Optimal design of negative emission hybrid renewable energy systems with biochar production☆
Graphical abstract
Introduction
As a proactive measure to reduce the risk and impacts of climate change, the Paris Agreement targets to hold the increase of the global average temperature to well below 2 °C compared with pre-industrial levels by the end of this century and pursues efforts to limit it to 1.5 °C [1]. It can be translated into a target of limiting the atmospheric greenhouse gas concentration to a range of 430–480 parts per million (ppm) equivalent carbon dioxide emissions (CO2-eq) [2]. The carbon dioxide emission generated from fossil fuel combustion and industrial processes accounted for 65% of global greenhouse gas emission in 2010, and the energy sector was found to be the largest source for greenhouse gas emission [3]. Although the penetration of renewable energies in the energy systems grows steadily in recent years [4], the energy sector is still a major contributor to global greenhouse gas emission [5] due to the continuous growth of energy demand and the high reliance on fossil fuels [6]. In the most recent assessment report published by IPCC, 65% of the carbon budget in keeping the 2 °C target has been consumed [7]. With an increase of 1.4% after three years’ plateau, the global energy-related greenhouse gas emission (GHGe) reached its highest value of 32.5 gigatonnes (Gt) in 2017 [8]. Therefore, substantial emission reductions in the energy system would be needed to meet the targets of the Paris Agreement [9].
Under this context, there is an on-going transition of the energy system towards sustainable, low-carbon, and affordable electricity derived from renewable energies. Renewable energy technologies, especially those utilizing solar, wind, and biomass resources, have been widely studied and developed as an alternative to fossil fuels for energy production. However, considering the rate of greenhouse gas emissions, the growth of energy demand, as well as the short of efficiency to promote emission mitigation strategies so far, it is challenging to meet Paris Agreement targets without negative emission technologies (NETs) [10]. NETs are technologies that remove greenhouse gas from the atmosphere [11]. As summarized in Table 1, various NETs have been proposed, including afforestation and reforestation (AR), soil carbon sequestration (SCS), biochar for soil amendment, bioenergy production with carbon capture and storage (BECCS), enhanced weathering (EW), direct air capture (DAC) of CO2 from ambient air and storage, and ocean fertilization (OF) [12]. In the meanwhile, negative emission technologies (NETs) are also widely investigated in recent years along with the traditional mitigation methods such as renewable energies [13].
Among the NETs, carbon sequestration by biochar is one of the most attractive options with high technical readiness, good carbon abatement potential, and moderate cost [18]. As a carbon-rich solid residue derived from gasification and pyrolysis, biochar has been recognized as an effective carbon abatement tool upon its application to soil. Since CO2 is absorbed during plant growth, there is a net zero emission of CO2 when biomass is combusted. Through converting biomass into biochar for soil application, a significant amount of carbon could be sequestrated, leading to a net removal of CO2 from the atmosphere. Additionally, biochar could also increase soil organic matter [19], microbial activity [20], water retention, and crop yields [21], while decrease fertilizer needs, soil greenhouse gas emissions, nutrient leaching, erosion, pollutant bioavailability and pollutant mobility [22], [23], [24], leading to an overall effect of indirect carbon abatement.
To produce both biochar and electricity, two waste-to-energy technologies are commonly applied: gasification and pyrolysis. Gasification could convert a carbonaceous feedstock into heat, biochar, and producer gas, which mainly consists of CO2, CO, H2, CH4, and N2 in a high-temperature, oxygen-insufficient environment [25]. In the pyrolysis process, organic material decomposes mainly into pyrolytic gas, pyrolytic oil, and biochar in the absence of oxygen [26]. In these processes, heat contained in the gas phase can be used to generate electricity. When more biochar is produced, less energy is contained in the gas phase, leading to a lower electricity generation rate and vice versa. As the amount of electricity and biochar productions are negatively correlated, there is a trade-off between them, which further leads to a trade-off between economic profit and carbon sequestration. Another thermal chemical process, combustion, can be employed to increase the energy utilization efficiency of the biomass. Complementary power generation methods can also be incorporated to target higher carbon sequestration goals while meeting the electricity demand in a system level. As solar and wind resources are abundant, endless, and accessible with no cost [27], solar photovoltaic (PV) and wind energy are recognized as two promising and popular choices for renewable power generation.
Therefore, it is important to select the most economical pathways and sizes of the system components to maximize the economic and environmental performance of the NEHRES while meeting the power supply requirements.
In order to effectively design a NEHRES that possesses carbon sequestration and cost-effectiveness while ensuring reliable energy supply, optimization methods can be used. In this study, we aim to propose a decision-support framework for the optimal design of a NEHRES. A case study on a standalone rural island where solar, wind and biomass resources are available will be conducted to demonstrate the proposed framework and the feasibility of a negative emission hybrid renewable energy system.
The remaining part of the paper is organized as follows. Section 2 presents a brief and general review of the state-of-the-art optimization methods for the design of renewable energy systems and the contribution of this work to the research field. A proposed stochastic multi-objective optimization framework for the design of the NEHRES is detailed in Section 3. The background of the case study will be introduced in Section 4. The results and discussion of the case study will be provided in Section 5 and the conclusions drawn in Section 6.
Section snippets
Literature review
A problem of renewable energy is that it is highly dependent on the environment, which can lead to an intermittent and fluctuating power generation. This problem poses challenges for the energy system to provide a stable and reliable electricity supply. Two solutions to this problem, energy storage and HRES, have been proposed by the researchers. An energy storage system, such as a battery bank, can be used to store excess electricity for later use. It is currently a necessary component for
Methodology
The proposed layout of the NEHRES is shown in Fig. 1. In the system, gasification and pyrolysis are the key contributors to negative emission through biochar production and application, while solar PV, wind, and biomass combustion technologies serve as main sources for electricity supply. The producer gas undergoes several processes and enters the turbine for electricity generation. When gasification and pyrolysis processes are chosen for biomass conversion, the liquid and solid products
Case study scenario setting
The NEHRES is suitable for an off-grid agriculture-based island, where there are abundant biomass resources in the form of agricultural residues throughout the year in addition to solar and wind resources. The application of the NEHRES concept and framework is examined using a case study based on Carabao Island (also called San Jose Island) in the Philippines.
Carabao Island is a rural island with a population of 10,881 people and an area of 22.05 km2 [55]. Currently, the electricity supply on
Results and discussion
The proposed multi-objective stochastic optimization framework for the NEHRES design is applied to the case study on Carabao Island. In the first place, the Pareto optimal solutions are obtained for the case study. Table 4 shows the values of the daily net cash flow and greenhouse gas emissions, as well as the capacities of the power generation and storage components in the NEHRES for each Pareto solutions.
In order to further interpret the results, the Pareto curve for the multi-objective
Conclusions and future work
In this paper, the concept, system modeling, and optimal design and assessment of a negative emission hybrid renewable energy system (NEHRES, a hybrid PV-wind-biomass renewable energy system with biochar production) have been presented. A multi-objective stochastic decision-support framework has been developed to maximize the daily net cash flow and minimize the greenhouse gas emissions of the whole system. Through a case study on a stand-alone rural island, we investigated the trade-off
Acknowledgement
This research is supported by the National Research Foundation, Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) programme.
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The short version of the paper was presented at ICAE2018, Aug 22–25, Hong Kong. This paper is a substantial extension of the short version of the conference paper.