两种手段提升太阳能新风热利用效率的研究

严寒地区居住建筑面临较为严重的冬季新风不足问题,太阳能新风系统是改善室内空气品质的一种重要手段。研究目的:了解空气流道厚度降低和使用双层玻璃盖板对于提升基础型太阳能新风集热器效率的实际效果。方法:通过搭建太阳能新风集热器实验平台,自制单层、双层玻璃盖板太阳能新风集热器,对比研究空气流道厚度(10mm和30mm)以及不同玻璃盖板层数对集热器效率的影响,并对不同工况下的出口温度和逐时效率进行监测和分析。结果表明:改小集热器空气流道厚度之后平均集热效率上升了10.8%,使用双层玻璃盖板之后平均集热效率上升了7.5%。双层玻璃盖板在低温下能够进一步发挥保温优势,减少了玻璃盖板表面的热损失。结论:减小空气流道厚度,采用双层玻璃盖板具有明显的实际效果,太阳能新风系统具有很好的推广应用潜力。     

Thermal performance analysis of the glass evacuated tube solar collector with U-tube

In this paper, based on the energy balance for the glass evacuated tube solar collector with U-tube, the thermal performance of the individual glass evacuated tube solar collector is investigated by analytical method. The solar collector considered in this study is a two-layered glass evacuated tube, and the absorber film is deposited in the outer surface of the absorber tube. The heat loss coefficient and heat efficiency factor are analyzed using one-dimensional analytical solution. And the influence of air layer between the absorber tube and the copper fin on the heat efficiency is also studied. The results show that the function relation of the heat loss coefficient of the glass evacuated tube solar collector with temperature difference between the absorbing coating surface and the ambient air is nonlinear. In the different ambient temperatures, the heat loss coefficient of the solar collector should be calculated by different expressions. The heat efficiency factor will be subject to influence of air layer between absorber tube and the copper fin. Specially, the influence is remarkable when the heat loss coefficient of the collector is large. When the synthetical conductance amounts to 5 W/m K, the solar collector efficiency decreases 10%, and the outlet fluid temperature decreases 16% compared with the case which the air thermal resistance is neglected. And the surface temperature of the absorbing coating increases 30 °C due to the effect of air thermal resistance. So the surface temperature of the absorbing coating is an important parameter to evaluate the thermal performance of the glass evacuated tube solar collector.     

冲缝吸热板渗透型太阳能空气集热器性能研究

介绍了冲缝吸热板渗透型太阳能空气集热器的结构,建立了传热数学模型。采用Matlab程序对传热数学模型进行求解,模拟研究了结构参数、运行参数对集热器热性能的影响。集热器出口空气温度的实测结果与模拟结果的平均偏差为0.99K,证明传热数学模型准确可靠。集热量随吸热板外表面发射率增大而降低,随集热器出口空气流量、太阳辐射强度的增大而升高,随环境温度的增大先降低后升高。集热器出口空气温度随吸热板外表面发射率、集热器出口空气流量的增大而降低,随太阳辐射强度、环境温度的增大而升高。     

Flame Heights and Heat Transfer in Façade System Ventilation Cavities

The design of buildings using multilayer constructions poses a challenge for fire safety and needs to be understood. Narrow air gaps and cavities are common in many constructions, e.g. ventilated façade systems. In these construction systems flames can enter the cavities and fire can spread on the interior surfaces of the cavities. An experimental program was performed to investigate the influence of the cavity width on the flame heights, the fire driven upward flow and the incident heat fluxes to the inner surfaces of the cavity. The experimental setup consisted of two parallel facing non-combustible plates (0.8 × 1.8 m) and a propane gas burner placed at one of the inner surfaces. The cavity width between the plates ranged from 0.02 m to 0.1 m and the burner heat release rate was varied from 16.5 kW to 40.4 kW per m of the burner length. At least three repeated tests were performed for each scenario. In addition, tests with a single plate were performed. The flame heights did not significantly change for Q′/W < 300 kW/m² (where Q′ is the heat release rate per unit length of the burner and W is the cavity width). For higher Q′/W ratios flame extensions up to 2.2 times were observed. When the distance between the plates was reduced or the heat release rate was increased, the incident heat fluxes to the inner surface increased along the entire height of the test setup. The results can be used for analysing methodologies for predicting heat transfer and fire spread in narrow air cavities.     

平行流换热器干工况下应用的可行性分析

阐述平行流换热器的构造特点以及传热与流体流动特性,分析其在干工况下使用时存在的差异和问题。与常用翅片管式换热器进行性能对比,可知平行流换热器代替翅片换热器在干工况下运行具有一定的可行性。     

Experimental energy and exergy analysis of a double-flow solar air heater having different obstacles on absorber plates

This paper presents an experimental energy and exergy analysis for a novel flat plate solar air heater (SAH) with several obstacles and without obstacles. For increasing the available heat-transfer area may be achieved if air is flowing simultaneously and separately over and under the different obstacle absorbing plates, instead of only flowing either over or under the different obstacle absorbing plates, leading to improved collector efficiency. The measured parameters were the inlet and outlet temperatures, the absorbing plate temperatures, the ambient temperature, and the solar radiation. Further, the measurements were performed at different values of mass flow rate of air and different levels of absorbing plates in flow channel duct. After the analysis of the results, the optimal value of efficiency is middle level of absorbing plate in flow channel duct for all operating conditions and the double-flow collector supplied with obstacles appears significantly better than that without obstacles. At the end of this study, the exergy relations are delivered for different SAHs. The results show that the largest irreversibility is occurring at the flat plate (without obstacles) collector in which collector efficiency is smallest.     

Experimental investigation on heat transfer characteristics of heat exchanger with bubble fin assistance

ABSTRACT

Heat transfer area per unit volume (m²/m³) of heat exchangers decides the size of the heat exchangers, over the period of years heat exchangers have undergone numerous development in this aspect. One such attractive design is plate heat exchanger which is very compact and has high thermal effectiveness, whereas the flow nature of fluid through this type of heat exchangers is complex. The most common type of plate heat exchanger is chevron type which suffers from large pressure drop (Δp) at higher heat transfer rates, to overcome this problem bubble finned heat exchangers are designed. In this paper the performance of bubble finned heat exchanger is studied with single phase fluid flow condition. The comparative study of finned and no fin configuration shows that the former has 1.8 times higher rate of heat transfer at lower value of Reynold’s number, further the flow remains laminar hence the Δp is minimum.     

U型管式全玻璃真空管太阳集热器的热性能

在U型管式全玻璃真空管集热器能量平衡分析的基础上,推导了集热器热损系数、效率因子等性能参数计算公式,其理论计算结果与实验数据吻合良好。计算分析表明：真空管热损系数与吸热管温和环境温度之差是非线性关系,将两者的计算关系式按环境温度分段整理将使计算结果更接近实际;涂层发射比对集热器效率影响较大,降低涂层发射比是提高集热器效率的有效途径;采取适当的措施降低吸热管与肋片接触热阻后,采用U型管连接方式不会对热水系统集热器效率造成太大影响。     

The use of a heavy internal wall with a ventilated air gap to store solar energy and improve summer comfort in timber frame houses

This paper considers supplementary heating and cooling within timber frame houses. The transmission of solar energy to an internal concrete cavity wall by air is analyzed. The objective of this work was initially to study the dynamic insulation in timber frame houses. The initial studies showed that it is more efficient to recover solar energy rather than heat losses, which is the principle of dynamic insulation. Clearly, the thermal regulations lead to lower heat losses through walls by conduction. Due to these factors we have decided to study a wall with an integrated solar air collector and a heavy ventilated internal wall. This internal wall, which is used to store solar energy will allow the reduction of heat demand in winter and will improve thermal comfort in summer because thermal mass increases and ventilation during the night will cool the internal wall. We have selected a closed loop air circulation system because, with an air to air heat exchanger, it can be proved to be more effective and the risk of unhealthy air pollution is reduced because the flow of fresh air will not pass through the ventilated air gap. We are constructing an integrated air collector prototype.     

低温地板辐射供暖特性参数的研究

建立了低温地板辐射供暖房间各围护结构的热平衡关系式，计算了地板表面温度、热流密度、室内空气温度和围护结构内表面温度。介绍了计算地板表面温度和围护结构内表面温度的耦合程序，测试了地板辐射供暖房间的特性参数。     

Experimental exergy analyses on fabricated parabolic solar collector with/without preheater and different collector materials

This is an experimental work, to get maximum performance by parabolic solar collector (PSC). Three different PSCs are opted with same dimensions for this experimental work; mirror, aluminium and preheater–aluminium collectors. Copper, aluminium, mild steel and brass are selected for various absorber pipe materials. By experimental analyses, fluid temperatures, convective heat transfer coefficients, overall heat loss coefficients, heat removal factors, collector efficiency factors, inlet exergies, outlet exergies, exergy gains and exergy efficiencies are obtained at a range of operating conditions for fabricated PSCs. Exergy analyses are conducted with various fluid flow rates and with different PSCs and then graphs are also generated for these analyses. After analyses, this work can be concluded as – the brass as absorber pipe material and preheater–aluminium sheet combination as PSC are the most excellent to achieve the most favourable performance from fabricated system. This combination gives maximum exergy efficiency which is near about 78%.     

Collector characterisation and heat loss tests on a novel batch solar water heater with CPC reflector for households

ABSTRACT

This solar water-heating unit is an integration of the older concept of batch water heating with the modern trends in solar water-heating technologies i.e. incorporating a concentrator in the design. The concentrator used is the compound parabolic type (CPC) which is a non-imaging device having wider acceptance angle (64°) and supported on a wooden cradle, which comprises the two arms of the parabola. To suppress the heat loss, an air gap has been introduced in the arms of the CPC. The collector is a single larger diameter drum which serves both as an absorber and storage unit positioned at the focus of CPC. The parametric study of the model showed the thermal efficiency of the collector as high as 38% and maximum water temperature attained was 53°C. Heat loss tests performed on the collector on a 24-hr cycle period showed good long time performance estimates. The response time of collector computed and performance characteristic curve plotted to predict system response under any given conditions of solar insolation and ambient temperature.     

制冷换热器肋片管的强化换热实验研究

以制冷换热器常用的矩表平肋片管作为比较对象，对四种不同片型的肋片管在吸风式直流风洞进行了空气外掠单排肋片管的对比性实验研究，得到了具有实用价值的结论。     

Study on the performance of a solar assisted air source heat pump system for building heating

This paper proposed a new solar assisted air source heat pump system with flexible operational modes to improve the performance of the heating system. A mathematical model was established on the solar assisted air source heat pump system for building heating with a heating capacity of 10 kW, and an air source heat pump unit was developed to validate the model. The effect of the solar collector area on the performance of the system running in Nanjing was studied. The results showed that the COP of the heat pump unit was enhanced with the increase of the solar radiation density during the typical sunny day in the heating season. In addition, the COP also increased in proportion to the solar collector area. Compared with the case when the solar collector area was 0 m², the COP increase of the heat pump and the energy-saving rate were 11.22% and 24% respectively when the solar collector area was 40 m². Meanwhile, the solar equivalent generation power efficiency could reach 11.8%.     

太阳能平板集热器关键参数对其热性能的影响

为了提升太阳能在建筑能源供给体系中的比重,形式多样的太阳能集热设备及其系统在建筑领域得到了广泛应用。基于机理分析法,建立了具有单层玻璃盖板的管板式太阳能平板集热器的稳态传热模型。并且针对集热管间距、集热管内径、工质入口温度和工质质量流量等关键参数对集热器集热效率的影响特性进行了数值模拟与分析。结果表明,建立的该稳态传热模型是可行的;此外,在其余参数值保持不变的情况下,减小集热管间距或增加集热管内径均可使集热器瞬时效率增大;增大工质入口温度会导致集热器瞬时效率下降;而增大工质质量流量会提升集热器瞬时效率。这些结论对于太阳能平板集热器在太阳能建筑一体化的实际应用中,具有一定的参考作用。     

Effect of flat plate collectors in parallel on the performance of the active solar still for Indian coastal climatic conditions

The effect of flat-plate collectors (FPCs) when connected in parallel on, in particular, the heat transfer coefficients and, in general, distillate output and the efficiency of solar still has been studied in the active mode. The solar still has an effective basin area of 1?m² and fixed cover inclination of 30°. Each FPC with an effective area of 2?m² is attached to the solar still. An effective collector inclination of 5° is used. Experiments have been conducted for 24?h in summer for the active solar distillation system at Kakinada (16°.93′N/83°.33′E), India, which has coastal climatic conditions. The aim of the present work is to study the variation in internal heat transfer coefficients with respect to the number of FPCs coupled with the solar still in parallel. It is understood that the heat transfer coefficients depend significantly on the size of the collector. The size of the collector is increased by increasing the number of collectors. It is also observed that this leads to an increase in distillate output but a decrease in efficiency of the solar still.     

竖直纵向肋片管自然对流传热特性试验研究及结构优化

利用搭建的试验台对竖直放置的纵向肋片管的传热特性进行试验,获得了相同结构参数下肋片形式分别是开放式、平端面封闭式和弧形端面封闭式下的散热量,单位质量散热量和金属热强度。对不同的肋片管,利用正交试验得到了各因素对散热量和单位质量散热量的影响程度,并获得了试验条件下肋片管的最优结构。     

Low-temperature baseboard heaters with integrated air supply – An analytical and numerical investigation

The functioning of a hydronic baseboard heating system with integrated air supply was analyzed. The aim was to investigate thermal performance of the system when cold outdoor (ventilation) airflow was forced through the baseboard heater. The performance of the system was evaluated for different ventilation rates at typical outdoor temperatures during the Swedish winter season. Three different analytical models and Computational Fluid Dynamics (CFD) were used to predict the temperature rise of the airflow inside the baseboard heater. Good agreement between numerical (CFD) and analytical calculations was obtained. Calculations showed that it was fully possible to pre-heat the incoming airflow to the indoor temperature and to cover transmission losses, using 45 °C supply water flow. The analytical calculations also showed that the airflow per supply opening in the baseboard heater needed to be limited to 7.0 l/s due to pressure losses inside the channel. At this ventilation rate, the integrated system with one air supply gave about 2.1 more heat output than a conventional baseboard heating system. CFD simulations also showed that the integrated system was capable of countering downdraught created by 2.0 m high glazed areas and a cold outdoor environment. Draught discomfort in the case with the conventional system was slightly above the recommended upper limit, but heat distribution across whole analyzed office space was uniform for both heating systems. It was concluded that low-temperature baseboard heating systems with integrated air supply can meet both international comfort requirements, and lead to energy savings in cold climates.     

Modeling and comparative thermal performance of ground air collector and earth air heat exchanger for heating of greenhouse

The potential of using the stored thermal energy of ground for space heating has been investigated with the help of two buried pipe systems, i.e., ground air collector and earth air heat exchanger, integrated with the greenhouse located in the premises of Indian Institute of Technology, Delhi, India. The total length of the buried pipes in both the arrangements was kept same for making a comparative study. A complete numerical model has been developed to predict and compare their thermal performance for choosing a suitable heating method in the composite climate of India. Experiments were conducted extensively during winter period from November 2002 to March 2003, but the model was validated against the clear and sunny days. Performance of these two arrangements was compared in terms of thermal load leveling and total heating potential. Temperatures of greenhouse air with ground air collector were observed to be 2–3 °C higher than those with earth air heat exchanger. The temperature fluctuations of greenhouse air were also less when operated with ground air collector as compared to earth air heat exchanger. Predicted and computed values of greenhouse air temperatures in both the systems exhibited fair agreement. Finally ground air collector was chosen as a suitable option for heating of greenhouse in the above climate.     

多元平行流细通道蒸发器

针对多元平行流蒸发器特殊的结构，选用合适的传热和压降关联式，建立了多元平行流蒸发器数学仿真模型，对管内制冷剂和空气侧的流动和换热进行数值仿真。运用该模型对影响蒸发器性能的各种重要参数进行了研究，可用于指导多元平行流蒸发器的设计和优化。