DCV…Grandma’s Kitchen Will Never Be The Same

Looking back I now realize this was my first exposure to the concept of resource optimization. While I doubt the words resource optimization ever crossed my grandfather's mind, clearly he was frustrated with my grandmother's insistence on cooking all that extra food in the event someone might show up — building the church for Easter Sunday as he would say — every day of the week. This was clearly wasteful to him, and looking back it is a clear example of not matching one's resources to the demand for those resources, the definition of inefficiency.

In hotels, the examples of resource optimization are numerous. Something as simple as a staff schedule that is modified to reflect the ebbs and flows of occupancy is a great example. Beyond manual systems, there are countless examples of the use of technology to help us match resources with the demand for those resources, such as occupancy sensing thermostats or motion detecting light switches that only allow for the consumption of the resource they control when the need for that resource is proven to be present. Today, the list of automated resource optimization tools available to hoteliers grows daily, and in this vein I offer the technology known as Demand Control Ventilation, or DCV.

DCV is a control system that works in concert with kitchen exhaust systems. It adjusts the rate of exhaust to coincide with the rate of cooking activity, speeding up and slowing down the rate of exhaust to coincide with the amount of air needed to eliminate the smoke and heat above cooking appliances at any given time…no more, no less. It accomplishes this feat using simple, well established component technologies such as high-temperature thermometers and variable frequency drives, along with proprietary software and control sequences. But while the concept is simple, the technology is quite advanced.

Properly installed DCV systems include a myriad of engineering and design calculations to ensure proper system performance, both of the DCV system as well as the kitchen's existing air handling system. DCV equipment controls not only the exhaust fan, but it also interfaces with supply air fans, fire suppression and alarm systems, and existing building automation systems, if desired. But this technology was not always so advanced. First generation DCV systems used photocell technology to "visually" identify the presence of smoke in kitchen hoods. But this methodology proved cumbersome and lacking in responsiveness to rapid changes in cooking activity, and all too often resulted in kitchens full of smoke or dining rooms filled with kitchen odors. Like so many good ideas, DCV just need some good old fashioned R&D to establish itself as an effective and reliable means of resource management, truly saving thousands of dollars in energy costs without any sacrifice in performance.

The numbers behind DCV have as great an impact as the technology itself. Consider this real world example. A full sized commercial kitchen operating in a major mid-Atlantic resort commences cooking operations at 5:30 AM every day in the bakery, and concludes at midnight on the In-Room Dining hotline. This results in a 19-hour operating cycle with exhaust systems operating at full speed for the duration of this cycle, regardless of lulls in activity. However, normal bakery operations cease midday, and there is very little demand for the kitchen's numerous hotlines from 2 to 4 PM, especially when there is a lull in hotel occupancy — but the exhaust systems run at full speed, all day, every day, since one never knows precisely when an order for hot food might come in. With DCV added to this typical exhaust set up, the consumption of energy plummets between meal periods (see fig. 1). Savings are generated not only from the reduction in electricity consumed by the exhaust fan's motor, but also the reduction in energy associated with heating and cooling the air introduced back to the kitchen to replace this exhaust air, known as make-up air. Modern DCV systems control make-up air fans, as well as the exhaust fans, so that increases and decreases in exhaust are mirrored by matching changes in the rate of make-up air — a critical step in the control sequence necessary to maintain a kitchen's air balance. Without it, kitchen odors can permeate the surrounding spaces, or unconditioned air can be pulled into the property, reducing the benefits of the DCV installation.

In great part the reason DCV systems can produce such prolific savings is due to the manner in which electric motors consume power. When the RPM of a motor decreases, it's power consumption decreases exponentially — not linearly — magnifying the savings. (See figure 2.) As a result, even modest decreases in motor speeds can produce substantial decreases in power consumption, potentially saving thousands of dollars annually per horsepower of motor size.

One could liken this phenomenon to driving car. Those of us with an instant mileage gauge know that our fuel economy drops at a much greater rate when we accelerate from 50 to 60 mph, than it does when accelerating from 20 to 30 mph. The relative increase in engine RPM requires a greater consumption of fuel at high speeds than equally sized increases at lower speeds. The difference between a car and a kitchen hood is we have a gas pedal in a car which we use to constantly modulate our speed in response to changing road conditions. In a kitchen, DCV provides this gas pedal — and the driver for that matter — automatically sensing changes in the amount of heat produced by cooking activities and speeding up or slowing down exhaust rates accordingly. Still, even Kit from the TV show Knight Rider had a nitrous oxide. Sometimes you just need to floor it. DCV's version of the nitrous button exists in a simple local override switch located adjacent to the exhaust hood. This allows a cook to instantly turn up the exhaust system to full speed for a predetermined amount of time should he or she deem it necessary — a rarely used but highly comforting feature of the DCV installation.

While the numbers in the example above are certainly dramatic enough, but there's perhaps no more compelling evidence of DCV as a legitimate energy saving technology than the existence of numerous rebate incentives. Utility companies around the country provide funding to mitigate the cost of DCV installations as a routine part of their incentive offerings. Remember, utility rebates are designed to help prevent power companies from having to build more generating capacity — a.k.a. power plants. Simply stated, it is cheaper to pay energy consumers to consume less energy than it is to build the power plants needed to meet the demand for consuming that energy. So utilities happily offer sizeable rebates for technologies proven to save them money, and DCV has clearly established itself as one of those technologies. Throughout the country, it is common to find rebates as high as $350 per horsepower for fan motors controlled by DCV.

DCV is offered by several major kitchen hood manufacturers as an option in new installations as well as a retrofit to existing systems, allowing for comparison shopping among savvy consumers. Is DCV right for any given hotel or restaurant? Properly specified and installed, it probably is and can save operations significant energy costs. Simple paybacks for typical installations are commonly less than two years, and often just a few months. Thereafter, they go on for years saving money, reducing carbon footprints, and giving a chef the comfort of knowing he can cook for a hundred unexpected diners whenever he likes — a luxury I think with which my grandmother would have surely been impressed.