Methodology using process integration for identifying suitable Organic Rankine Cycles for waste heat valorisation

The identification of a suitable combined electricity production cycle depends on the heating and cooling requirements of the respective industrial process. Due to the large number of options an exhaustive enumeration of all the possibilities is not realistic. An optimisation based approach has to be adopted; it should be multi-objective and non-linear. The Methodology proposed is systematically generating a list of competing options, it follows the logic: - Analyse – Identifying heating and cooling requirements of the process. - Generate – Generation of options and scenarios how requirements can be satisfied. - Evaluate – Classify the generated options. During the different steps, several models are used. In order to identify the heating and cooling requirements of the process, Pinch analysis and the integration of the process have to be applied. The generation of the various options is done by a number of models. First an evolutionary algorithm fixes the decision variables for one option within the pre-defined constraints; a flow sheeting tool is used to calculate the thermodynamic states with the help of thermodynamic models. From the results, thermodynamic key performance indicators are calculated. A further model calculates the optimal thermal integration of the cycle configuration into the process; the results of this integration are used by sizing models to calculate the sizing of needed equipment. With the help of the sizes, cost models are used in order to calculate cost indicators and environmental models are used to calculate key environmental and LCA-indicators. With all the indicators related to every single configuration, the pareto-optimal solutions in regard to the pre-defined objectives are identified. As decision variables we identify the thermodynamic cycle configurations, in which the working fluid, fluid mixtures and compositions, multi stage or single stage cycles are proposed. In addition a second level of continuous decision variables, define the operating conditions for the selected cycle configuration. In addition to the objective function a set of performance indicators is proposed for the choice of the cycle as a decision support: 1. Quantitative indicators like efficiency (first and second principle), cost, CO2-equivalents, ozone depletion potential (ODP), LCA-Indicators 2. Qualitative criteria like flammability, toxicity

Thermo-Economic Optimisation of Solar Tower Thermal Power Plants

James Spelling

Amidst a backdrop of rapidly increasing world-wide electricity demand, solar thermal power generation shows great potential for supplying the electricity needs of numerous countries in the Sun-belt regions of the world. In these regions, the absence of significant biomass, hydrological or geothermal reserves makes solar thermal power the most promising solution for meeting that most fundamental of demands: energy. Amidst the different options available for harnessing the Sun’s power, solar thermal technologies have been shown capable of producing electricity at the most economically viable rates [32]. However, all currently existing solar thermal power plants have been based around the use of Rankine, or steam turbine, cycles which are limited in the efficiencies they can achieve. In order to make better use of the Sun’s energy, higher efficiency cycles should be used. With their high concentration ratios, solar thermal power tower systems are amongst the best options for making use of the Sun’s potential. As the energy delivery temperature of central receiver systems is higher, they harness the solar radiation at a higher exergy level [4]. This higher temperature also opens the road to the use of more advanced thermodynamic cycles. Hybrid fossil-fuel – solar systems are also imaginable [13], as the energy delivery temperature is compatible with standard gas turbine cycles. Hybrid systems can help mitigate the requirements of thermal energy storage, as well as reducing the perceived risk when investing in new solar technology, which stimulates research and development, as well as providing employment. Recent developments in the field of high temperature volumetric receivers [10] along with rock- based packed-bed storage systems have opened up an interesting possibility. High temperature receivers allow the use of higher-efficiency combined-cycle setups, whereas packed-bed units offer the possibility of cheap storage. Larger storage volumes allow pure-solar systems to extend their power production into the night, making hybrid options less attractive. This is compounded by the common practice of guaranteeing a preferred tariff for electricity sales from pure-solar sources With this in mind, a complete dynamic model of a power plant based on the pure-solar concept has been elaborated. The performance of the setup is evaluated over a range of days, using solar insolation profiles obtained from satellite data. In order to examine different design options and their impact on the performance of the power plant, a multi-objective, thermo-economic optimisation of both the power plant superstructure and operating conditions was performed using the new, dynamic models. By means of an evolutionary algorithm [17], a family of Pareto-optimal points were obtained, representing the trade-off between increased power plant efficiency, and lower levelised energy costs. Through use of optimisation, it was shown that exergetic efficiencies in the region of 21-27% can be achieved, for relatively low power outputs of between 3 and 11 MWe. These configurations correspond with levelised electricity costs in the region of 14-21 UScts/kWhe. In most regions, the use of solar power is encouraged by the guarantee of a preferential tariff for electricity sales from renewable sources. Sale of electricity at a price of around 24 UScts/kWhe [32] should therefore be imaginable, ensuring the economic viability of solar thermal power tower systems. It can be concluded that, when properly designed, solar thermal power plants based on combined cycles are both economically and thermodynamically promising. By raising the efficiency of the plant, the size of the solar collector field is diminished, increasing the otherwise low energy densities of solar power systems. By reducing the levelised electricity costs, solar thermal power plants become more economically viable, accelerating their construction and thus, hopefully, reducing our dependence on fossil-fuels.

2009

THEORETICAL STUDY OF THERMALLY DRIVEN HEAT PUMPS BASED ON DOUBLE ORGANIC RANKINE CYCLE: WORKING FLUID COMPARISON AND OFF-DESIGN SIMULATION

Jonathan Demierre, Daniel Favrat

This study deals with a type of thermally driven heat pumps that consists of a reverse Rankine heat pump cycle, the compressor of which is driven by the turbine of a supercritical Organic Rankine Cycle (ORC). The application is residential heating (domestic hot water and floor heating) of small buildings requiring a cumulated thermal power of approximately 40 kW. An approach that enables the comparison of performance that can be expected with different working fluids is presented. It appears that R134a, R1234yf, R227ea and R236fa are among the best candidates. An off-design simulation tool of such thermally driven heat pumps has been implemented that includes both turbine and compressor off-design models. This thermally driven heat pump model has been used to predict the performance of an existing experimental prototype about to be tested.

2011

Thermo-Economic Optimisation of Solar Tower Thermal Power Plants

James Spelling

2009