The Thermal Spectrum Of Low-Temperature Energy Use In The United States
PROCEEDINGS, Thirty-Sixth Workshop on Geothermal Reservoir EngineeringStanford University, Stanford, California, January 31 - February 2, 2011SGP-TR-191THE THERMAL SPECTRUM OF LOW-TEMPERATURE ENERGY USE IN THE UNITEDSTATESDon B. Fox1, Daniel Sutter1, 2, and Jefferson W. Tester1,*1Atkinson Center for a Sustainable Future and the Cornell Energy InstituteCornell UniversityIthaca, NY 14853, USA.2Institute of Process Engineering, ETH ZurichSonneggstrasse 3, 8092 Zurich, Switzerland.*Corresponding author: jwt54@cornell.eduABSTRACTA detailed analysis of the U.S. energy consumptionwas performed as a function of its utilizationtemperature from 0 to 260 C. The study highlightsthe changes that have occurred in U.S. energy usesince the 1970s and suggests how renewable energycould provide a large fraction of energy now used fordirect use at low end-use temperatures that iscurrently mostly supplied by high grade fossil fuels.For example, most of the energy we need for waterand space heating used at low temperatures isprovided by combusting natural gas and oil at c potential of the fossil fuels forgenerating power resulting in large reduction of itsexergy or availability. By focusing attention on thethermodynamic losses inherent to our current energysystem, we suggest a paradigm shift in the way weview and use energy by strategically matching thesource providing the energy to the end-usetemperature of the application. Thermal energy below260oC could be supplied more sustainably withoutlarge exergetic or availability losses by geothermal orsolar thermal energy resources, or by waste heat. Inaddition, direct thermal use of geothermal energyalso has economic advantages over using it togenerate electricity by avoiding the inherently large2nd Law losses in converting geothermal energy toelectricity.Using the U.S. Energy Information Administrationdatabase as a primary source of information, wefound that the total thermal energy from 0 to 260 Cused in 2008 was 33.5 EJ (31.7 quads), about onethird of the entire U.S. demand. More than half of thethermal energy demand below 260 C (55%) comesfrom the residential sector, while the rest comes fromthe industrial (24%) and commercial (21%) sectors.Also quite importantly, almost 80% of 33.5 EJ isused to provide heat below 150 C. Space heating andwater heating have end-use temperatures of 40 to60 C and are responsible for 38% of the thermalenergy consumption below 260 C in the residentialand commercial sectors.BACKGROUND AND MOTIVATIONIn 2008, the U.S. used 104.8 EJ (99.3 quadrillionBtu) of primary energy, of which 84% came fromfossil fuels as reported by the U.S. EnergyInformation Administration (2009). The primaryenergy demand can generally be divided into demandfor mechanical drive and demand for thermal energy.About 40% of the total primary energy is consumedfor electricity generation (U.S. Energy InformationAdministration, 2009). Electricity, in turn, alsoprovides thermal or mechanical energy for specificend-uses. The primary energy demand for the U.S.can be divided into four demand sectors, residential,commercial, industrial, and transportation.Fossil fuels have remarkable properties as energycarriers, such as high specific energy (e.g. lowerheating value of gasoline is 43 MJ/kg) and highcombustion temperatures ranging from 1000 to2500 C, depending on fuel-air composition,combustor design, and other factors. Thesecharacteristics enable fossil fuels to meet extremeenergy demands of sophisticated machines such as jetengines and gas turbines and allow for highlyefficient energy conversion. However, many of theend-uses currently powered by fossil fuels do notnecessarily require these characteristics and the fossilfuel energy is downgraded to meet their demands,imposing exergy/availability losses. Exergy, oravailability, is the maximum work-producingpotential of an energy source. The combustiontemperatures of fossil fuels are too high for variousthermal end-uses. In U.S. residential buildings,93.5% of the energy used for space heating isprovided by natural gas, fuel oil, liquefied petroleum
gas, and kerosene (U.S. Energy InformationAdministration, 2009). A logical energy use schemewould close the gap between source and process enduse temperature and would reserve the high gradefossil energy sources for combustion processes thatrequire high temperatures. Correspondingly, a moreefficient energy utilization approach would be totailor the energy source used to the temperature needsof the process.The International Energy Agency (2007) reports, thatelectricity production has received most of theattention with regards to the use of renewable energy.As a result, policies aimed at encouraging directthermal use of renewables have not developed to thesame extent as policies for electricity generation. Inorder to evaluate the potential of the direct thermaluse of renewable energy, the actual demand for lowtemperature thermal energy as a function of processtemperature needs to be characterized. Byquantifying the thermal energy demand spectrumwith respect to required supply temperature, it ispossible to determine the potential market for lowtemperature thermal energy. With the rightinfrastructure, energy sources like geothermal, solar,and waste heat could provide a significant portion ofthe low temperature thermal demand for the U.S.These energy sources would be more valuable fromboth a sustainability and economic perspective whenused for direct thermal applications rather than incurthe losses of thermodynamically upgrading them toelectricity.Literature in the field of direct thermal energy usefrom renewable sources includes the famous Líndaldiagram shown in Figure 1. It presents a number ofpossible applications for direct thermal use in Icelandalong with typical end-use temperatures. The idea ofthe Líndal diagram has been taken up by otherauthors, such as Kalogirou (2003), who shows avariation with a focus on solar thermal rather thangeothermal. The Líndal diagram illustrates theopportunity but does not quantify the potential ofdirect use, since it does not document how muchthermal energy is used for the mentioned processes.The worldwide review of direct thermal applicationof geothermal energy (Lund et al., 2005) quantifiescurrent worldwide use, but again, misses the existingpotential. Vannoni et al. (2008) studied the potentialfor thermal energy from renewable sources but didnot specify the exact temperatures required by thepotential end-uses. Philibert (2006) reports that in2004, 40-50% of the total world-wide energy demandin residential, commercial, and industrial demandsectors was used for heating and cooling (as cited inInternational Energy Agency, 2007). Thus, there is aneed to not only quantify the thermal demand, butalso the demand temperatures to better understandhow much of the current demand can be replaced byrenewables such as geothermal and solar thermal.Thirty-five years ago, Reistad (1975) quantified thelow temperature thermal energy consumption andcorrelated it with end-use temperatures. Thisimportant study quantified the situation for the U.S.based on 1968 energy consumption data. Much haschanged with respect to energy demand in the last 42years due to an increase in population as well on, and housing sectors. In addition, enduse or process temperatures might have changed dueto technological development. A primary motivationfor our work was to update the situation as it nowexists in the United States.Figure 1: Industrial and other possible applicationsfor direct thermal use of geothermalenergy suggested by Líndal (1973) (ascited in Armstead, 1978).METHODOLOGYThis study focuses on the temperature range 0 to260 C (0 to 500 F) as “low temperature” becauseresources at higher temperatures would be used for
electricity production to exploit the higher 2nd Lawefficiencies before supplying lower temperature heatfor direct thermal use. A 20 C temperature bin sizewas chosen to allow for coarse graining and toacknowledge that certain processes can beundertaken over a range of temperatures rather thanrequiring a specific temperature.The EIA issues an Annual Energy Review (AER) forthe United States that defines four demand sectors,i.e. residential, commercial, industrial, andtransportation sector and provides information abouttheir yearly energy consumption (U.S. EnergyInformationAdministration, 2009).Thetransportation sector was left aside because itsconsumption of energy is for mechanical drive. Forall four demand sectors, the EIA issues sectorspecific reports in a rotating basis such that a reportfor each sector is released every four years (U.S.Energy Information Administration, a, b, c).While the AER reports useful general information,only the sector specific consumption surveys givedetails about end-use specific energy demand.Consequently, both reports were used for each sector.Extrapolation was applied because the sector specificsurveys are not issued in the same years. 2008 wasselected as the base year because it is the year of themost recent AER.The generally followed procedure is to break theenergy consumption down to the end-uses in order toquantify their energy demand, determine theirthermal and non-thermal character, and estimate theirrequired end-use temperature based on reportedpractices. The relative fractions of energy demand forthe end-uses can then be extrapolated to the baseyear, for which the total consumption of the demandsectors is given in the AER. For more detailedinformation on the methodology including all madeassumptions and numerical values for the energyconsumption, interested readers are invited to look atthe Cornell Energy Institute report #1, ThermalEnergy Use in the United States from 1968 to 2008below 260 C (2011).Residential SectorSpace heating, appliances, water heating, and airconditioning are the end-uses in the residential sector.The appliances were carefully examined to onlyconsider the ones requiring thermal energy for theirend-use. The appliances with thermal end-uses werefound to be refrigerators, clothes dryers, freezers,range tops, dishwashers, ovens, microwaves, clotheswashers, pools/hot tubs/spa heaters, coffee makers,waterbed heaters, large heated aquariums, andhumidifiers. The main source of information aboutthe specific energy use of common appliances wasthe “end-use consumption of electricity” report ly, temperatures around and above 50 C(122 F) are used in space heating, but usingtemperatures as low as 40 C (104 F) is feasible(Bloomquist, 2003). Therefore, space heating isincluded in the 40 to 60 C (104 to 140 F) range.Although many U.S. households set their waterheater to a higher temperatures, the U.S. Departmentof Energy (1995) points out that a temperature of48.9 C (120 F) is sufficient as opposed to mostmanufacturers’ setting of 60 C (140 F). Therefore,the temperature category for water heating is chosento be 40 to 60 C (104 to 140 F). Dishwasher thermalenergy contribution was also placed in the same binbecause dishwashing draws its energy primarily fromthe household hot water heaters (U.S. Department ofEnergy, 1995).The temperature assigned to air conditioning andrefrigeration is based on the temperaturerequirements to run a sorption cooling system. Forresidential air conditioning, a heat source temperatureof 60 to 80 C (140 to 176 F) is considered to besufficient to operate such systems (Wang et al, 2009).Higher temperatures would allow achieving highercoefficients of performance (COP), which increasesthe economic competitiveness.The temperature range for refrigeration and freezerswere based on a review of Fan et. al (2007) and waschosen to be 100 to 120 C (212 to 248 F) and 120 to140 C (248 to 284 F), respectively. The temperaturefor the clothes drying end-use varies from 40 C to120 C for different literature sources (Conde 1997,Bansal et al. 2001, Han and Deng 2003, Ng and Deng2008, Yadav and Moon 2008, Bansal et al. 2010).The temperature range 80 to 100 C (176 to 212 F)was ultimately chosen.Energy requirements of pools/hot tubs/spa heaters,waterbed heaters, and large aquariums werecollectively binned at 20 to 40 C (68 to 104 F). Forcooking applications, the range tops and conventionalovens were collectively placed in the 240 to 260 Crange (464 to 500 F) while microwave oven energyconsumption was binned at 100 to 120 C (212 to248 F) to reflect the nature of usage of themicrowave ovens. Due to their need to boil water,coffee makers and humidifiers were placed in the 100to 120 C (212 to 248 F) bin.Commercial SectorThe energy consumption of the commercial buildingssector is made up of space heating, lighting, airconditioning, water heating, refrigeration, ventilation,cooking computers, office equipment, and otherappliances. Of the listed end-uses, spacing heating,air conditioning, water heating, refrigeration, andcooking were considered thermal end-uses, similar tothat of the residential sector. The end-use temperature
Figure 2: Breakdown of the industrial sector energy consumption. The industrial sector’s energy demand is brokendown into 84% and 16% for manufacturing and non-manufacturing in 2006, respectively. The boxesoutlined in red were excluded in quantifying low temperature thermal energy demand.designations for the five thermal end-uses are thesame as for the residential sector.Industrial SectorAccording to the AER (U.S. Energy InformationAdministration 2009) the industrial sector consists ofthe following types of activity, categorized in theNorth American Industrial Classification System(NAICS): agriculture, forestry, fishing and hunting(NAICS code 11), mining (21), construction (23) manufacturing (31-33)The EIA provides a specific survey with the detailedinformation needed for this study only for themanufacturing sector, i.e. NAICS codes 31-33 in itsManufacturing Energy Consumption Survey (MECS)(U.S. Energy Information Administration, c). For thethree most recent MECS of 1998, 2002 and 2006, themanufacturing sector covers 84 to 90% of theindustrial consumption.Thermal energy consumption in the other industrialactivities was estimated (Fox, Sutter, and Tester,2011) and found to be small compared to themanufacturing energy consumption. The aim of thisstudy is to provide a conservative estimate for thepotential market for low temperature thermal energy.Due to the lack of detailed EIA data, onlymanufacturing activities were considered. Figure 2shows a graphical representation of the breakdown ofthe industrial sector energy consumption to the enduses of interest. For the quantification of the thermalenergy demand, only process steam, process cooling,and HVAC are investigated.The temperature distribution was investigated for thefive subsectors with the highest energy demand forprocess steam: (324) Petroleum and Coal ProductsManufacturing, (325) Chemical Manufacturing, (322) Paper Manufacturing, (311) Food Manufacturing, and (331) Primary Metals Manufacturing.The above manufacturing sectors are responsible for76% of the total manufacturing fuel consumption andfor 87% of the process steam fuel demand. Theirweighted (by steam energy demand) averagedistribution of process steam temperature is assumedto be representative for the remaining 16manufacturing subsectors and used to quantify thethermal energy distribution of the remaining “other”manufacturing sectors. Because process steam isoften used at different temperature ranges, theprocess steam temperature distribution for each of thefive main manufactures was individually determinedusing the sources of Resource Dynamic Corporation(2002), Ozalp and Hyman (2007), Brown et. al.(1985), and Lund et. al. (1980).Quantifying process cooling and HVAC, as withprocess steam, were done by consulting of theMECS. The HVAC contribution was broken down in
space heating and air condition whose temperaturebin was chosen to be the same as the residentialsector. For process cooling, the temperature rangewas increased to reflect industrial refrigerationprocesses demand for lower cooling temperatures onaverage than refrigeration in homes. Therefore, theend-use temperature for industrial cooling andrefrigeration was increased to 120 to 140 Ccompared to 100 to 120 C for residentialrefrigeration.Electrical System Energy LossesIf one Joule of electricity consumption is reduced, notonly would the one Joule of electrical energy itself besaved, but the entire fuel energy that had to beinvested for generating the electricity. Hence, lossesin the electricity generation system in the U.S. werealso considered in this work. The AER accounts forlosses in the generation, transmission, anddistribution of electricity in a category calledElectrical System Energy Losses (ESEL) (U.S.Energy Information Administration, 2009). Thecategory Electricity Retail Sales (ERS) covers theelectricity sold to the ultimate customers by theutilities (U.S. Energy Information Administration,2009). The overall system efficiency,, can becalculated based on ERS and ESEL:(1)The overall system efficiency for 2008 was 31.67%.RESULTS AND DISCUSSIONSFigure 3 illustrates the share of the total lowtemperature thermal energy use among the threedemand sectors. It is important to note that this workcalculated the consumption of primary energy toprovide heat for thermal end-uses in the year 2008.Covering the energy demand of these end-uses byother energy sources, such as geothermal, solarthermal or waste heat would not necessarily requirethe same amount of primary energy. Hence, in orderto estimate the amount of thermal energy that wouldhave to be generated from renewable sources to coverall low temperature thermal energy demands, thetechnological implementation would have to beinvestigated in detail for each end-use. For example,for sorption cooling, the COP would be required. Thepresented estimates for the actual energyconsumption however represent a valuable estimateof how much conventional energy could be saved byswitching to renewable thermal energy sources.Graph (b) in Figure 3 includes the 11.4 EJ (10.8quads) of Electrical System Energy Losses (ESEL)related to the 5.27 EJ (5.00 quads) of Net Electricityincluded in Graph (A). This means a 51% increase ofthe total. The area of the disks in Figure 3 comparesproportionally to the total. The 33.5 EJ (31.7 quads)in (b) represent 32% of the total U.S. energyFigure 3: Total low temperature thermal energydemand and share of the three demandsectors residential, commercial, andindustrial. (a) does not consider ElectricalSystem Energy Losses, (b) includesElectrical System Energy Losses.consumption of 105 EJ (99.3 quads) in 2008 (U.S.Energy Information Administration, 2009). As shownin the figure, the share of energy demand for theindustrial sector drops when ESEL is consideredwhile the residential and commercial sectors increase.The change in the share is due to the residential andcommercial sector having more end-uses powered byelectricity than the industrial sector, whose lowtemperature thermal demand is comprised mostly ofprocess steam.The residential sector has the highest potential fordirect use of low temperature thermal energy but thedecentralized nature of the residential sectorcomplicates large scale geothermal, solar or wasteheat recovery projects. Investments are too high forsingle households while infrastructure needs, such asdistrict heating networks, are enormous. Therefore,local communities should be encouraged to pursuesuch solutions. Geothermal heat pumps and flat platesolar collectors present small scale alternatives forsingle houses. The thermal end-uses in thecommercial sector have very similar requirements tothe residential sector. Commercial buildings aregenerally larger than residential buildings and morecentralized, which should result in lower complexityof a heat supply infrastructure.The EIA data shows that the residential andcommercial sectors’ energy demand has beenincreasing following an almost linear trend in the past40 years, whereas the industry’s demand decreasedfrom its 1997 high to reach a similar level as that ofthe early 1990s and mid 1970s, in 2008. The lowtemperature thermal energy demand of the threesectors does not necessarily follow the exact sametrends because the relative fraction of energy spenton each end-use also changes with time. The trends,
however, underline the relative importance of theresidential and commercial sector.Nonetheless, thermal energy intensive industries maywant to consider direct use of thermal energy fromalternative sources to supply part of their demand. In2008, 25% of the total manufacturing industry’senergy demand, including ESEL, powered thermalend-uses below 260 C. Additionally, regions in theU.S. with moderate to high geothermal temperaturegradients might be able to attract thermal energyintensive industries to create tax revenue and jobs. Aprominent example for this effect is aluminumproduction in Iceland. The electricity-intenseeconomy grew due to the availability of cheapelectricity from geothermal power plants and now,bauxite is shipped to the country to be processed intoaluminum (Hreinsson, 2007). Although this examplerelies on cheap electricity and hence indirect use ofgeothermal resources, abundant thermal energy couldpromote a similar effect.Figure 4 relates the thermal energy demand with itsassociated end-use temperature range with 20 C widetemperature bins. The industrial sector process steamdemand is sub-divided into the five keymanufacturing sectors and “Other Manufacturing”.The end-uses with the largest contributions areannotated.By far the largest energy use is in the temperaturerange from 40 to 60 C, with space and water heatingas major contributors. When ESEL are not isconsidered, space heating accounts for a demand of8.46 EJ (8.02 quads). This corresponds to 38% of thetotal energy demand for low temperature thermal useand 8% of the total U.S. energy consumption in 2008.Water heating makes up 2.80 EJ (2.66 quads). Figure5 incorporate ESEL to the appropriate end-uses suchas air conditioning, refrigeration, and cooking.Interestingly, most of the thermal energyconsumption in the investigated temperature rangefrom 0 to 260 C occurs in the lower half of thetemperature range. Besides cooking, all residentialand commercial contributions require end-usetemperatures below 140 C. Industrial process steamand cooking are the only end-uses above 140 C.Figure 6 shows a continuous functionalFigure 4: Thermal energy use temperature distribution from 0 to 260 C without Electrical System Energy Losses.The end-uses with the largest contribution are annotated. The total thermal energy demand from 0 to260 C in 2008 was 22.1 EJ (20.9 quads). See Appendix A for tabulated values.
Figure 5: Thermal energy use temperature distribution from 0 to 260 C with Electrical System Energy Losses(ESEL). The end-uses with the largest contribution are annotated. The total thermal energy demand from0 to 260 C in 2008 was 33.5 EJ (31.7 quads). See Appendix B for tabulated values.approximation of the discrete data of Figure 5. Thefigure approximates Figure 5 if the resolution was setto an infinitesimal value as opposed to the discrete20 C bins. A piecewise cubic Hermite polynomial(PCHIP) was used by employing the built-in pchipfunction from MATLAB (The MathWorks Inc,USA). The values of Figure 5 were plotted at themean value of each temperature bin and then aPCHIP was applied to obtain the smooth graph. Thecontinuous representation takes into account that theend-use temperature of each process might varyslightly and the temperature ranges are not sharplyconfined but rather overlap and merge. The total areaunder the curve represents the total thermal energydemand. The dashed horizontal line indicates themaximum economic drilling depth of 6 km (Tester etal., 2005). The temperature at this depth of the threetemperature gradients shown in the figure would be135 C (20 C/km gradient), 255 C (40 C/km), and375 C (60 C/km). Even with the lowest gradient, thetemperature at 6 km would be sufficient to meet thedemands of some of the largest contributors: spaceand water heating, air conditioning, and refrigeration.The cumulative thermal energy use is shown inFigure 7 for both with and without ESEL. Theprogress of the demand within the 20 C temperaturebins was assumed to be linear. For example, the valueat 20 C is zero, and the value linearly increases tomatch the whole demand of the 20 to 40 Ctemperature bin at 40 C. The scale on the right showsthe proportion of the U.S. total energy demandexcluding the transportation sector. Note that atemperature of 150 C would be sufficient to supplymore than 35% of the U.S. industrial, commercial,and residential energy demand. A temperature of150 C can be reached within the 6 km drilling depthlimit for most regions in the U.S. where averagegeothermal temperature gradients are higher than22 C/km.An alternative technology that has the capability tocover space heating, water heating, and cooling isgeothermal groundsource heat pumps (GHP).Typically, GHP have a coefficient of performance(COP) of 4 or more, meaning that 4 units of thermalenergy are transferred for every 1 unit of electricalenergy (Tester et al., 2005). Hence, providingresidential and commercial space heating, waterheating, and air conditioning demands by GHPsystems would replace significant amounts of fossilfuel use, but increase electricity consumption. Theproposed direct use of thermal energy in otherapplications would help to free up existing electricalcapacity and thus avoid that additional capacity hadto be built.
Demanding thermal energy at different temperaturesdoes not necessarily require multiple thermal energysources at different temperatures. A single thermalenergy source can supply end-uses at different, lowertemperatures in a cascaded heat system. Armsteadand Tester (1987) illustrate the general idea of such asystem and show how a source of thermal energy at agiven temperature would supply all of the thermalenergy demand of a hypothetical market along withelectricity production. They demonstrate theimportance of knowing the thermal energy demand ofa given market to effectively plan an energy usescheme. The same approach can be applied to theU.S. heat market. According to Figure 5 theresidential and commercial thermal energy demandfor space heating, water heating, and air conditioningmakes up 17.4 EJ (16.5 quads). A cascaded districtheating system would be able to serve all thesedemands, and at the same time provide electricitypowering applications whose energy use cannot bedisplaced with thermal energy (lighting, computers,etc.). Consider a heat source at 200 C. The workingfluid would first be introduced to a turbine togenerate electricity. To enable air conditioning in asorption cooling system, the fluid should leave theturbine at about 80 C, resulting in a Carnot efficiencyof ηc 0.25. The exhausted fluid would be used forcooling via a sorption cycle and could then cascadefurther down to provide any hot water needs.Depending on the seasonal need, the waste heat frompower production could also be used for spaceheating. The remaining energy left could be used forsnow melting, soil warming, or other end-uses below40 C. Similar cascaded systems have already beenimplemented, specifically the geothermal power plantin Neustadt-Glewe, Germany, which has an electricand thermal capacity of 230 kWe and 6MWt,respectively (Lund et al., 2005). Heat systems canalso be cascaded bottom up, i.e. for preheating ofcertain processes. Cascaded heat systems generallyminimize the earlier criticized gap between thesource of heat and the process temperature.As discussed in the industrial sector methodology,byproducts are used as fuel to help improve theeconomics of a process by reducing the amount ofimported energy. One might argue that this byproductuse should not be considered in the potential for thediscussed renewable, low temperature thermal energysources, because the byproducts should not bewasted. However, these byproducts could be used tomeet other energy requirements, especially those thatFigure 6: Continuous approximation of the energy demand distribution density with Electrical System EnergyLosses (ESEL). The energy demand is normalized by the temperature, i.e. the total area under the curverepresents the total thermal energy demand. The scale on the right-hand side indicates the depth neededto achieve the corresponding temperature for three different temperature gradients (20, 40, 60 C/km).The dashed horizontal line presents the current maximum economic drilling depth.
Figure 7: Cumulative thermal energy demand with and without Electrical System Energy Losses (ESEL).cannot be displaced by direct thermal energy use,such as electricity generation, machine drive, ordirect fired processes. Transportation fuel productionis another option for byproducts that becomeabundant, as their former thermal end-use is met bythe mentioned renewable sources. Forest industrybyproducts from paper manufacturing, for example,could be used to generate biofuels. The goal shouldbe to shift the byproduct use to applications thatmake use of their relatively high exergy andcombustion temperatures to generate electricity orpower mechanical drives as well as process heat incascaded or co-generation-systems, and thus,implement a more efficientenergy use scheme.CONCLUSIONS AND OUTLOOKWe evaluated the U.S. thermal energy consumptionover a range of temperatures from 0 to 260 C. Theresulting estimate for the total thermal energydemand below 260 C in 2008 is 33.5 EJ (31.7 quads)when electrical system energy losses are included.More than half of the demand (55%) is from theresidential sector. Detailed information about thedistribution of this energy consumption with end-usetemperatures is provided. Almost 80% of the total isused to p
thermal energy is used for the mentioned processes. The worldwide review of direct thermal application of geothermal energy (Lund et al., 2005) quantifies current worldwide use, but again, misses the existing potential. Vannoni et al. (2008) studied the potential for thermal energy from renewable sources but did
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