**1. Introduction**

The impact of integrating renewable energy sources from the environment solves one of the major problems regarding global warming and CO2 emissions. Fossil fuel resources, hydrocarbons, coal, and natural gas are all limited, cause pollution, and have a strong influence on carbon emissions, affecting the environment irreversibly [1].

Green, renewable energy sources, as an alternative to fossil fuels, are used by hybrid energy systems [2] based on high-efficiency technologies. For example, the solar concentrator that provides the energy needed to drive a Stirling engine [3], with a working cycle having the same name, created by the Scotsman Robert Stirling and patented in 1816 [4]. Thus, the Stirling engine appears as a promising option in the supply of energy compared to other traditional solutions based on renewable energy sources, with a reduced polluting effect similar to that of geothermal, solar, or biomass energy [5,6].

In the specialist literature, Zabalaga et al. presented in [7] an analysis in terms of energy efficiency and economic profitability of a hybrid energy system consisting of photovoltaic panels, a Stirling engine, and a battery system [8]. The dimensioning, using Homer software and, later, the simulation [9–11] of the starting and cooling sequences in MATLAB/Simulink T.M. 8.9 demonstrated a 69% reduction in greenhouse gas emissions, an 11% improvement in annual costs, and a 5% increase of energy efficiency compared to conventional diesel engine systems [12–14].

In [15], Islas et al. conducted a research study on the influence of several design variables and operating parameters over the performance of a 2 kW alpha-type Stirling engine, as well as methods for optimizing the major factors affecting engine efficiency. An important aspect is temperature and pressure control in the primary and secondary circuits of the Stirling engine.

Malali et al. discuss in [16] the effects of circumsolar radiation on the thermal efficiency of a Stirling engine–solar concentrator system, the mathematical model being realized in MATLAB. In the optimal case, the parameters thus obtained are used in the design of the hybrid system.

In [17], Zare and Saleh used the mathematical model of the Stirling engine and the Lyapunov method to predict the optimal operating conditions of a Stirling engine [18,19]. The starting sequences and the oscillations that may occur during operation were especially studied.

Buscemi et al. presented in [20] a hybrid energy system consisting of a solar concentrator and a 32 kW Stirling engine installed in Southern Italy. Their model is capable of predicting annual energy production, especially taking into account the level of dirt on the solar concentrator mirrors [21,22].

In [23], Arora et al. developed a genetic evolutionary algorithm in MATLAB for choosing the optimum values of the decision variables in order to obtain maximum power in terms of energy and heat, as well as an increased economic efficiency [24,25].

Based on the scientific literature, the novelty of this paper consists of the design and implementation of an automation and control system for the operation of a Stirling engine—this system being used so far only in other types of industrial installations.

Even though the use of programmable logic controllers (PLCs) and industrial automations is widely presented in literature, there are few to no cases where these are used in Stirling engine systems for the generation of both electrical and thermal energy, which increased the research team's interest in approaching this topic.

One particular element of this automation, in comparison to others from the industrial sector, is related to the specificity of the Stirling engine system, namely the adaptation to operating requirements, starting sequences, normal operation, control and valve actuation for loading and unloading the working fluid, operation of the fan and cooling pump, controlled shut-down, overload protection, actuation of the safety curtain of the primary exchanger, monitoring of working parameters, temperature, and speed.

Another aspect was to ensure the recording of the parameters, the global automation transmission through bidirectional ethernet connection, including the positioning of the solar concentrator on the maximum flux of solar thermal irradiation, according to the data collected by the Solys2 weather station, which determines the solar coordinates of the Stirling engine solar concentrator location.

In this context, the installation developed in the National Research and Development Institute for Cryogenic and Isotopic Technologies in Romania has integrated a Stirling engine working together with a solar concentrator with the purpose of cogeneration of electricity (10 kW electric) and thermal energy (25 kW thermal) at a low cost [26].

In this paper, we have designed, tested, and optimized the starting, operating, and stopping sequences of a Tedom-modified Stirling alpha V191 engine, representing the conversion unit of the Sunflower 35 solar concentrator [27], through an automation system. In the laboratory, we designed the electrical installation and the automation system, we implemented practically the automation system [28,29], and we programmed the PLCs in order to realize the starting sequences and the opening of the valves for both the cooling system of the compressor and the charging system with helium. The automation also provides the protection and alarm system, control of the protection curtain, and command and control of the electric generator, the heat exchanger, and fan control for the Stirling engine.

Programmable logic controllers (PLCs) emulate the electrical scheme and the computer equipment built around a processor and are capable of controlling, through digital and analog input/output modules, one or more pieces of industrial equipment based on dedicated software [30].

The specific objectives of the present study, based on the automation design, are the following:



The main objective and focus of this paper is to solve the starting, operating, and shut-down sequences in safe conditions, as well as monitor the working parameters. The performance indicators are given by the stabilization of the temperature of the heat exchanger towards the working temperature, of the engine speed, and of the working pressure according to the values given by the producer [27].

The main findings are as follows:


The solar concentrator using a Stirling engine offers a promising solution compared to other thermal installations that use solar energy, which is an unlimited renewable energy. The most-developed solar solutions currently used are the following:


Even though photovoltaic panels are a great development, the solar concentrators using a Stirling engine accomplish a higher conversion efficiency of solar energy into electricity—29.4% [31]. By comparison, solar collectors that have a parabolic trough or Fresnel-type system can achieve a solar-to-electricity conversion efficiency of only 18–22% [32]. We observe a significant advantage for the Stirling solar concentrators. The tower solar concentrators achieve a solar-to-electricity conversion efficiency of about 20–27% [32], closer to that of the Stirling solar concentrator, but with considerably higher execution costs.

Another revealing analysis is related to the cost of obtaining a kWh, as presented by Zayeda [33]. The Stirling solar concentrator has a promising cost of USD 0.2565/kWh, compared to USD 0.4/kWh for the solar tower concentrators, and USD 0.14–0.16/kWh for solar collectors with a parabolic trough or Fresnel system. The best cost is for the photovoltaic panels, at USD 0.06/kWh.

Awana has shown in [34] the advantages of solar concentrators over photovoltaic systems. That is, a 35.7% higher electricity production, but also a better capacity-use factor and a better solar-to-electric conversion efficiency.

It was also shown that Stirling solar concentrators have led to a significant reduction in the cost of electricity production and, in addition, that the use of Stirling solar concentrator systems working on cogeneration of electric and thermal energy achieved an efficiency of over 60%.

The article was structured as follows: in the first part, in the introduction, the main objectives of the work were stated, up -to-date information was presented relating to PLC automation and control strategies used in Stirling solar power systems and a comparative study was conducted regarding the conversion efficiency and the cost of the proposed system with respect to other solar solutions. The second chapter, "Structure of the Automation and Control system", presented two automation systems and identified the most suitable one, according to the equipment to be monitored/controlled. The third chapter, "Operating Principle", presented the electrical schemes of the main and secondary circuits of the Stirling engine, as well as the automation schemes of the proposed system. In chapter four, "Case Study", the experimental test stand was presented together with the data acquisition, control, and monitoring of the process parameters in tandem with the HMI interface. The results and discussions were presented in the fifth chapter and, finally, in the sixth chapter, the work was summarized and future perspectives were presented.
