Solar Calorimetry Laboratory The Potential and Challenges of 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 Calorimetry Laboratory 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 Calorimetry Laboratory 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. Solar Collector • 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 Hot Water Hot Water to Load to Load Building Wall Glycol/water Auxiliary Anti-freeze Auxiliary Heater Outdoor Fan-coil Circulation Loop Heater Evaporator Water Water Compressor Storage Storage Heat Tank Tank Outdoor Exchanger Ambient Air Condenser Cold Mains Expansion Valve Water Inlet Cold Mains Water Inlet Electric Pump Electric Pump Electric Pump Split Heat Pump (optional) (optional) Solar Calorimetry Laboratory Compact Air-source HPWH Indoor Fan-coil Evaporator • The use of compact air-source heat pump water heaters (HPWHs) is well Ambient Air established. Hot Water to Load • 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. Auxiliary Heater • 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 Wrap-around Condenser Cold Mains used or outdoor-air can be ducted into the indoor unit, however, cold Water Inlet outdoor temperatures can lower overall capacity and COP. Compact Heat Pump Water Heater Solar Calorimetry Laboratory WHY SB-HPWH HPWH Outdoor HPWH Indoor “Split” HPWH SB-HPWH Ambient Air Compressor Outdoor Ambient Air Expansion Valve Warm Climate Cold Climate Solar Calorimetry Laboratory Solar Boosted HPs Evaporator • Solar boosting HP output has been extensively studied and the benefits Solar Collector Roof Line in improved performance are well established but depend on climate Hot Water and the type of solar collector used. to Load Auxiliary Compressor Heater Water • Many configs: series vs parallel; direct and indirect; dual source etc. Storage Condenser Tank Expansion Valve Cold Mains • draw-backs to some SB-HPWH in terms of installation and operation, Water Inlet Heat Pump Electric Pump e.g., direct systems may need refrigeration connections on roof tops. (optional) • Indirect systems use conventional anti-freeze loops but require and an Solar additional pump Collector Roof Line • the most common configuration uses unglazed solar collectors that can Hot Water to Load collect ambient energy during low sun periods. Glycol/water Anti-freeze Auxiliary Circulation Loop Compressor Heater Water – These are simple and efficient but performance will drop during very Storage Evaporator Expansion Condenser Tank Valve cold or overcast periods. Cold Mains Water Inlet Electric Pump Heat Pump Electric Pump (optional) Solar Calorimetry Laboratory Motivation to Solar-boost a HP • Improvements to collector performance and HP COP • Cooling the solar collector Typical Efficiency Curves of Solar Collectors significantly improves its efficiency 1 • Increasing the HP’s evaporator 0.9 ΔT = 6, G=600W/mΔT = 230, G=600W/m2 temperature increases HP COP 0.8 0.7 ) • Collector area can be halved η 0.6 relative to Solar DHW. 0.5 0.4 • high-performance solar panels ( Efficiency 0.3 (with glazed and insulated 0.2 0.1 absorbers), may be used but limit 0 “non-solar”, air-source capacity; 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 reducing their benefit. (Tf -Ta)/GT Unglazed Collector Single Glazed, Flat Plate Evacuated Tube Solar Calorimetry Laboratory Example Performance comparison 70 60 60 SAHP-Unglazed SAHP-Unglazed 50 50 • Freeman and Bridgeman studied an SAHP-Glazed 40 Indirect Solar-Assisted HPWH 40 HP Base SAHP-Glazed 30 30 • Compared performance for various 20 SDHW Solar Fraction (%) Fraction Solar Solar Fraction (%) Fraction Solar 20 climates, e.g., Toronto, Montreal and SDHW 10 Vancouver. 10 0 0 2 4 6 8 0 1 2 3 4 5 6 7 8 9 10 11 12 • Collector area and overall cost was 2 CollectorCollector Area Area (m2) (m ) Month of the Year reduced 65 100 SAHP-Unglazed • System performance was more Vancouver uniform over year. 60 80 Toronto 60 SAHP-Glazed • Identified need for variable capacity 55 Montreal Solar fraction versus collector 40 SDHW compressor to accommodate seasonal (%) Fraction Solar 50 area for unglazed SAHP systems. operation 20 CollectorEfficiency (%) 45 0 1 2 3 4 5 6 7 8 0 2 4 6 8 Collector Area (m2) CollectorCollector Area Area (m2) (m2) Solar Calorimetry Laboratory 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., Venting channel allows new variable speed, high-efficiency compressors), fully integrated, energy collection during low- high performance, solar/HP hybrid water heaters are possible. solar 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 Dual Mode Vented Collector • By using an unglazed solar absorber it is • maximizes both ambient and solar possible to collect more heat from the ambient air (particularly during periods with a vented glazed solar collector with low solar input) however the solar that allow ambient air to circulate contribution will be less. next to the absorber plate • If collector temperature can be kept • The collector can reach higher very near or sub-ambient solar collector temperatures during sunny periods efficiency can be greater than 100%. (this was done for Team Ontario’s • The low temperature requirement may reduce heat pump COP and trade-off’s Solar Decathlon Entry, 2013) and need to be assessed. still draw heat from the ambient air. Solar Calorimetry Laboratory 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 Direct power to allows solar panel operation at low temperatures Compressor or Grid (even sub-ambient) increasing both electric and Direct DC thermal conversion efficiency. PV-Thermal power to Modules TEM HP • The COP of the Heat pump “leverages” the Natural Anti-freeze Convection electrical input of the system while increasing the Collector Loop Loop To Load rs thermal output of the system o Auxiliary t c e ll Compressor Heating o • The successful integration of these systems, their C Element Hot water r r e o t s storage a RHPefr igerant n r controls and the utilization of the PV generated e o Loop d p SDHW n a o v E electricity are areas requiring research and C development. Electric Pump Expansion Valve Water Mains Supply PVT Solar Boosted HP vs Air Source Source PVT V-C HP DHW Sink 2 1950 퐅퐄퐑 = 2000 = 97.5% x 3 m 3 x 2 450+1500 퐄퐟퐟 1900 퐜퐨퐥 = 2400 = 81% 2 퐀퐫퐞퐚퐂퐨퐥퐥퐞퐜퐭퐨퐫 = 3 m = 2400 W 2400 = Heat = 2000 W 2000 = Heat 1900 퐂퐎퐏 퐇퐏 = 450 = 4.2 Example W/m800 = Solar Energy Flows for a heating Air-SOURCE HP DHW (Split system) load of 2 kW AC Grid 650 W 1450 퐅퐄퐑 = 2000 = 72.5% 1900 Air- 1900 퐂퐎퐏 퐇퐏 = 650 = 2.9 source 1450 W W 2000 = Heat Outdoor Ambient Air Ambient Outdoor Solar Calorimetry Laboratory Competing Water Heating Options Domestic Hot Water Heating Approaches Storage Heaters On Demand (no storage) Traditional Solar PV Solar Thermal Central Home PV HP power fans, ICS (Integral Oil/Gas Fueled Heat Pump compressor etc. Collector Storage) Point of use Oil/gas and electric PV Power Electric Air Source HP Solar Source HP resistance Electric Resistance Resistance Htr. Split System Direct/Series Direct/No Freeze (Solar Evaporator) Protection Integrated System Indirect/Series Indirect/Freeze Heat Loop Protected Parallel System PV/Thermal Solar Panels (2 & 3 Mode) Solar Calorimetry Laboratory 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 Thank you! deliver: – high temps, energy efficiency, ease of installation, good cost performance, load shifting. • There is tremendous potential but still work to do! Solar Calorimetry Laboratory 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?.