Heating, Ventilation, and Air Conditioning (HVAC) systems account for a large share of the energy consumed in commercial buildings. Simple strategies such as adjusting HVAC set point temperatures can lead to significant energy savings by substantial reduction in the cooling load on HVAC. This is because buildings worldwide consume approximately 47 per cent of primary energy sources, making it the single largest energy consumption sector. The importance of improving a building’s energy performance was emphasised by the enforcement of sustainable building policies. The focus on energy performance of buildings states the importance of stimulating refurbishment of existing buildings into near zero-energy buildings. However, the effectiveness of the process depends on the basic building structure and the refurbishment designs. Hence, a method to find the effective strategies for retrofitting and modeling to predict energy reduction is vital.
The building surface provides a barrier among the indoor space and outside climate. The building external cooling load greatly depends upon exposure of wall, roof and windows to outdoor ambient. While the internal cooling load depends upon the heat and moisture produced by the occupants as well as appliances and activities associated with daily living. The cooling loads are what drive HVAC design and capacity. One important distinction between heating load and cooling load is that heating loads are based purely on thermal energy transfer driven by the temperature differences. Cooling loads, on the other hand, related two distinct components like as one thermal and the other moisture related. These components are described more preciously as the sensible and latent portions of the cooling load respectively. The sensible cooling load is the most dominant in residential HVAC applications. Three basic cooling load reduction strategies mostly employed to optimise HVAC design are air sealing, insulation and shading or reflectivity. The following Table 1 shows how each technique applied to control the cooling load of HVAC.
The roof represents one of the largest potential cooling loads in residence. Roof related cooling loads can be reduced by incorporating any of the above three strategies. But the metal roof provides some inherent reflectivity so only insulation and air sealing were addressed.
Several strategies for insulating the exterior walls are considered including the use of expanding polyurethane foam which could provide air sealing as well as insulation. The decision to do without insulation in the walls was driven by the fact that only 1.5 to 2 TR cooling load would be saved and that extensive deconstruction would be required. Adding insulation to the wall cavity may have unintended impacts on the moisture dynamics within the wall.
Insulation and air sealing are available options for reducing cooling load through the floor. Even left uninsulated, in summer time heat gained though the floor is relatively small compared to other cooling loads. Air leakage through the floor could have a significant impact on the air conditioner but would be difficult to quantify as separate load. Two load reduction alternatives that could still be implemented at a later date are primarily focused on reducing air leakage though floor. The first involves adding a layer of polyurethane foam insulation directly to the bottom of the floor which could provide an excellent seal as well as R-value. The second is more extensive requiring the crawl space to be enclosed with an insulation and air boundary and adding a vapour barrier on the ground beneath the home.
Window loads are typically one of the largest sources of heat gain in buildings. Now-a-days, window cooling loads are reduced by integration of simple and high-tech strategies including shading, air sealing, special coatings, and multiple panes of glass. The front porch windows are the only ones with a significant overhand which means shading have to be provided by surrounding trees and operable attachments to the windows. Strom shutters are useful shading devices, especially the type with operable louvers that can be adjusted to block direct sunlight while maximising the free area for air and light to pass through.
Duct leakages have a devastating impact on indoor air quality, building durability and air conditioner performance when not held in check. In small duct high velocity (SDHV) subject to higher system pressures, this higher pressure is more potential to leak air than in other standard systems, emphasising the need for a tight duct system. Upon maintenance, duct leakage to the outside within 3 to 5 per cent of air handler flow volume is permissible.
Infiltration or envelop leakage from roof, wall, floor and windows increase the load on HVAC greatly. So, buildings should be extensively renovated with almost the entire original breadboard replaced with gypsum wall board on the walls and ceilings. Modern double pane, vinyl framed windows were also installed throughout. Another major source of air leakage, the fire place, was blocked off.
The cooling load distribution among the cooling load components are shown in Figure 1.
Design of glazing should integrate the technical selection of windows and skylights, atriums, and other glazing units with important architectural issues, including view for the occupants, building appearance, day lighting, and passive solar heating. Ironically, the most important aspect of solar energy for many buildings is keeping sunlight out. Since the advent of modern architecture, with its vast expanses of bare glazing, designers have failed to deal effectively with the enormous increase in cooling load that is caused by such design. The air conditioning load caused by entry of sunlight through glass typically is the largest cause of high air conditioning cost in commercial buildings. In recent years, electricity demand charges have risen steeply as utilities seek to avoid building new power plants. Even in northern climates, heat gain through glass may cause a large fraction of annual energy costs, especially, in heavily glazed buildings. The heat content of direct sunlight is about 240 BTU per hour per square foot (about 0.7 kilowatts per square metre), measured perpendicular to the direction of the sunlight. Vertical glazing that is oriented toward the east, west, or south receives roughly 1,000 BTU per square foot (about 3 kilowatt-hours per square metre) per day in clear weather. This figure does not vary much throughout the middle latitudes, and it does not change much as the seasons change.
Many methods are available to reduce solar heat gain in virtually all types of buildings. The most powerful methods are based on shading. Exterior shading methods include parts of the building structure, including balconies, eaves, soffits, window insets, and other architectural features. External shading devices include awnings, louver materials, horizontal shelves, and other types of shading in almost unlimited configurations. Interior shading devices include venetian blinds, roller shades, draperies, louver blinds, diffusers, and a variety of other techniques that are classed as ‘window treatment.’ Some shading devices are built into windows. Plastic window films can be attached to existing windows to reduce solar heat substantially, as well as providing other benefits, such as breakage resistance. The ultimate shading technique is to reduce the area of glazing, which can be done in existing buildings as well as new construction. Windows, skylights, and other glazing should be selected to reduce both heating and cooling energy consumption. Selection factors involving the glass include the number of panes, the gap between panes, colour, low emissivity coatings, visible light transmission, infrared heat transmission, and breakage resistance.
Lighting system modification
The proper use of lighting is one of the most cost-effective ways to reduce carbon emission and application of existing technology could reduce electricity use for lighting by 50 per cent. A lighting system that includes automatic daylight dimmer in corridors and office zones as well as replacing existing lamps with high-efficiency LEDs. Luminance in the office zones were adjusted to 300 lux by the recommendations. Lighting operating schedule was proposed to accommodate the employees when the area is occupied.
The passive cooling measures also reduce the total cooling load on HVAC as shown in
In an architectural building design to reduce cooling load on HVAC, it is important that all main elements of the building should either block or reject solar heat gain and try to keep the building cool against the heat of summer. The architectural building design depends greatly on the climatic conditions of the area and should therefore, be oriented building accordingly. A passive building is often the key foundational element of a cost-effective zero energy termed as green building. In a hot and arid climate, most of the energy load is from mechanical systems, so this load could be reduced by adding elements to the building, such as louvered shading devices which can significantly reduce energy consumption by reducing the HVAC cooling load substantially.
Dr. D.B. Jani,
Government Engineering College,
Gujarat Technological University – GTU,
Ahmedabad, Gujarat, India.