Development of a spreadsheet to determine natural ventilation cooling hours for a commercial hybrid HVAC system

Achieving good indoor air quality in large residential and commercial buildings continues to be a top priority for designers, owners, and occupants. Maintaining good indoor air quality, which enables the occupants to obtain maximum comfort, is a great challenge. Many new materials, furnishings, processes, and products used in the construction further increase the release of air contaminants. Ventilation, air filtration, and source control are the common methods for achieving required indoor air quality. Of these methods, ventilation is most commonly used. Basically, ventilation is the process of replacing stale air from an enclosed place by fresh outdoor air. In ancient times, human beings were dependent only on natural ventilation. This did not prove to be a sustainable solution for many parts of the world that experienced severe climatic changes. To develop the sustainable solution for maintaining good indoor air quality, mechanical ventilation was introduced. Mechanical ventilation is costly because the outside air has to be heated or cooled depending on the season. Heating, ventilating, and air-conditioning (HVAC) systems are a major consumer of energy in both residential and commercial buildings. Issues such as global warming, the California energy crisis, and the blackout on August 15, 2003 in the United States and Canada have again diverted the focus on a major rethink of energy consumption by conventional systems. Researchers are trying to find energy-efficient solutions to replace the conventional systems without compromising indoor air quality. The possible solution could be the use of alternate modes, such as natural and mixed ventilation, which are more energy efficient and environmentally friendly. The application of these systems is growing very rapidly in the United States. Even though natural ventilation is energy efficient, it cannot be a remedy for all the issues related to indoor comfort, especially in severe climatic zones. Therefore, consideration should be made for the system that combines benefits of both natural and mechanical systems with minimized drawbacks. This new system is referred to as a “hybrid HVAC system.” The system would not be as energy efficient as natural ventilation, but would provide better control over usage that helps users to customize their environment and result in energy conservation. To reduce the energy consumption arising from HVAC systems, natural ventilation can be adopted for a suitable time period. Rather than depending totally on one type of ventilation (that is, natural or mechanical), a different system should be designed that will accommodate the benefits attributed to both types. The indoor temperature varies with the heat gains/losses because of various components of a building unit and because of the changes in the moisture content of both indoor and outdoor air. Thermal comfort is one of the top priorities of occupants. To maintain the thermal comfort, changes in indoor temperature should be studied carefully. Night ventilation is not only a power-saving measure for the time during which it is implemented, but also influences the consumption of energy in the daytime. It is clearly understood that night cooling can be a viable option for energy conservation because of its ease of use and economy. It is necessary to develop a hybrid HVAC system that will give users the choice of using natural or mechanical ventilation with adjustable air changes per hour. This research aims to help in designing such a hybrid ventilation system. This study expands the work done by Thomas et al. [1], which was targeted mainly for residential buildings. While developing the models in this study, additional heat gains/losses parameters (such as those attributed to motors, to human beings, and to lighting) and the effect of difference in indoor and outdoor moisture content, at specific relative humidity on indoor temperature, are considered. The duration for night ventilation is decided on the basis of indoor temperature and a spreadsheet is offered as a design aid in this report. In a building, outside climatic conditions affect heat gains by heat-transfer processes (see Figure 1). The whole concept behind developing this model is to calculate the time during the night for which total dependency on natural ventilation is possible. Inside temperature depends on the cooling, heat gains from various aspects such as windows, people working inside the plant, lighting, and motors as well as the properties of the walls and roof. The temperature difference between outside and inside temperatures is used to promote the natural ventilation that results in savings in electricity usage. The complete details on the model development are given by Pendse [2]. Factors affecting heat characteristics. The spreadsheet consists of four sheets: Input, Walls, Lighting, and Final. The sources of heat gains for the manufacturing plant model and the office model are identical, except that there will not be any motor inside the office. The heat transfer to the furnishings inside must also be taken into consideration in the case of an office building. Start time (in solar hours). A solar hour is the time measured from midnight to midnight (0 to 24). This system does not use a.m. or p.m. notations. (Think Quest Library [3]) Duration of cooling (in hours) End time (in solar hours) Volume of the plant (in ft3) Air changes per hour Design temperature (ASHRAE Handbook of Fundamentals [4]) Daily range (ASHRAE Handbook of Fundamentals [4]). The daily range is the mean difference between the daily maximum and minimum temperatures. Percentage of daily range (ASHRAE Handbook of Fundamentals [4]). The percentage of daily range is used to factor in the time of day into the calculation of the outdoor temperature. Indoor temperature before advent of night cooling Outside moisture content at designed wet bulb temperature (in gr/lb). The wet-bulb temperature is measured using a standard mercury-in-glass thermometer, with the thermometer bulb wrapped in muslin, which is kept wet. Inside moisture content at designed relative humidity (in gr/lb) Number of people performing various activities inside the plant: Light bench work Walking or light machine work Heavy work Heavy machine work (lifting) Number of people working inside the office (for Office Building) Motor specifications (for Manufacturing Plant only): Total horsepower of all the motors of type A (both motor and driven equipment inside) Total horsepower of all the motors of type B (motor outside and driven equipment outside) Total horsepower of all the motors of type C (motor inside and driven equipment outside) Internal mass inside the office building (in lb) (for Office Building only) Latitude and Longitude of the location under consideration Local standard time meridian Month (use combo box to select a month) Orientation of the walls in question (from a combo box) Exposed area of each wall Thermal conductivity of each wall Area of each window Thermal conductivity of the window Direct solar heat gain coefficient (SHGCθ) Diffuse solar heat gain coefficient (SHGCD) Once the inputs are entered, the direct normal irradiance (EDN), sky diffuse factor (Y), and direct solar irradiance (ED) are calculated by a single click on the compute button. Also available are navigation labels to display the other two sheets: Input and Final. In the “Lighting” sheet, combo buttons are provided to select the type of lighting. The user is required to provide information on the type of lighting with the help of the combo box, and input the number of bulbs of the same type. This worksheet provides information on lights, fixtures, and their characteristics. The output consists of the heat produced in Btu/h. The “Final” sheet calculates the final indoor temperature after night cooling and a part of the model equation. The initial indoor temperature and duration are obtained from the input sheet. The intermediate sheet gives the first part of the final model equation. The procedure for obtaining the duration of night cooling is discussed in the case study section. The spreadsheet can be downloaded from www.p2tools.utoledo.edu/. A users' guide is also available for learning the use of the spreadsheet. The original study was carried out for five cities from each zone as divided by the U.S. Department of Energy, the classification of which was on the basis of heating and cooling degree-days (see Table 1). Zone 1: Minneapolis, MN Zone 2: Toledo, OH Zone 3: Louisville, KY Zone 4: Raleigh, NC Zone 5: Brownsville, TX The cases were studied for a manufacturing plant as well as for an office building, assuming suitable data for each of them. The spreadsheet calculates the indoor temperature at the end of nighttime cooling for a given condition, which is then compared with the comfort conditions to check whether it lies in the comfort zone. The screen-shots of the design spreadsheet for a manufacturing plant located in Raleigh, NC are shown in Figures 2-6. These sheets show the input data required for the chosen case and the resulting indoor temperature at the end of nighttime cooling as output. Initially, the NOAA website was used to find mean monthly outdoor temperature for the area under consideration. Using this temperature, 80 and 90% thermal comfort ranges were found for indoor temperature using the chart given by Brager and de Dear [7]. The spreadsheet tool was run for different combinations of durations, start and end times, and air changes rates to obtain resultant indoor temperatures at the end of specified duration. These resultant temperatures were used for making recommendations. Two types of recommendations were made: conservative and nonconservative. For conservative recommendations, a 90% comfort range was used and duration for the resultant indoor temperature that falls at the center of the above-mentioned range was recommended. In this type, the importance was given to achieve the resultant temperature about the center of the 90% comfort range irrespective of duration. For nonconservative recommendations, an 80% comfort range was used and duration for the resultant indoor temperature providing maximum number of cooling hours was recommended. Here, the importance was given to achieve maximum cooling duration with the resultant temperature within the 80% comfort range. The results using this procedure are shown in Table 2. When there were two identical resultant indoor temperatures, the duration with higher cooling hours or/and lower air changes rate was recommended. Input sheet of Commercial HVAC System Design Spreadsheet. Walls sheet of Commercial HVAC System Design Spreadsheet. Lighting sheet of Commercial HVAC System Design Spreadsheet. Final sheet of Commercial HVAC System Design Spreadsheet User's Guide of Commercial HVAC System Design Spreadsheet. Whereas natural cooling for longer duration can be carried out, the truth remains that internal temperature must always be within the comfort range for occupants at the time of normal use. Hence, natural cooling cannot be carried out for months in which the indoor temperature falls below the comfort ranges recommended by Brager and de Dear [7]. By varying the cooling times and ventilation rates, internal temperature variation can be controlled to suit thermal comfort requirements. The variation of duration must be accompanied by optimal end time adjustments to ensure optimum derivation of the desired decrease or increase in indoor air temperature. The difference in the moisture content of indoor and outdoor air impacts the resultant temperature. As the designed relative humidity increases, a decrease in the value of resultant temperature is observed. To design an effective mixed-mode system, detailed study of all the input parameters must be made. Each case will have its own peculiarities and, consequently, its own specific natural ventilation opportunities. A designer must endeavor to carry out various permutations to arrive at a desirable plan for a hybrid system. A spreadsheet is available for the design of hybrid HVAC systems for a manufacturing facility and associated office building. It is easy to use and could be modified for a specific plant. It is hoped that the spreadsheet will be helpful in improving energy efficiency at your plant.

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