CUTTING INDUSTRIAL SOLAR SYSTEM COSTS IN HALF

Transcription

1 ESL-E CUTTNG NDUSTRAL SOLAR SYSTEM COSTS N HALF ABSTRACT R. C. Ness McQuay-Perfex nc. Staunton, Vrgna A. Wensten Wellngton Assocates Falls Church. Vrgna Whle there are techncal, socal, envronmental and nsttutonal barrers to the wdespread use of solar systems, the prncple barrer s economc. For commercal and ndustral frms to turn to ths alternate energy source, the frst cost must be sharply reduced so that the annual savngs that are achevable wll provde an attractve return on the ncremental nvestment. Ths paper dscusses one proven method of combnng the energy effcency of hgh temperature ndustral heat pumps wth solar collectors that result n an nstalled frst cost that approxmates one half of that of conventonal solar systems. Ths technology s now avalable for producng up to 220 F hot water for ndustral process heat, space heatng, and servce hot water heatng. The basc prncples of the technology are revewed, ncludng the typcal operatng characterstcs of the ndustral heat pumps and the solar collectors, plus the generc applcaton schematcs comparng ths approach wth conventonal solar collector only systems. Several case hstores are revewed, ncludng an ndustral plant, townhouse project, and hosptal. Not only s a lower frst cost demonstrated, but the combnaton uses small solar arrays, deal where roof area s lmted, and use less expensve solar collectors. NTRODUCTON n recent years, many commercal and ndustral solar systems have been desgned, constructed, and demonstrated for a varety of applcatons. Although there are some techncal and nsttutonal problems remanng, the prncple barrer to the wdespread use of solar energy contnues to be economc. Wth the Federal tax credt and accelerated deprecaton, solar water and space heatng can be economcally compettve when judged on the bass of three crtera (payback perod, years to recover down payment, and years to postve cash flow) only when compared to electrcty. but not compettve aganst current prces of gas or ol. n most cases. Snce the embargo. fossl fuel prces have ncreased much faster than those of electrc power. A crystal ball rather than a computer mght be more approprate to predct future energy costs. However. t can be expected that wth government plans to decontrol gas, more ncreases wll occur. The mpact on electrc rates s stll not expe~ted to be as great due to the utltes n- creased use of coal. actons to mprove load factors. and the effect of mbedded utlty nvestment. As gas and ol prces escalate. solar systems wll become more compettve. The reducton of solar system costs s also necessary to advance the tme when that occurs. Heat pumps can help. The combnaton of solar collectors wth heat pumps has been demonstrated n many applcatons as techncally feasble. For resdental syste!ms where ar-to-ar heat pumps are most commonly avalable and the ambent ar n addton to solar s a source of "free energy", t s shown n ref. (1) that a parallel system "s probably the most practcal solar-heat pump confguraton". For commercal and ndustral systems whch nvolve hgher delvery temperatures to the load. hgher performance and much hgher capacty water-towater heat pumps. a seres system s more practcal. The Solar-Asssted-Templfer, a hgh temperature heat pump dscussed n ref. (2). s such a system. t s sutable for commercal and ndustral space heatng and process hot water ap plcaton. THE SOLAR-ASSSTED-TEMPLFER CONCEPT The concept of a solar-asssted heat pump (ref. 2), whch s not new. s based on the recognton that t s sgnfcantly more effcent to collect solar energy at lower temperatures rangng from about 40-l20 F and then boost t to the requred temperature wth a heat pump. Most conventonal water-to-water heat pumps are lmted to a max1mum output temperature of about 120 F. Many applcatons, however, requre much hgher temperatures for process hot water, space heatng, lau~dres. servce hot water. etc. The Templfer Heat Pump (Temperature Amplfer) makes t possble to delver hot water at temperatures as hgh as 220 F. Producton unts are avalable n capactes from 100,000 Btu/hr to over 20,000,000 Btu/hr n a sngle unt. Recprocatng compressors are used below about Btu/hr and centrfugal compressors for larger eapacty unts. Dependng on the source water nlet temperature, the outlet hot water temperature can be as hgh as 220 F. The coeffcent of performance (C.O.P.) s a functon of the temperature lft as dsplayed n Fg. 1. For example. f the source water temperature s 9SoF, cooled to 8SoF n the Templfer. and an 669 Proceedngs from the Fourth ndustral Energy Technology Conference, Houston, TX, Aprl 4-7, 1982

2 outlet temperature of 140 F s requred, the C.O.P. would be 4. That s, for every Btu of equvalent electrc energy requred to drve the compressor, 4 Btu s delvered to the load. f an outlet temperature of 180 F s requred, the C.O.P. would be almost 3. Ths does not volate the frst law of thermodynamcs, snce the three "free" Btu can be obtaned ether from "waste" heat or solar collectors. ESL-E The capablty to use relatvely low temlerature water allows consderaton of lower per ormance and lower cost solar collectors. Fg.,2 shows performance curves for varous collecto~s and ther respectve approxmate costs. Notejthat at low Tl, the lowest cost collector has: hgher performance than the hghest cost collectdr. The estmated unnstalled 1979 collector co~ts per square foot, and the estmated total nstalled systems costs per square foot for each ype of collector are lsted n Table 1. TABLE 1 Unnstalled Cost Collector # ($/Ft 2) [ a 0 Source Water Outlet Temperature of Fg. 1. Templfer Heat Pump Performance A source temperature of 95 F can be obtaned wth hgh effcency from flat plate collectors as ndcated n the typcal Hottel-Whtter curve, Fg. 2. Under condtons of 50 F ambent and 250 Btu/ft 2 nsolaton, 50% collector effcency can be acheved even wth low performance, low cost collectors. f 180 F s requred by the load, the Templfer must then rase ths 95 F water from the collectors to 180 F (a temperature lft of 85 F) under whch condton, referrng to Fg. 1, the C.O.P. would be 3. Under hgher ambent condtons the collectors could delver temperatures of 120 F to the Templfer. Under such crcumstances the temperature lft would be only 60 F and the C.O.P. would ncrease to 3.6. Thus, lower collector effcences tend to be balanced by hgher Templfer C.O.P. 'so l ~ ~ O. 01 Flat P.r.. BlKk Chrome. 2 Co'l8fs. U3ltt'" nsl (,11 o 03 U, 0' Flat P!.le. 8lc1l. Chrome. 1 eo..,." '040m" nst (12) All "'-e. Povorooylene. t Cover. 530'''''' ns '.31 Fg. 2. Performance and Average nstalled Costs of Four Types of Lqud Solar Collectors As a consequence of beng able to operate!the solar collectors at hgher effcences when used as a source for the Solar-Asssted-Templfter, the total collector area requred to meet a specfed load can be sgnfcantly reduced wth a concomtant cost reducton. The amount of reduct~n s a functon of load requrements, ambent con4tons, nsolaton and alternatve energy costs. Every applcaton wll dffer and wll depen4 on the locaton. SOLAR-ASSSTED-TEMPLFER VS. SOLAR-ONLY TECHNCAL ANALYSS System Descrpton The Solar-Asssted-Templfer (S-A-T) ho~ water system s a combnaton of a solar energy,collecton system wth a Templfer heat pump. Trey are pped together physcally and operatonally. When solar energy s avalable, t s stored and used to satsfy the heatng load. Durng!xtended cloudy weather, the S-A-T system s shut d wn and the heatng load satsfed by a convento al system, n the same manner as n a solar-only ystem. The Templfer smply uses low temperaturl (75 F to 100 F) solar heat, collected at muc hgher collector effcences than n typcal so ar-only closed-loop systems, and amplfes t t hgher temperature levels. The heat from the solar source water s absorbed n the heat pump ~vaporator by the unt's workng flud, whch s then ncreased n both temperature and pressur~ by the compressor. From there t goes to the condenser where the heat s transferred to a delvefy flud to provde useful heat at temperatures up to 220 F. Performance Analyss A comparatve analyss between a typcal Solar-Asssted-Templfer (S-A-T) system and a conventonal solar system llustrates the dramatc frst cost savngs of the S-A-T system. 670 Proceedngs from the Fourth ndustral Energy Technology Conference, Houston, TX, Aprl 4-7, 1982

3 ESL-E Fgure 3 s a smplfed schematc of an S-A-T system and a closed-loop solar-only system, both desgned to delver 180 F hot water for ndustral process and other uses. Flat Plale 1Cover Solar Asssted Templfer (SAT) Storage Pae Flal@ Pr 950 (\ 0 0: llloof ;---' F ~ Solar Only PL Templfer 2 Covers.. ===:;~~~::_._ 1800F F Ps Storage Fgure 3 Closed Loop System Comparson The prncple advantage of usng a Templfer n a closed-loop system s that the return lne froln the evaporator provdes a contnuous cool water source (65 F to 90 F), allowng the collectors to operate wth hgh effcency at low temperatures. A solar-only closed-loop system must return temperatures up to 170 F, forcng the collectors to operate at hgh temperatures wth reduced effcency. The followng desgn condtons are assumed for comparson purposes: Desgn Load: 425,000 Btu/hr Operatng Perod: 16 hr/day; 264 days/yr Output Hot Water Temperature: 180 F Return Water Temperature: 170 F Thermal Storage: 8 hours For the Solar-Asssted-Templfer, a flat plate collector wth a sngle cover s adequate, snc.e temperatures of only 95 F are suffcent for use as the solar source nput. However, for the solar-only system t s necessary to use two covers n order to acheve the hgher output temperature of 180 F. A computer analyss of three load temperature cases yelds the strkng results 2. shown n Table TYPCAL NSTALLATONS A number of projects have been successfully nstalled and operated usng the S-A-T concept appled wth varous types of collectors. The followng dscusson revews several of these nstallatons. ndustral Plant n the manufacture of extruded alumnum parts, there are several areas that requre sgnfcant energy nput: the extruson press, heat-treatng oven, bllet heatng, and the anodzng lnes. General Extruson, nc. (GEl), Youngstown, Oho (Ref. 4) was faced, along wth everyone else, wth sharply ncreased costs of energy -- both n the prce of natural gas and n electrc rates. A project was started to fnd an alternate energy source that could be talored to ft ther needs. Ano,llzng lnes requre energy n two forms electrcty for the dc power requred for the anodzng process, and gas for drect-fred equpment needed to heat large volumes of water n tanks use;l for cleanng and colorng parts. Solarheated water seemed to offer the most lkely avenue for energy savngs. Beng n the alumnum ndustry, GEl decded to produce ther own collectors: smplcty of constructon and mantenance was to be stressed. The result was an acrylc-faced alumnum box contan- ng fve rows of half-parabolc reflector~-that concentrate the sunlght on alumnum-fnned tub- ng. TABLE 2 - CLOSED-LOOP SOLAR SYSTEM COMPARSON Load Temperature - OF S-A-T Collector Area Requred 4,700 ft 2 4,000 ft 2 4,500 ft 2 Solar-0nly Collector Area requred 8,000 ft 2 9,000 ft 2 11,500 ft 2 Reducton n Collector Areal 41% 49% 61% Potental Frst Cost Savng $179,000 $230,300 $337,000 ncreased annual electrc energy cost of S-A-T system over Solar-Only at,3c/kwh $3,300 $3,900 $4,700 c 1 Based on an assumed solar collecton system 1979 nstalled cost ncludng collectors, supports, storage, ppng and nsulaton of: S-A-T system collector and Solar-only system usng double cover flat usng sngle cover flat plate plate collector: $43/sq.ft. (To adjust ncludng the nstalled cost from the ahove 1979 prces to 1982 prces, of the Templfer and ts assocated pumps all fgures should be ncreased by approxand ppng: $35/sq.ft. rna tely 30%). 671 Proceedngs from the Fourth ndustral Energy Technology Conference, Houston, TX, Aprl 4-7, 1982

4 As wth m05t ndustral applcatons, lack of space rather than lack of use for the output lmted the amount of solar energy that could be used at GEL An array of 100 panels were nstalled on the roof of the plant. A smplfed dagram of the system s shown n Fg. 4. As can be seen, an ndustral solar applcaton nvolves more equpment than would be requred for water or space heatng n a home or commercal buldng. Frst of all, corroson s a greater problem; the atmosphere s more hostle, and the fluds handled are rarely just pure water. n the case of GEl, the tank chosen to be heated by solar energy holds 3500 gallons of soluton used for degreasng parts pror to anodzng. The flud s ether alkalne or acd, dependng on the product run. Ths medlum cannot be passed drectly through the solar VB ESL-E There are fve operatng modes n the GEl n~tallaton (see Fg. 4). n the frst, flud f~om the alkalne cleanng tank s heated drectly n the solar heat exchanger. n the second mode, rnse tank water s heated by the solar panels and used n the heat pump. Alkalne cleann~ tank water s heated n the alkalne heat exchanger. Modej 3 s smlar to mode 2, but the rnse tankls not ncluded. Ths confguraton s used whln the output temperature of the solar collectors s not hgh enough to heat the alkalne cleann tnk water to the requred temperature. Mode ~ could typcally be used when sunlght s no long~r drected at the solar array and the solar flud pump s "hut off. The heat stored n the rnse t~nk n mode 2 s used n the heat pump to heat the alkalne soluton. n the ffth mode, the! solar panels are used only to heat the rnse w ter n cases where there s no requrement for heat ng of the alkalne flud. Wth an ndustral heat pump and ts use 0 electrcty made the explotaton of solar teclnology n ths ndustral stuaton possble. The result was a reducton n the use of a dmnshng resource, natural gas -- gas that could be used to replace mported ol n power plants or for hollle heatng., O;~ Solar collector "\.: \ Fg. 4 Solar flud pump Smplfed Dagram of Solar System at General Extruson n~. panel tubng; a heat exchanger s requred. To mn mze lroblems assocatell w th crcula t n3 aqueous solutons, t was decded to use a lght ol as the heat transfer medum from the solar panels. The array 1:; capable of producng over 500,000 Btu per hour (146 kw). Thus, the transfer ol can be heated drectly at the panels to 190"F to acheve a soluton temperature of 170 F at the alkalne cleanng tank. But what happens when clouds [lass by? And where can excess heat be stored so that the use of the drect-fred equpment would not be requred mmedately when the sun's rays are no longer accepted by the reflectors? These consderatons led to the nstallaton of the hghtemperature, one-drecton ndustral heat pump, and the use of water n a rnse tank as a storage reservor. Townhouse Project Usually, solar nstallatons aren't ver well suted for northern clmates. n Cana a, for nstance, where wnters are severe and solar nsolaton s lmted, the drect use of sol'lr energy requ res large collectors and larse thermal storage capactes to meet any sgnfcant porton of the annual heatng load. Even f roofs were bg enough to carry the large number of panfls, the cost woulj be prohbtve. 0 The sun needs a lttle help. So, the owner! (Ref. 6) nstalled a lqud solar system asss ed by a heat pump to supply energy to 19 townho ses at Mnto Management's Heron Gate project n otlawa. For heatng hot water by solar energy, t~e heatpump-asssted solar system s really the ollly one feasble. t essent~a~ly d?ubles the solar systeon's capacty -- prov~d~ng tw~ce as many ll~u' s of heat. n ths case, 120 panels wth a total area of 2300 square feet provde the energy needed. The Heron Gate project conssts of mddle' ncome homes that have been occuped for about l years. t was deal for retrof ttng wth a solar system because t s desgned nto quadrants two blocks of 19 townhouse unts and two blocks, of 21 unts. Fan col heatng unts (heat exchanger and blower) transmt heat to the ndvdual townhouses. Proceedngs from the Fourth ndustral Energy Technology Conference, Houston, TX, Aprl 4-7,

5 ESL-E Solar collectors on top of- Heron Gate's The Templfer heal pump. The Heron Gate Project n Ottawa, Canada. townhouses. n wnter, the solar system provdes 40 percent of the space heatng and all of the hot water for the 19 unts. n the summer, overflow heat s used to heat all of the domestc hot water n two addtonal blocks, or 42 more townhouses. Snce the fourth quadrant (and ts heatng system) s dentcal to the frst, t's beng used to compare heatng costs and determne the savngs n energy. The ablty to use that extra heat n the summer gets around one of the bggest problems solar systems encounter n Canada and other areas north of the "sunbe1t" -- the collector space has to be so large to heat a dwellng n wnter that t presents a nusance problem durng hot months when the excess heat must be rejected or used. Fg. 5 Energy Flow Dagram Energy Flow Dagram of Heron Gate The solar system uses a Temp1fer hgh temperature heat pump to help ncrease ts effcency. Ths heat pump has a capacty of 230,000 Btu's per hour and a coeffcent of performance (C.O.P.) of 3.0. The system s really qute smple. The energy flow s dagramed n Fg. 5. Heat collected from solar energy s used to replace heat that would normally be suppled by the exstng hot water boler. The heat pump ncreases the temperature of the solar heat and makes collecton more effcent. The solar collectors consst of a flat copper plate (wth passages for hot water) encased n an alumnum frame, nsulated n back, and has a tempered glass panel n front. The heated water, whch vares from 70 F to 100 F, dependng on the tme of day, s then delvered to the storage tank. The Temp1fer permts the system to take the solar-heated water from the storage tank at 90 F and boosts t up to 180 F. The 7000-ga110n storage tank, located n the underground garage, s used to store the solar energy untl t s needed, smlar to a domestc hot water tank. t stores one or two days worth of thermal energy. The Temp1fer heat pump transfers or recovers heat from the low temperature storage tank to feed the hgh temperature hot water supply system. Of course, there s an energy cost, but the electrcal energy requred to accomplsh ths s only about 1/3 to 1/4 the amount of the useful heat energy whch s transferred. rhus, f the heat pump supples 230,000 Btu's per hour of heat at 180 F, t wll requre less than 78,000 Btu's per hour (about 23 klowatts) of electrcal energy. The hot water system for space heatng and dome& tc hot water already exsted wth the heat sup pled by a central hot water boler. ndvdual unts are heated by forced ar radators suppled wth hot water from ths central bouer. The so 1ar heat s added to the exstng system by smply passng the cooler return water through the hot sde of the heat pump. The return water tempera- ture s ncreased; and f there s suffcent so- 1ar heat avalable, the boler sn't needed. The domestc hot water s heated from the same water source as the space heatng equpment. hot Usng ths smple system, approxmately 40 percent of the energy prevously suppled by natural gas s now suppled by solar energy. Hosptal Project Mercy Hosptal n Pttsburgh has a 617 bed capacty and consequently, a large hot water need, normally heated wth gas. Concerned wth the rapdly escalatng fuel costs, the hosptal admnstraton decded to explore the possblty of retrofttng a solar system to heat the hot water, anrl appled to the Health Resources Admnstraton for a grant n response to that organzaton's request for proposals (Ref. 5). 673 Proceedngs from the Fourth ndustral Energy Technology Conference, Houston, TX, Aprl 4-7, 1982

6 ESL-E Collectors 110' F 40'F 40' 260Gpm "Buffer Tank" Hot Water Servce Fg. 6 Solar Asssted Heat Pump System at Mercy Hosptal, Pttsburgh Exstng Storage Tanks TABLE 3 Cost Estmates for Conventonal Solar System ntally, a conventonal solar system (wthout a Tank and assocated excavaton $ 50, heat pump) was desgned for Mercy Hosptal. t Collectors (@$17sq. t. delvered) 208, was calculated that 12,200 square feet of medum Truss Work (@$10sq.t.1 122, Pumps, ppng, nsulaton, heat exchanger temperature solar collector feedng nto a 21,000 and controls Archtectural cosmetc work , gallon storage tank was requred to provde 30% of the hosptal's annual hot water need. These col Structural framng for truss mountng , lectors would have requred the use of the roofs Mscellaneous equpment ncludng electrcal ' of two buldngs (Fg. 7) separated from each Roofng repar - materal and labor All other labor Crane rental and setup 20,000. other by 300 feet and four stores n heght. The cost of ppng up and between the two buldngs, Engneerng, admnstraton, deresl, draftng, etc as well as the aesthetc problems such an approach Tltal $ created, resulted n cost estmates of $980,000 as lsted n Table 3, Whch were consdered exces TABLE 4 sve. Had such costs been necessary, the project Solar Asssted Templfer Heat Pump - Cost Estmates would not have proceeded. nstead, a solar assst Collectors and truss work (@ $15/sq.lt. de~veredj ed heat puwp system, Fgs. 8, 9, 10, usng the $105, Templler@HeatPumplnstaledl Templfer heat pump, was consdered as an al Pumps, ppng, heat exchanger. controls and tank 90, ternatve. Such a system, desgned to perform the Roofng repar and truss framng 64, same functon, proved to be very much more accep Mscellaneous equpment ncludng electrcal l table for many reasons. n partcular, t de All other labor 25.ooo.0l creased collector area to 6600 square feet. These Crane rental and set up 10,ooo':a- Engneerng, admnstraton, clercal, draftng, etc could be nstalled on one roof and greatly reduce Total $ the supply and return ppng. n addton, t allowed the use of low cost polypropelenefberglass solar collectors wth prefabrcated alumnum supportng racks. Even wth the addtonal cost of the Templfer heat pump, the total system nstalled cost was reduced to $504,000, about one-half the non-heat pump system, as lsted n Table 4. Much of the costs are a consequence of the retroft nature of the project; on a new constructon bass, they would be substantally reduced. Fg. 7 Mercy Hosptal Buldngs (Arrow ponts to roof needed for solar only system collectors) 674 Proceedngs from the Fourth ndustral Energy Technology Conference, Houston, TX, Aprl 4-7, 1982

7 ESL-E The solar system heats up to 70 gallons per mnute of cty fed water to the requred temperature of l10 D F. Each year t's savng 2.8 bllon Btu of energy, equvalent to burnng over 20,000 gallons of ol. The annual electrc costs for operatng the Templfer Heat Pump are about $7,000, whch s small compared to the $484,000 nvestment cost saved through the use of ths heat pump system as compared to a conventonal solar system wthout a heat pump (the nterest on ths savng would be about $50,000 per year). The payback tme for fuel savngs to equal total nvestment was calculated to be 13 years. n vew of ol and gas deregulaton, the payback tme s expected to be sgnfcantly reduced. SUMMARY For the applcaton and typcal nstallatons examned, compared to solar-only systems, the Solar Asssted-Templfer: Requres about half the collector area to meet the same fracton of the load. Reduces ntal nvestment 30-60%. Reduces solar system nstalled costs per mllon Btu up to 60%. Reduces years to postve cash flow. Reduces payback perod by several years. Sgnfcantly ncreases return on nvestment. n several cases the S-A-T system s a cost effectve system today, whereas the conventon- al solar-only system s not. l: Can be used as a conventonal chller, further reducng solar costs. Normal Operatng Range Net Aperature Area Gross Area REFERENCES Fg at Solar Collector Thermal Performance (1) Freeman, T. L., Mtchell, J. W. and Audt, T. E. "Performance of Combned Solar-Heat Pump Systems", Solar Energy, Vol. 22, No.2, (1979) Page (2) Wensten, A., Duncan, R. T., Ness, R. C., Van Zuden, G. "Sawng Solar Costs n Half", Solar Heatng and Coolng, Vol. 3, No.5, October 78, Page (3) Wensten, A., Duncan, R. T., Ness, R. C., Van Zuden, G. "Applyng Heat Pump Engneerng to ndustral Hot Water Needs", Solar Engneerng, March 1979, Page (4) Berutt, A. "Savng Energy by Usng Electrcty", Electrcal Constructon and Mantenance, June (5) Wensten, A., Ness, R. C., Sero, S. "Mercy Hosptal Hot Water Heat Pump System", paper presented at 1981 Annual Meetng, Amercan Secton of the nternatonal Solar Energy Socety, Phladelpha, PA, May 28, Fg. 10 Mercy Hosptal Heat Pump (6) "Solar Heat Pumps: Soluton for Cold Clmates", Electrc Comfort Condtonng News, June Proceedngs from the Fourth ndustral Energy Technology Conference, Houston, TX, Aprl 4-7, 1982