Scroll expander Organic Rankine Cycle (ORC)efficiency boost of biogas engines

The most common energy application for biogas is on-site power generation using gas engine units. The introduction of a bottoming cycle based on Scroll expander Organic Rankine Cycle (ORC) technology within a biogas plant is of high interest since the excess heat produced by biogas engines can be used to produce additional power. This paper describes the retrofit of a plant consisting of a digester fed with green municipal wastes and two biogas engine units (200kWe each). The waste heat from the cooling jacket of the first biogas unit is used in cogeneration for the fermentation process and also for heating the buildings of the facility. The objective of the retrofit project is, in a first step, to convert the excess heat from the cooling jacket of the second biogas unit by means of Organic Rankine cycles. Onsite preliminary tests from the operation of the 7 kWe unit have been done, allowing performances to be measured over a broad range of conditions. Those tests confirmed a reasonable behavior of the ORC scroll expander and the interest of the concept. The measured efficiency of the ORC is about 7% with a heat source at around 90°C (i.e. 40% exergy efficiency).

Integration of Organic Rankine Cycles for Waste Heat Recovery in Industrial Processes

Matthias Bendig

Developing a methodology that allows identifying maximumelectricity production with the help of Organic Rankine Cycles (ORC) from the waste heat of an industrial process, at the lowest specific cost, without jeopardising the increase of the industrial process’s thermal efficiency. Such is the goal of this thesis. In order to reach this goal, a software tool which is able to identify the most suitable cycle for a heat source regarding electricity production and cost efficiency is developed and explained in detail. Threemain areas of research can be identified, the definition of waste heat, the analysis of experimental data with the help of data reconciliation and the identification of a suitable working fluid and operation parameters of an ORC for any given heat source. A method for identifying, characterising and quantifying the available waste heat, which can be converted into a useful form, for any industrial system, is presented. A distinction is made between avoidable and unavoidable waste heat. Combined pinch analysis and exergy analysis is used to characterise the waste heat potential of a given process. Based on the study of two industrial processes, it will become clear how the constraints of a waste heat analysis influence the outcome and the potential for the integration of ORCs. The studies illustrate how the increase in energy efficiency and degree of heat recovery and integration of a process can be contradictory to the production of electricity with ORCs. The concept of data reconciliation is needed for the measurements, collected from two ORC demonstrators. Apart from "classical" data reconciliation, a new method is presented, which consists in including parameters as virtual measurements and time dependency of measurements. This increases the redundancy of the system and thus the overall accuracy of the reconciled values. A methodology, capable of choosing the designpoint, a suitable working fluid and a cycle configuration, for the lowest specific investment cost while at the same time maximising the electricity output, for a given process environment is developed. The new methodology uses a multi objective optimisation (master optimisation) andMixed Integer Linear Programming (MILP) problem (slave optimisation). A novel approach of combining multi objective nonlinear master optimisations with single objective linear slave optimisations is introduced, increasing the reliability of the algorithm.

EPFL2015

Small Hybrid Solar Power System

Daniel Favrat, El Hadj Malick Kane, Diego Larrain

This paper introduces a novel of mini-hybrid solar power plant integrating a field of solar concentrators, two superposed Organic Rankine Cycles (ORC) and a (bio)Diesel engine. Turbines for the organic Rankine Cycles are hermetic scroll expander-generators. Sun tracking solar collectors are composed of rows of flat mirror bands (CEP) arranged in a plane, which focus the solar energy onto a collector tube similar to those used in SEGS plant in California. The waste heat from both the exhaust gases and block cooling of the thermal engine are also heat sources for the ORCs. Such units are considered to meet electricity, cooling and pumping needs of remote settlements in the sun belt areas or on special sites requiring low temperature heating such as swimming pools or greenhouses. The thermal engine guaranties a minimum level of both power and heat availability at night or during cloudy periods. Laboratory tests, made with the superposed ORCs only, confirmed adequate operational characteristics with good performances over a broad range of conditions. A few preliminary tests on the site of the solar power plant when coupled with the engine confirmed a reasonable behavior and the interest of the concept even at part load or during sharp variations of the thermal supply. The performances are satisfactory for a hybrid solar plant of this power range (10 to 25 kWe) and working so far with rather low temperature solar supply (165°C). First Law efficiencies of conversion in hybride mode is of the order of 41% when considerating only the fossil fuel input (total electrical power / LHV of the fuel). This already represents an efficiency increas of close to 50% compared to the Diesel engine alone. These results are encouraging particularly when one considers that excellent prospects exist for further increasing the supply temperature. However due to an over sizing of the turbines and a solar collector efficiency lower than expected, the tests reported here were made at very partial load and the efficiency in mode "solar only" was limited to 7.74%.

2001

Small Hybrid Solar Power System

Daniel Favrat, El Hadj Malick Kane, Diego Larrain

2001

Integration of Organic Rankine Cycles for Waste Heat Recovery in Industrial Processes

Matthias Bendig

EPFL2015

Small Hybrid Solar Power System

Daniel Favrat, El Hadj Malick Kane, Diego Larrain

2001