sizing curve for the design of thermal stabilizer of a diesel engine powered trigeneration system

This paper proposes a sizing methodology for sizing a thermal storage of diesel engine based trigeneration system integrated with hot water based absorption chiller via a thermal storage

1876-6102 © 2016 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of ICAER 2015

doi: 10.1016/j.egypro.2016.11.203

Energy Procedia 90 ( 2016 ) 360 – 369

ScienceDirect

5th International Conference on Advances in Energy Research, ICAER 2015, 15-17 December

2015, Mumbai, India Sizing curve for the design of thermal stabilizer of a diesel engine

powered trigeneration system Mahesh N Shelara** Prasad Gaikwada, G.N Kulkarnib

1 a Department of Mechanical Engineering,KKWagh Institute of Engineering,Nashik 422003,India

2 b Department of Mechanical Engineering,College of Engineering,Pune,411005,India

Abstract

Integrating absorption chillers with diesel engine generators makes possible the use of its rejected energy for meeting cooling energy demand The performance of such an absorption chiller is sensitive to the temperature at its desorber section In hot water driven double effect absorption system, the temperature of hot water is to be maintained between 170 0 C and 165 0 C A hot water storage tank is therefore proposed to be integrated so as to maintain the water temperature within allowable limits Sizing of such

a hot water storage referred to as a thermal stabilizer is therefore an important objective One such technique proposed is a sizing curve method based on thermal stability time A generalized methodology for generating a sizing curve for a cooling load is presented in this paper The method offers a simplified approach for sizing of thermal stabilizer for a given diesel generator catering to combined power and thermal demands This sizing methodology is illustrated for a 180 kW diesel engine based trigeneration system catering to fluctuating cooling and power loads

© 2016 The Authors Published by Elsevier Ltd

Peer-review under responsibility of the organizing committee of ICAER 2015

Keywords: Diesel generator,Trigeneration,Thermal stabilizer,Sizing curve

1* Corresponding author Tel.: 9822052351

E-mail address: mnshelar@kkwagh.edu.in

© 2016 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of ICAER 2015

1 Introduction

Trigeneration with reciprocating engines as prime movers involve recovery of energy rejected from engines and using this energy to satisfy cooling and heating demand of a given load This is known to be beneficial in reduction

in energy use and carbon emissions [1].Energy recovered from exhaust gases is commonly harnessed using suitably designed heat exchangers[2,3].Absorption chillers come in different configurations and types which affects its performance [4].Systems approach in analyzing trigeneration systems has been used in many studies indicating the desirability of trigeneration [5]

Trigeneration systems are required to satisfy the variable power demand or variable thermal demand depending

on the load curve and the mode of operation of diesel generator Diesel generators operating in electricity demand following mode would operate at part loads at times Part load behavior of engines would result in corresponding reduction in output of absorption chillers To compensate for this an hybridization strategy of using a compression chiller along with absorption chiller has been recommended [6] This strategy makes it possible to operate the diesel generator closer to its rated capacity while catering to the combined heating, cooling and power demand

For variable thermal demand, trigeneration systems should include a thermal storage A reduced thermal demand would require the surplus recovered energy to be dumped Thermal storage should be sized to reduce the energy to

be dumped Larger thermal storage volumes would however mean more surface heat losses Thermal storage sizing has to be done remembering these constraints This paper proposes a sizing methodology for sizing a thermal storage of diesel engine based trigeneration system integrated with hot water based absorption chiller via a thermal storage for applications with simultaneous heating, cooling and power demandHere introduce the paper, and put a nomenclature if necessary, in a box with the same font size as the rest of the paper

Nomenclature

Ast surface area of insulated storage tank [m2]

Cp specific heat of water [kJ/kgK]

Cpg specific heat of engine exhaust gases [kJ/kgK]

COP coefficient of performance

d diameter of storage tank [m]

DG diesel generator

h height of storage tank[m]

mg mass flow rate of exhaust gases of the engine [kg/s]

ms mass flow rate of hot water entering and exiting high temperature desorber [kg/s]

mf fuel consumption rate of engine [kg/s]

msr mass flow rate of hot water entering and exiting low temperature desorber[kg/s]

mw mass flow rate of engine jacket water [kg/s]

Pc Cooling load in TR

TR tons of refrigeration

Tg1 Temperature of exhaust gases entering exhaust gas heat exchanger [0C]

Tg2 Temperature of exhaust gases exiting exhaust gas heat exchanger [0C]

Ts Temperature of water in the low temperature storage tank [0C]

Tsh Temperature of water in the high temperature storage tank [0C]

Ta Temperature of ambient air [0C]

qstl rate of energy losses from storage [kJ/s]

qr rate of recovered energy [kJ/s]

ql rate of heat energy removed from the storage tank for input to the desorber [kJ/s]

Vsh storage volume of high temperature storage tank[m3]

ρ density of water [kg/m3]

2 Proposed Trigeneration configuration

Diesel engine trigeneration system considered for analysis is shown in Fig 1 The proposed trigeneration system would comprise of diesel generator, exhaust heat recovery unit, hot water storages and hot water fired single and double effect absorption chiller The absorption chillers are of lithium bromide water type The energy recovered from exhaust gases in the form of high temperature water is routed through a high temperature storage tank This high temperature hot water is input to the desorber of double effect chiller Similarly the energy recovered from the engine jacket water is routed through low temperature storage tank This low temperature hot water is input to the desorber of single effect chiller Thus, the high temperature storage tank is to be designed as a thermal stabilizer for double effect chiller Similarly the low temperature hot water storage would act as thermal stabilizer for single effect chiller The design of high temperature storage (thermal stabilizer) using design space approach and the related concepts are explained in the following text

Fig 1 Trigeneration with thermal storage

3 Thermal storage as stabilizer and its sizing

Thermal storage is an insulated hot water storage tank whose temperature is controlled within a desirable range For operating the absorption chiller at maximum COP controlling the input temperature at the desorber section of chiller is vital Thermal storage communicates with the desorber section of absorption chiller and performs this function It must be stated at the outset that the thermal storage is more of a stabilizer than an energy storage tank as

it does not allow the delinking of thermal load from production for a considerable period

Stabilization means maintaining the thermal storage temperature within allowable limit When the diesel engine

is integrated with hot water absorption chiller via the thermal storage, the temperature at the desorber inlet changes gradually w.r.t time This time for which the required temperature at its desorber can be maintained within the allowable limit is the function of storage volume of the water in the tank Increase in storage volume reduces the fluctuations in storage temperatures Stable temperature enables the operation of chiller Moreover increase in storage volume allows the energy to be stored which otherwise would have been rejected to the atmosphere as hot flue gases However increase in storage volume increases storage surface area and increases surface thermal losses

in addition to increase in capital costs

The design objective for trigeneration systems is the estimation of the thermal storage volume for meeting variable cooling demand Design space approach is used to size thermal storage This approach is unique in the sense that instead of giving a single design, it helps a designer to visualize all possible designs from which an optimum solution could be obtained Kulkarni et al (7) one of the authors proposed this approach while designing a

solar water heater P Arun et al (8) applied this approach for sizing diesel generator battery system

4 Mathematical model for high temperature stabilizer

Consider the well insulated thermal recovery tank with water It receives energy recovered from the engine and

discharges energy to the heating and refrigeration load when needed The change in internal energy of water in

thermal storage per unit time depends on the energy recovered from the diesel engine body or exhaust gases (which

increases the internal energy) and energy transferred to the absorption chiller and heating load (causing a decrease in

internal energy) and is expressed as (1) Energy Balance for storage volume can be represented by (1) The heat

losses from the storage tank are determined from the equation (2) The energy recovered from exhaust gases is

calculated by (3)

l stl p

sh

dt

dT C V

)

stl

Where stl 1 845 2 Vsh2/3

d

h

⎠

⎞

⎜

⎝

⎛ +

×

=

) ( g1 g2

pg g

5 Design Space and thermal stability time

The concept of design space involves identifying all possible and feasible designs of thermal stabilizer volume

subject to the availability of recovered energy and demand for hot water by absorption chiller So instead of

identifying a single design that is a single storage volume the design space approach identifies a range of storage

volumes for varying thermal loads The concept is illustrated for a trigeneration system whose parameters are

specified in Table 1 and 2

In the considered configuration, the volume of insulated hot water high temperature storage is required to be

sized The sizing criterion is the thermal stability time which is the time for which the high temperature storage can

be maintained in the allowable temperature range In the designed system, heat of exhaust gases of the diesel

generator is used to heat the water in the high temperature thermal stabilizer The hot water in thermal stabiliser is

used for providing heat to hot water fired vapour absorption machine The temperature of water is supposed to be in

between a certain temperature range for the vapour absorption machine to function at maximum COP When the

system operates with the thermal stabilizer, both the cooling loads and power demand are met by the diesel

generator based trigeneration system Even after assuming that the engine and the absorption chiller operate at the

rated capacity, the temperature in the storage tank would vary considering the impossible task of matching the

recovered energy with demand

The time for which the storage temperature can be maintained within the allowable temperature range when the

system caters to the cooling demand is the stability time In other words, the time required for water in stability tank

to exceed the upper temperature limit or fall below the lower limit is defined as stability time Once the stability

time is exceeded, it becomes necessary for the system to control the system by altering the storage temperature

These control strategies would involve dumping the exhaust gases to the atmosphere if the storage temperature is

exceeded or switching to an auxiliary heating source if the storage temperature falls below lower limit Short

stability times results in unsteady operation of system due to requirement of frequently dumping the energy of

exhaust gases to atmosphere or switching on the auxiliary source On the other hand, longer stability times would

necessitate use of larger storage volumes Choice of stability time depends on the nature of load curve and the

loading duration For example if the trigeneration is a backup system to the grid, lower stability times would be desirable In situations where trigeneration systems have longer and continuous hours of operation longer stability times might be attractive

Based on the concept of stability time, feasible and unfeasible storage volumes could be identified Thermal storage volume which results in storage temperature variation within allowable limits during stability time of an hour is the feasible volume If the temperature exceeds the limit it is considered unfeasible volume These concepts and the sizing methodology are explained first for constant loading conditions and then by considering variation in power and cooling loads

Table 1.System parameters and input data for illustrative example

Load Peak power demand is 180 kW and may vary Cooling demand vary after an hour

Heating load is zero

Engine V type turbocharged water cooled diesel engine with 180 kW capacity at 1500 rpm and

compression ratio of 16.7

Absorpti

on chiller

Thermax make, hot water operated double effect lithium bromide water system of maximum capacity 30 TR and COP=1.4.Single effect chiller has a COP =0.7

Storage Cylindrical, well mixed, always full, Mild steel, wall thickness 6 mm,h/d=1.2,insulated

with glass wool (k=0.4W/mK) and 0.2 m thick

Table 2.Specifications of Diesel engine generator at full load

Mass flow rate of jacket water 2.08kg/s

Heat carried away by jacket water 40 kW Mass flow rate of exhaust gases 0.25kg/s

Jacket water inlet temperature to DG 90°C

6 Sizing on the basis of thermal stability

Based on this criterion of a thermal stability time, sizing was done Stability time of a hour was considered Two terms, feasible and unfeasible storage volumes are defined In the illustrative example the allowable temperature of water is between 170oC and 165oC These constraints of allowable temperatures are introduced in the desorber

section of absorption chiller Expression (1) is employed for calculating the temperature of water in the storage for the considered time interval

For a given trigeneration load, if the storage volume is increased it would correspondingly reduce the storage temperature If this storage temperature falls below 1650C, chiller cannot operate Thus the minimum temperature of water at the absorption chiller desorber puts an upper limit on the storage volume On the other hand, for the given trigeneration load, when the storage volume is reduced, the storage temperature would increase and if that exceeds the saturation temperature, evaporation would result This is not desirable for hot water operated chiller This requirement puts a lower limit on storage volume Thus, for a given diesel generator set operating to satisfy a rated power demand and given cooling cum heating demand, there are two storage volume limits corresponding to 1650C and 1700C respectively

To illustrate this we assume a constant power and cooling load on trigeneration system closer to rated capacity For the diesel generator delivering 180 kW and loaded for an hour with 30 TR cooling load, trigeneration system with 2.6 m3 storage volumes (Fig 2a) would imply feasible design, but a 1.5 m3 storage volume becomes an infeasible case (Fig 2b) The lower limit volume when selected for 30 TR load would minimize surface losses from storage tank The upper limit means an increase in storage volume beyond it would make this trigeneration system infeasible/unacceptable As seen in Fig 3a, trigeneration system with storage volume of 11m3 with a 180 kW diesel generator subjected to load profile of 180 kW power, 30 TR cooling and negligible heating demand for an hour makes the design feasible (upper limit) This means that a trigeneration system can operate for one hour satisfying the cooling load of 30 TR even without exceeding the temperature range We can say that the storage acts as a stabilizer for an hour when the cooling load is 30 TR

However that does not mean that a larger storage volume would increase the stabilizer time indefinitely The reason is as follows An increase in storage volume would lead to increase in surface losses beyond an acceptable minimum Thus a storage volume of 20 m3 is an infeasible case (Fig 3b).Based on these thermal storage temperature limits, a design space region is identified and a sizing curve is obtained (Fig 4).The sizing curve has been obtained for the minimum stability time of 1 hour It therefore helps us chose a range of storage volumes under varying cooling loads based on stability criterion This sizing curve gives the range of feasible storage volumes when the cooling load on the absorption chiller capacity varies near its rated capacity Thus for an engine based trigeneration system when the cooling load changes from 30 TR to 25 TR in a hour then a 2.6 m3 storage volume becomes infeasible However if instead a 10 m3 storage is chosen this fluctuations in cooling load for a hour is stabilised Thus a 10m3 storage is the feasible storage volume if the cooling load fluctuates from 30 TR to 25 TR with a time interval of a hour Choice of storage volume would depend on the nature of load curve For applications where the load curve is variable, longer stability times would be necessary especially because then the stabilizers might also serve as thermal battery The role of thermal stabilizer as a thermal battery is explained in the following section

(a) (b)

Fig 2 (a) Variation of storage temperature 2.6m 3 storage, 30 TR cooling (b) Variation of storage temperature 1.5m 3 storage,30 TR cooling

366
Mahesh N Shelar et al / Energy Procedia 90 ( 2016 ) 360 – 369

(a) (b)

Fig 3 (a) Variation of storage temperature for 11m 3 storage, 30 TR cooling ; (b) Variation of storage temperature 20m 3 storage,30 TR cooling

Figure 4 Sizing Curve for high temperature stabilizer system

7 Thermal stabilizer as a battery

A hot water tank sized as a stabilizer also acts as a thermal battery for a smaller duration This becomes obvious from the data included in Table 3 The thermal battery time of a diesel engine integrated chiller system catering to

30 TR load increases with an increase in storage volume The upper limit volume of 11 m3 allows the thermal stabilizer to act as a thermal battery for more than half an hour This means the thermal stabilizer can cater to the cooling demand while the diesel generator takes a shut down for the time equal to battery time It however needs to

be emphasized that the storage cannot be sized to act as a thermal battery for a longer duration due to the lower specific heat capacity of water However the main advantage of introducing a storage is the capacity to withstand fluctuating cooling loads It is seen that a storage volume of 11 m3 originally sized for 30 TR system can cater to

varying loads with stable operation Choosing a 2.5 m3 size would not give this stability during fluctuating cooling loads

It would be of interest to size the thermal storage volume under variable loads The load curve under

consideration is Fig 5 and Fig 6 It is seen that there are variations in the power demand as well as cooling demand

per hour which results in part loading of diesel generator and absorption chiller This dynamic situation alters the feasible storage volumes because the rate of energy input and withdrawal from thermal storage changes As an example, consider a time interval of 4pm to 5 pm where the cooling load is 32TR.In this period, the recovered energy is 75kW due to part loading of diesel generator The desorber input required for cooling load of 32TR is greater than the recovered energy In this particular time interval thermal stabilizer functions as a battery and supplies the deficit heat energy required to be supplied to the desorber of chiller Similarly in the time interval of 6pm to 7 pm the recovered energy is 50kW due to part loading of diesel generator while the cooling load is 23TR.In this case, the recovered energy is less than the energy required to be supplied to absorption chiller, Hence the thermal stabilizer again functions as a battery and supplies deficit heat to absorption chiller.Based on the above, upper and lower feasible volume are to be identified For 32 TR peak load with similar hourly variation in cooling demand ,feasible storage volumes are in the range of 6m3 to 8m3.for the trigeneration system subjected to variable power and cooling demands Sizing curve is generated for variable load inputs assuming a similar nature of load curves with lower peaks (Fig 7)

Table 3.Stabilizer as a thermal battery for 30 TR chiller integrated with diesel generator

Volume(m 3 ) Stability time(hours)

DG ON , Chiller ON

Battery time(hours)

DG OFF, Chiller ON

Time for charging(hours)

DG ON, Chiller OFF

Fig 5 Typical power load curve to be met by diesel generator

Fig 6 Typical cooling load curve on Trigeneration

Fig 7.Sizing curve under variable loads

8 Conclusions

Trigeneration system with pressurized storage would aid in harnessing the exhaust energy in double effect absorption chiller resulting in better coefficient of performance Introducing a high temperature thermal storage would aid in managing the mismatch between the thermal demand and energy recovered Design space methodology for sizing such storage is described and the feasible design space is identified The design space gives an upper and lower storage volume limit for a stable and steady state operation of the system for a definite period termed as stability time The upper feasible storage volume gives a larger stability time period beyond which the system becomes unstable It was shown that the trigeneration system with a larger stabilizer size also serves as a thermal

battery for a smaller period

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