Just add boiling water...Direct Steam Generation using Parabolic Dish Collectors

A large percentage of the world's electricity is produced by steam turbines. Steam can be generated by a boiler that uses a heat source such as a coal furnace, a nuclear reactor or the Sun.Solar Thermal Power technology concentrates the light of the sun to generate large amounts of heat to use in steam generation and other industrial processes.

Introduction

Due to the varying nature of the Sun as a heat source, it is important to model and understand the transient behavior of the water boiling process inside the steam cavity receiver.Inputs such as solar irradiance, mass flow, ambient temperature and wind, are used to calculate the heat transfer from the receiver pipes to the water in them.

Dynamic Modeling of the Steam CavityReceiver

Find out more at http://solar-thermal.anu.edu.au

A 500 m² parabolic dish concentrator at the Australian National University is fitted with a mono-tube steam cavity receiver. Water is pumped into the cavity receiver and steam is obtained at 500 °C and 4.5 MPa which can be used to power a steam turbine.Although many solar thermal power plants around the world use special heat transfer fluids, direct steam generation is actively under research [1],[2] in an effort to reduce overall plant costs and complexity.

Direct Steam Generation at the ANU

In Solar Thermal Power plants, the mass flow of fluid sent to the receiver is the main variable to control the conversion from solar energy to thermal energy. For a fixed amount of solar radiation, a greater mass flow results in a lower output temperature and vice versa.The main purpose of this work is to develop a strategy to manipulate the mass flow of water to obtain a stable output steam temperature. Conditions such as short cloud coverage (a few minutes) and start up in the morning are of great interest. Longer term variations in the system such as mirror soiling are also considered, which requires a degree of robustness of the control algorithm.

Controlling the Steam Output

[1] S. Bendapudi, J. E. Braun, and E. A. Groll. A comparison of moving-boundary and finite-volume formulations for transients in centrifugal chillers. International Journal of Refrigeration, 31(8):1437 – 1452, 2008.[2] W. M. Conlon. Compact linear Fresnel reflector technology: Advances in superheated steam and applications for augmenting fossil-fired power generation. In Proceedings of the SolarPACES conference, 2010.[3] T. L. McKinley and A. G. Alleyne. An advanced nonlinear switched heat exchanger model for vapor compression cycles using the moving-boundary method. International Journal of Refrigeration, 31(7):1253 – 1264, 2008.[4] P. Meduri, C. Hanneman, and J. Pacheco. Performance characterisation and operation of esolar’s sierra sun tower power tower plant. In Proceedings of the SolarPACES Conference, 2010.[5] H. Tummescheit. Design and Implementation of Object-Oriented Model Libraries using Modelica. PhD thesis, Department of Automatic Control, LundInstitunte of Technology, August 2002.[6] J. Zapata, K. Lovegrove, and J. Pye. Steam receiver models for solar dish concentrators: Two models compared. In Proceedings of the SolarPACESConference, 2010.

References

Jose Zapata, Keith Lovegrove, John Pye, Greg Burgess, Solar Thermal Group, Australian National University

Feed water Tank Feed water Pump

Cooling Towers

Dish ConcentratorArray

Steam CavityReceiver

Sun

SteamTurbine/Engine

Generator

PowerGrid

A mathematical derivation using a technique called 'moving boundary formulation'[3] is used to model the behavior of the steam cavity receiver. A switching approach is incorporated [4],[5] to simulate the transition from liquid water to boiling steam.The advantage of this model versus finite element models is its simplicity and the fact it produces a set of ordinary differential equations that are suitable for control theory.The readily available 500 m² ANU dish is a great tool for the experimental validation of this model and to test its suitability for Solar Thermal Power systems.

Figure 1: Diagram of a Solar Thermal Steam Power Plant

Figure 2: Moving Boundary concept for a cavity receiver pipe

Figure 3: Cavity Receiver model response to a 5% increase in feed water mass-flow

Future work includes the selection and design of an appropriate control algorithm, a system simulation of the controlled system and experimental trials on the ANU 500 m² dish.

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