ASHRAE OR-16-C060-2016 Field Testing of a Prototype Residential Gas-Fired Heat Pump Water Heater.pdf

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1、 Paul Glanville is a Senior Engineer and Hillary Vadnal is a Principal Engineer, both with the Gas Technology Institute in Des Plaines, IL. Michael Garrabrant is the President of Ston e Mountain Technologies Inc., in Johnson City, TN. Field Testing of a Prototype Residential Gas-Fired Heat Pump Wate

2、r Heater Paul Glanville, PE Hillary Vadnal Michael Garrabrant, PE Member ASHRAE Member ASHRAE Member ASHRAE ABSTRACT HEADING Approximately half of water heaters sold in the U.S. and Canada for residential applications are natural gas fired storage water heaters, and for these products the maximum st

3、eady state thermal efficiency of available products is approximately 96%, with transient rated efficiencies much lower. To move beyond the thermal efficiency limits of standard condensing-efficiency residential gas water heating equipment, this paper describes an effort to develop an economic gas-fi

4、red ammonia-water absorption heat pump deployed as a packaged storage water heater. This new class of gas-fired heat pump water heaters are driven by a direct-fired 2.9 kW (10 kBtu/hr) air source absorption heat pump, which like vapor compression (electric) heat pump water heaters (EHPWH) utilize am

5、bient air to heat stored potable water. With a small 2 kW (6.3 kBtu/hr) gas burner, the packaged unit can exceed this efficiency limitation with Coefficients of Performance (COPs) in excess of 1.5. Unlike EHPWHs however, the prototype gas-fired heat pump water heater (GHPWH) heats potable water with

6、 a combination of output from the condenser, absorber, and heat recovery from the products of combustion, hence the evaporator load (space cooling effect) is 30-40% that of an equivalent vapor compression system. Concerning system cost, with a thermal input of 16% of storage GWHs and 3% of tankless

7、GWHs, the cost of GHPWH installation is minimized, requiring only small diameter gas and plastic vent piping, and standard electrical service. This GHPWH represents a step-change in energy efficiency at a projected competitive cost, a critical goal of both government agencies and utility energy effi

8、ciency programs. In this paper the authors report on data and findings from a preliminary field evaluation of this prototype GHPWH. INTRODUCTION With improving building envelopes reducing space heating loads and the continued growth of condensing efficiency warm air furnaces, estimated at over 50% o

9、f the U.S. furnace market, water heating represents a growing portion of the residential gas load on the west coast at 35% and growing, nearly 50% in California (EIA, 2009 and Seto, 2013). Despite this, of the approximately half of all residential water heaters sold in the U.S. and Canada that are n

10、atural gas-fired, the majority are minimum efficiency gas-fired storage water heaters with an average Energy Factor (EF) of 0.60. Highlighting that the majority of products sold are low-efficiency, recent data from the U.S. EnergyStar show that for the 4.3 million residential gas water heaters sold

11、in 2013, 161,000 were high-efficiency storage-type (condensing and non-condensing) and 397,000 were high-efficiency tankless-type, with the remaining 87% of gas products low-efficiency. Similarly, of the 4 million electric residential water heaters sold, only 43,000, or 1% of 2013 shipments were hig

12、h-efficiency electric heat pump water heaters (EHPWH) (EnergyStar, 2013). In general, residential hot water consumption has been on a slight decline over the past few decades, due in part to declining occupancies, broader deployment of water-efficient fixtures, and migration from colder to warmer cl

13、imates (a trend that has had a more pronounced effect on space heating loads). Similarly, domestic water heating patterns were found to be more distributed throughout the day, with fewer, shorter duration draws, than previously documented. This has broadly been discussed as it relates to the changin

14、g U.S. method of test for rating residential water heaters. In a recent meta-analysis of residential domestic hot water consumption, over 10 U.S. studies, Lutz et al. found that the actual daily median quantity is 61.6 hot water draws per day, versus six in the current rating method; and that the av

15、erage daily median hot water draw volume is 50.6 gallons (196 L), versus the 64.3 gallons(249 L) in the current method (Lutz, 2011). These have competing effects on electric and gas storage water heaters, both heat pump or conventional: Smaller daily draw volumes yield shorter water heater runtimes,

16、 thus the load from standby losses are a larger fraction of total output, yielding lower delivered efficiencies. Specific to gas products, this was recently illustrated in a field and laboratory evaluation of gas water heating products in California, with data summarized in Figure 1. Distributed dra

17、w patterns versus clustered draw patterns impact delivered efficiency through increased outlet water temperatures. For slower recovery HPWHs, this is more important and will be a focus of this study. Figure 1: Delivered Efficiency vs. Output for Gas Water Heating (Kosar, 2013) Opportunity for Gas He

18、at Pump Water Heaters Focusing on the retrofit market of the common, low-efficiency, gas-fired storage-type water heaters, an effort is underway to demonstrate a gas-fired heat pump water heater (GHPWH) with a projected EF of 1.3over twice that of standard gas-fired storage water heaters (Garrabrant

19、, 2013). This work looks beyond existing high-efficiency options for gas water heating customers, which have their own drawbacks and limitations, such as the known gap between the rated versus installed efficiency of gas tankless water heaters (Kosar, 2013). The packaged GHPWH heats the approximatel

20、y 75 gallon (285 L) of stored water with a nominal 10,000 Btu/hr output (2.9 kW) ammonia-water absorption heat pump, driven by a small 6,300 Btu/hr (1.9 kW) low-emission gas burner, and exceeds the thermal efficiency limitation of standard gas-fired products with Coefficients of Performance (COP) in

21、 excess of 1.5. The GHPWH represents a similar leap forward in water heating efficiency to the recent generation of residential EHPWHs, that have demonstrated delivered efficiencies at least twice that of standard electric resistance water heaters 0 50 100 150 200 250 300 350 400 45000.10.20.30.40.5

22、0.60.70.80.910 20 40 60 80 100 120Output (L/day, 19 C Rise) Estimated DeliveredEfficiency (Output/Input) Output (Gal/day, 67 F rise) Condensing StorageNon-condensing TanklessCondensing TanklessNon-condensing Storage(Glanville, 2012). Like the packaged EHPWHs, the GHPWH is comprised of three major co

23、mponents: a) storage tank, b) sealed system (set of heat exchangers containing the refrigerant), and c) supporting components such as the evaporator fan, combustion system, and controls. With the advent of the new U.S. minimum efficiency requirements (DOE, 2010) and the revised method of test (DOE,

24、2014), a technology like the GHPWH will benefit from these regulatory-driven market transformation as follows: a) Large volume storage tanks, above 55 gallons storage (213 L), have a higher allowable minimum efficiency than smaller storage tanks. With a required EF of approximately 0.75 for a 60 gal

25、lon tank (234 L), versus 0.62 for a 40 gallon tank (156 L), the large volume storage tank market will shift to condensing storage-type. With an installation cost of 2.5 to 3 times that of the 0.62 EF unitand even higher for retrofitsthese condensing storage-type units will struggle to justify the 20

26、% increase in operating efficiency. With a lower projected installation cost, end users will likely favor the GHPWH with a 110% operating efficiency increase. b) The new method of test, with a distributed draw pattern, including more, shorter duration, hot water draws, may result in a convergence be

27、tween the current EF ratings of tankless water heaters and those reductions employed by various authorities throughout the U.S., by up to 9%. c) The modified scope of the new method of test adds a new Gas Heat Pump Water Heater product category, in the recognition that a commercialized product is li

28、kely to be introduced during this regulatory cycle. This allows for ready categorization of the product, with a Uniform Energy Factor (UEF) and thus eligibility for EnergyStar. DESCRIBING THE GAS HEAT PUMP WATER HEATER The GHPWH is based on the vapor absorption refrigeration cycle, using the ammonia

29、-water working fluid pair, which an absorbent (water) is used as a carrier for the refrigerant (ammonia). While the refrigerant is still compressed by an electromechanical pump like EHPWHs, unlike a more typical vapor compression cycle, it is compressed as a liquid in solution with the absorbent. Li

30、fting the pressure of a liquid versus a vapor requires significantly less energy. For example, comparing a 1.3 COP heating ammonia-water heat pump to an 8.2 HSPF vapor compression heat pump, the absorption cycle solution pump requires less than 1.0% of the total energy input to the electric compress

31、or (Herold, 1996). C o n d e n s e rE v a p o r a t o rD e s o r b e rA b s o r b e rH e a t I n - A m b i e n tH e a t o u t - H y d r o n i cH e a t o u t - H y d r o n i cH e a t i n - C o m b u s t i o nE x p a n s i o n V a l v eS o l u t i o n P u m pH i g h P r e s s u r eL o w P r e s s u r

32、eThermalCompressorR e f r i g e r a n t ( N H 3 )R e f r i g e r a n t / A b s o r b e n t ( N H 3 / H 2 O )H e a t O u t F l u e C H XFigure 2: Simplified Diagram of the GHPWH Absorption Heat Pump Figure 2 shows a simplified diagram of the absorption heat pump, highlighting the common components wi

33、th vapor compression heat pumps (condenser, evaporator, expansion valve) and the components that comprise the “thermal compressor.” Like vapor compression-based EHPWHs, this refrigeration effect moves heat from ambient air, at the evaporator, to the stored water (via a hydronic loop), at the condens

34、er. While compression of the liquid refrigerant/absorbent solution is performed by the solution pump, thermal energy from the single-stage, 6,300 Btu/hr (1.9 kW) gas burner is required to drive the refrigerant vapor from its absorbed state in the desorber (sometimes referred to as a “generator”). Th

35、is desorption process occurs at an elevated temperature, 250-300F (121-149C), thus exiting flue gases still have useful heat, which is recovered in a separate condensing heat exchanger (CHX), submerged within the storage tank. As adequate refrigerant/absorbent pairs require high affinities and stabi

36、lity over a wide range of temperatures and pressures, they have significant heats of absorption. As such a portion of the desorption heat input is recovered at the absorber, from the heat of absorption of ammonia to water, providing a second heat output to the stored water (via a hydronic loop). Thu

37、s, the GHPWH heats the stored water via three inputs: condenser heat from the heat pump, recovered heat of absorption in the absorber, and heat recovery of the flue gases via the CHX. Like many of the EHPWHs currently available, the total output of the GHPWH is approximately 10,000 Btu/hr (2.9 kW) a

38、nd the portion of the heat delivered to the storage tank from the refrigeration effect is a fraction of total heat delivered, which combined yield the following unique features of this GHPWH design as compared to gas water heaters and EHPWHs: With a small combustion system, the gas piping does not r

39、equire an upsizing from ” piping (13 mm), typical for converting a low-efficiency storage water heater to a condensing storage or tankless unit. In fact, in new construction applications, smaller ” piping (6.4 mm) may be feasible for the GHPWH. As gas-fired equipment with maximum firing rates at or

40、below 10,000 Btu/hr (2.9 kW) are not common, neither is ” gas piping (6.4 mm). However, through the 2003 version of the International Fuel Gas Code (IFGC, 2003), sizing guidance exists for ” piping (6.4 mm) for pressures of 0.5 psi (3.4 kPa) or lower. While subsequent versions of the IFGC only have

41、guidance for 2.0 psi (13.8 kPa) operating pressures and below, with sizing guidance down to ” piping (13 mm), numerous authorities having jurisdiction retained this guidance for ” (6.4 mm) from this older version, such as for New York City did through the end of 2014 (NYC, 2014). Per this guidance,

42、a run of 150 ft. (45.7 m) of ” (6.4 mm) Schedule 40 pipe would suffice for the GHPWH, assuming an operating pressure of 0.5 psi (3.4 kPa) or less and a pressure drop of 0.3 “W.C (0.07 kPa). Also, the smaller GHPWH firing rate yields proportionately smaller output of flue gases, allowing for smaller

43、diameter venting of 1” or less. As a condensing efficiency water heater, lower cost venting materials such as PVC are suitable. Prior field installations of GHPWHs used ” PVC venting (see Figure 3). With a small heat pump, within the sealed system, the total ammonia charge is about 1.5 lbs (0.7 kg),

44、 lower than the 6.6 lb (3 kg) limit placed on its indoor use by ASHRAE Standard 15. The safe use of ammonia as a refrigerant for indoor equipment has been well demonstrated since the first widespread use of absorption refrigerators in the early 20th century to current times where the quieter absorpt

45、ion mini-refrigerators are preferred by large hotels for guest rooms. The absorption heat pump only partially heats stored water using ambient energy, cooling the room in which it is installed, however this cooling effect is between one third and one half that of EHPWHs, with the other heat outputs

46、to the water coming from the absorber and heat recovery of the flue products. Figure 3: Prototype GHPWH at TN Demo Site Compared to the most typical gas-fired water heater, the atmospherically-vented non-condensing storage water heater, the GHPWH and its smaller combustion system have a reduced impa

47、ct on the homes overall HVAC. Without the central flue common to these non-condensing gas storage water heaters, the standby heat loss from storage tank is not removed through the flue vent but rather warms the surrounding space, a minor benefit during the heating season. With a much smaller combust

48、ion air requirement and no draft hood for flue gas dilution (the GHPWH is a power vent water heater), this added ventilation load on the HVAC system is minimized and combustion safety issues with depressurization within tighter envelopes are eliminated. Like EHPWHs, the efficiency of the GHPWH is bo

49、th a function of the storage tank temperature, specifically the return temperature from the internal hydronic loop, and of the ambient air temperature. As a result, efficiency and heating capacity will vary seasonally, as ambient air temperatures vary in the conditioned or semi-conditioned installation space, and inlet water mains temperatures vary over the course of a typical heating cycle as hydronic return temperature increases until the thermostat is satisfied. Figure 4 shows this latter effect, with COP versus heat pump supply tempe