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May 2008

Profiting From Waste Heat
By: Mike Sherer

Simple technology can help you recover waste heat and save energy.

You've probably heard about waste heat recovery and how it can save you energy. Basically, recovery systems use the heat generated by your HVAC and refrigeration to heat water for your restaurants.

The technology to capture and use that waste heat has been around a long time, said Bernie Mittelstaedt, of Therma-Stor, when he spoke before the Multi-Unit Architects, Engineers & Construction Officers executive study group last fall. For example, he said, Schlitz Brewing installed a system in its Milwaukee brewery in 1916, and Eat'n Park has used waste heat recovery in its stores since '82.

Deciding whether or not to install waste heat recovery systems "comes down to practicality," Mittelstaedt said. "How much do you save versus how much does it cost?"

Take Out Pencil And Paper
The key questions are easier to consider than you might think. First you need to figure out how much energy you use to heat water, and how much that energy costs. Start with your fuel source and your water heater efficiency, Mittelstaedt said. Electric water heaters are 100% efficient except for some loss on standby. Gas heater efficiency varies depending on how much scale build-up there is in the tank. The U.S. Bureau of Statistics says that a heater with a 1/8" layer of scale uses 20% more fuel, while ¼" of scale requires 39% more fuel.

One way to calculate gallons of heated water to Btus of energy is to multiply the number of gallons of water you heat each day by 8.33 lbs./gal. Multiply that by the degree rise in temperature your heater uses to get the Btus. And divide that number by the efficiency percentage of your heater to get total Btus. From there you can apply the energy costs in your area.

For example, let's say you heat 100 gals. of water per day from 55ºF to 140ºF, and you have a gas water heater that's 60% efficient. You would multiply 100 gals. by 8.33 lbs./gal. to get 833 and multiply that by the 85ºF temperature rise to get 70,805 Btus. Divide that by 60% efficiency and you get 118,008 Btu to heat 100 gals. of water. If you're paying $1.00 a therm for gas, your energy cost is $1.18 to heat 100 gals. of water.

For an electric heater operating at 90% efficiency, you'd use 78,672 Btus to heat 100 gals. of water for an 85ºF rise. You'd then divide 78,672 by 3,413 (because 3,413 Btu equals 1 kWh) to figure how many kW you need, and then multiply that number—in this case, 23 kW—by 12 cents (hypothetical cost for electricity) and you get $2.77 to heat that 100 gals. of water from 55ºF to 140ºF.

A down and dirty way to calculate how much fuel your water heater is using, Mittelstaedt said, is to put a sensor on the flue that measures how many minutes a day the heater fires. If you multiply that by the Btu input of the heater, you've got total therms per day.

Save How Much?
The question then becomes how much energy you can reclaim from your refrigeration and/or air conditioning system. A 3-hp walk-in freezer and 2-hp walk-in cooler generate about 50,000 Btus of waste heat per hour, according to Mittelstaedt. At the per-therm costs of gas and electricity used above, that's about $.84 to $1.96 an hour, depending on fuel source.

How much of this waste heat you can reclaim depends on how much hot water you use in your store. Therma-Stor has an ROI calculator on its Web site that can help you determine your energy savings based on your hot water usage. Go to and click on the Return On Investment Calculation Form.

Two types of waste heat recovery systems are available: tube-in-tube and tank. Tube-in-tube systems, which essentially run your refrigeration lines inside hot water lines to reclaim heat, offer the advantage of flexibility of installation. Drawbacks, however, can include high maintenance costs and inefficiency.

Tank-type systems are passive systems in which refrigerant gas lines are rerouted to a waffle-shaped heat exchanger that wraps around a water tank like a blanket. As the water in the storage tank is heated, it feeds the cold water supply of your water heater. Tank-type units cost essentially nothing to maintain, last a long time—contributing to a low lifecycle cost—and are very efficient. Drawbacks include the amount of floor space they take up and the need to place them near compressors.

Installation costs of tank-type systems for restaurants typically run $2,500 for the system, $300 for copper pipe and labor costs for two people, two days. But Mittelstaedt cautioned MAECO attendees that contractors have charged as much as $28,000 to install the systems, so buyer beware.

You have to put pencil to, but figuring waste heat recovery is a good use of your time, Mittelstaedt said. An added bonus is that every 1ºF drop in condensing temperature increases the efficiency of your refrigeration system by 1%. If you cool your condenser down by 10ºF using waste heat recovery, you'll improve your efficiency 10%. Savings like that are hot.

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