorber tube of a collector.Absorptance and emittance measurements for a number of Sydney University evacuated collectors gave values of absorptance α = (0.92 ± 0.01) and emittance = 0.05 at 120°C for the selective surface utilized. Efficiency (η_o) measurements for Sydney University collectors in two simple mirror systems are also reported.     

Thermal performances of the active and passive water heating systems based on annual operation

In this paper, the year round thermal performance evaluation of active and passive solar water heating systems for rural/urban area have been studied. Two types experiments are the normalized draw-off temperature profile and the normalized draw-off mixing profile conducted in the months of April 2003–March 2004. Effect of morning, mid-afternoon, evening draw-off and effect of water mixing in the morning and evening are also studied during the experiments. Thermal efficiency for the system and collection efficiency for the collectors were calculated during the experiments. The values of F_R · (τα) of the collectors used in active and passive water heating systems were found 0.69 and 0.61 respectively. Ethylene glycol has a 50/50 glycol-to-water ratio used as a heat transfer fluid (HTF) in the systems due to the cold climate in South Korea.     

The steady state salt gradient solar pond

The three-zone salt gradient solar pond is analyzed as a steady-state flat-plate solar energy collector. The resultant efficiency equation is of the Hottel-Whillier-Bliss type commonly used for flat-plate collectors. The quantities that occur in this equation—the effective absorptivity-transmissivity product ατ, the loss factor U_L, the heat removal factor F_R, and the incident angle modifier θ(i)—are related to the physical properties and dimensions of the pond. For a given ΔT/H [(fluid inlet temperature—surface temperature)/insolation], the thickness of the nonconvective zone can be adjusted for maximum efficiency. U_L and ατ are smaller than the equivalent quantities for flat-plate collectors, while θ(i) and F_R are close to unity. As a consequence, steady-state salt-gradient solar ponds are less efficient than common flat-plate collectors at low ΔT/H, but they are more efficient at high ΔT/H.     

Thermodynamic analysis of monomethylamine–water solutions in a single-stage solar absorption refrigeration cycle at low generator temperatures

A theoretical analysis of the coefficient of performance COP was undertaken to examine the efficiency characteristics of the monomethylamine–water solutions for a single-stage absorption refrigeration machine, using low generator temperatures (60–80°C), which allows the use of flat plate solar collectors. The thermodynamic analysis considers both, basic and refined cycles. The refined absorption cycle included a sensible heat recover exchanger (that is a solution heat exchanger). The thermal coefficients of performance COP_h for the basis cycle and COP_SHE for the refined cycle were calculated using the enthalpies at various combinations, at the operating temperatures and concentrations. The flow ratio F_R has been calculated as additional optimization parameter. Due to the relative low pressure and the high coefficients of performance, the monomethylamine–water solutions present interesting properties for their application in solar absorption cycles at moderate condenser and absorber temperatures (25–35°C), with temperatures in the evaporator from −10°C to 10°C which are highly usable for food product preservation and for air conditioning in rural areas.     

Design application of the Hottel-Whillier-Bliss equation

A method is presented for experimentally determining the three factors that determine collector efficiency in the Hottel-Whillier-Bliss equation, Q_u = F_RA_c[(τα)_cI - U_L(T_f,i - T_a)]. These factors are: the collector heat removal factor, F_R; the effective transmittance-absorptance product, (τα)_e; and the overall heat loss coefficient, U_L. The method of testing requires: computation of (τα)_e from measurements of cover transmittance and collector reflectance; computation of F_R from a test in which the heat loss term equals zero; and computation of U_L from a test in which insolation equals zero. This method was applied to collectors used on Solar House I at Colorado State University, with experimental and theoretical results being in close agreement. The method can be used to experimentally evaluate collector performance and for optimization of collector design.     

Parameters of solar collectors tested with a KMITT solar simulator

This paper reports on the experimental performances of flat plate solar collectors tested with a solar simulator under steady-state conditions, in terms of collector efficiency, η, and ratio of temperature difference and solar radiation (T_fi-T_e)/I_T. T_e was the effective heat-sink temperature of the tested collector and could be evaluated from temperatures of the collector's cover, ambient and light source panel (or infrared filter). Techniques for converting values of the collector's parameters, F_RU_Le and F_R(τα)_e, obtained from the indoor tests to match outdoor results were demonstrated. The adjusted results agreed well with those of the outdoor data in the case of a collector having a flat glass cover. For a collector having a convex plastic cover, the estimated optical efficiency was lower than that of the outdoor result.     

Equations for estimating global solar radiation in data sparse regions

The knowledge of the amount of solar radiation in an area/region is very essential in the field of Solar Energy Physics. In this work two equations are put forward for estimating global solar radiation from common climate variables in data sparse regions. The first is the Hargreaves equation, R_s=0.16R_aT_d^0.5 where R_a is the extraterrestrial solar radiation and T_d is the temperature difference (maximum minus minimum), while the second is the Angstrom equation, R_s=R_a(0.28+0.39n/N) where n and N are the measured sunshine hours and the maximum daylight duration respectively. The global solar radiation estimated by the two equations for three sites, Owerri (5°28′N, 7°2′E), Umudike (5°29′N, 7°33′E) and Ilorin (8°32′N, 4°46′E), located in different climate zones of in Nigeria, West Africa, are in agreement with those of earlier workers and that from Photovoltaic Geographic Information System (PVGIS) project. The implication of this in solar photovoltaic applications has been stressed.     

Sol–gel deposition and optical characterization of multilayered SiO2/Ti1−xSixO2 coatings on solar collector glasses

Thin films of silicon oxide and silicon titanium mixed oxides are deposited on solar collector glazing in a sol–gel dip-coating process based on alcoxide precursors. Spectrophotometry is used to characterize the relation of film thickness and withdrawal speed for the precursor solutions, and to determine the refractive index of individual layers of the mixed oxides. The inferred dispersion relations n(λ) are compared to the predictions of effective medium theories. Based on the knowledge of the optical properties of individual layers, multilayer interference stacks are designed. Multilayered samples of superior quality are deposited by sol–gel dip-coating in a particle-free environment. The final optical performance of the multilayer stacks are characterized in terms of the visible reflectance R_VIS, CIE color coordinates, and the solar transmission T_sol. Values of up to 2.4 have been attained for the energy efficiency of the colored reflection M=R_VIS/(100%-T_sol). The produced coatings combine a bright colored reflection with an acceptable solar transmittance, and are thus well suited for the application in colored glazed thermal solar collectors. This novel type of colored glazing opens up new perspectives for the architectural integration of thermal solar collectors, e.g. as solar active glass facades.     

On the correlations between collector efficiency factor and material content of parallel flow flat-plate solar collectors

The collector efficiency factor F^′, besides the collector heat loss coefficient U_L, characterizes the thermal quality of a solar collector. As F^′ is strongly influenced by the tube distance w and the absorber plate thickness δ, F^′ is also correlated with the material content of absorber plus tubing. Due to the future mass production of collectors and to the restricted copper resources (in the literature, a range until 2026 is given), the role of material savings can be expected to become more and more important. This paper focuses on the correlations between F^′ and the material content of absorber and tubing for flat-plate collectors with the fin-and-tube geometry. The correlations between w, δ, F^′ and material content are presented in a new type of nomograph. This nomograph indicates the values of w and δ that minimize the material content (for a given F^′). For a typical absorber with F^′=0.90, material savings of 25% can theoretically be achieved without any deterioration of F^′, by reducing the absorber plate thickness and the tube distance. The resulting plate thickness is below 0.1 mm; the respective tube distance will be about 7 cm. Practical restrictions are discussed. In a sensitivity analysis, the influence of different parameters on F^′ is investigated. The most important parameters are w, U_L,δ and the Reynolds number. The technique chosen for contacting tube and absorber has only a minor influence on F^′.     

Silica aerogel granulate material for thermal insulation and daylighting

Silica aerogel granulate is a nanostructured material with high solar transmittance and low thermal conductivity. These properties offer exciting applications in building envelopes. One objective of the joint R&D project ISOTEG at ZAE Bayern was to develop and characterize a new glazing element based on granular silica aerogel. Heat transfer coefficients of less than 0.4 W/(m² K) and a total solar energy transmittance of 35% for the whole glazing unit were achieved. The glazing has a thickness of less than 50 mm. Another application for granular silica aerogel is, for example, in solar collectors.The thermal properties of the glazing as well as the optical and thermal properties of the granular aerogels are presented here. The solar transmittance of a 10 mm packed bed of silica aerogel was 53% for semi-translucent spheres and 88% for highly translucent granulate. In our heat transfer experiments the gas pressure, external pressure load, temperature and gas filling were varied. The various thermal conductivity values measured for the glazing and collector applications were compared to the values calculated using two different packed bed models. For the gas-dependent measurements the intergranular voids in the granulate were 1.0 ± 0.1 mm before loading the packed bed, 0.3 ± 0.1 mm at an external load of 3.2 bar (3.2 × 10⁵ Pa) and 0.6 ± 0.1 mm after release.A direct radiative conduction of λ_direct = 4.5 ± 0.5 × 10⁻³ W m⁻¹ K⁻¹ was obtained.     

Experimental testing of a box-type solar cooker using the standard procedure of cooking power

A box-type solar cooker with one (Model I) or four (Model II) cooking pots was constructed and tested under Tanta prevailing weather conditions. Experiments were performed during July 2002 using the cooker with or without load. The obtained results were employed to calculate the two figures of merit, F₁ and F₂, as well as the utilization efficiency η_u and the specific t_s and characteristic t_c boiling times. The obtained values of F₁ indicate that the cooker can be used twice a day for consecutive cooking. F₂ was found to increase almost linearly with the mass of the cooking fluid M_f. The cooker is able to boil 1 kg of water in 15 min when its aperture area equals 1 m². Furthermore, experiments also considered the requirements for the international standard test procedure for solar cookers. The cooking power P was correlated with the temperature difference ΔT between the cooking fluid and the ambient air. Linear correlations between P and ΔT had correlation coefficients higher than 0.90 satisfying the standard. The obtained values of the initial cooking power, heat loss coefficient and the cooking power at a temperature difference of 50 °C agree well with those obtained for small solar cookers. The present cooker is able to cook most kinds of food with an overall utilization efficiency of 26.7%.     

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.     

A criterion study of solar irradiation patterns for the performance testing of thermosyphon solar water heaters

A Taiwan test standard was established in 1989 using outdoor daily efficiency test methods. This test standard has been implemented for 12 years with satisfactory results. However, it was also found from field applications that the pattern of solar irradiation would affect the result of the performance test. In the present study, we used a distribution factor R_i defined as the ratio of the total irradiation in the morning to that in the afternoon to characterize this effect. R_i reflects the asymmetry of solar irradiation distribution in the morning and afternoon. A field study was carried out. The data collected from the daily efficiency tests were screened using the criterion of 0.5≤R_i≤1.6, in addition to the conditions defined in the Taiwan test standard. Two commercial products separately located in latitude 23° N and 25° N were tested. Data scattering occurs without using the R_i criterion. If we adopt test data using R_i, the results turn out to have a much better data correlation coefficient, from 0.915 to 0.969. The system characteristic efficiency η_s* changes significantly, from 0.479 to 0.514. There is a regulation that the commercial product should have a value of η_s* exceeding 0.5 in order to obtain a subsidy from the government in Taiwan. The performance test using the old standard is shown to result in a significant error, suggesting modification of the former test standard.     

Direct and diffuse insolation using approximation methods applied to horizontal surface insolation

Approximation methods estimate the A, B and C required to calculate the direct and diffuse solar insolation by the ASHRAE equations. Theoretical, experimental and meteorological records of horizontal surface insolation under clear sky conditions are used to study the behavior of the approximation methods in solving the equation I_H exp (BM) = A(sin α + C). Instantaneous data read from the chart trace of horizontal surface insolation gave better results than the cumulative horizontal surface radiation measured from solar noon. With careful selection of data, a probable error of ±5 per cent for A and ±10 per cent for B is not unreasonable.     

Design and test of non-evacuated solar collectors with compound parabolic concentrators

The intermediate range of concentration ratios (1.5X–10X) which can be achieved with CPCs without diurnal tracking provides both economic and thermal advantages for solar collector design even when used with non-evacuated absorbers. The present paper summarizes more than 3 yr of research on non-evacuated CPCs and reviews measured performance data and critical design considerations. Concentrations in the upper portions of the practical range (e.g. 6X) can provide good efficiency (40–50 per cent) in the 100–160°C temperature range with relatively frequent tilt adjustments (12–20 times per year). At lower concentrations (e.g. 3X) performance will still be substantially better than that for a double glazed flat plate collector above about 70°C and competitive below, while requiring only semi-annual adjustments for year round operation. In both cases the cost savings associated with inexpensive reflectors, and the optimal coupling to smaller, simple inexpensive absorbers (e.g. tubes, fins, etc.) can be as important an advantage as the improved thermal performance.The design problems for non-evacuated CPC collectors are entirely different from those for CPC collectors with evacuated receivers. For example, heat loss through the reflector can become critical, since ideal CPC optics demands that the reflector extend all the way to the absorber. Recent improvements in reflector surfaces and low cost antireflection coatings have made practical a double-glazed non-evacuated CPC design. It is calculated that a 1.5X version of such a collector would have an optical efficiency η_o = 0.71, a heat loss coefficient U = 2.2 W/m²°C and a heat extraction effciency factor F′ ≥ 0.98, while requiring no tilt adjustments.     

An autonomous solar ammonia-water refrigeration system

Solar refrigeration is especially attractive in isolated regions. The Danube Delta needs ice for fish preservation. This paper describes an intermittent single-stage H₂O---NH₃ solar absorption system of 46 MJ/cycle. Solar collectors heat the generator. Installation details and experimental results are presented. The system coefficient of performance (COP)_system varies between 0.152 and 0.09 in the period of May–September. Solar radiation availability and the theoretical (COP), also applicable to the Trombe-Foex system, are assessed. Reference is made to evacuated solar collectors with selective surfaces. Actual (COP)_system values of 0.25–0.30 can be achieved at generation and condensation temperatures of 80°C and 24.3°C respectively. For bigger capacities of 450–675 MJ/day, the pay-off period is estimated to be 6 and 4 years respectively and the life-time to 15–18 years.     

Thermal analysis of a flat-plate collector in multiphase flows, including superheat

A thermal analysis of the performance of a solar flat-plate collector operating in nonboiling, boiling, and superheated regimes is presented. The performance of the collector under these single and multiphase conditions is governed by the axial fractional channel lengths of the subcooled (nonboiling) and the superheated regions. The overall thermal loss coefficient, the dimensionless capacitance rate, and collector efficiency factors for various collector operating regions are defined. A new “Generalized Heat Removal Factor,” F_s, and a new overall thermal loss coefficient, U_L, for flat-plate collectors under any operation mode are developed. The thermal efficiency a flat-plate collector, whether under nonboiling, boiling, or superheated conditions, is evaluated using, F_s and U_L. It is shown that the value of F_s decreases and the value of U_L increases as the degree of superheat increases. Current applications of flat-plate collectors having multiphase flows are represented by those charged with refrigerants.     

Optimization of process parameters in a large-area hot-wire CVD reactor for the deposition of amorphous silicon (a-Si:H) for solar cell application with highly uniform material quality

Scale-up of a-Si:H-based thin film applications such as solar cells, entirely or partly prepared by hot-wire chemical vapor deposition (HWCVD), requires research on the deposition process in a large-area HWCVD system. The influence of gas supply and filament geometry on thickness uniformity has already been reported, but their influence on material quality is systematically studied for the first time. The optimization of deposition parameters for obtaining best material quality in our large-area HWCVD system resulted in an optimum filament temperature, T_fil≈1600°C, pressure, p=8 mTorr and silane flow, F(SiH₄)=100 sccm, keeping the substrate temperature at T_S=200°C. A special gas supply (gas shower with tiny holes of uniform size) and a filament grid, consisting of six filaments with an interfilament distance, d_fil=4 cm were used. The optimum filament-to-substrate distance was found to be d_fil–S=8.4 cm. While studying the influence of different d_fil and gas supply configurations on the material quality, the above-mentioned setup and parameters yield best results for both uniformity and material quality. With the setup mentioned, we could achieve device quality a-Si:H films with a thickness uniformity of ±2.5% on a circular area of 20 cm in diameter. The material, grown at a deposition rate of r_d≈4 Å/s, was characterized on nine positions of the 30 cm×30 cm substrate area, and revealed reasonable uniformity of the opto-electronic properties, e.g photosensitivity, σ_Ph/σ_D=(2.46±0.7)×10⁵, microstructure factor, R=0.17±0.05, defect densities, N_d(PDS)=(2.06±0.6)×10¹⁷ cm⁻³ and N_d(CPM)=(2.05±0.5)×10¹⁶ cm⁻³ (film properties are given as mean values and standard deviations). Finally, we fabricated pin solar cells, with the i-layer deposited on small-area p-substrates distributed over an area of 20 cm×20 cm in this large-area deposition system, and achieved high uniformity of the cell parameters with initial efficiencies of η=(6.1±0.2)% on the 20 cm×20 cm area.     

Screen printed conductive CuS-poly(acrylic acid) composite coatings

Copper sulfide (CuS) powder precipitated from two different chemical baths (citrate and triethanolamine baths) was dispersed in a poly(acrylic acid) aqueous solution and the resulting mixtures were screen-printed over glass slides. Compared to the coatings of the same composite materials obtained by casting, the screen-printed coatings show better homogeneity and an improved thermal stability (up to 300°C). Sheet resistances of 50 Ω/□ were typical in 20 μm thick coatings and these values remained stable even after baking in nitrogen or vacuum at 200–300°C. The X-ray diffraction pattern indicates degradation of CuS to Cu_1.8S and Cu_1.96S, particularly in air-baked samples at 250–300°C. The XRD, TGA and R_□ results show CuS(Cit)–PAA samples as superior conductive coatings.