The Potential and Challenges of Solar Boosted Heat …...2017/06/01 · The Potential and...

The Potential and Challengesof Solar Boosted Heat Pumps for

Domestic Hot Water Heating

Stephen Harrison Ph.D., P. Eng., Solar Calorimetry Laboratory,

Dept. of Mechanical and Materials Engineering, Queen’s University, Kingston, ON, Canada

Solar CalorimetryLaboratory


Background• As many groups try to improve energy efficiency in residences,

hot water heating loads remain a significant energy demand.

• Even in heating-dominated climates, energy use for hot water

production represents ~ 20% of a building’s annual energy

consumption.

• Many jurisdictions are imposing, or considering regulations,

specifying higher hot water heating efficiencies.

– New EU requirements will effectively require the use of either heat

pumps or solar heating systems for domestic hot water production

– In the USA, for storage systems above (i.e., 208 L) capacity, similar

regulations currently apply Canadian residential sector energy

consumption (Source: CBEEDAC)


Solar and HP water heaters

• Both solar-thermal and air-source heat pumps can achieve efficiencies above 100% based on their primary energy consumption.

• Both technologies are well developed, but have limitations in many climatic regions.

• In particular, colder ambient temperatures lower the performance of these units making them less attractive than alternative, more conventional, water heating approaches.

• Another drawback relates to the requirement to have an auxiliary heat source to supplement the solar or heat pump unit, particularly, during cold or overcast periods.

Solar

Collector

Glycol/water

Anti-freeze

Circulation Loop

Roof Line

Heat

Exchanger

Cold Mains

Water Inlet

Electric Pump

Hot Water

to Load

Electric Pump

(optional)

Auxiliary

Heater

Water

Storage

Tank

Outdoor Fan-coil

Evaporator

Building

Wall

Cold Mains

Water Inlet

Hot Water

to Load

Electric Pump

(optional)

Auxiliary

Heater

Condenser

Split Heat Pump

ExpansionValve

Compressor

Outdoor

Ambient Air

Water

Storage

Tank


Compact Air-source HPWH Indoor Fan-coil

Evaporator

Cold Mains

Water Inlet

Hot Water

to Load

Auxiliary

Heater

Compact Heat Pump

Water Heater

Ambient Air

Wrap-around

Condenser

• The use of compact air-source heat pump water heaters (HPWHs) is well

established.

• Most draw heat from the surrounding environment and are best suited for

mild climates where they may be placed outdoors or in an unheated garage.

• In cold regions, they must be located in a heated space to avoid

high standby-losses or freezing, thereby shifting the water heating load to

the space heating.

• To alleviate this, “split systems” with outdoor fan-coil evaporators can be

used or outdoor-air can be ducted into the indoor unit, however, cold

outdoor temperatures can lower overall capacity and COP.


WHY SB-HPWH

Ambient Air

Warm Climate

Cold Climate

Ambient Air

ExpansionValve

Compressor

Outdoor

Ambient Air

HPWH Outdoor HPWH Indoor “Split” HPWH SB-HPWH


Solar Boosted HPs

• Solar boosting HP output has been extensively studied and the benefits in improved performance are well established but depend on climate and the type of solar collector used.

• Many configs: series vs parallel; direct and indirect; dual source etc.

• draw-backs to some SB-HPWH in terms of installation and operation, e.g., direct systems may need refrigeration connections on roof tops.

• Indirect systems use conventional anti-freeze loops but require and an additional pump

• the most common configuration uses unglazed solar collectors that can collect ambient energy during low sun periods.

– These are simple and efficient but performance will drop during very

cold or overcast periods.

Solar

Collector

Glycol/water

Anti-freeze

Circulation Loop

Roof Line

Cold Mains

Water Inlet

Electric Pump

Hot Water

to Load

Electric Pump

(optional)

Auxiliary

Heater

Evaporator Condenser

Heat Pump

ExpansionValve

Compressor

Water

Storage

Tank

Evaporator

Solar Collector

Roof Line

Cold Mains

Water Inlet

Hot Water

to Load

Electric Pump

(optional)

Auxiliary

Heater

Condenser

Heat Pump

ExpansionValve

Compressor

Water

Storage

Tank

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14

Effi

cie

ncy

(η)

(Tf -Ta)/GT

Typical Efficiency Curves of Solar Collectors

Unglazed Collector Single Glazed, Flat Plate Evacuated Tube

• Improvements to collector performance and HP COP

ΔT = 30, G=600W/m2ΔT = 6, G=600W/m2

• Cooling the solar collector significantly improves its efficiency

• Increasing the HP’s evaporator temperature increases HP COP

• Collector area can be halved relative to Solar DHW.

• high-performance solar panels (with glazed and insulated absorbers), may be used but limit “non-solar”, air-source capacity; reducing their benefit.


Motivation to Solar-boost a HP


Example Performance comparison

• Freeman and Bridgeman studied an Indirect Solar-Assisted HPWH

• Compared performance for various climates, e.g., Toronto, Montreal and Vancouver.

• Collector area and overall cost was reduced

• System performance was more uniform over year.

• Identified need for variable capacity compressor to accommodate seasonal operation

0

10

20

30

40

50

60

70

0 1 2 3 4 5 6 7 8 9 10 11 12

Month of the Year

Sola

r F

racti

on

(%

)

SAHP-Glazed

SAHP-Unglazed

SDHW

0

20

40

60

80

100

0 2 4 6 8

Collector Area (m2)

Coll

ecto

r E

ffic

iency (

%)

SAHP-Glazed

SAHP-Unglazed

SDHW

10

20

30

40

50

60

0 2 4 6 8

Collector Area (m2)

Sola

r F

ract

ion

(%

)

SAHP-Glazed

SAHP-Unglazed

HP Base

SDHW

Collector Area (m2)

Collector Area (m2)

45

50

55

60

65

1 2 3 4 5 6 7 8

Collector Area (m2)

Sola

r F

racti

on (

%)

Toronto

Vancouver

Montreal

Solar fraction versus collector

area for unglazed SAHP systems.

Collector Area (m2)


• new systems are being studied that include dual- or tri-mode

solar collectors that act as efficient solar- or air-source

evaporators and may even include Photovoltaic/thermal

absorbers.

• Combined with new system configurations and components (e.g.,

new variable speed, high-efficiency compressors), fully integrated,

high performance, solar/HP hybrid water heaters are possible.

• PV/Thermal Panels offer many opportunities to “piggyback” on

existing PV infrastructure (installation and mounting, etc.)

• PV/Thermal panels offer high solar efficiency by delivering heat

and electricity.

New Approaches to SB-HPWH

Venting channel allows energy collection during low-

solar periods

Solar Boosted Heat Pump (Collector Options)

Unglazed Collector• By using an unglazed solar absorber it is

possible to collect more heat from the ambient air (particularly during periods with low solar input) however the solar contribution will be less.

• If collector temperature can be kept very near or sub-ambient solar collector efficiency can be greater than 100%.

• The low temperature requirement may reduce heat pump COP and trade-off’sneed to be assessed.

UnglazedSolar Thermal Collector

Dual Mode Vented Collector• maximizes both ambient and solar

with a vented glazed solar collector that allow ambient air to circulate next to the absorber plate

• The collector can reach higher temperatures during sunny periods (this was done for Team Ontario’s Solar Decathlon Entry, 2013) and still draw heat from the ambient air.


The Challenge: PV/Thermal HP Water heater• Solar conversion efficiency of PV/Thermal devices

can be very high as solar energy, not directly converted to electricity, is converted to heat and this can be extracted for heating purposes.

• The addition of a heat pump to this combination allows solar panel operation at low temperatures (even sub-ambient) increasing both electric and thermal conversion efficiency.

• The COP of the Heat pump “leverages” the electrical input of the system while increasing the thermal output of the system

• The successful integration of these systems, their controls and the utilization of the PV generated electricity are areas requiring research and development.

Compressor

Expansion Valve

Hot water

storage

Auxiliary

Heating

Element

To Load

Water Mains

Supply

Refrigerant

Loop

Natural

Convection

Loop

Electric Pump

Col

lect

ors

Anti-freeze

Collector Loop

Eva

po

rato

r

Co

nd

en

se

r

Direct power to Compressor or Grid

HP SDHW

PV-Thermal Modules

Direct DC power to TEM HP

PVT Solar Boosted HP vs Air Source

PVT V-C HP DHW

𝐅𝐄𝐑 =19502000 = 97.5%

𝐄𝐟𝐟𝐜𝐨𝐥 =450+1500

2400 = 81%

𝐂𝐎𝐏𝐇𝐏 =1900450 = 4.2

𝐀𝐫𝐞𝐚𝐂𝐨𝐥𝐥𝐞𝐜𝐭𝐨𝐫 = 3 m2

𝐅𝐄𝐑 =14502000 = 72.5%

𝐂𝐎𝐏𝐇𝐏 =1900650 = 2.9

Sola

r =

80

0 W

/m2

x 3

m2

= 2

40

0 W

SinkSource

Hea

t =

20

00

W

1900

Air-SOURCE HP DHW (Split system)

Hea

t =

20

00

W

AC Grid 650 W

Air-source1450 W

1900

Ou

tdo

or

Am

bie

nt

Air

Example Energy Flows for a heating load of 2 kW


Competing Water Heating Options

Domestic Hot Water Heating Approaches

Heat Pump

Solar Thermal

Direct/No Freeze Protection

ICS (Integral Collector Storage)

Indirect/Freeze

Protected

Electric Resistance

Oil/Gas Fueled

Storage Heaters

On Demand(no storage)

Central Home

Point of use

Air Source HP

Split System

Integrated System

Solar Source HP

Solar PV

Direct/Series

(Solar Evaporator)

Indirect/Series Heat Loop

PV/Thermal Solar Panels (2 & 3 Mode)

PV Power Electric Resistance Htr.

PV HP power fans,

compressor etc.

Traditional

Parallel System

Oil/gas and electric resistance


Conclusion

• As energy efficiency in buildings increases, space heating requirements may be reduced through improved building practices

• In the limit domestic hot water heating will persist as a significant load and major contributor to peak load demands.

• to be competitive Solar Boosted Heat Pumps must be able to deliver:– high temps, energy efficiency, ease of installation, good cost

performance, load shifting.

• There is tremendous potential but still work to do!

Thank you!

Questions:a) Is there a role for PV/Thermal technology for HPWH?b) Can solar boosted PVT HPWHs “piggyback” on PV infrastructure

to accelerate deployment?b) Should PVT-HPWHs send power to the “grid” or self-power?


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