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The CEA Chronicle
Make Net Zero Happen in Utah
Buildings exist to maintain an acceptable environment for occupants to perform a given function. Thus, HVAC should be viewed as the systems necessary for meeting the requirements and needs of people, and the goal should be to focus on providing the occupants' comfort in the most energy efficient and cost effective way possible.
These factors comprise the majority of local thermal discomfort issues in commercial buildings:
- Stratification (vertical air temperature distribution)
- Floor Surface Temperature
- Cyclical and Sporadic Temperature Variation(s)
- Drafts (unwanted)
- Asymmetrical Radiation Temperatures
Definitions for human thermal comfort are provided by ASHRAE Standard 55, where the threshold of a successful design is determined by satisfying 80% of occupants. Factors that are evaluated include:
- Air Temperature
- Air Speed
- Humidity
- Metabolic Rate
- Clothing Insulation
- Radiation Temperature
Consideration of all these variables toward thermal comfort, described as the adaptive model, is offered in Section 5 of Standard 55. Simple, user-friendly software is available to evaluate different thermal comfort design scenarios, using the adaptive model. More information is available at the following link.
http://apps1.eere.energy.gov/buildings/tools_directory/software.cfm/ID=371/pagename=alpha_list
Consider the following topics when designing for low energy thermal comfort approaches, using the adaptive model.
- Natural ventilation relies on pressure differences to move fresh air through buildings. By allowing this air movement, higher ambient temperatures are perceived as acceptable. Thus, cooling and ventilation energy can be saved, particularly in spring and fall months.
- Occupant control over their immediate environment, such as operable windows & adjustable diffusers, tends toward a broader range of tolerant conditions. This can translate into energy savings, but particular attention should be paid to the integration of these features with building control systems to avoid scenarios that might waste energy.
- Consider use of air movement, such as local work station fans, to increase the bounds on thermal comfort during cooling season. With low velocity air movement, higher acceptable space temperatures can be tolerated and the cost of operating cooling equipment can be eliminated in favor of increased fan energy consumption.
- In cooling applications, displacement ventilation can take advantage of temperature stratification by introducing cool air at the floor level and removing warm air at the ceiling. This reduces the total cooling load and the energy and cost associated with cooling. This approach can be particularly useful in large, densely occupied rooms, such as gymnasiums and auditoriums.
- Radiant floor heating supplies heat directly to the floor, which is then transferred to the room's occupants and objects. While the primary driver for this type of system is radiation, these systems also depend significantly on convection (air circulation) caused by heat rising from the floor. Utility and equipment costs are reduced through reduction of supply fans and duct losses. The lack of moving air, during heating season, also reduces potential for unwanted drafts which contribute to thermal discomfort.
- In drier climates, relative humidity can be considered comfortable at higher levels. Therefore, the benefits of evaporative cooling can be exploited without creating uncomfortable levels of relative humidity, within a space. Where direct and indirect evaporative cooling methods can be used, up to 90% of cooling energy consumption can be offset.
- Basic occupant education and understanding of the building's HVAC systems helps the occupants feel more connected to their built environment. Understanding how the building HVAC systems will respond to certain outdoor weather conditions allows the occupants to understand and adapt to these conditions. For example, educating occupants about use of natural ventilation will provide the occupant the ability to wear (seasonally) appropriate clothing, rather than experiencing discomfort and confusion.
- Consider the types of furniture and surface colors used for interior finishes and how it relates to the heating and cooling of spaces. Surface color choices can effect radiation between surfaces and occupants, as well as affect the perception of cool and warmth.
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