Recent Advances in Renewable Energy

During the last two decades, the use of biofuels has shown rapid growth, driven mostly by policies focused on increasing energy efficiency, and replacing fossil energy by renewable energy. There are different biomass raw materials that have been evaluated for the production of several added-value products. These raw materials have been classified into the first generation (agricultural and edible crops), second generation (inedible agroindustrial residues) and third generation (algae). The interest in the cultivation of microalgae has been increasing due to the high value products that can be obtained. Additionally, the oils present in microalgae are used for the production of biodiesel and the cake resulting after processing can be used for the production of bioethanol, biobutanol or energy. Based on this, this chapter first introduces the current uses and applications of multiple species of microalgae in terms of energy production and describes the technologies being used for the production of bioenergy using microalgae. Then, two specific cases are analyzed: cogeneration and biodiesel production. The performed analysis serves to conclude that in order to establish microalgae as an energy-producing feedstock, it is necessary to integrate their use to obtain multiple products simultaneously: metabolites due to their high value, oils to produce biodiesel and the dry cake for thermochemical production of energy. The extraction of multiple products can only be made possible if a biorefinery concept is applied.

Cultivation of microalgae for bioethanol production emerged as one of the potential solutions to drive green and sustainable biofuel production. Microalgae are photosynthetic microorganisms that can grow rapidly and are able to accumulate high content of carbohydrate within their cells. The carbohydrate is usually stored in the form of starch, which is easier to breakdown to simple reducing sugar compared to lignocellulosic biomass. In the present chapter, the process route to produce bioethanol from microalgae biomass is discussed, including the cultivation strategies to enhance microalgae carbohydrate productivity, biomass pre-treatment methods, hydrolysis and fermentation process.

Ethanol has increasingly attracted attention of researchers. Currently, ethanol is synthesized by different routes, such as production from the synthesis gas, ethylene hydration and fermentation of sugars. In addition, microalgae have been recognized as the potential sources for biofuel production, since they reach high biomass productivities. Recently, the use of microalgal biomass as feedstock for bioethanol production became an interest for researchers and then for the general public. Therefore, the aim of this chapter is to introduce the basics of algal-ethanol production and the current status of this emerging bioethanol source.

Anaerobic digestion (AD) is a biological process in which various types of biomasses are converted mainly into gaseous product called biogas. The importance of biogas lies in its composition: methane is the major compound, being used as a biofuel. Although the history of biogas dates back to more than two thousand years ago, the first biogas plant was built in the middle of the 19th century. AD started to be applied for wastewater treatment and biogas production in the early 1900s, and soon after, some researchers realised that biogas could be produced from the microalgae generated during the secondary treatment of wastewater. Afterwards, an increasing number of studies about the AD of microalgae have been published worldwide. This chapter introduces the basic principles of AD and describes the influence of some of the most important operating parameters during biogas production from microalgae.

In the search of alternative routes for energy, microalgal hydrogen production becomes an important topic as a renewable and sustainable candidate. Following the progresses in the knowledge related with cellular mechanisms and production processes, biohydrogen can have the chance to take a strong role in the future even if the current capacities are not comparable to the conventional fossil fuels. This chapter focuses on the state of the art for biohydrogen production from microalgae.

The design and construction of any algal photobioreactor is a complex multi-parametric problem, with successful examples requiring the consideration of many different factors. In practical terms, this means that most feasible cultivation platforms require a compromise between biotic, abiotic and economic factors. As a result, most of the successful designs are found commercially prompt to maximise favourable parameter ranges within practical cost limits. The end result is the construction and subsequent operation of the photobioreactor within a multi-parametric ‘sweet-spot’, which will vary dependent on the production process and end product. Such cost considerations are particularly important for the production of lower value commodities such as biofuels, which are yet to achieve economical production at scale. Considering these aforementioned constraints; 60 years of photobioreactor design and production have resulted in several general blue-prints for larger scale algal production platforms. Despite this fact, most of the photobioreactors in successful operation, today, can be categorised as having characteristics that are based on a relatively limited repertoire of basic designs and construction materials. The most common separation is between open and closed systems which differ fundamentally in their levels of parameter control and cost base. Further configurational commonalities can be seen in many examples and are most notable in their external appearances. The most common photobioreactor systems are horizontal tubular, vertical column, panel or plate and pond based systems.

Carbon dioxide (CO2) concentration in atmosphere has increased since the beginning of industrial age. CO2 biofixation by microalgae is a technology based on the use of solar energy through photosynthesis to capture and use this carbon source, mainly from industrial combustion flue gases. This CO2 mitigation technology enables microalgal cultivation with an alternative source of carbon, decreasing the emissions of industries (such as power generation) and making processes more sustainable and environmentally friendly. The chapter aimed to approach the implementation of using flue gas as a carbon source in microalgal cultures, as well as the metabolism of CO2 biofixation and the associated environmental issues.

Microalgae harvesting is an essential process for the production of almost all types of microalgae products. However, these harvesting, thickening and dewatering processes have a significant cost, despite extensive research efforts for cost reduction. Microalgal harvesting is especially challenging because these unicellular cells are small and have a similar density to water, and because the biomass concentration in culture is relatively low. The microalgal biomass (0.05% w/w) needs to be concentrated to a paste with 15-25% water content. This process is usually conducted in a multi-step scheme by primary concentration, thickening and dewatering of the biomass using a combination of various technologies. The most important implementation criteria are to avoid any biomass contamination, minimize any influence on biomass quality, and to allow water recycling to reduce the water footprint. This chapter gives an overview of several primary harvesting, thickening and dewatering methods with a focus on their advantages for implementation into a microalgae harvesting strategy.

Microalgae are an attractive source for biofuels due to their high growth rates and lipid content. However, up to the present time, the extraction of lipids from wet microalgae biomass remains an energy-intensive step, and an impediment for the economically viable large scale bio-oil production. During the extraction process, cell disruption and drying are two of the most energy-consuming steps. Several studies on genetic modification of microalgae towards lipid accumulation enhancement, or to reduce the harvesting and lipid extraction cost have been carried out successfully. This chapter aims to summarize the recent developments in microalgal oil extraction processes, including drying methods, cell disruption methods, conventional or supercritical solvent extraction methods, and recent approaches for direct biodiesel production. The current developments in metabolic engineering for lipids production are also reviewed. Hydrothermal liquefaction is also included as an alternative route to convert wet biomass into bio-crude oil.

Recent progress towards the implementation of renewable bioenergy production has included microalgae, which have potential to significantly contribute to a viable future bioeconomy. In a current challenging energy landscape, where an increased demand for renewable fuels is projected and accompanied by plummeting fossil fuels’ prices, economical production of algae-based fuels becomes more challenging. However, in the context of mitigating carbon emissions with the potential of algae to assimilate large quantities of CO2, there is a route to drive carbon sequestration and utilization to support a sustainable and secure global energy future. This chapter places international energy policy in the context of the current and projected energy landscape. The contribution that algae can make, is summarized as both a conceptual contribution as well as an overview of the commercial infrastructure installed globally. Some of the major recent developments and crucial technology innovations are the results of global government support for the development of algaebased bioenergy, biofuels and bioproduct applications, which have been awarded as public private partnerships and are summarized in this chapter.

The microalgae culture has a potential for obtaining bioproducts that can be used in various commercial segments. The microalgae industry generates interest in the investors who recognize good performance and suitability in the production of various microalgal products. In recent years, research projects and microalgal industries worldwide have been increased with a focus on the agriculture, aqualculture, human and animal nutrition, bulk and fine chemicals, cosmetics and biofuels. The microalgae biomass production has a limited scale, due to the problems presented in the scaling of the production process. The industrial production is dominated by open systems, allowing growth only of some microalgal species. The consolidation of microalgaebased products has been focused on improvements in the culture systems and production costs, making this industrial sector more competitive.

Nitrogen (N) and Phosphorus (P) in the wastewater are economical nutrient sources for algae growth. N and P can be assimilated into biomass. The mechanisms of N and P utilization by microalgae was discussed in this chapter. Diverse reactor configurations used for wastewater treatment and algae cultivation were developed to improve the biomass productivity and their settling ability. Enhanced algal-prokaryotic wastewater treatment systems are being designed to improve the wastewater treatment efficiency. A case study of a concentrated animal feeding operation (CAFO) integrating algae cultivation and anaerobic digestion indicated most N ad P are recycled in the form of biomass. The fixation of CO2 by microalgae can be the alternative methods for the CO2 mitigation. The CO2 mitigation rate is affected by the mass transfer and biomass assimilation. Flue gas can be used to reduce the cost of source gases and mitigate greenhouse gases emission. Membrane sparging systems provide fine bubbles and way higher mass transfer efficiency than airlift reactors. Design that improves light penetration should be considered for the scale-up of the complex membrane sparging systems.

The lipids accumulated inside microalgal biomass can be trans-esterified into biodiesel and have brought this type of feedstock to the forefront of the second generation renewable fuels research. Nevertheless, even though energy prices are generally increasing, fuel and energy products are of high volume with low value commodities and the production of microalgae-derived biofuels is not currently economically feasible when compared to fossil fuel energy. Thus, to achieve true sustainability in transferring microalgae into fuels, there is a need for integrated solutions which utilise low-value inputs to the process and exploit the full product portfolio achievable from microalgae. For example, microalgae have been shown to contain quantities of a vast array of biological materials such as exopolysaccharides, phycobiliproteins, potent antioxidants, polyunsaturated fatty acids, and mycosporinelike amino acids that have a range of high-value commercial applications in the cosmetic, pharmaceutical and nutraceutical industries. As a result, several research groups have introduced the microalgae bio-refinery approach to encapsulate this concept and maximise the full potential of an integrated microalgal production facility that harvests the high value components available and produces fuels as a secondary revenue stream.

A description of the importance of microalgae at industrial level and especially the case of Chlorella vulgaris are presented in this chapter. The cost and some existing technologies for microalgae oil production were analyzed. Furthermore, some of these technologies were selected for the design of microalgae oil production process using the simulation software Aspen Plus.

The environmental impacts associated to the use of fossil fuels have greatly accelerated the research and development of renewable fuels. This chapter reviews the technical and economic challenges for biofuel production from different feedstocks. Second generation lipid-based biomass feedstocks are considered to be cost-effective and eco-friendly for biofuel production, with microalgae being the most promising. The upgrading techniques of lipid-based feedstocks are also reviewed in terms of biodiesel and green diesel production, focusing on production chemistries and approaches, product application, and associated key technical challenges and opportunities. Due to the high polyunsaturated fatty acids (PUFAs) content, microalgae oil can be used to produce biodiesel through the insertion as a middle distillate feed stream into a hydrotreating unit of a petroleum refinery. The techno-economic assessment was performed for the production of hydrotreated algal oil (HTAO), animal feed and nutraceuticals. Base case calculations were for 10 000 barrels per day of HTAO. It was considered that nutraceuticals represented only 0.05% (w/w) of the raw algae oil. Algae doubling time has a strong influence on the sales price, which might be reduced if culture agitation is increased. Process economics can be improved taking into account the algal oil content, CAPEX, and moisture content of post-extracted algal residue. Considering the current operating parameter values and co-product credits, plant gate price was estimated by ~$10/gal. However, several improvements were also identified to enhance the process economics. The trade-off between oil content and area weight productivity favors oil content at constant oil area productivity (gal·acre-1yr-1). For the limit of 100% oil content, (i.e., no solid co-product), the sale price was estimated to be $7.90/gal.

Life cycle emissions of greenhouse gases associated with microalgal biofuels reported in peer- reviewed papers vary widely. For microalgal biodiesel, they range from -75 g CO2eq to + 534 g CO2eq MJ-1 biodiesel. Available studies on life cycle dealing with greenhouse gas emissions are consequential and subject to relatively large uncertainties. Thus, the results of life cycle assessments of specific combinations of product(s) and process(es) should be expressed as ranges rather than as one-point values. Choices about the application of credits, system boundaries, expected microalgal yields, allocation and the decarbonization of energy supply matter substantially to the results of life cycle assessments. The way emissions of methane are dealt with may also be important for estimated greenhouse gas emissions linked to microalgal biofuel lifecycles. Substantially reducing uncertainties in the outcomes of life cycle assessments awaits the availability of attributional life cycle assessments which deal with well-monitored existing commercial microalgal fuel productionconsumption chains.

Triple bottom line (TBL) accounting is a framework for assessing the sustainability performance of a product or an organization. The aim of this chapter is to present the results of a TBL assessment of algae bio-crude production. Algae are considered an ideal feedstock for biofuel production owing to their ability to grow on marginal land without the need for freshwater resources. This chapter presents an input-output based life-cycle assessment of algae bio-crude production. The following stages are considered in the analysis - cultivation of algae, extraction of bio-crude from algae, and transportation of algae bio-crude to a refinery for processing. A region in Australia ideal for the production of algae was chosen for this study. The results show that the supply chain of algae bio-crude production is more sustainable than that of crude oil production.