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.     

On the effects of solar radiation variations on solar heating system performances

The effect of solar radiation availability on the performance of different solar heating systems has been studied. The systems include a solar water heater, passive solar houses and district solar heating systems with seasonal heat storage. Also, different collector orientations and collector types have been investigated. The hourly radiation data were generated by a simple computational simulation procedure. It was found that district solar heating systems with concentrating collectors and passive solar houses showed the largest variations for the given conditions.     

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.     

Estimation of monthly solar radiation distribution for solar energy system analysis

The concept of probability density frequency, which is successfully used for analyses of wind speed and outdoor temperature distributions, is now modified and proposed for estimating solar radiation distributions for design and analysis of solar energy systems. In this study, global solar radiation distribution is comprehensively analyzed for photovoltaic (PV) panel and thermal collector systems. In this regard, a case study is conducted with actual global solar irradiation data of the last 15 years recorded by the Turkish State Meteorological Service. It is found that intensity of global solar irradiance greatly affects energy and exergy efficiencies and hence the performance of collectors.     

Performance of water-in-glass evacuated tube solar water heaters

The performance of water-in-glass evacuated tube solar water heaters is evaluated using experimental measurements of optical and heat loss characteristics and a simulation model of the thermosyphon circulation in single-ended tubes. The performance of water-in-glass evacuated tube solar collector systems are compared with flat plate solar collectors in a range of locations. The performance of a typical 30 tube evacuated tube array was found to be lower than a typical 2 panel flat plate array for domestic water heating in Sydney.     

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.     

The potential applications and advantages of powering solar air-conditioning systems using concentrator augmented solar collectors

Non-concentrated evacuated tube heat pipe solar collectors have been reported to show higher fluid temperatures with improved thermal performance in the low to medium temperature range (?60 °C) due to low heat losses but suffer higher heat losses at the medium to higher temperature range (?80 °C) which reduces their efficiency compared to concentrated evacuated tube heat pipe solar collectors. To operate as stand-alone systems capable of attaining temperatures in the range of 70-120 °C, an innovative concentrator augmented solar collector can be an attractive option. The performance of a combined low-concentrator augmented solar collector in an array of evacuated tube heat pipe solar collectors defined as concentrator augmented evacuated tube heat pipe array (CAETHPA) and an array of evacuated tube heat pipe collectors (ETHPC) were tested and compared and results presented in this paper. The analysis of the experimental data allows concluding that the use of a CAETHPA is a more efficient alternative for integrating renewable energy into buildings with higher fluid temperature response, energy collection and lower heat loss coefficient compared to the use of evacuated tube heat pipe collector array (ETHPA).     

Optimal tilt-angles of all-glass evacuated tube solar collectors

In this paper, a detailed mathematical procedure is developed to estimate daily collectible radiation on single tube of all-glass evacuated tube solar collectors based on solar geometry, knowledge of two-dimensional radiation transfer. Results shows that the annual collectible radiation on a tube is affected by many factors such as collector type, central distance between tubes, size of solar tubes, tilt and azimuth angles, use of diffuse flat reflector (DFR, in short); For collectors with identical parameters, T-type collectors (collectors with solar tubes tilt-arranged) annually collect slightly more radiation than H-type collectors (those with solar tubes horizontally arranged) do. The use of DFR can significantly improve the energy collection of collectors. Unlike the flat-plate collectors, all-glass evacuated tube solar collectors should be generally mounted with a tilt-angle less than the site latitude in order to maximize the annual energy collection. For most areas with the site latitude larger than 30° in China, T-type collectors should be installed with a tilt-angle about 10° less than the site latitude, whereas for H-type collectors without DFR, the reasonable tilt-angle should be about 20° less than the site latitude. Effects of some parameters on the annual collectible radiation on the collectors are also presented.     

A comparison between state-of-the-art and neural network modelling of solar collectors

The state-of-the-art modelling of solar collectors as described in the European Standard EN 12975-2 is based on equations describing the thermal behaviour of the collectors by characterising the physical phenomena, e.g. transmission of irradiance through transparent covers, absorption of irradiance by the absorber, temperature dependent heat losses and others. This approach leads to so called collector parameters that describe these phenomena, e.g. the zero-loss collector efficiency η₀ or the heat loss coefficients a₁ and a₂.Although the state-of-the-art approach in collector modelling and testing fits most of the collector types very well there are some collector designs (e.g. “Sydney” tubes using heat pipes and “water-in-glass” collectors) which cannot be modelled with the same accuracy than conventional collectors like flat plate or standard evacuated tubular collectors. The artificial neural network (ANN) approach could be an appropriate alternative to overcome this drawback.To compare the different approaches of modelling investigations for a conventional flat plate collector and an evacuated “Sydney” tubular collector have been carried out based on performance measurements according to the European Standard EN 12975-2. The investigations include the parameter identification (training), the comparisons between measured and modelled collector output and the simulated yearly collector yield for a solar domestic hot water system for both models.The obtained results show better agreement between measured and calculated collector output for the artificial neural network approach compared with the state-of-the-art modelling. The investigations also show that for the ANN approach special test sequences have to be designed and that the determination of the ANN that fits the thermal performance of the collector in the best way depends significantly on the expertise of the user.Nevertheless artificial neural networks have the potential to become an interesting alternative to the state-of-the-art collector models used today.     

Long term performance of evacuated tubular solar water heaters in Sydney, Australia

The performance of a domestic hot water system employing evacuated tubular collectors is compared with two others employing flat plate collectors over a period of one year. The efficiency of the evacuated tubular collector system was about 1.8 times that of a non selective flat plate system and about 1.3 times that of a high quality selective flat plate system. The superior performance of the evacuated system is explained in terms of the distribution of incident energy as a function of (ΔT/G) for domestic systems and the normal incidence collector efficiency curves.     

Advanced tubular solar energy collector— A state of the art

In this article, the state of the art of the evacuated tubular solar energy collector has been discussed. Evacuated collectors are one of the most promising alternatives for industrial process heating, air-conditioning and refrigeration, pumping of water, production of electric energy, etc., because of its high efficiency at sufficiently high temperatures and no tracking requirements. Here, the various designs, and special features which have distinguished it from conventional flat plate collectors have been high-lighted. An attempt has also been made to analyse the optical and thermal behaviour of this evacuated collector.     

Simulation and optimization of a LiBr solar absorption cooling system with evacuated tube collectors

Solar radiation is a clean form of energy, which is required for almost all natural processes on earth. Solar-powered air-conditioning has many advantages when compared to a conventional electrical system. This paper presents a solar cooling system that has been designed for Malaysia and similar tropical regions using evacuated tube solar collectors and LiBr absorption unit. The modeling and simulation of the absorption solar cooling system is carried out with TRNSYS program. The typical meteorological year file containing the weather parameters for Malaysia is used to simulate the system. The results presented show that the system is in phase with the weather, i.e. the cooling demand is large during periods that the solar radiation is high. In order to achieve continuous operation and increase the reliability of the system, a 0.8 m³ hot water storage tank is essential. The optimum system for Malaysia's climate for a 3.5 kW (1 refrigeration ton) system consists of 35 m² evacuated tubes solar collector sloped at 20°.     

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.     

The assessment of different models to predict the global solar radiation on a surface tilted to the south

Global and diffuse solar radiation intensities are, in general, measured on horizontal surfaces, whereas stationary solar conversion systems (both flat plate solar collector and PV) are tilted towards the sun in order to maximize the amount of solar radiation incident on the collector surface. Consequently, the solar radiation incident on a tilted surface must be determined by converting the solar radiation intensities measured on a horizontal surface to that incident on the tilted surface of interest. There exist a large number of models designed to perform such a conversion. 11 such models have been tested utilizing data measured in Beer Sheva, Israel. The data consist of hourly global and diffuse solar radiation on a horizontal surface, normal incidence beam and global radiation on a south-oriented surface tilted at 40°. The horizontal diffuse radiation measured using a shadow ring was corrected using four different correction models. This resulted in 44 model permutations. The individual model performance is assessed by an inter-comparison between the calculated and measured solar global radiation on the south-oriented surface tilted at 40° using both graphical and statistical methods. The relative performance of the different models under different sky conditions has been studied. Different grading systems have been applied in an attempt to score the relative performance of the models.     

The influence of climate and location on collector performance

The influence of annual climate variations on the performance of solar thermal collectors in the northern part of Europe has been investigated. The annual solar collector energy output has been calculated with the MINSUN simulation program using hourly, measured climatic data for the years 1983–98 for three cities situated in the south (Lund), central (Stockholm) and north (Luleå) of Sweden. A synthetic year created with the Meteonorm weather simulation program was also used in the simulations. Two solar thermal collectors were modelled: a flat plate solar collector and a tubular vacuum collector, both of commercial standard.The thermal energy output is strongly correlated to the annual global irradiation at a horizontal surface. The annual average energy delivered from the flat plate collector was 337 kWh/m² for Stockholm (337 for Lund and 298 for Luleå), and from the vacuum tube collector 668 kWh/m² for Stockholm (675 for Lund and 631 for Luleå) at an operating temperature of T=50°C. Maximum deviations from the average value for this 16-year period are around 20% for the flat plate and 15% for the vacuum tube collector, at T=50°C.The relation between global irradiation on a horizontal surface and the annually collected thermal energy at a constant operating temperature could be fitted to a linear equation: q_u=aG(0°)+bT, where q_u is the energy output from the collector, G(0°) the global irradiation at a horizontal surface, T the average temperature of the collector fluid, and a and b fitting parameters in a double linear regression analysis.     

直通式真空管空气集热器热性能实验及干燥应用

在直通式太阳能玻璃真空管空气集热器基础上改进联箱结构,并搭建测试平台对该种改进型空气集热器进行热性能实验研究。通过实验比较改进前后集热器的温升和效率,获得改进后集热器出口温度与太阳辐照度关系的线性回归方程,掌握不同空气质量流量对集热器出口温度和集热效率的影响规律,分析得到该种真空管的最佳串联个数,并对应用该种集热器的太阳能干燥系统的干燥效果进行初步测试分析。该研究结果可为太阳能空气集热干燥系统的设计及应用提供参考。     

Performance evaluation of a low-temperature solar Rankine cycle system utilizing R245fa

A low-temperature solar Rankine system utilizing R245fa as the working fluid is proposed and an experimental system is designed, constructed and tested. Both the evacuated solar collectors and the flat plate solar collectors are used in the experimental system, meanwhile, a rolling-piston R245fa expander is also mounted in the system. The new designed R245fa expander works stably in the experiment, with an average expansion power output of 1.73 kW and an average isentropic efficiency of 45.2%. The overall power generation efficiency estimated is 4.2%, when the evacuated solar collector is utilized in the system, and with the condition of flat plate solar collector, it is about 3.2%. The experimental results show that using R245fa as working fluid in the low-temperature solar power Rankine cycle system is feasible and the performance is acceptable.     

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.     

A device for measuring the angular distribution of incident radiation on tubular solar collectors

The calculation of the incident solar radiation falling upon tubular collectors presents difficulties which do not exist when this is carried out for flat plate collectors. Due to their cylindrical shape, the solar incident radiation on their surface is a function of angle and time. This paper presents experimental results which were obtained by means of a device specially designed in order to measure the distribution of such radiation. The device's sensor element (a photodiode) was characterised, calibrated and subsequently used to measure the angular distribution of radiation incident on the inside surface of the inner tube of an evacuated solar collector. It was found that the experimentally measured distribution agrees with that obtained by the ray-tracing method which appears in the literature.