Polar solar power plants – Investigating the potential and the design challenges

publishedVersio

Dynamic Modelling and Characterisation of a Solid Oxide Fuel Cell Integrated in a Gas Turbine Cycle

This thesis focuses on three main areas within the field of SOFC/GT-technology: • Development of a dynamic SOFC/GT model • Model calibration and sensitivity study • Assessment of the dynamic properties of a SOFC/GT power plant The SOFC/GT model developed in this thesis describes a pressurised tubular Siemens Westinghouse-type SOFC, which is integrated in a gas turbine cycle. The process further includes a plate-fin recuperator for stack air preheating, a prereformer, an anode exhaust gas recycling loop for steam/carbon-ratio control, an afterburner and a shell-tube heat exchanger for air preheating. The fuel cell tube, the recuperator and the shell-tube heat exchanger are spatially distributed models. The SOFC model is further thermally integrated with the prereformer. The compressor and turbine models are based on performance maps as a general representation of the characteristics. In addition, a shaft model which incorporates moment of inertia is included to account for gas turbine transients. The SOFC model is calibrated against experimentally obtained data from a single-cell experiment performed on a Siemens Westinghouse tubular SOFC. The agreement between the model and the experimental results is good. The sensitivity study revealed that the degree of prereforming is of great importance with respect to the axial temperature distribution of the fuel cell. Types of malfunctions are discussed prior to the dynamic behaviour study. The dynamic study of the SOFC/GT process is performed by simulating small and large load changes according to three different strategies; • Load change at constant mean fuel cell temperature • Load change at constant turbine inlet temperature • Load change at constant shaft speed Of these three strategies, the constant mean fuel cell temperature strategy appears to be the most rapid load change method. Furthermore, this strategy implies the lowest degree of thermal cycling, the smoothest fuel cell temperature distribution and the lowest current density at part-load. Thus, this strategy represents the overall lowest risk with respect to system malfunctions and degradation. In addition, the constant mean fuel cell temperature strategy facilitates high efficiency part-load operation. The constant turbine inlet temperature strategy proved to lead to unstable operation at low load, and thus it is considered to be the least adequate method for load change. For both the constant mean fuel cell temperature strategy and the constant TIT strategy, surge might be a problem for very large load reductions. The slowest response to load changes was found for the constant shaft speed strategy. Furthermore, this strategy leads to very low fuel cell temperatures at low loads. This in combination with a possible higher degradation rate makes the constant shaft speed strategy unsuited for large load variations. Nevertheless, operation at constant shaft speed may be facilitated by air bypass, VIGV or compressor blow off.Paper I is published with kind permission of Elsevier, Sciencedirect.co

Zero Village Bergen. Aggregated loads and PV generation profiles

The main findings can be summarized as follows: •The aggregated annual thermal load is approximately the same as the electric load (3.3 GWh) for the entire Zero Village Bergen; •The aggregated annual PV generation (2.9 GWh) covers ca. 90% of the electric load; •Even so, PV peak generation (2.9 MW) is ca. 4 times higher than the electric peak load (0.7 MW) giving a GM2 of ca. 4; •This implies that the local electric grid dimensioning capacity might be determined by the PV peak generation rather than by the peak load (depending on the choice of the thermal system and the expected load from electric vehicles, not considered here). The heating system for the Zero Village Bergen is not yet decided, since this will be the task in step three. However, the two most probable options on the design table are either an all-electric solution (with heat pumps in the buildings or at a local district heating station) or a thermal-carrier solution with a local district system (whether or not connected to the city district heating). The analysis of the energy balance and mismatch between loads and PV generation offers useful insights for the next step in the design phase: •All-electric solution If the thermal load is met by heat pumps the total electric load will be ca. 1/3 higher, assuming a seasonal COP of ca. 3 for the heat pump system, meaning that the ZEB-O target is not reached unless further generation (or load reduction) measures are considered3. The peak load can roughly be estimated at around 2 MW, giving a GM of ca. 1.5, and meaning that the local electric grid does not need to be largely over dimensioned due to the PV system. This might normally be regarded as a positive feature; • Thermal-carrier solution If the thermal load is met by a biomass based cogeneration system, this would provide at the same time a small additional load – counted in carbon emissions – and extra electricity generation, so that the overall ZEB-O goal may actually be reached. This will depend on the specific conversion factors used for biomass and electricity. The electric peak load would remain unchanged and so the GM. Having a high GM might not be a problem and may even be an advantage. It simply means that the dimensioning of the grid capacity is based on the PV peak generation in summer, while that capacity is free overnight year-round to be used for charging e-vehicles.publishedVersio

Zero Village Bergen. Aggregated loads and PV generation profiles

The main findings can be summarized as follows: •The aggregated annual thermal load is approximately the same as the electric load (3.3 GWh) for the entire Zero Village Bergen; •The aggregated annual PV generation (2.9 GWh) covers ca. 90% of the electric load; •Even so, PV peak generation (2.9 MW) is ca. 4 times higher than the electric peak load (0.7 MW) giving a GM2 of ca. 4; •This implies that the local electric grid dimensioning capacity might be determined by the PV peak generation rather than by the peak load (depending on the choice of the thermal system and the expected load from electric vehicles, not considered here). The heating system for the Zero Village Bergen is not yet decided, since this will be the task in step three. However, the two most probable options on the design table are either an all-electric solution (with heat pumps in the buildings or at a local district heating station) or a thermal-carrier solution with a local district system (whether or not connected to the city district heating). The analysis of the energy balance and mismatch between loads and PV generation offers useful insights for the next step in the design phase: •All-electric solution If the thermal load is met by heat pumps the total electric load will be ca. 1/3 higher, assuming a seasonal COP of ca. 3 for the heat pump system, meaning that the ZEB-O target is not reached unless further generation (or load reduction) measures are considered3. The peak load can roughly be estimated at around 2 MW, giving a GM of ca. 1.5, and meaning that the local electric grid does not need to be largely over dimensioned due to the PV system. This might normally be regarded as a positive feature; • Thermal-carrier solution If the thermal load is met by a biomass based cogeneration system, this would provide at the same time a small additional load – counted in carbon emissions – and extra electricity generation, so that the overall ZEB-O goal may actually be reached. This will depend on the specific conversion factors used for biomass and electricity. The electric peak load would remain unchanged and so the GM. Having a high GM might not be a problem and may even be an advantage. It simply means that the dimensioning of the grid capacity is based on the PV peak generation in summer, while that capacity is free overnight year-round to be used for charging e-vehicles.publishedVersio

Polar solar power plants – Investigating the potential and the design challenges

publishedVersio

CONTROL-RELEVANT SOFC MODELING AND MODEL EVALUATION

In this paper, a dynamic, lumped model of a Solide Oxide Fuel Cell (SOFC) is described, as a step towards developing control relevant models for a SOFC integrated in a gas turbine process. Several such lumped models can be aggregated to approximate the distributed nature of important variables of the SOFC. The model is evaluated against a distributed dynamic tubular SOFC model. The simulation results confirm that the simple model is able to capture the important dynamics of the SOFC. It is concluded that the simple model can be used for control and operability studies of the hybrid system