Air Conditioning - Essay Example

Air-conditioning Roll No: Teacher: 26th November Q1 In a multi-zoned building, energy requirements vary as a consequence of numerous factors. A building’s total energy requirement helps define the building load with all the functions/activities pertaining to the building taken into account. Over a period, there may be the need to observe rationalisation in energy consumption, depending on the nature or season of consumption. A consumption phase over which there is continuous or sustained demand for energy at a maximally high level is identifiable as the period of peak load for such buildings, which are usually multi-zoned in nature and thus necessitating various energy requirement levels. In Hong Kong, buildings are built close together, hence the external environment is considered as one of the variables in determining a building’s energy requirement. To identify cooling load, it is important to determine parameters which are closely associated, and which correspond to this, in a building. Such parameters, as noted by Hui (2009), include (1) determination of building location, orientation, external shading (e.g. adjacent buildings); (2) weather data and outdoor design conditions; (3) indoor design conditions(e.g. control limits, permissible variations); (4) characteristic of the building, it’s materials and components, etc; (5) indoor design conditions- permissible variations and control limits; (6) determination of internal thermal load through the assessment of occupants, internal equipment processes and appliances affecting thermal load; (7) lighting schedule; (8) space cooling determination at design conditions also helps towards identifying cooling load, as well as (9) determination of peak design load through assessment of cooling loads at several times, constituting a design day. Other vital factors for identifying a multi-zoned building’s cooling load, relate to the building’s thermal load. Such include the ceiling height, windows, stairways, the doors, the building’s orientation, and the use of space. Also closely associated with cooling load are the number or density of people (occupants), the nature of activity, the duration of occupancy, lighting, thermal storage, and continuous or intermittent operation. Cooling load can be further identified through its vital internal and external components in a building. The external components are recognisable as heat gain through partitions and interior doors, solar heat gain through the windows, conductive heat gain through the windows, and heat gain through exterior walls and roofs. Electric lights, equipment, people (occupants), air infiltration into a building through openings and/or cracks, constitute the internal components of a building’s cooling load. Cooling plant capacity is associated with fresh air supply/intake, as a part of the system load. In addition, useful factors will include enthalpy difference between the zone air and ambient air, as well as mass flow rate. A further identification parameter with respect to cooling plant capacity is the nature of the relationship between fresh air distribution and the heating/cooling loads. Whether the distribution is centrally handled, is another vital element or identifier of cooling plant capacity. In an air-conditioned building, cooling plant capacity is related to a variety of factors. The coefficient of performance (COP) of the chillers operating in the system is vital. The size of the floor area in the building requiring cooling, the number of chillers available and the voltage capacity driving them, as well as the temperature at which the chillers are to be supplied, are other important factors. Furthermore, the load condition adopted (i.e. whether part or peak), is quite relevant as well as the design inlet and outlet temperatures of the heat exchangers. The air handling terminals used for handling recycled air would be another factor, in an air-conditioned building. Alongside this, is the fact of whether or not ring-duct is used on the system of the building’s floors, as this facilitates the temperature at which primary air units handle external (outdoor) air. The time of use rate, used in Hong Kong for the reduction of electricity use during peak hours (Wang, Xu & Ma 2006), is also vital, affecting total cooling requirement, which impacts on cooling plant capacity available in an air-conditioned building. The source also notes the control of fresh air based on dynamic occupancy detection, as a useful element resulting in saved cooling load. It can also be inferred from Wang et al that the number of chillers, set point of supply cooling water temperature, set point of water pressure differential of the worst water loop, cooling tower pumps, and set point of supply chilled water temperature, are relevant factors. In addition, anon (2007) infers that cooling plant capacity will also be impacted by over-heating or over-cooling, which in turn is affected by independent airside system/control, which adjust zone temperatures to suit different occupancies. Small systems are exempted from this, according to the Hong Kong AC code. Q2 The utilisation of appropriated strategies for cooling load reduction needs to consider the comprehensive cooling load scenario. The total cooling load, which is generally the sum of the sensible and latent cooling load in the building, is a necessary feature in the calculations. This represents the sum of the sensible and total cooling loads for the building. This total cooling load must be treated as a composite load, subject to sophisticated and comprehensive reduction into components, around and within the building environment. As noted by Yu & Chan (2007), the number of steps of total cooling capacity, for instance, may be treated in the reduction strategy as a means of enabling a higher part load over a greater fraction or period of the total cooling hours. Further, as can be inferred from the source, the reduction strategy may approach consumption (i.e. cooling load demand required) as a quantity, which can be normalised by the total floor area of the building. Utilising reduction strategies for cooling load also can be considered in terms of increasing the number of steps of total cooling capacity as the frequency of chillers operating increases, where they are operating at or near full load. An increase or decrease in the annual pumping energy fluctuations serving the building is also relevant in a cooling load reduction strategy. Because the annual pumping energy serves as feeder to the annual electricity consumption of chillers, the pumping energy fluctuations affect the extent of drop in annual electricity consumption, and inevitably, cooling load reduction. Hong Kong’s thermal transfer value (OTTV) equations, note Chungloo, Limeechokchai, & Chungpaibulpatana (2001), have one of the highest levels of heat gain sensitivity to the parameters of window-wall ratio (WWR), Window SC, and annual building cooling loads. These are also the parameters of significance in efficient daylighting design, along with visible transmittance. External shading can be applied to buildings, and as the source notes, reduce the total annual energy consumption (with respect to cooling load energy requirement) by up to 2%. Natural ventilation can be applied in the appropriated strategy, to reduce the need for mechanical cooling through direct removal of intra-building warm air against the presence of cooler, incoming external air. Avoidance of excessive window area in an east-west direction is also a very relevant strategy in the reduction approach. For Hong Kong, a tropical humid environment, indirect evaporative cooling can be incorporated into the cooling load reduction approach. Further, as notes Lam Ngang Tung (2008), the application of integrated photovoltaics in buildings (BIPV) is appropriate to reduce solar heat gain (in addition to daylight penetration) and generate electricity. Further, the source notes, opaque and semi-transparent BIPV panels can serve as solar facade and window glazing of the buildings. Cheung, Luther & Fuller (2002) note that the annual energy consumption and peak load of the cooling system can be effectively reduced by thermal insulation. Peak load reduction of 25% - 31% has been achieved by 25mm of thermal insulation, when placed inside and in the middle of the external wall of the building respectively. Overall U-value, in the cooling load reduction strategy, is not so vulnerable to change in location of thermal insulation. However, there is an effect on internal thermal mass in the building. Cheung et al further note expectations of thermal insulation bringing about greater reduction in peak load than when paced outside or inside the wall, indicating the creation of excessive thermal mass through the placement of thermal insulation on the outside face of a reinforced concrete wall. This has a negative effect on annual cooling energy consumption and peak cooling load. Furthermore, the source asserts that middle-placement of thermal insulation in the reinforced concrete wall is the best option, taking peak cooling loads and annual cooling energy into consideration. Q3 Proposals based on analysis of peak time summer temperatures would appropriately draw from the concept where as noted by anon (2003), climatic design’s key objectives entail as much as possible (1) usage of ‘natural energy’ of a building instead of it’s mechanical system and power; (2) reduction of energy cost of a building and (3) provision of comfortable and healthy environment for occupants or people. Against this backdrop, during peak summer temperatures in Hong Kong, the country is susceptible to overloading, considering the fact that it is a low-latitude country. Vertical and horizontal shading is required throughout the year because of its climate, and a peak summer temperature is a particularly important period or season to take cognisance of this. Humidity can be very uncomfortable in this region (and period), hence appropriate ventilation is relevant to building concepts required. Hong Kong’s summer, occurring between May and September, is essentially of the tropical hot and humid type, with occasional showers and thunderstorms. In Hong Kong’s climate, transitional spaces within buildings, where movement of occupants take place, should have natural ventilation. This corresponds to the north and south sides of buildings, in this region. Further, a proposal would suggest that indoor temperatures should be hotter than peak summer outdoor temperatures. The period is also characterised by warm nights, hence there is a need to avoid undue increase of ceiling temperatures, which is achievable by using reflective surfaces for the underside and top of the ceiling, application of resistive insulation on ceilings, use of separate ceilings in buildings, use of reflective roof surface, and adequate ventilation in the attic. As earlier mentioned, peak summer temperatures also coincide with maximum humidity, and hence a need for windows facing one another in buildings- to facilitate cross-ventilation. Living areas should have an open-plan, to be appropriately suited to peak summer temperatures, in addition to high ceilings; the objective of this is the facilitation of air movement, and reduction of radiant heat to occupants. Furthermore, sleeping areas in buildings should have a long and narrow floor plan, to enhance maximum ‘through’ ventilation in bedrooms. The hot tropical climate in summer translates into warm nights; this means that walls should not be made of heat-retaining materials. Rather, material, which cools quicker, such as timber, would be appropriate. The building design should incorporate outdoor living areas, considering the hot, peak summer weather. This requirement serves to provide shade. Furthermore, the buildings and windows must be capable of reflecting the sun’s heat. For this reason, fixed overhangs (for shade) as well as shutters and canopies and/or verandas would be integral features of the building design. Natural cooling is desirable under peak summer temperature conditions; hence the need for shade against sunshine. In addition, it is vital for the comfort of the occupants, accelerating the evaporation of body sweat. The use of false ceilings for heat insulation is appropriate. The walls should be designed to have corners, also to provide shade. Shaded windows would also be useful, while attention should be strictly paid to minimise the number of east-west facing windows. Under peak summer conditions, air mass replacement is desirable in buildings, thus validating the appropriateness of high ceilings in the structure. Q4 Design solutions generally, should revolve around strategies (in building design) which, as noted by Levine et al (2007), make the most efficient use of ambient energy sources, efficient use of equipment, and effective control strategies. Design solutions must ensure that engineering and architectural elements work effectively together. In developing design solutions for summer or winter ventilation, the building envelope must be utilised effectively as a filter, in a flexible, appropriate manner. Therefore, ability to reject the summer solar radiation must be incorporated into its ability to perform the opposite function in winter. The same applies to outside air, in the context of building envelope as its filter. The need for cooling (summer) and heating (winter) should be factored into the building’s design solution. The need for lighting (greater in winter and less in summer), must feature in the design considerations along with an understanding of the need for appropriate summer ventilation, considering the humid conditions of Hong Kong. The building’s heat capacity must be brought into proper, full use in this (design solution) context. In summer or winter, for instance, the heat capacity must be used to shift the thermal load, on an appropriate time scale. This, according to Levine et al, ranges from hours to days. Further, this source notes that a design solution approach has to do with selection of a high-performance envelope and very efficient properly sized equipment. In addition, design solution entails, the source notes, full commissioning and maintenance of the building’s equipment as well as the incorporation of a building energy management system, which optimises equipment operation and human behaviour. These obviously affect the use of air-conditioning system used in ventilation in the building, as it constitutes part of the building’s equipment. The authors note that energy savings of the order of 35 – 50% are achievable, when these recommendations are incorporated in the design solution for a commercial building. In Hong Kong’s winter, building design solutions should take account of the fact the ground, aquifers and open water bodies, as well as ground water, can be selectively used as sources of heat. In summer, the design solution should appropriately focus on space cooling methods, which directly dissipate heat to natural heat sinks without usage of refrigeration cycles. Suggested heat dissipation techniques as noted by Levine et al, include earth-pipe cooling, radiative cooling to the night sky, and evaporative cooling. Furthermore, design solutions must incorporate the technicality of increasing the efficiency, the number and size of equipment/ appliances within conditioned spaces. Early in building design, key decisions to ultimately influence reduction in building energy usage need to be considered. These include the height/floor area ratio, self-shading, building forma and orientation. Building design solution must maximise the overall long-term thermal performance of the building, by means of choice of insulation material. In addition, as Levine et al note, consideration must be made of thermal bridges, in addition to other performance-damaging factors which include, for instance, water ingresses. Design concept for winter considerations may make use of the fact that thermal performance is greatly enhanced by application of multiple glazing layers as well as gases having low conductivity between the layers. Other useful design solution applications could be that of framing materials having very low conductivity. These, in addition to low emissivity coatings on one or more glazing surfaces. The winter incorporation of glazing that absorbs huge fraction of incident solar radiation, and in summer reflects an equally huge amount, is an ideal design consideration, making for cooling load reduction in summer. Q5 It can be suggested that, for the system in Q2.1, consideration should be given to the zoning requirement. The system constraints influence ventilation; the air-conditioning system serving the office floor could employ a (reheat) variable air volume mode, as this will regulate cooling loads in the conditioned space by varying the cooling air through a single duct. This may be even more appropriate, considering space availability for the air-conditioning system. Further, the system constraints influence the cooling load and the heating, which again are related to ventilation. The air-conditioning (equipment system) must not be situated to obstruct the conditioned space, the office floors, as this will compound the issue of architectural constraints. The terminal devices of the equipment need to feature in these considerations, in the context of non-intrusion into the office space, hence size of terminals must be appropriate. The suggested VAV (single-duct) system for this space will appropriately make use of diversity of lights to attract lower capital cost. This system further allows for longer running period of fans, at reduced volume. As an answer to the problem of architectural constraints in this conditioned space, the single-duct (reheat) VAV set-up will make for reduced noise level at off-peak loads. Furthermore, the suggested system will facilitate inexpensive temperature control, even across a multiple zoning arrangement in the conditioned space. Simultaneous heating-cooling flexibility may be appropriate in this space, considering the small difference between the outside and inside temperatures. Again, the recommended single duct VAV reheat system meets this requirement. The office-space temperature condition in winter and summer are factors inferable from the fact of its west-facing window, in a low-latitude region such as Hong Kong. Commercially, the suggested system is feasible as it permits lower capital cost due to diversity of lights, in addition to the earlier mentioned cost-saving longer running of fans at reduced volume. Furthermore, elimination of the need for seasonal changeover is another cost-saving feature of the single duct VAV system, even as simultaneous heating and cooling become possible in this space being conditioned. The earlier-mentioned advantage of reduced cooling load (courtesy of the system) also translate into lower energy consumption, and hence lower cost is there. Practically, space constraint would be a minimal threat equipment-wise, since it is a single duct system. Q6 One of the measure variables in this system is the airflow rate, as supplied to the conditioned space. This is a vital quantity in the ventilation and air-conditioning of the system. An indication of its extent of value is seen when we consider its impact on the ability (or otherwise) of the mechanical conditioning system to succeed or fail in bringing about desired ventilation or temperature conditions, simply due to availability/non-availability of the adequate air flow. Hence, the desirability of this variable’s measurability is there. Another important variable in the conditioning system is the energy efficiency of the system in the building. This, without doubt, is desirable considering its environmental implications, and equally valuable because of its cost-saving potential. Energy efficiency in the building system can mean permanent availability of energy to meet the conditioning needs of the building’s space, and at affordable cost, regardless of season. The earlier-mentioned variable of air supply flow rate is also closely associated with energy efficiency, and together they both are capable of bringing about energy saving, hence lower costs. Thermal comfort is an important variable in the building; it’s desirability in the conditioned space cuts across heating and cooling of the space in question. Indeed, it cuts across ventilation, because ventilation’s effects would pale into insignificance in the face of inappropriate thermal conditioning (temperature). Hence, the measurability of this variable is vital. Considerations of variables such as return space, return and exfiltration is of very high value, because of their potential to impact the effectiveness of the conditioning system and ventilation within the building space. It is highly desirable and recommended to be able to determine these parameters. Presently, there is versatility in the application of these variables with respect to the conditioned space in buildings. This is because such applications cut across single or dual duct VAV, or indeed other conditioning systems. Hence, the profoundness of the importance of their measurability is there. An invariable consequence of this is the fact that all building types require a close consideration of these measure variables, for attainment of ideal condition within their spaces. Bibliography Anon, “Guidelines on Energy efficiency of Air-conditioning Installations” (2007) 14 October 2010 Anon, “Analysis of Building Climatic Data” (2003) 20 October 2010 Cheung, C. K, Luther, M. B, Fuller, R. 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