Solutions for sustainable energy systems - Analyses of district heat optimization and microalgal biorefineries

Mikko Kouhia

Research output: ThesisDoctoral ThesisCollection of Articles


Energy systems are in transition, as fossil fuels are phased out and variable renewable energy generation is introduced in increasing amounts into electricity networks. As the energy provision bears high environmental impacts, attention has to be paid into the sustainability of current and planned systems. This dissertation investigates prospective technologies in the short and long term. District heat optimization and microalgal biorefineries are selected as the focus areas: the former represents current systems that are still mostly based on fossil and biomass combustion, and the latter an identified technology option to replace fossil fuels in the traffic sector. Optimization and process modelling are employed in the analysis. District heat networks are investigated with optimization methods; the district heat network energy storage capabilities are addressed with economic optimization, and the inclusion of sustainability metrics in the design is evaluated with multicriteria optimization. Microalgal biofuel and bioproduct generation are investigated in integrated contexts with process modelling. The results indicate that district heat system flexibility can be increased by letting the district heat supply temperature vary within given boundaries. This will reduce heat generation costs and facilitate increased variable renewable energy integration. Sustainability metrics can be included in the district heat supply design, to direct the heat supply into a more sustainable direction at the least possible cost. The energy returns of various microalgal biofuel generation alternatives are compared with each other. Lipid extraction with algal residue utilization is identified as the likely best alternative. High-solids supercritical water gasification offers high energy returns, but the technology is still under development. A multi-product mill concept is reviewed, where lipids, fertilizer and methane are generated from waste activated sludge, ash and flue gas inputs, with microalgae cultivation in the process. This concept appears technically feasible. The results of this dissertation may be employed in a more efficient and environmentally sound district heat system design and operation. The results also increase the understanding of microalgal biofuel and bioproduct concepts and help to direct further research efforts in enhancing the production economics.
Translated title of the contributionRatkaisuja kestäviin energiajärjestelmiin: analyysejä kaukolämmön optimoinnista ja mikroleviä hyödyntävistä biojalostamoista
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
  • Ahtila, Pekka, Supervising Professor
  • Holmberg, Henrik, Thesis Advisor
  • Laukkanen, Timo, Thesis Advisor
Print ISBNs978-952-60-3939-8
Electronic ISBNs978-952-60-3942-8
Publication statusPublished - 2020
MoE publication typeG5 Doctoral dissertation (article)


  • district heat
  • biorefinery
  • optimization
  • modelling
  • microalgae
  • sustainability

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