Effect of phase change material on passive thermal heating of a greenhouse

In this study, a periodic analysis of a greenhouse with combination of phase change material (PCM) and insulation as a north wall has been developed for thermal heating. The thermal model is based on Fourier analysis. Effect of distribution of PCM thickness on plant and room air temperature has been studied in detail. The plant and room air temperature have been evaluated with and without north wall. Numerical computations have been carried out for a typical winter day of New Delhi. On the basis of numerical results, it is inferred that (i) there is a significant effect of PCM north wall and heat capacity of plant temperature during off‐sunshine hour due to storage effect and (ii) the rate of heat flux inside greenhouse from north wall is maximum for least thickness of PCM. Copyright © 2005 John Wiley & Sons, Ltd.     

Numerical and experimental analysis of convection heat transfer in passive solar heating room with greenhouse and heat storage

In this paper, heat transfer and air flow in passive solar heating room with greenhouse and heat storage are studied. Thermal insulation of solar heating room has significant effects on temperature distribution and airflow in the heating chamber of this solar system. Heat transfer and air flow in a rock bed, which is used as solar absorber and storage layer, are also studied. If porosity is kept within certain range, increasing the rock size causes an increase of the capability of thermal storage and heating effects; increasing the porosity of thermal storage materials results in an increase of the bed temperature but a decrease of the rock mass. The specific heat capacity and thermal conductivity have a remarkable effect on the average temperature of rock bed. All these factors should be taken into account when designing a solar heating system.     

温棚系统中传热问题的理论与实验研究

给出了描述温棚系统中传热问题的数学物理模型及计算方法，并进行了实验研究。模型中考虑了气象环境参数、土壤的热湿物性以及塑料棚膜的太阳光学性质和辐射特性，可预报温棚内空气和土壤中的温度和含湿量的分布及变化规律。模型的数值模拟结果与实测值在数值和变化趋势上符合得较好，从而表明模型的正确性与可靠性。     

Thermal load levelling of heat flux through an insulated thermal storage water wall

This communication presents an investigation of the thickness distribution of a given total thickness of the insulation inside and outside a thermal storage water wall for acheiving the maximum load levelling of the heat flux entering through the wall. Analysis is based on the solution of the heat conduction equation for the temperature distribution in the insulated wall subjected to periodic solar radiation and atmospheric air on one side and in contact with room air at constant temperature (corresponding to air-conditioned rooms) on the other side. an explicit solution for a temperature distribution satisfying the apporpriate boundary conditions at the surface has been derived to obtaing a periodic heat flux through the storage water wall. It is found that for a given total thickness (cost) of insulation the thicknesses of outside and inside insulation must be equal for best load levelling. Moreover, more load levelling is achieved when the whole of the insulation is outside rather than inside the thermal storage water wall.     

Numerical Simulation of the Thermal Behavior of a Traditional Greenhouse by Using the Theory of the Green's Functions

In this article, we present a precise modeling translating the influence of the ground inertia in the thermal behavior of a greenhouse without vegetation. This work takes into account all the real mechanisms of exchanges (solar conduction, convection, radiations, thermal inertia) between the various elements of the system (cover, interior air, ground), but does not take into account the mass transfers (diffusions of moisture in the ground, evapotranspiration). We sought here to define a model constituting a core of procedure on which new extensions will be based. We show via the Green functions theory that the differential equations of the model are reduced to a system of integral equations on the ground surface. These equations implicitly take into account the heat propagation in the ground. This model carefully describes in detail the exchanges between the ground and the interior of the greenhouse. It also aims at defining the evolution of the greenhouse internal air temperature as well as that of the superficial temperature of its ground according to the following external data (power, exterior temperature). The mathematical study is completed by a digital simulation on an isolated greenhouse.     

Measuring and modeling vapor boundary layer growth during transient diffusion heat and moisture transfer in cellulose insulation

In this paper, an experimental test facility that permits continuous measurements of transient heat and moisture transfer in porous media is applied to study the vapor boundary layer in cellulose insulation. The experiment measures the relative humidity, temperature and moisture accumulation within the cellulose specimen with a fully developed flow of air at a controlled temperature and humidity provided above the surface. These experimental results are used to verify a mathematical model, which is used to develop an expression for moisture diffusivity (α_m) that is analogous to thermal diffusivity, and takes into consideration moisture storage. The moisture diffusivity is used to calculate the vapor density in the boundary layer and the size of vapor boundary layer in cellulose insulation. It is found that the moisture storage effect has a very significant effect on the vapor boundary layer and cannot be ignored. For cellulose insulation, the size of the vapor boundary layer may be over predicted by a factor of ten when moisture storage is neglected.     

应用于日光温室墙体的相变材料热物性优化研究

建立日光温室计算传热模型,以室内空气温度和墙体内表面温度为指标,通过实验方法验证了所建立的传热模型准确性,最后分析相变材料相变温度、相变焓、导热系数、密度等热物性对室内最低温度和相变蓄热率的影响规律,确定被动式相变蓄热墙体和主-被动式相变蓄热墙体的最佳相变材料热物性,阐明了实际应用时相变材料选择原则。研究结果表明,所建立的日光温室传热模型具有较高准确性,可用于日光温室墙体相变材料热物性优化;主-被动式相变蓄热墙体最佳相变材料的相变温度为27 ℃,相变焓为200 kJ/kg,导热系数为0.35 W/（m·K）,密度为440 kg/m³,被动式相变蓄热墙体最佳相变材料的相变温度为26 ℃,相变焓为200 kJ/kg,导热系数为0.35 W/（m·K）,密度为792 kg/m³;最佳相变材料热物性应用时,2种墙体室内最低温度均可达到15.0 ℃,但是被动式相变蓄热墙体的相变蓄热率较主-被动式相变蓄热墙体减小29.5%。本研究可为相变材料在日光温室的高效利用提供参考。     

Heat,air and moisture transfer through hollow porous blocks

The combined heat, air and moisture transfer in building hollow elements is of paramount importance in the construction area for accurate energy consumption prediction, thermal comfort evaluation, moisture growth risk assessment and material deterioration analysis. In this way, a mathematical model considering the combined two-dimensional heat, air and moisture transport through unsaturated building hollow bricks is presented. In the brick porous domain, the differential governing equations are based on driving potentials of temperature, moist air pressure and water vapor pressure gradients, while, in the air domain, a lumped approach is considered for modeling the heat and mass transfer through the brick cavity. The discretized algebraic equations are solved using the MTDMA (MultiTriDiagonal-Matrix Algorithm) for the three driving potentials. Comparisons in terms of heat and vapor fluxes at the internal boundary are presented for hollow, massive and insulating brick blocks. Despite most of building energy simulation codes disregard the moisture effect and the transport multidimensional nature, results show those hypotheses may cause great discrepancy on the prediction of hygrothermal building performance.     

Survey and evaluation of heating technologies for worldwide agricultural greenhouse applications

This paper reviews and evaluates all the available heating technologies for worldwide agricultural greenhouses and discusses the representative applications of each technology. Available information on the location, ground area, cover materials and performance of the greenhouses coupled with each technology such as water storage (including solar flat plate collectors, ground-source heat pumps and shallow solar ponds), rock bed storage, phase change material storage and earth-to-air heat exchanger system is collected and presented in detail. Using the collected information an attempt has been made to correlate the approximate storage volume or weight of the material required for a given greenhouse area for different storage media under various greenhouse cover materials. These relations can be of great importance to the potential users for the successful development of future applications. Apart from the above-mentioned technologies, a complete review and performance evaluation of other greenhouse heating technologies such as movable insulation, north wall storage, ground air collector and aquifer coupled cavity flow heat exchanger system is also presented. Heating potential (overall performance) of each system under different cover materials along with their advantages, limitations and suitability is also discussed.     

基于CFD技术的太阳能污泥干燥室内热湿环境研究

基于传热传质理论,建立一种太阳能污泥干燥室内热湿耦合传递的数学模型。综合考虑空气流动以及对流和辐射传热,利用CFD软件Fluent的k~ε湍流模型、组分输运模型及辐射模型,初步分析了在太阳辐射条件下不同干燥室结构、排风形式以及通风量对太阳能污泥干燥室内干燥区域的温度、相对湿度以及速度分布的影响。模拟结果表明:干燥室内温、湿度模拟值与实测值吻合较好,平均相对误差分别为3.55%和5.39%。对比分析不同结构下干燥室内的流场分布,两出口排风形式的太阳能污泥干燥室可以形成良好的干燥微环境。当两出口排风风速≥5 m/s时,室内干燥区域温度高于室外环境温度,同时相对湿度低于室外环境相对湿度,且增大出流风速,在干燥区域内空气扰流强度增强,有利于干燥室内污泥水分的蒸发。     

Experimental investigation of thermal and moisture behaviors of wet and dry soils with buried capillary heating system

An experimental study of thermal and moisture behaviors of dry and wet soils heated by buried capillary plaits was done. This study was carried out on a prototype similar to an agricultural tunnel greenhouse. The experimental procedure consisted on three different measuring phases distinguished by three different operational conditions of the capillary plaits: heating at 70 °C, heating at 40 °C and without heating in summer. During an experimental run, quantities measured are soil temperature, soil water content at various depths, soil surface heat flux, solar radiation under the plastic cover, internal relative humidity, internal and external air temperature. In unsaturated moist soils, the transport of heat is complicated by the fact that heat and mass transfer is a coupled process. During the daily soil temperature variation, it was found that the surface temperature amplitude was higher in wet soil than in dry soil. The water content increased during daytime and decreased during nighttime. The diurnal variation amplitude of water content was higher without underground heating and decreased with the buried heat source temperature.     

Assessment of Building External Wall Thermal Performance Based on Temperature Deviation Impact Factor under Discontinuous Radiant Heating

As the variation and timely meeting thermal environment requirement of indoor air temperature has a close relationship with the thermal performance of building external wall under discontinuous radiant heating condition, one appropriate assessment method or index for assessing the building external wall thermal performance is very necessary. In order to reasonably evaluate the thermal performance of external wall under discontinuous radiant heating condition and build the direct connections and interactions among the indoor air temperature, external wall inner surface temperature and outdoor air temperature, the first and second impact factors of temperature deviation were established, based on one mathematical model of room heat transfer. For one experimental room and four types of external walls under discontinuous radiant heating condition, both the influence of the external wall inner surface temperature deviation on the indoor air temperature and that of the outdoor air temperature deviation on the external wall inner surface temperature were determined effectively with the first and second impact factors of temperature deviation. In addition, favourable performance for the self-insulation and inner insulation walls were found, due to their superiority in effectively and timely improving the indoor thermal environment under discontinuous radiant heating condition.     

下沉式日光温室土质墙体的保温蓄热性能

为研究土质墙体下沉式日光温室的保温蓄热性能,对墙体温度及热流的变化进行了实验测试.测试结果表明:白天土质墙体接受太阳辐射并蓄热;夜间墙体内侧表面温度高于室内温度,墙体向室内放热.土质墙体具有良好的保温蓄热性能,可以满足作物生长的需要,.     

Analysis of flow and heat transfer characteristics of porous heat-storage wall in greenhouse

The flow and heat transfer characteristics of porous heat-storage wall in greenhouse are studied by using the one-dimensional steady energy two-equation model for saturated porous medium. The results show that the heat exchange between the air and the solid matrix of the porous heat-storage wall depends upon the inlet air velocity, the porosity and the permeability of porous medium, and the thermal conductivity of the solid matrix. Because the incidence of solar radiation on the porous heat-storage wall is not uniform, the new composite porous solar wall with different porosity is proposed to reduce the disadvantageous effect.     

辐射通道一维稳态温度场数值模拟

为得到辐射对流通道中的温度分布,依据能量守恒原理,建立了辐射、对流非线性边界条件下圆形管壁与管内空气的传热数学模型,提出了管壁温度、管内冷却空气温度一维稳态换热有限差分求解方法,其中辐射换热计算采用基于辐射传递系数的蒙特卡罗法。分析了相关参数对辐射通道温度分布的影响,所研究的参数包括辐射器表面温度、管道长度与半径比、管内冷却空气流速等。计算结果表明：辐射器表面温度是影响辐射通道最高温度的主要因素。此方法可为辐射通道精细的热工特性计算提供温度场数据。     

Analysis of passive heating concepts

Based on Fourier series solution of the heat conduction equation, a mathematical model has been developed to analyse the thermal performance of some typical passive heating concepts, namely the Trombe wall, Water wall and Solarium in two cases, (i) when the glazing is left uncovered thoughout and (ii) when the glazing is covered with an insulation during off sunshine hours. The model yields analytical expressions for the time dependent heat flux entering into the living space, which is assumed to be at a constant temperature corresponding to an air conditioned room. Numerical calculations corresponding to the meteorological data of a typical cold winter day in North America (Boulder, 13 January) show that from the point of view of thermal load levelling and average heat flux into the room, the solarium is best when the south glazing is not covered with a night insulation. In the case when night insulation is used a water wall is, however, found to be the best; a 0.10 m wide water wall followed by a 0.22 m thick concrete wall gives almost a constant heat flux into the living space.     

Influence of soil moisture content on soil solarization efficiency

Soil temperature under plastic cover is a function of incoming radiation and thermal characteristics of the mulching material and the soil. A field experiment was conducted at Fudhiliyah Agrometeorological research station to investigate how soil temperature is influenced by different soil moisture content regimes during soil solarization. Two soils, sandy loam and silty clay loam, were brought to five volumetric moisture regimes (M1–M5). The moistened soils were mulched with 180-μm-thick transparent polyethylene. Measurements of hourly soil temperatures for depths of 0.00, 0.05, 0.10 and 0.30 m were recorded from 1 June to 30 September. The results showed that the maximum soil temperatures decreased with increasing soil moisture content. The relationship between the hourly rise in soil temperature and heat flux were significantly affected by soil moisture content. Moreover, soil solarization efficiency decreased with increasing moisture content. The higher temperatures in both soils under the M4 irrigation regime resulted in faster eradication of the pathogenic fungus, Fusarium oxysporum (sp. lycopersici).     

Passive heat and moisture removal from a natural vented enclosure with a massive wall

Simultaneous transport of heat and moisture by conjugate natural convection in a partial enclosure with a solid wall is investigated numerically. Moist air motions are driven by the external temperature and concentration differences imposed across enclosures with different ambient moisture conditions. The Prandtl number and Schmidt number used are 0.7 and 0.6, respectively. The fluid, heat and moisture transports through the cavity and solid wall are, respectively, analyzed using the streamlines, heatlines and masslines, and the heat and mass transfer potentials are also explained by the variations of overall Nusselt and Sherwood numbers. The numerical simulations presented here span a wide range of the main parameters (heat and mass diffusion coefficient ratios, solid wall thickness and thermal Rayleigh numbers) in the domain of aiding and opposing buoyancy-driven flows. It is shown that the heat transfer potential, mass transfer potential, and volume flow rate can be promoted or inhibited, depending strongly on the wall materials and size, thermal and moisture Rayleigh numbers.     

Effect of the greenhouse design on the thermal behavior and microclimate distribution in greenhouses installed under semi‐arid climate

Greenhouse design and cover material properties may strongly impact greenhouse energy. To study the effect of these parameters, three typical unheated agricultural buildings equipped with rows of canopy were considered, and two‐dimensional simulations were conducted using the discrete ordinate model for simulating radiative transfers. Experiments were conducted to establish the boundary conditions and to validate the model. Two parametric studies were carried out: one for the nocturnal period when the energy performance of each type of greenhouse was investigated, and one for the diurnal period, when the sun path was simulated taking into account the type of the cover, its spectral optical and thermal properties. Results indicate that for the diurnal period, and for both plastic greenhouses equipped with fully opened side vents, the air located between the rows of canopy and ground surfaces remained very slow, not exceeding 0.2 m/s; for the Venlo glasshouse, the recirculation loop situated above the crop improved the air mixing and induced a good homogenization. Results also indicate that the cover material with the highest absorptivity, deteriorated the natural ventilation, increasing the air temperature by convection, enhancing local air temperature favoring the development of a secondary recirculation, and reduced the available photosynthetically active radiation. Concerning the nocturnal period, the ambient air temperature in the tunnel and in the vertical wall of the greenhouse was relatively homogenous and warmer than the temperature in the Venlo greenhouse. During the nocturnal period, the plastic greenhouse (in particular the one with the tunnel) performed better in regard to the homogenization of the climate and thermal energy storage.     

Numerical simulation of soil heat exchanger-storage systems for greenhouses

A numerical study was conducted for the thermal behavior of soil heat exchanger-storage systems (SHESSs) aimed at reducing the energy consumption of greenhouses. These systems consists of buried pipes circulating air for storing and removing heat from the soil. First, a transient fully three-dimensional heat transfer model resting on the coupled conservation equations of energy for the soil and the circulating air is presented. The model is validated with experimental data taken from a SHESS installed in a commercial type greenhouse. Next, the model is used to examine the effect of various design and operating parameters on the performance of SHESSs. Results, indicate that the total amount of energy stored or recovered daily per volume Q_v decreases exponentially with the pipe center-to-center distance and the pipe length. It increases with the air velocity and this effect is enhanced as the pipe center-to-center distance diminishes. Nevertheless, as a compromise between cost and performance, it appears that an air blowing of 4 m s^±1 is nearly optimal. As the moisture content of the soil increases, Q_v augments but its effect becomes negligible for large pipe lengths and small blowing velocities. Adding side insulation improves the performance of the SHESS but the beneficial effect of insulation underneath the bottom pipe row is significant. Finally, burying pipes deeper underground allows more energy to be stored during the day but less is recovered at night through the ground surface and the overall performance declines.