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The use of thermal energy storage for energy system based on cogeneration plant Anna Volkova, Andres Siirde

factor in selecting cogeneration plant capacity [1]. Heat demand usually is strongly time-varying or even discontinuous. Usage of thermal energy storage together with cogeneration technology provides an attractive solution by allowing the production of electricity in the periods, when heat load is low and later consumption of heat, when load is high [2]. Usually cogeneration plant produces thermal energy to provide base-load capacity and peak-load boilers have to be utilised additionally. These boilers can be partly replaced by thermal energy storage equipment charged during low-load time and utilised in the peak-load time [3]. Thermal energy storage as a possibility to increase the efficiency of cogeneration operation was discussed and analysed in various researches and studies including both short-term thermal storage [2-5] and long-term thermal storage [6] use together with cogeneration. There are cogeneration plants operating in Estonia, but the technology of thermal energy storage is not used on these plants. The perspectives for involvement of new cogeneration plants are high and the use of thermal storage will make these possibilities wider. At the moment there is only one research where possibility of thermal storage together with cogeneration plants in Estonia has been analysed [7]. There are three types of technologies which can be used for heat storage: storage in water, storage in the ground and rock and storage in form of chemical compound [6]. The most common type of thermal storage is an accumulator tank made of steel, which can maintain high temperature and has high efficiency. The purpose of the paper was to investigate the perspectives of using heat storage in the form of accumulator tank with cogeneration plants in Estonia. Case study is presented, where energy system including small-scale cogeneration plant, heat storage equipment and peak boilers, has been compared with traditional energy system including cogeneration plant and peak-load boilers. Alternatives were evaluated from technical and environmental point of view. As a case study, the heat load data for one typical little city in Estonia were used. It is planned to install in this city a new small-scale cogeneration plant based on gas engine technology. One of the possible solutions can be an installation of cogeneration plant coupled with thermal energy storage. The efficiency of this solution was evaluated using offered methodology.

Abstract—Usage of thermal energy storage together with cogeneration technology provides an attractive solution by allowing the production of electricity in the periods, when heat load is low and later consumption of heat, when load is high. The purpose of the paper was to investigate the perspectives of using heat storage in the form of accumulator tank with cogeneration plants in Estonia. Two alternatives were compared: the first alternative being an energy system, which included small-scale cogeneration plant, thermal energy storage equipment in the form of accumulator tank and peak boilers, was compared with the second alternative: the traditional energy system including only cogeneration plant and peak-load boilers. The methodology offered for evaluation of these two alternatives included the following steps: defining of the optimal capacity by heat produced in cogeneration maximisation method; simulation of district heating system operating; construction of heat load duration curve taking into account thermal energy storage; comparing of alternatives by technical and environmental parameters.

Keywords— CHP, cogeneration, energy efficiency, energy system, thermal storage. I. INTRODUCTION

R

egarding the agreements within the bounds of EU Directive 2004/8/EC on the promotion of cogeneration based on a useful heat demand in the internal energy market, Estonia should increase the share of electricity produced in the cogeneration plants of gross electricity consumption. According to this Directive strategic objectives for the Estonian electricity sector have to ensure that by 2020 the electricity produced in cogeneration plants forms 20% of the gross consumption and at least 18% by the year 2015. At the moment close to 15% from electricity gross consumption is produced in the cogeneration mode. Cogeneration is the simultaneous generation of heat and electricity, therefore it is important to use both types of produced energy appropriately. Concerning electricity, it may be both used on the spot and transported across great distances; heat, however, may only be used on site or very close to it. Thus, the heat load is considered the determining Manuscript received May 14, 2011. This work was supported by the European Social Fund within the researcher mobility programme MOBILITAS (2008-2015, 01140B/2009). A. Volkova is with the Department of Thermal Engineering, Tallinn University of Technology, Kopli 116, Tallinn, Estonia, 11712, phone +3725582866; e-mail: [email protected]. A.Siirde is with the Department of Thermal Engineering, Tallinn University of Technology, Kopli 116, Tallinn, Estonia, 11712, [email protected]

ISBN: 978-1-61804-022-0

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This method supposes that the optimisation criterion is calculated by equation (1).

II. METHODOLOGY When accumulator tank is used the cogeneration operating regime can be changed. Cogeneration can operate all day long with full capacity and boilers can be used less. It means that when the heat produced by cogeneration cannot be used at the same moment, it can be stored and then utilised when heat load is higher than the heat produced in cogeneration. The methodology algorithm was offered for evaluation of thermal energy storage use possibility together with cogeneration plant. This methodology included following steps: defining the optimal capacity by heat produced in cogeneration maximisation method; simulation of district heating system operating; construction of heat load duration curve taking into account thermal energy storage; comparing of alternatives by technical and environmental parameters.

Q·t→max, if Q=f(t)

Q=f(t) - heat load duration curve; Q - heat load, kW;

t load duration hours per year, h. The optimal cogeneration capacity is equal to such heat load when the amount of heat produced by cogeneration is maximal. B. Simulation of district heating system operating After defining the optimal heat capacity for installed cogeneration plant, the operating simulation was made for a district heating system, including consumer, cogeneration plant, peak boiler and thermal energy. Simulation model realised in the Excel software environment was used for this simulation. Heat load in thermal energy storage unit, i.e. accumulator tank was calculated using equation (2)

A. Defining the optimal capacity by heat produced in cogeneration maximisation method Before installation of a new cogeneration plant an energy producer faces the basic question of how high the plant’s installed capacity should be. One of the main issues for cogeneration efficiency is heat use by consumer. The risks of installed cogeneration capacity are related to two scenarios: if a load is used that exceeds the optimum setting, the station will not be able to operate during long period due to insufficient heat load, while, should the load be installed below optimum, the potential of utilising heat capacity will not be fully used [1]. According to the method used in previous research, the optimal capacity for cogeneration was selected based on the amount of heat demanded by consumers [1, 8]. Heat load values for residential buildings and load prevalence during the year are visualised with a heating load duration curve. The load for a district heating system consists of three main contributions: heating load, domestic hot water and distribution losses. The distribution losses are the heat losses from the pipes to environment and they stay fairly constant over the year. The domestic hot water load is also constant over the year; it is reduced only during the night time and during the summer months. The building heating load is the dominating load for most of the year and it follows the seasonal variations of the climate. There are two possibilities for construction of the heat load duration curve. The first method is when the heating load duration curve is derived from hourly loads to show all possible variations to the system. These data can be available from the existing boiler house operating data. The second method is heating load deriving from monthly degree-days. The optimal cogeneration facility is the one that is based on the heat duration curve and produces the maximum amount of heat year-round while working at full installed capacity. This means that the cogeneration plant capacity is determined by the largest area rectangle (maximum rectangle) inscribed in the heat duration curve.

ISBN: 978-1-61804-022-0

(1),

where

QTESi = QTESi −1 ⋅ (1 − lTES ) + (Qi − QCOGi ) if QTESi ≥ 0; QTESi −1 ≥ 0; Qi − QCOGi ≥ QTESi −1

(2),

where

QTESi is heat load in thermal energy storage during i hour, kW;

QTESi −1 is heat load in thermal energy storage during i-1 hour, kW; lTES is heat losses per hour from thermal energy storage;

Qi is consumer heat load during i hour, kW; QCOGi cogeneration heat load during i hour, kW. Heat load of peak boiler is calculated by equation (3)

Q PBi = Qi − QCOGi − QTESi −1 if

QTESi = 0; QTESi −1 ≥ 0; Qi − QCOGi ≤ QTESi −1

(3),

where

QPBi is heat load of peak boiler during i hour, kW. Analysis of hourly load is required for evaluating the thermal energy storage facility use. Typical winter, summer and spring/autumn day has been analysed after simulation. As a result of analysis, the operation seasons when cogeneration plant and thermal energy storage should be used, have been

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selected.

The following indicators were selected for comparing and evaluation: heat produced by cogeneration, cogeneration operating time per year, electricity produced by cogeneration, primary energy efficiency. For the calculation of both cases a gas engine was used as cogeneration technology. Gas engine based cogeneration plants are widely-distributed in the world, and particularly in Estonia. The main fuels for gas engines are natural gas and light fuel oil.

C. Construction of heat load duration curve taking into account the thermal energy storage After simulation of each operating hour using equations (23), a new heat load duration curve can be constructed. The heat load can be calculated using equation (4)

Qi = QCOGi + QPBi

(4).

III. RESULTS

The heat load duration curve, showing the cumulative duration for different loads in the system within a full year, can be cconstructed using the calculated hourly loads.

According to the part 2.B. the heat load duration curve for a reference city was constructed based on real data for the years 2009-2010 (Figure 1). The optimal capacity was defined. The solid line shows the heat load duration curve. As it can be seen from the Figure 1, the maximum heat load is 7300 KW. The dash line shows the optimal capacity defining process. The optimal capacity calculated by formula (1) is 2,58 MW, which is 0,353 from the maximal heat load.

D.Comparing of alternatives A district heating system consisting of cogeneration plant and thermal energy storage has been compared with a system including only cogeneration plant without any thermal energy storage.

Fig. 1 Heat load duration curve and optimal cogeneration capacity defining

The parameters used for calculations are shown in Table 1 As it can be seen from the Figure 2A, the heat load during winter days is always higher than the maximum cogeneration plant heat capacity. The heat load is covered by the heat produced by cogeneration and peak boiler. It means that during winter months the thermal energy storage utilities are not used. The Figure 2B shows typical spring/autumn day, when the heat load during the day becomes little higher or lower, than the installed cogeneration plant capacity. Using the thermal energy storage capacity it is possible to supply the required heat without the peak boilers. In this case the produced heat is used by heat consumers but actually later, than it has been produced.

For simulation of district heating system operating, including cogeneration plant coupled with thermal storage the following assumptions were made: -Losses of thermal energy storage are equal to 0.5% per hour; -Thermal energy maximum storage heat capacity is 5 MWh. TABLE 1 THE PARAMETERS OF COGENERATION PLANT AND PEAK BOILER

Capacity Efficiency Fuel

USED FOR SIMULATION Cogeneration plant (gas engine) 2,18 MWel ;2,58 MWth ηel=39%, ηtotal=85% natural gas

ISBN: 978-1-61804-022-0

Peak boiler 5 MWth 90% natural gas

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A

B

Fig. 2 Simulation for heat load, cogeneration plant , thermal energy storage and peak boilers A.Winter day B. Spring/autumn day

Using equation (4) the new heat load duration curve has been constructed. The Figure 3 shows this curve (the solid line). The dash line shows heat, produced by cogeneration.

As it can be seen, the cogeneration plant coupled with thermal energy storage equipment can operate during longer period that without it, and it can produce more heat and electricity.

Fig. 3 Heat load duration curve taking into account thermal energy storage

The different indicators were analysed for evaluating the thermal energy storage use in district heating system with cogeneration plant. The results of the evaluation are shown in Table 2. According to this table, the heat and electricity amount produced by cogeneration can be increased by more than 26% by using the thermal energy storage. The thermal energy storage use provides the primary energy efficiency increasing

ISBN: 978-1-61804-022-0

in comparison with separate power and heat production by 22.67 %, which is by 2.51% larger than in cogeneration without the thermal energy storage use. Despite the additional heat losses from the thermal energy storage unit this technology makes operation of cogeneration plant more effective. Besides, the reduction of CO2 emissions is larger by 26% with the thermal energy storage, than without it.

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TABLE 2 COMPARISON OF INDICATORS FOR COGENERATION WITH AND WITHOUT ACCUMULATOR TANK Cogeneration energy system

Cogeneration energy system with

without accumulator tank

accumulator tank

Heat, produced by cogeneration

12544 MWhth

15877 MWhth

3333 MWhth (+26.6%)

Electricity produced by cogeneration

10635 MWhel

13460 MWhel

2825 MWhel (+26.6%)

Heat produced by peak boilers

12173 MWhth

8873 MWhth

Fuel consumption

40800 MWh

44371 MW

3571 MWh (+8,8%)

20,16%

22,67%

2,51% (+12.45%)

2070 t

2615 t

545 (+26%)

Indicator

∆

-32441 MWhth (-27.1%)

Primary energy efficiency increasing in comparison with separate heat and power production CO2 reduction comparison with separate heat and power production

The installation of an accumulator tank is relatively cheap measure for increasing the efficiency of energy system, which can also be used in Estonia. According to Estonian local obstacles, the thermal energy storage can be mostly used during the spring and autumn time, when the heat load during the day could be higher and lower than the installed cogeneration plant capacity.

IV. CONCLUSIONS The usage of thermal energy storage together with cogeneration technology allows the production of electricity in the periods, when the thermal load is low and later consumption of heat, when the load is high. At the moment the thermal energy storage technologies are not used for efficient cogeneration plant operation in Estonia. The purpose of the paper was to analyse the perspectives of using the heat storage coupled with cogeneration plants in Estonia. The data about typical Estonian city were used for the evaluation and analysis. Two alternatives were compared: the first alternative being an energy system, which included smallscale cogeneration plant, thermal energy storage equipment in the form of accumulator tank and peak boilers, was compared with the second alternative: the traditional energy system including only cogeneration plant and peak-load boilers. The methodology offered for evaluation of these two alternatives included the following steps: defining of the optimal capacity by heat produced in cogeneration maximisation method; simulation of district heating system operating; construction of heat load duration curve taking into account thermal energy storage; comparing of alternatives by technical and environmental parameters. As results of the calculations showed, the thermal storage use increased the heat and electricity production by cogeneration plant by 26,6%. It was possible, because the usage of accumulator tank prolonged the operation time of cogeneration plant per year. The primary energy efficiency increase in comparison with separate heat and power production for cogeneration without thermal energy storage was 20,16%, but with the heat accumulator it became 22,67%.

ISBN: 978-1-61804-022-0

REFERENCES [1]

[2]

[3]

[4]

[5]

[6]

[7] [8]

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Volkova, A.; Hlebnikov, A.; Siirde, A. Defining of eligible capacity for biomass cogeneration plants in small towns in Estonia. Proceedings of International Conference on Renewable Energies and Power Quality, Granada (Spain), 23-25th March, 2010. Giorgio Pagliarinia Sara Rainieri, Modeling of a thermal energy storage system coupled with combined heat and power generation for the heating requirements of a University Campus, Applied Thermal Engineering Volume 30, Issue 10, July 2010, Pages 1255-1261 3. Željko Bogdan, Damir Kopjar Improvement of the cogeneration plant economy by using heat accumulator Energy, Volume 31, Issue 13, October 2006, Pages 2285-2292 K.H. Khana, M.G. Rasulb, M.M.K. Khanb, Energy conservation in buildings: cogeneration and cogeneration coupled with thermal-energy storage, Applied Energy 77 (2004) 15–34 Dries Haeseldonckx, Leen Peeters, Lieve Helsen,William D’haeseleer The impact of thermal storage on the operational behaviour of residential CHP facilities and the overall CO2 emissions Energy Conversion and Management 50 (2009) 639–647 Tor-Martin Tveit , Tuula Savola, Alemayehu Gebremedhin , Carl-Johan Fogelholm Multi-period MINLP model for optimising operation and structural changes to CHP plants in district heating networks with longterm thermal storage, Energy Conversion and Management, Volume 50, Issue 3, March 2009, Pages 639-647 Kuhi-Thalfeldt, R.; Valtin, J. Combined heat and power plants balancing wind power. Oil Shale, 26, 294 - 308.2009 Anna Voloshchuka (Volkova) Operational Analysis of Small Cogeneration Plants. Optimisation of Capacity Setting, Riga Technical University, 139 p., 2008