Experimental studies of a new solar water heater system using a solar water pump

The research goal was to develop a new solar water heater system (SWHS) that used a solar water pump instead of an electric pump. The pump was powered by the steam produced from a flat plate collector. Therefore, heat could be transferred downward from the collector to a hot water storage tank. The designed system consisted of four panels of flat plate solar collectors, an overhead tank installed at an upper level and a large water storage tank with a heat exchanger at a lower level. Discharge heads of 1, 1.5 and 2 m were tested. The pump could operate at the collector temperature of about 70–90 °C and vapor gage pressure of 7–14 kPa. It was found that water circulation within the SWHS ranged between 12 and 59 l/d depending on the incident solar intensity and system discharge head. The average daily pump efficiency was about 0.0014–0.0019%. Moreover, the SWHS could have a daily thermal efficiency of about 7–13%, whereas a conventional system had 30–60% efficiency. The present system was economically comparable to a conventional one.     

强制循环式太阳热水系统动态特性分析

基于集热器，水箱及换热器等部件热,地晴天无负荷条件下运行的强制循环式太阳能热水系统进行数值模拟，分析了贮热水箱内温度分层，水量，高径比和水流率等对瞬时集热效率和系统日效率的影响。特别探讨了带热交换器的复合回路系统在两种介质热容流率比值改变时，系统热性能变化规律。对设计和控制运行强制循环式太阳热水系统提出了一些建议。     

Thermosiphon circulation in solar water heaters incorporating evacuated tubular collectors and a novel water-in-glass manifold

A study is reported of thermosiphon circulation in solar water heaters incorporating glass tubular evacuated collectors and a water-in-glass manifold of extremely simple design. The manifold is characterised by the absence of partitioning of the inner volumes of the absorber tubes into inlet/outlet channels and buoyancy effects are utilised to convey heat to a header pipe at the open end of the tubes. Solar energy input to the tubes has been simulated by electric heating. The thermosiphon system design is unusual in that there are no risers within the collector tubes, thus, the pressure head responsible for thermosiphon flow originates entirely from relatively short pipe runs between manifold and storage tank. Thermosiphon flow has been measured for a number of system designs and a wide range of operating conditions. The relative impedances of the system components has been evaluated allowing optimization of the system design. An investigation of the effect of withdrawal of hot water from the storage tank, with associated injection of cold water to the bottom of the tank, has illustrated that the self-regulating nature of the thermosiphoning system results in a large proportion of heat stored in the wate filled collector tubes being effieciently transferred to the storage tank, providing some water is drawn off intermittently.     

Experimental investigation and performance analysis on a solar adsorption cooling system with/without heat storage

A solar adsorption cooling system which can be switched between a system with heat storage and a system without heat storage was designed. In the system with heat storage, a heat storage water tank was employed as the link between the solar collector circulation and the hot water circulation for the adsorption chillers. However, the heat storage water tank was isolated in the system without heat storage, and the hot water was directly circulated between the solar collector arrays and the adsorption chillers. It was found that the inlet and outlet temperatures for the solar collector arrays and the adsorption chillers in the system without heat storage were more fluctuant than those of the system with heat storage. Also found was that the system with heat storage operated stably because of the regulating effect by the heat storage water tank. However, under otherwise similar conditions, the cooling effect of the system without heat storage was similar to that of the system with heat storage. Compared with the system with heat storage, the system without heat storage has the advantages of higher solar collecting efficiency as well as higher electrical COP.     

Mantle heat exchangers for horizontal tank thermosyphon solar water heaters

This paper describes the characteristics of horizontal mantle heat exchangers for application in thermosyphon solar water heaters. A new correlation for heat transfer in horizontal mantle heat exchangers with bottom entry and exit ports was used to predict the overall heat transfer and stratification conditions in horizontal tanks with mantle heat exchangers. The model of a mantle heat exchanger tank was combined with the thermosyphon solar collector loop model in TRNSYS to develop a model of a thermosyphon solar water heater with collector loop heat exchanger. Predictions of stratification conditions in a horizontal mantle tank are compared with transient charging tests in a laboratory test rig. Predictions of daily energy gain in solar preheaters and in systems with in-tank auxiliary boosters are compared with extensive outdoor measurements and the model is found to give reliable results for both daily and long-term performance analysis.     

蓄热介质对太阳能热风发电集热器性能的影响

分别以沙子、石子以及石子与充水黑色密封管的组合作为蓄热介质,搭建了太阳能集热器试验装置并进行了相应的试验研究,得出了不同蓄热介质的吸放热特性及其对集热器内空气进出口温差、集热器效率的影响规律.结果表明:在相同的平均辐射强度下,石子与充水黑色密封管组合集热器的蓄热性能好,且集热器空气进出口温差及集热器效率最大,为太阳能热风发电系统集热器地面蓄热材料的选用提供了依据.     

The optimization of tank-volume-to-collector-area ratio for a thermosyphon solar water heater

Through the use of the TRNSYS simulation program, the performance of a domestic solar water heating system operating with natural circulation (thermosyphon) and a daily hot water load has been analysed. The effect of tank height on the annual solar fraction of the system has been investigated for different hot water load temperatures and storage tank volumes. Optimum values (values which maximize the annual solar fraction of the system) for storage tank height and volume are calculated for operating temperatures ranging from 50 to 80°C. The response of the system to the ratio of the storage tank volume to the collector area is investigated. The dependence of the solar fraction on tank height was observed to be more notable in the case of large tank volumes and high load temperatures. The results indicate the existence of an optimum value for the tank volume at a given tank height and a high load temperature. At lower temperatures, the solar fraction rises rapidly with tank volume to a nearly constant level. An optimum value of the storage-tank-volume-to-collector-area ratio was also observed at high load temperatures.     

Flat-plate collector construction and system configuration to optimize the thermosiphonic effect

Thermosiphon systems heat potable water or heat transfer fluid and use natural convection to transport it from the collector to storage. This type of technology is applied extensively in countries with good sunshine potential. One such example is Cyprus, which is currently the leading country in the world with respect to the application of solar water heaters for domestic applications, with more than 93% of the houses equipped with such a system. The performance of such a system depends on many factors including the collector construction and the arrangement of the system, mainly with respect to the distance between the top of the solar collector and the bottom of the storage tank and the solar collector slope, which affects both the energy collected and the hydrostatic pressure of the system. A typical system in Cyprus uses 3 m² of collectors, 160 l storage, its collectors are usually inclined at 45° from horizontal and has 15 mm copper riser tubes and header tubes with a diameter of 28 mm. The collector absorber plate is also made from copper. The main objective of this paper is to investigate through modeling and simulation possible configurations, which will optimize the performance of the system. For this purpose, a number of riser and header tube diameters were considered ranging from 6 mm to 35 mm, slopes from 20° to 90° and distances between the top of the collector to the bottom side of the storage tank ranging from ±15 cm. The system is modeled using TRNSYS and simulated with the Typical Meteorological Year (TMY) of Nicosia, Cyprus. The results showed that the best-optimized system is obtained for small header and riser pipe diameters and very close performance is obtained for various combinations. Therefore, the decision on the optimum system should depend on cost issues, which are currently very important because of the increased price of copper and operational problems depending on the hardness of the water in the area of installation, which could cause scale deposits that could clog the riser pipes. The optimum slope is found to be equal to the latitude plus 10°, i.e., 45°, although a smaller slope does not affect the performance a lot, and the optimum distance between the top of the collector and the bottom of the storage tank is −15 cm. These findings should prove valuable for the collector and systems designers and manufacturers.     

Optical performance of inclined south-north single-axis tracked solar panels

To investigate optical performance of the inclined south-north single-axis (ISN-axis, in short) tracked solar panels, a mathematical procedure to estimate the annual collectible radiation on fixed and tracked panels was suggested based on solar geometry and monthly horizontal radiation. For solar panels tracking about ISN-axis, the yearly optimal tilt-angle of ISN-axis for maximizing annual solar gain was about 3° deviating from the site latitude in most of China except in areas with poor solar resources, and the maximum annual collectible radiation on ISN-axis tracked panels was about 97–98% of that on dual-axis tracked panels; whereas for ISN-axis tracked panels with the tilt-angle of ISN-axis being adjusted four times in a year at three fixed tilt-angles, the annual collectible radiation was almost close to that on dual-axis tracked panels, the optimum date of tilt-angle adjustment of ISN-axis was 23 days from the equinoxes, and the optimum tilt-angle adjustment value for each adjustment was about 22°. Compared to fixed south-facing solar panels inclined at an optimal tilt-angle, the increase in the annual solar gain due to using ISN-axis sun tracking was above 30% in the areas with abundant solar resources and less than 20% in the areas with poor solar resources.     

Study on performance of solar assisted air source heat pump systems for hot water production in Hong Kong

This paper reports the investigation results on application of the solar assisted air source heat pump systems for hot water production in Hong Kong. A mathematical model of the system is developed to predict its operating performance under specified weather conditions. The optimum flow rate from the load water tank to the condenser is proposed considering both the appropriate outlet water temperature and system performance. The effect of various parameters, including circulation flow rate, solar collector area, tilt angle of solar collector array and initial water temperature in the preheating solar tank is investigated, and the results show that the system performance is governed strongly by the change of circulation flow rate, solar collector area and initial water temperature in the preheating solar tank.     

Integrated solar collector–storage tank system with thermal diode

Water heating by utilizing solar energy for domestic use is a well established technique. However, most systems consist of the solar collector and storage tank as separate units and require piping and extra thermal insulation for both. This work considers an integrated system, which is easy to manufacture or to modify the storage tank to operate as a solar collector as well as a storage tank. The system contains a thermal diode to prevent reverse circulation at night-time. A prototype is constructed and a mathematical model is developed to study the thermal performance of the integrated system. It is found that the thermal efficiency of the suggested system is comparable with conventional systems. Also, simulation indicated that the thermal diode significantly reduces heat losses at night-time.     

Optimal design of a forced circulation solar water heating system for a residential unit in cold climate using TRNSYS

An indirect forced circulation solar water heating systems using a flat-plate collector is modeled for domestic hot water requirements of a single-family residential unit in Montreal, Canada. All necessary design parameters are studied and the optimum values are determined using TRNSYS simulation program. The solar fraction of the entire system is used as the optimization parameter. Design parameters of both the system and the collector were optimized that include collector area, fluid type, collector mass flow rate, storage tank volume and height, heat exchanger effectiveness, size and length of connecting pipes, absorber plate material and thickness, number and size of the riser tubes, tube spacing, and the collector’s aspect ratio. The results show that by utilizing solar energy, the designed system could provide 83-97% and 30-62% of the hot water demands in summer and winter, respectively. It is also determined that even a locally made non-selective-coated collector can supply about 54% of the annual water heating energy requirement by solar energy.     

非跟踪低倍聚光热管式真空管集热器的实验研究

太阳能是一种取之不尽用之不竭的清洁能源,但存在分散性强、能量密度低、不稳定等特点,因此为了得到高能量密度和稳定的能量供应,需要解决聚光和储能两大问题。针对这两个问题,本文采用非跟踪低倍聚光的集热器和保温效果良好的储热油箱,提出了一种非跟踪低倍聚光热管式真空管集热器;基于几何光学原理,模拟了热管式真空管和半圆聚光器的不同放置方式和位置的聚光效率,制作了半圆形聚光热管式真空管集热器系统,选择了合适的储热油箱并进行了保温效果的理论计算;最后对该系统进行了集热性能测试实验。实验结果表明,在半圆形聚光器的聚光下,系统的瞬时效率截距为0.66,热损系数为2.53 W/(m2•℃)。该系统完全能够满足人们的日常生活用热的需求,具有良好的应用前景。     

太阳能地面采暖系统蓄热水箱容积分析

通过分析太阳能采暖系统所需蓄热鼍与建筑热负荷、太阳能集热量日变化规律之间的关系,得出太阳能采暖系统所需蓄热水箱容积的理论算式.根据拉萨、银川、西宁、西安等地的太阳辐射强度及建筑热负荷的日变化规律,模拟得出系统所需蓄热量变化规律;并对各种蓄热温差下对应的蓄热水箱容积进行了模拟分析,结果表明:太阳能采暖系统所需蓄热量随太阳集热器的集热量与建筑热负荷之间的差值增大而增加;蓄热水箱容积随蓄热温差增大而减小,当蓄热水温达到80℃时,在各种地面采暖系统取水温度下,单位集热器面积所需蓄热水箱容积趋于相等.     

一种太阳能集热器自动寻光系统设计

设计了一种能时时跟踪光线,保持最优集热效率的太阳能集热器自动寻光系统。集热管与保温水箱为分体式结构,89C55WD单片机通过温度和水位传感器控制电磁阀开闭保证系统高效运行。运用STC89C52单片机通过光敏电阻光强比较法和定时法确认光线角度,控制电动推杆调节集热器支架方位实现寻光,并对固定倾角、调整倾角和文章所设计集热器接收的太阳辐射量进行模拟对比及分析。     

Transient analysis of parallel plate forced circulation solar water heating system

In this paper, a transient analysis of a forced circulation solar water heating system with and without heat exchangers in the collector loop and storage tank has been presented for a parallel flat plate collector. The effect of various water heating system parameters on its performance have been studied. Numerical calculations have been made for a typical cold day viz. 26 January 1980 in Delhi.     

A new version of a solar water heating system coupled with a solar water pump

This research target was to improve the thermal efficiency of a solar water heating system (SWHS) coupled with a built-in solar water pump. The designed system consists of 1.58-m² flat plate solar collectors, an overhead tank placed at the top level, the larger water storage tank without a heat exchanger at the lower level, and a one-way valve for water circulation control. The discharge heads of 1 and 2 m were tested. The pump could operate at the collector temperature of about 70–90 °C and vapor gage pressure of 10–18 kPa. It was found that water circulation within the SWHS ranged between 15 and 65 l/d depending upon solar intensity and discharge head. Moreover, the max water temperature in the storage tank is around 59 °C. The max daily pump efficiency is about 0.0017%. The SWHS could have max daily thermal efficiency of about 21%. It is concluded that the thermal efficiency was successfully improved, except for the pump one. The new SWHS with 1 m discharge head or lower is suitable for residential use. It adds less weight to a building roof and saves electrical energy for a circulation pump. It has lower cost compared to a domestic SWHS.     

Comparative study of internal storage and external storage absorption cooling systems

This work presents a comparative study of the performance of absorption cooling systems with internal storage and with external storage. A full dynamic simulation model including the solar collector field, the absorption heat pump system and the building loads has been performed. The first system is composed by four heat pumps that store energy in the form of crystallized salts so that no external storage capacity is required. The second one is a conventional system composed of one liquid absorption pump and external storage in a water tank. Many batteries of simulations have been done to evaluate the performance of these cooling machines when varying solar field surface, solar collector’s efficiency curve and the storage capacity of the systems. Two different indices have been calculated to analyze the response of both systems: Solar Fraction and Primary Energy Ratio. The comparison between both absorption chillers indicates that in order to reach similar values of storage energy, conventional system has a greater room requirement than four units with internal storage working in parallel, requiring an external water tank of at least 15 m³.     

Water-in-glass evacuated tube solar water heaters

Evacuated tube solar collectors have better performance than flat-plate solar collectors, in particular for high temperature operations. A number of heat extraction methods from all-glass evacuated tubes have been developed and the water-in-glass concept has been found to be the most successful due to its simplicity and low manufacturing cost. In this paper, the performance of a water-in-glass evacuated tube solar pre-heater is investigated using the International Standard test method ISO 9459-2 for a range of locations. Factors influencing the operation of water-in-glass collector tubes are discussed and a numerical study of water circulation through long single-ended thermosyphon tubes is presented. Preliminary numerical simulations have shown the existence of inactive region near the sealed end of the tube which might influence the performance of the collector.     

Experimental study of the performance of a solar thermal-photovoltaic integrated system

In this work a self-contained solar heating forced water cooling unit was selected and assembled. It consists of three flat-plate solar collectors, each of area 1.2 m², a.d.c. pump, a photovoltaic module and a storage tank. The electrical power produced by the photovoltaic module operates the d.c. pump, which circulates the cooling water through the solar collectors to transfer the heat to the storage tank. The electrical voltage and current, the water rate of flow, and water temperature at inlet and outlet of the collector were all measured. Solar irradiation, wind speed and ambient temperature were also measured. Daily distribution of electrical current, water mass rate of flow, module efficiency and collector efficiency were plotted in figures. Module efficiency, pump efficiency and collector efficiency were taken as dependent variables, while the solar irradiation, ambient temperature and time were the independent variables. Optimum values were graphically indicated and related to each other in a clear discussion. An economic study and comparison of three different systems were carried out: a common thermosyphon system; an a.c. pump circulating system; and this system, which is a d.c. pump circulating system. Results revealed that collector efficiency reached a daily average value of 47% due to d.c. pump installation powered by photovoltaic electric output.