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Distributed generationliterature review and outline of the Swiss situation

Author(s): Koeppel, Gaudenz

Publication Date: 2003

Permanent Link: https://doi.org/10.3929/ethz-a-004619042

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eeh power systemslaboratory

Distributed Generation-

Literature Review and Outline of the Swiss Situation

Gaudenz Koeppel

Internal ReportZurich, November 2003

Abstract

This report intends to give an overview over various aspects of distributed generation. Distributed Generation wasfavoured in the last few years due to the liberalisation process of the electricity infrastructure as well as to the impulseto produce electricity independent of fossil fuels. The number of small decentral installations of e.g. wind turbines,photovoltaic systems or combined-heat-and-power plants therefore has increased significantly. This leads to changesin the topology of the existing network which will be discussed together with various other aspects as e.g. protection,forecasting, dispatching, combining of stochastically producing generator as well as cost distribution. In a second part,the focus will be put on Switzerland to determine the actual state as well as current and future needs and trendsregarding the embedding of distributed generation.

Contents

I Introduction 4

II The Definition of Distributed Generation 4

III Possible Forms of DG 4III-A Photovoltaics (PV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4III-B Solar Thermal Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5III-C Windenergy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5III-D Hydropower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6III-E Combined Heat and Power (CHP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7III-F Fuel Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7III-G Geothermal Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8III-H Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

IV Today’s Issues with DG 8IV-A Protection Needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9IV-B Forecast Importance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9IV-C Storage Needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10IV-D Dispatched Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11IV-E Virtual Power Plants (VPP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12IV-F Active Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13IV-G Connection Charges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

V DG in Switzerland - the current state 14

VI DG in Switzerland - a possible future state 15

VII Discussion 17

List of Figures

1 Economies-of-scale curve: per unit production costs vs. plant size . . . . . . . . . . . . . . . . . . . . . . 42 Renewable energy prospectus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 Average rating of installed wind turbines per year . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 Functional principle of the hot dry rock procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 Reduced breaker reach because of DG infeed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 Forecast and actual production matching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 Basic concept of a flywheel battery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 Example for the combination of generation and storage as a virtual power plant . . . . . . . . . . . . . . 129 Power plants > 10 MW installed in Switzerland by 2002 . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

List of Tables

I Characteristics of possible DG applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8II Characteristics of different storage devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11III Controllability of different types of DG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12IV Renewably produced electrical energy in Switzerland, 1990 and 2002; hydro power plants not included . . 14

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I. Introduction

The electricity infrastructure of most of the industri-alized countries today consists of large, centrally locatedpower plants connected to a highly meshed transmissionnetwork. In the majority of cases, these central powerplants are either nuclear-, fossil fuel- or hydro-powered.The rating of these plants lies in the range of several hun-dred MW’s to few GW’s, particularly favoured by the ther-mal power plant’s economies-of-scale when being built inthe second half of the 20th century.

New developments and technologies for thermal powerplants however led to a shift in the economies of scale inrecent years and smaller power plants with a few dozensof MW instead of more than one GW became more eco-nomical [1].

Figure 1: Economies-of-scale curve: per unit production costs vs.plant size1

At the same time has the application of electrical energygenerators with renewable sources as wind or sun becomeeconomically and technically feasible. First installations ofsmall power plants therefore started to take place duringthe 80s and 90s, mostly close to the customers, connectedto the distribution side of the network because of theirsmall rating. These installations therefore were denotedas embedded or distributed generators.

The increasing number of interconnections of distributedgeneration with the distribution network has raised newaspects of today’s power infrastructure as e.g. changedpower flow directions, different protection issues or howto manage raising connection costs.

As the power market liberalization in Europe advances,the number of smaller power producers joining the net-work is increasing; particularly in Denmark [2], the UK [3]and Germany [4], the proportion of distributed generationhas become considerable.

This report intends to give an overview over the cur-rent state and various aspects of distributed generationin general and in relation to Switzerland. In chapter II,a definition for distributed generation is provided, servingas a discussion basis for the subsequent chapters. Cur-rent and future technologies for distributed generators areoutlined in chapter III, whereas chapter IV discusses themost actual problems and questions related to distributed

1Figure taken from [1]

generation. In a second part, chapter V and VI, the sit-uation and future possibilities for distributed generatorsin Switzerland is outlined. Chapter VII then closes thereport.

II. The Definition of Distributed Generation

The increasing number of peripherally installed powergenerating facilities has stimulated the publication of var-ious papers, especially after the authorities started to rec-ognize the potential of distributed generation only re-cently. These discussions mostly use terms as embed-ded, decentralized or distributed generation, whereas ac-tual definitions are missing. This is why for this report adefinition from the 1st International Symposium on Dis-tributed Generation in 2001, given by Ackermann, Ander-sson and Soder [5], will serve as a common basis for thenext chapters:

Distributed Generation is an electric power sourceconnected directly to the distribution network oron the customer side of the meter

The definition given does purposely lack information con-cerning

- power rating and technology- environmental impact- ownership and delivery area- mode of operation

This allows a more general discussion of various aspects,since many issues regarding the interaction between DGand the existing grid - as e.g. deep or shallow connectioncharges or protection aspects - are similar for the differenttypes of DG.

To nevertheless achieve some graduation in this report,different technologies will be classified, as suggested bythe authors [5], by using prefixed terms as e.g. renewableDG. Power rating categories are additionally introducedas:

Micro distributed generation: < 5 kWSmall distributed generation: 5 kW - 5 MWMedium distributed generation: 5 MW - 50 MWLarge distributed generation: > 50 MW

Plants with a power rating exceeding 300 MW will not beconsidered as DG anymore.

III. Possible Forms of DG

This section intends to give an introduction to the cur-rently most discussed applications for DG and categorizesthem as suggested in the previous chapter. The chapteris closed with a summary of the main characteristics ofthe outlined technologies (chapter III-H).

A. Photovoltaics (PV)

The first Photovoltaic- or PV-modules became com-mercially available in the 70s, after originally being devel-oped for aerospace research programmes. Today, threemain types of solar cells are available whereas the siliconbased cells are the most widespread ones [6].

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PV systems make use of the so-called photovoltaic ef-fect to transform incident solar irradiation into electricity.The performance of a PV system however is sensitive andlittle shadow on the surface of such a system can alreadysignificantly reduce the output. PV systems consist ofseveral PV modules, which in turn consist of an array ofseveral single PV cells, strung together to achieve a higheroutput voltage.

PV systems can either be island systems for remote in-stallations and areas with no or marginal connections tothe transmission grid or so-called grid connected installa-tions. Since PV cells produce a DC output, the connectionto the grid must be performed through a DC/AC inter-face. Grid-connected systems have experienced an upturnin recent years because of large support programmes inJapan, Germany and the US. According to the 2003 edi-tion of the ’Renewables Information’ by the IEA [7], pho-tovoltaics - being small in absolute terms - experiencedthe largest global growth rate of all renewables form 1990to 2000: The average annual growth rate lay at 32 %,resulting in a growth from 16 GWh in 1990 to 339 GWhin 2001.

An advantage of PV systems - besides the almost main-tenance free and quiet operation - lies in its scalability:the efficiency of a system is independent of the size of theinstallation. PV systems therefore are well suited both forsmall installations on private houses as well as for largeindustrial installations. Prices for PV installations how-ever need to be significantly reduced before PV becomessuitable for competition [8].

The solar irradiation of around 1000 W/m2 in CentralEurope results together with the efficiency of the solar cellof at maximum 16 % and the efficiency of about 85 %of the power inverter in an average annual production ofsome 800 kWh/kWpinstalled [9].

By definition, PV hence can be found in the ranges ofMicro to Small renewable DG.

B. Solar Thermal Power

Unlike PV-Systems, which directly convert solar energyinto electricity, solar thermal power plants use the heatof the solar irradiation to produce steam which in turnpowers turbines. This is achieved by using sun-trackingmirrors which guide the sunlight to a focal point. Atthat focal point, either water is directly evaporated or athermal oil is heated which then passes the heat to waterin a heat exchangers. The most promising technologieswhich are either already in use or in test installations are[10]:

- parabolic trough collectors- solar tower receivers- dish-stirling systems- solar chimney power plants

Except for the solar chimney power plant, these gener-ators all work with concentrated sunlight, which impliesthat only the direct portion of the sunlight can be used.Since in Central Europe the proportion of direct radiationmakes up only approx. 50 %, these power plants are onlysuited for Mediterranean and North-African countries witha large amount of direct sunlight. For Central Europe,

these power plants are insofar interesting for scenarioswhere electricity is produced in North-African countriesand either transported directly to Europe or first trans-formed into hydrogen, which then in turn is transportedto Europe.

The total efficiency for the electricity generation liesaround 35 % today and is assumed to reach approx. 69% when installed in a cogeneration system [11]. By addinga regular fossil fueled burner, such a system can also beoperated with no sun irradiation; the advantage of beinga renewable generator however is lessened. Since solarthermal power plants are not suited for installations inCentral Europe, they will not further be discussed in thisreport.

C. Windenergy

Transforming the energy of wind into mechanical en-ergy has a long tradition and after the oil crisis in the 70sthe number of installed wind turbines for power genera-tion started to increase. Today, windenergy has becomea significant branch of industry and in Denmark, the in-stalled wind power capacity already exceeds the base loaddemand [2]. Because of fairly constant wind conditions,countries bordering a sea are well suited for wind harvest-ing. Thus did also Germany already cross the border of12’000 MW installed wind-power by the end of 2002 [12].

As figure 2 shows according to the 2001 Shell report,will wind energy take the highest share among the renew-able generation capacity [13].

Figure 2: Renewable energy prospectus2

Wind turbines built today are either constant speed orvariable speed wind turbines. Constant speed turbines aredirectly coupled to the grid and run synchronously withthe frequency of the grid (the so-called Danish concept).As the name says, these turbines run at a predefined speedand therefore reach their maximum efficiency only for onewind speed, predefined by the rotor design. Unless thewind regime at a particular site is highly peaked exactlyat that wind speed, the wind turbine will seldom operateat its optimum performance [14].

Variable speed wind turbines became possible and ef-ficient due to developments in power electronics. Eithera direct drive synchronous generator or a gear box and

2Figure taken from the ’Shell Global Scenarios’

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an induction generator is applied to transform the windenergy into electrical energy. The power electronic de-vices are then used to convert the variable frequency ACoutput into DC, which then in turn is converted into agrid-synchronous AC3. The rotor spins at the optimumratio of rotor-tip speed versus wind speed and achievesan overall production of 110 % of that of a fixed-speedturbine at the same location [14].

To prevent the turbines from exceeding their speedlimit, thereby destroying mechanical parts, two conceptsare used: stall and pitch control. With stall control, theblades are shaped in a way that with increasing wind speedthe wind creates turbulences on one side of the blade, de-creasing the performance and, in doing so, the speed ofthe rotor. Stall control is not an active control. Pitch con-trol on the other hand requires that each blade is hingedto a motor. This allows to turn the tips of the blades, de-creasing the working surface, leading to a slowing down ofthe rotor as well. Pitch control therefore is important forvariable speed turbines, allowing them to always operatewith the optimum rotor-tip speed vs. wind speed ratio[15].

Compared to constant speed wind turbines, variablespeed wind turbines are more expensive and the windspeed tracking rotor experiences rapid torque changes re-sulting in long-term fatigue damage, which is the majorfactor for failure [14]. The advantage of variable speedover constant speed turbines is however the better overallcontrollability particularly allowing to produce or to con-sume reactive power, helping to stabilize node voltages[16].

Besides several problems related to the power electronicinterface (see publications by Slootweg et al. for detaileddiscussion [16], [17], [18]), constitutes the forecasting ofwind power and associated generation a problem. Know-ing the amount of the next day production helps to moreeconomically operate renewable power generators. Thepower output of a windmill is proportional to the powerof three to the wind velocity; until recently, wind velocitymaps used graduation steps of 5 m/s; for a windmill how-ever, a velocity of 6 m/s or 10 m/s means no productionor full production [19]. These issues will be discussed inmore detail in chapter IV.

The largest variable-speed windmill available today wasinstalled by Enercon near Magdeburg, Germany, in the fallof 2002, and is rated 4.5 MW with a rotor diameter of112 m4. The average capacity of the in 2002 in Germanyinstalled wind turbines though is approximately 1.5 MW[12] (see figure 3).

Various small turbines with ratings starting at 400 Ware also available; they usually come with a built in AC/DCconverter to be used to charge small residential batterybanks5 [20].

Another aspect - however not applicable for Switzerland- is the question, to what extent large wind-parks can still

3For larger windparks, it would also be feasible to only use windturbines containing an AC-DC converter being connected to a DClink inside the park; few DC-AC converters would then provide theconnection between the windpark and the grid.

4http://www.enercon.de5http://www.windenergy.com6Figure taken from [12]

Pow

erper

Unit

kW

h/U

nit

20021999199619931987 19900

400

800

1200

1600

Figure 3: Average rating of installed wind turbines per year6

be considered as distributed generation. In Denmark, theHorns Rev Wind Farm with a capacity of 160 MW justwent on the grid, located up to 20 km into the NorthSea. Such an installation can by definition not anymore beconsidered as distributed generation; single wind turbineinstallations range from Micro to Small renewable DG,on-land windparks range up to Large renewable DG.

D. Hydropower

Hydropower plants are in use now for more than 100years. Some countries as e.g. Norway cover almost 100% of their power demand with hydropower. The poweroutput of a hydro plant is proportional to the differencein height between the water levels before and after theturbine, the so-called nominal head [10]. Countries withdistinctive altitude profiles therefore are better suited forhydropower than e.g. the Netherlands. Three differentkinds of hydropower plants exist:

- river power plants- water storage power plants- pump storage power plants

River power plants are installed among rivers, close toagglomeration-centers; they do not posses storage capa-bilities and therefore are supply-dependent. According toJenkins et.al.[21], the capacity factor for hydro was onlyapprox. 30 % for the UK in 19987. Water storage powerplants too are supply-dependent throughout the flow of ayear, but because of their storage capabilities, short-termfluctuations in water supply can be balanced. With pumpstorage power plants it must be distinguished whether anatural water inflow exists or not: plants without inflowmust be considered as regular storage installations andnot as installations transforming regenerative energy.

As water storage power plants are usually located inmountains, they are far from larger civilization parts andmoreover cannot be considered as distributed generationbecause of their power rating. River power plants on theother hand are rather small and widespread throughoutSwitzerland. In most Central European countries howeverthe capabilities for hydro power plants have been reachedand only small power improvements will be possible by

7The capacity factor defines the ratio of actually produced energyto potentially producible energy if the plant would run full-time withthe rated output

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eventually replacing existing turbines with newer and moreefficient ones [7].

Large hydro power plants can reach more than 1 GWand offer the possibility to accurately control the amountof production. This makes these power plants suited par-ticularly for load-following and peak-load supply. Besideslarger plants, it is also possible to install micro distributedgeneration by using small turbines, which can be installedinto existing mountain rivers - this is by all means an in-teresting option for remote farmers in the Alps [22].

As documented in a paper by Spreng, Wustenhagen andTruffer [23], concerning the perspectives of hydropower inSwitzerland, people do disagree on whether hydropower- despite the renewable source - can be considered asenvironmentally sound or not because of its impacts onnature as e.g. barriers to fish or the curtailing of thelandscape in general. Nevertheless, hydro power plantsare categorized as Micro to Large renewable DG becauseof their renewable source.

E. Combined Heat and Power (CHP)

Cogeneration or Combined Heat and Power (CHP) plantsare power plants where either heat is the primary prod-uct and electricity is generated as a byproduct, or whereelectricity is the primary product and accumulating heatis reused as a byproduct e.g. for district heating. Varioustechnologies used in CHP plants exist, and their heat-to-power ratio8 varies - depending on the implementedtechnologies - between 1.8:1 and 6.9:1 [21]. Today, fos-sil fueled turbines are used for the electricity production,with advancing fuel cell technologies however, CHP’s withfuel cells will become an interesting option9 [24].

Most CHP-plants serve for heat-production either indistrict-heating installations or providing process-heat forindustrial applications - these plants are therefore so-calledheat-controlled. Instead of simply burning some fuel andbuying electricity, the heat of the burned fuel is first usedto power a turbine to generate electricity and the remain-ing heat is then used for process heating; this processoverall reduces the electrical energy from the grid by pro-viding the supply itself. These systems are controlled in away that heat is always available when needed for the re-spective process; the electricity becomes a byproduct andit thus happens that electricity is produced at times whenit is not particularly needed. Although the amount of pro-duced electricity can be forecasted, these heat-controlledCHP plants are not suited for dispatch. Power-controlledCHP plants are in fact better suited for dispatch, the num-ber of installed systems is yet small compared to the num-ber of heat-controlled systems. This imbalance may be ex-plained by the fact that the efficiency of heat-controlledCHP plants is significantly higher than the efficiency ofpower-controlled CHP-plants [21].

In recent years, biomass fuel has become an attractivefuel for CHP plants because of its renewable properties.These biomass-fueled CHP’s are claimed to play an im-

8The heat-to-power ratio designates the ratio between per unitproduced heat and electrical power; a ratio of 4:1 would mean thatone unit of primary fuel can be transformed in 4 times as muchthermal energy than electrical energy at the same time.

9See also http://www.hexis.com

portant role in future distributed generation systems [10],[25], [26]. Raw materials for biomass generation are -among others - wood and farming products. Wood is aninteresting option for countries with large forests, howeverpossibly resulting in rather long transportation distancesbetween the harvesting area and the heat/electricity gen-eration and consumption area. The use of farming prod-ucts as e.g. straw or grass offers on the one hand an inter-esting financial option for farmers and on the other handthe possibility to generate biomass rather close to where itis needed. Jenbacher10, a large cogeneration plant manu-facturer, offers plants both for natural gas and for biogas;the rating is between 330 kW and 2’700 kW electricaland 380 kW and 2’800 kW thermal. The most econom-ical near-term use of biomass is so-called cofiring, wherebiomass is fired together with e.g. coal, thus reducing airemissions [20]

Another, only recently commercialized form of cogener-ation plants are the so-called Microturbines [27]. Micro-turbines are small gas turbines with a rating of 25-250 kWelectrical power. Since the technology is based on auto-mobile turbo charger technology, the manufacturing costscan be kept low. Applications are manifold, microturbinesrun on natural gas, landfill gas or on biogas11. The salesof microturbines have significantly increased over the lastfew years. [28].

Cogeneration plants can be defined as Small to LargeCHP DG, and if run exclusively on biogas, Small to Largerenewable CHP DG.

F. Fuel Cells

Fuel cells are systems where electricity and heat is gen-erated by electrochemically combining hydrogen and oxy-gen; water is generated as a ’waste’ product. The fuelcell thus is a system capable of producing electricity with-out any mechanical process, resulting in higher efficien-cies than regular thermo-mechanical systems and quieteroperation. They are still under development but variousprototype installations at authorities and in cars are beingtested12.

Fuel cells can be built from different materials, beingsuited for different application; the operating tempera-ture thereby lies between 50◦ C and 1000◦ C. The mostresearch is done on Proton Exchange Membrane (PEM)and Solid Oxide (SOFC) cell stacks. PEMs operate be-tween 50◦ and 80◦ C and thus are suitable for cars andmass transportation. Their efficiency is found to be about50 % [29]. Solid Oxide fuel cells on the other hand op-erate between 600◦ and 1000◦ C and are supposed tohave an efficiency of around 70 % if the heat is recov-ered. Current research focuses on reducing the operatingtemperature requiring expensive high temperature alloys.

Possible stationary forms include small systems for pri-vate homes, producing heat and electricity at the sametime13, large industrial applications as well as storage ap-plications where surplus electricity is transformed into hy-drogen by means of electrolysis. The stored hydrogen can

10http://www.jenbacher.com11http://www.microturbine.com12http://www.hyweb.de13See http://www.hexis.ch for first testing series

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at any time be transformed back into electricity throughthe fuel cell. Combination of these two processes are atthe moment tried to achieve through so-called regenera-tive or reversible PEM fuel cells [30].

Besides hydrogen it is also possible to use reformednatural gas, gasoline, methanol or alcohol, whereas thiswouldn’t be renewable generation anymore [31]. Fuel cellstherefore can only be considered as renewable DG if theused fuel were produced with renewably generated energy.

Fuel cells can be classified as Small to Medium DG [28],[32].

G. Geothermal Power

The last technology to be presented is the geothermalpower technology. Geothermal power technology makesuse of the thermal energy stored within the earth, mostlyoriginating from energy of the natural decomposition ofradioactive isotopes.

Geothermal energy can be used in several ways. Atsome places on earth, hot water is naturally transportedto the surface, which today is used - besides therapeu-tical applications - for powering turbines and generators.At some places have existing hot water sources been im-proved in efficiency and output by means of mine drilling.According to the 2003 IEA Renewables Information [7],does Iceland make use of its particular geographical ad-vantages and serves 17 % of its electricity demand withelectricity from geothermal CHP plants14 and 99 % of itsheat demand from geothermal plants. Another possibil-ity for using geothermal heat are heat-pumps with depthof 50 to 250 m which can be installed independently ofgeographical peculiarities, mostly serving for house andservice water heating.

TABLE I: Characteristics of possible DG applications

power connection development barriers

rating type stage

Photovoltaics micro-small =/≈17 mature price, forecasting

only suited forSolar thermal power small-medium ≈ prototypes

equatorial areas

Wind energy micro-small18 ≈/=/≈; ≈/=19 mature forecast

Hydropower micro-large ≈ mature location

CHP small-large ≈ mature -

Fuel cells small =/≈ test stage fuel infrastructure

Geothermal small-large ≈ mature location

If no natural sources are avail-able, it is possible to use geother-mal energy by building an artifi-cial water circulation. This is donewith the so-called hot-dry-rock ordeep heat mining procedure [33].A subterranean reservoir is built bydrilling a hole some few kilometersdeep and by expanding an existingabyss by so-called hydraulic frac-turing. The reservoir is filled withwater which is pumped back to theearth surface to pass the acquiredheat on to a heat exchanger for apower generation process (see fig-ure 4). Currently, several deep heat mining projects arerunning in Europe; in Basel, a probe drilling was per-formed to almost 3000 m depth15.

Another interesting form of geothermal energy use -particularly interesting for Switzerland - is the so-called

14The remaining 83 % are produced with hydro power plants.15For further information please go to http://www.dhm.ch16Figure taken from http://www.geothermal-energy.ch17= stands for DC, ≈ stands for AC; =/≈ thus stands for DC/AC

converter18Rating of a single wind turbine19In a wind park, turbines could also be connected to a DC-link

which in turn is connected to the grid through one DC/AC converter;the single wind turbines however are connected by DC then

Figure 4: Functional principle of the hot dry rock procedure16

tunnel water use; tunnels usually cause a draining of thetunneled mountain. This water is warmer than surround-ing air and is partially used for district heating in combi-nation with heat pumps [34].

However, for distributed generation of electricity, onlynatural sources (as existing e.g. in Iceland) or hot-dry-rock installations are suited. According to Williamson etal. [35], it is possible to commercially operate a geother-mal installation for 15-25 years. If the output rate wasreduced, thereby reducing the revenue, the operation timewould consequently be prolonged.

H. Summary

Table I summarizes the discussed characteristics of theabove presented DG applications; explanations to the sin-gle characteristics are found in the respective subchapters.

IV. Today’s Issues with DG

The growing number of distributed generation instal-lations and the grid penetration involved leads to variousnew security and maintenance aspects on the existing grid.As mentioned earlier, these aspects are mostly indepen-dent of the technical differences among the distributedgenerators. This chapter gives a short review of the cur-rently most discussed issues and shows some future trendsregarding the solution or improvement of these problems.It is possible to distinguish between technical (IV-A), op-erational (IV-B unto IV-F) and economical (IV-G) ques-tions.

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A. Protection Needs

Power flow in networks without distributed generationusually is unidirectional, from meshed high-voltage trans-mission networks down to radial low-voltage distributionnetworks. However, the growing number of distributedgenerators, operating in parallel with the existing grid,lead to a bi- or multidirectional power flow, thereby re-quiring new or redefined protection schemes [36]. Accord-ing to Ault et al. [37], the new protection requirementscan be grouped as generating unit protection, distribu-tion network protection and interface protection. Severalaspects of these areas are shortly discussed here.

Overcurrent protection is often done via fuses, breakersand reclosers; since most rural systems are constructedradially, opening and closing of one device is normallysufficient to clear a fault. Usually, breakers and recloserssense a certain distance on a feeder, the so-called reachof a device (see figure 5). In case of high peak load onthe net, when a breaker is already quite sensitive and notfar from opening the line, the connection with DG wouldmake sense to satisfy load demands. As the figure belowshows, this connection however also reduces the reach ofnearby devices and could allow high resistance faults tostay undetected until they become larger faults, possiblyresulting in larger damages [38].

Figure 5: Reduced breaker reach because of DG infeed20

Since the power flow in the network is not anymore ra-dial with DG present, Dugan and McDermott [38] suggestthat - in case of overcurrent - affected DG is disconnectedfrom the network to allow the network to clear the faultas it would do in a true radial configuration.

Furthermore - although DG can help to maintain powerquality and supply - in case of disconnection of DG toclear a fault, load curtailing can occur: if the power con-sumption of loads exceeds the maximum capacity of thetransfer station and only can be satisfied because of con-nected DG on the feeder, this demand cannot anymorebe satisfied if DG gets unconnected due to a fault. If thetransfer station was already at its maximum rating, it willnot be capable to satisfy the load demand until after DGhas reconnected a few minutes later-on. All the same,this does not mean that DG is worsening the supply andreliability quality but that it most often just mitigates anactually existing regulation or capacity problem. Duganand McDermott [38] therefore suggest to limit the capac-ity of interconnected DG to 5% of the capacity of theline, therefore avoiding significant modifications on theexisting feeder.

20Figure taken from [38]

Another issue to be mentioned is instantaneous reclos-ing. Usually, temporary faults are cleared by reclosing therespective breaker as soon as the arcs have cleared. Thatmeans that DG must detect the fault as early as possi-ble, preferably not to supply the arc, prolonging the faultstate and leading to another trough fault, shortening thelife of utility equipment. On the other hand, if the breakerhas reclosed before DG has disconnected completely, thiscould lead to serious damage on the DG because of out-of-phase switching. Reclose intervals often are as shortas 0.2 s to avoid the so-called ’blinking-clock’ problem,whereas a duration of 1 s would prevent DG equipmentfrom being damaged. It thus needs to be found a balancebetween degraded supply quality due to longer off-timesand potential damage to DG [36].

As this chapter tends to only give an overview overvarious issues regarding DG, it is referenced here to thebooks by Jenkins [21] and Dugan [39], who discuss thistopic in more detail.

B. Forecast Importance

As chapter III showed, distributed generation units trans-forming renewable energy are strongly supply-dependent.This becomes an important issue particularly with largerinstallations as in the case of wind-farms in Germany orDenmark. The power markets in these countries are al-ready liberalized and power is sold through a market. Inthe case of Denmark, deals are settled 24 h in advance,on an hourly basis. This means that the owner of a wind-farm must know up-front how much he will produce ev-ery hour of the next day. If the forecasted amount waswrong, balance energy has to be bought, which is neededto compensate for the accruing difference between agreedand actual production. Eltra (the transmission systemowner of western Denmark), whose installed wind powercapacity exceeds the off-peak demand significantly, stillis improving its forecast models, which are - as figure 6shows [2] - not always accurate.

Figure 6: Forecast and actual production matching21

According to Holtinnen, Giebel and Nielson [19], Eltralooses approx. a third of its possible production incomedue to regulation payments; if the bidding would takeplace only half a day in advance instead of 24 h, the earn-ings of Eltra would raise by approx. 15%. Forecastingtherefore is both of financial as well as of technical im-portance. If renewable DG is to be operated economically,accurate forecasting tools are essential. Forecasting mo-mentarily focuses particularly on wind speed predictions

21Figure taken from [2]

10

since photovoltaics are yet small in numbers of installedkWp.

Today, the lack of prediction exactness can be com-pensated by combining the renewable DG with a storagefacility (see also chapter IV-E) [18]. It is also discussedto produce hydrogen with surplus electricity, some paperseven suggest to directly produce only hydrogen at off-shore windparks and then to transport the hydrogen tofuel cells which in turn generate electricity when needed[40], [41]. This would reduce the otherwise remainingelectricity transmission losses between the off-shore wind-park and the on-land electrolysis facility. The next sub-chapter will give an overview in more detail over otherexisting and future storage technologies.

C. Storage Needs

According to the just mentioned difficulties concerningthe production forecasting of renewable DG as e.g. windor solar power plants, it seems obvious that storage willplay an important role in future energy networks to helpeven out the fluctuating output22 [18].

Several technologies in different development stages ex-ist, both for storing electrical and thermal energy. Thesestorage devices can roughly be characterized by time prop-erties as well as capacity and application feasibility, whereasthis chapter focusses on electrical storage devices23.

- Batteries (Lead Acid, NiMH, Flow)- Superconducting Magnetic Energy Storage (SMES)- Flywheels- Supercapacitors- Pump storage- Hydrogen- Compressed air energy storage (CAES)

Batteries are well known and have a high level of devel-opment stage in what concerns regular lead acid or NiMHbatteries. They are suited for short to medium term stor-age and their discharge leakage is as low as 0.1% to 0.5%per day [30]. Flow (also called Redox) batteries also storeelectrical energy through chemical reactions, but they arenot yet as developed as ’regular’ batteries are. Flow bat-teries consist of a membrane and two tanks containing twodifferent electrolytes. These electrolytes can be pumpedfrom the tanks into the membrane, producing electricitythere - flow batteries therefore are similar to regenera-tive fuel cells (see III-F). This process is fully reversible[43]. In contrast to ’regular’ batteries, flow batteries arecapable of storing large amounts of energy.

Superconducting Magnetic Energy Storage (SMES)devices work with the following principle: a coil is charged

22Recent advances in communications technology have also stim-ulated visions about networks without any storage [42]: both loadsand generators would communicate through the network, enablingeach other to match production and consumption, making storageredundant. A more realistic perception are distribution networkscontaining active control and protection devices as well as com-munication possibilities, thereby enabling supply and demand sidemanagement (see also chapter IV-F)

23Thermal storage devices are interesting for electricity generationonly if they are used to buffer heat e.g. in solar thermal power plants;accruing heat from the collectors is stored and later-on released topower the turbine, producing electrical energy when needed, e.g. atnight

with a current, thereby generating a magnetic field. Acryogenic system keeps the coils at a temperature wherethe material becomes superconductive, i.e. the resistanceof the coil material is zero. As soon as the charging cur-rent stops, the voltage over the coil becomes zero andbecause of no losses, the current stays constant. The dis-charge process works similar [44]. Through a power elec-tronic interface it is possible to store and discharge largequantities of power in little time at efficiencies of 80 to 90% [30]; the energy needed to keep the low temperatureis not respected in this efficiency. Momentarily, SMESis still expensive, first experiences however prove its helpparticularly for improving voltage stability, especially be-cause of its capability to dispense reactive or active power[45].

Flywheels are kinetic energy storage devices where anelectric motor is used to speed up a rotating mass, theflywheel. As soon as the charging stops, the flywheel con-tinues to spin until the process is reversed by connectingthe flywheel to a generator, thereby slowing down the fly-wheel (figure 7). To ensure a long runtime of the flywheel,it is usually mounted on magnetic bearings and inside ahousing with a vacuum, reducing air and frictional drags[44]; flywheels often spin with speeds of 10’000 to 60’000rpm.

Figure 7: Basic concept of a flywheel battery24

The storage holding time is in the range of hours, de-pending on the size of the flywheel. Depending on theapplication range, the size of a flywheel varies signifi-cantly, from satellite applications with pairwise installa-tion of small 2 kW flywheels to 400 kW power qualitysupporting flywheels and up to military applications with5-10 GW peak power capabilities [46].

The high efficiency as well as the exact knowledge aboutthe state of charge from the rotational velocity are clearadvantages of flywheels [30]. First installations of fly-wheels took already place, e.g. at the power supply sta-tion of a chip producer who is dependent on high qualitypower [46]. Because of their fast response time and rathershort charge holding time, flywheels could make an idealcombination with renewable DG, helping to bridge shortpeaks and sags due to the fluctuations of the primaryenergy supply [24].

Supercapacitors are capacitors with a high energy stor-age capacity. The working principle is similar to regularcapacitors, the electrodes however are highly perforated,thus resulting in a comparatively large effective plate sur-face. Consequently the capacitance can reach up to sev-eral 100 times the capacitance per unit of a conventionalelectrolytic capacitor [47]. Usually, activated carbon isused for the electrodes and sulfuric acid for the electrolyte[48]. This results in capacitors with a low energy den-sity and a high power density, thus particularly suited for

24Figure taken from [46]

11

short-time peak power applications [44]. Today, first su-percapacitors are used for power quality improvements,also well suited because of their ability to float at fullcharge for up to ten years and to quickly switch betweenfull charge and full discharge. In addition, experimentsshowed that up to more than 100’000 deep discharge cy-cles do not significantly age the material [47]. Ultraca-pacitors are therefore often said to eventually outreachregular lead acid batteries [48].

Pump storage was already discussed earlier in chap-ter III-D. Its advantages are low costs, good efficiencyas well as the possibility to accurately follow loads; hy-dro pump storage plants are therefore particularly suitedfor support in peak times. Unfortunately, pump storagedepends on geographical peculiarities, thus being only a

TABLE II: Characteristics of different storage devices

Storage Batteries SMES Flywheel Supercaps Pump Hydrogen CAES

device storage

Application - balancing - quality - quality - quality - peak load - long time - peak load

emphasis renewables - voltage - traffic - traffic - balancing - balancing - balancing

- traffic stability - stability - reserve renewables

Charge hold- seconds seconds up to up to

ing timedays

to hours to hoursseconds

months weeksdays

Efficiency 65-75 % 80-90 % 70-90 % 90+ % 70-80 % 30-60 % 42 %

Response

time1/10 sec 1/1000 sec 1/1000 sec 1/10 sec minutes - minutes

Storage

techniquechemical magnetic mechanical electrical mechanical chemical mechanical

storage option in com-bination with a virtualpower plant (see chapterIV-E).

The concept of usinghydrogen as an energycarrier for storage hasadvanced together withthe advancing develop-ment of fuel cell sys-tems. Using hydrogenfor storage purposes onlymakes sense when it canbe transformed back intoelectrical energy any timelater, and this is possibleeither with fuel cells orwith combustion engines.

Besides the electrolysis which separates water into hy-drogen and oxygen, it is also possible to reform fossil fuelsas gasoline or natural gas or to gasify coal [49]. How-ever, the only sustainable and renewable form of hydro-gen production is through the use of renewable electricalenergy and electrolysis. In chapter III-B it was alreadypointed out that some visions for future energy networksexpect large solar thermal power plants in northern Africa,producing hydrogen which then is transported to Europe[10] where it is used as fuel for fuel cells. Hydrogen willplay an important role as energy carrier in the future, asalso a widespread programme by the International EnergyAgency (IEA) proves [50].

Besides efforts to renewably produce hydrogen, the stor-age itself is a major issue. Possibilities are to store it asa gas like natural gas is stored, liquid at very low tem-peratures under high pressure or chemically bound in e.g.metal hydrides or methane [51]. Metal hydrides are theleast dangerous form, however being heavy and requiringhigh discharging temperatures25.

Compressed Air Energy Storage (CAES) is alreadylonger known but has only been rarely applied [24]. CAEStoday exists in two places worldwide in combination witha gas turbine. In low energy demand times, energy isused to pump air into existing cavities (e.g. former salt

25For further information please refer to the mentioned IEA Hy-drogen Agreement [50]

mines). In high energy demand times with more expen-sive fuel costs and higher revenues for produced electricity,the compressed air is used to power turbines. Their dis-advantage - besides the geographical dependency of thesesystems - is the loss of pressure because of the not com-pletely leak-proof storage room.

The above described storage devices can be classified ac-cording to their charge holding time, the response time,the efficiency as well as the suited application [30]. Ta-ble II shows that the storage devices are either suited forvery short storage time, helpful for balancing fluctuationsof the output of e.g. a windpark, or for longer term stor-age, helping to balance between day and night or betweenseasons. Batteries, CAES and pump storage are the only

technologies which are already thoroughly approved towork, whereas both CAES and pump storage are depen-dent of geographical particularities. Flywheels are sup-posed to have a strong future, also favoured by the factthat they can be built with standard devices as motors,power electronic interfaces and steel or carbon compos-ites for the wheel [24], [46]. Whether SMES will be usedfrequently cannot be determined yet; hydrogen storagehowever is definitely going to play an important role infuture energy systems [31].

D. Dispatched Operation

With renewable generators involved, the term dispatchedoperation denotes whether a generator can be shut off ornot. If both forecasting and storing fail or are not avail-able, it should at least be possible to turn a generatoron or off depending on the network situation trying tomaintain the voltage level. A thermal or hydro poweredgenerator usually can be well controlled and thus be dis-patched, in fact, gas-fueled and hydro plants can reactfast and accurately follow changing load demands. A re-newable generator on the other hand produces dependingon the inflow of the driving energy, which naturally can-not be influenced. With wind parks there is though thepossibility to turn some wind turbines off, thus reducingthe output and vice versa, to keep some turbines discon-nected until more energy is demanded. Wind turbineshowever need a certain time for starting up. With pitch

12

controlled turbines it is also possible to change the pitchangle and thus de- or increasing the power output. This byall means results in sacrificing energy in favor of controlla-bility; maximising the output of a wind park consequentlycontradicts a high controllability [16].

With the increasing amount of third-party owned DGon the net, due to the ongoing liberalisation process andstimulated by various subsidy programmes, it has there-fore become necessary to find mechanisms to combine anddispatch also renewable generators to avoid large balanc-ing energy demands.The following table lists the control-lability of the before presented types of DG (see chapterIII), whereas the possibility to simply turn a generator onor off is not considered as being controllable.

TABLE III: Controllability of different types of DG

adjustable controllable predictable

Photovoltaics xSolar thermal power x26 x27

Wind energy x28 xHydropower xCHP heat-controlled xCHP power-controlled xFuel cells x x29

Geothermal x

Another point to be discussed is, to what extent theutilities can demand that a third-party DG owner shuts itsgenerator off, thus helping to maintain the voltage level.The already addressed concept of combining renewablesources to avoid a large balancing energy demand is thetopic of the following chapter on virtual power plants.

E. Virtual Power Plants (VPP)

The term Virtual Power Plants (VPP) stands for aninteresting and auspicious concept of combining differ-ent types of renewable and non-renewable generators andstorage devices to be able to appear on the market as onepower plant with a defined hourly output. In other words,different power generation and storage devices with di-verse weaknesses (e.g. stochastic output) and strengths(e.g. high energy short term storage) are combined sothey cleverly complement.

A simple example would be to combine a wind farmwith a flywheel and pump storage, where the maximumoutput of the wind farm is defined as e.g. 70 % of thefull capacity. The flywheel would be used to compensateshort-time peaks and sags. Prolonged surplus produc-tion would be used to charge the pump storage whichin turn would counterbalance the production in times ofprolonged reduced output from the wind farm (figure 8).

Another possibility would be - as already addressed inthe chapter on dispatched operation (IV-D) - to keepsome wind turbines of a wind park disconnected from thegrid, still operating but charging a storage device and thusbeing able to immediately be connected to the grid when

26In combination with a secondary gas-burner27By e.g. turning away some of the tracking mirrors28With pitch-control29If installed in combination with a heat-controlled CHP

-400

-200

0

200

400

600

800

1000

1200

1400

0 5 10 15 20 25 30 35 40 45 50 55 60time

[kW

]

Pump storage

Flywheel

Wind turbine

targeted combined output

Exemplary power output of a wind turbine combinedwith idealised pump storage and flywheel

output without flywheel

Figure 8: Example for the combination of generation and storage asa virtual power plant

needed. Long start-up times would be avoided and lessenergy would be sacrificed than without storage.

Various, also more complicated, combinations of gener-ators and storage devices are thinkable, whereas it is notnecessary that these devices are locally close or ownedby the same party; situations are thinkable where e.g. awind park owner just has the permission to use a certainproportion of a pump storage plant for a certain price. Inany case will VPP strongly depend on a sophisticated in-formation system, a so-called Decentralised Energy Man-agement System (DEMS) as promoted e.g. by Siemens.

A DEMS generally consists of three levels: the prog-nosis, the resource planning and the online optimisation[52]. The prognosis includes the forecasting of the loadsand the forecasting of the DG; it is particularly importantto know whether some renewable generators will produceat all the next day. The resource planning involves deliveryand supply contracts, primary energy contracts as well asgeneration, storage and load scheduling, in considerationof the prognosis, further resource constraints and main-tenance outages. The online optimisation at last contin-uously adjusts dispatch and control parameters based onreal-time values for the load and the weather situation.Discrepancies between the actual situation and the prog-nosis are thus remedied. The such generated parametersare then used to control the existing devices of the VPP,depending on whether it is adjustable, controllable or onlypredictable DG (see table III).

The physical connection between the DEMS and theparticular component of a VPP can be established in sev-eral ways. Adjustable generators need an all-time con-nection either through the power line itself or with ISDN,ADSL or GSM to always be addressable. Controllablegenerators should be communicated with e.g. every 15 or30 minutes to transfer the new control parameters; thiscan be done through GSM or a simple dial-up connection.For solely predictable generators a dial-up connection issufficient for transferring online production data in inter-vals of e.g. 15 or 30 minutes. If a complete wind park ismonitored, it is sufficient to have one connection to thecontrol room of the wind park.

With GSM connections it just must be paid attention

13

to have a backup supply on-site; power lines for commu-nication are - although being manifest - difficult in regardto line disconnection after an occurring fault, thus dis-abling the DEMS to trip a generator. The internet is alsooften mentioned to be used as communication platform,the opinions concerning the reliability and safety of theinternet however still vary greatly.

Similar to Virtual Power Plants it is also possible togroup demand as one Virtual Customer by so-called demand-side management. This topic however represents rathera social problem than a technical or economical one: isit crucial - first of all - to maintain high quality powerfor all applications and is it justifiable to switch off cer-tain devices at certain times to better spread the load de-mand over the whole day, reducing peaks at certain hours?These points - although interesting - are not within thescope of this report and will not further be discussed.

F. Active Networks

In networks without connected DG, the high voltageand medium voltage transmission grids usually containdevices for the active control of power flow, frequencyand voltage as well as for protection. The distributionnetworks yet have no control mechanisms but protection.With a larger amount of DG devices connected to thedistribution grid, it will thus become necessary to controlthese generators. It is therefore important to adapt theknown control mechanisms from the high and mediumvoltage grids to the low voltage grid. Only if DG canbe controlled or at least dispatched, their advantages canfully be utilised (see chapter VII for a discussion of thevarious advantages). The term active networks standsfor this - already above addressed - extended distributionnetwork, containing control systems and communicationpossibilities [53].

With active networks and appropriate contracts betweenthe network operator and the DG owner, it should be pos-sible to protect the network, dispatch generators and tomaximise the use of active and reactive power generatedby DG. Some parts of the distribution networks could ob-tain so-called microgrid-similar characteristics. Van Over-beeke [54] even suggests to subdivide the distribution net-work into single cells, containing a certain amount of DGand loads, managing protection, power flow and voltagecontrol autonomously. Thus, distributed intelligence andcontrol would be added to DG. Several of those cells wouldbe included in a larger, higher-level cell, which cannotdirectly operate actuators in a cell; the higher-level cellcould only tell the respective cell the purpose of an ac-tion. The cell would then itself perform the necessaryoperations; a cell thus would be a small self-managingpart of the network.

Liew, Beddoes and Strbac [53] suggest introducing amarket for ancillary services on the distribution networkto achieve the goals of an active network. The transmis-sion operator hence would be provided with possibilitiesto dispatch generators and loads for system reserve and tocontrol voltage, frequency and active and reactive power.These control possibilities would also allow to connectmore DG onto the distribution network [55], being capa-ble of curtailing DG and of changing reactive and active

power levels. The maximum limit of connected DG capac-ity of 5 % of the infeed capacity as suggested by Duganand McDermott [38] could thus be extended (see chapterIV-A).

G. Connection Charges

This closing subchapter on the issues of distributed gen-eration deals with costs related to the connection of DGto the distribution network. The financial value DG couldbring because of better reliability, more reserve and lesstransmission loss on the grid will however not be discussedhere since no reliable data yet exists. It will though be fo-cused on costs concerning the initial connection (deep vs.shallow connection charges), costs emerging for the utili-ties because of these connections (stranded costs) as wellas costs for still maintaining the connection to the grid fortimes with DG outages (standby charges). Jenkins et al.[21] outline besides that the amount of charges dependson the voltage level of the connection; utilities prefer ahigher voltage level connection because the impact of DGon the power quality is thus reduced whereas the costsfor the physical connection usually are higher with highervoltage levels - this cost-influencing factor will also not betaken into account.

The connection of DG onto the network often has fur-ther influence on the grid than just at the connectionpoint; e.g. reinforcement or replacement of protectiondevices because of changed requirements. It is thereforediscussed whether a DG owner only has to pay for makingthe new connection (shallow costs) or also for indirectlyemerging necessary investments in the grid infrastructure(deep costs) [21]. Shallow costs are simple to be cal-culated since they just represent the costs for building atransmission line between the new DG and the connectionpoint. If however e.g. a new circuit breaker needs to beinstalled because of the impact of DG on the reach of thealready existing breakers (see chapter IV-A), these costscould be imposed on the DG owner, thus paying deepconnection charges. In any case, it must be ensured thatthe amount which the DG owner has to pay for the newbreaker is proportional to the DG capacity compared tothe total capacity covered by the breaker. In most cases,this share thus is rather small. A rather difficult questionis, to what extent future new-connecting DG owners needto pay for this now already past investment, however totheir benefit.

The term stranded costs is used to designate utilities’costs for investments, which cannot be amortised any-more because of connected DG. A simple example wouldbe that a utility builds a line exclusively to an industrial fa-cility which shortly after the completion of the line installsits own DG. The newly built line partly becomes redun-dant, because the customer will barely need the line (seebelow for standby charges). Utilities can therefore claimthat investments in generation, transmission and distri-bution were done under the assumption that they wouldamortise on the long run because of the served customers.Therefore utilities demand that DG owner must pay forthe lost investment although it has been noted that thecapacity of DG to be installed in the next years is unlikelyto exceed the demand growth, thus not making invest-

14

ments redundant [56].Two concepts exist, exit fees and the entailment of so-

called competitive transition charges (CTC). Exit fees arepaid once and reimburse the utility for past investments,now partly redundant because of DG connected. CTCs30

usually run for a limited period of time and are levied onall customers. In any case is it crucial that the imposedfees do not constitute a competitive disadvantage for DG[57]. This is why some states of the United States dictatethat exit fees may only be imposed on DG owners after theinstalled DG capacity exceeds a certain percentage of thetotal utility load in the state31, thus not discouraging newDG owners. With CTCs on the other hand it is difficultto determine, which party - the utility or the DG owner -is discriminated.

As already addressed, networks are not being decom-missioned because of stranded investments since DG own-ers usually still require a connection to the network intimes of planned or unplanned outages of their DG unit.For this maintenance or standby service the DG owner isbilled by the utility, usually depending on the monthly de-mand. As with stranded costs it must be found a balancefor these standby charges not to discourage DG ownersor to even make them completely disconnect from thenetwork32. Standby charges differ from stranded costs inthat they represent costs for a service and not for recover-ing past investments. Both types of charges neverthelessare meant to remedy investments originally undertaken onbehalf of the customer [56].

These presented costs definitively have an impact onthe attractiveness of DG and they should be regulatedthrough a policy framework ensuring a fair competition.

V. DG in Switzerland - the current state

In a first part of this chapter the current status andshare on the total electrical energy production of the inchapter III presented technologies are discussed. In a sec-ond part, it is elucidated which of these installations bydefinition can be considered as DG.

Until 1968, Switzerland generated 95.7 % of its to-tally produced electrical energy with hydro plants; therest was covered with non-nuclear thermal plants.33 Thepower consumption of 22’437 GWh in 1968 increased to54’029 GWh in 2002; together with transmission losses,pump storage consumption and import/export surpluses,this amounts to the total electrical power production of65’011 GWh in 2002. Hydro plants take a share on thatof 56.2 %, 39.5 % are produced by nuclear power plants

30CTCs have been used in the United States to pay for strandedgeneration investments during the restructuring

31E.g. 7.5 % in New Jersey and 10 % in Massachusetts32It is of course also possible to use DG to help the utilities satisfy

load demand at peak times, thus transferring the advantage of DGfrom the DG owner to the utility

33The here presented statistical data are taken from two publica-tions by the Bundesamt fur Energie (Swiss Federal Office of Energy,http://www.energie-schweiz.ch): The ”Swiss Electricity Statistic2002” [58] and the ”Swiss Statistic on renewable Energies in 2002”[59]

and 4.3 % by conventional thermal and renewable, non-hydro power plants [58]. These 4.3 % correspond to 2806GWh, which in turn are divided into 900.7 GWh renew-ably produced and 1905.3 GWh produced with conven-tional thermal power plants; 1.39 % of the total producedelectrical energy in Switzerland was thus produced fromother renewable sources than hydro [59].

Table IV shows how these 1.39 % are split among thedifferent generation forms and also how the share rosefrom 1990 to 2002.

TABLE IV: Renewably produced electrical energy in Switzerland,1990 and 2002; hydro power plants not included

1990 2002 increaseTechnology[GWh] [%] [GWh] [%] 1990-2002 [%]

Total 438.9 100.00 900.7 100.00 105.2Photovoltaics 1.1 0.25 13.8 1.53 1154.5Biomass 7.2 1.64 27.6 3.06 283.3Wind energy 0.0 0.0 5.4 0.6 .solid waste34 372 84.76 745.6 82.79 100.4liquid waste35 58.6 13.35 108.2 12.01 84.6

When considering hydro power plants as renewable gen-erators, it can be stated that the share of renewably pro-duced electrical energy makes up remarkable 57.5 % in2002 [59]. Not all of the contributing facilities howevercan be considered as DG either because of their rating(< 300 MW36) or because of the voltage level of theirconnection (distribution grid level or customer side of themeter36). Thus, it will be discussed for all contributingrenewable generators, whether they can be classified asDG or not.

Hydro plants produced an amount of 36’513 GWh in2002; 48.3 % were produced with river power plants, 51.7% with (pump) storage plants. Figure 9 shows the powerplants installed in Switzerland by 2002, showing the largeamount of river and (pump) storage plants.

A list of hydro power plants sorted by rivers and theirdrainage area - available on the homepage of the Swissassociation of water management37 - shows that mostpower plants have a capacity below 100 MW; few arebetween 100 and 400 MW, and Cleuson-Dixence with 1.2GW capacity is the only one rated above 400 MW. It cantherefore be argued that at least half of all installed hydropower plants are DG according to their rating, but notnecessarily because of their connection level.

In 1914 - as outlined in the ”Energy research conceptof the federal government for the period from 2004 to2007” [22] - around 7000 hydro power facilities were inoperation, over 90 % of them with a capacity below 300kW. By 1985, these were replaced or decommissioned infavour of large plants, totalling to approx. 1000 facilities.Since 1992 however, the government is supporting therecommissioning of the former abandoned decentralized

34Electricity produced from industrial biogas and landfill installa-tions as well as waste incineration facilities (318 GWh)

35Electricity produced from gasified industrial waste water andfrom sewage plants (58 GWh)

36according to chapter II37http://www.swv.ch/statistik.cfm

15

small water turbines, resulting in more than a dozen re-stored and reconnected river power plants per year, ratedat around 100 kW on the average38. These recommis-sioned river power plants can definitively be considered asDG.

Almost all of the in 2002 commissioned grid-connectedPV-systems have a capacity below 100 kWp, according

Figure 9: Power plants > 10 MW installed in Switzerland by 200139

to the 2002 solar energy statistic [60]. Mostprobably, this fact is also applicable for formeryears. Although not confirmed, it can be as-sumed that all PV-Systems in Switzerland canbe considered as DG, both because of theirrating as well as because of their connectionlevel. Since private owners of PV-systems areobliged to cover their own electricity demandfirst and to only feed surplus electricity intothe grid, the fed-in capacity still is low, havingno impact on voltage quality.

The most recent overview on biomass, re-spectively CHP plants is the statistics on com-bined heat and power plants in Switzerlandfrom 1990 to 2002 [61]. Biomass plants arediscussed together with CHP plants operatingwith solid and liquid waste. In 2002, a totalof 1016 CHP facilities were producing electri-cal energy. 37 of these CHP facilities havecapacities above 1 MWe and the remaining979 are mostly rated below 350 kWe. The37 large CHP plants consequently producedapproximately 69 % of the total electrical en-ergy generated with CHP plants. The largeCHP plants are mostly installed at industrial(chemical, paper and oil) facilities, whereas the small CHPplants operate either at waste water treatment facilities(29.6 %), with fossil fuels (64.8 %) or with biogas (5.6%). This leads to a share of 35 % of renewably fueledCHP plants, producing a share of 7.4 % of the total elec-trical energy generated with CHP plants. All CHP plantsare obviously not owned by utilities and can according totheir rating be considered as DG. Furthermore, it is inter-esting to observe that the amount of installed CHP plantsrated above 1 MWe has increased from 26 in 1990 to 37in 2002, whereas the amount of CHP plants rated below1 MWe has in the same time increased from 275 to 979facilities. This corresponds to a raise from a share of 13 %on the totally produced electrical energy to a share of 31% on the in 2002 with CHP plants produced 1684 GWh.

According to the research programme on wind by theBundesamt fur Energie (Swiss Federal Office of Energy)[62], was it the stated goal to produce 10 to 30 GWh ofelectricity per year by 2000 and up to 50 GWh annually by2010 with wind turbines (see also [22]). As table IV how-ever shows, was the target with 5.35 MW installed windcapacity missed as well in 2002 [59]. However, accordingto the mentioned research concept [62] are projects being

38http://www.smallhydro.ch/deutsch/kwk ch/statistik/KWK wirkung.asp

39Figure taken from http://www.strom.ch

processed, yielding an energy production of 60 GWh an-nually. This would result in a share of approximately 0.1% of the total Swiss electricity consumption.

The statistical data presented showed that a high shareof small power plants exists in Switzerland; since the con-nection levels are often unknown, most of the hydro powerplants cannot definitively be considered as DG. The next

chapter will discuss the expansion of the existing capacityand also discuss technologies as e.g. geothermal powerplants which did not yet contribute to the electrical powerproduction.

VI. DG in Switzerland - a possible future state

In a vote in the fall of 2000, the Swiss population turneddown a new law for the promotion of renewable energiesby applying a support tax on non-renewable generators.Two years later, a new law for the structured, step-wiseliberalisation of the electricity market was turned downas well. Both these new laws would have meant an en-couragement to the installation of new renewably drivenpower plants.

These two laws would have supported DG installations,particularly the second vote, regulating the liberalisation.Nevertheless will renewable energies be subsidised throughprogrammes existing for more than 10 years now. In 1992a programme was initiated to support the installation ofsolar collectors on school buildings. In 1996 this pro-gramme was extended from school to all buildings andincluded also support for photovoltaic installations with3000 CHF/kWp installed. The programme running inthe scope of ’Energie 2000’ finally had to be aborted 1.5years before the scheduled end; the provided funds weremeant for an installation of 1 MWp/year and thus wereovertaken by an installation of 1.7 MWp/year. Never-

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theless, a total installed capacity of 4.4 MWp could besupported by the programme, favoured also by variousnewly emerging solar power exchanges on canton levels[63]. This programme now has changed and was dele-gated to the cantons who share the 3000 CHF/kWp to-gether with the cantonal utilities. But according to thealready cited report ’Subventionsprogramm Photovoltaik1997-2001’ by the Bundesamt fur Energie [63], the sup-port amount should be around 6000-7000 CHF/kWp toreally spur on the installation of PV systems.

Besides these subsidies exclusively for solar poweredsystems, there exists a programme for the support of so-called pilot and demonstration installations, also applica-ble to other renewable generators as wind or geothermalinstallations40. Recent political decisions however intendto cut the money for the ’EnergieSchweiz’ programme by2006 [64].

Independent of these subsidy programmes, studies wereperformed to determine the theoretical, technical and eco-nomical potentials for photovoltaic, hydro and wind instal-lations in Switzerland. The results of these studies will bepresented here.

The photovoltaic potential of an area - e.g. a city, acanton or a whole state - often is estimated, assuming thatmost PV systems will be erected on existing buildings.Therefore, the total roof and facade surface area of allbuildings in the considered region is estimated. A certainratio is regarded as useless due to roof superstructuresas chimneys or windows (approx. one third) and due toclouding (approx. one fifth) [65]. The remaining surfacesare assessed according to their solar energy yield, whichdepends on the southward orientation, the inclination ofthe roof as well as the solar irradiation. All roofs, whichachieve a production of at least 80 % of the optimumroof are accounted as valid; i.e. only the 20 % best suitedroofs are taken into account.

In 1996, a study was performed by Gutschner [65], esti-mating the potential for building integrated PV systems inSwitzerland. The result varies between an annual electri-cal energy yield of 8.8 and 16 TWh; since a regionalisationof the solar irradation was not undertaken, these valuesresult as minimum and maximum yields. Compared tothe annual consumption of 48.7 TWh in 1996 [58], thepotential is considerable.

Based on the same principles, studies were carried outin 1998 for the city of Zurich [66] and for the canton of Fri-bourg [67]. The results are promising alike: Zurich’s con-sumption of 2.6 TWh in 1997 could be supported throughPV systems, contributing 0.44 TWh whereas 0.86 TWhof Fribourg’s total consumption of approx. 1.8 TWh in1996 could be produced with PV systems41.

An international study performed in 2002 by the ’Pho-tovoltaic Power Systems Programme (pvps)’ of the Inter-national Energy Agency IEA42 confirms the results of theearlier made studies [68]. The potential for the productionon roofs is estimated to be 15 TWh whereas the poten-tial for the production on facades is computed to be 3.4

40http://www.solarch.ch/index2.html41Both results only consider roofs and do not include facades42http://www.iea-pvps.org

TWh, totalling to a building integrated PV potential forSwitzerland of annually produced 18.4 TWh. Comparedto the before mentioned consumption of 54 TWh in 2002,does this potential deserve attention.

The potentials for hydro in Switzerland have been re-cently investigated and published in two independent stud-ies [69], [70]. In the study issued in the context of the’EuroWasser’ programme by the University of Kassel’s’Center for Environmental Systems Research’ [69], the so-called ’gross hydropower potential’ is calculated43. ForSwitzerland, the gross hydropower potential was calcu-lated to be at 128 TWh/a according to method A and at80.6 TWh/a according to method B.

The second study [70] was limited to technically andeconomically feasible potentials, resulting in a technicallyrealisable production yield of 41 TWh/a and an economicpotential of 35.5 TWh/a. Compared to the actual pro-duction of 36.5 TWh in 2002 [58], the economic poten-tial seems to be reached already today in Switzerland.According to the ”Swiss Electricity Statistic 2002” [58],only little extensions will be undertaken with the existingpower plants until 2009. The annual production expec-tations thus will not change considerable. It is howeverthinkable that the economic value of hydro will increasefurther, consequently increasing the amount of the eco-nomically feasible potential.

An interesting remark concerning the production of green-house gases, particularly of carbondioxide, can be foundin an article from 1996 [71]. The article outlines thatparticularly large hydropower storage lakes, where exist-ing forests are flooded, lead to a faster decaying processof the flooded vegetation, resulting in a large amount ofreleased carbondioxide and methane. Hydropower wouldthus become a carbondioxide and particularly a methaneproducing technology. Since however no other articlescould be found, confirming the stated observations, thesubject will not be followed further.

The potentials for wind power in Switzerland have beencalculated in a study in 1996, performed through the Bun-desamt fur Energie (Swiss Federal Office of Energy) [72].By the help of a geographical information system (GIS),Switzerland was divided into cells of 250 m x 250 m. Foreach cell, the wind conditions and the landscape protec-tion related conditions were aggregated44. It was thenassumed, that for each cell either one 500 kW or two

43This potential is defined as the ”annual energy that is poten-tially usable if all natural runoff at all locations were to be harnesseddown to the sea level (or to the border of the country)” (subsec-tion 8.3.1, 2nd paragraph, [69]). For that purpose, the country’ssurface was divided into cells and two methods were applied. Withmethod A, the total gross hydropower potential down to the borderof the country (in the case of Switzerland) - independent of wetherthe potential can be harnessed or not - is allocated to each cell.With method B, only that portion of the gross hydropower poten-tial, which can be locally harnessed down to the next downstreamcell, is allocated to the cell; thus both the inflowing discharge fromhigher adjoining cells and the runoff generated inside the cell mustbe considered. Method B consequently is the more realistic poten-tial estimation, considering only the utilisable amount of the totalpotential.

44See also the interactive wind map athttp://stratus.meteotest.ch/mme/winfo/presentation/winfo style/map.asp

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250 kW wind turbines can be installed. In total, ap-proximately 1% of the surface of Switzerland was foundto be suited, corresponding to 511 km2. The total po-tential was to calculated to be an annual production of1’628 GWh. Compared to the total annual consumptionof 54’029 GWh this corresponds to 3%. It could now beargumented that technology has advanced since 1996, to-day’s average wind turbines being rated above 1 MW (seefigure 3), thus making it possible that for each cell one 1MW turbine can be installed, thus doubling the potential.The increased height of the turbine however would lead toobjections by the ’Stiftung Schweizer Landschaftsschutz’(Foundation for the Protection of the Swiss Landscape)[72], consequently decreasing the number of suited cellsand thus again reducing the potential.

Altogether it can be stated that photovoltaics offer themost interesting potential from a technical point of view.Economically, PV is not yet capable of competing withhydro and wind. Hydro however has already reached itstoday economic limit and wind will not be able, for thementioned reasons, to significantly contribute to the Swisspower generation.

VII. Discussion

Chapter III and VI showed that several existing tech-nologies have been further developed over the last yearsand that they have become technically and economicallyoperable. Favoured by the economies of scale develop-ment, distributed generation will continue to take an in-creasing share of the power generation, despite the men-tioned not yet solved protection issues.

Throughout the report it however became clear thatthe applied definition is not always suited for denoting adistributed generator. The definition requires DG to beconnected either ’on the distribution level or on the cus-tomer side of the meter’. The customer side will howeverbe subject to various changes in coming liberalisation pro-cesses and it might not be necessary that DG is connectedat the customer’s side. It is therefore suggested that thedefinition should rather focus on the delivery area of thegenerator, representing the basic idea of distributed gener-ation, which there is to produce power where it is needed.Thus, a generator can be considered as a distributed gen-erator if - under normal operation conditions - all of theproduced energy is used locally, i.e. within a certain range,depending on the population-density. If however the mainshare of the generated energy has to be transported overa longer distance before it is consumed, the generationcannot anymore be considered as distributed.

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