14:00
Session 4D: Apps and Energy sources
Chair: George Kosmadakis
14:00
20 mins
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Comparison between Direct and Indirect ORCs for CSP Applications
Daminao Perusi, Marco Astolfi
Abstract: Concentrating solar power is still a niche market for ORCs counting several small applications but only few large scale plants. The main limit of solar ORCs with respect to PV systems is related to the complexity of this technology and the consequent higher specific cost. However, the possibility to adopt a thermal energy storage (TES) allows solar ORCs to providing dispatchable solar energy and to be possibly competitive with large PV systems integrated with electrochemical storage. The aim of this work is to define the best solar ORC power plant configuration for different maximum solar field temperatures by comparing conventional indirect power plants against non-conventional direct systems. Conventional indirect cycles consists of a solar field where the heat transfer fluid is heated up to maximum around 400°C, hot oil can be stored in the TES or used directly as heat input for the ORC generally designed as a subcritical saturated cycle. On the contrary, in direct cycles the working fluid flows directly into the solar field and stored in liquid phase. Hot working fluid is then throttled (partially or completely) and vapor fraction is expanded in a turbine. This configuration allows avoiding the HTF/working fluid heat exchanger possibly leading to a lower investment cost and higher efficiency. A Matlab code have been implemented in order to carry out the thermodynamic optimization of both indirect and direct solar ORCs. Indirect systems are optimized varying the evaporation and condensation pressure and the turbine inlet temperature while direct systems by varying both the condensation and the flash pressure. Both recuperative and non-recuperative configurations are investigated considering a large number of working fluids from Refprop database. The optimal working fluid is selected considering the system efficiency as figure of merit but optimal power plant selection considers also the size of the storage system. Respect to the scientific literature some novel aspects are implemented: (i) a model of the solar field in order to link pressure drops to fluid properties, (ii) a turbine efficiency dependent on the expansion volume ratio and (iii) an extensive sensibility analysis on different assumptions like minimum pressure and components performance.
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14:20
20 mins
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Flexible PVT-ORC Hybrid Solar-biomass Cogeneration Systems: the Case Study of the University Sports Centre in Bari, Italy
Kai Wang, Antonio Pantaleo, Oyeniyi Oyewunmi, Christos Markides
Abstract: The thermoeconomic feasibility of a hybrid solar-biomass renewable cogeneration systems based on a wood-chip boiler, photovoltaic-thermal (PVT) collectors and an organic Rankine cycle (ORC) engine is investigated for combined heat and power (CHP) provision in a sports-centre application. The PVT-based CHP (PVT-CHP) subsystem, integrated with thermal energy storage, is designed to meet most of the energy demands of the facility at high solar-irradiance conditions, while the biomass ORC-based CHP (ORC-CHP) subsystem, driven by a wood-chip boiler, is used to compensate the intermittent solar energy and match the onsite energy demand. A technoeconomic model is proposed to optimise the hybrid cogeneration system design and operation. Annual energy simulations are conducted in a case study focused on the provision of electricity, space heating, swimming pool heating and hot-water supply to the University Sports Centre (USC) of Bari, Italy. The size of the ORC engine is found to be critical to the performance of the PVT-ORC cogeneration system. With an installation area of 4,000 m2 for the PVT collectors and an ORC engine size of 40 kWe/310 kWt, the hybrid solar-biomass cogeneration system can provide 100% renewable energy supply to the USC with a payback time of 11.5 years, compared to 12.3 years for the solar-only PVT-CHP system. Although the biomass-only ORC-CHP system has much shorter payback time (5.0 years), the electricity output is insufficient to match the demand, accounting for only 27% of the electricity consumed onsite. This work shows that neither the solar-only nor biomass-only systems can provide full renewable energy supply to the USC, while their hybridisation makes this target attainable, with a moderate payback on investments.
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14:40
20 mins
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Thermodynamic and Economic Analysis of Geothermal Combined Heat and Power Based on a Double-stage Organic Rankine Cycle
Tim Eller, Florian Heberle, Dieter Brüggemann
Abstract: Geothermal energy is a suitable low-carbon energy resource for heat and power production. The techno-ecological potential for pure power production in Germany is estimated to 8.5 GW or 63.75 TWh/a. For low enthalpy reservoirs, the thermal power of the geothermal fluid is usually converted to electricity by binary power plants like Organic Rankine Cycle (ORC) power systems. Previous investigations show that the efficiency and the profitability of these power plants can be increased by an additional heat supply. The aim of this study is to investigate different concepts based on a double-stage Organic Rankine Cycle for geothermal combined heat and power (CHP) production.
The evaluation is based on annual return simulations, which are performed according to VDI 4655 [1]. For modelling and simulation the software Dymola [2] is used. The built-up model is based on the ThermoCycle library [3] and the fluid properties are calculated by the software CoolProp [4]. The operation mode of the combined heat and power plant is heat-driven, which means that the heat demand must be covered at any time by the geothermal resource. In this context, heat demand profiles are developed based on operational data of a real geothermal heat plant. Regarding the heat supply, a parallel configuration of ORC and district heating network (DHN) is considered. For the DHN, a supply temperature of 90 °C and a return temperature of 60 °C is assumed. For the peak load of the DHN a sensitivity analysis is conducted.
The results show that the efficiency and the profitability of the power plant can be significantly increased by supplying heat to a DHN with a peak load of 5 MW compared to the pure electricity production. By increasing the peak load of the DHN to 10 MW the average annual exergetic efficiency of the energy system can be further increased by 4.3 % to 51.7 %. In addition, the annual return is 1.6 million Euro higher than for a 5 MW DHN and 3.5 million Euro higher than for power production.
In ongoing work, the effect of potential future scenarios, like revision of feed-in tariffs, are economically evaluated.
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15:00
20 mins
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Optimization of System Operation and Heat Exchanger Sizing in Rankine Cycles - a case Study on Aluminium Smelter Heat-to-power Conversion
Monika Nikolaisen, Trond Andresen
Abstract: In industry processes worldwide, large amounts of heat in the temperature range 125-250 °C are rejected to the ambient. Direct re-use or upgrading is the most efficient and cost-effective utilization of this surplus heat. However, lack of sufficient local heat demand can make heat-to-power conversion an attractive option for significant surplus heat utilization. Cost-effective heat-to-power conversion is challenging in the temperature range considered and optimizing system design and operation is therefore important.
This work describes optimization of heat exchanger sizing and system operation of an ORC applied to heat-to-power conversion from aluminium smelter off-gas. The scenario involves 850 kW heat duty available at a temperature of 150°C. The overall system impact of considering heat source pressure drop was investigated in detail, as well as optimal distribution of heat transfer surface area between individual heat exchangers (HXs) in the system.
The ORC system model is relatively detailed, including geometrically described HXs discretized along the flow direction with local evaluations of fluid states, heat transfer coefficients and pressure gradients. The system was optimized with net power as objective function and by simultaneously optimizing process conditions and HX geometries. The total HX surface area was constrained to 750 m2 during optimization.
The effect of heat source pressure drop was investigated by calculating the fan work corresponding to the heat source HX pressure drop. Optimizing the system without accounting for the heat source pressure drop resulted in a net power production of 118 kW. Re-optimizing the system with a fan work penalty on pressure drop reduced net power by 21 %. The reduction was mostly caused by a lower heat transfer coefficient and higher exergy destruction in the heat recovery heat exchanger (HRHE).
Optimizing heat transfer area distribution showed that the HRHE should account for 68 % of total HX area to maximize net power in a scenario with no penalty on heat source pressure drop. The large HRHE area requirement was caused by a poor heat transfer performance on the gas side of the HX. The fan work penalty on heat source pressure drop further increased HRHE area requirement to 76 % of total HX area.
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15:20
20 mins
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Solar Thermal Energy Driven Organic Rankine Cycle Systems for Electricity and Fresh Water Generation
Nishith B Desai, Henrik Pranov, Fredrik Haglind
Abstract: For small to medium scale dispatchable (on demand) power and fresh water generation, solar photovoltaic with battery storage using reverse osmosis and diesel generator based systems can be used. Both options are costly, and in addition, while the former generates highly saline wastewater, the latter results in the production of greenhouse gasses. A more attractive option is, therefore, to use systems driven by solar thermal energy consisting of solar collectors, a power cycle and a thermal energy driven fresh water generation system. However, currently used concentrated solar power technologies (parabolic trough collector, linear Fresnel reflector, solar power tower) use heavy and very expensive glass mirrors and receivers. Recently, a novel polymer foil-based concentrating solar collector system, which avails the advantages of low installation cost, two-axis tracking and low operation and maintenance cost, has been proposed. A techno-economic analysis of a foil-based concentrating solar collector powered organic Rankine cycle based electricity and thermal energy driven fresh water generation system is presented in this paper. Specifically, the objective is to identify which is the more appropriate working fluid for such plant. The results indicate that cyclopentane is the more appropriate organic working fluid, compared to n-pentane, isopentane, hexamethyldisiloxane and toluene, for foil-based solar power plants.
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