TECHNOLOGY BRIEF | CLEAN ENERGY PSE |Physicians, Scientists,andEngineers for Healthy Energy present atanytemperature above absolute zero. counterintuitive, energy intheform ofheat isalways heat from thecold outdoor air. Whilethismayseem (GSHPs). Asthenameimplies, anASHP( heat (ASHPs) and ground-source heat pumps categories based onthesource ofheat:air-source Heat pumpsare generally classifiedinto two main for theresidential andcommercial sectors [1]. rival heat pumpsin efficiently delivering space heating hydropower. There isnotechnology thatcan currently renewable electricitygeneration from wind,solar, and can helpdecarbonize theheating sector by using gas to heat buildings. ofspace heating air volume systems, whichtypically burnoilornatural technologies suchaswarm-air andvariable ground, to theinterior, incontrast to more widespread electricity to transfer heat from the outside air, or the heating residential andcommercial use buildings.They Heat pumpsare ahighlyefficient technology for Overview July, 2017 Boris Lukanov,PhD Energy System Heat PumpsandTheirRoleinaClean areas. are now afamiliar sightinmanyresidential Figure 1: Source: WikipediaCommons. Air-source heat pumps (ASHPs) Fig. 1) transfers the year [2]. constant 40-60°F temperature oftheground throughout but alsomore efficientthanASHPs due to thenearly extract heat from theground. are They more expensive interior space. Similarly, ground-source heat pumps outside) andtransfers theabsorbedheat into the imperceptibly cooling thevast ofcold reservoir air An ASHPsimplyabsorbssomeofthatheat (thereby it condenses into asaturated liquidandgives offheat. superheated vapor thentravels to thecondenser, where is compressed to highpressure andtemperature. The 2 exchangers called theevaporator andthecondenser ( are acompressor, an expansion valve, andtwo heat its phasechanges. Themaincomponents ofaheat — arefrigerant —andtheheat stored orreleased during properties ofavolatile evaporating andcondensing fluid vapor-compression which exploits cycle, the physical most heat pumpsoperate ontheprinciple ofthe Efficiency heat, resulting intheheat pump’s (see highefficiency much less thantheamountof energy transferred as The amountofinputenergy neededisgenerally the opposite direction against atemperature gradient. a , electrical energy isusedto move heat in Heat naturally travels from warmer to colder places. In Operating principle ) [3].Whenrefrigerant vapor enters thecompressor, it One ofthebenefitselectricheat pumpsisthat flexibility onthedemandside. commercial sectors, whilealsooffering potential carbon heat generation in the residential and in production andprovide low- canthey take advantage oftheanticipated growth ). Similarto refrigerators andairconditioners, www.psehealthyenergy.org Fig.

TECHNOLOGY BRIEF | CLEAN ENERGY [2] andcan beaslow as3to 4years insomegeographic systems, thepayback time falls to between 5and15years [2,5]. By contrast, whenreplacing oilorelectrical heating replacing gas for heating buildingscan becloseto 30 years unit energy (atcurrent prices), thepayback periodsfor three to four timesmore expensive thannatural gas per maintenance costs [2].Because electricitytends to be andminimaloperationaltheir highefficiency and point required to inthelongrundue to save money expenditures, heat pumpshave passed thebreak-even required for underground piping.Despite thehighcapital greater compared to ASHPs due to the additional labor addition, theinstallation price tag for GSHPs ismuch higher than for conventional heating systems. In The upfront capital costs for heat pumpsare normally maintenance paybackCost, times,and becomes theevaporator. the condenser and the lower temperature indoor coil valve. Incooling mode(AC), theoutdoor coil becomes inverted to cool thebuildingby employing areversing In areversible heat pump, thewholeprocess can be vapor. surrounding outsideair(orground) andboilsagain to — enters theevaporator, where itabsorbsheat from the valve. Thenow-cold liquid—colder thantheoutsideair further lowered whenitexpands through anexpansion The temperature (and pressure) of the warm fluid is then Figure 2: Therefrigeration inheating cycle mode.Adapted from [3]. www.psehealthyenergy.org

explosion ornatural gas leakage. compared to conventional furnaces asthere isnoriskof locations. Heat pumpsrequire minimalmaintenance lower cost thanusing batteries, whilevastly increasing heat pumpscan beusedto store electricity asheat ata production withoutnoticeable lossofcomfort. In effect, thermal inertiainbuildingsallows for timedelaysinheat Well-insulated homescan function as“heat storage” — managed to supportthe realization ofasmartgrid[6]. on the flexible demandside that can be actively solar. Heat pumpsare asanenablingtechnology viewed electricity generation from renewables like windand is continuously adjusted to accommodate variable advance towards asystem where electricitydemand energy resources increases, thepower gridwillgradually As theshare ofelectricitygeneration from renewable smart grids Role indemand-sidemanagement and as upstate York New (see ASHPs economical inrelatively even cold climates such boil atlower temperatures, improvements thatmake efficient andadvanced that “cold-climate”[2]. Newer ASHPs, however, usemore suitable thanGSHPs for climates withextreme winters reduces ofASHPs theefficiency andmakes themless colder spellsandtheneedfor defrost This cycles. is thefreezing oftheoutdoor during One disadvantage ofASHPs incold anddampclimates Case Study ). 2

TECHNOLOGY BRIEF | CLEAN ENERGY CO Carbon emissions these problems. to interconnect broad geographic areas can helpalleviate large numbers ofheat pumpsandexpansion ofthegrid challenge. ofscalable Development smartcontrols for correlated with space heating demand, which adds to the correlated withairtemperature andtherefore anti- exacerbate thiseffect [1].Solarenergy production is the coldest months and the use of heat pumps can In suchclimates, electricitydemandtends to peak in on peak electricitydemandinareas withcold winters. At thesametime,akey issueistheeffect ofheat pumps the potential ofdemand-sidemanagement [2]. 2 emissionsfrom heat pumpsare necessarily linked similar to HSPF, issometimes usedinNorthAmerica specifically for cooling (AC) performance. = 3.412 × SCOP). To addto the confusing array of ratings, theSeasonal Energy Efficiency Ratio (SEER), a rating mix ofimperialandmetric units(Btu/watt-hours). Theconversion factor between HSPFandSCOP is3.412(HSPF COP (SCOP) isoften used, as well asanother rating called theHeating Seasonal Performance Factor (HSPF)— a depending onthegeographic location. To account for theseasonal variations intheCOP, aseasonally averaged ¹ an annualaverage ofT pumps have higherseasonal than air-source efficiency pumpsbecause ground temperatures fluctuate around heaters thanasdomestic water heaters (T (the so-called “lift”)needs to beassmallpossible. For this reason, heat pumpsare more efficientas space (T of a heat pump is inversely proportional to the temperature difference between the external source ofheat where from alllossesanddeviations theideal are cycle represented by expressed as 1 because alltheproduced heat iscreated from theinputelectrical energy. TheCOP ofaheat pumpcan alsobe COP intherange of4.2–5.2[4].By comparison, anelectrical resistance heater can have amaximumCOP ofonly several timeslarger thantheinputelectrical energy. Atypical ASHPhasaCOP of3.2–4.5,whileaGSHPhas Because heat pumpsmove heat asopposedto creating theheat it, energy transferred from theoutsidecan be (COP), definedastheheat delivered Q specificthermodynamicmeaning. Instead,a very acommonly usedmeasure istheCoefficientof Performance When measuring thetechnical performance ofheat pumps,itisbestto avoid theterm “efficiency,” whichhas Efficiency Using the Carnot efficiency ofaheatUsing theCarnotefficiency engine in reverse. cold ) andtheoutputheat (T 1

cold ~50°F, whereas ambientairtemperatures inthewinter can fall to below T hot ). To maximize theCOP, thedifference between insideandoutside temperatures www.psehealthyenergy.org HP dividedby theelectrical inputenergy W hot ~90°Ffor aircompared to 130°Ffor water), and ground-source CO option for theheating sector. Studies have shown that the attractiveness of heat pumps as a decarbonization point intimeandagiven location can greatly influence the marginal carbon intensity ofthegrid at agiven to thecarbon intensity use.Thus, oftheelectricitythey pump [9]. and needsto beminimized over thelife span of theheat pump operation, leakage isaseriousconcern CO warming potential (GWP) —about2,000timesthatof pumps istheiruseofrefrigerants highglobal withvery One concern regarding thecarbon footprint ofheat to displace 35%whenthey heating [2,7,8]. displace solidfuel, oil,orelectricheating, andby 10% by approximately 50%onaverage whenheat pumps 2 2 . Whilethere isnoimpact onclimate duringheat emissionsfrom domesticheating are reduced η [4].Thisshows thattheperformance HP : cold =0°F (2) (1) 3 TECHNOLOGY BRIEF | CLEAN ENERGY space heating renewable-grid ready, meaning that the weatherization of homes). Electrification also makes emissions intheresidential heating sector (inadditionto is themostpromising path towards reducing GHG Using heat pumpsto displace fossil fuel heating systems heating [11]. of thoseemissionscome from residential space conditioning andwater heating andthevast majority for over 60% ofthe emissions associated withspace imports). Theresidential sector itselfisresponsible Yorkof New State combined (includingelectricity for more CO [10]. Space conditioning andwater heating account accounting for 56%oftheresidential energy consumption single residential energy York end-useinNew State, residential heating sector. Space heating isthelargest substantial reductions intheuseoffossil fuels inthe In order to reach thisgoal, it isessential to achieve reductions York inNew State is40%below 1990levels. The 2030target for greenhouse gas (GHG)emissions Case Study:NewYorkState ⁵ ⁴ 3 2 CO Energy costs perMMBtu York inNew State: electricity=$49.82 [11];natural gas =$14.81,heating oil=$23.92(NYSERDA). Natural Source: IEER[11] Assuming anaverage annualthermalrequirement of90MMBtu for Upstate York New [11,12],90%efficiency for oiland gas Table 1: electricity in New Yorkelectricity inNew State for theperiod2015–2030 are estimated to be0.171kg CO gas andheating oilprices are averaged over 10years to account for highprice volatility. means millionBtu. 1kWh(electrical) =3,412Btu. heating systems, andaverage COP values of3.5and4.5for air-source andground-source heat pumpsrespectively. MMBtu Natural Gas Oil GSHP ASHP 2 emissions coefficients (kg CO 2 emissionsthantheentire electricitysector Costs,CO Installation cost² $24,000 $4,000 $4,000 $8,300 2 emissionsandpayback timesfor different heating York optionsinNew State

($) 2 /MMBtu) for fossil fuels: natural gas = 53.07, oil = 73.16 (EIA). The average CO Annual energy use³ (MMBtu) www.psehealthyenergy.org 100 100 20 26 Annual energy cost⁴ ($) $1,481 $2,392 $1,295 $996

of heating area, assumingtheretrofit takes place ata single unitstructures with1,500to 2,000square feet [10]. Theresidential buildingsconsidered here are 29% ofallresidential heating systems respectively fuels York inNew State, accounting for 57%and and heating oilare by far themostcommon heating natural gas andoilheating systems. Natural gas for cold-climate ASHPs andGSHPs compared to CO annual energy use,annualenergy costs, cumulative Table 1 renewables by 2030. increases towards York New State’s target of 50% automatically astheproportion ofrenewables emissions reductions willcontinue to improve projections offuture fuel costs due to thehigh These estimates donotincludediscount rates or replacement or, equivalently, construction. duringnew time whentheoilornatural gas needs 2 CO emissionsover 15years, andpayback times (metric tons) 15 years⁵ 2 emissions, 109.7 summarizes theestimated installation costs, 79.6 15.0 19.5 2 /kWh [11] Payback time vs Oil(years) 14.3 3.9 - - Payback time vs NG(years) 2 emissions of 41.3 23.2 - - 4

TECHNOLOGY BRIEF | CLEAN ENERGY quite different —thesimple payback timesbecome gas heating systems, however, theeconomics are over 15years. Whenitcomes to replacing natural former leads to significantly highernet benefits years for aGSHP. Thelower installation cost ofthe options: ~ 4 years for an ASHP and slightly over 14 times are quite different for the two heat pump a heat pumpsystem, althoughthebreak-even cost-effectiveit isvery to replace anoil furnace with advantage for GSHPs. Thenumbers alsosuggest that comparable emissionsreduction potential withaslight over 15 years. Thetwo heat pumpoptionshave a system can result insignificant CO from fossil fuel heating to either an ASHP or a GSHP Table 1 are alsonotconsidered inthissimpleanalysis. associated withnatural gas production anddistribution climate impacts ofupstream health benefitsofreducing airpollution,andthe benefits of reducing carbon emissions, the uncertainty innatural gas andoilprices. Thesocietal Figure 3: and payback timesfor ASHPs andGSHPs. and Estimated CO Fig. 3 above suggest thatswitching 2 emissionsreduction 2 emissionsreduction www.psehealthyenergy.org

the relatively stable cost ofelectricity. fuel prices due to their lower energy consumption and asahedge canis thatthey for serve volatile fossil into account. An additional benefit of heat pumps CO gas heating replacement unlesstheco-benefits of economical andGSHPs uneconomical asanatural the greater upfront costs make ASHPs marginally of natural gas compared to electricitycoupled with and electricityprices. Ingeneral, thecurrent low prices the heat pumpsize, and,mostimportantly, the fuel substantially dependingonthegeographic location, to keep inmindthattheseestimates can vary 25 to 30years would beunableto doso. Itisimportant nearly break andaGSHPwithlife even span of options. Withalife span of15to 20years, anASHPcan 23 and41years respectively for thetwo heat pump prices are taken into account. Meeting thegoal of emissions, airpollution,and thevolatility offossil fuel option for buildings,especiallywhenCO new infrastructure, ASHPs may still be the most beneficial to efficientheat pumpsystems. Inareas with natural gas YorkNew State residents can affordably switch directly systems. Inareas withoutnatural gas infrastructure, retrofitting ofalloil-heated homeswithefficient electrical In particular, thestate shouldfacilitate therapid incentives, cold climate concerns, lackofinformation). for heat pumpadoption (highupfront costs, split policies to overcome thevarious market barriers encouraged York inNew andthestate shouldprioritize pump systems —specifically ASHPs — should be fossil fuel space heating to efficient electrical heat The estimates above suggest thataswitch from source heat pumps. which improves theseasonal ofair- efficiency by themilderclimate York intheNew Cityarea, prices [13].Thesehighercosts are offset somewhat equipment andlaborcosts, andthehigherelectricity lower cost of natural gas and heating oil, the higher heating requirements for apartment buildings,the compared to Upstate York New dueto thelower slightly lesscost-effective for heat pumpadoption We note York thattheNew Cityarea isexpected to be 2 emissionsandairpollutionreduction are taken 5 2

TECHNOLOGY BRIEF | CLEAN ENERGY [14] "Technology roadmap 2011—energy-efficient buildings:Heating and cooling equipment." tech. rep., International [13] “Heat pumpspotential for energy York savings inNew State,” tech. rep., NYSERDA, 2015. [12] “Renewable heating andcooling framework: policy Optionsto advance growth andmarkets industry York,” inNew [11] A.Makhijani,“Makingresidential heating andcooling York climate-friendly inNew State,” tech. rep., IEER,2017. [10] [9] B.Greening, BandA.Azapagic, “Domesticheat pumps:Life environmental cycle impacts andpotential implications [8] D.Jenkins,R.Tucker, andR.Rawlings, “Modellingthecarbon-saving performance ofdomesticground-source heat [7] N.Kelly andJ. Cockroft, “Analysis of retrofit airsource heat pumpperformance: Results from detailed simulationsand [6] P. D.Lund, J. Lindgren, J. Mikkola, andJ. Salpakari, “Review ofenergy system flexibility measures to enablehighlevels [5] [4] D.FischerandH.Madani,“Onheat pumpsinsmartgrids: Areview,” [3] “Commercial earth energy systems: Abuyer’s guide,” 2002. [2] I.Staffell, D.Brett, N.Brandon, andA.Hawkes, “A ofdomesticheatreview pumps,” [1] T. Fawcett, N.Eyre, andR.Layberry, “Heat pumpsandglobal residential heating,” InP References energy by 2030isalsomet. by 2030,assumingthestate’s target of50%renewable existing natural gas furnaces withefficientheat pumps replacing allexisting oilfurnaces andover 20%of heating sector York inNew State by 2030would require 40% reduction inCO Renewable Heatandfrom CombinedHeatandPower Plants:Study andAnalysis. Energy Agency (IEA),2011. tech. rep., NYSERDA, 2017. for theUK,” pumps,” comparison to fieldtrialdata,” of variable renewable electricity,” 70, pp. 342–357,2017. vol. 5,no. 11,pp. 9291–9306,2012. Council for anEnergy EfficientEconomy SummerStudy Residential Energy ConsumptionSurvey (RECS) [email protected] Energy andBuildings 1440 Broadway, Suite 205 Energy, PSE Healthy Energy Oakland, CA94612 vol. 39,no. 1,pp. 205–217,2012. 2 Contact emissionsfrom theresidential , vol. 41,no. 6,pp. 587–595,2009. Energy andBuildings Renewable andSustainable Energy Reviews www.psehealthyenergy.org . U. S.Energy Information Administration (EIA),2009. , vol. 43,no. 1,pp. 239–245,2011. , 2015.

for market expansion needto beadopted. [14] energy performance standards, androadmaps financial incentives, utility programs, minimum agent problems, etc. —policiesinvolving energy indecision-making,principal- efficiency adoption —suchasupfront cost, low priority for market and behavioral barriers for heat pump complexities incolder climates. To overcome high initialcapital costs andaddedgrid systems. Some challenges remain, including lower heating costs over conventional heating systems, decarbonize theheating sector, and to effectively replace fossil fuel heating In summary: Renewable andSustainable Energy Reviews , vol. 45,pp. 785–807,2015. Heat pumpshave thepotential Energy &Environmental Science Future Energy Solutions,2005. roceedings oftheEuropean , vol. , vol. , 6