Calculation of Cooling Load for a Building

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Heat transfer makes a big part of thermal engineering and is focused on how heat is exchanged between different mediums. For countries that are prone to extreme temperatures, heating and cooling is a major consideration during construction. The design of any commercial building today requires that it allows easy installation of cooling and heating systems. The Heating and Ventilating Contractors’ Association (HVCA) requires that any building’s heating and ventilating systems be installed by qualified and licensed engineers to avoid risks that are associated with the process.

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Cooling load can be calculated in different ways depending with the building in question. Since cooling load calculations are sometimes very complex, different authors agree that software developments will go along way in helping achieve accuracy and reliability. To make these calculations easier, several assumptions have to be put into consideration. Different experts argue that making assumptions to make the calculations easier and then using complex formulations makes the process more unreliable.

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Introduction

Cooling load can be calculated in different ways. “The most basic of the available methods is the rule-of-thumb value, which is using square feet of floor area per tonne of cooling” (Grondzik, 2004). Such methods are very important in schematic designs and when an engineer is approximating equipment costs and sizes (United States Department of Energy, 2010). The main challenge for this method is the common assumption that the building’s design will not make a difference, while it has a significant influence on the final number. The other method is “using design equivalent temperature differences and design cooling load factor” (Ansari, Mokhtar, Abbas, & Adam, 2005). The method is more sophisticated but is more applicable in small families’ residences. The main challenge when attempting to use this formula is putting the many opaque components into one useful factor (Spitler, 2005). For big commercial building’s, such methods are not appropriate due to the high number of assumptions in them. For commercial buildings, the most applicable method is that developed by ASHRAE. With new technological innovations every day, computerized simulations are becoming increasingly common when estimating or calculating cooling loads for big buildings (Kreider, Peter, & Ari, 2002).

This paper explains how cooling load is calculated, parameters put into consideration and factors that affect the outcome. The paper highlights major assumptions when calculating cooling loads for buildings. It also looks at how they affect the calculations. The paper analyzes different views by different authors and seeks to understand their stand on the subject. It also examines the components of design cooling load and how different decisions affect them. Finally, the looks at how the high costs of heating and cooling can be minimized by utilizing new technologies.

Literature review

In their article on the approach for building cooling load estimation, Ansari, Mokhtar, Abbas and Adam, all professors at UPM, Malaysia, the authors demonstrate how very simple problems can be made complex due to different compulsions, many times academic or business. The professors also show how many simplifying assumptions can make it hard to handle a problem and many times result in a failed project. The article argues that dual policies in heat transfer calculations do not work, meaning that making too many assumptions to simplify a problem and using complex schemes to work it out at the same time will not work. The paper then presents an example to show how computer software can be used to make calculation simpler and more accurate. The example reveals how available technology can be used to easily investigate significant building parameters such as glass panes, roof types and glass shades just to mention a few. The authors conclude that computer software will make these calculations easier in future without compromising accuracy.

Walter Grondzik, a long time lecturer in heat transfer, explains design cooling load and its relationship with other parameters such as heat gains, heat loss and heating load. Walter also analyzes the different types of calculation methods and argues that the simple methods must not be used in calculating cooling load for commercial buildings. Such formulation according to him include “the rule of thumb, which uses square feet of floor per tonne of cooling to calculate the total cooling load for a building and the design equivalent temperature differences formula” (Grondzik 2004).

The United States Department of Energy, in its yearly publications, explains how building and investing in energy software will enable engineers have more accurate work when calculating the cooling load for commercial and residential buildings. The article explains different new tools for load calculations. These tools include the AEPs system planning used in calculating system performance, generation load storage calculations, and residential and commercial system sizing. The article also discusses tools such as AIRWIND Pro used for air conditioning load calculations.

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Omer is his article clearly states that heat pumps significantly reduce the amount of energy used for cooling or heating buildings (367). According to the author, ground source heating pumps can be used to reduce the need for primary fuels by more than 65% which would have increased the amount of emissions to the atmosphere. The article’s analysis is intended at estimating and comparing the warming effect of a GSHP and that of other cooling and heating systems. The paper studies the effects of heating on both commercial and residential buildings.

Analysis of data from the study reveals that conventional methods of heating and cooling emit up to 36% more greenhouse gasses than does GSHPs. The report also reveals that this technology is most effective in places such as Europe and other regions across the globe, which suffer extreme temperatures in different seasons. It concludes that it is unlikely the world is going to have a heating and cooling technology as friendly to the environment as the GSHP technology.

Phetteplace studies the technology behind heating and cooling buildings and explains how the technology operates by heat transfer from different sources to a building when it is cold and the reverse is true when it is hot. The journal is a simple explanation of the basic process which the readers would easily understand. It is also a comparison of the technology with other thermal technologies. The various types of heating and cooling together with their benefits are reviewed. A big part of the paper is the vertical borehole heat exchanger which is the most commonly used method of ground coupling (Phetteplace, 2007). The author discusses issues regarding sizing of heat exchangers and the various reasons for using each of the technologies.

Spittler argues how different technologies are being used as seasonal energy storage schemes. The paper uses the now common ground heating and cooling technologies and gives a general overview of how these technologies work. The paper gives a detailed overview of the buildings cooling technology since it was first mentioned in 1912. It is a discussion of how the technology has evolved and its current applicability in different parts of the world. The author also gives a highlight of what is expected of the technology in the few years to come. In regard to the current environmental concerns, more natural technologies appears to be the most favorable in minimizing environmental damages caused by conventional heating and cooling systems. The paper is also a brief research into how the GSHPs can be optimized to improve the technology’s thermal conductivity, a more environmental option for the cooling and heating processes.

Milenic, Petar, and Ana argue that environmental protection is conducted by use of renewable energy sources. Their article is a discussion of how the cooling and heating technology is being used in developed regions today such as Europe. Considering the number of homes and buildings put up in the western countries each year, the article argues that there is need for reduced energy consumption levels by utilizing technologies that can store heat while cooling and utilize it when heating is needed. Finally, Eicker and Christoph (2009) propose that geothermal heat exchangers would be the best technology as heat sinks for buildings energy produced during the hot seasons. An experiment carried out in this study is used to study how a building parameters, inlet and exit temperatures would influence the operation and effectiveness of heat transfer processes in a building.

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Information and analysis

Cooling or heat loss calculations are many times used to determine a building’s heating load, which then helps design its cooling system. Heat loss is simply gotten by summing up all the heat flows out of a given medium, in this case a building (Eicker, 2003). Heat gain on the other hand is calculated by adding up all the heat flows to the building and then summing it up with that generated internally (Ansari, Mokhtar, Abbas, & Adam, 2005).

The assumptions behind most design cooling loads are:

  • “Weather conditions are usually selected from statistical databases accumulated for a long period of time” (Eicker, 2003). “The weather conditions do not necessarily represent any specific year but the project always ensures that the figures used represent the buildings location” (Wiley, 2009). Such data has been tabulated by different researchers and groups such as ASHRAE. Engineers and designers must ensure they select the most appropriate weather for a building. There are specific codes for different locations, which allow accurate selection of data to avoid over or under-sized systems (Binggeli, 2003).
  • “For accurate calculations, solar loads for the most clear day of the month chosen for calculations are used” (Huber & Seeberg, 2006).
  • The building in question is always assumed to have a full design capacity occupancy (Santamouris, 1996).
  • “The building’s appliances, equipments and lighting system are assumed to be operating at the expected capacity as per the design” (Wittek & Kratzig, 2002).
  • Sensible and other loads as well as latent are considered during calculations.
  • Dynamic conditions are assumed when analyzing heat flow.
  • Heat storage in the building and other interior materials, which may affect heating, are put into consideration during calculations.

Such assumptions make it hard to calculate design cooling load than it is to calculate design heat loss. “Unfortunately, there is no way around them, especially in a cooling load dominant climate such as that in the southern United States” (Sawyer, 2000).

During calculations, the cooling load for the building must include heat through the buildings from other sources, as well that generated internally by the lighting system, people and other electrical equipments (Huber & Seeberg, 2006). The heat from outside, also known as external load, may come from different external environments depending with the building’s design and that of its heating system. Heat from within the building is known as internal load and will vary with the building’s location, structure and type of electrical and other equipments in the building (Chartered Institutional of Building Services Engineers, 2000). Buildings used for manufacturing purposes for example will have a bigger internal load than one used for administration purposes. Other types of loads include latent and sensible loads, which affect the dry bulb air temperature and absolute humidity respectively (Wittek & Kratzig, 2002).

Factors to consider during calculations

The first step in calculating heat loads usually involve establishing each zone’s peak heating and cooling load. The zones loads are then put together to get a building’s peak load (Wang, 1998). For these step, several factors need to be put into consideration. These factors include solar gains through windows and when there are glazed windows and glasses, they should be also put into consideration (Association of Energy Engineers, 2002). The next step is ensuring that all the buildings meet the required air requirements. “The amount of outside air is taken from ASHRAE Standards 62” (Wiley, 2009). Other important considerations during calculations include heat lost and gained through walls and roofs.

External and internal loads can be calculated using different equations:

External loads: Qs= (U)(A)(CLTD)

Where U= Overall coefficient of heat transfer

A= Surface area of element

CLTD= Cooling load temperature difference

For internal loads:

Qs= (Lamps wattage)(3.41)(Usage factor)(Ballast factor)(CLF)

Conclusion

Cooling and heating buildings has become a costly affair for many governments toady (Phetteplace, 2007). However, new technologies continue to make it easier for and more affordable to stay cool when it is hot and stay warm during winter. The latest technology is the ground source heating pumps, which supply heat to a building and keep it cool when it is hot. It is also proven that a building could use up to 70% less electricity by using GSHP technology (Grondzik, 2004). Its maintenance costs are half that of conventional systems while the cost of installing it can go as low as a quarter that of conventional systems.

The pipes are installed in such a way that they utilize the energy near the surface of the earth. This minimizes the time taken to transfer energy from and to a building. By tapping heat energy nearer to the surface, the length of piping is reduced which significantly saves on cost. During cooling, the excess heat in the building is absorbed by the earth through the pipes.

Omer (2008), estimates that an open-loop water source system for an average residential house would cost not more than $ 10,000 while the price may double for a closed-open loop system. The annual operating costs and maintenance costs amount to a far much less figures compared to other conventional heat exchange systems. The total cost of running a GSHP system average to $850, while a conventional system would amount to three time this (Milenic, Petar, & Ana, 2010). Even though the savings may be dependent on different factors such as climatic patterns of a place, lifestyle and size of a building, they cannot add up to the costs incurred when using conventional systems of cooling and heating buildings (Milenic, 2006). Economical benefits also arise from the fact that the technology does not incur any space for storage like in the case of boilers, cooling towers and generators (Eicker & Christoph, 2009). All the money saved can then go to savings and other economic developments in a society.

Reference list

Ansari, F.A., Mokhtar, A.S., Abbas, K.A., and Adam, N.M. (2005). A simple approach of building cooling load estimation. American Journal of Environmental Sciences, 1(3): 2009-212.

Association of Energy Engineers, (2002). Strategic planning for cogeneration and energy management. Atlanta, Ga: Fairmount Press.

Binggeli, C. (2003). Building systems for interior designers. Hoboken: J. Wiley & Sons.

Chartered Institutional of Building Services Engineers. (2000). Building services: The CIBSE journal. London: Builder Group Ltd.

Eicker, U. (2003). Solar technologies for buildings. Chichester: Wiley.

Eicker, U, & Christoph, V., (2009). Potential of Geothermal heat exchangers of office building climatization. Renewable Energy, 34 (13),1126-1133.

Grondzik, W. (2004). Environmental technology: Design cooling load. New York: Routledge Publishers.

Huber, M., & Seeberg, P. (2006). Limits and potentials of office building climatization with ambient air. Energy and Buildings, 38(6), 574-581

Kreider,J., Peter, C., & Ari, R. (2002). Heating and cooling buildings: Design for efficiency. New York: McGraw-Hill.

Milenic, D. (2006). The science of heating and cooling. New York: Routledge Publishers

Milenic, D, Petar V, & Ana, V, (2010). Criteria for use of groundwater as renewable energy source in geothermal heat pump systems for building heating/cooling purposes. Energy and Buildings, 42 (10), 649-657.

Omer, A. M. (2008). Ground-source heat pumps systems and applications. Renewable and Sustainable Energy Reviews 12(11), 344-371.

Phetteplace, G.(2007). Geothermal heat pumps. Journal of Energy Engineering 32(4), 32-38.

Santamouris, M. (1996). Passive cooling of buildings. London: James & James.

Sawyer, R. (2000). Calculating total power requirements for data centers. New York: Routledge Publishers.

Spitler, J. (2005). Ground-source heat pump system research: Past, present and future. HVAC & R Research, 11(2), 165-167.

United States Department of Energy. (2010). Load calculation. Web.

Wang, S. (1998). Dynamic simulation of a building central chilling system and evaluation of EMCS on-line control strategies. Building and Environment, 33 (1): 1- 20.

Wiley, U. (2009). Low energy cooling for sustainable buildings. Hoboken, NJ: John Wiley & Sons.

Wittek, U., & Kratzig, W.B. (2002). Natural drought cooling towers. Netherlands: Balkema Publishers.

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