Relevance of heat transfer and heat exchangers for development of sustainable energy systems

There are many reasons why heat transfer and heat exchangers play a key role in the development of sustainable energy systems as well as in the reduction of emissions and pollutants. In general, all attempts to achieve processes and thermodynamic cycles with high efficiency, low emissions, low costs include heat transfer and heat exchangers to a large extent. It is known that sustainable energy development can be achieved by three parallel approaches : reduce final energy consumption, improve overall conversion efficiency and make use of renewable energy sources [1]. Final energy consumption can be reduced through process integration, because process industry remains one of the biggest consumers of energy. A typical process consists of a chemical plant, a utility plant and a heat recovery network. Maximizing heat recovery in the heat recovery network can bring down the total energy consumption. Therefore, process integration is important, and it requires the optimization of the heat exchanger networks. Advanced heat changer technologies can improve their efficiency, including compact heat exchangers, multi-stream heat exchangers, heat transfer enhancement, mini- and micro- heat exchangers, etc. By doing so, current processes can be improved and the final energy demands can be reduced. There are many different ways to improve the conversion efficiency, but heat transfer and heat exchangers play a significant role in all means. Increasing the efficiency in thermal power plants requires an increase of the highest temperature in the process. To enable the materials of the equipment, e.g., in combustors and turbine blades in gas turbine units, to withstand such high temperatures cooling is needed. Also to achieve high thermodynamic efficiency, heat exchangers like recuperators and intercoolers are needed. The attempt to provide compact, efficient heat exchangers and at the same time allow a cheap and relatively simple manufacturing technique is a real challenge for research. In addition, a lot of research efforts are nowadays spent on fuel cells of various types in hybrid power conversion cycles. Several unique heat and mass transfer problems exist but have not been treated adequately so far. Hydro-power, biomass, wind and solar energy are regarded as the most important renewable energy sources. When using solar energy, external heat exchangers can improve the system performance significantly instead of using internal heat exchangers. Another important heat exchanger is the photovoltaic (PV) model, which is used to collect the solar energy, Such heat exchangers are subject to natural convection or radiation in addition to forced convection, and improved prediction of the performance must be provided. Similar heat transfer and heat exchanger issues exist for other renewable energy systems, but they are not fully addressed yet. In addition, heat transfer and heat exchangers are also important in protecting the environment by reducing the emissions and pollutants. Reduction of the final energy consumption and improvement of the power conversion efficiency means less prime energy consumption, resulting in overall emission reduction. Some renewable energy sources, solar, hydro-power and wind, have no emissions at all. In addition to these indirect effects, heat transfer and heat exchangers can also have direct effects. An example is the use of exhaust gas recycling in diesel combustion engines to reduce NO_x emissions. A heat exchanger is expected here in order to keep the same engine performance. In this paper, several examples are reviewed and illustrated, to demonstrate why heat transfer and heat exchangers are important in the development of sustainable energy systems. It can be concluded that the attempt to provide efficient, compact and cheap heat transfer methods and heat exchangers is a real challenge for research. In order to achieve this, both theoretical and experimental investigations must be conducted, and modern techniques must be adopted