The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.

Modelling Biomass-Fuelled Small-Scale CHP Plants for Process Synthesis Optimisation

Tuula Savola

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Mechanical Engineering for public examination and debate in Auditorium K216 at Helsinki University of Technology (Espoo, Finland) on the 15th of June, 2007, at 12 noon.

Overview in PDF format (ISBN 978-951-22-8802-1)   [1196 KB]
Dissertation is also available in print (ISBN 978-951-22-8801-4)


In this work possible process improvements for biomass-fuelled small-scale combined heat and power (CHP) plants are evaluated and a new mixed integer nonlinear programming (MINLP) model for process synthesis optimisation of these processes is presented. Small-scale (1-20 MWe) CHP plants are of interest, as in Finland the potential to increase the CHP production is in converting small heating units to CHP production. However, the profitability of these small-scale CHP investments should be higher than today. The small-scale CHP plants are usually operated according to the heat demand of a district heating network or an industrial process. Thus, the possibility to increase the profitability of these plants is in improving their power-to-heat ratios (α) or electrical efficiencies (ηe).

The possibilities to increase the power production of small-scale CHP plants are studied here with simulations and optimisation. Especially, a superstructure of the possible process improvements and a MINLP model including the special properties of small-scale CHP plants is developed for the process synthesis optimisation. Unlike previous models of CHP processes, the model includes the modelling of pressures and steam and water property functions that depend both on temperatures and pressures. Also, a new model for a back-pressure steam turbine is developed. This model takes into account the nonlinear efficiency changes in the regulation stage of the turbine, the changes caused by the exhaust losses at the end of the turbine, and the dependence of pressure on the steam mass flow through the turbine at part loads. The part load operation of the small-scale CHP plants is incorporated into the model with multiperiods.

With the developed simulation and optimisation models the profitability of the process changes in small-scale CHP processes based on existing plants are evaluated. With the addition of a steam reheater, a feed water preheater, and a two-stage district heat exchanger the simulation and optimisation models found profitable processes where α is increased from 0.23-0.50 to 0.45-0.50, depending on the size of the plant. Similarly, ηe is increased in a profitable way from 0.17-0.30 to 0.28-0.30. If also natural gas is used as fuel and a gas engine integrated to the process, the efficiencies are further improved. In general, the process alternatives and model formulations presented here can be useful in the design and planning of new efficient small-scale CHP processes. Some of the model formulations can be utilised also in the modelling of other energy related processes with similar challenges as in the modelling of small-scale CHP plants.

This thesis consists of an overview and of the following 5 publications:

  1. Savola, T. and Keppo, I. 2005. Off-design simulation and mathematical modeling of small-scale CHP plants at part loads. Applied Thermal Engineering 25 (8-9): 1219-1232.
  2. Savola, T. and Fogelholm, C.-J. 2006. Increased power to heat ratio of small scale CHP plants using biomass fuels and natural gas. Energy Conversion and Management 47 (18-19): 3105-3118.
  3. Savola, T. and Fogelholm, C.-J. 2007. MINLP optimisation model for increased power production in small-scale CHP plants. Applied Thermal Engineering 27 (1): 89-99.
  4. Tveit, T.-M., Savola, T. and Fogelholm, C.-J. 2005. Modelling of steam turbines for mixed integer nonlinear programming (MINLP) in design and off-design conditions of CHP plants. In: J. Amudsen, H. I. Andersson, E. Celledoni, T. Gravdahl, F. A. Michelsen, H. R. Nagel, T. Natvig (eds.), Proceedings of the 46th Conference on Simulation and Modeling (SIMS 2005), 13-14 October 2005, Trondheim, Norway. Trondheim, Tapir Academic Press: 335-344.
  5. Savola, T., Tveit, T.-M. and Fogelholm, C.-J. 2007. A MINLP model including the pressure levels and multiperiods for CHP process optimisation. Applied Thermal Engineering 27 (11-12): 1857-1867.

Keywords: energy systems, CHP, MINLP, modelling, simulation, optimisation

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© 2007 Helsinki University of Technology

Last update 2011-05-26