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.     

Consumers’ influence on the thermal performance of small SDHW systems—Theoretical investigations

Usually when SDHW systems are tested and modeled the daily hot-water consumption and consumption pattern are constant during all days of the test period and simulation period. This is not realistic in ‘real life’. Numerical simulations with detailed simulation models have been carried out to investigate the influence on the thermal performance of different hot-water consumptions and consumption patterns. Two different small SDHW systems are taken into the calculation, a low-flow system where the heat storage consists of a mantle tank and a high flow system with an internal heat exchanger spiral in the heat storage. Two different domestic hot water (DHW)-load profiles have been taken into the calculation. One of the DHW-load profiles has three draw-offs at equal time, size and duration every day while the other DHW-load profile is a realistic profile for a Danish family where the hot-water consumption and consumption pattern vary from day to day and furthermore weekends and holidays are taken into account in this profile. Different volumes of the tanks in the two systems are taken into the calculation in order to determine how the thermal performance of the systems is influenced by the tank volume for different hot-water consumptions. Furthermore it is investigated how the thermal performance of the systems are influenced by mixing in the solar tank during draw-offs.     

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.     

Optimization of a hybrid solar forced-convection water heating system

In this paper, a techno-economic model has been developed for a hybrid solar forced-convection water heating system. Two options of auxiliary energy use, viz. (A) an instant electric heater and (B) use of diesel as the auxiliary energy fuel, have been considered. Numerical calculations have been made for the climate of Delhi, India, corresponding to the two representative demand patterns, viz. (i) hot-water demand of big residential buildings and (ii) industrial hot-water demand. Taking into account the life, capital cost and the maintenance cost of the solar and auxiliary systems, the cost of useful energy has been calculated for different values of collector area and tank capacity. This exercise, thereby, yields the optimum values of collector area and tank capacity corresponding to the minimum cost of useful energy. The effect of government subsidy on the optimized values of collector area, tank capacity and cost of useful energy has also been investigated.     

Dimensioning of the solar heating system in the zero energy house in Denmark

The paper describes the project for a Zero Energy House constructed at the Technical University of Denmark. The house is designed and constructed in such a way that it can be heated all winter without any “artificial” energy supply, the main source being solar energy. With energy conservation arrangements, such as high-insulated constructions (30–40 cm mineral wool insulation), movable insulation of the windows and heat recovery in the ventilating system, the total heat requirement for space heating is calculated to 2300 kWh per year. For a typical, well insulated, one-storied, one-family house built in Denmark, the corresponding heat requirement is 20,000 kWh. The solar heating system is dimensioned to cover the heat requirements and the hot water supply for the Zero Energy House during the whole year on the basis of the weather data in the “Reference Year”. The solar heating system consists of a 42 m² flat-plate solar collector, a 30 m³ water storage tank (insulated with 60 cm of mineral wool), and a heat distribution system. A total heat balance is set up for the system and solved for each day of the “Reference Year”. Collected and accumulated solar energy in the system is about 7300 kWh per yr; 30 per cent of the collected energy is used for space heating, 30 per cent for hot water supply, and 40 per cent is heat loss from the accumulator tank. For the operation of the solar heating system, the pumps and valves need a conventional electric energy supply of 230 kWh per year (corresponding to 5 per cent of the useful solar energy).     

太阳能生活热水系统相变储能研究进展

该文阐述了太阳能生活热水（SDHW）系统用相变材料的选择与封装情况,综述了相变材料在SDHW系统水箱、太阳能集热器和SDHW系统循环中的储能及其改进与强化换热研究进展情况,并对今后SDHW系统相变材料应用发展方向提出建议和展望。研究表明,石蜡与三水醋酸钠应用于SDHW系统水箱和集热器的研究较为广泛,其封装形式主要是的塑料、铝、不锈钢的宏封装,形状为管、柱体、球等,且相变材料加入到传统的SDHW系统中均能提高太阳能生活热水器的储热性能,其储热性能还有较大的提升与改进空间。     

Analysis and performance of a low-cost solar heater

One of the alternatives to reduce the consumption of electricity for heating water is by popularizing the use of solar energy. This work contributes with studies on a Low-Cost Solar Heater (LCSH), a new concept of solar water heater made entirely of polymeric materials, which requires a relatively low investment and is user-assembled. The solar collector, which absorbs solar energy and transfers it to water in the form of heat, is composed of uncovered flat panels of rigid PVC. The storage tank that holds the water heated during the day is made of polyethylene coated with polystyrene. The results of the LCSH were compared with those of a conventional solar heater composed of a glass-covered copper collector and a stainless steel storage tank. The efficiency of the systems was evaluated by measuring the incident solar radiation and water temperature with the systems operating naturally (thermosiphon). The heat loss in the hot water storage tanks was measured to estimate the thermal performance of the solar heaters. Considering that the target temperature for the heated water is slightly above the ambient temperature, the results indicated that the LCSH showed a satisfactory global heat transfer coefficient for storage tanks and that it attained an excellent thermal performance, although it is not as efficient as the conventional heater.     

Improved design of mantle tanks for small low flow SDHW systems

Side‐by‐side tests of two small low flow solar domestic hot water (SDHW) systems based on mantle tanks have been carried out under the same test conditions in a laboratory test facility. The systems are identical with exception of the mantle tanks. One of the mantle tanks has the mantle inlet port located at the top of the mantle and the other mantle tank has the mantle inlet port moved 0.175 m down from the top of the mantle. The thermal performance is almost the same for the two systems in the measuring period of 252 days. The solar fractions were 0.66 and 0.68 for the two systems. The tests showed also that the system with the low mantle inlet perform better than the system with the high mantle inlet in periods with low solar fractions, that is in less sunny periods. Further, calculations with a simulation model for low flow SDHW systems based on mantle tanks showed that mantle tanks currently marketed can be greatly improved by relatively simple design changes: increasing the height/diameter ratio, reducing the mantle height and increasing the insulation thickness on the sides of the tank. Copyright © 2006 John Wiley & Sons, Ltd.     

Economic optimization of low-flow solar domestic hot water plants

A solar plant for hot-water production was investigated by the dynamic simulation code (TRNSYS). A typical daily home consumption for a 4 persons family was considered. The hot-water demand temperature (53 °C) is controlled by a conventional fuel auxiliary heater and a tempering valve. A heat-exchanger is considered between collector and storage tank. The fluids circulate by pumps activated by photovoltaic panels. This simplifies plant control systems and allows for stand-alone utilization of the plant. Annual energy performance, in terms of solar fraction, was calculated for three Italian localities. The economic viability of such a plant was evaluated with the life cycle savings (LCS) method, considering three conventional fuels (Gas-Oil, LPG and Electricity). Italian Government incentives show an economic viability only in comparison with electrical energy.     

Single- and multi-tank energy storage for solar heating systems: fundamentals

A multi-tank liquid-water system for storing low-temperature solar-derived heat is investigated experimentally and analytically. The motivation is a perceived economic advantage of the proposed system over single-tank systems in solar heating systems where the required total volume of water is rather large — say 2000 l or more. In the proposed system, the individual tanks in the multi-tank system (each with a volume of about 200 l) are interconnected by means of two strings of immersed-coil heat exchangers: the first string serially connects exchangers that have been immersed at the bottoms of the tanks, the second connects exchangers that have been immersed at the tops of the tanks. The first string will be connected to the solar collector loop and the second in the load loop. The present work experimentally demonstrates the degree of effective stratification that the multi-tank system can achieve, as well as a thermodynamically advantageous ‘thermal diode’ effect. It also describes a model for a multi-tank system as well as experiments that validate the model. Part of the multi-tank model is a model for a single tank with immersed coil heat exchanger, and this model drew upon a ‘reversion-elimination’ algorithm from the recent literature. The validity of the reversion-elimination algorithm is supported by the present experiments.     

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.     

Domestic hot water and solar energy in Ireland

Two systems are discussed which involve the use of solar energy to supply domestic hot-water requirements and their usefulness in Ireland is examined. the systems are evaluated for thermal performance and cost-effectiveness but the use of a computer simulation model of a system involving a typical commercially available solar panel. It is shown that such systems may be economically justified when compared with electricity, but only if the water supply is directly heated by solar panels and only if the installed cost of such panels is low. Further, it appears that the system performance is relatively insensitive to the panel orientation and consequently that retro-fit installations on existing houses are unlikely to cause difficulties.     

Performance prediction of a solar/gas driving double effect LiBr–H2O absorption system

This paper presents the performance prediction of a solar/gas driving double effect LiBr–H₂O absorption system. In order to use auxiliary energy more effectively and be less dependent on solar irradiation, a new kind of solar/gas driving double effect LiBr–H₂O absorption system is designed. In this system, the high-pressure generator is driven by conventional energy, natural gas, and solar energy together with water vapor generated in the high-pressure generator, which supplies energy to the low-pressure generator for a double effect absorption system. The temperature of hot water supplied to the low-pressure generator is close to 90 °C. Apart from refrigeration in summer, this system also supplies space heating in winter and hot water throughout the year for family daily use. Simulation results illustrate that this kind of system is feasible and economical. Economic evaluation of several systems is also given in this paper in order to get a clear knowledge of the energy consumption of the system.     

Optimization of a community solar heating system with a heat pump and seasonal storage

The optimization of a district solar heating system with an electric-driven heat pump and seasonal heat storage is discussed. The optimization process comprises thermal, economic and system control analyses. Thermal and economic optima have been derived for collector area and storage volume simultaneously. The effects of different collector types and building loads are also investigated. Summertime charging of the storage by off-peak electricity has been applied to avoid severe peaking of auxiliary in the winter and to reduce the yearly energy cost. The thermal co-storage of electric energy is emphasized with systems which fail to supply heat for the heat pump during the winter heating season.‡ It has been found that system cost-effectiveness is only slightly affected as storage volume is increased beyond the optimum size. Large variations in the optima for different system configurations were found. The minimum cost of heat supplied in an optimal 500-unit community with 90% solar fraction was estimated at 8.9 ¢ kWh⁻¹.     

太阳能与谷电互补供热系统设计

利用太阳能辅以低谷电加热,采用两个储热罐进行交替轮流集热、供热。当正常集热或低温预热时,两个储热罐的水温低者优先集热循环运行;若两储热罐水温相同,则设定储热罐优先集热循环运行。当正常供热、夜间防冻循环和低温维持运行时,太阳能集热系统优先于低谷电循环运行给储热罐加热。当假日集热或低温保温供热时,同时给两储热罐集热循环运行。供热循环水泵和用热循环水泵的启动运行,以供热时优先,其停止运行以用热系统的回水温度大于或等于取暖温度的最大设定值者优先。以可编程控制器和组态软件技术,实现了系统集热、供热时段的定温、定时、定温差。此互补供热系统可以取代传统的高耗能锅炉,节能率达30%以上。     

Solar multi-mode heating system based on latent heat thermal energy storage and its application

为克服太阳能间断性和不稳定性的缺点进而实现太阳能集热与采暖的能量供需调节和全天候连续供热,提出了基于相变储热的太阳能多模式采暖方法（太阳能集热直接采暖、太阳能集热采暖+相变储热、太阳能相变储热采暖）,并在西藏林芝市某建筑搭建了太阳能与相变储热相结合的采暖系统,该系统可根据太阳能集热温度和外界供热需求实现太阳能多模式采暖的自动控制和自动运行。实验研究表明：在西藏地区采用真空管太阳能集热器可以和中低温相变储热器很好地结合,白天储热器在储热过程中平均储热功率为10.63 kW,储热量达到92.67 kW·h,相变平台明显;晚上储热器在放热过程中供热量达85.23 kW·h,放热功率和放热温度平稳,储放热效率达92%,其储热密度是传统水箱的3.6倍,可连续供热时间长达10 h,从而实现了基于相变储热的太阳能全天候连续供热,相关研究结果对我国西藏地区实施太阳能采暖具有一定的指导作用。     

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.     

A new type of solar water heater

This investigation reports a new type of solar water heating system without water pipes on the collector surface or a separate storage tank. The water to be heated continuously flows perpendicularly from an upper transparent cover to a porous absorber and is stored in a small volume beneath this assembly. Three different systems were designed, manufactured and tested but only one proved to be successful; this design indicated higher thermal efficiency compared to conventional collectors at high flow rates whereas at low flow rates the opposite is true.     

Optimal insulation of solar heating system pipes and tanks

A compact and time-effective insulation design procedure for solar heating system piping and water-filled thermal storage tanks was developed. Recognizing the particular sensitivity of solar systems to cost, the economic aspect of the problem was treated by a comprehensive present-value life-cycle cost analysis. In the development of the method, a numerical sensitivity analysis was performed to determine the relative effects of all relevant independent variables (within their pertinent ranges) on piping and tank heat transfer coefficient values. For the acceptable error limits of ± 14% for pipes and ± 19% for tanks, it was found that one may assume that only the nominal pipe diameter (or tank diameter), the thermal conductivity of the insulation, and the insulation's thickness have an effect on the overall heat transfer coefficient. Based on this result, design graphs and tables are presented which can be used to determine the optimal insulation thickness and type, total annual heat losses, present-value annual costs of insulation and lost heat, and overall insulation R-values. The use of the method is illustrated by calculating all the above quantities for all piping and storage tanks for the University of Pennsylvania SolaRow House. The present method provided insulation thicknesses slightly greater than those obtained by the ETI technique.A major conclusion of the study is that the cost of insulation in solar systems is not insignificant (e.g., ~15% in SolaRow), and that heat losses through insulation could amount to an important percentage of the useful solar energy collected (e.g., 24% in SolaRow). This re-emphasizes the need for a careful design of insulation in solar systems.     

Performance and economics of annual storage solar heating systems

Annual storage is used with active solar heating systems to permit storage of summer-time solar heat for winter use. This paper presents the results of a comprehensive computer simulation study of the performance of active solar heating systems with long-term hot water storage. A unique feature of this study is the investigation of systems used to supply backup heat to passive solar and energy-conserving buildings, as well as to meet standard heating and hot water loads.

Findings show that system performance increases linearly as storage volume increases, up to the point where the storage tank is large enough to store all heat collected in summer. This point, the point of “unconstrained operation”, is the likely economic optimum. In contrast to diurnal storage systems, systems with annual storage show only slightly diminishing returns as system size increases. Annual storage systems providing nearly 100% solar space heat may cost the same or less per unit heat delivered as a 50 per cent diurnal solar system. Also in contrast to diurnal systems, annual storage systems perform efficiently in meeting the load of a passive or energy-efficient building. A breakeven cost 4¢–10¢/kWh is estimated for optimal 100 per cent solar heating in the U.S.A.