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September 2016
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AutomatedBuildings.com

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Buildings as Dynamical Systems

Part One: Impact of Weather on a Building
Dr Sophie Loire

Sophie Loire, Ph.D.,
Research and Technology Fellow
Ecorithm, Inc.

“The sun did not shine. It was too wet to play. So we sat in the house.  All that cold, cold, wet day.” ― Dr. Seuss, The Cat in the Hat

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Reliable Controls

Shelter is one of the basic necessities of human life and, in the 21st century, we live almost completely indoors. It is estimated that the average American spends approximately 93% of their life within the confines of a building.1 As society has advanced, buildings have evolved from mere shelter from the elements to a critical driver of human comfort, health, and productivity.

Buildings

As more of life has moved indoors, buildings have become amazingly complex, dynamical systems, which are increasingly difficult to monitor and manage within the constraints of tightening energy requirements and occupants’ comfort expectations. Unfortunately, comfort often has an inverse relationship to energy efficiency and/or maintenance costs making it a difficult task for building operators. To add to the challenge, the building environment is dramatically influenced by a variety of ever-changing external and internal factors, most notably weather and occupancy.  

“After three days, the temperature continues to plummet. The team is decimated and demoralized. Unless the weather miraculously changes, we’ll surely succumb to frostbite. This message is our last hope: please send an engineer to the 3rd floor conference room ASAP. Also, the room next to the kitchen smells musty.” ― Overly dramatic tenant complaint

The biggest impact that weather has on a building and the comfort of its occupants is through heat transfers that can occur in a variety of ways (conduction through the shell of the building, convection from airflow through openings, radiation from the sun, etc.).  Understanding these mechanisms of heat transfer and how well a building prevents or promotes them, informs us not only when to cool or when to heat but also by how much and by what method. 

Temperature is not the only concern when trying to maintain comfort, however, humidity and ventilation also play a large role.  The amount of moisture in the air can impact how an occupant perceives temperature and can also contribute to unwanted conditions such as poor indoor air quality.  Likewise, ventilation can impact how an occupant perceives temperature and can also be directly related to health and productivity through indoor air quality.  Humans require fresh outdoor air for ventilation, forcing the need to continuously condition outdoor air as it enters the building. 

To balance occupant comfort and energy efficiency, reliable analysis of the weather dynamic is needed but weather phenomena are complex and hard to predict. Consequently, HVAC systems are typically designed for the largest range of temperatures experienced by a given building based on its geographical location and most HVAC sequences are too loosely programmed to effectively and efficiently maintain comfort under the variety of temperature and weather cycles.

So how do we design a system that allows a building operator to utilize available building and weather data to control parameters such as high comfort and low energy expenditure?

Ecorithm’s True Analytics™ software solution applies advanced analytics to the intersecting cycles of weather and building operations to precisely define the impact of the weather on HVAC operation.2 True Analytics goes far beyond simple outdoor air temperature profiling and load estimations to extract the spectral patterns of both weather and HVAC equipment on an intraday, weekly, monthly, and seasonal basis. The intersection of these unique patterns enable True Analytics to identify and diagnose faulty responses to the outdoor environment with exceptional speed and accuracy.  A simple example is summarized below.

Graph

Figure 1: A building in Santa Barbara, which represents a typical building, has a simple control sequence based on outdoor air temperature or enthalpy.  In this example, Ecorithm was able to identify deficiencies in the control programming and made recommendations that increase both comfort and energy efficiency.

Reliable ControlsDespite its stochastic nature, weather is a dynamical system with known cyclical patterns from multi-year phenomena to seasonal variations to diurnal patterns not only in terms of temperature (hot summers and daytimes versus cooler winters and nights) but also in terms of precipitation, humidity, and cloud coverage.  Geographical locations like Santa Barbara, where Ecorithm is based, have seemingly ideal weather conditions year-round; however, there are still daily and seasonal patterns that emerge from data sets.

“There is no way that we can predict the weather six months ahead beyond giving the seasonal average” ― Stephen Hawking, Black Holes and Baby Universes

Ecorithm uses a unique approach of exploiting spectral content of weather data, rather than the data directly. By abstracting and understanding features of historical data and incorporating weather forecasting from a 3rd party API, Ecorithm can unlock the potential to estimate a building’s future dynamic response.  Improved accuracy in predictive analytics can allow building operators to better understand their buildings’ operations and enable proactive control of HVAC systems. Ultimately, understanding how buildings react to their environment allows smarter building design, construction and operations.   

This article is part one of a two-part series on “Buildings as Dynamical Systems”. Stay tuned next month where we discuss internal influences on buildings such as occupancy which plays a large role in the maintenance of comfort, energy savings, and operations and maintenance. 
________________________________________

1Environmental Protection Agency, Report to Congress on Indoor Air Quality: Volume II - Assessment and Control of Indoor Air Pollution.  1989. Preface Page i.

2The concept behind ETAS uses of weather were introduced by Ecorithm’s co-founder Dr. Igor Mezic in the 2010 paper "Decomposing building system data for model validation and analysis using the Koopman operator." Proceedings of the National IBPSA USA Conference
 


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