Passive solar heating of a non-airconditioned building with movable insulation over the roof pond

This paper presents a simple mathematical model for solar space heating in a non-airconditioned building with movable insulation over the roof pond. The building room considered is of rectangular shape (6 m × 5 m × 4 m) based on the ground. The effects of heat conduction to the ground, heat transfers to furnishings and heat losses due to air ventilation/infiltration have been taken into account in the general heat transfer analysis. The day-to-night change of insulation over the roof pond has been represented by a rectangular step function variation of the heat transfer coefficient at the pond's surface. An increase of 3 to 4°C in the room air temperature is achieved by means of movable insulation over the roof pond on a mild winter's day (17th February, 1982) in New Delhi.     

Nonlinear dynamic behavior of non-convective zone in salt gradient solar pond

Non-Convective Zone (NCZ) of salt gradient solar pond is a typical double diffusive system of salinity and temperature, and it is subjected to instable effects of adverse temperature gradient. The onset of instability may occur as an oscillatory motion because of the stabilizing effect of the salinity. In this paper, the marginal state between the steady state and the convection of the NCZ is studied. The stability of the Boussinesq approximation of the Navier-Stokes equations is analyzed by a perturbation approach. The marginal states for the onset of convection are obtained by analytical method, which is based on the linearization of the ordinary differential equations, and then numerical method is used to solve the nonlinear ordinary differential equations. Numerical results provide the trajectories of the temperature and velocity coefficients in the three-dimensional phase space, as well as the two-dimensional temperature, salinity and velocity fields in NCZ. The results demonstrate that the numerical study is in agreement with the marginal stability and the critical Rayleigh number derived from linear stability analysis. Both the linear and nonlinear studies indicate that oscillation is a narrow region above the stable region; however, the nonlinear numerical results indicate that the linear stability analysis leans to a larger upper boundary in the oscillatory regions.     

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.     

Improving the thermal performance of a roof pond system

A roof pond is an effective passive technique for the year-round heating and cooling of buildings. Data obtained from an experimental roof pond building show that the summer performance of such a building can be further improved by allowing vapours to escape from the pond into the atmosphere. Escape of vapours during winter operation can, however, cause deterioration in the performance. It is, therefore, suggeested that a control mechanism be used to allow the vapours to escape only when it is desirable     

Thermal analysis and measurement of a solar pond prototype to study the non-convective zone salt gradient stability

Solar ponds combine solar energy collection with long-term storage and can provide reliable thermal energy at temperature ranges from 50 to 90 °C. A solar pond consists of three distinct zones. The first zone, which is located at the top of the pond and contains the less dense saltwater mixture, is the absorption and transmission region, also known as the upper convective zone (UCZ). The second zone, which contains a variation of saltwater densities increasing with depth, is the gradient zone or non-convective zone (NCZ). The last zone is the storage zone or lower convective zone (LCZ). In this region, the density is uniform and near saturation. The stability of a solar pond prototype was experimentally performed. The setup is composed of an acrylic tube with a hot plate emulating the solar thermal energy input. A study of various salinity gradients was performed based on the Stability Margin Number (SMN) criterion, which is used to satisfy the dynamic stability criterion. It was observed that erosion of the NCZ was accelerated due to mass diffusion and convection in the LCZ. It can be determined that for this prototype the density of the NCZ is greatly affected as the SMN reaches 1.5.     

Experimental and theoretical temperature distributions in a solar pond

In this study, an experimental and theoretical investigation of temperature distributions in an insulated solar pond, particularly during daytimes and nighttimes, is presented. Several temperature sensors connected to a data acquisition are placed vertically inside and the bottom of the pond and also horizontally and vertically in the insulated side walls, and used to measure temperature changes with time and position. In addition, we model the solar pond to compute theoretical temperature distributions and compare with the experimental measurements, and hence a good agreement is found between experimental and theoretical temperature profiles. There is a large amount of heat losses between daytimes and nighttimes, depending upon the temperature difference, and these present a significant potential for energy savings and storage. During the months of January, May and August, it is found that the total heat losses from the inner surface of the pond and its bottom and side walls, as a function of temperature difference, are determined to account for 227.76 MJ (e.g., 84.94% from the inner surface, 3.93% from the bottom and 11.13% from the side walls, respectively).     

Experimental studies on a novel roof pond configuration for the cooling of buildings

A new evaporation based passive cooling technology was tested. The technology is based on the exposure of “floating” wetted cloth to the ambient air. It was compared to various other passive cooling techniques, with very favorable results. Two identical shallow ponds were constructed. One of them was covered with white cotton towels stretched on a densely perforated PVC panel supported by pieces of waterproof polystyrene keeping it just floating on the water surface. Five comparable experiments of different cooling techniques have been carried out. The results indicate that the new cooling technique performed slightly better than the pond with movable insulation, which is widely considered as one of the best roof cooling techniques based on evaporation. It seems that the higher efficiency of the tested technique is due to the thermal stratification created in the water inside the pond, which more effectively resists the transfer of heat gains from the sun and ambient air into the deep water of the pond. In turn, the water temperature near the floor of the pond is lower, thus heat flow from the building to the pond is increased. During the experiment, all the ponds which were compared were ranked according to performance (from best to worse): shaded pond with towels floated on it, pond with towels floated on it and pond with movable insulation, shaded open pond, open pond, covered pond.     

浅谈太阳能热水器与建筑结合的几种安装方式

一引言 随着全国各地太阳能与建筑结合图集的颁布与实施,太阳能热水器的推广应用范围不断扩大,各地已成功地建设了一批太阳能热水器与建筑应用结合的试点工程,同时也加强了太阳能热水器与建筑结合的管理.     

Equilibrium thermal characteristics of a building integrated photovoltaic tiled roof

Photovoltaic (PV) modules attain high temperatures when exposed to a combination of high radiation levels and elevated ambient temperatures. The temperature rise can be particularly problematic for fully building integrated PV (BIPV) roof tile systems if back ventilation is restricted. PV laminates could suffer yield degradation and accelerated aging in these conditions. This paper presents a laboratory based experimental investigation undertaken to determine the potential for high temperature operation in such a BIPV installation. This is achieved by ascertaining the dependence of the PV roof tile temperature on incident radiation and ambient temperature. A theory based correction was developed to account for the unrealistic sky temperature of the solar simulator used in the experiments. The particular PV roof tiles used are warranted up to an operational temperature of 85 °C, anything above this temperature will void the warranty because of potential damage to the integrity of the encapsulation. As a guide for installers, a map of southern Europe has been generated indicating locations where excessive module temperatures might be expected and thus where installation is inadvisable.     

Wind environment at a roof-mounted wind turbine on a peaked roof building

Knowledge of the wind resource above peaked roofs is necessary to determine whether installing small wind turbines on low-rise peaked roof buildings is feasible. The wind characteristics at a representative peaked roof barn in southern Ontario, Canada were investigated using a boundary layer wind tunnel and computational fluid dynamics. Field measurements at the barn were collected using sonic anemometers and compared with the simulation results. Wind speed amplification was confined to a region immediately above the roof and was relatively low for wind energy purposes. The presence of nearby trees or buildings adversely impacted wind speed amplification. Considering only wind-related factors, the placing of micro-wind turbines on roof peaks may be warranted. However, if sufficient space is available, it is recommended to place small turbines on a tower rather than on the peaked roof of a low-rise building.     

Optimal roof structure with multilayer cooling function materials for building energy saving

Both cool roof and phase change thermal storage are promising technologies in decreasing building energy consumption. Combining these two technologies is likely to further enhance the thermal comfort of the building as well as reduce air condition loads. In this paper, the cooling performance and energy-saving effects of four types of roof (normal roof, phase change material [PCM] roof, cool roof, and cool PCM roof [cool roof coupled with PCM]) were investigated under a simulated sunlight. Experimental results indicate that compared with normal roof, the other three roofs are able to narrow the indoor temperature fluctuation and decrease the heat flow entering into the room. Among them, cool PCM roof gave the best energy-saving effect that can lower the indoor temperature and heat entering into rooms by 6.6°C and 52.9%, respectively. Besides, the PCM location, PCM thickness, and insulation thickness exerted great impacts on the cooling performance of the roof. Placing the PCM on the internal layer beneath the extruded polystyrene (XPS) insulation board can make the indoor temperature 1.2°C lower than that on the middle layer. Although thicker PCM panels or insulation boards can provide a better thermal insulation, 5 mm in PCM thickness and 20 mm in insulation thickness are enough to guarantee the indoor temperature of cool PCM roof system at a comfortable range (22°C-28°C) for a whole day. These findings will give guidance in designing buildings with a light and compact roof structure to decrease energy consumption and improve comfort level.     

坡屋顶住宅台式太阳热水器的防雷措施

太阳热水器大都需要安装于屋脊上，其避雷防雷措施极为重要和迫切，施工难度也较大。本文仅就安装于坡屋顶住宅屋脊上台式太阳热水器的避雷防雷措施进行探讨，其它安装方式可以参照借鉴。屋顶装设的太阳热水器的避雷防雷包含两层意思：一是因太阳热水器和其金属支架高出屋脊（屋面）避雷带，在屋面覆盖，其本身需要避雷防雷保护；二是因太阳热水器与室内连接，既有管线的传导也有水的传导，室内用户需要人身避雷防雷。我们经过长期的施工实践，探索出一套相关施工技术措施，证明有效并得到了专家及用户的认可。以清华阳光太阳热水器安装为例…     

Determination of the optimum pond temperature for maximizing power production of a convecting solar pond thermal-energy conversion system

In order to maximize power production of a convecting solar pond power plant, the optimum pond temperature and the corresponding final conversion efficiency are determined numerically. As the heat sink temperature of the associated power plant was increased, the optimum pond temperature increased and the corresponding final conversion efficiency decreased. Further, the optimum final conversion efficiency of the present solar pond thermal-energy conversion system was found to be less than 3% under the meteorological conditions of Japan.     

Conjugate turbulent natural convection in the roof enclosure of a heavy construction building during winter

This paper presents the results of a study of conjugate turbulent natural convection inside a building attic in the shape of a rectangular enclosure bounded by realistic walls made from composite construction materials under winter day boundary conditions. The effects of cavity aspect ratio, Rayleigh number (Ra), depth of the external concrete beam, and external wall construction materials on the flow and heat transfer characteristics were the main focus of the investigation. The Shear stress transport k–ω turbulence model is implemented to calculate air-flow velocities and temperatures in a steady, turbulent, two-dimensional conjugate natural convection heat transfer inside an attic. The governing equations were solved by employing the line-by-line tri-diagonal matrix algorithm (TDMA) control volume method. For Ra ranging from 10⁷ to 10¹⁰, steady-state results of the streamline and temperature contours in addition to local and mean Nusselt numbers at all surfaces of the cavity were obtained. The results show that the values of Ra, attic aspect ratio and the composite wall materials have significant effect on the temperature and stream function contours within the enclosure, and the heat flux out of the room through the enclosure.     

Solar pond modeling with density and viscosity dependent on temperature and salinity

The paper presents a 2D numerical model where the behavior of a salt gradient solar pond (SGSP) is described in terms of temperature, salt concentration and velocity with the fluid density and viscosity dependent on temperature and salt concentration. The discretization of the governing equations is based on the respective weak formulations. The rectangular geometry allows for spectral type Galerkin approximations for which the essential homogeneous boundary conditions can easily be imposed. Taking into account the variation of density and viscosity with temperature and salinity improved the agreement between the numerical and the experimental results.     

Optimal heating control in a passive solar commercial building

A smart heating controller has a twofold objective: to save as much energy as possible while maintaining an acceptable comfort level in the building. Due to very large time constants in the building response, it has to anticipate internal and external disturbances. In the case of a passive solar commercial building, the need for anticipation is reinforced by important solar and internal gains. Indeed, large solar gains increase the energy savings potential but also the overheating risk. Optimal control theory presents an ideal formalism to solve this problem: its principle is to anticipate the building behaviour using a model and a forecasting of the disturbances in order to compute the control sequence that minimises a given cost function over the optimisation horizon. This cost function can combine comfort level and energy consumption. This paper presents the application of optimal control to auxiliary heating of a passive solar commercial building. Simulation-based and experimental results show that it can lead to significant energy savings while maintaining or improving the comfort level in this type of building.     

Maintenance strategy for a salt gradient solar pond coupled with an evaporation pond

In a previous study, the authors presented a simple mathematical model for predicting the ratio of the evaporation pond area to that of a salt gradient solar pond area. The evaporation pond idea provides a very attractive method of salt recycling by evaporation, especially in areas of high evaporation and low rates of rain as it is the case for North Africa.In this paper, the model was elaborated upon and applied to two types of surface water flushing (fresh water and seawater) under the prevailing conditions of Tripoli, Libya (latitude=32.86°N). All the results presented were predicted for the first three years of operation. The daily variations of brine concentration in the of Tajoura's Experimental Solar pond and those based on different designs were predicted and discussed under different scenarios. The quantities of brine provided by the evaporation pond and that required by were predicted for both cases of surface water flushing (fresh water and seawater) under the different design conditions. It was predicted that the can provide 20–40% during the first year and 45–95% during the third year depending on the design selected.     

Design methodology for a salt gradient solar pond coupled with an evaporation pond

This paper presents the results of a simple mathematical model for predicting the ratio of the evaporation pond (EP) area to that of a Salt Gradient Solar Pond (SGSP) area. The EP idea provides a very attractive method of salt recycling by evaporation, especially in areas of high rates of evaporation and low rates of rain as it is the case for North Africa. The model is applied for two types of surface water flushing (fresh water and seawater) under the prevailing conditions of Tripoli-Libya (Lat.=32.68°N) and for measured evaporation rates. Under the summer conditions and for the case of surface flushing by fresh water, the area ratio was estimated at about 0.17. While for the case of using seawater this ratio increases enormously to about 14.4. The time required for the salt concentration to increase from seawater concentration to a high concentrated brine, which can be injected at the bottom of the solar pond, is also presented. It was estimated that the time required to increase the salt concentration from 3.5 to 35% is about 120 to 250 days during the summer months and about 200 to 220 days during the winter months.     

Optimum pond temperature for maximizing power production of a non-convecting solar-pond heat-conversion system

In order to maximize the output from a non-convecting solar-pond powerplant, the optimum pond temperature and the corresponding final conversion efficiency are determined numerically for the meteorological conditions of Japan. In the analysis, the total depth of the pond is assumed to be 3 m, while the thickness of a non-convecting zone and the heat sink temperature of the power plant are varied. The optimum final conversion efficiency of the present non-convecting solar-pond thermal-energy conversion system is about half that of a convecting solar-pond power-plant.