Abstract:
The principal intention of this study was to come up with the most conducive internal environment in a building. At any given moment, the internal temperature of a building is a function of the amount of heat produced by sources therein and the net heat transfer into or out of the building. The four heat transfer modes are convection, radiation, conduction, and phase- change. In executing this project, the researchers sought the relevant literature to establish the thermal properties of various materials. The properties are critical in determining the amount of heat inflow and exit from the building. The researchers then came up with a combination of layering mechanisms that would optimize the overall heat resistance in a building. They found out that through glazing, one can minimize the rate of heat loss through glass windows.
The researchers also came up with the design parameters of an indoor installed condensing boiler to supply heat whenever the temperature in the house falls below the desired range. The investigators also found out that not all dimensions can save the costs of keeping a structure at the desired temperature. As such, the researchers came up with a combination of thicknesses of the various layers that would optimize the internal temperature of a house at the lowest possible costs. As such, the researchers sought to counterbalance between the initial costs and the maintenance costs of the wall materials and the boiler supplying heat during cold days. Conclusively, the investigators used the thermal properties of materials and a condensing boiler to come up with the best combination of wall layer dimensions at the lowest possible costs.
Introduction:
Heat transfer is the mechanism of thermal exchange in systems. Heat is conveyed from one system at a higher temperature to another at a lower temperature. This knowledge of heat transfer is useful in buildings to determine the design of the materials, and design passive and active system to regulate the thermal conditions inside the building (Sukhatme, 2005). There are four mechanisms of heat transfer in a building: conduction, convection, radiation, and phase-change.
Conduction is heat transfer by collisions of particles within an object due to the temperature difference. The temperature difference between inside and outside of the building induces heat transfer by conduction. Conduction is a major heat transfer method in internal cooling or heating of the building is expelled. Conduction heat transfer is barred by using materials with high thermal resistance (thermal insulators). The insulating property is also attained by thermal mass – ability to absorb, retain, and expel - of the building material (Modest, 2013).
Convection heat transfer is the transit of heat by the motion of fluid (air or liquid) by a massive molecular motion or diffusion. Convection is significant in designing buildings where fluid (air) movement is required to regulate the internal temperatures, remove accumulated moisture and band odors, and create comfort to the building occupants (Rathore, & Kapuno, 2011). Air into the building is either natural or forced using a pump. Heat can transfer within the building through mass transfer of fluids, for instance, hot or chilled water to provide heating or cooling respectively.
In radiation, heat moves in the form of electromagnetic waves. All bodies emit and absorb thermal radiation around their surroundings. Variation of the net radiation at a given moment results in heat transfer that induces a temperature change in the body. Thermal properties of materials – emissivity, transmittance, and reflectance are considered in the design of building materials (Rao, 2013). Heat is transferred when substances change phase. For instance, when heated liquid changes to gas, it absorbs heat bring a cooling effect. Phase changing materials are used in building construction to eliminate internal temperature.
The project aimed at giving insights of how heat transfer occurs in a building. The report intends to give a detailed description of how unwanted heat – heat causing discomfort – is reduced or eliminated from the building. In the report, ways of supplying heating to the building are discussed. Comparison of mechanisms of heat elimination and heat supply to the building is made.
Objective:
The report is intended to give building engineers the insight of what energy types are the best to consider when designing a building. The report also aims at helping the engineers determine the integrity, suitability, and affordability of the energy type of their considerations.
Methodology:
While executing this study, the researchers began by pursuing the existing literature to establish the thermal characteristics of the various construction materials. As such, they studied the various modes of heat transfer (conduction, convection, and radiation). They sought the various heat transfer parameters. Such parameters as the thermal conductivity of a material determine how easily a material can conduct heat through it. The researchers then proceeded to investigate the various mechanisms of eliminating undesired heat in a building. Such heat always causes discomfort to the inhabitants of a house.
Having found out the design approaches of eliminating the unnecessary heat, the researchers implemented mechanisms of supplying heat when the temperature of a building falls below the desired value. The reason is that a temperature below a particular range always causes discomfort to human beings who dwell in the building. The researchers thought of an indoor installed boiler to heat the internal atmosphere of the house. As such, they studied the various requirements such as fuel supply that would provide an optimal supply of heat energy. In this undertaking, the researcher considered a condensing boiler to serve in providing the heat.
Literature review:
In modern days, people provide thermal comfort in their rooms in winter by heating the whole air volume in the house. In ancient days, people then did know the concept of heating the entire room. Radiant heat sources that warmed just a portion of the room were used to create a microclimate of comfort. They barred large temperature differences with insulation furniture like folding screen and hooded chairs (Burmeister, 1993). They used heating sources that would warm a part of their bodies. Restoring this old method of warming with modern technology makes it more practical, safe and effective.
Heat transfer is the mechanism of thermal exchange in systems. Heat is conveyed from one system at higher temperature to another at a lower temperature. This knowledge of heat transfer is useful in buildings to determine the design of the materials, and design passive and active system to regulate the thermal conditions inside the building (Sukhatme, 2005). There are four mechanisms of heat transfer in a building: conduction, convection, radiation, and phase-change. During the investigation, the researchers sought to study the various modes of heat transfer and find out how to optimize each one of them.
Boiler heating system:
Almost all homes in Ireland consist of a heating source to provide comfort inside the house. They use boiler and radiator systems or electric storage heaters. Boilers heating systems are the most usual method of heating in Ireland. A boiler heats water, water pumped via pipes to a radiator. Boilers provide hot water to the kitchen and the bathroom as well. The boiler system uses main gas, LPG oil, coal, and wood. Main gas is the most used fuel because apart from its lower cost, it has very low carbon IV oxide emissions. The widely-used types of boilers are the condensing boilers. The boilers do have large heat exchanging volumes to recover heat fuels.
Gas or solid fueled boilers are located inside or outside the building. The boilers heat water that is distributed in the house by gravity or pumping mechanisms circulating to heat emitters. Boilers that are located outside the building and the hot water distribution pipes are well insulated to minimize heat loss. If the boilers are located inside the house, insulation is not necessary as the heat loss causes heating in the house. Figure 1 below shows an indoor installed condensing boiler (Knight, 2012). This type of boiler efficiently burns the fuel supplied to heat the house.
Condensing boilers are one of the most used types of boilers. The burning fuel condenses flue gasses and improves the efficiency of combustion. Compared to conventional boilers, condensing boilers are costlier, but their efficiency overpowers the cheap cost of the convectional boilers. Due to their low running costs, the cost difference is recovered ten to fifteen years down the line. The boilers are designed to run with maximum efficiency at lower temperatures and are ideal in under floor heating systems. During their operations, condensing boilers emit plumes of water vapor into the atmosphere. Figure 2 above shows a diagrammatic representation of a condensing boiler (Lochinvar, 2015).
In a boiler heating system, the more the return water temperature, the higher the efficiency of the boiler. In addition to this, condensing boiler systems have an excellent energy saving ability. The boiler fetches efficiency of about 95% in a normal operation. The graph below illustrates efficiency in boilers.
Wall insulation:
This technique involves adding a cover of insulating cladding to the either side of the wall in a building. In addition to insulating the building from large temperature differences, the cladding gives the building an aesthetic view (Wiley, et al., 2001). Several techniques are employed to insulate walls: batt insulation to fill the space using a glass fiber or mineral fiber; spray foam insulation using plastic resins; and loose-fill insulation using cellulose, glass or mineral fiber.
Glazing:
It is the method of furnishing or fitting with glass. Double glazed windows work effectively in minimizing the energy required to maintain temperatures of the building at the comfortable ranges for humans (Modest, 2013). Glazed windows and doors reduce more than 50% heat loss from the building as well as reducing heat transfer into the building. Making the window triple-glazed further reduces heat loss form the building.
Heat flow in a building:
Provision of thermal comfort with minimum space conditioning expenses is a brilliant practice. Thermal regulation is a significant aspect in many buildings. Knowing heat transfer and distribution of temperature in building materials is paramount to assess the energy consumption, thermal movement, and potential causes for moisture problems. Heat flow control in buildings requires insulating layers comprised of several thermal bridges, air barrier system, control of radiation, and regulation of the interior heat production.
Heat flow by conduction occurs by direct contact with objects through the successive collision of particles of the object. Thermal conductivity varies from one material to another. Various materials conduct and lose heat by conduction at different rates. Air, a poor conductor of heat, makes a perfect heat insulator (Lienhard, & Lienhard, 2011).
Heat flow by convection is enhanced by the flow of the fluid. Heat flows from one point of the fluid (hotter end) to the other end (colder end). A complete cycle of the movement sets up the convectional currents of the fluid. Heat flow by radiation involves an object emitting heat to the surrounding, for instance, heat flow from the sun.
In a building set up, most of the heat is lost through conduction and convection. Openings and air leaks in the building contribute to the largest pathways of heat loss. Figure 3 below shows air leaks in a building and the amount of loss. Heat loss in a building amounts to 10% of the total losses.
Surface Heat transfer in buildings:
The temperature variation between interior and exterior of the surface determines the rate of heat transfer through the surfaces. Surfaces in a building include walls, windows, the floor, doors, the roof, and ceiling, among others.
Determining Heat Loss in a Building:
The overall loss of heat in a building occurs through transmission, ventilation, and infiltration. The estimation of heat loss will account for the different parts of the building. Transmission of heat in the building is influenced by the type of material used to make the surface and the R-value (a measure of the ability of material reject heat transfer). Calculating the heat loss from the building involves a critical emphasis on the material used to make the roof, walls, windows and doors, as well as the method used to construct the house. Heat loss in a building can be calculated using the relation;
.. (1)
Where the H is the total loss; the Ht is loss because of transmission through the walls, the windows, the doors, the floors among others; Hv is loss through ventilation; and Hi is loss through infiltration.
Heat loss by building surfaces:
The normal heat load, or heat loss through the walls, the windows, and the like is estimated using the formula (2) below.
.. (2)
Where the Ht is the heat loss by transmission, A is the area of the displayed surface in square meters; U is the total heat transmission constant, ti is the air temperature inside the building, and to is the air temperature outside the building.
15% extra is added to heat loss through the roof because of radiations to the space. Equation (2) can be modified to equation (3); the walls and the floors must be modified with the temperature of the earth as shown in equation 4.
.. (3)
H = A U (ti - te) (4)
Where te represents the temperature of the earth. The coefficient U, can be estimated as shown in equation (5)
.. (5)
Where the Ci is the conductance of the inner wall; x is the material thickness in meters; k is the conductivity of the material; and Co is the conductance of the outside wall. The conductance of building element is shown as;
. (6)
The resistivity of the materials is reciprocal of conductance shown as;
.. (7)
Using equation (6) and equation (7), equation (5) is modified as:
. (8)
Where the Ri is the resistivity of the inside wall surface R1. Is the resistivity of separate wall or layers; the Ro is the resistivity of the surface of the outer wall.
Equation 8 can be modified regarding walls and floors against the earth.to yield equation (9) where Re is the thermal resistivity of the earth.
.. (9)
Heat loss by ventilations:
Heat loss of a house because of ventilation with no heat recovery is shown:
.. (10)
Here, the Hv is heat loss by ventilation in watts; cp is the heat constant; ρ is the density of the air; qv is the flow of air volume; ti is air temperature inside the house; to is air temperature outside the house.
If heat lost is recovered from loss by ventilation, it can be expressed as shown in equation (11) where β is the percentage efficiency of heat recovery.
.. (11)
For actual cross flow heat exchanger, approximately 50% efficiency of heat is recovered. In a rotating exchanger, the efficiency is higher, approximately 80%.
Heat loss through infiltration:
This is caused by leakages in the construction, routine opening of windows and doors, among others shifting the air inside the house. As the rule of the thumb suggests, count of air moves inside the building is found as 0.5/hour. This value relies on several factors – the speed of the wind, temperature gradient between the inside and the outside of the building, quality of the house among others. Heat loss by infiltration is estimated:
(12)
Here, Hi is the overall heat lost by infiltration; the cp is the specific constant of heat air; ρ is the air density; n is the count of the air shifts; V is the volume of the room (space in cubic meters); ti is air temperature inside the house; to is air temperature outside the house.
Example of heat loss estimation in wall, roof, and window set up:
Heat loss through the walls:
Varieties of materials are used in making building. Considering this variation in surface areas and K values, different materials have different heat loss. For instance, if the plaster inside the wall is 10-mm thick, and the wall is a solid block with 95-mm cavity and has an outer leaf of brick.
Calculating the resistances in parallel gives;
The overall resistance is;
Steady state heat loss by the wall;
Calculating the heat loss per surface area, the area of surface is divided by 0.144 m2
The area of both the front and back sides of the wall is 51 square meters. Windows occupy a total area of 10.15 square meters
51 – 10.15 = 40.85 m2
Therefore, the rate of heat loss through the wall is
Heat loss through windows:
If the estimated area of the glazing is 6.35 square meters
Thickness of individual glass pane is 0.025m and cavity thickness 0.010m
Assuming the K-value to be;
The overall resistance of the glazing is the sum of resistances calculated. It brings in the idea of how strong the wall is resistive to the flow of heat. The total resistance can be estimated as;
Heat loss through roof:
As hot air rises in the house, there is an appreciable increase of heat through the roof. Using the same method used to calculate heat loss by the wall surface, heat loss through air can be estimated. Assuming the R-value of the air to be 0.026, the estimation becomes;
The total resistance of the wall surface is;
The heat per square meter is 2.5. Total area of the roof calculated was 11.75 square meters. The total heat transfer through the roof is;
Heat Loss Analysis of the conduction, convection, and radiation:
Analysis of heat loss through the three modes is conducted by obtaining data from various buildings at different times of the year. Buildings with varied designs – with solid wall insulation, with internal cavity insulation, or with external cavity insulation Table 1 below presents data from the various models (HOLLADAY, 2012). The table reveals that a building with internal and cavity insulation experiences the lowest heat loss. A wall without insulation, on the other hand, experiences the highest heat loss due to a low total heat loss resistance.
Conclusion:
As designers (engineers), constructors (builders), and owners of buildings are striving to attain minimum heat loss as well as energy cost for their buildings; they must do it after evaluating exhaustively all the factors that lead to heat transfer in the region. They should adopt modern measures and techniques of controlling internal temperatures of the building to create a safe and comfortable place for humans. They must consider building materials and building design before they erect it.
As revealed by the report, heat loss through any surface in a building can be calculated easily using mathematical approach. Presenting the annual heating consumption in kWh/ square meter/year enables easy comparison of various sizes of houses. Modern building techniques such as fitting of ventilations and infiltrations enhance free air circulation in and out of the building thus help to create a comfortable environment inside the house. Passive house insulation methods are under review and may be included in buildings in future.
Bibliography
Burmeister,, R.U (1993).Convective heat loss. New York:
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HOLLADAY, MARTIN (2012,APR 27). How to Perform a Heat-Loss Calculation. Green Building Adivisor. [Online] Available at: >http://www.greenbuildingadvisor.com/blogs/dept/musings/how-perform-heat-loss-calculation-part-2.> [Accessed 23rd December.2016]
Knight, M (2012). Condensing Boilers. Boilers Review. Retrieved December 23,2016, from http://www.which.co.uk/reviews/boilers/article/condensing-boilers.
Lienhard, T. P, & Lienhard, M.R (2011).Heat transfer textbook. Mineola: N.Y Publications.
Modest, M. F. (2013).Radiative heat transfer. Oxford Academic.
Rao, Y. V. C. (2013).Heat transfer. Hyderabad: Universities Press.
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Sukhatme, L. K. (2005).A textbook about heat transfer. Hyderabad: India Press
Lochinvar (2015). CONDENSING BOILER. HIGH EFFICIENCY BOILERS AND WATER HEATERS, [Online] Available at: >http://www.lochinvar.com/products/default.aspx?type=productline&lineid=177> [Accessed 23rd December.2016]
