Refrigerant-charged integrated solar water heater

The performance of two R-11-charged integrated solar water heater collectors was investigated experimentally for forced and natural circulation water flows. The heat transfer from the refrigerant loop to the hot water storage tank took place through a condenser of novel design integrated within the collector frame. The effect of the condenser inclination on the system efficiency was remarkable for natural circulation water flow but had no significant effect for forced circulation flow. The difference in thermal response between the refrigerant and water loops caused buildup of stored energy in the condenser. This energy affected the buoyancy forces and generated flow pulsation that caused a harmonic-like variation of the system efficiency. This effect vanished with forced water circulation flow. The system efficiency varied between 20 and 50% depending on the solar insolation.     

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。     

A new heat-pipe type solar domestic hot water system

The performance of a new solar hot water system with an integrated heat-pipe is investigated theoretically and experimentally. The new system uses a wickless gravity assisted loop heat-pipe for the heat transfer from the collector–evaporator to the tank through a heat exchanger–condenser. A detailed heat transfer experimental study is performed, focused on the thermal behaviour of the different parts of the system. The results show that the system can reach satisfactory efficiencies which, in combination with manufacturing simplicity, absence of moving parts and good behaviour under freeze conditions, make it an interesting technological solution. Furthermore, a theoretical model for the collector is proposed and comparison with experimental results is performed. This model can be used for the optimisation of the system design.     

Flower-type pulsating heat pipe for a solar collector

Inspired by the sunflower, we report a new structure of a solar collector that integrates a pulsating heat pipe (PHP) into a flat-plate collector. The proposed flower-type PHP solar collector is designed after a sunflower with petals that absorb sunlight and transfer nutrients to the stem after photosynthesis. The evaporator section adopts the shape of a flower to absorb sunlight fully, and the condenser section is rolled into a cylinder and placed in the lower part of the structure. A systematic experimental study is conducted upon start-up, and the performance characteristics, with acetone as the working fluid, are evaluated. We also did a heat loss analysis, which has a deviation of 8%. The effects of the mass flow rate of cooling water, filling ratio, length of the condenser section, and solar intensity are assessed. As the temperature of the heat absorber plate increases, the thermal resistance of the PHP can decrease to a minimum of 0.14°C/W. Under sunny weather conditions, the instantaneous thermal efficiency of the system with a filling ratio of 50% reaches 50%. Besides, we discussed the unstable operation conditions and possible dryout phenomenon that happened inside the PHP.     

Simulated and experimental performance of a heat pipe assisted solar wall

Performance was evaluated for a passive solar space heating system utilizing heat pipes to transfer heat through an insulated wall from an absorber outside the building to a storage tank inside the building. The one-directional, thermal diode heat transfer effect of heat pipes make them ideal for passive solar applications. Gains by the heat pipe are not lost during cloud cover or periods of low irradiation. Simplified thermal resistance-based computer models were constructed to simulate the performance of direct gain, indirect gain, and integrated heat pipe passive solar systems in four different climates. The heat pipe system provided significantly higher solar fractions than the other passive options in all climates, but was particularly advantageous in cold and cloudy climates. Parametric sensitivity was evaluated for material and design features related to the collector cover, absorber plate, heat pipe, and water storage tank to determine a combination providing good thermal performance with diminishing returns for incremental parametric improvements. Important parameters included a high transmittance glazing, a high performance absorber surface and large thermal storage capacity.An experimental model of the heat pipe passive solar wall was also tested in a laboratory setting. Experimental variations included fluid fill levels, addition of insulation on the adiabatic section of the heat pipe, and fins on the outside of the condenser section. Filling the heat pipe to 120% of the volume of the evaporator section and insulating the adiabatic section achieved a system efficiency of 85%. Addition of fins on the condenser of the heat pipe did not significantly enhance overall performance.The computer model was validated by simulating the laboratory experiments and comparing experimental and simulated data. Temperatures across the system were matched by adjusting the model conductances, which resulted in good agreement with the experiment.     

Heat-pipe enhanced solar-assisted heat pump water heater

A heat-pipe enhanced solar-assisted heat pump water heater (HPSAHP) is studied. HPSAHP is a heat pump with dual heat sources that combines the performance of conventional heat pump and solar heat pipe collector. HPSAHP operates in heat-pump mode when solar radiation is low and in heat-pipe mode without electricity consumption when solar radiation is high. HPSAHP can thus achieve high energy efficiency. A prototype was designed and built in the present study. An outdoor test for a HPSAHP in the present study has shown that COP of the hybrid-mode operation can reach 3.32, an increase of 28.7% as compared to the heat-pump mode COP (2.58).     

Studies on semi-cylindrical solar tunnel dryers: year round collector performance

In this paper, the results of year round thermal performance of collector of semi-cylindrical solar tunnel dryers (STD) has been presented. The calculations have been made under natural circulation and forced circulation mode. The day-long average of solar irradiance, ambient temperature, rise in the inlet air temperature, natural circulation volume flow rates, and collector efficiency have been calculated for each month of the year. The effect of collector length, cover radius, collector inclination with horizontal have been made for rise in inlet air temperature, volume flow rate and for the efficiency. All calculations have been made for Delhi climate. © 1998 John Wiley & Sons, Ltd.     

集中供热热管太阳能集热系统

在原有平板集热器的基础上研制了一种与建筑结合的太阳能集中供热热管系统，并对策略热管的传热机理进行了分析，提出正常工况下该热管的集总参数模型，给出了热管的倾角范围。文中给出了该集热系统的理论预测和实验结果对比。热管太阳能集热器的理论模型以Duffie 和Beckman理论（1980）为基础，修改后用于能量传输。该文还给出了该集热器和传统的太阳能集热器的对比试验结果。热管太阳能集热器的瞬时效率在早上低于传统的太阳能集热器的，而当热管达到工作温度时，前者高于后者。     

The effect of condenser heat transfer on the energy performance of a plate heat pipe solar collector

For a novel prototype solar collector, using a plate heat pipe, condenser heat transfer was analysed in detail. The condenser has the shape of a rectangular channel. Flow and heat transfer of water in the rectangular channel was modelled and the heat transfer coefficient assessed, using the Fluent code. Under typical operating conditions a mixed convection situation occurs. The channel is inclined and heating is through one wall only (upper channel surface). The range of temperature differences considered was similar to the one verified under real operating conditions, covering a wide range of Grashof numbers. Results showed that the Nusselt number is significantly higher than the one for forced convection in a rectangular channel with fully developed boundary layers. In order to enhance heat transfer, a modification to the rectangular channel was analysed, using baffles to improve flow distribution and increase velocity. The effect of this modification on collector energy performance (efficiency) was assessed. Copyright © 2005 John Wiley & Sons, Ltd.     

A parabolic dish/AMTEC solar thermal power system and its performance evaluation

This paper proposes a parabolic dish/AMTEC solar thermal power system and evaluates its overall thermal–electric conversion performance. The system is a combined system in which a parabolic dish solar collector is cascaded with an alkali metal thermal to electric converter (AMTEC) through a coupling heat exchanger. A separate type heat-pipe receiver is selected to isothermally transfer the solar energy from the collector to the AMTEC. To assess the system’s overall thermal–electric conversion performance, a theoretical analysis has been undertaken in conjunction with a parametric investigation by varying relevant parameters, i.e., the average operating temperature and performance parameters associate with the dish collector and the AMTEC. Results show that the overall conversion efficiency of parabolic dish/AMTEC system could reach up to 20.6% with a power output of 18.54 kW corresponding to an operating temperature of 1280 K. Moreover, it is found that the optimal condenser temperature, corresponding to the maximum overall efficiency, is around 600 K. This study indicates that the parabolic dish/AMTEC solar power system exhibits a great potential and competitiveness over other solar dish/engine systems, and the proposed system is a viable solar thermal power system.     

太阳能有机朗肯循环发电系统分析

为高效利用太阳能,提出了太阳能平板集热器与有机朗肯循环相结合的发电系统。本文从太阳能低温发电系统的基本原理、工质选择及系统组成部分入手,分析了影响系统循环效率的因素。发现具有较大潜热与显热之比的干性或等熵有机工质适合本发电系统,并且膨胀机和冷凝器对系统循环效率影响较大。采用增加内部热交换器的再热循环、抽汽回热循环可以降低冷凝器的冷凝负荷,选择合适的再热度、抽汽量及抽汽压力可以提高系统循环效率。     

线性菲涅尔反射式太阳能集热系统的设计与试验研究

本文研究了线性菲涅尔反射式太阳能集热系统,基于几何光学原理计算模拟了线性菲涅尔反射镜镜场和复合抛物面的接收系统,在减少末端损失的基础上设计了系统的机械结构,制作了线性菲涅尔反射式太阳能集热系统的装置,并进行了集热性能的实验测试。测试结果表明,该系统在9倍聚焦倍率下,导热油的最高温度可达176.2℃,系统的平均瞬时热效率约为53%,很好地实现了其集热性能。     

冷却剂流量对聚光型太阳能光伏系统效率的影响

针对聚光型太阳能光伏电池工作中温度升高会导致发电效率降低的问题,在太阳能模组上铺设有机工质循环管路对光伏电池进行冷却,通过冷凝器对管内有机工质吸收的热量进行收集利用,构建聚光型太阳能光伏/光热综合利用系统。建立传热模型,计算不同日照强度下模组的输出效率并与实验数据进行对比。实验结果表明:发电效率随日照强度的增加先增加后减小;对光伏电池进行冷却可提升系统输出效率;太阳能光伏发电及散热量利用效率合计可达60%。     

Finite time thermal analysis of ground integrated‐collector‐storage solar water heater with transparent insulation cover

A transparent honeycomb insulated ground integrated‐collector‐storage system has been investigated for the engineering design and solar thermal performance. The system consists of a network of pipes embedded in a concrete slab whose surface is blackened and covered with transparent insulation materials (TIM) and the bottom is insulated by the ground. Heat may be retrieved by the flow of fluid through the pipe. A simulation model has been developed; it involves the solution of the two‐dimensional transient heat conduction equation using an explicit finite‐difference scheme. Computational results have been used to determine the effect of such governing parameters as depth as well as pitch of the pipe network and collector material on the thermal performance of the system. The pipe network depth of 10 cm and the TIM cover made of 5 cm compounded honeycomb seem suitable for the proposed system. Solar gain (solar collection efficiency of 30–50% corresponding to collection temperature of 40–60°C) and the diurnal heat storage characteristics of the system are found to be of the right order of magnitude for solar water heating applications. Copyright © 1999 John Wiley & Sons, Ltd.     

一种可蓄热太阳能睡床结构及性能分析

介绍了一种兼具蓄热与散热两种状态的太阳能供暖用睡床。该睡床的下部为蓄热水箱，可从太阳能集热板获取热量供给睡床。研究了基于该睡床的供暖系统在北京地区的应用情况，并分析了不同状态下床板上表面的散热量与被褥内的温度。结果表明：在全天散热状态下，典型年供暖季集热器效率为37.7 %，复合型睡床的有效供热量为4 390.2 MJ，太阳能保证率为80.7 %；在白天保温−夜间散热下，集热器效率为33.1 %，复合型睡床的有效供热量为4 441.1 MJ，太阳能保证率为81.8 %。     

Performance investigation on a thermal energy storage integrated solar collector system using nanofluid

The effect of Fe nanofluid on the performance enhancement on solar water heater integrated with thermal energy storage system is investigated experimentally. A 0.5% wt fraction of Fe nanoparticle was synthesized with the mixture of water/propylene‐glycol base fluid. The experimental implementation utilized 40‐nm‐size Fe nanoparticle, 15 ° collector tilt angle, and 1.5 kg/min mass flow rate heat‐transfer fluid circulation. The system efficiency reached 59.5% and 50.5% for with and without nanofluid. The water tank temperature was increased by 13 °C during night mode. The average water tank temperature at night mode was 47.5 °C, while the average ambient temperature was 26 °C. The Fe nanofluid improved the system working duration during night mode by an average of 5 h. The techno‐economic analysis results showed a yearly estimated cost savings of 28.5% using the Fe nanofluids as heat transfer fluid. The embodied energy emission rate, collector size, and weight can be reduced by 9.5% using nanofluids. Copyright © 2016 John Wiley & Sons, Ltd.     

Forced versus natural circulation solar water heaters: A comparative performance study

An experimental study has been carried out to compare the performance of natural and forced circulation domestic solar water heaters. Several measurments have been made for the two cases which included; the collector water inlet and outlet temperatures, the mass flow rate, the tank temperature, the ambient temperature and the solar insolation. The main parameters for the solar collector are calculated for the natural and forced circulation systems. These included; the top, back, and overall loss coefficients, the heat removal factor, the efficiency factor, the useful energy gain and the instantaneous efficiency. The comparison showed that the efficiency of the forced circulation system could be 35 to 80% higher.     

Optimal flow control of a forced circulation solar water heating system with energy storage units and connecting pipes

This paper focuses on pump flow rate optimization for forced circulation solar water heating systems with pipes. The system consists of: an array of flat plate solar collectors, two storage tanks for the circulation fluid and water, a heat exchanger, two pumps, and connecting pipes. The storage tanks operate in the fully mixed regime to avoid thermal stratification. The pipes are considered as separated components in the system so as to account for their thermal effects. The objective is to determine optimal flow rates in the primary and secondary loops in order to maximize energy transfer to the circulation fluid storage tank, while reaching user defined temperatures in the water storage tank to increase thermal comfort. A model is developed using mainly the first and second laws of thermodynamics. The model is used to maximize the difference between the energy extracted from the solar collector and the combined sum of the energy extracted by the heat exchanger and corresponding energies used by the pumps in the primary and secondary loops. The objective function maximizes the overall system energy gain whilst minimizing the sum of the energy extracted by the heat exchanger and corresponding pump energy in the secondary loop to conserve stored energy and meet the user requirement of water tank temperatures. A case study is shown to illustrate the effects of the model. When compared to other flow control techniques, in particular the most suitable energy efficient control strategy, the results of this study show a 7.82% increase in the amount of energy extracted. The results also show system thermal losses ranging between 5.54% and 7.34% for the different control strategies due to connecting pipe losses.     

改进型微热管平板太阳能空气集热器性能分析

近年来,以空气作为换热介质的太阳能集热器越来越受到重视。本文以微热管阵列为核心传热元件,设计并搭建了改进型微热管平板太阳能空气集热器性能测试系统。通过实验研究了不同空气流量和不同进口温度对集热器集热性能的影响,获得相应参量对集热器的出口空气温度、集热效率和微热管阵列蒸发段温度的影响特性,分析对比了改进前后集热器的集热性能,得到了集热器效率的归一化曲线。实验结果表明,改进型微热管平板太阳能空气集热器在夏季240 m³/h空气流量时集热性能最佳,改进后的集热器相比原集热器在夏季的平均集热效率最高同比提升13.8%;在240 m³/h风量下的平均集热效率最高达到了74%,对应集热器的压降为9.2 Pa。     

集热方式对全玻璃真空管太阳能热水器性能的影响

为对比分析自然循环竖排管、强制循环竖排管和强制循环横排管3种集热方式对全玻璃真空管太阳热水器有效集热量、平均有效集热效率和瞬时有效集热量等的影响,以3组30支Φ58×1800 mm的全玻璃真空管太阳热水器为研究对象,在兰州地区搭建试验平台,在太阳辐射为19.0~25.7 MJ/m²,风速为0.5~1.3 m/s,平均环境温度为8.9~12.0℃的情况下进行连续5 d的试验。试验结果表明:自然循环竖排管热水器有效集热量和平均有效集热效率比强制循环竖排管热水器分别高6.6~8.3 MJ、10.8%~14.8%;强制循环竖排管热水器有效集热量和平均有效集热效率比强制循环横排管热水器分别高0.9~3.4 MJ、1.6%~5.0%。从有效集热面积、真空管内部换热强度、循环管道热损失共3个方面解释不同集热方式产生差异的原因。