EMERGING TECHNOLOGIES This article was published in ASHRAE Journal, November 2011. Copyright 2011 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Posted at www.ashrae.org. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. For more information about ASHRAE Journal, visit www.ashrae.org. Drain Water Heat Recovery By Alissa Cooperman; John Dieckmann, Member ASHRAE; and James Brodrick, Ph.D., Member ASHRAE Energy consumption for water heating accounts for 17% of residential and 7% of commercial site energy consumed (13% of residential and 4% of commercial primary energy consumed). 1,2 A study conducted by the U.S. Department of Energy (DOE) found that approximately 350 billion kwh of energy is sent down the drain in the form of used hot water annually. 3 The use of drain water heat recovery (DWHR), also known as graywater heat exchanger (GWHE), technologies can reduce the energy consumed for water heating by recovering some of this energy, and recycling it to preheat incoming water. At the time it is sent down the drain, used hot water on average still contains 80% to 90% of the thermal energy it contained (relative to the cold supply water) when it left the water heater. By using a heat recovery heat exchanger between drain (graywater) and incoming fresh water, up to 40% of this wasted energy can be saved, reducing wasted energy to 49% to 54% of energy used to initially heat the hot water demanded. 4,5 DWHR systems use drain water to heat a building s incoming cold water supply. This is accomplished by a copper heat exchanger, which takes advantage of the film flow of drain water through vertical pipes and the unused heat energy in the drain water. DWHR units are available in two types: storage and on-demand. Storage units (Figure 1) consist of a submerged copper heat exchanger in a tank of fresh water. Warm drain water flows through the heat exchanger heating the tank water for later use. This enables the technology to be used with devices that do not continuously demand, or simultaneously demand and drain water, such as clothes washing machines and dishwashers. The warmed water in the Drain Water In Cold Water In Drain Water Heat Exchanger Figure 1: Drain water heat recovery storage unit. storage tank unit provides preheated inlet water for the water heater. 6 An on-demand DWHR unit (Figure 2) replaces a vertical segment of drain pipe. An inner copper pipe 3 in. to 4 in. (76 mm to 102 mm) in diameter wrapped in external 0.5 in. (13 mm) copper pipe(s) comprises the system.warm drain water flows down the inner pipe while incoming cold supply water flows up the external copper piping. The outer pipes are slightly flattened where they abut the inner pipe, and are soldered to the inner large diameter pipe to improve the thermal connection between the two conduits. On-demand units take advantage of the drain water film flow down the inner walls of the vertical pipe. Unlike horizontal drain pipes, in which the water flow touches half of the inner surface of the pipe, the water flow in vertical drain pipes coats all of the inner surface area in a thin film. The thin film results in a reasonably high heat transfer coefficient Water Storage Tank Warm Water Out Drain Water Out 58 ASHRAE Journal ashrae.org November 2011
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EMERGING TECHNOLOGIES to match the heat transfer coefficient on the fresh supply water side of the exchanger. 3 This type of GWHE is referred to as a gravity film heat exchanger. It is appropriate for devices where there is a continuous flow of hot or warm water and simultaneous flow down the drain, such as a shower. Three ways are commonly used to install the gravity film heat exchanger using either balanced or unbalanced flow. In balanced flow, the preheated water feeds both the water heater inlet and the cold water inlet of the shower. In this configuration, the temperature rise of the fresh water is equivalent to the temperature drop of the drain water. The flow rates between drain and fresh water are the same; they are balanced. Two unbalanced flow configurations are used. The heat exchanger is used to heat either the inlet water to the water heater or the inlet cold water to the shower. In this case the flows are unbalanced, there is more drain water flowing through the central pipe than fresh water flowing up and around it. The flow imbalance causes a larger temperature rise in the freshwater than the temperature drop in the drain water. Though this is the case, balanced flow installations are more efficient. Hot Water Faucet Hot Water Tank Preheated Cold Water To Plumbing Fixtures And To Water Heater Heat Exchanger Cold Water In Drain Water Figure 2: Drain water heat recovery on-demand unit. 4 Flow Configuration Heat Energy Savings Balanced ~50% Unbalanced 30% 45% (Savings Depend on Water End Use Temperature) Table 1: Heat energy savings based on HX flow configuration. 3 Advertisement formerly in this space. Advertisement formerly in this space. 60 ASHRAE Journal November 2011
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EMERGING TECHNOLOGIES Energy Consumed (kwh) Water Inlet Temperature Hot Water Tank Without GWHE Hot Water Tank With GWHE 35.24 F (1.8 C) 16.5 13.6 69.08 F (20.6 C) 10.6 9.4 Table 2: Energy consumed by hot water heaters with and without graywater heat exchanger. 7 There is less water being heated in the unbalanced flow configurations (Table 1, Page 60). 3 At least five manufacturers of DWHR units exist. Most are based in Canada and distribute their products to North America. DWHR units can also be bought at common hardware stores. However, it is recommended that a professional install the product. Energy Savings Drain water heat recovery has been shown to save significant amounts of energy. A study conducted in England in 1975 showed 31.5% savings on fuel used for water heating. Advertisement formerly in this space. Another study conducted in 1998 found 35% to 50% savings depending on the configuration (balanced or unbalanced) of the heat exchanger. A 1997 study of three manufacturers of DWHR units showed 16% energy savings on the energy load of the water heater. 7 A study conducted in Montreal concluded that 40% of the energy load for hot water is consumed by simultaneous flow, i.e., draining and demanding hot water at the same time. Thus, the reductions of hot water heater electricity demand with and without GWHE units were studied. Ten simulated electric hot water heater demand profiles were created, compared to 600 sets of experimental data, and averaged by hour of use. The experimental data sets confirmed the energy expenditures seen in the simulation data, and a daily peak of 1.3 kw was recorded. Table 2 summarizes the results of the study. It shows that colder inlet water temperature results in more energy being recovered. The same study demonstrated a 10.4% reduction in domestic water heater electrical demand in the morning and a 21.5% reduction in the evening. 7 A study conducted for the DOE found that DWHR units can save 800 kwh/yr to 2,300 kwh/yr per house based on the number installed. 3 Market Factors On-demand drain water heat recovery units are durable and easily integrated into existing and new construction. There are no moving parts, they replace existing segments of pipe, and fit into studded walls. These units provide benefits beyond energy and monetary savings. They extend the life of water heaters. Also, by increasing the capacity of the water heater they either allow for undersized water heaters to provide for above their rated capacity or allow for water heaters to be undersized in new construction or during a retrofit. 3 An average on-demand heat recovery unit costs $500 plus installation fees. It has been calculated that the payback time for one unit is two to five years. 5 Depending on the home owner and number of appliances that could benefit from DWHR units, one or more may be installed. Regardless, the technology will eventually pay for itself. References 1. DOE. 2010. 2009 Buildings Energy Data Book. Table 2.1.6. http://tinyurl.com/ table2-1-6. 2. DOE. 2010. 2009 Buildings Energy Data Book. Table 3.1.5. http://tinyurl.com/ table3-1-5. 3. DOE. 2005. Heat Recovery from Wastewater Using a Gravity-Film Heat Exchanger. Federal Energy Management Program. http:// tinyurl.com/gravity-film. 4. DOE. 2011. Drain Water Heat Recovery. http://tinyurl.com/drain-water. 5. Tomlinson, J.J. 2001. Heat Recovery From Wastewater Using a Gravity-Film Heat Exchanger. Federal Energy Management Program. Oak Ridge National Laboratory. http://tinyurl.com/tomlinson2001. 6. National Association of Home Builders. 2001. Drainwater Heat Recovery. http:// tinyurl.com/nahb2001. 7. Eslami-nejad, P. and M. Bernier. 2009. Impact of grey water heat recovery on the electrical demand of domestic hot water heaters. Eleventh International IBPSQA Conference. http://tinyurl.com/eslami. Alissa Cooperman is a technologist and John Dieckmann is a director in the Mechanical Systems Group of TIAX LLC, Lexington, Mass. James Brodrick, Ph.D., is a project manager with the Building Technologies Program, U.S. Department of Energy, Washington, D.C. 62 ASHRAE Journal November 2011
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