Category Archives: Sonar-Collecttors

Fresnel-Collectors in hybrid Solar Thermal Power Plants with high Solar Shares

Fig. 15 Constant pressure concept; external depressurization,

Saturated Steam

Fig. 16 Steam accumulator with integrated latent heat storage material.

Although steam accumulators exhibit only a small storage capacity, the availability of these buffer storage systems can contribute to reduce the investment costs for storage capacity if they are combined with storage systems intended for longer periods of discharge. By reducing the requirements regarding response time and discharge rate the specific costs for storage systems with several hours of heat capacity can be reduced.

Acknowledgement

Part of the work presented in this paper has been funded by the German Federal

Environment Ministry under the contract code PARASOL/WESPE and part by the European

Commission within the 5th Framework Programme on Research, Technological

Development and Demonstration under contract no. ENK5-CT-2001-00540.

The authors are responsible for the content of this publication.

References

[1] Tamme, R., Laing, D., Steinmann, W. D., Zunft, S., 2002, "Innovative Thermal Energy Storage Technology for Parabolic Trough Concentrating Solar Power Plants”, Proceedings EuroSun 2002, The 4th ISES Europe Solar Congress, Bologna, Italy

[2] Tamme, R., Steinmann, W. D., Laing, D., 2003, „High Temperature Thermal Energy Storage Technologies for Power Generation and Industrial Process Heat", Proceedings FUTURESTOCK 2003, 9th International Conference on Thermal Energy Storage, 1.-4. Sept. 2003, Warsaw, Poland.

[3] Tamme, R., Laing, D., Steinmann, W. D., 2004, „Advanced Thermal Energy Storage Technology for Parabolic Trough", ASME-J. of Solar Energy Engineering, Vol. 126, May 2004.

[4] Eck M., Zarza E., Eickhoff M., Rheinlander J., Valenzuela L.: Applied Research concerning the Direct Steam Generation in Parabolic Troughs, Solar Energy, Vol.

74 (2003) pp. 341-351

[5] Beckmann, G., Gilli, P. V. (1984): "Thermal Energy Storage", Springer Verlag

Hansjorg Lerchenmuller, Max Mertins, Gabriel Morin

Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, 79110 Freiburg e-mail: hansjoerg. lerchenmueller@ise. fraunhofer. de

Dr. Andreas Haberle

PSE GmbH, Solar Info Center, 79072 Freiburg, Germany e-mail:ah@pse. de

02

Dr. Stefan Bockamp, Dr. Markus Ewert, Matthias Fruth, Thomas Griestop E. ON Energie AG, Brienner Str. 40, 80333 Munich, Germany e-mail: markus. ewert@eon-energie. com

Dr. Jurgen Dersch

German Aerospace Centre (DLR), 51147 Cologne, Germany e-mail: juergen. dersch@dlr. de

Over the last few years Fresnel-Collectors have attracted a lot of attention within the solar thermal power sector. The main reason is comparatively low investment costs through simple components. The Fraunhofer Institute for Solar Energy Systems,

E. ON Energie AG and German Aerospace Centre (DLR) have carried out a feasibility study in order to assess the technology with respect to technical, economical and ecological aspects.

The mid to long term strategy of solar thermal electricity generation must aim at technical solutions with high solar shares. Thermal storage is not yet technically proven for direct steam generating systems. Therefore special configurations of hybrid operation are an interesting option from a technical and economical point of view. Full load hours of the power plant increase and allow for more stable plant operation. Based on Fresnel-Collectors, two different types of power plant configurations with low or zero CO2-emission are analysed in this paper:

• Hybrid operation of a solar field and a biomass vessel

• From the starting point of a Solar Only power plant, natural gas hybrid operation will be considered and the trade off between high solar share and low cost electricity production will be analysed in detail.

Calculations for this study were carried out in three steps:

• Thermodynamic calculations of the water/steam cycle were done with the commercial process simulation tool Ebsilon [1].

• Thermal and electrical yields were calculated with ColSim [2] for different solar field sizes and different options of hybridization. The simulations are based on the efficiencies of the power cycles — depending on ambient temperature and load — and hourly meteorological data for a site with a DNI of 2’247 kWh/(m2a) [3].

• Based on economic assumptions and on the results of the previous steps, calculations of levelised electricity costs (LEC) and profitability were carried out.

Geometry Optimization of Fresnel-Collectors with economic assessment

Dipl.-Ing. Max Mertins, University of Karlsruhe, Englerstr. 7, 76128 Karlsruhe Dipl.-Phys. Hansjorg Lerchenmuller, Fraunhofer ISE, Heidenhofstr. 2, 79110 Freiburg Dr. Andreas Haberle, PSE GmbH, Solar Info Center, 79072 Freiburg Dr. Ing. habil. Volker Heinzel, University of Karlsruhe, FZK, 76131 Karlsruhe

Abstract

The Fresnel solar collector is a promising concept to reduce the electricity cost price in solar thermal power plants. The optical performance of a Fresnel collec­tor depends on material properties, on its geometric layout and on the level of op­tical accuracy that can be obtained. A variety of geometric parameters, e. g. the height of the absorber, the number, size and distance of primary mirrors influence the shading and blocking of rays and the amount of rays missing the absorber. To evaluate the influence of the parameter variation regarding the electricity cost price and to yield an optimization, the optical performance is assessed with an annual simulation based on hourly weather-data. To permit a consideration of changes in collector cost according to different geometric layouts, cost factors where allocated to geometric parameters. The paper presents the method and the simulation re­sults of the optimization under different boundary conditions and shows how the developed simulation tool can lead to an optimum collector design with respect to cost price of electricity. The sensitivity of the results will be discussed.

Introduction

Similar to the parabolic trough system the linear Fresnel-collector, which is a piecewise approximation to the parabola, is suitable to produce steam for use in solar thermal power plants. The collector comprises of slightly elastic curved mirror-stripes, which reflect the sunlight to a fully stationary receiver (see figure 1).

secondary concentrator primary mirrors

insulation glass plane

-W

The receiver consists of a secondary CPC — type concentrator and a selective coated tubular absorber with no need of a vacuum insulation. Principally the collector is not lim­ited in aperture width[6], therefore a wide range primary mirrors glass plane —

of free geometry parameters is possible. The width of the primary mirrors B has to be

TOC o "1-5" h z coordinated with their gaps D, their number :

N and, the hight H of the receiver. Several studies ([1], [2]) have certified promising cost

perspectives of the linear Fresnel-concept ——- —

but at less specific energy yield. Hence an W ■

optimization of the geometry and the field Figure 1: Principle °f the linear Fresnel size is only meaningful with an economic collector

assessment.

The collector considered in this paper is intended to produce superheated steam at 440 °C

and 50 bar, therefore the collector-field is divided into sections for preheating, evaporating and superheating.

(a) cosine-losses (b) shading (c) blocking

Figure 2: Geometric losses of Fresnel type collectors

The main geometric losses of the Fresnel-concept are shown in figure 2. These effects can be decreased by heightening the absorber and by widening the gap between primary mirrors. On the other hand geometric changes cause losses due to inaccuracy of assembly and tracking of the primary mirrors.

Approach

For analyzing and optimizing the geometry, the optical efficiency is not of primary interest. The optimization for maximum performance of the collector at noon would differ from the one for lowest LEC[7]. The difference would be up to 10% in the LEC between the different optimization targets. Therefore an integral view on the whole system is essential. A link between the optical, thermodynamic and cost models is necessary to take the main influences into account. The electricity yield of the power-block is calculated via annual simulation based on hourly weather-data of a certain site under specific boundary conditions. After consideration of the specific cost the LEC is evaluated and chosen to assess the configuration. Hence arbitrary geometries and receiver-concepts can be investigated.

Biomass-Solar-Hybrid Plant

1.1 Technical Description

Within this chapter the integration of a solar collector into a 20 MWei biomass Rankine cycle will be presented with respect to technical and mainly economical aspects. The following benefits will result from the combination of both technologies:

• The plant is operated exclusively with renewable energy-sources

• Biomass is a limited source of renewable energy. With solar thermal electricity production the amount of biomass substituted by solar energy can be used in other energy applications

• No net-production of CO2 during operation

• Typical live steam parameters of a biomass plant correspond to those of the Fresnel-Collector (approximately 450°C and 70 bar)

• The plant can be operated 24 hours a day without the need for heat storage.

• Compared to Solar Only plants, the efficiency of converting solar radiation into elec­trical energy may be higher in hybrid power plants since the steam cycle always runs at full load.

Figure 1: 20 MWel Hybrid power cycle (biomass / solar thermal), calculation in Ebsilon, efficiency (steam->el, net): 34,8-36,4%, depending on cooling water temperature)

In order to use realistic assumptions for the power block within this study, its layout is based on existing biomass power plants of E. ON in Zolling or Landesbergen, Germany. However this kind of system would not be appropriate to be built in Germany because of the limited solar energy yields. The technology as described is applicable for example in South European regions.

Figure 1 shows the water / steam cycle of the hybrid plant. Both heat sources — solar field and biomass boiler — are connected in parallel. In order to supply a constant electricity output (20 MWel), the biomass boiler provides the amount of thermal energy that can not be supplied by

0 50.000 100.000 150.000 200.000

solar field size [m2]

Figure 2: Annual solar share depending on the solar field size (24 h/d operation)

the solar field. To ensure reliable operation, the biomass boiler runs at 50% or more of its nominal thermal load. Accordingly, the solar thermal input rises up to 50% depending on the availability of solar irradiation.

The size of the solar field is varied within these simulations from 0 m2 to 190’000 m2. The solar share, i. e. the percentage of the annual amount of solar steam fed into the turbine is pictured in figure 2. The solar field size 0 m2 corresponds to the pure biomass plant without

solar field.

The amount of solar energy depends approximately linearly on the solar field size between 0 and 65’000 m[1] [2]. Solar fields greater than 65’000 m2 lead to dumping of solar energy during hours with full irradiation, because the solar field must not provide more than 50% of the nominal steam mass flow for the turbine.

Optical Model

For the calculation of the optical performance at a designated sun position the model has to factor in the geometric layout of the primary field and of the secondary concentrator, the angular dependent material properties and the deviation from ideal reflection by optical errors and the sunshape[8]. For obvious reasons there is no closed form of an analytical formula available to calculate the optical performance of the linear Fresnel collector. A common numerical method is the raytrace algorithm, which is a bit slow to compute in view of the needed high amount of rays. For optimization and analysis purposes a time efficient algorithm is more advantageous. As one can treat the distributions of the sunshape 2sun(a) and of the optical errors [9] as statistically independent ([3]) they can lumped via convolution into one effective total relative radiance distribution Zeff[10]. Because of the linearity of the collector the convolution is needed only in one direction and can be expressed by equation 1.

2eff -0 )Tsun(o )dO (1)

-*k

The relative angular distribution of the optical errors is assumed to be standard normal with the standard deviation <7, which implies all errors of the primary field6.

(2)

‘Q2

XeffH

cos i

da-

Ip = Ib T

a1

Figure 3: Explanation of the optical model

(c) integral of If a)

For calculating the irradiance IP at a Point P (see Fig. 3) that is reflected from the sun at a mirror-segment M, only the bounding angles and a2 between the ideal reflection l’ of the sunvector l have to be regarded. To consider sunpositions that deviate from the transversal plane with the angle 9, the rayexpansion can be factored in with its cosine.

In order to compute the flux distribution of the reflection of a single mirror at the aperture of the secondary concentrator, the contribution of each finite mirror-segment has to be summed up for each point. As shown in figure 4 the raytrace algorithm reaches the smooth profile of the convolution algorithm only after a huge amount of rays. The profit in time without losses in accuracy is significant.

a ^ 0.00349 <7sun ^ 0.0041 50e3 rays

Figure 4: Comparison between raytrace and convolution algorithm

a ^ 0.00349 <7sun ^ 0.0041 5e5 rays

To describe the angular dependent optical performance of the collector, it is sufficient to calculate the incident angle modifiers (IAM) of the transversal Kx and the longitudinal K plane (see figure 5). A common interpolation formula is the factorization of the IAM [4]:

r? opt = Щ KX(0J K

(3)

Instead of 0 in formula 3, the incident angle 0^, which is the angle between the transversal plane and the sun-vector, is used in the model in order to get higher accuracy.

Beside the angular dependent material prop­erties the main influence on K is the increas­ing amount of rays missing the absorber due to the increasing length of reflected rays. The angle that influences the length of the rays is 9, therefore this angle is the best appropri­ated for the use in in the interpolation. With 0 the annual yield is underestimated in a range of 2 to up to 5% at higher latitudes.

Economic Optimization and Evaluation

First the economic assumptions in a conservative approach will be described before presenting the results of the LEC calculation. At last there will be an analysis under the economic conditions and the legal framework for renewable energies in Spain.

Assumptions

The economic assumptions are given in the following table: Table 1: Economic assumptions

Investment costs:

Specific Powerblock Investment 1

2’241 €/kW„l, net

Total Solar field investment 2

SF_Invest = 130€/m2 x 437’639m2 x (A / 437’639m2)093 ^ 127 €/m2 (617’000 m2) to 137 €/m2 (210’000 m2)

Annual costs:

Insurance

1% of total investment costs

Operation&Maintenance Power Block

6% of Power Block Investment

O&M solar field

O&M_SF = 3 €/(m2a) x 437.639 m2 x (A / 437’639 m2)072 ^ 2.7 €/(m2a) (617’000 m2) to 3.7 €/(m2a) (210’000 m2)

Biomass (fuel cost)

0.5 ct/kWh

Base load, 24h/d, 3 weeks revision in Jan.

Financial boundary conditions:

Interest rate

8%

Lifetime

25 years

Compensation (Spain):

Electricity (www. omel. es [4], average price March 03 — February 04)

3.74 ct/kWh

Bonus electricity Biomass (RD2818) [5]

3.05 ct/kWh

Bonus solar thermal elec’y (RD2818) [3]

12.02 ct/kWh

Cost assumptions apply for the second or third plant to be built, having under control the unexpected difficulties related to new technologies. The first demonstration plant will have considerably higher solar field costs.

SHAPE * MERGEFORMAT

Results

Solar Field [m2]

Figure 4: Average annual earnings (beyond the required 8%) as a function of the solar field size.

Provided that the bonus tariffs in Spain will be valid for hybrid operation, it is economically very attractive to integrate a solar field into a biomass plant. The solar field should ideally have a mirror surface of 68’000 m2 for a 20 MW biomass plant. The additional earnings between 0 and 120’000 m2 (see figure 4) indicate, that the solar field in a hybrid plant is profitable with the assumed bonus. If the bonus will decrease, the additional earnings will decrease until the additional solar costs equal the additional bonus. If the bonus increases above the assumed value of 12.02 ct/kWh — as being discussed these days by political decision makers in Spain — the maximum will shift to higher surface areas.

The resulting levelised electricity costs depending on the solar field size are given in Figure 3:

Solar field size [m2]

Figure 3: Levelised electricity costs of a 20 MW biomass solar hybrid plant, as a function of the solar field size [m2]

Due to high costs of the solar field, the levelised electricity costs of a biomass plant can apparently not be reduced by coupling it with a solar field.

In Spain the legal framework for renewable energies is defined in the so called »Real Decreto 2818/1998«. For biomass and solar thermal electricity a bonus on top of the market price is being paid to cover the additional electricity generating costs. The bonus as well as the average electricity price of the Spanish electricity exchange market (March 03 — February 04) are given in table 1. For the calculations it was assumed that the corresponding tariffs could be applied according to the solar and the biomass share. The criteria for evaluating the plant investment is the annuity method, that is: In case the realised cash flow leads to an internal rate of return higher than the required 8% (see table 1), the annuity is positive, otherwise negative.

Thermodynamic Model

For describing the thermodynamic behavior of the collector, a semi stationary model, based on an energy balance, is used. With equation 4 the reachable massflux M of a collector segment is calculated via the absorbed radiation Iabs = r? optIb, the heat of in- and outflow.

referred to direct horizontal irradi — ance Ibh

%

losses qloss and the enthalpy-difference zih

(0opt(0zCr) lb-q!

M

) Ac

4hi

The whole collector-field can only reach the lowest massflux of the three sections:

7в = arcsin(cos(7)sin(0z))

(a) 5 x 104 rays (b) 5 x 105 rays

M = min^, M2, M3) (5)

This way matching losses between the sections are taken into account. To make sure, that the collector is in operating conditions to produce its corresponding output, a dynamic thermal node at each end of collector-stage is used as shown in equation 6.

(mcp)l dt — (Ubs^loss) Wap (6)

The thermodynamic model is implemented in the simulation environment Co/S/m[5] which was developed at the Fraunhofer ISE to simulate solar-collector systems.

Power Block

(a) flow sheet

(b) part load of a simp/e fresh water coo/ed process

Figure 6: Powerblock Cost Model

The cost model is based on a first estimation for a starting configuration with N = 34,

H = 7.5 m, D = 0.075 m and an absorber tube with a diameter of 15 cm. The specific direct cost8 of this collector would be Cc = 90 €/m2 for the mentioned configuration. This figure corresponds to cost estimations of the Solarmundo collector. The cost estimations were evaluated and seem reasonable for a third plant. All changes in cost due to mutations of the geometry are assumed to be linear. The specific direct collector-cost Cc is expressed by equation 7.

Cm N + Ch (4m + H) + Cd (N-1) D + Ca

Cc ^ NB (7)

The whole investment Бр of the solar field, power block, infrastructure, land and engineering can be calculated with:

The power b/ock is assumed to be a simple process with only the fresh water tank as a preheater (see figure 6). The condenser is cooled by fresh water. The part load behavior is integrated into simulation via look-up-table.

TE — (CcAc + TO) x (1 + «e) + C|Ac(1 + -^^-21 + 2pb The LEC is calculated via annuity «a and the annual electricity yield.

(8)

LEC =

(«a + K. + ^Mt1 + «c)TE

Ja Eeldt

Table 1: 50 MW Faro DNI = 2247 kWtlm2a

Cc

90 €/m2

specifi c direct collector-cost (N = 34, H = 7.5 m, D = 0.075 m)

Cm

30.5 €/m

specifi c cost of a primary mirror (B = 0.5 m), incl. structure, tracking etc

Ch

13.8 €/m2

specifi c cost of the absorber tower

Cd

11.5 €/m2

specifi c cost of gap

Ca

304.0 €/m

specifi c cost of the absorber

Cl

3.0 €/m2

land and preparation

To

4002 t€

others, piping steam-separator

T

640.0 t€

Infrastructure

Tpb

33600 t€

powerblock 672 €/kW

a

9.368 %

annuity 25 a rate of interest 8%

«e

22.5 %

engineering, commissioning, project managing & license

«O&M

2 %

operation and maintenance

«i

1 %

insurance

c

5 %

additional mark-on for contingencies

The assumptions of the cost model are shown in table 1.

Solar Only Plant and Gas Co-firing

From the starting point of a 50 MWei Solar Only steam plant, it will be analysed how natural gas co-firing influences the electrical yield, the solar share and the costs of the plant. For this analysis 240 plant configurations with different solar fields (210’000 m2 — 617’000 m2) and boilers (0 MWth — 150 MWth) were examined.

2.1 Technical Description

Power Cycle

The 50 MWel power cycle is equipped with sea water cooling and the feed water storage tank as single preheater (see figure 5). Its net efficiency is 32.8% (at a water temperature of 14°C, for warmer water temperatures and at part load the efficiency drops). [4]

The steam is produced by the solar field. If it does not reach the maximum turbine load it is supported by a parallel gas boiler. As opposed to the biomass variant, it was taken into consideration that the solar share should remain high. The assumed operation mode is that gas will be co-fired, once the solar collector can provide more than one kg/s steam.[5] An exemplary day (July, 13) is given in figure 6. The boiler’s thermal energy production is limited either by its thermal capacity or by the nominal mass flow of the turbine. For technical reasons the boiler operates between 20% and 100% of its nominal capacity.

time

Figure 6: thermal energy to power block by solar field (453600 m2) and gas burner (Pth, n=70 MW), july, 13

Analysis and Optimization

The main parameters which influence the optical performance and solarfield investment are the hight H of the receiver, the gap D between primary mirrors and their number N. The influence of variation of each parameter on the LEC is shown in figure 7a-c. The calculation was made for three different optical errors. It is obvious that higher optical errors decrease the performance of the collector and increase the LEC. For small optical errors the optimal receiver height is displaced to higher values. The optimum gap between primary mirrors, which is assumed to be constant, is less dependent on the optical errors. The ground coverage is about 80%. Additional primary mirrors reduce the specific collector-cost but also decrease the specific performance.

As mentioned above the primary-mirrors are curved elastically, hence they have a distinct focal length f. The relative focal length f is defined as the ratio of the focal length and the distance from the mirror to the absorber. The influence of the relative focal length f on the LEC is shown in figure 7d[11]. The optimum of f is a bit higher than one, with less dependence to higher than to minor values.

H[m]

(a) variation of height

Figure 7: Variation of geometric parameters (D = 0.075 m, N = 34, H = 7.5 m)

gap [m]

(b) variation of gap

(c) variation of primary mirrors (d) variation of the focal length

An optimization of the three parameters corresponding to different optical errors leads to lower LEC shown in table 2. By reaching a high optical accuracy, a higher placed receiver enables more primary mirrors and hence the cost reduction potential is very high.

Table 2: LEC of the configuration opti — [mrad] 2.32 4.65 6.98

mized on the optical errors LEC 11.3 12.1 13.2

It is also possible that the gaps between primary mirrors are increased linearly rather than constantly. A linear increasing gap effectuates an optimum for lower receiver heights, because the blocking at high zenith angles is reduced. The difference at the boundary conditions presented here is very low. The LEC of a collector with constant gaps is about 3-8 0/00 higher. The decision whether to use linear or constant gaps depends on the specific cost of the receiver hight Ch and the expense of the linear increasing gap. With respect of simplicity a constant gap might be more favorable.

Conclusion

The linear Fresnel-concept has the potential of low electricity cost prices. Considering

the numerous possibilities of variation in geometry an optimization for a distinct purpose

in respect of reachable accuracy and specific cost factors is important. With the method

presented it is possible to evaluate intended improvements regarding the LEC.