Using HVAC and lighting demand response
Strategies for handling DR fall into two basic categories: standby generation and load curtailment.
Part I of our series on demand response (DR) strategies, introduced the goal of a commercial DR strategy. This article discusses how strategies for handling DR fall into two basic categories: standby generation and load curtailment (also known as load shed). Let's start with a discussion about load curtailment. According to a 2003 Dept. of Energy (DOE) survey, lighting and HVAC together account for two-thirds of the electrical power usage in a typical office building. That is good news, since lighting and HVAC also are best suited to being controlled by a responsive and programmable DR system, and should be first and second on a list of curtailment strategies.
Despite the fact that annually, lighting and HVAC electricity use is virtually equal, HVAC curtailment is often the only DR strategy used in a building. Theoretically, this makes perfect sense since demand peaks are usually associated with outside climate conditions that closely mirror the HVAC demand. In other words, the warmer the outside air temperature, the more electricity it takes to cool the building—and the cooler the outside temperature, the more electricity it takes to heat the building. Therefore, during peak demand HVAC typically accounts for the larger share of power usage.
This leads back to the risk-reward proposition that a faster reaction time equals greater reward. HVAC accounts for the larger share in a demand event, but with that come multiple DR management considerations including predictability and responsiveness. HVAC is weather-dependent, so predictability is limited. After all, it is difficult to predict how cold or warm it will be next April 3. And HVAC has to manage the relationship between three variables: temperature, ventilation, and humidity. HVAC does not respond either immediately or proportionately when you change settings. The thermal mass of the building is highly complex and has tremendous momentum: There is an extensive lag time between a change to the HVAC setting and the desired result. Reversing HVAC is like trying to reverse a moving train—it's a gradual process. HVAC also has a recovery time to account for. Radical adjustments can produce unintended peaks, which may be worse than no load shed at all.
Lighting control: Linear, responsive DR strategy
So what about lighting? As we mentioned before, lighting and HVAC are almost equal in yearly power usage, but lighting use is not climate driven. Day-to-day, lighting is essentially constant, much the same at 9 a.m. as it is at 3 p.m. with only slight deviations as a result of daylight harvesting. Even with daylight harvesting, algorithms can effectively calculate and account for the angle and arc of the sun in respect to a building's exact location, making lighting levels even more predictive. As opposed to HVAC, lighting is linear and highly responsive. Lighting power is simply the product of voltage and current, and since the voltage is steady there is only one variable—current. Reduce current and lights go down; increase current and lights go up. The speed at which you take current away or put it back is the speed at which the lights change, making lighting easier to manage than HVAC. It is the predictive nature of lighting along with its linear response that make it such a useful demand strategy, especially as a means of quickly contributing to response levels that HVAC can achieve only over time.
Occupant productivity is always an issue with demand events (see Figures 2 and 3). This is probably the biggest reason on-site generation is used more than curtailment. On-site generation consumes resources and contributes to faster equipment degradation but will not negatively impact productivity. Lighting, on the other hand, works in conjunction with the marvelous, innate qualities of the human eye. The pupil naturally expands to counter a decrease in light. A study by the Lighting Research Center, Rensselaer Polytechnic Institute, has shown that most occupants will not detect a gradual change in light level such as a 15%-20% decrease in light output. Gradual, slow, and steady changes over a few seconds are offset by the eyes’ natural capabilities and will have no impact on productivity. Part III of our series will cover federal and local legislation, and the future of DR.
Scott Ziegenfus is a senior applications engineer with Lutron Electronics Co. Inc. Ziegenfus has an electrical engineering degree from Lafayette College. He is an educational programs chair and board member for the Delaware Valley Chapter of the U.S. Green Building Council and is a certified LEED Study Guide Facilitator. Ziegenfus also serves on ASHRAE standards committees SPC 201P–Facility Smart Grid Information Model and SSPC 135–BACnet.
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