Energy-efficient heat exchanger networks at lower cost
Energy efficiency is central in responding to the global challenge presented by climate change. In large industrial processes, energy consumption can be made more efficient with heat exchanger networks, but this has to be done in an economically sustainable way.
In his doctoral dissertation examined at the Aalto University School of Engineering, Timo Laukkanen, D.Sc. (tech.), developed new Heat Exchanger Networks Synthesis approaches, which make the improvement of the energy efficiency of heat exchanger networks more economical and faster than previously.
̶ Industrial processes have several process streams that need to be either heated or cooled using for instance steam or electricity. If a heat exchanger network is constructed between the streams that need to be heated or cooled, such external heating is required much less than without the heat exchanger network and less energy is consumed.
Laukkanen explains that with heat exchanger networks, heat recovery can be enhanced. A real-life example of a small heat exchange network is the pre-heating of the outdoor air coming into a building using outgoing air. Building heat exchangers, however, costs money. To make the heat transfer as cost-effective as possible, the number and area of heat exchangers must be minimised when designing heat exchange networks.
̶ Cost effectiveness and energy efficiency are in direct competition, Laukkanen states. While the energy efficiency increases when the number and area of heat exchangers goes up, the cost effectiveness decreases.
Better and faster designing of heat exchanger networks
The mathematical bi-level optimisation method developed by Laukkanen helps to resolve the question of how efficient it is to construct a heat exchanger between streams in different situations. With the method it is possible to find solutions that can slightly improve both cost effectiveness and energy efficiency.
Laukkanen believes that the energy price hikes will get the industry to invest in heat exchange networks more heavily than at the moment.
̶ The design of heat exchanger networks involves a mathematically challenging problem, which is very time consuming to solve. With the method I developed, designing can be done much faster than before. As energy prices go up, the industry might become interested in using the method.
Mathematics to dig up all the options
Besides developing the bi-level optimisation method, Laukkanen applied an interactive multiobjective optimisation method to the design of heat exchanger networks. It can be used by the designers of heat exchanger networks and people, for instance industrialists, making decisions on them to find all the options on the Pareto optimal curve in relation to investments and energy efficiency.
̶ Mathematics will not answer the question of which option to apply because the choice will be made on the basis of values. Mathematics can, however, bring up all the possible options for us to look at.
According to Laukkanen, an interactive decision maker is at all times trying to decide which direction to take. If they do not want to make heavy investments they will ignore that part of the curve, looking for options in a different part. Therefore, a solution can be found without too much of a cognitive or computational load.
Timo Laukkanen’s doctoral dissertation ‘Multiobjective heat exchanger network synthesis based on grouping of process streams’ was examined at the Aalto University School of Engineering on 13 June 2012.
Aalto University Department on Energy Technology
timo.laukkanen [at] aalto [dot] fi (email@example.com