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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.solener.2009.06.019 Byline: Michel Y. Haller (a), Cynthia A. Cruickshank (b), Wolfgang Streicher (a), Stephen J. Harrison (b), Elsa Andersen (c), Simon Furbo (c) Keywords: Thermal stratification; Thermal energy storage; Solar heating systems; Thermocline Abstract: This paper reviews different methods that have been proposed to characterize thermal stratification in energy storages from a theoretical point of view. Specifically, this paper focuses on the methods that can be used to determine the ability of a storage to promote and maintain stratification during charging, storing and discharging, and represent this ability with a single numerical value in terms of a stratification efficiency for a given experiment or under given boundary conditions. Existing methods for calculating stratification efficiencies have been applied to hypothetical storage processes of charging, discharging and storing, and compared with the rate of entropy production caused by mixing calculated for the same experiments. The results depict that only one of the applied methods is in qualitative agreement with the rate of entropy production, however, none of the applied methods is in agreement with the rate of entropy production and also able to distinguish between the entropy production caused by mixing and the entropy changes due to heat losses. Author Affiliation: (a) Institute of Thermal Engineering, Graz University of Technology, Inffeldgasse 25/B, 8010 Graz, Austria (b) Mechanical and Materials Engineering, Queen's University, Kingston, Ontario, Canada (c) Department of Civil Engineering, Technical University of Denmark, Brovej, Building 118, DK-2800, Kgs. Lyngby, Denmark Article History: Received 16 June 2008; Revised 22 April 2009; Accepted 25 June 2009 Article Note: (miscellaneous) Communicated by: Associate Editor Halime Paksoy

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.solener.2008.10.009 Byline: Elsa Andersen, Simon Furbo Keywords: Measured weather data; Solar radiation variations; Solar heating systems; Solar collectors; Thermal performance; Utilized part of solar radiation Abstract: The thermal performances of solar collectors and solar combi systems with different solar fractions are studied under the influence of the Danish design reference year, DRY data file, and measured weather data from a solar radiation measurement station situated at the Technical University of Denmark in Kgs. Lyngby. The data from DRY data file are used for any location in Denmark. The thermal performances of the solar heating systems are calculated by means of validated computer models. The measured yearly solar radiation varies by approximately 23% in the period from 1990 until 2002, and the investigations show that it is not possible to predict the yearly solar radiation on a tilted surface based on the yearly global radiation. The annual thermal performance of solar combi systems cannot with reasonable approximation be fitted to a linear function of the annual total radiation on the solar collector or the annual global radiation. Solar combi systems with high efficient solar collectors are more influenced by weather variations from one year to another than systems with low efficient solar collectors. The annual thermal performance of solar collectors cannot be predicted from the global radiation, but both the annual thermal performance and the annual utilized solar energy can with a reasonable approximation be fitted to a linear function of the yearly solar radiation on the collector for both flat plate and evacuated tubular solar collectors. Also evacuated tubular solar collectors utilize less sunny years with large parts of diffuse radiation relatively better than flat plate collectors. Author Affiliation: Department of Civil Engineering, Technical University of Denmark, Building 118, DK-2800 Kgs. Lyngby, Denmark Article History: Received 24 May 2007; Revised 1 October 2008; Accepted 3 October 2008 Article Note: (miscellaneous) Communicated by: Associate Editor Yogi Goswami

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

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. Keywords: Solar heating system; Energy saving; System efficiency; Solar domestic hot water system

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

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.solener.2005.01.005 Byline: Simon Furbo, Elsa Andersen, Alexander Thur, Louise Jivan Shah, Karin Dyhr Andersen Keywords: SDHW systems; Solar combisystems; Heat storage tanks; Thermal stratification; Advanced discharge strategies Abstract: 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. Author Affiliation: Department of Civil Engineering, Technical University of Denmark, Building 118, DK-2800 Kgs. Lyngby, Denmark Article History: Received 13 July 2004; Revised 12 January 2005; Accepted 13 January 2005 Article Note: (miscellaneous) Communicated by: Associate Editor Erich Hahne

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.

Experimental and theoretical investigations of vertical mantle tanks for solar domestic hot water systems have been carried out. Differently designed mantle tanks have been evaluated in a laboratory test facility and a transient three-dimensional CFD-model of one of the mantle tanks is developed in the CFD-program CFX 4.1. The model is validated against the experimental tests and good agreement between measured and calculated results are achieved. The results from the CFD-calculations are used to illustrate the thermal behaviour and the fluid dynamics in the mantle and in the inner tank. The CFD-calculations are used to carry out a detailed analysis of the heat transfer from the solar collector fluid to the wall of the inner tank. The analysis has resulted in a local Nusselt-Rayleigh correlation for the heat transfer between the solar collector fluid and the wall of the inner tank.