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With energy prices reaching historical highs, biodiesel as an alternative fuel is increasingly attracting attention. Currently, biodiesel is made from a variety of feedstocks, including pure vegetable oils, waste cooking oils, and animal fat; however, the limited supply of these feedstocks impedes the further expansion of biodiesel production. Microalgae have long been recognized as potentially good sources for biofuel production because of their high oil content and rapid biomass production. In recent years, use of microalgae as an alternative biodiesel feedstock has gained renewed interest from researchers, entrepreneurs, and the general public. The objective of this publication is to introduce the basics of algal-biofuel production and the current status of this emerging biodiesel source. Current Feedstock for Biodiesel Production Biodiesel can be made from any oil/lipid source; the major components of these sources are tricylglycerol molecules (TAGs, figure 1). In general, biodiesel feedstock can be categorized into three groups:

The Stirling engine is one of the engines that has become popular due to its many advantages. Nowadays, lack of fossil fuels has led to a lot of attention to the use of renewable energies. For this reason, solar energy has drawn a lot of attention to be used as the Stirling engine heat source. The purpose of this paper is to investigate the performance of a proposed Stirling engine in different engine speeds, pressures, and working fluids. The proposed Stirling engine is of gamma-type. This engine is simulated using the Nlog code which is written for this engine in MATLAB work space. This code was validated by experimental data in our previous works. The investigated fluids in this study are helium, nitrogen, and hydrogen. It was found that the best fluid for the proposed Stirling engine is hydrogen, and the best engine speed is 1300 rpm. Also, the higher the engine pressure, the more the output work will be.

This article aims at coupling two Stirling machines together to produce cooling power from different heat sources. Two identical Gamma-type ST500 Stirling machines are coupled together. The first machine, named Stirling engine, is powered by burning natural gas and an electrical power supply as an auxiliary starter. The Stirling engine is connected to a second Stirling machine named Stirling refrigerator by a mechanical belt. The driving force is transferred to the Stirling refrigerator using the mechanical belt. Mechanical energy is then converted to cold energy in the Stirling refrigerator. This study presents the experimental and theoretical results by examining the whole system named duplex Stirling refrigerator system. Five experiments are performed on the system at different pressures to evaluate the results. Water circulation in copper coils is used to determine the cooling flux produced by the Stirling refrigerator. Finally, the coefficient of performance (COP) is calculated. The experimental data are validated using a MATLAB-based code developed based on the presented formulas in this study. The results show that the system's maximum COP and electrical efficiency, consisting of two similar 500 W Gamma-type Stirling machines, are between 0.21% and 70.8%, respectively. Setup theoretical and experimental efficiencies are 0.24 and 0.21, respectively, at the best case.The experimental results of the duplex Stirling refrigerator system used in this study are compared with the simulation results. Finally, the experimental results of this study are compared with some other reported results from different authors on the subject of the duplex Stirling refrigerator.