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The sun is the only source of renewable energy available to us, if geothermal energy is not taken into account. In the form of radiation (UV light, visible light, infrared light, Section 1.1) it sends us annually 178,000 terawatts (1 TW = 10¹² W; unit of power 1 W = 1 J s^–1 = 859.85 calories per hour), that is to say 15,000 times the energy consumed annually by humanity. Only 0.1% of the solar energy received by planet Earth is converted into plant biomass, i.e. 100 × 10⁹ tons per year which corresponds to ca. 180 × 10⁹ tons per year of CO₂ captured from the atmosphere. This CO₂ returns to the biosphere after the death of the plants. Consumption of fossil carbon emits ca. 35 × 10⁹ tons of CO₂ yearly. Biomass is the material produced by all living organisms (plants, animals, microorganisms, fungi)…

Recently, there has been an increase in public awareness on the health, environmental and safety hazards associated with the use of toxic organic solvents in food processing. The high cost of organic solvents, the increasingly stringent environmental regulations and the new requirements from the medical and food industries for ultrapure and high-added-value products have pointed out the need for the development of new and clean technologies for the processing of food products. On the other hand, the 12 principles of green chemistry emphasize the importance of sustainable processes, which minimize waste generation and energy consumption. The compliance of these concepts will lead to a cleaner environment and a more cost-effective use of resources. Thus, the solution passes by the development of more sustainable technologies that significantly increase the efficiency of chemical processes and reduce the operation time of such technologies. Conventional extraction techniques for solid vegetable matrices include Soxhlet, hydrodistillation, shake-flash methods, maceration and pressing. These methods present many disadvantages since they use toxic solvents, with the associated risks for human health and the environment, are time consuming, labor-intensive, present low yield and selectivity, and have high energy consumption. Therefore, the design of green, efficient and sustainable extraction techniques is mandatory. Thus, the green extraction processes should consider the minimization of the use of toxic solvents, reduce waste production, avoid the presence of toxic solvents in the final product and lower the energy consumption of the processes. Among the several techniques complying with these requirements, supercritical fluid extraction (SFE) is considered one of the most suitable for the extraction of compounds from vegetable matrices. Supercritical CO₂ extraction has provided an excellent alternative to the extraction and isolation of valuable compounds from natural products. In this section, SFE studies of compounds from microalgae and aromatic plants are described in more detail. Some of the most interesting experimental results obtained at the Experimental Thermodynamic Laboratory at the Technical University of Lisbon are presented and discussed.

Nannochloropsis gaditana is recognized as a source of valuable pigments and polyunsaturated fatty acids (PUFAs), in particular eicosapentaenoic acid (EPA, 20:5ω3). The duration of the light period is an important environmental factor that affects the production yield in photosynthetic microorganisms. This factor was found to determine the growth and biochemical composition of this microalga strain. Cell division occurs during the first hours of the light period. Whereas carbohydrates are accumulated in the light period and consumed at night, the cellular concentration of chlorophylls and proteins does not vary significantly. The storage lipids are consumed in the lag phase (after dilution of the culture) and they increase until the final growth period. The structural lipids reach a maximum at the end of the daily exponential phase, when the cells also reach a maximum density, in a semi-continuous culture with a daily replacement of fresh medium.

Carotenoids synthesis takes place in the chloroplast but is catalysed by enzymes encoded by nuclear genes. These enzymes are thus synthesised in the cytoplasm as precursor polypeptides with an amino-terminal extension, the transit peptide (tp), and targeted to their final location in the chloroplast. β-carotene oxygenase (BKT) mediates the addition of keto groups to the position C4 of β-ione-rings of carotenoids. We studied the effect of the transit peptide sequences of Rubisco small subunit (RbcS2), Ferredoxine (Fd) or Phytoene desaturase (Pds) from Chlamydomonas corresponding proteins and the absence of tp on the functionality of exogenous bkt1 in transgenic Chlamydomonas reinhardtii. RbcS, Fd and Pds are targeted to the chloroplast stroma, intertylakoid lumen and tylakoid membrane respectively. Our results indicate that the presence of a chloroplastic transit peptide is essential for the efficient production of ketocarotenoids in transgenic strains of the microalga Chlamydomonas reinhardtii.

Nannochloropsis oculata, a unicellular green microalgae has become a new prospective for biofuel feedstock which offers a long term of sustainability and energy security. N. oculata accumulate high oil content which is about 28.7-29% oil by biomass weight with the daily oil production of 25.8 mg/day. To enhance the economic feasibility of microalgal-based biofuel production, it is necessary to improve both biomass productivity and lipid content. This paper reviews previous works on microalgae undergo mixotrophic conditions and nitrogen deficient conditions in order to observe the biomass productivity and lipid content for biofuel production. Many microalgae are capable of using many types of metabolism, such as photoautotrophic, heterotrophic, and mixotrophic. In heterotrophic condition, microalgae growth in the absence of sunlight derives energy completely from outsource organic carbon. While in mixotrophic condition, microalgae obtain energy from both photosynthesis reaction in the presence of sunlight and outsource organic carbon. Two stage culture system, is proposed where N. oculata will be grown in various carbon sources such as glucose, glycerol and starch in the first stage and nitrogen deficient condition in the second stage for lipid accumulation.

The pyrolysis characteristics of Chlorella vulgaris (C. Vulgaris) (one kind of microalgae) by microwave oven are studied. The microwave pyrolysis of C. Vulgaris is carried out under different dosages of C. Vulgaris and different contents of activated carbon. The products and pyrolysis temperature are analyzed in order to obtain the optimal conditions. The results indicate that with increasing the dosage of C. Vulgaris, pyrolysis temperature drops and pyrolysis time increases, the average heating rate has the decreasing tendency. The maximum bio-oil yield and gas yield are achieved under 30g of C. Vulgaris and 25g of C. Vulgaris, respectively. The optimal content of activated carbon is 5% with the maximum gas yield of 60.25% and the largest weight loss of 87.47%.