Doerte Laing, Wolf-Dieter Steinmann, Rainer Tamme, Christoph Richter DLR — German Aerospace Centre — Institute of Technical Thermodynamics

Introduction

Economic thermal storage is a technological key issue for the future success of solar thermal power plant technologies. Storage technology becomes an urgent issue for the successful implementation of commercial solar thermal plants in the context of an accelerated promotion of such plants in those EU member countries, which are relying on solar power to reach the community goal of doubling the share of renewable energy in the EU energy balance from 6 % today to 12 % in 2010.

Without storage, a solar plant in Southern Europe will operate only 800 hours in real full load and over 2400 hours in part load, yielding about 2000 equivalent full load hours or a solar fraction of about 25% in a base load solar steam plant. With 5-6 hours of storage capacity, a solar plant in Southern Europe will operate about 4000 hours in real full load and reduce its part load operation to 400 hours. Due to improved part load efficiency and doubled utilisation of the power block, solar thermal production costs can be reduced with storage by up to 20%. Larger storage capacities may lead to further reduced costs.

At DLR different storage concepts are developed for application in commercial parabolic trough power plants:

> solid media sensible heat storage for parabolic trough power plants using thermal oil as heat transfer medium in the solar collectors

> phase change energy storage systems for direct steam generation in parabolic troughs

> steam accumulators as buffer storage system for compensation of fast transients in parabolic trough power plants

The temperature range for these storage concepts is between 150°C and 450°C.

Solid media sensible heat storage for thermal oil heat transfer media

Solid media sensible heat storage units have been developed in the project WESPE [1], funded by the German Government from December 2001 until December 2003. The focus of this project lay on the development of an efficient and cheap sensible storage material, on the optimization of the tube register heat exchanger and on the demonstration of this technology with a 350 kWh test unit.