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Thermal Energy Storage for Solar Powered ORC Engines steam quality. Both the model of the dry condenser and of the stratified storage tank embed an ideal pressure controller, i.e. able of maintaining always the imposed pressure value without any dynamic characteristic. This is appropriate for the level of detail of the present study, where the main focus is on the dynamics of the temperatures in the solar field and in the storage tank. The control strategy selected in this preliminary study aims at keeping the temperature at the outlet of the SF (Tout,SF) close to the nominal value under transient conditions. This ensures that the storage tank is always loaded from the top with fluid at the design temperature, thus avoiding as much as possible mixing phenomena that could reduce the efficiency of the downstream ORC system. The open-loop dynamic response of Tout,SF to variations of the pump flow rate strongly de- pends on the value of DNI: at low irradiation, the flow rate must be reduced to keep the outlet temperature constant, so the system dynamics become slower. However, the analysis of a linearized simplified model of this system (which is beyond the scope of this chapter) shows that its frequency response does not change much with DNI in a frequency range slightly above τ-1, where τ is the residence time of the fluid in the solar field at nominal DNI. This allows to tune a fixed-parameters proportional-integral (PI) controller with a crossover frequency ωc = 2τ−1 in that particular fre- quency range. In order to further improve the control performance, feed-forward compensation of the effects of DNI has been added to the controller output. The computation follows the assumption of negligi- ble heat losses from the SF to the ambient; this of course relies on the possibility that DNI readings from an accurate pyrheliometer are available to the control system. Finally, a lower saturation limit has been applied to the controller output, in order to prevent the flow rate from becoming too small for very low DNI, which could be dangerous in case of abrupt solar irradiance increases such as, e.g., when a cloud leaves the field. For the present study, the low limit is set at 1.0 kg s−1, with the nominal value being 2.7 kg s−1 and the maximum value being 4.5 kg s−1. 4.5.7 Dynamic Analysis Results The complete model introduced in sec. 4.5.5, and controlled according to the scheme described in sec. 4.5.6, is used to study the dynamic performance of the case-study plant working as outlined in sec. 4.5.1. As anticipated, the main goal is to assess if the whole system can be safely and efficiently operated through automatic control procedures. From the point of view of safe operation, the main concern regards the possibility of thermal decomposition of the working fluid to occur: for D4 the limit is close to 400◦C =Tmax [16]. Due to the favourable properties of silicon oils, the corresponding heat transfer coefficient is large enough to prevent, under all the foreseeable operating conditions, the wall temperature to exceed Tmax [46]. However, as a consequence of the adopted control strategy, a drop in the mass flow circulating in the SF follows a reduction of the solar input: a subsequent sharp increase in the DNI may thus cause the limit of Tmax to be exceeded somewhere along the absorber, on the internal wall surface (Twall). From the efficiency point of view, keeping the outlet temperature from the SF Tout,SF close to the nominal value allows to preserve the stratification in the storage vessel: the turnaround efficiency of the TES system is consequently increased, and the power block can be operated in conditions close to the design ones for a larger number of hours. The virtual plant is thus tested under a situation representative of extreme working conditions [65], whereby a series of clouds (3 in this example) causes the solar input to periodically drop, 103PDF Image | New Concepts FOR Organic Rankine Cycle Power Systems
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