distributed power from solar tower systems: a mius approach

Solar Energy Vol. 67, Nos. 4–6, pp. 249–264, 19992000 Elsevier Science Ltd

Pergamon PII: S0038 – 092X( 00 )00059 – 1 All rights reserved. Printed in Great Britain0038-092X/99/$ - see front matter

www.elsevier.com/ locate / solener

DISTRIBUTED POWER FROM SOLAR TOWER SYSTEMS: A MIUSAPPROACH

, ´ ´† ´ *MANUEL ROMERO* , MARıA J. MARCOS*, FELIX M. TELLEZ*, MANUEL BLANCO ,´VALERIO FERNANDEZ**, FRANCISCO BAONZA*** and SEBASTIEN BERGER****

*CIEMAT/DER-PSA, Avda. Complutense 22, E-28040 Madrid, Spain**AICIA, Escuela Superior de Ingenieros, Avda. de los Descubrimientos s /n. E-41092 Sevilla, Spain

´ ´ ´ ´***Dpto. Ingenierıa Mecanica, Escuela Politecnica Superior, Univ. Carlos III, E-28911 Leganes, Madrid,Spain

****Ecole Nat. des Ponts et Chaussees, 6–8 avenue Blaise Pascal-Champs-sur-Marne,´F-77455 Marne-la-Vallee Cedex 2, France

Received 6 September 1999; revised version accepted 27 March 2000

Communicated by LORIN VANT-HULL

Abstract—One of the short-term priorities for renewable energies in Europe is their integration intocommunities and energy islands for local power supply (blocks of buildings, new neighborhoods in residentialareas, shopping centers, hospitals, recreational areas, eco-parks, small rural areas or isolated ones such asislands or mountain communities). Following this strategy, the integration of small solar tower fields intoso-called MIUS (Modular Integrated Utility Systems) is proposed. This application strongly influences fieldconcepts leading to modular multi-tower systems able to more closely track demand, meet reliabilityrequirements with fewer megawatts of installed power and spread construction costs over time after output hasbegun. In addition, integration into single-cycle high-efficiency gas turbines plus waste-heat applicationsclearly increments the solar share. The main questions are whether solar towers can be redesigned for suchdistributed markets and how to make them feasible. This paper includes the design and performance analysisof a 1.36 MW plant and its integration in the MIUS system, as well as the expected cost of electricity and asensitivity analysis of the small tower plant’s performance with design parameters like heliostat configurationand tower height. A practical application is analyzed for a shopping center with a solar tower producingelectricity and waste heat for hot water and heating and cooling of spaces. 2000 Elsevier Science Ltd. Allrights reserved.

1. INTRODUCTION hybrid Solar Two concept (Kolb, 1998). Hybridi-zation today is coming up against two serious

Traditionally, Solar Power Tower Plant projectsbarriers for its application. On one hand, the trend

have been conceived as solar-only applications intoward large hybrid solar towers integrated in

dispatchable power markets. Under these circum-typical combined cycles faces resistance to con-

stances, large plants of from 100 to 200 MW havestruction of a first commercial plant, since the

been proposed in order for solar thermal electrici-annual solar share decreases to figures as low as

ty to become economically competitive and to8–16%. The use of a conventional Rankine cycle

optimize O&M costs (Chavez et al., 1993), butguarantees higher solar shares, but leads to poor

the high capital investment and lack of confidenceefficiencies. The second is that the search for a

(technology maturity) discourage investors fromniche has been in an obsolete electricity pro-

undertaking construction. A recent strategy forduction structure based on centralized utilities and

reducing the perception of risk in marketing solarlarge generation stations.

power tower systems has been hybridization.As market deregulation expands worldwide,

Some examples of water / steam may be found inenvironmental regulations become stricter and

the COLON SOLAR project (Silva et al., 1999),competition develops, the justification for central-

air in the PHOEBUS postfeasibility study 1Cized utilities weakens. Furthermore, compared to

(Schmitz-Goeb and Keintzel, 1997) and CONSO-transmission and distribution, the share of capital

LAR (Kribus et al., 1998) or molten salt in theinvestment in generation is decreasing. In the US,the fraction of total annual investment allocated to

† generation was 69% in 1985 versus 27% toAuthor to whom correspondence should be addressed. Tel.:transmission and distribution, in 1989 the ratio134-91-346-6487; fax: 134-91-346-6037; e-mail:

[email protected] was 50/50, in 1994 the situation changed drasti-

249

250 M. Romero et al.

cally to 38/51 and for 1997 the transmission and (Mills and Schramek, 1999). Modular systems aredistribution share rose to 80% of total utility able to more closely track demand and potentialconstruction outlays (Feinstein et al., 1997). growth in loads, meet reliability requirementsThese figures clearly indicate that Distributed with fewer megawatts of installed power andUtilities (DU) are becoming more and more spread construction costs over time after outputattractive when close to some specific application, has begun (Hoag and Terasawa, 1981). In addi-since the current oversizing of some grid and tion, integration into single-cycle high-efficiencydistribution networks designed for large peak gas turbines plus waste-heat applications likeloads is reduced. district heating, desalination or water treatment

In a fast-changing electrical sector evolving clearly increments the solar share. In other words,from the central station model to the distributed capital risk and the amount of initial investmentutility, centralized large solar thermal power may be reduced while simultaneously increment-plants face even greater difficulties for their ing conversion efficiency and solar share. To bedeployment. Therefore, a different strategy for viable, small cogeneration systems in industrialpenetration of the technology based on modularity applications must operate almost continuously,of design and integration in distributed utilities which is a clear drawback for the utilization of(MIUS) should be carefully assessed. In the solar tower plants. On the other hand applicationsWhite Paper for a Community Strategy and for buildings, shopping centers, residential areasAction Plan on Renewable Energies (European and communities in general typically require asCommission, 1997), one of the short-term initia- few as 4500 h per year (full load equivalent).tives foreseen is the integration of renewable Therefore a solar MIUS approach for this kind ofenergies for local power supply in 100 com- community represents a more favorable environ-munities (blocks of buildings, new neighborhoods ment with potential solar shares of up to 50%.in residential areas, recreational areas, shopping The main questions are whether solar towers cancenters, hospitals, small rural areas or isolated be redesigned for such distributed markets andones such as islands or mountain communities). how to make them feasible.Following this philosophy, the integration ofsmall tower fields into so-called MIUS (Modular

2. A REVIEW OF THE MIUS CONCEPTIntegrated Utility Systems) is proposed (Fig. 1).This kind of application strongly influences field Even though Distributed Utilities (DU) seem toconcepts leading to modular multi-tower systems be a brand new strategy to revitalize the renew-

Fig. 1. Schematic drawing of an example of a MIUS using a gas turbine and a solar tower. The energy balance corresponds tothe annual demand of a 450-unit apartment complex in Spain. The example makes use of electricity and waste heat produced bythe hybrid (solar /gas) turbine for domestic and auxiliary electricity, air-conditioning, domestic hot water and space heating.

Distributed power from solar tower systems: a MIUS approach 251

able electric industry (Aitken, 1997) related to the cial communities is by hybridizing them withsuccess of cogeneration, a historical review of the centralized prime movers (open or closed-cyclelast three decades reveals that the DU concept is gas turbines) to satisfy user needs during longbehind the origin of the existing solar thermal periods of time, even with poor solar radiationpower plants planned in the 1970s. The literature (McDonald, 1986).is full of MIUS-related terms like Total EnergySystems, Power Islands, DEUS, IEUS or even

3. SELECTED MIUS SOLAR TOWER DESIGNDistrict Heating, Energy Cascade and Cogenera-tion. Indeed the number of references is amazing. We propose the integration of small-size towerFor the purpose of this paper, the selected solar plants working in a fuel-saver mode. The follow-tower application is closer to the old concept of ing characteristics have been optimized for theMIUS as depicted in Fig. 1. reference solar tower plant:

Total Energy Systems are a particular applica- • Small tower and heliostats that reduce visualtion of cogeneration for large residential, commer- impact and can better guarantee receiver aper-cial or institutional building complexes (hospitals, ture fluxes and achieve higher field efficiencieshotels, etc.). The MRTS program which was (up to 4% more than large-area heliostats).started by the US Department of Housing and • Air as heat transfer media in a pressurizedUrban Development (HUD) in 1972 planned to volumetric receiver (3.4 MW thermal outlet).develop such total energy systems for use in • Use of an efficient (39.5%) small solar-gasresidential /commercial communities of 300 to turbine (1.36 kW) with intercooling, heat1000 dwellings, adding other typical utility ser- recovery and low working temperaturevices such as waste disposal, water treatment, etc. (8608C).(Rothenberg, 1976). MIUS combine the integra- • Waste heat (670 kW) at 2238C for watertion of renewable energies with heat and materials heating and space cooling /heating.which have been traditionally viewed as waste • As in the case of dish system parks, the smallbyproducts like exhaust heat from power genera- tower fields for distributed power should targettion, combustible solid wastes or treated waste- maximum unattended operation, to minimizewater, to produce light, heating, cooling, air O&M costs.conditioning, drying, process heat and power Economic viability of such small towers is the(Mixon, 1974). result of combining high efficiency turbines and

The prime movers usually suggested in MIUS reduced O&M costs. The recent success in theand DU are batteries, fuel cells, PV, wind tur- automatic operation of volumetric receivers (Gar-

´ ´bines, diesel engines and gas turbines (Brayton cıa-Martın et al., 1999) and the existing R&Dcycle engines). In general they are modular, programs on advanced control for volumetric-small-size units that supply unbundled services. receiver tower power plants makes possible toRegarding solar thermal integration into MIUS for foresee in the near future highly unattended/ re-communities application, the most extended tech- mote operation with integrated control systems.nologies are district heating and domestic hot We have fixed for the MIUS concept a target

21water with flat-plate collectors. Integration of O&M annual cost of $55 kW (gas consumptionhigh-temperature solar thermal concentrating sys- not included) as described in Table 3. Thistems into DU and MIUS seems not to be as challenging goal is part of a current project instraightforward. Building integrated concentrating collaboration with DLR, University of Seville andsystems have been suggested as a means of using INABENSA being planned for the year 2002 atthe roofs for production of high temperature the Plataforma Solar de Almeria.thermal energy (Gerics and Nicklas, 1996), but Today’s turbine /generator sets having capaci-the most serious attempts have been made with ties ranging from 500 kW to 25 MW are suitableparabolic dishes like the projects developed by for DU applications. Gas turbines are widely usedANU in Australia (Clark, 1990) or the STEP in cogeneration projects larger than 3–4 MWproject in the USA (Shenandoah Solar Total where there is a demand for high-pressure steam.Energy Project) where the energy-cascade con- Systems are also available in the 600–100 kWcept was applied to supply electricity, thermal range, but the electrical efficiency achieved at thisprocess energy, and chilled water for space con- scale is reduced from 30% to around 25% (Major,ditioning (Nelson and Heckes, 1989). 1995). Despite this, overall efficiency is 80–90%

The most efficient way to introduce solar with high-grade heat recovery which can be usedconcentrating systems into residential or commer- for medium and high pressure steam and for


direct heating or drying applications. For small turbine, are achievable by today’s solar centralgas-turbine generator sets (less than 10 MW), a receiver technology.cold startup to full load takes about 40 s. With Solarization of the H-1 is easily done bybattery backup for the starting motors, gas-turbine introducing hot air from the solar receivers paral-generator sets are capable of a ‘‘black start’’. lel to the high pressure combustor and lowGas-turbine generator sets are fully dispatchable pressure combustor (Fig. 2). The energy balanceand can be used to follow the load, but they are for solar-mode operation changes the parametersusually operated at rated power for optimum of the turbine. Fossil operation adds up to 0.09 kg

21efficiency (Goldstein, 1996). The cost of small s of fuel flow to the air, and the combustiongas turbine generator sets below 5 MW is in the process produces steam that improves both tur-

21 21$400 kW to $750 kW range (Gas Turbine bine efficiency and recuperator efficiency. TheWorld Handbook, 1995). Balance-of-plant costs pure solar mode results in a decrease of efficiency

21 21fall in the $50 kW to $120 kW range. estimated at 39.5%, power production of 1360 kWFor small gas turbine generators to enhance and lower efficiency at the recuperator. Therefore,

thermal efficiency up to 42% or more, the present the solar receiver should supply a maximumtrend is to increase the turbine inlet temperature thermal outlet of 3440 kW at design point.with higher combustion gas temperatures of Heliostat field layout, as depicted in Fig. 3,13508C. One such development is supported by tower height and receiver configuration have beenNEDO in Japan with the 300 kW CGT-301 and optimized by using a customized version of the302 (Ceramic Gas Turbine) developed by NGK well known DELSOL3 code (Kistler, 1987). Aand Kawasaki (CADDET Newsletter, 1998). A Windows95 version of DELSOL3 called WDEL-CGT 300 kW cogeneration system generating SOL has been developed that allows user-friendlypower and supplying domestic hot water in an generation of heliostat layout and visual infor-apartment complex of 400 dwellings is expected mation on flux distribution and heliostat fieldto save 7900–9700 GJ per year (equivalent to performance (Romero et al., 1999).200–250 million liters of oil). CGT 300 kW Optimization has taken noon of Julian day 172turbines may be of great interest for near future as the design point, and a latitude of 37.28

solar applications. Although near the lower limit corresponding to the city of Huelva (South Spain).of solar tower size, they still require additional Main characteristics of the optimized solar fieldtest operation. In addition, the required tempera- are listed in Table 2. Field layout optimization hasture is still a challenge for pressurized volumetric taken into consideration the visual impact pro-air receivers that today are limited to 8008C (Buck duced by the heliostat field and the tower. Be-et al., 2000). cause of that, a solar plant with a relatively short

For the present study we have selected a 1400 kW tower (26 m) has been created by increasing thetwo-shaft gas turbine with intercooling, heat land used. In fact, the land-use ratio is lowrecovery and two-stage combustion (Table 1). (17.4%). A different optimization strategy wouldThe study takes the promising first-of-a-kind H-1 lead to a much higher occupation of terrain withturbine developed by the Schelde Heron B.V. double tower height. For the field proposed, thecompany in The Netherlands (http: / /ww- small-area HELLAS heliostat produced by thew.heron.nl) as a baseline reference. The H-1 has a GHERSA company in Spain (Grimaldi et al.,remarkably high electrical efficiency of 42.9% at 1999) has been selected. This heliostat has aISO conditions, compared to 25–34% for other number of characteristics that make it suitable forsmall-size gas turbines. Relatively low turbine MIUS applications. It is modular so that theinlet temperatures (8608C) and the low pressure whole unit is easy to transport from the factory in

5before the turbine wheel, 8 3 10 Pa for the a few pieces. Pedestal and foundation are inte-5compression turbine and 3 3 10 Pa for the power grated in a single unit. Local control is connected

to the master control by radio-modem. The track-ing system makes use of cheap, standard recti-Table 1. Heron H-1 gas turbine technical specificationslinear actuators. The reflective surface is formed

Electrical power 1407 kWby only three easily canted facets verticallyThermal power 1200 kW

Fuel consumption 3280 kW assembled as depicted in Fig. 4, with individual21Heat rate 2.33 J J facet dimensions of 2 3 3.2 m. Beam quality

Electrical efficiency 42.9%supplied by HELLAS is 2.4 mrad. The installedThermal efficiency 36.6%

22Total efficiency 79.5% cost is $150 m .21NO emission ,20 g GJx The heliostat aspect ratio is 1.88, leading to a


Fig. 2. Selected two-shaft gas turbine of 1400 kW with intercooling, heat recuperation and two-stage combustion for integrationinto a solar tower field using a pressurized REFOS-type volumetric air receiver. The drawing shows the theoretical solarization ofa GasTurbine Heron H-1, leading in solar mode operation to an output power of 1360 kW. Cl and C2 are compression stages, CTis the compression turbine and PT the power turbine. HPC represents the high-pressure combustor that is set in parallel to a groupof six pressurized volumetric solar receivers (R1– R6). Other four solar receivers (R1–R10) are backed up by the low-pressurecombustor, LPC.

rather short vertical dimension and high heliostat- 85.1%, Shadowing 97.1%, Blocking, 98.9%, At-field design-point efficiency (74%). Best helios- mospheric Transmittance 98.0%, Receiver Aper-tat-field efficiency is calculated between October ture Spillage 98.8% and TOTAL 71.3%.and March with values up to 81%. The yearly The HELLAS heliostat’s small size minimizesaverage is as follows: Reflectivity 90%, Cosine spillage losses for the volumetric receiver pro-

Fig. 3. Heliostat field layout of the 1.36 MW plant obtained with the computer code WDELSOL (Design point noon, JD 172,latitude 37.28 in South Spain). The field has a North-shape configuration and 345 small-size heliostats. The large number ofheliostats and land area used has been a result of the optimization strategy since tower height minimization was given highpriority to reduce visual impact of the system.


2Table 2. Technical specifications of the MIUS solar tower posed with a peak flux limit of 630 kW m and aoptimized using the computer code WDELSOL. Solar field 2small aperture area of 16.5 m . The receiverarea and receiver dimensions have been designed for asolarized 1.36 MW Heron H-1 gas turbine consists of a cluster of 10 pressurized windowed

volumetric receivers of the type proposed in theLatitude (8) 37.2Tower optical height (m) 26 REFOS project which is successfully testing aNumber heliostats 345

2 350 kW unit at the Plataforma Solar de Almeria inHeliostat surface (m ) 19.22 a joint DLR–CIEMAT project (Buck et al.,Receiver surface (m ) 16.5

Receiver tilt angle(8) 30 2000). Six modules will be installed in parallel to2Land (m ) 38,000 the high pressure combustor (Fig. 2), raising air

temperatures from 5508C to 8608C. Two rows ofDesign point (Noon, JD 172) Power Efficiency21

2 three modules will divert 2.5 kg s of air each toDNI (Wm ) 875 –5

Power onto mirrors area (MW) 5.8 100% thecompressionturbineatapressureof8.4 3 10 Pa.Gross power onto receiver (MW) 4.3 74% Modules R3 and R6 will have a ceramic absorberPower to turbine (MW) 3.4 80%

and the rest are made of metal. The six module setGross electric power (MW) 1.4 39 %Total efficiency – 23% will supply 1.91 MW to the fluid before entering

the compression turbine.Investment ($) Four additional modules are installed in parallelHeliostats 995,765 5to the low pressure combustor working at 3.1 3 10Land 62,745Tower 104,575 Pa. Two rows of two modules each will raise theReceiver1Air circuit 484,750 temperature of the fluid from 6128C up to 8608C.Inst. & Control 107,000

Modules R8 and R10 will have a ceramic ab-Power block 1,146,000Fixed cost 65,350 sorber. The four-module set will supply 1.53 MW

to the air before entering the power turbine.Direct capital cost ($) 2,966,185

The disposition of four rows in parallel but21Installed cost ($ kW ) 2120 with modules connected in series allows a high

(including turbine set) controllability of the absorber temperature and airconditions.

Fig. 4. Structural configuration of the wireless small-size HELLAS heliostat selected for the MIUS solar tower. The reflectivesurface is formed by three 233.2 m facets vertically assembled and heliostat aspect ratio is 1.88. Pedestal and foundation areintegrated in a single unit. Drive mechanism consists of two rectilinear actuators.


24. SOLAR FIELD SENSITIVITY ANALYSIS (90 m ) produces 4% less energy per year andrequires 10% more land.

A sensitivity analysis to the solar field reflects Tower height has the inverse effect. The highersimilar performance to most solar thermal tower the tower, the less land is needed and annualplants. Efficiency of the plant is proportionally energy production is increased as depicted in Fig.influenced by yearly average reflectivity of the 6. As observed in Fig. 6, depending on theheliostat field as already determined in previous restrictions of the particular site (expensive land,

´analyses (Sanchez et al., 1997). A decrease of restricted area or visual impact), the tower can be10% in heliostat field reflectivity represents an used to minimize the effect. The effect of towersalmost proportional 12% decrease in annual elec- over 32 m tall on annual optical efficiency of thetricity production, therefore the use of low-iron heliostat field is not significant, but the amount ofhigh-reflectivity mirrors is recommended. Bi- land required is considerably less, decreasing

2weekly mirror washing would be sufficient to from 38,000 m with a 36-m-high tower, to2 2preserve up to a yearly average of 96% of initial 28,000 m with a 35-m tower and 21,000 m for a

reflectivity. The 345 small heliostats could easily 50-m tower.be washed in 8 h with a mechanized washingtruck.

5. SOLAR TOWER PERFORMANCEThe influence of beam quality is not so relevantCAPABILITIESin terms of annual electricity production. A drasticchange in HELLAS beam quality from 2 to The TRNSYS code was used for annual

4 mrad (today’s technology guarantees heliostats performance analysis of the solar thermal towerwith beam qualities on the order of 2.2–2.6 mrad) power plant. Heliostat field and solar-receiverproduces only a 5% impact in terms of annual performance were simulated by using the STECenergy production. library developed by the IEA/SolarPACES inter-

A relevant parameter for MIUS applications national cooperation project (Pitz-Paal and Jones,where there are land constraints and restrictions is 1999). New component models for the air com-the influence of heliostat size and tower height on pressor, the gas turbine and heat recuperator wereplant efficiency. As mentioned before, visual specifically formulated for the MIUS plant. Yearlyimpact has been considered in the proposed Direct Normal Insolation for the selected site is as

22 22design, selecting small tower and heliostats. Fig. 5 much as 7434 MJ m (2065 kWh m ). A2compares the HELLAS heliostat (19.2 m ) to typical design year corresponding to the city of

other sizes with the same aspect ratio and optical Huelva in southern Spain was used for thequality. A heliostat field with large-area heliostats detailed performance analysis. The weather file

Fig. 5. Sensitivity analysis of land area required (squares) and annual optical efficiency of the heliostat field (circles) versusheliostat reflective surface obtained with the computer code WDELSOL for the 1.36 MW tower plant. The figure compares the

2HELLAS heliostat area (19.2 m ) to other sizes with the same aspect ratio (1.88) and optical quality (2.4 mrad).


Fig. 6. Sensitivity analysis of land area required (squares) and annual optical efficiency of the heliostat field (circles) versus towerheight obtained with the computer code WDELSOL for the 1.36 MW tower plant. All cases are considering the use of theHELLAS heliostat represented in Fig. 4.

provides DNI in 15 min steps. Mathematical allows both use of electricity produced for inter-treatment of results has been done with nal consumption or sale to the grid, and use of

MATLAB . waste heat produced for internal consumption byBefore a particular application can be analyzed, hot water supply and space cooling.

the small tower system’s limits and capacities for As seen in Fig. 7, on a typical sunny day, theannual electricity production, conversion ef- electricity produced comes to between 21.6 andficiency and waste heat supply must be assessed. 39.6 GJ (6 and 11 MWh). Energy from waste heatTwo different operating modes were analyzed, is typically between 10.8 and 18 GJ (3 and 5 MWhsolar-only and hybrid with a maximum solar per day). A temperature gradient between 223 andshare. 958C has been considered for waste heat. The

annual electricity produced per year will be up to5.1. Solar-only 8320 GJ (2311 MWh) and the total amount of

This sun-tracking mode has no fossil backup. It waste heat available for heating and coolingis, in principle, recommended only for isolated purposes will be 4705 GJ (1307 MWh).communities or power islands. For those applica- Fig. 8 shows the relevance of partial loadtions, only oversized solar systems and appro- operation. The operating strategy assumed in thispriate energy storage would lead to practical example allows operation only with a 50–100%solutions. In any case, the stand-alone system is load to maintain high-efficiency electricity pro-worth assessment since it gives the lower-limit duction and to minimize sharp transients in criti-figures of merit for solar electricity production, cal components like the recuperator. With lesssince the gas turbine would essentially work on a than a 50% turbine load, the system stops. Fromnon-steady regime most of the time. the statistics, it can easily be inferred that the

The stand alone mode is of special interest for turbine is running about 1750 h per year with overSpain since the legal registration of solar thermal 65% loads. As a consequence, turbine dispat-power plants as renewable source power plants is chability under solar conditions is quite high, withbeing assessed by the Spanish authorities. A a relatively low impact on overall efficiency.combination of investment subsidies for a number As seen in Fig. 9, conversion efficiency is highof projects and a premium price added to the throughout the year, going from 18% to 22%, andmarket prices are under consideration between 6 decreasing in summer (2%), basically due to theand 18 cents per kWh. Because of that, the worse cosine factor in the heliostat field at thatoperating strategy for this MIUS plant in Spain time of the year. When efficiency in June is


Fig. 7. Distribution by frequency of daily electricity (dark) and usable waste heat (grey) produced by solar-only operation of theHeron H-1 gas turbine. On a typical sunny day, the electricity produced comes to between 21.6 and 39.6 GJ (6 and 11 MWh).Energy from waste heat is typically between 10.8 and 18 GJ (3 and 5 MWh per day). A temperature gradient between 223 and958C has been considered for usable waste heat estimation.

Fig. 8. Distribution of part load functioning of the solarized H-1 Heron gas turbine for the typical design year in Huelva (Spain)and operating in stand-alone mode. Weather file provides DNI in 15-min steps. In solar-only conditions, the operational strategyhas required the system to be stopped with less than 50% turbine load (2000 h). The system reveals high dispatchability and isrunning over 1750 h per year with over 65% loads.


Fig. 9. System efficiency during the year of the solarized H-1 Heron gas turbine combined with the 345-heliostat field optimizedwith the code WDELSOL and operating in solar-only mode. The figure represents evolution of solar-to-electricity conversionefficiency for a weather file providing DNI in 15-min steps for the city of Huelva (Spain). Efficiency moves between 18 and 22%,with 2% reduction in summer.

Fig. 10. Daily solar (grey) and fossil (dark) production of electricity compared to theoretical prediction (solid line) as calculatedfor the typical design year (sunrise-to-sunset operating mode). The curve represents daily electricity production by the solarized1.36 MW H-1 Heron gas turbine working in a fuel-saver mode at 100% load. The theoretical prediction of electricity production(5.6 GWh per year) is obtained by multiplying the nominal power (1.36 MW) by the number of hours of day between sunrise andsunset and integrating this value during the whole year. Daily electricity production moves between 12 and 19 MWh. Daily andyearly solar contributions to electricity production are about 50%.


compared with design-point efficiency, design- main advantages of fossil backup is the elimina-point performance is observed to be overestimated tion of partial turbine loads. This improves com-for better hybridization and minimization of ex- ponent lifetime and facilitates maximization ofcess solar energy. Even then, efficiencies are solar conversion. The opposite of solar-onlycomparable to those typically used in large-scale would be hybrid operation from sunrise to sunset.solar tower plants with steam Rankine conversion This kind of theoretical operation is represented incycles. Fig. 10. Solar production is maximized in this

way and can be sold entirely or partially to the5.2. Fuel-saver mode grid at the subsidized price.

This is the most appropriate operating mode for Annual SOLAR electricity production goes upthe plant. In communities, typical minimum usage to 10,440 GJ (2900 MWh); annual FOSSILis about 4500 h per year (51% capacity factor), electricity represents 9720 GJ (2700 MWh) and totalbasically, half of the time during sunny hours and electricity produced by the system is 20,160 GJhalf during dark or cloudy periods. One of the (5600 MWh), equivalent to the demand of a

typical 450-dwelling community in southernSpain (Fig. 1).

A statistical analysis in Fig. 10 demonstratesthat the solar share is 50% not only as a yearlyaverage, but also daily. The turbine can be seen tobe working many days with high solar dailyproduction and only a few days with mostly fossil(Fig. 11). This means that on most days, fossilbackup is limited.

6. APPLICATION TO A MIUS SCHEME

Two situations should be carefully analyzedwhen working with a MIUS tower field in solar-only mode: the part-load statistics for the turbineand mismatch between waste-heat offer and de-mand. The two previous analyses produced solarelectricity production limits between 8280 GJ(2.3 GWh) for solar-only mode and 10,440 GJ(2.9 GWh) with fossil full-load turbine operation.As seen in Fig. 1, the proposed solar-fossil small-tower would be enough to supply the communitydemands on a yearly basis with solar shares ofabout 50%, but complexity of the demand mayproduce daily differences between production andconsumption.

For a particular MIUS case, a more stableend-user with a simpler demand, a shoppingcenter, has been selected.

A shopping center is a good end-user for asolar thermal power plant since 85% of itselectricity demand is concentrated during thedaytime (Fig. 12) at typical shopping hours (inSpain from 7:00 to 19:00 solar time). This con-centration of consumption during the day repre-

Fig. 11. Distribution of frequencies on a daily basis for solar sents a more expensive kWh purchase. In addi-(grey) and fossil (dark) shares working on a hybrid mode tion, power demand has a uniform daily profilesunrise-to-sunset with the solarized H-1 Heron gas turbine with monthly differences between 1300 and 800 kW.(fuel-saver mode). As can be observed there is a higher

Electricity demand increases between June andamount of days with higher contribution of solar than fossil.October (peak in July) due basically to higherTypical daily electricity production from solar is 9 MWh and

from fossil is 6 MWh. cooling loads and lower demand between


Fig. 12. Power demand curve for a medium-size shopping center in Spain (real case), concentrating 85% electricity demand atshopping hours from 7:00 to 19:00 solar time. Depicted are profiles corresponding to typical days in February, July, Septemberand October. Daily demand and peak power at noon can be covered with hybrid solar–gas operation of the H-1 Heron gasturbine.

Fig. 13. Annual distribution on a daily basis of electricity demand (crosses line) corresponding to the selected shopping center inSouth Spain during shopping time between 7:00 and 19:00 h solar time. The solar tower system described in Table 2 and thesolarized 1.36 MW Heron H-1 gas turbine as represented in Fig. 2 are applied to cover electricity demand during shopping hours.The figure represents solar electricity production (grey), fossil electricity production (dark) and solar power excess (dark solidline) during shopping hours (from weather file of 15-min time step) for the referred scenario.


November and May with a minimum in February.(See solid line in Fig. 13). A second peak iscaused by Christmas.

The proposed operating scheme for the shop-ping center is night-time electricity consumptionfrom the grid and solar hybrid operation of the1.36 MW gas turbine during 14 h between 6:00and 20:00 in a power island mode. A detailedanalysis of annual performance with 15-min stepsconfirms that the 345-heliostat tower plant cansupply a significant part of the power demand.

As can be seen in Figs. 13 and 14, the bestperformance corresponds to the summer, wheresolar power represents a significant part of powerconsumption and no significant solar power ex-cess is estimated. In December and January noexcess is registered but again solar excess isnegligible. A third situation is noticed in equinoxwhere solar power is relevant (more in Springthan in Autumn) compared to demand but thistime solar excess is not negligible. This per-formance is a result of the design point selected.

The annual figures in Fig. 15 yield the finalresult:

Solar electricity production 5 8842 GJ

(2456 MWh)

Fossil electricity production 5 6811 GJ

(1892 MWh)

Solar electricity excess 5 1541 GJ (428 MWh)

The net solar electricity usable by the end-user is8842 GJ (equivalent to an energy saving of 683 toewhich is 56% of the solar share in terms of

Fig. 14. Performance of solar usable (black bars), solar excessconsumption, and 85% of the electricity to be (cross hatched bars) and fossil power (white bars) supplied byproduced as represented in Fig. 10 in ideal hybrid a small size solar tower plant and the solarized 1.36 MWsunrise-to-sunset full-load turbine operation Heron H-1 gas turbine to cover shopping center power demand

during working time for three selected days from the reference(10,440 GJ). Net production is not only affectedweather file. As can be observed in December (JD 365)by mismatch between production and demand indemand is high and therefore no solar power excess is

some periods of the year but also by partial registered. In addition the limited number of solar hours makesturbine loads demanded which in fact has been necessary more than 5 h with only-fossil operation of theoversized to absorb July peaks. In the part-load turbine in the afternoon. In the equinox the power demand

decreases and therefore a significant solar excess is producedprofile shown in Fig. 16 for the shopping-hours(JD 81). On the other hand the number of only-fossil hours inconsumption profile depicted in Fig. 12, there arethe afternoon is reduced to three. Finally the period between

very few hours with low loads (20%), which June and October (JD 161) registers a mixed performance withcorrespond to opening and closing hours and the a high demand like in December and therefore practically norest of the time the turbine is running at high solar excess (only some excess is noticed early in the morning)

and like in the equinox only 3 h of only fossil operation areloads (75% mean).registered in the afternoon.The usable waste heat supplied by the turbine,

calculated as the enthalpy decrement of thewater all year long. This demand is at presentexhaust gases between 2238C and 958C is repre-covered by a fossil boiler and waste heat from thesented in Fig. 17. The annual heat demand duringair conditioning system. Waste heat produced byshopping hours is 8827 GJ (2452 MWh), basicallythe turbine is 7711 GJ (2142 MWh, equivalent tofor space heating in winter and domestic hot


Fig. 15. Accumulated contributions of solar (grey) and fossil (dark) supplied by a small size solar tower plant and the solarized1.36 MW Heron H-1 gas turbine to the overall power demand of a selected shopping center in South Spain. Upper solid grey linerepresents total electricity demand and dark line the excess of solar power. Of the 2888 MWh total solar production, only428 MWh are not usable according to the power demand curve of the shopping center. Solar electricity contributes 56% to yearlypower demand of the user.

Fig. 16. Turbine operating mode (number of hours at partial load) obtained after analyzing performance of the small solar towerfield and the solarized 1.36 MW H-1 Heron gas turbine applied to the power demand curve of a shopping center in South Spain.Low loads (approx. 20%) correspond basically to opening and closing hours. Most of the working hours, the turbine is running athigh loads (approx. 75%).


Fig. 17. Monthly distribution of waste heat demand of a shopping center in South Spain (dark bars) and usable waste heatproduced by the solarized 1.36 MW H-1 Heron gas turbine performance in connection with a small solar tower field (grey bars).Heat demand and production are calculated during shopping hours (from 7:00 to 19:00 solar time).

8 toe) with a production excess of 25% as shown Heron. Regarding the solar receiver, we assumeda cost of $40,400 per module from privatein Fig. 17. As can be observed all the heatinformation supplied by DLR, currently develop-produced is lower than the demand except ining REFOS technology. The heliostat field costmonths 5–9 where there is an excess of 542 MWhwas quoted by the manufacturer GHERSA.not used by the shopping center. Solar is con-Breakdown of costs by subsystems is listed intributing to the waste heat produced with 4374 GJTable 2. Estimated LEC, assuming annuity of 10%(1215 MWh) and 4 toe that represents 49.5% of

21 21and O&M of $55 kW a year (plus $4.2 kWthe heat demand.23a year for gas consumption from $0.34 m )The investment cost for the proposed small

21 21would be $15.8 GJ ($0.057 kWh ). Operationsolar tower power plants is estimated at 2.97costs are assuming a highly automated control andmillion dollars. Costs of the conventional andpartly remote management, since the plant issolar parts were based on suppliers information.considered as an installation forming part of aCivil works costs, buildings and tower werelarger industrial facility (Table 3).kindly supplied by the company NECSO. Air

pipelines materials costs were obtained fromRATH, Rockwool and COTAINSA. Turbine set 7. CONCLUSIONSand regenerator costs were obtained from Schelde

A potential niche for the application of solartowers in Modular Integrated Utility Systems has

Table 3. Annual costs of the solar thermal tower power plant been identified. The solar field should be smallOperation and maintenance (annual) $ and modular to provide maximum flexibility forAdministration 6010 real systems.Operation /Maintenance 36,061

The maturity of the heliostat technology, theConsumables 4508Insurance 9015 recent developments in pressurized-air volumetricSolar extra cost 30% 16,678 receivers and new small gas turbines developed

for cogeneration with intercooling, heat recupera-Total O&M annual 72,272tion and relatively low working temperatures open

Total O&M annual (per kW installed) 55 the doors for small-scale systems of a few MW,21Gas annual consumption $4.2 kW maintaining relatively high conversion efficiencies


¨Deutsches Zentrum fur Luft und Raumfahrt, Cologne,of between 20 and 23%. A shopping center hasGermany.

been proposed as a design-case 1.36 MWapplication Hoag J. and Terasawa K. (1981) Some advantages of systems21at the realistic competitive cost of $2120 kW , with small module size. In Proceedings of the IASTED

Energy Symposia, 20 May 1981, San Francisco, Hainza M.which is a short-term objective even for largeH. (Ed.), pp. 45–48, ACTA Press, Anaheim, California.

solar tower plants. Kistler B.L. (1987) A User’s Manual for DELSOL3: AThe solar system can cope with 56% annual Computer Code for Calculating the Optical Performance

and Optimal Design for Solar Thermal Central Receiverelectricity demand during shopping hours andPlants. Sandia National Laboratories Livermore, SAND86-

49.5% of the heat demand contributing to an 8018.energy saving of 687 toe. Kolb G. J. (1998) Economic evaluation of solar-only and

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distributed power from solar tower systems: a mius approach

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