Test cell analysis of the use of a supply air window as a passive solar component

A series of experiments was conducted to determine the performance characteristic of a ‘double’ window used to pre-heat background room ventilation. A theoretical model of heat exchange conditions within the window was compared with results from a test cell. The test cell was used in different modes, firstly free-ventilated with the service room window and interconnecting duct to the cell left open, and then with forced ventilation at a consistent velocity to analyse the relative extent of direct solar and ventilation heat gain. A subsidiary study sought to determine the frequency of positive and negative air flow through trickle vents under real house conditions, this was compared with glass temperatures from the test cell measurements to assess the risk of condensation forming within the window. By reference to recent work on ventilated PV systems it was possible to derive a method of relating the U value and Solar Heat Gain Coefficient within the window cavity to a range of boundary conditions.     

A mathematical model of a solar chimney

A simple mathematical model of a solar chimney is proposed. The physical model is similar to the Trombe wall. One side of the chimney is provided with a glass cover which with the other three solid walls of the chimney form a channel through which the heated air could rise and flow by natural convection. Openings provided at the bottom and top of the chimney allow room air to enter and leave the channel. Steady state heat transfer equations were set up to determine the boundary temperatures at the surface of the glass cover, the rear solar heat absorbing wall and the air flow in the channel using a thermal resistance network. The equations were solved using a matrix-inversion solution procedure. The thermal performance of the solar chimney as determined from the glass, wall and air temperatures, air mass flow rate and instantaneous heat collection efficiency of the chimney are presented. Satisfactory correlation was obtained with experimental data from other investigators. Further experimental investigation is currently under way.     

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.     

Thermal modeling of integrated roof air heater for passive solar space heating of a non-air-conditioned building

This communication presents the periodic heat transfer analysis for solar space heating of an unconditioned building with an integrated roof air heater. The system consists of an air duct within the roof such that the air is continuously or intermittently forced to circulate the cooler room air through the inlet of the air duct. Time dependence of the air flow is represented by a step function of time for daily operation and, hence, has been expressed as a Fourier series in time. The analysis takes into account air ventilation, ground heat conduction and furnishings. The effects of depth of the air duct from the outer surface of the roof and the magnitude and duration of air flow rate on indoor air temperature have been studied for a typical cold winter day in Delhi. It is seen that a time dependent air flow through the duct is desirable from the point of view of increasing the indoor air temperature in the case of a bare roof. However, in the case of a blackened and glazed roof, continuous air flow is needed for increasing the room air temperature. The results are desirable from the point of view of efficient space heating of solar passive buildings.     

Heat losses from parabolic trough solar collectors

Parabolic trough solar collector usually consists of a parabolic solar energy concentrator, which reflects solar energy into an absorber. The absorber is a tube, painted with solar radiation absorbing material, located at the focal length of the concentrator, usually covered with a totally or partially vacuumed glass tube to minimize the heat losses. Typically, the concentration ratio ranges from 30 to 80, depending on the radius of the parabolic solar energy concentrator. The working fluid can reach a temperature up to 400°C, depending on the concentration ratio, solar intensity, working fluid flow rate and other parameters. Hence, such collectors are an ideal device for power generation and/or water desalination applications. However, as the length of the collector increases and/or the fluid flow rate decreases, the rate of heat losses increases. The length of the collector may reach a point that heat gain becomes equal to the heat losses; therefore, additional length will be passive. The current work introduces an analysis for the mentioned collector for single and double glass tubes. The main objectives of this work are to understand the thermal performance of the collector and identify the heat losses from the collector. The working fluid, tube and glass temperature's variation along the collector is calculated, and variations of the heat losses along the heated tube are estimated. It should be mentioned that the working fluid may experience a phase change as it flows through the tube. Hence, the heat transfer correlation for each phase is different and depends on the void fraction and flow characteristics. However, as a first approximation, the effect of phase change is neglected. Copyright © 2013 John Wiley & Sons, Ltd.     

Optical and solar parameters of irradiated lead–alkali–silicate glass

Lead–alkali–silicate glass that is used for a shielding window of hot cells in nuclear technology has been irradiated by a ⁶⁰Co radioisotope source between 0.998 and 35.939 kGray dose levels. Gamma rays can affect glass and change its several optical and solar parameters such as secondary internal heat transfer factor (q_i), direct solar transmittance (τ_e), solar factor (g) and shading coefficient via the absorbed dose. It is aimed to investigate the performance of the glass in terms of the shading coefficient, which is the most important parameter to view clearly inside of the hot cell. Furthermore, a comparative evaluation has been done with respect to the unexposed lead–alkali–silicate glass. Change in the shading coefficient with respect to absorbed dose is extremely important.     

PCM glazing systems

This paper presents a different approach for thermal effective windows, i.e. windows that reduce the energy transmitted into or out of a room. The idea is to use a double sealed glass filled with a phase change medium (PCM) whose fusion temperature is determined by solar–thermal calculations. The PCM used is polypropylene glycol. The investigation includes modelling of the heat and radiation transfer through a composite window and optical investigation of conventional and PCM filled windows, testing of the window and comparison with numerical simulations. A one-dimensional model for the composite glass window is developed to predict the thermal performance as a function of the geometrical parameters of the panel and the PCM used. Optical measurements were realized using photo-spectrometry to determine the transmittance, reflectance and absorptance. The specimens used include single glass of different thicknesses, double glass of different gap spacing and thicknesses filled with air or PCM, and finally coloured PCM. The results indicate big reductions in the energy transmitted, specially in the infra-red and ultraviolet regions, while maintaining a good visibility. © 1997 by John Wiley & Sons, Ltd.     

Theoretical approach of a flat-plate solar collector taking into account the absorption and emission within glass cover layer

A rigorous theoretical approach of a flat-plate solar collector with a black absorber considering the glass cover as an absorbing–emitting media is presented. The glass material is analyzed as a non-gray plane-parallel medium subjected to solar and thermal irradiations in one-dimensional case using the Radiation Element Method by Ray Emission Model (REM²). The optical constants of a clear glass window proposed by Rubin have been used. These optical constants, 160 values of real part n and imaginary part k of the complex refractive index of a clear glass, cover the range of interest for calculating the solar and thermal radiative transfer through the glass cover. The computational time for predicting the thermal behavior of solar collector was found to be prohibitively long for the non-gray calculation using 160 values of n and k. Therefore a suitable semi-gray model is proposed for rapid calculation. The profile of the efficiency curve obtained in the present study was found to be not linear in shape. Indeed, the heat loss from the collector is a combination of convection and radiation and highly non linear. The effect of the outside convective heat transfer on the efficiency curve is also studied. In fact, when the convection is the dominant heat transfer mode compared with the radiation one, the profile of the efficiency curve is more or less straight line. Consequently, the heat loss coefficient could be calculated using Klein model. It has been also shown that the effect of the wind speed on the glass cover mean temperature is very important. This effect increases with the increase of the mean absorber temperature.     

EFFECT OF OXYGEN ON PHOTOVOLTAIC PROPERTIES OF SCREEN-PRINTED CdS/CdTe SOLAR CELL AND MODULE Module Reliability and Oxygen Deficiency

An n-CdS/p-CdTe solar cell module constructed by complete seal of solar cell formed on a glass substrate by screen-printing and sintering process, was subjected to a heavy sunshine weather-meter test. The conversion efficiency of the module began to degrade after 50 days. Chemical analyses on gases in the sealed module showed that decrease in oxygen down to 2 vol.% had caused the degradation. Inversely, when sufficient oxygen was supplied to the solar cell by breaking the seal, the efficiency recovered up to its initial value within several days even at room temperature. Estimation of reaction velocity at room temperature suggested that the present recovery phenomenon was the same as increase in p-type characteristics by heat treatment in air in other fabrication process of CdS/CdTe junction. Easiness and reversibility of transformation from oxygen deficient to sufficient states in CdTe, strongly suggest that oxygen in CdTe induces a single acceptor which was confirmed in oxygen doped ZnSe.     

Semi-transparent solar collector window systems

Semi-transparent window solar collectors which can be integrated with opaque wall collectors for space heating and ventilation are described for buildings containing large quantities of glass. Simpler, inexpensive, retractable systems are also described. Such systems can have rapid impact on fuel savings. The achievement of 20 per cent grey-scale light transmission combined with 75 per cent thermal utilization of the total solar energy seems feasible. The collector system works by means of semi-transparent thin absorber and reflector coatings on double-glaze windows. Vents at the top and bottom of the double-coated window system permit convective flow of solar-heated air currents for space heating on cold sunny days, convective room ventilation on warm sunny days, and thermal insulation in the absence of sunshine. An account is given of experimental work on sunlight-absorbing and reflecting materials and coatings for implementation of the window systems. These metallic, (alloy or bilayer) and semi-conductor coatings also have applications to Schottky barrier and conventional solar cells and coatings for solar thermal collectors.     

Experimental study of the influence of collector height on the steady state performance of a passive solar air heater

Passive solar air heaters, such as solar chimneys and Trombe Walls, rely on solar-induced buoyancy-driven (natural) convection to produce the flow of air. Although buoyancy-driven convection is well understood for a single vertical plate, buoyancy-driven convection in an asymmetrically-heated channel is more problematic, and in particular, the effects of the channel height on the flow rate and heat transfer. This paper reports on experiments on test rigs resembling lightweight passive solar air-heating collectors. The test rigs were of heights 0.5, 1.0 and 2.0 m, with adjustable channel depths (20-150 mm) and heat inputs (up to 1000 W/m²). Measurements were made of the air, plate and cover temperatures, and air velocities. Results are presented as dimensionless correlations of mass flow (as Reynolds number) and efficiency against heat input (as Rayleigh number), channel depth and height. Thermal efficiency is shown to be a function of the heat input and the system height, but not of the channel depth; mass flow is shown to be a dependent on all three parameters.     

Evaluation of the thermal performance indices of a ventilated double window through experimental and analytical procedures: Uw-values

Simulations to evaluate energy demand for heating and cooling and thermal comfort are becoming more and more common place in the building design process, at least in the most complex cases. In all detailed or simplified calculations, to analyse heat transfer to and from a building, several input parameters are needed. The inputs for the simulation of a whole building are at least the building geometry, the building envelope thermal indices (like thermal transmittance or the solar heat gain coefficient) and typical local climatic data. In a ventilated double window, the air flow through the channel between the two windows makes its thermal performance highly dynamic and dependent on the air flow characteristics. For a whole building simulation, single coefficients or easily calculated coefficients are needed for each facade system, including ventilated systems. In this paper, equivalent thermal transmittance coefficients for a ventilated double window are assessed and presented. For that, experimental measurements in the absence of solar radiation (night period) were used to identify tendencies and validate calculations. Furthermore, simulations were done in order to estimate the U_w-values of the ventilated double window under different windows configuration and different air flow rates. These values can then be used in whole building simulation programmes.     

Control of conduction band offset in wide-gap Cu(In,Ga)Se2 solar cells

The effects of conduction band offset of window/Cu(In,Ga)Se₂ (CIGS) layers in wide-gap CIGS based solar cells are investigated. In order to control the conduction band offset, a Zn_1−xMg_xO film was utilized as the window layer. We fabricated CIGS solar cells consisting of an ITO/Zn_1−xMg_xO/CdS/CIGS/Mo/glass structure with various CIGS band gaps (E_g≈0.97–1.43 eV). The solar cells with CIGS band gaps wider than 1.15 eV showed higher open circuit voltages and fill factors than those of conventional ZnO/CdS/CIGS solar cells. The improvement is attributed to the reduction of the CdS/CIGS interface recombination, and it is also supported by the theoretical analysis using device simulation.     

Calculation of Escaped Solar Energy in Glazing Façade Buildings

ABSTRACT

In building's cooling load calculation, solar heat gain through transparent envelope is calculated by using solar heat gain coefficient which is a thermal performance parameter of window. In traditional buildings, window-wall ratio is small so it's is assumed that the incoming solar radiation can't escape through the window again. But this hypothesis isn't suitable for glazing façade buildings. To calculate the escaped solar energy ratio, a solar radiation model is established on the basis of radiosity-irradiation method and calculated by using the commercial software of Matlab. The impact of time, room geometric dimensioning and absorptance of interior surfaces are evaluated. The numerical calculation results show that the escaped solar radiation ratio varies according to solar radiation incident angle in different times and its maximum value is 8.85% in summer solstice; compare to the width, the depth and height of the room affect the ratio significantly; the reflectance of the floor has greater impact on the escaped solar energy ratio than of other internal surfaces. Finally a fitting formula of escaped solar energy ratio is provided as a function of the ratio between the window area and the internal surface area and of the internal surfaces' absorptance.     

Solar thermal modelling of a non-airconditioned building: Evaluation of overall heat flux

This paper presents an investigation of the thermal behaviour of a non-airconditioned building with walls/roof being exposed to periodic solar radiation and atmospheric air while the inside air temperature is controlled by an isothermal mass, window and door in the walls of the room. The effects of air ventilation and infiltration, the heat capacities of the isothermal storage mass inside air and walls/roof, heat loss into the ground, and the presence/absence of the window/door have been incorporated in the realistic time dependent periodic heat transfer analysis to evaluate the overall heat flux coming into the room and the inside air temperature. A numerical computer model using typical weather data for Delhi has been made to appreciate the analytical results quantitatively. It is found that the heat fluxes through different walls have different magnitudes and phase lags w.r.t. the corresponding solair temperatures. The overall heat flux coming into the room as well as the room air temperature are sensitive functions of the number of air changes per hour, closing/opening of the window and the door ventilation. The effects of the heat capacity of the isothermal mass and the basement ground are found to reduce the inside air temperature swing and the presence of a window is found to increase the inside air temperature even when the window area is much smaller than the wall/roof area. The model presented would be an aid to a building architect for good thermal design of non-airconditioned buildings.     

Solar houses/heating and cooling progress report

Performance data on seven solar homes are given. Solar Homes No. 1, 2, 3, and 4 are near Washington, D.C., 39° north latitude, where about half of the winter days are cloudy and temperatures drop far below freezing, sometimes to 0°F. These houses are described in the book Solar Houses and Solar House Models by Harry E. Thomason, published by Edmund Scientific Company, Barrington, New Jersey, 08007. Edmund Scientific Co. also publishes Solar House Plans, for building a house similar to Solar House No. 1, with improvements.

1. Solar House No. 1Solar House No. 1 has been in continuous operation for thirteen years. In its first year, solar heat supplied about 95 per cent of the heat requirements for home temperatures at 70°F, plus or minus 2°F. After 5 yr of operation, the heat collector was rebuilt. Longer-lived materials were used although efficiency was lowered somewhat. Also changes were made in the air conditioning system.

2. Solar House No. 2A number of changes were incorporated in House No. 2, built in 1960 and 1961. Cost for the original system was lowered, but the auxiliary heat cost ran slightly higher. An aluminum reflector was installed at the bottom of the solar heat collector to reflect additional sunlight onto the collector. The air conditioning system in House No. 2 is rather satisfactory, and that type of system is now in House No. 1 as well as in House No. 2.Summertime heat leakage from the solar heat collector into the closet space behind the collector was solved in House No. 2. The closet remains cool. However, at times the temperature in the closet drops too low and a new problem has to be solved by re-introducing heat to the closet.

3. Solar House No. 3The architectural appearance of House No. 3 was improved. Low-cost glazing with a minimum of glass breakage was achieved. The heat collector was moved entirely up to the roof so that winter sunshine enters the living room and built-in swimming pool on the south side. Improved air conditioning was installed.

4. Solar House No. 4A new type of solar heat collector (with asphalt shingles) and a new type of low-cost “Pancake” heat storage were incorporated into this A-frame house.

5. Solar House No. 5House No. 5, planned for a South Carolina firm, was never built due to insufficient funds.

6. Solar House No. 6House No. 6 has been completed in Mexico City, Mexico. The house and system were not constructed as the authors recommended so the solar heating system does not provide the major part of the heat load. Although Mexico City is quite far south (19° north latitude), the temperature drops below the freezing mark at times. (On December 8, 1970, the temperature dropped to 24°F.)

7. Solar House No. 7The authors have designed and engineered a new system, their Solaris “Sunny South Model,” with three principle innovations. (1) The system uses “Pancake” under-the-floor heat storage. (2) The system utilizes a shallow roof-pond solar heat collector with a reflector to intensify solar input. (3) The system allows the warm water from the roof-pond to drain each night to the under-floor “Pancake” heat storage area where it warms the floor and living space. During the summer the roof-pond helps minimize day-night temperature extremes by absorbing excess heat during the day and liberating it at night.

     

玻璃窗内加布帘后传热性能的研究

对玻璃窗内加布帘后的传热系数进行了测定。实验的窗系统分别为单层玻璃和双层玻璃内加布帘。实验中考虑了布帘边缘自然松弛和布布边缘密封的影响。通过对１００％玻璃的窗系统进行实验，得出该条件下传热系数的经验关系式，然后对窗框类型和环境风速的影响进行修正。修正后的传热系数经验公式可以方便地用于实际窗系统的瞬态传热计算。     

Natural convective heat transfer in trapezoidal enclosure of box-type solar cooker

This paper presents simple thermal analysis to evaluate the natural convective heat transfer coefficient, h_c12 for a trapezoidal absorber plate-inner glass cover enclosure of a double-glazed box-type solar cooker. Several indoor simulation experiments in steady state conditions have been performed to measure the temperatures of absorber plate, inner and outer glass covers, ambient air, electrical input supply and wind speed. The experimental data has been correlated by an equation of the form, Nu = CRaⁿ. The values of the constants C and n, obtained by linear regression analysis are used to calculate the convective heat transfer coefficient. The heat transfer analysis predicts that h_c12 varies from 4.84 to 6.23 W m^{−2 o}C⁻¹ for the absorber plate temperature from 54 to 141 ^oC. The results of h_c12 are compared with those of rectangular enclosure for the same absorber-inner glass cover temperatures and gap spacing. The study reveals that the values of convective heat transfer coefficient and top heat loss coefficient for rectangular enclosure are lower by 31–35% and 7% respectively.     

A glazing unit for solar control, daylighting and energy conservation

The glazing unit for solar control, daylighting and energy conservation is a system consisting of two prismatic panes. The prismatic ribs of the panes are inclined by a certain angle to the horizontal within the window plane, exhibit identical cross-sections in the shape of a rightangle-triangle with a certain basic prism angle, are facing each other and are positioned such that just a small gap remains between the two panes. The lower rib faces of the outer prismatic pane are coated with a specularly reflecting layer and the upper rib faces of the inner prismatic pane are coated with a diffusely reflecting layer. The prismatic glazing unit can be used for common window tilt angles and for window directions with significant solar irradiation at sites with a temperate climate. It does not reduce the view to the outside appreciably and achieves — in comparison to other window panes — relatively uniform illumination of a room with daylight. During the summer and the transitional seasons it provides improved protection against solar irradiation and distinctly reduced irradiated heat fluxes. The reflecting surfaces of the prismatic ribs do not create glare.     

Analytical optimization of interior PCM for energy storage in a lightweight passive solar room

Lightweight envelopes are widely used in modern buildings but they lack sufficient thermal capacity for passive solar utilization. An attractive solution to increase the building thermal capacity is to incorporate phase change material (PCM) into the building envelope. In this paper, a simplified theoretical model is established to optimize an interior PCM for energy storage in a lightweight passive solar room. Analytical equations are presented to calculate the optimal phase change temperature and the total amount of latent heat capacity and to estimate the benefit of the interior PCM for energy storage. Further, as an example, the analytical optimization is applied to the interior PCM panels in a direct-gain room with realistic outdoor climatic conditions of Beijing. The analytical results agree well with the numerical results. The analytical results show that: (1) the optimal phase change temperature depends on the average indoor air temperature and the radiation absorbed by the PCM panels; (2) the interior PCM has little effect on average indoor air temperature; and (3) the amplitude of the indoor air temperature fluctuation depends on the product of surface heat transfer coefficient h_in and area A of the PCM panels in a lightweight passive solar room.