Insulated solar storage tanks

This paper presents the theoretical and experimental investigation of an insulated parallelepiped, outdoor solar, water-filled storage tank of size 1 m × 0.5 m × 0.3 m, that is made from galvanized iron. The absorption coefficient of the insulating material has been determined. The effects of plastic covers and insulation thickness on the water temperature and the energy gained or lost by water are investigated. Moreover, the effects of insulation thickness on the temperature profiles of the insulating material are discussed. The results show that the absorption coefficient decreases as the insulation thickness increases. Also, it is found that the glass wool insulation of 2.5 cm thickness has the best results compared with the other thicknesses (5 cm, 7.5 cm, and 10 cm) as far as the water temperature and the energy gained by water are concerned.     

Large DHW solar systems with distributed storage tanks

The thermal behaviour of a central DHW solar system, the design of which is based on a new Central Collection-Separate Storing (CCSS) approach, has been investigated theoretically. The common practice for large DHW solar systems, of employing a central storage and delivery facility, has been shown in the past to exhibit a rather poor performance and considerable heat losses. This is due to the extensive lengths of pipework required for both the transfer of solar energy and the delivery of hot water. The CCSS solar system presented can overcome the above problems by employing separate storage tanks for each family, thus being best suited for multistory buildings. The simulation analysis has revealed a number of interesting features for the system performance: (i) the collected energy is distributed to all users in a fair manner, irrespective of their distance from the collector field and the daily hot water consumption profiles; (ii) an energy saving behaviour is most likely to evolve by most users, since the auxiliary energy consumptions are charged individually (unlike in large DHW solar systems with central water storage and delivery); and (iii) high values of solar fractions, comparable with those attained by thermosiphon systems, have been derived.     

Analysis of outdoor solar storage tanks having translucent insulation

A heat transfer model of a parallelepiped tank, full of water and covered with translucent insulation of diffuse material to solar energy, is developed considering the multiple absorptions and reflections to evaluate the heat gain or loss by water. The effect of the optical properties and thickness of insulation on that heat gained or lost by water is investigated. The results of a comparative study show that translucent insulation is more effective than opaque insulation and no insulation as far as the energy gained by water is concerned for outdoor solar storage tanks.     

Performance improvement by discharge from different levels in solar storage tanks

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 °C and 45 °C and an auxiliary volume temperature of 50.5 °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.     

New computation method for stratification pipes of solar storage tanks

The efficiency of low-flow solar systems is strongly influenced by the quality of the thermal stratification in the storage tank. The better a thermal stratification can be generated and maintained, the higher can be the yield of the solar system. Fluid mechanical charge systems are often used for this purpose, which cause, however, undesirable sucking effects. Therefore, the knowledge of the appearing fluid flows as well as the knowledge of the consequences of constructive changes are very important for the design of such charge systems. However, simulations with CFD (Computational Fluid Dynamics) are very costly and time-consuming. In this article a new and much simpler computation method is introduced making the determination of the individual fluid flows and the estimation of the effects of constructive changes possible. The computations can be carried out within short time. The comparison with CFD gives a qualitatively good agreement for a simple charge system. The results of a constructive modification of the charge system reducing the sucking effect are discussed. The remaining quantitative differences result from the discrepancies between the non-ideal behaviour of the real fluid and the model assumptions and point out improvement potentials.     

Thermal stratification in small solar domestic storage tanks caused by draw-offs

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.     

Diffuser design influence on the performance of solar thermal storage tanks

The effect of the inlet and outlet diffuser design on the performance of thermal stratification in a vertical water tank is investigated experimentally. Two sets of diffusers are used in the experiments, which are conducted with a moving thermocline (both up and down) for different flow rates. The results indicate that the preservation of the initial thermocline is excellent when using a settling mesh. It is also shown that the extraction efficiency of the tank is higher at low flow rates during charging, whereas it is lower at low flow rates during discharging.     

Analysis of solar aided heat pump systems with seasonal thermal energy storage in surface tanks

Annual periodic performance of a solar assisted ground-coupled heat pump space heating system with seasonal energy storage in a hemispherical surface tank is investigated using analytical and computational methods. The system investigated employs solar energy collection and dumping into a seasonal surface tank throughout the whole year with extraction of thermal energy from the tank for space heating during the winter season. A computational model is presented in this study for the prediction of the annual periodic transient behaviour of the system under investigation. The present computational model is based on a hybrid analytical–numerical procedure which facilitates determination of the annual variation of water temperature in the surface tank, the amounts of solar thermal energy collected during each month and the annual periodic performance of the solar aided space heating system.     

Optimization of hydrogen storage in metal-hydride tanks

The storage time of hydrogen in metal-hydride tanks (MHTs for short) is strongly influenced by the rate at which heat can be removed from the reaction bed. In the present work a two-dimensional mathematical model is developed and validated against experimental results. This model is used, first, to evaluate the impact of the tank wall thermal mass on the hydriding process. Walls in steel and in brass are tested and the obtained results show that there is no significant effect on hydrogen storage time. Then, the established model is used to study the dynamic behaviour inside various designs of MHTs: i) a cylindrical tank, ii) a cylindrical tank with external fins, iii) a cylindrical tank with a concentric tube filled with flowing cooling fluid and iv) a cylindrical tank with a concentric tube equipped with fins. Optimization results indicate that almost 80% improvement of the storage time can be achieved over the case where the tank is not optimized.     

Modeling of the warm water displacement from stratified tanks of integrated collector storage solar water heaters

The numerical modeling of warm water displacement from an inclined tank of an integrated collector storage solar water heater was performed, connection schemes of two heaters and the location of a supply and an extraction pipes were compared. It was shown that the parallel connection of heaters provides the stability of the stratification and a higher heat pickup than the series one, and the diagonal layout of the pipes in a stratified tank is preferable. The picture of thermocline dissipation was studied and the strong influence of the displacing fluid temperature on the degree of stratification was shown.     

太阳能供暖二级水箱变容积蓄热系统能量及?分析

太阳能供暖系统中的固定容积单水箱蓄热系统,在太阳能波动供给和建筑热负荷波动需求之间存在不匹配及灵活性不足的问题。为更高效地利用太阳能,本文对二级水箱温度分层变容积蓄热太阳能供暖系统建立了MATLAB数学模型,包括集热循环、充热循环、取热循环和供热循环四部分及相应的控制策略,并运用Trnsys进行了模型验证。提出了在某时间段内,实际参与充热、取热或同时充热与取热的水箱体积为有效蓄热体积的概念。定义了集热比、有效蓄热体积平均温度、水箱热量取充比和热损比等参数对系统进行了分析与评价。研究表明:与传统的太阳能供暖固定容积单水箱温度分层蓄热系统相比,在整个供暖季,二级水箱变容积蓄热系统的热损失减少了17.2%,取充比增加了6.3%,?效率提高了6.6%,辅热能耗减少了9.5%;在供暖初期,二级水箱变容积蓄热系统的水箱温度响应时间缩短了54.9%,可更灵活快速地用于供热。二级水箱变容积蓄热系统有利于调节供暖季不同时期的供需匹配,具有良好的节能效果,可进一步为太阳能供暖系统的设计与应用提供指导。     

Smart solar tanks for small solar domestic hot water systems

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.     

Multilayer fabric stratification pipes for solar tanks

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.     

Discharge mode for encapsulated PCMs in storage tanks

This paper is aimed at analysing the behaviour of encapsulated salt hydrates, used as latent energy storage in a heat transfer system of a domestic hot water tank. The salt is a eutectic mixture of hydrate nitrates of ammonium and magnesium, with low melting temperature, already tested for latent heat storage in domestic applications. In the discharge mode, cold water enters the tank and flows on the encapsulated melted PCM, which is cooled and solidified. In the initial condition the PCM is at its melting temperature. Suddenly its external surface is cooled to a constant temperature T₀; the duration of the solidification represents the time in which the latent heat is released to water. The discharge process of the phase change material (PCM) is analyzed analytically and its effectiveness is assessed, for constant surface temperature conditions, in three different geometrical configurations, i.e. considering the PCM encapsulated in slab, cylindrical or spherical polyethylene containers. The focus is on a model of the moving boundary within the phase-change material during the discharging mode, and the duration of the phenomenon. Results shown include transient position of the moving surface, temperature distribution, amount of solid PCM, energy released, and duration of complete solidification. The influence of the geometry and the Jacob number on the ending time of solidification is investigated. Among different geometrical configurations of the PCM, it is found that the shortest time for complete solidification is matched for small spherical capsules, with high Jacob numbers and thermal conductivity.     

Thermal stratification in ribbed liquid hydrogen storage tanks

A significant decrease in the degree of thermal stratification is demonstrated by improvising transverse wall ribs on the inner cylindrical surfaces of large liquid hydrogen storage tanks. The ribbed surfaces are modeled as fins and a conjugate transient heat transfer problem is formulated for predicting flow currents and heat transfer. Turbulent Rayleigh numbers between 1.2×10¹²$1.2\times {10}^{12}$ and 6×10¹⁶$6\times {10}^{16}$ are considered. A stratification parameter based on the moment of energy is defined to quantify the degree of stratification and this parameter is seen to be about 30% lower for the ribbed tanks. The degree of stratification is not sensitive to changes in the ratio of the rib height to the spacing between the ribs. The transient free convection is shown to be characterized by the parameter Fo×Ra^0.15$\mathrm{Fo}\times {\mathrm{Ra}}^{0.15}$. The process of stratification takes place more slowly in the ribbed tanks than in smooth-walled tanks. The free convective heat transfer coefficient for tanks having ribbed surface is also seen to be significantly lower. Incorporation of ribs over the inner surface of the insulated tanks is demonstrated to offer a simple means of reducing the stratification and boil-off losses.     

Optimal insulation of solar hot water tanks

An expression for the optimal thickness of insulation for maximum net savings (defined as the price of auxiliary energy saved minus the price of insulation) has been obtained. The optimal thicknesses and corresponding savings for popular insulations have been evaluated for Nicosia, Cyprus.     

ICS solar systems with two water tanks

Integrated collector storage (ICS) systems are compact solar water heaters, simple in construction, installation and operation. They are cheaper than flat plate thermosiphonic units, but their higher thermal losses make them suitable mainly for application in locations with favourable weather conditions. Aiming to the achievement of low system height and satisfactory water temperature stratification, new types of ICS systems with two horizontal cylindrical storage tanks, properly mounted in stationary CPC reflector troughs are suggested. The non-uniform distribution of solar radiation on the two absorbing surfaces is combined with the seasonal sun elevation, resulting to effective water heating. In addition, the inverted absorber concept design can be applied to ICS systems with two storage tanks. In this paper, we present the design and performance of double tank ICS solar systems, which are based on the combination of symmetric and asymmetric CPC reflectors with water storage tanks. The analytical equations of the collector geometry of all models are calculated with respect to the radius of the cylindrical water storage tank and the reflector rim angles. Experimental results for the variation of the water temperature inside storage tanks, the mean daily efficiency and the coefficient of thermal losses during night are given for all experimental models. The tests were performed without water draining and the results show that the double tank ICS systems are efficient in water temperature rise during day and satisfactory preservation of the hot water temperature during night, with the upper storage tank being more effective in performance in most of the studied models.     

Experimental study of temperature stratification in an integrated collector–storage solar water heater with two horizontal tanks

The effect of tank-interconnection geometry on temperature stratification in an integrated collector–storage solar water (ICSSW) heater with two horizontal cylindrical tanks has been studied. The tanks were parallel to each other, and separated horizontally and vertically, with the lower tank fitted directly below a glass cover, and half of the upper tank insulated. In addition, a truncated parabolic concentrator was fitted below the tanks, with its focal line along the axis of the upper tank. The heater was tested outdoors with the two tanks connected in parallel (P), and S1- and S2-series configurations, with and without hot water draw-off. Water temperature was monitored during solar collection and hot water draw-offs. For the heat charging process, it was found that only the lower tank exhibited temperature stratification in the P- and S1-tank modes of operation. There was satisfactory temperature stratification in both tanks in the S2-tank configuration. For the hot water draining process, the P-tank configuration exhibited some degree of stratification in both tanks. A significant loss of stratification was observed in the lower tank, with the upper tank exhibiting practical stratification, when the system was operated in the S1-tank mode. The S2-tank interconnection maintained a satisfactory degree of temperature stratification in both tanks. So, the S2-tank mode of operation was most effective in promoting practical temperature stratification in both tanks during solar collection and hot water draw-offs. Other results are presented and discussed in this paper.     

Design and performance of hot-oil storage tanks

Because the ratio of surface area to capacity decreases with increasing volume for a particular shaped hot-oil storage tank, there is a trend towards larger tanks in order to incur only relatively small heat losses per unit capacity. The present investigation suggests, for conditions encountered in the UK, that the aspect ratio (i.e. height-to-radius) for least heat losses from bare or fully insulated hot-oil tanks should be approximately 0·4. If the cylindrical walls of the tank are to be insulated, the roof remaining bare, then the optimal aspect ratio should be appropriately greater. However, when designing a tank, the running cost is only one of several considerations—although an increasingly important one as fuel costs inflate—the capital investment in the tank and site also radically affect the choice. The economically most favourable aspect ratio, i.e. that leading to the least total financial expenditure over the lifetime of the tank, is considerably in excess of the optimal aspect ratio corresponding to minimum rate of energy loss. This maximum energy thrift or minimum financial expenditure dichotomy is now serious when neither sufficient, cheap fuel nor adequate industrial investment is available.     

Heat-transfer considerations for large liquefied-natural-gas storage tanks

Natural gas is used worldwide as a practical energy source. In order to have a concentrated form of energy, natural gas is liquefied and stored under a pressure slightly above atmospheric and at a corresponding temperature just above its normal boiling point (112 K). This investigation presents a general steady-state study of the heat transfers into such a storage tank. Two mathematical models are proposed in order to help improve the thermal design process for such a tank. It is concluded that the aspect ratio (i.e. height-to-radius) of such a tank for the minimum rate of heat gains should be approximately unity. This value differs from the economically most favourable aspect ratio, i.e. that leading to the least total financial expenditure over the lifetime of the tank. However, the two models allow a complete simulation of the thermal costs. As the unit cost of fuel rises relative to other costs (e.g. for the construction of the tank), the overall most economic tank design approaches that of the optimal thermal design of tank as deduced in this investigation.