Energy Conversion and Management 51 (2010) 833–845

Contents lists available at ScienceDirect

Energy Conversion and Management

journal homepage: www.elsevier .com/ locate /enconman

Design of a combined heating, cooling and power system: Sizing, operationstrategy selection and parametric analysis

K.C. Kavvadias *, A.P. Tosios, Z.B. MaroulisSchool of Chemical Engineering, National Technical University of Athens, Zografou Campus, 15780 Athens, Greece

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

Article history:Received 12 September 2008Received in revised form 11 July 2009Accepted 21 November 2009Available online 16 December 2009

Keywords:TrigenerationResource planningPrimary energy savingsEnergy load profilesTariffsUtilization factor

0196-8904/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.enconman.2009.11.019

* Corresponding author. Tel.: +30 2107723149; faxE-mail address: kavadias@mail.ntua.gr (K.C. Kavva

This work addresses the problem of optimal design of trigeneration plants and discusses the factors thataffect the operation and the feasibility of investment. The effect of various operation parameters andenergy tariffs structures are studied and the performance of a trigeneration plant is presented and eval-uated versus the investment size. A new operation strategy is being introduced and examined along withthree existing strategies. The results incorporate various economical and performance indices that char-acterize the system, with reference to a case of a commercial hospital building.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Combined heating and power (CHP) technology has been in usein industrial applications from the end of 19th century. However,the rapid development of the technologies involved through thelast decades, made easier the penetration of CHP technology inbuildings, hotels, hospitals, schools, community heating or wastetreatment sites. Most recent advances incorporate the usage ofalternative fuels such as hydrogen or biomass, or the exploitationof excess heat converting it to cooling power, that is used in air-conditioning or in various industrial processes. The latter one isalso known as trigeneration plant and has become economicallyviable due to the commercial spread of absorption chillers [1] orother thermally activated technologies (technologies that usewaste heat as fuel to replace electric air conditioning or dehumid-ification loads). Trigeneration, is a very effective way of utilizingthe primary energy of a fuel more efficiently, economically, reliablyand with less harm to the environment than centralized, dedicatedelectric production [2–4].

The technology and design aspects of cogeneration systemshave been studied thoroughly. Thermoeconomic analysis [5,6],exergoeconomic [7] as well as multi-criteria analysis [8] while tak-ing into account environmental benefits [9] can conclude to a vitaltool to find the optimal design and operation strategy. The eco-nomic optimization as a part of feasibility study of a CHP unit is

ll rights reserved.

: +30 2107723155.dias).

usually performed through an examination of the investment crite-ria [10]. The results of all methods showed that CHP systems offerbetter performance than a conventional system, from both eco-nomic and energetic point of view.

Trigeneration systems have even bigger potential for economicand energy savings due to their further utilization of primary fuel.In a trigeneration system, as in a cogeneration system, heat, powerand cooling are mutually exclusive and the different loads cannotbe produced independent of each other [11]. On the other hand,trigeneration plants, needs more thorough study due to the higherdegree of freedom compared to simple CHP plants. Research oncommercial applications small scale of trigeneration systems is re-cent due to the unavailability of commercial size low temperatureand low cost absorption refrigeration systems.

RETscreen [12] has developed a spreadsheet model that evalu-ates the economic and environmental performance of CHP systemsfor different configurations of power heating and cooling supplies.The analysis is based on average monthly values of energy demandand do not include complicated tariff schemes, thermal storage op-tions and trigeneration-specific operation strategies.

Trigeneration specific analysis conducted from Cardona andPiacentino [1] present two operation strategies that aim to satisfythe maximum coincident heating and cooling demand. This analy-sis do not investigate the economic profitability and the effect ofthe tariff schemes.

Further work of Cardona et al. [13] and Piacentino and Cardona[14] includes operation strategies customized for trigeneration inwhich energy costs are compared on an hourly basis. This kind of

Nomenclature

AC absorption chiller nominal electrical power (kW)AOC annual operating cost (€)AOP annual operating profit (€)BL base load operation strategyC cost (€)Co cooling energy (kW)CO continuous operation strategyEl electricity (kW)ELF electrical-equivalent load following strategyF fuel (kW)mCp thermal capacity (kW=K)OE overall efficiency (%)P Prime Mover nominal electrical power (kW)PES primary energy savingsPS peak shaving strategyROI return on investment (%)T temperature (�C)Th thermal energy (kW)g efficiency (%)

SuperscriptsA:Chiller produced by absorption chillerBoiler produced by boilerBuffer buffer vesselCHP produced be combined heat, powerE:Chiller produced by electric chillerEndUse consumed in end usesGridIn imported from gridGridOut exported to gridn scale indexWaste wasted

Subscriptsk monthsmin corresponding to minimum capacitymax corresponding to maximum (rated) capacityset setpoint valuet hours

834 K.C. Kavvadias et al. / Energy Conversion and Management 51 (2010) 833–845

strategies can be applied only in specific tariffs (time-of-use) thathourly costs can be calculated on the fly.

Mixed integer programming models have also been used for theoptimal design of trigeneration plants [15–20]. Usually this kind ofmodels cannot formulate all the tariff characteristics (e.g. maxi-mum demand charges, peak hours) and the optimal solution sug-gest a theoretical optimal operation strategy instead of aapplicable way of operating the plant. Moreover these models can-not easily guarantee a global optimum solution due to the non-convexity of the solution space and usually they terminate witha solution, which is strongly dependent on the starting point[17]. Removing non-linearities concludes to a more inaccuratemodel, omitting parameters such as part load efficiency, heat stor-age, and specific tariff formulations.

The performance and the feasibility of an trigeneration invest-ment is dependent on many parameters, such as energy pricingschemes, and operation restrictions. The study of the effect of eachparameter and its contribution to the performance of a trigenera-tion plant is very important in order to understand the economical,energetic, and environmental benefits of this technology [13].

The scope of this work is to present a complete method ofdesigning a trigeneration plant taking into consideration all thecrucial parameters and to comparatively present the benefits ofdifferent configurations of it. A detailed hour-by-hour mathemati-cal model is used and the variance of various economical (opera-tional profit, return on investment) and performance parameters(system load coverage, primary energy savings, grid utilization fac-tor) versus the plant size is studied for various configurations andoperating parameters (strategy and tariff selection). The possiblebenefits and the sensitivity of the alternative solutions are pre-sented comparatively in order to show the magnitudes of theparameters’ variation.

2. Design parameters

2.1. Energy profile

Commercial buildings have demanding thermal and coolingloads due to heating, ventilating and air conditioning (HVAC) sys-tems. For that reason CHP technology and application matching inthe commercial sector is more difficult than industrial complexesas: (a) it has more fuzzy profiles, (b) on average they are operating

fewer hours per year so the payback period of the investment rises,and (c) are generally smaller than industrial sites which meansthat are less efficient having smaller economy scales. In contrastto industrial consumers, ambient temperature affect heavily thecommercial buildings. Of course, the most important factor is occu-pation and activity frequency. Both seasonal and daily variations ofenergy need to be considered for a more precise design result. Hos-pitals are energy intensive facilities with long operating hours,thus they are appropriate customers for a CHCP plant. Electricityis usually distributed to office applications and cooling devices.Thermal energy is used for space heating and other hospital pro-cesses, such as equipment sterilization, laundry, and kitchen gen-eral hot needs. Hospitals have more stable and predictable loadprofiles than other commercial buildings because they have a morestrict working schedule during a workday. For these reasons hospi-tals are considered to be an ideal commercial consumer and havebeen used extensively in literature as a trigeneration case studyplant. [13,21,20].

2.2. Energy tariffs

Electricity tariffs play a very important role in the decision-making process of the appropriate CHP system. The profitabilityand optimization of a CHCP investment is heavily dependent onthe structure and the pricing of energy that are applied beforeand after the installation of such a plant. Usually, there are threekinds of fees; fixed, volumetric, and maximum demand fees. Fixedfees apply to all consumers cannot be changed or improved. Volu-metric fees are in proportion to the electricity consumed eachmonth. Demand fees are charged depending on the maximumpower demand during the month, regardless of how often themaximum level occurs. Actual value of maximum demand is takenfrom a meter which aggregates kW h over a 15-min period. Maxi-mum demand is charged expensively because the public powercompany is forced to use less efficient generating sourced of en-ergy to meet demand, resulting in higher short-run operating costs.Usually there are two options for the end-users. Either choose atariff that charges expensively the max power but cheaper the en-ergy, or a tariff that charges expensively the energy and cheaperthe demand. Electricity consumed for industrial uses has adiscount. For the sell-back to grid, only volumetric tariffs apply,which are the same for all kinds of consumers, but they have a

K.C. Kavvadias et al. / Energy Conversion and Management 51 (2010) 833–845 835

lower price. Due to the structure of the above tariff, hourly spark-spread based optimization routines and operation strategies[14,18] cannot be applied, because of the nature of the monthlymaximum demand calculation.

Natural gas is the most common fuel used nowadays that is fedon a CHP system. Not only is the cleanest source of energy obtainedfrom fossil fuels, but it is the most cost-efficient fuel in terms of fi-nal energy use. Its price varies with the fuel oil cost and it is usuallycomparatively cheaper than diesel. Natural gas tariffs are not fixedand change every month according to various international indicesrelated to fuel prices and other geopolitical factors, which affectconsequently the profitability of the investment [22].

2.3. System description

The main principal of CHCP plant is that converts fuel energy di-rectly to mechanical shaft power which can drive a generator toproduce electricity. Waste heat can be recovered to cover the ther-mal demand or cooling demand via an absorption chiller. Thestructure of a CHCP plant can vary according to the technology thatis used. The most commonly used prime mover for medium-scale(100–5000 kW) applications and appropriate for commercialbuildings are reciprocate internal combustion engines (ICE), be-cause they have an almost flat efficiency curve above 30% of thenominal electrical power [23]. That means that they can work suc-cessfully on part loads and several operation strategies can be ap-plied successfully. Moreover, they have relatively high electricalefficiency (35%). An ICE-driven CHCP plant is shown in Fig. 1 anddescribed below:

A internal combustion engine is fed by air and natural gas asfuel. Through the combustion process, the chemical energy of thefuel is converted in mechanical shaft power which drives agenerator to produce electricity. ICEs are operating either accord-ing to the Otto or the Diesel cycle and waste heat is generated attwo temperature levels: by a low-temperature flow of coolant(90–125 �C) and by a medium temperature flow of exhaust gas(200–400 �C). The ICE exhaust gases can be used either directly inthermal processes or indirectly through a heat recovery steam gen-erator which produces superheated steam. In a commercial build-ing, there is no need for very high grade of thermal loads so it isassumed that there is no need to use the exhaust gases directly.

When the thermal output of the prime mover is not sufficient tocover the demand, a boiler is required to operate.

After the installation of a trigeneration plant cooling energy canbe ”produced” by two ways. By utilizing waste heat via an absorp-tion chiller or by utilizing electricity via an electric heat pump.Electric chillers use a mechanical compressor in order to take therefrigerant vapor from the lower evaporation pressure to the high-er condensation pressure. In absorption chillers this process is real-ized by means of a solution circuit, which serves as a thermalcompressor. Absorption chillers cycles are based on certain ther-modynamic properties of two fluids. One is the refrigerant andthe other the absorbent. The most common pairs of them foundin literature are:

� Ammonia is the refrigerant, water is the absorbent. Such a com-bination is chosen when low evaporation temperatures areneeded (below 0 �C).

� Water is the refrigerant and a solution of lithium bromide is theabsorbent.

Vapor generated in the evaporator is absorbed into the liquidabsorbent in the absorber. The absorber that has taken up refriger-ant is pumped to the generator. The refrigerant is released again asa vapor by waste heat from steam (or hot water) is to be condensed

in the condenser. The regenerated absorbent is led back to the ab-sorber to pick up refrigerant vapor [2].

In general, absorption chillers are fueled by the exhaust thermalenergy from the prime mover, reduce peak electric demand andelectricity charges by reducing the operating time of electric chill-ers and increase the electric to thermal load coincidence in thesummer months. It must be mentioned though that, when wasteheat is not available it is not always economically viable to gener-ate heat by burning fuel due to the small COP of the absorptionchiller (0.7–1.2) compared to the electrical one (2.5–5). Hence,absorption chiller should be preferred over the electrical chilleronly when waste heat is available; when cooling demand is signif-icantly bigger than heating demand. Moreover, absorption chillershave good partial load performance, low maintenance due to veryfew moving components but relatively high investment costs. Onthe other hand, compression chillers do not behave well on partialloads, have relatively high maintenance costs, high operation costsbut lower investment costs.

Finally a buffer vessel is utilized in order to balance the hourlyfluctuations of thermal demand, which will accumulate the heatproduced that is not needed at a specific moment ”smoothing” con-sequently the peaks. In case of a commercial building, the entire pip-ing system can be considered to have finite heat capacity andaccommodates the role of the buffer vessel. Energy is stored at timeswhen the available means of generation exceed demand and is re-turned when demand exceed supply. Thermal energy is, in practice,the only form of energy that can be stored by consumers. Electricitycan be stored locally, but at much higher cost than storage at thesupply side, and is at most cases not efficient. For that reason, a stor-age system for electricity (i.e. battery) is not examined.

2.4. Operation strategy

Operation strategy could be a simple decision in theory; operatethe plant when you can generate electricity at a lower cost thanyou would pay if purchasing the electricity from the grid. In prac-tice the operation strategies are part of the process control system,which is dependent on the following factors: demand for each kindof energy, prime mover nominal power, coefficients of perfor-mance and conversion factors for all energy conversions devicesinvolved. In literature the most common kinds of operation strat-egy are summarized as follows [24,25]:

1. Heat demand following: the system covers first the heat load andthen buys or sells electricity in order to integrate the rest ofenergy demand or offer, respectively.

2. Electricity demand following: the system covers first the electric-ity load and then produces (via the auxiliary boiler) or wastesthermal energy in order to integrate the rest of energy demandor offer, respectively. Instead of wasting thermal energy can bestored in a storage tank.

3. Continuous operation: the system operates only for a predefinedtime, e.g. two shifts, ignoring the energy demand. This appliesfor certain types of engines that their technology does not allowthem to operate on partial load and is usually met in energyproducers that sell power to the grid.

4. Peak shaving: the system operates for a limited amount of time,to cover a predefined part of the load during electricity peakconditions. As a result, the peak power bought from the gridis reduced.

5. Base load operation: the system is designed to cover only a con-stant amount of electricity load. This approach is very commonon large groups with poor dynamic performance [26].

Fig. 2 illustrates the latter two strategies. The time period ofeach adjustment can vary from a few seconds to few hours. This

Fuel

Air

G

Electricity to final uses

Auxiliary Boiler

Fuel

Air

GRID

BufferVessel

Electric Chiller

CoolingTower

Water

Oil Cooler

Jacket Water Cooler

A

G

Absorption Chiller

Internal Combustion

Engine

Exhaust Gases

Cold waterto final uses

Return

Return

ReturnHot water or

Steam to final uses

Fig. 1. Flow sheet of a typical trigeneration plant.

836 K.C. Kavvadias et al. / Energy Conversion and Management 51 (2010) 833–845

is usually done by either using a variable speed prime mover or bythrottling the fuel input. Of course, these fluctuations must be inthe allowed operation range of the prime mover.

However, the above strategies heat demand following or elec-tricity demand following that have been used widely in literaturecannot be applied successfully in a trigeneration system. Morespecifically:

� In heat demand following, the prime mover will always producethe heat needed, so there will not be any waste heat available tofeed the absorption chiller.

� In electricity demand following, the prime mover will alwaysproduce the electricity needed. However, it is not defined if cool-

ing is going to be covered via thermal or electrical load. If theaggregated electrical load is considered (cooling to be coveredfrom electrical chiller), the absorption chiller is going to be use-less. If electricity without the cooling load is considered, then allcooling is going to be covered from the absorption chiller, result-ing in various cases to the production of extra heat which isundesirable.

This problem makes more sense in applications with volatileheat to power ratio (excluding electricity that is used in cooling de-vices). To solve that problem a new kind of load following strategyis proposed: Electrical equivalent load following. The equivalent re-quired electricity ðEleqÞ is the electricity needed to cover electric

Fig. 2. Operation strategies.

K.C. Kavvadias et al. / Energy Conversion and Management 51 (2010) 833–845 837

demand and cooling demand that is not covered by the absorptionchiller. The main assumption of this strategy is that cooling energyis going to be produced from the absorption cooler only if wasteheat from the prime mover is available. In other words heat isnot allowed to be produced from the auxiliary boiler in order tooperate the absorption chiller. Hence, Eleq depends also on theprime mover type and size, as bigger prime mover with a biggerHPR will produce waste heat that can potentially be converted tocooling energy via an absorption chiller.

An example of this strategy is shown on Fig. 3. Given the de-mands for each kind of energy and the HPR of the prime mover,Eleq and thermal energy produced is plotted versus the primemover nominal size. It is clear that on the summer months Eleq de-

Fig. 3. An example of electrical-equ

clines as the prime mover size rises, because waste heat energyproduced by the prime mover is used to produce cooling. On win-ter the thermal demand is relatively bigger than cooling so thestrategy is similar to electrical load following because there is nowaste heat available from the prime mover to feed the absorptionchiller.

3. Process model

The simplified energy flows of the system described in Section2.3 is presented on Fig. 4. This energy system is modeled asfollows:

ivalent load following strategy.

CHP

Boiler

Absorption Chiller

Compression Chiller

Elec

Heat

CoolFuel

Storage

GridGrid

Waste

Fig. 4. Simplified energy flow diagram.

838 K.C. Kavvadias et al. / Energy Conversion and Management 51 (2010) 833–845

� A prime mover which produces electrical ðElCHPÞ and thermalðThCHPÞ energy and consumes fuel ðFCHPÞ. Most common technol-ogies are reciprocating engines, gas turbines and steam turbines.Each prime mover type is defined by its heat to power ratioðHPRÞ and its electrical efficiency gCHP

El

� �. From this figures know-

ing the maximum electrical nominal power ðPmaxÞ, the thermaland overall efficiency can be calculated.

� An auxiliary boiler which produces extra thermal energyðThBoilerÞwhen needed. Fuel ðFBoilerÞ is fed to the boiler and is con-verted to useful thermal energy with a known efficiency ðgBoilerÞ.

� An electrical chiller with a known COPE:Chiller and nominal poweroutput ðECmaxÞ, that is fed by electricity ðElE:ChillerÞ and producescooling energy ðCoE:ChillerÞ.

� An absorption chiller with a known COPA:Chiller and nominalpower output ðACmaxÞ, that is fed by thermal energy ðThA:ChillerÞand produces cooling energy ðCoA:ChillerÞ.

� A buffer vessel with a predefined heat capacity ðmCpÞ. Heat iswasted ðThWasteÞ if the vessel cannot store any more energy.

� The public power grid that sells ðElGridOutÞ or buys ðElGridInÞelectricity.

The energy balances for electricity, heat and cooling are calcu-lated for each hour ðtÞ by means of:

ElCHPt þ ElGridIn

t � ElE:Chillert � ElEndUse

t � ElGridOutt ¼ 0 ð1Þ

ThCHPt þ ThBoiler

t � ThA:Chillert � ThEndUse

t � ThWastet

¼ mCpðTt � Ttþ1Þ ð2ÞCoA:Chiller

t þ CoE:Chillert � CoEndUse

t ¼ 0 ð3Þ

The last term on Eq. (2) represent the accumulation of heat inthe heat buffer vessel (or the piping of the building) having a heatcapacity of mCp. This quasi-steady state equation can be appliedwith safety because the phenomena of temperature change is notdefinitive and the detailed study of such change is not desirablein this stage.

CHP specifications are defined by the following equations:

ThCHPt ¼ HPR � ElCHP

t ð4ÞElCHP

t ¼ gCHPEl FCHP

t ð5Þ

Similar specifications apply for the conversion of fuel to thermalenergy in the auxiliary boiler, the conversion of electrical energy tocooling in the electrical chiller, and the conversion of thermal en-ergy to cooling in the absorption chiller:

ThBoilert ¼ gBoilerFBoiler

t ð6Þ

CoE:Chillert ¼ COPE:ChillerElE:Chiller

t ð7Þ

CoA:Chillert ¼ COPA:ChillerThA:Chiller

t ð8Þ

The physical operation constraints consist of mainly capacityconstraints and are defined by the following inequalities:

Pmin 6 ElCHPt 6 Pmax ð9Þ

TBuffermin 6 TBuffer

t 6 TBuffermax ð10Þ

ACmin 6 ACt 6 ACmax ð11Þ

The operation of the prime mover and the production ofelectricity is regulated by the selection of the operationstrategy.

ElCHPt ¼

Pmax Continuous operation

Pmax if Eleq PPSset

0 if Eleq<PSset

(Peak shaving

BLset Base load operation

minfEleq;t;Pmaxg Electrical equivalent load following

8>>>>>>>>>><>>>>>>>>>>:

ð12Þ

The operating costs associated with the investment of a trigen-eration system are the costs of supplying fuel to the prime moverand the auxiliary boiler. Costs are calculated in a monthly ðkÞ levelas energy is priced monthly from the power and fuel supply com-panies. Excess electricity that is exported to the grid can also be ta-ken into consideration, if the consumer is permitted to sell. Eq. (13)calculates annual operating cost ðAOCÞ.

The objective of a trigeneration planning model is to minimizethe energy production and investment costs during the planninghorizon, achieving maximum investment returns [10,27]. The an-nual operational profit ðAOPÞ of the investment is calculated bysubtracting the AOC of the investment from the AOC of the plantbefore the investment (Eq. (14)) Eq. (15) calculates the capitalcost which is a function of the nominal power of the prime moverand the absorption chiller [28]. The cost exponent represent thescale index which is bigger for the prime mover as it is morecomplex unit and the economy scales do not have such an effect[29].

Fig. 5. Hospital loads.

Fig. 6. Typical winter and summer day.

Table 1Electricity tarrifs applied in Greece.

Tariff Power €kW

� �Energy €

kW h

� �Before 400 kW h

kW After 400 kW hkW

Electricity Com1 12.1 0.073 0.049Electricity Com2 4.5 0.096 0.096Electricity Sell – 0.072Natural gas – 0.045

Fig. 7. Tariff selection rules.

Fig. 8. The importance of electrical-equivalent load following strategy.

Table 2Design specifications.

Parameter Value

Prime mover gCHPel

33%

HPR 1.33CCHP 1 M€/MWn1 0.75

Absorption chiller COPA:Chiller 0.7

CAC 0.25 M€/MWn2 0.50

Electric chiller COPE:Chiller 2.5

Boiler gBoiler 85%Buffer vessel mCP 5000 kW/�C

Limits 40–95 �C

K.C. Kavvadias et al. / Energy Conversion and Management 51 (2010) 833–845 839

840 K.C. Kavvadias et al. / Energy Conversion and Management 51 (2010) 833–845

AOC ¼ MaintenanceCost þX12

CF FBoilerk þ FCHP

k

� �

þX12

CEl;GridInElGridInk

þmax ElGridInk �

X12

CEl;GridOutk ElGridOut

þX12

CTh;WasteThWastek ð13Þ

AOP ¼ AOC0 � AOCinvestment ð14ÞCapitalCost ¼ CCHPPn1

max þ CACACn2max ð15Þ

ROI% ¼ AOPCapitalCost

ð16Þ

Fig. 9. Coverage percentage of electricity, thermal and cooling energy from

The negative term of electricity exported to grid means that endusers are profited from that cost. Of course, several restrictions applyfor the electricity volume that an end-user can export to the grid.

The above criteria examine a CHCP investment from an eco-nomical point of view. For the assessment of the comparativeadvantage of CHP and the evaluation of the benefit of the new plantto global energy flows, two objective criteria have been defined:overall efficiency ðOEÞ and primary energy savings ðPESÞ. Accordingto EU directive [30] a CHP investment is acceptable when a 75–80%(depending on the type) overall annual efficiency is obtained and aPES of over 10% for large-scale systems (over 1MWe). Despite thefact that the above indexes describe the energy efficiency and sav-ing of a CHP plant, they cannot be applied successfully in a trigen-eration system as the above, because the savings of the absorptionchiller are not considered. Chicco and Mancarella [31] have pro-

the trigeneration plant for various strategies and strategy parameters.

K.C. Kavvadias et al. / Energy Conversion and Management 51 (2010) 833–845 841

posed a new generalized index that takes into account the coolingpower produced by the waste heat of the CHP prime mover and it isbased on a comparison with separate production means (Eq. (18)).

OE ¼ gel þ gth ¼ElCHP

FCHP þThCHP

FCHP ð17Þ

PES ¼ FSP � FCHP

FSP

¼ 1� FCHP

ElCHP

gSPelþ ThCHP

gSPthþ CoCHP

gSPel

COPE:Chiller

ð18Þ

Fig. 10. Primary energy savings ratio for va

Fig. 11. Effect of prime mover size

4. Case study

4.1. Case description

The energy needs for a 350 beds hospital complex have beenconsidered for a year. The current energy producing/convertingunits of the building consists of a natural gas boiler and an electri-cal chiller that cover the thermal and cooling needs, respectively.Thermal, electricity and cooling loads variations have been mea-sured on yearly and daily basis. Electricity consumed to the electricchiller is been subtracted and considered separately. Fig. 5 presents

rious strategies and investment sizes.

on the grid utilization factor.

842 K.C. Kavvadias et al. / Energy Conversion and Management 51 (2010) 833–845

the seasonal and daily fluctuations of the loads. These load patternsreflect the typical seasonal variability in outdoor temperature-re-lated loads for most commercial consumers. Combining the sea-sonal and daily profiles a separate daily load profile is beingproduced for each month. Fig. 6 depicts an average winter andsummer load.

The problem to be addressed here is stated as follows: Designproblem: Given the monthly energy demand profiles for each kindof energy (electricity, heat, cooling), along with their daily distribu-

Fig. 12. Effect of the nominal prime mover size

tions determine the optimal configuration and design of trigenera-tion system that will work along with the present equipment aswell as the optimal operation strategy and tariff.

4.2. Simulation parameters

The structure of the Greek gas and electricity pricing for commer-cial customers has been used. The rates that apply in Greece from Jul2008 are summarized in Table 1. Based on the above tariffs, Fig. 7

to the AOP for various strategies and tariffs.

K.C. Kavvadias et al. / Energy Conversion and Management 51 (2010) 833–845 843

illustrates a rule of thumb for the selection of the appropriate tarifffor each consumer: average price of electricity (including powerand energy charges) versus utilization factor which is expressed bythe percentage of the energy bought in each month in relation withthe energy that could be bought on maximum load. It is clear that ifthe consumer operates at loads near the maximum then cheappower – expensive energy tariff should be chosen. Natural gas priceis considered constant at 0.045 Euro/kW h although a small discountcould be given for trigeneration producers. The technical and costingdesign specifications used are shown in Table 2.

Fig. 13. Effect of the investment size to the return

The following cases have been considered:

� Four strategies– Electrical-equivalent load following (ELF) with a minimum

electricity load bought from the grid of 0%, 50% or 100% kWof the annual base load. That is, ELFset ¼ 0, 175 or 350 kW.

– Continuous operation (CO) for 8, 16, or 24 h simulating 1,2, or3 shifts.

– Peak shaving (PS) of PSset ¼ 70%, 80% or 90% of the monthlyelectricity load.

of investment for various strategies and tariffs.

Fig. 14. Effect of the buffer vessel to the AOP on the CO strategy.

844 K.C. Kavvadias et al. / Energy Conversion and Management 51 (2010) 833–845

– Base load (BL) operation on BLset ¼ 175, 350, 500 kW(50%,100%, 150% of the base load).

� The existence or not of a buffer vessel.� Two different tariffs: Com1 and Com2.

5. Results and discussion

With the above parameters 48 different simulations were cre-ated and were run for various nominal electrical powers of theprime mover.

5.1. Plant performance

Electrical equivalent load before and after the installation of a1MWe prime mover is illustrated on a Load Duration Curve inFig. 8. It is clear that the Eleq is reduced as the prime mover sizerises, because heat waste energy is utilized to produce cooling en-ergy. Moreover, the CHCP plant smooths the peak of requiredelectricity, because cooling energy demand on summer monthsis the cause of the abrupt peaks in electricity demand as shownin Fig. 8.

The percentage of energy loads covered from the new plant isbeing shown in Fig. 9 for various strategies and operation parame-ters as a function of prime mover nominal power. That includes:

� Electricity load that is produced from the prime mover and isutilized by the end user, not including cooling needs.

� Thermal load that is produced from the prime mover and is uti-lized by the consumer needs.

� Thermal load that is produced by the prime mover and is fed inthe absorption chiller in order to produce cooling energy utilizedby the consumer needs.

� Electricity load that is produced by the prime mover and fed inthe electrical chiller in order to produce cooling energy utilizedby the consumer needs.

The following conclusions can be derived:

� More than 80% of electrical,thermal and cooling requirementsare covered only in ELF, ELFset ¼ 0 and CO, Hrs=24 strategy witha prime mover nominal power of 0:75MWe.

� A 100% coverage percentage of thermal energy can be achievedonly with the CO strategy. In this case the ideal operation modeis 24 h.

� Percentage coverage on PS and BL are capped as expected, as theprime mover is not allowed to operate after a defined level. Insuch occasion, its pointless to use a bigger prime mover.

� An absorption chiller is not needed, in any case, when smallprime movers (<0:5MWe) are installed. This happens becausethere is not enough waste heat to feed the cooler.

Fig. 10 depicts how the selection of the nominal size affects theprimary energy savings ratio for various investment sizes. A PESRof 10% is desired for prime movers larger than 1MWe in order tobe considered high efficiency CHP Plant [30] and to be eligible forinvestment subsidies. Thus, strategies that do not achieve this per-centage such as PS or CO, Hrs = 8 (one shift continuous operation)may not be suitable.

Grid utilization factor versus nominal power is presented onFig. 11. Before the investment the utilization factor was 67%. Aspresented in Fig. 7 Com1 is more efficient for a utilization factorof over 50%. The tariffs that are based on the improvement of uti-lization factor, such as PS, must be accompanied with the selectionof Com1.

5.2. Plant economy

Figs. 12, 13 show the economic efficiency of such an investmentfor various strategies and tariffs. According to the particularity ofeach project, e.g. budget restriction, subsidies for energy savingsor for improving the utilization factor, each investor can choosethe optimal design of the plant according to its own criteria.

The best annual profit is being achieved from the proposed ELFstrategy and tariff Com2, but the same return on investment, butwith smaller profit can be achieved by using smaller investmentsize and other strategies such as base load operation. CO strategystops being profitable after a certain prime mover size as it pro-duces energy that is not needed by the plant and the cost of oper-ation and delivering this energy to the grid or the environment isgetting bigger.

Com2 tariff gives generally better economical results and its effi-ciency is connected to the grid utilization factor (Fig. 11). For thisreason ELF, ELFset ¼ 0 is more profitable in Com2 whereas ELF,ELFset ¼ 175 is more profitable in Com1. PS is more efficient inCom1 as it ”shaves” the expensive peaks and takes advantage ofthe low price of energy but generally is not very profitable becauseit is regulated to operate very few hours. However, it is more effi-cient than simple 1-shift CO strategy.

The buffer vessel affects the economy of the trigeneration plantonly if the continuous operation strategy is chosen, because in thisoperation strategy big amounts of waste heat are produced that arenot needed. Hence, storing them for later use increases the econ-omy of the investment. Fig. 14 illustrates how the annual profitis increased for less operation hours. That happens because, heatnot utilized in a specific time can be ”transferred” when needed.

6. Conclusion

A method of designing a trigeneration plant was presented. Themain design considerations were discussed and the economicalefficiency and performance characteristics of the plant were evalu-ated comparatively. The importance of energy tariff structures and

K.C. Kavvadias et al. / Energy Conversion and Management 51 (2010) 833–845 845

operation strategy selection was shown with reference to a case ofa hospital building.

The proposed electrical-equivalent load following strategy wassuperior than conventional strategies from both economic andenergetic point of view, when applied with maximum demand tar-iffs as it was proven to have two major benefits: better load coin-cidence and peak reduction. The selection of the best maximumdemand tariffs was proven to depend heavily on the grid utiliza-tion factor of the designed system which is affected by the sizeof the prime mover and its operation strategy.

This parametric analysis design method can be used by any con-sumer that intends to install a trigeneration plant, in order to de-fine the optimal investment size and compare the sensitivity andimportance of different operating variables.

References

[1] Cardona E, Piacentino A. A methodology for sizing a trigeneration plant inmediterranean areas. Appl Therm Eng 2003;23(13):1665–80. doi:10.1016/S1359-4311(03)00130-3. <http://dx.doi.org/10.1016/S1359-4311(03)00130-3>.

[2] Wu D, Wang R, cooling Combined. heating and power: a review. Prog EnergyCombust Sci 2006;32(5–6):459–95. doi:10.1016/j.pecs.2006.02.001. <http://dx.doi.org/10.1016/j.pecs.2006.02.001>.

[3] Hernández-Santoyo J, Sánchez-Cifuentes A. Trigeneration: an alternative forenergy savings. Appl Energy 2003;76(1–3):219–27. doi:10.1016/S0306-2619(03)00061-8. <http://dx.doi.org/10.1016/S0306-2619(03)00061-8>.

[4] Brown K, Minett S. History of chp developments and current trends. ApplEnergy 1996;53(1–2):11–22. doi:10.1016/0306-2619(95)00051-8. <http://dx.doi.org/10.1016/0306-2619(95)00051-8>.

[5] Moran A, Magoy PJ, Chamra LM. Thermoeconomic modeling of micro-chp(micro-cooling, heating, and power) for small commercial applications. Int JEnergy Res 2008;32(9):808–23. doi:10.1002/er.1395. <http://dx.doi.org/10.1002/er.1395>.

[6] Katsigiannis P, Papadopoulos D. A general technoeconomic and environmentalprocedure for assessment of small-scale cogeneration scheme installations:application to a local industry operating in thrace, greece, using microturbines.Energy Convers Manage 2005;46(20):3150–74. doi:10.1016/j.enconman.2005.03.005. <http://dx.doi.org/10.1016/j.enconman.2005.03.005>.

[7] Balli O, Aras H, Hepbasli A. Exergoeconomic analysis of a combined heat andpower (chp) system. Int J Energy Res 2008;32(4):273–89. doi:10.1002/er.1353.<http://dx.doi.org/10.1002/er.1353>.

[8] Pilavachi P, Roumpeas C, Minett S, Afgan N. Multi-criteria evaluation for chpsystem options. Energy Convers Manage 2006;47(20):3519–29. doi:10.1016/j.enconman.2006.03.004. <http://dx.doi.org/10.1016/j.enconman.2006.03.004>.

[9] Mancarella P, Chicco G. Assessment of the greenhouse gas emissions fromcogeneration and trigeneration systems. Part ii: analysis techniques andapplication cases. Energy 2008;33(3):418–30. doi:10.1016/j.energy.2007.10.008. <http://dx.doi.org/10.1016/j.energy.2007.10.008>.

[10] Biezma M, Cristóbal J. Investment criteria for the selection of cogenerationplants – a state of the art review. Appl Therm Eng 2006;26(5–6):583–8.doi:10.1016/j.applthermaleng.2005.07.006. <http://dx.doi.org/10.1016/j.applthermaleng.2005.07.006>.

[11] Bhatt M. Mapping of general combined heat and power systems. EnergyConvers Manage 2001;42(1):115–24. doi:10.1016/S0196-8904(00)00045-5.<http://dx.doi.org/10.1016/S0196-8904(00)00045-5>.

[12] RETScreen 4; 2008. <http://www.retscreen.net/> [accessed 30.05.09].[13] Cardona E, Piacentino A, Cardona F. Matching economical energetic and

environmental benefits: an analysis for hybrid chcp-heat pump systems.Energy Convers Manage 2006;47(20):3530–42. doi:10.1016/j.enconman.2006.02.027. <http://dx.doi.org/10.1016/j.enconman.2006.02.027>.

[14] Piacentino A, Cardona F. An original multi-objective criterion for the design ofsmall-scale polygeneration systems based on realistic operating conditions.Appl Therm Eng 2008;28(17–18):2391–404. doi:10.1016/j.applthermaleng.2008.01.017. <http://dx.doi.org/10.1016/j.applthermaleng.2008.01.017>.

[15] Hashemi R. A developed offline model for optimal operation of combinedheating and cooling and power systems. IEEE Trans Energy Convers 2009;24(1):222–9. doi:10.1109/TEC.2008.2002330. <http://dx.doi.org/10.1109/TEC.2008.2002330>.

[16] Cho H, Luck R, Eksioglu S, Chamra L. Cost-optimized real-time operation of chpsystems. Energy Build 2009;41(4):445–51. doi:10.1016/j.enbuild.2008.11.011.<http://dx.doi.org/10.1016/j.enbuild.2008.11.011>.

[17] Tveit T-M, Savola T, Gebremedhin A, Fogelholm C-J. Multi-period minlp modelfor optimising operation and structural changes to chp plants in district heatingnetworks with long-term thermal storage. Energy Convers Manage 2009;50(3):639–47. doi:10.1016/j.enconman.2008.10.010. <http://www.sciencedirect.com/science/article/B6V2P-4V5NSS1-1/2/93a1e3786cf4ccf62241a772c08f4a7c>.

[18] Piacentino A, Cardona F. Eabot – energetic analysis as a basis for robustoptimization of trigeneration systems by linear programming. Energy ConversManage 2008;49(11):3006–16. doi:10.1016/j.enconman.2008.06.015. <http://dx.doi.org/10.1016/j.enconman.2008.06.015>.

[19] Canova A, Cavallero C, Freschi F, Giaccone L. Comparative economical analysisof a small scale trigenerative plant: a case study. In: Industry applicationsconference; 2007. p. 1456–9. doi:doi:10.1109/IAS.2007.224. <http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=4347973>.

[20] Arcuri P, Florio G, Fragiacomo P. A mixed integer programming model foroptimal design of trigeneration in a hospital complex. Energy 2007;32(8):1430–47. doi:10.1016/j.energy.2006.10.023. <http://dx.doi.org/10.1016/j.energy.2006.10.023>.

[21] Ziher D, Poredos A. Economics of a trigeneration system in a hospital. ApplTherm Eng 2006;26(7):680–7. doi:10.1016/j.applthermaleng.2005.09.007.<http://linkinghub.elsevier.com/retrieve/pii/S1359431105002966>.

[22] Huangfu Y, Wu J, Wang R, Kong X, Wei B. Evaluation and analysis of novelmicro-scale combined cooling, heating and power (mcchp) system. EnergyConvers Manage 2007;48(5):1703–9. doi:10.1016/j.enconman.2006.11.008.<http://dx.doi.org/10.1016/j.enconman.2006.11.008>.

[23] Badami M, Mura M, Campanile P, Anzioso F. Design and performanceevaluation of an innovative small scale combined cycle cogeneration system.Energy 2008;33(8):1264–76. doi:10.1016/j.energy.2008.03.001. <http://dx.doi.org/10.1016/j.energy.2008.03.001>.

[24] Chicco G, Mancarella P. From cogeneration to trigeneration: profitablealternatives in a competitive market. IEEE Trans Energy Convers 2006;21(1):265–72. doi:10.1109/TEC.2005.858089. <http://dx.doi.org/10.1109/TEC.2005.858089>.

[25] Sanaye S, Meybodi M, Shokrollahi S. Selecting the prime movers and nominalpowers in combined heat and power systems. Appl Therm Eng 2008;28(10):1177–88. doi:10.1016/j.applthermaleng.2007.08.003. <http://dx.doi.org/10.1016/j.applthermaleng.2007.08.003>.

[26] Chicco G, Mancarella P. Planning aspects and performance indicators for small-scale trigeneration plants. In: International conference on future powersystems; 2005. p. 1–6. <http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=1600476>.

[27] Rong A, Lahdelma R. An efficient linear programming model and optimizationalgorithm for trigeneration. Appl Energy 2005;82(1):40–63. doi:10.1016/j.apenergy.2004.07.013. <http://dx.doi.org/10.1016/j.apenergy.2004.07.013>.

[28] Cardona E, Piacentino A. A new approach to exergoeconomic analysis anddesign of variable demand energy systems. Energy 2006;31(4):490–515.doi:10.1016/j.energy.2005.04.002. <http://dx.doi.org/10.1016/j.energy.2005.04.002>.

[29] Maroulis Z, Saravacos G. Food plant economics. CRC Press; 2007. <http://books.google.com/books?id=e19uGgAACAAJ>.

[30] EU, Directive 2004/8/ec of the european parliament and of the council, OfficialJ Eur Union L 2004; 52: 50–60. <http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX:32004L0008:EN:NOT>.

[31] Chicco G, Mancarella P. Trigeneration primary energy saving evaluation forenergy planning and policy development. Energy Policy 2007;35(12):6132–44.doi:10.1016/j.enpol.2007.07.016. <http://dx.doi.org/10.1016/j.enpol.2007.07.016>.