The Potential and Challenges of Solar Boosted Heat Pumps for Domestic Hot Water Heating Stephen Harrison Ph.D., P. Eng., Solar Calorimetry, Dept. of Mechanical and Materials Engineering, Queen s University, Kingston, ON, Canada
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 Solar Collector 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. Roof Line Glycol/water Anti-freeze Circulation Loop Heat Exchanger Electric Pump Electric Pump (optional) Water Storage Tank Hot Water to Load Auxiliary Heater Cold Mains Water Inlet Outdoor Ambient Air Outdoor Fan-coil Evaporator Building Wall Split Heat Pump Compressor Expansion Valve Condenser Electric Pump (optional) Water Storage Tank Hot Water to Load Auxiliary Heater Cold Mains Water Inlet
Compact Air-source HPWH Indoor Fan-coil Evaporator 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. Auxiliary Heater Wrap-around Condenser Ambient Air Hot Water to Load Cold Mains Water Inlet Compact Heat Pump Water Heater
WHY SB-HPWH HPWH Outdoor HPWH Indoor Split HPWH SB-HPWH Ambient Air Ambient Air Compressor Outdoor Ambient Air Expansion Valve Warm Climate Cold Climate
Solar Boosted HPs Solar Calorimetry 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. Evaporator Solar Collector Roof Line Roof Line Solar Collector Glycol/water Anti-freeze Circulation Loop Electric Pump Heat Pump Evaporator Compressor Expansion Valve Compressor Expansion Valve Heat Pump Condenser Electric Pump (optional) Condenser Electric Pump (optional) Water Storage Tank Water Storage Tank Hot Water to Load Auxiliary Heater Cold Mains Water Inlet Hot Water to Load Auxiliary Heater Cold Mains Water Inlet
Efficiency (η) Solar Calorimetry Motivation to Solar-boost a HP Improvements to collector performance and HP COP Cooling the solar collector significantly improves its efficiency 1 Typical Efficiency Curves of Solar Collectors Increasing the HP s evaporator temperature increases HP COP 0.9 0.8 0.7 ΔT = 6, G=600W/m ΔT = 2 30, G=600W/m 2 Collector area can be halved relative to Solar DHW. 0.6 0.5 0.4 high-performance solar panels (with glazed and insulated absorbers), may be used but limit non-solar, air-source capacity; reducing their benefit. 0.3 0.2 0.1 0 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 (T f -T a )/G T Unglazed Collector Single Glazed, Flat Plate Evacuated Tube
Example Performance comparison 70 Freeman and Bridgeman studied an Indirect Solar-Assisted HPWH Compared performance for various climates, e.g., Toronto, Montreal and Vancouver. Solar Fraction (%) 60 50 40 30 20 10 SAHP-Unglazed SAHP-Glazed SDHW HP Base Solar Fraction (%) 60 50 40 30 20 10 0 SAHP-Unglazed SAHP-Glazed SDHW Collector area and overall cost was reduced System performance was more uniform over year. Identified need for variable capacity compressor to accommodate seasonal operation Solar Fraction (%) 65 60 55 50 0 2 4 6 8 Collector Area (m2) ) Vancouver Toronto Montreal Solar fraction versus collector area for unglazed SAHP systems. Collector Efficiency (%) 100 80 60 40 20 0 1 2 3 4 5 6 7 8 9 10 11 12 Month of the Year SAHP-Unglazed SAHP-Glazed SDHW 45 0 1 2 3 4 5 6 7 8 0 2 4 6 8 Collector Area (m2) 2 ) Collector Area Area (m2) 2 )
New Approaches to SB-HPWH 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. Venting channel allows energy collection during lowsolar periods 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.
Solar Boosted Heat Pump (Collector Options) Unglazed Solar Thermal Collector 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 s need to be assessed. 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. PV-Thermal Modules Collectors Electric Pump Direct power to Compressor or Grid Direct DC power to TEM HP Anti-freeze Collector Loop Evaporator Compressor HP SDHW Refrigerant Loop Expansion Valve Natural Convection Loop Condenser Hot water storage To Load Auxiliary Heating Element Water Mains Supply
PVT Solar Boosted HP vs Air Source PVT V-C HP DHW Outdoor Ambient Air Heat = 2000 W Heat = 2000 W Source Sink Example Energy Flows for a heating load of 2 kw Solar = 800 W/m 2 x 3 m 2 = 2400 W AC Grid 650 W 1900 Air-SOURCE HP DHW (Split system) FER = 1950 2000 = 97.5% Eff col = 450+1500 2400 Area Collector = 3 m 2 COP HP = 1900 450 = 4.2 FER = 1450 2000 = 72.5% = 81% Airsource 1450 W 1900 COP HP = 1900 650 = 2.9
Competing Water Heating Options Domestic Hot Water Heating Approaches Storage Heaters On Demand (no storage) Traditional Solar PV Solar Thermal Central Home Oil/Gas Fueled PV HP power fans, compressor etc. Heat Pump ICS (Integral Collector Storage) Point of use Electric Resistance PV Power Electric Resistance Htr. Air Source HP Split System Solar Source HP Direct/Series (Solar Evaporator) Direct/No Freeze Protection Oil/gas and electric resistance Integrated System Indirect/Series Heat Loop Indirect/Freeze Protected Parallel System PV/Thermal Solar Panels (2 & 3 Mode)
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?