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Energy and environmental analysis of residential hot water systems: a study for Ontario, Canada.

INTRODUCTION

Domestic hot water (DHW) systems have been subjected to many comparison studies in the past. National Renewable Energy Laboratory (NREL) in Colorado, USA, carried out a performance comparison of residential hot water systems (Wiehagen et al., 2003). They presented performance testing and annual simulations of electric water-heating systems. In their research, test experiments were conducted to measure the energy performance of two different types of water heaters: electric storage tanks and electric on-demand tanks. Using the TRNSYS simulation model (TESS 2011), they showed that electrical energy savings for the on-demand system over the electric tank system were 34%. A recent study by Biaou and Bernier (2005) using TRNSYS compared four different DHW systems for net zero energy homes. Their results showed that heating water with solar thermal collectors with an electric backup is the best solution for net zero energy homes. Another experiment by Spur et al. (2006) studied the influence of the DHW daily draw-off profile. They concluded that realistic daily profiles should be used in the performance testing to reflect conditions experienced in the field. A case study by Jordan and Vajen (2000) analyzed the influence of DHW load profiles with a constant total yearly heat demand for a solar combi system. Using the TRNSYS model, they generated a more realistic profile on a one-minute time scale. They concluded that the influence of the DHW load profile might not be disregarded when combi stores are compared. Crawford and Treloar (2004) studied the net energy analysis of solar and conventional DHW systems. They concluded that the energy payback period of the electric and natural-gas-based solar hot water systems was 0.5 and 2 years, respectively, for the same fuel-based conventional hot water systems.

In the first part of this case study, conventional and solar-based DHW systems are simulated and their results are compared. Another feature of this study is to model and analyze the effect of time-of-use (TOU) pricing of electricity. The TOU electricity plan was developed by Ontario Energy Board (OEB) to provide stable and predictable electricity pricing, which also encourages conservation and ensures the prices consumers pay for electricity better reflect the actual costs of producing the electricity they use. Electricity prices charged per kilowatt-hour change throughout the day to better reflect the changes in the costs to produce the electricity at different times of the day (OEB 2008). As an initiative of the province of Ontario, all homes and businesses will be equipped with smart meters using TOU pricing by 2010. Our objective is to study the effects of the TOU feature by optimizing its use to reduce overall energy costs and greenhouse gas (GHG) emissions.

CASE STUDY, PART 1

This case study is based on an energy-efficient house built using The R-2000 Standard (NRCan 2005) located in Whit by, Ontario, Canada. The house is based on the design of the CCHT houses (CCHT 2004) in Ottawa, Ontario, Canada.

Following are the different DHW models studied:

1. Baseline--electric (94% efficiency)

2. Baseline--natural gas (56% efficiency)

3. Natural gas--83% efficiency--Test A

4. Ground-source heat pump (GSHP) for heat and hot water--Test B

5. Solar hot water preheat with gas (0.56 efficiency) backup--Test C

6. Solar hot water preheat with electric (0.94 efficiency) backup--Test D

7. High-efficiency (83%) on-demand gas hot water--Test E

8. High-efficiency (78%) on-demand modulating gas combo boiler--Test F

9. Electric hot water tank (0.94 efficiency) with timers off during peak times (7:00 a.m. until 10:00 p.m.)--Test G

10. Electric hot water tank (0.94 efficiency) with 122 cm (48 in.) drain-water heat recovery (0.6 efficiency) unit--Test H

11. Gas hot water tank (0.56 efficiency) with 122 cm (48 in.) drain-water heat recovery (0.6 efficiency) unit--Test I

12. Timers (off during peak times 7:00 a.m. until 10:00 p.m.) with solar preheat with TOU electrical backup (0.94 efficiency) secondary--Test J

13. TOU electric hot water (0.94 efficiency) tank (set at 65[degrees]C [149[degrees]F] off peak and 55[degrees]C [131[degrees]F] peak) with water heat recovery (0.6 efficiency) unit--Test K

14. TOU electric hot water (0.94 efficiency) tank (set at 70[degrees]C [158[degrees]F] off peak and 55[degrees]C [131[degrees]F] peak) with water heat recovery (0.6 efficiency) unit--Test L

15. Electric hot water tank timers (off during peak times 7:00 a.m. until 10:00 p.m.) with water heat recovery (0.6 efficiency) unit--Test M

16. High-efficiency (83%) on-demand gas hot water tank with gray-water heat recovery (0.6 efficiency) unit--Test N

17. High-efficiency on-demand modulating gas combo boiler (0.78 efficiency) with gray-water heat recovery (0.6 efficiency) unit--Test O

The monthly city water temperature profile was used as the input city water temperature. The TOU of electricity application was modeled in the systems involving timers. This feature sends control signals to the elements of the heating tank and uses off-peak rates of electricity for water heating, thus reducing the costs. TRNSYS output results provide in-depth hourly results; thus, analysis of system behavior throughout the year at any given time is feasible. Characteritics of the CCHT house are given in Table 1 (CCHT 2004).
Table 1. Characteristics of CCHT House

Component                              Characteristic

Construction standard  R-2000 Standard

Liveable area          210 [m.sup.2] (2260 f[t.sup.2]), 2 stories

Insulation             Attic: RSI8.6, Walls: RSI3.5

Basement               Poured concrete
                       35 [m.sup.2] (377 f[t.sup.2])

Window area            South facing: 16.2 [m.sup.2] (174 f[t.sup.2])


For the electric models, a variable hourly GHG emission factor for electricity generation in Ontario was used. GHG emission factors from research by Gordon and Fung (2007, 2009) were used. Yearly simulation tests were performed for all systems with 175 and 225 L (46 and 59 gal) of daily hot water demand. The feasibility analysis data were extracted from TRNSYS in Microsoft Excel format. The results were compared with baseline electric and natural gas models for analysis.

SOLAR DOMESTIC HOT WATER

A two-panel solar domestic hot water system (SDHW) is modeled (EnerWorks 2007). The system consists of two flat-plate solar collectors, a solar hot water collector, an external heat exchanger, a solar pump, a gray-water heat recovery heat exchanger, and a backup electric or natural gas auxiliary tank. Figure 1 shows the SDHW model.

Propylene glycol is used as the heat transfer fluid and is composed of 40% propylene glycol and 60% distilled water. The solar boiler module contains approximately 4 L (1.1 gal) of the propylene glycol/distilled water mixture. The system has an on/off differential controller (Type 2b). The solar pump is turned on/off by using this controller, which uses the temperature difference between the heat exchanger fluid exiting the solar collector array and the water flowing from the bottom of the solar storage tank and returning to the heat source (the temperature of the bottom node). The on/off differential controller has a 100[degrees]C (212[degrees]F) high limit cutoff temperature (monitoring temperature). The controller will set the solar pump to the off position if the temperature being monitored exceeds the high limit cutoff temperature. The temperature being monitored in this case is the temperature of the water flowing from the top of the solar storage tank to the load (the temperature of the top node). The controller will remain off until the monitored temperature falls below the high limit cutoff temperature of 100[degrees]C (212[degrees]F).

[FIGURE 1 OMITTED]

SDHW MODELING

A solar collector having a copper tube on an aluminium sheet absorber with absorptance 94% [+ or -] 2% and emittance 5% [+ or -] 2% was used for this study (EnerWorks 2007). The Type1b solar thermal collector is modelled from the TRNSYS library. The Type 1b models a quadratic efficiency equation. This collector type was used by Thevenard et al. (2004) and was compared with ESP-r model (ESRU 2011) results. They concluded that this model performs well in simulation models. Table 2 shows the characteristics of the solar hot water system. TRNSYS weather base data from METEONORM (METEO-TEST 2011) were used in solar models. The METEONORM weather database has more than 1000 locations in more than 150 countries. Solar collector thermal efficiency is given by Equation 1:

[[eta].sub.col] = m * [c.sub.p]([T.sub.o-col] - [T.sub.i-col] / [A.sub.col][I.sub.t] (1)

where

[[eta].sub.col] = collector efficiency

m = flow rate

[c.sub.p] = fluid specific heat

[T.sub.o-col] = outlet temperature of fluid from collector

[T.sub.i-col] = inlet temperature of fluid to collector

[A.sub.col] = collector area

[I.sub.t] = global radiation incident on the solar collector
Table 2. Characteristics of Solar Hot Water System

Mode                        Value

Collector                      5.382 [m.sup.2]
area                        (57.93 f[t.sup.2])

Fluid               3.747 kJ/kg-K (0.895 Btu/l
specific              [b.sup.m] *[degrees]R)
heat

Tested flow              26.756 kg/h*[m.sup.2]
rate            (5.48 l[b.sub.m]/h*f[t.sup.2])

Efficiency               4.063 W/[m.sup.2] * K
slope

Efficiency        0.0061 W/[m.sup.2]*[K.sup.2]
curvature

Collector       22.5[degrees]From horizontal
slope

Collector                                South
orientation

Solar hot                       300 L (79 gal)
water tank

Auxiliary                       227 L (60 gal)
tank

Auxiliary    60[degrees]C (140[degrees]F),
tank          80[degrees]C (176[degrees]F)
setpoint                           with mixing

Solar pump          72 kg/h (159 l[b.sub.m]/h)
rated flow
rate

Solar pump                                23 W
rated power

Solar pump                                 0.9
total
efficiency

Solar pump                                0.95
motor
efficiency

Heat                          3 (shell passes)
exchanger


CONVENTIONAL MODELS

The conventional electric hot water tank is modeled using Type 4a (stratified storage tank) in TRNSYS. The model optionally includes two electric resistance heating elements, subject to temperature and/or time control. The control option allows the addition of electrical energy to the tank during selected periods of each day. Both heaters are used to maximize the TOU electricity rate. One element set at 55[degrees]C (131[degrees] ) is used during on-peak and mid-peak hours, and a second set at 65[degrees]C or 70[degrees]C (149[degrees]F or 158[degrees]F) is used during off-peak hours to get the maximum benefit of the off-peak rate. Figure 2 shows the conventional electric hot water tank.

The conventional natural gas hot water tank is modeled using Type 60d (stratified storage tank) in TRNSYS. This model allows the tanks with a gas auxiliary heater. The model treats gas auxiliary energy the same as electric energy, so the heat rate when the burner is firing is the same as if it was electric power. Table 3 shows the characteristics of conventional DHW models.
Table 3. Characteristics of Conventional DHW Models

Mode                                       Value

Electric tank volume                             227 L (60 gal)
Electric tank efficiency                                   0.94
Maximum heating rate                                     3000 W
Natural gas tank volume                          227 L (60 gal)
Natural gas tank efficiency                                0.56
Natural gas tank height                         1.25 m (4.1 ft)
On-demand heater efficiency                                0.83
On-demand heating rate       10,800 kJ/h (10,236 Btu/h) maximum
Combo boiler rated capacity         100,000 kJ/h (94,782 Btu/h)
Boiler efficiency                                          0.78
Combustion efficiency                                      0.85


[FIGURE 2 OMITTED]

The on-demand gas hot water heater is modeled using Type 6 auxiliary heaters from the TRNSYS library. The heater is designed to add heat to the flow stream at a rate less than or equal to [Q.sub.max], which is a user-determined quantity. The modulating gas combo boiler is modeled using Type 700 from the TRNSYS library. In this model, the boiler efficiency and the combustion efficiency are supplied as inputs to the model.

WASTE HEAT RECOVERY

A zero-capacitance sensible heat exchanger (Type 91) with a constant effectiveness of 60% is modeled in TRNSYS. The same type is used for the study in the second part of this case study. For the constant effectiveness mode, the maximum possible heat transfer is calculated based on the minimum capacity rate fluid and the cold-side and hot-side fluid inlet temperatures. The waste temperature of the water coming out of house is assumed to be 37[degrees]C (99[degrees]F).

The following expressions are given to determine the maximum possible amount of heat transfer at a given time step:

If [C.sub.min] = [C.sub.h], [Q.sub.max] = [C.sub.h] [(T.sub.hi] - [T.sub.ci]) (2)

If [C.sub.min] = [C.sub.c], [Q.sub.max] = [C.sub.c] [(T.sub.hi] - [T.sub.ci]) (3)

The actual heat transfer then depends upon the user-specified [[effectiveness:

[Q.sub.T] = [epsilon] [Q.sub.max]

where

[C.sub.c] = capacity rate of fluid on cold side, [m.sub.c][C.sub.pc]

[C.sub.h] = capacity rate of fluid on hot side, [m.sub.h][C.sub.ph]

[C.SUBp.c] = specific heat of cold-side fluid

[C.sub.ph] = specific heat of hot-side fluid

[C.sub.min] = minimum capacity rate

[epsilon] = heat exchanger effectiveness

[m.sub.c] = fluid mass flow rate on cold side

[m.sub.h] = fluid mass flow rate on hot side

[Q.sub.t]= total heat transfer rate across heat exchanger

[Q.sub.max] = maximum heat transfer rate across exchanger

[T.sub.ci] = cold-side inlet temperature

[T.sub.hi] = hot-side inlet temperature

WATER DRAW PROFILE

The daily hot water draw profile has been subjected to many studies in the past. Perlman and Mills (1985) monitored the data from Canadian residences. They provided two sets of data, one for all families and one for typical families. They defined a typical family as two adults and two children, with a clothes washer and a dishwasher present. The typical hot water draw profile is most widely used. Becker and Stogsdill (1990) gathered, analyzed, and reported on nine different data sets consisting of more than three million data points on hot water use in residences. Their database included measurements from 110 single-family residences in both Canada and the U.S. Each of these data sets contains measured hot water use data of one year or greater in duration. Bouchelle et al. (2000) reported on hot water demand profiles for a large 204-home sample study conducted in central Florida. The hot water draw profile from Perlman and Mills (Figure 3) is used in this study as it is based solely on the Canadian data.

[FIGURE 3 OMITTED]

SIMULATION

DHW systems were simulated in TRNSYS (Type 56) for the whole year (8760 hours) using a one-hour time step. Daily hot water demands of 175 and 225 L (46 and 59 gal) were used for simulation. The results were validated using NRCan's HOT2000[TM] program (NRCan 2008). In the solar models, setpoint temperatures of 60[degrees]C (140[degrees]F) with no mixing and 80[degrees]C (176[degrees]F) with a mixing valve were used in the auxiliary tank for sensitivity analysis. The yearly results were extracted in Excel format using a one-hour time step. The hourly value of energy (in kilojoules) required to heat the water was analyzed. If water is not being heated at a particular hour, then the given value is zero. The temperature of hot water flowing into the house at any hour is also studied. The required water temperature for the house is 55[degrees]C (131[degrees]F). Monitoring the data of the hot water temperature to the house helps to conclude if the system will meet the requirements for a whole year. The energy required to heat the water is converted from kilojoules to kilowatt-hours in electric models and from kilo-joules to cubic meters in gas models. The GHG emissions are calculated using an hourly emission factor for electricity generation in Ontario (Gordon and Fung 2009). The yearly average emission factor is 226.35 (tons of [CO.sub.2]/total GWh generation). If no electricity is used to heat the water at a particular hour, then the corresponding [CO.sub.2] emission is zero. The constant emission factor of 1.856 kg/[m.sup.3] equivalent [CO.sub.2] was used in the gas models for GHG emission calculations.

TIME OF USE (TOU)

All TRNSYS models that involve timers are modeled considering TOU. There are different periods of TOU for winter and for summer (Toronto Hydro 2007). Winter timing is from November 1-April 30 and summer timing is from May 1-October 31. All weekends as well as all statutory holidays will cost off-peak rates for all hours of the day. To incorporate statutory holidays, the year 2005 calendar is used. Figure 4 shows the different rates (in Cents/kWh) during 24 hours.

[FIGURE 4 OMITTED]

ENERGY PRICES AND [CO.sub.2] FACTORS

Energy Prices

The following energy prices were used in the study. These prices are the final prices paid by the customer including delivery charges.

Electric (flat rate)--$0.10/kWh (Toronto Hydro 2007)

Electric (TOU)--$0.077/kWh off peak (Toronto Hydro 2007)

Electric (TOU)--$0.117/kWh mid peak (Toronto Hydro 2007)

Electric (TOU)--$0.147/kWh on peak (Toronto Hydro 2007)

Natural gas--$0.488[/m.sup.3] (Enbridge 2007)

[CO.sub.2] Factors

Natural gas--1.856 [kg/m.sup.3] equivalent [CO.sub.2] (NRCan 2006a) Electricity--variable for every hour of the year for electricity production in Ontario; the average emission factor is 226.35 (tons [CO.sub.2]/total GWh generation) (Gordon and Fung 2009).

RESULTS

Heat Transfer Rate

Figure 5 shows the total heat transfer rate between the fluids in the cross-flow heat exchanger of the solar model. The maximum heat transfer rate was 9372 kJ/h (8889 Btu/h) in the month of June. The average heat transfer rate for the whole year was 960 kJ/h (910 Btu/h).

Auxiliary Tank Heating

Figure 6 shows the heating rate of the TOU electric auxiliary tank. It demonstrates the effect of solar panels during the summer period. The solar model is able to keep up with the daily hot water demand during this period, thus hardly requiring use of the backup hot water electric tank. It was found that during the summer months (May 1-October 31), only 202 kWh of electricity is needed for the TOU auxiliary tank. This result is obtained assuming 225 L (59 gal) of daily hot water demand and 60[degrees]C (140[degrees]F) setpoint temperature in the auxiliary tank.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

GHG EMISSIONS

The heating of water for domestic purposes contributes 25% of total Canadian residential GHG emissions (NRCan 2006b). The effect of water heating on the environment has been subjected to studies in the past. Taborianski and Prado (2004) studied the GHG emissions related to electric, natural gas, liquefied petroleum gas (LPG), and solar water heaters for a 20-year life cycle. They concluded that the electric water heater is the worst GHG emitter. Their study was based on a constant input water temperature of 18[degrees]C (64[degrees]F) throughout the year and 80% efficiency for all conventional systems. Kalogirou (2004) compared solar models with electric models. The study concluded that solar-based systems have 74% GHG savings as compared to conventional systems.

[FIGURE 7 OMITTED]

Figure 7 shows the hourly GHG emissions related to the SDHW model with a TOU electric auxiliary tank having a 60[degrees]C (140[degrees]F) setpoint temperature and 225 L (59 gal) of daily hot water demand. It should be noted that there were only 43 kg (94 lb) of GHG emissions during the summer period (May 1-October 31). The solar system is able to provide the hot water for domestic uses, minimizing GHG emissions during this period.

Table 4 presents the GHG emissions for solar and conventional models. The solar system with a gray-water heat recovery unit and a TOU backup electric tank has the lowest emissions--this model has only 266 kg (586 lb) of GHG emissions for the whole year. The conventional natural gas hot water tank (0.56 efficiency) has the highest annual GHG emissions among the natural gas models: 1514 kg (3334 lb). The conventional electric hot water (0.94 efficiency) has the highest annual GHG emissions among the electric models: 1136 kg (2502 lb).
Table 4. GHG Emissions

Model                                           GHG Emissions, kg
                                                      (lb)

Baseline electric (0.94)                              1136 (2504)

Baseline natural gas (0.56)                           1515 (3340)

High-efficiency natural gas (0.83)                    1042 (2297)

GSHP for heat and hot water                            481 (1060)

Solar preheat with natural gas backup (0.56)           458 (1010)

Solar preheat with electric back up (0.94)              334 (736)

High-efficiency (83%) on-demand gas hot water          979 (2158)

High-efficiency (78%) on-demand modulating gas        1041 (2295)
combo boiler

Electric tank (0.94) with timers off during            873 (1925)
peak times (7:00 a.m. until 10:00 p.m.)

Electric tank (0.94) with 122 cm (48 in.)              709 (1563)
drain-water heat recovery (0.6) unit

Natural gas tank (0.56) with 122 cm (48 in.)           951 (2097)
drain-water heat recovery (0.6) unit

Timers (off during peak times 7:00 a.m. until           266 (586)
10:00 p.m.) with solar preheat with TOU
electric backup (0.94) secondary

TOU electric tank (0.94) set at 65[degrees]C           742 (1636)
(149[degrees]F) off peak and 55[degrees]C
(131[degrees]F) on peak with water
heat recovery (0.6) unit

TOU electric tank (0.94) set at 70[degrees]C           834 (1839)
(158[degrees]F) off peak and 55[degrees]C
(131[degrees]F) on peak with water
heat recovery (0.6) unit

Electric tank (0.94) with timers off during            567 (1250)
peak times 7:00 a.m. until 10:00 p.m. and
water heat recovery (0.6) unit

High-efficiency (83%) on-demand gas hot water          613 (1351)
tank with gray-water heat recovery (0.6) unit

High-efficiency on-demand modulating gas combo         652 (1437)
boiler (0.78) with gray-water heat recovery
(0.6) unit


FUEL CONSUMPTION

Fuel consumption results related to the all-residential hot water systems are shown in Table 5. Three different solar-based systems are I) solar preheat with a 0.56 efficiency natural gas backup tank, II) solar preheat with a 0.94 efficiency electric backup tank, and III) timers (off during peak times 7:00 a.m. until 10:00 p.m.) with solar preheat with TOU electric backup (0.94) secondary. Solar model III was found to have the lowest fuel consumption (4.38 GJ), whereas the basic natural gas tank (0.56 efficiency) has the highest fuel consumption (30.76 GJ). All models were simulated for 225 and 175 L (59 and 46 gal) daily hot water demand; the results shown in Table 5 correspond to the 225 L (59 gal) daily hot water demand. The three solar models have the lowest fuel consumption among all the models studied. The conventional electric hot water tank (0.94 efficiency) has the lowest fuel consumption (17.22 GJ) among the conventional models. The annual energy consumption for an on-demand gas water heater (0.83 efficiency) is 19.87 GJ.
Table 5. Fuel Consumption

Model                                            Annual Energy
                                                Consumption, GJ

Baseline electric (0.94)                                  17.22

Baseline natural gas (0.56)                               30.76

High-efficiency natural gas (0.83)                        21.16

GSHP for heat and hot water                                7.80

Solar preheat with natural gas backup (0.56)               9.30

Solar preheat with electric back up (0.94)                 5.21

High-efficiency (83%) on-demand gas hot water             19.87

High-efficiency (78%) on-demand modulating gas            24.15
combo boiler

Electric tank (0.94) with timers off during               14.82
peak times (7:00 a.m. until 10:00 p.m.)

Electric tank (0.94) with 122 cm (48 in.)                 10.82
drain-water heat recovery (0.6) unit

Natural gas tank (0.56) with 122 cm (48 in.)              19 31
drain-water heat recovery (0.6) unit

Timers (off during peak times 7:00 a.m.                    4.38
until10:00 p.m.) with solar preheat with TOU
electric backup (0.94) secondary

TOU electric tank (0.94) set at 65[degrees]C             12.48
(149[degrees]F) off peak and 55[degrees]C
(131[degrees]F) on peak with water heat
recovery (0.6) unit

TOU electric tank (0.94) set at 70[degrees]C             14.14
(158[degrees]F) off peak and 55[degrees]C
(131[degrees]F) on peak with water heat
recovery (0.6) unit

Electric tank (0.94) with timers off during                9.43
peak times 7:00 a.m. until 10:00 p.m. and
water heat recovery (0.6) unit

High-efficiency (83%) on-demand gas hot water             12.45
tank with gray-water heat recovery (0.6) unit

High-efficiency on-demand modulating gas combo            13.25
boiler (0.78) with gray-water heat recovery
(0.6) unit


LIFE-CYCLE COST

The life-cycle cost was calculated by the summation of 30-year equipment and replacement costs (EnterWorks 2007), maintenance costs, and fuel costs. Table 6 gives the capital cost of the equipment at installation. This cost includes installation cost and the cost of anti-scald equipment. In the case of solar systems, rebates given by federal and provincial governments are deducted from the capital cost. Table 7 shows the equipment life. Table 8 provides the 30-year life-cycle cost, which is the sum of the 30-year equipment capital and replacement, maintenance, and fuel costs. In the case of combo boilers and GSHPs, 30% of the equipment cost was allocated toward water heating while 70% was allocated for space heating. To calculate 30-year fuel costs, the following fuel escalation rates were used:
Table 6. Equipment Capital Cost

Model                                         Capital Cost, CADS

Baseline electric (0.94)                                    $780

Baseline natural gas (0.56)                               $1,030

High-efficiency natural gas (0.83)                        $1,355

GSHP for heat and hot water                               $9,930

Solar preheat with natural gas backup                     $8,445
(0.56)

Solar preheat with electric back up (0.94)                $8,195
High-efficiency (83%) on-demand gas hot                   $1,880
water

High-efficiency (78%) on-demand modulating                  $825
gas combo boiler

Electric tank (0.94) with timers off during                 $780
peak times (7:00 a.m. until 10:00 p.m.)

Electric tank (0.94) with 122 cm (48 in.)                  $1695
drain-water heat recovery (0.6) unit

Natural gas tank (0.56) with 122 cm (48 in.)               $1945
drain-water heat recovery (0.6) unit

Timers (off during peak times 7:00 a.m.                    $8195
until 10:00 p.m.) with solar preheat and
electrical (0.94) secondary

TOU electric tank (0.94) set at 65[degrees]                $1695
C (149[degrees]F) off peak and 55[degrees]
C (131[degrees]F) on peak with water heat
recovery (0.6) unit

TOU electric tank (0.94) set at 70[degrees]                $1695
C (158[degrees]F) off peak and 55[degrees]
C (131[degrees]F) on peak with water heat
recovery (0.6) unit

Electric tank (0.94) with timers off during                $1695
peak times 7:00 a.m. until 10:00 p.m. and
water heat recovery (0.6) unit

High-efficiency (83%) on-demand gas hot                    $2795
water tank with gray-water heat recovery
(0.6) unit

High-efficiency on-demand modulating gas                   $1740
combo boiler (0.78) with gray-water heat
recovery (0.6) unit

Table 7. Equipment Life

Model                                  Life, years

Electric tank                                   13
Natural gas tank                                21
GSHP                                            25
DHW solar system                                20
Gray-water heat recovery power pipe             50
On-demand gas hot water heater                  20
On-demand modulating gas combo boiler           20
Table 8. 30-Year Life Cycle Cost

Model                                      Life-Cycle Cost,
                                                 CADS

Baseline electric (0.94)                            $24,521

Baseline natural gas (0.56)                         $25,705

High-efficiency natural gas (0.83)                  $18,771

GSHP for heat and hot water                         $30,126

Solar preheat with natural gas backup               $28,075
(0.56)

Solar preheat with electric back up                 $27,996
(0.94)

High-efficiency (83%) on-demand gas hot             $19,223
water

High-efficiency (78%) on-demand                     $16,920
modulating gas combo boiler

Electric tank (0.94) with timers off                $17,828
during peak times (7:00 a.m. until 10:00
p.m.)

Electric tank (0.94) with 122 cm (48 in.)           $18,053
drain-water heat recovery (0.6) unit

Natural gas tank (0.56) with 122 cm (48             $18,648
in.) drain-water heat recovery (0.6)
unit

Timers (off during peak times 7:00 a.m.             $25,980
until 10:00 p.m.) with solar preheat and
electrical (0.94) secondary

TOU electric tank (0.94) set at 65[degrees]C        $16,668
(149[degrees]F) off peak and 55[degrees]C
(131[degrees]F) on peak with water heat
recovery (0.6) unit

TOU electric tank (0.94) set at 70[degrees]C        $18,135
(158[degrees]F) off peak and 55[degrees]C
(131[degrees]F) on peak with water heat
recovery (0.6) unit

Electric tank (0.94) with timers off                $13,956
during peak times 7:00 a.m. until 10:00
p.m. and water heat recovery (0.6) unit

High-efficiency (83%) on-demand gas hot             $14,967
water tank with gray-water heat recovery
(0.6) unit

High-efficiency on-demand modulating gas            $12,333
combo boiler (0.78) with gray-water heat
recovery (0.6) unit


Electricity--2.14% per year nominal rate (CEC 1996)

Natural gas--3.75% per year nominal rate (CEC 1996)

SYSTEM RANKINGS

Residential hot water systems are ranked by giving 25% consideration to each of the following four factors:

* Total annual energy consumption (GJ)

* Total annual GHG emissions (kg)

* Total annual energy cost (CAD$)

* Life-cycle cost (present value) (CAD$)

The ranking scores are in the 0-100 range (0-25 for each of the four factors). Table 9 provides the scores and rankings for all systems. The hot water system with timers (off during peak times 7:00 a.m. until 10:00 p.m.) with solar preheat with TOU electric backup (0.94) secondary is the first-ranked system, with a score of 80.97 out of a total 100. The electric tank (0.94) with timers off during peak times 7:00 a.m. until 10:00 p.m. and water heat recovery (0.6) unit is ranked second, with a score of 79.91, and the high-efficiency on-demand modulating gas combo boiler (0.78) with gray-water heat recovery (0.6) unit is ranked third, with a score of 78.81.
Table 9. System Rankings

%       Scenario               Baseline       Baseline     Test A
Weight                       (Electric)   (Natural Gas)

0.25    Total annual energy        17.22          30.76    21.16
        consumption, GJ

0.25    Total annual GHG     1136 (2504)    1515 (3340)     1042
        emissions, kg (lb)
                            (2297)

0.25    Total annual energy         $478           $398     $274
        cost, CAD$

0.25    Life-cycle cost          $24,521        $25,705  $18,771
        (present value),
        CAD$

Score   Total energy               12.83              0      9.1
        consumption

Score   Total annual GHG            7.59              0     9.46
        emissions

Score   Total annual energy            0           5.21    13.28
        cost

Score   Life-cycle cost             7.87           6.21    15.95
        (present value)

        Total Score                28.29          11.42     47.8

        Rank                          16             17       15

%       Scenario             Test B   Test C   Test D   Test E   Test F
Weight

0.25    Total annual energy     7.79      9.3      5.4    19.87    21.14
        consumption, GJ

0.25    Total annual GHG         481      458      334      979     1041
        emissions, kg (lb)    (1060)   (1010)    (736)   (2158)   (2295)

0.25    Total annual energy     $217     $120     $145     $257     $274
        cost, CAD$

0.25    Life-cycle cost      $30,125  $28,075  $27,996  $19,223  $16,920
        (present value),
        CAD$

Score   Total energy           21.77    20.34    24.03    10.32     9.12
        consumption

Score   Total annual GHG        20.7    21.15    23.64    10.73     9.48
        emissions

Score   Total annual energy    17.01    23.26    21.68    14.37     13.3
        cost

Score   Life-cycle cost            0     2.88     2.99    15.32    18.55
        (present value)

        Total Score            59.48    67.63    72.35    50.74    50.44

        Rank                       9        6        5       13       14

%       Scenario              Test G   Test H   Test I   Test J   Test K
Weight

0.25    Total annual energy    14.82    10.81    19.31     4.38    12.48
        consumption, GJ

0.25    Total annual GHG         873      709      951      266      742
        emissions, kg (lb)    (1925)   (1563)   (2097)    (586)   (1636)

0.25    Total annual energy     $317     $300     $250      $94     $267
        cost, CAD$

0.25    Life-cycle cost      $17,829  $18,053  $18,648  $25,880  $16,668
        (present value),
        CAD$

Score   Total energy           15.11    18.91    10.85       25    17.32
        consumption

Score   Total annual GHG       12.85    16.13    11.28       25    15.48
        emissions

Score   Total annual energy    10.49    11.57    14.84       25    13.74
        cost

Score   Life-cycle cost        17.28    16.96    16.13     5.97    18.91
        (present value)

        Total Score            55.72    63.57     53.1    80.97    65.45

        Rank                      11        8       12        1        7

%       Scenario             Test L   Test M   Test N   Test O
Weight

0.25    Total annual energy    14.13     9.42    12.44    13.24
        consumption, GJ

0.25    Total annual GHG         834      567      613      652
        emissions, kg (lb)    (1839)   (1250)   (1351)   (1437)

0.25    Total annual energy     $302     $202     $161     $172
        cost, CAD$

0.25    Life-cycle cost      $18,135  $13,956  $14,967  $12,333
        (present value),
        CAD$

Score   Total energy           15.76    20.22    17.36     16.6
        consumption

Score   Total annual GHG       13.63    18.98    18.05    17.27
        emissions

Score   Total annual energy    11.44    17.99    20.61    19.94
        cost

Score   Life-cycle cost        16.85    22.72     21.3       25
        (present value)

        Total Score            57.68    79.91    77.33    78.81

        Rank                      10        2        4        3


SOLAR DOMESTIC HOT WATER FOR SUI-NZEHH

SDHW systems have been subjected to many studies in the past. Druck et al. (2004) of University of Stuttgart, Germany, did a comparison test of different thermal solar systems for DHW. The systems were tested with regard to thermal performance as well as financial and environmental aspects. The sixteen thermal solar systems studied for DHW in their study had the effective collector area varied between 3.2 and 5.7 [m.sup.2] (34 and 61 f[t.sup.2]). Twelve systems were equipped with flat-plate collectors and four with vacuum-tube collectors. The effective usable storage volume of the DHW stores was in the range of 268 to 419 L (71 to 111 gal). For all sixteen systems, the solar energy was transferred to the DHW using a plain-tube heat exchanger. The systems were simulated for a single-family house located in Wurzburg, Germany, using TRNSYS simulation software. Their results showed that the system with a collector area of 3.2 [m.sup.2] (34 f[t.sup.2]) using vacuum-tube collectors has the lowest energy payback period (1.3 years). With regard to the assessment of the thermal performance, in total four SDHW systems obtained the ranking of "very good."

Biaou and Bernier (2005) of Ecole Polytechnique de Montreal, Quebec, Canada, examined different means of producing DHW in net zero energy homes. Four alternatives were examined: 1) a regular electric hot water tank, 2) the desuperheater of a GSHP with electric backup, 3) thermal solar collectors with electric backup, and 4) a heat pump water heater. The yearly simulation results showed that the alternative that uses thermal solar collectors is the best solution, with a yearly electric consumption of 1410 kWh.

Taborianski and Prado (2004) of University of Sao Paulo, Brazil, studied the contributions of several household water heating systems to global warming using 20-year life-cycle analysis. Four systems were studied: electric, natural gas, LPG, and solar water heaters. They concluded that the electric water heater is the worst GHG emitter.

The unique feature of this study is to model and analyze the effect of TOU pricing of electricity. The Ontario Energy Board Smart Price Pilot project, which was initiated in 2006, studied the impacts of consumer behavior in different time-sensitive price structures (OEB 2007).

CASE STUDY, PART 2

This part of the case study is based on the Sustainable Urbanism Initiative Net Zero Energy Healthy Housing (SUI-NZEHH) project located in Toronto, Ontario, Canada. SUI is an association of organizations, including Ryerson University faculty and students, with an interest in sustainable design. A net zero energy home is capable of producing an annual output of renewable energy that is equal to the total amount of its annual purchased energy. The SUI-NZEHH project consists of three 210 [m.sup.2] (2260 f[t.sup.2]) town-houses built with sustainable designs, methods, and materials. The area of the site is 0.0402 ha (402 [m.sup.2]). Each of the three units has from two to four bedrooms depending on the configuration. For the net zero energy house design, the goal was to achieve a net zero energy home by reducing the energy requirement enough so that a photovoltaic (PV) system would be able to generate enough electricity to meet the demands of the home on an annual basis. The heating and cooling of the house is provided by a GSHP and electricity is produced by using the PV system. The highly insulated envelope, which includes high-specification glazing, provides the low-energy design. The thermal mass effect is provided by using concrete walls separating the units and a concrete topping on the floors. The townhouses also use high-efficiency electrical appliances to save electricity. Table 10 provides the characteristics of the SUI-NZEHH houses (Masoumi and Fung 2007).
Table 10. Characteristics of a SUI-NZEHH House

Heated volume         685 [m.sup.3] (24191 f[t.sup.3]) per
                      unit

Heated area           210 [m.sup.2] (2260 f[t.sup.2]) per unit

Ceiling area          64.43 [m.sup.2] (694 f[t.sup.2]) per
                      unit

Exposed wall area     361.97 [m.sup.2] (3896 f[t.sup.2]) per
                      unit

Glazing area (north)  23.2 [m.sup.2](250 f[t.sup.2])

Glazing area (south)  33.6 [m.sup.2](362 f[t.sup.2])

Glazing area (east)   26.48 [m.sup.2](285 f[t.sup.2])

Glazing area (west)   30.34 [m.sup.2](327 f[t.sup.2])


The SDHW system was modeled using the TRNSYS simulation model. The variable hourly GHG emission factors for electricity generation in Ontario from a study by Gordon and Fung (2009) were used. The data for the sensitivity analysis was extracted from TRNSYS in Excel format. The savings in electricity and in GHG emissions were calculated by comparing the results with the baseline electric model.

The DHW is based on the solar hot water system used in the SUI-NZEHH houses (Thermo Dynamics 2007). The system consists of two solar thermal collectors, a PV-powered circulation pump, an external heat exchanger, a 454 L (120 gal) preheat tank, and a 227 L (60 gal) TOU electric backup hot water tank. Figure 1 shows the schematics of the SDHW model. Flat-plate solar collectors that are single glazed with tempered glass are used. The absorber consists of a single serpentine aluminium fin with an integral copper tube that is completely surrounded by the aluminium and metallurgically bonded with it. The shell-and-tube type heat exchanger is used in this model. The system has a solar loop that is closed from the atmosphere. The heat exchanger fluid, which is a mixture of 60% distilled water and 40% propylene glycol, circulates through the solar loop using a constant-pressure solar pump. The solar pump is a brass body displacement pump that has a maximum output pressure of 500 kPa (73 psi). The nominal flow rate is 72 L/h (19 gal/h). The solar boiler module contains approximately 4 L (1.1 gal) of the propylene glycol/distilled water mixture.

SDHW MODELING

A two-panel DHW system (Thermo Dynamics 2007) was modeled using TRNSYS. Individual components were selected from the TRNSYS library and connected with each other. Various technical parameters related to the DHW manufacturer were collected and input into these components.

Solar Collectors

The flat-plate collector (quadratic efficiency) Type 1b is selected from the TRNSYS component libraries. This component models the thermal performance of a flat-plate solar collector. The solar collector array consists of collectors connected in series. The number of modules in series and the characteristics of each module determine the thermal performance of the collector array.

TRNSYS models of solar collectors have different types of functions called parameters, inputs, and outputs. A parameter value function is specifically related to the manufacturer; such a value needs to be input according to manufacturer specifications. An input value function has a value that depends upon the test conditions, such as collector slope, orientation, etc. Some inputs and all outputs have a "linked" value, which means that the function has values that are linked to other components and will be determined by the real-time flow. Table 11 shows the characteristics of the solar collectors for SUI-NZEHH.
Table 11. Characteristics of Solar Collectors

Parameter  Collector area     5.56 [m.sup.2](60 f[t.sup.2])

Parameter  Fluid specific     3.747 kJ/kg-K (0.895 Btu/l[b.sub.m]
           heat               *[degrees]R)

Parameter  Tested flow rate   26.8432 kg/h * [m.sup.2]
                              (5.498 l[b.sub.m]/h * f[t.sup.2])

Parameter  Intercept          0.64
           efficiency

Parameter  Efficiency slope   4.65 W/[m.sup.2]*K

Input      Inlet temperature  Linked

Input      Inlet flow rate    Linked

Input      Ambient            Linked
           temperature

Input      Collector slope    20[degrees]

Input      Collector          37[degrees] west of south
           orientation

Output     Outlet             Linked
           temperature

Output     Outlet flow rate   Linked


Solar Pump

The solar pump is used to circulate the antifreeze fluid in the closed loop. It is powered by PV cells, which send the appropriate control signals when there is sufficient sunlight. Type 110 in the TRNSYS component libraries, which is a variable-speed pump that is able to maintain any outlet mass flow rate between zero and a rated value, is modeled. Table 12 shows the characteristics of the solar pump.
Table 12. Characteristics of Solar Pump

Parameter  Rated flow rate     72 kg/h (159 l[b.sub.m]/h)

Parameter  Fluid specific            3.747 kJ/kg-K (0.895
           heat             Btu/1[b.sub.m] * [degrees]R)

Parameter  Rated power                               36 W

Input      Total pump                                 0.9
           efficiency

Input      Motor                                     0.95
           efficiency


Heat Exchanger

A heat exchanger is used for the heat transfer between the water and antifreeze fluid, which is made of 40% propylene glycol and 60% distilled water. The shell-and-tube Type 5g from the TRNSYS component libraries is modeled. The hot-and cold-side inlet temperatures and flow rates are real-time values. The effectiveness is calculated for a given fixed value of the overall heat transfer coefficient. Table 13 shows the characteristics of the heat exchanger.
Table 13. Characteristics of Heat Exchanger

Parameter      Shell-and-tube mode                   7

Parameter  Specific heat of hot-side   3.747 kJ/kg-K (0.895
           fluid                       Btu/l[b.sub.]*[degrees]R)

Parameter  Specific heat of cold-side  4.19 kJ/kg-K (1.00
           fluid                       Btu/l[b.sub.]*[degrees]R)

Input      Overall heat transfer       834.48 kJ/h-K
           coefficient


Preheat Tank

Water is stored in the stratified preheat tank after it is warmed up with the heat exchange from the antifreeze fluid in the heat exchanger. Type 4a is modeled from the TRNSYS component library. This tank works based on the thermosiphon principle: the hot water flows upward in the tank and then is transferred to the auxiliary tank. Table 14 shows the characteristics of the preheat tank.
Table 14. Characteristics of Preheat Tank

Parameter  Tank volume     300/450/600 L (79/119/159 gal)

Parameter  Fluid specific  4.19 kJ/kg-K (1.00
           heat            Btu/l[b.sub.m]-[degrees]R)

Parameter  Fluid density   1000 kg/[m.sup.3]
                           (62.43 l[b.sub.m]/f[t.sup.3])

Parameter  Tank loss       1 kJ/h-[m.sup.2]-K
           coefficient


Auxiliary Tank

A 227 L (60 gal) stratified auxiliary tank is used in this model. This tank has TOU electrical power as backup. The thermal performance of a fluid-filled sensible energy storage tank, subject to thermal stratification, is modeled by assuming that the tank consists of eight fully mixed equal-volume segments. Type 4a is modeled from the TRNSYS component library. The model includes two electric resistance heating elements subject to temperature and time control. Table 15 shows the characteristics of the auxiliary tank.
Table 15. Characteristics of Auxiliary Tank

Parameter  Tank volume            227 L (60 gal)

Parameter  Fluid specific heat    4.19 kJ/kg*K (1.00 Btu/
                                  l[b.sub.m]*[degrees]R)

Parameter  Fluid density          1000 kg/[m.sup.3] (62.43
                                  l[b.sub.m]/f[t.sup.3])

Parameter  Setpoint               55[degrees]C (131[degrees]F),
           temperatures           60[degrees]C (140[degrees]F),
                                  65[degrees]C (149[degrees]F),
                                  and 70[degrees]C (158[degrees]
                                  F)

Parameter  Tank loss              0.5 kJ/h*[m.sup.2]*K
           coefficient

Parameter  Number of temperature  8
           levels


SENSITIVITY ANALYSIS

The purpose of the sensitivity analysis for the SUI-NZEHH was to study the effects of various components in the hybrid model on electricity consumption, GHG emissions per year, and energy costs. In total, 96 different scenarios in hybrid models were simulated for a whole year. The hybrid results were compared with NRCan's RETScreen[R] software (NRCan 2010). The model was simulated for a whole year (8760 hours) using a one-hour time step. The results were extracted in Excel format for each hour step of the whole year. The main results analyzed were the instantaneous value of energy (in kilojoules) required to heat the water at a particular hour. Another output is the temperature of the water going into the house at a particular hour. The required water temperature for the house is 55[degrees]C (131[degrees]F); this output helps to investigate if the system will meet the requirements at a particular hour. For the models with auxiliary setpoints of 60[degrees]C (140[degrees]F), 65[degrees]C (149[degrees]F), and 70[degrees]C (158[degrees]F), a mixer is used to deliver the water temperature at 55[degrees]C (131[degrees]F). Energy required to heat the water is converted from kilojoules to kilowatt-hours. For the GHG emissions calculation, the variable GHG emission factors for electricity generation in Ontario are used (Gordon and Fung 2007, 2009). Table 16 shows the criteria used for the sensitivity analysis.
Table 16. Criteria for Sensitivity Analysis

Daily water  100 L (26 gal), 155 L (41 gal), 190 L (50 gal), and 225
demand       L (59 gal)

Auxiliary    55[degrees]C (131[degrees]F), 60[degrees]C
setpoints    (140[degrees]F), 65[degrees]C (149[degrees]F), and
             70[degrees]C (158[degrees]F)

Preheat      300 L (79 gal), 450 L (119 gal), and 600 L (159 gal)
tank
volumes

Heat         Yes/No
recovery


RESULTS

Energy Consumption

Figure 8 shows the electricity demand for the backup auxiliary tank with a 60[degrees]C (140[degrees]F) setpoint temperature and a 100 L (26 gal) daily hot water demand. Figure 9 shows the electricity demand for hybrid systems with and without the gray-water heat recovery system. As daily hot water demand increases, the electricity demand between systems with and without gray-water heat recovery also increases. For 100 L of daily hot water demand, the hybrid system with gray-water heat recovery has an electricity demand of 479 kWh, while the system without gray-water heat recovery has 651 kWh. The demand increases to 1315 and 2067 kWh, respectively, for 225 L (59 gal) daily hot water demand.

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

GHG Emissions

Figure 10 shows the GHG emissions for the backup auxiliary tank with a 60[degrees]C (140[degrees]F) setpoint temperature and a 100 L (26 gal) daily hot water demand. Figure 11 shows the annual GHG emissions for hybrid systems with and without a gray-water heat recovery system. The GHG emissions of the hybrid system having 100 L (26 gal) daily hot water demand with gray-water heat recovery is107 kg (235 lb) and without gray-water heat recovery is 144 kg (316 lb). The difference in GHG emissions increases with the increase in daily hot water demand. The respective GHG emissions for 155 L (41 gal) daily hot water demand are 183 and 268 kg (403 and 590 lb), for 190 L (50 gal) are 235 and 359 kg (517 and 789 lb), and for 225 L (59 gal) are 287 and 448 kg (632 and 985 lb).

CONCLUSION

The first part of this paper studied the feasibility analysis of solar and conventional hot water systems for a detached house based on the design of the CCHT houses in Whitby, Ontario. The life-cycle cost analysis of solar models was compared with that of conventional models. The 30-year life-cycle cost of the solar system is comparable with the conventional models despite having high initial investment costs. Solar models have the lowest fuel consumption and GHG emissions compared to the conventional models. The SDHW system is a good alternative to conventional systems because of its environmental benefits. The future uncertainties related to fossil fuel availability as well as price fluctuations makes solar models an attractive choice.

Results indicate that consumers can take the price benefits of low-peak-period rates by using TOU electric tanks. The utilities will benefit from shifting the load from peak periods to off-peak periods. The solar system with a gray-water heat recovery unit and TOU electric backup tank has 202 kWh electric consumption and 43 kg (94 lb) GHG emissions during the summer period (May 1-October 31). The two-panel solar system is able to keep up with daily hot water demand, thus minimizing fuel consumption and GHG emissions during that period.

The second part of this study performed sensitivity analysis for one of the SUI-NZEHH houses with a two-panel SDHW system. The results concluded that the SDHW system with a gray-water heat recovery unit achieves up to 80% reduction in electricity cost and GHG emissions when compared with a conventional electrical tank without a gray-water heat recovery unit. The solar system with 225 L (59 gal) daily hot water demand and 60[degrees]C (140[degrees]F) temperature setpoint in the backup auxiliary tank has a yearly electricity cost of CAN $101 (US $98) and 287 kg (632 lb) GHG emissions. The conventional electrical tank with 225 L (59 gal) daily hot water demand has an electricity cost of CAN $497 (US $481) and 1135 kg (2500 lb) GHG emissions. The different preheat tank volumes of 300, 450, and 600 L (79, 119, and 159 gal) were used for sensitivity analysis. The results indicated that the hybrid system results are not sensitive to the preheat tank size. By using the TOU auxiliary hot water tank, the electric load can be shifted from peak periods to off-peak periods. This will decrease the strain on utilities and will also reduce GHG emissions related to peak periods. Homeowners can also benefit from off-peak electric pricing.

ACKNOWLEDGMENTS

The authors would like to thank NSERC Solar Building Research Network, NSERC Discovery Grant, and Summerhill Group, Toronto. This study was not possible without their support.

DISCUSSION

Carl Hiller, Dr., Applied Energy Technology Company, Davis, CA: Did you consider/analyze heat pump water heaters?

Gurjot S. Gill: I did model a ground-source heat pump (GSHP) for heating and hot water. The following data was obtained for 225 liters (59 U.S. gallons) daily hot water demand in GHSP:

Annual energy consumption (for hot water)--7.8 GJ

Annual GHG emissions (for hot water)--481 kg (1060 lb)

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Alan S. Fung, PhD, PEng Member ASHRAE

Gurjot S. Gill

Alan S. Fung is an associate professor in the Department of Mechanical and Industrial Engineering, Ryerson University, Toronto, Ontario, Canada. Gurjot S. Gill is a quality assurance analyst at Dahl Brothers Canada Ltd., Mississauga, Ontario, Canada.
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Date:Jul 1, 2011
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