Search

Theoretical and experimental investigations of the flow and temperature distribution in a 12.53 [m.sup.2] solar collector panel with an absorber consisting of two vertical manifolds interconnected by 16 parallel horizontal fins have been carried out. The investigations are focused on overheating and boiling problems in the collector panel. Single-phase liquid flow and heat transfer in the collector panel are studied by means of computational fuid dynamics (CFD) calculations. Differently designed collectors are investigated with different collector fluid volume flow rates. The effect of friction and the influence of the buoyancy effects are considered in the investigations. Further, experimental investigations of the solar collector panel are carried out. The flow distribution through the absorber is evaluated by means of temperature measurements on the back of the absorber tubes. The measured temperatures are compared to the temperatures determined by the CFD model and there is a good agreement between the measured and calculated temperatures. Calculations with the CFD model elucidate the flow and temperature distribution in the collector. The influences of collector fluid flow rate and inlet temperature on the flow and temperature distribution are shown. The flow distribution through the absorber tubes is uniform if a high flow rate of 10.0 l/min is used. By decreased collector fluid flow rate and by increased collector fluid inlet temperature, the flow distribution gets less uniform due to the influence of buoyancy force. If the collector fluid flow rate is small and the collector fluid inlet temperature is high enough, severe nonuniform flow distribution may happen with a small flow rate or even zero or reverse flow in the upper horizontal strips, resulting in overheating or boiling problems in the strips. The CFD calculations elucidate the flow and temperature distribution in the collector panels of different designs. Based on the investigations, recommendations are given in order to avoid overheating or boiling problems in the solar collector panel. [DOI: 10.1115/1.2840611] Keywords: flat-plate solar collector, buoyancy effects, flow distribution, computational fluid dynamics (CFD)

The objective of this work is to theoretically and experimentally investigate the flow and temperature distribution in a solar collector panel with an absorber consisting of horizontally inclined strips. Fluid flow and heat transfer in the collector panel are studied by means of computational fluid dynamics (CFD) calculations. Further, experimental investigations of a 12.5 [m.sup.2] solar collector panel with 16 parallel connected horizontal fins are carried out. The flow distribution through the absorber is evaluated by means of temperature measurements on the backside of the absorber tubes. The measured temperatures are compared to the temperatures determined by the CFD model and there is a good similarity between the measured and calculated results. Calculations with the CFD model elucidate the flow and temperature distribution in the collector. The influences of different operating conditions such as flow rate, properties of solar collector fluid, solar collector fluid inlet temperature and collector tilt angle are shown. The flow distribution through the absorber fins is uniform if high flow rates are used. By decreased flow rate and decreased content of glycol in the glycol/water mixture used as solar collector fluid, and by increased collector tilt and inlet temperature, the flow distribution gets worse resulting in an increased risk of boiling in the upper part of the collector panel. Keywords: Solar collector; Flow distribution; Computational fluid dynamics (CFD); Buoyancy effects

The thermal performance of solar heating systems is strongly influenced by the thermal stratification in the heat storage. The higher the degree of thermal stratification is, the higher the thermal performance of the solar heating systems. Thermal stratification in water storage can for instance be achieved by use of inlet stratifiers combined with low flow operation in the solar collector loop. In this paper, investigation of a number of different fabric stratification pipes is presented and compared to a non-flexible inlet stratifier. Additional, detailed investigation of the flow structure close to two fabric stratification pipes is presented for one set of operating conditions by means of the optical PIV (Particle Image Velocimetry) method. Keywords: Solar heating systems; Inlet stratifiers; PIV measurements

Heat transfer and flow structures inside all glass evacuated tubular collectors for different operating conditions are investigated by means of computational fluid dynamics. The investigations are based on a collector design with horizontal tubes connected to a vertical manifold channel. Three different tube lengths varying from 0.59 m to 1.47 m have been modelled with five different inlet mass flow rates varying from 0.05 kg/min to 10 kg/min with a constant inlet temperature of 333 K. Under these operating conditions the results showed that: * the collector with the shortest tube length achieved the highest efficiency, * the optimal inlet flow rate was around 0.4-1 kg/min, * the flow structures in the glass tubes were relatively uninfluenced by the inlet flow rate, Generally, the results showed only small variations in the efficiencies. This indicates that the collector design is well working for most operating conditions. Keywords: All glass evacuated tubular collector; CFD calculations; Flow structure

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.solener.2006.03.007 Byline: Alexander Thur, Simon Furbo, Louise Jivan Shah Keywords: Solar heating system; Energy saving; System efficiency; Solar domestic hot water system Abstract: In this paper the realistic behaviour and efficiency of heating systems were analysed, based on long term monitoring projects. Based on the measurements a boiler model used to calculate the boiler efficiency on a monthly basis was evaluated. Comparisons of measured and calculated fuel consumptions showed a good degree of similarity. With the boiler model, various simulations of solar domestic hot water heating systems were done for different hot water demands and collector sizes. The result shows that the potential of fuel reduction can be much higher than the solar gain of the solar thermal system. For some conditions the fuel reduction can be up to the double of the solar gain due to a strong increase of the system efficiency. As the monitored boilers were not older than 3 years, it can be assumed that the saving potential with older boilers could be even higher than calculated in this paper. Author Affiliation: Department of Civil Engineering, Technical University of Denmark, Building 118, DK-2800 Kgs. Lyngby, Denmark Article History: Received 15 November 2004; Revised 2 March 2006; Accepted 20 March 2006 Article Note: (miscellaneous) Communicated by: Associate Editor Klaus Vajen

The thermal advantages by utilizing discharge from different levels in solar storage tanks are investigated, both for a small SDHW system and for a solar combisystem. The investigations showed that it is possible to increase the thermal performance of both types of systems by using two draw-off levels from the solar tanks instead of one draw-off level at a fixed position. The best position of the second draw-off level is in the middle or just above the middle of the tank. For the investigated small SDHW system with a realistic draw off hot water temperature of 40[degrees]C and 45[degrees]C and an auxiliary volume temperature of 50.5[degrees]C the increase of the thermal performance by the second draw-off level is about 6%. For the investigated solar combisystem the increase in thermal performance by using one extra draw-off level, either for the domestic hot water heat exchanger or for the heating system, is about 3%, while an improvement of about 5% is possible by using a second draw-off level both for the domestic hot water heat exchanger and for the heating system. Keywords: SDHW systems; Solar combisystems; Heat storage tanks; Thermal stratification; Advanced discharge strategies

A new TRNSYS collector model for evacuated tubular collectors with tubular absorbers is developed. Traditional flat plate collector performance equations have been integrated over the whole absorber circumference. On each tube the model determines the size and position of the shadows caused by the neighbor tube. An all glass tubular collector with tubular absorbers with 14 tubes connected in parallel is investigated theoretically with the model and experimentally in an outdoor collector test facility. Performance calculations with the new model are compared with measured results and a good degree of similarity between the measured and calculated results is found. Further, it is illustrated how the model can be used for geometrical parameter studies both for constant collector mean fluid temperatures and for varying temperature conditions in a solar heating plant. These investigations are performed for two climates: Copenhagen (Denmark) and Uummannaq (Greenland). [DOI: 10.1115/1.1878872]

In year 2000 a 336 [m.sup.2] solar domestic hot water system was built in Sundparken, Elsinore, Denmark. The solar heating system is a low flow system with a 10000 l hot-water rank. Due to the orientation of the buildings half of the solar collectors are facing east, half of the solar collectors are facing west. The collector tilt is 15[degrees] from horizontal for all collectors. Both the east-facing and the west-facing collectors have their own solar collector loop, circulation pump, external heat exchanger and control system. The external heat exchangers are used to transfer the heat from the solar collector fluid to the domestic water. The domestic water is pumped from the bottom of the hot-water tank to the heat exchanger and back to the hot-water tank through stratification inlet pipes. The return flow from the DHW circulation pipe also enters the tank through stratification inlet pipes. The tank design ensures an excellent thermal stratification in the tank. The solar heating system was installed in May 2000. The thermal performance of the solar heating system has been measured in the first two years of operation. Compared to other large Danish solar domestic hot water systems the system is performing well in spite of the fact that the solar collectors are far from being orientated optimally. The utilization of the solar radiation on the collectors is higher, 46% in the second year of operation, than for any other system earlier investigated in Denmark, 16%-34%. The reason for the good thermal performance and for the excellent utilization of the solar radiation is the high hot-water consumption and the good system design making use of external heat exchangers and stratification inlet pipes. [DOI: 10.1115/1.1767190]

Storage tanks with different cold water inlet devices for small Solar Domestic Hot Water (SDHW) systems are compared. The objective of the investigation is to reveal the impact of the cold water inlet device on the thermal stratification in two marketed tanks and to evaluate the possible enhancement in the annual system performance of small solar heating systems. Two different marketed inlet designs are compared, one connected to a small curved plate placed above the inlet tube, the other one connected to a much larger flat plate. The cold domestic water enters the stores in vertical direction from the bottom of the tanks. Temperature measurements were carried out for different operating conditions. It was shown that the thermal stratification inside the two tanks depends differently on the flow rate, the draw-off volume, as well as the initial temperature in the storage tank. To carry out system simulations, a multi-node storage model was used and expanded by an additional input variable to model the mixing behaviour depending on the operating conditions. The inlet device with a comparatively large plate compared to the less favourable design results in an increase of the solar fraction of about 1-3%-points in annual system simulations with a solar fraction of about 60% and fairly large domestic hot water flow rates. This corresponds to a reduction of the auxiliary energy supply of the solar heating system of about 3-7% (58-155 MJ/year) for the investigated solar domestic hot water system. Keywords: Storage tank; Thermal stratification; Draw-off; Inlet device; Mixing; Domestic hot water; Pressure drop; Richardson number; TRNSYS simulations; Performance reduction rate

Investigation of small SDHW systems based on smart solar tanks are presented. The domestic water in a smart solar tank can be heated both by solar collectors and by means of an auxiliary energy supply system. The auxiliary energy supply system in this study electric heating elements heats up the hot-water tank from the top and the water volume heated by the auxiliary energy supply system is fitted to the hot-water consumption and consumption pattern. In periods with a large hot-water demand, the volume is large; in periods with a small hot-water demand, the volume is small. Two small SDHW systems, based on differently designed smart solar tanks and a traditional SDHW system were investigated by means of laboratory experiments and theoretical calculations. The investigations showed that the yearly thermal performance of SDHW systems with smart solar tanks is 5-35% higher than the thermal performance of traditional SDHW systems. Estimates indicate that the performance/cost ratio can be improved by up to 25% by using a smart solar tank instead of a traditional tank when the backup energy system is electric heating elements. Further, smart solar tanks are suitable for unknown, variable, large or small hot-water consumption and the risk of oversized solar heating systems and oversized tank volumes is reduced by using smart solar tanks. Based on the investigations it is recommended to start development of smart solar tank units with an oil-fired boiler or a natural gas burner as auxiliary energy supply system. Keywords: SDHW systems; Smart solar tanks; Smart control strategy; Experiments; Calculations

A theoretical and experimental analysis of water jets entering a solar storage tank is performed. CFD calculations of three inlet designs with different inlet flow rates were carried out to illustrate the varying behaviour of the thermal conditions in a solar store. The results showed the impact of the inlet design on the flow patterns in the tank and thus how the energy quality in a hot water tank is reduced with a poor inlet design. The numerical investigations were followed by experiments. A test solar store, similar to the store investigated by numerical modelling was constructed with cylindrical transparent walls so that the flow structures due to the inlet jets could be visualized. With the three inlets, nine draw-off tests with different inlet flow rates were carried out and the temperature stratification in the tank was measured during the draw-offs. The experimental results were used in an analysis using the first and second law of thermodynamics. The results showed how the entropy changes and the exergy changes in the storage during the drawoffs influenced by the Richardson number, the volume draw-off and the initial tank conditions.

Within the project PowerUp MyHouse (Project No: 2020-1-TR01-KA202-093467) partners from Turkey, Portugal, Sweden, Lithuania, and Denmark cooperate on preparation of teaching materials on PVT systems. The project aims to develop the know-how about PV/T systems in design aspects by comparing different climate conditions and to increase the vocational skills and knowledge on PV/T systems of renewable energy vocational teachers/students. This document summarizes all stages of the project and the final conclusions of all participants. [ABSTRACT FROM AUTHOR]