Design of the PID temperature controller for an alkaline electrolysis system with time delays_2

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Design of the PID temperature controller for an alkaline

electrolysis system with time delays

Ruomei Qia,∗

, Jiarong Lia,∗

, Jin Lina,b,∗∗

, Yonghua Songa,c, Jiepeng Wangd,e,

Qiangqiang Cuie, Yiwei Qiuf, Ming Tangb, Jian Wangb

aState Key Laboratory of Control and Simulation of Power Systems and Generation

Equipment, Department of Electrical Engineering, Tsinghua University, Beijing, China

bTsinghua-Sichuan Energy Internet Research Institute, Chengdu, China

cDepartment of Electrical and Computer Engineering, University of Macau, Macau,

China

dSchool of Materials Science and Engineering, Shanghai University, Shanghai, China

ePuriﬁcation Equipment Research Institute of CSIC, Handan, China

fCollege of Electrical Engineering, Sichuan University, Chengdu, China

Abstract

Electrolysis systems use proportional–integral–derivative (PID) temperature

controllers to maintain stack temperatures around set points. However, heat

transfer delays in electrolysis systems cause manual tuning of PID temper-

ature controllers to be time-consuming, and temperature oscillations often

occur. This paper focuses on the design of the PID temperature controller

for an alkaline electrolysis system to achieve fast and stable temperature con-

trol. A thermal dynamic model of an electrolysis system is established in the

frequency-domain for controller designs. Based on this model, the tempera-

ture stability is analysed by the root distribution, and the PID parameters

are optimized considering both the temperature overshoot and the settling

time. The performance of the optimal PID controllers is veriﬁed through

experiments. Furthermore, the simulation results show that the before-stack

temperature should be used as the feedback variable for small lab-scale sys-

tems to suppress stack temperature ﬂuctuations, and the after-stack temper-

ature should be used for larger systems to improve the economy. This study

is helpful in ensuring the temperature stability and control of electrolysis

∗These authors contributed equally.

∗∗ Corresponding author

Email address: linjin@tsinghua.edu.cn (Jin Lin )

Preprint submitted to International Journal of Hydrogen Energy October 4, 2022

arXiv:2210.00801v1 [eess.SY] 3 Oct 2022

systems.

Keywords: Electrolysis system, temperature control, PID controller.

Nomenclature

Parameters and variables

TAverage temperature

ˆyControl signal for valve opening

ρDensity

τTime delay

AArea

CThermal capacity

cSpeciﬁc heat capacity

ICurrent

kHeat transfer coeﬃcient

PElectricity power

QThermal power

RThermal resistance

TTemperature

tTime

TfTemperature feedback

UVoltage

uControl variable, u=yvalve

vVolume ﬂow rate

yvalve Valve opening

Superscripts and subscripts

* Steady-state

amb Ambient

c Cooling water

dis Heat dissipation

ele Electrolysis

sep Separator

th Thermal neutral

1. Introduction

Green hydrogen, produced by renewable energy, will play a critical role in

the decarbonization of the steel, chemical and transport sectors [1]. As the

core element of hydrogen production, it is important to ensure the safe and

eﬃcient operation of water electrolysis systems to achieve a steady hydrogen

supply. However, the temperature of the electrolysis system is often disturbed

by load and ambient temperature ﬂuctuations, which aﬀects both the system

eﬃciency and security. Temperatures lower than the rated temperature will

hinder the electrolysis reaction and lead to low eﬃciencies [3]; on the other

hand, high temperatures beyond the upper limit can harm the stack by

decreasing the corrosion resistance [4].

In existing commercial electrolysis systems, cooling devices are equipped

to maintain the temperature at a set point, and PID temperature controllers

are used to suppress the disturbances by regulating the cooling water ﬂow rate

[5, 6]. However, heat transfer delays in electrolysis systems make PID tuning

to be time-consuming, and the selected PID parameters often fail to achieve

satisfactory performance. For example, stack temperature oscillations occur

in [7] for both constant and intermittent power inputs caused by the improper

PID parameter setting. In [8], the stack temperature does not remain stable

under wind power inputs, and the temperature variation is approximately

8◦C with current ﬂuctuations between 40% to 100% rated. Restricted by

temperature variations, the electrolyte temperature is controlled at 65 ◦C in

[8], which is far from the allowable limit of 90 ◦C; thus, the system eﬃciency

is sacriﬁced.

For the temperature control of electrolysis systems, systematic modelling

and controller design methods are needed. Ulleberg [9] proposed a lumped

model to predict the operating temperature of an advanced alkaline electrol-

yser. This model considers the thermal balance among the heat generation,

heat loss and auxiliary cooling, which is widely used in thermal-related stud-

ies [10–14]. Y. Qiu presented an optimal production scheduling approach for

utility-scale P2H plants considering the dynamic thermal process through a

ﬁrst-order temperature model [15], whose parameters were estimated in [16].

The lumped model does not consider the temperature diﬀerence between the

stack and the auxiliary devices. Sakas et al. [6] and [17] used second-order

and third-order thermal models, respectively, taking into account the ther-

mal inertia of the gas-liquid separators. Time delays exist in heat transfer

processes and will aﬀect the accuracy of the model when analysing the tem-

perature of a speciﬁc component, e.g., the stack. Qi et al. [18] emphasized

the eﬀects of heat transfer delays on the thermal dynamic performance and

added two time delay terms for the stack and the cooling coil in the third-

order model.

There are few existing studies focusing on temperature control. Sakas et

al. [6] used a PID temperature controller in the simulation; however, the

dynamic performance of the temperature controller was not discussed. Qi et

al. [18] proposed two novel temperature controllers to reduce the temperature

overshoot: a current feed-forward PID controller and a model predictive

controller. As the most widely used temperature controller in commercial

electrolysis systems, the tuning process of the PID temperature controller is

a time-consuming task due to the multiple thermal inertia and time delay

terms, which have not been discussed yet.

The focus of this paper is on the thermal dynamic analysis and PID

controller design of an alkaline electrolysis system. The main contributions

are as follows.

1. A frequency-domain thermal model that considers the time delays in

the heat transfer process is ﬁrst proposed for the controller design of

electrolysis systems.

2. The temperature stability is analysed by the root distribution, and an

optimization model is proposed for parameter tuning considering both

fastness and security, which is veriﬁed through experiments.

3. Suggestions are given for system design to improve the thermal dynamic

performance. It is suggested to use the before-stack temperature as the

feedback variable for small lab-scale systems to suppress the tempera-

ture ﬂuctuation and use the after-stack temperature for larger systems

to improve the economy. In addition, time delays should be reduced to

improve the thermal dynamic performance by increasing the ﬂow rates

or using shorter channels.

This paper is organized as follows. In Section II, the complete thermal

model of an alkaline electrolysis system, which is linearized and transferred to

the frequency domain in Section III is introduced. In Section IV, a method

for PID tuning that considers the overshoot and setting time is provided.

The proposed PID tuning method is veriﬁed through experiments in Section

V. In Section VI, the PID temperature controllers are compared with before-

stack and after-stack temperature feedbacks. In Section VII, the inﬂuence of

time delays on the thermal dynamic performance is analysed.

2. Temperature control of an alkaline electrolysis system by the

PID temperature controller

2.1. System process description

The process of the analysed alkaline electrolysis system is shown in Fig.

1. The stack is the core element of the system, in which water is electrolyzed

to produce hydrogen and oxygen. The gas products mixed with electrolytes

enter the gas-liquid separators, in which the gas product is separated for sub-

sequent processing, and the remaining electrolyte from two sides are mixed

and circulated into the stack.

The electrolysis reaction in the stack is exothermic at room temperature

[9]. To maintain the stack temperature at the rated value, a cooling coil is

placed in the gas-liquid separator to cool down the electrolytes and indirectly

cool the stack. The cooling water ﬂow rate is controlled by the water valve

according to the command from the temperature controller. However, the

stack temperature tends to ﬂuctuate considerably in industrial practice due

to the inappropriate parameter setting of the temperature controller as well

as external disturbances, e.g., current and ambient temperature ﬂuctuations.

Thus, it becomes an important issue to obtain stable and fast temperature

control.

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DesignofthePIDtemperaturecontrollerforanalkalineelectrolysissystemwithtimedelaysRuomeiQia,,JiarongLia,,JinLina,b,,YonghuaSonga,c,JiepengWangd,e,QiangqiangCuie,YiweiQiuf,MingTangb,JianWangbaStateKeyLaboratoryofControlandSimulationofPowerSystemsandGenerationEquipment,DepartmentofElectricalEngineer...

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Design of the PID temperature controller for an alkaline electrolysis system with time delays_2

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