Thermocirculation characteristics of a Trombe wall passive test cell

Passive heating, using Trombe wall elements for solar energy collection, can be readily integrated into new building structures and can be retrofitted to existing buildings.

Although the basic physical principles underlying the natural operation of such air thermosiphon devices have been well known for many years now, there is still insufficient design information available to allow sensible sizing and installation decisions to be undertaken by architects and thermal system designers. In this experimental investigation, flow visualisation studies have given a deeper insight into the fundamental flow mechanisms; whilst air velocity and temperature measurements have been used to explore the natural convection heat transfer processes involved in the thermocirculation flow. Particular attention has been paid to the effects on operational performance of the wall parameters such as wall/glazing distance and vent size.

Adequate data correlation of the experimental results has been achieved by using expressions derived for natural (free) convection processes occuring between vertical paralled plates; in this case the vertical heated wall surface and the cooler vertical glazing panel. It is felt that this type of fundamental heat transfer information is vital so that the overall performance of Trombe wall systems can be adequately modelled using the large range of simulation techniques currently available for thermal network analysis.     

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.     

Performance of various design of solar air heaters for crop drying applications

Different heat sources are employed for the drying of agricultural products. However, in many rural locations in most developing countries, supplies of non-renewable sources of energy are either unavailable, unreliable or, for many farmers, too expensive. In renewable energy sources, solar energy is the most appropriate for drying systems. This energy allows independent systems to be constructed and possesses a thermal conversion mode which necessitates a simple technology which is adapted to the rural regions for crop drying applications. These systems are all based on the air heating flat plate solar collectors.Therefore, six different types of natural circulation air heating solar collectors (Model-1: single plastic glazing, black painted hardboard absorber and front-pass; Model-2: single plastic glazing, black painted flat plate absorber and front-pass; Model-3: single plastic glazing, black painted zigzag plate absorber and front-pass; Model-4: single plastic glazing, black painted flate plate absorber and back-pass; Model-5: single plastic glazing, black painted zigzag plate absorber and back-pass; Model-6: double plastic glazing, black painted flat plate absorber and back-pass) were designed, constructed and analysed for their performance in this study. Each collector mainly consisted of a frame constructed from hardboard, vent holes, hardboard insulation, absorbing surface made of black coated aluminium sheet and clear plastic glazing.All solar air heaters were mounted on a stand facing south at an inclination angle, and they were tested simultaneously under the same environmental conditions. The experimental setup was instrumented for the measurement of solar radiation, temperature and relative humidity of the atmosphere air, outlet air temperature, surface temperature of the back and edge insulator and absorber plate, air speed and wind velocity.It is understood from the results of the investigation that the performances of Model-1, Model-2, Model-3, Model-4, Model-5 and Model-6 are 42.11, 45.88, 44.23, 39.76, 39.05 and 36.94% respectively, and the performance of the most efficient collector (Model-2) is aproximately 9% more than the least efficient one (Model-6). In addition, it is seen that unlike number of glazing sheet and air pass method, the effect of the shape of the absorbing surface on the performance is considerably less.     

Determination of glazing area in direct gain systems for three different climatic zones

This study determines the glazing area in direct gain passive systems needed to ensure thermal comfort inside a building (room air temperature 20 ± 2°C). A 4 m × 4 m × 3 m single zone isolated house is analyzed in three different types of climates namely composite (8°C to 20°C, New Delhi), cold-cloudy (−2°C to 5°C, Srinagar), and cold-sunny (−14°C to −3°C, Leh). The analysis is based on the periodic solution of the heat conduction equations describing heat transmission in the building components, floor, walls, and roof, and the Fourier representation of the ambient temperature vnd the total solar radiation intercepted by the building envelope. Two types of construction are analyzed: the first type is a traditional construction with 22-cm-thick brick wall, plastered 15 mm on both the sides (U = 2.0 W m⁻² K⁻¹); and the second one is of the same type but with 10 cm of expanded polystyrene insulation on all the four walls and the roof (U = 0.31 W m⁻² K⁻¹). It is found that for traditional construction with U = 2.0 W m⁻² K⁻¹, the glazing U value has almost no effect on the room temperature even for large variation of the glazing area (10% to 40%, expressed in terms of percentage of floor area). For a well-insulated house (U = 0.31 W m⁻² K⁻¹), the glazing U value has no effect upon the room air temperature if the glazing area is small (less than 10%). The position of the insulation on the external surfaces is more effective in reducing large inroom air temperature. Finally, for an insulated house, we recommended glazing is 30%, 20%, and 10% for cold-sunny, cold-cloudy, and composite climates, respectively.     

Novel glazing technologies to mitigate energy consumption in low‐carbon buildings: a comparative experimental investigation

Buildings play a key role in total world energy consumption as a consequence of poor thermal insulation characteristics of facade materials. Among the elements of a typical building envelope, windows are responsible for the greatest energy loss because of their notably high overall heat transfer coefficients. About 60% of heat loss through the building fabric can be attributed to the glazed areas. In this respect, novel cost‐effective glazing technologies are needed to mitigate energy consumption, and thus to achieve the latest targets toward low/zero carbon buildings. Therefore in this study, three unique glazing products called vacuum tube window, heat insulation solar glass and solar pond window which have recently been developed at the University of Nottingham are introduced, and thermal performance analysis of each glazing technology is done through a comparative experimental investigation for the first time in literature. Standardized co‐heating test methodology is performed, and overall heat transfer coefficient (U‐value) is determined for each glazing product following the tests carried out in a calibrated environmental chamber. The research essentially aims at developing cost‐effective solutions to mitigate energy consumption because of windows. The results indicate that each glazing technology provides very promising U‐values which are incomparable with conventional commercial glazing products. Among the samples tested, the lowest U‐value is obtained from the vacuum tube window by 0.40 W/m²K, which corresponds to five times better thermal insulation ability compared to standard air filled double glazed windows. Copyright © 2015 John Wiley & Sons, Ltd.     

The use of a thermal energy recycle unit in conjunction with a basin-type solar still for enhanced productivity

A recently developed thermal energy recycling unit operating under forced air circulation was attached to a conventional, basin-type solar still to enhance overall still productivity. In this unit, a relatively large fraction of the latent heat of condensation of the distillate is utilized to preheat and evaporate the feedstock. The system performance was tested in the laboratory using a solar simulator. The solar still was double glazed and no condensation was observed on the inner glazing when operating in the thermal energy recycling mode. The overall system productivity was about three times that of a conventional (single-effect) basin-type solar still. The advantages of the proposed system design are the following: (i) the solar still productivity can be enhanced significantly and at a reasonable cost; (ii) non-wetting glazings (e.g. certain plastic glazings) can be utilized, since in this mode of operation the glazing does not function as a condensation surface; (iii) as a result, the thermal losses from the outer surface of the glazing to the ambient can be reduced significantly by the use of double glazings; (iv) the system is very adaptable to the utilization of an external waste energy source (e.g. wet steam or hot saturated air) for nocturnal distillation, viz. operation in the absence of solar radiation.     

The effect of glazing shape upon the thermal performance of buildings

The great incidence that glazing has in a building energy conservation makes it one of the most important parameters to be taken into account especially in commercial buildings, where the surface occupied by glass areas is very important. So, different shapes of glass areas and their influence in the energy consumption of a commercial building are studied in this paper. Horizontal glazing (with different heights) and vertical glazing (with the same area as the horizontal ones), separated by opaque areas are considered in a base case building. A traditional wall and a curtain-wall are considered, and the different annual consumptions per conditions unit surface, both in winter and summer, are obtained.     

Performance of PV-Trombe wall in winter correlated with south façade design

PV-Trombe wall (PVTW) is a novel version of Trombe-wall. Photovoltaic cells on the cover glazing of the PVTW can convert solar radiation into electricity and heat simultaneously. A window on the south façade can also introduce solar heat into the room in the winter season. Experiment has been conducted to study the temperature field of a building with both southern facing window and the PVTW. A dynamic numerical model is developed for the simulation of the whole building system. The temperature of the indoor air is found to be vertically stratified from the measurement. The nodal model is adopted to calculate the temperature profile in the room. The simulation results are in good agreement with the experimental data. The different south façade designs affect the thermal efficiency of the PVTW significantly from the numerical simulation. With a southern facing window, the thermal efficiency of the PVTW is reduced by 27% relatively. The increase of PV coverage on the glazing can reduce the thermal efficiency of the TW by up to 17%. By taking account of electric conversion, the total efficiency of solar utilization is reduced by 5% at most while the glazing is fully covered with PV cells. The electric conversion efficiency of the PVTW achieves 11.6%, and is slightly affected by south façade designs.     

Thermal transmittance of multiple glazing: computational fluid dynamics prediction

Computational fluid dynamics (CFD) is applied for predicting the convective heat transfer coefficient, thermal resistance and thermal transmittance for a double glazing unit. The predicted thermal resistance of glazing is compared with reference data and good agreement is achieved. The convective heat transfer coefficient and thermal transmittance vary with the air space width and the temperature difference across glazing. The CFD technique can be used to gain insight into multiple glazing performance and also optimise the design and operation of novel multiple glazing systems such as air flow windows or double skin facades in terms of energy efficiency and thermal comfort.     

Solar and thermal energy gain through windows in Jordan

Solar gain and thermal energy transfer through windows is studied for three different sites in Jordan using the TRNSYS computer program. Solar and thermal energy is calculated using the monthly average daily data for the above-mentioned three regions. Calculation of hourly radiation on a vertical plane is presented, and also the method of determination of the amount of radiation transmitted through the glazing layers is given. The effect of window orientation on the total solar gain is analysed. It is found that for all directions, solar gain is season-dependent, and this dependency varies from one direction to another. Calculations are carried out for two cases of glazing location: case 1, glazing flush with the outside of the wall; and case 2, glazing recessed by 15 cm from the outside wall, which represents a window with overhang and sidewalls. The number of glazing layers is taken as 1, 2 and 3 to observe the effect on solar gain as well as on the thermal energy exchange between the inside and outside of the building. During the calculations, the temperature of the inside is fixed at 22°C for the entire year. The results are tabulated to serve as a database for solar and thermal energy in Jordan.     

Thermal network predictions of the daily temperature fluctuations in a direct gain room

This paper studies the daily temperature fluctuations in a direct gain room measuring 6.1 × 4.57 × 2.44 m³ (20 ft × 15 ft × 8 ft). The room is assumed to have losses on three faces and 8.36 m² (90 ft²) of south glazing. Let r denote the ratio of the surface area of the mass wall to the area of the south facing glazing. The effect of r on mass walls 10.16, 20.32 and 30.48 cm (4, 8 and 12 in.) thick was determined for values of r equal to 1, 2, 4, 8 and 11.1. The temperature and solar insolation values for a typical January day in Nebraska were duplicated thirty consecutive times and were used for the weather data input to the modeling program. Using thirty consecutive like days has the advantage of damping out the transient solution to the point where it is negligible. It was found that for each thickness of wall, the temperature fluctuations over a day decrease as r increases, and it was also found that for a fixed value of r, the daily temperature fluctuations decrease as the wall thickness is increased. These results are consistent with those reported by Mazria.In addition cloudy day storage was considered for both the 10.16 and 20.32 cm (4 and 8 in.) walls, with r fixed at 4, runs were made with 20, 33, 50 and 100 per cent cloudy days. The results are presented in graphical form and indicate a rapid recovery of the system in most cases.     

Thermal conductance measurement on vacuum glazing

A method is described for measuring the thermal conductance of vacuum glazing that is well-suited for integration into the manufacturing process of such devices. The sample of vacuum glazing to be measured, initially at elevated temperature, is placed in contact with a second sample of vacuum glazing with a known thermal conductance. The external surfaces of the glazings are then cooled by forced flow of air at room temperature, and a measurement is made of the rate of decrease of the temperature of the contacting glass sheets of the two samples. The method is simple to implement, and can be automated. The results obtained with the method are quite reproducible. The measurement can be made as the production samples of vacuum glazing cool at the completion of the manufacturing process, resulting in significant savings in time and labour compared with other methods.     

Energy analysis of buildings employing thermal mass in Cyprus

In this paper the effects on the heating and cooling load resulting from the use of building thermal mass in Cyprus are presented. This is achieved by modelling and simulation with the TRNSYS program of a typical four-zone building with an insulated roof in which the south wall of one of the zones has been replaced by a thermal wall. Despite the fact that the diurnal temperature variations in Cyprus are ideal for the application of thermal mass, no such application is presently available. Therefore the main objective of this paper is to investigate the possible benefits resulting from such an application. The results of the simulation show that there is a reduction in the heating load requirement of the zone by about 47%, whereas at the same time a slight increase of the zone-cooling load is exhibited. Optimisations of the various construction parameters have also been carried out. The optimum overhang size is found to be equal to 1.2 m with minor variations in the range of 1 to 1.5 m. The effect of the air gap size between the glazing and the thermal wall is insignificant. The optimum value of wall thickness obtained is equal to 25 cm. The effect of roof insulation is investigated and it is found that insulation is a must for better comfort conditions. Also, the effect of applying ventilation whenever the ambient temperature is lower than the indoor temperature during summertime is investigated. A reduction of 7.5% is obtained when air at 3 air changes per hour is directed into the house. In conclusion it can be said that the thermal wall offers some advantages and should be used whenever buildings are erected with south-facing walls.     

A numerical study of solar chimney for natural ventilation of buildings with heat recovery

The performance of a glazed solar chimney for heat recovery in naturally-ventilated buildings was investigated using the CFD technique. The CFD program was validated against experimental data from the literature and good agreement between the prediction and measurement was achieved. The predicted ventilation rate increased with the chimney wall temperature. The effects of solar heat gain and glazing type were investigated. It was shown that in order to maximise the ventilation rate in a cold winter, double or even triple glazing should be used. Installing heat pipes in the chimney for heat recovery not only increased the flow resistance but also decreased the thermal buoyancy effect. To achieve the required air flow rates in naturally-ventilated buildings with heat recovery, use should be made of wind forces.     

Theoretical study of a composite Trombe-Michel wall solar collector system

In this study, the transfer of solar radiation in a composite Trombe-Michel wall solar collector system is studied theoretically. The composite system consists of a glazing, a massive wall, and an insulating wall put together without a convection channel between glazing and wall and with one between massive and insulating walls. It is an improvement over the simple nonconvective Trombe-Michel wall solar collector system with a relatively low thermal resistance, which is taken as a reference. The theoretical results indicate that the composite system can indeed perform better in cold and/or cloudy climates than the reference system and that optimum geometrical parameters can be determined depending on the dwelling type and climatic conditions of the area. The new system has a reduced massive wall thickness that is a desirable feature for lightweight constructions.     

Mathematical modeling of a box-type solar cooker employing an asymmetric compound parabolic concentrator

A novel design of solar cooker is introduced. The cooker is of box-type equipped with an asymmetric compound parabolic concentrator (CPC) as booster-reflector. It consists of an insulated box equipped with a vertical double glazing cover on a side, and a vertical absorber plate laid out just behind the transparent cover. The booster-reflector is fixed on the glazed side of the box. The absorber plate and the glazing form a vertical channel, open at the top and bottom, and enclosed at the sides. The two openings allow the inside air circulation. A mathematical model of the heat transfer processes involved with this solar cooker, containing a cooking pot loaded with water and deposited on the box floor; was developed and the effects of various parameters, such as solar radiation, load of water and clouds on the dynamic behavior of the cooker are studied.     

Thermal performance of an architectural window with chemically deposited SnS---CuxS solar control coating

A mathematical model enabling the prediction of the thermal performance of solar control glazings employing chemically deposited solar control coatings with or without a transparent protective polymer coating is presented. Differential energy balance for the glazing is set up assuming one-dimensional steady state case for normal incidence of air mass 2 solar radiation and by considering conductive heat transfer within the glazing and convective and radiative heat transfer into the interior and exterior of the building. Using the specific example of the optical properties of the already reported SnS---Cu_xS solar control coatings, the redistribution of the absorbed component of the solar radiation is evaluated for constant convective heat transfer coefficient and temperature in the interior and for exterior temperatures in the 0–50°C range. The results yield shading coefficient versus exterior temperature curves for two specific SnS---Cu_xS coatings without and with a protective transparent varnish and offering transmittance in the visible region of 27 and 21%.     

Performance of a solar chimney

A mathematical model of a solar chimney was proposed in order to predict its performance under varying ambient and geometrical features. Steady state heat transfer equations were set up using a thermal resistance network and solved using matrix inversion. Existing correlations of heat transfer coefficients were utilised. Property values for the air flow in the duct were based on mean bulk or film temperatures. The performance of the chimney was evaluated by predicting the temperatures of the glass glazing and the heat-absorbing wall and also the temperature and velocity of the induced air flow in the chimney. The effects of air gap and solar radiation intensity on the performance of different chimneys were investigated. In order to verify the theoretical model, experiments were conducted on a 2 m high×0.45 m wide physical model with air gaps of 0.1, 0.2 and 0.3 m. Experiments were carried out outdoors on the roof and the experimental model exposed to both direct and diffuse solar radiation. Air velocities between 0.25 m s⁻¹ and 0.39  m s⁻¹ for radiation intensity up to 650 W m⁻² were obtained. No reverse air flow circulation was observed even at the large gap of 0.3 m.     

Thermal performance assessment of an advanced glazing system

The four different techniques which were used to test an advanced, four-pane glazing system and standard double-glazed unit are described. The results from each test are compared. Where agreement is not good, explanations are suggested. The advanced glazing system was found to have a U-value of 0.9 W/m² K and a shading coefficient of 0.48. The glazing simulation models WINDOW (Lawrence Berkeley Laboratories, Berkeley, CA, U.S.) and MULTB (Pilkington Glass, U.K.) were used to predict glazing performance. Simulation of the two glazing systems which were experimentally assessed allows comparison between models, and between predicted and measured performance. Agreement was within the error bands associated with each assessment.