The off-gases discharged by various industrial processes, such as power stations and smelters, contribute to a large percent of the atmospheric CO₂ emissions. Moreover, large amounts of heat generated in the processes are discharged in off-gases. This heat is normally wasted and dissipated into the environment. In the past decade, using off-gases that are rich in CO₂ to enhance the growth of microalgae in order to produce biodiesel and help mitigate CO₂ emissions has aroused growing interest globally. However, there has been little or no discussion on also exploiting the heat in the gas stream for algae growing. Generators of off-gases are often located in regions, like Northern Europe and Canada, which experience long winter periods and extreme weather condition which might be very difficult to maintain a suitable cultivation pond temperature for microalgae growth all year around. Therefore, utilizing recovered heat from the off-gas to help maintain the water temperature in microalgae ponds is of significant importance and represents a novel approach to exploit this otherwise waste energy.

This study includes schemes for a smelter for using a heat exchanger that collects waste heat from the roaster off-gases and transfers it to a microalgae pond heating system to maintain a suitable water temperature for a whole year. A mathematic model is developed for describing the dynamic change of water temperature in the microalgae cultivation pond. A commercial program Matlab® was used for programming and simulations were carried out to evaluate pond temperatures in response to changes in heat transfer pipe diameter, length and pipe material;​ heat exchanger efficiency;​ heating fluids alternatives and inlet flow-rate.

The results demonstrate the feasibility of significantly extending the operation of a microalgae production facility through the year beyond what would be possible without the use of the off-gas.