Can you recommend some solutions for capturing waste heat in a commercial kitchen?
Commercial kitchens are very energy intensive places. At least some of that abundant energy can usually be recovered. The key is to identify waste heat sources, potential end uses, and times of day the waste heat is available. Here are some typical sources and uses for waste heat in commercial kitchens:
- Kitchen hood and dishwasher hood heat recovery used to preheat makeup air
- Dishwasher and pot sink drain water used to preheat domestic hot water
- Hot air in the kitchen used by a heat pump to preheat hot water
- Refrigeration heat recovery used to preheat hot water or makeup air
Let’s take these one at a time.
Commercial Kitchen Ventilation Heat Recovery
Class-one hoods (kitchen grease hoods) discharge between 200 - 600 cubic feet per minute (CFM) of warm air for each linear foot of hood, which must be replaced by makeup air from the outside. In a typical high-volume kitchen, this can easily add up to 10,000 CFM or more and represents a huge heating and cooling load.
While the exhaust air is usually quite warm, about 100°F., it is laden with grease which quickly coats the inside of the hood, filters, ductwork, fans and any heat exchanger surfaces that may be used for heat recovery—rendering them ineffective. While a few manufacturers have produced heat recovery ventilators with a water wash-down system built in to clean the heat exchanger, the high cost, energy consumption and maintenance of these units often precludes their use. Here’s an abstract from an ASHRAE presentation on the subject :
"Is it Possible or Practical to Recover Energy from Kitchen Hood Exhaust? Energy recovery from kitchen exhaust is not common despite its attractiveness for heating the makeup air and/or domestic water. The grease-laden characteristic of exhaust from cooking presents a technical and a local code challenge for the installation of energy recovery equipment. Although one manufacturer of kitchen ventilation equipment developed a heat recovery module, successful installations have not been documented. Similarly, engineered systems have been anecdotally reported for institutional kitchens in northern climate zones. Feedback from the design community on the potential for heat recovery from kitchen exhaust systems and the role of ASHRAE is sought."
APC Liaison: Kenneth M. Clark, P.E., Burns & McDonnell, Kansas City, MO.
Recently, several kitchen hood manufacturers have improved the grease-capturing efficiency of their hoods by redesigning the filter to better capture grease and by utilizing a new technology that uses ultraviolet light and ozone to destroy grease accumulations and food odors in the hood. These innovations may reduce grease accumulations in the heat exchanger enough to reduce the frequency of wash-down cycles allowing the systems to be cost effective. One unit I was able to find in my search was from Halton.
The same limitations on recovering heat from the ventilation system that apply to using the heat for makeup air also apply to other uses of the recovered heat, such as building heating.
While heat recovery from hoods is problematic, there are other ways of reducing the energy use of these systems besides recovering heat. I have included links to several general resources on kitchen energy efficiency.
Waste Water Heat Recovery
Another potential source of energy which can be recovered is the warm water draining from dishwashers. Pot sinks, kettle and other cooking appliance drains also provide warm drain water, but the volume and frequency of hot water drains is probably insufficient to make heat recovery economically practical.
If the heat from drain water is extracted, it can be used to pre-heat incoming cold water using a gravity-film drain-water heat exchanger or a specially designed horizontal heat exchanger designed for this purpose. The gravity-film heat exchangers consist of several feet of coiled copper tubing mounted vertically and arranged so that, as the warm drain water flows over the coils, heat is transferred to incoming cold water. Here are links to the manufacturers’ webpages for the two units currently on the market — the GFX heat exchanger and the Water Cycles Heat Exchanger.
In order for these systems to work, you need the vertical space to install the heat exchanger and a source of cold water circulating through the heat exchanger coinciding with the warm water out-flow in order to transfer heat. This is simple if you’re using the heat exchanger to recover heat from a shower drain because you can pipe the cold water going to the water heater through the heat exchanger so that whenever someone takes a shower, cold water will circulate. In the case of a dishwasher, this is not usually the case — dishwashers are usually filled at the beginning of the day and are drained hours later during cleanup. To use this system with a dishwasher it is necessary to add a pump and storage tank to recover the heat when it is available and store it until needed – this doubles cost of the system and reduces its cost effectiveness.
Since drain water usually includes significant amounts of grease, removing heat from the water can cause the dissolved grease to solidify in the piping downstream of the heat exchanger, creating a maintenance headache.
Heat Recovery from Kitchen Air
Many kitchens have "hot spots" — areas where heat builds up near cooking appliances. A special heat pump, designed to produce hot water is a perfect way to recover heat from hot kitchens and use it to heat or supplement hot water.
Heat Recovery from Refrigeration Equipment
Most kitchens have several pieces of refrigeration equipment—walk-in freezers and coolers, ice machines, etc. Heat can be extracted from the condensing units of this refrigeration equipment and used to pre-heat water or air during the heating season. If the refrigeration equipment is water cooled, heat recovery can be very cost effective and straightforward. The Natural Resources Canada factsheet Heat Recovery from Kitchen Refrigeration describes the technology.
If the refrigeration equipment is air cooled and their condensers are located within the kitchen, heat is recovered whenever the equipment runs—summer and winter. In summer this increases the cooling load and probably negates any wintertime energy savings. If the air-cooled condensers are located near an outside wall, it may be possible to locate them within an enclosed area and provide dampers and a fan to cool the enclosure with outside air in summer. In winter the dampers could be reset to close off the outside air and circulate the warm air into the kitchen.
Another way to recover heat from refrigeration systems is through a desuperheater—a small refrigerant-to-water heat exchanger designed to recover heat from the outlet of the refrigeration compressor. Desuperheaters can provide up to 140°F water, but since they are not a standard option on refrigeration skids, they must be field-installed, which increases their cost and reduces their economic viability considerably.
The California investor-owned utility-sponsored Food Service Technology Center has many articles on heat recovery in food service operations.
Another way to save energy in the kitchen is to buy energy-efficient appliances. The Energy Star Business & Government Commercial Food Service Equipment list covers appliances the meet Energy Star criteria for energy-efficient kitchen appliances.