Utility Scale PV and CSP Solar Power Plants

8/12/2019 Utility Scale PV and CSP Solar Power Plants

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Utility Scale PV and CSP Solar Power PlantsPerformance, Impact on the Territory

and Interaction with the Grid

Authors:

Martino BosatraFederico Fazi

Pier Franco Lionetto

Luca Travagnin

Foster Wheeler Italiana Milan, Italy

Presented at

Power-Gen Europe 2010

Rai, Amsterdam

The Netherlands

June 8 10, 2010

TP_SOLAR_10_01


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Utility-Scale PV and CSP solar power plants

Performance, impact on the territory and interaction with the grid

Martino Bosatra, Federico Fazi, Pier Franco Lionetto, Luca Travagnin

Foster Wheeler ItalianaCorsico (Milan- ITALY)

1

ABSTRACT

PV and CSP Solar Power Plants represent today one of the most booming segments within renewable

energy market. Both type of solar plants, boostered by favorable feed-in tariffs, are under

construction in different countries covering a range of size (up to 100MW) commonly reserved to

conventional technologies (thermal or combined cycle power plants). The report analyses the PV and

CSP Plant technologies and compares them with respect to landscape impact, performance and

impact on the grid (both in normal and transient condition). The relationship with the ancillary

services needed by the Grid to ensure

high service continuity and high quality level is investigated and possible future scenarios are

discussed starting from the requests of Grid-Codes.

2 INTRODUCTION

Utility-Scale photovoltaic (PV) plants and Concentrated Solar Power (CSP) plants are becoming a reality.

The uptake of such type of solar plant is expected to accelerate during the next decade especially in regions

such as Southern Europe due to the presence of high solar irradiance and the continuous pressure for the

implementation of numerous renewable energy generation technologies.

The growth of such plant is granted only if unlimited access to the HV and EHV grid is allowed. Besides

this issue, the higher penetration of renewable energies (especially wind and solar) harbors the risk of grid

instability in case the generating plants are not able to properly support the Grid. Thus leads to the search for

new developments with respect to the device design which should be armed with the task of supporting the

grid operation and stability. New interconnection requirements, especially those are related to Utility-Scale

PV plants, are coming into force in several European countries with the aim to serve as a planning document

and decision guidance for the project designer and for the PV equipment Manufacturers.

In the light of the above, a qualitative comparison has been carried out between an Utility-Scale PV plant

and a CSP plant with nominal capacity of about 50 MW, reasonably assumed as reference size for both

technologies (this size looks typical for most of the existing plants of both technologies). At once it has been

necessary to define a specific region (Southern Italy) for which were known solar beam (i.e. irradiance level)

and average ambient conditions, having these aspects huge influence on Plant design.

Then it have been analyzed pros and cons of the two PV and CSP plants in terms of performance, i.e.

efficiency and energy production, and impact on the territory, i.e. how many Hectares are needed and which

is the impact on the regions where installation of such plants seem to be more probable.


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This report would also outline which one of these two different solar plants may be nowadays considered as

less impacting on the Grid, which of them may or may not contribute to the grid stability during transients

and finally which would be the technological improvements that may be required to make them compliant

with the mandatory Grid Code requirements. The analysis do not refer to a specific Grid connection point

but refers anyway to the European HV network because the majority of the existing Utility-Scale PV Plants

and CSP plants are nowadays localized in Europe. Therefore the study refers to European Grid &

Transmission Codes and in particular makes reference to the German Transmission Code [1], which was the

first one, within the whole European Countries, had approached the topic of the renewable generating units

and their impact on the Grid.

3 SOLAR ENERGY RESOURCES GROWTH AND GLOBAL INSTALLED POWER

3.1 PV installations overview [1] [2]

The solar PV sector has been booming over the last decade and is forecast to confirm this trend in the

coming years. By the end of 2009 the Global worldwide cumulative capacity was approaching 15 GW.

Today, Europe is leading the way with more than 14 GW representing over 65% of the Global cumulative

PV installed capacity equal to almost seven times the installed power in Japan and ten times that of USA,

confirming as leader in the geographic area of PV installations. The growth in installations in Europe is even

more interesting when compared with the rest of the world, because in terms of market dynamics there was a

significant increase of so-called Utility-Scale PV plants.

However, this Europe leadership is the result of strong inhomogeneous in the growth of the States

composed, with some realities that are confirmed excellent market for PV, other that "marking time" and

others that seem to be well placed to play an important role in the near future.

These differences can be explained by the relative importance of plants segments (i.e. building installations,

industrial installations and multi-megawatt installations), and by policy choices of national governments,

resulting in rules and incentives to promote greater one segment respect to another and by the morphology

of the national territory that might be a limitation especially for larger plants.

3.2 CSP installations overview [3] [4] [5]

After almost 20 years of silence, in the early years of the new millennium, CSP gained interest again and

new plants have been built not only in USA but also in Europe.

At the moment the total worldwide installed power is approximately 655 MW producing 1400 GWh of

electric power in a year. Apart from USA and Spain, other countries where CSP plants are going to be built

or at least in project, are Australia, India, Central Asia, Mexico and the Mediterranean countries like Italy,

France, Greece, Turkey, Morocco, Libya, Algeria, Egypt and Israel.


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In contrast to the PV sector, the CSP industry is far less fragmented; with some of Europes best solar

resources and a favorable tariff regime, Spains solar CSP sector is well positioned, 22 projects under

construction for total 1,037 MWe out of the approximate 15 GW planned globally through 2014.

Italy is the first country following Spain to make a significant move in support of CSP projects with FITs. In

2008, Italian Government issued the legislative decree that set a new FIT for CSP; this decree requires that

all CSP plants, including hybrid plants, have to include thermal storage while in Spain storage technology is

allowed but not currently required for system approval.

4 SOLAR POWER PLANT TECHNOLOGIES

4.1 Utility-Scale PV plant

Utility-scale PV plant, sometimes called central station PV, acts more like a concentrated power plant,

producing energy and delivering it to the grid. Traditionally Utility-Scale PV plants use flat PV module

technology of either crystalline silicon (mono or poly-crystalline silicon types) or thin film modules (mainly

of cadmium tellurideCdTe type). Such PV modules are characterized by different sizes, i.e. 0.7 m2 to 1.7

m2, by different peak power, i.e. 70Wp to 250Wp, and by different efficiency ranges which typically are

lower for Thin Film PV modules, i.e. from 10% to 12%, than for crystalline silicon PV modules, whose

typical efficiency is in the range of 11% to 16%.

Another PV technology is represented by the concentrating photovoltaic (CPV) plant technology, which is

based on reflection of concentrated sunlight onto highly efficient photovoltaic cells (such as copper indium

gallium diselinide - CIGS, and thin film amorphous silicon). Nowadays such technology is used only in

smaller or prototype PV installations and therefore cannot yet be considered as a viable alternative to other

technologies for bigger Utility scale PV plant installations.

Since the Utility-Scale PV concept is relatively new, so these different technologies are to date competing

with no clear winners even if most ofthe biggest Utility Scale PV plants (i.e. within the range of 4060

MWp) have been realized by fixed system. The cost of PV tracking system is usually greater than the cost of

fixed PV system as well its performance is greater than the performance of the fixed PV system.

PV modules are installed on fixed metallic support structures arranged in long rows, adequately spaced

themselves, facing south (in the Northern Hemisphere) with an appropriate tilt, or deployed on tracking

devices to follow the sun. PV modules are electrically connected together in series and parallel and then

connected by DC cabling to the centralized inverters which convert DC power into AC power. Then the

Inverter are connected together, on a.c. side, to the plant Medium Voltage network, and then the produced

energy is delivered to the HV or EHV Grid by means of one or more step-up transformers.

4.2 CSP plant

All the CSP (Concentrating Solar Power) technologies produce heat or electricity using hundreds of mirrors

to concentrate the suns rays to high temperature (typically between 400C and 1000C).


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There are four different main CSP technologies depending if the solar radiation is concentrated on a linear

collector system (Parabolic trough and Linear Fresnell) or on a central focal point (Solar Tower and

Parabolic Dish).

Among the possible CSP Technologies the Parabolic Through seems to be the most commercially proven

and it appears to be, at present time, the favored by CSP plants developers. Though Power Tower lacks in

experience with few plants in operation, in medium to long term this configuration may produce electricity

at a lower cost than parabolic trough plants. On the other hand, Parabolic Dish and Linear Fresnell

systems have not progressed beyond their initial demonstration phase, though these technologies have the

potentiality to produce energy with lower capital cost respect the other two.

The CSP technology taken as reference for this study is based on Parabolic Through technology, mainly

constituted by:

the Solar Field, constituted by the Solar Collectors, which collect and concentrate the solar radiation,

and the by the Solar Receivers, which absorb the concentrated solar energy and convert it to useable

heat for the power block;

the heat Storage, which stores solar energy from solar field and dispatch it to the power block in case

of less solar radiation;

the Power Block (and Grid connection) which uses the heat collected from the sun to produce

electricity and it is constituted by a steam generator and a steam turbine coupled to a conventional

electrical generator (i.e. synchronous generator and excitation system) that generates energy which is

delivered to the HV or EHV Grid by means of its own step-up transformer.In a parabolic trough system, the solar field consists of hundred of linear solar collector assembly (SCA).

The collectors are horizontal modular array of long rectangular, curved (U-shaped) highly reflective mirrors

disposed in parallel rows. Each modular array are single-axis-tracking (typically North-South to be always

tilted toward the sun during the day) that focus solar energy onto a central receiver tube located along the

focal line of the mirrors where it is absorbed. The receiver tubes are particular high radiation absorbent

devices where in the inside flows a working fluid called heat transfer fluid (HTF), which has the scope to

exchange heat from the solar field to the power block. HTF is usually synthetic thermal oil (byphenil oil)

and it is heated to temperatures of around 400C. HTF pumped through a series of heat exchangers to

generate superheated steam for use in a conventional steam turbine generator or integrated in a combined

steam and gas turbine cycle.

To increase CSP Plant flexibility, the HTF can be stored in two tanks (hot and cold) that provides many

advantages (avoid dumping of solar energy, reduce over-size of the power block, increase CSP plant

operating hours, etc) as detailed in next section. Recent advanced researches are experimenting different

kind of HTF and developing more efficient solar collector and more sophisticated sun-tracking systems in

order to improve plant performance. In spite of solid experience in this technology and more than 500 MW

operating plants, research and development continue to play an important role to drive plant cost further

down.


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5 TECHNICAL COMPARISON

The technical comparison between the two solar PV and CSP plants has been performed on the basis of the

following main design bases:

50 MW size, reasonably assumed as reference size for both technologies

both Thin Film and Poly-crystalline PV module technologies, both fixed and tracker systems for

Utility-Scale PV Plant

Parabolic Through technology for CSP Plant

Location: southern Italy.

The main factors used for the comparison have been:

Landscape impact

performance

interaction with the Grid in steady state and transient condition

To compare a 50 MWp Utility-Scale grid connected PV plant with a 50 MWe CSP plant it is necessary to

define common design bases by siting the plant in a specific area with defined irradiation level. For the case,

this report localizes both plants in a southern region of Italy characterized by a yearly averaged Incident

Global Irradiation of approx. 1900 kWh/m2 year and 2200 kWh/m2 year, as respectively estimated for a

fixed PV field with an optimum tilt of 30C and for tracker PV s, and a yearly averaged Direct Normal

Irradiation of approx. 1800 kWh/m2year.

The yearly averaged Incident Global irradiation is the effective radiation reaching the tilted PV module

plane and it is the sum of incident direct radiation, Incident diffuse radiation (from sky) and incident albedo

radiation for a given reference year.

The yearly averaged Direct Normal Irradiation is the yearly average amount of solar radiation incident on a

surface oriented normal to the solar radiation for a given reference year

5.1 Landscape impact

5.1.1 Utility-Scale PV plant

The key factor in designing the Utility Scale PV plant is to gain, for any specific site, the optimal groundarea occupation ratio (GAOR) without valuable reduction of expected performance ratio.

The GAOR is calculated as ratio between the total surface of the PV modules used to realize the PV field

and the ground area occupied by the PV field. The GAOR is affected by the shed disposition of PV arrays

(i.e. by the distance between the rows of PV modules) which depends by PV array tilt angle and by limit

shading angle. PV array tilt angle mainly depends by physical dimension of PV modules and by their ability

to capture and transform as much as possible diffuse irradiance (thin film modules are usually better than

crystalline silicon modules). As first approximation, the optimal limit shading angle for southern Italylatitudes corresponds to the angle (approximately around 20) needed to avoid shadows between 10 a.m. and

2 p.m. on winter solstice. PV field performance slightly increased by increasing, for a given tilt angle, shed


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distance because thus reduces shadows (especially during winter time) and therefore helps to reduce

mismatch losses.

GAOR typical values, for a fixed installation localized in southern Italy, is in the range of 0.4 0.6, which

corresponds to a tilt angle range of 35 25 (lower tilt are often used for thin-film PV modules arrays due

to their better response to diffuse irradiance) while for the tracker system the GAOR is in the range of

0.20.3.

The corresponding ground area typically required for the same PV field, always localized in southern Italy,

amounts approx. to 2.2 2.5 Ha/MWpif thin film PV modules are used, while amounts approx. to 1.7 2.1

Ha/MWp if crystalline silicon PV modules are used (efficiency of crystalline silicon PV modules is better

than the efficiency of thin film PV modules; typical values are respectively in the range of 14 16% and

1012%).

On the basis of the above figures, the estimated ground area needed to build a 50MWpUtilityScale PV

plant amounts to approx. 120140Ha, for fixed PV field constituted by thin film PV arrays, and to

90110Ha for fixed PV field constituted by crystalline silicon PV arrays.

In case of a tracker PV field, the required ground area amounts approx. to 4 4.5 Ha/MWp , if thin film PV

module are used, while amounts approx. to 3 3.5 Ha/MWpif crystalline silicon PV modules are used.

Even if these different technologies and arrangements are nowadays competing with no clear winners, it

can be observed that, in terms of land impact, fixed PV field requires about half of the area necessary for a

Tracker PV system and as highlighted above the selection of PV modules may play an important role in

determining the area required by the plant. Therefore it can be concluded that for bigger size of Utility-ScalePV plant, the fixed PV field arrangement should be preferable when compared in terms of land impact.

5.1.2 CSP plant

The Key factor that affects the CSP plant size, performance and thus land occupation, is the direct normal

irradiation of the site which optimum value should be over than 1800 kWh/m2year.

The CSP plants land occupation depends by the total installed collector surface and also by the space

between each solar collector assembly required to minimize shadows and also to allow the maintenance of

the big curved reflecting mirrors.

Additional surface is required for the power block and heat storage, both usually located in the middle of the

plant area and with an occupation much smaller respect to the solar field.

Typical surface required by CSP plant localized in southern Italy as the PV plant above considered, with

Direct Normal Irradiance of 1800 kWh/m2year, is in the range of 34 Ha/MW. However each plants solar

field area, in a determinate location with a certain daily average direct normal irradiation, needs to be

optimized respect to solar collector area and heat storage capacity in order to reach the minimum levelized

cost of energy that is usually reached by installing an heat storage of approx. 6 hours.

Therefore the estimated area required for a 50MW CSP plant hypothetically localized in southern Italy and

based on Parabolic Through technology, amounts totally to approx. 160170 Ha


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5.2 Performance

5.2.1 Utility-Scale PV plants

To maximize sunlight exposition PV plants are designed by selecting the optimal tilt angle and the azimuth

angle and thus by avoiding, as much as possible, shading between PV arrays. The energy produced by a PV

system depends by:

solar radiation incident on the modules plane, which depends on :

latitude of the installation site

front surface reflectance of photovoltaic modules

exposure of modules: the tilt angle (angle between the horizontal plane and the plane of the

module surface) and the angle of orientation (azimuth angle)

any shade or fouling of photovoltaic modules

ambient temperature

the characteristics of the modules: power rating, temperature coefficient,

mismatch losses, due to inverter mismatch and/or due to non homogeneous module

characteristics connected in series and parallel, etc.;

the characteristics of the BOS such as efficiency of inverter, losses in the cables and diodes,

etc.

Variations in solar radiation and ambient temperature from month-to-month and year-to-year influence the

performance parameters.The electricity produced by the Utility-Scale PV plant can be determined according to the method defined

by the EN 61724 standard which defines the PV overall system performance with respect to the energy

production, solar resource, and overall effect of system losses. These parameters are the final PV system

yield Yf, the reference yield Yr, and the performance ratio PF.

The performance ratio PR (dimensionless) is the Yf divided by the Yr.

Yf represents the number of hours that the PV arrays would need to operate at its rated power to provide the

same energy and it is expressed in hours or kWh/kW while Yr represents an equivalent number of hours atthe reference irradiance.

Yrdefines the solar radiation resource for the PV system and it is a function of the location, orientation of the

PV array, and month-to-month and year-to-year weather variability.

The expected performance of a 50 MWpUtility scale PV plant, localized in Southern Italy, is shown in the

following table; the tracker PV system produces approximately 20% more energy than a fixed PV filed, on

yearly basis with the same nominal installed power.


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Tab.1

Utility Scale PV plant

Performance table

Fixed PV system Tracker PV system

Thin film PV

72.5 Wp

Poly-crystalline

220 Wp

Thin film PV

72.5 Wp

Incident Global Irradiation ~1900 kWh/m2year ~2150 kWh/m2year

Solar field total surface ~ 500000m2 ~ 370000 m2 ~ 500000 m2

Total Plant area ~ 120 Ha ~ 85 Ha ~200 Ha

Installed PV Peak Power 50 MWp

Energy production

(at inverter output terminals)~ 75 GWh/y ~ 90 GWh/y

PR (yearly average) ~ 0.79 ~ 0.83

5.2.2 CSP plant

As per Utility-scale PV plant, performance of a CSP plant is affected by variations in solar radiation,characteristics of solar collectors, and by geometrical factors such as shadows, etc.

For CSP plants it can be defined an efficiency factor so-called capacity factor of the plant. This is the ratio

of the energy produced by the plant over the course of the year to the output had the system operated at its

nameplate capacity (e.g. nominal size of the steam turbine for 8760 hours).

The capacity factor strongly depends on the solar resource available in a certain location and on the solar

field size, so on the mirrors area. Oversize the solar field allows the plant to provide more thermal energy to

the power block and consequently results in a higher capacity factor but also leads to an increase ofinvestment cost. On the other hand, if the solar field area is reduced, a consistent part of the potentially

available solar energy is dumped during high solar radiation days: the thermal energy to the power block is

beyond the steam turbine maximum admissible thermal input.

The expected performance of a CSP plant of this size, always localized in the same place of South Italy, is

shown in the following table.

Tab.2 - CSP Plant - Performance table Parabolic Trough

Averaged Direct Normal Irradiation 1800 kWh/m2 year

Solar field (collector surface) ~ 500000 m2

Total Plant area ~ 165

Thermal Storage 6 h

Thermal input at power block 140 MWth

Net Power Output 50 MWe

Net Solar Electric Energy Production ~ 120 GWhe/y

Capacity factor 27


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5.3 Interaction with the Grid in steady state and transient conditions

5.3.1 Solar plants and Grid exploitation constraints

The worldwide liberalization of the electrical power generation has brought the issue of market equilibrium

into the electricity power industry and therefore all the generating firms have to compete by exercising

market power trying to maximize their individual profit but the competition with solar power generation,

especially with Utility Scale PV plants, may affect the bidding strategies of the conventional generating

firms with potential impact on the nodal prices and profits.

The presence of solar power generation gives incentives to the conventional generating firms to exercise

market power in a different way than the case in which the entire generation output is based upon

conventional power plants only, resulting in a different market outcome; in the presence of solar generation,

the conventional generating firms would be forced to decrease their bidding parameters in order to cope with

the increased availability of supply. Hence, since the solar plants are operated in a competitive manner, the

market clearing price is reduced, thus the social welfare is increased. On the other hand, at the time whenthere is inadequate solar irradiance for the solar plants to generate power, the conventional generating firms

may take advantage of the lack of supply and alter their bidding strategies in order to force the nodal prices

to increase, thus increasing their profits. Considering that EU has set very ambitious targets regarding the

penetration of renewable power generation, it can be anticipated that the implementation of multi-megawatt

solar plants may cause some problems regarding the market prices.

This issue may be more remarkable if the network to which the solar plants are connected present some

bottle necks causing congestion (e.g. overload of lines that, due to safety reason, obliged connected solarplants to reduce their output). Furthermore, this effect would be more severe in Southern Europe regions (or

islands without suitable interconnection with the continental Grid) where unfavorable weather conditions

(especially for PV that is affected by temperature) and better irradiance can have a crucial impact on the

produced power.

Large-scale integration of multi-megawatt Solar Plants, especially Utility Scale PV plants, into Grid

operation would therefore lead to new operation constraints (e.g. power is produced during the day, when

the electricity demand is high, thus it is valuable peak current), for the entire HV distribution & transmission

system, that could result unacceptable in the next years in terms of system performance (grid and generating

plant). This fact would requires a rethinking of both the grid exploitation modality and of design of solar

plants, by adding for instance appropriate storage system.

a) CSP plant thermal Storage System

The optimum solution to keep a high capacity factor and a constant power production is to adopt the heat

storage technology.

With the heat storage, the energy excess from the solar field can be stored and dispatch to the power block

during solar radiation lack. Theoretically CSP plant with substantial heat storage can reach 100% capacity

factor and may provide base-load electricity as the conventional fossil fuel plants. In other words if sky is


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momentarily clouded or even during first hours of night time, CSP plant may operates at its regime putting

to use the solar energy previously stored.

Depending on the storage medium, the heat storage systems can be either direct (storage medium is the

same HTF circulating in the solar field concentrator, e.g. synthetic oil) or indirect (storage medium is

different from the HTF, e.g. synthetic oil as HTF and molten salts as storage medium).

Advanced heat storage systems are under development. Nowadays the most used and commercially

available heat storage technology for parabolic trough is the two-tank indirect thermal storage.

Two-tank indirect system is based on two tanks typically filled with molten salts (60% sodium nitrates and

40% potassium nitrates): one hot tank and one cold tank. The excess of solar energy from the solar field is

directed to a heat exchanger where cold molten salts are taken from the cold storage tank, heated from

approximately 290C to 390C and then stored in the hot tank. When energy is needed, the system operates

in reverse: the heated molten salts from the hot tank are pumped to the heat exchanger to reheat the HTF.

In the following graph it may be appreciate how during high radiation hours the solar energy is delivered to

the power block and to the heat storage. After filling the hot tank of the heat storage, the excess of energy

has to be dumped. During the first hours of night, when there is obviously no solar radiation, the heat storage

supplies energy to the power block.

0

100

200

300

400

500

600

700

800

900

1000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Hour of Day [h]

Direct Normal Irradiation

To steam turbine

Net electric output

Dumped energy

To heat storage

From heat storage

450

300

150

50

DirectNormalIrradiation[kWh/m2

day]

Power[MW]

Fig.1typical summer day CSP plants performance with thermal storage

b) PV plant battery storage System

PV plants are intermittent resources especially because performances are strictly related to irradiance

condition and ambient temperature which affect power output. Using battery storage which has the

capability to be quickly started or changed from charging to discharging in millisecond time frame, may

help to smooth spikes due to intermittent clouds over a PV system (which cause output to spike widely) and

make them more dispatchable, increasing their capacity factors.


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Battery size will depend on the amount of peak shaving desired; if the load profile to be guaranteed doesnt

allow peak shaving beyond a certain limit, the constraints being the depth of discharge limitations on the

battery itself. Of course is not so easy to determine the required daily battery discharge capacity since

producible energy is not constant all over the year and not easy to predict. Less storage may be required to

shave the peak day load shape during summer then what is required for winter peak day load. Although the

batteries have to be available to operate, they do not necessarily have to be operated on a daily

charge/discharge cycle to enhance power output capacity. Thus, relatively inexpensive light-duty batteries

would be adequate to enhance their capacity factors especially during the summer (cycling less than 50

times per year vs. heavy-duty and more costly batteries cycling daily up to 250 times per year).

0

5

10

15

20

25

30

35

0

200

400

600

800

1000

1200

0 4 8 12 16 20 24

Power[MW]

GlobalincidentirradianceinPVmoduletiltedplane

[kWh/m.day]

Hour of Day [h]

Global incident irradiance,

clear sky model

Global incident irradiance

in PV module tilted plane

Net Power at grid

connection Point

to battery storage

from battery storage

6

6.5

7

7.5

8

8.5

9

Power[MW]

Time of Day [hh:mm]

Fig.2typical summer day Utility-Scale PV plants performance with battery storage system

5.3.2 Solar plants and Grid Code rules

Multi-megawatt solar plants have to be connected either to HV or EHV networks. The grid connection point

and the voltage level to which the plant has or can be connected it depends by local network conditions and

also by different rules adopted by different countries; in some cases this can only be determined by acalculation of the network operator.

In the last couple of years, some new Directives and new Grid-Codes have been released by national

Authorities and by Transmission System Operators; such guidelines are not harmonized and therefore

inhomogeneous requirements are still required at national level (most probably due to the local Grid

situation). Facing the growth of multi-megawatt solar plants (most of them are Utility scale PV plants rather

than CSP) connected to the HV distribution systems, the year 2009 brings fundamental changes for new PV

systems in a number of European countries. While in the past, renewable generating units connected to thenetwork were commonly not required to take over an active role and had to disconnect at the first sign of

trouble, the new guidelines now require also the renewable generating units to actively support the grid


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during normal as well as disturbed conditions. This step is being regarded more and more as absolutely

necessary to guarantee reliability and quality of supply in the mid- to long-term. This new approach has

already been adopted in Germany and France and is being to be adopted also by other countries.

Although the new Directives and new Grid Codes introduced specific requirements for wind and solar

generating plants, CSP plants are not specifically mentioned. It is unclear whether they should be treated as

renewable generating units or should be considered, especially the Parabolic Through technology, in all

respects as conventional generating units (due to behavior of power block which is used to generate

electrical power from the solar field) and therefore have to comply with the relevant specific requirements.

Mandatory requirements specified by German Transmission Code [6] (German Operator is the first one,

within the whole European Countries, that has approached the topic of the renewable generating units and

their impact on the Grid) for renewable generating units are analyzed in the following chapters

a) Active power control (dispatching) and Remote set point control.

To avoid possible network congestion in case of line loss due to electrical fault any renewable generating

unit, such as Utility-Scale PV plant or and CSP plant, shall be able to reduce its power. Therefore the TSO is

in the position to require curtailment of the power output of solar power plants to face with the specific

critical system conditions. Different requirements may be applied for the CSP plants rather than the Utility

Scale PV plants because the CSP looks like a conventional generating plant using rotating synchronous

generating unit, while PV is using static inverter.

Power output of PV generating plants has to be reduced in steps of 10% per minute, under any operating

condition and from any working point to a maximum power value (target value) which could correspond

also to 100% power reduction, without disconnection of the plant from the network.

Such requirement might be applied also to CSP plants if they are considered in all respect as renewable

generating units, otherwise they have to fulfill requirements set for the conventional generating units, which

are required to reduce or to increase - their output with different ramp-rate (e.g. 1%/min. as required by

German Transmission [1]) between the minimum stable generation power and the continuous output.

CSP plants may fulfill the requirement set for the PV generating units provided that the set-point given by

TSO is compatible with the minimum operating load of the boiler and steam turbine. Being the

characteristics of the CSP plant similar to the traditional thermal power plant, the load reduction can reach

values around 40% of the nominal capacity, with limiting factor the stable operation of the heat exchangers.

The advantage of CSP is that, in case of temporary reduction (limited to few hours), the solar energy

captured by solar field is not loosen because could be stored in the thermal storage system (of course it

depends on specific storage capacity, by the actual operating condition, by sun condition, etc).

Also for the Utility Scale PV plants such requirement do not constitute an issue provided that an automatic

power sharing management system is installed and it is capable to modulate the production of the entire

plant, by acting on each inverter, through a communication based solution, by sending new power output set


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points or by sending shut down command to disconnect several inverters, or again, by combining the two

controls. Of course if battery storage system is not installed, the amount of reduced power is definitely lost.

So from technical point of view there are no barriers that may prevent CSP and utility Scale PV plants to be

compliant with such specific Grid-Code requirements, while a cost increase it may be envisaged for the PV

plants (rather than CSP plants) to implement the power sharing management system.

b)

Automatic reduction of active power generation according to active power droop characteristic in

situations of over-frequency

To avoid risk of unsafe system operation when the frequency rise over a certain value, any renewable

generating unit, such as Utility-Scale PV plant or and CSP plant, shall have the capability to reduce its

power generation when the grid frequency exceeds pre-set value. Reference value for the active power

reduction P would be relative bigger percentage of the currently available power generation value at the

point in time when the grid frequency is equal to 50.2Hz. This active power reference value must be reduced

according to a coefficient of some output percentage per Hz when the grid frequency deviates from the pre-

set value. The German Transmission Code requires, for all the renewable-based generating units, to reduce

the active power with a gradient of 40% of the plants instantaneously available capacity per Hz as shows in

the below fig.3. Requirements, in terms of power output adjustment and time duration, may be different

country by country and mainly depends by local network conditions.

Fig.3Active power reduction of renewable-based generating units in the case of overfrequency

Furthermore the generating units have to remain connected to the grid (without tripping) if either the gridfrequency increases to values equal to 51.5Hz or decreases to values equal to 47.5Hz; above 51.5Hz and

below 47.5Hz the plants can be disconnected.

These requirements can fairly easily be fulfilled by the Utility scale PV system. A new control scheme has to

be included in each inverter to control the operation point of the PV string and thus the power output. The

inverter will automatically reduce the power output and stays constant until frequency is decreased below

the pre-set value and after will increase automatically the power output switching to MPP (maximum power

point) tracking control.

CSP plants can easily fulfill such requirements and additionally could provide, since could be considered in

all respect as conventional generating units, the primary frequency control, provided that the whole plant

control (steam turbine, steam exchangers and thermal storage) is capable to operate in droop mode and with


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the required ramp rate. This coordinated regulation requires the contemporaneous reaction of steam turbine

control system (that acts on the inlet steam control valves) and of steam exchangers control system (that acts

on feed-water flow and hot fluid flow) to meet the fast response and wide variations required by the

frequency control. CSP plant may also provide ancillary services, such as secondary frequency control and

minute reserve, by means of appropriate thermal storage system.

c) Minimum power factor at connection point, reactive power control and set point control for voltage

stability, also remotely by network operator

Slow changes in network voltage have to be kept within acceptable limits. In case of operation requirements

and on demand of the system operator, any renewable generating unit, such as Utility-Scale PV plant or and

CSP plant, has to support network voltage by injecting on the Grid appropriate amount of reactive power, in

accordance with Network operator request.

Utility-scale PV plants nowadays installed are designed to produce active power only. Reactive power is

avoided due to losses in the inverter, lines and transformers. To meet the requirements of the grid codes, the

inverters of the Utility-Scale PV plant have to be designed bigger or a centralized static VAR compensation

system have to be installed. Reactive power has only to be provided during feed-in operation, so there is no

need to provide reactive power during the night. Overall, an increase of PV installation system costs can be

expected.

CSP plants can fulfill the minimum power factor and/or the reactive power control because their generating

units are constituted by synchronous generators equipped with excitation system capable to provide reactive

power as required. Therefore the amount of reactive power that can be delivered to the Grid mainly depends

by size of generator and relevant excitation system as for the conventional generating units.

In addition, CSP plants may be requested, since could be considered in all respect as conventional

generating units, to provide different amount of reactive power during different voltage situations. Apart

from the requirements to provide reactive power supply in the nominal design point of the generating unit

(P=Pn), there would be also requirements concerning operation at an active power output below the nominal

active power (P


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all respect as conventional generating units, they should meet requirement set for the conventional

generating units, while all the other renewable generating units, such as PV system, have to meet specific

requirements as described below.

In the event of network, with consequent voltage drop, any PV plant has to remain connected to the grid and

to inject a certain amount of short-circuit current (agreed case-by case with the network Operator) into the

network; furthermore it shall feed-in the same active power (and to absorb the same or less reactive power)

as soon as the fault is cleared.

The Code specifies the voltage drop that shall be ride through by any PV generating plant (Voltage through

capability) as shown in the following fig.4. Above borderline 1 the generating units must be remained

connected ; above borderline 2 and below borderline 1 generating units have to remain connected even if

not capable to support voltage network. Below the borderline 2, the generating units are allowed to be

disconnected.

Fig.4Limiting curves of voltage at the Grid connection point in the event of network fault

As shown in the next fig. 5, different fault-ride through capability requirementsare specified by different

Grid Codes within European countries [7]. The fault-ride through capability curves are quite similar, but

the dynamic requirements are different, mainly with respect to voltage support during the voltage drop.

.

Fig.5Comparison of fault-ride through capability required by different Grid Code in the event of network fault


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about 1416 months while to built a 50MWe CSP it would be required 2436 months. PV arrays are fairly

easy and quick to install while CSP really look like as conventional power plant, mainly for the part relevant

to the steam process part.

On the other side, in terms of Grid impact, Utility-Scale PV plants are not naturally suitable to supply

predictable energy and to provide network ancillary services unless special control features or additional

equipment (such as inverters frequency and voltage response capability, battery storage system, etc.) are

designed and installed, with consequent increase in capital cost.

On the contrary CSP generation is highly predictable and since any CSP plant is intrinsically coupled with

thermal storage this enables the CSP plant to easily meet, the load demand curve at any time, day and first

hours of nighttime, and can cover peak hours demand if they are scheduled. Furthermore CSP plants can

easily participate to primary and secondary grid frequency control so that they are able to support Grid

exploitation both during steady state and transient condition.

New grid-codes recently issued in some European countries have to be considered as an important step to

allow a reliable interaction between renewable generating plants and the electrical network, even if these

Grid Codes are not yet harmonized at European level. Different approaches are present with respect to

minimum mandatory requirement and ancillary service, depending on technical needs of the specific

power system, legal and organizational structure of different Transmission System Operator (TSO), which

historically established grid management procedures.

This fact is to day a barrier for the deployment of Utility Scale PV plants, that need, differently from CSP

plants, special and costly design to comply with different grid needs.As a conclusion of this paper it should be recommended and that all he technical needs are deeply

investigated inside CENELEC technical committees in order to achieve clearly defined and possibly

harmonized rules, at least within the whole European market, capable to bring in line the different interests

of manufacturers, power producers and network operators, minimizing capital costs.

7 REFERENCES

[1] EPIA, Annual Report 2009.[2] EPIA, Global Market outlook for Photovoltaic until 2013.

[3] Global Concentrated Solar Power Markets and Strategies, 20092020; Emerging Energy Research; April

2009.

[4] Global Concentrated Solar Power: Markets and Strategies, 20092020; Politecnico di Milano; Ed. 2009.

[5] Concentrating Solar Power, Global Outlook 09; ESTELA, IEA SolarPACES, Greenpeace International; Ed

2009.

[6] Transmission Code 2007Verband Der Netzbereiber VDN e.v. beim VDEW - Network and System Rules of

the German Transmission System OperatorsAugust 2007[7] Utility Scale PV system: Grid Connection requirements, test procedures and European harmonisation T.

Degner, G.Arnold, M.Braun, D.Geibel & W.Heckmann, Institut fr Solare Energieversorgungstechnick,

Kassel, Germany; R.Brndlinger, arsenal research, Electric Energy System, Vienna, Austria

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Utility Scale PV and CSP Solar Power Plants

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