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Heat Pump Water Heating Control Strategy Optimization for Cold Climates

Cynthia A. Cruickshank

August 21, 2015

Department of Mechanical and Aerospace Engineering

Smart Net Zero Resilient Buildings and Communities CZEBS-iiSBE-APEC Net Zero Built Environment 2015 Symposium | Concordia University, Montreal, QC

HPWH Control Strategy Optimization for Cold Climates

Background

- heat pump water heater (HPWH) units have had considerable popularity outside of North America, however they account for less than 1% of the hot water market in North America

Source: J. Maguire, X. Fang and a. E. Wilson, "Comparison of Advanced Residential Water Heating Technologies in the United States," NREL, Golden, 2013.


Background

- heat pump water heater (HPWH) units have had considerable popularity outside of North America, however they account for less than 1% of the hot water market in North America

- HPWH technology will likely see a significant rise in use due to new mandates from the U.S. Department of Energy

- this new mandate (effective 2015) prescribes the use of HPWH units in lieu of electric water heaters where the tank capacity is over 208 L (55 US Gal)

- due to Canada’s close relations with the US, it is likely that policy changes could occur to extend the mandate further north


- secondary energy use in Canada is dominated by space heating due to the prevalence of cold winters

- domestic water heating is the second largest end use, and as an end use with less load variance based on geography, provides an opportunity for widespread implementation of techno-logical advancements

Background

Source: Natural Resources Canada, "Energy Efficiency Trends in Canada 1990 to 2010," Natural Resources Canada, Ottawa, 2012.

Prof. C. A. Cruickshank, Carleton University


Sacrificial Anode

Condenser

Evaporator

Dehumidified Cool Air Out

Fan Warm Air In

Expansion Valve

Compressor

Hot Water Outlet

Electric Booster Element

Storage Tank

Cold Water Inlet

Operation of a Heat Pump Water Heater

- HPWHs use an air source heat pump to transfer thermal energy from the ambient air to the storage

- can be used as a stand-alone water heating system, or as a combined water heating and space conditioning system

- electric resistance element(s) are typically included to provide backup heating if the heat pump cannot provide sufficient heating capacity


Domestic Water Heater Market Share in Canada (2012)

- as a result of high electricity

prices (relative to natural gas per unit of energy), water heating units have trended downward in use over the last 25 years

- this provides a relevant niche for HPWH technology to infiltrate if proven to be less energy intensive than standard electric water heating

67.0%

25.3%

4.8% 2.3% 0.7%

GasElectricHeating OilWoodOther

- approximately one quarter of the domestic hot water heating systems found in Canadian homes are classified as having an electric energy source


Findings from Past Work (National Renewable Energy Laboratory)

The National Renewable Energy Laboratory in Golden, Colorado has investigated the performance of several HPWH units in numerous US climates, and found that:

- when used in homes with electrically powered space heating systems, HPWHs can reduce energy use with respect to electric water heaters

- relative to other water heating technologies, and in the absence of incentives, gas storage and HPWH units were dominant in minimizing energy use over most climates in the U.S.

- HPWH units are most cost effective in instances where they will see mid to high water daily water volume draws

- the wet bulb temperature had the largest impact on unit energy performance and for this reason, HPWH units tended to have the best performance and lowest life cycle cost in humid southern climates

- utility rates also have a large effect on the economics of a HPWH


Findings from Past Work (CanmetENERGY, Natural Resources Canada)

CanmetENERGY in Ottawa, Ontario investigated the performance of HPWH technologies when used in a typical Canadian climate.

Testing was conducted at the Canadian Centre for Housing Technologies (CCHT) to evaluate the impact of HPWHs on hot water energy consumption and whole house energy consumption. Two HPWHs were monitored for this study, and each HPWH was tested for a two week period.

The CCHT is a Test Facility Jointly owned and operated by NRC, CMHC and NRCan. The facility consists of two identical houses at the same location, with the same construction and appliances, and simulated occupation.


Findings from Past Work (CanmetENERGY, Natural Resources Canada)

During the heating season, it was found that: - the heat pump water heater used about one third of the

energy compared to an electrical resistance water heater; the furnace used about 6% more energy

- there was no significant change of the total house energy use (space heat + domestic hot water)

During the cooling season, it was found that: - the total house energy use was reduced by 18-25%

Further, it was found that HPWH was significantly effective for all electric homes. However, due to the lower unit energy cost of natural gas, HPWH do not reduce the utility cost in its current configuration.


HPWH Control Strategy Optimization Study

A recent study was conducted at Carleton University to generate a complete HPWH performance map to identify trends which could be exploited to minimize energy consumption.

Several set-point conditions were tested using an experimental apparatus to characterize the performance of both the heat pump and the electric booster heater.

HPWH controls were overridden by replacing the potentiometer controlled thermostats with relays, and control was facilitated by a National Instruments microcontroller via the LabVIEW interface.

Optimization using GenOpt black box parametric techniques was implemented with TRNSYS to test a wide range of multi-parameter scenarios, and determine the minimal energy use configurations.


Experimental Apparatus

Experimental Set-Up

Draw Control System

Measuring the Entering Air Temperature

Tempering Valve and Flow Meter

Experimental Apparatus



Experimental Procedure

The experimental procedure consisted of measuring:

- the electricity consumption of the heat pump, the booster heater, and the fan

- the heat delivered to the tank

- the draw volume (daily draw profiles as per CSA-F379.1)

- the coefficient of performance of the system (COP)

COP =∑∆𝐸therm∆𝐸elec

=∑ 𝜌𝜌𝑐p(𝑇n,i − 𝑇n−1,i)13i=1

∆𝐸elec

𝐸therm = the thermal energy delivered to the water in the tank 𝐸elec = the electric energy consumed by the water heating unit 𝑇n,i = the temperature at the respective node at time "n" 𝑇n−1,i = the temperature at the respective node at time "n− 1"


Experimental Results: Temperature Profile and Power Consumption


Experimental Results: COP and Power Consumption

A higher COP is observed when only the heat pump is running and a lower COP is observed when only the booster is running.


Experimental Results

The most energy efficient runs were found to be at lower set-points (e.g., 55oC for the booster, 50oC for the heat pump) and with smaller hot water draw profiles (150 L/day). This would take advantage of the tank's ability to store the heat added to the water and deliver it on demand.

HP/Booster Set-point

(oC/oC)

Energy Consumption

(MJ)

HP Activity Booster Activity

Both Activity

Inactivity

50/55 15.31 16.4% 7.1% 4.4% 72.0% 55/60 19.56 19.2% 4.8% 8.5% 67.3% 55/65 19.65 9.3% 6.6% 9.1% 74.8% 60/70 22.90 9.4% 6.4% 13.6% 70.5% 62/72 19.56 8.6% 7.3% 11.8% 72.2%

It becomes clear that the inefficient booster heater becomes more dominant as the booster set-point increases.


Varying Heat Pump Set-point (Booster Set-point at 70oC)

In this case, the heat pump begins to dominate energy consumption within 5oC of booster set-point.

These results correspond with the conclusion that having the booster and HP set-points close of each other yields the best overall energy performance.


TRNSYS Model and GenOpt

The TRNSYS model of the system contained a number of existing subroutines and was calibrated using experimental data.

Type 938 HPWH

Type 15 Weather File

Type 1237 Wraparound Coil Tank

Type 1502 Aquastat

Type 9 Data Reader Calculator

A series of simulations were executed to complete the performance map with the calibrated energy model. The TRNSYS model was then parametrically optimized in GenOpt using the hybrid particle swarm optimization and Hooke-Jeeves methods.

Design Optimization

The design parameter runs of GenOpt focused on 10 parameters – most of which could only be changed in unit manufacture and design – and provided guidelines for how to change the system properties to reduce energy use.


Booster Power

Booster Set-point

Booster Length

Cold Water Set-point

Heat Pump Power

Heat Pump Set-point

Heat Pump Length

Evaporator Air Flow

Tank Insulation

Tank Volume

all parameters related to the booster size shifted towards their minima, as expected when trying to reduce expensive electricity consumption

the heat pump shifted to a size that would likely be impractical for the installation space of a typical HPWH

the tank insulation shifted to its maximum value

a smaller tank was identified likely due to reduced surface area for thermal losses to the ambient


Design Optimization

Some of the observed trends could be incorporated into the HPWH design, but not to the magnitude of the GenOpt results for the sake of capital costs and installation practicality. As a result, performance optimizations were centered upon the 3 parameters of the physical system that could be varied in a lab setting after manufacture.

Booster Power

Booster Set-point

Booster Length

Cold Water Set-point

Heat Pump Power

Heat Pump Set-point

Heat Pump Length

Evaporator Air Flow

Tank Insulation

Tank Volume

results indicated that the minimum energy consumption occurred when the set-points were maintained as low as possible

results further indicated the air flow rate of the evaporator settled at optimal values at roughly 40% higher than the system design; this suggests that the fan was undersized

- a design decision to minimize fan costs? - a design decision to reduce turbulent air

flow in the adjacent space?


Design Optimization

GenOpt simulations were also conducted to determine the optimal utility rate ratios for the unit using heating and electricity utility rates.

Utility Rate Ratio = Heating Fuel Cost

Electricity Cost

The cost function shown was used to characterize the energy consumption of the HPWH system and achieve an optimal minimum.

Energy Cost = Electricity Cost * (Heat Pump Energy + Booster Energy) + Evaporator Energy * Heating Fuel Cost

Results indicated that energy consumption of the HPWH system would be at its lowest when the utility rate ratio was near the nominal COP, when using the indoor space as the thermal energy source.


Conclusions

HPWH offers the opportunity to reduce both water heating and space cooling energy use in the cooling season, and still provide a net reduction in energy consumption when compared to electric resistance water heating in the heating season.

Through multi-parametric optimization of HPWHs, it was determined that the operating costs for HPWH units are strongly dependent on local utility cost ratios between the heating fuel and electricity, and that similar set-points between the heat pump and booster heater provide the best cost performance for HPWH units.

Given the high initial costs of these units, further testing of alternate energy sources for the heat pump and more advanced control strategies may prove useful in adapting this technology for long Canadian winters.

Thank You.

Prof. C. A. Cruickshank, Carleton University

Dr. Cynthia A. Cruickshank Mechanical and Aerospace Engineering

Carleton University [email protected]

http://solar.carleton.ca

NSERC Smart Net-Zero Energy Buildings Strategic Research Network


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Heat Pump Water Heating Control Strategy Optimization for ...users.encs.concordia.ca/home/g/gparnis/CZEBS-iiSBE... · Heat Pump Water Heating Control Strategy Optimization for Cold