Applied Energy 88 (2011) 479–487

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Applied Energy

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A renewable energy system in Frederikshavn using low-temperature geothermalenergy for district heating

Poul Alberg Østergaard *, Henrik Lund 1

Department of Development and Planning, Aalborg University, Fibigerstræde 13, 9220 Aalborg Ø, Denmark

a r t i c l e i n f o a b s t r a c t

Article history:Received 24 September 2009Received in revised form 17 March 2010Accepted 23 March 2010Available online 20 April 2010

Keywords:GeothermalAbsorption heat pumpDistrict heatingEnergy systems analyses

0306-2619/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.apenergy.2010.03.018

* Corresponding author. Tel.: +45 99408424; fax: +E-mail addresses: poul@plan.aau.dk (P.A. Øs

(H. Lund).1 Tel.: +45 99408309; fax: +45 98153788.

The Danish city Frederikshavn is aiming at becoming a 100% renewable energy city. The city has anumber of energy resources including a potential for off-shore wind power, waste and low-temperaturegeothermal energy usable as heat source for heat pumps producing district heating.

In this article, a technical scenario is described and developed for the transition of Frederikshavn’senergy supply from being predominantly fossil fuelled to being fuelled by locally available renewableenergy sources. The scenario includes all aspects of energy demand in Frederikshavn i.e. electricitydemand, heat demand, industrial demand as well as the energy demand for transportation.

The locally available energy resources are deliberated and an energy system is designed and analysedwith an energy systems analysis model on an aggregate annual level as well as on an hourly basis.Particular attention is given to the use of geothermal energy in the area. It is shown, that the use ofgeothermal energy in combination with an absorption heat pump shows promise in a situation wherenatural gas supply to conventional cogeneration of heat and power (CHP) plants decreases radically.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

At the seminar Energy Camp 2006 in Denmark it was proposedby a number of participants that a Danish city should aim atbecoming a 100% renewable energy city. The city should functionas a testing site for innovative energy technologies as well as func-tion as a beacon for other cities and regions with similar ambitions.

Through a presence of attendees from Northern Jutland at theseminar, it was decided to proceed with the suggestion that Fred-erikshavn – located in Northern Jutland (see Fig. 1) – should be this100% renewable energy city. The ambition was subsequentlyadopted by the Frederikshavn Municipal Council.

Frederikshavn Municipal County later approved a scenario plan([1] – described in English in [2]). The short term (2015) objectiveof the scenario plan is simply to match aggregate demand with100% renewable energy sources on an annual basis. Energy maybe exchanged with surrounding areas – e.g. petrol for cars againstthe export of electricity from wind turbines. Biomass use in theshort term may exceed what would be available if nationally avail-able biomass resources were distributed according to populationand it is also noteworthy that the scenario plan includes wasteas a renewable energy source.

ll rights reserved.

45 98153788.tergaard), lund@plan.aau.dk

In the long term (undefined year), the system should function ina way that does not diminish the possibilities of other areas to havea 100% renewable energy supply. This entails a reduction in the useof biomass resources and an electricity system which does notmerely export hourly imbalances.

The permitted extent of electricity balancing with surroundingareas is not stated explicitly, only that the system of Frederikshavnmust be able to integrate into a national system based on 100%renewable energy. Using a term from [3], this may be termed ‘‘con-nected island mode” which gives ‘‘priority to the ability to operate inisland mode – but also [.. includes] a certain possibility of exchangewith the surroundings”. Different areas in Denmark each have a spe-cific point of departure in their specific set of geographic, industrialand demographic circumstances, however the potential energy re-source base does not vary that much within the country. Windpower and biomass are the main resources utilised presently [4]and both are likely candidates for important future roles. Thereare hence no significant synergies to be exploited between differ-ent areas in terms of load balancing as investigated in [5] andthe Frederikshavn system should therefore be able to operate in is-land mode to the highest extent possible.

The Frederikshavn project has already generated a number ofactivities. The largest Danish utility DONG Energy has been in-volved in the process due to their ownership of some of the exist-ing wind turbines in the Frederikshavn area. They have an interestin expanding off-shore wind power near Frederikshavn and havealso invested in a heat pump producing district heating while

Fig. 1. Denmark with Frederikshavn located in the very North.

480 P.A. Østergaard, H. Lund / Applied Energy 88 (2011) 479–487

helping integrating wind power. An additional 25 MW of off-shorewind power is planned of which 12 MW was planned to be erectedin 2009 but encountered delays.

The northern suburb of Strandby has already established alarge-scale solar collector for the Strandby district heating systemin combination with an absorption heat pump, and plans areunderway for the expansion of the local waste incineration plantas a consequence of lacking capacity and plans for shutting downa smaller installation in the city Skagen 35 km to the North (seeFig. 1). A biogas to natural gas upgrading facility has also beenproposed.

Potential energy resources are in limited supply however. Re-gions in the world with high penetrations of renewable energy typ-ically rely on single resources such as ample biomass resources,good wind resources or good potentials for hydro power – all com-bined with modest populations. There are limited biomass re-sources in the case of Frederikshavn, wind potentials are alsolimited, and there is no potential for hydro electricity. There ishowever an unrealised potential for geothermal heat which maybe combined with other sources to form a multi-tier renewable en-ergy system.

Investigating the scientific work on renewable energy systemsreveals a large focus on single-technology systems e.g. wind power[5–8], photo voltaic cells [9], biomass [10], biogas [11] and wasteresources [12,13]. All of the resources may serve as important fac-tors in 100% renewable energy systems, but energy systems mayalso benefit from synergies between the resources. Likewise, thereis focus on many of the potential technologies that may facilitate

100% renewable energy systems – compressed air energy storagesas deliberated by Lund and coworkers [14–16], hydrogen for trans-portation [17], electricity for transportation [18,19], hydrogen as astorage option [20] and conversion of individual heat systems todistrict heating systems [21]. In addition, renewable energysources may also be investigated from a spatial perspective[22,23] or even from an institutional or policy perspective [24–27]. Again, synergies may be exploited if a more integrative ap-proach is applied.

From an even more technical point are the issues of ancillaryservices, impacts on transmission grids and the electro-technicaldesign of electricity systems [5,28–33]. Such analyses are also rel-evant when designing future energy systems though beyond thescope of this article.

While much of the work referenced above investigates singletechnologies, most of the technologies are in a specified context– either a renewable energy context or a more traditional fossil fuelcontext. As noted in this article, as well as by e.g. Lund in [34], asingle-technology perspective does not suffice. Future 100% renew-able energy systems require a more integrated approach found innational analyses in [35–37] or local analyses as found in [38].However, of these examples, only [36] includes transportation inthe analyses.

Geothermal energy is widely utilised in many countries, how-ever most of the heating applications are using near-ground levelheat sources in combination with compression heat pumps [39].There is some research on using absorption heat pumps in combi-nation with geothermal energy, however most relate to cooling

P.A. Østergaard, H. Lund / Applied Energy 88 (2011) 479–487 481

demand and only few treat the combination from a space heatingobjective, as found in [40,41]. Even so, the perspective in said workis from single-technology perspective rather than from an inte-grated energy system perspective.

2. Scope of the article

This article outlines the energy situation in Frederikshavn. It de-tails an existing scenario developed in an iterative process betweenthe authors and Frederikshavn Municipality [42]. The scenariointegrates all electricity, heating and transportation demands.Based on this scenario, it is analysed what role geothermal energymay play in the context of Frederikshavn. The focus of the work isto investigate the impacts of geothermal energy on the dynamics ofthe energy system as well as analyse how such geothermal systemsare operated under the constraints of the given system.

3. Frederikshavn – geography and population

Frederikshavn, a city of some 25,000 inhabitants and the admin-istrative centre of Frederikshavn Municipality is located 35 kmsouth of the northernmost point of Denmark on the east coast ofthe peninsula Jutland. The Municipality with its population densityof approximately 97 persons per km2 has a low population densitycompared to Denmark in general which has a population density of128, however the energy city area is delimited to being mainly theurban area of the actual city Frederikshavn as well as some suburbswith a total area equalling 3741 ha.

The actual geographical delimitation follows the border of theelectricity grid company Frederikshavn Elnet A/S with some excep-tions made in order to fully encompass some suburbs within theareas of a neighbouring electricity grid company.

While the energy city mainly covers built-up areas, it also cov-ers some agricultural land and some small patches of forest. Inaddition, being a costal city, there are off-shore areas that maybe exploited for energy harvesting. Being a moraine landscape pro-duced by land rising, the depth of the water drops slowly with the10 m line being in a distance between 3000 and 6800 m from theshore. This may be compared to water depth across the Skagerrakin Norway, where the same distance off its bed-rock coast often isbeyond the 100 m curve. The water depth at the Danish Horns Reefoff-shore wind farm is between 6 and 14 m of depth so in terms ofgeology, the Frederikshavn area is well-suited for off-shore windpower. Protected areas reduce the potential though [43],while pre-valent westerly winds make the east coast location less opportunethan west coast locations. Compared to inland sites, full-load hoursare still favourable off the east coast.

4. The EnergyPLAN energy systems analysis model

The energy system is modelled using the EnergyPLAN model.This model has been developed over the last decade with theaim of creating a tool for the analysis of energy systems with manyinterdependencies i.e. through the use of combined heat andpower for district heating or through the use of energy technolo-gies tapping into fluctuating energy streams like wind, solar, andtidal. The model has been used in a number of studies ranging insize from national systems to small villages or in studies focusingon the performance of a single technology in complex energysystems. References include [3,5,6,12,14,15,30,44,45] and themodel is described in [46,47]. The model is available for free atwww.energyplan.eu.

EnergyPLAN is a deterministic model which calculates a year inone run on an hourly basis. For each hour, the model ensures that

there is balance between production and demand of heat, electric-ity, cooling and energy for transportation.

In addition to demands and production units, inputs includeeconomic factors and regulation strategies, where the user deter-mines how the controllable plants are to be operated based on fac-tors such as system imbalances and spot market prices. In theanalyses in this article, a technical regulation strategy has been ap-plied. CHP plants operate according to the momentary electricityand heat demand in the particular variant of technical regulationstrategies applied in this article. This is as opposed to e.g. CHPplants operating solely according to the heat balance. Among theoptions applied by the model is a temporal shift of heat productionfrom heat demand by the use of heat storages. The reason forchoosing a technical regulation strategy over an economic optimi-sation is to view the impacts on the system’s load following capa-bility. Once a system has a proper load following capability, it maybe analysed how to optimise its operation against e.g. an externalelectricity market, but the first step is simply to ensure sufficientflexibility to ensure security of supply. The technical regulationstrategy is also in line with the described goal of having a systemthat can work in connected island mode i.e. where priority is givento designing an energy that does not rely on neighbouring areas forbalancing purposes. Electricity imports and exports are thus mini-mised with this regulation strategy – and the magnitude of thesemay also be used to assess how well the system is designed.

Outputs of the model include hourly consumptions and produc-tions of the modelled types of technologies as well as aggregate an-nual data.

For the purpose of these analyses, the model has been expandedto enable the modelling of waste incineration & geothermalabsorption heat pump systems. In addition to varying degrees ofsteam output to the absorption heat pump, the model also includesa steam storage. Waste incineration plants typically operate con-tinuously in order to ensure a proper combustion of the waste.They are furthermore usually designed and sized from a wastehandling perspective leaving little or no excess capacity for loadshifting. With the organic fraction of waste degrading if stored[12], there is little possibility of introducing much flexibility at awaste incineration plant. By introducing the ability to model asteam storage in the EnergyPLAN model, the flexibility of the sys-tem is thereby increased.

5. Present energy situation in Frederikshavn

The present energy consumption in Frederikshavn is character-ised by the urban nature of the area with a much higher districtheating share of the final energy consumption than in Denmarkas an average. Electricity consumption also has a higher share ofthe final energy consumption whereas transportation, individualheating and industry are smaller compared to the national dataas indicated in Figs. 2 and 3.

The energy demand is presently covered by various sources. Theenergy demand for transportation, the fuel use for industrial pur-poses and to some extent individual heating is covered by fossilfuels imported into the area.

Electricity is partly produced on

� four existing near shore-wind turbines with a total capacity of10.6 MW,� a 2.5 MWe/10 MWth waste incineration CHP plant,� a large 17 MWe/31.5 MWth natural gas fired CHP plant, and� a small 2 MWe/2.3 MWth natural gas fired CHP plant.

In addition, electricity is being transmitted across the systemboundary of the energy city area. For the purpose of the electricity

Electricity demand 164 GWh

27%

District heating 195 GWh33%

Transport 165 GWh28%

Individual heating 37 GWh

6%

Industrial fuel use 36 GWh

6%

Fig. 2. Final energy consumption in Frederikshavn in 2007. Based on data fromAnne Dorthe Iversen, Frederikshavn Municipality and Poul Sørensen, Cowi.

Electricity demand 34 TWh

18%

District heating 29 TWh16%

Transport 62 TWh34%

Individual heating 30 TWh

16%

Industrial fuel use 30 TWh

16%

Fig. 3. Final energy consumption in Denmark in 2007. Based on data from [48].

482 P.A. Østergaard, H. Lund / Applied Energy 88 (2011) 479–487

system modelling, a condensing mode coal-fired power plant withan efficiency of 40% represents the outside world.

District heating is produced on the mentioned natural gas firedCHP plants, on 80 MW natural gas fired peak load boilers as well ason the newly installed solar collector and an absorption heat pumpin Strandby.

6. Energy system scenario for Frederikshavn

The scenario of Frederikshavn is developed with the technicalsystem integration in mind, and thus focuses on the aspects of fuelchanges and the load following capability of the system. The sce-nario includes a number of initiatives that are either foreseen orwhere decisions have already been made. These include:

� Wind power: additional 25 MW to be installed off-shore. In theanalyses, if nothing else is specified, a total of 46 MW windpower is modelled.� Biogas: a plant in Skagen will provide an annual 7 GWh for

transportation in Frederikshavn (compared to the presentdemand of 165 GWh)� Compression heat pump: a compression heat pump run on sew-

age heat with an annual production of 6 GWh at a COP (Coeffi-cient of Performance) of 3. The compression heat pump is not tobe mistaken for the absorption heat pump run on geothermalheat and steam.

� Waste incineration: a new waste incineration plant with awaste input of 186 GWh annually to be based on waste fromthe waste collecting area of the existing plant as well as froman extended area including Skagen.

In addition to these planned or foreseen initiatives, the follow-ing elements are also included:

� Low-temperature heat demand in the industry is changed todistrict heating thereby eliminating an annual fuel demand of31 GWh. The remaining industrial use of 5 GWh is replaced bybiomass.� Most of the individually heated dwellings within the project

area with an annual fuel demand of 26 GWh are connected tothe district heating system. A large area is supplied with naturalgas, but switching this to district heating adds considerable fuelflexibility and fuel efficiency to the system.� Additional district heating compression heat pumps with a COP

of 3 giving a total installed capacity of 10 MWth including thepreviously mentioned sewage-based heat pump. Again, thecompression heat pump is not to be mistaken for the absorptionheat pump run on geothermal heat and steam.� The remaining individually heated houses are supplied by

1 GWh of thermal solar collectors and 7 GWh from individualheat pumps per year. As with the other houses, the heat pumpsadd fuel flexibility to the system as they may e.g. be run onwind power.

Transportation is one of the difficult areas in the design of sus-tainable energy systems. In Denmark for instance, road transporta-tion accounts for 20% of the total primary energy consumption [48]– almost exclusively fossil fuelled. In the scenario, the followingsteps are taken:

� Petrol for mopeds and motorcycles and diesel for lorries andbusses is replaced by biogas, methanol or hydrogen (25 GWhfuel is replaced).� 7% of the cars are converted to biogas (10 GWh fuel is replaced).� 56% are converted to electricity (75 GWh fuel is replaced).� 19% are converted to fuel cells (26 GWh fuel is replaced).� 18% are converted to biogas in plug-in vehicles (25 GWh fuel is

replaced).

The timeframe for conversion of the energy supply for transpor-tation is not defined and relates to the long term perspective ad-dressed in the introduction of this article. The numbers are hencetentative as technological development most likely will induceshifts in favour of one or more of the mentioned technologies.Technologies like methanol cars and (natural) gas busses alreadyexist commercially, electric cars exist in a limited supply, whileyet other technologies like hydrogen vehicles or mopeds/motorcy-cles on alternative fuels remain at an experimental stage.

In order to produce biogas for transportation as well as supply-ing the existing gas turbine-based CHP plants with a renewablefuel, a biogas plant with an annual production of 150 GWh is in-cluded. The plant requires more bio-resources than what is avail-able from husbandry in Frederikshavn Municipality according toa resource assessment by Jakubovic et al. [49], so if technologicaldevelopment permits, a higher share of electric vehicles wouldbe preferred.

The large natural gas CHP is decreased in size from 17 MWe to3 MWe in order to leave a larger share of the biogas to other pur-poses including transportation.

Conventional biogas plants use biogas to produce heat and elec-tricity for auto-consumption, but these demands of 28 GWh and5 GWh respectively are included as demands to be covered by

Table 1Production on the waste-incineration/geothermal absorption heat pumpcombination.

Normaloperation(MW)

Absorptionheat pumpoperation(MW)

Fuel input to the CHP unit 21.0 21.0Electric output from the CHP unit 4.83 3.53District heating output from the CHP unit 13.48 1.58Steam output from the CHP unit 0.00 13.30

P.A. Østergaard, H. Lund / Applied Energy 88 (2011) 479–487 483

the overall energy system. The process heat demand is thus sup-plied via district heating. This increases the efficiency of the systemif the alternative is a biogas boiler for process heat generation.

Implementing all the changes described in the scenario willrender an energy system capable of covering electricity, heatingand transportation needs using only renewable energy sources.While some of the elements are already implemented, others aremerely applying known technologies, while yet others includetechnologies under development. Therefore, one cannot outline atime-line for the potential implementation.

District heating output from absorption heat pump 0.00 22.00

Total district heating output 13.48 23.58

Table 2Heat production on the different units in Frederikshavn Municipality.

2007Reference(GWh)

2015Withoutgeothermal(GWh)

2015 Withgeothermal(GWh)

Solar thermal collectors 0 3.92 3.92Small CHP plant 12.95 7.53 7.76Boiler at small CHP plant 7.04 8.54 8.32Industrial excess heat 0 17.00 17.00Waste incinerator heat 79.3 118.40 42.14Geothermal heat 0 0 141.38Large CHP plant 111.36 30.17 22.70Boiler at large CHP plant 28.67 85.24 38.53Compression heat pump at large

CHP plant0 57.77 46.71

Industrial heat generation for ownuse

28.80 4.80 4.80

Individual heat pumps 0 8.00 8.00Individual oil 4.90 0 0Individual natural gas 20.80 0 0Individual biomass 2.40 0 0

Total heat production 296.22 341.37 341.26

7. Geothermal energy in Frederikshavn

This article focuses on the applicability of using geothermal en-ergy in combination with an absorption heat pump run on steamfrom the waste incineration plant given the previously outlinedsystems configuration.

The Danish utility DONG Energy has analysed the potential forgeothermal energy in Frederikshavn, and while Denmark ingeneral is in a geologically stable region – and thus has poorgeothermal prospects, Frederikshavn is located atop a number ofdifferent geological formations; the upper two are with tempera-tures in the 20s. Further deep are the Gassum and the Skagerrakformations with temperatures at 32� and 40 �C respectively[50,51]. Temperatures are even higher deeper down and thus bet-ter for producing district heating. The DONG analyses are based ona temperature of 58 �C which corresponds to a well-depth ofapproximately 2000 m.

Even this temperature level, however, is clearly too low for di-rect heating purposes though very suitable as a heat source for aheat pump; a technology already applied in Thisted, 124 kmWest-South-West of Frederikshavn. In Thisted, warm water is ex-tracted from 1243 m below ground level and used as a low-tem-perature heat source in an absorption heat pump in combinationwith a high-temperature heat input from a boiler before beingre-injected into the ground [52].

The high-temperature heat source for the absorption heat pumpcould be in the form of e.g. 160 �C steam. In Frederikshavn, theabsorption heat pump may thus be operated using steam fromone of the CHP plants rather than from a boiler in order to add sys-tem flexibility and efficiency. The two existing natural gas firedCHP plants have a simple cycle gas turbine and a piston engine,so they are not configured to produce steam as opposed to thesteam-based waste incineration plant. The Danish engineeringconsultancy Rambøll has therefore been commissioned to analysethe prospects of using a geothermal source absorption heat pumpin combination with steam from the proposed waste incinerationplant, and while the electric and thermal outputs of the actualwaste incineration plant will be reduced, the added heat outputfrom the absorption heat pump compensates for this by far. Bleed-ing 13.3 MW steam from the turbine to the absorption heat pumplowers the electrical output by 1.3 MW and the district heatingoutput from the CHP unit by 11.9 MW. It also introduces a heatoutput from the absorption heat pump of 22 MW. The heat outputof the entire system thus increases by 10.1 MW or approximately75% as shown in Table 1.

The system is modelled similar to a back-pressure plant i.e. witha direct linear correlation between the outputs in the two points inTable 1. A varying degree of steam may thus be bled to the absorp-tion heat pump in order to ensure optimal system integration ofthe non-dispatchable productions e.g. wind.

There are no practical geological limits to how much heat maybe extracted from the ground in Frederikshavn; the source temper-ature will decrease in time, however if the temperature becomestoo low, the extraction point may simply be moved for extended

operation. Limits are rather economic as well as in terms of the re-quired high-temperature input for the absorption heat pump.

8. Results of energy systems analyses and discussion

The energy system as outlined previously has been modelled intwo different year 2015 situations; with and without a geothermalheat pump using the EnergyPLAN model. The system has also beenmodelled with varying degrees of wind power ranging from noneto an annual production corresponding to 100% of the annual elec-tricity demand in the area.

The main parameters used to asses the systems performance isfuel use and excess generation. See [3] for a thorough discussion ofvarious optimisation criteria.

The total heat demand increases slightly from the 2007 refer-ence to the two 2015 scenarios as shown in Table 2. The districtheating demand increases due to the conversion of previously indi-vidually heated houses to district heating and conversion of fueluse for industries to district heating use. The district heating de-mand in the 2015 scenarios also includes process heat for the bio-gas plant as mentioned previously.

Heat production on the large CHP plant is naturally loweredas the size is decreased. The introduced deficit is mainly coveredby a district heating boiler when geothermal energy is not uti-lised. This boiler production drops by 55% when geothermal en-ergy is used while still remaining above the 2007 referenceboiler production. More waste is available for the waste inciner-ation CHP plant in 2015 thereby increasing the potential heat

Table 3Primary energy use in Frederikshavn.

Reference2007(GWh)

2015withoutgeothermal(GWh)

2015 withgeothermal(GWh)

Wind power 34.45 149.49 149.49Waste use 112.00 185.00 185.00Natural gas use 319.16 174.27 160.87Coal use 110.15 �18.94 5.91Oil use 178.00 0.00 0.00Biomass use (ex waste) 4.00 112.55 54.17Solar heat 0.00 4.92 4.92Total primary energy supply 757.76 607.29 560.36Electricity exchange 0.21 0.11 0.10Carbon dioxide emissions (kt) 147.4 122.5 124.3Carbon dioxide emissions –

corrected for electricity trade (kt)147.2 29.3 35.0

484 P.A. Østergaard, H. Lund / Applied Energy 88 (2011) 479–487

production from 79 GWh to 118 GWh if geothermal heat is notused. Introducing geothermal energy lowers the direct heat effi-ciency of the waste incineration CHP plant as shown in Table 1causing the direct heat output to drop to 42 GWh. The 76 GWhdrop combined with a drop of 8.4 GWh in the electricity produc-tion does however generate an extra 142 GWh of heat from theabsorption heat pump.

Aggregate annual fuel use is shown in Table 3. The fuel use isapproximately 758 GWh before any changes are implemented.This is reduced to 607 GWh in 2015 without geothermal energyutilisation and to 560 GWh with geothermal energy. These num-bers are corrected for import and export of electricity using a con-densing mode coal-fired power plant with an efficiency of 40% asthe marginal production unit. Coal use and carbon dioxide emis-sions are higher with utilisation of geothermal energy in the2015 situation than without. This is due to this method of handlingimport and export of electricity. With use of geothermal energy,steam is being bled from the turbine to the absorption heat pump.Electricity production is hence lowered giving either an increasedimport or a decreased export. For comparison, carbon dioxideemissions are 33.0 kt and 35.8 kt with and without the utilisationof geothermal energy if the marginal production unit is carbondioxide emission free – irrespective of whether emissions are com-pensated for import/export or not.

0

10

20

30

40

50

60

70

80

90

100

0 732 1464 2196 2928 3660 4392 5124

Time of the year [hour

Pro

du

ctio

n a

nd

dem

and

[M

J/s]

Fig. 4. Hourly values of district heat production and consumpt

In the following, focus is on system behaviour with and withoutgeothermal energy in the 2015 scenarios.

Figs. 4 and 5 show the hourly production and consumption ofdistrict heating with and without the geothermal plant. It is clearto see by the degree to which boilers are used in both cases thatthe more energy efficient technologies like CHP units and heatpumps are unable to cover the demand most of the year. The boil-ers are used extensively and are only out of operation during thesummer months.

This also means that when geothermal energy is introduced, theabsorption heats pumps are not operated very flexibly but arerather applied more or less continuously apart from duringthe summer months. There is a demand for the heat productionthe geothermal plant is able to deliver. This is also evidentfrom the duration curve of the absorption pump shown in Fig. 6showing that the absorption heat pump runs full load 65% of theyear and only stops 8% of the year.

Exploiting geothermal heat for district heating purposes fromthe waste incineration CHP plant/geothermal absorption heatpump combination increases heat production as the reduction inthe direct district heating production is lower than the additionfrom the absorption heat pump.

Fuel use decreases when introducing geothermal based districtheating production due to the significantly higher efficiency of thegeothermal system compared to the boiler which is modelled withan efficiency of 80%. There is a fuel cost in terms of reduced elec-tricity generation from the waste incineration plant, but the fuelequivalence of this is significantly smaller than the avoided fueluse of the boiler as demonstrated in Fig. 7. The reduction is be-tween approximately 40 and 60 GWh depending on the modelledwind input. There is a certain seasonal variation though as thereduction in the electricity production is higher during the winterthan during the summer due to the higher heat demand duringwinter.

The reduced electricity generation by bleeding steam from thewaste incineration steam turbine to the absorption heat pump alsodecreases the forced export as indicated in Fig. 8. The export depictedin the figure is the export that needs to be exported regardless of theelectricity prices and regardless of requirements beyond the EnergyCity Frederikshavn boundaries – what is commonly referred to as ex-cess generation. It thus illustrates how well the system behaves inrelation to imbalances imposed on neighbouring regions.

5856 6588 7320 8052 8784

]

District heating demand

Solar

CHP

Boiler

Waste

Geothermal

Compression Heat Pump

ion starting January 1st at 46 MW installed wind capacity.

0

5

10

15

20

25

0 1000 2000 3000 4000 5000 6000 7000 8000 9000Duration [Hours]

Hea

t pro

duct

ion

[MW

]

Fig. 6. Duration curve for the absorption heat pump at 46 MW installed wind capacity.

0

100

200

300

400

500

600

700

800

900

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Wind production [Share of demand]

Fuel

use

[GW

h/ye

ar]

With geothermal

Without geothermal

Fig. 7. Fuel use excluding renewable energy sources as a function of wind input to the system.

0

10

20

30

40

50

60

70

80

90

100

0 732 1464 2196 2928 3660 4392 5124 5856 6588 7320 8052 8784

Time of the year [hour]

Pro

du

ctio

n a

nd

dem

and

[M

J/s]

District heating demand

Solar

CHP

Boiler

Waste

Geothermal

Compression Heat Pump

Fig. 5. Hourly values of district heat production and consumption starting January 1st at 46 MW installed wind capacity – without geothermal input.

P.A. Østergaard, H. Lund / Applied Energy 88 (2011) 479–487 485

As indicated in Fig. 8, export increases more without thegeothermal system as annual wind production increases beyondapproximately 30% of the annual electricity demand. The reductionamounts to approximately 1.1 GWh/year to 2.6 GWh/year between50% and 100% wind penetration. With wind penetrations above

80%, the geothermally induced export reduction starts to saturate.The export reduction from the geothermal plant at 90% windpower is only marginally higher than the export reduction fromthe geothermal plant at 80% wind power. This indicates that thereis a limit to what reductions a 4.8 MWe plant can bring about if

0

10

20

30

40

50

60

70

80

90

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%Wind production [Share of demand]

Expo

rt [G

Wh/

year

]

With geothermal

Without geothermal

Fig. 8. Electricity export as a function of wind input to the system.

0

20

40

60

80

100

120

140

160

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Wind production [Share of demand]

Con

dens

ing

mod

e op

erat

ion

[GW

h/ye

ar]

With geothermal

Without geothermal

Fig. 9. Electricity generation in condensing mode operation as a function of the wind input to the system.

486 P.A. Østergaard, H. Lund / Applied Energy 88 (2011) 479–487

installed wind power capacity exceeds this many fold. Belowapprox. 30% wind power, there are no problems with excess gener-ation and hence no reduction.

Likewise, Fig. 9 illustrates the required production in condens-ing mode operation within the system. Here there is a higher de-mand for condensing mode operation due to a lowered electricityproduction from the waste incineration plant. The difference be-tween the two curves ranges between 9.5 GWh/year at no windto 5.0 GWh/year at a wind production corresponding to the annualdemand.

The fact that the curves bend upwards at higher wind penetra-tions is due to system stability considerations, where it is assumedthat wind turbines are unable to supply the ancillary services re-quired [30] meaning that a high momentary wind generation willrequire a high production on ancillary service providing units atthe same moment – in this case condensing mode power plants.For comparison; had wind turbines been able to supply the fullrange of ancillary services, then the geothermal curve would haveended at approximately 47 GWh at 100% in Fig. 9.

9. Conclusions

This article has outlined how the primary energy consumptionfor Frederikshavn, Denmark can be reduced from 758 GWh to

560 GWh mainly through changes in the production system andthus without end-use savings. Carbon dioxide emissions are low-ered even more significantly through fuel substitution. They arelowered from 147.2 kt to 35.0 kt or 29.3 kt depending on whethergeothermal energy is used or not used. The higher emission withgeothermal energy utilisation than without is due to a loweredelectricity export and thus a lowered carbon dioxide compensationfor this export.

If the geothermal system is not implemented – whilst the otherchanges are – then much of the district heating production wouldneed to be covered by a boiler. This increases the fuel demand asthe efficiency of the boiler is much lower than the geothermalsystem. It also puts a drain on the available biomass resources;resources that have more use in either CHP plants of for transpor-tation or industry.

It should be noted though that the energy extracted from theground is not counted as a primary energy supply in this article.

Introducing an absorption heat pump which uses low-tempera-ture heat extracted by a geothermal plant and steam from a wasteincineration plant is an unconventional technology, however, it fitswell into the energy system in Frederikshavn. When energy re-sources with built in storage capacity are reserved for transporta-tion, and electricity to a large extent comes from wind power,CHP plants based on fuels that have a better use in transportationwill have to reduce their operation leaving a possible deficit in the

P.A. Østergaard, H. Lund / Applied Energy 88 (2011) 479–487 487

heat production. Geothermal plants have an important role to playunder such circumstances.

With waste incineration plants usually being operated at con-stant load, a variable steam output for an absorption heat pumpintroduces flexibility into the system. This flexibility, however, isnot fully exploited in the modelled system configuration due tothe near constant need for heat production. During 65% of theyear, the geothermal plant is producing at full load and it is onlyduring the summer months that the geothermal plant stopsoperating or operated at partial load.

The absorption heat pump reduces the electricity production ofthe waste incineration plant during the winter months where heatdemand is higher. Compared to the same system without thegeothermal plant, export is thus reduced between 1.1 and2.6 GWh per year. Condensing mode operation is increased how-ever – for the very same reason.

Acknowledgements

This is a revised version of an article originally presented at the5th Dubrovnik Conference on Sustainable Development of EnergyWater and Environment Systems, held in Dubrovnik, Croatia, Sep-tember 29–October 3, 2009. The work was supported by the Euro-pean Regional Development Fund.

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