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How does sensor accuracy impact building energy management?

<p> Keeping buildings warm or cool takes even more energy when the sensors don't work correctly. When you add in reporting, the problem snowballs. </p>

As per International Energy Agency (IEA), buildings contribute to about 32 percent of global energy consumption, much higher than transportation or any other category. In developed economies such as the U.S., this figure is much higher, going up to 47.6 percent in 2013.

In an increasingly energy deficient world, energy conservation in operating buildings have become a key focus area. Building automation and control systems help with this goal. However, their effectiveness relies heavily on the accuracy of sensing devices to provide inputs to the control system. These control systems take decisions around energy optimization in addition to comfort, convenience and safety.

Over the years, level of automation has risen steadily; but the sophistication in software and control systems has not always been able to achieve the desired energy management goals due to other limitations, sensing accuracy being one of the significant causes.

Control systems maintain proper building environment in a cost effective manner through four variables - temperature, humidity, pressure and ventilation. Usually control of built environment is executed by starting, stopping or regulating the HVAC system through a combination of hardware and software actions. The control process typically is composed of three steps:

1. Measure an environmental variable, such as temperature or humidity, at various points and collect the data

2. Process the data with other environmental and contextual information

3. Cause or trigger a control action, and if desired, feedback the impact to the control system

These steps are realized through sensors, controllers and software systems. You can see that if the sensors are not accurate, other actions will carry through the flaws, potentially even amplify them.

Run the numbers

Consider a typical commercial office building with an area of 100,000 square feet. The HVAC system is composed of many parts. The chillers in the central plant make up the high side of the system, and the air handling units in the various floors of the office make up the low side of the system. There are many other associated devices and components - pumps, cooling towers, etc-- but let's keep this example simple and limit it to the chillers and AHUs. There are usually sensors to monitor the temperature of chilled water entering and leaving the chillers.

If the two temperature sensors in this case are off by 0.5 degrees Celsius each, it could lead to an efficiency error of approximately 17 percent in the performance of the chiller. In the case of this particular building, we might end up paying about $29,000 excess in electricity charges per annum; two years of such losses almost equals the cost of the chiller itself. On the low side, a similar 0.5 C calibration difference at the various points supply air temperature is measured in the AHUs, could lead up to a 72 percent error in the reported efficiency performance.

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For every type of sensor, if there is an accuracy issue, we will see very similar deviations in actual and reported performance, which will completely mislead the control system and thereby cause significant energy efficiency loss and other operational challenges.

For building managers, many of the day-to-day issues reported by occupants can be traced to sensor faults. The following table captures some of the popular such faults and causes.

Faults / Issues

Contributing Cause

Corresponding Sensor Fault

High room temperature (under cooling)

High cooling coil entering temperature

Temperature sensor

Low cooling coil water flow

Temperature / pressure sensor

Excessive ambient temperature

Temperature sensor

High intake of outside air

Air-flow sensor

Insufficient supply air flow

Air-flow sensor / pressure sensor

Low room temperature (over cooling)

Low cooling coil entering temperature

Temperature sensor

High cooling coil water flow

Water flow sensor

Low outside air temperature

Temperature sensor

High intake of outside air

Air-flow sensor

High supply air flow

Air-flow sensor

Poor indoor air quality (IAQ)

High room CO2 value

CO2 sensor

High room CO (Carbon Monoxide - poisonous gas) value

CO sensor

High humidity

Room humidity not meeting the standard

Humidity sensor

High / low building static pressure

Building static pressure is higher or lower than the set value

Building static pressure sensor

 

Recent advancements have made automated fault detection and diagnostics very popular as a tool to optimize energy consumption and building performance. The sensor output information is critical for FDD. Sensor malfunction is amplified through the feedback loop and may trigger unrealistic fault alarms.

In addition to energy and financial losses, faulty sensors leading to incorrect control parameters also lead to deviations in building performance indicators such as energy benchmarking, energy forecasting, measurement and verification functions of guaranteed performance contracts or other statutory reporting, amongst others. Sustainability programs,corporations or even nations can be adversely impacted by something as simple as faulty sensors.

Failures in sensing devices are manifested in many different ways:

· Permanent: sensor output takes on an incorrect value permanently

· Offset: output consists of correct sensor values offset by a constant amount; this applies to both analog and digital sensors

· Erratic: output consists of correct sensor values offset by varying amounts in time

· Intermittent: occurs only occasionally, disrupting what is otherwise a valid sensor output

· Transient: usually due to failure of electronics associated with the sensor (too large current or voltage, interferences, e.g., lightning, radio-interference)

Sensor accuracy is increasingly becoming a topic of concern for various stakeholders in the sustainability ecosystem. Countries like Singapore are addressing this issue through regulatory actions implemented through its Building Construction Authority's Green Mark program, which specify limits for sensor accuracy in various building equipment and systems. Many progressive architects and designers are also incorporating sensor accuracy considerations in their building design. We need more education and collective action amongst the other players in this sustainability ecosystem to make a discernable impact.

In the next part of this series, I will explore various methods to identify sensor accuracy issues.

Top image of TK by TK via TK.

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