Solar thermal power plants Technology Technology Fundamentals published in Renewable Energy World 06/2003 pp !0"#!!3 $any $any peop people le asso associ ciat atee sola solarr elec electr tric icit ity y gene genera rati tion on dire direct ctly ly with with photo%oltaics and not with solar thermal power &et &et large' commercial' concentrating solar thermal power plants ha%e been generating electricity at reasonable costs (or more than !) years *ol+er ,uaschning describes the basics o( the most important types o( solar thermal power plants
$ost $ost techni techni-ues -ues (or generat generating ing electr electrici icity ty (rom (rom heat heat need need high high temper temperatu atures res to achie%e achie%e reason reasonabl ablee e((ic e((icien iencie cies s The output output temper temperatu atures res o( non#con non#concent centrat rating ing solar solar collec collector torss are limited to temperatures below 200. There(ore' concentrating systems must be used to produce higher temperatures ue to their high costs' lenses and burning glasses are not usually used (or large#scale power plants' and more cost#e((ecti%e alternati%es are used' including re(lecting concentrators The re(lector' which concentrates the sunlight to a (ocal line or (ocal point' has a parabolic shape1 such a re(lector must always be trac+ed n general terms' a distinction can be made between one#ais and two#ais trac+ing4 one#ais on e#ais trac+ing systems concentrate the sunlight onto an absorber tube in the (ocal line' while two#ais trac+ing systems do so onto a relati%ely small absorber sur(ace near the (ocal point 5see Figure !
F78RE ! oncentration o( sunlight using 5a parabolic trough collector 5b linear Fresnel collector 5c central recei%er system with dish collector and 5d central recei%er system with distributed re(lectors The theoretical maimum concentration (actor is 96'2!! t is (inite because the sun is not really a point radiation source The maimum theoretical concentration temperature that can be achie%ed is the sun:s sur(ace temperature o( ))00.1 i( the concentration ratio is lower' the maimum achie%able temperature decreases ;owe%er' real systems do not reach these theoretical maima This is because' on the one hand' it is not possible to build an absolutely eact system' and on the other' the technical systems which transport heat to the user also reduce the recei%er temperatures ( the heat trans(er process stops' though' the recei%er can reach critically high temperatures
power are installed in ali(ornia' and a plenty o( new plants are currently in the planning process in other locations The parabolic trough collector consists o( large cur%ed mirrors' which concentrate the sunlight by a (actor o( =0 or more to a (ocal line 600 metre long collector row' and a multitude o( parallel rows (orm the solar collector (ield The one#ais trac+ed collectors (ollow the sun The collector (ield can also be (ormed (rom %ery long rows o( parallel Fresnel collectors n the (ocal line o( these is a metal absorber tube' which is usually embedded in an e%acuated glass tube that reduces heat losses ? special high#temperature' resisti%e selecti%e coating additionally reduces radiation heat losses n the ali(ornian systems' thermo oil (lows through the absorber tube This tube heats up the oil to nearly 900.' and a heat echanger trans(ers the heat o( the thermal oil to a water steam cycle 5also called Ran+ine cycle ? (eedwater pump then puts the water under pressure Finally' an economi@er' %apori@er and superheater together produce superheated steam This steam epands in a two#stage turbine1 between the high#pressure and low#pressure parts o( this turbine is a reheater' which heats the steam again The turbine itsel( dri%es an electrical generator that con%erts the mechanical energy into electrical energy1 the condenser behind the turbine condenses the steam bac+ to water' which closes the cycle at the (eedwater pump t is also possible to produce superheated steam directly using solar collectors This ma+es the thermo oil unnecessary' and also reduces costs because the relati%ely epensi%e thermo oil and the heat echangers are no longer needed ;owe%er' direct solar steam generation is still in the prototype stage 7uaranteed apacity
n contrast to photo%oltaic systems' solar thermal power plants can guarantee capacity 5see Figure 2 uring periods o( bad weather or during the night' a parallel' (ossil (uel burner can produce steam1 this parallel burner can also be (ired by climate#compatible (uels such as biomass' or hydrogen produced by renewables With thermal storage' the solar thermal power plant can also generate electricity e%en i( there is no solar energy a%ailable
F78RE 2 Typical output o( a solar thermal power plant with two#hour thermal storage and bac+up heater to guarantee capacity ? pro%en (orm o( storage system operates with two tan+s The storage medium (or high# temperature heat storage is molten salt The ecess heat o( the solar collector (ield heats up the molten salt' which is pumped (rom the cold to the hot tan+ ( the solar collector (ield cannot produce enough heat to dri%e the turbine' the molten salt is pumped bac+ (rom the hot to the cold tan+' and heats up the heat trans(er (luid Figure 3 shows the principle o( the parabolic trough power plant with thermal storage
F78RE 3 Schematic o( a concentrated solar thermal trough power plant with thermal storage Trough
The e((iciency o( a solar thermal power plant is the product o( the collector e((iciency' (ield e((iciency and steam#cycle e((iciency The collector e((iciency depends on the angle o( incidence o( the sunlight and the temperature in the absorber tube' and can reach %alues up to A)B Field losses are usually below !0B ?ltogether' solar thermal trough power plants can reach annual e((iciencies o( about !)B1 the steam#cycle e((iciency o( about 3)B has the most signi(icant in(luence entral recei%er systems such as solar thermal tower plants can reach higher temperatures and there(ore achie%e higher e((iciencies Solar Thermal Tower
n solar thermal tower power plants' hundreds or e%en thousands o( large two#ais trac+ed mirrors are installed around a tower These slightly cur%ed mirrors are also called heliostats1 a computer calculates the ideal position (or each o( these' and a motor dri%e mo%es them into the sun The system must be %ery precise in order to ensure that sunlight is really (ocused on the top o( the tower t is here that the absorber is located' and this is heated up to temperatures o( !000. or more ;ot air or molten salt then transports the heat (rom the absorber to a steam generator1 superheated water steam is produced there' which dri%es a turbine and electrical generator' as described abo%e (or the parabolic trough power plants Cnly two types o( solar tower concepts will be described here in greater detail Cpen *olumetric ?ir Recei%er oncept
The (irst type o( solar tower is the open %olumetric recei%er concept 5see Figure 9a ? blower transports ambient air through the recei%er' which is heated up by the re(lected sunlight The recei%er consists o( wire mesh or ceramic or metallic materials in a honeyco mb structure' and air is drawn through this and heated up to temperatures between 6)0. and =)0. Cn the (ront side' cold' incoming air cools down the recei%er sur(ace There(ore' the %olumetric structure produces the highest temperatures inside the recei%er material' reducing the heat radiation losses on the recei%er sur(ace Det' the air reaches the heat boiler' where steam is produced ? duct burner and thermal storage can also guarantee capacity with this type o( solar thermal power plant
The %olumetric pressuri@ed recei%er concept 5see Figure 9b o((ers totally new opportunities (or solar thermal tower plants ? compressor pressuri@es air to about !) bar1 a transparent glass dome co%ers the recei%er and separates the absorber (rom the en%ironment nside the pressuri@ed recei%er' the air is heated to temperatures o( up to !!00.' and the hot air dri%es a gas turbine This turbine is connected to the compressor and a generator that produces
electricity The waste heat o( the gas turbine goes to a heat boiler and in addition to this dri%es a steam#cycle process The combined gas and steam turbine process can reach e((iciencies o( o%er )0B' whereas the e((iciency o( a simple steam turbine cycle is only 3)B There(ore' solar system e((iciencies o( o%er 20B are possible
F78RE 9 Schematic o( two types o( solar thermal tower power plant' showing 5a an open %olumetric recei%er with steam turbine cycle and 5b a pressuri@ed recei%er with combined gas and steam turbine cycle omparing Trough and Tower
n contrast to the parabolic trough power plants' no commercial tower power plant eists at present ;owe%er' prototype systems > in ?lmera' Spain' in arstow' ali(ornia' 8S' and in Reho%ot' srael > ha%e pro%en the (unctionality o( %arious tower power plant concepts The minimum si@e o( parabolic trough and solar tower power plants is in the range o( !0 $We elow this capacity' installation and CG$ costs increase and the system e((iciency decreases so much that smaller systems cannot usually operate economically n terms o( costs' the optimal system si@e is in the range o( )0>200 $We ish#Stirling Systems
So#called ish>Stirling systems can be used to generate electricity in the +ilowatts range ? parabolic conca%e mirror 5the dish concentrates sunlight1 the two#ais trac+ed mirror must (ollow the sun with a high degree o( accuracy in order to achie%e high e((iciencies n the (ocus is a recei%er which is heated up to 6)0. The absorbed heat dri%es a Stirling motor' which con%erts the heat into moti%e energy and dri%es a generator to produce electricity ( su((icient sunlight is not a%ailable' combustion heat (rom either (ossil (uels or bio(uels can also dri%e the Stirling engine and generate electricity The system e((iciency o( ish>Stirling systems can reach 20B or more Some ish>Stirling system prototypes ha%e been success(ully tested in a number o( countries ;owe%er' the electricity generation costs o( these systems are much higher than those (or trough or tower power plants' and only series production can achie%e (urther signi(icant cost reductions (or ish>Stirling systems
ish#Stirling prototype systems in Spain Solar himney
?ll three technologies described abo%e can only use direct normal irradiance ;owe%er' another solar thermal power plant concept > the solar chimney power plant > con%erts global irradiance
into electricity Since chimneys are o(ten associated negati%ely with ehaust gases' this concept is also +nown as the solar power tower plant' although it is totally di((erent (rom the tower concepts described abo%e ? solar chimney power plant has a high chimney 5tower' with a height o( up to !000 metres' and this is surrounded by a large collector roo(' up to !30 metres in diameter' that consists o( glass or resisti%e plastic supported on a (ramewor+ 5see artist:s impression Towards its centre' the roo( cur%es upwards to Hoin the chimney' creating a (unnel The sun heats up the ground and the air underneath the collector roo(' and the heated air (ollows the upward incline o( the roo( until it reaches the chimney There' it (lows at high speed through the chimney and dri%es wind generators at its bottom The ground under the collector roo( beha%es as a storage medium' and can e%en heat up the air (or a signi(icant time a(ter sunset The e((iciency o( the solar chimney power plant is below 2B' and depends mainly on the height o( the tower' and so these power plants can only be constructed on land which is %ery cheap or (ree Such areas are usually situated in desert regions ;owe%er' the whole power plant is not without other uses' as the outer area under the collector roo( can also be utili@ed as a greenhouse (or agricultural purposes ?s with trough and tower plants' the minimum economical si@e o( solar chimney power plants is also in the multi# megawatt range
?rtist:s impression o( a ) $W solar chimney power plant S;I?; ER7ER$?DD SCI?R 5SS 7$;' ST8TT7?RT wwwsbpde Electricity 7eneration osts
ue to the poor part#load beha%iour o( solar thermal power' plants should be installed in regions with a minimum o( around 2000 (ull#load hours This is the case in regions with a direct normal irradiance o( more than 2000 +Wh/m2 or a global irradiance o( more than !=00 +Wh/m2 These irradiance %alues can be (ound in the earth:s sunbelt1 howe%er' thermal storage can increase the number o( (ull#load hours signi(icantly The speci(ic system costs are between J2000/+W and J)000/+W depending on the system si@e' system concept and storage si@e ;ence' a )0 $We solar thermal power plant will cost J!00> 2)0 million ?t %ery good sites' today:s solar thermal power plants can generate electricity in the range o( J0!)/+Wh' and series production could soon bring down these costs below J0!0/+Wh The potential (or solar thermal power plants is enormous4 (or instance' about !B o( the area o( the Sahara desert co%ered with solar thermal power plants would theoretically be su((icient to meet the entire global electricity demand There(ore' solar thermal power systems will hope(ully play an important role in the world:s (uture electricity supply
