Condensing technology for home water heating.
A variety of condensing gas water heaters have come onto the market in the past several years. They are considerably more efficient than conventional gas storage water heaters because they extract more heat from the gas combustion products and lose less heat to standby losses than conventional gas water heaters.
Water Heating Technology
Storage water heaters are essentially an insulated tank of water with a heat source. Cold water enters the tank at its base (often via a dip tube from an external cold water supply connection on the top of the tank) and hot water is drawn from an outlet at its top.
Traditional gas-fired storage water heaters have a gas burner below the tank firing toward the bottom of the tank (Figure 1). The combustion gases are exhausted through a flue that runs up the center of the tank. As the combustion gas passes through the center flue, additional sensible heat is transferred to the surrounding water.
At steady state, the typical thermal efficiency is 75%. That means 75% of the higher heating value of the gas consumed is transferred to the water, while the rest remains in the combustion gas and is lost up the flue to ambient. During standby, when the water in the tank is at the temperature setpoint of the water heater, the center flue is a convective heat loss path, as room air is drawn into the flue, is warmed and flows out and up the flue by natural convection. Additional heat is lost through the finite insulation covering the tank.
Overall, only approximately 60% of the higher heating value of the gas consumed is contained as heat in the hot water delivered to the residence.
Through tank or heat exchange surface modifications, condensing water heaters extract more sensible heat from the combustion gas, lowering the temperature enough to cause moisture to condense out of the combustion gases, thus capturing a significant portion of the latent heat energy. Condensing water heaters extract energy from the combustion gases as both sensible and latent heat. By taking advantage of latent heat, the most energy possible is extracted from the fuel, with steady-state thermal efficiencies higher than 90%.
One manufacturer uses an internal helical combustion gas exhaust coil to maximize the surface area of interaction between gas and water (Figure 2). When the combustion gas in the coil passes water at or below 120[degrees]F (49[degrees]C), water vapor in the combustion gas condenses out, releasing latent heat. Another manufacturer achieves flue gas condensation with a firetube configuration.
Unlike conventional models, a down-fired burner and induced draft fan are used to pull the combustion gases through the tubes that are located throughout the water storage tank. As the combustion products flow through the submerged tubes they condense (Figure 3).
A third manufacturer produces a commercial sized unit that uses counterflow heat exchangers to extract energy from combustion gases. The combustion chamber is at the center of the heat exchanger, and the working fluid, water, circles it (Figure 4). The inlet water and exit combustion gas lines abut, thus creating condensation and preheating the inlet water.
Due to the increased removal of energy from the flue gases, the exhaust gases are at a lower temperature than found in traditional units. Venting can occur with PVC piping or similar plastic piping, minimizing construction costs.
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One of the most frequent causes of water heater failure is corrosion. Traditional water heaters solve this problem with glass-lined steel tanks and sacrificial anode rods, which protect the underlying steel from corrosion if there are flaws in the glass lining.
With the introduction of collected condensate, the issue of corrosion increases, as combustion gases contain sulfurous or C[O.sub.2] by-products which readily form acidic compounds when condensed along with the moisture from the combustion gas. Commercial and residential condensing water heaters on the market solve this problem through tank material selection and design refinement.
One manufacturer takes the traditional approach. The helical flue is glass lined inside and out, as is the tank (Figure 2). Additionally, two protective anode rods are added to the tank. Unlike the traditional method, the second manufacturer has constructed a tank that is stainless steel on the combustion side, and copper on the water side. For added protection, corrosion resistant coatings are used to prevent corrosion and scaling. The third manufacturer uses all stainless steel components, which are higher in cost, but reliably corrosion-resistant.
The majority of the 79 million single-family homes and the 21 million multifamily homes in the U.S. use hot water heaters. There are a total of approximately 103 million residential hot water heaters--some residences have multiple units. Fifty-three percent of the units are gas-fired and 40% are electric. The remaining 7% use other fuel sources, e.g., LPG and fuel oil. (3)
Water heaters are the second-largest energy consumer in the home, with annual individual household energy bills for water heating ranging between $200 to $600. As noted previously, residential water heating consumes 2.45 quads of primary energy (1.48 quads for electric and 1.13 quads for natural gas and LPG) and commercial water heating consumes 1.09 quads of primary energy. On average, 8% of hot water heaters are replaced each year. Reasons for replacement are most commonly unit failure (often due to corrosion though the tank wall and resulting water leakage), unacceptable performance, or owner choice to upgrade. With this significant turnover rate and installation of condensing gas water heaters, savings introduced by more efficient hot water heaters would produce large energy savings in the building sector. (3)
Standards established under the National Appliance Energy Conservation Act by the U.S. Department of Energy (DOE) in 1987, and effective from 1990, required an Energy Factor (EF) of 0.525 for a 50 gallon (190 L) gas-fired water heater.
EF covers efficiency of energy transfer between source and water, and heat losses from stored hot water in the tank, at a specified set of test conditions, at a representative daily hot water usage rate (64.3 gal/day), and is indicative of the overall energy efficiency (for gas-fired water heaters, the heat delivered in the hot water divided by the higher heating value of the fuel consumed) with which the water heater delivers hot water at the tank outlet. (5,6) Subsequent revisions of the standard have required hot water heaters to be increasingly more efficient. Table 1 shows the evolution of the standard for gas and electric hot water heaters, for 50 gallon (190 L) capacity tanks.
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DOE most recently revised the water heating efficiency standard in 2010, with the new minimum efficiencies taking effect in 2013. The new minimum EF of 0.600 (for a 50 gallon [190 L] capacity tank) is close to the maximum that a conventional, non-condensing gas water heater can attain, due the combination of losses described earlier. The EF of condensing gas water heaters ranges between 0.8 and 0.9, saving about 25% to 30% relative to the conventional tank heater. (3) If all of the installed base of natural gas-fired and LPG-fired water heaters were replaced by condensing gas water heaters, approximately 0.55 quads of energy would be saved annually.
Condensing gas waters have been introduced only recently to the market and account for a small share of water heaters produced. Two basic market issues are important: the high installed cost of a condensing model and the venting arrangement required.
The installed cost of a condensing gas water heater in a typical replacement situation is approximately $2,000, more than $1,000 more than the installed cost of a conventional gas water heater.
From the perspective of simple payback period, the condensing water heater will be more attractive to a heavy user of hot water, where the simple payback period could be less than five years. As production volumes of condensing water heaters increase and as the industry becomes more familiar with them, the installed cost should decrease.
In a situation where a water heater is being replaced, a condensing gas water heater may require an alternative flue gas venting configuration. Large numbers of existing conventional gas storage water heaters are vented though a masonry chimney and flue.
With the relatively low thermal efficiency of a conventional gas water heater and the standby flow of warmed air through the center flue of the tank, the masonry flue is maintained at a high enough temperature to prevent condensation of moisture from the combustion gas.
Condensed moisture from the combustion gases, which will be acidic, would cause the rapid deterioration of a masonry chimney. Therefore, a condensing gas water heater that replaces a conventional gas water heater cannot use the existing masonry flue.
As described earlier, the low temperature of the exhaust combustion gas from a condensing water heater enables the venting to be done via PVC pipe, which can be run though a nearby exterior wall to the outside of the building. In some replacement situations, this may not be possible or may be costly.
For new construction, the avoided cost of a masonry chimney could be considerably more than the cost premium for the condensing water heater.
The Federal Energy Management Program (FEMP) requires that federal construction adhere to specific energy efficient standards in order to reduce the nation's energy use, and set a model example.
Gas water heaters purchased must be either ENERGY STAR rated or within the top 25% energy-efficient technologies. As a result, the qualifying gas fueled water heaters are all condensing models. (8) This will provide a boost to the adoption of condensing gas water heaters.
(1.) DOE. 2010. Building Energy Data Book, Table 2.1.6: Residential Energy End-Use Splits, by Fuel Type (Quadrillion Btu)." http://tinyurl.com/table216.
(2.) DOE. 2010. Building Energy Data Book, Table 3.1.5, Commercial Energy End-Use Splits, by Fuel Type (Quadrillion Btu). http://tinyurl.com/table315.
(3.) DOE. 2009. "New Technologies New Savings: Water Heater Market Profile 2009." http://tinyurl.com/WHprofile.
(4.) Flex Your Power Efficiency Partnership. 2010. "Commercial Water Heaters." http://tinyurl.com/CWH-Flex.
(5.) DOE. 2010. "Determining Energy Efficiency of Storage, Demand, and Heat Pump Water Heaters." http://tinyurl.com/ efficiency-WH.
(6.) DOE. "10 CFR Part 430: Energy Conservation Program: Energy Conservation Standards for Residential Water Heaters, Direct Heating Equipment, and Pool Heaters; Final Rule." Federal Register.
(7.) SustainableBusiness.com. 2010. "Energy Efficiency Advocates Praise New DOE Water Heater Standards." http://tinyurl. com/2010Water.
(8.) DOE. 2010. "FEMP Designated Product: Commercial Gas Water Heaters." U.S. Department of Energy, Energy Efficiency & Renewable Energy. http://tinyurl.com/ femp-WH.
By Alissa Cooperman; John Dieckmann, Member ASHRAE; James Brodrick, Ph.D., Member ASHRAE
Alissa Cooperman is a technologist and John Dieckmann is a director in the Mechanical Systems Group of TIAX LLC, Cambridge, Mass. James Brodrick, Ph. D., is a project manager with the Building Technologies Program, U.S. Department of Energy, Washington, D.C.
Table 1: Evolution of the Minimum Energy Factor. (3,6) Fuel Type 1990 2004 2013 Gas 0.525 0.575 0.600 Electric 0.864 0.904 0.945
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|Title Annotation:||EMERGING TECHNOLOGIES|
|Author:||Cooperman, Alissa; Dieckmann, John; Brodrick, James|
|Date:||Jan 1, 2011|
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