Total Heat Gain Research Articles

高反射率塗料の日射反射性能に関する研究

Reflective paint is used to reduce the solar heat gain of buildings in hot areas. When the reflectivity of building envelops in urban areas are increased, the total heat gain in the city is decreased and the heat island phenomena can be restrained. In this paper, the characteristics of reflective paints, which are available in Japan, are studied. Firstly the spectral reflectance is measured by spectrophotometer for 61 paints that include 22 white paints, 19 grey ones and 20 black ones. There exists a large difference of reflectance for grey and black paints. On the other hand, no remarkable difference is observed for white paints. Secondly the relationship between incidence angle and solar reflectance is studied by the experiment in which 44 metal boards covered with reflective paint are exposed to sunshine and the surface temperature of each board is measured. The influence of the incidence angle is not large.

Optimum Overhang Geometry for Building Energy Saving in Tropical Climates

abstractIn hot and humid climates, one draw back of using shading devices is the risk of reducing daylight level which in turn increases the use of artificial lighting. It is important to understand the magnitude of energy consumption for cooling and lighting when shading devices are adapted in order to propose optimum external horizontal shading strategies as design solutions. This study investigates the effect of six different alternatives of external horizontal shading devices on incident solar radiation, transmitted solar heat gains, natural-light penetration and energy consumption. The study was carried out using a standard, single fenestration perimeter office room in a typical high-rise office building. The investigation is conducted using eQUEST-3, which is a dynamic energy simulation program supported by DOE2.2 calculation engine. The results showed several optimum geometry of the external horizontal shading device depending on incident direct solar radiation, transmitted solar heat gains, natural-light penetration and energy consumption. This study concludes, considering the trade off between total heat gain and natural-light penetration to optimize the total energy consumption as the best option in designing external solar shading in hot and humid climates.

Simplified model for a ventilated glass window under forced air flow conditions

This paper presents a study on a ventilated window composed of two glass sheets separated by a spacing through which air is forced to flow. The proposed model is one dimensional and unsteady based upon global energy balance over the glass sheets and the flowing fluid. The external glass sheet of the cavity is subjected to variable heat flow due to the solar radiation as well as variable external ambient temperature. The exchange of radiation energy (infrared radiation) between the glass sheets is also included in the formulation. Effects of the spacing between the glass sheets, variation of the forced mass flow rate on the total heat gain and the shading coefficients are investigated. The results show that the effect of the increase of the mass flow rate is found to reduce the mean solar heat gain and the shading coefficients while the increase of the fluid entry temperature is found to deteriorate the window thermal performance.

Modeling and simulation of a simple glass window

This paper presents a mathematical model with numerical simulations of the heat transfer across a simple glass window. The model is two-dimensional, transient based upon the energy equation with a source term to account for the solar radiation absorbed through the glass sheet. Variable incident solar radiation and external ambient temperature are considered in the numerical simulations. The governing equations and the associated boundary conditions are discretized by the finite difference approach and the ADI scheme. Numerical simulations are realized for the cases of clear and absorbing glass to show the effect of the glass thickness on the total heat gain, the solar heat gain and the shading coefficient.

An analysis of absorbed radiation by domed and vaulted roofs as compared with flat roofs

Domed or vaulted roofs have been extensively used in buildings throughout the Middle East and other hot arid regions. Many studies have focused on climate-related considerations, but very little quantitative research exists in the literature. In this study, an attempt was made to calculate the insolation absorbed by these roofs, as compared with flat roofs, based on angular dependence of absorptance and solar geometry. Results showed that a domed or vaulted roof will absorb more solar radiation than its corresponding flat roof. The ratios K b, K tot increase with the increase of the half dome or vault angle but are insignificantly affected by climate characteristics and latitude of the location. It was also seen that a south–north facing vaulted roof both reduces the solar heat gain of buildings in summer months and increases solar heat gain in winter months compared to one which faces east–west; the greater the proportion of area exposed to the sun is, the smaller the amount of beam radiation that will be absorbed by a curved roof. Furthermore, results showed that even if absorptance is assumed to be constant this would affect total solar heat gain of the roofs studied by less than 4%.

Steady-Periodic Three-Dimensional Foundation Heat Transfer From Refrigerated Structures

Analytical solutions for steady-periodic ground-coupled heat conduction problems for cylindrical and three-dimensional rectangular slab-on-grade floors beneath refrigerated structures with uniform and partial insulation are presented in this paper. The solutions provide the soil temperature field, and the total slab heat gain. A parametric analysis is conducted to determine the effect of thermal insulation U-value and insulation width on the mean, amplitude, and phase lag of total slab heat gain. In particular, it was found that the mean and the amplitude of the total slab heat gain are effectively independent of its shape but are strongly affected by the slab size and thermal characteristics. [S0199-6231(00)00502-5]

A generalized methodology for determining the total heat gain through building envelopes

We present a generalized methodology for determining the annual total heat gain through external walls and proofs of large air-conditioned buildings. The methodology is based on the concept of the overall thermal transfer value (OTTV). Respective OTTV equations for building envelopes and roofs are developed through parametric simulations using the DOE-2 computer code. The equations are valid for buildings having different aspect ratios and wall masses. Appropriate coefficients for heat conduction through fenestrations and opaque walls and solar correction factors for wall facades of different orientations are computed from local weather data. The equations allow building designers to make accurate estimates of the total heat gain for the purpose of evaluating energy-efficient building envelope components and air-conditioning systems and plant options. The methodology is validated using DOE-2 computed heat gain results and can be applied to different classes of buildings, construction types and locations.

A simplified overall thermal transfer value equation for building envelopes

We describe a methodology for determining a simplified overall thermal transfer value (OTTV) equation for air-conditioned buildings in Singapore. The OTTV equation was formulated by modelling a generic commercial building and using results from DOE-2 computer simulations as a database of heat gains. The new equation increases the weight of the solar heat gain as compared to the currently used OTTV equation. This change yields an improved relation between the OTTV and the total heat gain through the building envelope.

Unsteady heat transfers through a multi-layer wall

This paper presents a numerical analysis of the heat transfers through a multi-layer wall subjected to complicated time-dependent boundary conditions. A layer of insulating material at the exterior side or at the interior side has significant influences on the daily total heat gain or loss of the associated building. Averaged total incident solar radiation and ambient temperature functions can be used to estimate the heat transfer rate through the wall.

Natural production, storage, and utilization of ice in deep ponds for summer air conditioning

A covered ice pond was analysed for the gradual production, storage, and utilization of ice for air conditioning of a building with an annual cooling demand of 1.2 TJ. Winter ambient air, blown into the pond for ice production, provided the major sources of coolness. The ice production rate and the total height of ice which can be produced were determined. The total heat gain by the ice pond was estimated by a two-dimensional heat conduction analysis, employing a finite difference technique. A coolness recovery factor of 0.84 and a coolness utilization factor of 23 were obtained for the system. An economic analysis showed that both initial and operating costs of the system are lower than those of a vapor compression refrigeration machine of comparable capacity.

Influence of heating rate on control of heat loss from the tail in mice.

Although heating rate is important for stimulating thermoregulatory reflexes, it is not known if the control system differentiates between total heat gain and rate of heat gain. Exposing animals to microwaves inside a waveguide permits continuous monitoring of whole-body heat absorption. Tail skin temperature of restrained mice was recorded during whole-body exposure to 2,450-MHz microwave radiation at specific absorption rates (SAR) of either 11.5, 21.7, or 43.5 W . kg-1 and whole-body heat loads of 0.3-14 J . g-1. The integration of tail skin temperature with time, defined as the skin temperature index (STI), was measured as a function of absorbed heat load. At ambient temperatures of 20 and 25 degrees C the STI, averaged with respect to heat load, increased significantly with SAR. Depending on SAR, the sensitivity of heat loss from the tail to microwave exposure increased 32-71% per 1 degree C elevation in ambient temperature. The data indicate that heat loss from the tail increases with the whole-body heat load accrued from microwave exposure. When heat loss is averaged with respect to heat load, the rate of heat absorption and ambient temperature increase the sensitivity of thermoregulatory centers that control peripheral heat loss from the tail of mice.

Effect of solar radiation on soil temperature in pots housed in cold frames

Heat gain in pot soil within cold frames covered with glass and without an artificial supply of heat and ventilation may take place during the day: (a) by advection from the cold frame air, as the air is at a higher temperature than the pot soil; (b) by advection from the inner glass surface, as this surface is at a higher temperature than the cold frame air; (c) by radiation from the inner glass surface; (d) by solar radiation. A theoretical consideration of the heat gain by advection from the cold frame air resulted in an equation which expresses the relationship between pot soil temperature and cold frame air. An application of this equation to experimental data showed that the heat gain by advection from the air is only 10–20% of the total heat gain. The heat gain by advection and radiation from the inner glass surface is very small compared with the solar radiation transmitted through glass and can be omitted. Therefore the main source of heat in cold frames without an artificial supply of heat is solar radiation since (a), (b) and (c) above have been proved negligible. The pot, acting as a black body, accumulates the solar energy falling on it. Using a theoretical approach the effect of solar radiation on soil temperature was found Θ sj − Θ so = A ss r G C (R sun t j) The parameters involved in this equation are; an initial temperature of pot soil ( Θ so), the pot soil temperature ( Θ s j ) at any given time ( t j ) before daytime maximum, the pot soil surface area ( A ss), the transmission coefficient of glass ( r G), the thermal capacity of pot soil ( C) and finally the net energy from sun ( R sun t j ) integrated from the initial time up to time t j . From the above equation a prediction of pot soil temperature from solar radiation can be made with a good degree of accuracy, since the experimental data fit this equation satisfactorily.

Frequency- and time-domain thermal response of dwellings

A system of algorithms is developed for calculation of the frequency- and time-domain responses of the heat transfer equations for a dwelling. Conduction through the walls, considered as multi-layered slabs, as well as convection to the inside air, infiltration, solar radiation deposition in the space, and radiative interaction between the walls is included in the calculation. The first part of this system of algorithms provides a means whereby a set of frequency responses of the dwelling as a whole may be calculated, one frequency response for each input-output pair, considering internal temperature or heat flow as the output, and the sol-air temperatures on each surface, infiltration, and the solar heat gain to the interior as inputs. The second part of the system of algorithms is then used to obtain from these frequency responses sets of digital transfer function coefficients of the type presented in the ASHRAE Handbook of Fundamentals, but relating the total dwelling heat gain, rather than that for a single wall, with specified internal temperatures, or internal temperature with specified heat gains, to the sol-air temperatures on each surface, infiltration, and solar heat deposited in the interior. The method is shown to produce rapidly calculatable, accurate results in the time-domain. Further the plots of the frequency responses themselves are expected to prove useful in seeking optimal thermal designs.