Application of Solid Oxide Fuel Cell－Auxiliary Power Unit on Different Trucks in Northeast China

ZHOU Su(周蘇)，WANG Shi－jia(王士嘉)

1 School of Automotive Studies，Tongji University，Shanghai201804，China

2 Chinesisch－Deutsche Hochschulkolleg，Tongji University，Shanghai200092，China

3 Sino－German School，Tongji Zhejiang College，Jiaxing 314051，China

Application of Solid Oxide Fuel Cell－Auxiliary Power Unit on Different Trucks in Northeast China

ZHOU Su(周蘇)1，2，3*，WANG Shi－jia(王士嘉)1

1 School of Automotive Studies，Tongji University，Shanghai201804，China

2 Chinesisch－Deutsche Hochschulkolleg，Tongji University，Shanghai200092，China

3 Sino－German School，Tongji Zhejiang College，Jiaxing 314051，China

A diesel engine of conventional trucks has a low efficiency under the idling condition，leading to a high cost for heating or cooling in the cab during night．The solution to this problem will have great significance on energy conservation and em ission reduction．A new auxiliary power unit of solid oxide fuel cell(SOFCAPU)with high efficiency solves this problem perfectly．Heat pump air conditioner is considered as a prom ising device for the application of SOFC-APU with a high cooling and heating efficiency．To make a quantitative analysis for the application of SOFC-APU，a model is built in M atlab/Simulink．The diesel engine model and SOFC-APU model are fitted based on some experimental data of SOFC-APU and diesel engine during the idling operation．An analysis of the app lication of SOFC-APU on different trucks in Northeast China is comprehensively made，including efficiency and em ission．

diesel engine;truck;heating and cooling power; emissions;auxiliary power unit of solid oxide fuel cell(SOFC－APU)

Introduction

Heating and cooling in the cab when a driver sleeps overnight is necessary in Northeast China.A diesel engine operates to heat or cool under the idling condition with low efficiency and high em ission.Trucks often waste a lot of diesel on the idling condition and much pollution is produced at the same time.Cooling on refrigerated trucks also has the same problem.

A new auxiliary power unitof solid oxide fuel cell(SOFCAPU)with a high generation efficiency solves this problem perfectly［1］.SOFC is a device which transfers chemical energy into electric energy directly with high efficiency，zero pollution and zero noise.SOFC can be fuelled by oil.Therefore as an APU on vehicle it obtains a high attention around the world. Lutsey et al．［2］applied Monte Carlo sampling and net present value(NPV)econom ic analysis to ADVISOR－based vehicle fuel consumptionmodel and estimated the APU market size as a function of APU costs.They forecasted that SOFC－APU on truck would lead to a 30%reduction of fuel use during overnight idling and a 40%reduction in oxides of nitrogen (NOx)produced during idling.Sorrentino and Pianese［3］built a 5 kW SOFC－APU controlorientedmodel.The core partof the model is the SOFC stack，surrounded by a number of ancillary devices:air compressor/blower，regulating pressure valves， heat exchangers，pre－reformer，and after－burner.Simulation result shows that SOFC－APU achieves a potential reduction in fuel consumption of up to 70%.Up to now，there have few research results about SOFC－APU in domestic.

There are some problems that need to be solved for the application of SOFC－APU on the truck.At present，waste heat from diesel engine is used to heat the cab directly and in this way the energy utilization ratio is improved.As a result，the air conditioning system on conventional truck only has the function of cooling.In addition，air compressor in the truck's cooling system uses the engine asa power source.So when the engine is not working on the idling condition(then the truck is parked during night)，SOFC－APU cannot supply power directly to the air conditioning system.

Heat pump air conditioner is considered as a promising solution to these problems.It can use the electric energy directly.Heat pump has a high cooling and heating efficiency on the conventional truck.

The distribution of the energy from SOFC－APU is shown in Fig.1.Part of the electric energy generated from SOFC－APU is used to supply heat and the other supplies the additional electrical power consumers such as lights and radio.

Fig.1 The distribution of energy generated from SOFC－APU

There are four types of trucks in China.Classification standard and ownership in Northeast China are listed in Table 1［4－5］.The analysis below is based on these four types of trucks.

Table 1 Standard of classification for trucks in China and its ownership in Northeast China

To make a quantitative analysis for the application of SOFC－APU on every kind of truckmentioned above，amodel is built in Matlab/Simulink.It consists of SOFC－APU module，heat pump module，diesel enginemodule，and the heating and cooling power module.The SOFC－APU module and diesel enginemodule were fitted based on some experimental data of SOFC－APU and diesel engine during the idling condition.

The prospect of the application of SOFC－APU on trucks is comprehensively analyzed based on statistic data in Northeast Chinamarket including efficiency and emission.An analysis is alsomade for its promising application on refrigerated trucks.

It should be noted that the current trucks in China are not equipped with the appropriate living and entertainment device，such as refrigerator and television，and this situation will change as living standards improve.As a result，the simulation analysis is divided into three parts:the current situation，the future scenarios and an application example on refrigerated trucks. Heating and cooling power is considered.Then the paper takes all the electric devices on truck into consideration in future scenarios analysis based on some statistics in America.

1 Simulation Model

1．1 SOFC-APU

Figure 2 shows the power flows of 1 kW and 5 kW SOFCAPUs，respectively.As can be seen from the figure，diesel fuel passes through the reformer.H2and a small amount of CO are produced and flow into the stack to react.A smallamountof the fuel gas which isn't involved in the process of reaction flows into the after－burner.Then part of the thermal energy generated from the combustion reaction is used tomaintain the temperature in the stack，and the other combines with the waste heat from the stack and add to the heat exchanger for recycling.

Air will be supplied in after－burner so that the gas fully burns.After combustion reaction in the after－burner，all the fuel gas completely transforms to CO2and H2O.As a result，SOFCAPU doesn't produce any pollutant and it can transform all diesels into CO2and H2O.

Fig.2 Power flow diagrams of 1 and 5 kW SOFC－APUs

The information above can be summarized into Table 2. The SOFC－APU hasa higher efficiency than the dieselengine on the idling condition and produces CO2only in terms of green house gas emissions.Part of waste heat from the stack is also used to supply heat in order to have a higher efficiency during w inter.

The generation efficiency of SOFC－APU is about 30% which is higher than the diesel engine on the idling condition. The SOFC－APU model is built based on Table 2 with the method of fitting.

1．2 Heat pump air conditioner

The structure of automotive heat pump system shown in Fig.3 is sim ilar to the conventional automotive air－conditioning system［6－7］.

When heating，the SOFC－APU drives the motor and the compressor.Refrigerant at high pressure and high temperature comes out from the compressor and enters into the heat exchanger 1 which is near to the cab.A fan blows the air into the heat exchanger 1.The warm air is formed in the heat exchanger 1 and used to heat the cab.Refrigerant comes out from the heat exchanger 1，and through the expansion valve comes into the external heat exchanger 2.Refrigerant expands into the heat exchanger 2 and exchanges heat with an external source，and then is sucked into the compressor again for the next round of cycle.

The cycle of refrigeration is just the opposite of the heating process.

Fig.3 Scheme of heat pump heating

Coefficient of performance(COP)is a ratio between output power and input power for an air conditioner.The COP of heat pump air conditioning system is about 2.9 when cooling，and about2.3 when heating.

There are several features to use the SOFC－APU and heatpump on trucks:

Table 2 The experimental data of SOFC－APU

(1)having the function of refrigerating and heating with a high COP and good reliability;

(2)a perfect solution to the problem that air conditioning systems“grab”engine power on the conventional truck;

(3)air conditioning system on the truck needs to be replaced completely which leads to a high refit costs.

1．3 Diesel enginemodel

Experimental data of three kinds of diesel engines in China is shown in Table 3，the emissions ofwhichmeet the demand of CountryⅣ(China).All the experimental data are derived from a diesel engine company in China.

Table 3 Experimental data of diesel engine during the idling condition in China

In this paper，the data listed in the table is used to simulate the diesel engine during the idling condition with themethod of fitting.The model describes the relationships between fuel consumption，em issions and the rated power.

1．4 Calculation of heating and cooling power

The basic formula of wall heat transfer is［7］

where K is the wall heat transfer coefficient in W/(m2·K)，A is the wall heat transfer area in m2，andΔt is the difference between the temperatures in the cab and outside in℃.

Heating power consists of the follow ing components，which is shown in Fig.4:

where PHis the total heating power in W，and Proof，Pb，Pfloor，Pair，and Ppare the powers of heatdissipation on the roof，from doors and w indows，on the floor，from cold air infiltration，and from the body of drivers in W，respectively.

Fig.4 Sketch of heat dissipation of the cab

Cooling power is similar to the heating and consists of the follow ing components:

The only difference from the heating power formula is that the sign of heat dissipation power from drivers'body is the opposite.

1．5 Model in Simulink

The model in Simulink consists of SOFC－APU module，heat pump module，diesel enginemodule，and the heating and cooling power module，which is shown in Fig.5.Power of diesel engine，temperature in the cab，temperature outside，members in the cab，and size of cab(a，b，c)are the inputs of themodel.Then cooling and heating power needed in cab can be calculated.Simulation results on the idling condition for four types of truckswill be available in diesel engine and SOFC－APU module.Combined with statistic data in Northeast China，a comparison between diesel engine and SOFC－APU＆heat pump on the ability of heating will bemade.

Fig.5 Diagram of the simulation model

1．6 Other simulation data

Other simulation data needed in the Simulink model are listed in Table 4.

Table 4 Simulation data

So far there is no relevant agency to collect statistical data of annual operating trucks in China.According to the data from a transport company in China，a truck runs an average of 300 d per year normally.Heating time per year in Northeast China is five and a halfmonths.So the time of a truck in China needed to be heated overnight per year is about138 d.There is also 75 d when the temperature ismore than 30℃in summer.Therefore the number of cooling days in the cab is62.

At present，the law of sleeping overnight for the driver in China is still in the exploratory stage，and has not yetenacted a legal regulation.While some regions have demanded the drivers to rest3 h overnight and it is used in current situation analysis.

2 Simulation Results and Analyses

Simulation result analysis is divided into two parts.The first one is current situation analysis and only cooling and heating are considered;the second part shows the future potential of the SOFC－APU application on truck in China.

2．1 Heating and cooling capacity

As shown in Fig.6，for the different temperature outside in the three provinces of Northeast China during w inter and summer，the heating and cooling powers needed vary with location and truck type.

Fig.6 Heating and cooling power on different trucks

A heavy truck will need a power of 5－6 kW to heat and less than 1.5 kW to cool which are both higher than medium and light trucks.The m ini truck with more member needs the higher paver.

2．2 Current situation

Heating and cooling need is considered in current scenarios in China.Figure 7 makes an analysis about the savings of diesel，CO2and other emissions by the application of SOFCAPU per truck，which is compared to the truck idling case.For a heavy truck，1.5 t diesels and nearly 5.2 t CO2will be saved by the application of SOFC－APU every year，while it has the worst effect on amini truck.

Fig.7 Savings of diesel，CO2and other em issions by the application of SOFC－APU per truck per year

2．3 Saving m oney by the app lication of SOFCAPU on different types of truck

Savingmoney on different types of truck with SOFC－APU per year is shown in Fig.8.

Fig.8 Saving money by the application of SOFC－APU on different types of truck per year

At present，diesel price in China is about7.32 yuan/L and diesel density of diesel is about0.84 g/cm3.So 1 kg diesel is equal to 1.19 L and its price is 8.71 yuan/kg.With the saving of diesel on trucks，the savingmoney can be calculated.

An SOFC－APU with heatpump air conditionerwill save the most(almost14 000 yuan/a)on a heavy truck，followed by medium and light trucks.

For themini truck，4 710 yuan will be saved by using an SOFC－APU per year.Itwill take several years to pay back the cost of SOFC－APU system.

In fact，there are few companies and people using a mini truck as a long－distance transport in Northeast China，and it is impossible for amini truck to stay out overnight.So an SOFCAPU is not suitable for themini truck in Northeast China.

2．4 Future scenarios

Other appliances besides heating and cooling are considered in future scenarios in China.Rest time in the cab will be 6 h in the future according to a survey in America.

The data in Table 5 is the estimation for average appliance power on truck idling［8］.Althoughmostof the appliances listed in the table are notequipped on truck in China now，all of them will be useful several years later when drivers change their way of life on the truck.

Table 5 Estimations for average appliances power on truck idling

The savings of a truck by the application of SOFC－APU per year in future is shown in Fig.9.For a heavy truck，3.2 t dieselswill be saved every year which means 68%of fuel is saved in the idling condition during night.A medium truck is similar to a light truck.

Fig.9 Savings of diesel，CO2and other em issions by the application of SOFC－APU per truck per year in future scenarios

Figure 10 shows the saving money for different types of truck with the application of SOFC－APU per year in future scenarios.

Fig.10 Saving money by the application of SOFC－APU on different types of truck per year in future scenarios

More than 27 000 yuan will be saved on a heavy truck with an SOFC－APU in future scenarioswhich is almost twice asmuch as the current situation.There will be a big market for the SOFC－APU used on trucks in Northeast China in the future when its price is low enough.

3 An Application Exam ple on Refrigerated T ruck

At the end of 2012 there are 70 000 refrigerated trucks in China with an extremely uneven distribution［9］.The load capacity of refrigerated trucks in China changes from 0.5 to 17 t，and 95%of refrigerated trucks'load capacity is less than 10 t.Environmental and operational conditions in various regions vary greatly.Large refrigerated trucks aren't w idely used.

Currently，few research institutions are dealt with SOFCAPU application on refrigerated vehicles，and relevant statistical data are extremely scarce.Based on the abovemodel，amultitemperature refrigerated truck(carriage is divided into three temperature zones:frozen zone，－18℃;refrigerated zone，0－5℃;normal temperature zone)with length of6 500mm，w idth of2 465mm，heightof2 500mm is used as an example to have a quantitative analysis of the SOFC－APU application on a refrigerated vehicle.

The calculation method of demanded power on a refrigerated vehicle is different from the demanded power in cab.Demanded power on a refrigerated truck consists of carriage wall heat load(P1)，goods heat release(P2)，ventilation heat load(P3)，motor thermal load(P4)and operations management heat load(P5)．

The total cooling power is P=P1+P2+P3+P4+P5= 3 764．34W．

The case of a combination of these models，the ambient temperature is 30℃，the load rate is 0.7，and the simulation results are shown in Table 6.

Table 6 Simulation results on a refrigerated truck

Due to the lack of experimental data of refrigerated trucks under idling condition and the vast majority of refrigerated trucks in China ismedium－sized lorry，so the experimental data of themedium truck are listed.

Aforementioned heating and cooling power in the cab are based on the overnight idling condition.The power demand time of refrigerated trucks is not always consistentwith it.As a result，it isanalyzed respectively in the paper.Demanded power on the refrigerated trucks is almost tw ice than that on the ordinary trucks.It is of great significance for the application of SOFC－APU on refrigerated trucks.

4 Conclusions

The simulation results are shown as follows.

(1)The application of SOFC－APU on trucks in Northeast China has an annual savings of 1 800 000 t diesels and 5 700 000 t CO2emissions in the currentmarket.The number will be 3 600 000 t diesels and 11 600 000 tof CO2when all the appliances are sw itched to the SOFC－APU.

(2)The average savings on a truck is about 1.3 t diesels and 4.3 t CO2emissions in current situation.Pay－back years in future will be half less than the current situation.

(3)The application of SOFC－APU on heavy trucks and refrigerated trucks will have the greatest significance for energy saving while an SOFC－APU is not suitable formini truck.

(4)The SOFC－APU market in China has significant potential but still needs promotion.The APU system should have a lower price，more supporting policy and higher public safety awareness.

Acknow ledgement

The authors thanks to AVL LIST GMBH(A－8020 Graz，Hans－List－Platz 1)for its funding.

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TM911

A

1672－5220(2015)04－0565－06

date:2014－03－03

*Correspondence should be addressed to ZHOU Su，E－mail:suzhou@#edu.cn