Exergo-Economic Analysis of Solar-Driven Ammonia Production System for a Sustainable Energy Carrier ^†

Abstract

The industrial sector’s movement toward decarbonization is regarded as essential for governments. This paper assesses a system that uses only solar energy to synthesize liquid hydrogen and ammonia as energy carriers. Photovoltaic modules deliver electrical power, while parabolic dish collectors are responsible for directing thermal energy to the solid oxide electrolyzer for hydrogen production, which then mixes with nitrogen to produce ammonia after a number of compression stages. To investigate the proposed system, comprehensive thermodynamic and exergo-economic studies are performed using an engineering equation solver and ASPEN PLUS software.

1. Introduction

Decarbonizing energy carriers is crucial for power-to-fuel energy systems. The 2015 Paris Agreement aims to achieve net-zero carbon emissions by 2050 and restrict the rise in world temperatures to less than 2 °C [1]. By increasing the production of green hydrogen and its products, improving the efficiency of energy conversion systems, and creating more power from renewable resources, the International Renewable Energy Agency (IRENA) estimates that taking this course will result in an annual reduction in CO₂ emissions of 37 Gt [2]. It is inevitable that various forms of renewable energy will be developed to satisfy the requirements of the global energy structure transition, and solar energy will undoubtedly play a significant role in the future [3]. The intermittent nature of renewable energy systems may increase their dependability despite their high installation costs. The primary means of storing excess energy with a comparatively high safety and energy density are batteries; however, their capacity to store energy is limited, and their efficiency is diminished after multiple cycles of charging and discharging. In this sense, hydrogen and its more energetic derivatives are important for the future. However, there are certain limitations associated with hydrogen storage, including safety and cost. The low density of hydrogen (about 0.083 kg/m³) is the main issue associated with its storage and transportation. Hydrogen can be stored or transported efficiently in two ways: either it should be liquid at −250 °C and atmospheric pressure, or it should be pressurized to 100–300 bar [4]. However, both of these methods need a lot of energy. Converting hydrogen into other derivatives, such as ammonia, is one way to solve this problem. Ammonia is a zero-carbon emission fuel and an important substance for the food and energy nexus as it contains almost 50% more hydrogen than pure hydrogen at the same volume.

Numerous studies have assessed the viability of producing ammonia and green hydrogen from an economic and environmental perspective. In order to produce hydrogen and ammonia, Tukenmez et al. [5] assessed solar and biomass-powered systems. Pozo et al. [6] examined two green electricity and blue ammonia production facilities from an economic perspective and suggested two low and zero-carbon emission systems for ammonia synthesis. Eco-friendly ammonia synthesis using hydroelectric electricity and alkaline electrolyzers was studied by Rivarolo et al. [7]. According to the findings, the price of ammonia was less than EUR 400 per ton, which is comparable to the cost of traditional fossil fuel-based plants that produce ammonia. Ikaheimo et al. [8] proposed a novel means of combining district heating, electricity, and long-term energy storage with power-to-ammonia technology. The suggested system could be a cost-effective alternative if the price of natural gas surpasses EUR 70/MW·h. or the carbon tax exceeds EUR 200/ton CO₂. A thermodynamic analysis of a new solar and wind-powered hydrogen generation device was carried out by Sorgulu and Dincer [9]. However, by electrolyzing water, high-temperature solid-oxide electrolysis (SOEC) can be used to store renewable energy in the form of hydrogen and oxygen. In addition to having greater electrical efficiency than low-temperature electrolysis technologies, SOEC makes it possible to integrate heat into the downstream synthesis process.

A thorough analysis of the literature reveals that more study is required to create a thorough thermodynamic and exergo-economic model for renewable systems, with a particular emphasis on the generation of hydrogen and ammonia. Numerous studies in this field have been carried out individually for ammonia production and liquid hydrogen. Their assumptions, background and conditions are quite different (system configurations, supply and demand profile, and nature of operation). Therefore, a pure solar-based (thermal and PV) hydrogen and ammonia production system via SOEC is proposed, developed and assessed from an energy, exergy and economic point of view by considering the effects of crucial parameters on the performance of the system. The novel aspects of the proposed system configuration are as follows:

• This is a pure solar thermal and PV-based system for hydrogen and ammonia utilizing high-temperature SOEC.
• This system supplies freshwater from reverse osmosis desalination for the community and supports SOEC for hydrogen production.
• This study conducts a comprehensive exergy and exergy destruction analysis for the whole system.
• This study performs sensitivity analysis to investigate the impact of more efficient parameters.

2. System Description

Figure 1 depicts the solar-assisted hydrogen and ammonia production system. PV modules provide electric power, while solar dish collectors are responsible for directing thermal power to the SOEC for hydrogen production. Following multi-compression, the stream is injected into the ammonia synthesis reactor. The nitrogen is supplied from the nitrogen cylinder and the flow rate is regulated to maintain a 1:3 N2:H2 molar ratio. The hydrogen and nitrogen gasses undergo a reversible reaction inside the reactor using a catalyst. Therefore, after going through the heat exchanger, the mixture of synthetic ammonia, unreacted hydrogen, and nitrogen gases is sent through an ammonia trap. Here, unreacted gasses are sent back to the synthesis reactor and ammonia is stored in the tank for further use. Figure 2 shows the flow chart of the thermodynamic model developed and simulated with the Aspen Plus V11 software. The design input parameters are available in

Table 1. Design input parameters for all the sub-systems proposed.

Parameter	Value	Unit
Direct normal irradiance (DNI)	1000	W/m2
Dish aperture area	300	m2
Area of PV module	800	m2
Normal cell temperature	317.5	K
TSOEC	750	°C
PSOEC	1	bar
Operating current density, J	5000	A/m2
Area of SOEC	0.530660	m2
Ammonia synthesis reaction pressure	100	bar
Ammonia synthesis reaction temperature	665	K

.

3. Methodology and Thermodynamic Analysis

In this section, a thermodynamic analysis of the investigated system is presented. Engineering equation solver (ees) and Aspen Plus software are used for the analysis and simulation. The energy, exergy and mass flow balances can be evaluated as follows:

$$\dot{Q}-\dot{W}=\sum {\left(\dot{m}·h\right)}_{in}-\sum {\left(\dot{m}·h\right)}_{out}$$

(1)

$$\sum {\dot{m}}_{in}-\sum {\dot{m}}_{out}=0$$

(2)

$${\dot{Ex}}_{heat}-\dot{W}=\sum {\left(\dot{m}.ex\right)}_{in}-\sum {\left(\dot{m}.ex\right)}_{out}+{\dot{Ex}}_{dest}$$

(3)

The parabolic dish collector (PDC), PV module, SOEC and ammonia synthesis are taken from reference [10] and analyzed by the following relations. The thermal and exergy efficiencies of PDC can be given as follows [11]:

$${\eta }_{th}={{\dot{Q}}_u/\dot{Q}}_{sun}$$

(4)

$${\eta }_{ex}={{\dot{Ex}}_u/\dot{Ex}}_{sun}$$

(5)

Furthermore, the electrical efficiency can be computed as follows:

$${\eta }_{en}={\displaystyle \frac{P_{max}}{A\times G_b}}$$

(6)

The current–voltage characteristics (I–V) of the solar cell are as follows:

$$I=I_I-I_O\times {exp}^{\left[{\displaystyle \frac{q\times \left(V-IRs\right)}{A\times K\times T}}\right]}$$

(7)

The electrical exergy of the PV module is written as follows:

$${\dot{Ex}}_{elec}=V_{OC}\times I_{SC}\times FF$$

(8)

The equation used for the modeling of SOEC is taken from the authors’ previous work [12]. The Solid Oxide Electrolysis Cell is essentially made up of three layers. H₂O molecules in the porous cathode diffuse to the triple-phase boundary (TPB) via the porous electrode, where they undergo reduction to H₂. To create oxygen, the electrolyte transports the oxygen ions to the anode.

$$H_{2}O+2e^{-}\to H_2+O^{2-}$$

(9)

$$2O^{2-}\to O_2+4e^{-}$$

(10)

The Gibbs function serves as the foundation for energy conservation, and the power needed for the cell P_SOEC is computed by the following:

$$∆H=∆G+T∆S$$

(11)

$$P_{SOEC}=jE$$

(12)

The electrical efficiency of ${\eta }_{SOEC}$ is as follows:

$${\eta }_{en,SOEC}={\displaystyle \frac{{LHV}_{{\mathrm{H}}_2}\times {\dot{m}}_{{\mathrm{H}}_2}}{P_{SOEC}}}$$

(13)

In ammonia synthesis, the H₂ generated from SOEC and N₂ from the air enter the ammonia synthesis reactor after a number of compressor stages to produce ammonia under a reversible reaction. A nickel-based (NiO-Al₂O₃) catalyst was used to increase the conversion rate of ammonia.

$$N_2+3H_2\leftrightarrow 2{NH}_3$$

(14)

The solar-to-fuel efficiency can be computed as follows:

$${\eta }_{StF}={\displaystyle \frac{{LHV}_{ammonia}\times {\dot{m}}_{ammonia}}{{\dot{Q}}_{sun}}}$$

(15)

4. Model Validation, Results and Discussion

This section presents the model validation of SOEC in the present study using the published literature [13] and the results obtained by the simulation and analysis. The cell potential values at various current densities at an operating temperature of 750 °C and the other input parameters indicated in the graph are compared in Figure 3. The data’s reliability is demonstrated by an R² value of 0.9758. The correctness of our model is demonstrated by the very good fit between the model results and the reference study.

Figure 4 shows the variation in the SOEC temperature from 600 °C to 100 °C and its impact on the SOEC and solar thermal efficiencies. It can be seen that the solar thermal efficiency decreases, while the SOEC efficiency increase. The increase in the of η_STF is because of a higher SOEC efficiency, which is boosted by a higher operating temperature. The decrease in the solar thermal efficiency is due to the irreversibility at higher surface temperatures and re-radiation loss, which play a crucial role in reducing the efficiency. Moreover, Figure 5 depicts the effect of the current density (J) on the cell voltage and SOEC efficiencies. The current density is a key factor that affects the cell voltage and SOEC efficiency. Cell voltage is the combination of four potentials (Nernst potential, activation, ohmic and concentration polarization). As the activation, ohmic and concentration polarization increase with the current density, the cell voltage of the SOEC is increased. However, the SOEC efficiency monotonically decreases with a rise in the current density, indicating that the energy loss caused by polarization plays a significant role in deteriorating the SOEC efficiency.

The impact of the synthesis reaction temperature on the ammonia production rate and solar-to-ammonia efficiency is plotted in Figure 6. This shows that increasing the reaction temperature to between 550 K and 900 K reduces the conversion rate, which has a negative impact on the generated ammonia and fuel efficiency. The ammonia production rate is decreased from 1.3 kg/s to 0.13 kg/s, while the solar-to-ammonia efficiency reduces to approximately 23.59%. This is due to catalyst deactivation at higher temperatures, which lowers the ammonia production rate. In addition, a higher temperature leads to irreversibility, causing more heat loss that will further decrease the efficiency. In addition, Figure 7 shows that the exergy efficiency of the solar dish collector is considerably higher (40%) than the other sub-systems of the proposed cycle, followed by the SOEC system (22%). The lowest exergy efficiency is observed in the ammonia synthesis unit, and is almost 10%.

5. Conclusions

The present work performs a comprehensive analysis of solar-powered ammonia production by high-temperature SOEC. Energy and exergy analysis are conducted by varying influential operating parameters like the current density, SOEC temperature, and reactor synthesis temperature. The performance parameters are the solar thermal efficiency, SOEC efficiency, solar-to-ammonia conversion efficiency and ammonia production rate. Parabolic dish collectors had the highest exergy efficiency (40%), followed by SOEC and PV modules. The solar-to-ammonia efficiency and ammonia production rate decrease with the reactor synthesis temperature. There are certain challenges associated with ammonia, as it is toxic and requires careful handling and measures. Its conversion back to hydrogen is an energy-intensive process that causes a reduction in energy efficiency. Moreover, future studies can consider electrochemical ammonia synthesis with chemical kinetics and their effect on the system’s performance.